REGULATED EXPRESSION OF FIBRONECTIN, LAMININ AND RELATED INTEGRIN RECEPTORS DURING THE EARLY CHONDROCYTE DIFFERENTIATION

We have investigated the expression and localization of fibronectin, l aminin, and their receptors, and we used an in vitro chick chondrocyte differentiation model to define a time hierarchy for their appearance in early chondrogenesis and to determine their role in the cell conde nsation process, By serum fibronectin depletion/reconstitution, or GRG DSP peptide competition experiments, we show that fibronectin contribu tes to the initial cell-cell interactions that occur during condensati on, In later stages, a downregulation of both fibronectin and of its a lpha(5) beta(1) integrin receptor occur, as demonstrated by mRNA and p rotein kinetics, Immunolocalisation studies suggest that the reduction of fibronectin in discrete areas is involved in local activation of t he cell differentiation program, Furthermore, we show that laminin is expressed during the in vitro cell condensation process in areas that are negative for fibronectin staining, The types of laminin as well as the timing of expression have been determined by northern blot and RT -PCR analyses, The highest levels of expression are coincident with ma ximal cell aggregation, The alpha(3) beta(1) laminin receptor, highly expressed in dedifferentiated cells, follows later on the ligand trend . During in vitro chondrogenesis, a down-regulation in the B isoform, and an up-regulation of the A isoform, of the alpha subunit of the alp ha(6) beta(1) laminin receptor occurs, Immunolocalisation studies sugg est that laminin is involved in the definition of differentiating area s as opposed to non differentiating areas of the condensed region, i.e . the periphery, which eventually gives rise to the perichondrium.