Germline‑style mtDNA purging via TORC1 inhibition and CrAT conformational gating rescues sarcopenic muscle

Intermittent TORC1 inhibition combined with transient acetyl-CoA elevation can recreate the germline mtDNA quality‑control cycle in skeletal muscle. The germline maintains mitochondrial fidelity through two sequential steps: (1) a mtDNA bottleneck that amplifies heteroplasmy variance, and (2) programmed germline mitophagy (PGM) that eliminates defective organelles via BNIP3‑dependent autophagy when TORC1 is suppressed at meiosis entry. Somatic muscle lacks both mechanisms, allowing deleterious mtDNA mutations to accumulate and impair β‑oxidation, a deficit exacerbated by reduced L‑carnitine‑mediated fatty‑acid import and dysregulated carnitine acetyltransferase (CrAT) activity. Recent work shows that pre‑binding of CoA or acetyl‑CoA inhibits CrAT catalysis ~100‑fold through a conformational shift, suggesting that acute rises in acetyl‑CoA could transiently suppress CrAT flux and signal metabolic stress.

We hypothesize that pulsatile elevation of acetyl‑CoA (via short‑term sodium acetate administration) during periods of TORC1 inhibition (using low‑dose rapamycin) will mimic the germline’s variance‑amplification and selective purge in aged skeletal muscle. Specifically, acetyl‑CoA‑mediated CrAT inhibition will reduce acetyl‑CoA consumption, raising the acetyl‑CoA/CoA ratio and activating AMPK, which in turn upregulates BNIP3 and drives mitophagy independently of the PINK1/Parkin pathway. This surge in BNIP3‑dependent autophagy will preferentially remove mitochondria bearing high mutational loads, because the preceding bottleneck‑like effect of reduced CrAT activity lowers mitochondrial acetyl‑CoA utilization, increasing the relative impact of mtDNA defects on ATP production and thus exposing them to autophagic surveillance. After the purge phase, exogenous L‑carnitine supplementation will restore fatty‑acid import in the renovated mitochondrial pool, improving β‑oxidation and muscle contractile function.

A testable prediction follows: In 24‑month‑old C57BL/6 mice, a regimen of rapamycin (1 mg/kg, i.p., twice weekly) plus sodium acetate (1 g/kg, oral, 30 min before each rapamycin dose) for four weeks, followed by L‑carnitine (500 mg/kg, oral, daily) for an additional two weeks, will (a) increase BNIP3‑dependent mitophagy markers (LC3‑II/BNIP3 colocalization, mitochondrial DNA clearance) in gastrocnemius tissue, (b) reduce mtDNA heteroplasmy levels measured by duplex sequencing, (c) elevate CPT1‑mediated palmitoyl‑carnitine formation and ATP production rates in permeabilized fibers, and (d) improve grip strength and treadmill endurance relative to rapamycin‑only, acetate‑only, or vehicle controls. The hypothesis is falsifiable: If BNIP3 knockout in muscle abolishes the heteroplasmy reduction and functional improvements despite identical drug treatment, the proposed germline‑like purge mechanism is invalid.

This approach links a kinetic peculiarity of CrAT to organelle quality control, offering a mechanistic bridge between metabolic signaling and the evolutionary strategy that keeps the germline mutation‑free. Success would indicate that somatic tissues can be endowed with a ‘germline‑grade editing budget’ through temporally controlled metabolic stress, opening a new avenue for treating age‑related metabolic decline.