Hypothesis: Vesicular Packaging Gates MOTS-c Bioavailability

Core idea Exercise triggers mitochondrial-derived vesicle (MDV) release that encapsulates MOTS-c, shielding it from degradation and enabling targeted delivery to metabolic tissues. Free MOTS-c measured by most ELISA kits reflects a rapidly cleared cytosolic pool, explaining the discordant assay ranges and null clinical signals.

Testable predictions

1. In healthy volunteers, acute exercise will increase exosome-associated MOTS-c by >10-fold, while total circulating MOTS-c (free + vesicle) shows the previously reported wide variability.
2. Immunodepletion of exosomes from plasma will abolish the exercise‑induced rise in AMPK phosphorylation in cultured myotubes, whereas adding back purified exosomes restores signaling.
3. Individuals with type-2 diabetes will exhibit a blunted exercise‑induced increase in vesicular MOTS-c despite normal elevations in total peptide, correlating with impaired insulin sensitivity improvement.
4. Longitudinal tracking of vesicular MOTS-c, but not free MOTS-c, will predict improvements in HbA1c over a 12-week exercise intervention.

Mechanistic rationale MOTS-c is synthesized within mitochondria and, under energetic stress, is sorted into MDVs via the machinery involving MIRO1 and TRAF2 (analogous to mitochondrial antigen shedding). Vesicular encapsulation confers resistance to peptidases and directs uptake through HSPG-mediated endocytosis, placing the peptide in proximity to cytosolic AMPK activators (LKB1, CaMKKβ). This pathway bypasses the need for high free concentrations that are rapidly cleared by renal filtration, accounting for the nanomolar-to-picomolar assay spread reported across kits [3].

The hypothesis directly addresses the disconnect: rodent studies that administered synthetic MOTS-c systemically may have overwhelmed clearance mechanisms, whereas physiological signaling depends on the vesicular fraction that current assays poorly detect [2]. By isolating the vesicle‑bound pool, we can reconcile robust preclinical benefits with absent human therapeutic signals.

Experimental approach

• Collect plasma before and after a standardized bout of cycling (60% VO2max, 30 min) from n=30 participants.
• Separate exosomes via ultracentrifugation or size-exclusion chromatography; quantify MOTS-c in vesicle lysates and supernatant using a validated immuno-MS assay (to avoid antibody-based variability).
• Treat primary human myotubes with isolated vesicles vs. free peptide; measure p-AMPK, p-ACC, and mitochondrial respiration.
• Correlate vesicular MOTS-c changes with clinical endpoints (HOMA-IR, HbA1c) in a parallel cohort of prediabetic subjects undergoing 12-week aerobic training.

Falsifiability If exercise fails to raise vesicular MOTS-c, or if vesicular MOTS-c does not predict metabolic improvements despite changes in free peptide, the hypothesis is refuted. Conversely, a consistent vesicle-specific signal linked to functional outcomes would support a revised view of MOTS-c as a mitokine whose bioavailability hinges on mitochondrial vesicle trafficking.