Changes in the cytoskeleton of 3T3 fibroblasts induced by the phosphatase inhibitor, calyculin-A

Vimentin Intermediate Filament Rings Deform the Nucleus During the First Steps of Adhesion

During cell spreading, cells undergo many changes to their architecture and their mechanical properties. Vimentin, as an integral part of the cell architecture, and its mechanical stability must adapt to the new state of the cell. This study focuses on the structures formed by vimentin during the first steps of cell adhesion. Very early, ball-like structures, or "knots," are seen and often vimentin filaments emerge in the shape of rings around the nucleus. Although intermediate filaments are not known to be associated to motor proteins to form contractile systems, these rings can nonetheless strongly deform the cell nucleus. In the first 6 to 12 h of adhesion, these vimentin knots and rings disappear, and the intermediate filament network returns to the state seen before detachment of the cells. As these vimentin structures are very transient in the early steps of cell spreading, they have rarely been described in the literature. However, they can also be seen during mitosis, which is an event that involves partial detachment and re-spreading of the cells. Interestingly, the turnover dynamics of vimentin are reduced in both the knots and rings, compared to vimentin in the lamellipodia. It remains to define how the force is transmitted from the ball-like structures to the rings, and to measure the impact of such strong nuclear deformation on gene expression during cell re-spreading and the rearrangement of the vimentin network.

Cellular orientation is guided by strain gradients

The strain-induced reorientation response of cyclically stretched cells has been well characterized in uniform strain fields. In the present study, we comprehensively analyse the behaviour of human fibroblasts subjected to a highly non-uniform strain field within a polymethylsiloxane microdevice. Our results indicate that the strain gradient amplitude and direction regulate cell reorientation through a coordinated gradient avoidance response. We provide critical evidence that strain gradient is a key physical cue that can guide cell organization. Specifically, our work suggests that cells are able to pinpoint the location under the cell of multiple physical cues and integrate this information (strain and strain gradient amplitudes and directions), resulting in a coordinated response. To gain insight into the underlying mechanosensing processes, we studied focal adhesion reorganization and the effect of modulating myosin-II contractility. The extracted focal adhesion orientation distributions are similar to those obtained for the cell bodies, and their density is increased by the presence of stretching forces. Moreover, it was found that the myosin-II activity promoter calyculin-A has little effect on the cellular response, while the inhibitor blebbistatin suppresses cell and focal adhesion alignment and reduces focal adhesion density. These results confirm that similar internal structures involved in sensing and responding to strain direction and amplitude are also key players in strain gradient mechanosensing and avoidance.

Tissue constructs: platforms for basic research and drug discovery

The functions, form and mechanical properties of cells are inextricably linked to their extracellular environment. Cells from solid tissues change fundamentally when, isolated from this environment, they are cultured on rigid two-dimensional substrata. These changes limit the significance of mechanical measurements on cells in two-dimensional culture and motivate the development of constructs with cells embedded in three-dimensional matrices that mimic the natural tissue. While measurements of cell mechanics are difficult in natural tissues, they have proven effective in engineered tissue constructs, especially constructs that emphasize specific cell types and their functions, e.g. engineered heart tissues. Tissue constructs developed as models of disease also have been useful as platforms for drug discovery. Underlying the use of tissue constructs as platforms for basic research and drug discovery is integration of multiscale biomaterials measurement and computational modelling to dissect the distinguishable mechanical responses separately of cells and extracellular matrix from measurements on tissue constructs and to quantify the effects of drug treatment on these responses. These methods and their application are the main subjects of this review.

Cell Death Inducing Microbial Protein Phosphatase Inhibitors--Mechanisms of Action

Okadaic acid (OA) and microcystin (MC) as well as several other microbial toxins like nodularin and calyculinA are known as tumor promoters as well as inducers of apoptotic cell death. Their intracellular targets are the major serine/threonine protein phosphatases. This review summarizes mechanisms believed to be responsible for the death induction and tumor promotion with focus on the interdependent production of reactive oxygen species (ROS) and activation of Ca²⁺/calmodulin kinase II (CaM-KII). New data are presented using inhibitors of specific ROS producing enzymes to curb nodularin/MC-induced liver cell (hepatocyte) death. They indicate that enzymes of the arachidonic acid pathway, notably phospholipase A2, 5-lipoxygenase, and cyclooxygenases, may be required for nodularin/MC-induced (and presumably OA-induced) cell death, suggesting new ways to overcome at least some aspects of OA and MC toxicity.

In vivo roles for myosin phosphatase targeting subunit-1 phosphorylation sites T694 and T852 in bladder smooth muscle contraction

KEY POINTS

Force production and maintenance in smooth muscle is largely controlled by myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. MYPT1 is the regulatory subunit of MLCP that biochemically inhibits MLCP activity via T694 or T852 phosphorylation in vitro. Here we separately investigated the contribution of these two phosphorylation sites in bladder smooth muscles by establishing two single point mutation mouse lines, T694A and T852A, and found that phosphorylation of MYPT1 T694, but not T852, mediates force maintenance via inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. Our findings reveal the role of MYPT1 T694/T852 phosphorylation in vivo in regulation of smooth muscle contraction.

ABSTRACT

Force production and maintenance in smooth muscle is largely controlled by different signalling modules that fine tune myosin regulatory light chain (RLC) phosphorylation, which relies on a balance between Ca(2+)/calmodulin-dependent myosin light chain kinase (MLCK) and myosin light chain phosphatase (MLCP) activities. To investigate the regulation of MLCP activity in vivo, we analysed the role of two phosphorylation sites on MYPT1 (regulatory subunit of MLCP) that biochemically inhibit MLCP activity in vitro. MYPT1 is constitutively phosphorylated at T694 by unidentified kinases in vivo, whereas the T852 site is phosphorylated by RhoA-associated protein kinase (ROCK). We established two mouse lines with alanine substitution of T694 or T852. Isolated bladder smooth muscle from T852A mice displayed no significant changes in RLC phosphorylation or force responses, but force was inhibited with a ROCK inhibitor. In contrast, smooth muscles containing the T694A mutation showed a significant reduction of force along with reduced RLC phosphorylation. The contractile responses of T694A mutant smooth muscle were also independent of ROCK activation. Thus, phosphorylation of MYPT1 T694, but not T852, is a primary mechanism contributing to inhibition of MLCP activity and enhancement of RLC phosphorylation in vivo. The constitutive phosphorylation of MYPT1 T694 may provide a mechanism for regulating force maintenance of smooth muscle.

Cytoskeleton reorganization as an alternative mechanism of store-operated calcium entry control in neuroendocrine-differentiated cells

Neuroendocrine differentiation (NED) is a hallmark of advanced androgen-independent prostate cancer, for which no successful therapy exists. NED tumour cells escape apoptotic cell death by alterations of Ca(2+) homeostasis where the store-operated Ca(2+) entry (SOCE) is known to be a key event. We have previously shown that the downregulation of Orai1 protein representing the major molecular component of endogenous SOCE in human prostate cancer cells, and constituting the principal source of Ca(2+) influx used by the cell to trigger apoptosis, contributes to the establishment of an apoptosis-resistant phenotype (Cell Death Dis. 2010 Sep 16;1:e75.). Here, we report for the first time that the decrease of SOCE during NED may be caused by alternative NED-induced mechanism involving cytoskeleton reorganisation. NED induced by androgen deprivation resulted in a decrease of SOCE due to cortical F-actin over-polymerization which inhibits thapsigargin-induced SOCE. The disruption of F-actin polymerization by Cytochalasin D in NED cells restored SOCE, while the induction of F-actin polymerization by jasplakinolide or calyculin A diminished SOCE without changing the expression of key SOCE players: Orai1, STIM1, and TRPC1. Our data suggest that targeting cytoskeleton-induced pathways of malignant cells together with SOCE-involved channels may prove a useful strategy in the treatment of advanced prostate cancer.

Effects of stress fiber contractility on uniaxial stretch guiding mitosis orientation and stress fiber alignment

It has been documented that mitosis orientation (MO) is guided by stress fibers (SFs), which are perpendicular to exogenous cyclic uniaxial stretch. However, the effect of mechanical forces on MO and the mechanism of stretch-induced SFs reorientation are not well elucidated to date. In the present study, we used murine 3T3 fibroblasts as a model, to investigate the effects of uniaxial stretch on SFO and MO utilizing custom-made stretch device. We found that cyclic uniaxial stretch induced both SFs and mitosis directions orienting perpendicularly to the stretch direction. The F-actin and myosin II blockages, which resulted in disoriented SFs and mitosis directions under uniaxial stretch, suggested a high correlation between SFO and MO. Y27632 (10 μM), ML7 (50 μM, or 75 μM), and blebbistatin (50 μM, or 75 μM) treatments resulted in SFO parallel to the principle stretch direction. Upon stimulating and inhibiting the phosphorylation of myosin light chain (p-MLC), we observed a monotonic proportion of SFO to the level of p-MLC. These results suggested that the level of cell contraction is crucial to the response of SFs, either perpendicular or parallel, to the external stretch. Showing the possible role of cell contractility in tuning SFO under external stretch, our experimental data are valuable to understand the predominant factor controlling SFO response to exogenous uniaxial stretch, and thus helpful for improving mechanical models.

Calyculin A, an enhancer of myosin, speeds up anaphase chromosome movement

Actin and myosin inhibitors often blocked anaphase movements in insect spermatocytes in previous experiments. Here we treat cells with an enhancer of myosin, Calyculin A, which inhibits myosin-light-chain phosphatase from dephosphorylating myosin; myosin thus is hyperactivated. Calyculin A causes anaphase crane-fly spermatocyte chromosomes to accelerate poleward; after they reach the poles they often move back toward the equator. When added during metaphase, chromosomes at anaphase move faster than normal. Calyculin A causes prometaphase chromosomes to move rapidly up and back along the spindle axis, and to rotate. Immunofluorescence staining with an antibody against phosphorylated myosin regulatory light chain (p-squash) indicated increased phosphorylation of cleavage furrow myosin compared to control cells, indicating that calyculin A indeed increased myosin phosphorylation. To test whether the Calyculin A effects are due to myosin phosphatase or to type 2 phosphatases, we treated cells with okadaic acid, which inhibits protein phosphatase 2A at concentrations similar to Calyculin A but requires much higher concentrations to inhibit myosin phosphatase. Okadaic acid had no effect on chromosome movement. Backward movements did not require myosin or actin since they were not affected by 2,3-butanedione monoxime or LatruculinB. Calyculin A affects the distribution and organization of spindle microtubules, spindle actin, cortical actin and putative spindle matrix proteins skeletor and titin, as visualized using immunofluorescence. We discuss how accelerated and backwards movements might arise.

Methods for the detection of paxillin post-translational modifications and interacting proteins by mass spectrometry

Methods for the simultaneous identification of interacting proteins and post-translational modifications of the focal adhesion adapter protein, paxillin, are presented. The strategy includes (1) lower-level, transient transfection of FLAG-GFP-Paxillin into HEK293 cells, (2) incubation of cells with phosphatase inhibitors prior to lysis, (3) purification of paxillin by anti-FLAG immunoprecipitation, (4) analysis of peptides generated from on-beads digestion using LTQ-FT or LTQ-ETD mass spectrometry, and (5) enrichment of phosphopeptide methyl esters with IMAC. Using the above strategies, we identify 29 phosphorylation sites (19 novel and 10 previously reported) and a novel glycosylation site on Ser 74. Furthermore, with this method, we simultaneously detect 10 co-purifying proteins which are present in focal adhesion complexes. Extension of these methods to other substrates should facilitate generation of global phosphorylation maps and protein-protein interactions for any protein of interest.

Phosphorylation of Thr695 and Thr850 on the myosin phosphatase target subunit: inhibitory effects and occurrence in A7r5 cells

Major sites for Rho-kinase on the myosin phosphatase target subunit (MYPT1) are Thr695 and Thr850. Phosphorylation of Thr695 inhibits phosphatase activity but the role of phosphorylation at Thr850 is not clear and is evaluated here. Phosphorylation of both Thr695 and Thr850 by Rho-kinase inhibited activity of the type 1 phosphatase catalytic subunit. Rates of phosphorylation of the two sites were similar and efficacy of inhibition following phosphorylation was equivalent for each site. Phosphorylation of each site on MYPT1 was detected in A7r5 cells, but Thr850 was preferred by Rho-kinase and Thr695 was phosphorylated by an unidentified kinase(s).

Simultaneous stretching and contraction of stress fibers in vivo

To study the dynamics of stress fiber components in cultured fibroblasts, we expressed alpha-actinin and the myosin II regulatory myosin light chain (MLC) as fusion proteins with green fluorescent protein. Myosin activation was stimulated by treatment with calyculin A, a serine/threonine phosphatase inhibitor that elevates MLC phosphorylation, or with LPA, another agent that ultimately stimulates phosphorylation of MLC via a RhoA-mediated pathway. The resulting contraction caused stress fiber shortening and allowed observation of changes in the spacing of stress fiber components. We have observed that stress fibers, unlike muscle myofibrils, do not contract uniformly along their lengths. Although peripheral regions shortened, more central regions stretched. We detected higher levels of MLC and phosphorylated MLC in the peripheral region of stress fibers. Fluorescence recovery after photobleaching revealed more rapid exchange of myosin and alpha-actinin in the middle of stress fibers, compared with the periphery. Surprisingly, the widths of the myosin and alpha-actinin bands in stress fibers also varied in different regions. In the periphery, the banding patterns for both proteins were shorter, whereas in central regions, where stretching occurred, the bands were wider.

Actin-rich spherical extrusion induced in okadaic acid-treated K562 cells by crosslinking of membrane microdomains

Interconnection between surface microdomains and the actin cytoskeleton is vital to various cellular activities. We studied the responses of okadaic acid (OKA)-treated K562 leukemia cells to crosslinking of membrane microdomains. Although OKA alone induced clustering of surface-bound F-actin, addition of a biotinylated poly(ethylene glycol) derivative of cholesterol (bPEG-Chol) and subsequent binding of streptavidin (SA) further induced accumulation of the clusters, resulting in the formation of a spherical cell extrusion. This extrusion was also induced by direct crosslinking of a raft marker, CD59, and ganglioside GM1. In addition, we found that knockout of the gene encoding Fyn kinase inhibited formation of the spherical extrusion in murine T-cells. In bPEG-Chol/SA-treated cells, CD59, ganglioside GM1, and clathrin/AP-2 were all accumulated on the surface of the actin-rich extrusion, whereas dynamin and transferrin receptors were unaffected. Intermediate filaments, mitochondria, and other vesicles also accumulated. These results suggest that crosslinking of membrane domains exaggerates the linkage between actin and a defined set of membrane proteins in OKA-treated cells.

Calyculin-A induces focal adhesion assembly and tyrosine phosphorylation of p125(Fak), p130(Cas), and paxillin in Swiss 3T3 cells

Treatment of intact Swiss 3T3 cells with calyculin-A, an inhibitor of myosin light chain (MLC) phosphatase, induces tyrosine phosphorylation of p125(Fak) in a sharply concentration- and time-dependent manner. Maximal stimulation was 4.2 +/- 2.1-fold (n = 14). The stimulatory effect of calyculin-A was observed at low nanomolar concentrations (<10 nM); at higher concentrations (>10 nM) tyrosine phosphorylation of p125(Fak) was strikingly decreased. Calyculin-A induced tyrosine phosphorylation of p125(Fak) through a protein kinase C- and Ca(2+)-independent pathway. Exposure to either cytochalasin-D or latrunculin-A, which disrupt actin organization by different mechanisms, abolished tyrosine phosphorylation of p125(Fak) in response to calyculin-A. Treatment with high concentrations of platelet-derived growth factor (20 ng/ml) which also disrupt actin stress fibers, completely inhibited tyrosine phosphorylation of p125(Fak) in response to calyculin-A. This agent also induced tyrosine phosphorylation of the focal adhesion-associated proteins p130(Cas) and paxillin. These tyrosine phosphorylation events were associated with a striking increase in the assembly of focal adhesions. The Rho kinase (ROK) inhibitor HA1077 that blocked focal adhesion formation by bombesin, had no effect on the focal adhesion assembly induced by calyculin-A. Thus, calyculin-A induces transient focal adhesion assembly and tyrosine phosphorylation of p125(Fak), p130(Cas), and paxillin, acting downstream of ROK.

Calyculin-A, an inhibitor for protein phosphatases, induces cortical contraction in unfertilized sea urchin eggs

When an unfertilized sea urchin egg was exposed to calyculin-A (CL-A), an inhibitor of protein phosphatases, for a short period and then lysed, the cortex contracted to exclude cytoplasm and became a cup-shaped mass. We call the contracted cortex "actin cup" since actin filaments were major structural components. Electron microscopic observation revealed that the cup consisted of inner electron-dense layer, middle microfilamentous layer, and outermost granular region. Microfilaments were heavily accumulated in the inner electron-dense layer. The middle layer also contained numerous microfilaments, which were determined to be actin filaments by myosin S1 decoration, and they were aligned so that their barbed ends directed toward the outermost region. Myosin II, Arp2, Arp3, and spectrin were concentrated in the actin cup. Immuno-electron microscopy revealed that myosin II was localized to the electron-dense layer. We further found that the cortical tension of the egg increased just after application of CL-A and reached maximum within 10 min. Cytochalasin B or butanedione monoxime blocked the contraction, which suggested that both actin filaments and myosin ATPase activity were required for the contraction. Myosin regulatory light chain (MRLC) in the actin cup was shown to be phosphorylated at the activation sites Ser-19 and Thr-18, by immunoblotting with anti-phosphoepitope antibodies. The phosphorylation of MRLC was also confirmed by a (32)P in vivo labeling experiment. The CL-A-induced cortical contraction may be a good model system for studying the mechanism of cytokinesis.

Cell mechanics studied by a reconstituted model tissue

Tissue models reconstituted from cells and extracellular matrix (ECM) simulate natural tissues. Cytoskeletal and matrix proteins govern the force exerted by a tissue and its stiffness. Cells regulate cytoskeletal structure and remodel ECM to produce mechanical changes during tissue development and wound healing. Characterization and control of mechanical properties of reconstituted tissues are essential for tissue engineering applications. We have quantitatively characterized mechanical properties of connective tissue models, fibroblast-populated matrices (FPMs), via uniaxial stretch measurements. FPMs resemble natural tissues in their exponential dependence of stress on strain and linear dependence of stiffness on force at a given strain. Activating cellular contractile forces by calf serum and disrupting F-actin by cytochalasin D yield "active" and "passive" components, which respectively emphasize cellular and matrix mechanical contributions. The strain-dependent stress and elastic modulus of the active component were independent of cell density above a threshold density. The same quantities for the passive component increased with cell number due to compression and reorganization of the matrix by the cells.

Mode-specific inhibition of sodium-calcium exchange during protein phosphatase blockade

The effects of the protein phosphatase inhibitors calyculin A and okadaic acid on Na(+)/Ca(2+) exchange activity were examined in transfected Chinese hamster ovary cells expressing the bovine cardiac Na(+)/Ca(2+) exchanger. Incubating the cells for 5-10 min with 100 nM calyculin A reduced exchange-mediated (45)Ca(2+) uptake or Ba(2+) influx by 50-75%. Half-maximal inhibition of (45)Ca(2+) uptake was observed at 15 nM calyculin A. The nonselective protein kinase inhibitors K252a and staurosporine provided partial protection against the effects of calyculin A. Okadaic acid, another protein phosphatase inhibitor, nearly completely blocked exchange-mediated Ba(2+) influx. Chinese hamster ovary cells expressing a mutant exchanger in which 420 out of 520 amino acid residues were deleted from the central hydrophilic domain of the exchanger remained sensitive to the inhibitory effects of calyculin A and okadaic acid. Surprisingly, Na(o)(+)-dependent Ca(2+) efflux appeared to be only modestly inhibited, if at all, by calyculin A or okadaic acid. We conclude that protein hyperphosphorylation during protein phosphatase blockade selectively inhibits the Ca(2+) influx mode of Na(+)/Ca(2+) exchange, probably by an indirect mechanism that does not involve phosphorylation of the exchanger itself.

Expression, subcellular localization, and cloning of the 130-kDa regulatory subunit of myosin phosphatase in porcine aortic endothelial cells

In endothelial cells in situ and in primary culture, immunoblot analysis revealed an expression of the 130-kDa subunit of myosin phosphatase, similar to the myosin phosphatase targeting subunit (MYPT) of smooth muscle. Screening of an endothelial cell cDNA library yielded a clone encoding an NH2-terminal fragment of 89.6 kDa, closely related to smooth muscle MYPT1. Two isoforms differing by a central insert of 56 residues were detected. In growing cells, MYPT1 was localized on stress fiber, but at confluence the localization pattern changed and MYPT1 was distributed close to the cell membrane and at cell-cell contacts. The membrane localization of MYPT1 suggested a target other than myosin and raised the possibility that MYPT1 may be involved in dephosphorylation of alternative substrate(s). These distinct mechanisms would also be dependent on the growth state of the endothelial cells, i.e., regulation of actin-myosin interactions in growing cells and an unknown function in cells at confluence.

Changes in morphology of human skin fibroblasts induced by local anaesthetics: role of actomyosin contraction

Local anaesthetics block action potentials in the membranes of excitable cells but their effects on non-excitable cells are less well known. Some local anaesthetics are applied directly onto the skin, and for this reason the effect of procaine (p-aminobenzoic acid diethylamino-etyl ester hydrochloride) and tetracaine (4-[butylamino]benzoic acid 2-[dimethylamino]ethyl ester) upon the morphology and cytoskeleton organisation of human skin fibroblasts was investigated. The time lapse video recording of fibroblasts cultured in serum-enriched medium revealed that the cells rapidly change shape after the addition of the anaesthetic. These effects were fully reversible. The microscopic observations were confirmed by quantitative analysis of projected cell area and cell shape parameters. Local anaesthetics significantly changed the actin cytoskeleton organisation, inducing total disappearance of stress fibres. Serum-starvation or myosin light chain kinase inhibitors, KT 5926 inhibitor (8R*,9S*,11S*)-(-)-9-hydroxy-9-methoxycarbonyl-8-methyl-14-n-propoxy-2,3 ,9, 10-tetrahydro-8,11-epoxy,1H,8H,11H-2,7b,11a-triazadibenzo[a,g]c ycloocta[cde] trinden-1-one or wortmannin, which induce the 'relaxed' morphology of the cells, prevent both the anaesthetic-induced changes in cell shape and the disassembly of stress fibres. Together, the observations suggest that local anaesthetics affect the actomyosin system, inducing contraction.

Fluid-phase markers in the basolateral endocytic pathway accumulate in response to the actin assembly-promoting drug Jasplakinolide

To investigate the role of filamentous actin in the endocytic pathway, we used the cell-permeant drug Jasplakinolide (JAS) to polymerize actin in intact polarized Madin-Darby canine kidney (MDCK) cells. The uptake and accumulation of the fluid-phase markers fluorescein isothiocyanate (FITC)-dextran and horseradish peroxidase (HRP) were followed in JAS-treated or untreated cells with confocal fluorescence microscopy, biochemical assays, and electron microscopy. Pretreatment with JAS increased the uptake and accumulation of fluid-phase markers in MDCK cells. JAS increased endocytosis in a polarized manner, with a marked effect on fluid-phase uptake from the basolateral surface but not from the apical surface of polarized MDCK cells. The early uptake of FITC-dextran and HRP was increased more than twofold in JAS-treated cells. At later times, FITC-dextran and HRP accumulated in clustered endosomes in the basal and middle regions of JAS-treated cells. The large accumulated endosomes were similar to late endosomes but they were not colabeled for other late endosome markers, such as rab7 or mannose-6-phosphate receptor. JAS altered transport in the endocytic pathway at a later stage than the microtubule-dependent step affected by nocodazole. JAS also had a notable effect on cell morphology, inducing membrane bunching at the apical pole of MDCK cells. Although other studies have implicated actin in endocytosis at the apical cell surface, our results provide novel evidence that filamentous actin is also involved in the endocytosis of fluid-phase markers from the basolateral membrane of polarized cells.

Low concentrations of the phosphatase inhibitor okadaic acid stop tumor cell locomotion

The phosphatase inhibitor okadaic acid exerted a biphasic effect on the shape of spontaneously polarized Walker carcinosarcoma cells. At lower concentrations, the drug suppressed cell polarity (IC50 = 0.14 microM) and the cells reverted to a spherical shape. At higher concentrations (> 0.25 microM), cells developed large blebs (IC50 = 0.4 microM). Furthermore, 0.2 microM okadaic acid completely suppressed spontaneous cell locomotion. Two specific inhibitors of protein kinase C did not prevent the actions of okadaic acid on cell shape, showing that this enzyme is very likely not involved. Another phosphatase inhibitor, calyculin A, also suppressed polarity (IC50 = 60 nM) and produced blebbing cells (IC50 = 70 nM). 1 microM okadaic acid induced a 40- to 70-fold increase in phosphorylation of the intermediate filament protein vimentin in intact cells. Increased phosphorylation of this major phosphoprotein correlated with the generation of blebbing cells, rather than with inhibition of polarity and may thus be involved in generating the marked shape changes. We conclude that constitutive phosphatase activity is required for motility and control of shape in Walker carcinosarcoma cells.

Protein phosphatases maintain the organization and structural interactions of hepatic keratin intermediate filaments

The importance of protein phosphatases in the maintenance of cytoskeletal structure is supported by the serious liver injury caused by microcystin-LR, a hepatotoxic inhibitor of type-1 and type-2A serine/threonine protein phosphatases. We used the microcystin-LR-induced cell injury as a model to study the roles of protein dephosphorylation in maintaining cytoskeletal structure and cellular interactions in primary rat hepatocyte cultures. Confocal microscopy revealed that the first visible effect of microcystin-LR is disruption of desmoplakin organization at the cell surface, indicating dissociation of desmosomes. This effect is followed by a dramatic reorganization of both the intermediate filament (keratins 8 and 18) and microfilament networks, resulting in a merged structure in which the intermediate filaments are organized around a condensed actin core. Keratin 8, keratin 18 and desmoplakin I/II are the major cytoskeleton-associated targets for microcystin-LR-induced phosphorylation. Hyperphosphorylation of keratin 8 and 18 is accompanied by an increased keratin solubility, which correlates with the observed morphological effects. Phosphopeptide mapping shows that four specific tryptic phosphopeptides are highly phosphorylated predominantly in the soluble pool of keratin 18, whereas keratin 8 shows no indications of such assembly state-specific sites. Phosphopeptide maps of keratins phosphorylated in vivo and in vitro indicate that Ca2+/calmodulin-dependent kinase may be involved in regulating the serine-specific phosphorylation of both keratin 8 and keratin 18, while cAMP-dependent protein kinase does not seem to play a major role in this context. Taken together, our results show that the interactions between keratin intermediate filaments and desmosomes as well as the assembly states of their main constituent proteins, are directly regulated by serine/threonine kinase/phosphatase equilibria.

Endothall thioanhydride inhibits protein phosphatases-1 and -2A in vivo

The objective of this study was to relate the toxicity of several cantharidin-derivative pesticides with their abilities to inhibit protein phosphatases-1 (PP1) and -2A (PP2A). Cantharidin (CA), endothall, and endothall thioanhydride (ETA) inhibited the activity of PP1 and PP2A, and the potency sequence was CA > endothall > ETA in vitro. We determined the inhibitory potency of these pesticides on hepatic protein phosphatases by administration of the toxins into the portal vein of rats. The potency sequence of ETA > CA > endothall was established for the inhibition of PP1 and PP2A in vivo and shows close correlation with the sequence of relative toxicity. ETA predominantly targets PP1 for inhibition in liver, as revealed by assays specific for PP1 or PP2A. Studies using 3T3 fibroblasts showed that only ETA, but not CA or endothall, induced marked morphological changes. These effects included cell rounding and detachment as well as extensive reorganization of actin filaments and are characteristic for the cell-permeable phosphatase-inhibitory toxins. It is suggested that the in vivo effectiveness is related to enhanced uptake of ETA, because this is permeable across the plasmalemma.

Involvement of protein phosphatase-1 in cytoskeletal organization of cultured endothelial cells

The phosphorylation and dephosphorylation of cytoskeletal proteins regulate the shape of eukaryotic cells. To elucidate the role of serine/threonine protein phosphatases (PP) in this process, we studied the effects of calyculin A (CLA), a potent and specific inhibitor of protein phosphatases 1 (PP-1) and 2A (PP-2A) on the cytoskeletal structure of cultured human umbilical vein endothelial cells (HUVECs). The addition of CLA (5 min) caused marked alterations in cell morphology, such as cell constriction and bleb formation. Microtubules and F-actin were reorganized, becoming markedly condensed around the nucleus. Although the fluorescence intensity of phosphoamino acids was not significantly different according to immunocytochemistry between cells with and without CLA, polypeptides of 135, 140, 158, and 175 kDa were specifically phosphorylated on serine and/or threonine residues. There was no significant effect on tyrosine residues. The effects of CLA on cytoskeletal changes and protein phosphorylation were almost completely inhibited by the non-selective kinase inhibitor, K-252a. The effect of CLA on cell morphology was at least 100 times more potent than that of okadaic acid, consistent with the inhibitory potency against PP-1. The catalytic subunit of PP-1 was also identified in HUVECs by Western blotting with its monoclonal antibody antibody. These results suggest that PP-1 is closely involved in sustaining the normal structure of the cytoskeleton.

Alterations in microtubules, intermediate filaments, and microfilaments induced by microcystin-LR in cultured cells

Microcystin-LR (MCLR) is a cyanobacterial hepatotoxin that inhibits intracellular serine/threonine protein phosphatases causing disruption of actin microfilaments (MFs) and intermediate filaments (IFs) in hepatocytes. This study compared the effects of MCLR on the organization of MFs, IFs, and microtubules (MTs) in hepatocytes and nonhepatocyte cell lines and determined the sequence of toxin-induced changes in these cytoskeletal components. Rat renal epithelial cells and fibroblasts were incubated with MCLR at 100 or 200 microM for 6-18 hr. Rat hepatocytes in primary culture were exposed to the toxin at 1 or 10 microM for 2-64 min. Cells were fixed and incubated with primary antibodies against beta-tubulin, actin, and vimentin or cytokeratin IFs, followed by gold-labeled secondary antibodies with silver enhancement of the gold probe. The fraction of fibroblasts and hepatocytes with altered cytoskeletal morphology was evaluated as a function of MCLR dose and exposure time to assess the sequence of changes in cytoskeletal components. Changes in fibroblasts and some hepatocytes were characterized initially by disorganization of IFs, followed rapidly by disorganization of MTs, with the progressive collapse of both cytoskeletal components around cell nuclei. Many hepatocytes exhibited MT changes prior to effects on IF structure. Alterations in MFs occurred later and included initial aggregation of actin under the plasma membrane, followed by condensation into rosette-like structures and eventual complete collapse into a dense perinuclear bundle. The similarity of effects among different cell types suggests a common mechanism of action, but the independent kinetics of IF and MT disruption in hepatocytes suggests that there may be at least 2 sites of phosphorylation that lead to cytoskeletal alterations.

Cytoskeletal reorganization of human platelets induced by the protein phosphatase 1/2 A inhibitors okadaic acid and calyculin A

Okadaic acid (OA) and calyculin A (CLA), which are potent and specific inhibitors of serine/threonine protein phosphatases type 1 and 2A, have been shown to induce drastic changes in platelet morphology. The aim of this study was to analyse the molecular mechanisms of OA- or CLA-induced cytoskeletal reorganization, with a specific focus on microtubules and actin filaments. Confocal fluorescence microscopy revealed that OA or CLA altered the distribution of microtubules from marginal band arrangements to homogeneous patterns, consistent with the transmission-electron-microscopic finding that microtubules were fragmented and redistributed into pseudopod-like processes. In thrombin-activated platelets, OA or CLA induced extremely long pseudopods containing an array of microtubules and actin filaments, and a condensed mass of actin filaments in the centre of platelets. OA or CLA induced the constriction of actin filaments without an increase in filamentous (F)-actin, and also rather significantly inhibited actin polymerization in thrombin-activated platelets. Furthermore, neither OA or CLA enhanced phosphorylation of myosin light chain (MLC). By immunoprecipitation of platelet lysate with anti-alpha-tubulin antibody, a 90 kDa protein was co-precipitated with tubulin and was predominantly phosphorylated in the presence of OA. As the time-dependent phosphorylation of 90 kDa protein correlated well with the reorganization of microtubules, these data suggest that phosphorylation and dephosphorylation of this protein might play a role in the regulation of microtubule organization. These findings indicate that OA or CLA induces reorganization of microtubules and actin filaments via the phosphorylation of a microtubule-associated 90 kDa protein and an MLC-phosphorylation-independent mechanism. mechanism.

Changes in the cytoskeletal structure of cultured smooth muscle cells induced by calyculin-A

Changes in the cytoskeletal structure of cultured A10 smooth muscle cells induced by calyculin-A (CL-A), a potent inhibitor of types 1 and 2A protein phosphatases, were analyzed using indirect fluorescence techniques. In the presence of 1 × 10(−7) M CL-A the cells became round and subsequently detached from the substratum. The effect of CL-A was inhibited by a non-selective kinase inhibitor, K-252a, but not by EGTA. In rounded cells stress fibers were absent and staining for F-actin appeared in patches. Vinculin, one of the components of focal contacts, was localized at the periphery of control cells. CL-A treatment moved the focal contacts towards the inside of the cell along the stress fibers, and this was followed by the rounding up of the cell. In addition, rapid and marked changes in microtubule structure were observed in CL-A-treated cells. Many ‘nicks’ or ‘gaps’ were observed along the microtubules in the attached, spread cells. A filamentous network of microtubules was not observed in the detached cells, i.e. after longer exposure to CL-A. These results suggest that CL-A may change the structure of focal contacts, resulting in the rounding up of the cell, and inducing a microtubule-severing activity. These effects were independent of the external Ca2+ concentration. The changes in cytoskeletal structure may be caused by disturbing the balance of phosphorylation and dephosphorylation in the cell.

Okadaic acid-induced actin assembly in neutrophils: role of protein phosphatases

Activation of neutrophils results in morphological and functional alterations including changes in cell shape and initiation of motile behavior that depend on assembly and reorganization of the actin cytoskeleton. Phosphoproteins are thought to be key intermediates in the regulation of cytoskeletal alterations and whereas much attention has been directed at the role of protein kinases, relatively little information is available on the importance of phosphatases. To elucidate the role of protein phosphatases, we studied the effects of the phosphatase inhibitors okadaic acid and calyculin A on the actin cytoskeleton of human neutrophils. Exposure of cells to okadaic acid resulted in assembly and spatial redistribution of actin, which peaked at 25 min and returned to baseline levels by 45 min, as assessed by flow cytometric analysis of NBD-phallacidin stained cells and confocal fluorescence microscopy, respectively. These effects correlated with an increase in protein phosphorylation, determined by incorporation of 32P into cellular proteins using SDS-PAGE and autoradiography. Similar but more rapid responses were observed in electropermeabilized cells treated with okadaic acid or calyculin A. The dose dependence of these effects was compatible with a role for phosphatase type 1 as the target enzyme. These findings also suggested the presence of constitutively active protein kinases capable of effecting actin polymerization. Phosphorylation of myosin light chain (MLC) has been postulated to promote actin assembly, but myosin light chain kinase (MLCK) appeared not to be involved because: (1) the effect of okadaic acid was not inhibited by the MLCK inhibitor KT5926 and (2) in permeabilized cells suspended in medium with free calcium [Ca2+] < 10 nM (conditions under which MLCK is inactive), the effect of okadaic acid persisted. The role of phosphatases in stimulus-induced actin assembly was assessed in cells preincubated with okadaic acid for 45 min, after F-actin levels had returned to baseline. Under these conditions, okadaic acid completely abrogated actin assembly induced by phorbol myristate acetate, platelet activating factor, and leukotriene B4, whereas the effects of the chemotactic peptide fMLP and opsonized zymosan (OpZ) were unaffected. We conclude that serine and threonine phosphatases exert a tonic negative influence on actin assembly and organization. Furthermore, divergent pathways seem to mediate the response to lipidic stimuli, on one hand, and fMLP and OpZ, on the other, as evidenced by the differential susceptibility to inhibition by okadaic acid.