Neuronal m6A Deficiency Drives Gut Dysbiosis via Altered Neuropeptide Secretion, Closing the Inflammaging Loop

Hypothesis

Age‑dependent loss of METTL3/14‑mediated m6A methylation in hippocampal neurons reduces translation of genes that encode gut‑regulating neuropeptides (e.g., PYY, GLP‑1, substance P). This neuronal output shift alters vagal signaling and mucosal secretion, fostering a dysbiotic microbiome that further depletes methyl donors, thereby exacerbating neuronal epitranscriptomic collapse. Restoring neuronal m6A or supplying methyl donors breaks this bidirectional loop.

Mechanistic Rationale

m6A modification enhances cytoplasmic translation of specific transcripts without changing their abundance. In aged hippocampi, METTL3/14 decline preferentially targets mRNAs with DRACH motifs encoding secreted neuropeptides that modulate gut motility, mucus production, and immune tone. Reduced neuropeptide release diminishes vagal afferent signaling and alters enteroendocrine cell activity, leading to decreased secretion of antimicrobial peptides and altered bile‑acid pools. These changes favor expansion of proteobacteria and depletion of methyl‑donor‑producing taxa (e.g., certain Bifidobacterium and Lactobacillus strains). The resulting microbiome produces less folate, choline, and betaine, lowering hepatic SAM synthesis and neuronal m6A capacity—a self‑reinforcing circuit.

Experimental Design

1. Neuronal METTL3 manipulation: Use AAV‑Cre to delete METTL3 specifically in hippocampal excitatory neurons of young adult mice (3 mo) and overexpress METTL3 in aged mice (20 mo). Include GFP‑only controls.
2. Readouts at 8 weeks post‑injection:
   • Hippocampal m6A‑seq (MeRIP‑seq) focusing on neuropeptide transcripts.
   • Quantitative PCR and ELISA for PYY, GLP‑1, substance P in hippocampal homogenates and plasma.
   • Vagal nerve activity recorded via ex vivo electrophysiology.
   • Fecal 16S rRNA sequencing to assess microbiota composition, with particular attention to methyl‑donor‑synthesizing taxa.
   • Gut barrier integrity measured by FITC‑dextran permeability and mucin‑2 staining.
   • Cognitive performance via Morris water maze.
3. Rescue arms: Sub‑cohorts receive choline‑bitartrate (0.5 % w/w) in drinking water or a defined probiotic blend enriched for folate‑producing strains.
4. Falsifiable predictions:
   • Neuronal METTL3 loss will decrease m6A on neuropeptide mRNAs, lower circulating neuropeptide levels, increase gut permeability, and shift microbiota toward a pro‑inflammatory profile.
   • Overexpressing METTL3 in aged mice will restore neuropeptide output, improve barrier function, rebalance methyl‑donor‑producing taxa, and rescue spatial memory.
   • Methyl‑donor supplementation will ameliorate gut pathology but will not fully restore neuronal m6A or cognition if neuronal METTL3 remains deficient, confirming the primal role of neuronal epitranscriptomics.

Expected Outcomes and Falsifiability

If the hypothesis is correct, neuronal m6A status will predict gut‑derived methyl‑donor availability and vice versa, demonstrating a causal, bidirectional link. Failure to observe correlated changes in neuropeptide secretion, microbiota composition, or barrier integrity after neuronal METTL3 manipulation would falsify the proposed mechanism. Successful rescue by methyl donors only when neuronal m6A is intact would further validate the hierarchy of control.