Chemotropic guidance of retinal growth cones

• Main Author: Campbell, D. S.
• Published: University of Cambridge 2002
• Subjects: 617.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597251

Axons navigate long distances to their targets in the embryonic brain using a series of membrane bound and diffusible guidance cues along their pathway. This study presents an investigation of the <I>in vitro</I> effect of semaphorin 3A (Sema3A) on <I>Xenopus</I> retinal growth cones and into the role of local protein synthesis and proteasome-dependent proteolysis in mediating chemotropic responses of retinal growth cones. Growth cones acquire responsiveness to Sema3A with age and the onset of responsiveness correlates with the appearance of the Sema3A receptor component neuropilin-1(NP-1) immunoreactivity. Growth cones from "old" (stage 35/36) retinal explants collapse rapidly (5-10 minutes) in response to Sema3A and turn away from a gradient of Sema3A whereas 'young' growth cones (stage 24) are insensitive to Sema3A. Moreover, transfection of full length NP-1 into young neurones confers premature Sema3A-sensitivity. When young neurones are aged in culture they develop Sema3A-sensitivity in parallel with those <I>in vivo</I> suggesting that an intrinsic mechanism of NP-1 regulation mediates this age-dependent change. Sema3A-induced collapse is transient and upon recovery approximately 30% of growth cones extend new branches within 1 hour implicating Sema3A as a branching factor. Pharmacological inhibitors were used to investigate whether these three Sema3A-induced behaviours (collapse, turning and branching) use distinct second messenger signalling pathways. All three behaviours were found to be mediated via cGMP. In the presence of translation inhibitors, retinal growth cones fail to collapse in response to Sema3A and are unable to turn attractively or repulsively to Sema3A or netrin-1. Furthermore, Sema3A and netrin-1 stimulate rapid rises in phosphorylation of translation initiation proteins (eIF-4E and eIF-4EBP1) and in protein synthesis in isolated growth cones. Pharmacological inhibitors were used to determine that the effects on chemotropic responses of Sema3A and netrin-1 differ in their requirement for Phosphatidylinositol-3 kinase (PI-3 kinase) and share a common target of rapamycin (TOR) sensitive pathway leading to the initiation of translation. These effects occur within minutes suggesting that guidance molecules may steer axon growth by triggering local translation in growth cones.