Myosin light chain kinase-driven myosin II turnover regulates actin cortex contractility during mitosis

Force generation by the molecular motor myosin II (MII) at the actin cortex is a universal feature of animal cells. Despite its central role in driving cell shape changes, the mechanisms underlying MII regulation at the actin cortex remain incompletely understood. Here we show that myosin light chain kinase (MLCK) promotes MII turnover at the mitotic cortex. Inhibition of MLCK resulted in an alteration of the relative levels of phosphorylated regulatory light chain (RLC), with MLCK preferentially creating a short-lived pRLC species and Rho-associated kinase (ROCK) preferentially creating a stable ppRLC species during metaphase. Slower turnover of MII and altered RLC homeostasis on MLCK inhibition correlated with increased cortex tension, driving increased membrane bleb initiation and growth, but reduced bleb retraction during mitosis. Taken together, we show that ROCK and MLCK play distinct roles at the actin cortex during mitosis; ROCK activity is required for recruitment of MII to the cortex, while MLCK activity promotes MII turnover. Our findings support the growing evidence that MII turnover is an essential dynamic process influencing the mechanical output of the actin cortex.

Precise Tuning of Cortical Contractility Regulates Cell Shape during Cytokinesis

The mechanical properties of the actin cortex regulate shape changes during cell division, cell migration, and tissue morphogenesis. We show that modulation of myosin II (MII) filament composition allows tuning of surface tension at the cortex to maintain cell shape during cytokinesis. Our results reveal that MIIA generates cortex tension, while MIIB acts as a stabilizing motor and its inclusion in MII hetero-filaments reduces cortex tension. Tension generation by MIIA drives faster cleavage furrow ingression and bleb formation. We also show distinct roles for the motor and tail domains of MIIB in maintaining cytokinetic fidelity. Maintenance of cortical stability by the motor domain of MIIB safeguards against shape instability-induced chromosome missegregation, while its tail domain mediates cortical localization at the terminal stages of cytokinesis to mediate cell abscission. Because most non-muscle contractile systems are cortical, this tuning mechanism will likely be applicable to numerous processes driven by myosin-II contractility.

Nonmuscle myosin-2 contractility-dependent actin turnover limits the length of epithelial microvilli

Brush border microvilli enable functions that are critical for epithelial homeostasis, including solute uptake and host defense. However, the mechanisms that regulate the assembly and morphology of these protrusions are poorly understood. The parallel actin bundles that support microvilli have their pointed-end rootlets anchored in a filamentous meshwork referred to as the "terminal web." Although classic electron microscopy studies revealed complex ultrastructure, the composition and function of the terminal web remain unclear. Here we identify nonmuscle myosin-2C (NM2C) as a component of the terminal web. NM2C is found in a dense, isotropic layer of puncta across the subapical domain, which transects the rootlets of microvillar actin bundles. Puncta are separated by ∼210 nm, the expected size of filaments formed by NM2C. In intestinal organoid cultures, the terminal web NM2C network is highly dynamic and exhibits continuous remodeling. Using pharmacological and genetic perturbations in cultured intestinal epithelial cells, we found that NM2C controls the length of growing microvilli by regulating actin turnover in a manner that requires a fully active motor domain. Our findings answer a decades-old question on the function of terminal web myosin and hold broad implications for understanding apical morphogenesis in diverse epithelial systems.

A Myosin II-Based Nanomachine Devised for the Study of Ca2+-Dependent Mechanisms of Muscle Regulation

The emergent properties of the array arrangement of the molecular motor myosin II in the sarcomere of the striated muscle, the generation of steady force and shortening, can be studied in vitro with a synthetic nanomachine made of an ensemble of eight heavy-meromyosin (HMM) fragments of myosin from rabbit psoas muscle, carried on a piezoelectric nanopositioner and brought to interact with a properly oriented actin filament attached via gelsolin (a Ca²⁺-regulated actin binding protein) to a bead trapped by dual laser optical tweezers. However, the application of the original version of the nanomachine to investigate the Ca²⁺-dependent regulation mechanisms of the other sarcomeric (regulatory or cytoskeleton) proteins, adding them one at a time, was prevented by the impossibility to preserve [Ca²⁺] as a free parameter. Here, the nanomachine is implemented by assembling the bead-attached actin filament with the Ca²⁺-insensitive gelsolin fragment TL40. The performance of the nanomachine is determined both in the absence and in the presence of Ca²⁺ (0.1 mM, the concentration required for actin attachment to the bead with gelsolin). The nanomachine exhibits a maximum power output of 5.4 aW, independently of [Ca²⁺], opening the possibility for future studies of the Ca²⁺-dependent function/dysfunction of regulatory and cytoskeletal proteins.

Unilateral Cleavage Furrows in Multinucleate Cells

Multinucleate cells can be produced in Dictyostelium by electric pulse-induced fusion. In these cells, unilateral cleavage furrows are formed at spaces between areas that are controlled by aster microtubules. A peculiarity of unilateral cleavage furrows is their propensity to join laterally with other furrows into rings to form constrictions. This means cytokinesis is biphasic in multinucleate cells, the final abscission of daughter cells being independent of the initial direction of furrow progression. Myosin-II and the actin filament cross-linking protein cortexillin accumulate in unilateral furrows, as they do in the normal cleavage furrows of mononucleate cells. In a myosin-II-null background, multinucleate or mononucleate cells were produced by cultivation either in suspension or on an adhesive substrate. Myosin-II is not essential for cytokinesis either in mononucleate or in multinucleate cells but stabilizes and confines the position of the cleavage furrows. In fused wild-type cells, unilateral furrows ingress with an average velocity of 1.7 µm × min-1, with no appreciable decrease of velocity in the course of ingression. In multinucleate myosin-II-null cells, some of the furrows stop growing, thus leaving space for the extensive broadening of the few remaining furrows.

Physical Constraints and Forces Involved in Phagocytosis

Phagocytosis is a specialized process that enables cellular ingestion and clearance of microbes, dead cells and tissue debris that are too large for other endocytic routes. As such, it is an essential component of tissue homeostasis and the innate immune response, and also provides a link to the adaptive immune response. However, ingestion of large particulate materials represents a monumental task for phagocytic cells. It requires profound reorganization of the cell morphology around the target in a controlled manner, which is limited by biophysical constraints. Experimental and theoretical studies have identified critical aspects associated with the interconnected biophysical properties of the receptors, the membrane, and the actin cytoskeleton that can determine the success of large particle internalization. In this review, we will discuss the major physical constraints involved in the formation of a phagosome. Focusing on two of the most-studied types of phagocytic receptors, the Fcγ receptors and the complement receptor 3 (αMβ2 integrin), we will describe the complex molecular mechanisms employed by phagocytes to overcome these physical constraints.

Direct visualization of human myosin II force generation using DNA origami-based thick filaments

The sarcomere, the minimal mechanical unit of muscle, is composed of myosins, which self-assemble into thick filaments that interact with actin-based thin filaments in a highly-structured lattice. This complex imposes a geometric restriction on myosin in force generation. However, how single myosins generate force within the restriction remains elusive and conventional synthetic filaments do not recapitulate the symmetric bipolar filaments in sarcomeres. Here we engineered thick filaments using DNA origami that incorporate human muscle myosin to directly visualize the motion of the heads during force generation in a restricted space. We found that when the head diffuses, it weakly interacts with actin filaments and then strongly binds preferentially to the forward region as a Brownian ratchet. Upon strong binding, the two-step lever-arm swing dominantly halts at the first step and occasionally reverses direction. Our results illustrate the usefulness of our DNA origami-based assay system to dissect the mechanistic details of motor proteins.

Repellent and Attractant Guidance Cues Initiate Cell Migration by Distinct Rear-Driven and Front-Driven Cytoskeletal Mechanisms

Attractive and repulsive cell guidance is essential for animal life and important in disease. Cell migration toward attractants dominates studies [1, 2, 3, 4, 5, 6, 7, 8], but migration away from repellents is important in biology yet relatively little studied [5, 9, 10]. It is widely held that cells initiate migration by protrusion of their front [11, 12, 13, 14, 15], yet this has not been explicitly tested for cell guidance because cell margin displacement at opposite ends of the cell has not been distinguished for any cue. We argue that protrusion of the front, retraction of the rear, or both together could in principle break cell symmetry and start migration in response to guidance cues [16]. Here, we find in the Dictyostelium model [6] that an attractant—cAMP—breaks symmetry by causing protrusion of the front of the cell, whereas its repellent analog—8CPT—breaks symmetry by causing retraction of the rear. Protrusion of the front of these cells in response to cAMP starts with local actin filament assembly, while the delayed retraction of the rear is independent of both myosin II polarization and of motor-based contractility. On the contrary, myosin II accumulates locally in the rear of the cell in response to 8CPT, anticipating retraction and required for it, while local actin assembly is delayed and couples to delayed protrusion at the front. These data reveal an important new concept in the understanding of cell guidance.

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In attractant, cell front protrusion breaks cell symmetry and starts migration

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In repellent, cell rear retraction breaks cell symmetry and starts migration

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Myosin II motor is not required for front-driven migration toward attractant

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Biased myosin II motor contractility drives rear-driven migration away from repellent

Study of the influence of actin-binding proteins using linear analyses of cell deformability

The actin cytoskeleton plays a key role in the deformability of the cell and in mechanosensing. Here we analyze the contributions of three major actin cross-linking proteins, myosin II, α-actinin and filamin, to cell deformability, by using micropipette aspiration of Dictyostelium cells. We examine the applicability of three simple mechanical models: for small deformation, linear viscoelasticity and drop of liquid with a tense cortex; and for large deformation, a Newtonian viscous fluid. For these models, we have derived linearized equations and we provide a novel, straightforward methodology to analyze the experiments. This methodology allowed us to differentiate the effects of the cross-linking proteins in the different regimes of deformation. Our results confirm some previous observations and suggest important relations between the molecular characteristics of the actin-binding proteins and the cell behavior: the effect of myosin is explained in terms of the relation between the lifetime of the bond to actin and the resistive force; the presence of α-actinin obstructs the deformation of the cytoskeleton, presumably mainly due to the higher molecular stiffness and to the lower dissociation rate constants; and filamin contributes critically to the global connectivity of the network, possibly by rapidly turning over cross-links during the remodeling of the cytoskeletal network, thanks to the higher rate constants, flexibility and larger size. The results suggest a sophisticated relationship between the expression levels of actin-binding proteins, deformability and mechanosensing.

Nonmuscle myosin II is a critical regulator of clathrin-mediated endocytosis

Variable requirements for actin during clathrin-mediated endocytosis (CME) may be related to regional or cellular differences in membrane tension. To compensate, local regulation of force generation may be needed to facilitate membrane curving and vesicle budding. Force generation is assumed to occur primarily through actin polymerization. Here we examine the role of myosin II using loss of function experiments. Our results indicate that myosin II acts on cortical actin scaffolds primarily in the plane of the plasma membrane (bottom arrow) to generate changes that are critical for enhancing CME progression.

Rotational model for actin filament alignment by myosin

Dynamics of the actomyosin cytoskeleton regulate cellular processes such as secretion, cell division, cell motility, and shape change. Actomyosin dynamics are themselves regulated by proteins that control actin filament polymerization and depolymerization, and myosin motor contractility. Previous theoretical work has focused on translational movement of actin filaments but has not considered the role of filament rotation. Since filament rotational movements are likely sources of forces that direct cell shape change and movement we explicitly model the dynamics of actin filament rotation as myosin II motors traverse filament pairs, drawing them into alignment. Using Monte Carlo simulations we find an optimal motor velocity for alignment of actin filaments. In addition, when we introduce polymerization and depolymerization of actin filaments, we find that alignment is reduced and the filament arrays exist in a stable, asynchronous state. Further analysis with continuum models allows us to investigate factors contributing to the stability of filament arrays and their ability to generate force. Interestingly, we find that two different morphologies of F-actin arrays generate the same amount of force. We also identify a phase transition to alignment which occurs when either polymerization rates are reduced or motor velocities are optimized. We have extended our analysis to include a maximum allowed stretch of the myosin motors, and a non-uniform length for filaments leading to little change in the qualitative results. Through the integration of simulations and continuum analysis, we are able to approach the problem of understanding rotational alignment of actin filaments by myosin II motors.

An integrated in vitro and in situ study of kinetics of myosin II from frog skeletal muscle

A new efficient protocol for extraction and conservation of myosin II from frog skeletal muscle made it possible to preserve the myosin functionality for a week and apply single molecule techniques to the molecular motor that has been best characterized for its mechanical, structural and energetic parameters in situ.With the in vitro motility assay, we estimated the sliding velocity of actin on frog myosin II (VF) and its modulation by pH, myosin density, temperature (range 4-30◦C) and substrate concentration. VF was 8.88 ± 0.26 μms⁻¹ at 30.6◦C and decreased to 1.60 ± 0.09 μms⁻¹ at 4.5◦C. The in vitro mechanical and kinetic parameters were integrated with the in situ parameters of frog muscle myosin working in arrays in each half-sarcomere. By comparing VF with the shortening velocities determined in intact frog muscle fibres under different loads and their dependence on temperature, we found that VF is 40-50% less than the fibre unloaded shortening velocity (V0) at the same temperature and we determined the load that explains the reduced value of VF. With this integrated approach we could define fundamental kinetic steps of the acto-myosin ATPase cycle in situ and their relation with mechanical steps. In particular we found that at 5◦C the rate of ADP release calculated using the step size estimated from in situ experiments accounts for the rate of detachment of motors during steady shortening under low loads.

The functions of myosin II and myosin V homologs in tip growth and septation in Aspergillus nidulans

Because of the industrial and medical importance of members of the fungal genus Aspergillus, there is considerable interest in the functions of cytoskeletal components in growth and secretion in these organisms. We have analyzed the genome of Aspergillus nidulans and found that there are two previously unstudied myosin genes, a myosin II homolog, myoB (product = MyoB) and a myosin V homolog, myoE (product = MyoE). Deletions of either cause significant growth defects. MyoB localizes in strings that coalesce into contractile rings at forming septa. It is critical for septation and normal deposition of chitin but not for hyphal extension. MyoE localizes to the Spitzenkörper and to moving puncta in the cytoplasm. Time-lapse imaging of SynA, a v-SNARE, reveals that in myoE deletion strains vesicles no longer localize to the Spitzenkörper. Tip morphology is slightly abnormal and branching occurs more frequently than in controls. Tip extension is slower than in controls, but because hyphal diameter is greater, growth (increase in volume/time) is only slightly reduced. Concentration of vesicles into the Spitzenkörper before incorporation into the plasma membrane is, thus, not required for hyphal growth but facilitates faster tip extension and a more normal hyphal shape.

The role of actin turnover in retrograde actin network flow in neuronal growth cones

The balance of actin filament polymerization and depolymerization maintains a steady state network treadmill in neuronal growth cones essential for motility and guidance. Here we have investigated the connection between depolymerization and treadmilling dynamics. We show that polymerization-competent barbed ends are concentrated at the leading edge and depolymerization is distributed throughout the peripheral domain. We found a high-to-low G-actin gradient between peripheral and central domains. Inhibiting turnover with jasplakinolide collapsed this gradient and lowered leading edge barbed end density. Ultrastructural analysis showed dramatic reduction of leading edge actin filament density and filament accumulation in central regions. Live cell imaging revealed that the leading edge retracted even as retrograde actin flow rate decreased exponentially. Inhibition of myosin II activity before jasplakinolide treatment lowered baseline retrograde flow rates and prevented leading edge retraction. Myosin II activity preferentially affected filopodial bundle disassembly distinct from the global effects of jasplakinolide on network turnover. We propose that growth cone retraction following turnover inhibition resulted from the persistence of myosin II contractility even as leading edge assembly rates decreased. The buildup of actin filaments in central regions combined with monomer depletion and reduced polymerization from barbed ends suggests a mechanism for the observed exponential decay in actin retrograde flow. Our results show that growth cone motility is critically dependent on continuous disassembly of the peripheral actin network.

Differential effects of nutritional and non-nutritional therapies on intestinal barrier function in an in vitro model

BACKGROUND

Diminished intestinal epithelial barrier function contributes to the pathogenesis of Crohn's disease. Clinical and experimental studies propose that increased tumor necrosis factor (TNF)-α promotes barrier dysfunction. The aim of this study was to investigate the effects of nutritional and other therapies upon intestinal barrier function in the presence of TNF-α in an in vitro model.

METHODS

Caco-2 monolayers were grown to confluence on membrane supports and then exposed to TNF-α in the presence of polymeric formula, hydrocortisone or infliximab. Monolayer permeability was evaluated by measuring epithelial resistance, short-circuit current and horseradish peroxidase flux in an Ussing chamber. Tight junction and myosin II regulatory light-chain kinase gene expression was analysed by real-time PCR, with protein expression and localization analysed by Western blot and immunofluorescence.

RESULTS

TNF-α increased monolayer permeability and diminished tight junction integrity. However both polymeric formula and infliximab completely abrogated the effects of TNF-α. These monolayers displayed unchanged permeability and tight junction integrity compared to untreated cells (media-no-TNF-α controls). In contrast, hydrocortisone only partially abrogated the effects of TNF-α, with these monolayers having increased permeability and altered tight junction integrity compared to media-no-TNF-α controls.

CONCLUSIONS

Both polymeric formula and infliximab completely prevent epithelial barrier dysfunction in the presence of TNF-α, whereas hydrocortisone partially prevents barrier dysfunction. These results provide evidence that superior mucosal healing can be achieved with both polymeric formula and infliximab compared to hydrocortisone.

Blood flow dynamics of one cardiac cycle and relationship to mechanotransduction and trabeculation during heart looping

Analyses of form-function relationships during heart looping are directly related to technological advances. Recent advances in four-dimensional optical coherence tomography (OCT) permit observations of cardiac dynamics at high-speed acquisition rates and high resolution. Real-time observation of the avian stage 13 looping heart reveals that interactions between the endocardial and myocardial compartments are more complex than previously depicted. Here we applied four-dimensional OCT to elucidate the relationships of the endocardium, myocardium, and cardiac jelly compartments in a single cardiac cycle during looping. Six cardiac levels along the longitudinal heart tube were each analyzed at 15 time points from diastole to systole. Using image analyses, the organization of mechanotransducing molecules, fibronectin, tenascin C, α-tubulin, and nonmuscle myosin II was correlated with specific cardiac regions defined by OCT data. Optical coherence microscopy helped to visualize details of cardiac architectural development in the embryonic mouse heart. Throughout the cardiac cycle, the endocardium was consistently oriented between the midline of the ventral floor of the foregut and the outer curvature of the myocardial wall, with multiple endocardial folds allowing high-volume capacities during filling. The cardiac area fractional shortening is much higher than previously published. The in vivo profile captured by OCT revealed an interaction of the looping heart with the extra-embryonic splanchnopleural membrane providing outside-in information. In summary, the combined dynamic and imaging data show the developing structural capacity to accommodate increasing flow and the mechanotransducing networks that organize to effectively facilitate formation of the trabeculated four-chambered heart.

Duration of fusion pore opening and the amount of hormone released are regulated by myosin II during kiss-and-run exocytosis

Since the fusion pore of the secretory vesicle is resealed before complete dilation during 'kiss-and-run' exocytosis, their cargoes are not completely released. Although the transient fusion pore is kept open for several seconds, the precise mechanisms that control fusion pore maintenance, and their physiological significance, are not well understood. Using dual-colour TIRF (total internal reflection fluorescence) microscopy in neuroendocrine PC12 cells, we show that myosin II regulates the fusion pore dynamics during kiss-and-run exocytosis. The release kinetics of mRFP (monomeric red fluorescent protein)-tagged tPA (tissue plasminogen activator) and Venus-tagged BDNF (brain-derived neurotrophic factor), which show slower release kinetics than NPY (neuropeptide Y)-mRFP and insulin-mRFP, were prolonged by the overexpression of a wild-type form of the RLC (myosin II regulatory light chain). In contrast, overexpression of a dominant-negative form of RLC shortened the release kinetics. Using spH (synapto-pHluorin), a green fluorescent protein-based pH sensor inside the vesicles, we confirmed that the modulation of the release kinetics by myosin II is due to changes in the duration of fusion pore opening. In addition, we revealed that the amount of hormone released into the extracellular space upon stimulation was increased by overexpression of wild-type RLC. We propose that the duration of fusion pore opening is regulated by myosin II to control the amount of hormone released from a single vesicle.

Transient frictional slip between integrin and the ECM in focal adhesions under myosin II tension

BACKGROUND

The spatiotemporal regulation of adhesion to the extracellular matrix is important in metazoan cell migration and mechanosensation. Although adhesion assembly depends on intracellular and extracellular tension, the biophysical regulation of force transmission between the actin cytoskeleton and extracellular matrix during this process remains largely unknown.

RESULTS

To elucidate the nature of force transmission as myosin II tension is applied to focal adhesions, we correlated the dynamics of focal adhesion proteins and the actin cytoskeleton to local traction stresses. Under low extracellular tension, newly formed adhesions near the cell periphery underwent a transient retrograde displacement preceding elongation. We found that myosin II-generated tension drives this mobility, and we determine the interface of differential motion, or "slip," to be between integrin and the ECM. The magnitude and duration of both adhesion slip and associated F-actin dynamics is strongly modulated by ECM compliance. Traction forces are generated throughout the slip period, and adhesion immobilization occurs at a constant tension.

CONCLUSIONS

We have identified a tension-dependent, extracellular "clutch" between integrins and the extracellular matrix; this clutch stabilizes adhesions under myosin II driven tension. The current work elucidates a mechanism by which force transmission is modulated during focal adhesion maturation.

Mechanical tugging force regulates the size of cell-cell junctions

Actomyosin contractility affects cellular organization within tissues in part through the generation of mechanical forces at sites of cell-matrix and cell-cell contact. While increased mechanical loading at cell-matrix adhesions results in focal adhesion growth, whether forces drive changes in the size of cell-cell adhesions remains an open question. To investigate the responsiveness of adherens junctions (AJ) to force, we adapted a system of microfabricated force sensors to quantitatively report cell-cell tugging force and AJ size. We observed that AJ size was modulated by endothelial cell-cell tugging forces: AJs and tugging force grew or decayed with myosin activation or inhibition, respectively. Myosin-dependent regulation of AJs operated in concert with a Rac1, and this coordinated regulation was illustrated by showing that the effects of vascular permeability agents (S1P, thrombin) on junctional stability were reversed by changing the extent to which these agents coupled to the Rac and myosin-dependent pathways. Furthermore, direct application of mechanical tugging force, rather than myosin activity per se, was sufficient to trigger AJ growth. These findings demonstrate that the dynamic coordination of mechanical forces and cell-cell adhesive interactions likely is critical to the maintenance of multicellular integrity and highlight the need for new approaches to study tugging forces.

Myosin II motor proteins with different functions determine the fate of lamellipodia extension during cell spreading

Non-muscle cells express multiple myosin-II motor proteins myosin IIA, myosin IIB and myosin IIC transcribed from different loci in the human genome. Due to a significant homology in their sequences, these ubiquitously expressed myosin II motor proteins are believed to have overlapping cellular functions, but the mechanistic details are not elucidated. The present study uncovered a mechanism that coordinates the distinctly localized myosin IIA and myosin IIB with unexpected opposite mechanical roles in maneuvering lamellipodia extension, a critical step in the initiation of cell invasion, spreading, and migration. Myosin IIB motor protein by localizing at the front drives lamellipodia extension during cell spreading. On the other hand, myosin IIA localizes next to myosin IIB and attenuates or retracts lamellipodia extension. Myosin IIA and IIB increase cell adhesion by regulating focal contacts formation in the spreading margins and central part of the spreading cell, respectively. Spreading cells expressing both myosin IIA and myosin IIB motor proteins display an organized actin network consisting of retrograde filaments, arcs and central filaments attached to focal contacts. This organized actin network especially arcs and focal contacts formation in the spreading margins were lost in myosin IIÂ cells. Surprisingly, myosin IIB̂ cells displayed long parallel actin filaments connected to focal contacts in the spreading margins. Thus, with different roles in the regulation of the actin network and focal contacts formation, both myosin IIA and IIB determine the fate of lamellipodia extension during cell spreading.

Resolving the role of actoymyosin contractility in cell microrheology

Einstein's original description of Brownian motion established a direct relationship between thermally-excited random forces and the transport properties of a submicron particle in a viscous liquid. Recent work based on reconstituted actin filament networks suggests that nonthermal forces driven by the motor protein myosin II can induce large non-equilibrium fluctuations that dominate the motion of particles in cytoskeletal networks. Here, using high-resolution particle tracking, we find that thermal forces, not myosin-induced fluctuating forces, drive the motion of submicron particles embedded in the cytoskeleton of living cells. These results resolve the roles of myosin II and contractile actomyosin structures in the motion of nanoparticles lodged in the cytoplasm, reveal the biphasic mechanical architecture of adherent cells—stiff contractile stress fibers interdigitating in a network at the cell cortex and a soft actin meshwork in the body of the cell, validate the method of particle tracking-microrheology, and reconcile seemingly disparate atomic force microscopy (AFM) and particle-tracking microrheology measurements of living cells.

Mechanosensing through cooperative interactions between myosin II and the actin crosslinker cortexillin I

BACKGROUND

Mechanosensing governs many processes from molecular to organismal levels, including during cytokinesis where it ensures successful and symmetrical cell division. Although many proteins are now known to be force sensitive, myosin motors with their ATPase activity and force-sensitive mechanical steps are well poised to facilitate cellular mechanosensing. For a myosin motor to experience tension, the actin filament must also be anchored.

RESULTS

Here, we find a cooperative relationship between myosin II and the actin crosslinker cortexillin I where both proteins are essential for cellular mechanosensory responses. Although many functions of cortexillin I and myosin II are dispensable for cytokinesis, all are required for full mechanosensing. Our analysis demonstrates that this mechanosensor has three critical elements: the myosin motor where the lever arm acts as a force amplifier, a force-sensitive bipolar thick-filament assembly, and a long-lived actin crosslinker, which anchors the actin filament so that the motor may experience tension. We also demonstrate that a Rac small GTPase inhibits this mechanosensory module during interphase, allowing the module to be primarily active during cytokinesis.

CONCLUSIONS

Overall, myosin II and cortexillin I define a cellular-scale mechanosensor that controls cell shape during cytokinesis. This system is exquisitely tuned through the enzymatic properties of the myosin motor, its lever arm length, and bipolar thick-filament assembly dynamics. The system also requires cortexillin I to stably anchor the actin filament so that the myosin motor can experience tension. Through this cross-talk, myosin II and cortexillin I define a cellular-scale mechanosensor that monitors and corrects shape defects, ensuring symmetrical cell division.

Local cortical tension by myosin II guides 3D endothelial cell branching

A key feature of angiogenesis is directional control of endothelial cell (EC) morphogenesis and movement [1]. During angiogenic sprouting, endothelial "tip cells" directionally branch from existing vessels in response to biochemical cues such as VEGF or hypoxia and migrate and invade the surrounding extracellular matrix (ECM) in a process that requires ECM remodeling by matrix metalloproteases (MMPs) [2-4]. Tip EC branching is mediated by directional protrusion of subcellular pseudopodial branches [5, 6]. Here, we seek to understand how EC pseudopodial branching is locally regulated to directionally guide angiogenesis. We develop an in vitro 3D EC model system in which migrating ECs display branched pseudopodia morphodynamics similar to those in living zebrafish. Using this system, we find that ECM stiffness and ROCK-mediated myosin II activity inhibit EC pseudopodial branch initiation. Myosin II is dynamically localized to the EC cortex and is partially released under conditions that promote branching. Local depletion of cortical myosin II precedes branch initiation, and initiation can be induced by local inhibition of myosin II activity. Thus, local downregulation of myosin II cortical contraction allows pseudopodium initiation to mediate EC branching and hence guide directional migration and angiogenesis.

Experimental investigation of the seesaw mechanism of the relay region that moves the myosin lever arm

A seesaw-like movement of the relay region upon the recovery step of myosin was recently simulated in silico. In this model the relay helix tilts around its pivoting point formed by a phenylalanine cluster (Phe(481), Phe(482), and Phe(652)), which moves the lever arm of myosin. To study the effect of the elimination of the proposed pivoting point, these phenylalanines were mutated to alanines in two Dictyostelium myosin II motor domain constructs (M(F481A, F482A) and M(F652A)). The relay movement was followed by the fluorescence change of Trp(501) located in the relay region. The steady-state and transient kinetic fluorescence experiments showed that the lack of the phenylalanine fulcrum perturbs the formation of the "up" lever arm state, and only moderate effects were found in the nucleotide binding, the formation of the "down" lever arm position, and the ATP hydrolysis steps. We conclude that the lack of the fulcrum decouples the distal part of the relay from the nucleotide binding site upon the recovery step. Our molecular dynamics simulations also showed that the conformation of the motor is not perturbed by the mutation in the down lever arm state, however, the lack of the pivoting point rearranges the dynamic pattern of the kink region of the relay helix.

Stable and dynamic microtubules coordinately shape the myosin activation zone during cytokinetic furrow formation

The cytokinetic furrow arises from spatial and temporal regulation of cortical contractility. To test the role microtubules play in furrow specification, we studied myosin II activation in echinoderm zygotes by assessing serine19-phosphorylated regulatory light chain (pRLC) localization after precisely timed drug treatments. Cortical pRLC was globally depressed before cytokinesis, then elevated only at the equator. We implicated cell cycle biochemistry (not microtubules) in pRLC depression, and differential microtubule stability in localizing the subsequent myosin activation. With no microtubules, pRLC accumulation occurred globally instead of equatorially, and loss of just dynamic microtubules increased equatorial pRLC recruitment. Nocodazole treatment revealed a population of stable astral microtubules that formed during anaphase; among these, those aimed toward the equator grew longer, and their tips coincided with cortical pRLC accumulation. Shrinking the mitotic apparatus with colchicine revealed pRLC suppression near dynamic microtubule arrays. We conclude that opposite effects of stable versus dynamic microtubules focuses myosin activation to the cell equator during cytokinesis.

Self-organized podosomes are dynamic mechanosensors

Podosomes are self-organized, dynamic, actin-containing structures that adhere to the extracellular matrix via integrins [1-5]. Yet, it is not clear what regulates podosome dynamics and whether podosomes can function as direct mechanosensors, like focal adhesions [6-9]. We show here that myosin-II proteins form circular structures outside and at the podosome actin ring to regulate podosome dynamics. Inhibiting myosin-II-dependent tension dissipated podosome actin rings before dissipating the myosin-ring structure. As podosome rings changed size or shape, tractions underneath the podosomes were exerted onto the substrate and were abolished when myosin-light-chain activity was inhibited. The magnitudes of tractions were comparable to those generated underneath focal adhesions, and they increased with substrate stiffness. The dynamics of podosomes and of focal adhesions were different. Torsional tractions underneath the podosome rings were generated with rotations of podosome rings in a nonmotile, nonrotating cell, suggesting a unique feature of these circular structures. Stresses applied via integrins at the apical surface directly displaced podosomes near the basal surface. Stress-induced podosome displacements increased nonlinearly with applied stresses. Our results suggest that podosomes are dynamic mechanosensors in which interactions of myosin tension and actin dynamics are crucial for regulating these self-organized structures in living cells.

The Rho-Rock-Myosin signaling axis determines cell-cell integrity of self-renewing pluripotent stem cells

Background

Embryonic stem (ES) cells self-renew as coherent colonies in which cells maintain tight cell-cell contact. Although intercellular communications are essential to establish the basis of cell-specific identity, molecular mechanisms underlying intrinsic cell-cell interactions in ES cells at the signaling level remain underexplored.

Methodology/Principal Findings

Here we show that endogenous Rho signaling is required for the maintenance of cell-cell contacts in ES cells. siRNA-mediated loss of function experiments demonstrated that Rock, a major effector kinase downstream of Rho, played a key role in the formation of cell-cell junctional assemblies through regulation of myosin II by controlling a myosin light chain phosphatase. Chemical engineering of this signaling axis by a Rock-specific inhibitor revealed that cell-cell adhesion was reversibly controllable and dispensable for self-renewal of mouse ES cells as confirmed by chimera assay. Furthermore, a novel culture system combining a single synthetic matrix, defined medium, and the Rock inhibitor fully warranted human ES cell self-renewal independent of animal-derived matrices, tight cell contacts, or fibroblastic niche-forming cells as determined by teratoma formation assay.

Conclusions/Significance

These findings demonstrate an essential role of the Rho-Rock-Myosin signaling axis for the regulation of basic cell-cell communications in both mouse and human ES cells, and would contribute to advance in medically compatible xeno-free environments for human pluripotent stem cells.

Role of myosin II activity and the regulation of myosin light chain phosphorylation in astrocytomas

The generation of contractile force mediated by actin-myosin interactions is essential for cell motility. Myosin activity is promoted by phosphorylation of myosin light chain (MLC). MLC phosphorylation in large part is controlled by kinases that are effectors of Rho family GTPases. Accordingly, in this study we examined the effects of ROCK and Rac1 inhibition on MLC phosphorylation in astrocytoma cells. We found that low concentrations of the ROCK inhibitor Y27632 increased the phosphorylation state of the Triton X-100 soluble fraction of MLC, whereas higher concentrations of Y27632 decreased soluble phospho-MLC. These effects of Y27632 were dependent on Rac1. The soluble form of phospho-MLC comprises about 10% of total phospho-MLC in control cells. Interestingly, ROCK inhibition led to a decrease in the phosphorylation state of total MLC, whereas Rac1 inhibition had little effect. Thus, the soluble form of MLC is differentially regulated by ROCK and Rac1 compared with MLC examined in a total cell extract. We also observed that astrocytoma migration is stimulated by low concentrations of the myosin II inhibitor blebbistatin. However, higher concentrations of blebbistatin inhibit migration leading us to believe that migration has a biphasic dependence on myosin II activity. Taken together, our data show that modulation of myosin II activity is important in determining optimal astrocytoma migration. In addition, these findings suggest that there are at least two populations of MLC that are differentially regulated.

Myosin II and Rho kinase activity are required for melanosome aggregation in fish retinal pigment epithelial cells

In the retinal pigment epithelium (RPE) of fish, melanosomes (pigment granules) migrate long distances through the cell body into apical projections in the light, and aggregate back into the cell body in the dark. RPE cells can be isolated from the eye, dissociated, and cultured as single cells in vitro. Treatment of isolated RPE cells with cAMP or the phosphatase inhibitor, okadaic acid (OA), stimulates melanosome aggregation, while cAMP or OA washout in the presence of dopamine triggers dispersion. Previous studies have shown that actin filaments are both necessary and sufficient for aggregation and dispersion of melanosomes within apical projections of isolated RPE. The role of myosin II in melanosome motility was investigated using the myosin II inhibitor, blebbistatin, and a specific rho kinase (ROCK) inhibitor, H-1152. Blebbistatin and H-1152 partially blocked melanosome aggregation triggered by cAMP in dissociated, isolated RPE cells and isolated sheets of RPE. In contrast, neither drug affected melanosome dispersion. In cells exposed to either blebbistatin or H-1152, then triggered to aggregate using OA, melanosome aggregation was completely inhibited. These results demonstrate that (1) melanosome aggregation and dispersion occur through different, actin-dependent mechanisms; (2) myosin II and ROCK activity are required for full melanosome aggregation, but not dispersion; (3) partial aggregation that occurred despite myosin II or ROCK inhibition suggests a second component of aggregation that is dependent on cAMP signaling, but independent of ROCK and myosin II.

Reversal of cell polarity and actin-myosin cytoskeleton reorganization under mechanical and chemical stimulation

To study reorganization of the actin system in cells that invert their polarity, we stimulated Dictyostelium cells by mechanical forces from alternating directions. The cells oriented in a fluid flow by establishing a protruding front directed against the flow and a retracting tail. Labels for polymerized actin and filamentous myosin-II marked front and tail. At 2.1 Pa, actin first disassembled at the previous front before it began to polymerize at the newly induced front. In contrast, myosin-II slowly disappeared from the previous tail and continuously redistributed to the new tail. Front specification was myosin-II independent and accumulation of polymerized actin was even more focused in mutants lacking myosin-II heavy chains. We conclude that under mechanical stimulation, the inversion of cell polarity is initiated by a global internal signal that turns down actin polymerization in the entire cell. It is thought to be elicited at the most strongly stimulated site of the cell, the incipient front region, and to be counterbalanced by a slowly generated, short-range signal that locally activates actin polymerization at the front. Similar pattern of front and tail interconversion were observed in cells reorienting in strong gradients of the chemoattractant cyclic AMP.

Myosin II co-chaperone general cell UNC-45 overexpression is associated with ovarian cancer, rapid proliferation, and motility

Both tumor cell proliferation and metastasis are dependent on myosin II. Because UNC-45 is required to chaperone the assembly of a functional myosin II motor, we examined the expression of the general cell (GC) UNC-45 isoform in ovarian tumors. Serous carcinoma expressed elevated levels of GC UNC-45 compared with normal ovarian surface epithelium and benign cystadenoma. High-stage exhibited greater GC UNC-45 expression than low-stage serous carcinoma. Similarly, GC UNC-45 transcripts and protein levels were higher in ovarian cell lines than in immortalized ovarian surface epithelial cells. Elevation of GC UNC-45 levels by ectopic expression enhanced the rate of ovarian cancer cell proliferation, whereas siRNA knockdown of GC UNC-45 suppressed proliferation without altering myosin II levels. GC UNC-45 and myosin II were diffuse within the cytoplasm of confluent interphase cells, but both accumulated together at the cleavage furrow during cytokinesis. GC UNC-45 and myosin II also trafficked to the leading edges of ovarian cancer cells induced to move in a scratch assay. Knockdown of GC UNC-45 reduced the spreading ability of ovarian cancer cells whereas it was enhanced by GC UNC-45 overexpression. In sum, these findings implicate elevated GC UNC-45 protein expression in ovarian carcinoma proliferation and metastasis.

The Structural Coupling between ATPase Activation and Recovery Stroke in the Myosin II Motor

Before the myosin motor head can perform the next power stroke, it undergoes a large conformational transition in which the converter domain, bearing the lever arm, rotates approximately 65 degrees . Simultaneous with this "recovery stroke," myosin activates its ATPase function by closing the Switch-2 loop over the bound ATP. This coupling between the motions of the converter domain and of the 40 A-distant Switch-2 loop is essential to avoid unproductive ATP hydrolysis. The coupling mechanism is determined here by finding a series of optimized intermediates between crystallographic end structures of the recovery stroke (Dictyostelium discoideum), yielding movies of the transition at atomic detail. The successive formation of two hydrogen bonds by the Switch-2 loop is correlated with the successive see-saw motions of the relay and SH1 helices that hold the converter domain. SH1 helix and Switch-2 loop communicate via a highly conserved loop that wedges against the SH1-helix upon Switch-2 closing.

A microfabricated platform probing cytoskeleton dynamics using multidirectional topographical cues

Cell migration, which involves complicated coordination of cytoskeleton elements and regulatory molecules, plays a central role in a large variety of biological processes from development, immune response to tissue regeneration. However, conventional methods to study in vitro cell migration are often limited to stimulating a cell along a single direction or at a single location. This restriction prevents a deeper understanding of the fundamental mechanisms that control the spatio-temporal dynamics of cytoskeleton. Here we report a novel microfabricated platform that enables a multi-directional stimulation to a cell using topographical cues. In this device, cells were seeded on a grid-patterned topographically structured surface composed of 2 microm wide and 2 microm high straight ridges. Because the size of a unit grid was smaller than a single cell, each cell was simultaneously experiencing contact guidance leading to different directions. The device showed that healthy cells preferred to align and migrate in the direction of the longer side of the grid. But cells with impaired intracelluar tension force generation exhibited multiple uncoordinated cell protrusions along guiding ridges in all directions. Our results demonstrate the importance of actomyosin network in long-range communication and regulation of local actin polymerization activities. This platform will find wide applications in investigations of signal transduction and regulation process in cell migration.

Traction force microscopy in Dictyostelium reveals distinct roles for myosin II motor and actin-crosslinking activity in polarized cell movement

Continuous cell movement requires the coordination of protrusive forces at the leading edge with contractile forces at the rear of the cell. Myosin II is required to generate the necessary contractile force to facilitate retraction; however, Dictyostelium cells that lack myosin II (mhcA–) are still motile. To directly investigate the role of myosin II in contractility we used a gelatin traction force assay to measure the magnitude and dynamic redistribution of traction stresses generated by randomly moving wild-type, myosin II essential light chain null (mlcE–) and mhcA– cells. Our data show that for each cell type, periods of rapid, directed cell movement occur when an asymmetrical distribution of traction stress is present, in which traction stresses at the rear are significantly higher than those at the front. We found that the major determinants of cell speed are the rate and frequency at which traction stress asymmetry develops, not the absolute magnitude of traction stress. We conclude that traction stress asymmetry is important for rapid, polarized cell movement because high traction stresses at the rear promote retraction, whereas low traction at the front allows protrusion. We propose that myosin II motor activity increases the rate and frequency at which traction stress asymmetry develops, whereas actin crosslinking activity is important for stabilizing it.

Myosin II and mechanotransduction: a balancing act

Adherent cells respond to mechanical properties of the surrounding extracellular matrix. Mechanical forces, sensed at specialized cell-matrix adhesion sites, promote actomyosin-based contraction within the cell. By manipulating matrix rigidity and adhesion strength, new roles for actomyosin contractility in the regulation of basic cellular functions, including cell proliferation, migration and stem cell differentiation, have recently been discovered. These investigations demonstrate that a balance of forces between cell adhesion on the outside and myosin II-based contractility on the inside of the cell controls many aspects of cell behavior. Disturbing this balance contributes to the pathogenesis of various human diseases. Therefore, elaborate signaling networks have evolved that modulate myosin II activity to maintain tensional homeostasis. These include signaling pathways that regulate myosin light chain phosphorylation as well as myosin II heavy chain interactions.

On a pulsating Brownian motor and its characterization

As a model of Brownian motor we consider the jump diffusion motion of a particle in the presence of an asymmetric periodic potential with a unique minimum and subject to half-period space shifts at the instants of occurrence of two Poisson processes. The relevant quantities, i.e., probability current, effective driving force, stall force, power and efficiency of the motor are explicitly calculated as averages of certain functions of the random variable representing the particle position.

Roles of type II myosin and a tropomyosin isoform in retrograde actin flow in budding yeast

Retrograde flow of cortical actin networks and bundles is essential for cell motility and retrograde intracellular movement, and for the formation and maintenance of microvilli, stereocilia, and filopodia. Actin cables, which are F-actin bundles that serve as tracks for anterograde and retrograde cargo movement in budding yeast, undergo retrograde flow that is driven, in part, by actin polymerization and assembly. We find that the actin cable retrograde flow rate is reduced by deletion or delocalization of the type II myosin Myo1p, and by deletion or conditional mutation of the Myo1p motor domain. Deletion of the tropomyosin isoform Tpm2p, but not the Tpm1p isoform, increases the rate of actin cable retrograde flow. Pretreatment of F-actin with Tpm2p, but not Tpm1p, inhibits Myo1p binding to F-actin and Myo1p-dependent F-actin gliding. These data support novel, opposing roles of Myo1p and Tpm2 in regulating retrograde actin flow in budding yeast and an isoform-specific function of Tpm1p in promoting actin cable function in myosin-driven anterograde cargo transport.

Nonequilibrium mechanics of active cytoskeletal networks

Cells both actively generate and sensitively react to forces through their mechanical framework, the cytoskeleton, which is a nonequilibrium composite material including polymers and motor proteins. We measured the dynamics and mechanical properties of a simple three-component model system consisting of myosin II, actin filaments, and cross-linkers. In this system, stresses arising from motor activity controlled the cytoskeletal network mechanics, increasing stiffness by a factor of nearly 100 and qualitatively changing the viscoelastic response of the network in an adenosine triphosphate-dependent manner. We present a quantitative theoretical model connecting the large-scale properties of this active gel to molecular force generation.

A role for non-muscle myosin II function in furrow maturation in the early zebrafish embryo

Cytokinesis in early zebrafish embryos involves coordinated changes in the f-actin- and microtubule-based cytoskeleton, and the recruitment of adhesion junction components to the furrow. We show that exposure to inhibitors of non-muscle myosin II function does not affect furrow ingression during the early cleavage cycles but interferes with the recruitment of pericleavage f-actin and cortical beta-catenin aggregates to the furrow, as well as the remodeling of the furrow microtubule array. This remodeling is in turn required for the distal aggregation of the zebrafish germ plasm. Embryos with reduced myosin activity also exhibit at late stages of cytokinesis a stabilized contractile ring apparatus that appears as a ladder-like pattern of short f-actin cables, supporting a role for myosin function in the disassembly of the contractile ring after furrow formation. Our studies support a role for myosin function in furrow maturation that is independent of furrow ingression and which is essential for the recruitment of furrow components and the remodeling of the cytoskeleton during cytokinesis.

Mitosis-specific mechanosensing and contractile-protein redistribution control cell shape

Because cell-division failure is deleterious, promoting tumorigenesis in mammals, cells utilize numerous mechanisms to control their cell-cycle progression. Though cell division is considered a well-ordered sequence of biochemical events, cytokinesis, an inherently mechanical process, must also be mechanically controlled to ensure that two equivalent daughter cells are produced with high fidelity. Given that cells respond to their mechanical environment, we hypothesized that cells utilize mechanosensing and mechanical feedback to sense and correct shape asymmetries during cytokinesis. Because the mitotic spindle and myosin II are vital to cell division, we explored their roles in responding to shape perturbations during cell division. We demonstrate that the contractile proteins myosin II and cortexillin I redistribute in response to intrinsic and externally induced shape asymmetries. In early cytokinesis, mechanical load overrides spindle cues and slows cytokinesis progression while contractile proteins accumulate and correct shape asymmetries. In late cytokinesis, mechanical perturbation also directs contractile proteins but without apparently disrupting cytokinesis. Significantly, this response only occurs during anaphase through cytokinesis, does not require microtubules, and is independent of spindle orientation, but is dependent on myosin II. Our data provide evidence for a mechanosensory system that directs contractile proteins to regulate cell shape during mitosis.

Nonmuscle myosin II generates forces that transmit tension and drive contraction in multiple tissues during dorsal closure

BACKGROUND

The morphogenic movements that characterize embryonic development require the precise temporal and spatial control of cell-shape changes. Drosophila dorsal closure is a well-established model for epithelial sheet morphogenesis, and mutations in more than 60 genes cause defects in closure. Closure requires that four forces, derived from distinct tissues, be precisely balanced. The proteins responsible for generating each of the forces have not been determined.

RESULTS

We document dorsal closure in living embryos to show that mutations in nonmuscle myosin II (encoded by zipper; zip/MyoII) disrupt the integrity of multiple tissues during closure. We demonstrate that MyoII localization is distinct from, but overlaps, F-actin in the supracellular purse string, whereas in the amnioserosa and lateral epidermis each has similar, cortical distributions. In zip/MyoII mutant embryos, we restore MyoII function either ubiquitously or specifically in the leading edge, amnioserosa, or lateral epidermis and find that zip/MyoII function in any one tissue can rescue closure. Using a novel, transgenic mosaic approach, we establish that contractility of the supracellular purse string in leading-edge cells requires zip/MyoII-generated forces; that zip/MyoII function is responsible for the apical contraction of amnioserosa cells; that zip/MyoII is important for zipping; and that defects in zip/MyoII contractility cause the misalignment of the lateral-epidermal sheets during seam formation.

CONCLUSIONS

We establish that zip/MyoII is responsible for generating the forces that drive cell-shape changes in each of the force-generating tissues that contribute to closure. This highly conserved contractile protein likely drives cell-sheet movements throughout phylogeny.

The role of myosin II motor activity in distributing myosin asymmetrically and coupling protrusive activity to cell translocation

Nonmuscle myosin IIA and IIB distribute preferentially toward opposite ends of migrating endothelial cells. To understand the mechanism and function of this behavior, myosin II was examined in cells treated with the motor inhibitor, blebbistatin. Blebbistatin at > or = 30 microM inhibited anterior redistribution of myosin IIA, with 100 microM blebbistatin causing posterior accumulation. Posterior accumulation of myosin IIB was unaffected. Time-lapse cinemicrography showed myosin IIA entering lamellipodia shortly after their formation, but failing to move into lamellipodia in blebbistatin. Thus, myosin II requires motor activity to move forward onto F-actin in protrusions. However, this movement is inhibited by myosin filament assembly, because whole myosin was delayed relative to a tailless fragment. Inhibiting myosin's forward movement reduced coupling between protrusive activity and translocation of the cell body: In untreated cells, body movement followed advancing lamellipodia, whereas blebbistatin-treated cells extended protrusions without displacement of the body or with a longer delay before movement. Anterior cytoplasm of blebbistatin-treated cells contained disorganized bundles of parallel microfilaments, but anterior F-actin bundles in untreated cells were mostly oriented perpendicular to movement. Myosin II may ordinarily move anteriorly on actin filaments and pull crossed filaments into antiparallel bundles, with the resulting realignment pulling the cell body forward.

Differential localization of unconventional myosin I and nonmuscle myosin II during B cell spreading

Cross-linking of CD44 in vitro promotes chemokinesis and actin-based dendrite formation in T and B cells. However, the mechanisms by which the adhesion molecule CD44 induces cytoskeleton activation in lymphocytes are still poorly understood. In this study, we have investigated whether myosin isoforms are involved in CD44-dependent dendrite formation in activated B cells. Pharmacological inhibition of myosin with 2,3-butanedione monoxime strongly affected spreading and dendrite formation, suggesting that these cellular motors may participate in these phenomena. Furthermore, immunofluorescence analysis showed differences in subcellular localization of class I and class II myosin during B cell spreading. In response to CD44 cross-linking, myosin-1c was polarized to lamellipodia, where F-actin was high. In contrast, the distribution of cytosplasmic nonmuscle class II myosin was not altered. Expressions of myosin-1c and II were also demonstrated in B cells by Western blot. Although the inhibition of PLCgamma, PI3K and MEK-1 activation affected the spreading and dendrite formation in activated B cells, only PLCgamma and MEK-1 inhibition correlated with absence of myosin-1c polarization. Additionally, myosin-1c polarization was observed upon cross-linking of other surface molecules, suggesting a common mechanism for B cell spreading. This work shows that class I and class II myosin are expressed in B cells, are differentially distributed, and may participate in the morphological changes of these cells.

The RhoGAP crossveinless-c links trachealess and EGFR signaling to cell shape remodeling in Drosophila tracheal invagination

A major issue in morphogenesis is to understand how the activity of genes specifying cell fate affects cytoskeletal components that modify cell shape and induce cell movements. Here, we approach this question by investigating how a group of cells from an epithelial sheet initiate invagination to ultimately form the Drosophila tracheal tubes. We describe tracheal cell behavior at invagination and show that it is associated with, and requires, a distinct recruitment of Myosin II to the apical surface of cells at the invaginating edge. We show that this process is achieved by the activity of crossveinless-c, a gene coding for a RhoGAP and whose specific transcriptional activation in the tracheal cells is triggered by both the trachealess patterning gene and the EGF Receptor (EGFR) signaling pathway. Our results identify a developmental pathway linking cell fate genes and cell signaling pathways to intracellular modifications during tracheal cell invagination.

Spatiotemporal feedback between actomyosin and focal-adhesion systems optimizes rapid cell migration

Cells exhibit a biphasic migration-velocity response to increasing adhesion strength, with fast migration occurring at intermediate extracellular matrix (ECM) concentration and slow migration occurring at low and high ECM concentration. A simple mechanical model has been proposed to explain this observation, in which too little adhesion does not provide sufficient traction whereas too much adhesion renders cells immobile. Here we characterize a phenotype for rapid cell migration, which in contrast to the previous model reveals a complex interdependence of subcellular systems that mediates optimal cell migration in response to increasing adhesion strength. The organization and activity of actin, myosin II, and focal adhesions (FAs) are spatially and temporally highly variable and do not exhibit a simple correlation with optimal motility rates. Furthermore, we can recapitulate rapid migration at a nonoptimal ECM concentration by manipulating myosin II activity. Thus, the interplay between actomyosin and FA dynamics results in a specific balance between adhesion and contraction, which induces maximal migration velocity.

Temporal change in local forces and total force all over the surface of the sea urchin egg during cytokinesis

We determined the tension over the entire surface of the sea urchin eggs during cytokinesis, on the basis of the intracellular pressure and cell shape. This allowed us to determine the temporal changes in both the distribution of local forces and the total force produced in the whole cell cortex. A spike-like peak at anaphase and a broader peak at the onset of furrowing were observed in the time-course of the total force. Treatment of the eggs with cytochalasin D, blebbistatin, ML-9, or ML-7 significantly lowered the total force when they inhibited cytokinesis, suggesting that the tension results mainly from the interaction between intact actin filaments and activated myosin II. Myosin II would function as a motor, not only in the furrow region, but over a wide area of the cell surface, because the sum of the tensions outside the furrow region was larger than that inside the furrow region throughout cytokinesis. The distribution of the local force revealed that a global increase in the cortical force started well before the onset of furrowing, and that the force inside the furrow region continued to increase despite the decrease in the force outside the furrow region after the onset of furrowing. The spatial and temporal patterns of the force over the entire surface support the hypothesis that there are two separate but coordinated actomyosin activation mechanisms, one of which induces global activation of the cortex and the other of which then maintains the contractility only inside the furrow region.

Rho kinase, myosin-II, and p42/44 MAPK control extracellular matrix-mediated apical bile canalicular lumen morphogenesis in HepG2 cells

The molecular mechanisms that regulate multicellular architecture and the development of extended apical bile canalicular lumens in hepatocytes are poorly understood. Here, we show that hepatic HepG2 cells cultured on glass coverslips first develop intercellular apical lumens typically formed by a pair of cells. Prolonged cell culture results in extensive organizational changes, including cell clustering, multilayering, and apical lumen morphogenesis. The latter includes the development of large acinar structures and subsequent elongated canalicular lumens that span multiple cells. These morphological changes closely resemble the early organizational pattern during development, regeneration, and neoplasia of the liver and are rapidly induced when cells are cultured on predeposited extracellular matrix (ECM). Inhibition of Rho kinase or its target myosin-II ATPase in cells cultured on glass coverslips mimics the morphogenic response to ECM. Consistently, stimulation of Rho kinase and subsequent myosin-II ATPase activity by lipoxygenase-controlled eicosatetranoic acid metabolism inhibits ECM-mediated cell multilayering and apical lumen morphogenesis but not initial apical lumen formation. Furthermore, apical lumen remodeling but not cell multilayering requires basal p42/44 MAPK activity. Together, the data suggest a role for hepatocyte-derived ECM in the spatial organization of hepatocytes and apical lumen morphogenesis and identify Rho kinase, myosin-II, and MAPK as potentially important players in different aspects of bile canalicular lumen morphogenesis.

A novel guanine nucleotide exchange factor MyoGEF is required for cytokinesis

The cleavage furrow is created by an actomyosin contractile ring that is regulated by small GTPase proteins such as Rac1 and RhoA. Guanine nucleotide exchange factors (GEFs) are positive regulators of the small GTPase proteins and have been implicated as important factors in regulating cytokinesis. However, it is still unclear how GEFs regulate the contractile ring during cytokinesis in mammalian cells. Here we report that a novel GEF, which is termed MyoGEF (myosin-interacting GEF), interacts with non-muscle myosin II and exhibits activity toward RhoA. MyoGEF and non-muscle myosin II colocalize to the cleavage furrow in early anaphase cells. Disruption of MyoGEF expression in U2OS cells by RNA interference (RNAi) results in the formation of multinucleated cells. These results suggest that MyoGEF, RhoA, and non-muscle myosin II act as a functional unit at the cleavage furrow to advance furrow ingression during cytokinesis.

Host tissue invasion by Entamoeba histolytica is powered by motility and phagocytosis

During amebiasis, E. histolytica motility is a key factor to achieve its progression across tissues. The pathogenicity of E. histolytica includes its capacity to phagocyte human cells. Motility requires polarization of E. histolytica that involves protrusion of a pseudopod containing actin and associated proteins [myosin IB, ABP-120 and a p21-activated kinase (PAK)] and whole-cell propulsion after contraction of the rear of the cell, where myosin II and F-actin are concentrated. An interesting characteristic of this parasite is the presence of two unique myosins (myosin II and unconventional myosin IB), in contrast to several actin genes. Little is known about the regulation of the actin-myosin cytoskeleton dynamics of E. histolytica, and a better understanding of signaling pathways that stimulate and coordinate regulators protein and cytoskeleton elements will provide new insight into the cell biology of the parasite and in amebiasis. Here we summarize the pleiotropic functions described for myosin II and PAK in E. histolytica. We propose that survival and pathogenicity of E. histolytica require an active actin-myosin cytoskeleton to cap surface receptors, to adhere to host components, to migrate through tissues, to phagocyte human cells and to form liver abscesses.