Association of calnexin with wild type and mutant AVPR2 that causes nephrogenic diabetes insipidus

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.

Trafficking-defective mutant PROKR2 cycles between endoplasmic reticulum and Golgi to attenuate endoplasmic reticulum stress

The endoplasmic reticulum (ER) possesses a quality control system that prevents misfolded proteins from leaving the ER for routing to the ER-associated degradation pathway. Some misfolded proteins can escape the ER to reach the Golgi, where they are then retrieved from the Golgi back to the ER for degradation, but why this occurs needs to be clarified. Studying a mutant prokineticin receptor 2 identified in patients with hypogonadotropic hypogonadism as a model, we find that the post-ER retrieval system provides another layer of quality control and also lowers the load of misfolded proteins in the ER to reduce ER stress. Our findings reveal the importance of a post-ER quality control mechanism in contributing to cellular homeostasis.

G protein–coupled receptors (GPCRs) play crucial roles in numerous physiological and pathological processes. Mutations in GPCRs that result in loss of function or alterations in signaling can lead to inherited or acquired diseases. Herein, studying prokineticin receptor 2 (PROKR2), we initially identify distinct interactomes for wild-type (WT) versus a mutant (P290S) PROKR2 that causes hypogonadotropic hypogonadism. We then find that both the WT and mutant PROKR2 are targeted for endoplasmic reticulum (ER)-associated degradation, but the mutant is degraded to a greater extent. Further analysis revealed that both forms can also leave the ER to reach the Golgi. However, whereas most of the WT is further transported to the cell surface, most of the mutant is retrieved to the ER. Thus, the post-ER itinerary plays an important role in distinguishing the ultimate fate of the WT versus the mutant. We have further discovered that this post-ER itinerary reduces ER stress induced by the mutant PROKR2. Moreover, we extend the core findings to another model GPCR. Our findings advance the understanding of disease pathogenesis induced by a mutation at a key residue that is conserved across many GPCRs and thus contributes to a fundamental understanding of the diverse mechanisms used by cellular quality control to accommodate misfolded proteins.

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.

Novel AVPR2 mutations and clinical characteristics in 28 Chinese families with congenital nephrogenic diabetes insipidus

AIMS

To investigate genotype and phenotype of congenital nephrogenic diabetes insipidus caused by AVPR2 mutations, which is rare and limitedly studied in Chinese population.

METHODS

88 subjects from 28 families with NDI in a department (Beijing, PUMCH) were screened for AVPR2 mutations. Medical records were retrospectively reviewed and characterized. Genotype and phenotype analysis was performed.

RESULTS

23 AVPR2 mutations were identified, including six novel mutations (p.Y117D, p.W208R, p.L313R, p.S127del, p.V162Sfs*30 and p.G251Pfs*96). The onset-age ranged from 1 week to 3 years. Common presentations were polydipsia and polyuria (100%) and intermittent fever (57%). 21% and 14% of patients had short stature and mental impairment. Urine SG and osmolality were decreased, while serum osmolality and sodium were high. Urological ultrasonography results showed hydronephrosis of the kidney (52%), dilation of the ureter (48%), and thickened bladder wall or increased residual urine (32%), led to intermittent urethral catheterization (7%), cystostomy (11%) and binary nephrostomy (4%). Urological defects were developed in older patients. Genotype and phenotype analysis revealed patients with non-missense mutations had higher levels of serum sodium than missense mutations.

CONCLUSION

In the first and largest case series of NDI caused by AVPR2 mutations in Chinese population, we established genetic profile and characterized clinical data, reporting six novel mutations. Further, we found genotype was associated with phenotype. This knowledge broadens genotype and phenotype spectrum of rare congenital NDI caused by AVPR2 mutations, and provides basis for studying molecular biology of AVPR2.

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.

Molecular chaperones and G protein-coupled receptor maturation and pharmacology

G protein-coupled receptors (GPCRs) are highly conserved versatile signaling molecules located at the plasma membrane that respond to diverse extracellular signals. They regulate almost all physiological processes in the vertebrates. About 35% of current drugs target these receptors. Mutations in these genes have been identified as causes of numerous diseases. The seven transmembrane domain structure of GPCRs implies that the folding of these transmembrane proteins is extremely complicated and difficult. Indeed, many wild type GPCRs are not folded optimally. The most common defect in genetic diseases caused by GPCR mutations is misfolding and failure to reach the plasma membrane where it functions. General molecular chaperones aid the folding of all proteins, including GPCRs, by preventing aggregation, promoting folding and disaggregating small aggregates. Some GPCRs need additional receptor-specific chaperones to assist their folding. Many of these receptor-specific chaperones interact with additional receptors and alter receptor pharmacology, expanding the understanding of these chaperone proteins.

Mechanisms of the anterograde trafficking of GPCRs: Regulation of AT1R transport by interacting proteins and motifs

Anterograde cell surface transport of nascent G protein-coupled receptors (GPCRs) en route from the endoplasmic reticulum (ER) through the Golgi apparatus represents a crucial checkpoint to control the amount of the receptors at the functional destination and the strength of receptor activation-elicited cellular responses. However, as compared with extensively studied internalization and recycling processes, the molecular mechanisms of cell surface trafficking of GPCRs are relatively less defined. Here, we will review the current advances in understanding the ER-Golgi-cell surface transport of GPCRs and use angiotensin II type 1 receptor as a representative GPCR to discuss emerging roles of receptor-interacting proteins and specific motifs embedded within the receptors in controlling the forward traffic of GPCRs along the biosynthetic pathway.

Degradation routes of trafficking-defective VLDLR mutants associated with Dysequilibrium syndrome

Low density lipoprotein receptor (LDLR) family members are involved in signaling in the developing brain. Previously we have reported that missense mutations in the Very Low Density Lipoprotein Receptor gene (VLDLR), causing Dysequilibrium syndrome (DES), disrupt ligand-binding, due to endoplasmic reticulum (ER) retention of the mutants. We explored the degradation routes of these VLDLR mutants in cultured cells. Our results indicate that VLDLR mutants are retained in the ER for prolonged periods which could be facilitated by association with the ER-resident chaperone calnexin. The mutants were prone to aggregation and capable of eliciting ER stress. The VLDLR mutants were found to be degraded predominantly by the proteasomal pathway, since ubiquitinated VLDLR was found to accumulate in response to proteasomal inhibition. Further, the mutants were found to interact with the ER degradation adaptor protein SEL1L. The degradation of VLDLR wild type and mutant were delayed in CRISPR/Cas9 edited SEL1L knock-out cells which was reversed by exogenous expression of SEL1L. In summary, ER retention of pathogenic VLDLR mutants involves binding to calnexin, elevated ER stress, and delayed degradation which is dependent on SEL1L. Since core LDLR family members share common structural domains, common mechanisms may be involved in their ER processing.

Intracellular Trafficking of Gonadotropin Receptors in Health and Disease

Gonadotropin receptors belong to the highly conserved subfamily of the G protein-coupled receptor (GPCR) superfamily, the so-called Rhodopsin-like family (class A), which is the largest class of GPCRs and currently a major drug target. Both the follicle-stimulating hormone receptor (FSHR) and the luteinizing hormone/chorionic gonadotropin hormone receptor (LHCGR) are mainly located in the gonads where they play key functions associated to essential reproductive functions. As any other protein, gonadotropin receptors must be properly folded into a mature tertiary conformation compatible with quaternary assembly and endoplasmic reticulum export to the cell surface plasma membrane. Several primary and secondary structural features, including presence of particular amino acid residues and short motifs and in addition, posttranslational modifications, regulate intracellular trafficking of gonadotropin receptors to the plasma membrane as well as internalization and recycling of the receptor back to the cell surface after activation by agonist. Inactivating mutations of gonadotropin receptors may derive from receptor misfolding and lead to absent or reduced plasma membrane expression of the altered receptor, thereby manifesting an array of phenotypical abnormalities mostly characterized by reproductive failure and/or abnormal or absence of development of secondary sex characteristics. In this chapter we review the structural requirements necessary for intracellular trafficking of the gonadotropin receptors, and describe how mutations in these receptors may lead to receptor misfolding and disease in humans.

The yeast Ste2p G protein-coupled receptor dimerizes on the cell plasma membrane

Dimerization of G protein-coupled receptors (GPCR) may play an important role in maturation, internalization, signaling and/or pharmacology of these receptors. However, the location where dimerization occurs is still under debate. In our study, variants of Ste2p, a yeast mating pheromone GPCR, were tagged with split EGFP (enhanced green fluorescent protein) fragments inserted between transmembrane domain seven and the C-terminus or appended to the C-terminus. Bimolecular Fluorescence Complementation (BiFC) assay was used to determine where receptor dimerization occurred during protein trafficking by monitoring generation of EGFP fluorescence, which occurred upon GPCR dimerization. Our results suggest that these tagged receptors traffic to the membrane as monomers, undergo dimerization or higher ordered oligomerization predominantly on the plasma membrane, and are internalized as dimers/oligomers. This study is the first to provide direct in vivo visualization of GPCR dimerization/oligomerization, during trafficking to and from the plasma membrane.

Monitoring G protein-coupled receptor and β-arrestin trafficking in live cells using enhanced bystander BRET

Endocytosis and intracellular trafficking of receptors are pivotal to maintain physiological functions and drug action; however, robust quantitative approaches are lacking to study such processes in live cells. Here we present new bioluminescence resonance energy transfer (BRET) sensors to quantitatively monitor G protein-coupled receptors (GPCRs) and β-arrestin trafficking. These sensors are based on bystander BRET and use the naturally interacting chromophores luciferase (RLuc) and green fluorescent protein (rGFP) from Renilla. The versatility and robustness of this approach are exemplified by anchoring rGFP at the plasma membrane or in endosomes to generate high dynamic spectrometric BRET signals on ligand-promoted recruitment or sequestration of RLuc-tagged proteins to, or from, specific cell compartments, as well as sensitive subcellular BRET imaging for protein translocation visualization. These sensors are scalable to high-throughput formats and allow quantitative pharmacological studies of GPCR trafficking in real time, in live cells, revealing ligand-dependent biased trafficking of receptor/β-arrestin complexes.

Cellular signaling processes often involve trafficking of receptors and other proteins between subcellular compartments. Here the authors demonstrate a method based on the concept of Enhanced bystander Bioluminescence Resonance Energy Transfer (EbBRET) that allows efficient real time monitoring of endocytosis and trafficking.

Regulation of GPCR Anterograde Trafficking by Molecular Chaperones and Motifs

G protein-coupled receptors (GPCRs) make up a superfamily of integral membrane proteins that respond to a wide variety of extracellular stimuli, giving them an important role in cell function and survival. They have also proven to be valuable targets in the fight against various diseases. As such, GPCR signal regulation has received considerable attention over the last few decades. With the amplitude of signaling being determined in large part by receptor density at the plasma membrane, several endogenous mechanisms for modulating GPCR expression at the cell surface have come to light. It has been shown that cell surface expression is determined by both exocytic and endocytic processes. However, the body of knowledge surrounding GPCR trafficking from the endoplasmic reticulum to the plasma membrane, commonly known as anterograde trafficking, has considerable room for growth. We focus here on the current paradigms of anterograde GPCR trafficking. We will discuss the regulatory role of both the general and "nonclassical private" chaperone systems in GPCR trafficking as well as conserved motifs that serve as modulators of GPCR export from the endoplasmic reticulum and Golgi apparatus. Together, these topics summarize some of the known mechanisms by which the cell regulates anterograde GPCR trafficking.

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.

An ER complex of ODR-4 and ODR-8/Ufm1 specific protease 2 promotes GPCR maturation by a Ufm1-independent mechanism

Despite the importance of G-protein coupled receptors (GPCRs) their biogenesis is poorly understood. Like vertebrates, C. elegans uses a large family of GPCRs as chemoreceptors. A subset of these receptors, such as ODR-10, requires the odr-4 and odr-8 genes to be appropriately localized to sensory cilia. The odr-4 gene encodes a conserved tail-anchored transmembrane protein; the molecular identity of odr-8 is unknown. Here, we show that odr-8 encodes the C. elegans ortholog of Ufm1-specific protease 2 (UfSP2). UfSPs are cysteine proteases identified biochemically by their ability to liberate the ubiquitin-like modifier Ufm1 from its pro-form and protein conjugates. ODR-8/UfSP2 and ODR-4 are expressed in the same set of twelve chemosensory neurons, and physically interact at the ER membrane. ODR-4 also binds ODR-10, suggesting that an ODR-4/ODR-8 complex promotes GPCR folding, maturation, or export from the ER. The physical interaction between human ODR4 and UfSP2 suggests that this complex's role in GPCR biogenesis may be evolutionarily conserved. Unexpectedly, mutant versions of ODR-8/UfSP2 lacking catalytic residues required for protease activity can rescue all odr-8 mutant phenotypes tested. Moreover, deleting C. elegans ufm-1 does not alter chemoreceptor traffic to cilia, either in wild type or in odr-8 mutants. Thus, UfSP2 proteins have protease- and Ufm1-independent functions in GPCR biogenesis.

Despite the importance of G-protein coupled receptors (GPCRs), we know little about their biogenesis. Olfactory receptors form a large and divergent group of GPCRs. We investigate their biogenesis in C. elegans. We show that maturation of a subset of these GPCRs, including the diacetyl receptor ODR-10, requires Ufm1 specific protease 2 (UfSP2), which corresponds to odr-8. Biochemical studies suggest mouse UfSP2 activates the Ubiquitin-like molecule Ufm1 and cleaves it from protein conjugates. However, neither the protease active site nor ufm-1 is required for UfSP2/ODR-8 to promote ODR-10 maturation. C. elegans UfSP2 is expressed in the same chemosensory neurons as ODR-4, a tail-anchored transmembrane protein also required for ODR-10 maturation. ODR-4 resides in the endoplasmic reticulum (ER); UfSP2 is cytosolic but associates with ER membranes. In odr-4 and odr-8 mutants ODR-10-GFP is retained in the ER, suggesting these genes are required to fold GPCRs or traffic them out of the ER. ODR-4 interacts biochemically with ODR-8 and ODR-10 to form an ER complex. ODR-4 and UfSP2 are conserved from plants to man, and human ODR4 can bind human UfSP2 and recruit it to ER membranes. Both proteins are expressed widely in mammals, suggesting a broader role in GPCR biogenesis.

Targeting opioid receptors with pharmacological chaperones

G protein-coupled receptors (GPCRs) are polytopic membrane proteins that have a pivotal role in cellular signaling. Like other membrane proteins, they fold in the endoplasmic reticulum (ER) before they are transported to the plasma membrane. The ER quality control monitors the folding process and misfolded proteins and slowly folding intermediates are targeted to degradation in the cytosol via the ubiquitin-proteasome pathway. The high efficiency of the quality control machinery may lead to the disposal of potentially functional receptors. This is the major underlying course for loss-of-function conformational diseases, such as retinitis pigmentosa, nephrogenic diabetes insipidus and early onset obesity, which involve mutant GPCRs. During the past decade, it has become increasingly evident that small-molecular lipophilic and pharmacologically selective receptor ligands, called pharmacological chaperones (PCs), can rescue these mutant receptors from degradation by stabilizing newly synthesized receptors in the ER and enhancing their transport to the cell surface. This has raised the interesting prospect that PCs might have therapeutic value for the treatment of conformational diseases. At the same time, accumulating evidence has indicated that wild-type receptors might also be targeted by PCs, widening their therapeutic potential. This review focuses on one GPCR subfamily, opioid receptors that have been useful models to unravel the mechanism of action of PCs. In contrast to most other GPCRs, compounds that act as PCs for opioid receptors, including widely used opioid drugs, target wild-type receptors and their common natural variants.

Species sequence differences determine the interaction of GnRH receptor with the cellular quality control system

Plasma membrane expression (PME) of the human GnRHR (hGnRHR) is regulated by a primate-specific Lys(191) which destabilizes a Cys(14)-Cys(200) bridge required by the cellular quality control system (QCS). A 4-amino, non-contiguous "motif" (Leu(112), Gln(208), Leu(300), Asp(302)) is required for this effect. The hGnRHR sequence, with or without Lys(191), decreases PME and inositol phosphate (IP) production when co-expressed with calnexin, a QCS chaperone. WT rat GnRHR, decreases PME and IP production, when co-expressed with calnexin, but to a lesser degree than hGnRH. When the human sequence contains the rat motif, IP production is closer to that of rat GnRHR. When Lys(191) is deleted from hGnRHR and co-expressed with calnexin, IP production is similar to the rat sequence. When rat GnRHR containing Lys(191) and the human motif is co-expressed with calnexin, IP production is similar to cells expressing the hGnRHR. The motif sequence appears to be a determinant of calnexin recognition.

Calnexin-assisted biogenesis of the neuronal glycine transporter 2 (GlyT2)

The neuronal transporter GlyT2 is a polytopic, 12-transmembrane domain, plasma membrane glycoprotein involved in the removal and recycling of synaptic glycine from inhibitory synapses. Mutations in the human GlyT2 gene (SLC6A5) that cause deficient glycine transport or defective GlyT2 trafficking are the second most common cause of hyperekplexia or startle disease. In this study we examined several aspects of GlyT2 biogenesis that involve the endoplasmic reticulum chaperone calnexin (CNX). CNX binds transiently to an intermediate under-glycosylated transporter precursor and facilitates GlyT2 processing. In cells expressing GlyT2, transporter accumulation and transport activity were attenuated by siRNA-mediated CNX knockdown and enhanced by CNX overexpression. GlyT2 binding to CNX was mediated by glycan and polypeptide-based interactions as revealed by pharmacological approaches and the behavior of GlyT2 N-glycan-deficient mutants. Moreover, transporter folding appeared to be stabilized by N-glycans. Co-expression of CNX and a fully non-glycosylated mutant rescues glycine transport but not mutant surface expression. Hence, CNX discriminates between different conformational states of GlyT2 displaying a lectin-independent chaperone activity. GlyT2 wild-type and mutant transporters were finally degraded in the lysosome. Our findings provide further insight into GlyT2 biogenesis, and a useful framework for the study of newly synthesized GlyT2 transporters bearing hyperekplexia mutations.

Interactions of vasopressin and oxytocin receptors with vasopressin analogues substituted in position 2 with 3,3'-diphenylalanine--a molecular docking study

Vasopressin and oxytocin receptors belong to the superfamily of G protein-coupled receptors and play an important role in many physiological functions. They are also involved in a number of pathological conditions being important drug targets. In this work, four vasopressin analogues substituted at position 2 with 3,3'-diphenylalanine have been docked into partially flexible vasopressin and oxytocin receptors. The bulky residue at position 2 acts as a structural restraint much stronger in the oxytocin receptor (OTR) than in the vasopressin V2 receptor (V2R), resulting in a different location of the analogues in these receptors. This explains the different, either agonistic or antagonistic, activities of the analogues in V2R and OTR, respectively. In all complexes, the conserved polar residues serve as anchor points for the ligand both in OTR and V2R. Strong interactions of the C-terminus of analogue II ([Mpa(1) ,d-Dpa(2) ,Val(4) ,d-Arg(8) ]VP) with extracellular loop 3 may be responsible for its highest activity at V2R. It also appears that V2R adapts more readily to the docking analogues by conformational changes in the aromatic side chains triggering receptor activation. A weak activity at V1a vasopressin receptor appears to be caused by weak receptor-ligand interactions. Results of this study may facilitate a rational design of new analogues with the highest activity/selectivity at vasopressin and OTRs.

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Pharmacoperones: a new therapeutic approach for diseases caused by misfolded G protein-coupled receptors

G Protein-coupled receptors (GPCRs) are cell membrane proteins that recognize specific chemical signals such as drugs and hormones and transduce these signals into cellular responses by activating G-proteins. As is the case for all newly synthesized proteins, GPCRs are subjected to conformational scrutiny at the endoplasmic reticulum prior to processing and trafficking to the cell surface membrane. Because of this stringent quality control screening mechanism, mutations that result in protein misfolding frequently lead to retention in the endoplasmic reticulum, aggregation or other misrouting and, eventually, to disease. This article reviews some patents and new therapeutic opportunities based on the misfolding and retention of otherwise functional GPCRs that represent promising approaches to correct conformational abnormalities leading to distinct disease states.

Pathomechanistic characterization of two exonic L1CAM variants located in trans in an obligate carrier of X-linked hydrocephalus

Mutations in the gene encoding the neural cell adhesion molecule L1CAM cause several neurological disorders collectively referred to as L1 syndrome. We report here a family case of X-linked hydrocephalus in which an obligate female carrier has two exonic L1CAM missense mutations in trans substituting amino acids in the first (p.W635C) or second (p.V768I) fibronectin-type III domains. We performed various biochemical and cell biological in vitro assays to evaluate the pathogenicity of these variants. Mutant L1-W635C protein accumulates in the endoplasmic reticulum (ER), is not transported into axons, and fails to promote L1CAM-mediated cell-cell adhesion as well as neurite growth. Immunoprecipitation experiments show that L1-W635C associates with the molecular ER chaperone calnexin and is modified by poly-ubiquitination. The mutant L1-V768I protein localizes at the cell surface, is not retained in the ER, and promotes neurite growth similar to wild-type L1CAM. However, the p.V768I mutation impairs L1CAM-mediated cell-cell adhesion albeit less severe than L1-W635C. These data indicate that p.W635C is a novel loss-of-function L1 syndrome mutation. The p.V768I mutation may represent a non-pathogenic variant or a variant associated with low penetrance. The poly-ubiquitination of L1-W635C and its association with the ER chaperone calnexin provide further insights into the molecular mechanisms underlying defective cell surface trafficking of L1CAM in L1 syndrome.

Pharmacological chaperones correct misfolded GPCRs and rescue function: protein trafficking as a therapeutic target

G-protein-coupled receptors (GPCRs) are a large superfamily of plasma membrane proteins that play central roles in transducing endocrine, neural and -sensory signals. In humans, more than 30 disorders are associated with mutations in GPCRs and these proteins are common drug development targets, with 30-50% of drugs targeting them. GPCR mutants are frequently misfolded, recognized as defective by the cellular quality control system, retained in the endoplasmic reticulum and do not traffic to the plasma membrane. The use of small molecules chaperones (pharmacological chaperones or "pharmacoperones") to rescue misfolded GPCRs has provided a new approach for treatment of human diseases caused by misfolding and misrouting. This chapter provides an overview of the molecular basis of this approach using the human gonadotropin-releasing hormone receptor (hGnRHR) as model for treatment of conformational diseases provoked by -misfolded GPCRs.

Effects of cellular, chemical, and pharmacological chaperones on the rescue of a trafficking-defective mutant of the ATP-binding cassette transporter proteins ABCB1/ABCB4

The ATP-binding cassette transporter ABCB4 is a phosphatidylcholine translocator specifically expressed at the bile canalicular membrane in hepatocytes, highly homologous to the multidrug transporter ABCB1. Variations in the ABCB4 gene sequence cause progressive familial intrahepatic cholestasis type 3. We have shown previously that the I541F mutation, when reproduced either in ABCB1 or in ABCB4, led to retention in the endoplasmic reticulum (ER)/Golgi. Here, Madin-Darby canine kidney cells expressing ABCB1-GFP were used as a model to investigate this mutant. We show that ABCB1-I541F is not properly folded and is more susceptible to in situ protease degradation. It colocalizes and coprecipitates with the ER chaperone calnexin and coprecipitates with the cytosolic chaperone Hsc/Hsp70. Silencing of calnexin or overexpression of Hsp70 have no effect on maturation of the mutant. We also tested potential rescue by chemical and pharmacological chaperones. Thapsigargin and sodium 4-phenyl butyrate were inefficient. Glycerol improved maturation and exit of the mutant from the ER. Cyclosporin A, a competitive substrate for ABCB1, restored maturation, plasma membrane expression, and activity of ABCB1-I541F. Cyclosporin A also improved maturation of ABCB4-I541F in Madin-Darby canine kidney cells. In HepG(2) cells transfected with ABCB4-I541F cDNA, cyclosporin A allowed a significant amount of the mutant protein to reach the membrane of bile canaliculi. These results show that the best strategy to rescue conformation-defective ABCB4 mutants is provided by pharmacological chaperones that specifically target the protein. They identify cyclosporin A as a potential novel therapeutic tool for progressive familial intrahepatic cholestasis type 3 patients.

GnRH receptor mutations in isolated gonadotropic deficiency

GnRH and its receptor GnRHR are key regulators of the hypothalamo-pituitary axis. They modulate the secretion of LH and FSH gonadotropins and therefore, the development and maturation of gonads in fetal life as well as after birth. Congenital functional defect of this axis results in isolated hypogonadotropic hypogonadism (IHH). Several natural mutations causing IHH without anosmia have now been identified in GnRHR or GnRH genes. These mutations inactivate GnRHR or its ligand function and cause highly variable phenotypes, ranging from partial to complete gonadotropic deficiencies. The present review describes the published natural GnRHR mutations and tries to correlate them with the corresponding phenotypes according to the different steps of the GnRH system development.

Functional characterization of novel loss-of-function mutations in the vasopressin type 2 receptor gene causing nephrogenic diabetes insipidus

BACKGROUND

X-linked nephrogenic diabetes insipidus (NDI) is a rare polyuric disorder caused by inactivating mutations in the arginine vasopressin receptor Type 2 (AVPR2) gene.

METHODS

NDI patients from six unrelated families were subjected to mutational analysis of the AVPR2 gene. In-depth in vitro characterization of novel AVPR2 mutants by a combination of functional and immunological techniques provided further insight into molecular mechanisms causing receptor dysfunction.

RESULTS

Mutational analysis revealed four novel (A89P, G107R, Q174R, W208X) and three recurrent (V277A, R337X, ΔR247-G250) mutations within the AVPR2 gene. One family carried the missense mutation R337X and a 12-bp deletion (ΔR247-G250), corresponding to a fragment in the third intracellular loop (ICL3), which was not genetically linked to R337X. The functionally tested missense mutations A89P, G107R and Q174R led to reduced receptor cell surface expression in transfected COS-7 cells, most probably due to misfolding and intracellular retention, and consequently to reduction or loss of agonist-mediated cyclic adenosine monophosphate formation. Deletion of R247-G250 had no effect on receptor function in vitro. Comparison with other mammalian AVPR2 orthologs showed that this part of the ICL3 is structurally not conserved and, therefore, less relevant for receptor function. In contrast, all missense mutations (A89P, G107R, Q174R, V277A) affect receptor positions that were fully preserved during mammalian evolution.

CONCLUSION

Our results provide valuable information about residues critical for AVPR2 folding, trafficking and function and proof that these mutations are responsible for causing NDI in the affected subjects.

Calnexin improves the folding efficiency of mutant rhodopsin in the presence of pharmacological chaperone 11-cis-retinal

The lectin chaperone calnexin (Cnx) is important for quality control of glycoproteins, and the chances of correct folding of a protein increase the longer the protein interacts with Cnx. Mutations in glycoproteins increase their association with Cnx, and these mutant proteins are retained in the endoplasmic reticulum. However, until now, the increased interaction with Cnx was not known to increase the folding of mutant glycoproteins. Because many human diseases result from glycoprotein misfolding, a Cnx-assisted folding of mutant glycoproteins could be beneficial. Mutations of rhodopsin, the glycoprotein pigment of rod photoreceptors, cause misfolding resulting in retinitis pigmentosa. Despite the critical role of Cnx in glycoprotein folding, surprisingly little is known about its interaction with rhodopsin or whether this interaction could be modulated to increase the folding of mutant rhodopsin. Here, we demonstrate that Cnx preferentially associates with misfolded mutant opsins associated with retinitis pigmentosa. Furthermore, the overexpression of Cnx leads to an increased accumulation of misfolded P23H opsin but not the correctly folded protein. Finally, we demonstrate that increased levels of Cnx in the presence of the pharmacological chaperone 11-cis-retinal increase the folding efficiency and result in an increase in correct folding of mutant rhodopsin. These results demonstrate that misfolded rather than correctly folded rhodopsin is a substrate for Cnx and that the interaction between Cnx and mutant, misfolded rhodopsin, can be targeted to increase the yield of folded mutant protein.

Identification and functional analysis of novel variants of the human melanocortin 1 receptor found in melanoma patients

The melanocortin 1 receptor, a Gs protein-coupled receptor expressed in epidermal melanocytes, is a major determinant of skin pigmentation and phototype and an important contributor to melanoma risk. MC1R activation stimulates synthesis of black, strongly photoprotective eumelanin pigments. Several MC1R alleles are associated with red hair, fair skin, increased sensitivity to ultraviolet radiation, and increased skin cancer risk. The MC1R gene is highly polymorphic, but only a few naturally occurring alleles have been functionally characterized, which complicates the establishment of accurate correlations between the signaling properties of mutant alleles and defined cutaneous phenotypes. We report the functional characterization of six MC1R alleles found in Spanish melanoma patients. Two variants (c.152T>C, p.Val51Ala and c.865T>C, p.Cys289Arg) have never been described, and the others (c.112G>A, p.Val38Met; c.122C>T, p.Ser41Phe; c.383T>C, p.Met128Thr; and c.842A>G, p.Asn281Ser) have not been analyzed for function. p.Asn281Ser corresponds to a functionally silent polymorphism. The other mutations are associated with varying degrees of loss of function (LOF), from moderate decreases in coupling to the cAMP pathway (p.Val38Met and p.Val51Ala) to nearly complete absence of functional coupling (p.Ser41Phe, p.Met128Thr, and p.Cys289Arg). The LOF p.Met128Thr and p.Cys289Arg mutants are trafficked to the cell surface, but are unable to bind agonists efficiently. Conversely, LOF of p.Val38Met, p.Ser41Phe, and p.Val51Ala is due to reduced cell surface expression as a consequence of retention in the endoplasmic reticulum (ER). Therefore, LOF of MC1R alleles is frequently associated with aberrant forward trafficking and accumulation within the ER or with inability to bind properly the activatory ligand.

Functional rescue of beta-adrenoceptor dimerization and trafficking by pharmacological chaperones

Site-directed mutagenesis guided by evolutionary trace analysis revealed that substitution of V179 and W183 within a cluster of evolutionarily important residues on the surface of the fourth transmembrane domain of the beta(1)-adrenergic receptor (beta(1)AR) significantly reduced the propensity of the receptor to self-assemble into homodimers as assessed by bioluminescence resonance energy transfer in living cells. These results suggest that mutation of V179 and W183 result in conformational changes that reduce homodimerization either directly by interfering with the dimerization interface or indirectly by causing local misfolding that result in reduced self-assembly. However, the mutations did not cause a general misfolding of the beta(1)AR as they did not prevent heterodimerization with the beta(2)AR. The homodimerization-compromised mutants were significantly retained in the endoplasmic reticulum (ER) and could not be properly matured and trafficked to the cell surface. Lipophilic beta-adrenergic ligands acted as pharmacological chaperones by restoring both dimerization and plasma membrane trafficking of the ER-retained dimerization-compromised beta(1)AR mutants. These results clearly indicate that homodimerization occurs early in the biosynthetic process in the ER and that pharmacological chaperones can promote both dimerization and cell surface targeting, most likely by stabilizing receptor conformations compatible with the two processes.

mu-Opioid receptor cell surface expression is regulated by its direct interaction with Ribophorin I

The trafficking of the mu-opioid receptor (MOR), a member of the rhodopsin G protein-coupled receptor (GPCR) family, can be regulated by interaction with multiple cellular proteins. To determine the proteins involved in receptor trafficking, using the targeted proteomic approach and mass spectrometry analysis, we have identified that Ribophorin I (RPNI), a component of the oligosaccharide transferase complex, could directly interact with MOR. RPNI can be shown to participate in MOR export by the intracellular retention of the receptor after small interfering RNA knockdown of endogenous RPNI. Overexpression of RPNI rescued the surface expression of the MOR 344KFCTR348 deletion mutant independent of calnexin. Furthermore, RPNI regulation of MOR trafficking is dependent on the glycosylation state of the receptor, as reflected by the inability of overexpression of RPNI to affect the trafficking of the N-glycosylation-deficient mutants, or GPCRs that have minimal glycosylation sites. Hence, this novel RPNI chaperone activity is a consequence of N-glycosylation-dependent direct interaction with MOR.

Anterograde trafficking of G protein-coupled receptors: function of the C-terminal F(X)6LL motif in export from the endoplasmic reticulum

We have reported previously that the F(X)(6)LL motif in the C termini is essential for export of alpha(2B)-adrenergic (alpha(2B)-AR) and angiotensin II type 1 receptors (AT1Rs) from the endoplasmic reticulum (ER). Here, we further demonstrate that mutation of the F(X)(6)LL motif similarly abolished the cell-surface expression of alpha(2B)-AR, AT1R, alpha(1B)-AR, and beta(2)-AR, suggesting that the F(X)(6)LL motif plays a general role in ER export of G protein-coupled receptors (GPCRs). Mutation of Phe to Val, Leu, Trp, and Tyr, and mutation of LL to FF and VV, markedly inhibited alpha(2B)-AR transport, indicating that the F(X)(6)LL function cannot be fully substituted by other hydrophobic residues. The structural analysis revealed that the Phe residue in the F(X)(6)LL motif is buried in the transmembrane domains and possibly interacts with Ile58 in beta(2)-AR and Val42 in alpha(2B)-AR, whereas the LL motif is exposed to the cytosolic space. Indeed, mutation of Ile58 in beta(2)-AR and Val42 in alpha(2B)-AR markedly disrupted cell surface transport of the receptors. It is noteworthy that the Val and Ile residues are highly conserved among the GPCRs carrying the F(X)(6)LL motif. Furthermore, the Phe mutant exhibited a stronger interaction with ER chaperones and was more potently rescued by physical and chemical treatments than the LL mutant. These data suggest that the Phe residue is probably involved in folding of alpha(2B)-AR and beta(2)-AR, possibly through interaction with other hydrophobic residues in neighboring domains. These data also provide the first evidence implying crucial roles of the C termini possibly through modulating multiple events in anterograde trafficking of GPCRs.

N-glycan-mediated quality control in the endoplasmic reticulum is required for the expression of correctly folded delta-opioid receptors at the cell surface

A great majority of G protein-coupled receptors are modified by N-glycosylation, but the functional significance of this modification for receptor folding and intracellular transport has remained elusive. Here we studied these phenomena by mutating the two N-terminal N-glycosylation sites (Asn(18) and Asn(33)) of the human delta-opioid receptor, and expressing the mutants from the same chromosomal integration site in stably transfected inducible HEK293 cells. Both N-glycosylation sites were used, and their abolishment decreased the steady-state level of receptors at the cell surface. However, pulse-chase labeling, cell surface biotinylation, and immunofluorescence microscopy revealed that this was not because of intracellular accumulation. Instead, the non-N-glycosylated receptors were exported from the endoplasmic reticulum with enhanced kinetics. The results also revealed differences in the significance of the individual N-glycans, as the one attached to Asn(33) was found to be more important for endoplasmic reticulum retention of the receptor. The non-N-glycosylated receptors did not show gross functional impairment, but flow cytometry revealed that a fraction of them was incapable of ligand binding at the cell surface. In addition, the receptors that were devoid of N-glycans showed accelerated turnover and internalization and were targeted for lysosomal degradation. The results accentuate the importance of protein conformation-based screening before export from the endoplasmic reticulum, and demonstrate how the system is compromised when N-glycosylation is disrupted. We conclude that N-glycosylation of the delta-opioid receptor is needed to maintain the expression of fully functional and stable receptor molecules at the cell surface.

Role of N-linked oligosaccharides in the biosynthetic processing of the cystic fibrosis membrane conductance regulator

The epithelial chloride channel CFTR is a glycoprotein that is modified by two N-linked oligosaccharides. The most common mutant CFTR protein in patients with cystic fibrosis, DeltaF508, is misfolded and retained by ER quality control. As oligosaccharide moieties of glycoproteins are known to mediate interactions with ER lectin chaperones, we investigated the role of N-linked glycosylation in the processing of wild-type and DeltaF508 CFTR. We found that N-glycosylation and ER lectin interactions are not major determinants of trafficking of wild-type and DeltaF508 from the ER to the plasma membrane. Unglycosylated CFTR, generated by removal of glycosylation sites or treatment of cells with the N-glycosylation inhibitor tunicamycin, did not bind calnexin, but did traffic to the cell surface and exhibited chloride channel activity. Most importantly, unglycosylated DeltaF508 CFTR still could not escape quality control in the early secretory pathway and remained associated with the ER. However, the absence of N-linked oligosaccharides did reduce the stability of wild-type CFTR, causing significantly more-rapid turnover in post-ER compartments. Surprisingly, the individual N-linked carbohydrates do not play equivalent roles and modulate the fate of the wild-type protein in different ways in its early biosynthetic pathway.

The multifunctional protein GC1q-R interacts specifically with the i3 loop arginine cluster of the vasopressin V2 receptor

In this study, we identified the multifunctional protein GC1q-R as a novel vasopressin V(2) receptor (V(2)R) interacting protein. For this purpose, we have developed a proteomic approach combining pull-down assays using a cyclic peptide mimicking the third intracellular loop of V(2)R as a bait and mass spectrometry analyses of proteins isolated from either rat or human kidney tissues or the HEK 293 cell line. Co-immunoprecipitation of GC1q-R with the c-Myc-tagged h-V(2)R expressed in a HEK cell line confirmed the existence of a specific interaction between GC1q-R and the V(2) receptor. Then, construction of a mutant receptor in i3 loop allowed us to identify the i3 loop arginine cluster of the vasopressin V(2) receptor as the interacting determinant for GC1q-R interaction. Using purified receptor as a bait and recombinant (74-282) GC1q-R, we demonstrated a direct and specific interaction between these two proteins via the arginine cluster.

G Protein-Coupled Receptor Trafficking in Health and Disease: Lessons Learned to Prepare for Therapeutic Mutant Rescue in Vivo

G protein-coupled receptors (GPCR) comprise the largest family of drug targets. This is not surprising as many signaling systems rely on this class of receptor to convert external and internal stimuli to intracellular responses. As is the case with other membrane proteins, GPCRs are subjected to a stringent quality control mechanism at the endoplasmic reticulum, which ensures that only correctly folded proteins enter the secretory pathway. Because of this quality control system, point mutations resulting in protein sequence variations may result in the production of misfolded and disease-causing proteins that are unable to reach their functional destinations in the cell. There is now a wealth of information demonstrating the functional rescue of misfolded mutant receptors by small nonpeptide molecules originally designed to serve as receptor antagonists; these small molecules ("pharmacoperones") serve as molecular templates, promoting correct folding and allowing the mutants to pass the scrutiny of the cellular quality control system and be expressed at the cell surface membrane. Two of these systems are especially well characterized: the gonadotropin-releasing hormone and the vasopressin type 2 receptors, which play important roles in regulating reproduction and water homeostasis, respectively. Mutations in these receptors can lead to well defined diseases that are recognized as being caused by receptor misfolding that may potentially be amenable to treatment with pharmacoperones. This review is focused on protein misfolding and misrouting related to various disease states, with special emphasis on these two receptors, which have proved to be of value for development of drugs potentially useful in regulating GPCR trafficking in healthy and disease states.

Pharmacological chaperones in nephrogenic diabetes insipidus: possibilities for clinical application

The antidiuretic hormone arginine-vasopressin regulates water homeostasis in the human body by binding to its vasopressin type 2 receptor (V2R). Mutations in AVPR2, the gene encoding V2R, lead to the X-linked congenital form of nephrogenic diabetes insipidus (NDI), a disease characterized by the inability to concentrate urine in response to vasopressin; often this involves missense mutations or deletion of one or a few amino acids. In vitro V2R expression studies revealed that the function of most of these receptors is not disturbed, but due to their misfolding, the quality control mechanism of the endoplasmic reticulum (ER) retains these receptors inside the cell, thereby preventing their functioning at the plasma membrane. This review summarizes our current knowledge on ER retention of V2R mutants, and describes the different approaches that have been undertaken to restore the plasma membrane expression and function of V2R mutants in NDI in vitro and in vivo. The use of cell permeable receptor ligands (called 'pharmacological chaperones') appears promising for the treatment of NDI in a subset of patients.

Opioid receptor pharmacological chaperones act by binding and stabilizing newly synthesized receptors in the endoplasmic reticulum

Accumulating evidence has indicated that membrane-permeable G protein-coupled receptor ligands can enhance cell surface targeting of their cognate wild-type and mutant receptors. This pharmacological chaperoning was thought to result from ligand-mediated stabilization of immature receptors in the endoplasmic reticulum (ER). In the present study, we directly tested this hypothesis using wild-type and mutant forms of the human delta-opioid receptor as models. ER-localized receptors were isolated by expressing the receptors in HEK293 cells under tightly controlled tetracycline induction and blocking their ER export with brefeldin A. The ER-retained delta-opioid receptor precursors were able to bind [(3)H]diprenorphine with high affinity, and treatment of cells with an opioid antagonist naltrexone led to a 2-fold increase in the number of binding sites. After removing the transport block, the antagonist-mediated increase in the number of receptors was detectable at the cell surface by flow cytometry and cell surface biotinylation assay. Importantly, opioid ligands, both antagonists and agonists, were found to stabilize the ER-retained receptor precursors in an in vitro heat inactivation assay and the treatment enhanced dissociation of receptor precursors from the molecular chaperone calnexin. Thus, we conclude that pharmacological chaperones facilitate plasma membrane targeting of delta-opioid receptors by binding and stabilizing receptor precursors, thereby promoting their release from the stringent ER quality control.

Molecular genetic study of congenital nephrogenic diabetes insipidus and rescue of mutant vasopressin V2 receptor by chemical chaperones

AIM

X-linked nephrogenic diabetes insipidus is a rare disease caused by mutations in the arginine vasopressin V2 receptor (AVPR2) gene, which encodes vasopressin V2 receptor (V2R). More than a half of reported mutations in AVPR2 are missense mutations, and a large number of missense mutant receptors fail to fold properly and therefore are not routed to the cell surface.

METHODS

We analysed the AVPR2 gene in 14 unrelated patients with X-linked nephrogenic diabetes insipidus, and found 13 different mutations including eight missense point mutations. The cellular expression patterns of three missense mutant (A98P, L274P and R113W) and wild-type V2R were determined in transfected COS-7 cells.

RESULTS

In contrast to wild-type V2R, the cell-surface expressions of mutant receptors were totally (A98P and L274P) or partially (R113W) absent. Instead, they were retained intracellularly. However, treatment of cells with two chemical chaperones (100 mmol/L trimethylamine oxide or 2% dimethyl sulfoxide) or incubation at 26 degrees C restored the cell-surface expressions of mutant receptors.

CONCLUSION

These data show that some chemical chaperones correct the mistrafficking of misfolded A98P, L274P and R113W V2R. Thus, we believe that a therapeutic strategy based on chemical chaperones in patients with these mutations is worth trying.

D1 and D2 dopamine receptor expression is regulated by direct interaction with the chaperone protein calnexin

As for all proteins, G protein-coupled receptors (GPCRs) undergo synthesis and maturation within the endoplasmic reticulum (ER). The mechanisms involved in the biogenesis and trafficking of GPCRs from the ER to the cell surface are poorly understood, but they may involve interactions with other proteins. We have now identified the ER chaperone protein calnexin as an interacting protein for both D(1) and D(2) dopamine receptors. These protein-protein interactions were confirmed using Western blot analysis and co-immunoprecipitation experiments. To determine the influence of calnexin on receptor expression, we conducted assays in HEK293T cells using a variety of calnexin-modifying conditions. Inhibition of glycosylation either through receptor mutations or treatments with glycosylation inhibitors partially blocks the interactions with calnexin with a resulting decrease in cell surface receptor expression. Confocal fluorescence microscopy reveals the accumulation of D(1)-green fluorescent protein and D(2)-yellow fluorescent protein receptors within internal stores following treatment with calnexin inhibitors. Overexpression of calnexin also results in a marked decrease in both D(1) and D(2) receptor expression. This is likely because of an increase in ER retention because confocal microscopy revealed intracellular clustering of dopamine receptors that were co-localized with an ER marker protein. Additionally, we show that calnexin interacts with the receptors via two distinct mechanisms, glycan-dependent and glycan-independent, which may underlie the multiple effects (ER retention and surface trafficking) of calnexin on receptor expression. Our data suggest that optimal receptor-calnexin interactions critically regulate D(1) and D(2) receptor trafficking and expression at the cell surface, a mechanism likely to be of importance for many GPCRs.

The endoplasmic reticulum Ca2+-pump SERCA2b interacts with G protein-coupled receptors and enhances their expression at the cell surface

Calcium (Ca(2+)) plays a pivotal role in both cellular signaling and protein synthesis. However, it is not well understood how calcium metabolism and synthesis of secreted and membrane-bound proteins are related. Here we demonstrate that the sarco(endo)plasmic reticulum Ca(2+) ATPase 2b (SERCA2b), which maintains high Ca(2+) concentration in the lumen of the endoplasmic reticulum, interacts specifically with the human delta opioid receptor during early steps of receptor biogenesis in human embryonic kidney 293 cells. The interaction involves newly synthesized incompletely folded receptor precursors, because the association between the delta opioid receptor and SERCA2b (i) was short-lived and took place soon after receptor translation, (ii) was not affected by misfolding of the receptor, and (iii) decreased if receptor folding was enhanced by opioid receptor pharmacological chaperone. The physical association with SERCA2b was found to be a universal feature among G protein-coupled receptors within family A and was shown to occur also between the endogenously expressed luteinizing hormone receptor and SERCA2b in rat ovaries. Importantly, active SERCA2b rather than undisturbed Ca(2+) homeostasis was found to be essential for delta opioid receptor biogenesis, as inhibition of its Ca(2+) pumping activity by thapsigargin reduced the interaction and impaired the efficiency of receptor maturation, two phenomena that were not affected by a Ca(2+) ionophore A23187. Nevertheless, inhibition of SERCA2b did not compromise the functionality of receptors that were able to mature. Thus, we propose that the association with SERCA2b is required for efficient folding and/or membrane integration of G protein-coupled receptors.

Regulation and dysregulation of aquaporins in water balance disorders

The discovery of aquaporin-1 (AQP1) explained the long-standing biophysical question of how water specifically crosses biological membranes. These studies led to the identification of a whole new family of membrane proteins, the aquaporin water channels. At present, at least eight aquaporins are expressed at distinct sites in the kidney and four members of this family (AQP1-4) have been demonstrated to play pivotal roles in the physiology and pathophysiology for renal regulation of body water balance. In the present review, a number of inherited and acquired conditions characterized by urinary concentration defects as well as common diseases associated with severe water retention are discussed with relation to the role of aquaporins in regulation and dysregulation of renal water transport.

Molecular basis and clinical features of nephrogenic diabetes insipidus

Maintenance of water and electrolyte homeostasis is central to mammalian survival and, therefore, under stringent hormonal control. Water homeostasis is achieved by balancing fluid intake with water excretion, governed by the antidiuretic action of arginine vasopressin. Arginine vasopressin stimulation of renal V₂ vasopressin receptors in the basolateral membrane of principal cells induces aquaporin-2-mediated water reabsorption in the kidney. The importance of this system is apparent when mutations inactivate V₂ vasopressin receptors and aquaporin-2 and cause the clinical phenotype of nephrogenic diabetes insipidus. To date, over 190 mutations in the V₂ vasopressin receptors gene (AVPR2) and approximately 38 mutations in the aquaporin-2 gene have been identified in patients with inherited nephrogenic diabetes insipidus. Extensive in vitro expression and mutagenesis studies of V₂ vasopressin receptors and aquaporin-2 have provided detailed insights into the molecular mechanisms of G-protein-coupled receptor and water channel dysfunction per se. Targeted deletions of AVPR2 and AQP2 in mice have extended the knowledge of nephrogenic diabetes insipidus pathophysiology and have stimulated testing of old and new ideas to therapeutically restore normal kidney function in animal models and patients with this disease. In this review, we summarize the current knowledge relevant to understand the molecular basis of inherited nephrogenic diabetes insipidus forms and the rationales for the current pharmacological treatment of patients with this illness.

Regulation of G protein-coupled receptor export trafficking

G protein-coupled receptors (GPCRs) constitute a superfamily of cell-surface receptors which share a common topology of seven transmembrane domains and modulate a variety of cell functions through coupling to heterotrimeric G proteins by responding to a vast array of stimuli. The magnitude of cellular response elicited by a given signal is dictated by the level of GPCR expression at the plasma membrane, which is the balance of elaborately regulated endocytic and exocytic trafficking. This review will cover recent advances in understanding the molecular mechanism underlying anterograde transport of the newly synthesized GPCRs from the endoplasmic reticulum (ER) through the Golgi to the plasma membrane. We will focus on recently identified motifs involved in GPCR exit from the ER and the Golgi, GPCR folding in the ER and the rescue of misfolded receptors from within, GPCR-interacting proteins that modulate receptor cell-surface targeting, pathways that mediate GPCR traffic, and the functional role of export in controlling GPCR signaling.

Inactivating mutations of G protein-coupled receptors and diseases: Structure-function insights and therapeutic implications

Since the discovery of the first rhodopsin mutation that causes retinitis pigmentosa in 1990, significant progresses have been made in elucidating the pathophysiology of diseases caused by inactivating mutations of G protein-coupled receptors (GPCRs). This review aims to compile the compelling evidence accumulated during the past 15 years demonstrating the etiologies of more than a dozen diseases caused by inactivating GPCR mutations. A generalized classification scheme, based on the life cycle of GPCRs, is proposed. Insights gained through detailed studies of these naturally occurring mutations into the structure-function relationship of these receptors are reviewed. Therapeutic approaches directed against the different classes of mutants are being developed. Since intracellular retention emerges as the most common defect, recent progresses aimed at correcting this defect through membrane permeable pharmacological chaperones are highlighted.

Cell biological aspects of the vasopressin type-2 receptor and aquaporin 2 water channel in nephrogenic diabetes insipidus

In the renal collecting duct, water reabsorption is regulated by the antidiuretic hormone vasopressin (AVP). Binding of this hormone to the vasopressin V2 receptor (V2R) leads to insertion of aquaporin-2 (AQP2) water channels in the apical membrane, thereby allowing water reabsorption from the pro-urine to the interstitium. The disorder nephrogenic diabetes insipidus (NDI) is characterized by the kidney's inability to concentrate pro-urine in response to AVP, which is mostly acquired due to electrolyte disturbances or lithium therapy. Alternatively, NDI is inherited in an X-linked or autosomal fashion due to mutations in the genes encoding V2R or AQP2, respectively. This review describes the current knowledge of the cell biological causes of NDI and how these defects may explain the patients' phenotypes. Also, the increased understanding of these cellular defects in NDI has opened exciting initiatives in the development of novel therapies for NDI, which are extensively discussed in this review.