Role of transmembrane 4 superfamily (TM4SF) proteins CD9 and CD81 in muscle cell fusion and myotube maintenance

The role of transmembrane 4 superfamily (TM4SF) proteins during muscle cell fusion has not been investigated previously, Here we show that the appeara nce of TM4SF protein, CD9, and the formation of CD9-beta 1 integrin complex es were both regulated in coordination with murine C2C12 myoblast cell diff erentiation. Also, anti-CD9 and anti-CD81 monoclonal antibodies substantial ly inhibited and delayed conversion of C2C12 cells to elongated myotubes, w ithout affecting muscle-specific protein expression. Studies of the human m yoblast-derived RD sarcoma cell line further demonstrated that TM4SF protei ns have a role during muscle cell fusion. Ectopic expression of CD9 caused a four- to eightfold increase in RD cell syncytia formation, whereas anti-C D9 and anti-CD81 antibodies markedly delayed RD syncytia formation. Finally , anti-CD9 and anti-CD81 monoclonal antibodies triggered apoptotic degenera tion of C2C12 cell myotubes after they were formed. In summary,TM4SF protei ns such as CD9 and CD81 appear to promote muscle cell fusion and support my otube maintenance.