Function, Pharmacology, Evolution and Anatomical Localization of G Protein-Coupled Receptors and Solute Carriers

Abstract: The G protein-coupled receptors (GPCRs) and solute carriers (SLC) are two large families of membrane-bound proteins. The aim of this study was to characterize these two families in terms of evolution and function. The melanocortin (MC) receptors belong to the Rhodopsin family of GPCRs and we cloned the MC4 and MC5 receptors from the rainbow trout, MC3 and MC5 from the spiny dogfish and MCa and MCb from the river lamprey. Pharmacological characterization of the cloned MC receptors demonstrated higher affinity for adrenocorticotropic hormone (ACTH) compared to melanocyte stimulating hormone (MSH) peptides (alpha-, beta- and gamma-MSH). We performed expression analysis with reverse transcription PCR, which showed that the MC4 and MC5 receptors in the rainbow trout are expressed centrally as well as in peripheral tissues. The dogfish MC3 and MC5 receptors were expressed in the brain, while the lamprey MCa and MCb receptors were expressed in the periphery. An extensive tissue localization analysis was performed for the entire family of Adhesion GPCRs in the rat and mouse. Using quantitative real-time PCR (qRT-PCR) we discovered that the majority of GPCRs were expressed either specifically in the CNS or ubiquitously in the CNS and peripheral tissues. We identified all non-olfactory GPCRs in the dog and classified them into Adhesion, Frizzled, Glutamate, Rhodopsin and Secretin families. The dog GPCR repertoire seemed to be more similar to the human repertoire than to the repertoires in rodents. Solute carrier family 25 includes mitochondrial membrane transporters. Using bioinformatics techniques we identified 14 novel members of the SLC25 family, which now has 46 members. We identified orthologs of the novel SLC25 family members in yeast and performed expression analysis of 9 of them with qRT-PCR on a panel containing 30 central and peripheral tissues from the rat. To conclude, this study has expanded our knowledge of the repertoire of genes coding for membrane-bound proteins and provided information about their functional roles.