Regulation of the p75 neurotrophin receptor in a rat myogenic cell line (L6)

Neurotrophins are expressed in muscle cells both during development and pos tnatally. Furthermore, during development muscle cells express high levels of the common p75 neurotrophin receptor, which binds all neurotrophins. Onl y fragmentary and controversial data are available regarding the responsive ness of muscle cells to neurotrophins and the importance of low-affinity p7 5 receptor in muscle development. The present study investigates in vitro t he immunocytochemical expression of p75 in a rat myogenic cell line (L6) at various time points and in response to different coating substrates as a f irst step in elucidating the regulation of p75 in muscle. We found that in L6 myoblasts, p75 is expressed only at very early stages of maturation and its levels of expression are regulated by the nature of the coating substra tes. p75 expression decreases in cells growing on substrates more suitable for myoblast fusion into myotubes. Time course analysis indicates a reverse correlation between myoblast fusion into myotubes and the levels of p75 ex pression. Myotubes were always p75 negative. Substrates not suitable for th e fusion process induced a prolonged presence of p75 in myoblasts with an i ncrease of their apoptosis. We conclude that expression of p75, at least in this in vitro condition, is regulated by the stages of myoblast differenti ation and the nature of the coating substrates. According to the observed t ime- and substrate-related evidences, future studies should investigate in vivo both the regulation of p75 in the myoblast fusion and the effects and the importance of neurotrophins binding during myoblast differentiation.