Biology of cortical bone of long bones and calvarium: Role of Sfrp4 in periosteal bone formation

骨膜在皮质骨扩张和内环境稳定中起着关键作用，具有再生能力， 对合成代谢药物有反应然而，尽管它的临床意义，我们的基本理解， 骨膜细胞特性、局部或旁分泌调节因子以及 哪种合成代谢药物调节它，仍然是个谜。由于强大的新测序技术， 组织蛋白酶K（Ctsk+）标记的骨膜细胞群，包括真正的骨膜干细胞 最近在长骨中发现了PSCs和非干骨膜祖细胞（PP1和PP2）， 和颅骨，提供了一个很好的机会，研究的调节和生理相关性， 骨膜在骨骼表型。在这种情况下，我们最近的研究已经确定， Wnt抑制剂SFRP 4的突变导致派尔病，这是一种罕见的骨骼疾病，其特征在于肢体 畸形和脆性骨折。使用Sfrp4-/-小鼠模型，我们发现Sfrp4有助于皮层神经元的生长。 通过调节骨内膜重塑和骨膜形成来实现骨扩张和体内平衡。研究 这里提出的重点是骨膜表面的生物学。我们的目标是确定 Sfrp4介导的信号转导调节其活性。我们发现骨膜中的Sfrp4+细胞构成了 Ctsk +"骨膜"祖细胞（PP1和PP2）的特定池，Sfrp4缺失改变了PSC的% PP2细胞目标1中提出的研究目标是解决Sfrp4是否调节扩增， 骨膜干细胞和祖细胞的分化和/或功能，以及这种分化和/或功能 发生。鉴于PTH是一种有效的骨膜骨形成刺激剂， 干细胞，表达PTH受体，以及PTH正调节Sfrp4的发现， 骨膜是重要的，并可能有临床后果的治疗派尔病 患者因此，目的2中提出的研究的目标是探索Sfrp4-Wnt介导的 信号传导与PTH协同调节骨膜形成。令人信服的证据表明， 颅骨缝和骨膜中的干细胞作为骨形成的生长中心， 发育和支持骨修复，以应对损伤。在颅骨中，PSC主要位于骨缝内， 而PP2细胞主要存在于颅骨缝外的骨膜中， 网站.尽管有经典和非经典的Wnt信号传导激活，Sfrp4的缺乏导致缺乏新的Wnt信号传导。 骨形成是对损伤的反应。目标3中提出的研究目的是探索机制 Sfrp4+细胞通过其促进干细胞/祖细胞的骨再生潜能， 作为损伤修复模型。调控骨膜干细胞和祖细胞的信号通路分析 可能为与骨脆性相关的人类疾病开辟新的和特异性的治疗选择， 骨愈合和骨再生受损。

The periosteum plays a critical role in cortical bone expansion and homeostasis, has regenerative capabilities and responds to anabolic drugs. However, in spite of its clinical significance our basic understanding of periosteal cellular characteristics, local or paracrine regulatory factors as well as the specific mechanisms by which anabolic drugs regulate it, remain elusive. Thanks to powerful new sequencing technologies, a periosteal population of cells labeled by Cathepsin K (Ctsk+), which includes bona fide periosteal stem cells (PSCs) and non-stem periosteal progenitor (PP1 and PP2) cells has been recently identified in the long bones and calvarium, providing a great opportunity for studying the regulation and physiological relevance of the periosteum in skeletal phenotypes. In this context, our recent studies have established that loss of function mutations of the Wnt inhibitor SFRP4 lead to Pyle’s disease, a rare skeletal disease characterized by limb deformity and fragility fractures. Using the Sfrp4-/- mouse model we uncovered that Sfrp4 contributes to cortical bone expansion and homeostasis by regulating endosteal remodeling and periosteal formation. The studies proposed here are focused on the biology of the periosteal surface. Our objective is to identify the role of Sfrp4-mediated signaling in regulating its activity. We have found that Sfrp4+ cells in the periosteum constitute a specific pool of Ctsk+ “periosteal” progenitors (PP1 and PP2) and that Sfrp4 deletion alters the % of PSCs and PP2 cells. The goal of the studies proposed in Aim 1 is to address whether Sfrp4 regulates the expansion, differentiation and/or function of periosteal stem cells and progenitors and the mechanisms by which this occurs. Given that PTH is a potent stimulator of periosteal bone formation, the findings that PSCs, bona fide stem cells, express the PTH receptor together with the findings that PTH positively regulates Sfrp4 in the periosteum are of significance and may have clinical consequences for the treatment of Pyle’s disease patients. The goal of the studies proposed in Aim 2 is therefore to explore whether Sfrp4-Wnt mediated signaling cooperate with PTH to regulate periosteal formation. Compelling evidences have demonstrated that stem cells in the calvarium suture and periosteum serve as growth centers for bone formation during development and support bone repair in response to injury. In the calvarium PSCs are mainly within the suture, while PP2 cells are predominant in the calvarial periosteum outside of the suture and PP1 are found at both sites. Despite canonical and non-canonical Wnt signaling activation, the absence of Sfrp4 results in lack of new bone formation in response to injury. The goal of the studies proposed in Aim 3 is to explore the mechanism(s) by which Sfrp4+ cells contribute to the bone regenerative potential of stem cells/progenitors using calvarial defects as injury repair model. Analysis of signaling pathways regulating periosteal stem cells and progenitors may open novel and specific therapeutic options for human diseases associated with bone fragility and impaired bone healing and bone regeneration.