Local Pulses of Rhoa Activation Assemble Polarized Network Architectures for Efficient Actomyosin Contractility

Abstract

Spatiotemporal patterning of actomyosin contractility plays a key role in cell and tissue morphogenesis during early development. In embryonic cells, actomyosin arrays are highly dynamic structures that remodel on a time scale of 10s of seconds, through a combination of local actomyosin turnover and rapid spatial redistribution of filaments and motors caused by myosin activity or actin polymerization. Because of these dynamic and active properties, contractility is complex and intrinsically self-organizing.We used the C. elegans early embryo to understand how cells pattern force generation through local modulation of self-organized contractility, focusing on pulsed contractility in the C. elegans embryo.We combined two-color fluorescence imaging, live single-molecule imaging, particle tracking, image analysis, and numerical modeling to tease apart the mechanisms of pulse initiation and termination. Our results demonstrate that the mechanical component (advection) played little role in pulse initiation or termination, and that the process was mostly governed by Actin and Myosin turnover. In our system, autocatalytic RhoA activation/recruitment is responsible for pulse initiation, while the delayed recruitment of a RhoA inactivator (RGA-3/4) onto Actin filaments drives pulse termination. Using single-molecule imaging, we further show that this mode of actin assembly drives the formation of structural units displaying a characteristically polarized architecture with (1) a gradient of myosin accumulation, myosin being recruited at the center of the pulses and (2) a transient anisotropic organization of the actin network, where filaments elongate from the instide of the pulse, barbed ends pointing outwards, as visualized by displacement of the actin-barbed-end-tracking formins that processively elongate actin filaments.Based on these results, we propose here that pulsed contractions represent a mode of actomyosin assembly which structures the actomyosin network in functional modules to drive efficient cell contractility.