The actin cytoskeleton

Nanozyme-Engineered Hydrogels for Anti-Inflammation and Skin Regeneration

Inflammatory skin disorders can cause chronic scarring and functional impairments, posing a significant burden on patients and the healthcare system. Conventional therapies, such as corticosteroids and nonsteroidal anti-inflammatory drugs, are limited in efficacy and associated with adverse effects. Recently, nanozyme (NZ)-based hydrogels have shown great promise in addressing these challenges. NZ-based hydrogels possess unique therapeutic abilities by combining the therapeutic benefits of redox nanomaterials with enzymatic activity and the water-retaining capacity of hydrogels. The multifaceted therapeutic effects of these hydrogels include scavenging reactive oxygen species and other inflammatory mediators modulating immune responses toward a pro-regenerative environment and enhancing regenerative potential by triggering cell migration and differentiation. This review highlights the current state of the art in NZ-engineered hydrogels (NZ@hydrogels) for anti-inflammatory and skin regeneration applications. It also discusses the underlying chemo-mechano-biological mechanisms behind their effectiveness. Additionally, the challenges and future directions in this ground, particularly their clinical translation, are addressed. The insights provided in this review can aid in the design and engineering of novel NZ-based hydrogels, offering new possibilities for targeted and personalized skin-care therapies.

Cytochalasans and Their Impact on Actin Filament Remodeling

The eukaryotic actin cytoskeleton comprises the protein itself in its monomeric and filamentous forms, G- and F-actin, as well as multiple interaction partners (actin-binding proteins, ABPs). This gives rise to a temporally and spatially controlled, dynamic network, eliciting a plethora of motility-associated processes. To interfere with the complex inter- and intracellular interactions the actin cytoskeleton confers, small molecular inhibitors have been used, foremost of all to study the relevance of actin filaments and their turnover for various cellular processes. The most prominent inhibitors act by, e.g., sequestering monomers or by interfering with the polymerization of new filaments and the elongation of existing filaments. Among these inhibitors used as tool compounds are the cytochalasans, fungal secondary metabolites known for decades and exploited for their F-actin polymerization inhibitory capabilities. In spite of their application as tool compounds for decades, comprehensive data are lacking that explain (i) how the structural deviances of the more than 400 cytochalasans described to date influence their bioactivity mechanistically and (ii) how the intricate network of ABPs reacts (or adapts) to cytochalasan binding. This review thus aims to summarize the information available concerning the structural features of cytochalasans and their influence on the described activities on cell morphology and actin cytoskeleton organization in eukaryotic cells.

Protrusion growth driven by myosin-generated force

Actin-based protrusions extend from the surface of all eukaryotic cells, where they support diverse activities essential for life. Models of protrusion growth hypothesize that actin filament assembly exerts force for pushing the plasma membrane outward. However, membrane-associated myosin motors are also abundant in protrusions, although their potential for contributing, growth-promoting force remains unexplored. Using an inducible system that docks myosin motor domains to membrane-binding modules with temporal control, we found that application of myosin-generated force to the membrane is sufficient for driving robust protrusion elongation in human, mouse, and pig cell culture models. Protrusion growth scaled with motor accumulation, required barbed-end-directed force, and was independent of cargo delivery or recruitment of canonical elongation factors. Application of growth-promoting force was also supported by structurally distinct myosin motors and membrane-binding modules. Thus, myosin-generated force can drive protrusion growth, and this mechanism is likely active in diverse biological contexts.

The Taspase1/Myosin1f-axis regulates filopodia dynamics

The unique threonine protease Tasp1 impacts not only ordered development and cell proliferation but also pathologies. However, its substrates and the underlying molecular mechanisms remain poorly understood. We demonstrate that the unconventional Myo1f is a Tasp1 substrate and unravel the physiological relevance of this proteolysis. We classify Myo1f as a nucleo-cytoplasmic shuttle protein, allowing its unhindered processing by nuclear Tasp1 and an association with chromatin. Moreover, we show that Myo1f induces filopodia resulting in increased cellular adhesion and migration. Importantly, filopodia formation was antagonized by Tasp1-mediated proteolysis, supported by an inverse correlation between Myo1f concentration and Tasp1 expression level. The Tasp1/Myo1f-axis might be relevant in human hematopoiesis as reduced Tasp1 expression coincided with increased Myo1f concentrations and filopodia in macrophages compared to monocytes and vice versa. In sum, we discovered Tasp1-mediated proteolysis of Myo1f as a mechanism to fine-tune filopodia formation, inter alia relevant for cells of the immune system.

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Myosin1f is a nucleo-cytoplasmic shuttle protein temporarily located in the nucleus

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Myosin1f induces filopodia resulting in increased cellular adhesion and migration

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The protease Taspase1 cleaves Myosin1f, thereby impairing its function

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Taspase1 and Myosin1f inversely correlate in immune cell differentiation

Confinement Geometry Tunes Fascin-Actin Bundle Structures and Consequently the Shape of a Lipid Bilayer Vesicle

Depending on the physical and biochemical properties of actin-binding proteins, actin networks form different types of membrane protrusions at the cell periphery. Actin crosslinkers, which facilitate the interaction of actin filaments with one another, are pivotal in determining the mechanical properties and protrusive behavior of actin networks. Short crosslinkers such as fascin bundle F-actin to form rigid spiky filopodial protrusions. By encapsulation of fascin and actin in giant unilamellar vesicles (GUVs), we show that fascin-actin bundles cause various GUV shape changes by forming bundle networks or straight single bundles depending on GUV size and fascin concentration. We also show that the presence of a long crosslinker, α-actinin, impacts fascin-induced GUV shape changes and significantly impairs the formation of filopodia-like protrusions. Actin bundle-induced GUV shape changes are confirmed by light-induced disassembly of actin bundles leading to the reversal of GUV shape. Our study contributes to advancing the design of shape-changing minimal cells for better characterization of the interaction between lipid bilayer membranes and actin cytoskeleton.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Loss of Ena/VASP interferes with lamellipodium architecture, motility and integrin-dependent adhesion

Cell migration entails networks and bundles of actin filaments termed lamellipodia and microspikes or filopodia, respectively, as well as focal adhesions, all of which recruit Ena/VASP family members hitherto thought to antagonize efficient cell motility. However, we find these proteins to act as positive regulators of migration in different murine cell lines. CRISPR/Cas9-mediated loss of Ena/VASP proteins reduced lamellipodial actin assembly and perturbed lamellipodial architecture, as evidenced by changed network geometry as well as reduction of filament length and number that was accompanied by abnormal Arp2/3 complex and heterodimeric capping protein accumulation. Loss of Ena/VASP function also abolished the formation of microspikes normally embedded in lamellipodia, but not of filopodia capable of emanating without lamellipodia. Ena/VASP-deficiency also impaired integrin-mediated adhesion accompanied by reduced traction forces exerted through these structures. Our data thus uncover novel Ena/VASP functions of these actin polymerases that are fully consistent with their promotion of cell migration.

Extracellular Matrix in Regulation of Contractile System in Cardiomyocytes

The contractile apparatus of cardiomyocytes is considered to be a stable system. However, it undergoes strong rearrangements during heart development as cells progress from their non-muscle precursors. Long-term culturing of mature cardiomyocytes is also accompanied by the reorganization of their contractile apparatus with the conversion of typical myofibrils into structures of non-muscle type. Processes of heart development as well as cell adaptation to culture conditions in cardiomyocytes both involve extracellular matrix changes, which appear to be crucial for the maturation of contractile apparatus. The aim of this review is to analyze the role of extracellular matrix in the regulation of contractile system dynamics in cardiomyocytes. Here, the remodeling of actin contractile structures and the expression of actin isoforms in cardiomyocytes during differentiation and adaptation to the culture system are described along with the extracellular matrix alterations. The data supporting the regulation of actin dynamics by extracellular matrix are highlighted and the possible mechanisms of such regulation are discussed.

Interrogating Synaptic Architecture: Approaches for Labeling Organelles and Cytoskeleton Components

Synaptic transmission has been studied for decades, as a fundamental step in brain function. The structure of the synapse, and its changes during activity, turned out to be key aspects not only in the transfer of information between neurons, but also in cognitive processes such as learning and memory. The overall synaptic morphology has traditionally been studied by electron microscopy, which enables the visualization of synaptic structure in great detail. The changes in the organization of easily identified structures, such as the presynaptic active zone, or the postsynaptic density, are optimally studied via electron microscopy. However, few reliable methods are available for labeling individual organelles or protein complexes in electron microscopy. For such targets one typically relies either on combination of electron and fluorescence microscopy, or on super-resolution fluorescence microscopy. This review focuses on approaches and techniques used to specifically reveal synaptic organelles and protein complexes, such as cytoskeletal assemblies. We place the strongest emphasis on methods detecting the targets of interest by affinity binding, and we discuss the advantages and limitations of each method.

Impacts of the combined exposure to seawater acidification and arsenic on the proteome of Crassostrea angulata and Crassostrea gigas

Proteomic analysis was performed to compare the effects of Arsenic (As), seawater acidification (Low pH) and the combination of both stressors (Low pH + As) on Crassostrea angulata and Crassostrea gigas juveniles in the context of global environmental change. This study aimed to elucidate if two closely related Crassostrea species respond similarly to these environmental stressors, considering both single and combined exposures, to infer if the simultaneous exposure to both stressors induced a differentiated response. Identification of the most important differentially expressed proteins between conditions revealed marked differences in the response of each species towards single and combined exposures, evidencing species-related differences towards each experimental condition. Moreover, protein alterations observed in the combined exposure (Low pH + As) were substantially different from those observed in single exposures. Identified proteins and their putative biological functions revealed an array of modes of action in each condition. Among the most important, those involved in cellular structure (Actin, Atlastin, Severin, Gelsolin, Coronin) and extracellular matrix modulation (Ependymin, Tight junction ZO-1, Neprilysin) were strongly regulated, although in different exposure conditions and species. Data also revealed differences regarding metabolic modulation capacity (ATP β, Enolase, Aconitate hydratase) and oxidative stress response (Aldehyde dehydrogenase, Lactoylglutathione, Retinal dehydrogenase) of each species, which also depended on single or combined exposures, illustrating a different response capacity of both oyster species to the presence of multiple stressors. Interestingly, alterations of piRNA abundance in C. angulata suggested genome reconfiguration in response to multiple stressors, likely an important mode of action related to adaptive evolution mechanisms previously unknown to oyster species, which requires further investigation. The present findings provide a deeper insight into the complexity of C. angulata and C. gigas responses to environmental stress at the proteome level, evidencing different capacities to endure abiotic changes, with relevance regarding the ecophysiological fitness of each species and competitive advantages in a changing environment.

Leptin affects filopodia and cofilin in NK-92 cells in a dose- and time-dependent manner

Hyperleptinemia, associated with obesity, is related with immune dysfunction and carcinogenesis. Natural Killer (NK) cells, a major component of the innate immune system are mediators of anti-tumor immunity and the most actively migrating cells among leukocytes. Actin rearrangement, promoted by cofilin plays a central role in cellular migration. Leptin affects the phosphorylation-dependent activity of cofilin and thus actin remodeling. We used human NK-92 cells to explore the in vitro effects of leptin on co-localization of cofilin and F-actin and on morphological changes in NK cells. NK-92 cells were incubated with different leptin concentrations (10 and 100 ng/mL) for 30 min and 24 h and immunocytochemically stained. Results demonstrate a dose- and time-dependent influence of leptin on cellular morphology. Utilizing confocal microscopy, we observed that the co-localization of cofilin-1 and F-actin was slightly influenced by leptin. In summary, the present study demonstrates an impact of a physiological leptin stimulation on the filopodia length, and a time-dependent effect on the co-localization of cofilin and F-actin in NK-92 cells.

Melatonin Represses Metastasis in Her2-Postive Human Breast Cancer Cells by Suppressing RSK2 Expression

The importance of the circadian/melatonin signal in suppressing the metastatic progression of breast and other cancers has been reported by numerous laboratories including our own. Currently, the mechanisms underlying the antimetastatic actions of melatonin have not been well established. In the present study, the antimetastatic actions of melatonin were evaluated and compared on the ERα-negative, Her2-positive SKBR-3 breast tumor cell line and ERα-positive MCF-7 cells overexpressing a constitutively active HER2.1 construct (MCF-7Her2.1 cells). Activation of Her2 is reported to induce the expression and/or phosphorylation-dependent activation of numerous kinases and transcription factors that drive drug resistance and metastasis in breast cancer. A key signaling node activated by the Her2/Mapk/Erk pathway is Rsk2, which has been shown to induce numerous signaling pathways associated with the development of epithelial-to-mesenchymal transition (EMT) and metastasis including: Creb, Stat3, cSrc, Fak, Pax, Fascin, and actin polymerization. The data demonstrate that melatonin (both endogenous and exogenous) significantly represses this invasive/metastatic phenotype through a mechanism that involves the suppression of EMT, either by promoting mesenchymal-to-epithelial transition, and/or by inhibiting key signaling pathways involved in later stages of metastasis. These data, combined with our earlier in vitro studies, support the concept that maintenance of elevated and extended duration of nocturnal melatonin levels plays a critical role in repressing the metastatic progression of breast cancer.

IMPLICATIONS

Melatonin inhibition of Rsk2 represses the metastatic phenotype in breast cancer cells suppressing EMT or inhibiting other mechanisms that promote metastasis; disruption of the melatonin signal may promote metastatic progression in breast cancer. Mol Cancer Res; 14(11); 1159-69. ©2016 AACR.

A mechanism of leading-edge protrusion in the absence of Arp2/3 complex

Cells employ protrusive leading edges to navigate and promote their migration in diverse physiological environments. Classical models of leading-edge protrusion rely on a treadmilling dendritic actin network that undergoes continuous assembly nucleated by the Arp2/3 complex, forming ruffling lamellipodia. Recent work demonstrated, however, that, in the absence of the Arp2/3 complex, fibroblast cells adopt a leading edge with filopodia-like protrusions (FLPs) and maintain an ability to move, albeit with altered responses to different environmental signals. We show that formin-family actin nucleators are required for the extension of FLPs but are insufficient to produce a continuous leading edge in fibroblasts lacking Arp2/3 complex. Myosin II is concentrated in arc-like regions of the leading edge in between FLPs, and its activity is required for coordinated advancement of these regions with formin-generated FLPs. We propose that actomyosin contraction acting against membrane tension advances the web of arcs between FLPs. Predictions of this model are verified experimentally. The dependence of myosin II in leading-edge advancement helps explain the previously reported defect in directional movement in the Arpc3-null fibroblasts. We provide further evidence that this defect is cell autonomous during chemotaxis.

Denbinobin, a Phenanthrene from Dendrobium nobile, Impairs Prostate Cancer Migration by Inhibiting Rac1 Activity

Prostate cancer is the most prevalent type of cancer in the United States. The most common site of prostate cancer metastasis is bone. CXCL12 is preferentially expressed in bone and is targeted by prostate cancer cells, which over-express the receptor for CXCL12, CXCR4. In response to CXCL12 stimulation, Rac1, a GTPase, along with its effectors, regulates actin polymerization to form lamellipodia, which is a critical event for cell migration. Cortactin, an actin-binding protein, is recruited to the lamellipodia and is phosphorylated at tyrosine residues. The phosphorylated cortactin is also involved in cell migration. The inhibition of Rac1 activity using a dominant negative Rac1 impairs lamellipodial protrusion as well as cortactin translocation and cortactin phosphorylation. Denbinobin, a substance extracted from Dendrobium nobile, has anticancer effects in many cancer cell lines. Whether denbinobin can inhibit prostate cancer cell migration is not clear. Here, we report that denbinobin inhibited Rac1 activity. The inhibition of Rac1 activity prevented lamellipodial formation. Cortactin phosphorylation and translocation to the lamellipodia were also impaired, and PC3 cells were unable to migrate. These results indicate that denbinobin prevents CXCL12-induced PC3 cell migration by inhibiting Rac1 activity.

The prometastatic ribosomal S6 kinase 2-cAMP response element-binding protein (RSK2-CREB) signaling pathway up-regulates the actin-binding protein fascin-1 to promote tumor metastasis

Metastasis is the leading cause of death in patients with breast, lung, and head and neck cancers. However, the molecular mechanisms underlying metastases in these cancers remain unclear. We found that the p90 ribosomal S6 kinase 2 (RSK2)-cAMP response element-binding protein (CREB) pathway is commonly activated in diverse metastatic human cancer cells, leading to up-regulation of a CREB transcription target Fascin-1. We also observed that the protein expression patterns of RSK2 and Fascin-1 correlate in primary human tumor tissue samples from head and neck squamous cell carcinoma patients. Moreover, knockdown of RSK2 disrupts filopodia formation and bundling in highly invasive cancer cells, leading to attenuated cancer cell invasion in vitro and tumor metastasis in vivo, whereas expression of Fascin-1 significantly rescues these phenotypes. Furthermore, targeting RSK2 with the small molecule RSK inhibitor FMK-MEA effectively attenuated the invasive and metastatic potential of cancer cells in vitro and in vivo, respectively. Taken together, our findings for the first time link RSK2-CREB signaling to filopodia formation and bundling through the up-regulation of Fascin-1, providing a proinvasive and prometastatic advantage to human cancers. Therefore, protein effectors of the RSK2-CREB-Fascin-1 pathway represent promising biomarkers and therapeutic targets in the clinical prognosis and treatment of metastatic human cancers.

The formin Daam1 and fascin directly collaborate to promote filopodia formation

Filopodia are slender cellular protrusions that dynamically extend and retract to facilitate directional cell migration, pathogen sensing, and cell-cell adhesion. Each filopodium contains a rigid and organized bundle of parallel actin filaments, which are elongated at filopodial tips by formins and Ena/VASP proteins. However, relatively little is known about how the actin filaments in the filopodial shaft are spatially organized to form a bundle with appropriate dimensions and mechanical properties. Here, we report that the mammalian formin Daam1 (Disheveled-associated activator of morphogenesis 1) is a potent actin-bundling protein and localizes all along the filopodial shaft, which differs from other formins that localize specifically to the tips. Silencing of Daam1 led to severe defects in filopodial number, integrity, and architecture, similar to silencing of the bundling protein fascin. This led us to investigate the potential relationship between Daam1 and fascin. Fascin and Daam1 coimmunoprecipitated from cell extracts, and silencing of fascin led to a striking loss of Daam1 localization to filopodial shafts, but not tips. Furthermore, purified fascin bound directly to Daam1, and multicolor single-molecule TIRF imaging revealed that fascin recruited Daam1 to and stabilized Daam1 on actin bundles in vitro. Our results reveal an unanticipated and direct collaboration between Daam1 and fascin in bundling actin, which is required for proper filopodial formation.

A high resolution view of the fly actin cytoskeleton lacking a functional WAVE complex

The development of multicellular organisms involves a series of morphogenetic processes coordinating a highly dynamic and organized interplay between cells and their environment. Thus, the generation of forces that drive cellular and intracellular movements is prerequisite to shape single cells into tissues and organs. The actin cytoskeleton represents a highly dynamic filamentous system providing cell structure and mechanical forces to drive membrane protrusion, cell migration and vesicle trafficking. Here, we apply the structured-illumination microscopy (SIM) technique to analyse the actin cytoskeleton in fixed Drosophila Schneider (S2R+) cells, both in wild type and in cells depleted for WAVE, a major activator of Arp2/3 mediated actin polymerization. In addition, we demonstrate that live cell SIM imaging also allows the visualization of actin-driven lamellipodial membrane dynamics at high spatial resolution in S2R+ cells. Three dimensional (3D) SIM images of up to 70 μm thick Drosophila wild-type and abi-mutant egg chambers further enables us to resolve changes of actin structures in a multicellular context with an impressive lateral and axial resolution, which is not possible with conventional confocal microscopy. Thus, the combination of superresolution 3D microscopy with Drosophila genetics and cell biology allows detailed insights into the structural and molecular requirements of different actin-dependent processes.

The cytoskeletal mechanisms of cell-cell junction formation in endothelial cells

Cell–cell contact is initiated by lamellipodia, followed by filopodia-like structure formation. Filopodia-like bridges maintain cell–cell contact through adherens junctions. Although bridges are structurally similar to filopodia, they are formed via a unique mechanism. Myosin II activity is important for bridge formation and cadherin accumulation.

The actin cytoskeleton and associated proteins play a vital role in cell–cell adhesion. However, the procedure by which cells establish adherens junctions remains unclear. We investigated the dynamics of cell–cell junction formation and the corresponding architecture of the underlying cytoskeleton in cultured human umbilical vein endothelial cells. We show that the initial interaction between cells is mediated by protruding lamellipodia. On their retraction, cells maintain contact through thin bridges formed by filopodia-like protrusions connected by VE-cadherin–rich junctions. Bridges share multiple features with conventional filopodia, such as an internal actin bundle associated with fascin along the length and vasodilator-stimulated phosphoprotein at the tip. It is striking that, unlike conventional filopodia, transformation of actin organization from the lamellipodial network to filopodial bundle during bridge formation occurs in a proximal-to-distal direction and is accompanied by recruitment of fascin in the same direction. Subsequently, bridge bundles recruit nonmuscle myosin II and mature into stress fibers. Myosin II activity is important for bridge formation and accumulation of VE-cadherin in nascent adherens junctions. Our data reveal a mechanism of cell–cell junction formation in endothelial cells using lamellipodia as the initial protrusive contact, subsequently transforming into filopodia-like bridges connected through adherens junctions. Moreover, a novel lamellipodia-to-filopodia transition is used in this context.

Dicing with dogma: de-branching the lamellipodium

The primary event in the movement of a migrating eukaryotic cell is the extension of cytoplasmic sheets termed lamellipodia composed of networks of actin filaments. Lamellipodia networks are thought to arise through the branching of new filaments from the sides of old filaments, producing a dendritic array. Recent studies by electron tomography have revealed the three dimensional organization of lamellipodia and show, contrary to previous evidence, that actin filaments do not form dendritic arrays in vivo. These findings signal a reconsideration of the structural basis of protrusion and about the roles of the different actin nucleating and elongating complexes involved in the process.

Mechanical integration of actin and adhesion dynamics in cell migration

Directed cell migration is a physical process that requires dramatic changes in cell shape and adhesion to the extracellular matrix. For efficient movement, these processes must be spatiotemporally coordinated. To a large degree, the morphological changes and physical forces that occur during migration are generated by a dynamic filamentous actin (F-actin) cytoskeleton. Adhesion is regulated by dynamic assemblies of structural and signaling proteins that couple the F-actin cytoskeleton to the extracellular matrix. Here, we review current knowledge of the dynamic organization of the F-actin cytoskeleton in cell migration and the regulation of focal adhesion assembly and disassembly with an emphasis on how mechanical and biochemical signaling between these two systems regulate the coordination of physical processes in cell migration.

Electron tomography reveals unbranched networks of actin filaments in lamellipodia

Eukaryotic cells can initiate movement using the forces exerted by polymerizing actin filaments to extend lamellipodial and filopodial protrusions. In the current model, actin filaments in lamellipodia are organized in a branched, dendritic network. We applied electron tomography to vitreously frozen 'live' cells, fixed cells and cytoskeletons, embedded in vitreous ice or in deep-negative stain. In lamellipodia from four cell types, including rapidly migrating fish keratocytes, we found that actin filaments are almost exclusively unbranched. The vast majority of apparent filament junctions proved to be overlapping filaments, rather than branched end-to-side junctions. Analysis of the tomograms revealed that actin filaments terminate at the membrane interface within a zone several hundred nanometres wide at the lamellipodium front, and yielded the first direct measurements of filament densities. Actin filament pairs were also identified as lamellipodium components and bundle precursors. These data provide a new structural basis for understanding actin-driven protrusion during cell migration.

The actin cytoskeleton in whole mount preparations and sections

In non-muscle cells, the actin cytoskeleton plays a key role by providing a scaffold contributing to the definition of cell shape, force for driving cell motility, cytokinesis, endocytosis, and propulsion of pathogens, as well as tracks for intracellular transport. A thorough understanding of these processes requires insight into the spatial and temporal organisation of actin filaments into diverse higher-order structures, such as networks, parallel bundles, and contractile arrays. Transmission and scanning electron microscopy can be used to visualise the actin cytoskeleton, but due to the delicate nature of actin filaments, they are easily affected by standard preparation protocols, yielding variable degrees of ultrastructural preservation. In this chapter, we describe different conventional and cryo-approaches to visualise the actin cytoskeleton using transmission electron microscopy and discuss their specific advantages and drawbacks. In the first part, we present three different whole mount techniques, which allow visualisation of actin in the peripheral, thinly spread parts of cells grown in monolayers. In the second part, we describe specific issues concerning the visualisation of actin in thin sections. Techniques for three-dimensional visualisation of actin, protein localisation, and correlative light and electron microscopy are also included.

Coordination of membrane and actin cytoskeleton dynamics during filopodia protrusion

Leading edge protrusion of migrating cells involves tightly coordinated changes in the plasma membrane and actin cytoskeleton. It remains unclear whether polymerizing actin filaments push and deform the membrane, or membrane deformation occurs independently and is subsequently stabilized by actin filaments. To address this question, we employed an ability of the membrane-binding I-BAR domain of IRSp53 to uncouple the membrane and actin dynamics and to induce filopodia in expressing cells. Using time-lapse imaging and electron microscopy of IRSp53-I-BAR-expressing B16F1 melanoma cells, we demonstrate that cells are not able to protrude or maintain durable long extensions without actin filaments in their interior, but I-BAR-dependent membrane deformation can create a small and transient space at filopodial tips that is subsequently filled with actin filaments. Moreover, the expressed I-BAR domain forms a submembranous coat that may structurally support these transient actin-free protrusions until they are further stabilized by the actin cytoskeleton. Actin filaments in the I-BAR-induced filopodia, in contrast to normal filopodia, do not have a uniform length, are less abundant, poorly bundled, and display erratic dynamics. Such unconventional structural organization and dynamics of actin in I-BAR-induced filopodia suggests that a typical bundle of parallel actin filaments is not necessary for generation and mechanical support of the highly asymmetric filopodial geometry. Together, our data suggest that actin filaments may not directly drive the protrusion, but only stabilize the space generated by the membrane deformation; yet, such stabilization is necessary for efficient protrusion.

Filopodia: Complex models for simple rods

Filopodia are prominent cell surface projections filled with bundles of linear actin filaments that drive their protrusion. These structures are considered important sensory organelles, for instance in neuronal growth cones or during the fusion of sheets of epithelial tissues. In addition, they can serve a precursor function in adhesion site or stress fibre formation. Actin filament assembly is essential for filopodia formation and turnover, yet the precise molecular mechanisms of filament nucleation and/or elongation are controversial. Indeed, conflicting reports on the molecular requirements of filopodia initiation have prompted researchers to propose different types and/or alternative or redundant mechanisms mediating this process. However, recent data shed new light on these questions, and they indicate that the balance of a limited set of biochemical activities can determine the structural outcome of a given filopodium. Here we focus on discussing our current view of the relevance of these activities, and attempt to propose a molecular mechanism of filopodia assembly based on a single core machinery.

Filopodia formation induced by active mDia2/Drf3

Filopodia are rod-shaped cell surface protrusions composed of a parallel bundle of actin filaments. Since filopodia frequently emanate from lamellipodia, it has been proposed that they form exclusively by the convergence and elongation of actin filaments generated in lamellipodia networks. However, filopodia form without Arp2/3-complex, which is essential for lamellipodia formation, indicating that actin filaments in filopodia may be generated by other nucleators. Here we analyzed the effects of ectopic expression of GFP-tagged full length or a constitutively active variant of the human formin mDia2/Drf3. By contrast to the full-length molecule, which did not affect cell behaviour and was entirely cytosolic, active Drf3 lacking the C-terminal regulatory region (Drf3DeltaDAD) induced the formation of filopodia and accumulated at their tips. Low expression of Drf3DeltaDAD induced rod-shaped or tapered filopodia, whereas over-expression resulted in multiple, club-shaped filopodia. The clubs were filled with densely bundled actin filaments, whose number but not packing density decreased further away from the tip. Interestingly, clubs frequently increased in width after protrusion beyond the cell periphery, which correlated with increased amounts of Drf3DeltaDAD at their tips. These data suggest Drf3-induced filopodia form and extend by de novo nucleation of actin filaments instead of convergent elongation. Finally, Drf3DeltaDAD also induced the formation of unusual, lamellipodia-like structures, which contained both lamellipodial markers and the prominent filopodial protein fascin. Microarray analyses revealed highly variable Drf3 expression levels in different commonly used cell lines, reflecting the need for more detailed analyses of the functions of distinct formins in actin cytoskeleton turnover and different cell types.

Mechanisms of actin stress fibre assembly

Stress fibres are contractile acto-myosin structures found from many types of non-muscle cells, where they are involved in adhesion, motility and morphogenesis. Stress fibres typically display a periodic alpha-actinin-myosin II pattern and are thus suggested to resemble the sarcomeric actin filament structures of muscle cells. Mammalian cells contain three categories of stress fibres: ventral stress fibres that are attached to focal adhesions at both ends, dorsal stress fibres that are attached to focal adhesions typically at one end and transverse arcs that are curved acto-myosin bundles, which do not directly attach to focal adhesions. In this review, we discuss the definition of stress fibres, organization of actin filaments and other components within these contractile structures, and the mechanisms of stress fibre assembly.

Unravelling the structure of the lamellipodium

Summary Pushing at the cell front is the business of lamellipodia and understanding how lamellipodia function requires knowledge of their structural organization. Analysis of extracted, critical-point-dried cells by electron microscopy has led to a current dogma that the lamellipodium pushes as a branched array of actin filaments, with a branching angle of 70 degrees , defined by the Arp2/3 complex. Comparison of different preparative methods indicates that the critical-point-drying-replica technique introduces distortions into actin networks, such that crossing filaments may appear branched. After negative staining and from preliminary studies by cryo-electron tomography, no clear evidence could be found for actin filament branching in lamellipodia. From recent observations of a sub-class of actin speckles in lamellipodia that exhibit a dynamic behaviour similar to speckles in the lamella region behind, it has been proposed that the lamellipodium surfs on top of the lamella. Negative stain electron microscopy and cryo-electron microscopy of fixed cells, which reveal the entire complement of filaments in lamellipodia show, however, that there is no separate, second array of filaments beneath the lamellipodium network. From present data, we conclude that the lamellipodium is a distinct protrusive entity composed of a network of primarily unbranched actin filaments. Cryo-electron tomography of snap-frozen intact cells will be required to finally clarify the three-dimensional arrangement of actin filaments in lamellipodia in vivo.

Cortactin branches out: roles in regulating protrusive actin dynamics

Since its discovery in the early 1990's, cortactin has emerged as a key signaling protein in many cellular processes, including cell adhesion, migration, endocytosis, and tumor invasion. While the list of cellular functions influenced by cortactin grows, the ability of cortactin to interact with and alter the cortical actin network is central to its role in regulating these processes. Recently, several advances have been made in our understanding of the interaction between actin and cortactin, providing insight into how these two proteins work together to provide a framework for normal and altered cellular function. This review examines how regulation of cortactin through post-translational modifications and interactions with multiple binding partners elicits changes in cortical actin cytoskeletal organization, impacting the regulation and formation of actin-rich motility structures.

On the Rho'd: the regulation of membrane protrusions by Rho-GTPases

Cell migration entails the formation of cellular protrusions such as lamellipodia or filopodia, the growth of which is powered by the polymerisation of actin filaments abutting the plasma membrane. Specific Rho-GTPase subfamilies are able to drive different types of protrusions. However, significant crosstalk between Rho-family members and the interplay of distinct Rho-effectors regulating or modulating actin reorganization in protrusions complicate the picture of how precisely they are initiated and maintained. Here, we briefly sketch our current knowledge on structure and dynamics of different protrusions as well as their regulation by Rho-GTPases. We also comment on topical, unresolved controversies in the field, with special emphasis on the interrelation of different protrusion types, and on the composition of the nanomachineries driving them.

Differentially oriented populations of actin filaments generated in lamellipodia collaborate in pushing and pausing at the cell front

Eukaryotic cells advance in phases of protrusion, pause and withdrawal. Protrusion occurs in lamellipodia, which are composed of diagonal networks of actin filaments, and withdrawal terminates with the formation of actin bundles parallel to the cell edge. Using correlated live-cell imaging and electron microscopy, we have shown that actin filaments in protruding lamellipodia subtend angles from 15-90 degrees to the front, and that transitions from protrusion to pause are associated with a proportional increase in filaments oriented more parallel to the cell edge. Microspike bundles of actin filaments also showed a wide angular distribution and correspondingly variable bilateral polymerization rates along the cell front. We propose that the angular shift of filaments in lamellipodia serves in adapting to slower protrusion rates while maintaining the filament densities required for structural support; further, we suggest that single filaments and microspike bundles contribute to the construction of the lamella behind and to the formation of the cell edge when protrusion ceases. Our findings provide an explanation for the variable turnover dynamics of actin filaments in lamellipodia observed by fluorescence speckle microscopy and are inconsistent with a current model of lamellipodia structure that features actin filaments branching at 70 degrees in a dendritic array.

Correlated light and electron microscopy of the cytoskeleton

The cytoskeleton of cultured cells can be most easily visualized in the electron microscope by simultaneous extraction and fixation with Triton-glutaraldehyde mixtures, followed by negative staining. Actin filaments are better preserved by stabilization with phalloidin, either during or after the primary fixation step. A technique is described for the combination of this procedure with live cell microscopy. Optimal conditions for light microscopy are achieved by culturing cells on coverslips coated with formvar film. For cell relocation a gold finder grid pattern is embossed on the film by evaporation through a tailor-made mask. After video microscopy and fixation, the film is floated from the coverslip and an electron microscope grid added to the film with the central hole of the grid over the region of interest. Accurate positioning is achieved under a dissecting microscope, using forceps mounted in a micromanipulator. Examples are shown of the changes in organization of actin filaments in the lamellipodia of migrating melanoma cells resulting from changes in protrusion rate. The technique is applicable to alternative processing procedures after fixation, including cryoelectron tomography.

Sequential roles for myosin-X in BMP6-dependent filopodial extension, migration, and activation of BMP receptors

Endothelial cell migration is an important step during angiogenesis, and its dysregulation contributes to aberrant neovascularization. The bone morphogenetic proteins (BMPs) are potent stimulators of cell migration and angiogenesis. Using microarray analyses, we find that myosin-X (Myo10) is a BMP target gene. In endothelial cells, BMP6-induced Myo10 localizes in filopodia, and BMP-dependent filopodial assembly decreases when Myo10 expression is reduced. Likewise, cellular alignment and directional migration induced by BMP6 are Myo10 dependent. Surprisingly, we find that Myo10 and BMP6 receptor ALK6 colocalize in a BMP6-dependent fashion. ALK6 translocates into filopodia after BMP6 stimulation, and both ALK6 and Myo10 possess intrafilopodial motility. Additionally, Myo10 is required for BMP6-dependent Smad activation, indicating that in addition to its function in filopodial assembly, Myo10 also participates in a requisite amplification loop for BMP signaling. Our data indicate that Myo10 is required to guide endothelial migration toward BMP6 gradients via the regulation of filopodial function and amplification of BMP signals.

Imaging of focal contacts of chicken heart muscle cells by high-frequency acoustic microscopy

A study of the adhesion of embryonic chicken heart muscle cells was conducted with a newly developed time-resolved acoustic microscope, which operates in the GHz-frequency range. The interpretation of the acoustical images of the heart muscle cells was done in combination with the fluorescence optical microscopy. A comparison between the acoustical images of chicken heart muscle cells and optical images of the same cells after staining showed that the actin fibers end inside the dark streaks in the acoustical images and thus represent the focal contacts (FCs). For cell biology applications, this demonstrates (a) the use of SAM as a tool for studying the dynamics of the FCs of living cells without any chemical staining and (b) that the combination of acoustic and optical microscopes allows interpretation of the acoustical images by using the wide variety of techniques available in optical microscopy.

Ena/VASP proteins have an anti-capping independent function in filopodia formation

Filopodia have been implicated in a number of diverse cellular processes including growth-cone path finding, wound healing, and metastasis. The Ena/VASP family of proteins has emerged as key to filopodia formation but the exact mechanism for how they function has yet to be fully elucidated. Using cell spreading as a model system in combination with small interfering RNA depletion of Capping Protein, we determined that Ena/VASP proteins have a role beyond anticapping activity in filopodia formation. Analysis of mutant Ena/VASP proteins demonstrated that the entire EVH2 domain was the minimal domain required for filopodia formation. Fluorescent recovery after photobleaching data indicate that Ena/VASP proteins rapidly exchange at the leading edge of lamellipodia, whereas virtually no exchange occurred at filopodial tips. Mutation of the G-actin-binding motif (GAB) partially compromised stabilization of Ena/VASP at filopodia tips. These observations led us to propose a model where the EVH2 domain of Ena/VASP induces and maintains clustering of the barbed ends of actin filaments, which putatively corresponds to a transition from lamellipodial to filopodial localization. Furthermore, the EVH1 domain, together with the GAB motif in the EVH2 domain, helps to maintain Ena/VASP at the growing barbed ends.

Role of fascin in filopodial protrusion

In this study, the mechanisms of actin-bundling in filopodia were examined. Analysis of cellular localization of known actin cross-linking proteins in mouse melanoma B16F1 cells revealed that fascin was specifically localized along the entire length of all filopodia, whereas other actin cross-linkers were not. RNA interference of fascin reduced the number of filopodia, and remaining filopodia had abnormal morphology with wavy and loosely bundled actin organization. Dephosphorylation of serine 39 likely determined cellular filopodia frequency. The constitutively active fascin mutant S39A increased the number and length of filopodia, whereas the inactive fascin mutant S39E reduced filopodia frequency. Fluorescence recovery after photobleaching of GFP-tagged wild-type and S39A fascin showed that dephosphorylated fascin underwent rapid cycles of association to and dissociation from actin filaments in filopodia, with t_1/2 < 10 s. We propose that fascin is a key specific actin cross-linker, providing stiffness for filopodial bundles, and that its dynamic behavior allows for efficient coordination between elongation and bundling of filopodial actin filaments.

Structure and locomotion of adult in vitro regenerated spiral ganglion growth cones – A study using video microscopy and SEM

Neuronal development and neurite regeneration depends on the locomotion and navigation of nerve growth cones (GCs). There are few detailed descriptions of the GC function and structure in the adult auditory system. In this study, GCs of adult dissociated and cultured spiral ganglion (SG) neurons were analyzed in vitro utilizing combined high resolution scanning electron microscopy (SEM) and time lapse video microscopy (TLVM). Axon kinesis was assessed on planar substratum with growth factors BDNF, NT-3 and GDNF. At the nano-scale level, lamellipodial abdomen of the expanding GC was found to be decorated with short surface specializations, which at TLVM were considered to be related to their crawling capacity. Filopodia were devoid of these surface structures, supporting its generally described sensory role. Microspikes appearing on lamellipodia and axons, showed circular adhesions, which at TLVM were found to provide anchorage of the navigating and turning axon. Neurons and GCs expressed the DCC-receptor for the guidance molecule netrin-1. Asymmetric ligand-based stimulation initiated turning responses suggest that this attractant cue influences steering of GC in adult regenerating auditory neurites. Hopefully, these findings may be used for ensuing tentative navigation of spiral ganglion neurons to induce regenerative processes in the human ear.

Filopodia formation in the absence of functional WAVE- and Arp2/3-complexes

Cell migration is initiated by plasma membrane protrusions, in the form of lamellipodia and filopodia. The latter rod-like projections may exert sensory functions and are found in organisms as distant in evolution as mammals and amoeba such as Dictyostelium discoideum. In mammals, lamellipodia protrusion downstream of the small GTPase Rac1 requires a multimeric protein assembly, the WAVE-complex, which activates Arp2/3-mediated actin filament nucleation and actin network assembly. A current model of filopodia formation postulates that these structures arise from a dendritic network of lamellipodial actin filaments by selective elongation and bundling. Here, we have analyzed filopodia formation in mammalian cells abrogated in expression of essential components of the lamellipodial actin polymerization machinery. Cells depleted of the WAVE-complex component Nck-associated protein 1 (Nap1), and, in consequence, of lamellipodia, exhibited normal filopodia protrusion. Likewise, the Arp2/3-complex, which is essential for lamellipodia protrusion, is dispensable for filopodia formation. Moreover, genetic disruption of nap1 or the WAVE-orthologue suppressor of cAMP receptor (scar) in Dictyostelium was also ineffective in preventing filopodia protrusion. These data suggest that the molecular mechanism of filopodia formation is conserved throughout evolution from Dictyostelium to mammals and show that lamellipodia and filopodia formation are functionally separable.

Mechanistic differences in actin bundling activity of two mammalian formins, FRL1 and mDia2

Formin proteins are regulators of actin dynamics, mediating assembly of unbranched actin filaments. These multidomain proteins are defined by the presence of a Formin Homology 2 (FH2) domain. Previous work has shown that FH2 domains bind to filament barbed ends and move processively at the barbed end as the filament elongates. Here we report that two FH2 domains, from mammalian FRL1 and mDia2, also bundle filaments, whereas the FH2 domain from mDia1 cannot under similar conditions. The FH2 domain alone is sufficient for bundling. Bundled filaments made by either FRL1 or mDia2 are in both parallel and anti-parallel orientations. A novel property that might contribute to bundling is the ability of the dimeric FH2 domains from both FRL1 and mDia2 to dissociate and recombine. This property is not observed for mDia1. A difference between FRL1 and mDia2 is that FRL1-mediated bundling is competitive with barbed end binding, whereas mDia2-mediated bundling is not. Mutation of a highly conserved isoleucine residue in the FH2 domain does not inhibit bundling by either FRL1 or mDia2, but inhibits barbed end activities. However, the severity of this mutation varies between formins. For mDia1 and mDia2, the mutation strongly inhibits all effects of barbed end binding, but affects FRL1 much less strongly. Furthermore, our results suggest that the Ile mutation affects processivity. Taken together, our data suggest that the bundling activities of FRL1 and mDia2, while producing phenotypically similar bundles, differ in mechanistic detail.

Fibroblast spreading induced by connective tissue stretch involves intracellular redistribution of alpha- and beta-actin

Mechanical stretching of connective tissue occurs with normal movement and postural changes, as well as treatments including physical therapy, massage and acupuncture. Connective tissue fibroblasts were recently shown to respond actively to short-term mechanical stretch (minutes to hours) with reversible cytoskeletal remodeling, characterized by extensive cell spreading and lamellipodia formation. In this study, we have examined the effect of tissue stretch on the distribution of alpha- and beta-actin in subcutaneous tissue fibroblasts ex vivo. Normal fibroblasts uniformly exhibited alpha-smooth muscle actin (alpha-SMA) immunoreactivity. Unlike cultured fibroblasts and smooth muscle cells, alpha-SMA in these fibroblasts was not in F-actin form (indicated by lack of phalloidin co-localization) nor was it organized into distinct stress fibers. The lack of stress fibers and fibronexus was confirmed by electron microscopy, indicating that these cells were not myofibroblasts. In unstretched tissue, the pattern of alpha-actin was diffuse and granular. With tissue stretch (30 min), alpha-actin formed a star-shaped pattern centered on the nucleus, while beta-actin extended throughout the cytoplasm including lamellipodia and cell cortex. This dual response pattern of alpha- and beta-actin may be an important component of cellular mechanotransduction mechanisms relevant to physiologic and therapeutic mechanical forces applied to connective tissue.

Mechanics and dynamics of actin-driven thin membrane protrusions

Motile cells explore their surrounding milieu by extending thin dynamic protrusions, or filopodia. The growth of filopodia is driven by actin filament bundles that polymerize underneath the cell membrane. We compute the mechanical and dynamical features of the protrusion growth process by explicitly incorporating the flexible plasma membrane. We find that a critical number of filaments are needed to generate net filopodial growth. Without external influences, the filopodium can extend indefinitely up to the buckling length of the F-actin bundle. Dynamical calculations show that the protrusion speed is enhanced by the thermal fluctuations of the membrane; a filament bundle encased in a flexible membrane grows much faster. The protrusion speed depends directly on the number and spatial arrangement of the filaments in the bundle and whether the filaments are tethered to the membrane. Filopodia also attract each other through distortions of the membrane. Spatially close filopodia will merge to form a larger one. Force-velocity relationships mimicking micromanipulation experiments testing our predictions are computed.

Computer-assisted analysis of filopod formation and the role of myosin II heavy chain phosphorylation in Dictyostelium

To investigate the role played by filopodia in the motility and chemotaxis of amoeboid cells, a computer-assisted 3D reconstruction and motion analysis system, DIAS 4.0, has been developed. Reconstruction at short time intervals of Dictyostelium amoebae migrating in buffer or in response to chemotactic signals, revealed that the great majority of filopodia form on pseudopodia, not on the cell body; that filopodia on the cell body originate primarily on pseudopodia and relocate; and that filopodia on the uropod are longer and more stable than those located on other portions of the cell. When adjusting direction through lateral pseudopod formation in a spatial gradient of chemoattractant, the temporal and spatial dynamics of lateral pseudopodia suggest that filopodia may be involved in stabilizing pseudopodia on the substratum while the decision is being made by a cell either to turn into a pseudopodium formed in the correct direction (up the gradient) or to retract a pseudopodium formed in the wrong direction (down the gradient). Experiments in which amoebae were treated with high concentrations of chemoattractant further revealed that receptor occupancy plays a role both in filopod formation and retraction. As phosphorylation-dephosphorylation of myosin II heavy chain (MHC) plays a role in lateral pseudopod formation, turning and chemotaxis, the temporal and spatial dynamics of filopod formation were analyzed in MHC phosphorylation mutants. These studies revealed that MHC phosphorylation-dephosphorylation plays a role in the regulation of filopod formation during cell migration in buffer and during chemotaxis. The computer-assisted technology described here for reconstructing filopodia at short time intervals in living cells, therefore provides a new tool for investigating the role filopodia play in the motility and chemotaxis of amoeboid cells.

Actin filament organization in aligned prefusion myoblasts

The organization of the actin cytoskeleton in prefusion aligning myoblasts is likely to be important for their shape and interaction. We investigated actin filament organization and polarity by transmission electron microscopy (TEM) in these cells. About 84% of the filaments counted were either found in a subplasmalemma sheet up to 0.5 microm thick that was aligned with the long axis of the cell, or in protrusions. The remaining filaments were found in the cytoplasm, where they were randomly orientated and not organized into bundles. The polarity of the subplasmalemma filaments changed progressively from one end of the cell to the other. At the ends of the cells and in protrusions, the majority of filaments were organized such that their barbed ends faced the tip of the protrusion. We did not find any actin filament bundles or stress fibres in these cells. Time-lapse phase microscopy demonstrated that aligned cells were still actively migrating at the time of our TEM observations, and their direction of movement was restricted to the long axis of the cell group. The ability of these cells to locomote actively in the absence of actin filament bundles suggests that in these cells the subplasmalemma actin sheet contributes not only to cell shape but also to cell locomotion.

S1P and LPA trigger Schwann cell actin changes and migration

The processes by which a Schwann cell (SC) migrates towards, wraps around and, in some cases, myelinates an axon are incompletely understood. The complex morphological rearrangements involved in these events require fundamental changes in the actin cytoskeleton. Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are two modulators of the actin cytoskeleton, and receptors for these signalling lipids are expressed on SCs at the time of differentiation. Previous work has revealed a role for LPA in SC survival, morphology and differentiation, but the effects of S1P have received less attention. Here we show that S1P and LPA both cause major rearrangements to the actin cytoskeleton in primary rat SCs and the SCL4.1/F7 rat SC line. S1P and LPA caused formation of lamellipodia and a circular geodesic actin network. We also show that S1P and LPA increased cell migration. The small GTPases RhoA and Rac1 were both activated by S1P/LPA treatment, but the actin rearrangements were dependent on Rac1 and not RhoA. These effects of S1P/LPA could be mimicked by SCL4.1/F7 cell-conditioned medium, which was found to contain S1P. Reduction in cellular synthesis of S1P by adding the sphingosine kinase inhibitor dimethyl sphingosine during medium conditioning reduced the ability of conditioned medium to cause actin rearrangements. These results support a role for S1P as an autocrine signal regulating the actin cytoskeleton during Schwann cell development.

Regulation of dendritic branching and filopodia formation in hippocampal neurons by specific acylated protein motifs

Although neuronal axons and dendrites with their associated filopodia and spines exhibit a profound cell polarity, the mechanism by which they develop is largely unknown. Here, we demonstrate that specific palmitoylated protein motifs, characterized by two adjacent cysteines and nearby basic residues, are sufficient to induce filopodial extensions in heterologous cells and to increase the number of filopodia and the branching of dendrites and axons in neurons. Such motifs are present at the N-terminus of GAP-43 and the C-terminus of paralemmin, two neuronal proteins implicated in cytoskeletal organization and filopodial outgrowth. Filopodia induction is blocked by mutations of the palmitoylated sites or by treatment with 2-bromopalmitate, an agent that inhibits protein palmitoylation. Moreover, overexpression of a constitutively active form of ARF6, a GTPase that regulates membrane cycling and dendritic branching reversed the effects of the acylated protein motifs. Filopodia induction by the specific palmitoylated motifs was also reduced upon overexpression of a dominant negative form of the GTPase cdc42. These results demonstrate that select dually lipidated protein motifs trigger changes in the development and growth of neuronal processes.

A new look at the cellular scaffold by embedment-free electron microscopy method

The basic scaffold of most cells is afforded by the cytoskeleton (comprising microfilaments, intermediate filaments and the microtubules). The conventional methods of electron microscopy fail to visualize filamentous cell structure. They can show only these filaments lying at the section surface. Heavy metal staining (I), and the optical properties of the resins used for embedding are similar to those of proteins hence most proteinaceous structures remain unresolved and the cytoplasm seems to be quite homogenous (II). Aldehyde fixation could cross-link proteins and lead to the emergence of artificial structures (III). These limitations may be overcome by the use of the embedment-free electron microscopy (EF-EM). This technique present cellular scaffold as a purified, isolated, three-dimensional network with various thickness of filaments. Our study on the dynamic aspect of cellular scaffold indicate that the thickness and arrangement of filaments depend on cell type and both physiological or pathological environments. Thank also to the adaptation of immunocytochemistry to EF-EM it was possible to understand the nuclear matrix and cytomatrix structure in relation to function. Thus, combination these methods revealed findings suggesting the nuclear homing of proapoptotic proteins and their association with intermediate filaments.

SWAP-70 identifies a transitional subset of actin filaments in motile cells

Functionally different subsets of actin filament arrays contribute to cellular organization and motility. We report the identification of a novel subset of loose actin filament arrays through regulated association with the widely expressed protein SWAP-70. These loose actin filament arrays were commonly located behind protruding lamellipodia and membrane ruffles. Visualization of these loose actin filament arrays was dependent on lamellipodial protrusion and the binding of the SWAP-70 PH-domain to a 3'-phosphoinositide. SWAP-70 with a functional pleckstrin homology-domain lacking the C-terminal 60 residues was targeted to the area of the loose actin filament arrays, but it did not associate with actin filaments. The C-terminal 60 residues were sufficient for actin filament association, but they provided no specificity for the subset of loose actin filament arrays. These results identify SWAP-70 as a phosphoinositide 3-kinase signaling-dependent marker for a distinct, hitherto unrecognized, array of actin filaments. Overexpression of SWAP-70 altered the actin organization and lamellipodial morphology. These alterations were dependent on a proper subcellular targeting of SWAP-70. We propose that SWAP-70 regulates the actin cytoskeleton as an effector or adaptor protein in response to agonist stimulated phosphatidylinositol (3,4)-bisphosphate production and cell protrusion.

Observation of keratin particles showing fast bidirectional movement colocalized with microtubules

Keratin intermediate filament networks were observed in living cultured epithelial cells using the incorporation of fluorescently tagged keratin from a transfected enhanced green fluorescent protein (EGFP) construct. In steady-state conditions EGFP-keratin exists not only as readily detectable intermediate filaments, but also as small particles, of which there are two types: a less mobile population (slow or static S particles) and a highly dynamic one (fast or F particles). The dynamic F particles move around the cell very fast and in a non-random way. Their movement is composed of a series of steps, giving an overall characteristic zig-zag trajectory. The keratin particles are found all over the cell and their movement is aligned with microtubules; treatment of cells with nocodazole has an inhibitory effect on keratin particle movement, suggesting the involvement of microtubule motor proteins. Double-transfection experiments to visualize tubulin and keratin together suggest that the movement of keratin particles can be bidirectional, as particles are seen moving both towards and away from the centrosome area. Using field emission scanning and transmission electron microscopy combined with immunogold labelling, we also detected particulate keratin structures in untransfected epithelial cells, suggesting that keratin particles may be a natural component of keratin filament dynamics in living cells.