Molecular mechanism of "transportsomes" involved in Ca^<2+-> activated Cl^- transport

人们已经认识到，除了离子渗透外，细胞膜上表达的阴离子通道（CFTR、CLC等）还可以调节多种生理功能。其中，CLCA蛋白已被证明是多功能的，具有分泌、细胞粘附、分化和肿瘤抑制等作用。我们假设rCLCA与接头或支架蛋白形成不同的分子复合物（转运体）来发挥这种多功能。我们发现大鼠CLCA同源物（rCLCAl-f）全长负责调节Ca^<2+>-依赖的Cl^-转运。最近，我们提出了rCLCA1-f在大鼠颌下腺（SMG）导管系统中经上皮Cl^-运输中的生理作用的体内证据。该异构体在早期和循环的核内体中表达，这些核内体被其特定的标记物标记。然后，我们描述了大鼠rCLCA剪接异构体（rCLCAl-t）的独特表达和生理功能。rCLCAl-t比rCLCAl-f更大程度地削弱细胞粘附，并且发现与RACK1（活化C激酶受体）相关联，据报道RACK1与β_1-整合素相互作用并促进细胞粘附。免疫组织化学显示rCLCA1-t位于大鼠SMG排泄管基底未分化细胞的核周区域。这些结果表明，细胞特异性的rCLCA mRNA剪接可能导致了上皮细胞之间不同类型转运体的形成，从而导致了生理功能的差异。

It has been recognized that various physiological function, in addition to ion permeation, can be regulated by anion channels (CFTR, CLC etc.) expressed on cell membrane. Among them, CLCA proteins have been shown to be multifunctional, with roles in, for example, secretion, cell adhesion, differentiation, and tumor suppressing action. We hypothesized that rCLCA make distinct molecular complexes (transportsome) with adaptor or scaffold proteins to exert such multifunctions. We found that full-length of a rat CLCA homologue (rCLCAl-f) is responsible for modulating Ca^<2+>-dependent Cl^- transport. More recently, we presented in vivo evidence of a physiological role for rCLCA1-f in transepithelial Cl^- transport in the ductal system of rat submandibular gland (SMG). This isoform was expressed in early and recycling endosomes labeled by their specific markers. We then described a distinct expression and physiological function of a splicing isoform of rat rCLCA (rCLCAl-t). rCLCAl-t weakened cell attachment to a greater extent than rCLCAl-f did, and was found to associate with RACK1 (receptor for activated C kinase) which is reported to interact with β_1-integrin and to promote cell adhesion. Immunohistochemistry revealed rCLCA1-t to be located in the perinuclear region of the basal, undifferentiated cells of the rat SMG excretory duct. These suggest that cell-specific splicing of rCLCA mRNA may contribute to the differences in physiological function probably due to formation of distinct types of transportsome among epithelial cells.