Extracellular Cl− regulates human SO4 2−/anion exchanger SLC26A1 by altering pH sensitivity of anion transport

Abstract

Genetic deficiency of the SLC26A1 anion exchanger in mice is known to be associated with hyposulfatemia and hyperoxaluria with nephrolithiasis, but many aspects of human SLC26A1 function remain to be explored. We report here the functional characterization of human SLC26A1, a DIDS-sensitive, electroneutral sodium-independent anion exchanger transporting sulfate, oxalate, bicarbonate, thiosulfate and (with divergent properties) chloride. Human SLC26A1-mediated anion exchange differs from that of its rodent orthologs in its stimulation by alkaline pHo and inhibition by acidic pHo but not pHi, and in its failure to transport glyoxylate. SLC26A1-mediated transport of sulfate and oxalate is highly dependent on allosteric activation by extracellular chloride or nonsubstrate anions. Extracellular chloride stimulates apparent Vmax of human SLC26A1-mediated sulfate uptake by conferring a two-log decrease in sensitivity to inhibition by extracellular protons, without changing transporter affinity for extracellular sulfate. In contrast to SLC26A1-mediated sulfate transport, SLC26A1-associated chloride transport is activated by acid pHo, shows reduced sensitivity to DIDS, and exhibits cation-dependence of its DIDS-insensitive component. Human SLC26A1 resembles SLC26 paralogs in its inhibition by phorbol ester activation of PKC, which differs in its undiminished polypeptide abundance at or near the oocyte surface. Mutation of SLC26A1 residues corresponding to candidate anion binding site-associated residues in avian SLC26A5/prestin altered anion transport in patterns resembling those of prestin. However, rare SLC26A1 polymorphic variants from a patient with renal Fanconi Syndrome and from a patient with nephrolithiasis/calcinosis exhibited no loss-of-function phenotypes consistent with disease pathogenesis.

Absence of the sulfate transporter SAT-1 has no impact on oxalate handling by mouse intestine and does not cause hyperoxaluria or hyperoxalemia

“…2D) and distal colon (Fig. 4D), plus SAT-1 has been determined to be an electroneutral transporter (52,90). We are unable to account for this current in terms of changes to net Cl Ϫ flux (Fig.…”

“…On another set of tissues from a separate group of mice, sulfate and Cl Ϫ fluxes were measured simultaneously across these same three intestinal segments using Na2 35 SO 4 2Ϫ and H 36 Cl, respectively. The use of 36 Cl Ϫ as a tracer for Cl Ϫ was included to determine whether SAT-1 might also contribute to intestinal Cl Ϫ fluxes, particularly since it has been reported to transport Cl Ϫ by some studies (55,68,90), but not all (32,61,91). Furthermore, because Cl Ϫ is one of the major substrates associated with intestinal oxalate transport, information on the resultant Cl Ϫ fluxes may also help to interpret the anticipated impacts on oxalate and sulfate fluxes in SAT-1-KO tissues relative to those from WT mice.…”

“…For instance, oxalate and sulfate are frequently transported in parallel with Cl Ϫ (23, 24, 36 -38), and with at least two basolateral sulfate antiporters described in the ileum, this arrangement may allow for sulfate recycling across the membrane to potentially facilitate Cl Ϫ /HCO 3 Ϫ exchange (49). Furthermore, when expressed in Xenopus oocytes, SAT-1 can also transport Cl Ϫ (55,68,90), and the experiments with BLMVs from the SAT-1 KO mouse intestine were performed with an outwardly directed Cl Ϫ gradient (17), thus depicting SAT-1 as an oxalate/Cl Ϫ exchanger. Having simultaneously measured Cl Ϫ fluxes in these experiments we were able to evaluate, for the first time, the contribution of SAT-1 to intestinal Cl Ϫ transport.…”

“…When expressed in vitro, SAT-1 can operate as a sulfate (SO 4 2Ϫ )/HCO 3 exchanger (27,52,67,90), although this was not a characteristic of oxalate secretion across the mouse duodenum, and the SAT-1-KO mouse was subsequently used to rule against the participation of SAT-1 (51). Nevertheless, it remains an open question whether SAT-1 contributes to transepithelial oxalate secretion elsewhere along the GI tract, in particular the distal ileum and large intestine, where the SAT-1 protein was shown to be functionally expressed (17).…”

Profiling solute-carrier transporters in key metabolic tissues during the postpartum evolution of mammary epithelial cells from nonsecretory to secretory

“…SLC26a1 is important for sulfate homeostasis and proteoglycan synthesis (3). This specific transporter is also linked to effluxion of oxalates from the hepatocytes for excretion via the kidneys (106). SLC29a1 is a facilitative nucleoside transporter that is ubiquitously expressed in the body, and its levels are altered in diverse cancers (109).…”