HIFs in osteocytes

项目摘要 社会经济和医学上迫切需要能够替代丢失的骨骼的合成代谢疗法。 在骨质疏松症等疾病中大量存在。具有惊人的高骨量(HBM)的人类和小鼠可以帮助 确定促进骨合成代谢的新机制。值得注意的例子包括激活突变 在Wnt共同受体LRP5和硬化症患者中(SOST表达降低)。 我们发现，VHL在骨细胞中的缺失，而不是成骨细胞(Dmp1-cre；Vhlf/f)的缺失产生了强大的 骨骼表型，以皮质和骨小梁微结构显著增加为特征 与年龄匹配的野生型小鼠相比。VHL靶向缺氧诱导因子-α(HIF-a)亚单位 泛素化和随后的蛋白酶体在常氧条件下的降解，被认为是 HIF活性的主要调节者。与之前在成骨细胞中删除HIF1a的工作不同，我们没有观察到 缺乏HIF1a的骨细胞的倒数，低骨量表型。我们的数据表明，骨细胞可能只是 需要HIF-2a来转导VHL信号，为识别和利用未知的信号提供了新的可能性 在骨细胞中的通路，可以被利用来改善骨骼健康。 成骨细胞和骨细胞可能使用不同的信号机制来 转导一种特定的信号，因为许多基因在转录过程中经历上调或下调 成骨细胞到骨细胞的转变。越来越多的证据表明，虽然HIF-1a和HIF-2a都是 在骨细胞中表达，它们的稳定性受到差异调节，并诱导不同基因的转录 目标。虽然许多人关注HIF-1a在骨骼中的作用，但对HIF-2a知之甚少。 我们的长期目标是阐明HIF--一种对骨骼发育有贡献的异构体，HIF-a异构体 功能冗余，与Wnt/b-catenin信号的整合，以及对VHL/HIF-a表达的IF操作 防止卵巢摘除(OVX)引起的骨丢失。我们的总体假设是骨细胞需要HIF-2a， 而不是HIF-1a，以调节对骨骼的影响。在内部，我们将评估以下基本要求 Ocy HIF-2a在纵向骨生长中的作用，以及HIF-一种异构体特异性，以概括和维持 VHL CKO HBM表型(目标1)--b-catenin在VHL CKO HBM表型中的上位关系 小鼠(目标2)，以及靶向HIF-a在去卵巢小鼠模型中改善骨属性的效用(目标3)。 这些研究将确定VHL的HIF-a依赖功能在骨细胞中的作用，从而推动骨细胞获得 HBM。了解这些信号通路可能有助于识别新的治疗靶点，从而导致 骨积聚和逆转伴随衰老和更年期的骨质疏松症。这样做会缓解 导致不可避免的骨折的费用和相关的生活质量问题是如此常见，没有 伴随着当前治疗的相关并发症。

Project Summary There is a critical socioeconomic and medical need for anabolic therapies capable of replacing lost bone mass in diseases such as osteoporosis. Humans and mice with striking high bone mass (HBM) can aid in the identification of novel mechanisms to promote osteoanabolism. Notable examples include activating mutations in the Wnt co-receptor Lrp5 and individuals with sclerosteosis (decreased SOST expression). We have found that deletion of Vhl in osteocytes but not osteoblasts (Dmp1-cre;Vhlf/f) produces a robust skeletal phenotype, characterized by dramatic increases in both cortical and trabecular microarchitecture compared to age-matched wild-type mice. Vhl targets hypoxia inducible factor-alpha (HIF-a) subunits for ubiquitination and subsequent proteasomal degradation under normoxic conditions, and is considered a master regulator of HIF activity. In contrast to previous work deleting Hif1a in osteoblasts, we do not observe a reciprocal, low bone mass phenotype in osteocytes lacking Hif1a. Our data suggest that osteocytes may only require HIF-2a to transduce Vhl signaling, presenting new possibilities to identify and exploit yet-unknown pathways in osteocytes, that could be harnessed to improve bone health. It is not that surprising that osteoblasts and osteocytes might use different signaling machinery to transduce a particular signal, as a multitude of genes undergo upregulation or downregulation during the osteoblast-to-osteocyte transition. Increasing evidence shows that while HIF-1a and HIF-2a are both expressed in bone cells, their stability is differentially regulated, and they induce transcription of distinct gene targets. While much attention has focused on the role of HIF-1a in bone, very little is known about HIF-2a. Our long-term goal is to elucidate HIF-a isoform contribution to skeletal development, HIF-a isoform functional redundancy, integration with Wnt/b-catenin signaling, and if manipulation of Vhl/HIF-a expression prevents ovariectomy (OVX)-induced bone loss. Our overall hypothesis is that osteocytes require HIF-2a, rather than HIF-1a, to mediate effects on the skeleton. Within, we will evaluate the fundamental requirement of OCY HIF-2a in longitudinal bone growth, as well as HIF-a isoform specificity to recapitulate and maintain the Vhl cKO HBM phenotype (Aim 1), the epistatic relationship of b-catenin in the HBM phenotype of Vhl cKO mice (Aim 2), and the utility of targeting HIF-a for improving bone properties in an OVX mouse model (Aim 3). These studies will define the role of HIF-a-dependent functions of Vhl in osteocytes that drive acquisition of HBM. Understanding these signaling pathways may allow identification of novel therapeutic targets leading to bone accrual and reversing the osteoporosis that accompanies aging and menopause. Doing so will alleviate the costs and associated quality of life issues that result in the inevitable fractures that are so common, without the associated complications that accompany current therapy.