Probing Cadherin Interactions in Zebrafish with E- and N-Cadherin Missense Mutants.

Cadherins are cell-cell adhesion molecules that regulate numerous adhesive interactions during embryonic development and adult life. Consistent with these functions, when their expression goes astray cells loose their normal adhesive properties resulting in defective morphogenesis, disease, and even metastatic cancer. In general, classical cadherins exert their effect by homophilic interactions via their five characteristic extracellular (EC) repeats. The EC1 repeat provides the mechanism for cadherins to dimerize with each other whereas the EC2 repeat may facilitate dimerization. Less is known about the other EC repeats. Here we show that a zebrafish missense mutation in the EC5 repeat of N-cadherin is a dominant gain-of-function mutation and demonstrate that this mutation alters cell-cell adhesion almost to the same degree as a zebrafish missense mutation in the EC1 repeat of N-cadherin. We also show that E- and N-cadherin missense mutations genetically interact. Perturbation of cell-cell adhesion in embryos heterozygous mutant at both loci is similar to that observed in single homozygous mutants. Introducing an E-cadherin dominant gain-of-function allele into the homozygous N-cadherin dominant gain-of-function mutant more radically affects morphogenesis causing synergistic phenotypes consistent with interdependent functions being disrupted. Our studies indicate that a functional EC5 repeat is critical for cadherin-mediated cell-cell affinity suggesting that its role may be more important than previously thought. These results also suggest the possibility that E- and N-cadherin have heterophilic interactions during early morphogenesis of the embryo. Interactions that might help balance the variety of cell affinities needed during embryonic development.