The Legionella pneumophila effector SidJ is required for efficient recruitment of endoplasmic reticulum proteins to the bacterial phagosome

The Legionella pneumophila replication vacuole: making a cosy niche inside host cells

The pathogenesis of Legionella pneumophila is derived from its growth within lung macrophages after aerosols are inhaled from contaminated water sources. Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it a model for intravacuolar pathogens. Establishment of the replication compartment requires a specialized translocation system that transports a large cadre of protein substrates across the vacuolar membrane. These substrates regulate vesicle traffic and survival pathways in the host cell. This Review focuses on the strategies that L. pneumophila uses to establish intracellular growth and evaluates why this microorganism has accumulated an unprecedented number of translocated substrates that are targeted at host cells.

Large-scale identification of Legionella pneumophila Dot/Icm substrates that modulate host cell vesicle trafficking pathways

The bacterial pathogen Legionella pneumophila replicates in a specialized vacuole within host cells. Establishment of the replication vacuole depends on the Dot/Icm translocation system that delivers a large number of protein substrates into the host cell. The functions of most substrates are unknown. Here, we analysed a defined set of 127 confirmed or candidate Dot/Icm substrates for their effect on host cell processes using yeast as a model system. Expression of 79 candidates caused significant yeast growth defects, indicating that these proteins impact essential host cell pathways. Notably, a group of 21 candidates interfered with the trafficking of secretory proteins to the yeast vacuole. Three candidates that caused yeast secretory defects (SetA, Ceg19 and Ceg9) were investigated further. These proteins impinged upon vesicle trafficking at distinct stages and had signals that allowed translocation into host cells by the Dot/Icm system. Ectopically produced SetA, Ceg19 and Ceg9 localized to secretory organelles in mammalian cells, consistent with a role for these proteins in modulating host cell vesicle trafficking. Interestingly, the ability of SetA to cause yeast phenotypes was dependent upon a functional glycosyltransferase domain. We hypothesize that SetA may glycosylate a component of the host cell vesicle trafficking machinery during L. pneumophila infection.

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.

Identification of a hypervariable region containing new Legionella pneumophila Icm/Dot translocated substrates by using the conserved icmQ regulatory signature

Legionella pneumophila is an intracellular pathogen that has been shown to utilize the Icm/Dot type IV secretion system for pathogenesis. This system was shown to be composed of Icm/Dot complex components, accessory proteins, and a large number of translocated substrates. In this study, comparison of the icmQ regulatory regions from many Legionella species revealed a conserved regulatory sequence that includes the icmQ -10 promoter element. Mutagenesis of this conserved regulatory element indicated that each of the nucleotides in it affects the level of expression of the icmQ gene but not in a uniform fashion. A genomic analysis discovered that four additional genes in L. pneumophila contain this conserved regulatory sequence, which was found to function similarly in these genes as well. Examination of these four genes indicated that they are dispensable for intracellular growth, but two of them were found to encode new Icm/Dot translocated substrates (IDTS). Comparison of the genomic regions encoding these two IDTS among the four available L. pneumophila genomic sequences indicated that one of these genes is located in a hypervariable genomic region, which was shown before to contain an IDTS-encoding gene. Translocation analysis that was performed for nine proteins encoded from this hypervariable genomic region indicated that six of them are new IDTS which are translocated into host cells in an Icm/Dot-dependent manner. Furthermore, a bioinformatic analysis indicated that additional L. pneumophila genomic regions that contain several neighboring IDTS-encoding genes are hypervariable in gene content.

Legionella eukaryotic-like type IV substrates interfere with organelle trafficking

Legionella pneumophila, the causative agent of Legionnaires' disease, evades phago-lysosome fusion in mammalian and protozoan hosts to create a suitable niche for intracellular replication. To modulate vesicle trafficking pathways, L. pneumophila translocates effector proteins into eukaryotic cells through a Type IVB macro-molecular transport system called the Icm-Dot system. In this study, we employed a fluorescence-based translocation assay to show that 33 previously identified Legionella eukaryotic-like genes (leg) encode substrates of the Icm-Dot secretion system. To assess which of these proteins may contribute to the disruption of vesicle trafficking, we expressed each gene in yeast and looked for phenotypes related to vacuolar protein sorting. We found that LegC3-GFP and LegC7/YlfA-GFP caused the mis-secretion of CPY-Invertase, a fusion protein normally restricted to the yeast vacuole. We also found that LegC7/YlfA-GFP and its paralog LegC2/YlfB-GFP formed large structures around the yeast vacuole while LegC3-GFP localized to the plasma membrane and a fragmented vacuole. In mammalian cells, LegC2/YlfB-GFP and LegC7/YlfA-GFP were found within large structures that co-localized with anti-KDEL antibodies but excluded the lysosomal marker LAMP-1, similar to what is observed in Legionella-containing vacuoles. LegC3-GFP, in contrast, was observed as smaller structures which had no obvious co-localization with KDEL or LAMP-1. Finally, LegC3-GFP caused the accumulation of many endosome-like structures containing undigested material when expressed in the protozoan host Dictyostelium discoideum. Our results demonstrate that multiple Leg proteins are Icm/Dot-dependent substrates and that LegC3, LegC7/YlfA, and LegC2/YlfB may contribute to the intracellular trafficking of L. pneumophila by interfering with highly conserved pathways that modulate vesicle maturation.

An in vivo gene deletion system for determining temporal requirement of bacterial virulence factors

Analysis of phenotypes associated with specific mutants has been instrumental in determining the roles of a bacterial gene in a biological process. However, this technique does not allow one to address whether a specific gene or gene set is necessary to maintain such a process once it has been established. In the study of microbial pathogenesis, it is important but difficult to determine the temporal requirement of essential pathogenic determinants in the entire infection cycle. Here we report a Cre/loxP-based genetic system that allowed inducible deletion of specific bacterial genes after the pathogen had been phagocytosed by host cells. Using this system, we have examined the temporal requirement of the Dot/Icm type IV protein transporter of Legionella pneumophila during infection. We found that deletion of single essential dot/icm genes did not prevent the internalized bacteria from completing one cycle of intracellular replication. Further analyses indicate that the observed phenotypes were due to the high stability of the examined Dot/Icm protein. However, postinfection deletion within 8 h of the gene coding for the Dot/Icm substrate, SdhA, abolishes intracellular bacterial growth. This result indicates that the Dot/Icm transporter is important for intracellular bacterial growth after the initial biogenesis of the vacuole. Our study has provided a technical concept for analyzing the temporal requirement of specific bacterial proteins or protein complexes in infection or development.

Type IV secretion systems: tools of bacterial horizontal gene transfer and virulence

Type IV secretion systems (T4SSs) are multisubunit cell-envelope-spanning structures, ancestrally related to bacterial conjugation machines, which transfer proteins and nucleoprotein complexes across membranes. T4SSs mediate horizontal gene transfer, thus contributing to genome plasticity and the evolution of pathogens through dissemination of antibiotic resistance and virulence genes. Moreover, T4SSs are also used for the delivery of bacterial effector proteins across the bacterial membrane and the plasmatic membrane of eukaryotic host cell, thus contributing directly to pathogenicity. T4SSs are usually encoded by multiple genes organized into a single functional unit. Based on a number of features, the organization of genetic determinants, shared homologies and evolutionary relationships, T4SSs have been divided into several groups. Type F and P (type IVA) T4SSs resembling the archetypal VirB/VirD4 system of Agrobacterium tumefaciens are considered to be the paradigm of type IV secretion, while type I (type IVB) T4SSs are found in intracellular bacterial pathogens, Legionella pneumophila and Coxiella burnetii. Several novel T4SSs have been identified recently and their functions await investigation. The most recently described GI type T4SSs play a key role in the horizontal transfer of a wide variety of genomic islands derived from a broad spectrum of bacterial strains.

The Legionella pneumophila IcmSW complex interacts with multiple Dot/Icm effectors to facilitate type IV translocation

Many Gram-negative pathogens use a type IV secretion system (T4SS) to deliver effector proteins into eukaryotic host cells. The fidelity of protein translocation depends on the efficient recognition of effector proteins by the T4SS. Legionella pneumophila delivers a large number of effector proteins into eukaryotic cells using the Dot/Icm T4SS. How the Dot/Icm system is able to recognize and control the delivery of effectors is poorly understood. Recent studies suggest that the IcmS and IcmW proteins interact to form a stable complex that facilitates translocation of effector proteins by the Dot/Icm system by an unknown mechanism. Here we demonstrate that the IcmSW complex is necessary for the productive translocation of multiple Dot/Icm effector proteins. Effector proteins that were able to bind IcmSW in vitro required icmS and icmW for efficient translocation into eukaryotic cells during L. pneumophila infection. We identified regions in the effector protein SidG involved in icmSW-dependent translocation. Although the full-length SidG protein was translocated by an icmSW-dependent mechanism, deletion of amino terminal regions in the SidG protein resulted in icmSW-independent translocation, indicating that the IcmSW complex is not contributing directly to recognition of effector proteins by the Dot/Icm system. Biochemical and genetic studies showed that the IcmSW complex interacts with a central region of the SidG protein. The IcmSW interaction resulted in a conformational change in the SidG protein as determined by differences in protease sensitivity in vitro. These data suggest that IcmSW binding to effectors could enhance effector protein delivery by mediating a conformational change that facilitates T4SS recognition of a translocation domain located in the carboxyl region of the effector protein.

Intracellular pathogens often manipulate the activities of the eukaryotic host cell in which they reside by using a specialized transport apparatus known as a type IV secretion system to deliver proteins that directly manipulate host cell processes. How proteins to be delivered into eukaryotic cells are recognized by a type IV section system is not well understood. For Legionella pneumophila, the bacterium that causes a severe pneumonia known as Legionnaires disease, a type IV system called Dot/Icm is used to deliver an estimated 150 different proteins into host cells during infection. In this study, we demonstrate that a complex consisting of the proteins IcmS and IcmW bind many of the substrate proteins transported into eukaryotic host cells by the Dot/Icm system. Binding of the IcmSW complex to Dot/Icm substrate proteins enhanced the efficiency by which the substrate proteins were transported into cells by a process that involved altering the conformation of the substrate protein. Thus, this work defines a step that is important for the type IV secretion process and provides new molecular details on substrate protein recognition by type IV secretion systems.

Host cell processes that influence the intracellular survival of Legionella pneumophila

Key to the pathogenesis of intracellular pathogens is their ability to manipulate host cell processes, permitting the establishment of an intracellular replicative niche. In turn, the host cell deploys defence mechanisms that limit intracellular infection. The bacterial pathogen Legionella pneumophila, the aetiological agent of Legionnaire's Disease, has evolved virulence mechanisms that allow it to replicate within protozoa, its natural host. Many of these tactics also enable L. pneumophila's survival and replication inside macrophages within a membrane-bound compartment known as the Legionella-containing vacuole. One of the virulence factors indispensable for L. pneumophila's intracellular survival is a type IV secretion system, which translocates a large repertoire of bacterial effectors into the host cell. These effectors modulate multiple host cell processes and in particular, redirect trafficking of the L. pneumophila phagosome and mediate its conversion into an ER-derived organelle competent for intracellular bacterial replication. In this review, we discuss how L. pneumophila manipulates host cells, as well as host cell processes that either facilitate or impede its intracellular survival.

Molecular mechanisms of host-pathogen interactions and their potential for the discovery of new drug targets

Vaccines and chemotherapy have undeniably been the discoveries in the field of biomedical research that have exerted the biggest impact on the improvement of public health. Nevertheless, the development of bacterial resistance to antibiotics has co-evolved over time with the discovery of new drugs. This entails the necessity for continuous research on new anti-infectious agents. The current review highlights recent discoveries in the molecular mechanisms of specific host pathogen interactions and their potential for drug discovery. The focus is on facultative and obligate intracellular pathogens (Mycobacterium, Chlamydia and Legionella) and their manipulation of host cells in regard to inhibition of phagosome maturation and cell death. Furthermore, the composition and role of the SecA2 and the ESX-1 secretion pathways in bacterial virulence and manipulation of infected host cells is discussed. The central hypothesis proposed in this review is that the characterization of bacterial proteins and lipids involved in host cell manipulation (modulins) will provide an abundance of new drug targets. One advantage of targeting such bacterial modulins for drug development is that these anti-modulin drugs will not disrupt the beneficial host microflora and therefore have fewer side effects.

The response regulator CpxR directly regulates expression of several Legionella pneumophila icm/dot components as well as new translocated substrates

Legionella pneumophila has been shown to utilize the icm/dot type IV secretion system for pathogenesis. This system was shown to be composed of icm/dot complex components and accessory proteins, as well as a large number of translocated substrates. Bioinformatic analysis of the regulatory regions of all the genes revealed that several icm/dot genes, as well as two genes encoding icm/dot translocated substrates, contain the conserved CpxR regulatory element, a regulator that has been shown previously to control the expression of the icmR gene. An experimental analysis, which included a comparison of gene expression in a L. pneumophila wild-type strain and gene expression in a cpxR deletion mutant, construction of mutants with mutations in the CpxR conserved regulatory elements, controlled expression studies, and mobility shift assays, demonstrated the direct relationship between the CpxR regulator and the expression of the genes. Furthermore, genomic analysis identified nine additional genes that contain a putative CpxR regulatory element; five of these genes (two legA genes and three ceg genes) were suggested previously to be putative icm/dot translocated substrates. The three ceg genes identified, which were shown previously to contain a putative PmrA regulatory element, were found here to be regulated by both CpxR and PmrA. The other six genes (two legA genes and four new genes products were found to be regulated by CpxR. Moreover, using the CyaA translocation assay, these nine gene products were found to be translocated into host cells in an Icm/Dot-dependent manner. Our results establish that the CpxR regulator is a fundamental regulator of the icm/dot type IV secretion system in L. pneumophila.

The type II secretion system of Legionella pneumophila elaborates two aminopeptidases, as well as a metalloprotease that contributes to differential infection among protozoan hosts

Legionella pneumophila, the agent of Legionnaires' disease, is an intracellular parasite of aquatic amoebae and human macrophages. A key factor for L. pneumophila in intracellular infection is its type II protein secretion system (Lsp). In order to more completely define Lsp output, we recently performed a proteomic analysis of culture supernatants. Based upon the predictions of that analysis, we found that L. pneumophila secretes two distinct aminopeptidase activities encoded by the genes lapA and lapB. Whereas lapA conferred activity against leucine, phenylalanine, and tyrosine aminopeptides, lapB was linked to the cleavage of lysine- and arginine-containing substrates. To assess the role of secreted aminopeptidases in intracellular infection, we examined the relative abilities of lapA and lapB mutants to infect human U937 cell macrophages as well as Hartmannella vermiformis and Acanthamoeba castellanii amoebae. Although these experiments identified a dispensable role for LapA and LapB, they uncovered a previously unrecognized role for the type II-dependent ProA (MspA) metalloprotease. Whereas proA mutants were not defective for macrophage or A. castellanii infection, they (but not their complemented derivatives) were impaired for growth upon coculture with H. vermiformis. Thus, ProA represents the first type II effector implicated in an intracellular infection event. Furthermore, proA represents an L. pneumophila gene that shows differential importance among protozoan infection models, suggesting that the legionellae might have evolved some of its factors to especially target certain of their protozoan hosts.

Effector proteins translocated by Legionella pneumophila: strength in numbers

The Gram-negative bacterium Legionella pneumophila is a parasite of eukaryotic cells. It has evolved to survive and replicate in a wide range of protozoan hosts and can also infect human alveolar macrophages as an opportunistic pathogen. Crucially for the infection process, L. pneumophila uses a type IV secretion system called Dot/Icm to translocate bacterial proteins into host cells. In recent years a large number of Dot/Icm-translocated proteins have been identified. The study of these proteins, referred to as effectors, is providing valuable insight into the mechanism by which an intracellular pathogen can manipulate eukaryotic cellular processes to traffic and replicate in host cells.

A bacterial ecto-triphosphate diphosphohydrolase similar to human CD39 is essential for intracellular multiplication of Legionella pneumophila

As part of its pathogenesis, Legionella pneumophila persists within human alveolar macrophages in non-acidified organelles that do not mature into phagolysosomes. Two L. pneumophila genes, lpg0971 and lpg1905, are predicted to encode ecto-nucleoside triphosphate diphosphohydrolases (ecto-NTPDases) that share sequence similarity with human CD39/NTPDase1. The predicted products possess five apyrase conserved domains that are typical of eukaryotic ecto-NTPDases. In this study, we found that an lpg1905 mutant was recovered in lower numbers from macrophages, alveolar epithelial cells and the amoeba, Hartmannella vermiformis compared with wild-type L. pneumophila and an lpg0971 mutant. Similar to human CD39, recombinant purified Lpg1905 exhibited ATPase and ADPase activity and possessed the ability to inhibit platelet aggregation. Mutation of a conserved Glu159 residue that is essential for CD39 activity inhibited ATPase and ADPase activity of Lpg1905. In addition, enzyme activity was inhibited in the presence of the specific ecto-NTPDase inhibitor, ARL67156. The entry and replication defect of the lpg1905 mutant was reversed upon transcomplementation with lpg1905 but not lpg1905E159A encoding an enzymatically inactive form of the protein. Although several protozoan parasites exhibit ecto-NTPDase activity, including Toxoplasma gondii, Trichomonas vaginalis and Trypanosoma cruzi, this is the first time a bacterial ecto-NTPDase has been implicated in virulence.