Hypothesis

It's plausible that age‑related shifts in hepatic bile acid synthesis remodel the gut microbiota toward bile‑acid‑resistant pathobionts, increasing circulating secondary bile acids that activate neuronal TGR5 and microglial inflammasome pathways, thereby accelerating neuroinflammation and memory loss.

Mechanistic Rationale

It's known that the liver’s declining expression of CYP7A1 and CYP8B1 with age reduces primary bile acid pool diversity, causing an overrepresentation of deoxycholic acid (DCA) and lithocholic acid (LCA) produced by gut microbes. These secondary bile acids act as ligands for TGR5 on enteroendocrine cells, triggering serotonin release that alters vagal afferent firing. Simultaneously, DCA/LCA can cross a leaky barrier and bind microglial TGR5, biasing the NLRP3 inflammasome toward IL‑1β release. The resulting cytokine surge feeds back to the hypothalamus, suppressing vagal output and further impairing gut barrier integrity—a self‑reinforcing loop that mirrors the bidirectional gut‑brain dysregulation described in prior work.[https://www.medicalnewstoday.com/articles/gut-microbiome-driving-aging-memory-loss-vagus-nerve][https://pmc.ncbi.nlm.nih.gov/articles/PMC12832290/][https://pmc.ncbi.nlm.nih.gov/articles/PMC12995645/]

Testable Predictions

We expect that aged mice exhibiting elevated serum DCA/LCA will show heightened microglial NLRP3 activation and poorer performance in hippocampal‑dependent tasks compared with age‑matched controls. We predict that pharmacological inhibition of microbial bile‑acid 7α‑dehydroxylation (using a specific inhibitor) will normalize secondary bile acid levels, reduce vagal afferent dysrhythmia, and rescue memory deficits. We anticipate that supplementation with tauroursodeoxycholic acid (TUDCA), a hydrophilic bile acid that antagonizes TGR5‑mediated inflammasome signaling, will decrease serum LPS, improve vagal tone, and restore cognitive function in aged mice, an effect absent in vagotomized animals. We hypothesize that germ‑free mice colonized with microbiota from aged donors fed a cholic‑acid‑deficient diet will develop exaggerated secondary bile acid accumulation and accelerated cognitive decline, whereas colonization with the same microbiota plus a bile‑acid‑sequestrant will blunt the phenotype.

Experimental Approach

We'll use 24‑month‑old C57BL/6 mice and age‑matched 3‑month‑old young mice. Measure hepatic CYP7A1/CYP8B1 mRNA, serum primary/secondary bile acids (LC‑MS), fecal microbiota composition (16S rRNA), intestinal permeability (FITC‑dextran), vagal afferent firing (ex vivo vagus nerve electrophysiology), microglial NLRP3 activation (Western blot for cleaved caspase‑1), and behavior (Morris water maze, novel object recognition). Intervention groups receive either a bile‑acid‑7α‑dehydroxylase inhibitor, TUDCA (500 mg/kg/day oral), or vehicle for 8 weeks. A subset undergoes subdiaphragmatic vagotomy to test vagal dependence. Germ‑free recipients are colonized with fecal transplants from aged donors and fed either standard chow or a cholic‑acid‑free diet supplemented with a sequestrant (cholestyramine). Cognitive assays are performed after 4 weeks. Statistical analysis employs two‑way ANOVA with post‑hoc Tukey; significance set at p<0.05.