Calcium and cAMP signals differentially regulate cAMP-responsive element-binding protein function via a Rap1-extracellular signal-regulated kinase pathway

Two major intracellular signals that regulate neuronal function are calcium and cAMP, In many cases, the actions of these two second messengers involv e long term changes in gene expression. One well studied target of both cal cium and cAMP signaling is the transcription factor cAMP-responsive element -binding protein (CREB), Multiple signaling pathways have been shown to con tribute to the regulation of CREB-dependent transcription, including both p rotein kinase A (PKA)- and mitogen-activated protein (MAP) kinase/extracell ular signal-regulated kinase (ERK)-dependent kinase cascades. We have previ ously described a mechanism by which cAMP and calcium influx may stimulate ERKs in neuronal cells, This pathway involves the PKA-dependent activation of the Ras-related small G-protein, Rap1, and subsequent stimulation of the neuronal Raf isoform, B-Raf, In this study, we examined the contribution o f the Rap1-ERK pathway to the control of gene transcription by calcium infl ux and cAMP, Using the PC12 cell model system, we found that both calcium i nflux and cAMP stimulated CREB-dependent transcription via a Rap1-ERK pathw ay, but this regulation occurred through distinct mechanisms. Calcium-media ted phosphorylation of CREB through the PKA-Rap1-ERK pathway. In contrast, cAMP phosphorylated CREB via PKA directly but required a Rap1-ERK pathway t o activate a component downstream of CREB phosphorylation and CREB-binding protein recruitment, These data suggest that the Rap 1/B-Raf signaling path way may have an important role in the regulation of CREB-dependent gene exp ression.