A monoclonal antibody that distinguishes between temporal and nasal retinal axons

A monoclonal antibody was developed that recognizes an antigen with an asymmetric distribution in the chick retina. Immunohistochemistry showed that this antigen, temporal retinal axon protein (TRAP), was present on most if not all axons that arose from the temporal side of the retina. Very few of the axons from the nasal side of the retina were positive for TRAP. The nasal-temporal difference appeared to be in the number of axons that stained with this antibody rather than in the intensity of staining. The transition between nasal and temporal retina based on TRAP distribution appeared to be a vertical line centered on the optic fissure. A competition-based ELISA was developed to quantify the average amount of TRAP on axons in different regions of the retina. This assay also suggested that the pattern of TRAP distribution across the retina was a step function, though the results did not completely rule out the possibility of a continuous concentration gradient oriented circumferentially around the retina. Explants of embryonic nasal and temporal retina had a similar dichotomy in TRAP expression during the first 1 or 2 d in culture. The antibody to TRAP bound to retinal neurites in culture without the cell membrane being made permeable, which suggests that TRAP is a cell-surface molecule. In culture, TRAP was also expressed on the growth cones. Immunoblots showed that TRAP is trypsin sensitive and has an approximate molecular mass of 135 kDa. This is the first molecule identified with an asymmetric distribution in the nasal-temporal axis of the retina. Its distribution correlates with previous studies that demonstrated differences in the activity of axons in culture from the nasal and temporal sides of the retina. It is possible that TRAP has a role in development of a topographic pattern of axonal connections between the eye and central visual nuclei in the brain.