Sef (comparable 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. from mice showed enhanced FGF2-induced activation of the ERK pathway, whereas bone marrow cells from Sef transgenic mice showed decreased FGF2-induced signaling. FGF2-induced acetylation and stability of Runx2 was enhanced in bone marrow cells, whereas overexpression of Sef inhibited Runx2-responsive luciferase reporter activity. Bone marrow from mice showed enhanced hematopoietic lineage-dependent and osteoblast-dependent osteoclastogenesis and increased bone resorptive activity relative to wild type controls in in vitro assays, while overexpression of Sef inhibited osteoclast differentiation. Taken together, these studies indicate that Sef has specific roles in osteoblast and osteoclast lineages, and that its absence results in increased osteoblast and osteoclast activity with a net increase in cortical bone mass. gene in mice results in decreased bone mass and bone formation (4). Conversely, overexpression of FGF2 in transgenic mice leads to skeletal dwarfism (5). Deletion of in mice results in increased endochondral bone formation (6, 7), and tissue specific deletion of in osteo-chondro-progenitor cells results delayed osteoblast differentiation (8). Comparable studies in which was deleted in the mouse osteo-chondro-progenitor lineage resulted in skeletal dwarfism and decreased bone mineral density (9). In humans, mutations in and cause Leukadherin 1 craniofacial abnormalities (10, 11), whereas mutations in are associated with dwarfism (12C14). It is apparent from these studies that there is a critical threshold of FGF signaling for normal skeletal growth, above or below which leads to skeletal abnormities. Recent studies show that there are several mechanisms by which FGF signaling is usually attenuated. Members of the Sprouty (Spry) family of proteins are feedback inhibitors of receptor tyrosine kinase (RTK) signaling, including FGF signaling, by inhibiting the Ras-Raf-ERK pathway (15, 16) and Sef (comparable expression to fgf genes) which appears to target FGFRs specifically (17C20). Sef was identified as an Rabbit Polyclonal to CAMK2D inhibitor of FGF signaling in zebrafish (17, 20), and was shown to physically associate with FGFR1 and FGFR2 and to inhibit FGF-induced receptor tyrosine phosphorylation, resulting in inhibition of both ERK and Akt signaling (18). Furthermore, Sef does not inhibit ERK Leukadherin 1 activation by epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) in NIH3T3 cells, suggesting its function may be restricted to FGFR signaling (18). Gene targeting studies of in the mouse revealed that there are no significant embryonic phenotypic abnormalities, however, one study showed that disruption of by a gene trap approach produced defects in auditory brainstem development (21C23). Because FGF signaling is usually important to skeletal growth and maintenance, and because Sef is an inhibitor of FGF signaling, we sought to investigate its role in skeletal growth and homeostasis. Here we show that Sef loss-of-function results in postnatal increases in cortical bone mass relative to wild type mice. In vitro, loss-of- Leukadherin 1 function of Sef results increased osteoblast and osteoclast differentiation and increased activation of the ERK pathway in osteoblasts in response to FGF2. These results suggest that regulation of the FGF pathway by Sef contributes to the Leukadherin 1 regulation of the postnatal skeleton by balancing FGF signaling. Materials and Methods Mice The Institutional Animal Care and Use Committee at Maine Medical Center approved all experiments involving the use of mice. Sef transgenic mice were generated by using a CAGCAT-Z vector made up of a chicken -actin gene (CAG) promoter-on an FVB background. Upon Cre-mediated recombination, Sef expression is induced, with concomitant loss of GFP reporter gene expression due to Cre-mediated excision of the.