From branched networks of actin filaments to bundles

Liquid-like condensates mediate competition between actin branching and bundling

Cellular remodeling of actin networks underlies cell motility during key morphological events, from embryogenesis to metastasis. In these transformations, there is an inherent competition between actin branching and bundling, because steric clashes among branches create a mechanical barrier to bundling. Recently, liquid-like condensates consisting purely of proteins involved in either branching or bundling of the cytoskeleton have been found to catalyze their respective functions. Yet in the cell, proteins that drive branching and bundling are present simultaneously. In this complex environment, which factors determine whether a condensate drives filaments to branch or become bundled? To answer this question, we added the branched actin nucleator, Arp2/3, to condensates composed of VASP, an actin bundling protein. At low actin to VASP ratios, branching activity, mediated by Arp2/3, robustly inhibited VASP-mediated bundling of filaments, in agreement with agent-based simulations. In contrast, as the actin to VASP ratio increased, addition of Arp2/3 led to formation of aster-shaped structures, in which bundled filaments emerged from a branched actin core, analogous to filopodia emerging from a branched lamellipodial network. These results demonstrate that multi-component, liquid-like condensates can modulate the inherent competition between bundled and branched actin morphologies, leading to organized, higher-order structures, similar to those found in motile cells.

Liquid-like condensates mediate competition between actin branching and bundling

Cellular remodeling of actin networks underlies cell motility during key morphological events, from embryogenesis to metastasis. In these transformations there is an inherent competition between actin branching and bundling, because steric clashes among branches create a mechanical barrier to bundling. Recently, liquid-like condensates consisting purely of proteins involved in either branching or bundling of the cytoskeleton have been found to catalyze their respective functions. Yet in the cell, proteins that drive branching and bundling are present simultaneously. In this complex environment, which factors determine whether a condensate drives filaments to branch versus becoming bundled? To answer this question, we added the branched actin nucleator, Arp2/3, to condensates composed of VASP, an actin bundling protein. At low actin to VASP ratios, branching activity, mediated by Arp2/3, robustly inhibited VASP-mediated bundling of filaments, in agreement with agent-based simulations. In contrast, as the actin to VASP ratio increased, addition of Arp2/3 led to formation of aster-shaped structures, in which bundled filaments emerged from a branched actin core, analogous to filopodia emerging from a branched lamellipodial network. These results demonstrate that multi-component, liquid-like condensates can modulate the inherent competition between bundled and branched actin morphologies, leading to organized, higher-order structures, similar to those found in motile cells.

Fascin-induced actin protrusions are suppressed by dendritic networks in giant unilamellar vesicles

The interactions between actin networks and cell membrane are immensely important for eukaryotic cell functions including cell shape changes, motility, polarity establishment, and adhesion. Actin-binding proteins are known to compete and cooperate using a finite amount of actin monomers to form distinct actin networks. How actin-bundling protein fascin and actin-branching protein Arp2/3 complex compete to remodel membranes is not entirely clear. To investigate fascin- and Arp2/3-mediated actin network remodeling, we applied a reconstitution approach encapsulating bundled and dendritic actin networks inside giant unilamellar vesicles (GUVs). Independently reconstituted, membrane-bound Arp2/3 nucleation forms an actin cortex in GUVs, whereas fascin mediates formation of actin bundles that protrude out of GUVs. Coencapsulating both fascin and Arp2/3 complex leads to polarized dendritic aggregates and significantly reduces membrane protrusions, irrespective of whether the dendritic network is membrane bound or not. However, reducing Arp2/3 complex while increasing fascin restores membrane protrusion. Such changes in network assembly and the subsequent interplay with membrane can be attributed to competition between fascin and Arp2/3 complex to utilize a finite pool of actin.

Mechanics of the cell: Interaction mechanisms and mechanobiological models

The importance of cell mechanics has long been recognized for the cell development and function. Biomechanics plays an important role in cell metabolism, regulation of mechanotransduction pathways and also modulation of nuclear response. The mechanical properties of the cell are likely determined by, among many others, the cytoskeleton elasticity, membrane tension and cell-substrate adhesion. This coordinated but complex mechanical interplay is required however, for the cell to respond to and influence in a reciprocal manner the chemical and mechanical signals from the extracellular matrix (ECM). In an effort to better and more fully understand the cell mechanics, the role of nuclear mechanics has emerged as an important contributor to the overall cellular mechanics. It is not too difficult to appreciate the physical connection between the nucleus and the cytoskeleton network that may be connected to the ECM through the cell membrane. Transmission of forces from ECM through this connection is essential for a wide range of cellular behaviors and functions such as cytoskeletal reorganization, nuclear movement, cell migration and differentiation. Unlike the cellular mechanics that can be measured using a number of biophysical techniques that were developed in the past few decades, it still remains a daunting challenge to probe the nuclear mechanics directly. In this paper, we therefore aim to provide informative description of the cell membrane and cytoskeleton mechanics, followed by unique computational modeling efforts to elucidate the nucleus-cytoskeleton coupling. Advances in our knowledge of complete cellular biomechanics and mechanotransduction may lead to clinical relevance and applications in mechano-diseases such as atherosclerosis, stem cell-based therapies, and the development of tissue engineered products.

Finger-like membrane protrusions are favored by heterogeneities in the actin network

Finger-like protrusions in cells are mostly generated by an active actin cytoskeleton pushing against the cell membrane. Conventional filopodia, localized at the leading edge of the cells, are long and thin protrusions composed of parallel actin filaments that emanate from a branched actin network. In contrast, dendritic filopodia, precursors of dendritic spines in neurons, are entirely filled in with a branched actin network. Here, we investigate in vitro how the dynamics of branched actin structures, polymerized at a membrane surface, trigger the formation of both protrusion types. Using supported bilayers and liposomes, we show that a decrease in the amount of activation sites at the membrane surface leads to the appearance of heterogeneities in the actin network coverage. Such heterogeneities promote the formation of membrane protrusions, and the size of heterogeneity patches matches the one of the protrusion base. Protrusion shape, cylindrical or conical, directly correlates with the absence or the presence of actin branches, respectively.

Efficient simulation of thermally fluctuating biopolymers immersed in fluids on 1-micron, 1-second scales

The combination of fluid-structure interactions with stochasticity, due to thermal fluctuations, remains a challenging problem in computational fluid dynamics. We develop an efficient scheme based on the stochastic immersed boundary method, Stokeslets, and multiple timestepping. We test our method for spherical particles and filaments under purely thermal and deterministic forces and find good agreement with theoretical predictions for Brownian Motion of a particle and equilibrium thermal undulations of a semi-flexible filament. As an initial application, we simulate bio-filaments with the properties of F-actin. We specifically study the average time for two nearby parallel filaments to bundle together. Interestingly, we find a two-fold acceleration in this time between simulations that account for long-range hydrodynamics compared to those that do not, suggesting that our method will reveal significant hydrodynamic effects in biological phenomena.

Dynamic reorganization of the actin cytoskeleton

Cellular processes, including morphogenesis, polarization, and motility, rely on a variety of actin-based structures. Although the biochemical composition and filament organization of these structures are different, they often emerge from a common origin. This is possible because the actin structures are highly dynamic. Indeed, they assemble, grow, and disassemble in a time scale of a second to a minute. Therefore, the reorganization of a given actin structure can promote the formation of another. Here, we discuss such transitions and illustrate them with computer simulations.

Reconstitution of actin-based motility by vasodilator-stimulated phosphoprotein (VASP) depends on the recruitment of F-actin seeds from the solution produced by cofilin

Vasodilator-stimulated phosphoprotein (VASP) is active in many filopodium-based and cytoskeleton reorganization processes. It is not fully understood how VASP directly functions in actin-based motility and how regulatory proteins affect its function. Here, we combine bead motility assay and single filament experiments. In the presence of a bundling component, actin bundles that grow from the surface of WT-VASP-coated beads induced movement of the beads. VASP promotes actin-based movement alone, in the absence of other actin nucleators. We propose that at physiological salt conditions VASP nucleation activity is too weak to promote motility and bundle formation. Rather, VASP recruits F-actin seeds from the solution and promotes their elongation. Cofilin has a crucial role in the nucleation of these F-actin seeds, notably under conditions of unfavorable spontaneous actin nucleation. We explored the role of multiple VASP variants. We found that the VASP-F-actin binding domain is required for the recruitment of F-actin seeds from the solution. We also found that the interaction of profilin-actin complexes with the VASP-proline-rich domain and the binding of the VASP-F-actin binding domain to the side of growing filaments is critical for transforming actin polymerization into motion. At the single filament level, profilin mediates both filament elongation rate and VASP anti-capping activity. Binding of profilin-actin complexes increases the polymerization efficiency by VASP but decreases its efficiency as an anti-capper; binding of free profilin creates the opposite effect. Finally, we found that an additional component such as methylcellulose or fascin is required for actin bundle formation and motility mediated by VASP.

Assembly kinetics determine the architecture of α-actinin crosslinked F-actin networks

The actin cytoskeleton is organized into diverse meshworks and bundles that support many aspects of cell physiology. Understanding the self-assembly of these actin-based structures is essential for developing predictive models of cytoskeletal organization. Here we show that the competing kinetics of bundle formation with the onset of dynamic arrest arising from filament entanglements and cross-linking determine the architecture of reconstituted actin networks formed with α-actinin cross-links. Cross-link mediated bundle formation only occurs in dilute solutions of highly mobile actin filaments. As actin polymerization proceeds, filament mobility and bundle formation are arrested concomitantly. By controlling the onset of dynamic arrest, perturbations to actin assembly kinetics dramatically alter the architecture of biochemically identical samples. Thus, the morphology of reconstituted F-actin networks is a kinetically determined structure similar to those formed by physical gels and glasses. These results establish mechanisms controlling the structure and mechanics in diverse semi-flexible biopolymer networks.

Filopodia initiation: focus on the Arp2/3 complex and formins

Filopodia are long, slender, actin-rich cellular protrusions, which recently have become a focus of cell biology research because of their proposed roles as sensory and exploratory organelles that allow for "intelligent" cell behavior. Actin nucleation, elongation and bundling are believed to be essential for filopodia formation and functions. However, the identity of actin filament nucleators responsible for the initiation of filopodia remains controversial. Two alternative models, the convergent elongation and tip nucleation, emphasize two different actin filament nucleators, the Arp2/3 complex or formins, respectively, as key players during filopodia initiation. Although these two models in principle are not mutually exclusive, it is important to understand which of them is actually employed by cells. In this review, we discuss the existing evidence regarding the relative roles of the Arp2/3 complex and formins in filopodia initiation.

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.

Model of myosin node aggregation into a contractile ring: the effect of local alignment

Actomyosin bundles frequently form through aggregation of membrane-bound myosin clusters. One such example is the formation of the contractile ring in fission yeast from a broad band of cortical nodes. Nodes are macromolecular complexes containing several dozens of myosin-II molecules and a few formin dimers. The condensation of a broad band of nodes into the contractile ring has been previously described by a search, capture, pull and release (SCPR) model. In SCPR, a random search process mediated by actin filaments nucleated by formins leads to transient actomyosin connections among nodes that pull one another into a ring. The SCPR model reproduces the transport of nodes over long distances and predicts observed clump-formation instabilities in mutants. However, the model does not generate transient linear elements and meshwork structures as observed in some wild-type and mutant cells during ring assembly. As a minimal model of node alignment, we added short-range aligning forces to the SCPR model representing currently unresolved mechanisms that may involve structural components, cross-linking and bundling proteins. We studied the effect of the local node alignment mechanism on ring formation numerically. We varied the new parameters and found viable rings for a realistic range of values. Morphologically, transient structures that form during ring assembly resemble those observed in experiments with wild-type and cdc25-22 cells. Our work supports a hierarchical process of ring self-organization involving components drawn together from distant parts of the cell followed by progressive stabilization.

Fascin is a predictor for invasiveness and recurrence of urothelial carcinoma of bladder

OBJECTIVES

To evaluate the expression of fascin in bladder urothelial carcinoma, and to analyze its association with clinicopathologic features and prognosis of urinary bladder urothelial carcinoma.

MATERIALS AND METHODS

Immunohistochemistry was used to detect the expression of fascin, Ki-67, p53, CK20, and multidrug resistance gene (MDR) in 111 bladder urothelial carcinoma and 42 normal epithelial tissues. The association between fascin expression and clinicopathologic parameters and prognostic factors on tumor recurrence was analyzed by Kaplan-Meier method, log-rank test, and Cox proportional hazards model.

RESULTS

Ninety-four of 111 cases of bladder urothelial carcinoma showed positive fascin expression, while no fascin expression was detected in normal transitional epithelium. There was a significant difference in the expression of fascin in normal epithelium and bladder urothelial carcinoma (P = 0.000). Fascin expression was positively correlated with pT stage (P = 0.001) and tumor size (P = 0.011), while it had no association with age, gender, and tumor grade (P > 0.05). pT stage and the expression of fascin, Ki-67, p53, and CK20 were significantly correlated with urothelial carcinoma recurrence, and fascin expression was an independent factor predicting tumor recurrence.

CONCLUSIONS

Fascin expression was up-regulated in bladder urothelial carcinoma. Over-expression of fascin might play an important role in invasiveness and recurrence of bladder urothelial carcinoma. Fascin may be used as a prognostic marker and a new target for the treatment of bladder urothelial carcinoma.