Erk pathway and activator protein 1 play crucial roles in FGF2‐stimulated premature cranial suture closure

Abstract

Cranial sutures are an important growth center of the cranial bones, and the suture space must be maintained to permit the cranial adjustments needed to accommodate brain growth. Craniosynostosis, characterized by premature suture closure, mainly results from mutations that generate constitutively active fibroblast growth factor (FGF) receptors. FGF signaling, thus, is responsible for the pathogenesis of craniosynostosis. Even though FGF activates many different signaling pathways, the one involved in premature suture closure has not been defined. We observed that placing FGF2-soaked bead on the osteogenic fronts of cultured mouse calvaria accelerates cranial suture closure and strongly induces the expression of osteopontin, an early marker of differentiated osteoblasts. FGF2 treatment also induced fos and jun mRNAs and later increased the nuclear levels of activator protein 1 (AP1).

Erk1/2 signaling is required for Tgf‐β2–induced suture closure

“…Because the DMEM culture medium was supplemented with several factors, including glutamine, insulin, selenium, and transferrin, the culture medium itself or one or more of these molecules may have been responsible for transient stimulation of Erk1/2-P. Up-regulation of Erk1/2-P by the culture conditions could have been responsible for the gradual narrowing of control sutures in culture; however, it was not sufficient to induce suture closure comparable to Tgf-␤2 exposure. Similar gradual narrowing of control sutures was seen in other culture experiments (Kim et al, 2003). Suture narrowing could have been due to sutural cells behaving like fibroblastic cells, which contract in culture medium, a process that is Erk1/2-dependent (Langholz et al, 1997;Hirano et al, 2002).…”

“…Erk1/2 activity is a critical regulator of Fgf2 signaling during suture closure, as blocking Erk1/2-P abrogated the effect of Fgf2 on suture closure (Kim et al, 2003). Tgf-␤2 also induced premature suture closure both in vitro and in vivo (Opperman et al, 2000;Moursi et al, 2003); Mooney et al, manuscript submitted for publication).…”

“…To test whether Tgf-␤2 induced suture closure is dependent on Erk1/2 signaling, fetal day 17.5 (E17.5) mouse calvariae were cultured in a well-established explant model system (Opperman et al, 1995(Opperman et al, , 1999(Opperman et al, , 2000Kim et al, 1998Kim et al, , 2003, with Tgf-␤2 and PD, an inhibitor of Erk1/2 activity. Calvariae were photographed during culture (Fig.…”

“…Fgf2 treatment has been shown to induce premature suture closure (Kim et al, 1998(Kim et al, , 2003Ignelzi et al, 2003), and the map kinase Erk1/2 signaling pathway was required for Fgf-stimulated suture closure (Kim et al, 2003). In both in vitro and in vivo experiments, Tgf-␤3 rescued sutures from obliteration, whereas Tgf-␤2 induced suture obliteration (Opperman et al, 2000(Opperman et al, , 2002Chong et al, 2003;Moursi et al, 2003).…”

“…Erk1/2 signaling is critical for normal expression of the osteoblast phenotype (Lai and Cheng, 2002). The Erk1/2 pathway is downstream to growth factor signaling such as Egf and Fgf and is critical for Fgf2-stimulated premature cranial suture closure, as blocking Erk1/2 phosphorylation prevents Fgf2-induced suture closure (Kim et al, 2003). It was hypothesized, therefore, that Tgf-␤2-induced suture closure occurred by means of an Erk1/2-dependent signaling pathway.…”