Ubiquitous and kidney-specific subunits of vacuolar H+-ATPase are differentially expressed during nephrogenesis

Intercalated cell function, kidney innate immunity, and urinary tract infections

Intercalated cells (ICs) in the kidney collecting duct have a versatile role in acid-base and electrolyte regulation along with the host immune defense. Located in the terminal kidney tubule segment, ICs are among the first kidney cells to encounter bacteria when bacteria ascend from the bladder into the kidney. ICs have developed several mechanisms to combat bacterial infections of the kidneys. For example, ICs produce antimicrobial peptides (AMPs), which have direct bactericidal activity, and in many cases are upregulated in response to infections. Some AMP genes with IC-specific kidney expression are multiallelic, and having more copies of the gene confers increased resistance to bacterial infections of the kidney and urinary tract. Similarly, studies in human children demonstrate that those with history of UTIs are more likely to have single-nucleotide polymorphisms in IC-expressed AMP genes that impair the AMP's bactericidal activity. In murine models, depleted or impaired ICs result in decreased clearance of bacterial load following transurethral challenge with uropathogenic E. coli. A 2021 study demonstrated that ICs even act as phagocytes and acidify bacteria within phagolysosomes. Several immune signaling pathways have been identified in ICs which may represent future therapeutic targets in managing kidney infections or inflammation. This review's objective is to highlight IC structure and function with an emphasis on current knowledge of IC's diverse innate immune capabilities.

Generation of Atp6v1g3-Cre mice for investigation of intercalated cells and the collecting duct

Kidney intercalated cells (ICs) maintain acid-base homeostasis and recent studies have demonstrated that they function in the kidney's innate defense. To study kidney innate immune function, ICs have been enriched using vacuolar ATPase (V-ATPase) B1 subunit (Atp6v1b1)-Cre (B1-Cre) mice. Although Atp6v1b1 is considered kidney specific, it is expressed in multiple organ systems, both in mice and humans, raising the possibility of off-target effects when using the Cre-lox system. We have recently shown using single-cell RNA sequencing that the gene that codes for the V-ATPase G3 subunit (mouse gene: Atp6v1g3; human gene: ATP6V1G3; protein abbreviation: G3) mRNA is selectively enriched in human kidney ICs. In this study, we generated Atp6v1g3-Cre (G3-Cre) reporter mice using CRISPR/CAS technology and crossed them with Tdtomatoflox/flox mice. The resultant G3-Cre+Tdt+ progeny was evaluated for kidney specificity in multiple tissues and found to be highly specific to kidney cells with minimal or no expression in other organs evaluated compared with B1-Cre mice. Tdt+ cells were flow sorted and were enriched for IC marker genes on RT-PCR analysis. Next, we crossed these mice to ihCD59 mice to generate an IC depletion mouse model (G3-Cre+ihCD59+/+). ICs were depleted in these mice using intermedilysin, which resulted in lower blood pH, suggestive of a distal renal tubular acidosis phenotype. The G3-Cre mice were healthy, bred normally, and produce regular-sized litter. Thus, this new "IC reporter" mice can be a useful tool to study ICs.NEW & NOTEWORTHY This study details the development, validation, and experimental use of a new mouse model to study the collecting duct and intercalated cells. Kidney intercalated cells are a cell type increasingly recognized to be important in several human diseases including kidney infections, acid-base disorders, and acute kidney injury.

Na+-Retaining Action of COX-2 (Cyclooxygenase-2)/EP1 Pathway in the Collecting Duct via Activation of Intrarenal Renin-Angiotensin-Aldosterone System and Epithelial Sodium Channel

BACKGROUND

The collecting duct (CD) is a major site of both biosynthesis and action of prostaglandin E₂ as highlighted by the predominant expression of COX-2 (cyclooxygenase-2) and some E-prostanoid (EP) subtypes at this nephron site. The purpose of this study was to determine the relevance and mechanism of CD COX-2/prostaglandin E₂/EP₁ signaling for the regulation of Na⁺ hemostasis during Na⁺ depletion.

METHODS

Mice with Aqp2Cre-driven deletion of COX-2 (COX-2^fl/flAqp2Cre⁺) or the EP₁ subtype (EP₁^fl/flAqp2Cre⁺) were generated and the Na⁺-wasting phenotype of these mice during low-salt (LS) intake was examined. EP subtypes responsible for prostaglandin E₂-induced local renin response were analyzed in primary cultured mouse inner medullary CD cells.

RESULTS

Following 28-day LS intake, COX-2^fl/flAqp2Cre⁺ mice exhibited a higher urinary Na⁺ excretion and lower cumulative Na⁺ balance, accompanied with suppressed intrarenal renin, AngII (angiotensin II), and aldosterone, expression of CYP11B2 (cytochrome P450 family 11 subfamily B member 2), and blunted expression of epithelial sodium channel subunits compared to floxed controls (COX-2^fl/flAqp2Cre^-), whereas no differences were observed for indices of systemic renin-angiotensin-aldosterone system. In cultured CD cells, exposure to prostaglandin E₂ stimulated release of soluble (pro)renin receptor, prorenin/renin and aldosterone and the stimulation was more sensitive to antagonism of EP₁ as compared other EP subtypes. Subsequently, EP₁^fl/flAqp2Cre⁺ mice largely recapitulated Na⁺-wasting phenotype seen in COX-2^fl/flAqp2Cre⁺ mice.

CONCLUSIONS

The study for the first time reports that CD COX-2/EP₁ pathway might play a key role in maintenance of Na⁺ homeostasis in the face of Na⁺ depletion, at least in part, through activation of intrarenal renin-angiotensin-aldosterone-system and epithelial sodium channel.

Cardiovascular aspects of the (pro)renin receptor: Function and significance

Cardiovascular diseases (CVDs), including all types of disorders related to the heart or blood vessels, are the major public health problems and the leading causes of mortality globally. (Pro)renin receptor (PRR), a single transmembrane protein, is present in cardiomyocytes, vascular smooth muscle cells, and endothelial cells. PRR plays an essential role in cardiovascular homeostasis by regulating the renin-angiotensin system and several intracellular signals such as mitogen-activated protein kinase signaling and wnt/β-catenin signaling in various cardiovascular cells. This review discusses the current evidence for the pathophysiological roles of the cardiac and vascular PRR. Activation of PRR in cardiomyocytes may contribute to myocardial ischemia/reperfusion injury, cardiac hypertrophy, diabetic or alcoholic cardiomyopathy, salt-induced heart damage, and heart failure. Activation of PRR promotes vascular smooth muscle cell proliferation, endothelial cell dysfunction, neovascularization, and the progress of vascular diseases. In addition, phenotypes of animals transgenic for PRR and the hypertensive actions of PRR in the brain and kidney and the soluble PRR are also discussed. Targeting PRR in local tissues may offer benefits for patients with CVDs, including heart injury, atherosclerosis, and hypertension.

Kidney intercalated cells and the transcription factor FOXi1 drive cystogenesis in tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is caused by mutations in TSC1 or TSC2 gene and affects multiple organs, including the kidney, where it presents with angiomyolipomata and cysts that can result in kidney failure. The factors promoting cyst formation and tumor growth in TSC are incompletely understood. Current studies demonstrate that kidney cyst epithelia in TSC mouse models and in humans with TSC are composed of hyperproliferating intercalated cells, along with activation of H⁺-ATPase and carbonic anhydrase 2. Interfering with intercalated cell proliferation completely inhibited and inactivating carbonic anhydrase 2 significantly protected against cyst formation in TSC. Targeting the acid base and/or electrolyte transporters of intercalated cells may provide a therapeutic approach for the treatment of kidney cysts in TSC.

Tuberous sclerosis complex (TSC) is caused by mutations in either TSC1 or TSC2 genes and affects multiple organs, including kidney, lung, and brain. In the kidney, TSC presents with the enlargement of benign tumors (angiomyolipomata) and cysts, which eventually leads to kidney failure. The factors promoting cyst formation and tumor growth in TSC are incompletely understood. Here, we report that mice with principal cell-specific inactivation of Tsc1 develop numerous cortical cysts, which are overwhelmingly composed of hyperproliferating A-intercalated (A-IC) cells. RNA sequencing and confirmatory expression studies demonstrated robust expression of Forkhead Transcription Factor 1 (Foxi1) and its downstream targets, apical H⁺-ATPase and cytoplasmic carbonic anhydrase 2 (CAII), in cyst epithelia in Tsc1 knockout (KO) mice but not in Pkd1 mutant mice. In addition, the electrogenic 2Cl⁻/H⁺ exchanger (CLC-5) is significantly up-regulated and shows remarkable colocalization with H⁺-ATPase on the apical membrane of cyst epithelia in Tsc1 KO mice. Deletion of Foxi1, which is vital to intercalated cells viability and H⁺-ATPase expression, completely abrogated the cyst burden in Tsc1 KO mice, as indicated by MRI images and histological analysis in kidneys of Foxi1/Tsc1 double-knockout (dKO) mice. Deletion of CAII, which is critical to H⁺-ATPase activation, caused significant reduction in cyst burden and increased life expectancy in CAII/Tsc1 dKO mice vs. Tsc1 KO mice. We propose that intercalated cells and their acid/base/electrolyte transport machinery (H⁺-ATPase/CAII/CLC-5) are critical to cystogenesis, and their inhibition or inactivation is associated with significant protection against cyst generation and/or enlargement in TSC.

Kidney intercalated cells are phagocytic and acidify internalized uropathogenic Escherichia coli

Kidney intercalated cells are involved in acid-base homeostasis via vacuolar ATPase expression. Here we report six human intercalated cell subtypes, including hybrid principal-intercalated cells identified from single cell transcriptomics. Phagosome maturation is a biological process that increases in biological pathway analysis rank following exposure to uropathogenic Escherichia coli in two of the intercalated cell subtypes. Real time confocal microscopy visualization of murine renal tubules perfused with green fluorescent protein expressing Escherichia coli or pHrodo Green E. coli BioParticles demonstrates that intercalated cells actively phagocytose bacteria then acidify phagolysosomes. Additionally, intercalated cells have increased vacuolar ATPase expression following in vivo experimental UTI. Taken together, intercalated cells exhibit a transcriptional response conducive to the kidney's defense, engulf bacteria and acidify the internalized bacteria. Intercalated cells represent an epithelial cell with characteristics of professional phagocytes like macrophages.

Developmental loss, but not pharmacological suppression, of renal carbonic anhydrase 2 results in pyelonephritis susceptibility

Carbonic anhydrase II knockout (Car2^-/-) mice have depleted numbers of renal intercalated cells, which are increasingly recognized to be innate immune effectors. We compared pyelonephritis susceptibility following reciprocal renal transplantations between Car2^-/- and wild-type mice. We examined the effect of pharmacological CA suppression using acetazolamide in an experimental murine model of urinary tract infection. Car2^-/- versus wild-type mice were compared for differences in renal innate immunity. In our transplant scheme, mice lacking CA-II in the kidney had increased pyelonephritis risk. Mice treated with acetazolamide had lower kidney bacterial burdens at 6 h postinfection, which appeared to be due to tubular flow from diuresis because comparable results were obtained when furosemide was substituted for acetazolamide. Isolated Car2^-/- kidney cells enriched for intercalated cells demonstrated altered intercalated cell innate immune gene expression, notably increased calgizzarin and insulin receptor expression. Intercalated cell number and function along with renal tubular flow are determinants of pyelonephritis risk.

TRPV4 is associated with central rather than nephrogenic osmoregulation

TRPV4 is a polymodal cation channel expressed in osmosensitive neurons of the hypothalamus and in the mammalian nephron. The segmental distribution and role(s) of TRPV4 in osmoregulation remain debated. We investigated the renal distribution pattern of TRPV4 and the functional consequences of its disruption in mouse models. Using qPCR on microdissected segments, immunohistochemistry, and a LacZ reporter mouse, we found that TRPV4 is abundantly expressed in the proximal tubule, the late distal convoluted tubule, and throughout the connecting tubule and collecting duct, including principal and intercalated cells. TRPV4 was undetectable in the glomeruli and thick ascending limb and weakly abundant in the early distal convoluted tubule. Metabolic studies in Trpv4 (+/+) and Trpv4 (-/-) littermates revealed that the lack of TRPV4 did not influence activity, food and water intake, renal function, and urinary concentration at baseline. The mice showed a similar response to furosemide, water loading and deprivation, acid loading, and dietary NaCl restriction. However, Trpv4 (-/-) mice showed a significantly lower vasopressin synthesis and release after water deprivation, with a loss of the positive correlation between plasma osmolality and plasma vasopressin levels, and a delayed water intake upon acute administration of hypertonic saline. Specific activation of TRPV4 in primary cultures of proximal tubule cells increased albumin uptake, whereas no effect of TRPV4 deletion could be observed at baseline. These data reveal that, despite its abundant expression in tubular segments, TRPV4 does not play a major role in the kidney or is efficiently compensated when deleted. Instead, TRPV4 is critical for the release of vasopressin, the sensation of thirst, and the central osmoregulation.

Investigation of ATP6V1B1 and ATP6V0A4 genes causing hereditary hearing loss associated with distal renal tubular acidosis in Iranian families

BACKGROUND

Hearing defects are the most common sensory disorders, affecting 1 out of every 500 newborns. ATP6V1B mutations are associated with early sensorineural hearing loss, whereas ATP6V0A4 mutations are classically associated with either late-onset sensorineural hearing loss or normal hearing. ATP6V1B1 and ATP6V0A4 genetic mutations cause recessive forms of distal renal tubular acidosis.

METHOD

Ten unrelated deaf Iranian families with distal renal tubular acidosis were referred to the Genetics Research Centre, University of Social Welfare and Rehabilitation Sciences, Tehran. All exons of the ATP6V1B1 and ATP6V0A4 genes were sequenced in affected family members.

RESULTS

We identified a previously reported ATP6V1B1 frameshift mutation (P385fsX441) in two families and a nucleotide substitution in exon 10 (P346R) in three families. In addition, one patient was homozygous for a novel nucleotide substitution in exon 3.

CONCLUSION

ATP6V1B1 genetic mutations were detected in more than half of the families studied. Mutations in this gene therefore seem to be the most common causative factors in hearing loss associated with distal renal tubular acidosis in these families.

Vacuolar H+-ATPase: An Essential Multitasking Enzyme in Physiology and Pathophysiology

Vacuolar H+-ATPases (V-ATPases) are large multisubunit proton pumps that are required for housekeeping acidification of membrane-bound compartments in eukaryotic cells. Mammalian V-ATPases are composed of 13 different subunits. Their housekeeping functions include acidifying endosomes, lysosomes, phagosomes, compartments for uncoupling receptors and ligands, autophagosomes, and elements of the Golgi apparatus. Specialized cells, including osteoclasts, intercalated cells in the kidney and pancreatic beta cells, contain both the housekeeping V-ATPases and an additional subset of V-ATPases, which plays a cell type specific role. The specialized V-ATPases are typically marked by the inclusion of cell type specific isoforms of one or more of the subunits. Three human diseases caused by mutations of isoforms of subunits have been identified. Cancer cells utilize V-ATPases in unusual ways; characterization of V-ATPases may lead to new therapeutic modalities for the treatment of cancer. Two accessory proteins to the V-ATPase have been identified that regulate the proton pump. One is the (pro)renin receptor and data is emerging that indicates that V-ATPase may be intimately linked to renin/angiotensin signaling both systemically and locally. In summary, V-ATPases play vital housekeeping roles in eukaryotic cells. Specialized versions of the pump are required by specific organ systems and are involved in diseases.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Deletion of the prorenin receptor from the ureteric bud causes renal hypodysplasia

The role of the prorenin receptor (PRR) in the regulation of ureteric bud (UB) branching morphogenesis is unknown. Here, we investigated whether PRR acts specifically in the UB to regulate UB branching, kidney development and function. We demonstrate that embryonic (E) day E13.5 mouse metanephroi, isolated intact E11.5 UBs and cultured UB cells express PRR mRNA. To study its role in UB development, we conditionally ablated PRR in the developing UB (PRR^UB−/−) using Hoxb7^Cre mice. On E12.5, PRR^UB−/− mice had decreased UB branching and increased UB cell apoptosis. These defects were associated with decreased expression of Ret, Wnt11, Etv4/Etv5, and reduced phosphorylation of Erk1/2 in the UB. On E18.5, mutants had marked kidney hypoplasia, widespread apoptosis of medullary collecting duct cells and decreased expression of Foxi1, AE1 and H⁺-ATPase α4 mRNA. Ultimately, they developed occasional small cysts in medullary collecting ducts and had decreased nephron number. To test the functional consequences of these alterations, we determined the ability of PRR^UB−/− mice to acidify and concentrate the urine on postnatal (P) day P30. PRR^UB−/− mice were polyuric, had lower urine osmolality and a higher urine pH following 48 hours of acidic loading with NH₄Cl. Taken together, these data show that PRR present in the UB epithelia performs essential functions during UB branching morphogenesis and collecting duct development via control of Ret/Wnt11 pathway gene expression, UB cell survival, activation of Erk1/2, terminal differentiation and function of collecting duct cells needed for maintaining adequate water and acid-base homeostasis. We propose that mutations in PRR could possibly cause renal hypodysplasia and renal tubular acidosis in humans.

Zebrafish as an animal model to study ion homeostasis

Zebrafish (Danio rerio) possesses several advantages as an experimental organism, including the applicability of molecular tools, ease of in vivo cellular observation and functional analysis, and rapid embryonic development, making it an emerging model for the study of integrative and regulatory physiology and, in particular, the epithelial transport associated with body fluid ionic homeostasis. Zebrafish inhabits a hypotonic freshwater environment, and as such, the gills (or the skin, during embryonic stages) assume the role of the kidney in body fluid ionic homeostasis. Four types of ionocyte expressing distinct sets of transporters have been identified in these organs: H⁺-ATPase-rich, Na⁺-K⁺-ATPase-rich, Na⁺-Cl⁻ cotransporter-expressing and K⁺-secreting cells; these ionocytes perform transepithelial H⁺ secretion/Na⁺ uptake/NH₄⁺ excretion, Ca²⁺ uptake, Na⁺/Cl⁻ uptake, and K⁺ secretion, respectively. Zebrafish ionocytes are analogous to various renal tubular cells, in terms of ion transporter expression and function. During embryonic development, ionocyte progenitors develop from epidermal stem cells and then differentiate into different types of ionocyte through a positive regulatory loop of Foxi3a/-3b and other transcription factors. Several hormones, including cortisol, vitamin D, stanniocalcin-1, calcitonin, and isotocin, were found to participate in the control pathways of ionic homeostasis by precisely studying the target ion transport pathways, ion transporters, or ionocytes of the hormonal actions. In conclusion, the zebrafish model not only enhances our understanding of body fluid ion homeostasis and hormonal control in fish but also informs studies on mammals and other animal species, thereby providing new insights into related fields.

Atp6v0a4 knockout mouse is a model of distal renal tubular acidosis with hearing loss, with additional extrarenal phenotype

Autosomal recessive distal renal tubular acidosis (dRTA) is a severe disorder of acid-base homeostasis, often accompanied by sensorineural deafness. We and others have previously shown that mutations in the tissue-restricted a4 and B1 subunits of the H(+)-ATPase underlie this syndrome. Here, we describe an Atp6v0a4 knockout mouse, which lacks the a4 subunit. Using β-galactosidase as a reporter for the null gene, developmental a4 expression was detected in developing bone, nose, eye, and skin, in addition to that expected in kidney and inner ear. By the time of weaning, Atp6v0a4(-/-) mice demonstrated severe metabolic acidosis, hypokalemia, and early nephrocalcinosis. Null mice were hypocitraturic, but hypercalciuria was absent. They were severely hearing-impaired, as shown by elevated auditory brainstem response thresholds and absent endocochlear potential. They died rapidly unless alkalinized. If they survived weaning with alkali supplementation, treatment could later be withdrawn, but -/- animals remained acidotic with alkaline urine. They also had an impaired sense of smell. Heterozygous animals were biochemically normal until acid-challenged, when they became more acidotic than +/+ animals. This mouse model recapitulates the loss of H(+)-ATPase function seen in human disease and can provide additional insights into dRTA and the physiology of the a4 subunit.

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.

Embryonic frog epidermis: a model for the study of cell-cell interactions in the development of mucociliary disease

Specialised epithelia such as mucociliary, secretory and transporting epithelia line all major organs, including the lung, gut and kidney. Malfunction of these epithelia is associated with many human diseases. The frog embryonic epidermis possesses mucus-secreting and multiciliated cells, and has served as an excellent model system for the biogenesis of cilia. However, ionic regulation is important for the function of all specialised epithelia and it is not clear how this is achieved in the embryonic frog epidermis. Here, we show that a third cell type develops alongside ciliated and mucus-secreting cells in the tadpole skin. These cells express high levels of ion channels and transporters; therefore, we suggest that they are analogous to ionocytes found in transporting epithelia such as the mammalian kidney. We show that frog ionocytes express the transcription factor foxi1e, which is required for the development of these cells. Depletion of ionocytes by foxi1e knockdown has detrimental effects on the development of multiciliated cells, which show fewer and aberrantly beating cilia. These results reveal a newly identified role for ionocytes and suggest that the frog embryonic skin is a model system that is particularly suited to studying the interactions of different cell types in mucociliary, as well as in secretory and transporting, epithelia.

The transcription factor HNF1α regulates expression of chloride-proton exchanger ClC-5 in the renal proximal tubule

The Cl−/H+ exchanger ClC-5 is essential for the endocytic activity of the proximal tubule cells and the tubular clearance of proteins filtered in the glomeruli. The mechanisms that regulate the expression of ClC-5 in general and its specific expression in the proximal tubule are unknown. In this study, we investigated the hypothesis that the hepatocyte nuclear transcription factor HNF1α, which is predominantly expressed in proximal tubule segments, may directly regulate the expression of ClC-5. In situ hybridization demonstrated that the expression of Clcn5 overlaps with that of Hnf1α in the developing kidney as well as in absorptive epithelia, including the digestive tract and yolk sac. Multiple binding sites for HNF1 were mapped in the 5′-regulatory sequences of the mouse and human Clcn5/CLCN5 genes. The transactivation of the Clcn5/CLCN5 promoter by HNF1α was verified in vitro, and the binding of HNF1α to the Clcn5 promoter in vivo was confirmed by chromatin immunoprecipitation in mouse kidney. The expression of Clcn5 was reduced in the proximal tubule segments of HNF1α-null kidneys, and it was rescued upon transfection of HNF1α-null cells with wild-type but not with mutant HNF1α. These data demonstrate that HNF1α directly regulates the expression of ClC-5 in the renal proximal tubule and yield insights into the mechanisms governing epithelial differentiation and specialized transport activities in the kidney.

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.

Cellular physiology of the renal H+ATPase

PURPOSE OF REVIEW

Vacuolar-type H+ATPases are multisubunit macromolecules that play an essential role in renal acid-base homeostasis. Other cellular processes also rely on the proton pumping ability of H+ATPases to acidify organellar or lumenal spaces. Several diseases, including distal renal tubular acidosis, osteoporosis and wrinkly skin syndrome, are due to mutations in genes encoding alternate subunits that make up the H+ATPase. This review highlights recent key articles in this research area.

RECENT FINDINGS

Further insights into the structure, expression and regulation of H+ATPases have been elucidated, within the kidney and elsewhere. This knowledge may enhance the potential for future drug targeting.

SUMMARY

Novel findings concerning tissue-specific subunits of the H+ATPase that are important in the kidney and more general lessons of H+ATPase function and regulation are slowly emerging, though the paucity of cellular tools available has to date limited progress.

The yeast lysosome-like vacuole: endpoint and crossroads

Fungal vacuoles are acidic organelles with degradative and storage capabilities that have many similarities to mammalian lysosomes and plant vacuoles. In the past several years, well-developed genetic, genomic, biochemical and cell biological tools in S. cerevisiae have provided fresh insights into vacuolar protein sorting, organelle acidification, ion homeostasis, autophagy, and stress-related functions of the vacuole, and these insights have often found parallels in mammalian lysosomes. This review provides a broad overview of the defining features and functions of S. cerevisiae vacuoles and compares these features to mammalian lysosomes. Recent research challenges the traditional view of vacuoles and lysosomes as simply the terminal compartment of biosynthetic and endocytic pathways (i.e. the "garbage dump" of the cell), and suggests instead that these compartments are unexpectedly dynamic and highly regulated.

Role of SNAREs and H+-ATPase in the targeting of proton pump-coated vesicles to collecting duct cell apical membrane

Recycling of H(+)-ATPase to the apical plasma membrane, mediated by vesicular exocytosis and endocytosis, is an important mechanism for controlling H(+) secretion by the collecting duct. We hypothesized that SNAREs (soluble N-ethylmaleimide-sensitive factor attachment proteins) may be involved in the targeting of H(+)-ATPase-coated vesicles. Using a tissue culture model of collecting duct H(+) secretory cells (inner medullary collecting duct (IMCD) cells), we demonstrated that they express the proteins required for SNARE-mediated exocytosis and form SNARE-fusion complexes upon stimulation of H(+)-ATPase exocytosis. Furthermore, exocytic amplification of apical H(+)-ATPase is sensitive to clostridial toxins that cleave SNAREs and thereby inhibit secretion. Thus, SNAREs are critical for H(+)-ATPase cycling to the plasma membrane. The process in IMCD cells has a feature distinct from that of neuronal cells: the SNARE complex includes and requires the vesicular cargo (H(+)-ATPase) for targeting. Using chimeras and truncations of syntaxin 1, we demonstrated that there is a specific cassette within the syntaxin 1 H3 domain that mediates binding of the SNAREs and a second distinct H3 region that binds H(+)-ATPase. Utilizing point mutations of the B1 subunit of the H(+)-ATPase, we document that this subunit contains specific targeting information for the H(+)-ATPase itself. In addition, we found that Munc-18-2, a regulator of exocytosis, plays a multifunctional role in this system: it regulates SNARE complex formation and the affinity of syntaxin 1 for H(+)-ATPase.

Tissue specific expression of the splice variants of the mouse vacuolar proton-translocating ATPase a4 subunit

We have identified splicing variants of the mouse a4 subunit which have the same open reading frame but have a different 5'-noncoding sequence. Further determination of the 5'-upstream region of the a4 gene in mouse indicated the presence of two first exons (exon 1a and exon 1b) which include the 5'-noncoding sequence of each variant. The mRNAs of both splicing variants (a4-I and a4-II) show a similar expression pattern in mouse kidney by in situ hybridization. However, tissue and developmental expression patterns of the variants are different. In addition to strong expression in kidney, a4-I expression was detected in heart, lung, skeletal muscle, and testis, whereas a4-II is expressed in lung, liver, and testis. During development, a4-I was expressed beginning with the early embryonic stage, but a4-II mRNA was detected from day 17. These results suggest that each a4 variant has both a tissue and developmental stage specific function.

Compensatory membrane expression of the V-ATPase B2 subunit isoform in renal medullary intercalated cells of B1-deficient mice

Mice deficient in the ATP6V1B1 ("B1") subunit of the vacuolar proton-pumping ATPase (V-ATPase) maintain body acid-base homeostasis under normal conditions, but not when exposed to an acid load. Here, compensatory mechanisms involving the alternate ATP6V1B2 ("B2") isoform were examined to explain the persistence of baseline pH regulation in these animals. By immunocytochemistry, the mean pixel intensity of apical B2 immunostaining in medullary A intercalated cells (A-ICs) was twofold greater in B1-/- mice than in B1+/+ animals, and B2 was colocalized with other V-ATPase subunits. No significant upregulation of B2 mRNA or protein expression was detected in B1-/- mice compared with wild-type controls. We conclude that increased apical B2 staining is due to relocalization of B2-containing V-ATPase complexes from the cytosol to the plasma membrane. Recycling of B2-containing holoenzymes between these domains was confirmed by the intracellular accumulation of B1-deficient V-ATPases in response to the microtubule-disrupting drug colchicine. V-ATPase membrane expression is further supported by the presence of "rod-shaped" intramembranous particles seen by freeze fracture microscopy in apical membranes of normal and B1-deficient A-ICs. Intracellular pH recovery assays show that significant (28-40% of normal) V-ATPase function is preserved in medullary ICs from B1-/- mice. We conclude that the activity of apical B2-containing V-ATPase holoenzymes in A-ICs is sufficient to maintain baseline acid-base homeostasis in B1-deficient mice. However, our results show no increase in cell surface V-ATPase activity in response to metabolic acidosis in ICs from these animals, consistent with their inability to appropriately acidify their urine under these conditions.

Foxi3 transcription factors and Notch signaling control the formation of skin ionocytes from epidermal precursors of the zebrafish embryo

Ionocytes are specialized epithelial cell types involved in the maintenance of osmotic homeostasis. In amniotes, they are present in the renal system, while in water-living embryos of lower vertebrates additional ionocytes are found in the skin. Thus far, relatively little has been known about the mechanisms of ionocyte development. Here we demonstrate that skin ionocytes of zebrafish embryos derive from the same precursor cells as keratinocytes. Carrying out various combinations of gain- and loss-of-function studies, we show that the segregation of ionocytes from the epidermal epithelium is governed by an interplay between Notch signaling and two Forkhead-box transcription factors, Foxi3a and Foxi3b. The two foxi3 genes are expressed in ionocyte precursors and are required both for ionocyte-specific expression of the Notch ligand Jagged2a, and for ionocyte differentiation, characterized by the production of particular ATPases. Ionocytic Notch ligands, in turn, signal to neighboring cells, where activated Notch1 leads to a repression of foxi3 expression, allowing those cells to become keratinocytes. A model for ionocyte versus keratinocyte development will be presented, postulating additional thus far unidentified pro-ionocyte factors.

PKD1 haploinsufficiency causes a syndrome of inappropriate antidiuresis in mice

Mutations in PKD1 are associated with autosomal dominant polycystic kidney disease. Studies in mouse models suggest that the vasopressin (AVP) V2 receptor (V2R) pathway is involved in renal cyst progression, but potential changes before cystogenesis are unknown. This study used a noncystic mouse model to investigate the effect of Pkd1 haploinsufficiency on water handling and AVP signaling in the collecting duct (CD). In comparison with wild-type littermates, Pkd1(+/-) mice showed inappropriate antidiuresis with higher urine osmolality and lower plasma osmolality at baseline, despite similar renal function and water intake. The Pkd1(+/-) mice had a decreased aquaretic response to both a water load and a selective V2R antagonist, despite similar V2R distribution and affinity. They showed an inappropriate expression of AVP in brain, irrespective of the hypo-osmolality. The cAMP levels in kidney and urine were unchanged, as were the mRNA levels of aquaporin-2 (AQP2), V2R, and cAMP-dependent mediators in kidney. However, the (Ser256) phosphorylated AQP2 was upregulated in Pkd1(+/-) kidneys, with AQP2 recruitment to the apical plasma membrane of CD principal cells. The basal intracellular Ca(2+) concentration was significantly lower in isolated Pkd1(+/-) CD, with downregulated phosphorylated extracellular signal-regulated kinase 1/2 and decreased RhoA activity. Thus, in absence of cystic changes, reduced Pkd1 gene dosage is associated with a syndrome of inappropriate antidiuresis (positive water balance) reflecting decreased intracellular Ca(2+) concentration, decreased activity of RhoA, recruitment of AQP2 in the CD, and inappropriate expression of AVP in the brain. These data give new insights in the potential roles of polycystin-1 in the AVP and Ca(2+) signaling and the trafficking of AQP2 in the CD.

Cystic fibrosis is associated with a defect in apical receptor-mediated endocytosis in mouse and human kidney

Inactivation of the chloride channel cystic fibrosis transmembrane conductance regulator (CFTR) causes cystic fibrosis (CF). Although CFTR is expressed in the kidney, no overwhelming renal phenotype has been documented in patients with CF. This study investigated the expression, subcellular distribution, and processing of CFTR in the kidney; used various mouse models to assess the role of CFTR in proximal tubule (PT) endocytosis; and tested the relevance of these findings in patients with CF. The level of CFTR mRNA in mouse kidney approached that found in lung. CFTR was located in the apical area of PT cells, with a maximal intensity in the straight part (S3) of the PT. Fractionation showed that CFTR co-distributed with the chloride/proton exchanger ClC-5 in PT endosomes. Cftr(-/-) mice showed impaired (125)I-beta(2)-microglobulin uptake, together with a decreased amount of the multiligand receptor cubilin in the S3 segment and a significant loss of cubilin and its low molecular weight (LMW) ligands into the urine. Defective receptor-mediated endocytosis was found less consistently in Cftr(DeltaF/DeltaF) mice, characterized by a large phenotypic heterogeneity and moderate versus mice that lacked ClC-5. A significant LMW proteinuria (and particularly transferrinuria) also was documented in a cohort of patients with CF but not in patients with asthma and chronic lung inflammation. In conclusion, CFTR inactivation leads to a moderate defect in receptor-mediated PT endocytosis, associated with a cubilin defect and a significant LMW proteinuria in mouse and human. The magnitude of the endocytosis defect that is caused by CFTR versus ClC-5 loss likely reflects functional heterogeneity along the PT.

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.

Kidney collecting duct acid-base "regulon"

Kidneys are essential for acid-base homeostasis, especially when organisms cope with changes in acid or base dietary intake. Because collecting ducts constitute the final site for regulating urine acid-base balance, we undertook to identify the gene network involved in acid-base transport and regulation in the mouse outer medullary collecting duct (OMCD). For this purpose, we combined kidney functional studies and quantitative analysis of gene expression in OMCDs, by transcriptome and candidate gene approaches, during metabolic acidosis. Furthermore, to better delineate the set of genes concerned with acid-base disturbance, the OMCD transcriptome of acidotic mice was compared with that of both normal mice and mice undergoing an adaptative response through potassium depletion. Metabolic acidosis, achieved through an NH4Cl-supplemented diet for 3 days, not only induced acid secretion but also stimulated the aldosterone and vasopressin systems and triggered cell proliferation. Accordingly, metabolic acidosis increased the expression of genes involved in acid-base transport, sodium transport, water transport, and cell proliferation. In particular, >25 transcripts encoding proteins involved in urine acidification (subunits of H-ATPase, kidney anion exchanger, chloride channel Clcka, carbonic anhydrase-2, aldolase) were co-regulated during acidosis. These transcripts, which cooperate to achieve a similar function and are co-regulated during acidosis, constitute a functional unit that we propose to call a "regulon".

A novel cellular survival factor--the B2 subunit of vacuolar H+-ATPase inhibits apoptosis

The ubiquitous vacuolar H(+)-ATPase, a multisubunit proton pump, is essential for intraorganellar acidification. Disruption of its function leads to disturbances of organelle function and cell death. Here, we report that overexpression of the B2 subunit of the H(+)-ATPase inhibits apoptosis. This antiapoptotic effect is not mediated by an increase in H(+)-ATPase activity but through activation of the Ras-mitogen-activated protein kinase (MAPK)-signaling pathway that results in the serine phosphorylation of Bad at residues 112 and 155. Increased Bad phosphorylation reduces its translocation to mitochondria, limits the release of mitochondrial cytochrome c and apoptosis-inducing factor and increases the resistance of the B2 overexpressing cells to apoptosis. Screening experiments of kinase inhibitors, including inhibitors of cAMP-activated protein kinase, protein kinase C, protein kinase B, (MAPK/extracellular signal-regulated (ERK) kinase) MEK and Ste-MEK1(13), a cell permeable ERK activation inhibitor peptide, revealed that the B2 subunit of H(+)-ATPase acts upstream of MEK activation in the MEK/ERK pathway to ameliorate apoptosis.