Physics-Biology interface seminar : Michael Murrell



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Mechanical Force Generation and Turnover in the Cell Cytoskeleton

Michael Murrell (University of Wisconsin, Madison)

Myosin II motors drive contractility of the cortical actin network, enabling shape change and cytoplasmic flows underlying diverse physiological processes ranging from cell division and migration to tissue morphogenesis. Yet, despite its importance, the mechanisms that describe contractility and the generation of mechanical forces within the cortex are not well understood. We recapitulate contractility in vitro, through the development of a minimal model of the cell actomyosin cortex by coupling a two-dimensional, cross-linked F-actin network decorated by myosin thick filaments to a model cell membrane. Myosin motors generate both compressive and tensile stresses on F-actin and consequently, induce large bending fluctuations. Over a large range of crosslinking, we show the extent of network contraction corresponds exactly to the extent of individual F-actin shortening via buckling. This demonstrates an essential role of buckling in facilitating local compression to enable mesoscale network contraction of up to 80% strain. Buckled F-actin at high curvatures are prone to severing and thus, compressive stresses mechanically coordinate contractility with F-actin severing, the initial step of F-actin turnover. Finally, the F-actin curvature acquired by myosin-induced stresses can be further constrained by adhesion of the network to a membrane, accelerating filament severing but inhibiting the long-range transmission of the stresses necessary for network contractility. Thus, the extent of membrane adhesion can regulate the coupling between network contraction and F-actin severing. These data demonstrate the essential role of the non-linear response of Factin to compressive stresses in potentiating both myosin-mediated contractility and filament dynamics.

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