Impact of Wnt1 mutation on skeletal remodeling, bone matrix quality, fracture healing and response to mechanical stimulation

了解成骨细胞的分子调控具有巨大的临床意义，因为它可能为开发新的骨合成代谢治疗方案铺平道路。Wnt信号通路对控制成骨细胞活性的关键影响已经通过几种手段得到证明，包括遗传数据，这些数据不仅在小鼠身上获得，在患有遗传性骨骼疾病的人类中也得到了证实。我们和其他人最近在早发性骨质疏松症患者中发现了WNT1杂合子突变，而WNT1纯合子突变被发现会导致更严重的儿童期骨病，被归类为成骨不全X型(OI-XV)。我们还可以证明，在成骨细胞系细胞中WNT1的失活会导致低骨量并伴随骨折，而在成骨细胞中可诱导的WNT1表达会在转基因小鼠模型中引起快速的骨合成反应。对于本项目，我们将重点放在另一个小鼠模型上，在那里我们将G177C突变引入到小鼠WNT1基因中，该突变以前在OI-XV患者中被发现。结果得到的Wnt1G177C/G177C小鼠与Wnt1-/-小鼠不同，没有表现出可检测到的脑异常，但它们的骨量在4周大时就已经显著减少。在6个月大时，我们观察到Wnt1G177C/G177C小鼠自发性骨折的发生率很高，对骨折骨痂的组织学分析表明，脂肪细胞异常聚集。同样值得注意的是，Wnt1G177C/G177C小鼠的骨折发生率很高，尽管只有中度的骨量减少，因此表明Wnt1突变深刻地影响骨基质的组成和质量。我们两个小组的合作目标是：1)了解WNT1在成骨细胞中的分子功能；2)确定介导WNT1对骨合成代谢影响的负责受体(复合体)；3)在已建立的截骨模型中评估WNT1突变对骨折愈合的影响；以及4)分析WNT1作为机械负荷下骨合成反应的介体所起的作用。我们的共同努力将产生更多关于WNT1在骨架中的关键作用的分子知识。这可能为开发新的药物铺平道路，特别是在生理性骨重建或骨再生的背景下，增加成骨细胞的活性和骨质量。

Understanding the molecular regulation of bone-forming osteoblasts is of tremendous clinical importance, as it might pave the way to develop novel osteoanabolic treatment options. The key impact of the Wnt signaling pathway for controlling osteoblast activity has been demonstrated by several means, including genetic data, obtained not only in mice, but also in humans with inherited bone disorders. We, and others have recently identified heterozygous WNT1 mutations in individuals with early-onset osteoporosis, whereas homozygous WNT1 mutations were found to cause a more severe childhood bone disorder, classified as osteogenesis imperfecta type XV (OI-XV). We could also show that inactivation of Wnt1 in osteoblast lineage cells causes low bone mass with accompanying fractures, whereas inducible Wnt1 expression in osteoblasts causes a rapid osteoanabolic response in a transgenic mouse model.For the present project we will focus on another mouse model, where we have introduced the G177C mutation, previously identified in a patient with OI-XV, into the murine Wnt1 gene. The resulting Wnt1G177C/G177C mice, unlike Wnt1-/- mice, did not display a detectable brain abnormality, but their bone mass was significantly reduced already at 4 weeks of age. At the age of 6 months we observed a high incidence of spontaneous fractures in Wnt1G177C/G177C mice, and histological analysis of the fracture callus indicated an unusual accumulation of adipocytes. It is also noteworthy that the high fracture incidence of the Wnt1G177C/G177C mice occurred despite only moderate osteopenia, thereby suggesting that Wnt1 mutation profoundly affects bone matrix composition and quality. The collaboration between our two groups aims at i) understanding the molecular function of Wnt1 in osteoblasts, ii) defining the responsible receptor (complex) mediating the osteoanabolic influence of Wnt1, iii) assessing the impact of Wnt1 mutation on fracture healing in an established osteotomy model, and iv) analyzing the putative role of Wnt1 as a mediator of the osteoanabolic response towards mechanical loading. Our combined efforts will generate additional molecular knowledge on the critical role of Wnt1 in the skeleton. This might pave the way for the development of novel drugs specifically increasing osteoblast activity and bone quality in the context of physiological bone remodeling or bone regeneration.