Function and mechanism of LGR4 and LGR5 in modulation of Wnt signaling

摘要LGR4和LGR5是视紫红质样7TM受体超家族的两个相关受体。LGR5特异性表达于整个胃肠道快速循环的隐窝干细胞和凸起的毛囊干细胞中，而LGR4对于隐窝干细胞的存活至关重要。在小鼠中完全敲除LGR4或LGR5都会导致胚胎/新生儿的完全致死。在视紫红质家族的数百个7TM受体中，LGR4和LGR5是少数对发育至关重要的受体。就在最近，我们发现LGR4/5作为R-spondins的配体，R-spondins是一组分泌蛋白，在胚胎发育、se测定、指甲形成和隐窝干细胞生长中具有重要功能。研究表明，通过R-spondins激活LGR4/5可以增强Wnt/b-catenin信号，这是一个众所周知的对干细胞发育和存活至关重要的系统。R-spondins作为LGR4/5配体的发现，为Wnt/b-catenin通路的干细胞特异性作用以及LGR4/5和R-spondins在发育和干细胞存活中的多效性功能提供了分子基础。然而，RSPO-LGR4/5配体受体系统的确切功能和信号传导机制尚不清楚。未发现LGR4/5与异三聚体G蛋白或b-阻滞蛋白偶联的证据。因此，LGR4/5可能具有独特的信号转导器和机制，将LGR4/5的激活传递到增加的Wnt/b-catenin信号，从而将它们与数百种其他视紫红质样受体区分开来。在本提案中，我们计划使用各种方法识别和表征LGR4和LGR5的换能器和机制。了解LGR4和LGR5的机制不仅对干细胞的运作原理、性别决定和器官发育的分子过程，而且对信号转导的一般领域都有重要的意义。

DESCRIPTION (provided by applicant): Function and mechanism of LGR4 and LGR5 in modulation of Wnt signaling Abstract LGR4 and LGR5 are two related receptors of the rhodopsin-like 7TM receptor superfamily. LGR5 is specifically expressed in the rapidly cycling crypt stem cells of the entire gastrointestinal tract and hair follicle stem cells in the bulge whie LGR4 is essential for survival of the crypt stem cells. Complete knockout of either LGR4 or LGR5 in the mouse leads total embryonic/neonatal lethality. Among the hundreds of 7TM receptors in the rhodopsin family, LGR4 and LGR5 are among the few that are essential for development. Just recently, we discovered that LGR4/5 function as ligands of R-spondins, a group of secreted proteins with vital functions in embryonic development, se determination, nail formation and growth of crypt stem cells. Activation of LGR4/5 by R-spondins were shown to robustly potentiate Wnt/b-catenin signaling, a system that is well known to be essential for development and for survival of stem cells. The discovery of R-spondins as ligand of LGR4/5 provides a molecular basis for stem cell-specific effect of the Wnt/b-catenin pathway and for the pleiotropic functions LGR4/5 and R- spondins has in development and stem cell survival. However, the exact functions and signaling mechanisms of the RSPO-LGR4/5 ligand-receptor system remain unknown. No evidence of LGR4/5 coupling to heterotrimeric G proteins or to b-arrestin was found. Thus LGR4/5 may have unique signal transducers and mechanisms that relay activation of LGR4/5 to increased Wnt/b-catenin signaling and thus differentiate them from hundreds of other rhodopsin-like receptors. In this proposal, we plan to identify and characterize the transducers and mechanisms of LGR4 and LGR5 using a variety of approaches. Understanding of the mechanism of LGR4 and LGR5 will provide important insights not only to the operating principles of stem cells, the molecular processes of sex determination and organ development but also to the general field of signal transduction. PUBLIC HEALTH RELEVANCE: Stem cells are critical to the maintenance and repair of normal tissues. Understanding of the maintenance and growth of stem cells is critical to the development of stem cell-based therapeutics for the treatment of degenerative diseases. LGR4 and LGR5 are two genes with essential function in the stem cells of intestine by functioning as receptors for stem cell growth factors. However, little is known about the signaling mechanisms of LG4 and LGR5 in stem cells. The goal of this project is to identify and characterize signal transducers and mechanism of LGR4 and LGR5 that enable them to perform their essential function in stem cells. The research will lead to a better understanding of the biology of adult stem cells and may provide important knowledge to the development of therapeutics for regenerative medicine.