Molecular Diversity in Zebrafish NCAM Family: Three Members with Different VASE Usage and Distinct Localization

Abstract

NCAM in vertebrates and its related molecules, apCAM in Aplysia, fasciclin II in Drosophila, and OCAM in mammals, play key roles in various aspects of brain development and functions. In this study, we have identified and characterized three members of the NCAM gene family in zebrafish, designated as zNCAM, zOCAM, and zPCAM. Three molecules exhibit similar domain organization: an amino-terminal signal peptide, five immunoglobulin-like domains, two fibronectin type III-like domains, a transmembrane segment, and a carboxy-terminal cytoplasmic region. A novel molecule zPCAM is most closely related to zNCAM with 66% amino acid identity. Diversity in the extracellular region of zPCAM is generated by insertion of two different types of variable alternatively spliced exons. In situ hybridization analysis revealed that three molecules were specifically expressed by the central and peripheral nervous systems from early developmental stages in region-specific and cell-type-specific manners. For example, zPCAM showed a neuromere-specific segmental expression pattern, while zOCAM first appeared in specific clusters of secondary neurons in the forebrain. These results suggest that each member of the NCAM gene family plays distinct roles in the formation and maintenance of functional neuronal networks in the zebrafish nervous system.