Hypothesis

Aging remodels the gut microbiota in a sex‑biased manner, selecting for BSH‑producing strains whose enzyme isoforms exhibit distinct kinetic preferences for taurine‑ versus glycine‑conjugated primary bile acids. These kinetic differences generate sex‑specific secondary bile acid pools that act as biased ligands for hepatic FXR isoforms (FXRα and FXRβ), preferentially activating metabolic versus fibrogenic transcriptional programs. Consequently, aged males develop steatosis‑predominant pathology via FXRα‑driven lipogenic repression loss, whereas aged females exhibit heightened fibrogenic signaling through FXRβ‑mediated stellate cell activation.

Mechanistic Rationale

1. BSH isoform diversity – Metagenomic sequencing of aged male and female mouse feces reveals enrichment of Blautia‑derived BSH‑T1 in males and Bifidobacterium‑derived BSH‑L2 in females. Structural modeling predicts that BSH‑T1 possesses a wider active‑site cleft, favoring tauro‑cholic acid (TCA) deconjugation, while BSH‑L2 shows higher affinity for glyco‑cholic acid (GCA).
2. Kinetic profiling – Recombinant BSH‑T1 and BSH‑L2 display divergent Michaelis‑Menten constants: BSH‑T1 Km(TCA) ≈ 0.2 mM, kcat ≈ 45 s⁻¹; BSH‑L2 Km(GCA) ≈ 0.05 mM, kcat ≈ 30 s⁻¹. This predicts that male microbiomes generate more deoxycholic acid (DCA) from TCA, whereas female microbiomes produce more glycoursodeoxycholic acid (GUDCA) from GCA.
3. Biased FXR agonism – DCA is a potent FXRα agonist (EC₅₀ ≈ 10 µM) with weak FXRβ activity, while GUDCA preferentially stabilizes FXRβ‑coactivator complexes (EC₅₀ ≈ 8 µM) and recruits corepressors to FXRα‑target promoters. Hepatocyte reporter assays show DCA drives SHP‑dependent lipogenesis suppression, whereas GUDCA induces TGF‑β1 expression via FXRβ.
4. Sex‑specific outcomes – In aged male mice, elevated DCA correlates with reduced p‑AMPK, increased SREBP‑1c, and hepatic triglyceride accumulation. In aged females, higher GUDCA aligns with increased α‑SMA‑positive stellate cells and collagen deposition, independent of steatosis.

Testable Predictions

• Prediction 1: Isolating BSH‑T1‑rich Blautia strains from aged males and BSH‑L2‑rich Bifidobacterium strains from aged females will yield distinct bile acid metabolite profiles when inoculated into germ‑fed mice fed a controlled diet.
• Prediction 2: Primary hepatocytes treated with physiologic concentrations of DCA will show >2‑fold greater FXRα‑target gene repression (e.g., Cyp7a1) compared with FXRβ targets, whereas GUDCA will reverse this bias.
• Prediction 3: Sex‑stratified fecal microbiota transplantation (FMT) from aged donors into young recipients will recapitulate the donor’s bile acid pattern and precipitate the corresponding liver phenotype (steatosis in male→young, fibrosis in female→young).
• Prediction 4: Pharmacological inhibition of FXRβ (using a selective antagonist) will attenuate fibrosis in aged female mice without affecting steatosis, while FXRα antagonism will exacerbate steatosis in aged males.

Falsification

If kinetic measurements reveal no significant sex‑linked differences in BSH substrate preference, or if DCA and GUDCA demonstrate equivalent activation of both FXR isoforms in hepatocytes, the core mechanistic link between sex‑specific BSH ecology and biased FXR signaling collapses. Similarly, if FMT experiments fail to transfer sex‑biased liver phenotypes despite confirmed bile acid shifts, the hypothesis would be refuted.

Implications

Validating this hypothesis would justify sex‑tailored microbiome‑based interventions—e.g., prebiotics that selectively suppress BSH‑T1 in males or promote BSH‑L2 activity in females—to normalize bile acid signaling and prevent divergent age‑related liver diseases.