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Myosin II functions in actin-bundle turnover in neuronal growth cones

Abstract

Retrograde actin flow works in concert with cell adhesion to generate traction forces that are involved in axon guidance in neuronal growth cones. Myosins have been implicated in retrograde flow, but identification of the specific myosin subtype(s) involved has been controversial. Using fluorescent speckle microscopy (FSM) to assess actin dynamics, we report that inhibition of myosin II alone decreases retrograde flow by 51% and the remaining flow can be almost fully accounted for by the 'push' of plus-end actin assembly at the leading edge of the growth cone. Interestingly, actin bundles that are associated with filopodium roots elongated by approximately 83% after inhibition of myosin II. This unexpected result was due to decreased rates of actin-bundle severing near their proximal (minus or pointed) ends which are located in the transition zone of the growth cone. Our study reveals a mechanism for the regulation of actin-bundle length by myosin II that is dependent on actin-bundle severing, and demonstrate that retrograde flow is a steady state that depends on both myosin II contractility and actin-network treadmilling.

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Figure 1: Myosin II localizes to the transition zone of the growth cone where actin-bundle severing occurs.
Figure 2: Blebbistatin treatment reversibly depletes myosin II bound in the transition zone.
Figure 3: Myosin II function in peripheral-domain retrograde flow and actin-arc translocation.
Figure 4: Actin bundles grow as a result of decreased severing after inhibition of myosin II.
Figure 6: Myosin II regulates leading-edge dynamics.
Figure 5: Actin polymerization drives retrograde flow in the absence of myosin II activity.
Figure 7: Role for myosin II contractility in actin-bundle recycling.

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Acknowledgements

The authors would like to thank members of the Forscher lab, in particular, A. Schaefer and X. Zhang, for helpful comments and discussion. We thank M. Lord and M. Tyska for technical advice and M. Mooseker, H. Keshishian and D. Wells for critical comments and discussion. This work was supported by a National Institutes of Health grant (RO1-NS28695) to P.F. and the Nikon Partners-in-Research Program.

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Correspondence to Paul Forscher.

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Medeiros, N., Burnette, D. & Forscher, P. Myosin II functions in actin-bundle turnover in neuronal growth cones. Nat Cell Biol 8, 216–226 (2006). https://doi.org/10.1038/ncb1367

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