Differential influence on bone remodeling by regulators of Wnt-dependent signal transduction

自从在骨硬化症和骨质疏松症患者中分别发现LRP5基因的激活和失活突变以来，Wnt信号被认为是成骨细胞的主要调控途径。事实上，LRP5已被描述为Wnt家族配体的共同受体和内源性Wnt信号拮抗剂的分子靶点。鉴于这些发现在骨代谢治疗骨骼疾病方面的潜在相关性，在分子水平上了解LRP5在细胞表面的复杂相互作用以及由此调节的成骨细胞内事件是基础科学的主要目标。对于Wnt信号的特定配体、受体和拮抗剂与骨形成相关的问题尤其如此，因为这些分子可以替代地用作骨合成治疗的分子靶点。基于我们在这方面的初步工作，我们得出了三个重要结论：1)Wnt受体Fzd9是骨形成的正向调节因子。2)作为Wnt拮抗剂Dkk1受体的跨膜蛋白Krm2是骨形成的负性调节因子，它还能激活破骨细胞的生成。3)Krm2抑制β-连环素依赖的Wnt信号通路，而Fzd9缺乏只影响所谓的非典型Wnt信号通路。为了进一步研究这些潜在的相关差异，本项目旨在建立用于组织学和分子分析的转基因小鼠模型，从而解决至少三个问题：1)Fzd9是LRP5的生理上相关的相互作用伙伴吗？2)Krm2的哪些分子效应是通过与LRP5的相互作用来介导的？3)Sost的作用是通过Wnt信号拮抗？

Since the discovery of activating and inactivating mutations of the gene LRP5 in individuals with osteosclerosis and osteoporosis, respectively, Wnt signaling is considered a major regulatory pathway in bone-forming osteoblasts. Indeed, Lrp5 has been described as a co-receptor for ligands of the Wnt family and as a molecular target for endogenous antagonists of Wnt signaling. Given the potential relevance of these findings in terms of an osteoanabolic treatment of skeletal disorders, it is a major goal of basic science to understand the complex interactions of Lrp5 at the cell surface and the thereby regulated intracellular events in osteoblasts at the molecular level. This is particularly true for the question, which specific ligands, receptors and antagonists of Wnt signaling are relevant for bone formation, since these molecules could alternatively be utilized as molecular targets for an osteoanabolic therapy.Based on our own preliminary work in this context we came to three important conclusions: 1) The Wnt receptor Fzd9 is a positive regulator of bone formation. 2) The transmembrane protein Krm2, which functions as a receptor of the Wnt antagonist Dkk1, is a negative regulator of bone formation that additionally activates osteoclastogenesis. 3) While Krm2 inhibits ß-catenin-dependent Wnt signaling, Fzd9-deficiency only affects so-called non-canonical Wnt signaling pathways. To further study these potentially relevant discrepancies, this project aims at the generation of genetically modified mouse models for histological and molecular analyses, thereby addressing at least three questions: 1) Is Fzd9 a physiologically relevant interaction partner of Lrp5? 2) Which molecular effects of Krm2 are mediated by an interaction with Lrp5? 3) Is the action of Sost, whose mutational inactivation causes osteosclerosis, mediated by Wnt signaling antagonism?