Runx2 and Axin2 Interactions During Bone Formation

描述（申请人提供）：本项目探讨颅面发育、骨修复/再生和最佳骨量积累和维持的两个关键调节因子Runx2和Axin2之间的分子和生理相互作用。Runx2 （Cbfa1）是成骨细胞发生和软骨细胞肥大所需的转录因子。runx2缺乏症在出生后是致命的，而runx2单倍体不足会导致锁骨颅发育不良（CCD）和骨质减少。唯一已知的至少部分挽救Runx2小鼠CCD表型的生物学方法是抑制Gsk32。Axin2是一种限制浓度的支架蛋白，它将Gsk32、2-catenin和其他组分组装成2-catenin破坏复合物。Axin2是典型Wnt信号的负反馈调节因子，可减缓成骨细胞的增殖。有趣的是，Axin2敲除小鼠有颅缝闭合（CS）和高小梁骨量密度。在人类中，AXIN2突变与家族性牙齿发育有关。因此，axin2缺乏和Runx2-单倍不足导致相反的颅面骨和小梁骨表型。我们发现，在runx2缺陷细胞中，Axin2水平升高。该项目的中心假设是Runx2主动抑制Axin2以增强骨细胞中的Wnt/ 2-catenin信号。我们的目的是确定Runx2-/-小鼠成骨细胞祖细胞中的典型Wnt信号是否发生改变，确定Runx2调节Axin2转录的分子机制，并定量评估“双突变”Runx2: Axin2-/-小鼠相对于野生型、Runx2和Axin2-/-小鼠的骨骼表型。

DESCRIPTION (provided by applicant): This project explores the molecular and physiological interactions between two crucial regulators of craniofacial development, bone repair/regeneration, and optimal bone mass accrual and maintenance: Runx2 and Axin2. Runx2 (Cbfa1) is a transcription factor required for osteoblastogenesis and chondrocytes hypertrophy. Runx2-deficiency is postnatally lethal, whereas Runx2-haploinsufficiency causes cleidocranial dysplasia (CCD) and osteopenia. The only known biologic means to at least partially rescue the CCD phenotype in Runx2 mice is by inhibiting Gsk32. Axin2 is the concentration- limiting scaffolding protein that assembles Gsk32, 2-catenin and other components into the 2-catenin destruction complex. Axin2 is a negative feedback regulator of canonical Wnt signaling and slows osteoblast proliferation. Interestingly, Axin2 knockout mice have craniosynostosis (CS) and high trabecular bone mass density. In humans, AXIN2 mutations are linked to familial tooth agenesis. Thus, Axin2-deficiency and Runx2- haploinsufficiency cause opposing craniofacial and trabecular bone phenotypes. We show that Axin2 levels are increased in Runx2-deficient cells. The central hypothesis of this project is that Runx2 actively represses Axin2 to enhance Wnt/ 2-catenin signaling in bone cells. We aim to determine if canonical Wnt signaling is altered in osteoblast progenitors from Runx2-/- mice, define the molecular mechanisms whereby Runx2 regulates Axin2 transcription, and to quantitatively assess the skeletal phenotypes of "double mutant" Runx2: Axin2-/- mice relative to wild type, Runx2 and Axin2-/- mice. PUBLIC HEALTH RELEVANCE: Bone disorders or damage can afflict humans of any age. One in 750 live births is associated with a craniofacial abnormality and approximately 1 in 2000 children develops a form of craniosynostosis. During their lifetimes, half of all women and a quarter of men can expect to suffer an osteoporosis-related fracture. This project explores the molecular interactions between two crucial regulators of suture closure, bone repair/regeneration, and optimal bone mass accrual and maintenance: Runx2 and Axin2. The proposed project is significant because Runx2 is required for osteoblastogenesis and Axin2 is a crucial negative regulator of the Wnt-2-catenin signaling pathway, which is a target for new anabolic agents. Because Axin2 is an intracellular inhibitor of 2-catenin and Lrp5 signaling, its activity could theoretically decrease the efficiency of emerging anabolic therapies that neutralize extracellular Wnt/Lrp5/6 inhibitors (e.g. anti-Sclerostin or anti-Dkk1 antibodies). The work will have collective impact because Runx2 and Axin2 also contribute to chondrocyte maturation and are tumor suppressors.