Origin of Slow Stress Relaxation in the Cytoskeleton

Abstract

Dynamically crosslinked semiflexible biopolymers such as the actin cytoskeleton govern the mechanical behavior of living cells. Semiflexible biopolymers nonlinearly stiffen in response to mechanical loads, whereas the crosslinker dynamics allow for stress relaxation over time.Here we show, through rheology and theoretical modeling, that the combined nonlinearity in time and stress leads to an unexpectedly slow stress relaxation, similar to the dynamics of disordered systems close to the glass transition. Our work suggests that transient crosslinking combined with internal stress can explain prior reports of soft glassy rheology of cells, in which the shear modulus increases weakly with frequency.

Pre-stress of actin cortices is important for the viscoelastic response of living cells

“…Here, we can confirm that the source of the pre-stress responsible for low β values of the cortex of living cells are indeed myosin motors. In the absence of motor activity and therefore low pre-stress T 0 , β peaks around ≈ 0.5 − 0.6 as theoretically expected for reversibly cross-linked actin filaments [17] reflecting the broad spectrum of relaxation times from the unbinding and rebinding of various crosslinkers. However, at larger pre-stress the network indeed solidifies as predicted by Koenderink and coworkers [17].…”

“…We found that an increase in internal stress due to higher myosin activity is accompanied by a reduction of the power law coefficient (Fig. 3B) suggesting that cells with a stiffer, more [17] suggesting that the glassy dynamics of the cortex are a natural consequence of transient cross-links combined with intrinsic pre-stress. Here, we can confirm that the source of the pre-stress responsible for low β values of the cortex of living cells are indeed myosin motors.…”

“…The challenge lies in closing the gap between the rheological properties found for living cells and those of artificial actin cortices. A large leap forward was recently made when Mulla et al could show for reversibly linked artificial actin networks that transient cross-linking combined with internal stress can explain the low power law coefficients observed for cells [17]. Our goal is to obtain fundamental insight into the universal impact of internal stress on the viscoelastic properties of cells.…”

Weak catch bonds make strong networks

“…Weak catch bonds make strong networks Yuval Mulla 1,2 , Mario J Avellaneda , Antoine Roland 1 , Lucia Baldauf 1,3 , Sander J Tans 1,3 *, Gijsje H Koenderink 1,3 *…”

“…1. Catch bond 7 linkers in low tension areas rapidly unbind, increasing the pool of unbound linkers(2). As a result, there is increased binding everywhere in the network (3), at the expense of only the linkers in low tension areas.…”