Essential role of NHE3 in facilitating formate-dependent NaCl absorption in the proximal tubule

The Na+/H+ Exchanger 3 in the Intestines and the Proximal Tubule of the Kidney: Localization, Physiological Function, and Key Roles in Angiotensin II-Induced Hypertension

The sodium (Na⁺)/hydrogen (H⁺) exchanger 3 (NHE3) is one of the most important Na⁺/H⁺ antiporters in the small intestines of the gastrointestinal tract and the proximal tubules of the kidney. The roles of NHE3 in the regulation of intracellular pH and acid-base balance have been well established in cellular physiology using in vitro techniques. Localized primarily on the apical membranes in small intestines and proximal tubules, the key action of NHE3 is to facilitate the entry of luminal Na⁺ and the extrusion of intracellular H⁺ from intestinal and proximal tubule tubular epithelial cells. NHE3 is, directly and indirectly, responsible for absorbing the majority of ingested Na⁺ from small and large intestines and reabsorbing >50% of filtered Na⁺ in the proximal tubules of the kidney. However, the roles of NHE3 in the regulation of proximal tubular Na⁺ transport in the integrative physiological settings and its contributions to the basal blood pressure regulation and angiotensin II (Ang II)-induced hypertension have not been well studied previously due to the lack of suitable animal models. Recently, novel genetically modified mouse models with whole-body, kidney-specific, or proximal tubule-specific deletion of NHE3 have been generated by us and others to determine the critical roles and underlying mechanisms of NHE3 in maintaining basal body salt and fluid balance, blood pressure homeostasis, and the development of Ang II-induced hypertension at the whole-body, kidney, or proximal tubule levels. The objective of this invited article is to review, update, and discuss recent findings on the critical roles of intestinal and proximal tubule NHE3 in maintaining basal blood pressure homeostasis and their potential therapeutic implications in the development of angiotensin II (Ang II)-dependent hypertension.

Glucose-induced decrease of cystathionine β-synthase mediates renal injuries

Exogenous hydrogen sulfide (H₂ S) protects kidneys from diabetic injuries in animal models. In order to explore the role of endogenous H₂ S in diabetic nephropathy, we determined the renal H2S producing enzymes in vivo and in vitro. In diabetic mice, H₂ S levels in blood and kidney were decreased while cystathionine β-synthase (CBS), mainly located in mouse renal proximal convoluted tubules (PCT), was reduced selectively. In cultured mouse PCT cells treated with high glucose, CBS protein and activity was reduced while ubiquitinated CBS was increased, which was abolished by a proteasome inhibitor MG132 at 1 hour; high glucose drove CBS colocalized with proteasome 26S subunit ATPase6, indicating an involvement of ubiquitination proteasome degradation. At 48 hours, high glucose also selectively decreased CBS protein, concentration-dependently, but increased the ubiquitination of CBS; silence of CBS by siRNA increased nitrotyrosine, a marker for protein oxidative injury. Nitrotyrosine was also increased by high glucose treatments. The increases of nitrotyrosine either by cbs-siRNA or by glucose were restored by GYY4137, indicating that the H₂ S donor may protect kidney from oxidative injury induced by CBS deficiency. In diabetic kidneys, ubiquitinated CBS and nitrotyrosine were increased but restored by GYY4137. The treatment also ameliorated albuminuria and renal morphologic changes in diabetic mice. Our findings suggest that high glucose induces reduction of renal CBS protein and activity in vivo and in vitro that is critical to the pathogenesis of diabetic kidney disease.

Molecular mechanisms and regulation of urinary acidification

The H(+) concentration in human blood is kept within very narrow limits, ~40 nmol/L, despite the fact that dietary metabolism generates acid and base loads that are added to the systemic circulation throughout the life of mammals. One of the primary functions of the kidney is to maintain the constancy of systemic acid-base chemistry. The kidney has evolved the capacity to regulate blood acidity by performing three key functions: (i) reabsorb HCO3(-) that is filtered through the glomeruli to prevent its excretion in the urine; (ii) generate a sufficient quantity of new HCO3(-) to compensate for the loss of HCO3(-) resulting from dietary metabolic H(+) loads and loss of HCO3(-) in the urea cycle; and (iii) excrete HCO3(-) (or metabolizable organic anions) following a systemic base load. The ability of the kidney to perform these functions requires that various cell types throughout the nephron respond to changes in acid-base chemistry by modulating specific ion transport and/or metabolic processes in a coordinated fashion such that the urine and renal vein chemistry is altered appropriately. The purpose of the article is to provide the interested reader with a broad review of a field that began historically ~60 years ago with whole animal studies, and has evolved to where we are currently addressing questions related to kidney acid-base regulation at the single protein structure/function level.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Chloride transport

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

Determinants of kidney oxygen consumption and their relationship to tissue oxygen tension in diabetes and hypertension

The high renal oxygen (O(2) ) demand is associated primarily with tubular O(2) consumption (Qo(2) ) necessary for solute reabsorption. Increasing O(2) delivery relative to demand via increased blood flow results in augmented tubular electrolyte load following elevated glomerular filtration, which, in turn, increases metabolic demand. Consequently, elevated kidney metabolism results in decreased tissue oxygen tension. The metabolic efficiency for solute transport (Qo(2) /T(Na) ) varies not only between different nephron sites, but also under different conditions of fluid homeostasis and disease. Contributing mechanisms include the presence of different Na(+) transporters, different levels of oxidative stress and segmental tubular dysfunction. Sustained hyperglycaemia results in increased kidney Qo(2) , partly due to mitochondrial dysfunction and reduced electrolyte transport efficiency. This results in intrarenal tissue hypoxia because the increased Qo(2) is not matched by a similar increase in O(2) delivery. Hypertension leads to renal hypoxia, mediated by increased angiotensin receptor tonus and oxidative stress. Reduced uptake in the proximal tubule increases load to the thick ascending limb. There, the increased load is reabsorbed, but at greater O(2) cost. The combination of hypertension, angiotensin II and oxidative stress initiates events leading to renal damage and reduced function. Tissue hypoxia is now recognized as a unifying pathway to chronic kidney disease. We have gained good knowledge about major changes in O(2) metabolism occurring in diabetic and hypertensive kidneys. However, further efforts are needed to elucidate how these alterations can be prevented or reversed before translation into clinical practice.

Proximal tubule Na+/H+ exchanger activity in adult NHE8-/-, NHE3-/-, and NHE3-/-/NHE8-/- mice

NHE3 is the predominant Na(+)/H(+) exchanger on the brush-border membrane (BBM) of the proximal tubule in adults. However, NHE3 null mice still have significant renal BBM Na(+)/H(+) activity. NHE8 has been localized to the BBM of proximal tubules and is more highly expressed in neonates than adult animals. The relative role of NHE8 in adult renal H(+) transport is unclear. This study examined whether there was compensation by NHE8 in NHE3(-/-) mice and by NHE3 in NHE8(-/-) mice. NHE3(-/-) mice had significant metabolic acidosis, and renal BBM NHE8 protein abundance was greater in NHE3(-/-) mice than control mice, indicating that there may be compensation by NHE8 in NHE3(-/-) mice. NHE8(-/-) mice had serum bicarbonate levels and pH that were not different from controls. NHE3 protein expression on the BBM was greater in NHE8(-/-) mice than in wild-type mice, indicating that there may be compensation by NHE3 in NHE8(-/-) mice. Both BBM NHE3 and NHE8 protein abundance increased in response to acidosis. Blood pressure and Na(+)/H(+) exchanger activity were comparable in NHE8(-/-) mice to that of controls, but both were significantly lower in NHE3(-/-) mice compared with control mice. Compared with NHE3(-/-) mice, NHE3(-/-)/NHE8(-/-) mice had lower blood pressures. While serum bicarbonate was comparable in NHE3(-/-) mice and NHE3(-/-)/NHE8(-/-) mice, proximal tubule Na(+)/H(+) exchange activity was less in NHE3(-/-)/NHE8(-/-) mice compared with NHE3(-/-) mice. In conclusion, NHE3 is the predominant Na(+)/H(+) exchanger in adult mice. NHE8 may play a compensatory role in renal acidification and blood pressure regulation in NHE3(-/-) mice.

Renal outer medullary potassium channel knockout models reveal thick ascending limb function and dysfunction

The renal outer medullary potassium channel (ROMK) is an adenosine triphosphate-sensitive inward-rectifier potassium channel (Kir1.1 or KCNJ1) highly expressed in the cortical and medullary thick ascending limbs (TAL), connecting segment (CNT) and cortical collecting duct (CCD) in the mammalian kidney, where it serves to recycle potassium (K(+)) across the apical membrane in TAL and to secrete K(+) in the CNT and CCD. ROMK channel mutations cause type II Bartter's syndrome with salt wasting and dehydration, and ROMK knockout mice display a similar phenotype of Bartter's syndrome in humans. Studies from ROMK null mice indicate that ROMK is required to form both the small-conductance (30pS, SK) K channels and the 70pS (IK) K channels in the TAL. The availability of ROMK(-/-) mice has made it possible to study electrolyte transport along the nephron in order to understand the TAL function under physiological conditions and the compensatory mechanisms of salt and water transport under the conditions of TAL dysfunction. This review summarizes previous progress in the study of K(+) channel activity in the TAL and CCD, ion transporter expression and activities along the nephron, and renal functions under physiological and pathophysiological conditions using ROMK(-/-) mice.

Loss of the NHE2 Na+/H+ exchanger in mice results in dilation of folliculo-stellate cell canaliculi

Genetic ablation of the NHE2 Na⁺/H⁺ exchanger causes gastric achlorhydria, absorptive defects in kidney and colon, and low fertility. Here we show that NHE2 is expressed in the pituitary, with the highest mRNA expression in pars distalis and lower expression in pars intermedia. In pars distalis of NHE2-null mice, prominent cyst-like dilatations of folliculo-stellate (FS) cell canaliculi developed with age, and there were increased FS cell area, accumulation of lipid in FS cell cytoplasm, redundancies in FS cell basement membrane, and other changes. The expansion of the canaliculi indicates that NHE2 is a major absorptive Na⁺/H⁺ exchanger in the luminal membranes lining the extensive network of channels formed by FS cells, which may provide a means of intrapituitary communication. The results suggest that NHE2 contributes to homeostatic regulation of the volume and composition of the canalicular fluid and may counter the secretory activity of the CFTR Cl⁻ channel, which is known to be expressed in pituitary.

Reduced NHE3-mediated Na+ absorption increases survival and decreases the incidence of intestinal obstructions in cystic fibrosis mice

In cystic fibrosis, impaired secretion resulting from loss of activity of the cystic fibrosis transmembrane conductance regulator (CFTR) causes dehydration of intestinal contents and life-threatening obstructions. Conversely, impaired absorption resulting from loss of the NHE3 Na+/H+ exchanger causes increased fluidity of the intestinal contents and diarrhea. To test the hypothesis that reduced NHE3-mediated absorption could increase survival and prevent some of the intestinal pathologies of cystic fibrosis, Cftr/Nhe3 double heterozygous mice were mated and their offspring analyzed. Cftr-null mice lacking one or both copies of the NHE3 gene exhibited increased fluidity of their intestinal contents, which prevented the formation of obstructions and increased survival. Goblet cell hyperplasia was eliminated, but not the accumulation of Paneth cell granules or increased cell proliferation in the crypts. Microarray analysis of small intestine RNA from Cftr-null, NHE3-null, and double-null mice all revealed downregulation of genes involved in xenobiotic metabolism, including a cohort of genes involved in glutathione metabolism. Expression of energy metabolism genes was altered, but there were no changes in genes involved in inflammation. Total intracellular glutathione was increased in the jejunum of all of the mutants and the ratio of reduced to oxidized glutathione was reduced in Cftr-null mutants, indicating that CFTR deficiency affects intestinal glutathione metabolism. The data establish a major role for NHE3 in regulating the fluidity of the intestinal contents and show that reduced NHE3-mediated absorption reverses some of the intestinal pathologies of cystic fibrosis, thus suggesting that it may serve as a potential therapeutic target.

Reduced NHE3-mediated Na+ absorption increases survival and decreases the incidence of intestinal obstructions in cystic fibrosis mice

In cystic fibrosis, impaired secretion resulting from loss of activity of the cystic fibrosis transmembrane conductance regulator (CFTR) causes dehydration of intestinal contents and life-threatening obstructions. Conversely, impaired absorption resulting from loss of the NHE3 Na+/H+ exchanger causes increased fluidity of the intestinal contents and diarrhea. To test the hypothesis that reduced NHE3-mediated absorption could increase survival and prevent some of the intestinal pathologies of cystic fibrosis, Cftr/Nhe3 double heterozygous mice were mated and their offspring analyzed. Cftr-null mice lacking one or both copies of the NHE3 gene exhibited increased fluidity of their intestinal contents, which prevented the formation of obstructions and increased survival. Goblet cell hyperplasia was eliminated, but not the accumulation of Paneth cell granules or increased cell proliferation in the crypts. Microarray analysis of small intestine RNA from Cftr-null, NHE3-null, and double-null mice all revealed downregulation of genes involved in xenobiotic metabolism, including a cohort of genes involved in glutathione metabolism. Expression of energy metabolism genes was altered, but there were no changes in genes involved in inflammation. Total intracellular glutathione was increased in the jejunum of all of the mutants and the ratio of reduced to oxidized glutathione was reduced in Cftr-null mutants, indicating that CFTR deficiency affects intestinal glutathione metabolism. The data establish a major role for NHE3 in regulating the fluidity of the intestinal contents and show that reduced NHE3-mediated absorption reverses some of the intestinal pathologies of cystic fibrosis, thus suggesting that it may serve as a potential therapeutic target.

Regulation of anion exchanger Slc26a6 by protein kinase C

SLC26A6 (CFEX, PAT1) is an anion exchanger expressed in several tissues including renal proximal tubule, pancreatic duct, small intestine, liver, stomach, and heart. It has recently been reported that PKC activation inhibits A6-mediated Cl/HCO3exchange by disrupting binding of carbonic anhydrase to A6. However, A6 can operate in HCO3-independent exchange modes of physiological importance, as A6-mediated Cl/oxalate exchange plays important roles in proximal tubule NaCl reabsorption and intestinal oxalate secretion. We therefore examined whether PKC activation affects HCO3-independent exchange modes of Slc26a6 functionally expressed in Xenopus oocytes. We found that PKC activation inhibited Cl/formate exchange mediated by Slc26a6 but failed to inhibit the related anion exchanger pendrin (SLC26A4) under identical conditions. PKC activation inhibited Slc26a6-mediated Cl/formate exchange, Cl/oxalate exchange, and Cl/Cl exchange to a similar extent. The inhibitor sensitivity profile and the finding that PMA-induced inhibition was calcium independent suggested a potential role for PKC-δ. Indeed, the PKC-δ-selective inhibitor rottlerin significantly blocked PMA-induced inhibition of Slc26a6 activity. Localization of Slc26a6 by immunofluorescence microscopy demonstrated that exposure to PKC activation led to redistribution of Slc26a6 from the oocyte plasma membrane to the intracellular compartment immediately below it. We also observed that PMA decreased the pool of Slc26a6 available to surface biotinylation but had no effect on total Slc26a6 expression. The physiological significance of these findings was supported by the observation that PKC activation inhibited mouse duodenal oxalate secretion, an effect blocked by rottlerin. We conclude that multiple modes of anion exchange mediated by Slc26a6 are negatively regulated by PKC-δ activation.

Specificity and Regulation of Renal Sulfate Transporters

Sulfate is essential for normal cellular function. The kidney plays a major role in sulfate homeostasis. Sulfate is freely filtered and then undergoes net reabsorption in the proximal tubule. The apical membrane Na(+)/sulfate cotransporter NaS1 (SLC13A1) has a major role in mediating proximal tubule sulfate reabsorption, as demonstrated by the findings of hyposulfatemia and hypersulfaturia in Nas1-null mice. The anion exchanger SAT1 (SLC26A1), the founding member of the SLC26 sulfate transporter family, mediates sulfate exit across the basolateral membrane to complete the process of transtubular sulfate reabsorption. Another member of this family, CFEX (SLC26A6), is present at the apical membrane of proximal tubular cells. It also can transport sulfate by anion exchange, which probably mediates backflux of sulfate into the lumen. Knockout mouse studies have demonstrated a major role of CFEX as an apical membrane Cl(-)/oxalate exchanger that contributes to NaCl reabsorption in the proximal tubule. Several additional SLC26 family members mediate sulfate transport and show some level of renal expression (e.g., SLC26A2, SLC26A7, SLC26A11). Their roles in mediating renal tubular sulfate transport are presently unknown. This paper reviews current data available on the function and regulation of three sulfate transporters (NaS1, SAT1, and CFEX) and their physiological roles in the kidney.

Characterization of the regulation of renal Na+/H+ exchanger NHE3 by insulin

Insulin receptors are widely distributed in the kidney and affect multiple aspects of renal function. In the proximal tubule, insulin regulates volume and acid-base regulation through stimulation of the Na(+)/H(+) exchanger NHE3. This paper characterizes the signaling pathway by which insulin stimulates NHE3 in a cell culture model [opossum kidney (OK) cell]. Insulin has two distinct phases of action on NHE3. Chronic insulin (24 h) activates NHE3 through the classic phosphatidylinositol 3-kinase-serum- and glucocorticoid-dependent kinase 1 (PI3K-SGK1) pathway as insulin stimulates SGK1 phosphorylation and the insulin effect can be blocked by the PI3K inhibitor wortmannin or a dominant-negative SGK1. We showed that SGK1 transcript and protein are expressed in rat proximal tubule and OK cells. We previously showed that glucocorticoids augment the effect of insulin on NHE3 (Klisic J, Hu MC, Nief V, Reyes L, Fuster D, Moe OW, Ambuhl PM. Am J Physiol Renal Physiol 283: F532-F539, 2002). Part of this can be mediated via induction of SGK1 by glucocorticoids, and indeed the insulin effect on NHE3 can also be amplified by overexpression of SGK1. We next addressed the acute effect of insulin (1-2 h) on NHE3 by systematically examining the candidate signaling cascades and activation mechanisms of NHE3. We ruled out the PI3K-SGK1-Akt and TC10 pathways, increased surface NHE3, NHE3 phosphorylation, NHE3 association with calcineurin homologous protein 1 or megalin as mechanisms of acute activation of NHE3 by insulin. In summary, insulin stimulates NHE3 acutely via yet undefined pathways and mechanisms. The chronic effect of insulin is mediated by the classic PI3K-SGK1 route.

Essential roles of CFEX-mediated Cl(-)-oxalate exchange in proximal tubule NaCl transport and prevention of urolithiasis

The majority of the Na(+) and Cl(-) filtered by the kidney is reabsorbed in the proximal tubule. In this nephron segment, a significant fraction of Cl(-) is transported via apical membrane Cl(-)-base exchange: Cl(-)-formate exchange, Cl(-)-oxalate exchange, Cl(-)-OH(-) exchange, and Cl(-)-HCO(3)(-) exchange. A search for the transporter responsible for apical membrane Cl(-)-formate exchange in the proximal tubule led to the identification of CFEX (SLC26A6). Functional expression studies in Xenopus oocytes demonstrated that CFEX is capable of mediating not only Cl(-)-formate exchange but also Cl(-)-oxalate exchange, Cl(-)-OH(-) exchange, and Cl(-)-HCO(3)(-) exchange. Studies in CFEX-null mice have begun to elucidate which of the anion exchange activities mediated by CFEX is important for renal physiology and pathophysiology in vivo. Measurements of transport in renal brush border vesicles isolated from CFEX-null mice demonstrated that CFEX primarily mediates Cl(-)-oxalate exchange rather than Cl(-)-formate exchange. Microperfusion studies in CFEX-null mice revealed that CFEX plays an essential role in mediating oxalate-dependent NaCl absorption in the proximal tubule. CFEX-null mice were found to have hyperoxaluria and a high incidence of calcium oxalate urolithiasis. The etiology of hyperoxaluria in CFEX-null mice was observed to be a defect in oxalate secretion in the intestine, leading to enhanced net absorption of ingested oxalate and elevation of plasma oxalate. Thus, by virtue of its function as a Cl(-)-oxalate exchanger, CFEX plays essential roles both in proximal tubule NaCl transport and in the prevention of hyperoxaluria and calcium oxalate nephrolithiasis.

Immunolocalization of anion transporter Slc26a7 in mouse kidney

Previous studies have indicated that a major fraction of the filtered Cl−is reabsorbed via apical membrane Cl−/base exchange in the proximal tubule. Recent studies in Slc26a6 null mice have suggested that this transporter mediates only a portion of proximal tubule Cl−/base exchange, raising the possibility that one or more unidentified apical membrane transporters may additionally contribute. Recent studies have identified Slc26a7 as another Cl−/base exchanger expressed in the kidney. We therefore generated Slc26a7-specific polyclonal and monoclonal antibodies to examine cellular and subcellular sites of expression in mouse kidney. The specificity of each antibody was verified by immunoblotting and immunofluorescence of COS-7 cells transiently transfected with mouse Slc26a7. Immunofluorescence microscopy of mouse kidney detected the expression of Slc26a7 subapically in proximal tubule cells, and on the basolateral surface of thick ascending limb cells. Similar staining patterns were demonstrated with two antibodies shown to react with different epitopes on Slc26a7. Immunolocalization of Slc26a7 to proximal tubule and thick ascending limb was also observed in rat kidney. We conclude that Slc26a7 is expressed in the proximal tubule and thick ascending limb of the loop of Henle, and it may therefore contribute to anion transport in these nephron segments.

Kidney oxygen consumption, carbonic anhydrase, and proton secretion

Oxygen consumed by the kidney (Q(O(2))) is primarily obligated to sodium reabsorption (T(Na)). The relationship of Q(O(2)) to T(Na) (Q(O(2))/T(Na)) may be altered by hormones and autacoids. To examine whether Q(O(2))/T(Na) depends on the mechanism of sodium reabsorption, we first evaluated the effects on Q(O(2)) and Q(O(2))/T(Na) of benzolamide (BNZ), a proximal diuretic that works by inhibiting membrane carbonic anhydrase. During BNZ infusion in anesthetized rats, Q(O(2)) increased by 50% despite a 25% decline in T(Na). However, BNZ failed to increase Q(O(2))/T(Na) when given along with the adenosine A1 receptor blocker, DPCPX, which inhibits basolateral Na-bicarbonate cotransport (NBC1), or EIPA, which inhibits sodium-hydrogen exchange (NHE). Incubating freshly harvested rat proximal tubules with BNZ also caused Q(O(2))to increase by 62%, an effect that was prevented by blocking the apical NHE3 with S3226. Blocking NBC1 or NHE3 in the proximal tubule will have opposite effects on cell pH, but both maneuvers should reduce active chloride transport. In conclusion, inhibiting membrane carbonic anhydrase in the proximal tubule increases Q(O(2)) and reduces the energy efficiency of sodium reabsorption by the kidney. This is not purely due to shifting the burden of reabsorption to a more expensive site downstream from the proximal tubule. Instead, increased cost may be incurred within the proximal tubule as the result of increased active chloride transport.

Role of PDZK1 in membrane expression of renal brush border ion exchangers

Na-H exchanger NHE3 and Cl-anion exchanger CFEX (SLC26A6, PAT1) play principal roles in the reabsorption of Na and Cl in the proximal tubule of the mammalian kidney. The mechanisms by which NHE3 and CFEX are localized to and maintained in the brush border of the proximal tubule are largely unknown. To investigate the possible interaction of NHE3 and CFEX with the PDZ-domain-containing scaffolding protein PDZK1, we performed a series of in vitro interaction assays with GST-fusion proteins and native brush border membrane proteins. These studies demonstrated that, not only were NHE3 and CFEX capable of directly interacting with PDZK1, but that this interaction was mediated through their C-terminal PDZ-interaction sites. To determine whether PDZK1 interaction is essential for brush border localization of NHE3 and CFEX in vivo, we examined the expression of NHE3 and CFEX in kidneys of wild-type and PDZK1-null mutant mice by both Western analysis and immunocytochemistry. These studies indicated that, although brush border expression of NHE3 was unaffected by the loss of PDZK1, the expression of CFEX was markedly reduced. Finally, we assayed CFEX functional activity as Cl-oxalate exchange in brush border membrane vesicles and oxalate-stimulated volume absorption in microperfused proximal tubules. Consistent with the observed decrease in CFEX protein expression, both measures of CFEX functional activity were dramatically reduced in PDZK1-null animals. In conclusion, the scaffolding protein PDZK1 is essential for the normal expression and function of Cl-anion exchanger CFEX in the proximal tubule of the mammalian kidney.

Renal and intestinal transport defects in Slc26a6-null mice

SLC26A6 (PAT1, CFEX) is an anion exchanger that is expressed on the apical membrane of the kidney proximal tubule and the small intestine. Modes of transport mediated by SLC26A6 include Cl-/formate exchange, Cl-/HCO3- exchange, and Cl-/oxalate exchange. To study its role in kidney and intestinal physiology, gene targeting was used to prepare mice lacking Slc26a6. Homozygous mutant Slc26a6-/- mice appeared healthy and exhibited a normal blood pressure, kidney function, and plasma electrolyte profile. In proximal tubules microperfused with a low-HCO3-/high-Cl- solution, the baseline rate of fluid absorption (Jv), an index of NaCl transport under these conditions, was the same in wild-type and null mice. However, the stimulation of Jv by oxalate observed in wild-type mice was completely abolished in Slc26a6-null mice (P<0.05). Formate stimulation of Jv was partially reduced in null mice, but the difference from the response in wild-type mice did not reach statistical significance. Apical membrane Cl-/base exchange activity, assayed with the pH-sensitive dye BCPCF in microperfused proximal tubules, was decreased by 58% in Slc26a6-/- animals (P<0.001 vs. wild types). In the duodenum, the baseline rate of HCO3- secretion measured in mucosal tissue mounted in Ussing chambers was decreased by approximately 30% (P<0.03), whereas the forskolin-stimulated component of HCO3- secretion was the same in wild-type and Slc26a6-/- mice. We conclude that Slc26a6 mediates oxalate-stimulated NaCl absorption, contributes to apical membrane Cl-/base exchange in the kidney proximal tubule, and also plays an important role in HCO3- secretion in the duodenum.

Immunolocalization of NHE8 in rat kidney

In situ hybridization studies demonstrated that Na+/H+ exchanger NHE8 is expressed in kidney proximal tubules. Although membrane fractionation studies suggested apical brush-border localization, precise membrane localization could not be definitively established. The goal of the present study was to develop isoform-specific NHE8 antibodies as a tool to directly establish the localization of NHE8 protein in the kidney by immunocytochemistry. Toward this goal, two sets of antibodies that label different NHE8 epitopes were developed. Monoclonal antibody 7A11 and polyclonal antibody Rab65 both specifically labeled NHE8 by Western blotting as well as by immunofluorescence microscopy. The immunolocalization pattern in the kidney seen with both antibodies was the same, thereby validating NHE8 specificity. In particular, NHE8 expression was observed on the apical brush-border membrane of all proximal tubules from S1 to S3. The most intense staining was evident in proximal tubules in the deeper cortex and medulla with a significant but somewhat weaker staining in superficial proximal tubules. Colocalization studies with gamma-glutamyltranspeptidase and megalin indicated expression of NHE8 on both the microvillar surface membrane and the coated-pit region of proximal tubule cells, suggesting that NHE8 may be subject to endocytic retrieval and recycling. Although colocalizing in the proximal tubule with NHE3, no significant alteration in NHE8 protein expression was evident in NHE3-null mice. We conclude that NHE8 is expressed on the apical brush-border membrane of proximal tubule cells, where it may play a role in mediating or regulating ion transport in this nephron segment.

Chloride transport in the renal proximal tubule

The renal proximal tubule is responsible for most of the renal sodium, chloride, and bicarbonate reabsorption. Micropuncture studies and electrophysiological techniques have furnished the bulk of our knowledge about the physiology of this tubular segment. As a consequence of the leakiness of this epithelium, paracellular ionic transport--in particular that of Cl(-)--is of considerable importance in this first part of the nephron. It was long accepted that proximal Cl(-) reabsorption proceeds solely paracellularly, but it is now known that transcellular Cl(-) transport also exists. Cl(-) channels and Cl(-)-coupled transporters are involved in transcellular Cl(-) transport. In the apical membrane, Cl(-)/anion (formate, oxalate and bicarbonate) exchangers represent the first step in transcellular Cl(-) reabsorption. A basolateral Cl(-)/HCO(3)(-) exchanger, involved in HCO(3)(-) reclamation, participates in the rise of intracellular Cl(-) activity above its equilibrium value, and thus also contributes to the creation of an outwardly directed electrochemical Cl(-) gradient across the cell membranes. This driving force favours Cl(-) diffusion from the cell to the lumen and to the interstitium. In the basolateral membrane, the main mechanism for transcellular Cl(-) reabsorption is a Cl(-) conductance, but a Na(+)-driven Cl(-)/HCO(3)(-) exchanger may also participate in Cl(-) reabsorption.

Kidneys sans glomeruli

The evolution of the vertebrate kidney records three occasions, each separated by about 50 million years, when fish have abandoned glomeruli to produce urine by tubular mechanisms. The recurring dismissal of glomeruli suggests a mechanism of aglomerular urine formation intrinsic to renal tubules. Indeed, the transepithelial secretion of organic solutes and of inorganic solutes such as sulfate, phosphate, and magnesium can all drive secretory water flow in renal proximal tubules of fish. However, the secretion of NaCl via secondary active transport of Cl is the primary mover of secretory water flow in, surprisingly, proximal tubules of both glomerular and aglomerular fish. In filtering kidneys, the tubular secretion of solute and water is overshadowed by reabsorptive transport activities, but secretion progressively comes to light as glomerular filtration decreases. Thus the difference between glomerular and aglomerular urine formation is more a difference of degree than of kind. At low rates of glomerular filtration in seawater fish, NaCl-coupled water secretion serves to increase the renal excretory capacity by increasing the luminal volume into which waste, excess, and toxic solutes can be secreted. The reabsorption of NaCl and water in the distal nephron and urinary bladder concentrates unwanted solutes for excretion while minimizing renal water loss. In aglomerular fish, NaCl-coupled water secretion across proximal tubules replaces glomerular filtration to increase renal excretory capacity. A review of the literature suggests that tubular secretion of NaCl and water is an early function of the vertebrate proximal tubule that has been retained throughout evolution. Active transepithelial Cl secretion takes place in gall bladders studied as models of the mammalian proximal tubule and in proximal tubules of amphibians and apparently also of mammals. The tubular secretion of Cl is also observed in mammalian distal tubules. The evidence consistent with and for Cl secretion in, respectively, proximal and distal tubules of the mammalian kidney calls for a reexamination of basic assumptions in renal physiology that may lead to new opportunities for managing some forms of renal disease.

Activation of the apical Na+/H+ exchanger NHE3 by formate: a basis of enhanced fluid and electrolyte reabsorption by formate in the kidney

Formate stimulates sodium chloride and fluid reabsorption in kidney proximal tubule; however, the exact cellular mechanism of this effect remains unknown. We hypothesized that the primary target of formate is the apical Na(+)/H(+) exchanger. Here, we demonstrate that formate directly enhances the apical Na(+)/H(+) exchanger (NHE3) activity in mouse kidney proximal tubule. In the absence of CO(2)/HCO(3)(-), addition of formate (500 microM) to the bath and lumen of microperfused mouse kidney proximal tubule caused significant intracellular alkalinization, with intracellular pH (pH(i)) increasing from baseline levels 7.17 +/- 0.01 to 7.55 +/- 0.01 (P < 0.001, n = 14), with a Delta pH of 0.38 +/- 0.02. Removal of luminal chloride did not block cell pH alkalinization by formate (baseline pH of 7.26 +/- 0.01 to 7.53 +/- 0.01 with formate, P < 0.001, n = 10), indicating that the apical Cl(-)/OH(-) exchanger was not the primary mediator of the effect of formate on cell pH. However, removal of sodium from the lumen or addition of EIPA completely prevented cell pH alkalinization. Addition of formate to the lumen and bath in the outer medullary collecting duct, which does not express any apical Na(+)/H(+) exchanger, did not cause any cell pH alkalinization. At lower concentrations (50 microM), formate caused significant pH(i) alkalinization in proximal tubule cells, with pH(i) increasing from baseline levels 7.15 +/- 0.02 to 7.36 +/- 0.02 (P < 0.02, n = 11). Acetate, at 50 microM, had no effect on pH(i). Formate's effect was observed both in the absence and presence of CO(2)/HCO(3)(-) in the media. We conclude that formate stimulates the apical Na(+)/H(+) exchanger NHE3 in the kidney proximal tubule. We propose that formate stimulation of chloride reabsorption in the proximal tubule is indirect and is secondary to the activation of apical Na(+)/H(+) exchanger NHE3, which then leads to the stimulation of the apical chloride/base exchanger.

Mathematical models of renal fluid and electrolyte transport: acknowledging our uncertainty

Mathematical models of renal tubular function, with detail at the cellular level, have been developed for most nephron segments, and these have generally been successful at capturing the overall bookkeeping of solute and water transport. Nevertheless, considerable uncertainty remains about important transport events along the nephron. The examples presented include the role of proximal tubule tight junctions in water transport and in regulation of Na(+) transport, the mechanism by which axial flow in proximal tubule modulates solute reabsorption, the effect of formate on proximal Cl(-) transport, the assessment of potassium transport along collecting duct segments inaccessible to micropuncture, the assignment of pathways for peritubular Cl(-) exit in outer medullary collecting duct, and the interaction of carbonic anhydrase-sensitive and -insensitive pathways for base exit from inner medullary collecting duct. Some of these uncertainties have had intense experimental interest well before they were cast as modeling problems. Indeed, many of the renal tubular models have been developed based on data acquired over two or three decades. Nevertheless, some uncertainties have been delineated as the result of model exploration and represent communications from the modelers back to the experimental community that certain issues should not be considered closed. With respect to model refinement, incorporating more biophysical detail about individual transporters will certainly enhance model reliability, but ultimate confidence in tubular models will still be contingent on experimental development of critical information at the tubular level.

Formate-stimulated NaCl absorption in the proximal tubule is independent of the pendrin protein

A significant fraction of active chloride reabsorption across the apical membrane of the proximal tubule is mediated by a chloride/formate exchange process, whereby intracellular formate drives the transport of chloride into the cell. When chloride/formate exchange operates in parallel with Na(+)/H(+) exchange and H(+)-coupled recycling of formate, the net result is electroneutral NaCl reabsorption. Pendrin is the protein product of the PDS gene (SLC26A4) and functions in several different anion exchange modes, including chloride/formate exchange. Pendrin is expressed in the kidney and may serve as the transporter responsible for formate-dependent NaCl reabsorption. In the present study, Pds-knockout mice were used to determine the role of pendrin in proximal tubule chloride reabsorption. We show that formate-dependent NaCl absorption in microperfused proximal tubules is similar between wild-type and pendrin-deficient mice. In addition, there is no difference in the rate of formate-mediated chloride transport in brush-border membrane vesicles isolated from wild-type and pendrin-deficient mice. These studies demonstrate that pendrin is not responsible for formate-dependent NaCl reabsorption in the proximal tubule.

Molecular characterization of the murine Slc26a6 anion exchanger: functional comparison with Slc26a1

We report the molecular and functional characterization of murine Slc26a6, the putative apical chloride-formate exchanger of the proximal tubule. The Slc26a6 transcript is expressed in several tissues, including kidney. Alternative splicing of the second exon generates two distinct isoforms, denoted Slc26a6a and Slc26a6b, which differ in the inclusion of a 23-residue NH(2)-terminal extension. Functional comparison with murine Slc26a1, the basolateral oxalate exchanger of the proximal tubule, reveals a number of intriguing differences. Whereas Slc26a6 is capable of Cl(-), SO, formate, and oxalate uptake when expressed in Xenopus laevis oocytes, Slc26a1 transports only SO and oxalate. Measurement of intracellular pH during the removal of extracellular Cl(-) in the presence and absence of HCO indicates that Slc26a6 functions as both a Cl(-)/HCO and a Cl(-)/OH(-) exchanger; simultaneous membrane hyperpolarization during these experimental maneuvers reveals that HCO and OH(-) transport mediated by Slc26a6 is electrogenic. Cis-inhibition and efflux experiments indicate that Slc26a6 can mediate the exchange of both Cl(-) and SOwith a number of substrates, including formate and oxalate. In contrast, SO and oxalate transport by Slc26a1 are mutually cis-inhibited but activated significantly by extracellular halides, lactate, and formate. The data indicate that Slc26a6 encodes an apical Cl(-)/formate/oxalate and Cl(-)/base exchanger and reveal significant mechanistic differences between apical and basolateral oxalate exchangers of the proximal tubule.

Insulin activates Na(+)/H(+) exchanger 3: biphasic response and glucocorticoid dependence

Insulin is an important regulator of renal salt and water excretion, and hyperinsulinemia has been implicated to play a role in hypertension. One of the target proteins of insulin action in the kidney is Na(+)/H(+) exchanger 3 (NHE3), a principal Na(+) transporter responsible for salt absorption in the mammalian proximal tubule. The molecular mechanisms involved in activation of NHE3 by insulin have not been studied so far. In opossum kidney (OK) cells, insulin increased Na(+)/H(+) exchange activity in a time- and concentration-dependent manner. This effect is due to activation of NHE3 as it persisted after pharmacological inhibition of NHE1 and NHE2. In the early phase of stimulation (2-12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, enhanced NHE3 activity was accompanied by an increase in total and cell surface NHE3 protein and NHE3 mRNA abundance. All the effects of insulin on NHE3 activity, protein, and mRNA were amplified in the presence of hydrocortisone. These results suggest that insulin stimulates renal tubular NHE3 activity via a biphasic mechanism involving posttranslational factors and an increase in NHE3 gene expression and the effects are dependent on the permissive action of hydrocortisone.

Genetic diseases of acid-base transporters

Genetic disorders of acid-base transporters involve plasmalemmal and organellar transporters of H(+), HCO3(-), and Cl(-). Autosomal-dominant and -recessive forms of distal renal tubular acidosis (dRTA) are caused by mutations in ion transporters of the acid-secreting Type A intercalated cell of the renal collecting duct. These include the AE1 Cl(-)/HCO3(-) exchanger of the basolateral membrane and at least two subunits of the apical membrane vacuolar (v)H(+)-ATPase, the V1 subunit B1 (associated with deafness) and the V0 subunit a4. Recessive proximal RTA with ocular disease arises from mutations in the electrogenic Na(+)-bicarbonate cotransporter NBC1 of the proximal tubular cell basolateral membrane. Recessive mixed proximal-distal RTA accompanied by osteopetrosis and mental retardation is associated with mutations in cytoplasmic carbonic anhydrase II. The metabolic alkalosis of congenital chloride-losing diarrhea is caused by mutations in the DRA Cl(-)/HCO3(-) exchanger of the ileocolonic apical membrane. Recessive osteopetrosis is caused by deficient osteoclast acid secretion across the ruffled border lacunar membrane, the result of mutations in the vH(+)-ATPase V0 subunit or in the CLC-7 Cl(-) channel. X-linked nephrolithiasis and engineered deficiencies in some other CLC Cl(-) channels are thought to represent defects of organellar acidification. Study of acid-base transport disease-associated mutations should enhance our understanding of protein structure-function relationships and their impact on the physiology of cell, tissue, and organism.