The Eph receptor tyrosine kinase ligand, ephrinB1 (EfnB1) is important for correct skeletal and cartilage development, however, the role of EfnB1 in fracture repair is unknown. This study investigated the role of EfnB1 during fracture repair where EfnB1 expression increased significantly at 1 and 2 weeks post fracture in C57Bl/6 wildtype mice, coinciding with the haematoma, soft callus formation/remodelling stages, respectively. To investigate the specific role of EfnB1 within the osteogenic lineage during fracture repair, male mice with a conditional deletion of EfnB1 in the osteogenic lineage (EfnB1OBfl/O), driven by the Osterix (Osx) promoter, and their male Osx:Cre counterparts were subject to a femoral fracture with internal fixation. Two weeks post fracture micro computed tomography (μCT) analysis revealed that EfnB1OBfl/O mice displayed a significant decrease in bone volume relative to tissue volume within the fracture callus. This was attributed to an alteration in the distribution of osteoclasts within the fracture site, a significant elevation in cartilaginous tissue and reduction in the osteoprogenitor population and calcein labelled bone within the fracture site of EfnB1OBfl/O mice. Supportive in vitro studies demonstrated that under osteogenic conditions, cultured EfnB1OBfl/O stromal cells derived from the 2 week fracture site exhibited a reduced capacity to produce mineral and decreased expression of the osteogenic gene, Osterix, when compared to Osx:Cre controls. These findings suggest that the loss of EfnB1 delays the fracture repair process. The present study confirmed that EFNB1 activation in human BMSC, following stimulation with soluble-EphB2 resulted in de-phosphorylation of TAZ, demonstrating similarities in EfnB1 signalling between human and mouse stromal populations. Overall, the present study provides evidence that loss of EfnB1 in the osteo/chondrogenic lineages delays the soft callus formation/remodelling stages of the fracture repair process.

中文翻译：

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成骨祖细胞中ephrinB1的条件性敲除延迟了骨折修复过程中的软骨内骨化过程

Eph 受体酪氨酸激酶配体 ephrinB1 (EfnB1) 对正确的骨骼和软骨发育很重要，然而，EfnB1 在骨折修复中的作用尚不清楚。本研究调查了 EfnB1 在骨折修复过程中的作用，其中 EfnB1 表达在 C57Bl/6 野生型小鼠骨折后 1 周和 2 周显着增加，分别与血肿、软骨痂形成/重塑阶段相吻合。为了研究 EfnB1 在骨折修复过程中成骨谱系中的特定作用，由 Osterix (Osx) 启动子驱动的成骨谱系中 EfnB1 条件性缺失 (EfnB1OBfl/O) 的雄性小鼠及其雄性 Osx:Cre 对应物是股骨骨折内固定。骨折后两周显微计算机断层扫描 (μCT) 分析显示，EfnB1OBfl/O 小鼠的骨体积相对于骨折愈伤组织内的组织体积显着减少。这归因于骨折部位破骨细胞分布的改变、软骨组织显着升高以及 EfnB1OBfl/O 小鼠骨折部位内骨祖细胞和钙黄绿素标记骨的减少。支持性体外研究表明，在成骨条件下，与 Osx:Cre 对照相比，来自 2 周骨折部位的培养的 EfnB1OBfl/O 基质细胞产生矿物质的能力降低，成骨基因 Osterix 的表达降低。这些发现表明 EfnB1 的缺失延迟了骨折修复过程。本研究证实，在用可溶性 EphB2 刺激后，人 BMSC 中的 EFNB1 激活导致 TAZ 去磷酸化，证明了人和小鼠基质群之间 EfnB1 信号传导的相似性。总体而言，本研究提供的证据表明，骨/软骨形成谱系中 EfnB1 的缺失延迟了骨折修复过程的软骨痂形成/重塑阶段。