Ras-GDP is inactive, but exchange Selleckchem PR-171 of GTP for GDP induces conformational changes that enable Ras to activate effectors. Signals disseminated by Ras regulate cell proliferation and differentiation. Ras also mediates signaling in nondividing, terminally differentiated cells, such as neurons. The Ras-ERK pathway is essential for optimal synaptic transmission, synaptic plasticity and the creation
of certain types of memory (Thomas and Huganir, 2004). Mutations in proteins comprising the Ras-ERK and Ras-phosphatidylinositol 3-kinase (PI3K) signaling cascades cause human learning deficiencies and mental retardation (Krab et al., 2008). Mechanisms governing Ras activation in adult neurons are poorly understood. Several GTP/GDP exchange factors (GEFs) catalyze Ras activation (Bos et al., 2007 and Buday and Downward, 2008). SOS is an abundant Ras activator in embryonic and postnatal neurons. However, SOS protein declines at puberty and its GTP exchange capacity is low in adult neurons (Tian et al., 2004). RasGRF1, a Ca2+-calmodulin activated GEF, associates with dendritic plasma membrane in mature
neurons and promotes Ras activation at certain postsynaptic sites. Little is known about GEFs that control presynaptic Ras activation or calmodulin-independent, postsynaptic Ras-ERK signaling. Ras guanine nucleotide releasing proteins (RasGRPs) are candidates to fulfill these roles. Four genes encode mammalian RasGRPs, which load GTP onto Ras and Rap1 (Buday and Downward,
2008 and Stone, 2006). The GEFs are expressed in platelets, T cells, Selleck LGK-974 B cells, and mast cells, where they regulate clotting, positive T cell isothipendyl selection and differentiation, IgG production, and inflammation. RasGRPs accumulate in many types of neurons (Toki et al., 2001), but functions of neuronal RasGRPs are unknown. RasGRPs contain a predicted diacylglycerol (DAG)/phorbol 12-myristate-13-acetate (PMA)-binding C1 domain, two putative Ca2+-binding EF hands, a conserved protein kinase C (PKC) phosphorylation site and a CDC25-related catalytic domain (Stone, 2006). PMA, DAG, Ca2+, and phosphorylation stimulate RasGRP activity in cultured cells. Thus, RasGRPs may integrate signals generated by multiple stimuli. Activation of RasGRP by DAG can trigger signaling that bypasses DAG-regulated PKCs. This expands the range of cell functions controlled by DAG and places Ras effectors under control of hormones and neurotransmitters (NTs) that activate phospholipase C (PLC) β, γ, and ɛ. PLCs produce DAG and inositol 3, 4, 5-trisphosphate (IP3) by hydrolyzing phosphatidylinositol 4, 5-bisphosphate (PI4,5P2). IP3 binds a receptor/channel in endoplasmic reticulum (ER), thereby eliciting Ca2+ efflux into cytoplasm. DAG enhances NT and neuropeptide (NP) release during synaptic transmission in mammalian and C. elegans nervous systems ( de Jong and Verhage, 2009, Lackner et al., 1999 and Sieburth et al., 2007).