Signaling by fibroblast growth factors (FGF) and fibroblast growth factor receptor 2 (FGFR2)-activating mutations blocks mineralization and induces apoptosis in osteoblasts

Fibroblast growth factors (FGF) play a critical role in bone growth and dev elopment affecting both chondrogenesis and osteogenesis, During the process of intramembranous ossification, which leads to the formation of the flat bones of the skull, unregulated FGF signaling can produce premature suture closure or craniosynostosis and other craniofacial deformities. Indeed, man y human craniosynostosis disorders have been linked to activating mutations in FGF receptors (FGFR) 1 and 2, but the precise effects of FGF on the pro liferation, maturation and differentiation of the target osteoblastic cells are still unclear. In this report, we studied the effects of FGF treatment on primary murine calvarial osteoblast, and on OB1,a newly established ost eoblastic cell line. We show that FGF signaling has a dual effect on osteob last proliferation and differentiation, FGFs activate the endogenous FGFRs leading to the formation of a Grb2/FRS2/Shp2 complex and activation of MAP kinase, However, immature osteoblasts respond to FGF treatment with increas ed proliferation: whereas in differentiating cells FGF does not induce DNA synthesis but causes apoptosis. When either primary or OB1 osteoblasts are induced to differentiate. FGF signaling inhibits expression of alkaline pho sphatase, and blocks mineralization,To study the effect of craniosynostosis -linked mutations in osteoblasts, we introduced FGFR2 carrying either the C 342Y (Crouzon syndrome) or the S252W (Apert syndrome) mutation in OB1 cells . Both mutations inhibited differentiation, while dramatically inducing apo ptosis. Furthermore, we could also show that overexpression of FGF2 in tran sgenic mice leads to increased apoptosis in their calvaria, These data prov ide the first biochemical analysis of FGF signaling in osteoblasts, and sho w that FGF can act as a cell death inducer with distinct effects in prolife rating and differentiating osteoblasts.