Hypothesis

The epigenetic age of the gut microbiome, quantified by a microbiome‑specific clock, predicts circulating phenylacetic acid (PAA) levels and induces endothelial senescence in cerebral vessels even when host age is held constant. Reversing microbial epigenetic age through targeted phage depletion of PAA‑producing strains will lower PAA, protect brain endothelium, and improve cognitive performance in aged animals.

Mechanistic Basis

Age‑associated shifts enrich phenylalanine‑catabolizing bacteria that raise PAA, which directly triggers endothelial senescence (1). Microbiota also exhibit an epigenetic drift analogous to host aging, measurable by microbiome‑specific CpG methylation patterns (2). We propose that this microbial epigenetic age, rather than host age, determines the functional output of phenylalanine pathways and thus PAA production.

Predictions

1. Older mice with a youthful microbiome epigenetic profile (low microbiome‑age score) will exhibit lower plasma PAA, reduced cerebral endothelial senescence markers (p16^INK4a^, SA‑β‑gal), and better performance in hippocampal‑dependent tasks than age‑matched controls with an aged microbiome.
2. Transplanting fecal microbiota from epigenetically aged donors into young germ‑free recipients will accelerate microbiome epigenetic aging, elevate PAa, induce endothelial senescence, and impair cognition despite the recipients’ young host age.
3. Phage‑mediated depletion of the PAA‑producing clade (e.g., Clostridium spp. enriched in PAA pathways) will decrease the microbiome‑age score, lower PAa, rescue endothelial integrity, and restore cognitive function in naturally aged mice.

Experimental Design

• Microbiome‑age profiling: Perform whole‑genome shotgun sequencing of fecal DNA from young (3 mo) and aged (24 mo) mice; compute microbiome‑age using a published microbial epigenetic clock (2).
• Cohort groups: (i) aged mice receiving autologous FMT (control); (ii) aged mice receiving FMT from young donors with low microbiome‑age; (iii) young germ‑free mice colonized with microbiota from aged donors; (iv) aged mice treated with PAA‑specific phage cocktail; (v) aged mice receiving sham phage.
• Readouts: Plasma PAa via LC‑MS; cerebral endothelial senescence (immunohistochemistry for p16^INK4a^, SA‑β‑gal); BBB integrity (Evans blue extravasation); cytokine panel (IL‑6, TNF‑α); behavioral assays (Morris water maze, novel object recognition).
• Statistical plan: Two‑way ANOVA (host age × microbiome age) with post‑hoc Tukey; falsification if no significant interaction or if phage treatment fails to reduce PAa or improve endpoints.

Potential Outcomes

If the hypothesis holds, microbial epigenetic age will emerge as an independent driver of brain vascular aging, offering a biomarker and therapeutic target distinct from host chronological age. Failure to observe microbiome‑age‑dependent effects would challenge the notion that microbial epigenetic drift dictates inflammaging and redirect focus toward host‑centric mechanisms.