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伤害，可导致严重的发病率、高昂的医疗费用和显着的死亡率，尤其是老年人。愈合失败，包括骨不连、继发性移位和股骨头缺血性坏死，需要重复进行侵入性手术，而后续手术的成功率会降低。这种骨修复失败的病因尚不清楚，但可以通过更好地了解调节软骨内骨化的分子机制来了解，包括间充质浓缩、软骨细胞分化和肥大，形成软骨原基，然后被血管侵入，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。我们实验室的工作确定了 Creb 家族 Atf4 的转录因子，可以调节软骨细胞增殖， 分别通过激活印度刺猬蛋白 (Ihh)、核因子-κB 配体受体激活剂 (Rankl) 和骨钙素 (Ocn) 的转录来促进分化和成骨细胞成熟。 Atf4 整体缺乏（Atf4-/-）会导致小鼠侏儒症和严重骨质减少。令人惊讶的是，Atf4-/-小鼠软骨细胞中的 Atf4 表达不仅完全恢复 挽救了他们的身材，也挽救了他们的骨质缺陷。这一发现强烈表明，软骨细胞中的 Atf4 (chAtf4)，而不是成骨细胞 (obAtf4) 中的 Atf4，在软骨内骨化的控制中发挥着不可或缺的作用，并且 chAtf4 对于生长板伸长和出生后骨量增长之间的耦合至关重要。仍有待了解的是 chAtf4 与 obAtf4 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。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稳态。我们提出以下具体目标，以使用动物模型来解决这一假设。目标 1. 确定 chAtf4 是否通过调节 Mmp13 和 Rankl 转录来控制软骨向骨的转化。目标 2. 阐明 obAtf4 是否是成人骨重塑所必需的。目标 3. 确定成人软骨内骨修复各阶段之间的正确过渡是否需要 Atf4。影响。在成骨细胞分化因子中，Atf4 对于关注软骨向骨转变调节因子的研究来说是独一无二的，因为它的无效突变体在出生后仍能存活，并且在所有软骨细胞亚群中表达。因此，Atf4 的时间和空间功能将为我们提供一个独特的机会来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潜在机制。

 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.