Effect of acid/base balance on H-ATPase 31 kD subunit mRNA levels in collecting duct cells

Zebrafish as a Model System for Investigating the Compensatory Regulation of Ionic Balance during Metabolic Acidosis

Zebrafish (Danio rerio) have become an important model for integrative physiological research. Zebrafish inhabit a hypo-osmotic environment; to maintain ionic and acid-base homeostasis, they must actively take up ions and secrete acid to the water. The gills in the adult and the skin at larval stage are the primary sites of ionic regulation in zebrafish. The uptake of ions in zebrafish is mediated by specific ion transporting cells termed ionocytes. Similarly, in mammals, ion reabsorption and acid excretion occur in specific cell types in the terminal region of the renal tubules (distal convoluted tubule and collecting duct). Previous studies have suggested that functional regulation of several ion transporters/channels in the zebrafish ionocytes resembles that in the mammalian renal cells. Additionally, several mechanisms involved in regulating the epithelial ion transport during metabolic acidosis are found to be similar between zebrafish and mammals. In this article, we systemically review the similarities and differences in ionic regulation between zebrafish and mammals during metabolic acidosis. We summarize the available information on the regulation of epithelial ion transporters during acidosis, with a focus on epithelial Na⁺, Cl- and Ca2+ transporters in zebrafish ionocytes and mammalian renal cells. We also discuss the neuroendocrine responses to acid exposure, and their potential role in ionic compensation. Finally, we identify several knowledge gaps that would benefit from further study.

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".

The kidney-specific WNK1 isoform is induced by aldosterone and stimulates epithelial sodium channel-mediated Na+ transport

WNK1 belongs to a unique family of Ser/Thr kinases that have been implicated in the control of blood pressure. Intronic deletions in the WNK1 gene result in its overexpression and lead to pseudohypoaldosteronism type II, a disease with salt-sensitive hypertension and hyperkalemia. How overexpression of WNK1 leads to Na(+) retention and hypertension is not entirely clear. Similarly, there is no information on the hormonal regulation of expression of WNK kinases. There are two main WNK1 transcripts expressed in the kidney: the originally described "long" WNK1 and a shorter transcript that is specifically expressed in the kidney (KS-WNK1). The goal of this study was to determine the effect of aldosterone, the main hormonal regulator of Na(+) homeostasis, on the transcription of WNK1 isoforms in renal target cells, by using an unique mouse cortical collecting duct cell line that stably expresses functional mineralocorticoid receptors. Our results demonstrate that aldosterone, at physiological concentrations, rapidly induces the expression of the KS-WNK1 but not that of the long-WNK1 in these cells. Importantly, stable overexpression of KS-WNK1 significantly increases transepithelial Na(+) transport in cortical collecting duct cells. Similarly, coexpression of KS-WNK1 and the epithelial Na(+) channel in Fischer rat thyroid epithelial cells also stimulates Na(+) current, suggesting that KS-WNK1 affects the subcellular location or activity but not the expression of epithelial Na(+) channel. These observations suggest that stimulation of KS-WNK1 expression might be an important element of aldosterone-induced Na(+) retention and hypertension.

Gene regulation of ENaC subunits by serum- and glucocorticoid-inducible kinase-1

Aldosterone is a key regulator of epithelial Na+ channels (ENaC) in renal cortical collecting ducts (CCD). The goal of this study was to examine whether serum- and glucocorticoid-inducible kinase-1 (SGK1), an aldosterone-induced gene, is vital to the delayed effect of aldosterone by increasing the gene expression of ENaC subunits. To test this hypothesis, we compared the levels of ENaC mRNA in mouse CCD cells that stably express either full-length (FL)-SGK1 or a kinase-dead dominant negative (K127M)-SGK1. Our results revealed that SGK1 regulates gene expression of ENaC, whether cells are maintained in steroid-free media or in the presence of corticosteroids (CS) and/or other growth factors. Under all conditions, the loss of function of SGK1 caused a significant decrease in the expression of alpha- and beta-ENaC, but not gamma-ENaC. Compared with cells expressing FL-SGK1, K127M-SGK1 decreased the expression of alpha- and beta-subunit mRNA by approximately 45 and approximately 90%, respectively. Next, to determine whether SGK1 is one of the proteins mediating the induction of alpha-ENaC mRNA by CS, we compared steroid induction of alpha-ENaC in cells expressing K127M-SGK1 vs. FL-SGK1. The maximum level of alpha-ENaC mRNA levels following CS was significantly (approximately 45%) higher in FL-SGK1- vs. K127M-SGK1-expressing cells, although the fold-induction by CS was similar in both FL-SGK1- and K127M-SGK1-expressing cells. In summary, we report for the first time that SGK1 regulates transcription of ENaC subunits. We propose that the effect of SGK1 on ENaC transcription is mediated by the activation of unidentified transcription factors.

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.

Molecular cloning and characterization of Atp6v1b1, the murine vacuolar H+ -ATPase B1-subunit

The multisubunit vacuolar-type proton-translocating ATPases (H(+)-ATPases) mediate the acidification of various intracellular organelles. In a subset of tissues, they also mediate H(+) secretion at the plasma membrane. Two isoforms of the H(+)-ATPase B-subunit exist in humans; we have shown that mutations in ATP6V1B1, encoding the B1-isoform, cause the clinical condition distal renal tubular acidosis. Here we report the cloning and characterization of murine Atp6v1b1, which encodes a 513-amino acid (aa) protein with 93% identity to human ATP6V1B1. Genomic organization is conserved between the murine and human H(+)-ATPase B1-subunits, and Atp6v1b1 maps to a region of mouse chromosome 6 syntenic to human 2p13, the location of ATP6V1B1. Northern blotting detects a 2.2-kb Atp6v1b1 transcript in the kidney and testis, but not other major organs. In mouse kidney, the B1-subunit localizes to intercalated cells of the cortical and medullary collecting duct. B1 protein levels were not increased in either mouse renal cortex or medulla after either 2 or 7 days of oral acid loading. These results demonstrate that Atp6v1b1 encodes the murine ortholog of human ATP6V1B1 and provides a tool for future development of animal models based on manipulation of the Atp6v1b1 genomic locus.

Localization and Regulation of the ATP6V0A4 (a4) Vacuolar H+-ATPase Subunit Defective in an Inherited Form of Distal Renal Tubular Acidosis

ABSTRACT. Vacuolar-type H+-ATPases (V-H+-ATPases) are the major H+-secreting protein in the distal portion of the nephron and are involved in net H+secretion (bicarbonate generation) or H+reabsorption (net bicarbonate secretion). In addition, V-H+-ATPases are involved in HCO3−reabsorption in the proximal tubule and distal tubule. V-H+-ATPases consist of at least 13 subunits, the functions of which have not all been elucidated. Mutations in the accessory ATP6V0A4 (a4 isoform) subunit have recently been shown to cause an inherited form of distal renal tubular acidosis in humans. Here, the localization of this subunit in human and mouse kidney was studied and the regulation of expression and localization of this subunit in mouse kidney in response to acid-base and electrolyte intake was investigated. Reverse transcription-PCR on dissected mouse nephron segments amplified a4-specific transcripts in proximal tubule, loop of Henle, distal convoluted tubule, and cortical and medullary collecting duct. a4 protein was localized by immunohistochemistry to the apical compartment of the proximal tubule (S1/S2 segment), the loop of Henle, the intercalated cells of the distal convoluted tubule, the connecting segment, and all intercalated cells of the entire collecting duct in human and mouse kidney. All types of intercalated cells expressed a4. NH4Cl or NaHCO3loading for 24 h, 48 h, or 7 d as well as K+depletion for 7 and 14 d had no influence on a4 protein expression levels in either cortex or medulla as determined by Western blotting. Immunohistochemistry, however, demonstrated a subcellular redistribution of a4 in response to the different stimuli. NH4Cl and K+depletion led to a pronounced apical staining in the connecting segment, cortical collecting duct, and outer medullary collecting duct, whereas NaHCO3loading caused a stronger bipolar staining in the cortical collecting duct. Taken together, these results demonstrate a4 expression in the proximal tubule, loop of Henle, distal tubule, and collecting duct and suggest that under conditions in which increased V-H+-ATPase activity is required, a4 is regulated by trafficking but not protein expression. This may allow for the rapid adaptation of V-H+-ATPase activity to altered acid-base intake to achieve systemic pH homeostasis. The significance of a4 expression in the proximal tubule in the context of distal renal tubular acidosis will require further clarification.

Expression and immunolocalization of AQP6 in intercalated cells of the rat kidney collecting duct

The expression and localization of AQP6 were examined in rat kidneys. In the kidney compartments, the expression was more intense in the outer medulla than in the cortex or inner medulla, and was negative in the glomerulus. During development, the AQP6 mRNA expression in the kidney was not detected in the fetus, but was recognized at birth, increased gradually by 4 weeks of age, and was unchanged thereafter. In situ hybridization demonstrated significant signals for AQP6 mRNA along the outer and inner medullary collecting ducts. Since the localization of the AQP6 mRNA-expressing cells was comparable to that of immunoreactive H+ ATPase-bearing cells in the collecting duct, they were identified as intercalated cells. No AQP6 mRNA signals were recognizable in other cells in the kidneys, including glomerular cells. No glomerular expression of AQP6 mRNA was confirmed by RT-PCR using total RNA extracted from the glomeruli. Immunohistochemistry using an antibody raised against recombinant rat AQP6 protein could localize the immunoreactivity in a population of collecting duct cells. Serial section observations indicated that the AQP6-immunoreactive cells corresponded to H+ ATPase bearing intercalated cells.

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.

sgk is an aldosterone-induced kinase in the renal collecting duct. Effects on epithelial na+ channels

The early phase of the stimulatory effect of aldosterone on sodium reabsorption in renal epithelia is thought to involve activation of apical sodium channels. However, the genes initiating this effect are unknown. We used a combination of polymerase chain reaction-based subtractive hybridization and differential display techniques to identify aldosterone-regulated immediate early genes in renal mineralocorticoid target cells. We report here that aldosterone rapidly increases mRNA levels of a putative Ser/Thr kinase, sgk (or serum- and glucocorticoid-regulated kinase), in its native target cells, i.e. in cortical collecting duct cells. The effect occurs within 30 min of the addition of aldosterone, is mediated through mineralocorticoid receptors, and does not require de novo protein synthesis. The full-length sequences of rabbit and mouse sgk cDNAs were determined. Both cDNAs show significant homology to rat and human sgk (88-94% at the nucleotide level, and 96-99% at the amino acid level). Coexpression of the mouse sgk in Xenopus oocytes with the three subunits of the epithelial Na+ channel results in a significantly enhanced Na+ current. These results suggest that sgk is an immediate early aldosterone-induced gene, and this protein kinase plays an important role in the early phase of aldosterone-stimulated Na+ transport.

Ganglioside GM2-activator protein and vesicular transport in collecting duct intercalated cells

This study describes the molecular characterization of an antigen defined by an autoantibody from a woman with habitual abortion as GM2-activator protein. The patient showed no disorder of renal function. Accidentally with routine serum screening for autoantibodies, an immunoreactivity was found in kidney collecting duct intercalated cells. Three distinct patterns of immunostaining of intercalated cells were observed: staining of the apical pole, basolateral pole, and diffuse cytoplasmic labeling. Ultrastructurally, the immunoreactivity was associated with "studs," which represent the cytoplasmic domain of the vacuolar proton pump in intercalated cells. This pump is subjected to a shuttling mechanism from cytoplasmic stores to the cell membrane, which exclusively occurs in intercalated cells. Peptide sequences of a 23-kD protein purified from rat kidney cortex showed complete identity with corresponding sequences of GM2-activator protein. In the brain, GM2-activator protein is required for hexosaminidase A to split a sugar from ganglioside GM2. Because neither ganglioside GM2 nor GM1 (its precursor) is present in significant amounts in the kidney, the previous finding that this tissue contains the highest level of activator protein in the body was confusing. In this study, a novel role for GM2-activator protein in intercalated cells is proposed, and possible roles in the shuttling mechanism are discussed.

Adaptation of the outer medullary collecting duct to metabolic acidosis in vitro

Metabolic acidosis in vivo, as well as in vitro (1 h at pH 6.8 followed by 2 h at pH 7.4) stimulates H+-ATPase-dependent H+ secretion in outer medullary collecting ducts from the inner stripe (OMCDi) (S. Tsuruoka and G. J. Schwartz. J. Clin. Invest. 99: 1420-1431, 1997). Another group has shown that the adaptation to metabolic acidosis in vivo is mediated by an apical polarization of H+ pumps without an increase in total H+ pump mRNA or protein (B. Bastani, H. Purcell, P. Hemken, D. Trigg, and S. Gluck. J. Clin. Invest. 88: 126-136, 1991). To further address the mechanism of adaptation, we measured net HCO-3 absorption before and after applying protein/RNA synthesis and signal transduction inhibitors during the 1 h of low pH and a cytoskeletal inhibitor during the entire 3-h incubation. Net HCO-3 transport, measured by microcalorimetry, increased approximately 33% after in vitro acidosis. This increase was prevented by application during the first hour of anisomycin (10 microM) or actinomycin D (4 microM), but not by anisomycin applied during the 2-h incubation at pH 7.4. Similar results were obtained with the cell calcium chelator, 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM, 20 microM), the calmodulin antagonist, calmidazolium (30 nM), the endoplasmic reticulum Ca-ATPase inhibitor, thapsigargin (100 nM), and the protein kinase C (PKC) inhibitor, staurosporine (100 nM), applied during the 1 h at pH 6.8, but not with BAPTA-AM or thapsigargin used during the 2-h incubation at pH 7. 4. Colchicine (10 microM) applied during the entire 3-h incubation also prevented this adaptive increase in H+ secretion, whereas lumicolchicine (10 microM, the inactive congener) did not. Colchicine also reversibly prevented any adaptive increases in transepithelial positive voltage. Thus the adaptation to acidosis in vitro required RNA and protein synthesis, changes in intracellular calcium and PKC activity, and intact microtubules. Time was required for the adaptation to occur, as the increase in HCO-3 transport was small after <3-h incubation. Protein synthesis and changes in cell calcium were critical during the initial period of low pH but not once the acid stimulus had been removed. Exocytosis of H+ pumps appears to occur continually during the entire 3-h incubation. These data would suggest that the synthesis and regulation of proteins involved in shuttling H+ pumps in cytoplasmic vesicles to the apical membrane via exocytosis are important for the OMCDi to adapt to low pH in vitro and probably to metabolic acidosis in vivo.

Metabolic acidosis stimulates H+ secretion in the rabbit outer medullary collecting duct (inner stripe) of the kidney

The outer medullary collecting duct (OMCD) absorbs HCO3- at high rates, but it is not clear if it responds to metabolic acidosis to increase H+ secretion. We measured net HCO3- transport in isolated perfused OMCDs taken from deep in the inner stripes of kidneys from control and acidotic (NH4Cl-fed for 3 d) rabbits. We used specific inhibitors to characterize the mechanisms of HCO3- transport: 10 microM Sch 28080 or luminal K+ removal to inhibit P-type H+,K+-ATPase activity, and 5-10 nM bafilomycin A1 or 1-10 nM concanamycin A to inhibit H+-ATPase activity. The results were comparable using either of each pair of inhibitors, and allowed us to show in control rabbits that 65% of net HCO3- absorption depended on H+-ATPase (H flux), and 35% depended on H+,K+-ATPase (H,K flux). Tubules from acidotic rabbits showed higher rates of HCO3- absorption (16.8+/-0.3 vs. 12.8+/-0.2 pmol/min per mm, P < 0.01). There was no difference in the H,K flux (5.9+/-0.2 vs. 5.8+/-0.2 pmol/min per mm), whereas there was a 61% higher H flux in segments from acidotic rabbits (11.3+/-0.2 vs. 7.0+/-0.2 pmol/min per mm, P < 0.01). Transport was then measured in other OMCDs before and after incubation for 1 h at pH 6.8, followed by 2 h at pH 7.4 (in vitro metabolic acidosis). Acid incubation in vitro stimulated HCO3- absorption (12.3+/-0.3 to 16.2+/-0.3 pmol/min per mm, P < 0.01), while incubation at pH 7.4 for 3 h did not change basal rate (11.8+/-0.4 to 11.7+/-0.4 pmol/min per mm). After acid incubation the H,K flux did not change, (4.7+/-0.4 to 4.6+/-0.4 pmol/min per mm), however, there was a 60% increase in H flux (6.6+/-0.3 to 10.8+/-0.3 pmol/min per mm, P < 0.01). In OMCDs from acidotic animals, and in OMCDs incubated in acid in vitro, there was a higher basal rate and a further increase in HCO3- absorption (16.7+/-0.4 to 21.3+/-0.3 pmol/min per mm, P < 0.01) because of increased H flux (11.5+/-0.3 to 15.7+/-0.2 pmol/min per mm, P < 0.01) without any change in H,K flux (5.4+/-0.3 to 5.6+/-0.3 pmol/min per mm). These data indicate that HCO3- absorption (H+ secretion) in OMCD is stimulated by metabolic acidosis in vivo and in vitro by an increase in H+-ATPase-sensitive HCO3- absorption. The mechanism of adaptation may involve increased synthesis and exocytosis to the apical membrane of proton pumps. This adaptation helps maintain homeostasis during metabolic acidosis.

Cellular localization and regulation of CHIF in kidney and colon

Channel inducing factor (CHIF) is a novel cDNA recently cloned from a rat distal colon cDNA library of dexamethasone-treated animals. While its expression in Xenopus oocytes evokes a potassium channel activity similar to that induced by Isk (minK), its cellular role is not clear. CHIF exhibits significant homologies with proteins that are putatively regulatory (phospholemman, gamma-subunit of Na(+)-K(+)-ATPase, Mat-8) while it differs from the small-conductance potassium channel Isk. We have studied the tissue specificity of CHIF expression in rat by in situ hybridization. CHIF is selectively present in the distal parts of the nephron (medullary and papillary collecting ducts and end portions of cortical collecting tubule) and in the epithelial cells of the distal colon. No expression of CHIF was found in renal proximal tubule, loop of Henle and distal tubule, proximal colon, small intestine, lung, choroid plexus, salivary glands, or brain. To gain some insight into CHIF function, we have investigated, using in situ hybridization and ribonuclease protection assay, whether CHIF mRNA expression could be altered in some situations. In the distal colon, corticosteroid hormones, sodium restriction, low-potassium diet, and metabolic acidosis significantly increased CHIF mRNA expression. In the kidney, metabolic acidosis was the only condition that showed an increase in CHIF mRNA expression. Some of these treatments also altered the expression of the colonic H(+)-K(+)-ATPase mRNA. In summary, CHIF mRNA is selectively expressed in the medullary collecting duct of the kidney and in the epithelium of the distal colon; its expression varies differently in these two target tissues after alterations in corticosteroid status, potassium depletion, and metabolic acidosis. The precise cell-specific functions of CHIF remain to be established.