Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

WDR1 promotes prostate cancer progression through Wnt/β-catenin signaling

Prostate cancer (PCa) is the second most common cancer and the fifth leading cause of cancer-related death among men. A comprehensive understanding of PCa progression is crucial for the development of innovative therapeutic strategies for its treatment. While WDR1 (WD-repeat domain 1) serves as a significant cofactor of actin-depolymerizing factor/cofilin, its role in PCa progression remains unknown. In this study, we demonstrated that knockdown of WDR1 in various PCa cells substantially inhibited cell proliferation, migration, and invasion in vitro, as confirmed at both the cellular and molecular levels. Moreover, the overexpression of WDR1 promoted PCa cell proliferation and metastasis in vitro. Mechanistically, we showed that the application of lithium chloride, an activator of the Wnt/β-Catenin signaling pathway, restored the suppressive effects of WDR1 deficiency on cell proliferation and migration in PCa cells. Our findings suggest that the WDR1-β-Catenin axis functions as an activator of the malignant phenotype and represents a promising therapeutic target for PCa treatment.

Vanillin Promotes Osteoblast Differentiation, Mineral Apposition, and Antioxidant Effects in Pre-Osteoblasts

Antioxidant vanillin (4-hydroxy-3-methoxybenzaldehyde) is used as a flavoring in foods, beverages, and pharmaceuticals. Vanillin possesses various biological effects, such as antioxidant, anti-inflammatory, antibacterial, and anticancer properties. This study aimed to investigate the biological activities of vanillin purified from Adenophora triphylla var. japonica Hara on bone-forming processes. Vanillin treatment induced mineralization as a marker for mature osteoblasts, after stimulating alkaline phosphatase (ALP) staining and activity. The bone-forming processes of vanillin are mainly mediated by the upregulation of the bone morphogenetic protein 2 (BMP2), phospho-Smad1/5/8, and runt-related transcription factor 2 (RUNX2) pathway during the differentiation of osteogenic cells. Moreover, vanillin promoted osteoblast-mediated bone-forming phenotypes by inducing migration and F-actin polymerization. Furthermore, we validated that vanillin-mediated bone-forming processes were attenuated by noggin and DKK1. Finally, we demonstrated that vanillin-mediated antioxidant effects prevent the death of osteoblasts during bone-forming processes. Overall, vanillin has bone-forming properties through the BMP2-mediated biological mechanism, indicating it as a bone-protective compound for bone health and bone diseases such as periodontitis and osteoporosis.

Cyclase-associated protein interacts with actin filament barbed ends to promote depolymerization and formin displacement

Cyclase-associated protein (CAP) has emerged as a central player in cellular actin turnover, but its molecular mechanisms of action are not yet fully understood. Recent studies revealed that the N terminus of CAP interacts with the pointed ends of actin filaments to accelerate depolymerization in conjunction with cofilin. Here, we use in vitro microfluidics-assisted TIRF microscopy to show that the C terminus of CAP promotes depolymerization at the opposite (barbed) ends of actin filaments. In the absence of actin monomers, full-length mouse CAP1 and C-terminal halves of CAP1 (C-CAP1) and CAP2 (C-CAP2) accelerate barbed end depolymerization. Using mutagenesis and structural modeling, we show that these activities are mediated by the WH2 and CARP domains of CAP. In addition, we observe that CAP collaborates with profilin to accelerate barbed end depolymerization and that these effects depend on their direct interaction, providing the first known example of CAP-profilin collaborative effects in regulating actin. In the presence of actin monomers, CAP1 attenuates barbed end growth and promotes formin dissociation. Overall, these findings demonstrate that CAP uses distinct domains and mechanisms to interact with opposite ends of actin filaments and drive turnover. Further, they contribute to the emerging view of actin barbed ends as sites of dynamic molecular regulation, where numerous proteins compete and cooperate with each other to tune polymer dynamics, similar to the rich complexity seen at microtubule ends.

The α-arrestin SUP-13/ARRD-15 promotes isoform turnover of actin-interacting protein 1 in Caenorhabditis elegans striated muscle

Precise arrangement of actin, myosin, and other regulatory components in a sarcomeric pattern is critical for producing contractile forces in striated muscles. Actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), is one of essential factors that regulate sarcomeric assembly of actin filaments. In the nematode Caenorhabditis elegans, mutation in unc-78, encoding one of the two AIP1 isoforms, causes severe disorganization of sarcomeric actin filaments and near paralysis, but mutation in sup-13 suppresses the unc-78-mutant phenotypes to restore nearly normal sarcomeric actin organization and worm motility. Here, we identified that sup-13 is a nonsense allele of arrd-15 encoding an α-arrestin. The sup-13/arrd-15 mutation suppressed the phenotypes of unc-78 null mutant but required aipl-1 that encodes a second AIP1 isoform. aipl-1 was normally expressed highly in embryos and downregulated in mature muscle. However, in the sup-13/arrd-15 mutant, the AIPL-1 protein was maintained at high levels in adult muscle to compensate for the absence of the UNC-78 protein. The sup-13/arrd-15 mutation caused accumulation of ubiquitinated AIPL-1 protein, suggesting that a normal function of sup-13/arrd-15 is to enhance degradation of ubiquitinated AIPL-1, thereby promoting transition of AIP1 isoforms from AIPL-1 to UNC-78 in developing muscle. These results suggest that α-arrestin is a novel factor to promote isoform turnover by enhancing protein degradation.

The actin cytoskeleton in hair bundle development and hearing loss

Inner ear hair cells assemble mechanosensitive hair bundles on their apical surface that transduce sounds and accelerations. Each hair bundle is comprised of ∼ 100 individual stereocilia that are arranged into rows of increasing height and width; their specific and precise architecture being necessary for mechanoelectrical transduction (MET). The actin cytoskeleton is fundamental to establishing this architecture, not only by forming the structural scaffold shaping each stereocilium, but also by composing rootlets and the cuticular plate that together provide a stable foundation supporting each stereocilium. In concert with the actin cytoskeleton, a large assortment of actin-binding proteins (ABPs) function to cross-link actin filaments into specific topologies, as well as control actin filament growth, severing, and capping. These processes are individually critical for sensory transduction and are all disrupted in hereditary forms of human hearing loss. In this review, we provide an overview of actin-based structures in the hair bundle and the molecules contributing to their assembly and functional properties. We also highlight recent advances in mechanisms driving stereocilia elongation and how these processes are tuned by MET.

Correlation between chemical denaturation and the unfolding energetics of Acanthamoeba actophorin

The actin filament network is in part remodeled by the action of a family of filament severing proteins that are responsible for modulating the ratio between monomeric and filamentous actin. Recent work on the protein actophorin from the amoeba Acanthamoeba castellani identified a series of site-directed mutations that increase the thermal stability of the protein by 22°C. Here, we expand this observation by showing that the mutant protein is also significantly stable to both equilibrium and kinetic chemical denaturation, and employ computer simulations to account for the increase in thermal or chemical stability through an accounting of atomic-level interactions. Specifically, the potential of mean force (PMF) can be obtained from steered molecular dynamics (SMD) simulations in which a protein is unfolded. However, SMD can be inefficient for large proteins as they require large solvent boxes, and computationally expensive as they require increasingly many SMD trajectories to converge the PMF. Adaptive steered molecular dynamics (ASMD) overcomes the second of these limitations by steering the particle in stages, which allows for convergence of the PMF using fewer trajectories compared with SMD. Use of the telescoping water scheme within ASMD partially overcomes the first of these limitations by reducing the number of waters at each stage to only those needed to solvate the structure within a given stage. In the PMFs obtained from ASMD, the work of unfolding Acto-2 was found to be higher than the Acto-WT by approximately 120 kCal/mol and reflects the increased stability seen in the chemical denaturation experiments. The evolution of the average number of hydrogen bonds and number of salt bridges during the pulling process provides a mechanistic view of the structural changes of the actophorin protein as it is unfolded, and how it is affected by the mutation in concert with the energetics reported through the PMF.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

A condensate dynamic instability orchestrates actomyosin cortex activation

A key event at the onset of development is the activation of a contractile actomyosin cortex during the oocyte-to-embryo transition^1-3. Here we report on the discovery that, in Caenorhabditis elegans oocytes, actomyosin cortex activation is supported by the emergence of thousands of short-lived protein condensates rich in F-actin, N-WASP and the ARP2/3 complex^4-8 that form an active micro-emulsion. A phase portrait analysis of the dynamics of individual cortical condensates reveals that condensates initially grow and then transition to disassembly before dissolving completely. We find that, in contrast to condensate growth through diffusion⁹, the growth dynamics of cortical condensates are chemically driven. Notably, the associated chemical reactions obey mass action kinetics that govern both composition and size. We suggest that the resultant condensate dynamic instability¹⁰ suppresses coarsening of the active micro-emulsion¹¹, ensures reaction kinetics that are independent of condensate size and prevents runaway F-actin nucleation during the formation of the first cortical actin meshwork.

Soft substrates promote direct chemical reprogramming of fibroblasts into neurons

Fibroblasts can be directly reprogrammed via a combination of small molecules to generate induced neurons (iNs), bypassing intermediate stages. This method holds great promise for regenerative medicine; however, it remains inefficient. Recently, studies have suggested that physical cues may improve the direct reprogramming of fibroblasts into neurons, but the underlying mechanisms remain to be further explored, and the physical factors reported to date do not exhibit the full properties of the extracellular matrix (ECM). Previous in vitro studies mainly used rigid polystyrene dishes, while one of the characteristics of the native in-vivo environment of neurons is the soft nature of brain ECM. The reported stiffness of brain tissue is very soft ranging between 100 Pa and 3 kPa, and the effect of substrate stiffness on direct neuronal reprogramming has not been explored. Here, we show for the first time that soft substrates substantially improved the production efficiency and quality of iNs, without needing to co-culture with glial cells during reprogramming, producing more glutamatergic neurons with electrophysiological functions in a shorter time. Transcriptome sequencing indicated that soft substrates might promote glutamatergic neuron reprogramming through integrins, actin cytoskeleton, Hippo signalling pathway, and regulation of mesenchymal-to-epithelial transition, and competing endogenous RNA network analysis provided new targets for neuronal reprogramming. We demonstrated that soft substrates may promote neuronal reprogramming by inhibiting microRNA-615-3p-targeting integrin subunit beta 4. Our findings can aid the development of regenerative therapies and help improve our understanding of neuronal reprogramming. STATEMENT OF SIGNIFICANCE: : First, we have shown that low stiffness promotes direct reprogramming on the basis of small molecule combinations. To the best of our knowledge, this is the first report on this type of method, which may greatly promote the progress of neural reprogramming. Second, we found that miR-615-3p may interact with ITGB4, and the soft substrates may promote neural reprogramming by inhibiting microRNA (miR)-615-3p targeting integrin subunit beta 4 (ITGB4). We are the first to report on this mechanism. Our findings will provide more functional neurons for subsequent basic and clinical research in neurological regenerative medicine, and will help to improve the overall understanding of neural reprogramming. This work also provides new ideas for the design of medical biomaterials for nerve regeneration.

Transcriptomic and Proteomic Analysis of Marine Nematode Litoditis marina Acclimated to Different Salinities

Salinity is a critical abiotic factor for all living organisms. The ability to adapt to different salinity environments determines an organism’s survival and ecological niches. Litoditis marina is a euryhaline marine nematode widely distributed in coastal ecosystems all over the world, although numerous genes involved in its salinity response have been reported, the adaptive mechanisms underlying its euryhalinity remain unexplored. Here, we utilized worms which have been acclimated to either low-salinity or high-salinity conditions and evaluated their basal gene expression at both transcriptomic and proteomic levels. We found that several conserved regulators, including osmolytes biosynthesis genes, transthyretin-like family genes, V-type H⁺-transporting ATPase and potassium channel genes, were involved in both short-term salinity stress response and long-term acclimation processes. In addition, we identified genes related to cell volume regulation, such as actin regulatory genes, Rho family small GTPases and diverse ion transporters, which might contribute to hyposaline acclimation, while the glycerol biosynthesis genes gpdh-1 and gpdh-2 accompanied hypersaline acclimation in L. marina. This study paves the way for further in-depth exploration of the adaptive mechanisms underlying euryhalinity and may also contribute to the study of healthy ecosystems in the context of global climate change.

Structure and activity of a thermally stable mutant of Acanthamoeba actophorin

Actophorin, which was recently tested for crystallization under microgravity on the International Space Station, was subjected to mutagenesis to identify a construct with improved biophysical properties that were expected to improve the extent of diffraction. First, 20 mutations, including one C-terminal deletion of three residues, were introduced individually into actophorin, resulting in modest increases in thermal stability of between +0.5°C and +2.2°C. All but two of the stabilizing mutants increased both the rates of severing F-actin filaments and of spontaneous polymerization of pyrenyl G-actin in vitro. When the individual mutations were combined into a single actophorin variant, Acto-2, the overall thermal stability was 22°C higher than that of wild-type actophorin. When an inactivating S2P mutation in Acto-2 was restored, Acto-2/P2S was more stable by 20°C but was notably more active than the wild-type protein. The inactivating S2P mutation reaffirms the importance that Ser2 plays in the F-actin-severing reaction. The crystal structure of Acto-2 was solved to 1.7 Å resolution in a monoclinic space group, a first for actophorin. Surprisingly, despite the increase in thermal stability, the extended β-turn region, which is intimately involved in interactions with F-actin, is disordered in one copy of Acto-2 in the asymmetric unit. These observations emphasize the complex interplay among protein thermal stability, function and dynamics.

Expression and Functional Analysis of cofilin1-like in Craniofacial Development in Zebrafish

Pharyngeal pouches, a series of outgrowths of the pharyngeal endoderm, are a key epithelial structure governing facial skeleton development in vertebrates. Pouch formation is achieved through collective cell migration and rearrangement of pouch-forming cells controlled by actin cytoskeleton dynamics. While essential transcription factors and signaling molecules have been identified in pouch formation, regulators of actin cytoskeleton dynamics have not been reported yet in any vertebrates. Cofilin1-like (Cfl1l) is a fish-specific member of the Actin-depolymerizing factor (ADF)/Cofilin family, a critical regulator of actin cytoskeleton dynamics in eukaryotic cells. Here, we report the expression and function of cfl1l in pouch development in zebrafish. We first showed that fish cfl1l might be an ortholog of vertebrate adf, based on phylogenetic analysis of vertebrate adf and cfl genes. During pouch formation, cfl1l was expressed sequentially in the developing pouches but not in the posterior cell mass in which future pouch-forming cells are present. However, pouches, as well as facial cartilages whose development is dependent upon pouch formation, were unaffected by loss-of-function mutations in cfl1l. Although it could not be completely ruled out a possibility of a genetic redundancy of Cfl1l with other Cfls, our results suggest that the cfl1l expression in the developing pouches might be dispensable for regulating actin cytoskeleton dynamics in pouch-forming cells.

Cofilin‑1 as a potential biomarker for Mycobacterium tuberculosis infection

Tuberculosis (TB) induced by Mycobacterium tuberculosis (M. tb), is one of the deadliest human infections worldwide. Our previous studies demonstrated cofilin-1 (CFL1) expression was significantly increased in exosomes from Mycobacterium avium (M. avium)-infected macrophages. The expression of CFL1 protein in M. tb infected hosts was investigated in the present study to predict whether CFL1 could have potential as a biomarker for M. tb infection. In the present study, the mRNA and protein expression levels of CFL1 in M. avium-infected macrophages and supernatants were analyzed via reverse transcription-quantitative PCR and western blotting. Furthermore, CFL1 expression in macrophages was knocked down in vivo, and then CFL1 expression levels in M. avium-infected macrophages and supernatant were detected via western blotting and ELISA. In addition, CFL1 was detected in the peripheral blood mononuclear cells and plasma of patients with TB using western blotting and ELISA. The specificity and sensitivity of CFL1 as a biomarker and the association between TB infection and normal individuals were compared and analyzed using GraphPad Prism 5. CFL1 protein expression levels were significantly increased in M. avium-infected macrophages and supernatant. Meanwhile, CFL1 was upregulated in patients with TB. Bioinformatics statistics indicated the high specificity and sensitivity of CFL1 in patients with TB. Thus, these results suggest that CFL1 may act as a potential biomarker of TB infection.

Improved resolution crystal structure of Acanthamoeba actophorin reveals structural plasticity not induced by microgravity

Actophorin, a protein that severs actin filaments isolated from the amoeba Acanthamoeba castellanii, was employed as a test case for crystallization under microgravity. Crystals of purified actophorin were grown under microgravity conditions aboard the International Space Station (ISS) utilizing an interactive crystallization setup between the ISS crew and ground-based experimenters. Crystals grew in conditions similar to those grown on earth. The structure was solved by molecular replacement at a resolution of 1.65 Å. Surprisingly, the structure reveals conformational changes in a remote β-turn region that were previously associated with actophorin phosphorylated at the terminal residue Ser1. Although crystallization under microgravity did not yield a higher resolution than crystals grown under typical laboratory conditions, the conformation of actophorin obtained from solving the structure suggests greater flexibility in the actophorin β-turn than previously appreciated and may be beneficial for the binding of actophorin to actin filaments.

Ca2+-dependent binding of S100A6 to cofilin-1 regulates actin filament polymerization-depolymerization dynamics

S100A6 is a Ca²⁺-binding protein belonging to the S100 family. Many reports indicate that S100A6 is involved in actin filament organization, however the mechanism of S100A6 action in this process is not fully understood. By screening S100A6 binding partners in NIH3T3 mouse fibroblasts, we have found that S100A6 binds cofilin-1, a protein required for the dynamics of actin polymerization and depolymerization. By applying various biochemical and cell biology assays, we have shown that S100A6 bound to cofilin-1 in a Ca²⁺-dependent manner and increased cofilin-1 affinity for F-actin. Microscopic analysis indicated that S100A6 significantly decreased severing of the actin filaments induced by cofilin-1. Moreover, in the presence of cofilin-1, S100A6 stabilized the filaments by inhibiting their depolymerization. When S100A6 was present at sub-stoichiometric concentrations in relation to actin, polymerization of G-actin accelerated by cofilin-1 was increased. At higher S100A6:actin ratios the polymerization rate was decreased. Altogether, these results show that S100A6 regulates actin filament dynamics by controlling activity of cofilin-1 and suggest that this regulation is Ca²⁺ -dependent.

Cellular Uptake Pathways of Nanoparticles: Process of Endocytosis and Factors Affecting Their Fate

BACKGROUND

Efficient and controlled internalization of NPs into the cells depends on their physicochemical properties and dynamics of the plasma membrane. NPs-cell interaction is a complex process that decides the fate of NPs internalization through different endocytosis pathways.

OBJECTIVE

The aim of this review is to highlight the physicochemical properties of synthesized nanoparticles (NPs) and their interaction with the cellular-dynamics and pathways like phagocytosis, pinocytosis, macropinocytosis, clathrin, and caveolae-mediated endocytosis and the involvement of effector proteins domain such as clathrin, AP2, caveolin, Arf6, Cdc42, dynamin and cell surface receptors during the endocytosis process of NPs.

METHOD

An electronic search was performed to explore the focused reviews and research articles on types of endocytosis and physicochemical properties of nanoparticles and their impact on cellular internalizations. The search was limited to peer-reviewed journals in the PubMed database.

RESULTS

This article discusses in detail how different types of NPs and their physicochemical properties such as size, shape, aspect ratio, surface charge, hydrophobicity, elasticity, stiffness, corona formation, surface functionalization changes the pattern of endocytosis in the presence of different pharmacological blockers. Some external forces like a magnetic field, electric field, and ultrasound exploit the cell membrane dynamics to permeabilize them for efficient internalization with respect to fundamental principles of membrane bending and pore formation.

CONCLUSION

This review will be useful to attract and guide the audience to understand the endocytosis mechanism and their pattern with respect to physicochemical properties of NPs to improve their efficacy and targeting to achieve the impactful outcome in drug-delivery and theranostics applications.

Enhancing single-cell bioconversion efficiency by harnessing nanosecond pulsed electric field processing

Nanosecond pulsed electric field (nsPEF) processing is gaining momentum as a physical means for single-cell bioconversion efficiency enhancement. The technology allows biomass yields per substrate (Y_X/S) to be leveraged and poses a viable option for stimulating intracellular compound production. NsPEF processing thus resonates with myriad domains spanning the pharmaceutical and medical sectors, as well as food and feed production. The exact working mechanisms underlying nsPEF-based enhancement of bioconversion efficiency, however, remain elusive, and a better understanding would be pivotal for leveraging process control to broaden the application of nsPEF and scale-up industrial implementation. To bridge this gap, the study provides the electrotechnological and metabolic fundamentals of nsPEF processing in the bio-based domain to enable a critical evaluation of pathways underlying the enhancement of single-cell bioconversion efficiency. Evidence suggests that treating cells during the rapid proliferating and thus the early to mid-exponential state of cellular growth is critical to promoting bioconversion efficiency. A combined effect of transient intracellular and sublethal stress induction and effects caused on the plasma membrane level result in an enhancement of cellular bioconversion efficiency. Congruency exists regarding the involvement of transient cytosolic Ca²⁺ hubs in nsPEF treatment responses, as well as that of reactive oxygen species formation culminating in the onset of cellular response pathways. A distinct assignment of single effects and their contributions to enhancing bioconversion efficiency, however, remains challenging. Current applications of nsPEF processing comprise microalgae, bacteria, and yeast biorefineries, but these endeavors are in their infancies with limitations associated with a lack of understanding of the underlying treatment mechanisms, an incomplete reporting, insufficient characterization, and control of processing parameters. The study aids in fostering the upsurge of nsPEF applications in the bio-based domain by providing a basis to gain a better understanding of cellular mechanisms underlying an nsPEF-based enhancement of cellular bioconversion efficiency and suggests best practice guidelines for nsPEF documentation for improved knowledge transfer. Better understanding and reporting of processes parameters and consequently improved process control could foster industrial-scale nsPEF realization and ultimately aid in perpetuating nsPEF applicability within the bio-based domain.

Electric fields regulate cellular elasticity through intracellular Ca2+ concentrations

Cellular elasticity is a key factor related to a broad range of physiological and pathological processes. The elasticity of a single cell has thus emerged as a potential biomarker to characterize the cellular state. Both internal and external stimuli affect cellular elasticity, and changes in elasticity can cause alterations in cellular characteristics or function. The application of electric fields (EFs) is a promising method that can be used to change cellular elasticity; however, the mechanisms underlying its effect remain unknown. Here, we demonstrate EFs-induced elasticity changes in human dermal fibroblasts and discuss the underlying mechanism related to actin polymerization. Cellular elasticity increases after EF (50 mV/mm) stimulation, reaching a maximum at 30 min before decreasing between 30 and 120 min. The cellular elasticity under EF stimulation, regardless of stimulation time, is higher than that of the control. F-actin regulates the elasticity of cells through gelsolin activation. We show changes in intracellular Ca²⁺ caused by EFs, which induced gelsolin activation and F-actin content changes. This result demonstrates a series of processes in which external electrical stimulation conditions regulate cellular elasticity.

The structure of the actin filament uncapping complex mediated by twinfilin

Uncapping of actin filaments is essential for driving polymerization and depolymerization dynamics from capping protein-associated filaments; however, the mechanisms of uncapping leading to rapid disassembly are unknown. Here, we elucidated the x-ray crystal structure of the actin/twinfilin/capping protein complex to address the mechanisms of twinfilin uncapping of actin filaments. The twinfilin/capping protein complex binds to two G-actin subunits in an orientation that resembles the actin filament barbed end. This suggests an unanticipated mechanism by which twinfilin disrupts the stable capping of actin filaments by inducing a G-actin conformation in the two terminal actin subunits. Furthermore, twinfilin disorders critical actin-capping protein interactions, which will assist in the dissociation of capping protein, and may promote filament uncapping through a second mechanism involving V-1 competition for an actin-binding surface on capping protein. The extensive interactions with capping protein indicate that the evolutionary conserved role of twinfilin is to uncap actin filaments.

Active site metals mediate an oligomeric equilibrium in Plasmodium M17 aminopeptidases

M17 leucyl aminopeptidases are metal-dependent exopeptidases that rely on oligomerization to diversify their functional roles. The M17 aminopeptidases from Plasmodium falciparum (PfA-M17) and Plasmodium vivax (Pv-M17) function as catalytically active hexamers to generate free amino acids from human hemoglobin and are drug targets for the design of novel antimalarial agents. However, the molecular basis for oligomeric assembly is not fully understood. In this study, we found that the active site metal ions essential for catalytic activity have a secondary structural role mediating the formation of active hexamers. We found that PfA-M17 and Pv-M17 exist in a metal-dependent dynamic equilibrium between active hexameric species and smaller inactive species that can be controlled by manipulating the identity and concentration of metals available. Mutation of residues involved in metal ion binding impaired catalytic activity and the formation of active hexamers. Structural resolution of Pv-M17 by cryoelectron microscopy and X-ray crystallography together with solution studies revealed that PfA-M17 and Pv-M17 bind metal ions and substrates in a conserved fashion, although Pv-M17 forms the active hexamer more readily and processes substrates faster than PfA-M17. On the basis of these studies, we propose a dynamic equilibrium between monomer ↔ dimer ↔ tetramer ↔ hexamer, which becomes directional toward the large oligomeric states with the addition of metal ions. This sophisticated metal-dependent dynamic equilibrium may apply to other M17 aminopeptidases and underpin the moonlighting capabilities of this enzyme family.

Emerging Functions of Actins and Actin Binding Proteins in Trypanosomatids

Actin is the major protein constituent of the cytoskeleton that performs wide range of cellular functions. It exists in monomeric and filamentous forms, dynamics of which is regulated by a large repertoire of actin binding proteins. However, not much was known about existence of these proteins in trypanosomatids, till the genome sequence data of three important organisms of this class, viz. Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, became available. Here, we have reviewed most of the findings reported to date on the intracellular distribution, structure and functions of these proteins and based on them, we have hypothesized some of their functions. The major findings are as follows: (1) All the three organisms encode at least a set of ten actin binding proteins (profilin, twinfilin, ADF/cofilin, CAP/srv2, CAPz, coronin, two myosins, two formins) and one isoform of actin, except that T. cruzi encodes for three formins and several myosins along with four actins. (2) Actin 1 and a few actin binding proteins (ADF/cofilin, profilin, twinfilin, coronin and myosin13 in L. donovani; ADF/cofilin, profilin and myosin1 in T. brucei; profilin and myosin-F in T.cruzi) have been identified and characterized. (3) In all the three organisms, actin cytoskeleton has been shown to regulate endocytosis and intracellular trafficking. (4) Leishmania actin1 has been the most characterized protein among trypanosomatid actins. (5) This protein is localized to the cytoplasm as well as in the flagellum, nucleus and kinetoplast, and in vitro, it binds to DNA and displays scDNA relaxing and kDNA nicking activities. (6) The pure protein prefers to form bundles instead of thin filaments, and does not bind DNase1 or phalloidin. (7) Myosin13, myosin1 and myosin-F regulate endocytosis and intracellular trafficking, respectively, in Leishmania, T. brucei and T. cruzi. (8) Actin-dependent myosin13 motor is involved in dynamics and assembly of Leishmania flagellum. (9) Leishmania twinfilin localizes mostly to the nucleolus and coordinates karyokinesis by effecting splindle elongation and DNA synthesis. (10) Leishmania coronin binds and promotes actin filament formation and exists in tetrameric form rather than trimeric form, like other coronins. (11) Trypanosomatid profilins are essential for survival of all the three parasites.

Reduced allergic lung inflammation and airway responsiveness in mice lacking the cytoskeletal protein gelsolin

Airway smooth muscle hyperresponsiveness associated with chronic airway inflammation leads to the typical symptoms of asthma including bronchoconstriction and wheezing. Asthma severity is associated with airway inflammation; therefore reducing airway inflammation is an important therapeutic target. Gelsolin is an actin capping and severing protein that has been reported to be involved in modulation of the inflammatory response. Using mice genetically lacking gelsolin, we evaluated the role of gelsolin in the establishment of house dust mite (HDM) antigen-induced allergic lung inflammation. The genetic absence of gelsolin was found to be protective against HDM sensitization, resulting in reduced lung inflammation, inflammatory cytokines and Muc5AC protein in bronchoalveolar lavage (BAL) fluid. The number of eosinophils, lymphocytes and interstitial macrophages in the BAL were increased after HDM sensitization in wild type mice, but were attenuated in gelsolin null mice. The observed attenuation of inflammation may be partly due to delayed migration of immune cells, because the reduced eosinophils in the BALs from gelsolin null mice compared to controls occurred despite similar amounts of the chemoattractant eotaxin. Splenic T cells demonstrated similar proliferation rates, but ex vivo alveolar macrophage migration was delayed in gelsolin null mice. In vivo, the reduced lung inflammation after HDM sensitization in gelsolin null mice was associated with significantly diminished airway resistance to inhaled methacholine compared with HDM-treated wild type mice. Our results suggest that modulation of gelsolin expression or function in selective inflammatory cell types that modulate allergic lung inflammation could be a therapeutic approach for asthma.

AimB Is a Small Protein Regulator of Cell Size and MreB Assembly

The MreB actin-like cytoskeleton assembles into dynamic polymers that coordinate cell shape in many bacteria. In contrast to most other cytoskeleton systems, few MreB-interacting proteins have been well characterized. Here, we identify a small protein from Caulobacter crescentus, an assembly inhibitor of MreB (AimB). AimB overexpression mimics inhibition of MreB polymerization, leading to increased cell width and MreB delocalization. Furthermore, aimB appears to be essential, and its depletion results in decreased cell width and increased resistance to A22, a small-molecule inhibitor of MreB assembly. Molecular dynamics simulations suggest that AimB binds MreB at its monomer-monomer protofilament interaction cleft and that this interaction is favored for C. crescentus MreB over Escherichia coli MreB because of a closer match in the degree of opening with AimB size, suggesting coevolution of AimB with MreB conformational dynamics in C. crescentus. We support this model through functional analysis of point mutants in both AimB and MreB, photo-cross-linking studies with site-specific unnatural amino acids, and species-specific activity of AimB. Together, our findings are consistent with AimB promoting MreB dynamics by inhibiting monomer-monomer assembly interactions, representing a new mechanism for regulating actin-like polymers and the first identification of a non-toxin MreB assembly inhibitor. Because AimB has only 104 amino acids and small proteins are often poorly characterized, our work suggests the possibility of more bacterial cytoskeletal regulators to be found in this class. Thus, like FtsZ and eukaryotic actin, MreB may have a rich repertoire of regulators to tune its precise assembly and dynamics.

A glance on the role of actin in osteogenic and adipogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) have the capacity to differentiate into multiple lineages including osteogenic and adipogenic lineages. An increasing number of studies have indicated that lineage commitment by MSCs is influenced by actin remodeling. Moreover, actin has roles in determining cell shape, nuclear shape, cell spreading, and cell stiffness, which eventually affect cell differentiation. Osteogenic differentiation is promoted in MSCs that exhibit a large spreading area, increased matrix stiffness, higher levels of actin polymerization, and higher density of stress fibers, whereas adipogenic differentiation is prevalent in MSCs with disrupted actin networks. In addition, the mechanical properties of F-actin empower cells to sense and transduce mechanical stimuli, which are also reported to influence differentiation. Various biomaterials, mechanical, and chemical interventions along with pathogen-induced actin alteration in the form of polymerization and depolymerization in MSC differentiation were studied recently. This review will cover the role of actin and its modifications through the use of different methods in inducing osteogenic and adipogenic differentiation.

Mislocalization of SMN from the I-band and M-band in human skeletal myofibers in spinal muscular atrophy associates with primary structural alterations of the sarcomere

Spinal muscular atrophy (SMA) is caused by a deletion or mutation of the survival motor neuron 1 (SMN1) gene. Reduced SMN levels lead to motor neuron degeneration and muscular atrophy. SMN protein localizes to the cytoplasm and Cajal bodies. Moreover, in myofibrils from Drosophila and mice, SMN is a sarcomeric protein localized to the Z-disc. Although SMN participates in multiple functions, including the biogenesis of spliceosomal small nuclear ribonucleoproteins, its role in the sarcomere is unclear. Here, we analyzed the sarcomeric organization of SMN in human control and type I SMA skeletal myofibers. In control sarcomeres, we demonstrate that human SMN is localized to the titin-positive M-band and actin-positive I-band, and to SMN-positive granules that flanked the Z-discs. Co-immunoprecipitation assays revealed that SMN interacts with the sarcomeric protein actin, α-actinin, titin, and profilin2. In the type I SMA muscle, SMN levels were reduced, and atrophic (denervated) and hypertrophic (nondenervated) myofibers coexisted. The hypertrophied myofibers, which are potential primary targets of SMN deficiency, exhibited sites of focal or segmental alterations of the actin cytoskeleton, where the SMN immunostaining pattern was altered. Moreover, SMN was relocalized to the Z-disc in overcontracted minisarcomeres from hypertrophic myofibers. We propose that SMN could have an integrating role in the molecular components of the sarcomere. Consequently, low SMN levels might impact the normal sarcomeric architecture, resulting in the disruption of myofibrils found in SMA muscle. This primary effect might be independent of the neurogenic myopathy produced by denervation and contribute to pathophysiology of the SMA myopathy.

Upregulation of cyclase-associated actin cytoskeleton regulatory protein 2 in epithelial ovarian cancer correlates with aggressive histologic types and worse outcomes

OBJECTIVE

Cyclase-associated actin cytoskeleton regulatory protein 2 (CAP2) regulates actin dynamics to control cell cycles and cell migration. CAP2 overexpression contributes to cancer progression in several tumor types; however, the role of CAP2 expression in ovarian cancer remains unclear. This study aimed to clarify the significance of CAP2 expression in epithelial ovarian tumor.

METHODS

We evaluated CAP2 expression in ovarian cancer cell lines using quantitative real-time polymerase chain reaction, western blotting and immunocytochemistry and examined the effect of CAP2 silencing in migration and proliferation assays. CAP2 immunohistochemistry was conducted using tissue specimens from 432 ovarian carcinoma patients; a further 55 borderline and benign 65 lesions were analyzed. CAP2 expression levels were defined as low, intermediate or high, for correlation analysis with clinicopathological factors.

RESULTS

CAP2 expression was significantly higher in cell lines from Type II ovarian cancer than in those in Type I, and knockdown of CAP2 showed decreased migration and proliferation. Higher levels of CAP2 expression in human tissues were associated with Type II histology, residual lesion, lymph node metastasis, ascites cytology and higher clinical stage. High CAP2 expression levels were observed in 26 (23.4%) of 111 Type II ovarian cancers and in 16 (5.0%) of 321 Type I cancers but not in any borderline or benign lesions. Multivariate analyses showed that CAP2 expression in ovarian cancer is an independent prognostic factor for recurrence-free survival (P = 0.019).

CONCLUSION

CAP2 expression is upregulated in aggressive histologic types of epithelial ovarian cancer and serves as a novel prognostic biomarker for patient survival.

AIP1 and Cofilin control the actin dynamics to modulate the asymmetric division and cytokinesis in mouse oocytes

Actin-interacting protein 1 (AIP1), also known as WD repeat-containing protein 1 (WDR1), is ubiquitous in eukaryotic organisms, and it plays critical roles in the dynamic reorganization of the actin cytoskeleton. However, the biological function and mechanism of AIP1 in mammalian oocyte maturation is still largely unclear. In this study, we demonstrated that AIP1 boosts ADF/Cofilin activity in mouse oocytes. AIP1 is primarily distributed around the spindle region during oocyte maturation, and its depletion impairs meiotic spindle migration and asymmetric division. The knockdown of AIP1 resulted in the gathering of a large number of actin-positive patches around the spindle region. This effect was reduced by human AIP1 (hAIP1) or Cofilin (S3A) expression. AIP1 knockdown also reduced the phosphorylation of Cofilin near the spindle, indicating that AIP1 interacts with ADF/Cofilin-decorated actin filaments and enhances filament disassembly. Moreover, the deletion of AIP1 disrupts Cofilin localization in metaphase I (MI) and induces cytokinesis defects in metaphase II (MII). Taken together, our results provide evidence that AIP1 promotes actin dynamics and cytokinesis via Cofilin in the gametes of female mice.

Dynamic Phosphorylation and Dephosphorylation of Cyclase-Associated Protein 1 by Antagonistic Signaling through Cyclin-Dependent Kinase 5 and cAMP Are Critical for the Protein Functions in Actin Filament Disassembly and Cell Adhesion

Cyclase-associated protein 1 (CAP1) is a conserved actin-regulating protein that enhances actin filament dynamics and also regulates adhesion in mammalian cells. We previously found that phosphorylation at the Ser307/Ser309 tandem site controls its association with cofilin and actin and is important for CAP1 to regulate the actin cytoskeleton. Here, we report that transient Ser307/Ser309 phosphorylation is required for CAP1 function in both actin filament disassembly and cell adhesion. Both the phosphomimetic and the nonphosphorylatable CAP1 mutant, which resist transition between phosphorylated and dephosphorylated forms, had defects in rescuing the reduced rate of actin filament disassembly in the CAP1 knockdown HeLa cells. The phosphorylation mutants also had defects in alleviating the elevated focal adhesion kinase (FAK) activity and the enhanced focal adhesions in the knockdown cells. In dissecting further phosphoregulatory cell signals for CAP1, we found that cyclin-dependent kinase 5 (CDK5) phosphorylates both Ser307 and Ser309 residues, whereas cAMP signaling induces dephosphorylation at the tandem site, through its effectors protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac). No evidence supports an involvement of activated protein phosphatase in executing the dephosphorylation downstream from cAMP, whereas preventing CAP1 from accessing its kinase CDK5 appears to underlie CAP1 dephosphorylation induced by cAMP. Therefore, this study provides direct cellular evidence that transient phosphorylation is required for CAP1 functions in both actin filament turnover and adhesion, and the novel mechanistic insights substantially extend our knowledge of the cell signals that function in concert to regulate CAP1 by facilitating its transient phosphorylation.

Overexpression of CFL1 in gastric cancer and the effects of its silencing by siRNA with a nanoparticle delivery system in the gastric cancer cell line

Gastric adenocarcinoma, like other cancers, is a multifactorial genetic disease, and metastasis of cancer cells is one of the main features of this illness. The expression levels of the CFL1 gene have been modulated in this pathway. Using small interfering RNA (siRNA) in the treatment of gastric cancer is considered a hopeful gene therapeutic approach. The present study reported the level of CFL1 genes between tumor and margin and healthy tissue of gastric cancer. Also, the features of a cationic nanoparticle with a polymer coating containing polyacrylic acid and polyethyleneimine that were used in the delivery of CFL1 siRNA, were shown. Then the cytotoxicity, cellular uptake, and gene silencing efficiency of this nanoparticle were evaluated with CFL1siRNA.

METHOD

In this study, the CFL1 gene expression was measured in 40 gastric adenocarcinoma, marginal and 15 healthy biopsy samples by a real-time polymerase chain reaction. Physicochemical characteristics, apoptosis, and inhibition of migration of the delivery of CFL1 siRNA by nanoparticle and lipofectamine were investigated in gastric cancer cells.

RESULT

The CFL1 expression was remarkably increased in gastric cancer tissues in comparison with the marginal samples and normal tissues (p < .05) and the biomarker index for CFL1 was obtained as 0.94, then this gene can be probably used as a biomarker for gastric cancer. After treatment of the AGS cell line by CFL1 siRNA, the CFL1 expression level of mRNA and migration in AGS cells were remarkably suppressed after transfection. Furthermore, the amount of apoptosis increased (p < .05).

CONCLUSION

Our results demonstrated that CFL1 downregulation in AGS cells can interdict cell migration. Finally, our outcomes propose that CFL1 can function as an oncogenic gene in gastric cancer and would be considered as a potential purpose of gene therapy for gastric cancer treatment.

Oligomerization Affects the Ability of Human Cyclase-Associated Proteins 1 and 2 to Promote Actin Severing by Cofilins

Actin-depolymerizing factor (ADF)/cofilins accelerate actin turnover by severing aged actin filaments and promoting the dissociation of actin subunits. In the cell, ADF/cofilins are assisted by other proteins, among which cyclase-associated proteins 1 and 2 (CAP1,2) are particularly important. The N-terminal half of CAP has been shown to promote actin filament dynamics by enhancing ADF-/cofilin-mediated actin severing, while the central and C-terminal domains are involved in recharging the depolymerized ADP-G-actin/cofilin complexes with ATP and profilin. We analyzed the ability of the N-terminal fragments of human CAP1 and CAP2 to assist human isoforms of "muscle" (CFL2) and "non-muscle" (CFL1) cofilins in accelerating actin dynamics. By conducting bulk actin depolymerization assays and monitoring single-filament severing by total internal reflection fluorescence (TIRF) microscopy, we found that the N-terminal domains of both isoforms enhanced cofilin-mediated severing and depolymerization at similar rates. According to our analytical sedimentation and native mass spectrometry data, the N-terminal recombinant fragments of both human CAP isoforms form tetramers. Replacement of the original oligomerization domain of CAPs with artificial coiled-coil sequences of known oligomerization patterns showed that the activity of the proteins is directly proportional to the stoichiometry of their oligomerization; i.e., tetramers and trimers are more potent than dimers, which are more effective than monomers. Along with higher binding affinities of the higher-order oligomers to actin, this observation suggests that the mechanism of actin severing and depolymerization involves simultaneous or consequent and coordinated binding of more than one N-CAP domain to F-actin/cofilin complexes.

Heavy Water (D2O) Containing Preservation Solution Reduces Hepatic Cold Preservation and Reperfusion Injury in an Isolated Perfused Rat Liver (IPRL) Model

Background: Heavy water (D₂O) has many biological effects due to the isotope effect of deuterium. We previously reported the efficacy of D₂O containing solution (Dsol) in the cold preservation of rat hearts. Here, we evaluated whether Dsol reduced hepatic cold preservation and reperfusion injury. Methods: Rat livers were subjected to 48-hour cold storage in University of Wisconsin (UW) solution or Dsol, and subsequently reperfused on an isolated perfused rat liver. Graft function, injury, perfusion kinetics, oxidative stress, and cytoskeletal integrity were assessed. Results: In the UW group, severe ischemia and reperfusion injury (IRI) was shown by histopathology, higher liver enzymes leakage, portal resistance, and apoptotic index, oxygen consumption, less bile production, energy charge, and reduced glutathione (GSH)/oxidized glutathione (GSSG) ratio (versus control). The Dsol group showed that these injuries were significantly ameliorated (versus the UW group). Furthermore, cytoskeletal derangement was progressed in the UW group, as shown by less degradation of α-Fodrin and by the inactivation of the actin depolymerization pathway, whereas these changes were significantly suppressed in the Dsol group. Conclusion: Dsol reduced hepatic IRI after extended cold preservation and subsequent reperfusion. The protection was primarily due to the maintenance of mitochondrial function, cytoskeletal integrity, leading to limiting oxidative stress, apoptosis, and necrosis pathways.

Hippocampal Wdr1 Deficit Impairs Learning and Memory by Perturbing F-actin Depolymerization in Mice

WD repeat protein 1 (Wdr1), known as a cofactor of actin-depolymerizing factor (ADF)/cofilin, is conserved among eukaryotes, and it plays a critical role in the dynamic reorganization of the actin cytoskeleton. However, the function of Wdr1 in the central nervous system remains elusive. Using Wdr1 conditional knockout mice, we demonstrated that Wdr1 plays a significant role in regulating synaptic plasticity and memory. The knockout mice exhibited altered reversal spatial learning and fear responses. Moreover, the Wdr1 CKO mice showed significant abnormalities in spine morphology and synaptic function, including enhanced hippocampal long-term potentiation and impaired long-term depression. Furthermore, we observed that Wdr1 deficiency perturbed actin rearrangement through regulation of the ADF/cofilin activity. Taken together, these results indicate that Wdr1 in the hippocampal CA1 area plays a critical role in actin dynamics in associative learning and postsynaptic receptor availability.

ADF/Cofilin-Mediated Actin Turnover Promotes Axon Regeneration in the Adult CNS

Injured axons fail to regenerate in the adult CNS, which contrasts with their vigorous growth during embryonic development. We explored the potential of re-initiating axon extension after injury by reactivating the molecular mechanisms that drive morphogenetic transformation of neurons during development. Genetic loss- and gain-of-function experiments followed by time-lapse microscopy, in vivo imaging, and whole-mount analysis show that axon regeneration is fueled by elevated actin turnover. Actin depolymerizing factor (ADF)/cofilin controls actin turnover to sustain axon regeneration after spinal cord injury through its actin-severing activity. This pinpoints ADF/cofilin as a key regulator of axon growth competence, irrespective of developmental stage. These findings reveal the central role of actin dynamics regulation in this process and elucidate a core mechanism underlying axon growth after CNS trauma. Thereby, neurons maintain the capacity to stimulate developmental programs during adult life, expanding their potential for plasticity. Thus, actin turnover is a key process for future regenerative interventions.

Rho GTPases-Emerging Regulators of Glucose Homeostasis and Metabolic Health

Rho guanosine triphosphatases (GTPases) are key regulators in a number of cellular functions, including actin cytoskeleton remodeling and vesicle traffic. Traditionally, Rho GTPases are studied because of their function in cell migration and cancer, while their roles in metabolism are less documented. However, emerging evidence implicates Rho GTPases as regulators of processes of crucial importance for maintaining metabolic homeostasis. Thus, the time is now ripe for reviewing Rho GTPases in the context of metabolic health. Rho GTPase-mediated key processes include the release of insulin from pancreatic β cells, glucose uptake into skeletal muscle and adipose tissue, and muscle mass regulation. Through the current review, we cast light on the important roles of Rho GTPases in skeletal muscle, adipose tissue, and the pancreas and discuss the proposed mechanisms by which Rho GTPases act to regulate glucose metabolism in health and disease. We also describe challenges and goals for future research.

Studies of postpartum mammary gland involution reveal novel pro-metastatic mechanisms

Postpartum involution is the process by which the lactating mammary gland returns to the pre-pregnant state after weaning. Expression of tumor-promotional collagen, upregulation of matrix metalloproteinases, infiltration of M2 macrophages, and remodeling of blood and lymphatic vasculature are all characteristics shared by the involuting mammary gland and breast tumor microenvironment. The tumor promotional nature of the involuting mammary gland is perhaps best evidenced by cases of postpartum breast cancer (PPBC), or those cases diagnosed within 10 years of most recent childbirth. Women with PPBC experience more aggressive disease and higher risk of metastasis than nulliparous patients and those diagnosed outside the postpartum window. Semaphorin 7a (SEMA7A), cyclooxygenase-2 (COX-2), and collagen are all expressed in the involuting mammary gland and, together, predict for decreased metastasis free survival in breast cancer. Studies investigating the role of these proteins in involution have been important for understanding their contributions to PPBC. Postpartum involution thus represents a valuable model for the identification of novel molecular drivers of PPBC and classical cancer hallmarks. In this review, we will highlight the similarities between involution and cancer in the mammary gland, and further define the contribution of SEMA7A/COX-2/collagen interplay to postpartum involution and breast tumor progression and metastasis.

Real-Time Single-Molecule Kinetic Analyses of AIP1-Enhanced Actin Filament Severing in the Presence of Cofilin

Rearrangement of actin filaments by polymerization, depolymerization, and severing is important for cell locomotion, membrane trafficking, and many other cellular functions. Cofilin and actin-interacting protein 1 (AIP1; also known as WDR1) are evolutionally conserved proteins that cooperatively sever actin filaments. However, little is known about the biophysical basis of the actin filament severing by these proteins. Here, we performed single-molecule kinetic analyses of fluorescently labeled AIP1 during the severing process of cofilin-decorated actin filaments. Results demonstrated that binding of a single AIP molecule was sufficient to enhance filament severing. After AIP1 binding to a filament, severing occurred with a delay of 0.7 s. Kinetics of binding and dissociation of a single AIP1 molecule to/from actin filaments followed a second-order and a first-order kinetics scheme, respectively. AIP1 binding and severing were detected preferentially at the boundary between the cofilin-decorated and bare regions on actin filaments. Based on the kinetic parameters explored in this study, we propose a possible mechanism behind the enhanced severing by AIP1.

Characterization of septin expression in normal and fibrotic kidneys

Chronic kidney disease (CKD) is characterized by the loss of nephrons and worsening organ-fibrosis that leads to deterioration and ultimately the total breakdown of kidney function. Renal fibrosis has become a major public health problem worldwide and necessitates hemodialysis and kidney transplantation in affected patients. CKD is mainly characterized by the activation and proliferation of interstitial fibroblasts and by excessive synthesis and accumulation of extracellular matrix components, causing the disruption of the normal tissue architecture of the kidney. Septins are GTPase proteins associated with membranes, actin filaments, and microtubules and are undoubtedly crucial for cytoskeleton organization. Although some septins are involved in liver fibrosis, they have not been investigated in the context of renal fibrosis. Here, we show that numerous septins are expressed in the healthy kidney and demonstrate in fibrotic mouse kidneys that various septins are remarkably up-regulated in the tubulointerstitium compared to contralateral control kidneys. We observed the same findings in human fibrotic kidneys. In both healthy and fibrotic kidneys, septins are coexpressed with extracellular matrix components, reinforcing the structural function of septins as cytoskeletal components. Furthermore, we could show in septin 8-deficient mice that septin 8 is dispensable for the formation of renal fibrosis, and that no other septin was compensatory changed in kidneys compared to wild-type mice.

Cofilin-1 phosphorylation catalyzed by ERK1/2 alters cardiac actin dynamics in dilated cardiomyopathy caused by lamin A/C gene mutation

Hyper-activation of extracellular signal-regulated kinase (ERK) 1/2 contributes to heart dysfunction in cardiomyopathy caused by mutations in the lamin A/C gene (LMNA cardiomyopathy). The mechanism of how this affects cardiac function is unknown. We show that active phosphorylated ERK1/2 directly binds to and catalyzes the phosphorylation of the actin depolymerizing factor cofilin-1 on Thr25. Cofilin-1 becomes active and disassembles actin filaments in a large array of cellular and animal models of LMNA cardiomyopathy. In vivo expression of cofilin-1, phosphorylated on Thr25 by endogenous ERK1/2 signaling, leads to alterations in left ventricular function and cardiac actin. These results demonstrate a novel role for cofilin-1 on actin dynamics in cardiac muscle and provide a rationale on how increased ERK1/2 signaling leads to LMNA cardiomyopathy.

Species-Specific Functions of Twinfilin in Actin Filament Depolymerization

Twinfilin is a highly conserved member of the actin depolymerization factor homology (ADF-H) protein superfamily, which also includes ADF/Cofilin, Abp1/Drebrin, GMF, and Coactosin. Twinfilin has a unique molecular architecture consisting of two ADF-H domains joined by a linker and followed by a C-terminal tail. Yeast Twinfilin, in conjunction with yeast cyclase-associated protein (Srv2/CAP), increases the rate of depolymerization at both the barbed and pointed ends of actin filaments. However, it has remained unclear whether these activities extend to Twinfilin homologs in other species. To address this, we purified the three mouse Twinfilin isoforms (mTwf1, mTwf2a, mTwf2b) and mouse CAP1, and used total internal reflection fluorescence microscopy assays to study their effects on filament disassembly. Our results show that all three mouse Twinfilin isoforms accelerate barbed end depolymerization similar to yeast Twinfilin, suggesting that this activity is evolutionarily conserved. In striking contrast, mouse Twinfilin isoforms and CAP1 failed to induce rapid pointed end depolymerization. Using chimeras, we show that the yeast-specific pointed end depolymerization activity is specified by the C-terminal ADF-H domain of yeast Twinfilin. In addition, Tropomyosin decoration of filaments failed to impede depolymerization by yeast and mouse Twinfilin and Srv2/CAP, but inhibited Cofilin severing. Together, our results indicate that Twinfilin has conserved functions in regulating barbed end dynamics, although its ability to drive rapid pointed end depolymerization appears to be species-specific. We discuss the implications of this work, including that pointed end depolymerization may be catalyzed by different ADF-H family members in different species.

Detection of sputum cofilin-1 as indicator of malignancy

Cofilin-1 (CFL1), a small protein of 18 kDa, has been studied as a biomarker due to its involvement in tumor cell migration and invasion. Our aim was to evaluate CFL1 as an indicator of malignancy and aggressiveness in sputum samples. CFL1 was analyzed by ELISA immunoassay in the sputum of 73 lung cancer patients, 13 cancer-free patients, and 6 healthy volunteers. Statistical analyses included ANOVA, ROC curves, Spearman correlation, and logistic regression. Sputum CFL1 levels were increased in cancer patients compared to cancer-free patients and volunteers (P<0.05). High expression of sputum CFL1 was correlated to T4 stage (P=0.01) and N stage (P=0.03), tobacco history (P=0.01), and squamous cell carcinoma histologic type (P=0.04). The accuracy of sputum CFL1 in discriminating cancer patients from cancer-free patients and healthy volunteers were 0.78 and 0.69, respectively. CFL1 at a cut-off value of 415.25 pg/mL showed sensitivity/specificity of 0.80/0.70 in differentiating between healthy volunteers and cancer patients. Sputum CFL1 was also able to identify cancer-free patients from patients with lung cancer. The AUC was 0.70 and, at a cut-off point ≥662.63 pg/mL, we obtained 60% sensitivity and 54% specificity. Logistic regression analysis controlled for tobacco history, histologic types, and N stage showed that cancer cell-associated CFL1 was an independent predictor of death. Smoker patients with squamous cell carcinoma, lymph node metastasis and sputum CFL1>1.475 pg/mL showed augmented chance of death, suggesting lung cancer aggressiveness. CFL1 presented diagnostic value in detecting lung cancer and was associated to tumor aggressiveness.

Structural basis for cofilin binding and actin filament disassembly

Actin depolymerizing factor (ADF) and cofilin accelerate actin dynamics by severing and disassembling actin filaments. Here, we present the 3.8 Å resolution cryo-EM structure of cofilactin (cofilin-decorated actin filament). The actin subunit structure of cofilactin (C-form) is distinct from those of F-actin (F-form) and monomeric actin (G-form). During the transition between these three conformations, the inner domain of actin (subdomains 3 and 4) and the majority of subdomain 1 move as two separate rigid bodies. The cofilin–actin interface consists of three distinct parts. Based on the rigid body movements of actin and the three cofilin–actin interfaces, we propose models for the cooperative binding of cofilin to actin, preferential binding of cofilin to ADP-bound actin filaments and cofilin-mediated severing of actin filaments.

Cofilin is a small actin-binding protein that accelerates actin turnover by disassembling actin filaments. Here the authors present the 3.8 Å cryo-EM structure of a cofilin-decorated actin filament and discuss mechanistic implications.

Structure, dynamics, and biochemical characterization of ADF/cofilin Twinstar from Drosophilamelanogaster

BACKGROUND

Twinstar is an ADF/cofilin family protein, which is expressed by the tsr gene in Drosophila melanogaster. Twinstar is one of the main regulators of actin cytoskeleton remodelling and is essential for vital cellular processes like cytokinesis and endocytosis.

METHODS

We have characterized the structure and dynamics of Twinstar by solution NMR spectroscopy, the interaction of Twinstar with rabbit muscle actin by ITC, and biochemical activities of Twinstar through different biochemical assays using fluorescence spectroscopy and ultra-centrifugation.

RESULTS

The solution structure of Twinstar shows characteristic ADF-H fold with well-formed G/F-site and F-site for interaction with actin. The structure possesses an extended F-loop, which is rigid at the base, but flexible towards its apical region. Twinstar shares similar dynamics for the G/F-site with C. elegans homologs, UNC-60A and UNC-60B. However, the dynamics of its F-loop are different from its C. elegans homologs. Twinstar shows strong affinity for ADP-G-Actin and ATP-G-Actin with K_ds of ~7.6 nM and ~0.4 μM, respectively. It shows mild F-actin depolymerizing activity and stable interaction with F-actin with a K_d of ~5.0 μM. It inhibits the rate of the nucleotide exchange in a dose dependent manner.

CONCLUSION

On the basis of structure, dynamics, and biochemical activity, Twinstar can be taken to execute its biochemical role by facilitating directional growth and maintenance of length of actin filaments.

GENERAL SIGNIFICANCE

This study characterizes the structure, backbone dynamics, and biochemical activities of Twinstar of Drosophila, which provides an insight into the regulation of actin dynamics in the member of phylum insecta.