Identification of a specific α-synuclein peptide (α-Syn 29-40) capable of eliciting microglial superoxide production to damage dopaminergic neurons
© The Author(s). 2016
Received: 3 December 2015
Accepted: 1 June 2016
Published: 21 June 2016
Misfolded α-synuclein (α-Syn) aggregates participate in the pathogenesis of synucleinopathies, such as Parkinson’s disease. Whereas much is known about how the various domains within full-length α-Syn (FL-α-Syn) contribute to the formation of α-Syn aggregates and therefore to their neurotoxicity, little is known about whether the individual peptides that can be generated from α-syn, possibly as intermediate metabolites during degradation of misfolded α-Syn aggregates, are neurotoxic themselves.
A series of synthesized α-Syn peptides, corresponding to the locus in FL-α-Syn containing alanine 30, substitution of which with a proline causes a familial form of Parkinson’s disease, were examined for their capacity of inducing release of microglial superoxide. The neurotoxicity of these peptides was measured according to their influence on the ability of neuroglial cultures deficient in gp91 phox , the catalytic unit of NADPH oxidase (Nox2), or wild-type cultures to take up 3H-labeled dopamine and on the number of tyrosine hydroxylase-staining-positive neurons. Western blots and confocal images were utilized to analyze membrane translocation of p47 phox and p67 phox , phosphorylation of p47 phox and Erk1/2 kinase, and binding of α-Syn peptides to gp91 phox . Activation of brain microglia in mice injected with α-Syn peptides was demonstrated by immunostaining for major histocompatibility complex (MHC)-II along with qPCR for Iba-1 and MHC-II.
We report α-Syn (29-40) as a specific peptide capable of activating microglial Nox2 to produce superoxide and cause dopaminergic neuronal damage. Administered to mice, this peptide also activated brain microglia to increase expression of MHC-II and Iba-1 and stimulated oxidation reaction. Exploring the underlying mechanisms showed that α-Syn (29-40) peptide triggered Nox2 to generate extracellular superoxide and its metabolite H2O2 by binding to the catalytic unit gp91 phox of Nox2; diffusing into cytosol, H2O2 activated Erk1/2 kinase to phosphorylate p47 phox and p67 phox and further activated Nox2, establishing a positive feedback loop to amplify the Nox2-mediated response.
Collectively, our study suggests novel information regarding how α-Syn causes neuronal injury, possibly including mechanisms involving abnormal metabolites of α-Syn aggregates.
Keywordsα-Synuclein gp91 phox Superoxide Neuronal damage Microglia
Aggregation of α-synuclein (α-Syn) in the central nervous system (CNS) is a fundamentally important pathological process involved in the development and progression of synucleinopathies including Parkinson’s disease (PD) . α-Syn aggregates, predominantly in oligomeric forms, are thought to be capable of causing neurodegeneration either by directly damaging neurons or by activating microglia [2–5], immune cells in the CNS, to produce inflammatory mediators (e.g., tumor necrosis factor-α (TNF-α)) and reactive oxygen species (ROS) that are neurotoxic [3, 6].
Native α-Syn, composed of 140 amino acids, is traditionally divided into three domains: residues 1–60 form the apolipoprotein-binding domains; residues 61–95 make up a central hydrophobic fragment with a non-amyloid-β component (NAC) region; and residues 96–140 construct a proline-rich region . The physiological function of α-Syn, though it remains to be characterized further, includes the regulation of vesicular transport and neurotransmitter release [8, 9]. In pathological conditions, α-Syn can aggregate, leading to neurodegeneration, and it appears that various regions of α-Syn contribute to the process following different mechanisms of action. Specifically, the NAC region tends to form β-sheets, via a nucleation-dependent kinetic mechanism [10–12]. The apolipoprotein-binding domain also plays a role in aggregation, especially when mutated. For instance, the familial PD-related A30P (alanine 30 to proline-mutated α-synuclein) and A53T (alanine 53 to threonine-mutated α-synuclein) mutations, in which the alanine (A) 30 is replaced by a proline (P) and the alanine (A) 53 by a threonine (T), respectively, dramatically accelerate the initial oligomerization of α-Syn [13–15]. Remarkably, our previous experiments supported the notion that the A30P or A53T forms of the full-length protein (FL-α-Syn) activated microglia more potently than their corresponding wild type  and led to greater dopaminergic (DA) neuronal damage. This finding suggests that specific sequences of amino acids in α-Syn could be essential to enhanced neurotoxicity by activating microglia more effectively.
To begin testing this hypothesis, in this investigation, we compared the ability of various peptides corresponding to the A30P region to activate microglia and identified a specific peptide α-Syn (29–40) (i.e., consisting of amino acids A29-V40 of α-Syn) that can directly activate microglial NADPH oxidase (Nox2) to produce superoxide by binding to gp91 phox , the catalytic subunit of this enzyme. The discovery was further validated in vivo, and the underlying mechanisms were found to be related to the phosphorylation of p47 phox by Erk1/2 kinase, which established a positive feedback loop to achieve a full activation of Nox2.
Mice lacking in gp91 phox (gp91 phox−/−) and their wild-type (WT) controls with a C57BL/6J background were provided by the Jackson Laboratory. Animal housing, breeding, and experiments were approved by the Institutional Animal Care Committee of the National Institute of Environmental Health Sciences and performed by strictly following the National Institutes of Health guidelines.
To determine whether the tested α-Syn peptide can pass across the blood-brain barrier (BBB) and diffuse into the interstitial tissues in the brain, mice were injected with saline or fluorescein isothiocyanate (FITC)-labeled α-Syn peptide (5 mg/kg) (ABI Scientific, Inc. Sterling, VA) along with Texas red-labeled lectin (30 μl/mouse) (Vector Laboratories, Inc. Burlingame, CA) for 6 h. The brain tissues were sectioned and stained by using anti-ionized calcium-binding adaptor molecule 1 (Iba-1) antibody (Wako Chemicals USA, Richmond, VA) and 4′,6-diamidino-2-phenylindole, dihydrochloride (DAPI, a fluorescent dye for nuclei) (Sigma-Aldrich, St. Louis, MO). To examine whether injection of α-Syn peptide leads to microglial activation, mice were injected with saline or unlabeled α-Syn peptide (5 mg/kg) for 24 h and further injected with Texas red-labeled lectin 10 min before sacrifice. Brain tissues were harvested and sectioned (30 μm in thickness) for staining nuclei, Iba-1 (using 019-19741 Ab; Wako Chemicals, Richmond, VA), and major histocompatibility complex II (MHC-II) (using M5/114.15.2 Ab; BioLegend, San Diego, CA) or for biochemical assays (e.g., real-time quantitative PCR (RT-qPCR) and malondialdehyde (MDA)). See below for details.
Preparation of α-Syn aggregates
Dissolved in water at 2 mg/ml, recombinant human α-Syn (rPeptide, Bogart, GA) (endotoxin level: <0.024 EU/μg) was incubated with agitation at 37 °C for 7 days to allow for α-Syn to form aggregates [4, 17]. The concentration of α-Syn aggregates is calculated in terms of the initial concentration of synuclein.
Design and synthesis of α-Syn peptides
Peptides comprising 12 amino acid residues were chemically synthesized (ABI Scientific, Sterling, VA) according to the amino acid sequence covering the alanine (A) 30-containing loci, which are located within the N-terminal amphipathic region. In the synthesized mutant α-Syn peptides, the alanine (A) 30 was replaced by a P (A30P mutants). The first mutated peptide spans A19 to P30. Shifts occur every two amino acid residues between adjacent peptides. The corresponding amino acid sequences (with the replaced amino acids marked in italics) are AEKTKQGVAEAP for A19-P30 (A30P), KTKQGVAEAPGK for K21-K32 (A30P), KQGVAEAPGKTK for K23-K34 (A30P), GVAEAPGKTKEG for G25-G36 (A30P), AEAPGKTKEGVL for A27-L38 (A30P), and APGKTKEGVLYV for A29-V40 (A30P). In the screening test using O2 • − production assay, raw peptides with a purity of approximately 75 % were used individually or together (referred to as “AP pooled”) to stimulate mouse microglia. The experiments were performed following a double blind strategy. Based on the results of screening tests, specific peptides with a purity >98 % were selected to validate the screening test.
Isolation of microglia
Microglia were prepared as previously described . Briefly, brain cells of mouse pups were collected on postnatal day one (P1) and seeded into poly-d-lysine-coated (Sigma-Aldrich, St. Louis, MO) T175 flasks which were filled with Dulbecco’s modified Eagle medium (DMEM)-F12 (Life Technologies, Grand Island, NY) media containing 10 % fetal bovine serum, 100 U/ml penicillin and streptomycin, 2.0 mM glutamine, 2.0 mM nonessential amino acids, and 1.0 mM sodium pyruvate. Two weeks later, microglia were suspended and harvested by shaking the flasks at 180 rpm for 30 min.
Primary neuron-glia, neuron-astrocyte, and reconstituted neuron-glia cultures
Neuron-glia cultures were established by following the procedure described previously . The cells of mesencephalic tissues were collected from mouse or rat embryos on embryonic day fourteen (E14) and seeded onto poly-d-lysine-coated 24-well plates in DMEM media (Gibco, Grand Island, NY) supplemented with 10 % fetal bovine serum, 10 % horse serum, 2.0 mM glutamine, 2.0 mM nonessential amino acids, 100 U/ml penicillin and streptomycin, 1.0 mM sodium pyruvate, and 5.6 mM glucose. Typically, immunostaining demonstrated that intact neuron-glia cultures were composed of approximately 40 % neurons (NeuN immunopositive cells), 50 % astrocytes (GFAP immunopositive cells), and 10 % microglia (OX-42 immunopositive cells) . To make neuron-astrocyte cultures, the cells of mesencephalic tissues were collected and seeded into the wells of 24-well plates exactly as in the preparation of neuron-glia cultures. Forty-eight hours after seeding the cells, 2 mM l-leucine methyl ester (Sigma-Aldrich, St. Louis, MO), a compound that is capable of removing microglia but has no influence on the growth of other cell types, was added to each well. Six days after the initial cell seeding, neuron-astrocyte cultures were achieved, which usually contain 44 % neurons, 55 % astrocytes, and less than 0.5 % microglia. To prepare reconstituted neuron-glia cultures, an identical number of freshly isolated WT or gp91 phox−/− mouse microglia were loaded to rat neuron-astrocyte cultures on day 6 after seeding cells, allowing the newly added microglia to establish cell-cell interactions with neurons and astrocytes. These reconstituted neuron-glia cultures have the same cellular composition as that in the original neuron-glia cultures, including approximately 40 % neurons, 50 % astrocytes, and 10 % microglia. The next day, these reconstituted neuron-glia cultures along with neuron-glia cultures and neuron-astrocyte cultures were treated by adding 150 EU/ml lipopolysaccharide (LPS) (Calbiochem, San Diego, CA) or 1 μM α-Syn peptides. In some experiments, dimethyl sulfoxide (DMSO) or 0.25 mM apocynin (Apo) was added to the cultures 30 min before loading LPS or α-Syn peptides. Seven days after the treatments, all of the cultures were either subjected to dopamine (DA) uptake measurements or immunochemically stained for tyrosine hydroxylase (TH).
O 2 • − production assay
O 2 • − produced by mouse microglia was detected based on the superoxide dismutase (SOD)-inhibitable reduction of the tetrazolium salt WST-1 (Santa Cruz Biotech. Dallas, TX) . Briefly, seeded onto 96-well plates overnight, mouse microglia were washed with Hanks’ balanced salt solution (HBSS) without phenol red followed by adding to each well with 50 μl of HBSS with or without 50 U/ml SOD (Sigma-Aldrich, St. Louis, MO), 50 μl of HBSS containing 1 mM WST-1, and 50 μl of HBSS containing the vehicle control or 1 μM of each α-Syn peptide. The absorbance at 450 nm of each sample was immediately measured using a spectrophotometer (Molecular Devices, Sunnyvale, CA) every 2 min for 30 min. The level of O2 • − production was calculated in terms of the difference between the absorbance values in the presence and absence of SOD.
Measurement of DA uptake
After being stimulated with 150 EU/ml LPS, 250 nM α-Syn aggregates, or 1.0 μM α-Syn peptides, neuron-glia, neuron-astrocyte, and reconstituted neuron-glia cultures were incubated for 20 min at 37 °C with 3H-labeled DA (PerkinElmer Life Sciences, Santa Clara, CA) in Krebs-Ringer buffer (16 mM sodium phosphate, 119 mM NaCl, 4.7 mM KCl, 1.8 mM CaCl2, 1.2 mM MgSO4, 1.3 mM EDTA, and 5.6 mM glucose; pH 7.4). Cells were washed with cold Krebs-Ringer buffer and lysed in 1 N NaOH. The radioactivity of the lysates was measured using a liquid scintillation counter (PerkinElmer, Santa Clara, CA) and calculated by subtracting nonspecific DA uptake detected in samples in the presence of 10 μM mazindol (Sigma-Aldrich, St. Louis, MO) from the results of samples that did not include mazindol. Data were presented as the percentage of controls in the same batch of cell cultures .
IC or IF staining
For immunochemical (IC) staining, neuron-glia, neuron-astrocyte, and reconstituted neuron-glia cultures were fixed after stimulation with either 150 EU/ml LPS or 1 μM α-Syn peptides for 7 days. Endogenous peroxidase in the fixed cultures was neutralized with 1 % hydrogen peroxide for 20 min followed by sequentially incubating the cultures together with a TH antibody (Chemicon International, Billerica, MA), biotinylated secondary antibody, and ABC reagents (Dako, Carpinteria, CA). The reaction was revealed by adding 3,3-diaminobenzidine (Thermo Scientific, Waltham, MA) and analyzed using a microscopy coupled with a ×10 objective (Nikon, Tokyo, Japan). The amount of TH-positive cells, which represent dopaminergic neurons, was calculated as the percentage of controls from the same batch of cell cultures . For immunofluorescence (IF) staining of p47 phox and CD11b, live microglia were first stimulated by 100 nM phorbol 12-myristate 13-acetate (PMA) or 1 μM α-Syn peptides for 30 min at 37 °C. Cells were further incubated on ice with rat anti-mouse CD16/32 IgG (BD Biosciences, San Jose, CA), which blocks Fc receptors on microglia, followed by immunostaining with an APC-conjugated anti-mouse CD11b Ab (BioLegend, San Diego, CA) for 30 min. After being fixed in 4.0 % paraformaldehyde and permeabilized in phosphate-buffered saline (PBS) containing 0.05 % Triton-X, cells were stained for p47 phox using Alexa 488-labeled anti-mouse antibody (Bioss Inc. Woburn, MA) and were further stained by DAPI to visualize the nuclei. To demonstrate the localization of gp91 phox and A29-V40 peptide or A19-A30 peptide, freshly isolated WT or gp91 phox−/− microglia were fixed in 4 % paraformaldehyde and permeabilized by 0.05 % Triton-X. After preincubation with rat anti-mouse CD16/32 IgG, cells were reacted with mouse anti-gp91 phox Ab (Santa Cruz Biotech. Dallas, TX), followed by adding Alexa 568-labeled secondary Ab. Samples were further incubated with FITC-labeled A29-V40 peptide or FITC-labeled A19-A30 peptide along with DAPI and analyzed with a Zeiss 510 confocal microscopy (Carl Zeiss, Pleasanton, CA). For immunofluorescent staining in thick tissues , brain slices (30 μm in thickness) were fixed in 4 % paraformaldehyde for 2 h and blocked in PBS containing 0.5 % Triton-X, 5 % fetal bovine serum, and 5 % goat serum at room temperature (RT) for 2 h. The tissues were further incubated with an anti-Iba-1 Ab (Wako Chemicals, Richmond, VA) in cold room overnight. After multiple washes, samples were incubated with an Alexa 647-conjugated goat anti-rabbit secondary Ab (Life Technologies, Grand Island, NY) along with an Alexa 488-labeled rat anti-mouse MHC-II Ab (M5/114.15.2 Ab; BioLegend, San Diego, CA) and DAPI at RT for 2 h. After washes, tissues were imaged by using a Zeiss 510 confocal microscopy (Carl Zeiss, Pleasanton, CA).
Detection of membrane translocation of p47phox and p67phox via Western blot
Highly aggressively proliferating immortalized (HAPI) cells were stimulated using 0.1 % BSA, 100 nM PMA, or 1 μM each α-Syn peptide and then lysed in a hypotonic lysis buffer (1 mM Tris, 1 mM KCl, 1 mM EGTA, 1 mM EDTA, and 0.1 mM DTT) supplemented with 10 mg/ml of the protease inhibitor mixture (Thermo Scientific, Waltham, MA) and 1 mM PMSF (Thermo Scientific, Waltham, MA). After dounce homogenization (20–25 St, tight pestle A), the cell lysates were centrifuged at 1600×g for 15 min, and the supernatants were further centrifuged at 40,000×g for 2 h. The high-speed centrifugation supernatants, which contain the cytosolic fraction, were then harvested. Meanwhile, the plasma membrane fragments were collected by dissolving the pellets in 1 % nonidet P-40 hypotonic lysis buffer. Proteins in both the high-speed supernatants and the re-suspended pellets were separated by running SDS/PAGE gels and were transferred to polyvinylidene difluoride (PVDF) membranes, followed by immune-blotting using anti-p47 phox , anti-p67 phox , and anti-gp91 phox Abs (Cell Signaling, Danvers, MA). The relative amount of each protein was quantified using the Quantity One program (Bio-Rad, Redmond, WA). The glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in whole-cell lysates were immunoblotted by an Ab (Cell Signaling, Danvers, MA) as a loading control .
Flag-fused α-Syn peptides (ABI Scientific Inc., Sterling, VA) and cell lysates, which were collected from WT or gp91 phox−/− mixed glial cultures or enriched microglia by using IP lysis buffer (Thermo Scientific, Waltham, MA), were mixed in a cold room overnight in the presence of protease inhibitor cocktail (Thermo Scientific, Waltham, MA). Simultaneously, goat anti-gp91 phox polyclonal antibodies (Santa Cruz Biotech., Dallas, TX) or control IgGs were conjugated to protein G magnetic beads using a Dynabeads protein G immunoprecipitation kit (Life Technologies, Grand Island, NY) at RT for 10 min. Next, the α-Syn peptides and cell lysate mixtures were incubated with the antibody- or control IgG-conjugated protein G magnetic beads at RT for 30 min. After washing, the beads were boiled in Laemmli sample buffer and the supernatants were resolved using SDS-PAGE. The proteins were transferred to Nitrocellulose membranes (0.2 μm, Bio-Rad Laboratories Inc., Redmond, WA) and immunoblotted using mouse antibodies against Flag (Origene, Rockville, MD), gp91 phox (Santa Cruz Biotech. Dallas, TX), or tubulin (Abcam, Cambridge, MA).
Isolation of total RNA and qPCR
Isolation of total RNA from the brain tissues was performed by using TRIzol according to the manufacturer’s instructions (Life Technologies, Grand Island, NY). Quality and quantity of total RNA were measured by using the Agilent 2100 Bioanalyzer (Agilent Technologies, Inc., Santa Clara, CA). Reverse transcriptase reactions were run on 1 μg of total messenger RNA (mRNA) by using the reverse transcription system (Promega, Madison, WI). Quantitative PCR was performed by using a Light Cycler 480 (Roche, Indianapolis, IN) and the SYBR Green I Master (Roche, Indianapolis, IN) according to the manufacturer’s instructions. Primers for mouse Iba-1 were 5′-GGATTTGCAGGGAGGAAAAG-3′ and 5′-TGGGATCATCGAGGAATTG-3′; primers for mouse MHC-II were 5′-CCG TCA CAG GAG TCA GAA AGG-3′ and 5′-CGG AGC AGA GAC ATT CAG GTC-3′; and primers for β-actin were 5′-CCCAAGGCCAACCGCGAGAAGAT-3′ and 5′-GTCCCGGCCAGCCAGGTCCAG-3′.
Mouse brain tissues were homogenized on ice using 300 μl of the MDA lysis buffer according to the manufacturer’s guidance (Sigma-Aldrich, St. Louis, MO) (with 3 μl of butylated hydroxytoluene [100X]), followed by centrifugation (13,000×g, 10 min) to remove insoluble materials. The supernatant (200 μl) from each homogenized sample was placed into a micro-centrifuge tube and incubated with 600 μl of thiobarbituric acid solution at 95 °C for 60 min. Cooled to RT in an ice bath for 10 min, 200 μl of mixtures was pipetted into a 96-well microplate and analyzed by using a fluorescence plate reader (Ex: 532 nm; Em: 553 nm) (Molecular Devices, Sunnyvale, CA).
Presented as the means ± SEM, data were evaluated using analysis of variance (ANOVA) followed by multiple comparison tests. If the P value is less than 0.05, results are considered statistically significant.
A29-V40 (A30P) α-Syn peptide stimulates microglia to produce superoxide (O 2 • −)
These findings suggest that the regions covering the A30P locus possibly contribute to activation of microglia and the ensuing neuroinflammation-mediated dopaminergic neuronal damage. This hypothesis led us to systematically examine the capacity of peptides corresponding to the amino acid sequences in the proximity of alanine 30 to activate microglia and induce neuronal damage. For this purpose, we synthesized six peptides consisting of 12 amino acids, all of which included the A30P substitution, following the strategy illustrated in Fig. 1c. For each consecutive peptide within this set, the first included amino acid was shifted by two amino acids toward the C-terminus. These peptides with a purity of 75 %, either individually or together, were added to stimulate primary mouse microglia. Superoxide released from microglia was significantly increased (149.4 ± 23.2 % increase) after exposure to the mixture of six peptides harboring the A30P mutation. However, when each of these peptides was added individually to mouse microglial cultures, A29-V40 (A30P, APGKTKEGVLYV) was the only peptide capable of stimulating the release of microglial superoxide (Fig. 1d) approximately 158.0 ± 21.4 % more than the BSA control. In another set of experiments, we tested whether the A30P substitution was critical for generating the elevated microglial response using highly purified peptides (purity >98 %) and found that WT and A30P-mutated A29-V40 peptides were equipotent in activating microglia to release superoxide (128.6 ± 22.2 % and 192.6 ± 47.1 % increase, respectively, compared to the BSA control), suggesting that at the concentration of 1.0 μM, the presence of A30P mutation does not enhance peptide-induced microglial release of superoxide (Fig. 1e). This is different from our previous finding that the increase of superoxide production induced by A30P-mutated FL-α-Syn aggregates (≈250 %) was significantly greater than by its WT form (≈90 %) .
Unlike FL-α-Syn aggregates, WT A29-V40 peptide exhibits similar toxicity to dopaminergic neurons as its A30P mutant
Nox2 activity is essential to A29-V40 peptide-induced damage of dopaminergic neurons
Nox2 activation in microglia rather than in neuron-astrocytes results in A29-V40 peptide-induced neurotoxicity
A29-V40 activates Nox2 by binding to gp91 phox , which is located on cell membranes
As a membrane-permeable compound, PMA can enter the cytosol and directly activate PKC to phosphorylate p47 phox and p67 phox , eventually leading to Nox2 activation via an “inside to outside” signal pathway . However, A29-V40 peptide and PMA share no similarity in biochemical characteristics, prompting us to speculate that PMA and A29-V40 peptide activate Nox2 through distinct pathways. In this regard, previous studies have reported that some small molecules [28, 29] can directly target and activate the Nox2 catalytic unit gp91 phox . Therefore, we tested whether A29-V40 peptide can bind to gp91 phox . Supporting this idea, when gp91 phox antibody-conjugated magnetic beads were incubated with a mixture of Flag-tagged A29-V40 peptide and WT microglial lysates, both A29-V40 peptide and gp91 phox were precipitated, while neither was precipitated by beads conjugated to control IgG. However, the beads failed to pull down A29-V40 peptide from the mixture composed of Flag-tagged A29-V40 peptide and gp91 phox−/− cell lysates (Fig. 5f) or precipitate A19-A30 control peptide from the mixture composed of Flag-tagged A19-A30 peptide and WT cell lysates (Fig. 5g), suggesting that A29-V40 peptide is a specific peptide capable of binding to gp91 phox . This idea was further confirmed by the observation that FITC-labeled A29-V40 peptide could stick to the cell surface of WT but not gp91 phox−/− microglia whereas A19-A30 peptide failed to bind to the cell surface of WT microglia (Fig. 5h).
Exposure to A29-V40 peptide activates Erk1/2 to phosphorylate p47 phox and to amplify Nox2 activation, dependent on the presence of H2O2
As key components in the Ras-Raf-MEK-ERK signal transduction pathway, ERK1 and ERK2 are related to protein-serine/threonine kinases that may cause phosphorylation of p47 phox . Therefore, we sought to determine whether Erk1/2 phosphorylation is necessary for p47 phox phosphorylation. Pretreatment with the Erk1/2 inhibitor U0126  abolished Erk1/2 phosphorylation in PMA or A29-V40 peptide-stimulated WT microglia to levels even lower than controls. This inhibitor also led to a lower level of p47 phox phosphorylation in A29-V40 peptide-stimulated WT microglia but had no effect on PMA-induced p47 phox phosphorylation (Fig. 6e, g). Consistent with data shown in Fig. 6b, d, exposure of gp91 phox−/− microglia to A29-V40 peptide could neither activate Erk1/2 nor induce p47 phox phosphorylation while stimulation by PMA led to phosphorylation of both Erk1/2 and p47 phox . This suggests that Erk1/2 activation is upstream of p47 phox phosphorylation in A29-V40 peptide-initiated response, but PMA activates Erk1/2 and p47 phox independent of gp91 phox . Using U0126 to inhibit Erk1/2 activity did not cause any additive suppression of p47 phox phosphorylation in A29-V40 peptide-treated gp91 phox−/− microglia, compared to the DMSO control. This observation suggests that binding of A29-V40 peptide and gp91 phox is the initial event that activates Nox2 to produce superoxide and hence H2O2; H2O2 then enters cells and induces activation of Erk1/2, upstream of p47 phox phosphorylation, which may amplify A29-V40 peptide-induced Nox2 activation. Despite the suppression of Erk1/2 activity in U0126-pretreated gp91 phox−/− microglia, PMA still induced p47 phox phosphorylation (Fig. 6f, h), indicating that this compound activates Nox2 in an Erk1/2-independent manner.
A29-V40 peptide activates microglia and elicits redox reactions in vivo
α-Syn, particularly in its oligomeric form, can damage neurons and therefore plays a critical role in the pathogenesis of synucleinopathies, including PD and dementia with Lewy bodies. Exploring the mechanism by which α-Syn forms aggregates has demonstrated that the apolipoprotein-binding domain clearly contributes to its aggregation and associated toxicity. Furthermore, studies including ours have shown that a single amino acid substitution in this region, e.g., at alanine 30, which is associated with the development and progression of familial-PD , activates microglia more potently, exacerbating the α-Syn aggregate-induced neuronal damage [16, 35]. These findings prompted us to test the capability of a series of synthesized peptides, starting from those which cover the alanine 30 locus, to induce microglial superoxide production and neuronal damage. Our study revealed that a specific peptide, A29-V40, is capable of directly promoting the release of superoxide by microglia and inducing dopaminergic neuronal damage via a unique mechanism that is different from that caused by FL-α-Syn aggregates.
The pursuit began with comparing WT FL-α-Syn with that containing the A30P substitution, based on the relationship of this mutant form with familial PD, and the finding that FL-α-Syn featuring this change forms aggregates more quickly and becomes more toxic to neurons compared with the WT form. In our initial experiments, examining the neurotoxicity of peptides covering the alanine 30 locus, only A29-V40 was identified as initiating specific neurotoxic effects. However, when comparing the effects of the same peptides with or without the A30P alteration, the WT and mutant forms of the A29-V40 peptide were similar in terms of their toxicity to neurons. Because the effects of both the A30P and WT peptides were quite similar (Fig. 2), further experiments identifying microglia as the cells mediating the toxic effects on neurons focused on the WT peptide. Therefore, it is theoretically possible that the A30P peptide may induce a greater effect on astrocytes, while the WT peptide induces toxicity primarily through microglia, as observed (Fig. 4). However, this outcome would require a reciprocal decrease in microglial sensitivity to A30P, balancing to an equal toxicity to neurons (as in Fig. 2), a mechanism probably less likely than the proposed model of a similar, microglia-mediated response to both peptides. Nonetheless, further effects on astrocytes are an area of interest for future studies. An interesting implication of the similar microglial response to both WT and A30P peptides is that it suggests that distinct mechanisms are responsible for the neurotoxicity induced by FL-α-Syn aggregates and the peptide A29-V40. In this regard, it has been suggested that extracellular FL-α-Syn aggregates may activate microglia to generate inflammatory mediators, such as TNF-α and superoxide, to induce neuronal damage, a response that is mediated by the binding of FL-α-Syn aggregates to microglial surface receptors such as TLR2 and CD11b [3, 17]. It is possible that FL-α-Syn aggregates formed by mutant α-Syn monomers (e.g., A30P) create a conformation that is distinct from that of WT FL-α-Syn aggregates, resulting in an altered binding affinity of FL-α-Syn aggregates for their microglial receptors and allows mutant FL-α-Syn aggregates to more effectively activate microglia. In contrast, the peptides used here include only twelve amino acids, and whether they recapitulate the folded structures of the equivalent regions of the full-length protein, or represent structures with potentially very different affinities from FL-α-Syn is not known.
Two obvious questions remain to be answered. First, does this specific peptide exist in vivo? Clearly, the direct relevance of this particular peptide and its interaction with microglia to PD depends on its existence (and relative abundance) in normal and diseased human brain. However, even if the exact peptide is not present in vivo, it can be hypothesized that similar peptides generated as intermediate metabolites during the degradation of α-Syn aggregates could behave similarly to A29-V40 peptide. The rationale behind this argument is that, in theory, α-Syn aggregates are foreign materials to the immune cells, and foreign proteins are typically cleaved into peptides when recognized as antigens by the antigen processing cells (e.g., microglia and macrophages), which usually contain around nine amino acids , and the newly generated peptides, along with MHC-I molecule, would be further presented to T cell receptors . Indeed, a series of autoantibodies against α-Syn  and T cells, including both CD4+ and CD8+ phenotypes, have been found in the bodily fluids and the brain tissues, respectively, of patients with PD [39–42].
The second question centers on what precisely mediates the direct interaction between the peptide and Nox2 enzyme. In this regard, it has been reported that some small peptides (e.g., substance P)  or small compounds (e.g., rotenone)  seem to preferentially bind to gp91 phox , the catalytic unit of Nox2, while activating Nox2 to produce superoxide. Our data demonstrating that A29-V40-induced dopaminergic neuronal damage depends on the presence of gp91 phox in microglia and that A29-V40 is capable of precipitating gp91 phox indicate that gp91 phox might also act as a receptor for this α-Syn peptide. Clearly, in future investigations, direct binding of the peptide with gp91 phox needs to be demonstrated by more advanced technology, e.g., mass spectrometry mapping of critical amino acid sequence needed for Nox2 activation after crosslinking the peptide with gp91 phox . However, this process is quite challenging because the purified gp91 phox that maintains its physiologically functional structure is currently unavailable.
The biological significance of microglial activation is of course neurodegeneration. In in vitro assays, the V29-V40 peptide exhibits toxicity to neurons via activating microglia to produce superoxide. Utilizing this peptide to in vivo models, we also found that it could activate microglia and stimulate the oxidation reaction. However, whether this peptide can damage dopaminergic neurons in vivo remains unknown, which warrants further investigation. Furthermore, besides the A29-V40 peptide, in future investigations, it would be interesting to examine whether specific neurotoxic peptides also exist in the regions that correspond to the glutamic acid 46- or the alanine 53-containing locus.
In summary, we identified the α-Syn peptide, A29-V40, as a neuronal toxicant that is capable of directly inducing the release of microglial superoxide leading to dopaminergic neurodegeneration. These findings provide novel mechanistic information about how α-Syn may cause neuronal injury, possibly via the abnormal metabolites of α-Syn aggregates.
A30P, alanine 30 to proline-mutated α-synuclein; A53T, alanine 53 to threonine-mutated α-synuclein; ANOVA, analysis of variance; Apo, apocynin; BBB, blood-brain barrier; CNS, central nervous system; DA, dopamine; DAPI, 4′,6-diamidino-2-phenylindole, dihydrochloride; DMEM, Dulbecco’s modified Eagle medium; FL-α-Syn, full-length α-synuclein; HBSS, Hank’s balanced salt solution; LPS, lipopolysaccharide; MDA, malondialdehyde; MHC-II, major histocompatibility complex II; NAC, non-amyloid-β component; Nox2, NADPH oxidase; PBS, phosphate-buffered saline; PD, Parkinson’s disease; PMA, phorbol 12-myristate 13-acetate; ROS, reactive oxygen species; SOD, superoxide dismutase; TH, tyrosine hydroxylase; TNF-α, tumor necrosis factor α; WT, wild type; α-Syn, α-synuclein
We appreciate the technical support from Mr. Jeff Tucker at the Imaging Center of the National Institute of Environmental Health Science. We are also grateful to Dr. Ru-Band Lu (National Cheng Kung University, Tainan, Taiwan) for providing the certain reagents used in this work.
This study was supported by the intramural funding from the NIEHS (to JSH) and the National Institutes of Health Grants R01 ES019277 and R01 ES016873 (to JZ).
Availability of data and materials
Materials used in this paper are publicly available. Upon acceptance for publication, the data supporting the conclusions contained in this manuscript will be deposited in machine-readable formats on FigShare for public access.
SW conceived the signaling pathway and managed the experiments. SW, CHC, MG, LJ, HN, WZ, BW, LY, and TS performed the experiments. SW, JZ, and JSH analyzed and interpreted the data. SW wrote the paper. JZ and JSH provided the support for the study. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
All experiments and protocols involving animal use were approved by the Institutional Animal Care Committee of the National Institute of Environmental Health Sciences and performed by strictly following the National Institutes of Health guidelines.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. 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.
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