Department of Biology, Faculty of Science, Shizuoka University, Shizuoka 422-8529, Japan.
J Neuroendocrinol. 2010 May;22(5):407-12. doi: 10.1111/j.1365-2826.2010.01981.x. Epub 2010 Feb 12.
Anuran amphibians represent the first vertebrates that adapted to terrestrial environments, and are successfully distributed around the world, even to forests and arid deserts. Many adult anurans have specialised osmoregulatory organs, in addition to the kidney (i.e. the ventral pelvic skin to absorb water from the external environments and a urinary bladder that stores water and reabsorbs it in times of need). Aquaporin (AQP), a water channel protein, plays a fundamental role in these water absorption/reabsorption processes. The anuran AQP family consists of at least AQP0-AQP5, AQP7-AQP10 and two anuran-specific types, designated as AQPa1 and AQPa2. For the three osmoregulatory organs, AQP3 is constitutively located in the basolateral membrane of the tight-junctioned epithelial cells, allowing water transport between the cytoplasm of these cells and the neighbouring tissue fluid at all times. On the other hand, AQPs at the apical side of the tight epithelial cells are different among these organs, and are named kidney-type AQP2, ventral pelvic skin-type AQPa2 and urinary bladder-type AQPa2. All of them show translocation from the cytoplasmic pool to the apical plasma membrane in response to arginine vasotocin, thereby regulating water transport independently in each osmoregulatory organ. It was further revealed that, in terrestrial and arboreal anurans, the bladder-type AQPa2 is expressed in the pelvic skin, together with the pelvic skin-type AQPa2, potentially facilitating water absorption from the pelvic skin. By contrast, Xenopus has lost the ability to efficiently produce pelvic skin-type AQPa2 (AQP-x3) because Cys-273 of AQP-x3 and/or Cys-273-coding region of AQPx3 mRNA attenuate gene expression at a post-transcriptional step, presumably leading to the prevention of excessive water influx in this aquatic species. Collectively, the acquisition of two forms of AQPa2 and the diversified regulation of their gene expression appears to provide the necessary mechanisms for the evolutionary adaptation of anurans to a wide variety of ecological environments.
蛙类两栖动物是第一批适应陆地环境的脊椎动物，成功分布在世界各地，甚至包括森林和干旱沙漠。许多成年蛙类除了肾脏（即从外部环境中吸收水分的腹侧盆腔皮肤和储存水分并在需要时再吸收水分的膀胱）之外，还有专门的渗透调节器官。水通道蛋白 (AQP) 是一种水通道蛋白，在这些水吸收/再吸收过程中起着至关重要的作用。蛙类 AQP 家族至少由 AQP0-AQP5、AQP7-AQP10 和两种蛙类特异性类型组成，分别命名为 AQPa1 和 AQPa2。对于这三种渗透调节器官，AQP3 位于紧密连接上皮细胞的基底外侧膜上，始终允许这些细胞的细胞质与相邻组织液之间进行水运输。另一方面，在上皮细胞的顶端侧，这些器官中的 AQPs 不同，被命名为肾脏型 AQP2、腹侧盆腔皮肤型 AQPa2 和膀胱型 AQPa2。所有这些 AQP 在受到精氨酸加压素的刺激后都会从细胞质池转移到顶端质膜，从而独立调节每个渗透调节器官的水运输。进一步表明，在陆地和树栖蛙类中，膀胱型 AQPa2 与盆腔皮肤型 AQPa2 一起表达在盆腔皮肤中，有助于从盆腔皮肤吸收水分。相比之下，非洲爪蟾已经失去了有效产生盆腔皮肤型 AQPa2 (AQP-x3) 的能力，因为 AQP-x3 的 Cys-273 和/或 AQPx3 mRNA 的 Cys-273 编码区在转录后步骤减弱了基因表达，可能导致这种水生物种防止过度水流入。总的来说，AQPa2 的两种形式的获得及其基因表达的多样化调节似乎为蛙类对各种生态环境的进化适应提供了必要的机制。
