Nonlinear contractile response of actomyosin active gels to control signals

TL;DR

This study investigates the nonlinear contractile response of reconstituted actomyosin gels to control signals, revealing a scale-dependent transfer function and nonlinear dynamics crucial for understanding cellular mechanotransduction.

Contribution

It provides the first quantitative characterization of the nonlinear impulse response of actomyosin active gels to control signals, highlighting a density-dependent mechanism.

Findings

Nonlinear impulse response quantified via transfer function.

Scaling relation $rac{ riangle ext{strain}}{ riangle ext{energy}} \\sim g^{-0.3}$.

Decreased energy-to-strain conversion efficiency with increased activity.

Abstract

Biological systems tightly regulate their physiological state using control signals. This includes the actomyosin cytoskeleton, a contractile active gel that consumes chemical free energy to drive many examples of cellular mechanical behavior. Upstream regulatory pathways activate or inhibit actomyosin activity. However, the contractile response of the actomyosin cytoskeleton to control signals remains poorly characterized. Here we employ reconstituted actomyosin active gels and subject them to step and pulsatile activation inputs. We find evidence for a nonlinear impulse response, which we quantify via a transfer function $\delta \varepsilon / \delta g$ that relates input free-energy pulses $\delta g$ to output strain pulses $\delta \varepsilon$. We find a scaling relation $\delta \varepsilon / \delta g \sim g^{-0.3}$. The negative sign of the exponent represents a decreased effectiveness of a contracting gel in converting energy to strain. We ascribe nonlinearity in our system to a density-dependent mechanism, which contrasts strain-stiffening nonlinear responses to external stresses. Contractile response to control signals is an essential step toward understanding how information from mechanical signaling processes flow through actomyosin networks in living, and likely also synthetic, cells.