Hormonal Control of Chloride Cell Differentiation in Fish Gills

本研究旨在阐明环境盐度变化对鱼鳃中氯细胞分化的内分泌调控作用。在过去两年中取得的成果如下：利用虹鳟皮质醇受体激素结合区引物，从罗非鱼肝脏mRNA中克隆出453bp的cDNA。Northern blot分析表明，淡水鱼的受体信息强于海水鱼。该受体mRNA在受精后的卵黄中就已经存在，表明其在早期发育过程中对细胞和组织分化具有重要作用。在大马哈鱼鳃丝中鉴定出两种类型的氯细胞。片层状氯离子细胞主要存在于淡水环境的鱼类体内，在适应海水的过程中消失，这表明它们在淡水环境中是离子吸收的场所。另一方面，在适应海水的过程中发育出了长丝氯细胞，这表明它们是海水中盐排泄的一个场所。原位杂交检测了鲑鱼苗中两种氯细胞中皮质醇受体基因的表达，表明皮质醇是调节氯细胞功能的重要因素之一。皮质醇受体也存在于未分化的细胞中，也提示其参与了氯细胞的功能分化。从罗非鱼肝脏mRNA中克隆了泌乳素受体基因，并对哺乳动物和窄盐罗非鱼的细胞外受体区域进行了引物克隆。竞争PCR结果显示，该受体在受精后2 d的胚胎中存在，且淡水胚中该基因的表达强于海水胚。越来越清楚的是，各种鱼类的卵黄中含有皮质醇和甲状腺激素等多种激素，可能影响细胞和组织的分化。催乳素和甲状腺激素是否特异性参与鳃氯细胞的分化有待进一步研究。少

The aim of this study is to clarify endocrine control of differentiation of chloride cells in fish gills in response to the changes in environmental salinity. The results obtained during the past two years are as follows.1. A cDNA of 453 bp was cloned from liver mRNA of tilapia with primers designed for the hormone-binding domain of cortisol receptor of the rainbow trout. The receptor message by Northern blot analysis was stronger in fish in fresh water than in those in seawater. The receptor mRNA was already present in the yolk just after fertilization, indicating its importance in cell and tissue differentiation during the early development.2. Two types of chloride cells were identified in the gill filament of chum salmon. Lamellar chloride cells were mainly present in the fish in fresh water and disappeared during seawater adaptation, suggesting their role as a site for ion uptake in fresh water. On the other hand, filament chloride cells were developed during seawater adaptation, i … More ndicating their role as a site of salt excretion in seawater.3. Cortisol receptor gene expression, examined by in situ hybridization, were found in the two types of chloride cells in the chum salmon fry, indicating that cortisol is one of the important factors regulating chloride cell functions.Cortisol receptors were also present in undifferentiated cells, also suggesting its involvement in the functional differentiation of chloride cells.4. Prolactin receptor gene was cloned from the liver mRNA of tilapia with primers for the extracellular domain of the receptors of mammalian and stenohaline tilapia. Competitive PCR revealed the presence of the receptor in the embryos 2 days after fertilization, and the gene expression was stronger in the embryos developing in fresh water than those in seawater.5. It has becaome increasingly, clear that various hormones such as cortisol and thyroid hormones are contained in the yolk of various fish species, possibly affecting the cell and tissue differentiation. Further studies are called for to examine whether prolactin and thyroid hormones are involved specifically in the differentiation of the gill chloride cells. Less