The actomyosin complex is a protein structure mainly composed of semi-flexible actin filaments and myosin motors. Myosin motors pull on the actin filaments, which makes the actomyosin complex contractile. In skeletal muscles, the actomyosin complex is highly ordered and stable, forming contractile units called sarcomeres, which are the basis for understanding muscle contraction. In non-muscle cells, the actomyosin complex appears as a highly disordered meshwork of actin filaments, myosin motors, and crosslinkers that are highly dynamic and undergo permanent assembly and disassembly. It composes the cell cortex, the principal regulator of the cell’s shape, enabling cell motility and force generation.
The cell cortex
The cell cortex, a thin layer beneath the plasma membrane, imparts mechanical resistance to animal cells and drives their shape changes. My previous work involved developing a homogenized theory based on constitutive equations for incompressible active gels. Through numerical implementation using the finite-element method, I studied osmotic shocks and cell division. To achieve a more comprehensive model of the cell surface, it is crucial to consider the mechanical contributions of the plasma membrane, which possesses surface tension and bending rigidity. In a recent study, I simplified the cortex-membrane system into a single composite model to show how cell protrusion and local contraction generates long-range membrane tension transmission. The future challenge lies in the development of a comprehensive model of the cortex-membrane structure encompassing both mechanical and regulatory elements (for instance, the membrane can carry regulators of actin nucleation and growth). Preserving the complexity of this interaction, especially considering the fractal structure of the cortex and membrane, presents a challenge in capturing the essential features within the homogenized model
References
2023
Cell protrusions and contractions generate long-range membrane tension propagation
Henry De Belly, Shannon Yan, Hudson Borja da Rocha, and 8 more authors
Membrane tension is thought to be a long-range integrator of cell physiology. Membrane tension has been proposed to enable cell polarity during migration through front-back coordination and long-range protrusion competition. These roles necessitate effective tension transmission across the cell. However, conflicting observations have left the field divided as to whether cell membranes support or resist tension propagation. This discrepancy likely originates from the use of exogenous forces that may not accurately mimic endogenous forces. We overcome this complication by leveraging optogenetics to directly control localized actin-based protrusions or actomyosin contractions while simultaneously monitoring the propagation of membrane tension using dual-trap optical tweezers. Surprisingly, actin-driven protrusions and actomyosin contractions both elicit rapid global membrane tension propagation, whereas forces applied to cell membranes alone do not. We present a simple unifying mechanical model in which mechanical forces that engage the actin cortex drive rapid, robust membrane tension propagation through long-range membrane flows.
2022
A viscous active shell theory of the cell cortex
Hudson Borja da Rocha, Jeremy Bleyer, and Hervé Turlier
Journal of the Mechanics and Physics of Solids, Mar 2022
The cell cortex is a thin layer beneath the plasma membrane that gives animal cells mechanical resistance and drives most of their shape changes, from migration, division to multicellular morphogenesis. It is mainly composed of actin filaments, actin binding proteins, and myosin molecular motors. Constantly stirred by myosin motors and under fast renewal, this material may be well described by viscous and contractile active-gel theories. Here, we assume that the cortex is a thin viscous shell with non-negligible curvature and use asymptotic expansions to find the leading-order equations describing its shape dynamics, starting from constitutive equations for an incompressible viscous active gel. Reducing the three-dimensional equations leads to a Koiter-like shell theory, where both resistance to stretching and bending rates are present. Constitutive equations are completed by a kinematical equation describing the evolution of the cortex thickness with turnover. We show that tension and moment resultants depend not only on the shell deformation rate and motor activity but also on the active turnover of the material, which may also exert either contractile or extensile stress. Using the finite-element method, we implement our theory numerically to study two biological examples of drastic cell shape changes: osmotic shocks and cell division. Our work provides a numerical implementation of thin active viscous layers and a generic theoretical framework to develop shell theories for slender active biological structures.
