The Role of Sef (Similar Expression to FGF Genes) in Skeletal Development and Bone Homeostasis

Sef (similar expression to fgf genes) is a feedback inhibitor of fibroblast growth factor (FGF) signaling and functions in part by binding to FGF receptors and inhibiting their activation. Because FGF signaling is critical to skeletal growth and maintenance, we investigated whether Sef had a function in bone. Adult mice deficient in Sef showed an increase in cortical bone thickness and volume compare to wild type mice. Bone marrow stromal cells (BMSC) from Sef-/- mice grown in osteogenic medium showed increased proliferation; increase alkaline phosphatase and Alizarin red staining and increased expression of osteoblast differentiation genes compared to wild type BMSC. Overexpression of Sef in BMSC inhibited proliferation, alkaline phosphatase and Alizarin red staining and inhibited the expression of osteoblast differentiation genes. BMSC from Sef-/- mice showed enhanced FGF2-induced ERK activation, whereas BMSC from Sef transgenic mice showed decreased FGF2-induced ERK signaling. We also observed that FGF2-induced acetylation and stability of Runx2 was enhanced in Sef -/- BMSC. Overexpression of Sef inhibited Runx2-responsive luciferase reporter activity to an extent similar to a MEK inhibitor, and the effect of Sef overexpression was rescued by the HDAC inhibitor Trichostatin A. Histomorphometric analysis showed that Sef-/- mice trended toward increased trabecular osteoblasts and osteoclasts compared to wild type, and BMSC from Sef-/- mice showed increased osteoclast formation and bone resorption in vitro. Taken together, these studies indicate that Sef has specific roles in osteoblast and osteoclast lineages, and that its absence causes increased FGF signaling, increased Runx2 acétylation and transcriptional activity, and increased bone mass