Hypothesis

Rapamycin extends lifespan only when mitochondrial carnitine acetyltransferase (CAT) can buffer excess acetyl‑CoA generated by heightened fatty acid oxidation. When CAT activity declines with age, acetyl‑CoA accumulates, inhibiting key β‑oxidation enzymes and abolishing the metabolic shift that underlies rapamycin’s longevity effect.

Mechanistic Rationale

• Rapamycin boosts CPT1/CPT2 expression, increasing fatty acid import and β‑oxidation flux【3†L1-L3】.
• Enhanced β‑oxidation raises mitochondrial acetyl‑CoA, which must be exported as acetylcarnitine via the CAT‑OCTN1 shuttle to prevent product inhibition【2†L1-L4】.
• CAT buffers acetyl‑CoA by converting it to acetylcarnitine, maintaining a low acetyl‑CoA/CoA ratio that sustains LCAD and other acyl‑CoA dehydrogenases【2†L5-L7】.
• Age‑related decline in CAT expression or catalytic efficiency (hypothesized Km increase, Vmax decrease) would raise acetyl‑CoA levels, suppressing β‑oxidation despite elevated CPT1/2【4†L1-L3】.
• Thus rapamycin’s benefit is contingent on a functional CAT buffer; without it, the drug merely mimics scarcity without delivering the metabolic shift.

Predictions & Experimental Design

1. Measure CAT kinetics in young vs. old mouse liver and muscle. Expect higher Km and lower Vmax in old tissue.
2. Genetically overexpress CAT in aged mice. Predict restoration of acetylcarnitine efflux and rescued fatty acid oxidation under rapamycin treatment.
3. Pharmacologically inhibit CAT (e.g., with mild‑dose etomoxir analogues that spare CPT1/2) in young mice on rapamycin. Predict loss of the 60% FAO increase and shortened lifespan relative to rapamycin‑only controls.
4. Quantify mitochondrial acetyl‑CoA, acetylcarnitine, and downstream β‑oxidation markers (ACADM, CPT2) across groups. Rapamycin‑treated old mice with CAT overexpression should show low acetyl‑CoA, high acetylcarnitine, and elevated β‑oxidation; CAT‑inhibited young mice should show the opposite.
5. Survival assay. Lifespan extension by rapamycin should be significant only in cohorts with intact or boosted CAT activity.

Potential Outcomes & Falsifiability

• Support: CAT overexpression rescues rapamycin‑induced FAO and lifespan gain in aged mice; CAT inhibition blunts these effects in young mice.
• Refutation: Altering CAT levels does not change rapamycin’s effect on FAO or longevity, indicating that CAT is not the gating factor.

This framework directly tests whether the carnitine acetyltransferase step is the mechanistic checkpoint that converts mTOR inhibition into a genuine metabolic re‑programming rather than a superficial stress mimic.