Catch and slip bond dynamics of cytoskeletal cross-linkers in live cells – a new approach to understand principles of cytoskeletal material engineering
Final Report Abstract
Within animal cells, the protein actin assembles into a biopolymer network thereby weaving an essential material of life – the actin cytoskeleton. The actin cortex is a thin layer of interweaved actin filaments located beneath the plasma membrane of the cell. This cortical material layer acts as a dynamic shield for a cell, regulating cellular shape through various stages of its lifecycle. This unique structure must strike a delicate balance, being both flexible enough to enable changes in its form and composition and robust enough to prevent cell rupture or damage. Until before our project, the mechanisms governing the actin cortex's response to tension peaks remained a mystery. Within the actin cytoskeleton, actin filaments are cross-linked through transient binding of active and passive cross-linker proteins. This cross-linking substantially strengthens the actin cytoskeleton mechanically. Correspondingly, actin cross-linkers have been found to be essential for biological function. In this project, we developed a comprehensive investigation of cortical composition and dynamics upon induced changes of active and passive deformation-induced tension. As a biological model system, we used mitotic HeLa cells expressing fluorescently labelled cortical proteins of interest. Tension changes were either imposed i) through cell squeezing with the cantilever of an atomic force microscope inducing passive mechanical tension peaks or ii) through chemical manipulation of myosin motor activity inducing peaks in active tension. We focused on the mechanosensitive response of the scaffold proteins f-actin and myosin II as well as of the cross-linker proteins a-actinin-1, a-actinin-4 and filamins A and B. Our findings disclose that the cortex exhibits a twofold mechanosensitive response that includes i) a short-term direct mechanosensitivity of the actin cortex likely mediated by a catch bond behaviour of passive cross-linker proteins and ii) an indirect cortical mechanosensitivity that triggers long-term cortex reinforcement through increased actin polymerisation in particular after active tension peaks. Our findings provide an explanation as to how the actin cortex can stay structurally intact in the presence of high tension while maintaining fast molecular turnover and molecular adaptability. Finally, we confirmed key aspects of our results for the actin cytoskeleton of interphase cells. We conclude that when subjected to mechanical stress, the actin cortex enhances its structural integrity by generating additional actin filaments and cross-link components. We baptized this mechanism "molecular safety belt mechanism" for cells, since the actin cortex strengthens itself in response to mechanical tension similar to the safety belt in a car.
Publications
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Binding Dynamics of α-Actinin-4 in Dependence ofActin Cortex Tension. Biophysical Journal, 119(6), 1091-1107.
Hosseini, Kamran; Sbosny, Leon; Poser, Ina & Fischer-Friedrich, Elisabeth
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Twofold Mechanosensitivity Ensures Actin Cortex Reinforcement upon Peaks in Mechanical Tension. Advanced Physics Research, 2(12).
Ruffine, Valentin; Hartmann, Andreas; Frenzel, Annika; Schlierf, Michael & Fischer‐Friedrich, Elisabeth
