Cross-linking of actin filaments by myosin II is a major contributor to cortical integrity and cell motility in restrictive environments

Differential regulation of protrusion and polarity by PI3K during neutrophil motility in live zebrafish

Cell polarity is crucial for directed migration. Here we show that phosphoinositide 3-kinase (PI(3)K) mediates neutrophil migration in vivo by differentially regulating cell protrusion and polarity. The dynamics of PI(3)K products PI(3,4,5)P(3)-PI(3,4)P(2) during neutrophil migration were visualized in living zebrafish, revealing that PI(3)K activation at the leading edge is critical for neutrophil motility in intact tissues. A genetically encoded photoactivatable Rac was used to demonstrate that localized activation of Rac is sufficient to direct migration with precise temporal and spatial control in vivo. Similar stimulation of PI(3)K-inhibited cells did not direct migration. Localized Rac activation rescued membrane protrusion but not anteroposterior polarization of F-actin dynamics of PI(3)K-inhibited cells. Uncoupling Rac-mediated protrusion and polarization suggests a paradigm of two-tiered PI(3)K-mediated regulation of cell motility. This work provides new insight into how cell signaling at the front and back of the cell is coordinated during polarized cell migration in intact tissues within a multicellular organism.

Extracellular matrix with the rigidity of adipose tissue helps 3T3-L1 adipocytes maintain insulin responsiveness

Despite the popularity of 3T3-L1 adipocytes as a model system of adipocytes in vivo, they do not carry all of the cellular functions of adipocytes in vivo. In this study, we investigated the effect of extracellular matrix (ECM) rigidity on insulin signal transduction in 3T3-L1 adipocytes. On 250 Pa polyacrylamide gel (soft gel) laminated with a mixture of collagen type 1 and fibronectin, whose rigidity matches that of adipose tissue, expression of the insulin receptor, IRS-1 and AKT was upregulated and their insulin-stimulated phosphorylation was enhanced. Furthermore, the expression of GLUT1 was downregulated, whereas the expression of GLUT4 was unaffected as ECM rigidity decreased. Insulin-stimulated GLUT4 recruitment to the plasma membrane was significantly enhanced in cells seeded on soft gel. These results suggest that adjusting the ECM rigidity to that of adipose tissue augments insulin signaling in 3T3-L1 adipocytes and enhances insulin-stimulated GLUT4 recruitment to the plasma membrane.

Myosin IIA participates in docking of Glut4 storage vesicles with the plasma membrane in 3T3-L1 adipocytes

In adipocytes and myocytes, insulin stimulation translocates glucose transporter 4 (Glut4) storage vesicles (GSVs) from their intracellular storage sites to the plasma membrane (PM) where they dock with the PM. Then, Glut4 is inserted into the PM and initiates glucose uptake into these cells. Previous studies using chemical inhibitors demonstrated that myosin II participates in fusion of GSVs and the PM and increase in the intrinsic activity of Glut4. In this study, the effect of myosin IIA on GSV trafficking was examined by knocking down myosin IIA expression. Myosin IIA knockdown decreased both glucose uptake and exposures of myc-tagged Glut4 to the cell surface in insulin-stimulated cells, but did not affect insulin signal transduction. Interestingly, myosin IIA knockdown failed to decrease insulin-dependent trafficking of Glut4 to the PM. Moreover, in myosin IIA knockdown cells, insulin-stimulated binding of GSV SNARE protein, vesicle-associated membrane protein 2 (VAMP2) to PM SNARE protein, syntaxin 4 was inhibited. These data suggest that myosin IIA plays a role in insulin-stimulated docking of GSVs to the PM in 3T3-L1 adipocytes through SNARE complex formation.

Myosin II is essential for the spatiotemporal organization of traction forces during cell motility

Amoeboid motility results from pseudopod protrusions and retractions driven by traction forces of cells. We propose that the motor and actin-crosslinking functions of MyoII differentially control the temporal and spatial distribution of the traction forces, and establish mechanistic relationships between these distributions, enabling cells to move.

Amoeboid motility requires spatiotemporal coordination of biochemical pathways regulating force generation and consists of the quasi-periodic repetition of a motility cycle driven by actin polymerization and actomyosin contraction. Using new analytical tools and statistical methods, we provide, for the first time, a statistically significant quantification of the spatial distribution of the traction forces generated at each phase of the cycle (protrusion, contraction, retraction, and relaxation). We show that cells are constantly under tensional stress and that wild-type cells develop two opposing “pole” forces pulling the front and back toward the center whose strength is modulated up and down periodically in each cycle. We demonstrate that nonmuscular myosin II complex (MyoII) cross-linking and motor functions have different roles in controlling the spatiotemporal distribution of traction forces, the changes in cell shape, and the duration of all the phases. We show that the time required to complete each phase is dramatically increased in cells with altered MyoII motor function, demonstrating that it is required not only for contraction but also for protrusion. Concomitant loss of MyoII actin cross-linking leads to a force redistribution throughout the cell perimeter pulling inward toward the center. However, it does not reduce significantly the magnitude of the traction forces, uncovering a non–MyoII-mediated mechanism for the contractility of the cell.

The effects of extracellular calcium on motility, pseudopod and uropod formation, chemotaxis, and the cortical localization of myosin II in Dictyostelium discoideum

Extracellular Ca(++), a ubiquitous cation in the soluble environment of cells both free living and within the human body, regulates most aspects of amoeboid cell motility, including shape, uropod formation, pseudopod formation, velocity, and turning in Dictyostelium discoideum. Hence it affects the efficiency of both basic motile behavior and chemotaxis. Extracellular Ca(++) is optimal at 10 mM. A gradient of the chemoattractant cAMP generated in the absence of added Ca(++) only affects turning, but in combination with extracellular Ca(++), enhances the effects of extracellular Ca(++). Potassium, at 40 mM, can partially substitute for Ca(++). Mg(++), Mn(++), Zn(++), and Na(+) cannot. Extracellular Ca(++), or K(+), also induce the cortical localization of myosin II in a polar fashion. The effects of Ca(++), K(+) or a cAMP gradient do not appear to be similarly mediated by an increase in the general pool of free cytosolic Ca(++). These results suggest a model, in which each agent functioning through different signaling systems, converge to affect the cortical localization of myosin II, which in turn effects the behavioral changes leading to efficient cell motility and chemotaxis. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

How a cell crawls and the role of cortical myosin II

The movements of Dictyostelium discoideum amoebae translocating on a glass surface in the absence of chemoattractant have been reconstructed at 5-second intervals and motion analyzed by employing 3D-DIAS software. A morphometric analysis of pseudopods, the main cell body, and the uropod provides a comprehensive description of the basic motile behavior of a cell in four dimensions (4D), resulting in a list of 18 characteristics. A similar analysis of the myosin II phosphorylation mutant 3XASP reveals a role for the cortical localization of myosin II in the suppression of lateral pseudopods, formation of the uropod, cytoplasmic distribution of cytoplasm in the main cell body, and efficient motility. The results of the morphometric analysis suggest that pseudopods, the main cell body, and the uropod represent three motility compartments that are coordinated for efficient translocation. It provides a contextual framework for interpreting the effects of mutations, inhibitors, and chemoattractants on the basic motile behavior of D. discoideum. The generality of the characteristics of the basic motile behavior of D. discoideum must now be tested by similar 4D analyses of the motility of amoeboid cells of higher eukaryotic cells, in particular human polymorphonuclear leukocytes.

Mechanical modes of 'amoeboid' cell migration

The morphological term 'amoeboid' migration subsumes a number of rather distinct biophysical modes of cellular locomotion that range from blebbing motility to entirely actin-polymerization-based gliding. Here, we discuss the diverse principles of force generation and force transduction that lead to the distinct amoeboid phenotypes. We argue that shifting the balance between actin protrusion, actomyosin contraction, and adhesion to the extracellular substrate can explain the different modes of amoeboid movement and that blebbing and gliding are barely extreme variants of one common migration strategy. Depending on the cell type, physiological conditions or experimental manipulation, amoeboid cells can adopt the distinct mechanical modes of amoeboid migration.

Myosin II regulates extension, growth and patterning in the mammalian cochlear duct

The sensory epithelium of the mammalian cochlea comprises mechanosensory hair cells that are arranged into four ordered rows extending along the length of the cochlear spiral. The factors that regulate the alignment of these rows are unknown. Results presented here demonstrate that cellular patterning within the cochlea, including the formation of ordered rows of hair cells, arises through morphological remodeling that is consistent with the mediolateral component of convergent extension. Non-muscle myosin II is shown to be expressed in a pattern that is consistent with an active role in cellular remodeling within the cochlea, and genetic or pharmacological inhibition of myosin II results in defects in cellular patterning that are consistent with a disruption in convergence and extension. These results identify the first molecule, myosin II, which directly regulates cellular patterning and alignment within the cochlear sensory epithelium. Our results also provide insights into the cellular mechanisms that are required for the formation of highly ordered cellular patterns.

Myosin-IIA and ICAM-1 regulate the interchange between two distinct modes of T cell migration

How T cells achieve rapid chemotactic motility under certain circumstances and efficient cell surface surveillance in others is not fully understood. We show that T lymphocytes are motile in two distinct modes: a fast "amoeboid-like" mode, which uses sequential discontinuous contacts to the substrate; and a slower mode using a single continuously translating adhesion, similar to mesenchymal motility. Myosin-IIA is necessary for fast amoeboid motility, and our data suggests that this occurs via cyclical rear-mediated compressions that eliminate existing adhesions while licensing subsequent ones at the front of the cell. Regulation of Myosin-IIA function in T cells is thus a key mechanism to regulate surface contact area and crawling velocity within different environments. This can provide T lymphocytes with motile and adhesive properties that are uniquely suited toward alternative requirements for immune surveillance and response.

Regulation of Dictyostelium morphogenesis by RapGAP3

Rap1 is a key regulator of cell adhesion and cell motility in Dictyostelium. Here, we identify a Rap1-specific GAP protein (RapGAP3) and provide evidence that Rap1 signaling regulates cell-cell adhesion and cell migration within the multicellular organism. RapGAP3 mediates the deactivation of Rap1 at the late mound stage of development and plays an important role in regulating cell sorting during apical tip formation, when the anterior-posterior axis of the organism is formed, by controlling cell-cell adhesion and cell migration. The loss of RapGAP3 results in a severely altered morphogenesis of the multicellular organism at the late mound stage. Direct measurement of cell motility within the mound shows that rapGAP3(-) cells have a reduced speed of movement and, compared to wild-type cells, have a reduced motility towards the apex. rapGAP3(-) cells exhibit some increased EDTA/EGTA sensitive cell-cell adhesion at the late mound stage. RapGAP3 transiently and rapidly translocates to the cell cortex in response to chemoattractant stimulation, which is dependent on F-actin polymerization. We suggest that the altered morphogenesis and the cell-sorting defect of rapGAP3(-) cells may result in reduced directional movement of the mutant cells to the apex of the mound.

Morphogenetic movements driving neural tube closure in Xenopus require myosin IIB

Vertebrate neural tube formation involves two distinct morphogenetic events--convergent extension (CE) driven by mediolateral cell intercalation, and bending of the neural plate driven largely by cellular apical constriction. However, the cellular and molecular biomechanics of these processes are not understood. Here, using tissue-targeting techniques, we show that the myosin IIB motor protein complex is essential for both these processes, as well as for conferring resistance to deformation to the neural plate tissue. We show that myosin IIB is required for actin-cytoskeletal organization in both superficial and deep layers of the Xenopus neural plate. In the superficial layer, myosin IIB is needed for apical actin accumulation, which underlies constriction of the neuroepithelial cells, and that ultimately drive neural plate bending, whereas in the deep neural cells myosin IIB organizes a cortical actin cytoskeleton, which we describe for the first time, and that is necessary for both normal neural cell cortical tension and shape and for autonomous CE of the neural tissue. We also show that myosin IIB is required for resistance to deformation ("stiffness") in the neural plate, indicating that the cytoskeleton-organizing roles of this protein translate in regulation of the biomechanical properties of the neural plate at the tissue-level.

Dependence of myoblast fusion on a cortical actin wall and nonmuscle myosin IIA

Cell-cell fusion is a fundamental cellular process that is essential for development as well as fertilization. Myoblast fusion to form multinucleated skeletal muscle myotubes is a well studied, yet incompletely understood example of cell-cell fusion that is essential for formation of contractile skeletal muscle tissue. Studies in this report identify several novel cytoskeletal events essential to an early phase of myoblast fusion among cultured murine myoblasts. During myoblast pairing and alignment, cortical actin filaments organize into a dense actin wall structure that parallels and extends the length of the plasma membrane of the bipolar, aligned cells. As fusion progresses, gaps appear within the actin wall at sites of vesicle accumulation, the vesicles pair across the aligned myoblasts, cell-cell contacts and fusion pores form. Inhibition of nonmuscle myosin IIA (NM-MHC-IIA) motor activity prevents formation of this cortical actin wall, as well as the appearance of vesicles at a membrane proximal location, and myoblast fusion. These results suggest that early formation of a subplasmalemmal actin wall during myoblast alignment is a critical event for myoblast fusion that supports bipolar membrane alignment and temporally regulates trafficking of vesicles to the nascent fusion sites during skeletal muscle myoblast differentiation.

Up-regulation of Rho/ROCK signaling in sarcoma cells drives invasion and increased generation of protrusive forces

Tumor cell invasion is the most critical step of metastasis. Determination of the mode of invasion within the particular tumor is critical for effective cancer treatment. Protease-independent amoeboid mode of invasion has been described in carcinoma cells and more recently in sarcoma cells on treatment with protease inhibitors. To analyze invasive behavior, we compared highly metastatic sarcoma cells with parental nonmetastatic cells. The metastatic cells exhibited a functional up-regulation of Rho/ROCK signaling and, similarly to carcinoma cells, an amoeboid mode of invasion. Using confocal and traction force microscopy, we showed that an up-regulation of Rho/ROCK signaling leads to increased cytoskeletal dynamics, myosin light chain localization, and increased tractions at the leading edge of the cells and that all of these contributed to increased cell invasiveness in a three-dimensional collagen matrix. We conclude that cells of mesenchymal origin can use the amoeboid nonmesenchymal mode of invasion as their primary invading mechanism and show the dependence of ROCK-mediated amoeboid mode of invasion on the increased capacity of cells to generate force.

Moving towards a better understanding of chemotaxis

Eukaryotic cells are thought to move across supporting surfaces through a combination of coordinated processes: polarisation; extension of dynamic protrusions from a leading edge; adhesion-associated stabilisation of some protrusions; centripetal pulling against those leading adhesions; and de-adhesion at the rear. Gradients of extracellular ligands can be detected by cells and then used to guide them either towards the source (in the case of a chemoattractant) or away from the source (in the case of a chemorepellent)--such migration is termed chemotaxis. Recent work suggests that chemotaxis probably emerges from the ability of cells to spatially encode extracellular gradients of ligands, a process for which phosphoinositide 3'-kinase (PI3K) signals alone are insufficient, and to use that vectorial information to bias movement by enhancing the survival, and not the formation, of the protrusions that experience the greatest stimulation.

Dual role for myosin II in GLUT4-mediated glucose uptake in 3T3-L1 adipocytes

Insulin-stimulated glucose uptake requires the activation of several signaling pathways to mediate the translocation and fusion of GLUT4 vesicles to the plasma membrane. Our previous studies demonstrated that GLUT4-mediated glucose uptake is a myosin II-dependent process in adipocytes. The experiments described in this report are the first to show a dual role for the myosin IIA isoform specifically in regulating insulin-stimulated glucose uptake in adipocytes. We demonstrate that inhibition of MLCK but not RhoK results in impaired insulin-stimulated glucose uptake. Furthermore, our studies show that insulin specifically stimulates the phosphorylation of the RLC associated with the myosin IIA isoform via MLCK. In time course experiments, we determined that GLUT4 translocates to the plasma membrane prior to myosin IIA recruitment. We further show that recruitment of myosin IIA to the plasma membrane requires that myosin IIA be activated via phosphorylation of the RLC by MLCK. Our findings also reveal that myosin II is required for proper GLUT4-vesicle fusion at the plasma membrane. We show that once at the plasma membrane, myosin II is involved in regulating the intrinsic activity of GLUT4 after insulin stimulation. Collectively, our results are the first to reveal that myosin IIA plays a critical role in mediating insulin-stimulated glucose uptake in 3T3-LI adipocytes, via both GLUT4 vesicle fusion at the plasma membrane and GLUT4 activity.

Changing directions in the study of chemotaxis

Chemotaxis--the guided movement of cells in chemical gradients--probably first emerged in our single-celled ancestors and even today is recognizably similar in neutrophils and amoebae. Chemotaxis enables immune cells to reach sites of infection, allows wounds to heal and is crucial for forming embryonic patterns. Furthermore, the manipulation of chemotaxis may help to alleviate disease states, including the metastasis of cancer cells. This review discusses recent results concerning how cells orientate in chemotactic gradients and the role of phosphatidylinositol-3,4,5-trisphosphate, what produces the force for projecting pseudopodia and a new role for the endocytic cycle in movement.

Convergence and extension at gastrulation require a myosin IIB-dependent cortical actin network

Force-producing convergence (narrowing) and extension (lengthening) of tissues by active intercalation of cells along the axis of convergence play a major role in axial morphogenesis during embryo development in both vertebrates and invertebrates, and failure of these processes in human embryos leads to defects including spina bifida and anencephaly. Here we use Xenopus laevis, a system in which the polarized cell motility that drives this active cell intercalation has been related to the development of forces that close the blastopore and elongate the body axis, to examine the role of myosin IIB in convergence and extension. We find that myosin IIB is localized in the cortex of intercalating cells, and show by morpholino knockdown that this myosin isoform is essential for the maintenance of a stereotypical, cortical actin cytoskeleton as visualized with time-lapse fluorescent confocal microscopy. We show that this actin network consists of foci or nodes connected by cables and is polarized relative to the embryonic axis, preferentially cyclically shortening and lengthening parallel to the axis of cell polarization, elongation and intercalation, and also parallel to the axis of convergence forces during gastrulation. Depletion of MHC-B results in disruption of this polarized cytoskeleton, loss of the polarized protrusive activity characteristic of intercalating cells, eventual loss of cell-cell and cell-matrix adhesion, and dose-dependent failure of blastopore closure, arguably because of failure to develop convergence forces parallel to the myosin IIB-dependent dynamics of the actin cytoskeleton. These findings bridge the gap between a molecular-scale motor protein and tissue-scale embryonic morphogenesis.

Actin-cytoskeleton dynamics in non-monotonic cell spreading

The spreading of motile cells on a substrate surface is accompanied by reorganization of their actin network. We show that spreading in the highly motile cells of Dictyostelium is non-monotonic, and thus differs from the passage of spreading cells through a regular series of stages. Quantification of the gain and loss of contact area revealed fluctuating forces of protrusion and retraction that dominate the interaction of Dictyostelium cells with a substrate. The molecular basis of these fluctuations is elucidated by dual-fluorescence labeling of filamentous actin together with proteins that highlight specific activities in the actin system. Front-to-tail polarity is established by the sorting out of myosin-II from regions where dense actin assemblies are accumulating. Myosin-IB identifies protruding front regions, and the Arp2/3 complex localizes to lamellipodia protruded from the fronts. Coronin is used as a sensitive indicator of actin disassembly to visualize the delicate balance of polymerization and depolymerization in spreading cells. Short-lived actin patches that co-localize with clathrin suggest that membrane internalization occurs even when the substrate-attached cell surface expands. We conclude that non-monotonic cell spreading is characterized by spatiotemporal patterns formed by motor proteins together with regulatory proteins that either promote or terminate actin polymerization on the scale of seconds.

Depolymerization-driven flow in nematode spermatozoa relates crawling speed to size and shape

Cell crawling is an inherently physical process that includes protrusion of the leading edge, adhesion to the substrate, and advance of the trailing cell body. Research into advance of the cell body has focused on actomyosin contraction, with cytoskeletal disassembly regarded as incidental, rather than causative; however, extracts from nematode spermatozoa, which use Major Sperm Protein rather than actin, provide at least one example where cytoskeletal disassembly apparently generates force in the absence of molecular motors. To test whether depolymerization can explain force production during nematode sperm crawling, we constructed a mathematical model that simultaneously describes the dynamics of both the cytoskeleton and the cytosol. We also performed corresponding experiments using motile Caenorhabditis elegans spermatozoa. Our experiments reveal that crawling speed is an increasing function of both cell size and anterior-posterior elongation. The quantitative, depolymerization-driven model robustly predicts that cell speed should increase with cell size and yields a cytoskeletal disassembly rate that is consistent with previous measurements. Notably, the model requires anisotropic elasticity, with the cell being stiffer along the direction of motion, to accurately reproduce the dependence of speed on elongation. Our simulations also predict that speed should increase with cytoskeletal anisotropy and disassembly rate.

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.

Spatio-temporal analysis of eukaryotic cell motility by improved force cytometry

Cell motility plays an essential role in many biological systems, but precise quantitative knowledge of the biophysical processes involved in cell migration is limited. Better measurements are needed to ultimately build models with predictive capabilities. We present an improved force cytometry method and apply it to the analysis of the dynamics of the chemotactic migration of the amoeboid form of Dictyostelium discoideum. Our explicit calculation of the force field takes into account the finite thickness of the elastic substrate and improves the accuracy and resolution compared with previous methods. This approach enables us to quantitatively study the differences in the mechanics of the migration of wild-type (WT) and mutant cell lines. The time evolution of the strain energy exerted by the migrating cells on their substrate is quasi-periodic and can be used as a simple indicator of the stages of the cell motility cycle. We have found that the mean velocity of migration v and the period of the strain energy T cycle are related through a hyperbolic law v = L/T, where L is a constant step length that remains unchanged in mutants with adhesion or contraction defects. Furthermore, when cells adhere to the substrate, they exert opposing pole forces that are orders of magnitude higher than required to overcome the resistance from their environment.

PTEN plays a role in the suppression of lateral pseudopod formation during Dictyostelium motility and chemotaxis

It has been suggested that the phosphatydylinositol (3,4,5)-trisphosphate [PtdIns(3,4,5)P(3)] phosphatase and tensin homolog PTEN plays a fundamental role in Dictyostelium discoideum chemotaxis. To identify that role, the behavior of a pten(-) mutant was quantitatively analyzed using two-dimensional and three-dimensional computer-assisted methods. pten(-) cells were capable of polarizing and translocating in the absence of attractant, and sensing and responding to spatial gradients, temporal gradients and natural waves of attractant. However, all of these responses were compromised (i.e. less efficient) because of the fundamental incapacity of pten(-) cells to suppress lateral pseudopod formation and turning. This defect was equally manifested in the absence, as well as presence, of attractant. PTEN, which is constitutively localized in the cortex of polarized cells, was found essential for the attractant-stimulated increase in cortical myosin II and F-actin that is responsible for the increased suppression of pseudopods during chemotaxis. PTEN, therefore, plays a fundamental role in the suppression of lateral pseudopod formation, a process essential for the efficiency of locomotion and chemotaxis, but not in directional sensing.

Signaling pathways mediating chemotaxis in the social amoeba, Dictyostelium discoideum

Chemotaxis, or cell migration guided by chemical cues, is critical for a multitude of biological processes in a diverse array of organisms. Dictyostelium discoideum amoebae rely on chemotaxis to find food and to survive starvation conditions, and we have taken advantage of this system to study the molecular regulation of this vital cell behavior. Previous work has identified phosphoinositide signaling as one mechanism which may contribute to directional sensing and actin polymerization during chemotaxis; a mechanism which is conserved in mammalian neutrophils. In this review, we will discuss recent data on genes and pathways governing directional sensing and actin polymerization, with a particular emphasis on contributions from our laboratory.

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.

Revealing early steps of alpha2beta1 integrin-mediated adhesion to collagen type I by using single-cell force spectroscopy

We have characterized early steps of alpha(2)beta(1) integrin-mediated cell adhesion to a collagen type I matrix by using single-cell force spectroscopy. In agreement with the role of alpha(2)beta(1) as a collagen type I receptor, alpha(2)beta(1)-expressing Chinese hamster ovary (CHO)-A2 cells spread rapidly on the matrix, whereas alpha(2)beta(1)-negative CHO wild-type cells adhered poorly. Probing CHO-A2 cell detachment forces over a contact time range of 600 s revealed a nonlinear adhesion response. During the first 60 s, cell adhesion increased slowly, and forces associated with the smallest rupture events were consistent with the breakage of individual integrin-collagen bonds. Above 60 s, a fraction of cells rapidly switched into an activated adhesion state marked by up to 10-fold increased detachment forces. Elevated overall cell adhesion coincided with a rise of the smallest rupture forces above the value required to break a single-integrin-collagen bond, suggesting a change from single to cooperative receptor binding. Transition into the activated adhesion mode and the increase of the smallest rupture forces were both blocked by inhibitors of actomyosin contractility. We therefore propose a two-step mechanism for the establishment of alpha(2)beta(1)-mediated adhesion as weak initial, single-integrin-mediated binding events are superseded by strong adhesive interactions involving receptor cooperativity and actomyosin contractility.

The role of actin, actomyosin and microtubules in defining cell shape during the differentiation of Naegleria amebae into flagellates

Differentiation of Naegleria amebae into flagellates was used to examine the interaction between actin, actomyosin and microtubules in defining cell shape. Amebae, which lack microtubules except during mitosis, differentiate into flagellates with a fixed shape and a complex microtubule cytoskeleton in 120 min. Based on earlier models of ameboid motility it has been suggested that actomyosin is quiescent in flagellates. This hypothesis was tested by following changes in the cytoskeleton using three-dimensional reconstructions prepared by confocal microscopy of individual cells stained with antibodies against actin and tubulin as well as with phalloidin and DNase I. F-actin as defined by phalloidin staining was concentrated in expanding pseudopods. Most phalloidin staining was lost as cells rounded up before the onset of flagellum formation. Actin staining with a Naegleria-specific antibody that recognizes both F- and G-actin was confined to the cell cortex of both amebae and flagellates. DNase I demonstrated G-actin throughout all stages. Most of the actin in the cortex was not bound by phalloidin yet was resistant to detergent extraction suggesting that it was polymerized. The microtubule cytoskeleton of flagellates was intimately associated with this actin cortex. Treatment of flagellates with cytochalasin D produced a rapid loss of flagellate shape and the appearance of phalloidin staining while latrunculin A stabilized the flagellate shape. These results suggest that tension produced by an actomyosin network is required to maintain the flagellate shape. The rapid loss of the flagellate shape induced by drugs, which specifically block myosin light chain kinase, supports this hypothesis.

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.

Dissection of amoeboid movement into two mechanically distinct modes

The current dominant model of cell locomotion proposes that actin polymerization pushes against the membrane at the leading edge producing filopodia and lamellipodia that move the cell forward. Despite its success, this model does not fully explain the complex process of amoeboid motility, such as that occurring during embryogenesis and metastasis. Here, we show that Dictyostelium cells moving in a physiological milieu continuously produce `blebs' at their leading edges, and demonstrate that focal blebbing contributes greatly to their locomotion. Blebs are well-characterized spherical hyaline protrusions that occur when a patch of cell membrane detaches from its supporting cortex. Their formation requires the activity of myosin II, and their physiological contribution to cell motility has not been fully appreciated. We find that pseudopodia extension, cell body retraction and overall cell displacement are reduced under conditions that prevent blebbing, including high osmolarity and blebbistatin, and in myosin-II-null cells. We conclude that amoeboid motility comprises two mechanically different processes characterized by the production of two distinct cell-surface protrusions, blebs and filopodia-lamellipodia.

The regulation of myosin II in Dictyostelium

Dictyostelium conventional myosin (myosin II) is an abundant protein that plays a role in various cellular processes such as cytokinesis, cell protrusion and development. This review will focus on the signal transduction pathways that regulate myosin II during cell movement. Myosin II appears to have two modes of action in Dictyostelium: local stabilization of the cytoskeleton by myosin filament association to the actin meshwork (structural mode) and force generation by contraction of actin filaments (motor mode). Some processes, such as cell movement under restrictive environment, require only the structural mode of myosin. However, cytokinesis in suspension and uropod retraction depend on motor activity as well. Myosin II can self-assemble into bipolar filaments. The formation of these filaments is negatively regulated by heavy chain phosphorylation through the action of a set of novel alpha kinases and is relatively well understood. However, only recently it has become clear that the formation of bipolar filaments and their translocation to the cortex are separate events. Translocation depends on filamentous actin, and is regulated by a cGMP pathway and possibly also by the cAMP phosphodiesterase RegA and the p21-activated kinase PAKa. Myosin motor activity is regulated by phosphorylation of the regulatory light chain through myosin light chain kinase A. Unlike conventional light chain kinases, this enzyme is not regulated by calcium but is activated by cGMP-induced phosphorylation via an upstream kinase and subsequent autophosphorylation.

Myosin heavy-chain kinase A from Dictyostelium possesses a novel actin-binding domain that cross-links actin filaments

Myosin heavy-chain kinase A (MHCK A) catalyses the disassembly of myosin II filaments in Dictyostelium cells via myosin II heavy-chain phosphorylation. MHCK A possesses a 'coiled-coil'-enriched domain that mediates the oligomerization, cellular localization and actin-binding activities of the kinase. F-actin (filamentous actin) binding by the coiled-coil domain leads to a 40-fold increase in MHCK A activity. In the present study we examined the actin-binding characteristics of the coiled-coil domain as a means of identifying mechanisms by which MHCK A-mediated disassembly of myosin II filaments can be regulated in the cell. Co-sedimentation assays revealed that the coiled-coil domain of MHCK A binds co-operatively to F-actin with an apparent K(D) of approx. 0.5 muM and a stoichiometry of approx. 5:1 [actin/C(1-498)]. Further analyses indicate that the coiled-coil domain binds along the length of the actin filament and possesses at least two actin-binding regions. Quite surprisingly, we found that the coiled-coil domain cross-links actin filaments into bundles, indicating that MHCK A can affect the cytoskeleton in two important ways: (1) by driving myosin II-filament disassembly via myosin II heavy-chain phosphorylation, and (2) by cross-linking/bundling actin filaments. This discovery, along with other supporting data, suggests a model in which MHCK A-mediated bundling of actin filaments plays a central role in the recruitment and activation of the kinase at specific sites in the cell. Ultimately this provides a means for achieving the robust and highly localized disruption of myosin II filaments that facilitates polarized changes in cell shape during processes such as chemotaxis, cytokinesis and multicellular development.

Shroom4 (Kiaa1202) is an actin-associated protein implicated in cytoskeletal organization

All animal cells utilize a specialized set of cytoskeletal proteins to determine their overall shape and the organization of their intracellular compartments and organelles. During embryonic development, the dynamic nature of the actin cytoskeleton is critical for virtually all morphogenic events requiring changes in cell shape, migration, adhesion, and division. The behavior of the actin cytoskeleton is modulated by a myriad of accessory proteins. Shroom3 is an actin binding protein that regulates neural tube morphogenesis by eliciting changes in cell shape through a myosin II-dependent pathway. The Shroom-related gene SHROOM4 (formerly called KIAA1202) has also been implicated in neural development, as mutations in this gene are associated with human X-linked mental retardation. To better understand the function of Shrm4 in embryonic development, we have cloned mouse Shroom4 and characterized its protein product in vivo and in vitro. Shroom4 is expressed in a wide range of cell types during mouse development, including vascular endothelium and the polarized epithelium of the neural tube and kidney. In endothelial cells and embryo fibroblasts, endogenous Shroom4 co-distributes with myosin II to a distinct cytoplasmic population of F-actin and ectopic expression of Shroom4 in multiple cell types enhances or induces the formation of this actin-based structure. This localization is mediated, at least in part, by the direct interaction of Shroom4 and F-actin. Our results suggest that Shroom4 is a regulator of cytoskeletal architecture that may play an important role in vertebrate development.

Cytokinesis depends on the motor domains of myosin-II in fission yeast but not in budding yeast

Budding yeast possesses one myosin-II, Myo1p, whereas fission yeast has two, Myo2p and Myp2p, all of which contribute to cytokinesis. We find that chimeras consisting of Myo2p or Myp2p motor domains fused to the tail of Myo1p are fully functional in supporting budding yeast cytokinesis. Remarkably, the tail alone of budding yeast Myo1p localizes to the contractile ring, supporting both its constriction and cytokinesis. In contrast, fission yeast Myo2p and Myp2p require both the catalytic head domain as well as tail domains for function, with the tails providing distinct functions (Bezanilla and Pollard, 2000). Myo1p is the first example of a myosin whose cellular function does not require a catalytic motor domain revealing a novel mechanism of action for budding yeast myosin-II independent of actin binding and ATPase activity.

Signaling to cytoskeletal dynamics during chemotaxis

We review insights in signaling pathways controlling cell polarization and cytoskeletal organization during chemotactic movement in Dictyostelium amoebae and neutrophils. We compare and contrast these insights with our current understanding of pathways controlling chemotactic movements in more-complex multicellular developmental contexts.

A novel role for myosin II in insulin-stimulated glucose uptake in 3T3-L1 adipocytes

Insulin-stimulated glucose uptake requires the activation of several signaling pathways to mediate the translocation and fusion of GLUT4 vesicles from an intracellular pool to the plasma membrane. The studies presented here show that inhibition of myosin II activity impairs GLUT4-mediated glucose uptake but not GLUT4 translocation to the plasma membrane. We also show that adipocytes express both myosin IIA and IIB isoforms, and that myosin IIA is recruited to the plasma membrane upon insulin stimulation. Taken together, the data presented here represent the first demonstration that GLUT4-mediated glucose uptake is a myosin II-dependent process in adipocytes. Based on our findings, we hypothesize that myosin II is activated upon insulin stimulation and recruited to the cell cortex to facilitate GLUT4 fusion with the plasma membrane. The identification of myosin II as a key component of GLUT4-mediated glucose uptake represents an important advance in our understanding of the mechanisms regulating glucose homeostasis.

Leukocytes on the move with phosphoinositide 3-kinase and its downstream effectors

Cell signalling mediators derived from membrane phospholipids are frequent participants in biological processes. The family of phosphoinositide 3-kinases (PI3Ks) phosphorylate the membrane lipid phosphatidylinositol, generating second messengers that direct diverse responses. These PI3K products are fundamental for leukocyte migration or chemotaxis, a pivotal event during the immune response. This system is therefore of significant biomedical interest. This review focuses on the biochemistry and signalling pathways of PI3K, with particular emphasis on chemokine (chemotactic cytokine)-directed responses. The key objectives of chemotaxis are motility and direction. The latter--direction--requires distinct events at the front and back of a cell. In light of this, the coordinated localisation of signalling factors, an event choreographed by a sharp intracellular gradient of PI3K-derived products, is a common theme.

Subsecond reorganization of the actin network in cell motility and chemotaxis

Actin networks are continuously reorganized in cells that rapidly change their shape. Applying total internal reflection fluorescence microscopy at acquisition rates of 10-20 Hz, we measured an average growth rate of 3 microm.sec(-1) for filamentous actin structures throughout the entire substrate-attached cortex of Dictyostelium cells. New filaments often proceed along preexisting ones, resulting in bundle formation concurrent with filament growth. In cells that orientate in a gradient of chemoattractant, prominent assemblies of actin enriched in the Arp2/3 complex are inserted into the network, primarily at the base of filopods that point into the direction of the gradient. We propose that high turnover rates of actin filaments confer the plasticity to the cell cortex that is required for rapid accommodation to external stimuli.

RasGEF-containing proteins GbpC and GbpD have differential effects on cell polarity and chemotaxis in Dictyostelium

The regulation of cell polarity plays an important role in chemotaxis. Previously, two proteins termed GbpC and GbpD were identified in Dictyostelium, which contain RasGEF and cyclic nucleotide binding domains. Here we show that gbpC-null cells display strongly reduced chemotaxis, because they are unable to polarise effectively in a chemotactic gradient. However, gbpD-null mutants exhibit the opposite phenotype: cells display improved chemotaxis and appear hyperpolar, because cells make very few lateral pseudopodia, whereas the leading edge is continuously remodelled. Overexpression of GbpD protein results in severely reduced chemotaxis. Cells extend many bifurcated and lateral pseudopodia, resulting in the absence of a leading edge. Furthermore, cells are flat and adhesive owing to an increased number of substrate-attached pseudopodia. This GbpD phenotype is not dependent on intracellular cGMP or cAMP, like its mammalian homolog PDZ-GEF. Previously we showed that GbpC is a high-affinity cGMP-binding protein that acts via myosin II. We conclude that cGMP activates GbpC, mediating the chemoattractant-induced establishment of cell polarity through myosin. GbpD induces the formation of substrate-attached pseudopodia, resulting in increased attachment and suppression of polarity.

Acto-myosin cytoskeleton dependent viscosity and shear-thinning behavior of the amoeba cytoplasm

The mechanical behavior of the human parasite Entamoeba histolytica plays a major role in the invasive process of host tissues and vessels. In this study, we set up an intracellular rheological technique derived from magnetic tweezers to measure the viscoelastic properties within living amoebae. The experimental setup combines two magnetic fields at 90 degrees from each other and is adapted to an inverted microscope, which allows monitoring of the rotation of pairs of magnetic phagosomes. We observe either the response of the phagosome pair to an instantaneous 45 degrees rotation of the magnetic field or the response to a permanent uniform rotation of the field at a given frequency. By the first method, we concluded that the phagosome pairs experience a soft viscoelastic medium, represented by the same mechanical model previously described for the cytoplasm of Dictyostelium discoideum [Feneberg et al. in Eur Biophys J 30(4):284-294 2001]. By the second method, the permanent rotation of a pair allowed us to apply a constant shear rate and to calculate the apparent viscosity of the cytoplasm. As found for entangled polymers, the viscosity decreases with the shear rate applied (shear-thinning behavior) and exhibits a power-law-type thinning, with a corresponding exponent of 0.65. Treatment of amoeba with drugs that affect the actin polymer content demonstrated that the shear-thinning behavior of the cytoplasm depends on the presence of an intact actin cytoskeleton. These data present a physiologic relevance for Entamoeba histolytica virulence. The shear-thinning behavior could facilitate cytoplasm streamings during cell movement and cell deformation, under important shear experienced by the amoeba during the invasion of human tissues. In this study, we also investigated the role of the actin-based motor myosin II and concluded that myosin II stiffens the F-actin gel in living parasites likely by its cross-linking activity.

Actin Activation of Myosin Heavy Chain Kinase A in Dictyostelium

Studies in Dictyostelium discoideum have established that the cycle of myosin II bipolar filament assembly and disassembly controls the temporal and spatial localization of myosin II during critical cellular processes, such as cytokinesis and cell locomotion. Myosin heavy chain kinase A (MHCK A) is a key enzyme regulating myosin II filament disassembly through myosin heavy chain phosphorylation in Dictyostelium. Under various cellular conditions, MHCK A is recruited to actin-rich cortical sites and is preferentially enriched at sites of pseudopod formation, and thus MHCK A is proposed to play a role in regulating localized disassembly of myosin II filaments in the cell. MHCK A possesses an aminoterminal coiled-coil domain that participates in the oligomerization, cellular localization, and actin binding activities of the kinase. In the current study, we show that the interaction between the coiled-coil domain of MHCK A and filamentous actin leads to an approximately 40-fold increase in the initial rate of kinase catalytic activity. Actin-mediated activation of MHCK A involves increased rates of kinase autophosphorylation and requires the presence of the coiled-coil domain. Structure-function analyses revealed that the coiled-coil domain alone binds to actin filaments (apparent K(D) = 0.9 microm) and thus mediates the direct interaction with F-actin required for MHCK A activation. Collectively, these results indicate that MHCK A recruitment to actin-rich sites could lead to localized activation of the kinase via direct interaction with actin filaments, and thus this mode of kinase regulation may represent an important mechanism by which the cell achieves localized disassembly of myosin II filaments required for specific changes in cell shape.

Evidence for tension-based regulation of Drosophila MAL and SRF during invasive cell migration

Cells migrating through a tissue exert force via their cytoskeleton and are themselves subject to tension, but the effects of physical forces on cell behavior in vivo are poorly understood. Border cell migration during Drosophila oogenesis is a useful model for invasive cell movement. We report that this migration requires the activity of the transcriptional factor serum response factor (SRF) and its cofactor MAL-D and present evidence that nuclear accumulation of MAL-D is induced by cell stretching. Border cells that cannot migrate lack nuclear MAL-D but can accumulate it if they are pulled by other migrating cells. Like mammalian MAL, MAL-D also responds to activated Diaphanous, which affects actin dynamics. MAL-D/SRF activity is required to build a robust actin cytoskeleton in the migrating cells; mutant cells break apart when initiating migration. Thus, tension-induced MAL-D activity may provide a feedback mechanism for enhancing cytoskeletal strength during invasive migration.