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Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

We have investigated how Bacillus anthracis lethal toxin (LT) triggers caspase-3 activation and the formation of thick actin cables in human endothelial cells. By DNA array analysis we show that LT has a major impact on the cell transcriptome and we identify key host genes involved in LT cytotoxic effects. Indeed, upregulation of TRAIL and downregulation of XIAP both participate in LT-induced caspase-3 activation. LT induces a downregulation of the immediate early gene and master regulator of transcription egr1. Importantly, its re-expression in LT-intoxicated cells blocks caspase-3 activation. In parallel, we found that the formation of actin cables induced by LT occurs in the absence of direct activation of RhoA/ROCK signalling. We show that knock-down of cortactin and rhophilin-2 under conditions of calponin-1 expression defines the minimal set of genes regulated by LT for actin cable formation. Together our data establish that the modulation of the cell transcriptome by LT plays a key role in triggering human endothelial cell toxicity.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

As a major virulence factor, Bacillus anthracis synthesizes a tripartite toxin composed of the protective antigen (PA), the oedema factor (EF) and the lethal factor (LF) (Collier and Young, 2003; Moayeri and Leppla, 2004). The toxin encoding genes are borne by the plasmid pXO1. LF and/or EF can associate with heptamers of PA to form, respectively, the lethal toxin (LT for PA+LF) or the oedema toxin (ET for PA+EF). Heptamers of PA transport LF and/or EF into host cell endosomes and promote their injection into the cytosol (Abrami et al., 2006; Sun et al., 2008a). Two receptors for PA, which contain a Willebrand factor A-like domain, have been characterized: the tumour endothelial marker 8 (TEM8) and capillary morphogenesis protein 2 (CMG2) (Bradley et al., 2001; Scobie et al., 2003). TEM8 is highly expressed in epithelial cells lining B. anthracis entry sites and tumour vasculature, whereas CMG2 is expressed in most human cells (Rmali et al., 2004; Bonuccelli et al., 2005). Upon reaching the cytosol, LF uncouples MAP kinase signalling cascades by cleavage of the amino-terminal part of MKKs (Duesbery et al., 1998). EF produces an increase in the intracellular level of cyclic-AMP through its calcium/calmodulin-dependent adenylate cyclase activity (Moayeri and Leppla, 2004). Deletion of EF or LF leads to a 10- and 1000-fold attenuation of bacterial virulence respectively (Pezard et al., 1991).

LT (PA+LF) produces vascular damages and septic shock-like manifestations, pointing to LT as a major determinant of B. anthracis-induced vascular dysfunctions (Moayeri and Leppla, 2004; Gozes et al., 2006; Stearns-Kurosawa et al., 2006; Bolcome et al., 2008). Theprogressive and belated increase in vascular permeability induced by LT is determined by its enzymatic activity (Rolando et al., 2009). Recent studies have unravelled how LT triggers a caspase-dependent cell death of both primary human umbilical vein endothelial cells (HUVECs) and primary human lung or dermal microvascular endothelial cells (HPMVECs and HDMVECs) (Kirby, 2004; Warfel et al., 2005; Gozes et al., 2006). One study also describes that LT caused the formation of actin cables in endothelial cells (Warfel et al., 2005). The re-organization of the actin cytoskeleton induced a rupture of the barrier formed by endothelial cell monolayers (Warfel et al., 2005). How MAPK pathway inhibition by LT produces a re-organization of the human endothelial cell actin cytoskeleton and caspase-3 activation remains to be defined.

Recent studies point to host cell transcription as an important target of LT. Transcriptome responses of immortalized endothelial cells 6 h after infection by either wild-type or pXO1-deficient B. anthracis Sterne were measured (van Sorge et al., 2008). Wild-type B. anthracis primarily reduces host gene expression, a phenomenon likely controlled by pXO1-borne toxin (van Sorge et al., 2008). In parallel, it has been determined that LT can block inflammatory gene expression by interfering with MKK-regulated histone modifications at their promoter (van Sorge et al., 2008; Raymond et al., 2009). Other possible consequences of corrupting the host transcriptome in LT-induced cytotoxic effects remain to be defined.

Contractile actomyosin cables have a major function in regulating endothelium permeability, owing to their ability to destabilize intercellular junctions (Wojciak-Stothard and Ridley, 2003). Activation of the Rho kinase (ROCK) by RhoA drives actin filament (F-actin) cross-linking into contractile actin cables (Jaffe and Hall, 2005). This effect results primarily from a downstream phosphorylation of the Myosin Light Chain (MLC) (Watanabe et al., 1999; Jaffe and Hall, 2005). The RhoA/ROCK pathway, as well as the Rac/Cdc42-activated kinases PAK, also stabilizes actin filaments (F-actin) via a LIM kinase-induced phosphorylation and inactivation of cofilin (Jaffe and Hall, 2005). In parallel to the RhoA/ROCK pathway, several F-actin regulators with capping, severing or cross-linking activities modulate the organization of the actin cytoskeleton (Huang et al., 2004; Le Clainche and Carlier, 2008). Importantly, MAP kinases also control the activity of actin regulators (Huang et al., 2004). Inhibition of the p38-MAPK signalling by LT can block Hsp27 phosphorylation and the induction of actin filament assembly at the leading edge of motile neutrophils, as well as actin-based motility in HeLa cells (During et al., 2005; 2007). Recent studies suggest that the control of gene expression by MAP kinases interferes with actin cytoskeleton regulation. For instance, the serum response factor transcription factor and its MAP kinase- and RhoA-regulated cofactors play a crucial role in shaping the actin cytoskeleton of skeletal and smooth muscle cells (Wang et al., 2004; Posern and Treisman, 2006; Long et al., 2007; Miano et al., 2007). How MAP kinase pathways signal to the endothelial cell actin cytoskeleton for its organization is a key question.

Here we have identified key host factors that are regulated by LT at the mRNA level and are involved in its cytotoxicity on human endothelial cells. We propose that modulation of the host cell transcriptome plays a key role in LT-induced human endothelial actin cytoskeleton re-organization and caspase-3 activation.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

LT triggers progressive cytotoxic effects on human endothelial cells

In order to determine the molecular basis of actin cytoskeleton remodelling and induction of human endothelial cell death by LT, we established the kinetics of appearance of these cytotoxic effects (Fig. 1 and Videos S1–S3). Control monolayers organized in stable cobblestone architecture (Fig. 1A and Video S4). Between six to eight hours after addition of the toxin, we observed that some cells started to elongate (Video S1). This phenomenon progressively affected individual cells in the monolayer to reach a maximum effect at 24 h of intoxication (Video S2). Also, following 12 h of intoxication, individual cells progressively underwent blebbing and detachment leading to the formation of large intercellular gaps in the endothelial cell monolayer (Fig. 1A and Videos S2 and S3). This correlated with a progressive increase in the activity of caspase-3, starting 8 h after toxin addition (Fig. 1B). We also measured an increase in caspase-8 and -9 activities (Fig. S1A). Moreover, we measured, after 24 h of intoxication, a threefold increase in early apoptotic cells (Fig. S1B). LT-induced elongation of cells correlated with a re-organization of the actin cytoskeleton, which forms thick actin cables parallel to each other starting around 6 h and best visualized at 24 h of intoxication (Fig. 1C). In addition, we observed a loss of the cortical F-actin network and a redistribution of the major adherens junction protein VE-cadherin (Fig. 1C). This redistribution of VE-cadherin is consistent with the formation of contractile actin cables and cell elongation resulting in cell stretching and opening of intercellular gaps (Wojciak-Stothard and Ridley, 2003). Monolayer treatment with the catalytically inactive mutant LTm (LFE687A + PA), with PA alone or LF alone, neither produced MEK2 cleavage nor cytotoxicity (Fig. S1C and D, and data not shown). LT purified from B. anthracis produced similar cytotoxic effects on the actin cytoskeleton and caspase-3 activity (Fig. S1E and F). Our data established that LT induced progressive and belated cytotoxic effects, as opposed to the early inactivation of the MAP kinase signalling (Fig. S2A). Treatment of endothelial cell monolayers with a mix of MKK/MAP kinase inhibitors (MKI) was found to phenocopy the cytotoxic effects of LT, although with a lower efficiency likely due to the lesser inhibition of MAPK phosphorylation (Fig. 1C and Fig. S1G).

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Figure 1. Gradual cytotoxic effects on endothelium monolayers intoxicated by LT. HUVEC monolayers were treated with LT (PA+LF, 3 + 1 µg ml−1), a mix of MKK/MAP kinase inhibitors (MKI: 10 µM U0126, 20 µM SB203580 and 10 µM SP600125) or zVAD (20 µM). A. Pictures taken from Video S1–S4, at indicated times. Figure shows the gradual effect of LT on an endothelium monolayer. Control corresponds to a monolayer left untreated for 24 h. Scale bar, 100 µm. B. Measure of caspase-3 activities. zVAD was added to evaluate the specificity of caspase activation by LT. Activities are expressed as arbitrary units (A.U.) per mg of protein. Mean values of n > 3 independent biological replicates per condition ± SEM (*P < 0.05 versus control cells). C. Actin cytoskeleton re-organization in intoxicated monolayers at the indicated times. F-actin was labelled with FITC-phalloidin (green) and VE-cadherin with anti-cadherin5 (red). Insets show a close-up of actin cytoskeleton and intercellular junctions. Scale bar, 10 µm.

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Importantly, these new observations establish that LT and MAPK pathway inhibition induce a progressive and belated activation of caspase-3 and re-organization of the actin cytoskeleton.

LT reprograms gene expression of the TRAIL signalling pathway

We rationalized that a corruption of gene expression by LT might be involved in this delayed induction of cytotoxicity. Transcriptomes of endothelial cell monolayers intoxicated for 4 and 8 h by LT were compared with those of control monolayers (GEO Series Accession No. GSE12131). Most notably, at 8 h of intoxication, LT dampened levels of 2341 transcripts. In agreement with a recent study (van Sorge et al., 2008), the immediate early gene egr1 (early growth response-1) appeared as the most downregulated transcript (not shown and Fig. 2A). In contrast, a cluster of genes showed a sustained increase of expression at 4 and 8 h of intoxication (Fig. S2B). This cluster of genes comprises the TNF-related apoptosis-inducing ligand (TRAIL or APO2L). TRAIL is a type-II transmembrane protein, eventually released as a soluble cytokine, which can associate with death receptors of the TNF receptor superfamily for activation (Guicciardi and Gores, 2009). Interestingly, several studies showed that MAP kinase inhibition sensitizes cells to TRAIL-induced apoptosis and conversely that ERK activation inhibits TRAIL-induced cell death (Guicciardi and Gores, 2009). Other factors involved in the TRAIL pathway were identified: we noticed a decrease of xiap (X-linked Inhibitor of Apoptosis Protein) and an increase of xaf1 (XIAP associated factor-1) mRNA. Kinetics of modulation of egr1, trail, xiap, xaf1, as well as other LT regulated genes were established by qRT-PCR (Fig. 2A). Inhibition of MAP kinase cascades with MKI recapitulated the modulation of expression of these genes (Fig. 2B). Also LT purified from B. anthracis had a similar effect on egr1, trail, xiap and xaf1 mRNA expression (Fig. S1H). Finally, we found that addition of actinomycin (actD) blocked the induction by LT of the genes tested (data not shown).

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Figure 2. Gene regulation in endothelium monolayers intoxicated by LT. HUVEC monolayers were intoxicated with recombinant LT or a mix of MKK/MAP kinase inhibitors (MKI). Mean values of n > 6 independent biological replicates per condition ± SEM (*P < 0.05 versus control cells). A. Kinetics of modulation of transcript levels induced by LT were assessed by qRT-PCR at the indicated times. Results are expressed as fold variations compared with untreated monolayers. B. Gene transcript modulations upon LT versus MKI treatments were assessed by qRT-PCR. Results are presented as fold variation between intoxicated and untreated monolayers.

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Thus, LT triggers a sustained modulation of expression of a large number of genes. This comprises the immediate early gene egr1, as well as genes of the TRAIL signalling pathway.

LT-induced caspase-3 activation requires trail, xiap and egr-1 modulation of expression

Kinetics of TRAIL induction and down-modulation of XIAP were established at the protein level. This shows a threefold upregulation of TRAIL and a 40% downregulation of XIAP at 8 h of intoxication, concomitant with the increase in caspase-3 activity (Fig. 1B, Fig. 3A and B). The induction of TRAIL and the decrease of XIAP further progress over the first 24 h of intoxication. We next tested whether TRAIL inhibition and XIAP re-expression reverted LT-induced caspase-3 activation. Addition of TRAIL inhibitory antibodies, as well as expression of XIAP reduced the activation of caspase-3 by the toxin (Fig. 3C and D). We verified that in these conditions LT still induced MEK2 cleavage (data not shown).

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Figure 3. Role of TRAIL, XIAP and Egr1 in activation of caspase-3 by LT. HUVECs monolayers were treated with LT (PA+LF, 3 + 1 µg ml−1), with a catalytically inactive toxin (LTm: PA+LFE687A, 3 + 1 µg ml−1), with a mix of MKK/MAP kinase inhibitors (MKI) or with zVAD (20 µM). A. Graph shows levels of TRAIL assessed by ELISA in cell lysates. Mean values of 5 independent experiments ± SEM (*P < 0.05 versus untreated cells). B. Immunoblots showing the decrease of XIAP protein levels. Values indicated below represent XIAP normalized to actin (XIAP/Actin) in three independent experiments. C and D. Measure of caspase-3 activity. (C) HUVECs were left untreated (Ctrl) or treated 24 h as indicated with LT, zVAD, anti-TRAIL or control anti-GST antibodies (irrelevant Ab). Values correspond to fold induction compared with control in n ≥ 3 experiments ± SEM (*P < 0.05 versus indicated points). (D) HUVECs transfected for 24 h with empty vector or XIAP expression vector were next intoxicated 24 h with LT or untreated. Values are expressed in arbitrary units (A.U.) per mg of proteins (mean values of n > 6 experiments ± SEM; *P < 0.05 versus indicated points). E. Immunoblot showing the decrease of Egr1 in LT- or MKI-treated HUVEC monolayers, as quantified below (Egr1 normalized to actin: Egr1/Actin). F. Measure of caspase-3 activity. HUVECs transfected for 24 h with empty vector or Egr1 expression vector were next intoxicated 24 h with LT or untreated. Values are expressed in arbitrary units (A.U.) per mg of proteins (mean values of n ≥ 3 experiments ± SEM; *P < 0.05 versus indicated points).

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We next analysed whether deregulation of an upstream transcription factor might account for LT-induced cytotoxic effects. We focused our attention on egr1 given that it is an immediate early gene acting as a master regulator of transcription (Fu et al., 2003) and that it is down-modulated by B. anthracis in endothelial cells. We observed that, consistent with the rapid decrease of egr1 mRNA induced by LT (Fig. 2A), its protein level dropped rapidly to 50% after cell intoxication (Fig. 3E). Interestingly, we measured that re-expression of Egr1 in toxin-treated cells significantly restored xiap expression (data not shown). We thus assessed whether Egr1 re-expression interferes with the activation of caspase-3 by LT. Indeed, we found that re-expression of Egr1 in LT-treated cells reduced the activation of caspase-3 (Fig. 3F).

Collectively, our data show that the modulation of TRAIL and XIAP expression downstream of Egr-1 participates to the activation of caspase-3 by LT.

LT induces an unconventional formation of actin cables

Activation of the Rho kinase ROCK by membrane-associated RhoA controls the formation of actin cables. We thus investigated whether LT might have an impact on this pathway. Surprisingly, control and LT-intoxicated monolayers showed an equivalent activity and membrane association of RhoA (Fig. 4A and B). Similar results were obtained for Rac and Cdc42 (Fig. S2C–E). We further investigated the activity of ROCK by measuring the phosphorylation of downstream targets. Control and LT-intoxicated monolayers showed an equivalent phosphorylation of both myosin light chain and cofilin (Fig. 4C and D). Complementary to these findings, we observed that direct inhibition of RhoA by ADP-ribosylation with the exotoxin EDIN of Staphylococcus aureus disrupted the actin cables produced by LT (Fig. 4E) (Wilde et al., 2003). Actin cables were also disrupted by inhibition of the RhoA effector kinase ROCK using Y27632 (Fig. 4E) (Ishizaki et al., 2000). EDIN and Y27632 did not block MEK2 cleavage by LT (Fig. S2F). Collectively, these data show that the RhoA/ROCK pathway controls the stability of these actin cables but is likely not directly modulated by LT. Our results establish that LT triggers an unconventional formation of actin cables in human endothelial cells.

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Figure 4. Unconventional formation of actin cables by LT. HUVEC monolayers were left untreated or treated with PA (3 µg ml−1), LT (PA+LF, 3 + 1 µg ml−1), as indicated. A. Measure of active RhoA in cells treated for different times by PA, LT or the RhoA activating toxin CNF1 at 10−9 M. Immunoblotting anti-RhoA shows levels of active RhoA (RhoAGTP) and total RhoA (RhoA). Values of RhoAGTP normalized to total RhoA (RhoAGTP/RhoA) are indicated below. B. Immunoblots showing levels of RhoA in membrane and cytosolic fractions. Cells were intoxicated 24 h with LT. Membrane fractionation was controlled by immunoblotting anti-RhoGDI (cytosol) and anti-transferrin receptor (membrane). Quantities of RhoA in cytosol or membrane were normalized to total fraction (RhoA/Total) as indicated. C and D. Immunoblots showing the absence of variation of phosphorylation of myosin light chain (pp-MLC) (C) and cofilin (p-S3-cofilin) (D) in LT treated cell monolayers. Quantities of phosphorylated forms were normalized to total forms as indicated. E and F. Actin cytoskeleton re-organization at 24 h of monolayer treatment. F-actin was labelled with FITC-phalloidin (green) and VE-cadherin with anti-cadherin5 (red). Scale bar, 10 µm. (E) LT and EDIN (100 µg ml−1) or Y27632 (20 µM). (F) LT and actD (1 µg ml−1). Insets show close-up of actin cables.

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Strikingly, we also made the observation that the global inhibition of transcription, using actD or alpha-amanitin, triggered formation of thin actin filaments in endothelial cells (Fig. 4F). Moreover, inhibition of transcription prevented the bundling of actin filaments into thick actin cables in the presence of LT (Fig. 4F and not shown). ActD had no effect on the activity of ERK1/2 and did not block MEK2 cleavage by LT (data not shown). These results led us to investigate whether LT-mediated re-organization of the actin cytoskeleton might be mediated at the transcriptional level.

Modulation of gene expression by LT participates to actin cable formation

Following our observation that LT induces an unconventional re-organization of the actin cytoskeleton, we searched for LT-modulated genes encoding actin regulators using gene ontology of molecular function (Table 1). We found 38 repressed genes and three induced genes. We next established the kinetics of gene expression of the two most upregulated and downregulated actin regulators and verified their MAP kinase-dependent modulation of expression (Fig. 5A). LT treatment significantly decreases the expression of cttn (cortactin) and rhpn2 (rhophilin-2) and increases cnn1 (calponin-1) and dmn (desmuslin) mRNA at 24 h of intoxication. Addition of actD blocked LT-induced expression of cnn1 (calponin-1) and dmn (desmuslin) mRNA (data not shown). Cortactin could be detected by immunoblotting, showing its downregulation in LT intoxicated cells (Fig. 5B).

Table 1.  Actin-regulators modulated by LT identified by DNA array analysis.
Gene symbolRefSeqLT 4 h/0 hLT 8 h/4 hDescription
  1. Table shows the values (Log2) obtained in DNA array results between 0 and 4 h (4 h/0 h) and between 4 and 8 h (8 h/4 h) in HUVECs intoxicated by LT (PA+LF, 3 + 1 µg ml−1). Gene variations verified by qRT-PCR are indicated in bold.

Induced    
 CNN1NM_0012991.91.9Calponin-1
 DMNNM_0152860.951.65Desmuslin/alpha-synemin
 SPTBN1NM_00312801.7Spectrin, beta, non-erythrocytic 1
Repressed    
 RHPN2NM_033103−1.4−5Rhophilin, Rho GTPase binding protein 2
 CTTNNM_005231−0.2−2.85Cortactin
 PALLDNM_016081−2.35−2.6Palladin, Cytoskeletal associated protein
 CMYA3NM_152381−2.8−2.2Cardiomyopathy associated 3
 FDG6NM_018351−0.35−2.1FYVE, RhoGEF and PH domain containing 6
 COBLL1NM_014900−2.65−1.65COBL-like 1
 CDC42EP3NM_006449−2.3−1.6CDC42 effector protein (Rho GTPase binding) 3
 DCAMKL1NM_004734−0.1−1.35Doublecortin and CaM kinase-like 1
 MYLKNM_005965−0.45−1.25Myosin light polypeptide kinase
 ARHGAP30NM_001025598−1.4−1.22Rho GTPase activating protein 30
 ARPC5LNM_030978−1.4−0.9Actin-related protein 2/3 complex. subunit 5-like
 MRCL3NM_006471−1.6−0.75Myosin regulatory light chain MRCL3
 LIMK2NM_001031801−1.35−0.65LIM domain kinase 2
 ARHGAP29NM_004815−1.45−0.65Rho GTPase activating protein 29
 FILIP1NM_015687−1.35−0.6Filamin A interacting protein 1
 FMNL2NM_052905−1.3−0.6Formin-like 2
 ARHGAP29NM_004815−1.7−0.55Rho GTPase activating protein 29
 FRMD4ANM_018027−1.3−0.55FERM domain containing 4A
 CDC42EP2NM_006779−1.45−0.5CDC42 effector protein (Rho GTPase binding) 2
 MYO5ANM_000259−1.4−0.45Myosin VA (heavy polypeptide 12. myoxin)
 ARHGAP22NM_021226−1.2−0.4Rho GTPase activating protein 22
 TPM3NM_152263−1.1−0.4Tropomyosin 3
 ARHGAP18NM_033515−1−0.4Rho GTPase activating protein 18
 ARHGAP5NM_001030055−1.25−0.55Rho GTPase activating protein 5
 ARHGAP18NM_033515−1.45−0.3Rho GTPase activating protein 18
 SPET11NM_018243−1.15−0.3Septin 11
 ARHGEF2NM_004723−1.2−0.25Rho/rac guanine nucleotide exchange factor (GEF) 2
 LMCD1NM_014583−1.55−0.15LIM and cysteine-rich domains 1
 FNBP4NM_015308−1.4−0.15Formin binding protein 4
 STRNNM_003162−1.15−0.15Striatin, Calmodulin binding protein
 PHACTR2NM_014721−1.05−0.1Phosphatase and actin regulator 2
 MYO IXBNM_004145−1−0.1Myosin IXB
 FNBP1LNM_001024948−1.6−0.05Formin binding protein 1-like
 ROCK2NM_004850−1.650Rho-associated coiled-coil containing protein kinase 2
 SSH2NM_033389−1.250Slingshot homologue 2 (Drosophila)
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Figure 5. Role of LT target genes in LT-induced actin cable formation. A–D. HUVECs were left untreated or treated with LT (PA+LF, 3 + 1 µg ml−1). A. Graph of transcript modulations assessed by qRT-PCR in monolayers intoxicated by LT or a mix of MKK/MAP kinase inhibitors (MKI). Mean values expressed as fold modulation in n > 6 independent biological replicates per condition ± SEM (*P < 0.05; versus control cells). B. Immunoblot showing the decrease of cortactin (CTTN) in cells treated with LT or with a mix of MKK/MAP kinase inhibitors (MKI). Values of CTTN normalized to actin (CTTN/Actin) are indicated below. C. Graph of the percentage of transfected cells with thick actin cables. HUVECs were transfected 48 h with expression plasmids of cortactin (CTTN), rhophilin-2 (RHPN2), GFP-calponin-1 (GFP-CNN1) or shRNA expression plasmids. Cells were next intoxicated with LT 24 h or untreated. A total of 300 cells were counted in three independent experiments (*P < 0.05 versus indicated points). D. Immunofluorescence images of HUVECs were transfected with GFP-rhophilin-2 (GFP-RHPN2) or cotransfected with expression plasmids of GFP together with cortactin (CTTN), or calponin-1 shRNA (shCNN1). Cells were next treated 48 h with LT. Cortactin was stained with anti-cortactin antibodies (green). F-actin was labelled with TRITC-phalloidin (red). Scale bar, 10 µm. E. Immunofluorescence images of HUVECs co-transfected 48 h with shRNA targeting cortactin (shCTTN) and rhophilin-2 (shRHPN2), as well as GFP-calponin-1 expression plasmid. F-actin was labelled with TRITC-phalloidin (red). Scale bar, 10 µm.

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We next analysed the functional implication of actin regulators identified. Cortactin integrates signals at the membrane to drive cortical F-actin polymerization (Selbach and Backert, 2005). Rhophilin-2 triggers a loss of F-actin from stress fibres by a mechanism, which remains to be established, but that is independent of Rho inactivation (Peck et al., 2002). Calponin-1 is a calcium binding protein, which inhibits myosin ATPase activity to stabilize actin cables (Danninger and Gimona, 2000). Induction of calponin-1 might thus account for the bundling of F-actin into thick actin cables. We performed a functional analysis to determine whether these three proteins contribute to the formation of actin cables induced by LT. This was assessed by measuring the percentage of transfected cells containing thick actin cables (Fig. 5C). Consistent with our hypothesis, expression of cortactin in LT-intoxicated cells produced a 20% decrease in the number of cells containing thick actin cables and restored the network of cortical actin in LT-intoxicated cells (Fig. 5C and D). Expression of rhophilin-2 reduced the number of LT-treated cells containing thick actin cables by 50% (Fig. 5C and D). Finally, we observed that calponin-1 knock-down by shRNA reduced the formation of thick actin cables in LT-intoxicated cells (Fig. 5C and D). We next assessed whether the concomitant modulation of cellular levels of these three proteins might phenocopy LT effects. At first, we observed that individual expression of rhophilin-2 shRNA, cortactin shRNA or calponin-1 had only a minor effect on actin cable formation (Fig. 5C). In contrast, the combined expression of rhophilin-2 shRNA and cortactin shRNA, together with GFP-calponin-1, produced thick actin cables in 50% of GFP positive cells (Fig. 5C and E).

Our functional analysis thus defines three key genes, whose expression is modulated by LT to contribute to the formation of thick actin cables by the toxin.

LT-induced actin cable formation is reduced upon egr1 re-expression

We rationalized that egr1 repression could also contribute to LT-induced actin cable formation. We measured that re-expression of Egr1 in toxin-treated cells significantly interferes with the modulation of expression of cortactin and rhophilin-2 (Fig. 6A). Re-expression of Egr1 in LT-treated cells also had a major inhibitory effect on the formation of actin cables and restored the formation of cortical actin (Fig. 6B). Indeed, under these conditions we measured 83.5% cells with actin cables in mock-transfected cells and only 31.2% in Egr1-transfected cells (*P < 0.05 versus control cells) (Fig. 6C).

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Figure 6. Modulation of Egr1 in LT-induced actin re-organization. A. HUVECs transfected 48 h with empty vector or with Egr1 expression plasmid were left untreated or intoxicated 24 h with LT (PA+LF, 3 + 1 µg ml−1). Transcript levels were assessed by qRT-PCR (mean values of n ≥ 3 experiments ± SEM, *P < 0.05 versus indicated points). B. Immunofluorescence pictures of HUVECs transfected with GFP and Egr1 expression plasmids followed by 24 h of intoxication with LT (PA+LF, 3 + 1 µg ml−1). F-actin was labelled with TRITC-phalloidin (red). Scale bar, 10 µm. C. Graph showing percentage of transfected cells with thick actin cables. Cells were transfected with empty or Egr1 expression vectors. Cells were next intoxicated 24 h with LT (PA+LF, 3 + 1 µg ml−1) or untreated. A total of 300 cells were counted in three independent experiments (*P < 0.05 versus indicated points).

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Taken collectively, our data identify a group of host factors transcriptionally regulated by Egr1, which play key roles in LT-induced actin cable formation.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

It was of importance to determine the molecular basis of actin cytoskeleton remodelling and induction of caspase-3 activity by LT in human endothelial cells. Here, we identify key factors, whose modulation at the mRNA level is implicated in LT-induced cytotoxicity of human endothelial cells. We show that the inhibitory effect of LT on MAPK signalling is sufficient to account for these cytotoxic effects and that both actin re-organization and caspase-3 activation require host transcript modulations. Indeed, down-modulation of rhophilin-2 and cortactin, together with an upregulation of calponin-1, is sufficient to produce actin cables. We show that modulation of TRAIL and XIAP expression participates in caspase-3 activation. At last, we show that re-expression of the immediate early gene and master regulator of transcription Egr1 in LT-intoxicated cells blocks both actin re-organization and caspase-3 activation. Collectively, this establishes that modulation of host gene expression is crucial for the cytotoxic effects of LT in human endothelial cells.

We report that LT has a major impact on the cell transcriptome. Our findings are consistent with the previously reported observation that factors borne by the plasmid pXO1 are responsible for the modulation of 90% of host genes altered by pathogenic B. anthracis during endothelial cell infection (van Sorge et al., 2008). Interestingly, wild-type B. anthracis reduces the steady-state expression of most affected gene transcripts (van Sorge et al., 2008). Here we extend these findings by showing that LT alone reduces the steady-state expression of 84% of affected gene transcripts. Furthermore, we show that LT down-modulates the expression of the immediate-early gene egr1, a master regulator of transcription, as does wild-type B. anthracis in endothelial cells (van Sorge et al., 2008). Importantly, expression of egr1 is deregulated and participates in a large number of cardiovascular diseases (Fu et al., 2003; Khachigian, 2006). Down-modulation of egr-1 alone by RNAi knock down is not sufficient to phenocopy LT-effects on gene expression and endothelial cell cytotoxicity (not shown). This is likely due to redundancy between Egr1 family members and/or due to the involvement of other transcriptional regulators controlled by MAP kinases (Fu et al., 2003; van Sorge et al., 2008). Nevertheless, we demonstrate that re-expression of Egr-1 in LT-intoxicated cells has a protective effect on human endothelial cells as it reduces the toxin-induced cytotoxic effects on actin re-organization and caspase-3 activation. We also show that Egr1 controls the expression of several genes involved in caspase-3 activation and actin cytoskeleton re-organization.

We have identified host factors whose expression is corrupted by LT and plays key roles in LT-induced caspase-3 activation in endothelial cells. Indeed, we define that modulation of the expression of trail and the downstream factor xiap regulate caspase-3 activation. In support of our observation that LT controls apoptosis at the transcriptional level, a recent study performed on a B. anthracis infected monocytic cell line reports that pXO1 alters transcription of a number of genes encoding regulators of the extrinsic pathway of apoptosis (Bradburne et al., 2008). This delays early apoptosis of infected human macrophages. One interesting hypothesis is that LT may have cell type-specific effects as a consequence of distinct profiles of genes being regulated by MAP kinases in these cell types (Posern and Treisman, 2006). XIAP is a potent regulator of TRAIL-induced apoptosis (Rigaud et al., 2006). We found that Egr1 expression in LT-treated cells produced an upregulation of XIAP (not shown) and blocks the activation of caspase-3. Other host factors regulated by LT, such as XIAP-associated Factor-1 (XAF1), might also contribute to caspase-3 activation by the toxin. The binding of XAF1 to XIAP inhibits its anti-caspase activity (Liston et al., 2001). In conclusion, the differential regulation of signalling molecules of the TRAIL pathway, such as XIAP, and potentially XAF1, illustrates the essential role of transcriptional remodelling in LT-induced cytotoxicity on endothelial cells.

We have also identified several actin cytoskeleton regulators whose gene expression is modulated by LT in human endothelial cells (Table 1). Importantly, we unravel that LT produces an unconventional formation of actin cables requiring host gene transcription in absence of direct activation of the RhoA/ROCK pathway. Consistent with our findings, it was recently reported that LT induced formation of actin cables in absence of RhoA activation and cofilin phosphorylation in human bronchial epithelial cells (Lehmann et al., 2009). Here we provide evidence that gene expression is required for the induction of actin cables by LT. We have analysed the function of actin regulatory factors acting in parallel to RhoA/ROCK. This does not exclude that LT might also directly corrupt signalling pathways controlling actin regulators (During et al., 2005; 2007). Nevertheless, we provide functional evidence that the downregulation of cortactin and rhophilin-2 in combination with upregulation of calponin-1 is sufficient to produce actin cables. Our findings suggest that transcriptional mechanisms are essential in the remodelling of the endothelial actin cytoskeleton, and suggest that Egr1 is an important actor in this process.

In conclusion, we have identified key host factors whose gene expression is controlled by MAP kinase cascades and that are exploited by B. anthracis LT to trigger human endothelial cell cytotoxicity.

Experimental procedures

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

Cell culture and reagents

The HUVECs (PromoCell, Heidelberg, Germany) were cultured and transfected, as previously described (Doye et al., 2006). Cells were used between passages two and five. Kindly provided protein expression vectors are pCI-HA-XIAP from Dr S. Plenchette and Dr R.G. Korneluk (Children's Hospital of Eastern Ontario Research Institute, Ottawa, Canada); pcDNA3-cortactin from Dr G. Tran Van Nhieu (Institut Pasteur, Paris, France); pEGFP-calponin1 from Dr M. Gimona (Consorzio Mario Negri Sud, Santa Maria Imbaro, Italy); pRSV2.1-Egr1 from Dr S. Tartare-Deckert (University of Nice, France). Rhophilin-2 cDNA (Dr I. Pirson, Institute of Interdisciplinary Research, Brussels, Belgium) was cloned in pcDNA-DESTGFP vector (Invitrogen). To generate shRNA expression plasmids (shCTTN for cortactin, shRHPN2 for rhophilin-2 and shCNN1 for calponin-1), sequences corresponding to cortactin (5′-GACTGGTTTTGGAGGCAAA), to rhophilin-2 (5′-CGCCCCCGTTCAGGTTCAC) to calponin-1 (5′-ACTACTACAATTCCGCCTA) and to Egr1 (5′-GAGGCATACCAAAATCCAT) were cloned in the pSUPER RNAi system (Invitrogen), according to the manufacturer's instructions. We measured by qRT-PCR that 48 h of expression of shCTTN, shRHPN2, reduced mRNA level of cortactin and rhophilin-2 by 60% and 65% respectively (data not shown). We measured by immunoblotting that shEgr1 reduced by 40% Egr1 protein level at 48 h post transfection. The activity of shCNN1 was verified by co-transfection with pEGFP-calponin1 (data not shown).

Monoclonal antibodies used are mouse anti-Myc ([clone 9E10], Roche), anti-beta-actin ([clone AC-74], Sigma), anti-cortactin ([clone 4F11], Upstate biotechnology), anti-HA ([clone 16-B12], Covance), anti-RhoGDI ([clone A-20], Santa Cruz) and anti-transferrin receptor-1 ([clone H68.4], Zymed). From BD Biosciences: anti-RhoA [clone 26C4], anti-Rac1 [clone 102], anti-CDC42 [clone 44], anti-XIAP [clone 28] and anti-cadherin5 [clone 75]. Polyclonal antibodies used are rabbit: anti-acidic-calponin (Covance), anti-phospho-S3-cofilin courtesy of Dr Bambourg JR (Colorado State University, USA), anti-cofilin (Cytoskeleton). Rabbit anti-MEK2-N20, anti-MLC, anti-ROCK-I, anti-ROCK-II and anti-Egr1 antibodies were purchased from Santa Cruz Technologies. Rabbit immunopurified anti-TRAIL and anti-GST antibodies were purchased from Abcam and Chemicon respectively. Rabbit anti-p-ERK1/2 (pERK), anti-pT18-pS19-MLC and anti-ERK1/2 were purchased from Cell Signalling. For immunofluorescence we used Texas red-conjugated anti-mouse antibodies (Vector, Biovalley), Texas red-conjugated anti-rabbit antibodies (Jackson Immunoresearch Laboratories) or FITC-conjugated anti-rabbit antibodies (Biosys). For immunoblotting we used goat anti-mouse or anti-rabbit horseradish peroxidase-conjugated secondary antibodies (DAKO) followed by chemiluminescence using Immobilon Western (Millipore). Chemiluminescence signals were recorded on a FUJIFILM LAS-3000 and data quantified with the software MultiGauge V3.0. We used Human TRAIL/TNFSF10 Immunoassay (Quantikine, R&D Systems).

Y27632 (ROCK inhibitor, used at 20 µM), U0126 (MEK1, 2 inhibitor, used at 10 µM), SB203580 (P38α, β inhibitor, used at 20 µM) and SP600125 (JNK1, 2, 3 inhibitor, used at 10 µM) were purchased from Calbiochem. Z-Val-Ala-Asp(Ome)-FMK (zVAD) and actinomycin D were purchased from MP Biomedicals. Other biochemical reagents were purchased from Sigma Aldrich.

Recombinant toxin production

Protective antigen DNA was PCR-amplified with primers 5′-GGATCCCGAATTAAACAGGAGAACCGG and 5′-GAGCTCTGTCCTATCTCATAGCCTTTTTTAGA and cloned BamHI-SacI into pET22 (Novagen). LF gene was PCR-amplified with primers 5′-GGATCCATGGCGGGCGGTCATGGTGAT and 5′-GAGCTCTTATGAGTTAATAATGAACTTAAT and cloned BamHI-SacI into the pQE30 (Qiagen). Amplification of PA and LF was performed on genomic DNA of B. anthracis strain Sterne (a kind gift of Patrice Boquet, Nice, France). Catalytically inactive mutant LFE687A was obtained using the QuickChange Site Directed Mutagenesis kit (Stratagene). Plasmids were transformed into Escherichia coli BL21-DE3. Clarified supernatants containing LF were purified on nickel column (Amersham Bioscience). Fractions of LF, wild-type and mutant were dialysed against buffer A [25 mM Tris-HCl (pH 7.4), 50 mM NaCl] and purified on MonoQ (Amersham) using a gradient of NaCl 50–300 mM in buffer A. PA was prepared from the periplasm using standard techniques and purified on Q sepharose fast flow column (Amersham Biosciences) with a gradient of 50–300 mM NaCl in buffer B (Hepes 20 mM, pH 7). Fractions of PA were sieved on a BioSep-Sec-S column (Phenomenex) and purified on ResourceQ (Amersham Biosciences) with a gradient of 50–300 mM NaCl in buffer B. CNF1 and EDIN were purified as described (Doye et al., 2002; Boyer et al., 2006). All proteins were applied on EndoTrap Red column and absence of endotoxin was assessed using the Limulus Amebocyte Lysate QCl-1000 (Cambrex).

Biochemical measurements

Active RhoA, Rac and Cdc42 present in lysates were determined by affinity chromatography, as described in Doye et al. (2006). Membrane/cytosol fractionation was performed as described (Doye et al., 2002).

Transcriptome analysis and qRT-PCR

For GeneChip hybridization and statistical analysis RNA were prepared from HUVEC monolayers either control (two samples) or intoxicated by LT (PA+LF, 3 + 1 µg ml−1) for 4 and 8 h. Transcripts expression was analysed by Aros Applied Biotechnology ApS (http://www.arosab.com), using the Affymetrix Human GeneChip U133A Plus 2.0, as recommended by the manufacturer. The cRNA synthesis, hybridization and labelling, as well as statistical analysis were carried out as previously described (Munro et al., 2004). Gene expression in control condition showing variations −1 ≤ Log2 ≥ 1 were removed prior to comparison with expression at 4 and 8 h of intoxication. Microarray data have been deposited in the National Center for Biotechnology Information Gene Expression Omnibus (http://www.ncbi.nlm.nih.gov/projects/geo) and are accessible through GEO Series Accession No. GSE12131.

RNA were isolated using the RNeasy Mini kit (QIAGEN). Reverse transcriptase reactions were performed on 1 µg RNA samples using the SuperScript II reverse transcriptase (Invitrogen) and oligo(dT)15 primers (Invitrogen). Primers were defined using the primer express software and their specificity verified (Fig. S3 and data not shown). Quantitative PCR were performed in 25 µl containing 5 µl of cDNA, primers (0.4 µM each) and 12.5 µl of SYBR Green Master-mix (Eurogentec). Reactions were run on an ABI-Prism 7000 instrument (Applied Biosystems). Relative quantification of gene expression was performed using the comparative Ct method. Results were normalized using the human 36B4-housekeeping gene.

Immunofluorescence and time-lapse microscopy

Immunofluorecence studies were performed on cells fixed in 4% paraformaldehyde (Sigma). Actin cytoskeleton was labelled using 1 µg ml−1 FITC or TRITC-conjugated phalloidin (Sigma). Immunosignals were analysed with a LSM510-Meta confocal microscope (Carl Zeiss) at 63× magnification lens. Each picture represents the projection of six serial confocal sections. Cells were analysed by video microscopy on an Axiovert 200 microscope equipped with shutter-controlled illumination (Carl Zeiss) and a cooled, digital CCD camera (Roper Scientific). Images were processed using MetaMorph 2.0 image (Molecular Devices) and QuickTime pro 7 (Apple) softwares.

Caspase activity and apoptosis measurements

Caspase activity measurements were performed at various time points at 37°C using 35 µg of protein together with 0.5 mM of Ac-DEVD-AMC (caspase-3 substrate), Ac-IETD-AMC (casp-8) or Ac-LEHD-AMC (casp-9) (Alexis Biomedicals) in the presence or absence of 10 µM of Ac-DEVD-CHO (casp-3), Ac-IETD-CHO (casp-8) or Ac-LEHD-CHO (casp-9) (Alexis Biomedicals). Measures were performed using an Ascent Fluoroskan (Thermo Labsystem). For early apoptotic cell determination HUVEC monolayers were simultaneously stained with annexin-V-Alexa Fluor and propidium iodide (Roche). The cell suspensions were adjusted to 106 cells ml−1 before acquisition and analysed on a fluorescence-activated cell sorting (FACS) flow cytometer (FACSCalibur; BD Biosciences). Data acquisition was performed with the CellQuest software (BD Biosciences).

Statistical analysis

The non-parametric anova analogue test of Kruskal–Wallis was used for qRT-PCR data statistical analysis. anova with Bonferroni post hoc was used for other statistical analyses. Analyses were performed with Prism V5.0b software.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

We thank M. Moayeri and Stephen H. Leppla for providing us with LT toxin purified from B. anthracis, Dr J.R. Bamburg for phospho-cofilin antibodies, Dr D. Paulin and Z. Xue for alpha-synemin antibodies. We are grateful to S. Bourdoulous, P. Cossart, A. Eychène, M. Maddugoda, C. Pouponnot and T. Thibaut for fruitful discussions. We are grateful to A. Doye, F. Prodon, P. Colosetti, F. Massiera, V. Giordanengo, S. Tartare-Deckert and V. Lavallard for technical help. We thank the Conseil Régional PACAC and the Conseil Général des Alpes-Maritimes for their financial support at the microscopy facility platform MiCorBio. U.R.R., M.R. and A.G. were supported by the international DFG research training group 1141 Würzburg/Nice (GCWN) and the Franco-German University (ED-31-04). Our laboratory is supported by an institutional funding from INSERM, a grant (ARC 3800) from the Association pour la Recherche sur le Cancer, a grant and fellowship to M.R. from the Agence Nationale de la Recherche (ANR RPV07055ASA).

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Results
  5. Discussion
  6. Experimental procedures
  7. Acknowledgements
  8. References
  9. Supporting Information

Fig. S1. Control of the cytotoxicity induced by LT. A. Measure of LT-induced activation of caspase-3, 8 and 9 in HUVEC monolayers. zVAD 20 µM was added to evaluate the specificity of caspase activation by LT. Activities are expressed as arbitrary units (A.U.) per mg of proteins (mean values of n = 3 experiments ± SEM). B. HUVECs were either left untreated or intoxicated with LT (PA+LF, 3 + 1 µg ml−1) for 24 h. Graph shows the distribution of annexin V-FITC-positive cells (x-axis) and propidium iodide positive cells (y-axis). Values correspond to the percentages of distribution of each cell population. C and D. Control immunoblots of LT activity on MEK2 cleavage. Western blot anti-actin shows equal protein loading. (C) HUVEC monolayers intoxication for 24 h with PA 3 µg ml−1 alone or together with LF at various concentrations (µg ml−1). (D) HUVEC monolayers intoxication with LT (PA+LF, 3 + 1 µg ml−1) or a catalytically inactive mutant LTm (PA+LFE687A 3 + 1 µg ml−1). E. HUVEC monolayers were intoxicated 24 h with recombinant LT purified from E. coli (PA+LF, 3 + 1 µg ml−1) or with B. anthracis purified native toxin (LT Ba: PA+LF, 3 + 1 µg ml−1). F-actin was labelled with FITC-phalloidin and VE-cadherin with anti-cadherin5. Scale bar, 10 µm. F. Caspase-3 activity in HUVEC monolayers intoxicated 24 h with recombinant LT purified from E. coli (PA+LF, 3 + 1 µg ml−1) or with B. anthracis purified native toxin (LT Ba: PA+LF, 3 + 1 µg ml−1). zVAD 20 µM was added to evaluate the specificity of caspase activation by LT. Activities are expressed as arbitrary units (A.U.) per mg of proteins (one representative experiment). G. Caspase-3 activity in HUVEC monolayers treated 24 h with a mix of MKK/MKI (10 µM U0126, 20 µM SB203580 and SP600125 10 µM) or with LT (PA+LF, 3 + 1 µg ml−1). Activities are expressed as arbitrary units (A.U.) per mg of proteins (mean values of n ≥ 2 experiments ± SEM). Immunoblots show the effect of LT and MKI on ERK phosphorylation (pERK) (inset). H. Gene transcript modulations assessed by qRT-PCR. HUVEC monolayers were intoxicated 24 h with B. anthracis purified native toxin (LT Ba: PA+LF, 3 + 1 µg ml−1). Mean values of n ≥ 3 independent biological replicates per condition ± SEM (*P < 0.05 versus control cells).

Fig. S2. Control of LT activity on gene expression and Rho signalling pathway. A. HUVEC monolayers were treated with either LT (PA+LF, 3 + 1 µg ml−1) or LTm (PA+LFE687A, 3 + 1 µg ml−1). At the indicated times the activity of LT on MEK2 cleavage and ERK phosphorylation was assessed by anti-MEK2N20 or anti-p-ERK1/2 immunoblotting. Immunoblot anti-actin and anti-ERK1/2 shows equal protein loading. B. Cluster of genes induced by LT (Log2 ≥ 1, Green) at both 4 and 8 h. Genes found twice or more are indicated in bold. C and D. Immunoblots show the absence of impact of LT on the activation of Rac and Cdc42. HUVEC monolayers were intoxicated with LT (PA+LF, 3 + 1 µg ml−1) for the indicated times. CNF1 toxin 10−9 M was used as positive control of Rho protein activation. Levels of active-Rac and active-Cdc42 were determined by GST-p21pak RBD pull down (RacGTP and Cdc42GTP). Immunoblots anti-Rac and anti-Cdc42 performed on total cell lysate show total protein levels. Immunoblot anti-actin shows equal loading. E. Immunoblots showing the levels of Rac and Cdc42 in membrane and cytosolic fractions. Cells were intoxicated 24 h with LT. Membrane fractionation was controlled by immunoblotting anti-RhoGDI (cytosol) and anti-transferrin receptor (membrane). F. Immunoblots anti-MEK2N20 showing the absence of interference of either EDIN or Y-27632 on LT-induced MEK2 cleavage. HUVEC monolayers were intoxicated 24 and 48 h with LT (PA+LF, 3 + 1 µg ml−1) together with EDIN (100 µg ml−1) or Y-27632 (20 µM).

Fig. S3. List of primers used for qRT-PCR experiments. Table shows primers used for qRT-PCR and expression vectors constructs. The asterisk ‘*’ indicates primers previously described by (Indraccolo et al., 2007).

Video S1. Endothelium monolayer intoxicated from 0 to 12 h by LT. HUVEC monolayer recorded from 0 to 12 h after intoxication with LT (PA+LF, 3 + 1 µg ml−1), using a 20× lens. Movie recorded at 1 frame per minute and played at 1 frame per 1/30 s.

Video S2. Endothelium monolayer intoxicated from 12 to 24 h by LT. HUVEC monolayer recorded from 12 to 24 h after intoxication with LT (PA+LF, 3 + 1 µg ml−1), using a 20× lens. Movie recorded at 1 frame per minute and played at 1 frame per 1/30 s.

Video S3. Endothelium monolayer intoxicated from 24 to 36 h by LT. HUVEC monolayer recorded from 24 to 36 h after intoxication with LT (PA+LF, 3 + 1 µg ml−1), using a 20× lens. Movie recorded at 1 frame per minute and played at 1 frame per 1/30 s.

Video S4. Control monolayer. HUVEC monolayer recorded from 0 to 24 h using a 20× lens. Movie recorded at 1 frame per minute and played at 1 frame per 1/30 s.

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