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Reply: Visualizing branched actin filaments in lamellipodia by electron tomography

Nature Cell Biology volume 13, pages 1013–1014 (2011)Cite this article

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Small et al . reply:

Cell migration is initiated by the extension of cytoplasmic sheets, termed lamellipodia. In a currently accepted model of lamellipodial dynamics, the cell front is pushed by short and stiff actin filaments branching from the sides of other filaments in a dendritic array1. The model is based on the finding that the Arp2/3 complex induces actin branching in vitro and the observation of high frequency branching of actin filaments in anterior regions of lamellipodia cytoskeletons prepared for conventional transmission electron microscopy by the critical-point drying/platinum-replica procedure2. Using electron tomography, we recently presented the first three-dimensional images of lamellipodia3 in cells in vitreous ice and in fixed cells contrasted by the negative-staining method. Focusing attention on the front of lamellipodia, we reported that putative branch points were seldom when compared with filament overlaps, and attributed the reported high concentration of branch points, within 20–50 nm of each other at the front of keratocyte lamellipodia2, to the difficulty of distinguishing incidences of branching from filament overlaps in images obtained by the platinum-replica method. In the accompanying letter, Yang and Svitkina report a re-analysis of one of our published tomograms of a negatively stained lamellipodium of a 3T3 cell spreading on poly-L-lysine, and note the presence of actin branches similar in morphology to those described in negatively stained preparations of actin branches formed in vitro by the Arp2/3 complex4. In light of their analysis and our ongoing studies we agree with them that actin branches are present in the lamellipodium network, but at a very different frequency and distribution from that envisioned in the current dendritic model1.

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Figure 1: Projection of three-dimensional model of the published tomogram3 of a lamellipodium of a 3T3 cell spread on poly-L-lysine.

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Acknowledgements

This work was supported by grants from the Austrian Science Fund (FWF project P21292) and from the Vienna Science and Technology Fund (WWTF project MA09_004).

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Authors and Affiliations

  1. Institute of Molecular Biotechnology of the Austrian Academy of Sciences, Dr Bohr Gasse 3–7, Vienna, 1030, Austria

    J. Victor Small & Marlene Vinzenz

  2. RICAM, Austrian Academy of Sciences Vienna, Altenbergstrasse 69, Linz, 4040, Austria

    Christoph Winkler & Christian Schmeiser

  3. Faculty of Mathematics, University of Vienna, Nordbergstrasse 15, Vienna, 1090, Austria

    Christian Schmeiser

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Correspondence to J. Victor Small.

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Small, J., Winkler, C., Vinzenz, M. et al. Reply: Visualizing branched actin filaments in lamellipodia by electron tomography. Nat Cell Biol 13, 1013–1014 (2011). https://doi.org/10.1038/ncb2322

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