Hypothesis

Restoring a youthful gut microbiome composition that produces rhythmic, pulsatile butyrate release—combined with targeted vagal nerve stimulation—will reverse senescence in aged cardiac c-Kit+ progenitor cells (CPCs) by stabilizing HIF‑1α, enhancing autophagic flux, and suppressing the SASP, thereby rescuing their proliferative and paracrine capacity.

Rationale

• Aged CPCs exhibit high p16^INK4a^ and SA‑β‑gal positivity, reduced proliferation, and deficient secretion of reparative factors {pubmed.ncbi.nlm.nih.gov/28737214/}.
• Butyrate, a microbiota‑derived short‑chain fatty acid, activates GPR41/43, inhibits HDACs, and promotes IL‑10‑producing Tregs, which dampen cardiac inflammation {frontiersin.org/journals/cardiovascular-medicine/articles/10.3389/fcvm.2025.1572948/full}.
• Vagal tone correlates with microbiota diversity and inversely with systemic IL‑6 and CRP {pubmed.ncbi.nlm.nih.gov/36246995/}; acetylcholine released from vagal terminals engages α7nAChR on macrophages to curb NF‑κB signaling {tandfonline.com/doi/full/10.1080/19490976.2023.2290331}.
• HDAC inhibition by butyrate can stabilize HIF‑1α under normoxia, driving transcription of BNIP3 and LC3, which promote mitophagy and autophagy—a pathway already shown to improve aged CPC function when PI3K‑Akt is inhibited {pmc.ncbi.nlm.nih.gov/articles/PMC11414664/}.

Thus, the gut‑brain‑vagal axis may act as a metabolic‑epigenetic rheostat that sets the autophagic tone of CPCs; dysbiosis blunts butyrate pulses and vagal signaling, leading to HIF‑1α decline, autophagic insufficiency, and senescence.

Novel Mechanistic Insight

We propose that pulsatile—rather than constant—butyrate exposure creates intermittent HDAC inhibition, which generates oscillatory HIF‑1α stabilization. These oscillations trigger autophagic bursts that preferentially clear damaged mitochondria and reduce ROS‑induced DNA damage signals (γH2AX) in CPCs. Concurrently, vagal acetylcholine signaling lowers IL‑6‑STAT3 activity, removing a transcriptional driver of p16^INK4a^. The combined effect is a “metabolic reset” that shifts CPCs from a senescent, SASP‑high state to a quiescent‑ready state capable of rapid proliferation and paracrine factor release upon injury.

Experimental Plan (Testable & Falsifiable)

1. Microbiota‑Defined Butyrate Pulses

   • Colonize germ‑free aged mice with a defined consortium of butyrate‑producing strains (e.g., Faecalibacterium prausnitzii, Roseburia spp.) engineered to release butyrate in a circadian‑gated manner using a inducible promoter.
   • Control groups: (a) aged mice receiving same consortium without inducible control (continuous low butyrate), (b) aged mice receiving vehicle.

2. Vagal Stimulation

   • Apply transcutaneous vagal nerve stimulation (tVNS) at 0.5 mA, 20 Hz, 1 h/day to half of each microbiota group.
   • Include bilateral cervical vagotomy sub‑cohort to confirm vagal dependence.

3. Readouts (4‑week intervention)

   • CPC Isolation: Flow‑sort c-Kit^+^ cells from heart tissue.
   • Senescence: Quantify p16^INK4a^, p21^Cip1^, SA‑β‑gal, and SASP cytokines (IL‑6, IL-1β) by qPCR/ELISA.
   • Autophagy: Measure LC3‑II/I ratio, p62 degradation, and mitochondrial mass (MitoTracker) via western blot and flow cytometry.
   • HIF‑1α: Nuclear HIF‑1α protein levels and BNIP3 expression.
   • Function: Ex vivo proliferation (EdU incorporation), differentiation into cardiomyocytes (α‑actinin^+^), and secretory capacity (VEGF, HGF).
   • In Vivo Cardiac Repair: Induce MI, assess ejection fraction by echo, scar size (Masson’s trichrome), and engraftment of labeled CPCs.

4. Falsification Criteria

   • If pulsed butyrate + tVNS fails to reduce senescence markers (<10 % change vs. controls) or does not improve autophagic flux and HIF‑1α stabilization, the hypothesis is falsified.
   • If vagotomy abolishes the benefits despite pulsed butyrate, confirming vagal necessity.
   • If continuous low butyrate yields equal or superior effects, the pulsatile component is not essential.

Expected Outcome

We anticipate a synergistic reduction in CPC senescence (≥50 % decrease in p16^INK4a^+ cells), restoration of proliferative index to youthful levels (~2‑fold increase), and enhanced paracrine factor secretion, translating to improved post‑MI cardiac function (EF ↑15 %). This would demonstrate that targeting the gut‑brain‑vagal axis can rejuvenate the cardiac progenitor niche by exploiting the inherent bidirectional messiness of the system as a therapeutic lever.