Hypothesis

The disconnect between healthspan improvements and lifespan extension observed with mitochondrial-derived peptides (MDPs) stems from the transient, exercise‑like nature of their signaling rather than a lack of biological activity. Acute pulses of MOTS-c—such as those generated by a single bout of exercise—activate AMPK/SIRT1 pathways and improve glucose homeostasis, yet they do not produce the persistent epigenetic reprogramming needed to slow aging at the organismal level. Sustained, low‑level elevation of MOTS-c in circulation (or targeted delivery to key tissues) would be necessary to translate metabolic healthspan gains into measurable lifespan extension.

Mechanistic Basis

MDPs like humanin, SHLP2, and MOTS-c act as mitokines that signal from mitochondria to the nucleus, engaging FOXO, AMPK, and SIRT1 to regulate stress resistance and metabolism[1][2]. However, the field lacks data on receptor binding, dose‑response, and tissue‑specific expression, which obscures why healthspan and lifespan effects diverge[2]. We propose that MOTS-c’s primary action is a rapid, post‑translational modification of metabolic enzymes (e.g., increasing Complex IV activity) that improves insulin sensitivity without altering the mitochondrial‑nuclear retrograde transcriptional program that governs longevity[3]. In contrast, chronic elevation would allow MOTS-c to accumulate in the nucleus, where it could modulate histone acetylation or directly influence FOXO‑dependent transcription of longevity genes, a step missed in acute‑pulse experiments.

Testable Predictions

1. Chronic, low‑dose subcutaneous infusion of MOTS-c via osmotic pump in mice will significantly increase median lifespan compared with vehicle controls.
2. Acute elevation of MOTS-c mimicking post‑exercise spikes (e.g., repeated bolus injections) will improve insulin tolerance and cognitive performance but will not alter lifespan.
3. Muscle‑specific knockout of TFAM (reducing mitochondrial MOTS-c production) will abolish the healthspan benefits of exercise without affecting baseline lifespan, indicating that tissue‑derived MOTS-c is sufficient for metabolic improvements but not for longevity.

Experimental Approach

• Animal cohorts: C57BL/6J mice, n=50 per group, powered for survival analysis (α=0.05, power=0.8).
• Treatment groups: (a) vehicle, (b) intermittent MOTS-c bolus (matching post‑exercise 11.9‑fold increase measured by Western blot)[4], (c) continuous low‑dose MOTS-c infusion (0.5 µg/kg/h) via Alzet pump, (d) muscle‑TFAM KO + intermittent MOTS-c, (e) muscle‑TFAM KO + continuous MOTS-c.
• Readouts: Survival curves, glucose tolerance tests, indirect calorimetry, cognitive assays (novel object recognition), tissue‑specific MOTS-c levels (ELISA), and nuclear FOXO target gene expression (qPCR).
• Statistical analysis: Kaplan‑Meier with log‑rank test for lifespan; two‑way ANOVA for metabolic endpoints; correction for multiple comparisons.

Potential Outcomes

If continuous MOTS-c extends lifespan while intermittent treatment does not, the hypothesis is supported, indicating that duration of signaling—not mere presence—determines longevity outcomes. Conversely, if neither regimen affects lifespan, the hypothesis would be falsified, suggesting that MOTS-c’s role is strictly metabolic and that other mitochondrial signals (e.g., NAD⁺ metabolites) drive lifespan effects. Tissue‑specific rescue experiments would further clarify whether muscle‑derived MOTS-c is sufficient for healthspan but requires systemic or nuclear action for lifespan extension.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC7343442/ [2] https://www.jci.org/articles/view/158449 [3] https://pmc.ncbi.nlm.nih.gov/articles/PMC7817689/ [4] https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2025.1602271/full