Identification of renal transporters involved in sulfate excretion in marine teleost fish

Sulfate (SO₄^2-) is the second most abundant anion in seawater (SW), and excretion of excess SO₄^2- from ingested SW is essential for marine fish to survive. Marine teleosts excrete SO ₄^2- via the urine produced in the kidney. The SO ₄^2- transporter that secretes and concentrates SO ₄^2- in the urine has not previously been identified. Here, we have identified and characterized candidates for the long-sought transporters. Using sequences from the fugu database, we have cloned cDNA fragments of all transporters belonging to the Slc13 and Slc26 families from mefugu (Takifugu obscurus). We compared Slc13 and Slc26 mRNA expression in the kidney between freshwater (FW) and SW mefugu. Among 14 clones examined, the expression of a Slc26a6 paralog (mfSlc26a6A) was the most upregulated (30-fold) in the kidney of SW mefugu. Electrophysiological analyses of Xenopus oocytes expressing mfSlc26a6A, mfSlc26a6B, and mouse Slc26a6 (mSlc26a6) demonstrated that all transporters mediate electrogenic Cl^-/SO₄ ^2-, Cl^-/oxalate^2-, and Cl^-/nHCO ₃^- exchanges and electroneutral Cl^-/ formate^- exchange. Two-electrode voltage-clamp experiments demonstrated that the SO₄^2--elicited currents of mfSlc26a6A is quite large (∼35 μA at +60 mV) and 50- to 200-fold higher than those of mfSlc26a6B and mSlc26a6. Conversely, the currents elicited by oxalate and HCO₃^- are almost identical among mfSlc26a6A, mfSlc26a6B, and mSlc26a6. Kinetic analysis revealed that mfSlc26a6A has the highest SO₄^2- affinity as well as capacity. Immunohistochemical analyses demonstrated that mfSlc26a6A localizes to the apical (brush-border) region of the proximal tubules. Together, these findings suggest that mfSlc26a6A is the most likely candidate for the major apical SO₄^2- transporter that mediates SO₄ ^2- secretion in the kidney of marine teleosts.