Regulation of murine hepatic hydroxysteroid sulfotransferase expression in hyposulfatemic mice and in a cell model of 3'-phosphoadenosine-5'-phosphosulfate deficiency

The cytosolic sulfotransferases (SULTs) catalyze the sulfate conjugation of nucleophilic substrates, and the cofactor for sulfonation, 3'-phosphoadenosine-5'-phosphosulfate (PAPS), is biosynthesized from sulfate and ATP. The phenotype of male knockout mice for the NaS1 sodium sulfate cotransporter includes hyposulfatemia and increased hepatic expression of mouse cytoplasmic sulfotransferase Sult2a and Sult3a1. Here we report that in 8-week-old female NaS1-null mice, hepatic Sult2a1 mRNA levels were ∼51-fold higher than they were in a wild-type liver but expression of no other Sult was affected. To address whether hyposulfatemia-inducible Sult2a1 expression might be due to reduced PAPS levels, we stably knocked down PAPS synthases 1 and 2 in HepG2 cells (shPAPSS1/2 cells). When a reporter plasmid containing at least 233 nucleotides (nt) of Sult2a1 5'-flanking sequence was transfected into shPAPSS1/2 cells, reporter activity was significantly increased relative to the activity that was seen for reporters containing 179 or fewer nucleotides. Mutation of an IR0 (inverted repeat of AGGTCA, with 0 intervening bases) nuclear receptor motif at nt -191 to 180 significantly attenuated the PAPSS1/2 knockdown-mediated increase. PAPSS1/2 knockdown significantly activated farnesoid X receptor (FXR), retinoid-related orphan receptor, and pregnane X receptor responsive reporters, and treatment with the FXR agonist GW4064 [3-(2,6-dichlorophenyl)-4-(3'-carboxy-2-chlorostilben-4-yl)oxymethyl-5-isopropylisoxazole] increased Sult2a1 promoter activity when the IR0 was intact. Transfection of shPAPSS1/2 cells with FXR small interfering RNA (siRNA) significantly reduced the Sult2a1 promoter activity. The impact of PAPSS1/2 knockdown on Sult2a1 promoter activity was recapitulated by knocking down endogenous SULT2A1 expression in HepG2 cells. We propose that hyposulfatemia leads to hepatic PAPS depletion, which causes loss of SULT2A1 activity and results in accumulation of nonsulfated bile acids and FXR activation.

Sodium-sulfate/carboxylate cotransporters (SLC13)

The SLC13 gene family is comprised of five sequence related proteins that are found in animals, plants, yeast and bacteria. Proteins encoded by the SLC13 genes are divided into the following two groups of transporters with distinct anion specificities: the Na(+)-sulfate (NaS) cotransporters and the Na(+)-carboxylate (NaC) cotransporters. Members of this gene family (in ascending order) are: SLC13A1 (NaS1), SLC13A2 (NaC1), SLC13A3 (NaC3), SLC13A4 (NaS2) and SLC13A5 (NaC2). SLC13 proteins encode plasma membrane polypeptides with 8-13 putative transmembrane domains, and are expressed in a variety of tissues. They are all Na(+)-coupled symporters with strong cation preference for Na(+), and insensitive to the stilbene 4, 4'-diisothiocyanatostilbene-2, 2'-disulphonic acid (DIDS). Their Na(+):anion coupling ratio is 3:1, indicative of electrogenic properties. They have a substrate preference for divalent anions, which include tetra-oxyanions for the NaS cotransporters or Krebs cycle intermediates (including mono-, di- and tricarboxylates) for the NaC cotransporters. This review will describe the molecular and cellular mechanisms underlying the biochemical, physiological and structural properties of the SLC13 gene family.

Slc13a1 and Slc26a1 KO models reveal physiological roles of anion transporters

Anion transporters NaS1 (SLC13A1) and Sat1 (SLC26A1) mediate sulfate (re)absorption across renal proximal tubule and small intestinal epithelia, thereby regulating blood sulfate levels. Disruption of murine NaS1 and Sat1 genes leads to hyposulfatemia and hypersulfaturia. Sat1-null mice also exhibit hyperoxalemia, hyperoxaluria, and calcium oxalate urolithiasis. This review will highlight the current pathophysiological features of NaS1- and Sat1-null mice resulting from alterations in circulating sulfate and oxalate anion levels.

Sat1 is dispensable for active oxalate secretion in mouse duodenum

Mice deficient for the apical membrane oxalate transporter SLC26A6 develop hyperoxalemia, hyperoxaluria, and calcium oxalate stones due to a defect in intestinal oxalate secretion. However, the nature of the basolateral membrane oxalate transport process that operates in series with SLC26A6 to mediate active oxalate secretion in the intestine remains unknown. Sulfate anion transporter-1 (Sat1 or SLC26A1) is a basolateral membrane anion exchanger that mediates intestinal oxalate transport. Moreover, Sat1-deficient mice also have a phenotype of hyperoxalemia, hyperoxaluria, and calcium oxalate stones. We, therefore, tested the role of Sat1 in mouse duodenum, a tissue with Sat1 expression and SLC26A6-dependent oxalate secretion. Although the active secretory flux of oxalate across mouse duodenum was strongly inhibited (>90%) by addition of the disulfonic stilbene DIDS to the basolateral solution, secretion was unaffected by changes in medium concentrations of sulfate and bicarbonate, key substrates for Sat1-mediated anion exchange. Inhibition of intracellular bicarbonate production by acetazolamide and complete removal of bicarbonate from the buffer also produced no change in oxalate secretion. Finally, active oxalate secretion was not reduced in Sat1-null mice. We conclude that a DIDS-sensitive basolateral transporter is involved in mediating oxalate secretion across mouse duodenum, but Sat1 itself is dispensable for this process.

Physiological roles of renal anion transporters NaS1 and Sat1

This review will briefly summarize current knowledge on the renal anion transporters sodium-sulfate cotransporter-1 (NaS1; Slc13a1) and sulfate-anion transporter-1 (Sat1; Slc26a1). NaS1 and Sat1 mediate renal proximal tubular sulfate reabsorption and thereby regulate blood sulfate levels. Sat1 also mediates renal oxalate transport and controls blood oxalate levels. Targeted disruption of murine NaS1 and Sat1 leads to hyposulfatemia and hypersulfaturia. Sat1 null mice also exhibit hyperoxalemia, hyperoxaluria, and calcium oxalate urolithiasis. NaS1 and Sat1 null mice also have other phenotypes that result due to changes in blood sulfate and oxalate levels. Experimental data indicate that NaS1 is essential for maintaining sulfate homeostasis, whereas Sat1 controls both sulfate and oxalate homeostasis in vivo.

Fetal loss and hyposulfataemia in pregnant NaS1 transporter null mice

Sulfate is important for growth and development, and is supplied from mother to fetus throughout pregnancy. We used NaS1 sulfate transporter null (Nas1(-/-)) mice to investigate the role of NaS1 in maintaining sulfate homeostasis during pregnancy and to determine the physiological consequences of maternal hyposulfataemia on fetal, placental and postnatal growth. We show that maternal serum (≤0.5 mM), fetal serum (<0.1 mM) and amniotic fluid (≤0.5 mM) sulfate levels were significantly lower in pregnant Nas1(-/-) mice when compared with maternal serum (≍2.0 mM), fetal serum (≍1.5 mM) and amniotic fluid (≍1.7 mM) sulfate levels in pregnant Nas1(+/+) mice. After 12 days of pregnancy, fetal reabsorptions led to markedly reduced (by ≥50%) fetal numbers in Nas1(-/-) mice. Placental labyrinth and spongiotrophoblast layers were increased (by ≍140%) in pregnant Nas1(-/-) mice when compared to pregnant Nas1(+/+) mice. Birth weights of progeny from female Nas1(-/-) mice were increased (by ≍7%) when compared to progeny of Nas1(+/+) mice. These findings show that NaS1 is essential to maintain high maternal and fetal sulfate levels, which is important for maintaining pregnancy, placental development and normal birth weight.

Physiological roles of mammalian sulfate transporters NaS1 and Sat1

This review summarizes the physiological roles of the renal sulfate transporters NaS1 (Slc13a1) and Sat1 (Slc26a1). NaS1 and Sat1 encode renal anion transporters that mediate proximal tubular sulfate reabsorption and thereby regulate blood sulfate levels. Targeted disruption of murine NaS1 and Sat1 leads to hyposulfatemia and hypersulfaturia. Sat1 null mice also exhibit hyperoxalemia, hyperoxaluria and calcium oxalate urolithiasis. Dysregulation of NaS1 and Sat1 leads to hypersulfaturia, hyposulfatemia and liver damage. Loss of Sat1 leads additionally to hyperoxaluria with hyperoxalemia, nephrocalcinosis and calcium oxalate urolithiasis. These data indicate that the renal anion transporters NaS1 and Sat1 are essential for sulfate and oxalate homeostasis, respectively.

WITHDRAWN: NaS1 sulfate transporter is linked to hyposulfatemia and longevity

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Urolithiasis and hepatotoxicity are linked to the anion transporter Sat1 in mice

Urolithiasis, a condition in which stones are present in the urinary system, including the kidneys and bladder, is a poorly understood yet common disorder worldwide that leads to significant health care costs, morbidity, and work loss. Acetaminophen-induced liver damage is a major cause of death in patients with acute liver failure. Kidney and urinary stones and liver toxicity are disturbances linked to alterations in oxalate and sulfate homeostasis, respectively. The sulfate anion transporter-1 (Sat1; also known as Slc26a1) mediates epithelial transport of oxalate and sulfate, and its localization in the kidney, liver, and intestine suggests that it may play a role in oxalate and sulfate homeostasis. To determine the physiological roles of Sat1, we created Sat1-/- mice by gene disruption. These mice exhibited hyperoxaluria with hyperoxalemia, nephrocalcinosis, and calcium oxalate stones in their renal tubules and bladder. Sat1-/- mice also displayed hypersulfaturia, hyposulfatemia, and enhanced acetaminophen-induced liver toxicity. These data suggest that Sat1 regulates both oxalate and sulfate homeostasis and may be critical to the development of calcium oxalate urolithiasis and hepatotoxicity.

Reduced mucin sulfonation and impaired intestinal barrier function in the hyposulfataemic NaS1 null mouse

OBJECTIVE

Sulfate (SO(4)(2-)) is an abundant component of intestinal mucins and its content is decreased in certain gastrointestinal diseases, including inflammatory bowel disease. In this study, the hyposulfataemic NaS1 sulfate transporter null (Nas1(-/-)) mice were used to investigate the physiological consequences of disturbed sulfate homeostasis on (1) intestinal sulfomucin content and mRNA expression; (2) intestinal permeability and proliferation; (3) dextran sulfate sodium (DSS)-induced colitis; and (4) intestinal barrier function against the bacterial pathogen, Campylobacter jejuni.

METHODS

Intestinal sulfomucins and sialomucins were detected by high iron diamine staining, permeability was assessed by fluorescein isothiocyanate (FITC)-dextran uptake, and proliferation was assessed by 5-bromodeoxyuridine (BrdU) incorporation. Nas1(-/-) and wild-type (Nas1(+/+)) mice received DSS in drinking water, and intestinal damage was assessed by histological, clinical and haematological measurements. Mice were orally inoculated with C jejuni, and intestinal and systemic infection was assessed. Ileal mRNA expression profiles of Nas1(-/-) and Nas1(+/+) mice were determined by cDNA microarrays and validated by quantitative real-time PCR.

RESULTS

Nas1(-/-) mice exhibited reduced intestinal sulfomucin content, enhanced intestinal permeability and DSS-induced colitis, and developed systemic infections when challenged orally with C jejuni. The transcriptional profile of 41 genes was altered in Nas1(-/-) mice, with the most upregulated gene being pancreatic lipase-related protein 2 and the most downregulated gene being carbonic anhydrase 1 (Car1).

CONCLUSION

Sulfate homeostasis is essential for maintaining a normal intestinal metabolic state, and hyposulfataemia leads to reduced intestinal sulfomucin content, enhanced susceptibility to toxin-induced colitis and impaired intestinal barrier to bacterial infection.

Kidney transcriptome reveals altered steroid homeostasis in NaS1 sulfate transporter null mice

Sulfate is essential for human growth and development, and circulating sulfate levels are maintained by the NaS1 sulfate transporter which is expressed in the kidney. Previously, we generated a NaS1-null (Nas1(-/-)) mouse which exhibits hyposulfatemia. In this study, we investigated the kidney transcriptome of Nas1(-/-) mice. We found increased (n=25) and decreased (n=60) mRNA levels of genes with functional roles that include sulfate transport and steroid metabolism. Corticosteroid-binding globulin was the most up-regulated gene (110% increase) in Nas1(-/-) mouse kidney, whereas the sulfate anion transporter-1 (Sat1) was among the most down-regulated genes (>or=50% decrease). These findings led us to investigate the circulating and urinary steroid levels of Nas1(-/-) and Nas1(+/+) mice, which revealed reduced blood levels of corticosterone ( approximately 50% decrease), dehydroepiandrosterone (DHEA, approximately 30% decrease) and DHEA-sulfate ( approximately 40% decrease), and increased urinary corticosterone ( approximately 16-fold increase) and DHEA ( approximately 40% increase) levels in Nas1(-/-) mice. Our data suggest that NaS1 is essential for maintaining a normal metabolic state in the kidney and that loss of NaS1 function leads to reduced circulating steroid levels and increased urinary steroid excretion.

Functional and structural characterization of the zebrafish Na+-sulfate cotransporter 1 (NaS1) cDNA and gene (slc13a1)

Sulfate plays an essential role during growth, development and cellular metabolism. In this study, we characterized the function and structure of the zebrafish ( Danio rerio) Na+-sulfate cotransporter 1 (NaS1) cDNA and gene ( slc13a1). Zebrafish NaS1 encodes a protein of 583 amino acids with 13 putative transmembrane domains. Expression of zebrafish NaS1 protein in Xenopus oocytes led to Na+-sulfate cotransport, which was significantly inhibited by thiosulfate, selenate, molybdate, and tungstate. Zebrafish NaS1 transport kinetics were: Vmax = 1,731.670 ± 92.853 pmol sulfate/oocyte·hour and Km = 1.414 ± 0.275 mM for sulfate and Vmax = 307.016 ± 32.992 pmol sulfate/oocyte·hour, Km = 24.582 ± 4.547 mM and n (Hill coefficient) = 1.624 ± 0.354 for sodium. Zebrafish NaS1 mRNA is developmentally expressed in embryos from day 1 postfertilization and in the intestine, kidney, brain, and eye of adult zebrafish. The zebrafish NaS1 gene slc13a1 contains 15 exons spanning 8,716 bp. Characterization of the zebrafish NaS1 contributes to a greater understanding of sulfate transporters in a well-defined genetic model and will allow the elucidation of evolutionary and functional relationships among vertebrate sulfate transporters.

Monitoring protein-protein interactions between the mammalian integral membrane transporters and PDZ-interacting partners using a modified split-ubiquitin membrane yeast two-hybrid system

PDZ-binding motifs are found in the C-terminal tails of numerous integral membrane proteins where they mediate specific protein-protein interactions by binding to PDZ-containing proteins. Conventional yeast two-hybrid screens have been used to probe protein-protein interactions of these soluble C termini. However, to date no in vivo technology has been available to study interactions between the full-length integral membrane proteins and their cognate PDZ-interacting partners. We previously developed a split-ubiquitin membrane yeast two-hybrid (MYTH) system to test interactions between such integral membrane proteins by using a transcriptional output based on cleavage of a transcription factor from the C terminus of membrane-inserted baits. Here we modified MYTH to permit detection of C-terminal PDZ domain interactions by redirecting the transcription factor moiety from the C to the N terminus of a given integral membrane protein thus liberating their native C termini. We successfully applied this "MYTH 2.0" system to five different mammalian full-length renal transporters and identified novel PDZ domain-containing partners of the phosphate (NaPi-IIa) and sulfate (NaS1) transporters that would have otherwise not been detectable. Furthermore this assay was applied to locate the PDZ-binding domain on the NaS1 protein. We showed that the PDZ-binding domain for PDZK1 on NaS1 is upstream of its C terminus, whereas the two interacting proteins, NHERF-1 and NHERF-2, bind at a location closer to the N terminus of NaS1. Moreover NHERF-1 and NHERF-2 increased functional sulfate uptake in Xenopus oocytes when co-expressed with NaS1. Finally we used MYTH 2.0 to demonstrate that the NaPi-IIa transporter homodimerizes via protein-protein interactions within the lipid bilayer. In summary, our study establishes the MYTH 2.0 system as a novel tool for interactive proteomics studies of membrane protein complexes.

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.

Molecular cloning and characterization of the mouse Na+ sulfate cotransporter gene (Slc13a4): Structure and expression

Sulfate is an essential ion required for numerous functions in mammalian physiology. Due to its hydrophilic nature, cells require sulfate transporters on their plasma membranes to allow entry of sulfate into cells. In this study, we identified a new mouse Na(+)-sulfate cotransporter (mNaS2), characterized its tissue distribution and determined its cDNA and gene (Slc13a4) structures. mNaS2 mRNA was expressed in placenta, brain, lung, eye, heart, testis, thymus and liver. The mouse NaS2 cDNA spans 3384 nucleotides and its open frame encodes a protein of 624 amino acids. Slc13a4 maps to mouse chromosome 6B1 and contains 16 exons, spanning over 40 kb in length. Its 5'-flanking region contains CAAT- and GC-box motifs and a number of putative transcription factor binding sites, including GATA-1, MTF-1, STAT6 and HNF4 consensus sequences. This is the first study to define the tissue distribution of mNaS2 and resolve its cDNA and gene structures, which will allow us to investigate mNaS2 gene expression in vivo and determine its role in mammalian physiology.

Quaternary structure and apical membrane sorting of the mammalian NaSi-1 sulfate transporter in renal cell lines

NaSi-1 encodes a Na(+)-sulfate cotransporter expressed on the apical membrane of renal proximal tubular cells, which is responsible for body sulfate homeostasis. Limited information is available on NaSi-1 protein structure and the mechanisms controlling its apical membrane sorting. The aims of this study were to biochemically determine the quaternary structure of the rat NaSi-1 protein and to characterize its expression in renal epithelial cell lines. Hexahistidyl-tagged NaSi-1 (NaSi-1-His) proteins expressed in Xenopus oocytes, appeared as two bands of about 60 and 75 kDa. PNGase F treatment shifted both bands to 57 kDa while endoglycosidase H treatment led to a downward shift of the lower molecular mass band only. Mutagenesis of a putative N-glycosylation site (N591S) produced a single band that was not shifted by endoglycosidase H or PNGase F, confirming a single glycosylation site at residue 591. Blue native-PAGE and cross-linking experiments revealed dimeric complexes, suggesting the native form of NaSi-1 to be a dimer. Transient transfection of EGFP/NaSi-1 in renal epithelial cells (OK, LLC-PK1 and MDCK) demonstrated apical membrane sorting, which was insensitive to tunicamycin. Transfection of the EGFP/NaSi-1 N591S glycosylation mutant also showed apical expression, suggesting N591 is not essential for apical sorting. Treatment with cholesterol depleting compounds did not disrupt apical sorting, but brefeldin A led to misrouting to the basolateral membrane, suggesting that NaSi-1 sorting is through the ER to Golgi pathway. Our data demonstrates that NaSi-1 forms a dimeric protein which is glycosylated at N591, whose sorting to the apical membrane in renal epithelial cells is brefeldin A-sensitive and independent of lipid rafts or glycosylation.

Disruption of NaS1 sulfate transport function in mice leads to enhanced acetaminophen-induced hepatotoxicity

Sulfate is required for detoxification of xenobiotics such as acetaminophen (APAP), a leading cause of liver failure in humans. The NaS1 sulfate transporter maintains blood sulfate levels sufficiently high for sulfonation reactions to work effectively for drug detoxification. In the present study, we identified two loss-of-function polymorphisms in the human NaS1 gene and showed the Nas1-null mouse to be hypersensitive to APAP hepatotoxicity. APAP treatment led to increased liver damage and decreased hepatic glutathione levels in the hyposulfatemic Nas1-null mice compared with that in normosulfatemic wild-type mice. Analysis of urinary APAP metabolites revealed a significantly lower ratio of APAP-sulfate to APAP-glucuronide in the Nas1-null mice. These results suggest hyposulfatemia increases sensitivity to APAP-induced hepatotoxicity by decreasing the sulfonation capacity to metabolize APAP. In conclusion, the results of this study highlight the importance of plasma sulfate level as a key modulator of acetaminophen metabolism and suggest that individuals with reduced NaS1 sulfate transporter function would be more sensitive to hepatotoxic agents.

Transcriptional profile reveals altered hepatic lipid and cholesterol metabolism in hyposulfatemic NaS1 null mice

Sulfate plays an essential role in human growth and development, and its circulating levels are maintained by the renal Na+-SO42- cotransporter, NaS1. We previously generated a NaS1 knockout (Nas1-/-) mouse, an animal model for hyposulfatemia, that exhibits reduced growth and liver abnormalities including hepatomegaly. In this study, we investigated the hepatic gene expression profile of Nas1-/- mice using oligonucleotide microarrays. The mRNA expression levels of 92 genes with known functional roles in metabolism, cell signaling, cell defense, immune response, cell structure, transcription, or protein synthesis were increased (n = 51) or decreased (n = 41) in Nas1-/- mice when compared with Nas1+/+ mice. The most upregulated transcript levels in Nas1-/- mice were found for the sulfotransferase genes, Sult3a1 (approximately 500% increase) and Sult2a2 (100% increase), whereas the metallothionein-1 gene, Mt1, was among the most downregulated genes (70% decrease). Several genes involved in lipid and cholesterol metabolism, including Scd1, Acly, Gpam, Elov16, Acsl5, Mvd, Insig1, and Apoa4, were found to be upregulated (> or = 30% increase) in Nas1-/- mice. In addition, Nas1-/- mice exhibited increased levels of hepatic lipid (approximately 16% increase), serum cholesterol (approximately 20% increase), and low-density lipoprotein (approximately 100% increase) and reduced hepatic glycogen (approximately 50% decrease) levels. In conclusion, these data suggest an altered lipid and cholesterol metabolism in the hyposulfatemic Nas1-/- mouse and provide new insights into the metabolic state of the liver in Nas1-/- mice.

Sulfate transport by SLC26 transporters

Sulfate is the fourth most abundant anion in human plasma that is essential for numerous physiological functions, including biotransformation of xenobiotics, steroids, non-steroidal anti-inflammatory drugs (NSAIDs), adrenergic stimulants/blockers and analgesics. Sulfate is also required for activation of many endogenous compounds (heparin, heparan sulfate, dermatan sulfate, bile acids) and utilized in the metabolism of neurotransmitters. Sulfation of structural components, including glycosaminoglycans and cerebroside sulfate, is essential for the maintenance of normal structure and function of tissues. Due to its hydrophilic nature, sulfate cannot readily cross the lipid bilayer of cells, thus plasma membrane proteins, known as sulfate transporters, are required for the movement of sulfate into/out of cells. Sulfate transporters can be divided into two distinct groups: Na+-dependent sulfate transporters belonging to the SLC13 gene family and Na+-independent sulfate transporters (antiporters, exchangers) belonging to SLC26 gene family. There are 11 members of the SLC26 family (including Sat1, DTDST, CLD, pendrin, prestin, cfex) whose structures and functions have been only partially characterized. In this presentation, the current information on the structures and functions of the sulfate transporters in the SLC26 gene family will be described and the issue that certain members of this family are unable to transport sulfate, will be addressed.

Impaired memory and olfactory performance in NaSi-1 sulphate transporter deficient mice

In the present study, NaSi-1 sulphate transporter knock-out (Nas1-/-) mice, an animal model of hyposulphataemia, were examined for spatial memory and learning in a Morris water maze, and for olfactory function in a cookie test. The Nas1-/- mice displayed significantly (P<0.05) increased latencies to find an escape platform in the reversal learning trials at 2 days but not 1 day after the last acquisition trial in a Morris water maze test, suggesting that Nas1-/- mice may have proactive memory interference. While the wild-type (Nas1+/+) mice showed a significant (P<0.02) decrease in time to locate a hidden food reward over four trials after overnight fasting, Nas1-/- mice did not change their performance, resulting in significantly (P<0.05) higher latencies when compared to their Nas1+/+ littermates. There were no significant differences between Nas1-/- and Nas1+/+ mice in the cookie test after moderate food deprivation. In addition, both Nas1-/- and Nas1+/+ mice displayed similar escape latencies in the acquisition phase of the Morris water maze test, suggesting that learning, motivation, vision and motor skills required for the task may not be affected in Nas1-/- mice. This is the first study to demonstrate an impairment in memory and olfactory performance in the hyposulphataemic Nas1-/- mouse.

NaSi-1 and Sat-1: structure, function and transcriptional regulation of two genes encoding renal proximal tubular sulfate transporters

Sulfate (SO(4)2-) is an important anion regulating many metabolic and cellular processes. Maintenance of SO4(2-) homeostasis occurs in the renal proximal tubule via membrane transport proteins. Two SO(4)2- transporters that have been characterized and implicated in regulating serum SO4(2-) levels are: NaSi-1, a Na+-SO(4)2- cotransporter located at the brush border membrane and Sat-1, a SO4(2-)-anion exchanger located on the basolateral membranes of proximal tubular cells. Unlike Sat-1, for which very few studies have looked at regulation of its expression, NaSi-1 has been shown to be regulated by various hormones and dietary conditions in vivo. To study this further, NaSi-1 (SLC13A1) and Sat-1 (SLC26A1) gene structures were determined and recent studies have characterized their respective gene promoters. This review presents the current understanding of the transcriptional regulation of NaSi-1 and Sat-1, and describes possible pathogenetic implications which arise as a consequence of altered SO(4)2- homeostasis.

Pathogenetics of the human SLC26 transporters

Over the past decade, 11 human genes belonging to the solute linked carrier (SLC) 26 family of transporters, have been identified. The SLC26 proteins, which include SAT-1, DTDST, DRA/CLD, pendrin, prestin, PAT-1/CFEX and Tat-1, are structurally related and have been shown to transport one or more of the following substrates: sulfate, chloride, bicarbonate, iodide, oxalate, formate, hydroxyl or fructose. Special interest has focused on four members of the SLC26 family that are associated with distinct recessive diseases: (i) Mutations in SLC26A2 lead to four different chondrodysplasias (diastrophic dysplasia, atelosteogenesis type II, achondrogenesis type IB and multiple epiphyseal dysplasia); (ii) SLC26A3 is associated with congenital chloride diarrhea; (iii) SLC26A4 is associated with Pendred syndrome and non-syndromic deafness, DFNB4; and (iv) SLC26A5 is defective in non-syndromic hearing impairment. This review article summarizes current information on the pathophysiological consequences of mutations in the human SLC26A2 to A5 genes.

The rat Na+-sulfate cotransporter rNaS2: functional characterization, tissue distribution, and gene (slc13a4) structure

Inorganic sulfate is essential for numerous functions in mammalian physiology. In the present study, we characterized the functional properties of the rat Na+-sulfate cotransporter NaS2 (rNaS2), determined its tissue distribution, and identified its gene (slc13a4) structure. Expression of rNaS2 protein in Xenopus oocytes led to a Na+-dependent transport of sulfate that was inhibited by phosphate, thiosulfate, tungstate, selenate, oxalate, and molybdate, but not by citrate, succinate, or DIDS. Transport kinetics of rNaS2 determined a K(M) for sulfate of 1.26 mM. Na+ kinetics determined a Hill coefficient of n=3.0+/-0.7, suggesting a Na+:SO4 (2-) stoichiometry of 3:1. rNaS2 mRNA was highly expressed in placenta, with lower levels found in the brain and liver. slc13a4 maps to rat chromosome 4 and contains 17 exons, spanning over 46 kb in length. This gene produces two alternatively spliced transcripts, of which the transcript lacking exon 2 is the most abundant form. Its 5' flanking region contains CAAT- and GC-box motifs and a number of putative transcription factor binding sites, including GATA-1, SP1, and AP-2 consensus sequences. This is the first study to characterize rNaS2 transport kinetics, define its tissue distribution, and resolve its gene (slc13a4) structure and 5' flanking region.

Control of ion transport in mammalian airways by protease activated receptors type 2 (PAR-2)

Protease-activated receptors (PARs) are widely distributed in human airways. They couple to G- proteins and are activated after proteolytic cleavage of the N terminus of the receptor. Evidence is growing that PAR subtype 2 plays a pivotal role in inflammatory airway diseases, such as allergic asthma or bronchitis. However, nothing is known about the effects of PAR-2 on electrolyte transport in the native airways. PAR-2 is expressed in airway epithelial cells, where they are activated by mast cell tryptase, neutrophil proteinase 3, or trypsin. Recent studies produced conflicting results about the functional consequence of PAR-2 stimulation. Here we report that stimulation of PAR-2 receptors in mouse and human airways leads to a change in electrolyte transport and a shift from absorption to secretion. Although PAR-2 appears to be expressed on both sides of the epithelium, only basolateral stimulation results in inhibition of amiloride sensitive Na+ conductance and stimulation of both luminal Cl- channels and basolateral K+ channels. The present data indicate that these changes occur through activation of phospholipase C and increase in intracellular Ca2+, which activates basolateral SK4 K+ channels and luminal Ca2+-dependent Cl- channels. In addition, the present data suggest a PAR-2 mediated release of prostaglandin E2, which may contribute to the secretory response. In conclusion, these results provide further evidence for a role of PAR-2 in inflammatory airway disease: stimulation of these receptors may cause accumulation of airway surface liquid, which, however, may help to flush noxious stimuli away from the affected airways.

Functional characterization and genomic organization of the human Na(+)-sulfate cotransporter hNaS2 gene (SLC13A4)

Sulfate plays an essential role in human growth and development. Here, we characterized the functional properties of the human Na(+)-sulfate cotransporter (hNaS2), determined its tissue distribution, and identified its gene (SLC13A4) structure. Expression of hNaS2 protein in Xenopus oocytes led to a Na(+)-dependent transport of sulfate that was inhibited by thiosulfate, phosphate, molybdate, selenate and tungstate, but not by oxalate, citrate, succinate, phenol red or DIDS. Transport kinetics of hNaS2 determined a K(m) for sulfate of 0.38mM, suggestive of a high affinity sulfate transporter. Na(+) kinetics determined a Hill coefficient of n=1.6+/-0.6, suggesting a Na:SO(4)(2-) stoichiometry of 2:1. hNaS2 mRNA was highly expressed in placenta and testis, with intermediate levels in brain and lower levels found in the heart, thymus, and liver. The SLC13A4 gene contains 16 exons, spanning over 47kb in length. Its 5'-flanking region contains CAAT- and GC-box motifs, and a number of putative transcription factor binding sites, including GATA-1, AP-1, and AP-2 consensus sequences. This is the first study to characterize hNaS2 transport kinetics, define its tissue distribution, and resolve its gene (SLC13A4) structure and 5' flanking region.

Behavioural abnormalities of the hyposulphataemic Nas1 knock-out mouse

We recently generated a sodium sulphate cotransporter knock-out mouse (Nas1-/-) which has increased urinary sulphate excretion and hyposulphataemia. To examine the consequences of disturbed sulphate homeostasis in the modulation of mouse behavioural characteristics, Nas1-/- mice were compared with Nas1+/- and Nas1+/+ littermates in a series of behavioural tests. The Nas1-/- mice displayed significantly (P < 0.001) decreased marble burying behaviour (4.33 +/- 0.82 buried) when compared to Nas1+/+ (7.86 +/- 0.44) and Nas1+/- (8.40 +/- 0.37) animals, suggesting that Nas1-/- mice may have decreased object-induced anxiety. The Nas1-/- mice also displayed decreased locomotor activity by moving less distance (1.53 +/- 0.27 m, P < 0.05) in an open-field test when compared to Nas1+/+ (2.31 +/- 0.24 m) and Nas1+/- (2.15 +/- 0.19 m) mice. The three genotypes displayed similar spatiotemporal and ethological behaviours in the elevated-plus maze and open-field test, with the exception of a decreased defecation frequency by the Nas1-/- mice (40% reduction, P < 0.01). There were no significant differences between Nas1-/- and Nas1+/+ mice in a rotarod performance test of motor coordination and in the forced swim test assessing (anti-)depressant-like behaviours. This is the first study to demonstrate behavioural abnormalities in the hyposulphataemic Nas1-/- mice.

Purinergic inhibition of the epithelial Na+ transport via hydrolysis of PIP2

Stimulation of purinergic receptors inhibits amiloride-sensitive Na+ transport in epithelial tissues by an unknown mechanism. Because previous studies excluded the role of intracellular Ca2+ or protein kinase C, we examined whether purinergic regulation of Na+ absorption occurs via hydrolysis of phospholipid such as phosphatidylinositol-bisphosphates (PIP2). Inhibition of amiloride-sensitive short-circuit currents (Isc-Amil) by adenine 5'-triphosphate (ATP) in native tracheal epithelia and M1 collecting duct cells was suppressed by binding neomycin to PIP2, and recovery from ATP inhibition was abolished by blocking phosphatidylinositol-4-kinase or diacylglycerol kinase. Stimulation by ATP depleted PIP2 from apical membranes, and PIP2 co-immunoprecipitated the beta subunit of ENaC. ENaC was inhibited by ATP stimulation of P2Y2 receptors in Xenopus oocytes. Mutations in the PIP2 binding domain of betaENaC but not gammaENaC reduced ENaC currents without affecting surface expression. Collectively, these data supply evidence for a novel and physiologically relevant regulation of ENaC in epithelial tissues. Although surface expression is controlled by its C terminus, N-terminal binding of betaENaC to PIP2 determines channel activity.

Slc26a6: a cardiac chloride-hydroxyl exchanger and predominant chloride-bicarbonate exchanger of the mouse heart

Bicarbonate facilitate more than 50% of pH recovery in the acidotic myocardium, and have roles in cardiac hypertrophy and steady-state pH regulation. To determine which bicarbonate transporters are responsible for this activity, we measured the expression levels of all known HCO3(-)-anion exchange proteins in mouse heart, by quantitative real time RT-PCR. Bicarbonate-anion exchangers are members of either the SLC4A or the SLC26A gene families. In neonatal and adult myocardium, AE1 (Slc4a1), AE2 (Slc4a2), AE3 (Slc4a3) (AE3fl and AE3c variants), Slc26a3 and Slc26a6 were expressed. Adult hearts expressed Slc26a3 and Slc4a1-3 mRNAs at similar levels, while Slc26a6 mRNA was about seven-fold higher than AE3, which was more abundant than any other. Immunohistochemistry revealed that Slc26a6 and AE3 are present in the plasma membrane of ventricular myocytes. Slc26a6 expression levels were higher in ventricle than atrium, whereas AE3 was detected only in ventricle. Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange activity of SLC26A6 and AE3 were investigated in transfected HEK293 cells, using intracellular fluorescence measurements of 2',7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), to monitor intracellular pH (pH(i)). Rates of pH(i) change were measured under HCO3(-)-containing (Cl(-)-HCO(3)(-)) or nominally HCO3(-)-free (Cl(-)-OH(-)) conditions. HCO3(-) fluxes were similar for cells expressing AE3fl, SLC26A6 or Slc26a3, suggesting that they have similar transport activity. However, only SLC26A6 and Slc26a3 functioned as Cl(-)-OH(-) exchangers. Activation of alpha-adrenergic receptors, which stimulates protein kinase C, inhibited SLC26A6 Cl(-)-HCO(3)(-) exchange activity. We conclude that Slc26a6 is the predominant Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchanger of the myocardium and that Slc26a6 is negatively regulated upon alpha-adrenergic stimulation.

Acute effects of parainfluenza virus on epithelial electrolyte transport

Parainfluenza viruses are important causes of respiratory disease in both children and adults. In particular, they are the major cause of the serious childhood illness croup (laryngotracheobronchitis). The infections produced by parainfluenza viruses are associated with the accumulation of ions and fluid in the respiratory tract. It is not known, however, whether this accumulation is because of a direct effect of the viruses on ion and fluid transport by the respiratory epithelium. Here we show that a model parainfluenza virus (the Sendai virus), in concentrations observed during respiratory infections, activates Cl- secretion and inhibits Na+ absorption across the tracheal epithelium. It does so by binding to a neuraminidase-insensitive glycolipid, possibly asialo-GM1, triggering the release of ATP, which then acts in an autocrine fashion on apical P2Y receptors to produce the observed changes in ion transport. These findings indicate that fluid accumulation in the respiratory tract associated with parainfluenza virus infection is attributable, at least in part, to direct effects of the virus on ion transport by the respiratory epithelium.

Characterization of the human renal Na(+)-sulphate cotransporter gene ( NAS1) promoter

Sulphate (SO(4)(2-)) plays an essential role during growth, development, and cellular metabolism. Recently, we have isolated the human renal Na(+)-SO(4)(2-) cotransporter (hNaSi-1) that is implicated in the regulation of serum SO(4)(2-) levels. To gain an insight into hNaSi-1 regulation, our aims were to clone and characterize functionally the hNaSi-1 gene ( NAS1) promoter. We PCR-amplified 3742 bp of the NAS1 5'-flanking region, which is 64% AT-rich and contains numerous putative cis-acting elements. The NAS1 transcription start site was mapped to 25 bp upstream from the translation start site. NAS1 promoter truncations fused to luciferase gene constructs transfected into renal LLC-PK1, MDCK and OK cells allowed us to establish that the first 169 bp of the NAS1 promoter are sufficient for basal transcription. Furthermore, the NAS1 promoter conferred responsiveness to the polycyclic aromatic hydrocarbon 3-methylcholanthrene (3-MC), but not to thyroid hormone (T(3)) or vitamin D [1,25-(OH)(2)D(3)]. Site-directed mutagenesis of the NAS1 promoter identified a functional xenobiotic response element at -2,052, which conferred 3-MC responsiveness. The human NAS1 gene promoter is not responsive to Vitamin D or T(3), unlike the mouse Nas1 promoter with which it shares approximately 40% sequence similarity, but is transactivated by 3-MC, suggesting that the control of renal SO(4)(2-) reabsorption via the regulation of NAS1 transcription may be important for maintaining the sulphation potential for kidney polycyclic aromatic hydrocarbon metabolism.

A dileucine motif targets the sulfate anion transporter sat-1 to the basolateral membrane in renal cell lines

The sat-1 transporter mediates sulfate/bicarbonate/oxalate anion exchange in vivo at the basolateral membrane of the kidney proximal tubule. In the present study, we show two renal cell lines [Madin-Darby canine kidney (MDCK) and porcine proximal tubular kidney (LLC-PK1) cells] that similarly target sat-1 exclusively to the basolateral membrane. To identify possible sorting determinants, we generated truncations of the sat-1 cytoplasmic COOH terminus, fused to enhanced green fluorescence protein (EGFP) or the human IL-2 receptor alpha-chain (Tac) protein, and both fusion constructs were transiently transfected into MDCK cells. Confocal microscopy revealed that removal of the last three residues on the sat-1 COOH terminus, a putative PDZ domain, had no effect on basolateral sorting in MDCK cells or on sulfate transport in Xenopus oocytes. Removal of the last 30 residues led to an intracellular expression for the GFP fusion protein and an apical expression for the Tac fusion protein, suggesting that a possible sorting motif lies between the last 3 and 30 residues of the sat-1 COOH terminus. Elimination of a dileucine motif at position 677/678 resulted in the loss of basolateral sorting, suggesting that this motif is required for sat-1 targeting to the basolateral membrane. This posttranslational mechanism may be important for the regulation of sulfate reabsorption and oxalate secretion by sat-1 in the kidney proximal tubule.

The SLC13 gene family of sodium sulphate/carboxylate cotransporters

The SLC13 gene family consist of five sequence-related members that have been identified in a variety of animals, plants, yeast and bacteria. Proteins encoded by these genes are divided into two functionally unrelated groups: the Na(+)-sulphate (NaS) cotransporters and the Na(+)-carboxylate (NaC) cotransporters. Members of this family include the renal Na(+)-dependent inorganic sulphate transporter-1 (NaSi-1, SLC13A1), the Na(+)-dependent dicarboxylate transporters NaDC-1/SDCT1 (SLC13A2), NaDC-3/SDCT2 (SLC13A3), the sulphate transporter-1 (SUT-1, SLC13A4) and the Na(+)-coupled citrate transporter (NaCT, SLC13A5). The general characteristics of the SLC13 proteins are that they encode multi-spanning proteins with 8-13 transmembrane domains, have a wide tissue distribution with most being expressed in the epithelial cells of the kidney and the gastrointestinal tract. They are Na(+)-coupled symporters, DIDS-insensitive, with strong cation preference for Na(+), with a Na(+):anion coupling ratio of around 3:1 and have a substrate preference for divalent anions, which include tetraoxyanions (for the NaS cotransporters) or Krebs cycle intermediates, including mono-, di-, and tri-carboxylates (for the NaC cotransporters). The purpose of this review is to provide an update on the most recent advances and to summarize the biochemical, physiological and structural aspects of the vertebrate SLC13 gene family.

Hyposulfatemia, growth retardation, reduced fertility, and seizures in mice lacking a functional NaSi-1 gene

Inorganic sulfate is required for numerous functions in mammalian physiology, and its circulating levels are proposed to be maintained by the Na+-SO42- cotransporter, (NaSi-1). To determine the role of NaSi-1 in sulfate homeostasis and the physiological consequences in its absence, we have generated a mouse lacking a functional NaSi-1 gene, Nas1. Serum sulfate concentration was reduced by >75% in Nas1-/- mice when compared with Nas1+/+ mice. Nas1-/- mice exhibit increased urinary sulfate excretion, reduced renal and intestinal Na+-SO42- cotransport, and a general growth retardation. Nas1-/- mouse body weight was reduced by >20% when compared with Nas1+/+ and Nas1+/- littermates at 2 weeks of age and remained so throughout adulthood. Nas1-/- females had a lowered fertility, with a 60% reduction in litter size. Spontaneous clonic seizures were observed in Nas1-/- mice from 8 months of age. These data demonstrate NaSi-1 is essential for maintaining sulfate homeostasis, and its expression is necessary for a wide range of physiological functions.

Effects of purinergic stimulation, CFTR and osmotic stress on amiloride-sensitive Na+ transport in epithelia and Xenopus oocytes

Both stimulation of purinergic receptors by ATP and activation of the cystic fibrosis transmembrane conductance regulator (CFTR) inhibit amiloride-sensitive Na+ transport and activate Cl- secretion. These changes in ion transport may well affect cell volume. We therefore examined whether cell shrinkage or cell swelling do affect amiloride-sensitive Na+ transport in epithelial tissues or Xenopus oocytes and whether osmotic stress interferes with regulation of Na+ transport by ATP or CFTR. Stimulation of purinergic receptors by ATP/UTP or activation of CFTR by IBMX and forskolin inhibited amiloride-sensitive transport in mouse trachea and colon, respectively, by a mechanism that was Cl- dependent. When exposed to a hypertonic but not hypotonic bath solution, amiloride-sensitive Na+ transport was inhibited in mouse trachea and colon, independent of the extracellular Cl- concentration. Both inhibition of Na+ transport by hypertonic bath solution and ATP were additive. When coexpressed in Xenopus oocytes, activation of CFTR by IBMX and forskolin inhibited the epithelial Na+ channel (ENaC) in a Cl- dependent fashion. However, both hypertonic and hypotonic bath solutions showed only minor effects on amiloride-sensitive conductance, independent of the bath Cl- concentration. Moreover, CFTR-induced inhibition of ENaC could be detected in oocytes even after exposure to hypertonic or hypotonic bath solutions. We conclude that amiloride-sensitive Na+ absorption in mouse airways and colon is inhibited by cell shrinkage by a mechanism that does not interfere with purinergic and CFTR-mediated inhibition of ENaC.

Isoforms of SLC26A6 mediate anion transport and have functional PDZ interaction domains

The solute carrier gene family SLC26 consists of tissue-specific anion exchanger genes, three of them associated with distinct human recessive disorders. By a genome-driven approach, several new SLC26 family members have been identified, including a kidney- and pancreas-specific gene, SLC26A6. We report the functional characterization of SLC26A6 and two new alternatively spliced variants, named SLC26A6c and SLC26A6d. Immunofluorescence studies on transiently transfected cells indicated membrane localization and indicated that both NH(2)- and COOH-terminal tails of the SLC26A6 variants are located intracellularly, suggesting a topology with an even number of transmembrane domains. Functional expression of the three proteins in Xenopus oocytes demonstrated Cl(-) and SO(4)(2-) transport activity. In addition, the transport of SO(4)(2-) and Cl(-) was inhibited by DIDS and HCO(3)(-). We demonstrated also that the COOH terminus of SLC26A6 binds to the first and second PDZ domains of the Na(+)/H(+) exchanger (NHE)3 kinase A regulatory protein (E3KARP) and NHE3 regulatory factor (NHERF) proteins in vitro. Truncation of the last three amino acids (TRL) of SLC26A6 abrogated the interaction but did not affect transport function. These results demonstrate that SLC26A6 and its two splice variants can function as anion transporters linked to PDZ-interaction pathways. Our results support the general concept of microdomain organization for ion transport and suggest a mechanism for cystic fibrosis transmembrane regulator (CFTR)-mediated SLC26A6 upregulation in pancreatic duct cells.

Transcriptional regulation of the sodium-sulfate cotransporter NaS(i)-1 gene

Inorganic sulfate is one of the most abundant anions in mammalian plasma and is essential for proper cell growth and development, as well as detoxification and activation of many biological compounds. To date, little is understood how physiological levels of sulfate are maintained in the body. Our studies, and of others, have identified the NAS(i)-1 protein to be a functional sulfate transporter in the kidney and intestine, and due to this localization, constitutes a strong candidate gene for maintaining body sulfate homeostasis. Several factors, including hormones and metabolic conditions, have been shown to alter NAS(i)-1 mRNA and protein levels in vivo. In this study, we describe the transcriptional regulation of NaS(i)-1, with a focus on the mouse NaS(i)-1 gene (Nas1) that was recently cloned in our laboratory. Vitamin D (1,25-(OH)2D3) and thyroid hormone (T3) led to an increase in Nas1 promoter activity in OK cells. Mutational analysis of the Nas1 promoter resulted in identification of a direct repeat 6-type vitamin-D-responsive element (DR6 VDRE) at -525 to -508 and an imperfect inverted repeat 0-type T3 responsive element (IRO T3RE) at -426 to -425 which conferred 1,25-(OH)2D3 and T3 responsiveness respectively. These findings suggest for vitamin D and thyroid hormone regulation of NaS(i)-1, may provide important clues to the physiological control of sulfate homeostasis.

Characterization of the human sulfate anion transporter (hsat-1) protein and gene (SAT1; SLC26A1)

Sulfate plays an essential role during growth, development, bone/cartilage formation, and cellular metabolism. In this study, we have isolated the human sulfate anion transporter cDNA (hsat-1; SCL26A1) and gene (SAT1), determined its protein function in Xenopus oocytes and characterized SAT1 promoter activity in mammalian renal cell lines. hsat-1 encodes a protein of 75 kDa, with 12 putative transmembrane domains, that induces sulfate, chloride, and oxalate transport in Xenopus oocytes. hsat-1 mRNA is expressed most abundantly in the kidney and liver, with lower levels in the pancreas, testis, brain, small intestine, colon, and lung. The SAT1 gene is comprised of four exons stretching 6 kb in length, with an alternative splice site formed from an optional exon. SAT1 5' flanking region led to promoter activity in renal OK and LLC-PK1 cells. Using SAT1 5' flanking region truncations, the first 135 bp was shown to be sufficient for basal promoter activity. Mutation of the activator protein-1 (AP-1) site at position -52 in the SAT1 promoter led to loss of transcriptional activity, suggesting its requirement for SAT1 basal expression. This study represents the first functional characterization of the human SAT1 gene and protein encoded by the anion transporter hsat-1.

The mouse sulfate anion transporter gene Sat1 (Slc26a1): cloning, tissue distribution, gene structure, functional characterization, and transcriptional regulation thyroid hormone

Sulfate (SO(4)(2-)) is required for bone/cartilage formation and cellular metabolism. sat-1 is a SO(4)(2-) anion transporter expressed on basolateral membranes of renal proximal tubules, and is suggested to play an important role in maintaining SO(4)(2-) homeostasis. As a first step towards studying its tissue-specific expression, hormonal regulation, and in preparation for the generation of knockout mice, we have cloned and characterized the mouse sat-1 cDNA (msat-1), gene (sat1; Slc26a1) and promoter region. msat-1 encodes a 704 amino acid protein (75.4 kDa) with 12 putative transmembrane domains that induce SO(4)(2-) (also oxalate and chloride) transport in Xenopus oocytes. msat-1 mRNA was expressed in kidney, liver, cecum, calvaria, brain, heart, and skeletal muscle. Two distinct transcripts were expressed in kidney and liver due to alternative utilization of the first intron, corresponding to an internal portion of the 5'-untranslated region. The Sat1 gene (~6 kb) consists of 4 exons. Its promoter is ~52% G + C rich and contains a number of well-characterized cis-acting elements, including sequences resembling hormone responsive elements T(3)REs and VDREs. We demonstrate that Sat1 promoter driven basal transcription in OK cells was stimulated by tri-iodothyronine. Site-directed mutagenesis identified an imperfect T(3)RE at -454-bp in the Sat1 promoter to be responsible for this activity. This study represents the first characterization of the structure and regulation of the Sat1 gene encoding a SO(4)(2-)/chloride/oxalate anion transporter.

The role of putative phosphorylation sites in the targeting and shuttling of the aquaporin-2 water channel

In renal collecting ducts, a vasopressin-induced cAMP increase results in the phosphorylation of aquaporin-2 (AQP2) water channels at Ser-256 and its redistribution from intracellular vesicles to the apical membrane. Hormones that activate protein kinase C (PKC) proteins counteract this process. To determine the role of the putative kinase sites in the trafficking and hormonal regulation of human AQP2, three putative casein kinase II (Ser-148, Ser-229, Thr-244), one PKC (Ser-231), and one protein kinase A (Ser-256) site were altered to mimic a constitutively non-phosphorylated/phosphorylated state and were expressed in Madin-Darby canine kidney cells. Except for Ser-256 mutants, seven correctly folded AQP2 kinase mutants trafficked as wild-type AQP2 to the apical membrane via forskolin-sensitive intracellular vesicles. With or without forskolin, AQP2-Ser-256A was localized in intracellular vesicles, whereas AQP2-S256D was localized in the apical membrane. Phorbol 12-myristate 13-acetate-induced PKC activation following forskolin treatment resulted in vesicular distribution of all AQP2 kinase mutants, while all were still phosphorylated at Ser-256. Our data indicate that in collecting duct cells, AQP2 trafficking to vasopressin-sensitive vesicles is phosphorylation-independent, that phosphorylation of Ser-256 is necessary and sufficient for expression of AQP2 in the apical membrane, and that PMA-induced PKC-mediated endocytosis of AQP2 is independent of the AQP2 phosphorylation state.

Regulation of the mouse Nas1 promoter by vitamin D and thyroid hormone

The renal sodium-sulfate cotransporter, NaS(i)-1, a protein implicated to control serum sulfate levels, has been shown to be regulated in vivo by 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and tri-iodothyronine (T(3)). Recently, we cloned the mouse NaS(i)-1 gene ( Nas1) and in the present study identified a 1,25-(OH)(2)D(3)- and T(3)-responsive element located within the Nas1 promoter. Mutational analysis of the Nas1 promoter resulted in identification of a direct repeat 6-type vitamin-D-responsive element (DR6 VDRE) at -525 to -508 and an imperfect inverted repeat 0-type T(3)-responsive element (IR0 T(3)RE) at -436 to -425 which conferred 1,25-(OH)(2)D(3) and T(3) responsiveness, respectively. In summary, we have identified responsive elements that mediate the enhanced transcription of Nas1 by 1,25-(OH)(2)D(3) and T(3), and these mechanisms may provide important clues to the physiological control of sulfate homeostasis.

Functional characterization of three novel tissue-specific anion exchangers SLC26A7, -A8, and -A9

A second distinct family of anion exchangers, SLC26, in addition to the classical SLC4 (or anion exchanger) family, has recently been delineated. Particular interest in this gene family is stimulated by the fact that the SLC26A2, SLC26A3, and SLC26A4 genes have been recognized as the disease genes mutated in diastrophic dysplasia, congenital chloride diarrhea, and Pendred syndrome, respectively. We report the expansion of the SLC26 gene family by characterizing three novel tissue-specific members, named SLC26A7, SLC26A8, and SLC26A9, on chromosomes 8, 6, and 1, respectively. The SLC26A7-A9 proteins are structurally very similar at the amino acid level to the previous family members and show tissue-specific expression in kidney, testis, and lung, respectively. More detailed characterization by immunohistochemistry and/or in situ hybridization localized SLC26A7 to distal segments of nephrons, SLC26A8 to developing spermatocytes, and SLC26A9 to the lumenal side of the bronchiolar and alveolar epithelium of lung. Expression of SLC26A7-A9 proteins in Xenopus oocytes demonstrated chloride, sulfate, and oxalate transport activity, suggesting that they encode functional anion exchangers. The functional characterization of the novel tissue-specific members may provide new insights to anion transport physiology in different parts of body.