Neuronal Calcium Activates a Rap1 and B-Raf Signaling Pathway via the Cyclic Adenosine Monophosphate-dependent Protein Kinase

Abstract

Activity-dependent regulation of neuronal events such as cell survival and synaptic plasticity is controlled by increases in neuronal calcium levels. These actions often involve stimulation of intracellular kinase signaling pathways. For example, the mitogen-activated protein kinase, or extracellular signal-regulated kinase (ERK), signaling cascade has increasingly been shown to be important for the induction of gene expression and long term potentiation. However, the mechanisms leading to ERK activation by neuronal calcium are still unclear. In the present study, we describe a protein kinase A (PKA)-dependent signaling pathway that may link neuronal calcium influx to ERKs via the small G-protein, Rap1, and the neuronal Raf isoform, B-Raf. Thus, in PC12 cells, depolarization-mediated calcium influx led to the activation of B-Raf, but not Raf-1, via PKA. Furthermore, depolarization also induced the PKA-dependent stimulation of Rap1 and led to the formation of a Rap1/B-Raf signaling complex. In contrast, depolarization did not lead to the association of Ras with B-Raf. The major action of PKA-dependent Rap1/B-Raf signaling in neuronal cells is the activation of ERKs. Thus, we further show that, in both PC12 cells and hippocampal neurons, depolarization-induced calcium influx stimulates ERK activity in a PKA-dependent manner. Given the fact that both Rap1 and B-Raf are highly expressed in the central nervous system, we suggest that this signaling pathway may regulate a number of activity-dependent neuronal functions.

Crk family adaptors–signalling complex formation and biological roles

“…An involvement of PI3 kinase in NGF-induced Rap1 activation has also been described (York et al, 2000). PC12 cells also have NGF-and Crk-independent means of activating Rap1 and B-Raf (Vossler et al, 1997;Grewal et al, 2000). On the other hand, support for the NGF ± TrkA ± Crk ± C3G ± Rap1 ± B-Raf ± MEK ± MAPK cascade model now comes from another study just published (Kao et al, 2001).…”

“…Overexpression of the tyrosine kinase GTK (also called BSK, IYK, FRK or RAK) or other proteins like Shb in PC12 cells apparently favours Crk-dependent Rap1 activation Lu et al, 2000,). An involvement of PI3 kinase in NGF-induced Rap1 activation has also been described (York et al, 2000). PC12 cells also have NGF-and Crk-independent means of activating Rap1 and B-Raf (Vossler et al, 1997;Grewal et al, 2000).…”

“…The downstream targets of GTP-loaded Rap1 are in most cases not well characterized, with one exception: the cytoplasmic kinase B-Raf has been implicated as a Rap1-target in many studies (Ohtsuka et al, 1996;Vossler et al, 1997;York et al, 1998;Dugan et al, 1999;Czyzyk et al, 2000;Grewal et al, 2000;Schmitt and Stork, 2000;and others). This leads to activation of the MAPK/Erk kinases and subsequent activation of the transcription factor Elk-1.…”

SynGAP regulates synaptic strength and mitogen-activated protein kinases in cultured neurons

“…Indeed, there is precedent for promiscuity among classes of GAPs and small G proteins (29), and SynGAP has recently been shown to regulate Rap1 in vitro (13). Interestingly, calcium influx stimulates a Rap1͞BRAF signaling pathway that leads to ERK activation in neurons (30), and the expression of an inhibitory Rap1 isoform in the forebrain results in suppression of ERK1͞2 (31), suggesting multiple independent pathways for this type of signaling.…”

Differential Neurotrophic Regulation of Sodium and Calcium Channels in an Adult Sympathetic Neuron

“…One is via cAMP/PKA/B-raf and the other via cAMP/Epac/rap1/B-raf (de Rooij et al 1998;Grewal et al 2000;Kawasaki et al 1998;Vossler et al 1997). The pituitary adenylate cyclase-activating polypeptide (PACAP) receptor Pac1 stimulates both a cAMP pathway and the PLC pathway to activate rap-1 and ras.…”