Recent advances in our understanding of intercalated cells

Chloride and Potassium Assessment Is a Helpful Tool for Differential Diagnosis of Thiazide-Associated Hyponatremia

CONTEXT

Differential diagnosis of thiazide-associated hyponatremia (TAH) is challenging. Patients can either have volume depletion or a syndrome of inappropriate antidiuresis (SIAD)-like presentation.

OBJECTIVE

To evaluate the impact of the simplified apparent strong ion difference in serum (aSID; sodium + potassium - chloride) as well as the urine chloride and potassium score (ChU; chloride - potassium in urine) in the differential diagnosis of TAH, in addition to assessment of fractional uric acid excretion (FUA).

METHODS

Post hoc analysis of prospectively collected data from June 2011 to August 2013 from 98 hospitalized patients with TAH < 125 mmol/L enrolled at University Hospital Basel and University Medical Clinic Aarau, Switzerland. Patients were categorized according to treatment response in volume-depleted TAH requiring volume substitution or SIAD-like TAH requiring fluid restriction. We computed sensitivity analyses with ROC curves for positive predictive value (PPV) and negative predictive value (NPV) of aSID, ChU, and FUA in differential diagnosis of TAH.

RESULTS

An aSID > 42 mmol/L had a PPV of 79.1% in identifying patients with volume-depleted TAH, whereas a value < 39 mmol/L excluded it with a NPV of 76.5%. In patients for whom aSID was inconclusive, a ChU < 15 mmol/L had a PPV of 100% and a NPV of 83.3%, whereas FUA < 12% had a PPV of 85.7% and a NPV of 64.3% in identifying patients with volume-depleted TAH.

CONCLUSION

In patients with TAH, assessment of aSID, potassium, and chloride in urine can help identifying patients with volume-depleted TAH requiring fluid substitution vs patients with SIAD-like TAH requiring fluid restriction.

Tsc2 mutation induces renal tubular cell nonautonomous disease

TSC renal cystic disease is poorly understood and has no approved treatment. In a new principal cell-targeted murine model of Tsc cystic disease, the renal cystic epithelium is mostly composed of type A intercalated cells with an intact Tsc2 gene confirmed by sequencing, although these cells exhibit a Tsc-mutant disease phenotype. We used a newly derived targeted murine model in lineage tracing and extracellular vesicle (EV) characterization experiments and a cell culture model in EV characterization and cellular induction experiments to understand TSC cystogenesis. Using lineage tracing experiments, we found principal cells undergo clonal expansion but contribute very few cells to the cyst. We determined that cystic kidneys contain more interstitial EVs than noncystic kidneys, excrete fewer EVs in urine, and contain EVs in cyst fluid. Moreover, the loss of Tsc2 gene in EV-producing cells greatly changes the effect of EVs on renal tubular epithelium, such that the epithelium develops increased secretory and proliferative pathway activity. We demonstate that the mTORC1 pathway activity is independent form the EV production, and that the EV effects for a single cell line can vary significantly. TSC cystogenesis involves significant contribution from genetically intact cells conscripted to the mutant phenotype by mutant cell derived EVs.

Renal cystic disease in tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is associated with TSC1 or TSC2 gene mutations resulting in hyperactivation of the mTORC1 pathway. This mTORC1 activation is associated with abnormal tissue development and proliferation such that in the kidney there are both solid tumors and cystic lesions. This review summarizes recent advances in tuberous sclerosis complex nephrology and focuses on the genetics and cell biology of tuberous sclerosis complex renal disease, highlighting a role of extracellular vesicles and the innate immune system in disease pathogenesis.

Hepatobiliary acid-base homeostasis: Insights from analogous secretory epithelia

Many epithelia secrete bicarbonate-rich fluid to generate flow, alter viscosity, control pH and potentially protect luminal and intracellular structures from chemical stress. Bicarbonate is a key component of human bile and impaired biliary bicarbonate secretion is associated with liver damage. Major efforts have been undertaken to gain insight into acid-base homeostasis in cholangiocytes and more can be learned from analogous secretory epithelia. Extrahepatic examples include salivary and pancreatic duct cells, duodenocytes, airway and renal epithelial cells. The cellular machinery involved in acid-base homeostasis includes carbonic anhydrase enzymes, transporters of the solute carrier family, and intra- and extracellular pH sensors. This pH-regulatory system is orchestrated by protein-protein interactions, the establishment of an electrochemical gradient across the plasma membrane and bicarbonate sensing of the intra- and extracellular compartment. In this review, we discuss conserved principles identified in analogous secretory epithelia in the light of current knowledge on cholangiocyte physiology. We present a framework for cholangiocellular acid-base homeostasis supported by expression analysis of publicly available single-cell RNA sequencing datasets from human cholangiocytes, which provide insights into the molecular basis of pH homeostasis and dysregulation in the biliary system.

Metabolic Alkalosis: A Brief Pathophysiologic Review

Metabolic alkalosis is a very commonly encountered acid-base disorder that may be generated by a variety of exogenous and/or endogenous, pathophysiologic mechanisms. Multiple mechanisms are also responsible for the persistence, or maintenance, of metabolic alkalosis. Understanding these generation and maintenance mechanisms helps direct appropriate intervention and correction of this disorder. The framework utilized in this review is based on the ECF volume-centered approach popularized by Donald Seldin and Floyd Rector in the 1970s.  Although many subsequent scientific discoveries have advanced our understanding of the pathophysiology of metabolic alkalosis, that framework continues to be a valuable and relatively straightforward diagnostic and therapeutic model.

Insights into cellular and molecular basis for urinary tract infection in autosomal-dominant polycystic kidney disease

Urinary tract infection (UTI) is a broad term referring to an infection of the kidneys, ureters, bladder, and/or urethra. Because of its prevalence, frequent recurrence, and rising resistance to antibiotics, UTI has become a challenge in clinical practice. Autosomal-dominant polycystic kidney disease (ADPKD) is the most common monogenic disorder of the kidney and is characterized by the growth of fluid-filled cysts in both kidneys. Progressive cystic enlargement, inflammation, and interstitial fibrosis result in nephron loss with subsequent decline in kidney function. ADPKD patients frequently develop UTI; however, the cellular and molecular mechanisms responsible for the high UTI incidence in ADPKD patients remain virtually unaddressed. Emerging evidence suggests that α-intercalated cells (α-ICs) of the collecting ducts function in the innate immune defense against UTI. α-ICs inhibit bacterial growth by acidifying urine and secreting neutrophil gelatinase-associated lipocalin (NGAL) that chelates siderophore-containing iron. It is necessary to determine, therefore, if ADPKD patients with recurrent UTI have a reduced number and/or impaired function of α-ICs. Identification of the underlying cellular and molecular mechanisms may lead to the development of novel strategies to reduce UTI in ADPKD.

Development of the Mammalian Kidney

The basic unit of kidney function is the nephron. In the mouse, around 14,000 nephrons form in a 10-day period extending into early neonatal life, while the human fetus forms the adult complement of nephrons in a 32-week period completed prior to birth. This review discusses our current understanding of mammalian nephrogenesis: the contributing cell types and the regulatory processes at play. A conceptual developmental framework has emerged for the mouse kidney. This framework is now guiding studies of human kidney development enabled in part by in vitro systems of pluripotent stem cell-seeded nephrogenesis. A near future goal will be to translate our developmental knowledge-base to the productive engineering of new kidney structures for regenerative medicine.

Dot1l deficiency leads to increased intercalated cells and upregulation of V-ATPase B1 in mice

The collecting duct in the mammalian kidney consists of principal cells (PCs) and intercalated cells (ICs), which regulate electrolyte/fluid and acid/base balance, respectively. The epigenetic regulators of PC and IC differentiation remain obscure. We previously used Aqp2 and V-ATPase B1B2 to label PCs and ICs, respectively. We found that mice with histone H3 K79 methyltransferase Dot1l disrupted in Aqp2-expressing cells (Dot1l(AC)) vs. Dot1l(f/f) possessed ~20% more ICs coupled with a similar decrease in PCs. Here, we performed multiple double immunofluorescence staining using various PC and IC markers and confirmed that this finding. Both α-IC and β-IC populations were significantly expanded in Dot1l(AC) vs. Dot1l(f/f). These changes are associated with significantly upregulated V-ATPase B1 and B2, but not Aqp2, AE1, and Pendrin. Chromatin immunoprecipitation assay unveiled a significant reduction of Dot1l and H3K79 di-methylation bound at the Atp6v1b1 5' flanking region. Overexpression of Dot1a significantly downregulated a stably-transfected luciferase reporter driven by the Atp6v1b1 promoter in IMCD3 cells. This downregulation was impaired, but not completely abolished when a methyltransferase-dead mutant was overexpressed. Taken together, our data suggest that Dot1l is a new epigenetic regulator of PC and IC differentiation and Atp6v1b1 is a new transcriptional target of Dot1l.

Crosstalk between (Pro)renin receptor and COX-2 in the renal medulla during angiotensin II-induced hypertension

Angiotensin II (AngII) is an octapeptide hormone that plays a central role in regulation of sodium balance, plasma volume, and blood pressure. Its role in the pathogenesis of hypertension is highlighted by the wide use of inhibitors of the renin-angiotensin system (RAS) as the first-line antihypertensive therapy. However, despite intensive investigation, the mechanism of AngII-induced hypertension is still incompletely understood. Although diverse pathways are likely involved, increasing evidence suggests that the activation of intrarenal RAS may represent a dominant mechanism of AngII-induced hypertension. (Pro)renin receptor (PRR), a potential regulator of intrarenal RAS, is expressed in the intercalated cells of the collecting duct (CD) and induced by AngII, in parallel with increased renin in the principal cells of the CD. Activation of PRR elevated PGE2 release and COX-2 expression in renal inner medullary cells whereas COX-2-derived PGE2via the EP4 receptor mediates the upregulation of PRR during AngII infusion, thus forming a vicious cycle. The mutually stimulatory relationship between PRR and COX-2 in the distal nephron may play an important role in mediating AngII-induced hypertension.

Kidney α-intercalated cells and lipocalin 2: defending the urinary tract

A growing body of evidence indicates that the kidneys contribute substantially to immune defense against pathogens in the urinary tract. In this issue, Paragas et al. report that α-intercalated cells (A-ICs) within the nephron collecting duct sense infecting Gram-negative bacteria, resulting in simultaneously secretion of the iron chelating protein lipocalin 2 (LCN2) and protons, which acidify the urine. A-IC-specific LCN2 and proton secretion markedly reduced the ability of infecting uropathogenic E. coli (UPEC) to grow and sustain infection. The capacity of A-ICs to sense and actively promote clearance of infecting bacteria in the lower urinary tract represents a novel function for these specialized kidney cells, which are best known for their role in modulating acid-base homeostasis.

Adam10 mediates the choice between principal cells and intercalated cells in the kidney

A disintegrin and metalloproteinase domain 10 (Adam10), a member of the ADAM family of cell membrane-anchored proteins, has been linked to the regulation of the Notch, EGF, E-cadherin, and other signaling pathways. However, it is unclear what role Adam10 has in the kidney in vivo. In this study, we showed that Adam10 deficiency in ureteric bud (UB) derivatives leads to a decrease in urinary concentrating ability, polyuria, and hydronephrosis in mice. Furthermore, Adam10 deficiency led to a reduction in the percentage of aquaporin 2 (Aqp2)(+) principal cells (PCs) in the collecting ducts that was accompanied by a proportional increase in the percentage of intercalated cells (ICs). This increase was more prominent in type A ICs than in type B ICs. Foxi1, a transcription factor important for the differentiation of ICs, was upregulated in the Adam10 mutants. The observed reduction of Notch activity in Adam10 mutant collecting duct epithelium and the similar reduction of PC/IC ratios in the collecting ducts in mice deficient for mindbomb E3 ubiquitin protein ligase 1, a key regulator of the Notch and Wnt/receptor-like tyrosine kinase signaling pathways, suggest that Adam10 regulates cell fate determination through the activation of Notch signaling, probably through the regulation of Foxi1 expression. However, phenotypic differences between the Adam10 mutants, the Mib1 mutants, and the Foxi1 mutants suggest that the functions of Adam10 in determining the fate of collecting duct cells are more complex than those of a simple upstream factor in a linear pathway involving Notch and Foxi1.

SLC22A13 catalyses unidirectional efflux of aspartate and glutamate at the basolateral membrane of type A intercalated cells in the renal collecting duct

SLC22A13 is specifically expressed in renal type A intercalated cells where it mediates the basolateral expulsion of aspartate and glutamate.

Chloride transport

Chloride transport along the nephron is one of the key actions of the kidney that regulates extracellular volume and blood pressure. To maintain steady state, the kidney needs to reabsorb the vast majority of the filtered load of chloride. This is accomplished by the integrated function of sequential chloride transport activities along the nephron. The detailed mechanisms of transport in each segment generate unique patterns of interactions between chloride and numerous other individual components that are transported by the kidney. Consequently, chloride transport is inextricably intertwined with that of sodium, potassium, protons, calcium, and water. These interactions not only allow for exquisitely precise regulation but also determine the particular patterns in which the system can fail in disease states.

Expression of the ammonia transporter family member, Rh B Glycoprotein, in the human kidney

The ammonia transporter family member, Rh B Glycoprotein (RhBG/Rhbg), is essential for ammonia transport by the rodent kidney, but in the human kidney mRNA but not protein expression has been reported. Because ammonia transport is fundamental for acid-base homeostasis, the current study addressed RhBG expression in the human kidney. Two distinct RhBG mRNA sequences have been reported, with different numbers of consecutive cytosines at nt1265 and thus encoding different carboxy-tails. Sequencing the region of difference in both human kidney and liver mRNA showed eight sequential cytosines, not seven as in some reports. Knowing the correct mRNA sequence for RhBG, we then assessed RhBG protein expression using antibodies against the correct amino acid sequence. Immunoblot analysis demonstrated RhBG protein expression in human kidney and immunohistochemistry identified basolateral RhBG in connecting segment (CNT) and the cortical and outer medullary collecting ducts. Colocalization of RhBG with multiple cell-specific markers demonstrated that that CNT cells and collecting duct type A intercalated cells express high levels of RhBG, and type B intercalated cells and principal cells do not express detectable RhBG. Thus, these studies identify the correct mRNA and thus protein sequence for human RhBG and show that the human kidney expresses basolateral RhBG protein in CNT, type A intercalated cells, and non-A, non-B cells. We conclude that RhBG can mediate an important role in human renal ammonia transport.

Aqp2-expressing cells give rise to renal intercalated cells

The mammalian collecting duct comprises principal and intercalated cells, which maintain sodium/water and acid/base balance, respectively, but the epigenetic contributors to the differentiation of these cell types remain unknown. Here, we investigated whether the histone H3 K79 methyltransferase Dot1l, which is highly expressed in principal cells, participates in this process. Taking advantage of the distribution of aquaporin 2 (Aqp2), which localizes to principal cells of the collecting duct, we developed mice lacking Dot1l in Aqp2-expressing cells (Dot1l(AC)) and found that these mice had approximately 20% fewer principal cells and 13%-16% more intercalated cells than control mice. This deletion of Dot1l in principal cells abolished histone H3 K79 methylation in these cells, but unexpectedly, most intercalated cells also had undetectable di-methyl K79, suggesting that Aqp2(+) cells give rise to intercalated cells. These Aqp2(+) cell-derived intercalated cells were present in both developing and mature kidneys. Furthermore, compared with control mice, Dot1l(AC) mice had 40% higher urine volume and 18% lower urine osmolarity with relatively normal electrolyte and acid-base homeostasis. In conclusion, these data suggest that Dot1l deletion facilitates the differentiation of some α- and β-intercalated cells from Aqp2-expressing progenitor cells or mature principal cells.

Regulation of paracellular transport in the distal nephron

PURPOSE OF REVIEW

Claudins play a major role in the regulation of paracellular electrolyte reabsorption in the kidney. This review describes the recent findings of the physiological function of claudins underlying the paracellular transport mechanisms for Cl(-) reabsorption in the collecting duct.

RECENT FINDINGS

There are two parallel mechanisms for transepithelial Cl(-) reabsorption in the collecting duct that utilize the Na-driven Cl-bicarbonate exchanger (NDCBE) and the claudin-based paracellular channel. Histological studies have demonstrated the renal localization of claudin-3, claudin-4, claudin-7, and claudin-8 in the collecting duct. Molecular analyses using several collecting duct cell models have come to the conclusion that claudin-4 functions as a paracellular Cl(-) channel. The channel function of claudin-4 is conferred by a charged lysine residue (K65) in its extracellular loop. Claudin-8 is required for paracellular Cl(-) permeation through its interaction with and recruitment of claudin-4 during tight junction assembly. Claudin-7 provides the basic barrier function to the collecting duct. Genetic ablation of claudin-7 in animals results in systemic dehydration owing to the loss of extracellular ions and fluid in the kidney.

SUMMARY

The paracellular pathway in the collecting duct is an important route for transepithelial Cl(-) reabsorption that determines the extracellular NaCl content and the blood pressure. In the collecting duct cells, claudin-4 and claudin-8 interact to form a paracellular Cl(-) channel, whereas claudin-7 maintains the transepithelial resistance. Different subsets of the claudin family proteins fulfill diverse aspects of the tight junction function that will be fundamental to understanding the physiology of the paracellular pathway.

Altered regulation of renal Acid base transporters in response to ammonium chloride loading in rats

The role of the kidney in combating metabolic acidosis has been a subject of considerable interest for many years. The present study was aimed to determine whether there is an altered regulation of renal acid base transporters in acute and chronic acid loading. Male Sprague-Dawley rats were used. Metabolic acidosis was induced by administration of NH₄Cl for 2 days (acute) and for 7days (chronic). The serum and urinary pH and bicarbonate were measured. The protein expression of renal acid base transporters [type 3 Na⁺/H⁺ exchanger (NHE3), type 1 Na⁺/HCO₃^- cotransporter (NBC1), Na-K⁺ ATPase, H⁺-ATPase, anion exchanger-1 (AE-1)] was measured by semiquantitative immunoblotting. Serum bicarbonate and pH were decreased in acute acid loading rats compared with controls. Accordingly, urinary pH decreased. The protein expression of NHE3, H⁺-ATPase, AE-1 and NBC1 was not changed. In chronic acid loading rats, serum bicarbonate and pH were not changed, while urinary pH was decreased compared with controls. The protein expression of NHE3, H⁺-ATPase was increased in the renal cortex of chronic acid loading rats. These results suggest that unaltered expression of acid transporters combined with acute acid loading may contribute to the development of acidosis. The subsequent increased expression of NHE3, H⁺-ATPase in the kidney may play a role in promoting acid excretion in the later stage of acid loading, which counteract the development of metabolic acidosis.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Differentiation of intercalated cells in the kidney

The intercalated cell of collecting ducts of the kidney is of two forms, the α form secretes acid, whereas the β form secretes HCO(3). Here, we review recent work that shows that the α form is derived from the β form and that the pathway is mediated by an extracellular matrix protein called hensin/DMBT1.

Insulin receptor-related receptor as an extracellular alkali sensor

The insulin receptor-related receptor (IRR), an orphan receptor tyrosine kinase of the insulin receptor family, can be activated by alkaline media both in vitro and in vivo at pH >7.9. The alkali-sensing property of IRR is conserved in frog, mouse, and human. IRR activation is specific, dose-dependent and quickly reversible and demonstrates positive cooperativity. It also triggers receptor conformational changes and elicits intracellular signaling. The pH sensitivity of IRR is primarily defined by its L1F extracellular domains. IRR is predominantly expressed in organs that come in contact with mildly alkaline media. In particular, IRR is expressed in the cell subsets of the kidney that secrete bicarbonate into urine. Disruption of IRR in mice impairs the renal response to alkali loading attested by development of metabolic alkalosis and decreased urinary bicarbonate excretion in response to this challenge. We therefore postulate that IRR is an alkali sensor that functions in the kidney to manage metabolic bicarbonate excess.

Clinical and molecular characteristics of Pendred syndrome

Pendred syndrome is an autosomal recessive disorder defined by sensorineural deafness, goiter and a partial defect in the organification of iodide. It is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a multifunctional anion exchanger. At the level of the inner ear, pendrin is important for the creation of a normal endolymph composition and the maintenance of the endocochlear potential. In the thyroid, pendrin is expressed at the apical membrane of thyroid follicular cells and it appears to be involved in mediating iodide efflux into the lumen and/or maintenance of the follicular pH. Goiter development and hypothyroidism vary among affected individuals and seem to be partially dependent on nutritional iodide intake. In the kidney, pendrin functions as a chloride/bicarbonate exchanger. Elucidation of the molecular basis of Pendred syndrome and the function of pendrin has provided unexpected novel insights into the pathophysiology of the inner ear, thyroid hormone synthesis, and chloride/bicarbonate exchange in the kidney.

Specification of ion transport cells in the Xenopus larval skin

Specialized epithelial cells in the amphibian skin play important roles in ion transport, but how they arise developmentally is largely unknown. Here we show that proton-secreting cells (PSCs) differentiate in the X. laevis larval skin soon after gastrulation, based on the expression of a `kidney-specific' form of the H(+)v-ATPase that localizes to the plasma membrane, orthologs of the Cl(-)/HCO(-)(3) antiporters ae1 and pendrin, and two isoforms of carbonic anhydrase. Like PSCs in other species, we show that the expression of these genes is likely to be driven by an ortholog of foxi1, which is also sufficient to promote the formation of PSC precursors. Strikingly, the PSCs form in the skin as two distinct subtypes that resemble the alpha- and beta-intercalated cells of the kidney. The alpha-subtype expresses ae1 and localizes H(+)v-ATPases to the apical plasma membrane, whereas the beta-subtype expresses pendrin and localizes the H(+)v-ATPase cytosolically or basolaterally. These two subtypes are specified during early PSC differentiation by a binary switch that can be regulated by Notch signaling and by the expression of ubp1, a transcription factor of the grainyhead family. These results have implications for how PSCs are specified in vertebrates and become functionally heterogeneous.

Deletion of hensin/DMBT1 blocks conversion of beta- to alpha-intercalated cells and induces distal renal tubular acidosis

Acid-base transport in the renal collecting tubule is mediated by two canonical cell types: the β-intercalated cell secretes HCO(3) by an apical Cl:HCO(3) named pendrin and a basolateral vacuolar (V)-ATPase. Acid secretion is mediated by the α-intercalated cell, which has an apical V-ATPase and a basolateral Cl:HCO(3) exchanger (kAE1). We previously suggested that the β-cell converts to the α-cell in response to acid feeding, a process that depended on the secretion and deposition of an extracellular matrix protein termed hensin (DMBT1). Here, we show that deletion of hensin from intercalated cells results in the absence of typical α-intercalated cells and the consequent development of complete distal renal tubular acidosis (dRTA). Essentially all of the intercalated cells in the cortex of the mutant mice are canonical β-type cells, with apical pendrin and basolateral or diffuse/bipolar V-ATPase. In the medulla, however, a previously undescribed cell type has been uncovered, which resembles the cortical β-intercalated cell in ultrastructure, but does not express pendrin. Polymerization and deposition of hensin (in response to acidosis) requires the activation of β1 integrin, and deletion of this gene from the intercalated cell caused a phenotype that was identical to the deletion of hensin itself, supporting its critical role in hensin function. Because previous studies suggested that the conversion of β- to α-intercalated cells is a manifestation of terminal differentiation, the present results demonstrate that this differentiation proceeds from HCO(3) secreting to acid secreting phenotypes, a process that requires deposition of hensin in the ECM.

Genetic variation in solute carrier genes is associated with preeclampsia

OBJECTIVE

The objective of the study was to evaluate allelic variation in 26 members of the solute carrier (SLC) gene family for an association with preeclampsia.

STUDY DESIGN

Preeclampsia cases were women with mild or severe preeclampsia. Controls were enrolled from women without hypertension-related complications who presented for delivery at term (≥37 weeks). The association between preeclampsia and SLC gene single-nucleotide polymorphisms (SNPs) and haplotypes was evaluated by logistic regression models.

RESULTS

Rs4957061 in SLC9A3 was significantly associated with a reduced risk of preeclampsia in whites (T allele, P = .002; odds ratio [OR], 0.33; 95% confidence interval [CI], 0.16-0.67). For SLC4A1 in blacks, the G allele of rs2074107 (P = .004; OR, 1.56; 95% CI, 1.15-2.12) and the A allele of rs2857078 (P < .001; OR, 1.67; 95% CI, 1.27-2.17) were significantly associated with preeclampsia. Also in blacks, rs10066650 in SLCO4C1 (G allele, P = .002; OR, 1.72; 95% CI, 1.21-2.46) was significantly associated with increased risk. Sliding window haplotype analyses identified significantly associated haplotypes in these genes.

CONCLUSION

SNPs and haplotypes in SLC9A3 in whites and SLC4A1 and SLCO4C1 in blacks are significantly associated with preeclampsia.

Claudin-4 forms paracellular chloride channel in the kidney and requires claudin-8 for tight junction localization

Tight junctions (TJs) play a key role in mediating paracellular ion reabsorption in the kidney. The paracellular pathway in the collecting duct of the kidney is a predominant route for transepithelial chloride reabsorption that determines the extracellular NaCl content and the blood pressure. However, the molecular mechanisms underlying the paracellular chloride reabsorption in the collecting duct are not understood. Here we showed that in mouse kidney collecting duct cells, claudin-4 functioned as a Cl(-) channel. A positively charged lysine residue at position 65 of claudin-4 was critical for its anion selectivity. Claudin-4 was observed to interact with claudin-8 using several criteria. In the collecting duct cells, the assembly of claudin-4 into TJ strands required its interaction with claudin-8. Depletion of claudin-8 resulted in the loss of paracellular chloride conductance, through a mechanism involving its recruitment of claudin-4 during TJ assembly. Together, our data show that claudin-4 interacts with claudin-8 and that their association is required for the anion-selective paracellular pathway in the collecting duct, suggesting a mechanism for coupling chloride reabsorption with sodium reabsorption in the collecting duct.

Unilateral ureteral obstruction alters expression of acid-base transporters in rat kidney

PURPOSE

Unilateral ureteral obstruction is a common clinical problem that is often associated with a urinary acidification defect caused by decreased net H(+) secretion and/or HCO(3)(-) reabsorption. To clarify the molecular mechanisms of these defects we examined expression levels of key acid-base transporters along the renal nephron segments and collecting duct.

MATERIALS AND METHODS

Wistar rats (Møllegard Breeding Centre, Eiby, Denmark) underwent 24-hour unilateral ureteral obstruction, unilateral ureteral obstruction release followed for 4 days or unilateral ureteral obstruction release followed for 4 days plus experimental acidosis induced by NH(4)Cl oral administration. After sacrifice kidneys were processed for immunoblotting and immunohistochemistry.

RESULTS

Semiquantitative immunoblotting revealed that unilateral ureteral obstruction caused significant mean +/- SE down-regulation of type 3 Na(+)/H(+) exchanger to 53% +/- 9%, electrogenic Na(+)/HCO(3)(-) cotransporter to 60% +/- 9%, type 1 bumetanide sensitive Na(+)-K(+)(NH(4)(+)) -2Cl(-) cotransporter to 64% +/- 7%, electroneutral Na(+)/HCO(3)(-) cotransporter to 43% +/- 4% and anion exchanger (pendrin) to 53% +/- 10% in the obstructed kidney, which was confirmed by immunohistochemistry. After release of unilateral ureteral obstruction down-regulation of these transporters persisted together with marked down-regulation of H(+)-adenosine triphosphatase in the obstructed kidney. In rats with unilateral ureteral obstruction release followed for 4 days with experimental acidosis induced by NH(4)Cl oral administration plasma pH and HCO(3)(-) were dramatically decreased in response to NH(4)Cl for 2 days compared with those in sham operated rats with acid loading, indicating a defect in H(+) excretion and HCO(3)(-) reabsorption after obstruction release. Expression of these transporters did not change in the contralateral nonobstructed kidney of rats with unilateral ureteral obstruction and unilateral ureteral obstruction release followed for 4 days.

CONCLUSIONS

The expression of renal acid-base transporters is markedly decreased in the obstructed kidney, which may be responsible for the contribution of impaired renal H(+) excretion and HCO(3)(-) reabsorption to the urinary acidification defect in response to unilateral ureteral obstruction.

Age-dependent renal expression of acid-base transporters in neonatal ureter obstruction

Congenital obstructive nephropathy accounts for a major proportion of renal insufficiency in infancy and childhood. In an earlier investigation we demonstrated that bilateral complete ureteral obstruction (BUO) in rats is associated with inadequate urinary acidification [Am J Physiol Renal Physiol. 295(2):F497-506, 2008]. The aim of the study reported here was to determine whether this defect is also associated with unilateral ureteral obstruction (UUO), which is clinically more common than BUO. The time-course of the changes in protein expression levels of major renal acid-base transporters was examined at 7 and 14 weeks in rats with neonatally induced partial unilateral ureteral obstruction (PUUO), which was performed within the first 48 h of life. We observed that protein expression of the renal acid-base transporters NHE3, NBC1, NBCn1, pendrin and Na(+)-K(+)-ATPase was increased in both obstructed and non-obstructed kidneys 7 weeks after the induction of neonatal PUUO. This was confirmed by immunocytochemistry. In contrast, 14 weeks after the induction of PUUO, there was a significant downregulation of the renal acid-base transporters NBC1, NBCn1 and Na(+)-K(+)-ATPase in the obstructed kidneys. These time/age-dependent changes in protein expression were associated with parallel changes in renal function resulting in urine acidification in response to exogenous acid loading. In conclusion, these results show that downregulation of protein expression is a time/age-dependent response to PUUO, which could contribute to the decreased net acid excretion and development of metabolic acidosis in neonatal rats with PUUO.

Diverse transport modes by the solute carrier 26 family of anion transporters

The solute carrier 26 (SLC26) transporters are anion transporters with diverse substrate specificity. Several members are ubiquitous while others show limited tissue distribution. They are expressed in many epithelia and to the extent known, play a central role in anion secretion and absorption. Members of the family are primarily Cl- transporters, although some members transport mainly SO(4)2-, Cl-, HCO(3)- or I-. A defining feature of the family is their functional diversity. Slc26a1 and Slc26a2 function as specific SO(4)2- transporters while Slc26a4 functions as an electroneutral Cl-/I-/HCO(3)- exchanger. Slc26a3 and Slc26a6 function as coupled electrogenic Cl-/HCO(3)- exchangers or as bona fide anion channels. SLC26A7 and SLC26A9 function exclusively as Cl- channels. This short review discusses the functional diversity of the SLC26 transporters.

Human kidney anion exchanger 1 localisation in MDCK cells is controlled by the phosphorylation status of two critical tyrosines

An important question in renal physiology is how the alpha-intercalated cells of the kidney regulate the distribution of the basolateral kidney anion exchanger 1 (kAE1) according to systemic acid-base status. Previous work using a MDCKI model system demonstrated that kAE1 basolateral targeting requires an N-terminal determinant and a critical C-terminal tyrosine (Y904). Here, we show that the N-terminal determinant is residue Y359, because a Y359A substitution mutant was mistargeted to the apical membrane. Further determinants might exist because a range of N-terminal kAE1 truncations that contained Y359 were incorrectly targeted to the TGN. Y359 and Y904 in kAE1 are phosphorylated upon pervanadate treatment and this phosphorylation is sensitive to specific Src kinase family inhibitors. We tested a range of stimuli on this model system and only the application of high nonphysiological concentrations of extracellular bicarbonate, and to a lesser extent hypertonicity or hyperosmolarity, induced tyrosine phosphorylation of kAE1. Treatment with pervanadate caused internalisation of kAE1 from the plasma membrane, but treatment with high concentrations of bicarbonate did not, because of the hypertonicity of the solution. We propose that alpha-intercalated cells control the distribution of kAE1 by reversible phosphorylation of tyrosine residues Y359 and Y904.

Pendred syndrome and iodide transport in the thyroid

Pendred syndrome is an autosomal recessive disorder characterized by sensorineural hearing impairment, presence of goiter, and a partial defect in iodide organification, which may be associated with insufficient thyroid hormone synthesis. Goiter development and development of hypothyroidism are variable and depend on nutritional iodide intake. Pendred syndrome is caused by biallelic mutations in the SLC26A4 gene, which encodes pendrin, a transporter of chloride, bicarbonate and iodide. This review discusses the controversies surrounding the potential role of pendrin in mediating apical iodide efflux into the lumen of thyroid follicles, and discusses its functional role in the kidney and the inner ear.

The Slc26a4 transporter functions as an electroneutral Cl-/I-/HCO3- exchanger: role of Slc26a4 and Slc26a6 in I- and HCO3- secretion and in regulation of CFTR in the parotid duct

Transcellular Cl(-) and HCO(3)(-) transport is a vital function of secretory epithelia and exit across the luminal membrane is mediated by members of the SLC26 transporters in conjunction with cystic fibrosis transmembrane conductance regulator (CFTR) channel. Typically, secretory epithelia express several SLC26 transporters in the same tissue; however, how their specific function is determined in vivo is not known. In the present work we used the parotid gland duct which expressed Slc26a4 and Slc26a6 and the model systems of Slc26a4(-/-) and Slc26a6(-/-) mice to study the role and regulation of these SLC26 transporters. We examined the transport modes of SLC26A4 expressed in Xenopus oocytes and report that SLC26A4 functions as a coupled, electroneutral I(-)/Cl(-), I(-)/HCO(3)(-) and Cl(-)/HCO(3)(-) exchanger with 1: 1 stoichiometry, with I(-) as the preferred anion. In the duct, Slc26a4 is expressed in the luminal membrane and mainly mediates I(-) secretion with minimal role in luminal HCO(3)(-) transport. By contrast, Slc26a6 mediates luminal Cl(-)/HCO(3)(-) exchange activity with minimal role in I(-) secretion. Furthermore, silencing of CFTR altered Cl(-)/HCO(3)(-) exchange by Slc26a6, but had no effect on I(-) secretion by Slc26a4. Accordingly, deletion of Slc26a6, but not deletion of Slc26a4, results in dysregulation of CFTR. These findings provide the first evidence for a selective role of the SLC26 transporters expressed in the same tissue in epithelial anion transport and suggest that transport specificity is achieved by both the properties of the transporters and the composition of the complexes they form.

The solute carrier 26 family of proteins in epithelial ion transport

Transepithelial Cl(-) and HCO(3)(-) transport is critically important for the function of all epithelia and, when altered or ablated, leads to a number of diseases, including cystic fibrosis, congenital chloride diarrhea, deafness, and hypotension (78, 111, 119, 126). HCO(3)(-) is the biological buffer that maintains acid-base balance, thereby preventing metabolic and respiratory acidosis (48). HCO(3)(-) also buffers the pH of the mucosal layers that line all epithelia, protecting them from injury (2). Being a chaotropic ion, HCO(3)(-) is essential for solubilization of ions and macromolecules such as mucins and digestive enzymes in secreted fluids. Most epithelia have a Cl(-)/HCO(3) exchange activity in the luminal membrane. The molecular nature of this activity remained a mystery for many years until the discovery of SLC26A3 and the realization that it is a member of a new family of Cl(-) and HCO(3)(-) transporters, the SLC26 family (73, 78). This review will highlight structural features, the functional diversity, and several regulatory aspects of the SLC26 transporters.

Ureter obstruction alters expression of renal acid-base transport proteins in rat kidney

Urinary tract obstruction impairs renal function and is often associated with a urinary acidification defect caused by diminished net H+ secretion and/or HCO3- reabsorption. To identify the molecular mechanisms of these defects, protein expression of key acid-base transporters were examined along the renal nephron and collecting duct of kidneys from rats subjected to 24-h bilateral ureteral obstruction (BUO), 4 days after release of BUO (BUO-R), or BUO-R rats with experimentally induced metabolic acidosis (BUO-A). Semiquantitative immunoblotting revealed that BUO caused a significant reduction in the expression of the type 3 Na+/H+ exchanger (NHE3) in the cortex (21 +/- 4%), electrogenic Na+/HCO3- cotransporter (NBC1; 71 +/- 5%), type 1 bumetanide-sensitive Na+-K+-2Cl- cotransporter (NKCC2; 3 +/- 1%), electroneutral Na+/HCO3- cotransporter (NBCn1; 46 +/- 7%), and anion exchanger (pendrin; 87 +/- 2%). The expression of H+-ATPase increased in the inner medullary collecting duct (152 +/- 13%). These changes were confirmed by immunocytochemistry. In BUO-R rats, there was a persistent downregulation of all the acid-base transporters including H+-ATPase. Two days of NH4Cl loading reduced plasma pH and HCO3- levels in BUO-A rats. The results demonstrate that the expression of multiple renal acid-base transporters are markedly altered in response to BUO, which may be responsible for development of metabolic acidosis and contribute to the urinary acidification defect after release of the obstruction.

Angiotensin II activates H+-ATPase in type A intercalated cells

We reported previously that angiotensin II (AngII) increases net Cl(-) absorption in mouse cortical collecting duct (CCD) by transcellular transport across type B intercalated cells (IC) via an H(+)-ATPase-and pendrin-dependent mechanism. Because intracellular trafficking regulates both pendrin and H(+)-ATPase, we hypothesized that AngII induces the subcellular redistribution of one or both of these exchangers. To answer this question, CCD from furosemide-treated mice were perfused in vitro, and the subcellular distributions of pendrin and the H(+)-ATPase were quantified using immunogold cytochemistry and morphometric analysis. Addition of AngII in vitro did not change the distribution of pendrin or H(+)-ATPase within type B IC but within type A IC increased the ratio of apical plasma membrane to cytoplasmic H(+)-ATPase three-fold. Moreover, CCDs secreted bicarbonate under basal conditions but absorbed bicarbonate in response to AngII. In summary, angiotensin II stimulates H(+) secretion into the lumen, which drives Cl(-) absorption mediated by apical Cl(-)/HCO(3)(-) exchange as well as generates more favorable electrochemical gradient for ENaC-mediated Na(+) absorption.

IL-18 is expressed in the intercalated cell of human kidney

We determined the cellular location of interleukin-18 (IL-18) and caspase-1 and the purinergic receptor P2X7, two proteins necessary for its activation and secretion. The mRNA and protein of IL-18 were detectable in normal human kidney by means of polymerase chain reaction (PCR), in situ hybridization, and Western blot. Immunohistochemistry located IL-18 to nephron segments containing calbinbin-D28k or aquaporin-2 that suggest location in the distal convoluted and the connecting tubule and to parts of the collecting duct. IL-18 was not detected in the thick ascending limb of Henle. Confocal microscopy showed that IL-18 was expressed in cells negative for calbindin-D28k and for aquaporin-2 but positive for the vacuolar H(+)-ATPase. This demonstrates that the intercalated cells produce IL-18. These segments were also positive for caspase-1 and P2X7 that are essential for IL-18 secretion. Our results show that IL-18 is constitutively expressed by intercalated cells of the late distal convoluted tubule, the connecting tubule, and the collecting duct of the healthy human kidney. Since IL-18 is an early component of the inflammatory cytokine cascade, its location suggests that renal intercalated cells may contribute to immediate immune response of the kidney.

SLC26A9 is a Cl(-) channel regulated by the WNK kinases

SLC26A9 is a member of the SLC26 family of anion transporters, which is expressed at high levels in airway and gastric surface epithelial cells. The transport properties and regulation of SLC26A9, and thus its physiological function, are not known. Here we report that SLC26A9 is a highly selective Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability that is regulated by the WNK kinases. Expression in Xenopus oocytes and simultaneous measurement of membrane potential or current, intracellular pH (pH(i)) and intracellular Cl(-) (Cl(-)(i)) revealed that expression of SLC26A9 resulted in a large Cl(-) current. SLC26A9 displays a selectivity sequence of I(-) > Br(-) > NO(3)(-) > Cl(-) > Glu(-), but it conducts Br(-) > Cl(-) > I(-) > NO(3)(-) > Glu(-), with NO(3)(-) and I(-) inhibiting the Cl(-) conductance. Similarly, expression of SLC26A9 in HEK cells resulted in a large Cl(-) current. Although detectable, OH(-) and HCO(3)(-) fluxes in oocytes expressing SLC26A9 were very small. Moreover, HCO(3)(-) had no discernable effect on the Cl(-) current, the reversal potential in the presence or absence of Cl(-)(o) and, importantly, HCO(3)(-) had no effect on Cl(-) fluxes. These findings indicate that SLC26A9 is a Cl(-) channel with minimal OH(-)/HCO(3)(-) permeability. Co-expression of SLC26A9 with the WNK kinases WNK1, WNK3 or WNK4 inhibited SLC26A9 activity, and the inhibition was independent of WNK kinase activity. Immunolocalization in oocytes and cell surface biotinylation in HEK cells indicated that the WNK-mediated inhibition of SLC26A9 activity is caused by reduced SLC26A9 surface expression. Expression of SLC26A9 in the airway and the response of the WNKs to homeostatic stress raise the possibility that SLC26A9 serves to mediate the response of the airway to stress.

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K+cycling and endolymphatic K+, Na+, Ca2+, and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K+cycling include K+channels, Na+-2Cl−-K+cotransporter, Na+/K+-ATPase, Cl−channels, connexins, and K+/Cl−cotransporters. Furthermore, endolymphatic Na+and Ca2+homeostasis depends on Ca2+-ATPase, Ca2+channels, Na+channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H+-ATPase and Cl−/HCO3−exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K+channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na+-2Cl−-K+cotransporter (SLC12A2), K+/Cl−cotransporters (KCC3 and KCC4), Cl−channels (BSND and CLCNKA + CLCNKB), and H+-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na+-coupled transport (KCNE1/KCNQ1), impaired K+secretion (KCNMA1), limited HCO3−elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Mechanisms of acid–base regulation in the African lungfishProtopterus annectens

African lungfish Protopterus annectens utilized both respiratory and metabolic compensation to restore arterial pH to control levels following the imposition of a metabolic acidosis or alkalosis. Acid infusion (3 mmol kg–1 NH4Cl) to lower arterial pH by 0.24 units increased both pulmonary (by 1.8-fold) and branchial (by 1.7-fold) ventilation frequencies significantly, contributing to 4.8-fold and 1.9-fold increases in,respectively, aerial and aquatic CO2 excretion. This respiratory compensation appeared to be the main mechanism behind the restoration of arterial pH, because even though net acid excretion(JnetH+) increased following acid infusion in 7 of 11 fish, the mean increase in net acid excretion, 184.5±118.5μmol H+ kg–1 h–1 (mean± s.e.m., N=11), was not significantly different from zero. Base infusion (3 mmol kg–1 NaHCO3) to increase arterial pH by 0.29 units halved branchial ventilation frequency, although pulmonary ventilation frequency was unaffected. Correspondingly, aquatic CO2 excretion also fell significantly (by 3.7-fold) while aerial CO2 excretion was unaffected. Metabolic compensation consisting of negative net acid excretion (net base excretion) accompanied this respiratory compensation, with JnetH+ decreasing from 88.5±75.6 to –337.9±199.4 μmol H+kg–1 h–1 (N=8). Partitioning of net acid excretion into renal and extra-renal (assumed to be branchial and/or cutaneous) components revealed that under control conditions, net acid excretion occurred primarily by extra-renal routes. Finally, several genes that are involved in the exchange of acid–base equivalents between the animal and its environment (carbonic anhydrase, V-type H+-ATPase and Na+/HCO –3 cotransporter) were cloned, and their branchial and renal mRNA expressions were examined prior to and following acid or base infusion. In no case was mRNA expression significantly altered by metabolic acid–base disturbance. These findings suggest that lungfish, like tetrapods, alter ventilation to compensate for metabolic acid–base disturbances, a mechanism that is not employed by water-breathing fish. Like fish and amphibians, however, extra-renal routes play a key role in metabolic compensation.

A positive regulatory loop between foxi3a and foxi3b is essential for specification and differentiation of zebrafish epidermal ionocytes

BACKGROUND

Epidermal ionocytes play essential roles in the transepithelial transportation of ions, water, and acid-base balance in fish embryos before their branchial counterparts are fully functional. However, the mechanism controlling epidermal ionocyte specification and differentiation remains unknown.

METHODOLOGY/PRINCIPAL FINDINGS

In zebrafish, we demonstrated that Delta-Notch-mediated lateral inhibition plays a vital role in singling out epidermal ionocyte progenitors from epidermal stem cells. The entire epidermal ionocyte domain of genetic mutants and morphants, which failed to transmit the DeltaC-Notch1a/Notch3 signal from sending cells (epidermal ionocytes) to receiving cells (epidermal stem cells), differentiates into epidermal ionocytes. The low Notch activity in epidermal ionocyte progenitors is permissive for activating winged helix/forkhead box transcription factors of foxi3a and foxi3b. Through gain- and loss-of-function assays, we show that the foxi3a-foxi3b regulatory loop functions as a master regulator to mediate a dual role of specifying epidermal ionocyte progenitors as well as of subsequently promoting differentiation of Na(+),K(+)-ATPase-rich cells and H(+)-ATPase-rich cells in a concentration-dependent manner.

CONCLUSIONS/SIGNIFICANCE

This study provides a framework to show the molecular mechanism controlling epidermal ionocyte specification and differentiation in a low vertebrate for the first time. We propose that the positive regulatory loop between foxi3a and foxi3b not only drives early ionocyte differentiation but also prevents the complete blockage of ionocyte differentiation when the master regulator of foxi3 function is unilaterally compromised.

Molecular regulation and physiology of the H+,K+ -ATPases in kidney

Two H(+), K(+)-adenosine triphosphatase (ATPase) proteins participate in K(+) absorption and H(+) secretion in the renal medulla. Both the gastric (HKalpha(1)) and colonic (HKalpha(2)) H(+),K(+)-ATPases have been localized and characterized by a number of techniques, and are known to be highly regulated in response to acid-base and electrolyte disturbances. Both ATPases are dimers of composition alpha/beta that localize to the apical membrane and both interact with the tetraspanin protein CD63. Although CD63 interacts with the carboxy-terminus of the alpha-subunit of the colonic H(+),K(+)-ATPase, it interacts with the beta-subunit of the gastric H(+),K(+)-ATPase. Pharmacologically, both ATPases are distinct; for example, the gastric H(+),K(+)-ATPase is inhibited by Sch-28080, but the colonic H(+),K(+)-ATPase is inhibited by ouabain (a classic inhibitor of the Na(+)-pump) and is completely insensitive to Sch-28080. The alpha-subunit of the colonic H(+),K(+)-ATPase is the only subunit of the X(+),K(+)-ATPase superfamily that has 3 different splice variants that emerge by deletion or elongation of the amino-terminus. The messenger RNA and protein of one of these splice variants (HKalpha(2C)) is specifically up-regulated in newborn rats and becomes undetectable in adult rats. Therefore, HKalpha(2), in addition to its role in potassium and acid-base homeostasis, appears to play a significant role in early growth and development. Finally, because chronic hypokalemia appears to be the most potent stimulus for upregulation of HKalpha(2), we propose that the HKalpha(2) participates importantly in the maintenance of chronic metabolic alkalosis.

Angiotensin II increases chloride absorption in the cortical collecting duct in mice through a pendrin-dependent mechanism

Pendrin (Slc26a4) localizes to type B and non-A, non-B intercalated cells in the distal convoluted tubule, the connecting tubule, and the cortical collecting duct (CCD), where it mediates apical Cl(-)/HCO(3)(-) exchange. The purpose of this study was to determine whether angiotensin II increases transepithelial net chloride transport, J(Cl), in mouse CCD through a pendrin-dependent mechanism. J(Cl) and transepithelial voltage, V(T), were measured in CCDs perfused in vitro from wild-type and Slc26a4 null mice ingesting a NaCl-replete diet or a NaCl-replete diet and furosemide. In CCDs from wild-type mice ingesting a NaCl-replete diet, V(T) and J(Cl) were not different from zero either in the presence or absence of angiotensin II (10(-8) M) in the bath. Thus further experiments employed mice given the high-NaCl diet and furosemide to upregulate renal pendrin expression. CCDs from furosemide-treated wild-type mice had a lumen-negative V(T) and absorbed Cl(-). With angiotensin II in the bath, Cl(-) absorption doubled although V(T) did not become more lumen negative. In contrast, in CCDs from furosemide-treated Slc26a4 null mice, Cl(-) secretion and a V(T) of approximately 0 were observed, neither of which changed with angiotensin II application. Inhibiting ENaC with benzamil abolished V(T) although J(Cl) fell only approximately 50%. Thus substantial Cl(-) absorption is observed in the absence of an electromotive force. Attenuating apical anion exchange with the peritubular application of the H(+)-ATPase inhibitor bafilomycin abolished benzamil-insensitive Cl(-) absorption. In conclusion, angiotensin II increases transcellular Cl(-) absorption in the CCD through a pendrin- and H(+)-ATPase-dependent process.

Kidney Vacuolar H+-ATPase: Physiology and Regulation

The vacuolar H(+)-ATPase is a multisubunit protein consisting of a peripheral catalytic domain (V(1)) that binds and hydrolyzes adenosine triphosphate (ATP) and provides energy to pump H(+) through the transmembrane domain (V(0)) against a large gradient. This proton-translocating vacuolar H(+)-ATPase is present in both intracellular compartments and the plasma membrane of eukaryotic cells. Mutations in genes encoding kidney intercalated cell-specific V(0) a4 and V(1) B1 subunits of the vacuolar H(+)-ATPase cause the syndrome of distal tubular renal acidosis. This review focuses on the function, regulation, and the role of vacuolar H(+)-ATPases in renal physiology. The localization of vacuolar H(+)-ATPases in the kidney, and their role in intracellular pH (pHi) regulation, transepithelial proton transport, and acid-base homeostasis are discussed.

Expression of the ammonia transporter, rh C glycoprotein, in normal and neoplastic human kidney

Recent studies have identified the presence of a novel Mep/Amt/Rh glycoprotein family of proteins that may play an important role in transmembrane ammonia transport. One of the mammalian members of this family, Rh C glycoprotein (RhCG), transports ammonia, is expressed in distal nephron sites that are critically important for ammonia secretion, exhibits increased expression in response to chronic metabolic acidosis, and originally was cloned as a tumor-related protein. The purpose of our studies was to determine the localization of RhCG in the normal and neoplastic human kidney. Immunoblot analysis of human renal cortical protein lysates demonstrated RhCG protein expression with a molecular weight of approximately 52 kD. Immunohistochemistry revealed both apical and basolateral Rhcg expression in the distal convoluted tubule, connecting segment, and initial collecting tubule and throughout the collecting duct. Co-localization with calbindin-D28k, H(+)-ATPase, aquaporin-2, and pendrin showed that distal convoluted tubule and connecting segment cells, A-type intercalated cells, and non-A, non-B cells express RhCG and that B-type intercalated cells, principal cells, and inner medullary collecting duct cells do not. In renal neoplasms, RhCG was expressed by chromophobe renal cell carcinoma and renal oncocytoma but not by clear cell renal cell carcinoma or by papillary renal cell carcinomas. These studies suggest that RhCG contributes to both apical and basolateral membrane ammonia transport in the human kidney. Furthermore, renal chromophobe renal cell carcinoma and renal oncocytoma seem to originate from the A-type intercalated cell.

Functional characterization of wild-type and a mutated form of SLC26A4 identified in a patient with Pendred syndrome

BACKGROUND

Malfunction of the SLC26A4 protein leads to prelingual deafness often associated with mild thyroid dysfunction and goiter. It is assumed that SLC26A4 acts as a chloride/anion exchanger responsible for the iodide organification in the thyroid gland, and conditioning of the endolymphatic fluid in the inner ear.

METHODS

Chloride uptake studies were made using HEK293-Phoenix cells expressing human wild type SLC26A4 (pendrin) and a mutant (SLC26A4(S28R)) we recently described in a patient with hypothyroidism, goiter and sensorineural hearing loss.

RESULTS

Experiments are summarized showing the functional characterization of wild type SLC26A4 and a mutant (S28R), which we described recently. This mutant protein is transposed towards the cell membrane, however, its transport capability is markedly reduced if compared to wild-type SLC26A4. Furthermore, we show that the SLC26A4 induced chloride uptake in HEK293-Phoenix cells competes with iodide, and, in addition, that the chloride uptake can be blocked by NPPB and niflumic acid, whereas DIDS is ineffective.

CONCLUSIONS

The functional characteristics of SLC26A4(S28R) we describe here, are consistent with the clinical phenotype observed in the patient from which the mutant was derived.