Hypothesis

Age-related shifts in the gut microbiota reduce production of the secondary bile acid deoxycholic acid (DCA) and increase circulating lipopolysaccharide (LPS), which together activate microglial TGR5 and TLR4 receptors to trigger IKKβ/NF-κB signaling in the hypothalamus, suppressing GnRH and driving systemic inflammaging.

Mechanistic Rationale

The hypothalamus acts as an aging pacemaker through microglial TNF-α induced IKKβ/NF-κB activation that suppresses GnRH[1]. While peripheral inflammation can feed this process, the specific microbial ligands that bridge gut dysbiosis to hypothalamic microglia remain undefined. DCA, a microbiota-derived bile acid, is known to bind the G-protein coupled receptor TGR5 on immune cells, modulating NF-κB activity[2]. Conversely, LPS engages TLR4 to potentiate IKKβ phosphorylation[3]. In aging, diminished SCFA‑producing taxa lower butyrate levels, which normally inhibit HDAC activity and restrain NF-κB-driven transcription; loss of this brake permits heightened NF-κB signaling. Combined, low DCA/TGR5 anti‑tonic signaling and high LPS/TLR4 pro‑inflammatory signaling shift microglia toward a primed state, amplifying TNF-α release and subsequent neuronal IKKβ/NF-κB activation. This dual ligand model explains why merely measuring systemic cytokines fails to capture the gut‑originating trigger.

Testable Predictions

1. Aged mice supplemented with DCA will show decreased hypothalamic microglial IκB phosphorylation, restored GnRH expression, and extended median lifespan compared with vehicle controls.
2. Colonizing young germ‑free mice with microbiota from aged donors will elevate plasma LPS, reduce colonic DCA, increase hypothalamic microglial TNF-α, and prematurely suppress GnRH.
3. Pharmacological blockade of TGR5 in aged mice will abolish the protective effects of DCA supplementation, confirming receptor specificity.
4. Microglial‑specific TLR4 knockout will prevent LPS‑induced IKKβ/NF-κB activation despite high circulating endotoxin, rescuing GnRH levels.

Experimental Approach

• Cohort design: Young (3 mo), aged (20 mo), and aged + DCA (0.5 % w/w in diet) mice; germ‑free young mice colonized with aged or young microbiota.
• Readouts: Hypothalamic microglial IκBα phosphorylation (Western blot), TNF-α ELISA, GnRH mRNA (qPCR), serum LPS (LAL assay), fecal DCA (LC‑MS), lifespan monitoring, neurogenesis (BrdU labeling in dentate gyrus).
• Controls: Vehicle, TGR5 antagonist, microglial‑specific TGR5 or TLR4 conditional knockouts.
• Statistical plan: Power analysis for n = 10 per group; ANOVA with post‑hoc Tukey; survival curves compared by log‑rank test.

If DCA replenishment normalizes microglial NF-κB and extends life, while aged microbiota transfers inflammation to young hosts, the hypothesis gains support. Failure to observe these effects would falsify the proposed gut‑derived ligand axis.