| The motor behavior of vertebrates requires and relies on the reciprocal communications between motor neurons and muscle fibers. Such communications occur at the specialized sites termed neuromuscular junctions (NMJs). The maturation of NMJ is initiated and co-ordinated by a series of trophic factors secreted from motor neurons and muscle fibers. Much efforts have been put in the identification of these trophic factors. However, the interaction between these factors is still unclear and the molecular mechanism by which they trigger the formation of NMJ is largely unknown. The present study aimed to explore the functional roles and interactions of several key trophic factors, including neurotrophins, agrin and neuregulins during the development of NMJ. We found that neurotrophin-3 (NT-3) derived from muscle could modulate the differentiation of the post-synaptic apparatus, i.e., enhancing the acetylcholine receptor (AChR) aggregation on muscle cells. This effect of NT-3 was mediated by inducing the expression of pre-synaptic proteins, acetylcholinesterase (AChE) and agrin. Agrin is a well characterized molecule responsible for inducing the aggregation of AChRs on muscle. As part of an effort to elucidate the mechanism of agrin action, we explored the involvement of a recently identified muscle specific kinase (MuSK) in the agrin signaling pathway. We found that MuSK served as one of the receptor component for agrin. To expand our knowledge about agrin and MuSK from mammalian to amphibian systems, we identified and functionally characterized the MuSK ortholog in Xenopus. Furthermore, the action of agrin during the maturation of NMJ requires the co-operation of another nerve-derived factor, neuregulins, which regulate the transcription of post-synaptic genes such as AChRs. We have identified several neuregulin-regulated genes using RNA fingerprinting by arbitrarily primed-PCR. Taken together, our study provides important information in elucidating the molecular mechanism and interactions of these key trophic factors in the formation of NMJ. |
