A neuron's reach can exceed its grasp: expression of a cell adhesion molecule in the zebrafish embryo

Neurons make long distance connections with their targets by extending axons along stereotyped paths during embryonic development. One important mechanism that guides growing axons is selective fasciculation with other axons. Cell surface proteins expressed on a subset of embryonic axons can mediate axon guidance through selective fasciculation. I have used the zn-5 monoclonal antibody to study a candidate axon guidance protein in the zebrafish embryo. I have shown that the antigen recognized by the zn-5 antibody is zebrafish DM-GRASP, a membrane spanning protein of the immunoglobulin superfamily that is a candidate molecule for mediating intercellular adhesion. This conclusion is based upon partial peptide sequence of the zn-5 antigen, cross-reactivity of zn-5 and DM-GRASP antibodies, and matching zn-5 labeling and DM-GRASP mRNA expression patterns. To understand the function of this protein in neural development, I examined its expression pattern in the embryo. Subsets of neurons including retinal ganglion cells and spinal secondary motoneurons express DM-GRASP. DM-GRASP is restricted to the cell bodies and fasciculated regions of axons during the period of axonal growth. The distal, unfasciculated branches and terminals of neurons do not localize the protein. All neurons that express DM-GRASP project axons in tracts over long distances. However, not all neurons with axons in long projecting tracts express DM-GRASP. Some tracts contain axons of DM-GRASP expressing and non-expressing neurons. The spinal segmental nerves contain the axons of secondary and primary motoneurons. Secondary motoneurons strongly express DM-GRASP, yet primary motoneurons, which extend the earliest axons into the spinal nerves, do not express DM-GRASP. Specific non-neural cells, including the floor plate of the spinal cord and precursors of slow (red) muscle also express DM-GRASP. The regions of these cells that are in contact with each other localize the protein. The patterns of DM-GRASP expression are consistent with its potential role in specific intercellular adhesion. This dissertation includes both my previously published and unpublished co-authored materials.