A role of cofilin/destrin in reorganization of actin cytoskeleton in response to stresses and cell stimuli

Bioenergetic and excitotoxic determinants of cofilactin rod formation

Cofilactin rods (CARs), which are 1:1 aggregates of cofilin-1 and actin, lead to neurite loss in ischemic stroke and other disorders. The biochemical pathways driving CAR formation are well-established, but how these pathways are engaged under ischemic conditions is less clear. Brain ischemia produces both ATP depletion and glutamate excitotoxicity, both of which have been shown to drive CAR formation in other settings. Here, we show that CARs are formed in cultured neurons exposed to ischemia-like conditions: oxygen-glucose deprivation (OGD), glutamate, or oxidative stress. Of these conditions, only OGD produced significant ATP depletion, showing that ATP depletion is not required for CAR formation. Moreover, the OGD-induced CAR formation was blocked by the glutamate receptor antagonists MK-801 and kynurenic acid; the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase inhibitors GSK2795039 and apocynin; as well as an ROS scavenger. The findings identify a biochemical pathway leading from OGD to CAR formation in which the glutamate release induced by energy failure leads to activation of neuronal glutamate receptors, which in turn activates NADPH oxidase to generate oxidative stress and CARs.

Characterization of a Human Neuronal Culture System for the Study of Cofilin-Actin Rod Pathology

Cofilactin rod pathology, which can initiate synapse loss, has been extensively studied in rodent neurons, hippocampal slices, and in vivo mouse models of human neurodegenerative diseases such as Alzheimer's disease (AD). In these systems, rod formation induced by disease-associated factors, such as soluble oligomers of Amyloid-β (Aβ) in AD, utilizes a pathway requiring cellular prion protein (PrPC), NADPH oxidase (NOX), and cytokine/chemokine receptors (CCR5 and/or CXCR4). However, rod pathways have not been systematically assessed in a human neuronal model. Here, we characterize glutamatergic neurons differentiated from human-induced pluripotent stem cells (iPSCs) for the formation of rods in response to activators of the PrPC-dependent pathway. Optimization of substratum, cell density, and use of glial-conditioned medium yielded a robust system for studying the development of Aβ-induced rods in the absence of glia, suggesting a cell-autonomous pathway. Rod induction in younger neurons requires ectopic expression of PrPC, but this dependency disappears by Day 55. The quantification of proteins within the rod-inducing pathway suggests that increased PrPC and CXCR4 expression may be factors in the doubling of the rod response to Aβ between Days 35 and 55. FDA-approved antagonists to CXCR4 and CCR5 inhibit the rod response. Rods were predominantly observed in dendrites, although severe cytoskeletal disruptions prevented the assignment of over 40% of the rods to either an axon or dendrite. In the absence of glia, a condition in which rods are more readily observed, neurons mature and fire action potentials but do not form functional synapses. However, PSD95-containing dendritic spines associate with axonal regions of pre-synaptic vesicles containing the glutamate transporter, VGLUT1. Thus, our results identified stem cell-derived neurons as a robust model for studying cofilactin rod formation in a human cellular environment and for developing effective therapeutic strategies for the treatment of dementias arising from multiple proteinopathies with different rod initiators.

Formation of Cytoplasmic Actin-Cofilin Rods is Triggered by Metabolic Stress and Changes in Cellular pH

Actin dynamics plays a crucial role in regulating essential cell functions and thereby is largely responsible to a considerable extent for cellular energy consumption. Certain pathological conditions in humans, like neurological disorders such as Alzheimer’s disease or amyotrophic lateral sclerosis (ALS) as well as variants of nemaline myopathy are associated with cytoskeletal abnormalities, so-called actin-cofilin rods. Actin-cofilin rods are aggregates consisting mainly of actin and cofilin, which are formed as a result of cellular stress and thereby help to ensure the survival of cells under unfavorable conditions. We have used Dictyostelium discoideum, an established model system for cytoskeletal research to study formation and principles of cytoplasmic actin rod assembly in response to energy depletion. Experimentally, depletion of ATP was provoked by addition of either sodium azide, dinitrophenol, or 2-deoxy-glucose, and the formation of rod assembly was recorded by live-cell imaging. Furthermore, we show that hyperosmotic shock induces actin-cofilin rods, and that a drop in the intracellular pH accompanies this condition. Our data reveal that acidification of the cytoplasm can induce the formation of actin-cofilin rods to varying degrees and suggest that a local reduction in cellular pH may be a cause for the formation of cytoplasmic rods. We hypothesize that local phase separation mechanistically triggers the assembly of actin-cofilin rods and thereby influences the material properties of actin structures.

AD-Related N-Terminal Truncated Tau Is Sufficient to Recapitulate In Vivo the Early Perturbations of Human Neuropathology: Implications for Immunotherapy

The NH₂tau 26-44 aa (i.e., NH₂htau) is the minimal biologically active moiety of longer 20-22-kDa NH₂-truncated form of human tau-a neurotoxic fragment mapping between 26 and 230 amino acids of full-length protein (htau40)-which is detectable in presynaptic terminals and peripheral CSF from patients suffering from AD and other non-AD neurodegenerative diseases. Nevertheless, whether its exogenous administration in healthy nontransgenic mice is able to elicit a neuropathological phenotype resembling human tauopathies has not been yet investigated. We explored the in vivo effects evoked by subchronic intracerebroventricular (i.c.v.) infusion of NH₂htau or its reverse counterpart into two lines of young (2-month-old) wild-type mice (C57BL/6 and B6SJL). Six days after its accumulation into hippocampal parenchyma, significant impairment in memory/learning performance was detected in NH₂htau-treated group in association with reduced synaptic connectivity and neuroinflammatory response. Compromised short-term plasticity in paired-pulse facilitation paradigm (PPF) was detected in the CA3/CA1 synapses from NH₂htau-impaired animals along with downregulation in calcineurin (CaN)-stimulated pCREB/c-Fos pathway(s). Importantly, these behavioral, synaptotoxic, and neuropathological effects were independent from the genetic background, occurred prior to frank neuronal loss, and were specific because no alterations were detected in the control group infused with its reverse counterpart. Finally, a 2.0-kDa peptide which biochemically and immunologically resembles the injected NH₂htau was endogenously detected in vivo, being present in hippocampal synaptosomal preparations from AD subjects. Given that the identification of the neurotoxic tau species is mandatory to develop a more effective tau-based immunological approach, our evidence can have important translational implications for cure of human tauopathies.

Identification of biomarkers for childhood obesity based on expressional correlation and functional similarity

The aim of the current study was to identify potential biomarkers of childhood obesity, and investigate molecular mechanisms and candidate agents in order to improve therapeutic strategies for childhood obesity. The GSE9624 gene expression profile was downloaded from the Gene Expression Omnibus database. The differentially expressed genes (DEGs) in omental adipose tissues were analyzed with limma package by comparing samples from obese and normal control children. Two-way hierarchical clustering was applied using the pheatmap package. The co-expression (CE) analysis was performed using online CoExpress software. Subsequent to functional classification via the GOSim package, the gene network enriched by DEGs was visualized using the Cytoscape package. The codon usage bias of the DEGs was then examined using the CAI program from the European Molecular Biology Open Software Suite. In total, 583 DEGs (273 upregulated genes and 310 downregulated genes) were observed in the omental adipose tissues between samples from obese and normal control children. Hierarchical clustering identified a significant difference between samples from obese and normal control children. Subsequent to CE analysis, 130 DEGs, which were classified into 4 clusters, were selected. The following 3 upregulated and 2 downregulated genes were identified to be significant: Upregulated genes, microtubule-associated protein tau (MAPT), destrin (actin depolymerizing factor) (DSTN) and spectrin, β, non-erythrocytic 1 (SPTBN1); downregulated genes, Rho/Rac guanine nucleotide exchange factor 2 (ARHGEF2) and spindle and kinetochore associated complex subunit 1 (SKA1). The top 3 amino acids were identified to be glycine, leucine and serine with a high bias. The DEGs MAPT, DSTN, SPTBN1, ARHGEF2 and SKA1 are suggested to be candidate biomarkers for childhood obesity.

Cofilin-1 and Other ADF/Cofilin Superfamily Members in Human Malignant Cells

Identification of actin-depolymerizing factor homology (ADF-H) domains in the structures of several related proteins led first to the formation of the ADF/cofilin family, which then expanded to the ADF/cofilin superfamily. This superfamily includes the well-studied cofilin-1 (Cfl-1) and about a dozen different human proteins that interact directly or indirectly with the actin cytoskeleton, provide its remodeling, and alter cell motility. According to some data, Cfl-1 is contained in various human malignant cells (HMCs) and is involved in the formation of malignant properties, including invasiveness, metastatic potential, and resistance to chemotherapeutic drugs. The presence of other ADF/cofilin superfamily proteins in HMCs and their involvement in the regulation of cell motility were discovered with the use of various OMICS technologies. In our review, we discuss the results of the study of Cfl-1 and other ADF/cofilin superfamily proteins, which may be of interest for solving different problems of molecular oncology, as well as for the prospects of further investigations of these proteins in HMCs.

The actin depolymerizing factor (ADF)/cofilin signaling pathway and DNA damage responses in cancer

The actin depolymerizing factor (ADF)/cofilin protein family is essential for actin dynamics, cell division, chemotaxis and tumor metastasis. Cofilin-1 (CFL-1) is a primary non-muscle isoform of the ADF/cofilin protein family accelerating the actin filamental turnover in vitro and in vivo. In response to environmental stimulation, CFL-1 enters the nucleus to regulate the actin dynamics. Although the purpose of this cytoplasm-nucleus transition remains unclear, it is speculated that the interaction between CFL-1 and DNA may influence various biological responses, including DNA damage repair. In this review, we will discuss the possible involvement of CFL-1 in DNA damage responses (DDR) induced by ionizing radiation (IR), and the implications for cancer radiotherapy.

RanBP9 overexpression reduces dendritic arbor and spine density

RanBP9 is a multi-domain scaffolding protein known to integrate extracellular signaling with intracellular targets. We previously demonstrated that RanBP9 enhances Aβ generation and amyloid plaque burden which results in loss of specific pre- and postsynaptic proteins in vivo in a transgenic mouse model. Additionally, we showed that the levels of spinophilin, a marker of dendritic spines were inversely proportional to the RanBP9 protein levels within the synaptosomes isolated from AD brains. In the present study, we found reduced dendritic intersections within the layer 6 pyramidal neurons of the cortex as well as the hippocampus of RanBP9 transgenic mice compared to age-matched wild-type (WT) controls at 12 months of age but not at 6months. Similarly, the dendritic spine numbers were reduced in the cortex at only 12 months of age by 30% (p<0.01), but not at 6months. In the hippocampus also the spine densities were reduced at 12 months of age (38%, p<0.01) in the RanBP9 transgenic mice. Interestingly, the levels of phosphorylated form of cofilin, an actin binding protein that plays crucial role in the regulation of spine numbers were significantly decreased in the cortical synaptosomes at only 12months of age by 26% (p<0.01). In the hippocampal synaptosomes, the decrease in cofilin levels were 36% (p<0.01) at 12 months of age. Thus dendritic arbor and spine density were directly correlated to the levels of phosphorylated form of cofilin in the RanBP9 transgenic mice. Similarly, cortical synaptosomes showed a 20% (p<0.01) reduction in the levels of spinophilin in the RanBP9 transgenic mice. These results provided the physical basis for the loss of synaptic proteins by RanBP9 and most importantly it also explains the impaired spatial learning and memory skills previously observed in the RanBP9 transgenic mice.

Signaling from the podocyte intercellular junction to the actin cytoskeleton

Observations of hereditary glomerular disease support the contention that podocyte intercellular junction proteins are essential for junction formation and maintenance. Genetic deletion of most of these podocyte intercellular junction proteins results in foot process effacement and proteinuria. This review focuses on the current understanding of molecular mechanisms by which podocyte intercellular junction proteins such as the nephrin-neph1-podocin-receptor complex coordinate cytoskeletal dynamics and thus intercellular junction formation, maintenance, and injury-dependent remodeling.

Enhanced cellular radiosensitivity induced by cofilin-1 over-expression is associated with reduced DNA repair capacity

PURPOSE

A previous report has indicated that over-expression of cofilin-1 (CFL-1), a member of the actin depolymerizing factor (ADF)/cofilin protein family, enhances cellular radiosensitivity. This study explores the involvement of various DNA damage responses and repair systems in the enhanced cellular radiosensitivity as well as assessing the role of CFL-1 phosphorylation in radiosensitivity.

MATERIALS AND METHODS

Human non-small lung cancer H1299 cells harboring a tet-on gene expression system were used to induce exogenous expression of wild-type CFL-1. Colony formation assays were used to determine cell survival after γ-ray exposure. DNA damage levels were determined by Comet assay. DNA repair capacity was assessed by fluorescence-based DNA repair analysis and antibody detection of various repair proteins. The effects of CFL-1 phosphorylation on radiation responses were explored using two mutant CFL-1 proteins, S3D and S3A. Finally, endogenous CFL-1 phosphorylation levels were investigated using latrunculin A (LA), cytochalasin B (CB) and Y27632.

RESULTS

When phosphorylatable CFL-1 was expressed, radiosensitivity was enhanced after exposure to γ-rays and this was accompanied by DNA damage. Phosphorylated histone H2AX (γ-H2AX) and p53-binding protein-1 (53BP1) foci, as well as Chk1/2 phosphorylation, were apparently suppressed, although ataxia telangiectasia mutated (ATM) kinase activation was apparently unaffected. In addition, two radiation-induced double-strand break (DSB) repair systems, namely homologous recombination repair (HRR) and non-homologous end joining (NHEJ), were suppressed. Moreover, over-expression of CFL-1 S3D and CFL-1 S3A both enhanced radiosensitivity. However, enhanced radiosensitivity and reduced γ-H2AX expression were only detected in cells treated with LA which increased endogenous phospho-CFL-1, and not in cells treated with Y27632, which dephosphorylates CFL-1.

CONCLUSION

CFL-1 over-expression enhances radiosensitivity and this is associated with reduced DNA repair capacity. Although phosphorylated CFL-1 seems to be involved in radiosensitivity, further studies are required to address the importance of CFL-1 activity to the regulation of radiosensitivity.

Comparative proteomics analysis of mice lymphocytes in early stages of infection by different strains of rabies virus

The CNS immune response to rabies virus has been shown to be influenced by virulence of the virus strains. There is no comprehensive report of the peripheral immune response against different strains of rabies virus. In this report we used a comparative proteome analysis to find the early events in the spleen lymphocytes of mice infected by a street strain and an attenuated strain of the rabies virus. Differentially expressed proteins were identified which play important biological roles such as T and B lymphocyte activation (coronin 1), antiviral activity (peroxiredoxin 1), and cytoskeletal reorganization (cofilin 1). These results could be strong hints of early divergence on peripheral immune response under influence of viral strain and their pathogenicity.

A requirement for polymerized actin in DNA double-strand break repair

Nuclear actin is involved in several nuclear processes from chromatin remodeling to transcription. Here we examined the requirement for actin polymerization in DNA double-strand break repair. Double-strand breaks are considered the most dangerous type of DNA lesion. Double-strand break repair consists of a complex set of events that are tightly regulated. Failure at any step can have catastrophic consequences such as genomic instability, oncogenesis or cell death. Many proteins involved in this repair process have been identified and their roles characterized. We discovered that some DNA double-strand break repair factors are capable of associating with polymeric actin in vitro and specifically, that purified Ku70/80 interacts with polymerized actin under these conditions. We find that the disruption of polymeric actin inhibits DNA double strand break repair both in vitro and in vivo. Introduction of nuclear targeted mutant actin that cannot polymerize, or the depolymerization of endogenous actin filaments by the addition of cytochalasin D, alters the retention of Ku80 at sites of DNA damage in live cells. Our results suggest that polymeric actin is required for proper DNA double-strand break repair and may function through the stabilization of the Ku heterodimer at the DNA damage site.

Nuclear actin and lamins in viral infections

Lamins are the best characterized cytoskeletal components of the cell nucleus that help to maintain the nuclear shape and participate in diverse nuclear processes including replication or transcription. Nuclear actin is now widely accepted to be another cytoskeletal protein present in the nucleus that fulfills important functions in the gene expression. Some viruses replicating in the nucleus evolved the ability to interact with and probably utilize nuclear actin for their replication, e.g., for the assembly and transport of capsids or mRNA export. On the other hand, lamins play a role in the propagation of other viruses since nuclear lamina may represent a barrier for virions entering or escaping the nucleus. This review will summarize the current knowledge about the roles of nuclear actin and lamins in viral infections.

Cofilin under control of β-arrestin-2 in NMDA-dependent dendritic spine plasticity, long-term depression (LTD), and learning

Dendritic spines are dynamic, actin-rich structures that form the postsynaptic sites of most excitatory synapses in the brain. The F-actin severing protein cofilin has been implicated in the remodeling of dendritic spines and synapses under normal and pathological conditions, by yet unknown mechanisms. Here we report that β-arrestin-2 plays an important role in NMDA-induced remodeling of dendritic spines and synapses via translocation of active cofilin to dendritic spines. NMDAR activation triggers cofilin activation through calcineurin and phosphatidylinositol 3-kinase (PI3K)-mediated dephosphorylation and promotes cofilin translocation to dendritic spines that is mediated by β-arrestin-2. Hippocampal neurons lacking β-arrestin-2 develop mature spines that fail to remodel in response to NMDA. β-Arrestin-2-deficient mice exhibit normal hippocampal long-term potentiation, but significantly impaired NMDA-dependent long-term depression and spatial learning deficits. Moreover, β-arrestin-2-deficient hippocampal neurons are resistant to Aβ-induced dendritic spine loss. Our studies demonstrate unique functions of β-arrestin-2 in NMDAR-mediated dendritic spine and synapse plasticity through spatial control over cofilin activation.

Mass spectrometry-based analysis of proteomic changes in the root tips of flooded soybean seedlings

Flooding injury is a major problem in soybean cultivation. A proteomics approach was used to clarify the occurrence of changes in protein expression level and phosphorylation in soybeans under flooding stress. Two-day-old seedlings were flooded for 1 day, proteins were extracted from root tips of the seedlings and digested with trypsin, and their expression levels and phosphorylation states were compared to those of untreated controls using mass spectrometry-based proteomics techniques. Phosphoproteins were enriched using a phosphoprotein purification column prior to digestion and mass spectrometry. The expression of proteins involved in energy production increased as a result of flooding, while expression of proteins involved in protein folding and cell structure maintenance decreased. Flooding induced changes of phosphorylation status of proteins involved in energy generation, protein synthesis and cell structure maintenance. The response to flooding stress may be regulated by both modulation of protein expression and phosphorylation state. Energy-demanding and production-related metabolic pathways may be particularly subject to regulation by changes in protein phosphorylation during flooding.

Spatial and temporal expression of actin depolymerizing factors ADF7 and ADF10 during male gametophyte development in Arabidopsis thaliana

The actin cytoskeleton plays a crucial role in many aspects of plant cell development. During male gametophyte development, the actin arrays are conspicuously remodeled both during pollen maturation in the anther and after pollen hydration on the receptive stigma and pollen tube elongation. Remodeling of actin arrays results from the highly orchestrated activities of numerous actin binding proteins (ABPs). A key player in actin remodeling is the actin depolymerizing factor (ADF), which increases actin filament treadmilling rates. We prepared fluorescent protein fusions of two Arabidopsis pollen-specific ADFs, ADF7 and ADF10. We monitored the expression and subcellular localization of these proteins during male gametophyte development, pollen germination and pollen tube growth. ADF7 and ADF10 were differentially expressed with the ADF7 signal appearing in the microspore stage and that of ADF10 only during the polarized microspore stage. ADF7 was associated with the microspore nucleus and the vegetative nucleus of the mature grain during less metabolically active stages, but in germinating pollen grains and elongating pollen tubes, it was associated with the subapical actin fringe. On the other hand, ADF10 was associated with filamentous actin in the developing gametophyte, in particular with the arrays surrounding the apertures of the mature pollen grain. In the shank of elongating pollen tubes, ADF10 was associated with thick actin cables. We propose possible specific functions of these two ADFs based on their differences in expression and localization.

Functional genomics of the muscle response to restraint and transport in chickens

In the present study, we used global approaches (proteomics, transcriptomics, and metabolomics) to assess the molecular basis of the muscle response to stress in chickens. A restraint test, combined with transport for 2 h (RT test) was chosen as the potentially stressful situation. Chickens (6 wk old) were either nontreated (control chickens) or submitted to the RT test (treated chickens). The RT test induced a 6-fold increase in corticosterone concentrations, suggesting hypothalamic-pituitary-adrenal axis activation. The RT test decreased the relative abundance of several hexose phosphates [glucose-1-P (G1P), glucose-6-P (G6P), fructose-6-P (F6P), and mannose-6-P (M6P)] in thigh muscle. In addition, 55 transcripts, among which 39 corresponded to unique annotated genes, were significantly up- (12 genes) or downregulated (27 genes) by treatment. Similarly, 45 proteic spots, among which 29 corresponded to unique annotated proteins, were overexpressed (11 proteins), underexpressed (14 proteins), or only expressed in treated chickens. Integrative analysis of differentially expressed genes and proteins showed that most transcripts and proteins belong to 2 networks whose genes were mainly related with cytoskeleton structure or carbohydrate metabolism. Whereas the decrease in energetic metabolites suggested an activation of glycogenolysis and glycolysis in response to the RT test, the reduced expression of genes and proteins involved in these pathways suggested the opposite. We hypothesized that the prolonged RT test resulted in a repression of glycogenolysis and glycolysis in thigh muscle of chickens. The down-expression of genes and proteins involved in the formation of fiber stress after the RT test suggests a reinforcement of myofibrils in response to stress.

Distinct roles for Rho versus Rac/Cdc42 GTPases downstream of Vav2 in regulating mammary epithelial acinar architecture

Non-malignant mammary epithelial cells (MECs) undergo acinar morphogenesis in three-dimensional Matrigel culture, a trait that is lost upon oncogenic transformation. Rho GTPases are thought to play important roles in regulating epithelial cell-cell junctions, but their contributions to acinar morphogenesis remain unclear. Here we report that the activity of Rho GTPases is down-regulated in non-malignant MECs in three-dimensional culture with particular suppression of Rac1 and Cdc42. Inducible expression of a constitutively active form of Vav2, a Rho GTPase guanine nucleotide exchange factor activated by receptor tyrosine kinases, in three-dimensional MEC culture activated Rac1 and Cdc42; Vav2 induction from early stages of culture impaired acinar morphogenesis, and induction in preformed acini disrupted the pre-established acinar architecture and led to cellular outgrowths. Knockdown studies demonstrated that Rac1 and Cdc42 mediate the constitutively active Vav2 phenotype, whereas in contrast, RhoA knockdown intensified the Vav2-induced disruption of acini, leading to more aggressive cell outgrowth and branching morphogenesis. These results indicate that RhoA plays an antagonistic role to Rac1/Cdc42 in the control of mammary epithelial acinar morphogenesis.

Generally detected proteins in comparative proteomics--a matter of cellular stress response?

The specificity of proteins identified by proteomics as biomarkers for defined conditions or as components of biological processes and pathways is crucial. We critically reviewed differentially expressed proteins from comparative proteomic studies identified by 2-DE followed by MS, especially with MALDI technique. Based on 66 of those studies, a list of 44 proteins is presented as generally detected proteins regardless of species, in vivo or in vitro conditions, tissues and organs, and experimental objective. Similarly, a list of 28 generally detected protein families is presented. The enriched functions linked to these generally detected proteins reveal that there are some common biological features beyond the technical limitations. Cellular stress response can be the universal reason as to why these proteins are generally expressed differentially. Using those proteins as biomarkers for cellular processes other than stress response should be done with caution. In future proteomic studies more profound approaches should be applied to look beyond these proteins to find specific biomarkers. Our results are discussed in relation to a recent viewpoint publication by Petrak et al. [Proteomics 2008, 8, 1744-1749].

Actin dynamics and functions in the interphase nucleus: moving toward an understanding of nuclear polymeric actin

Actin exists as a dynamic equilibrium of monomers and polymers within the nucleus of living cells. It is utilized by the cell for many aspects of gene regulation, including mRNA processing, chromatin remodelling, and global gene expression. Polymeric actin is now specifically linked to transcription by RNA polymerase I, II, and III. An active process, requiring both actin polymers and myosin, appears to drive RNA polymerase I transcription, and is also implicated in long-range chromatin movement. This type of mechanism brings activated genes from separate chromosomal territories together, and then participates in their compartmentalization near nuclear speckles. Nuclear speckle formation requires polymeric actin, and factors promoting polymerization, such as profilin and PIP2, are concentrated there. A review of the literature shows that a functional population of G-actin cycles between the cytoplasm and the nucleoplasm. Its nuclear concentration is dependent on the cytoplasmic G-actin pool, as well as on the activity of import and export mechanisms and the availability of interactions that sequester it within the nucleus. The N-WASP-Arp2/3 actin polymer-nucleating mechanism functions in the nucleus, and its mediators, including NCK, PIP2, and Rac1, can be found in the nucleoplasm, where they likely influence the kinetics of polymer formation. The actin polymer species produced are tightly regulated, and may take on conformations not easily recognized by phalloidin. Many of the factors that cleave F-actin in the cytoplasm are present at high levels in the nucleoplasm, and are also likely to affect actin dynamics there. The absolute and relative G-actin content in the nucleoplasm and the cytoplasm of a cell contains information about the homeostatic state of that cell. We propose that the cycling of G-actin between the nucleus and cytoplasm represents a signal transduction mechanism that can function through both extremes of global cellular G-actin content. MAL signalling within the serum response factor pathway, when G-actin levels are low, represents a well-studied example of actin functioning in signal transduction. The translocation of NCK into the nucleus, along with G-actin, during dissolution of the cytoskeleton in response to DNA damage represents another instance of a unique signalling mechanism operating when G-actin levels are high.

A proteomic study of the aortic media in human thoracic aortic dissection: implication for oxidative stress

OBJECTIVE

The aortic media lesion is a key pathologic feature in thoracic aortic dissection. To identify key proteins in aortic media lesions that may contribute to its pathogenesis, we performed proteomic studies to find differentially expressed proteins in the media from diseased and normal thoracic aorta.

METHODS

Ascending aortic segments were obtained from patients with thoracic aortic dissection (n = 8) and age-matched normal donors (n = 6). The differentially expressed proteins of their media tissues were analyzed by 2-dimensional electrophoresis and mass spectrometry, and verified by Western blotting. Oxidative stress was measured by functional assays in a larger sample size (15 patients and 10 controls).

RESULTS

Image analysis of the protein profiles from 2-dimensional gels revealed 126 differentially expressed proteins, of which 26 were identified by mass spectrometry. Among them, extracellular superoxide dismutase, an enzyme involved in oxidative stress, was selected for further studies. Western blotting showed that extracellular superoxide dismutase expression was more than 50% lower in patient samples than in controls (P < .001). Superoxide dismutase activity was consistently decreased (P < .001) and lipid peroxidation was increased (P = .019) in patient media homogenates compared with that in controls.

CONCLUSION

Our results indicate that protein expression profiles in the aortic media from thoracic aortic dissection differ significantly from that of controls, which may provide important insights into the disease mechanisms. This study also suggests that increased oxidative stress may play an important role in the disease.

Lysophosphatidic acid stimulation of platelets rapidly induces Ca2+-dependent dephosphorylation of cofilin that is independent of dense granule secretion and aggregation

Cofilin is an actin dynamizing protein and inactivated after Ser3 phosphorylation by LIM-kinases (LIMKs). We studied whether in platelets stimulated by lysophosphatidic acid (LPA), Rho-kinase or p21-activated kinase (PAK) mediates LIMK-1 activation leading to subsequent phosphorylation and inactivation of cofilin and the increase of F-actin. During LPA (0.1 microM)-induced shape change, a rapid Rho-kinase activation and a slower activation of PAK were observed. Rho-kinase activation led to rapid LIMK-1 (Thr508) phosphorylation. Despite of LIMK-1 activation, cofilin net phosphorylation was not increased. Cofilin rapidly associated with F-actin and preceded the F-actin increase. Pretreatment with the Rho-kinase inhibitor Y-27632 inhibited LIMK-1 phosphorylation, unmasked cofilin dephosphorylation and inhibited the reversible F-actin increase during shape change. In the presence of fibrinogen, LPA (10 microM) induced ATP-secretion from dense granules and aggregation, and cofilin was rapidly dephosphorylated and then rephosphorylated in a Rho-kinase/LIMK-1-dependent manner. In the absence of fibrinogen, cofilin de- and rephosphorylation after LPA (10 microM) was unchanged, but secretion and aggregation were absent. Cofilin dephosphorylation was completely blocked by BAPTA-AM indicating that it was mediated by an increase of cytosolic Ca(2+). We conclude that in LPA-stimulated platelets, Rho-kinase-dependent LIMK-1 activation mediates the F-actin increase during shape change without enhancing cofilin net phosphorylation. However, a rapid dephosphorylation of cofilin occurs during secretion and aggregation, which is Ca(2+)-dependent, upstream of secretion and aggregation and might regulate these platelet responses.

Conformation-specific antibodies reveal distinct actin structures in the nucleus and the cytoplasm

For many years the existence of actin in the nucleus has been doubted because of the lack of phalloidin staining as well as the failure to document nuclear actin filaments by electron microscopy. More recent findings reveal actin to be a component of chromatin remodeling complexes and of the machinery involved in RNA synthesis and transport. With distinct functions for nuclear actin emerging, the quest for its conformation and oligomeric/polymeric structure in the nucleus has resumed importance. We used chemically cross-linked 'lower dimer' (LD) to generate mouse monoclonal antibodies specific for different actin conformations. One of the resulting antibodies, termed 1C7, recognizes an epitope that is buried in the F-actin filament, but is surface-exposed in G-actin as well as in the LD. In immunofluorescence studies with different cell lines, 1C7 selectively reacts with non-filamentous actin in the cytoplasm. In addition, it detects a discrete form of actin in the nucleus, which is different from the nuclear actin revealed by the previously described 2G2 [Gonsior, S.M., Platz, S., Buchmeier, S., Scheer, U., Jockusch, B.M., Hinssen, H., 1999. J. Cell Sci. 112, 797]. Upon latrunculin-induced disassembly of the filamentous cytoskeleton in Rat2 fibroblasts, we observed a perinuclear accumulation of the 1C7-reactive actin conformation. In addition, latrunculin treatment led to the assembly of phalloidin-staining actin structures in chromatin-free regions of the nucleus in these cells. Our results indicate that distinct actin conformations and/or structures are present in the nucleus and the cytoplasm of different cell types and that their distribution varies in response to external signals.

Enhancement of radiosensitivity in H1299 cancer cells by actin-associated protein cofilin

Cofilin is an actin-associated protein that belongs to the actin depolymerization factor/cofilin family and is important for regulation of actin dynamics. Cofilin can import actin monomers into the nucleus under certain stress conditions, however the biological effects of nuclear transport are unclear. In this study, we found that over-expression of cofilin led to increased radiation sensitivity in human non-small lung cancer H1299 cells. Cell survival as determined by colony forming assay showed that cells over-expressing cofilin were more sensitive to ionizing radiation (IR) than normal cells. To determine whether the DNA repair capacity was altered in cofilin over-expressing cells, comet assays were performed on irradiated cells. Repair of DNA damage caused by ionizing radiation was detected in cofilin over-expressing cells after 24 h of recovery. Consistent with this observation, the key components for repair of DNA double-strand breaks, including Rad51, Rad52, and Ku70/Ku80, were down-regulated in cofilin over-expressing cells after IR exposure. These findings suggest that cofilin can influence radiosensitivity by altering DNA repair capacity.

The NEIBank project for ocular genomics: data-mining gene expression in human and rodent eye tissues

NEIBank is a project to gather and organize genomic resources for eye research. The first phase of this project covers the construction and sequence analysis of cDNA libraries from human and animal model eye tissues to develop an overview of the repertoire of genes expressed in the eye and a resource of cDNA clones for further studies. The sequence data are grouped and identified using the tools of bioinformatics and the results are displayed through a web site where they can be interrogated by keyword search, chromosome location, by Blast (sequence comparison) or by alignment on completed genomes. Many novel proteins and novel splice forms of known genes have already emerged from analysis of the accumulating data. This review provides an overview of the current state of the database for human eye tissues, with specific comparisons to some parallel data from mouse and rat, and with illustrative examples of the kinds of insights and discoveries these data can produce. One of the major themes that emerges is that at the molecular level human eye tissues have significant differences from those of rodents, encompassing species specific genes, alternative splice forms and great variation in levels of gene expression. These point to specific adaptations and mechanisms in the human eye and emphasize that care needs to be taken in the application of appropriate animal model systems.

Proteomic Profiling Identifies an UV-Induced Activation of Cofilin-1 and Destrin in Human Epidermis

The human skin is the only line of defense against UV radiation. A series of responses to protect the skin are induced by UV radiation. In this study, a proteomic approach was used to study these responses. We have performed high-resolution two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) analysis of (solar simulated) UV-exposed reconstructed skin equivalents as well as native skin. Differentially expressed proteins were processed for mass spectrometric analysis, when consistent differences were observed in all individual human skin equivalents. In addition to proteins known to be involved in UV responses (HSP27, MnSOD, and PDX-2), we identified two novel proteins that were downregulated following UV exposure. Further analysis revealed that these proteins were the phosphorylated forms of the actin cytoskeleton modulators cofilin-1 and destrin. The de-phosphorylation of cofilin-1 was confirmed using western blotting of UV-exposed skin equivalents and ex vivo skin protein extracts. In conclusion, our study indicates the potency of a proteomic approach to study UV-induced changes in a tissue culture system mimicking human skin as well as excised human skin.

Proteome changes in ovarian epithelial cells derived from women with BRCA1 mutations and family histories of cancer

Malignant transformation of the ovarian surface epithelium (OSE) accounts for most ovarian carcinoma. Detection of preneoplastic changes in the OSE leading to overt malignancy is important in prevention and management of ovarian cancer. We identified OSE proteins with altered expression derived from women with a family history (FH) of ovarian and/or breast cancer and mutations in the BRCA1 tumor suppressor gene. Proteins from SV-40-transformed FH-OSE cell lines and control OSE lines derived from women without such histories (non-family history) were separated by two-dimensional PAGE. Gels were analyzed, a protein data base was created, and proteins were characterized according to their molecular weight, isoelectric point, and relative abundance. Mass spectrometry was performed on tryptic protein digests, and data bases were searched for known proteins with the same theoretical tryptic peptide masses. Several proteins showed altered expression in the FH-OSE cells. Beta-tubulin and to a lesser extent ubiquitin carboxyl-terminal hydrolase and glyoxalase 1 appeared to be up-regulated. In contrast, proteins suppressed in FH lines include the 27-kDa heat shock protein, translationally controlled tumor protein, and several proteins associated with actin modification such as actin prepeptide, F-actin capping protein alpha subunit, and cofilin. Sequencing of several cofilin gel spots revealed phosphorylation of serine 3, a post-translational modification associated with decreased actin binding and cytoskeletal reorganization. Two-dimensional Western blots probed with cofilin antibody showed multiple protein spots with isoelectric points of 6-9 pH units. Blots of one-dimensional gels showed a significant reduction in cofilin expression in three FH lines when compared with three non-family history lines (p < or = 0.05). Identification of these and other OSE proteins may be useful in detecting changes suggestive of increased risk of developing preneoplastic disease and defining the possible role(s) of the BRCA1 gene in regulation of OSE cell function.

Actin-filled nuclear invaginations indicate degree of cell de-differentiation

For years the existence of nuclear actin has been heavily debated, but recent data have clearly demonstrated that actin, as well as actin-binding proteins (ABPs), are located in the nucleus. We examined live EGFP-actin-expressing cells using confocal microscopy and saw the presence of structures strongly resembling actin filaments in the nuclei of MDA-MB-231 human mammary epithelial tumor cells. Many nuclei had more than one of these filamentous structures, some of which appeared to cross the entire nucleus. Extensive analysis, including fluorescence recovery after photobleaching (FRAP), showed that all EGFP-actin in the nucleus is monomeric (G-actin) rather than filamentous (F-actin) and that the apparent filaments seen in the nucleus are invaginations of cytoplasmic monomeric actin. Immunolocalization of nuclear pore complex proteins shows that similar invaginations are seen in cells that are not overexpressing EGFP-actin. To determine whether there is a correlation between increased levels of invagination in the cell nuclei and the state of de-differentiation of the cell, we examined a variety of cell types, including live Xenopus embryonic cells. Cells that were highly de-differentiated, or cancerous, had an increased incidence of invagination, while cells that were differentiated had few nuclear invaginations. The nuclei of embryonic cells that were not yet differentiated underwent multiple shape changes throughout interphase, and demonstrated numerous transient invaginations of varying sizes and shapes. Although the function of these actin-filled invaginations remains speculative, their presence correlates with cells that have increased levels of nuclear activity.

Adhesion, migration, transcriptional, interferon-inducible, and other signaling molecules newly implicated in cancer susceptibility and resistance of JB6 cells by cDNA microarray analyses

Relative expression levels of 9500 genes were determined by cDNA microarray analyses in mouse skin JB6 cells susceptible (P+) and resistant (P-) to 12-O-tetradecanoyl phorbol-13 acetate (TPA)-induced neoplastic transformation. Seventy-four genes in 6 functional classes were differentially expressed: (I) extracellular matrix (ECM) and basement membrane (BM) proteins (20 genes). P+ cells express higher levels than P- cells of several collagens and proteases, and lower levels of protease inhibitors. Multiple genes encoding adhesion molecules are expressed preferentially in P- cells, including six genes implicated in axon guidance and adhesion. (II) Cytoskeletal proteins (13 genes). These include actin isoforms and regulatory proteins, almost all preferentially expressed in P- cells. (III) Signal transduction proteins (12 genes). Among these are Ras-GTPase activating protein (Ras-GAP), the deleted in oral cancer-1 and SLIT2 tumor suppressors, and connexin 43 (Cx43) gap junctional protein, all expressed preferentially in P- cells. (IV) Interferon-inducible proteins (3 genes). These include interferon-inducible protein (IFI)-16, an Sp1 transcriptional regulator expressed preferentially in P- cells. (V) Other transcription factors (4 genes). Paired related homeobox gene 2 (Prx2)/S8 homeobox, and retinoic acid (RA)-regulated nur77 and cellular retinoic acid-binding protein II (CRABPII) transcription factors are expressed preferentially in P- cells. The RIN-ZF Sp-transcriptional suppressor exhibits preferential P+ expression. (VI) Genes of unknown functions (22 sequences). Numerous mesenchymal markers are expressed in both cell types. Data for multiple genes were confirmed by real-time PCR. Overall, 26 genes were newly implicated in cancer. Detailed analyses of the functions of the genes and their interrelationships provided converging evidence for their possible roles in implementing genetic programs mediating cancer susceptibility and resistance. These results, in conjunction with cell wounding and phalloidin staining data, indicated that concerted genetic programs were implemented that were conducive to cell adhesion and tumor suppression in P- cells and that favored matrix turnover, cell motility, and abrogation of tumor suppression in P+ cells. Such genetic programs may in part be orchestrated by Sp-, RA-, and Hox-transcriptional regulatory pathways implicated in this study.

Control of actin reorganization by Slingshot, a family of phosphatases that dephosphorylate ADF/cofilin

The ADF (actin-depolymerizing factor)/cofilin family is a stimulus-responsive mediator of actin dynamics. In contrast to the mechanisms of inactivation of ADF/cofilin by kinases such as LIM-kinase 1 (LIMK1), much less is known about its reactivation through dephosphorylation. Here we report Slingshot (SSH), a family of phosphatases that have the property of F actin binding. In Drosophila, loss of ssh function dramatically increased levels of both F actin and phospho-cofilin (P cofilin) and disorganized epidermal cell morphogenesis. In mammalian cells, human SSH homologs (hSSHs) suppressed LIMK1-induced actin reorganization. Furthermore, SSH and the hSSHs dephosphorylated P cofilin in cultured cells and in cell-free assays. Our results strongly suggest that the SSH family plays a pivotal role in actin dynamics by reactivating ADF/cofilin in vivo.

Cofilin-2, a novel type of cofilin, is expressed specifically at aggregation stage of Dictyostelium discoideum development

BACKGROUND

A conventional cofilin, cofilin-1 in Dictyostelium discoideum plays significant roles in cell proliferation, phagocytosis, chemotactic movement and macropinocytosis.

RESULTS

We identified a new member of the cofilin family, named cofilin-2 in D. discoideum. Cofilin-2 shows significant homology to a conventional Dictyostelium cofilin, cofilin-1, through its entire sequence, and contains residues conserved among the cofilin family that are responsible for actin-binding. On the other hand, several residues that are conserved among the cofilin family are missing from cofilin-2. Purified cofilin-2 depolymerized actin filaments in a dose- and pH-dependent manner and reduced the apparent viscosity of an actin solution, although they did not co-sediment with actin filaments at all. Cofilin-2 was not expressed in vegetative cells, but was transiently induced during the aggregation stage of development, whereas cofilin-1 was predominantly expressed in vegetative cells. Immunocytochemistry revealed that cofilin-2 localizes at substrate adhesion sites, where cofilin-1 is almost completely excluded. Disruption of the cofilin-2 gene caused an increase in actin accumulation at the substrate adhesion sites. We also found that cofilin-2 did not rescue Deltacof1 yeast cells, whereas cofilin-1 did.

CONCLUSIONS

Cofilin-2 may play a distinct role from that of cofilin-1 in destabilization of the actin cytoskeleton during Dictyostelium development.

Regulation of the formation of tumor cell pseudopodia by the Na(+)/H(+) exchanger NHE1

The Na(+)/H(+) exchanger NHE1 is involved in intracellular pH homeostasis and cell volume regulation and accumulates with actin in the lamellipodia of fibroblasts. In order to determine the role of NHE1 following epithelial transformation and the acquisition of motile and invasive properties, we studied NHE1 expression in polarized MDCK cells, Moloney Sarcoma virus (MSV) transformed MDCK cells and an invasive MSV-MDCK cell variant (MSV-MDCK-INV). Expression of NHE1 was significantly increased in MSV-MDCK-INV cells relative to MSV-MDCK and MDCK cells. NHE1 was localized with b-actin to the tips of MSV-MDCK-INV cell pseudopodia by immunofluorescence. Sensitivity of NHE1-mediated (22)Na uptake to ethylisopropylamiloride, a specific inhibitor of NHE1, was increased in MSV-MDCK cells relative to MDCK cells. Changes in intracellular pH induced upon EIPA treatment were also of higher magnitude in MSV-MDCK and MSV-MDCK-INV cells compared to wild-type MDCK cells, especially in Hepes-buffered DMEM medium. Inhibition of NHE1 by 50 microM ethylisopropylamiloride induced the disassembly of actin stress fibers and redistribution of the actin cytoskeleton in all cell types. However, in MSV-MDCK-INV cells, the effect of ethylisopropylamiloride treatment was more pronounced and associated with the increased reversible detachment of the cells from the substrate. Videomicroscopy of MSV-MDCK-INV cells revealed that within 20 minutes of addition, ethylisopropylamiloride induced pseudopodial retraction and inhibited cell motility. The ability of ethylisopropylamiloride to prevent nocodazole-induced formation of actin stress fibers in MSV-MDCK cells was more pronounced in Hepes medium relative to NaHCO(3) medium, showing that NHE1 can regulate actin stress fiber assembly in transformed MSV-MDCK cells via its intracellular pH regulatory effect. These results implicate NHE1 in the regulation of the actin cytoskeleton dynamics necessary for the adhesion and pseudopodial protrusion of motile, invasive tumor cells.

Proteins of the ADF/cofilin family: essential regulators of actin dynamics

Ubiquitous among eukaryotes, the ADF/cofilins are essential proteins responsible for the high turnover rates of actin filaments in vivo. In vertebrates, ADF and cofilin are products of different genes. Both bind to F-actin cooperatively and induce a twist in the actin filament that results in the loss of the phalloidin-binding site. This conformational change may be responsible for the enhancement of the off rate of subunits at the minus end of ADF/cofilin-decorated filaments and for the weak filament-severing activity. Binding of ADF/cofilin is competitive with tropomyosin. Other regulatory mechanisms in animal cells include binding of phosphoinositides, phosphorylation by LIM kinases on a single serine, and changes in pH. Although vertebrate ADF/cofilins contain a nuclear localization sequence, they are usually concentrated in regions containing dynamic actin pools, such as the leading edge of migrating cells and neuronal growth cones. ADF/cofilins are essential for cytokinesis, phagocytosis, fluid phase endocytosis, and other cellular processes dependent upon actin dynamics.

Dictyostelium as model system for studies of the actin cytoskeleton by molecular genetics

The actin cytoskeleton is an essential structure for most movements at the cellular and intracellular level. Whereas for contraction a muscle cell requires a rather static organisation of cytoskeletal proteins, cell motility of amoeboid cells relies on a tremendously dynamic turnover of filamentous networks in a matter of seconds and at distinct regions inside the cell. The best model system for studying cell motility is Dictyostelium discoideum. The cells live as single amoebae but can also start a developmental program that leads to multicellular stages and differentiation into simple types of tissues. Thus, cell motility can be studied on single cells and on cells in a tissue-like aggregate. The ability to combine protein purification and biochemistry with fairly easy molecular genetics is a unique feature for investigation of the cytoskeleton and cell motility. The actin cytoskeleton in Dictyostelium harbours essentially all classes of actin-binding proteins that have been found throughout eukaryotes. By conventional mutagenesis, gene disruption, antisense approaches, or gene replacements many genes that code for cytoskeletal proteins have been disrupted, and altered phenotypes in transformants that lacked one or more of those cytoskeletal proteins allowed solid conclusions about their in vivo function. In addition, tagging the proteins or selected domains with green fluorescent protein allows the monitoring of protein redistribution during cell movement. Gene tagging by restriction enzyme mediated integration of vectors and the ongoing international genome and cDNA sequencing projects offer the chance to understand the dynamics of the cytoskeleton by identification and functional characterisation of all proteins involved.

Herbimycin A inhibits both dephosphorylation and translocation of cofilin induced by opsonized zymosan in macrophagelike U937 cells

We previously reported that a 21-kDa phosphoprotein may play an important role in superoxide production through dephosphorylation by neutrophillike differentiated HL-60 cells (Suzuki et al., 1995, Biochim Biophys Acta 1266: 261-267). The phosphoprotein was identified as cofilin, an actin-binding protein, and the activation-induced changes in its intracellular distribution have been described elsewhere (Suzuki et al., 1995, J Biol Chem 270:19551-19556). However, the physiologic roles of cofilin in phagocytes remain to be established, and the regulatory mechanisms for dephosphorylation and translocation of cofilin are unknown. In the present study, we investigated the roles of cofilin in the opsonized zymosan (OZ)-activated macrophagelike U937 cells by using herbimycin A, an inhibitor for protein tyrosine kinase. In the individual adherent phagocytes, OZ induced many events: 1) production of superoxide, 2) phagocytosis of the insoluble particles OZ, 3) dephosphorylation of cofilin, 4) translocation of cofilin from cytosol to plasma membrane regions, 5) decrease in intracellular pH from 7.4 to aprroximately 6.8, and 6) rapid and transient increase in filamentous actin at the cell periphery. All of these events were inhibited or reduced significantly by herbimycin A. OZ increased phosphorylation of tyrosine in 110-, 50-, 34-, and 29-kDa proteins, whereas herbimycin A inhibited it. These results suggest that tyrosine kinase plays an essential role upstream of these events through phosphorylation of such proteins. Furthermore, microinjection of anti-cofilin antibody to the differentiated U937 cells caused inhibition of the phagocytosis. These results suggest that cofilin plays critical roles in phagocytic functions through changes in cytoskeletal organization.

Hyperosmotic stress-induced reorganization of actin bundles in Dictyostelium cells over-expressing cofilin

BACKGROUND

Cofilin is a low-molecular weight actin-modulating protein, which binds to, severs, and depolymerizes actin filaments in vitro. Aip1, an actin-interacting protein, was recently identified as a product of a gene on a multicopy plasmid which suppresses the temperature-sensitive phenotype of a cofilin mutant in Saccharomyces cerevisiae. Actin cytoskeleton plays an essential role in resistance to hyperosmotic stress in Dictyostelium discoideum. The roles of cofilin and Aip1 in this resistance are not known.

RESULTS

In response to hyperosmotic stress, D. discoideum cells round up. This stress-induced morphological change involves the redistribution of cofilin, together with actin filaments, into cortical contractile portions of the cells, followed by their contraction. Over-expression of cofilin increases and thickens cortical actin bundles in cells. The bundles become tight and are reorganized into a ring-shaped structure in response to hyperosmotic stress. The ring structure of actin bundles had two characteristic bands across them; bright and dark bands, heavily stained and not stained with phalloidin. In the bundles, straight filaments with a diameter of 5.3-nm were aligned parallel by cross-bridge structures. In cells lacking the myosin-II heavy chain, the bundles, which were induced by an over-expression of cofilin, shortened and became straight following hyperosmotic stress, forming a polygonal structure. D. discoideum Aip1/Wrp2 enhanced the severing of actin filaments by cofilin in vitro and colocalized with cofilin in cells, including those that were over-expressing cofilin before and after exposure to hyperosmotic stress.

CONCLUSIONS

Cofilin plays a pivotal role in concert with Aip1/Wrp2 in the reorganization of actin architectures into bundles that contract in a myosin-II-independent manner, in response to hyperosmotic stress.

Cofilin and gelsolin segment-1: molecular dynamics simulation and biochemical analysis predict a similar actin binding mode

An understanding of the actin-depolymerizing function attributed to members of the ADF/cofilin/destrin superfamily requires a structural model of these proteins in complex with actin. As a step toward defining actin-cofilin interactions, the complex of yeast cofilin with monomeric actin was predicted, starting with the actin-gelsolin segment-1 binding mode recently suggested for the actin-destrin complex. After refinement by molecular dynamics simulation, the structure of cofilin converged in a new binding mode that required only minimal changes induced in the actin-cofilin interface. The predicted complex exhibits strong interactions between the N termini of actin and cofilin, mediated by a salt bridge of cofilin Arg3 with actin Asp1. The forming of this salt bridge could be prevented by the phosphorylation of cofilin Ser4, which is believed to inhibit cofilin depolymerization activity. Recent mutagenesis studies, crosslinking experiments and peptide binding studies are consistent with the predicted model of the actin-cofilin complex. The structural homology between cofilin and gelsolin segment-1 binding to actin was confirmed experimentally by two types of competitive binding assays.