Hypothesis

Age-related loss of bile salt hydrolase (BSH) activity depletes secondary bile acids such as deoxycholic acid (DCA), reducing FXR activation in liver and brain. This creates a self‑reinforcing loop of metabolic inflexibility and neuronal rigidity that manifests as cognitive aging. Restoring secondary bile acid signaling should re‑introduce controlled uncertainty, reset FXR‑dependent transcriptional dynamics, and improve cognitive flexibility.

Mechanistic Rationale

• Hepatic FXR downregulation with age and Western diet reduces mitochondrial oxidative phosphorylation and locks metabolic inflexibility [3].
• FXR deficiency accelerates age‑related liver pathology including NASH‑like damage and cholestasis [4].
• Secondary bile acids like DCA are potent FXR agonists; their age‑related depletion through reduced BSH activity likely drives transcriptional rigidity (via altered FXR‑dependent gene regulation) and epigenetic consolidation (5hmC marks that restrict dynamic responses) [1].
• Eight BSH phylotypes with distinct substrate specificity decline with age, and microbiota remodeling partially restores bile acid ratios and reduces inflammation [2].
• FXR influences neuron differentiation and cytoskeleton organization [3], linking hepatic metabolic state to neuronal plasticity.
• Aging gut‑brain axis shows reduced cognitive plasticity and blunted vagal signaling driven by dysbiotic metabolites like medium‑chain fatty acids from Parabacteroides goldsteinii [5].

We propose that diminished hepatic FXR signaling lowers vagal afferent tone, reducing brainstem‑to‑forebrain neuromodulatory input. Consequently, neuronal FXR activity falls, leading to decreased expression of genes required for synaptic remodeling and increased reliance on existing predictive models—i.e., over‑consolidation of neural circuits.

Predictions

1. Aged mice with low hepatic DCA will show reduced FXR target gene expression (e.g., Shp, Srebp‑1c) and elevated 5hmC at promoters of metabolic flexibility genes.
2. These mice will exhibit blunted vagal nerve firing in response to a mixed‑meal challenge and lower hippocampal BDNF and dendritic spine density.
3. Behavioral assays (e.g., reversal learning, novel object recognition) will reveal impaired cognitive flexibility correlating with hepatic FXR activity.
4. Oral DCA supplementation or administration of a BSH‑enriched probiotic will restore hepatic DCA, increase FXR signaling, normalize vagal tone, rescue hippocampal plasticity, and improve reversal learning without altering total bile acid pool size.
5. If hepatic FXR is genetically knocked out in adult mice, DCA supplementation will fail to improve cognition, confirming liver‑brain mediation.

Experimental Design

• Subjects: 24‑month‑old C57BL/6J mice (n=10 per group) and age‑matched young controls (3‑month).
• Groups: (1) Vehicle, (2) DCA (0.5% w/w in diet), (3) BSH‑probiotic blend (Lactobacillus spp. engineered for high BSH activity), (4) DCA + FXR antagonist (glycine‑β‑muricholic acid) to test specificity, (5) Liver‑specific FXR knockout + DCA.
• Measurements (after 4 weeks):
  • Serum and hepatic bile acid profiling (LC‑MS).
  • Hepatic FXR target mRNA (qPCR) and 5hmC levels (dot‑blot).
  • Vagal afferent firing (ex vivo vagus nerve electrophysiology post‑meal).
  • Hippocampal BDNF, PSD‑95, and spine density (Western blot, confocal microscopy).
  • Cognitive flexibility: Barnes maze reversal, attentional set‑shifting task.
• Analysis: Two‑way ANOVA with post‑hoc Tukey; correlation analysis between hepatic FXR activity and behavioral scores.

Potential Outcomes and Falsifiability

• Support: DCA or BSH treatment raises hepatic DCA, increases FXR target expression, reduces 5hmC at flexibility genes, enhances vagal firing, rescues hippocampal plasticity, and improves reversal learning. Liver‑specific FXR knockout abolishes these benefits.
• Refute: DCA/BSH treatment alters bile acids but does not change FXR signaling, vagal tone, hippocampal markers, or cognition; or improvements occur despite FXR antagonism, indicating an FXR‑independent mechanism.

This hypothesis is directly testable, integrates liver‑brain axis biology, and offers a clear intervention strategy: re‑introduce controlled microbial‑derived chemical uncertainty to break metabolic and neural over‑consolidation in aging.