Skip to main content

Potential of activated microglia as a source of dysregulated extracellular microRNAs contributing to neurodegeneration in amyotrophic lateral sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is the most common form of motor neuron degeneration in adults, and several mechanisms underlying the disease pathology have been proposed. It has been shown that glia communicate with other cells by releasing extracellular vesicles containing proteins and nucleic acids, including microRNAs (miRNAs), which play a role in the post-transcriptional regulation of gene expression. Dysregulation of miRNAs is commonly observed in ALS patients, together with inflammation and an altered microglial phenotype. However, the role of miRNA-containing vesicles in microglia-to-neuron communication in the context of ALS has not been explored in depth. This review summarises the evidence for the presence of inflammation, pro-inflammatory microglia and dysregulated miRNAs in ALS, then explores how microglia may potentially be responsible for this miRNA dysregulation. The possibility of pro-inflammatory ALS microglia releasing miRNAs which may then enter neuronal cells to contribute to degeneration is also explored. Based on the literature reviewed here, microglia are a likely source of dysregulated miRNAs and potential mediators of neurodegenerative processes. Therefore, dysregulated miRNAs may be promising candidates for the development of therapeutic strategies.

Introduction

Amyotrophic lateral sclerosis (ALS) is the most severe and most common form of motor neuron degeneration in adults, with an estimated worldwide prevalence of 5 cases per 100,000 population and an incidence of approximately 2 per 100,000 individuals per year [1]. It targets the motor cortex, brainstem and spinal cord and involves the death of upper and lower motor neurons that control voluntary muscles. This results in symptoms such as muscle stiffness and twitching, limb weakness due to a gradual decrease in muscle size, and difficulty swallowing or speaking. Additionally, up to half of the patients develop frontotemporal dementia (FTD) [2] which is characterised by progressive degeneration in the frontal and temporal lobes, behavioural and personality changes and language and executive function decline [3, 4]. In most cases, during the late stages of the disease, the weakening of the diaphragm and intercostal muscles results in death by respiratory failure [5]. ALS is a heterogeneous disease in age of onset and progression rate, with a median survival time of 3 to 5 years from diagnosis [6].

Around 10% of ALS cases are due to inherited genetic mutations (familial ALS; fALS). These cases are associated with over 25 genes, of which the most common are chromosome 9 open reading frame 72 (C9orf72) [7], superoxide dismutase 1 (SOD1) [8], fused in sarcoma (FUS) [9] and transactive response DNA-binding protein 43 (TARDBP; TDP-43) [10]. The remaining 90% of cases are sporadic (sALS) and arise without any family history, but around 10% of patients with sALS have genetic mutations like those in fALS [11].

Motor neurons and surrounding oligodendrocytes in ALS spinal cord, cerebellum, hippocampus, and frontal and temporal cortices have characteristic protein-rich cytoplasmic inclusions and aggregates [12, 13]. The most common are ubiquitinated-protein aggregates known as Lewy body-like or skein-like inclusions, characterised by randomly orientated filaments covered by fine granules. TDP-43, which is predominantly found in the nucleus, is hyper-phosphorylated, cleaved and mislocalised into these cytoplasmic inclusions in most sALS cases and most SOD1-negative fALS patients [14,15,16,17,18,19]. FUS-positive inclusions are similarly mislocalised to the cytoplasm and are also observed in the spinal cord of sALS and SOD1-negative fALS patients [20,21,22]. In FUS mutation carriers, there is a normal TDP-43 distribution, but FUS-positive cytoplasmic inclusions are observed in lower motor neurons [20], whereas in SOD1 mutation carriers, SOD1 protein aggregates are observed [23, 24]. The exact pathological effects of these inclusions remain largely unknown, and whether mislocalisation leads to impaired cellular function due to the proteins being sequestered and unable to function properly is unclear.

Several mechanisms underlying the disease-mediated toxicity seen in fALS have been proposed such as glutamate excitotoxicity [25], endoplasmic reticulum (ER) stress [26,27,28], inhibition of the proteasome [29, 30], mitochondrial damage [31,32,33], extracellular toxicity of misfolded proteins [34, 35], aberrant superoxide production [36], microhaemorrhages of spinal capillaries [37] and axonal disorganisation and disrupted axonal transport [38,39,40,41]. Furthermore, the identification of mutations in the FUS and TARDBP genes, which are involved in the production of proteins that take part in RNA splicing, translation, transport and microRNA (miRNA) biogenesis, suggests a potential role of altered RNA expression and metabolism in the disease [42]. Indeed, post-mortem pathological analysis of both fALS and sALS cases has shown the presence of abnormal levels of RNA and RNA-binding proteins in both motor neurons and glial cells [12].

Although neurodegenerative diseases were traditionally considered as having cell autonomous mechanisms (i.e. damage within a population of neurons being enough to cause disease), the death of motor neurons in ALS is influenced by non-neuronal cells such as astrocytes and microglia [43,44,45], and non-cell-autonomous mechanisms appear to play significant roles in the disease onset and/or progression. In fact, motor neuron degeneration appears to be dependent on neighbouring glia expressing mutant proteins [46, 47]. For example, studies have shown that high expression of mutant SOD1 in either most or all motor neurons of mice is insufficient for disease onset [46, 47], whereas mutant SOD1 expressed within microglia is required for disease to occur [48, 49]. Additionally, extracellular mutant SOD1 from the SOD1G93A mouse model of ALS does not cause detectable direct killing of motoneurons in culture, but it activates microglia which then release toxic factors that lead to motor neuron death [35]. In line with this, reducing the expression of mutant SOD1 within microglia slows disease progression [43]. Similarly, deletion of mutant SOD1 within astrocytes, oligodendrocytes and NG2 glial cells delays disease progression and improves survival [44, 50, 51]. Therefore, although ALS was once considered a motor neuron disease, it is now known as a multi-cellular and multi-systemic disease [52, 53], with motor neuron death being primarily driven by glial cell pathology as well as a convergence of other damaging mechanisms such as inflammatory conditions [54, 55].

Recently, it has been documented that glia also communicate with other cells by releasing extracellular vesicles containing proteins and nucleic acids, including miRNAs. However, the involvement of miRNA-containing vesicles in microglia-to-neuron communication in the context of ALS has not been explored in depth. Hence, this review will first summarise the evidence for the presence of inflammation, pro-inflammatory microglia and dysregulated miRNAs in the disease, then explore how microglia may potentially be responsible for this miRNA dysregulation by presenting evidence for a coexistence of an altered miRNA expression and a neurodegeneration-related microglial state in ALS. Finally, the possibility of pro-inflammatory ALS microglia releasing dysregulated miRNAs, which may then enter neuronal cells to cause degeneration, will be explored by reviewing studies that show how glia-to-neuron transfer of nucleic acids leads to functional changes within recipient neurons.

Inflammation is present in ALS

The levels of several pro-inflammatory cytokines are altered in ALS, suggesting the presence of inflammation. For example, an increase in the protein expression levels of tumour necrosis factor (TNF), interleukin (IL)-8, IL-12, IL-17(A), interferon (IFN)-γ and monocyte chemoattractant protein (MCP)-1 in the blood serum and/or cerebrospinal fluid (CSF) of ALS patients has been observed by at least two independent studies [56,57,58,59,60,61,62]. Moreover, increased expression of several chemokines was found in the CSF of ALS patients [62]. Although the expression of IL-1β, IL-2, IL-6 and IL-15 has been found upregulated in the serum or CSF of ALS patients in some studies, it remained unchanged in other studies [56, 57, 59,60,61,62,63,64,65], likely due to the different detection methods employed. Furthermore, increased production of TNF and IL-1β, as well as reactive oxygen species (ROS) and prostanoids, was observed in spinal cord tissue from ALS patients [66]. It has also been shown that chronic administration of IL-1β results in neurodegeneration [67], whereas IL-1β depletion or IL-1 receptor antagonism attenuates inflammation and prolongs the lifespan of ALS mouse models [68], providing further evidence of the importance of inflammation in the pathology of ALS. Moreover, increased levels of lymphocyte function-associated molecule 1 (LFA-1) and leukocyte common antigen (LCA), which are found on lymphocytes, suggest an infiltration of peripheral immune cells into the central nervous system (CNS). Increased levels of complement receptors CR3 and CR4 [69, 70], which are mostly expressed on monocytes and macrophages, as well as increased mRNA and protein levels of the complement components C1q, C3, C4 and C5b-9, have also been reported in the spinal cord and motor cortex of ALS patients [71], suggesting increased inflammation and phagocytosis in these areas. Additionally, upregulation of cyclooxygenase 2 (COX2), a common target of anti-inflammatory drugs that normally contributes to memory consolidation, synaptic activity and functional hyperaemia, is observed in ALS tissues, again suggesting an increase in neuroinflammation [72].

Changes in glial cells also play a role in ALS inflammation. For example, increased expression of Toll-like receptor 4 (TLR4) mRNA and protein is detected in astrocytes in both the grey and white matter of the spinal cord in ALS, whereas increased expression of TLR2 mRNA and protein is observed in microglia [73]. Significant upregulation of TLR7 (the murine orthologue of human TLR8) mRNA was observed in the anterior lumbar spinal cord of ALS patients, together with an increase in the number of astrocytes and activated microglia [65]. These receptors constitute some of the “sensors” of the immune system and mediate the continued glial reactivity seen in pathological conditions [74]. Furthermore, increased IFN-γ immunoreactivity, a cytokine important for macrophage activation and induction of major histocompatibility complex (MHC) class II molecule expression on immune cells, is observed in glia and neurons in the ventral horn of the spinal cord in ALS, compared to controls [75], suggesting a sustained activation of the innate immune response. Therefore, a plethora of studies confirm the presence of inflammation in ALS, even though ALS is not considered an inflammatory disease.

Inflammation in ALS involves the activation of microglia

Microglia are distributed throughout the brain and spinal cord parenchyma and account for 10–20% of the total glial population [76, 77]. They are the main defence cells in the CNS against invading bacteria, viruses and prions [78, 79], and they are responsible for maintaining brain homeostasis [80]. They are phagocytic cells that secrete several pro-inflammatory cytokines [80] and other neurotoxic substances such as nitric oxide (NO) [81] and reactive oxygen intermediates [82]. Under homeostatic conditions, microglia display surveillance behaviour, evidenced by their low mobility, small cell bodies and extensive, highly motile processes constantly scanning the CNS environment [83, 84]. This “resting” state is commonly referred to as the neutral or “M0” state [85] and is characterised by a low expression of macrophage-related surface markers, such as CD45 and MHC II [77].

Microglia are extremely sensitive to physiological changes in their environment and become “activated” following exposure to specific cytokines and growth factors that indicate infection, trauma, neuronal insult or inflammation [77]. Exposure to pro-inflammatory cytokines induces “classical” activation of microglia and switching to a pro-inflammatory phenotype or “M1” state, which is neurotoxic [86]. On the other hand, exposure to anti-inflammatory cytokines such as IL-4 induces “alternative” activation and switching to an anti-inflammatory phenotype or “M2” state which promotes tissue repair [85]. Interestingly, M1- and M2-associated genes can be co-expressed [87], suggesting that the two states are not mutually exclusive and that this binary classification is insufficient.

Increased numbers of activated microglia have been observed in the CNS of ALS mouse models and human ALS patients [88, 89]. Moreover, studies on post-mortem tissues from human patients and ALS mouse models have shown increased levels of activated microglia in areas of the brain with neuronal loss [43, 90]. For example, extracellular ATP binding to the P2X7 receptor on microglia is known to induce a pro-inflammatory response [91], and the P2X7 receptor was found elevated in the microglia of ALS patients [92]. Activation of this receptor by the agonist BzATP (2′3′-O-(benzoyl-benzoyl) ATP) in primary microglia from SOD1G93A mice enhances production of several pro-inflammatory mediators which may lead to neuronal degeneration [93,94,95]. It was also shown that the level of microglial activation parallels motor neuron degeneration in ALS patients [96, 97], and that microglia expressing mutant SOD1G93A in mice are more activated than wild-type microglia [98]. Furthermore, mice overexpressing SOD1 show an increase in M1-like microglia [99], and SOD1G93A mutations in rat microglia result in accelerated disease progression, compared to wild-type microglia [100].

The transcription factor NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) is induced by certain pro-inflammatory cytokines and regulates genes responsible for the innate and adaptive immune response. Several studies have shown that this transcription factor is upregulated in glial cells of both sALS and fALS patients [101,102,103]. Interestingly, the ALS-associated gene optineurin negatively regulates pro-inflammatory-mediated NF-κB activation [104], and loss-of-function mutations in this gene are seen in some ALS patients [102]. Consistent with this, SOD1G93A mice exhibit NF-κB hyperactivation in microglia, while NF-κB inhibition extends the survival of these mice by slowing disease progression [101].

In addition to the altered state of microglia observed in ALS, evidence also suggests microglial degeneration in the disease. For example, mononuclear phagocytes (which include both CNS-resident microglia and infiltrating monocytes from the periphery) were shown to degenerate in transgenic mutant SOD1 rats [105] and mice [106, 107]. A similar degeneration of microglia has also been observed in the brains of Alzheimer’s disease (AD) patients [108]. However, peripheral monocyte infiltration in ALS remains a controversial topic; several lines of evidence suggest that peripheral monocytes invade the CNS in ALS [107, 109, 110], whereas others have shown no contribution of peripheral monocytes to the disease [87, 111]. Therefore, whether degenerating immune cells in ALS include CNS-resident microglia, infiltrating monocytes or both requires further investigation.

A unique population of immune cells has recently been identified and was termed “disease-associated microglia” (DAM). DAM not only express microglial markers such as Hexb, Iba1 and Cst3, but also downregulate genes commonly associated with homeostatic microglia, such as Cx3cr1, P2ry12/13 and Tmem119. DAM also upregulate the expression of neurodegeneration-specific genes, such as Trem2, Apoe, Tyrobp, Lpl and Ctsd [112]. This transcriptionally distinct microglial population has primarily been associated with AD mouse models [112,113,114,115,116], but a DAM-like phenotype has also been observed in ageing [112, 116,117,118], tauopathy models [119] and ALS [87, 112, 113, 116, 120]. It has therefore been proposed that DAM are not part of a specific disease aetiology, but are rather a common occurrence following CNS pathology [121]. The transition of homeostatic microglia to DAM, their role in health and disease and the potential impact of their discovery in the development of therapies have previously been discussed [121, 122] and are beyond the focus of this review. Interestingly, at least in AD, the role of DAM may be protective, since gain-of-function mutations in genes downregulated in DAM and loss-of-function mutations in genes upregulated in DAM are associated with an increased risk of developing AD [121]. Nonetheless, whether DAM have a similarly protective role in ALS is unclear and warrants further investigation. In conclusion, there is overwhelming evidence that the inflammation seen in ALS involves the pro-inflammatory activation of microglia, and an abundance of research suggests the presence of transcriptionally distinct microglia in ALS.

Causes of microglial activation in ALS

Several hypotheses as to how microglial activation occurs in ALS have been proposed. Studies suggest that exposure to low levels of systemic signalling molecules associated with ageing and chronic inflammation (i.e. microglial “priming”) can exacerbate the microglial response to a second local stimulus, such as the presence of protein aggregates characteristic of neurodegenerative diseases, potentiating tissue damage [123]. Additionally, it was recently shown that IL-1β-mediated activation of astrocytes overexpressing wild-type FUS alter their cross-talk with microglia so that microglia acquire a pro-inflammatory profile resembling the phenotype seen in ALS [124]. It was further suggested that the mechanism of this activation involved an increase in the level of prostanoids released by these astrocytes; however, no evidence was found to support this. Instead, other pro-inflammatory cytokines such as IL-5, IL-6, IL-7, IL-15 and other molecules under astrocytic NF-κB transcriptional control have been suggested as likely candidates driving microglial activation [124]. It was also proposed that T-cells interacting with microglia may cause their activation, since spinal cord infiltration of Th and Tc cells increases over time in ALS mice, compared to controls. Furthermore, expression of humoral immune response, oxidative phosphorylation and ROS genes in microglia was found to correlate with Th cell numbers, whereas expression of genes involved in phagocytosis and coenzyme metabolism in microglia was correlated with Tc cell numbers [87].

Another hypothesis is that sub-clinical infections that activate the immune system could activate microglia and lead to the inflammation seen in ALS. However, a recent study using RNA sequencing to locate common parasitic and bacterial genomes in areas of activated microglia in CNS tissues from ALS patients found no supporting evidence [125]. Nevertheless, only a limited list of infectious agents was examined, and there are several limitations with the study, such as the possibility of loss of infectious agent transcripts as the disease progresses (since the tissues were from end-stage cases only), as well as RNA decay during frozen sectioning and variable post-mortem intervals that could have interfered with the RNA sequencing. Therefore, the possibility of sub-clinical infections causing chronic microglial activation and neurodegeneration requires further investigation.

Another possible activation route is via changes in gut microbiota, since these can reportedly control the maturation and function of microglia [126]. However, studies linking ALS with alterations in gut microbiota in patients and mouse models (reviewed in [127, 128]) are limited, and even though they suggest some instances of gut dysbiosis, whether microglia are activated in those cases has not been examined.

The most convincing evidence of how microglia may become activated in ALS comes from studies showing that extracellular misfolded proteins such as oxidised or mutant SOD1 secreted from astrocytes or neurons through neuroendocrine pathways can activate microglia and subsequently cause motor neuron death [34, 129, 130]. Additionally, mutant SOD1 proteins are not directly neurotoxic in the absence of microglia [35]. Similarly, five forms of extracellular TDP-43 (TDP-43-WT, TDP-43-M337V, TDP-25-WT, TDP-25-M337V, TDP-43-A315T) can activate microglia, with lower doses of the mutant proteins having a greater activating effect than the wild-type proteins [131]. This activation is the result of TDP-43 proteins interacting with microglial CD14, a pattern recognition receptor that also interacts with mutant SOD1 [35]. Again, in the absence of microglia, extracellular mutant TDP-43 proteins are not neurotoxic. Furthermore, incubation of microglia with both wild-type and mutant TDP-43 causes an increase in phosphorylated p65 (one of the subunits of NF-κB and an index of NF-κB activation) and phosphorylated p38 mitogen-activated protein kinase (MAPK; a mediator of the MAPK pathway that controls responses to stress and cytokines). Additionally, IκB—an inhibitor of NF-κB activation and pro-inflammatory cytokine production—decreases in microglia incubated with wild-type or mutant TDP-43 [131].

Interestingly, upregulation of TDP-43 within microglia can also enhance the microglial response via activation of the NF-κB pathway that results in the release of increased TNF, NOX2, IL-1β, IL-6 and associated ROS and reactive nitrogen species (RNS) that are toxic to neighbouring neurons [103]. TDP-43 may also promote microglial activation through interactions with activator protein 1 (AP-1), a transcription factor regulating gene expression in response to cytokines, stress, growth factors and other stimuli. Indeed, SR11302, an AP-1 inhibitor, was able to block IL-1β protein expression in microglia treated with wild-type TDP-43 [131]. Therefore, it is likely that the microglial activation in ALS is primarily driven by misfolded proteins expressed within the cells or by extracellular mutant proteins targeting microglia. Other mechanisms of microglial activation could exist; however, more studies are needed to examine this.

Possible mechanisms of microglia-mediated neurodegeneration in ALS

Despite the evidence for the presence of inflammation in ALS, exactly how glia-mediated neuroinflammation contributes to disease progression is still unclear. A possible mechanism of microglia-mediated neurotoxicity is via the production of ROS, RNS and pro-inflammatory cytokines, which may result in tissue injury and neurodegeneration [132,133,134,135]. Importantly, increased ROS and NO release from activated microglia is seen in some fALS patients with SOD1 mutations and correlates with neuronal cell death [136, 137]. Furthermore, some of the soluble factors released by microglia, which alter neuron excitability and affect synaptic function, are also involved in neuroinflammatory disorders. These factors regulate the expression of important molecules for synaptic plasticity such as cofilin or CREB (cAMP response element-binding protein) [138] or modulate the properties and expression of synaptic channels [139,140,141,142,143]. Studies have also shown that microglia regulate neuronal synapses via contact-dependent mechanisms such as synaptic element engulfment, leading to synapse loss during CNS inflammation [144,145,146]. However, it is likely that microglia-mediated synapse elimination is an appropriate response to remove a “diseased” synapse that is abnormally inactive due to causes unrelated to microglia (for example, because of degenerating neurons).

Glutamate excitotoxicity has also been suggested to play a role in microglia-mediated neurodegeneration in ALS. For example, increased levels of glutamate are detectable in the CSF of some ALS patients [147]. A reduction of extracellular glutamate uptake by astrocytic glutamate transporters has been observed in ALS, and this contributes to motor neuron death [138, 148]. Activated microglia increase the susceptibility of motor neurons to glutamate toxicity, through reducing glutamate uptake by astrocytes [149]. Furthermore, TNF-dependent glutamate release by activated microglia induces cortical neuron death [150], whereas blockade of excessive glutamate release by activated microglia suppresses neuronal loss in the spinal cord of ALS mouse models [151]. Evidence also suggests that microglia expressing mutant SOD1 mediate neuronal death via an overproduction of D-serine, a co-agonist at N-methyl-d-aspartate (NMDA) receptors [152]. Furthermore, it has been proposed that activated microglia release increased amounts of quinolinic acid, which may bind to NMDA receptors on motor neurons to cause excitotoxicity [153]. Nevertheless, glutamate excitotoxicity may play only a minor role in ALS pathogenesis since several tested drugs targeting glutamatergic transmission either did not work, or had only a modest effect on life span (reviewed in [154]).

The levels of certain miRNAs are altered in ALS

MiRNAs are small, approximately 22 nucleotides-long, non-coding RNAs (ncRNAs) transcribed from intergenic regions or from introns of protein-coding genes by RNA polymerases [155] and are involved in the regulation of translation [156]. They bind to complementary mRNA sequences, resulting in gene silencing via degradation of the mRNA or translational repression [157]. The process of mRNA degradation following interaction with a miRNA involves deadenylation and decapping, followed by 5′ to 3′ exonucleolytic digestion [158,159,160]. In the case of translational repression, the function of the ribosomes during the elongation step can be hindered, and the recognition of the eukaryotic translation initiation factor 4F (eIF4F) cap can be inhibited [161,162,163]. A single miRNA usually targets many different genes, and often a set of miRNAs synergistically target a single gene [164]. The brain has the highest expression of tissue-specific miRNAs [165,166,167], with a specific set of them localised to dendrites, where they play a role in adult neuronal plasticity [168, 169] and dendritic spine morphology [170].

The ALS genes FUS and TARDBP are directly involved in miRNA processing, by enhancing production through Drosha recruitment—a ribonuclease enzyme involved in miRNA biogenesis [171]—and by promoting the interaction between Drosha and Dicer—an RNase endonuclease involved in pre-miRNA processing [172]. Furthermore, TDP-43 protein is normally involved in the post-transcriptional maturation of certain miRNAs in the cytoplasm and nucleus. Consequently, its mislocalisation in cytoplasmic aggregates in ALS has been associated with a decrease in Dicer and Drosha processing of TDP-43-regulated miRNAs [172].

The role of miRNAs in ALS pathology was evidenced when a differential miRNA expression profile was observed between ALS patients and healthy controls in the CSF and blood serum and plasma [107, 173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189], giving rise to the opportunity of using them as potential biomarkers [190, 191]. However, the source of these miRNAs is unknown, and further studies are required to identify if these are released by degenerating motor neurons, atrophied muscles, activated astrocytes and microglia, or other cell types. Researchers have so far investigated astrocytes and neurons as a potential source. Using human patient-induced astrocytes, it was shown that a dysregulated release of 137 miRNAs occurs in culture [192]. However, of the 85 upregulated miRNAs released by these cells, only two have also been found upregulated in the CSF of ALS patients [176, 188] and none correspond to those upregulated in the blood of patients. Similarly, of the 52 downregulated miRNAs released by induced patient astrocytes, only three were also identified as downregulated in the CSF or blood of patients [188, 193]. Therefore, the contribution of astrocytes to extracellular miRNAs in ALS requires further investigation. In terms of neurons as a source of extracellular miRNAs, a recent study identified 30 differentially expressed miRNAs in neuron-derived extracellular vesicles in the plasma of ALS patients [179]. Surprisingly, none of these correspond to previously identified dysregulated miRNAs in plasma [174, 183, 184]. Therefore, the current limited evidence suggests that astrocytes and neurons are an unlikely source of the circulating dysregulated miRNAs in ALS.

In theory, miRNA upregulation within cells may result in their concurrent upregulated release. Therefore, investigating the miRNA expression of specific cell types in ALS tissues may provide a starting point of identifying the source of dysregulated circulating miRNAs. In fact, over 100 miRNAs involved in cell death pathways, inflammation, immune responses and defence responses have been found dysregulated in the spinal cord of ALS patients [194, 195] (summarised in Additional file 1). It was suggested that these changes are primarily due to altered miRNA expression within motor neurons. However, a recent study observed a downregulation of only four miRNAs in motor neurons [196], of which only one was previously found to be downregulated in spinal cord tissue. Similarly, miRNAs downregulated in patient-induced motor neuron progenitors in culture [197] do not correspond to any of those downregulated in biological fluids. Circulating miRNAs may also be released by atrophied muscles during the progression of ALS. Indeed, studies have shown differential miRNA expression in muscle tissue from ALS patients and mouse models, compared to healthy controls ([198,199,200,201,202,203] and reviewed in [204]). However, only a small percentage of the miRNAs dysregulated in muscles are also differentially expressed in biological fluids (Additional file 1). These findings suggest that cell types other than motor neurons and muscle cells may be responsible for the release of miRNAs in the disease. Furthermore, the cell types responsible for the differential miRNA expression in the spinal cords of patients remain unknown.

Microglial activation is associated with miRNA dysregulation in ALS

Recent evidence suggests that dysregulation of certain miRNAs is linked specifically to the microglial activation seen in ALS. For example, miR-22-3p, miR-125b-5p, miR-146b-5p, miR-155-5p, miR-214-3p and miR-365-3p are overexpressed in microglia from SOD1G93A mice, and these modulate inflammatory genes linked to ALS, such as the IL-6 pathway which determines the transcription of TNF [205]. Moreover, in vitro activation of wild-type microglia results in the upregulation of several miRNAs, including miR-22-3p, miR-125b-5p, miR-146b-5p and miR-155-5p, which are similarly upregulated in SOD1G93A-overexpressing microglia.

MiR-155-5p overexpression is also seen prior to disease onset and throughout disease progression in spinal cord tissue from both sALS and fALS patients [206], as well as in end-stage SOD1G93A rat and mouse spinal cords. This overexpression is accompanied by an elevation of all inflammatory miRNAs, as well as several genes associated with neuroinflammation, astrogliosis and microglial activation [207]. Furthermore, genetic ablation of miR-155-5p in SOD1G93A mice decreases the number of resident microglia in the mouse spinal cord and also prolongs survival, enhances performance on motor tasks and delays disease onset [208]. Interestingly, even though microglia have normal phagocytic ability in miR-155-5p−/− mice, SOD1G93A/miR-155-5p+/− mice produce microglia impaired in their phagocytic ability and thus are unable to clear dead neurons. In parallel, most of the direct gene targets of miR-155-5p are no longer repressed in miR-155-5p−/− mice. Similarly, central administration of anti-miR-155-5p in SOD1 mouse models de-represses miR-155-5p gene targets within neurons, astrocytes and microglia, paralleled by increased survival and improved motor function. These findings are consistent with reports that miR-155-5p is found upregulated in ALS patients and that anti-miR-155-5p extends survival in mice with SOD1 mutations [207]. Therefore, miR-155-5p appears to play a role in the pathology of ALS through modulating microglial function.

Given the evidence that microglial activation and miRNA dysregulation in ALS co-occur, it is possible that microglia are a possible source of circulating miRNAs found dysregulated in the blood and CSF of ALS patients (Table 1). Nonetheless, no studies have investigated this possibility so far. Furthermore, apart from two studies showing upregulation of certain miRNAs within microglia in ALS mouse models [107, 205], no studies have confirmed whether this is also the case in human patients. Of the 29 miRNAs upregulated in microglia from ALS mouse models, six are also upregulated in the CSF and/or serum of human patients (Table 1). These six miRNAs collectively target over 10,000 experimentally validated genes. Gene ontology analysis of these genes showed enrichment in several biological processes, including processes that are disrupted in ALS, such as cytoskeletal dynamics, oxidative stress, inflammation, RNA regulation and organelle transport (Additional file 2).

Table 1 List of miRNAs upregulated in the blood and CSF of ALS patients

Released miRNAs as a potential mechanism of microglia-mediated neurodegeneration

Another microglia-mediated mechanism of neurotoxicity is likely to be via the release of miRNAs into the extracellular environment. In biological fluids such as blood and CSF, miRNAs can be found in vesicles (exosomes, ectosomes/microvesicles and apoptotic bodies) [209, 210], or they can be bound to proteins that increase their stability in the extracellular space, such as AGO2 [209, 211]. Furthermore, it is known that microglia communicate with neighbouring neurons via the secretion of extracellular vesicles [212, 213] carrying a defined cargo of lipids, RNAs and proteins [214]. Reports also indicate a direct miRNA transfer between co-cultured macrophages and hepato-carcinoma cells via gap junctions [215]. Since gap junctions also exist between microglia and neurons [216], it would be interesting to investigate whether miRNAs can be transferred between these cells in a similar way. In addition, extracellular miRNAs can be endocytosed by target cells and bind to intracellular TLR7 to activate downstream signalling pathways [217]. Moreover, it was recently shown that microglia-derived let-7 leads to ethanol-induced neurotoxicity by activating neuronal TLR7 in rat brain slices [218]. These findings prompted the idea of “miRceptors” [219], and it would be interesting to investigate whether other known receptors can function as miRceptors. In summary, miRNAs released by glia may function as endocrine, paracrine and/or autocrine regulators and they may be endocytosed by target cells to modulate cellular function. Such an endocrine function has already been described for miRNAs from hypothalamic neural stem cells (hNSCs). Exosomal miRNAs from hNSCs were found circulating in the CSF of young mice, but several of these miRNA species were significantly reduced in the CSF of aged mice [220]. Subsequent treatment of aged mice with exosomal miRNAs from young mice resulted in an anti-ageing effect and an associated decrease in inflammatory mRNAs, such as TNF, IL-1β and IL-6.

Uptake of released miRNAs by nearby cells was previously observed when oligodendroglial exosomes carrying miRNAs could be endocytosed by neurons to improve their viability under stress conditions [221]. Additionally, exosome-mediated miRNA transfer to neurons has been shown when haematopoietic cells enter the brain under inflammatory conditions and release exosomes that are taken up by Purkinje cells [222]. In the context of ALS, transfer of miRNAs to motor neurons has so far been shown only from astrocytes [192]. In this study, exosomal miRNAs released by induced astrocytes from ALS patients with C9orf72 mutations affected the maintenance and survival of wild-type mouse motor neurons. In a different study, it was shown that glia-to-neuron shuttling of bioactive miRNAs is possible via extracellular vesicles and that this transfer is responsible for gene expression changes within receiving neurons [223]. Pro-inflammatory activation of microglia resulted in the upregulation and release of miR-146a-5p in extracellular vesicles, which then fused with the neuronal membrane of cultured hippocampal neurons to transfer their miRNA cargo inside the cells. This resulted in a downregulation of Syt1 and Nlg1, which are targets of miR-146a-5p, in the cell body and proximal dendrites, accompanied by a reduction in dendritic spine density and miniature synaptic currents.

Evidence for microglial miRNAs targeting neurons has also been proposed for other neurodegenerative diseases. For example, increased levels of let-7b have been found in the CSF of AD patients and it was shown to cause neurodegeneration by targeting neuronal TLR7 in AD mouse models [224]. Given that let-7 upregulation is also observed in activated microglia of SOD1 transgenic mice [107], it is not unlikely that its neurodegenerative effects are exerted in a similar way in ALS. Furthermore, depending on the identity of the miRNA released by microglia, its transfer into neurons can also be neuroprotective. For example, following a traumatic brain injury (TBI), there is an acute inflammatory response that persists chronically, including a pro-inflammatory activation of microglia, ultimately leading to neurodegeneration [225]. These microglia upregulate certain miRNAs, including miR-124-3p, both inside the cells and in released exosomes [226]. Treatment of injured cultured neurons with microglia-derived exosomes containing miR-124-3p results in inhibition of the inflammatory response by promoting IL-10 expression and suppressing IL-1β, IL-6 and TNF expression as a result of reduced mTOR (mammalian target of rapamycin) signalling. This also restores the number of neurite branches and total neurite length by suppressing the expression of neurodegenerative proteins such as phosphorylated Tau and amyloid-β peptide [226]. These effects are paralleled in vivo, with additional improvement in neurologic outcome in mice with TBI. On the other hand, downregulation of miR-124-3p in microglial exosomes has the opposite effect [226].

The aforementioned studies provide evidence that microglia release miRNAs that can influence neighbouring neurons in the context of neurodegeneration, and although similar findings have been observed with other glial cell types in ALS, there is a gap in our knowledge about the role of microglia-derived miRNAs in ALS pathology. It is plausible that miRNAs of microglial origin are transferred to motor neurons and elicit functional changes relevant to neurodegeneration and/or neuroprotection in ALS (Fig. 1). This is particularly relevant to miRNAs that have been dysregulated due to a pro-inflammatory activation of microglia, or even those released by the transcriptionally distinct DAM population. To examine this, it would be necessary to determine which miRNAs are released by microglia and then validate their target genes in motor neurons. Moreover, it would be interesting to investigate whether these miRNAs correspond to those detected in the CSF and blood of ALS patients. This could be done by examining whether microglial markers are present on exosomes containing the circulating miRNAs. Such experiments would be the first step towards understanding whether microglia-mediated neurodegeneration in ALS involves the release of miRNAs.

Fig. 1
figure1

Different routes via which microglia-derived miRNAs could affect gene expression in neurons. Of these different mechanisms, transfer via extracellular vesicles (represented by a circle; a) is the best characterised transfer route between microglia and neurons. This has been shown through let-7b binding to TLR7 receptors (blue) within endosomes (double line circle) to cause neurodegeneration, miR-146-5p binding to target genes in hippocampal rat neurons to alter synaptic excitability properties, and miR-124-3p inhibiting mTOR signalling and activating a neuronal inflammation phenotype. Alternatively, miRNAs could affect gene expression in neurons through interactions with high-density lipoproteins (HDL; hexagon; b). These miRNAs released by microglia bound to proteins in HDL particles are endocytosed into the cell when binding to HDL receptors (orange) on neurons. These miRNAs are then released and regulate target genes. Alternatively, miRNAs could be directly transferred between microglia and motor neurons through gap junctions (green; c) where the two cell types are directly connected through a channel

Microglia and associated miRNAs as therapeutic targets

Given the evidence of microglial activation as well as an altered miRNA expression profile in ALS patients and mouse models, they both appear as likely candidates for the development of therapeutic strategies. Many studies have explored the inhibition of specific microglial pro-inflammatory cytokines as a therapeutic strategy; however, this has so far proven unsuccessful (for a review see [227]). This is likely because of an absence of temporal and spatial specificity, but also due to simultaneously targeting the beneficial anti-inflammatory microglial factors. Indeed, it was previously shown that during disease progression, in addition to neurotoxic factors, ALS microglia concurrently show induction of neuroprotective factors such as insulin-like growth factor 1 (IGF1), progranulin (GRN) and triggering receptor expressed on myeloid cells 2 (TREM2) [87]. These responses are likely mediated by extrinsic regulatory factors such as signals released by dying motor neurons. In line with this, reducing proliferating microglia in ALS mice has no effect on survival [228], hence reflecting the multifaceted role of microglia in the disease. Therefore, neutralising the effect of activated microglia may not be a suitable therapeutic avenue.

Instead, miRNA modulation may be a promising strategy when used with the appropriate gene therapy tools, since a single miRNA can target several proteins involved in a signalling pathway associated with the disease. For example, miRNA knockdown may be used to reduce the expression of an upregulated miRNA. This can be done either by genomic editing, by targeting the transcription or processing of the miRNA, or by the use of antagomirs, which are chemically engineered oligonucleotides complementary to miRNAs; the use of locked nucleic acids in this last approach has technical benefits. By hybridising to miRNA and thus preventing its binding to the target mRNA, antagomirs allow normal translation of the target mRNA. On the other hand, downregulated miRNAs can be replaced with miRNA mimics that have the same sequence as the miRNA and thus bind to the mRNA targets to repress their translation. However, a problem with using miRNA mimics is the possibility of off-target effects because of the large number of mRNAs modulated by a single miRNA. Nevertheless, miRNA-based therapies have been tested in human clinical trials for other diseases, such as hepatitis C viral infection [229] and cancer [230], although, for neurodegenerative diseases, the challenge is the delivery of these reagents into the CNS and to the target cells. Finally, reprogramming ALS microglia towards a beneficial phenotype associated with the release of neuroprotective miRNAs may be another possible therapeutic avenue. In fact, redirecting microglia from a neurotoxic to a pro-regenerative phenotype has already been achieved by drugs targeting cell metabolism (reviewed in [231]), and is one of the current pharmacological approaches for the treatment of neuroinflammatory diseases associated with microglial activation, such as multiple sclerosis.

Conclusion

In conclusion, given the evidence of an altered miRNA expression profile in ALS patients, as well as the presence of inflammation and the associated microglial reactivity, microglia are a likely source of dysregulated miRNAs, which are potential mediators of neurodegenerative processes. Furthermore, as miRNA dysregulation may be involved in the mechanisms of neurodegeneration and since a single miRNA can affect the expression of several genes, their modulation could change cellular phenotypes, thus representing a potential target for therapeutic intervention in the hopes of attenuating some of their detrimental functions and improving disease outcomes. Finally, although minimally invasive diagnostic tools and effective therapeutics for most CNS diseases are lacking, miRNAs associated with immune cells are promising candidates both for the development of biomarkers and treatments. Full understanding of the microglia-associated miRNA regulation/dysregulation and release is therefore crucial towards a comprehensive understanding of their role in ALS pathology and their therapeutic potential.

Availability of data and materials

Not applicable.

Abbreviations

AD:

Alzheimer’s disease

ALS:

Amyotrophic lateral sclerosis

AP-1:

Activator Protein 1

BzATP:

2′-3′-O-(benzoyl-benzoyl) ATP

C9orf72:

Chromosome 9 open reading frame 72

CNS:

Central nervous system

COX2:

Cyclooxygenase 2

CREB:

cAMP Response Element-Binding protein

CSF:

Cerebrospinal fluid

DAM:

Disease-associated microglia

eIF4F:

Eukaryotic translation Initiation Factor 4F

ER:

Endoplasmic reticulum

fALS:

Familial amyotrophic lateral sclerosis

FTD:

Frontotemporal dementia

FUS:

Fused in sarcoma

GRN:

Progranulin

HDL:

High-Density Lipoprotein

IFN:

Interferon

IGF1:

Insulin-like Growth Factor 1

IL:

Interleukin

LCA:

Leukocyte Common Antigen

LFA:

Lymphocyte Function-Associated molecule

MAPK:

Mitogen-Activated Protein Kinase

MCP:

Monocyte Chemoattractant Protein

MHC:

Major Histocompatibility Complex

miRNA:

MicroRNA

ncRNA:

Non-coding RNA

NF-κB:

Nuclear Factor kappa-light-chain-enhancer of activated B cells

NO:

Nitric oxide

RNS:

Reactive nitrogen species

ROS:

Reactive oxygen species

sALS:

Sporadic Amyotrophic Lateral Sclerosis

SOD1:

Superoxide Dismutase 1

TARDBP:

Transactive Response DNA-Binding Protein

TBI:

Traumatic brain injury

TDP-43:

Transactive Response DNA-binding Protein 43

TLR:

Toll-Like Receptor

TNF:

Tumour Necrosis Factor

TREM2:

Triggering Receptor Expressed on Myeloid cells 2

References

  1. 1.

    Chiò A, Logroscino G, Traynor BJ, Collins J, Simeone JC, Goldstein LA, et al. Global epidemiology of amyotrophic lateral sclerosis: a systematic review of the published literature. Neuroepidemiology. 2013;41(2):118–30.

    PubMed  PubMed Central  Article  Google Scholar 

  2. 2.

    Giordana MT, Ferrero P, Grifoni S, Pellerino A, Naldi A, Montuschi A. Dementia and cognitive impairment in amyotrophic lateral sclerosis: a review. Neurol Sci. 2011;32(1):9–16.

    PubMed  Article  PubMed Central  Google Scholar 

  3. 3.

    Gorno-Tempini ML, Hillis AE, Weintraub S, Kertesz A, Mendez M, Cappa SF, et al. Classification of primary progressive aphasia and its variants. Neurology. 2011;76(11):1006–14.

    PubMed  PubMed Central  Article  Google Scholar 

  4. 4.

    Rascovsky K, Hodges JR, Knopman D, Mendez MF, Kramer JH, Neuhaus J, et al. Sensitivity of revised diagnostic criteria for the behavioural variant of frontotemporal dementia. Brain. 2011;134(9):2456–77.

    PubMed  PubMed Central  Article  Google Scholar 

  5. 5.

    Kiernan MC, Vucic S, Cheah BC, Turner MR, Eisen A, Hardiman O, et al. Amyotrophic lateral sclerosis. Lancet. 2011;377(9769):942–55.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  6. 6.

    Swinnen B, Robberecht W. The phenotypic variability of amyotrophic lateral sclerosis. Nat Rev Neurol. 2014;10:661.

    PubMed  Article  PubMed Central  Google Scholar 

  7. 7.

    Haeusler AR, Donnelly CJ, Periz G, Simko EAJ, Shaw PG, Kim M-S, et al. C9orf72 nucleotide repeat structures initiate molecular cascades of disease. Nature. 2014;507:195.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  8. 8.

    Rosen DR, Siddique T, Patterson D, Figlewicz DA, Sapp P, Hentati A, et al. Mutations in Cu/Zn superoxide dismutase gene are associated with familial amyotrophic lateral sclerosis. Nature. 1993;362(6415):59–62.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  9. 9.

    Vance C, Rogelj B, Hortobágyi T, De Vos KJ, Nishimura AL, Sreedharan J, et al. Mutations in FUS, an RNA processing protein, cause familial amyotrophic lateral sclerosis type 6. Science. 2009;323(5918):1208.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  10. 10.

    Sreedharan J, Blair IP, Tripathi VB, Hu X, Vance C, Rogelj B, et al. TDP-43 Mutations in familial and sporadic amyotrophic lateral sclerosis. Science. 2008;319(5870):1668.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  11. 11.

    van Es MA, Hardiman O, Chio A, Al-Chalabi A, Pasterkamp RJ, Veldink JH, et al. Amyotrophic lateral sclerosis. Lancet. 2017;390(10107):2084–98.

    PubMed  Article  PubMed Central  Google Scholar 

  12. 12.

    Peters OM, Ghasemi M, Brown RH Jr. Emerging mechanisms of molecular pathology in ALS. J Clin Invest. 2015;125(5):1767–79.

    PubMed  PubMed Central  Article  Google Scholar 

  13. 13.

    Saberi S, Stauffer JE, Schulte DJ, Ravits J. Neuropathology of amyotrophic lateral sclerosis and its variants. Neurol Clin. 2015;33(4):855–76.

    PubMed  PubMed Central  Article  Google Scholar 

  14. 14.

    Robinson JL, Geser F, Stieber A, Umoh M, Kwong LK, Van Deerlin VM, et al. TDP-43 skeins show properties of amyloid in a subset of ALS cases. Acta Neuropathol. 2013;125(1):121–31.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  15. 15.

    Lin W-L, Dickson DW. Ultrastructural localization of TDP-43 in filamentous neuronal inclusions in various neurodegenerative diseases. Acta Neuropathol. 2008;116(2):205–13.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  16. 16.

    Hasegawa M, Arai T, Nonaka T, Kametani F, Yoshida M, Hashizume Y, et al. Phosphorylated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Ann Neurol. 2008;64(1):60–70.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  17. 17.

    Tan C-F, Eguchi H, Tagawa A, Onodera O, Iwasaki T, Tsujino A, et al. TDP-43 immunoreactivity in neuronal inclusions in familial amyotrophic lateral sclerosis with or without SOD1 gene mutation. Acta Neuropathol. 2007;113(5):535–42.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  18. 18.

    Mackenzie IRA, Bigio EH, Ince PG, Geser F, Neumann M, Cairns NJ, et al. Pathological TDP-43 distinguishes sporadic amyotrophic lateral sclerosis from amyotrophic lateral sclerosis with SOD1 mutations. Ann Neurol. 2007;61(5):427–34.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  19. 19.

    Neumann M, Sampathu DM, Kwong LK, Truax AC, Micsenyi MC, Chou TT, et al. Ubiquitinated TDP-43 in frontotemporal lobar degeneration and amyotrophic lateral sclerosis. Science. 2006;314(5796):130.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  20. 20.

    Deng H-X, Zhai H, Bigio EH, Yan J, Fecto F, Ajroud K, et al. FUS-immunoreactive inclusions are a common feature in sporadic and non-SOD1 familial amyotrophic lateral sclerosis. Ann Neurol. 2010;67(6):739–48.

    CAS  PubMed  PubMed Central  Google Scholar 

  21. 21.

    Ikenaka K, Ishigaki S, Iguchi Y, Kawai K, Fujioka Y, Yokoi S, et al. Characteristic features of FUS inclusions in spinal motor neurons of sporadic amyotrophic lateral sclerosis. Journal of Neuropathology & Experimental Neurology. 2020.

  22. 22.

    Tyzack GE, Luisier R, Taha DM, Neeves J, Modic M, Mitchell JS, et al. Widespread FUS mislocalization is a molecular hallmark of amyotrophic lateral sclerosis. Brain. 2019;142(9):2572–80.

    PubMed  PubMed Central  Article  Google Scholar 

  23. 23.

    Wang J, Xu G, Li H, Gonzales V, Fromholt D, Karch C, et al. Somatodendritic accumulation of misfolded SOD1-L126Z in motor neurons mediates degeneration: αB-crystallin modulates aggregation. Hum Mol Genet. 2005;14(16):2335–47.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  24. 24.

    Karch CM, Prudencio M, Winkler DD, Hart PJ, Borchelt DR. Role of mutant SOD1 disulfide oxidation and aggregation in the pathogenesis of familial ALS. Proc Natl Acad Sci U S A. 2009;106(19):7774–9.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  25. 25.

    Boston-Howes W, Gibb SL, Williams EO, Pasinelli P, Brown RH, Trotti D. Caspase-3 cleaves and inactivates the glutamate transporter EAAT2. J Biol Chem. 2006;281(20):14076–84.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  26. 26.

    Kikuchi H, Almer G, Yamashita S, Guégan C, Nagai M, Xu Z, et al. Spinal cord endoplasmic reticulum stress associated with a microsomal accumulation of mutant superoxide dismutase-1 in an ALS model. Proc Natl Acad Sci. 2006;103(15):6025.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  27. 27.

    Nishitoh H, Kadowaki H, Nagai A, Maruyama T, Yokota T, Fukutomi H, et al. ALS-linked mutant SOD1 induces ER stress- and ASK1-dependent motor neuron death by targeting Derlin-1. Genes Dev. 2008;22(11):1451–64.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  28. 28.

    Saxena S, Cabuy E, Caroni P. A role for motoneuron subtype–selective ER stress in disease manifestations of FALS mice. Nat Neurosci. 2009;12:627.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  29. 29.

    Bruijn LI, Houseweart MK, Kato S, Anderson KL, Anderson SD, Ohama E, et al. Aggregation and motor neuron toxicity of an ALS-linked SOD1 mutant independent from wild-type SOD1. Science. 1998;281(5384):1851.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  30. 30.

    Hoffman EK, Wilcox HM, Scott RW, Siman R. Proteasome inhibition enhances the stability of mouse Cu Zn superoxide dismutase with mutations linked to familial amyotrophic lateral sclerosis. J Neurol Sci. 1996;139(1):15–20.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  31. 31.

    Cassina P, Cassina A, Pehar M, Castellanos R, Gandelman M, de León A, et al. Mitochondrial dysfunction in SOD1-G93A-bearing astrocytes promotes motor neuron degeneration: prevention by mitochondrial-targeted antioxidants. J Neurosci. 2008;28(16):4115.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  32. 32.

    Liu J, Lillo C, Jonsson PA, Velde CV, Ward CM, Miller TM, et al. Toxicity of Familial ALS-linked SOD1 mutants from selective recruitment to spinal mitochondria. Neuron. 2004;43(1):5–17.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  33. 33.

    Vande Velde C, Miller TM, Cashman NR, Cleveland DW. Selective association of misfolded ALS-linked mutant SOD1 with the cytoplasmic face of mitochondria. Proc Natl Acad Sci. 2008;105(10):4022.

    PubMed  Article  PubMed Central  Google Scholar 

  34. 34.

    Urushitani M, Sik A, Sakurai T, Nukina N, Takahashi R, Julien J-P. Chromogranin-mediated secretion of mutant superoxide dismutase proteins linked to amyotrophic lateral sclerosis. Nat Neurosci. 2006;9(1):108–18.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  35. 35.

    Zhao W, Beers DR, Henkel JS, Zhang W, Urushitani M, Julien J-P, et al. Extracellular mutant SOD1 induces microglial-mediated motoneuron injury. Glia. 2010;58(2):231–43.

    PubMed  PubMed Central  Article  Google Scholar 

  36. 36.

    Harraz MM, Marden JJ, Zhou W, Zhang Y, Williams A, Sharov VS, et al. SOD1 mutations disrupt redox-sensitive Rac regulation of NADPH oxidase in a familial ALS model. J Clin Invest. 2008;118(2):659–70.

    CAS  PubMed  PubMed Central  Google Scholar 

  37. 37.

    Zhong Z, Deane R, Ali Z, Parisi M, Shapovalov Y, O'Banion MK, et al. ALS-causing SOD1 mutants generate vascular changes prior to motor neuron degeneration. Nat Neurosci. 2008;11:420.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  38. 38.

    Murakami T, Nagano I, Hayashi T, Manabe Y, Shoji M, Setoguchi Y, et al. Impaired retrograde axonal transport of adenovirus-mediated E. coli LacZ gene in the mice carrying mutant SOD1 gene. Neurosci Lett. 2001;308(3):149–52.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  39. 39.

    Perlson E, Jeong G-B, Ross JL, Dixit R, Wallace KE, Kalb RG, et al. A switch in retrograde signaling from survival to stress in rapid-onset neurodegeneration. J Neurosci. 2009;29(31):9903.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  40. 40.

    Williamson TL, Cleveland DW. Slowing of axonal transport is a very early event in the toxicity of ALS–linked SOD1 mutants to motor neurons. Nat Neurosci. 1999;2(1):50–6.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  41. 41.

    De Vos KJ, Hafezparast M. Neurobiology of axonal transport defects in motor neuron diseases: opportunities for translational research? Neurobiol Dis. 2017;105:283–99.

    PubMed  PubMed Central  Article  Google Scholar 

  42. 42.

    Paez-Colasante X, Figueroa-Romero C, Sakowski SA, Goutman SA, Feldman EL. Amyotrophic lateral sclerosis: mechanisms and therapeutics in the epigenomic era. Nat Rev Neurol. 2015;11:266.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  43. 43.

    Boillée S, Yamanaka K, Lobsiger CS, Copeland NG, Jenkins NA, Kassiotis G, et al. Onset and progression in inherited ALS determined by motor neurons and microglia. Science. 2006;312(5778):1389.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  44. 44.

    Kang SH, Li Y, Fukaya M, Lorenzini I, Cleveland DW, Ostrow LW, et al. Degeneration and impaired regeneration of gray matter oligodendrocytes in amyotrophic lateral sclerosis. Nat Neurosci. 2013;16:571.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  45. 45.

    Yamanaka K, Komine O. The multi-dimensional roles of astrocytes in ALS. Neurosci Res. 2018;126:31–8.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  46. 46.

    Lino MM, Schneider C, Caroni P. Accumulation of SOD1 mutants in postnatal motoneurons does not cause motoneuron pathology or motoneuron disease. J Neurosci. 2002;22(12):4825.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  47. 47.

    Pramatarova A, Laganière J, Roussel J, Brisebois K, Rouleau GA. Neuron-specific expression of mutant superoxide dismutase 1 in transgenic mice does not lead to motor impairment. J Neurosci. 2001;21(10):3369.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  48. 48.

    Clement AM, Nguyen MD, Roberts EA, Garcia ML, Boillée S, Rule M, et al. Wild-type nonneuronal cells extend survival of SOD1 mutant motor neurons in ALS mice. Science. 2003;302(5642):113.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  49. 49.

    Yamanaka K, Boillee S, Roberts EA, Garcia ML, McAlonis-Downes M, Mikse OR, et al. Mutant SOD1 in cell types other than motor neurons and oligodendrocytes accelerates onset of disease in ALS mice. Proc Natl Acad Sci. 2008;105(21):7594.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  50. 50.

    Yamanaka K, Chun SJ, Boillee S, Fujimori-Tonou N, Yamashita H, Gutmann DH, et al. Astrocytes as determinants of disease progression in inherited amyotrophic lateral sclerosis. Nat Neurosci. 2008;11:251.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  51. 51.

    Wang L, Gutmann DH, Roos RP. Astrocyte loss of mutant SOD1 delays ALS disease onset and progression in G85R transgenic mice. Hum Mol Genet. 2010;20(2):286–93.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  52. 52.

    Nardo G, Pozzi S, Pignataro M, Lauranzano E, Spano G, Garbelli S, et al. Amyotrophic lateral sclerosis multiprotein biomarkers in peripheral blood mononuclear cells. Plos One. 2011;6(10):e25545.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  53. 53.

    Verde F, Del Tredici K, Braak H, Ludolph A. The multisystem degeneration amyotrophic lateral sclerosis - neuropathological staging and clinical translation. Arch Ital Biol. 2017;155(4):118–30.

    PubMed  PubMed Central  Google Scholar 

  54. 54.

    Beers DR, Zhao W, Liao B, Kano O, Wang J, Huang A, et al. Neuroinflammation modulates distinct regional and temporal clinical responses in ALS mice. Brain Behav Immun. 2011;25(5):1025–35.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  55. 55.

    Grosskreutz J, Van Den Bosch L, Keller BU. Calcium dysregulation in amyotrophic lateral sclerosis. Cell Calcium. 2010;47(2):165–74.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  56. 56.

    Fiala M, Chattopadhay M, La Cava A, Tse E, Liu G, Lourenco E, et al. IL-17A is increased in the serum and in spinal cord CD8 and mast cells of ALS patients. J Neuroinflammation. 2010;7(1):76.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  57. 57.

    Moreau C, Devos D, Brunaud-Danel V, Defebvre L, Perez T, Destée A, et al. Elevated IL-6 and TNF-α levels in patients with ALS: inflammation or hypoxia? Neurology. 2005;65(12):1958.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  58. 58.

    Kuhle J, Lindberg RLP, Regeniter A, Mehling M, Steck AJ, Kappos L, et al. Increased levels of inflammatory chemokines in amyotrophic lateral sclerosis. Eur J Neurol. 2009;16(6):771–4.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  59. 59.

    Mitchell RM, Freeman WM, Randazzo WT, Stephens HE, Beard JL, Simmons Z, et al. A CSF biomarker panel for identification of patients with amyotrophic lateral sclerosis. Neurology. 2009;72(1):14.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  60. 60.

    Rentzos M, Rombos A, Nikolaou C, Zoga M, Zouvelou V, Dimitrakopoulos A, et al. Interleukin-15 and interleukin-12 are elevated in serum and cerebrospinal fluid of [atients with amyotrophic lateral sclerosis. Eur Neurol. 2010;63(5):285-90.

  61. 61.

    Sekizawa T, Openshaw H, Ohbo K, Sugamura K, Itoyama Y, Niland JC. Cerebrospinal fluid interleukin 6 in amyotrophic lateral sclerosis: immunological parameter and comparison with inflammatory and non-inflammatory central nervous system diseases. J Neurol Sci. 1998;154(2):194–9.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  62. 62.

    Tateishi T, Yamasaki R, Tanaka M, Matsushita T, Kikuchi H, Isobe N, et al. CSF chemokine alterations related to the clinical course of amyotrophic lateral sclerosis. J Neuroimmunol. 2010;222(1):76–81.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  63. 63.

    Italiani P, Carlesi C, Giungato P, Puxeddu I, Borroni B, Bossù P, et al. Evaluating the levels of interleukin-1 family cytokines in sporadic amyotrophic lateral sclerosis. J Neuroinflammation. 2014;11(1):94.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  64. 64.

    Krieger C, Perry TL, Ziltener HJ. Amyotrophic Lateral Sclerosis: Interleukin-6 Levels in cerebrospinal fluid. Canadian Journal of Neurological Sciences / Journal Canadien des Sciences Neurologiques. 1992;19(3):357–9.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  65. 65.

    Berjaoui S, Povedano M, Garcia-Esparcia P, Carmona M, Aso E, Ferrer I. Complex inflammation mRNA-related response in ALS is region dependent. Neural Plasticity. 2015;2015:11.

    Article  CAS  Google Scholar 

  66. 66.

    Glass CK, Saijo K, Winner B, Marchetto MC, Gage FH. Mechanisms underlying inflammation in neurodegeneration. Cell. 2010;140(6):918–34.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  67. 67.

    Song C, Zhang Y, Dong Y. Acute and subacute IL-1β administrations differentially modulate neuroimmune and neurotrophic systems: possible implications for neuroprotection and neurodegeneration. J Neuroinflammation. 2013;10(1):826.

    Article  CAS  Google Scholar 

  68. 68.

    Meissner F, Molawi K, Zychlinsky A. Mutant superoxide dismutase 1-induced IL-1β accelerates ALS pathogenesis. Proc Natl Acad Sci. 2010;107(29):13046.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  69. 69.

    Kawamata T, Akiyama H, Yamada T, McGeer PL. Immunologic reactions in amyotrophic lateral sclerosis brain and spinal cord tissue. Am J Pathol. 1992;140(3):691–707.

    CAS  PubMed  PubMed Central  Google Scholar 

  70. 70.

    Papadimitriou D, Le Verche V, Jacquier A, Ikiz B, Przedborski S, Re DB. Inflammation in ALS and SMA: sorting out the good from the evil. Neurobiol Di. 2010;37(3):493–502.

    CAS  Article  Google Scholar 

  71. 71.

    Sta M, Sylva-Steenland RMR, Casula M, de Jong JMBV, Troost D, Aronica E, et al. Innate and adaptive immunity in amyotrophic lateral sclerosis: evidence of complement activation. Neurobiol Dis. 2011;42(3):211–20.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  72. 72.

    Almer G, Guégan C, Teismann P, Naini A, Rosoklija G, Hays AP, et al. Increased expression of the pro-inflammatory enzyme cyclooxygenase-2 in amyotrophic lateral sclerosis. Ann Neurol. 2001;49(2):176–85.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  73. 73.

    Casula M, Iyer AM, Spliet WGM, Anink JJ, Steentjes K, Sta M, et al. Toll-like receptor signaling in amyotrophic lateral sclerosis spinal cord tissue. Neuroscience. 2011;179:233–43.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  74. 74.

    Kielian T. Toll-like receptors in central nervous system glial inflammation and homeostasis. J Neurosci Res. 2006;83(5):711–30.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  75. 75.

    Aebischer J, Moumen A, Sazdovitch V, Seilhean D, Meininger V, Raoul C. Elevated levels of IFNγ and LIGHT in the spinal cord of patients with sporadic amyotrophic lateral sclerosis. Eur J Neurol. 2012;19(5):752–9.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  76. 76.

    Luo X-G, Chen S-D. The changing phenotype of microglia from homeostasis to disease. Translational Neurodegeneration. 2012;1(1):9.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  77. 77.

    Lynch MA. The multifaceted profile of activated microglia. Mol Neurobiol. 2009;40(2):139–56.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  78. 78.

    Ousman SS, Kubes P. Immune surveillance in the central nervous system. Nat Neurosci. 2012;15:1096.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  79. 79.

    Rock RB, Gekker G, Hu S, Sheng WS, Cheeran M, Lokensgard JR, et al. Role of microglia in central nervous system infections. Clin Microbiol Rev. 2004;17(4):942.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  80. 80.

    Kettenmann H, Hanisch U-K, Noda M, Verkhratsky A. Physiology of microglia. Physiol Rev. 2011;91(2):461–553.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  81. 81.

    Dawson VL, Dawson TM, Bartley DA, Uhl GR, Snyder SH. Mechanisms of nitric oxide-mediated neurotoxicity in primary brain cultures. J Neurosci. 1993;13(6):2651.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  82. 82.

    Tanaka M, Sotomatsu A, Yoshida T, Hirai S, Nishida A. Detection of superoxide production by activated microglia using a sensitive and specific chemiluminescence assay and microglia-mediated PC12h cell death. J Neurochem. 1994;63(1):266–70.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  83. 83.

    Davalos D, Grutzendler J, Yang G, Kim JV, Zuo Y, Jung S, et al. ATP mediates rapid microglial response to local brain injury in vivo. Nat Neurosci. 2005;8(6):752–8.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  84. 84.

    Nimmerjahn A, Kirchhoff F, Helmchen F. Resting microglial cells are highly dynamic surveillants of brain parenchyma in vivo. Science. 2005;308(5726):1314.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  85. 85.

    Timmerman R, Burm SM, Bajramovic JJ. An overview of in vitro methods to study microglia. Front Cell Neurosci. 2018;12:242.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  86. 86.

    Hu X, Leak RK, Shi Y, Suenaga J, Gao Y, Zheng P, et al. Microglial and macrophage polarization—new prospects for brain repair. Nat Rev Neurol. 2015;11(1):56–64.

    PubMed  Article  PubMed Central  Google Scholar 

  87. 87.

    Chiu Isaac M, Morimoto Emiko TA, Goodarzi H, Liao Jennifer T, O’Keeffe S, Phatnani Hemali P, et al. A neurodegeneration-specific gene-expression signature of acutely isolated microglia from an amyotrophic lateral sclerosis mouse model. Cell Rep. 2013;4(2):385–401.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  88. 88.

    Hall ED, Oostveen JA, Gurney ME. Relationship of microglial and astrocytic activation to disease onset and progression in a transgenic model of familial ALS. Glia. 1998;23(3):249–56.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  89. 89.

    McGeer PL, Kawamata T, Walker DG, Akiyama H, Tooyama I, McGeer EG. Microglia in degenerative neurological disease. Glia. 1993;7(1):84–92.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  90. 90.

    Polazzi E, Monti B. Polazzi, E and Monti, B. Microglia and neuroprotection: from in vitro studies to therapeutic applications. Prog Neurobiol 92: 293-315. Prog Neurobiol. 2010;92:293–315.

    PubMed  Article  PubMed Central  Google Scholar 

  91. 91.

    North RA. Molecular physiology of P2X receptors. Physiol Rev. 2002;82(4):1013–67.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  92. 92.

    Yiangou Y, Facer P, Durrenberger P, Chessell IP, Naylor A, Bountra C, et al. COX-2, CB2 and P2X7-immunoreactivities are increased in activated microglial cells/macrophages of multiple sclerosis and amyotrophic lateral sclerosis spinal cord. BMC Neurol. 2006;6(1):12.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  93. 93.

    D'Ambrosi N, Finocchi P, Apolloni S, Cozzolino M, Ferri A, Padovano V, et al. The proinflammatory action of microglial P2 receptors is enhanced in SOD1 models for amyotrophic lateral sclerosis. J Immunol. 2009;183(7):4648.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  94. 94.

    Apolloni S, Parisi C, Pesaresi MG, Rossi S, Carrì MT, Cozzolino M, et al. The NADPH oxidase pathway is dysregulated by the P2X7 receptor in the SOD1-G93A microglia model of amyotrophic lateral sclerosis. J Immunol. 2013;190(10):5187.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  95. 95.

    Parisi C, Napoli G, Pelegrin P, Volonté C. M1 and M2 functional imprinting of primary microglia: role of P2X7 activation and miR-125b. Mediators of Inflamm. 2016;2016:9.

    Article  CAS  Google Scholar 

  96. 96.

    Corcia P, Tauber C, Vercoullie J, Arlicot N, Prunier C, Praline J, et al. Molecular imaging of microglial activation in amyotrophic lateral sclerosis. PLOS ONE. 2013;7(12):e52941.

    Article  CAS  Google Scholar 

  97. 97.

    Turner MR, Cagnin A, Turkheimer FE, Miller CCJ, Shaw CE, Brooks DJ, et al. Evidence of widespread cerebral microglial activation in amyotrophic lateral sclerosis: an [11C](R)-PK11195 positron emission tomography study. Neurobiol Dis. 2004;15(3):601–9.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  98. 98.

    Xiao Q, Zhao W, Beers DR, Yen AA, Xie W, Henkel JS, et al. Mutant SOD1G93A microglia are more neurotoxic relative to wild-type microglia. J Neurochem. 2007;102(6):2008–19.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  99. 99.

    Liao B, Zhao W, Beers DR, Henkel JS, Appel SH. Transformation from a neuroprotective to a neurotoxic microglial phenotype in a mouse model of ALS. Exp Neurol. 2012;237(1):147–52.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  100. 100.

    Beers DR, Henkel JS, Xiao Q, Zhao W, Wang J, Yen AA, et al. Wild-type microglia extend survival in PU.1 knockout mice with familial amyotrophic lateral sclerosis. Proc Natl Acad Sci. 2006;103(43):16021.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  101. 101.

    Frakes Ashley E, Ferraiuolo L, Haidet-Phillips Amanda M, Schmelzer L, Braun L, Miranda Carlos J, et al. Microglia induce motor neuron death via the classical NF-κB pathway in amyotrophic lateral sclerosis. Neuron. 2014;81(5):1009–23.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  102. 102.

    Maruyama H, Morino H, Ito H, Izumi Y, Kato H, Watanabe Y, et al. Mutations of optineurin in amyotrophic lateral sclerosis. Nature. 2010;465:223.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  103. 103.

    Swarup V, Phaneuf D, Dupré N, Petri S, Strong M, Kriz J, et al. Deregulation of TDP-43 in amyotrophic lateral sclerosis triggers nuclear factor κB–mediated pathogenic pathways. J Exp Med. 2011;208(12):2429.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  104. 104.

    Zhu G, Wu C-J, Zhao Y, Ashwell JD. Optineurin negatively regulates TNFα- induced NF-κB activation by competing with NEMO for ubiquitinated RIP. Curr Biol. 2007;17(16):1438–43.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  105. 105.

    Fendrick SE, Xue Q-S, Streit WJ. Formation of multinucleated giant cells and microglial degeneration in rats expressing a mutant Cu/Zn superoxide dismutase gene. J Neuroinflamm. 2007;4(1):9.

    Article  CAS  Google Scholar 

  106. 106.

    El Oussini H, Bayer H, Scekic-Zahirovic J, Vercruysse P, Sinniger J, Dirrig-Grosch S, et al. Serotonin 2B receptor slows disease progression and prevents degeneration of spinal cord mononuclear phagocytes in amyotrophic lateral sclerosis. Acta Neuropathol. 2016;131(3):465–80.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  107. 107.

    Butovsky O, Siddiqui S, Gabriely G, Lanser AJ, Dake B, Murugaiyan G, et al. Modulating inflammatory monocytes with a unique microRNA gene signature ameliorates murine ALS. J Clin Invest. 2012;122(9):3063–87.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  108. 108.

    Sanchez-Mejias E, Navarro V, Jimenez S, Sanchez-Mico M, Sanchez-Varo R, Nuñez-Diaz C, et al. Soluble phospho-tau from Alzheimer’s disease hippocampus drives microglial degeneration. Acta Neuropathol. 2016;132(6):897–916.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  109. 109.

    Lewis C-AB, Solomon JN, Rossi FM, Krieger C. Bone marrow-derived cells in the central nervous system of a mouse model of amyotrophic lateral sclerosis are associated with blood vessels and express CX3CR1. Glia. 2009;57(13):1410–9.

    PubMed  Article  PubMed Central  Google Scholar 

  110. 110.

    Zondler L, Müller K, Khalaji S, Bliederhäuser C, Ruf WP, Grozdanov V, et al. Peripheral monocytes are functionally altered and invade the CNS in ALS patients. Acta Neuropathol. 2016;132(3):391–411.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  111. 111.

    Kunis G, Baruch K, Miller O, Schwartz M. Immunization with a myelin-derived antigen activates the brain's choroid plexus for recruitment of immunoregulatory cells to the CNS and attenuates disease progression in a mouse model of ALS. J Neurosci. 2015;35(16):6381.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  112. 112.

    Keren-Shaul H, Spinrad A, Weiner A, Matcovitch-Natan O, Dvir-Szternfeld R, Ulland TK, et al. A unique microglia type associated with restricting development of Alzheimer’s disease. Cell. 2017;169(7):1276–90.e17.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  113. 113.

    Krasemann S, Madore C, Cialic R, Baufeld C, Calcagno N, El Fatimy R, et al. The TREM2-APOE pathway drives the transcriptional phenotype of dysfunctional microglia in neurodegenerative diseases. Immunity. 2017;47(3):566–81.e9.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  114. 114.

    Ofengeim D, Mazzitelli S, Ito Y, DeWitt JP, Mifflin L, Zou C, et al. RIPK1 mediates a disease-associated microglial response in Alzheimer’s disease. Proc Natl Acad Sci. 2017;114(41):E8788.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  115. 115.

    Kamphuis W, Kooijman L, Schetters S, Orre M, Hol EM. Transcriptional profiling of CD11c-positive microglia accumulating around amyloid plaques in a mouse model for Alzheimer's disease. Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease. 2016;1862(10):1847–60.

    CAS  Article  Google Scholar 

  116. 116.

    Holtman IR, Raj DD, Miller JA, Schaafsma W, Yin Z, Brouwer N, et al. Induction of a common microglia gene expression signature by aging and neurodegenerative conditions: a co-expression meta-analysis. Acta Neuropathologica Communications. 2015;3(1):31.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  117. 117.

    Mrdjen D, Pavlovic A, Hartmann FJ, Schreiner B, Utz SG, Leung BP, et al. High-dimensional single-cell mapping of central nervous system immune cells reveals distinct myeloid subsets in health, aging, and disease. Immunity. 2018;48(2):380–95.e6.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  118. 118.

    Olah M, Patrick E, Villani A-C, Xu J, White CC, Ryan KJ, et al. A transcriptomic atlas of aged human microglia. Nat Commun. 2018;9(1):539.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  119. 119.

    Friedman BA, Srinivasan K, Ayalon G, Meilandt WJ, Lin H, Huntley MA, et al. Diverse brain myeloid expression profiles reveal distinct microglial activation states and aspects of Alzheimer’s disease Not Evident in Mouse Models. Cell Rep. 2018;22(3):832–47.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  120. 120.

    Spiller KJ, Restrepo CR, Khan T, Dominique MA, Fang TC, Canter RG, et al. Microglia-mediated recovery from ALS-relevant motor neuron degeneration in a mouse model of TDP-43 proteinopathy. Nat Neurosci. 2018;21(3):329–40.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  121. 121.

    Deczkowska A, Keren-Shaul H, Weiner A, Colonna M, Schwartz M, Amit I. Disease-associated microglia: a universal immune sensor of neurodegeneration. Cell. 2018;173(5):1073–81.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  122. 122.

    Butovsky O, Weiner HL. Microglial signatures and their role in health and disease. Nat Rev Neurosci. 2018;19(10):622–35.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  123. 123.

    Cribbs DH, Berchtold NC, Perreau V, Coleman PD, Rogers J, Tenner AJ, et al. Extensive innate immune gene activation accompanies brain aging, increasing vulnerability to cognitive decline and neurodegeneration: a microarray study. J Neuroinflammation. 2012;9(1):179.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  124. 124.

    Ajmone-Cat MA, Onori A, Toselli C, Stronati E, Morlando M, Bozzoni I, et al. Increased FUS levels in astrocytes leads to astrocyte and microglia activation and neuronal death. Sci Rep. 2019;9(1):4572.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  125. 125.

    Bennett JP, Keeney PM, Brohawn DG. RNA sequencing reveals small and variable contributions of infectious agents to transcriptomes of postmortem nervous tissues from amyotrophic lateral sclerosis, Alzheimer’s disease and Parkinson’s disease subjects, and increased expression of genes from disease-activated microglia. Front Neurosci. 2019;13:235.

    PubMed  PubMed Central  Article  Google Scholar 

  126. 126.

    Erny D. Hrabě de Angelis AL, Jaitin D, Wieghofer P, Staszewski O, David E, et al. Host microbiota constantly control maturation and function of microglia in the CNS. Na Neurosci. 2015;18(7):965–77.

    CAS  Article  Google Scholar 

  127. 127.

    Wright ML, Fournier C, Houser MC, Tansey M, Glass J, Hertzberg VS. Potential role of the gut microbiome in ALS: a systematic review. Biol Res Nurs. 2018;20(5):513–21.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  128. 128.

    McCombe PA, Henderson RD, Lee A, Lee JD, Woodruff TM, Restuadi R, et al. Gut microbiota in ALS: possible role in pathogenesis? Expert Review of Neurotherapeutics. 2019:1–21.

  129. 129.

    Ezzi SA, Urushitani M, Julien J-P. Wild-type superoxide dismutase acquires binding and toxic properties of ALS-linked mutant forms through oxidation. J Neurochem. 2007;102(1):170–8.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  130. 130.

    Roberts K, Zeineddine R, Corcoran L, Li W, Campbell IL, Yerbury JJ. Extracellular aggregated Cu/Zn superoxide dismutase activates microglia to give a cytotoxic phenotype. Glia. 2013;61(3):409–19.

    PubMed  Article  PubMed Central  Google Scholar 

  131. 131.

    Zhao W, Beers DR, Bell S, Wang J, Wen S, Baloh RH, et al. TDP-43 activates microglia through NF-κB and NLRP3 inflammasome. Exp Neurol. 2015;273:24–35.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  132. 132.

    Block ML, Zecca L, Hong J-S. Microglia-mediated neurotoxicity: uncovering the molecular mechanisms. Nat Rev Neurosci. 2007;8:57.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  133. 133.

    Heneka MT, Kummer MP, Latz E. Innate immune activation in neurodegenerative disease. Nature Reviews Immunology. 2014;14:463.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  134. 134.

    Lijia Z, Zhao S, Wang X, Wu C, Yang J. A self-propelling cycle mediated by reactive oxide species and nitric oxide exists in LPS-activated microglia. Neurochem Int. 2012;61(7):1220–30.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  135. 135.

    Neniskyte U, Brown GC. Analysis of microglial production of reactive oxygen and nitrogen species. In: Joseph B, Venero JL, editors. Microglia: methods and protocols. Totowa, NJ: Humana Press; 2013. p. 103–11.

    Google Scholar 

  136. 136.

    Kang J, Rivest S. MyD88-deficient bone marrow cells accelerate onset and reduce survival in a mouse model of amyotrophic lateral sclerosis. J Cell Biol. 2007;179(6):1219–30.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  137. 137.

    Trudler D, Farfara D, Frenkel D. Toll-like receptors expression and signaling in glia cells in neuro-amyloidogenic diseases: towards future therapeutic application. Mediators Inflammation. 2010;2010:12.

    Article  CAS  Google Scholar 

  138. 138.

    Tong L, Prieto GA, Kramár EA, Smith ED, Cribbs DH, Lynch G, et al. Brain-derived neurotrophic factor-dependent synaptic plasticity is suppressed by interleukin-1β via p38 mitogen-activated protein kinase. J Neurosci. 2012;32(49):17714.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  139. 139.

    Centonze D, Muzio L, Rossi S, Cavasinni F, De Chiara V, Bergami A, et al. Inflammation triggers synaptic alteration and degeneration in experimental autoimmune encephalomyelitis. J Neurosci. 2009;29(11):3442.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  140. 140.

    Pascual O, Ben Achour S, Rostaing P, Triller A, Bessis A. Microglia activation triggers astrocyte-mediated modulation of excitatory neurotransmission. Proc Natl Acad Sci. 2012;109(4):E197.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  141. 141.

    Rossi S, Muzio L, De Chiara V, Grasselli G, Musella A, Musumeci G, et al. Impaired striatal GABA transmission in experimental autoimmune encephalomyelitis. Brain Behav Immun. 2011;25(5):947–56.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  142. 142.

    Viviani B, Bartesaghi S, Gardoni F, Vezzani A, Behrens MM, Bartfai T, et al. Interleukin-1β enhances NMDA receptor-mediated intracellular calcium increase through activation of the Src family of kinases. J Neurosci. 2003;23(25):8692.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  143. 143.

    Weber A, Wasiliew P, Kracht M. Interleukin-1 (IL-1) pathway. Sci Signal. 2010;3(105):cm1.

    PubMed  PubMed Central  Google Scholar 

  144. 144.

    Tremblay M-È, Stevens B, Sierra A, Wake H, Bessis A, Nimmerjahn A. The role of microglia in the healthy brain. J Neurosci. 2011;31(45):16064.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  145. 145.

    Wake H, Moorhouse AJ, Jinno S, Kohsaka S, Nabekura J. Resting microglia directly monitor the functional state of synapses in vivo and determine the fate of ischemic terminals. J Neurosci. 2009;29(13):3974.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  146. 146.

    Wu Y, Dissing-Olesen L, MacVicar BA, Stevens B. Microglia: dynamic mediators of synapse development and plasticity. TrendsImmunol. 2015;36(10):605–13.

    Google Scholar 

  147. 147.

    Rothstein JD, Tsai G, Kuncl RW, Clawson L, Cornblath DR, Drachman DB, et al. Abnormal excitatory amino acid metabolism in amyotrophic lateral sclerosis. Ann Neurol. 1990;28(1):18–25.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  148. 148.

    Rothstein JD, Van Kammen M, Levey AI, Martin LJ, Kuncl RW. Selective loss of glial glutamate transporter GLT-1 in amyotrophic lateral sclerosis. Ann Neurol. 1995;38(1):73–84.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  149. 149.

    Zhao W, Xie W, Le W, Beers DR, He Y, Henkel JS, et al. Activated microglia initiate motor neuron injury by a nitric oxide and glutamate-mediated mechanism. J Neuropathol Exp Neurol. 2004;63(9):964–77.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  150. 150.

    Takeuchi H, Jin S, Wang J, Zhang G, Kawanokuchi J, Kuno R, et al. Tumor necrosis factor-α induces neurotoxicity via glutamate release from hemichannels of activated microglia in an autocrine manner. J Biol Chem. 2006;281(30):21362–8.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  151. 151.

    Takeuchi H, Mizoguchi H, Doi Y, Jin S, Noda M, Liang J, et al. Blockade of gap junction hemichannel suppresses disease progression in mouse models of amyotrophic lateral sclerosis and Alzheimer's disease. PloS One. 2011;6(6):e21108.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  152. 152.

    Sasabe J, Chiba T, Yamada M, Okamoto K, Nishimoto I, Matsuoka M, et al. D-serine is a key determinant of glutamate toxicity in amyotrophic lateral sclerosis. EMBO J. 2007;26(18):4149–59.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  153. 153.

    Chen Y, Stankovic R, Cullen KM, Meininger V, Garner B, Coggan S, et al. The kynurenine pathway and inflammation in amyotrophic lateral sclerosis. Neurotox Res. 2010;18(2):132–42.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  154. 154.

    Petrov D, Mansfield C, Moussy A, Hermine O. ALS clinical trials review: 20 years of failure. are we any closer to registering a new treatment? Front Aging Neurosci. 2017;9:–68.

  155. 155.

    Krol J, Loedige I, Filipowicz W. The widespread regulation of microRNA biogenesis, function and decay. Nat Rev Genet. 2010;11:597.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  156. 156.

    Filipowicz W, Bhattacharyya SN, Sonenberg N. Mechanisms of post-transcriptional regulation by microRNAs: are the answers in sight? Nat Rev Genet. 2008;9:102.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  157. 157.

    Bartel DP. MicroRNAs: target recognition and regulatory functions. Cell. 2009;136(2):215–33.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  158. 158.

    Lykke-Andersen J. Identification of a human decapping complex associated with hUpf proteins in nonsense-mediated decay. Mol Cell Biol. 2002;22(23):8114–21.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  159. 159.

    Parker R, Song H. The enzymes and control of eukaryotic mRNA turnover. Nat Struct Mol Biol. 2004;11(2):121–7.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  160. 160.

    van Dijk E, Cougot N, Meyer S, Babajko S, Wahle E, Séraphin B. Human Dcp2: a catalytically active mRNA decapping enzyme located in specific cytoplasmic structures. EMBO J. 2002;21(24):6915–24.

    PubMed  PubMed Central  Article  Google Scholar 

  161. 161.

    Bagga S, Bracht J, Hunter S, Massirer K, Holtz J, Eachus R, et al. Regulation by let-7 and lin-4 miRNAs Results in Target mRNA Degradation. Cell. 2005;122(4):553–63.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  162. 162.

    Guo H, Ingolia NT, Weissman JS, Bartel DP. Mammalian microRNAs predominantly act to decrease target mRNA levels. Nature. 2010;466:835.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  163. 163.

    Pillai RS, Bhattacharyya SN, Artus CG, Zoller T, Cougot N, Basyuk E, et al. Inhibition of translational initiation by Let-7 microRNA in human cells. Science. 2005;309(5740):1573.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  164. 164.

    Thomson DW, Bracken CP, Goodall GJ. Experimental strategies for microRNA target identification. Nucleic Acids Res. 2011;39(16):6845–53.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  165. 165.

    Babak T, Zhang W, Morris Q, Blencowe BJ, Hughes TR. Probing microRNAs with microarrays: tissue specificity and functional inference. RNA. 2004;10(11):1813–9.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  166. 166.

    Schonrock N, Ke YD, Humphreys D, Staufenbiel M, Ittner LM, Preiss T, et al. Neuronal microRNA deregulation in response to Alzheimer's disease amyloid-β. PLOS ONE. 2010;5(6):e11070.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  167. 167.

    Sempere LF, Freemantle S, Pitha-Rowe I, Moss E, Dmitrovsky E, Ambros V. Expression profiling of mammalian microRNAs uncovers a subset of brain-expressed microRNAs with possible roles in murine and human neuronal differentiation. Genome Biol. 2004;5(3):R13.

    PubMed  PubMed Central  Article  Google Scholar 

  168. 168.

    Gao F-B. Context-dependent functions of specific microRNAs in neuronal development. Neural Dev. 2010;5(1):25.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  169. 169.

    McNeill E, Van Vactor D. microRNAs shape the neuronal landscape. Neuron. 2012;75(3):363–79.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  170. 170.

    Schratt G. microRNAs at the synapse. Nat Rev Neurosci. 2009;10:842.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  171. 171.

    Morlando M, Dini Modigliani S, Torrelli G, Rosa A, Di Carlo V, Caffarelli E, et al. FUS stimulates microRNA biogenesis by facilitating co-transcriptional Drosha recruitment. EMBO J. 2012;31(24):4502–10.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  172. 172.

    Kawahara Y, Mieda-Sato A. TDP-43 promotes microRNA biogenesis as a component of the Drosha and Dicer complexes. Proc Natl Acad Sci. 2012;109(9):3347.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  173. 173.

    Benigni M, Ricci C, Jones AR, Giannini F, Al-Chalabi A, Battistini S. Identification of miRNAs as potential biomarkers in cerebrospinal fluid from amyotrophic lateral sclerosis patients. Neuro Mol Med. 2016;18(4):551–60.

    CAS  Article  Google Scholar 

  174. 174.

    de Andrade HMT, de Albuquerque M, Avansini SH, de S. Rocha C, Dogini DB, Nucci A, et al. MicroRNAs-424 and 206 are potential prognostic markers in spinal onset amyotrophic lateral sclerosis. J Neurol Sci. 2016;368:19–24.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  175. 175.

    De Felice B, Annunziata A, Fiorentino G, Borra M, Biffali E, Coppola C, et al. miR-338-3p is over-expressed in blood, CFS, serum and spinal cord from sporadic amyotrophic lateral sclerosis patients. Neurogenetics. 2014;15(4):243–53.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  176. 176.

    Freischmidt A, Müller K, Ludolph AC, Weishaupt JH. Systemic dysregulation of TDP-43 binding microRNAs in amyotrophic lateral sclerosis. Acta Neuropathologica Communications. 2013;1:42.

    PubMed  PubMed Central  Article  Google Scholar 

  177. 177.

    Freischmidt A, Müller K, Zondler L, Weydt P, Mayer B, von Arnim CAF, et al. Serum microRNAs in sporadic amyotrophic lateral sclerosis. Neurobiol Aging. 2015;36(9):2660.e15–20.

    CAS  Article  Google Scholar 

  178. 178.

    Freischmidt A, Müller K, Zondler L, Weydt P, Volk AE, Božič AL, et al. Serum microRNAs in patients with genetic amyotrophic lateral sclerosis and pre-manifest mutation carriers. Brain. 2014;137(11):2938–50.

    PubMed  Article  PubMed Central  Google Scholar 

  179. 179.

    Katsu M, Hama Y, Utsumi J, Takashina K, Yasumatsu H, Mori F, et al. MicroRNA expression profiles of neuron-derived extracellular vesicles in plasma from patients with amyotrophic lateral sclerosis. Neurosci Lett. 2019;708:134176.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  180. 180.

    Matamala JM, Arias-Carrasco R, Sanchez C, Uhrig M, Bargsted L, Matus S, et al. Genome-wide circulating microRNA expression profiling reveals potential biomarkers for amyotrophic lateral sclerosis. Neurobiol Aging. 2018;64:123–38.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  181. 181.

    Raheja R, Regev K, Healy BC, Mazzola MA, Beynon V, Von Glehn F, et al. Correlating serum micrornas and clinical parameters in amyotrophic lateral sclerosis. Muscle Nerve. 2018;58(2):261–9.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  182. 182.

    Saucier D, Wajnberg G, Roy J, Beauregard A-P, Chacko S, Crapoulet N, et al. Identification of a circulating miRNA signature in extracellular vesicles collected from amyotrophic lateral sclerosis patients. Brain Res. 1708;2019:100–8.

    Google Scholar 

  183. 183.

    Sheinerman KS, Toledo JB, Tsivinsky VG, Irwin D, Grossman M, Weintraub D, et al. Circulating brain-enriched microRNAs as novel biomarkers for detection and differentiation of neurodegenerative diseases. Alzheimer's Res Ther. 2017;9(1):89.

    Article  CAS  Google Scholar 

  184. 184.

    Takahashi I, Hama Y, Matsushima M, Hirotani M, Kano T, Hohzen H, et al. Identification of plasma microRNAs as a biomarker of sporadic amyotrophic lateral sclerosis. Mol Brain. 2015;8(1):67.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  185. 185.

    Tasca E, Pegoraro V, Merico A, Angelini C. Circulating microRNAs as biomarkers of muscle differentiation and atrophy in ALS. Clin Neuropathol. 2016;35(1):22–30.

    PubMed  Article  PubMed Central  Google Scholar 

  186. 186.

    Toivonen JM, Manzano R, Oliván S, Zaragoza P, García-Redondo A, Osta R. MicroRNA-206: a potential circulating biomarker candidate for amyotrophic lateral sclerosis. PLOS ONE. 2014;9(2):e89065.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  187. 187.

    Waller R, Goodall EF, Milo M, Cooper-Knock J, Da Costa M, Hobson E, et al. Serum miRNAs miR-206, 143-3p and 374b-5p as potential biomarkers for amyotrophic lateral sclerosis (ALS). Neurobiol Aging. 2017;55:123–31.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  188. 188.

    Waller R, Wyles M, Heath PR, Kazoka M, Wollff H, Shaw PJ, et al. Small RNA sequencing of sporadic amyotrophic lateral sclerosis cerebrospinal fluid reveals differentially expressed miRNAs related to neural and glial activity. Front Neurosci. 2018;11:731.

    PubMed  PubMed Central  Article  Google Scholar 

  189. 189.

    Xu Q, Zhao Y, Zhou X, Luan J, Cui Y, Han J. Comparison of the extraction and determination of serum exosome and miRNA in serum and the detection of miR-27a-3p in serum exosome of ALS patients. Intractable Rare Dis Res. 2018;7(1):13–8.

    PubMed  PubMed Central  Article  Google Scholar 

  190. 190.

    Joilin G, Leigh PN, Newbury SF, Hafezparast M. An overview of microRNAs as biomarkers of ALS. Front Neurol. 2019;10:186.

    PubMed  PubMed Central  Article  Google Scholar 

  191. 191.

    Ricci C, Marzocchi C, Battistini S. MicroRNAs as biomarkers in amyotrophic lateral sclerosis. Cells. 2018;7(11).

  192. 192.

    Varcianna A, Myszczynska MA, Castelli LM, O'Neill B, Kim Y, Talbot J, et al. Micro-RNAs secreted through astrocyte-derived extracellular vesicles cause neuronal network degeneration in C9orf72 ALS. EBioMedicine. 2019;40:626–35.

    PubMed  PubMed Central  Article  Google Scholar 

  193. 193.

    Liguori M, Nuzziello N, Introna A, Consiglio A, Licciulli F, D’Errico E, et al. Dysregulation of MicroRNAs and target genes networks in peripheral blood of patients with sporadic amyotrophic lateral sclerosis. Front Mol Neurosci. 2018;11:288.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  194. 194.

    Campos-Melo D, Droppelmann CA, He Z, Volkening K, Strong MJ. Altered microRNA expression profile in amyotrophic lateral sclerosis: a role in the regulation of NFL mRNA levels. Mol Brain. 2013;6(1):26.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  195. 195.

    Figueroa-Romero C, Hur J, Lunn JS, Paez-Colasante X, Bender DE, Yung R, et al. Expression of microRNAs in human post-mortem amyotrophic lateral sclerosis spinal cords provides insight into disease mechanisms. Mol Cell Neurosci. 2016;71:34–45.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  196. 196.

    Emde A, Eitan C, Liou L-L, Libby RT, Rivkin N, Magen I, et al. Dysregulated miRNA biogenesis downstream of cellular stress and ALS-causing mutations: a new mechanism for ALS. EMBO J. 2015;34(21):2633–51.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  197. 197.

    Rizzuti M, Filosa G, Melzi V, Calandriello L, Dioni L, Bollati V, et al. MicroRNA expression analysis identifies a subset of downregulated miRNAs in ALS motor neuron progenitors. Sci Rep. 2018;8(1):10105.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  198. 198.

    Bruneteau G, Simonet T, Bauché S, Mandjee N, Malfatti E, Girard E, et al. Muscle histone deacetylase 4 upregulation in amyotrophic lateral sclerosis: potential role in reinnervation ability and disease progression. Brain. 2013;136(8):2359–68.

    PubMed  Article  PubMed Central  Google Scholar 

  199. 199.

    Jensen L, Jørgensen LH, Bech RD, Frandsen U, Schrøder HD. Skeletal muscle remodelling as a function of disease progression in amyotrophic lateral sclerosis. Biomed Res Int. 2016;2016:5930621.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  200. 200.

    Kovanda A, Leonardis L, Zidar J, Koritnik B, Dolenc-Groselj L, Ristic Kovacic S, et al. Differential expression of microRNAs and other small RNAs in muscle tissue of patients with ALS and healthy age-matched controls. Sci Rep. 2018;8(1):5609.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  201. 201.

    Pegoraro V, Merico A, Angelini C. Micro-RNAs in ALS muscle: differences in gender, age at onset and disease duration. J Neurolog Sci. 2017;380:58–63.

    CAS  Article  Google Scholar 

  202. 202.

    Russell AP, Wada S, Vergani L, Hock MB, Lamon S, Léger B, et al. Disruption of skeletal muscle mitochondrial network genes and miRNAs in amyotrophic lateral sclerosis. Neurobiol Dis. 2013;49:107–17.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  203. 203.

    Williams AH, Valdez G, Moresi V, Qi X, McAnally J, Elliott JL, et al. MicroRNA-206 delays ALS progression and promotes regeneration of neuromuscular synapses in mice. Science (New York, NY). 2009;326(5959):1549–54.

    CAS  Article  Google Scholar 

  204. 204.

    Di Pietro L, Lattanzi W, Bernardini C. Skeletal muscle microRNAs as key players in the pathogenesis of amyotrophic lateral sclerosis. Int J Mol Sci. 2018;19(5):1534.

    PubMed Central  Article  CAS  Google Scholar 

  205. 205.

    Parisi C, Arisi I, D'Ambrosi N, Storti AE, Brandi R, D'Onofrio M, et al. Dysregulated microRNAs in amyotrophic lateral sclerosis microglia modulate genes linked to neuroinflammation. Cell Death Dis. 2013;4(12):e959.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  206. 206.

    Cunha C, Santos C, Gomes C, Fernandes A, Correia AM, Sebastião AM, et al. Downregulated glia interplay and increased miRNA-155 as promising markers to track ALS at an early stage. Mol Neurobiol. 2018;55(5):4207–24.

    CAS  PubMed  PubMed Central  Google Scholar 

  207. 207.

    Koval ED, Shaner C, Zhang P, du Maine X, Fischer K, Tay J, et al. Method for widespread microRNA-155 inhibition prolongs survival in ALS-model mice. Hum Mol Genet. 2013;22(20):4127–35.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  208. 208.

    Butovsky O, Jedrychowski MP, Cialic R, Krasemann S, Murugaiyan G, Fanek Z, et al. Targeting miR-155 restores abnormal microglia and attenuates disease in SOD1 mice. Ann Neurol. 2015;77(1):75–99.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  209. 209.

    Gallo A, Tandon M, Alevizos I, Illei GG. The majority of microRNAs detectable in serum and saliva is concentrated in exosomes. PLOS ONE. 2012;7(3):e30679.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  210. 210.

    Iftikhar H, Carney GE. Evidence and potential in vivo functions for biofluid miRNAs: from expression profiling to functional testing. BioEssays. 2016;38(4):367–78.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  211. 211.

    Turchinovich A, Weiz L, Langheinz A, Burwinkel B. Characterization of extracellular circulating microRNA. Nucleic Acids Res. 2011;39(16):7223–33.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  212. 212.

    Budnik V, Ruiz-Cañada C, Wendler F. Extracellular vesicles round off communication in the nervous system. Nat Rev Neurosci. 2016;17:160.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  213. 213.

    Krämer-Albers E-M, Hill AF. Extracellular vesicles: interneural shuttles of complex messages. Curr Opin Neurobiol. 2016;39:101–7.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  214. 214.

    Cocucci E, Meldolesi J. Ectosomes and exosomes: shedding the confusion between extracellular vesicles. Trends Cell Biol. 2015;25(6):364–72.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  215. 215.

    Aucher A, Rudnicka D, Davis DM. MicroRNAs transfer from human macrophages to hepato-carcinoma cells and inhibit proliferation. J Immunol. 2013;191(12):6250.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  216. 216.

    Dobrenis K, Chang HY, Pina-Benabou MH, Woodroffe A, Lee SC, Rozental R, et al. Human and mouse microglia express connexin36, and functional gap junctions are formed between rodent microglia and neurons. J Neurosci Res. 2005;82(3):306–15.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  217. 217.

    Fabbri M, Paone A, Calore F, Galli R, Gaudio E, Santhanam R, et al. MicroRNAs bind to toll-like receptors to induce prometastatic inflammatory response. Proc Natl Acad Sci. 2012;109(31):E2110.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  218. 218.

    Coleman LG Jr, Zou J, Crews FT. Microglial-derived miRNA let-7 and HMGB1 contribute to ethanol-induced neurotoxicity via TLR7. J Neuroinflammation. 2017;14(1):22.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  219. 219.

    Fabbri M. MicroRNAs and miRceptors: a new mechanism of action for intercellular communication. Philosophical Transactions of the Royal Society B: Biological Sciences. 2018;373(1737):20160486.

    Article  CAS  Google Scholar 

  220. 220.

    Zhang Y, Kim MS, Jia B, Yan J, Zuniga-Hertz JP, Han C, et al. Hypothalamic stem cells control ageing speed partly through exosomal miRNAs. Nature. 2017;548(7665):52–7.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  221. 221.

    Frühbeis C, Fröhlich D, Kuo WP, Amphornrat J, Thilemann S, Saab AS, et al. Neurotransmitter-triggered transfer of exosomes mediates oligodendrocyte–neuron communication. PLOS Biol. 2013;11(7):e1001604.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  222. 222.

    Ridder K, Keller S, Dams M, Rupp A-K, Schlaudraff J, Del Turco D, et al. Extracellular vesicle-mediated transfer of genetic information between the hematopoietic system and the brain in response to inflammation. PLOS Biol. 2014;12(6):e1001874.

    PubMed  PubMed Central  Article  CAS  Google Scholar 

  223. 223.

    Prada I, Gabrielli M, Turola E, Iorio A, D’Arrigo G, Parolisi R, et al. Glia-to-neuron transfer of miRNAs via extracellular vesicles: a new mechanism underlying inflammation-induced synaptic alterations. Acta Neuropathol. 2018;135(4):529–50.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  224. 224.

    Lehmann SM, Krüger C, Park B, Derkow K, Rosenberger K, Baumgart J, et al. An unconventional role for miRNA: let-7 activates Toll-like receptor 7 and causes neurodegeneration. Nat Neurosci. 2012;15:827.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  225. 225.

    Ramlackhansingh AF, Brooks DJ, Greenwood RJ, Bose SK, Turkheimer FE, Kinnunen KM, et al. Inflammation after trauma: microglial activation and traumatic brain injury. Ann Neurol. 2011;70(3):374–83.

    PubMed  Article  PubMed Central  Google Scholar 

  226. 226.

    Huang S, Ge X, Yu J, Han Z, Yin Z, Li Y, et al. Increased miR-124-3p in microglial exosomes following traumatic brain injury inhibits neuronal inflammation and contributes to neurite outgrowth via their transfer into neurons. FASEB J. 2017;32(1):512–28.

    PubMed  Article  CAS  PubMed Central  Google Scholar 

  227. 227.

    Philips T, Robberecht W. Neuroinflammation in amyotrophic lateral sclerosis: role of glial activation in motor neuron disease. Lancet Neurol. 2011;10(3):253–63.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  228. 228.

    Gowing G, Philips T, Van Wijmeersch B, Audet J-N, Dewil M, Van Den Bosch L, et al. Ablation of proliferating microglia does not affect motor neuron degeneration in amyotrophic lateral sclerosis caused by mutant superoxide dismutase. J Neurosci. 2008;28(41):10234.

    CAS  PubMed  PubMed Central  Article  Google Scholar 

  229. 229.

    Janssen HLA, Reesink HW, Lawitz EJ, Zeuzem S, Rodriguez-Torres M, Patel K, et al. Treatment of HCV infection by targeting microRNA. New England J Med. 2013;368(18):1685–94.

    CAS  Article  Google Scholar 

  230. 230.

    Beg MS, Brenner AJ, Sachdev J, Borad M, Kang Y-K, Stoudemire J, et al. Phase I study of MRX34, a liposomal miR-34a mimic, administered twice weekly in patients with advanced solid tumors. Invest New Drugs. 2017;35(2):180–8.

    CAS  PubMed  Article  PubMed Central  Google Scholar 

  231. 231.

    Fumagalli M, Lombardi M, Gressens P, Verderio C. How to reprogram microglia toward beneficial functions. Glia. 2018;66(12):2531–49.

    PubMed  PubMed Central  Article  Google Scholar 

Download references

Acknowledgements

We would like to thank Marion Brownridge for her generous support during the writing of this review.

Funding

EC was supported by the Sussex Neuroscience 4-year PhD programme. GJ and MH were supported by the Motor Neurone Disease Association (Hafezparast/Apr15/836-791 and Hafezparast/Apr18/861-791).

Author information

Affiliations

Authors

Contributions

EC drafted and edited the manuscript and prepared the table and additional files. GJ performed gene ontology analysis and prepared the figure. GJ and MH critically reviewed the manuscript. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Majid Hafezparast.

Ethics declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

Additional information

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Additional file 1.

List of miRNAs dysregulated in ALS. Dysregulated miRNAs in plasma, serum, cerebrospinal fluid (CSF), spinal cord tissue, skeletal muscle tissue, microglia and other cell types in ALS patients and animal models.

Additional file 2.

List of genes and biological processes targeted by dysregulated miRNAs. List of genes targeted by the six miRNAs found concurrently upregulated in the biological fluids of ALS patients and inside microglia from SOD1G93A mice. These are only the genes that have been experimentally validated to be targeted by each miRNA. Gene ontology analysis of these genes revealed enrichment in several biological processes, including processes commonly affected in ALS.

Rights and permissions

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Christoforidou, E., Joilin, G. & Hafezparast, M. Potential of activated microglia as a source of dysregulated extracellular microRNAs contributing to neurodegeneration in amyotrophic lateral sclerosis. J Neuroinflammation 17, 135 (2020). https://doi.org/10.1186/s12974-020-01822-4

Download citation

Keywords

  • Microglia
  • microRNA
  • Amyotrophic lateral sclerosis
  • Neurodegeneration