Analysis of N-cadherin function in limb mesenchymal chondrogenesis in vitro

During embryonic limb development, cartilage formation is presaged by a crucial mesenchymal cell condensation phase. N-Cadherin, a Ca2+ -dependent cell-cell adhesion molecule, is expressed in embryonic chick limb buds in a spatiotemporal pattern suggestive of its involvement during cellular condensation; functional blocking of N-cadherin homotypic binding, by using a neutralizing monoclonal antibody, results in perturbed chondrogenesis in vitro and in vivo. In high-density micromass cultures of embryonic limb mesenchymal cells, N-cadherin expression level is high during days 1 and 2, coincident with active cellular condensation, and decreases upon overt chondrogenic differentiation from day 3 on. In this study, we have used a transfection approach to evaluate the effects of gain- and loss-of-function expression of N-cadherin constructs on mesenchymal condensation and chondrogenesis in vitro. Chick limb mesenchymal cells were transfected by electroporation with recombinant expression plasmids encoding wild-type or two mutant extracellular/cytoplasmic deletion forms of N-cadherin. Expression of the transfected N-cadherin forms showed a transient profile, being high on days 1-2 of culture, and decreasing by day 3, fortuitously coincident with the temporal profile of endogenous N-cadherin gene expression. Examined by means of peanut agglutinin (PNA) staining for condensing precartilage mesenchymal cells, cultures overexpressing wild-type N-cadherin showed enhanced cellular condensation on culture days 2 and 3, whereas expression of the deletion mutant forms (extracellular/cytoplasmic) of N-cadherin resulted in a decrease in PNA staining, suggesting that a complete N-cadherin protein is required for normal cellular condensation to occur. Subsequent chondrogenesis was also affected. Cultures overexpressing the wild-type N-cadherin protein showed enhanced chondrogenesis, indicated by increased production of cartilage matrix (sulfated proteoglycans, collagen type II, and cartilage proteoglycan link protein), as well as increased cartilage nodule number and size of individual nodules, compared with control cultures and cultures transfected with either of the two mutant N-cadherin constructs. These results demonstrate that complete N-cadherin function, at the levels of both extracellular homotypic binding and cytoplasmic linkage to the cytoskeleton by means of the catenin complex, is required for chondrogenesis by mediating functional mesenchymal cell condensation.