Actin, a central player in cell shape and movement

Mis-localization of Arp2 mRNA impairs persistence of directional cell migration

Arp2/3 complex is an actin polymerization nucleator and localized in the leading protrusions of migrating cells. It has been unclear how this complex is targeted to the protrusions and whether its localization is functionally important. We previously demonstrated that mRNAs encoding for the subunits of the complex were localized in the protrusions of fibroblasts, suggesting a mechanism to target the complex to the protrusions. We here present data demonstrating the importance of Arp2/3 complex mRNA localization in directional cell migration. Using a novel mechanism by which Dia1 mRNA is targeted to the perinuclear endoplasmic reticulum, we redirected the mRNA encoding Arp2, a subunit of the Arp2/3 complex, to the perinuclear region in fibroblasts. Knockdown of Arp2 alone caused dramatic reduction of the complex and resulted in narrow protrusions, increased random cell migration speed and loss of directionality. Rescue with a protrusion-localizing Arp2 mRNA restored normal cell migration behavior, whereas rescue with a mis-localizing Arp2 mRNA failed to restore speed and directionality. These results demonstrate that localization of Arp2/3 complex mRNAs in the leading protrusions is functionally important for directional cell migration.

Purification of actin from fission yeast Schizosaccharomyces pombe and characterization of functional differences from muscle actin

Fission yeast Schizosaccharomyces pombe is an important genetic model organism for studying the mechanisms of endocytosis and cytokinesis. However, most work on the biochemical properties of fission yeast actin-binding proteins has been done with skeletal muscle actin for matters of convenience. When simulations of mathematical models of the mechanism of endocytosis were compared with events in live cells, some of the reactions appeared to be much faster than observed in biochemical experiments with muscle actin. Here, we used gelsolin affinity chromatography to purify actin from fission yeast. S. pombe actin shares many properties with skeletal muscle actin but has higher intrinsic nucleotide exchange rate, faster trimer nucleus formation, faster phosphate dissociation rate from polymerized actin, and faster nucleation of actin filaments with Arp2/3 complex. These properties close the gap between the biochemistry and predictions made by mathematical models of endocytosis in S. pombe cells.

Actin dynamics in plant cells: a team effort from multiple proteins orchestrates this very fast-paced game

Gazing at a giant redwood tree in the Pacific Northwest, that has grown to enormous heights over centuries, does little to convince one that plants are built for speed and versatility. Even at the cellular level, a system of polymers-the cell skeleton or cytoskeleton-integrates signals and generates subcellular structures spanning scales of a few nanometers to hundreds of micrometers that coordinate cell growth. The term cytoskeleton itself connotes a stable structure. Clearly, this is not the case. Recent studies using advanced imaging modalities reveal the plant actin cytoskeleton to be a highly dynamic, ever changing assemblage of polymers. These insights along with growing evidence about the biochemical/biophysical properties of plant cytoskeletal polymers, especially those obtained by single filament imaging and reconstituted systems of purified proteins analyzed by total internal reflection fluorescence microscopy, allow the generation of a unique model for the dynamic turnover of actin filaments, termed stochastic dynamics. Here, we review several significant advances and highlight opportunities that will position plants at the vanguard of research on actin organization and turnover. A challenge for the future will be to apply the power of reverse-genetics in several model organisms to test the molecular details of this new model.

Estrogen receptor-{alpha} promotes endothelial cell motility through focal adhesion kinase

Sex steroids play a key role in cell movement and tissue organization. Cell migration requires the integration of events that induce changes in cell structure such as protrusion, polarization and traction toward the direction of migration. These actions are driven by actin remodeling and are stabilized by the development of adhesion sites to extracellular matrix via transmembrane receptors linked to the actin cytoskeleton. Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that facilitates cell migration via the control of the turnover of focal adhesion complexes. In this work, we demonstrated that 17β-estradiol (E(2)) regulates actin remodeling and cell movement in human umbilical vein endothelial cells through the recruitment of FAK. E(2) induces phosphorylation of FAK and its translocation toward membrane sites where focal adhesion complexes are assembled. This process is triggered via a Gα/Gβ protein-dependent, rapid extra-nuclear signaling of estrogen receptor-α (ERα) that interacts in a multiprotein complex with c-Src, phosphatidylinositol 3-OH kinase and FAK. Phosphorylation of FAK is fundamental for its activation, translocation to the plasmatic membrane and the subsequent formation of focal adhesion complexes. In conclusion, we found that ERα enhances endothelial cell motility through the dynamic control of actin arrangement and the formation of focal adhesion complexes. The identification of these processes broadens the understanding of the actions of estrogens on endothelial cells and could be relevant in physiological or pathological settings.

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.

I-BAR domain proteins: linking actin and plasma membrane dynamics

Dynamic plasma membrane rearrangements occur during many cellular processes including endocytosis, morphogenesis, and migration. Actin polymerization together with proteins that directly deform membranes, such as the BAR superfamily proteins, is essential for generation of membrane invaginations during endocytosis. Importantly, recent studies revealed that direct membrane deformation contributes also to the formation of plasma membrane protrusions such as filopodia and lamellipodia. Inverse BAR (I-BAR) domain proteins bind phosphoinositide-rich membrane with high affinity and generate negative membrane curvature to induce plasma membrane protrusions. I-BAR domain proteins, such as IRSp53, MIM, ABBA, and IRTKS also harbor many protein-protein interaction modules that link them to actin dynamics. Thus, I-BAR domain proteins may connect direct membrane deformation to actin polymerization in cell morphogenesis and migration.

Characterization of engineered actin binding proteins that control filament assembly and structure

BACKGROUND

Eukaryotic cells strictly regulate the structure and assembly of their actin filament networks in response to various stimuli. The actin binding proteins that control filament assembly are therefore attractive targets for those who wish to reorganize actin filaments and reengineer the cytoskeleton. Unfortunately, the naturally occurring actin binding proteins include only a limited set of pointed-end cappers, or proteins that will block polymerization from the slow-growing end of actin filaments. Of the few that are known, most are part of large multimeric complexes that are challenging to manipulate.

METHODOLOGY/PRINCIPAL FINDINGS

We describe here the use of phage display mutagenesis to generate of a new class of binding protein that can be targeted to the pointed-end of actin. These proteins, called synthetic antigen binders (sABs), are based on an antibody-like scaffold where sequence diversity is introduced into the binding loops using a novel "reduced genetic code" phage display library. We describe effective strategies to select and screen for sABs that ensure the generated sABs bind to the pointed-end surface of actin exclusively.

CONCLUSIONS/SIGNIFICANCE

From our set of pointed-end binders, we identify three sABs with particularly useful properties to systematically probe actin dynamics: one protein that caps the pointed end, a second that crosslinks actin filaments, and a third that severs actin filaments and promotes disassembly.

Intracellular transport of viruses and their components: utilizing the cytoskeleton and membrane highways

Plant viruses are obligate organisms that require host components for movement within and between cells. A mechanistic understanding of virus movement will allow the identification of new methods to control virus systemic spread and serve as a model system for understanding host macromolecule intra- and intercellular transport. Recent studies have moved beyond the identification of virus proteins involved in virus movement and their effect on plasmodesmal size exclusion limits to the analysis of their interactions with host components to allow movement within and between cells. It is clear that individual virus proteins and replication complexes associate with and, in some cases, traffic along the host cytoskeleton and membranes. Here, we review these recent findings, highlighting the diverse associations observed between these components and their trafficking capacity. Plant viruses operate individually, sometimes within virus species, to utilize unique interactions between their proteins or complexes and individual host cytoskeletal or membrane elements over time or space for their movement. However, there is not sufficient information for any plant virus to create a complete model of its intracellular movement; thus, more research is needed to achieve that goal.

Dynamic regulation of sarcomeric actin filaments in striated muscle

In striated muscle, the actin cytoskeleton is differentiated into myofibrils. Actin and myosin filaments are organized in sarcomeres and specialized for producing contractile forces. Regular arrangement of actin filaments with uniform length and polarity is critical for the contractile function. However, the mechanisms of assembly and maintenance of sarcomeric actin filaments in striated muscle are not completely understood. Live imaging of actin in striated muscle has revealed that actin subunits within sarcomeric actin filaments are dynamically exchanged without altering overall sarcomeric structures. A number of regulators for actin dynamics have been identified, and malfunction of these regulators often result in disorganization of myofibril structures or muscle diseases. Therefore, proper regulation of actin dynamics in striated muscle is critical for assembly and maintenance of functional myofibrils. Recent studies have suggested that both enhancers of actin dynamics and stabilizers of actin filaments are important for sarcomeric actin organization. Further investigation of the regulatory mechanism of actin dynamics in striated muscle should be a key to understanding how myofibrils develop and operate.

Human Ermin (hErmin), a new oligodendrocyte-specific cytoskeletal protein related to epileptic seizure

During the maturation of oligodendrocytes, cells are characterized by their morphological changes such as the number of process extensions and sheet-like membranes. This process relies critically on cytoskeleton rearrangement, but the molecular mechanisms underlying this are still unclear. Here, we identify human Ermin (hErmin), a novel cytoskeletal molecule that is expressed exclusively in oligodendrocytes in human brain, as a regulator of cytoskeleton rearrangement. In vitro, full-length hErmin expression, but not its truncated mutants lacking the actin-binding domain, promote arborization of cultured COS-7 cells and induce marked changes in cell morphology. The most important is that expression of hErmin in specimens of epileptic patients is much lower than that of control, implying that hErmin may be involved in epileptogenic process. These findings suggest a role for hErmin as a novel cytoskeleton-related oligodendroglial protein in human brain myelination and human epileptogenesis, and provide new evidence for the relationship between oligodendrocytes and epilepsy.

Direct dynamin-actin interactions regulate the actin cytoskeleton

The large GTPase dynamin assembles into higher order structures that are thought to promote endocytosis. Dynamin also regulates the actin cytoskeleton through an unknown, GTPase-dependent mechanism. Here, we identify a highly conserved site in dynamin that binds directly to actin filaments and aligns them into bundles. Point mutations in the actin-binding domain cause aberrant membrane ruffling and defective actin stress fibre formation in cells. Short actin filaments promote dynamin assembly into higher order structures, which in turn efficiently release the actin-capping protein (CP) gelsolin from barbed actin ends in vitro, allowing for elongation of actin filaments. Together, our results support a model in which assembled dynamin, generated through interactions with short actin filaments, promotes actin polymerization via displacement of actin-CPs.

Keratins in health and cancer: more than mere epithelial cell markers

Keratins are the intermediate filament (IF)-forming proteins of epithelial cells. Since their initial characterization almost 30 years ago, the total number of mammalian keratins has increased to 54, including 28 type I and 26 type II keratins. Keratins are obligate heteropolymers and, similarly to other IFs, they contain a dimeric central α-helical rod domain that is flanked by non-helical head and tail domains. The 10-nm keratin filaments participate in the formation of a proteinaceous structural framework within the cellular cytoplasm and, as such, serve an important role in epithelial cell protection from mechanical and non-mechanical stressors, a property extensively substantiated by the discovery of human keratin mutations predisposing to tissue-specific injury and by studies in keratin knockout and transgenic mice. More recently, keratins have also been recognized as regulators of other cellular properties and functions, including apico-basal polarization, motility, cell size, protein synthesis and membrane traffic and signaling. In cancer, keratins are extensively used as diagnostic tumor markers, as epithelial malignancies largely maintain the specific keratin patterns associated with their respective cells of origin, and, in many occasions, full-length or cleaved keratin expression (or lack there of) in tumors and/or peripheral blood carries prognostic significance for cancer patients. Quite intriguingly, several studies have provided evidence for active keratin involvement in cancer cell invasion and metastasis, as well as in treatment responsiveness, and have set the foundation for further exploration of the role of keratins as multifunctional regulators of epithelial tumorigenesis.

Evolutionary selection of new breast cancer cell-targeting peptides and phages with the cell-targeting peptides fully displayed on the major coat and their effects on actin dynamics during cell internalization

Filamentous phage as a bacteria-specific virus can be conjugated with an anticancer drug and has been proposed to serve as a carrier to deliver drugs to cancer cells for targeted therapy. However, how cell-targeting filamentous phage alone affects cancer cell biology is unclear. Phage libraries provide an inexhaustible reservoir of new ligands against tumor cells and tissues that have potential therapeutic and diagnostic applications in cancer treatment. Some of these identified ligands might stimulate various cell responses. Here we identified new cell internalizing peptides (and the phages with such peptides fused to each of ~3900 copies of their major coat protein) using landscape phage libraries and for the first time investigated the actin dynamics when selected phages are internalized into the SKBR-3 breast cancer cells. Our results show that phages harboring VSSTQDFP and DGSIPWST peptides could selectively internalize into the SKBR-3 breast cancer cells with high affinity, and also show rapid involvement of membrane ruffling and rearrangements of actin cytoskeleton during the phage entry. The actin dynamics was studied by using live cell and fluorescence imaging. The cell-targeting phages were found to enter breast cancer cells through energy dependent mechanism and phage entry interferes with actin dynamics, resulting in reorganization of actin filaments and increased membrane rufflings in SKBR-3 cells. These results suggest that, when phage enters epithelial cells, it triggers transient changes in the host cell actin cytoskeleton. This study also shows that using multivalent phage libraries considerably increases the repertoire of available cell-internalizing ligands with potential applications in targeted drug delivery, imaging, molecular monitoring and profiling of breast cancer cells.

Cell-matrix interactions in mammary gland development and breast cancer

The mammary gland is an organ that at once gives life to the young, but at the same time poses one of the greatest threats to the mother. Understanding how the tissue develops and functions is of pressing importance in determining how its control mechanisms break down in breast cancer. Here we argue that the interactions between mammary epithelial cells and their extracellular matrix (ECM) are crucial in the development and function of the tissue. Current strategies for treating breast cancer take advantage of our knowledge of the endocrine regulation of breast development, and the emerging role of stromal-epithelial interactions (Fig. 1). Focusing, in addition, on the microenvironmental influences that arise from cell-matrix interactions will open new opportunities for therapeutic intervention. We suggest that ultimately a three-pronged approach targeting endocrine, growth factor, and cell-matrix interactions will provide the best chance of curing the disease.

Conformational dynamics of actin: effectors and implications for biological function

Actin is a protein abundant in many cell types. Decades of investigations have provided evidence that it has many functions in living cells. The diverse morphology and dynamics of actin structures adapted to versatile cellular functions is established by a large repertoire of actin-binding proteins. The proper interactions with these proteins assume effective molecular adaptations from actin, in which its conformational transitions play essential role. This review attempts to summarise our current knowledge regarding the coupling between the conformational states of actin and its biological function.

Expression of the actin-binding proteins indicates that cofilin and fascin are related to breast tumour size

This study was designed to investigate the expression of four actin-binding proteins, alpha-actinin-4, cofilin 1, fascin and elongation factor 1-alpha 2 (eEF1A2), in samples of breast cancer from 112 patients with different stages of breast cancer (stages T0 - T1, T2 and T3) compared with normal control tissues (n = 33). Levels of eEF1A2 and alpha-actinin-4 mRNA appeared to be unrelated to tumour size, except for a significant down-regulation of alpha-actinin-4 mRNA in T3 cases. Significant up-regulation of cofilin 1 mRNA was associated with stages T0 - T1 and T2; up-regulation seen at stage T3 was not significant compared with control tissue. Fascin mRNA levels were significantly reduced at all three tumour stages (T0 - T1, T2 and T3) compared with control tissue. In conclusion, some components of the actin cytoskeletal system might hold significant potential as targets in future cancer therapies.

A QUICK screen for Lrrk2 interaction partners--leucine-rich repeat kinase 2 is involved in actin cytoskeleton dynamics

Mutations in human leucine-rich repeat kinase 2 (Lrrk2), a protein of yet unknown function, are linked to Parkinson's disease caused by degeneration of midbrain dopaminergic neurons. The protein comprises several domains including a GTPase and a kinase domain both affected by several pathogenic mutations. To elucidate the molecular interaction network of endogenous Lrrk2 under stoichiometric constraints, we applied QUICK (quantitative immunoprecipitation combined with knockdown) in NIH3T3 cells. The identified interactome reveals actin isoforms as well as actin-associated proteins involved in actin filament assembly, organization, rearrangement, and maintenance, suggesting that the biological function of Lrrk2 is linked to cytoskeletal dynamics. In fact, we demonstrate Lrrk2 de novo binding to F-actin and its ability to modulate its assembly in vitro. When tested in intact cells, knockdown of Lrrk2 causes morphological alterations in NIH3T3 cells. In developing dopaminergic midbrain primary neurons, Lrrk2 knockdown results in shortened neurite processes, indicating a physiological role of Lrrk2 in cytoskeletal organization and dynamics of dopaminergic neurons. Hence, our results demonstrate that molecular interactions as well as the physiological function of Lrrk2 are closely related to the organization of the actin-based cytoskeleton, a crucial feature of neuronal development and neuron function.

Self-assembly of filopodia-like structures on supported lipid bilayers

Filopodia are finger-like protrusive structures, containing actin bundles. By incubating frog egg extracts with supported lipid bilayers containing phosphatidylinositol 4,5 bisphosphate, we have reconstituted the assembly of filopodia-like structures (FLSs). The actin assembles into parallel bundles, and known filopodial components localize to the tip and shaft. The filopodia tip complexes self-organize--they are not templated by preexisting membrane microdomains. The F-BAR domain protein toca-1 recruits N-WASP, followed by the Arp2/3 complex and actin. Elongation proteins, Diaphanous-related formin, VASP, and fascin are recruited subsequently. Although the Arp2/3 complex is required for FLS initiation, it is not essential for elongation, which involves formins. We propose that filopodia form via clustering of Arp2/3 complex activators, self-assembly of filopodial tip complexes on the membrane, and outgrowth of actin bundles.

Which way to go? Cytoskeletal organization and polarized transport in neurons

To establish and maintain their polarized morphology, neurons employ active transport driven by cytoskeletal motor proteins to sort cargo between axons and dendrites. These motors can move in a specific direction over either microtubules (kinesins, dynein) or actin filaments (myosins). The basic traffic rules governing polarized transport on the neuronal cytoskeleton have long remained unclear, but recent work has revealed several fundamental sorting principles based on differences in the cytoskeletal organization in axons versus dendrites. We will highlight the basic characteristics of the neuronal cytoskeleton and review existing evidence for microtubule and actin based traffic rules in polarized neuronal transport. We will propose a model in which polarized sorting of cargo is established by recruiting or activating the proper subset of motor proteins, which are subsequently guided to specific directions by the polarized organization of the neuronal cytoskeleton.

Evolution of diverse cell division and vesicle formation systems in Archaea

Recently a novel cell division system comprised of homologues of eukaryotic ESCRT-III (endosomal sorting complex required for transport III) proteins was discovered in the hyperthermophilic crenarchaeote Sulfolobus acidocaldarius. On the basis of this discovery, we undertook a comparative genomic analysis of the machineries for cell division and vesicle formation in Archaea. Archaea possess at least three distinct membrane remodelling systems: the FtsZ-based bacterial-type system, the ESCRT-III-based eukaryote-like system and a putative novel system that uses an archaeal actin-related protein. Many archaeal genomes encode assortments of components from different systems. Evolutionary reconstruction from these findings suggests that the last common ancestor of the extant Archaea possessed a complex membrane remodelling apparatus, different components of which were lost during subsequent evolution of archaeal lineages. By contrast, eukaryotes seem to have inherited all three ancestral systems.

Physiological activation of synaptic Rac>PAK (p-21 activated kinase) signaling is defective in a mouse model of fragile X syndrome

The abnormal spine morphology found in fragile X syndrome (FXS) is suggestive of an error in the signaling cascades that organize the actin cytoskeleton. We report here that physiological activation of the small GTPase Rac1 and its effector p-21 activated kinase (PAK), two enzymes critically involved in actin management and functional synaptic plasticity, is impaired at hippocampal synapses in the Fmr1-knock-out (KO) mouse model of FXS. Theta burst afferent stimulation (TBS) caused a marked increase in the number of synapses associated with phosphorylated PAK in adult hippocampal slices from wild-type, but not Fmr1-KO, mice. Stimulation-induced activation of synaptic Rac1 was also absent in the mutants. The polymerization of spine actin that occurs immediately after theta stimulation appeared normal in mutant slices but the newly formed polymers did not properly stabilize, as evidenced by a prolonged vulnerability to a toxin (latrunculin) that disrupts dynamic actin filaments. Latrunculin also reversed long-term potentiation when applied at 10 min post-TBS, a time point at which the potentiation effect is resistant to interference in wild-type slices. We propose that a Rac>PAK signaling pathway needed for rapid stabilization of activity-induced actin filaments, and thus for normal spine morphology and lasting synaptic changes, is defective in FXS.

Coarse-Grained Representations of Large Biomolecular Complexes from Low-Resolution Structural Data

High-resolution atomistic structures of many large biomolecular complexes have not yet been solved by experiments, such as X-ray crystallography or NMR. Often however low-resolution information is obtained by alternative techniques, such as cryo-electron microscopy or small-angle X-ray scattering. Coarse-grained (CG) models are an appropriate choice to computationally study these complexes given the limited resolution experimental data. One of the important questions therefore is how to define CG representations from these low-resolution density maps. This work provides a space-based essential dynamics coarse-graining (ED-CG) method to define a CG representation from a density map without detailed knowledge of its underlying atomistic structure and primary sequence information. This method is demonstrated on G-actin (both the atomic structure and its density map). It is then applied to the density maps of the Escherichia coli 70S ribosome and the microtubule. The results indicate that the method can define highly CG models that still preserve functionally important dynamics of large biomolecular complexes.

Involvement of cytoskeleton-associated proteins in the commitment of C3H10T1/2 pluripotent stem cells to adipocyte lineage induced by BMP2/4

The developmental pathway that gives rise to mature adipocytes involves two distinct stages: commitment and terminal differentiation. Although the important proteins/factors contributing to terminal adipocyte differentiation have been well defined, the proteins/factors in the commitment of mesenchymal stem cells to the adipocyte lineage cells have not. In this study, we applied proteomics analysis profiling to characterize differences between uncommitted C3H10T1/2 pluripotent stem cells and those that have been committed to the adipocyte lineage by BMP4 or BMP2 with the goal to identify such proteins/factors and to understand the molecular mechanisms that govern the earliest stages of adipocyte lineage commitment. Eight proteins were found to be up-regulated by BMP2, and 27 proteins were up-regulated by BMP4, whereas five unique proteins were up-regulated at least 10-fold by both BMP2/4, including three cytoskeleton-associated proteins (i.e. lysyl oxidase (LOX), translationally controlled tumor protein 1 (TPT1), and αB-crystallin). Western blotting further confirmed the induction of the expression of these cytoskeleton-associated proteins in the committed C3H10T1/2 induced by BMP2/4. Importantly, knockdown of LOX expression totally prevented the commitment, whereas knockdown of TPT1 and αB-crystallin expression partially inhibited the commitment. Several published reports suggest that cell shape can influence the differentiation of partially committed precursors of adipocytes, osteoblasts, and chondrocytes. We observed a dramatic change of cell shape during the commitment process, and we showed that knockdown of these cytoskeleton-associated proteins prevented the cell shape change and restored F-actin organization into stress fibers and inhibited the commitment to the adipocyte lineage. Our studies indicate that these differentially expressed cytoskeleton-associate proteins might determine the fate of mesenchymal stem cells to commit to the adipocyte lineage through cell shape regulation.

Trypanosoma cruzi: multiple actin isovariants are observed along different developmental stages

The expression and biological role of actin during the Trypanosoma cruzi life cycle remains largely unknown. Polyclonal antibodies against a recombinant T. cruzi actin protein were used to confirm its expression in epimastigotes, trypomastigotes, and amastigotes. Although the overall levels of expression were similar, clear differences in the subcellular distribution of actin among the developmental stages were identified. The existence of five actin variants in each developmental stage with distinct patterns of expression were uncovered by immunoblotting of protein extracts separated 2D-SDS gels. The isoelectric points of the actin variants in epimastigotes ranged from 4.45 to 4.9, whereas they ranged from 4.9 to 5.24 in trypomastigotes and amastigotes. To determine if the actin variants found could represent previously unidentified actins, we performed a genomic survey of the T.cruzi GeneDB database and found 12 independent loci encoding for a diverse group of actins and actin-like proteins that are conserved among trypanosomatids.

Toxic effects of iron oxide nanoparticles on human umbilical vein endothelial cells

Iron oxide nanoparticles (IONPs) have been employed for hyperthermia treatments, stem cell therapies, cell labeling, and imaging modalities. The biocompatibility and cytotoxic effects of iron oxide nanoparticles when used in biomedical applications, however, are an ongoing concern. Endothelial cells have a critical role in this research dealing with tumors, cardiovascular disease and inflammation. However, there is little information dealing with the biologic effects of IONPs on the endothelial cell. This paper deals with the influence of dextran and citric acid coated IONPs on the behavior and function of human umbilical vein endothelial cells (HUVECs). After exposing endothelial cells to IONPs, dose-dependent effects on HUVECs viability, cytoskeleton and function were determined. Both citric acid and dextran coated particles appeared to be largely internalized by HUVECs through endocytosis and contribute to eventual cell death possibly by apoptosis. Cytoskeletal structures were greatly disrupted, as evidenced by diminished vinculin spots, and disorganized actin fiber and tubulin networks. The capacity of HUVECs to form a vascular network on Matrigel™ diminished after exposure to IONPs. Cell migration/invasion were inhibited significantly even at very low iron concentrations (0.1 mM). The results of this study indicate the great importance of thoroughly understanding nanoparticle-cell interactions, and the potential to exploit this understanding in tumor therapy applications involving IONPs as thermo/chemoembolization agents.

A new method for direct detection of the sites of actin polymerization in intact cells and its application to differentiated vascular smooth muscle

Here we report and validate a new method, suitable broadly, for use in differentiated cells and tissues, for the direct visualization of actin polymerization under physiological conditions. We have designed and tested different versions of fluorescently labeled actin, reversibly attached to the protein transduction tag TAT, and have introduced this novel reagent into intact differentiated vascular smooth muscle cells (dVSMCs). A thiol-reactive version of the TAT peptide was synthesized by adding the amino acids glycine and cysteine to its NH(2)-terminus and forming a thionitrobenzoate adduct: viz. TAT-Cys-S-STNB. This peptide reacts readily with G-actin, and the complex is rapidly taken up by freshly enzymatically isolated dVSMC, as indicated by the fluorescence of a FITC tag on the TAT peptide. By comparing different versions of the construct, we determined that the optimal construct for biological applications is a nonfluorescently labeled TAT peptide conjugated to rhodamine-labeled actin. When TAT-Cys-S-STNB-tagged rhodamine actin (TSSAR) was added to live, freshly enzymatically isolated cells, we observed punctae of incorporated actin at the cortex of the cell. The punctae are indistinguishable from those we have previously reported to occur in the same cell type when rhodamine G-actin is added to permeabilized cells. Thus this new method allows the delivery of labeled G-actin into intact cells without disrupting the native state and will allow its further use to study the effect of physiological intracellular Ca(2+) concentration transients and signal transduction on actin dynamics in intact cells.

Arabidopsis VILLIN5, an actin filament bundling and severing protein, is necessary for normal pollen tube growth

A dynamic actin cytoskeleton is essential for pollen germination and tube growth. However, the molecular mechanisms underlying the organization and turnover of the actin cytoskeleton in pollen remain poorly understood. Villin plays a key role in the formation of higher-order structures from actin filaments and in the regulation of actin dynamics in eukaryotic cells. It belongs to the villin/gelsolin/fragmin superfamily of actin binding proteins and is composed of six gelsolin-homology domains at its core and a villin headpiece domain at its C terminus. Recently, several villin family members from plants have been shown to sever, cap, and bundle actin filaments in vitro. Here, we characterized a villin isovariant, Arabidopsis thaliana VILLIN5 (VLN5), that is highly and preferentially expressed in pollen. VLN5 loss-of-function retarded pollen tube growth and sensitized actin filaments in pollen grains and tubes to latrunculin B. In vitro biochemical analyses revealed that VLN5 is a typical member of the villin family and retains a full suite of activities, including barbed-end capping, filament bundling, and calcium-dependent severing. The severing activity was confirmed with time-lapse evanescent wave microscopy of individual actin filaments in vitro. We propose that VLN5 is a major regulator of actin filament stability and turnover that functions in concert with oscillatory calcium gradients in pollen and therefore plays an integral role in pollen germination and tube growth.

The kinetics of cooperative cofilin binding reveals two states of the cofilin-actin filament

The interaction of cofilin with actin filaments displays positive cooperativity. The equilibrium binding and associated thermodynamic properties of this interaction are well described by a simple, one-dimensional Ising model with nearest neighbor interactions. Here we evaluate the kinetic contributions to cooperative binding and the ability of this model to account for binding across a wide range of cofilin concentrations. A Monte Carlo-based simulation protocol that allows for nearest-neighbor interactions between adjacent binding sites was used to globally fit time courses of human cofilin binding to human nonmuscle (beta-, gamma-) actin filaments. Several extensions of the one-dimensional Ising model were tested, and a mechanism that includes isomerization of the actin filament was found to best account for time courses of association as well as irreversible dissociation from a saturated filament. This model predicts two equilibrium states of the cofilin-actin, or cofilactin, filament, and the resulting set of binding parameters are in agreement with equilibrium thermodynamic parameters. We conclude that despite its simplicity, this one-dimensional Ising model is a reliable model for analyzing and interpreting the energetics and kinetics of cooperative cofilin-actin filament interactions. The model predicts that severing activity associated with boundaries between bare and decorated segments will not be linear, but display a transient burst at short times on cofilin activation then dissipate due to a kinetic competition between severing activity and cofilin binding. A second peak of severing activity is predicted to arise from irreversible cofilin dissociation on inactivation. These behaviors predict what we believe to be novel mechanisms of cofilin severing and spatial regulation of actin filament turnover in cells. The methods developed for this system are generally applicable to the kinetic analysis of cooperative ligand binding to linear polymers.

Pushing and pulling in prokaryotic DNA segregation

In prokaryotes, DNA can be segregated by three different types of cytoskeletal filaments. The best-understood type of partitioning (par) locus encodes an actin homolog called ParM, which forms dynamically unstable filaments that push plasmids apart in a process reminiscent of mitosis. However, the most common type of par locus, which is present on many plasmids and most bacterial chromosomes, encodes a P loop ATPase (ParA) that distributes plasmids equidistant from one another on the bacterial nucleoid. A third type of par locus encodes a tubulin homolog (TubZ) that forms cytoskeletal filaments that move rapidly with treadmill dynamics.

Directed 3D cell alignment and elongation in microengineered hydrogels

Organized cellular alignment is critical to controlling tissue microarchitecture and biological function. Although a multitude of techniques have been described to control cellular alignment in 2D, recapitulating the cellular alignment of highly organized native tissues in 3D engineered tissues remains a challenge. While cellular alignment in engineered tissues can be induced through the use of external physical stimuli, there are few simple techniques for microscale control of cell behavior that are largely cell-driven. In this study we present a simple and direct method to control the alignment and elongation of fibroblasts, myoblasts, endothelial cells and cardiac stem cells encapsulated in microengineered 3D gelatin methacrylate (GelMA) hydrogels, demonstrating that cells with the intrinsic potential to form aligned tissues in vivo will self-organize into functional tissues in vitro if confined in the appropriate 3D microarchitecture. The presented system may be used as an in vitro model for investigating cell and tissue morphogenesis in 3D, as well as for creating tissue constructs with microscale control of 3D cellular alignment and elongation, that could have great potential for the engineering of functional tissues with aligned cells and anisotropic function.

Delta1 family members are involved in filopodial actin formation and neuronal cell migration independent of Notch signaling

Delta family proteins are transmembrane molecules that bind Notch receptors and activate downstream signaling events in neighboring cells. In addition to serving as Notch ligands, Notch-independent roles for Delta have been suggested but are not fully understood. Here, we demonstrate a previously unrecognized role for Delta in filopodial actin formation. Delta1 and Delta4, but not Delta3, exhibit filopodial protrusive activity, and this activity is independent of Notch signaling. The filopodial activity of Delta1 does not depend on the PDZ-binding domain at the C-terminus; however, the intracellular membrane-proximal region that is anchored to the plasma membrane plays an important role in filopodial activity. We further identified a Notch-independent role of DeltaD in neuronal cell migration in zebrafish. These findings suggest a possible functional link between Notch-independent filopodial activity of Delta and the control of cell motility.

Structures of actin-bound Wiskott-Aldrich syndrome protein homology 2 (WH2) domains of Spire and the implication for filament nucleation

Three classes of proteins are known to nucleate new filaments: the Arp2/3 complex, formins, and the third group of proteins that contain ca. 25 amino acid long actin-binding Wiskott-Aldrich syndrome protein homology 2 domains, called the WH2 repeats. Crystal structures of the complexes between the actin-binding WH2 repeats of the Spire protein and actin were determined for the Spire single WH2 domain D, the double (SpirCD), triple (SpirBCD), quadruple (SpirABCD) domains, and an artificial Spire WH2 construct comprising three identical D repeats (SpirDDD). SpirCD represents the minimal functional core of Spire that can nucleate actin filaments. Packing in the crystals of the actin complexes with SpirCD, SpirBCD, SpirABCD, and SpirDDD shows the presence of two types of assemblies, "side-to-side" and "straight-longitudinal," which can serve as actin filament nuclei. The principal feature of these structures is their loose, open conformations, in which the sides of actins that normally constitute the inner interface core of a filament are flipped inside out. These Spire structures are distant from those seen in the filamentous nuclei of Arp2/3, formins, and in the F-actin filament.

Towards the application of cytoskeletal motor proteins in molecular detection and diagnostic devices

Over the past ten years, great advancements have been made towards using biomolecular motors for nanotechnological applications. In particular, devices using cytoskeletal motor proteins for molecular transport are maturing. First efforts towards designing such devices used motor proteins attached to micro-structured substrates for the directed transport of microtubules and actin filaments. Soon thereafter, the specific capture, transport and detection of target analytes like viruses were demonstrated. Recently, spatial guiding of the gliding filaments was added to increase the sensitivity of detection and allow parallelization. Whereas molecular motor powered devices have not yet demonstrated performance beyond the level of existing detection techniques, the potential is great: Replacing microfluidics with transport powered by molecular motors allows integration of the energy source (ATP) into the assay solution. This opens up the opportunity to design highly integrated, miniaturized, autonomous detection devices. Such devices, in turn, may allow fast and cheap on-site diagnosis of diseases and detection of environmental pathogens and toxins.

Class XI myosins are required for development, cell expansion, and F-Actin organization in Arabidopsis

The actomyosin system is conserved throughout eukaryotes. Although F-actin is essential for cell growth and plant development, roles of the associated myosins are poorly understood. Using multiple gene knockouts in Arabidopsis thaliana, we investigated functional profiles of five class XI myosins, XI-K, XI-1, XI-2, XI-B, and XI-I. Plants lacking three myosins XI showed stunted growth and delayed flowering, whereas elimination of four myosins further exacerbated these defects. Loss of myosins led to decreased leaf cell expansion, with the most severe defects observed in the larger leaf cells. Root hair length in myosin-deficient plants was reduced approximately 10-fold, with quadruple knockouts showing morphological abnormalities. It was also found that trafficking of Golgi and peroxisomes was entirely myosin dependent. Surprisingly, myosins were required for proper organization of F-actin and the associated endoplasmic reticulum networks, revealing a novel, architectural function of the class XI myosins. These results establish critical roles of myosin-driven transport and F-actin organization during polarized and diffuse cell growth and indicate that myosins are key factors in plant growth and development.

Actin complexes in the cell nucleus: new stones in an old field

Actin is a well-known protein that has shown a myriad of activities in the cytoplasm. However, recent findings of actin involvement in nuclear processes are overwhelming. Actin complexes in the nucleus range from very dynamic chromatin-remodeling complexes to structural elements of the matrix with single partners known as actin-binding proteins (ABPs). This review summarizes the recent findings of actin-containing complexes in the nucleus. Particular attention is given to key processes like chromatin remodeling, transcription, DNA replication, nucleocytoplasmic transport and to actin roles in nuclear architecture. Understanding the mechanisms involving ABPs will definitely lead us to the principles of the regulation of gene expression performed via concerting nuclear and cytoplasmic processes.

A potential yeast actin allosteric conduit dependent on hydrophobic core residues val-76 and trp-79

Intramolecular allosteric interactions responsible for actin conformational regulation are largely unknown. Previous work demonstrated that replacing yeast actin Val-76 with muscle actin Ile caused decreased nucleotide exchange. Residue 76 abuts Trp-79 in a six-residue linear array beginning with Lys-118 on the surface and ending with His-73 in the nucleotide cleft. To test if altering the degree of packing of these two residues would affect actin dynamics, we constructed V76I, W79F, and W79Y single mutants as well as the Ile-76/Phe-79 and Ile-76/Tyr-79 double mutants. Tyr or Phe should decrease crowding and increase protein flexibility. Subsequent introduction of Ile should restore packing and dampen changes. All mutants showed decreased growth in liquid medium. W79Y alone was severely osmosensitive and exhibited vacuole abnormalities. Both properties were rescued by Ile-76. Phe-79 or Tyr decreased the thermostability of actin and increased its nucleotide exchange rate. These effects, generally greater for Tyr than for Phe, were reversed by introduction of Ile-76. HD exchange showed that the mutations caused propagated conformational changes to all four subdomains. Based on results from phosphate release and light-scattering assays, single mutations affected polymerization in the order of Ile, Phe, and Tyr from least to most. Introduction of Ile-76 partially rescued the polymerization defects caused by either Tyr-79 or Phe-79. Thus, alterations in crowding of the 76-79 residue pair can strongly affect actin conformation and behavior, and these results support the theory that the amino acid array in which they are located may play a central role in actin regulation.

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.

Cytoskeletal dynamics in fission yeast: a review of models for polarization and division

We review modeling studies concerning cytoskeletal activity of fission yeast. Recent models vary in length and time scales, describing a range of phenomena from cellular morphogenesis to polymer assembly. The components of cytoskeleton act in concert to mediate cell-scale events and interactions such as polarization. The mathematical models reduce these events and interactions to their essential ingredients, describing the cytoskeleton by its bulk properties. On a smaller scale, models describe cytoskeletal subcomponents and how bulk properties emerge.

Two competing orientation patterns explain experimentally observed anomalies in growing actin networks

The lamellipodium of migrating animal cells protrudes by directed polymerization of a branched actin network. The underlying mechanisms of filament growth, branching, and capping can be studied in in vitro assays. However, conflicting results have been reported for the force-velocity relation of such actin networks, namely both convex and concave shapes as well as history dependencies. Here we model branching as a reaction that is independent of the number of existing filaments, in contrast to capping, which is assumed to be proportional to the number of existing filaments. Using both stochastic network simulations and deterministic rate equations, we show that such a description naturally leads to the stability of two qualitatively different stationary states of the system, namely a +/- 35 degrees and a +70/0/-70 degrees orientation pattern. Changes in network growth velocity induce a transition between these two patterns. For sufficiently different protrusion efficiency of the two network architectures, this leads to hysteresis in the growth velocity of actin networks under force. Dependent on the history of the system, convex and concave regimes are obtained for the force-velocity relation. Thus a simple generic model can explain the experimentally observed anomalies, with far reaching consequences for cell migration.

RNA localization in neurite morphogenesis and synaptic regulation: current evidence and novel approaches

It is now generally accepted that RNA localization in the central nervous system conveys important roles both during development and in the adult brain. Of special interest is protein synthesis located at the synapse, as this potentially confers selective synaptic modification and has been implicated in the establishment of memories. However, the underlying molecular events are largely unknown. In this review, we will first discuss novel findings that highlight the role of RNA localization in neurons. We will focus on the role of RNA localization in neurotrophin signaling, axon outgrowth, dendrite and dendritic spine morphogenesis as well as in synaptic plasticity. Second, we will briefly present recent work on the role of microRNAs in translational control in dendrites and its implications for learning and memory. Finally, we discuss recent approaches to visualize RNAs in living cells and their employment for studying RNA trafficking in neurons.

Use of virtual cell in studies of cellular dynamics

The Virtual Cell (VCell) is a unique computational environment for modeling and simulation of cell biology. It has been specifically designed to be a tool for a wide range of scientists, from experimental cell biologists to theoretical biophysicists. The models created with VCell can range from the simple, to evaluate hypotheses or to interpret experimental data, to complex multilayered models used to probe the predicted behavior of spatially resolved, highly nonlinear systems. In this chapter, we discuss modeling capabilities of VCell and demonstrate representative examples of the models published by the VCell users.