Pulsatile contractions and pattern formation in excitable actomyosin cortex

Abstract

The actin cortex is an active adaptive material, embedded with complex regulatory networks that can sense, generate and transmit mechanical forces. The cortex can exhibit a wide range of dynamic behaviours, from generating pulsatory contractions and traveling waves to forming highly organised structures such as ordered fibers, contractile rings and networks that must adapt to the local cellular environment. Despite the progress in characterising the biochemical and mechanical components of the actin cortex, our quantitative understanding of the emergent dynamics of this mechanochemical system is limited. Here we develop a mathematical model for the RhoA signalling network, the upstream regulator for actomyosin assembly and contractility, coupled to an active polymer gel, to investigate how the interplay between chemical signalling and mechanical forces govern the propagation of contractile stresses and patterns in the cortex. We demonstrate that mechanical feedback in the excitable RhoA system, through dilution and concentration of chemicals, acts to destabilise homogeneous states and robustly generate pulsatile contractions. While moderate active stresses generate spatial propagation of contraction pulses, higher active stresses assemble localised contractile structures. Moreover, mechanochemical feedback induces memory in the active gel, enabling long-range propagation of transient local signals.

Size-dependent transition from steady contraction to waves in actomyosin networks with turnover

“…While in some cases, the function of the pulsatile dynamics is unclear, in other cases the pulsatile dynamics are essential, enabling large-scale force generation while maintaining tissue integrity 8, 9 , and facilitating cellular functions such as active transport and motility 6 . Typically, these periodic or aperiodic waves emerge from complex nonlinear reactions in coupled mechano-chemical systems, involving regulators of the actomyosin machinery such as Rho GTPases 10, 11, 36, 39, 40 . Our work shows that waves can also emerge in essentially mechanical systems, with turnover but without complex biochemical regulation.…”

Mechanochemical Principles of Spatial and Temporal Patterns in Cells and Tissues

“…For example, an active elastomer model with strain-induced unbinding of MyoII can account for experimentally observed MyoII pulses in Drosophila (Banerjee et al 2017). These models can also be extended to include the associated Rho pulses (Staddon et al 2021). Theoretical work can help identify the importance of mechanics in such systems.…”