Separate Cyclic AMP Sensors for Neuritogenesis, Growth Arrest, and Survival of Neuroendocrine Cells*

Background: Three cAMP sensors (PKA, Epac1/2, and NCS/Rapgef2) coexist in neuroendocrine cells. Their roles in differentiation require elucidation. Results: Epac2, PKA, and NCS/Rapgef2 independently gate signaling for growth arrest, cell survival, and neuritogenesis after GPCR-G(s) engagement in PC12 cells. Conclusion: Parallel, insulated pathways effect cAMP-dependent neuroendocrine cell differentiation. Significance: Assays for parcellated cAMP signaling in neuroendocrine cells have a broad application for CNS drug discovery. Dividing neuroendocrine cells differentiate into a neuronal-like phenotype in response to ligands activating G protein-coupled receptors, leading to the elevation of the second messenger cAMP. Growth factors that act at receptor tyrosine kinases, such as nerve growth factor, also cause differentiation. We report here that two aspects of cAMP-induced differentiation, neurite extension and growth arrest, are dissociable at the level of the sensors conveying the cAMP signal in PC12 and NS-1 cells. Following cAMP elevation, neuritogenic cyclic AMP sensor/Rapgef2 is activated for signaling to ERK to mediate neuritogenesis, whereas Epac2 is activated for signaling to the MAP kinase p38 to mediate growth arrest. Neither action of cAMP requires transactivation of TrkA, the receptor for NGF. In fact, the differentiating effects of NGF do not require activation of any of the cAMP sensors protein kinase A, Epac, or neuritogenic cyclic AMP sensor/Rapgef2 but, rather, depend on ERK and p38 activation via completely independent signaling pathways. Hence, cAMP- and NGF-dependent signaling for differentiation are also completely insulated from each other. Cyclic AMP and NGF also protect NS-1 cells from serum withdrawal-induced cell death, again by two wholly separate signaling mechanisms, PKA-dependent for cAMP and PKA-independent for NGF.