Profiling skeletal muscle-derived secretome with differentiation and acute contractile activity

Extracellular vesicles (EVs) released from all cells, are essential to cellular communication, and contain biomolecular cargo that can affect recipient cell function. Studies on the effects of contractile activity (exercise) on EVs usually rely on plasma/serum-based assessments, which contain EVs from many different cells. To specifically characterize skeletal muscle-derived vesicles and the effect of acute contractile activity, we used an in vitro model where C2C12 mouse myoblasts were differentiated to form myotubes. EVs were isolated from conditioned media from muscle cells, pre-differentiation (myoblasts) and post-differentiation (myotubes), as well as from acutely stimulated myotubes (1hr @ 14V, C-Pace EM, IonOptix) using total exosome isolation reagent (TEI, ThermoFisher, referred to as extracellular particles [EPs]) and differential ultracentrifugation (dUC; EVs). Myotube-EPs (~98 nm) were 41% smaller than myoblast-EPs (~167 nm, p<0.001, N=8-10). Two-way ANOVA showed a significant main effect for size distribution of myotube vs. myoblast-EPs (p<0.01, N=10-13). Myoblast-EPs displayed a bimodal size distribution profile with peaks at <200 nm and 400-600 nm, compared to myotube-EPs that were largely 50-300 nm in size. Total protein yield from myotube-EPs was nearly 15-fold higher than myoblast-EPs, (p<0.001 N=6-9). Similar biophysical characteristics were observed when EVs were isolated using dUC: myotube-EVs (~195 nm) remained 41% smaller in average size than myoblast-EVs (~330 nm, p=0.07, N=4-6) and had comparable size distribution profiles as EPs isolated via TEI. Myotube-EVs also had 4.7-fold higher protein yield vs. myoblast EVs (p<0.05, N=4-6). Myotube-EPs had significantly decreased expression of exosomal marker proteins TSG101, CD63, ALIX and CD81 compared to myoblast-EPs (p<0.05, N=7-12). Conversely, microvesicle marker ARF6, and lipoprotein marker APO-A1was only found in the myotube-EPs (p<0.05, N=4-12). There was no effect of acute stimulation on myotube-EP biophysical characteristics (N=7), nor on expression of TSG101, ARF6 or CD81 (N=5-6). Myoblasts treated with control or acute stimulation-derived EPs (13 g/well) for 48hrs and 72hrs showed no changes in mitochondrial mass (MitoTracker Red), cell viability or cell count (N=3-4). Myoblasts treated with EP-depleted media (72hrs) had ~90% lower cell counts (p<0.01, N=3). Our data show that EVs differ in size, distribution, protein yield and expression of subtype markers pre- vs. post-skeletal muscle differentiation. There was no effect of acute stimulation on biophysical profile or protein markers in EPs. Acute stimulation-derived EPs did not alter mitochondrial mass nor cell count/viability. Further investigation into the effects of chronic contractile activity on the biophysical characteristics and cargo of skeletal muscle-specific EVs are warranted.