Defective bone mineralization and osteopenia in young adult FGFR3-/- mice

Abstract

Mutations that cause constitutive activation of fibroblast growth factor receptor 3 (FGFR3) result in skeletal disorders that are characterized by short-limbed dwarfism and premature closure of cranial sutures. In previous work, it was shown that congenital deficiency of FGFR3 led to skeletal overgrowth. Using a combination of imaging, classic histology and molecular cell biology we now show that young adult FGFR3(-/-) mice are osteopenic due to reduced cortical bone thickness and defective trabecular bone mineralization. The reduction in mineralized bone and lack of trabecular connectivity observed by micro-computed tomography were confirmed in histological and histomorphometric analyses, which revealed a significant decrease in calcein labelling of mineralizing surfaces and a significant increase in osteoid in the long bones of 4-month-old FGFR3(-/-) mice. These alterations were associated with increased staining for recognized markers of differentiated osteoblasts and increased numbers of tartrate-resistant acid phsophatase postitive osteoclasts. Primary cultures of adherent bone marrow-derived cells from FGFR3(-/-) mice expressed markers of differentiated osteoblasts but developed fewer mineralized nodules than FGFR3(+/+) cultures of the same age. Our observations reveal a role for FGFR3 in post-natal bone growth and remodelling, which identifies it as a potential therapeutic target for osteopenic disorders and those associated with defective bone mineralization.

Fibroblast growth factor signaling in skeletal development and disease

“…Consistently, mice conditionally lacking Fgfr2 or harboring a mutation in Fgfr2c, the mesenchymal splice variant of Fgfr2, show decreased Runx2 expression, retarded ossification, and decreased bone mineral density, suggesting that Fgfr2 is a positive regulator of osteoblast maturation (Eswarakumar et al 2002;Yu et al 2003). Although FGFR3 is mainly expressed in chondrocytes, young adult Fgfr3-null mice are osteopenic due to reduced osteoblast maturation and defective trabecular bone mineralization, suggesting a role for FGFR3 in postnatal bone metabolism (Valverde-Franco et al 2004). Osteoblasts derived from Stat1 −/− mice have decreased expression of Fgfr3 and increased expression of Fgf18, which acts through FGFR1 or FGFR2, suggesting that changes in the repertoire of Fgfr expression affects feedback loops that control FGF-dependent osteoblastogenesis (Xiao et al 2004).…”

T cell protein tyrosine phosphatase deficiency results in spontaneous synovitis and subchondral bone resorption in mice

“…Left femurs were embedded in polymethylmethacrylate, and 5-m sections were cut for staining with tartrate-resistant acid phosphatase (TRAP), alkaline phosphatase (AP), and von Kossa's/toluidine blue stain, as previously described (12). Osteoclast and osteoblast counts as well as bone volume measurements were performed using a Scanscope XT (Aperio).…”

Mesenchymal stem cell transplantation to promote bone healing

“…Whole bone marrow was extracted from the long bones of three 4-6 month old FGFR3 þ/þ mice and the MSC isolated by adherence to tissue culture plastic (TCP) as described previously. 13,14 Discs measuring 21 mm in diameter and 1.5 mm thick were fabricated by Changzhou Kanghui Joint Implants Company (China) from implant grade titanium and polished to generate a surface with an average roughness comparable to TCP. Prior to use, all discs were cleaned in 100% ethanol, passivated in 30% nitric acid at room temperature for 30 min, rinsed repeatedly in distilled water and steam sterilized.…”

“…Reverse transcription and semi quantitative PCR analyses were performed essentially as described previously 13,14 to examine the expression of recognized markers of osteoblast differentiation. PCR products were obtained in the linear range of amplification and quantified by scanning densitometry with Image J software (NIH).…”

“…Detailed analysis of the skeletons of young adult FGFR3 À/À defective MSC differentiation, identifying them as a model to examine MSC transplantation as a mechanism to enhance bone formation. 13 …”