Hypothesis

Clearing senescent cells in the aged intestine while supplying exogenous short‑chain fatty acids (SCFAs) will re‑establish gut‑to‑brain vagal afferent signaling, reduce neuroinflammation, and rescue age‑related cognitive decline.

Rationale

• Aging intestines accumulate senescent epithelial and immune cells that secrete a proinflammatory SASP, driving barrier leakiness and microbial dysbiosis (3).
• This milieu diminishes luminal SCFA production and allows endotoxin translocation, which activates microglial ROS and impairs hippocampal plasticity (4).
• SCFAs such as butyrate cross the blood‑brain barrier and inhibit histone deacetylases, promoting neuroprotective gene expression (5).
• Vagal afferents sense gut‑derived metabolites (SCFAs, bile acids) and relay signals to the nucleus tractus solitarius, influencing mood and cognition (1, 2).

We propose that senescent gut cells aberrantly modulate bile acid pools and SCFA synthesis, blunting vagal tone. Removing these cells normalizes metabolite production; supplemental SCFAs then ensure sufficient ligand availability for vagal receptors (e.g., FFAR2, TGR5).

Experimental Design

Model: 24‑month‑old C57BL/6 mice (n=10 per group).

Groups:

1. Vehicle control
2. Gut‑targeted senolytic (dasatinib + quercetin, oral formulation that preferentially accumulates in intestinal epithelium)
3. SCFA supplement (sodium butyrate in drinking water, 150 mM)
4. Combined senolytic + butyrate

Intervention duration: 8 weeks.

Readouts

• Vagal afferent activity: c‑Fos immunostaining in the nucleus tractus solitarius after intestinal SCFA gavage; ex vivo vagal nerve electrophysiology.
• Gut barrier integrity: FITC‑dextran serum levels, Claudin‑1 immunostaining.
• Metabolomics: luminal SCFA concentrations (GC‑MS), serum bile acid profile.
• Neuroinflammation: Iba1 and CD68 staining in hippocampus, microglial ROS (DHE assay).
• Epigenetic mark: Hippocampal H3K27ac levels (ChIP‑qPCR) as readout of HDAC inhibition.
• Cognition: Morris water maze (spatial memory) and novel object recognition.

Predictions

• Senolytic alone will modestly improve barrier function and SCFA levels but not fully restore vagal signaling due to insufficient ligand.
• Butyrate alone will increase HDAC inhibition but vagal tone remains low because afferent receptors are desensitized by chronic SASP.
• The combined group will show: (a) normalized luminal SCFA and secondary bile acids, (b) ↑ vagal c‑Fos and firing rate, (c) ↓ serum FITC‑dextran and hippocampal microglial activation, (d) ↑ hippocampal H3K27ac and BDNF expression, (e) significant improvement in maze latency and discrimination index vs. controls.

Falsifiability If the combined treatment fails to increase vagal afferent activity or improve cognitive performance relative to both monotherapies and control, the hypothesis is refuted. Conversely, a selective improvement only in the combined group supports the upstream gut‑to‑brain model.

Potential Impact

Demonstrating that gut senolysis coupled with metabolic supplementation can reset vagal afferent tone would shift longevity interventions from neurocentric to gut‑centric strategies, offering a combinatorial approach that targets the root cause of inflammaging‑driven cognitive decline.