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1. Myosin and Alpha-actinin regulation of stress fiber contractility under tensile stress

   https://doi.org/10.1038/s41598-023-35675-7  

   Ni, Haoran; Ni, Qin; Papoian, Garegin A; Trache, Andreea; Jiang, Yi (December 2023, Scientific Reports)

   Abstract Stress fibers are actomyosin bundles that regulate cellular mechanosensation and force transduction. Interacting with the extracellular matrix through focal adhesion complexes, stress fibers are highly dynamic structures regulated by myosin motors and crosslinking proteins. Under external mechanical stimuli such as tensile forces, the stress fiber remodels its architecture to adapt to external cues, displaying properties of viscoelastic materials. How the structural remodeling of stress fibers is related to the generation of contractile force is not well understood. In this work, we simulate mechanochemical dynamics and force generation of stress fibers using the molecular simulation platform MEDYAN. We model stress fiber as two connecting bipolar bundles attached at the ends to focal adhesion complexes. The simulated stress fibers generate contractile force that is regulated by myosin motors and$$\alpha$$ $\alpha$ -actinin crosslinkers. We find that stress fibers enhance contractility by reducing the distance between actin filaments to increase crosslinker binding, and this structural remodeling ability depends on the crosslinker turnover rate. Under tensile pulling force, the stress fiber shows an instantaneous increase of the contractile forces followed by a slow relaxation into a new steady state. While the new steady state contractility after pulling depends only on the overlap between actin bundles, the short-term contractility enhancement is sensitive to the tensile pulling distance. We further show that this mechanical response is also sensitive to the crosslinker turnover rate. Our results provide new insights into the stress fiber mechanics that have significant implications for understanding cellular adaptation to mechanical signaling. 

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2. Subcellular context-specific tuning of actomyosin ring contractility within a common cytoplasm

   https://doi.org/10.1101/2024.08.08.607200  

   Linehan, John B; Zampetaki, Alexandra; Werner, Michael E; Heck, Bryan; Maddox, Paul S; Fürthauer, Sebastian; Maddox, Amy S (August 2024, bioRxiv)

   Summary The non-muscle actomyosin cytoskeleton generates contractile force through the dynamic rearrangement of its constituent parts. Actomyosin rings are a specialization of the non-muscle actomyosin cytoskeleton that drive cell shape changes during division, wound healing, and other events. Contractile rings throughout phylogeny and in a range of cellular contexts are built from conserved components including non-muscle myosin II (NMMII), actin filaments (F-actin), and crosslinking proteins. However, it is unknown whether diverse actomyosin rings close via a single unifying mechanism. To explore how contractile forces are generated by actomyosin rings, we studied three instances of ring closure within the common cytoplasm of theC. elegansoogenic germline: mitotic cytokinesis of germline stem cells (GSCs), apoptosis of meiotic compartments, and cellularization of oocytes. We found that each ring type closed with unique kinetics, protein density and abundance dynamics. These measurements suggested that the mechanism of contractile force generation varied across the subcellular contexts. Next, we formulated a physical model that related the forces generated by filament-filament interactions to the material properties of these rings that dictate the kinetics of their closure. Using this framework, we related the density of conserved cytoskeletal proteins anillin and NMMII to the kinematics of ring closure. We fitted model rings to in situ measurements to estimate parameters that are currently experimentally inaccessible, such as the asymmetric distribution of protein along the length of F-actin, which occurs naturally due to differences in the dimensions of the crosslinker and NMMII filaments. Our work predicted that the role of NMMII varies across these ring types, due in part to its distribution along F-actin and motoring. Our model also predicted that the degree of contractility and the impact of ring material properties on contractility differs among ring types. 

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3. A tug of war between filament treadmilling and myosin induced contractility generates actin rings

   https://doi.org/10.7554/eLife.82658  

   Ni, Qin; Wagh, Kaustubh; Pathni, Aashli; Ni, Haoran; Vashisht, Vishavdeep; Upadhyaya, Arpita; Papoian, Garegin A (October 2022, eLife)

   In most eukaryotic cells, actin filaments assemble into a shell-like actin cortex under the plasma membrane, controlling cellular morphology, mechanics, and signaling. The actin cortex is highly polymorphic, adopting diverse forms such as the ring-like structures found in podosomes, axonal rings, and immune synapses. The biophysical principles that underlie the formation of actin rings and cortices remain unknown. Using a molecular simulation platform called MEDYAN, we discovered that varying the filament treadmilling rate and myosin concentration induces a finite size phase transition in actomyosin network structures. We found that actomyosin networks condense into clusters at low treadmilling rates or high myosin concentrations but form ring-like or cortex-like structures at high treadmilling rates and low myosin concentrations. This mechanism is supported by our corroborating experiments on live T cells, which exhibit ring-like actin networks upon activation by stimulatory antibody. Upon disruption of filament treadmilling or enhancement of myosin activity, the pre-existing actin rings are disrupted into actin clusters or collapse towards the network center respectively. Our analyses suggest that the ring-like actin structure is a preferred state of low mechanical energy, which is, importantly, only reachable at sufficiently high treadmilling rates. 

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4. Controlling pinned and coupled actomyosin contraction

   https://doi.org/10.1515/ot-2026-0008  

   Lee, Hyunjae; Clarke, James; Wong, Kyla; Glenn, Julia; Marne, Aniket; Miyahara, Yoichi; Alvarado, José (January 2026, Open Transport)

   Abstract Actin and myosin drive many instances of force generation, deformation, and shape change in cells, tissues, and organisms. In particular, cytoskeletal actomyosin is remarkable in its adaptive architecture, responding to a host of actin-binding proteins. Equally important, however, is actomyosin’s interaction with its mechanical environment. Actomyosin contractility and environmental properties, such as geometry and stiffness, are inherently coupled. To understand this coupling, novel experimental techniques are needed. Here we describe methods to spatially control the anchoring of reconstituted contractile actomyosin networks to two, opposing surfaces (“transverse anchoring”). This differs from current experimental approaches that produce tangential contractile forces, as transverse anchoring enables axial force generation and loading as the actomyosin gel pulls the boundaries perpendicularly. The two surfaces can be either rigid (“pinned contraction”), or one of the surfaces may be compliant (“coupled contraction”). We introduce compliance by manufacturing flexure hinges, and describe their calibration. Calibration permits a direct measurement of the contractile force and mechanical work that actomyosin exerts on the environment. The methods described here provide an avenue toward a more complete characterization of actomyosin’s role as an actuator, an essential property in its context of driving deformation and shape change in living organisms. 

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5. Chemo-Mechanical Factors That Limit Cellular Force Generation

   https://doi.org/10.3389/fphy.2022.831776  

   Vazquez-Hidalgo, Esteban; Farris, Carly M.; Rowat, Amy C.; Katira, Parag (February 2022, Frontiers in Physics)

   Cellular traction forces that are dependent on actin-myosin activity are necessary for numerous developmental and physiological processes. As traction force emerges as a promising cancer biomarker there is a growing need to understand force generation in response to chemical and mechanical cues. Our goal is to present a unified modeling framework that integrates actin-myosin activity, substrate stiffness, integrin bond type, and adhesion complex dynamics to explain how force develops under specific conditions. Our simulation results show that substrate stiffness and number of myosin motors contribute to the maximum actin-myosin forces that can be generated but do not solely control the force transmitted by the cells to the surface, i.e., the traction force. The kinetics of the bonds between the cell and the substrate plays an equally important role. Overall, we find that while the cell can generate large actin-myosin forces in individual stress fibers ( > 300 pN), the maximum force transmitted to the surface per cell-substrate attachment only reaches a fraction of these values (approx. 50 pN). Traction stress, the sum of forces transferred by all cell-substrate attachments in a unit area, is biphasic or sigmoidal with increasing substrate stiffness depending on the number of active myosin motors generating forces. Finally, we conclude that adhesions < 1  μm 2 generate widely variable traction forces and that impulse, the magnitude and duration of a force generating event, is a key limiting factor in traction stress. 

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