Chemoattractant-stimulated polymorphonuclear leukocytes contain two populations of actin filaments that differ in their spatial distributions and relative stabilities

Amoeboid-like migration ensures correct horizontal cell layer formation in the developing vertebrate retina

Migration of cells in the developing brain is integral for the establishment of neural circuits and function of the central nervous system. While migration modes during which neurons employ predetermined directional guidance of either preexisting neuronal processes or underlying cells have been well explored, less is known about how cells featuring multipolar morphology migrate in the dense environment of the developing brain. To address this, we here investigated multipolar migration of horizontal cells in the zebrafish retina. We found that these cells feature several hallmarks of amoeboid-like migration that enable them to tailor their movements to the spatial constraints of the crowded retina. These hallmarks include cell and nuclear shape changes, as well as persistent rearward polarization of stable F-actin. Interference with the organization of the developing retina by changing nuclear properties or overall tissue architecture hampers efficient horizontal cell migration and layer formation showing that cell-tissue interplay is crucial for this process. In view of the high proportion of multipolar migration phenomena observed in brain development, the here uncovered amoeboid-like migration mode might be conserved in other areas of the developing nervous system.

Cytochalasin B Modulates Nanomechanical Patterning and Fate in Human Adipose-Derived Stem Cells

Cytoskeletal proteins provide architectural and signaling cues within cells. They are able to reorganize themselves in response to mechanical forces, converting the stimuli received into specific cellular responses. Thus, the cytoskeleton influences cell shape, proliferation, and even differentiation. In particular, the cytoskeleton affects the fate of mesenchymal stem cells (MSCs), which are highly attractive candidates for cell therapy approaches due to their capacity for self-renewal and multi-lineage differentiation. Cytochalasin B (CB), a cyto-permeable mycotoxin, is able to inhibit the formation of actin microfilaments, resulting in direct effects on cell biological properties. Here, we investigated for the first time the effects of different concentrations of CB (0.1–10 μM) on human adipose-derived stem cells (hASCs) both after 24 h (h) of CB treatment and 24 h after CB wash-out. CB influenced the metabolism, proliferation, and morphology of hASCs in a dose-dependent manner, in association with progressive disorganization of actin microfilaments. Furthermore, the removal of CB highlighted the ability of cells to restore their cytoskeletal organization. Finally, atomic force microscopy (AFM) revealed that cytoskeletal changes induced by CB modulated the viscoelastic properties of hASCs, influencing their stiffness and viscosity, thereby affecting adipogenic fate.

Context-Dependent Role of Vinculin in Neutrophil Adhesion, Motility and Trafficking

Neutrophils are innate immune effector cells that traffic from the circulation to extravascular sites of inflammation. β2 integrins are important mediators of the processes involved in neutrophil recruitment. Although neutrophils express the cytoskeletal protein vinculin, they do not form mature focal adhesions. Here, we characterize the role of vinculin in β2 integrin-dependent neutrophil adhesion, migration, mechanosensing, and recruitment. We observe that knockout of vinculin attenuates, but does not completely abrogate, neutrophil adhesion, spreading, and crawling under static conditions. However, we also found that vinculin deficiency does not affect these behaviors in the presence of forces from fluid flow. In addition, we identify a role for vinculin in mechanosensing, as vinculin-deficient neutrophils exhibit attenuated spreading on stiff, but not soft, substrates. Consistent with these findings, we observe that in vivo neutrophil recruitment into the inflamed peritoneum of mice remains intact in the absence of vinculin. Together, these data suggest that while vinculin regulates some aspects of neutrophil adhesion and spreading, it may be dispensable for β2 integrin-dependent neutrophil recruitment in vivo.

Gingipains disrupt F-actin and cause osteoblast apoptosis via integrin β1

BACKGROUND AND OBJECTIVE

The aim of this study was to explore the cellular mechanisms underlying gingipain-caused changes in cell morphology and apoptosis of osteoblasts.

MATERIAL AND METHODS

Human calvarial osteoblasts and mouse osteoblasts MC3T3-E1 were treated with gingipain extracts from Porphyromonas gingivalis stain W83. Apoptosis was detected with annexin V and propidium iodide flow cytometry analysis or terminal deoxynucleotidyl transferase mediated dUTP nick-end labeling staining. F-actin was determined by immunostaining. Western blotting was used to detect protein expression. Knocking down and overexpressing approaches were used to determine the role of integrin β1.

RESULTS

Osteoblasts exposed to gingipain extracts displayed increased apoptosis, accompanied by loss of F-actin integrity and cell shrinkage. The effects of gingipain extracts were abolished by the cysteine protease inhibitor N-tosyl-l-lysyl chloromethyl-ketone. Notably, gingipain extracts resulted in reduction of integrin β1, accompanied by diminished active RhoA whereas without effect on the total RhoA. Knockdown of integrin β1 resembled those seen in gingipain-treated osteoblasts. By contrast, the effects of gingipain extracts were abrogated by either overexpression of integrin β1 or presence of RhoA agonist CN03.

CONCLUSION

Gingipain-induced F-actin disruption and apoptosis are mediated by the degradation of integrin β1 and inhibition of RhoA activity, which account for osteoblast apoptosis.

Role of the Cytoskeleton in Myeloid Cell Function

During an innate immune response, myeloid cells undergo complex morphological adaptations in response to inflammatory cues, which allow them to exit the vasculature, enter the tissues, and destroy invading pathogens. The actin and microtubule cytoskeletons are central to many of the most essential cellular functions including cell division, cell morphology, migration, intracellular trafficking, and signaling. Cytoskeletal structure and regulation are crucial for many myeloid cell functions, which require rapid and dynamic responses to extracellular signals. In this chapter, we review the roles of the actin and microtubule cytoskeletons in myeloid cells, focusing primarily on their roles in chemotaxis and phagocytosis. The role of myeloid cell cytoskeletal defects in hematological disorders is highlighted throughout.

Role of Actin Cytoskeleton During Mammalian Sperm Acrosomal Exocytosis

Mammalian sperm require to undergo an exocytotic process called acrosomal exocytosis in order to be able to fuse with the oocyte. This ability is acquired during the course of sperm capacitation. This review is focused on one aspect related to this acquisition: the role of the actin cytoskeleton. Evidence from different laboratories indicates that actin polymerization occurs during capacitation, and the detection of several actin-related proteins suggests that the cytoskeleton is involved in important sperm functions. In other mammalian cells, the cortical actin network acts as a dominant negative clamp which blocks constitutive exocytosis but, at the same time, is necessary to prepare the cell to undergo regulated exocytosis. Thus, F-actin stabilizes structures generated by exocytosis and supports the physiological progression of this process. Is this also the case in mammalian sperm? This review summarizes what is currently known about actin and its related proteins in the male gamete, with particular emphasis on their role in acrosomal exocytosis.

Motile Human Neutrophils Sense Ligand Density Over Their Entire Contact Area

Neutrophils are key components of the immune system and motility is central their function during the inflammatory response. We have previously demonstrated that neutrophils are capable of switching their motile phenotype between amoeboid-like and keratocyte-like in response to the ligand density of adhesion molecules (Henry et al. in Int Biol 6:348-356, 2014). In this study, we engineered planar micropatterned surfaces that presented adhesion molecules in local islands of high density, separated by regions largely devoid of ligands. By controlling the geometry of islands we made arrays in which the local (on island) adhesion density was high but the global (multi-island) adhesion density over the entire cell-substrate interface was low. Neutrophils in contact with these island arrays assumed a well-spread and directionally-persistent motile phenotype (keratocyte-like) in contrast to the classical amoeboid morphology they display on uniform fields of high adhesion density. By virtue of our rationally designed substrates, we were able to conclude that neutrophils were integrating the stimulation received across their entire contact interface; furthermore, they were able to mount this whole cell response on the timescale of seconds. This work demonstrates the capacity of adhesive microenvironments to direct the phenotype of cell motility, which has broader implications in physiologic processes such as inflammation and cancer metastasis.

Temperature oscillations drive cycles in the activity of MMP-2,9 secreted by a human trabecular meshwork cell line

PURPOSE

Aqueous humor inflow falls 50% during sleeping hours without proportional fall in IOP, partly reflecting reduced outflow facility. The mechanisms underlying outflow facility cycling are unknown. One outflow facility regulator is matrix metalloproteinase (MMP) release from trabecular meshwork (TM) cells. Because anterior segment temperature must oscillate due to core temperature cycling and eyelid closure during sleep, we tested whether physiologically relevant temperature oscillations drive cycles in the activity of secreted MMP.

METHODS

Temperature of transformed normal human TM cells (hTM5 line) was fixed or alternated 12 hours/12 hours between 33°C and 37°C. Activity of secreted MMP-2 and MMP-9 was measured by zymography, and gene expression by RT-PCR and quantitative PCR.

RESULTS

Raising temperature to 37°C increased, and lowering to 33°C reduced, activity of secreted MMP. Switching between 37°C and 33°C altered MMP-9 by 40% ± 3% and MMP-2 by 22% ± 2%. Peripheral circadian clocks did not mediate temperature-driven cycling of MMP secretion because MMP-release oscillations did not persist at constant temperature after 3 to 6 days of alternating temperatures, and temperature cycles did not entrain clock-gene expression in these cells. Furthermore, inhibiting heat shock transcription factor 1, which links temperature and peripheral clock-gene oscillations, inhibited MMP-9 but not MMP-2 temperature-driven MMP cycling. Inhibition of heat-sensitive TRPV1 channels altered total MMP secretion but not temperature-induced modulations. Inhibiting cold-sensitive TRPM-8 channels had no effect.

CONCLUSIONS

Physiologically relevant temperature oscillations drive fluctuations of secreted MMP-2 and MMP-9 activity in hTM5 cells independent of peripheral clock genes and temperature-sensitive TRP channels.

Ligand density elicits a phenotypic switch in human neutrophils

Neutrophils are mediators of innate immunity and motility is critical to their function. We used microcontact printing to investigate the relationship between density of adhesive ligands and the dynamics of neutrophil motility. We show that neutrophils adopt a well-spread morphology without a uropod on moderate densities of adhesion ligand. As density is increased, the morphology switches to a classic amoeboid shape. In addition to the morphological differences, the dynamics of motility were quantitatively distinct. Well-spread cells without uropods glide slowly with high persistence, while amoeboid cells made frequent directional changes migrating quickly with low persistence. Using an antibody panel against various integrin chains, we show that adhesion and motility on fibronectin are mediated by MAC-1 (αMβ2). The phenotypic switch could be generalized to other surface ligands, such as bovine serum albumin, to which the promiscuous MAC-1 also binds. These results suggest that neutrophils are capable of displaying multiple modes of motility as dictated by their adhesive environment.

Phenotypic analysis of chemokine-driven actin reorganization in primary human neutrophils

The chemokine-driven activation of CXC-type chemokine receptors 1/2 (CXCR1/2) and the subsequent reorganization of the neutrophilic actin are early key events in the induction of neutrophil migration toward centers of inflammation. In this study, an image analysis algorithm was developed to detect subtle chemokine-induced changes in the actin cytoskeleton of primary human neutrophils. By this means, a discrete early step of neutrophil activation was dissected that could be initiated by concentrations of growth-related oncogen α (Gro-α) or interleukin-8 (IL-8) just above their resting-state plasma levels. The associated half-maximal effective concentration (EC50) values for Gro-α and IL-8 of 8 and 22 pM, respectively, are between two and three orders of magnitude below the so-far reported EC50 values of these chemokines for the induction of neutrophilic calcium release, integrin expression, degranulation, and receptor internalization. Sch527123, a known inhibitor of CXCR2 (KD=49 pM) and with a lower potency/affinity also of CXCR1 (KD=3.9 nM), antagonized actin remodeling with half-maximal inhibitory concentration (IC50) values of 400 pM for the CXCR2-specific agonist Gro-α and of 36 nM for the CXCR1/2-promiscuous agonist IL-8. This observation indicates that the here-described early step of chemokine-driven actin reorganization is modulated by both CXCR1 and CXCR2. Thus, the imaging-based assay format, as developed in this work, may be employed in a phenotypic screening campaign to identify inhibitors of an early step in CXCR1/2-induced neutrophilic chemotaxis.

Actin dynamics rapidly reset chemoattractant receptor sensitivity following adaptation in neutrophils

Neutrophils are cells of the innate immune system that hunt and kill pathogens using directed migration. This process, known as chemotaxis, requires the regulation of actin polymerization downstream of chemoattractant receptors. Reciprocal interactions between actin and intracellular signals are thought to underlie many of the sophisticated signal processing capabilities of the chemotactic cascade including adaptation, amplification and long-range inhibition. However, with existing tools, it has been difficult to discern actin's role in these processes. Most studies investigating the role of the actin cytoskeleton have primarily relied on actin-depolymerizing agents, which not only block new actin polymerization but also destroy the existing cytoskeleton. We recently developed a combination of pharmacological inhibitors that stabilizes the existing actin cytoskeleton by inhibiting actin polymerization, depolymerization and myosin-based rearrangements; we refer to these processes collectively as actin dynamics. Here, we investigated how actin dynamics influence multiple signalling responses (PI3K lipid products, calcium and Pak phosphorylation) following acute agonist addition or during desensitization. We find that stabilized actin polymer extends the period of receptor desensitization following agonist binding and that actin dynamics rapidly reset receptors from this desensitized state. Spatial differences in actin dynamics may underlie front/back differences in agonist sensitivity in neutrophils.

Brodde M. State of the art in platelet function testing

Platelets perform many functions in hemostasis but also in other areas of physiology and pathology. Therefore, it is obvious that many different function tests have been developed, each one conceived and standardized for a special purpose. This review will summarize the different fields in which platelet function testing is currently in use; diagnostics of patients with bleeding disorders, monitoring patients' response to anti-platelet therapy, monitoring in transfusion medicine (blood donors, platelet concentrates, and after transfusion), and monitoring in perioperative medicine to predict bleeding tendency. The second part of the review outlines different methods for platelet function testing, spanning bleeding time, and platelet counting as well as determining platelet adhesion, platelet secretion, platelet aggregation, platelet morphology, platelet signal transduction, platelet procoagulant activity, platelet apoptosis, platelet proteomics, and molecular biology.

Promotion of Ccn2 expression and osteoblastic differentiation by actin polymerization, which is induced by laminar fluid flow stress

Fluid flow stress (FSS) is a major mechanical stress that induces bone remodeling upon orthodontic tooth movement, whereas CCN family protein 2 (CCN2) is a potent regenerator of bone defects. In this study, we initially evaluated the effect of laminar FSS on Ccn2 expression and investigated its mechanism in osteoblastic MC3T3-E1 cells. The Ccn2 expression was drastically induced by uniform FSS in an intensity dependent manner. Of note, the observed effect was inhibited by a Rho kinase inhibitor Y27632. Moreover, the inhibition of actin polymerization blocked the FSS-induced activation of Ccn2, whereas inducing F-actin formation using cytochalasin D and jasplakinolide enhanced Ccn2 expression in the same cells. Finally, F-actin formation was found to induce osteoblastic differentiation. In addition, activation of cyclic AMP-dependent kinase, which inhibits Rho signaling, abolished the effect of FSS. Collectively, these findings indicate the critical role of actin polymerization and Rho signaling in CCN2 induction and bone remodeling provoked by FSS.

Regulation of glucose transport by ROCK1 differs from that of ROCK2 and is controlled by actin polymerization

A role of Rho-associated coiled-coil-containing protein kinase (ROCK)1 in regulating whole-body glucose homeostasis has been reported. However, cell-autonomous effects of ROCK1 on insulin-dependent glucose transport in adipocytes and muscle cells have not been elucidated. To determine the specific role of ROCK1 in glucose transport directly, ROCK1 expression in 3T3-L1 adipocytes and L6 myoblasts was biologically modulated. Here, we show that small interfering RNA-mediated ROCK1 depletion decreased insulin-induced glucose transport in adipocytes and myoblasts, whereas adenovirus-mediated ROCK1 expression increased this in a dose-dependent manner, indicating that ROCK1 is permissive for glucose transport. Inhibition of ROCK1 also impaired glucose transporter 4 translocation in 3T3-L1 adipocytes. Importantly, the ED₅₀ of insulin for adipocyte glucose transport was reduced when ROCK1 was expressed, leading to hypersensitivity to insulin. These effects are dependent on actin cytoskeleton remodeling, because inhibitors of actin polymerization significantly decreased ROCK1's effect to promote insulin-stimulated glucose transport. Unlike ROCK2, ROCK1 binding to insulin receptor substrate (IRS)-1 was not detected by immunoprecipitation, although cell fractionation demonstrated both ROCK isoforms localize with IRS-1 in low-density microsomes. Moreover, insulin's ability to increase IRS-1 tyrosine 612 and serine 632/635 phosphorylation was attenuated by ROCK1 suppression. Replacing IRS-1 serine 632/635 with alanine reduced insulin-stimulated phosphatidylinositol 3-kinase activation and glucose transport in 3T3-L1 adipocytes, indicating that phosphorylation of these serine residues of IRS-1, which are substrates of the ROCK2 isoform in vitro, are crucial for maximal stimulation of glucose transport by insulin. Our studies identify ROCK1 as an important positive regulator of insulin action on glucose transport in adipocytes and muscle cells.

Membrane tension maintains cell polarity by confining signals to the leading edge during neutrophil migration

Little is known about how neutrophils and other cells establish a single zone of actin assembly during migration. A widespread assumption is that the leading edge prevents formation of additional fronts by generating long-range diffusible inhibitors or by sequestering essential polarity components. We use morphological perturbations, cell-severing experiments, and computational simulations to show that diffusion-based mechanisms are not sufficient for long-range inhibition by the pseudopod. Instead, plasma membrane tension could serve as a long-range inhibitor in neutrophils. We find that membrane tension doubles during leading-edge protrusion, and increasing tension is sufficient for long-range inhibition of actin assembly and Rac activation. Furthermore, reducing membrane tension causes uniform actin assembly. We suggest that tension, rather than diffusible molecules generated or sequestered at the leading edge, is the dominant source of long-range inhibition that constrains the spread of the existing front and prevents the formation of secondary fronts.

Disruption of the actin cytoskeleton induces fluorescent glucose accumulation on the rat hepatocytes Clone 9

BACKGROUND

Glucose transport and metabolism are highly specialized in hepatocytes. Actin cytoskeleton is fundamental to the maintenance of their morphology as well as to ensure their functionality. Here we study the effect of the actin disrupting natural compounds cytochalasin B and latrunculin A on the glucose metabolism of the Clone 9 rat hepatocytes once the glucose molecule is inside them and the effects of two hormones which main function is regulating the glucose metabolism on the actin cytoskeleton of Clone 9 cells.

METHODS

F-actin was labeled by using Oregon Green 514 ® phalloidin and glucose inside cells was monitored with the fluorescent D-glucose derivative; 2-NBDG. Observations and measurements were carried out by using a confocal microscope.

RESULTS

Nor insulin neither glucagon was able to induce any significant effect in the quantity of F-actin present on Clone 9 cells. But insulin triggers a strong reorganization on the pattern of distribution of F-actin. However, the actin cytoskeleton disruption induced by CB and more efficiently by Lat A caused accumulation of 2-NBDG in cells.

CONCLUSION

These results state that disruption of the actin cytoskeleton induces fluorescent glucose accumulation on the rat hepatocytes Clone 9 suggesting that actin disrupting agents cause a blockage in the glycolytic pathway of Clone 9 hepatocytes.

Distribution of dystrophin- and utrophin-associated protein complexes during activation of human neutrophils

OBJECTIVE

Dystrophins, utrophins, and their associated proteins are involved in structural and signaling roles in nonmuscle tissues; however, description of these proteins in neutrophils remained unexplored. Therefore we characterize the pattern expression, and the cellular distribution of dystrophin and utrophin gene products and dystrophin-associated proteins (i.e., beta-dystroglycan, alpha-syntrophin, and alpha-dystrobrevins) in relation to actin filaments in resting and activated with formyl-methionyl-leucyl-phenylalanine human neutrophils.

MATERIALS AND METHODS

Resting and fMLP-activated human neutrophils were analyzed by immunoblot and by confocal microscopy analysis. Immunoprecipitation assays were performed to corroborate the presence of protein complexes.

RESULTS

Immunoprecipitation assays and confocal analysis demonstrated the presence of two dystrophin-associated protein complexes in resting and activated neutrophils: the former formed by Dp71d/Dp71Delta(110)(m) and dystrophin-associated proteins (beta-dystroglycan, alpha-syntrophin, alpha-dystrobrevin-1, and -2), while the latter contains Up400, instead of Dp71d/Dp71Delta(110)(m), as a central component of the dystrophin-associated protein complexes (DAPC). Confocal analysis also showed the subcellular redistribution of Dp71d/Dp71Delta(110)(m) approximately DAPC and Up400 approximately DAPC in F-actin-based structures displayed during activation process with fMLP.

CONCLUSIONS

Our study showed the existence of two protein complexes formed by Dp71d/Dp71Delta(110)(m) or Up400 associated with DAPs in resting and fMLP-treated human polymorphonuclears. The interaction of these complexes with the actin cytoskeleton is indicative of their dynamic participation in the chemotaxis process.

Identification of a benzamide derivative that inhibits stress-induced adrenal corticosteroid synthesis

Elevated serum glucocorticoid levels contribute to the progression of many diseases, including depression, Alzheimer’s disease, hypertension, and acquired immunodeficiency syndrome. Here we show that the benzamide derivative N-[2-(4-cyclopropanecarbonyl-3-methyl-piperazin-1-yl)-1-(tert-butyl-1H-indol-3-yl-methyl)-2-oxo-ethyl]-4-nitrobenzamide (SP-10) inhibits dibutyryl cyclic AMP (dbcAMP)-induced corticosteroid synthesis in a dose-dependent manner in Y-1 adrenal cortical mouse tumor cells, without affecting basal steroid synthesis and reduced stress-induced corticosterone increases in rats without affecting the physiological levels of the steroid in blood. SP-10 did not affect cholesterol transport and metabolism by the mitochondria but was unexpectedly found to increase 3-hydroxy-3-methylglutaryl-coenzyme A, low density lipoprotein receptor, and scavenger receptor class B type I (SR-BI) expression. However, it also markedly reduced dbcAMP-induced NBD-cholesterol uptake, suggesting that this is a compensatory mechanism aimed at maintaining cholesterol levels. SP-10 also induced a redistribution of filamentous (F-) and monomeric (G-) actin, leading to decreased actin levels in the submembrane cytoskeleton suggesting that SP-10-induced changes in actin distribution might prevent the formation of microvilli– cellular structures required for SR-BI-mediated cholesterol uptake in adrenal cells.

p53-cofactor JMY is a multifunctional actin nucleation factor

Many cellular structures are assembled from networks of actin filaments, and the architecture of these networks depends on the mechanism by which the filaments are formed. Several classes of proteins are known to assemble new filaments, including the Arp2/3 complex, which creates branched filament networks, and Spire, which creates unbranched filaments. We find that JMY, a vertebrate protein first identified as a transcriptional co-activator of p53, combines these two nucleating activities by both activating Arp2/3 and assembling filaments directly using a Spire-like mechanism. Increased levels of JMY expression enhance motility, whereas loss of JMY slows cell migration. When slowly migrating HL-60 cells are differentiated into highly motile neutrophil-like cells, JMY moves from the nucleus to the cytoplasm and is concentrated at the leading edge. Thus, JMY represents a new class of multifunctional actin assembly factor whose activity is regulated, at least in part, by sequestration in the nucleus.

Clocking the Lyme spirochete

In order to clear the body of infecting spirochetes, phagocytic cells must be able to get hold of them. In real-time phase-contrast videomicroscopy we were able to measure the speed of Borrelia burgdorferi (Bb), the Lyme spirochete, moving back and forth across a platelet to which it was tethered. Its mean crossing speed was 1,636 µm/min (N = 28), maximum, 2800 µm/min (N = 3). This is the fastest speed recorded for a spirochete, and upward of two orders of magnitude above the speed of a human neutrophil, the fastest cell in the body. This alacrity and its interpretation, in an organism with bidirectional motor capacity, may well contribute to difficulties in spirochete clearance by the host.

Reorganization of the human neutrophil plasma membrane is associated with functional priming: implications for neutrophil preparations

Changes in the functional and plasma membrane organizational states of human neutrophils were examined using two isolation procedures, which may simulate altered physiological states in vivo. A gelatin-based method of blood-neutrophil isolation was used to model in vivo priming, and neutrophils isolated by this method were compared with control populations prepared by a pyrogen-free, dextran-based method. Gelatin-prepared neutrophils were functionally primed for adherence and agonist-stimulated superoxide generation relative to unprimed, control neutrophils. The organizational state of the membrane cortex was examined by mapping the subcellular distribution of select cortical and transmembrane proteins by several methods, including subcellular fractionation, indirect immunofluorescence, and compositional analysis of Triton X-100-insoluble membrane skeleton preparations. Filamentous actin, fodrin, and the fodrin anchor, CD45, were largely cytoplasmic in unprimed neutrophils but translocated to plasma membranes upon priming, whereas CD43 and ezrin were exclusively surface-associated in both populations. Isopycnic sucrose density gradient analysis of N(2)-cavitated neutrophils revealed a major shift in the distribution of surface-associated transmembrane and membrane cortical components relative to the plasma membrane marker alkaline phosphatase in primed but not unprimed neutrophils. Similar results were obtained after neutrophil stimulation with known priming agents, LPS, TNF-alpha, or GM-CSF. Together, these results may suggest that priming of suspended, circulating neutrophils is associated with a large-scale reorganization of the plasma membrane and associated membrane cortex in a process that is independent of cellular adhesion and gross morphologic polarization.

Gliotoxin from Aspergillus fumigatus affects phagocytosis and the organization of the actin cytoskeleton by distinct signalling pathways in human neutrophils

Gliotoxin is a mycotoxin having a considerable number of immuno-suppressive actions and is produced by several moulds such as Aspergillus fumigatus. In this study, we investigated its toxic effects on human neutrophils at concentrations corresponding to those found in the blood of patients with invasive aspergillosis. Incubation of the cells for 10min with 30-100ng/ml of gliotoxin inhibited phagocytosis of either zymosan or serum-opsonized zymosan without affecting superoxide production or the exocytosis of specific and azurophil granules. Gliotoxin also induced a significant re-organization of the actin cytoskeleton which collapsed around the nucleus leading to cell shrinkage and the disappearance of filopodia. This gliotoxin-induced actin phenotype was reversed by the cAMP antagonist Rp-cAMP and mimicked by pCPT-cAMP indicating that it probably resulted from the deregulation of intracellular cAMP homeostasis as previously described for gliotoxin-induced apoptosis. By contrast, gliotoxin-induced inhibition of phagocytosis was not reversed by Rp-cAMP but by arachidonic acid, another member of a known signalling pathway affected by the toxin. This suggests that gliotoxin can affect circulating neutrophils and favour the dissemination of A. fumigatus by inhibiting phagocytosis and the consequent killing of conidia.

Mechanics of neutrophil phagocytosis: experiments and quantitative models

To quantitatively characterize the mechanical processes that drive phagocytosis, we observed the FcγR-driven engulfment of antibody-coated beads of diameters 3 μm to 11 μm by initially spherical neutrophils. In particular, the time course of cell morphology, of bead motion and of cortical tension were determined. Here, we introduce a number of mechanistic models for phagocytosis and test their validity by comparing the experimental data with finite element computations for multiple bead sizes. We find that the optimal models involve two key mechanical interactions: a repulsion or pressure between cytoskeleton and free membrane that drives protrusion, and an attraction between cytoskeleton and membrane newly adherent to the bead that flattens the cell into a thin lamella. Other models such as cytoskeletal expansion or swelling appear to be ruled out as main drivers of phagocytosis because of the characteristics of bead motion during engulfment. We finally show that the protrusive force necessary for the engulfment of large beads points towards storage of strain energy in the cytoskeleton over a large distance from the leading edge (∼0.5 μm), and that the flattening force can plausibly be generated by the known concentrations of unconventional myosins at the leading edge.

Presence of a novel inhibitor of capping protein in neutrophil extract

Capping of actin filament barbed ends regulates the duration of filament elongation and the steady-state level of actin polymerization. We find that the specific capping activity (capping activity per milligram protein) increased when a high speed supernatant of lysed neutrophils was diluted with buffer. The specific capping activity also increased when the concentration of barbed ends increased. This suggested the presence of a capping protein inhibitor that dissociates from capping protein upon dilution and that competes with barbed ends for binding to capping protein. Gel filtration of supernatant revealed a fraction of low-molecular-weight inhibitor (separated from capping protein) that both inhibited and reversed capping of barbed ends by pure capping protein. The properties and molecular weight of this inhibitor do not match with those of other inhibitors including V-1, VASP, or CARMIL. Thus, this inhibitor must either be a modified version of a known inhibitor or a novel inhibitor of capping.

Perturbation of actin dynamics induces NF-kappaB activation in myelomonocytic cells through an NADPH oxidase-dependent pathway

Although several reports showed the effect of compounds disrupting microtubules on NF-kappaB (nuclear factor kappaB) activation, nothing is known about agents perturbing actin dynamics. In the present study, we have shown that actin cytoskeleton disruption induced by actin-depolymerizing agents such as cytochalasin D and latrunculin B and actin-polymerizing compounds such as jasplakinolide induced NF-kappaB activation in myelomonocytic cells. The transduction pathway involved the IkappaB (inhibitory kappaB) kinase complex and a degradation of IkappaBalpha. We have shown that NF-kappaB activation in response to the perturbation of actin dynamics required reactive oxygen species, as demonstrated by the effect of antioxidants. Actin cytoskeleton disruption by cytochalasin D induced O2- release from human monocytes, through the activation of the NADPH oxidase, as confirmed by the phosphorylation and by the membrane translocation of p47phox. NF-kappaB activation after actin cytoskeleton disruption could be physiologically relevant during monocyte activation and/or recruitment into injured tissues, where cellular attachment, migration and phagocytosis result in cyclic shifts in cytoskeletal organization and disorganization.

Involvement of Actin in Agonist-induced Endocytosis of the G Protein-coupled Receptor for Thromboxane A2

The role of actin in endocytosis of G protein-coupled receptors is poorly defined. In the present study, we demonstrate that agents that depolymerize (latrunculin B and cytochalasin D) or stabilize (jasplakinolide) the actin cytoskeleton blocked agonist-induced endocytosis of the beta isoform of the thromboxane A(2) receptor (TPbeta) in HEK293 cells. This suggests that endocytosis of TPbeta requires active remodeling of the actin cytoskeleton. On the other hand, disruption of microtubules with colchicine did not affect endocytosis of the receptor. Expression of wild-type and mutant forms of the small GTPases RhoA and Cdc42 potently inhibited endocytosis of TPbeta, further indicating a role for the dynamic regulation of the actin cytoskeleton in this pathway. Agonist treatment of TPbeta in HEK293 cells resulted in the formation of actin stress fibers through Galpha(q/11) signaling. Because we previously showed that endocytosis of TPbeta is dependent on arrestins, we decided to explore the relation between arrestin-2 and -3 and actin in endocytosis of this receptor. Interestingly, we show that the inhibition of TPbeta endocytosis by the actin toxins in HEK293 cells was overcome by the overexpression of arrestin-3, but not of arrestin-2. These results indicate that the actin cytoskeleton is not essential in arrestin-3-mediated endocytosis of TPbeta. However, arrestin-3 could not promote endocytosis of the TPbetaY339A and TPbetaI343A carboxyl-terminal mutants when the actin cytoskeleton was disrupted. Our data provide new evidence that the actin cytoskeleton plays an essential role in TPbeta endocytosis. Furthermore, our work suggests the existence of actin-dependent and -independent arrestin-mediated pathways of endocytosis.

Beginning and ending an actin filament: control at the barbed end

Dynamic actin filaments contribute to cell migration, organelle movements, memory, and gene regulation. These dynamic processes are often regulated by extracellular and?or cell cycle signals. Regulation targets, not actin itself, but the factors that determine it's dynamic properties. Thus, filament nucleation, rate and duration of elongation, and depolymerization are each controlled with regard to time and?or space. Two mechanisms exist for nucleating filaments de novo, the Arp23 complex and the formins; multiple pathways regulate each. A new filament elongates rapidly but transiently before its barbed end is capped. Rapid capping allows the cell to maintain fine temporal and spatial control over F-actin distribution. Modulation of capping protein activity and its access to barbed ends is emerging as a site of local regulation. Finally, to maintain a steady state filaments must depolymerize. Depolymerization can limit the rate of new filament nucleation and elongation. The activity of ADF?cofilin, which facilitates depolymerization, is also regulated by multiple inputs. This chapter describes (1) mechanism and regulation of new filament formation, (2) mechanism of enhancing elongation at barbed ends, (3) capping proteins and their regulators, and (4) recycling of actin monomers from filamentous actin (F-actin) back to globular actin (G-actin).

Cytoskeletal reorganization during process of apoptosis induced by cytostatic drugs in K-562 and HL-60 leukemia cell lines

The aim of the present study was to investigate the reorganization of F-actin, vimentin and tubulin in K-562 and HL-60 cell lines during apoptosis induced by etoposide, doxorubicin and taxol. The distribution of cytoskeletal proteins was analyzed by fluorescence microscopy. Actin was also studied by confocal microscopy and at the ultrastructural level. Changes in the distribution of cytoskeletal proteins were found to be dose-dependent and appeared to be more intense in HL-60 cells. Etoposide- and doxorubicin-treated cells showed similar changes in the distribution of F-actin, vimentin and tubulin. The reorganization of cytoskeletal proteins seemed to be consistent with features of apoptosis. An increase in bright staining of F-actin, vimentin and tubulin at the site of apoptotic bodies formation was observed. Immunogold labeling of actin in HL-60 cells was associated with features typical for apoptosis, i.e. compaction and margination of nuclear chromatin. K-562 cells showed cytoplasmic actin-positivity in the cytoplasm. Significant changes in morphology of HL-60 cells were found in the following concentrations: etoposide 20, 200 microM; doxorubicin 5, 10 microM and taxol 2-10 microM. The investigated proteins seemed to be involved in the above-reported apoptotic changes. Bright staining of F-actin, vimentin and tubulin, concentrated at the site of apoptotic bodies formation might suggested importance of these proteins for this process. Moreover, the increase in actin labeling in areas of chromatin compaction and margination of nuclear chromatin especially in HL-60 cells, which are more susceptible to apoptosis might implicate that actin might be involved in the chromatin remodeling during apoptosis.

Increased focal Kv4.2 channel expression at the plasma membrane is the result of actin depolymerization

Voltage-dependent potassium channel trafficking and localization are regulated by proteins of the cytoskeleton, but the mechanisms by which these occur are still unclear. Using human embryonic kidney (HEK) cells as a heterologous expression system, we tested the role of the actin cytoskeleton in modulating the function of Kv4.2 channels. Pretreatment (>or=1 h) of HEK cells with 5 microM cytochalasin D to disrupt the actin microfilaments greatly augmented whole cell Kv4.2 currents at potentials positive to -20 mV. However, no changes in the voltage dependence of activation and inactivation of macroscopic currents were observed to account for this increase. Similarly, single channel recordings failed to reveal any significant changes in the single channel conductance, open probability, and kinetics. However, the mean patch current was increased from 0.9 +/- 0.2 pA in control to 6.7 +/- 3.0 pA in the presence of cytochalasin D. Imaging experiments revealed a clear increase in the surface expression of the channels and the appearance of "bright spot" features, suggesting that large numbers of channels were being grouped at specific sites. Our data provide clear evidence that increased numbers and altered distribution of Kv4.2 channels at the cell surface are primarily the result of reorganization of the actin cytoskeleton.

The mechanics of neutrophils: synthetic modeling of three experiments

Much experimental data exist on the mechanical properties of neutrophils, but so far, they have mostly been approached within the framework of liquid droplet models. This has two main drawbacks: 1), It treats the cytoplasm as a single phase when in reality, it is a composite of cytosol and cytoskeleton; and 2), It does not address the problem of active neutrophil deformation and force generation. To fill these lacunae, we develop here a comprehensive continuum-mechanical paradigm of the neutrophil that includes proper treatment of the membrane, cytosol, and cytoskeleton components. We further introduce two models of active force production: a cytoskeletal swelling force and a polymerization force. Armed with these tools, we present computer simulations of three classic experiments: the passive aspiration of a neutrophil into a micropipette, the active extension of a pseudopod by a neutrophil exposed to a local stimulus, and the crawling of a neutrophil inside a micropipette toward a chemoattractant against a varying counterpressure. Principal results include: 1), Membrane cortical tension is a global property of the neutrophil that is affected by local area-increasing shape changes. We argue that there exists an area dilation viscosity caused by the work of unfurling membrane-storing wrinkles and that this viscosity is responsible for much of the regulation of neutrophil deformation. 2), If there is no swelling force of the cytoskeleton, then it must be endowed with a strong cohesive elasticity to prevent phase separation from the cytosol during vigorous suction into a capillary tube. 3), We find that both swelling and polymerization force models are able to provide a unifying fit to the experimental data for the three experiments. However, force production required in the polymerization model is beyond what is expected from a simple short-range Brownian ratchet model. 4), It appears that, in the crawling of neutrophils or other amoeboid cells inside a micropipette, measurement of velocity versus counterpressure curves could provide a determination of whether cytoskeleton-to-cytoskeleton interactions (such as swelling) or cytoskeleton-to-membrane interactions (such as polymerization force) are predominantly responsible for active protrusion.

Membrane lipid organization is critical for human neutrophil polarization

In response to chemoattractants neutrophils extend an actin-rich pseudopod, which imparts morphological polarity and is required for migration. Even when stimulated by an isotropic bath of chemoattractant, neutrophils exhibit persistent polarization and continued lamellipod formation at the front, suggesting that the cells establish an internal polarity. In this report, we show that perturbing lipid organization by depleting plasma membrane cholesterol levels reversibly inhibits cell polarization and migration. Among other receptor-mediated responses, beta(2) integrin up-regulation was unaffected, and initial calcium mobilization was only partially reduced by cholesterol depletion, indicating that this treatment did not abrogate initial receptor-mediated signal transduction. Interestingly, cholesterol depletion did not prevent initial activation of the GTPase Rac or an initial burst of actin polymerization, but rather it inhibited prolonged activation of Rac and sustained actin polymerization. Collectively, these findings support a model in which the plasma membrane is organized into domains that aid in amplifying the chemoattractant gradient and maintaining cell polarization.

Rac and Cdc42 play distinct roles in regulating PI(3,4,5)P3 and polarity during neutrophil chemotaxis

Neutrophils exposed to chemoattractants polarize and accumulate polymerized actin at the leading edge. In neutrophil-like HL-60 cells, this asymmetry depends on a positive feedback loop in which accumulation of a membrane lipid, phosphatidylinositol (PI) 3,4,5-trisphosphate (PI[3,4,5]P3), leads to activation of Rac and/or Cdc42, and vice versa. We now report that Rac and Cdc42 play distinct roles in regulating this asymmetry. In the absence of chemoattractant, expression of constitutively active Rac stimulates accumulation at the plasma membrane of actin polymers and of GFP-tagged fluorescent probes for PI(3,4,5)P3 (the PH domain of Akt) and activated Rac (the p21-binding domain of p21-activated kinase). Dominant negative Rac inhibits chemoattractant-stimulated accumulation of actin polymers and membrane translocation of both fluorescent probes and attainment of morphologic polarity. Expression of constitutively active Cdc42 or of two different protein inhibitors of Cdc42 fails to mimic effects of the Rac mutants on actin or PI(3,4,5)P3. Instead, Cdc42 inhibitors prevent cells from maintaining a persistent leading edge and frequently induce formation of multiple, short lived leading edges containing actin polymers, PI(3,4,5)P3, and activated Rac. We conclude that Rac plays a dominant role in the PI(3,4,5)P3-dependent positive feedback loop required for forming a leading edge, whereas location and stability of the leading edge are regulated by Cdc42.

Products of phosphoinositide specific phospholipase C can trigger dephosphorylation of cofilin in chemoattractant stimulated neutrophils

The signal transduction pathways that trigger dephosphorylation of cofilin in neutrophils stimulated with the chemoattractant fMet-Leu-Phe (fMLP) were investigated with a phospho-specific antibody that recognized cofilin only when this protein was phosphorylated on ser-3. Unlike earlier studies that monitored changes in (32)P-labeled cofilin, this Ab allowed us to monitor changes in the total mass of phosphorylated cofilin during neutrophil stimulation. Neutrophils stimulated with fMLP (1.0 microM) for 1.0 min exhibited a massive loss (> 85%) of phosphate from cofilin, which was blocked by an antagonist of phosphoinositide-specific phospholipase C (PI-PLC) (1.0 microM U73122). Products of PI-PLC, sn-1,2-diglyceride and inositol (1,4,5)-trisphosphate, are known to activate protein kinase C (PKC) and increase intracellular Ca(2+), respectively. Treatment of neutrophils with agents that selectively activate PKC [4beta-phorbol 12-myristate 13-acetate (PMA) ] or cellular Ca(2+) (ionophore A23187) also triggered dephosphorylation of cofilin. Both a nonspecific (100 nM staurosporine) and a highly selective antagonist of PKC (200 nM bisindolylmaleimide I) blocked dephosphorylation of cofilin in neutrophils stimulated with PMA but not with fMLP or ionophore A23187. The calmodulin (CaM) antagonists trifluoperazine (15 microM) and W-7 (50 microM) blocked dephosphorylation of cofilin in stimulated neutrophils whereas inactive/less-active analogs of these inhibitors (15 microM promethazine, 50 microM W-5) were substantially less effective. Calyculin A (40 nM), an antagonist of type 1 and 2A protein phosphatases, also triggered a massive dephosphorylation of cofilin in unstimulated neutrophils through a pathway that was insensitive to inhibitors of type 2B phosphatases. These data suggest that both PKC-dependent and independent pathways can trigger dephosphorylation of cofilin in neutrophils with the latter pathway predominating in fMLP-stimulated cells. These pathways may also contain CaM and a type 2C and/or novel phosphatase (e.g., slingshot).

System y+ localizes to different membrane subdomains in the basolateral plasma membrane of epithelial cells

We report here that the system y+ cationic amino acid transporter ATRC1 localized to clusters within the basolateral membrane of polarized Madin-Darby canine kidney and human embryonic kidney (HEK) cells, suggesting that the transporters are restricted to discrete membrane microdomains in epithelial cells. Based on solubility in nonionic detergents, two populations of ATRC1 molecules existed: approximately half of the total ATRC1 in HEK cells associated with the actin membrane cytoskeleton, whereas another one-fourth resided in detergent-resistant membranes (DRM). In agreement with these findings, cytochalasin D reduced the amount of ATRC1 associated with the actin membrane cytoskeleton. Although some ATRC1 clusters in HEK cells colocalized with caveolin, the majority of ATRC1 did not colocalize with this marker protein for a type of DRM called caveolae. This distribution of ATRC1 is somewhat different from that reported for pulmonary artery endothelial cells in which transporters cluster predominantly in caveolae, suggesting that differences in the proportion of ATRC1 in specific membrane microdomains correlate with differences in the physiological role of the transporter in polarized kidney epithelial vs. vascular endothelial cells.

Localised depletion of polymerised actin at the front of Walker carcinosarcoma cells increases the speed of locomotion

Spontaneously migrating Walker carcinosarcoma cells usually form lamellipodia at the front. Combined treatment with 10(-5)M colchicine and 10(-7)M latrunculin A produces large defects in the cortical F-actin layer at the leading front and suppresses lamellipodia. However, the cortical actin layer at the rear is intact and shows myosin IIA accumulation. These cells, showing no or little detectable cortical F-actin at the front and no morphologically recognisable protrusions, migrate faster than control cells with lamellipodia and an intact cortical actin layer. This documents that the cortical actin layer or actin-powered force generation at the front is redundant for locomotion. Colchicine and latrunculin A have synergistic effects in compromising the cortical layer at the front and in increasing the speed of locomotion, but antagonistic effects on the relative amount of F-actin per cell. Colchicine but not latrunculin A, can increase the proportion of polarised and locomoting cells under appropriate conditions. Locomotion and polarity of cells treated with latrunculin A and colchicine is inhibited at latrunculin A concentrations >10(-7)M, by the myosin inhibitor BDM or the ROCK inhibitor Y-27632. Colchicine and Y-27632 have antagonistic effects on polarity and the speed of locomoting cells. The data show that locomotion of metazoan cells, which normally form lamellipodia, can be driven by actomyosin contraction behind the front (cell body, uropod). They are best compatible with a cortical contraction/frontal expansion model, but they are not compatible with models implying that actin polymerisation or actomyosin contraction at the front drive locomotion of the cells studied.

Possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation in Ehrlich ascites tumor cells

Osmotic shrinkage of Ehrlich ascites tumor cells (EATC) elicited translocation of myosin II from the cytosol to the cortical region, and swelling elicits concentration of myosin II in the Golgi region. Rho kinase and p38 both appeared to be involved in shrinkage-induced myosin II reorganization. In contrast, the previously reported shrinkage-induced actin polymerization [Pedersen et al. (1999) Exp. Cell Res. 252, 63-74] was independent of Rho kinase, p38, myosin light chain kinase (MLCK), and protein kinase C (PKC), which thus do not exert their effects on the shrinkage-activated transporters via effects on F-actin. The subsequent F-actin depolymerization, however, appeared MLCK- and PKC-dependent, and the initial swelling-induced F-actin depolymerization was MLCK-dependent; both effects were apparently secondary to kinase-mediated effects on cell volume changes. NHE1 in EATC is activated both by osmotic shrinkage and by the serine/threonine phosphatase inhibitor Calyculin A (CL-A). Both stimuli caused Rho kinase-dependent myosin II relocation to the cortical cytoplasm, but in contrast to the shrinkage-induced F-actin polymerization, CL-A treatment elicited a slight F-actin depolymerization. Moreover, Rho kinase inhibition did not significantly affect NHE1 activation, neither by shrinkage nor by CL-A. Implications for the possible interrelationship between changes in F-actin and myosin II, protein phosphorylation, and cell volume regulation are discussed.

An unexplained sequestration of latrunculin A is required in neutrophils for inhibition of actin polymerization

Latrunculin A (LatA) is a toxic natural product that causes disruption of the actin cytoskeleton in many eukaryotic cells at submicromolar concentrations. LatA has been found to bind G-actin with a dissociation constant of 0.2 microM, and more recently to bind profilin-G-actin and, weakly, thymosin beta4-G-actin. A number of investigators have used LatA as a G-actin sequestering agent. Thus, we studied neutrophil chemotaxis and its requisite conversion of G-actin to F-actin, supported by an extensive pool of G-actin, mainly bound to thymosin beta4. Calculations suggest that the affinity of LatA is insufficient to cause significant sequestration of this pool, and the pool's buffering action should protect neutrophils from depletion of productive G-actin species by submicromolar LatA. Nonetheless, we found that both chemoattractant stimulated migration and F-actin polymerization in neutrophils were inhibited by LatA at these concentrations. The latter effect was accompanied by sequestration of LatA and showed a cell density dependence that was consistent with G-actin sequestration. The apparent contradiction between the calculations and the experimental observations could be reconciled by assuming the presence of an accessory species, of unknown normal function, which forms a high affinity ternary complex with LatA and G-actin, thus causing the cells to concentrate LatA. Other models that could not be ruled out also invoke new actions of LatA, suggesting caution in the interpretation of its effects on cells.

G protein-coupled receptor/arrestin3 modulation of the endocytic machinery

Nonvisual arrestins (arr) modulate G protein-coupled receptor (GPCR) desensitization and internalization and bind to both clathrin (CL) and AP-2 components of the endocytic coated pit (CP). This raises the possibility that endocytosis of some GPCRs may be a consequence of arr-induced de novo CP formation. To directly test this hypothesis, we examined the behavior of green fluorescent protein (GFP)-arr3 in live cells expressing beta2-adrenergic receptors and fluorescent CL. After agonist stimulation, the diffuse GFP-arr3 signal rapidly became punctate and colocalized virtually completely with preexisting CP spots, demonstrating that activated complexes accumulate in previously formed CPs rather than nucleating new CP formation. After arr3 recruitment, CP appeared larger: electron microscopy analysis revealed an increase in both CP number and in the occurrence of clustered CPs. Mutant arr3 proteins with impaired binding to CL or AP-2 displayed reduced recruitment to CPs, but were still capable of inducing CP clustering. In contrast, though constitutively present in CPs, the COOH-terminal moiety of arr3, which contains CP binding sites but lacks receptor binding, did not induce CP clustering. Together, these results indicate that recruitment of functional arr3-GPCR complexes to CP is necessary to induce clustering. Latrunculin B or 16 degrees C blocked CP rearrangements without affecting arr3 recruitment to CP. These results and earlier studies suggest that discrete CP zones exist on cell surfaces, each capable of supporting adjacent CPs, and that the cortical actin membrane skeleton is intimately involved with both the maintenance of existing CPs and the generation of new structures.

Oxidized low density lipoprotein-induced LFA-1-dependent adhesion and transendothelial migration of monocytes via the protein kinase C pathway

Inflammatory and immune responses are highly relevant processes in the pathogenesis of atherosclerosis, as illustrated by the central event of monocyte accumulation in atherosclerotic plaques. Integrin LFA-1-mediated adhesion of circulating monocytes to the endothelium is a prerequisite for recruitment of monocytes to these areas. Integrin-mediated adhesion is tightly regulated and integrins are only functional in response to particular monocyte activation stimuli. We investigated the role of oxidized low-density lipoprotein (LDL) in adhesion of resting monocytes prepared by elutriation from endothelium. Our results showed that: (1) oxidized LDL (and MCP-1) induced both LFA-1-mediated adhesion of monocytes to endothelial cells and transendothelial migration of monocytes; (2) oxidized LDL functionally transformed monocyte LFA-1 to an activated form; (3) oxidized LDL induced F-actin polymerization and cytoskeletal rearrangement within seconds; and (4) the LDL-associated antioxidant, alpha-tocopherol, but not beta-tocopherol, inhibited both F-actin polymerization and LFA-1-mediated adhesion of monocytes, which paralleled the effect of protein kinase C (PKC) inhibitors. Our results indicate that oxidized LDL plays a pivotal role in triggering LFA-1 activation and LFA-1-mediated adhesion and transmigration of monocytes to sites of atherosclerotic plaques, via the PKC pathway.

Actin filament turnover removes bundles fromDrosophilabristle cells

Drosophila bristle cells form enormous extensions that are supported by equally impressive scaffolds of modular, polarized and crosslinked actin filament bundles. As the cell matures and support is taken over by the secreted cuticle, the actin scaffold is completely removed. This removal begins during cell elongation and proceeds via an orderly series of steps that operate on each module. Using confocal and electron microscopy, we found that the ∼500-filament modules are fractured longitudinally into 25-50-filament subbundles, indicating that module breakdown is the reverse of assembly. Time-lapse confocal analysis of GFP-decorated bundles in live cells showed that modules were shortened by subunit removal from filament barbed ends, again indicating that module breakdown is the reverse of assembly. Module shortening takes place at a fairly slow rate of ∼1μm/hour,implying that maximally crosslinked modules are not rapidly depolymerized. Barbed-end depolymerization was prevented with jasplakinolide and accelerated with cycloheximide, indicating that barbed-end maintenance requires continuous protein synthesis. Subbundle adhesion was lost in the presence of cytochalasin, indicating that continuous actin polymerization is required. Thus, these polarized actin filament bundles are dynamic structures that require continuous maintenance owing to protein and actin filament turnover. We propose that after cell elongation, maintenance falls behind turnover,resulting in the removal of this modular cytoskeleton.

The cytoskeleton and cell volume regulation

Although the precise mechanisms have yet to be elucidated, early events in osmotic signal transduction may involve the clustering of cell surface receptors, initiating downstream signaling events such as assembly of focal adhesion complexes, and activation of, e.g. Rho family GTPases, phospholipases, lipid kinases, and tyrosine- and serine/threonine protein kinases. In the present paper, we briefly review recent evidence regarding the possible relation between such signaling events, the F-actin cytoskeleton, and volume-regulatory membrane transporters, focusing primarily on our own work in Ehrlich ascites tumer cells (EATC). In EATC, cell shrinkage is associated with an increase, and cell swelling with a decrease in F-actin content, respectively. The role of the F-actin cytoskeleton in cell volume regulation in various cell types has largely been investigated using cytochalasins to disrupt F-actin and highly varying effects have been reported. Findings in EATC show that the effect of cytochalasin treatment cannot always be assumed to be F-actin depolymerization, and that, moreover, there is no well-defined correlation between effects of cytochalasins on F-actin content and their effects on F-actin organization and cell morphology. At a concentration verified to depolymerize F-actin, cytochalasin B (CB), but not cytochalasin D (CD), inhibited the regulatory volume decrease (RVD) and regulatory volume increase (RVI) processes in EATC. This suggests that the effect of CB is related to an effect other than F-actin depolymerization, possibly its F-actin severing activity.

Molecular mechanisms controlling actin filament dynamics in nonmuscle cells

We review how motile cells regulate actin filament assembly at their leading edge. Activation of cell surface receptors generates signals (including activated Rho family GTPases) that converge on integrating proteins of the WASp family (WASp, N-WASP, and Scar/WAVE). WASP family proteins stimulate Arp2/3 complex to nucleate actin filaments, which grow at a fixed 70 degrees angle from the side of pre-existing actin filaments. These filaments push the membrane forward as they grow at their barbed ends. Arp2/3 complex is incorporated into the network, and new filaments are capped rapidly, so that activated Arp2/3 complex must be supplied continuously to keep the network growing. Hydrolysis of ATP bound to polymerized actin followed by phosphate dissociation marks older filaments for depolymerization by ADF/cofilins. Profilin catalyzes exchange of ADP for ATP, recycling actin back to a pool of unpolymerized monomers bound to profilin and thymosin-beta 4 that is poised for rapid elongation of new barbed ends.

Involvement of actin filaments and integrins in the binding step in collagen phagocytosis by human fibroblasts

In physiological conditions, collagen degradation by fibroblasts occurs primarily via phagocytosis, an intracellular pathway that is thought to require collagen receptors and actin assembly for fibril internalization and degradation. Currently it is unclear which specific steps of collagen phagocytosis in fibroblasts involve actin filament assembly. As studies of phagocytosis in fibroblasts are complicated by the relatively slow rate of particle internalization compared to professional phagocytes, we have examined the role of collagen receptors and actin only in the initial collagen binding step. Prior to the binding of collagen-coated fluorescent beads by human gingival fibroblasts, a cell type that is avidly phagocytic in vitro, cells were treated with cytochalasin D (actin filament barbed-end capping) or swinholide A (actin dimer sequestering and severing) or latrunculin B (actin monomer sequestering). Bead binding and immunostaining of (alpha)(2)(beta)(1) and (alpha)(3)(beta)(1) integrin collagen receptors were measured by flow cytometry. After 1–3 hours of coincubation with beads, cytochalasin D or swinholide A eliminated actin filaments stained by rhodamine-phalloidin and inhibited collagen bead binding (reductions of 25% and 50%, respectively), possibly because of cell rounding and restricted interactions with beads. In contrast, latrunculin enhanced binding dose-dependently over controls (twofold at 1 microM) and induced the formation of brightly staining aggregates of actin and the retention of long cytoplasmic extensions. Latrunculin also reduced surface (beta)(1), (alpha)(2) and (alpha)(3) integrin staining up to 40% in bead-free and bead-loaded cells, indicating that latrunculin enhanced collagen receptor internalization. As determined by fluorescence recovery after photobleaching, latrunculin increased the mobility of surface-bound (beta)(1) integrin. The stimulatory effect of latrunculin on collagen bead binding was reduced to control levels by treatment with a (beta)(1) integrin inactivating antibody while a (beta)(1) integrin blocking antibody abrogated both bead binding and the latrunculin-induced stimulation. Immunoblotting of bead-associated proteins showed that latrunculin completely eliminated binding of (beta)-actin to collagen beads but did not affect (beta)(1) integrin binding. These data indicate that latrunculin-induced sequestration of actin monomers facilitates the disengagement of actin from (beta)(1) integrin receptors, increases collagen bead binding and enhances collagen receptor mobility. We suggest that these alterations increase the probability of adhesive bead-to-cell interactions.

Requirement of cortical actin organization for bombesin, endothelin, and EGF receptor internalization

The role of actin organization in occupancy-induced receptor internalization remains poorly defined. Here we report that treatment of mouse Swiss 3T3 cells with latrunculin A, a potent inhibitor of actin polymerization (including cortical actin), inhibited the internalization of the endogenous bombesin/gastrin-releasing peptide (GRP) receptor, as judged by uptake of (125)I-labeled GRP or fluorescent Cy3-labeled bombesin. In contrast, cells pretreated with cytochalasin D showed minimal inhibition of bombesin/GRP receptor internalization. Similarly, pretreatment of Swiss 3T3 cells with the potent Rho-kinase inhibitor HA-1077, at concentrations (10-20 microM) that abrogated bombesin-mediated stress fiber formation, did not significantly alter receptor-mediated internalization of (125)I-GRP. These results indicate that bombesin/GRP receptor internalization depends on latrunculin A-sensitive cortical actin rather than on rapidly turning over actin stress fibers that are disrupted by either cytochalasin D or HA-1077. The rates and total levels of internalization of the endogenously expressed endothelin A receptor and epidermal growth factor receptor were also markedly reduced by latrunculin A in Swiss 3T3 cells. The potency of latrunculin A for inhibiting G protein-coupled receptor endocytosis was comparable to that for reducing internalization of the epidermal growth factor tyrosine kinase receptor. We conclude that cortical actin structures, disrupted by latrunculin A, are necessary for occupancy-induced receptor internalization in animal cells.

Effect of cytochalasins on F-actin and morphology of Ehrlich ascites tumor cells

Cytochalasins have been used extensively to probe the role of F-actin in different aspects of cellular function. Most of the data obtained are interpreted on the basis of the well-established depolymerizing effects of cytochalasins on F-actin preparations in vitro. However, some evidence indicates that, in intact cells, different cytochalasins can have varying effects on cell morphology and F-actin content and organization. To examine this problem in more detail, we analyzed the effects of cytochalasins on the cell morphology of and F-actin content and organization in Ehrlich ascites tumor (EAT) cells. After a 3-min exposure to 0.5 microM cytochalasin D, B, or E, F-actin content was equally reduced in all cases and this correlated with a reduction in the amount of cortical F-actin associated with the EAT cell membrane. However, only with CE was cell morphology markedly altered, with the appearance of numerous blebs. At 10 microM, blebbing was present in all conditions and the organization of cortical F-actin was disrupted. F-actin content, however, was not further reduced by this higher concentration and in CD it was identical to control levels. Exposure of EAT cells to similar concentrations of cheatoglobosin C, an analog of the cytochalasins that has little to no affinity for F-actin, resulted in a loss of F-actin content, a reduction in F-actin fluorescence, but no change in cell morphology, including a complete lack of bleb formation. Myosin II immunoreactivity, concentrated in the cortical cytoplasm colocalized with F-actin and in an area associated with the Golgi, was reduced by the high-dose cytochalasin. These results demonstrate that caution must be exercised in the use of cytochalasins to probe the role of F-actin in cellular function and that several parameters must be analyzed to obtain an accurate assessment of the effect of cytochalasin on the actin filament system.

Endocytosis of an HIV-derived lipopeptide into human dendritic cells followed by class I-restricted CD8(+) T lymphocyte activation

CD8(+) T lymphocytes, which are major immune effectors, require primary stimulation by dendritic cells (DC) presenting MHC class I molecule-bound epitopes. Sensitization to exogenous protein epitopes that are not synthesized in DC, such as cross-priming, is obtained through pathways leading to their association with MHC class I. To follow class I-restricted pathways in human DC, we have tracked a lipopeptide derived from the conserved HLA-A*0201-restricted HIV-1 reverse transcriptase 476-484 epitope, by N-terminal addition of an Nepsilon-palmytoyl-lysine. Indeed, lipopeptides elicit cytotoxic responses from CD8(+) T lymphocytes, whereas peptides without a lipid moiety do not. The lipopeptide and its parent peptide were labeled unequivocally by rhodamine to study their entry into immature monocyte-derived human DC by confocal microscopy. The lipid moiety induced endocytosis of the lipopeptide, assessed by rapid entry into vesicles, colocalization with Dextran-FITC and dependence on energy. Internalization occurred even when actin filaments were depolymerized by Cytochalasin B. This internalization induced functional stimulation of specific CD8(+) T lymphocytes in IFN-gamma ELISPOT assays. The peptide alone was not visualized inside the DC and was presented through direct surface association to HLA-A*0201. Therefore, lipopeptides are a unique opportunity to define precisely the pathways that lead exogenous proteins to associate with MHC class I molecules in DC. The results will also be useful to design lipopeptide vaccines.

Profilin enhances Cdc42-induced nucleation of actin polymerization

We find that profilin contributes in several ways to Cdc42-induced nucleation of actin filaments in high speed supernatant of lysed neutrophils. Depletion of profilin inhibited Cdc42-induced nucleation; re-addition of profilin restored much of the activity. Mutant profilins with a decreased affinity for either actin or poly-l-proline were less effective at restoring activity. Whereas Cdc42 must activate Wiskott-Aldrich Syndrome protein (WASP) to stimulate nucleation by the Arp2/3 complex, VCA (verpolin homology, cofilin, and acidic domain contained in the COOH-terminal fragment of N-WASP) constitutively activates the Arp2/3 complex. Nucleation by VCA was not inhibited by profilin depletion. With purified N-WASP and Arp2/3 complex, Cdc42-induced nucleation did not require profilin but was enhanced by profilin, wild-type profilin being more effective than mutant profilin with reduced affinity for poly-l-proline. Nucleation by the Arp2/3 complex is a function of the free G-actin concentration. Thus, when profilin addition decreased the free G-actin concentration, it inhibited Cdc42- and VCA-induced nucleation. However, when profilin was added with G-actin in a ratio that maintained the initial free G-actin concentration, it increased the rate of both Cdc42- and VCA-induced nucleation. This enhancement, also seen with purified proteins, was greatest when the free G-actin concentration was low. These data suggest that under conditions present in intact cells, profilin enhances nucleation by activated Arp2/3 complex.