FGF and FGFR signaling in chondrodysplasias and craniosynostosis

The first experimental mouse model for FGF2 in bone dysplasia was made serendipitously by overexpression of FGF from a constitutive promoter. The results were not widely accepted, rightfully drew skepticism, and were difficult to publish; because of over 2,000 studies published on FGF-2 at the time (1993), only a few reported a role of FGF-2 in bone growth and differentiation. However, mapping of human dwarfisms to mutations of the FGFRs shortly, thereafter, made the case that bone growth and remodeling was a major physiological function for FGF. Subsequent production of numerous transgenic and targeted null mice for several genes in the bone growth and remodeling pathways have marvelously elucidated the role of FGFs and their interactions with other genes. Indeed, studies of the FGF pathway present one of the best success stories for use of experimental genetics in functionally parsing morphogenetic regulatory pathways. What remains largely unresolved is the pleiotropic nature of FGF-2. How does it accelerate growth in one cell then stimulate apoptosis or retard growth for another cell in the same type of tissue? Some of the answers may come through distinguishing the FGF-2 protein isoforms, made from alternative translation start sites, these appear to have substantially different functions. Although we have made substantial progress, there is still much to be learned regarding FGF-2 as a most complex, enigmatic protein. Studies of genetic models in mice and human FGFR mutations have provided strong evidence that FGFRs are important modulators of osteoblast function during membranous bone formation. However, there is some controversy regarding the effects of FGFR signaling in human and murine genetic models. Although significant progress has been made in our understanding of FGFR signaling, several questions remain concerning the signaling pathways involved in osteoblast regulation by activated FGFR. Additionally, little is known about the specific role of FGFR target genes involved in cranial bone formation. These issues need to be addressed in future in in vitro and in vivo approaches to better understand the molecular mechanisms of action of FGFR signaling in osteoblasts that result in anabolic effects in bone formation.