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Filopodial actin bundles are not necessary for microtubule advance into the peripheral domain of Aplysia neuronal growth cones

Nature Cell Biology volume 9, pages 1360–1369 (2007)Cite this article

Abstract

Filopodial actin bundles guide microtubule assembly in the growth cone peripheral (P) domain and retrograde actin-network flow simultaneously transports microtubules rearward. Therefore, microtubule-end position is determined by the sum of microtubule assembly and retrograde transport rates. However, how filopodia actually affect microtubule assembly dynamics is unknown. To address this issue we quantitatively assessed microtubule and actin dynamics before and after selective removal of filopodia. Filopodium removal had surprisingly little effect on retrograde actin-flow rates or underlying network structures, but resulted in an approximate doubling of peripheral microtubule density and deeper penetration of microtubules into the P domain. The latter stemmed from less efficient coupling of microtubules to remaining actin networks and not from a change in microtubule polymer dynamics. Loss of filopodia also resulted in increased lateral microtubule movements and a more randomized microtubule distribution in the P domain. In summary, filopodia do not seem to be formally required for microtubule advance; however, their presence ensures radial distribution of microtubules in the P domain and facilitates microtubule transport by retrograde flow. The resulting dynamic steady state has interesting implications for rapid microtubule-positioning responses in the P domain.

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Figure 1: Retrograde flow is a barrier to microtubule advance.
Figure 2: Filopodial actin bundles are removed after low cytochalasin B treatment.
Figure 3: Low cytochalasin B removes filopodia actin bundles from the P domain leaving the less polarized network intact.
Figure 4: Low cytochalasin B randomizes microtubule spatial distribution in the P domain.
Figure 5: Filopodium removal increases myosin II dependence of retrograde flow.
Figure 6: Microtubule behaviour in the presence and absence of filopodial actin bundles.

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Acknowledgements

The authors would like to thank members of the Forscher lab for helpful comments and discussion. We thank B. Piekos for training D.B. in electron microscopy. We also thank A. Koleske and T. Pollard for critical reading and their insightful comments. This work was supported NIH grants RO1-NS28695 and RO1-NS051786 to P.F., RO1-GM67230 and U54-GM64346 (Cell Migration Consortium) to G.D. and the Nikon Partners-in-Research Program.

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  1. Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, 06511, CT, USA

    Dylan T. Burnette, Andrew W. Schaefer & Paul Forscher

  2. Department of Cell Biology, Scripps Research Institute (TSRI), La Jolla, 92037, CA, USA

    Lin Ji & Gaudenz Danuser

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  1. Dylan T. Burnette
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Correspondence to Paul Forscher.

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Burnette, D., Schaefer, A., Ji, L. et al. Filopodial actin bundles are not necessary for microtubule advance into the peripheral domain of Aplysia neuronal growth cones. Nat Cell Biol 9, 1360–1369 (2007). https://doi.org/10.1038/ncb1655

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