Microparticulate Caspase 1 Regulates Gasdermin D and Pulmonary Vascular Endothelial Cell Injury

Lung endothelial cell apoptosis and injury occur throughout all stages of acute lung injury/acute respiratory distress syndrome and impact disease progression. Caspases 1, 4, and 5 are essential for completion of the apoptotic program known as pyroptosis that also involves proinflammatory cytokines. Because gasdermin D (GSDMD) mediates pyroptotic death and is essential for pore formation, we hypothesized that it might direct caspase 1-encapsulated microparticle (MP) release and mediate endothelial cell death. Our present work provides evidence that GSDMD is released by LPS-stimulated THP-1 monocytic cells, where it is packaged into microparticles together with active caspase 1. Furthermore, only MP released from stimulated monocytic cells that contain both cleaved GSDMD and active caspase 1 induce endothelial cell apoptosis. MPs pretreated with caspase 1 inhibitor Y-VAD or pan-caspase inhibitor Z-VAD do not contain cleaved GSDMD. MPs from caspase 1-knockout cells are also deficient in p30 active GSDMD, further confirming that caspase 1 regulates GSDMD function. Although control MPs contained cleaved GSDMD without caspase 1, these fractions were unable to induce cell death, suggesting that encapsulation of both caspase 1 and GSDMD is essential for cell death induction. Release of microparticulate active caspase 1 was abrogated in GSDMD knockout cells, although cytosolic caspase 1 activation was not impaired. Last, higher concentrations of microparticulate GSDMD were detected in the plasma of septic patients with acute respiratory distress syndrome than in that of healthy donors. Taken together, these findings suggest that GSDMD regulates the release of microparticulate active caspase 1 from monocytes essential for induction of cell death and thereby may play a critical role in sepsis-induced endothelial cell injury.

Clostridium difficile TcdB toxin differentially regulates pyrin via differences in activity of RhoA between proliferating and non-proliferating mammalian cells

Background

Pyrin is a member of the TRIM family of proteins. Pyrin senses RhoA posttranslational modifications that inactivate RhoA, resulting in the formation of a pyrin inflammasome. Clostridium difficile TcdB toxin inactivates RhoA by glycation of threonine 37. Previous studies have shown that active RhoA predominates in proliferating cells. Thus, TcdB may differentially regulate the pyrin inflammasome in proliferating versus non-proliferating mammalian cells.

Methods

We used a RhoA activation assay (Rhotekin) to test this hypothesis in proliferating monocytic cells (THP1), PMA-differentiated THP1 cells and non-proliferating human monocytes. Pyrin function was determined by its interaction with 14-3-3ɛ and IL-1β release.

Results

We noted that TcdB did not inactivate RhoA in proliferating THP1 cells. However, the toxin drastically reduced the level of active RhoA in non-proliferating human monocytes and PMA-differentiated THP1 cells. TcdB synergized with LPS to induce IL1β release from TcdB- and LPS-stimulated THP1 and PMA-differentiated cells. In contrast, TcdB alone was sufficient to generate a robust IL1β release from TcdB-stimulated human monocytes. TcdB disrupted pyrin’s interaction with 14-3-3ɛ in human monocytes and differentiated THP1 cells, suggesting a decrease in the level of phosphorylated pyrin.

Conclusion

Our data indicate that TcdB is capable of turning off active RhoA in non-proliferating cells but not in proliferating cells. Hence, cellular proliferation may impair pyrin inflammasome assembly and activation.

MPLEx: a method for simultaneous pathogen inactivation and extraction of samples for multi-omics profiling

The continued emergence and spread of infectious agents is of great concern, and systems biology approaches to infectious disease research can advance our understanding of host-pathogen relationships and facilitate the development of new therapies and vaccines. Molecular characterization of infectious samples outside of appropriate biosafety containment can take place only subsequent to pathogen inactivation. Herein, we describe a modified Folch extraction using chloroform/methanol that facilitates the molecular characterization of infectious samples by enabling simultaneous pathogen inactivation and extraction of proteins, metabolites, and lipids for subsequent mass spectrometry-based multi-omics measurements. This single-sample metabolite, protein and lipid extraction (MPLEx) method resulted in complete inactivation of clinically important bacterial and viral pathogens with exposed lipid membranes, including Yersinia pestis, Salmonella Typhimurium, and Campylobacter jejuni in pure culture, and Yersinia pestis, Campylobacter jejuni, and West Nile, MERS-CoV, Ebola, and influenza H7N9 viruses in infection studies. In addition, >99% inactivation, which increased with solvent exposure time, was also observed for pathogens without exposed lipid membranes including community-associated methicillin-resistant Staphylococcus aureus, Clostridium difficile spores and vegetative cells, and adenovirus type 5. The overall pipeline of inactivation and subsequent proteomic, metabolomic, and lipidomic analyses was evaluated using a human epithelial lung cell line infected with wild-type and mutant influenza H7N9 viruses, thereby demonstrating that MPLEx yields biomaterial of sufficient quality for subsequent multi-omics analyses. Based on these experimental results, we believe that MPLEx will facilitate systems biology studies of infectious samples by enabling simultaneous pathogen inactivation and multi-omics measurements from a single specimen with high success for pathogens with exposed lipid membranes.

Molecular characterization of the Dot/Icm-translocated AnkH and AnkJ eukaryotic-like effectors of Legionella pneumophila

Although most Dot/Icm-translocated effectors of Legionella pneumophila are not required for intracellular proliferation, the eukaryotic-like ankyrin effectors, AnkH and AnkJ are required for intracellular proliferation. In this report, we show that the IcmSW chaperones are essential for translocation of AnkJ but not AnkH. The 10 C-terminal residues and the ANK domains of AnkH and AnkJ are required for translocation. Our data indicate that the two ANK domains of AnkH are critical domains required for the function of the effector in intracellular replication of L. pneumophila. The ankH and ankJ mutants are severely defective in intrapulmonary proliferation in mice. Expression of AnkH and AnkJ fusions within HEK293 cells show a punctuate distribution in the cytosol but no association with endocytic vesicles, the Golgi apparatus or the endoplasmic reticulum. Interestingly, the defect in intracellular proliferation of the ankH or ankJ mutants is rescued in HEK293 cells expressing the respective protein. We conclude that AnkH and AnkJ are effectors translocated by the Dot/Icm system by distinct mechanisms and modulate distinct cytosolic processes in the host cell.

Molecular mimicry by an F-box effector of Legionella pneumophila hijacks a conserved polyubiquitination machinery within macrophages and protozoa

The ability of Legionella pneumophila to proliferate within various protozoa in the aquatic environment and in macrophages indicates a remarkable evolution and microbial exploitation of evolutionarily conserved eukaryotic processes. Ankyrin B (AnkB) of L. pneumophila is a non-canonical F-box-containing protein, and is the only known Dot/Icm-translocated effector of L. pneumophila essential for intra-vacuolar proliferation within both macrophages and protozoan hosts. We show that the F-box domain of AnkB and the ⁹L¹⁰P conserved residues are essential for intracellular bacterial proliferation and for rapid acquisition of polyubiquitinated proteins by the Legionella-containing vacuole (LCV) within macrophages, Dictyostelium discoideum, and Acanthamoeba. Interestingly, translocation of AnkB and recruitment of polyubiquitinated proteins in macrophages and Acanthamoeba is rapidly triggered by extracellular bacteria within 5 min of bacterial attachment. Ectopically expressed AnkB within mammalian cells is localized to the periphery of the cell where it co-localizes with host SKP1 and recruits polyubiquitinated proteins, which results in restoration of intracellular growth to the ankB mutant similar to the parental strain. While an ectopically expressed AnkB-⁹L¹⁰P/AA variant is localized to the cell periphery, it does not recruit polyubiquitinated proteins and fails to trans-rescue the ankB mutant intracellular growth defect. Direct in vivo interaction of AnkB but not the AnkB-⁹L¹⁰P/AA variant with the host SKP1 is demonstrated. Importantly, RNAi-mediated silencing of expression of SKP1 renders the cells non-permissive for intracellular proliferation of L. pneumophila. The role of AnkB in exploitation of the polyubiquitination machinery is essential for intrapulmonary bacterial proliferation in the mouse model of Legionnaires' disease. Therefore, AnkB exhibits a novel molecular and functional mimicry of eukaryotic F-box proteins that exploits conserved polyubiquitination machinery for intracellular proliferation within evolutionarily distant hosts.

Legionella pneumophila is abundantly found in the aquatic environment within various protozoa and can cause a severe pneumonia called Legionnaires' disease when it invades human macrophages in the lung. The ability of L. pneumophila to invade and proliferate within macrophages and protozoa is dependent on the translocation of specific proteins into the invaded cell via a specialized secretory device, and these proteins modulate various host cell processes. Of these translocated proteins, AnkB is indispensable for intracellular growth of L. pneumophila within macrophages and protozoa. Here we show that AnkB is essential for establishing a favorable intracellular replicative niche by promoting the decoration of the Legionella containing vacuole (LCV) with polyubiquitinated proteins. The AnkB effector achieves this by mimicking the action of host cell F-box proteins, a highly conserved component of the SCF ubiquitin ligase complex that is found in both unicellular organisms and mammalian cells. Our study provides new insights into the ability of intracellular pathogens to hijack evolutionarily conserved host cell processes through molecular mimicry to establish a favorable replicative niche within various hosts and to cause disease in mammals.

Temporal and spatial trigger of post-exponential virulence-associated regulatory cascades by Legionella pneumophila after bacterial escape into the host cell cytosol

During late stages of infection and prior to lysis of the infected macrophages or amoeba, the Legionella pneumophila-containing phagosome becomes disrupted, followed by bacterial escape into the host cell cytosol, where the last few rounds of bacterial proliferation occur prior to lysis of the plasma membrane. This coincides with growth transition into the post-exponential (PE) phase, which is controlled by regulatory cascades including RpoS and the LetA/S two-component regulator. Whether the temporal expression of flagella by the regulatory cascades at the PE phase is exhibited within the phagosome or after bacterial escape into the host cell cytosol is not known. We have utilized fluorescence microscopy-based phagosome integrity assay to differentiate between vacuolar and cytosolic bacteria/or bacteria within disrupted phagosomes. Our data show that during late stages of infection, expression of FlaA is triggered after bacterial escape into the macrophage cytosol and the peak of FlaA expression is delayed for few hours after cytosolic residence of the bacteria. Importantly, bacterial escape into the host cell cytosol is independent of flagella, RpoS and the two-component regulator LetA/S, which are all triggered by L. pneumophila upon growth transition into the PE phase. Disruption of the phagosome and bacterial escape into the cytosol of macrophages is independent of the bacterial pore-forming activity, and occurs prior to the induction of apoptosis during late stages of infection. We conclude that the temporal and spatial engagement of virulence-associated regulatory cascades by L. pneumophila at the PE phase is temporally and spatially triggered after phagosomal escape and bacterial residence in the host cell cytosol.

A Dot/Icm-translocated ankyrin protein of Legionella pneumophila is required for intracellular proliferation within human macrophages and protozoa

The Dot/Icm type IV secretion system of Legionella pneumophila translocates numerous bacterial effectors into the host cell and is essential for bacterial proliferation within macrophages and protozoa. We have recently shown that L. pneumophila strain AA100/130b harbours 11 genes encoding eukaryotic-like ankyrin (Ank) proteins, a family of proteins involved in various essential eukaryotic cellular processes. In contrast to most Dot/Icm-exported substrates, which have little or no detectable role in intracellular proliferation, a mutation in ankB results in a severe growth defect in intracellular replication within human monocyte-derived macrophages (hMDMs), U937 macrophages and Acanthamoeba polyphaga. Single cell analyses of coinfections of hMDMs have shown that the intracellular growth defect of the ankB mutant is totally rescued in cis within communal phagosomes harbouring the wild type strain. Interestingly, distinct from dot/icm structural mutants, the ankB mutant is also rescued in trans within cells harbouring the wild type strain in a different phagosome, indicating that AnkB is a trans-acting secreted effector. Using adenylate cyclase fusions to AnkB, we show that AnkB is translocated into the host cell via the Dot/Icm secretion system in an IcmSW-dependent manner and that the last three C-terminal amino acid residues are essential for translocation. Distinct from the dot/icm structural mutants, the ankB mutant-containing phagosomes exclude late endosomal and lysosomal markers and their phagosomes are remodelled by the rough endoplasmic reticulum. We show that at the postexponential phase of growth, the LetA/S and PmrA/B Two Component Systems confer a positive regulation on expression of the ankB gene, whereas RpoS, LetE and RelA suppress its expression. Our data show that the eukaryotic-like AnkB protein is a Dot/Icm-exported effector that plays a major role in intracellular replication of L. pneumophila within macrophages and protozoa, and its expression is temporally controlled by regulators of the postexponential phase of growth.

Role for the Ankyrin eukaryotic-like genes of Legionella pneumophila in parasitism of protozoan hosts and human macrophages

Legionella pneumophila is a ubiquitous organism in the aquatic environment where it is capable of invasion and intracellular proliferation within various protozoan species and is also capable of causing pneumonia in humans. In silico analysis showed that the three sequenced L. pneumophila genomes each contained a common multigene family of 11 ankyrin (ank) genes encoding proteins with approximately 30-35 amino acid tandem Ankyrin repeats that are involved in protein-protein interactions in eukaryotic cells. To examine whether the ank genes are involved in tropism of protozoan hosts, we have constructed isogenic mutants of L. pneumophila in ten of the ank genes. Among the mutants, the DeltaankH and DeltaankJ mutants exhibit significant defects in robust intracellular replication within A. polyphaga, Hartmanella vermiformis and Tetrahymena pyriformis. A similar defect is also exhibited in human macrophages. Most of the ank genes are upregulated by L. pneumophila upon growth transition into the post-exponential phase in vitro and within Acanthamoeba polyphaga, and this upregulation is mediated, at least in part, by RpoS. Single-cell analyses have shown that upon co-infection of the wild-type strain with the ankH or ankJ mutant, the replication defect of the mutant is rescued within communal phagosomes harbouring the wild-type strain, similar to dot/icm mutants. Therefore, at least two of the L. pneumophila eukaryotic-like Ank proteins play a role in intracellular replication of L. pneumophila within amoeba, ciliated protozoa and human macrophages. The Ank proteins may not be involved in host tropism in the aquatic environment. Many of the L. pneumophila eukaryotic-like ank genes are triggered upon growth transition into post-exponential phase in vitro as well as within A. polyphaga. Our data suggest a role for AnkH and AnkJ in modulation of phagosome biogenesis by L. pneumophila independent of evasion of lysosomal fusion and recruitment of the rough endoplasmic reticulum.