p75 NTR ‐mediated signaling promotes the survival of myoblasts and influences muscle strength

During muscle development, the p75 NTR is expressed transiently on myoblasts. The temporal expression pattern of the receptor raises the possibility that the receptor is influencing muscle development. To test this hypothesis, p75 NTR ‐deficient mutant mice were tested for muscle strength by using a standard wire gripe strength test and were found to have significantly decreased strength relative to that of normal mice. When normal mybolasts were examined in vivo for expression of NGF receptors, p75 NTR was detected on myoblasts but the high affinity NGF receptor, trk A, was not co‐expressed with p75 NTR . In vitro, proliferating C2C12 and primary myoblasts co‐expressed the p75 NTR and MyoD, but immunofluorescent analysis of primary myoblasts and RT‐PCR analysis of C2C12 mRNA revealed that myoblasts were devoid of trk A. In contrast to the cell death functions that characterize the p75 NTR in neurons, p75 NTR ‐positive primary and C2C12 myoblasts did not differentiate or undergo apoptosis in response to neurotrophins. Rather, myoblasts survived and even proliferated when grown at subconfluent densities in the presence of the neurotrophins. Furthermore, when myoblasts treated with NGF were lysed and immunoprecipitated with antibodies against phosphorylated I‐κB and AKT, the cells contained increased levels of both phospho‐proteins, both of which promote cell survival. By contrast, neurotrophin‐treated myoblasts did not induce phosphorylation of Map Kinase p42/44 or p38, indicating the survival was not mediated by the trk A receptor. Taken together, the data indicate that the p75 NTR mediates survival of myoblasts prior to differentiation and that the activity of this receptor during myogenesis is important for developing muscle. © 2005 Wiley‐Liss, Inc.