Loss of ephrinB1 in osteogenic progenitor cells impedes endochondral ossification and compromises bone strength integrity during skeletal development.

ABSTRACT

The EphB receptor tyrosine kinase family and their ephrinB ligands have been implicated as mediators of skeletal development and bone homeostasis in humans, where mutations in ephrinB1 contribute to frontonasal dysplasia and coronal craniosynostosis. In mouse models, ephrinB1 has been shown to be a critical factor mediating osteoblast function. The present study examined the functional importance of ephrinB1 during endochondral ossification using the Cre recombination system with targeted deletion of ephrinB1 (EfnB1fl/fl) in osteogenic progenitor cells, under the control of the osterix (OsxCre) promoter. The OsxEfnB1-/- mice displayed aberrant bone growth during embryonic and postnatal skeletal development up to 4weeks of age, when compared to the OsxCre controls. Furthermore, compared to the OsxCre control mice, the OsxEfnB1-/- mice exhibited significantly weaker and less rigid bones, with a reduction in trabecular/ cortical bone formation, reduced trabecular architecture and a reduction in the size of the growth plates at the distal end of the femora from newborn through to 4weeks of age. The aberrant bone formation correlated with increased numbers of tartrate resistant acid phosphatase positive osteoclasts and decreased numbers of bone lining osteoblasts in 4week old OsxEfnB1-/- mice, compared to OsxCre control mice. Taken together, these observations demonstrate the importance of ephrinB1 signalling between cells of the skeleton required for endochondral ossification.