G protein-coupled receptors can control the Hippo/YAP pathway through Gq signaling

The Hippo pathway is an evolutionarily conserved kinase cascade involved in the control of tissue homeostasis, cellular differentiation, proliferation, and organ size, and is regulated by cell-cell contact, apical cell polarity, and mechanical signals. Miss-regulation of this pathway can lead to cancer. The Hippo pathway acts through the inhibition of the transcriptional coactivators YAP and TAZ through phosphorylation. Among the various signaling mechanisms controlling the hippo pathway, activation of G12/13 by G protein-coupled receptors (GPCR) recently emerged. Here we show that a GPCR, the ghrelin receptor, that activates several types of G proteins, including G12/13, Gi/o, and Gq, can activate YAP through Gq/11 exclusively, independently of G12/13. We revealed that a strong basal YAP activation results from the high constitutive activity of this receptor, which can be further increased upon agonist activation. Thus, acting on ghrelin receptor allowed to modulate up-and-down YAP activity, as activating the receptor increased YAP activity and blocking constitutive activity reduced YAP activity. Our results demonstrate that GPCRs can be used as molecular switches to finely up- or down-regulate YAP activity through a pure Gq pathway.

HTRF® Total and Phospho-YAP (Ser127) Cellular Assays

The YAP protein is a co-transcription factor increasing the expression of genes involved in cell proliferation and repressing the expression of genes important for cell differentiation and apoptosis. It is regulated by several inputs, like the Hippo pathway, through the action of kinases that phosphorylate YAP on several residues. The level of phosphorylation of the residues serine 127 (S127) of YAP is generally assessed in cellular models, native tissues, and organs, as a marker of YAP activity and location, and is regulated by numerous partners. This phosphorylation event is classically detected using a western blot technical approach. Here, we describe a novel approach to detect both the relative amount of total YAP (T-YAP assay) and the phosphorylation of the residue S127 of YAP (S127-P-YAP assay) using a HTRF^®-based method. This easy-to-run method can easily be miniaturized and allows for a high-throughput analysis in 96/384-well plate format, requiring less cellular material and being more rapid than other approaches.

Functional Characterisation of Eel Dopamine D2 Receptors and Involvement in the Direct Inhibition of Pituitary Gonadotrophins

In various vertebrate species, dopamine (DA) exerts an inhibitory action on reproduction. In the European eel, DA plays a pivotal role in the inhibitory control of gonadotroph function and the blockade of puberty. In vivo studies have suggested that this effect is mediated by receptors pharmacologically related to the D2 family. In the European eel, two distinct D2 receptor (D2-R) paralogous genes have been identified (D2A-R and D2B-R) and both were shown to be expressed in the pituitary. We investigated the potential role of each paralogue in the control of gonadotroph function in this species. Eel recombinant D2A-R or D2B-R were expressed in HEK 293 cells, with a universal Gα subunit, and receptor activation was followed by inositol phosphate production. Recombinant D2-Rs exhibited a comparable affinity for DA, although they had differential affinities for mammalian D2-R agonists and antagonists, supporting subtle structure/activity differences. Furthermore, using eel pituitary cell primary cultures, the expression by gonadotroph cells of both native eel D2-R paralogues was examined by in situ hybridisation of D2A-R or D2B-R transcripts, coupled with immunofluorescence of luteinising hormone (LH)β or follicle-stimulating (FSH)β. LH and to a lesser extent, FSH cells expressed both D2-R transcripts but with a clear predominance of D2B-R. Notably, D2B-R transcripts were detected for the majority of LH cells. Accordingly, using these cultures, we showed that DA potently inhibited basal and testosterone-stimulated LHβ expression and less potently basal and activin-stimulated FSHβ expression. We also tested some D2-R antagonists, aiming to select the most adequate one to be used in innovative protocols for induction of eel sexual maturation. We identified eticlopride as the most potent inhibitor of DA action on basal and stimulated LH expression in vitro. Our data suggest a differential functionalisation of the duplicated receptor genes and demonstrate that mainly D2B-R is involved in the dopaminergic inhibitory control of eel gonadotroph function.

Further insights into GPR179: expression, localization, and associated pathogenic mechanisms leading to complete congenital stationary night blindness

PURPOSE

Mutations in GPR179, which encodes the G protein-coupled receptor 179, lead to autosomal recessive complete (c) congenital stationary night blindness (CSNB), which is characterized by an ON-bipolar retinal cell dysfunction. This study further defined the exact site of Gpr179 expression and its protein localization in human retina and elucidated the pathogenic mechanism of the reported missense and splice site mutations.

METHODS

RNA in situ hybridization was performed with mouse retinal sections. A commercially available antibody was validated with GPR179-overexpressing COS-1 cells and applied to human retinal sections. Live-cell extracellular staining along with subsequent intracellular immunolocalization and ELISA studies were performed using mammalian cells overexpressing wild-type or missense mutated GPR179. Wild-type and splice site-mutated mini-gene constructs were transiently transfected, and RNA was extracted. RT-PCR-amplified products were cloned, and Sanger sequenced.

RESULTS

Mouse Gpr179 transcript was expressed in the upper part of the inner nuclear layer, and the respective human protein localized at the dendritic tips of bipolar cells in human retina. The missense mutations p.Tyr220Cys, p.Gly455Asp, and p.His603Tyr led to severely reduced cell surface localization, whereas p.Asp126His did not. The mutated splice donor site altered GPR179 splicing.

CONCLUSIONS

Our findings indicate that the site of expression and protein localization of human and mouse GPR179 is similar to that of other proteins implicated in cCSNB. For most of the mutations identified so far, loss of the GPR179 protein function seems to be the underlying pathogenic mechanism leading to this form of cCSNB.

Trans-activation between 7TM domains: implication in heterodimeric GABAB receptor activation

Seven-transmembrane domain (7TM) receptors have important functions in cell-cell communication and can assemble into dimers or oligomers. Such complexes may allow specific functional cross-talk through trans-activation of interacting 7TMs, but this hypothesis requires further validation. Herein, we used the GABAB receptor, which is composed of two distinct subunits, GABAB1, which binds the agonist, and GABAB2, which activates G proteins, as a model system. By using a novel orthogonal-labelling approach compatible with time-resolved FRET and based on ACP- and SNAP-tag technologies to verify the heterodimerization of wild-type and mutated GABAB subunits, we demonstrate the existence of a direct allosteric coupling between the 7TMs of GABAB heterodimers. Indeed, a GABAB receptor, in which the GABAB2 extracellular domain was deleted, was still capable of activating G proteins. Furthermore, synthetic ligands for the GABAB2 7TM could increase agonist affinity at the GABAB1 subunit in this mutated receptor. In addition to bringing new information on GABAB receptor activation, these data clearly demonstrate the existence of direct trans-activation between the 7TM of two interacting proteins.

Crosstalk between GABAB and mGlu1a receptors reveals new insight into GPCR signal integration

G protein-coupled receptors (GPCRs) have critical functions in intercellular communication. Although a wide range of different receptors have been identified in the same cells, the mechanism by which signals are integrated remains elusive. The ability of GPCRs to form dimers or larger hetero-oligomers is thought to generate such signal integration. We examined the molecular mechanisms responsible for the GABA(B) receptor-mediated potentiation of the mGlu receptor signalling reported in Purkinje neurons. We showed that this effect does not require a physical interaction between both receptors. Instead, it is the result of a more general mechanism in which the betagamma subunits produced by the Gi-coupled GABA(B) receptor enhance the mGlu-mediated Gq response. Most importantly, this mechanism could be generally applied to other pairs of Gi- and Gq-coupled receptors and the signal integration varied depending on the time delay between activation of each receptor. Such a mechanism helps explain specific properties of cells expressing two different Gi- and Gq-coupled receptors activated by a single transmitter, or properties of GPCRs naturally coupled to both types of the G protein.

Common structural requirements for heptahelical domain function in class A and class C G protein-coupled receptors

G protein-coupled receptors (GPCRs) are key players in cell communication. Several classes of such receptors have been identified. Although all GPCRs possess a heptahelical domain directly activating G proteins, important structural and sequence differences within receptors from different classes suggested distinct activation mechanisms. Here we show that highly conserved charged residues likely involved in an interaction network between transmembrane domains (TM) 3 and 6 at the cytoplasmic side of class C GPCRs are critical for activation of the gamma-aminobutyric acid type B receptor. Indeed, the loss of function resulting from the mutation of the conserved lysine residue into aspartate or glutamate in the TM3 of gamma-aminobutyric acid type B(2) can be partly rescued by mutating the conserved acidic residue of TM6 into either lysine or arginine. In addition, mutation of the conserved lysine into an acidic residue leads to a nonfunctional receptor that displays a high agonist affinity. This is reminiscent of a similar ionic network that constitutes a lock stabilizing the inactive state of many class A rhodopsin-like GPCRs. These data reveal that despite their original structure, class C GPCRs share with class A receptors at least some common structural feature controlling G protein activation.

Coupling of agonist binding to effector domain activation in metabotropic glutamate-like receptors

Many membrane receptors are made of a ligand binding domain and an effector domain mediating intracellular signaling. This is the case for the metabotropic glutamate-like G-protein-coupled receptors. How ligand binding leads to the active conformation of the effector domain in such receptors is largely unknown. Here, we used an evolutionary trace analysis and mutagenesis to identify critical residues involved in the allosteric coupling between the Venus flytrap ligand binding domain (VFT) and the heptahelical G-protein activating domain of the metabotropic glutamate-like receptors. We have shown that a conserved interdomain disulfide bridge is required for this allosteric interaction. Taking into account that these receptors are homodimers, this finding provides important new information explaining how the different conformations of the dimer of VFT lead to different signaling of such dimeric receptors.

Among the twenty classical L-amino acids, only glutamate directly activates metabotropic glutamate receptors

Under pathophysiological conditions, cellular amino acids can be profusely released from cells into the cerebral interstitial space. Because several class-C G protein coupled receptors (GPCRs) display a broad natural ligand spectrum, being sensitive to more than one endogenous ligand, we wondered whether the related metabotropic glutamate (mGlu) receptors could be modulated by various types of L-amino acids, allowing them to sense large increase in extracellular amino acid concentration. Here, the agonist, antagonist and allosteric effects of the twenty classical L-amino acids were evaluated on the eight mGlu receptor subtypes. We show that, in addition to glutamate (Glu), cysteine, aspartate and asparagine also lead to the activation of mGlu3, 4 and 5. Interestingly, our data demonstrate that the effect of these three amino acids did not result from a direct activation of the receptors, but from an indirect action involving Glu-transporters/exchangers. These data first demonstrate that mGlu receptors, unlike other class-C GPCRs, display an extremely high selectivity towards one ligand. Moreover, our results also show that Glu transport systems allow mGlu receptors to sense large increase in the extracellular concentration of some amino acids. Such a system will certainly lead to a large increase in some mGlu receptor activity under pathological conditions, such as seizure, ischemia or other brain injuries.

Evidence for a single heptahelical domain being turned on upon activation of a dimeric GPCR

G-protein-coupled receptors (GPCRs) have been shown to form dimers, but the relevance of this phenomenon in G-protein activation is not known. Among the large GPCR family, metabotropic glutamate (mGlu) receptors are constitutive dimers. Here we examined whether both heptahelical domains (HDs) are turned on upon full receptor activation. To that aim, we measured G-protein coupling efficacy of dimeric mGlu receptors in which one subunit bears specific mutations. We show that a mutation in the third intracellular loop (i3 loop) known to prevent G-protein activation in a single subunit decreases coupling efficacy. However, when a single HD is blocked in its inactive state using an inverse agonist, 2-methyl-6-(phenylethynyl)pyridine (MPEP), no decrease in receptor activity is observed. Interestingly, in a receptor dimer in which the subunit that binds MPEP is mutated in its i3 loop, MPEP enhances agonist-induced activity, reflecting a 'better' activation of the adjacent HD. These data are consistent with a model in which a single HD is turned on upon activation of such homodimeric receptors and raise important issues in deciphering the functional role of GPCR dimer formation for G-protein activation.

Heptahelical domain of metabotropic glutamate receptor 5 behaves like rhodopsin-like receptors

Although agonists bind directly in the heptahelical domain (HD) of most class-I rhodopsin-like G protein coupled receptors (GPCRs), class-III agonists bind in the extracellular domain of their receptors. Indeed, the latter possess a large extracellular domain composed of a cysteine-rich domain and a Venus flytrap module. Both the low sequence homology and the structural organization of class-III GPCRs raised the question of whether or not the HD of these receptors functions the same way as rhodopsin-like GPCRs. Here, we show that the HD of metabotropic glutamate receptor 5 (mGlu(5)) displays the same agonist-independent constitutive activity as the wild-type receptor. Moreover, we show that the noncompetitive antagonist MPEP [2-methyl-6-(phenylethynyl)-pyridine hydrochloride] and the positive allosteric modulator DFB (3,3'-difluorobenzaldazine) act as inverse agonist and full agonist, respectively, on the mGlu(5) HD in the absence of the extracellular domain. This finding illustrates that, like rhodopsin-like receptors, the HD of mGluRs can constitutively couple to G proteins and be negatively and positively regulated by ligands. These data show that the HD of mGluRs behave like any other class-I GPCRs in terms of G protein coupling and regulation by various types of ligands.

Up-regulation of fatty acid metabolizing-enzymes mRNA in rat spinal cord during persistent peripheral local inflammation

Persistent peripheral inflammation is associated with repetitive painful inputs into the spinal cord, leading to a chronic pain state. Related dramatic changes occur in the central nervous system (CNS) including central sensitization, which results in hyperalgesia. This neural plasticity involves in part fatty acids as functional and structural compounds. We hypothesized that central modification of fatty acids metabolism might occur after prolonged peripheral noxious stimulation. In the present study, the regulation of genes involved in fatty acids metabolism in the rat CNS was investigated during a chronic pain state. Using semiquantitative RT-PCR, we explored in the neuraxis the mRNA expression of brain acyl-CoA synthetases (ACS) and acyl-CoA oxidase (ACO), which are major fatty acid-metabolizing enzymes, following complete Freund's adjuvant (CFA) injection into a hind paw. Similar spinal up-regulation of the isoforms ACS2, ACS3, ACS4, and of ACO was detected early after 30 min, reaching a maximal after 6 h post-injection. Other peaks were also observed after 4 and 21 days post-inoculation, corresponding to the acute and chronic inflammation, respectively. Induction occurred only in the lumbar spinal cord ipsilaterally to the inflamed paw and was completely inhibited by a local anaesthesia of the sciatic nerve, suggesting a neural transmission of the inducing signal. Moreover, intrathecal injection of MK801, a noncompetitive NMDA antagonist, partially prevented these inductions, highlighting the involvement of the neurotransmitter glutamate in the central ACS and ACO up-regulation. These findings suggest that the fatty metabolism is stimulated in the CNS during a chronic pain state.