||Cyclin-dependent kinase 5 (Cdk5), a member of the Cdks family, is identified to play a critical role in the development of CNS. Association with one of its neuronal activators, p35 or p39, is required for Cdk5 to elicit its diverse effects in the nervous system such as neuronal migration and neurite outgrowth. Many efforts have been put on the understanding of Cdk5 functions, the mechanism in regulating its kinase activity is still not clearly elucidated. The first part of this study is to characterize the expression and localization of Cdk5 and its activators in a human embryonal carcinoma cell line, NT2, during neuronal differentiation. NT2 cells with the characteristics of embryonic stem cell provide a model system for us to understand the regulatory mechanism of Cdk5 signaling in the CNS. In addition to its well-studied effects in neuronal migration, increasing evidence suggests that Cdk5 plays important role in the maturation or maintenance of synapses in CNS. The second part of this study is to explore the novel function of Cdk5 at the neuromuscular synapse. The formation of neuromuscular junction (NMJ) is mediated by the induction of AChR clustering and the up-regulation of AChR and other synaptic proteins. To explore the function of Cdk5 at the NMJ, the expression and regulation of Cdk5 and its activators, p35 and p39, in muscle during development were characterized. Surprisingly, the neuron specific activators of Cdk5, p35 and p39, were prominently expressed in embryonic muscle, and concentrated at NMJ in adulthood. The expression of Cdk5 and its activators in muscle were regulated by neural derived factors and electrical activity. The temporal profiles for the regulation of these transcripts were different, suggesting that the changes in gene transcription might be regulated by different mechanisms. The kinase activity of Cdk5 in muscle was also regulated after nerve denervation and NRG treatment. This study also describes a fbnctional role of Cdk5 in regulating the gene expression of acetylcholine receptors at neuromuscular synapse. Co-immunoprecipitation studies revealed the association between Cdk5, p35 and ErbB receptors in muscle. Inhibition of Cdk5 activity not only blocked the NRG-induced AChR transcription, but also attenuated ErbB activation in cultured myotubes. In light of the present finding that overexpression of p35 or p39 alone led to an increase in AChR promoter activity in muscle, Cdk5 activation is sufficient to mediate the up-regulation of AChR gene expression. Taken together, these results reveal the unexpected involvement of CdkS kinase in neuregulin signaling at the neuromuscular synapse. This is the demonstration of Cdk5 in the formation of synapse.