Regulation of Neuronal Function by Ras-GRF Exchange Factors

Abstract

Ras-GRF1 (GRF1) and Ras-GRF2 (GRF2) constitute a family of guanine nucleotide exchange factors (GEFs). The main isoforms, p140-GRF1 and p135-GRF2, have 2 GEF domains that give them the capacity to activate both Ras and Rac GTPases in response to signals from a variety of neurotransmitter receptors. GRF1 and GRF2 proteins are found predominantly in adult neurons of the central nervous system, although they can also be detected in a limited number of other tissues. p140-GRF1 and p135-GRF2 contain calcium/calmodulin-binding IQ domains that allow them to act as calcium sensors to mediate the actions of NMDA-type and calcium-permeable AMPA-type glutamate receptors. p140-GRF1 also mediates the action of dopamine receptors that signal through cAMP. Although p140-GRF1 and p135-GRF2 have similar functional domains, studies of GRF knockout mice show that they can play strikingly different roles in regulating MAP kinase family members, neuronal synaptic plasticity, specific forms of learning and memory, and behavioral responses to psychoactive drugs. In addition, the function of GRF proteins may vary in different regions of the brain. Alternative splice variants yielding smaller GRF1 gene isoforms with fewer functional domains also exist; however, their distinct roles in neurons have not been revealed. Continuing studies of these proteins should yield important insights into the biochemical basis of brain function as well as novel concepts to explain how complex signal transduction proteins, like Ras-GRFs, integrate multiple upstream signals into specific downstream outputs to control brain function.

Keywords: Ras; Ras-GRF; neurons; synaptic plasticity.

Figure 1.

Functional domains of p140-GRF1 and p135-GRF2. Both proteins have N-terminal pleckstrin homology (PH) domains, coiled-coil (CC), and IQ domains. These are followed by Dbl homology (DH) and PH domains that lead to the activation of the Rac GTPase and its effector proteins. Their C-terminal regions contain Ras-activating CDC25 domains and their associated Ras exchange motifs (REMs). p140-GRF1 differs from p135-GRF2 in that it contains a “neuronal domain” that binds to the 2B subunit of NMDA-type glutamate receptors. In addition, the CDC25 domain of GRF1, but not that of GRF2, has the ability to activate R-Ras in addition to Ras proteins.

Figure 2.

Gene organization of GRF1 and GRF2 including mRNA splice variants. (A) Exons of the GRF1 gene along with splice variants that lead to 3 different GRF1 protein isoforms. (B) Functional domains found in p140-GRF1 (isoform 1), p55-GRF1 (isoform 2), and p20-GRF1 (isoform 3). (C) Exons of the GRF2 gene and splice variants that potentially lead to 2 different GRF2 proteins. (D) Functional domains found in p135-GRF2 and p60-GRF2 (not definitively identified).

Figure 3.

Properties of transgenic mice used in studies to reveal the function of GRF proteins.

Figure 4.

p140-GRF1 and p135-GRF2 mediate opposite forms of synaptic plasticity in the CA1 region of the hippocampus of 1-month-old mice. p135-GRF2 mediates the ability of NR2A-containing NMDARs to promote LTP at least in part through its ability to preferentially activate Ras and Erk MAP kinase. In contrast, p140-GRF1 mediates the ability of NR2B receptors to promote LTD at least in part through its ability to preferentially activate Rac and p38 MAP kinase. Some of this signaling specificity difference between GRF family members may derive from the local association of regulators of signaling downstream of GRF proteins, such as the negative regulator of Ras, Syn-GAP, which is known to associate with NR2B-containing NMDARs.

Figure 5.

p140-GRF1 integrates glutamate and dopamine signaling in the striatum. GRF1 mediates Ras/Erk activation by both glutamate and dopamine receptors. The former is thought to function through calcium (Ca²⁺) entering through NR2B-containing NMDARs and the calmodulin (CaM)–binding domain of p140-GRF1 and/or p55-GRF1 (not shown). Dopamine is thought to function through cAMP/PKA to phosphorylate GRF1. The role of the DH domain in this process has not been investigated.