Confining geometry determines the contracted shape of the active cytoskeleton

In a cell, the cytoskeleton serves a multitude of purposes; in particular, actin filaments and myosin molecular motors, which together form the actomyosin complex, are responsible for regulating the cell shape, motility as well as division, among many other functions. We are interested in uncovering the underlying physical principles that govern such varied orderings of the actomyosin complex.

While there have been numerous studies on how actomyosin self-organizes inside cell-sized confinements, the effect that the cell geometry itself has on self-organization is relatively less explored. Our work aims to address this point by confining cytoplasmic extracts containing the contractile actomyosin complex inside non-circular microwells. We find that in these microwells, the actomyosin network contracts to form a nucleus-like cluster, and that the cluster shape changes with the microwell shape.

To clarify the mechanism by which cell boundary shapes are transferred to internal clusters, we utilize an active gel theory of actomyosin and find that the numerical simulation agrees with the experiments, meaning our active gel model is sufficient to explain the geometric effect in cluster formation. Hence, this work reveals how the feedback due to the interplay between the asymmetric confinement and the actomyosin network leads to self-organization previously unseen in circular confinements.