Regulation of Skeletal Growth by Nuclear Retinoid Receptors

 描述(申请人提供)：软骨内骨化是胚胎骨骼发育和纵向骨生长的主要过程。在这个过程中，每个骨骼元素中的软骨细胞都会组织成生长板，并经历成熟和肥大。在肥大之后，细胞进入最后的晚期肥大阶段，在此阶段，它们促进软骨基质的重塑和血管的矿化和侵袭，导致钙化的软骨被骨取代。这种表型改变的序列需要许多系统和局部因素的严格控制，而这种调节机制的改变是先天性和获得性骨骼病理的基础。视黄酸受体α，β和γ(RARα、RARβ和RARγ)是调节基本生物过程的核激素受体。在之前的资金周期中，我们研究了RAR信号在生长板功能中的作用，发现(1)RARγ是生长板上的主要受体；(2)在无维甲酸的条件下，RARγ的表达促进了培养的软骨细胞凝集素的产生；(3)RARγ主要定位于增生区和肥大前期，但活性形式的维甲酸(AtRA)在这些区域几乎检测不到；以及(4)RARγ缺乏导致骨骼生长迟缓。这些发现使我们得出结论，RARγ作为一种细胞因子在软骨基质的产生中起着重要的作用。 生长板中的无配体转录抑制因子。令人惊讶的是，RARγ的免疫组织化学分析显示，RARγ蛋白定位于存在全反式维甲酸的肥大带，表明与配体结合的RARγ作用于肥大的软骨细胞。我们还发现，选择性的RARγ激动剂显著刺激培养的原代软骨细胞与晚期肥大功能相关的基因表达水平，并且RARγ激动剂的应用加速了小鼠软骨细胞的肥大并导致生长板关闭。这些发现和其他发现导致了我们的中心假设，即肥大带中的配体结合的RARγ促进了软骨细胞的晚期肥大分化和软骨内成骨，并且RARγ通路的调节可能具有治疗价值。我们的具体目标是(1)明确RARγ在调节生长板软骨细胞晚期肥大分化和从软骨向骨过渡中的作用；(2)确定RARγ调节软骨细胞晚期肥大分化的分子机制；(3)研究RARγ在长骨病理性生长停滞中的作用，并测试选择性抑制RARγ是否可以改善这种情况。我们将利用转基因小鼠系统和/或药物操作来修饰肥大软骨细胞中的RARγ信号，并阐明RARγ在生长板中的生理意义。这一结果还可能为治疗性早熟生长板关闭和阻止骨生长提供一种新的途径，而目前大多数病例还没有这种治疗方法。

 DESCRIPTION (provided by applicant): Endochondral ossification is the major process of embryonic skeletal development and longitudinal bone growth. During this process chondrocytes within each skeletal element become organized into growth plates and undergo maturation and hypertrophy. Following hypertrophy, the cells reach a final late-hypertrophic stage during which they facilitate cartilage matrix remodeling and mineralization and invasion of blood vessels, leading to replacement of calcified cartilage by bone. This sequence of phenotypic changes requires tight control by numerous systemic and local factors, and alterations of such regulatory mechanisms underlie congenital and acquired skeletal pathologies. The retinoic acid receptors α, β and γ (RARα, RARβ and RARγ) are nuclear hormone receptors that regulate fundamental biological processes. During the previous funding cycle, we studied the roles of RAR signaling in growth plate function and found that (1) RARγ is the dominant receptor in growth plate; (2) expression of RARγ enhances aggrecan production in cultured chondrocytes under retinoid-free condition; (3) RARγ mRNA is predominantly localized in proliferative and prehypertrophic zones but the active form of retinoic acid (atRA) is hardly detectable in these zones; and (4) RARγ deficiency induces skeletal growth retardation. These findings led us to conclude that RARγ exerts an essential role in cartilage matrix production as a ligand-less transcriptional repressor in growth plate. Surprisingly, immuno- histochemical analysis of RARγ revealed that RARγ protein was localized to the hypertrophic zone where atRA was present, indicating that ligand-bound RARγ acts on hypertrophic chondrocytes. We found also that selective RARγ agonists markedly stimulated gene expression levels linked to late-hypertrophic function in cultured primary chondrocytes and that administration of RARγ agonist accelerated chondrocyte hypertrophy and caused growth-plate closure in mouse. These and other findings lead to our central hypothesis that ligand-bound RARγ in hypertrophic zone stimulates late-hypertrophic differentiation of chondrocytes and endochondral ossification and that modulations of RARγ pathway could have therapeutic value. Our specific goals are (1) To define the role of RARγ in regulation of late-hypertrophic differentiation of growth plate chondrocytes and transition from cartilage to bone; (2) To determine the molecular mechanisms by which RARγ regulates late-hypertrophic differentiation of chondrocytes; and (3) To examine the role of RARγ in pathological growth-arrest of long bones and test if selective inhibition of RARγ ameliorates the condition. We will modify RARγ signaling in hypertrophic chondrocytes by use of genetically modified mouse systems and/or pharmacological manipulations and clarify physiological importance of RARγ in growth plate. The results could also provide a new avenue to therapy for precocious growth plate closure and arrest of bone growth which is not currently available for most cases.