ATF4 in Chondrocytes Regulates Cartilage to Bone Transition

 描述（由申请人提供）：骨折在美国是一种常见的损伤，可导致严重的发病率、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，需要重复侵入性手术，后续手术的成功率降低。这种骨修复失败的病因尚不清楚，但可以通过更好地理解调节软骨内骨化的分子机制来了解，所述软骨内骨化包括间充质凝聚、软骨细胞分化和肥大，以形成软骨原基，然后血管侵入软骨原基，从而使成骨细胞和破骨细胞形成骨。软骨内骨化过程中的每一步都受到许多生长激素、细胞因子和转录因子的严格控制。我们实验室的工作鉴定了调节软骨细胞增殖的Creb家族Atf 4的转录因子， 分化和成骨细胞成熟分别通过激活Indian hedgehog（Ihh）、核因子-κB配体受体激活剂（Rankl）和骨钙素（Ocn）的转录。 整体Atf 4缺陷（Atf 4-/-）导致小鼠侏儒症和严重的骨质减少。令人惊讶的是，Atf 4-/-小鼠软骨细胞中Atf 4表达的特异性恢复不仅完全 拯救了他们的体型，也挽救了他们的骨量缺陷。这一发现有力地表明，Atf 4在软骨细胞（chAtf 4），而不是它在成骨细胞（obAtf 4），在控制软骨内骨化中起着不可或缺的作用，chAtf 4是关键的生长板伸长和出生后骨量增加之间的耦合。尚待了解的是chAtf 4与obAtf 4如何影响参与骨建模和重塑的激素、生长因子、细胞因子和转录因子的集合。我们的中心假设是，chAtf 4是一个关键的转录稳态之间的和谐耦合软骨形成和骨建模在发展和骨修复。我们提出了以下具体目标，以解决这一假设使用动物模型。目标1.确定chAtf 4是否通过调节Mmp 13和Rankl转录来控制软骨向骨的转化。目标2.阐明obAtf 4是否是成人骨重建所必需的。目标3。确定成人是否需要Atf 4在软骨内骨修复阶段之间进行适当的过渡。冲击在成骨细胞分化因子中，Atf 4对于专注于软骨向骨过渡的调节剂的研究是独特的，因为其无效突变体在出生后存活，并且在所有软骨细胞亚群中表达。因此，Atf 4的时空功能将为我们提供一个独特的机会，以深入了解骨骼发育过程中的生理过程和骨折修复，骨不连，骨关节炎的病理条件下的机制。

 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.