Harvard School of Dental Medicine, Boston, MA, USA.
Harvard School of Dental Medicine, Boston, MA, USA; University of Dallas, Irving, TX, USA.
Bone. 2021 Feb;143:115738. doi: 10.1016/j.bone.2020.115738. Epub 2020 Nov 11.
Bone is a dynamic organ that is continuously modified during development, load-induced adaptation, and fracture repair. Understanding the cellular and molecular mechanisms for natural fracture healing can lead to therapeutics that enhance the quality of newly formed tissue, advance the rate of healing, or replace the need for invasive surgical procedures. Prx1-expressing cells in the periosteum are thought to supply the majority of osteoblasts and chondrocytes in the fracture callus, but the exact mechanisms for this behavior are unknown. The primary cilium is a sensory organelle that is known to mediate several signaling pathways involved in fracture healing and required for Prx1-expressing cells to contribute to juvenile bone development and adult load-induced bone formation. We therefore investigated the role of Prx1-expressing cell primary cilia in fracture repair by developing a mouse model that enabled us to simultaneously track Prx1 lineage cell fate and disrupt Prx1-expressing cell primary cilia in vivo. The cilium KO mice exhibited abnormally large calluses with significantly decreased bone formation and persistent cartilage nodules. Analysis of mRNA expression in the early soft callus revealed downregulation of osteogenesis, Hh signaling, and Wnt signaling, and upregulation of chondrogenesis and angiogenesis. The mutant mice also exhibited decreased Osx and Periostin but increased αSMA and PECAM-1 protein expression in the hard callus. We further used a Gli1 reporter and found that Hh signaling was significantly upregulated in the mutant callus at later stages of healing. Interestingly, altered protein expression and Hh signaling did not correlate with labeled Prx1-lineage cells, suggesting loss of cilia altered Hh signaling non-autonomously. Overall, cilium KO mice demonstrated severely delayed and incomplete fracture healing, and our findings suggest Prx1-expressing cell primary cilia are necessary to tune Hh signaling for proper fracture repair.
骨骼是一种动态器官，在发育、负荷诱导适应和骨折修复过程中不断被修饰。了解自然骨折愈合的细胞和分子机制可以导致治疗方法的出现，这些方法可以增强新形成组织的质量、加速愈合速度，或者取代需要进行侵入性手术的需求。骨膜中表达 Prx1 的细胞被认为提供了骨折痂中的大部分成骨细胞和软骨细胞，但这种行为的确切机制尚不清楚。初级纤毛是一种感觉细胞器，已知它可以介导几个参与骨折愈合的信号通路，并为表达 Prx1 的细胞促进幼年骨骼发育和成年负荷诱导的骨骼形成所必需。因此，我们通过开发一种能够同时追踪 Prx1 谱系细胞命运和体内破坏表达 Prx1 的细胞初级纤毛的小鼠模型，研究了表达 Prx1 的细胞初级纤毛在骨折修复中的作用。纤毛 KO 小鼠表现出异常大的骨痂，骨形成明显减少，软骨结节持续存在。对早期软骨痂中的 mRNA 表达分析显示，成骨作用、Hh 信号和 Wnt 信号下调，软骨生成和血管生成上调。突变小鼠在硬骨痂中的 Osx 和 Periostin 表达减少，但 αSMA 和 PECAM-1 蛋白表达增加。我们进一步使用 Gli1 报告基因，发现 Hh 信号在愈合后期的突变骨痂中显著上调。有趣的是，改变的蛋白表达和 Hh 信号与标记的 Prx1 谱系细胞不相关，表明纤毛的丧失改变了 Hh 信号的非自主性。总的来说，纤毛 KO 小鼠表现出严重延迟和不完全的骨折愈合，我们的研究结果表明，表达 Prx1 的细胞初级纤毛对于适当的骨折修复是必要的，以调节 Hh 信号。
