Filopodia: molecular architecture and cellular functions

CRP1, a protein localized in filopodia of growth cones, is involved in dendritic growth

The cysteine-rich protein (CRP) family is a subgroup of LIM domain proteins. CRP1, which cross-links actin filaments to make actin bundles, is the only CRP family member expressed in the CNS with little known about its function in nerve cells. Here, we report that CRP1 colocalizes with actin in the filopodia of growth cones in cultured rat hippocampal neurons. Knockdown of CRP1 expression by short hairpin RNA interference results in inhibition of filopodia formation and dendritic growth in neurons. Overexpression of CRP1 increases filopodia formation and neurite branching, which require its actin-bundling activity. Expression of CRP1 with a constitutively active form of Cdc42, a GTPase involved in filopodia formation, increases filopodia formation in COS-7 cells, suggesting cooperation between the two proteins. Moreover, we demonstrate that neuronal activity upregulates CRP1 expression in hippocampal neurons via Ca²⁺ influx after depolarization. Ca²⁺/calmodulin-dependent protein kinase IV (CaMKIV) and cAMP response element binding protein mediate the Ca²⁺-induced upregulation of CRP1 expression. Furthermore, CRP1 is required for the dendritic growth induced by Ca²⁺ influx or CaMKIV. Together, these data are the first to demonstrate a role for CRP1 in dendritic growth.

Dystroglycan promotes filopodial formation and process branching in differentiating oligodendroglia

During central nervous system (CNS) development, individual oligodendrocytes myelinate multiple axons, thus requiring the outgrowth and extensive branching of oligodendroglial processes. Laminin (Lm)-deficient mice have a lower percentage of myelinated axons, which may indicate a defect in the ability to properly extend and branch processes. It remains unclear, however, to what extent extracellular matrix (ECM) receptors contribute to oligodendroglial process remodeling itself. In the current study, we report that the ECM receptor dystroglycan is necessary for Lm enhancement of filopodial formation, process outgrowth, and process branching in differentiating oligodendroglia. During early oligodendroglial differentiation, the disruption of dystroglycan-Lm interactions, via blocking antibodies or dystroglycan small interfering RNA (siRNA), resulted in decreased filopodial number and length, decreased process length, and decreased numbers of primary and secondary processes. Later in oligodendrocyte differentiation, dystroglycan-deficient cells developed fewer branches, thus producing less complex networks of processes as determined by Sholl analysis. In newly differentiating oligodendroglia, dystroglycan was localized in filopodial tips, whereas, in more mature oligodendrocytes, dystroglycan was enriched in focal adhesion kinase (FAK)-positive focal adhesion structures. These results suggest that dystroglycan-Lm interactions influence oligodendroglial process dynamics and therefore may regulate the myelination capacity of individual oligodendroglia.

Dictyostelium ACAP-A is an ArfGAP involved in cytokinesis, cell migration and actin cytoskeleton dynamics

ACAPs and ASAPs are Arf-GTPase-activating proteins with BAR, PH, GAP and ankyrin repeat domains and are known to regulate vesicular traffic and actin cytoskeleton dynamics in mammalian cells. The amoeba Dictyostelium has only two proteins with this domain organization instead of six in human, enabling a more precise functional analysis. Genetic invalidation of acapA, resulted in multinucleated cells with cytokinesis defects. Mutant acapA− cells were hardly motile and their multicellular development was significantly delayed. In addition, formation of filopodial protrusions was deficient in these cells. Conversely, re-expression of ACAP-A-GFP resulted in numerous and long filopodia-like protrusions. Mutagenesis studies showed that ACAP-A actin remodeling function was dependent on its ability to activate its substrate, the small GTPase ArfA. Likewise, the expression of a constitutively active ArfA•GTP mutant in wild-type cells led to a significant reduction of filopodia length. Together our data support a role for ACAP-A in the control of the actin cytoskeleton organization and dynamics through an ArfA-dependent mechanism.

Dyrk1A negatively regulates the actin cytoskeleton through threonine phosphorylation of N-WASP

Neural Wiskott-Aldrich syndrome protein (N-WASP) is involved in tight regulation of actin polymerization and dynamics. N-WASP activity is regulated by intramolecular interaction, binding to small GTPases and tyrosine phosphorylation. Here, we report on a novel regulatory mechanism; we demonstrate that N-WASP interacts with dual-specificity tyrosine-phosphorylation-regulated kinase 1A (Dyrk1A). In vitro kinase assays indicate that Dyrk1A directly phosphorylates the GTPase-binding domain (GBD) of N-WASP at three sites (Thr196, Thr202 and Thr259). Phosphorylation of the GBD by Dyrk1A promotes the intramolecular interaction of the GBD and verprolin, cofilin and acidic (VCA) domains of N-WASP, and subsequently inhibits Arp2/3-complex-mediated actin polymerization. Overexpression of either Dyrk1A or a phospho-mimetic N-WASP mutant inhibits filopodia formation in COS-7 cells. By contrast, the knockdown of Dyrk1A expression or overexpression of a phospho-deficient N-WASP mutant promotes filopodia formation. Furthermore, the overexpression of a phospho-mimetic N-WASP mutant significantly inhibits dendritic spine formation in primary hippocampal neurons. These findings suggest that Dyrk1A negatively regulates actin filament assembly by phosphorylating N-WASP, which ultimately promotes the intramolecular interaction of its GBD and VCA domains. These results provide insight on the mechanisms contributing to diverse actin-based cellular processes such as cell migration, endocytosis and neuronal differentiation.

Actin branching in the initiation and maintenance of lamellipodia

Using correlated live cell imaging and electron tomography we found that actin branch junctions in protruding and treadmilling lamellipodia are not concentrated at the front as previously supposed, but link actin filament subsets in which there is a continuum of distances from a junction to the filament plus ends, up to at least 1 µm. When branch sites were observed closely spaced on the same filament their separation was commonly a multiple of the actin helical repeat of 36 nm. Image averaging of branch junctions in the tomograms yielded a model for the in vivo branch at 2.9 nm resolution, which compared closely to that derived for the in vitro actin - Arp2/3 complex. Lamellipodia initiation was monitored in an intracellular wound-healing model and involved branching from the sides of actin filaments oriented parallel to the plasmalemma. Many filament plus ends, presumably capped, terminated behind the lamellipodium tip and localized on the dorsal and ventral surfaces of the actin network. These findings reveal how branching events initiate and maintain a network of actin filaments of variable length and provide the first structural model of the branch junction in vivo. A possible role of filament capping in generating the lamellipodium leaflet is discussed and a mathematical model of protrusion is also presented.

mDia1 and WAVE2 proteins interact directly with IRSp53 in filopodia and are involved in filopodium formation

Filopodia are dynamic actin-rich cell surface protrusions involved in cell migration, axon guidance, and wound healing. The RhoGTPase Cdc42 generates filopodia via IRSp53, a multidomain protein that links the processes of plasma membrane deformation and actin dynamics required for their formation in mammalian cells. The Src homology 3 domain of IRSp53 binds to the actin regulators Mena, Eps8, WAVE1, WAVE2, mDia1, and mDia2. We show that mDia1 and WAVE2 synergize with IRSp53 to form filopodia. IRSp53 also interacts directly with these two proteins within filopodia, as observed in acceptor photobleaching FRET studies. Measurement of filopodium formation by time-lapse imaging of live cells also revealed that depleting neuronal cells of either mDia1 or WAVE2 protein decreases the ability of IRSp53 to induce filopodia. In contrast, IRSp53 does not appear to partner WAVE1 or mDia2 to give rise to these structures. In addition, although all three isoforms of mDia are capable of inducing filopodia, IRSp53 requires only mDia1 to do so. These findings suggest that mDia1 and WAVE2 are important Src homology 3 domain partners of IRSp53 in forming filopodia.

Soluble THSD7A is an N-glycoprotein that promotes endothelial cell migration and tube formation in angiogenesis

Background

Thrombospondin type I domain containing 7A (THSD7A) is a novel neural protein that is known to affect endothelial migration and vascular patterning during development. To further understand the role of THSD7A in angiogenesis, we investigated the post-translational modification scheme of THS7DA and to reveal the underlying mechanisms by which this protein regulates blood vessel growth.

Methodology/Principal Findings

Full-length THSD7A was overexpressed in human embryonic kidney 293T (HEK293T) cells and was found to be membrane associated and N-glycosylated. The soluble form of THSD7A, which is released into the cultured medium, was harvested for further angiogenic assays. We found that soluble THSD7A promotes human umbilical vein endothelial cell (HUVEC) migration and tube formation. HUVEC sprouts and zebrafish subintestinal vessel (SIV) angiogenic assays further revealed that soluble THSD7A increases the number of branching points of new vessels. Interestingly, we found that soluble THSD7A increased the formation of filopodia in HUVEC. The distribution patterns of vinculin and phosphorylated focal adhesion kinase (FAK) were also affected, which implies a role for THSD7A in focal adhesion assembly. Moreover, soluble THSD7A increased FAK phosphorylation in HUVEC, suggesting that THSD7A is involved in regulating cytoskeleton reorganization.

Conclusions/Significance

Taken together, our results indicate that THSD7A is a membrane-associated N-glycoprotein with a soluble form. Soluble THSD7A promotes endothelial cell migration during angiogenesis via a FAK-dependent mechanism and thus may be a novel neuroangiogenic factor.

The cytoskeletal and signaling mechanisms of axon collateral branching

During development, axons are guided to their appropriate targets by a variety of guidance factors. On arriving at their synaptic targets, or while en route, axons form branches. Branches generated de novo from the main axon are termed collateral branches. The generation of axon collateral branches allows individual neurons to make contacts with multiple neurons within a target and with multiple targets. In the adult nervous system, the formation of axon collateral branches is associated with injury and disease states and may contribute to normally occurring plasticity. Collateral branches are initiated by actin filament– based axonal protrusions that subsequently become invaded by microtubules, thereby allowing the branch to mature and continue extending. This article reviews the current knowledge of the cellular mechanisms of the formation of axon collateral branches. The major conclusions of this review are (1) the mechanisms of axon extension and branching are not identical; (2) active suppression of protrusive activity along the axon negatively regulates branching; (3) the earliest steps in the formation of axon branches involve focal activation of signaling pathways within axons, which in turn drive the formation of actin-based protrusions; and (4) regulation of the microtubule array by microtubule-associated and severing proteins underlies the development of branches. Linking the activation of signaling pathways to specific proteins that directly regulate the axonal cytoskeleton underlying the formation of collateral branches remains a frontier in the field.

Building distinct actin filament networks in a common cytoplasm

Eukaryotic cells generate a diversity of actin filament networks in a common cytoplasm to optimally perform functions such as cell motility, cell adhesion, endocytosis and cytokinesis. Each of these networks maintains precise mechanical and dynamic properties by autonomously controlling the composition of its interacting proteins and spatial organization of its actin filaments. In this review, we discuss the chemical and physical mechanisms that target distinct sets of actin-binding proteins to distinct actin filament populations after nucleation, resulting in the assembly of actin filament networks that are optimized for specific functions.

Membrane shaping by the Bin/amphiphysin/Rvs (BAR) domain protein superfamily

BAR domain superfamily proteins have emerged as central regulators of dynamic membrane remodeling, thereby playing important roles in a wide variety of cellular processes, such as organelle biogenesis, cell division, cell migration, secretion, and endocytosis. Here, we review the mechanistic and structural basis for the membrane curvature-sensing and deforming properties of BAR domain superfamily proteins. Moreover, we summarize the present state of knowledge with respect to their regulation by autoinhibitory mechanisms or posttranslational modifications, and their interactions with other proteins, in particular with GTPases, and with membrane lipids. We postulate that BAR superfamily proteins act as membrane-deforming scaffolds that spatiotemporally orchestrate membrane remodeling.

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.

Target identification of bioactive compounds

To fully understand the regulation of cellular events, functional analysis of each protein involved in the regulatory systems is required. Among a variety of methods to uncover protein function, chemical genetics is a remarkable approach in which small molecular compounds are used as probes to elucidate protein functions within signaling pathways. However, identifying the target of small molecular bioactive compounds isolated by cell-based assays represents a crucial hurdle that must be overcome before chemical genetic studies can commence. A variety of methods and technologies for identifying target proteins have been reported. This review therefore aims to describe approaches for identifying these molecular targets.

The Rho kinase inhibitor Fasudil up-regulates astrocytic glutamate transport subsequent to actin remodelling in murine cultured astrocytes

BACKGROUND AND PURPOSE

Glutamate transporters play a major role in maintaining brain homeostasis and the astrocytic transporters, EAAT1 and EAAT2, are functionally dominant. Astrocytic excitatory amino acid transporters (EAATs) play important roles in various neuropathologies wherein astrocytes undergo cytoskeletal changes. Astrocytic plasticity is well documented, but the interface between EAAT function, actin and the astrocytic cytoskeleton is poorly understood. Because Rho kinase (ROCK) is a key determinant of actin polymerization, we investigated the effects of ROCK inhibitors on EAAT activity and astrocytic morphology.

EXPERIMENTAL APPROACH

The functional activity of glutamate transport was determined in murine cultured astrocytes after exposure to the ROCK inhibitors Fasudil (HA-1077) and Y27632 using biochemical, molecular and morphological approaches. Cytochemical analyses assessed changes in astrocytic morphology, F-/G-actin, and localizations of EAAT1/2.

RESULTS

Fasudil and Y27632 increased [(3)H]-D-aspartate (D-Asp) uptake into astrocytes, and the action of Fasudil was time-dependent and concentration-related. The rapid stellation of astrocytes (glial fibrillary acidic protein immunocytochemistry) induced by Fasudil was accompanied by reduced phalloidin staining of F-actin and increased V(max) for [(3)H]-D-Asp uptake. Immunoblotting after biotinylation demonstrated that Fasudil increased the expression of EAAT1 and EAAT2 on the cell surface. Immunocytochemistry indicated that Fasudil induced prominent labelling of astrocytic processes by EAAT1/2.

CONCLUSION AND IMPLICATIONS

These data show for the first time that ROCK plays a major role in determining the cell surface expression of EAAT1/2, providing new evidence for an association between transporter function and astrocytic phenotype. ROCK inhibitors, via the actin cytoskeleton, effect a consequent elevation of glutamate transporter function - this activity profile may contribute to their beneficial actions in neuropathologies.

Cell cytoskeleton and tether extraction

We perform a detailed investigation of the force × deformation curve in tether extraction from 3T3 cells by optical tweezers. Contrary to conventional wisdom about tethers extracted from cells, we find that actin filaments are present within them, so that a revised theory of tether pulling from cells is called for. We also measure steady and maximum tether force values significantly higher than previously published ones for 3T3 cells. Possible explanations for these differences are investigated. Further experimental support of the theory of force barriers for membrane tube extension is obtained. The potential of studies on tether pulling force × deformation for retrieving information on membrane-cytoskeleton interaction is emphasized.

Structural basis of the myosin X PH1(N)-PH2-PH1(C) tandem as a specific and acute cellular PI(3,4,5)P(3) sensor

The first PH domain of the myosin X cargo-binding domain is split into halves by insertion of another PH domain forming a PH1N-PH2-PH1C tandem. This tandem forms a rigid supramodule with the two lipid-binding pockets juxtaposed for cooperative binding to PI(3,4,5)P3-containing lipid membranes.

Myosin X (MyoX) is an unconventional myosin that is known to induce the formation and elongation of filopodia in many cell types. MyoX-induced filopodial induction requires the three PH domains in its tail region, although with unknown underlying molecular mechanisms. MyoX's first PH domain is split into halves by its second PH domain. We show here that the PH1_N-PH2-PH1_C tandem allows MyoX to bind to phosphatidylinositol (3,4,5)-triphosphate [PI(3,4,5)P₃] with high specificity and cooperativity. We further show that PH2 is responsible for the specificity of the PI(3,4,5)P₃ interaction, whereas PH1 functions to enhance the lipid membrane–binding avidity of the tandem. The structure of the MyoX PH1_N-PH2-PH1_C tandem reveals that the split PH1, PH2, and the highly conserved interdomain linker sequences together form a rigid supramodule with two lipid-binding pockets positioned side by side for binding to phosphoinositide membrane bilayers with cooperativity. Finally, we demonstrate that disruption of PH2-mediated binding to PI(3,4,5)P₃ abolishes MyoX's function in inducing filopodial formation and elongation.

Role of actin depolymerizing factor in the development of methamphetamine-induced place preference in mice

The present study investigated role of actin depolymerizing factor (ADF) in methamphetamine-induced place preference using ADF mutant (ADFm) and wild-type mice (WT). Whereas methamphetamine developed dose-dependently place preference in wild-type mice, methamphetamine-induced place preference was significantly attenuated by ADF mutation. Moreover, the administration of phalloidin, an F-actin stabilizer, dose-dependently inhibited methamphetamine-induced place preference. Methamphetamine-conditioning significantly increased the protein levels of ADF, cofilin, G-actin and F-actin in the limbic forebrain of both WT and ADFm, though the degree of enhancement of G- and F-actins by methamphetamine in ADFm was significantly smaller than in WT. Immunohistochemical study showed the co-localization of synaptophysin and tyrosine hydroxylase in WT and the potentiation of the immunoreactivity of these proteins by methamphetamine-conditioning, whereas such potentiation was suppressed by ADF mutation. In addition, the protein levels of synaptophysin, post-synaptic density 95 (PSD95), and neuroligin1 significantly increased in the limbic forebrain of WT showing methamphetamine-induced place preference, but not in ADFm. These findings indicate that ADF play an important role in the development of methamphetamine-induced place preference via regulation of actin dynamics.

β3 integrin-EGF receptor cross-talk activates p190RhoGAP in mouse mammary gland epithelial cells

Active RhoA localizes to plasma membrane, where it stimulates formation of focal adhesions and stress fibers. RhoA activity is inhibited by p190RhoGAP following integrin-mediated cell attachment to allow sampling of new adhesive environments. p190RhoGAP is itself activated by Src-dependent tyrosine phosphorylation, which facilitates complex formation with p120RasGAP. This complex then translocates to the cell surface, where p190RhoGAP down-regulates RhoA. Here we demonstrate that the epidermal growth factor receptor (EGFR) cooperates with β3 integrin to regulate p190RhoGAP activity in mouse mammary gland epithelial cells. Adhesion to fibronectin stimulates tyrosine phosphorylation of the EGFR in the absence of receptor ligands. Use of a dominant inhibitory EGFR mutant demonstrates that fibronectin-activated EGFR recruits p120RasGAP to the cell periphery. Expression of an inactive β3 integrin subunit abolishes p190RhoGAP tyrosine phosphorylation, demonstrating a mechanistic link between β3 integrin-activated Src and EGFR regulation of the RhoA inhibitor. The β3 integrin/EGFR pathway also has a positive role in formation of filopodia. Together our data suggest that EGFR constitutes an important intrinsic migratory cue since fibronectin is a key component of the microenvironment in normal mammary gland development and breast cancer. Our data also suggest that EGFR expressed at high levels has a role in eliciting cell shape changes associated with epithelial-to-mesenchymal transition.

DAAM family members leading a novel path into formin research

Formins are an important and evolutionarily well conserved class of actin binding proteins with essential biological functions. Although their molecular roles in actin regulation have been clearly demonstrated in vitro, their functions at the cellular or organism levels are still poorly understood. To illustrate this problem, but also to demonstrate potential ways forward, we focus here on the DAAM group of formins. In vertebrates, DAAM group members have been demonstrated to be important regulators of cellular and tissue morphogenesis but, as for all formins, the molecular mechanisms underlying these morphogenetic functions remain to be uncovered. The genome of the fruitfly Drosophila encodes a single DAAM gene that is evolutionarily highly conserved. Recent work on dDAAM has already provided a unique combination of observations and experimental opportunities unrivalled by any other Drosophila formin. These comprise in vitro actin polymerisation assays, subcellular studies in culture and in vivo, and a range of developmental phenotypes revealing a role in tracheal morphogenesis, axonal growth and muscle organization. At all these levels, future work on dDAAM will capitalize on the power of fly genetics, raising unique opportunities to advance our understanding of dDAAM at the systems level, with obvious implications for other formins.

The role of actin bundling proteins in the assembly of filopodia in epithelial cells

The goal of this review is to highlight how emerging new models of filopodia assembly, which include tissue specific actin-bundling proteins, could provide more comprehensive representations of filopodia assembly that would describe more adequately and effectively the complexity and plasticity of epithelial cells.  This review also describes how the true diversity of actin bundling proteins must be considered to predict the far-reaching significance and versatile functions of filopodia in epithelial cells.

Illuminating epidermal growth factor receptor densities on filopodia through plasmon coupling

Filopodia have been hypothesized to act as remote sensors of the cell environment, but many details of the sensor function remain unclear. We investigated the distribution of the epidermal growth factor (EGF) receptor (EGFR) density on filopodia and on the dorsal cell membrane of A431 human epidermoid carcinoma cells using a nanoplasmonic enabled imaging tool. We targeted cell surface EGFR with 40 nm diameter Au nanoparticles (NPs) using a high affinity multivalent labeling strategy and determined relative NP binding affinities spatially resolved through plasmon coupling. Distance-dependent near-field interactions between the labels generated a NP density (ρ)-dependent spectral response that facilitated a spatial mapping of the EGFR density distribution on subcellular length scales in an optical microscope in solution. The measured ρ values were significantly higher on filopodia than on the cellular surface, which is indicative of an enrichment of EGFR on filopodia. A detailed characterization of the spatial distribution of the NP immunolabels through scanning electron microscopy (SEM) confirmed the findings of the all-optical plasmon coupling studies and provided additional structural details. The NPs exhibited a preferential association with the sides of the filopodia. We calibrated the ρ-dependent spectral response of the Au immunolabels through correlation of optical spectroscopy and SEM. The experimental dependence of the measured plasmon resonance wavelength (λ(res)) of the interacting immunolabels on ρ was well described by the fit λ(res) = 595.0 nm - 46.36 nm exp(-ρ/51.48) for ρ ≤ 476 NPs/μm(2). The performed correlated spectroscopic/SEM studies pave the way toward quantitative immunolabeling studies of EGFR and other important cell surface receptors in an optical microscope.

Reflectivity and topography of cells grown on glass-coverslips measured with phase-shifted laser feedback interference microscopy

In spite of the advantages associated with the molecular specificity of fluorescence imaging, there is still a significant need to augment these approaches with label-free imaging. Therefore, we have implemented a form of interference microscopy based upon phase-shifted, laser-feedback interferometry and developed an algorithm that can be used to separate the contribution of the elastically scattered light by sub-cellular structures from the reflection at the coverslip-buffer interface. The method offers an opportunity to probe protein aggregation, index of refraction variations and structure. We measure the topography and reflection from calibration spheres and from stress fibers and adhesions in both fixed and motile cells. Unlike the data acquired with reflection interference contrast microscopy, where the reflection from adhesions can appear dark, our approach demonstrates that these regions have high reflectivity. The data acquired from fixed and live cells show the presence of a dense actin layer located ≈ 100 nm above the coverslip interface. Finally, the measured dynamics of filopodia and the lamella in a live cell supports retrograde flow as the dominate mechanism responsible for filopodia retraction.

Actin dynamics and turnover in cell motility

Cell migration is a highly coordinated process involving a multitude of separable but intertwined phenomena traditionally studied in multiple cell types, tissues and model systems. In spite of the multitude of mechanisms and modes of motility described in all these different systems, the ability to dynamically reorganize the actin cytoskeleton is common to all of them. However, defining the key molecular players in motility and their precise molecular functions continues to be challenging, last not least owing to robustness and flexibility common to complex biological phenomena. Here we will draft the future steps essential for achieving true progress towards the goal to increase our understanding of actin cytoskeleton dynamics driving cell migration.

Effects of surface nano-topography on human osteoblast filopodia

Anchorage-dependent cells growing over a substratum require stable adhesion areas on the surface for the next cellular activities. The adhesion is achieved by some contact points called focal adhesions. Because focal adhesions were distributed randomly, a trial to control the positions of focal adhesion with a specific order may cause interesting effects like as cytoskeleton rearrangement, which may induce and transfer new signals to the nucleus. Here, we cultured human osteoblasts over two sorts of nanopatterned surfaces with different pattern densities fabricated by using laser interference lithography and the nanoimprinting technique. Of the two nanopatterns, cells over the nanopattern with low pattern density showed relatively higher adaptation to the topography with guided filopodia protrusion. However, cells over the dense nanopattern showed difficulty in finding suitable paths for migration, as judged from the activities of filopodium formation and the presence of a shovel-like feature at the tip of each filopodium.

The Eps8/IRSp53/VASP network differentially controls actin capping and bundling in filopodia formation

There is a body of literature that describes the geometry and the physics of filopodia using either stochastic models or partial differential equations and elasticity and coarse-grained theory. Comparatively, there is a paucity of models focusing on the regulation of the network of proteins that control the formation of different actin structures. Using a combination of in-vivo and in-vitro experiments together with a system of ordinary differential equations, we focused on a small number of well-characterized, interacting molecules involved in actin-dependent filopodia formation: the actin remodeler Eps8, whose capping and bundling activities are a function of its ligands, Abi-1 and IRSp53, respectively; VASP and Capping Protein (CP), which exert antagonistic functions in controlling filament elongation. The model emphasizes the essential role of complexes that contain the membrane deforming protein IRSp53, in the process of filopodia initiation. This model accurately accounted for all observations, including a seemingly paradoxical result whereby genetic removal of Eps8 reduced filopodia in HeLa, but increased them in hippocampal neurons, and generated quantitative predictions, which were experimentally verified. The model further permitted us to explain how filopodia are generated in different cellular contexts, depending on the dynamic interaction established by Eps8, IRSp53 and VASP with actin filaments, thus revealing an unexpected plasticity of the signaling network that governs the multifunctional activities of its components in the formation of filopodia.

Cells move and interact with the environment by forming migratory structures composed of self organized polymers of actin. These protrusions can be flat and short surfaces, the lamellipodia, or adopt an elongated, finger-like shape called filopodia. In this article, we analyze the ‘computation’ performed by cells when they opt to form filopodia. We focus our attention on some initiators of filopodia that play an essential role due to their interaction with the cell membrane. We analyze the formation of these filopodia initiators in different genotypes, thus providing a way to rationalize the behaviors of different cells in terms of tendency to form filopodia. Our results, based on the combination of experimental and computational approaches, suggest that cells have developed molecular networks that are extremely flexible in their capability to follow the path leading to filopodia formation. In this sense the role of an element of the network, Eps8, is paradigmatic, as this protein can both induce or inhibit the formation of filopodia depending on the cellular context.

New approaches to targeting the actin cytoskeleton for chemotherapy

The actin cytoskeleton is indispensable for normal cellular function. In particular, several actin-based structures coordinate cellular motility, a process hijacked by tumor cells in order to facilitate their propagation to distant sites. The actin cytoskeleton, therefore, represents a point for chemotherapeutic intervention. The challenge in disrupting the actin cytoskeleton is in preserving actin-driven contraction of cardiac and skeletal muscle. By targeting actin-binding proteins with altered expression in malignancy, it may be possible to achieve tumor-specific toxicity. A number of actin-binding proteins act cooperatively and synergistically to regulate actin structures required for motility. The actin cytoskeleton is characterized by a significant degree of plasticity. Targeting specific actin-binding proteins for chemotherapy will only be successful if no other compensatory mechanisms exist.

On the mechanical stabilization of filopodia

We studied force-induced elongation of filopodia by coupling magnetic tweezers to the tip through the bacterial coat protein invasin, which couples the force generator to the actin bundles (through myosin X), thus impeding the growth of the actin plus end. Single force pulses (15-30 s) with amplitudes between 20 and 600 pN and staircase-like force scenarios (amplitudes, ∼50 pN; step widths, 30 s) were applied. In both cases, the responses consist of a fast viscoelastic deflection followed by a linear flow regime. The deflections are reversible after switching off the forces, suggesting a mechanical memory. The elongation velocity exhibits an exponential distribution (half-width <v(1/2)>, ∼0.02 μm s(-1)) and did not increase systematically with the force amplitudes. We estimate the bending modulus (0.4 × 10(-23) J m) and the number of actin filaments (∼10) by analyzing filopodium bending fluctuations. Sequestering of intracellular Ca(2+) by BAPTA caused a strong reduction in the amplitude of elongation, whereas latrunculin A resulted in loss of the elastic response. We attribute the force-independent velocity to the elongation of actin bundles enabled by the force-induced actin membrane uncoupling and the reversibility by the treadmilling mechanism and an elastic response.

The 'ins' and 'outs' of podosomes and invadopodia: characteristics, formation and function

Podosomes and invadopodia are actin-based dynamic protrusions of the plasma membrane of metazoan cells that represent sites of attachment to - and degradation of - the extracellular matrix. The key proteins in these structures include the actin regulators cortactin and neural Wiskott-Aldrich syndrome protein (N-WASP), the adaptor proteins Tyr kinase substrate with four SH3 domains (TKS4) and Tyr kinase substrate with five SH3 domains (TKS5), and the metalloprotease membrane type 1 matrix metalloprotease (MT1MMP; also known as MMP14). Many cell types can produce these structures, including invasive cancer cells, vascular smooth muscle and endothelial cells, and immune cells such as macrophages and dendritic cells. Recently, progress has been made in our understanding of the regulatory and functional aspects of podosome and invadopodium biology and their role in human disease.

Prenylated c17orf37 induces filopodia formation to promote cell migration and metastasis

Post-translational modification by covalent attachment of isoprenoid lipids (prenylation) regulates the functions and biological activities of several proteins implicated in the oncogenic transformation and metastatic progression of cancer. The largest group of prenylated proteins contains a CAAX motif at the C-terminal that serves as a substrate for a series of post-translational modifications that convert these otherwise hydrophilic proteins to lipidated proteins, thus facilitating membrane association. C17orf37 (chromosome 17 open reading frame 37), also known as C35/Rdx12/MGC14832, located in the 17q12 amplicon, is overexpressed in human cancer, and its expression correlates with the migratory and invasive phenotype of cancer cells. Here we show that C17orf37 contains a functional CAAX motif and is post-translationally modified by protein geranylgeranyltransferase-I (GGTase-I). Geranylgeranylation of C17orf37 at the CAAX motif facilitates association of the protein to the inner leaflet of plasma membrane, enhances migratory phenotype of cells by inducing increased filopodia formation, and potentiates directional migration. A prenylation-deficient mutant of C17orf37 is functionally inactive and fails to trigger dissemination of tail vein-injected cells in a mouse model of metastasis. These findings demonstrate that prenylation is required for the function of the C17orf37 protein in cancer cells and imply that the post-translational modification may functionally regulate metastatic progression of disease.

TM4SF1: a tetraspanin-like protein necessary for nanopodia formation and endothelial cell migration

Transmembrane-4-L-six-family-1 (TM4SF1) is a tetraspanin-like membrane protein that is highly and selectively expressed by cultured endothelial cells (EC) and, in vivo, by EC lining angiogenic tumor blood vessels. TM4SF1 is necessary for the formation of unusually long (up to a 50 μm), thin (~100-300 nm wide), F-actin-poor EC cell projections that we term 'nanopodia'. Immunostaining of nanopodia at both the light and electron microsopic levels localized TM4SF1 in a regularly spaced, banded pattern, forming TM4FS1-enriched domains. Live cell imaging of GFP-transduced HUVEC demonstrated that EC project nanopodia as they migrate and interact with neighboring cells. When TM4SF1 mRNA levels in EC were increased from the normal ~90 mRNA copies/cell to ~400 copies/cell through adenoviral transduction, EC projected more and longer nanopodia from the entire cell circumference but were unable to polarize or migrate effectively. When fibroblasts, which normally express TM4SF1 at ~5 copies/cell, were transduced to express TM4SF1 at EC-like levels, they formed typical TM4SF1-banded nanopodia, and broadened, EC-like lamellipodia. Mass-spectrometry demonstrated that TM4SF1 interacted with myosin-10 and β-actin, proteins involved in filopodia formation and cell migration. In summary, TM4SF1, like genuine tetraspanins, serves as a molecular organizer that interacts with membrane and cytoskeleton-associated proteins and uniquely initiates the formation of nanopodia and facilitates cell polarization and migration.

Initial steps of metastasis: cell invasion and endothelial transmigration

Metastasis is the leading cause of cancer mortality. The metastatic cascade represents a multi-step process which includes local tumor cell invasion, entry into the vasculature followed by the exit of carcinoma cells from the circulation and colonization at the distal sites. At the earliest stage of successful cancer cell dissemination, the primary cancer adapts the secondary site of tumor colonization involving the tumor–stroma crosstalk. The migration and plasticity of cancer cells as well as the surrounding environment such as stromal and endothelial cells are mandatory. Consequently, the mechanisms of cell movement are of utmost relevance for targeted intervention of which three different types have been reported. Tumor cells can migrate either collectively, in a mesenchymal or in an amoeboid type of movement and intravasate the blood or lymph vasculature. Intravasation by the interaction of tumor cells with the vascular endothelium is mechanistically poorly understood. Changes in the epithelial plasticity enable carcinoma cells to switch between these types of motility. The types of migration may change depending on the intervention thereby increasing the velocity and aggressiveness of invading cancer cells. Interference with collective or mesenchymal cell invasion by targeting integrin expression or metalloproteinase activity, respectively, resulted in an amoeboid cell phenotype as the ultimate exit strategy of cancer cells. There are little mechanistic details reported in vivo showing that the amoeboid behavior can be either reversed or efficiently inhibited. Future concepts of metastasis intervention must simultaneously address the collective, mesenchymal and amoeboid mechanisms of cell invasion in order to advance in anti-metastatic strategies as these different types of movement can coexist and cooperate. Beyond the targeting of cell movements, the adhesion of cancer cells to the stroma in heterotypic circulating tumor cell emboli is of paramount relevance for anti-metastatic therapy.

Theoretical model for cellular shapes driven by protrusive and adhesive forces

The forces that arise from the actin cytoskeleton play a crucial role in determining the cell shape. These include protrusive forces due to actin polymerization and adhesion to the external matrix. We present here a theoretical model for the cellular shapes resulting from the feedback between the membrane shape and the forces acting on the membrane, mediated by curvature-sensitive membrane complexes of a convex shape. In previous theoretical studies we have investigated the regimes of linear instability where spontaneous formation of cellular protrusions is initiated. Here we calculate the evolution of a two dimensional cell contour beyond the linear regime and determine the final steady-state shapes arising within the model. We find that shapes driven by adhesion or by actin polymerization (lamellipodia) have very different morphologies, as observed in cells. Furthermore, we find that as the strength of the protrusive forces diminish, the system approaches a stabilization of a periodic pattern of protrusions. This result can provide an explanation for a number of puzzling experimental observations regarding cellular shape dependence on the properties of the extra-cellular matrix.

Cells have highly varied and dynamic shapes, which are determined by internal forces generated by the cytoskeleton. These forces include protrusive forces due to the formation of new internal fibers and forces produced due to attachment of the cell to an external substrate. A long standing challenge is to explain how the myriad components of the cytoskeleton self-organize to form the observed shapes of cells. We present here a theoretical study of the shapes of cells that are driven only by protrusive forces of two types; one is the force due to polymerization of actin filaments which acts as an internal pressure on the membrane, and the second is the force due to adhesion between the membrane and external substrate. The key property is that both forces are localized on the cell membrane by protein complexes that have convex spontaneous curvature. This leads to a positive feedback that destabilizes the uniform cell shape and induces the spontaneous formation of patterns. We compare the resulting patterns to observed cellular shapes and find good agreement, which allows us to explain some of the puzzling dependencies of cell shapes on the properties of the surrounding matrix.

Phospholipids regulate localization and activity of mDia1 formin

Diaphanous-related formins (DRFs) are large multi-domain proteins that nucleate and assemble linear actin filaments. Binding of active Rho family proteins to the GTPase-binding domain (GBD) triggers localization at the membrane and the activation of most formins if not all. In recent years GTPase regulation of formins has been extensively studied, but other molecular mechanisms that determine subcellular distribution or regulate formin activity have remained poorly understood. Here, we provide evidence that the activity and localization of mouse formin mDia1 can be regulated through interactions with phospholipids. The phospholipid-binding sites of mDia1 are clusters of positively charged residues in the N-terminal basic domain (BD) and at the C-terminal region. Upon binding to the lipid bilayer the N-terminal region of mDia1 induces strong clustering of phosphatidylinositol-4,5-bisphosphate (PIP(2)) and subsequently inserts into the membrane bilayer thus anchoring mDia1 to the reconstituted plasma membrane. In addition, an interaction of phospholipids with the C-terminal region of mDia1 causes a drastic reduction of its actin filament assembly activity. Our data suggest that the N-terminal phospholipid-binding sites help to anchor formins at the plasma membrane, and the interaction with phospholipids in the C-terminus functions as a switch for transient inactivation.

Cdc42-interacting protein 4 is a Src substrate that regulates invadopodia and invasiveness of breast tumors by promoting MT1-MMP endocytosis

Invadopodia are actin-rich membrane protrusions that promote extracellular matrix degradation and invasiveness of tumor cells. Src protein-tyrosine kinase is a potent inducer of invadopodia and tumor metastases. Cdc42-interacting protein 4 (CIP4) adaptor protein interacts with actin regulatory proteins and regulates endocytosis. Here, we show that CIP4 is a Src substrate that localizes to invadopodia in MDA-MB-231 breast tumor cells expressing activated Src (MDA-SrcYF). To probe the function of CIP4 in invadopodia, we established stable CIP4 knockdown in MDA-SrcYF cell lines by RNA interference. Compared with control cells, CIP4 knockdown cells degrade more extracellular matrix (ECM), have increased numbers of mature invadopodia and are more invasive through matrigel. Similar results are observed with knockdown of CIP4 in EGF-treated MDA-MB-231 cells. This inhibitory role of CIP4 is explained by our finding that CIP4 limits surface expression of transmembrane type I matrix metalloprotease (MT1-MMP), by promoting MT1-MMP internalization. Ectopic expression of CIP4 reduces ECM digestion by MDA-SrcYF cells, and this activity is enhanced by mutation of the major Src phosphorylation site in CIP4 (Y471). Overall, our results identify CIP4 as a suppressor of Src-induced invadopodia and invasion in breast tumor cells by promoting endocytosis of MT1-MMP.

Propagating cell-membrane waves driven by curved activators of actin polymerization

Cells exhibit propagating membrane waves which involve the actin cytoskeleton. One type of such membranal waves are Circular Dorsal Ruffles (CDR) which are related to endocytosis and receptor internalization. Experimentally, CDRs have been associated with membrane bound activators of actin polymerization of concave shape. We present experimental evidence for the localization of convex membrane proteins in these structures, and their insensitivity to inhibition of myosin II contractility in immortalized mouse embryo fibroblasts cell cultures. These observations lead us to propose a theoretical model which explains the formation of these waves due to the interplay between complexes that contain activators of actin polymerization and membrane-bound curved proteins of both types of curvature (concave and convex). Our model predicts that the activity of both types of curved proteins is essential for sustaining propagating waves, which are abolished when one type of curved activator is removed. Within this model waves are initiated when the level of actin polymerization induced by the curved activators is higher than some threshold value, which allows the cell to control CDR formation. We demonstrate that the model can explain many features of CDRs, and give several testable predictions. This work demonstrates the importance of curved membrane proteins in organizing the actin cytoskeleton and cell shape.

The small GTPase Cdc42 promotes membrane protrusion during polar body emission via ARP2-nucleated actin polymerization

Polar body emission is a specialized cell division throughout the animal kingdom, serving to reduce chromosome ploidy while preserving the egg cytoplasm. Critical to polar body emission are the asymmetric positioning of the meiotic spindle prior to anaphase, with one pole attached to the oocyte cortex, and the simultaneous membrane protrusion during subsequent cytokinesis. We have shown that, during Xenopus oocyte maturation, the small GTPase Cdc42 promotes membrane protrusion while a classical RhoA contractile ring forms and constricts at the base of the protrusion. We report here that treating oocytes with low concentrations of nocodazole diminished the size of metaphase I spindles and prevented polar body emission, and yet an active Cdc42 cap of correspondingly diminished size still developed, on time, atop of the spindle pole. Conversely, treating oocytes with low concentrations of taxol resulted in a spindle with multiple poles attached to the cortex, but still each of these poles were associated with activated cortical Cdc42 at the appropriate time. Therefore, the asymmetric positioning of the meiotic spindle with one pole anchored to the cortex is a prerequisite for Cdc42 activation. Furthermore, we demonstrated that the Cdc42-regulated F-actin nucleator ARP2/3 complex was similarly localized at the cortex of the protruding polar body membrane, suggesting that Cdc42 promotes membrane protrusion through an F-actin meshwork mechanism. Finally, we demonstrated that Cdc42 and RhoA formed similarly complementary activity zones during egg activation and that inhibition of Cdc42 prevented second polar body emission. Therefore, Cdc42 activation likely promotes membrane protrusion during polar body emission in widespread systems.

Cell adhesion on nanotextured slippery superhydrophobic substrates

In this work, the response of Saos2 cells to polymeric surfaces with different roughness/density of nanometric dots produced by a tailored plasma-etching process has been studied. Topographical features have been evaluated by atomic force microscopy, while wetting behavior, in terms of water-surface adhesion energy, has been evaluated by measurements of drop sliding angle. Saos2 cytocompatibility has been investigated by scanning electron microscopy, fluorescent microscopy, and optical microscopy. The similarity in outer chemical composition has allowed isolation of the impact of the topographical features on cellular behavior. The results indicate that Saos2 cells respond differently to surfaces with different nanoscale topographical features, clearly showing a certain inhibition in cell adhesion when the nanoscale is particularly small. This effect appears to be attenuated in surfaces with relatively bigger nanofeatures, though these express a more pronounced slippery/dry wetting character.

Effects of intervertebral disc cells on neurite outgrowth from dorsal root ganglion explants in culture

STUDY DESIGN

An experimental study investigating the effect of disc cells on neurite outgrowth in a rat dorsal root ganglion (DRG) culture system.

OBJECTIVE

To examine the effects of the 2 nucleus pulposus (NP) cell populations, notochordal cells (NC) and chondrocyte-like cells (CC) on neurite outgrowth from DRGs.

SUMMARY OF BACKGROUND DATA

NP consists of at least 2 cell populations, NC and CC. The cells in NP have been shown to be responsible for negative effects on neurite outgrowth in vitro and on nerve tissue in vivo. It is unknown whether 1 cell type or combinations of the 2 cell types are responsible for the reported effects.

METHODS

A total of 939 DRGs from newborn Sprague Dawley rats were harvested and placed in culture dishes. After 24 hours, the neurite outgrowth was measured. NP was harvested from tail discs of adult rats and the NP cells were separated into 2 populations, NC and CC. The cell populations were applied to the DRG culture in different cell concentrations and combinations, and compared to medium. After 24 hours of exposure, the neurite outgrowth was reassessed and expressed as the ratio between the outgrowth at 48 and 24 hours culture.

RESULTS

NC in intermediate and high concentration and CC in high concentration induced a significant inhibition of the neurite outgrowth compared to culture medium. Further, one of the combinations (low NC and high CC concentration) resulted in a significant inhibition of the neurite outgrowth.

CONCLUSION

The present study demonstrated negative effects of NP cells on nerve tissue culture explants. The combination of low NC and high CC concentrations may mimic the situation in humans, where we have an increased proportion of chondrocyte-like cells with age. The results from this study may provide a biologic explanation for the large variation of symptoms in disc herniation patients despite similar mechanical influence on nerve tissue.

In vivo assembly of an archaeal virus studied with whole-cell electron cryotomography

We applied whole-cell electron cryotomography to the archaeon Sulfolobus infected by Sulfolobus turreted icosahedral virus (STIV), which belongs to the PRD1-Adeno lineage of dsDNA viruses. STIV infection induced the formation of pyramid-like protrusions with sharply defined facets on the cell surface. They had a thicker cross-section than the cytoplasmic membrane and did not contain an exterior surface protein layer (S-layer). Intrapyramidal bodies often occupied the volume of the pyramids. Mature virions, procapsids without genome cores, and partially assembled particles were identified, suggesting that the capsid and inner membrane coassemble in the cytoplasm to form a procapsid. A two-class reconstruction using a maximum likelihood algorithm demonstrated that no dramatic capsid transformation occurred upon DNA packaging. Virions tended to form tightly packed clusters or quasicrystalline arrays while procapsids mostly scattered outside or on the edges of the clusters. The study revealed vivid images of STIV assembly, maturation, and particle distribution in cell.

Dissecting regulatory networks of filopodia formation in a Drosophila growth cone model

F-actin networks are important structural determinants of cell shape and morphogenesis. They are regulated through a number of actin-binding proteins. The function of many of these proteins is well understood, but very little is known about how they cooperate and integrate their activities in cellular contexts. Here, we have focussed on the cellular roles of actin regulators in controlling filopodial dynamics. Filopodia are needle-shaped, actin-driven cell protrusions with characteristic features that are well conserved amongst vertebrates and invertebrates. However, existing models of filopodia formation are still incomplete and controversial, pieced together from a wide range of different organisms and cell types. Therefore, we used embryonic Drosophila primary neurons as one consistent cellular model to study filopodia regulation. Our data for loss-of-function of capping proteins, enabled, different Arp2/3 complex components, the formin DAAM and profilin reveal characteristic changes in filopodia number and length, providing a promising starting point to study their functional relationships in the cellular context. Furthermore, the results are consistent with effects reported for the respective vertebrate homologues, demonstrating the conserved nature of our Drosophila model system. Using combinatorial genetics, we demonstrate that different classes of nucleators cooperate in filopodia formation. In the absence of Arp2/3 or DAAM filopodia numbers are reduced, in their combined absence filopodia are eliminated, and in genetic assays they display strong functional interactions with regard to filopodia formation. The two nucleators also genetically interact with enabled, but not with profilin. In contrast, enabled shows strong genetic interaction with profilin, although loss of profilin alone does not affect filopodia numbers. Our genetic data support a model in which Arp2/3 and DAAM cooperate in a common mechanism of filopodia formation that essentially depends on enabled, and is regulated through profilin activity at different steps.

Free edges in epithelia as cues for motility

One of the primary functions of any epithelium is to act as a barrier. To maintain integrity, epithelia migrate rapidly to cover wounds, and there is intense interest in understanding how wounds are detected. Numerous soluble factors are present in the wound environment and epithelia can sense the presence of adjacent denuded extracellular matrix. However, the presence of such cues is expected to be highly variable, and here we focus on the presence of edges in the epithelial sheets as a stimulus, since they are universally and continuously present in wounds. Using a novel tissue culture model, free edges in the absence of any other identifiable cues were found to trigger activation of the epidermal growth factor receptor and increase cell motility. Edges bordered by inert physical barriers do not activate the receptor, indicating that activation is related to mechanical factors rather than to specific cell cell interactions.