Hypothesis

Aged female rodents exhibit increased ileal ASBT expression and activity, leading to heightened reabsorption of conjugated primary bile acids (CPBAs). This intestinal hyperabsorption elevates serum CPBA levels, which cross the blood‑brain barrier and activate microglial TGR5 receptors. Chronic TGR5 signaling triggers NF‑κB‑mediated neuroinflammation, resulting in hippocampal synaptic loss and cognitive decline. The effect is female‑specific because estrogen‑ERα signaling upregulates ASBT transcription in aged enterocytes, while concomitant age‑related shifts in the gut microbiota increase bile‑salt hydrolase (BSH) activity, favoring the production of neurotoxic conjugated species.

Mechanistic Rationale

1. Estrogen‑Dependent ASBT Upregulation – ERα binds to estrogen‑response elements in the Asbt promoter, a mechanism demonstrated in hepatocytes and extrapolated to intestinal epithelium. In aging females, circulating estradiol fluctuates but retains sufficient nuclear ERα activity to sustain Asbt transcription, explaining the sex‑dimorphic increase observed in [1].
2. Microbiota‑Mediated Bile Acid remodeling – Aging is associated with a rise in bile‑salt hydrolase‑producing bacteria (e.g., Lactobacillus, Bifidobacterium). Enhanced BSH deconjugates bile acids, yet the rapid reuptake via overexpressed ASBT preferentially recaptures conjugated species before deconjugation can occur, shunting more CPBAs into the portal circulation ([3]).
3. Brain Delivery and TGR5 Activation – Conjugated bile acids are more hydrophilic yet retain sufficient lipophilicity to cross the BBB via passive diffusion or organic anion transporters. Within the parenchyma, they bind TGR5 on microglia, stimulating cAMP production and paradoxically activating NF‑κB through cross‑talk with TLR4 signaling, a pathway linked to synaptic pruning ([4]).
4. Outcome: Synaptic Loss and Cognitive Impairment – Persistent microglial activation leads to complement‑mediated phagocytosis of synapses, measurable as reduced PSD‑95 and synaptophysin levels, correlating with the hippocampal deficits reported in [1].

Testable Predictions

• Prediction 1: Pharmacological inhibition of ileal ASBT (e.g., elobixibat) in 18‑month‑old female C57BL/6 mice will reduce serum and hippocampal concentrations of taurocholate and glycocholate by ≥30 % compared with vehicle, while having no significant effect in age‑matched males.
• Prediction 2: ASBT inhibition will attenuate microglial TGR5‑dependent NF‑κB activation (measured by p‑p65 immunoreaction) and restore hippocampal synaptic protein levels to those seen in 6‑month‑old mice.
• Prediction 3: Germ‑free aged females colonized with a BSH‑deficient microbiota (Δbsh Lactobacillus) will show blunted CPBA elevation despite high ASBT expression, indicating microbiota dependence.
• Prediction 4: Administering a TGR5 antagonist centrally (intracerebroventricular) will replicate the cognitive benefits of ASBT inhibition, confirming the receptor’s mediating role.

Experimental Approach

1. Groups: Aged female and male mice (24 mo) receiving elobixibat (10 mg/kg/day) or control for 8 weeks; subsets germ‑free or colonized with defined microbiota.
2. Readouts: Serum and hippocampal bile‑acid profiling (LC‑MS/MS), qPCR/Western for Asbt, Ostβ, Mrp3, ERα; microglial isolation for TGR5, p‑p65, Iba1; behavioral assays (Morris water maze, novel object recognition); synaptic markers (PSD‑95, synaptophysin).
3. Statistical Power: n = 12 per group to detect 25 % differences with α = 0.05, power = 0.8.

Falsifiability

If ASBT blockade fails to lower hippocampal CPBA concentrations, does not modify microglial TGR5/NF‑κB signaling, or does not improve cognitive performance in aged females, the core mechanistic link between ileal hyperabsorption and brain aging would be refuted. Likewise, demonstrating that estrogen receptor antagonism does not reduce Asbt upregulation in aged female enterocytes would invalidate the sex‑specific transcriptional hypothesis.

Implications

Validating this hypothesis would reposition ileal ASBT—not hepatic synthesis—as a druggable gateway to mitigate age‑related cognitive decline, particularly in women. It would also motivate repurposing existing ASBT inhibitors for preventive neurology, guided by sex‑stratified clinical trials.