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

骨膜在皮质骨的扩张和动态平衡中起着关键作用，具有再生能力。 对合成代谢药物有反应。然而，尽管它具有临床意义，但我们对 骨膜细胞特性、局部或旁分泌调节因子及其作用机制 究竟是哪种合成代谢药物来调节它，仍然难以捉摸。多亏了强大的新测序技术， 组织蛋白酶K(CTSK+)标记的细胞的骨膜群，包括真正的骨膜干细胞 最近在长骨中发现了(PSCs)和非干骨膜前体细胞(PP1和PP2) 和颅骨，为研究颅骨的调节和生理相关性提供了一个很好的机会。 骨骼表型中的骨膜。在这种情况下，我们最近的研究证实了功能丧失 Wnt抑制因子sFRP4突变导致以肢体为特征的罕见骨骼疾病--派尔氏病 畸形和脆性骨折。利用sFRP4-/-小鼠模型，我们发现sFRP4对大脑皮质 通过调节骨内膜重塑和骨膜形成实现骨扩张和动态平衡。这些研究 这里提出的是骨膜表面的生物学研究。我们的目标是确定 SFRP4介导的信号调节其活性。我们发现骨膜中的sFRP4+细胞构成 一个特定的CTSK+“骨膜”祖细胞池(PP1和PP2)和sFRP4缺失改变了PSCs的百分比 和PP2细胞。目标1中提出的研究的目标是解决sFRP4是否调控扩张， 骨膜干细胞和祖细胞的分化和/或功能及其机制 发生。鉴于甲状旁腺激素是一种强有力的骨膜成骨刺激因子，PSCs，真正的 干细胞表达PTH受体，并发现PTH正向调节sFRP4在 骨膜对于派尔氏病的治疗具有重要意义并可能产生临床后果 病人。因此，目标2中提出的研究的目标是探索sFRP4-Wnt是否介导了 信号与甲状旁腺激素协同调节骨膜形成。令人信服的证据表明 颅骨骨缝和骨膜中的干细胞是骨形成的生长中心 发展和支持骨骼修复，以应对伤害。在颅骨中，PSCs主要在缝合口内， 而PP2细胞主要分布在缝线外的颅骨骨膜中，PP1细胞在两处均可见 网站。尽管有规范和非规范的Wnt信号激活，但缺少sFRP4会导致缺乏新的 对损伤作出反应的骨形成。目标3中提出的研究的目标是探索这种机制(S) SFRP4+细胞通过颅骨促进干细胞/祖细胞的骨再生潜能 缺陷作为损伤修复模型。骨膜干细胞及其前体细胞调控的信号通路分析 可能会为与骨脆性和骨质疏松症相关的人类疾病开辟新的和特定的治疗选择 影响骨愈合和骨再生。

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.