Expression and involvement of gicerin, a cell adhesion molecule, in the development of chick optic tectum

Gicerin is a cell adhesion molecule belonging to the immunoglobulin superfamily. It has both a homophilic binding activity and a heterophilic binding activity to neurite outgrowth factor (NOF) a molecule belonging to the laminin family. We have reported many studies on the heterophilic activity of gicerin and NOF, but the function of its homophilic binding activity in vivo had been unclear. In the retina, gicerin is expressed in retinal ganglion cells only when they extend neurites to the optic tectum. In this report we have found that gicerin is also transiently expressed in the optic tectum during this time. First, cell aggregation assays were used to show that gicerin expressed in the optic tectum displays homophilic binding activity. Then, explant cultures of embryonic day 6 chick optic tectum on gicerin-Fc chimeric protein-coated dishes and NOF-coated dishes were carried out. It was found that gicerin-gicerin homophilic interactions promoted cell migration, whereas heterophilic interactions with NOF induced neurite formation. Furthermore, when anti-gicerin antibodies were injected in order to examine the effect of gicerin protein in the formation of the tectal layer in ovo, cell migration was strongly inhibited. These data suggest that homophilic interaction of gicerin participates in the migration of neural cells during the layer formation and plays a crucial role in the organization of the optic tectum.