Tuning cellular contractility by assembly of subcellular actomyosin structures

Cells require mechanical forces to perform dynamic processes, such as cell migration. Mechanical output of the cell is tuned by the actomyosin cytoskeleton, where myosin motors pull on F-actin networks in cells to generate contractile forces. However, it is unclear how forces at the cellular scale can be regulated by the subcellular arrangement of actomyosin structures. To this end, we aim to quantitatively characterize how actomyosin assemblies remodel in cells leading to varying levels of contractility. Using quantitative microscopy, we observe that myosin appears as punctate structures in cells, suggesting that myosin locally accumulates in clusters. The number of myosin in these clusters shows a heavy-tailed distribution, with larger clusters located on thick F-actin bundles. In more contractile cells, for example by activating RhoA, both the number and the size of myosin clusters increase. This reflects an increase in the density of myosin motors at the subcellular scale, which correlates well with the measured mechanical energy density produced by the cell. With a simple simulation, we further show that the distribution of myosin clusters can be altered by the F-actin network. Our results can provide insight on how actomyosin assemblies tune cellular contractility.