Peptide transporter 1 (PepT1, alias Slc15a1) mediates the uptake of dietary di/tripeptides in all vertebrates. However, in teleost fish, more than one PepT1-type transporter might function, due to specific whole genome duplication event(s) that occurred during their evolution leading to a more complex paralogue gene repertoire than in higher vertebrates (tetrapods). Here, we describe a novel di/tripeptide transporter in the zebrafish (Danio rerio), i.e., the zebrafish peptide transporter 1a (PepT1a; also known as Solute carrier family 15 member a1, Slc15a1a), which is a paralogue (78% similarity, 62% identity at the amino acid level) of the previously described zebrafish peptide transporter 1b (PepT1b, alias PepT1; also known as Solute carrier family 15 member 1b, Slc15a1b). Also, we report a basic analysis of the pept1a (slc15a1a) mRNA expression levels in zebrafish adult tissues/organs and embryonic/early larval developmental stages. As assessed by expression in Xenopus laevis oocytes and two-electrode voltage clamp measurements, zebrafish PepT1a, as PepT1b, is electrogenic, Na+-independent, and pH-dependent and functions as a low-affinity system, with K0.5 values for Gly-Gln at − 60 mV of 6.92 mmol/L at pH 7.6 and 0.24 mmol/L at pH 6.5 and at − 120 mV of 3.61 mmol/L at pH 7.6 and 0.45 mmol/L at pH 6.5. Zebrafish pept1a mRNA is highly expressed in the intestine and ovary of the adult fish, while its expression in early development undergoes a complex trend over time, with pept1a mRNA being detected 1 and 2 days post-fertilization (dpf), possibly due to its occurrence in the RNA maternal pool, decreasing at 3 dpf (~ 0.5-fold) and increasing above the 1–2 dpf levels at 4 to 7 dpf, with a peak (~ 7-fold) at 6 dpf. We show that the zebrafish PepT1a-type transporter is functional and co-expressed with pept1b (slc15a1b) in the adult fish intestine. Its expression is also confirmed during the early phases of development when the yolk syncytial layer is present and yolk protein resorption processes are active. While completing the missing information on PepT1-type transporters function in the zebrafish, these results open to future investigations on the similar/differential role(s) of PepT1a/PepT1b in zebrafish and teleost fish physiology.

中文翻译：

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斑马鱼斑马鱼的肽转运蛋白 1a，一种新兴的营养基因组学和营养研究模型：分子特征、功能特性和表达分析

肽转运蛋白 1（PepT1，别名 Slc15a1）介导所有脊椎动物对膳食二肽/三肽的摄取。然而，在硬骨鱼中，可能有不止一种 PepT1 型转运蛋白起作用，这是由于在进化过程中发生的特定全基因组复制事件导致比高等脊椎动物（四足动物）更复杂的旁系同源基因库。在这里，我们描述了斑马鱼 (Danio rerio) 中的一种新型二肽/三肽转运蛋白，即斑马鱼肽转运蛋白 1a (PepT1a；也称为溶质载体家族 15 成员 a1，Slc15a1a)，它是一个旁系同源物（78% 相似性，先前描述的斑马鱼肽转运蛋白 1b（PepT1b，别名 PepT1；也称为溶质载体家族 15 成员 1b，Slc15a1b）的氨基酸水平的 62% 同一性。还，我们报告了斑马鱼成体组织/器官和胚胎/早期幼虫发育阶段中 pept1a (slc15a1a) mRNA 表达水平的基本分析。通过非洲爪蟾卵母细胞中的表达和双电极电压钳测量评估，斑马鱼 PepT1a 与 PepT1b 一样，具有电性、Na+ 非依赖性和 pH 依赖性，并作为低亲和力系统发挥作用，Gly- 的 K0.5 值Gln 在 - 60 mV 时为 6.92 mmol/L，pH 7.6 和 0.24 mmol/L 在 pH 6.5 和 -120 mV 时 3.61 mmol/L 在 pH 7.6 和 0.45 mmol/L 在 pH 6.5。斑马鱼 pept1a mRNA 在成鱼的肠道和卵巢中高度表达，而其在早期发育中的表达随着时间的推移呈现出复杂的趋势，在受精后 1 天和 2 天 (dpf) 检测到 pept1a mRNA，可能是由于其发生在 RNA 母体池中，在 3 dpf (~ 0. 5 倍）并在 4 到 7 dpf 时增加到 1-2 dpf 水平以上，在 6 dpf 时达到峰值（~ 7 倍）。我们表明斑马鱼 PepT1a 型转运蛋白在成鱼肠道中具有功能性并与 pept1b (slc15a1b) 共同表达。当卵黄合胞层存在且卵黄蛋白再吸收过程活跃时，它的表达也在发育的早期阶段得到证实。在完成有关斑马鱼中 PepT1 型转运蛋白功能的缺失信息的同时，这些结果为未来研究 PepT1a/PepT1b 在斑马鱼和硬骨鱼生理学中的相似/差异作用打开了大门。当卵黄合胞层存在且卵黄蛋白再吸收过程活跃时，它的表达也在发育的早期阶段得到证实。在完成有关斑马鱼中 PepT1 型转运蛋白功能的缺失信息的同时，这些结果为未来研究 PepT1a/PepT1b 在斑马鱼和硬骨鱼生理学中的相似/差异作用打开了大门。当卵黄合胞层存在且卵黄蛋白再吸收过程活跃时，它的表达也在发育的早期阶段得到证实。在完成有关斑马鱼中 PepT1 型转运蛋白功能的缺失信息的同时，这些结果为未来研究 PepT1a/PepT1b 在斑马鱼和硬骨鱼生理学中的相似/差异作用打开了大门。