Transient expression of phosphatidylserine at cell-cell contact areas is required for myotube formation

Cell surface exposure of phosphatidylserine (PS) is shown to be part of nor mal physiology of skeletal muscle development and to mediate myotube format ion. A transient exposure of PS was observed on mouse embryonic myotubes; a t E13, at a stage of development when primary myotubes are formed. The stud y of this process in cell cultures of differentiating C2C12 and H9C2 myobla sts also reveals a transient expression of PS at the cell surface. This exp osure of PS locates mainly at cell-cell contact areas and takes place at a stage when the structural organization of the sarcomeric protein titin is i nitiated, prior to actual fusion of individual myoblast into multinucleated myotubes. Myotube formation in vitro can be inhibited by the PS binding pr otein annexin V, in contrast to its mutant M1234, which lacks the ability t o bind to PS. Although apoptotic myoblasts also expose PS, differentiating muscle cells show neither loss of mitochondrial membrane potential nor dete ctable levels of active caspase-3 protein. Moreover, myotube formation and exposure of PS cannot be blocked by the caspase inhibitor zVAD(OMe)-fmk. Ou r findings indicate that different mechanisms regulate PS exposure during a poptosis and muscle cell differentiation, and that surface exposed PS plays a crucial role in the process of myotube formation.