Showcase of Undergraduate Research and Creative Endeavors (SOURCE)

Investigating the Responsiveness of Embryonic Chick Retinal Ganglion Cells to Semaphorin-3A

During embryonic development, neural connections are made when axons grow over long distances to find their synaptic targets, guided by terminal structures called growth cones. Growth cones either continue to grow or collapse, retracting and growing in a different direction, in response to attractive or repulsive cues. One such axon guidance cue is Semaphorin-3A (Sema-3A). Luo et al. (1993) showed that Sema-3A causes growth cone collapse from embryonic chick dorsal root ganglion cells (DRGs) but not from chick retinal ganglion cells (RGCs). However, the experiment was repeated in Dr. Birgbauer’s lab, and we found that Sema-3A can cause growth cone collapse of chick RGCs. There are several possible hypotheses on why the results of those two experiments differed. There could be a dose-dependent difference or an age-dependent difference. There could also be a time-dependent difference, since the previous research assay was for 60 minutes while we used a 15-minute assay. We have tested these hypotheses and found a dose-dependent growth cone collapse in chick RGCs. Also, the time response of Sema-3A was investigated, revealing that RGCs exhibited a peak of collapse after 15 minutes, and that they started regenerating at 20 minutes. We also examined the responsiveness of different embryonic ages to Sema-3A, and it was found that as the chick embryo develops, the chick RGCs maintain a significant level of responsiveness (p < 0.05). We have also investigated the expression of Sema-3A receptors, Neuropilin-1 and Neuropilin-2, as well as their co-receptors in the chick retina by RT-PCR. We found that all of these are expressed except for Class B plexins and Neuropilin-1. In conclusion, we have found strong evidence that Sema-3A induces growth cone collapse in embryonic chick RGCs. We found that the retinal growth cone collapse is dose-dependent and developmentally relevant. Also, embryonic chick RGC growth cones rapidly desensitize to Sema-3A, explaining the difference from Luo et al.