Introduction
Parkinson’s disease (PD) is a multisystem neurodegenerative disease characterized by the appearance of Lewy bodies containing misfolded fibrillar α-synuclein and the selective loss of midbrain dopaminergic neurons in the substantia nigra pars compacta (SNpc), leading to postural instability, bradykinesia and tremor [
1,
2]. Although little is known about the etiology of PD processes, autosomal dominant α-synuclein gene amplifications or mutations directly link α-synuclein dysfunction to PD causation [
3,
4]. Moreover, dysfunctional α-synuclein pathology is present in both sporadic and familial PD patients and the distribution of α-synuclein correlates with the clinical symptoms of PD. Aggregated α-synuclein is the major component of Lewy bodies and is associated with neurodegenerative diseases, including PD and dementia with Lewy bodies [
5]. The α-synuclein can be transmitted between neurons by a progressive “prion-like” mechanism as a response to stimulation or cellular stress [
6,
7]. Neighboring glia cells can take up and clear extracellular α-synuclein to maintain α-synuclein homeostasis in the brain. The uptake and degradation processes of glia cells are thus key to regulating the spread and deposition of α-synuclein, and even to altering the pathological progression of PD.
Microglia, which are the prototypical scavenger cells in the brain, show the highest efficiency for ingesting and degrading extracellular α-synuclein in vitro [
8]
. It has been reported that α-synuclein released from neurons activates microglia, which then engulf and degrade α-synuclein via autophagy, both in vitro and in vivo [
9]. However, the efficiency of the microglial autophagy–lysosome degradation system is reduced in PD, leading to accumulation of misfolded α-synuclein and degeneration of dopaminergic neurons [
3,
10]. Increasing microglial phagocytosis and degradation of α-synuclein by autophagy may thus represent an important therapeutic target for PD.
Transient receptor potential vanilloid 1 (TRPV1) channels are nonselective ligand-gated cation channels that have been proposed as neuroprotective targets in neurodegenerative diseases such as Alzheimer’s disease and PD [
11‐
13]. The TRPV1 agonist capsaicin was shown to prevent degeneration of dopaminergic neurons by inhibiting oxidative stress and neuroinflammation caused by activation of glia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of PD [
14]. Capsaicin was also shown to restore rotarod performance, as well as dopaminergic signaling, in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of PD [
15]. It was further demonstrated that the neuroprotective effects of TRPV1 activation using capsaicin were mediated through endogenous ciliary neurotrophic factor and its receptors [
12].
Here, we identified defects in energy metabolism, mitophagy and phagocytosis in microglia with chronic tolerance to α-synuclein preformed fibril (PFF). Metabolic boosting with capsaicin reversed metabolic impairments and mitophagy defection in PFF-tolerant microglia by modulating the AKT–mTOR–HIF-1α pathway. We showed that behavioral deficits and loss of dopaminergic neurons were accelerated in the α-synuclein PFF TRPV1flox/flox; Cx3cr1Cre mouse model of sporadic PD. In light of its beneficial effects, TRPV1 activation should be evaluated for the treatment of PD and related neurodegenerative disorders characterized by activation of microglia.
Materials and methods
Mice
TRPV1flox/flox mice were obtained from the Shanghai Model Organisms Center, Inc. (Shanghai, China). Cx3cr1Cre transgenic mice (no. 021160) were purchased from the Jackson Laboratory (Bar Harbor, ME, USA). TRPV1flox/flox mice were bred with Cx3cr1Cre mice to generate TRPV1flox/flox; Cx3cr1Cre mice. Eight-week-old male and female TRPV1flox/flox; Cx3cr1Cre mice were injected intraperitoneally with tamoxifen (sc-208414, Santa Cruz Biotechnology, Inc., Santa Cruz, CA, USA) at a dose of 75 mg/kg/day for 5 consecutive days. The mice were housed at room temperature (22 ± 1 °C) under a 12-h light/dark cycle. The protocols for all animal experiments were approved by the Animal Experimentation Ethics Committee of Shanghai Jiao Tong University School of Medicine (Shanghai, China).
Preparation of α-synuclein PFF
Recombinant human α-synuclein proteins were purchased from ATgen (SNA2001L, ATgen, Seongnam, South Korea). α-Synuclein monomers were dissolved in phosphate-buffered saline (PBS) at a concentration of 5 mg/ml, and incubated for 7 days (1000 rpm at 37℃) [
16]. Aggregates should be shipped and stored at − 80℃. Samples were diluted to the desired concentration and sonicated for 30 s at 10% amplitude immediately prior to use [
17]; solutions were mixed between injections and used within 4 h. Successful generation of α-synuclein PFF was validated by transmission electron microscopy (TEM) and the ability to generate p-α-syn (ser129) pathology.
Transmission electron microscopy
The morphology of α-synuclein monomers, PFF and sonicated PFF was monitored by TEM. Specimens were prepared by depositing on 75 mesh copper grids coated with Formvar and carbon. The grids were washed after 5 min and negatively stained with 2% uranyl acetate for 30 s. Excess liquid was wicked away using a filter paper and the grids were dried in the air at room temperature for 20 min. Images was captured using an H-7650 transmission electron microscope (Hitachi, Tokyo, Japan).
Stereotaxic injection of α-synuclein PFF
Littermates of TRPV1
flox/flox and TRPV1
flox/flox; Cx3cr1
Cre were randomly allocated to experimental groups at the age of 8–12 weeks. For PFF delivery, mice were anaesthetized and placed in a stereotaxic frame (RWD Life Science Co., Ltd., Shenzhen, China). A solution of PFF in PBS (2 μL, 2.5 μg/μL) or an equal volume of PBS was injected unilaterally into the striatum (right hemisphere) at a rate of 0.2 µL/min using an injection pump. In keeping with a previous report [
17], the injection coordinates were: mediolateral, 2.0 mm from bregma; anteroposterior, 0.2 mm; dorsoventral, 2.6 mm. The needle was left in the striatum for 10 min to allow the injectant to diffuse before it was slowly removed and the wound sutured.
Behavioral test
In order to evaluate the impact of PFF on motor coordination, mice were assessed by rotarod test and open field test 4 months after stereotactic injection of PBS and PFF into striatum. All the in vivo experiments were double-blind. The open field test and rotarod test was carried out as described previously [
18].
Open field test
The animals were acclimated to the environment for at least 1 day prior to initiating experiments. Then, the test mice were placed in the center of the open field room (40 cm × 40 cm × 40 cm) and was permitted to move freely for 5 min. Their locomotor activity was recorded by a video camera and analyzed via a tracking system (Noldus Ethovision, Wageningen, Netherlands). At the end of each trail, the chamber was cleaned with 75% ethanol solution to remove urine or odor.
Rotarod test
After the open field test, mice were acclimated to the rotarod apparatus (IITC Life Science, Woodland Hills, CA, USA) for 150 s on two consecutive days at a low rotation speed of 8 rpm and 12 rpm. While on the seven consecutive testing days, the mice were placed on the rotating rods at 8, 12, 16, 20, 24, 28 rpm. The maximum time for each trail was 150 s. The time spent on the rod was recorded and the area under the curve was calculated to evaluate the motor coordination. Mice were rested for 30 min between trails to avoid influence of exhaustion and stress.
RNA sequencing analysis
In this study, total RNA was extracted from collected hemibrain tissues. The mRNA with poly A structure were enriched by Oligo (dT) magnetic beads in the total RNA, and then interrupted about 300 bp fragment by ion interruption. The first strand of cDNA was synthesized by using reverse transcriptase as template, 6 base random primers, and the second strand cDNA was synthesized using the first-strand cDNA as a template. The library fragments were enriched by PCR amplification after cDNA libraries construction, and the libraries were selected according to the fragment size (450 bp). The cDNA libraries were validated using the Agilent 2100 Bioanalyzer (Agilent Technologies, Palo Alto, CA, USA). Libraries were sequenced on the Illumina sequencing platform by Next-Generation Sequencing. The FASTQ raw data were filtered to remove the reads with adapters and low-quality sequences, and mapped to the mouse reference genome using HISAT2 (
http://ccb.jhu.edu/software/hisat2/index.shtml). Gene expression counts were calculated according to comparison results. R pheatmap package (
https://cran.rproject.org/web/packages/pheatmap/index.html) was used to perform bi-directional clustering analysis of all different genes of samples. Then, we used DESeq. (1.30.0) to identify the differential gene expression between samples according to screening conditions: absolute value of log2 (fold change) larger than 1 and significance
p value < 0.05. Volcano plots were generated using R ggplots2 package. Gene enrichment analyses were performed using Kyoto Encyclopedia of Genes and Genomes (KEGG) (
http://www.kegg.jp/) with default parameters to determine the biological functions of the differential expression genes.
Weighted gene co-expression network analyses
Weighted gene co-expression network analyses (WGCNA) were performed to identify the effects of microglial TRPV1 on PD-like pathology induced by PFF using the R WGCNA package [
19] (WGCNA version 1.66,
https://cran.r-project.org/src/contrib/Archive/WGCNA/). Clustering analyses were performed on the gene expression. Afterward, the TOMSimilarity module was used to calculate the gene–gene co-expression similarity coefficient, which was converted to connections among the genes by the pickSoftThreshold function, and to realize the functional connection of genes. Next, a weighted co-expression network model was established to divide thousands of genes into several modules. The correlation coefficient and
p value were calculated and screened modules related to the biological characteristics. Genes in several modules were significantly enriched into pathways of both Gene Ontology (GO) and KEGG using Metascape (
https://metascape.org).
Immunohistochemistry
The mice were anaesthetized and transcardially perfused with PBS. Coronal sections of the SNpc (10 μm) were incubated with rabbit anti-p-α-syn (Ser129) (1:500, 23706S; Cell Signaling Technology, Beverly, MA, USA) at 4 ℃ overnight. The slices were then incubated with biotinylated anti-rabbit IgG, treated with avidin–biotin peroxidase complex, and visualized using 0.05% DAB + 0.03% H2O2. Images were captured using a DM6 B microscope (Leica Microsystems, Wetzlar, Germany), and p-α-syn (Ser129) positive cells in the brain region were counted by ImageJ software (NIH, Bethesda, USA).
Immunofluorescence and quantitative analysis
Brain tissues of the SNpc were permeabilized with 0.3% Triton X-100, and blocked with 10% goat serum in 0.01 M PBS for 1 h at room temperature. Brain sections were incubated with the following primary antibodies at 4℃ overnight: rabbit anti-TH (1:500, ab112, Abcam, Cambridge, UK), mouse anti-Iba-1 (1:500, GB12105, Servicebio Technology Co., Ltd., Wuhan, China), rabbit anti-p-α-syn (Ser129) (1:500, 23706S; Cell Signaling Technology). The slices were next incubated with Alexa Fluor 647 goat anti-mouse (1:1000, A32728; ThermoFisher Scientific, Waltham, MA, USA) and Alexa Fluor 568 goat anti-rabbit (1:500, A21428, ThermoFisher Scientific), and then stained with 4′,6-diamidino-2-phenylindole (DAPI). Fluorescent images were captured using a TCS SP8 confocal laser scanning microscope (Leica Microsystems). Three-dimensional reconstructions of microglia were performed using Imaris software (BitPlane Scientific Software, Zürich, Switzerland), and subsequent analyses were conducted using Fiji software (NIH, Bethesda, USA), as previously described [
20]. Analysis of microglial morphology was conducted using the Skeletonize plug-in for Fiji.
Culture and stimulation of primary microglia
Primary cultures of microglia were prepared from the cerebral cortices and hippocampi of newborn (P0–P2) C57BL/6 mice. The brain tissues were dissociated into single cells by treatment with 0.25% trypsin for 10 min at 37 ℃, and the cells were then cultured in high glucose Dulbecco’s Modified Eagle’s medium (DMEM) (SH30022.01B; Hyclone, Beijing, China), supplemented with 10% fetal bovine serum (S711-001S; Shuangru Biotechnology Co., Ltd., Shanghai, China), 1% penicillin/streptomycin (C0222; Beyotime Institute of Biotechnology, Shanghai, China) and 2 mM L-glutamine (C0212; Beyotime Institute of Biotechnology) at 37℃ in an incubator with 5% CO2. On day 14, the microglia were harvested from the mixed glial culture by shaking at 200 rpm for 4 h at 37 ℃. Primary microglia were plated and incubated for 3–4 days before using.
Primary microglia were stimulated with 100 ng/ml lipopolysaccharide (LPS) (055: B5; Sigma-Aldrich, St. Louis, MO, USA), or 1 μg/ml PFF for 24 h. In some experiments, microglia were pretreated with 30 nM rapamycin (TargetMol, Shanghai, China) to block the mTOR pathway or 10 μM capsaicin (TargetMol, Shanghai, China) to activate the TRPV1 channel before addition of PFF. Experiments using the tolerant model were used to mimic chronic disease. Primary microglia were incubated with 1 μg/mL PFF for 24 h, the medium was removed and replaced with culture medium and the cells were incubated for a further 3–5 days. The cells were then restimulated with 1 μg/mL PFF for 24 h. For treatment experiments, tolerant microglia were pretreated with 10 μM capsaicin for 30 min and then 1 μg/mL PFF was added. Primary microglia-conditioned medium (MCM) from cells pretreated with 10 μM capsaicin and then treated with 1 μg/ml PFF was collected and stored at − 80 ℃.
Primary culture of cortical neurons
Primary cortical neurons were isolated from postnatal Day 0 C57BL/6 J mouse pups as described previously [
18]. Briefly, mouse cortices were dissected in ice-cold DMEM and then dissociated into single-cell suspensions by treatment with papain (A501612-0025; Sangon Biotech, Shanghai, China). After dissociation, the neurons were seeded on poly-L-lysine-coated dishes (E607015; Sangon Biotech). After 6–8 h, plating medium (DMEM supplemented with 10% horse serum (E510006-0100; Sangon Biotech), 1% glutamax (35,050,061; Gibco, Grand Island, NY, USA) and 1% penicillin/streptomycin) was replaced with Neurobasal medium (T710KJ; BasalMedia Technologies Co., Ltd., Shanghai, China) containing 2% B27 supplement (S440J7; BasalMedia Technologies) and 1% glutamax. Neurons were grown in vitro for 7 days, with the medium changed every 3 days. Primary cortical neurons were pretreated with 5 μg/ml PFF for 5 days and then treated with 50% PFF MCM for 24 h.
Quantitative real-time PCR
Total RNA was isolated from microglia using TRIzol reagent (R0016; Beyotime Institute of Biotechnology). The first-strand of cDNA was synthesized by reverse transcription using a cDNA synthesis kit (6210A; Takara Ltd., Otsu, Japan) after examining total RNA using a BioDrop spectrophotometer (Biochrom Ltd, Cambridge, UK). Quantitative real-time PCR (qPCR) with 10 ng diluted cDNAs was performed on a LightCycler 480II (Roche Applied Science, Mannheim, Germany), using BeyoFast™ SYBR Green qPCR Mix (D7265, Beyotime Institute of Biotechnology). The qPCR was carried out with a hold step at 95 ℃ for 2 min; 40 cycles of denaturation at 95 ℃ for 15 s, annealing at 60 ℃ for 20 s and extension at 72 ℃ for 30 s. Melting curve analysis was performed from 60 to 95 ℃. Expression levels of target genes were normalized to GAPDH and calculated using the 2−ΔΔCt method. The primer sequences were as follows: for IL-1β, 5'-TCCAGGATGAGGACATGAGCAC-3' (forward) and 5'-GAACGTCACACACCAGCAGGTTA-3' (reverse); for TNF-α, 5'-CAGGAGGGAGAACAGAAACTCCA-3' (forward) and 5'-CCTGGTTGGCTGCTTGCTT-3' (reverse); for GAPDH, 5'-TTGATGGCAACAATCTCCAC-3' (forward) and 5'-CGTCCCGTAGACAAAATGGT-3' (reverse).
Western blotting
Primary microglia and the SNpc region of mouse brain were homogenized and prepared in radioimmunoprecipitation lysis buffer (P0013B, Beyotime Institute of Biotechnology) on ice containing phenylmethanesulphonyl fluoride. Lysates containing equal concentration of 50 μg proteins were separated by 8%, 12% or 15% (w/v) sodium dodecyl sulfate–polyacrylamide gel electrophoresis and transferred to a polyvinylidene difluoride membrane, which was then blocked in 5% nonfat milk at room temperature for 1 h. The membranes were incubated at 4℃ overnight with indicated antibodies as following: TH (1:1000, ab112, Abcam), Iba-1 (1:1000, 019-19741, Wako Pure Chemical Industries, Ltd., Osaka, Japan), TRPV1 (1:200, ACC-030, Alomone labs, Jerusalem, Israel), p-α-syn (Ser129) (1:1000, 23706S, Cell Signal Technology), α-synuclein (1:1000, 4179T, Cell Signal Technology), GFAP (GA5) (1:1000, 3670T, Cell Signal Technology), Phospho-mTOR (Ser2448) (1:1000, 5536T, Cell Signal Technology), mTOR (1:1000, 2983T, Cell Signal Technology), Phospho-Akt (Thr308) (1:1000, 2965P, Cell Signal Technology), Akt (1:1000, 4691P, Cell Signal Technology), Phospho-AMPKα (Thr172) (1:1000, 2535T, Cell Signal Technology), LC3B (1:1000, 2775, Cell Signal Technology), SQSTM1 (1:1000, 5114S, Cell Signal Technology), Atg3 (1:1000, 3415T, Cell Signal Technology), β-actin (1:1000, 3700S, Cell Signal Technology), HIF1-α (1:500, AH339-1, Beyotime Institute of Biotechnology), Pink1 (1:1000, AF7755, Beyotime Institute of Biotechnology), Parkin (1:1000, AF7680, Beyotime Institute of Biotechnology), p-TBK1/NAK (Ser 172) (1:500, AF5959, Beyotime Institute of Biotechnology). Afterward, the membranes were incubated with the peroxidase-conjugated anti-mouse (1:1000, A0216, Beyotime Institute of Biotechnology) and anti-rabbit (1:1000, A0208, Beyotime Institute of Biotechnology) IgG at room temperature for 1 h. Immunoreactive proteins were visualized using an enhanced chemiluminescent substrate (36222ES76, Yeasen, Shanghai, China) and the Image Studio Lite Ver 5.2 software (LI-COR Biosciences, Lincoln, NE, USA).
Bioenergetic analysis of microglia was performed using a Seahorse XFe96 analyzer (Seahorse Bioscience, Billerica, MA, USA) on XF96 cell culture microplates (102416-100; Seahorse Bioscience). Before the assay, primary microglia were seeded at a density of 10,000 cells/well and cultured overnight. After drug treatment, the medium was replaced with Seahorse XF Base medium (102353-100; Seahorse Bioscience) containing 25 mM glucose (G7528, Sigma-Aldrich), 200 mM glutamine (2503164; Gibco) and 1 mM pyruvate (11360070; Gibco), and the microglia were incubated in a 37℃ incubator without CO2. Mitochondrial stress was induced by sequential injection of 5 μM oligomycin, 10 μM carbonyl cyanide p-(trifluoromethoxy) phenyl-hydrazone and 10 μM rotenone and antimycin A (103015-100; Seahorse Bioscience). Glycolysis stress was detected by sequential injection of 10 mM glucose, 0.5 μM oligomycin and 50 mM 2-deoxy-glucose (103020-100; Seahorse Bioscience). Raw data were analyzed and exported using Wave 2.6.0 version (Agilent Technologies, Inc., Santa Clara, CA, USA).
Measurement of reactive oxygen species
Primary microglia were incubated with 1 μg/mL PFF, with or without 10 μM capsaicin, for 24 h. Production of reactive oxygen species (ROS) was detected by 2′,7′-dichlorofluorescein diacetate (10 μM) staining, using a fluorescence microplate reader (ThermoFisher Scientific) with detection at 450 nm.
Measurement of mitochondrial membrane potential
Microglia were seeded and cultured overnight in glass bottom dishes at a density of 1 × 105 cells/well. Mitochondrial membrane potential was measured using a MitoProbe™ JC-1 Assay Kit (C2005; Beyotime Institute of Biotechnology). Microglia were incubated with a 10 μM solution of JC-1 dye for 20 min at 37 °C (shielded from light) and washed with PBS prior to assessment using a TCS SP8 confocal laser scanning microscope. In normal mitochondria, JC-1 forms aggregates, which emit a red fluorescence (561 nm). Following decline or loss of mitochondrial membrane potential, aggregated JC-1 was released into the cytoplasm as the monomeric form, which emits a green fluorescence (488 nm). Mitochondrial membrane potential was calculated as the red/green ratio using ImageJ software.
Cellular uptake of fluorescent beads
Phagocytic ability was measured using a fluorescent beads uptake assay, as previously described [
21]. Fluorescent latex beads (1 μM, L2778; Sigma-Aldrich) were pretreated with 50% fetal bovine serum, centrifuged at 1,000 rpm for 2 min, and then diluted with serum-free DMEM. The preprocessed beads were loaded onto microglia and incubated at 37 °C for 4 h. Residual beads were washed off the cells with PBS at the end of incubation period and the microglia were then fixed and stained with DAPI. Microglial phagocytic uptake of the fluorescent latex beads was determined using a TCS SP8 confocal laser scanning microscope with ImageJ software.
Measurement of autophagy flux
Tandem fluorescent mRFP-GFP-LC3 plasmids were used to label and track LC3 and measure the autophagic flux. In brief, cultured primary microglia were transfected with mRFP-GFP-LC3 plasmids (Addgene, Watertown, MA, USA), according to the manufacturer’s protocol, and incubated for 48 h. The microglia were then treated with PFF or capsaicin for 24 h before fixing and staining with DAPI. The locations of mRFP and GFP were tracked using a TCS SP8 confocal laser scanning microscope. The autophagic flux was evaluated by counting the puncta of different colors with ImageJ software.
Measurement of mitochondrial autophagy
After PFF or capsaicin stimulation, cells were incubated in the dark with 200 nM MitoTracker Red CMXRos (C1049B; Beyotime Institute of Biotechnology) at 37 °C for 20 min. The microglia were blocked with 10% goat serum in 0.01 M PBS, and incubated with mouse anti-LC3B (1:400, 83506S; Cell Signaling Technology) or rabbit anti-Parkin (1:200, AF7680; Beyotime Institute of Biotechnology) at 4 °C overnight. The cells were stained with Alexa Fluor 647 goat anti-mouse (1:1000, A32728; ThermoFisher Scientific) or Alexa Fluor 647 goat anti-rabbit (1:500, A21245; ThermoFisher Scientific), and then stained with DAPI. Colocalization of fluorescence was captured using a TCS SP8 confocal laser scanning microscope and analyzed by ImageJ software.
Immunofluorescence of primary cortical neurons
Primary cortical neurons were treated with 5 μg/mL PFF for 5 days and then incubated with 50% MCM for 24 h. The cells were blocked with 10% goat serum, and incubated with mouse anti-MAP2 (1:500, ab254143; Abcam) and rabbit anti-p-α-syn (Ser129) (1:500, 23706S; Cell Signaling Technology) at 4 °C overnight. The cells were stained with Alexa Fluor 647 goat anti-mouse (1:1,000, A32728; ThermoFisher Scientific) and Alexa Fluor 568 goat anti-rabbit (1:500, A21428; ThermoFisher Scientific), and then stained with DAPI. Fluorescent images were captured using a TCS SP8 confocal laser scanning microscope and analyzed by ImageJ software.
Statistical analysis
All data are expressed as mean ± SD. Statistical analyses were performed using GraphPad Prism 7.0 (GraphPad Software, Inc., La Jolla, CA, USA). The Kolmogorov–Smirnov normality test was performed to test if the values fit a Gaussian distribution. The Student’s t-test was used to compare two independent groups. One-way analysis of variance (ANOVA) and Tukey’s multiple comparisons test were used to compare three or more independent groups. Two-way ANOVA with a Bonferroni post-test was used to compare multiple factors. Statistical significance was set at p < 0.05. Sample sizes were estimated from pilot experiments.
Discussion
Microglia-specific TRPV1 deficiency accelerated PFF-induced behavioral impairment and loss of dopaminergic neurons in vivo. TRPV1 deficiency in microglia exacerbated defects in microglial phagocytosis and impairment of immune function in the SNpc of PFF-injected TRPV1flox/flox; Cx3cr1Cre mice. Using metabolic profiling, we also found that acute stimulation with PFF led to an active microglial phenotype and a switch in cellular metabolism from oxidative phosphorylation to glycolysis via the mTOR–AKT–HIF-1α pathway. Chronic exposure of microglia to PFF induced innate immune tolerance and metabolic defects, including changes in oxidative phosphorylation and aerobic glycolysis. Capsaicin rescued impaired cellular metabolism, mTOR signaling and immune functions of PFF-tolerant microglia. These results indicated that TRPV1 was an important therapeutic target for restoring microglial metabolic immune function for the treatment of PD.
The brains of PD patients contain intracellular aggregated α-synuclein, a 140-amino-acid cytoplasmic protein that is located in presynaptic nerve terminals and participates in the assembly of SNARE complexes [
43]. In PD, α-synuclein phosphorylated at residue Ser 129 polymerizes to form protein aggregates that deposit within the brain. Because mutations of the gene encoding α-synuclein cause early-onset PD, α-synuclein is believed to be involved in the pathogenic processes of synucleinopathy. Mounting evidence has shown that α-synuclein can be transmitted between neurons by a progressive “prion-like” mechanism, leading to protein aggregates within the brain [
44].
Microglia, the brain’s innate immune cells, play a key role in ingesting and degrading α-synuclein released by neurons [
29,
38,
39]. The efficiency of the microglial autophagy–lysosome degradation system is decreased in PD, promoting accumulation of misfolded α-synuclein and causing degeneration of dopaminergic neurons [
10]. p-α-Syn (Ser129) protein was clearly present in microglia at the striatum of TRPV1
flox/flox + PFF mice and the volume of microglial p-α-synuclein was larger in TRPV1
flox/flox + PFF mice than in TRPV1
flox/flox; Cx3cr1
Cre + PFF mice (Fig.
7a, b). These data showed that microglia-specific TRPV1 deficiency exacerbated defects in microglial phagocytosis and immune impairment in vivo.
Pathological aggregates of α‐synuclein disrupt synaptic protein trafficking, as well causing neuroinflammation and deterioration in the functions of autophagy–lysosomes and mitochondria [
45‐
47]. Aggregated α‐synuclein can be released from a donor cell to a neighboring neuron or glial cell; both intracellular and intercellular toxicity of aggregated α‐synuclein then accelerate cellular damage [
48‐
53]. Regulation of lipid rafts, endocytosis, trans-synaptic transmission and direct penetration by Parkin have been shown to mediate the spread of aggregated α‐synuclein [
45,
54]. Autophagic dysfunction has also been suggested to accelerate intercellular transfer of α‐synuclein [
55‐
57]. Expression of both the mitochondrial protein kinase Pink1 and the cytoplasmic ubiquitin ligase Parkin was downregulated in PFF-tolerant microglia compared with acutely stimulated microglia. Activation of TBK1 by phosphorylation on its activation loop site (Ser172), a feed-forward amplification mechanism to promote mitochondrial clearance, was downregulated in PFF-tolerant microglia. Chronic treatment with PFF reduced TRPV1 expression in microglia (Fig.
2f) and treatment of PFF-tolerant microglia with capsaicin increased the expression of Pink1, Parkin, p-TBK1/NAK (Ser172) and TRPV1 (Fig.
2g).
It was reported that AMPK involved in oxidative stress process as a redox-sensitive protein [
58,
59]. As shown in Fig.
2d, ROS production was significantly upregulated in both acute and chronic PFF-stimulated microglia (Fig.
2d). Previous studies have demonstrated that AMPK was sensitive to cellular stress and phosphorylation of AMPKα at Thr172 was decreased in a ROS-dependent manner during aging and obesity [
60,
61]. This might explain that phosphorylation of AMPKα at Thr172 was significantly downregulated in chronic PFF-induced tolerance (Additional file
1: Fig. S1a, b).
Metabolic dysfunction and redox stress have been shown to be the key mediators of proteotoxicity associated with PD. Mitochondria are important targets of α‐synuclein, and mitochondrial function was perturbed in both cell culture and transgenic mouse models of PD [
62‐
64]. After exposure to the pesticides Maneb and paraquat, aggregated α‐synuclein induced release of cytochrome c from mitochondria to the cytosol [
65]. When microglial gene expression profiles were assessed in the brains of TRPV1
flox/flox; Cx3cr1
cre + PFF mice and TRPV1
flox/flox + PFF mice using RNA-seq analysis, we observed that microglia-specific TRPV1 deficiency resulted in greater repression of genes associated with immune responses, antigen processing and presentation, and cell motility in TRPV1
flox/flox; Cx3cr1
Cre + PFF mice compared with TRPV1
flox/flox + PFF mice (Fig.
7i).
Loss of key mitophagy proteins that are involved in mitochondrial clearance has been shown to cause PD, as well as a significant proportion of genes encoding PD proteins associated with the autophagy–lysosomal pathway. Enhancement of mitophagy has also been reported to abolish the neuronal hyperphosphorylation of tau in Alzheimer’s disease and rescue memory impairment in transgenic mice.
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.