Protocadherin-18a has a role in cell adhesion, behavior and migration in zebrafish development

Protocadherin-18a (Pcdh18a) belongs to the delta2-protocadherins, which constitute the largest subgroup within the cadherin superfamily. Here we present isolation of a full-length zebrafish cDNA that encodes a protein highly similar to human and mouse Pcdh18. Zebrafish pcdh18a is expressed in a complex and dynamic pattern in the nervous system from gastrula stages onward, with lesser expression in mesodermal derivatives. Pcdh18a-eGFP fusion protein is expressed in a punctate manner on the membranes between cells. Overexpression of pcdh18a in embryos caused cyclopia, mislocalization of hatching gland tissue, and duplication or splitting of the neural tube. Most neural markers tested were expressed in an approximately correct A-P pattern. By cell transplantation we showed that overexpression of pcdh18a causes diminished cell migration and reduced cell protrusions, resulting in a tendency of cells to stay more firmly aggregated, probably due to increased cell adhesion. In contrast, knockdown of pcdh18a by a morpholino oligonucleotide caused defects in epiboly, and led to reduced cell adhesion as shown by cell dissociation, sorting and transplantation experiments. These results suggest a role for Pcdh18a in cell adhesion, migration and behavior but not cell specification during gastrula and segmentation stages of development.