Background

Aging reshapes the gut microbiome in a sex‑biased manner, altering the repertoire of bile salt hydrolase (BSH) enzymes that convert primary to secondary bile acids. In old male mice, BSH‑T1/T7‑enriched taxa such as Blautia and Roseburia increase while BSH‑T4‑rich Bifidobacterium declines; females show the inverse pattern. These shifts elevate the primary‑to‑secondary bile acid ratio, perturbing hepatic FXR signaling and allowing microbial‑derived secondary bile acids to cross the blood‑brain barrier and engage neuronal FXR/TGR5 receptors, thereby modulating neuroinflammation and microglial activation.

Hypothesis

We propose that the sex‑specific BSH isoform profile—not merely the overall bile acid pool—directly programs distinct aging trajectories in the liver and brain by differentially activating FXR versus TGR5 signaling in each organ. In males, enrichment of BSH‑T1/T7 favors production of deoxycholic acid (DCA), a potent TGR5 agonist that drives hepatic stellate cell activation and microglial priming via a TGR5‑cAMP‑PKA cascade, accelerating steatohepatitis and neuroinflammatory phenotypes. In females, dominance of BSH‑T4 yields higher levels of lithocholic acid (LCA) and ursodeoxycholic acid (UDCA), which preferentially activate FXR, promoting hepatoprotective anti‑fibrotic pathways but concurrently suppressing neuronal FXR‑dependent BDNF expression, leading to cognitive decline without pronounced liver fibrosis.

Predictions

1. Metabolomic profiling of portal and systemic blood from sex‑stratified aged mice will reveal male‑biased elevations of DCA and female‑biased elevations of LCA/UDCA, correlating with organ‑specific signaling readouts (p‑PKA in liver microglia, p‑ERK in hepatic stellate cells; FXR‑target gene expression in hepatocytes vs. neuronal BDNF).
2. Genetic ablation of microbial BSH‑T1/T7 (using CRISPR‑edited Blautia strains) in aged males will reduce DCA, attenuate TGR5‑cAMP signaling in liver Kupffer cells and brain microglia, and improve both hepatic steatosis and spatial memory performance.
3. Conversely, supplementation with a BSH‑T4‑enriched Bifidobacterium strain in aged females will increase LCA/UDCA, boost hepatic FXR‑dependent SHP expression, lower fibrosis markers, but exacerbate hippocampal synaptic loss due to reduced neuronal FXR‑BDNF signaling.
4. Co‑housing young male mice with aged female donors will transfer the female BSH‑T4 profile, conferring hepatic protection without worsening cognition, whereas the reverse co‑housing will transmit the male BSH‑T1/T7 profile, worsening liver pathology while sparing cognitive decline.

Experimental Design

• Use germ‑free mice colonized with defined synthetic communities: (i) male‑associated BSH‑T1/T7 consortium, (ii) female‑associated BSH‑T4 consortium, (iii) control wild‑type microbiome.
• Age cohorts (6, 12, 18 months) of each sex; collect liver, brain, serum, and fecal samples.
• Quantify bile acids via LC‑MS/MS, assess FXR/TGR5 pathway activation (Western blot, reporter assays), measure hepatic fibrosis (hydroxyproline, Sirius Red), steatosis (Oil Red O), and neuroinflammation (Iba1, GFAP, cytokine panels).
• Behavioral testing: Morris water maze for memory, open‑field for anxiety.
• Perform rescue experiments with organ‑specific antagonists (TGR5 antagonist for males, FXR agonist for females) to confirm causality.

Significance

If validated, this hypothesis reframes the gut‑brain‑liver axis in aging as a set of isoform‑specific enzymatic switches that can be selectively tuned to mitigate sex‑biased organ decline. It moves beyond broad microbiome remodeling toward precision editing of microbial BSH activities, offering a mechanistic route to disentangle liver‑centric from brain‑centric aging interventions.