Self-organization of stress patterns drives state transitions in actin cortices

The Schmidt group (project A02) developed model actomyosin cortices in water-in-oil emulsion droplets containing Xenopus egg extract, which, in the presence of rapid turnover, display distinct steady states, each distinguished by characteristic order and dynamics as a function of network connectivity. The different states arise from a subtle interaction between mechanical percolation of the actin network and myosin-generated stresses. Remarkably, myosin motors generate actin architectures, which in turn force the emergence of ordered stress patterns. Reminiscent of second order phase transitions, the emergence of order is accompanied by a critical regime characterized by strongly enhanced strain fluctuations. The striking dynamics in the critical regime were revealed using fluorescent single-walled carbon nanotubes (SWCNTs) as novel probes of cortical dynamics (Tan et al., Sci. Adv. (2018) 4:eaar2847).



Signatures of distinct steady states. Network percolation can be induced by increasing density of cross-linkers (α-actinin). Three distinct steady states can be identified based on qualitative differences in collective network dynamics imaged in the bottom cortical layer in the emulsion droplets. (A) Low cross-linking; (B) intermediate cross-linking; (C) high cross-linking. (A-C) Top left: Fluorescence images of inserted SWCNT probes. Top right: Confocal images of the actin distribution. Bottom: Individual SWCNTs were tracked in movies of 200 s total length with 2000 frames. Tracks of individual SWCNTs color-coded for progression in time. (Scale bar = 20 μm).