Biased agonist pharmacochaperones of the AVP V2 receptor may treat congenital nephrogenic diabetes insipidus

Therapeutic potentials of nonpeptidic V2R agonists for partial cNDI-causing V2R mutants

Loss-of-function mutations in the type 2 vasopressin receptor (V2R) are a major cause of congenital nephrogenic diabetes insipidus (cNDI). In the context of partial cNDI, the response to desmopressin (dDAVP) is partially, but not entirely, diminished. For those with the partial cNDI, restoration of V2R function would offer a prospective therapeutic approach. In this study, we revealed that OPC-51803 (OPC5) and its structurally related V2R agonists could functionally restore V2R mutants causing partial cNDI by inducing prolonged signal activation. The OPC5-related agonists exhibited functional selectivity by inducing signaling through the Gs-cAMP pathway while not recruiting β-arrestin1/2. We found that six cNDI-related V2R partial mutants (V882.53M, Y1283.41S, L1614.47P, T2736.37M, S3298.47R and S3338.51del) displayed varying degrees of plasma membrane expression levels and exhibited moderately impaired signaling function. Several OPC5-related agonists induced higher cAMP responses than AVP at V2R mutants after prolonged agonist stimulation, suggesting their potential effectiveness in compensating impaired V2R-mediated function. Furthermore, docking analysis revealed that the differential interaction of agonists with L3127.40 caused altered coordination of TM7, potentially contributing to the functional selectivity of signaling. These findings suggest that nonpeptide V2R agonists could hold promise as potential drug candidates for addressing partial cNDI.

Oxytocin and vasopressin signaling in health and disease

Neurohypophysial peptides are ancient and evolutionarily highly conserved neuropeptides that regulate many crucial physiological functions in vertebrates and invertebrates. The human neurohypophysial oxytocin/vasopressin (OT/VP) signaling system with its four receptors has become an attractive drug target for a variety of diseases, including cancer, pain, cardiovascular indications, and neurological disorders. Despite its promise, drug development faces hurdles, including signaling complexity, selectivity and off-target concerns, translational interspecies differences, and inefficient drug delivery. In this review we dive into the complexity of the OT/VP signaling system in health and disease, provide an overview of relevant pharmacological probes, and discuss the latest trends in therapeutic lead discovery and drug development.

The Psychological Burden of Families with Diabetic Children: A Literature Review Focusing on Quality of Life and Stress

Chronic diseases, such as childhood diabetes mellitus (DM), are a complex and continuous struggle as well as a great challenge both for the children who face the disease and for their parents. DM is characterized by the complex management of therapeutic treatments, thus causing physical and psychological complications infamily members. There are many families who, upon hearing the diagnosis of their child with DM, stand still in front of these new facts as their lives change. All these unprecedented conditions cause parents intense stress and discomfort, leading them to a mental burden, as the only thing that concerns them upon diagnosis is how the family will survive in the face of the current conditions they are experiencing as well as the future of the sick child. The purpose of this brief literature review is to present the research findings related to the psychological burden of families withchildren with DM, focusing on the quality of life and stress.

V2 vasopressin receptor mutations: future personalized therapy based on individual molecular biology

The diluting and concentrating function of the kidney plays a crucial role in regulating the water homeostasis of the body. This function is regulated by the antidiuretic hormone, arginine vasopressin through the type 2 vasopressin receptor (V2R), allowing the body to adapt to periods of water load or water restriction. Loss-of-function mutations of the V2R cause X-linked nephrogenic diabetes insipidus (XNDI), which is characterized by polyuria, polydipsia, and hyposthenuria. Gain-of-function mutations of the V2R lead to nephrogenic syndrome of inappropriate antidiuresis disease (NSIAD), which results in hyponatremia. Various mechanisms may be responsible for the impaired receptor functions, and this review provides an overview of recent findings about the potential therapeutic interventions in the light of the current experimental data.

Genetic basis of nephrogenic diabetes insipidus

Nephrogenic diabetes insipidus is defined as an inability to concentrate urine due to a complete or partial alteration of the renal tubular response to arginine vasopressin hormone, resulting in excessive diluted urine excretion. Hereditary forms are caused by molecular defects in the genes encoding either of the two main renal effectors of the arginine vasopressin pathway: the AVPR2 gene, which encodes for the type 2 vasopressin receptor, or the AQP2 gene, which encodes for the water channel aquaporin-2. About 90% of cases of nephrogenic diabetes insipidus result from loss-of-function variants in the AVPR2 gene, which are inherited in a X-linked recessive manner. The remaining 10% of cases result from loss-of-function variants in the AQP2 gene, which can be inherited in either a recessive or a dominant manner. The main symptoms of the disease are polyuria, chronic dehydration and hypernatremia. These symptoms usually occur in the first year of life, although some patients present later. Diagnosis is based on abnormal response in urinary osmolality after water restriction and/or administration of exogenous vasopressin. Treatment involves ensuring adequate water intake on demand, possibly combined with thiazide diuretics, non-steroidal anti-inflammatory drugs, and a low-salt and protein diet. In this review, we provide an update on current understanding of the molecular basis of inherited nephrogenic insipidus diabetes.

Structures of the arginine-vasopressin and oxytocin receptor signaling complexes

Arginine-vasopressin (AVP) and oxytocin (OT) are neurohypophysial hormones which share a high sequence and structure homology. These are two cyclic C-terminally amidated nonapeptides with different residues at position 3 and 8. In mammals, AVP and OT exert their multiple biological functions through a specific G protein-coupled receptor family: four receptors are identified, the V1a, V1b, V2 receptors (V1aR, V1bR and V2R) and the OT receptor (OTR). The chemical structure of AVP and OT was elucidated in the early 1950s. Thanks to X-ray crystallography and cryo-electron microscopy, it took however 70 additional years to determine the three-dimensional structures of the OTR and the V2R in complex with their natural agonist ligands and with different signaling partners, G proteins and β-arrestins. Today, the comparison of the different AVP/OT receptor structures gives structural insights into their orthosteric ligand binding pocket, their molecular mechanisms of activation, and their interfaces with canonical Gs, Gq and β-arrestin proteins. It also paves the way to future rational drug design and therapeutic compound development. Indeed, agonist, antagonist, biased agonist, or pharmacological chaperone analogues of AVP and OT are promising candidates to regulate different physiological functions and treat several pathologies.

Structure of the vasopressin hormone-V2 receptor-β-arrestin1 ternary complex

Arrestins interact with G protein-coupled receptors (GPCRs) to stop G protein activation and to initiate key signaling pathways. Recent structural studies shed light on the molecular mechanisms involved in GPCR-arrestin coupling, but whether this process is conserved among GPCRs is poorly understood. Here, we report the cryo-electron microscopy active structure of the wild-type arginine-vasopressin V2 receptor (V2R) in complex with β-arrestin1. It reveals an atypical position of β-arrestin1 compared to previously described GPCR-arrestin assemblies, associated with an original V2R/β-arrestin1 interface involving all receptor intracellular loops. Phosphorylated sites of the V2R carboxyl terminus are clearly identified and interact extensively with the β-arrestin1 N-lobe, in agreement with structural data obtained with chimeric or synthetic systems. Overall, these findings highlight a notable structural variability among GPCR-arrestin signaling complexes.

Functional Rescue of a Nephrogenic Diabetes Insipidus Causing Mutation in the V2 Vasopressin Receptor by Specific Antagonist and Agonist Pharmacochaperones

The urine concentrating function of the kidney is essential to maintain the water homeostasis of the human body. It is mainly regulated by the arginine-vasopressin (AVP), which targets the type 2 vasopressin receptor (V2R) in the kidney. The inability of V2R to respond to AVP stimulation leads to decreased urine concentration and congenital nephrogenic diabetes insipidus (NDI). NDI is characterized by polyuria, polydipsia, and hyposthenuria. In this study, we identified a point mutation (S127F) in the AVPR2 gene of an NDI patient, and we characterized the impaired function of the V2R mutant in HEK293 cells. Based on our data, the S127F-V2R mutant is almost exclusively located intracellularly in the endoplasmic reticulum (ER), and very few receptors were detected at the cell surface, where the receptor can bind to AVP. The overexpressed S127F-V2R mutant receptor has negligible cAMP generation capability compared to the wild-type receptor in response to AVP stimulation. Since certain misfolded mutant proteins, that are retained in the ER, can be rescued by pharmacological chaperones, we examined the potential rescue effects of two pharmacochaperones on the S127F-V2R. We found that pretreatment with both tolvaptan (an established V2R inverse agonist) and MCF14 compound (a cell-permeable high-affinity agonist for the V2R) were capable of partially restoring the cAMP generating function of the receptor in response to vasopressin stimulation. According to our data, both cell permeant agonists and antagonists can function as pharmacochaperones, and serve as the starting compounds to develop medicines for patients carrying the S127F mutation.

Targeting trafficking as a therapeutic avenue for misfolded GPCRs leading to endocrine diseases

G protein-coupled receptors (GPCRs) are plasma membrane proteins associated with an array of functions. Mutations in these receptors lead to a number of genetic diseases, including diseases involving the endocrine system. A particular subset of loss-of-function mutant GPCRs are misfolded receptors unable to traffic to their site of function (i.e. the cell surface plasma membrane). Endocrine disorders in humans caused by GPCR misfolding include, among others, hypo- and hyper-gonadotropic hypogonadism, morbid obesity, familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism, X-linked nephrogenic diabetes insipidus, congenital hypothyroidism, and familial glucocorticoid resistance. Several in vitro and in vivo experimental approaches have been employed to restore function of some misfolded GPCRs linked to endocrine disfunction. The most promising approach is by employing pharmacological chaperones or pharmacoperones, which assist abnormally and incompletely folded proteins to refold correctly and adopt a more stable configuration to pass the scrutiny of the cell's quality control system, thereby correcting misrouting. This review covers the most important aspects that regulate folding and traffic of newly synthesized proteins, as well as the experimental approaches targeted to overcome protein misfolding, with special focus on GPCRs involved in endocrine diseases.

Functional characterization of a loss-of-function mutant I324M of arginine vasopressin receptor 2 in X-linked nephrogenic diabetes insipidus

X-linked nephrogenic diabetes insipidus (X-linked NDI) is a rare inherited disease mainly caused by lost-of-function mutations in human AVPR2 gene encoding arginine vasopressin receptor 2 (V2R). Our focus of the current study is on exploration of the functional and biochemical properties of Ile324Met (I324M) mutation identified in a pedigree showing as typical recessive X-linked NDI. We demonstrated that I324M mutation interfered with the conformation of complex glycosylation of V2R. Moreover, almost all of the I324M-V2R failed to express on the cell surface due to being captured by the endoplasmic reticulum control system. We further examined the signaling activity of DDAVP-medicated cAMP and ERK1/2 pathways and the results revealed that the mutant receptor lost the ability in response to DDAVP stimulation contributed to the failure of accumulation of cAMP and phosphorylated ERK1/2. Based on the characteristics of molecular defects of I324M mutant, we selected two reagents (SR49059 and alvespimycin) to determine whether the functions of I324M-V2R can be restored and we found that both compounds can significantly “rescue” I324M mutation. Our findings may provide further insights for understanding the pathogenic mechanism of AVPR2 gene mutations and may offer some implications on development of promising treatments for patients with X-linked NDI.

Dielectric Spectroscopy Based Detection of Specific and Nonspecific Cellular Mechanisms

Using radiofrequency dielectric spectroscopy, we have investigated the impact of the interaction between a G protein-coupled receptor (GPCR), the sterile2 α-factor receptor protein (Ste2), and its cognate agonist ligand, the α-factor pheromone, on the dielectric properties of the plasma membrane in living yeast cells (Saccharomyces cerevisiae). The dielectric properties of a cell suspension containing a saturating concentration of α-factor were measured over the frequency range 40Hz–110 MHz and compared to the behavior of a similarly prepared suspension of cells in the absence of α-factor. A spherical three-shell model was used to determine the electrical phase parameters for the yeast cells in both types of suspensions. The relative permittivity of the plasma membrane showed a significant increase after exposure to α-factor (by 0.06 ± 0.05). The equivalent experiment performed on yeast cells lacking the ability to express Ste2 showed no change in plasma membrane permittivity. Interestingly, a large change also occurred to the electrical properties of the cellular interior after the addition of α-factor to the cell suspending medium, whether or not the cells were expressing Ste2. We present a number of different complementary experiments performed on the yeast to support these dielectric data and interpret the results in terms of specific cellular reactions to the presence of α-factor.

Cryo-electron microscopy structure of the antidiuretic hormone arginine-vasopressin V2 receptor signaling complex

The antidiuretic hormone arginine-vasopressin (AVP) forms a signaling complex with the V2 receptor (V2R) and the G_s protein, promoting kidney water reabsorption. Molecular mechanisms underlying activation of this critical G protein-coupled receptor (GPCR) signaling system are still unknown. To fill this gap of knowledge, we report here the cryo-electron microscopy structure of the AVP-V2R-G_s complex. Single-particle analysis revealed the presence of three different states. The two best maps were combined with computational and nuclear magnetic resonance spectroscopy constraints to reconstruct two structures of the ternary complex. These structures differ in AVP and G_s binding modes. They reveal an original receptor-G_s interface in which the Gα_s subunit penetrates deep into the active V2R. The structures help to explain how V2R R137H or R137L/C variants can lead to two severe genetic diseases. Our study provides important structural insights into the function of this clinically relevant GPCR signaling complex.

[Chaperone molecules: The example of Fabry disease]

Fabry disease is due to mutations in the GLA gene that cause a deficiency of the activity of the lysosomal enzyme alpha-galactosidase A (α-gal A) resulting in intra-tissue accumulation of globotriaosylceramide. Recently, a novel therapeutic approach based on the pharmacological chaperone migalastat has been developed. It binds, in a specific and reversible manner, to the catalytic site of α-gal A mutants, to prevent their degradation by the quality control system of the endoplasmic reticulum and allow them to catabolize globotriaosylceramide in the lysosomes. This treatment concerns approximately 35% of the GLA gene mutations recognized as sensitive to migalastat according to an in vitro pharmacogenetic test. Two pivotal Phase III studies, FACETS: migalastat vs. placebo and ATTRACT: migalastat vs. enzyme replacement therapy analyzed the in vivo effects of migalastat. Despite some methodological limitations, promising results were found. Migalastat seems to be more effective than enzyme replacement therapy in reducing left ventricular mass index in case of cardiac hypertrophy and has comparable renal effects. This oral treatment is the first personalized treatment, based on the genetic profile of Fabry patients and opens a new era in the management of conformational diseases.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Mutations in G Protein-Coupled Receptors: Mechanisms, Pathophysiology and Potential Therapeutic Approaches

There are approximately 800 annotated G protein-coupled receptor (GPCR) genes, making these membrane receptors members of the most abundant gene family in the human genome. Besides being involved in manifold physiologic functions and serving as important pharmacotherapeutic targets, mutations in 55 GPCR genes cause about 66 inherited monogenic diseases in humans. Alterations of nine GPCR genes are causatively involved in inherited digenic diseases. In addition to classic gain- and loss-of-function variants, other aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, pseudogenes, gene fusion, and gene dosage, contribute to the repertoire of GPCR dysfunctions. However, the spectrum of alterations and GPCR involvement is probably much larger because an additional 91 GPCR genes contain homozygous or hemizygous loss-of-function mutations in human individuals with currently unidentified phenotypes. This review highlights the complexity of genomic alteration of GPCR genes as well as their functional consequences and discusses derived therapeutic approaches. SIGNIFICANCE STATEMENT: With the advent of new transgenic and sequencing technologies, the number of monogenic diseases related to G protein-coupled receptor (GPCR) mutants has significantly increased, and our understanding of the functional impact of certain kinds of mutations has substantially improved. Besides the classical gain- and loss-of-function alterations, additional aspects, such as biased signaling, trans-signaling, ectopic expression, allele variants of GPCRs, uniparental disomy, pseudogenes, gene fusion, and gene dosage, need to be elaborated in light of GPCR dysfunctions and possible therapeutic strategies.

Assistance for Folding of Disease-Causing Plasma Membrane Proteins

An extensive catalog of plasma membrane (PM) protein mutations related to phenotypic diseases is associated with incorrect protein folding and/or localization. These impairments, in addition to dysfunction, frequently promote protein aggregation, which can be detrimental to cells. Here, we review PM protein processing, from protein synthesis in the endoplasmic reticulum to delivery to the PM, stressing the main repercussions of processing failures and their physiological consequences in pathologies, and we summarize the recent proposed therapeutic strategies to rescue misassembled proteins through different types of chaperones and/or small molecule drugs that safeguard protein quality control and regulate proteostasis.

V2 vasopressin receptor mutations

V2 vasopressin receptor (V2R) is a member of the G protein-coupled receptor (GPCR) family in which many disease-causing mutations have been identified and thus generated much interest. Loss-of-function V2R mutations cause nephrogenic diabetes insipidus (NDI) whereas gain-of-function mutations cause nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The mechanisms underlying a V2R loss-of-function can be theoretically classified as either protein expression, localization (ER retention) or functional disorders. Functional analyses have revealed however that these mechanisms are likely to be complex. Strikingly, V2R mutations at the same site can result in opposite phenotypes, e.g., R137H and R137L/C cause NDI and NSIAD, respectively. These findings support the notion that the constitutive activation of GPCRs might be often associated with their instability and denaturation. Thus, functional analysis of disease-causing V2R mutations may not only reveal potential new treatment strategies using pharmacochaperones for NDI and inverse agonists for NSIAD, but also provide a greater understanding of the physiological functions of GPCRs and highlight the new paradigms, i.e., biased agonism and protean agonism.

New insights into the transcriptional regulation of aquaporin-2 and the treatment of X-linked hereditary nephrogenic diabetes insipidus

The kidney collecting duct (CD) is a tubular segment of the kidney where the osmolality and final flow rate of urine are established, enabling urine concentration and body water homeostasis. Water reabsorption in the CD depends on the action of arginine vasopressin (AVP) and a transepithelial osmotic gradient between the luminal fluid and surrounding interstitium. AVP induces transcellular water reabsorption across CD principal cells through associated signaling pathways after binding to arginine vasopressin receptor 2 (AVPR2). This signaling cascade regulates the water channel protein aquaporin-2 (AQP2). AQP2 is exclusively localized in kidney connecting tubules and CDs. Specifically, AVP stimulates the intracellular translocation of AQP2-containing vesicles to the apical plasma membrane, increasing the osmotic water permeability of CD cells. Moreover, AVP induces transcription of the Aqp2 gene, increasing AQP2 protein abundance. This review provides new insights into the transcriptional regulation of the Aqp2 gene in the kidney CD with an overview of AVP and AQP2. It summarizes current therapeutic approaches for X-linked nephrogenic diabetes insipidus caused by AVPR2 gene mutations.

Metadynamics simulations of ligand binding to GPCRs

Recent developments in metadynamics simulation techniques for ligand binding to Class A GPCRs are described and the results obtained elucidated. The computational protocol makes good use of modern massively parallel hardware, making simulations of the binding/unbinding process routine. The simulations reveal unprecedented details of the ligand-binding pathways, including multiple binding sites in many cases. Free energies of binding are reproduced very well and the simulations allow prediction of the efficacy (agonist, antagonist etc.) of ligands.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.

Modulation of proteostasis and protein trafficking: a therapeutic avenue for misfolded G protein-coupled receptors causing disease in humans

Proteostasis refers to the process whereby the cell maintains in equilibrium the protein content of different compartments. This system consists of a highly interconnected network intended to efficiently regulate the synthesis, folding, trafficking, and degradation of newly synthesized proteins. Molecular chaperones are key players of the proteostasis network. These proteins assist in the assembly and folding processes of newly synthesized proteins in a concerted manner to achieve a three-dimensional structure compatible with export from the endoplasmic reticulum to other cell compartments. Pharmacologic interventions intended to modulate the proteostasis network and tackle the devastating effects of conformational diseases caused by protein misfolding are under development. These include small molecules called pharmacoperones, which are highly specific toward the target protein serving as a molecular framework to cause misfolded mutant proteins to fold and adopt a stable conformation suitable for passing the scrutiny of the quality control system and reach its correct location within the cell. Here, we review the main components of the proteostasis network and how pharmacoperones may be employed to correct misfolding of two G protein-coupled receptors, the vasopressin 2 receptor and the gonadotropin-releasing hormone receptor, whose mutations lead to X-linked nephrogenic diabetes insipidus and congenital hypogonadotropic hypogonadism in humans respectively.

Nephrogenic Diabetes Insipidus

Body fluid homeostasis is essential for normal life. In the maintenance of water balance, the most important factor and regulated process is the excretory function of the kidneys. The kidneys are capable to compensate not only the daily fluctuations of water intake but also the consequences of fluid loss (respiration, perspiration, sweating, hemorrhage). The final volume and osmolality of the excreted urine is set in the collecting duct via hormonal regulation. The hormone of water conservation is the vasopressin (AVP), and a large volume of urine is produced and excreted in the absence of AVP secretion or if AVP is ineffective in the kidneys. The aquaporin-2 water channel (AQP2) is expressed in the principal cells, and it plays an essential role in the reabsorption of water in the collecting ducts via type 2 vasopressin receptor (V2R)-mediated mechanism. If neural or hormonal regulation fails to operate the normal function of AVP-V2R-AQP2 system, it can result in various diseases such as diabetes insipidus (DI) or nephrogenic syndrome of inappropriate diuresis (NSIAD). The DI is characterized by excessive production of hyposmotic urine ("insipidus" means tasteless) due to the inability of the kidneys to concentrate urine. In this chapter, we focus and discuss the pathophysiology of nephrogenic DI (NDI) and the potential therapeutic interventions in the light of the current experimental data.

Chemical validation and optimization of pharmacoperones targeting vasopressin type 2 receptor mutant

A series of compounds formerly identified by high-throughput screening was studied for their ability to serve as pharmacoperones for the vasopressin type 2 receptor (V2R) mutant L83Q, which is known to cause nephrogenic diabetes insipidus (NDI). Three compounds were particularly effective in rerouting the mutant receptor in a concentration-dependent manner, were neither agonists nor antagonists, and displayed low cellular toxicity. Compound 1 was most effective and can be used as a molecular probe for future studies of how small molecules may affect NDI caused by mutant V2R. These compounds, however, failed to rescue the V2R Y128S mutant, indicating that the compounds described may not work in the rescue of all known mutants of V2R. Taken collectively, the present studies have now identified a promising lead compound that could function as a pharmacoperone to correct the trafficking defect of the NDI-associated mutant V2R L83Q and thus has the therapeutic potential for the treatment of NDI.

Pharmacological Chaperones: Beyond Conformational Disorders

Pharmacological chaperones (PCs) are small molecules that bind to nascent protein targets to facilitate their biogenesis. The ability of PCs to assist in the folding and subsequent forward trafficking of disease-causative protein misfolding mutants has opened new avenues for the treatment of conformational diseases such as cystic fibrosis and lysosomal storage disorders. In this chapter, an overview of the use of PCs for the treatment of conformational disorders is provided. Beyond the therapeutic application of PCs for the treatment of these disorders, pharmacological chaperoning of wild-type integral membrane proteins is discussed. Central to this discussion is the notion that the endoplasmic reticulum is a reservoir of viable but inefficiently processed wild-type protein folding intermediates whose biogenesis can be facilitated by PCs to increase functional pools. To date, the potential therapeutic use of PCs to enhance the biogenesis of wild-type proteins has received little attention. Here the rationale for the development of PCs that target WT proteins is discussed. Also considered is the likelihood that some commonly used therapeutic agents may exert unrecognized pharmacological chaperoning activity on wild-type targets in patient populations.

Pharmacological Chaperones as Potential Therapeutic Strategies for Misfolded Mutant Vasopressin Receptors

Pharmacological chaperones recently opened new possibilities in G protein-coupled receptor drug discovery. Even more interestingly, some unique ligands combine pharmacological chaperoning and biased agonism properties, boosting their therapeutic interest in many human diseases resulting from G protein-coupled receptor mutation and misfolding. These compounds displaying dual characteristics would constitute a perfect treatment for congenital Nephrogenic Diabetes Insipidus, a typical conformational disease. This X-linked genetic pathology is mostly associated with inactivating mutations of the renal arginine-vasopressin V2 receptor leading to misfolding and intracellular retention of the receptor, causing the inability of patients to concentrate their urine in response to the antidiuretic hormone. Cell-permeable pharmacological chaperones have been successfully challenged to restore plasma membrane localization of many V2 receptor mutants. In addition, different classes of specific ligands such as antagonists, agonists as well as biased agonists of the V2 receptor have proven their usefulness in rescuing mutant receptor function. This is particularly relevant for small-molecule biased agonists which only trigger Gs protein activation and cyclic adenosine monophosphate production, the V2-induced signaling pathway responsible for water reabsorption. In parallel, high-throughput screening assays based on receptor trafficking rescue approaches have been developed to discover novel V2 pharmacological chaperone molecules from different chemical libraries. These new hit compounds, which still need to be pharmacologically characterized and functionally tested in vivo, represent promising candidates for the treatment of congenital Nephrogenic Diabetes Insipidus.

Hereditary Nephrogenic Diabetes Insipidus: Pathophysiology and Possible Treatment. An Update

Under physiological conditions, excessive loss of water through the urine is prevented by the release of the antidiuretic hormone arginine-vasopressin (AVP) from the posterior pituitary. In the kidney, AVP elicits a number of cellular responses, which converge on increasing the osmotic reabsorption of water in the collecting duct. One of the key events triggered by the binding of AVP to its type-2 receptor (AVPR2) is the exocytosis of the water channel aquaporin 2 (AQP2) at the apical membrane the principal cells of the collecting duct. Mutations of either AVPR2 or AQP2 result in a genetic disease known as nephrogenic diabetes insipidus, which is characterized by the lack of responsiveness of the collecting duct to the antidiuretic action of AVP. The affected subject, being incapable of concentrating the urine, presents marked polyuria and compensatory polydipsia and is constantly at risk of severe dehydration. The molecular bases of the disease are fully uncovered, as well as the genetic or clinical tests for a prompt diagnosis of the disease in newborns. A real cure for nephrogenic diabetes insipidus (NDI) is still missing, and the main symptoms of the disease are handled with s continuous supply of water, a restrictive diet, and nonspecific drugs. Unfortunately, the current therapeutic options are limited and only partially beneficial. Further investigation in vitro or using the available animal models of the disease, combined with clinical trials, will eventually lead to the identification of one or more targeted strategies that will improve or replace the current conventional therapy and grant NDI patients a better quality of life. Here we provide an updated overview of the genetic defects causing NDI, the most recent strategies under investigation for rescuing the activity of mutated AVPR2 or AQP2, or for bypassing defective AVPR2 signaling and restoring AQP2 plasma membrane expression.

Pharmacoperone rescue of vasopressin 2 receptor mutants reveals unexpected constitutive activity and coupling bias

Pharmacoperones are small molecules that diffuse into cells and rescue misfolded, mistrafficked protein mutants, restoring their function. These substances act with high target specificity, serving as templates to fold (or refold) receptors, enzymes, ion channels or other proteins and enable them to pass the scrutiny of the cellular quality control system ("rescue"). In the present study we demonstrate that a rescued mutant (L83Q) of the vasopressin type 2 receptor (V2R), shows a strong bias for Gs coupling unlike the WT V2 receptor, which couples to both Gs and Gq/11. Failure of the mutant to couple to Gq/11 was not due to a limiting quantity of G-proteins since other Gq/11-coupled receptors (WT V2R, histamine receptor and muscarinic receptor) responded appropriately to their ligands. Transfection with DNA encoding Gq enabled the V2 receptor mutant to couple to this G protein, but only modestly compared with the WT receptor. Fourteen V2R mutant pharmacoperones, of multiple chemical classes, obtained from a high throughput screen of a 660,000 structure library, and one V2R peptidomimetic antagonist rescues L83Q. The rescued mutant shows similar bias with all pharmacoperones identified, suggesting that the bias is intrinsic to the mutant protein's structure, rather than due to the chemical class of the pharmacoperone. In the case of V2R mutant Y128S, rescue with a pharmacoperone revealed constitutive activity, also with bias for Gs, although both IP and cAMP were produced in response to agonist. These results suggest that particular rescued receptor mutants show functional characteristics that differ from the WT receptor; a finding that may be important to consider as pharmacoperones are developed as therapeutic agents.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.

An Efficient Metadynamics-Based Protocol To Model the Binding Affinity and the Transition State Ensemble of G-Protein-Coupled Receptor Ligands

A generally applicable metadynamics scheme for predicting the free energy profile of ligand binding to G-protein-coupled receptors (GPCRs) is described. A common and effective collective variable (CV) has been defined using the ideally placed and highly conserved Trp6.48 as a reference point for ligand-GPCR distance measurement and the common orientation of GPCRs in the cell membrane. Using this single CV together with well-tempered multiple-walker metadynamics with a funnel-like boundary allows an efficient exploration of the entire ligand binding path from the extracellular medium to the orthosteric binding site, including vestibule and intermediate sites. The protocol can be used with X-ray structures or high-quality homology models (based on a high-quality template and after thorough refinement) for the receptor and is universally applicable to agonists, antagonists, and partial and reverse agonists. The root-mean-square error (RMSE) in predicted binding free energies for 12 diverse ligands in five receptors (a total of 23 data points) is surprisingly small (less than 1 kcal mol^-1). The RMSEs for simulations that use receptor X-ray structures and homology models are very similar.

Biased G Protein-Coupled Receptor Signaling: New Player in Modulating Physiology and Pathology

G protein-coupled receptors (GPCRs) are a family of cell-surface proteins that play critical roles in regulating a variety of pathophysiological processes and thus are targeted by almost a third of currently available therapeutics. It was originally thought that GPCRs convert extracellular stimuli into intracellular signals through activating G proteins, whereas β-arrestins have important roles in internalization and desensitization of the receptor. Over the past decade, several novel functional aspects of β-arrestins in regulating GPCR signaling have been discovered. These previously unanticipated roles of β-arrestins to act as signal transducers and mediators of G protein-independent signaling have led to the concept of biased agonism. Biased GPCR ligands are able to engage with their target receptors in a manner that preferentially activates only G protein- or β-arrestin-mediated downstream signaling. This offers the potential for next generation drugs with high selectivity to therapeutically relevant GPCR signaling pathways. In this review, we provide a summary of the recent studies highlighting G protein- or β-arrestin-biased GPCR signaling and the effects of biased ligands on disease pathogenesis and regulation.

Analysis of the V2 Vasopressin Receptor (V2R) Mutations Causing Partial Nephrogenic Diabetes Insipidus Highlights a Sustainable Signaling by a Non-peptide V2R Agonist

Disease-causing mutations in G protein-coupled receptor (GPCR) genes, including the V2 vasopressin receptor (V2R) gene, often cause misfolded receptors, leading to a defect in plasma membrane trafficking. A novel V2R mutation, T273M, identified in a boy with partial nephrogenic diabetes insipidus (NDI), shows intracellular localization and partial defects similar to the two mutants we described previously (10). Although non-peptide V2R antagonists have been shown to rescue the membrane localization of V2R mutants, their level of functional rescue is weak. Interestingly, it has been reported that a non-peptide agonist, OPC51803, activates misfolded V2R mutants intracellularly without degradation, thus potentially serving as a therapeutic agent against NDI (14). In our current experiments, however, a peptide antagonist blocked arginine vasopressin (AVP)- or OPC51803-stimulated cAMP accumulation both in COS-7 and MDCK cells, suggesting that OPC51803 mainly stimulates cell surface V2R mutants. In addition, our analyses revealed that OPC51803 works not only as a non-peptide agonist that causes activation/β-arrestin-dependent desensitization of V2R mutants expressed at the plasma membrane but also as a pharmacochaperone that promotes the endoplasmic reticulum-retained mutant maturation and trafficking to the plasma membrane. The ratio of the pharmacochaperone effect to the desensitization effect likely correlates negatively with the residual function of the tested mutants, suggesting that OPC5 has a more favorable effect on the V2R mutants with a less residual function. We speculated that the canceling of the desensitization effect of OPC51803 by the pharmacochaperone effect after long-term treatment may produce sustainable signaling, and thus pharmacochaperone agonists such as OPC51803 may serve as promising therapeutics for NDI caused by misfolded V2R mutants.

Genetic forms of nephrogenic diabetes insipidus (NDI): Vasopressin receptor defect (X-linked) and aquaporin defect (autosomal recessive and dominant)

Nephrogenic diabetes insipidus (NDI), which can be inherited or acquired, is characterized by an inability to concentrate urine despite normal or elevated plasma concentrations of the antidiuretic hormone, arginine vasopressin (AVP). Polyuria with hyposthenuria and polydipsia are the cardinal clinical manifestations of the disease. About 90% of patients with congenital NDI are males with X-linked NDI who have mutations in the vasopressin V2 receptor (AVPR2) gene encoding the vasopressin V2 receptor. In less than 10% of the families studied, congenital NDI has an autosomal recessive or autosomal dominant mode of inheritance with mutations in the aquaporin-2 (AQP2) gene. When studied in vitro, most AVPR2 and AQP2 mutations lead to proteins trapped in the endoplasmic reticulum and are unable to reach the plasma membrane. Prior knowledge of AVPR2 or AQP2 mutations in NDI families and perinatal mutation testing is of direct clinical value and can avert the physical and mental retardation associated with repeated episodes of dehydration.

A novel therapeutic effect of statins on nephrogenic diabetes insipidus

Statins competitively inhibit hepatic 3-hydroxy-3-methylglutaryl-coenzyme A reductase, resulting in reduced plasma total and low-density lipoprotein cholesterol levels. Recently, it has been shown that statins exert additional ‘pleiotropic’ effects by increasing expression levels of the membrane water channels aquaporin 2 (AQP2). AQP2 is localized mainly in the kidney and plays a critical role in determining cellular water content. This additional effect is independent of cholesterol homoeostasis, and depends on depletion of mevalonate-derived intermediates of sterol synthetic pathways, i.e. farnesylpyrophosphate and geranylgeranylpyrophosphate. By up-regulating the expression levels of AQP2, statins increase water reabsorption by the kidney, thus opening up a new avenue in treating patients with nephrogenic diabetes insipidus (NDI), a hereditary disease that yet lacks high-powered and limited side effects therapy. Aspects related to water balance determined by AQP2 in the kidney, as well as standard and novel therapeutic strategies of NDI are discussed.

[Vasopressin V2 receptor-related pathologies: congenital nephrogenic diabetes insipidus and nephrogenic syndrome of inappropiate antidiuresis]

Congenital nephrogenic diabetes insipidus is a rare hereditary disease with mainly an X-linked inheritance (90% of the cases) but there are also autosomal recessive and dominant forms. Congenital nephrogenic diabetes insipidus is characterized by a resistance of the renal collecting duct to the action of the arginine vasopressin hormone responsible for the inability of the kidney to concentrate urine. The X-linked form is due to inactivating mutations of the vasopressin 2 receptor gene leading to a loss of function of the mutated receptors. Affected males are often symptomatic in the neonatal period with a lack of weight gain, dehydration and hypernatremia but mild phenotypes may also occur. Females carrying the mutation may be asymptomatic but, sometimes, severe polyuria is found due to the random X chromosome inactivation. The autosomal recessive and dominant forms, occurring in both genders, are linked to mutations in the aquaporin-2 gene. The treatment remains difficult, especially in infants, and is based on a low osmotic diet with increased water intake and the use of thiazides and indomethacin. The main goal is to avoid hypernatremic episodes and maintain a good hydration state. Potentially, specific treatment, in some cases of X-linked congenital nephrogenic diabetes insipidus, with pharmacological chaperones such as non-peptide vasopressin-2 receptor antagonists will be available in the future. Conversely, the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is linked to a constitutive activation of the V(2)-receptor due to activating mutations with clinical and biological features of inappropriate antidiuresis but with low or undetectable plasma arginine vasopressin hormone levels.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

Altered agonist sensitivity of a mutant v2 receptor suggests a novel therapeutic strategy for nephrogenic diabetes insipidus

Loss-of-function mutations of the type 2 vasopressin receptor (V2R) in kidney can lead to nephrogenic diabetes insipidus (NDI). We studied a previously described, but uncharacterized, mutation of the V2R (N321K missense mutation) of a patient with NDI. The properties of the mutant receptor were evaluated. We constructed a highly sensitive Epac-based bioluminescence resonance energy transfer biosensor to perform real-time cAMP measurements after agonist stimulation of transiently transfected HEK293 cells with V2Rs. β-Arrestin binding of the activated receptors was examined with luciferase-tagged β-arrestin and mVenus-tagged V2Rs using the bioluminescence resonance energy transfer technique. Cell surface expression levels of hemagglutinin-tagged receptors were determined with flow cytometry using anti-hemagglutinin-Alexa 488 antibodies. Cellular localization examinations were implemented with fluorescent tagged receptors visualized with confocal laser scanning microscopy. The effect of various vasopressin analogs on the type 1 vasopressin receptor (V1R) was tested on mouse arteries by wire myography. The N321K mutant V2R showed normal cell surface expression, but the potency of arginine vasopressin for cAMP generation was low, whereas the clinically used desmopressin was not efficient. The β-arrestin binding and internalization properties of the mutant receptor were also different than those for the wild type. The function of the mutant receptor can be rescued with administration of the V2R agonist Val(4)-desmopressin, which had no detectable side effects on V1R in the effective cAMP generating concentrations. Based on these findings we propose a therapeutic strategy for patients with NDI carrying the N321K mutation, as our in vivo experiments suggest that Val(4)-desmopressin could rescue the function of the N321K-V2R without significant side effects on the V1R.

Pharmacological chaperoning: a primer on mechanism and pharmacology

Approximately forty percent of diseases are attributable to protein misfolding, including those for which genetic mutation produces misfolding mutants. Intriguingly, many of these mutants are not terminally misfolded since native-like folding, and subsequent trafficking to functional locations, can be induced by target-specific, small molecules variably termed pharmacological chaperones, pharmacoperones, or pharmacochaperones (PCs). PC targets include enzymes, receptors, transporters, and ion channels, revealing the breadth of proteins that can be engaged by ligand-assisted folding. The purpose of this review is to provide an integrated primer of the diverse mechanisms and pharmacology of PCs. In this regard, we examine the structural mechanisms that underlie PC rescue of misfolding mutants, including the ability of PCs to act as surrogates for defective intramolecular interactions and, at the intermolecular level, overcome oligomerization deficiencies and dominant negative effects, as well as influence the subunit stoichiometry of heteropentameric receptors. Not surprisingly, PC-mediated structural correction of misfolding mutants normalizes interactions with molecular chaperones that participate in protein quality control and forward-trafficking. A variety of small molecules have proven to be efficacious PCs and the advantages and disadvantages of employing orthostatic antagonists, active-site inhibitors, orthostatic agonists, and allosteric modulator PCs are considered. Also examined is the possibility that several therapeutic agents may have unrecognized activity as PCs, and this chaperoning activity may mediate/contribute to therapeutic action and/or account for adverse effects. Lastly, we explore evidence that pharmacological chaperoning exploits intrinsic ligand-assisted folding mechanisms. Given the widespread applicability of PC rescue of mutants associated with protein folding disorders, both in vitro and in vivo, the therapeutic potential of PCs is vast. This is most evident in the treatment of lysosomal storage disorders, cystic fibrosis, and nephrogenic diabetes insipidus, for which proof of principle in humans has been demonstrated.

Combination of secretin and fluvastatin ameliorates the polyuria associated with X-linked nephrogenic diabetes insipidus in mice

X-linked nephrogenic diabetes insipidus (X-NDI) is a disease caused by inactivating mutations of the vasopressin (AVP) type 2 receptor (V2R) gene. Loss of V2R function prevents plasma membrane expression of the AQP2 water channel in the kidney collecting duct cells and impairs the kidney concentration ability. In an attempt to develop strategies to bypass V2R signaling in X-NDI, we evaluated the effects of secretin and fluvastatin, either alone or in combination, on kidney function in a mouse model of X-NDI. The secretin receptor was found to be functionally expressed in the kidney collecting duct cells. Based on this, X-NDI mice were infused with secretin for 14 days but urinary parameters were not altered by the infusion. Interestingly, secretin significantly increased AQP2 levels in the collecting duct but the protein primarily accumulated in the cytosol. Since we previously reported that fluvastatin treatment increased AQP2 plasma membrane expression in wild-type mice, secretin-infused X-NDI mice received a single injection of fluvastatin. Interestingly, urine production by X-NDI mice treated with secretin plus fluvastatin was reduced by nearly 90% and the urine osmolality was doubled. Immunostaining showed that secretin increased intracellular stores of AQP2 and the addition of fluvastatin promoted AQP2 trafficking to the plasma membrane. Taken together, these findings open new perspectives for the pharmacological treatment of X-NDI.

Renal aquaporins and water balance disorders

BACKGROUND

Aquaporins (AQPs) are a family of proteins that can act as water channels. Regulation of AQPs is critical to osmoregulation and the maintenance of body water homeostasis. Eight AQPs are expressed in the kidney of which five have been shown to play a role in body water balance; AQP1, AQP2, AQP3, AQP4 and AQP7. AQP2 in particular is regulated by vasopressin.

SCOPE OF REVIEW

This review summarizes our current knowledge of the underlying mechanisms of various water balance disorders and their treatment strategies.

MAJOR CONCLUSIONS

Dysfunctions of AQPs are involved in disorders associated with disturbed water homeostasis. Hyponatremia with increased AQP levels can be caused by diseases with low effective circulating blood volume, such as congestive heart failure, or osmoregulation disorders such as the syndrome of inappropriate secretion of antidiuretic hormone. Treatment consists of fluid restriction, demeclocycline and vasopressin type-2 receptor antagonists. Decreased AQP levels can lead to diabetes insipidus (DI), characterized by polyuria and polydipsia. In central DI, vasopressin production is impaired, while in gestational DI, levels of the vasopressin-degrading enzyme vasopressinase are abnormally increased. Treatment consists of the vasopressin analogue dDAVP. Nephrogenic DI is caused by the inability of the kidney to respond to vasopressin and can be congenital, but is most commonly acquired, usually due to lithium therapy. Treatment consists of sufficient fluid supply, low-solute diet and diuretics.

GENERAL SIGNIFICANCE

In recent years, our understanding of the underlying mechanisms of water balance disorders has increased enormously, which has opened up several possible new treatment strategies. This article is part of a Special Issue entitled Aquaporins.

Global analysis of the effects of the V2 receptor antagonist satavaptan on protein phosphorylation in collecting duct

Satavaptan (SR121463) is a vasopressin V2 receptor antagonist that has been shown to improve hyponatremia in patients with cirrhosis, congestive heart failure, and syndrome of inappropriate antidiuresis. While known to inhibit adenylyl cyclase-mediated accumulation of intracellular cyclic AMP and potentially recruit β-arrestin in kidney cell lines, very little is known regarding the signaling pathways that are affected by this drug. To this end, we carried out a global quantitative phosphoproteomic analysis of native rat inner medullary collecting duct cells pretreated with satavaptan or vehicle control followed by the V2 receptor agonist desmopressin (dDAVP) for 0.5, 2, 5, or 15 min. A total of 2,449 unique phosphopeptides from 1,160 proteins were identified. Phosphopeptides significantly changed by satavaptan included many of the same kinases [protein kinase A, phosphoinositide 3-kinase, mitogen-activated protein kinase kinase kinase 7 (TAK1), and calcium/calmodulin-dependent kinase kinase 2] and channels (aquaporin-2 and urea transporter UT-A1) regulated by vasopressin. Time course clustering and kinase motif analysis suggest that satavaptan blocks dDAVP-mediated activation of basophilic kinases, while also blocking dDAVP-mediated inhibition of proline-directed kinases. Satavaptan affects a variety of dDAVP-mediated processes including regulation of cell-cell junctions, actin cytoskeleton dynamics, and signaling through Rho GTPases. These results demonstrate that, overall, satavaptan acts as a selective V2 receptor antagonist and affects many of the same signaling pathways regulated by vasopressin. This study represents the first "systems-wide" analysis of a "vaptan"-class drug and provides a wealth of new data regarding the effects of satavaptan on vasopressin-mediated phosphorylation events.

Vasopressin receptors and pharmacological chaperones: from functional rescue to promising therapeutic strategies

Conformational diseases result from protein misfolding and/or aggregation and constitute a major public health problem. Congenital Nephrogenic Diabetes Insipidus is a typical conformational disease. In most of the cases, it is associated to inactivating mutations of the renal arginine-vasopressin V2 receptor gene leading to misfolding and intracellular retention of the receptor, causing the inability of patients to concentrate their urine in response to the antidiuretic hormone. Cell-permeable pharmacological chaperones have been successfully challenged to restore plasma membrane localization of the receptor mutants and to rescue their function. Interestingly, different classes of specific ligands such as antagonists (vaptans), agonists as well as biased agonists of the V2 receptor have proven their usefulness as efficient pharmacochaperones. These compounds represent a potential therapeutic treatment of this X-linked genetic pathology.

Inverse agonist and pharmacochaperone properties of MK-0524 on the prostanoid DP1 receptor

Prostaglandin D₂ (PGD₂) acts through two G protein-coupled receptors (GPCRs), the prostanoid DP receptor and CRTH2 also known as DP1 and DP2, respectively. Several previously characterized GPCR antagonists are now classified as inverse agonists and a number of GPCR ligands are known to display pharmacochaperone activity towards a given receptor. Here, we demonstrate that a DP1 specific antagonist, MK-0524 (also known as laropiprant), decreased basal levels of intracellular cAMP produced by DP1, a Gα(s)-coupled receptor, in HEK293 cells. This reduction in cAMP levels was not altered by pertussis toxin treatment, indicating that MK-0524 did not induce coupling of DP1 to Gα(i/o) proteins and that this ligand is a DP1 inverse agonist. Basal ERK1/2 activation by DP1 was not modulated by MK-0524. Interestingly, treatment of HEK293 cells expressing Flag-tagged DP1 with MK-0524 promoted DP1 cell surface expression time-dependently to reach a maximum increase of 50% compared to control after 24 h. In contrast, PGD₂ induced the internalization of 75% of cell surface DP1 after the same time of stimulation. The increase in DP1 cell surface targeting by MK-0524 was inhibited by Brefeldin A, an inhibitor of transport from the endoplasmic reticulum-Golgi to the plasma membrane. Confocal microscopy confirmed that a large population of DP1 remained trapped intracellularly and co-localized with calnexin, an endoplasmic reticulum marker. Redistribution of DP1 from intracellular compartments to the plasma membrane was observed following treatment with MK-0524 for 24 h. Furthermore, MK-0524 promoted the interaction between DP1 and the ANKRD13C protein, which we showed previously to display chaperone-like effects towards the receptor. We thus report that MK-0524 is an inverse agonist and a pharmacochaperone of DP1. Our findings may have important implications during therapeutic treatments with MK-0524 and for the development of new molecules targeting DP1.

Selective and potent agonists and antagonists for investigating the role of mouse oxytocin receptors

The neuropeptides oxytocin (OT) and vasopressin (AVP) have been shown to play a central role in social behaviors; as a consequence, they have been recognized as potential drugs to treat neurodevelopmental and psychiatric disorders characterized by impaired social interactions. However, despite the basic and preclinical relevance of mouse strains carrying genetic alterations in the OT/AVP systems to basic and preclinical translational neuroscience, the pharmacological profile of mouse OT/AVP receptor subtypes has not been fully characterized. To fill in this gap, we have characterized a number of OT and AVP agonists and antagonists at three murine OT/AVP receptors expressed in the nervous system as follows: the oxytocin (mOTR) and vasopressin V1a (mV1aR) and V1b (mV1bR) subtypes. These three receptors were transiently expressed in vitro for binding and intracellular signaling assays, and then a homology model of the mOTR structure was constructed to investigate how its molecular features compare with human and rat OTR orthologs. Our data indicate that the selectivity profile of the natural ligands, OT and AVP, is conserved in humans, rats, and mice. Furthermore, we found that the synthetic peptide [Thr(4)Gly(7)]OT (TGOT) is remarkably selective for the mOTR and, like the endogenous OT ligand, activates Gq and Gi and recruits β-arrestins. Finally, we report three antagonists that exhibit remarkably high affinities and selectivities at mOTRs. These highly selective pharmacological tools will contribute to the investigation of the specific physiologic and pathologic roles of mOTR for the development of selective OT-based therapeutics.

Nephrogenic diabetes insipidus: essential insights into the molecular background and potential therapies for treatment

The water channel aquaporin-2 (AQP2), expressed in the kidney collecting ducts, plays a pivotal role in maintaining body water balance. The channel is regulated by the peptide hormone arginine vasopressin (AVP), which exerts its effects through the type 2 vasopressin receptor (AVPR2). Disrupted function or regulation of AQP2 or the AVPR2 results in nephrogenic diabetes insipidus (NDI), a common clinical condition of renal origin characterized by polydipsia and polyuria. Over several years, major research efforts have advanced our understanding of NDI at the genetic, cellular, molecular, and biological levels. NDI is commonly characterized as hereditary (congenital) NDI, arising from genetic mutations in the AVPR2 or AQP2; or acquired NDI, due to for exmple medical treatment or electrolyte disturbances. In this article, we provide a comprehensive overview of the genetic, cell biological, and pathophysiological causes of NDI, with emphasis on the congenital forms and the acquired forms arising from lithium and other drug therapies, acute and chronic renal failure, and disturbed levels of calcium and potassium. Additionally, we provide an overview of the exciting new treatment strategies that have been recently proposed for alleviating the symptoms of some forms of the disease and for bypassing G protein-coupled receptor signaling.