Structure of the third intracellular loop of the vasopressin V2 receptor and conformational changes upon binding to gC1qR

Bringing GPCR Structural Biology to Medical Applications: Insights from Both V2 Vasopressin and Mu-Opioid Receptors

G-protein coupled receptors (GPCRs) are versatile signaling proteins that regulate key physiological processes in response to a wide variety of extracellular stimuli. The last decade has seen a revolution in the structural biology of clinically important GPCRs. Indeed, the improvement in molecular and biochemical methods to study GPCRs and their transducer complexes, together with advances in cryo-electron microscopy, NMR development, and progress in molecular dynamic simulations, have led to a better understanding of their regulation by ligands of different efficacy and bias. This has also renewed a great interest in GPCR drug discovery, such as finding biased ligands that can either promote or not promote specific regulations. In this review, we focus on two therapeutically relevant GPCR targets, the V2 vasopressin receptor (V2R) and the mu-opioid receptor (µOR), to shed light on the recent structural biology studies and show the impact of this integrative approach on the determination of new potential clinical effective compounds.

Structure, dynamics and lipid interactions of serotonin receptors: excitements and challenges

Serotonin (5-hydroxytryptamine, 5-HT) is an intrinsically fluorescent neurotransmitter found in organisms spanning a wide evolutionary range. Serotonin exerts its diverse actions by binding to distinct cell membrane receptors which are classified into many groups. Serotonin receptors are involved in regulating a diverse array of physiological signaling pathways and belong to the family of either G protein-coupled receptors (GPCRs) or ligand-gated ion channels. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions such as sleep, mood, pain, anxiety, depression, aggression, and learning. Serotonin receptors act as drug targets for a number of diseases, particularly neuropsychiatric disorders. The signaling mechanism and efficiency of serotonin receptors depend on their amazing ability to rapidly access multiple conformational states. This conformational plasticity, necessary for the wide variety of functions displayed by serotonin receptors, is regulated by binding to various ligands. In this review, we provide a succinct overview of recent developments in generating and analyzing high-resolution structures of serotonin receptors obtained using crystallography and cryo-electron microscopy. Capturing structures of distinct conformational states is crucial for understanding the mechanism of action of these receptors, which could provide important insight for rational drug design targeting serotonin receptors. We further provide emerging information and insight from studies on interactions of membrane lipids (such as cholesterol) with serotonin receptors. We envision that a judicious combination of analysis of high-resolution structures and receptor-lipid interaction would allow a comprehensive understanding of GPCR structure, function and dynamics, thereby leading to efficient drug discovery.

Complement component 1q subcomponent binding protein in the brain of the rat

Complement component 1q subcomponent binding protein (C1qbp) is a multifunctional protein involved in immune response, energy homeostasis of cells as a plasma membrane receptor, and a nuclear, cytoplasmic or mitochondrial protein. Recent reports suggested its neuronal function, too, possibly in axon maintenance, synaptic function, and neuroplasticity. Therefore, we addressed to identify C1qbp in the rat brain using in situ hybridization histochemistry and immunolabelling at light and electron microscopic level. C1qbp has a topographical distribution in the brain established by the same pattern of C1qbp mRNA-expressing and protein-containing neurons with the highest abundance in the cerebral cortex, anterodorsal thalamic nucleus, hypothalamic paraventricular (PVN) and arcuate nuclei, spinal trigeminal nucleus. Double labelling of C1qbp with the neuronal marker NeuN, with the astrocyte marker S100, and the microglia marker Iba1 demonstrated the presence of C1qbp in neurons but not in glial cells in the normal brain, while C1qbp appeared in microglia following their activation induced by focal ischemic lesion. Only restricted neurons expressed C1qbp, for example, in the PVN, magnocellular neurons selectively contained C1qbp. Further double labelling by using the mitochondria marker Idh3a antibody suggested the mitochondrial localization of C1qbp in the brain, confirmed by correlated light and electron microscopy at 3 different brain regions. Post-embedding immunoelectron microscopy also suggested uneven C1qbp content of mitochondria in different brain areas but also heterogeneity within single neurons. These data suggest a specific function of C1qbp in the brain related to mitochondria, such as the regulation of local energy supply in neuronal cells.

Molecular cloning, sequencing and phylogeny of vasotocin receptor genes in the air-breathing catfish Heteropneustes fossilis with sex dimorphic and seasonal variations in tissue expression

Vasotocin (VT) is the ortholog of vasopressin (VP) in non-mammalian vertebrates and is known for multiple functions. Teleost fishes have a complete repertoire of known VP/VT receptor subtypes (vasopressin type, VR): two V1A subtypes (V1Aa and V1Ab or V1a1 and V1a2) and five V2 subtypes (V2A1, V1A2, V2B1, V2B2 and V2C). Full-length cDNAs of v1a1, v1a2 and v2 (v2a1) with ORFs of 1,308, 1,137 and 1,527 bp, respectively, were cloned and characterized in the catfish Heteropneustes fossilis (Siluriformes, Ostariophysi). BLAST analysis revealed that the genes encoded three VT receptors, V1a1, V1a2 and V2 of 436, 379 and 509 amino acid residues, respectively. The predicted proteins showed typical features of the seven-transmembrane domain receptor core structure with hallmark triplets Asp-Arg-Tyr/Asp-Arg-His (DRY/DRH) and the variable intracellular loop III of vertebrate neurohypophysial hormone receptors. Phylogenetic analysis of the deduced protein sequences revealed that they clustered with the V1Aa, V1Ab and V2A1, respectively, of other teleosts. The V2R has a sequence identity of 70-76% with V2A1 than with the V2B type (sequence identity 43-49%). Semiquantitative PCR analysis showed that the receptor gene transcripts were expressed ubiquitously in the tissues examined (brain, pituitary, gonads, liver, muscle, kidney and gills) and displayed sex and seasonal fluctuations in a tissue-specific manner. The results form a basis for functional studies on the VT receptors in the catfish.

Membrane proteins structure and dynamics by nuclear magnetic resonance

Membrane proteins represent a challenging class of biological systems to study. They are extremely difficult to crystallize and in most cases they retain their structure and functions only in membrane environments. Therefore, commonly used diffraction methods fail to give detailed molecular structure and other approaches have to be utilized to obtain biologically relevant information. Nuclear magnetic resonance (NMR) spectroscopy, however, can provide powerful structural and dynamical constraints on these complicated systems. Solution- and solid-state NMR are powerful methods for investigating membrane proteins studies. In this work, we briefly review both solution and solid-state NMR techniques for membrane protein studies and illustrate the applications of these methods to elucidate proteins structure, conformation, topology, dynamics, and function. Recent advances in electronics, biological sample preparation, and spectral processing provided opportunities for complex biological systems, such as membrane proteins inside lipid vesicles, to be studied faster and with outstanding quality. New analysis methods therefore have emerged, that benefit from the combination of sample preparation and corresponding specific high-end NMR techniques, which give access to more structural and dynamic information.

Structure and dynamic properties of membrane proteins using NMR

Integral membrane proteins are one of the most challenging groups of macromolecules despite their apparent conformational simplicity. They manage and drive transport, circulate information, and participate in cellular movements via interactions with other proteins and through intricate conformational changes. Their structural and functional decoding is challenging and has imposed demanding experimental development. Solution nuclear magnetic resonance (NMR) spectroscopy is one of the techniques providing the capacity to make a significant difference in the deciphering of the membrane protein structure-function paradigm. The method has evolved dramatically during the last decade resulting in a plethora of new experiments leading to a significant increase in the scientific repertoire for studying membrane proteins. Besides solving the three-dimensional structures using state-of-the-art approaches, a large variety of developments of well-established techniques are available providing insight into membrane protein flexibility, dynamics, and interactions. Inspired by the speed of development in the application of new strategies, by invention of methods to measure solvent accessibility and describe low-populated states, this review seeks to introduce the vast possibilities solution NMR can offer to the study of membrane protein structure-function analyses with special focus on applicability.

Relevance of rhodopsin studies for GPCR activation

Rhodopsin, the dim-light photoreceptor present in the rod cells of the retina, is both a retinal-binding protein and a G protein-coupled receptor (GPCR). Due to this conjunction, it benefits from an arsenal of spectroscopy techniques that can be used for its characterization, while being a model system for the important family of Class A (also referred to as "rhodopsin-like") GPCRs. For instance, rhodopsin has been a crucial player in the field of GPCR structural biology. Until 2007, it was the only GPCR for which a high-resolution crystal structure was available, so all structure-activity analyses on GPCRs, from structure-based drug discovery to studies of structural changes upon activation, were based on rhodopsin. At present, about a third of currently available GPCR structures are still from rhodopsin. In this review, I show some examples of how these structures can still be used to gain insight into general aspects of GPCR activation. First, the analysis of the third intracellular loop in rhodopsin structures allows us to gain an understanding of the structural and dynamic properties of this region, which is absent (due to protein engineering or poor electron density) in most of the currently available GPCR structures. Second, a detailed analysis of the structure of the transmembrane domains in inactive, intermediate and active rhodopsin structures allows us to detect early conformational changes in the process of ligand-induced GPCR activation. Finally, the analysis of a conserved ligand-activated transmission switch in the transmembrane bundle of GPCRs in the context of the rhodopsin activation cycle, allows us to suggest that the structures of many of the currently available agonist-bound GPCRs may correspond to intermediate active states. While the focus in GPCR structural biology is inevitably moving away from rhodopsin, in other aspects rhodopsin is still at the forefront. For instance, the first studies of the structural basis of disease mutants in GPCRs, or the most detailed analysis of cellular GPCR signal transduction networks using a systems biology approach, have been carried out in rhodopsin. Finally, due again to its unique properties among GPCRs, rhodopsin will likely play an important role in the application of X-ray free electron laser crystallography to time-resolved structural biology in membrane proteins. Rhodopsin, thus, still remains relevant as a model system to study the molecular mechanisms of GPCR activation. This article is part of a Special Issue entitled: Retinal Proteins-You can teach an old dog new tricks.

Expression, purification and NMR characterization of the cyclic recombinant form of the third intracellular loop of the vasopressin type 2 receptor

The vasopressin type 2 (V2R) receptor belongs to the class of G-protein coupled receptors. It is mainly expressed in the membrane of kidney tubules, where it is activated by the extracellular arginine vasopressin. In men, inactivating and activating mutations cause nephrogenic diabetes insipidus and the nephrogenic syndrome of inappropriate antidiuresis respectively. Like most GPCRs, V2R's third intracellular loop (V2R-i3) is involved in the binding and activation of its major effector, the GαS protein. We overexpressed the V2R₂₂₄₋₂₇₄ fragment corresponding to V2R-i3 as a fusion protein with thioredoxin A at the N-terminus and a hexahistidine tag between the two proteins. Recombinant V2R-i3 was designed to harbor N- and C-terminal cysteines, in order to introduce a disulfide bond between N- and C-terminal extremities and hence reproduce the hairpin fold presumably present in the full-length receptor. The fusion protein was produced as inclusion bodies in Escherichia coli and purified by nickel affinity chromatography under denaturing conditions. After a refolding step, thioredoxin and hexahistidine tags were specifically cleaved with the tobacco etch virus protease. The hydrolysis yield, initially very low, increased up to 80% thanks to optimization of buffers and refolding methods. The cleaved fragment, V2₂₂₄₋₂₇₄, devoid of any tag, was then eluted with low imidazole concentrations in a second nickel affinity chromatography in denaturing conditions. The final yield was sufficient to prepare a ¹⁵N-¹³C labeled NMR sample suitable for triple resonance experiments. We assigned all NMR resonances and confirmed the correct peptide sequence. As expected, the peptide forms a hairpin stabilized by a disulfide bond between its N- and C-terminal parts, thus mimicking its native structure in the full-length receptor. This study may provide a strategy for producing and studying the structure/function relationship of GPCR fragments.

Molecular cloning and characterization of V2-type receptor in two ray-finned fish, gray bichir, Polypterus senegalus and medaka, Oryzias latipes

In tetrapods, vasopressin (VP) and vasotocin (VT) are involved in various aspects of physiology and behavior including osmoregulation, cardiovascular function, reproduction, stress response and social behavior. Pharmacological and molecular studies have identified three types of VP/VT receptors, V1a-type (V1aR), V1b-type (V1bR) and V2-type (V2R). On the other hand, only V1aR has so far been identified in teleosts. In the present study, we successfully cloned V2Rs from two ray-finned fish, gray bichir and medaka. Phylogenetic analysis showed that the cloned receptors belong to the V2R group of lobe-finned fish and tetrapods. The amino acid sequences of bichir V2R and medaka V2R were high identity (60-65.5% and 53.2-80.9%, respectively) with other known V2R members, respectively. Reverse transcriptase PCR revealed that ray-finned fish V2R transcripts have been detected in various tissues including brain, gill, heart, liver, kidney and reproductive organs, suggesting that ray-finned fish V2R might mediate multiple functions of VT. In functional analysis, the cells transfected with the cloned receptors responded with the accumulation of intracellular cAMP in a concentration-dependent manner following VT stimulation, but not respond with [Ca(2+)]i. Furthermore, pretreatment with mammalian V2R antagonist (OPC-31260) to the cells transfected with medaka V2R significantly inhibited an increase of the VT-induced intracellular cAMP. These results suggest that ray-finned fish possess a functional V2R linked to adenylate cyclase and the cAMP signaling pathway as well as V2Rs of lobe-finned fish and tetrapods. Thus, the present study suggests that functional V2R evolved prior to the divergence of the ray- and lobe-finned fish lineages.

Identification of multiple vasotocin receptor cDNAs in teleost fish: sequences, phylogenetic analysis, sites of expression, and regulation in the hypothalamus and gill in response to hyperosmotic challenge

Vasopressin and its homolog vasotocin regulate hydromineral balance, stress responses, and social behaviors in vertebrates. In mammals, the functions of vasopressin are mediated via three classes of membrane-bound receptors: V1a-type, V1b-type and V2-type. To date, however, only a single class of vasotocin receptor has been identified in teleost fish. Here, cDNAs encoding three putative vasotocin receptors - two distinct V1a-type receptor paralogs (V1a1 and V1a2) and a previously undescribed V2-type receptor (V2) - and a single isotocin receptor were isolated and sequenced from the Amargosa pupfish (Cyprinodon nevadensis amargosae). RT-PCR revealed that mRNAs for these receptors differed in expression patterns with V1a1 mRNAs abundant in the brain, pituitary and testis, V1a2 transcripts at greatest levels in brain, heart and muscle, V2 transcripts most common in the gills, heart and kidney, and isotocin receptor mRNAs abundant in the midbrain, pituitary and gonads. In response to an acute hyperosmotic challenge, pro-vasotocin and V2 mRNA levels in the hypothalamus decreased, while transcripts of V1a1 in the hypothalamus and V1a2 in the gills increased. Partial transcripts for structurally related V2-type, as well as multiple V1a-type, receptors were also identified in other teleosts, suggesting that multiple vasotocin receptors may be present in many Actinopterygii fishes.

Diverse vasopressin V2 receptor functionality underlying partial congenital nephrogenic diabetes insipidus

X-linked congenital nephrogenic diabetes insipidus (CNDI) is characterized by a defective renal response to the antidiuretic hormone (AVP) due to variations in the arginine vasopressin receptor 2 (AVPR2) gene. In a unique group of patients, the renal insensitivity to the effects of AVP is incomplete resulting in a partial phenotype. To investigate the molecular defects, two previously published variations in the AVPR2 gene, known to cause a partial CNDI phenotype, were expressed in transiently transfected human embryonic kidney cells. One variation (p.Arg104Cys) is located in the first extracellular loop and the other variation (p.Ser329Arg) is located in the intracellular COOH terminal of the receptor protein. Western blotting showed almost equal amounts of WT-V2R and Arg104Cys-V2R protein at steady state, whereas the level of Ser329Arg-V2R protein was lower. Confocal microscopy established that WT-V2R and Arg104Cys-V2R are localized on the cellular surface while the Ser329Arg-V2R primarily accumulates within the endoplasmic reticulum resulting in reduced surface expression. Ligand binding analysis demonstrated that the B(max) for cells expressing Arg104Cys-V2R and Ser329Arg-V2R were 14.8- and 2.5-fold lower than B(max) for WT-V2R, respectively. AVP affinity (1/K(d)) for WT-V2R and the Ser329Arg-V2R was similar while 1/K(d) for Arg104Cys-V2R was increased. cAMP assay revealed that cells expressing p.Arg104Cys-V2R or p.Ser329Arg-V2R produced 1.7- and 6.8-fold lower amounts of cAMP compared with WT-V2R, respectively. In conclusion, ligand binding and signal transduction capability are dependent on localization of the amino acid variation. Striking divergences at the level of receptor functionality may thus underlie similar clinical phenotypes in CNDI.

Comparative sequence and structural analyses of G-protein-coupled receptor crystal structures and implications for molecular models

BACKGROUND

Up until recently the only available experimental (high resolution) structure of a G-protein-coupled receptor (GPCR) was that of bovine rhodopsin. In the past few years the determination of GPCR structures has accelerated with three new receptors, as well as squid rhodopsin, being successfully crystallized. All share a common molecular architecture of seven transmembrane helices and can therefore serve as templates for building molecular models of homologous GPCRs. However, despite the common general architecture of these structures key differences do exist between them. The choice of which experimental GPCR structure(s) to use for building a comparative model of a particular GPCR is unclear and without detailed structural and sequence analyses, could be arbitrary. The aim of this study is therefore to perform a systematic and detailed analysis of sequence-structure relationships of known GPCR structures.

METHODOLOGY

We analyzed in detail conserved and unique sequence motifs and structural features in experimentally-determined GPCR structures. Deeper insight into specific and important structural features of GPCRs as well as valuable information for template selection has been gained. Using key features a workflow has been formulated for identifying the most appropriate template(s) for building homology models of GPCRs of unknown structure. This workflow was applied to a set of 14 human family A GPCRs suggesting for each the most appropriate template(s) for building a comparative molecular model.

CONCLUSIONS

The available crystal structures represent only a subset of all possible structural variation in family A GPCRs. Some GPCRs have structural features that are distributed over different crystal structures or which are not present in the templates suggesting that homology models should be built using multiple templates. This study provides a systematic analysis of GPCR crystal structures and a consistent method for identifying suitable templates for GPCR homology modelling that will help to produce more reliable three-dimensional models.