Vocalfold mTORC1 activity as a tissue‑specific civilization‑survival dial governing presbyphonia
Hypothesis
We hypothesize that the basal mTORC1 signaling level in vocal fold thyroarytenoid muscle (TAM) and lamina propria fibroblasts is lower than that in limb skeletal muscle, placing these tissues on the survival‑biased side of the mTOR civilization‑versus‑survival dial. This setpoint preserves autophagic proteostasis under constant phonatory demand, but age‑associated mTORC1 elevation shifts the balance toward anabolic excess, driving the collagen‑rich, elastin‑poor ECM changes seen in presbyphonia[1].
Mechanistic Rationale
Continuous vibration of the vocal folds creates repetitive mechanical stress that requires rapid turnover of structural proteins. Autophagy, kept active by low mTORC1 activity, clears damaged proteins and organelles, preventing maladaptive ECM remodeling. In limb muscle, intermittent high‑intensity contractions favor bursts of mTORC1‑driven hypertrophy, which can become maladaptive with age[2]. In the larynx, constitutive low mTORC1 sustains Smad7 expression, which antagonizes TGF‑β/Smad signaling and limits fibroblast‑to‑myofibroblast transition. When mTORC1 rises with age, Smad7 declines, TGF‑β signaling increases, and fibroblasts deposit excess collagen while elastin synthesis falls. This mechanistic link explains why functional electrical stimulation can reverse TAM atrophy without fiber‑type shift: it transiently lifts mTORC1 enough to stimulate protein synthesis without triggering the fibrotic cascade[3].
Testable Predictions
- Young rodent vocal fold TAM will show significantly lower phospho‑S6K1 and phospho‑4E‑BP1 levels than age‑matched limb muscle (p<0.01).
- Aged vocal fold tissue will exhibit a measurable increase in phospho‑S6K1 relative to young, yet remain below limb muscle levels.
- Pharmacological inhibition of mTORC1 with rapamycin in aged vocal fold explants will reduce collagen I deposition and increase the LC3‑II/I ratio compared with vehicle.
- Forced activation of mTORC1 via AAV‑mediated Rheb overexpression in vocal fold fibroblasts will decrease Smad7, elevate phospho‑Smad2/3, raise collagen content, and lower autophagic flux.
Experimental Approach
- Harvest TAM and limb skeletal muscle from 3‑month and 24‑month rats. Perform Western blot for p‑S6K1, p‑4E‑BP1, total S6K1, LC3‑I/II, p62, Smad7, collagen I, and elastin.
- Culture primary TAM fibroblasts and treat with rapamycin (20 nM) or MHY1485 (10 µM) for 48 h; assess collagen secretion by Sircol assay and autophagic flux by GFP‑LC3 puncta.
- Inject AAV9‑Rheb unilaterally into the vocal folds of 12‑month rats; after 4 weeks measure in vivo phonatory frequency and amplitude, then harvest tissue for the same biochemical markers.
- Statistical analysis using two‑way ANOVA (age × treatment) with post‑hoc Tukey; significance set at p<0.05.
If the data confirm lower basal mTORC1 in young vocal fold tissue and an age‑dependent shift toward higher signaling that correlates with ECM deterioration and reduced autophagy, the hypothesis is supported. Conversely, if mTORC1 activity is equal to or higher than limb muscle in young tissue, or if manipulating mTORC1 does not alter collagen or autophagy as predicted, the hypothesis is falsified.