Cxcl12-Hedgehog signaling in cranial bone regeneration

摘要： 骨骼干细胞如何被激活以扩张，迁移到损伤部位，并成为成骨细胞 修复受损的骨骼是骨骼再生的核心问题，而骨骼再生是维持 正常的骨骼系统。颅面骨主要通过膜内骨化形成，有限 位于缝合线中的骨髓间隙和骨骼干细胞，缝合线是一种特殊的纤维关节，连接 头盖骨和头盖骨一起，已经被确定为主要的细胞群，需要 颅面部骨骼动态平衡与损伤修复。在人类和老鼠身上进行的大量研究强调了 干细胞的重要性，干细胞栖息的微环境，也调节干细胞 行为。当颅面骨骼包裹大脑并保护它时，颅骨中的颅骨缺损，如果 与骨组织愈合不好，从而发生纤维性骨不连，与高发病率有关 和死亡率。因此，颅骨缺损，特别是临界大小的颅骨缺损的重建和再生， 不断地摆出一种未被满足的治疗挑战。G蛋白刺激性α亚单位(GAS)，由 GNAS基因，从G蛋白偶联受体(GPCRs)传递信号，已成为一个关键的 通过抑制Hedgehog(HH)信号调节成骨细胞分化。两种小鼠模型的研究 人类遗传病支持GAS和HH信号在调节骨骼干细胞中的关键作用 多个上下文中的单元格。研究表明，通过失去气体激活HH信号是一种常见的途径 与异位骨化一样，对颅骨形成中的膜内骨化起关键作用 (HO)，我们在未发表的颅骨骨损伤模型的初步研究中也有了新的发现。 定向缝合干细胞(SuSCs)向损伤部位的迁移与上调表达 趋化因子(C-X-C基序)配体12(CXCL12)和Sonic Hedgehog(Shh)，程度较小的印度刺猬 (IHH)，在SuSC利基。此外，抑制与GAI偶联的CXCL12同源受体CXCR4 中和气体信号，严重损害颅骨再生。气体增强Shh的损失 表达，诱导成骨细胞分化。因此，我们假设CXCL12和Shh是关键的 颅骨损伤诱导的生态位因子协同促进SuSC迁移、扩张和 成骨细胞分化，所有这些都是颅骨损伤修复所必需的。这一假设将是 在三个具体目标中进行测试：1)更好地定义SuSCs并确定CXCL12和GAI/Gas的作用 在颅骨再生过程中引导SuSC迁移到损伤部位的信号；2)确定 GAI/GAS和Shh信号在颅骨中SuSC扩增和成骨细胞分化中的作用 3)研究CXCL12和Shh信号在骨再生过程中的相互作用。 颅骨损伤。我们希望发现新的交互式CXCL12和Shh信号来利用内在调节 巢和SuSCs之间的回路，以促进颅骨再生和治疗颅骨缺陷。

Abstract: How skeletal stem cells are activated to expand, migrate to the injury site, and become osteoblast cells to restore damaged bone are central questions in skeletal regeneration, which is key for the maintenance of a functional skeletal system. Craniofacial bones mainly form through intramembranous ossification with limited bone marrow space and skeletal stem cells that reside in the suture, a special fibrous joint that connects calvarial bones of the skull together, have been identified as a major cell population that are required for craniofacial bone homeostasis and injury repair. Numerous studies in both human and mice have highlighted the importance of stem cell niche, a microenvironment where stem cells reside and also regulate stem cell behaviors. As the craniofacial skeleton encases the brain and protects it, calvarial bone defects in the skull, if failed to heal with bony tissue and consequently fibrous non-unions occur, are associated with high morbidity and mortality. Therefore, reconstruction and regeneration of calvarial defects, in particular critical-size defects, continuously poses as an unmet therapeutic challenge. The G protein stimulatory α-subunit (Gas), encoded by GNAS gene, transduces signals from G protein coupled receptors (GPCRs) and has emerged as a critical regulator of osteoblast differentiation by inhibiting Hedgehog (Hh) signaling. Studies in both mouse models and human genetic diseases supports the critical roles of both Gas and Hh signaling in regulating skeletal stem cells in multiple contexts. Having showed that activation of Hh signaling by loss of Gas is a common pathway that critically regulates intramembranous ossification in calvarial bone formation as in heterotopic ossification (HO), we have also made novel findings in unpublished preliminary studies in an calvarial bone injury model that directed suture stem cells (SuSCs) migration to the injury site correlates with upregulated expression of chemokine (C-X-C motif) ligand 12 (Cxcl12) and Sonic Hedgehog (Shh), to a less extent Indian Hedgehog (Ihh), in SuSC niche. Further, inhibition of the Cxcl12 cognate receptor Cxcr4, which is coupled to Gai that counteracts Gas signaling, severely impaired calvarial bone regeneration. Loss of Gas enhanced Shh expression, which induced osteoblast differentiation. We therefore hypothesize that Cxcl12 and Shh are critical niche factors that are induced by calvarial injury and coordinately promote SuSC migration, expansion and osteoblast differentiation, all of which are essential for calvarial bone injury repair. This hypothesis will be tested in three specific aims: 1) To better define SuSCs and determine the roles of Cxcl12 and Gai/Gas signaling in directing SuSC migration to the injury site during calvarial bone regeneration; 2) To determine the roles of Gai/Gas and Shh signaling in SuSC expansion and osteoblast differentiation during calvarial bone regeneration; 3) To determine the interaction of Cxcl12 and Shh signaling during bone regeneration after calvarial injury. We expect to identify novel interactive Cxcl12 and Shh signaling to harness intrinsic regulatory circuitry between niche and SuSCs to promote calvarial bone regeneration and treat calvarial defects.