Role of Hypoxia-inducible factor-2α signaling in age-induced bone loss

PROJECT SUMMARY Osteoporotic fractures due to age-associated bone loss impart tremendous economic burden and impair quality and longevity of life. There remains an undeniable need to identify mechanisms that underlie bone deterioration with age, for the targeted development of novel therapies that can reverse bone loss to improve bone quality, fracture resistance, and quality of life. Changes in bone mass during aging occur via dysregulated skeletal remodeling. Under healthy conditions, skeletal remodeling maintains healthy bone mass and biomechanical integrity. Homeostatic remodeling balances osteoclastic (OC) bone resorption and osteoblastic (OB) bone formation coordinated by osteocytes (OCYs). OCYs are terminally differentiated OBs embedded within the bony matrix that live for decades and are the most abundant cell type in bone. OCY hypoxia inducible factor (HIF) signaling is an obligate regulatory pathway in skeletal health and maintenance. HIFs are transcription factors composed of an oxygen-dependent α subunit (transcriptionally active isoforms: HIF-1α, HIF-2α) and constitutively stable ß subunit (HIF- ß). HIF-α isoforms stabilize under hypoxia and induce skeletal changes. Skeletal Hif2a expression increases with age, concomitant with senile osteoporosis, and heterozygous Hif2a depletion abrogates senile osteoporotic bone loss. Cellular senescence associates with age-induced osteoporotic bone loss. Senescence is characterized by irreversible growth arrest and secretion of pro-inflammatory and matrix-degrading factors, creating a toxic microenvironment that drives age-related tissue dysfunction. Hif2a levels increase with age, concomitant with expression of canonical senescence markers p16 (Cdkn2a) and p21 (Cdkn1a) in bone, suggesting mechanistic coupling between HIF-2α and cell senescence. Furthermore, HIF-2α can bind to hypoxia response elements (HREs) in p16 and p21 promoters and directly regulate their expression. While OCYs from aged mice express senescent markers, whether HIF- 2α controls OCY senescence is unknown. Our overall hypothesis is that HIF-2α expression increases with age in OCYs and directly contributes to age-induced osteoporosis by promoting senescence-associated gene expression. We will test this hypothesis by establishing the relationship between HIF-2α and senescent gene signatures in OCYs during skeletal aging (Aim 1) and demonstrate that OCY HIF-2α stabilization drives a senescent gene signature in OCYs using in vivo and in vitro approaches (Aim 2). Using micro-computed tomography, biomechanics, and histology, we will demonstrate that Hif2a deletion prevents age-related bone loss and preserves bone strength in mice (Aim 3). Our overall goal is to elucidate HIF-2α’s role in driving OCY senescence and osteoporosis, revealing novel targets that can be harnessed pharmaceutically to prevent age- induced senescent-related bone loss. This proposal describes a comprehensive research training plan, supported by innovative facilities at UC Davis that will foster my scientific and professional development toward my long-term goal of pursuing an academic research position.