Sialin (SLC17A5) functions as a nitrate transporter in the plasma membrane

In vivo recycling of nitrate (NO ₃⁻) and nitrite (NO ₂⁻) is an important alternative pathway for the generation of nitric oxide (NO) and maintenance of systemic nitrate–nitrite–NO balance. More than 25% of the circulating NO ₃⁻ is actively removed and secreted by salivary glands. Oral commensal bacteria convert salivary NO ₃⁻ to NO ₂⁻, which enters circulation and leads to NO generation. The transporters for NO ₃⁻ in salivary glands have not yet been identified. Here we report that sialin (SLC17A 5), mutations in which cause Salla disease and infantile sialic acid storage disorder (ISSD), functions as an electrogenic 2NO ₃⁻/H ⁺ cotransporter in the plasma membrane of salivary gland acinar cells. We have identified an extracellular pH-dependent anion current that is carried by NO ₃⁻ or sialic acid (SA), but not by Br ⁻, and is accompanied by intracellular acidification. Both responses were reduced by knockdown of sialin expression and increased by the plasma membrane-targeted sialin mutant (L22A-L23A). Fibroblasts from patients with ISSD displayed reduced SA- and NO ₃⁻-induced currents compared with healthy controls. Furthermore, expression of disease-associated sialin mutants in fibroblasts and salivary gland cells suppressed the H ⁺-dependent NO ₃⁻ conductance. Importantly, adenovirus-dependent expression of the sialinH183R mutant in vivo in pig salivary glands decreased NO ₃⁻ secretion in saliva after intake of a NO ₃⁻-rich diet. Taken together, these data demonstrate that sialin mediates nitrate influx into salivary gland and other cell types. We suggest that the 2NO ₃⁻/H ⁺ transport function of sialin in salivary glands can contribute significantly to clearance of serum nitrate, as well as nitrate recycling and physiological nitrite-NO homeostasis.