Hypothesis

Chronic mTORC1 hyperactivation drives divergent fates in cholinergic versus nitrergic myenteric neurons during aging, leading to selective cholinergic loss and nitrergic hypertrophy/swelling via distinct downstream effectors.

Mechanistic Model

• In cholinergic neurons, high basal oxidative phosphorylation sustains elevated mTORC1 activity, which phosphorylates p70S6K and suppresses autophagy, accumulating damaged mitochondria and triggering senescence‑associated secretory phenotype (SASP) that promotes apoptosis.
• In nitrergic neurons, reliance on glycolysis keeps mTORC1 activity at a moderate level; repeated stress‑induced mTORC1 activation phosphorylates Drp1 (Ser616), enhancing mitochondrial fission and causing axonal swelling, while sustained protein synthesis drives somatic hypertrophy.
• Enteric glia sense neuronal stress via ATP release; mTORC1 inhibition in glia reduces NF‑κB‑mediated IL‑1β release, breaking a vicious cycle of neuroinflammation that exacerbates cholinergic vulnerability.

Novel Insight

The dichotomy arises from subtype‑specific metabolic wiring that dictates how mTORC1 signals are partitioned between anabolic growth (S6K) and organelle quality control (Drp1, autophagy). Thus, the same kinase acts as a civilization‑versus‑survival dial whose output depends on the neuron's metabolic set‑point.

Predictions

1. Aged mice will show increased p‑S6K and p‑Drp1 (Ser616) in myenteric plexi, with cholinergic neurons exhibiting higher p‑S6K/p‑AMPK ratio and nitrergic neurons showing higher p‑Drp1/total Drp1.
2. Rapamycin treatment will normalize p‑S6K in cholinergic neurons, reducing apoptosis (cleaved caspase‑3) and preserving ChAT+ cell numbers.
3. Rapamycin will decrease p‑Drp1 in nitrergic neurons, rescuing axonal diameter and preventing somatic hypertrophy without abolishing nNOS expression.
4. Glial IL‑1β levels will drop with rapamycin, correlating with improved cholinergic survival.
5. Functional readout: colonic transit time will improve preferentially in rapamycin‑treated aged mice, correlating with the cholinergic neuron rescue rather than nitrergic morphology.

Experimental Approach

• Use 24‑month‑old C57BL/6 mice, administer rapamycin (8 mg/kg diet) or control for 8 weeks.
• Immunofluorescence on whole‑mount colon: ChAT, nNOS, cleaved caspase‑3, p‑S6K, p‑Drp1 (Ser616), GFAP, IL‑1β.
• Electron microscopy to quantify axonal swelling and mitochondrial morphology in nitrergic fibers.
• Measure colonic transit using carmine red assay.
• Statistical analysis: two‑way ANOVA (age × treatment) with post‑hoc tests; n ≥ 6 per group.

Falsifiability

If rapamycin fails to reduce p‑S6K in cholinergic neurons or does not preserve ChAT+ counts, or if nitrergic hypertrophy persists despite lowered p‑Drp1, the hypothesis is refuted. Conversely, selective preservation of cholinergic neurons without changes in nitrergic parameters would support a more uniform mTOR effect, contradicting the subtype‑specific model.