Evolving roles and dynamics for catch and slip bonds during adhesion cluster maturation.

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Focal adhesions are the loci of cellular adhesion to the extracellular matrix. At these sites, various integrins forge connections between the intracellular cytoskeleton and the outside world; large patches of multiple types of integrins together grip hold of collagen, fibronectin, and other extracellular matrix components. A single focal adhesion will likely contain bonds whose lifetime increases with applied load (catch bonds), and bonds whose lifetime decreases with applied load (slip bonds). Prior work suggests that the combination of different types of integrins is essential for focal adhesion stability and mechanosensory functionality. In the present work, we investigate numerically the interplay between two distinct types of bonds, and we ask how the presence of slip bonds, in the same focal integrin cluster, augments the collective behavior of the catch bonds. We show that mixing these two components may increase the low-force mechanical integrity that may be lacking in pure-catch adhesions, while preserving the potential to strengthen the entire adhesion when a force is applied. We investigate the spatial distribution in mixed-integrin focal adhesions, and we show that the differential response to loading leads, via an excluded volume interaction, to a dependence of the individual integrin diffusivities on the applied load, an effect that has been reported in experiments.