Diffusion, capture and recycling of SCAR/WAVE and Arp2/3 complexes observed in cells by single-molecule imaging

The WAVE complex forms linear arrays at negative membrane curvature to instruct lamellipodia formation

Cells generate a wide range of actin-based membrane protrusions for various cell behaviors. These protrusions are organized by different actin nucleation promoting factors. For example, N-WASP controls finger-like filopodia, whereas the WAVE complex controls sheet-like lamellipodia. These different membrane morphologies likely reflect different patterns of nucleator self-organization. N-WASP phase separation has been successfully studied through biochemical reconstitutions, but how the WAVE complex self-organizes to instruct lamellipodia is unknown. Because WAVE complex self-organization has proven refractory to cell-free studies, we leverage in vivo biochemical approaches to investigate WAVE complex organization within its native cellular context. With single molecule tracking and molecular counting, we show that the WAVE complex forms highly regular multilayered linear arrays at the plasma membrane that are reminiscent of a microtubule-like organization. Similar to the organization of microtubule protofilaments in a curved array, membrane curvature is both necessary and sufficient for formation of these WAVE complex linear arrays, though actin polymerization is not. This dependency on negative membrane curvature could explain both the templating of lamellipodia and their emergent behaviors, including barrier avoidance. Our data uncover the key biophysical properties of mesoscale WAVE complex patterning and highlight an integral relationship between NPF self-organization and cell morphogenesis.

Local monomer levels and established filaments potentiate non-muscle myosin 2 assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet direct biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here, we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the static actin architecture plays a less clear role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin-driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes filament stacks prior to partitioning into clusters that feed higher-order networks. Together, these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Controlling Physical and Biochemical Parameters of Actin Nucleation Using a Patterned Model Lipid Membrane

Self-assembly of nanoscale actin cytoskeletal proteins into filamentous networks requires organizing actin nucleation areas on the plasma membrane through recruiting actin nucleators and nucleation-promoting factors (NPFs) to the areas. To investigate impacts of the nucleation geometry on actin network assembly, we localized NPF or nucleator on defined micropatterns of laterally mobile lipid bilayers confined in a framework of a polymerized lipid bilayer. We demonstrated that actin network assembly in purified protein mixtures was confined on NPF- or nucleator-localized fluid bilayers. By controlling the shape and size of nucleation areas as well as the density and types of localized NPFs and nucleators, we showed that these parameters regulate actin network architectures. Actin network assembly in Xenopus egg extracts was also spatially controlled by patterning bilayers containing phosphatidylinositol 4,5-bisphoshate (PI(4,5)P2), an essential lipid signaling mediator. Therefore, the system provides a promising platform to investigate the physical and biochemical principles for actin network assembly.

PPP2R1A regulates migration persistence through the NHSL1-containing WAVE Shell Complex

The RAC1-WAVE-Arp2/3 signaling pathway generates branched actin networks that power lamellipodium protrusion of migrating cells. Feedback is thought to control protrusion lifetime and migration persistence, but its molecular circuitry remains elusive. Here, we identify PPP2R1A by proteomics as a protein differentially associated with the WAVE complex subunit ABI1 when RAC1 is activated and downstream generation of branched actin is blocked. PPP2R1A is found to associate at the lamellipodial edge with an alternative form of WAVE complex, the WAVE Shell Complex, that contains NHSL1 instead of the Arp2/3 activating subunit WAVE, as in the canonical WAVE Regulatory Complex. PPP2R1A is required for persistence in random and directed migration assays and for RAC1-dependent actin polymerization in cell extracts. PPP2R1A requirement is abolished by NHSL1 depletion. PPP2R1A mutations found in tumors impair WAVE Shell Complex binding and migration regulation, suggesting that the coupling of PPP2R1A to the WAVE Shell Complex is essential to its function.

Local Monomer Levels and Established Filaments Potentiate Non-Muscle Myosin 2 Assembly

The ability to dynamically assemble contractile networks is required throughout cell physiology, yet the biophysical mechanisms regulating non-muscle myosin 2 filament assembly in living cells are lacking. Here we use a suite of dynamic, quantitative imaging approaches to identify deterministic factors that drive myosin filament appearance and amplification. We find that actin dynamics regulate myosin assembly, but that the actin architecture plays a minimal direct role. Instead, remodeling of actin networks modulates the local myosin monomer levels and facilitates assembly through myosin:myosin driven interactions. Using optogenetically controlled myosin, we demonstrate that locally concentrating myosin is sufficient to both form filaments and jump-start filament amplification and partitioning. By counting myosin monomers within filaments, we demonstrate a myosin-facilitated assembly process that establishes sub-resolution filament stacks prior to partitioning into clusters that feed higher-order networks. Together these findings establish the biophysical mechanisms regulating the assembly of non-muscle contractile structures that are ubiquitous throughout cell biology.

Nucleation causes an actin network to fragment into multiple high-density domains

Actin networks rely on nucleation mechanisms to generate new filaments because spontaneous nucleation is kinetically disfavored. Branching nucleation of actin filaments by actin-related protein (Arp2/3), in particular, is critical for actin self-organization. In this study, we use the simulation platform for active matter MEDYAN to generate 2000 s long stochastic trajectories of actin networks, under varying Arp2/3 concentrations, in reaction volumes of biologically meaningful size (>20 μm3). We find that the dynamics of Arp2/3 increase the abundance of short filaments and increases network treadmilling rate. By analyzing the density fields of F-actin, we find that at low Arp2/3 concentrations, F-actin is organized into a single connected and contractile domain, while at elevated Arp2/3 levels (10 nM and above), such high-density actin domains fragment into smaller domains spanning a wide range of volumes. These fragmented domains are extremely dynamic, continuously merging and splitting, owing to the high treadmilling rate of the underlying actin network. Treating the domain dynamics as a drift-diffusion process, we find that the fragmented state is stochastically favored, and the network state slowly drifts toward the fragmented state with considerable diffusion (variability) in the number of domains. We suggest that tuning the Arp2/3 concentration enables cells to transition from a globally coherent cytoskeleton, whose response involves the entire cytoplasmic network, to a fragmented cytoskeleton, where domains can respond independently to locally varying signals.

A mechanism with severing near barbed ends andannealing explains structure and dynamics of dendriticactin networks

Single molecule imaging has shown that part of actin disassembles within a few seconds after incorporation into the dendritic filament network in lamellipodia, suggestive of frequent destabilization near barbed ends. To investigate the mechanisms behind network remodeling, we created a stochastic model with polymerization, depolymerization, branching, capping, uncapping, severing, oligomer diffusion, annealing, and debranching. We find that filament severing, enhanced near barbed ends, can explain the single molecule actin lifetime distribution, if oligomer fragments reanneal to free ends with rate constants comparable to in vitro measurements. The same mechanism leads to actin networks consistent with measured filament, end, and branch concentrations. These networks undergo structural remodeling, leading to longer filaments away from the leading edge, at the +/- 35𝑜 orientation pattern. Imaging of actin speckle lifetimes at sub-second resolution verifies frequent disassembly of newly-assembled actin. We thus propose a unified mechanism that fits a diverse set of basic lamellipodia phenomenology.

ERK signaling for cell migration and invasion

The RAS - Extracellular signal-regulated kinase (RAS-ERK) pathway plays a conserved role in promoting cell migration and invasion. Growth factors, adhesion, and oncogenes activate ERK. While historically studied with respect to its control of cell proliferation and differentiation, the signaling pattern and effectors specific for cell migration are now coming to light. New advances in pathway probes have revealed how steady-state ERK activity fluctuates within individual cells and propagates to neighboring cells. We review new findings on the different modes of ERK pathway stimulation and how an increased baseline level of activity promotes single cell and collective migration and invasion. We discuss how ERK drives actin polymerization and adhesion turnover for edge protrusion and how cell contraction stimulates cell movement and ERK activity waves in epithelial sheets. With the steady development of new biosensors for monitoring spatial and temporal ERK activity, determining how cells individually interpret the multiple in vivo signals to ERK is within reach.

Forces generated by lamellipodial actin filament elongation regulate the WAVE complex during cell migration

Actin filaments generate mechanical forces that drive membrane movements during trafficking, endocytosis and cell migration. Reciprocally, adaptations of actin networks to forces regulate their assembly and architecture. Yet, a demonstration of forces acting on actin regulators at actin assembly sites in cells is missing. Here we show that local forces arising from actin filament elongation mechanically control WAVE regulatory complex (WRC) dynamics and function, that is, Arp2/3 complex activation in the lamellipodium. Single-protein tracking revealed WRC lateral movements along the lamellipodium tip, driven by elongation of actin filaments and correlating with WRC turnover. The use of optical tweezers to mechanically manipulate functional WRC showed that piconewton forces, as generated by single-filament elongation, dissociated WRC from the lamellipodium tip. WRC activation correlated with its trapping, dwell time and the binding strength at the lamellipodium tip. WRC crosslinking, hindering its mechanical dissociation, increased WRC dwell time and Arp2/3-dependent membrane protrusion. Thus, forces generated by individual actin filaments on their regulators can mechanically tune their turnover and hence activity during cell migration.

The WAVE complex associates with sites of saddle membrane curvature

How local interactions of actin regulators yield large-scale organization of cell shape and movement is not well understood. Here we investigate how the WAVE complex organizes sheet-like lamellipodia. Using super-resolution microscopy, we find that the WAVE complex forms actin-independent 230-nm-wide rings that localize to regions of saddle membrane curvature. This pattern of enrichment could explain several emergent cell behaviors, such as expanding and self-straightening lamellipodia and the ability of endothelial cells to recognize and seal transcellular holes. The WAVE complex recruits IRSp53 to sites of saddle curvature but does not depend on IRSp53 for its own localization. Although the WAVE complex stimulates actin nucleation via the Arp2/3 complex, sheet-like protrusions are still observed in ARP2-null, but not WAVE complex-null, cells. Therefore, the WAVE complex has additional roles in cell morphogenesis beyond Arp2/3 complex activation. Our work defines organizing principles of the WAVE complex lamellipodial template and suggests how feedback between cell shape and actin regulators instructs cell morphogenesis.

Paradoxical activation of c-Src as a drug-resistant mechanism

ATP-competitive inhibitors have been developed as promising anti-cancer agents. However, drug-resistance frequently occurs, and the underlying mechanisms are not fully understood. Here, we show that the activation of c-Src and its downstream phosphorylation cascade can be paradoxically induced by Src-targeted and RTK-targeted kinase inhibitors. We reveal that inhibitor binding induces a conformational change in c-Src, leading to the association of the active form c-Src with focal adhesion kinase (FAK). Reduction of the inhibitor concentration results in the dissociation of inhibitors from the c-Src-FAK complex, which allows c-Src to phosphorylate FAK and initiate FAK-Grb2-mediated Erk signaling. Furthermore, a drug-resistant mutation in c-Src, which reduces the affinity of inhibitors for c-Src, converts Src inhibitors into facilitators of cell proliferation by enhancing the phosphorylation of FAK and Erk in c-Src-mutated cells. Our data thus reveal paradoxical enhancement of cell growth evoked by target-based kinase inhibitors, providing potentially important clues for the future development of effective and safe cancer treatment.

Cell-substrate adhesion drives Scar/WAVE activation and phosphorylation by a Ste20-family kinase, which controls pseudopod lifetime

The Scar/WAVE complex is the principal catalyst of pseudopod and lamellipod formation. Here we show that Scar/WAVE's proline-rich domain is polyphosphorylated after the complex is activated. Blocking Scar/WAVE activation stops phosphorylation in both Dictyostelium and mammalian cells, implying that phosphorylation modulates pseudopods after they have been formed, rather than controlling whether they are initiated. Unexpectedly, phosphorylation is not promoted by chemotactic signaling but is greatly stimulated by cell:substrate adhesion and diminished when cells deadhere. Phosphorylation-deficient or phosphomimetic Scar/WAVE mutants are both normally functional and rescue the phenotype of knockout cells, demonstrating that phosphorylation is dispensable for activation and actin regulation. However, pseudopods and patches of phosphorylation-deficient Scar/WAVE last substantially longer in mutants, altering the dynamics and size of pseudopods and lamellipods and thus changing migration speed. Scar/WAVE phosphorylation does not require ERK2 in Dictyostelium or mammalian cells. However, the MAPKKK homologue SepA contributes substantially-sepA mutants have less steady-state phosphorylation, which does not increase in response to adhesion. The mutants also behave similarly to cells expressing phosphorylation-deficient Scar, with longer-lived pseudopods and patches of Scar recruitment. We conclude that pseudopod engagement with substratum is more important than extracellular signals at regulating Scar/WAVE's activity and that phosphorylation acts as a pseudopod timer by promoting Scar/WAVE turnover.

Actin dynamics in cell migration

Cell migration is an essential process, both in unicellular organisms such as amoeba and as individual or collective motility in highly developed multicellular organisms like mammals. It is controlled by a variety of activities combining protrusive and contractile forces, normally generated by actin filaments. Here, we summarize actin filament assembly and turnover processes, and how respective biochemical activities translate into different protrusion types engaged in migration. These actin-based plasma membrane protrusions include actin-related protein 2/3 complex-dependent structures such as lamellipodia and membrane ruffles, filopodia as well as plasma membrane blebs. We also address observed antagonisms between these protrusion types, and propose a model - also inspired by previous literature - in which a complex balance between specific Rho GTPase signaling pathways dictates the protrusion mechanism employed by cells. Furthermore, we revisit published work regarding the fascinating antagonism between Rac and Rho GTPases, and how this intricate signaling network can define cell behavior and modes of migration. Finally, we discuss how the assembly of actin filament networks can feed back onto their regulators, as exemplified for the lamellipodial factor WAVE regulatory complex, tightly controlling accumulation of this complex at specific subcellular locations as well as its turnover.

Single-Turnover Activation of Arp2/3 Complex by Dip1 May Balance Nucleation of Linear versus Branched Actin Filaments

Arp2/3 complex nucleates branched actin filaments important for cellular motility, endocytosis, meiosis, and cellular differentiation [1-4]. Wiskott-Aldrich syndrome proteins (WASPs), the prototypical Arp2/3 complex activators, activate Arp2/3 complex only once it is bound to the side of an actin filament [5, 6]. This ensures WASP-activated Arp2/3 complex only nucleates branched actin filaments but means branched actin networks must be seeded with an initial preformed filament. Dip1 and other WISH/DIP/SPIN90 family proteins activate Arp2/3 complex without preformed filaments [7], creating seed filaments that activate WASP-bound Arp2/3 complex [8]. Importantly, Dip1-mediated activation of Arp2/3 complex creates linear filaments instead of branches [7]. Cells may therefore need to limit Dip1 activity relative to WASP to preserve the dendritic nature of actin networks, although it is unclear whether such regulatory mechanisms exist. Here, we use total internal reflection fluorescence (TIRF) microscopy to show that Dip1 causes actin assembled with WASP and Arp2/3 complex to form disconnected networks with many linear filaments rather than highly branched arrays. We discover a key biochemical difference between Dip1 and WASP that may limit linear filament nucleation in cells; although WASP must be released for nucleation, Dip1 stays associated with Arp2/3 complex on the pointed ends of nucleated actin filaments, so Dip1 is consumed in the reaction. Using live-cell imaging of fission yeast, we provide evidence that Dip1 is a single-turnover activator of Arp2/3 complex in vivo, revealing a mechanism by which Dip1 can initiate branched actin networks at endocytic sites without disrupting their branched architectures.

Joining forces: crosstalk between biochemical signalling and physical forces orchestrates cellular polarity and dynamics

Dynamic processes like cell migration and morphogenesis emerge from the self-organized interaction between signalling and cytoskeletal rearrangements. How are these molecular to sub-cellular scale processes integrated to enable cell-wide responses? A growing body of recent studies suggest that forces generated by cytoskeletal dynamics and motor activity at the cellular or tissue scale can organize processes ranging from cell movement, polarity and division to the coordination of responses across fields of cells. To do so, forces not only act mechanically but also engage with biochemical signalling. Here, we review recent advances in our understanding of this dynamic crosstalk between biochemical signalling, self-organized cortical actomyosin dynamics and physical forces with a special focus on the role of membrane tension in integrating cellular motility.This article is part of the theme issue 'Self-organization in cell biology'.

Building a dendritic actin filament network branch by branch: models of filament orientation pattern and force generation in lamellipodia

We review mathematical and computational models of the structure, dynamics, and force generation properties of dendritic actin networks. These models have been motivated by the dendritic nucleation model, which provided a mechanistic picture of how the actin cytoskeleton system powers cell motility. We describe how they aimed to explain the self-organization of the branched network into a bimodal distribution of filament orientations peaked at 35° and - 35° with respect to the direction of membrane protrusion, as well as other patterns. Concave and convex force-velocity relationships were derived, depending on network organization, filament, and membrane elasticity and accounting for actin polymerization at the barbed end as a Brownian ratchet. This review also describes models that considered the kinetics and transport of actin and diffuse regulators and mechanical coupling to a substrate, together with explicit modeling of dendritic networks.

New insights into the formation and the function of lamellipodia and ruffles in mesenchymal cell migration

Lamellipodia and ruffles are veil-shaped cell protrusions composed of a highly branched actin filament meshwork assembled by the Arp2/3 complex. These structures not only hallmark the leading edge of cells adopting the adhesion-based mesenchymal mode of migration but are also thought to drive cell movement. Although regarded as textbook knowledge, the mechanism of formation of lamellipodia and ruffles has been revisited in the last years leveraging new technologies. Furthermore, recent observations have also challenged our current view of the function of lamellipodia and ruffles in mesenchymal cell migration. Here, I review this literature and compare it with older studies to highlight the controversies and the outstanding open issues in the field. Moreover, I outline simple and plausible explanations to reconcile conflicting results and conclusions. Finally, I integrate the mechanisms regulating actin-based protrusion in a unifying model that accounts for random and ballistic mesenchymal cell migration.

Actin Cross-Linking Toxin Is a Universal Inhibitor of Tandem-Organized and Oligomeric G-Actin Binding Proteins

Delivery of bacterial toxins to host cells is hindered by host protective barriers. This obstruction dictates a remarkable efficiency of toxins, a single copy of which may kill a host cell. Efficiency of actin-targeting toxins is further hampered by an overwhelming abundance of their target. The actin cross-linking domain (ACD) toxins of Vibrio species and related bacterial genera catalyze the formation of covalently cross-linked actin oligomers. Recently, we reported that the ACD toxicity can be amplified via a multivalent inhibitory association of actin oligomers with actin assembly factors formins, suggesting that the oligomers may act as secondary toxins. Importantly, many proteins involved in nucleation, elongation, severing, branching, and bundling of actin filaments contain G-actin-binding Wiskott-Aldrich syndrome protein (WASP)-homology motifs 2 (WH2) organized in tandem and therefore may act as a multivalent platform for high-affinity interaction with the ACD-cross-linked actin oligomers. Using live-cell single-molecule speckle (SiMS) microscopy, total internal reflection fluorescence (TIRF) microscopy, and actin polymerization assays, we show that, in addition to formins, the oligomers bind with high affinity and potently inhibit several families of actin assembly factors: Ena/vasodilator-stimulated phosphorprotein (VASP); Spire; and the Arp2/3 complex, both in vitro and in live cells. As a result, ACD blocks the actin retrograde flow and membrane dynamics and disrupts association of Ena/VASP with adhesion complexes. This study defines ACD as a universal inhibitor of tandem-organized G-actin binding proteins that overcomes the abundance of actin by redirecting the toxicity cascade toward less abundant targets and thus leading to profound disorganization of the actin cytoskeleton and disruption of actin-dependent cellular functions.

Membrane shape-mediated wave propagation of cortical protein dynamics

Immune cells exhibit stimulation-dependent traveling waves in the cortex, much faster than typical cortical actin waves. These waves reflect rhythmic assembly of both actin machinery and peripheral membrane proteins such as F-BAR domain-containing proteins. Combining theory and experiments, we develop a mechanochemical feedback model involving membrane shape changes and F-BAR proteins that render the cortex an interesting dynamical system. We show that such cortical dynamics manifests itself as ultrafast traveling waves of cortical proteins, in which the curvature sensitivity-driven feedback always constrains protein lateral diffusion in wave propagation. The resulting protein wave propagation mainly reflects the spatial gradient in the timing of local protein recruitment from cytoplasm. We provide evidence that membrane undulations accompany these protein waves and potentiate their propagation. Therefore, membrane shape change and protein curvature sensitivity may have underappreciated roles in setting high-speed cortical signal transduction rhythms.

Traveling waves in the cell cortex can propagate much faster than actin waves, and the mechanism is unknown. Here the authors propose a mechanochemical feedback model for traveling waves that incorporates membrane shape changes and recruitment of F-BAR proteins that enables fast wave propagation.

The Arp2/3 Regulatory System and Its Deregulation in Cancer

The Arp2/3 complex is an evolutionary conserved molecular machine that generates branched actin networks. When activated, the Arp2/3 complex contributes the actin branched junction and thus cross-links the polymerizing actin filaments in a network that exerts a pushing force. The different activators initiate branched actin networks at the cytosolic surface of different cellular membranes to promote their protrusion, movement, or scission in cell migration and membrane traffic. Here we review the structure, function, and regulation of all the direct regulators of the Arp2/3 complex that induce or inhibit the initiation of a branched actin network and that controls the stability of its branched junctions. Our goal is to present recent findings concerning novel inhibitory proteins or the regulation of the actin branched junction and place these in the context of what was previously known to provide a global overview of how the Arp2/3 complex is regulated in human cells. We focus on the human set of Arp2/3 regulators to compare normal Arp2/3 regulation in untransformed cells to the deregulation of the Arp2/3 system observed in patients affected by various cancers. In many cases, these deregulations promote cancer progression and have a direct impact on patient survival.

Cell protrusion and retraction driven by fluctuations in actin polymerization: A two-dimensional model

Animal cells that spread onto a surface often rely on actin-rich lamellipodial extensions to execute protrusion. Many cell types recently adhered on a two-dimensional substrate exhibit protrusion and retraction of their lamellipodia, even though the cell is not translating. Travelling waves of protrusion have also been observed, similar to those observed in crawling cells. These regular patterns of protrusion and retraction allow quantitative analysis for comparison to mathematical models. The periodic fluctuations in leading edge position of XTC cells have been linked to excitable actin dynamics using a one-dimensional model of actin dynamics, as a function of arc-length along the cell. In this work we extend this earlier model of actin dynamics into two dimensions (along the arc-length and radial directions of the cell) and include a model membrane that protrudes and retracts in response to the changing number of free barbed ends of actin filaments near the membrane. We show that if the polymerization rate at the barbed ends changes in response to changes in their local concentration at the leading edge and/or the opposing force from the cell membrane, the model can reproduce the patterns of membrane protrusion and retraction seen in experiment. We investigate both Brownian ratchet and switch-like force-velocity relationships between the membrane load forces and actin polymerization rate. The switch-like polymerization dynamics recover the observed patterns of protrusion and retraction as well as the fluctuations in F-actin concentration profiles. The model generates predictions for the behavior of cells after local membrane tension perturbations.

Overview of Single-Molecule Speckle (SiMS) Microscopy and Its Electroporation-Based Version with Efficient Labeling and Improved Spatiotemporal Resolution

Live-cell single-molecule imaging was introduced more than a decade ago, and has provided critical information on remodeling of the actin cytoskeleton, the motion of plasma membrane proteins, and dynamics of molecular motor proteins. Actin remodeling has been the best target for this approach because actin and its associated proteins stop diffusing when assembled, allowing visualization of single-molecules of fluorescently-labeled proteins in a state specific manner. The approach based on this simple principle is called Single-Molecule Speckle (SiMS) microscopy. For instance, spatiotemporal regulation of actin polymerization and lifetime distribution of actin filaments can be monitored directly by tracking actin SiMS. In combination with fluorescently labeled probes of various actin regulators, SiMS microscopy has contributed to clarifying the processes underlying recycling, motion and remodeling of the live-cell actin network. Recently, we introduced an electroporation-based method called eSiMS microscopy, with high efficiency, easiness and improved spatiotemporal precision. In this review, we describe the application of live-cell single-molecule imaging to cellular actin dynamics and discuss the advantages of eSiMS microscopy over previous SiMS microscopy.

Actin-Regulator Feedback Interactions during Endocytosis

Endocytosis mediated by clathrin, a cellular process by which cells internalize membrane receptors and their extracellular ligands, is an important component of cell signaling regulation. Actin polymerization is involved in endocytosis in varying degrees depending on the cellular context. In yeast, clathrin-mediated endocytosis requires a pulse of polymerized actin and its regulators, which recruit and activate the Arp2/3 complex. In this article, we seek to identify the main protein-protein interactions that 1) cause actin and its regulators to appear in pulses, and 2) determine the effects of key mutations and drug treatments on actin and regulator assembly. We perform a joint modeling/experimental study of actin and regulator dynamics during endocytosis in the budding yeast Saccharomyces cerevisiae. We treat both a stochastic model that grows an explicit three-dimensional actin network, and a simpler two-variable Fitzhugh-Nagumo type model. The models include a negative-feedback interaction of F-actin onto the Arp2/3 regulators. Both models explain the pulse time courses and the effects of interventions on actin polymerization: the surprising increase in the peak F-actin count caused by reduced regulator branching activity, the increase in F-actin resulting from slowing of actin disassembly, and the increased Arp2/3 regulator lifetime resulting from latrunculin treatment. In addition, they predict that decreases in the regulator branching activity lead to increases in accumulation of regulators, and we confirmed this prediction with experiments on yeast harboring mutations in the Arp2/3 regulators, using quantitative fluorescence microscopy. Our experimental measurements suggest that the regulators act quasi-independently, in the sense that accumulation of a particular regulator is most strongly affected by mutations of that regulator, as opposed to the others.

Model of turnover kinetics in the lamellipodium: implications of slow- and fast- diffusing capping protein and Arp2/3 complex

Cell protrusion through polymerization of actin filaments at the leading edge of motile cells may be influenced by spatial gradients of diffuse actin and regulators. Here we study the distribution of two of the most important regulators, capping protein and Arp2/3 complex, which regulate actin polymerization in the lamellipodium through capping and nucleation of free barbed ends. We modeled their kinetics using data from prior single molecule microscopy experiments on XTC cells. These experiments have provided evidence for a broad distribution of diffusion coefficients of both capping protein and Arp2/3 complex. The slowly diffusing proteins appear as extended 'clouds' while proteins bound to the actin filament network appear as speckles that undergo retrograde flow. Speckle appearance and disappearance events correspond to assembly and dissociation from the actin filament network and speckle lifetimes correspond to the dissociation rate. The slowly diffusing capping protein could represent severed capped actin filament fragments or membrane-bound capping protein. Prior evidence suggests that slowly diffusing Apr2/3 complex associates with the membrane. We use the measured rates and estimates of diffusion coefficients of capping protein and Arp2/3 complex in a Monte Carlo simulation that includes particles in association with a filament network and diffuse in the cytoplasm. We consider two separate pools of diffuse proteins, representing fast and slowly diffusing species. We find a steady state with concentration gradients involving a balance of diffusive flow of fast and slow species with retrograde flow. We show that simulations of FRAP are consistent with prior experiments performed on different cell types. We provide estimates for the ratio of bound to diffuse complexes and calculate conditions where Arp2/3 complex recycling by diffusion may become limiting. We discuss the implications of slowly diffusing populations and suggest experiments to distinguish among mechanisms that influence long range transport.

Local Arp2/3-dependent actin assembly modulates applied traction force during apCAM adhesion site maturation

In growth cones, local Arp 2/3-dependent actin assembly mechanically buffers apCAM adhesions from retrograde flow–associated traction forces. The resulting propulsive forces drive the exploratory motility of inductopodia. Increasing the stiffness of apCAM targets induces an extensive 3D actin cup to form at the adhesion during evoked growth responses.

Homophilic binding of immunoglobulin superfamily molecules such as the Aplysia cell adhesion molecule (apCAM) leads to actin filament assembly near nascent adhesion sites. Such actin assembly can generate significant localized forces that have not been characterized in the larger context of axon growth and guidance. We used apCAM-coated bead substrates applied to the surface of neuronal growth cones to characterize the development of forces evoked by varying stiffness of mechanical restraint. Unrestrained bead propulsion matched or exceeded rates of retrograde network flow and was dependent on Arp2/3 complex activity. Analysis of growth cone forces applied to beads at low stiffness of restraint revealed switching between two states: frictional coupling to retrograde flow and Arp2/3-dependent propulsion. Stiff mechanical restraint led to formation of an extensive actin cup matching the geometric profile of the bead target and forward growth cone translocation; pharmacological inhibition of the Arp2/3 complex or Rac attenuated F-actin assembly near bead binding sites, decreased the efficacy of growth responses, and blocked accumulation of signaling molecules associated with nascent adhesions. These studies introduce a new model for regulation of traction force in which local actin assembly forces buffer nascent adhesion sites from the mechanical effects of retrograde flow.

Direct interaction between exocyst and Wave complexes promotes cell protrusions and motility

Coordination between membrane trafficking and actin polymerization is fundamental in cell migration, but a dynamic view of the underlying molecular mechanisms is still missing. The Rac1 GTPase controls actin polymerization at protrusions by interacting with its effector, the Wave regulatory complex (WRC). The exocyst complex, which functions in polarized exocytosis, has been involved in the regulation of cell motility. Here, we show a physical and functional connection between exocyst and WRC. Purified components of exocyst and WRC directly associate in vitro, and interactions interfaces are identified. The exocyst-WRC interaction is confirmed in cells by co-immunoprecipitation and is shown to occur independently of the Arp2/3 complex. Disruption of the exocyst-WRC interaction leads to impaired migration. By using time-lapse microscopy coupled to image correlation analysis, we visualized the trafficking of the WRC towards the front of the cell in nascent protrusions. The exocyst is necessary for WRC recruitment at the leading edge and for resulting cell edge movements. This direct link between the exocyst and WRC provides a new mechanistic insight into the spatio-temporal regulation of cell migration.

β-Arrestin 1-dependent regulation of Rap2 is required for fMLP-stimulated chemotaxis in neutrophil-like HL-60 cells

β-Arrestins have emerged as key regulators of cytoskeletal rearrangement that are required for directed cell migration. Whereas it is known that β-arrestins are required for formyl-Met-Leu-Phe receptor (FPR) recycling, less is known about their role in regulating FPR-mediated neutrophil chemotaxis. Here, we show that β-arrestin 1 (ArrB1) coaccumulated with F-actin within the leading edge of neutrophil-like HL-60 cells during chemotaxis, and its knockdown resulted in markedly reduced migration within fMLP gradients. The small GTPase Ras-related protein 2 (Rap2) was found to bind ArrB1 under resting conditions but dissociated upon fMLP stimulation. The FPR-dependent activation of Rap2 required ArrB1 but was independent of Gα_i activity. Significantly, depletion of either ArrB1 or Rap2 resulted in reduced chemotaxis and defects in cellular repolarization within fMLP gradients. These data strongly suggest a model in which FPR is able to direct ArrB1 and other bound proteins that are required for lamellipodial extension to the leading edge in migrating neutrophils, thereby orientating and directing cell migration.

Membrane Tension Acts Through PLD2 and mTORC2 to Limit Actin Network Assembly During Neutrophil Migration

For efficient polarity and migration, cells need to regulate the magnitude and spatial distribution of actin assembly. This process is coordinated by reciprocal interactions between the actin cytoskeleton and mechanical forces. Actin polymerization-based protrusion increases tension in the plasma membrane, which in turn acts as a long-range inhibitor of actin assembly. These interactions form a negative feedback circuit that limits the magnitude of membrane tension in neutrophils and prevents expansion of the existing front and the formation of secondary fronts. It has been suggested that the plasma membrane directly inhibits actin assembly by serving as a physical barrier that opposes protrusion. Here we show that efficient control of actin polymerization-based protrusion requires an additional mechanosensory feedback cascade that indirectly links membrane tension with actin assembly. Specifically, elevated membrane tension acts through phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin nucleation. In the absence of this pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling suggests roles for both the direct (mechanical) and indirect (biochemical via PLD2 and mTORC2) feedback loops in organizing cell polarity and motility—the indirect loop is better suited to enable competition between fronts, whereas the direct loop helps spatially organize actin nucleation for efficient leading edge formation and cell movement. This circuit is essential for polarity, motility, and the control of membrane tension.

A mechanosensory biochemical cascade involving phospholipase D2 and mTORC2 coordinates physical forces and cytoskeletal rearrangements to allow efficient polarization and migration of neutrophils.

How cells regulate the size and number of their protrusions for efficient polarity and motility is a fundamental question in cell biology. We recently found that immune cells known as neutrophils use physical forces to regulate this process. Actin polymerization-based protrusion stretches the plasma membrane, and this increased membrane tension acts as a long-range inhibitor of actin-based protrusions elsewhere in the cell. Here we investigate how membrane tension limits protrusion. We demonstrate that the magnitude of actin network assembly in neutrophils is determined by a mechanosensory biochemical cascade that converts increases in membrane tension into decreases in protrusion. Specifically, we show that increasing plasma membrane tension acts through a pathway containing the phospholipase D2 (PLD2) and the mammalian target of rapamycin complex 2 (mTORC2) to limit actin network assembly. Without this negative feedback pathway, neutrophils exhibit larger leading edges, higher membrane tension, and profoundly defective chemotaxis. Mathematical modeling indicates that this feedback circuit is a favorable topology to enable competition between protrusions during neutrophil polarization. Our work shows how biochemical signals, physical forces, and the cytoskeleton can collaborate to generate large-scale cellular organization.

Gβ Regulates Coupling between Actin Oscillators for Cell Polarity and Directional Migration

For directional movement, eukaryotic cells depend on the proper organization of their actin cytoskeleton. This engine of motility is made up of highly dynamic nonequilibrium actin structures such as flashes, oscillations, and traveling waves. In Dictyostelium, oscillatory actin foci interact with signals such as Ras and phosphatidylinositol 3,4,5-trisphosphate (PIP₃) to form protrusions. However, how signaling cues tame actin dynamics to produce a pseudopod and guide cellular motility is a critical open question in eukaryotic chemotaxis. Here, we demonstrate that the strength of coupling between individual actin oscillators controls cell polarization and directional movement. We implement an inducible sequestration system to inactivate the heterotrimeric G protein subunit Gβ and find that this acute perturbation triggers persistent, high-amplitude cortical oscillations of F-actin. Actin oscillators that are normally weakly coupled to one another in wild-type cells become strongly synchronized following acute inactivation of Gβ. This global coupling impairs sensing of internal cues during spontaneous polarization and sensing of external cues during directional motility. A simple mathematical model of coupled actin oscillators reveals the importance of appropriate coupling strength for chemotaxis: moderate coupling can increase sensitivity to noisy inputs. Taken together, our data suggest that Gβ regulates the strength of coupling between actin oscillators for efficient polarity and directional migration. As these observations are only possible following acute inhibition of Gβ and are masked by slow compensation in genetic knockouts, our work also shows that acute loss-of-function approaches can complement and extend the reach of classical genetics in Dictyostelium and likely other systems as well.

Coupling of individual oscillators regulates biological functions ranging from crickets chirping in unison to the coordination of pacemaker cells of the heart. This study finds that a similar concept—coupling between actin oscillators—is at work within single slime mold cells to establish polarity and guide their direction of migration.

The actin cytoskeleton of motile cells is comprised of highly dynamic structures. Recently, small oscillating actin foci have been discovered around the periphery of Dictyostelium cells. These oscillators are thought to enable pseudopod formation, but how their dynamics are regulated for this is unknown. Here, we demonstrate that the strength of coupling between individual actin oscillators controls cell polarization and directional movement. Actin oscillators are weakly coupled to one another in wild-type cells, but they become strongly synchronized after acute inactivation of the signaling protein Gβ. This global coupling impairs sensing of internal cues during spontaneous polarization and sensing of external cues during directional motility. Supported by a mathematical model, our data suggest that wild-type cells are tuned to an optimal coupling strength for patterning by upstream cues. These observations are only possible following acute inhibition of Gβ, which highlights the value of revisiting classical mutants with acute loss-of-function perturbations.

ERK reinforces actin polymerization to power persistent edge protrusion during motility

Cells move through perpetual protrusion and retraction cycles at the leading edge. These cycles are coordinated with substrate adhesion and retraction of the cell rear. We tracked spatial and temporal fluctuations in the molecular activities of individual moving cells to elucidate how extracellular signal-regulated kinase (ERK) signaling controlled the dynamics of protrusion and retraction cycles. ERK is activated by many cell surface receptors, and we found that ERK signaling specifically reinforced cellular protrusions so that they translated into rapid, sustained forward motion of the leading edge. Using quantitative fluorescent speckle microscopy and cross-correlation analysis, we showed that ERK controlled the rate and timing of actin polymerization by promoting the recruitment of the actin nucleator Arp2/3 to the leading edge. These findings support a model in which surges in ERK activity induced by extracellular cues enhance Arp2/3-mediated actin polymerization to generate protrusion power phases with enough force to counteract increasing membrane tension and to promote sustained motility.

The role of reactive oxygen species in microvascular remodeling

The microcirculation is a portion of the vascular circulatory system that consists of resistance arteries, arterioles, capillaries and venules. It is the place where gases and nutrients are exchanged between blood and tissues. In addition the microcirculation is the major contributor to blood flow resistance and consequently to regulation of blood pressure. Therefore, structural remodeling of this section of the vascular tree has profound implications on cardiovascular pathophysiology. This review is focused on the role that reactive oxygen species (ROS) play on changing the structural characteristics of vessels within the microcirculation. Particular attention is given to the resistance arteries and the functional pathways that are affected by ROS in these vessels and subsequently induce vascular remodeling. The primary sources of ROS in the microcirculation are identified and the effects of ROS on other microcirculatory remodeling phenomena such as rarefaction and collateralization are briefly reviewed.

Steering cell migration: lamellipodium dynamics and the regulation of directional persistence

Membrane protrusions at the leading edge of cells, known as lamellipodia, drive cell migration in many normal and pathological situations. Lamellipodial protrusion is powered by actin polymerization, which is mediated by the actin-related protein 2/3 (ARP2/3)-induced nucleation of branched actin networks and the elongation of actin filaments. Recently, advances have been made in our understanding of positive and negative ARP2/3 regulators (such as the SCAR/WAVE (SCAR/WASP family verprolin-homologous protein) complex and Arpin, respectively) and of proteins that control actin branch stability (such as glial maturation factor (GMF)) or actin filament elongation (such as ENA/VASP proteins) in lamellipodium dynamics and cell migration. This Review highlights how the balance between actin filament branching and elongation, and between the positive and negative feedback loops that regulate these activities, determines lamellipodial persistence. Importantly, directional persistence, which results from lamellipodial persistence, emerges as a critical factor in steering cell migration.

Nanoscale segregation of actin nucleation and elongation factors determines dendritic spine protrusion

Actin dynamics drive morphological remodeling of neuronal dendritic spines and changes in synaptic transmission. Yet, the spatiotemporal coordination of actin regulators in spines is unknown. Using single protein tracking and super-resolution imaging, we revealed the nanoscale organization and dynamics of branched F-actin regulators in spines. Branched F-actin nucleation occurs at the PSD vicinity, while elongation occurs at the tip of finger-like protrusions. This spatial segregation differs from lamellipodia where both branched F-actin nucleation and elongation occur at protrusion tips. The PSD is a persistent confinement zone for IRSp53 and the WAVE complex, an activator of the Arp2/3 complex. In contrast, filament elongators like VASP and formin-like protein-2 move outwards from the PSD with protrusion tips. Accordingly, Arp2/3 complexes associated with F-actin are immobile and surround the PSD. Arp2/3 and Rac1 GTPase converge to the PSD, respectively, by cytosolic and free-diffusion on the membrane. Enhanced Rac1 activation and Shank3 over-expression, both associated with spine enlargement, induce delocalization of the WAVE complex from the PSD. Thus, the specific localization of branched F-actin regulators in spines might be reorganized during spine morphological remodeling often associated with synaptic plasticity.

Self-organization of protrusions and polarity during eukaryotic chemotaxis

Many eukaryotic cells regulate their polarity and motility in response to external chemical cues. While we know many of the linear connections that link receptors with downstream actin polymerization events, we have a much murkier understanding of the higher order positive and negative feedback loops that organize these processes in space and time. Importantly, physical forces and actin polymerization events do not simply act downstream of chemotactic inputs but are rather involved in a web of reciprocal interactions with signaling components to generate self-organizing pseudopods and cell polarity. Here we focus on recent progress and open questions in the field, including the basic unit of actin organization, how cells regulate the number and speed of protrusions, and 2D versus 3D migration.

Single-molecule analysis of cell surface dynamics in Caenorhabditis elegans embryos

We describe a general, versatile and minimally invasive method to image single molecules near the cell surface that can be applied to any GFP-tagged protein in Caenorhabditis elegans embryos. We exploited tunable expression via RNAi and a dynamically exchanging monomer pool to achieve fast, continuous single-molecule imaging at optimal densities with signal-to-noise ratios adequate for robust single-particle tracking (SPT). We introduce a method called smPReSS, single-molecule photobleaching relaxation to steady state, that infers exchange rates from quantitative analysis of single-molecule photobleaching kinetics without using SPT. Combining SPT and smPReSS allowed for spatially and temporally resolved measurements of protein mobility and exchange kinetics. We used these methods to (i) resolve distinct mobility states and spatial variation in exchange rates of the polarity protein PAR-6 and (ii) measure spatiotemporal modulation of actin filament assembly and disassembly. These methods offer a promising avenue to investigate dynamic mechanisms that pattern the embryonic cell surface.

New single-molecule speckle microscopy reveals modification of the retrograde actin flow by focal adhesions at nanometer scales

This paper introduces a new, easy-to-use method of fluorescence single-molecule speckle microscopy for actin with nanometer-scale accuracy. This new method reveals that actin flows in front of mature focal adhesions (FAs) are fast and biased toward FAs, suggesting that mature FAs are actively engaged in pulling and remodeling the local actin network.

Speckle microscopy directly visualizes the retrograde actin flow, which is believed to promote cell-edge protrusion when linked to focal adhesions (FAs). However, it has been argued that, due to rapid actin turnover, the use of green fluorescent protein–actin, the lack of appropriate analysis algorithms, and technical difficulties, speckle microscopy does not necessarily report the flow velocities of entire actin populations. In this study, we developed a new, user-friendly single-molecule speckle (SiMS) microscopy using DyLight dye-labeled actin. Our new SiMS method enables in vivo nanometer-scale displacement analysis with a low localization error of ±8–8.5 nm, allowing accurate flow-velocity measurement for actin speckles with lifetime <5 s. In lamellipodia, both short- and long-lived F-actin molecules flow with the same speed, indicating they are part of a single actin network. These results do not support coexistence of F-actin populations with different flow speeds, which is referred to as the lamella hypothesis. Mature FAs, but not nascent adhesions, locally obstruct the retrograde flow. Interestingly, the actin flow in front of mature FAs is fast and biased toward FAs, suggesting that mature FAs attract the flow in front and actively remodel the local actin network.

Actin dynamics rapidly reset chemoattractant receptor sensitivity following adaptation in neutrophils

Neutrophils are cells of the innate immune system that hunt and kill pathogens using directed migration. This process, known as chemotaxis, requires the regulation of actin polymerization downstream of chemoattractant receptors. Reciprocal interactions between actin and intracellular signals are thought to underlie many of the sophisticated signal processing capabilities of the chemotactic cascade including adaptation, amplification and long-range inhibition. However, with existing tools, it has been difficult to discern actin's role in these processes. Most studies investigating the role of the actin cytoskeleton have primarily relied on actin-depolymerizing agents, which not only block new actin polymerization but also destroy the existing cytoskeleton. We recently developed a combination of pharmacological inhibitors that stabilizes the existing actin cytoskeleton by inhibiting actin polymerization, depolymerization and myosin-based rearrangements; we refer to these processes collectively as actin dynamics. Here, we investigated how actin dynamics influence multiple signalling responses (PI3K lipid products, calcium and Pak phosphorylation) following acute agonist addition or during desensitization. We find that stabilized actin polymer extends the period of receptor desensitization following agonist binding and that actin dynamics rapidly reset receptors from this desensitized state. Spatial differences in actin dynamics may underlie front/back differences in agonist sensitivity in neutrophils.

Arp2/3 complex is essential for actin network treadmilling as well as for targeting of capping protein and cofilin

Acute suppression of Arp2/3 complex activity in lamellipodia demonstrates its essential role in actin network treadmilling and filament organization and geometry. Arp2/3 complex activity also defines the recruitment of crucial independent factors, including capping protein and cofilin, and is essential for lamellipodia-based keratocyte migration.

Lamellipodia are sheet-like protrusions formed during migration or phagocytosis and comprise a network of actin filaments. Filament formation in this network is initiated by nucleation/branching through the actin-related protein 2/3 (Arp2/3) complex downstream of its activator, suppressor of cAMP receptor/WASP-family verprolin homologous (Scar/WAVE), but the relative relevance of Arp2/3-mediated branching versus actin filament elongation is unknown. Here we use instantaneous interference with Arp2/3 complex function in live fibroblasts with established lamellipodia. This allows direct examination of both the fate of elongating filaments upon instantaneous suppression of Arp2/3 complex activity and the consequences of this treatment on the dynamics of other lamellipodial regulators. We show that Arp2/3 complex is an essential organizer of treadmilling actin filament arrays but has little effect on the net rate of actin filament turnover at the cell periphery. In addition, Arp2/3 complex serves as key upstream factor for the recruitment of modulators of lamellipodia formation such as capping protein or cofilin. Arp2/3 complex is thus decisive for filament organization and geometry within the network not only by generating branches and novel filament ends, but also by directing capping or severing activities to the lamellipodium. Arp2/3 complex is also crucial to lamellipodia-based migration of keratocytes.

A continuum model of actin waves in Dictyostelium discoideum

Actin waves are complex dynamical patterns of the dendritic network of filamentous actin in eukaryotes. We developed a model of actin waves in PTEN-deficient Dictyostelium discoideum by deriving an approximation of the dynamics of discrete actin filaments and combining it with a signaling pathway that controls filament branching. This signaling pathway, together with the actin network, contains a positive feedback loop that drives the actin waves. Our model predicts the structure, composition, and dynamics of waves that are consistent with existing experimental evidence, as well as the biochemical dependence on various protein partners. Simulation suggests that actin waves are initiated when local actin network activity, caused by an independent process, exceeds a certain threshold. Moreover, diffusion of proteins that form a positive feedback loop with the actin network alone is sufficient for propagation of actin waves at the observed speed of . Decay of the wave back can be caused by scarcity of network components, and the shape of actin waves is highly dependent on the filament disassembly rate. The model allows retraction of actin waves and captures formation of new wave fronts in broken waves. Our results demonstrate that a delicate balance between a positive feedback, filament disassembly, and local availability of network components is essential for the complex dynamics of actin waves.

Unifying autocatalytic and zeroth-order branching models for growing actin networks

The directed polymerization of actin networks is an essential element of many biological processes, including cell migration. Different theoretical models considering the interplay between the underlying processes of polymerization, capping, and branching have resulted in conflicting predictions. One of the main reasons for this discrepancy is the assumption of a branching reaction that is either first order (autocatalytic) or zeroth order in the number of existing filaments. Here we introduce a unifying framework from which the two established scenarios emerge as limiting cases for low and high filament numbers. A smooth transition between the two cases is found at intermediate conditions. We also derive a threshold for the capping rate above which autocatalytic growth is predicted at sufficiently low filament number. Below the threshold, zeroth-order characteristics are predicted to dominate the dynamics of the network for all accessible filament numbers. Together, these mechanisms allow cells to grow stable actin networks over a large range of different conditions.

Actin polymerization controls the organization of WASH domains at the surface of endosomes

Sorting of cargoes in endosomes occurs through their selective enrichment into sorting platforms, where transport intermediates are generated. The WASH complex, which directly binds to lipids, activates the Arp2/3 complex and hence actin polymerization onto such sorting platforms. Here, we analyzed the role of actin polymerization in the physiology of endosomal domains containing WASH using quantitative image analysis. Actin depolymerization is known to enlarge endosomes. Using a novel colocalization method that is insensitive to the heterogeneity of size and shape of endosomes, we further show that preventing the generation of branched actin networks induces endosomal accumulation of the WASH complex. Moreover, we found that actin depolymerization induces a dramatic decrease in the recovery of endosomal WASH after photobleaching. This result suggests a built-in turnover, where the actin network, i.e. the product of the WASH complex, contributes to the dynamic exchange of the WASH complex by promoting its detachment from endosomes. Our experiments also provide evidence for a role of actin polymerization in the lateral compartmentalization of endosomes: several WASH domains exist at the surface of enlarged endosomes, however, the WASH domains coalesce upon actin depolymerization or Arp2/3 depletion. Branched actin networks are thus involved in the regulation of the size of WASH domains. The potential role of this regulation in membrane scission are discussed.