HIFs in osteocytes

项目摘要 有一个关键的社会经济和医疗需要的合成代谢疗法能够取代失去的骨 骨质疏松症等疾病中的肿块。具有惊人高骨量（HBM）的人类和小鼠可以帮助 鉴定促进骨愈合的新机制。值得注意的例子包括激活突变 在Wnt共受体Lrp5和患有硬化性骨质疏松症的个体中（SOST表达降低）。 我们已经发现，在骨细胞而不是成骨细胞中Vhl的缺失（Dmp 1-cre; Vhlf/f）产生了强有力的骨诱导分化。 骨骼表型，其特征在于皮质和小梁微结构的显著增加 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）亚基， 泛素化和随后的蛋白酶体降解在常氧条件下，并被认为是一个 HIF活性的主要调节因子。与以前在成骨细胞中删除Hif1a的工作相反，我们没有观察到 在缺乏Hif1a的骨细胞中，低骨量表型是相互的。我们的数据表明，骨细胞可能只 需要HIF-2a来抑制Vhl信号，这为识别和利用未知的 骨细胞中的通路，可以利用它来改善骨骼健康。 这并不奇怪，成骨细胞和骨细胞可能使用不同的信号机制， 这是一个特定的信号，因为在细胞周期中，许多基因经历上调或下调。 成骨细胞向骨细胞的转变。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，它们的稳定性受到不同的调节，并且它们诱导不同基因的转录， 目标的虽然人们对HIF-1a在骨中的作用关注很多，但对HIF-2a的了解很少。 我们的长期目标是阐明HIF-a亚型对骨骼发育的作用， 功能冗余，与Wnt/β-连环蛋白信号传导的整合，以及Vhl/HIF-α表达的操纵 防止卵巢切除术（OVX）引起的骨质流失。我们的总体假设是骨细胞需要HIF-2a， 而不是HIF-1a来调节对骨骼的影响。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-α亚型特异性重演和维持骨生长的作用。 Vhl cKO HBM表型（Aim 1），β-连环蛋白在Vhl cKO HBM表型中的上位关系 小鼠（目标2），以及靶向HIF-α用于改善OVX小鼠模型中骨性质的效用（目标3）。 这些研究将确定Vhl的HIF-α依赖性功能在骨细胞中的作用，该功能驱动骨细胞的获得。 HBM。了解这些信号通路可能有助于识别新的治疗靶标， 骨质增生和扭转骨质疏松症，伴随着老化和更年期。这样做将缓解 成本和相关的生活质量问题，导致不可避免的骨折是如此普遍， 伴随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.