Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Reversible stress softening of actin networks


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.

This is a preview of subscription content, access via your institution

Access options

Rent or buy this article

Prices vary by article type



Prices may be subject to local taxes which are calculated during checkout

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.


  1. Pollard, T. D. & Borisy, G. G. Cellular motility driven by assembly and disassembly of actin filaments. Cell 112, 453–465 (2003)

    Article  CAS  PubMed  Google Scholar 

  2. Janmey, P. A. & Weitz, D. A. Dealing with mechanics: mechanisms of force transduction in cells. Trends Biochem. Sci. 29, 364–370 (2004)

    Article  CAS  PubMed  Google Scholar 

  3. Satcher, R. L. & Dewey, C. F. Theoretical estimates of mechanical properties of the endothelial cell cytoskeleton. Biophys. J. 71, 109–118 (1996)

    Article  PubMed  PubMed Central  Google Scholar 

  4. Xu, J. Y., Tseng, Y. & Wirtz, D. Strain hardening of actin filament networks—Regulation by the dynamic cross-linking protein α-actinin. J. Biol. Chem. 275, 35886–35892 (2000)

    Article  CAS  PubMed  Google Scholar 

  5. Gardel, M. L. et al. Elastic behavior of cross-linked and bundled actin networks. Science 304, 1301–1305 (2004)

    Article  ADS  CAS  PubMed  Google Scholar 

  6. Storm, C., Pastore, J. J., MacKintosh, F. C., Lubensky, T. C. & Janmey, P. A. Nonlinear elasticity in biological gels. Nature 435, 191–194 (2005)

    Article  ADS  CAS  PubMed  Google Scholar 

  7. Gardel, M. L. et al. Prestressed F-actin networks cross-linked by hinged filamins replicate mechanical properties of cells. Proc. Natl Acad. Sci. USA 103, 1762–1767 (2006)

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  8. Mackintosh, F. C., Kas, J. & Janmey, P. A. Elasticity of semiflexible biopolymer networks. Phys. Rev. Lett. 75, 4425–4428 (1995)

    Article  ADS  CAS  PubMed  Google Scholar 

  9. Mullins, R. D., Heuser, J. A. & Pollard, T. D. The interaction of Arp2/3 complex with actin: Nucleation, high affinity pointed end capping, and formation of branching networks of filaments. Proc. Natl Acad. Sci. USA 95, 6181–6186 (1998)

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  10. Svitkina, T. M. & Borisy, G. G. Arp2/3 complex and actin depolymerizing factor cofilin in dendritic organization and treadmilling of actin filament array in lamellipodia. J. Cell Biol. 145, 1009–1026 (1999)

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  11. Bustamante, C., Marko, J. F., Siggia, E. D. & Smith, S. Entropic elasticity of λ-phage DNA. Science 265, 1599–1600 (1994)

    Article  ADS  CAS  PubMed  Google Scholar 

  12. Landau, L. D. & Lifshitz, E. M. Theory of Elasticity (Butterworth-Heinemann, Oxford, 1986)

    MATH  Google Scholar 

  13. Gittes, F., Mickey, B., Nettleton, J. & Howard, J. Flexural rigidity of microtubules and actin-filaments measured from thermal fluctuations in shape. J. Cell Biol. 120, 923–934 (1993)

    CAS  PubMed  Google Scholar 

  14. Kroy, K. & Frey, E. Force-extension relation and plateau modulus for wormlike chains. Phys. Rev. Lett. 77, 306–309 (1996)

    Article  ADS  CAS  PubMed  Google Scholar 

  15. Head, D. A., Levine, A. J. & MacKintosh, E. C. Deformation of cross-linked semiflexible polymer networks. Phys. Rev. Lett. 91, 108102 (2003)

    Article  ADS  PubMed  Google Scholar 

  16. Parekh, S. H., Chaudhuri, O., Theriot, J. A. & Fletcher, D. A. Loading history determines the velocity of actin-network growth. Nature Cell Biol. 7, 1119–1123 (2005)

    Article  CAS  Google Scholar 

  17. Cameron, L. A., Svitkina, T. M., Vignjevic, D., Theriot, J. A. & Borisy, G. G. Dendritic organization of actin comet tails. Curr. Biol. 11, 130–135 (2001)

    Article  CAS  PubMed  Google Scholar 

  18. Cameron, L. A., Footer, M. J., van Oudenaarden, A. & Theriot, J. A. Motility of ActA protein-coated microspheres driven by actin polymerization. Proc. Natl Acad. Sci. USA 96, 4908–4913 (1999)

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  19. Mahaffy, R. E., Shih, C. K., MacKintosh, F. C. & Kas, J. Scanning probe-based frequency-dependent microrheology of polymer gels and biological cells. Phys. Rev. Lett. 85, 880–883 (2000)

    Article  ADS  CAS  PubMed  Google Scholar 

  20. Alcaraz, J. et al. Microrheology of human lung epithelial cells measured by atomic force microscopy. Biophys. J. 84, 2071–2079 (2003)

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  21. Stamenovic, D., Suki, B., Fabry, B., Wang, N. & Fredberg, J. J. Rheology of airway smooth muscle cells is associated with cytoskeletal contractile stress. J. Appl. Physiol. 96, 1600–1605 (2004)

    Article  PubMed  Google Scholar 

  22. Fabry, B. et al. Scaling the microrheology of living cells. Phys. Rev. Lett. 87, 148102 (2001)

    Article  ADS  CAS  PubMed  Google Scholar 

  23. Marcy, Y., Prost, J., Carlier, M. F. & Sykes, C. Forces generated during actin-based propulsion: A direct measurement by micromanipulation. Proc. Natl Acad. Sci. USA 101, 5992–5997 (2004)

    Article  ADS  CAS  PubMed  PubMed Central  Google Scholar 

  24. Tseng, Y. & Wirtz, D. Dendritic branching and homogenization of actin networks mediated by Arp2/3 complex. Phys. Rev. Lett. 93, 258104 (2004)

    Article  ADS  PubMed  Google Scholar 

  25. Nakamura, F., Osborn, E., Janmey, P. A. & Stossel, T. P. Comparison of filamin A-induced cross-linking and Arp2/3 complex-mediated branching on the mechanics of actin filaments. J. Biol. Chem. 277, 9148–9154 (2002)

    Article  CAS  PubMed  Google Scholar 

  26. Onck, P. R., Koeman, T., van Dillen, T. & van der Giessen, E. Alternative explanation of stiffening in cross-linked semiflexible networks. Phys. Rev. Lett. 95, 178102 (2005)

    Article  ADS  CAS  PubMed  Google Scholar 

  27. Coughlin, M. F. & Stamenovic, D. A tensegrity model of the cytoskeleton in spread and round cells. J. Biomech. Eng. Trans. Asme 120, 770–777 (1998)

    Article  CAS  Google Scholar 

  28. Gibson, L. J. & Ashby, M. F. Cellular Solids: Structure and Properties (Pergamon Press, Cambridge, 1988)

    MATH  Google Scholar 

  29. Brangwynne, C. P. et al. Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement. J. Cell Biol. 173, 733–741 (2006)

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  30. Pollard, T. D., Blanchoin, L. & Mullins, R. D. Molecular mechanisms controlling actin filament dynamics in nonmuscle cells. Annu. Rev. Biophys. Biomol. Struct. 29, 545–576 (2000)

    Article  CAS  PubMed  Google Scholar 

Download references


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.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Daniel A. Fletcher.

Ethics declarations

Competing interests

Reprints and permissions information is available at The authors declare no competing financial interests.

Supplementary information

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)

Rights and permissions

Reprints and permissions

About this article

Cite this article

Chaudhuri, O., Parekh, S. & Fletcher, D. Reversible stress softening of actin networks. Nature 445, 295–298 (2007).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing