Disorders Of Water Balance Research Articles

The Enigma That Is ROHHAD Syndrome: Challenges and Future Strategies

Rapid-onset obesity with hypoventilation, hypothalamic dysfunction, and autonomic dysregulation (ROHHAD) is a rare syndrome presenting in early childhood associated with a high risk of mortality between 50 and 60%. It is characterised by rapid, early onset of obesity between 1.5–7 years, along with central hypoventilation and hypothalamic dysfunction, such as central hypothyroidism, hyperprolactinemia, disorders of sodium and water balance, growth hormone deficiency, adrenocortical insufficiency, or disorders of puberty and features of autonomic dysregulation. Up to half of cases have neural crest tumours, most commonly ganglioneuromas or ganglioneuroblastomas. The incidence of ROHHAD syndrome in any population is unknown. Currently, there is no specific diagnostic or genetic biomarker for ROHHAD, and diagnosis is based on clinical signs and symptoms, which is often challenging, and consequently may be delayed or unrecognised. Early diagnosis is important, as without intervention, ROHHAD is associated with high morbidity and mortality. Aetiology remains unclear; an autoimmune origin has been postulated, with immunosuppressive agents being used with variable benefit. With no cure, multidisciplinary management is largely supportive. Therefore, there are many unanswered questions in ROHHAD syndrome. In this review article, we outline the challenges posed by ROHHAD syndrome, including aetiology, genetics, diagnosis, screening, management, and prognosis. We present research priorities to tackle these issues to improve outcomes.

The biology of water homeostasis.

Water homeostasis is controlled by a brain-kidney axis that consists of central osmoreceptors, synthesis and secretion of arginine vasopressin (AVP) and AVP-responsive aquaporin-2 (AQP2) water channels in kidney collecting duct principal cells that facilitate water reabsorption. In addition to AVP, thirst represents a second line of defense to maintain water balance. Water balance disorders arise because of deficiency, resistance or inappropriate secretion of AVP or disturbances in thirst sensation (hypodipsia, polydipsia). People with water balance disorders are prone to develop hyponatremia or hypernatremia, which expose cells to osmotic stress and activate cell volume regulation mechanisms. This review covers several recent insights that have expanded our understanding of central osmoregulation, AQP2 regulation and cell volume regulation. This includes the role of with-no-lysine kinase 1 (WNK1) as a putative central osmolality sensor and, more generally, as an intracellular crowding sensor that coordinates the cell volume rescue response by activating sodium and potassium cotransporters. Furthermore, several new regulators of AQP2 have been identified, including for AVP-dependent AQP2 regulation (yes-associated protein, nuclear factor of activated T-cells, microRNAs) and AVP-independent AQP2 regulation (epidermal growth factor receptor, fluconazole, prostaglandin E2). It is also becoming increasingly clear that long-term cell volume adaptation to chronic hypotonicity through release of organic osmolytes comes at the expense of compromised organ function. This potentially explains the complications of chronic hyponatremia, including cognitive impairment, bone loss and vascular calcification. This review will illustrate why these new insights derived from basic science are also relevant for developing new approaches to treat water balance disorders.

Focal Adhesion Kinase (FAK) inhibition induces membrane accumulation of aquaporin2 (AQP2) through endocytosis inhibition and actin depolymerization in renal epithelial cells.

Cellular trafficking of the water channel aquaporin 2 (AQP2) is regulated by the actin cytoskeleton in collecting duct principal cells (PC) to maintain proper water balance in animals. Critical actin depolymerization/polymerization events are involved in both constitutive AQP2 recycling, and the pathway stimulated by vasopressin receptor signaling. Focal adhesion kinase (FAK) plays an important role in modulating the actin cytoskeleton through inhibiting small GTPases, and multiple studies have shown the involvement of FAK in insulin and cholesterol trafficking through actin regulation. To understand whether FAK contributes to water reabsorption by the kidney, we performed a series of in vitro experiments to examine the involvement of FAK and its signaling in mediating AQP2 trafficking in cultured renal epithelial cells. Our data showed that FAK inhibition by specific inhibitors caused membrane accumulation of AQP2 in AQP2expressing LLCPK1 cells by immunofluorescence staining. AQP2 membrane accumulation induced by FAK inhibition is associated with significantly reduced endocytosis of AQP2 via the clathrin-mediated endocytosis pathway. Moreover, AQP2 membrane accumulation induced by FAK inhibition also occurred in cells expressing the constitutive dephosphorylation mutant of AQP2, S256A. This was confirmed by immunoblotting using a specific antibody against phospho-serine 256 AQP2, supporting a phosphorylation independent mechanism. Finally, we demonstrated that inhibition of FAK caused reduced RhoA signaling and promoted F-actin depolymerization. In conclusion, our study identifies FAK signaling as a pathway that could provide a novel therapeutical avenue for AQP2 trafficking regulation in water balance disorders.

Fluid therapy for dysnatraemia

Dysnatraemias (water balance disorders, which present as hypo- or hypernatraemia) are a relatively common primary or comorbidity in hospitalised patients and are associated with increased mortality in dogs and cats, particularly if an appropriate therapy is not instituted. Sodium alterations may be caused by derangements in total body sodium, water or both. Determining patient volume status and osmolality is important in establishing differential diagnoses. The goal of treatment is to return serum sodium levels to normal by correcting sodium and water derangements at an appropriate rate to prevent rapid movement of water in or out of the intracranial space and the potential neurological consequences. Development of an appropriate fluid therapy plan and frequent monitoring of sodium levels are essential for successful treatment of these disorders.

Stichoposide C and Rhizochalin as Potential Aquaglyceroporin Modulators.

Aquaporins (AQPs) are a family of integral membrane proteins that selectively transport water and glycerol across the cell membrane. Because AQPs are involved in a wide range of physiological functions and pathophysiological conditions, AQP-based therapeutics may have the broad potential for clinical utility, including for disorders of water and energy balance. However, AQP modulators have not yet been developed as suitable candidates for clinical applications. In this study, to identify potential modulators of AQPs, we screened 31 natural products by measuring the water and glycerol permeability of mouse erythrocyte membranes using a stopped-flow light scattering method. None of the tested natural compounds substantially affected the osmotic water permeability. However, several compounds considerably affected the glycerol permeability. Stichoposide C increased the glycerol permeability of mouse erythrocyte membranes, whereas rhizochalin decreased it at nanomolar concentrations. Immunohistochemistry revealed that AQP7 was the main aquaglyceroporin in mouse erythrocyte membranes. We further verified the effects of stichoposide C and rhizochalin on aquaglyceroporins using human AQP3-expressing keratinocyte cells. Stichoposide C, but not stichoposide D, increased AQP3-mediated transepithelial glycerol transport, whereas the peracetyl aglycon of rhizochalin was the most potent inhibitor of glycerol transport among the tested rhizochalin derivatives. Collectively, stichoposide C and the peracetyl aglycon of rhizochalin might function as modulators of AQP3 and AQP7, and suggests the possibility of these natural products as potential drug candidates for aquaglyceroporin modulators.

EGFR inhibition with erlotinib induces membrane accumulation of aquaporin-2 via PDK1 and RSK activation

In the canonical aquaporin-2 (AQP2) signaling pathway, vasopressin (VP) binds to its V2 receptor, which in turn activates adenylate cyclase and protein kinase A (PKA), resulting in AQP2 S256 phosphorylation and accumulation of AQP2 in the membrane. The epidermal growth factor receptor (EGFR) inhibitor erlotinib also increases AQP2 S256 phosphorylation and membrane accumulation, yet it does not increase cAMP or PKA activity. We hypothesize that the ribosomal s6 kinase (RSK), a downstream effector in the EGFR-MAPK/ERK pathway, is the terminal kinase phosphorylating AQP2 in this novel PKA-independent pathway activated by erlotinib, and that the phosphoinositide dependent kinase-1 (PDK1), a well-established activator of RSK, is indispensable for erlotinib-induced AQP2 activation. Using our LLC-AQP2 cell model, we show that erlotinib-induced AQP2 membrane accumulation and S256 phosphorylation are abolished by the specific RSK inhibitor BI-D1870. RSK knockdown with siRNA also blocked AQP2 S256 phosphorylation and membrane accumulation, as did RSK knockout with CRISPR/cas9 editing. Next, rat kidney slices were incubated in Hank’s buffer with or without BI-D1870 and treated with erlotinib. BI-D1870 inhibited AQP2 membrane accumulation in collecting duct principal cells, supporting the relevance of this pathway in kidney cells in situ. An in-vitro kinase assay using purified proteins demonstrated that RSK directly phosphorylates AQP2 at the S256 residue. Additionally, erlotinib consistently increased phosphorylation of RSK T359, a residue associated with RSK’s active state. In canonical RSK activation, a phosphorylation cascade results in PDK1 binding to and activating RSK. We found that PDK1 inhibition with an experimental compound PS-222 also blocked erlotinib-induced AQP2 membrane accumulation and S256 phosphorylation. This implicates PDK1 along with RSK in the novel erlotinib-induced pathway of AQP2 traffcking. Our data show that RSK and PDK1 are terminal effectors in the novel, PKA-independent pathway of AQP2 activation induced by erlotinib. Further elaboration of this new non-canonical pathway promises to uncover potential pharmacological targets for the treatment of water balance disorders. This work was supported by the National Institutes of Health (NIH) grant DK096586 (D. Brown). P. W. Cheung was supported was supported by NIH K-award DK115901. Richard Babicz is the recipient of the Ben J Lipps Research Fellowship (American Society of Nephrology). Additional support for the Program in Membrane Biology Microscopy Core came from the Boston Area Diabetes and Endocrinology Research Center (DK057521) and the Massachusetts General Hospital (MGH) Center for the Study of Inflammatory Bowel Disease (DK043351). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Focal adhesion kinase (FAK) inhibition induces aquaporin 2 (AQP2) plasma membrane accumulation in renal epithelial cells by remodeling the actin cytoskeleton

The actin cytoskeleton is involved in the cellular traffcking of the AQP2 water channel in kidney collecting duct (CD) principal cells (PC): this plays an important role in water homeostasis. Focal adhesion kinase (FAK) is known to modulate the actin cytoskeleton through small GTPase-mediated pathways and we, therefore, investigated the potential role of FAK in AQP2 traffcking. We and others have previously shown that AQP2 constitutively traffcs and recycles independently of vasopressin (VP) and that this involves actin depolymerization/polymerization; therefore, FAK could regulate AQP2 traffcking through actin remodeling, bypassing the VP-V2R (cAMP/PKA) signaling pathway. To examine this, we used small molecule FAK inhibitors, VS-4718, GSK2256098 (GSK), or Defactinib-Hydrochloride (DH). We used immunofluorescence staining, a transferrin endocytosis assay ± cold block treatment, a fluorometric exocytosis assay using cells expressing secreted yellow fluorescent protein (ssYFP), a RhoA activity assay, and western blotting. Inhibiting FAK with 1mM VS-4718 for 30 min increased AQP2 membrane accumulation in LLCPK1 and IMCD kidney epithelial cells expressing AQP2. VS-4718 resulted in reduced clathrin-mediated endocytosis, as well as increased vesicle exocytosis, both mechanisms contributing to increased AQP2 membrane accumulation. This effect of FAK inhibition was independent of vasopressin/cAMP signaling and did not require phosphorylation of the Serine 256 residue of AQP2, since AQP2 membrane accumulation also occurred in cells expressing the constitutive dephosphorylation AQP2 S256A mutant. The FAK inhibitor VS-4718 promoted actin depolymerization (phalloidin staining assay) which would facilitate AQP2 vesicle exocytosis into the plasma membrane. We then examined the activity of the small GTPase, RhoA, whose activity increases actin polymerization and vesicle traffcking in the exocytic and endocytic pathways. We found that FAK inhibition by VS-4718 downregulated RhoA activity using GST-RBD, a substrate that binds to active RhoA, as seen by western blotting using phospho-specific antibodies. Similar results were obtained with GSK and DH. In summary, our study revealed that the interaction between FAK and cytoskeletal architecture plays a crucial role in regulating AQP2 recycling and its subsequent entry into the endocytic pathway. Thus, targeting FAK could be a potential therapeutic strategy for water balance disorders involving defective AQP2 reabsorption, such as nephrogenic diabetes insipidus. This work was supported by NIDDK R01DK096015 (JHL) and DK096586 (DB). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Early Postoperative Serum Sodium Level and Urine Output Volume Profile after Endoscopic Endonasal Transsphenoidal Surgery in Macroadenoma Pituitary Tumor

Introduction: Surgery is still the initial treatment of choice to for the majority of date with pituitary tumors. One of the most commonly performed minimally invasive neurosurgical procedures is transsphenoidal endoscopic endonasal surgery (ETS) and microscopic transsphenoidal surgery (MTS), used commonly for the resection of pituitary tumors. Neurosurgical operations for pituitary and suprasellar tumors may result in postoperative complications due to the crucial anatomical location of these tumors. The resulting postoperative complications can manifest as anterior or posterior pituitary dysfunction, particularly sodium disturbances, due to the changes in antidiuretic hormone (ADH) secretion, which remains one of the most frequent postoperative reasons for hospital readmission. Method: The aim of this study is to describe and determine the frequency distribution of patient characteristics who underwent endoscopic endonasal transsphenoidal resection of pituitary adenoma at Universitas Sumatera Utara Hospital between 2021 and 2022. Result: The findings of this study showed the frequency distribution of patient characteristics who underwent endoscopic endonasal transsphenoidal resection and postoperative sodium serum level profile as well as postoperative urine output profile. Discussion: The discussion section briefly discusses the findings and their implications. It mentions the high prevalence of headache and visual deficits among the patients, which aligns with previous studies. It also provides an overview of normal water balance and the pathophysiology of water balance disorders after pituitary surgery.

#3642 SERUM APELIN AND RENAL PROGNOSIS IN PATIENTS WITH NON-DIALYSIS DEPENDENT CHRONIC KIDNEY DISEASE

Abstract Background and Aims The apelin/APJ system is a novel pleiotropic system with an essential role in renal and cardiovascular physiology and disease. A role for the apelin/APJ system in diverse pathological states, including disorders of sodium and water balance, hypertension, heart failure, pre-eclampsia, acute kidney injury, sepsis and diabetic nephropathy, has recently been reported. Especially, the impact of apelin/APJ system on cardiovascular system has been attracting attention, but scarce data is available in chronic kidney disease (CKD). Method We measured serum apelin-36 by ELISA in 411 outpatients with non-dialysis dependent (NDD) CKD and analyzed factors associated with serum apelin. Furthermore, we examined the association of serum apelin with renal outcome (renal replacement therapy, RRT) using multivariate Cox proportional hazards model. Results Median age and eGFR were 71 years and 21.7 ml/min/1.73 m2, respectively and the prevalence of male gender and diabetes was 26.8% and 50.8%, respectively. Median serum apelin was 0.81pg/ml. There was no significant difference in serum apelin level between male and female patients, or those with and without diabetes. Moreover, serum apelin level had no significant association with age and eGFR; however, there was a significant relationship between serum apelin and brain natriutetic peptide. During the follow-up period of 38 months, 126 patients started RRT. eGFR and age were associated with the time to RRT induction. Serum apelin was not associated with renal outcome (HR, 1.05; 95%CI, 0.64-1.71; P,0.86); however, this association was interacted by diabetes and gender (P for interaction, 0.073 and 0.070, respectively). Conclusion Serum apelin could be a predictive marker of renal prognosis in male and non-diabetic CKD patients.

Evidence-based hyponatremia management in liver disease.

Hyponatremia is primarily a water balance disorder associated with high morbidity and mortality. The pathophysiological mechanisms behind hyponatremia are multifactorial, and diagnosing and treating this disorder remains challenging. In this review, the classification, pathogenesis, and step-by-step management approaches for hyponatremia in patients with liver disease are described based on recent evidence. We summarize the five sequential steps of the traditional diagnostic approach: 1) confirm true hypotonic hyponatremia, 2) assess the severity of hyponatremia symptoms, 3) measure urine osmolality, 4) classify hyponatremia based on the urine sodium concentration and extracellular fluid status, and 5) rule out any coexisting endocrine disorder and renal failure. Distinct treatment strategies for hyponatremia in liver disease should be applied according to the symptoms, duration, and etiology of disease. Symptomatic hyponatremia requires immediate correction with 3% saline. Asymptomatic chronic hyponatremia in liver disease is prevalent and treatment plans should be individualized based on diagnosis. Treatment options for correcting hyponatremia in advanced liver disease may include water restriction; hypokalemia correction; and administration of vasopressin antagonists, albumin, and 3% saline. Safety concerns for patients with liver disease include a higher risk of osmotic demyelination syndrome.

Protocolo de valoración de las alteraciones hidroelectrolíticas en el paciente con enfermedad renal

The gradual loss of kidney function leads to adaptive and compensatory changes that make it possible to maintain homeostasis with glomerular filtration rates of down to 10-25ml/min. Under 10ml/min disorders arise in the internal medium and these have clinical consequences. It is therefore important to properly evaluate alterations in the water balance, sodium, potassium and acid-basic disorders in chronic kidney disease.

Autophagy and regulation of aquaporins in the kidneys.

Aquaporins (AQPs) are water channel proteins that facilitate the transport of water molecules across cell membranes. To date, seven AQPs have been found to be expressed in mammal kidneys. The cellular localization and regulation of the transport properties of AQPs in the kidney have been widely investigated. Autophagy is known as a highly conserved lysosomal pathway, which degrades cytoplasmic components. Through basal autophagy, kidney cells maintain their functions and structure. As a part of the adaptive responses of the kidney, autophagy may be altered in response to stress conditions. Recent studies revealed that autophagic degradation of AQP2 in the kidney collecting ducts leads to impaired urine concentration in animal models with polyuria. Therefore, the modulation of autophagy could be a therapeutic approach to treat water balance disorders. However, as autophagy is either protective or deleterious, it is crucial to establish an optimal condition and therapeutic window where autophagy induction or inhibition could yield beneficial effects. Further studies are needed to understand both the regulation of autophagy and the interaction between AQPs and autophagy in the kidneys in renal diseases, including nephrogenic diabetes insipidus.

Proteomics and AQP2 regulation.

The advent of modern quantitative protein mass spectrometry techniques around the turn of the 21st century has contributed to a revolution in biology referred to as 'systems biology'. These methods allow identification and quantification of thousands of proteins in a biological specimen, as well as detection and quantification of post-translational protein modifications including phosphorylation. Here, we discuss these methodologies and show how they can be applied to understand the effects of the peptide hormone vasopressin to regulate the molecular water channel aquaporin-2. The emerging picture provides a detailed framework for understanding the molecular mechanisms involved in water balance disorders.

Hyponatraemia – diagnostic difficulties in 2.5-year-old boy with nephrogenic syndrome of inappropriate antidiuresis. Case report

Normal sodium levels in the extracellular fluid are in the range of 135–145 mmol/L. Low serum sodium (hyponatraemia) is a common water and electrolyte balance disorder. Dehydration and overhydration are the most common causes of hyponatraemia in children. This paper describes a case of a 2.5-year-old boy admitted to the paediatric ward due to pneumonia. The child showed reluctance to fluid intake, hypoosmolar hyponatraemia with hypouricaemia, and increased urinary sodium excretion in the absence of abnormalities in hydration status. He had a family history of persistent hyponatraemia and hypertension in his grandfather. After excluding pituitary, thyroid, adrenal cortex pathologies, kidney disfunction, and proliferative processes in the differential diagnosis, cerebral salt wasting syndrome, the syndrome of inappropriate secretion of antidiuretic hormone and the renal-related syndrome of inappropriate antidiuresis were taken into account. The aim of the study was to discuss the differential diagnosis of chronic euvolemic hyponatraemia and to draw attention to the need for genetic testing for the nephrogenic syndrome of inappropriate antidiuretic hormone secretion caused by activating point mutations of the vasopressin 2 receptor gene (V2R).

Experimental assessment of the possibility of removing foreign particles from the respiratory tract using total liquid ventilation

Introduction. In the structure of occupational diseases associated with the effects of chemical factors, the pathology of the respiratory organs due to the action of dust, occupies the first place. Among the recommended methods of treating occupational dust pathology, there are no procedures for removing foreign particles from the respiratory tract aimed at eliminating the cause of the disease. Despite the effectiveness of total bronchoalveolar lavage in the treatment of professional dust bronchitis, the method has a number of disadvantages due to the need to use large volumes of saline solution, which can lead to undesirable side effects associated with water balance disorders in the body caused by the absorption of fluid by the lungs during the procedure. In this regard, attention is drawn to the possibility of using perfluorocarbon compounds with low bioavailability for the elimination of foreign microparticles from the lungs by bronchoalveolar lavage or total liquid ventilation (TLV). The study aims to give an experimental assessment of a possible method for the treatment of occupational lung diseases of dust etiology, based on the removal of dust foreign particles from the respiratory tract using perfluorocarbon compounds. Materials and methods. Scientists performed a study on male rabbits of the Soviet Chinchilla breed aged 3-4 months, weighing 2.8-3.6 kg. They used a device of their own design with an open circuit to carry out the procedure of TLV, in which the liquid "washed" the lungs and drained into a special container. Perfluorodecalin was used as a respiratory fluid. At the first stage, rabbits on TLV inhaled for 3 minutes microparticles of silica gel of the LL-254 brand (5/40 microns) marked with a luminescent indicator. At the second stage, after the end of inhalation, the rabbits of the control group continued to be on artificial ventilation with air, and the rabbits of the experimental group were transferred to the TVL for 5 minutes. At the third stage, artificial ventilation of the lungs with air was performed until the end of the action of the muscle relaxant and the restoration of independent breathing. Estimated indicators: the presence of luminous particles on the surface of the trachea, bronchi, on sections of lung tissue and in perfluorodecalin passed through the rabbit's lungs; histomorphological picture of the lungs; indicators of respiratory gases and acid-base state in arterial blood. Results. Elimination of silica gel from airways by TLV procedure was detected by luminescent visualization and histomorphologically. Complete recovery respiratory gases and acid-base status or arterial blood in one hour after procedure proved safety of TLV procedure. Conclusion. TLV procedure is able to remove foreign particles from the respiratory tract. The researchers proved the effectiveness of the procedure according to three criteria: presence of luminescent labeled particles in waste perfluorodecalin after TLV; decrease of luminescent labeled silica gel macroscopic visualization in trachea and bronchi; decrease of silica gel amount in lung tissue detected by microscopic histomorphological methothods; Perfluorodecalin application for foreign (including radioactive) particles removing from respiratory tract is more preferably than saline, because of saline lungs absorption that potentiate particles incorporation in tissue. Five-minutes TLV procedure does not induce markable disturbances of gas exchange monitored by respiratory gases and acid-base status or arterial blood. Complete recovery of these parameters was detected within one hour after procedure. Ethics. The scientists conducted studies involving laboratory animals in compliance with the necessary regulations (the Helsinki Declaration of 2000 on Humane Treatment of Animals and the "Rules for carrying out work using experimental animals" (Order of the Ministry of Health of the USSR No. 755 of 12.08.1977)). The protocol of the study was approved by the Ethics Committee of the Izmerov Research Institute of Occupational Health.

Altered Serum Uric Acid Levels in Kidney Disorders.

Serum uric acid levels are altered by kidney disorders because the kidneys play a dominant role in uric acid excretion. Here, major kidney disorders which accompany hyperuricemia or hypouricemia, including their pathophysiology, are discussed. Chronic kidney disease (CKD) and hyperuricemia are frequently associated, but recent clinical trials have not supported the pathogenic roles of hyperuricemia in CKD incidence and progression. Diabetes mellitus (DM) is often associated with hyperuricemia, and hyperuricemia may be associated with an increased risk of diabetic kidney disease in patients with type 2 DM. Sodium-glucose cotransporter 2 inhibitors have a uricosuric effect and can relieve hyperuricemia in DM. Autosomal dominant tubulointerstitial kidney disease (ADTKD) is an important hereditary kidney disease, mainly caused by mutations of uromodulin (UMOD) or mucin-1 (MUC-1). Hyperuricemia and gout are the major clinical manifestations of ADTKD-UMOD and ADTKD-MUC1. Renal hypouricemia is caused by URAT1 or GLUT9 loss-of-function mutations and renders patients susceptible to exercise-induced acute kidney injury, probably because of excessive urinary uric acid excretion. Hypouricemia derived from renal uric acid wasting is a component of Fanconi syndrome, which can be hereditary or acquired. During treatment for human immunodeficiency virus, hepatitis B or cytomegalovirus, tenofovir, adefovir, and cidofovir may cause drug-induced renal Fanconi syndrome. In coronavirus disease 2019, hypouricemia due to proximal tubular injury is related to disease severity, including respiratory failure. Finally, serum uric acid and the fractional excretion of uric acid are indicative of plasma volume status; hyperuricemia caused by the enhanced uric acid reabsorption can be induced by volume depletion, and hypouricemia caused by an increased fractional excretion of uric acid is the characteristic finding in syndromes of inappropriate anti-diuresis, cerebral/renal salt wasting, and thiazide-induced hyponatremia. Molecular mechanisms by which uric acid transport is dysregulated in volume or water balance disorders need to be investigated.

Disorders of Salt and Water Balance After Pituitary Surgery.

Transsphenoidal surgery is the first-line treatment for many clinically significant pituitary tumors and sellar lesions. Although complication rates are low when performed at high-volume centers, disorders of salt and water balance are relatively common postoperatively. Both, or either, central diabetes insipidus (recently renamed arginine vasopressin deficiency - AVP-D), caused by a deficiency in production and/or secretion of arginine vasopressin, and hyponatremia, most commonly secondary to the syndrome of inappropriate antidiuresis, may occur. These conditions can extend hospital stay and increase the risk of readmission. This article discusses common presentations of salt and water balance disorders following pituitary surgery, the pathophysiology of these conditions, and their diagnosis and management.

Genetic deletion of the nuclear factor of activated T cells 5 in collecting duct principal cells causes nephrogenic diabetes insipidus.

Water homeostasis is tightly regulated by the kidneys via the process of urine concentration. During reduced water intake, the antidiuretic hormone arginine vasopressin (AVP) binds to the vasopressin receptor type II (V2R) in the kidney to enhance countercurrent multiplication and medullary osmolality, and increase water reabsorption via aquaporin-2 (AQP2) water channels. The importance of this AVP, V2R, and AQP2 axis is highlighted by low urine osmolality and polyuria in people with various water balance disorders, including nephrogenic diabetes insipidus (NDI). ELF5 and nuclear factor of activated T cells 5 (NFAT5) are two transcription factors proposed to regulate Aqp2 expression, but their role is poorly defined. Here we generated two novel mouse lines with principal cell (PC)-specific deletion of ELF5 or NFAT5 and phenotyped them in respect to renal water handling. ELF5-deficient mice (ELF5PC-KO ) had a very mild phenotype, with no clear differences in AQP2 abundance, and mild differences in renal water handling and maximal urinary concentrating capacity. In contrast, NFAT5 (NFAT5PC-KO ) mice had significantly higher water intake and their 24 h urine volume was almost 10-fold greater than controls. After challenging with dDAVP or 8h water restriction, NFAT5PC-KO mice were unable to concentrate their urine, demonstrating that they suffer from NDI. The abundance of AQP2, other AQPs, and the urea transporter UT-A1 were greatly decreased in NFAT5PC-KO mice. In conclusion, NFAT5 is a major regulator of not only Aqp2 gene transcription, but also other genes important for water homeostasis and its absence leads to the development of NDI.

Acute pyelonephritis: Increased plasma membrane targeting of renal aquaporin‐2

Aquaporin-2 (AQP2) shuttling between intracellular vesicles and the apical plasma membrane is pivotal in arginine vasopressin-mediated urine concentration and is dysregulated in multiple diseases associated with water balance disorders. Children and adults with acute pyelonephritis have a urinary concentration defect and studies in children revealed increased AQP2 excretion in the urine. This study aimed to analyse AQP2 trafficking in response to acute pyelonephritis. Immunofluorescence analysis was used to evaluate subcellular localization of AQP2 and AQP2-S256A (mimicking non-phosphorylated AQP2 on serine 256) in cells stimulated with bacterial lysates and of AQP2 and pS256-AQP2 in a mouse model at day 5 of acute pyelonephritis. Western blotting was used to evaluate AQP2 levels and AQP2 phosphorylation on S256 upon incubation with bacterial lysates. Time-lapse imaging was used to measure intracellular cAMP levels in response to incubation with bacterial lysates. In cell cultures, lysates from both uropathogenic and nonpathogenic bacteria-mediated AQP2 plasma membrane targeting and increased AQP2 phosphorylation on serine 256 (pS256) without increasing cAMP levels. Both bacterial lysates induced plasma membrane targeting of AQP2-S256A. Immunofluorescence analysis of renal sections from mice after 5days of acute pyelonephritis revealed apical plasma membrane targeting of AQP2 and pS256-AQP2 in inner medullary collecting ducts. Uropathogenic bacteria induce AQP2 plasma membrane targeting in vitro and in vivo. cAMP levels were not elevated by the bacterial lysates and AQP2 plasma membrane targeting could occur without S256 phosphorylation. This may explain increased AQP2 excretion in the urine during acute pyelonephritis.

Updates and Perspectives on Aquaporin-2 and Water Balance Disorders.

Ensuring the proper amount of water inside the body is essential for survival. One of the key factors in the maintenance of body water balance is water reabsorption in the collecting ducts of the kidney, a process that is regulated by aquaporin-2 (AQP2). AQP2 is a channel that is exclusively selective for water molecules and impermeable to ions or other small molecules. Impairments of AQP2 result in various water balance disorders, including nephrogenic diabetes insipidus (NDI), which is a disease characterized by a massive loss of water through the kidney and consequent severe dehydration. Dysregulation of AQP2 is also a cause of water retention with hyponatremia in heart failure, hepatic cirrhosis, and syndrome of inappropriate antidiuretic hormone secretion (SIADH). Antidiuretic hormone vasopressin is an upstream regulator of AQP2. Its binding to the vasopressin V2 receptor promotes AQP2 targeting to the apical membrane and thus enables water reabsorption. Tolvaptan, a vasopressin V2 receptor antagonist, is effective and widely used for water retention with hyponatremia. However, there are no studies showing improvement in hard outcomes or long-term prognosis. A possible reason is that vasopressin receptors have many downstream effects other than AQP2 function. It is expected that the development of drugs that directly target AQP2 may result in increased treatment specificity and effectiveness for water balance disorders. This review summarizes recent progress in studies of AQP2 and drug development challenges for water balance disorders.