Myosin I contributes to the generation of resting cortical tension

PakB binds to the SH3 domain of Dictyostelium Abp1 and regulates its effects on cell polarity and early development

Dictyostelium p21-activated kinase B (PakB) phosphorylates and activates class I myosins. PakB colocalizes with myosin I to actin-rich regions of the cell, including macropinocytic and phagocytic cups and the leading edge of migrating cells. Here we show that residues 1-180 mediate the cellular localization of PakB. Yeast two-hybrid and pull-down experiments identify two proline-rich motifs in PakB-1-180 that directly interact with the SH3 domain of Dictyostelium actin-binding protein 1 (dAbp1). dAbp1 colocalizes with PakB to actin-rich regions in the cell. The loss of dAbp1 does not affect the cellular distribution of PakB, whereas the loss of PakB causes dAbp1 to adopt a diffuse cytosolic distribution. Cosedimentation studies show that the N-terminal region of PakB (residues 1-70) binds directly to actin filaments, whereas dAbp1 exhibits only a low affinity for filamentous actin. PakB-1-180 significantly enhances the binding of dAbp1 to actin filaments. When overexpressed in PakB-null cells, dAbp1 completely blocks early development at the aggregation stage, prevents cell polarization, and significantly reduces chemotaxis rates. The inhibitory effects are abrogated by the introduction of a function-blocking mutation into the dAbp1 SH3 domain. We conclude that PakB plays a critical role in regulating the cellular functions of dAbp1, which are mediated largely by its SH3 domain.

Compression and dilation of the membrane-cortex layer generates rapid changes in cell shape

A cyclic process of membrane-cortex compression and dilation generates a traveling wave of cortical actin density that in turn generates oscillations in cell morphology.

Rapid changes in cellular morphology require a cell body that is highly flexible yet retains sufficient strength to maintain structural integrity. We present a mechanism that meets both of these requirements. We demonstrate that compression (folding) and subsequent dilation (unfolding) of the coupled plasma membrane–cortex layer generates rapid shape transformations in rounded cells. Two- and three-dimensional live-cell images showed that the cyclic process of membrane-cortex compression and dilation resulted in a traveling wave of cortical actin density. We also demonstrate that the membrane-cortex traveling wave led to amoeboid-like cell migration. The compression–dilation hypothesis offers a mechanism for large-scale cell shape transformations that is complementary to blebbing, where the plasma membrane detaches from the actin cortex and is initially unsupported when the bleb extends as a result of cytosolic pressure. Our findings provide insight into the mechanisms that drive the rapid morphological changes that occur in many physiological contexts, such as amoeboid migration and cytokinesis.

Actin cortex mechanics and cellular morphogenesis

The cortex is a thin, crosslinked actin network lying immediately beneath the plasma membrane of animal cells. Myosin motors exert contractile forces in the meshwork. Because the cortex is attached to the cell membrane, it plays a central role in cell shape control. The proteic constituents of the cortex undergo rapid turnover, making the cortex both mechanically rigid and highly plastic, two properties essential to its function. The cortex has recently attracted increasing attention and its functions in cellular processes such as cytokinesis, cell migration, and embryogenesis are progressively being dissected. In this review, we summarize current knowledge on the structural organization, composition, and mechanics of the actin cortex, focusing on the link between molecular processes and macroscopic physical properties. We also highlight consequences of cortex dysfunction in disease.

Cytokinesis mechanics and mechanosensing

Cytokinesis shape change occurs through the interfacing of three modules, cell mechanics, myosin II-mediated contractile stress generation and sensing, and a control system of regulatory proteins, which together ensure flexibility and robustness. This integrated system then defines the stereotypical shape changes of successful cytokinesis, which occurs under a diversity of mechanical contexts and environmental conditions.

Separation anxiety: stress, tension and cytokinesis

Cytokinesis, the physical separation of a mother cell into two daughter cells, progresses through a series of well-defined changes in morphology. These changes involve distinct biochemical and mechanical processes. Here, we review the mechanical features of cells during cytokinesis, discussing both the material properties as well as sources of stresses, both active and passive, which lead to the observed changes in morphology. We also describe a mechanosensory feedback control system that regulates protein localization and shape progression during cytokinesis.

In vitro mesothelial clearance assay that models the early steps of ovarian cancer metastasis

Ovarian cancer is the fifth leading cause of cancer related deaths in the United States(1). Despite a positive initial response to therapies, 70 to 90 percent of women with ovarian cancer develop new metastases, and the recurrence is often fatal(2). It is, therefore, necessary to understand how secondary metastases arise in order to develop better treatments for intermediate and late stage ovarian cancer. Ovarian cancer metastasis occurs when malignant cells detach from the primary tumor site and disseminate throughout the peritoneal cavity. The disseminated cells can form multicellular clusters, or spheroids, that will either remain unattached, or implant onto organs within the peritoneal cavity(3) (Figure 1, Movie 1). All of the organs within the peritoneal cavity are lined with a single, continuous, layer of mesothelial cells(4-6) (Figure 2). However, mesothelial cells are absent from underneath peritoneal tumor masses, as revealed by electron micrograph studies of excised human tumor tissue sections(3,5-7) (Figure 2). This suggests that mesothelial cells are excluded from underneath the tumor mass by an unknown process. Previous in vitro experiments demonstrated that primary ovarian cancer cells attach more efficiently to extracellular matrix than to mesothelial cells(8), and more recent studies showed that primary peritoneal mesothelial cells actually provide a barrier to ovarian cancer cell adhesion and invasion (as compared to adhesion and invasion on substrates that were not covered with mesothelial cells)(9,10). This would suggest that mesothelial cells act as a barrier against ovarian cancer metastasis. The cellular and molecular mechanisms by which ovarian cancer cells breach this barrier, and exclude the mesothelium have, until recently, remained unknown. Here we describe the methodology for an in vitro assay that models the interaction between ovarian cancer cell spheroids and mesothelial cells in vivo (Figure 3, Movie 2). Our protocol was adapted from previously described methods for analyzing ovarian tumor cell interactions with mesothelial monolayers(8-16), and was first described in a report showing that ovarian tumor cells utilize an integrin -dependent activation of myosin and traction force to promote the exclusion of the mesothelial cells from under a tumor spheroid(17). This model takes advantage of time-lapse fluorescence microscopy to monitor the two cell populations in real time, providing spatial and temporal information on the interaction. The ovarian cancer cells express red fluorescent protein (RFP) while the mesothelial cells express green fluorescent protein (GFP). RFP-expressing ovarian cancer cell spheroids attach to the GFP-expressing mesothelial monolayer. The spheroids spread, invade, and force the mesothelial cells aside creating a hole in the monolayer. This hole is visualized as the negative space (black) in the GFP image. The area of the hole can then be measured to quantitatively analyze differences in clearance activity between control and experimental populations of ovarian cancer and/ or mesothelial cells. This assay requires only a small number of ovarian cancer cells (100 cells per spheroid X 20-30 spheroids per condition), so it is feasible to perform this assay using precious primary tumor cell samples. Furthermore, this assay can be easily adapted for high throughput screening.

Constitutively active ezrin increases membrane tension, slows migration, and impedes endothelial transmigration of lymphocytes in vivo in mice

ERM (ezrin, radixin moesin) proteins in lymphocytes link cortical actin to plasma membrane, which is regulated in part by ERM protein phosphorylation. To assess whether phosphorylation of ERM proteins regulates lymphocyte migration and membrane tension, we generated transgenic mice whose T-lymphocytes express low levels of ezrin phosphomimetic protein (T567E). In these mice, T-cell number in lymph nodes was reduced by 27%. Lymphocyte migration rate in vitro and in vivo in lymph nodes decreased by 18% to 47%. Lymphocyte membrane tension increased by 71%. Investigations of other possible underlying mechanisms revealed impaired chemokine-induced shape change/lamellipod extension and increased integrin-mediated adhesion. Notably, lymphocyte homing to lymph nodes was decreased by 30%. Unlike most described homing defects, there was not impaired rolling or sticking to lymph node vascular endothelium but rather decreased migration across that endothelium. Moreover, decreased numbers of transgenic T cells in efferent lymph suggested defective egress. These studies confirm the critical role of ERM dephosphorylation in regulating lymphocyte migration and transmigration. Of particular note, they identify phospho-ERM as the first described regulator of lymphocyte membrane tension, whose increase probably contributes to the multiple defects observed in the ezrin T567E transgenic mice.

The role of adhesion energy in controlling cell-cell contacts

Recent advances in microscopy techniques and biophysical measurements have provided novel insight into the molecular, cellular and biophysical basis of cell adhesion. However, comparably little is known about a core element of cell–cell adhesion—the energy of adhesion at the cell–cell contact. In this review, we discuss approaches to understand the nature and regulation of adhesion energy, and propose strategies to determine adhesion energy between cells in vitro and in vivo.

Identification of calmodulin and MlcC as light chains for Dictyostelium myosin-I isozymes

Dictyostelium discoideum express seven single-headed myosin-I isozymes (MyoA-MyoE and MyoK) that drive motile processes at the cell membrane. The light chains for MyoA and MyoE were identified by expressing Flag-tagged constructs consisting of the motor domain and the two IQ motifs in the neck region in Dictyostelium. The MyoA and MyoE constructs both copurified with calmodulin. Isothermal titration calorimetry (ITC) showed that apo-calmodulin bound to peptides corresponding to the MyoA and MyoE IQ motifs with micromolar affinity. In the presence of calcium, calmodulin cross-linked two IQ motif peptides, with one domain binding with nanomolar affinity and the other with micromolar affinity. The IQ motifs were required for the actin-activated MgATPase activity of MyoA but not MyoE; however, neither myosin exhibited calcium-dependent activity. A Flag-tagged construct consisting of the MyoC motor domain and the three IQ motifs in the adjacent neck region bound a novel 8.6 kDa two EF-hand protein named MlcC, for myosin light chain for MyoC. MlcC is most similar to the C-terminal domain of calmodulin but does not bind calcium. ITC studies showed that MlcC binds IQ1 and IQ2 but not IQ3 of MyoC. IQ3 contains a proline residue that may render it nonfunctional. Each long-tailed Dictyostelium myosin-I has now been shown to have a unique light chain (MyoB-MlcB, MyoC-MlcC, and MyoD-MlcD), whereas the short-tailed myosins-I, MyoA and MyoE, have the multifunctional calmodulin as a light chain. The diversity in light chain composition is likely to contribute to the distinct cellular functions of each myosin-I isozyme.

Myosin-1C associates with microtubules and stabilizes the mitotic spindle during cell division

The mitotic spindle in eukaryotic cells is composed of a bipolar array of microtubules (MTs) and associated proteins that are required during mitosis for the correct partitioning of the two sets of chromosomes to the daughter cells. In addition to the well-established functions of MT-associated proteins (MAPs) and MT-based motors in cell division, there is increasing evidence that the F-actin-based myosin motors are important mediators of F-actin-MT interactions during mitosis. Here, we report the functional characterization of the long-tailed class-1 myosin myosin-1C from Dictyostelium discoideum during mitosis. Our data reveal that myosin-1C binds to MTs and has a role in maintenance of spindle stability for accurate chromosome separation. Both myosin-1C motor function and tail-domain-mediated MT-F-actin interactions are required for the cell-cycle-dependent relocalization of the protein from the cell periphery to the spindle. We show that the association of myosin-1C with MTs is mediated through the tail domain. The myosin-1C tail can inhibit kinesin motor activity, increase the stability of MTs, and form crosslinks between MTs and F-actin. These data illustrate that myosin-1C is involved in the regulation of MT function during mitosis in D. discoideum.

Single-molecule adhesion forces and attachment lifetimes of myosin-I phosphoinositide interactions

Phosphoinositides regulate the activities and localization of many cytoskeletal proteins involved in crucial biological processes, including membrane-cytoskeleton adhesion. Yet little is known about the mechanics of protein-phosphoinositide interactions, or about the membrane-attachment mechanics of any peripheral membrane proteins. Myosin-Ic (myo1c) is a molecular motor that links membranes to the cytoskeleton via phosphoinositide binding, so it is particularly important to understand the mechanics of its membrane attachment. We used optical tweezers to measure the strength and attachment lifetime of single myo1c molecules as they bind beads coated with a bilayer of 2% phosphatidylinositol 4,5-bisphosphate and 98% phosphatidylcholine. Adhesion forces measured under ramp-load ranged between 5.5 and 16 pN at loading rates between 250 and 1800 pN/s. Dissociation rates increased linearly with constant force (0.3-2.5 pN), with rates exceeding 360 s(-1) at 2.5 pN. Attachment lifetimes calculated from adhesion force measurements were loading-rate-dependent, suggesting nonadiabatic behavior during pulling. The adhesion forces of myo1c with phosphoinositides are greater than the motors stall forces and are within twofold of the force required to extract a lipid molecule from the membrane. However, attachment durations are short-lived, suggesting that phosphoinositides alone do not provide the mechanical stability required to anchor myo1c to membranes during multiple ATPase cycles.

Target-specific mechanics of phagocytosis: protrusive neutrophil response to zymosan differs from the uptake of antibody-tagged pathogens

The physical mechanisms that control target-specific responses of human neutrophils to distinct immune threats are poorly understood. Using dual-micropipette manipulation, we have quantified and compared the time courses of neutrophil phagocytosis of two different targets: zymosan (a prominent model of fungal infection), and antibody-coated (Fc) particles. Our single-live-cell/single-target approach exposes surprising differences between these two forms of phagocytosis. Unlike the efficient uptake of 3-μm Fc targets (within ~66 seconds), the engulfment of similarly sized zymosan is slow (~167 seconds), mainly due to the formation of a characteristic pedestal that initially pushes the particle outwards by ~1 μm. Despite a roughly twofold difference in maximum cortical tensions, the top 'pull-in' speeds of zymosan and Fc targets are indistinguishable at ~33 nm/second. Drug inhibition shows that both actin as well as myosin II partake in the regulation of neutrophil cortical tension and cytoplasmic viscosity; other than that, myosin II appears to play a minor role in both forms of phagocytosis. Remarkably, an intact actin cytoskeleton is required to suppress, in antibody-mediated phagocytosis, the initially protrusive deformation that distinguishes the neutrophil response to zymosan.

Mechanics and regulation of cell shape during the cell cycle

Many cell types undergo dramatic changes in shape throughout the cell cycle. For individual cells, a tight control of cell shape is crucial during cell division, but also in interphase, for example during cell migration. Moreover, cell cycle-related cell shape changes have been shown to be important for tissue morphogenesis in a number of developmental contexts. Cell shape is the physical result of cellular mechanical properties and of the forces exerted on the cell. An understanding of the causes and repercussions of cell shape changes thus requires knowledge of both the molecular regulation of cellular mechanics and how specific changes in cell mechanics in turn effect global shape changes. In this chapter, we provide an overview of the current knowledge on the control of cell morphology, both in terms of general cell mechanics and specifically during the cell cycle.

Physical model of cellular symmetry breaking

Cells can polarize in response to external signals, such as chemical gradients, cell-cell contacts, and electromagnetic fields. However, cells can also polarize in the absence of an external cue. For example, a motile cell, which initially has a more or less round shape, can lose its symmetry spontaneously even in a homogeneous environment and start moving in random directions. One of the principal determinants of cell polarity is the cortical actin network that underlies the plasma membrane. Tension in this network generated by myosin motors can be relaxed by rupture of the shell, leading to polarization. In this article, we discuss how simplified model systems can help us to understand the physics that underlie the mechanics of symmetry breaking.

Spatial organization of adhesion: force-dependent regulation and function in tissue morphogenesis

Integrin- and cadherin-mediated adhesion is central for cell and tissue morphogenesis, allowing cells and tissues to change shape without loosing integrity. Studies predominantly in cell culture showed that mechanosensation through adhesion structures is achieved by force-mediated modulation of their molecular composition. The specific molecular composition of adhesion sites in turn determines their signalling activity and dynamic reorganization. Here, we will review how adhesion sites respond to mecanical stimuli, and how spatially and temporally regulated signalling from different adhesion sites controls cell migration and tissue morphogenesis.

Computational model for cell morphodynamics

We develop a computational model, based on the phase-field method, for cell morphodynamics and apply it to fish keratocytes. Our model incorporates the membrane bending force and the surface tension and enforces a constant area. Furthermore, it implements a cross-linked actin filament field and an actin bundle field that are responsible for the protrusion and retraction forces, respectively. We show that our model predicts steady state cell shapes with a wide range of aspect ratios, depending on system parameters. Furthermore, we find that the dependence of the cell speed on this aspect ratio matches experimentally observed data.

Dynamical organization of the cytoskeletal cortex probed by micropipette aspiration

Bleb-based cell motility proceeds by the successive inflation and retraction of large spherical membrane protrusions ("blebs") coupled with substrate adhesion. In addition to their role in motility, cellular blebs constitute a remarkable illustration of the dynamical interactions between the cytoskeletal cortex and the plasma membrane. Here we study the bleb-based motions of Entamoeba histolytica in the constrained geometry of a micropipette. We construct a generic theoretical model that combines the polymerization of an actin cortex underneath the plasma membrane with the myosin-generated contractile stress in the cortex and the stress-induced failure of membrane-cortex adhesion. One major parameter dictating the cell response to micropipette suction is the stationary cortex thickness, controlled by actin polymerization and depolymerization. The other relevant physical parameters can be combined into two characteristic cortex thicknesses for which the myosin stress (i) balances the suction pressure and (ii) provokes membrane-cortex unbinding. We propose a general phase diagram for cell motions inside a micropipette by comparing these three thicknesses. In particular, we theoretically predict and experimentally verify the existence of saltatory and oscillatory motions for a well-defined range of micropipette suction pressures.

Leveraging the membrane - cytoskeleton interface with myosin-1

Class 1 myosins are small motor proteins with the ability to simultaneously bind to actin filaments and cellular membranes. Given their ability to generate mechanical force, and their high prevalence in many cell types, these molecules are well positioned to carry out several important biological functions at the interface of membrane and the actin cytoskeleton. Indeed, recent studies implicate these motors in endocytosis, exocytosis, release of extracellular vesicles, and the regulation of tension between membrane and the cytoskeleton. Many class 1 myosins also exhibit a load-dependent mechano-chemical cycle that enables them to maintain tension for long periods of time without hydrolyzing ATP. These properties put myosins-1 in a unique position to regulate dynamic membrane-cytoskeleton interactions and respond to physical forces during these events.

A myosin IK-Abp1-PakB circuit acts as a switch to regulate phagocytosis efficiency

Actin dynamics and myosin (Myo) contractile forces are necessary for formation and closure of the phagocytic cup. In Dictyostelium, the actin-binding protein Abp1 and myosin IK are enriched in the closing cup and especially at an actin-dense constriction furrow formed around the neck of engulfed budded yeasts. This phagocytic furrow consists of concentric overlapping rings of MyoK, Abp1, Arp3, coronin, and myosin II, following an order strikingly reminiscent of the overall organization of the lamellipodium of migrating cells. Mutation analyses of MyoK revealed that both a C-terminal farnesylation membrane anchor and a Gly-Pro-Arg domain that interacts with profilin and Abp1 were necessary for proper localization in the furrow and efficient phagocytosis. Consequently, we measured the binding affinities of these interactions and unraveled further interactions with profilins, dynamin A, and PakB. Due to the redundancy of the interaction network, we hypothesize that MyoK and Abp1 are restricted to regulatory roles and might affect the dynamic of cup progression. Indeed, phagocytic uptake was regulated antagonistically by MyoK and Abp1. MyoK is phosphorylated by PakB and positively regulates phagocytosis, whereas binding of Abp1 negatively regulates PakB and MyoK. We conclude that a MyoK-Abp1-PakB circuit acts as a switch regulating phagocytosis efficiency of large particles.

Biology and physics of cell shape changes in development

Together with cell growth, division and death, changes in cell shape are of central importance for tissue morphogenesis during development. Cell shape is the product of a cell's material and active properties balanced by external forces. Control of cell shape, therefore, relies on both tight regulation of intracellular mechanics and the cell's physical interaction with its environment. In this review, we first discuss the biological and physical mechanisms of cell shape control. We next examine a number of developmental processes in which cell shape change - either individually or in a coordinated manner - drives embryonic morphogenesis and discuss how cell shape is controlled in these processes. Finally, we emphasize that cell shape control during tissue morphogenesis can only be fully understood by using a combination of cellular, molecular, developmental and biophysical approaches.

Myosin 1G (Myo1G) is a haematopoietic specific myosin that localises to the plasma membrane and regulates cell elasticity

Immune cells navigate through different environments where they experience different mechanical forces. Responses to external forces are determined by the mechanical properties of a cell and they depend to a large extent on the actin-rich cell cortex. We report here that Myo1G, a previously uncharacterised member of class I myosins, is expressed specifically in haematopoietic tissues and cells. It is associated with the plasma membrane. This association is dependent on a conserved PH-domain-like myosin I tail homology motif and the head domain. However, the head domain does not need to be a functional motor. Knockdown of Myo1G in Jurkat cells decreased cell elasticity significantly. We propose that Myo1G regulates cell elasticity by deformations of the actin network at the cell cortex.

Role of cortical tension in bleb growth

Blebs are spherical membrane protrusions often observed during cell migration, cell spreading, cytokinesis, and apoptosis, both in cultured cells and in vivo. Bleb expansion is thought to be driven by the contractile actomyosin cortex, which generates hydrostatic pressure in the cytoplasm and can thus drive herniations of the plasma membrane. However, the role of cortical tension in bleb formation has not been directly tested, and despite the importance of blebbing, little is known about the mechanisms of bleb growth. In order to explore the link between cortical tension and bleb expansion, we induced bleb formation on cells with different tensions. Blebs were nucleated in a controlled manner by laser ablation of the cortex, mimicking endogenous bleb nucleation. Cortical tension was modified by treatments affecting the level of myosin activity or proteins regulating actin turnover. We show that there is a critical tension below which blebs cannot expand. Above this threshold, the maximal size of a bleb strongly depends on tension, and this dependence can be fitted with a model of the cortex as an active elastic material. Together, our observations and model allow us to relate bleb shape parameters to the underlying cellular mechanics and provide insights as to how bleb formation can be biochemically regulated during cell motility.

Control of cell membrane tension by myosin-I

All cell functions that involve membrane deformation or a change in cell shape (e.g., endocytosis, exocytosis, cell motility, and cytokinesis) are regulated by membrane tension. While molecular contacts between the plasma membrane and the underlying actin cytoskeleton are known to make significant contributions to membrane tension, little is known about the molecules that mediate these interactions. We used an optical trap to directly probe the molecular determinants of membrane tension in isolated organelles and in living cells. Here, we show that class I myosins, a family of membrane-binding, actin-based motor proteins, mediate membrane/cytoskeleton adhesion and thus, make major contributions to membrane tension. These studies show that class I myosins directly control the mechanical properties of the cell membrane; they also position these motor proteins as master regulators of cellular events involving membrane deformation.

Pulsed-laser creation and characterization of giant plasma membrane vesicles from cells

Femtosecond-pulsed laser irradiation was found to initiate giant plasma membrane vesicle (GPMV) formation on individual cells. Laser-induced GPMV formation resulted from intracellular cavitation and did not require the addition of chemical stressors to the cellular environment. The viscosity, structure, and contents of laser-induced GPMVs were measured with fluorescence microscopy and single-particle tracking. These GPMVs exhibit the following properties: (1) GPMVs grow fastest immediately after laser irradiation; (2) GPMVs contain barriers to free diffusion of incorporated fluorescent beads; (3) materials from both the cytoplasm and surrounding media flow into the growing GPMVs; (4) the GPMVs are surrounded by phospholipids, including phosphatidylserine; (5) F-actin is incorporated into the vesicles; and (6) caspase activity is not essential for GPMV formation. The effective viscosity of 65 nm polystyrene nanoparticles within GPMVs ranged from 32 to 434 cP. The nanoparticle diffusion was commonly affected by relatively large, macromolecular structures within the bleb.

Unconventional myosins acting unconventionally

Unconventional myosins are proteins that bind actin filaments in an ATP-regulated manner. Because of their association with membranes, they have traditionally been viewed as motors that function primarily to transport membranous organelles along actin filaments. Recently, however, a wealth of roles for myosins that are not obviously related to organelle transport have been uncovered, including organization of F-actin, mitotic spindle regulation and gene transcription. Furthermore, it has also become apparent that the motor domains of different myosins vary strikingly in their biophysical attributes. We suggest that the assumption that most unconventional myosins function primarily as organelle transporters might be misguided.

Cortical factor feedback model for cellular locomotion and cytofission

Eukaryotic cells can move spontaneously without being guided by external cues. For such spontaneous movements, a variety of different modes have been observed, including the amoeboid-like locomotion with protrusion of multiple pseudopods, the keratocyte-like locomotion with a widely spread lamellipodium, cell division with two daughter cells crawling in opposite directions, and fragmentations of a cell to multiple pieces. Mutagenesis studies have revealed that cells exhibit these modes depending on which genes are deficient, suggesting that seemingly different modes are the manifestation of a common mechanism to regulate cell motion. In this paper, we propose a hypothesis that the positive feedback mechanism working through the inhomogeneous distribution of regulatory proteins underlies this variety of cell locomotion and cytofission. In this hypothesis, a set of regulatory proteins, which we call cortical factors, suppress actin polymerization. These suppressing factors are diluted at the extending front and accumulated at the retracting rear of cell, which establishes a cellular polarity and enhances the cell motility, leading to the further accumulation of cortical factors at the rear. Stochastic simulation of cell movement shows that the positive feedback mechanism of cortical factors stabilizes or destabilizes modes of movement and determines the cell migration pattern. The model predicts that the pattern is selected by changing the rate of formation of the actin-filament network or the threshold to initiate the network formation.

Modeling cellular deformations using the level set formalism

Background

Many cellular processes involve substantial shape changes. Traditional simulations of these cell shape changes require that grids and boundaries be moved as the cell's shape evolves. Here we demonstrate that accurate cell shape changes can be recreated using level set methods (LSM), in which the cellular shape is defined implicitly, thereby eschewing the need for updating boundaries.

Results

We obtain a viscoelastic model of Dictyostelium cells using micropipette aspiration and show how this viscoelastic model can be incorporated into LSM simulations to recreate the observed protrusion of cells into the micropipette faithfully. We also demonstrate the use of our techniques by simulating the cell shape changes elicited by the chemotactic response to an external chemoattractant gradient.

Conclusion

Our results provide a simple but effective means of incorporating cellular deformations into mathematical simulations of cell signaling. Such methods will be useful for simulating important cellular events such as chemotaxis and cytokinesis.

Regulation of sinusoidal perfusion in portal hypertension

Portal hypertension, a major complication of cirrhosis, is caused by both increased portal blood flow and augmented intrahepatic vascular resistance. Even though the latter is primarily caused by anatomical changes, it has become clear that dynamic factors contribute to the increased hepatic vascular resistance. The hepatic sinusoid is the narrowest vascular structure within the liver and is the principal site of blood flow regulation. The anatomical location of hepatic stellate cells, which embrace the sinusoids, provides a favorable arrangement for sinusoidal constriction, and for control of sinusoidal vascular tone and blood flow. Hepatic stellate cells possess the essential contractile apparatus for cell contraction and relaxation. Moreover, the mechanisms of stellate cell contraction are better understood, and many substances which influence contractility have been identified, providing a rationale and opportunity for targeting these cells in the treatment of portal hypertension in cirrhosis.

Moesin controls cortical rigidity, cell rounding, and spindle morphogenesis during mitosis

BACKGROUND

During mitosis, animal cells undergo a complex sequence of morphological changes, from retraction of the cell margin and cell rounding at the onset of mitosis to axial elongation and cytokinesis at mitotic exit. The molecular mechanisms driving the early changes in mitotic cell form and their functional significance, however, remain unknown. Here we identify Moesin as a key player. Moesin is the sole Drosophila member of the ERM proteins, which, once activated via phosphorylation, crosslink actin filaments to the cytoplasmic tails of plasma membrane proteins.

RESULTS

We find that the Moesin is activated upon entry into mitosis, is necessary for the accompanying increase in cortical rigidity and cell rounding and, when artificially activated, is sufficient to induce both processes in interphase cells, independently of Myosin II. This phospho-Moesin-induced increase in cortical rigidity plays an important role during mitotic progression, because spindle morphogenesis and chromosome alignment are compromised in Moesin RNAi cells. Significantly, however, the spindle defects observed in soft metaphase cells can be rescued by the re-establishment of cortical tension from outside the cell.

CONCLUSIONS

These data show that changes in the activity and localization of Moesin that accompany mitotic progression contribute to the establishment of a stiff, rounded cortex at metaphase and to polar relaxation at anaphase and reveal the importance of this Moesin-induced increase in cortical rigidity for spindle morphogenesis and orderly chromosome segregation. In doing so, they help to explain why dynamic changes in cortical architecture are a universal feature of mitosis in animal cells.

Tensile forces govern germ-layer organization in zebrafish

Understanding the factors that direct tissue organization during development is one of the most fundamental goals in developmental biology. Various hypotheses explain cell sorting and tissue organization on the basis of the adhesive and mechanical properties of the constituent cells. However, validating these hypotheses has been difficult due to the lack of appropriate tools to measure these parameters. Here we use atomic force microscopy (AFM) to quantify the adhesive and mechanical properties of individual ectoderm, mesoderm and endoderm progenitor cells from gastrulating zebrafish embryos. Combining these data with tissue self-assembly in vitro and the sorting behaviour of progenitors in vivo, we have shown that differential actomyosin-dependent cell-cortex tension, regulated by Nodal/TGFbeta-signalling (transforming growth factor beta), constitutes a key factor that directs progenitor-cell sorting. These results demonstrate a previously unrecognized role for Nodal-controlled cell-cortex tension in germ-layer organization during gastrulation.

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.

Toward the structure of dynamic membrane-anchored actin networks: an approach using cryo-electron tomography

In the cortex of a motile cell, membrane-anchored actin filaments assemble into structures of varying shape and function. Filopodia are distinguished by a core of bundled actin filaments within finger-like extensions of the membrane. In a recent paper by Medalia et al(1) cryo-electron tomography has been used to reconstruct, from filopodia of Dictyostelium cells, the 3-dimensional organization of actin filaments in connection with the plasma membrane. A special arrangement of short filaments converging toward the filopod's tip has been called a "terminal cone". In this region force is applied for protrusion of the membrane. Here we discuss actin organization in the filopodia of Dictyostelium in the light of current views on forces that are generated by polymerizing actin filaments, and on the resistance of membranes against deformation that counteracts these forces.

Effect of temperature on tether extraction, surface protrusion, and cortical tension of human neutrophils

Neutrophil rolling on endothelial cells, the initial stage of its migrational journey to a site of inflammation, is facilitated by tether extraction and surface protrusion. Both phenomena have been studied extensively at room temperature, which is considerably lower than human body temperature. It is known that temperature greatly affects cellular mechanical properties such as viscosity. Therefore, we carried out tether extraction, surface protrusion, and cortical tension experiments at 37 degrees C with the micropipette aspiration technique. The experimental temperature was elevated using a custom-designed microscope chamber for the micropipette aspiration technique. To evaluate the constant temperature assumption in our experiments, the temperature distribution in the whole chamber was computed with finite element simulation. Our simulation results showed that temperature variation around the location where our experiments were performed was less than 0.2 degrees C. For tether extraction at 37 degrees C, the threshold force required to pull a tether (40 pN) was not statistically different from the value at room temperature (51 pN), whereas the effective viscosity (0.75 pN.s/microm) decreased significantly from the value at room temperature (1.5 pN.s/microm). Surface protrusion, which was modeled as a linear deformation, had a slightly smaller spring constant at 37 degrees C (40 pN/microm) than it did at room temperature (56 pN/microm). However, the cortical tension at 37 degrees C (5.7+/-2.2 pN/microm) was substantially smaller than that at room temperature (23+/-8 pN/microm). These data clearly suggest that neutrophils roll differently at body temperature than they do at room temperature by having distinct mechanical responses to shear stress of blood flow.

The effect of cellular cholesterol on membrane-cytoskeleton adhesion

Whereas recent studies suggest that cholesterol plays important role in the regulation of membrane proteins, its effect on the interaction of the cell membrane with the underlying cytoskeleton is not well understood. Here, we investigated this by measuring the forces needed to extract nanotubes (tethers) from the plasma membrane, using atomic force microscopy. The magnitude of these forces provided a direct measure of cell stiffness, cell membrane effective surface viscosity and association with the underlying cytoskeleton. Furthermore, we measured the lateral diffusion constant of a lipid analog DiIC12, using fluorescence recovery after photobleaching, which offers additional information on the organization of the membrane. We found that cholesterol depletion significantly increased the adhesion energy between the membrane and the cytoskeleton and decreased the membrane diffusion constant. An increase in cellular cholesterol to a level higher than that in control cells led to a decrease in the adhesion energy and the membrane surface viscosity. Disassembly of the actin network abrogated all the observed effects, suggesting that cholesterol affects the mechanical properties of a cell through the underlying cytoskeleton. The results of these quantitative studies may help to better understand the biomechanical processes accompanying the development of atherosclerosis.

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.

Cracking up: symmetry breaking in cellular systems

The shape of animal cells is, to a large extent, determined by the cortical actin network that underlies the cell membrane. Because of the presence of myosin motors, the actin cortex is under tension, and local relaxation of this tension can result in cortical flows that lead to deformation and polarization of the cell. Cortex relaxation is often regulated by polarizing signals, but the cortex can also rupture and relax spontaneously. A similar tension-induced polarization is observed in actin gels growing around beads, and we propose that a common mechanism governs actin gel rupture in both systems.

Active mechanical stabilization of the viscoplastic intracellular space of Dictyostelia cells by microtubule-actin crosstalk

The micro-viscoelasticity of the intracellular space of Dictyostelium discoideum cells is studied by evaluating the intracellular transport of magnetic force probes and their viscoelastic responses to force pulses of 20-700 pN. The role of the actin cortex, the microtubule (MT) aster and their crosstalk is explored by comparing the behaviour of wild-type cells, myosin II null mutants, latrunculin A and benomyl treated cells. The MT coupled beads perform irregular local and long range directed motions which are characterized by measuring their velocity distributions (P(v)). The correlated motion of the MT and the centrosome are evaluated by microfluorescence of GFP-labelled MTs. P(v) can be represented by log-normal distributions with long tails and it is determined by random sweeping motions (v approximately 0.5 microm/s) of the MTs (caused by tangential forces on the filament ends coupled to the actin cortex) and by intermittent bead transports parallel to the MTs (v(max) approximately 1.5 microm/s). The tails are due to spontaneous filament deflections (with speeds up to 10 microm/s) attributed to pre-stressing of the MT by local cortical tensions, generated by dynactin motors generating plus-end directed forces in the MTs. The viscoelastic responses are strongly non-linear and are mostly directed opposite or perpendicular to the force, showing that the cytoplasm behaves as an active viscoplastic body with time and force dependent drag coefficients. Nano-Newton loads exerted on the soft MT are balanced by traction forces arising at the MT ends coupled to the actin cortex and the centrosome, respectively. The mechanical coupling between the soft microtubules and the viscoelastic actin cortex provides cells with high mechanical stability despite the softness of the cytoplasm.

Modulation of the reaction rate of regulating protein induces large morphological and motional change of amoebic cell

Morphologies of moving amoebae are categorized into two types. One is the "neutrophil" type in which the long axis of cell roughly coincides with its moving direction. This type of cell extends a leading edge at the front and retracts a narrow tail at the rear, whose shape has been often drawn as a typical amoeba in textbooks. The other one is the "keratocyte" type with widespread lamellipodia along the front side arc. Short axis of cell in this type roughly coincides with its moving direction. In order to understand what kind of molecular feature causes conversion between two types of morphologies, and how two typical morphologies are maintained, a mathematical model of amoebic cells is developed. This model describes movement of cell and intracellular reactions of activator, inhibitor and actin filaments in a unified way. It is found that the producing rate of activator is a key factor of conversion between two types. This model also explains the observed data that the keratocyte type cells tend to rapidly move along a straight line. The neutrophil type cells move along a straight line when the moving velocity is small, but they show fluctuated motions deviating from a line when they move as fast as the keratocyte type cells. Efficient energy consumption in the neutrophil type cells is predicted.

Continuous membrane-cytoskeleton adhesion requires continuous accommodation to lipid and cytoskeleton dynamics

The plasma membrane of most animal cells conforms to the cytoskeleton and only occasionally separates to form blebs. Previous studies indicated that many weak interactions between cytoskeleton and the lipid bilayer kept the surfaces together to counteract the normal outward pressure of cytoplasm. Either the loss of adhesion strength or the formation of gaps in the cytoskeleton enables the pressure to form blebs. Membrane-associated cytoskeleton proteins, such as spectrin and filamin, can control the movement and aggregation of membrane proteins and lipids, e.g., phosphoinositol phospholipids (PIPs), as well as blebbing. At the same time, lipids (particularly PIPs) and membrane proteins affect cytoskeleton and signaling dynamics. We consider here the roles of the major phosphatidylinositol-4,5-diphosphate (PIP2) binding protein, MARCKS, and PIP2 levels in controlling cytoskeleton dynamics. Further understanding of dynamics will provide important clues about how membrane-cytoskeleton adhesion rapidly adjusts to cytoskeleton and membrane dynamics.

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.

Dictyostelium myosin II mechanochemistry promotes active behavior of the cortex on long time scales

Cell cortices rearrange dynamically to complete cytokinesis, crawlin response to chemoattractant, build tissues, and make neuronal connections. Highly enriched in the cell cortex, actin, myosin II, and actin crosslinkers facilitate cortical movements. Because cortical behavior is the consequence of nanoscale biochemical events, it is essential to probe the cortex at this level. Here, we use high-resolution laser-based particle tracking to examine how myosin II mechanochemistry and dynacortin-mediated actin crosslinking control cortex dynamics in Dictyostelium. Consistent with its low duty ratio, myosin II does not directly drive active bead motility. Instead, myosin II and dynacortin antagonistically regulate other active processes in the living cortex.

Brownian motion of nucleated cell envelopes impedes adhesion

We demonstrate that composite envelopes of nucleated cells exhibit pronounced short wavelength (<or=0.5 microm) bending excitations of approximately 10 nm root mean square amplitudes at physiological temperatures, which impede strong adhesion due to entropic repulsion forces. Quantitative microinterferometric analysis of the dynamic cell surface roughness of macrophages in terms of the theory of statistical surfaces suggests that the membrane excitations are mainly thermally driven Brownian motions (although active driving forces may contribute substantially). We determine the effective bending modulus of the cell envelope (approximately 1000kBT), the cortical tension (approximately 10(-4) N m-1), and the work of adhesion (approximately 10(-5) J m-2).

Mechanical models for living cells--a review

As physical entities, living cells possess structural and physical properties that enable them to withstand the physiological environment as well as mechanical stimuli occurring within and outside the body. Any deviation from these properties will not only undermine the physical integrity of the cells, but also their biological functions. As such, a quantitative study in single cell mechanics needs to be conducted. In this review, we will examine some mechanical models that have been developed to characterize mechanical responses of living cells when subjected to both transient and dynamic loads. The mechanical models include the cortical shell-liquid core (or liquid drop) models which are widely applied to suspended cells; the solid model which is generally used for adherent cells; the power-law structural damping model which is more suited for studying the dynamic behavior of adherent cells; and finally, the biphasic model which has been widely used to study musculoskeletal cell mechanics. Based upon these models, future attempts can be made to develop even more detailed and accurate mechanical models of living cells once these three factors are adequately addressed: structural heterogeneity, appropriate constitutive relations for each of the distinct subcellular regions and components, and active forces acting within the cell. More realistic mechanical models of living cells can further contribute towards the study of mechanotransduction in cells.

Identification and characterization of an 8-kDa light chain associated with Dictyostelium discoideum MyoB, a class I myosin

Dictyostelium discoideum MyoB is a single-headed class I myosin. Analysis of purified MyoB by SDS-PAGE indicated the presence of an approximately 9-kDa light chain. A tryptic digest of MyoB yielded a partial sequence for the light chain that exactly matched a sequence in a 73-amino acid, 8,296-Da protein (dictyBase number DDB0188713). This protein, termed MlcB, contains two EF-hand motifs and shares approximately 30% sequence identity with the N- and C-terminal lobes of calmodulin. FLAG-MlcB expressed in Dictyostelium co-immunoprecipitated with MyoB but not with the related class myosins and MyoD. Recombinant MlcB bound Ca2+ with a Kd value of 0.2 microm and underwent a Ca2+-induced change in conformation that increased alpha-helical content and surface hydrophobicity. Mutational analysis showed that the first EF-hand was responsible for Ca2+ binding. In the presence and absence of Ca2+ MlcB was a monomer in solution and bound to a MyoB IQ motif peptide with a Kd value of approximately 0.5 microm. A MyoB head-neck construct with a Ser to Glu mutation at the TEDS site bound MlcB and displayed an actin-activated Mg2+ ATPase activity that was insensitive to Ca2+. We conclude that MlcB represents a novel type of small myosin light chain that binds to IQ motifs in a manner comparable with a single lobe of a typical four-EF-hand protein.

Role of fission yeast myosin I in organization of sterol-rich membrane domains

Specialized membrane domains containing lipid rafts are thought to be important for membrane processes such as signaling and trafficking. An unconventional type I myosin has been shown to reside in lipid rafts and function to target a disaccharidase to rafts in brush borders of intestinal mammalian cells. In the fission yeast Schizosaccharomyces pombe, distinct sterol-rich membrane domains are formed at the cell division site and sites of polarized cell growth at cell tips. Here, we show that the sole S. pombe myosin I, myo1p, is required for proper organization of these membrane domains. myo1 mutants lacking the TH1 domain exhibit a uniform distribution of sterol-rich membranes all over the plasma membrane throughout the cell cycle. These effects are independent of endocytosis because myo1 mutants exhibit no endocytic defects. Conversely, overexpression of myo1p induces ectopic sterol-rich membrane domains. Myo1p localizes to nonmotile foci that cluster in sterol-rich plasma membrane domains and fractionates with detergent-resistant membranes. Because the myo1p TH1 domain may bind directly to acidic phospholipids, these findings suggest a model for how type I myosin contributes to the organization of specialized membrane domains.

Myosins: tails (and heads) of functional diversity

The myosin family of actin filament-based molecular motors consists of at least 20 structurally and functionally distinct classes. The human genome contains nearly 40 myosin genes, encoding 12 of these classes. Myosins have been implicated in a variety of intracellular functions, including cell migration and adhesion; intracellular transport and localization of organelles and macromolecules; signal transduction; and tumor suppression. In this review, recent insights into the remarkable diversity in the mechanochemical and functional properties associated with this family of molecular motors are discussed.