Cellular Mechanisms Underlying Retinoic Acid-Induced Growth Cone Guidance During Neuronal Regeneration

Abstract

During the period of neuronal development, neurons must make correct synaptic connections with their appropriate targets. The intricate connections of the nervous system are established in part by growth cones, located at the tips of extending neurites. These unique structures are essential for axon pathfinding and target cell selection by sensing and integrating numerous guidance cues from their environment. Retinoic acid, the active metabolite of vitamin A, is an important regulator of neurite outgrowth during vertebrate development, but there is substantial evidence that it also plays a role in axon guidance. Previous studies have provided preliminary evidence of a potential role of retinoid receptors in mediating the chemotropic effects of retinoic acid. In this study, I demonstrated that a synthetic retinoic acid receptor agonist was able to mimic the effects of retinoic acid in inducing growth cone turning. I also examined the intracellular pathways activated by retinoic acid that induce changes in growth cone behaviour. Previously it has been shown that retinoic acid-induced growth cone turning of invertebrate motorneurons requires local protein synthesis and calcium influx, similar to other known guidance cues in the central nervous system. However, the signalling pathways that link calcium influx to the regulation of cytoskeletal dynamics involved in growth cone turning are not currently known. Here, I examined potential effectors downstream of retinoic acid and have provided evidence that the intracellular pathways likely involve the Rho GTPases, Rac and Cdc42. I demonstrated that the inhibition of Rac or Cdc42 prevented growth cone turning towards retinoic acid. However, it was shown that the involvement of Rac differed depending on whether the growth cones maintained communication with the cell body or not. Moreover, the inhibition of Cdc42 not only blocked growth cone turning towards retinoic acid, but also induced a switch in growth cone responsiveness from attraction to repulsion. Overall, these studies provide new knowledge of the mechanisms underlying growth cone pathfinding by retinoids during nervous system development and regeneration.