Roles of Slc13a1 and Slc26a1 sulfate transporters of eel kidney in sulfate homeostasis and osmoregulation in freshwater

Abstract

We have shown that the renal sulfate transport system has dual roles in euryhaline eel, namely, maintenance of sulfate homeostasis and osmoregulation of body fluids. To clarify the physiological roles of sulfate transporters in teleost fish, we cloned orthologs of the mammalian renal sulfate transporters Slc13a1 (NaSi-1) and Slc26a1 (Sat-1) from eel (Anguilla japonica) and assessed their functional characteristics, tissue localization, and regulated expression. Full-length cDNAs coding for ajSlc13a1 and ajSlc26a1 were isolated from a freshwater eel kidney cDNA library. Functional expression in Xenopus oocytes revealed the expected sulfate transport characteristics; furthermore, both transporters were inhibited by mercuric chloride. Northern blot analysis, in situ hybridization, and immunohistochemistry demonstrated robust apical and basolateral expression of ajSlc13a1 and ajSlc26a1, respectively, within the proximal tubule of freshwater eel kidney. Expression was dramatically reduced after the transfer of eels from freshwater to seawater; the circulating sulfate concentration in eels was in turn markedly elevated in freshwater compared with seawater conditions (19 mM vs. 1 mM). The reabsorption of sulfate via the apical ajSlc13a1 and basolateral ajSlc26a1 transporters may thus contribute to freshwater osmoregulation in euryhaline eels, via the regulation of circulating sulfate concentration. freshwater adaptation; immunohistochemistry; sulfate transporter; renal proximal tubule SULFATE IS ESSENTIAL for a variety of metabolic and cellular processes, including production of highly sulfated proteoglycans by chondrocytes, detoxification, and elimination of xenobiotics and endogenous compounds by sulfoconjugation in the liver and kidney, and biosynthesis of sulfated hormones such as gastrin and cholecystokinin (27). Mechanisms regulating the levels of plasma sulfate are therefore essential for the maintenance of normal physiology. In mammals, the regulation of sulfate homeostasis is largely determined by the kidney, with the major fraction of filtered sulfate being reabsorbed in the proximal tubule. Two sulfate transporters have been identified that are involved in this reabsorption of sulfate from the glomerular ultrafiltrate: solute carrier family 13a1 (Slc13a1; NaSi-1) and solute carrier family 26a1 (Slc26a1; Sat-1). Slc13a1 is an electrogenic Na ϩ -dependent sulfate transporter (alternatively called Na ϩ -SO 4 2Ϫ cotransporter) that is located in the apical membrane of renal proximal tubule cells and mediates entry of Na ϩ -SO 4 2Ϫ with a stoichiometry of 3:1 (5, 24). Slc26a1 is sulfate/anion exchanger mediating SO 4 2Ϫ efflux across the basolateral membrane in exchange for HCO 3 Ϫ (17). Physiological significance of this regulatory system is well established in mammals through targeted disruption of the Slc13a1 gene (9).Physiological studies on sulfate homeostasis also have been conducted in nonmammalian systems, including those of birds (12,40,44,45), bivalve (11), and fish (10, 36, 37, 43, 46 -49). However, li...

Environmental factors responsible for switching on the SO42−excretory system in the kidney of seawater eels

“…The unique SO 4 2Ϫ accumulation in FW eel plasma was reported previously by Nakada et al (23). Thus the eel exhibits drastic changes in SO 4 2Ϫ regulation in the kidney after transfer from FW to SW and thus serves as an excellent model to investigate the switching mechanism of SO 4 2Ϫ regulation.…”

“…(6,21), but obligatory influx of SO 4 2Ϫ was nullified by excretion via the kidney in marine teleosts (3,7,8,28). We recently showed that the eel is unique in SO 4 2Ϫ regulation compared with other euryhaline teleosts because it has much higher plasma SO 4 2Ϫ in FW (ϳ6 mM) than in SW (ϳ1 mM) (35) as reported previously (23). We also showed that 85% of the SO 4 2Ϫ in SW is taken up by the gills and 97% of SO 4 2Ϫ that enter the circulation is excreted by the kidney in SW eels (Watanabe T, Takei Y, unpublished data).…”

“…In this study, we investigated such factors that trigger the change in SO 4 2Ϫ regulation using plasma and urine concentrations and renal SO 4 2Ϫ transporters as markers. As for the transporters, we chose Slc13a1 and Slc26a6a because they are known to be expressed only in FW and SW eels, respectively (23,35). The higher expression of the Slc26a6a gene in SW is also reported very recently in the euryhaline pufferfish, Takifugu obscurus (13).…”

“…It seems that major ions in SW are responsible for changes in the transporter gene expression, but it remains to be determined which ion(s) actually switch the renal SO 4 2Ϫ regulation to a SW type. We used the eel as a model species because it changes the SO 4 2Ϫ regulation most drastically among euryhaline species (23,35). Initially, we examined the time course changes in SO 4 2Ϫ regulation after transfer of FW eels to SW using plasma and urine SO 4 2Ϫ concentrations and expression of Slc13a1 and Slc26a6a genes…”

“…In teleost fish, Slc13a1, Slc26a1, Slc26a3, and Slc26a6a,b,c have been cloned in the eel (23), Slc26a1 in the rainbow trout (14), Slc13a1 in the zebrafish (19), and Slc26a6a,b,c in the pufferfish Takifugu obscurus (13). Among them, Slc13a1 and Slc26a1 play key roles in SO 4 2Ϫ reabsorption at the renal proximal tubule of mammals (5) and FW eels (23). On the other hand, Slc26a6 and Slc26a1 are suggested to be involved in renal SO 4 2Ϫ secretion in mammals (16,20).…”