How Exercise Turns Damaged Mitochondria Into a Muscle-Boosting Signal Molecule

Endurance exercise is one of the most powerful interventions for metabolic health and longevity, yet the molecular mechanisms driving adaptation in specific muscle fiber types have remained poorly understood. This study from Peking University, published in Autophagy (2025), reveals a novel pathway by which the cellular recycling of stressed mitochondria generates a bioactive lipid signal that orchestrates the beneficial remodeling of slow-twitch skeletal muscle.

Using single-fiber transcriptomics (SMART-seq), the researchers compared gene expression in isolated slow-twitch (type I) and fast-twitch (type IIb) myofibers from trained and untrained mice. Endurance training caused far greater transcriptomic remodeling in slow-twitch fibers, with upregulation of pathways governing mitochondrial function, fatty acid oxidation, and — notably — sphingolipid metabolism. Genes encoding SPHK1, S1PR1, and S1PR2 were specifically enriched in slow-twitch fibers in both mice and humans, a finding confirmed by protein quantification, qPCR, co-immunostaining, and analysis of the GTEx human dataset.

The mechanistic core of the study shows that endurance exercise causes ceramides — pro-apoptotic sphingolipids — to accumulate on stressed mitochondria. These ceramide-laden mitochondria are selectively eliminated through mitophagy. During lysosomal degradation, ceramides are converted into sphingosine, which SPHK1 then phosphorylates into sphingosine-1-phosphate (S1P). This mitophagy-dependent rise in S1P activates S1PR1 and S1PR2 on slow-twitch fiber membranes, triggering downstream signaling that promotes mitochondrial biogenesis via PPARGC1A/PGC-1α and enhances endurance performance. Inhibiting mitophagy (using chloroquine) or knocking out SPHK1 or S1PR1/S1PR2 blunted these adaptive responses, while exogenous S1P administration rescued endurance capacity in atrophied mice, mimicking the benefits of exercise.

The study also used BXD mouse genetic reference populations and GTEx human data to show that expression levels of SPHK1 and S1PR2 correlate positively with exercise performance indicators such as post-training VO2 max, reinforcing the physiological relevance of this axis. Importantly, the pathway was not active in fast-twitch fibers, explaining why slow-twitch muscles disproportionately benefit from aerobic training.

These findings position the SPHK1-S1P-S1PR1/S1PR2 axis as a central, fiber-type-specific mediator of endurance exercise adaptation and open a new therapeutic window. Diseases characterized by impaired autophagy and muscle atrophy — including DMD and sarcopenia — may be amenable to S1P-based interventions that recapitulate exercise benefits without requiring physical activity.