Seawater fish use an electrogenic boric acid transporter, Slc4a11A, for boric acid excretion by the kidney

海洋鱼类利用Slc4a11A生电硼酸转运机制，经肾脏排出硼酸

Seawater fish use an electrogenic boric acid transporter, Slc4a11A, for boric acid excretion by the kidney

作者：Akira Kato, Yuuri Kimura, Yukihiro Kurita, Min-Hwang Chang, Koji Kasai, Toru Fujiwara, Taku Hirata, Hiroyuki Doi, Shigehisa Hirose, Michael F Romero.

期刊：Journal of Biological Chemistry

Boric acid is a vital micronutrient in animals; however, excess amounts are toxic to them. Little is known about whole-body boric acid homeostasis in animals. Seawater (SW) contains 0.4 mM boric acid, and since marine fish drink SW, their urinary system was used here as a model of the boric acid excretion system. We determined that the bladder urine of a euryhaline pufferfish (river pufferfish, Takifugu obscurus) acclimated to fresh water and SW contained 0.020 and 19 mM of boric acid, respectively (a 950-fold difference), indicating the presence of a powerful excretory renal system for boric acid. Slc4a11 is a potential animal homolog of the plant boron transporter BOR1; however, mammalian Slc4a11 mediates H⁺ (OH^-) conductance but does not transport boric acid. We found that renal expression of the pufferfish paralog of Slc4a11, Slc4a11A, was markedly induced after transfer from fresh water to SW, and Slc4a11A was localized to the apical membrane of kidney tubules. When pufferfish Slc4a11A was expressed in Xenopus oocytes, exposure to media containing boric acid and a voltage clamp elicited whole-cell outward currents, a marked increase in pH_i, and increased boron content. In addition, the activity of Slc4a11A was independent of extracellular Na⁺. These results indicate that pufferfish Slc4a11A is an electrogenic boric acid transporter that functions as a B(OH)₄^- uniporter, B(OH)₃-OH^- cotransporter, or B(OH)₃/H⁺ exchanger. These observations suggest that Slc4a11A is involved in the kidney tubular secretion of boric acid in SW fish, probably induced by the negative membrane potential and low pH of urine.

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Fig. 2 Renal expression of Slc4a11A.A, phylogenetic tree of boric acid transporters in relation to the other human SLC4 family members. The boric acid transport activity of Takifugu Slc4a11A is shown in this study. The scale bar represents 0.1 amino acid substitution per site. B, tissue distribution of Slc4a11A and Slc4a11B. Semiquantitative RT–PCR was performed on various tissues of river pufferfish. Numbers indicate PCR cycles. Results from 27 PCR cycles show tissues with relatively high expression of the indicated genes, and those of 32 cycles show all tissues expressing the indicated genes from low to high levels (111, 162, 65). C, real-time PCR quantification of mRNAs for Slc4a11A and Slc4a11B in the kidneys of river pufferfish acclimated to FW and SW. Values are expressed relative to GAPDH. Dots represent individual data. Bar graphs represent means ± SD, n = 5. ∗p < 0.05. D, in situ hybridization of Slc4a11A and Slc4a11B in the kidney of river pufferfish in SW. Sense probes did not show labeling (data not shown). AE, anion exchanger; c, chicken; F, FW; NBC, Na⁺-HCO₃^– cotransporter; NDCBE, Na⁺-driven Cl^–/HCO₃^– exchanger; S, SW; SLC4, solute carrier family 4; z, zebrafish.

硼酸是动物体内重要的微量营养素，但若过量则会产生毒性。然而，人们对动物全身的硼酸内稳态知之甚少。海水（SW）含有0.4 mM的硼酸，且海洋鱼类会吸入海水，因此，本研究将海洋鱼类的泌尿系统作为硼酸排出系统的模型。我们对一种广盐性河鲀（河豚/暗纹东方鲀）进行了研究，发现其适应淡水时膀胱尿液含有0.020mM的硼酸，适应海水时则是19 mM（后者是前者的950倍），这表明存在强大的泌尿排泄系统可排出硼酸。Slc4a11存在于动物体内，是编码植物硼转运蛋白BOR1同源的蛋白质的未表征基因；然而，哺乳动物的Slc4a11虽能调节H⁺ (OH^-)的传导，但无法转运硼酸。将河豚从淡水转移到海水后，我们发现其Slc4a11旁系基因Slc4a11A的肾脏表达明显受到诱导，且Slc4a11A定位于肾小管的顶端膜。当河豚的Slc4a11A基因在非洲爪蟾卵母细胞中表达时，接触含有硼酸的介质和电压钳导致产生全细胞外向电流，pH_i值明显增加，硼含量增加。此外，Slc4a11A的活性与胞外Na⁺无关。以上结果表明河豚Slc4a11A是一种可生电的硼酸转运机制，与B(OH)₄^-单转运蛋白、B(OH)₃-OH^-协同转运蛋白或B(OH)₃/H⁺交换蛋白的功能相同。这些观察表明，Slc4a11A参与了海洋鱼类肾小管的硼酸排出，原因或为负膜电位和低尿液pH值。

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（实习生申澳编译）