Hypothesis

The mTOR activity gradient along the intestinal crypt is not only a rheostat for stem cell fate but also a regulator of Wnt ligand secretion from Paneth cells via a mechanosensitive Hippo‑YAP axis. We propose that sustained mTORC1 hyperactivation in stem cells elevates cortical actin tension, which inhibits YAP nuclear translocation in neighboring Paneth cells, reducing their Wnt output. This creates a feedback loop that collapses the spatial mTOR‑Wnt gradient, driving niche erosion during aging. Restoring periodic mTOR oscillations, rather than static suppression, should preserve YAP‑mediated Wnt secretion and delay crypt atrophy.

Mechanistic Model

1. mTORC1‑driven cytoskeletal remodeling – High mTORC1 stimulates S6K‑dependent phosphorylation of PTEN and RhoA, increasing F‑actin polymerization and cortical tension in ISCs.
2. Mechanical transmission to Paneth cells – Elevated tension propagates through the basement membrane and adherens junctions, activating integrin‑FAK signaling in adjacent Paneth cells.
3. Hippo pathway activation – FAK signaling promotes LATS1/2 phosphorylation, leading to YAP cytoplasmic sequestration and transcriptional repression of Wnt ligands (Wnt3a, Wnt9b).
4. Gradient collapse – Reduced Wnt lowers β‑catenin activity in ISCs, further diminishing mTORC1 heterogeneity because Wnt signaling normally amplifies mTORC1 in the crypt base via Rheb activation.
5. Aging exacerbation – Age‑related mitochondrial ROS amplify RhoA activity, locking the system in a high‑tension, low‑Wnt state.

Testable Predictions

• Pharmacological inhibition of ROCK (downstream of RhoA) will restore YAP nuclear localization in Paneth cells of aged mice without altering global mTORC1 levels, and will increase crypt Wnt target expression.
• Optogenetic pulsatile activation of mTORC1 (using a light‑ inducible TSC2 mutant) in the crypt base will maintain YAP activity in Paneth cells and preserve the mTOR gradient, whereas constant mTORC1 inhibition will flatten the gradient and impair regeneration.
• Deleting YAP specifically in Paneth cells will mimic the effect of chronic mTORC1 hyperactivation: reduced Wnt secretion, crypt shortening, and loss of Olfm4+ ISCs, even when mTORC1 activity is normal.
• In human colonic organoids, serum from old donors will increase actin tension and decrease YAP nuclear signal in Paneth‑like cells; adding a ROCK inhibitor will rescue Wnt secretion.

Experimental Design

• Mouse models: Use Villin‑CreERT2; Tsc2fl/fl for inducible mTORC1 hyperactivation, combined with a Paneth‑cell‑specific YAP knockout (Spp1‑Cre; Yap1fl/fl). Measure crypt depth, Ki67, Olfm4, and Wnt3a expression by immunofluorescence and qPCR at 3, 12, and 24 months.
• Pharmacology: Treat aged mice with the ROCK inhibitor fasudil (30 mg/kg i.p. daily) for 4 weeks; assess Paneth cell YAP localization (immunostaining) and Wnt target gene expression (Axinin2, Lgr5).
• Optogenetics: Generate a Rosa26‑LSL‑CRY2‑TSC2 line; expose crypts to 470 nm light pulses (5 min on/off) ex vivo; compare mTOR activity (p‑S6), Wnt signaling (β‑catenin nuclear), and organoid budding efficiency.
• Human validation: Isolate colonic crypts from colonoscopy biopsies of young (<35) and old (>65) donors; treat with fasudil or DMSO; quantify YAP nuclear/cytoplasmic ratio and Wnt ligand secretion via ELISA.

Possible Confounds

• ROCK inhibition may affect smooth muscle contraction and alter crypt mechanics independently of YAP; control with a YAP‑null background to isolate the epithelial effect.
• Light‑induced mTORC1 pulsatility could trigger stress responses; include a dark‑exposed CRISPR‑dead TSC2 control.
• Age‑related changes in microbiota could influence Wnt secretion; co‑house mice or use germ‑free recipients to rule out microbial confounding.

If these experiments show that modulating cytoskeletal tension or YAP activity rescues Wnt secretion and preserves the mTOR gradient without globally suppressing mTORC1, the hypothesis will be supported. Conversely, failure of ROCK or YAP manipulation to restore Wnt gradients despite normalizing mTORC1 dynamics would falsify the proposed mechanical feedback and suggest that mTOR influences niche integrity through alternative pathways.