Transcriptional responses of murine macrophages to the adenylate cyclase toxin of Bordetella pertussis

Conquering the host: Bordetella spp. and Pseudomonas aeruginosa molecular regulators in lung infection

When bacteria sense cues from the host environment, stress responses are activated. Two component systems, sigma factors, small RNAs, ppGpp stringent response, and chaperones start coordinate the expression of virulence factors or immunomodulators to allow bacteria to respond. Although, some of these are well studied, such as the two-component systems, the contribution of other regulators, such as sigma factors or ppGpp, is increasingly gaining attention. Pseudomonas aeruginosa is the gold standard pathogen for studying the molecular mechanisms to sense and respond to environmental cues. Bordetella spp., on the other hand, is a microbial model for studying host-pathogen interactions at the molecular level. These two pathogens have the ability to colonize the lungs of patients with chronic diseases, suggesting that they have the potential to share a niche and interact. However, the molecular networks that facilitate adaptation of Bordetella spp. to cues are unclear. Here, we offer a side-by-side comparison of what is known about these diverse molecular mechanisms that bacteria utilize to counteract host immune responses, while highlighting the relatively unexplored interactions between them.

Structure-Function Relationships Underlying the Capacity of Bordetella Adenylate Cyclase Toxin to Disarm Host Phagocytes

Bordetellae, pathogenic to mammals, produce an immunomodulatory adenylate cyclase toxin-hemolysin (CyaA, ACT or AC-Hly) that enables them to overcome the innate immune defense of the host. CyaA subverts host phagocytic cells by an orchestrated action of its functional domains, where an extremely catalytically active adenylyl cyclase enzyme is delivered into phagocyte cytosol by a pore-forming repeat-in-toxin (RTX) cytolysin moiety. By targeting sentinel cells expressing the complement receptor 3, known as the CD11b/CD18 (α_Mβ₂) integrin, CyaA compromises the bactericidal functions of host phagocytes and supports infection of host airways by Bordetellae. Here, we review the state of knowledge on structural and functional aspects of CyaA toxin action, placing particular emphasis on signaling mechanisms by which the toxin-produced 3',5'-cyclic adenosine monophosphate (cAMP) subverts the physiology of phagocytic cells.

Membrane-Pore Forming Characteristics of the Bordetella pertussis CyaA-Hemolysin Domain

Previously, the 126-kDa Bordetella pertussis CyaA pore-forming/hemolysin (CyaA-Hly) domain was shown to retain its hemolytic activity causing lysis of susceptible erythrocytes. Here, we have succeeded in producing, at large quantity and high purity, the His-tagged CyaA-Hly domain over-expressed in Escherichia coli as a soluble hemolytically-active form. Quantitative assays of hemolysis against sheep erythrocytes revealed that the purified CyaA-Hly domain could function cooperatively by forming an oligomeric pore in the target cell membrane with a Hill coefficient of ~3. When the CyaA-Hly toxin was incorporated into planar lipid bilayers (PLBs) under symmetrical conditions at 1.0 M KCl, 10 mM HEPES buffer (pH 7.4), it produced a clearly resolved single channel with a maximum conductance of ~35 pS. PLB results also revealed that the CyaA-Hly induced channel was unidirectional and opened more frequently at higher negative membrane potentials. Altogether, our results first provide more insights into pore-forming characteristics of the CyaA-Hly domain as being the major pore-forming determinant of which the ability to induce such ion channels in receptor-free membranes could account for its cooperative hemolytic action on the target erythrocytes.

Bordetella pertussis Adenylate Cyclase Toxin Blocks Induction of Bactericidal Nitric Oxide in Macrophages through cAMP-Dependent Activation of the SHP-1 Phosphatase

The adenylate cyclase toxin-hemolysin (CyaA) plays a key role in the virulence of Bordetella pertussis. CyaA penetrates complement receptor 3-expressing phagocytes and catalyzes uncontrolled conversion of cytosolic ATP to the key second messenger molecule cAMP. This paralyzes the capacity of neutrophils and macrophages to kill bacteria by complement-dependent oxidative burst and opsonophagocytic mechanisms. We show that cAMP signaling through the protein kinase A (PKA) pathway activates Src homology domain 2 containing protein tyrosine phosphatase (SHP) 1 and suppresses production of bactericidal NO in macrophage cells. Selective activation of PKA by the cell-permeable analog N(6)-benzoyladenosine-3',5'-cyclic monophosphate interfered with LPS-induced inducible NO synthase (iNOS) expression in RAW264.7 macrophages, whereas inhibition of PKA by H-89 largely restored the production of iNOS in CyaA-treated murine macrophages. CyaA/cAMP signaling induced SHP phosphatase-dependent dephosphorylation of the c-Fos subunit of the transcription factor AP-1 and thereby inhibited TLR4-triggered induction of iNOS gene expression. Selective small interfering RNA knockdown of SHP-1, but not of the SHP-2 phosphatase, rescued production of TLR-inducible NO in toxin-treated cells. Finally, inhibition of SHP phosphatase activity by NSC87877 abrogated B. pertussis survival inside murine macrophages. These results reveal that an as yet unknown cAMP-activated signaling pathway controls SHP-1 phosphatase activity and may regulate numerous receptor signaling pathways in leukocytes. Hijacking of SHP-1 by CyaA action then enables B. pertussis to evade NO-mediated killing in sentinel cells of innate immunity.

Allosteric activation of Bordetella pertussis adenylyl cyclase by calmodulin: molecular dynamics and mutagenesis studies

Adenylyl cyclase (AC) toxin is an essential toxin that allows Bordetella pertussis to invade eukaryotic cells, where it is activated after binding to calmodulin (CaM). Based on the crystal structure of the AC catalytic domain in complex with the C-terminal half of CaM (C-CaM), our previous molecular dynamics simulations (Selwa, E., Laine, E., and Malliavin, T. (2012) Differential role of calmodulin and calcium ions in the stabilization of the catalytic domain of adenyl cyclase CyaA from Bordetella pertussis. Proteins 80, 1028–1040) suggested that three residues (i.e. Arg(338), Asn(347), and Asp(360)) might be important for stabilizing the AC/CaM interaction. These residues belong to a loop-helix-loop motif at the C-terminal end of AC, which is located at the interface between CaM and the AC catalytic loop. In the present study, we conducted the in silico and in vitro characterization of three AC variants, where one (Asn(347); ACm1A), two (Arg(338) and Asp(360); ACm2A), or three residues (Arg(338), Asn(347), and Asp(360); ACm3A) were substituted with Ala. Biochemical studies showed that the affinities of ACm1A and ACm2A for CaM were not affected significantly, whereas that of ACm3A was reduced dramatically. To understand the effects of these modifications, molecular dynamics simulations were performed based on the modified proteins. The molecular dynamics trajectories recorded for the ACm3AC-CaM complex showed that the calcium-binding loops of C-CaM exhibited large fluctuations, which could be related to the weakened interaction between ACm3A and its activator. Overall, our results suggest that the loop-helix-loop motif at the C-terminal end of AC is crucial during CaM binding for stabilizing the AC catalytic loop in an active configuration.

Bordetella pertussis adenylate cyclase toxin translocation across a tethered lipid bilayer

Numerous bacterial toxins can cross biological membranes to reach the cytosol of mammalian cells, where they exert their cytotoxic effects. Our model toxin, the adenylate cyclase (CyaA) from Bordetella pertussis, is able to invade eukaryotic cells by translocating its catalytic domain directly across the plasma membrane of target cells. To characterize its original translocation process, we designed an in vitro assay based on a biomimetic membrane model in which a tethered lipid bilayer (tBLM) is assembled on an amine-gold surface derivatized with calmodulin (CaM). The assembled bilayer forms a continuous and protein-impermeable boundary completely separating the underlying calmodulin (trans side) from the medium above (cis side). The binding of CyaA to the tBLM is monitored by surface plasmon resonance (SPR) spectroscopy. CyaA binding to the immobilized CaM, revealed by enzymatic activity, serves as a highly sensitive reporter of toxin translocation across the bilayer. Translocation of the CyaA catalytic domain was found to be strictly dependent on the presence of calcium and also on the application of a negative potential, as shown earlier in eukaryotic cells. Thus, CyaA is able to deliver its catalytic domain across a biological membrane without the need for any eukaryotic components besides CaM. This suggests that the calcium-dependent CyaA translocation may be driven in part by the electrical field across the membrane. This study's in vitro demonstration of toxin translocation across a tBLM provides an opportunity to explore the molecular mechanisms of protein translocation across biological membranes in precisely defined experimental conditions.

Calcium influx rescues adenylate cyclase-hemolysin from rapid cell membrane removal and enables phagocyte permeabilization by toxin pores

Bordetella adenylate cyclase toxin-hemolysin (CyaA) penetrates the cytoplasmic membrane of phagocytes and employs two distinct conformers to exert its multiple activities. One conformer forms cation-selective pores that permeabilize phagocyte membrane for efflux of cytosolic potassium. The other conformer conducts extracellular calcium ions across cytoplasmic membrane of cells, relocates into lipid rafts, translocates the adenylate cyclase enzyme (AC) domain into cells and converts cytosolic ATP to cAMP. We show that the calcium-conducting activity of CyaA controls the path and kinetics of endocytic removal of toxin pores from phagocyte membrane. The enzymatically inactive but calcium-conducting CyaA-AC⁻ toxoid was endocytosed via a clathrin-dependent pathway. In contrast, a doubly mutated (E570K+E581P) toxoid, unable to conduct Ca²⁺ into cells, was rapidly internalized by membrane macropinocytosis, unless rescued by Ca²⁺ influx promoted in trans by ionomycin or intact toxoid. Moreover, a fully pore-forming CyaA-ΔAC hemolysin failed to permeabilize phagocytes, unless endocytic removal of its pores from cell membrane was decelerated through Ca²⁺ influx promoted by molecules locked in a Ca²⁺-conducting conformation by the 3D1 antibody. Inhibition of endocytosis also enabled the native B. pertussis-produced CyaA to induce lysis of J774A.1 macrophages at concentrations starting from 100 ng/ml. Hence, by mediating calcium influx into cells, the translocating conformer of CyaA controls the removal of bystander toxin pores from phagocyte membrane. This triggers a positive feedback loop of exacerbated cell permeabilization, where the efflux of cellular potassium yields further decreased toxin pore removal from cell membrane and this further enhances cell permeabilization and potassium efflux.

The adenylate cyclase toxin (CyaA) of pathogenic Bordetellae eliminates the first line of host innate immune defense by inhibiting the oxidative burst and complement-mediated opsonophagocytic killing of bacteria. The toxin penetrates myeloid phagocytes, such as neutrophil, macrophage or dendritic cells, and subverts their signaling by catalyzing a rapid and massive conversion of intracellular ATP to the key signaling molecule cAMP. In parallel, the toxin forms cation-selective pores and permeabilizes the cytoplasmic membrane of phagocytes. This so-called ‘hemolysin’ activity synergizes with the enzymatic AC activity of CyaA in promoting apoptotic or necrotic cell death, depending on the toxin dose. Moreover, the pore-forming activity promotes activation of NALP3 inflammasome and release of interleukin IL-1β. We show here that the capacity of CyaA to permeabilize phagocytes depends on its ability to mediate influx of extracellular calcium ions into cells. This enables bystander CyaA pores to escape rapid macropinocytic removal from cell membrane and exacerbate the permeabilization of cells. These observations set a new paradigm for the mechanism of action of pore-forming RTX leukotoxins on phagocytes.

Identification of a region that assists membrane insertion and translocation of the catalytic domain of Bordetella pertussis CyaA toxin

The adenylate cyclase (CyaA) toxin, one of the virulence factors secreted by Bordetella pertussis, the pathogenic bacteria responsible for whooping cough, plays a critical role in the early stages of respiratory tract colonization by this bacterium. The CyaA toxin is able to invade eukaryotic cells by translocating its N-terminal catalytic domain directly across the plasma membrane of the target cells, where, activated by endogenous calmodulin, it produces supraphysiological levels of cAMP. How the catalytic domain is transferred from the hydrophilic extracellular medium into the hydrophobic environment of the membrane and then to the cell cytoplasm remains an unsolved question. In this report, we have characterized the membrane-interacting properties of the CyaA catalytic domain. We showed that a protein covering the catalytic domain (AC384, encompassing residues 1-384 of CyaA) displayed no membrane association propensity. However, a longer polypeptide (AC489), encompassing residues 1-489 of CyaA, exhibited the intrinsic property to bind to membranes and to induce lipid bilayer destabilization. We further showed that deletion of residues 375-485 within CyaA totally abrogated the toxin's ability to increase intracellular cAMP in target cells. These results indicate that, whereas the calmodulin dependent enzymatic domain is restricted to the amino-terminal residues 1-384 of CyaA, the membrane-interacting, translocation-competent domain extends up to residue 489. This thus suggests an important role of the region adjacent to the catalytic domain of CyaA in promoting its interaction with and its translocation across the plasma membrane of target cells.

Modulation of the host interferon response and ISGylation pathway by B. pertussis filamentous hemagglutinin

Bordetella pertussis filamentous hemagglutinin (FHA) is a surface-associated and secreted protein that serves as a crucial adherence factor, and displays immunomodulatory activity in human peripheral blood mononuclear cells (PBMCs). In order to appreciate more fully the role of secreted FHA in pathogenesis, we analyzed FHA-induced changes in genome-wide transcript abundance in human PBMCs. Among the 683 known unique genes with greater than 3-fold change in transcript abundance following FHA treatment, 125 (18.3%) were identified as interferon (IFN)-regulated. Among the latter group were genes encoding several members of the IFN type I response, as well as 3 key components of the ISGylation pathway. Using real-time RT-PCR, we confirmed FHA-associated increases in transcript abundance for the genes encoding ubiquitin-like protein, ISG15, and its specific protease USP18. Western-blot analysis demonstrated the presence of both, free ISG15 and several ISGylated conjugates in FHA-stimulated PBMC lysates, but not in unstimulated cells. Intracellular FACS analysis provided evidence that monocytes and a natural killer-enriched cell population were the primary producers of ISG15 in PBMCs after FHA stimulation. Our data reveal previously-unrecognized effects of B. pertussis FHA on host IFN and ISGylation responses, and suggest previously-unsuspected mechanisms by which FHA may alter the outcome of the host-pathogen interaction.

Reversible inhibition of murine cytomegalovirus replication by gamma interferon (IFN-γ) in primary macrophages involves a primed type I IFN-signaling subnetwork for full establishment of an immediate-early antiviral state

Activated macrophages play a central role in controlling inflammatory responses to infection and are tightly regulated to rapidly mount responses to infectious challenge. Type I interferon (alpha/beta interferon [IFN-α/β]) and type II interferon (IFN-γ) play a crucial role in activating macrophages and subsequently restricting viral infections. Both types of IFNs signal through related but distinct signaling pathways, inducing a vast number of interferon-stimulated genes that are overlapping but distinguishable. The exact mechanism by which IFNs, particularly IFN-γ, inhibit DNA viruses such as cytomegalovirus (CMV) is still not fully understood. Here, we investigate the antiviral state developed in macrophages upon reversible inhibition of murine CMV by IFN-γ. On the basis of molecular profiling of the reversible inhibition, we identify a significant contribution of a restricted type I IFN subnetwork linked with IFN-γ activation. Genetic knockout of the type I-signaling pathway, in the context of IFN-γ stimulation, revealed an essential requirement for a primed type I-signaling process in developing a full refractory state in macrophages. A minimal transient induction of IFN-β upon macrophage activation with IFN-γ is also detectable. In dose and kinetic viral replication inhibition experiments with IFN-γ, the establishment of an antiviral effect is demonstrated to occur within the first hours of infection. We show that the inhibitory mechanisms at these very early times involve a blockade of the viral major immediate-early promoter activity. Altogether our results show that a primed type I IFN subnetwork contributes to an immediate-early antiviral state induced by type II IFN activation of macrophages, with a potential further amplification loop contributed by transient induction of IFN-β.

The Bordetella pertussis adenylate cyclase toxin binds to T cells via LFA-1 and induces its disengagement from the immune synapse

The Bordella pertussis toxin CyaA binds to LFA-1 on T cells and disrupts the immune synapse.

The Bordetella pertussis adenylate cyclase toxin (CyaA) assists infection by potently suppressing the host immune response. Although CyaA effectively targets T lymphocytes, its putative receptor on these cells is unknown. Here, we show that CyaA binds to T cells via the β₂ integrin LFA-1 in its active conformation. CyaA clusters with LFA-1 at the immune synapse (IS), from which it induces the premature disengagement of LFA-1 concomitant with the dissipation of talin, which tethers the integrin to the underlying actin cytoskeleton. The CyaA-induced redistribution of LFA-1 was cAMP- and protein kinase A (PKA)–dependent. These results not only identify LFA-1 as a CyaA receptor on T cells but unveil a novel mechanism of immunosuppression whereby the toxin parasitizes its interaction with LFA-1 to inhibit signaling at the IS through the local production of cAMP. The data also provide novel insights into the role of cAMP/PKA signaling in controlling the dynamics of the IS.

Peptide-tags for enhanced DNA microarray performance

DNA microarrays are powerful tools for gene expression analysis and genotyping studies in research and diagnostic applications. A high sensitivity and short time-to-result are prerequisites for their practical application in the clinic. The hybridization efficiency of DNA microarrays depends on the probe density and the probe orientation and thus their accessibility for target molecules. In order to find an optimal probe immobilization procedure a set of different oligonucleotide modifications was tested on epoxy silane functionalized glass slides. It was found that histidine-tagged oligonucleotides resulted in the highest amount of bound probe and by far the best hybridization efficiencies. The detection limit obtained with histidine-tagged probes was up to two orders of magnitude lower compared to commonly used probe modifications. In order to further investigate the binding mechanism of histidine-tags towards functionalized glass substrates a set of different peptide-tags with and without free terminal amino-groups and with different amino acid compositions was tested. The results indicate an impact of the terminal amino group on the covalent surface binding and of aromatic amino acid residues on the enhanced hybridisation efficiency.

Functional significance of the highly conserved Glu(570) in the putative pore-forming helix 3 of the Bordetella pertussis haemolysin toxin

Adenylate cyclase-haemolysin toxin (CyaA) is a virulence factor secreted from the etiologic agent of whooping cough, Bordetella pertussis. Previously, the haemolysin or pore-forming domain (CyaA-PF) has been shown to cause cell lysis of sheep erythrocytes independently, and the predicted helix 3((570-593)) within the PF-hydrophobic stretch could be a pore-lining constituent. Here, a plausible involvement in haemolytic activity of polar or charged residues (Glu(570), Gln(574), Glu(581), Ser(584) and Ser(585)) lining the hydrophilic side of CyaA-PF helix 3 was investigated via single-alanine substitutions. All the 126-kDa mutant proteins over-expressed in Escherichia coli were verified for toxin acylation as the results are corresponding to the wild-type toxin. When haemolytic activity of E. coli lysates containing soluble mutant proteins was tested against sheep erythrocytes, the importance of Glu(570), which is highly conserved among the pore-forming RTX cytotoxin family, was revealed for pore formation, conceivably for a general pore-lining residue involved in ion conduction.

Pertussis toxin and adenylate cyclase toxin: key virulence factors of Bordetella pertussis and cell biology tools

Pertussis toxin and adenylate cyclase toxin are two important virulence factors of Bordetella pertussis, the bacterial cause of the respiratory disease pertussis or whooping cough. In addition to studies on the structure, function and role in pathogenesis of these two toxins, they are both used as cell biology tools for a variety of applications owing to their ability to enter mammalian cells, perform enzymatic activities and modify cell signaling events. In this article, recent data from the research literature that enhance our understanding of the nature of these two toxins, their role in the pathogenesis of B. pertussis infection and disease, particularly in modulating host immune responses, and their use as tools for other areas of research will be outlined.