The cell as a material
Original entry: Nefeli Georgoulia, APPHY 226, Spring 2009
Authors: K.E. Kasza, A.C.Rowat, J. Liu, T.A. Angelini, C.P. Brangwynne, G.H. Koenderink & D.A. Weitz
Source: Current Opinion in Cell Biology, Vol 19, 101-107, (2007)
Soft Matter key words: rheology, elastic behavior, viscous behavior, prestress
This review paper summarizes the advances made in probing and recording the material properties of cells. Experiments in this field can be divided in two broad categories: the shearing of purified cytoskeletal filament networks and the probing of whole cells. Results indicate that the cell is a viscoelastic material. Rheology of semi-flexible biopolymer networks reveals stress-stiffening behavior: an increase in applied stress increases the network's elastic modulus (figure1). This is thought to be a consequence of the 'pulling out' of filament thermal fluctuations at high stress. Although purified filament networks have a linear elasticity much lower than cells, prestressed networks of such filaments display an elasticity similar to that of cells. According to the authors, this suggests that cells themselves are prestressed into a non-linear regime, possibly by molecular motors such as myosin. Cellular prestress has been experimentally confirmed by traction force microscopy, and it is thought to enable cellular response to external mechanical stimuli. In the last part of the paper, all this information is integrated into two competing models that account for cell mechanical behavior.
Soft Matter Snippet
The two models suggested to account for cell mechanical behavior are of soft matter interest:
1) The tensegrity model: According to tensegrity, some components of the cells are under tension, and these forces are balanced by other components of the cell which are under compression. Stress fibers (actin-myosin fibrillar assemblies) are thought to be the tensile components, while microtubules have been shown to bear compressive cellular loads. In fact, figure 2 demonstrates how cutting a stress fiber with laser nanoscissors causes it to snap back. The tensegrity model highlights the role of prestress in determining cell elasticity. It was architecturally inspired and parallels the mechanical behavior of cells to that of buildings!
2) The soft glassy rheology model: This model suggests that the cell is a soft solid composed of an elastic solid with some non-thermal relaxation processes, such as those generated by molecular motors. The predicted mechanical response displays a characteristic timescale dependance that is set by the effective 'temperature' of these non-thermal fluctuations. Experimental evidence that justify this model include applying large shear stresses on cells by magnetic bead cytometry. In these experiments magnetic beads are attached on the cell membrane and the application of stress induces cell softening, much like the shear-induced melting that characterizes soft glasses.