Legionella pneumophila exploits PI(4)P to anchor secreted effector proteins to the replicative vacuole

Interactions of legionella effector proteins with host phosphoinositide lipids

By means of the Icm/Dot type IV secretion system Legionella pneumophila translocates several effector proteins into host cells, where they anchor to the cytoplasmic face of the LCV membrane by binding to phosphoinositide (PI) lipids. Thus, phosphatidylinositol-4-phosphate anchors the effector proteins SidC and SidM, which promote the interaction of LCVs with the ER and the secretory vesicle trafficking -pathway. In this chapter, we describe protocols to (1) identify PI-binding proteins in Legionella lysates using PI-beads, (2) determine PI-binding specificities and affinities of recombinant Legionella effector proteins by protein-lipid overlays, and (3) use Legionella effectors to identify cellular PI lipids.

Quantitative analysis of phagocytosis and phagosome maturation

Phagocytosis and phagosome maturation lead to killing and digestion of bacteria by protozoans and innate immune phagocytes. Phagocytosis of particles expressing or coupled to various fluorescent reporters and sensors can be used to monitor quantitatively various parameters of this central biological process. In this chapter we detail different labeling techniques of bacteria and latex beads used to measure adhesion and uptake by FACS analysis. We also describe methods to use fluorescent reporter dyes (FITC or DQgreen) coupled to silica beads to measure the kinetics of acidification and proteolysis. Measurements can be performed either at the single-cell level, using live microscopy, or for a whole cell population, with a fluorescence microplate reader.

Phosphoinositide lipids and the Legionella pathogen vacuole

Subversion of vesicle trafficking is vital for intracellular survival of Legionella pneumophila within host cells. L. pneumophila produces several type IV-translocated effector proteins that modify components of the phagosomal membrane, in particular the phosphoinositide (PI) lipids. Within eukaryotic cells PIs co-define subcellular compartments and membrane dynamics. The generation, half-life, and localization of PI lipids are not only tightly regulated by the host cell, but also targeted and modulated by a number of L. pneumophila effectors. These effectors either anchor to PIs, directly modify the lipids, or recruit PI-metabolizing enzymes to the LCV membrane. Together, PI-subverting L. pneumophila effectors act jointly to promote the formation of a replication-permissive niche inside the host.

Cytotoxic glucosyltransferases of Legionella pneumophila

Legionella is a gram-negative bacterium and the causative pathogen of legionellosis-a severe pneumonia in humans. A large number of Legionella effectors interfere with numerous host cell functions, including intracellular vacuole trafficking and maturation, phospholipid metabolism, protein ubiquitination, pro-/anti-apoptotic balances or inflammatory responses. Moreover, eukaryotic protein synthesis is affected by L. pneumophila glucosyltransferases Lgt1, Lgt2, and Lgt3. Structurally, these enzymes are similar to large clostridial cytotoxins, use UDP-glucose as a co-substrate and modify a conserved serine residue (Ser-53) in elongation factor 1A (eEF1A). The ternary complex consisting of eEF1A, GTP, and aminoacylated-tRNA seems to be the substrate for Lgts. Studies with Saccharomyces cerevisiae corroborated that eEF1A is the major target responsible for Lgt-induced cytotoxic activity. In addition to Lgt proteins, Legionella produces other effector glycosyltransferase, including the modularly composed protein SetA, which displays tropism for early endosomal compartments, subverts host cell vesicle trafficking and demonstrates toxic activities toward yeast and mammalian cells. Here, our current knowledge about both groups of L. pneumophila glycosylating effectors is reviewed.

Identification of two Legionella pneumophila effectors that manipulate host phospholipids biosynthesis

The intracellular pathogen Legionella pneumophila translocates a large number of effector proteins into host cells via the Icm/Dot type-IVB secretion system. Some of these effectors were shown to cause lethal effect on yeast growth. Here we characterized one such effector (LecE) and identified yeast suppressors that reduced its lethal effect. The LecE lethal effect was found to be suppressed by the over expression of the yeast protein Dgk1 a diacylglycerol (DAG) kinase enzyme and by a deletion of the gene encoding for Pah1 a phosphatidic acid (PA) phosphatase that counteracts the activity of Dgk1. Genetic analysis using yeast deletion mutants, strains expressing relevant yeast genes and point mutations constructed in the Dgk1 and Pah1 conserved domains indicated that LecE functions similarly to the Nem1-Spo7 phosphatase complex that activates Pah1 in yeast. In addition, by using relevant yeast genetic backgrounds we examined several L. pneumophila effectors expected to be involved in phospholipids biosynthesis and identified an effector (LpdA) that contains a phospholipase-D (PLD) domain which caused lethal effect only in a dgk1 deletion mutant of yeast. Additionally, LpdA was found to enhance the lethal effect of LecE in yeast cells, a phenomenon which was found to be dependent on its PLD activity. Furthermore, to determine whether LecE and LpdA affect the levels or distribution of DAG and PA in-vivo in mammalian cells, we utilized fluorescent DAG and PA biosensors and validated the notion that LecE and LpdA affect the in-vivo levels and distribution of DAG and PA, respectively. Finally, we examined the intracellular localization of both LecE and LpdA in human macrophages during L. pneumophila infection and found that both effectors are localized to the bacterial phagosome. Our results suggest that L. pneumophila utilize at least two effectors to manipulate important steps in phospholipids biosynthesis.

Legionella pneumophila is an intracellular pathogen that causes a severe pneumonia known as Legionnaires' disease. Following infection, the bacteria use a Type-IVB secretion system to translocate multiple effector proteins into macrophages and generate the Legionella-containing vacuole (LCV). The formation of the LCV involves the recruitment of specific bacterial effectors and host cell factors to the LCV as well as changes in its lipids composition. By screening L. pneumophila effectors for yeast growth inhibition, we have identified an effector, named LecE, that strongly inhibits yeast growth. By using yeast genetic tools, we found that LecE activates the yeast lipin homolog – Pah1, an enzyme that catalyzes the conversion of diacylglycerol to phosphatidic acid, these two molecules function as bioactive lipid signaling molecules in eukaryotic cells. In addition, by using yeast deletion mutants in genes relevant to lipids biosynthesis, we have identified another effector, named LpdA, which function as a phospholipase-D enzyme. Both effectors were found to be localized to the LCV during infection. Our results reveal a possible mechanism by which an intravacuolar pathogen might change the lipid composition of the vacuole in which it resides, a process that might lead to the recruitment of specific bacterial and host cell factors to the vacoule.

Evaluation of phosphatidylinositol-4-kinase IIIα as a hepatitis C virus drug target

Phosphatidylinositol-4-kinase IIIα (PI4KIIIα) is an essential host cell factor for hepatitis C virus (HCV) replication. An N-terminally truncated 130-kDa form was used to reconstitute an in vitro biochemical lipid kinase assay that was optimized for small-molecule compound screening and identified potent and specific inhibitors. Cell culture studies with PI4KIIIα inhibitors demonstrated that the kinase activity was essential for HCV RNA replication. Two PI4KIIIα inhibitors were used to select cell lines harboring HCV replicon mutants with a 20-fold loss in sensitivity to the compounds. Reverse genetic mapping isolated an NS4B-NS5A segment that rescued HCV RNA replication in PIK4IIIα-deficient cells. HCV RNA replication occurs on specialized membranous webs, and this study with PIK4IIIα inhibitor-resistant mutants provides a genetic link between NS4B/NS5A functions and PI4-phosphate lipid metabolism. A comprehensive assessment of PI4KIIIα as a drug target included its evaluation for pharmacologic intervention in vivo through conditional transgenic murine lines that mimic target-specific inhibition in adult mice. Homozygotes that induce a knockout of the kinase domain or knock in a single amino acid substitution, kinase-defective PI4KIIIα, displayed a lethal phenotype with a fairly widespread mucosal epithelial degeneration of the gastrointestinal tract. This essential host physiologic role raises doubt about the pursuit of PI4KIIIα inhibitors for treatment of chronic HCV infection.

Structural basis for substrate recognition by a unique Legionella phosphoinositide phosphatase

Legionella pneumophila is an opportunistic intracellular pathogen that causes sporadic and epidemic cases of Legionnaires' disease. Emerging data suggest that Legionella infection involves the subversion of host phosphoinositide (PI) metabolism. However, how this bacterium actively manipulates PI lipids to benefit its infection is still an enigma. Here, we report that the L. pneumophila virulence factor SidF is a phosphatidylinositol polyphosphate 3-phosphatase that specifically hydrolyzes the D3 phosphate of PI(3,4)P(2) and PI(3,4,5)P(3). This activity is necessary for anchoring of PI(4)P-binding effectors to bacterial phagosomes. Crystal structures of SidF and its complex with its substrate PI(3,4)P(2) reveal striking conformational rearrangement of residues at the catalytic site to form a cationic pocket that specifically accommodates the D4 phosphate group of the substrate. Thus, our findings unveil a unique Legionella PI phosphatase essential for the establishment of lipid identity of bacterial phagosomes.

Lipids at the interface of virus-host interactions

Viruses physically and metabolically remodel the host cell to establish an optimal environment for their replication. Many of these processes involve the manipulation of lipid signaling, synthesis, and metabolism. An emerging theme is that these lipid-modifying pathways are also linked to innate antiviral responses and can be modulated to inhibit viral replication.

Bacterial Type IV secretion systems: versatile virulence machines

Many bacterial pathogens employ multicomponent protein complexes to deliver macromolecules directly into their eukaryotic host cell to promote infection. Some Gram-negative pathogens use a versatile Type IV secretion system (T4SS) that can translocate DNA or proteins into host cells. T4SSs represent major bacterial virulence determinants and have recently been the focus of intense research efforts designed to better understand and combat infectious diseases. Interestingly, although the two major classes of T4SSs function in a similar manner to secrete proteins, the translocated 'effectors' vary substantially from one organism to another. In fact, differing effector repertoires likely contribute to organism-specific host cell interactions and disease outcomes. In this review, we discuss the current state of T4SS research, with an emphasis on intracellular bacterial pathogens of humans and the diverse array of translocated effectors used to manipulate host cells.

Legionella pneumophila pathogenesis in the Galleria mellonella infection model

Legionella pneumophila is a facultative intracellular human pathogen and the etiological agent of severe pneumonia known as Legionnaires' disease. Its virulence depends on protein secretion systems, in particular, the Dot/Icm type IV secretion system (T4SS), which is essential to establish a replication-permissive vacuole in macrophages. The analysis of the role of these systems and their substrates for pathogenesis requires easy-to-use models which approximate human infection. We examined the effectiveness of the larvae of the wax moth Galleria mellonella as a new model for L. pneumophila infection. We found that the L. pneumophila strains 130b, Paris, and JR32 caused mortality of the G. mellonella larvae that was strain, infectious dose, growth phase, and T4SS dependent. Wild-type L. pneumophila persisted and replicated within the larvae, whereas T4SS mutants were rapidly cleared. L. pneumophila strain Lp02, which is attenuated in the absence of thymidine but has a functional T4SS, resisted clearance in G. mellonella up to 18 h postinfection without inducing mortality. Immunofluorescence and transmission electron microscopy revealed that L. pneumophila resided within insect hemocytes in a vacuole that ultrastructurally resembled the Legionella-containing vacuole (LCV) observed in macrophages. The vacuole was decorated with the T4SS effector and LCV marker SidC. Infection caused severe damage to the insect organs and triggered immune responses, including activation of the phenoloxidase cascade leading to melanization, nodule formation, and upregulation of antimicrobial peptides. Taken together, these results suggest that G. mellonella provides an effective model to investigate the interaction between L. pneumophila and the host.

Exploitation of evolutionarily conserved amoeba and mammalian processes by Legionella

Legionella pneumophila proliferates within various protists and metazoan cells, where a cadre of ∼300 effectors is injected into the host cell by the defect in organelle trafficking/intracellular multiplication (Dot/Icm) type IVB translocation system. Interkingdom horizontal gene transfer of genes of protists and their subsequent convergent evolution to become translocated effectors has probably enabled L. pneumophila to adapt to the intracellular life within various protists and metazoan cells through exploitation of evolutionarily eukaryotic processes, such as endoplasmic reticulum-to-Golgi vesicle traffic, phosphoinositol metabolism, AMPylation, deAMPylation, prenylation, polyubiquitination, proteasomal degradation and cytosolic amino- and oligo-peptidases. This is highlighted by the ankyrin B (AnkB) F-box effector that exploits multiple conserved eukaryotic machineries to generate high levels of free amino acids as sources of carbon and energy essential for intracellular proliferation in protists and metazoan cells and for manifestation of pulmonary disease in mammals.

Secretive bacterial pathogens and the secretory pathway

Eukaryotic cells possess two extensive endomembrane systems, each consisting of several sub-compartments connected by vesicular trafficking. One of these systems, the endocytic pathway, serves incoming traffic, and the other system, the secretory pathway (SP), is responsible for surface-bound traffic of intracellularly formed vesicles. Compartments derived of either system can be colonized by intracellular pathogens. In this review, we discuss the interactions between the SP and prominent intracellular bacterial pathogens of the genera Legionella, Brucella, Chlamydia and Salmonella. We emphasize secreted bacterial effector proteins, which directly manipulate host components of this pathway.

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.

Legionella pneumophila secretes a mitochondrial carrier protein during infection

The Mitochondrial Carrier Family (MCF) is a signature group of integral membrane proteins that transport metabolites across the mitochondrial inner membrane in eukaryotes. MCF proteins are characterized by six transmembrane segments that assemble to form a highly-selective channel for metabolite transport. We discovered a novel MCF member, termed Legionellanucleotide carrier Protein (LncP), encoded in the genome of Legionella pneumophila, the causative agent of Legionnaire's disease. LncP was secreted via the bacterial Dot/Icm type IV secretion system into macrophages and assembled in the mitochondrial inner membrane. In a yeast cellular system, LncP induced a dominant-negative phenotype that was rescued by deleting an endogenous ATP carrier. Substrate transport studies on purified LncP reconstituted in liposomes revealed that it catalyzes unidirectional transport and exchange of ATP transport across membranes, thereby supporting a role for LncP as an ATP transporter. A hidden Markov model revealed further MCF proteins in the intracellular pathogens, Legionella longbeachae and Neorickettsia sennetsu, thereby challenging the notion that MCF proteins exist exclusively in eukaryotic organisms.

Mitochondrial carrier proteins evolved during endosymbiosis to transport substrates across the mitochondrial inner membrane. As such the proteins are associated exclusively with eukaryotic organisms. Despite this, we identified putative mitochondrial carrier proteins in the genomes of different intracellular bacterial pathogens, including Legionella pneumophila, the causative agent of Legionnaire's disease. We named the mitochondrial carrier protein from L. pneumophila LncP and determined that the protein is translocated into host cells during infection by the bacterial Dot/Icm type IV secretion system. From there, LncP accesses the classical mitochondrial import pathway and is incorporated into the mitochondrial inner membrane as an integral membrane protein. Remarkably, LncP crosses five biological membranes to reach its final location. Biochemically, LncP is a unidirectional nucleotide transporter similar to Aac1 in yeast. Although not essential for intracellular replication, the high carriage rate of lncP among isolates of L. pneumophila suggests that the ability of the pathogen to manipulate mitochondrial ATP transport assists survival of the bacteria in an intracellular environment.

Role of lipids in Coxiella burnetii infection

Lipids are essential components of both eukaryotic and prokaryotic cells, serving diverse functions including energy metabolism and membrane structure. Intracellular vacuolar pathogens such as Coxiella burnetii require lipids for both normal bacterial functions as well as formation of the acidic, phagolysosomal-like parasitophorous vacuole (PV) surrounding the bacteria. As an intracellular pathogen, C. burnetii can acquire lipid through both de novo bacterial synthesis and subversion of host cell pools. The C. burnetii genome encodes enzymes required for de novo synthesis of fatty acids and phospholipids. The high percentage of branched fatty acids suggests C. burnetii modifies these molecules to generate a bacterial cell envelope that can resist the harsh environment of the PV, such as the acidic pH. In addition to fatty acids and their derivatives, C. burnetii requires isoprenoids, particularly sterols as the PV membrane is cholesterol-rich. With the exception of two eukaryote-like sterol reductases, C. burnetii does not have the capability to generate cholesterol, suggesting sterols are actively diverted from the host cell. While C. burnetii utilizes host cell lipids for membrane biogenesis and possibly energy, bacterial manipulation of host cell lipid signaling pathways may support establishment of the intracellular niche. For example, effectors secreted by the C. burnetii Type IV secretion system may either directly or indirectly modify host cell lipids. Further understanding of the lipid biosynthetic capabilities of C. burnetii, along with C. burnetii's manipulation of host cell lipids, will provide insight into the host-pathogen relationship.

Identification of lipid-binding effectors

In recent years, the functional roles of effectors from a wide variety of fungal and oomycete pathogens have begun to emerge. As a product of this work, the importance of effector-lipid interactions has been made apparent. Phospholipids are not only important signaling molecules, but they also play important roles in the trafficking of endosomes and the localization of proteins. Characterizing effector-lipid interactions can provide novel information regarding the functions of effectors relevant to their cellular and subcellular targeting and their potential effects on host signaling and vesicle trafficking. We present here two techniques that can be used to screen for and validate protein-lipid interactions without the need to access highly specialized machinery. We describe in detail how to perform lipid filter and liposome-binding assays and provide suggestions for troubleshooting potential problems with these assays.

Legionella secreted effectors and innate immune responses

Legionella pneumophila is a facultative intracellular pathogen capable of replicating in a wide spectrum of cells. Successful infection by Legionella requires the Dot/Icm type IV secretion system, which translocates a large number of effector proteins into infected cells. By co-opting numerous host cellular processes, these proteins function to establish a specialized organelle that allows bacterial survival and proliferation. Even within the vacuole, L. pneumophila triggers robust immune responses. Recent studies reveal that a subset of Legionella effectors directly target some basic components of the host innate immunity systems such as phagosome maturation. Others play essential roles in engaging the host innate immune surveillance system. This review will highlight recent progress in our understanding of these interactions and discuss implications for the study of the immune detection mechanisms.

RpkA, a highly conserved GPCR with a lipid kinase domain, has a role in phagocytosis and anti-bacterial defense

RpkA (Receptor phosphatidylinositol kinase A) is an unusual seven-helix transmembrane protein of Dictyostelium discoideum with a G protein coupled receptor (GPCR) signature and a C-terminal lipid kinase domain (GPCR-PIPK) predicted as a phosphatidylinositol-4-phosphate 5-kinase. RpkA-homologs are present in all so far sequenced Dictyostelidae as well as in several other lower eukaryotes like the oomycete Phytophthora, and in the Legionella host Acanthamoeba castellani. Here we show by immunofluorescence that RpkA localizes to endosomal membranes and is specifically recruited to phagosomes. RpkA interacts with the phagosomal protein complex V-ATPase as proteins of this complex co-precipitate with RpkA-GFP as well as with the GST-tagged PIPK domain of RpkA. Loss of RpkA leads to a defect in phagocytosis as measured by yeast particle uptake. The uptake of the pathogenic bacterium Legionella pneumophila was however unaltered whereas its intra-cellular replication was significantly enhanced in rpkA^-. The difference between wild type and rpkA^- was even more prominent when L. hackeliae was used. When we investigated the reason for the enhanced susceptibility for L. pneumophila of rpkA^- we could not detect a difference in endosomal pH but rpkA^- showed depletion of phosphoinositides (PIP and PIP₂) when we compared metabolically labeled phosphoinositides from wild type and rpkA^-. Furthermore rpkA^- exhibited reduced nitrogen starvation tolerance, an indicator for a reduced autophagy rate. Our results indicate that RpkA is a component of the defense system of D. discoideum as well as other lower eukaryotes.

Comparative and functional genomics of legionella identified eukaryotic like proteins as key players in host-pathogen interactions

Although best known for its ability to cause severe pneumonia in people whose immune defenses are weakened, Legionella pneumophila and Legionella longbeachae are two species of a large genus of bacteria that are ubiquitous in nature, where they parasitize protozoa. Adaptation to the host environment and exploitation of host cell functions are critical for the success of these intracellular pathogens. The establishment and publication of the complete genome sequences of L. pneumophila and L. longbeachae isolates paved the way for major breakthroughs in understanding the biology of these organisms. In this review we present the knowledge gained from the analyses and comparison of the complete genome sequences of different L. pneumophila and L. longbeachae strains. Emphasis is given on putative virulence and Legionella life cycle related functions, such as the identification of an extended array of eukaryotic like proteins, many of which have been shown to modulate host cell functions to the pathogen’s advantage. Surprisingly, many of the eukaryotic domain proteins identified in L. pneumophila as well as many substrates of the Dot/Icm type IV secretion system essential for intracellular replication are different between these two species, although they cause the same disease. Finally, evolutionary aspects regarding the eukaryotic like proteins in Legionella are discussed.

Salmonella typhimurium is pathogenic for Dictyostelium cells and subverts the starvation response

In unicellular amoebae, such as Dictyostelium discoideum, bacterial phagocytosis is a food hunting device, while in higher organisms it is the first defence barrier against microbial infection. In both cases, pathogenic bacteria exploit phagocytosis to enter the cell and multiply intracellularly. Salmonella typhimurium, the agent of food-borne gastroenteritis, is phagocytosed by both macrophages and Dictyostelium cells. By using cell biological assays and global transcriptional analysis with DNA microarrays covering the Dictyostelium genome, we show here that S. typhimurium is pathogenic for Dictyostelium cells. Depending on the degree of virulence, which in turn depended on bacterial growth conditions, Salmonella could kill Dictyostelium cells or inhibit their growth and development. In the early phase of infection in non-nutrient buffer, the ingested bacteria escaped degradation, induced a starvation-like transcriptional response but inhibited selectively genes required for chemotaxis and aggregation. This way differentiation of the host cells into spore and stalk cells was blocked or delayed, which in turn is likely to be favourable for the establishment of a replicative niche for Salmonella. Inhibition of the aggregation competence and chemotactic streaming of aggregation-competent cells in the presence of Salmonella suggests interference with cAMP signalling.

The professional phagocyte Dictyostelium discoideum as a model host for bacterial pathogens

The use of simple hosts such as Dictyostelium discoideum in the study of host pathogen interactions offers a number of advantages and has steadily increased in recent years. Infection-specific genes can often only be studied in a very limited way in man and even in the mouse model their analysis is usually expensive, time consuming and technically challenging or sometimes even impossible. In contrast, their functional analysis in D. discoideum and other simple model organisms is often easier, faster and cheaper. Because host-pathogen interactions necessarily involve two organisms, it is desirable to be able to genetically manipulate both the pathogen and its host. Particularly suited are those hosts, like D. discoideum, whose genome sequence is known and annotated and for which excellent genetic and cell biological tools are available in order to dissect the complex crosstalk between host and pathogen. The review focusses on host-pathogen interactions of D. discoideum with Legionella pneumophila, mycobacteria, and Salmonella typhimurium which replicate intracellularly.

Membrane-trafficking sorting hubs: cooperation between PI4P and small GTPases at the trans-Golgi network

Cell polarity in eukaryotes requires constant sorting, packaging and transport of membrane-bound cargo within the cell. These processes occur in two sorting hubs: the recycling endosome for incoming material and the trans-Golgi network for outgoing material. Phosphatidylinositol 3-phosphate and phosphatidylinositol 4-phosphate are enriched at the endocytic and exocytic sorting hubs, respectively, where they act together with small GTPases to recruit factors to segregate cargo and regulate carrier formation and transport. In this review, we summarize the current understanding of how these lipids and GTPases regulate membrane trafficking directly, emphasizing the recent discoveries of phosphatidylinositol 4-phosphate functions at the trans-Golgi network.

Manipulation of host membranes by bacterial effectors

Bacterial pathogens interact with host membranes to trigger a wide range of cellular processes during the course of infection. These processes include alterations to the dynamics between the plasma membrane and the actin cytoskeleton, and subversion of the membrane-associated pathways involved in vesicle trafficking. Such changes facilitate the entry and replication of the pathogen, and prevent its phagocytosis and degradation. In this Review, we describe the manipulation of host membranes by numerous bacterial effectors that target phosphoinositide metabolism, GTPase signalling and autophagy.

Endosomal and secretory markers of the Legionella-containing vacuole

The Gram-negative opportunistic pathogen Legionella pneumophila replicates in phagocytes within a specific compartment, the Legionella-containing vacuole (LCV). Formation of LCVs is a complex process requiring the bacterial Icm/Dot type IV secretion system and more than 100 translocated effector proteins, which putatively subvert cellular signaling and vesicle trafficking pathways. Phosphoinositide (PI) glycerolipids are pivotal regulators of signal transduction and membrane dynamics in eukaryotes. Recently, a number of Icm/Dot substrates were found to anchor to the LCV membrane by binding to PIs. One of these effectors, SidC, specifically interacts with phosphatidylinositol-4 phosphate [PtdIns(4)P]. Using an antibody against SidC and magnetic beads coupled to a secondary antibody, intact LCVs were purified by immuno-magnetic separation, followed by density centrifugation. This purification strategy is in principle applicable to any pathogen vacuole that carries specific markers. The LCV proteome determined by LC-MS/MS revealed 566 host proteins, including novel components of the endosomal pathway, as well as the early and late secretory trafficking pathways. Thus, LCV formation is a robust process that involves many (functionally redundant) Icm/Dot substrates, as well as the interaction with different host cell vesicle trafficking pathways.

Legionella pneumophila infection is enhanced in a RacH-null mutant of Dictyostelium

Recently we reported that Dictyostelium cells ingest Legionella pneumophila by macropinocytosis, whereas other bacteria, such as Escherichia coli, Mycobacterium avium, Neisseria meningitidis or Salmonella typhimurium, are taken up by phagocytosis.1 In contrast to phagocytosis, macropinocytosis is partially inhibited by PI3K or PTEN inactivation, whereas both processes are sensitive to PLC inhibition. Independently from reduced uptake, L. pneumophila proliferates more efficiently in PI3K-null than in wild-type cells. PI3K inactivation also neutralizes resistance to infection conferred by constitutively expressing the endo-lysosomal iron transporter Nramp1. We have shown this to be due to altered recruitment of the V-H(+) ATPase, but not Nramp1, in the Legionella-containing vacuole (LCV) early during infection.1 As further evidence for impaired LCV acidification we examine here the effects of disrupting the small G protein RacH on Legionella infection.

Global cellular changes induced by Legionella pneumophila infection of bone marrow-derived macrophages

The nucleotide-binding oligomerization domain (Nod)-like receptor (NLR) family member Naip5 plays an essential role in restricting Legionella pneumophila growth inside primary macrophages. Upon interaction with bacterial flagellin, the intracellular receptor Naip5 forms a multi-protein complex, the inflammasome, which activation has a protective role against infection. The A/J mouse strain carries a Naip5 allele (Naip5(A/J)), which renders its macrophages susceptible to Legionella infection. However, Naip5(A/J) is still competent for inflammasome activation suggesting that an as yet unidentified signaling pathway located downstream of Naip5 and defective in Naip5(A/J) macrophages regulates macrophage defenses against Legionella. Therefore, transcriptional profiling experiments with macrophages from C57BL/6J mice (B6), and from congenic mice (BcA75) carrying the partial loss-of-function A/J-derived allele (Naip5(A/J)) on a B6 background, infected or not with wild-type L. pneumophila or flagellin-deficient mutant were carried out to identify genes regulated by flagellin and by Naip5. Both the Legionella infection per se and the presence of flagellin had very strong effects on transcriptional responses of macrophages, 4h following infection, including modulation of cellular pathways associated with inflammatory response and cell survival. On the other hand, the presence of wild type or partial loss of function allele (Naip5(A/J)) at Naip5 did not cause large effects on transcriptional responses of macrophages to infection. We also examined in L. pneumophila infected macrophages, the effect of Naip5 alleles on expression and phosphorylation of 524 phosphoproteins, kinases and phosphatases involved in cell proliferation, immune response, stress and apoptosis. Naip5 alleles had an effect on the TLR-Il1R signaling pathway, the cell cycle and the caveolin-mediated response to pathogen. The results of transcriptome and proteome analyses were organized into cellular pathways in macrophages that are modulated in response to Legionella infection.

Legionella spp. outdoors: colonization, communication and persistence

Bacteria of the genus Legionella persist in a wide range of environmental habitats, including biofilms, protozoa and nematodes. Legionellaceae are 'accidental' human pathogens that upon inhalation cause a severe pneumonia termed 'Legionnaires' disease'. The interactions of L. pneumophila with eukaryotic hosts are governed by the Icm/Dot type IV secretion system (T4SS) and more than 150 'effector proteins', which subvert signal transduction pathways and promote the formation of the replication-permissive 'Legionella-containing vacuole'. The Icm/Dot T4SS is essential to infect free-living protozoa, such as the amoeba Dictyostelium discoideum, as well as the nematode Caenorhabditis elegans, or mammalian macrophages. To adapt to different niches, L. pneumophila not only responds to exogenous cues, but also to endogenous signals, such as the α-hydroxyketone compound LAI-1 (Legionella autoinducer-1). The long-term adaptation of Legionella spp. is based on extensive horizontal DNA transfer. In fact, Legionella spp. have acquired canonical 'genomic islands' of prokaryotic origin, but also a number of eukaryotic genes. Since many aspects of Legionella virulence against environmental predators and immune phagocytes are similar, an understanding of Legionella ecology provides valuable insights into the pathogenesis of legionellaceae for humans.

Anchors for effectors: subversion of phosphoinositide lipids by legionella

The facultative intracellular pathogen Legionella pneumophila replicates in free-living amoebae and macrophages within a distinct compartment, the “Legionella-containing vacuole” (LCV). LCV formation involves phosphoinositide (PI) glycerolipids, which are key factors controlling vesicle trafficking pathways and membrane dynamics of eukaryotic cells. To govern the interactions with host cells, L. pneumophila employs the Icm/Dot type IV secretion system and more than 250 translocated “effector proteins” that presumably subvert host signaling and vesicle trafficking pathways. Some of the effector proteins anchor through distinct PIs to the cytosolic face of LCVs and promote the interaction with host vesicles and organelles, catalyze guanine nucleotide exchange of small GTPases, or bind to PI-metabolizing enzymes, such as OCRL1. The PI 5-phosphatase OCRL1 and its Dictyostelium homologue Dd5P4 restrict intracellular growth of L. pneumophila. Moreover, OCRL1/Dd5P4, PI 3-kinases (PI3Ks), and PI4KIIIβ regulate LCV formation and localization of the effector protein SidC, which selectively decorates the LCV membrane. SidC and its 20-kDa “P4C” fragment are robust and specific probes for phosphatidylinositol-4-phosphate, and SidC can be targeted to purify intact LCVs by immuno-magnetic separation. Taken together, bacterial PI-binding effectors as well as host PIs and PI-modulating enzymes play a pivotal role for intracellular replication of L. pneumophila, and the PI-binding effectors are valuable tools for the analysis of eukaryotic PI lipids.

Campylobacter jejuni survival within human epithelial cells is enhanced by the secreted protein CiaI

Although it is known that Campylobacter jejuni invade the cells that line the human intestinal tract, the bacterial proteins that enable this pathogen to survive within Campylobacter-containing vacuoles (CCV) have not been identified. Here, we describe the identification and characterization of a protein that we termed CiaI for Campylobacter invasion antigen involved in intracellular survival. We show that CiaI harbours an amino-terminal type III secretion sequence and is secreted from C. jejuni through the flagellar type III secretion system. In addition, the ciaI mutant was impaired in intracellular survival when compared with a wild-type strain, as judged by the gentamicin-protection assay. Fluorescence microscopy examination of epithelial cells infected with the C. jejuni ciaI mutant revealed that the CCV were more frequently co-localized with Cathepsin D (a lysosomal marker) than the CCV in cells infected with a C. jejuni wild-type strain. Ectopic expression of CiaI-GFP in epithelial cells yielded a punctate phenotype not observed with the other C. jejuni genes, and this phenotype was abolished by mutation of a dileucine motif located in the carboxy-terminus of the protein. Based on the data, we conclude that CiaI contributes to the ability of C. jejuni to survive within epithelial cells.

Control of host cell phosphorylation by legionella pneumophila

Phosphorylation is one of the most frequent modifications in intracellular signaling and is implicated in many processes ranging from transcriptional control to signal transduction in innate immunity. Many pathogens modulate host cell phosphorylation pathways to promote growth and establish an infectious disease. The intracellular pathogen Legionella pneumophila targets and exploits the host phosphorylation system throughout the infection cycle as part of its strategy to establish an environment beneficial for replication. Key to this manipulation is the L. pneumophila Icm/Dot type IV secretion system, which translocates bacterial proteins into the host cytosol that can act directly on phosphorylation cascades. This review will focus on the different stages of L. pneumophila infection, in which host kinases and phosphatases contribute to infection of the host cell and promote intracellular survival of the pathogen. This includes the involvement of phosphatidylinositol 3-kinases during phagocytosis as well as the role of phosphoinositide metabolism during the establishment of the replication vacuole. Furthermore, L. pneumophila infection modulates the NF-κB and mitogen-activated protein kinase pathways, two signaling pathways that are central to the host innate immune response and involved in regulation of host cell survival. Therefore, L. pneumophila infection manipulates host cell signal transduction by phosphorylation at multiple levels.

Protein kinase LegK2 is a type IV secretion system effector involved in endoplasmic reticulum recruitment and intracellular replication of Legionella pneumophila

Legionella pneumophila is the etiological agent of Legionnaires' disease. Crucial to the pathogenesis of this intracellular pathogen is its ability to subvert host cell defenses, permitting intracellular replication in specialized vacuoles within host cells. The Dot/Icm type IV secretion system (T4SS), which translocates a large number of bacterial effectors into host cell, is absolutely required for rerouting the Legionella phagosome. Many Legionella effectors display distinctive eukaryotic domains, among which are protein kinase domains. In silico analysis and in vitro phosphorylation assays identified five functional protein kinases, LegK1 to LegK5, encoded by the epidemic L. pneumophila Lens strain. Except for LegK5, the Legionella protein kinases are all T4SS effectors. LegK2 plays a key role in bacterial virulence, as demonstrated by gene inactivation. The legK2 mutant containing vacuoles displays less-efficient recruitment of endoplasmic reticulum markers, which results in delayed intracellular replication. Considering that a kinase-dead substitution mutant of legK2 exhibits the same virulence defects, we highlight here a new molecular mechanism, namely, protein phosphorylation, developed by L. pneumophila to establish a replicative niche and evade host cell defenses.

Two small ncRNAs jointly govern virulence and transmission in Legionella pneumophila

To transit from intra- to extracellular environments, Legionella pneumophila differentiates from a replicative/non-virulent to a transmissive/virulent form using the two-component system LetA/LetS and the global repressor protein CsrA. While investigating how both regulators act co-ordinately we characterized two ncRNAs, RsmY and RsmZ, that link the LetA/LetS and CsrA regulatory networks. We demonstrate that LetA directly regulates their expression and show that RsmY and RsmZ are functional in Escherichia coli and are able to bind CsrA in vitro. Single mutants have no (ΔrsmY) or a little (ΔrsmZ) impact on virulence, but the ΔrsmYZ strain shows a drastic defect in intracellular growth in Acanthamoeba castellanii and THP-1 monocyte-derived macrophages. Analysis of the transcriptional programmes of the ΔletA, ΔletS and ΔrsmYZ strains revealed that the switch to the transmissive phase is partially blocked. One major difference between the ΔletA, ΔletS and ΔrsmYZ strains was that the latter synthesizes flagella. Taken together, LetA activates transcription of RsmY and RsmZ, which sequester CsrA and abolish its post-transcriptional repressive activity. However, the RsmYZ-CsrA pathway appears not to be the main or only regulatory circuit governing flagella synthesis. We suggest that rather RpoS and LetA, by influencing LetE and probably cyclic-di-GMP levels, regulate motility in L. pneumophila.

The yeast Arf GTPase-activating protein Age1 is regulated by phospholipase D for post-Golgi vesicular transport

Vesicular transport shuttles cargo among intracellular compartments. Several stages of vesicular transport are mediated by the small GTPase Arf, which is controlled in a cycle of GTP binding and hydrolysis by Arf guanine-nucleotide exchange factors and Arf GTPase-activating proteins (ArfGAPs), respectively. In budding yeast the Age2 + Gcs1 ArfGAP pair facilitates post-Golgi transport. We have found the AGE1 gene, encoding another ArfGAP, can in high gene-copy number alleviate the temperature sensitivity of cells carrying mutations affecting the Age2 + Gcs1 ArfGAP pair. Moreover, increased AGE1 gene dosage compensates for the complete absence of the otherwise essential Age2 + Gcs1 ArfGAP pair. Increased dosage of SFH2, encoding a phosphatidylinositol transfer protein, also allows cell growth in the absence of the Age2 + Gcs1 pair, but good growth in this situation requires Age1. The ability of Age1 to overcome the need for Age2 + Gcs1 depends on phospholipase D activity that regulates lipid composition. We show by direct assessment of Age1 ArfGAP activity that Age1 is regulated by lipid composition and can provide ArfGAP function for post-Golgi transport.

Phosphoinositides differentially regulate bacterial uptake and Nramp1-induced resistance to Legionella infection in Dictyostelium

Membrane phosphatidylinositides recruit cytosolic proteins to regulate phagocytosis, macropinocytosis and endolysosomal vesicle maturation. Here, we describe effects of inactivation of PI3K, PTEN or PLC on Escherichia coli and Legionella pneumophila uptake by the professional phagocyte Dictyostelium discoideum. We show that L. pneumophila is engulfed by macropinocytosis, a process that is partially sensitive to PI3K inactivation, unlike phagocytosis of E. coli. Both processes are blocked by PLC inhibition. Whereas E. coli is rapidly digested, Legionella proliferates intracellularly. Proliferation is blocked by constitutively expressing Nramp1, an endolysosomal iron transporter that confers resistance against invasive bacteria. Inactivation of PI3K, but not PTEN or PLC, enhances Legionella infection and suppresses the protective effect of Nramp1 overexpression. PI3K activity is restricted to early infection and is not mediated by effects on the actin cytoskeleton; rather L. pneumophila, in contrast to E. coli, subverts phosphoinositide-sensitive fusion of Legionella-containing macropinosomes with acidic vesicles, without affecting Nramp1 recruitment. A model is presented to explain how Legionella escapes fusion with acidic vesicles and Nramp1-induced resistance to pathogens.

Antibodies protect against intracellular bacteria by Fc receptor-mediated lysosomal targeting

The protective effect of antibodies (Abs) is generally attributed to neutralization or complement activation. Using Legionella pneumophila and Mycobacterium bovis bacillus Calmette-Guérin as a model, we discovered an additional mechanism of Ab-mediated protection effective against intracellular pathogens that normally evade lysosomal fusion. We show that Fc receptor (FcR) engagement by Abs, which can be temporally and spatially separated from bacterial infection, renders the host cell nonpermissive for bacterial replication and targets the pathogens to lysosomes. This process is strictly dependent on kinases involved in FcR signaling but not on host cell protein synthesis or protease activation. Based on these findings, we propose a mechanism whereby Abs and FcR engagement subverts the strategies by which intracellular bacterial pathogens evade lysosomal degradation.

Modulation of host cell function by Legionella pneumophila type IV effectors

Macrophages and protozoa ingest bacteria by phagocytosis and destroy these microbes using a conserved pathway that mediates fusion of the phagosome with lysosomes. To survive within phagocytic host cells, bacterial pathogens have evolved a variety of strategies to avoid fusion with lysosomes. A virulence strategy used by the intracellular pathogen Legionella pneumophila is to manipulate host cellular processes using bacterial proteins that are delivered into the cytosolic compartment of the host cell by a specialized secretion system called Dot/Icm. The proteins delivered by the Dot/Icm system target host factors that play evolutionarily conserved roles in controlling membrane transport in eukaryotic cells, which enables L. pneumophila to create an endoplasmic reticulum-like vacuole that supports intracellular replication in both protozoan and mammalian host cells. This review focuses on intracellular trafficking of L. pneumophila and describes how bacterial proteins contribute to modulation of host processes required for survival within host cells.

Legionella pneumophila strain 130b possesses a unique combination of type IV secretion systems and novel Dot/Icm secretion system effector proteins

Legionella pneumophila is a ubiquitous inhabitant of environmental water reservoirs. The bacteria infect a wide variety of protozoa and, after accidental inhalation, human alveolar macrophages, which can lead to severe pneumonia. The capability to thrive in phagocytic hosts is dependent on the Dot/Icm type IV secretion system (T4SS), which translocates multiple effector proteins into the host cell. In this study, we determined the draft genome sequence of L. pneumophila strain 130b (Wadsworth). We found that the 130b genome encodes a unique set of T4SSs, namely, the Dot/Icm T4SS, a Trb-1-like T4SS, and two Lvh T4SS gene clusters. Sequence analysis substantiated that a core set of 107 Dot/Icm T4SS effectors was conserved among the sequenced L. pneumophila strains Philadelphia-1, Lens, Paris, Corby, Alcoy, and 130b. We also identified new effector candidates and validated the translocation of 10 novel Dot/Icm T4SS effectors that are not present in L. pneumophila strain Philadelphia-1. We examined the prevalence of the new effector genes among 87 environmental and clinical L. pneumophila isolates. Five of the new effectors were identified in 34 to 62% of the isolates, while less than 15% of the strains tested positive for the other five genes. Collectively, our data show that the core set of conserved Dot/Icm T4SS effector proteins is supplemented by a variable repertoire of accessory effectors that may partly account for differences in the virulences and prevalences of particular L. pneumophila strains.

High-affinity binding of phosphatidylinositol 4-phosphate by Legionella pneumophila DrrA

The DrrA protein of Legionella pneumophila is involved in mistargeting of endoplasmic reticulum-derived vesicles to Legionella-containing vacuoles through recruitment of the small GTPase Rab1. To this effect, DrrA binds specifically to phosphatidylinositol 4-phosphate (PtdIns(4)P) lipids on the cytosolic surface of the phagosomal membrane shortly after infection. In this study, we present the atomic structure of the PtdIns(4)P-binding domain of a protein (DrrA) from a human pathogen. A detailed kinetic investigation of its interaction with PtdIns(4)P reveals that DrrA binds to this phospholipid with, as yet unprecedented, high affinity, suggesting that DrrA can sense a very low abundance of the lipid.

Molecular pathogenesis of infections caused by Legionella pneumophila

The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease, a severe form of acute pneumonia. L. pneumophila is an intracellular pathogen, and as part of its pathogenesis, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole that exhibits many characteristics of the endoplasmic reticulum (ER). The formation of the unusual L. pneumophila vacuole is a feature of its interaction with the host, yet the mechanisms by which the bacteria avoid classical endosome fusion and recruit markers of the ER are incompletely understood. Here we review the factors that contribute to the ability of L. pneumophila to infect and replicate in human cells and amoebae with an emphasis on proteins that are secreted by the bacteria into the Legionella vacuole and/or the host cell. Many of these factors undermine eukaryotic trafficking and signaling pathways by acting as functional and, in some cases, structural mimics of eukaryotic proteins. We discuss the consequences of this mimicry for the biology of the infected cell and also for immune responses to L. pneumophila infection.

Manipulation of host membrane machinery by bacterial pathogens

Subversion of host membrane machinery is important for the uptake, survival, and replication of bacterial pathogens. Understanding how pathogens manipulate host membrane transport pathways provides mechanistic insight into how infection occurs and is also revealing new information on biochemical processes involved in the functioning of eukaryotic cells. In this review we discuss several of the canonical host pathways targeted by bacterial pathogens and emerging areas of investigation in this exciting field.

Isolation of Legionella-containing vacuoles by immuno-magnetic separation

The environmental bacterium Legionella pneumophila naturally parasitizes free-living amoebae. L. pneumophila is an opportunistic human pathogen that grows in macrophages, thus causing a life-threatening pneumonia termed Legionnaires' disease. The bacteria replicate intracellularly in environmental and immune phagocytes within a unique compartment, the Legionella-containing vacuole (LCV). Formation of LCVs is a complex and robust process involving >150 secreted bacterial effector proteins, which are believed to subvert host cell signaling and vesicle trafficking pathways. This unit describes a simple approach to purify intact LCVs from Dictyostelium discoideum amoebae. The method comprises a two-step purification protocol that includes immuno-magnetic separation by means of an antibody against an effector protein specifically binding to LCVs, followed by density gradient centrifugation. The use of D. discoideum producing a fluorescent LCV marker and fluorescently labeled L. pneumophila allow tracking the enrichment of LCVs by light microscopy.

Inhibition of host vacuolar H+-ATPase activity by a Legionella pneumophila effector

Legionella pneumophila is an intracellular pathogen responsible for Legionnaires' disease. This bacterium uses the Dot/Icm type IV secretion system to inject a large number of bacterial proteins into host cells to facilitate the biogenesis of a phagosome permissive for its intracellular growth. Like many highly adapted intravacuolar pathogens, L. pneumophila is able to maintain a neutral pH in the lumen of its phagosome, particularly in the early phase of infection. However, in all cases, the molecular mechanisms underlying this observation remain unknown. In this report, we describe the identification and characterization of a Legionella protein termed SidK that specifically targets host v-ATPase, the multi-subunit machinery primarily responsible for organelle acidification in eukaryotic cells. Our results indicate that after being injected into infected cells by the Dot/Icm secretion system, SidK interacts with VatA, a key component of the proton pump. Such binding leads to the inhibition of ATP hydrolysis and proton translocation. When delivered into macrophages, SidK inhibits vacuole acidification and impairs the ability of the cells to digest non-pathogenic E. coli. We also show that a domain located in the N-terminal portion of SidK is responsible for its interactions with VatA. Furthermore, expression of sidK is highly induced when bacteria begin to enter new growth cycle, correlating well with the potential temporal requirement of its activity during infection. Our results indicate that direct targeting of v-ATPase by secreted proteins constitutes a virulence strategy for L. pneumophila, a vacuolar pathogen of macrophages and amoebae.

Structural basis of wedging the Golgi membrane by FAPP pleckstrin homology domains

Overduin and colleagues present the NMR structures of free, micelle and PtdIns(4)P-bound FAPP1-PH domain. The micelle-bound structure reveals how its prominent wedge independently tubulates Golgi membranes by leaflet penetration. A hydrophobic element inserts into and bends membranes, and is conserved in pleckstrin homology domains of CERT and OSBP proteins.

The mechanisms underlying Golgi targeting and vesiculation are unknown, although the responsible phosphatidylinositol 4-phosphate (PtdIns(4)P) ligand and four-phosphate-adaptor protein (FAPP) modules have been defined. The micelle-bound structure of the FAPP1 pleckstrin homology domain reveals how its prominent wedge independently tubulates Golgi membranes by leaflet penetration. Mutations compromising the exposed hydrophobicity of full-length FAPP2 abolish lipid monolayer binding and compression. The trafficking process begins with an electrostatic approach, phosphoinositide sampling and perpendicular penetration. Extensive protein contacts with PtdIns(4)P and neighbouring phospholipids reshape the bilayer and initiate tubulation through a conserved wedge with features shared by diverse protein modules.

Multiple host proteins that function in phosphatidylinositol-4-phosphate metabolism are recruited to the chlamydial inclusion

Chlamydiae replicate within a nonacidified vacuole, termed an inclusion. As obligate intracellular bacteria, chlamydiae actively modify their vacuole to exploit host signaling and trafficking pathways. Recently, we demonstrated that several Rab GTPases are actively targeted to the inclusion. To define the biological roles of inclusion localized Rab GTPases, we have begun to identify inclusion-localized Rab effectors. Here we demonstrate that oculocerebrorenal syndrome of Lowe protein 1 (OCRL1), a Golgi complex-localized phosphatidylinositol (PI)-5-phosphatase that binds to multiple Rab GTPases, localizes to chlamydial inclusions. By examining the intracellular localization of green fluorescent protein (GFP) fusion proteins that bind to unique phosphoinositide species, we also demonstrate that phosphatidylinositol-4-phosphate (PI4P), the product of OCRL1, is present at the inclusion membrane. Furthermore, two additional host proteins, Arf1, which together with PI4P mediates the recruitment of PI4P-binding proteins to the Golgi complex, and PI4KII alpha, a major producer of Golgi complex-localized PI4P, also localize to chlamydial inclusions. Depletion of OCRL1, Arf1, or PI4KII alpha by small interfering RNA (siRNA) decreases inclusion formation and the production of infectious progeny. Infectivity is further decreased in cells simultaneously depleted for all three host proteins, suggesting partially overlapping functions in infected cells. Collectively, these data demonstrate that Chlamydia species create a unique replication-competent vacuolar environment by modulating both the Rab GTPase and the PI composition of the chlamydial inclusion.

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.

Analysis of the Legionella longbeachae genome and transcriptome uncovers unique strategies to cause Legionnaires' disease

Legionella pneumophila and L. longbeachae are two species of a large genus of bacteria that are ubiquitous in nature. L. pneumophila is mainly found in natural and artificial water circuits while L. longbeachae is mainly present in soil. Under the appropriate conditions both species are human pathogens, capable of causing a severe form of pneumonia termed Legionnaires' disease. Here we report the sequencing and analysis of four L. longbeachae genomes, one complete genome sequence of L. longbeachae strain NSW150 serogroup (Sg) 1, and three draft genome sequences another belonging to Sg1 and two to Sg2. The genome organization and gene content of the four L. longbeachae genomes are highly conserved, indicating strong pressure for niche adaptation. Analysis and comparison of L. longbeachae strain NSW150 with L. pneumophila revealed common but also unexpected features specific to this pathogen. The interaction with host cells shows distinct features from L. pneumophila, as L. longbeachae possesses a unique repertoire of putative Dot/Icm type IV secretion system substrates, eukaryotic-like and eukaryotic domain proteins, and encodes additional secretion systems. However, analysis of the ability of a dotA mutant of L. longbeachae NSW150 to replicate in the Acanthamoeba castellanii and in a mouse lung infection model showed that the Dot/Icm type IV secretion system is also essential for the virulence of L. longbeachae. In contrast to L. pneumophila, L. longbeachae does not encode flagella, thereby providing a possible explanation for differences in mouse susceptibility to infection between the two pathogens. Furthermore, transcriptome analysis revealed that L. longbeachae has a less pronounced biphasic life cycle as compared to L. pneumophila, and genome analysis and electron microscopy suggested that L. longbeachae is encapsulated. These species-specific differences may account for the different environmental niches and disease epidemiology of these two Legionella species.

Recent insights into host-pathogen interactions from Dictyostelium

To protect themselves from predation by amoebae and protozoa in the natural environment, some bacteria evolved means of escaping killing. The same mechanisms allow survival in mammalian phagocytes, producing opportunistic human pathogens. The social amoeba Dictyostelium discoideum is a powerful system for analysis of conserved host-pathogen interactions. This report reviews recent insights gained for several bacterial pathogens using Dictyostelium as host.