Conditional Deletion of HIF-2 alpha in Mouse Nucleus Pulposus Reduces Fibrosis and Provides Mild and Transient Protection From Age-Dependent Structural Changes in Intervertebral Disc

Hypoxia-inducible factors (HIFs) are critical to the development and homeostasis of hypoxic tissues. Although HIF-2 alpha, one of the main HIF-alpha isoforms, is expressed in nucleus pulposus (NP) cells, its functions remain unknown. We deleted HIF-2 alpha in the NP tissue using a notochord-specific FoxA2(Cre) allele to study HIF-2 alpha function in the adult intervertebral disc. Unlike observations in HIF-1 alpha(cKO) mice, fate mapping studies using Rosa26-mTmG reporter showed that HIF-2 alpha loss in NP did not negatively impact cell survival or affect compartment development. Rather, loss of HIF-2 alpha resulted in slightly better attributes of NP morphology in 14-month-old HIF-2 alpha(cKO) mice as evident from lower scores of degeneration. These 14-month-old HIF-2 alpha(cKO) mice also exhibited significant reduction in NP tissue fibrosis and lower collagen turnover in the annulus fibrosis (AF) compartment. Imaging-Fourier transform-infrared (FTIR) analyses showed decreased collagen and protein content in the NP and maintained chondroitin sulfate levels in 14-month-old HIF-2 alpha(cKO). Mechanistically, global transcriptomic analysis showed enrichment of differentially expressed genes with Gene Ontology (GO) terms related to metabolic processes and cell development, molecular functions concerned with histone and protein binding, and associated pathways, including oxidative stress. Noteworthy, these morphological differences were not apparent in 24-month-old HIF-2 alpha(cKO), indicating that aging is the dominant factor in governing disc health. Together these data suggest that loss of HIF-2 alpha in the NP compartment is not detrimental to the intervertebral disc development but rather mitigates NP tissue fibrosis and offers mild but transient protection from age-dependent early degenerative changes. (c) 2022 American Society for Bone and Mineral Research (ASBMR).