Biology subjects

Livne, G.

Publications and source records attributed to Livne, G..

3 recordsLinked to original sources

bioRxiv2025

Investigating active dynamics of contractile actomyosin gels with Micro Particle Image Velocimetry (Micro-PIV) analysis

Micro Particle Image Velocimetry (Micro-PIV), an advanced imaging technique, enables high-resolution velocity field measurements by tracking fluorescent tracers in microscopic environments. Here, we adapt conventional micro-PIV to study the rapidly contractile dynamics of active poroelastic gels. We demonstrate how frame-to-frame correlation improves signal-to-noise ratios and how the elastic nature of the solid phase of the gel can be included in the analysis. To do this, we average the gel displacement data under an axisymmetric assumption to extract radial strain profiles that reliably reveal local deformations of the gel. By analyzing gels of varying shapes, we further show that our method extends robustly to gels that are not completely circular or that do not displace symmetrically towards their geometric center. The analysis reveals common underlying features in the radial profiles of gel deformation. These strain profiles will allow the inference of the spatial and orientational distribution of motor-generated active stresses with appropriate constitutive models for the gel mechanics. Our findings emphasize the importance of tailored micro-PIV methodologies for analyzing complex fluids, particularly autonomously contracting poroelastic materials. This approach significantly enhances understanding of cytoskeletal dynamics and self-organization processes, with broad implications for cell motility, morphogenesis, and active matter physics.

biophysics↗

bioRxiv2024

Actin turnover and myosin contractility determine emergent Thickness Robustness of Cell-Mimicking Cortex

Cell shape deformation in eukaryotes, is primarily determined by the actin cortex, a thin network of actin filaments and myosin motors. This network is attached to the plasma membrane endowing cells with their mechanical stability and structural integrity. Since inside the cells hundreds of proteins are associated with the actin cortex, identifying the roles of the individual components and the mechanisms maintaining a functional cortex is highly challenging. Here, using a minimal set of components, we succeeded in recreating long-lasting dynamic treadmilling in an artificial actomyosin cortex on a bilayer membrane with cell-mimicking characteristics, both with and without myosin-II motors, replicating features observed in cells, such as myosin-induced increased cortical thickness and stress-dependent dynamics. We mechanistically reveal how treadmilling is regulated by actin network disassembly factors and myosin contractility, and the emergence of robustness of the cortical thickness to concentration fluctuations in cytoskeletal components. The robustness and responsiveness of the actin cortex which we found has a fundamental significance for the ability of Eukaryotic cells to maintain the mechanical integrity of their membrane, despite concentration fluctuation of the cytoskeleton components. Unlike complex biological cells, this system enables high-resolution and systematic studies of cortical dynamics under controlled conditions. It thus provides a cell-mimicking artificial system for designing interventions to modify the cellular cortex to improve our understanding of the basic mechanisms driving fundamental biological processes as well as for potential medical applications. SignificanceWe report a breakthrough in understanding the actin cortex, a key structure governing cell shape, stability, and motility. By reconstructing in-vitro a treadmilling actomyosin cortex on a lipid bilayer using minimal components, we replicate essential cellular features. This system reveals how cortical thickness is maintained despite concentration fluctuations, highlighting an actin turnover mechanism that buffers variability. The inclusion of myosin motors is found to modify the cortical dynamics, mimicking cellular properties such as the increased thickness and stress-dependent dynamics. Unlike complex living cells, this artificial system enables systematic studies of cortical behavior, with fine control over the components and high-resolution imaging. Our artificial system provides a platform for designing interventions to modify cortical dynamics, with broad implications for biology and medicine. One-Sentence SummaryIn-vitro recreation of a cell-like, membrane-bound cortical actin skeleton composed of a minimal set of building-blocks with continuous turnover and emergent robustness.

biophysics↗

bioRxiv2023

Cytoskeletal networks are adaptive active elastic filamentous materials that design their own shape in response to system geometry

Living systems adopt a diversity of curved and highly dynamic shapes. These diverse morphologies appear on many length-scales, from cells to tissues and organismal scales. The common driving force for these dynamic shape changes are contractile stresses generated by myosin motors in the cell cytoskeleton, an intrinsically active filamentous material, while converting chemical energy into mechanical work. A good understanding of how contractile stresses in the cytoskeleton arise into different 3D shapes and what are the selection rules that determine their final configurations still lacks. Aiming to identify the selection rules governing the shapes formed by contractile forces in living systems, we recreate the actomyosin cytoskeleton in-vitro, with precisely controlled composition and initial geometry. A set of actomyosin gel discs, intrinsically identical but of variable initial geometry, spontaneously self-organize into a family of 3D shapes. This process occurs through robust distinct dynamical pathways, without specific pre-programming and additional regulation. Shape selection is encoded in the initial disc radius to thickness aspect ratio, and thus scale-free. This may indicate a universal process of shape selection, that works across scales, from cells to tissues and organelles. Finally, our results suggest that, while the dynamical pathways may depend on the detailed interactions of the different microscopic components within the gel, the final selected shapes obey the general theory of elastic deformations of thin sheets. Altogether, these results provide novel insights on the mechanically induced spontaneous shape transitions in active contractile matter and uncover new mechanisms that drive shape selections in living systems across scales. Significance statementLiving systems adopt a diversity of curved and highly dynamic shapes. These diverse morphologies appear on many length-scales, from cells to organismal scales, and are commonly driven by contractile stresses generated by myosin motors in the cell cytoskeleton. By recreating the actomyosin cytoskeleton in-vitro, with precisely controlled composition and initial geometry, we identify the shape selection rules that determine the final adopted configuration. Specifically, we find that shape selection is scale-free, which may indicate a universal process of shape selection, that works across scales, from cells to tissues and organelles. Altogether, our results provide novel insights on the mechanically induced spontaneous shape transitions in contractile active matter and uncover new mechanisms that drive shape selections in living systems.

biophysics↗