Retinoic Acid and the Underlying Cellular Mechanisms involved in Neurite Outgrowth and Growth Cone Turning during Regeneration

Abstract

The involvement of retinoic acid (RA) in nervous system regeneration has been well documented, though the precise cellular and molecular mechanisms have not yet been fully determined. During regeneration, RA can exert trophic support for cells, as well as tropic effects to guide neurite outgrowth. Cultured neurons of the Lymnaea central nervous system (CNS) have previously been used to investigate the role of RA in neurite outgrowth and growth cone guidance. Recently however, a novel phenomenon has been identified, in which neurite outgrowth occurs from cut nerves of Lymnaea CNS “floating” on the surface of cultured medium. In this study, I examined whether RA could induce or guide neurite outgrowth from this novel preparation. Unlike previous findings with cultured neurons, there was a lack of consistent effects of 9-cis and all-trans RA in promoting neurite outgrowth from the floating CNS. However, the growth cones of these floating neurites were found to turn toward a local source of RA, indicating for the first time, that they are capable of responding to guidance cues, even in the absence of adhesion to a solid substrate. The cellular mechanisms of RA-induced growth cone turning were then conducted on cultured neurons. I demonstrated that various retinoid receptor agonists could mimic the chemotropic effects of RA, providing further evidence for a potential role of retinoid X receptor and retinoic acid receptors in growth cone turning. I also examined potential downstream effectors involved in this chemotropic response to RA and provided the first evidence that the intracellular signaling pathway likely involves the Rho GTPase, Rac. However, it was also shown that the involvement of Rac in mediating the RA-induced growth cone turning differed depending on whether the turning was induced by endogenous or synthetic retinoids, and also whether the growth cones were still attached to the cell body or not. Overall, these data provide new insights into the mechanisms by which retinoids affect neurite outgrowth and growth cone behaviour during regeneration of the nervous system.