Aging is associated with a numerical expansion of Paneth cells yet a functional decline marked by elevated Notum secretion that flattens the Wnt3 gradient at the crypt base [1][2]. While mTORC1 hyperactivity in Paneth cells has been implicated in Notum upregulation, the upstream trigger linking age‑altered luminal composition to this pathway remains undefined. We hypothesize that age‑related shifts in the gut microbiota increase circulating secondary bile acids, particularly deoxycholic acid (DCA), which activate the G‑protein‑coupled receptor TGR5 on Paneth cells. TGR5 signaling stimulates mTORC1 activity, leading to transcriptional up‑regulation of Notum and subsequent Wnt ligand disengagement from Lrp5/6 receptors, thereby suppressing β‑catenin stabilization in ISCs. This mechanistic chain explains both the numerical rise (Paneth cells proliferate in response to bile acid–mediated signaling) and the functional collapse (Notum‑driven Wnt antagonism) observed with age.

Testable predictions

1. Aged mice will exhibit higher luminal and serum DCA levels compared with young cohorts, correlating with increased TGR5 expression in Paneth cells.
2. Pharmacological inhibition of TGR5 (e.g., with SBI‑115) or genetic deletion of Tgr5 in Paneth cells will reduce mTORC1 activity (measured by p‑S6K), lower Notum secretion, and restore the Wnt3 gradient without altering Paneth cell numbers.
3. Conversely, exogenous DCA administration to young mice will recapitulate the aged phenotype: increased Paneth cell Notum, flattened Wnt gradient, and impaired organoid formation, reversible by TGR5 antagonism or mTORC1 inhibition (rapamycin).
4. Conditioned medium from Tgr5‑deficient aged Paneth cells will rescue Wnt/β‑catenin signaling in wild‑type ISC organoids, whereas medium from wild‑type aged Paneth cells will suppress it; adding a Notum-neutralizing antibody to the latter will restore signaling.

Experimental approach

• Measure bile acid profiles in feces and serum of young (3 mo) and aged (24 mo) mice using LC‑MS/MS.
• Perform immunofluorescence and flow cytometry for TGR5, p‑S6K, and Notum in isolated Paneth cells.
• Generate Paneth‑cell‑specific Tgr5 knockout mice (Vil1‑CreERT2;Tgr5^fl/^fx) and assess Paneth cell number, Notum expression (ELISA), Wnt3 gradient (immunostaining of β‑catenin), and ISC organoid efficiency.
• Treat aged wild‑type mice with SBI‑115 or vehicle for 4 weeks; evaluate the same endpoints.
• Conduct rescue experiments: culture aged ISCs with Wnt3a plus conditioned medium from Tgr5‑deficient Paneth cells ± Notum antibody.

Falsifiability If aged mice do not show elevated DCA or TGR5 activation, or if TGR5 loss fails to reduce Notum and restore Wnt signaling despite mTORC1 inhibition, the hypothesis would be refuted. Additionally, if DCA supplementation in young mice does not increase Notum or impair ISC function, the proposed bile‑acid‑TGR5 axis would be insufficient to drive niche erosion.

This framework integrates microbiota‑derived metabolites, Paneth cell stress signaling, and Wnt gradient dynamics, offering a mechanistic niche‑centric explanation for age‑related intestinal decline that can be directly tested with existing pharmacological and genetic tools.