ROCK1/2 signaling contributes to corticosteroid-refractory acute graft-versus-host disease

Patients with corticosteroid-refractory acute graft-versus-host disease (aGVHD) have a low one-year survival rate. Identification and validation of novel targetable kinases in patients who experience corticosteroid-refractory-aGVHD may help improve outcomes. Kinase-specific proteomics of leukocytes from patients with corticosteroid-refractory-GVHD identified rho kinase type 1 (ROCK1) as the most significantly upregulated kinase. ROCK1/2 inhibition improved survival and histological GVHD severity in mice and was synergistic with JAK1/2 inhibition, without compromising graft-versus-leukemia-effects. ROCK1/2-inhibition in macrophages or dendritic cells prior to transfer reduced GVHD severity. Mechanistically, ROCK1/2 inhibition or ROCK1 knockdown interfered with CD80, CD86, MHC-II expression and IL-6, IL-1β, iNOS and TNF production in myeloid cells. This was accompanied by impaired T cell activation by dendritic cells and inhibition of cytoskeletal rearrangements, thereby reducing macrophage and DC migration. NF-κB signaling was reduced in myeloid cells following ROCK1/2 inhibition. In conclusion, ROCK1/2 inhibition interferes with immune activation at multiple levels and reduces acute GVHD while maintaining GVL-effects, including in corticosteroid-refractory settings.

Formin-mediated nuclear actin at androgen receptors promotes transcription

Steroid hormone receptors are ligand-binding transcription factors essential for mammalian physiology. The androgen receptor (AR) binds androgens mediating gene expression for sexual, somatic, and behavioral functions, and is involved in various conditions including androgen-insensitivity-syndrome (AIS) or prostate cancer ¹. Here we identified functional AIS-patient mutations in the formin and actin nucleator DAAM2. DAAM2 was enriched in the nucleus, where its localization correlated with that of the AR to form actin-dependent transcriptional droplets in response to dihydrotestosterone. DAAM2-AR-droplets ranged from 0.02 to 0.06 µm³ in size and associated with active RNA polymerase II. DAAM2 polymerized actin directly at the AR to promote droplet coalescence in a highly dynamic manner and nuclear actin polymerization is required for prostate-specific antigen expression in cancer cells. Our data uncover signal-regulated nuclear actin assembly at a steroid hormone receptor necessary for transcription.

Spatiotemporal Regulation of FMNL2 by N-Terminal Myristoylation and C-Terminal Phosphorylation Drives Rapid Filopodia Formation

The actin nucleating and polymerizing formin-like 2 (FMNL2) is upregulated in several cancers and has been shown to play important roles in cell migration, invasion, cell–cell adhesion and filopodia formation. Here, using structured illumination microscopy we show that FMNL2 promotes rapid and highly dynamic filopodia formation in epithelial cells while remaining on the tip of the growing filopodia. This filopodia tip localization depends fully on its N-terminal myristoylation. We further show that FMNL2-dependent filopodia formation requires its serine 1072 phosphorylation within the diaphanous-autoregulatory domain (DAD) by protein kinase C (PKC) α. Consistent with this, filopodia formation depends on PKC activity and PKCα localizes to the base of growing filopodia. Thus, a PKCα–FMNL2 signaling module spatiotemporally controls dynamic filopodia formation.

Vesicle-Associated Actin Assembly by Formins Promotes TGFβ-Induced ANGPTL4 Trafficking, Secretion and Cell Invasion

Vesicle trafficking has emerged as an important process driving tumor progression through various mechanisms. Transforming growth factor beta (TGFβ)-mediated secretion of Angiopoietin-like 4 (ANGPTL4) is important for cancer development. Here, Formin-like 2 (FMNL2) is identified to be necessary for ANGPTL4 trafficking and secretion in response to TGFβ. Protein kinase C (PKC)-dependent phosphorylation of FMNL2 downstream of TGFβ stimulation is required for cancer cell invasion as well as ANGPTL4 vesicle trafficking and secretion. Moreover, using super resolution microscopy, ANGPTL4 trafficking is actin-dependent with FMNL2 directly polymerizing actin at ANGPTL4-containing vesicles, which are associated with Rab8a and myosin Vb. This work uncovers a formin-controlled mechanism that transiently polymerizes actin directly at intracellular vesicles to facilitate their mobility. This mechanism may be important for the regulation of cancer cell metastasis and tumor progression.

Inhibition of Arp2/3 Complex after ADP-Ribosylation of Arp2 by Binary Clostridioides Toxins

Clostridioides bacteria are responsible for life threatening infections. Here, we show that in addition to actin, the binary toxins CDT, C2I, and Iota from Clostridioides difficile, botulinum, and perfrigens, respectively, ADP-ribosylate the actin-related protein Arp2 of Arp2/3 complex and its additional components ArpC1, ArpC2, and ArpC4/5. The Arp2/3 complex is composed of seven subunits and stimulates the formation of branched actin filament networks. This activity is inhibited after ADP-ribosylation of Arp2. Translocation of the ADP-ribosyltransferase component of CDT toxin into human colon carcinoma Caco2 cells led to ADP-ribosylation of cellular Arp2 and actin followed by a collapse of the lamellipodial extensions and F-actin network. Exposure of isolated mouse colon pieces to CDT toxin induced the dissolution of the enterocytes leading to luminal aggregation of cellular debris and the collapse of the mucosal organization. Thus, we identify the Arp2/3 complex as hitherto unknown target of clostridial ADP-ribosyltransferases.

BCAT1 redox function maintains mitotic fidelity

The metabolic enzyme branched-chain amino acid transaminase 1 (BCAT1) drives cell proliferation in aggressive cancers such as glioblastoma. Here, we show that BCAT1 localizes to mitotic structures and has a non-metabolic function as a mitotic regulator. Furthermore, BCAT1 is required for chromosome segregation in cancer and induced pluripotent stem cells and tumor growth in human cerebral organoid and mouse syngraft models. Applying gene knockout and rescue strategies, we show that the BCAT1 CXXC redox motif is crucial for controlling cysteine sulfenylation specifically in mitotic cells, promoting Aurora kinase B localization to centromeres, and securing accurate chromosome segregation. These findings offer an explanation for the well-established role of BCAT1 in promoting cancer cell proliferation. In summary, our data establish BCAT1 as a component of the mitotic apparatus that safeguards mitotic fidelity through a moonlighting redox functionality.

Spatiotemporal sequence of mesoderm and endoderm lineage segregation during mouse gastrulation

Anterior mesoderm (AM) and definitive endoderm (DE) progenitors represent the earliest embryonic cell types that are specified during germ layer formation at the primitive streak (PS) of the mouse embryo. Genetic experiments indicate that both lineages segregate from Eomes-expressing progenitors in response to different Nodal signaling levels. However, the precise spatiotemporal pattern of the emergence of these cell types and molecular details of lineage segregation remain unexplored. We combined genetic fate labeling and imaging approaches with single-cell RNA sequencing (scRNA-seq) to follow the transcriptional identities and define lineage trajectories of Eomes-dependent cell types. Accordingly, all cells moving through the PS during the first day of gastrulation express Eomes AM and DE specification occurs before cells leave the PS from Eomes-positive progenitors in a distinct spatiotemporal pattern. ScRNA-seq analysis further suggested the immediate and complete separation of AM and DE lineages from Eomes-expressing cells as last common bipotential progenitor.

A tight grip on differentiation: Nuclear constriction by microtubules regulates hematopoietic stem cells

Maintenance of the mature blood cells requires controlled cell fate decisions by hematopoietic stem and progenitor cells (HSPCs). While our knowledge of the gene expression changes that facilitate differentiation has made a leap forward, less is known about the cellular triggers that induce them. Biedzinski et al (2020) now uncover a new intracellular mechanism that drives myeloid differentiation: Microtubule bundles squeeze the nucleus of HSPCs and form large invaginations, thus causing changes in chromatin organization. These microtubule-induced nuclear shape changes result in gene expression profiles that favor myeloid differentiation.

Clostridioides difficile binary toxin (CDT) is recognized by the TLR2/6 heterodimer to induce an NF-κB response

Clostridioides difficile infection (CDI) represents a significant burden on the health care system, one that is exacerbated by the emergence of binary toxin (CDT)-producing hypervirulent C. difficile strains. Previous work from our lab has shown that TLR2 recognizes CDT to induce inflammation. Here we explore the interactions of CDT with TLR2 and the impact on host immunity during CDI. We found that the TLR2/6 heterodimer, not TLR2/1, is responsible for CDT recognition, and that gene pathways including NF-κB and MAPK downstream of TLR2/6 are upregulated in mice with intact TLR2/6 signaling during CDI.

Postmitotic expansion of cell nuclei requires nuclear actin filament bundling by α-actinin 4

The actin cytoskeleton operates in a multitude of cellular processes including cell shape and migration, mechanoregulation, and membrane or organelle dynamics. However, its filamentous properties and functions inside the mammalian cell nucleus are less well explored. We previously described transient actin assembly at mitotic exit that promotes nuclear expansion during chromatin decondensation. Here, we identify non‐muscle α‐actinin 4 (ACTN4) as a critical regulator to facilitate F‐actin reorganization and bundling during postmitotic nuclear expansion. ACTN4 binds to nuclear actin filament structures, and ACTN4 clusters associate with nuclear F‐actin in a highly dynamic fashion. ACTN4 but not ACTN1 is required for proper postmitotic nuclear volume expansion, mediated by its actin‐binding domain. Using super‐resolution imaging to quantify actin filament numbers and widths in individual nuclei, we find that ACTN4 is necessary for postmitotic nuclear actin reorganization and actin filament bundling. Our findings uncover a nuclear cytoskeletal function for ACTN4 to control nuclear size and chromatin organization during mitotic cell division.

Human peptide α-defensin-1 interferes with Clostridioides difficile toxins TcdA, TcdB, and CDT

The human pathogenic bacterium Clostridioides difficile produces two exotoxins TcdA and TcdB, which inactivate Rho GTPases thereby causing C. difficile-associated diseases (CDAD) including life-threatening pseudomembranous colitis. Hypervirulent strains produce additionally the binary actin ADP-ribosylating toxin CDT. These strains are hallmarked by more severe forms of CDAD and increased frequency and severity. Once in the cytosol, the toxins act as enzymes resulting in the typical clinical symptoms. Therefore, targeting and inactivation of the released toxins are of peculiar interest. Prompted by earlier findings that human α-defensin-1 neutralizes TcdB, we investigated the effects of the defensin on all three C. difficile toxins. Inhibition of TcdA, TcdB, and CDT was demonstrated by analyzing toxin-induced changes in cell morphology, substrate modification, and decrease in transepithelial electrical resistance. Application of α-defensin-1 protected cells and human intestinal organoids from the cytotoxic effects of TcdA, TcdB, CDT, and their combination which is attributed to a direct interaction between the toxins and α-defensin-1. In mice, the application of α-defensin-1 reduced the TcdA-induced damage of intestinal loops in vivo. In conclusion, human α-defensin-1 is a specific and potent inhibitor of the C. difficile toxins and a promising agent to develop novel therapeutic options against C. difficile infections.

Inverse control of Rab proteins by Yersinia ADP-ribosyltransferase and glycosyltransferase related to clostridial glucosylating toxins

We identified a glucosyltransferase (YGT) and an ADP-ribosyltransferase (YART) in Yersinia mollaretii, highly related to glucosylating toxins from Clostridium difficile, the cause of antibiotics-associated enterocolitis. Both Yersinia toxins consist of an amino-terminal enzyme domain, an autoprotease domain activated by inositol hexakisphosphate, and a carboxyl-terminal translocation domain. YGT N-acetylglucosaminylates Rab5 and Rab31 at Thr⁵² and Thr³⁶, respectively, thereby inactivating the Rab proteins. YART ADP-ribosylates Rab5 and Rab31 at Gln⁷⁹ and Gln⁶⁴, respectively. This activates Rab proteins by inhibiting GTP hydrolysis. We determined the crystal structure of the glycosyltransferase domain of YGT (YGT^G) in the presence and absence of UDP at 1.9- and 3.4-Å resolution, respectively. Thereby, we identified a previously unknown potassium ion-binding site, which explains potassium ion-dependent enhanced glycosyltransferase activity in clostridial and related toxins. Our findings exhibit a novel type of inverse regulation of Rab proteins by toxins and provide new insights into the structure-function relationship of glycosyltransferase toxins.

Synchronizing Protein Traffic to the Primary Cilium

The primary cilium is able to maintain a specific protein composition, which is critical for its function as a signaling organelle. Here we introduce a system to synchronize biosynthetic trafficking of ciliary proteins that is based on conditional aggregation domains (CADs). This approach enables to create a wave of ciliary proteins that are transported together, which opens novel avenues for visualizing and studying ciliary import mechanisms. By using somatostatin receptor 3 (SSTR3) as model protein we studied intracellular transport and ciliary import with high temporal and spatial resolution in epithelial Madin-Darby canine kidney (MDCK) cells. This yielded the interesting discovery that SSTR3, besides being transported to the primary cilium, is also targeted to the basolateral plasma membrane. In addition, we found a similar behavior for another ciliary protein, nephrocystin-3 (NPHP3), thus suggesting a potential correlation between ciliary and basolateral trafficking. Furthermore, our CAD-based system allowed assembling a large dataset in which apical and basolateral surface SSTR3 signals could be compared to ciliary SSTR3 signals on a single cell level. This enabled to generate novel complementary evidence for the previously proposed lateral import mechanism of SSTR3 into the cilium along the plasma membrane.

Identification of a novel anoikis signalling pathway using the fungal virulence factor gliotoxin

Anoikis is a form of apoptosis induced by cell detachment. Integrin inactivation plays a major role in the process but the exact signalling pathway is ill-defined. Here we identify an anoikis pathway using gliotoxin (GT), a virulence factor of the fungus Aspergillus fumigatus, which causes invasive aspergillosis in humans. GT prevents integrin binding to RGD-containing extracellular matrix components by covalently modifying cysteines in the binding pocket. As a consequence, focal adhesion kinase (FAK) is inhibited resulting in dephosphorylation of p190RhoGAP, allowing activation of RhoA. Sequential activation of ROCK, MKK4/MKK7 and JNK then triggers pro-apoptotic phosphorylation of Bim. Cells in suspension or lacking integrin surface expression are insensitive to GT but are sensitised to ROCK-MKK4/MKK7-JNK-dependent anoikis upon attachment to fibronectin or integrin upregulation. The same signalling pathway is triggered by FAK inhibition or inhibiting integrin αV/β3 with Cilengitide. Thus, GT can target integrins to induce anoikis on lung epithelial cells.

Salmonella Typhimurium effector SseI inhibits chemotaxis and increases host cell survival by deamidation of heterotrimeric Gi proteins

Salmonella enterica serotype Typhimurium (S. Typhimurium) is one of the most frequent causes of food-borne illness in humans and usually associated with acute self-limiting gastroenteritis. However, in immunocompromised patients, the pathogen can disseminate and lead to severe systemic diseases. S. Typhimurium are facultative intracellular bacteria. For uptake and intracellular life, Salmonella translocate numerous effector proteins into host cells using two type-III secretion systems (T3SS), which are encoded within Salmonella pathogenicity islands 1 (SPI-1) and 2 (SPI-2). While SPI-1 effectors mainly promote initial invasion, SPI-2 effectors control intracellular survival and proliferation. Here, we elucidate the mode of action of Salmonella SPI-2 effector SseI, which is involved in control of systemic dissemination of S. Typhimurium. SseI deamidates a specific glutamine residue of heterotrimeric G proteins of the Gα_i family, resulting in persistent activation of the G protein. G_i activation inhibits cAMP production and stimulates PI3-kinase γ by Gα_i-released Gβγ subunits, resulting in activation of survival pathways by phosphorylation of Akt and mTOR. Moreover, SseI-induced deamidation leads to non-polarized activation of Gα_i and, thereby, to loss of directed migration of dendritic cells.

Salmonella Typhimurium is one of the most common causes of gastroenteritis in humans. In immunocompromised patients, the pathogen can cause systemic infections. Crucial virulence factors are encoded on two Salmonella pathogenicity islands SPI-1 and SPI-2. While SPI-1 encodes virulence factors essential for host cell invasion, intracellular proliferation of the pathogen depends mainly on SPI-2 effectors. Here, we elucidate the mode of action of Salmonella SPI-2 effector SseI. SseI activates heterotrimeric G proteins of the Gα_i family by deamidation of a specific glutamine residue. Deamidation blocks GTP hydrolysis by Gα_i, resulting in a persistently active G protein. G_i activation inhibits cAMP production and stimulates PI3Kγ by Gα_i-released Gβγ subunits, resulting in activation of survival pathways by phosphorylation of Akt and mTOR. Moreover, deamidation of Gα_i leads to a loss of directed migration in dendritic cells. The data offers a new perspective in the understanding of the actions of SseI.

Loss of LSR affects epithelial barrier integrity and tumor xenograft growth of CaCo-2 cells

The lipolysis-stimulated lipoprotein receptor (LSR) is a lipoprotein receptor, serves as host receptor for clostridial iota-like toxins and is involved in the formation of tricellular contacts. Of particular interest is the role of LSR in progression of various cancers. Here we aimed to study the tumor growth of LSR-deficient colon carcinoma-derived cell lines HCT116 and CaCo-2 in a mouse xenograft model. Whereas knockout of LSR had no effect on tumor growth of HCT116 cells, we observed that CaCo-2 LSR knockout tumors grew to a smaller size than their wild-type counterparts. Histological analysis revealed increased apoptotic and necrotic cell death in a tumor originating from LSR-deficient CaCo-2 cells. LSR-deficient CaCo-2 cells exhibited increased cell proliferation in vitro and an altered epithelial morphology with impaired targeting of tricellulin to tricellular contacts. In addition, loss of LSR reduced the transepithelial electrical resistance of CaCo-2 cell monolayers and increased permeability for small molecules. Moreover, LSR-deficient CaCo-2 cells formed larger cysts in 3D culture than their wild-type counterparts. Our study provides evidence that LSR affects epithelial morphology and barrier formation in CaCo-2 cells and examines for the first time the effects of LSR deficiency on the tumor growth properties of colon carcinoma-derived cell lines.

Binary Clostridium difficile toxin (CDT) - A virulence factor disturbing the cytoskeleton

Clostridium difficile infection causes antibiotics-associated diarrhea and pseudomembranous colitis. Major virulence factors of C. difficile are the Rho-glucosylating toxins TcdA and TcdB. In addition, many, so-called hypervirulent C. difficile strains produce the binary actin-ADP-ribosylating toxin CDT. CDT causes depolymerization of F-actin and rearrangement of the actin cytoskeleton. Thereby, many cellular functions, which depend on actin, are altered. CDT disturbs the dynamic balance between actin and microtubules in target cells. The toxin increases microtubule polymerization and induces the formation of microtubule-based protrusions at the plasma membrane of target cells. Moreover, CDT causes a redistribution of vesicles from the basolateral side to the apical side, where extracellular matrix proteins are released. These processes may increase the adherence of clostridia to target cells. Here, we review the effects of the action of CDT on the actin cytoskeleton and on the microtubule system.

ADP-Ribosylation and Cross-Linking of Actin by Bacterial Protein Toxins

Actin and the actin cytoskeleton play fundamental roles in host-pathogen interactions. Proper function of the actin cytoskeleton is crucial for innate and acquired immune defense. Bacterial toxins attack the actin cytoskeleton by targeting regulators of actin. Moreover, actin is directly modified by various bacterial protein toxins and effectors, which cause ADP-ribosylation or cross-linking of actin. Modification of actin can result in inhibition or stimulation of actin polymerization. Toxins, acting directly on actin, are reviewed.

Clostridium difficile Toxin Biology

Clostridium difficile is the cause of antibiotics-associated diarrhea and pseudomembranous colitis. The pathogen produces three protein toxins: C. difficile toxins A (TcdA) and B (TcdB), and C. difficile transferase toxin (CDT). The single-chain toxins TcdA and TcdB are the main virulence factors. They bind to cell membrane receptors and are internalized. The N-terminal glucosyltransferase and autoprotease domains of the toxins translocate from low-pH endosomes into the cytosol. After activation by inositol hexakisphosphate (InsP6), the autoprotease cleaves and releases the glucosyltransferase domain into the cytosol, where GTP-binding proteins of the Rho/Ras family are mono-O-glucosylated and, thereby, inactivated. Inactivation of Rho proteins disturbs the organization of the cytoskeleton and affects multiple Rho-dependent cellular processes, including loss of epithelial barrier functions, induction of apoptosis, and inflammation. CDT, the third C. difficile toxin, is a binary actin-ADP-ribosylating toxin that causes depolymerization of actin, thereby inducing formation of the microtubule-based protrusions. Recent progress in understanding of the toxins' actions include insights into the toxin structures, their interaction with host cells, and functional consequences of their actions.

The Hsp90 machinery facilitates the transport of diphtheria toxin into human cells

Diphtheria toxin kills human cells because it delivers its enzyme domain DTA into their cytosol where it inhibits protein synthesis. After receptor-mediated uptake of the toxin, DTA translocates from acidic endosomes into the cytosol, which might be assisted by host cell factors. Here we investigated the role of Hsp90 and its co-chaperones during the uptake of native diphtheria toxin into human cells and identified the components of the Hsp90 machinery including Hsp90, Hsp70, Cyp40 and the FK506 binding proteins FKBP51 and FKBP52 as DTA binding partners. Moreover, pharmacological inhibition of the chaperone activity of Hsp90 and Hsp70 and of the peptidyl-prolyl cis/trans isomerase (PPIase) activity of Cyps and FKBPs protected cells from intoxication with diphtheria toxin and inhibited the pH-dependent trans-membrane transport of DTA into the cytosol. In conclusion, these host cell factors facilitate toxin uptake into human cells, which might lead to development of novel therapeutic strategies against diphtheria.

Actin ADP-ribosylation at Threonine148 by Photorhabdus luminescens toxin TccC3 induces aggregation of intracellular F-actin

Intoxication of eukaryotic cells by Photorhabdus luminescens toxin TccC3 induces cell rounding and detachment from the substratum within a few hours and compromises a number of cell functions like phagocytosis. Here, we used morphological and biochemical procedures to analyse the mechanism of TccC3 intoxication. Life imaging of TccC3-intoxicated HeLa cells transfected with AcGFP-actin shows condensation of F-actin into large aggregates. Life cell total internal reflection fluorescence (TIRF) microscopy of identically treated HeLa cells confirmed the formation of actin aggregates but also disassembly of F-actin stress fibres. Recombinant TccC3 toxin ADP-ribosylates purified skeletal and non-muscle actin at threonine148 leading to a strong propensity to polymerize and F-actin bundle formation as shown by TIRF and electron microscopy. Native gel electrophoresis shows strongly reduced binding of Thr148-ADP-ribosylated actin to the severing proteins gelsolin and its fragments G1 and G1-3, and to ADF/cofilin. Complexation of actin with these proteins inhibits its ADP-ribosylation. TIRF microscopy demonstrates rapid polymerization of Thr148-ADP-ribosylated actin to curled F-actin bundles even in the presence of thymosin β4, gelsolin or G1-3. Thr148-ADP-ribosylated F-actin cannot be depolymerized by gelsolin or G1-3 as verified by TIRF, co-sedimentation and electron microscopy and shows reduced treadmilling as indicated by a lack of stimulation of its ATPase activity after addition of cofilin-1.

Formation of Nanotube-Like Protrusions, Regulation of Septin Organization and Re-guidance of Vesicle Traffic by Depolymerization of the Actin Cytoskeleton Induced by Binary Bacterial Protein Toxins

A large group of bacterial protein toxins, including binary ADP-ribosylating toxins, modify actin at arginine-177, thereby actin polymerization is blocked and the actin cytoskeleton is redistributed. Modulation of actin functions largely affects other components of the cytoskeleton, especially microtubules and septins. Here, recent findings about the functional interconnections of the actin cytoskeleton with microtubules and septins, affected by bacterial toxins, are reviewed.

Cyclophilin-facilitated membrane translocation as pharmacological target to prevent intoxication of mammalian cells by binary clostridial actin ADP-ribosylated toxins

Clostridium botulinum C2 toxin, Clostridium perfringens iota toxin and Clostridium difficile CDT belong to the family of binary actin ADP-ribosylating toxins and are composed of a binding/translocation component and a separate enzyme component. The enzyme components ADP-ribosylate G-actin in the cytosol of target cells resulting in depolymerization of F-actin, cell rounding and cell death. The binding/translocation components bind to their cell receptors and form complexes with the respective enzyme components. After receptor-mediated endocytosis, the binding/translocation components form pores in membranes of acidified endosomes and the enzyme components translocate through these pores into the cytosol. This step is facilitated by the host cell chaperone heat shock protein 90 and peptidyl-prolyl cis/trans isomerases including cyclophilin A. Here, we demonstrate that a large isoform of cyclophilin A, the multi-domain enzyme cyclophilin 40 (Cyp40), binds to the enzyme components C2I, Ia and CDTa in vitro. Isothermal titration calorimetry revealed a direct binding to C2I with a calculated affinity of 101 nM and to Ia with an affinity of 1.01 μM. Closer investigation for the prototypic C2I revealed that binding to Cyp40 did not depend on its ADP-ribosyltransferase activity but was stronger for unfolded C2I. The interaction of C2I with Cyp40 was also demonstrated in lysates from C2-treated cells by pull-down. Treatment of cells with a non-immunosuppressive cyclosporine A derivative, which still binds to and inhibits the peptidyl-prolyl cis/trans isomerase activity of cyclophilins, protected cells from intoxication with C2, iota and CDT toxins, offering an attractive approach for development of novel therapeutic strategies against binary actin ADP-ribosylating toxins.

Tailored cyclodextrin pore blocker protects mammalian cells from clostridium difficile binary toxin CDT

Some Clostridium difficile strains produce, in addition to toxins A and B, the binary toxin Clostridium difficile transferase (CDT), which ADP-ribosylates actin and may contribute to the hypervirulence of these strains. The separate binding and translocation component CDTb mediates transport of the enzyme component CDTa into mammalian target cells. CDTb binds to its receptor on the cell surface, CDTa assembles and CDTb/CDTa complexes are internalised. In acidic endosomes, CDTb mediates the delivery of CDTa into the cytosol, most likely by forming a translocation pore in endosomal membranes. We demonstrate that a seven-fold symmetrical positively charged β-cyclodextrin derivative, per-6-S-(3-aminomethyl)benzylthio-β-cyclodextrin, which was developed earlier as a potent inhibitor of the translocation pores of related binary toxins of Bacillus anthracis, Clostridium botulinum and Clostridium perfringens, protects cells from intoxication with CDT. The pore blocker did not interfere with the CDTa-catalyzed ADP-ribosylation of actin or toxin binding to Vero cells but inhibited the pH-dependent membrane translocation of CDTa into the cytosol. In conclusion, the cationic β-cyclodextrin could serve as the lead compound in a development of novel pharmacological strategies against the CDT-producing strains of C. difficile.

Lu/BCAM adhesion glycoprotein is a receptor for Escherichia coli Cytotoxic Necrotizing Factor 1 (CNF1)

The Cytotoxic Necrotizing Factor 1 (CNF1) is a protein toxin which is a major virulence factor of pathogenic Escherichia coli strains. Here, we identified the Lutheran (Lu) adhesion glycoprotein/basal cell adhesion molecule (BCAM) as cellular receptor for CNF1 by co-precipitation of cell surface molecules with tagged toxin. The CNF1-Lu/BCAM interaction was verified by direct protein-protein interaction analysis and competition studies. These studies revealed amino acids 720 to 1014 of CNF1 as the binding site for Lu/BCAM. We suggest two cell interaction sites in CNF1: first the N-terminus, which binds to p37LRP as postulated before. Binding of CNF1 to p37LRP seems to be crucial for the toxin's action. However, it is not sufficient for the binding of CNF1 to the cell surface. A region directly adjacent to the catalytic domain is a high affinity interaction site for Lu/BCAM. We found Lu/BCAM to be essential for the binding of CNF1 to cells. Cells deficient in Lu/BCAM but expressing p37LRP could not bind labeled CNF1. Therefore, we conclude that LRP and Lu/BCAM are both required for toxin action but with different functions.

The chaperone Hsp90 and PPIases of the cyclophilin and FKBP families facilitate membrane translocation of Photorhabdus luminescens ADP-ribosyltransferases

TccC3 and TccC5 from Photorhabdus luminescens are ADP-ribosyltransferases, which modify actin and Rho GTPases, respectively, thereby inducing polymerization and clustering of actin. The bacterial proteins are components of the Photorhabdus toxin complexes, consisting of the binding and translocation component TcdA1, a proposed linker component TcdB2 and the enzymatic component TccC3/5. While the action of the toxins on target proteins is clearly defined, uptake and translocation of the toxins into the cytosol of target cells are not well understood. Here we show by using pharmacological inhibitors that heat shock protein 90 (Hsp90) and peptidyl prolyl cis/trans isomerases (PPIases) including cyclophilins and FK506-binding proteins (FKBPs) facilitate the uptake of the ADP-ribosylating toxins into the host cell cytosol. Inhibition of Hsp90 and/or PPIases resulted in decreased intoxication of target cells by Photorhabdus toxin complexes determined by cell rounding and reduction of transepithelial electrical resistance of cell monolayers. ADP-ribosyltransferase activity of toxins and toxin-induced pore formation were notimpaired by the inhibitors of Hsp90 and PPIases. The Photorhabdus toxins interacted with Hsp90, FKBP51, Cyp40 and CypA, suggesting a role of these host cell factors in translocation and/or refolding of the ADP-ribosyltransferases.

Molecular characteristics of Clostridium perfringens TpeL toxin and consequences of mono-O-GlcNAcylation of Ras in living cells

TpeL is a member of the family of clostridial glucosylating toxins produced by Clostridium perfringens type A, B, and C strains. In contrast to other members of this toxin family, it lacks a C-terminal polypeptide repeat domain, which is suggested to be involved in target cell binding. It was shown that the glucosyltransferase domain of TpeL modifies Ras in vitro by mono-O-glucosylation or mono-O-GlcNAcylation (Nagahama, M., Ohkubo, A., Oda, M., Kobayashi, K., Amimoto, K., Miyamoto, K., and Sakurai, J. (2011) Infect. Immun. 79, 905-910). Here we show that TpeL preferably utilizes UDP-N-acetylglucosamine (UDP-GlcNAc) as a sugar donor. Change of alanine 383 of TpeL to isoleucine turns the sugar donor preference from UDP-GlcNAc to UDP-glucose. In contrast to previous studies, we show that Rac is a poor substrate in vitro and in vivo and requires 1-2 magnitudes higher toxin concentrations for modification by TpeL. The toxin is autoproteolytically processed in the presence of inositol hexakisphosphate (InsP(6)) by an intrinsic cysteine protease domain, located next to the glucosyltransferase domain. A C-terminally extended TpeL full-length variant (TpeL1-1779) induces apoptosis in HeLa cells (most likely by mono-O-GlcNAcylation of Ras), and inhibits Ras signaling including Ras-Raf interaction and ERK activation. In addition, TpeL blocks Ras signaling in rat pheochromocytoma PC12 cells. TpeL is a glucosylating toxin, which modifies Ras and induces apoptosis in target cells without having a typical C-terminal polypeptide repeat domain.

FK506-binding protein 51 interacts with Clostridium botulinum C2 toxin and FK506 inhibits membrane translocation of the toxin in mammalian cells

The binary Clostridium botulinum C2 toxin consists of the binding/translocation component C2IIa and the separate enzyme component C2I. C2IIa delivers C2I into the cytosol of eukaryotic target cells where C2I ADP-ribosylates actin. After receptor-mediated endocytosis of the C2IIa/C2I complex, C2IIa forms pores in membranes of acidified early endosomes and unfolded C2I translocates through the pores into the cytosol. Membrane translocation of C2I is facilitated by the activities of host cell chaperone Hsp90 and the peptidyl-prolyl cis/trans isomerase (PPIase) cyclophilin A. Here, we demonstrated that Hsp90 co-precipitates with C2I from lysates of C2 toxin-treated cells and identified the FK506-binding protein (FKBP) 51 as a novel interaction partner of C2I in vitro and in intact mammalian cells. Prompted by this finding, we used the specific pharmacological inhibitor FK506 to investigate whether the PPIase activity of FKBPs plays a role during membrane translocation of C2 toxin. Treatment of cells with FK506 protected cultured cells from intoxication with C2 toxin. Moreover, FK506 inhibited the pH-dependent translocation of C2I across membranes into the cytosol but did not interfere with the enzyme activity of C2I or binding of C2 toxin to cells. Furthermore, FK506 treatment delayed intoxication with the related binary actin ADP-ribosylating toxins from Clostridium perfringens (iota toxin) and Clostridium difficile (CDT) but not with the Rho-glucosylating Clostridium difficile toxin A (TcdA). In conclusion, our results support the hypothesis that clostridial binary actin-ADP-ribosylating toxins share a specific FKBP-dependent translocation mechanism during their uptake into mammalian cells.

Domain organization of Legionella effector SetA

Legionella pneumophila is a human pathogen causing severe pneumonia called Legionnaires' disease. Multiple Legionella effectors are type IV-secreted into the host cell to establish a specific vesicular compartment for pathogen replication. Recently, it has been reported that the Legionella effector SetA shares sequence similarity with glycosyltransferases and interferes with vesicular trafficking of host cells. Here we show that SetA possesses glycohydrolase and mono-O-glucosyltransferase activity by using UDP-glucose as a donor substrate. Whereas the catalytic activity is located at the N terminus of SetA, the C terminus (amino acids 401-644) is essential for guidance of SetA to vesicular compartments of host cells. EGFP-SetA expressed in HeLa cells localizes to early endosomes by interacting with phosphatidylinositol 3-phosphate. EGFP-SetA, transiently expressed in RAW 264.7 macrophages, associates with early phagosomes after infection with Escherichia coli and L. pneumophila. Only the combined expression of the C- and N-terminal domains induces growth defects in yeast similar to full-length SetA. The data indicate that SetA is a multidomain protein with an N-terminal glucosyltransferase domain and a C-terminal phosphatidylinositol 3-phosphate-binding domain, which guides the Legionella effector to the surface of the Legionella-containing vacuole. Both, the localization and the glucosyltransferase domains of SetA are crucial for cellular functions.

Cholesterol- and sphingolipid-rich microdomains are essential for microtubule-based membrane protrusions induced by Clostridium difficile transferase (CDT)

Clostridium difficile toxin (CDT) is a binary actin-ADP-ribosylating toxin that causes depolymerization of the actin cytoskeleton and formation of microtubule-based membrane protrusions, which are suggested to be involved in enhanced bacterial adhesion and colonization of hypervirulent C. difficile strains. Here, we studied the involvement of membrane lipid components of human colon adenocarcinoma (Caco-2) cells in formation of membrane protrusions. Depletion of cholesterol by methyl-β-cyclodextrin inhibited protrusion formation in a concentration-dependent manner but had no major effect on the toxin-catalyzed modification of actin in target cells. Repletion of cholesterol reconstituted formation of protrusions and increased velocity and total amount of protrusion formation. Methyl-β-cyclodextrin had no effect on the CDT-induced changes in the dynamics of microtubules. Formation of membrane protrusions was also inhibited by the cholesterol-binding polyene antibiotic nystatin. Degradation or inhibition of synthesis of sphingolipids by sphingomyelinase and myriocin, respectively, blocked CDT-induced protrusion formation. Benzyl alcohol, which increases membrane fluidity, prevented protrusion formation. CDT-induced membrane protrusions were stained by flotillin-2 and by the fluorescent-labeled lipid raft marker cholera toxin subunit B, which selectively interacts with GM1 ganglioside mainly located in lipid microdomains. The data suggest that formation and especially the initiation of CDT-induced microtubule-based membrane protrusions depend on cholesterol- and sphingolipid-rich lipid microdomains.

Actin as target for modification by bacterial protein toxins

Various bacterial protein toxins and effectors target the actin cytoskeleton. At least three groups of toxins/effectors can be identified, which directly modify actin molecules. One group of toxins/effectors causes ADP-ribosylation of actin at arginine-177, thereby inhibiting actin polymerization. Members of this group are numerous binary actin-ADP-ribosylating exotoxins (e.g. Clostridium botulinum C2 toxin) as well as several bacterial ADP-ribosyltransferases (e.g. Salmonella enterica SpvB) which are not binary in structure. The second group includes toxins that modify actin to promote actin polymerization and the formation of actin aggregates. To this group belongs a toxin from the Photorhabdus luminescens Tc toxin complex that ADP-ribosylates actin at threonine-148. A third group of bacterial toxins/effectors (e.g. Vibrio cholerae multifunctional, autoprocessing RTX toxin) catalyses a chemical crosslinking reaction of actin thereby forming oligomers, while blocking the polymerization of actin to functional filaments. Novel findings about members of these toxin groups are discussed in detail.

From cosubstrate similarity to inhibitor diversity--inhibitors of ADP-ribosyltransferases from kinase inhibitor screening

ADP-ribosyltransferases (ADP-RTs) use NAD(+) to transfer an ADP-ribosyl group to target proteins. Although some ADP-RTs are bacterial toxins only few inhibitors are known. Here we present the development of fluorescence-based assays and a focussed library screening using kinase inhibitors as a new approach towards inhibitors of ADP-RTs. Different screening setups were established using surrogate small molecule substrates or the quantitation of the cofactor NAD(+). Proof-of-principle screening experiments were performed using a kinase inhibitor library in order to target the NAD(+) binding pockets. This led to the discovery of structurally different lead inhibitors for the mono-ADP-ribosyltransferases Mosquitocidal toxin (MTX) from Bacillus sphaericus SSII-1, C3bot toxin from Clostridium botulinum and CDTa from Clostridium difficile. The interaction of the inhibitors with the toxin proteins was analyzed by means of docking and binding free energy calculations. Binding at the nicotinamide subpocket, which shows a significant difference in the three enzymes, is used to explain the selectivity of the identified inhibitors and offers an opportunity for further development of potent and selective inhibitors.

Pleiotropic role of Rac in mast cell activation revealed by a cell permeable Bordetella dermonecrotic fusion toxin

To activate the GTPase Rac in rat basophilic leukemia (RBL) cells and mouse bone marrow-derived mast cells (BMMC) a TAT fusion toxin of Bordetella dermonecrotic toxin (DNT-TAT) was constructed. The fusion toxin activated Rac1 and RhoA in vitro but only Rac in RBL cells and BMMC. DNT-TAT caused an increase in inositol phosphate formation, calcium mobilization, ERK activation and degranulation of mast cells. All these effects were inhibited by the Rho GTPase-inactivating Clostridium difficile toxin B and Clostridium sordellii lethal toxin. Also the calcium ionophore A23187 caused mast cell activation, including ERK phosphorylation, by processes involving an activation of Rac. The data indicate pleiotropic functions of Rac in mast cell activation.

Clostridium difficile toxin CDT induces formation of microtubule-based protrusions and increases adherence of bacteria

Clostridium difficile causes antibiotic-associated diarrhea and pseudomembranous colitis by production of the Rho GTPase-glucosylating toxins A and B. Recently emerging hypervirulent Clostridium difficile strains additionally produce the binary ADP-ribosyltransferase toxin CDT (Clostridium difficile transferase), which ADP-ribosylates actin and inhibits actin polymerization. Thus far, the role of CDT as a virulence factor is not understood. Here we report by using time-lapse- and immunofluorescence microscopy that CDT and other binary actin-ADP-ribosylating toxins, including Clostridium botulinum C2 toxin and Clostridium perfringens iota toxin, induce redistribution of microtubules and formation of long (up to >150 µm) microtubule-based protrusions at the surface of intestinal epithelial cells. The toxins increase the length of decoration of microtubule plus-ends by EB1/3, CLIP-170 and CLIP-115 proteins and cause redistribution of the capture proteins CLASP2 and ACF7 from microtubules at the cell cortex into the cell interior. The CDT-induced microtubule protrusions form a dense meshwork at the cell surface, which wrap and embed bacterial cells, thereby largely increasing the adherence of Clostridia. The study describes a novel type of microtubule structure caused by less efficient microtubule capture and offers a new perspective for the pathogenetic role of CDT and other binary actin-ADP-ribosylating toxins in host–pathogen interactions.

Clostridium difficile is responsible for ∼20 percent of antibiotic-related cases of diarrhea and nearly all cases of pseudomembranous colitis. The pathogens produce two protein toxins (toxins A and B), which inactivate Rho-GTPases of host cells by glucosylation. Recently emerging hypervirulent strains of C. difficile release higher amounts of toxins A and B, are resistant towards fluoroquinolones and produce an additional protein toxin called C. difficile transferase (CDT). CDT is a binary toxin, which modifies G-actin by ADP-ribosylation, thereby inhibiting actin polymerization. So far the pathogenetic role of CDT is not clear. Here we studied the effects of CDT on human colon carcinoma cells and show that the toxin causes rearrangement of microtubules and formation of long cellular protrusions. The microtubule-based protrusions form a dense meshwork at the cell surface, which wrap and embed Clostridia, thereby increasing adherence of the pathogens. We observed similar effects with other members of the family of binary actin-ADP-ribosylating toxins like C. botulinum C2 toxin and C. perfringens iota toxin. Our findings show a novel type of microtubule structures induced by actin-ADP-ribosylating toxins and propose an important role of these toxins in host–pathogen interactions by their effects on adherence and colonization of Clostridia.

[Severe ethanol-intoxication mimics symptoms of intracranial hypertension]

We report on a patient intoxicated with ethanol, who was found unconscious with fixed dilated pupils, deviation of eyes, absent cough and corneal reflexes, with her arms in decerebrate posture. Because of an orbital haematoma a traumatic intracranial hemorrhage with elevated intracranial pressure and compression or concussion of the brain stem was suspected. A CT-scan of the head did not reveal an intracranial mass or any radiographic signs of intracranial hypertension. Drug screening, blood gas analysis and other laboratory tests as well anamnestic data were not indicative of other intoxications or a metabolic decompensation. These unusual neurological findings were caused by a life-threatening ethanol intoxication with a blood-alcohol concentration of 700 mg/dl (151.8 mmol/l). The patient was extubated after several hours of mechanical ventilation in the intensive care unit and was discharged to a regular ward on the following day.

Antioxidative capacity in patients with common variable immunodeficiency

Highly reactive oxygen species (ROS) are involved in T-cell activation and in the defense against environmental pathogens. An imbalance of ROS generation, detoxifying scavenger enzymes, and molecules with antioxidant capacity could contribute to the increased susceptibility to cancer and infections in severe humoral immunodeficiency. We studied antioxidant status, i.e., plasma antioxidant capacity (TEAC), retinol, alpha-to-copherol, ubiquinol, and the number of activated T cells in 16 patients with common variable immunodeficiency (CVID) compared to age-matched healthy controls. As expected, patients showed significantly increased levels of activated HLA-DR and CD45RO-expressing T cells. Plasma levels of the endogenous ROS scavenger ubiquinol (Q 10) were significantly lower in patients as compared to controls. However, patients showed only slightly reduced levels of TEAC as well as the exogenous antioxidants retinol and alpha-tocopherol. Although no correlation of the number of activated T cells and antioxidant capacity could be demonstrated, an increase in ROS and a diminished reactive oxygen scavenger capacity may be involved in the disease process in patients with common variable immunodeficiency.

Gingerol, isoproterenol and ouabain normalize impaired post-rest behavior but not force-frequency relation in failing human myocardium

OBJECTIVE

Rest- and stimulation frequency-dependent potentiation of contractile force is blunted in failing human myocardium. These alterations have been related to reduced sarcoplasmic reticulum (SR) Ca(2+)-reuptake and enhanced transsarcolemmal Ca(2+)-elimination by Na+/Ca(2+)-exchange. We investigated whether inotropic interventions that enhance SR Ca(2+)-uptake, or reduce Ca(2+)-elimination by Na+/Ca(2+)-exchange, normalize impaired post-rest and force-frequency behavior in left ventricular muscle strips from failing human hearts.

METHODS

We tested the influence of [10]-gingerol which activates SR Ca(2+)-ATPase (10 mumol/l; n = 13), and isoproterenol which activates cAMP-dependent pathways (0.01, 0.1, 1 mumol/l; n = 40) on post-rest and force-frequency behavior. Ouabain which blocks Na+/K(+)-ATPase (0.03 mumol/l; n = 16) was used to test the effects of inhibiting Ca(2+)-elimination by Na+/Ca(2+)-exchange. For comparison, the effects of blocking SR Ca(2+)-uptake by thapsigargin (10 mumol/l; n = 14) were tested. In addition, Ca(2+)-uptake in myocardial homogenates was measured for gingerol (10 mumol/l; n = 6).

RESULTS

Gingerol, isoproterenol (0.1, 1 mumol/l) and ouabain exerted significant positive inotropic effects under basal experimental conditions and normalized post-rest behavior. In contrast, force-frequency relation was only slightly improved by gingerol and isoproterenol (0.01 mumol/l). Ouabain and isoproterenol (1 mumol/l) further deteriorated force-frequency relation due to frequency-dependent significant increases in diastolic tension. Thapsigargin exerted negative inotropic effects and significantly deteriorated post-rest and force-frequency behavior. In addition, gingerol increased SR Ca(2+)-uptake significantly in myocardial homogenates.

CONCLUSIONS

Inotropic interventions that stimulate SR Ca(2+)-ATPase or inhibit Na+/Ca(2+)-exchange normalize impaired post-rest behavior. Force-frequency behavior is only slightly improved by stimulation of SR Ca(2+)-ATPase but not by inhibition of Na+/Ca(2+)-exchange. This dissociation between post-rest and force-frequency behavior results from diastolic dysfunction at high stimulation rates.

[Postdural puncture headache: diagnosis, prevention and therapy]

Lumbar puncture (LP) is a routine technique performed for a variety of procedures, e.g. diagnosis, administration of drugs, myelography and spinal anaesthesia. Postdural puncture headache is a common complication (30-40% in diagnostic LP). Prevention can be accomplished by using small-gauge needles (< or = 25 G) or pencil-point needles (22 G). Therapy should be carried out in a stepwise approach. The first step is bedrest, use of analgetics, i.v. fluids and an adequate guidance of the patient. The second step comprises special drug therapy. Several methods of pharmacologic management have been presented in the literature, but most of these are case reports. There is a lack of large double-blind placebo-controlled studies. Theophylline, caffeine, ACTH and sumatriptan are potentially promising agents for the treatment of postdural puncture headache. The efficacy of theophylline has been proven in a double-blind and placebo-controlled study. There are a few studies and case reports reporting that caffeine p.o. and i.v. is effective in the treatment of postdural puncture headache, but recurrence of headache after caffeine therapy is frequent. ACTH acts on a complex hormonal system. The treatment with sumatriptan has been reported in only a few case reports. The third step, and one of the most effective treatments of postdural puncture headache, is the epidural blood patch. The success rate ranges between 80 and 97%.

Anti-oxidative capacity in patients with ataxia telangiectasia

Highly reactive oxygen species (ROS) are involved in T-cell activation and in the defense against environmental pathogens. An imbalance of ROS generation and detoxifying scavenger enzymes could contribute to the increased susceptibility to cancer and infections in ataxia telangiectasia. We studied oxidative status, i.e. plasma total antioxidant capacity (TEAC), retinol, alpha-tocopherol, ubiquinol, and the number of activated T cells in 10 patients with ataxia telangiectasia (AT) compared to age-matched healthy controls. As expected, patients showed significantly increased levels of activated human leukocyte antigen-DR and CD45RO expressing T cells. TEAC levels as well as the exogenous antioxidants retinol and alpha-tocopherol were significantly reduced in patients. In addition, patients showed slightly reduced plasma levels of the endogenous ROS scavenger enzyme ubiquinol (Q10). Although no correlation between number of activated T-cells and antioxidant capacity could be demonstrated, an increase in ROS and a diminished reactive oxygen scavenger capacity may be involved in the disease process of patients with AT.