Osteogenic and Angiogenic Response to Skeletal Loading

描述(申请人提供)：骨骼的机械负荷是新骨形成(成骨)的强大刺激。在许多情况下(如发育、骨折愈合)骨的形成严重依赖于新血管的形成(血管生成)，然而血管生成在支持机械负荷诱导的成骨中的作用尚不清楚。衰老和糖尿病导致的血管缺陷可能会损害骨骼对机械刺激的反应，从而可能导致骨质疏松性骨折的风险。利用大鼠前肢负荷模型，我们最近量化了在骨质破坏疲劳负荷后，编织骨形成之前血管的快速增加。我们还量化了疲劳负荷后1小时内血管生成基因(如血管内皮生长因子)和骨诱导基因BMP-2(骨形态发生蛋白-2)的上调，意外发现BMP-2首先在血管细胞中表达，然后在骨细胞中表达。在功能上，编织骨的形成与疲劳损伤的程度成正比，全骨强度在加载两周后恢复--无论最初的损伤程度如何。值得注意的是，血管生成抑制显著降低了疲劳负荷后编织骨的反应。一个基本的未知问题--是什么生物因素导致疲劳负荷后的编织骨形成与合成代谢负荷后的板层骨形成？在下一个项目期间，我们将使用(大鼠和小鼠)前肢压缩的活体加载模型，以实现确定机械加载后导致快速非软骨内骨形成的机械生物路径的长期目标。在目标1中，我们将比较疲劳负荷和合成负荷下的分子、血管和骨形成反应，并确定发育成骨(特别是Hedgehog信号)和血管生成在负荷诱导编织和板层骨形成中的作用。在目标2中，我们将确定是否需要BMP信号(特别是内皮BMP-2)来诱导负荷诱导的成骨和血管生成。综上所述，这些研究将建立重要的机械-生物因素，调节板层和编织骨形成的不同过程，并阐明血管生成的作用。它们将扩大我们对骨骼应力损伤(疲劳性骨折)的成骨反应的理解，并可能作为未来开发快速骨形成策略的基础，以增强脆弱或受损的骨骼。与公共卫生相关：骨骼在一定程度上受到机械力的调节。这个项目的目标是了解机械刺激后控制骨形成的生物途径。这些信息可能会导致未来增加骨量和降低骨骼脆性的策略。

DESCRIPTION (provided by applicant): Mechanical loading of the skeleton is a powerful stimulus for new bone formation (osteogenesis). Bone formation in many contexts (e.g., development, fracture healing) depends critically on the formation of new blood vessels (angiogenesis), yet the role of angiogenesis in supporting mechanical loading-induced osteogenesis is not known. Deficits in vascularity due to aging and diabetes may compromise the skeletal response to mechanical stimuli and thus may contribute to osteoporotic fracture risk. Using the rat forelimb loading model, we recently quantified a rapid increase in vascularity after damaging fatigue loading of bone, prior to woven bone formation. We also quantified the upregulation of angiogenic genes (e.g., VEGF, vascular endothelial growth factor) and the osteoinductive gene BMP-2 (bone morphogenetic protein-2) within 1 hr after fatigue loading, with the unexpected finding that BMP-2 expression occurred first in vascular cells then in bone cells. Functionally, woven bone formation occurred in proportion to the level of fatigue damage and whole-bone strength was restored two weeks after loading - regardless of the level of initial damage. Notably, angiogenic inhibition markedly reduced the woven bone response after fatigue loading. A fundamental unknown remains - what are the biological factors that cause woven bone formation after fatigue loading versus lamellar bone formation after anabolic loading? In the next project period we will use the in vivo loading model of forelimb compression (in rats and mice) as we work toward the long term goal of determining the mechano-biological pathways that lead to rapid non-endochondral bone formation after mechanical loading. In Aim 1, we will compare molecular, vascular and bone formation responses to fatigue loading versus anabolic loading, and determine the role of developmental osteogenesis (in particular Hedgehog signaling) and angiogenesis in loading-induced woven and lamellar bone formation. In Aim 2, we will determine if BMP signaling (and in particular endothelial BMP-2) is required for loading-induced osteogenesis and angiogenesis. Taken together, these studies will establish important mechano-biological factors that mediate the distinct processes of lamellar versus woven bone formation and clarify the role of angiogenesis. They will extend our understanding of the osteogenic response to skeletal stress injuries (fatigue fractures) and may serve as a basis for future development of strategies for rapid bone formation to augment weak or damaged bones. PUBLIC HEALTH RELEVANCE: The skeleton is regulated in part by mechanical forces. The goal of this project is to understand the biological pathways that control bone formation following mechanical stimulation. This information may lead to future strategies to increase bone mass and reduce skeletal fragility.