Biology subjects

Gardel, M. L.

Publications and source records attributed to Gardel, M. L..

At least 19 recordsLinked to original sources

bioRxiv2023

Cracked actin filaments as mechanosensitive receptors

Actin filament networks are exposed to mechanical stimuli, but the effect of strain on actin filament structure has not been well-established in molecular detail. This is a critical gap in understanding because the activity of a variety of actin-binding proteins have recently been determined to be altered by actin filament strain. We therefore used all-atom molecular dynamics simulations to apply tensile strains to actin filaments and find that changes in actin subunit organization are minimal in mechanically strained, but intact, actin filaments. However, a conformational change disrupts the critical D-loop to W-loop connection between longitudinal neighboring subunits, which leads to a metastable cracked conformation of the actin filament, whereby one protofilament is broken prior to filament severing. We propose that the metastable crack presents a force-activated binding site for actin regulatory factors that specifically associate with strained actin filaments. Through protein-protein docking simulations, we find that 43 evolutionarily-diverse members of the dual zinc finger containing LIM domain family, which localize to mechanically strained actin filaments, recognize two binding sites exposed at the cracked interface. Furthermore, through its interactions with the crack, LIM domains increase the length of time damaged filaments remain stable. Our findings propose a new molecular model for mechanosensitive binding to actin filaments. SIGNIFICANCE STATEMENTCells continually experience mechanical strain, which has been observed to alter the interactions between actin filaments and mechanosensitive actin-binding proteins in recent experimental studies. However, the structural basis of this mechanosensitivity is not well understood. We used molecular dynamics and protein-protein docking simulations to investigate how tension alters the actin filament binding surface and interactions with associated proteins. We identified a novel metastable cracked conformation of the actin filament, whereby one protofilament breaks before the other, presenting a unique strain-induced binding surface. Mechanosensitive LIM domain actin-binding proteins can then preferentially bind the cracked interface, and this association stabilizes damaged actin filaments.

biophysics↗

bioRxiv2023

Direct detection of deformation modes on varying length scales in active biopolymer networks

Correlated flows and forces that emerge from active matter orchestrate complex processes such as shape regulation and deformations in biological cells and tissues. The active materials central to cellular mechanics are cytoskeletal networks, where molecular motor activity drives deformations and remodeling. Here, we investigate deformation modes in contractile actin networks driven by the molecular motor myosin II through quantitative fluorescence microscopy. We examine the deformation anisotropy at different length scales in networks of sparsely cross-linked and bundled actin. In sparsely cross-linked networks, we find myosin-dependent biaxial buckling modes across length scales. Interestingly, both long and short-wavelength buckling may contribute to network contractility. In cross-linked bundled networks, uniaxial contraction predominates on long length scales, while the uniaxial or biaxial nature of the deformation depends on bundle microstructure at shorter length scales. The anisotropy of deformations may provide insight to the mechanical origins of contractility in actin networks and regulation of collective behavior in a variety of active materials.

biophysics↗

bioRxiv2022

Confluence and tight junction dependence of volume regulation in epithelial tissue

Epithelial cell volume regulation is a key component to tissue stability and dynamics. In particular, how cells respond to osmotic stresses is of significant physiological interest in kidney epithelial tissue. For individual mammalian cells, it is well established that Na-K-2Cl cotransporter (NKCC) channels mediate cell volume homeostasis in response to hyperosmotic stress. However, whether mature epithelium respond similarly is not well known. Here we show that while small colonies of MDCK epithelial cells behave similarly to single cells and exhibit volume homeostasis that is dependent on the NKCC channel function, mature epithelial tissue does not. Instead, the cell volume decreases by 33% when confluent monolayers or acini formed from MDCK are subjected to hyperosmotic stress. We show that the tight junction protein, ZO-1, and Rho-associated kinase (ROCK) are essential for osmotic regulation of cell volume in mature epithelium. Since these both are known to be essential for tight junction assembly, this strongly suggest a role for tight junctions in changing volume response in mature epithelium. Thus, tight junctions act either directly or indirectly in osmotic pressure response of epithelial tissue to suppress volume homeostasis common to isolated epithelial cells.

biophysics↗

bioRxiv2021

TES-1/Tes protects junctional actin networks under tension from self-injury during epidermal morphogenesis in the C. elegans embryo

During embryonic morphogenesis, the integrity of epithelial tissues depends on the ability of cells in tissue sheets to undergo rapid changes in cell shape while preventing self-injury to junctional actin networks. LIM domain-containing repeat (LCR) proteins are recruited to sites of strained actin filaments in cultured cells [1-3], and are therefore promising candidates for mediating self-healing of actin networks, but whether they play similar roles in living organisms has not been determined. Here, we establish roles for Caenorhabditis elegans TES-1/Tes, an actin-binding LCR protein present at apical junctions, during epithelial morphogenesis. TES-1::GFP is recruited to apical junctions during embryonic elongation, when junctions are under tension; in embryos in which stochastic failure of cell elongation occurs, TES-1 is only strongly recruited to junctions in cells that successfully elongate, and recruitment is severely compromised in genetic backgrounds in which cell shape changes do not successfully occur. tes-1 mutant embryos display junctional F-actin defects, and loss of TES-1 strongly enhances tension-dependent injury of junctional actin networks in hypomorphic mutant backgrounds for CCC components, suggesting that TES-1 helps to prevent self-injury of junctional actin networks during rapid cell shape change. Consistent with such role, a fragment of TES-1 containing its LIM domains localizes to stress fiber strain sites (SFSS) in cultured vertebrate cells. Together, these data establish TES-1 as a tension-sensitive stabilizer of the junctional actin cytoskeleton during embryonic morphogenesis.

developmental biology↗

bioRxiv2021

Actin filament alignment causes mechanical hysteresis in cross-linked networks

Cells dynamically control their material properties through remodeling of the actin cytoskeleton, an assembly of cross-linked networks and bundles formed from the biopolymer actin. We recently found that cross-linked networks of actin filaments reconstituted in vitro can exhibit adaptive behavior and thus serve as a model system to understand the underlying mechanisms of mechanical adaptation of the cytoskeleton. In these networks, training, in the form of applied shear stress, can induce asymmetry in the nonlinear elasticity. Here, we explore control over this mechanical hysteresis by tuning the concentration and mechanical properties of cross-linking proteins in both experimental and simulated networks. We find that this effect depends on two conditions: the initial network must exhibit nonlinear strain stiffening, and filaments in the network must be able to reorient during training. Hysteresis depends strongly and non-monotonically on cross-linker concentration, with a peak at moderate concentrations. In contrast, at low concentrations, where the network does not strain stiffen, or at high concentrations, where filaments are less able to rearrange, there is little response to training. Additionally, we investigate the effect of changing cross-linker properties and find that longer or more flexible cross-linkers enhance hysteresis. Remarkably plotting hysteresis against alignment after training yields a single curve regardless of the physical properties or concentration of the cross-linkers.

biophysics↗

bioRxiv2020

Actin bundle architecture and mechanics regulate myosin II force generation

The actin cytoskeleton is a soft, structural material that underlies biological processes such as cell division, motility, and cargo transport. The cross-linked actin filaments self-organize into a myriad of architectures, from disordered meshworks to ordered bundles, which are hypothesized to control the actomyosin force generation that regulates cell migration, shape, and adhesion. Here, we use fluorescence microscopy and simulations to investigate how actin bundle architectures with varying polarity, spacing, and rigidity impact myosin II dynamics and force generation. Microscopy reveals that mixed polarity bundles formed by rigid cross-linkers support slow, bidirectional myosin II filament motion, punctuated by periods of stalled motion. Simulations reveal that these locations of stalled myosin motion correspond to sustained, high forces in regions of balanced actin filament polarity. By contrast, mixed polarity bundles formed by compliant, large cross-linkers support fast, bidirectional motion with no traps. Simulations indicate that trap duration is directly related to force magnitude, and that the observed increased velocity corresponds to lower forces resulting from both the increased bundle compliance and filament spacing. Our results indicate that the properties of actin structures regulate the dynamics and magnitude of myosin II forces, highlighting the importance of architecture and mechanics in regulating forces in biological materials.

biophysics↗

bioRxiv2020

Evolutionary diverse LIM domain-containing proteins bind stressed actin filaments through a conserved mechanism

The actin cytoskeleton assembles into diverse load-bearing networks including stress fibers, muscle sarcomeres, and the cytokinetic ring to both generate and sense mechanical forces. The LIM (Lin11, Isl-1 & Mec-3) domain family is functionally diverse, but most members can associate with the actin cytoskeleton with apparent force-sensitivity. Zyxin rapidly localizes via its LIM domains to failing stress fibers in cells, known as strain sites, to initiate stress fiber repair and maintain mechanical homeostasis. The mechanism by which these LIM domains associate with stress fiber strain sites is not known. Additionally, it is unknown how widespread strain sensing is within the LIM protein family. We observe that many, but not all, LIM domains from functionally diverse proteins localize to spontaneous or induced stress fiber strain sites in mammalian cells. Additionally, the LIM domain region from the fission yeast protein paxillin like 1 (Pxl1) also localizes to stress fiber strain sites in mammalian cells, suggesting that the strain sensing mechanism is ancient and highly conserved. Sequence analysis and mutagenesis of the LIM domain region of zyxin indicate a requirement of tandem LIM domains, which contribute additively, for sensing stress fiber strain sites. In vitro, purified LIM domains from mammalian zyxin and fission yeast Pxl1 bind to mechanically stressed F-actin networks but do not associate with relaxed actin filaments. We propose that tandem LIM domains recognize an F-actin conformation that is rare in the relaxed state but is enriched in the presence of mechanical stress.

cell biology↗

bioRxiv2019

Tuning shape and internal structure of protein droplets via biopolymer filaments

Macromolecules can phase separate to form liquid condensates, which are emerging as critical compartments in fields as diverse as intracellular organization and soft materials design. A myriad of macromolecules, including the protein FUS, form condensates which behave as isotropic liquids. Here, we investigate the influence of filament dopants on the material properties of protein liquids. We find that the short, biopolymer filaments of actin spontaneously partition into FUS droplets to form composite liquid droplets. As the concentration of the filament dopants increases, the coalescence time decreases, indicating that the dopants control the relative surface tension to viscosity. The droplet shape is tunable ranging from spherical to tactoid as the filament length or concentration is increased. We find that the tactoids are well described by a model of a bipolar liquid crystal droplet, where nematic order from the anisotropic actin filaments competes with isotropic interfacial energy from the FUS, controlling droplet shape in a size-dependent manner. Our results demonstrate a versatile approach to construct tunable, anisotropic macromolecular liquids.

biophysics↗

bioRxiv2019

Cell division Rate Controls Cell Shape Remodeling in Epithelia

Epithelia have distinct cellular architectures, which are established in development, re-established after wounding, and maintained during tissue homeostasis despite cell turnover and mechanical perturbations. In turn, cell shape also controls tissue function as a regulator of cell differentiation, proliferation, and motility. Here we investigate cell shape changes in a model epithelial monolayer. After the onset of confluence, cells continue to proliferate and change shape over time, eventually leading to a final architecture characterized by arrested motion and more regular cell shapes. Such monolayer remodeling is robust, with qualitatively similar evolution in cell shape and dynamics observed across disparate perturbations. Here we quantify differences in monolayer remodeling guided by the active vertex model to identify underlying order parameters controlling epithelial architecture. When monolayers are formed atop extracellular matrix with varied stiffness, we find the cell density at which motion arrests varies significantly but the cell shape remains constant, consistent with the onset of tissue rigidity. In contrast, pharmacological perturbations can significantly alter the cell shape at which tissue dynamics is arrested, consistent with varied amounts of active stress within the tissue. Across all experimental conditions the final cell shape is well correlated to the cell proliferation rate, and cell cycle inhibition immediately arrests cell motility. Finally, we demonstrate cell-cycle variation in junctional tension as a source of active stress within the monolayer. Thus, the architecture and mechanics of epithelial tissue can arise from an interplay between cell mechanics and stresses arising from cell cycle dynamics. SIGNIFICANCE STATEMENTThe morphology of biological tissue is determined by the shape and density of constituent cells. Here we measure the dynamics of cells in model epithelial tissues to study the evolution of their shape and density over time. Guided by a mathematical model, we find that cell shape is controlled by rigidity and active stresses within the tissue. We then show that cell cycle dynamics are the source of active stress that drives epithelial remodeling.

biophysics↗

bioRxiv2019

Mechanosensitive junction remodelling promotes robust epithelial morphogenesis

Morphogenesis of epithelial tissues requires tight spatiotemporal coordination of cell shape changes. In vivo, many tissue-scale shape changes are driven by pulsatile contractions of intercellular junctions, which are rectified to produce irreversible deformations. The functional role of this pulsatory ratchet and its mechanistic basis remain unknown. Here we combine theory and biophysical experiments to show that mechanosensitive tension remodelling of epithelial cell junctions promotes robust epithelial shape changes via ratcheting. Using optogenetic control of actomyosin contractility, we find that epithelial junctions show elastic behaviour under low contractile stress, returning to their original lengths after contraction, but undergo irreversible deformation under higher magnitudes of contractile stress. Existing vertex-based models for the epithelium are unable to capture these results, with cell junctions displaying purely elastic or fluid-like behaviours, depending on the choice of model parameters. To describe the experimental results, we propose a modified vertex model with two essential ingredients for junction mechanics: thresholded tension remodelling and continuous strain relaxation. First, a critical strain threshold for tension remodelling triggers irreversible junction length changes for sufficiently strong contractions, making the system robust to small fluctuations in contractile activity. Second, continuous strain relaxation allows for mechanical memory removal, enabling frequency-dependent modulation of cell shape changes via mechanical ratcheting. Taken together, the combination of mechanosensitive tension remodelling and junctional strain relaxation provides a robust mechanism for large-scale morphogenesis.

biophysics↗

bioRxiv2019

RhoA mediates epithelial cell shape changes via mechanosensitive endocytosis

Morphogenetic movements require tight spatiotemporal control over cell-cell junction lengths. Contractile forces, acting at adherens junctions, alter cell-cell contact lengths in a cyclic fashion as a mechanical ratchet. Pulsatile RhoA activity is thought to drive ratcheting through acute periods of junction contraction followed by stabilization. Currently, we lack a mechanistic understanding of if and how RhoA activity governs junction length and subsequent cell shape within epithelia. In this study we use optogenetics to exogenously control RhoA activity in model Caco-2 epithelium. We find that at short timescales, RhoA activation drives reversible junction contraction. Sustained RhoA activity drives irreversible junction shortening but the amount of shortening saturates for a single pulse. To capture these data, we develop a vertex model modified to include strain-dependent junction length and tension remodeling. We find that, to account for experimental data, tension remodeling requires a strain-dependent threshold. Our model predicts that temporal structuring of RhoA activity allows for subsequent tension remodeling events to overcome the limited shortening within a single pulse and this is confirmed by our experimental data. We find that RhoA-mediated junction remodeling requires activities of formin and dynamin, indicating the closely inter-connected activities of contractility, E-cadherin clustering, and endocytosis. Junction length is therefore regulated by the coordinated action of RhoA-mediated contractility, membrane trafficking, and adhesion receptor remodeling. Altogether these data provide insights into the underlying molecular and biophysical mechanisms of RhoA-mediated regulation of epithelial cell shape.

cell biology↗

bioRxiv2018

Self-organizing motors divide active liquid droplets

The cytoskeleton is a collection of protein assemblies that dynamically impose spatial structure in cells and coordinate processes such as cell division and mechanical regulation. Biopolymer filaments, cross-linking proteins, and enzymatically active motor proteins collectively self-organize into various precise cytoskeletal assemblies critical for specific biological functions. An outstanding question is how the precise spatial organization arises from the component macromolecules. We develop a new system to investigate simple physical mechanisms of self-organization in biological assemblies. Using a minimal set of purified proteins, we create droplets of cross-linked biopolymer filaments. Through the addition of enzymatically active motor proteins we construct composite assemblies, evocative of cellular structures such as spindles, where the inherent anisotropy drives motor self-organization and droplet deformation. These results suggest that simple physical principles underlie the self-organization in complex biological assemblies and inform bio-inspired materials design.

biophysics↗

bioRxiv2018

Force localization modes in dynamic epithelial colonies

Collective cell behaviors, including tissue remodeling, morphogenesis and cancer metastasis rely on dynamics between cells, their neighbors and the extracellular matrix. The lack of quantitative models precludes understanding of how cell-cell and cell-matrix interactions regulate tissue-scale force transmission to guide morphogenic processes. We integrate biophysical measurements on model epithelial tissues and computational modelling to explore how cell-level dynamics alter mechanical stress organization at multicellular scales. We show that traction stress distribution in epithelial colonies can vary widely for identical geometries. For colonies with peripheral localization of traction stresses, we recapitulate previously described mechanical behavior of cohesive tissues with a continuum model. By contrast, highly motile cells within colonies produce traction stresses that fluctuate in space and time. To predict the traction force dynamics, we introduce an Active Adherent Vertex Model (AAVM) for epithelial monolayers. AAVM predicts that increased cellular motility and reduced intercellular mechanical coupling localize traction stresses in the colony interior, in agreement with our experimental data. Furthermore, the model captures a wide spectrum of localized stress production modes that arise from individual cell activities including cell division, rotation, and polarized migration. This approach provides a robust quantitative framework to study how cell-scale dynamics influence force transmission in epithelial tissues.

biophysics↗

bioRxiv2018

Tuning molecular motor transport through cytoskeletal filament network organization

The interaction of motor proteins with intracellular filaments is required for transport processes and force generation in cells. Within a cell, crosslinking proteins organize cytoskeletal filaments both temporally and spatially to create dynamic, and structurally diverse networks. The architecture of these networks changes both the mechanics as well as the transport dynamics; however, the effects on transport are less well understood. Here, we compare the transport dynamics of myosin II motor proteins moving on model cytoskeletal networks created by common crosslinking proteins. We observe that motor dynamics change predictably based on the microstructure of the underlying networks and discuss how this can be utilized by cells to achieve specific transport goals.

biophysics↗

bioRxiv2017

The lamellipodium is a myosin independent mechanosensor

The ability of adherent cells to sense changes in the mechanical properties of their extracellular environments is critical to numerous aspects of their physiology. It has been well documented that cell attachment and spreading are sensitive to substrate stiffness. Here we demonstrate that this behavior is actually biphasic, with a transition that occurs around a Youngs modulus of [~]7 kPa. Furthermore, we demonstrate that, contrary to established assumptions, this property is independent of myosin II activity. Rather, we find that cell spreading on soft substrates is inhibited due to reduced nascent adhesion formation within the lamellipodium. Cells on soft substrates display normal leading edge protrusion activity, but these protrusions are not stabilized due to impaired adhesion assembly. Enhancing integrin-ECM affinity through addition of Mn2+ recovers nascent adhesion assembly and cell spreading on soft substrates. Using a computational model to simulate nascent adhesion assembly, we find that biophysical properties of the integrin-ECM bond are optimized to stabilize interactions above a threshold matrix stiffness that is consistent with the experimentally observations. Together these results suggest that myosin II-independent forces in the lamellipodium are responsible for mechanosensation by regulating new adhesion assembly, which in turn, directly controls cell spreading. This myosin II-independent mechanism of substrate stiffness sensing could potentially regulate a number of other stiffness sensitive processes.\n\nSignificance StatementCell physiology can be regulated by the mechanics of the extracellular environment. Here, we demonstrate that cell spreading is a mechanosensitive process regulated by weak forces generated at the cell periphery and independent of motor activity. We show that stiffness sensing depends on the kinetics of the initial adhesion bonds that are subjected to forces driven by protein polymerization. This work demonstrates how the binding kinetics of adhesion molecules are sensitively tuned to a range of forces that enable mechanosensation.

biophysics↗

bioRxiv2017

Cofilin Drives Rapid Turnover and Fluidization of Entangled F-actin

The shape of most animal cells is controlled by the actin cortex, a thin, isotropic network of dynamic actin filaments (F-actin) situated just beneath the plasma membrane. The cortex is held far from equilibrium by both active stresses and turnover: Myosin-II molecular motors drive deformations required for cell division, migration, and tissue morphogenesis, while turnover of the molecular components of the actin cortex relax stress and facilitate network reorganization. While many aspects of F-actin network viscoelasticity are well-characterized in the presence and absence of motor activity, a mechanistic understanding of how non-equilibrium actin turnover contributes to stress relaxation is still lacking. To address this, we developed a reconstituted in vitro system wherein the steady-state length and turnover rate of F-actin in entangled solutions are controlled by the actin regulatory proteins cofilin, profilin, and formin, which sever, recycle, and nucleate filaments, respectively. Cofilin-mediated severing accelerates the turnover and spatial reorganization of F-actin, without significant changes to filament length. Microrheology measurements demonstrate that cofilin-mediated severing is a single-timescale mode of stress relaxation that tunes the low-frequency viscosity over two orders of magnitude. These findings serve as the foundation for understanding the mechanics of more physiological F-actin networks with turnover, and inform an updated microscopic model of single-filament turnover. They also demonstrate that polymer activity, in the form of ATP hydrolysis on F-actin coupled to nucleotide-dependent cofilin binding, is sufficient to generate a form of active matter wherein asymmetric filament disassembly preserves filament number in spite of sustained severing.\n\nSignificance StatementWhen an animal cell moves or divides, a disordered network of actin filaments (F-actin) plays a central role in controlling the resulting changes in cell shape. While it is known that continual turnover of F-actin by cofilin-mediated severing aids in reorganization of the cellular cytoskeleton, it is unclear how the turnover of structural elements alters the mechanical properties of the network. Here we show that severing of F-actin by cofilin results in a stress relaxation mechanism in entangled solutions characterized by a single-timescale set by the severing rate. Additionally, we identify ATP hydrolysis and nucleotide-dependent cofilin binding as sufficient ingredients to generate a non-equilibrium steady-state in which asymmetric F-actin disassembly preserves filament number in spite of sustained severing.

biophysics↗

bioRxiv2017

Partitioning and Enhanced Self-Assembly of Actin in Polypeptide Coacervates

Biomolecules exist and function in cellular micro-environments that control their spatial organization, local concentration and biochemical reactivity. Due to the complexity of native cytoplasm, the development of artificial bioreactors and cellular mimics to compartmentalize, concentrate and control the local physicochemical properties is of great interest. Here, we employ self-assembling polypeptide coacervates to explore the partitioning of the ubiquitous cytoskeletal protein actin into liquid polymer-rich droplets. We find that actin spontaneously partitions into coacervate droplets and is enriched by up to {approx}30-fold. Actin polymerizes into micrometer-long filaments and, in contrast to the globular protein BSA, these filaments localize predominately to the droplet periphery. We observe up to a 50-fold enhancement in the actin filament assembly rate inside coacervate droplets, consistent with the enrichment of actin within the coacervate phase. Together these results suggest that coacervates can serve as a versatile platform in which to localize and enrich biomolecules to study their reactivity in physiological environments.\n\nSIGNIFICANCE STATEMENTLiving cells harbor many protein-rich membrane-less organelles, the biological functions of which are defined by compartment composition and properties. Significant differences between the physico-chemical properties of these crowded compartments and the dilute solutions in which biochemical reactions are traditionally studied pose a major challenge for understanding regulation of organelle composition and component activity. Here, we report the spontaneous partitioning and accelerated polymerization of the cytoskeletal protein actin inside model polypeptide coacervates as a proof-of-concept demonstration of coacervates as bioreactors for studying biomolecular reactions in cell-like environments. Our work introduces exciting avenues for the use of synthetic polymers to control the physical and biological properties of bioreactors in vitro, enabling studies of biochemical reactions in cell-like micro-environments.

biophysics↗

bioRxiv2017

Filament Rigidity And Connectivity Tune The Deformation Modes Of Active Biopolymer Networks

Molecular motors embedded within collections of actin and microtubule filaments underlie the dynamic behaviors of cytoskeletal assemblies. Understanding the physics of such motor-filament materials is critical to developing a physical model of the cytoskeleton and the design of biomimetic active materials. Here, we demonstrate through experiments and simulations that the rigidity and connectivity of filaments in active biopolymer networks regulates the anisotropy and the length scale of the underlying deformations, yielding materials with varying contractility. Semi-flexible filaments that can be compressed and bent by motor stresses undergo deformations that are predominantly biaxial. By contrast, rigid filament bundles contract via actomyosin sliding deformations that are predominantly uniaxial. Networks dominated by filament buckling are robustly contractile under a wide range of connectivities, while networks dominated by actomyosin sliding can be tuned from contractile to extensile through reduced connectivity via cross-linking. These results identify physical parameters that control the forces generated within motor-filament arrays, and provide insight into the self-organization and mechanics of cytoskeletal assemblies.

biophysics↗