Roles of Inhibitory Smads in Endochondral Bone Formation

项目概要/摘要 软骨内骨化涉及软骨细胞分化、增殖、成熟和肥大的高度协调程序。软骨细胞的细胞活动在生长板中的不同区域中组织，使得这种组织独特地适合于体内研究，因为突变的影响可以与特定的细胞活动相关联。骨形态发生蛋白（BMP）和转化生长因子β（TGF β）/激活素途径是这些过程的重要调节因子。已经确定了细胞外拮抗剂作为BMP和TGF β/激活素信号传导的持续时间、强度和程度的调节剂的重要性。例如，缺乏BMP受体拮抗剂头蛋白的小鼠表现出软骨过度生长，同时伴有过量的BMP活性。然而，很少有人知道BMP和TGF β/激活素途径的细胞内抑制剂，如抑制性Smad（I-Smad）蛋白，Smad 6和Smad 7的作用。功能获得和丧失研究揭示了I-Smads在体外软骨细胞中的潜在作用，但尚未进行体内研究。该项目的长期目标是了解I-Smads如何通过BMP和TGF β/激活素信号的细胞内调节来控制软骨形成，这是利用BMP实现软骨维护和修复的更有效策略的先决条件。为了实现这一目标，重要的是要确定I-Smads是否对软骨形成至关重要，并确定软骨形成的具体方面，以及I-Smads影响的机制。因此，本提案中要解决的具体假设是：（1）I-Smads对于正常的软骨内骨形成至关重要，（2）I-Smads的缺失影响生长板中TGF β/激活素和BMP途径之间的信号串扰平衡，这将影响软骨内骨化的进展，以及（3）TGF β信号转导通过Smad 7与生长板中的关键存活/应激途径相互作用。

Project Summary/Abstract Endochondral ossification involves a highly coordinated program of chondrocyte differentiation, proliferation, maturation and hypertrophy. The cellular activities of chondrocytes are organized in distinct zones in the growth plate, making this tissue uniquely suited for in vivo studies, as effects of mutations can be linked to specific cellular activities. The bone morphogenetic protein (BMP) and transforming growth factor beta (TGFbeta)/activin pathways are important regulators of these processes. The importance of extracellular antagonists as regulators of the duration, intensity and extent of BMP and TGFbeta/activin signaling has been defined. For example, mice lacking the BMP receptor antagonist Noggin exhibit cartilage overgrowth concurrent with excess BMP activity. However, very little is known about the roles of intracellular inhibitors of BMP and TGFbeta/activin pathways, such as the inhibitory Smad (I-Smad) proteins, Smad6 and Smad7. Gain and loss of function studies have revealed potential roles for I-Smads in chondrocytes in vitro, but no in vivo studies have been performed. The long-term goal of this project is to understand how intracellular regulation of BMP and TGFbeta/activin signaling by I-Smads controls chondrogenesis as a prerequisite to more efficient strategies utilizing BMPs to achieve cartilage maintenance and repair. To achieve this goal, it is important to determine whether I-Smads are critical for chondrogenesis and to identify specific aspects of chondrogenesis, as well as mechanisms, impacted by I-Smads. Hence, the specific hypotheses to be addressed in this proposal are (1) I-Smads are essential for normal endochondral bone formation, (2) loss of I-Smads impacts the balance of signaling cross-talk between TGFbeta/activin and BMP pathways in the growth plate that will affect the progression of endochondral ossification, and (3) TGFbeta signaling interact via Smad7 with key survival/stress pathways in the growth plate.