Mettl3/Nrf2 Axis Suppresses Parkinson's Disease Progression via Inhibiting NLRP3-Induced Pyroptosis in Serotonin Neurons

Parkinson's disease (PD) ranks as the second most prevalent neurodegenerative disorder among the elderly, affecting around 2-3% of individuals aged 65 and older, which poses a significant burden on both families and society¹. Although the precise mechanisms behind PD remain partially understood, accumulating evidence indicates a connection between PD development and challenges in neuronal transmission, along with neuroinflammation driven by microglial cells, which is a common trait seen in aging brains and various neurodegenerative diseases, including PD^2,3,4,5. Microglia serve as the innate immune cells within the central nervous system (CNS) and are vital for sustaining brain homeostasis⁶. However, persistent overactivation of these microglia can trigger chronic neuroinflammatory responses, ultimately contributing to neurodegenerative disease progression.

Both central and peripheral inflammatory processes significantly influence the pathology of PD^7,8. The activation of microglial cells prompts inflammatory responses that impact neuronal survival⁹, with emerging evidence highlighting its priming by ubiquitin ligases¹⁰. Among various inflammatory pathways, NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasome activation serves as a primary contributor to microglial inflammatory regulation¹¹. Activation leads to NLRP3 expression and subsequent assembly of the inflammasome complex composed of the caspase activation and recruitment domain (CARD) adapter protein and pro-caspase-1, culminating in protein cleavage and cytokine release¹². Elevated NLRP3 activation has been observed in PD patients and various animal models of the disease, resulting in neuronal death. Notably, inhibiting NLRP3 has demonstrated protective effects against PD pathology in mouse models, underscoring the NLRP3 inflammasome's crucial role in PD onset^13,14.

Serotonin (5-HT) signaling is a significant mechanism of neural regulation, influencing numerous behaviors and physiological functions through interactions with at least 14 postsynaptic receptor subtypes^15,16, including roles in CNS pathology as detailed in comprehensive overviews¹⁷. The extensive neuromodulatory influence of the 5-HT system is governed by approximately 26,000 neurons in the rodent brain¹⁸. While substantial literature associates PD predominantly with dopaminergic neuron loss, the relationship between 5-HT neurons and PD is less thoroughly examined.

N6-methyladenosine (m6A) modification, the most prevalent mRNA modification in eukaryotic cells, is key to regulating mRNA splicing, stability, and export, thereby influencing various cellular activities¹⁹. m6A levels are modulated by methyltransferases and demethylases. Elevated m6A modifications in the brain have been linked to neurodevelopment, with its dysregulation being closely tied to neurodegenerative conditions^20,21, including altered METTL3 expression in Alzheimer's models²². For example, the accumulation of methyltransferase-like 3 (METTL3) in the insoluble fraction of post-mortem brain tissues from Alzheimer's patients has been positively correlated with levels of insoluble Tau protein²³. Furthermore, a significant decrease in m6A levels in the striatum can lead to a substantial decline in dopamine neurotransmitter levels²⁴. Notably, twelve m6A-related single-nucleotide polymorphisms have shown significant associations with PD susceptibility²⁵. This protocol establishes comprehensive methodologies for investigating how METTL3-mediated m6A modifications regulate microglial pyroptosis through the Nrf2/NLRP3 axis, ultimately affecting serotonin neuron survival in PD models. The acute MPTP in vivo model and LPS in vitro model were selected for their robust induction of neuroinflammatory responses, though chronic paradigms could complement future studies as discussed below.

All animal experiments were conducted under the approval of the Institutional Animal Care and Use Committee of the Feicheng People's Hospital (approval number IACUC-2024-118) and performed in strict accordance with institutional guidelines and established ethical principles for laboratory animal research. The study protocols ensured humane care and treatment of all animals, with particular emphasis on minimizing suffering and distress, while adhering to both institutional standards and ARRIVE guidelines for responsible animal research practices.

1. Cell culture and microglial activation model

1. BV2 cell preparation and maintenance
   1. Rapidly thaw frozen BV2 microglial cell stocks in a 37 °C water bath for 2 min, ensuring gentle swirling to prevent thermal shock.
   2. Centrifuge the thawed cell suspension at 300 × g for 5 min at room temperature (RT) to remove cryoprotectant.
   3. Discard the supernatant and resuspend the cell pellet in 10 mL of complete high-glucose DMEM medium supplemented with 10% FBS, 1% penicillin-streptomycin, and 2 mM L-glutamine.
   4. Perform cell viability assessment using the trypan blue exclusion method (mix 10 µL of cell suspension with 10 µL of 0.4% trypan blue, count using hemocytometer), ensuring >95% viability before proceeding.
   5. Plate cells in T75 culture flasks at a density of 1 × 10⁶ cells per flask and maintain in a humidified cell culture incubator at 37°C with 5% CO₂.
   6. Passage cells every 48 h when reaching 80-85% confluency using standard trypsinization procedures.
      NOTE Maintain consistent passage numbers (passages 3-8) to ensure reproducible inflammatory responses. All cell culture experiments were independently repeated at least three times, with three technical replicates per condition to minimize variability.
2. LPS-induced microglial activation
   1. Seed BV2 cells in 6-well plates at 2 × 10⁵ cells per well in 2 mL of complete medium 24 h before treatment.
   2. Prepare LPS stock solution at 1 mg/mL in sterile PBS, filter-sterilize through a 0.22 µm filter, and store in single-use aliquots at -20 °C.
   3. Validate LPS activity using Limulus Amebocyte Lysate (LAL) assay to confirm endotoxin concentration before each experiment²⁶.
   4. Dilute LPS stock to a working concentration of 1 µg/mL in serum-free DMEM immediately before use.
   5. Remove culture medium from wells and wash cells twice with sterile PBS.
   6. Add 2 mL of LPS-containing medium (1 µg/mL final concentration) to treatment wells and serum-free DMEM to control wells.
   7. Incubate cells for 24 h at 37 °C with 5% CO₂ for inflammatory activation.
   8. Include positive control wells treated with 100 ng/mL tumor necrosis factor (TNF)-α and negative control wells with vehicle only (PBS) to validate the inflammatory response.
      NOTE: Prepare fresh LPS solution for each experiment to maintain consistent bioactivity. If the inflammatory response is weak (e.g., <2-fold cytokine increase), check reagent stability or extend incubation to 48 h.
3. Mettl3 overexpression and knockdown
   1. Design and synthesize small interfering RNA (siRNA) sequences targeting Mettl3 and scrambled control sequences using standard oligonucleotide synthesis protocols. Select target sequences from the coding region of Mettl3 mRNA (GenBank accession: NM_019721) with 19-21 nucleotide length, ensuring GC content of 40-60%²⁷.
   2. Validate siRNA sequences using BLAST analysis to confirm specificity and avoid off-target effects (ensure <70% homology with non-target genes).
   3. Prepare overexpression vectors containing full-length Mettl3 cDNA cloned into the pcDNA3.1 vector using restriction sites EcoRI and XhoI.
   4. Transfect cells at 70-80% confluency using a cationic lipid transfection reagent:
      1. Mix 2 µL of cationic lipid transfection reagent with 50 pmol siRNA or 2 µg of plasmid DNA in 100 µL of Opti-MEM medium.
      2. Incubate mixture for 15 min at RT.
      3. Add the transfection complex dropwise to cells and incubate for 4-6 h before changing to fresh complete medium.
   5. Incubate transfected cells for 48 h before LPS treatment to allow adequate protein expression changes.
   6. Confirm transfection efficiency using fluorescent reporter co-transfection (100 ng of pEGFP-C1 per well), targeting >70% efficiency by fluorescence microscopy before proceeding.

2. Animal model development and experimental design

1. Animal preparation and randomization
   1. Obtain C57BL/6J mice from Vital River Laboratory, ensuring equal male-to-female ratios, aged 8 weeks, weighing 19-26 g.
   2. House animals in specific pathogen-free (SPF) conditions with 12:12 h light-dark cycle, controlled temperature (22 ± 2°C), and humidity (50-60%).
   3. Allow a 7-day acclimatization period with ad libitum access to standard chow and water.
   4. Perform baseline behavioral assessments during acclimatization: Conduct comprehensive behavioral evaluations, including forelimb placement test (10 trials per side), accelerating rotarod assessment (4-40 rpm over 5 min), and open field locomotor activity analysis (10-min sessions in a 50 cm × 50 cm × 50 cm apparatus)^28,29,30.
   5. Randomly assign animals to experimental groups (n = 6 per group) using computerized randomization to minimize bias: sham, Model, Model+Nrf2-KD, Model+oe-Nrf2.
   6. Implement a blinded experimental design where investigators performing behavioral assessments are unaware of group assignments.
2. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced Parkinson's disease model
   1. Prepare MPTP solution at 30 mg/kg in sterile saline immediately before injection by dissolving 15 mg of MPTP hydrochloride in 5 mL of sterile saline and adjusting volume based on animal weight.
      ​CAUTION: MPTP is highly neurotoxic. Handle in a chemical fume hood with appropriate personal protective equipment, including double gloves and a lab coat.
   2. Administer MPTP via intraperitoneal injection at 30 mg/kg body weight (injection volume 10 mL/kg) for three consecutive days at the same time each day (09:00 ± 30 min).
   3. Inject sham group animals with equivalent volumes of sterile saline following an identical injection schedule.
   4. Monitor animals for signs of distress, weight loss (>15% considered endpoint), or adverse reactions daily during the MPTP administration period using standardized scoring sheets.
   5. Maintain animals for 8 days post-final injection to allow neurodegeneration development.
      NOTE: Animals can be housed normally during this period with continued monitoring.
3. Stereotactic viral injection
   1. Prepare AAV9 viral vectors by triple transfection of HEK293T cells using polyethylenimine (PEI; 1:3 DNA:PEI ratio), harvest at 72 h post-transfection, purify using iodixanol gradient ultracentrifugation (177,000 × g for 2 h), and concentrate using centrifugal filter devices to achieve 1 × 10¹² genome copies/mL.
   2. Validate viral titer via quantitative polymerase chain reaction (PCR) using primers targeting the ITR regions, ensuring titers reach 1 × 10¹² genome copies/mL with standard curve analysis.
   3. Validate viral titer using SYBR Green-based quantitative PCR with ITR-specific primers, performing serial dilutions of known standards and calculating genome copies using standard curve analysis. Confirm absence of replication-competent virus contamination using p24 ELISA.
   4. Anesthetize mice with isoflurane (3% induction, 1.5-2% maintenance) delivered at 0.8-1.0 L/min oxygen flow rate through a nose cone, monitoring respiratory rate (60-100 breaths/min) and toe pinch reflex throughout the procedure. Secure the mouse in the stereotactic frame and apply ophthalmic ointment to prevent corneal drying.
   5. Secure the mouse in the stereotactic frame and apply ophthalmic ointment to prevent corneal drying.
   6. Make a 1.5 cm midline scalp incision using a sterile scalpel blade and identify the bregma landmark using stereotactic coordinates with 0.1 mm precision.
   7. Calculate injection coordinates for striatum: anterior-posterior -0.5 mm, medial-lateral ±2.0 mm, dorsal-ventral -3.0 mm from bregma.
   8. Perform pilot injections with fluorescent tracer (e.g., green fluorescent protein [GFP]) to validate targeting accuracy, confirming >80% co-localization with microglial markers (Iba1) via immunohistochemistry before experimental injections.
   9. Lower injection needle (33-G) to target depth at 1 mm/min rate and inject 2 µL of viral suspension at 0.2 µL/min using a micro syringe pump with an automated controller. Lower the injection needle to the target depth and inject 2 µL of viral suspension at a rate of 0.2 µL/min using a microsyringe pump.
   10. Maintain needle position for 5 min post-injection to prevent backflow.
   11. Slowly withdraw the needle at a 0.5 mm/min rate, close the incision with 4-0 silk sutures using an interrupted pattern, and allow recovery from anesthesia on a heated recovery pad.
   12. Administer post-operative analgesia (carprofen 5 mg/kg subcutaneously once daily for 3 days) and monitor recovery for 24 h with detailed observation sheets.

3. Molecular biology techniques and validation

1. RNA extraction and quality control
   1. Harvest cells or striatal brain tissue samples immediately after experimental endpoints by euthanizing animals humanely via CO₂ inhalation followed by cervical dislocation; dissect tissue on ice, mince finely, and enzymatically dissociate with 0.25% trypsin for 10 min at 37 °C if harvesting cells.
   2. Extract total RNA using TRIzol reagent (1 mL per 1 10⁶ cells or 100 mg of tissue) with mechanical homogenization (20-25 strokes in a dounce homogenizer) and chloroform phase separation (0.2 mL of chloroform per 1 mL of TRIzol).
   3. Assess RNA quality using spectrophotometry (A260/A280 ratio 1.8-2.0) and gel electrophoresis (1% agarose) to confirm 28S and 18S ribosomal bands.
   4. Quantify RNA concentration using a spectrophotometer.
   5. Store RNA samples at -80 °C in nuclease-free water at -80 °C with the addition of RNase inhibitor (40 units/µL) until analysis.
2. Nuclear-cytoplasmic fractionation
   1. Harvest cells (2 × 10⁶ minimum) and wash twice with ice-cold PBS (5 mL per wash, 5 min centrifugation at 300 × g).
   2. Utilize a commercial cell fractionation kit with the following protocol.
      1. Resuspend the cell pellet in 200 µL of cytoplasmic extraction buffer with protease inhibitors.
      2. Incubate on ice for 10 min with vortexing every 3 min.
      3. Centrifuge at 16,000 × g for 5 min at 4 °C and collect supernatant (cytoplasmic fraction).
      4. Wash the pellet twice with cytoplasmic extraction buffer.
      5. Resuspend the pellet in 100 µL of nuclear extraction buffer.
      6. Incubate on ice for 40 min with vortexing every 10 min.
      7. Centrifuge at 16,000 × g for 10 min at 4 °C and collect the supernatant (nuclear fraction)
   3. Validate fractionation efficiency by analyzing nuclear marker (U6) and cytoplasmic marker (GAPDH) distribution using Western blot.
   4. Extract RNA from nuclear and cytoplasmic fractions separately.
   5. Analyze Nrf2 mRNA levels in each fraction using RT-qPCR with fraction-specific reference genes.
3. Quantitative real-time PCR (RT-qPCR)
   1. Synthesize cDNA from 1 µg of total RNA using a reverse transcription reagent kit with random primers.
   2. Perform qPCR using Premix Ex Taq II kit with gene-specific primers on a real-time PCR system.
   3. Validate primer specificity using melt curve analysis and confirm a single amplicon by gel electrophoresis.
   4. Use the following thermal cycling conditions: initial denaturation at 95 °C for 10 min, followed by 40 cycles of 95 °C for 15 s, 60 °C for 30 s, and 72 °C for 30 s.
   5. Calculate relative mRNA expression levels using the 2^-ΔΔCt method with β-actin as an internal reference.
   6. Include no-template controls and validate reference gene stability across experimental conditions using geNorm analysis (stability value M < 0.5).
      NOTE: Reproducibility scores: intra-assay coefficient of variation (CV) <10% across three independent runs.
4. MeRIP-qPCR for m6A modification analysis
   1. Extract total RNA (minimum 20 µg) and fragment to 100-200 nucleotides using RNA fragmentation reagent (10 mM ZnCl₂, 10 mM Tris-HCl, pH 7.0) at 70 °C for 5 min.
   2. Perform immunoprecipitation using m6A-specific antibody (2 µg per 20 µg of RNA) overnight at 4 °C with rotation (10 rpm) in immunoprecipitation buffer (50 mM Tris-HCl, pH 7.4, 750 mM NaCl, 0.5% NP-40).
   3. Validate antibody specificity using known m6A-modified and unmodified RNA controls.
   4. Wash immunoprecipitated complexes five times with washing buffer.
   5. Elute bound RNA and reverse transcribe using random primers.
   6. Reverse transcribes eluted RNA using random primers and perform qPCR analysis using Nrf2-specific primers covering potential m6A sites.
   7. Perform qPCR analysis using Nrf2-specific primers and calculate enrichment relative to input RNA.
      NOTE: If detection is inconsistent, optimize fragmentation time (4-6 min) to improve reproducibility; quantitative benchmark: signal-to-noise ratio >15:1 compared to IgG controls.

4. Protein analysis and validation

1. Protein extraction and quantification
   1. Lyse cells (1 × 10⁶ cells) or homogenize tissue samples (100 mg of striatal tissue) in 200 µL of radioimmunoprecipitation assay (RIPA) buffer containing protease inhibitors (1 mM phenylmethylsulfonyl fluoride (PMSF), 1 µg/mL leupeptin, 1 µg/mL pepstatin A) and phosphatase inhibitors (1 mM sodium orthovanadate, 5 mM sodium fluoride).
   2. Incubate lysates on ice for 30 min with periodic vortexing (every 10 min for 10 s).
   3. Centrifuge at 12,000 × g for 15 min at 4 °C to remove cellular debris and collect supernatant.
   4. Quantify protein concentration using bicinchoninic acid (BCA) assay with bovine serum albumin standards (0, 25, 125, 250, 500, 750, 1000, 1500, 2000 µg/mL) in triplicate measurements.
   5. Validate protein integrity by analyzing housekeeping protein levels (GAPDH, expected MW: 36 kDa) across all samples using Western blot.
2. Western blot analysis
   1. Prepare protein samples with equal loading (30-50 µg) in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) loading buffer (final concentrations: 62.5 mM Tris-HCl, pH 6.8, 2% SDS, 10% glycerol, 5% β-mercaptoethanol, 0.003% bromophenol blue) and denature at 95 °C for 5 min.
   2. Separate proteins by SDS-PAGE using 10% polyacrylamide gels, running at 80 V for 30 min followed by 120 V for 90 min in Tris-glycine-SDS buffer.
   3. Transfer proteins to polyvinylidene fluoride (PVDF) membranes using a semi-dry transfer system (400 mA for 90 min) in transfer buffer (25 mM Tris, 192 mM glycine, 20% methanol).
   4. Confirm transfer efficiency using reversible protein staining (Ponceau S: 0.1% in 5% acetic acid for 5 min) before proceeding to antibody incubation.
   5. Block membranes with 5% non-fat milk in TBST (TBS + 0.1% Tween-20) for 1 h at RT with gentle agitation.
   6. Incubate with primary antibodies diluted 1:1000 in blocking buffer overnight at 4 °C with gentle agitation.
   7. Wash membranes three times with TBST (10 min each) and incubate with HRP-conjugated secondary antibodies (1:5000 dilution) for 1 h at RT.
   8. Detect protein bands using enhanced chemiluminescence and quantify using densitometry.
   9. Normalize target protein expression to loading control (GAPDH) and validate antibody specificity using appropriate controls (peptide blocking for primary antibodies, secondary-only controls).

5. Behavioral assessment and functional analysis

1. Forelimb placement test
   1. Allow the mouse to acclimate to the testing environment (quiet room, 22 °C, dim lighting) for 30 min before assessment.
   2. Grasp the mouse gently by the dorsal skin, suspending all four limbs approximately 0.5 cm above the table edge while ensuring the mouse cannot see the table surface.
   3. Brush vibrissae on each side against the table edge to trigger the placing reflex and record successful forelimb placement within 2 s.
   4. Perform 10 trials per side with 30-s intervals between trials, alternating left and right sides.
   5. Perform testing in consistent lighting conditions (50-100 lux) and time of day (13:00-17:00) to minimize circadian influences.
   6. Calculate success rate as a percentage of successful placements: (successful placements/total trials) × 100.
2. Accelerating the rotarod test
   1. Set the rotarod apparatus to an initial speed of 4 rpm with linear acceleration to 40 rpm over 5 min duration.
   2. Train mice on the rotarod for 3 consecutive days (3 trials per day, 15-min intervals) before testing to minimize learning effects.
   3. Standardize testing conditions, including RT (22 ± 2 °C), lighting (100 lux), and background noise levels (<50 dB).
   4. Place the mouse on the rotating rod facing forward and start the acceleration protocol immediately.
   5. Record latency to fall (time from start until mouse falls or completes 5-min trial) for each trial, testing 5 times per animal with 15-min rest intervals.
   6. Calculate average latency from the three longest trials to minimize variability due to motivational factors.
3. Open field test
   1. Use an open field apparatus (50 cm × 50 cm × 50 cm clear acrylic box) with a video tracking system positioned 1 m above the arena.
   2. Clean apparatus with 70% ethanol between animals (spray and wipe, 5-min air dry) to eliminate odor cues.
   3. Place the mouse in the center of the apparatus and record activity for 10 min under consistent lighting (200 lux) with video capture at 30 fps.
   4. Analyze multiple parameters using automated tracking software:
      Total distance traveled (cm)
      Center zone time (defined as 10 cm from walls, reported in s)
      Movement velocity (cm/s)
      Time spent in periphery vs center zones
4. Define the central zone as 10 cm from the walls and quantify time spent and distance traveled in the center versus the periphery zones.

6. Advanced microscopy and imaging

1. DHE staining for ROS detection
   1. Prepare fresh dihydroethidium (DHE) solution at 10 µM in PBS immediately before use (protect from light).
   2. Perform co-immunofluorescence staining combining DHE with tryptophan hydroxylase 2 (TPH2) antibody (1:500 dilution) to specifically identify serotonin neurons.
      ​NOTE: This dual-labeling approach enables discrimination of ROS production within TPH2-positive neuronal cell bodies from oxidative stress in surrounding glial cells, based on the principle that DHE, upon oxidation by superoxide, forms membrane-impermeant fluorescent products that remain localized within the cell of origin.
   3. Incubate tissue sections (20 µm thickness) with DHE solution for 30 min at 37 °C in dark conditions in a humidified chamber.
   4. Include negative controls (no DHE) and positive controls (100 µM H₂O₂ treatment for 30 min) to validate ROS detection specificity.
   5. Wash sections three times with PBS and mount with anti-fade mounting medium.
   6. Image using fluorescence microscopy (DHE: 518 nm excitation/605 nm emission, TPH2: 488 nm excitation/525 nm emission) with consistent exposure settings (200 ms) across samples; imaging systems were calibrated weekly using standard fluorescence beads (signal-to-noise ratio > 20:1).
   7. Quantify fluorescence intensity in TPH2-positive cells only using ImageJ software with standardized analysis protocols (threshold: 50-255, particle size: 10-∞ pixels²). Establish ROI boundaries based on TPH2 immunoreactivity to ensure measurement of oxidative stress specifically within serotonergic neurons while excluding signal from adjacent microglia, astrocytes, or other cell types. For each animal, analyze a minimum of 50 TPH2-positive neurons across multiple tissue sections.
2. Immunohistochemistry
   1. Fix tissue sections in 4% paraformaldehyde in PBS for 24 h at 4 °C with gentle agitation.
   2. Dehydrate through a graded ethanol series (70%, 80%, 90%, 95%, 100% × 2, 1 h each) and embed in paraffin using an automated processor.
   3. Cut 5 µm sections using a microtome and mount on charged glass slides, ensuring 3-4 sections per slide.
   4. Deparaffinize sections using xylene (2 × 10 min) and rehydrate through graded ethanol to water.
   5. Perform antigen retrieval using citrate buffer (10 mM sodium citrate, pH 6.0) in a microwave (750 W for 10 min with 2-min cooling intervals).
   6. Validate antibody specificity using appropriate negative controls (no primary antibody) and positive control tissues (brain sections known to express target proteins).
   7. Block endogenous peroxidase activity with 3% hydrogen peroxide in methanol for 10 min at RT.
   8. Apply primary antibodies overnight at 4 °C in a humidified chamber.
   9. Detect using HRP-conjugated secondary antibodies (1:500 dilution, 1 h at RT) and DAB chromogen (incubate until brown color develops, typically 2-5 min).
   10. Counterstain with hematoxylin (30 s), dehydrate, clear, and mount with permanent mounting medium.
   11. Quantify positive cells using stereological methods with systematic random sampling (every 5th section, 3 counting frames per section, 40× magnification).

7. Statistical analysis and data validation

1. Statistical design and analysis
   1. Perform power analysis to determine adequate sample sizes for detecting biologically meaningful differences (effect size ≥ 0.8, power ≥ 0.80, α = 0.05) using G*Power 3.1.9.7 software.
   2. Validate data normality using the Shapiro-Wilk test and assess homogeneity of variance using Levene's test before parametric analysis .
   3. Analyze data using a statistical software with appropriate statistical tests based on experimental design and data distribution.
   4. Apply one-way analysis of variance (ANOVA) for single-factor comparisons and two-way ANOVA for factorial designs (e.g., treatment × time interactions).
   5. Use Tukey's post hoc test for multiple comparisons when ANOVA shows significance (p < 0.05), applying Bonferroni correction when appropriate.
   6. Set statistical significance threshold at p < 0.05 for all analyses with additional notation for p < 0.01 and p < 0.001.
   7. Report effect sizes (Cohen's d or η²) and 95% confidence intervals alongside p-values to provide a comprehensive statistical interpretation.
   8. Present data as mean ± SEM from a minimum of three independent experiments, with individual data points shown when n ≤ 10.
      NOTE: All cell culture experiments should be independently repeated at least three times, with each experiment including appropriate controls and multiple technical replicates, and observed variability (e.g., CV <15% for behavioral data) was minimized through blinding and standardized timing.

The protocol successfully demonstrates that Mettl3 expression decreases in LPS-treated microglial cells (Figure 1), as evidenced by both mRNA and protein level reductions compared to control groups (Figure 1B,C). ELISA analysis confirms successful LPS-mediated inflammatory activation through elevated IL-6 and TNF-α levels in culture supernatants (Figure 1A).

MeRIP-qPCR analysis reveals that Mettl3 overexpression enhances Nrf2 mRNA m6A methylation, which paradoxically increases protein stability and expression rather than promoting degradation, consistent with emerging evidence that m6A modifications can enhance translation efficiency under specific cellular contexts (Figure 2A,B). Nuclear-cytoplasmic fractionation experiments demonstrate that while Nrf2 mRNA distribution remains unchanged across cellular compartments, protein levels are significantly modulated by Mettl3 expression status (Figure 2C,D).

In vivo experiments using MPTP-induced PD models show that behavioral deficits correlate with molecular changes in the Mettl3/Nrf2 axis. All tissue samples were obtained from the striatal region (coordinates: +0.5 to -0.5 mm from bregma, ±1.5 to ±2.5 mm lateral, -2.5 to -3.5 mm ventral). Mice in the Model group exhibit reduced forelimb placement accuracy, decreased rotarod performance, and diminished open field activity compared to sham controls (Figure 3A). Nrf2 knockdown exacerbates these deficits, while Nrf2 overexpression provides partial protection. Immunohistochemical analysis reveals decreased microglial populations (Iba1-positive cells) in striatal regions of PD models, with Nrf2 modulation affecting microglial survival accordingly (Figure 3B).

Pro-inflammatory cytokine levels (IL-1β, IL-18, TNF-α) increase significantly in disease models as expected, with the Model group showing elevated levels compared to Sham controls, and Nrf2 overexpression providing anti-inflammatory effects (Figure 3C). Molecular analysis demonstrates elevated pyroptosis markers, including GSDMD-N, cleaved-caspase-1, and NLRP3 in MPTP-treated animals, with corrected Western blot results showing proper molecular weight markers (Figure 3D). Scanning electron microscopy confirms increased pyroptotic body formation in diseased animals, with Nrf2 modulation affecting the extent of pyroptotic cell death (Figure 3E).

DHE staining combined with TPH2 immunofluorescence reveals elevated ROS levels specifically in serotonin neurons of PD models (Figure 4A). The co-localization of DHE oxidation products within TPH2-positive cell bodies indicates endogenous superoxide generation in these neurons, consistent with inflammatory cytokine-induced mitochondrial dysfunction and impaired antioxidant defenses. The spatial distribution of the DHE signal matches neuronal morphology rather than diffuse tissue oxidation, supporting cell-type-specific oxidative stress. Immunohistochemistry shows decreased expression of serotonin neuron markers TPH2 and SLC6A4 in Model and Model+Nrf2-KD groups, with protective effects observed in the Model+oe-Nrf2 group (Figure 4B). These findings indicate that microglial pyroptosis leads to oxidative stress and subsequent serotonin neuron damage.

The overall mechanistic pathway is summarized in Figure 5, which illustrates how Mettl3-mediated m6A modification of Nrf2 suppresses microglial pyroptosis and protects serotonin neurons.

Successful experiments typically show clear dose-response relationships between Mettl3/Nrf2 expression levels and downstream inflammatory markers. Suboptimal results may occur due to incomplete viral transduction, inadequate LPS activation, or technical issues during tissue processing. Control experiments consistently demonstrate proper antibody specificity and absence of non-specific binding in immunohistochemical analyses. Viral infection efficiency in the striatum was validated through co-localization with Iba1 and NeuN markers, confirming predominant microglial targeting.

Figure 1: Under-expression of Mettl3 in LPS-induced microglial cells. (A) ELISA measurement of IL-6 and TNF-α levels in LPS-treated microglial cells. (B) RT-qPCR measurement of Mettl3 mRNA levels in LPS-treated microglial cells. (C) Western blot measurement of Mettl3 protein levels in LPS-treated microglial cells showing Mettl3 at 64 kDa and GAPDH at 36 kDa. **Data represent mean ± SEM, n = 6 per group. *p < 0.05, p < 0.01 vs. Control group. Please click here to view a larger version of this figure.

Figure 2: Mettl3 affects Nrf2 translation through m6A modification in LPS-induced microglial cells. (A) RT-qPCR measurement of Nrf2 mRNA levels in oe-NC and oe-Mettl3 groups. (B) MeRIP-qPCR measurement of Nrf2 methylation levels in oe-NC and oe-Mettl3 groups. (C) RT-qPCR measurement of Nrf2 mRNA levels in the nucleus and cytoplasm of experimental groups. (D) Western blot measurement of Nrf2 protein levels in experimental groups showing Nrf2 at 110 kDa and GAPDH at 36 kDa. **Data represent mean ± SEM, n = 6 per group. *p < 0.05, p < 0.01 vs. Control group. Please click here to view a larger version of this figure.

Figure 3: In vivo demonstration of Mettl3/Nrf2 axis triggering microglial pyroptosis through NLRP3 inflammasome. (A) Behavioral assessments including forelimb placement, rotarod, and open field tests. (B) Immunohistochemical detection of Iba protein expression in brain tissue (scale bar = 50 µm). (C) ELISA detection of inflammatory cytokines in brain tissue showing expected elevation in Model groups with reduction upon Nrf2 overexpression. (D) Western blot detection of pyroptosis markers with molecular weight annotations: NLRP3 at 110 kDa, cleaved-Caspase-1 at 22 kDa, GSDMD-N at 31 kDa, and GAPDH at 36 kDa. (E) Scanning electron microscopy detection of pyroptotic bodies (indicated by white arrows showing characteristic 1-5 µm membrane-bound vesicles). Quantification based on 5 fields per section, n = 6 animals/group. **Data represent mean ± SEM, n = 6 per group. *p < 0.05, p < 0.01 vs. Sham group. Please click here to view a larger version of this figure.

Figure 4: Microglial pyroptosis causes mitochondrial damage and promotes 5-HT neuron apoptosis. (A) DHE staining combined with TPH2 immunofluorescence for ROS detection specifically in 5-HT neurons (scale bar = 50 µm). The co-localization approach enables discrimination of oxidative stress within TPH2-positive neuronal cell bodies from surrounding glial cells. (B) Immunohistochemical detection of TPH2 and SLC6A4 protein expression in 5-HT neurons (scale bar = 50 µm). **Data represent mean ± SEM, n = 6 per group. *p < 0.05, p < 0.01 vs. Sham group. Please click here to view a larger version of this figure.

Figure 5: Schematic representation of Mettl3-mediated suppression of Parkinson's disease progression through m6A modification of Nrf2 and inhibition of 5-HT neuronal death. Please click here to view a larger version of this figure.

There are several critical steps that require careful attention to ensure reproducible results. The LPS activation timing is crucial - inflammatory responses peak at 24 h post-treatment, and we strongly recommend preparing fresh LPS solution for each experiment, as stored solutions can lose bioactivity^19,20. For the MeRIP-qPCR analysis, antibody quality is paramount. We have found that validating antibodies using known positive and negative controls before experimental samples prevents many troubleshooting issues later. During stereotactic viral injection, maintaining precise coordinates and injection speed (0.2 µL/min) is critical for targeting accuracy. The 5-min waiting period post-injection prevents backflow, which can significantly reduce transduction efficiency.

Several modifications can optimize experimental outcomes based on specific research questions. For studies investigating chronic neuroinflammation, we have successfully used lower LPS concentrations (100 ng/mL) over extended periods (72-96 h), which better simulate chronic disease conditions observed in human patients^7,8. We have also tested alternative viral vectors (AAV1, AAV2) for different cell-type targeting, though AAV9 provides the most reliable broad CNS transduction in our hands. Common troubleshooting issues include inadequate viral transduction, which can usually be resolved by increasing the titer to 2 × 10¹² gc/mL, and variable behavioral responses, which we address by extending the acclimatization period to 10-14 days. For the m6A analysis, inconsistent detection often results from suboptimal RNA fragmentation - strictly controlling the fragmentation time and temperature significantly improves reproducibility^25,24.

While this approach provides robust tools for investigating Mettl3/Nrf2 signaling, several important limitations must be acknowledged. The acute MPTP model, while allowing precise temporal control of neurodegeneration, may not fully recapitulate the chronic progressive nature of human Parkinson's disease^1,15. We chose this model because it provides standardized, reproducible results within a reasonable timeframe, but researchers interested in chronic progression should consider complementary approaches. The use of BV2 microglial cells provides excellent experimental control and reproducibility, but primary microglia may respond differently to the same stimuli^2,6. Our focus on striatal tissue analysis was chosen because this region shows the most consistent changes in pilot studies, but important disease-relevant changes may occur in other brain regions, such as the substantia nigra^15,18. Additionally, the current experimental design cannot definitively distinguish between direct effects of Mettl3/Nrf2 manipulation on serotonin neurons versus indirect effects mediated through microglial signaling.

While the co-localization approach provides strong evidence for serotonergic neuronal oxidative stress, we acknowledge that DHE fluorescence within TPH2-positive neurons could potentially include contributions from both endogenously generated ROS and extracellular ROS that enter the neuron. However, several lines of evidence support predominantly endogenous neuronal superoxide generation: (1) the Nrf2 rescue effect demonstrates that enhancing neuronal antioxidant capacity reduces DHE signal, (2) the spatial pattern of DHE fluorescence matches neuronal morphology rather than diffuse extracellular distribution, and (3) the mechanistic plausibility of inflammatory cytokine-induced neuronal mitochondrial dysfunction. Future studies employing genetically encoded ROS sensors with subcellular localization could further refine the understanding of ROS sources and dynamics.

This work advances beyond existing approaches by providing the first comprehensive framework for investigating epitranscriptomic regulation of neuroinflammation in Parkinson's disease models. Unlike previous studies that examined individual components of this pathway^11,31, our integrated approach allows simultaneous analysis of m6A modifications, protein expression, inflammatory signaling, and behavioral outcomes within the same experimental system. The inclusion of serotonin neuron analysis addresses a significant gap in the field, as most PD research focuses exclusively on dopaminergic systems^1,15,16. We have found that the combined in vitro and in vivo approach provides both mechanistic insights and physiological relevance that neither approach could achieve alone. The emphasis on validating epitranscriptomic modifications through multiple complementary techniques (MeRIP-qPCR, Western blot, functional assays) strengthens the reliability of findings compared to studies relying on single analytical approaches.

The experimental framework establishes a foundation for multiple future research directions with significant potential for therapeutic development. The techniques can be readily adapted to investigate other RNA modifications (m1A, m5C, pseudouridine) in neurodegeneration - we have successfully used similar approaches for m1A analysis with minor modifications^21,25. The viral delivery system can be easily modified to test therapeutic interventions, including small molecule enhancers of Nrf2 activity or specific NLRP3 inhibitors^32,33. We have already begun testing several compounds using this behavioral assessment battery as a screening platform. The demonstration that serotonin neuron dysfunction contributes to PD pathology opens new therapeutic avenues, as serotonin systems may be more amenable to pharmacological manipulation than dopaminergic systems^15,16. The modular design facilitates adaptation for other neurodegenerative diseases - we are currently adapting these techniques for Alzheimer's disease research with promising preliminary results.

The identification of Mettl3 as a potential therapeutic target is particularly exciting given the growing interest in RNA-modifying enzymes as druggable targets^31,34,35. However, successful clinical translation will require addressing several challenges, including developing brain-penetrant compounds capable of modulating m6A pathways and ensuring long-term safety of epitranscriptomic interventions. When planning experiments using these techniques, researchers should carefully consider their specific research questions and modify the procedures accordingly. We recommend starting with the basic approach as described and then introducing modifications once the standard procedures are well-established in the laboratory.