Neurolin, the goldfish homolog of DM-GRASP, is involved in retinal axon pathfinding to the optic disk.

Young axons of new retinal ganglion cells (RGCs) in the continuously growing goldfish retina fasciculate with one another and their immediate forerunners on their path toward the optic disk and along the optic nerve. They express the immunoglobulin superfamily cell adhesion molecules (CAMs) neurolin (DM-GRASP) and the L1-like E587 antigen. Repeated injections of Fab fragments from polyclonal antisera against neurolin (neurolin Fabs) into the eye of 3. 4-cm-long and rapidly growing goldfish caused highly aberrant pathways of young RGC axon subfascicles in the dorsal retina. Many axons grew in circles and failed to reach the optic disk. In contrast, E587 Fabs, used in parallel experiments, disrupted the fascicles but did not interfere with the disk-directed growth. Neurolin Fabs also disturbed axonal fasciculation in vivo as well as in vitro but less severely than E587 Fabs. Coinjections of both Fabs increased defasciculation of the dorsal axons in both aberrant and disk-directed routes. They also disrupted the order of young RGC axons in the optic nerve more severely than E587 Fabs alone. This demonstrates that the development of tight and orderly fascicles in the dorsal retina and in the optic nerve requires both E587 antigen and neurolin. More importantly, our results suggest an involvement of neurolin in RGC axonal guidance from the retinal periphery to the optic disk. Because disrupted fascicles and errant axon routes were found only in the dorsal retinal half, a cooperation with so-called positional markers may be conceived.