Once introduced into the very early eukaryotic blueprint, seven-transmembrane receptors soon became the central and versatile components of the evolutionary highly successful G protein-coupled transmembrane signaling mechanism. In contrast to all other components of this signal transduction pathway, G protein-coupled receptors (GPCR) evolved in various structural families, eventually comprising hundreds of members in vertebrate genomes. Their functional diversity is in contrast to the conserved transmembrane core and the invariant set of intracellular signaling mechanisms, and it may be the interplay of these properties that is the key to the evolutionary success of GPCR. The GPCR repertoires retrieved from extant vertebrate genomes are the recent endpoints of this long evolutionary process. But the shaping of the fine structure and the repertoire of GPCR is still ongoing, and signatures of recent selection acting on GPCR genes can be made visible by modern population genetic methods. The very dynamic evolution of GPCR can be analyzed from different perspectives: at the levels of sequence comparisons between species from different families, orders and classes, and at the level of populations within a species. Here, we summarize the main conclusions from studies at these different levels with a specific focus on the more recent evolutionary dynamics of GPCR.

The family of trace amine-associated receptors (TAAR) comprises 9 mammalian TAAR subtypes, with intact gene and pseudogene numbers differing considerably even between closely related species. To date the best characterized subtype is TAAR1, which activates the Gs protein/adenylyl cyclase pathway upon stimulation by trace amines and psychoactive substances like MDMA or LSD. Recently, chemosensory function involving recognition of volatile amines was proposed for murine TAAR3, TAAR4 and TAAR5. Humans can smell volatile amines despite carrying open reading frame (ORF) disruptions in TAAR3 and TAAR4. Therefore, we set out to study the functional and structural evolution of these genes with a special focus on primates. Functional analyses showed that ligands activating the murine TAAR3, TAAR4 and TAAR5 do not activate intact primate and mammalian orthologs, although they evolve under purifying selection and hence must be functional. We also find little evidence for positive selection that could explain the functional differences between mouse and other mammals. Our findings rather suggest that the previously identified volatile amine TAAR3–5 agonists reflect the high agonist promiscuity of TAAR, and that the ligands driving purifying selection of these TAAR in mouse and other mammals still await discovery. More generally, our study points out how analyses in an evolutionary context can help to interpret functional data generated in single species.

Mammals adapted to a great variety of habitats with different accessibility to water. In addition to changes in kidney morphology, e.g. the length of the loops of Henle, several hormone systems are involved in adaptation to limited water supply, among them the renal-neurohypophysial vasopressin/vasopressin receptor system. Comparison of over 80 mammalian V2 vasopressin receptor (V2R) orthologs revealed high structural and functional conservation of this key component involved in renal water reabsorption. Although many mammalian species have unlimited access to water there is no evidence for complete loss of V2R function indicating an essential role of V2R activity for survival even of those species. In contrast, several marsupial V2R orthologs show a significant increase in basal receptor activity. An increased vasopressin-independent V2R activity can be interpreted as a shift in the set point of the renal-neurohypophysial hormone circuit to realize sufficient water reabsorption already at low hormone levels. As found in other desert mammals arid-adapted marsupials show high urine osmolalities. The gain of basal V2R function in several marsupials may contribute to the increased urine concentration abilities and, therefore, provide an advantage to maintain water and electrolyte homeostasis under limited water supply conditions.

Chemokine receptors control leukocyte chemotaxis and cell-cell communication but have also been associated with pathogen entry. GPR33, an orphan member of the chemokine-like receptor family, is a pseudogene in most humans. After the appearance of GPR33 in first mammalian genomes, this receptor underwent independent pseudogenization in humans, other hominoids and some rodent species. It was speculated that a likely cause of GPR33 inactivation was its interplay with a rodenthominoid-specific pathogen. Simultaneous pseudogenization in several unrelated species within the last 1 million years (myr) caused by neutral drift appears to be very unlikely suggesting selection on the GPR33 null-allele. Although there are no signatures of recent selection on human GPR33 we found a significant increase in the pseudogene allele frequency in European populations when compared with African and Asian populations. Because its role in the immune system was still hypothetical expression analysis revealed that GPR33 is highly expressed in dendritic cells (DC). Murine GPR33 expression is regulated by the activity of toll-like receptors (TLR) and AP-1/NF-κB signaling pathways in cell culture and in vivo. Our data indicate an important role of GPR33 function ⋆ The sequences reported in this paper have been deposited in the GenBank (http://www.ncbi.nlm.nih.gov/Genbank/) database (Accession

Our results provide valuable information about residues critical for AVPR2 folding, trafficking and function and proof that these mutations are responsible for causing NDI in the affected subjects.