Heparin Variants Differ in Their Effect on Fibroblast Growth Factor 23 and Fibroblast Growth Factor Receptor 4 Binding

Background: Fibroblast growth factor (FGF) 23 is a bone-derived hormone that targets the kidney via FGF receptor (FGFR) isoform 1c and klotho and increases phosphate excretion. In Chronic Kidney Disease (CKD) patients, renal FGF23-responsiveness is impaired, leading to high serum FGF23 levels that clinical studies have shown to be associated with cardiac hypertrophy and cardiovascular mortality. In previous studies we found that elevated FGF23 can directly target cardiac myocytes and activate FGFR4 in a klotho-independent manner, resulting in hypertrophic cell growth. Surprisingly, we also found that unfractionated heparin (UFH) increases the FGF23:FGFR4 binding affinity and when injected into mice with elevated FGF23 levels aggravates cardiac hypertrophy. Heparin exists in different variants with a large range of structures and charges. In this study, we investigate whether heparin variants differ in their ability to modulate FGF23:FGFR4 binding. Methods: FGF23:FGFR4 binding affinity was analyzed using our plate-based binding assay. 96-well plates were coated with recombinant FGF23 protein and sequentially incubated with heparin variants, Fc-tagged FGFR4, horse radish peroxidase (HRP)-coupled anti-Fc antibody, and HRP substrate. Reactions were analyzed using a plate reader at 450 nm wavelength. All samples were run in triplicates. Results: FGF23:FGFR4 binding affinity was increased by large UFHs, including dialysis UFH, and by low-molecular heparins. In contrast, smaller heparin variants, like the pentasaccharide fondaparinux sodium, did not increase FGF23:FGFR4 binding. Desulfated heparins only had a minor impact on FGF23:FGFR4 binding. Conclusion: Our study indicates that the ability of heparin to mediate FGF23:FGFR4 binding depends on its length and negative charges. Small and uncharged heparin variants do not increase FGF23:FGFR4 binding affinity. Next, we will test the effects of heparin variants on FGF23-induced cardiac hypertrophy in cell culture and animal models. Most patients with end-stage kidney disease undergo hemodialysis, during which UFH is commonly administered as an anticoagulant. Since hemodialysis does not lower serum FGF23 levels, we postulate that frequent UFH infusions contribute to cardiac injury and high mortality rates and that replacing UFH with small-chain heparin variants might extend the lifespan of hemodialysis patients. Funding: NIDDK Support, Private Foundation Support