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Aquaporin-mediated oocyte hydration is a developmentally regulated adaptive mechanism that co-occurs with meiosis resumption in marine teleosts. It provides the early embryos with vital water until osmoregulatory systems develop, and in the majority of marine teleosts causes their eggs to float. Recent studies have shown that the subdomains of two water channels (Aqp1ab1 and Aqp1ab2) encoded in a teleost-specific aquaporin-1 cluster (TSA1C) co-evolved with duplicated Ywhaz-like (14-3-3ζ-like) binding proteins to differentially control their membrane trafficking for maximal egg hydration. Here, we report that in species that encode the full TSA1C, in-frame intronic splice variants of Aqp1ab1 result in truncated proteins that cause dominant-negative inhibition of the canonical channel trafficking to the plasma membrane. The inhibition likely occurs through hetero-oligomerization and retention in the endoplasmic reticulum (ER) and ultimate degradation. Conversely, in species that only encode the Aqp1ab2 channel we found an in-frame intronic splice variant that results in an intact protein with an extended extracellular loop E, and an out-of frame intronic splice variant with exon readthrough that results in a truncated protein. Both isoforms cause dominant-negative enhancement of the degradation pathway. However, the extended and truncated Aqp1ab2-type variants can also partially escape from the ER to reach the oocyte plasma membrane, where they dominantly-negatively inhibit water flux. The ovarian follicular expression ratios of the Aqp1ab2 isoforms in relation to the canonical channel are lowest during oocyte hydration, but subsequently highest when the canonical channel is recycled, thus leaving the eggs endowed with >90% water. These findings suggest that the expression of inhibitory isoforms of Aqp1ab1 and Aqp1ab2 may represent a new regulatory mechanism through which the cell-surface expression and the activity of the canonical channels can be physiologically modulated during oocyte hydration in marine teleosts.

水通道蛋白（Aquaporin）介导的卵母细胞（oocyte）水化是一种受发育调控的适应性机制，与海洋硬骨鱼（marine teleosts）的减数分裂恢复同步发生。该机制可为早期胚胎提供关键水分，直至其渗透压调节系统发育完善；在多数海洋硬骨鱼中，该过程还可使鱼卵保持漂浮状态。近期研究表明，硬骨鱼特异性水通道蛋白-1基因簇（teleost-specific aquaporin-1 cluster, TSA1C）所编码的两类水通道蛋白（Aqp1ab1与Aqp1ab2）的亚结构域，与复制产生的Ywhaz样（14-3-3ζ样）结合蛋白协同进化，以差异化调控其膜运输过程，实现卵母细胞的最大化水化。我们在此报道：在完整编码TSA1C的物种中，Aqp1ab1的内含子框内剪接变体可产生截短蛋白，该蛋白会对经典通道向质膜的运输产生显性负调控抑制效应。这种抑制作用可能通过异源寡聚化、将通道滞留于内质网（endoplasmic reticulum, ER）并最终使其降解而实现。与之相反，在仅编码Aqp1ab2通道的物种中，我们发现两类内含子剪接变体：其一为框内内含子剪接变体，可产生带有延长细胞外环E的完整蛋白；其二为移码内含子剪接变体，伴随外显子通读（exon readthrough），最终产生截短蛋白。两种变体均可通过增强降解通路实现显性负调控。不过，带有延长环的Aqp1ab2型变体与截短的Aqp1ab2型变体均可部分逃逸内质网，抵达卵母细胞质膜，并在该处通过显性负调控作用抑制水通量。Aqp1ab2各变体与经典通道的卵巢滤泡表达比例，在卵母细胞水化阶段处于最低水平，而在经典通道被回收利用时达到峰值，此时鱼卵含水量可达90%以上。上述研究结果表明，Aqp1ab1与Aqp1ab2的抑制性变体的表达，可能代表了一种全新的调控机制：在海洋硬骨鱼的卵母细胞水化过程中，该机制可生理性调控经典通道的细胞表面表达与活性。