Department of Orthopaedic Surgery, University of California at San Francisco, San Francisco, CA, USA.
Bone. 2013 Jan;52(1):111-9. doi: 10.1016/j.bone.2012.09.018. Epub 2012 Sep 23.
Like other tissue injuries, bone fracture triggers an inflammatory response, which plays an important role in skeletal repair. Inflammation is believed to have both positive and negative effects on bone repair, but the underlying cellular mechanisms are not well understood. To assess the role of inflammation on skeletal cell differentiation, we used mouse models of fracture repair that stimulate either intramembranous or endochondral ossification. In the first model, fractures are rigidly stabilized leading to direct bone formation, while in the second model, fracture instability causes cartilage and bone formation. We compared the inflammatory response in these two mechanical environments and found changes in the expression patterns of inflammatory genes and in the recruitment of inflammatory cells and osteoclasts. These results suggested that the inflammatory response could influence skeletal cell differentiation after fracture. We then exploited matrix metalloproteinase 9 (MMP9) that is expressed in inflammatory cells and osteoclasts, and which we previously showed is a potential regulator of cell fate decisions during fracture repair. Mmp9(-/-) mice heal stabilized fractures via endochondral ossification, while wild type mice heal via intramembranous ossification. In parallel, we observed increases in macrophages and T cells in the callus of Mmp9(-/-) compared to wild type mice. To assess the link between the profile of inflammatory cells and skeletal cell fate functionally, we transplanted Mmp9(-/-) mice with wild type bone marrow, to reconstitute a wild type hematopoietic lineage in interaction with the Mmp9(-/-) stroma and periosteum. Following transplantation, Mmp9(-/-) mice healed stabilized fractures via intramembranous ossification and exhibited a normal profile of inflammatory cells. Moreover, Mmp9(-/-) periosteal grafts healed via intramembranous ossification in wild type hosts, but healed via endochondral ossification in Mmp9(-/-) hosts. We observed that macrophages accumulated at the periosteal surface in Mmp9(-/-) mice, suggesting that cell differentiation in the periosteum is influenced by factors such as BMP2 that are produced locally by inflammatory cells. Taken together, these results show that MMP9 mediates indirect effects on skeletal cell differentiation by regulating the inflammatory response and the distribution of inflammatory cells, leading to the local regulation of periosteal cell differentiation.
像其他组织损伤一样，骨折会引发炎症反应，而炎症反应在骨骼修复中起着重要作用。炎症反应被认为对骨修复既有积极影响，也有消极影响，但其中的细胞机制尚不清楚。为了评估炎症反应对骨骼细胞分化的作用，我们使用了刺激膜内成骨或软骨内成骨的骨折修复小鼠模型。在第一个模型中，骨折被刚性固定，导致直接骨形成，而在第二个模型中，骨折不稳定导致软骨和骨形成。我们比较了这两种机械环境中的炎症反应，发现炎症基因的表达模式以及炎症细胞和破骨细胞的募集都发生了变化。这些结果表明，炎症反应可能会影响骨折后的骨骼细胞分化。然后，我们利用了基质金属蛋白酶 9（MMP9），它在炎症细胞和破骨细胞中表达，我们之前的研究表明，它是骨折修复过程中细胞命运决定的潜在调节因子。Mmp9(-/-) 小鼠通过软骨内成骨愈合稳定的骨折，而野生型小鼠则通过膜内成骨愈合。与此同时，我们观察到 Mmp9(-/-) 小鼠的骨痂中巨噬细胞和 T 细胞增多。为了从功能上评估炎症细胞的特征与骨骼细胞命运之间的联系，我们将 Mmp9(-/-) 小鼠的骨髓移植到野生型小鼠中，在 Mmp9(-/-) 基质和成骨细胞的相互作用下重建野生型造血谱系。移植后，Mmp9(-/-) 小鼠通过膜内成骨愈合稳定的骨折，并表现出正常的炎症细胞特征。此外，Mmp9(-/-) 骨膜移植物在野生型宿主中通过膜内成骨愈合，但在 Mmp9(-/-) 宿主中通过软骨内成骨愈合。我们观察到 Mmp9(-/-) 小鼠的骨膜表面聚集了巨噬细胞，这表明骨膜细胞的分化受到局部产生的 BMP2 等因素的影响，这些因素由炎症细胞产生。综上所述，这些结果表明，MMP9 通过调节炎症反应和炎症细胞的分布，对骨骼细胞分化产生间接影响，从而导致骨膜细胞分化的局部调节。
