Nephrogenic syndrome of inappropriate antidiuresis: a novel disorder in water balance in pediatric patients

Aquaporins and diseases pathogenesis: From trivial to undeniable involvements, a disease-based point of view

Aquaporins (AQPs), as transmembrane proteins, were primarily identified as water channels with the ability of regulating the transmission of water, glycerol, urea, and other small-sized molecules. The classic view of AQPs involvement in therapeutic plan restricted them and their regulators into managing only a narrow spectrum of the diseases such as diabetes insipidus and the syndrome of inappropriate ADH secretion. However, further investigations performed, especially in the third millennium, has found that their cooperation in water transmission control can be manipulated to handle other burden-imposing diseases such as cirrhosis, heart failure, Meniere's disease, cancer, bullous pemphigoid, eczema, and Sjögren's syndrome.

Vasopressin receptor 2 mutations in the nephrogenic syndrome of inappropriate antidiuresis show different mechanisms of constitutive activation for G protein coupled receptors

Vasopressin receptor 2 (V2R) mutations causing the nephrogenic syndrome of inappropriate antidiuresis (NSIAD) can generate two constitutively active receptor phenotypes. One type results from residue substitutions in several V2R domains and is sensitive to vaptan inverse agonists. The other is only caused by Arg 137 replacements and is vaptan resistant. We compared constitutive and agonist-driven interactions of the vaptan-sensitive F229V and vaptan-resistant R137C/L V2R mutations with β-arrestin 1, β-arrestin 2, and Gαs, using null fibroblasts reconstituted with individual versions of the ablated transduction protein genes. F229V displayed very high level of constitutive activation for Gs but not for β-arrestins, and enhanced or normal responsiveness to agonists and inverse agonists. In contrast, R137C/L mutants exhibited maximal levels of constitutive activation for βarrestin 2 and Gs, minimal levels for β-arrestin 1, but a sharp decline of ligands sensitivity at all transducer interactions. The enhanced constitutive activity and reduced ligand sensitivity of R137 mutants on cAMP signaling persisted in cells lacking β-arrestins, indicating that these are intrinsic molecular properties of the mutations, not the consequence of altered receptor trafficking. The results suggest that the two groups of NSIAD mutations represent two distinct molecular mechanisms of constitutive activation in GPCRs.

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.

Nephrogenic syndrome of inappropriate antidiuresis secondary to an activating mutation in the arginine vasopressin receptor AVPR2

CONTEXT

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD), resulting from activating mutations in the arginine vasopressin receptor type 2 (AVPR2), is a rare cause of hyponatraemia. However, its true prevalence may be underestimated and it should be considered in the investigation of unexplained hyponatraemia, with implications for management and targeted gene testing.

OBJECTIVE

We describe a structured approach to the investigation of hyponatraemia in a young patient, which allowed a diagnosis of NSIAD to be made. We review current knowledge of NSIAD and use a structural modelling approach to further our understanding of the potential mechanisms by which the causative mutation leads to a constitutively active AVPR2.

DESIGN

Clinical and biochemical investigation of hyponatraemia; a formal water load test with measurement of arginine vasopressin levels (AVP); sequencing of AVPR2; and computed structural modelling of the wild-type and constitutively activated mutant receptors.

RESULTS

A 38-year-old man presented with intermittent confusion and nausea associated with hyponatraemia and a biochemical picture consistent with syndrome of inappropriate antidiuretic hormone (SIADH). Adrenocortical and thyroid function and an acute intermittent porphyria screen were normal. Cross-sectional imaging of the head, chest and abdomen did not identify an underlying cause and so we proceeded to a water load test. This demonstrated a marked inability to excrete a free water load (just 15% of a 20 ml/kg oral load by 240 min postingestion), with the onset of hyponatraemia (Na(+) 125 mmol/l, urine osmolality 808 mOsm/kg). However, AVP levels were low throughout the test (0·4-0·9 pmol/l), consistent with a diagnosis of NSIAD. AVPR2 sequencing revealed a previously described hemizygous activating mutation (p.Arg137Cys). Through structural modelling of AVPR2, we suggest that disruption of a hydrogen bond between residues Thr269 and Arg137 may promote stabilization of the receptor in its active conformation. Since diagnosis, the patient has adhered to modest fluid restriction and remained well, with no further episodes of hyponatraemia.

CONCLUSION

NSIAD should be considered in young patients with unexplained hyponatraemia. A water load test with AVP measurement is a potentially informative investigation, while AVPR2 sequencing provides a definitive molecular genetic diagnosis and a rationale for long-term fluid restriction.

Angiotensin II induces membrane trafficking of natively expressed transient receptor potential vanilloid type 4 channels in hypothalamic 4B cells

Transient receptor potential vanilloid family type 4 (TRPV4) channels are expressed in central neuroendocrine neurons and have been shown to be polymodal in other systems. We previously reported that in the rodent, a model of dilutional hyponatremia associated with hepatic cirrhosis, TRPV4 expression is increased in lipid rafts from the hypothalamus and that this effect may be angiotensin dependent. In this study, we utilized the immortalized neuroendocrine rat hypothalamic 4B cell line to more directly test the effects of angiotensin II (ANG II) on TRPV4 expression and function. Our results demonstrate the expression of corticotropin-releasing factor (CRF) transcripts, for sex-determining region Y (SRY) (male genotype), arginine vasopressin (AVP), TRPV4, and ANG II type 1a and 1b receptor in 4B cells. After a 1-h incubation in ANG II (100 nM), 4B cells showed increased TRPV4 abundance in the plasma membrane fraction, and this effect was prevented by the ANG II type 1 receptor antagonist losartan (1 μM) and by a Src kinase inhibitor PP2 (10 μM). Ratiometric calcium imaging experiments demonstrated that ANG II incubation potentiated TRPV4 agonist (GSK 1016790A, 20 nM)-induced calcium influx (control 18.4 ± 2.8% n = 5 and ANG II 80.5 ± 2.4% n = 5). This ANG II-induced increase in calcium influx was also blocked by 1 μM losartan and 10 μM PP2 (losartan 26.4 ± 3.8% n = 5 and PP2 19.7 ± 3.9% n = 5). Our data suggests that ANG II can increase TRPV4 channel membrane expression in 4B cells through its action on AT1R involving a Src kinase pathway.

Lack of responsiveness to 1-desamino-D arginin vasopressin (desmopressin) in male patients with nephrogenic syndrome of inappropriate antidiuresis: from bench to bedside

BACKGROUND

Nephrogenic syndrome of inappropriate antidiuresis (NSIAD) is a recently described entity, linked to gain-of-function mutations (R137C and R137L) in arginine vasopressin (AVP) gene leading to chronic activation of tubular V2 AVP receptor (V2R) and thus free water reabsorption. In addition to collecting duct cells, the V2R is also expressed in endothelial cells, where it mediates the rise in circulating levels of von Willebrand factor (vWF) and coagulation factor VIII (fVIII). Recent in vitro data showed that these mutant receptors are resistant to vasopressin-stimulated cAMP production. We aimed to explore by clinical observations the sensitivity to vasopressin of the R137C-V2R mutant in vivo.

MATERIAL AND METHODS

We performed a stimulation test with 1-desamino-D arginin vasopressin (dDAVP) 0·3 μg/kg of bodyweight in three patients (two hemizygous male and one heterozygous female) with NSIAD with R137C mutation and measured on the one hand the levels of vWF and fVIII and the other hand urine osmolality and albumin excretion (UAE).

RESULTS

Whereas the female heterozygous patient displayed normal response to simulation by dDAVP (except for UAE), no increase in vWF, fVIII, urinary osmolality and UAE was observed among hemizygous male patients.

CONCLUSIONS

Coherent with in vitro observation in transfected cells, our clinical observations demonstrate that the R137C-V2R mutant is resistant to vasopressin stimulation in its physiological sites of expression.

Syndrome of inappropriate antidiuretic hormone secretion: Revisiting a classical endocrine disorder

Hyponatremia occurs in about 30% of hospitalized patients and syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common cause of hyponatremia. SIADH should be differentiated from other causes of hyponatremia like diuretic therapy, hypothyroidism and hypocortisolism. Where possible, all attempts should be made to identify and rectify the cause of SIADH. The main problem in SIADH is fluid excess, and hyponatremia is dilutional in nature. Fluid restriction is the main stay in the treatment of SIADH; however, cerebral salt wasting should be excluded in the clinical setting of brain surgeries, subarachnoid hemorrhage, etc. Fluid restriction in cerebral salt wasting can be hazardous. Sodium correction in chronic hyponatremia (onset >48 hours) should be done slowly to avoid deleterious effects in brain.

Familial forms of diabetes insipidus: clinical and molecular characteristics

Over the past two decades, the genetic and molecular basis of familial forms of diabetes insipidus has been elucidated. Diabetes insipidus is a clinical syndrome characterized by the excretion of abnormally large volumes of diluted urine (polyuria) and increased fluid intake (polydipsia). The most common type of diabetes insipidus is caused by lack of the antidiuretic hormone arginine vasopressin (vasopressin), which is produced in the hypothalamus and secreted by the neurohypophysis. This type of diabetes insipidus is referred to here as neurohypophyseal diabetes insipidus. The syndrome can also result from resistance to the antidiuretic effects of vasopressin on the kidney, either at the level of the vasopressin 2 receptor or the aquaporin 2 water channel (which mediates the re-absorption of water from urine), and is referred to as renal or nephrogenic diabetes insipidus. Differentiation between these two types of diabetes insipidus and primary polydipsia can be difficult owing to the existence of partial as well as complete forms of vasopressin deficiency or resistance. Seven different familial forms of diabetes insipidus are known to exist. The clinical presentation, genetic basis and cellular mechanisms responsible for them vary considerably. This information has led to improved methods of differential diagnosis and could provide the basis of new forms of therapy.

[Current considerations in syndrome of inappropriate secretion of antidiuretic hormone/syndrome of inappropriate antidiuresis]

The syndrome of inappropriate secretion of antidiuretic hormone (SIADH)/syndrome of inappropriate antidiuresis is characterized by a hypotonic hyponatremia, with an insufficiently diluted urine given the plasmatic hypoosmolality, in the absence of hypovolemia (with or without a third space), hypotension, renal or heart failure, cirrhosis of the liver, hypothyroidism, adrenal insufficiency, vomiting, or other non-osmotic stimuli of ADH secretion. The response of ADH to the infusion of hypertonic saline divides SIADH into 4 different types. In type D, there is no alteration in ADH secretion. Rather, the defect is the maintained permeability of kidney aquaporin-2 channels to water. Activating mutations of the V2 receptor have been identified. The most frequent cause of SIADH is the use of drugs that induce secretion of the hormone. Old age is per se a risk factor for its development. SIADH is underdiagnosed, and hospitalization often worsens the clinical situation, due to an iatrogenic excess in the use of oral and i.v. liquids, often hypotonic, together with a reduction in salt intake. Treatment is directed towards normalization of natremia when possible, together with the avoidance of both hyponatremic encephalopathy as well as the osmotic demyelinization syndrome. Cases of "appropriate" secretion of ADH with normovolemic hyponatremia and high mortality rates should be treated with the same urgency as SIADH--such is the case of post-surgical hyponatremia.

Vaptans for the treatment of hyponatremia

The vaptans constitute a new class of pharmaceuticals developed for the treatment of the hypervolemic and euvolemic forms of hyponatremia. These agents are nonpeptide vasopressin antagonists that interfere with the antidiuretic effect of the hormone by competitively binding to V(2) receptors in the kidney. This blockade results in water diuresis (aquaresis) that, if not offset by increased fluid intake, reduces body water content and raises plasma sodium levels. Probably as a result of this rise in plasma sodium, thirst and plasma vasopressin concentration increase, potentionally limiting the effects of the vasopressin antagonists. Nonetheless, vaptans are particularly useful to treat hypervolemic hyponatremia associated with severe congestive heart failure or chronic liver failure, as the only other treatments currently available, such as fluid restriction and diuretics, are slow-acting and minimally effective. Vaptans are also useful for treating euvolemic hyponatremia associated with the syndrome of inappropriate antidiuretic hormone (SIADH), at least when it is chronic and/or minimally symptomatic. However, because their effects vary unpredictably from patient to patient, vaptans are less useful than hypertonic saline infusion in cases of acute, severe and symptomatic hyponatremia. Vaptan therapy is absolutely contraindicated in hypovolemic hyponatremia (in which total body water is reduced) and is ineffective in the vasopressin-independent form of inappropriate antidiuresis caused by constitutive activating mutations of V(2) receptors.

Body fluids and salt metabolism - part II

There is a high frequency of diarrhea and vomiting in childhood. As a consequence the focus of the present review is to recognize the different body fluid compartments, to clinically assess the degree of dehydration, to know how the equilibrium between extracellular fluid and intracellular fluid is maintained, to calculate the effective blood osmolality and discuss both parenteral fluid maintenance and replacement.

Nephrogenic syndrome of inappropriate antidiuresis: a novel cause of euvolemic hypotonic hyponatremia in newborns. Diagnosis and practical management

Hyponatremia, defined by a serum sodium concentration of less than 135 mmmol/l, is a complex clinical occurrence frequently manifested in newborns admitted to the neonatal intensive care unit. The pathogenetic mechanisms and clinical timing underlying the onset of hyponatremia have not been well established in the newborn. Aim of this review is to present a practical approach and management of hypotonic hyponatremia in newborns, with particular emphasis on nephrogenic syndrome of inappropriate antidiuresis, recently described by us for the first time in the literature in a newborn.

Agonist-independent interactions between beta-arrestins and mutant vasopressin type II receptors associated with nephrogenic syndrome of inappropriate antidiuresis

Nephrogenic syndrome of inappropriate antidiuresis is a recently identified genetic disease first described in two unrelated male infants with severe symptomatic hyponatremia. Despite undetectable arginine vasopressin levels, patients have inappropriately concentrated urine resulting in hyponatremia, hypoosmolality, and natriuresis. It was found that each infant had a different mutation of the vasopressin type II receptor (V2R) at codon 137 where arginine was converted to cysteine or leucine (R137C or R137L), resulting in constitutive signaling. Interestingly, a missense mutation at the same codon, converting arginine to histidine (R137H), leads to the opposite disease phenotype with a loss of the kidney's ability to concentrate urine resulting in nephrogenic diabetes insipidus. This mutation is associated with impaired signaling, although whether this is predominantly due to impaired trafficking to the plasma membrane, agonist-independent internalization, or G protein uncoupling is currently unclear. Using bioluminescence resonance energy transfer and confocal microscopy, we demonstrate that both V2R-R137C and V2R-R137L mutants interact with beta-arrestins in an agonist-independent manner resulting in dynamin-dependent internalization. This phenotype is similar to that observed for V2R-R137H, which is intriguing considering that it is accompanied by constitutive rather than impaired signaling. Consequently, it would seem that agonist-independent internalization per se is unlikely to be the major determinant of impaired V2R-R137H signaling. Our findings indicate that the V2R-R137C and V2R-R137L mutants traffic considerably more efficiently to the plasma membrane than V2R-R137H, identifying this as a potentially important mutation-dependent difference affecting V2R function.

Altered central TRPV4 expression and lipid raft association related to inappropriate vasopressin secretion in cirrhotic rats

Inappropriate vasopressin (AVP) release causes dilutional hyponatremia in many pathophysiological states such as cirrhosis. The central molecular mechanisms that mediate inappropriate AVP release are unknown. We tested the hypothesis that changes in the expression or trafficking of TRPV4 in the central nervous system may contribute to inappropriate AVP release in the bile duct ligation (BDL) model of cirrhosis in the rat. Four weeks after surgery, BDL rats demonstrated significantly increased plasma vasopressin and plasma renin activity (PRA), hypervolemia, and decreased plasma osmolality. These effects were blocked by providing BDL rats with 2% saline to drink for 15 days. TRPV4 protein expression was significantly increased in brain punches from BDL rats containing the supraoptic nucleus (SON) of the hypothalamus (100% +/- 11 to 157% +/- 4.8), and this effect was blocked in BDL rats given saline. Immunohistochemistry demonstrated a significant increase in TRPV4-positive cells and the percentage of AVP neurons that also were TRPV4-positive in the SON of BDL rats. In the hypothalamus of BDL rats, TRPV4 lipid raft association increased compared with sham (from 100% +/- 2.1 to 326.1% +/- 16). This effect was significantly attenuated in BDL rats given 2% saline to drink (174% +/- 11). In the brain stem, TRPV4 lipid raft association was reduced by BDL and inversely related to plasma AVP and PRA. We speculate that changes in TRPV4 expression and compartmentalization within lipid rafts could contribute to a feed-forward mechanism related to AVP release in cirrhosis.

Neonatal onset of nephrogenic syndrome of inappropriate antidiuresis

This paper describes the manifestation in a child of a new syndrome characterized by unusual, severe, persistent hyponatremia associated with hyposmolarity, euvolemia, inappropriately concentrated urine and elevated natriuresis. This is the fourth case of this syndrome reported to date, and the first to be reported in a neonate. The clinical features resemble those typically observed in patients with inappropriate antidiuretic hormone secretion, although high arginine vasopressin (AVP) levels are lacking. The findings led the authors to hypothesise a nephrogenic syndrome of inappropriate antidiuresis (NSIAD). The previously described R137C gain-of-function mutation was detected by means of mutation analysis of the V2R gene. Our results indicate that NSIAD is already present during the neonatal period and that molecular analysis of the V2R receptor should therefore be carried out, in all newborns with prolonged euvolemic hyponatremia with hypo-osmolarity, high urinary sodium and normal/low or undetectable AVP levels.

Intermittent severe, symptomatic hyponatraemia due to the nephrogenic syndrome of inappropriate antidiuresis

A 20-year-old fit male soldier presented on two separate occasions 16 months apart with severe, symptomatic hyponatraemia and a clinical and biochemical picture consistent with the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). In the intervening period, repeated plasma sodium values were in the reference range. Intensive investigation failed to reveal a cause for SIADH that was initially considered idiopathic. The description of a family comprising several adults with intermittent or water load induced-hyponatraemia associated with an activating mutation in the arginine vasopressin (AVP) receptor type 2 (AVPR2) raised the question of whether our patient could have a similar ‘nephrogenic syndrome of inappropriate antidiuresis’. Mutational screening of AVPR2 in our patient revealed a single missense mutation (R137C) in the second intracellular loop, which has been associated with constitutive activation of the AVPR2. In conclusion, adults with intermittent, severe hyponatraemia may have a constitutively activating mutation in the AVPR2 with resultant nephrogenic syndrome of inappropriate antidiuresis. Patients with idiopathic SIADH, particularly those with unmeasurable circulating AVP concentrations, should be considered for mutational screening of AVPR2.

Regulation of arginine vasopressin in the syndrome of inappropriate antidiuresis

The syndrome of inappropriate antidiuresis (SIAD) is a disorder of sodium and water balance characterized by hypotonic hyponatremia and impaired water excretion in the absence of renal insufficiency, adrenal insufficiency, or any recognized stimulus for the antidiuretic hormone arginine vasopressin (AVP). Hyponatremia is primarily a result of excessive water retention caused by a combination of excessive intake and inappropriate antidiuresis. It is sometimes aggravated by a sodium deficiency caused by decreased intake and/or a secondary natriuresis triggered by and largely corrective of the increase in extracellular volume. Hence, there is neither edema nor signs of hypovolemia. Inappropriate antidiuresis is usually due to administration or endogenous production of AVP or another vasopressin receptor agonist such as desmopressin. Endogenous production can be either ectopic (from a tumor) or eutopic (from the neurohypophysis). The latter apparently is induced by a wide variety of diseases, drugs, or injuries and is divisible into 3 different types of abnormal AVP release during hypertonic saline infusion: high, erratic fluctuations unrelated to increases in plasma sodium (type A); a slow constant "leak" that is also unaffected by increases in plasma sodium (type B); and rapid progressive increases in plasma AVP that correlate closely with plasma sodium as it rises toward the normal range (type C or "reset osmostat"). In 5% to 10% of patients, there is no demonstrable abnormality in the osmoregulation of AVP (type D) and the cause of inappropriate antidiuresis is unclear. In some children it appears to be due to an activating mutation of the V2 receptor (V2R). In other patients, it may be due to abnormal control of aquaporin-2 water channels in renal collecting tubules or production of an antidiuretic principle other than AVP. These different types of osmoregulatory dysfunction underlying SIAD may result in marked differences in clinical presentation or response to therapy with fluid restriction, hypertonic saline infusion, or vasopressin antagonists.