Convergence of the BMP and EGF signaling pathways on Smad1 in the regulation of chondrogenesis

Bone morphogenetic protein 4 (BMP4) induces, whereas epidermal growth facto r (EGF) inhibits chondrogenesis. We hypothesize that BMP4 and EGF mediated intracellular signals are both coupled in the regulation of Meckel's cartil age development. Two chondrogenic experimental model systems were employed to test the hypothesis: (1) an ex vivo, serum-free, organ culture system fo r mouse embryonic mandibular processes, and (2) a micromass culture system for chicken embryonic mandibular processes. Chondrogenesis was assayed by a lcian blue staining and expression of Sox9 and type II collagen. Exogenous EGF inhibited and BMP4 induced ectopic cartilage in a dose-dependent manner . When BMP4- and EGF-soaked beads were implanted in juxtaposition within em bryonic day 10 mouse mandibular processes, the incidence and amount of ecto pic cartilage, and Sox9 and type II collagen expression induced by BMP4, we re significantly reduced as the concentration of EGF was increased. Similar ly, in chicken serum-free micromass cultures, expression of a constitutivel y active BMP receptor type IB by replication competent avian retrovirus sys tem promoted the rate and extent of chondrogenesis; however, exogenous EGF attenuated this effect. In micromass cultures, BMP signaling resulted in nu clear translocation and accumulation of the signaling molecule Smad1, where as the addition of EGF inhibited this event. Our results suggest that BMP4 and EGF function antagonistically, yet are coupled in the regulation of ini tial chondrogenesis. Smad1 serves as a point of convergence for the integra tion of two different growth factor signaling pathways during chondrogenesi s..