Sellers Zachary M, Mann Elizabeth, Smith Anders, Ko Kwang Hyun, Giannella Ralph, Cohen Mitchell B, Barrett Kim E, Dong Hui
Department of Medicine, Division of Gastroenterology, University of California, San Diego, La Jolla, California, USA.
FASEB J. 2008 May;22(5):1306-16. doi: 10.1096/fj.06-7540com. Epub 2007 Dec 20.
The heat-stable enterotoxin of Escherichia coli (STa) is a potent stimulant of intestinal chloride and bicarbonate secretion. Guanylyl cyclase C (GC-C) has been shown to be the primary receptor involved in mediating this response. However, numerous studies have suggested the existence of an alternative STa-binding receptor. The aims of this study were to determine whether a non-GC-C receptor exists for STa and what is the functional relevance of this for intestinal bicarbonate secretion in mice. (125)I-STa-binding experiments were performed with intestinal mucosae from GC-C knockout (KO) and wild type (WT) mice. Subsequently, the functional relevance of an alternative STa-binding receptor was explored by examining STa-, uroguanylin-, and guanylin-stimulated duodenal bicarbonate secretion (DBS) in GC-C KO mice in vitro and in vivo. Significant (125)I-STa-binding occurred in the proximal small intestines of GC-C KO and WT mice. Analysis of binding coefficients and pH dependence showed that (125)I-STa-binding in GC-C KO mice involved a receptor distinct from that of WT mice. Functionally, STa, uroguanylin, and guanylin all stimulated a significant increase in DBS in GC-C KO mice. Uroguanylin- and guanylin-stimulated DBS were significantly inhibited by glibenclamide, but not by 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). However, STa-stimulated DBS was unaffected by glibenclamide but inhibited by DIDS. Taken together, our results suggest that alternative, non-GC-C, receptors likely exist for STa, uroguanylin, and guanylin in the intestines of mice. While uroguanylin- and guanylin-stimulated DBS are cystic fibrosis transmembrane conductance regulator (CFTR) dependent, STa-stimulated DBS is CFTR independent. Further understanding of this alternative receptor and its signaling pathway may provide important insights into rectification of intestinal bicarbonate secretion in cystic fibrosis.
大肠杆菌的热稳定肠毒素（STa）是肠道氯离子和碳酸氢盐分泌的强效刺激物。已证明鸟苷酸环化酶C（GC-C）是介导这种反应的主要受体。然而，大量研究表明存在一种替代性的STa结合受体。本研究的目的是确定STa是否存在非GC-C受体，以及这对小鼠肠道碳酸氢盐分泌的功能相关性是什么。用GC-C基因敲除（KO）小鼠和野生型（WT）小鼠的肠黏膜进行了（125）I-STa结合实验。随后，通过在体外和体内检测GC-C KO小鼠中STa、尿鸟苷素和鸟苷素刺激的十二指肠碳酸氢盐分泌（DBS），探讨了替代性STa结合受体的功能相关性。在GC-C KO小鼠和WT小鼠的近端小肠中发生了显著的（125）I-STa结合。结合系数和pH依赖性分析表明，GC-C KO小鼠中的（125）I-STa结合涉及一种与WT小鼠不同的受体。在功能上，STa、尿鸟苷素和鸟苷素均刺激GC-C KO小鼠的DBS显著增加。格列本脲可显著抑制尿鸟苷素和鸟苷素刺激的DBS，但4,4'-二异硫氰酸根合芪-2,2'-二磺酸（DIDS）则无此作用。然而，STa刺激的DBS不受格列本脲影响，但受DIDS抑制。综上所述，我们的结果表明，小鼠肠道中可能存在STa、尿鸟苷素和鸟苷素的替代性非GC-C受体。虽然尿鸟苷素和鸟苷素刺激的DBS依赖于囊性纤维化跨膜传导调节因子（CFTR），但STa刺激的DBS不依赖于CFTR。对这种替代性受体及其信号通路的进一步了解可能为纠正囊性纤维化患者的肠道碳酸氢盐分泌提供重要见解。
