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==The mechanics of the cytoskeleton and cell adhesions== Robert M. McMeeking Department of Mechanical Engineering & Department of Materials University of California, Santa Barbara Abstract: The mechanical characteristics of eukaryotic cells arise largely due to the cytoskeleton, which assembles and dissociates in response to biochemical signals. Actin protein chains form the most significant structural element in the cytoskeleton, with myosin motor proteins endowing such stress-fibers with contractility. It is notable that tensile stress seems to stabilize stress fibers against depolymerization, so that there is an intimate coupling between the mechanics of the cytoskeleton and its biochemistry. In addition, stress-fibers interact with integrins that are the principal proteins in the adhesions transducting contractile force to the cell’s substrate or extracellular matrix. A model is presented for the processes of cytoskeleton stress-fiber formation, dissociation, contractility and their interaction with focal adhesions and mobile integrins. In the model, the polymerization of the stress fiber network is driven by a signal that rises quickly and then decays exponentially, causing the stress-fiber formation process to be self-limited. Subsequent depolymerization of the stress-fibers occurs spontaneously, unless it is inhibited by tension, which is generated by the constrained contractility of the actomyosin fibers. Consequently, the stress-fiber component of the cytoskeleton is most robust when it is able to generate tensile stress by contracting against an external constraint. An additional feature of the model is that the integrins are mechano-sensitive, and induce the growth of focal adhesion plaques when they are subject to tensile forces applied by stress-fibers. Examples characterizing the model are given, illustrating the mechanical features and behavior of eukaryotic cells. The model is able to simulate the cell’s differing behavior when attached to a stiff substrate compared to its activity on a compliant substrate. In addition, a cell’s response to a measurement protocol can be predicted with the model, such as when a bead is attached to the cell and displaced outwards by an external force, causing a reconfiguration of the cytoskeleton. Additional phenomena that are successfully simulated with use of the model include the orientation of the stress-fibers in cyclically strained cells, and their orientations when a cell is adhered to a shaped pattern of fibronectin.
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