Aim: The EphB receptor tyrosine kinase family and their ephrinB ligands have predominantly been recognised as mediators of skeletal development and bone homeostasis. Particularly human mutations of ephrinB1 contribute to frontonasal dysplasia and coronal craniosynostosis. Mouse models of ephrinB1 demonstrate its importance for correct cartilage segmentation, ossification patterning, osteoblast and osteoclast function. The present study aimed to identify the functional role of ephrinB1 during skeletal development and maturation.
Method: The Cre recombination system under the control of the osterix (Osx:Cre) promoter was used for the targeted deletion of ephrinB1 (EfnB1fl/fl) by osteogenic progenitor. Homozygote females (Osx:EfnB1-/-), hemizyogte males (Osx:EfnB1-/O) and Osx:Cre control mice were assessed by alizarin red/alcian blue staining, biomechanical testing, micro-computational tomography (µCT) and histomorphometry at embryonic day (E) E16.5, P0 and 4 weeks of age.
Results: The Osx:EfnB1-/- mice exhibited perturbed bone growth during embryonic and postnatal skeletal development up to 4 weeks of age, when compared to the Osx:Cre controls. Osx:EfnB1-/- newborn mice showed an increase in bone formation when compared to Osx:Cre control mice. Conversely, by 4 weeks of age Osx:EfnB1-/- mice displayed significantly weaker and less rigid bones. This aligned with a reduction in trabecular bone formation and architecture; and a reduction in cortical bone formation. Correlating with increased numbers of TRAP positive osteoclasts and decreased numbers of bone lining osteoblasts in Osx:EfnB1-/- mice when compared to Osx:Cre control mice. Furthermore, the growth plate of newborn and 4 weeks old mice was significantly shorter, presenting a perturbed structure in Osx:EfnB1-/- mice when compared to Osx:Cre controls.
Conclusions: This study showed that ephrinB1 promotes growth plate formation and mineral formation through reverse signalling, while activation of EphB forward signalling by ephrinB1 inhibits osteoclast function. Taken together, these observations demonstrate that ephrinB1, expressed by osteogenic progenitors, contributes to endochondral ossification and bone modelling.