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The neuregulins (NRGs) constitute a family of trophic factors that are known to play critical roles during neural development. We recently reported that G subunit regulates NRG-mediated signaling and gene transcription in cultured C2C12 myotubes. In this study, we demonstrated that NRG treatment of C2C12 myotubes stimulates a rapid translocation of Gz protein to the plasma membranes. In addition, Gz protein is localized to the postsynaptic regions at adult neuromuscular junctions and is prominently expressed in rat skeletal muscle during early postnatal stages. Interestingly, we found that expression of the constitutively activated Gz in C2C12 myoblasts attenuates myogenic differentiation. Taken together, our observations reveal an unanticipated role of Gz in mediating the actions of NRG during neural development.

Cyclin-dependent kinase 5 (Cdk5) is a serine/threonine kinase that plays important roles during central nervous system development. Cdk5 kinase activity depends on its regulatory partners, p35 or p39, which are prominently expressed in the central nervous system. We have previously demonstrated the involvement of Cdk5 in the regulation of acetylcholine receptor expression at the neuromuscular junction, suggesting a novel functional role of Cdk5 at the synapse. Here we report the identification of Pctaire1, a member of the Cdk-related kinase family, as a p35-interacting protein in muscle. Binding of Pctaire1 to p35 can be demonstrated by in vitro binding assay and co-immunoprecipitation experiments. Pctaire1 is associated with p35 in cultured myotubes and skeletal muscle, and is concentrated at the neuromuscular junction. Furthermore, Pctaire1 can be phosphorylated by the Cdk5/p25 complex, and serine 95 is the major phosphorylation site. In brain and muscle of Cdk5 null mice, Pctaire1 activity is significantly reduced. Moreover, Pctaire1 activity is increased following preincubation with brain extracts and phosphorylation by the Cdk5/p25 complex. Taken together, our findings demonstrate that Pctaire1 interacts with p35, both in vitro and in vivo, and that phosphorylation of Pctaire1 by Cdk5 enhances its kinase activity.

Cyclin-dependent kinase 5 (Cdk5) was originally identified as a serinethreonine kinase and subsequently demonstrated to play a critical role in the development of CNS. We recently reported the novel function of Cdk5 in the neuregulin signaling pathway during the development of neuromuscular junction (NMJ). Here, we report the regulation of Cdk5 and p35 in rat skeletal muscle after nerve injury. Northern blot analysis revealed that Cdk5 and p35 transcripts were up-regulated in muscle after nerve denervation. The temporal profiles for the regulation of Cdk5 and p35 transcripts were different, suggesting that these changes in gene transcription might be regulated by different mechanism. Our finding on the ability of tetrodotoxin to induce p35 transcript in muscle suggested that electrical activity could regulate p35 expression. In addition to the induction of mRNA expression, the total Cdk5 and p35-associated kinase activity in muscle increased prominently after nerve denervation. Taken together, our findings suggest that Cdk5 and p35 may play important physiological roles in muscle regeneration following nerve injury.