Cease-fire at the leading edge: new perspectives on actin filament branching, debranching, and cross-linking

Direct observation of cortactin protecting Arp2/3-actin filament branch junctions from GMF-mediated destabilization

How cells tightly control the formation and turnover of branched actin filament arrays to drive cell motility, endocytosis, and other cellular processes is still not well understood. Here, we investigated the mechanistic relationship between two binding partners of the Arp2/3 complex, glia maturation factor (GMF) and cortactin. Individually, GMF and cortactin have opposite effects on the stability of actin filament branches, but it is unknown how they work in concert with each other to govern branch turnover. Using TIRF microscopy, we observe that GMF's branch destabilizing activities are potently blocked by cortactin (IC50 = 1.3 nM) and that this inhibition requires direct interactions of cortactin with Arp2/3 complex. The simplest model that would explain these results is competition for binding Arp2/3 complex. However, we find that cortactin and GMF do not compete for free Arp2/3 complex in solution. Further, we use single molecule analysis to show that cortactin's on-rate (3 ×107 s-1 M-1) and off-rate (0.03 s-1) at branch junctions are minimally affected by excess GMF. Together, these results show that cortactin binds with high affinity to branch junctions, where it blocks the destabilizing effects of GMF, possibly by a mechanism that is allosteric in nature. In addition, the affinities we measure for cortactin at actin filament branch junctions (Kd = 0.9 nM) and filament sides (Kd = 206 nM) are approximately 20-fold stronger than previously reported. These observations contribute to an emerging view of molecular complexity in how Arp2/3 complex is regulated through the integration of multiple inputs.

Mechanisms of actin disassembly and turnover

Cellular actin networks exhibit a wide range of sizes, shapes, and architectures tailored to their biological roles. Once assembled, these filamentous networks are either maintained in a state of polarized turnover or induced to undergo net disassembly. Further, the rates at which the networks are turned over and/or dismantled can vary greatly, from seconds to minutes to hours or even days. Here, we review the molecular machinery and mechanisms employed in cells to drive the disassembly and turnover of actin networks. In particular, we highlight recent discoveries showing that specific combinations of conserved actin disassembly-promoting proteins (cofilin, GMF, twinfilin, Srv2/CAP, coronin, AIP1, capping protein, and profilin) work in concert to debranch, sever, cap, and depolymerize actin filaments, and to recharge actin monomers for new rounds of assembly.

Getting cells into shape by calcium-dependent actin cross-linking proteins

The actin cytoskeleton represents a highly dynamic filament system providing cell structure and mechanical forces to drive a variety of cellular processes. The dynamics of the actin cytoskeleton are controlled by a number of conserved proteins that maintain the pool of actin monomers, promote actin nucleation, restrict the length of actin filaments and cross-link filaments into networks or bundles. Previous work has been established that cytoplasmic calcium is an important signal to rapidly relay information to the actin cytoskeleton, but the underlying mechanisms remain poorly understood. Here, we summarize new recent perspectives on how calcium fluxes are transduced to the actin cytoskeleton in a physiological context. In this mini-review we will focus on three calcium-binding EF-hand-containing actin cross-linking proteins, α-actinin, plastin and EFHD2/Swiprosin-1, and how these conserved proteins affect the cell's actin reorganization in the context of cell migration and wound closure in response to calcium.

Calcium bursts allow rapid reorganization of EFhD2/Swip-1 cross-linked actin networks in epithelial wound closure

Changes in cell morphology require the dynamic remodeling of the actin cytoskeleton. Calcium fluxes have been suggested as an important signal to rapidly relay information to the actin cytoskeleton, but the underlying mechanisms remain poorly understood. Here, we identify the EF-hand domain containing protein EFhD2/Swip-1 as a conserved lamellipodial protein strongly upregulated in Drosophila macrophages at the onset of metamorphosis when macrophage behavior shifts from quiescent to migratory state. Loss- and gain-of-function analysis confirm a critical function of EFhD2/Swip-1 in lamellipodial cell migration in fly and mouse melanoma cells. Contrary to previous assumptions, TIRF-analyses unambiguously demonstrate that EFhD2/Swip-1 proteins efficiently cross-link actin filaments in a calcium-dependent manner. Using a single-cell wounding model, we show that EFhD2/Swip-1 promotes wound closure in a calcium-dependent manner. Mechanistically, our data suggest that transient calcium bursts reduce EFhD2/Swip-1 cross-linking activity and thereby promote rapid reorganization of existing actin networks to drive epithelial wound closure.

The expression of human testis-specific actin capping protein predicts in vitro fertilization outcomes: A novel biomarker of sperm function for assisted reproductive technology

PURPOSE

Sperm function tests do not adequately assess fertilization potential, and new indices are required. We have previously reported that human testis-specific actin capping proteins may be involved in both sperm morphology and function. This study aimed to determine whether testis-specific actin capping proteins can be a predictive marker of IVF success.

METHODS

Ninety-seven infertile couples who underwent IVF at an infertility clinic were included. Sperm were immunohistochemically stained to evaluate capping protein expression, and the percentage of sperms with normal staining was calculated. The relationship between actin capping protein expression and IVF outcomes was examined.

RESULTS

The couples were divided into four groups according to the percentage of normally expressing actin capping protein as follows: ≥90% Group Ⅰ, 80%-90% Group Ⅱ, 70%-80% Group Ⅲ, and <70% Group Ⅳ. Multiple regression analysis showed a significant trend in fertilization rates across the 4 groups (p for trend =0.008).There was no significant trend in pregnancy rates (p for trend =0.276).

CONCLUSION

The human testis-specific actin capping protein may be a marker of male contributing factors that predict IVF outcomes.

A barbed end interference mechanism reveals how capping protein promotes nucleation in branched actin networks

Heterodimeric capping protein (CP/CapZ) is an essential factor for the assembly of branched actin networks, which push against cellular membranes to drive a large variety of cellular processes. Aside from terminating filament growth, CP potentiates the nucleation of actin filaments by the Arp2/3 complex in branched actin networks through an unclear mechanism. Here, we combine structural biology with in vitro reconstitution to demonstrate that CP not only terminates filament elongation, but indirectly stimulates the activity of Arp2/3 activating nucleation promoting factors (NPFs) by preventing their association to filament barbed ends. Key to this function is one of CP's C-terminal "tentacle" extensions, which sterically masks the main interaction site of the terminal actin protomer. Deletion of the β tentacle only modestly impairs capping. However, in the context of a growing branched actin network, its removal potently inhibits nucleation promoting factors by tethering them to capped filament ends. End tethering of NPFs prevents their loading with actin monomers required for activation of the Arp2/3 complex and thus strongly inhibits branched network assembly both in cells and reconstituted motility assays. Our results mechanistically explain how CP couples two opposed processes-capping and nucleation-in branched actin network assembly.

Unified control of amoeboid pseudopod extension in multiple organisms by branched F-actin in the front and parallel F-actin/myosin in the cortex

The trajectory of moving eukaryotic cells depends on the kinetics and direction of extending pseudopods. The direction of pseudopods has been well studied to unravel mechanisms for chemotaxis, wound healing and inflammation. However, the kinetics of pseudopod extension-when and why do pseudopods start and stop- is equally important, but is largely unknown. Here the START and STOP of about 4000 pseudopods was determined in four different species, at four conditions and in nine mutants (fast amoeboids Dictyostelium and neutrophils, slow mesenchymal stem cells, and fungus B.d. chytrid with pseudopod and a flagellum). The START of a first pseudopod is a random event with a probability that is species-specific (23%/s for neutrophils). In all species and conditions, the START of a second pseudopod is strongly inhibited by the extending first pseudopod, which depends on parallel filamentous actin/myosin in the cell cortex. Pseudopods extend at a constant rate by polymerization of branched F-actin at the pseudopod tip, which requires the Scar complex. The STOP of pseudopod extension is induced by multiple inhibitory processes that evolve during pseudopod extension and mainly depend on the increasing size of the pseudopod. Surprisingly, no differences in pseudopod kinetics are detectable between polarized, unpolarized or chemotactic cells, and also not between different species except for small differences in numerical values. This suggests that the analysis has uncovered the fundament of cell movement with distinct roles for stimulatory branched F-actin in the protrusion and inhibitory parallel F-actin in the contractile cortex.

Physiological role of actin regulation in male fertility: Insight into actin capping proteins in spermatogenic cells

BACKGROUND

During spermatogenesis, cytoskeletal elements are essential for spermatogenic cells to change morphologically and translocate in the seminiferous tubule. Actin filaments have been revealed to be concentrated in specific regions of spermatogenic cells and are regulated by a large number of actin-binding proteins. Actin capping protein is one of the essential actin regulatory proteins, and a recent study showed that testis-specific actin capping protein may affect male infertility.

METHODS

The roles of actin during spermatogenesis and testis-specific actin capping protein were reviewed by referring to the previous literature.

MAIN FINDINGS RESULTS

Actin filaments are involved in several crucial phases of spermatogenesis including acrosome biogenesis, flagellum formation, and nuclear processes such as the formation of synaptonemal complex. Besides, an implication for capacitation and acrosome reaction was also suggested. Testis-specific actin capping proteins are suggested to be associated with the removal of excess cytoplasm in mice. By the use of high-throughput sperm proteomics, lower protein expression of testis-specific actin capping protein in infertile men was also reported.

CONCLUSION

Actin is involved in the crucial phases of spermatogenesis, and the altered expression of testis-specific actin capping proteins is suggested to be a cause of male infertility in humans.

Force Generation by Myosin Motors: A Structural Perspective

Generating force and movement is essential for the functions of cells and organisms. A variety of molecular motors that can move on tracks within cells have evolved to serve this role. How these motors interact with their tracks and how that, in turn, leads to the generation of force and movement is key to understanding the cellular roles that these motor-track systems serve. This review is focused on the best understood of these systems, which is the molecular motor myosin that moves on tracks of filamentous (F-) actin. The review highlights both the progress and the limits of our current understanding of how force generation can be controlled by F-actin-myosin interactions. What has emerged are insights they may serve as a framework for understanding the design principles of a number of types of molecular motors and their interactions with their tracks.

GMF as an Actin Network Remodeling Factor

Glia maturation factor (GMF) has recently been established as a regulator of the actin cytoskeleton with a unique role in remodeling actin network architecture. Conserved from yeast to mammals, GMF is one of five members of the ADF-H family of actin regulatory proteins, which includes ADF/cofilin, Abp1/Drebrin, Twinfilin, and Coactosin. GMF does not bind actin, but instead binds the Arp2/3 complex with high affinity. Through this association, GMF catalyzes the debranching of actin filament networks and inhibits actin nucleation by Arp2/3 complex. Here, we discuss GMF's emerging role in controlling actin filament spatial organization and dynamics underlying cell motility, endocytosis, and other biological processes. Further, we attempt to reconcile these functions with its earlier characterization as a cell differentiation factor.

Load Adaptation of Lamellipodial Actin Networks

Actin filaments polymerizing against membranes power endocytosis, vesicular traffic, and cell motility. In vitro reconstitution studies suggest that the structure and the dynamics of actin networks respond to mechanical forces. We demonstrate that lamellipodial actin of migrating cells responds to mechanical load when membrane tension is modulated. In a steady state, migrating cell filaments assume the canonical dendritic geometry, defined by Arp2/3-generated 70° branch points. Increased tension triggers a dense network with a broadened range of angles, whereas decreased tension causes a shift to a sparse configuration dominated by filaments growing perpendicularly to the plasma membrane. We show that these responses emerge from the geometry of branched actin: when load per filament decreases, elongation speed increases and perpendicular filaments gradually outcompete others because they polymerize the shortest distance to the membrane, where they are protected from capping. This network-intrinsic geometrical adaptation mechanism tunes protrusive force in response to mechanical load.

Recent advances in understanding Listeria monocytogenes infection: the importance of subcellular and physiological context

The bacterial pathogen Listeria monocytogenes ( Lm) is the causative agent of listeriosis, a rare but fatal foodborne disease. During infection, Lm can traverse several host barriers and enter the cytosol of a variety of cell types. Thus, consideration of the extracellular and intracellular niches of Lm is critical for understanding the infection process. Here, we review advances in our understanding of Lm infection and highlight how the interactions between the host and the pathogen are context dependent. We discuss discoveries of how Lm senses entry into the host cell cytosol. We present findings concerning how the nature of the various cytoskeleton components subverted by Lm changes depending on both the stage of infection and the subcellular context. We present discoveries of critical components required for Lm traversal of physiological barriers. Interactions between the host gut microbiota and Lm will be briefly discussed. Finally, the importance of Lm biodiversity and post-genomics approaches as a promising way to discover novel virulence factors will be highlighted.

Quantitative computational models of molecular self-assembly in systems biology

Molecular self-assembly is the dominant form of chemical reaction in living systems, yet efforts at systems biology modeling are only beginning to appreciate the need for and challenges to accurate quantitative modeling of self-assembly. Self-assembly reactions are essential to nearly every important process in cell and molecular biology and handling them is thus a necessary step in building comprehensive models of complex cellular systems. They present exceptional challenges, however, to standard methods for simulating complex systems. While the general systems biology world is just beginning to deal with these challenges, there is an extensive literature dealing with them for more specialized self-assembly modeling. This review will examine the challenges of self-assembly modeling, nascent efforts to deal with these challenges in the systems modeling community, and some of the solutions offered in prior work on self-assembly specifically. The review concludes with some consideration of the likely role of self-assembly in the future of complex biological system models more generally.

Structural Basis of Arp2/3 Complex Inhibition by GMF, Coronin, and Arpin

The evolutionarily conserved Arp2/3 complex plays a central role in nucleating the branched actin filament arrays that drive cell migration, endocytosis, and other processes. To better understand Arp2/3 complex regulation, we used single-particle electron microscopy to compare the structures of Arp2/3 complex bound to three different inhibitory ligands: glia maturation factor (GMF), Coronin, and Arpin. Although the three inhibitors have distinct binding sites on Arp2/3 complex, they each induced an "open" nucleation-inactive conformation. Coronin promoted a standard (previously described) open conformation of Arp2/3 complex, with the N-terminal β-propeller domain of Coronin positioned near the p35/ARPC2 subunit of Arp2/3 complex. GMF induced two distinct open conformations of Arp2/3 complex, which correlated with the two suggested binding sites for GMF. Furthermore, GMF synergized with Coronin in inhibiting actin nucleation by Arp2/3 complex. Arpin, which uses VCA-related acidic (A) motifs to interact with the Arp2/3 complex, induced the standard open conformation, and two new masses appeared at positions near Arp2 and Arp3. Furthermore, Arpin showed additive inhibitory effects on Arp2/3 complex with Coronin and GMF. Together, these data suggest that Arp2/3 complex conformation is highly polymorphic and that its activities can be controlled combinatorially by different inhibitory ligands.

Connecting the cytoskeleton to the endoplasmic reticulum and Golgi

A tendency in cell biology is to divide and conquer. For example, decades of painstaking work have led to an understanding of endoplasmic reticulum (ER) and Golgi structure, dynamics, and transport. In parallel, cytoskeletal researchers have revealed a fantastic diversity of structure and cellular function in both actin and microtubules. Increasingly, these areas overlap, necessitating an understanding of both organelle and cytoskeletal biology. This review addresses connections between the actin/microtubule cytoskeletons and organelles in animal cells, focusing on three key areas: ER structure and function; ER-to-Golgi transport; and Golgi structure and function. Making these connections has been challenging for several reasons: the small sizes and dynamic characteristics of some components; the fact that organelle-specific cytoskeletal elements can easily be obscured by more abundant cytoskeletal structures; and the difficulties in imaging membranes and cytoskeleton simultaneously, especially at the ultrastructural level. One major concept is that the cytoskeleton is frequently used to generate force for membrane movement, with two potential consequences: translocation of the organelle, or deformation of the organelle membrane. While initially discussing issues common to metazoan cells in general, we subsequently highlight specific features of neurons, since these highly polarized cells present unique challenges for organellar distribution and dynamics.

Emerging roles of sumoylation in the regulation of actin, microtubules, intermediate filaments, and septins

Sumoylation is a powerful regulatory system that controls many of the critical processes in the cell, including DNA repair, transcriptional regulation, nuclear transport, and DNA replication. Recently, new functions for SUMO have begun to emerge. SUMO is covalently attached to components of each of the four major cytoskeletal networks, including microtubule-associated proteins, septins, and intermediate filaments, in addition to nuclear actin and actin-regulatory proteins. However, knowledge of the mechanisms by which this signal transduction system controls the cytoskeleton is still in its infancy. One story that is beginning to unfold is that SUMO may regulate the microtubule motor protein dynein by modification of its adaptor Lis1. In other instances, cytoskeletal elements can both bind to SUMO non-covalently and also be conjugated by it. The molecular mechanisms for many of these new functions are not yet clear, but are under active investigation. One emerging model links the function of MAP sumoylation to protein degradation through SUMO-targeted ubiquitin ligases, also known as STUbL enzymes. Other possible functions for cytoskeletal sumoylation are also discussed.

Actin-interacting protein 1 controls assembly and permeability of intestinal epithelial apical junctions

Adherens junctions (AJs) and tight junctions (TJs) are crucial regulators of the integrity and restitution of the intestinal epithelial barrier. The structure and function of epithelial junctions depend on their association with the cortical actin cytoskeleton that, in polarized epithelial cells, is represented by a prominent perijunctional actomyosin belt. The assembly and stability of the perijunctional cytoskeleton is controlled by constant turnover (disassembly and reassembly) of actin filaments. Actin-interacting protein (Aip) 1 is an emerging regulator of the actin cytoskeleton, playing a critical role in filament disassembly. In this study, we examined the roles of Aip1 in regulating the structure and remodeling of AJs and TJs in human intestinal epithelium. Aip1 was enriched at apical junctions in polarized human intestinal epithelial cells and normal mouse colonic mucosa. Knockdown of Aip1 by RNA interference increased the paracellular permeability of epithelial cell monolayers, decreased recruitment of AJ/TJ proteins to steady-state intercellular contacts, and attenuated junctional reassembly in a calcium-switch model. The observed defects of AJ/TJ structure and functions were accompanied by abnormal organization and dynamics of the perijunctional F-actin cytoskeleton. Moreover, loss of Aip1 impaired the apico-basal polarity of intestinal epithelial cell monolayers and inhibited formation of polarized epithelial cysts in 3-D Matrigel. Our findings demonstrate a previously unanticipated role of Aip1 in regulating the structure and remodeling of intestinal epithelial junctions and early steps of epithelial morphogenesis.

Novel roles for actin in mitochondrial fission

Mitochondrial dynamics, including fusion, fission and translocation, are crucial to cellular homeostasis, with roles in cellular polarity, stress response and apoptosis. Mitochondrial fission has received particular attention, owing to links with several neurodegenerative diseases. A central player in fission is the cytoplasmic dynamin-related GTPase Drp1, which oligomerizes at the fission site and hydrolyzes GTP to drive membrane ingression. Drp1 recruitment to the outer mitochondrial membrane (OMM) is a key regulatory event, which appears to require a pre-constriction step in which the endoplasmic reticulum (ER) and mitochondrion interact extensively, a process termed ERMD (ER-associated mitochondrial division). It is unclear how ER-mitochondrial contact generates the force required for pre-constriction or why pre-constriction leads to Drp1 recruitment. Recent results, however, show that ERMD might be an actin-based process in mammals that requires the ER-associated formin INF2 upstream of Drp1, and that myosin II and other actin-binding proteins might be involved. In this Commentary, we present a mechanistic model for mitochondrial fission in which actin and myosin contribute in two ways; firstly, by supplying the force for pre-constriction and secondly, by serving as a coincidence detector for Drp1 binding. In addition, we discuss the possibility that multiple fission mechanisms exist in mammals.

Capping protein regulators fine-tune actin assembly dynamics

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament. In addition, a diverse and unrelated group of proteins interact with CP through a conserved 'capping protein interaction' (CPI) motif. These proteins, including CARMIL (capping protein, ARP2/3 and myosin I linker), CD2AP (CD2-associated protein) and the WASH (WASP and SCAR homologue) complex subunit FAM21, recruit CP to specific subcellular locations and modulate its actin-capping activity via allosteric effects.

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.

Visualization of uncorrelated, tandem symmetry mismatches in the internal genome packaging apparatus of bacteriophage T7

Motor-driven packaging of a dsDNA genome into a preformed protein capsid through a unique portal vertex is essential in the life cycle of a large number of dsDNA viruses. We have used single-particle electron cryomicroscopy to study the multilayer structure of the portal vertex of the bacteriophage T7 procapsid, the recipient of T7 DNA in packaging. A focused asymmetric reconstruction method was developed and applied to selectively resolve neighboring pairs of symmetry-mismatched layers of the portal vertex. However, structural features in all layers of the multilayer portal vertex could not be resolved simultaneously. Our results imply that layers with mismatched symmetries can join together in several different relative orientations, and that orientations at different interfaces assort independently to produce structural isomers, a process that we call combinatorial assembly isomerism. This isomerism explains rotational smearing in previously reported asymmetric reconstructions of the portal vertex of T7 and other bacteriophages. Combinatorial assembly isomerism may represent a new regime of structural biology in which globally varying structures assemble from a common set of components. Our reconstructions collectively validate previously proposed symmetries, compositions, and sequential order of T7 portal vertex layers, resolving in tandem the 5-fold gene product 10 (gp10) shell, 12-fold gp8 portal ring, and an internal core stack consisting of 12-fold gp14 adaptor ring, 8-fold bowl-shaped gp15, and 4-fold gp16 tip. We also found a small tilt of the core stack relative to the icosahedral fivefold axis and propose that this tilt assists DNA spooling without tangling during packaging.

Distributed actin turnover in the lamellipodium and FRAP kinetics

Studies of actin dynamics at the leading edge of motile cells with single-molecule speckle (SiMS) microscopy have shown a broad distribution of EGFP-actin speckle lifetimes and indicated actin polymerization and depolymerization over an extended region. Other experiments using FRAP with the same EGFP-actin as a probe have suggested, by contrast, that polymerization occurs exclusively at the leading edge. We performed FRAP experiments on XTC cells to compare SiMS to FRAP on the same cell type. We used speckle statistics obtained by SiMS to model the steady-state distribution and kinetics of actin in the lamellipodium. We demonstrate that a model with a single diffuse actin species is in good agreement with FRAP experiments. A model including two species of diffuse actin provides an even better agreement. The second species consists of slowly diffusing oligomers that associate to the F-actin network throughout the lamellipodium or break up into monomers after a characteristic time. Our work motivates studies to test the presence and composition of slowly diffusing actin species that may contribute to local remodeling of the actin network and increase the amount of soluble actin.

Controlling the cortical actin motor

Actin is the essential force-generating component of the microfilament system, which powers numerous motile processes in eukaryotic cells and undergoes dynamic remodeling in response to different internal and external signaling. The ability of actin to polymerize into asymmetric filaments is the inherent property behind the site-directed force-generating capacity that operates during various intracellular movements and in surface protrusions. Not surprisingly, a broad variety of signaling pathways and components are involved in controlling and coordinating the activities of the actin microfilament system in a myriad of different interactions. The characterization of these processes has stimulated cell biologists for decades and has, as a consequence, resulted in a huge body of data. The purpose here is to present a cellular perspective on recent advances in our understanding of the microfilament system with respect to actin polymerization, filament structure and specific folding requirements.

UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph polarize F-actin during embryonic morphogenesis by regulating the WAVE/SCAR actin nucleation complex

Many cells in a developing embryo, including neurons and their axons and growth cones, must integrate multiple guidance cues to undergo directed growth and migration. The UNC-6/netrin, SLT-1/slit, and VAB-2/Ephrin guidance cues, and their receptors, UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph, are known to be major regulators of cellular growth and migration. One important area of research is identifying the molecules that interpret this guidance information downstream of the guidance receptors to reorganize the actin cytoskeleton. However, how guidance cues regulate the actin cytoskeleton is not well understood. We report here that UNC-40/DCC, SAX-3/Robo, and VAB-1/Eph differentially regulate the abundance and subcellular localization of the WAVE/SCAR actin nucleation complex and its activator, Rac1/CED-10, in the Caenorhabditis elegans embryonic epidermis. Loss of any of these three pathways results in embryos that fail embryonic morphogenesis. Similar defects in epidermal enclosure have been observed when CED-10/Rac1 or the WAVE/SCAR actin nucleation complex are missing during embryonic development in C. elegans. Genetic and molecular experiments demonstrate that in fact, these three axonal guidance proteins differentially regulate the levels and membrane enrichment of the WAVE/SCAR complex and its activator, Rac1/CED-10, in the epidermis. Live imaging of filamentous actin (F-actin) in embryos developing in the absence of individual guidance receptors shows that high levels of F-actin are not essential for polarized cell migrations, but that properly polarized distribution of F-actin is essential. These results suggest that proper membrane recruitment and activation of CED-10/Rac1 and of WAVE/SCAR by signals at the plasma membrane result in polarized F-actin that permits directed movements and suggest how multiple guidance cues can result in distinct changes in actin nucleation during morphogenesis.

Actin stress fibers--assembly, dynamics and biological roles

Actin filaments assemble into diverse protrusive and contractile structures to provide force for a number of vital cellular processes. Stress fibers are contractile actomyosin bundles found in many cultured non-muscle cells, where they have a central role in cell adhesion and morphogenesis. Focal-adhesion-anchored stress fibers also have an important role in mechanotransduction. In animal tissues, stress fibers are especially abundant in endothelial cells, myofibroblasts and epithelial cells. Importantly, recent live-cell imaging studies have provided new information regarding the mechanisms of stress fiber assembly and how their contractility is regulated in cells. In addition, these studies might elucidate the general mechanisms by which contractile actomyosin arrays, including muscle cell myofibrils and cytokinetic contractile ring, can be generated in cells. In this Commentary, we discuss recent findings concerning the physiological roles of stress fibers and the mechanism by which these structures are generated in cells.