Ca2+-Dependent Routes to Ras: Mechanisms for Neuronal Survival, Differentiation, and Plasticity?

through voltage-dependent ion channels activates Ras/ MAPK. Subsequently, four Ca1-dependent pathways p21 (Ras) is a small guanine nucleotide–binding proto Ras have been described; however, the functional tein that functions in signal transduction cascades that independence of the pathways is unclear, since cross– mediate cellular growth and differentiation. Ras is highly talk between different pathways probably occurs (Figure expressed in the developing and adult nervous systems 1). One Ca1-dependent pathway to Ras involves the and plays an important role mediating growth factor tyrosine kinase PYK2 (Lev et al., 1995). Attempts to idenresponses. Nearly 2 years ago, it was shown that volttify a function for PYK2 revealed that increases in cytoage-gated Ca1 influx can also activate Ras. Since plasmic Ca increased PYK2 tyrosine phosphorylation growth factor and Ca1 signaling cascades were beand kinase activity. Overexpression of PYK2 proportionlieved to be distinct, the mechanisms by which Ca ally increases phosphorylation and activation of MAPK, activated Ras were expected to be novel. Until recently, suggesting that PYK2 may activate Ras. One answer to these mechanisms have remained a mystery. However, how PYK2 might activate Ras comes from analysis of several recent papers provided evidence for no less than a structurally homologous protein, the focal adhesion four Ca-dependent pathways that lead to Ras. These kinase (p125 or FAK). FAK is a tyrosine kinase that pathways may play critical roles linking activity-induced mediates signaling by the integrin family of extracellular changes in cytoplasmic Ca levels and long-term matrix receptors. One model posits that FAK autophochanges in neuronal differentiation, survival, and synapsphorylation and activation results when the cytotic strength. Apparent differences in the subcellular loplasmic domains of activated integrin receptors cluster. calization of certain Ca-dependent paths to Ras sugFAK autophosphorylation at tyrosine (Y397) enables gest that they may play distinct roles in preand FAK to bind avidly the SH2 domain of the tyrosine kinase postsynaptic plasticity mechanisms. The existence of Src. The interaction between FAK and Src may induce multiple pathways could also be important for generata conformational change that activates Src. Src then ing specific Ca21 responses and for integrating the activphosphorylates FAK on a specific tyrosine (Y925) that ity of Ca1-independent signaling pathways. serves as a binding site for Grb2. Thus, activated FAK The Ras/MAPK Cascade may localize the Grb2–Sos complex to the inner surface Four genes comprise the mammalian ras family: Ki-ras, of the plasma membrane where it can activate Ras. H-ras, N-ras, and R-ras. These genes encode small guaSince FAK and PYK2 are structurally homologous and nine nucleotide–binding proteins that localize to the inthe FAK tyrosine residues Y397 and Y925 are conserved ner face of the plasma membrane and function as moin PYK2, it seems likely that PYK2 also activates Ras lecular switches that transmit receptor signals to by a Src-dependent mechanism. Support for this model downstream mitogen-activated protein kinase (MAPK) comes from a recent study by Halegoua and colleagues cascades (Seger and Krebs, 1995; Herskowitz, 1995). in Neuron demonstrating that voltage-dependent Ca21 Prior to growth factor stimulation or cytosolic Ca eleinflux activates Src in PC12 cells (Rusanescu et al., vations, Ras resides at the plasma membrane in an inac1995). tive state, bound to GDP. Growth factors, upon binding, Halegoua and coworkers showed that Ca influx induce their cognate receptors to dimerize and undergo through L-type voltage-dependent channels increases tyrosine autophosphorylation within their cytoplasmic Src kinase activity and induces a redistribution of Src domains (Ng and Shooter, 1993). Certain tyrosine phosfrom the cytosol to the plasma membrane (Rusanescu phorylations create docking sites for proteins that conet al., 1995). They also developed a PC12 subline that tain highly conserved Src homology 2 (SH2) domains. stably expressed a dominant-interfering form of Src Two such SH2-containing proteins are the adapter pro(SrcDN). In contrast with the parent PC12 line, SrcDN teins known as Grb2 and Shc. Grb2 exists as a complex cells did not undergo depolarization-induced MAPK with the guanine nucleotide exchange factor (GEF) phosphorylation or transcription of the immediate early called Son of sevenless (Sos). Shc can bind the Grb2– gene NGFI-A. Since factors that regulate NGFI-A tranSos complex. GEFs activate Ras by inducing inactive scription are downstream targets of the Ras/MAPK Ras to exchange GDP for GTP. Thus, growth factor pathway, these results suggest that Ca21 activates the receptors activate Ras by bringing the Ras–GEF, Sos, Ras/MAPK pathway via Src. In this study, Rusanescu to the membrane through binding Grb2 or Shc. Once et al. also found evidence for Shc involvement in Src active, Ras transmits its signal to the MAPK cascade signaling. In PC12 or RasDN cells, Ca1 influx triggered