Hypothesis

Partial mTOR inhibition preserves lamina-associated domains (LADs) and prevents ectopic enhancer-promoter contacts that drive stem cell geroconversion, thereby uncoupling biosynthetic activity from chromatin architectural decay.

Rationale

• mTORC1 activity raises protein synthesis and nucleolar expansion, increasing nuclear crowding and mechanical tension on the lamina.
• Hyperactive mTOR in aged stem cells correlates with loss of intra‑TAD insulation and enhancer‑promoter looping (see 2 and 3).
• The nuclear lamina acts as a mechanosensitive scaffold; its attachment to chromatin organizes topologically associating domains (TADs) and represses lineage‑inappropriate genes.
• We propose that mTOR‑driven biosynthetic flux alters lamina phosphorylation (via ERK/MAPK downstream of mTOR) reducing LAD stability, allowing super‑enhancer loops to rewire toward inflammatory and differentiation genes.

Novel Mechanistic Insight

mTORC1 signaling modulates the activity of Lamin B1 kinase (MKK2) and phosphatases that regulate Lamin A/C phosphorylation state. When mTOR is high, increased CDK1‑like activity hyper‑phosphorylates lamins, weakening lamina‑chromatin tethering. This loosens the nuclear periphery, permitting enhancer‑promoter contacts that were previously insulated, leading to loss of stem‑cell identity and geroconversion. Partial inhibition tunes lamin phosphorylation to a "sweet spot" that maintains enough peripheral heterochromatin to preserve TAD insulation while still permitting necessary anabolic processes.

Testable Predictions

1. Graded rapamycin treatment (0, 5, 20, 100 nM) in young and aged muscle stem cells will produce a biphasic curve of Lamin B1 phosphorylation (p‑LMNB1) and LAD occupancy (DamID‑seq), with maximal LAD retention at ~20 nM.
2. At the dose preserving LADs, enhancer‑promoter loops measured by promoter‑capture Hi‑C will show restored intra‑TAD insulation and reduced ectopic contacts with inflammatory genes (e.g., Il6, Cxcl1).
3. Cells treated with the optimal rapamycin dose will retain quiescent markers (Pax7+, Ki67−) and resist senescence (low SA‑β‑gal, p21) despite continued moderate protein synthesis (OP‑PUAz incorporation).
4. Genetic ablation of Lamin B1 or expression of a phospho‑mimetic Lamin B1 mutant will abolish the protective effect of partial mTOR inhibition, causing loop decay and geroconversion even at low rapamycin.

Experimental Design

• Isolate murine muscle stem cells (young 3 mo, aged 24 mo).
• Treat with rapamycin series; collect at 24 h for:
  • Western blot for p‑LMNB1 (Ser395) and total LMNB1.
  • DamID‑seq for Lamin B1 to quantify LAD strength.
  • Promoter‑capture Hi‑C (or HiChIP for H3K27ac) to assess enhancer‑promoter contacts.
  • Flow cytometry for Pax7, Ki67, SA‑β‑gal.
  • OP‑PUAz labeling for nascent protein synthesis.
• Include controls: Torin1 (mTORC1/2 inhibitor) to test specificity, and Lamin B1 CRISPRi or phospho‑mutant rescue.

Potential Outcomes & Falsification

• Support: Biphasic LAD retention correlating with preserved loop insulation and stemness at intermediate rapamycin doses; loss of protection when lamin phosphorylation is forced high.
• Falsify: No change in LAD occupancy or lamina phosphorylation across rapamycin doses, or protection persists despite lamin B1 loss/mimic, indicating mTOR’s effect on chromatin is lamina‑independent.

By linking mTOR’s metabolic output to nuclear mechanical state via the lamina, this hypothesis extends the civilization‑versus‑survival dial into a concrete, testable biophysical mechanism linking growth signaling to 3D genome architecture and stem cell fate.