Heterogeneities in stress buildup and relaxation in contractile actomyosin networks pinned at the boundaries

Actomyosin gels, consisting of f-actin, myosin motors, and other auxiliary proteins, undergo contraction in a variety of contexts in animal cells to generate motion. These gels can be reconstituted in vitro to recreate the contraction. In the in vivo context, questions still remain on the interactions between the gel and its boundary and their effect on contraction. To answer these questions, we prepare a two-dimensional actin gel whose parallel edges are pinned to the boundary. Upon the addition of myosin to the gel, the strain due to contraction is quantified using PIV from which stress and energy dissipation are estimated. Concurrently, we model the gel as a crosslinked network of semiflexible filaments and simulate the response of the network to active and discrete contraction events. In both experiments and simulations, the network depins from the boundary if the local stress exceeds a threshold. The gel depins only after some time has elapsed during which non-zero stresses develop and propagate through the network. The resulting evolution of the gel displays spatial heterogeneities and intermittency in strain. Overall, this study sheds light on how stresses develop and relax in a pinned actomyosin network undergoing contraction.