Deficiency of Sef Is Associated With Increased Postnatal Cortical Bone Mass by Regulating Runx2 Activity

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. Genetic studies in mice and humans indicate an important role for fibroblast growth factor signaling in bone growth and homeostasis. We, therefore, investigated whether Sef had a function role in skeletal acquisition and remodeling. Sef expression is increased during osteoblast differentiation in vitro, and LacZ staining of Sef+/? mice showed high expression of Sef in the periosteum and chondro‐osseous junction of neonatal and adult mice. Mice with a global deletion of Sef showed increased cortical bone thickness, bone volume, and increased periosteal perimeter by micro‐computed tomography (micro‐CT). Histomorphometric analysis of cortical bone revealed a significant increase in osteoblast number. Interestingly, Sef?/? mice showed very little difference in trabecular bone by micro‐CT and histomorphometry compared with wild‐type mice. Bone marrow cells from Sef?/? mice grown in osteogenic medium showed increased proliferation and increased osteoblast differentiation compared with wild‐type bone marrow cells. Bone marrow cells from Sef?/? 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 Sef?/? bone marrow cells, whereas overexpression of Sef inhibited Runx2‐responsive luciferase reporter activity. Bone marrow from Sef?/? mice showed enhanced hematopoietic lineage‐dependent and osteoblast‐dependent osteoclastogenesis and increased bone resorptive activity relative to wild‐type controls in in vitro assays, whereas 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. © 2014 American Society for Bone and Mineral Research.     

CRELD2 Is a Novel LRP1 Chaperone That Regulates Noncanonical WNT Signaling in Skeletal Development

Cysteine-rich with epidermal growth factor (EGF)-like domains 2 (CRELD2) is an endoplasmic reticulum (ER)-resident chaperone highly activated under ER stress in conditions such as chondrodysplasias; however, its role in healthy skeletal development is unknown. We show for the first time that cartilage-specific deletion of Creld2 results in disrupted endochondral ossification and short limbed dwarfism, whereas deletion of Creld2 in bone results in osteopenia, with a low bone density and altered trabecular architecture. Our study provides the first evidence that CRELD2 promotes the differentiation and maturation of skeletal cells by modulating noncanonical WNT4 signaling regulated by p38 MAPK. Furthermore, we show that CRELD2 is a novel chaperone for the receptor low-density lipoprotein receptor-related protein 1 (LRP1), promoting its transport to the cell surface, and that LRP1 directly regulates WNT4 expression in chondrocytes through TGF-β1 signaling. Therefore, our data provide a novel link between an ER-resident chaperone and the essential WNT signaling pathways active during skeletal differentiation that could be applicable in other WNT-responsive tissues. © 2020 American Society for Bone and Mineral Research. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research..     

TGFβ Inducible Early Gene‐1 Plays an Important Role in Mediating Estrogen Signaling in the Skeleton

TGFβ Inducible Early Gene‐1 (TIEG1) knockout (KO) mice display a sex‐specific osteopenic phenotype characterized by low bone mineral density, bone mineral content, and overall loss of bone strength in female mice. We, therefore, speculated that loss of TIEG1 expression would impair the actions of estrogen on bone in female mice. To test this hypothesis, we employed an ovariectomy (OVX) and estrogen replacement model system to comprehensively analyze the role of TIEG1 in mediating estrogen signaling in bone at the tissue, cell, and biochemical level. Dual‐energy X‐ray absorptiometry (DXA), peripheral quantitative computed tomography (pQCT), and micro‐CT analyses revealed that loss of TIEG1 expression diminished the effects of estrogen throughout the skeleton and within multiple bone compartments. Estrogen exposure also led to reductions in bone formation rates and mineralizing perimeter in wild‐type mice with little to no effects on these parameters in TIEG1 KO mice. Osteoclast perimeter per bone perimeter and resorptive activity as determined by serum levels of CTX‐1 were differentially regulated after estrogen treatment in TIEG1 KO mice compared with wild‐type littermates. No significant differences were detected in serum levels of P1NP between wild‐type and TIEG1 KO mice. Taken together, these data implicate an important role for TIEG1 in mediating estrogen signaling throughout the mouse skeleton and suggest that defects in this pathway are likely to contribute to the sex‐specific osteopenic phenotype observed in female TIEG1 KO mice. © 2014 American Society for Bone and Mineral Research.