HIFs in osteocytes

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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.