Plasticity in epithelial polarity of renal intercalated cells: targeting of the H(+)-ATPase and band 3

The study of intercalated cells using ex vivo techniques: primary cell culture, cell lines, kidney slices, and organoids

This review summarizes methods to study kidney intercalated cell (IC) function ex vivo. While important for acid-base homeostasis, IC dysfunction is often not recognized clinically until it becomes severe. The advantage of using ex vivo techniques is that they allow for the differential evaluation of IC function in controlled environments. Although in vitro kidney tubular perfusion is a classical ex vivo technique to study IC, here we concentrate on primary cell cultures, immortalized cell lines, and ex vivo kidney slices. Ex vivo techniques are useful in evaluating IC signaling pathways that allow rapid responses to extracellular changes in pH, CO2, and bicarbonate (HCO3-). However, these methods for IC work can also be challenging, as cell lines that recapitulate IC do not proliferate easily in culture. Moreover, a "pure" IC population in culture does not necessarily replicate its collecting duct (CD) environment, where ICs are surrounded by the more abundant principal cells (PCs). It is reassuring that many findings obtained in ex vivo IC systems signaling have been largely confirmed in vivo. Some of these newly identified signaling pathways reveal that ICs are important for regulating NaCl reabsorption, thus suggesting new frontiers to target antihypertensive treatments. Moreover, recent single-cell characterization studies of kidney epithelial cells revealed a dual developmental origin of IC, as well as the presence of novel CD cell types with certain IC characteristics. These exciting findings present new opportunities for the study of IC ex vivo and will likely rediscover the importance of available tools in this field.NEW & NOTEWORTHY The study of kidney intercalated cells has been limited by current cell culture and kidney tissue isolation techniques. This review is to be used as a reference to select ex vivo techniques to study intercalated cells. We focused on the use of cell lines and kidney slices as potential useful models to study membrane transport proteins. We also review how novel collecting duct organoids may help better elucidate the role of these intriguing cells.

The H+-ATPase (V-ATPase): from proton pump to signaling complex in health and disease

A primary function of the H⁺-ATPase (or V-ATPase) is to create an electrochemical proton gradient across eukaryotic cell membranes, which energizes fundamental cellular processes. Its activity allows for the acidification of intracellular vesicles and organelles, which is necessary for many essential cell biological events to occur. In addition, many specialized cell types in various organ systems such as the kidney, bone, male reproductive tract, inner ear, olfactory mucosa, and more, use plasma membrane V-ATPases to perform specific activities that depend on extracellular acidification. It is, however, increasingly apparent that V-ATPases are central players in many normal and pathophysiological processes that directly influence human health in many different and sometimes unexpected ways. These include cancer, neurodegenerative diseases, diabetes, and sensory perception, as well as energy and nutrient-sensing functions within cells. This review first covers the well-established role of the V-ATPase as a transmembrane proton pump in the plasma membrane and intracellular vesicles and outlines factors contributing to its physiological regulation in different cell types. This is followed by a discussion of the more recently emerging unconventional roles for the V-ATPase, such as its role as a protein interaction hub involved in cell signaling, and the (patho)physiological implications of these interactions. Finally, the central importance of endosomal acidification and V-ATPase activity on viral infection will be discussed in the context of the current COVID-19 pandemic.

Identifying drug targets in tissues and whole blood with thermal-shift profiling

Monitoring drug-target interactions with methods such as the cellular thermal-shift assay (CETSA) is well established for simple cell systems but remains challenging in vivo. Here we introduce tissue thermal proteome profiling (tissue-TPP), which measures binding of small-molecule drugs to proteins in tissue samples from drug-treated animals by detecting changes in protein thermal stability using quantitative mass spectrometry. We report organ-specific, proteome-wide thermal stability maps and derive target profiles of the non-covalent histone deacetylase inhibitor panobinostat in rat liver, lung, kidney and spleen and of the B-Raf inhibitor vemurafenib in mouse testis. In addition, we devised blood-CETSA and blood-TPP and applied it to measure target and off-target engagement of panobinostat and the BET family inhibitor JQ1 directly in whole blood. Blood-TPP analysis of panobinostat confirmed its binding to known targets and also revealed thermal stabilization of the zinc-finger transcription factor ZNF512. These methods will help to elucidate the mechanisms of drug action in vivo.

Mechanisms underlying the regulation of intracellular and luminal pH in vaginal epithelium

The vagina provides a characteristic low-Na⁺ and low-pH fluid microenvironment that is considered generally protective. Previous studies have shown that various types of epithelial cells harbor the capacity of intracellular pH (pHi) regulation. However, it remains elusive whether vaginal epithelium could actively regulate pHi by transporting acid-base ions. In this study, we verified that after transient exposure to NH₄ Cl, the pHi values could rapidly recover from acidification via Na⁺ -H⁺ exchanger (NHE), Na⁺ -HCO₃ ^- cotransporter (NBC), and carbonic anhydrase in human vaginal epithelial cell line VK2/E6E7. Positive expression of the main acid-base transporters including NHE1-2, NBCe1-2, and NBCn1 mRNA was also detected in VK2/E6E7 cells. Moreover, the in vivo study further showed that interfering with the function of V-type H⁺ -ATPase, NHE or NBC expressed in vagina impaired vaginal luminal pH homeostasis in rats. Taken together, our study reveals the property of pH regulation in vaginal epithelial cells, which might provide novel insights into the potential role of vaginal epithelium in the formation of the vaginal acidic microenvironment.

Intercalated Cells of the Kidney Collecting Duct in Kidney Physiology

The epithelium of the kidney collecting duct (CD) is composed mainly of two different types of cells with distinct and complementary functions. CD principal cells traditionally have been considered to have a major role in Na+ and water regulation, while intercalated cells (ICs) were thought to largely modulate acid-base homeostasis. In recent years, our understanding of IC function has improved significantly owing to new research findings. Thus, we now have a new model for CD transport that integrates mechanisms of salt and water reabsorption, K+ homeostasis, and acid-base status between principal cells and ICs. There are three main types of ICs (type A, type B, and non-A, non-B), which first appear in the late distal convoluted tubule or in the connecting segment in a species-dependent manner. ICs can be detected in CD from cortex to the initial part of the inner medulla, although some transport proteins that are key components of ICs also are present in medullary CD, cells considered inner medullary. Of the three types of ICs, each has a distinct morphology and expresses different complements of membrane transport proteins that translate into very different functions in homeostasis and contributions to CD luminal pro-urine composition. This review includes recent discoveries in IC intracellular and paracrine signaling that contributes to acid-base regulation as well as Na+, Cl-, K+, and Ca2+ homeostasis. Thus, these new findings highlight the potential role of ICs as targets for potential hypertension treatments.

Hyperkalemic Forms of Renal Tubular Acidosis: Clinical and Pathophysiological Aspects

In contrast to distal type I or classic renal tubular acidosis (RTA) that is associated with hypokalemia, hyperkalemic forms of RTA also occur usually in the setting of mild-to-moderate CKD. Two pathogenic types of hyperkalemic metabolic acidosis are frequently encountered in adults with underlying CKD. One type, which corresponds to some extent to the animal model of selective aldosterone deficiency (SAD) created experimentally by adrenalectomy and glucocorticoid replacement, is manifested in humans by low plasma and urinary aldosterone levels, reduced ammonium excretion, and preserved ability to lower urine pH below 5.5. This type of hyperkalemic RTA is also referred to as type IV RTA. It should be noted that the mere deficiency of aldosterone when glomerular filtration rate is completely normal only causes a modest decline in plasma bicarbonate which emphasizes the importance of reduced glomerular filtration rate in the development of the hyperchloremic metabolic acidosis associated with SAD. Another type of hyperkalemic RTA distinctive from SAD in which plasma aldosterone is not reduced is referred to as hyperkalemic distal renal tubular acidosis because urine pH cannot be reduced despite acidemia or after provocative tests aimed at increasing sodium-dependent distal acidification such as the administration of sodium sulfate or loop diuretics with or without concurrent mineralocorticoid administration. This type of hyperkalemic RTA (also referred to as voltage-dependent distal renal tubular acidosis) has been best described in patients with obstructive uropathy and resembles the impairment in both hydrogen ion and potassium secretion that are induced experimentally by urinary tract obstruction and when sodium transport in the cortical collecting tubule is blocked by amiloride.

Relative contribution of clear cells and principal cells to luminal pH in the mouse epididymis

While spermatozoa undergo epididymal maturation, they remain quiescent thanks to the establishment of a low luminal pH. This study is aimed at determining how epithelial cells lining the epididymal lumen work together to maintain and regulate this acidic milieu. In particular, we examined the relative contribution of clear cells (CCs) and principal cells (PCs) to this process. Functional analysis in the mouse cauda epididymidis (Cd) perfused in vivo showed that the pH of a control solution remained constant at pH 6.6 after perfusion through the Cd lumen. In contrast, the pH of both an acidic (pH 5.8) and alkaline (pH 7.8) perfusate was progressively restored toward the control acidic pH. Pharmacological studies indicated the contribution of cystic fibrosis transmembrane regulator, previously shown to be present in the apical membrane of PCs, to the recovery from an acidic pH of 5.8. In addition, we found that CCs and PCs equally contribute to the recovery from an alkaline of 7.8, via the H+ pumping vacuolar ATPase (V-ATPase) located in CCs, and the Na+/H+ exchanger type 3 (NHE3) located in PCs. Immunofluorescence labeling showed apical membrane accumulation of the V-ATPase in CCs at pH 7.8, and its internalization at pH 5.8 compared to pH 6.6. Immunofluorescence showed expression of NHE3, but absence of NHE2, in PCs located in the Cd. RT-PCR and western blotting showed expression of NHE3 in all epididymal regions. Luminal 8-(4-chlorophenylthio)adenosine 3΄,5΄-cyclic monophosphate (cpt-cAMP) partially inhibited luminal pH recovery from pH 7.8. However, cpt-cAMP induced an increase in V-ATPase apical membrane accumulation at this pH. Cell fractionation studies showed the apical accumulation of NHE3 from intracellular vesicles at pH 7.8 versus 6.6, and prevention of this effect by cpt-cAMP. These results indicate the participation of both CCs and PCs in the regulation of luminal pH in the epididymis. Our study also shows the dual role of PCs in HCO3− and H+ secretion, and that this switch from base to acid secretion depends on the luminal environment. Characterization of the respective roles of CCs and PCs in the regulation of the optimal luminal condition for epididymal sperm maturation should provide new frameworks for the evaluation and treatment of male infertility.

Changes in V-ATPase subunits of human urinary exosomes reflect the renal response to acute acid/alkali loading and the defects in distal renal tubular acidosis

In the kidney, final urinary acidification is achieved by V-ATPases expressed in type A intercalated cells. The B1 subunit of the V-ATPase is required for maximal urinary acidification, while the role of the homologous B2 subunit is less clear. Here we examined the effect of acute acid/alkali loading in humans on B1 and B2 subunit abundance in urinary exosomes in normal individuals and of acid loading in patients with distal renal tubular acidosis (dRTA). Specificities of B1 and B2 subunit antibodies were verified by yeast heterologously expressing human B1 and B2 subunits, and murine wild-type and B1-deleted kidney lysates. Acute ammonium chloride loading elicited systemic acidemia, a drop in urinary pH, and increased urinary ammonium excretion. Nadir urinary pH was achieved at four to five hours, and exosomal B1 abundance was significantly increased at two through six hours after ammonium chloride loading. After acute equimolar sodium bicarbonate loading, blood and urinary pH rose rapidly, with a concomitant reduction of exosomal B1 abundance within two hours, which remained lower throughout the test. In contrast, no change in exosomal B2 abundance was found following acid or alkali loading. In patients with inherited or acquired distal RTA, the urinary B1 subunit was extremely low or undetectable and did not respond to acid loading in urine, whereas no change in B2 subunit was found. Thus, both B1 and B2 subunits of the V-ATPase are detectable in human urinary exosomes, and acid and alkali loading or distal RTA cause changes in the B1 but not B2 subunit abundance in urinary exosomes.

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.

Vacuolar-ATPase (V-ATPase) Mediates Progesterone-Induced Uterine Fluid Acidification in Rats

We hypothesized that progesterone-induced decrease in uterine fluid pH involves V-ATPase. In this study, expression and functional activity of V-ATPase in uterus were investigated under progesterone influence. Ovariectomized adult female rats received subcutaneous injection of estradiol-17β (1 µg/kg/day) or progesterone (20 mg/kg/day) for 3 days or 3 days estradiol-17β followed by 3 days vehicle, progesterone, or estradiol-17β plus progesterone. Mifepristone, a progesterone receptor blocker, was concomitantly given to the rats which received progesterone. A day after last injection, rate of uterine fluid secretion, its HCO3 (-) concentration, and pH were determined via in vivo uterine perfusion in rats under anesthesia. V-ATPase inhibitor, bafilomycin, was introduced into the perfusion buffer, and changes in these parameters were observed. Expression of V-ATPase A1 and B1/2 proteins and mRNAs in uterus were quantified by Western blotting and real-time PCR, respectively. Distribution of these proteins was observed by immunohistochemistry. Our findings showed that under progesterone influence, uterine fluid secretion rate, HCO3 (-) concentration, and pH were significantly reduced. Administration of bafilomycin did not cause significant changes in fluid secretion rate; however, HCO3 (-) concentration and pH were significantly elevated. In parallel with these changes, expression of V-ATPase A1 and B1/2 proteins and mRNAs were significantly increased with these proteins highly distributed in uterine luminal and glandular epithelia. In conclusion, increased expression and functional activity of V-ATPase were most likely responsible for the decreased in uterine fluid pH observed under progesterone influence.

Collecting duct intercalated cell function and regulation

Intercalated cells are kidney tubule epithelial cells with important roles in the regulation of acid-base homeostasis. However, in recent years the understanding of the function of the intercalated cell has become greatly enhanced and has shaped a new model for how the distal segments of the kidney tubule integrate salt and water reabsorption, potassium homeostasis, and acid-base status. These cells appear in the late distal convoluted tubule or in the connecting segment, depending on the species. They are most abundant in the collecting duct, where they can be detected all the way from the cortex to the initial part of the inner medulla. Intercalated cells are interspersed among the more numerous segment-specific principal cells. There are three types of intercalated cells, each having distinct structures and expressing different ensembles of transport proteins that translate into very different functions in the processing of the urine. This review includes recent findings on how intercalated cells regulate their intracellular milieu and contribute to acid-base regulation and sodium, chloride, and potassium homeostasis, thus highlighting their potential role as targets for the treatment of hypertension. Their novel regulation by paracrine signals in the collecting duct is also discussed. Finally, this article addresses their role as part of the innate immune system of the kidney tubule.

Regulation of luminal acidification by the V-ATPase

Specialized cells in the body express high levels of V-ATPase in their plasma membrane and respond to hormonal and nonhormonal cues to regulate extracellular acidification. Mutations in or loss of some V-ATPase subunits cause several disorders, including renal distal tubular acidosis and male infertility. This review focuses on the regulation of V-ATPase-dependent luminal acidification in renal intercalated cells and epididymal clear cells, which are key players in these physiological processes.

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.

Role of BK channels in hypertension and potassium secretion

PURPOSE OF REVIEW

This review summarizes recent studies of hypertension associated with a defect in renal K excretion due to genetic deletions of various components of the large, Ca-activated K channel (BK), and describes new evidence and theories regarding K secretory roles of BK in intercalated cells.

RECENT FINDINGS

Isolated perfused tubule methods have revealed the importance of BK in flow-induced K secretion. Subsequently, mice with genetically deleted BK subunits revealed the complexities of BK-mediated K secretion. Deletion of BKα results in extreme aldosteronism, hypertension, and an absence of flow-induced K secretion. Deletion of the BKβ1 ancillary subunit results in decreased handling of a K load, increased plasma K, mild aldosteronism and hypertension that is exacerbated by a high K diet. Deletion of BKβ4 (β4KO) leads to insufficient K handling, high plasma K, fluid retention, but with milder hypertension. Fluid retention in β4KO may be the result of insufficient flow-induced secretion of adenosine triphosphate (ATP), which normally inhibits epithelial Na channels (ENaCs).

SUMMARY

Classical physiological analysis of electrolyte handling in knockout mice has enlightened our understanding of the mechanism of handling K loads by renal K channels. Studies have focused on the different roles of BK-α/β1 and BK-α/β4 in the kidney. BKβ1 hypertension may be a 'three-hit' hypertension, involving a K secretory defect, elevated production of aldosterone, and increased vascular tone. The disorders observed in BK knockout mice have shed new insights on the importance of proper renal K handling for maintaining volume balance and blood pressure.

AQP5 is expressed in type-B intercalated cells in the collecting duct system of the rat, mouse and human kidney

We screened human kidney-derived multipotent CD133+/CD24+ ARPCs for the possible expression of all 13 aquaporin isoforms cloned in humans. Interestingly, we found that ARPCs expressed both AQP5 mRNA and mature protein. This novel finding prompted us to investigate the presence of AQP5 in situ in kidney. We report here the novel finding that AQP5 is expressed in human, rat and mouse kidney at the apical membrane of type-B intercalated cells. AQP5 is expressed in the renal cortex and completely absent from the medulla. Immunocytochemical analysis using segment- and cell type-specific markers unambiguously indicated that AQP5 is expressed throughout the collecting system at the apical membrane of type-B intercalated cells, where it co-localizes with pendrin. No basolateral AQPs were detected in type-B intercalated cells, suggesting that AQP5 is unlikely to be involved in the net trans-epithelial water reabsorption occurring in the distal tubule. An intriguing hypothesis is that AQP5 may serve an osmosensor for the composition of the fluid coming from the thick ascending limb. Future studies will unravel the physiological role of AQP5 in the kidney.

Proliferation of acid-secretory cells in the kidney during adaptive remodelling of the collecting duct

The renal collecting duct adapts to changes in acid-base metabolism by remodelling and altering the relative number of acid or alkali secreting cells, a phenomenon termed plasticity. Acid secretory A intercalated cells (A-IC) express apical H(+)-ATPases and basolateral bicarbonate exchanger AE1 whereas bicarbonate secretory B intercalated cells (B-IC) express basolateral (and apical) H(+)-ATPases and the apical bicarbonate exchanger pendrin. Intercalated cells were thought to be terminally differentiated and unable to proliferate. However, a recent report in mouse kidney suggested that intercalated cells may proliferate and that this process is in part dependent on GDF-15. Here we extend these observations to rat kidney and provide a detailed analysis of regional differences and demonstrate that differentiated A-IC proliferate massively during adaptation to systemic acidosis. We used markers of proliferation (PCNA, Ki67, BrdU incorporation) and cell-specific markers for A-IC (AE1) and B-IC (pendrin). Induction of remodelling in rats with metabolic acidosis (with NH(4)Cl for 12 hrs, 4 and 7 days) or treatment with acetazolamide for 10 days resulted in a larger fraction of AE1 positive cells in the cortical collecting duct. A large number of AE1 expressing A-IC was labelled with proliferative markers in the cortical and outer medullary collecting duct whereas no labeling was found in B-IC. In addition, chronic acidosis also increased the rate of proliferation of principal collecting duct cells. The fact that both NH(4)Cl as well as acetazolamide stimulated proliferation suggests that systemic but not urinary pH triggers this response. Thus, during chronic acidosis proliferation of AE1 containing acid-secretory cells occurs and may contribute to the remodelling of the collecting duct or replace A-IC due to a shortened life span under these conditions.

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.

Mitochondria-rich cells in amphibian skin epithelium: Relationship of immuno- and peanut lectin labeling pattern and transport functions

Mitochondria-rich cells are an integral component of the epidermis of amphibian skin and play a functional role. Whereas the principal cell compartment of the epithelium is specialized almost exclusively for active uptake of sodium, the mitochondria-rich cells perform other diverse ion-transport functions, including transport of Cl(-), H(+), HCO(3)(-) and organic molecules. These transporting functions differ in different species. Antibodies, such as those directed against band 3, H(+)-ATPase, and also peanut lectin (PNA), bind specifically to the mitochondria-rich cells, but do so differently in various species. Examination of these immunolocalizations and lectin labeling in the skin of over 10 amphibian species, including both Anurans and Urodeles, illustrate species-specific differences. The binding pattern and the transport capabilities of the skin in the various species do not show a universal correlation, they appear to be species specific and do not permit construction of a general scheme common to all the species studied. The mitochondria-rich cells of heterocellular epithelia and their roles in ion transport remain a subject that requires further studies to elucidate their particular functions within the framework of the whole epithelium.

PKA regulates vacuolar H+-ATPase localization and activity via direct phosphorylation of the a subunit in kidney cells

The vacuolar H(+)-ATPase (V-ATPase) is a major contributor to luminal acidification in epithelia of Wolffian duct origin. In both kidney-intercalated cells and epididymal clear cells, cAMP induces V-ATPase apical membrane accumulation, which is linked to proton secretion. We have shown previously that the A subunit in the cytoplasmic V(1) sector of the V-ATPase is phosphorylated by protein kinase A (PKA). Here we have identified by mass spectrometry and mutagenesis that Ser-175 is the major PKA phosphorylation site in the A subunit. Overexpression in HEK-293T cells of either a wild-type (WT) or phosphomimic Ser-175 to Asp (S175D) A subunit mutant caused increased acidification of HCO(3)(-)-containing culture medium compared with cells expressing vector alone or a PKA phosphorylation-deficient Ser-175 to Ala (S175A) mutant. Moreover, localization of the S175A A subunit mutant expressed in HEK-293T cells was more diffusely cytosolic than that of WT or S175D A subunit. Acute V-ATPase-mediated, bafilomycin-sensitive H(+) secretion was up-regulated by a specific PKA activator in HEK-293T cells expressing WT A subunit in HCO(3)(-)-free buffer. In cells expressing the S175D mutant, V-ATPase activity at the membrane was constitutively up-regulated and unresponsive to PKA activators, whereas cells expressing the S175A mutant had decreased V-ATPase activity that was unresponsive to PKA activation. Finally, Ser-175 was necessary for PKA-stimulated apical accumulation of the V-ATPase in a polarized rabbit cell line of collecting duct A-type intercalated cell characteristics (Clone C). In summary, these results indicate a novel mechanism for the regulation of V-ATPase localization and activity in kidney cells via direct PKA-dependent phosphorylation of the A subunit at Ser-175.

Loss of the apical V-ATPase a-subunit VHA-6 prevents acidification of the intestinal lumen during a rhythmic behavior in C. elegans

In Caenorhabditis elegans, oscillations of intestinal pH contribute to the rhythmic defecation behavior, but the acid-base transport mechanisms that facilitate proton movement are not well understood. Here, we demonstrate that VHA-6, an intestine-specific a-subunit of the H(+)-K(+)-ATPase complex (V-ATPase), resides in the apical membrane of the intestinal epithelial cells and is required for luminal acidification. Disruption of the vha-6 gene led to early developmental arrest; the arrest phenotype could be complemented by expression of a fluorescently labeled vha-6 transgene. To study the contribution of vha-6 to pH homeostasis in larval worms, we used a partial reduction of function through postembryonic single-generation RNA interference. We demonstrate that the inability to fully acidify the intestinal lumen coincides with a defect in pH recovery of the intestinal epithelial cells, suggesting that VHA-6 is essential for proton pumping following defecation. Moreover, intestinal dipeptide accumulation and fat storage are compromised by the loss of VHA-6, suggesting that luminal acidification promotes nutrient uptake in worms, as well as in mammals. Since acidified intracellular vesicles and autofluorescent storage granules are indistinguishable between the vha-6 mutant and controls, it is likely that the nutrient-restricted phenotype is due to a loss of plasma membrane V-ATPase activity specifically. These data establish a simple genetic model for proton pump-driven acidification. Since defecation occurs at 45-s intervals in worms, this model represents an opportunity to study acute regulation of V-ATPase activity on a short time scale and may be useful in the study of alternative treatments for acid-peptic disorders.

Regulation of the V-ATPase in kidney epithelial cells: dual role in acid-base homeostasis and vesicle trafficking

The proton-pumping V-ATPase is a complex, multi-subunit enzyme that is highly expressed in the plasma membranes of some epithelial cells in the kidney, including collecting duct intercalated cells. It is also located on the limiting membranes of intracellular organelles in the degradative and secretory pathways of all cells. Different isoforms of some V-ATPase subunits are involved in the targeting of the proton pump to its various intracellular locations, where it functions in transporting protons out of the cell across the plasma membrane or acidifying intracellular compartments. The former process plays a critical role in proton secretion by the kidney and regulates systemic acid-base status whereas the latter process is central to intracellular vesicle trafficking, membrane recycling and the degradative pathway in cells. We will focus our discussion on two cell types in the kidney: (1) intercalated cells, in which proton secretion is controlled by shuttling V-ATPase complexes back and forth between the plasma membrane and highly-specialized intracellular vesicles, and (2) proximal tubule cells, in which the endocytotic pathway that retrieves proteins from the glomerular ultrafiltrate requires V-ATPase-dependent acidification of post-endocytotic vesicles. The regulation of both of these activities depends upon the ability of cells to monitor the pH and/or bicarbonate content of their extracellular environment and intracellular compartments. Recent information about these pH-sensing mechanisms, which include the role of the V-ATPase itself as a pH sensor and the soluble adenylyl cyclase as a bicarbonate sensor, will be addressed in this review.

GDF15 triggers homeostatic proliferation of acid-secreting collecting duct cells

Although adult kidney cells are quiescent, enlargement of specific populations of epithelial cells occurs during repair and adaptive processes. A prerequisite to the development of regenerative therapeutics is to identify the mechanisms and factors that control the size of specific populations of renal cells. Unfortunately, in most cases, it is unknown whether the growth of cell populations results from transdifferentiation or proliferation and whether proliferating cells derive from epithelial cells or from circulating or resident progenitors. In this study, the mechanisms underlying the enlargement of the acid-secreting cell population in the mouse kidney collecting duct in response to metabolic acidosis was investigated. Acidosis led to two phases of proliferation that preferentially affected the acid-secreting cells of the outer medullary collecting duct. All proliferating cells displayed polarized expression of functional markers. The first phase of proliferation, which started within 24 h and peaked at day 3, was dependent on the overexpression of growth differentiation factor 15 (GDF15) and cyclin D1 and was abolished when phosphatidylinositol-3 kinase and mammalian target of rapamycin were inhibited. During this phase, cells mostly divided along the tubular axis, contributing to tubule lengthening. The second phase of proliferation was independent of GDF15 but was associated with induction of cyclin D3. During this phase, cells divided transversely. In summary, acid-secreting cells proliferate as the collecting duct adapts to metabolic acidosis, and GDF15 seems to be an important determinant of collecting duct lengthening.

Genome-wide gene expression profiling reveals renal genes regulated during metabolic acidosis

Production and excretion of acids are balanced to maintain systemic acid-base homeostasis. During metabolic acidosis (MA) excess acid accumulates and is removed from the body, a process achieved, at least in part, by increasing renal acid excretion. This acid-secretory process requires the concerted regulation of metabolic and transport pathways, which are only partially understood. Chronic MA causes also morphological remodeling of the kidney. Therefore, we characterized transcriptional changes in mammalian kidney during MA to gain insights into adaptive pathways. Total kidney RNA from control and 2- and 7-days NH(4)Cl treated mice was subjected to microarray gene profiling. We identified 4,075 transcripts significantly (P < 0.05) regulated after 2 and/or 7 days of treatment. Microarray results were confirmed by qRT-PCR. Analysis of candidate genes revealed that a large group of regulated transcripts was represented by different solute carrier transporters, genes involved in cell growth, proliferation, apoptosis, water homeostasis, and ammoniagenesis. Pathway analysis revealed that oxidative phosphorylation was the most affected pathway. Interestingly, the majority of acutely regulated genes after 2 days, returned to normal values after 7 days suggesting that adaptation had occurred. Besides these temporal changes, we detected also differential regulation of selected genes (SNAT3, PEPCK, PDG) between early and late proximal tubule. In conclusion, the mammalian kidney responds to MA by temporally and spatially altering the expression of a large number of genes. Our analysis suggests that many of these genes may participate in various processes leading to adaptation and restoration of normal systemic acid-base and electrolyte homeostasis.

Thyroid hormone deficiency alters expression of acid-base transporters in rat kidney

Hypothyroidism in humans is associated with incomplete distal renal tubular acidosis, presenting as the inability to respond appropriately to an acid challenge by excreting less acid. Here, we induced hypothyroidism in rats with methimazole (HYPO) and in one group substituted with l-thyroxine (EU). After 4 wk, acid-base status was similar in both groups. However, after 24 h acid loading with NH(4)Cl HYPO rats displayed a more pronounced metabolic acidosis. The expression of the Na(+)/H(+) exchanger NHE3, the Na(+)-phosphate cotransporter NaPi-IIa, and the B2 subunit of the vacuolar H(+)-ATPase was reduced in the brush-border membrane of the proximal tubule of the HYPO group, paralleled by a lower abundance of the Na(+)/HCO(3)(-) cotransporter NBCe1 and a higher expression of the acid-secretory type A intercalated cell-specific Cl(-)/HCO(3)(-) exchanger AE1. In contrast to control conditions, the expression of NBCe1 was increased in the HYPO group during metabolic acidosis. In addition, net acid excretion was similar in both groups. The relative number of type A intercalated cells was increased in the connecting tubule and cortical collecting duct of the HYPO group during acidosis. Thus thyroid hormones modulate the renal response to an acid challenge and alter the expression of several key acid-base transporters. Mild hypothyroidism is associated only with a very mild defect in renal acid handling, which appears to be mainly located in the proximal tubule and is compensated by the distal nephron.

New insights into the regulation of V-ATPase-dependent proton secretion

The vacuolar H(+)-ATPase (V-ATPase) is a key player in several aspects of cellular function, including acidification of intracellular organelles and regulation of extracellular pH. In specialized cells of the kidney, male reproductive tract and osteoclasts, proton secretion via the V-ATPase represents a major process for the regulation of systemic acid/base status, sperm maturation and bone resorption, respectively. These processes are regulated via modulation of the plasma membrane expression and activity of the V-ATPase. The present review describes selected aspects of V-ATPase regulation, including recycling of V-ATPase-containing vesicles to and from the plasma membrane, assembly/disassembly of the two domains (V(0) and V(1)) of the holoenzyme, and the coupling ratio between ATP hydrolysis and proton pumping. Modulation of the V-ATPase-rich cell phenotype and the pathophysiology of the V-ATPase in humans and experimental animals are also discussed.

Epididymal expression of the forkhead transcription factor Foxi1 is required for male fertility

An essential aspect of male reproductive capacity is the immediate availability of fertilization-ready spermatozoa. To ensure this, most mammals rely on post-testicular sperm maturation. In epididymis, germ cells are matured and stored in a quiescent state that readily can be altered to produce active spermatozoa. This depends on active proton secretion into the epididymal lumen. We have identified Foxi1 as an important regulator of gene expression in narrow and clear cells-the major proton secretory cells of epididymal epithelia. Foxi1 appears to be required for the expression of the B1-subunit of the vacuolar H+ -ATPase proton pump and for carbonic anhydrase II as well as the chloride/bicarbonate transporter pendrin. Using transfection experiments, we have identified a Foxi1 binding cis-element in the ATP6V1B1 (encoding the B1-subunit) promoter that is critical for reporter gene activation. When this site is mutated to eliminate Foxi1 binding, activation is also abolished. As a consequence of defect Foxi1-dependent epididymal sperm maturation, we demonstrate that spermatozoa from Foxi1 null males fail to reach the female genital tract in sufficient number to allow fertilization.

Selective assembly of V-ATPase subunit isoforms in mouse kidney

The kidney plays vital roles in acid-base homeostasis, and the reabsorption of water, ions, and proteins. These processes are achieved through acidification of urine and endosomes of proximal tubule epithelial cells. Multisubunit vacuolar-type proton ATPase (V-ATPase) is one of the major acidification-machinery proteins that localizes to the apical or basolateral plasma membranes of intercalated cells in collecting ducts and the endosomal region at the base of brush border microvilli in proximal tubules. Multiple subunit isoforms of V-ATPase, which are expressed in kidney, have been identified. One obvious question is whether the pumps at different locations in the kidney have their own unique subunit identities. We have used a combination of methods to study this enzyme in kidney including immunocytochemical staining and immunoprecipitation analyses. The subunit isoforms of V-ATPase exhibited selective association/assembly in kidney: kidney-specific isoforms predominantly formed the intercalated cell proton pump, whereas the pump located in the brush border comprised ubiquitously expressed counterparts.

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".

Early, H+-V-ATPase-dependent proton flux is necessary for consistent left-right patterning of non-mammalian vertebrates

Biased left-right asymmetry is a fascinating and medically important phenomenon. We provide molecular genetic and physiological characterization of a novel, conserved, early, biophysical event that is crucial for correct asymmetry: H+ flux. A pharmacological screen implicated the H+-pump H+-V-ATPase in Xenopus asymmetry, where it acts upstream of early asymmetric markers. Immunohistochemistry revealed an actin-dependent asymmetry of H+-V-ATPase subunits during the first three cleavages. H+-flux across plasma membranes is also asymmetric at the four- and eight-cell stages, and this asymmetry requires H+-V-ATPase activity. Abolishing the asymmetry in H+ flux, using a dominant-negative subunit of the H+-V-ATPase or an ectopic H+ pump, randomized embryonic situs without causing any other defects. To understand the mechanism of action of H+-V-ATPase, we isolated its two physiological functions, cytoplasmic pH and membrane voltage (Vmem) regulation. Varying either pH or Vmem, independently of direct manipulation of H+-V-ATPase, caused disruptions of normal asymmetry, suggesting roles for both functions. V-ATPase inhibition also abolished the normal early localization of serotonin, functionally linking these two early asymmetry pathways. The involvement of H+-V-ATPase in asymmetry is conserved to chick and zebrafish. Inhibition of the H+-V-ATPase induces heterotaxia in both species; in chick, H+-V-ATPase activity is upstream of Shh; in fish, it is upstream of Kupffer's vesicle and Spaw expression. Our data implicate H+-V-ATPase activity in patterning the LR axis of vertebrates and reveal mechanisms upstream and downstream of its activity. We propose a pH- and Vmem-dependent model of the early physiology of LR patterning.

The V-type H+ ATPase: molecular structure and function, physiological roles and regulation

It was nearly 30 years before the V-type H+ ATPase was admitted to the small circle of bona fide transport ATPases alongside F-type and P-type ATPases. The V-type H+ ATPase is an ATP-driven enzyme that transforms the energy of ATP hydrolysis to electrochemical potential differences of protons across diverse biological membranes via the primary active transport of H+. In turn, the transmembrane electrochemical potential of H+ is used to drive a variety of (i) secondary active transport systems via H+-dependent symporters and antiporters and (ii) channel-mediated transport systems. For example, expression of Cl- channels or transporters next to the V-type H+ ATPase in vacuoles of plants and fungi and in lysosomes of animals brings about the acidification of the endosomal compartment, and the expression of the H+/neurotransmitter antiporter next to the V-type H+ ATPase concentrates neurotransmitters in synaptic vesicles. First found in association with endosomal membranes, the V-type H+ ATPase is now also found in increasing examples of plasma membranes where the proton pump energizes transport across cell membranes and entire epithelia. The molecular details reveal up to 14 protein subunits arranged in (i) a cytoplasmic V1 complex, which mediates the hydrolysis of ATP, and (ii) a membrane-embedded V0 complex, which translocates H+ across the membrane. Clever experiments have revealed the V-type H+ ATPase as a molecular motor akin to F-type ATPases. The hydrolysis of ATP turns a rotor consisting largely of one copy of subunits D and F of the V1 complex and a ring of six or more copies of subunit c of the V0 complex. The rotation of the ring is thought to deliver H+ from the cytoplasmic to the endosomal or extracellular side of the membrane, probably via channels formed by subunit a. The reversible dissociation of V1 and V0 complexes is one mechanism of physiological regulation that appears to be widely conserved from yeast to animal cells. Other mechanisms, such as subunit-subunit interactions or interactions of the V-type H+ ATPase with other proteins that serve physiological regulation, remain to be explored. Some diseases can now be attributed to genetic alterations of specific subunits of the V-type H+ ATPase.

Proton pump-rich cell secretes acid in skin of zebrafish larvae

The mammalian kidney excretes its metabolic acid load through the proton-transporting cells, intercalated cells, in the distal nephron and collecting duct. Fish excrete acid through external organs, gill, or skin; however, the cellular function is still controversial. In this study, molecular and electrophysiological approaches were used to identify a novel cell type secreting acid in skin of zebrafish ( Danio rerio) larvae. Among keratinocytes covering the larval surface, novel proton-secreting ionocytes, proton pump (H+-ATPase)-rich cells, were identified to generate strong outward H+flux. The present work demonstrates for the first time, with a noninvasive technique, H+-secreting cells in an intact animal model, the zebrafish, showing it to be a suitable model in which to study the functions of vertebrate transporting epithelia in vivo.

Role of acid/base transporters in the male reproductive tract and potential consequences of their malfunction

Acid/base transporters play a key role in establishing an acidic luminal environment for sperm maturation and storage in the male reproductive tract. Impairment of the acidification capacity of the epididymis, via either genetic mutations or exposure to environmental factors, may have profound consequences on male fertility.

Generation of a vector system facilitating cloning of DMBT1 variants and recombinant expression of functional full-length DMBT1

Deleted in malignant brain tumours 1 (DMBT1) codes for a approximately 340kDa glycoprotein with highly repetitive scavenger receptor cysteine-rich (SRCR) domains. DMBT1 was implicated in cancer, defence against viral and bacterial infections, and differentiation of epithelial cells. Recombinant expression and purification of DMBT1 is an essential step for systematic standardized functional research and towards the evaluation of its therapeutical potential. So far, DMBT1 is obtained from natural sources such as bronchioalveolar lavage or saliva, resulting in time consuming sample collection, low yields, and protein preparations which may substantially vary due to differential processing and genetic polymorphism, all of which impedes functional research on DMBT1. Cloning of DMBT1 cDNAs is hampered because of the size and the 13 highly homologous SRCR exons. In this study, we report on the setup of a vector system that facilitates cloning of DMBT1 variants. We demonstrate applicability of the vector system by expression of the largest DMBT1 variant in a tetracycline-inducible mammalian expression system using the Chinese hamster ovary cell line. Yields up to 30 mg rDMBT1 per litre of cell culture supernatant could be achieved with an optimized production procedure. By harnessing the specific bacteria-binding property of DMBT1 we established an affinity purification procedure which allows the isolation of more than 3 mg rDMBT1 with a purity of about 95%. Although the glycosylation moieties of rDMBT1 are different from DMBT1(SAG) isolated from saliva, we demonstrate that rDMBT1 is functionally active in aggregating Gram-positive and Gram-negative bacteria and binding to C1q and lactoferrin, which represent two known endogenous DMBT1 ligands.

V-ATPase B1-subunit promoter drives expression of EGFP in intercalated cells of kidney, clear cells of epididymis and airway cells of lung in transgenic mice

The kidney, epididymis, and lungs are complex organs with considerable epithelial cell heterogeneity. This has limited the characterization of pathophysiological transport processes that are specific for each cell type in these epithelia. The purpose of the present study was to develop new tools to study cell-specific gene and protein expression in such complex tissues and organs. We report the production of a transgenic mouse that expresses enhanced green fluorescent protein (EGFP) in a subset of epithelial cells that express the B1 subunit of vacuolar H(+)-ATPase (V-ATPase) and are actively involved in proton transport. A 6.5-kb portion of the V-ATPase B1 promoter was used to drive expression of EGFP. In two founders, quantitative real-time RT-PCR demonstrated expression of EGFP in kidney, epididymis, and lung. Immunofluorescence labeling using antibodies against the B1 and E subunits of V-ATPase and against carbonic anhydrase type II (CAII) revealed specific EGFP expression in all renal type A and type B intercalated cells, some renal connecting tubule cells, all epididymal narrow and clear cells, and some nonciliated airway epithelial cells. No EGFP expression was detected in collecting duct principal cells (identified using an anti-AQP2 antibody) or epididymal principal cells (negative for V-ATPase or CAII). This EGFP-expressing mouse model should prove useful in future studies of gene and protein expression and their physiological and/or developmental regulation in distinct cell types that can now be separated using fluorescence-assisted microdissection, fluorescence-activated cell sorting, and laser capture microdissection.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Plasticity of mouse renal collecting duct in response to potassium depletion

Plasticity of mouse renal collecting duct in response to potassium depletion. —Renal collecting ducts are the main sites for regulation of whole body potassium balance. Changes in dietary intake of potassium induce pleiotropic adaptations of collecting duct cells, which include alterations of ion and water transport properties along with an hypertrophic response. To study the pleiotropic adaptation of the outer medullary collecting duct (OMCD) to dietary potassium depletion, we combined functional studies of renal function (ion, water, and acid/base handling), analysis of OMCD hypertrophy (electron microscopy) and hyperplasia (PCNA labeling), and large scale analysis of gene expression (transcriptome analysis). The transcriptome of OMCD was compared in mice fed either a normal or a potassium-depleted diet for 3 days using serial analysis of gene expression (SAGE) adapted for downsized extracts. SAGE is based on the generation of transcript-specific tag libraries. Approximately 20,000 tags corresponding to 10,000 different molecular species were sequenced in each library. Among the 186 tags differentially expressed ( P < 0.05) between the two libraries, 120 were overexpressed and 66 were downregulated. The SAGE expression profile obtained in the control library was representative of different functional classes of proteins and of the two cell types (principal and α-intercalated cells) constituting the OMCD. Combined with gene expression analysis, results of functional and morphological studies allowed us to identify candidate genes for distinct physiological processes modified by potassium depletion: sodium, potassium, and water handling, hyperplasia and hypertrophy. Finally, comparison of mouse and human OMCD transcriptomes allowed us to address the question of the relevance of the mouse as a model for human physiology and pathophysiology.

Transport protein trafficking in polarized cells

In order to carry out their physiological functions, ion transport proteins must be targeted to the appropriate domains of cell membranes. Regulation of ion transport activity frequently involves the tightly controlled delivery of intracellular populations of transport proteins to the plasma membrane or the endocytic retrieval of transport proteins from the cell surface. Transport proteins carry signals embedded within their structures that specify their subcellular distributions and endow them with the capacity to participate in regulated membrane trafficking processes. Recently, a great deal has been learned about the biochemical nature of these signals, as well as about the cellular machinery that interprets them and acts upon their messages.

Decreased amiloride-sensitive Na+ absorption in collecting duct principal cells isolated from BPK ARPKD mice

The main feature of polycystic kidney diseases (PKD) is formation and progressive enlargement of renal cysts. Alterations in epithelial cell proliferation, extracellular matrix, and ion transport are thought to contribute to cyst enlargement and loss of renal function. Abnormal Cl- secretion is implicated in cyst enlargement in autosomal dominant PKD (ADPKD), but little is known about transport abnormalities in autosomal recessive PKD (ARPKD). We developed a method to isolate collecting duct (CD) principal cells (site of the lesion in ARPKD) from normal and ARPKD mice. A transgenic mouse (Hoxb7/GFP) in which enhanced green fluorescent protein (GFP) is expressed in CDs was bred with an ARPKD mouse (BPK), and GFP-positive cells from normal and cystic mice were selected by fluorescence-activated cell sorting. GFP-positive CD cells (>95 +/- 3%) obtained from either normal or cystic mice formed high-resistance, polarized epithelial monolayers. Expression patterns for marker proteins and the presence of a central cilium confirmed that the monolayers are composed of principal cells. Under basal conditions, the Cl- secretory responses elicited by elevation of cAMP or calcium were not significantly different between normal and cystic monolayers. In contrast, the amiloride-sensitive short-circuit current was significantly reduced in monolayers of cells isolated from cystic mice (12.9 +/- 1.6 microA/cm2; n = 10) compared with monolayers of cells isolated from normal mice (27.3 +/- 3.4 microA/cm2; n = 12). The results of these studies suggest that epithelial sodium channel-mediated sodium absorption is decreased in principal cells of ARPKD CD cysts and that the reduction in sodium absorption may contribute to the accumulation of luminal fluid.

Early embryonic expression of ion channels and pumps in chick and Xenopus development

An extensive body of literature implicates endogenous ion currents and standing voltage potential differences in the control of events during embryonic morphogenesis. Although the expression of ion channel and pump genes, which are responsible for ion flux, has been investigated in detail in nervous tissues, little data are available on the distribution and function of specific channels and pumps in early embryogenesis. To provide a necessary basis for the molecular understanding of the role of ion flux in development, we surveyed the expression of ion channel and pump mRNAs, as well as other genes that help to regulate membrane potential. Analysis in two species, chick and Xenopus, shows that several ion channel and pump mRNAs are present in specific and dynamic expression patterns in early embryos, well before the appearance of neurons. Examination of the distribution of maternal mRNAs reveals complex spatiotemporal subcellular localization patterns of transcripts in early blastomeres in Xenopus. Taken together, these data are consistent with an important role for ion flux in early embryonic morphogenesis; this survey characterizes candidate genes and provides information on likely embryonic contexts for their function, setting the stage for functional studies of the morphogenetic roles of ion transport.

AE4 is a DIDS-sensitive Cl(-)/HCO(-)(3) exchanger in the basolateral membrane of the renal CCD and the SMG duct

The renal cortical collecting duct (CCD) plays an important role in systemic acid-base homeostasis. The beta-intercalated cells secrete most of the HCO(-)(3), which is mediated by a luminal, DIDS-insensitive, Cl(-)/HCO(-)(3) exchange. The identity of the luminal exchanger is a matter of debate. Anion exchanger isoform 4 (AE4) cloned from the rabbit kidney was proposed to perform this function (Tsuganezawa H et al. J Biol Chem 276: 8180-8189, 2001). By contrast, it was proposed (Royaux IE et al. Proc Natl Acad Sci USA 98: 4221-4226, 2001) that pendrin accomplishes this function in the mouse CCD. In the present work, we cloned, localized, and characterized the function of the rat AE4. Northern blot and RT-PCR showed high levels of AE4 mRNA in the CCD. Expression in HEK-293 and LLC-PK(1) cells showed that AE4 is targeted to the plasma membrane. Measurement of intracellular pH (pH(i)) revealed that AE4 indeed functions as a Cl(-)/HCO(-)(3) exchanger. However, AE4 activity was inhibited by DIDS. Immunolocalization revealed species-specific expression of AE4. In the rat and mouse CCD and the mouse SMG duct AE4 was in the basolateral membrane. By contrast, in the rabbit, AE4 was in the luminal and lateral membranes. In both, the rat and rabbit CCD AE4 was in alpha-intercalated cells. Importantly, localization of AE4 was not affected by the systemic acid-base status of the rats. Therefore, we conclude that expression and possibly function of AE4 is species specific. In the rat and mouse AE4 functions as a Cl(-)/HCO(-)(3) exchanger in the basolateral membrane of alpha-intercalated cells and may participate in HCO(-)(3) absorption. In the rabbit AE4 may contribute to HCO(-)(3) secretion.

Inherited disorders of the H+-ATPase

PURPOSE OF REVIEW

The alpha-intercalated cell in the distal nephron shares a number of molecular features with the osteoclast, including site-limited proton pumps that are present at high density. These are multisubunit H -ATPases, which are essential for acid-base homeostasis and for the maintenance of normal bone turnover. In recent years it has become evident that some rare inherited human disorders are due to pump dysfunction in kidney or in bone; these are reviewed here.

RECENT FINDINGS

The present review provides an overview of acid secretion in both kidney and bone, and describes the recently identified diseases that are associated with mutations in tissue-specific subunits of these pumps.

SUMMARY

Elucidation of the molecular bases of a number of inherited renal acidopathies and bone disorders raises the possibility that additional tissue-specific subunits of these important pumps will be identified, gives hope for a better understanding of normal function at the molecular level, and may have implications for future therapeutic development.

The organizing potential of sphingolipids in intracellular membrane transport

Eukaryotes are characterized by endomembranes that are connected by vesicular transport along secretory and endocytic pathways. The compositional differences between the various cellular membranes are maintained by sorting events, and it has long been believed that sorting is based solely on protein-protein interactions. However, the central sorting station along the secretory pathway is the Golgi apparatus, and this is the site of synthesis of the sphingolipids. Sphingolipids are essential for eukaryotic life, and this review ascribes the sorting power of the Golgi to its capability to act as a distillation apparatus for sphingolipids and cholesterol. As Golgi cisternae mature, ongoing sphingolipid synthesis attracts endoplasmic reticulum-derived cholesterol and drives a fluid-fluid lipid phase separation that segregates sphingolipids and sterols from unsaturated glycerolipids into lateral domains. While sphingolipid domains move forward, unsaturated glycerolipids are retrieved by recycling vesicles budding from the sphingolipid-poor environment. We hypothesize that by this mechanism, the composition of the sphingolipid domains, and the surrounding membrane changes along the cis-trans axis. At the same time the membrane thickens. These features are recognized by a number of membrane proteins that as a consequence of partitioning between domain and environment follow the domains but can enter recycling vesicles at any stage of the pathway. The interplay between protein- and lipid-mediated sorting is discussed.

Molecular physiology of the renal chloride-formate exchanger

Renal apical chloride-base exchangers are essential to electrolyte and acid-base homeostasis. Different functional isoforms of apical anion exchangers have been identified in kidney proximal tubule and cortical collecting duct. Included amongst these are the following: chloride-formate, chloride-oxalate, and chloride-hydroxyl exchangers in proximal tubule; and chloride-bicarbonate exchanger in cortical collecting duct. Chloride-formate exchange, which was first identified in kidney proximal tubule, works in parallel with the apical sodium-hydrogen exchanger, and is thought to reabsorb the bulk of luminal chloride. Despite numerous studies, the molecular identities of apical chloride-base exchangers have remained unknown. Recent studies have identified a new class of anion exchangers, including pendrin (encoded by the PDS gene) and downregulated in adenoma (DRA, encoded by the DRA gene). Pendrin is expressed in the kidney, whereas DRA is not. Functional studies indicate that pendrin can function in chloride-formate and chloride-base exchange modes. It is unlikely that pendrin is the apical chloride-formate exchanger in the kidney proximal tubule. However, it is the only molecule that has been shown to mediate chloride-formate exchange. In the present review, recent studies regarding the renal distribution and membrane localization of pendrin, and its functional properties, including its roles in chloride reabsorption and base excretion, are addressed.

Pendrin, encoded by the Pendred syndrome gene, resides in the apical region of renal intercalated cells and mediates bicarbonate secretion

Pendrin is an anion transporter encoded by the PDS/Pds gene. In humans, mutations in PDS cause the genetic disorder Pendred syndrome, which is associated with deafness and goiter. Previous studies have shown that this gene has a relatively restricted pattern of expression, with PDS/Pds mRNA detected only in the thyroid, inner ear, and kidney. The present study examined the distribution and function of pendrin in the mammalian kidney. Immunolocalization studies were performed using anti-pendrin polyclonal and monoclonal antibodies. Labeling was detected on the apical surface of a subpopulation of cells within the cortical collecting ducts (CCDs) that also express the H(+)-ATPase but not aquaporin-2, indicating that pendrin is present in intercalated cells of the CCD. Furthermore, pendrin was detected exclusively within the subpopulation of intercalated cells that express the H(+)-ATPase but not the anion exchanger 1 (AE1) and that are thought to mediate bicarbonate secretion. The same distribution of pendrin was observed in mouse, rat, and human kidney. However, pendrin was not detected in kidneys from a Pds-knockout mouse. Perfused CCD tubules isolated from alkali-loaded wild-type mice secreted bicarbonate, whereas tubules from alkali-loaded Pds-knockout mice failed to secrete bicarbonate. Together, these studies indicate that pendrin is an apical anion transporter in intercalated cells of CCDs and has an essential role in renal bicarbonate secretion.

Remodeling the cellular profile of collecting ducts by chronic carbonic anhydrase inhibition

Factors regulating the differentiated phenotype of principal cells (PC) and A- and B-intercalated cells (IC) in kidney collecting ducts are poorly understood. However, we have shown previously that carbonic anhydrase II (CAII)-deficient mice have no IC in their medullary collecting ducts, suggesting a potential role for this enzyme in determining the cellular composition of this tubule segment. We now report that the cellular profile of the collecting ducts of adult rats can be remodeled by inhibiting CA activity in rats by using osmotic pumps containing acetazolamide. The 31-kDa subunit of the vacuolar H(+)-ATPase, the sodium/hydrogen exchanger regulatory factor NHE-RF, and the anion exchanger AE1 were used to identify IC subtypes by immunofluorescence staining, while aquaporin 2 and aquaporin 4 were used to identify PC. In the cortical collecting ducts of animals treated with acetazolamide for 2 wk, the percentage of B-IC decreased significantly (18 +/- 2 vs. 36 +/- 4%, P < 0.01) whereas the percentage of A-IC increased (82 +/- 2 vs. 64 +/- 4%, P < 0.01) with no change in the percentage of total IC in the epithelium. In some treated rats, B-IC were virtually undetectable. In the inner stripe of the outer medulla, the percentage of IC increased in treated animals (48 +/- 2 vs. 37 +/- 3%, P < 0.05) and the percentage of PC decreased (52 +/- 2 vs. 63 +/- 3%, P < 0.05). Moreover, IC appeared bulkier, protruded into the lumen, and showed a significant increase in the length of their apical (20.8 +/- 0.5 vs. 14.6 +/- 0.4 microm, P < 0.05) and basolateral membranes (25.8 +/- 0.4 vs. 23.8 +/- 0.5 microm, P < 0.05) compared with control rats. In the inner medullary collecting ducts of treated animals, the number of IC in the proximal third of the papilla was reduced compared with controls (11 +/- 4 vs. 40 +/- 11 IC/mm(2), P < 0.05). These data suggest that CA activity plays an important role in determining the differentiated phenotype of medullary collecting duct epithelial cells and that the cellular profile of collecting ducts can be remodeled even in adult rats. The relative depletion of cortical B-IC and the relative increase in number and hyperplasia of A-IC in the medulla may be adaptive processes that would tend to correct or stabilize the metabolic acidosis that would otherwise ensue following systemic carbonic anhydrase inhibition.

The B1 subunit of the H+ATPase is a PDZ domain-binding protein. Colocalization with NHE-RF in renal B-intercalated cells

The 56-kDa B1 subunit of the vacuolar H(+)ATPase has a C-terminal DTAL amino acid motif typical of PDZ-binding proteins that associate with the PDZ protein, NHE-RF (Na(+)/H(+) exchanger regulatory factor). This B1 isoform is amplified in renal intercalated cells, which play a role in distal urinary acid-base transport. In contrast, proximal tubules express the B2 isoform that lacks the C-terminal PDZ-binding motif. Both the B1 56-kDa subunit and the 31-kDa (E) subunit of the H(+)ATPase are pulled down by glutathione S-transferase NHE-RF bound to GSH-Sepharose beads. These subunits associate in vivo as part of the cytoplasmic V1 portion of the H(+)ATPase, and the E subunit was co-immunoprecipitated from rat kidney cytosol with NHE-RF antibodies. The interaction of H(+)ATPase subunits with NHE-RF was inhibited by a peptide derived from the C terminus of the B1 but not the B2 isoform. NHE-RF colocalized with H(+)ATPase in either the apical or the basolateral region of B-type intercalated cells, whereas NHE-RF staining was undetectable in A-intercalated cells. In proximal tubules, NHE-RF was located in the apical brush border. In contrast, H(+)ATPase was concentrated in a distinct membrane domain at the base of the brush border, from which NHE-RF was absent, consistent with the expression of the truncated B2 subunit isoform in this tubule segment. The colocalization of NHE-RF and H(+)ATPase in B- but not A-intercalated cells suggests a role in generating, maintaining, or modulating the variable H(+)ATPase polarity that characterizes the B-cell phenotype.