Cyclic AMP-dependent protein kinase A and CREB are involved in neuregulin-induced synapse-specific expression of acetylcholine receptor gene

Neuregulin is reported to stimulate synapse-specific transcription of acetylcholine receptor (AChR) genes in the skeletal muscle fiber by multiple signaling pathways such as ERK, PI3K, and JNK. The co-localization of PKA mRNA with AChR and ErbBs, receptors for neuregulin, at the confined region of synapse implicates the putative role of PKA in neuregulin-induced AChR gene expression. In the present study, we found that mRNA and protein of a regulatory subunit of PKA (PKARIα) were concentrated at synaptic sites of the rat sternomastoid muscle fiber, while those of ERK and PI3K were uniformly distributed throughout the muscle fiber. Neuregulin (100ng/ml) increased both PKA activity in the nucleus and AChRδ subunit gene transcription in cultured Sol8 myotubes. These increases were significantly blocked by a specific PKA inhibitor H-89 (100nM) and an adenylcyclase inhibitor SQ 22536 (200μM) (72.5% and 60.1%, respectively). Furthermore, neuregulin phosphorylated CREB, a well-known down-stream transcription factor of PKA. While H-89 inhibited CREB phosphorylation, H-89 and PD098059 (50μM), a specific MEK1/2 inhibitor, did not inhibit the phosphorylation of ERK and CREB, respectively, suggesting no cross-talk between PKA and ERK pathways. In conclusion, neuregulin increases AChRδ subunit gene transcription, in part, by the activation of PKA/CREB, an alternative route to the previously reported ERK signaling pathway.