Identification and proximal tubular localization of the Mg²⁺ transporter, Slc41a1, in a seawater fish

The second most abundant cation in seawater (SW), Mg²⁺, is present at concentrations of ~53 mM. Marine teleosts maintain plasma Mg²⁺ concentration at 1-2 mM by excreting Mg²⁺ into the urine. Urine Mg²⁺ concentrations of SW teleosts exceed 70 mM, most of which is secreted by the renal tubular epithelial cells. However, molecular mechanisms of the Mg²⁺ secretion have yet to be clarified. To identify transporters involved in Mg²⁺ secretion, we analyzed the expression of fish homologs of the Slc41 Mg²⁺ transporter family in various tissues of SW pufferfish torafugu (Takifugu rubripes) and its closely related euryhaline species mefugu (Takifugu obscurus). Takifugu genome contained five members of Slc41 genes, and only Slc41a1 was highly expressed in the kidney. Renal expression of Slc41a1 was markedly elevated when mefugu were transferred from fresh water (FW) to SW. In situ hybridization analysis and immunohistochemistry at the light and electron microscopic levels revealed that Slc41a1 is localized to vacuoles in the apical cytoplasm of the proximal tubules. These results suggest that pufferfish Slc41a1 is a Mg²⁺ transporter involved in renal tubular transepithelial Mg²⁺ secretion by mediating Mg²⁺ transport from the cytosol to the vacuolar lumen, and support the hypothesis that Mg^{2 +} secretion is mediated by exocytosis of Mg²⁺-rich vacuoles to the lumen.