Phylogenomic analysis of metabotrophic P2Y receptor family and its expression in zebrafish, Danio rerio

G-protein-coupled receptors (GPCRs) constitute the largest and most divergent class of cell surface proteins. GPCRs can be activated by extracellular signals as diverse as light, peptides, proteins, lipids, odorants, tastants, nucleotides and nucleosides. Currently, all the known chemosensory receptor genes, such as odorant, taste and pheromone receptors belong to the GPCR family, with many of them being class A or class-A related genes. Metabotropic P2Y receptors belong to class A GPCRs and are activated by extracellular nucleotides. Nucleotides are released to the environment when the organism is injured and therefore serve as one of the food stimuli. As such, it would be interesting to see if P2Y receptors play any role in olfaction in zebrafish. To date, eight functional human P2Y receptors and 25 related orphan receptors have been found. I performed extensive data mining in drosophila, ascidian, jawless, cartilaginous and bony fish, frog and human genomes to delineate the P2Y family and investigate its evolutionary origin. The P2Y family originates early in the vertebrate lineage, reflected by the presence of lamprey and the absence of ascidian orthologues. Consistent with these findings, no P2Y receptor is found in invertebrates. In total, 38 subfamilies can be distinguished within the P2Y family, at least two third of which are already present in the shark genome. Two subfamilies, p2yl-3 and p2yl-4, are lost in the human lineage and only GPR 87 subfamily is lost in all teleost species. Zebrafish has 68 P2Y receptor genes, the most of any fish species, and almost double as many than mammals. The teleost P2Y genes are widely distributed in the genome as small cluster and singletons. The vast majority of P2Y genes are intronless while the remaining genes contain up to five introns. In the teleost lineage, the genomic arrangement of P2Y genes is preserved to a large extent, and some synteny is found even with the elephant shark and human genome, possibly reflecting the functional importance of these genes. Selective pressure on teleost P2Y genes generally is high, as evidenced by a preponderance of negative selection. However, a few genes exhibit positive selection at individual sites. In early development, P2Y genes are expressed in many tissues and organs, notably the central and peripheral nervous system, pharyngeal arches, otic vesicle and kidney, suggesting an important role in the development of many tissues. However, no expression is detected in larval olfactory epithelium. In contrast, olfactory epithelium of adult fish does express several P2Y genes as shown by RT-PCR. A possible explanation would be a late onset of expression in the olfactory epithelium. In situ hybridisation of adult olfactory epithelium established an ubiquitous distribution, both in the sensory and non-sensory region, which seems to argue against a role of P2Y genes in nucleotide odor detection. However, further studies will be necessary to give a definitive answer to that question