Latex beads as probes of cell surface-extracellular matrix interactions during chondrogenesis: evidence for a role for amino-terminal heparin-binding domain of fibronectin.

Fibronectin-rich mesenchymal condensations form at sites of incipient chondrogenesis in the developing vertebrate limb, and in cultures of limb bud mesenchyme. We have used 6 microns polystyrene latex beads coated with various substances as probes for adhesive interactions that may mediate the formation of these condensations. Beads coated with heparin, chondroitin sulfate, or poly L-lysine, that were mixed with limb bud mesenchymal cells were centripetally conveyed into fibronectin-rich regions of cell condensation over a period of several days. Beads coated with dextran sulfate remained uniformly dispersed throughout the cultures during the same period. A monoclonal antibody directed against the amino-terminal heparin-binding domain of fibronectin completely inhibited accumulation of heparin-coated beads at condensing foci, but monoclonal antibodies directed against the collagen- or cell-binding domains of fibronectin were not inhibitory. Accumulation of chondroitin sulfate- or poly L-lysine-coated beads at condensing foci was unaffected by the antibody against the fibronectin amino terminus. Peptides with the sequence arg-gly-asp-ser or gly-arg-gly-asp-ser, which inhibit adhesive interactions mediated by the integrin-binding domain of fibronectin, had no effect on conveyance or accumulation of heparin-coated beads, but the peptide with the sequence gly-arg-gly, a repeated motif in the amino-terminal heparin-binding domain was completely inhibitory. These findings indicate that the amino-terminal heparin-binding domain of fibronectin can, within a tissue microenvironment, interact adhesively with heparin-like components on the surfaces of polystyrene beads, and by implication, on mesenchymal cells themselves. This interaction may therefore be a component of the condensation-forming mechanism in chondrogenic mesenchyme.