Trinity Centre for Bioengineering, Trinity Biomedical Sciences Institute, Trinity College Dublin, Dublin 2, Ireland.
J Orthop Res. 2013 Oct;31(10):1585-96. doi: 10.1002/jor.22396. Epub 2013 Jun 15.
It is often difficult to decouple the relative importance of different factors in regulating MSC differentiation. Genetically modified mice provide model systems whereby some variables can be manipulated while others are kept constant. Fracture repair in thrombospondin-2 (TSP2)-null mice is characterized by reduced endochondral ossification and enhanced intramembranous bone formation. The proposed mechanism for this shift in MSC fate is that increased vascular density and hence oxygen availability in TSP2-null mice regulates differentiation. However, TSP2 is multifunctional and regulates other aspects of the regenerative cascade, such as MSC proliferation. The objective of this study is to use a previously developed computational model of tissue differentiation, in which substrate stiffness and oxygen tension regulate stem cell differentiation, to simulate potential mechanisms which may drive alterations in MSC fate in TSP2-null mice. Four models (increased cell proliferation, increased numbers of MSCs in the marrow decreased cellular oxygen consumption, and an initially stiffer callus) were not predictive of experimental observations in TSP2-null mice. In contrast, increasing the rate of angiogenic progression led to a prediction of greater intramembranous ossification, diminished endochondral ossification, and a reduced region of hypoxia in the fracture callus similar to that quantified experimentally by the immunohistochemical detection of pimonidazole adducts that develop with hypoxia. This study therefore provides further support for the hypothesis that oxygen availability during early fracture healing is a key regulator of MSC bipotential differentiation, and furthermore, it highlights the advantages of integrating computational models with genetically modified mouse studies for further elucidating mechanisms regulating stem cell fate.
在调节 MSC 分化的过程中，不同因素的相对重要性往往难以分离。基因修饰小鼠提供了模型系统，通过该系统可以操纵某些变量，同时保持其他变量不变。在血小板反应蛋白 2（TSP2）缺失小鼠中，骨折修复的特征是软骨内骨化减少和膜内骨形成增强。这种 MSC 命运转变的提出机制是，TSP2 缺失小鼠中增加的血管密度和因此增加的氧气供应调节分化。然而，TSP2 具有多功能性，可调节再生级联的其他方面，如 MSC 增殖。本研究的目的是使用先前开发的组织分化计算模型，其中基质硬度和氧张力调节干细胞分化，模拟可能导致 TSP2 缺失小鼠 MSC 命运改变的潜在机制。四个模型（细胞增殖增加、骨髓中 MSC 数量增加、细胞耗氧量减少和初始骨痂更硬）不能预测 TSP2 缺失小鼠的实验观察结果。相比之下，增加血管生成进展的速度导致对更多膜内骨形成、软骨内骨化减少和骨折骨痂中缺氧区域减少的预测，这与通过缺氧发展的 pimonidazole 加合物的免疫组织化学检测在实验上定量的结果相似。因此，本研究进一步支持了这样的假设，即在早期骨折愈合过程中氧气供应是 MSC 双潜能分化的关键调节剂，并且进一步强调了将计算模型与基因修饰小鼠研究相结合以进一步阐明调节干细胞命运的机制的优势。
