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Reversible stress softening of actin networks

Abstract

The mechanical properties of cells play an essential role in numerous physiological processes. Organized networks of semiflexible actin filaments determine cell stiffness and transmit force during mechanotransduction, cytokinesis, cell motility and other cellular shape changes1,2,3. Although numerous actin-binding proteins have been identified that organize networks, the mechanical properties of actin networks with physiological architectures and concentrations have been difficult to measure quantitatively. Studies of mechanical properties in vitro have found that crosslinked networks of actin filaments formed in solution exhibit stress stiffening arising from the entropic elasticity of individual filaments or crosslinkers resisting extension4,5,6,7,8. Here we report reversible stress-softening behaviour in actin networks reconstituted in vitro that suggests a critical role for filaments resisting compression. Using a modified atomic force microscope to probe dendritic actin networks (like those formed in the lamellipodia of motile cells), we observe stress stiffening followed by a regime of reversible stress softening at higher loads. This softening behaviour can be explained by elastic buckling of individual filaments under compression that avoids catastrophic fracture of the network. The observation of both stress stiffening and softening suggests a complex interplay between entropic and enthalpic elasticity in determining the mechanical properties of actin networks.

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Figure 1: AFM-based microrheology of growing dendritic actin networks.
Figure 2: Frequency dependence of elastic (filled triangles, E′ ) and viscous (open triangles, E′′ ) moduli.
Figure 3: Dendritic actin networks exhibit stress stiffening and reversible stress softening.
Figure 4: Stress stiffening and stress softening can arise in dendritic networks owing to filaments resisting extension and buckling of filaments resisting compression.

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Acknowledgements

We thank J. W. Shaevitz, M. J. Rosenbluth, S. Pronk, P. L. Geissler and J. Alcaraz for discussions and reading of the manuscript as well as the entire Fletcher laboratory for support. We are also grateful to R. L. Jeng and M. J. Footer for assistance in protein preparation. This work was supported by an ASEE NDSEG Fellowship to O.C., an ARCS Fellowship to S.H.P., and an NSF Career Award and NIH grants to D.A.F.

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Correspondence to Daniel A. Fletcher.

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Supplementary Information

This file contains Supplementary Methods; Supplementary Notes; Supplementary Figures 1-3 with legends . Supplementary Methods are shown as Supplementary Information A. Supplementary Information B describes control experiments showing cantilever-surface interaction to be negligible and includes Figure S1 and Figure S2. Supplementary Information C describes the normalization method used to determine the power law and includes Figure S3. Finally myosin inhibition experiments are detailed in Supplementary Information D, and demonstrate that there was no myosin dependent prestressing of the dendritic actin networks studied here. (PDF 256 kb)

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Chaudhuri, O., Parekh, S. & Fletcher, D. Reversible stress softening of actin networks. Nature 445, 295–298 (2007). https://doi.org/10.1038/nature05459

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