Role of RANK-RANKL-OPG axis in cranial suture homeostasis.

Craniosynostosis is a significant disorder affecting 1 in 2500 live births worldwide. Although a large body of work has focused on dural regulation and the contributions of molecular mediators such as fibroblast growth factor, bone morphogenetic protein, and transforming growth factor β, minimal attention has been directed toward osteoclast function in cranial suture biology. Receptor activator of nuclear factor κB (RANK) is an essential mediator of osteoclastogenesis and osteoclast activation. In this study, physiologic fusion of posterior frontal sutures in murine development correlated with decreasing protein expression of RANK in comparison to age-matched coronal and sagittal sutures via immunohistochemical survey. However, RANK mRNA did not exhibit a similar pattern suggesting that RANK is regulated at the protein level. Fused cranial sutures in nonsyndromic craniosynostotic children also showed decreased levels of RANK staining in immunohistochemistry in comparison to patent sutures from the same patients. Immunohistochemistry with a RANK ligand antibody did not show differences in fused or patent sutures. Moreover, RANK knockdown in calvarial strip suture cultures displayed increased bone density specifically in the suture line after infection with small interfering RANK viruses. Cranial suture biology, similar to bone biology in general, likely depends on a complex interplay between osteoblasts and osteoclasts. We now report a temporospatial correlation between RANK expression and suture morphology that suggests that osteoclast activity is important in maintenance of cranial suture patency in normal physiology and disease. Furthermore, RANK downregulation promoted suture fusion establishing a causal relationship between the presence of RANK and patency.