MODULATION OF LIMB BUD CHONDROGENESIS BY RETINOIC ACID AND RETINOIC ACID RECEPTORS

An excess of retinoic acid (RA) in the mouse embryo in utero produces hypochondrogenesis and severe limb bone deformities. Since one of the RA receptors - RAR-beta 2, is specifically induced in the limb bud cel ls upon treatment of embryos with teratogenic doses of RA, we investig ated if this receptor played a role in teratogenesis by regulating the process of chondrogenesis. In micromass cultures of mouse limb bud me senchymal cells, we found that a downregulation of RAR-beta 2 as well as several other RAR isoforms by supplementation of the culture medium with specific antisense oligodeoxynucleotides stimulated chondrogenes is: cartilage nodule number, sulfated proteoglycans, and synthesis of collagen type IIB were all enhanced in a dose-dependent manner. Howeve r, only the antisense RAR-beta 2 probe efficiently prevented the stron g inhibitory effects of exogenous RA on chondrogenesis in these cells. The data suggest that the RAR-RA complexes play a role in position-de pendent patterning of the limb skeleton in normal development and that , in particular, RAR-beta 2 serves to prevent the mesenchymal cells fr om expressing their chondrogenic bias. Our results further strengthen the argument that RA-dependent elevation in RAR-beta 2 levels plays a unique role in RA-induced teratogenesis.