The DRY motif as a molecular switch of the human oxytocin receptor

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

Targeting the Oxytocin Receptor for Breast Cancer Management: A Niche for Peptide Tracers

Breast cancer is a leading cause of death in women, and its management highly depends on early disease diagnosis and monitoring. This remains challenging due to breast cancer's heterogeneity and a scarcity of specific biomarkers that could predict responses to therapy and enable personalized treatment. This Perspective describes the diagnostic landscape for breast cancer management, molecular strategies targeting receptors overexpressed in tumors, the theranostic potential of the oxytocin receptor (OTR) as an emerging breast cancer target, and the development of OTR-specific optical and nuclear tracers to study, visualize, and treat tumors. A special focus is on the chemistry and pharmacology underpinning OTR tracer development, preclinical in vitro and in vivo studies, challenges, and future directions. The use of peptide-based tracers targeting upregulated receptors in cancer is a highly promising strategy complementing current diagnostics and therapies and providing new opportunities to improve cancer management and patient survival.

Mechanism of negative membrane curvature generation by I-BAR domains

The membrane sculpting ability of BAR domains has been attributed to the intrinsic curvature of their banana-shaped dimeric structure. However, there is often a mismatch between this intrinsic curvature and the diameter of the membrane tubules generated. I-BAR domains are especially mysterious since they are almost flat but generate high negative membrane curvature. Here, we use atomistic implicit-solvent computer modeling to show that the membrane bending of the IRSp53 I-BAR domain is dictated by its higher oligomeric structure, whose curvature is completely unrelated to the intrinsic curvature of the dimer. Two other I-BARs give similar results, whereas a flat F-BAR sheet develops a concave membrane-binding interface, consistent with its observed positive membrane curvature generation. Laterally interacting helical spirals of I-BAR dimers on tube interiors are stable and have an enhanced binding energy that is sufficient for membrane bending to experimentally observed tubule diameters at a reasonable surface density.

Hypothalamic Neuropeptide Brain Protection: Focus on Oxytocin

Oxytocin (OXT) is hypothalamic neuropeptide synthetized in the brain by magnocellular and parvo cellular neurons of the paraventricular (PVN), supraoptic (SON) and accessory nuclei (AN) of the hypothalamus. OXT acts in the central and peripheral nervous systems via G-protein-coupled receptors. The classical physiological functions of OXT are uterine contractions, the milk ejection reflex during lactation, penile erection and sexual arousal, but recent studies have demonstrated that OXT may have anti-inflammatory and anti-oxidant properties and regulate immune and anti-inflammatory responses. In the pathogenesis of various neurodegenerative diseases, microglia are present in an active form and release high levels of pro-inflammatory cytokines and chemokines that are implicated in the process of neural injury. A promising treatment for neurodegenerative diseases involves new therapeutic approaches targeting activated microglia. Recent studies have reported that OXT exerts neuroprotective effects through the inhibition of production of pro-inflammatory mediators, and in the development of correct neural circuitry. The focus of this review is to attribute a new important role of OXT in neuroprotection through the microglia–OXT interaction of immature and adult brains. In addition, we analyzed the strategies that could enhance the delivery of OXT in the brain and amplify its positive effects.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

Molecular modeling and docking studies of the oxytocin receptor

Aim: Low oxytocin (OT) level is involved in a number of psychiatric diseases, indicating that OT could be used to aid treating these disorders. OT itself is unable to cross the blood–brain barrier, and development of new small nonpeptide drugs targeting the OT receptor (OXTR) may be beneficial for treating mental disorders. Results & methodology: Three OXTR models were constructed based on crystallized homologous proteins (Protein Data Bank [PDB]: 2Y00, PDB: 4BVN and PDB: 4LDE). The abilities of the models to discriminate between true binders and decoys were analyzed using receiver operating characteristics curves, and the 4LDE-based model gave the best result. Conclusion: The present study demonstrates that the 4LDE-based model may be suitable as a tool for the development of novel drugs targeting OXTR.

Molecular Basis of Oxytocin Receptor Signalling in the Brain: What We Know and What We Need to Know

Oxytocin (OT), a hypothalamic neuropeptide involved in regulating the social behaviour of all vertebrates, has been proposed as a treatment for a number of neuropsychiatric disorders characterised by deficits in the social domain. Over the last few decades, advances focused on understanding the social effects of OT and its role in physiological conditions and brain diseases, but much less has been done to clarify the molecular cascade of events involved in mediating such effects and in particular the cellular and molecular pharmacology of OT and its target receptor (OTR) in neuronal and glial cells.The entity and persistence of OT activity in the brain is closely related to the expression and regulation of the OTR expressed on the cell surface, which transmits the signal intracellularly and permits OT to affect cell function. Understanding the various signalling mechanisms mediating OTR-induced cell responses is crucial to determine the different responses in different cells and brain regions, and the success of OT and OT-derived analogues in the treatment of neurodevelopmental and psychiatric diseases depends on how well we can control such responses. In this review, we will consider the most important aspects of OT/OTR signalling by focusing on the molecular events involved in OT binding and coupling, on the main signalling pathways activated by the OTR in neuronal cells and on intracellular and plasma membrane OTR trafficking, all of which contribute to the quantitative and qualitative features of OT responses in the brain.

Carbetocin is a Functional Selective Gq Agonist That Does Not Promote Oxytocin Receptor Recycling After Inducing β-Arrestin-Independent Internalisation

Carbetocin, a long-acting oxytocin analogue, has been reported to elicit interesting and peculiar behavioural effects. The present study investigated the molecular pharmacology of carbetocin, aiming to better understand the molecular basis of its action in the brain. Using bioluminescence resonance energy transfer biosensors, we characterised the effects of carbetocin on the three human oxytocin/vasopressin receptors expressed in the nervous system: the oxytocin receptor (OXTR) and the vasopressin V1a (V1aR) and V1b (V1bR) receptors. Our results indicate that (i) carbetocin activates the OXTR but not the V1aR and V1bR at which it may act as an antagonist; (ii) carbetocin selectively activates only the OXTR/Gq pathway displaying a strong functional selectivity; (iii) carbetocin is a partial agonist at the OXTR/Gq coupling; (iv) carbetocin promotes OXTR internalisation via a previously unreported β-arrestin-independent pathway; and (v) carbetocin does not induce OXTR recycling to the plasma membrane. Altogether, these molecular pharmacology features identify carbetocin as a substantially different analogue compared to the endogenous oxytocin and, consequently, carbetocin is not expected to mimic oxytocin in the brain. Whether these unique features of carbetocin could be exploited therapeutically remains to be established.

Role of FQQI motif in the internalization, trafficking, and signaling of guanylyl-cyclase/natriuretic peptide receptor-A in cultured murine mesangial cells

Binding of the cardiac hormone atrial natriuretic peptide (ANP) to transmembrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), produces the intracellular second messenger cGMP in target cells. To delineate the critical role of an endocytic signal in intracellular sorting of the receptor, we have identified a FQQI (Phe(790), Gln(791), Gln(792), and Ile(793)) motif in the carboxyl-terminal region of NPRA. Mouse mesangial cells (MMCs) were transiently transfected with the enhanced green fluorescence protein (eGFP)-tagged wild-type (WT) and mutant constructs of eGFP-NPRA. The mutation FQQI/AAAA, in the eGFP-NPRA cDNA sequence, markedly attenuated the internalization of mutant receptors by almost 49% compared with the WT receptor. Interestingly, we show that the μ1B subunit of adaptor protein-1 binds directly to a phenylalanine-based FQQI motif in the cytoplasmic tail of the receptor. However, subcellular trafficking indicated that immunofluorescence colocalization of the mutated receptor with early endosome antigen-1 (EEA-1), lysosome-associated membrane protein-1 (LAMP-1), and Rab 11 marker was decreased by 57% in early endosomes, 48% in lysosomes, and 42% in recycling endosomes, respectively, compared with the WT receptor in MMCs. The receptor containing the mutated motif (FQQI/AAAA) also produced a significantly decreased level of intracellular cGMP during subcellular trafficking than the WT receptor. The coimmunoprecipitation assay confirmed a decreased level of colocalization of the mutant receptor with subcellular compartments during endocytic processes. The results suggest that the FQQI motif is essential for the internalization and subcellular trafficking of NPRA during the hormone signaling process in intact MMCs.

A structure-based approach to understanding somatostatin receptor-4 agonism (sst4)

It has been reported that somatostatin receptor subtypes 4 and 5 would be high-impact templates for homology modeling if their 3D structures became available. We have generated a homology model of the somatostatin receptor subtype 4 (sst4), using the newest active state β(2) adrenoreceptor crystal structure, and subsequently docked a variety of agonists into the model-built receptor to elucidate the binding modes of reported agonists. Using experimental restraints, we were able to explain observed activity profiles. We propose two binding modes that can consistently explain findings for high-affinity agonists and reason why certain structures display low affinities for the receptor.

Functional selective oxytocin-derived agonists discriminate between individual G protein family subtypes

We used a bioluminescence resonance energy transfer biosensor to screen for functional selective ligands of the human oxytocin (OT) receptor. We demonstrated that OT promoted the direct engagement and activation of G(q) and all the G(i/o) subtypes at the OT receptor. Other peptidic analogues, chosen because of specific substitutions in key OT structural/functional residues, all showed biased activation of G protein subtypes. No ligand, except OT, activated G(oA) or G(oB), and, with only one exception, all of the peptides that activated G(q) also activated G(i2) and G(i3) but not G(i1), G(oA), or G(oB), indicating a strong bias toward these subunits. Two peptides (DNalOVT and atosiban) activated only G(i1) or G(i3), failed to recruit β-arrestins, and did not induce receptor internalization, providing the first clear examples of ligands differentiating individual G(i/o) family members. Both analogs inhibited cell proliferation, showing that a single G(i) subtype-mediated pathway is sufficient to prompt this physiological response. These analogs represent unique tools for examining the contribution of G(i/o) members in complex biological responses and open the way to the development of drugs with peculiar selectivity profiles. This is of particular relevance because OT has been shown to improve symptoms in neurodevelopmental and psychiatric disorders characterized by abnormal social behaviors, such as autism. Functional selective ligands, activating a specific G protein signaling pathway, may possess a higher efficacy and specificity on OT-based therapeutics.

New, potent, selective, and short-acting peptidic V1a receptor agonists

[Arg(8)]vasopressin (AVP) produces vasoconstriction via V(1a) receptor (V(1a)R)-mediated vascular smooth muscle cell contraction and is being used to increase blood pressure in septic shock, a form of vasodilatory hypotension. However, AVP also induces V(2) receptor (V(2)R)-mediated antidiuresis, vasodilation, and coagulation factor release, all deleterious in septic shock. The V(1a)R agonist terlipressin (H-Gly(3)[Lys(8)]VP) also lacks selectivity vs the V(2)R and has sizably longer duration of action than AVP, preventing rapid titration of its vasopressor effect in the clinic. We designed and synthesized new short acting V(1a)R selective analogues of general structure [Xaa(2),Ile(3),Yaa(4),Zaa(8)]VP. The most potent and selective compounds in in vitro functional assays (e.g., [Phe(2),Ile(3),Asn(Me(2))(4),Orn(8)]VP (31), [Phe(2),Ile(3),Asn((CH(2))(3)OH)(4),Orn(8)]VP (34), [Phe(2),Ile(3),Hgn(4),Orn(iPr)(8)]VP (45), [Phe(2),Ile(3),Asn(Et)(4),Dab(8)]VP (49), [Thi(2),Ile(3),Orn(iPr)(8)]VP (59), [Cha(2),Ile(3),Asn(4),Orn(iPr)(8)]VP (68)) were tested by intravenous bolus in rats for duration of vasopressive action. Analogues 31, 34, 45, and 49 were as short-acting as AVP. Compound 45, FE 202158, is currently undergoing clinical trials in septic shock.

Prolactin-releasing peptide receptor expressed in the pituitary in Mozambique tilapia Oreochromis mossambicus: an aspect of prolactin regulatory mechanisms

Prolactin (PRL)-releasing peptide (PrRP) has been reported as a strong candidate for a stimulating factor of both PRL secretion and expression in teleost species; however, there is no information available on its receptor. Here we report cDNA cloning and characterization of PrRP receptor expressed in the pituitary of Mozambique tilapia Oreochromis mossambicus. The deduced amino acid sequence of cDNA for tilapia PrRP receptor shared 50-83% homology with other vertebrate homologs. Intracellular calcium mobilization assay revealed that PrRP receptor responded to as low as 1nM order of tilapia PrRP, indicating its high affinity to PrRP. The expression of PrRP receptor was detected in the brain, pituitary, heart, spleen, kidney and rectum of freshwater (FW)- and seawater (SW)-adapted fish. There was no significant difference between FW and SW fish in transcription levels of PrRP receptor in the rostral pars distalis (RPD) of the pituitary. Similarly, the PrRP expression level in the whole brain was not changed by environmental salinity. Immunohistochemistry with a specific antibody showed that PrRP receptor was mainly localized in the cells of the RPD and neurohypophysis in the pituitary of both FW and SW tilapia. We also examined the effects of PrRP on PRL expression in primary-incubated PRL cells of FW tilapia; PrRP failed to stimulate PRL expression in PRL cells in vitro. These results suggest that in vivo stimulatory effects of PrRP on PRL gene expression reported in teleosts are presumably mediated by an unknown regulator secreted from the neurohypophysis expressing PrRP receptor.

Human Cytomegalovirus US28: a functionally selective chemokine binding receptor

Chemokines are small cytokines that are part of a large family of molecules that bind to G-protein coupled receptors, which, as a family, are the most widely targeted group of molecules in the treatment of disease. Chemokines are critical for recruiting and activating the cells of the immune system during inflammation especially during viral infections. However, a number of viruses including the large herpes virus human cytomegalovirus (HCMV) encode mechanisms to impede the effects of chemokines or has gained the ability to use these molecules to its own advantage. The Human Cytomegalovirus (HCMV)-encoded chemokine receptor US28 is the best characterized of the four unique chemokine receptor-like molecules found in the HCMV genome. US28 has been studied as an important virulence factor for HCMV-mediated vascular disease and, more recently, in models of HCMV-associated malignancy. US28 is a rare multi-chemokine family binding receptor with the ability to bind ligands from two distinct chemokine classes. Ligand binding to US28 activates cell-type and ligand-specific signaling pathways leading to cellular migration, which is an important example of receptor functional selectivity. Additionally, US28 has been demonstrated to constitutively activate phospholipase C (PLC) and NF-kB signaling pathways. Understanding the structure/function relationships between US28, its ligands and intracellular signaling molecules will provide essential clues for effective pharmacological targeting of this multifunctional chemokine receptor.

Src phosphorylation of micro-receptor is responsible for the receptor switching from an inhibitory to a stimulatory signal

Recent studies have revealed that in G protein-coupled receptor signalings switching between G protein- and beta-arrestin (betaArr)-dependent pathways occurs. In the case of opioid receptors, the signal is switched from the initial inhibition of adenylyl cyclase (AC) to an increase in AC activity (AC activation) during prolonged agonist treatment. The mechanism of such AC activation has been suggested to involve the switching of G proteins activated by the receptor, phosphorylation of signaling molecules, or receptor-dependent recruitment of cellular proteins. Using protein kinase inhibitors, dominant negative mutant studies and mouse embryonic fibroblast cells isolated from Src kinase knock-out mice, we demonstrated that mu-opioid receptor (OPRM1)-mediated AC activation requires direct association and activation of Src kinase by lipid raft-located OPRM1. Such Src activation was independent of betaArr as indicated by the ability of OPRM1 to activate Src and AC after prolonged agonist treatment in mouse embryonic fibroblast cells lacking both betaArr-1 and -2. Instead the switching of OPRM1 signals was dependent on the heterotrimeric G protein, specifically Gi2 alpha-subunit. Among the Src kinase substrates, OPRM1 was phosphorylated at Tyr336 within NPXXY motif by Src during AC activation. Mutation of this Tyr residue, together with mutation of Tyr166 within the DRY motif to Phe, resulted in the complete blunting of AC activation. Thus, the recruitment and activation of Src kinase by OPRM1 during chronic agonist treatment, which eventually results in the receptor tyrosine phosphorylation, is the key for switching the opioid receptor signals from its initial AC inhibition to subsequent AC activation.

Identification of Tyr(290(6.58)) of the human gonadotropin-releasing hormone (GnRH) receptor as a contact residue for both GnRH I and GnRH II: importance for high-affinity binding and receptor activation

Molecular modeling showed interactions of Tyr (290(6.58)) in transmembrane domain 6 of the GnRH receptor with Tyr (5) of GnRH I, and His (5) of GnRH II. The wild-type receptor exhibited high affinity for [Phe (5)]GnRH I and [Tyr (5)]GnRH II, but 127- and 177-fold decreased affinity for [Ala (5)]GnRH I and [Ala (5)]GnRH II, indicating that the aromatic ring in position 5 is crucial for receptor binding. The receptor mutation Y290F decreased affinity for GnRH I, [Phe (5)]GnRH I, GnRH II and [Tyr (5)]GnRH II, while Y290A and Y290L caused larger decreases, suggesting that both the para-OH and aromatic ring of Tyr (290(6.58)) are important for binding of ligands with aromatic residues in position 5. Mutating Tyr (290(6.58)) to Gln increased affinity for Tyr (5)-containing GnRH analogues 3-12-fold compared with the Y290A and Y290L mutants, suggesting a hydrogen-bond between Gln of the Y290Q mutant and Tyr (5) of GnRH analogues. All mutations had small effects on affinity of GnRH analogues that lack an aromatic residue in position 5. These results support direct interactions of the Tyr (290(6.58)) side chain with Tyr (5) of GnRH I and His (5) of GnRH II. Tyr (290(6.58)) mutations, except for Y290F, caused larger decreases in GnRH potency than affinity, indicating that an aromatic ring is important for the agonist-induced receptor conformational switch.

Gonadotropin-releasing hormone analog structural determinants of selectivity for inhibition of cell growth: support for the concept of ligand-induced selective signaling

GnRH and its receptor are expressed in human reproductive tract cancers, and direct antiproliferative effects of GnRH analogs have been demonstrated in cancer cell lines. The intracellular signaling responsible for this effect differs from that mediating pituitary gonadotropin secretion. The GnRH structure-activity relationship is different for the two effects. Here we report a structure-activity relationship study of GnRH agonist antiproliferative action in model cell systems of rat and human GnRH receptors stably expressed in HEK293 cells. GnRH II was more potent than GnRH I in inhibiting cell growth in the cell lines. In contrast, GnRH I was more potent than GnRH II in stimulating inositol phosphate production, the signaling pathway in gonadotropes. The different residues in GnRH II (His(5), Trp(7), Tyr(8)) were introduced singly or in pairs into GnRH I. Tyr(5) replacement by His(5) produced the highest increase in the antiproliferative potency of GnRH I. Tyr(8) substitution of Arg(8) produced the most selective analog, with very poor inositol phosphate generation but high antiproliferative potency. In nude mice bearing tumors of the HEK293 cell line, GnRH II and an antagonist administration was ineffective in inhibiting tumor growth, but D-amino acid stabilized analogs (D-Lys(6) and D-Arg(6)) ablated tumor growth. Docking of GnRH I and GnRH II to the human GnRH receptor molecular model revealed that Arg(8) of GnRH I makes contact with Asp(302), whereas Tyr(8) of GnRH II appears to make different contacts, suggesting these residues stabilize different receptor conformations mediating differential intracellular signaling and effects on gonadotropin and cell growth. These findings provide the basis for the development of selective GnRH analog cancer therapeutics that directly target tumor cells or inhibit pituitary gonadotropins or do both.

Biological properties and functional determinants of the urotensin II receptor

The urotensin II receptor (UT) is a member of the G protein-coupled receptor (GPCR) family and binds the cyclic undecapeptide urotensin II (U-II) as well as the octapeptide urotensin II-related peptide (URP). The active UT mediates pleiotropic effects through various signal transduction pathways, including coupling to G proteins and activating the mitogen-activated protein kinase pathway. Several highly conserved residues and motifs of class A GPCRs that are important for activity are found in UT. This review highlights some of the putative roles of these motifs in the binding, activation and desensitization of UT.

Functional analysis of the murine cytomegalovirus chemokine receptor homologue M33: ablation of constitutive signaling is associated with an attenuated phenotype in vivo

The murine cytomegalovirus (MCMV) M33 gene is conserved among all betaherpesviruses and encodes a homologue of seven-transmembrane receptors (7TMR) with the capacity for constitutive signaling. Previous studies have demonstrated that M33 is important for MCMV dissemination to or replication within the salivary glands. In this study, we probed N- and C-terminal regions of M33 as well as known 7TMR signature motifs in transmembrane (TM) II and TM III to determine the impact on cell surface expression, constitutive signaling, and in vivo phenotype. The region between amino acids R(340) and A(353) of the C terminus was found to be important for CREB- and NFAT-mediated signaling, although not essential for phosphatidylinositol turnover. Tagging or truncation of the N terminus of M33 resulted in loss of cell surface expression. Within TM II, an F79D mutation abolished constitutive signaling, demonstrating a role, as in other cellular and viral 7TMR, of TM II in receptor activation. In TM III, the arginine (but not the asparagine) residue of the NRY motif (the counterpart of the common DRY motif in cellular 7TMR) was found to be essential for constitutive signaling. Selected mutations incorporated into recombinant MCMV showed that disruption of constitutive signaling for a viral 7TMR homologue resulted in a reduced capacity to disseminate to or replicate in the salivary glands. In addition, HCMV UL33 was found to partially compensate for the lack of M33 in vivo, suggesting conserved biological roles of the UL33 gene family.

Diversity of actions of GnRHs mediated by ligand-induced selective signaling

Geoffrey Wingfield Harris' demonstration of hypothalamic hormones regulating pituitary function led to their structural identification and therapeutic utilization in a wide spectrum of diseases. Amongst these, Gonadotropin Releasing Hormone (GnRH) and its analogs are widely employed in modulating gonadotropin and sex steroid secretion to treat infertility, precocious puberty and many hormone-dependent diseases including endometriosis, uterine fibroids and prostatic cancer. While these effects are all mediated via modulation of the pituitary gonadotrope GnRH receptor and the G(q) signaling pathway, it has become increasingly apparent that GnRH regulates many extrapituitary cells in the nervous system and periphery. This review focuses on two such examples, namely GnRH analog effects on reproductive behaviors and GnRH analog effects on the inhibition of cancer cell growth. For both effects the relative activities of a range of GnRH analogs is distinctly different from their effects on the pituitary gonadotrope and different signaling pathways are utilized. As there is only a single functional GnRH receptor type in man we have proposed that the GnRH receptor can assume different conformations which have different selectivity for GnRH analogs and intracellular signaling proteins complexes. This ligand-induced selective-signaling recruits certain pathways while by-passing others and has implications in developing more selective GnRH analogs for highly specific therapeutic intervention.

Modeling the human oxytocin receptor for drug discovery efforts

The oxytocin receptor belongs to class A receptors within the great family of G protein-coupled receptors. The endogenous ligand oxytocin is a nonapeptide hormone that induces the uterine contractions at parturition and is used to induce the labor. The peptide oxytocin and, even more, its non-peptide antagonist, could be valuable tools in tocolysis. The knowledge of the three-dimensional structure of the oxytocin receptor and the determination of the main interaction points between the receptor and the ligands may help to develop selective oxytocin agonists and antagonist. This review summarizes the knowledge about the mapping of the binding domain of the oxytocin receptor and the efforts in the field of molecular modeling studies related to oxytocin receptor-ligand interactions.

G protein-coupled receptor-mediated ERK1/2 phosphorylation: towards a generic sensor of GPCR activation

The development of new analytical methods, aimed at profiling G protein-coupled receptor (GPCR) ligands, regardless of the G protein-coupling pattern of their respective receptor, remains a key goal in drug discovery. Considerable evidence has recently revived the central role that could be played by extracellular-signal-regulated kinase (ERK), the cornerstone protein kinase of the first tyrosine kinase receptor-mediated pathway identified, in response to the activation of various types of GPCRs. Here we reveal a conceptual study in which the potential of ERK phosphorylation is evaluated as a generic readout in response to three different receptors activating three main classes of G proteins: Galphas, Galphai and Galphaq. GPCR-mediated ERK phosphorylation was compared with different readouts such as GTPgammaS, CAMP, or Ca2 +. We propose the measurement of GPCR-activated ERK phosphorylation as an alternative assay to better understand the molecular pharmacology of ligands of promiscuous GPCRs.

Structural determinants for ligand-receptor conformational selection in a peptide G protein-coupled receptor

G protein coupled receptors (GPCRs) modulate the majority of physiological processes through specific intermolecular interactions with structurally diverse ligands and activation of differential intracellular signaling. A key issue yet to be resolved is how GPCRs developed selectivity and diversity of ligand binding and intracellular signaling during evolution. We have explored the structural basis of selectivity of naturally occurring gonadotropin-releasing hormones (GnRHs) from different species in the single functional human GnRH receptor. We found that the highly variable amino acids in position 8 of the naturally occurring isoforms of GnRH play a discriminating role in selecting receptor conformational states. The human GnRH receptor has a higher affinity for the cognate GnRH I but a lower affinity for GnRH II and GnRHs from other species possessing substitutions for Arg(8). The latter were partial agonists in the human GnRH receptor. Mutation of Asn(7.45) in transmembrane domain (TM) 7 had no effect on GnRH I affinity but specifically increased affinity for other GnRHs and converted them to full agonists. Using molecular modeling and site-directed mutagenesis, we demonstrated that the highly conserved Asn(7.45) makes intramolecular interactions with a highly conserved Cys(6.47) in TM 6, suggesting that disruption of this intramolecular interaction induces a receptor conformational change which allosterically alters ligand specific binding sites and changes ligand selectivity and signaling efficacy. These results reveal GnRH ligand and receptor structural elements for conformational selection, and support co-evolution of GnRH ligand and receptor conformations.

Amino acids in the COOH-terminal region of the oxytocin receptor third intracellular domain are important for receptor function

Previously, residue K6.30 in the COOH-terminal region of the third intracellular domain (3iC) of the oxytocin (OT) receptor (OTR) was identified as important for receptor function leading to phospholipase C activation in both OTR and the vasopressin V(2) receptor (V(2)R) chimera V(2)ROTR3iC. Substitution of either A6.28K or V6.30K in wild-type V(2)R did not recapitulate the increase in phosphatidylinositide (PI) turnover observed in V(2)ROTR3iC. Hence, the role of K6.30 may be context-specific. Deletion of two NH(2)-terminal OTR3iC segments in the V(2)ROTR3iC chimera did not diminish vasopressin-stimulated PI turnover, whereas deletion of RVSSVKL (residues 6.19-6.25) reduced receptor expression. Deletion of this sequence in wild-type OTR reduced expression by 50% without affecting affinity for [(3)H]OT. This OTR mutant was unable to activate PI turnover or extracellular signal-regulated kinase 1/2 phosphorylation. The effects of alanine substitution for individual residues in RVSSVKL indicated differential importance for OTR function. The R6.19A substitution lost high-affinity sites for [(3)H]OT and the ability to stimulate PI turnover. Affinity for [(3)H]OT and membrane expression was not affected by any other substitutions. OTR-V6.20A and OTR-K6.24A mutants functioned as well as wild-type OTR, whereas OTR S6.21A, S6.22A, and V6.23A mutants exhibited impaired abilities to activate PI turnover (20-40% of OTR), and the OTR-L6.25A mutant exhibited constitutive activity. In conclusion, specific amino acids in the RVSSVKL segment in the COOH-terminal region of the third intracellular domain of OTR influence the ability of OTR to activate G protein-mediated actions.

The highly conserved DRY motif of class A G protein-coupled receptors: beyond the ground state

Despite extensive study of heptahelical G protein-coupled receptors (GPCRs), the precise mechanism of G protein activation is unknown. The role of one highly conserved stretch of residues, the amino acids glutamic acid/aspartic acid-arginine-tyrosine (i.e., the E/DRY motif), has received considerable attention with respect to regulating GPCR conformational states. In the consensus view, glutamic acid/aspartic acid maintains the receptor in its ground state, because mutations frequently induce constitutive activity (CA). This hypothesis has been confirmed by the rhodopsin ground-state crystal structure and by computational modeling approaches. However, some class A GPCRs are resistant to CA, suggesting alternative roles for the glutamic acid/aspartic acid residue and the E/DRY motif. Here, we propose two different subgroups of receptors within class A GPCRs that make different use of the E/DRY motif, independent of the G protein type (G(s), G(i), or G(q)) to which the receptor couples. In phenotype 1 receptors, nonconservative mutations of the glutamic acid/aspartic acid-arginine residues, besides inducing CA, increase affinity for agonist binding, retain G protein coupling, and retain an agonist-induced response. In contrast, in second phenotype receptors, the E/DRY motif is more directly involved in governing receptor conformation and G protein coupling/recognition. Hence, mutations of the glutamic acid/aspartic acid residues do not induce CA. Conversely, nonconservative mutations of the arginine of the E/DRY motif always impair agonist-induced receptor responses and, generally, reduce agonist binding affinity. Thus, it is essential to look beyond the rhodopsin ground-state model of conformational activation to clarify the role of this highly conserved triplet in GPCR activation and function.

Inactive and active states and supramolecular organization of GPCRs: insights from computational modeling

Herein we make an overview of the results of our computational experiments aimed at gaining insight into the molecular mechanisms of GPCR functioning either in their normal conditions or when hit by gain-of-function or loss-of-function mutations. Molecular simulations of a number of GPCRs in their wild type and mutated as well as free and ligand-bound forms were instrumental in inferring the structural features, which differentiate the mutation- and ligand-induced active from the inactive states. These features essentially reside in the interaction pattern of the E/DRY arginine and in the degree of solvent exposure of selected cytosolic domains. Indeed, the active states differ from the inactive ones in the weakening of the interactions made by the highly conserved arginine and in the increase in solvent accessibility of the cytosolic interface between helices 3 and 6. Where possible, the structural hallmarks of the active and inactive receptor states are translated into molecular descriptors useful for in silico functional screening of novel receptor mutants or ligands. Computational modeling of the supramolecular organization of GPCRs and their intracellular partners is the current challenge toward a deep understanding of their functioning mechanisms.

Mutations of CB1 T210 produce active and inactive receptor forms: correlations with ligand affinity, receptor stability, and cellular localization

Human cannabinoid receptor 1 (CB(1)) has attracted substantial interest as a potential therapeutic target for treating obesity and other obsessive disorders. An understanding of the mechanism governing the transition of the CB(1) receptor between its inactive and active states is critical for understanding how therapeutics can selectively regulate receptor activity. We have examined the importance of the Thr at position 210 in CB(1) in this transition, a residue predicted to be on the same face of the helix as the Arg of the DRY motif highly conserved in the G protein-coupled receptor superfamily. This Thr was substituted with Ile and Ala via mutagenesis, and the receptors, T210I and T210A, were expressed in HEK 293 cells. The T210I receptor exhibited enhanced agonist and diminished inverse agonist affinity relative to the wild type, consistent with a shift toward the active form. However, treatment with GTPgammaS to inhibit G protein coupling diminished the affinity change for the inverse agonist SR141716A. The decreased thermal stability of the T210I receptor and increased level of internalization of a T210I receptor-GFP chimera were also observed, consistent with constitutive activity. In contrast, the T210A receptor exhibited the opposite profile: diminished agonist and enhanced inverse agonist affinity. The T210A receptor was found to be more thermally stable than the wild type, and high levels of a T210A receptor-GFP chimera were localized to the cell surface as predicted for an inactive receptor form. These results suggest that T210 plays a key role in governing the transition between inactive and active CB(1) receptor states.