Les récepteurs couplés aux protéines G (RCPG) constituent l'une des plus grandes familles de gènes du génome des mammifères. Ils sont impliqués dans la plupart des processus physiologiques et physiopathologiques ce qui en a fait des cibles thérapeutiques de choix. GPR158 est un récepteur orphelin, dont on ne connaît pas de ligand, de la classe C des RCPG. Il partage 20% d'identité de séquence entre son domaine transmembranaire (TM) et celui du récepteur GABAB mais son domaine N-terminal est dépourvu du domaine Venus Flytrap (VFT), le domaine de liaison du ligand caractéristique des RCPG de la classe C, ce qui suggère que cette protéine a développé un autre mode de liaison de son ligand endogène (s'il existe). GPR158 est exprimé majoritairement au niveau du cerveau. De manière intéressante, son expression a été aussi décrite, dans des cribles à plus ou moins grande échelle, comme étant associée ou modifiée dans différentes conditions pathologiques dont 50% sont des maladies cancéreuses.À ce jour, on ne connait ni la fonction physiologique, ni les voies de signalisation de GPR158. Dans un premier temps, notre objectif était de comprendre la fonctionnalité de GPR158 en cherchant une activité constitutive de ce récepteur ou en essayant de générer des mutants, des récepteurs tronqués ou des récepteurs chimériques constitutivement actifs. Jusqu'à présent, nous n'avons pas réussi à avoir un récepteur actif, malgré les résidus des boucles intracellulaires et des domaines TM importants pour le couplage aux protéines G et pour l'activation des autres RCPG, qui sont conservés dans GPR158. Ce qui suggère que ce récepteur pourrait ne pas avoir une signalisation en lui même, et il régulerait ainsi l'activité d'autres RCPG. Dans le cas inverse, GPR158 aurait un mode de signalisation très original qui reste à découvrir. Puis nous avons cherché à comprendre le rôle physiologique lié aux trois motifs VCPWE, que nous avons identifiés au niveau du domaine C-terminal (C-ter), très long, de ce récepteur. Ces motifs sont bien conservés chez les différentes espèces et joueraient ainsi des rôles fonctionnels importants. À ce sujet, nous avons montré que le troisième motif s'associe spécifiquement avec la sous-unité Gαo, probablement activée, des protéines G. Et nous avons également identifié, un site d'interaction d'un régulateur de l'activité des protéines G, RGS7, au niveau du domaine C-ter de GPR158, en amont des motifs. Vu que Gαo est le substrat de RGS7, nous suggérons que Gαo lierait en même temps le domaine RGS de la protéine RGS7, et ils formeraient ainsi avec GPR158 un complexe de régulation de l'activité du récepteur orphelin ainsi que d'autres RCPG présents dans le nano-environnement de GPR158. Enfin, afin de mieux comprendre la fonction et les possibles voies de signalisation de GPR158, une analyse protéomique des complexes multi-protéiques bâtis autour du domaine C-ter de GPR158, a été menée. Après purification du récepteur orphelin et des protéines associées par immunoprécipitation, l'identification par spectrométrie de masse des protéines présentes a permis d'identifier 6 nouveaux partenaires potentiels. Parmi eux, quatre protéines, p53, PPM1G, Sgt1 et SIRT1, sont des régulateurs du facteur de transcription suppresseur de tumeur p53, et deux protéines, SIRT1 et TRIM58, sont impliquées dans le processus de vieillissement cellulaire. Par conséquent, une implication dans la transcription, la régulation du cycle cellulaire, la réparation de l'ADN, la prolifération, l'apoptose, la tumorigenèse et le vieillissement, du récepteur orphelin GPR158 peut être envisagée. / G protein-coupled receptors (GPCR) are known to form the largest family of cell communication proteins, and to participate to all functions of the body, making them high potential therapeutic targets. However, lots of these proteins are still orphan receptors, for which no ligand, neither function have been described, although some could be of very high interest, like GPR158, a class C orphan GPCR. The seven transmembrane domain (7TM) of this orphan receptor was related to class C GPCR (GPR158 and GABAB share 20% sequence identity in the TM core region) but its N-terminal domain was not homologous to the typical Venus Flytrap (VFT) known to bind the ligands in most of class C receptors. Which suggests that GPR158 has developed different ligand binding mode. GPR158 is expressed mainly in the brain. Interestingly, the expression of this receptor has been found in many cells and tissues to be potentially regulated in pathological conditions, of which 50% are cancerous diseases. We thus intended to decipher its cellular function and partners, to understand its potential physiological and physiopathological roles. Initially, our goal was to determine the functionality of GPR158, and the possible signaling and cellular mechanisms it was involved in, by looking for some constitutive activity for this orphan GPCR, in the absensce of any ligand. Curiously, we could not detect any G protein coupling, like constitutive G protein stimulation by overexpression of wild type, mutated, truncated and chimeric receptors. This despite the residues of intracellular loops and TM domain, important for the G protein coupling and for the activation of other GPCR, which are conserved in GPR158. This suggests that GPR158 in itself might not have a signalization, and thus it would regulate the activity of other GPCR. Alternatively, GPR158 would have an original way of signaling to be discovered with more sophisticated techniques.Then, we tried to understand the role of three VCPWE specific motifs that we have identified at the long C-terminal (C-ter) domain of GPR158. These motifs are well conserved among different species and thus would play important functional roles. Therefore, we have shown that the third motif indeed binds G protein alpha o subunit, likely in active state. Interestingly, we have also shown that RGS7 that deactivated alpha o, interacts constitutively with the C-terminal domain of GPR158 upstream of VCPWE motifs. Thus, RGS7 would regulate the alpha subunit association with GPR158. Hence, GPR158 would act as a signaling regulatory platform, controlling G protein pathways by binding active alpha subunit and RGS7. This would be of great importance as a local signaling regulatory mechanism. Finally, to better understand the function and possible signaling pathways of GPR158, a proteomic analysis of multi-protein complexes built around the C-ter domain of GPR158, was conducted. After purification of the orphan receptor and its associated proteins by immunoprecipitation, the identification by mass spectrometry of GPR158 interacting proteins led to the identification of six potential new partners. Among them, four proteins, p53, PPM1G, SGT1 and SIRT1, are regulators of the p53 tumor suppressor protein widely known for its role as a transcription factor that regulates the expression of stress response genes, and two proteins, SIRT1 and TRIM58 are involved in cellular aging process. Therefore, GPR158 could be involved in transcription, cell cycle regulation, DNA repair, proliferation, apoptosis, tumorigenesis and cell aging.

The R7 regulators of G protein signaling (R7 RGS), namely RGS6, RGS7, RGS9 and RGS11, are expressed in the retina along with its binding partner Gβ5. The RGS9-1 isoform is expressed only in retinal photoreceptors and rate-limits the recovery of rod phototransduction by acting as a member of the transducin GAP complex (RGS9-1/Gβ5L/R9AP). The Gβ5L isoform is also only expressed in retinal photoreceptors and acts by stabilizing the GAP complex. The Gβ5S isoform differs from Gβ5L by the absence of exon 1 due to alternative splicing and is expressed in many other retinal cells. Gβ5L is barely detectable in RGS9-/- mice suggesting that Gβ5L has a protein degradation signal conferring instability in the absence of RGS9. To study the role of exon 1 of Gβ5L, we replaced Gβ5L with Gβ5S in rods by expressing transgenic Gβ5S under the control of the rhodopsin promoter within a Gβ5-/- mouse and determined that exon 1 of Gβ5L has two previously unidentified functions: (1) to increase the capacity of Gβ5L to bind to RGS9-1 and (2) to serve as a signal for rapid degradation of Gβ5L in RGS9-/- photoreceptors. Several groups have reported that RGS7 and RGS11 with Gβ5S reside in the dendritic tips of depolarizing bipolar cells (DBCs) and that they are involved in the mGluR6/Gαo/TRPM1 pathway, which mediates DBC light responses. The exact role of RGS7 in DBCs has not been unequivocally determined. We have contributed by making a true RGS7 null mouse line and found the RGS7-/- mice are viable and fertile, but have a small body size. Electroretinogram (ERG) b-wave implicit time in young RGS7-/- mice is prolonged at eye opening, but the phenotype disappears by 2 months of age. Expression levels of RGS6 and RGS11 are unchanged in RGS7-/- retina, but the Gβ5S level is significantly reduced. We further generated a RGS7 and RGS11 double knockout (711dKO) mouse line and found that Gβ5S expression in the retinal outer plexiform layer is eliminated, as well as the ERG b-wave. Ultrastructural defects similar to those of Gβ5-/- mice are present in 711dKO. Furthermore, in retinas of mice lacking RGS6, RGS7, and RGS11, Gβ5S becomes undetectable, while the photoreceptor-specific Gβ5L remains unaffected. Whereas RGS6 alone sustains a significant amount of Gβ5S expression in the retina, the DBC-related defects found in Gβ5-/- mice appear to be caused solely by a combined loss of RGS7 and RGS11. The notion that the role of Gβ5 in the retina, and likely in the entire nervous system, is mediated exclusively by R7 RGS proteins is firmly established in this work. The availability of all four R7 RGS single knockout mouse lines enables future studies to further elucidate the roles of R7 RGS proteins in vision.