The role of pendrin in blood pressure regulation

Role of Kir5.1 (Kcnj16) Channels in Regulating Renal Ammonia Metabolism during Metabolic Acidosis in Dahl Salt-Sensitive Rats

Maintaining acid-base homeostasis is critical for normal physiological function. The kidneys are essential for regulating acid-base homeostasis through maintaining systemic bicarbonate concentration. Chronic metabolic acidosis is an independent risk factor for chronic kidney diseases. Renal inwardly rectifying potassium channel Kir5.1 plays an essential role in maintaining resting membrane potential. Patients with loss-of-function mutations in the KCNJ16 gene, which encodes Kir5.1, may have tubulopathy with hypokalemia, salt wasting, and hearing loss. Importantly, these mutations also disrupt acid-base balance, particularly causing metabolic acidosis. This study aimed to use Dahl salt-sensitive rats with a knockout of the Kcnj16 gene (SSKcnj16-/-) to investigate how the deletion of Kir5.1 affects the regulation of acid-base balance in salt-sensitive hypertension. SSKcnj16-/- rats displayed metabolic acidosis under a normal salt diet. Further analysis using RNA sequencing and Western blot analyses showed unchanged expression of proteins responsible for ammonia metabolism in the kidney of SSKcnj16-/- rats despite observed acidosis. However, there was a significant increase in the expression of bicarbonate transporter NBCe1, where there was a significant decrease in pendrin. In conclusion, the current study demonstrated that the loss of Kir5.1 impairs the sensitivity of ammonia metabolism in the kidney in response to metabolic acidosis, which provides mechanistic insights into developing potential therapeutics for conditions involving hypokalemia and acid-base abnormalities.

SLC26A4-AP-2 mu2 interaction regulates SLC26A4 plasma membrane abundance in the endolymphatic sac

Decreased presence or activity of human SLC26A4 at the plasma membrane is a common cause of hearing loss. SLC26A4 (Pendrin) is necessary for normal reabsorption of endolymph, the fluid bathing the inner ear. We identified the μ2 subunit of adaptor protein 2 (AP-2) complex required for clathrin-mediated endocytosis as a protein-partner of SLC26A4 involved in regulating its plasma membrane abundance. We showed that, in the endolymphatic sac, where fluid reabsorption occurs, SLC26A4 is localized along the apical microvilli of mitochondria-rich cells, in contact with the endolymph, and associated with clathrin-coated pits where μ2 and AP-2 are present. Based on SLC26A4 structure, the elements involved in SLC26A4-μ2 interaction were identified and validated experimentally, allowing modeling of this interaction at the atomic level. Pharmacological inhibition of clathrin-mediated endocytosis led to an increased plasma membrane abundance of hemagglutinin-tagged SLC26A4 virally or endogenously expressed in mitochondria-rich cells. These results indicate that the SLC26A4-μ2 interaction regulates SLC26A4 abundance at the apical surface of mitochondria-rich cells.

Chloride/Multiple Anion Exchanger SLC26A Family: Systemic Roles of SLC26A4 in Various Organs

Solute carrier family 26 member 4 (SLC26A4) is a member of the SLC26A transporter family and is expressed in various tissues, including the airway epithelium, kidney, thyroid, and tumors. It transports various ions, including bicarbonate, chloride, iodine, and oxalate. As a multiple-ion transporter, SLC26A4 is involved in the maintenance of hearing function, renal function, blood pressure, and hormone and pH regulation. In this review, we have summarized the various functions of SLC26A4 in multiple tissues and organs. Moreover, the relationships between SLC26A4 and other channels, such as cystic fibrosis transmembrane conductance regulator, epithelial sodium channel, and sodium chloride cotransporter, are highlighted. Although the modulation of SLC26A4 is critical for recovery from malfunctions of various organs, development of specific inducers or agonists of SLC26A4 remains challenging. This review contributes to providing a better understanding of the role of SLC26A4 and development of therapeutic approaches for the SLC26A4-associated hearing loss and SLC26A4-related dysfunction of various organs.

Novel functions of the anion exchanger AE4 (SLC4A9)

The kidney plays a crucial role in acid-base homeostasis. In the distal nephron, α-intercalated cells contribute to urinary acid (H+) secretion and β-intercalated cells accomplish urinary base (HCO3-) secretion. β-intercalated cells regulate the acid base status through modulation of the apical Cl-/HCO3- exchanger pendrin (SLC26A4) activity. In this review, we summarize and discuss our current knowledge of the physiological role of the renal transporter AE4 (SLC4A9). The AE4, as cation-dependent Cl-/HCO3- exchanger, is exclusively expressed in the basolateral membrane of β-intercalated cells and is essential for the sensing of metabolic acid-base disturbances in mice, but not for renal sodium reabsorption and plasma volume control. Potential intracellular signaling pathways are discussed that might link basolateral acid-base sensing through the AE4 to apical pendrin activity.

The pathophysiology of distal renal tubular acidosis

The kidneys have a central role in the control of acid-base homeostasis owing to bicarbonate reabsorption and production of ammonia and ammonium in the proximal tubule and active acid secretion along the collecting duct. Impaired acid excretion by the collecting duct system causes distal renal tubular acidosis (dRTA), which is characterized by the failure to acidify urine below pH 5.5. This defect originates from reduced function of acid-secretory type A intercalated cells. Inherited forms of dRTA are caused by variants in SLC4A1, ATP6V1B1, ATP6V0A4, FOXI1, WDR72 and probably in other genes that are yet to be discovered. Inheritance of dRTA follows autosomal-dominant and -recessive patterns. Acquired forms of dRTA are caused by various types of autoimmune diseases or adverse effects of some drugs. Incomplete dRTA is frequently found in patients with and without kidney stone disease. These patients fail to appropriately acidify their urine when challenged, suggesting that incomplete dRTA may represent an intermediate state in the spectrum of the ability to excrete acids. Unrecognized or insufficiently treated dRTA can cause rickets and failure to thrive in children, osteomalacia in adults, nephrolithiasis and nephrocalcinosis. Electrolyte disorders are also often present and poorly controlled dRTA can increase the risk of developing chronic kidney disease.

Control of sodium and potassium homeostasis by renal distal convoluted tubules

Distal convoluted tubules (DCT), which contain the Na-Cl cotransporter (NCC) inhibited by thiazide diuretics, undergo complex modulation to preserve Na+ and K+ homeostasis. The lysine kinases 1 and 4 (WNK1 and WNK4), identified as hyperactive in the hereditary disease pseudohypoaldosteronism type 2, are responsible for activation of NCC and consequent hypokalemia and hypertension. WNK4, highly expressed in DCT, activates the SPAK/OSR1 kinases, which phosphorylate NCC and other regulatory proteins and transporters in the distal nephron. WNK4 works as a chloride sensor through a Cl- binding site, which acts as an on/off switch at this kinase in response to changes of basolateral membrane electrical potential, the driving force of cellular Cl- efflux. High intracellular Cl- in hyperkalemia decreases NCC phosphorylation and low intracellular Cl- in hypokalemia increases NCC phosphorylation and activity, which makes plasma K+ concentration a central modulator of NCC and of K+ secretion. The WNK4 phosphorylation by cSrc or SGK1, activated by angiotensin II or aldosterone, respectively, is another relevant mechanism of NCC, ENaC, and ROMK modulation in states such as volume reduction, hyperkalemia, and hypokalemia. Loss of NCC function induces upregulation of electroneutral NaCl reabsorption by type B intercalated cells through the combined activity of pendrin and NDCBE, as demonstrated in double knockout mice (KO) animal models, Ncc/pendrin or Ncc/NDCBE. The analysis of ks-Nedd-4-2 KO animal models introduced the modulation of NEDD4-2 by intracellular Mg2+ activity as an important regulator of NCC, explaining the thiazide-induced persistent hypokalemia.

Blood pressure effects of sodium transport along the distal nephron

The mammalian distal nephron is a target of highly effective antihypertensive drugs. Genetic variants that alter its transport activity are also inherited causes of high or low blood pressure, clearly establishing its central role in human blood pressure regulation. Much has been learned during the past 25 years about salt transport along this nephron segment, spurred by the cloning of major transport proteins and the discovery of disease-causing genetic variants. Recognition is increasing that substantial cellular and segmental heterogeneity is present along this segment, with electroneutral sodium transport dominating more proximal segments and electrogenic sodium transport dominating more distal segments. Coupled with recent insights into factors that modulate transport along these segments, we now understand one important mechanism by which dietary potassium intake influences sodium excretion and blood pressure. This finding has solved the aldosterone paradox, by demonstrating how aldosterone can be both kaliuretic, when plasma potassium is elevated, and anti-natriuretic, when extracellular fluid volume is low. However, what also has become clear is that aldosterone itself only stimulates a portion of the mineralocorticoid receptors along this segment, with the others being activated by glucocorticoid hormones instead. These recent insights provide an increasingly clear picture of how this short nephron segment contributes to blood pressure homeostasis and have important implications for hypertension prevention and treatment.

Effect of luminal flow on doming of mpkCCD cells in a 3D perfusable kidney cortical collecting duct model

The cortical collecting duct (CCD) of the mammalian kidney plays a major role in the maintenance of total body electrolyte, acid/base, and fluid homeostasis by tubular reabsorption and excretion. The mammalian CCD is heterogeneous, composed of Na⁺-absorbing principal cells (PCs) and acid-base-transporting intercalated cells (ICs). Perturbations in luminal flow rate alter hydrodynamic forces to which these cells in the cylindrical tubules are exposed. However, most studies of tubular ion transport have been performed in cell monolayers grown on or epithelial sheets affixed to a flat support, since analysis of transepithelial transport in native tubules by in vitro microperfusion requires considerable expertise. Here, we report on the generation and characterization of an in vitro, perfusable three-dimensional kidney CCD model (3D CCD), in which immortalized mouse PC-like mpkCCD cells are seeded within a cylindrical channel embedded within an engineered extracellular matrix and subjected to luminal fluid flow. We find that a tight epithelial barrier composed of differentiated and polarized PCs forms within 1 wk. Immunofluorescence microscopy reveals the apical epithelial Na⁺ channel ENaC and basolateral Na⁺/K⁺-ATPase. On cessation of luminal flow, benzamil-inhibitable cell doming is observed within these 3D CCDs consistent with the presence of ENaC-mediated Na⁺ absorption. Our 3D CCD provides a geometrically and microphysiologically relevant platform for studying the development and physiology of renal tubule segments.

Molecular Pathophysiology of Acid-Base Disorders

Acid-base balance is critical for normal life. Acute and chronic disturbances impact cellular energy metabolism, endocrine signaling, ion channel activity, neuronal activity, and cardiovascular functions such as cardiac contractility and vascular blood flow. Maintenance and adaptation of acid-base homeostasis are mostly controlled by respiration and kidney. The kidney contributes to acid-base balance by reabsorbing filtered bicarbonate, regenerating bicarbonate through ammoniagenesis and generation of protons, and by excreting acid. This review focuses on acid-base disorders caused by renal processes, both inherited and acquired. Distinct rare inherited monogenic diseases affecting acid-base handling in the proximal tubule and collecting duct have been identified. In the proximal tubule, mutations of solute carrier 4A4 (SLC4A4) (electrogenic Na⁺/HCO₃^--cotransporter Na⁺/bicarbonate cotransporter e1 [NBCe1]) and other genes such as CLCN5 (Cl^-/H⁺-antiporter), SLC2A2 (GLUT2 glucose transporter), or EHHADH (enoyl-CoA, hydratase/3-hydroxyacyl CoA dehydrogenase) causing more generalized proximal tubule dysfunction can cause proximal renal tubular acidosis resulting from bicarbonate wasting and reduced ammoniagenesis. Mutations in adenosine triphosphate ATP6V1 (B1 H⁺-ATPase subunit), ATPV0A4 (a4 H⁺-ATPase subunit), SLC4A1 (anion exchanger 1), and FOXI1 (forkhead transcription factor) cause distal renal tubular acidosis type I. Carbonic anhydrase II mutations affect several nephron segments and give rise to a mixed proximal and distal phenotype. Finally, mutations in genes affecting aldosterone synthesis, signaling, or downstream targets can lead to hyperkalemic variants of renal tubular acidosis (type IV). More common forms of renal acidosis are found in patients with advanced stages of chronic kidney disease and are owing, at least in part, to a reduced capacity for ammoniagenesis.

Body Fluid Compartments, Cell Membrane Ion Transport, Electrolyte Concentrations, and Acid-Base Balance

Measurements made in the blood, part of the extracellular compartment, are used in the clinical assessment of acid-base disorders; however, intracellular events determine much of the metabolic importance of these disorders. Intracellular and interstitial compartment acid-base balance is complex and varies in different tissues. This review considers the determination of extracellular pH in the context of ion transport processes at the interface of cells and the interstitial fluid, and between epithelial cells lining the transcellular contents of the gastrointestinal and urinary tracts that open to the external environment. A further consideration is the role of these membrane transporters in the generation of acid-base disorders and the associated disruption of electrolyte balance. This review suggests a process of internal and external balance for pH regulation similar to that of potassium, and considers the role of secretory gastrointestinal epithelia and renal epithelia with respect to normal pH homeostasis and clinical disorders. Electroneutrality of electrolytes in the extracellular fluid is a fundamental feature of reciprocal changes in Cl^- or non-Cl^- anions and HCO₃^-. Normal mechanisms for protecting cell pH and producing normal gastrointestinal and renal secretions in healthy states also may result in disease when abnormal. In a similar manner, organic anions such as ketoacid anions and lactate, normally transported as fuels between organs, result in acid-base disturbances in disease. Understanding the genomic basis of these transporters may contribute to specific treatments.

A Vaccinomics Approach for the Identification of Tick Protective Antigens for the Control of Ixodes ricinus and Dermacentor reticulatus Infestations in Companion Animals

Ticks and tick-borne pathogens affect health and welfare of companion animals worldwide, and some human tick-borne diseases are associated with exposure to domestic animals. Vaccines are the most environmentally friendly alternative to acaracides for the control of tick infestations, and to reduce the risk for tick-borne diseases affecting human and animal health. However, vaccines have not been developed or successfully implemented for most vector-borne diseases. The main limitation for the development of effective vaccines is the identification of protective antigens. To address this limitation, in this study we used an experimental approach combining vaccinomics based on transcriptomics and proteomics data with vaccination trials for the identification of tick protective antigens. The study was focused on Ixodes ricinus and Dermacentor reticulatus that infest humans, companion animals and other domestic and wild animals, and transmit disease-causing pathogens. Tick larvae and adult salivary glands were selected for analysis to target tick organs and developmental stages playing a key role during tick life cycle and pathogen infection and transmission. Two I. ricinus (heme lipoprotein and uncharacterized secreted protein) and five D. reticulatus (glypican-like protein, secreted protein involved in homophilic cell adhesion, sulfate/anion exchanger, signal peptidase complex subunit 3, and uncharacterized secreted protein) proteins were identified as the most effective protective antigens based on the criteria of vaccine E > 80%. The putative function of selected protective antigens, which are involved in different biological processes, resulted in vaccines affecting multiple tick developmental stages. These results suggested that the combination of some of these antigens might be considered to increase vaccine efficacy through antigen synergy for the control of tick infestations and potentially affecting pathogen infection and transmission. These antigens were proposed for commercial vaccine development for the control of tick infestations in companion animals, and potentially in other hosts for these tick species.

"I don't get no respect": the role of chloride in acute kidney injury

Acute kidney injury (AKI) is a major public health problem that complicates 10-40% of hospital admissions. Importantly, AKI is independently associated with increased risk of progression to chronic kidney disease, end-stage renal disease, cardiovascular events, and increased risk of in-hospital and long-term mortality. The chloride content of intravenous fluid has garnered much attention over the last decade, as well as its association with excess use and adverse outcomes, including AKI. Numerous studies show that changes in serum chloride concentration, independent of serum sodium and bicarbonate, are associated with increased risk of AKI, morbidity, and mortality. This comprehensive review details the complex renal physiology regarding the role of chloride in regulating renal blood flow, glomerular filtration rate, tubuloglomerular feedback, and tubular injury, as well as the findings of clinical research related to the chloride content of intravenous fluids, changes in serum chloride concentration, and AKI. Chloride is underappreciated in both physiology and pathophysiology. Although the exact mechanism is debated, avoidance of excessive chloride administration is a reasonable treatment option for all patients and especially in those at risk for AKI. Therefore, high-risk patients and those with "incipient" AKI should receive balanced solutions rather than normal saline to minimize the risk of AKI.

Extracellular Acid-Base Balance and Ion Transport Between Body Fluid Compartments

Clinical assessment of acid-base disorders depends on measurements made in the blood, part of the extracellular compartment. Yet much of the metabolic importance of these disorders concerns intracellular events. Intracellular and interstitial compartment acid-base balance is complex and heterogeneous. This review considers the determinants of the extracellular fluid pH related to the ion transport processes at the interface of cells and the interstitial fluid, and between epithelial cells lining the transcellular contents of the gastrointestinal and urinary tracts that open to the external environment. The generation of acid-base disorders and the associated disruption of electrolyte balance are considered in the context of these membrane transporters. This review suggests a process of internal and external balance for pH regulation, similar to that of potassium. The role of secretory gastrointestinal epithelia and renal epithelia with respect to normal pH homeostasis and clinical disorders are considered. Electroneutrality of electrolytes in the ECF is discussed in the context of reciprocal changes in Cl^- or non Cl^- anions and [Formula: see text].

PGE2 EP1 receptor inhibits vasopressin-dependent water reabsorption and sodium transport in mouse collecting duct

PGE2 regulates glomerular hemodynamics, renin secretion, and tubular transport. This study examined the contribution of PGE2 EP1 receptors to sodium and water homeostasis. Male EP1-/- mice were bred with hypertensive TTRhRen mice (Htn) to evaluate blood pressure and kidney function at 8 weeks of age in four groups: wildtype (WT), EP1-/-, Htn, HtnEP1-/-. Blood pressure and water balance were unaffected by EP1 deletion. COX1 and mPGE2 synthase were increased and COX2 was decreased in mice lacking EP1, with increases in EP3 and reductions in EP2 and EP4 mRNA throughout the nephron. Microdissected proximal tubule sglt1, NHE3, and AQP1 were increased in HtnEP1-/-, but sglt2 was increased in EP1-/- mice. Thick ascending limb NKCC2 was reduced in the cortex but increased in the medulla. Inner medullary collecting duct (IMCD) AQP1 and ENaC were increased, but AVP V2 receptors and urea transporter-1 were reduced in all mice compared to WT. In WT and Htn mice, PGE2 inhibited AVP-water transport and increased calcium in the IMCD, and inhibited sodium transport in cortical collecting ducts, but not in EP1-/- or HtnEP1-/- mice. Amiloride (ENaC) and hydrochlorothiazide (pendrin inhibitor) equally attenuated the effect of PGE2 on sodium transport. Taken together, the data suggest that EP1 regulates renal aquaporins and sodium transporters, attenuates AVP-water transport and inhibits sodium transport in the mouse collecting duct, which is mediated by both ENaC and pendrin-dependent pathways.

Versatility of NaCl transport mechanisms in the cortical collecting duct

The cortical collecting duct (CCD) forms part of the aldosterone-sensitive distal nephron and plays an essential role in maintaining the NaCl balance and acid-base status. The CCD epithelium comprises principal cells as well as different types of intercalated cells. Until recently, transcellular Na⁺ transport was thought to be restricted to principal cells, whereas (acid-secreting) type A and (bicarbonate-secreting) type B intercalated cells were associated with the regulation of acid-base homeostasis. This review describes how this traditional view has been upended by several discoveries in the past decade. A series of studies has shown that type B intercalated cells can mediate electroneutral NaCl reabsorption by a mechanism involving Na⁺-dependent and Na⁺-independent Cl^-/[Formula: see text] exchange, and that is energetically driven by basolateral vacuolar H⁺-ATPase pumps. Other research indicates that type A intercalated cells can mediate NaCl secretion, through a bumetanide-sensitive pathway that is energized by apical H⁺,K⁺-ATPase type 2 pumps operating as Na⁺/K⁺ exchangers. We also review recent findings on the contribution of the paracellular route to NaCl transport in the CCD. Last, we describe cross-talk processes, by which one CCD cell type impacts Na⁺/Cl^- transport in another cell type. The mechanisms that have been identified to date demonstrate clearly the interdependence of NaCl and acid-base transport systems in the CCD. They also highlight the remarkable versatility of this nephron segment.

Aldosterone regulates a 5' variant sgk1 transcript via a shared hormone response element in the sgk1 5' regulatory region

We previously identified a 5ʹ variant alternate transcript of Sgk1 (Sgk1_v3) encoding an NH₂‐terminal variant Sgk1 isoform, Sgk1_i3 that, like Sgk1, is expressed in the distal convoluted tubule, connecting tubule and collecting duct and can stimulate epithelial Na⁺ transport (Am J Physiol Renal Physiol 303: F1527–F1533, 2012). We now demonstrate that, similar to Sgk1, aldosterone and glucocorticoids stimulate Sgk1_v3 expression in cell lines from the collecting duct and airway epithelia. In mice, short term aldosterone infusion and maneuvers that increase endogenous aldosterone secretion including dietary Na⁺ deprivation and K⁺ loading increases distal nephron Sgk1_v3 expression in vivo. Although Sgk1_v3 has a different 5ʹ proximal regulatory region from Sgk1, the transcription start sites are less than 1000 bp apart. We cloned the 5ʹ regulatory region for Sgk1 and Sgk_v3 upstream of a luciferase gene and by deletion and reporter gene analysis we localized the corticosteroid regulatory region for Sgk1_v3 to a glucocorticoid response element (GRE) that had previously been identified for Sgk1 (Am J Physiol Endo Metab 283: E971–E979, 2002). We tested this element with MR in an MR‐null cell line and demonstrate that aldosterone stimulates Sgk1 and Sgk1_v3 via this GRE. We conclude that corticosteroids stimulate Sgk1 and Sgk1_v3 expression in epithelial cells via activation of a common conserved GRE in the 5ʹ flanking region of Sgk1.

Pendred syndrome

Pendred syndrome is an autosomal recessive disorder that is classically defined by the combination of sensorineural deafness/hearing impairment, goiter, and an abnormal organification of iodide with or without hypothyroidism. The hallmark of the syndrome is the impaired hearing, which is associated with inner ear malformations such as an enlarged vestibular aqueduct (EVA). The thyroid phenotype is variable and may be modified by the nutritional iodine intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4/PDS gene, which encodes the multifunctional anion exchanger pendrin. Pendrin has affinity for chloride, iodide, and bicarbonate, among other anions. In the inner ear, pendrin functions as a chloride/bicarbonate exchanger that is essential for maintaining the composition and the potential of the endolymph. In the thyroid, pendrin is expressed at the apical membrane of thyroid cells facing the follicular lumen. Functional studies have demonstrated that pendrin can mediate iodide efflux in heterologous cells. This, together with the thyroid phenotype observed in humans (goiter, impaired iodine organification) suggests that pendrin could be involved in iodide efflux into the lumen, one of the steps required for thyroid hormone synthesis. Iodide efflux can, however, also occur in the absence of pendrin suggesting that other exchangers or channels are involved. It has been suggested that Anoctamin 1 (ANO1/TMEM16A), a calcium-activated anion channel, which is also expressed at the apical membrane of thyrocytes, could participate in mediating apical efflux. In the kidney, pendrin is involved in bicarbonate secretion and chloride reabsorption. While there is no renal phenotype under basal conditions, severe metabolic alkalosis has been reported in Pendred syndrome patients exposed to an increased alkali load. This review provides an overview on the clinical spectrum of Pendred syndrome, the functional data on pendrin with a focus on its potential role in the thyroid, as well as the controversy surrounding the relative physiological roles of pendrin and anoctamin.