ATF4 in Chondrocytes Regulates Cartilage to Bone Transition

 描述(申请人提供)：骨折在美国是一种常见的损伤，可能会导致严重的发病率、高昂的医疗费用和显著的死亡率，尤其是在老年人中。愈合失败，包括骨不连、继发性移位和缺血性坏死，需要反复进行侵入性手术，后续手术的成功率会降低。这种骨修复失败的原因尚不清楚，但通过更好地了解调控软骨内成骨的分子机制，包括间充质凝聚、软骨细胞分化和肥大形成软骨原，然后软骨原被血管侵入，使成骨细胞和破骨细胞形成骨，可以了解这一点。软骨内成骨过程中的每一步都受到一些生长激素、细胞因子和转录因子的严格控制。我们实验室的工作发现了一种调控软骨细胞增殖的Creb家族ATF4转录因子， 分别通过激活核因子受体激活剂κB配体(RANKL)和骨钙素(OCN)的转录，促进成骨细胞的分化和成熟。 全球ATF4缺乏(ATF4-/-)导致小鼠侏儒症和严重的骨量减少。令人惊讶的是，ATF4在ATF4-/-小鼠软骨细胞中特异性表达的恢复不仅完全 不仅挽救了他们的体型，还挽救了他们的骨量缺陷。这一发现有力地表明，软骨细胞(ChAtf4)中的ATF4而不是成骨细胞(ObAtf4)中的ATF4在软骨内成骨的控制中起着不可或缺的作用，并且chAtf4对于生长板延长和出生后骨量增加之间的耦合至关重要。尚需了解的是chAtf4和obAtf4如何影响参与骨骼建模和重建的激素、生长因子、细胞因子和转录因子的集合。我们的中心假设是，chAtf4是发育和骨修复过程中软骨形成和骨建模之间和谐耦合的关键转录恒定。我们提出了以下具体目标，以利用动物模型来解决这一假设。目的1.探讨chAtf4是否通过调控Mmp13和RANKL转录调控软骨向骨的转化。目的2.阐明obAtf4是否是成人骨重建所必需的。目的3.确定成人软骨内骨修复阶段之间的适当过渡是否需要ATF4。冲击力。在成骨细胞分化因子中，ATF4在软骨向骨转化的调控研究中是独一无二的，因为它的零突变在出生后存活，并且在所有软骨细胞亚群中都有表达。因此，ATF4的时间和空间功能将为我们提供一个独特的机会，让我们深入了解骨骼发育过程中潜在的生理过程以及骨折修复、骨不连、骨关节炎等病理情况。

 DESCRIPTION (provided by applicant): Fractures are a common injury in the United States and can lead to severe morbidity, high medical costs and significant mortality, especially in the elderly. Failures of healing, including cases of nonunion, secondary displacement and avascular necrosis, require repeated invasive surgery with decreased success rate with subsequent surgeries. The etiology underlying such bone repair failures is unknown but can be informed by a better understanding of the molecular mechanisms regulating endochondral ossification that includes mesenchymal condensation, chondrocyte differentiation and hypertrophy to form a cartilaginou anlagen that is then invaded by blood vessels, which brings osteoblasts and osteoclasts to form bone. Each step during endochondral ossification is tightly controlled by a number of growth hormones, cytokines and transcription factors. Work from our laboratory identified a transcription factor of the Creb family Atf4 that regulates chondrocyte proliferation, differentiation, and osteoblast maturation via activating the transcription of Indian hedgehog (Ihh), receptor activator of nuclear factor-κB ligand (Rankl) and osteocalcin (Ocn), respectively. Global Atf4 deficiency (Atf4-/-) led to dwarfism and severe osteopenia in mice. Surprisingly, restoration of Atf4 expression specifically in chondrocytes of the Atf4-/- mice not only completely rescued their size but also their bone mass defects. This finding strongly suggests that Atf4 in chondrocytes (chAtf4), but not it in osteoblasts (obAtf4), plays an indispensible role in the control of endochondral ossification and the chAtf4 is critical to the coupling between growth plate elongation and bone mass accrual after birth. What remains to be understood is how the chAtf4 versus the obAtf4, influences the set of hormones, growth factors, cytokines, and transcription factors involved in bone modeling and remodeling. Our central hypothesis is that chAtf4 is a critical transcriptional homeostat for the harmonious coupling between cartilage formation and bone modeling during development and bone repair. We propose the following specific aims to address this hypothesis using animal models. Aim 1. To determine whether chAtf4 controls the conversion of cartilage to bone via its regulation of Mmp13 and Rankl transcription. Aim 2. To elucidate whether obAtf4 is necessary for bone remodeling in adults. Aim 3. To determine whether Atf4 is required in adults for proper transition between the phases of endochondral bone repair. Impact. Among the osteoblast differentiation factors Atf4 is unique for studies focusing on the regulators of cartilage to bone transition because its null mutants survive postnatally and it is expressed in all the subpopulation of chondrocytes. Thus the temporal and spatial function of Atf4 will provides us a unique opportunity to gain insights into mechanisms underlying the physiological processes during skeletal development and pathological conditions in fracture repair, nonunion, osteoarthritis.