Actin switches in phagocytosis

Abstract

Exposure of phagocytes to non-spherical particles has provided evidence for multiple actions of the actin system in force generation. For the uptake of long cylindrical particles, a “motile actin clamp” mechanism is proposed. When a phagocyte is engaged with an hourglass-shaped particle, it exerts contractile activity alternatively at the far end of the particle or at its concave region. Phagocytes can switch within seconds between these different strategies of taking up a particle. This response switching is based on reprogramming the pattern of actin polymerization and depolymerization. The choice between different strategies of interaction with a particle increases the probability of engulfing the entire particle or at least a portion of it. Finally, a switch to actin disassembly enables a phagocyte to release a particle that turns out to be too big to be enclosed. 

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Acknowledgements

I would like to acknowledge support of the Max Planck Society, and to thank Margaret Clarke, the first author of the paper to which this Addendum applies, for comments.

Figures and Tables

Figure 1 Interaction of a phagocyte with particles of different shapes. The diagram illustrates the uptake of three types of particles: spherical ones with a convex surface of constant curvature (A), bi-lobed particles with a concave neck (B) and long cylindrical particles with parallel contour aspect (C). (A) the internalization of a sphere occurs in two phases. First, the phagocytic cup needs to expand without the application of contractile forces, since otherwise the particle would be repelled (left part). Second, the cup has to contract around the particle (middle) until it can close on top of the particle for separation of the phagosome from the cell surface (right). (B) a bi-lobed particle imposes a conflict on the phagocyte: should the concave neck be taken as the end of the particle (left possibility in the split image) or should the cup continue to progress in search of a more distal end (right possibility). (C) a rod-shaped particle will be drawn into the phagocyte by gliding movement of the phagocytic cup along the particle's constant perimeter.

Addendum to: