The primary focus of the section is to further our understanding of the molecular basis of signaling between G protein coupled receptors and voltage gated ion channels in neurons using electrophysiological, molecular, and imaging techniques. A current project involves the investigation of a recently """"""""de-orphanized"""""""" G-protein coupled receptor termed GPR41 or FFA3 in rodent sympathetic neurons. These receptors use short-chained fatty acids (SCFA;e.g., acetate and propionate) as endogenous ligands but little else is known. We have discovered that GPR41 is natively expressed and functionally coupled to calcium channels in sympathetic neurons (primarily the celiac/superior mesenteric ganglia). We have also generated evidence that receptor expression is highest in paravertebral and prevertebral sympathetic neurons. Finally, our data indicate that a ketone body, beta-hydroxybutyrate, acts as an agonist at this receptor. These findings are potentially important for several reasons. First, GPR41 may be expressed in neurons that innervate adipose tissue and thus be an important target for anti-obesity drugs. Second, we proposed that beta-hydroxybutyrate may serve as the primary endogenous ligand for sympathetic ganglia. This finding has implications for conditions in which plasma beta-hydroxybutyrate is elevated such as diabetic ketoacidosis or ketogenic diets. Third, acetate is a major metabolite of ethanol. Hence, GPR41 may be involved in both the response to ethanol (especially hangover symptoms) and addiction. It is possible that polymorphisms in GPR41 and primate-specific segmental duplication GPR42 contribute to susceptibility to alcoholism in humans. Fourth, as GPR41 is likely capable of detecting ethanol indirectly via blood acetate levels, it may be possible to leverage this property to interrupt neural pathways contributing to addiction in model organisms using genetic techniques. Won Y-J, Lu VB, Puhl HL, Ikeda SR (2013) Beta-Hydroxybutyrate Modulates N-Type Calcium Channels in Rat Sympathetic Neurons by Acting as an Agonist for the G-Protein-Coupled Receptor FFA3. J Neurosci 33:1931419325. As a logical extension to this study, we are examining the sequence of the human FFA3 gene. FFA3 is segmentally duplicated in primates including humans giving rise to a second gene termed GPR42. Our preliminary work indicates extreme variability in the open reading frames of both FFA3 and GPR42. Moreover, we find that GPR42 is both transcribed and functional when heterologously expressed thus challenging long held beliefs that GPR42 is non-functional or a pseudogene. Finally, we have generated evidence for a common structural variation in this region that results in a GPR42 copy number variation. Together, these results indicate unexpected variability in FFA3/GPR42 genotype and copy number that could potential impact functions dependent on this receptor class. A second project investigated the RGK proteins (Gem, Rad, Rem1, and Rem2), members of the Ras superfamily of small GTP-binding proteins that interact with calcium channel beta subunits to modify voltage-gated calcium channel function. In addition, RGK proteins affect several cellular processes such as cytoskeletal rearrangement, neuronal dendritic complexity, and synapse formation. To probe the phylogenetic origins of RGK proteincalcium channel interactions, we identified potential RGK-like protein homologs in genomes of genetically diverse organisms from both the deuterostome and protostome animal superphyla. RGK-like protein homologs cloned from Danio rerio (zebrafish) and Drosophila melanogaster (fruit flies) expressed in mammalian sympathetic neurons decreased calcium current density as reported for expression of mammalian RGK proteins. Sequence alignments from evolutionarily diverse organisms spanning the protostome/ deuterostome divide revealed conservation of residues within the RGK G-domain involved in RGK proteincalcium channel beta subunit interaction. In addition, the C-terminal eleven residues were highly conserved and constituted a signature sequence unique to RGK proteins but of unknown function. Taken together, these data suggest that RGK proteins, and the ability to modify calcium channel function, arose from an ancestor predating the protostomes split from deuterostomes approximately 550 million years ago. Puhl HL, Lu VB, Won Y-J, Sasson Y, Hirsch JA, Ono F, Ikeda SR (2014) Ancient origins of RGK protein function: modulation of voltage-gated calcium channels preceded the protostome and deuterostome split. PLoS ONE 9:e100694.
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