Molecular and cellular mechanisms of heterotopic ossification

异位骨化（HO）是骨骼肌和软组织中异位软骨内骨的形成，是引起关节不动和疼痛的重要原因。导致HO的确切机制尚不清楚；然而，它与术后和创伤后环境的关联表明了一个损伤修复过程的紊乱。进一步了解HO的原因可以从先天性HO综合征进行性骨化纤维发育不良（FOP）中获得。FOP是由骨形态发生蛋白（BMP） I型受体ALK2的“组成性激活”突变引起的，这种突变导致轻微创伤或炎症引发进行性和广泛的关节骨化。FOP和获得性HO都缺乏有效的治疗方法。事实上，有大量证据表明，FOP和HO都是由BMP信号通路的不当激活引起的。目前尚不清楚增强的BMP信号是如何偏离损伤修复程序的，也不知道哪些细胞群介导了增强的BMP信号的作用。为了解决这些问题，我们开发了一种小鼠FOP模型，其中ALK2的组成活性突变形式（caALK2）被诱导表达。与受影响的人类相似，该基因的表达不会自发诱导HO，但在炎症和肌肉损伤的额外刺激下，会发生剧烈的骨化和关节融合。我们对该模型的后续研究表明，这些caALK2蛋白可能不像之前认为的那样具有组成活性，但可能使细胞对传统的配体介导的BMP信号敏感。在本研究的目标1中，我们将使用该模型来识别caALK2使细胞对BMP信号敏感的机制，测试caALK2是否需要配体介导的信号传导或独立的功能。为了确定介导BMP信号增强作用的细胞祖细胞，以及导致异位骨病变的细胞祖细胞，在Aim 2中，我们将突变体caALK2的表达靶向到几个候选祖细胞谱系。利用基于细胞、基因靶向和过继移植技术的补充，我们将系统地确定在具有已知成骨潜力的腔室中表达caALK2的影响，包括骨骼肌卫星细胞、血管周细胞以及骨髓来源的谱系。在Aim 3中，我们在该模型中研究了先天免疫信号在异位骨发育中的作用。了解caALK2突变如何改变BMP信号传导的结果，可以为HO的治疗提供新的分子或细胞靶点。本研究旨在阐明增强的BMP信号如何影响间充质祖细胞，并在损伤、炎症和再生的界面上调控适应性和非适应性成骨。

Heterotopic ossification (HO), the formation of ectopic endochondral bone in skeletal muscle and soft tissues, is a significant cause of morbidity from joint immobility and pain. The precise mechanisms responsible for HO are not known; however, its association with postsurgical and posttraumatic contexts suggests a process of disordered injury repair. Further insights into the causes of HO may be gained from a congenital HO syndrome, fibrodysplasia ossificans progressiva (FOP). FOP is caused by "constitutively-activating" mutations in the bone morphogenetic protein (BMP) type I receptor ALK2, which result in progressive and widespread joint ossifications triggered by minimal trauma or inflammation. Both FOP and acquired forms of HO lack effective therapies. In fact, there is significant evidence that both FOP and HO are caused by inappropriate activation of the BMP signaling pathway. It is not known how enhanced BMP signaling deviates the injury repair program, or which populations of cells mediate the effects of enhanced BMP signaling. To address these questions we have developed a mouse model of FOP in which a constitutively-active mutant form of ALK2 (caALK2) is inducibly expressed. Similar to affected humans, expression of this gene does not spontaneously induce HO, but vigorous ossification and joint fusion occur with additional stimuli of inflammation and muscle injury. Our subsequent studies with this model suggest that these caALK2 proteins may not be constitutively-active, as previously thought, but may sensitize cells to traditional ligand-mediated BMP signals. In Aim 1 of this grant, we will employ this model to discern the mechanisms by which caALK2 sensitizes cells to BMP signals, testing whether caALK2 requires ligand-mediated signaling or functions independently. To identify the cellular progenitors which mediate the effects of enhanced BMP signaling, and which contribute to the ectopic bone lesions, in Aim 2 we have targeted the expression of mutant caALK2 to several candidate progenitor lineages. Using a complement of cell-based, genetic targeting, and adoptive transfer techniques, we will systematically determine the impact of expressing caALK2 in compartments with known osteogenic potential, including skeletal muscle satellite cells, vascular pericytes, as well as bone-marrow derived lineages. In Aim 3, we examine the role of innate immune signaling in the development of ectopic bone in this model. Understanding how caALK2 mutations alter the consequences of BMP signaling could highlight novel molecular or cellular targets for management of HO. This proposal seeks to elucidate how enhanced BMP signaling impacts mesenchymal progenitors and governs adaptive and maladaptive osteogenesis at the interface of injury, inflammation, and regeneration.