Cytoskeleton Dynamics

How do actin filaments act as a force-sensing conduit for both internal and external forces?

Internal forces

The orientation of individual actin filaments in the cytoskeleton is a force-driven evolutionary process [1] that contributes to the elastic behavior of the network and influences whether a filament will deform by compression, bending or extension [2][3]. Cross-linked actin networks initially become more elastic under low force as a result of filament resistance to the direction of compression [4]. As the force increases, individual filaments inherently resist being compressed and/or cross-linking proteins become more extended, which causes the cytoskeleton network to become more rigid [5][6]; cell stiffening has also been correlated with actin recruitment [7]. Each of these processes in turn, can influence mechanosensors that are linked to the actin network to induce subsequent mechanotransduction events. Although forces ~ 400 pN along the filament axis will break actin filaments, much higher forces are generated in migrating cells (reviewed in [8] ); this raises the question – how does a cell alleviate the forces on the internal actin network?? The answer lies in the proteins that are linked to the actin: extreme stress on the filaments causes buckling and reduced elasticity of the network in a process known as ‘stress softening’ [9]. Softening is attributed to either filament fracturing, which leads to new uncapped ends for filament assembly/disassembly, or to softening, which may be due to unbinding of flexible protein crosslinkers [4][10]. In addition, because the filaments are interconnected to the rest of the cytoskeleton, buckled filaments don’t collapse and the process can be reversed when the stress is removed [11] (reviewed in [12][13]).

External or applied forces

The actin cytoskeleton is physically connected with the cell exterior, e.g., ECM or other cells, through a multiprotein complex known as the focal adhesion complex. Interactions between cell surface molecules, e.g., integrins, and the actin cytoskeleton are bidirectional, with the focal adhesion complex forming the link between them [14]. Actin filaments and their associated focal adhesion complexes act as information handling machines or mechanosensors: they convert both the strength of the adhesion and the tensile forces along the linked network of actin filaments (and associated proteins) into biochemical signals that control actin extension and cell migration (reviewed in [13][15][16][17][18][19]). Focal adhesions are subject to continuous pulling forces [20][21] and the force differences due to external stress [22] or the chemical nature (reviewed in [23]), rigidity [24][25], and topography of the exterior components [26][27][28] (reviewed in [29][30][31]) will influence the assembly and organization of the actin cytoskeleton [28][32][33][34][35].

References

1. Maly IV, and Borisy GG. Self-organization of a propulsive actin network as an evolutionary process. Proc. Natl. Acad. Sci. U.S.A. 2001; 98(20):11324-9. [PMID: 11572984]
2. Loisel TP, Boujemaa R, Pantaloni D, and Carlier MF. Reconstitution of actin-based motility of Listeria and Shigella using pure proteins. Nature 1999; 401(6753):613-6. [PMID: 10524632]
3. Kroy , and Frey . Force-Extension Relation and Plateau Modulus for Wormlike Chains. Phys. Rev. Lett. 1996; 77(2):306-309. [PMID: 10062418]
4. Gardel ML, Shin JH, MacKintosh FC, Mahadevan L, Matsudaira P, and Weitz DA. Elastic behavior of cross-linked and bundled actin networks. Science 2004; 304(5675):1301-5. [PMID: 15166374]
5. Footer MJ, Kerssemakers JWJ, Theriot JA, and Dogterom M. Direct measurement of force generation by actin filament polymerization using an optical trap. Proc. Natl. Acad. Sci. U.S.A. 2007; 104(7):2181-6. [PMID: 17277076]
6. Johnson CP, Tang H, Carag C, Speicher DW, and Discher DE. Forced unfolding of proteins within cells. Science 2007; 317(5838):663-6. [PMID: 17673662]
7. Icard-Arcizet D, Cardoso O, Richert A, and Hénon S. Cell stiffening in response to external stress is correlated to actin recruitment. Biophys. J. 2008; 94(7):2906-13. [PMID: 18178644]
8. Khan S, and Sheetz MP. Force effects on biochemical kinetics. Annu. Rev. Biochem. 1997; 66:785-805. [PMID: 9242924]
9. Chaudhuri O, Parekh SH, and Fletcher DA. Reversible stress softening of actin networks. Nature 2007; 445(7125):295-8. [PMID: 17230186]
10. Gardel ML, Nakamura F, Hartwig JH, Crocker JC, Stossel TP, and Weitz DA. Prestressed F-actin networks cross-linked by hinged filamins replicate mechanical properties of cells. Proc. Natl. Acad. Sci. U.S.A. 2006; 103(6):1762-7. [PMID: 16446458]
11. Ingber DE, and Folkman J. Mechanochemical switching between growth and differentiation during fibroblast growth factor-stimulated angiogenesis in vitro: role of extracellular matrix. J. Cell Biol. 1989; 109(1):317-30. [PMID: 2473081]
12. Ingber DE. Tensegrity-based mechanosensing from macro to micro. Prog. Biophys. Mol. Biol. 2008; 97(2-3):163-79. [PMID: 18406455]
13. Ingber DE. Cellular mechanotransduction: putting all the pieces together again. FASEB J. 2006; 20(7):811-27. [PMID: 16675838]
14. Goetz JG. Bidirectional control of the inner dynamics of focal adhesions promotes cell migration. Cell Adh Migr 2009; 3(2):185-90. [PMID: 19398887]
15. Bershadsky A, Kozlov M, and Geiger B. Adhesion-mediated mechanosensitivity: a time to experiment, and a time to theorize. Curr. Opin. Cell Biol. 2006; 18(5):472-81. [PMID: 16930976]
16. Bershadsky AD, Ballestrem C, Carramusa L, Zilberman Y, Gilquin B, Khochbin S, Alexandrova AY, Verkhovsky AB, Shemesh T, and Kozlov MM. Assembly and mechanosensory function of focal adhesions: experiments and models. Eur. J. Cell Biol. 2005; 85(3-4):165-73. [PMID: 16360240]
17. Chen CS. Mechanotransduction – a field pulling together? J. Cell. Sci. 2008; 121(Pt 20):3285-92. [PMID: 18843115]
18. Lock JG, Wehrle-Haller B, and Strömblad S. Cell-matrix adhesion complexes: master control machinery of cell migration. Semin. Cancer Biol. 2007; 18(1):65-76. [PMID: 18023204]
19. Geiger B, Spatz JP, and Bershadsky AD. Environmental sensing through focal adhesions. Nat. Rev. Mol. Cell Biol. 2009; 10(1):21-33. [PMID: 19197329]
20. Griffin MA, Sen S, Sweeney HL, and Discher DE. Adhesion-contractile balance in myocyte differentiation. J. Cell. Sci. 2004; 117(Pt 24):5855-63. [PMID: 15522893]
21. Gibson MC, and Perrimon N. Extrusion and death of DPP/BMP-compromised epithelial cells in the developing Drosophila wing. Science 2005; 307(5716):1785-9. [PMID: 15774762]
22. Helmke BP, and Davies PF. The cytoskeleton under external fluid mechanical forces: hemodynamic forces acting on the endothelium. Ann Biomed Eng 2002; 30(3):284-96. [PMID: 12051614]
23. Hersel U, Dahmen C, and Kessler H. RGD modified polymers: biomaterials for stimulated cell adhesion and beyond. Biomaterials 2003; 24(24):4385-415. [PMID: 12922151]
24. Discher DE, Janmey P, and Wang Y. Tissue cells feel and respond to the stiffness of their substrate. Science 2005; 310(5751):1139-43. [PMID: 16293750]
25. Engler AJ, Sen S, Sweeney HL, and Discher DE. Matrix elasticity directs stem cell lineage specification. Cell 2006; 126(4):677-89. [PMID: 16923388]
26. Curtis A, and Wilkinson C. New depths in cell behaviour: reactions of cells to nanotopography. Biochem. Soc. Symp. 1999; 65:15-26. [PMID: 10320930]
27. Dalby MJ, Riehle MO, Johnstone H, Affrossman S, and Curtis ASG. In vitro reaction of endothelial cells to polymer demixed nanotopography. Biomaterials 2002; 23(14):2945-54. [PMID: 12069336]
28. Parker KK, Brock AL, Brangwynne C, Mannix RJ, Wang N, Ostuni E, Geisse NA, Adams JC, Whitesides GM, and Ingber DE. Directional control of lamellipodia extension by constraining cell shape and orienting cell tractional forces. FASEB J. 2002; 16(10):1195-204. [PMID: 12153987]
29. Vogel V, and Sheetz M. Local force and geometry sensing regulate cell functions. Nat. Rev. Mol. Cell Biol. 2006; 7(4):265-75. [PMID: 16607289]
30. Curtis A, and Riehle M. Tissue engineering: the biophysical background. Phys Med Biol 2001; 46(4):R47-65. [PMID: 11324976]
31. Spatz JP, and Geiger B. Molecular engineering of cellular environments: cell adhesion to nano-digital surfaces. Methods Cell Biol. 2007; 83:89-111. [PMID: 17613306]
32. Keren K, Pincus Z, Allen GM, Barnhart EL, Marriott G, Mogilner A, and Theriot JA. Mechanism of shape determination in motile cells. Nature 2008; 453(7194):475-80. [PMID: 18497816]
33. Singhvi R, Kumar A, Lopez GP, Stephanopoulos GN, Wang DI, Whitesides GM, and Ingber DE. Engineering cell shape and function. Science 1994; 264(5159):696-8. [PMID: 8171320]
34. Chen CS, Mrksich M, Huang S, Whitesides GM, and Ingber DE. Geometric control of cell life and death. Science 1997; 276(5317):1425-8. [PMID: 9162012]
35. Parekh SH, Chaudhuri O, Theriot JA, and Fletcher DA. Loading history determines the velocity of actin-network growth. Nat. Cell Biol. 2005; 7(12):1219-23. [PMID: 16299496]