Mechanisms of GDF-5 action during skeletal development

Mutations in GDF-5, a member of the TGF-beta superfamily, result in the aut osomal recessive syndromes brachypod (bp) in mice and Hunter-Thompson and G rebe-type chondrodysplasias in humans. These syndromes are all characterise d by the shortening of the appendicular skeleton and loss or abnormal devel opment of some joints. To investigate how GDF-5 controls skeletogenesis, we overexpressed GDF-5 during chick limb development using the retrovirus, RC ASBP, This resulted in up to a 37.5% increase in length of the skeletal ele ments, which was predominantly due to an increase in the number of chondroc ytes. By injecting virus at different stages of development, we show that G DF-5 can increase both the size of the early cartilage condensation and the later developing skeletal element. Using in vitro micromass cultures as a model system to study the early steps of chondrogenesis, we show that GDF-5 increases chondrogenesis in a dose-dependent manner. We did not detect cha nges in proliferation, However, cell suspension cultures showed that GDF-5 might act at these stages by increasing cell adhesion, a critical determina nt of early chondrogenesis. In contrast, pulse labelling experiments of GDF -5-infected limbs showed that at later stages of skeletal development GDF-5 can increase proliferation of chondrocytes. Thus, here we show two mechani sms of how GDF-5 may control different stages of skeletogenesis. Finally, o ur data show that levels of GDF-5 expression/activity are important in cont rolling the size of skeletal elements and provides a possible explanation f or the variation in the severity of skeletal defects resulting from mutatio ns in GDF-5.