Reorganization of the actin cytoskeleton in the protruding lamellae of human fibroblasts

Cytoskeleton-associated gelsolin responds to the midgut distention process in saline meal-fed Aedes aegypti and affects arbovirus dissemination from the midgut

The mosquito, Aedes aegypti, is the principal vector for several arboviruses. The mosquito midgut is the initial tissue that gets infected with an arbovirus acquired along with a blood meal from a vertebrate host. Blood meal ingestion leads to midgut tissue distention thereby increasing the pore size of the surrounding basal lamina. This allows newly synthesized virions to exit the midgut by traversing the distended basal lamina to infect secondary tissues of the mosquito. We conducted a quantitative label-free proteomic time course analysis with saline meal-fed Ae. aegypti females to identify host factors involved in midgut tissue distention. Around 2000 proteins were detected during each of the seven sampling time points and 164 of those were uniquely expressed. Forty-five of 97 differentially expressed proteins were upregulated during the 96-h time course and most of those were involved in cytoskeleton modulation, metabolic activity, and vesicle/vacuole formation. The F-actin-modulating Ae. aegypti (Aa)-gelsolin was selected for further functional studies. Stable knockout of Aa-gelsolin resulted in a mosquito line, which showed distorted actin filaments in midgut-associated tissues likely due to diminished F-actin processing by gelsolin. Zika virus dissemination from the midgut of these mosquitoes was diminished and delayed. The loss of Aa-gelsolin function was associated with an increased induction of apoptosis in midgut tissue indicating an involvement of Aa-gelsolin in apoptotic signaling in mosquitoes. Here, we used proteomics to discover a novel host factor, Aa-gelsolin, which affects the midgut escape barrier for arboviruses in mosquitoes and apoptotic signaling in the midgut.

A defective mechanosensing pathway affects fibroblast-to-myofibroblast transition in the old male mouse heart

The cardiac fibroblast interacts with an extracellular matrix (ECM), enabling myofibroblast maturation via a process called mechanosensing. Although in the aging male heart, ECM is stiffer than in the young mouse, myofibroblast development is impaired, as demonstrated in 2-D and 3-D experiments. In old male cardiac fibroblasts, we found a decrease in actin polymerization, α-smooth muscle actin (α-SMA), and Kindlin-2 expressions, the latter an effector of the mechanosensing. When Kindlin-2 levels were manipulated via siRNA interference, young fibroblasts developed an old-like fibroblast phenotype, whereas Kindlin-2 overexpression in old fibroblasts reversed the defective phenotype. Finally, inhibition of overactivated extracellular regulated kinases 1 and 2 (ERK1/2) in the old male fibroblasts rescued actin polymerization and α-SMA expression. Pathological ERK1/2 overactivation was also attenuated by Kindlin-2 overexpression. In contrast, old female cardiac fibroblasts retained an operant mechanosensing pathway. In conclusion, we identified defective components of the Kindlin/ERK/actin/α-SMA mechanosensing axis in aged male fibroblasts.

Lipid signaling affects primary fibroblast collective migration and anchorage in response to stiffness and microtopography

Cell migration is regulated by several mechanotransduction pathways, which consist of sensing and converting mechanical microenvironmental cues to internal biochemical cellular signals, such as protein phosphorylation and lipid signaling. While there has been significant progress in understanding protein changes in the context of mechanotransduction, lipid signaling is more difficult to investigate. In this study, physical cues of stiffness (10, 100, 400 kPa, and glass), and microrod or micropost topography were manipulated in order to reprogram primary fibroblasts and assess the effects of lipid signaling on the actin cytoskeleton. In an in vitro wound closure assay, primary cardiac fibroblast migration velocity was significantly higher on soft polymeric substrata. Modulation of PIP2 availability through neomycin treatment nearly doubled migration velocity on 10 kPa substrata, with significant increases on all stiffnesses. The distance between focal adhesions and the lamellar membrane (using wortmannin treatment to increase PIP2 via PI3K inhibition) was significantly shortest compared to untreated fibroblasts grown on the same surface. PIP2 localized to the leading edge of migrating fibroblasts more prominently in neomycin-treated cells. The membrane-bound protein, lamellipodin, did not vary under any condition. Additionally, fifteen micron-high micropost topography, which blocks migration, concentrates PIP2 near to the post. Actin dynamics within stress fibers, measured by fluorescence recovery after photobleaching, was not significantly different with stiffness, microtopography, nor with drug treatment. PIP2-modulating drugs delivered from microrod structures also affected migration velocity. Thus, manipulation of the microenvironment and lipid signaling regulatory drugs might be beneficial in improving therapeutics geared toward wound healing.

Gelsolin activity controls efficient early HIV-1 infection

Background

HIV-1 entry into target lymphocytes requires the activity of actin adaptors that stabilize and reorganize cortical F-actin, like moesin and filamin-A. These alterations are necessary for the redistribution of CD4-CXCR4/CCR5 to one pole of the cell, a process that increases the probability of HIV-1 Envelope (Env)-CD4/co-receptor interactions and that generates the tension at the plasma membrane necessary to potentiate fusion pore formation, thereby favouring early HIV-1 infection. However, it remains unclear whether the dynamic processing of F-actin and the amount of cortical actin available during the initial virus-cell contact are required to such events.

Results

Here we show that gelsolin restructures cortical F-actin during HIV-1 Env-gp120-mediated signalling, without affecting cell-surface expression of receptors or viral co-receptor signalling. Remarkably, efficient HIV-1 Env-mediated membrane fusion and infection of permissive lymphocytes were impaired when gelsolin was either overexpressed or silenced, which led to a loss or gain of cortical actin, respectively. Indeed, HIV-1 Env-gp120-induced F-actin reorganization and viral receptor capping were impaired under these experimental conditions. Moreover, gelsolin knockdown promoted HIV-1 Env-gp120-mediated aberrant pseudopodia formation. These perturbed-actin events are responsible for the inhibition of early HIV-1 infection.

Conclusions

For the first time we provide evidence that through its severing of cortical actin, and by controlling the amount of actin available for reorganization during HIV-1 Env-mediated viral fusion, entry and infection, gelsolin can constitute a barrier that restricts HIV-1 infection of CD4+ lymphocytes in a pre-fusion step. These findings provide important insights into the complex molecular and actin-associated dynamics events that underlie early viral infection. Thus, we propose that gelsolin is a new factor that can limit HIV-1 infection acting at a pre-fusion step, and accordingly, cell-signals that regulate gelsolin expression and/or its actin-severing activity may be crucial to combat HIV-1 infection.

Regulation of actin dynamics by protein kinase R control of gelsolin enforces basal innate immune defense

Primary resistance to pathogens is reliant on both basal and inducible immune defenses. To date, research has focused upon inducible innate immune responses. In contrast to resistance via cytokine induction, basal defense mechanisms are less evident. Here we showed that the antiviral protein kinase R (PKR) inhibited the key actin-modifying protein gelsolin to regulate actin dynamics and control cytoskeletal cellular functions under homeostatic conditions. Through this mechanism, PKR controlled fundamental innate immune, actin-dependent processes that included membrane ruffling and particle engulfment. Accordingly, PKR counteracted viral entry into the cell. These findings identify a layer of host resistance, showing that the regulation of actin-modifying proteins during the innate immune response bolsters first-line defense against intracellular pathogens and has a sustained effect on virus production. Moreover, these data provide proof of principle for a concept in which the cell cytoskeleton could be targeted to elicit broad antiviral protection.

Direct dynamin-actin interactions regulate the actin cytoskeleton

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

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

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

H-Ras and PI3K are required for the formation of circular dorsal ruffles induced by low-power laser irradiation

The formation of circular dorsal ruffles upon growth factor stimulation facilitates the static cells for subsequent motility. Low-power laser irradiation (LPLI) has been shown to exert some promotive effects on migration and proliferation in various cell types. It is unclear whether LPLI could induce the formation of circular ruffles. In this study, using confocal fluorescence microscope, we for the first time demonstrated that LPLI could induce the production of circular ruffle structures in COS-7 cells. These structures were proved to be actin-based and originated from membrane microdomains enriched in cholesterol. Ras was shown to be activated by LPLI and expression of YFP-H-Ras (N17), a dominant negative H-Ras, blocked the generation of circular ruffles induced by LPLI. Wortmannin, PI3K inhibitor, potently suppressed the formation of LPLI-induced circular ruffles in a dose-dependent manner. However, blocking the activation of PKC, which was activated during LPLI-induced cell proliferation in our previous study, had no effect on the formation of circular ruffles. Thus, both H-Ras and PI3K were required for the formation of circular ruffles induced by LPLI and the generation of circular ruffles provides new information for the mechanisms of biological effects of LPLI. J. Cell. Physiol. 219: 535-543, 2009. (c) 2009 Wiley-Liss, Inc.

One step ahead: role of filopodia in adhesion formation during cell migration of keratinocytes

Cell adhesion is an essential prerequisite for cell function and movement. It depends strongly on focal adhesion complexes connecting the extracellular matrix to the actin cytoskeleton. Especially in moving cells focal adhesions are highly dynamic and believed to be formed closely behind the leading edge. Filopodia were thought to act mainly as guiding cues using their tip complexes for elongation. Here we show for keratinocytes a strong dependence of lamellipodial adhesion sites on filopodia. Upon stable contact of the VASP-containing tip spot to the substrate, a filopodial focal complex (filopodial FX) is formed right behind along the filopodia axis. These filopodial FXs are fully assembled, yet small adhesions containing all adhesion markers tested. Filopodial FXs when reached by the lamellipodium are just increased in size resulting in classical focal adhesions. At the same time most filopodia regain their elongation ability. Blocking filopodia inhibits development of new focal adhesions in the lamellipodium, while focal adhesion maturation in terms of vinculin exchange dynamics remains active. Our data therefore argue for a strong spatial and temporal dependence of focal adhesions on filopodial focal complexes in keratinocytes with filopodia not permanently initiated via new clustering of actin filaments to induce elongation.

Injectable iron-modified apatitic bone cement intended for kyphoplasty: cytocompatibility study

In this study, the cytocompatibility of human ephitelial (HEp-2) cells cultured on new injectable iron-modified calcium phosphate cements (IM-CPCs) has been investigated in terms of cell adhesion, cell proliferation, and morphology. Quantitative MTT-assay and scanning electron microscopy (SEM) showed that cell adhesion and viability were not affected with culturing time by iron concentration in a dose-dependent manner. SEM-cell morphology showed that HEp-2 cells, seeded on IM-CPCs, were able to adhere, spread, and attain normal morphology. These results showed that the new injectable IM-CPCs have cytocompatible features of interest to the intended kyphophasty application, for the treatment of osteoporotic vertebral compression fractures.

Altered gene expression in Choristoneura fumiferana and Manduca sexta in response to sublethal intoxication by Bacillus thuringiensis Cry1Ab toxin

In order to understand how lepidopteran insects react physiologically to Bacillus thuringiensis crystal toxin ingestion, transcriptional profiling of Choristoneura fumiferana larvae exposed to sublethal doses of Cry1Ab protoxin were monitored using a C. fumiferana-specific cDNA microarray derived from a protoxin-specific subtractive library. Differential gene expression occurred primarily between 2 and 5 h postingestion. Metabolic enzymes such as lipases and proteases were generally repressed, whereas genes involved in detoxification, immune system regulation or general stress response were upregulated. A similar protoxin-specific transcriptional pattern was also observed with Manduca sexta larvae, using three upregulated genes (serpin, cytochrome P450 and carboxyl/cholinesterase) and one downregulated gene (beta-glucosidase), suggesting that a susceptible larval response to Cry toxin exposure might be universal among lepidopterous insects.

Get off my back! Rapid receptor internalization through circular dorsal ruffles

Internalization and subsequent trafficking of receptor tyrosine kinases (RTKs) play an important role in the modulation of growth factor-stimulated signaling events that affect different cellular processes, from cell growth and mitosis to motility and invasion. The intracellular transport of these receptors has traditionally been viewed as being initiated via clathrin-coated pits. However, nonclathrin pathways have been implicated as well, although these remain poorly understood. Most recently, the formation of dynamic, transient endocytic membrane structures termed circular dorsal ruffles or "dorsal waves" have been reported to selectively sequester and internalize a large percentage of a specific RTK from the surface of growth factor-stimulated cells. This process is dependent on dynamin and cortactin, two endocytic proteins that are also associated with the actin cytoskeleton, whereas it is independent of traditional coat proteins, such as clathrin and caveolin. Additionally, dorsal wave formation requires the participation and remodeling of a dynamic actin cytoskeleton. Most importantly, the formation of these structures may be less frequent in tumor cells and thereby have significant effects on receptor signaling and cell growth.

A novel endocytic mechanism of epidermal growth factor receptor sequestration and internalization

Cells form transient, circular dorsal ruffles or "waves" in response to stimulation of receptor tyrosine kinases, including epidermal growth factor receptor (EGFR) or platelet-derived growth factor receptor. These dynamic structures progress inward on the dorsal surface and disappear, occurring concomitantly with a marked reorganization of F-actin. The cellular function of these structures is largely unknown. Here we show that EGF-induced waves selectively sequester and internalize approximately 50% of ligand-bound EGFR from the cell surface. This process requires receptor phosphorylation, active phosphatidylinositol 3-kinase, and dynamin 2, although clathrin-coated pits or caveolae are not required. Epithelial and fibroblast cells stimulated with EGF sequestered EGFR rapidly into waves that subsequently generated numerous receptor-positive tubular-vesicular structures. Electron microscopy confirmed that waves formed along the dorsal membrane surface and extended numerous tubules into the cytoplasm. These findings characterize a structure that selectively sequesters large numbers of activated EGFR for their subsequent internalization, independent of traditional endocytic mechanisms such as clathrin pits or caveolae.