Integrins at the neuromuscular junction are important for motoneuron survival

During development motoneurons depend on target contact for their survival. Following injury to the sciatic nerve in neonatal rats, a large proportion of motoneurons die. However, the same injury inflicted at 5 days of age re sults in no loss of motoneurons. This critical period of postnatal developm ent coincides with the time during which there is a significant increase in the release of transmitter from the nerve terminals at the neuromuscular j unction. We have proposed that the role of the target muscle cell during th is period is to induce this up-regulation of transmitter release from motor nerve terminals. It has been shown that stretch-induced increase in transm itter release from frog motor nerve terminals is accomplished via an integr in-dependent mechanism. In this study we examined the role of integrins at the rat neuromuscular junction in motoneuron survival. We found that blocki ng integrin binding at the developing neuromuscular junction delayed the in crease in choline acetyltransferase activity that normally takes place duri ng the early postnatal period, and resulted in motoneuron death. Furthermor e, the maturation of those motoneurons that survived was delayed so they re mained susceptible to subsequent nerve injury. These results support the po ssibility that integrins, by their involvement in modulating transmitter re lease, can influence motoneuron survival.