Hypothesis

Sex‑specific expression of the SCFA receptor FFAR3 in vagal afferent neurons gates the gut‑to‑brain longevity signal of acarbose, explaining the male‑biased lifespan extension.

Rationale

• Acarbose raises colonic SCFAs that activate vagal afferents via FFAR2 and FFAR3 (citations).
• Estrogen signaling in females upregulates a microRNA (miR‑200c) that represses FFAR3 transcription in nodose ganglia, dampening SCFA‑evoked vagal firing.
• Males, with lower estrogen, retain higher FFAR3 levels, yielding stronger brainstem NF‑κB inhibition and systemic metabolic improvements.
• This predicts that manipulating FFAR3 or estrogen will invert or equalize the sex difference.

Testable Predictions

1. Vagal FFAR3 knockdown in male mice will reduce acarbose‑mediated lifespan extension to female levels (~5%).
2. Ovariectomy (or estrogen receptor antagonist) in female mice will increase FFAR3 expression, boost vagal SCFA signaling, and extend lifespan by ≈15% with acarbose.
3. FFAR3 over‑expression in female vagal neurons will rescue the longevity benefit without altering estrogen levels.
4. Vagotomy will abolish the lifespan extension in both sexes only when FFAR3 is intact, confirming the afferent route.
5. Pharmacologic FFAR3 agonist (e.g., MK‑8566) combined with low‑dose acarbose will mimic the full effect in females.

Experimental Design

• Use CRISPR‑Cas9 or AAV‑shRNA to target FFAR3 specifically in Phox2b⁺ vagal neurons.
• Monitor SCFA levels, vagal electrophysiology, hypothalamic NF‑κB activity, and metabolic readouts.
• Survival cohorts (n≥80 per group) under ITP‑standard diet with acarbose (1000 ppm).
• Include hormone assays and miRNA quantification to confirm mechanism.

Potential Outcomes & Interpretation

If FFAR3 loss in males abolishes the benefit while its gain in females restores it, the hypothesis is supported. Conversely, if lifespan changes occur without altered vagal signaling, the gut‑to‑brain direction would be challenged, suggesting alternative peripheral pathways.