Inhibition of Chondrogenesis byWntGene Expressionin Vivoandin Vitro

AbstractThe Wnt family of secreted signaling proteins are implicated in regulating morphogenesis and tissue patterning in a wide variety of organ systems. SeveralWntgenes are expressed in the developing limbs and head, implying roles in skeletal development. To explore these functions, we have used retroviral gene transfer to expressWnt-1 ectopically in the limb buds and craniofacial region of chick embryos. Infection of wing buds at stage 17 and tissues in the head at stage 10 resulted in skeletal abnormalities whose most consistent defects suggested a localized failure of cartilage formation. To test this hypothesis, we infected micromass cultures of prechondrogenic mesenchymein vitroand found that expression ofWnt-1 caused a severe block in chondrogenesis.Wnt-7a, a gene endogenously expressed in the limb and facial ectoderm, had a similar inhibitory effect. Further analysis of this phenomenonin vitroshowed thatWnt-1 andWnt-7a had mitogenic effects only in early prechondrogenic mesenchyme, that cell aggregation and formation of the prechondrogenic blastema occurred normally, and that the block to differentiation was at the late-blastema/early-chondroblast stage. These results indicate that Wnt signals can have specific inhibitory effects on cytodifferentiation and suggest that one function of endogenous Wnt proteins in the limbs and face may be to influence skeletal morphology by localized inhibition of chondrogenesis