Translating Cues into Action

Abstract

Axonal pathfinding depends on local protein translation, but the identities of the newly synthesized proteins--and exactly how they mediate growth cone turning--have been unclear (see Ming). Two groups have now implicated asymmetric localization and translation of β-actin in growth cones in pathfinding. Yao et al . showed that both attractive turning of cultured Xenopus laevis spinal neurons to brain-derived neurotrophic factor (BDNF), which depends on Ca 2+ , and repulsive turning to BDNF, which occurs when protein kinase A (PKA) is inhibited, were blocked by inhibitors of protein synthesis, although extension was not affected. Turning in response to a localized increase in [Ca 2+ ] i after photolysis of caged Ca 2+ also depended on protein synthesis. Bath application of BDNF increased abundance of β-actin mRNA in the growth cone and colocalization of β-actin mRNA with Vg1RBP, the Xenopus homolog of the zipcode binding protein ZBP1, effects that were blocked by antisense oligonucleotides to the β-actin 3′ untranslated region zipcodes. Localized application of BDNF led to an increase in colocalized β-actin mRNA-ZBP1 puncta on the side of the growth cone exposed to higher concentrations of BDNF. The antisense oligonucleotides that blocked the zipcode interaction also blocked attractive and repulsive turning responses to BDNF or localized increases in [Ca 2+ ] i . Immunofluorescence analysis revealed that local BDNF elicited an asymmetric increase in growth cone β-actin and in phosphorylated Src on the side near BDNF, which depended on protein synthesis. An asymmetric decrease in β-actin and phosphorylated Src distribution on that side was apparent when PKA activity was inhibited. Using live cell imaging to visualize Vg1RBP tagged with enhanced green fluorescent protein (Vg1RBP-eGFP) in Xenopus retinal growth cones, Leung et al . found that Vg1RBP-eGFP-containing granules (and colocalized β-actin mRNA) moved from the central region of growth cones to the filopodia in response to bath application of netrin-1. Like BDNF, netrin-1 elicited a translation-dependent increase in growth cone β-actin. Exposure to a netrin-1 gradient led to polarized transport of Vg1RBP-eGFP to the near side, to an asymmetric increase in phosphorylation of the translation initiation factor 4EBP, and to a translation-dependent asymmetric increase in β-actin (on the near side). Inhibition of β-actin synthesis prevented growth cone attraction to netrin-1 but not repulsive turning to Sema3a or to netrin-1 with growth cones grown on a substrate with a high concentration of laminin. Thus, both studies suggest that asymmetric translocation and translation of β-actin mediate growth cone turning toward attractive cues. For reasons that remain to be determined, the studies differ on whether β-actin is involved in repulsive turning. J. Yao, Y. Sasaki, Z. Wen, G. J. Bassell, J. Q. Zheng, An essential role for β-actin mRNA localization and translation in Ca 2+ -dependent growth cone guidance. Nat. Neurosci. 9 , 1265-1273 (2006). [PubMed] K.-M. Leung, F. P. G. van Horck, A. C. Lin, R. Allison, N. Standart, C. E. Holt, Asymmetrical β-actin mRNA translation in growth cones mediates attractive turning to netrin-1. Nat. Neurosci. 9 , 1247-1256 (2006). [PubMed] G.-L. Ming, Turning by asymmetric actin. Nat. Neurosci . 9 , 1201-1203 (2006). [PubMed]