Role of osteogenic stem cells in bone regeneration and repair in vivo

描述（由申请人提供）：骨系细胞在骨的终身再生过程中具有特别重要的意义。它们在骨髓微环境中也至关重要，在骨髓微环境中它们控制造血，并可能参与白血病和癌症转移。然而，尽管成骨细胞在维持骨骼和造血中起着关键作用，但这些细胞的体内起源、寿命和动力学仍然是一个悬而未决的问题。因此，我们的目的是确定骨形成成骨细胞的寿命及其在稳态和损伤条件下的替代来源。我们最近利用遗传脉冲追踪技术结合延时动物荧光成像技术的研究表明，功能性成骨细胞和成骨细胞前体具有较短的寿命，并且在体内由其祖细胞持久维持。此外，我们发现“间充质干细胞”（根据体外多系能力定义）在体内没有这种功能。相反，它们在正常的体内平衡中作为限制骨系的祖细胞。这些成骨祖细胞可以通过黏液病毒抗性-1 （Mx1）启动子的短暂激活进行遗传标记。我们证明了mx1标记的成骨祖细胞具有血管周围起源，能够有效地迁移到损伤部位，并提供血管内皮细胞

DESCRIPTION (provided by applicant): Osteo-lineage cells are of particular importance in the life-long regenerative processes of bone. They are also critical in the bone marrow microenvironment where they control hematopoiesis and likely participate in leukemia and cancer metastasis. However, despite the pivotal roles of osteogenic cells in the maintenance of bones and hematopoiesis, the in vivo origin, lifespan, and dynamics of these cells remain fundamentally unanswered questions. Therefore, we aimed to define the lifespan of bone-forming osteoblasts and their source of replacement under homeostatic and injury conditions. Our recent studies using genetic pulse-chase techniques in combination with time-lapse live-animal fluorescent imaging have revealed that functional osteoblasts and osteoblast precursors have a short lifespan and are durably maintained by their progenitor cells in vivo. Additionally, we found that the 'mesenchymal stem cells' (as defined by in vitro multilineage capacity) have no such function in vivo. Rather, they serve as osteolineage-restricted progenitors in normal homeostasis. These osteogenic progenitors can be genetically marked by transient activation of the myxovirus resistance-1 (Mx1) promoter. We demonstrated that Mx1-labeled osteogenic progenitor cells have a perivascular origin, migrate efficiently to sites of injury, and supply the majority of new osteoblasts in fracture healing. Further, they overlap with a subset of perivascular nestin� MSCs. We now have distinct stage-specific inducible models that can label osteogenic stem/progenitors, pre-osteoblasts and mature osteoblasts. The goal of this study is to understand the role of mature osteoblasts and their stem/progenitor cells in the context of bone regeneration and repair in vivo. We will also explore the clinical relevance of osteogenic stem cells with the goal of reversing conditions of degenerative bone disease. We thus propose to: 1. Test if Mx1 and nestin double labeling specify osteogenic stem cells in vivo. 2. Test if te transplantation of osteogenic stem cells improves the restoration of bone fracture. 3. Test the role of osteogenic stem cells in bone loss diseases. By studying three specific, clinically relevan questions, we will further define new biological and clinical insights of osteogenic stem cells in physiological conditions.