Hypothesis

If intestinal senescent cells are the primary source of circulating pro‑aging metabolites such as phenylacetic acid (PAA), then selective removal of these cells will lower systemic PAA levels, attenuate vascular endothelial senescence, and improve cognitive performance in aged mice, independent of direct brain‑targeted senolytics.

Mechanistic Rationale

Recent work shows that aged microbiota produce excess PAA from dietary phenylalanine, which enters circulation and induces endothelial senescence via mitochondrial H2O2 and SASP upregulation [2]. Concurrently, loss of intestinal amino‑acid transporters impairs nutrient sensing and longevity [3]. We propose that intestinal senescent enterocytes and enteroendocrine cells amplify this loop by:

1. Secreting higher amounts of PAA due to upregulated phenylalanine‑hydroxylase activity in a senescent‑associated secretory phenotype (SASP).
2. Releasing extracellular vesicles enriched in miR‑34a that suppress SLC7A5 expression in neighboring epithelial cells, further reducing phenylalanine uptake and creating a vicious cycle of PAA overproduction.
3. Activating vagal afferents through ATP release, which alters nucleus tractus solitarius signaling and sets a pro‑inflammatory baseline in the brain.

By clearing these senescent gut cells, we expect a rapid drop in portal PAA, restoration of normal amino‑acid transport, and reduced vesicular miR‑34a transfer, thereby breaking the gut‑to‑vasculature‑to‑brain aging axis.

Experimental Plan

1. Model: Use progeroid Ercc1-/- mice and naturally aged C57BL/6 mice (24 months).
2. Intervention: Employ a galactose‑conjugated senolytic (e.g., navitoclax‑galactose) that preferentially targets senescent enterocytes after oral gavage, validated by flow cytometry for p16^Ink4a^+ CD45^- EpCAM^+ cells.
3. Controls: Vehicle gavage, and a brain‑penetrant senolytic (dasatinib + quercetin) administered intraperitoneally to compare systemic vs. central clearance.
4. Readouts (at 1 week and 4 weeks post‑treatment):
   • Plasma PAA quantified by LC‑MS/MS.
   • Vascular endothelial senescence: SA-beta-gal staining in aorta, p21^Cip1^ expression, mitochondrial H2O2 (MitoSOX).
   • Cognitive performance: Novel object recognition and Y‑maze spontaneous alternation.
   • Gut histology: p16^Ink4a^+ cell counts, SLC7A5 mRNA (qPCR), vesicular miR‑34a levels in isolated exosomes.
   • Vagal tone: heart‑rate variability and c‑Fos in nucleus tractus solitarius.
5. Predicted Outcome: Gut‑targeted senolysis will reduce plasma PAA by ≥40%, decrease aortic endothelial SA-beta-gal positivity by ≥30%, and improve cognitive scores to levels comparable to young controls, whereas brain‑penetrant senolytics will improve cognition but not significantly affect PAA or vascular senescence.

Falsifiability

If intestinal senescent cell clearance fails to lower circulating PAA or does not rescue vascular senescence and cognition despite efficient gut p16^Ink4a^+ cell reduction, the hypothesis that the gut is the primary upstream driver of systemic aging via PAA export would be refuted. Conversely, a significant improvement in vascular and cognitive endpoints without changes in brain senescence would support a gut‑centric model of aging.

Broader Impact

Confirming this hypothesis would shift the longevity intervention paradigm from reactive systemic senolytics to preventive gut‑focused strategies, potentially combining dietary phenylalanine restriction, pre‑biotic modulation of microbiota, and gut‑restricted senolytics to break the aging cascade at its source.