Arginine vasopressin secretion with mutants of wild-type and Brattleboro rats AVP gene

Kidney collecting duct-derived vasopressin is not essential for appropriate concentration or dilution of urine

We have previously shown that kidney collecting ducts make vasopressin. However, the physiological role of collecting duct-derived vasopressin is uncertain. We hypothesized that collecting duct-derived vasopressin is required for the appropriate concentration of urine. We developed a vasopressin conditional knockout (KO) mouse model wherein Cre recombinase expression induces deletion of arginine vasopressin (Avp) exon 1 in the distal nephron. We then used age-matched 8- to 12-wk-old Avp fl/fl;Ksp-Cre(-) [wild type (WT)] and Avp fl/fl;Ksp-Cre(+) mice for all experiments. We collected urine, serum, and kidney lysates at baseline. We then challenged both WT and knockout (KO) mice with 24-h water restriction, water loading, and administration of the vasopressin type 2 receptor agonist desmopressin (1 µg/kg ip) followed by the vasopressin type 2 receptor antagonist OPC-31260 (10 mg/kg ip). We performed immunofluorescence and immunoblot analysis at baseline and confirmed vasopressin KO in the collecting duct. We found that urinary osmolality (UOsm), plasma Na+, K+, Cl-, blood urea nitrogen, and copeptin were similar in WT vs. KO mice at baseline. Immunoblots of the vasopressin-regulated proteins Na+-K+-2Cl- cotransporter, NaCl cotransporter, and water channel aquaporin-2 showed no difference in expression or phosphorylation at baseline. Following 24-h water restriction, WT and KO mice had no differences in UOsm, plasma Na+, K+, Cl-, blood urea nitrogen, or copeptin. In addition, there were no differences in the rate of urinary concentration or dilution as in WT and KO mice UOsm was nearly identical after desmopressin and OPC-31260 administration. We conclude that collecting duct-derived vasopressin is not essential to appropriately concentrate or dilute urine.NEW & NOTEWORTHY Hypothalamic vasopressin is required for appropriate urinary concentration. However, whether collecting duct-derived vasopressin is involved remains unknown. We developed a novel transgenic mouse model to induce tissue-specific deletion of vasopressin and showed that collecting duct-derived vasopressin is not required to concentrate or dilute urine.

Rethinking Vasopressin: New Insights into Vasopressin Signaling and Its Implications

Vasopressin is a highly conserved peptide hormone that has been traditionally associated with water homeostasis. There is accumulating evidence in both humans and animal models that vasopressin is implicated in the regulation of metabolism. This review focuses on the effects that vasopressin exerts on the regulation of glucose and fatty acids with a particular emphasis on the potential repercussions of metabolic dysregulation in kidney disease.

Characterization of dietary and herbal sourced natural compounds that modulate SEL1L-HRD1 ERAD activity and alleviate protein misfolding in the ER

Dysregulated production of peptide hormones is the key pathogenic factor of various endocrine diseases. Endoplasmic reticulum (ER) associated degradation (ERAD) is a critical machinery in maintaining ER proteostasis in mammalian cells by degrading misfolded proteins. Dysfunction of ERAD leads to maturation defect of many peptide hormones, such as provasopressin (proAVP), which results in the occurrence of Central Diabetes Insipidus. However, drugs targeting ERAD to regulate the production of peptide hormones are very limited. Herbal products provide not only nutritional sources, but also alternative therapeutics for chronic diseases. Virtual screening provides an effective and high-throughput strategy for identifying protein structure-based interacting compounds extracted from a variety of dietary or herbal sources, which could be served as (pro)drugs for preventing or treating endocrine diseases. Here, we performed a virtual screening by directly targeting SEL1L of the most conserved SEL1L-HRD1 ERAD machinery. Further, we analyzed 58 top-ranked compounds and demonstrated that Cryptochlorogenic acid (CCA) showed strong affinity with the binding pocket of SEL1L with HRD1. Through structure-based docking, protein expression assays, and FACS analysis, we revealed that CCA enhanced ERAD activity and promoted the degradation of misfolded proAVP, thus facilitated the secretion of well-folded proAVP. These results provide us with insights into drug discovery strategies targeting ER protein homeostasis, as well as candidate compounds for treating hormone-related diseases.

Vasopressin stimulates the proliferation and differentiation of red blood cell precursors and improves recovery from anemia

Arginine vasopressin (AVP) made by hypothalamic neurons is released into the circulation to stimulate water resorption by the kidneys and restore water balance after blood loss. Patients who lack this antidiuretic hormone suffer from central diabetes insipidus. We observed that many of these patients were anemic and asked whether AVP might play a role in red blood cell (RBC) production. We found that all three AVP receptors are expressed in human and mouse hematopoietic stem and progenitor cells. The AVPR1B appears to play the most important role in regulating erythropoiesis in both human and mouse cells. AVP increases phosphorylation of signal transducer and activator of transcription 5, as erythropoietin (EPO) does. After sublethal irradiation, AVP-deficient Brattleboro rats showed delayed recovery of RBC numbers compared to control rats. In mouse models of anemia (induced by bleeding, irradiation, or increased destruction of circulating RBCs), AVP increased the number of circulating RBCs independently of EPO. In these models, AVP appears to jump-start peripheral blood cell replenishment until EPO can take over. We suggest that specific AVPR1B agonists might be used to induce fast RBC production after bleeding, drug toxicity, or chemotherapy.

The secretion patterns and roles of cardiac and circulating arginine vasopressin during the development of heart failure

OBJECTIVE

The aim of this study is to investigate local cardiac and circulating AVP secretion during heart failure and to determine whether AVP mediates ventricular remodeling.

METHODS

We assessed cardiac function and AVP levels of post-myocardial infarction (MI) heart-failure rats 3 weeks (n = 10), 4 weeks (n = 10), 6 weeks (n = 10), 9 weeks (n = 15) after the proximal left anterior descending coronary artery (LAD) ligation. Ten sham-operated rats were used as the control group. In vitro, cardiac microvascular endothelial cells (CMECs) were initiated from isolated Wistar rat hearts and subjected to Ang II to induce AVP expression and secretion. Besides, the effects of AVP stimulation on CMECs and cardiac fibroblasts (CFs) were studied using methylthiazol tetrazolium assay, Western blotting and real-time PCR.

RESULTS

With cardiac dysfunction, plasma and local cardiac AVP, aldosterone levels increased over time, peaking at 9 weeks post-MI. AVP levels were negatively correlated with serum Na(+) and LVEF but positively correlated with LVEDD and myocardial hydroxyproline. In CMECs treated with Ang II, AVP mRNA and protein expression increased. In addition, AVP promoted CFs proliferation and up-regulated the expression of endothelin-1 and connective tissue growth factor.

CONCLUSION

CMECs are the cellular sources of elevated local heart AVP stimulated with Ang II/AT1. An intrinsic cardiac AVP system exists. Local cardiac and circulating AVP secretion were enhanced by deteriorating cardiac function. AVP may promote ventricular remodeling. Thus, AVP could be an important mediator of myocardial fibrosis in late-stage heart failure.

Arginine vasopressin neuronal loss results from autophagy-associated cell death in a mouse model for familial neurohypophysial diabetes insipidus

Familial neurohypophysial diabetes insipidus (FNDI) characterized by progressive polyuria is mostly caused by mutations in the gene encoding neurophysin II (NPII), which is the carrier protein of the antidiuretic hormone, arginine vasopressin (AVP). Although accumulation of mutant NPII in the endoplasmic reticulum (ER) could be toxic for AVP neurons, the precise mechanisms of cell death of AVP neurons, reported in autopsy studies, remain unclear. Here, we subjected FNDI model mice to intermittent water deprivation (WD) in order to promote the phenotypes. Electron microscopic analyses demonstrated that, while aggregates are confined to a certain compartment of the ER in the AVP neurons of FNDI mice with water access ad libitum, they were scattered throughout the dilated ER lumen in the FNDI mice subjected to WD for 4 weeks. It is also demonstrated that phagophores, the autophagosome precursors, emerged in the vicinity of aggregates and engulfed the ER containing scattered aggregates. Immunohistochemical analyses revealed that expression of p62, an adapter protein between ubiquitin and autophagosome, was elicited on autophagosomal membranes in the AVP neurons, suggesting selective autophagy induction at this time point. Treatment of hypothalamic explants of green fluorescent protein (GFP)-microtubule-associated protein 1 light chain 3 (LC3) transgenic mice with an ER stressor thapsigargin increased the number of GFP-LC3 puncta, suggesting that ER stress could induce autophagosome formation in the hypothalamus of wild-type mice as well. The cytoplasm of AVP neurons in FNDI mice was occupied with vacuoles in the mice subjected to WD for 12 weeks, when 30–40% of AVP neurons are lost. Our data thus demonstrated that autophagy was induced in the AVP neurons subjected to ER stress in FNDI mice. Although autophagy should primarily be protective for neurons, it is suggested that the organelles including ER were lost over time through autophagy, leading to autophagy-associated cell death of AVP neurons.

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

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

Effects of diabetes insipidus mutations on neurophysin folding and function

Mechanisms underlying the pathogenicity of diabetes insipidus mutations were probed by studying their effects on the properties of bovine oxytocin-related neurophysin. The mutations G17V, DeltaE47, G57S, G57R, and C67STOP were each shown to have structural consequences that would diminish the conformational stability and folding efficiency of the precursors in which they were incorporated, and factors contributing to the origins of these property changes were identified. Effects of the mutations on dimerization of the folded proteins were similarly analyzed. The projected relative impact of the above mutations on precursor folding properties qualitatively parallels the reported relative severity of their effects on the biological handling of the human vasopressin precursor, but quantitative differences between thermodynamic effects and biological impact are noted and explored. The sole mutation for which no clear thermodynamic basis was found for its pathogenicity was 87STOP, suggesting that the region of the precursor deleted by this mutation plays a role in targeting independent from effects on folding, or participates in stabilizing interactions unique to the human vasopressin precursor.

Water-losing and water-retaining states: role of water channels and vasopressin receptor antagonists

Alterations in water metabolism are present in conditions such as diabetes insipidus, syndrome of inappropriate antidiuretic hormone secretion, cardiac failure, cirrhosis, and pregnancy. Recent advances in molecular biology have enhanced our understanding of disordered water metabolism in these conditions. This review examines the roles of central vasopressin synthesis and release and collecting duct vasopressin V2 receptor and aquaporin-2 water channel regulation in water-losing and water-retaining states.

A diabetes insipidus vasopressin prohormone altered outside the central core of neurophysin accumulates in the endoplasmic reticulum

Over 20 mutations affecting the neurophysin moiety of the vasopressin prohormone, have been identified in families suffering from familial neurohypophysial diabetes insipidus (FNDI). Only one of these, NP87E-->stop, is located outside the central conserved domain implicated in sorting of the vasopressin prohormone. To obtain clues about the mechanism of induction of FNDI by this atypical mutant we stably expressed wild type and NP87E-->stop vasopressin prohormones in (neuro)endocrine cell lines. Metabolic labeling and immunoprecipitation demonstrated reduced processing of the mutant prohormone to neurophysin. In addition, evoked secretion of neurophysin and vasopressin was diminished, suggesting that part of the mutant is retained in another intracellular compartment than the secretory granules. Indeed, immunofluorescence demonstrated accumulation of the truncated vasopressin prohormone in the endoplasmic reticulum. We conclude that the presence of the vasopressin moiety and the central conserved core of the neurophysin domain suffices for sorting and processing, but not for efficient endoplasmic reticulum exit of the vasopressin-neurophysin molecule.

Effects of various mutations in the neurophysin/glycopeptide portion of the vasopressin gene on vasopressin expression in vitro

The vasopressin gene encodes three polypeptides besides the signal peptide: vasopressin, neurophysin II (neurophysin), and the carboxy-terminal glycopeptide (glycopeptide). Although the function of vasopressin is well characterized, those of the latter two are not completely understood. In the present study, we investigated the effects of various mutations within the neurophysin/glycopeptide portion of the vasopressin gene on vasopressin secretion in vitro, to clarify the role of each peptide in vasopressin biosynthesis. Expression vectors containing the vasopressin gene, either wild-type or various mutants, were transiently transfected into AtT20 cells, which are known to have the enzymes necessary for the proper processing of the vasopressin precursor protein. The amount of vasopressin secreted into the culture medium was estimated by specific radioimmunoassay. Variable degrees of decreased vasopressin secretion were observed with mutant vasopressin genes harboring deletions or amino acid substitutions in neurophysin. The naturally-occurring frame-shift mutation in the hereditary diabetes insipidus (Brattleboro) rat completely eliminated vasopressin expression. In contrast, a missense mutation found in patients with familial neurogenic diabetes insipidus only partially decreased vasopressin secretion. Finally, the mutant vasopressin gene lacking the N-linked glycosylation site in glycopeptide had no effect on vasopressin expression. Our data suggest that 1) intact neurophysin is not indispensable for vasopressin expression, although an altered structure of neurophysin significantly affects the secretion of the hormone; 2) the pathogenesis of diabetes insipidus with the two naturally-occurring mutations found in the rat (Brattleboro rat) and human (familial central diabetes insipidus) seem to be different; and 3) glycosylation of the carboxy-terminal glycopeptide is not essential for the expression of vasopressin.

Trafficking of the vasopressin and oxytocin prohormone through the regulated secretory pathway

The trafficking of prohormones and of regulated secretory proteins in general has been studied extensively in the last decades of the last century. Prohormone trafficking starts with correct folding and subsequently efficient sorting into the secretory granule of the regulated secretory pathway. The vasopressin/oxytocin prohormone is particularly interesting for studying protein trafficking, because the physicochemical properties and three-dimensional structure have been largely elucidated. In the case of pro-vasopressin and pro-oxytocin, folding and sorting depend completely on both intramolecular and intermolecular interactions. Proper folding is guided by the hormone-neurophysin association and the sorting event relies on the aggregative properties of the neurophysin domain in the prohormone, as well as a specific sorting signal, which is revealed when the aggregative property of the neurophysin domain is deleted. A comprehensive mechanism for trafficking of the vasopressin/oxytocin prohormone from the endoplasmic reticulum to the secretory granule is proposed.