The goal of this research is to understand the molecular regulation of cartilage differentiation and cartilage-specific gene expression in the developing chick limb bud. A critical event in limb cartilage differentiation is a transient cellular condensation process in which prechondrogenic limb mesenchymal cells become closely juxtaposed, round- up, and interact with one another prior to initiating cartilage-specific gene expression. A cDNA that encodes a membrane-intercalated proteoglycan (PG) termed syndecan-3 that is expressed in high amounts during this critical condensation process has been isolated. Syndecan-3 contains a cytoplasmic domain potentially associated with the cytoskeleton, and an ectodomain containing several putative GAG attachment sites, as well as an extensive mucin-like domain potentially containing numerous O-linked oligosaccharide chains. The multiple functional domains of syndecan-3 provide potential sites for mediating the adhesive cell-matrix interactions and cytoskeletal reorganization involved in the critical condensation phase of chondrogenesis. Studies will include a full characterization of the structure of syndecan-3, its temporal and spatial pattern of expression during limb chondrogenesis, and the possible regulation of its expression at the onset of chondrogenesis by TGF-beta. The role of syndecan-3 and other matrix molecules in mediating precartilage condensation formation will be studied. It will be determined if phosphorylation of the cytoplasmic domain of syndecan-3 and/or the shedding of its ectodomain from the cell surface relate to the cytoskeletal reorganization and cell shape changes that occur during condensation. The binding of syndecan-3 to other molecules expressed during condensation (PG-M, tenascin, cell surface galactosyltransferases (GalTase) , fibronectin, type I collagen) and the roles of PG-M, tenascin, and cell surface GalTase in condensation formation will be studied.
