Extracoagulative Role of FVIII: Impact on in Vitro and In Vivo Murine Bone Cell Differentiation and Function

Introduction: The complex interplay between hemophilia A (HA) and skeletal health has been demonstrated in several pre-clinical and clinical studies. Patients with HA exhibit low bone mineral density (BMD) and heightened fracture risk, and osteopenia and osteoporosis have been reported as early as 12 years of age. Deficiency of factor VIII (FVIII) could be contributing to an imbalance in bone metabolism, leading to reduced BMD. The reduction in BMD may be caused by altered signaling between cells responsible for bone remodeling: osteoblasts for building and osteoclasts for resorption. However, the exact impact of FVIII deficiency on skeletal health remains unclear.

Aim: To explore the direct effect of FVIII on the maturation and activity of osteoblasts and osteoclasts in vitro, and to track ex vivo bone structure in a murine model of severe HA.

Methods: HA and wild-type (wt) osteoblast precursors were isolated from the bones of 3-week-old mice by collagenase treatment while myelomonocytic progenitors were flushed out from the bone marrow (BM) of 8-week-old mice. Osteoblast differentiation was induced with ascorbic acid and β-glycerophosphate whereas osteoclastogenesis was promoted by macrophage colony-stimulating factor and receptor activator of nuclear factor kappa-B ligand (RANKL). The direct effect of FVIII was assessed by adding recombinant (r)FVIII for the duration of the differentiating cultures. Osteoclastogenesis was assessed by tartrate-resistant acid phosphatase staining, while osteoblast differentiation and mineralization efficiency were evaluated by Alkaline Phosphates and Alizarin-Red-S respectively. Quantitative polymerase chain reaction was performed to quantify the expression level of osteoblast-specific genes, such as RUNX2, collagen type 1 alpha 1 chain (COL1A1), RANKL and osteoprotegerin (OPG). Ex vivo longitudinal assessments of bone parameters using 3D morphometric analysis by micro-computed tomography were performed on tibiae and femurs obtained from mice at different time points (3, 8, 24 and 52 weeks).

Results: In vitro differentiation and Ca²⁺ deposition of osteoblasts was compromised in HA compared to wt cells. Aligned with compromised differentiation and Ca²⁺ deposition, expression of the main genes involved in osteoblastic differentiation (RUNX2) and bone matrix deposition (COL1A1) were downregulated in HA osteoblasts. Addition of rFVIII restored osteoblast differentiation and Ca²⁺ deposition and increased RUNX2 expression in both HA and wt pre-osteoblasts. RANKL/OPG expression ratio in HA osteoblasts was shifted towards a strong promotion of osteoclastogenesis which was re-balanced with in vitro rFVIII supplementation. HA-derived BM progenitors displayed greater in vitro osteoclastogenesis that was unaffected by FVIII supplementation. HA mice displayed significantly lower ex vivo trabecular bone volume and number compared with wt mice at all measured time points while cortical parameters were not affected.

Conclusion: These results highlight an impaired balance of in vitro osteoblast and osteoclast maturation in the absence of FVIII, in favor of bone resorption. Moreover, rFVIII administration restored adequate osteoblast differentiation. These in vitro data were supported by the ex vivo analysis revealing a long-term decrease of trabecular bone parameters. Studies are ongoing to further dissect the role of FVIII in bone homeostasis.