Liu Chia-Chen, Pearson Chelsea, Bu Guojun
Department of Pediatrics, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Biol Chem. 2009 May 29;284(22):15299-307. doi: 10.1074/jbc.M807285200. Epub 2009 Mar 31.
Wnt/beta-catenin signaling controls cell growth during development, and its misregulation in adults can cause human diseases. LRP6, the essential co-receptor for the Wnt pathway, consists of four beta-propeller domains flanked by epidermal growth factor repeats in its extracellular region. To understand the maturation and ligand-binding properties of individual BP domains, we generated soluble receptor consisting of individual BPs, as well as combinations of these domains. We show that BP1, BP2, and BP4 each can be folded and secreted, and their secretion was enhanced by co-expression of Mesd, a molecular chaperone essential for LRP6 folding and maturation. BP3 is not secreted when expressed on its own or in combination with BP2 or BP1 and 2 (BP12); however, folding and secretion of BP3 is vastly enhanced when expressed together with BP4. Similar cooperative folding and maturation was observed between BP1 and BP2. These results suggest that BP1 forms a functional folding unit with BP2, whereas BP3 folds together with BP4. Using these BP constructs, we also found that BP12 and BP34 constitute independent ligand-binding domains capable of binding Wnt3a, Dkk1, and Mesd. The ability of Mesd to block the binding of both Wnt3a and Dkk1 to LRP6 enables this specialized chaperone to function as a Wnt signaling modulator. Together, our studies reveal unique properties of the LRP6 BP domains and provide novel tools to understand LRP6 function in ligand binding and Wnt signaling. Our results also support the development of soluble LRP6 receptors and Mesd as potential therapeutic molecules that target Wnt signaling.
Wnt/β-连环蛋白信号通路在发育过程中控制细胞生长，其在成体中的失调可导致人类疾病。LRP6是Wnt信号通路的必需共受体，其细胞外区域由四个β-螺旋桨结构域组成，两侧为表皮生长因子重复序列。为了解单个β-螺旋桨（BP）结构域的成熟和配体结合特性，我们构建了由单个BP结构域以及这些结构域组合而成的可溶性受体。我们发现，BP1、BP2和BP4各自都能折叠并分泌，并且分子伴侣Mesd（LRP6折叠和成熟所必需的）的共表达增强了它们的分泌。单独表达BP3或与BP2或BP1和2（BP12）组合表达时，BP3不分泌；然而，与BP4一起表达时，BP3的折叠和分泌大大增强。在BP1和BP2之间也观察到了类似的协同折叠和成熟。这些结果表明，BP1与BP2形成一个功能性折叠单元，而BP3与BP4一起折叠。利用这些BP构建体，我们还发现BP12和BP34构成了能够结合Wnt3a、Dkk1和Mesd的独立配体结合结构域。Mesd能够阻断Wnt3a和Dkk1与LRP6的结合，这使得这种特殊的分子伴侣能够作为Wnt信号调节剂发挥作用。总之，我们的研究揭示了LRP6的BP结构域的独特特性，并提供了新的工具来理解LRP6在配体结合和Wnt信号传导中的功能。我们的结果还支持开发可溶性LRP6受体和Mesd作为靶向Wnt信号的潜在治疗分子。
