The leading edge is a lipid diffusion barrier

A plasma membrane template for macropinocytic cups

Macropinocytosis is a fundamental mechanism that allows cells to take up extracellular liquid into large vesicles. It critically depends on the formation of a ring of protrusive actin beneath the plasma membrane, which develops into the macropinocytic cup. We show that macropinocytic cups in Dictyostelium are organised around coincident intense patches of PIP3, active Ras and active Rac. These signalling patches are invariably associated with a ring of active SCAR/WAVE at their periphery, as are all examined structures based on PIP3 patches, including phagocytic cups and basal waves. Patch formation does not depend on the enclosing F-actin ring, and patches become enlarged when the RasGAP NF1 is mutated, showing that Ras plays an instructive role. New macropinocytic cups predominantly form by splitting from existing ones. We propose that cup-shaped plasma membrane structures form from self-organizing patches of active Ras/PIP3, which recruit a ring of actin nucleators to their periphery.

The physical basis of total internal reflection fluorescence (TIRF) microscopy and its cellular applications

Total internal reflection fluorescence (TIRF) microscopy has gained popularity in recent years among cell biologists due to its ability to clearly visualize events that occur at the adherent plasma membrane of cells. TIRF microscopy systems are now commercially available from nearly all microscope suppliers. This review aims to give the reader an introduction to the physical basis of TIRF and considerations that need to be made when purchasing a commercial system. We explain how TIRF can be combined with other microscopy modalities and describe how to use TIRF to study processes such as endocytosis, exocytosis, and focal adhesion dynamics. Finally, we provide a step-by-step guide to imaging and analyzing focal adhesion dynamics in a migrating cell using TIRF microscopy.

Computational estimates of membrane flow and tension gradient in motile cells

All parts of motile cells, including the plasma membrane, have to translocate in the direction of locomotion. Both directed intracellular membrane transport coupled with polarized endo- and exocytosis and fluid flow in the plane of the plasma membrane can contribute to this overall plasma membrane translocation. It remains unclear how strong a force is required to generate this flow. We numerically solve Stokes equations for the viscous membrane flow across a flat plasma membrane surface in the presence of transmembrane proteins attached to the cytoskeleton and find the membrane tension gradient associated with this flow. This gradient is sensitive to the size and density of the transmembrane proteins attached to the cytoskeleton and can become significant enough to slow down cell movement. We estimate the influence of intracellular membrane transport and actin growth and contraction on the tension gradient, and discuss possible ‘tank tread’ flow at ventral and dorsal surfaces.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

A spatial model for integrin clustering as a result of feedback between integrin activation and integrin binding

Integrins are transmembrane adhesion receptors that bind extracellular matrix (ECM) proteins and signal bidirectionally to regulate cell adhesion and migration. In many cell types, integrins cluster at cell-ECM contacts to create the foundation for adhesion complexes that transfer force between the cell and the ECM. Even though the temporal and spatial regulation of these integrin clusters is essential for cell migration, how cells regulate their formation is currently unknown. It has been shown that integrin cluster formation is independent of actin stress fiber formation, but requires active (high-affinity) integrins, phosphoinositol-4,5-bisphosphate (PIP2), talin, and immobile ECM ligand. Based on these observations, we propose a minimal model for initial formation of integrin clusters, facilitated by localized activation and binding of integrins to ECM ligands as a result of biochemical feedback between integrin binding and integrin activation. By employing a diffusion-reaction framework for modeling these reactions, we show how spatial organization of bound integrins into clusters may be achieved by a local source of active integrins, namely protein complexes formed on the cytoplasmic tails of bound integrins. Further, we show how such a mechanism can turn small local increases in the concentration of active talin or active integrin into integrin clusters via positive feedback. Our results suggest that the formation of integrin clusters by the proposed mechanism depends on the relationships between production and diffusion of integrin-activating species, and that changes to the relative rates of these processes may affect the resulting properties of integrin clusters.

Ruffles limit diffusion in the plasma membrane during macropinosome formation

In murine macrophages stimulated with macrophage-colony-stimulating factor (M-CSF), signals essential to macropinosome formation are restricted to the domain of plasma membrane enclosed within cup-shaped, circular ruffles. Consistent with a role for these actin-rich structures in signal amplification, microscopic measures of Rac1 activity determined that disruption of actin polymerization by latrunculin B inhibited ruffling and the localized activation of Rac1 in response to M-CSF. To test the hypothesis that circular ruffles restrict the lateral diffusion of membrane proteins that are essential for signaling, we monitored diffusion of membrane-tethered, photoactivatable green fluorescent protein (PAGFP-MEM) in ruffling and non-ruffling regions of cells. Although diffusion within macropinocytic cups was not inhibited, circular ruffles retained photoactivated PAGFP-MEM inside cup domains. Confinement of membrane molecules by circular ruffles could explain how actin facilitates positive feedback amplification of Rac1 in these relatively large domains of the plasma membrane, thereby organizing the contractile activities that close macropinosomes.

HaCaT keratinocytes exhibit a cholesterol and plasma membrane viscosity gradient during directed migration

Keratinocyte migration plays an important role in cutaneous wound healing by supporting the process of reepithelialisation. During directional migration cells develop a polarised shape with an asymmetric distribution of a variety of signalling molecules in their plasma membrane. Here, we investigated front-to-back differences of the physical properties of the plasma membrane of migrating keratinocyte-like HaCaT cells. Using FRAP and fluorescence lifetime analysis, both under TIR illumination, we demonstrate a reduced viscosity of the plasma membrane in the lamellipodia of migrating HaCaT cells compared with the cell rears. This asymmetry is most likely caused by a reduced cholesterol content of the lamellipodia as demonstrated by filipin staining. siRNA-mediated silencing of the cholesterol transporter ABCA1, which is known to redistribute cholesterol from rafts to non-raft regions, as well as pharmacological inhibition of this transporter with glibenclamide, strongly diminished the viscosity gradient of the plasma membrane. In addition, HaCaT cell migration was inhibited by glibenclamide treatment. These data suggest a preferential role of non-raft cholesterol in the establishment of the asymmetric plasma membrane viscosity.

Cell motility: the integrating role of the plasma membrane

The plasma membrane is of central importance in the motility process. It defines the boundary separating the intracellular and extracellular environments, and mediates the interactions between a motile cell and its environment. Furthermore, the membrane serves as a dynamic platform for localization of various components which actively participate in all aspects of the motility process, including force generation, adhesion, signaling, and regulation. Membrane transport between internal membranes and the plasma membrane, and in particular polarized membrane transport, facilitates continuous reorganization of the plasma membrane and is thought to be involved in maintaining polarity and recycling of essential components in some motile cell types. Beyond its biochemical composition, the mechanical characteristics of the plasma membrane and, in particular, membrane tension are of central importance in cell motility; membrane tension affects the rates of all the processes which involve membrane deformation including edge extension, endocytosis, and exocytosis. Most importantly, the mechanical characteristics of the membrane and its biochemical composition are tightly intertwined; membrane tension and local curvature are largely determined by the biochemical composition of the membrane and the biochemical reactions taking place; at the same time, curvature and tension affect the localization of components and reaction rates. This review focuses on this dynamic interplay and the feedbacks between the biochemical and biophysical characteristics of the membrane and their effects on cell movement. New insight on these will be crucial for understanding the motility process.

Characterizing system performance in total internal reflection fluorescence microscopy

Total internal reflection fluorescence microscopy (TIRF-M) has become an increasingly popular tool to study events in close proximity to the cell cortex, such as cell adhesion (Axelrod, J Cell Biol 89:141-145, 1981; Gingell et al., J Cell Biol 100:1334-1338, 1985; Patel et al., J Cell Sci 121:1159-1164, 2008), actin (Bretschneider et al., Curr Biol 14:1-10, 2004; Gerisch, Biophys J 87:3493-3503, 2004; Merrifield et al., Nat Cell Biol 4:691-698, 2002), and membrane dynamics (Oheim et al., Eur Biophys J 27:83-98, 1998; Steyer et al., Nature 388:474-478, 1997; Weisswange et al., J Cell Sci 118:4375-4380, 2005). In TIRF-M, dim fluorescence from cortical structures can be imaged with high contrast despite large cytoplasmic background from the bulk of the cell body. With any imaging method, standard samples are required to ensure correct alignment and monitor system performance over time. Here, we describe procedures for the production and use of a test sample to characterise and optimize TIRF system performance.

Spatial restriction of receptor tyrosine kinase activity through a polarized endocytic cycle controls border cell migration

Border cell migration is a stereotyped migration occurring during the development of the Drosophila egg chamber. During this process, a cluster composed of six to eight follicle cells migrates between nurse cells toward the oocyte. Receptor tyrosine kinases (RTKs) are enriched at the leading edge of the follicle cells and establish the directionality of their migration. Endocytosis has been shown to play a role in the maintenance of this polarization; however, the mechanisms involved are largely unknown. In this study, we show that border cell migration requires the function of the small GTPases Rab5 and Rab11 that regulate trafficking through the early and the recycling endosome, respectively. Expression of a dominant negative form of rab11 induces a loss of the polarization of RTK activity, which correlates with a severe migration phenotype. In addition, we demonstrate that the exocyst component Sec15 is distributed in structures that are polarized during the migration process in a Rab11-dependent manner and that the down-regulation of different subunits of the exocyst also affects migration. Together, our data demonstrate a fundamental role for a plasma membrane-endosome trafficking cycle in the maintenance of active RTK at the leading edge of border cells during their migration.

Mobility of G proteins is heterogeneous and polarized during chemotaxis

The interaction of G-protein-coupled receptors with G proteins is a key event in transmembrane signal transduction that leads to vital decision-making by the cell. Here, we applied single-molecule epifluorescence microscopy to study the mobility of both the Gbetagamma and the Galpha2 subunits of the G protein heterotrimer in comparison with the cAMP receptor responsible for chemotactic signaling in Dictyostelium discoideum. Our experimental results suggest that approximately 30% of the G protein heterotrimers exist in receptor-precoupled complexes. Upon stimulation in a chemotactic gradient, this complex dissociates, subsequently leading to a linear diffusion and collision amplification of the external signal. We further found that Gbetagamma was partially immobilized and confined in an agonist-, F-actin- and Galpha2-dependent fashion. This led to the hypothesis that functional nanometric domains exist in the plasma membrane, which locally restrict the activation signal, and in turn, lead to faithful and efficient chemotactic signaling.

Hierarchical organization of the plasma membrane: investigations by single-molecule tracking vs. fluorescence correlation spectroscopy

Single-molecule tracking and fluorescence correlation spectroscopy (FCS) applied to the plasma membrane in living cells have allowed a number of unprecedented observations, thus fostering a new basic understanding of molecular diffusion, interaction, and signal transduction in the plasma membrane. It is becoming clear that the plasma membrane is a heterogeneous entity, containing diverse structures on nano-meso-scales (2-200 nm) with a variety of lifetimes, where certain membrane molecules stay together for limited durations. Molecular interactions occur in the time-dependent inhomogeneous two-dimensional liquid of the plasma membrane, which might be a key for plasma membrane functions.

Septins and the lateral compartmentalization of eukaryotic membranes

Eukaryotic cells from neurons and epithelial cells to unicellular fungi frequently rely on cellular appendages such as axons, dendritic spines, cilia, and buds for their biology. The emergence and differentiation of these appendages depend on the formation of lateral diffusion barriers at their bases to insulate their membranes from the rest of the cell. Here, we review recent progress regarding the molecular mechanisms and functions of such barriers. This overview underlines the importance and conservation of septin-dependent diffusion barriers, which coordinately compartmentalize both plasmatic and internal membranes. We discuss their role in memory establishment and the control of cellular aging.

Measuring diffusion of lipid-like probes in artificial and natural membranes by raster image correlation spectroscopy (RICS): use of a commercial laser-scanning microscope with analog detection

The heterogeneity in composition and interaction within the cellular membrane translates into a wide range of diffusion coefficients of its constituents. Therefore, several complementary microfluorimetric techniques such as fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP) and single-particle tracking (SPT) have to be applied to explore the dynamics of membrane components. The recently introduced raster image correlation spectroscopy (RICS) offers a much wider dynamic range than each of these methods separately and allows for spatial mapping of the dynamic properties. RICS is implemented on a confocal laser-scanning microscope (CLSM), and the wide dynamic range is achieved by exploiting the inherent time information carried by the scanning laser beam in the generation of the confocal images. The original introduction of RICS used two-photon excitation and photon counting detection. However, most CLSM systems are based on one-photon excitation with analog detection. Here we report on the performance of such a commercial CLSM (Zeiss LSM 510 META) in the study of the diffusion of the fluorescent lipid analog 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indodicarbocyanine perchlorate (DiI-C(18)(5)) both in giant unilamellar vesicles and in the plasma membrane of living oligodendrocytes, i.e., the myelin-producing cells of the central nervous system. It is shown that RICS on a commercial CLSM with analog detection allows for reliable results in the study of membrane diffusion by removal of unwanted correlations introduced by the analog detection system. The results obtained compare well with those collected by FRAP and FCS.

Real-time study of E-cadherin and membrane dynamics in living animals: implications for disease modeling and drug development

The ability of tumor cells to invade and metastasize requires deregulation of interactions with adjacent cells and the extracellular matrix. A major challenge of cancer biology is to observe the dynamics of the proteins involved in this process in their functional and physiologic context. Here, for the first time, we have used photobleaching and photoactivation to compare the mobility of cell adhesion and plasma membrane probes in vitro and in tumors grown in mice (in vivo). We find differences between in vitro and in vivo recovery dynamics of two key molecules, the tumor suppressor E-cadherin and the membrane-targeting sequence of H-Ras. Our data show that E-cadherin dynamics are significantly faster in vivo compared with cultured cells, that the ratio of E-cadherin stabilized in cell-cell junctions is significantly higher in vivo, and that E-cadherin mobility correlates with cell migration. Moreover, quantitative imaging has allowed us to assess the effects of therapeutic intervention on E-cadherin dynamics using dasatinib, a clinically approved Src inhibitor, and show clear differences in the efficacy of drug treatment in vivo. Our results show for the first time the utility of photobleaching and photoactivation in the analysis of dynamic biomarkers in living animals. Furthermore, this work highlights critical differences in molecular dynamics in vitro and in vivo, which have important implications for the use of cultured disease models as surrogates for living tissue.

Fluctuations of intracellular forces during cell protrusion

We present a model to estimate intracellular force variations from live-cell images of actin filament (F-actin) flow during protrusion-retraction cycles of epithelial cells in a wound healing response. To establish a mechanistic relationship between force development and cytoskelal dynamics, force fluctuations were correlated with fluctuations in F-actin turnover, flow and F-actin-vinculin coupling. Our analyses suggest that force transmission at focal adhesions requires binding of vinculin to F-actin and integrin (indirectly), which is modulated at the vinculin-integrin but not the vinculin-F-actin interface. Force transmission at focal adhesions is colocalized in space and synchronized in time with transient increases in the boundary force at the cell edge. Surprisingly, the maxima in adhesion and boundary forces lag behind maximal edge advancement by about 40 s. Maximal F-actin assembly was observed about 20 s after maximal edge advancement. On the basis of these findings, we propose that protrusion events are limited by membrane tension and that the characteristic duration of a protrusion cycle is determined by the efficiency in reinforcing F-actin assembly and adhesion formation as tension increases.

Diffusion coefficient of fluorescent phosphatidylinositol 4,5-bisphosphate in the plasma membrane of cells

Phosphatidylinositol 4,5-bisphosphate (PIP(2)) controls a surprisingly large number of processes in cells. Thus, many investigators have suggested that there might be different pools of PIP(2) on the inner leaflet of the plasma membrane. If a significant fraction of PIP(2) is bound electrostatically to unstructured clusters of basic residues on membrane proteins, the PIP(2) diffusion constant, D, should be reduced. We microinjected micelles of Bodipy TMR-PIP(2) into cells, and we measured D on the inner leaflet of fibroblasts and epithelial cells by using fluorescence correlation spectroscopy. The average +/- SD value from all cell types was D = 0.8 +/- 0.2 microm(2)/s (n = 218; 25 degrees C). This is threefold lower than the D in blebs formed on Rat1 cells, D = 2.5 +/- 0.8 microm(2)/s (n = 26). It is also significantly lower than the D in the outer leaflet or in giant unilamellar vesicles and the diffusion coefficient for other lipids on the inner leaflet of these cell membranes. The simplest interpretation is that approximately two thirds of the PIP(2) on inner leaflet of these plasma membranes is bound reversibly.

Fluorescence lifetime imaging: association of cortical actin with a PIP3-rich membrane compartment

We have used fluorescence lifetime imaging (FLIM) to study actin and plasma membrane dynamics in B16-F1 melanoma cells. In the absence of a FRET acceptor, significant changes in the fluorescence lifetime of GFP were induced simply by linking the fluorophore to different functional probes, including beta-actin, the PH domains of PLCdelta and Akt, the Ras farnesylation signal, and the neuromodulin palmitoylation signal (MEM). In contrast, the lifetime of GFP-actin was constant despite the many different local environments of G- and F-actin within the cell. Treatment with cytochalasin D but not latrunculin A significantly shortened the lifetime of GFP-beta-actin in the absence of a FRET acceptor. Robust lifetime shifts were observed using either a GFP-RFP chimera or co-transfection of GFP-MEM with RFP-MEM. In contrast to previous reports we observed a photobleaching-dependent change in the lifetime of GFP which could complicate the interpretation of FRET experiments. Of the membrane probes tested only the fluorescence lifetime of GFP-Akt was influenced by the presence of mRFP-actin, suggesting that the cortical actin meshwork is associated with a PIP3-enriched compartment of the plasma membrane. These results will aid in the design of new FRET-based approaches to study cytoskeletal interactions at the molecular level.

Arp2/3 controls the motile behavior of N-WASP-functionalized GUVs and modulates N-WASP surface distribution by mediating transient links with actin filaments

Spatially controlled assembly of actin in branched filaments generates cell protrusions or the propulsion of intracellular vesicles and pathogens. The propulsive movement of giant unilamellar vesicles (GUVs) functionalized by N-WASP (full-length or truncated) is reconstituted in a biochemically controlled medium, and analyzed using phase contrast and fluorescence microscopy to elucidate the links between membrane components and the actin cytoskeleton that determine motile behavior. Actin-based propulsion displays a continuous regime or a periodic saltatory regime. The transition between the two regimes is controlled by the concentration of Arp2/3 complex, which branches filaments by interacting with N-WASP at the liposome surface. Saltatory motion is linked to cycles in the distribution of N-WASP at the membrane between a homogeneous and a segregated state. Comparison of the changes in distribution of N-WASP, Arp2/3, and actin during propulsion demonstrates that actin filaments bind to N-WASP, and that these bonds are transitory. This interaction, mediated by Arp2/3, drives N-WASP segregation. VC-fragments of N-WASP, that interact more weakly than N-WASP with the Arp2/3 complex, segregate less than N-WASP at the rear of the GUVs. GUV propulsion is inhibited by the presence of VCA-actin covalent complex, showing that the release of actin from the nucleator is required for movement. The balance between segregation and free diffusion determines whether continuous movement can be sustained. Computed surface distributions of N-WASP, derived from a theoretical description of this segregation-diffusion mechanism, account satisfactorily for the measured density profiles of N-WASP, Arp2/3 complex, and actin.

Dynamics of single potassium channel proteins in the plasma membrane of migrating cells

Cell migration is an important physiological process among others controlled by ion channel activity. Calcium-activated potassium channels (K(Ca)3.1) are required for optimal cell migration. Previously, we identified single human (h)K(Ca)3.1 channel proteins in the plasma membrane by means of quantum dot (QD) labeling. In the present study, we tracked single-channel proteins during migration to classify their dynamics in the plasma membrane of MDCK-F cells. Single hK(Ca)3.1 channels were visualized with QD- or Alexa488-conjugated antibodies and tracked at the basal cell membrane using time-lapse total internal reflection fluorescence (TIRF) microscopy. Analysis of the trajectories allowed the classification of channel dynamics. Channel tracks were compared with those of free QD-conjugated antibodies. The size of the label has a pronounced effect on hK(Ca)3.1 channel diffusion. QD-labeled channels have a (sub)diffusion coefficient D(QDbound) = 0.067 microm(2)/s(alpha), whereas that of Alexa488-labeled channels is D(Alexa) = 0.139 microm(2)/s. Free QD-conjugated antibodies move much faster: D(QDfree) = 2.163 microm(2)/s(alpha). Plotting the mean squared distances (msd) covered by hK(Ca)3.1 channels as a function of time points to the mode of diffusion. Alexa488-labeled channels diffuse normally, whereas the QD-label renders hK(Ca)3.1 channel diffusion anomalous. Free QD-labeled antibodies also diffuse anomalously. Hence, QDs slow down diffusion of hK(Ca)3.1 channels and change the mode of diffusion. These results, referring to the role of label size and properties of the extracellular environment, suggest that the pericellular glycocalyx has an important impact on labels used for single molecule tracking. Thus tracking fluorescent particles within the glycocalyx opens up a possibility to characterize the pericellular nanoenvironment.

Phase transitions of the coupled membrane-cytoskeleton modify cellular shape

Formation of protrusions and protein segregation on the membrane is of a great importance for the functioning of the living cell. This is most evident in recent experiments that show the effects of the mechanical properties of the surrounding substrate on cell morphology. We propose a mechanism for the formation of membrane protrusions and protein phase separation, which may lay behind this effect. In our model, the fluid cell membrane has a mobile but constant population of proteins with a convex spontaneous curvature. Our basic assumption is that these membrane proteins represent small adhesion complexes, and also include proteins that activate actin polymerization. Such a continuum model couples the membrane and protein dynamics, including cell-substrate adhesion and protrusive actin force. Linear stability analysis shows that sufficiently strong adhesion energy and actin polymerization force can bring about phase separation of the membrane protein and the appearance of protrusions. Specifically, this occurs when the spontaneous curvature and aggregation potential alone (passive system) do not cause phase separation. Finite-size patterns may appear in the regime where the spontaneous curvature energy is a strong factor. Different instability characteristics are calculated for the various regimes, and are compared to various types of observed protrusions and phase separations, both in living cells and in artificial model systems. A number of testable predictions are proposed.

Model of polarization and bistability of cell fragments

Directed cell motility is preceded by cell polarization-development of a front-rear asymmetry of the cytoskeleton and the cell shape. Extensive studies implicated complex spatial-temporal feedbacks between multiple signaling pathways in establishing cell polarity, yet physical mechanisms of this phenomenon remain elusive. Based on observations of lamellipodial fragments of fish keratocyte cells, we suggest a purely thermodynamic (not involving signaling) quantitative model of the cell polarization and bistability. The model is based on the interplay between pushing force exerted by F-actin polymerization on the cell edges, contractile force powered by myosin II across the cell, and elastic tension in the cell membrane. We calculate the thermodynamic work produced by these intracellular forces, and show that on the short timescale, the cell mechanics can be characterized by an effective energy profile with two minima that describe two stable states separated by an energy barrier and corresponding to the nonpolarized and polarized cells. Cell dynamics implied by this energy profile is bistable-the cell is either disk-shaped and stationary, or crescent-shaped and motile-with a possible transition between them upon a finite external stimulus able to drive the system over the macroscopic energy barrier. The model accounts for the observations of the keratocyte fragments' behavior and generates quantitative predictions about relations between the intracellular forces' magnitudes and the cell geometry and motility.

Protein binding onto surfactant-based synthetic vesicles

Synthetic vesicles were prepared by mixing anionic and cationic surfactants, aqueous sodium dodecylsulfate with didodecyltrimethylammonium or cetyltrimethylammonium bromide. The overall surfactant content and the (anionic/cationic) mole ratios allow one to obtain negatively charged vesicles. In the phase diagram, the vesicular region is located between a solution phase, a lamellar liquid crystalline dispersion, and a precipitate area. Characterization of the vesicles was performed by electrophoretic mobility, NMR, TEM, and DLS and we determined their uni-lamellar character, size, stability, and charge density. Negatively charged vesicular dispersions, made of sodium dodecylsulfate/didodecyltrimethylammonium bromide or sodium dodecylsulfate/cetyltrimethylammonium bromide, were mixed with lysozyme, to form lipoplexes. Depending on the protein/vesicle charge ratio, binding, surface saturation, and lipoplexes flocculation, or precipitation, occurs. The free protein in excess remains in solution, after binding saturation. The systems were investigated by thermodynamic (surface tension and solution calorimetry), DLS, CD, TEM, 1H NMR, transport properties, electrophoretic mobility, and dielectric relaxation. The latter two methods give information on the vesicle charge neutralization by adsorbed protein. Binding is concomitant to modifications in the double layer thickness of vesicles and in the surface charge density of the resulting lipoplexes. This is also confirmed by developing the electrophoretic mobility results in terms of a Langmuir-like adsorption isotherm. Charges in excess with respect to the amount required to neutralize the vesicle surface promote lipoplexes clustering and/or flocculation. Protein-vesicle interactions were observed by DLS, indicating changes in particle size (and in their distribution functions) upon addition of LYSO. According to CD, the bound protein retains its native conformation, at least in the SDS/CTAB vesicular system. In fact, changes in the alpha-helix and beta-sheet conformations are moderate, if any. Calorimetric methods indicate that the maximum heat effect for LYSO binding occurs at charge neutralization. They also indicate that enthalpic are by far the dominant contributions to the system stability. Accordingly, energy effects associated with charge neutralization and double-layer contributions are much higher than counterion exchange and dehydration terms.

Mechanically induced actin-mediated rocketing of phagosomes

Actin polymerization can be induced in Dictyostelium by compressing the cells to bring phagosomes filled with large particles into contact with the plasma membrane. Asymmetric actin assembly results in rocketing movement of the phagosomes. We show that the compression-induced assembly of actin at the cytoplasmic face of the plasma membrane involves the Arp2/3 complex. We also identify two other proteins associated with the mechanically induced actin assembly. The class I myosin MyoB accumulates at the plasma membrane-phagosome interface early during the initiation of the response, and coronin is recruited as the actin filaments are disassembling. The forces generated by rocketing phagosomes are sufficient to push the entire microtubule apparatus forward and to dislocate the nucleus.

Actin polymerization serves as a membrane domain switch in model lipid bilayers

The ability of cells to mount localized responses to external or internal stimuli is critically dependent on organization of lipids and proteins in the plasma membrane. Involvement of the actin cytoskeleton in membrane organization has been documented, but an active role for actin networks that directly links internal organization of the cytoskeleton with membrane organization has not yet been identified. Here we show that branched actin networks formed on model lipid membranes enriched with the lipid second messenger PIP(2) trigger both temporal and spatial rearrangement of membrane components. Using giant unilamellar vesicles able to separate into two coexisting liquid phases, we demonstrate that polymerization of dendritic actin networks on the membrane induces phase separation of initially homogenous vesicles. This switch-like behavior depends only on the PIP(2)-N-WASP link between the membrane and actin network, and we find that the presence of a preexisting actin network spatially biases the location of phase separation. These results show that dynamic, membrane-bound actin networks alone can control when and where membrane domains form and may actively contribute to membrane organization during cell signaling.

Endocytic recycling pathways: emerging regulators of cell migration

The past five years have seen a steady accumulation of data reinforcing the view that Rab-regulated recycling pathways contribute to cell migration. In particular, detailed descriptions have emerged of the mechanisms that recruit integrins and growth factor receptors to Rab4- and Rab11-driven pathways. Recent work provides new insight into the importance of particular recycling events in cell migration within a variety of physiological contexts.

On the edge: modeling protrusion

Actin-based protrusion is the first step in cell crawling. In the last two decades, the studies of actin networks in the lamellipodium and Listeria's comet tail advanced so far that the last goal of the reductionist agenda - reconstitution of protrusion from purified components in vitro and in silico - became viable. Earlier models dealt with growth of and force generation by a single actin filament. Modern models of tethered ratchet, autocatalytic branching, end-tracking motor action and elastic- and nano- propulsion have recently helped to elucidate dynamics and forces in complex actin networks. By considering these models, their limitations and their relationships to recent biophysical data, progress is being made toward a unified model of protrusion.