Identification and characterization of the microvesicles released by muscle cells.

Several cell types have the capacity to secrete small lipid vesicles that contain peculiar transmembrane proteins, and enclose cytosolic components otherwise excluded from the secretory pathway, because of their lack of signal sequences. These vesicles function in signalling and in the horizontal transfer of their membrane proteins and/or cargo molecules, either proteins or mRNAs. Two distinct mechanisms for the release of two distinctive types of signalling vesicle exist: the exocytosis of multivesicular bodies, with the consequent release of the exosomes; and the direct budding from the plasma membrane of small vesicles, the shedding vesicles. The molecular composition differs between the two types of these microvesicles, and reflects the specialised functions of their cellular origin. Here, we demonstrate that muscle cells secrete microvesicles, which we characterized mainly as exosomes. We describe the isolation and the partial characterisation of exosomes released by proliferating and differentiated striated muscle cells; their exosome nature was confirmed by biochemical means and by electron microscopy. Analysis of the vesicular protein contents allowed us to identify the presence of proteins such as: the E3 ubiquitin-ligase Ozz, the multifunctional protein Alix/AIP1, the Enolase, and Chaperones (hsp70). Alix is considered as an exosome marker, since it is one of the most abundant exosome protein, as demonstrated in B cells and dendritic cell-derived exosomes (Mathivanan, S. and Simpson, 2009). Muscle exosomes could represent a previously undiscovered cellular communication system, which could play an important role in the maintenance of striated muscle homeostasis by synchronizing the proliferation of myoblast, or the alignment and the fusion of myotubes during muscle development and regeneration. Mathivanan, S. and Simpson, R.J. ExoCarta: A compendium of exosomal proteins and RNA. Proteomics. 2009. 21, 4997-5000.