Department of Biological Sciences, University of Arkansas, SCEN 601, Fayetteville, AR 72701, USA.
Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark.
Int J Mol Sci. 2020 Mar 8;21(5):1853. doi: 10.3390/ijms21051853.
When euryhaline fish move between fresh water (FW) and seawater (SW), the intestine undergoes functional changes to handle imbibed SW. In Japanese medaka, the potential transcellular aquaporin-mediated conduits for water are paradoxically downregulated during SW acclimation, suggesting paracellular transport to be of principal importance in hyperosmotic conditions. In mammals, intestinal claudin-15 (CLDN15) forms paracellular channels for small cations and water, which may participate in water transport. Since two paralogs, and , have previously been identified in medaka, we examined the salinity effects on their mRNA expression and immunolocalization in the intestine. In addition, we analyzed the drinking rate and intestinal water handling by adding non-absorbable radiotracers, 51-Cr-EDTA or 99-Tc-DTPA, to the water. The drinking rate was >2-fold higher in SW than FW-acclimated fish, and radiotracer experiments showed anterior accumulation in FW and posterior buildup in SW intestines. Salinity had no effect on expression of , while was approximately 100-fold higher in FW than SW. Despite differences in transcript dynamics, Cldn15a and Cldn15b proteins were both similarly localized in the apical tight junctions of enterocytes, co-localizing with occludin and with no apparent difference in localization and abundance between FW and SW. The stability of the Cldn15 protein suggests a physiological role in water transport in the medaka intestine.
当广盐性鱼类从淡水（FW）转移到海水（SW）时，肠道会发生功能变化，以处理摄入的 SW。在日本青鳉中，潜在的水介导的细胞旁 Aquaporin 通道在 SW 适应过程中被下调，这表明在高渗条件下细胞旁转运是主要的。在哺乳动物中，肠道 Claudin-15（CLDN15）形成用于小阳离子和水的细胞旁通道，可能参与水的转运。由于先前在青鳉中鉴定出两个 paralogs 和 ，我们研究了它们在肠道中的 mRNA 表达和免疫定位对盐度的影响。此外，我们通过向水中添加不可吸收的放射性示踪剂 51-Cr-EDTA 或 99-Tc-DTPA 来分析摄水率和肠道水的处理。SW 中鱼的摄水率比 FW 适应组高 2 倍以上，放射性示踪剂实验表明 FW 中前肠积聚，SW 中后肠积聚。盐度对 的表达没有影响，而 在 FW 中的表达比 SW 高约 100 倍。尽管转录动态存在差异，但 Cldn15a 和 Cldn15b 蛋白都在肠上皮细胞的顶端紧密连接处被类似地定位，与紧密连接蛋白 occludin 共定位，在 FW 和 SW 之间没有明显的定位和丰度差异。Cldn15 蛋白的稳定性表明其在青鳉肠道水转运中具有生理作用。
