Nephrogenic diabetes insipidus

The Interaction of Vasopressin with Hormones of the Hypothalamo-Pituitary-Adrenal Axis: The Significance for Therapeutic Strategies in Cardiovascular and Metabolic Diseases

A large body of evidence indicates that vasopressin (AVP) and steroid hormones are frequently secreted together and closely cooperate in the regulation of blood pressure, metabolism, water-electrolyte balance, and behavior, thereby securing survival and the comfort of life. Vasopressin cooperates with hormones of the hypothalamo-pituitary-adrenal axis (HPA) at several levels through regulation of the release of corticotropin-releasing hormone (CRH), adrenocorticotropic hormone (ACTH), and multiple steroid hormones, as well as through interactions with steroids in the target organs. These interactions are facilitated by positive and negative feedback between specific components of the HPA. Altogether, AVP and the HPA cooperate closely as a coordinated functional AVP-HPA system. It has been shown that cooperation between AVP and steroid hormones may be affected by cellular stress combined with hypoxia, and by metabolic, cardiovascular, and respiratory disorders; neurogenic stress; and inflammation. Growing evidence indicates that central and peripheral interactions between AVP and steroid hormones are reprogrammed in cardiovascular and metabolic diseases and that these rearrangements exert either beneficial or harmful effects. The present review highlights specific mechanisms of the interactions between AVP and steroids at cellular and systemic levels and analyses the consequences of the inappropriate cooperation of various components of the AVP-HPA system for the pathogenesis of cardiovascular and metabolic diseases.

Lithium-induced Nephrogenic Diabetes Insipidus with Efficacy of Desmopressin in Combination with Thiazide Diuretics and Non-steroidal Anti-inflammatory Drugs: A Case Report with a Review of the Literature

Nephrogenic diabetes insipidus (NDI) is characterized by excessive urination and an inability to concentrate urine. Lithium is the most common cause of acquired NDI. Treatment typically involves thiazide diuretics and nonsteroidal anti-inflammatory drugs (NSAIDs). However, the efficacy of desmopressin in NDI remains unclear. We herein report a case of lithium-induced NDI in a 71-year-old woman with lithium-induced NDI. Thiazide diuretics and NSAIDs reduced the urine output by approximately 40% compared to pretreatment, while the addition of desmopressin reduced it by approximately 70%. This case suggests that desmopressin can be a viable treatment option for lithium-induced NDI.

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.

Gαs and Gαq/11 protein coupling bias of two AVPR2 mutants (R68W and V162A) that cause nephrogenic diabetes insipidus

Loss-of-function mutations of the arginine vasopressin receptor 2 gene (AVPR2) cause Nephrogenic diabetes insipidus (NDI). AVPR2 is a kind of G protein coupled receptor (GPCR) and mainly couples with Gαs protein leading to cAMP accumulation in the cell as a secondary messenger. Recent studies showed that some AVPR2 mutations could cause biased Gαq/11 protein coupling rather than Gαs. Investigation into the characterization of biased receptors may give insights into the relationship between the conformational change of the receptor because of the mutation and related downstream signaling. In this study, R68W and V162A were analyzed to whether they show a bias to Gαs or Gαq/11 proteins. Their functionality in terms of cAMP production via Gαs protein coupling was decreased compared to the wild-type receptor. On the other hand, they showed the ability to couple with Gαq/11 protein and make Ca²⁺ mobilization at different levels in the cell. R68W showed bias to coupling with Gαq/11 protein rather than V162A and wild-type receptor. Studies about the Gα protein coupling bias of mutant AVPR2s may broaden our understanding of the relationship between the changed conformation of the receptor and consequently activated signaling pathways, and also may shed light on the development of more effective new therapeutics.

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.

A Novel Missense Mutation of Arginine Vasopressin Receptor 2 in a Chinese Family with Congenital Nephrogenic Diabetes Insipidus: X-Chromosome Inactivation in Female CNDI Patients with Heterozygote 814A>G Mutation

Background

To identify novel clinical phenotypic signatures of congenital nephrogenic diabetes insipidus (CNDI).

Methods

A Chinese family with CNDI was recruited for participation in this study. The proband and one of his uncles suffered from polydipsia and polyuria since infancy. The results of clinical testing indicated the diagnosis of CNDI. 10 family members had similar symptoms but did not seek medical advice. Genetic testing of mutations in the coding region of the aquaporin 2 (AQP2) gene and the arginine vasopressin receptor 2 (AVPR2) gene were carried out in 11 family members. Somatic DNA from 5 female family members was used to test for methylation of polymorphic CAG repeats in the human androgen receptor (AR) gene, as an index for X-chromosome inactivation pattern (XCIP).

Results

AQP2 gene mutations were not found in any family members, but a novel missense mutation (814th base A>G) in exon 2 of the AVPR2 gene was identified in 10 individuals. This mutation leads to a Met 272 Val (GAT-GGT) amino acid substitution. Skewed X-chromosome inactivation patterns of the normal X allele were observed in 4 females with the AVPR2 gene mutation and symptoms of diabetes insipidus, but not in an asymptomatic female with the AVPR2 gene mutation.

Conclusions

Met 272 Val mutation of the AVPR2 gene was identified as a novel genetic risk factor for CDNI. The clinical NDI phenotype of female carriers with heterozygous AVPR2 mutation may be caused by X-chromosome inactivation induced by dominant methylation of the normal allele of AVPR2 gene.

The effect of electron dose on positive polymethyl methacrylate resist for nanolithography of gold bowtie nanoantennas

Plasmonic structures, such as bowtie nanoantennas, may be used in Surface Enhanced Raman Spectroscopy (SERS). Nanoantennas can be employed to amplify the biomolecular and chemical reactions, which is useful for biomedical applications. The electric field created by nanoantennas are optimized when the resonant wavelength of the probed laser light closely matches the resonant wavelength of the plasmonic structure. In this work, we fabricated several bowtie nanoantennas with varying geometric spacing for use with a 532 nm wavelength laser line in Raman Spectroscopy. The fabrication utilized nanolithography by electron beam lithography on a Raith Voyager, development, deposition, and metal lift-off. This study explored a specific bowtie nanoantenna geometry of 270 nm equilateral sides triangle pairs with 3 varying gap sizes, 50 nm, 20 nm, and 10 nm, and the effect of varying electron beam doses on the final structure of the nanoantenna. The results presented here, will show that the working dose factor range is 6.5-10.3 (650-10,300 μC/cm2) for 120 nm thick polymethyl methacrylate (PMMA), and with a 44.78% increase in dose, the footprint area increases between 5.9% and 10.7%.

Nephrogenic diabetes insipidus: a comprehensive overview

Nephrogenic diabetes insipidus (NDI) is characterized by the inability to concentrate urine that results in polyuria and polydipsia, despite having normal or elevated plasma concentrations of arginine vasopressin (AVP). In this study, we review the clinical aspects and diagnosis of NDI, the various etiologies, current treatment options and potential future developments. NDI has different clinical manifestations and approaches according to the etiology. Hereditary forms of NDI are mainly caused by mutations in the genes that encode key proteins in the AVP signaling pathway, while acquired causes are normally associated with specific drug exposure, especially lithium, and hydroelectrolytic disorders. Clinical manifestations of the disease vary according to the degree of dehydration and hyperosmolality, being worse when renal water losses cannot be properly compensated by fluid intake. Regarding the diagnosis of NDI, it is important to consider the symptoms of the patient and the diagnostic tests, including the water deprivation test and the baseline plasma copeptin measurement, a stable surrogate biomarker of AVP release. Without proper treatment, patients may developcomplications leading to high morbidity and mortality, such as severe dehydration and hypernatremia. In that sense, the treatment of NDI consists in decreasing the urine output, while allowing appropriate fluid balance, normonatremia, and ensuring an acceptable quality of life. Therefore, therapeutic options include nonpharmacological interventions, including sufficient water intake and a low-sodium diet, and pharmacological treatment. The main medications used for NDI are thiazide diuretics, nonsteroidal anti-inflammatory drugs (NSAIDs), and amiloride, used isolated or in combination.

[Physiological role of the apelin receptor: implication in body fluid homeostasis and hyponatremia]

Apelin, a vasoactive neuropeptide, its receptor and arginine-vasopressin (AVP, antidiuretic hormone) are co-localized in magnocellular vasopressinergic neurons. In the kidney, the apelin receptor is present in glomerular arterioles and the collecting duct (CD) where the AVP type 2 (V2-R) receptors are located. Apelin exerts an aquaretic action both by its inhibitory effect on the phasic electrical activity of vasopressinergic neurons and the secretion of AVP into the bloodstream and by its direct actions at the kidney level resulting in an increase in the renal microcirculation and the inhibition of the antidiuretic effect of AVP mediated by V2-R in the CD. Plasma apelin and AVP are conversely regulated by osmotic stimuli in both humans and rodents, showing that apelin is involved with AVP in maintaining body fluid homeostasis. Clinically, in patients with inappropriate antidiuresis syndrome (SIAD), the apelin/AVP balance is altered, which contributes to water metabolism defect. Activation of the apelin receptor by the metabolically stable apelin-17 analog, that increases aqueous diuresis and moderately water intake and gradually corrects hyponatremia, may constitute a new approach for the treatment of SIAD.

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.

Hereditary tubulopathies accompanying polyuia

Tubulopathies are a group of heterogeneous diseases that are manifested in the malfunction of the renal tubules. This review addresses tubulopathies associated with polyuria syndrome, namely renal glucosuria syndrome, nephrogenic diabetes insipidus and pseudohyperaldosteronism. Types of renal glucosuria are described, namely: type A, type B and the most severe type 0. Type A is characterized by a low filtration threshold and low glucose reabsorption. The type of inheritance is autosomal recessive. Type B, autosomal dominant, is characterized by uneven activity of glucose transport, in which its reabsorption is reduced only in some nephrons. That is, normal reabsorption of glucose is maintained, but the filtration threshold of the latter is reduced. Type 0 with a severe course is characterized by complete inability of epithelial cells of the proximal tubules to reabsorb glucose. Nephrogenic diabetes insipidus is a rare inherited disease caused by impaired response of the renal tubules to antidiuretic hormone (ADH). Depending on the degree of inability to concentrate urine, there are complete and partial forms. It is divided into nephrogenic diabetes insipidus type I (X-linked recessive); nephrogenic diabetes insipidus type II (autosomal recessive and autosomal dominant) and nephrogenic diabetes insipidus syndrome with dementia and intracerebral calcifications (type of inheritance remains unknown). Children with autosomal recessive type of inheritance suffer from the more severe disease course. Pseudohypoaldosteronism is characterized by a special condition of the renal tubules which is due to insufficient sensitivity of the tubular epithelium to aldosterone, which in turn leads to hyperaldosteronism, the development of hyponatremia, metabolic acidosis with hyperkalemia, polydipsia and polyuria, decreased sodium reabsorption and retardation of the child's physical development. The classification includes three syndromes of pseudohypoaldosteronism, namely: type I (PHA1), which is divided into PHA1A (autosomal dominant, renal), PHA1B (autosomal recessive, systemic); type II (PHA2; Gordon’s syndrome), type III (secondary), which develops as a result of renal pathology.

Rapid development of vasopressin resistance in dietary K+ deficiency

The association between diabetes insipidus (DI) and chronic dietary K+ deprivation is well known, but it remains uncertain how the disorder develops and whether it is influenced by the sexual dimorphism in K+ handling. Here, we determined the plasma K+ (PK) threshold for DI in male and female mice and ascertained if DI is initiated by polydipsia or by a central or nephrogenic defect. C57BL6J mice were randomized to a control diet or to graded reductions in dietary K+ for 8 days, and kidney function and transporters involved in water balance were characterized. We found that male and female mice develop polyuria and secondary polydipsia. Altered water balance coincided with a decrease in aquaporin-2 (AQP2) phosphorylation and apical localization despite increased levels of the vasopressin surrogate marker copeptin. No change in the protein abundance of urea transporter-A1 was observed. The Na+-K+-2Cl- cotransporter decreased only in males. Desmopressin treatment failed to reverse water diuresis in K+-restricted mice. These findings indicate that even a small fall in PK is associated with nephrogenic DI (NDI), coincident with the development of altered AQP2 regulation, implicating low PK as a causal trigger of NDI. We found that PK decreased more in females, and, consequently, females were more prone to develop NDI. Together, these data indicate that AQP2 regulation is disrupted by a small decrease in PK and that the response is influenced by sexual dimorphism in K+ handling. These findings provide new insights into the mechanisms linking water and K+ balances and support defining the disorder as "potassium-dependent NDI."NEW & NOTEWORTHY This study shows that aquaporin-2 regulation is disrupted by a small fall in plasma potassium levels and the response is influenced by sexual dimorphism in renal potassium handling. The findings provided new insights into the mechanisms by which water balance is altered in dietary potassium deficiency and support defining the disorder as "potassium-dependent nephrogenic diabetes insipidus."

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.

Characterization of five novel vasopressin V2 receptor mutants causing nephrogenic diabetes insipidus reveals a role of tolvaptan for M272R-V2R mutation

Nephrogenic diabetes insipidus (NDI) is a rare tubulopathy characterized by urinary concentration defect due to renal resistance to vasopressin. Loss-of-function mutations of vasopressin V2 receptor (V2R) gene (AVPR2) is the most common cause of the disease. We have identified five novel mutations L86P, R113Q, C192S, M272R, and W323_I324insR from NDI-affected patients. Functional characterization of these mutants revealed that R113Q and C192S were normally localized at the basolateral membrane of polarized Madin-Darby Canine Kidney (MDCK) cells and presented proper glycosylation maturation. On the other side, L86P, M272R, and W323_I324insR mutants were retained in endoplasmic reticulum and exhibited immature glycosylation and considerably reduced stability. All five mutants were resistant to administration of vasopressin analogues as evaluated by defective response in cAMP release. In order to rescue the function of the mutated V2R, we tested VX-809, sildenafil citrate, ibuprofen and tolvaptan in MDCK cells. Among these, tolvaptan was effective in rescuing the function of M272R mutation, by both allowing proper glycosylation maturation, membrane sorting and response to dDAVP. These results show an important proof of concept for the use of tolvaptan in patients affected by M272R mutation of V2R causing NDI.

Specific TBC Domain-Containing Proteins Control the ER-Golgi-Plasma Membrane Trafficking of GPCRs

G-protein-coupled receptors (GPCRs) constitute the largest superfamily of cell surface signaling proteins. However, the molecular mechanisms underlying their cell surface delivery after synthesis remain poorly understood. Here, we screen the TBC domain-containing proteins, putative Rab GTPase-activating proteins (GAPs), in the intracellular trafficking of GPCRs and identify several TBC proteins that activity-dependently regulate the anterograde transport, en route from the endoplasmic reticulum to the Golgi or from the Golgi to the cell surface, of several prototypic GPCR members without affecting other plasma membrane proteins. We also show that TBC1D6 functions as a GAP for Rab26, physically associates with Rab26, and attenuates Rab26 interaction with GPCRs. Furthermore, both overexpression and depletion of TBC1D6 inhibit the post-Golgi traffic of GPCRs. These data demonstrate important roles of the TBC proteins in forward trafficking of nascent GPCRs and reveal regulatory mechanisms of GPCR targeting to the functional destination.

Wei et al. report that several TBC proteins specifically and activity-dependently regulate ER-Golgi-plasma membrane transport of nascent GPCRs. They also show that TBC1D6 is a GAP for Rab26 and controls GPCR post-Golgi traffic. Their results reveal crucial roles of TBC proteins in and provide regulatory mechanisms of GPCR trafficking.

Gain-of-function mutations of the V2 vasopressin receptor in nephrogenic syndrome of inappropriate antidiuresis (NSIAD): a cell-based assay to assess constitutive water reabsorption

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently identified chromosome X-linked disease associated with gain-of-function mutations of the V2 vasopressin receptor (V2R), a G-protein-coupled receptor. It is characterized by inability to excrete a free water load, hyponatremia, and undetectable vasopressin-circulating levels. Hyponatremia can be quite severe in affected male children. To gain a deeper insight into the functional properties of the V2R active mutants and how they might translate into the pathological outcome of NSIAD, in this study, we have expressed the wild-type V2R and three constitutively active V2R mutants associated with NSIAD (R137L, R137C, and the F229V) in MCD4 cells, a cell line derived from renal mouse collecting duct, stably expressing the vasopressin-sensitive water channel aquaporin-2 (AQP2). Our findings indicate that in cells expressing each active mutant, AQP2 was constitutively localized to the apical plasma membrane in the absence of vasopressin stimulation. In line with these observations, under basal conditions, osmotic water permeability in cells expressing the constitutively active mutants was significantly higher compared to that of cells expressing the wild-type V2R. Our findings demonstrate a direct link between activating mutations of the V2R and the perturbation of water balance in NSIAD. In addition, this study provides a useful cell-based assay system to assess the functional consequences of newly discovered activating mutations of the V2R on water permeability in kidney cells and to screen the effect of drugs on the mutated receptors.

Involvement of PDZ-SAP97 interactions in regulating AQP2 translocation in response to vasopressin in LLC-PK1 cells

Arginine-vasopressin (AVP)-mediated translocation of aquaporin-2 (AQP2) protein-forming water channels from storage vesicles to the membrane of renal collecting ducts is critical for the renal conservation of water. The type-1 PDZ-binding motif (PBM) in AQP2, "GTKA," is a critical barcode for its translocation, but its precise role and that of its interacting protein partners in this process remain obscure. We determined that synapse-associated protein-97 (SAP97), a membrane-associated guanylate kinase protein involved in establishing epithelial cell polarity, was an avid binding partner to the PBM of AQP2. The role of PBM and SAP97 on AQP2 redistribution in response to AVP was assessed in LLC-PK1 renal collecting cells by confocal microscopy and cell surface biotinylation techniques. These experiments indicated that distribution of AQP2 and SAP97 overlapped in the kidneys and LLC-PK1 cells and that knockdown of SAP97 inhibited the translocation of AQP2 in response to AVP. Binding between AQP2 and SAP97 was mediated by specific interactions between the second PDZ of SAP97 and PBM of AQP2. Mechanistically, inactivation of the PBM of AQP2, global delocalization of PKA, or knockdown of SAP97 inhibited AQP2 translocation as well as AVP- and forskolin-mediated phosphorylation of Ser256 in AQP2, which serves as the major translocation barcode of AQP2. These results suggest that the targeting of PKA to the microdomain of AQP2 via SAP97-AQP2 interactions in association with cross-talk between two barcodes in AQP2, namely, the PBM and phospho-Ser256, plays an important role in the translocation of AQP2 in the kidney.

Cardio-renal safety of non-steroidal anti-inflammatory drugs

Non-steroidal anti-inflammatory drugs (NSAIDs) are among the most widely used therapeutic class in clinical medicine. These are sub-divided based on their selectivity for inhibition of cyclooxygenase (COX) isoforms (COX-1 and COX-2) into: (1) non-selective (ns-NSAIDs), and (2) selective NSAIDs (s-NSAIDs) with preferential inhibition of COX-2 isozyme. The safety and pathophysiology of NSAIDs on the renal and cardiovascular systems have continued to evolve over the years following short- and long-term treatment in both preclinical models and humans. This review summarizes major learnings on cardiac and renal complications associated with pharmaceutical inhibition of COX-1 and COX-2 with focus on preclinical to clinical translatability of cardio-renal data.

Hydration, Arginine Vasopressin, and Glucoregulatory Health in Humans: A Critical Perspective

Glucoregulatory diseases, such as type 2 diabetes are currently a key public health priority. Public health messages have started to include the addition of water in their dietary guidelines. Such guidelines however are not based on causal evidence pertaining to the health effects of increased water intake, but rather more heavily based upon non-causal or mechanistic data. One line of thinking linking fluid intake and health is that hypohydration induces elevated blood concentrations of arginine vasopressin (AVP). Research in the 1970s and 1980s implicated AVP in glucoregulation, supported by observational evidence. This important area of research subsequently appeared to stop until the 21st century during which interest in hypertonic saline infusion studies, animal AVP receptor knockout models, dietary and genetic associations, and human interventions manipulating hydration status have resurged. This narrative review briefly describes and critically evaluates the usefulness of the current AVP-glucoregulatory research. We offer suggestions on how to test the independent glucoregulatory effects of body water changes compared to elevated circulating AVP concentrations, such as investigating hydration manipulations using 3,4-Methylenedioxymethamphetamine. Whilst much research is still needed before making firm conclusions, the current evidence suggests that although AVP may be partially implicated in glucoregulation, more ecologically valid models using human participants suggests this effect might be independent of the hydration status. The key implication of this hypothesis if confirmed in future research is that manipulating the hydration status to reduce circulating AVP concentrations may not be an effective method to improve glucoregulatory health.

Focus on neonatal and infantile onset of nephrogenic syndrome of inappropriate antidiuresis: 12 years later

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD), first described in 2005, is a rare genetic X-linked disease, presenting with hyponatremia, hyposmolarity, euvolemia, inappropriately concentrated urine, increased natriuresis, and undetectable or very low arginine-vasopressine (AVP) circulating levels. It can occur in neonates, infants, or later in life. NSIAD must be early recognized and treated to prevent severe hyponatremia, which can show a dangerous impact on neonatal outcome. In fact, it potentially leads to death or, in case of survival, neurologic sequelae. This review is an update of NSIAD 12 years after the first description, focusing on reported cases of neonatal and infantile onset. The different molecular patterns affecting the AVP receptor 2 (V2R) and determining its gain of function are reported in detail; moreover, we also provide a comparison between the different triggers involved in the development of hyponatremia, the evolution of the symptoms, and modality and efficacy of the different treatments available.

Inhibition of non-receptor tyrosine kinase Src induces phosphoserine 256-independent aquaporin-2 membrane accumulation

KEY POINTS

Aquaporin-2 (AQP2) is crucial for water homeostasis, and vasopressin (VP) induces AQP2 membrane trafficking by increasing intracellular cAMP, activating PKA and causing phosphorylation of AQP2 at serine 256, 264 and 269 residues and dephosphorylation of serine 261 residue on the AQP2 C-terminus. It is thought that serine 256 is the master regulator of AQP2 trafficking, and its phosphorylation has to precede the change of phosphorylation state of other serine residues. We found that Src inhibition causes serine 256-independent AQP2 membrane trafficking and induces phosphorylation of serine 269 independently of serine 256. This targeted phosphorylation of serine 269 is important for Src inhibition-induced AQP2 membrane accumulation; without serine 269, Src inhibition exerts no effect on AQP2 trafficking. This result helps us better understand the independent pathways that can target different AQP2 residues, and design new strategies to induce or sustain AQP2 membrane expression when VP signalling is defective.

ABSTRACT

Aquaporin-2 (AQP2) is essential for water homeostasis. Upon stimulation by vasopressin, AQP2 is phosphorylated at serine 256 (S256), S264 and S269, and dephosphorylated at S261. It is thought that S256 is the master regulator of AQP2 trafficking and membrane accumulation, and that its phosphorylation has to precede phosphorylation of other serine residues. In this study, we found that VP reduces Src kinase phosphorylation: by suppressing Src using the inhibitor dasatinib and siRNA, we could increase AQP2 membrane accumulation in cultured AQP2-expressing cells and in kidney collecting duct principal cells. Src inhibition increased exocytosis and inhibited clathrin-mediated endocytosis of AQP2, but exerted its effect in a cAMP, PKA and S256 phosphorylation (pS256)-independent manner. Despite the lack of S256 phosphorylation, dasatinib increased phosphorylation of S269, even in S256A mutant cells in which S256 phosphorylation cannot occur. To confirm the importance of pS269 in AQP2 re-distribution, we expressed an AQP2 S269A mutant in LLC-PK1 cells, and found that dasatinib no longer induced AQP2 membrane accumulation. In conclusion, Src inhibition causes phosphorylation of S269 independently of pS256, and induces AQP2 membrane accumulation by inhibiting clathrin-mediated endocytosis and increasing exocytosis. We conclude that S269 can be phosphorylated without pS256, and pS269 alone is important for AQP2 apical membrane accumulation under some conditions. These data increase our understanding of the independent pathways that can phosphorylate different residues in the AQP2 C-terminus, and suggest new strategies to target distinct AQP2 serine residues to induce membrane expression of this water channel when VP signalling is defective.

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.

A Novel Mutation in the AVPR2 Gene Causing Congenital Nephrogenic Diabetes Insipidus

OBJECTIVE

Congenital nephrogenic diabetes insipidus (CNDI) is a rare inherited disorder characterized by a renal insensitivity to arginine vasopressin (AVP). In the majority of the cases, CNDI is caused by mutations in the arginine vasopressin receptor 2 (AVPR2) gene. Our objective is to report a novel mutation in the AVPR2 gene causing CNDI in a 6-year-old boy, presenting with growth failure and dull normal cognitive functions.

METHODS

The proband was the third off-spring of non-consanguineous parents and had polyuria (4.3 L/day), polydipsia (5 L/day). The diagnosis of CNDI was established by a water-deprivation test and a desmopressin challenge test. Genetic studies were also carried out in the mother, siblings and affected family members, since excessive fluid intake and diuresis were also reported in these individuals. All exons of the AVPR2 gene for all participants were amplified and sequenced. Bioinformatics analysis for wild-type and mutant AVPR2 were obtained with Swiss-Model and UCSF Chimera 1.10.2.

RESULTS

A novel, hemizygous, missense mutation was identified at the position 80^th in exon 2 (p.H80Y) of AVPR2 in the proband. The proband’s mother, maternal aunt and grandmother were heterozygous and his maternal uncle was hemizygous for this mutation. Bioinformatic analysis indicates this mutation would cause significant conformational changes in protein structure.

CONCLUSION

p.H80Y mutation will cause inappropriate folding of the protein compromising water homeostasis via AVPR2 and AVP and leading to diabetes insipidus. We suggest that future functional investigations of the H80Y mutation may provide a basis for understanding the pathophysiology of the NDI in patients with this variant.

A novel small molecule chaperone of rod opsin and its potential therapy for retinal degeneration

Rhodopsin homeostasis is tightly coupled to rod photoreceptor cell survival and vision. Mutations resulting in the misfolding of rhodopsin can lead to autosomal dominant retinitis pigmentosa (adRP), a progressive retinal degeneration that currently is untreatable. Using a cell-based high-throughput screen (HTS) to identify small molecules that can stabilize the P23H-opsin mutant, which causes most cases of adRP, we identified a novel pharmacological chaperone of rod photoreceptor opsin, YC-001. As a non-retinoid molecule, YC-001 demonstrates micromolar potency and efficacy greater than 9-cis-retinal with lower cytotoxicity. YC-001 binds to bovine rod opsin with an EC50 similar to 9-cis-retinal. The chaperone activity of YC-001 is evidenced by its ability to rescue the transport of multiple rod opsin mutants in mammalian cells. YC-001 is also an inverse agonist that non-competitively antagonizes rod opsin signaling. Significantly, a single dose of YC-001 protects Abca4 -/- Rdh8 -/- mice from bright light-induced retinal degeneration, suggesting its broad therapeutic potential.

AKAPs-PKA disruptors increase AQP2 activity independently of vasopressin in a model of nephrogenic diabetes insipidus

Congenital nephrogenic diabetes insipidus (NDI) is characterized by the inability of the kidney to concentrate urine. Congenital NDI is mainly caused by loss-of-function mutations in the vasopressin type 2 receptor (V2R), leading to impaired aquaporin-2 (AQP2) water channel activity. So far, treatment options of congenital NDI either by rescuing mutant V2R with chemical chaperones or by elevating cyclic adenosine monophosphate (cAMP) levels have failed to yield effective therapies. Here we show that inhibition of A-kinase anchoring proteins (AKAPs) binding to PKA increases PKA activity and activates AQP2 channels in cortical collecting duct cells. In vivo, the low molecular weight compound 3,3′-diamino-4,4′-dihydroxydiphenylmethane (FMP-API-1) and its derivatives increase AQP2 activity to the same extent as vasopressin, and increase urine osmolality in the context of V2R inhibition. We therefore suggest that FMP-API-1 may constitute a promising lead compound for the treatment of congenital NDI caused by V2R mutations.

Patients suffering from congenital nephrogenic diabetes insipidus (NDI) fail to concentrate urine due to mutations in vasopressin type 2 receptor (V2R). Here Ando et al. show that agents disrupting the interaction between PKA and AKAPs restore aquaporin-2 activity downstream of V2R, offering a therapeutic approach for the treatment of NDI.

Determination of the renal concentration capacity following intravenous administration of dDAVP in healthy humans

The synthetic AVP analogue 1-desamino-8-d-arginine-vasopressin (dDAVP) is used for treatment of polyuric disorders. Lack of commercially available assays limits the usefulness of dDAVP as a diagnostic tool in the assessment of renal concentrating capacity. We aimed to develop a specific radioimmunoassay (RIA) for determination of plasma dDAVP (pdDAVP) in order to investigate the relationship between pdDAVP levels and urine osmolality (Uosm). Further, we aimed to determine the onset, duration, and maximum concentrating capacity following intravenous (i.v.) bolus dDAVP injection. The dDAVP assay was based on a well-established RIA for measurements of AVP. Fourteen healthy subjects (aged 15-18 years) participated. Blood and urine samples were collected prior to and after i.v. bolus of 0.03 µg/kg dDAVP. Diuresis and Uosm was measured for nine hours following dDAVP administration. PdDAVP and Uosm were analyzed.We established a specific RIA for the measurement of pdDAVP. All subjects reached maximal pdDAVP concentration (C_max) 30 minutes following infusion, and a rise in Uosm after 60 minutes. Maximal Uosm varied between subjects, with no direct correlation to the achieved pdDAVP levels. We found no significant intra-individual variation between two dDAVP infusions and the effect was reproducible in terms of C_max and maximal Uosm. We characterized the relationship between pdDAVP and Uosm after dDAVP bolus injection in healthy adolescents using our dDAVP assay. Maximal Uosm achieved correlated with the baseline Uosm levels and seemed unrelated to achieved pdDAVP levels. The urine concentrating response was maintained at least eight hours.

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.

Functional characterization of AVPR2 mutants found in Turkish patients with nephrogenic diabetes insipidus

Diabetes insipidus is a rare disorder characterized by an impairment in water balance because of the inability to concentrate urine. While central diabetes insipidus is caused by mutations in the AVP, the reason for genetically determined nephrogenic diabetes insipidus can be mutations in AQP2 or AVPR2 After release of AVP from posterior pituitary into blood stream, it binds to AVPR2, which is one of the receptors for AVP and is mainly expressed in principal cells of collecting ducts of kidney. Receptor activation increases cAMP levels in principal cells, resulting in the incorporation of AQP2 into the membrane, finally increasing water reabsorption. This pathway can be altered by mutations in AVPR2 causing nephrogenic diabetes insipidus. In this study, we functionally characterize four mutations (R68W, ΔR67-G69/G107W, V162A and T273M) in AVPR2, which were found in Turkish patients. Upon AVP stimulation, R68W, ΔR67-G69/G107W and T273M showed a significantly reduced maximum in cAMP response compared to wild-type receptor. All mutant receptor proteins were expressed at the protein level; however, R68W, ΔR67-G69/G107W and T273M were partially retained in the cellular interior. Immunofluorescence studies showed that these mutant receptors were trapped in ER and Golgi apparatus. The function of V162A was indistinguishable from the indicating other defects causing disease. The results are important for understanding the influence of mutations on receptor function and cellular trafficking. Therefore, characterization of these mutations provides useful information for further studies addressing treatment of intracellularly trapped receptors with cell-permeable antagonists to restore receptor function in patients with 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.

Cognate Ligand Chaperoning: a Novel Mechanism for the Post-translational Regulation of Neurotransmitter Receptor Biogenesis

The functional unit for inter-neuronal communication in the central nervous system is the neuronal synapse. The number of postsynaptic neurotransmitter receptors at the cell surface is an important determinant of synaptic efficacy and plasticity. A diverse array of post-translational processes regulate postsynaptic receptor number, including receptor exocytosis, lateral diffusion, surface stabilization, endocytosis, and recycling, thus highlighting the importance of mechanisms that control postsynaptic receptor levels. Another putative post-translational mechanism for regulating receptor surface expression is cognate ligand chaperoning. It has been proposed that neurotransmitters function as cognate ligand chaperones by binding, within the endoplasmic reticulum (ER) lumen, to their nascent neurotransmitter receptors and facilitating receptor biogenesis. Here we discuss proof-of-concept evidence that small molecules can selectively facilitate the biogenesis of their targets and examine the specific evidence in support of cognate ligand chaperoning of neurotransmitter receptor biogenesis.

Regulation of α2B-Adrenergic Receptor Cell Surface Transport by GGA1 and GGA2

The molecular mechanisms that control the targeting of newly synthesized G protein-coupled receptors (GPCRs) to the functional destinations remain poorly elucidated. Here, we have determined the role of Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding proteins 1 and 2 (GGA1 and GGA2) in the cell surface transport of α_2B-adrenergic receptor (α_2B-AR), a prototypic GPCR, and studied the underlying mechanisms. We demonstrated that knockdown of GGA1 and GGA2 by shRNA and siRNA significantly reduced the cell surface expression of inducibly expressed α_2B-AR and arrested the receptor in the perinuclear region. Knockdown of each GGA markedly inhibited the dendritic expression of α_2B-AR in primary cortical neurons. Consistently, depleting GGA1 and GGA2 attenuated receptor-mediated signal transduction measured as ERK1/2 activation and cAMP inhibition. Although full length α_2B-AR associated with GGA2 but not GGA1, its third intracellular loop was found to directly interact with both GGA1 and GGA2. More interestingly, further mapping of interaction domains showed that the GGA1 hinge region and the GGA2 GAE domain bound to multiple subdomains of the loop. These studies have identified an important function and revealed novel mechanisms of the GGA family proteins in the forward trafficking of a cell surface GPCR.

Outside the mainstream: novel collecting duct proteins regulating water balance

Body water balance is critical to survival and, therefore, very tightly regulated by the hypothalamus and kidney. A key mechanism involved in this process, the arginine vasopressin-mediated phosphorylation and apical membrane insertion of aquaporin 2 in the collecting duct, has been extensively studied; however, with the increased availability of conditional knockout animals, several novel collecting duct proteins have recently been implicated in water homeostasis. In this Mini-Review, we briefly discuss these novel proteins and their roles in the regulation of water homeostasis.

Receptor antagonism/agonism can be uncoupled from pharmacoperone activity

Pharmacoperones rescue misrouted mutants of the vasopressin receptor type 2 (V2R) and enable them to traffic to the correct biological locus where they function. Previously, a library of nearly 645,000 structures was interrogated with a high throughput screen; pharmacoperones were identified for V2R mutants with a view toward correcting the underlying mutational defects in nephrogenic diabetes insipidus. In the present study, an orthologous assay was used to evaluate hits from the earlier study. We found no consistent relation between antagonism or agonism and pharmacoperone activity. Active pharmacoperones were identified which had minimal antagonistic activity. This increases the therapeutic reach of these drugs, since virtually all pharmacoperone drugs reported to date were selected from peptidomimetic antagonists. Such mixed-activity drugs have a complex pharmacology limiting their therapeutic utility and requiring their removal prior to stimulation of the receptor with agonist.

AKAP220 manages apical actin networks that coordinate aquaporin-2 location and renal water reabsorption

Filtration through the kidney eliminates toxins, manages electrolyte balance, and controls water homeostasis. Reabsorption of water from the luminal fluid of the nephron occurs through aquaporin-2 (AQP2) water pores in principal cells that line the kidney-collecting duct. This vital process is impeded by formation of an "actin barrier" that obstructs the passive transit of AQP2 to the plasma membrane. Bidirectional control of AQP2 trafficking is managed by hormones and signaling enzymes. We have discovered that vasopressin-independent facets of this homeostatic mechanism are under the control of A-Kinase Anchoring Protein 220 (AKAP220; product of the Akap11 gene). CRISPR/Cas9 gene editing and imaging approaches show that loss of AKAP220 disrupts apical actin networks in organoid cultures. Similar defects are evident in tissue sections from AKAP220-KO mice. Biochemical analysis of AKAP220-null kidney extracts detected reduced levels of active RhoA GTPase, a well-known modulator of the actin cytoskeleton. Fluorescent imaging of kidney sections from these genetically modified mice revealed that RhoA and AQP2 accumulate at the apical surface of the collecting duct. Consequently, these animals are unable to appropriately dilute urine in response to overhydration. We propose that membrane-proximal signaling complexes constrained by AKAP220 impact the actin barrier dynamics and AQP2 trafficking to ensure water homeostasis.

Mutations of Vasopressin Receptor 2 Including Novel L312S Have Differential Effects on Trafficking

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a genetic disease first described in 2 unrelated male infants with severe symptomatic hyponatremia. Despite undetectable arginine vasopressin levels, patients have inappropriately concentrated urine resulting in hyponatremia, hypoosmolality, and natriuresis. Here, we describe and functionally characterize a novel vasopressin type 2 receptor (V2R) gain-of-function mutation. An L312S substitution in the seventh transmembrane domain was identified in a boy presenting with water-induced hyponatremic seizures at the age of 5.8 years. We show that, compared with wild-type V2R, the L312S mutation results in the constitutive production of cAMP, indicative of the gain-of-function NSIAD profile. Interestingly, like the previously described F229V and I130N NSIAD-causing mutants, this appears to both occur in the absence of notable constitutive β-arrestin2 recruitment and can be reduced by the inverse agonist Tolvaptan. In addition, to understand the effect of various V2R substitutions on the full receptor "life-cycle," we have used and further developed a bioluminescence resonance energy transfer intracellular localization assay using multiple localization markers validated with confocal microscopy. This allowed us to characterize differences in the constitutive and ligand-induced localization and trafficking profiles of the novel L312S mutation as well as for previously described V2R gain-of-function mutants (NSIAD; R137C and R137L), loss-of-function mutants (nephrogenic diabetes insipidus; R137H, R181C, and M311V), and a putative silent V266A V2R polymorphism. In doing so, we describe differences in trafficking between unique V2R substitutions, even at the same amino acid position, therefore highlighting the value of full and thorough characterization of receptor function beyond simple signaling pathway analysis.

Gatekeepers Controlling GPCR Export and Function

Regulated export of G protein-coupled receptors (GPCRs) from intracellular stores involves chaperones and escort proteins, which promote their progression to the cell surface, and gatekeepers, which retain them in intracellular compartments. Functional γ-aminobutyric acid (GABA)B receptors, the paradigm of this phenomenon, comprise GB1 and GB2 subunits forming a heterodimer. GB1 is retained in the endoplasmic reticulum (ER) in the absence of GB2. A specific ER-resident gatekeeper, prenylated Rab acceptor family 2 (PRAF2), is involved in GB1 retention and prevents its progression into the biosynthetic pathway. GB1 can be released from PRAF2 only on competitive interaction with GB2. PRAF2 is ubiquitous and belongs to a subgroup of the mammalian Ypt-interacting protein (Yip) family. Several other GPCRs are likely to be regulated by Yip proteins, which might be involved in the pathophysiology of human diseases that are associated with impaired receptor targeting to the cell surface.

GGA3 Interacts with a G Protein-Coupled Receptor and Modulates Its Cell Surface Export

Molecular mechanisms governing the anterograde trafficking of nascent G protein-coupled receptors (GPCRs) are poorly understood. Here, we have studied the regulation of cell surface transport of α2-adrenergic receptors (α2-ARs) by GGA3 (Golgi-localized, γ-adaptin ear domain homology, ADP ribosylation factor-binding protein 3), a multidomain clathrin adaptor protein that sorts cargo proteins at the trans-Golgi network (TGN) to the endosome/lysosome pathway. By using an inducible system, we demonstrated that GGA3 knockdown significantly inhibited the cell surface expression of newly synthesized α2B-AR without altering overall receptor synthesis and internalization. The receptors were arrested in the TGN. Furthermore, GGA3 knockdown attenuated α2B-AR-mediated signaling, including extracellular signal-regulated kinase 1/2 (ERK1/2) activation and cyclic AMP (cAMP) inhibition. More interestingly, GGA3 physically interacted with α2B-AR, and the interaction sites were identified as the triple Arg motif in the third intracellular loop of the receptor and the acidic motif EDWE in the VHS domain of GGA3. In contrast, α2A-AR did not interact with GGA3 and its cell surface export and signaling were not affected by GGA3 knockdown. These data reveal a novel function of GGA3 in export trafficking of a GPCR that is mediated via a specific interaction with the receptor.

Vasopressin regulates renal calcium excretion in humans

Antidiuretic hormone or arginine vasopressin (AVP) increases water reabsorption in the collecting ducts of the kidney. Three decades ago, experimental models have shown that AVP may increase calcium reabsorption in rat kidney. The objective of this study was to assess whether AVP modulates renal calcium excretion in humans. We analyzed calcium, potassium, and sodium fractional excretion in eight patients affected by insipidus diabetes (nephrogenic or central) under acute vasopressin receptor agonist action and in 10 patients undergoing oral water load test affected or not by inappropriate antidiuretic hormone secretion (SIADH). Synthetic V2 receptor agonist (dDAVP) reduced significantly calcium fractional excretion from 1.71% to 0.58% (P < 0.05) in patients with central diabetes insipidus. In patients with nephrogenic diabetes insipidus (resistant to AVP), calcium fractional excretion did not change significantly after injection (0.48–0.68%, P = NS). In normal subjects undergoing oral water load test, calcium fractional excretion increased significantly from 1.02% to 2.54% (P < 0.05). Patients affected by SIADH had a high calcium fractional excretion at baseline that remained stable during test from 3.30% to 3.33% (P = NS), possibly resulting from a reduced calcium absorption in renal proximal tubule. In both groups, there was a significant correlation between urine output and calcium renal excretion. In humans, dDAVP decreases calcium fractional excretion in the short term. Conversely, water intake, which lowers AVP concentration, increases calcium fractional excretion. The correlation between urine output and calcium excretion suggests that AVP-related antidiuresis increases calcium reabsorption in collecting ducts.

Novel mutations associated with nephrogenic diabetes insipidus. A clinical-genetic study

Molecular diagnosis is a useful diagnostic tool in primary nephrogenic diabetes insipidus (NDI), an inherited disease characterized by renal inability to concentrate urine. The AVPR2 and AQP2 genes were screened for mutations in a cohort of 25 patients with clinical diagnosis of NDI. Patients presented with dehydration, polyuria-polydipsia, failure to thrive (mean ± SD; Z-height -1.9 ± 2.1 and Z-weight -2.4 ± 1.7), severe hypernatremia (mean ± SD; Na 150 ± 10 mEq/L), increased plasma osmolality (mean ± SD; 311 ± 18 mOsm/Kg), but normal glomerular filtration rate. Genetic diagnosis revealed that 24 male patients were hemizygous for 17 different putative disease-causing mutations in the AVPR2 gene (each one in a different family). Of those, nine had not been previously reported, and eight were recurrent. Moreover, we found those same AVPR2 changes in 12 relatives who were heterozygous carriers. Further, in one female patient, AVPR2 gene study turned out to be negative and she was found to be homozygous for the novel AQP2 p.Ala86Val alteration.

CONCLUSION

Genetic analysis presumably confirmed the diagnosis of nephrogenic diabetes insipidus in every patient of the studied cohort. We emphasize that we detected a high presence (50 %) of heterozygous females with clinical NDI symptoms.

WHAT IS KNOWN

• In most cases (90 %), inherited nephrogenic diabetes insipidus (NDI) is an X-linked disease, caused by mutations in the AVPR2 gene. • In rare occasions (10 %), it is caused by mutations in the AQP2 gene. What is new: • In this study, we report 10 novel mutations associated with NDI. • We have detected a high presence (50 %) of heterozygous carriers with clinical NDI symptoms.

Polyuria in relation to dysnatraemias in neurocritical care

INTRODUCTION

Polyuria has the potential to cause severe water and sodium imbalance. We studied the epidemiology of polyuria in association with dysnatraemias and whether polyuria is an independent risk factor for higher mortality and poorer outcome in neurocritical care.

METHODS

We performed an analysis of a 3-year prospective database containing 902 neurocritical care patients. Polyuria was defined as diuresis above 4000 ml/day, hyponatraemia as a serum sodium (SeNa) < 135 mmol/l and hypernatraemia as SeNa > 150 mmol/l.

RESULTS

We identified polyuria in 236 (26.2%) patients (639 days). Polyuric patients stayed in the neurointensive care unit (NICU) longer than those without polyuria (mean: 10.7 vs. 3.5 days, p < 0.001). These patients also had more frequent cerebral complications (p < 0.001) and a poorer outcome upon discharge from the NICU (p = 0.032). NICU mortality had borderline significance in relation to whether the patients were polyuric (p = 0.055). There were only 49 (20.8%) patients with dysnatraemia who were shown to have a significantly higher NICU mortality (p = 0.006). There were no differences in frequency between hyponatraemic and hypernatraemic polyuric patients (p = 0.127). Polyuric patients with hypernatraemia suffered poorer outcomes (p = 0.009) and higher NICU mortality (p < 0.001), but they had a lower Glasgow Coma Scale or GCS recorded at the onset of polyuria (p < 0.001). Cerebral salt wasting (CSW) was thought to be the cause of polyuria in 7 (3.0%) patients and central diabetes insipidus (CDI) in another 5 (2.1%) patients. Univariate models showed polyuria to be a risk factor for poor outcome (odds ratio [OR] = 1.39, p = 0.032) and had a borderline significance for mortality during their NICU stay (OR = 1.83, p = 0.055). These factors did not appear as significant following multivariate logistic regression analysis.

CONCLUSION

Polyuria often occurred in neurocritical care patients, but was not usually associated with sodium imbalance, CSW or CDI. We did not find that polyuria was a significant predictor of increased mortality or poorer outcome in NICU patients.

Assay strategies for identification of therapeutic leads that target protein trafficking

Receptors, enzymes, and ion channels are traditional targets of therapeutic development. A common strategy is to target these proteins with agents that either activate or suppress their activity with ligands or substrates that occupy orthosteric sites or have allosteric interactions. An alternative approach involves regulation of protein trafficking. In principle, this approach enables 'rescue' of misfolded and misrouted mutant proteins to restore function, 'shipwrecking' of undesirable proteins by targeting them for destruction, and regulation of levels of partially expressed wild type (WT) proteins at their functional sites of action. Here, we present drug discovery strategies that identify 'pharmacoperones', which are small molecules that serve as molecular templates and cause otherwise misfolded mutant proteins to fold and route correctly.

Neuronal gamma-aminobutyric acid (GABA) type A receptors undergo cognate ligand chaperoning in the endoplasmic reticulum by endogenous GABA

GABA_A receptors mediate fast inhibitory neurotransmission in the brain. Dysfunction of these receptors is associated with various psychiatric/neurological disorders and drugs targeting this receptor are widely used therapeutic agents. Both the efficacy and plasticity of GABA_A receptor-mediated neurotransmission depends on the number of surface GABA_A receptors. An understudied aspect of receptor cell surface expression is the post-translational regulation of receptor biogenesis within the endoplasmic reticulum (ER). We have previously shown that exogenous GABA can act as a ligand chaperone of recombinant GABA_A receptors in the early secretory pathway leading us to now investigate whether endogenous GABA facilitates the biogenesis of GABA_A receptors in primary cerebral cortical cultures. In immunofluorescence labeling experiments, we have determined that neurons expressing surface GABA_A receptors contain both GABA and its degradative enzyme GABA transaminase (GABA-T). Treatment of neurons with GABA-T inhibitors, a treatment known to increase intracellular GABA levels, decreases the interaction of the receptor with the ER quality control protein calnexin, concomittantly increasing receptor forward-trafficking and plasma membrane insertion. The effect of GABA-T inhibition on the receptor/calnexin interaction is not due to the activation of surface GABA_A or GABA_B receptors. Consistent with our hypothesis that GABA acts as a cognate ligand chaperone in the ER, immunogold-labeling of rodent brain slices reveals the presence of GABA within the rough ER. The density of this labeling is similar to that present in mitochondria, the organelle in which GABA is degraded. Lastly, the effect of GABA-T inhibition on the receptor/calnexin interaction was prevented by pretreatment with a GABA transporter inhibitor. Together, these data indicate that endogenous GABA acts in the rough ER as a cognate ligand chaperone to facilitate the biogenesis of neuronal GABA_A receptors.

X-Linked Recessive form of Nephrogenic Diabetes Insipidus in a 7-Year-Old Boy

Nephrogenic diabetes insipidus (NDI) is caused by the inability of renal collecting duct cells to respond to arginine vasopressin (AVP)/antidiuretic hormone (ADH). We present the case of a 7-year-old boy with a history of excretion of large amounts of dilute urine and polydipsia since infancy. The boy had several vomiting episodes with mild dehydration during the first 3 years of life. There was no evidence of headaches, dizziness or visual problems. He drinks between 2 and 3 L/day and has 24-hour diuresis of 2 liters, now. He has prepubertal appearance with appropriate weight [+0.85 standard deviation score (SDS)] and height (+0.15 SDS) for his age. His intelligence was also normal. The water deprivation test showed low urine osmolality after 8 hours of dehydration. After desmopressin administration, urine osmolality remained low. Serum osmolality was in the normal range for sex and age before and after desmopressin administration. This indicated a nephrogenic form of diabetes insipidus. Molecular analyses revealed a P286L [p.Pro(CCC)286Leu(CTC)] mutation in the AVPR2 gene, that was inherited from his mother. This patient is the first case with genetically confirmed X-linked inherited form of NDI in the Republic of Macedonia. Molecular analysis confirmed the clinical diagnosis and enabled genetic advice for this family.