Differential Effects of cAMP in Neurons and Astrocytes

Abstract

Mitogen-activated protein kinase (MAPK) activation provides cell type-specific signals important for cellular differentiation, proliferation, and survival. Cyclic AMP (cAMP) has divergent effects on MAPK activity depending on whether signaling is through Ras/Raf-1 or Rap1/ B-raf. We found that central nervous system-derived neurons, but not astrocytes, express B-raf. In neurons, cAMP activated MAPK in a Rap1/B-raf-dependent manner, while in astrocytes, cAMP decreased MAPK activity. Inhibition of MAPK in neurons decreased neuronal growth factor-mediated survival, and activation of MAPK by cAMP analogues rescued neurons from death. Furthermore, constitutive expression of B-raf in astrocytoma cells increased MAPK activation, as seen in neurons, and enhanced proliferation. These data provide the first experimental evidence that B-raf is the molecular switch which dominantly permits differential cAMPdependent regulation of MAPK in neurons versus astrocytes, with important implications for both survival and proliferation.Recent studies have demonstrated that cyclic AMP (cAMP) provides a powerful survival signal for neurons. The short-term survival of spinal motor neurons in vitro is greatly enhanced by elevated intracellular cAMP (1). In the absence of peptide growth factors, the majority of motor neurons extended processes and survived for 1 week in response to cAMP, while the combination of multiple peptide trophic factors with cAMP elevation extended neuronal survival in serum-free media for as much as 3 weeks. Similarly, survival of superior cervical ganglion neurons (2-4) and cerebellar granule cells (5, 6) can be supported by increasing cAMP levels. However, the inability of phosphatidylinositol 3-kinase (PI3K) 1 inhibitors to block the pro-survival effects of cAMP in both neuronal types suggests that cAMP may promote neuronal survival by additional non-PI3K/protein kinase B (Akt) mechanisms (4, 6). cAMP elevation can increase recruitment of the trkB receptor to the plasma membrane in retinal ganglion cells (7), suggesting that cAMP may promote neuronal survival, in part, by increasing neurotrophin receptor availability and signaling.The traditional receptor tyrosine kinase signaling pathway activates mitogen-activated protein kinase (MAPK) by Ras-dependent recruitment of Raf-1, which subsequently phosphorylates MAPK kinase (MEK) and results in activation of MAPK (8, 9). However, an alternative pathway was recently described in PC12 cells, which preferentially utilizes Rap1 and 95-kDa Braf, instead of Ras/Raf-1, to activate MEK (10). These two pathways, Ras/Raf-1 and Rap1/B-raf, differ in their response to cAMP, with cAMP inhibiting MAPK when signaling is through Ras/Raf-1, and activating MAPK when signaling is through Rap1/B-raf (10). This divergent signaling has been reported to reflect cAMP regulation of Rap1 activity and selective interaction of Rap1, instead of Ras, with 95-kDa B-raf (11). Although cAMP effects on Rap1 were attributed to activation of protein kinase A (PKA) in that study, a recent report su...

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