Lipopolysaccharide-induced neuroinflammation leads to the accumulation of ubiquitinated proteins and increases susceptibility to neurodegeneration induced by proteasome inhibition in rat hippocampus
© Pintado et al.; licensee BioMed Central Ltd. 2012
Received: 10 January 2012
Accepted: 12 March 2012
Published: 4 May 2012
Neuroinflammation and protein accumulation are characteristic hallmarks of both normal aging and age-related neurodegenerative diseases. However, the relationship between these factors in neurodegenerative processes is poorly understood. We have previously shown that proteasome inhibition produced higher neurodegeneration in aged than in young rats, suggesting that other additional age-related events could be involved in neurodegeneration. We evaluated the role of lipopolysaccharide (LPS)-induced neuroinflammation as a potential synergic risk factor for hippocampal neurodegeneration induced by proteasome inhibition.
Young male Wistar rats were injected with 1 μL of saline or LPS (5 mg/mL) into the hippocampus to evaluate the effect of LPS-induced neuroinflammation on protein homeostasis. The synergic effect of LPS and proteasome inhibition was analyzed in young rats that first received 1 μL of LPS and 24 h later 1 μL (5 mg/mL) of the proteasome inhibitor lactacystin. Animals were sacrificed at different times post-injection and hippocampi isolated and processed for gene expression analysis by real-time polymerase chain reaction; protein expression analysis by western blots; proteasome activity by fluorescence spectroscopy; immunofluorescence analysis by confocal microscopy; and degeneration assay by Fluoro-Jade B staining.
LPS injection produced the accumulation of ubiquitinated proteins in hippocampal neurons, increased expression of the E2 ubiquitin-conjugating enzyme UB2L6, decreased proteasome activity and increased immunoproteasome content. However, LPS injection was not sufficient to produce neurodegeneration. The combination of neuroinflammation and proteasome inhibition leads to higher neuronal accumulation of ubiquitinated proteins, predominant expression of pro-apoptotic markers and increased neurodegeneration, when compared with LPS or lactacystin (LT) injection alone.
Our results identify neuroinflammation as a risk factor that increases susceptibility to neurodegeneration induced by proteasome inhibition. These results highlight the modulation of neuroinflammation as a mechanism for neuronal protection that could be relevant in situations where both factors are present, such as aging and neurodegenerative diseases.
Neuroinflammation has distinct features that are shared in aging and in neurodegenerative diseases. Microglia are the main immune cell in the brain, playing a role in both physiological and pathological conditions [1–5]. Although acute neuroinflammation plays a protective role [6–8], chronic neuroinflammation is frequently considered detrimental and damaging to nervous tissue [2, 3]. Thus, whether neuroinflammation has beneficial or harmful outcomes in the brain may critically depend on both the duration of the inflammatory response and the kind of microglial activation . As the primary source for proinflammatory cytokines, microglia are implicated as a pivotal mediator of neuroinflammation and can induce or modulate a broad spectrum of cellular responses .
In relation to protein homeostasis, some proinflammatory cytokines, such as IFN-γ and TNF-α, can alter the proteolytic activity of the proteasome, leading to the switch to immunoproteasome (i-proteasome) [11, 12]. The proteasome is a molecular complex that controls intracellular protein homeostasis by degrading misfolded and/or regulatory proteins. It is made up of the 20 S-proteasome, a central unit carrying the catalytic activities, and several regulatory complexes such as PA700/19 S or PA28/11 S . The 26 S-proteasome (19 S-20 S-19 S) is responsible for the catalysis of the ATP-dependent degradation of polyubiquitinated proteins formed by a cascade of E1, E2 and E3 enzymes, which activate, conjugate and transfer, respectively, multiple ubiquitin molecules to protein substrates, thus targeting these for degradation [13–15].
As mentioned before, following IFN-γ or TNF-α stimulation, or after LPS injection, the constitutive catalytic subunits β1, β2 and β5 are replaced by the inducible catalytic subunits β1i, β2i and β5i, in order to form the i-proteasome that associates with the regulatory complex PA28/11 S [12, 16, 17]. Because substrates of proteasomes are short-lived regulatory proteins involved in cell differentiation, cell-cycle regulation, transcriptional regulation or apoptosis , a rapid and efficient elimination of proteins by the ubiquitin proteasome system (UPS) is essential under stress conditions that cause the accumulation of misfolded or partially denatured proteins .
Despite neuroinflammation and proteasome dysfunction being two significant hallmarks in many neurodegenerative diseases, the relationship between the factors is poorly explored. Here, we have evaluated the potential role of neuroinflammation on the UPS. Our results provide strong evidence supporting a synergic effect of neuroinflammation and proteasome dysfunction for hippocampal neurodegeneration.
Material and methods
Young (3 to 4 months) and aged (24 to 26 months) male Wistar rats were provided by the animal care facility of the University of Seville. All experiments were approved by local ethical committees and complied with international animal welfare guidelines.
Young male Wistar rats (200 to 250 g; n = 58) were processed for surgery as previously described [20, 21]. Different groups of animals were established, rats injected with LPS, rats injected with saline + LPS or saline + LT or LPS + LT, and control rats injected with saline only.
For the rats injected with LPS, the LPS (Sigma-Aldrich, St Louis, MO, USA) was dissolved (5 mg/mL) in a solution of sterilized PBS and 1 μL was injected into both hippocampi. The rats were anesthetized with 400 mg/kg chloral hydrate and positioned in a stereotaxic apparatus (Kopf Instruments, Tujunga, CA, USA). According to Paxinos’ atlas, the coordinates were: 3.3 mm posterior, 1.6 mm lateral and 3.2 mm ventral to the bregma and 4.8 mm posterior, 5.5 mm lateral and 6.0 mm ventral to the bregma. The injections were delivered over a period of 2 min and the needle was left in situ for an additional 5 min to avoid reflux along the injection track. Animals were decapitated at 3 hours, 6 hours, 14 hours, 24 hours, 3 days and 7 days after LPS injection and brains were quickly removed. Control animals were processed similarly but received 1 μL of sterilized PBS in both hippocampi.
The procedure for rats injected with saline + LPS or saline + LT or LPS + LT, the LT (Sigma-Aldrich) was dissolved (5 mg/mL) in a solution of sterilized PBS and 1 μL was injected into both hippocampi. For each case, saline or LPS was first administered and 24 hours later, LPS or LT was injected through the same drilled hole. Finally, animals were sacrificed 48 hours after the last injection. In addition, male Wistar aged rats (24-month-old, n = 3) were included in the saline + LT-injected group. Animals were processed similarly but the coordinates were 6.0 mm posterior, ± 4.6 mm lateral and 4.6 mm ventral to the bregma as previously shown .
Both hippocampi were dissected, frozen in liquid N2 and stored at −80°C until use. Hippocampi were homogenized in 700 μL of ice cold sucrose buffer (0.25 M sucrose, 1 mM ethylenediaminetetraacetic acid, 10 mM Tris–HCl, pH 7.4) supplemented with a protease inhibitor cocktail (Sigma-Aldrich). Three hundred microliters were separated and used for RNA isolation (see below). The remaining homogenized solution (400 μL) was centrifuged at 15,000 × g for 30 min at 4°C and the supernatant was recovered and stored at −80°C until use. Protein concentration was determined by the Lowry method.
RNA extraction, reverse transcription and real-time PCR
Total RNA extraction and reverse transcription was carried out with 300 μL of each homogenized hippocampi sample as previously described . Real-time PCR was performed in an ABI Prism 7000 sequence detector (Applied Biosystems, Madrid, Spain) using cDNA diluted in sterile water as a template. Analyzed genes were amplified using specific Taqman probes supplied by Applied Biosystems. Threshold cycle (Ct) values were calculated using the software supplied by Applied Biosystems.
Proteasome activity assay
Proteasome activity was determined in hippocampal samples using specific fluorogenic substrates for the chymotrypsin activity of the proteasome. Proteasome activity was abolished in the presence of 10 μM MG-132 .
Antibodies and immunoblots
The following primary antibodies were used in this study. Rabbit polyclonal anti-inducible nitric oxide synthase (iNOS; BD Bioscience, San José, CA, USA), anti-ubiquitin (Dako, Glostrup, Denmark); anti-β5i subunit (Abcam, Cambridge, UK), anti-proteasome maturation protein (POMP; Biomol, Madrid, Spain), anti-Bax, anti-Bak, anti-B-cell lymphoma extra large (Bcl-XL) and anti-Bcl-2 (Cell Signaling, Danvers, MA, USA), and anti-caspase-3 (Stressgen, Ann Arbor, MI, USA); mouse monoclonal anti-β-actin (Sigma-Aldrich) and anti-neuronal nuclei (NeuN; Chemicon, Billerica, MD, USA); horseradish-peroxidase-conjugated corresponding secondary antibodies (Dako); and secondary antibodies conjugated to DyLight fluorophores (Jackson Inmunoresearch, Madrid, Spain). Immunoblots were performed as previously described [20, 22].
Immunofluorescence and confocal microscopy
Animals were transcardially perfused with 4% paraformaldehyde and brains were processed as previously described . Sections 25 μm-thick were cut on a cryostat and mounted on gelatin-coated slides, permeabilized with 0.5% Triton (Sigma-Aldrich) overnight at room temperature, incubated with primary antibody anti-ubiquitin for 1 h at room temperature and overnight at 4°C and, finally, with the appropriate DyLightTM-conjugated-secondary antibodies for 1 h. Nuclei were counterstained with 4′-6-diamidino-2-phenylindole (DAPI) at a final concentration of 1 ng/μL after secondary antibody labeling. Control staining included omission of primary antibodies or irrelevant primary antibodies of the same isotype. Then, sections were washed and coverslipped with 0.01 M PBS containing 50% glycerin and 2.5% triethylenediamine and examined under a motorized upright wide-field microscope (Leica DM6000B). Confocal images were captured using a TCS SP5 Confocal Leica laser scanning microscope equipped with a DMI60000 microscope and an HCX PL APO lambda blue 63× 1.4 oil objective at 22°C. Maximum projection image was obtained.
Statistical analysis was performed using the Statgraphics plus (v 3.1) software. The differences between groups in the time-course experiments were assessed by one-way analysis of variance followed by Turkey’s test. The data comparison between the saline and saline + LT, saline + LPS and LPS + LT animals was carried out using two-tailed t-test. The significance was set at P <0.05. Significant differences are indicated by an asterisk.
LPS injection increases the content of ubiquitinated proteins in hippocampal neurons
LPS injection up-regulates the mRNA expression of the E2 ubiquitin-conjugating enzyme UB2L6, decreases proteasome activity and increases immunoproteasome biogenesis
LPS-induced neuroinflammation increases neurodegeneration produced by proteasome inhibition
Because LPS injection increased the content of ubiquitinated proteins in neurons and decreased proteasome activity, we wondered whether neuroinflammation could increase susceptibility to cellular death induced by proteasome inhibition. For that, we produced proteasome inhibition 24 h after LPS injection, exactly when ubiquitinated proteins accumulated and proteasome activity was decreased (see above).
In the present work we have evaluated the potential role of neuroinflammation as a synergic risk factor for hippocampal neurodegeneration induced by proteasome inhibition. Our results demonstrated that LPS injection, in addition to the classical neuroinflammatory response characterized by the production of proinflammatory mediators, also altered protein homeostasis. In fact, LPS injection produced neuronal accumulation of ubiquitinated proteins; de novo i-proteasome biogenesis; a transient decrease of the proteasome activity; and a robust and sustained transcriptional up-regulation of the E2 ubiquitin-conjugating enzyme UB2L6. Thus, the LPS-induced accumulation of ubiquitinated proteins in hippocampal neurons could be consequence of both increased ubiquitin conjugation activity, to meet the substrate demands of a strongly up-regulated antigen presentation machinery, and the shift from constitutive to i-proteasome upon neuroinflammation, to increase the peptide supply for antigen presentation [21, 23, 26–29]. Taken together, these data indicate that LPS-induced modification of protein homeostasis could be part of a more general neuroinflammatory response to increase the production of peptides for antigen presentation [17, 30]. In support of this, viability of neurons was well-preserved during the adaptation to this short phase of reduced proteolytic activity, as judge by the absence of Fluoro-Jade B staining and the predominant expression of pro-survival proteins (see also ). However, under this scenario (neuroinflammatory response), neurons become more vulnerable to proteasome inhibition. Indeed, proteasome inhibition after neuroinflammation (LPS + LT) leads to an early and predominant expression of pro-apoptotic proteins, followed by a qualitative increase in the number of degenerating neurons. Moreover, this combined treatment produced the formation of aggresome-like structures exclusively in the LPS + LT-treated rats. Similar structures have been previously observed in vitro after treatment with IFN-γ in cells lacking i-proteasome, suggesting that i-proteasome plays a pivotal role in both cytokine-mediated inflammation and the clearance of damage proteins and aggresome-like structures . In this sense, our in vivo results are supporting this view. LPS injection produced a sustained expression of i-proteasome and the accumulation of ubiquitinated proteins in hippocampal neurons without the appearance of aggresome-like structures. However, proteasome inhibition in rats expressing a higher proportion of i-proteasome (that is, following LPS injection) led to the formation of neuronal aggresome-like structures. Importantly, all these biochemical and cellular modifications were not observed when proteasome inhibition or neuroinflammation were induced separately. Thus, present data strongly indicate that neuroinflammation is acting as a synergic risk factor for intracellular protein accumulation and neurodegeneration.
Having in mind that acute neuroinflammation induced by LPS injection in young animals is not at all similar to neuroinflammatory processes occurring during normal aging (chronic neuroinflammation), present findings could be relevant in the context of hippocampal aging. In this sense, aged rat hippocampus is characterized by chronic neuroinflammation [2, 4, 31]; decreased proteasome activity [32, 33]; accumulation of ubiquitinated proteins [20, 22]; higher proportion of i-proteasome ; and absence of significant neurodegeneration of pyramidal neurons . Interestingly, LT injection alone in aged hippocampus reproduced protein accumulation observed in young rats injected with LPS + LT. Moreover, as we have previously shown, LT injection in aged rats leads to a predominant expression of pro-apoptotic proteins Bax, Bak and caspase-3 in addition to a higher neurodegeneration compared to young rats subjected to LT injection .
All of these age-related modifications can be reproduced in young animals only when proteasome was inhibited during the development of a neuroinflammatory response (LPS + LT). Thus, present and previous data support the idea that chronic neuroinflammation, as occurs in normal aging and in age-related neurodegenerative diseases (see below), should be considered as a synergic risk factor for neurodegeneration under situations of proteasome dysfunction (see  for a detailed review). In consequence, the modulation of neuroinflammation could represent an attractive therapeutic target in order to delay the onset and/or progression of neurodegeneration associated to the age-related neurodegenerative diseases . Neuronal i-proteasome expression, which is almost absent in young healthy human brains, has been detected in the hippocampus from elderly patients as well as in patients affected by Alzheimer’s disease . i-proteasome expression has also been observed in neurons localized in different brain areas from patients with Huntington’s disease, multiple sclerosis and temporal lobe epilepsy, diseases coursing with chronic neuroinflammation [36–38]. This raises the question whether i-proteasome expression has a protective role, as a homeostatic attempt of neurons to cope with the progressive accumulation of damage proteins, or, by contrast, has a deleterious effect. In this sense, present and previous data show evidence for both possibilities: i-proteasome expression may have a protective role in the context of acute neuroinflammation, but a detrimental effect when neuroinflammation become chronic as occurs in the majority of neurodegenerative diseases.
The mechanisms underlying the neuroprotective role of i-proteasome in the context of acute neuroinflammation and its detrimental properties in the context of chronic neuroinflammation are currently unknown. The comprehension of the physiological role of i-proteasome in neuroinflammation and its participation in neurodegenerative diseases coursing with neuroinflammation is an expanding area in biomedical research. In this sense, the combination of neuroinflammation and proteasome inhibition may be a plausible model for the study of the physiological role of i-proteasome upon neuroinflammation and their involvement in diseases with a neuroinflammatory component. In summary, we report evidence supporting the idea that the two main hallmarks of age-related neurodegenerative diseases may form a neurodegenerative loop.
Authors thank Paloma Dominguez (from CABIMER) for technical assistance in confocal microscopy and Ana Ruano for technical support. This work was supported by grants PS09/00848 (to DR), PS09/00151 (to JV) and PS09/00099 (to AG) from the Carlos III Health Institute, Spain; CVI-3199 (to RMR) and SAS PI0496/2009 (to AG) from the Junta de Andalucia. CP was recipient of a fellowship from the Spanish Ministry of Education and Science, Spain. EG is supported by a fellowship from JA and MPG by a Juan de la Cierva contract from MICINN, Spain.
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