Gurmessa, B. J. | bioRxiv Science

bioRxiv2025

Motor-driven modulation of actin network mechanics across linear and nonlinear regimes

Cytoskeletal networks enable cells to dynamically regulate their mechanical properties in response to internal forces and external cues. Here, we investigate how motor activity influences the structure and mechanics of actomyosin networks reconstituted in vitro from filamentous actin, myosin II minifilaments, and transient -actinin cross-linkers. By varying the myosin-to-actin molar ratio (RMA), we observe a transition from isotropic actin meshes to contractile, coarsened architectures marked by bundled filaments and increasing spatial correlation lengths ({xi}z,{xi} t). Optical tweezers microrheology reveals a nonmonotonic mechanical response: at low RMA, networks fluidize, with reductions in the plateau modulus (G0), zero-shear viscosity (0), and fast relaxation timescales ([Formula],{tau} 1). At higher motor levels, the networks stiffen and retain internal stress, reflecting contractile reinforcement. Notably,{tau} 1 exhibits a minimum when plotted against{xi} z, suggesting that intermediate levels of coarsening facilitate efficient local stress dissipation. These results identify distinct mechanical regimes governed by motor-induced remodeling and highlight a structural basis for the dual roles of myosin in fluidization and reinforcement.

biophysics↗

bioRxiv2024

Switchable microscale stress response of actin-vimentin composites emerges from scale-dependent interactions

The mechanical properties of the mammalian cell regulate many cellular functions and are largely dictated by the cytoskeleton, a composite network of protein filaments, including actin, microtubules, and intermediate filaments. Interactions between these distinct filaments give rise to emergent mechanical properties that are difficult to generate synthetically, and recent studies have made great strides in advancing our understanding of the mechanical interplay between actin and microtubule filaments. While intermediate filaments play critical roles in the stress response of cells, their effect on the rheological properties of the composite cytoskeleton remains poorly understood. Here, we use optical tweezers microrheology to measure the linear viscoelastic properties and nonlinear stress response of composites of actin and vimentin with varying molar ratios of actin to vimentin. We reveal a surprising, nearly opposite effect of actin-vimentin network mechanics compared to single-component networks in the linear versus nonlinear regimes. Namely, the linear elastic plateau modulus and zero-shear viscosity are markedly reduced in composites compared to single-component networks of actin or vimentin, whereas the initial response force and stiffness are maximized in composites versus single-component networks in the nonlinear regime. While these emergent trends are indicative of distinct interactions between actin and vimentin, nonlinear stiffening and longtime stress response appear to both be dictated primarily by actin, at odds with previous bulk rheology studies. We demonstrate that these complex, scale-dependent effects arise from the varied contributions of network density, filament stiffness, non-specific interactions, and poroelasticity to the mechanical response at different spatiotemporal scales. Cells may harness this complex behavior to facilitate distinct stress responses at different scales and in response to different stimuli to allow for their hallmark multifunctionality.

biophysics↗