Gain-of-function mutation in FGFR3 in mice leads to decreased bone mass by affecting both osteoblastogenesis and osteoclastogenesis

Abstract

Achondroplasia (ACH) is a short-limbed dwarfism resulting from gain-of-function mutations in fibroblast growth factor receptor 3 (FGFR3). Previous studies have shown that ACH patients have impaired chondrogenesis, but the effects of FGFR3 on bone formation and bone remodeling at adult stages of ACH have not been fully investigated. Using micro-computed tomography and histomorphometric analyses, we found that 2-month-old Fgfr3 G369C/1 mice (mouse model mimicking human ACH) showed decreased bone mass due to reduced trabecular bone volume and bone mineral density, defect in bone mineralization and increased osteoclast numbers and activity. Compared with primary cultures of bone marrow stromal cells (BMSCs) from wild-type mice, Fgfr3 G369C/1 cultures showed decreased cell proliferation, increased osteogenic differentiation including up-regulation of alkaline phosphatase activity and expressions of osteoblast marker genes, and reduced bone matrix mineralization. Furthermore, our studies also suggest that decreased cell proliferation and enhanced osteogenic differentiation observed in Fgfr3 G369C/1 BMSCs are caused by upregulation of p38 phosphorylation and that enhanced Erk1/2 activity is responsible for the impaired bone matrix mineralization. In addition, in vitro osteoclast formation and bone resorption assays demonstrated that osteoclast numbers and bone resorption area were increased in cultured bone marrow cells derived from Fgfr3 G369C/1 mice. These findings demonstrate that gain-of-function mutation in FGFR3 leads to decreased bone mass by regulating both osteoblast and osteoclast activities. Our studies provide new insight into the mechanism underlying the development of ACH.

Chondrocyte FGFR3 Regulates Bone Mass by Inhibiting Osteogenesis

“…regulate osteogenesis by promoting or inhibiting osteogenic differentiation and mineralization of bone marrow stromal cells (BMSCs) (31,33). In this study, we further demonstrate that FGFR3, in addition to its direct regulation of osteogenesis, can also regulate bone formation and bone mass indirectly through its regulation of chondrogenesis.…”

“…In addition, FGFR3 also regulates bone mass. Previous studies showed that adult Fgfr3 and Fgfr3IIIc knock-out mice displayed decreased bone mass (31,32), and ubiquitous gain-of-function mutations in FGFR3 also led to decreased bone mass in mice (33,35). FGFR3 can directly FIGURE 5.…”

“…Adult Fgfr3 and Fgfr3IIIc null mutant mice showed osteopenia (31,32). However, mice with activated mutation of FGFR3, G369C (mimicking human ACH), also displayed decreased bone mass and increased differentiation of osteoblasts (25,33). The seemingly contradictory data indicate that the underlying mechanisms for the effect of FGFR3 on bone formation are complicated.…”

Fibroblast growth factor signaling in skeletal development and disease

“…Osteoclasts with FGFR1 deficiency (LysM-Cre mice) displayed decreased expression of genes related to osteoclast activity, including TRAP and MMP9, and exhibited abnormal bone remodeling with reduced osteoclast number and impaired osteoclast function ). On the other hand, osteoclast numbers and bone resorption area were increased in cultured bone marrow cells derived from Fgfr3 G369C/+ mice, suggesting that gain-of-function mutations in FGFR3 increase osteoclast activity, which contributes to decreased bone mass in these mice (Su et al 2010). …”

“…Indeed, chondrocyte-specific activation of Fgfr3 in mice was found to induce premature suture closure and indirectly enhance osteoblast differentiation through MAPK activation and BMP signaling (Matsushita et al 2009b). In long bones, gain-of-function FGFR3 mutations in chondrocytes, but not in osteoblasts, caused increased osteoclast recruitment, defective mineralization, and decreased trabecular bone mass, indicating that overactive FGFR3 signaling indirectly affects trabecular bone development (Su et al 2010;Mugniery et al 2012).…”