Linking Osteocyte Oxygen Sensing and Biomineralization via FGG23: Implications for Chronic Kidney Disease

Abstract

FGF23 is an osteocyte produced hormone necessary for maintaining systemic phosphate handling, and thus bone structure and function in both rare and common disorders such as chronic kidney disease (CKD). FGF23 is a critical factor in CKD, with elevated levels causing alterations in mineral metabolism and increased odds for mortality. However, the mechanisms directing the production of key modulators of skeletal homeostasis and biomineralization within osteocytes, and how this is altered in chronic kidney disease, remain unclear. The experimental focus of this dissertation was to dissect the molecular systems and role of oxygen sensing in the regulated production of FGF23. In CKD, up to 75% of patients have anemia and concomitant marked elevations in FGF23, increasing mortality odds. Anemia is a potent driver of FGF23 secretion, therefore, current and emerging therapies, including recombinant EPO and the hypoxia inducible factorprolyl hydroxylase inhibitors (HIF-PHI) FG-4592 and BAY 85-3934, were used to improve anemia in the adenine diet-induced mouse model of CKD. In the mice with CKD, iFGF23 was markedly elevated in control mice but was attenuated by 65-85% after delivery of EPO or HIF-PHI, with no changes in serum phosphate. This was associated with improved systemic iron utilization and reductions in mRNA markers of renal fibrosis. In osteocyte-like cell cultures treated with HIF-PHI, integrative RNAseq and ATACseq analysis identified candidate genes upregulated in response to mimicked hypoxia, concomitant with elevated Fgf23 expression. These genes were found to be downregulated in CKD bone, therefore, knock-out cells were generated using CRISPR/Cas9 technology. These cells were found to be functionally similar to in vivo conditional knockout models that have enhanced bone mass and elevated FGF23. Taken together, these results further define novel factors involved in the regulation of FGF23 and identify new therapeutic targets.