Abstract
During development, growth cones are essential for axon pathfinding by sensing numerous guidance cues in their environment. Retinoic acid, the metabolite of vitamin A, is important for neurite outgrowth during vertebrate development, but may also play a role in axon guidance, though little is known of the cellular mechanisms involved. Our previous studies showed that retinoid-induced growth cone turning of invertebrate motorneurons requires local protein synthesis and calcium influx. However, the signalling pathways that link calcium influx to cytoskeletal dynamics involved in retinoid-mediated growth cone turning are not currently known. The Rho GTPases, Cdc42 and Rac, are known regulators of the growth cone cytoskeleton. Here, we demonstrated that inhibition of Cdc42 or Rac not only prevented growth cone turning toward retinoic acid but could also induce a switch in growth cone responsiveness to chemorepulsion or growth cone collapse. However, the effects of Cdc42 or Rac inhibition on growth cone responsiveness differed, depending on whether the turning was induced by the all-trans or 9-cis retinoid isomer. The effects also differed depending on whether the growth cones maintained communication with the cell body. These data strongly suggest that Cdc42 and Rac are downstream effectors of retinoic acid during growth cone guidance.
Highlights
Retinoic acid has been implicated in the development [1,2] and regeneration [3,4] of various organ systems in several species
As Ras homolog (Rho) GTPases are known downstream effectors of calcium and are involved in growth cone turning responses to other guidance cues [30,31], we first investigated the role of the Rho GTPase, cell division control protein 42 (Cdc42), in retinoid-mediated growth cone turning
Similar to many other guidance cues, we have previously shown that retinoid-induced growth cone attraction requires both local protein synthesis and calcium influx [16]
Summary
Retinoic acid has been implicated in the development [1,2] and regeneration [3,4] of various organ systems in several species. The ability of retinoic acid to exert chemotropic effects was first demonstrated in developing neurites of chick embryonic neural tube cells [17], and later in regenerating newt spinal cord explants [8], where neurites grew toward the source of retinoic acid (chemoattraction). It was using regenerating cultured motorneurons from the mollusc Lymnaea stagnalis that it was determined that the chemoattractant effects of retinoic acid were non-genomic in nature [16]. The growth cones of regenerating molluscan neurons can be physically transected from the cell bodies and continue to Biomolecules 2019, 9, 460; doi:10.3390/biom9090460 www.mdpi.com/journal/biomolecules
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