UNCORRECTED PROOF 1 In-depth analysis of the secretome identifies three major 2 independent secretory pathways in differentiating

22 Article history:23 Received 18 June 201224 Accepted 8 September 20122526 Efficient muscle regeneration requires cross talk between multiple cell types via secreted27 signalingmolecules.However,asyettherehasbe ennocomprehensiveanalysisofthissecreted28 signaling network in order to understand how it regulates myogenesis in humans.29 Using integrated proteomic and genomic strategies, we show that human muscle cells30 release not only soluble secreted proteins through conventional secretory mechanisms but31 also complex protein and nucleic acid cargos via membrane microvesicle shedding. The32 soluble secretome of muscle cells contains 253 conventionally secreted signaling proteins,33 including 43 previously implicated in myogenesis, while others are known to modulate34 various cell types thus implying a much broader role for myoblasts in muscle remodeling.35 We also isolated and characterized two types of secreted membrane-derived vesicles:36 nanovesicles harboring typical exosomal features and larger, morphologically distinct,37 microvesicles. While they share some common features, their distinct protein and RNA38 cargos suggest independent functions in myogenesis. We further demonstrate that both39 types of microvesicles can dock and fuse with adjacent muscle cells but also deliver40 functional protein cargo.45 Keywords:46 Skeletal muscle47 Myoblast differentiation48 Secretome49 Proteomics50 Secreted factors51 Secreted membrane microvesicles