RasGRP and Ras signaling in neurons:
Ras proteins are an important class of the small GTPase signal trasduction factors. In neurons, Ras acts to control critical cellular functions of survival, synaptic plasticity and differentiation. In turn, Ras activities are under the control of a variety of Ras regulatory proteins in the brain.
Our colleague, Jim Stone, has recently discovered an new Ras activating protein he calls RasGRP (Ras guanyl nucleotide-releasing protein), that is expressed at high levels in brain. RasGRP possesses a pair of "EF hands" that bind calcium and a C1 domain which binds diacylglycerol. We believe that RasGRP activation and Ras signaling are linked to calcium transients and diacylglycerol messengers in neurons. Our goals are to uncover the exact roles for this novel signaling pathway in control of brain functions.
RasGRP is concentrated in the dendrites of neurons
In situ hybridization with RasGRP mRNA probes indicated that RasGRP is widely expressed in the adult rat brain, with particularly high levels in neurons of the hippocampus and associated structures (Pierret et al., 2000). This expression in adult hippocampal neurons suggests that RasGRP may be involved in neuronal signaling processes critical to learning and memory. Using antibodies to RasGRP, we have recently shown that RasGRP is found predominantly in the dendrites of neurons in the hippocampus and entorhinal cortex (Pierret et al., 2001). Because much of the signal processing and integration in neurons occurs in the dendrite, this result suggests that RasGRP must contribute to the control of dendritic function.
RasGRP: Future studies.
Our studies on RasGRP have only just begun, our plan is to address the question of RasGRP and its role in dendritic function. Using the Semliki Forest Virus expression system, we are developing vectors to introduce recombinant, mutant RasGRP genes into hippocampal neurons. The activities of the recombinant RasGRP proteins will be tested for effects on synaptic signaling, ion channel activities and cell survival. Using the yeast two hybrid protein-protein interaction screen, we are looking for novel proteins that interact with RasGRP in dendrites. The long term goals of our studies are to determine how the RasGRP signal transduction pathway acts to regulate the activities of neurons in the brain.
References:
Ebinu JO, Bottorff DA, Chan EYW, Stang SL, Dunn RJ and Stone JC (1998) RasGRP, a ras guanyl nucleotide releasing protein with calcium and diacylglycerol binding motifs. Science 280, 1082-1086
Pierret P, Dunn RJ, Djordjevic B, Stone JC and Richardson PM (2000) Expression of Ras-GRP in selected neurons of adult rat brain. J. Neurocytol. 29: 485-497.
Pierret P, Vallee A, Mechawar N, Dower NA, Stone JC, Richardson PM and Dunn RJ (2001) Cellular and subcellular localization of ras guanyl nucleotide-releasing protein in the rat hippocampus. Neuroscience (in press).
