FGF23 Neutralizing Antibody Partially Improves Bone Mineralization Defect of HMWFGF2 Isoforms in Transgenic Female Mice

Abstract

Mice overexpressing high molecular weight FGF2 isoforms (HMWTg) in osteoblast lineage phenocopy human X-linked hypophosphatemic rickets (XLH) and a Hyp murine model of XLH demonstrating increased FGF23/FGF receptor signaling and hypophosphatemic rickets/osteomalacia. Because HMWFGF2 was upregulated in bones of Hyp mice and abnormal FGF23 signaling is important in XLH, HMWTg mice were used to examine the effect of the FGF23 neutralizing antibody (FGF23Ab). Eight-week-old female Vector control mice and HMWTg mice were treated with FGF23Ab or control IgG. A single injection of FGF23Ab rescued abnormal hypophosphatemia in HMWTg. The decreased type II sodium-dependent phosphate co-transporter (Npt2a) was rescued by FGF23Ab treatment. Inappropriately low serum 1,25(OH) D in HMWTg mice was normalized by FGF23Ab treatment, which is accompanied by increased anabolic vitamin D hydroxylase Cyp27b1 and decreased catabolic vitamin D hydroxylase Cyp24 mRNA in kidney. Long-term treatment with FGF23Ab normalized femur length and significantly increased vertebrae BMD and BMC, and femur BMC in HMWTg mice compared to IgG-treated HMWTg mice. Micro-computed tomography (μCT) revealed increased cortical porosity and decreased cortical apparent density in the HMWTg-IgG group compared with the Vector-IgG group; however, FGF23Ab treatment rescued defective cortical mineralization, decreased porosity, and increased apparent density in HMWTg mice. Bone histomorphometry analysis showed FGF23Ab treatment decreased osteoid volume, increased intra-label thickness, mineralization apposition rate, and bone formation rate in HMWTg mice. FGF23Ab improved disorganized double labeling in femurs from HMWTg mice. Quantitative real-time PCR analysis of tibia shafts showed FGF23Ab treatment normalized the osteocalcin (Ocn) mRNA expression in HMWTg mice, but further increased expression of SIBLING protein-related and pyrophosphate-related genes that are important in matrix mineralization, suggesting that HMWFGF2 modulates these genes independent of FGF23. We conclude that FGF23Ab partially rescued hypophosphatemic osteomalacia in HMWTg. However, long-term treatment with FGF23Ab further increased SIBLING protein-related genes and pyrophosphate-related genes in bone that could contribute to incomplete rescue of the mineralization defect. © 2018 American Society for Bone and Mineral Research.

Fibroblast growth factor 23 and α-Klotho co-dependent and independent functions

“…In addition, HMW-FGF2, which is derived from alternative translation, functions as an intracellular ligand for FGFR1 in Hyp mice [28]. Transgenic HMW-FGF2 reproduce the phenotype of Hyp mice [26,29].…”

What Can We Learn from FGF-2 Isoform-Specific Mouse Mutants? Differential Insights into FGF-2 Physiology In Vivo

“…Meanwhile, serum PTH and 1,25D were not affected by this treatment, which probably was mediated through simultaneous upregulation of both 1,25D regulating genes, renal 25-hydroxyvitamin D 1 α-hydroxylase (CYP27B1) and renal 25-hydroxyvitamin D 24-hydroxylase (CYP24). These results were mainly confirmed by a study in female FGF-2HMWtg mice comparing single or long-term treatment of FGF-23 neutralizing antibody [79]. However, female FGF-2HMWtg had higher serum 1,25D concentrations, even though degrading CYP24 expression was further increased.…”

“…FGF-2HMWtg mice developed the most severe dysgenesis phenocopying the Hyp mouse, an established model for human X-linked hypophosphatemic rickets [45,81]. Independent of sex, those mice analyzed developed dwarfism, along with reduced bone mineral density and content and also rickets/osteomalacia [45,79,82,83]. Histomorphometric investigations supported this phenotype, demonstrating predominant bone resorption processes, while a decreased bone formation was registered [45].…”

“…Additionally, pro-mineralization genes like dentin-matrix phosphoprotein 1 (Dmp1) belonging to the small integrin-binding ligand N-linked glycoprotein (SIBLING) family, and its interaction phosphate-regulating neutral endopeptidase X-linked gene (Phex) were also increased in bone documenting exaggerated osteoblast function [78]. Consequentially, reduced Fgf-23 mRNA expression was detected in bone compared to wt littermates, influenced by Phex and Dmp1 through FGFR-1, which, in turn, was upregulated [79,80]. Meanwhile, serum levels of FGF-23 were not changed compared to wt, which was also true for phosphate, calcium, and parathyroid hormone (PTH) levels in FGF-2HMWko, as well as FGF-2LMWtg mice [47,78].…”

Fibroblast growth factors signaling in bone metastasis

“…Cell-based and GEM studies implicate FGF2 (one of the most studied FGF ligands) in osteogenesis (Montero et al 2000. However, different FGF2 isoforms (low molecular weight (LMW) and high molecular weight (HMW)) seem to have opposite effects on bone mass (Xiao et al 2010(Xiao et al , 2018. Briefly, LMW-FGF2 induces bone mass in mice by modulating the Wnt/β-catenin signaling pathway (Xiao et al 2009).…”