Ileal oxalate absorption and urinary oxalate excretion are enhanced in Slc26a6 null mice

Parsing apical oxalate exchange in Caco-2BBe1 monolayers: siRNA knockdown of SLC26A6 reveals the role and properties of PAT-1

The purpose of this investigation was to quantitate the contribution of the anion exchanger PAT-1 (putative anion transporter-1), encoded by SLC26A6, to oxalate transport in a model intestinal epithelium and to discern some characteristics of this exchanger expressed in its native environment. Control (Con) Caco-2 BBe1 monolayers, 6-8 days postseeding, were compared with those transfected with a small interfering RNA targeted to SLC26A6 (A6KD). Radiotracer and Ussing chamber techniques were used to determine the transepithelial unidirectional fluxes of Ox(2-), Cl(-), and SO(4)(2-) whereas fluorometric/BCECF measurements of intracellular pH were used to assess HCO(3)(-) exchange. PAT-1 was functionally targeted to the apical membrane, and SLC26A6 knockdown reduced PAT-1 protein (>60%) and mRNA (>75%) expression in A6KD. No net flux of Ox(2-), Cl(-), or SO(4)(2-) was detected in Con or A6KD monolayers, yet the unidirectional fluxes in A6KD were reduced 50, 35, and 15%, respectively. Cl(-)-dependent HCO(3)(-) efflux from A6KD was reduced 50% compared with Con. The difference between Con and A6KD properties represents that mediated solely by PAT-1, and by this approach we found that PAT-1-mediated oxalate influx and efflux are inhibited equally by mucosal DIDS (EC(50) approximately 5 microM) and that mucosal Cl(-) inhibits oxalate uptake with an EC(50) < 20 mM. Transepithelial Cl(-) gradients supported large, DIDS-sensitive net absorptive or secretory fluxes of oxalate in a direction opposite that of the imposed Cl(-) gradient. The overall symmetry of PAT-1-mediated oxalate exchange suggests that vectorial oxalate transport observed in vivo is principally dependent on the magnitude and direction of counterion gradients.

Genetic basis of renal cellular dysfunction and the formation of kidney stones

Nephrolithiasis is a result of formation and retention of crystals within the kidneys. The driving force behind crystal formation is urinary supersaturation with respect to the stone-forming salts, which means that crystals form when the concentrations of participating ions are higher than the thermodynamic solubility for that salt. Levels of supersaturation are kept low and under control by proper functioning of a variety of cells including those that line the renal tubules. It is our hypothesis that crystal deposition, i.e., formation and retention in the kidneys, is a result of impaired cellular function, which may be intrinsic and inherent or triggered by external stimuli and challenges. Cellular impairment or dysfunction affects the supersaturation, by influencing the excretion of participating ions such as calcium, oxalate and citrate and causing hypercalciuria, hyperoxaluria or hypocitraturia. The production and excretion of macromolecular promoters and inhibitors of crystallization is also dependent upon proper functioning of the renal epithelial cells. Insufficient or ineffective crystallization modulators such as osteopontin, Tamm-Horsfall protein, bikunin, etc. are most likely produced by the impaired cells.

Functional coupling of the downregulated in adenoma Cl-/base exchanger DRA and the apical Na+/H+ exchangers NHE2 and NHE3

Non-nutrient-dependent salt absorption across the brush-border membrane of intestinal epithelial cells is primarily mediated by coupled apical Na(+)/H(+) (aNHE) and anion exchange transport, with the latter suspected of being mediated by DRA (downregulated in adenoma; SLC26A3) that is defective in congenital chloridorrhea. To investigate DRA in greater detail and determine whether DRA and NHE activities can be coupled, we measured (22)Na(+) and (36)Cl(-) uptake in Caco2BBE colon cells infected with the tet-off-inducible DRA transgene. Under basal conditions, DRA activity was low in normal and infected Caco2BBE cells in the presence of tetracycline, whereas NHE activities could be easily detected. When apical NHE activity was increased by transfection or serum-induced expression of the aNHE isoforms NHE2 and NHE3, increased (36)Cl(-) uptake was observed. Inhibition of DRA activity by niflumic acid was greater than that by DIDS as well as by the NHE inhibitor dimethylamiloride and the carbonic anhydrase inhibitor methazolamide. DRA activity was largely aNHE-dependent, whereas a component of DRA-independent aNHE uptake continued to be observed. Coupled aNHE and DRA activities were inhibited by increased cellular cAMP and calcium and were associated with synaptotagmin I-dependent, clathrin-mediated endocytosis. In summary, these data support the role of DRA in electroneutral NaCl absorption involving functional coupling of Cl(-)/base exchange and apical NHE.

Pharmacology of stone disease

Kidney stone disease remains a major health and economic burden on the nation. It has been increasingly recognized that nephrolithiasis can be both a chronic or systemic illness. There have been major limitations in the development of new drugs for the prevention and management of this disease, largely due to our lack of understanding of the complex pathophysiologic mechanisms involving the interaction of three major target organs: the kidney, bone, and intestine. We also do not yet understand the molecular genetic basis of this polygenic disorder. These limitations are coupled with the incorrect perception that kidney stone disease is solely an acute illness, and the lack of reliable tests to assess outcome measures. All of these factors combined have diminished the willingness of the pharmaceutical industry to engage in the development of novel drugs.

Prospects for dietary therapy of recurrent nephrolithiasis

The goal of this article is to propose a randomized controlled trial (RCT) that tests a hypothesis that dietary manipulation prevents recurrent kidney stones. Dietary interventions based on epidemiologic and pathophysiologic data are reviewed. The only diet trial successful in preventing stones showed that calcium intake of 1,200 mg/d, accompanied by restriction of animal protein, salt, and oxalate ingestion, was superior to 400 mg of calcium and restricted oxalate intake. This study may be worth repeating in women and in a society in which salt restriction might be less effective (eg, United States). The net result of diet trials establishes significant positive effects on urine chemistries, but these have not yet shown efficacy with regard to stone recurrence. Oxalate restriction alone could be effective, but many questions regarding which populations to study are not defined, and dietary oxalate's contribution to stone formation is disputed. Would such a study be limited to patients identified as having high dietary oxalate intake or high intestinal oxalate absorption? Would colonization with Oxalobacter formigenes influence the result? The increased prevalence of stones is linked to weight gain and obesity, making weight loss a possible therapy to prevent stones. Randomized trials show that diets consisting of low-fat content or low-caloric content cause modest weight loss and might be effective in reducing stone formation. Because the efficacy of thiazides in the prevention of stones in patients with hypercalciuria is clear, I propose dietary comparison of higher calcium intake to thiazides for the prevention of calcium-based kidney stones.

Recent advances in the pathophysiology of nephrolithiasis

Over the past 10 years, major progress has been made in the pathogenesis of uric acid and calcium stones. These advances have led to our further understanding of a pathogenetic link between uric acid nephrolithiasis and the metabolic syndrome, the role of Oxalobacter formigenes in calcium oxalate stone formation, oxalate transport in Slc26a6-null mice, the potential pathogenetic role of Randall's plaque as a precursor for calcium oxalate nephrolithiasis, and the role of renal tubular crystal retention. With these advances, we may target the development of novel drugs including (1) insulin sensitizers; (2) probiotic therapy with O. formigenes, recombinant enzymes, or engineered bacteria; (3) treatments that involve the upregulation of intestinal luminal oxalate secretion by increasing anion transporter activity (Slc26a6), luminally active nonabsorbed agents, or oxalate binders; and (4) drugs that prevent the formation of Randall's plaque and/or renal tubular crystal adhesions.

Phenotypic and functional analysis of human SLC26A6 variants in patients with familial hyperoxaluria and calcium oxalate nephrolithiasis

BACKGROUND

Urinary oxalate is a major risk factor for calcium oxalate stones. Marked hyperoxaluria arises from mutations in 2 separate loci, AGXT and GRHPR, the causes of primary hyperoxaluria (PH) types 1 (PH1) and 2 (PH2), respectively. Studies of null Slc26a6(-/-) mice have shown a phenotype of hyperoxaluria, hyperoxalemia, and calcium oxalate urolithiasis, leading to the hypothesis that SLC26A6 mutations may cause or modify hyperoxaluria in humans.

STUDY DESIGN

Cross-sectional case-control.

SETTING & PARTICIPANTS

Cases were recruited from the International Primary Hyperoxaluria Registry. Control DNA samples were from a pool of adult subjects who identified themselves as being in good health.

PREDICTOR

PH1, PH2, and non-PH1/PH2 genotypes in cases.

OUTCOMES & MEASURES

Homozygosity or compound heterozygosity for SLC26A6 variants. Functional expression of oxalate transport in Xenopus laevis oocytes.

RESULTS

80 PH1, 6 PH2, 8 non-PH1/PH2, and 96 control samples were available for SLC26A6 screening. A rare variant, c.487C-->T (p.Pro163Ser), was detected solely in 1 non-PH1/PH2 pedigree, but this variant failed to segregate with hyperoxaluria, and functional studies of oxalate transport in Xenopus oocytes showed no transport defect. No other rare variant was identified specifically in non-PH1/PH2. Six additional missense variants were detected in controls and cases. Of these, c.616G-->A (p.Val206Met) was most common (11%) and showed a 30% reduction in oxalate transport. To test p.Val206Met as a potential modifier of hyperoxaluria, we extended screening to PH1 and PH2. Heterozygosity for this variant did not affect plasma or urine oxalate levels in this population.

LIMITATIONS

We did not have a sufficient number of cases to determine whether homozygosity for p.Val206Met might significantly affect urine oxalate.

CONCLUSIONS

SLC26A6 was effectively ruled out as the disease gene in this non-PH1/PH2 cohort. Taken together, our studies are the first to identify and characterize SLC26A6 variants in patients with hyperoxaluria. Phenotypic and functional analysis excluded a significant effect of identified variants on oxalate excretion.

Partitioning of 14C-oxalate excretion in rats during a persistent oxalate challenge

This study was done to resolve published discrepancies in oxalate excretion between humans and rats and to characterize oxalate partitioning in rats during persistent severe hyperoxaluria, such as that seen in many bariatric patients. Osmotic minipumps dispensing 360 micromole/day KOx + 3.9 +/- 0.14 microCi/day (14)C-oxalate were implanted subcutaneously. All excreta were collected. Rats were killed on day 13 and carcasses were dissected, ground, dissolved in HCl, and subjected to scintillation counting, and 92.1 +/- 3.9% of total oxalate administered was recovered. This was partitioned among the skin complex (38.2 +/- 7.7%), carcass complex (24.5 +/- 5.9%), and excreta (29.5 +/- 1.9%). The distribution of oxalate in the skin and carcass complexes led us to infer that only 29.5 +/- 1.9% of the administered oxalate entered the circulation. Of the circulated oxalate, 98.4 +/- 0.4% was excreted (total urine 78.9 +/- 1.7%; raw feces 21.0 +/- 1.7%). Thus, most oxalate that does enter the circulation is promptly excreted in rats, as in humans. Consequently, even after a large, persistent oxalate challenge, very little oxalate had accumulated in the internal organs, muscle, and skeleton. Unlike humans, however, rats excrete a significant fraction of oxalate in the feces.

CFTR and its key role in in vivo resting and luminal acid-induced duodenal HCO3- secretion

BACKGROUND AND AIMS

We investigated the role of the recently discovered, villous-expressed anion exchanger Slc26a6 (PAT1) and the predominantly crypt-expressed cystic fibrosis transmembrane regulator (CFTR) in basal and acid-stimulated murine duodenal HCO(3)(-) secretion in vivo, and the influence of blood HCO(3)(-) concentration on both.

METHODS

The proximal duodenum of anaesthetized mice was perfused in situ, and HCO(3)(-) secretion was determined by back-titration. Duodenal mucosal permeability was assessed by determining (51)Cr-EDTA leakage from blood to lumen.

RESULTS

Compared with wild type (WT) littermates basal duodenal HCO(3)(-) secretory rates were slightly reduced in Slc26-deficient mice at low ( approximately 21 mm), and markedly reduced at high blood HCO(3)(-) concentration ( approximately 29 mm). In contrast, basal HCO(3)(-) secretion was markedly reduced in CFTR-deficient mice compared with WT littermates both at high and low blood HCO(3)(-) concentration. A short-term application of luminal acid increased duodenal HCO(3)(-) secretory rate in Slc26a6-deficient and WT mice to the same degree, but had no stimulatory effect in the absence of CFTR. Luminal acidification to pH 2.5 did not alter duodenal permeability.

CONCLUSIONS

The involvement of Slc26a6 in basal HCO(3)(-) secretion in murine duodenum in vivo is critically dependent on the systemic acid/base status, and this transporter is not involved in acid-stimulated HCO(3)(-) secretion. The presence of CFTR is essential for basal and acid-induced HCO(3)(-) secretion irrespective of acid/base status. This suggests a coupled action of Slc26a6 with CFTR for murine basal duodenal HCO(3)(-) secretion, but not acid-stimulated secretion, in vivo.

Fructose-induced hypertension: essential role of chloride and fructose absorbing transporters PAT1 and Glut5

Increased dietary fructose in rodents recapitulates many aspects of the Metabolic Syndrome with hypertension, insulin resistance and dyslipidemia. Here we show that fructose increased jejunal NaCl and water absorption which was significantly decreased in mice whose apical chloride/base exchanger Slc26a6 (PAT1, CFEX) was knocked out. Increased dietary fructose intake enhanced expression of this transporter as well as the fructose-absorbing transporter Slc2a5 (Glut5) in the small intestine of wild type mice. Fructose feeding decreased salt excretion by the kidney and resulted in hypertension, a response almost abolished in the knockout mice. In parallel studies, a chloride-free diet blocked fructose-induced hypertension in Sprague Dawley rats. Serum uric acid remained unchanged in animals on increased fructose intake with hypertension. We suggest that fructose-induced hypertension is likely caused by increased salt absorption by the intestine and kidney and the transporters Slc26a6 and Slc2a5 are essential in this process.

Nephrolithiasis after bariatric surgery for obesity

Surgical intervention has become an accepted therapeutic alternative for the patient with medically complicated obesity. Multiple investigators have reported significant and sustained weight loss after bariatric surgery that is associated with improvement of many weight-related medical comorbidities, and statistically significant decreased overall mortality for surgically treated as compared with medically treated subjects. Although the Roux-en-Y gastric bypass (RYGB) is considered an acceptably safe treatment, an increasing number of patients are being recognized with nephrolithiasis after this, the most common bariatric surgery currently performed. The main risk factor appears to be hyperoxaluria, although low urine volume and citrate concentrations may contribute. The incidence of these urinary risk factors among the total post-RYGB population is unknown, but may be more than previously suspected based on small pilot studies. The etiology of the hyperoxaluria is unknown, but may be related to subtle and seemingly subclinical fat malabsorption. Clearly, further study is needed, especially to define better treatment options than the standard advice for a low-fat, low-oxalate diet, and use of calcium as an oxalate binder.

The roles and mechanisms of intestinal oxalate transport in oxalate homeostasis

The mammalian intestine has an important role in the dynamics of oxalate exchange and thereby is significant in the etiology of calcium oxalate nephrolithiasis. Here we review some of the phenomenologic observations that have led to the conclusion that anion exchangers (antiporters) are important mediators of secondarily active, net oxalate transport along the intestine (both absorptive and secretory). Understanding the mechanisms of transepithelial oxalate transport has been advanced radically in recent years by the identification of the solute-linked carrier (SLC)26 family of anion transporters, which has facilitated the identification of specific proteins mediating individual apical or basolateral oxalate transport pathways. Moreover, identification of specific exchangers has underscored their relative importance to oxalate homeostasis as revealed by using knockout mouse models and has facilitated studies of oxalate transport regulation in heterologous expression systems. Finally, the significance of oxalate degrading bacteria to oxalate homeostasis is considered from basic and applied perspectives.

Enteric oxalate secretion is not directly mediated by the human CFTR chloride channel

The secretion of the oxalate anion by intestinal epithelia is a functionally significant component of oxalate homeostasis and hence a relevant factor in the etiology and management of calcium oxalate urolithiasis. To test the hypothesis that human cystic fibrosis transmembrane conductance regulator (hCFTR) can directly mediate the efflux of the oxalate anion, we compared cAMP-stimulated 36Cl-, 14C-oxalate, and 35SO(4)2- efflux from Xenopus oocytes expressing hCFTR with water-injected control oocytes. hCFTR-expressing oocytes exhibited a large, reversible cAMP-dependent increase in whole cell conductance measured using a two-electrode voltage clamp and a 13-fold increase in rate of cAMP-stimulated 36Cl- efflux. In contrast, the rate constants of oxalate and sulfate efflux were low and unaffected by cAMP in either control or hCFTR-expressing oocytes. We conclude that the human CFTR gene product does not directly mediate oxalate efflux in secretory epithelia and hence is not directly involved in oxalate homeostasis in humans.

Oxalate in renal stone disease: the terminal metabolite that just won't go away

The incidence of calcium oxalate nephrolithiasis in the US has been increasing throughout the past three decades. Biopsy studies show that both calcium oxalate nephrolithiasis and nephrocalcinosis probably occur by different mechanisms in different subsets of patients. Before more-effective medical therapies can be developed for these conditions, we must understand the mechanisms governing the transport and excretion of oxalate and the interactions of the ion in general and renal physiology. Blood oxalate derives from diet, degradation of ascorbate, and production by the liver and erythrocytes. In mammals, oxalate is a terminal metabolite that must be excreted or sequestered. The kidneys are the primary route of excretion and the site of oxalate's only known function. Oxalate stimulates the uptake of chloride, water, and sodium by the proximal tubule through the exchange of oxalate for sulfate or chloride via the solute carrier SLC26A6. Fecal excretion of oxalate is stimulated by hyperoxalemia in rodents, but no similar phenomenon has been observed in humans. Studies in which rats were treated with (14)C-oxalate have shown that less than 2% of a chronic oxalate load accumulates in the internal organs, plasma, and skeleton. These studies have also demonstrated that there is interindividual variability in the accumulation of oxalate, especially by the kidney. This Review summarizes the transport and function of oxalate in mammalian physiology and the ion's potential roles in nephrolithiasis and nephrocalcinosis.

Calcium oxalate crystal deposition in kidneys of hypercalciuric mice with disrupted type IIa sodium-phosphate cotransporter

The most common theories about the pathogenesis of idiopathic kidney stones consider precipitation of calcium phosphate (CaP) within the kidneys critical for the development of the disease. We decided to test the hypothesis that a CaP substrate can promote the deposition of calcium oxalate (CaOx) in the kidneys. Experimental hyperoxaluria was induced by feeding glyoxylate to male mice with knockout (KO) of NaP(i) IIa (Npt2a), a sodium-phosphate cotransporter. Npt2a KO mice are hypercalciuric and produce CaP deposits in their renal tubules. Experimental hyperoxaluria led to CaOx crystalluria in both the hypercalciuric KO mice and the normocalciuric control B6 mice. Only the KO mice produced CaOx crystal deposits in their kidneys, but the CaOx crystals deposited separately from the CaP deposits. Perhaps CaP deposits were not available for a CaOx overgrowth. These results also validate earlier animal model observations that showed that CaP substrate is not required for renal deposition of CaOx and that other factors, such as local supersaturation, may be involved. The absence of CaOx deposition in the B6 mice despite extreme hyperoxaluria also signifies the importance of both calcium and oxalate in the development of CaOx nephrolithiasis.

Use of sevelamer hydrochloride as an oxalate binder

PURPOSE

We tested the hypothesis that the cationic phosphate binder sevelamer hydrochloride could reduce hyperoxaluria and calcium oxalate supersaturation in patients with enteric hyperoxaluria by binding fatty acids, binding phosphate and rendering calcium free to bind oxalate, and/or directly binding oxalate. A secondary objective was to assess changes in the urinary excretion of other substances associated with nephrolithiasis.

MATERIALS AND METHODS

Ten patients with enteric hyperoxaluria were enrolled in a nonrandomized, open label trial of sevelamer hydrochloride (3,200 mg 3 times daily for 7 days).

RESULTS

With treatment mean urinary oxalate decreased 17% (0.84 to 0.70 mmol per day) and the urinary oxalate-to-creatinine ratio decreased 11% (0.055 to 0.049 mmol/mmol, p not significant for both). Urinary calcium increased 25% (p not significant). Urinary citrate decreased 23% (p = 0.01) and urinary phosphorus decreased 44% (p = 0.0001). Mean supersaturation of calcium oxalate, brushite, hydroxyapatite, uric acid and sodium urate did not change significantly. However, the decrease in brushite supersaturation approached statistical significance (p = 0.07). Mean serum phosphorus was 3.6 mg/dl at baseline and 3.3 mg/dl with therapy (p not significant). Hypophosphatemia did not develop in any patients. One patient dropped out of study due to abdominal pain.

CONCLUSIONS

Sevelamer hydrochloride dramatically decreased urinary phosphorus excretion with a lesser effect on urinary oxalate. Supersaturation of calcium oxalate did not decrease due to countervailing effects on other constituents including an increase in urinary calcium and a decrease in urinary citrate. Although sevelamer hydrochloride may not be an ideal agent for correcting hyperoxaluria, its potential to reduce calcium phosphate supersaturation merits further investigation.

Species differences in Cl- affinity and in electrogenicity of SLC26A6-mediated oxalate/Cl- exchange correlate with the distinct human and mouse susceptibilities to nephrolithiasis

The mouse is refractory to lithogenic agents active in rats and humans, and so has been traditionally considered a poor experimental model for nephrolithiasis. However, recent studies have identified slc26a6 as an oxalate nephrolithiasis gene in the mouse. Here we extend our earlier demonstration of different anion selectivities of the orthologous mouse and human SLC26A6 polypeptides to investigate the correlation between species-specific differences in SLC26A6 oxalate/anion exchange properties as expressed in Xenopus oocytes and in reported nephrolithiasis susceptibility. We find that human SLC26A6 mediates minimal rates of Cl(-) exchange for Cl(-), sulphate or formate, but rates of oxalate/Cl(-) exchange roughly equivalent to those of mouse slc2a6. Both transporters exhibit highly cooperative dependence of oxalate efflux rate on extracellular [Cl(-)], but whereas the K(1/2) for extracellular [Cl(-)] is only 8 mM for mouse slc26a6, that for human SLC26A6 is 62 mM. This latter value approximates the reported mean luminal [Cl(-)] of postprandial human jejunal chyme, and reflects contributions from both transmembrane and C-terminal cytoplasmic domains of human SLC26A6. Human SLC26A6 variant V185M exhibits altered [Cl(-)] dependence and reduced rates of oxalate/Cl(-) exchange. Whereas mouse slc26a6 mediates bidirectional electrogenic oxalate/Cl(-) exchange, human SLC26A6-mediated oxalate transport appears to be electroneutral. We hypothesize that the low extracellular Cl(-) affinity and apparent electroneutrality of oxalate efflux characterizing human SLC26A6 may partially explain the high human susceptibility to nephrolithiasis relative to that of mouse. SLC26A6 sequence variant(s) are candidate risk modifiers for nephrolithiasis.

Heat-stable enterotoxin of Escherichia coli (STa) can stimulate duodenal HCO3(-) secretion via a novel GC-C- and CFTR-independent pathway

The heat-stable enterotoxin of Escherichia coli (STa) is a potent stimulant of intestinal chloride and bicarbonate secretion. Guanylyl cyclase C (GC-C) has been shown to be the primary receptor involved in mediating this response. However, numerous studies have suggested the existence of an alternative STa-binding receptor. The aims of this study were to determine whether a non-GC-C receptor exists for STa and what is the functional relevance of this for intestinal bicarbonate secretion in mice. (125)I-STa-binding experiments were performed with intestinal mucosae from GC-C knockout (KO) and wild type (WT) mice. Subsequently, the functional relevance of an alternative STa-binding receptor was explored by examining STa-, uroguanylin-, and guanylin-stimulated duodenal bicarbonate secretion (DBS) in GC-C KO mice in vitro and in vivo. Significant (125)I-STa-binding occurred in the proximal small intestines of GC-C KO and WT mice. Analysis of binding coefficients and pH dependence showed that (125)I-STa-binding in GC-C KO mice involved a receptor distinct from that of WT mice. Functionally, STa, uroguanylin, and guanylin all stimulated a significant increase in DBS in GC-C KO mice. Uroguanylin- and guanylin-stimulated DBS were significantly inhibited by glibenclamide, but not by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). However, STa-stimulated DBS was unaffected by glibenclamide but inhibited by DIDS. Taken together, our results suggest that alternative, non-GC-C, receptors likely exist for STa, uroguanylin, and guanylin in the intestines of mice. While uroguanylin- and guanylin-stimulated DBS are cystic fibrosis transmembrane conductance regulator (CFTR) dependent, STa-stimulated DBS is CFTR independent. Further understanding of this alternative receptor and its signaling pathway may provide important insights into rectification of intestinal bicarbonate secretion in cystic fibrosis.

Renal physiology of SLC26 anion exchangers

PURPOSE OF REVIEW

The multifunctional anion exchanger family (Slc26) encompasses 11 identified genes, but only 10 encode real proteins (Slc26a10 is a pseudogene). Most of the Slc26 proteins function primarily as anion exchangers, exchanging sulfate, iodide, formate, oxalate, hydroxyl ion, and bicarbonate anions, whereas other Slc26 proteins function as chloride ion channels or anion-gated molecular motors. The aim of this review is to present recent studies on the molecular function of the Slc26 family and its role in renal physiology and pathophysiology.

RECENT FINDINGS

In proximal tubules, Slc26a1 (Sat-1) mediates sulfate and oxalate transport across the basolateral membrane, while Slc26a6 (CFEX, Pat-1) mediates a variety of anion exchange at the apical membrane to facilitate transcellular sodium chloride absorption. Targeted deletion of murine Slc26a6 leads to intestinal hyperabsorption of oxalate, hyperoxaluria, and kidney stones. Slc26a4 (pendrin) and Slc26a7 are expressed in intercalated cells, and are involved in acid-base homeostasis and blood pressure regulation. Messenger RNA for Slc26a2, Slc26a9, and Slc26a11 is also present in the kidney, yet the roles of these family members in renal physiology or pathophysiology are not clear.

SUMMARY

Members of this multifunctional anion transporter family play evolving roles in the etiology of nephrolithiasis (Slc26a6) and hypertension (Slc26a4 and Slc26a6). Other Slc26 family members (Slc26a2, Slc26a9, Slc26a11) express mRNA in the kidney but their roles in renal physiology are not yet known.

Variability of Oxalobacter formigenes and oxalate in stool samples

PURPOSE

The intestinal organism Oxalobacter formigenes is unique in using oxalate as its primary carbon and energy source. Intestinal colonization with O. formigenes may have clinical significance by decreasing intestinal oxalate and its absorption, thereby influencing the concentration of oxalate in plasma and urine, and the development of calcium oxalate stone disease. Because the oxalate content of the diet varies considerably, we hypothesized that the number of O. formigenes and amount of oxalate would vary in feces.

MATERIALS AND METHODS

To enumerate the number of O. formigenes in feces an accurate and reproducible real-time polymerase chain reaction assay was developed to quantify O. formigenes DNA. Stool samples were obtained from 10 colonized individuals to determine the levels of O. formigenes by this assay and the oxalate content by ion chromatography.

RESULTS

Concentrations of O. formigenes ranged from lower than the limit of detection of 5 x 10(3) to 1.04 x 10(9) cells per gm stool. The total oxalate content of stool samples varied from 0.1 to 1.8 mg/gm and fecal water oxalate varied from 60 to 600 microM. All parameters measured varied within each stool collection, among stool collections on different days and among individuals. Notably in 7 of 10 individuals at least 1 stool sample contained no detectable O. formigenes. In addition, 7 of 10 subjects had a fecal colonization of less than 4 x 10(4) per gm stool.

CONCLUSIONS

This study demonstrates that there is intrastool and interstool sample variability in the amount of O. formigenes measured by real-time polymerase chain reaction that did not correlate with the quantity of oxalate in stool. Most subjects had a fecal colonization of less than 4 x 10(4) per gm stool.

Bariatric surgery, hyperoxaluria, and nephrolithiasis: a plea for close postoperative management of risk factors

Hyperoxaluria contributes significant risk for kidney stone development. It can result from disordered metabolism, excessive intake of oxalate or other nutrients, or alterations of bowel flora and function. Sinha et al. show that individuals undergoing Roux-en-Y bypass have high rates of stone disease, both before and after their procedure. Whether or not the incidence of stone events is increased after surgery, hyperoxaluria should be addressed. After Roux-en-Y, it is important for patients to maintain high urine outputs, to limit fat intake, and to ingest adequate amounts of calcium.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Nonmammalian orthologs of prestin (SLC26A5) are electrogenic divalent/chloride anion exchangers

Individual members of the mammalian SLC26 anion transporter family serve two fundamentally distinct functions. Whereas most members transport different anion substrates across a variety of epithelia, prestin (SLC26A5) is special, functioning as a membrane-localized motor protein that generates electrically induced motions (electromotility) in auditory sensory hair cells of the mammalian inner ear. The transport mechanism of SLC26 proteins is not well understood, and a mechanistic relation between anion transport and electromotility has been suggested but not firmly established so far. To address these questions, we have cloned prestin orthologs from chicken and zebrafish, nonmammalian vertebrates that presumably lack electromotility in their auditory systems. Using patch-clamp recordings, we show that these prestin orthologs, but not mammalian prestin, generate robust transport currents in the presence of the divalent anions sulfate or oxalate. Transport is blocked by salicylate, an inhibitor of electromotility generated by mammalian prestin. The dependence of transport equilibrium potentials on sulfate and chloride concentration gradients shows that the prestin orthologs are electrogenic antiporters, exchanging sulfate or oxalate for chloride in a strictly coupled manner with a 1:1 stoichiometry. These data identify transport mode and stoichiometry of electrogenic divalent/monovalent anion exchange and establish a reliable and simple method for the quantitative determination of the various transport modes that have been proposed for other SLC26 transport proteins. Moreover, the sequence conservation between mammalian and nonmammalian prestin together with a common pharmacology of electromotility and divalent antiport suggest that the molecular mechanism behind electromotility is closely related to an anion transport cycle.

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.

Treatment of the primary hyperoxalurias: a new chapter

Despite advances in the enzymology, molecular genetics, and clinical knowledge of the primary hyperoxalurias, few treatments are available. Oxalobacter formigenes is a promising new therapy with potential to induce secretion of oxalate into the intestinal lumen, where it can be degraded by the bacteria.

Slc26a6 regulates CFTR activity in vivo to determine pancreatic duct HCO3- secretion: relevance to cystic fibrosis

Fluid and HCO(3)(-) secretion are vital functions of the pancreatic duct and other secretory epithelia. CFTR and Cl(-)/HCO(3)(-) exchange activity at the luminal membrane are required for these functions. The molecular identity of the Cl(-)/HCO(3)(-) exchangers and their relationship with CFTR in determining fluid and HCO(3)(-) secretion are not known. We show here that the Cl(-)/HCO(3)(-) exchanger slc26a6 controls CFTR activity and ductal fluid and HCO(3)(-) secretion. Unexpectedly, deletion of slc26a6 in mice and measurement of fluid and HCO(3)(-) secretion into sealed intralobular pancreatic ducts revealed that deletion of slc26a6 enhanced spontaneous and decreased stimulated secretion. Remarkably, inhibition of CFTR activity with CFTR(inh)-172, knock-down of CFTR by siRNA and measurement of CFTR current in WT and slc26a6(-/-) duct cells revealed that deletion of slc26a6 resulted in dis-regulation of CFTR activity by removal of tonic inhibition of CFTR by slc26a6. These findings reveal the intricate regulation of CFTR activity by slc26a6 in both the resting and stimulated states and the essential role of slc26a6 in pancreatic HCO(3)(-) secretion in vivo.

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.

Calcium oxalate urolithiasis in mice lacking anion transporter Slc26a6

Urolithiasis is one of the most common urologic diseases in industrialized societies. Calcium oxalate is the predominant component in 70-80% of kidney stones, and small changes in urinary oxalate concentration affect the risk of stone formation. SLC26A6 is an anion exchanger expressed on the apical membrane in many epithelial tissues, including kidney and intestine. Among its transport activities, SLC26A6 mediates Cl(-)-oxalate exchange. Here we show that mutant mice lacking Slc26a6 develop a high incidence of calcium oxalate urolithiasis. Slc26a6-null mice have significant hyperoxaluria and elevation in plasma oxalate concentration that is greatly attenuated by dietary oxalate restriction. In vitro flux studies indicated that mice lacking Slc26a6 have a defect in intestinal oxalate secretion resulting in enhanced net absorption of oxalate. We conclude that the anion exchanger SLC26A6 has a major constitutive role in limiting net intestinal absorption of oxalate, thereby preventing hyperoxaluria and calcium oxalate urolithiasis.