During cytokinesis’ onset the recruitment of multiple proteins is orchestrated by specific signaling events while reproducible cortical actin flows have been observed. This brings up the question of the contribution of the mechanics during this initial phase of ring assembly. In this study we investigate how myosin induced cortical flows impact actin network organization and filament alignment, thereby directly initiating stable ingression by mechanically remodeling the architecture.
We perform our analysis in the early C. elegans zygote. In this organism, a pseudocleavage furrow is formed during the phase of polarizing cortical flows, at a time when the biochemical triggers involved in cytokinesis are not yet present. In this system, we can thus uncouple the mechanical contributions to furrow formation from the biochemical effects involved in the later stages of cytokinesis. We observed that actin filaments align in converging and compressive flow. Hence, alignment and order arises in a disordered network in response to compression by flow and orthogonally to its direction. We show that this partially ingressing furrow is a direct consequence of actin filament alignment induced by the initial flow phase. Quantitative analysis of the dynamics of flows, filaments orientation and cell shape changes, together with theoretical modeling in the framework of a thin film of active and nematic fluid, allowed for a precise characterization of how deformation and shear in the cortical flow gives rise to ordering of actin filaments. Notably, such cortical dynamics and pattern formation arise in a very similar manner both during pseudocleavage and cytokinesis phases.
Taken together, our work paints a simple picture of how furrow ingression arises: the presence of a contractility gradient induces the formation of convergent flows, which in turn result in the local alignment of filaments. Filament alignment by compressive flow in turn drives asymmetric stress generation and furrow ingression. We thus identify the key physical principles that lead to the generation of an ingression for cytokinesis.
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