Cell biology of infection by Legionella pneumophila

The Legionella longbeachae Icm/Dot substrate SidC selectively binds phosphatidylinositol 4-phosphate with nanomolar affinity and promotes pathogen vacuole-endoplasmic reticulum interactions

Legionella spp. cause the severe pneumonia Legionnaires' disease. The environmental bacteria replicate intracellularly in free-living amoebae and human alveolar macrophages within a distinct, endoplasmic reticulum (ER)-derived compartment termed the Legionella-containing vacuole (LCV). LCV formation requires the bacterial Icm/Dot type IV secretion system (T4SS) that translocates into host cells a plethora of different "effector" proteins, some of which anchor to the pathogen vacuole by binding to phosphoinositide (PI) lipids. Here, we identified by unbiased pulldown assays in Legionella longbeachae lysates a 111-kDa SidC homologue as the major phosphatidylinositol 4-phosphate [PtdIns(4)P]-binding protein. The PI-binding domain was mapped to a 20-kDa P4C [PtdIns(4)P binding of SidC] fragment. Isothermal titration calorimetry revealed that SidC of L. longbeachae (SidC(Llo)) binds PtdIns(4)P with a K(d) (dissociation constant) of 71 nM, which is 3 to 4 times lower than that of the SidC orthologue of Legionella pneumophila (SidC(Lpn)). Upon infection of RAW 264.7 macrophages with L. longbeachae, endogenous SidC(Llo) or ectopically produced SidC(Lpn) localized in an Icm/Dot-dependent manner to the PtdIns(4)P-positive LCVs. An L. longbeachae ΔsidC deletion mutant was impaired for calnexin recruitment to LCVs in Dictyostelium discoideum amoebae and outcompeted by wild-type bacteria in Acanthamoeba castellanii. Calnexin recruitment was restored by SidC(Llo) or its orthologues SidC(Lpn) and SdcA(Lpn). Conversely, calnexin recruitment was restored by SidC(Llo) in L. pneumophila lacking sidC and sdcA. Together, biochemical, genetic, and cell biological data indicate that SidC(Llo) is an L. longbeachae effector that binds through a P4C domain with high affinity to PtdIns(4)P on LCVs, promotes ER recruitment to the LCV, and thus plays a role in pathogen-host interactions.

Sulfonamide inhibition studies of two β-carbonic anhydrases from the bacterial pathogen Legionella pneumophila

Two β-carbonic anhydrases (CAs, EC 4.2.1.1) were identified, cloned and purified in the pathogenic bacterium Legionella pneumophila, denominated LpCA1 and LpCA2. They efficiently catalyze CO2 hydration to bicarbonate and protons, with kcat in the range of (3.4-8.3) × 10(5)s(-1) and kcat/Km of (4.7-8.5) × 10(7)M(-1)s(-1), and are inhibited by sulfonamides and sulfamates. The best LpCA1 inhibitors were aminobenzolamide and structurally similar sulfonylated aromatic sulfonamides, as well as acetazolamide and ethoxzolamide(KIs in the range of 40.3-90.5 nM). The best LpCA2 inhibitors belonged to the same class of sulfonylated sulfonamides, together with acetazolamide, methazolamide and dichlorophenamide (KIs in the range of 25.2-88.5 nM). As these enzymes may be involved in pH regulation in the phagosome during Legionella infection, their inhibition may lead to antibacterials with a novel mechanism of action.

Anion inhibition studies of two new β-carbonic anhydrases from the bacterial pathogen Legionella pneumophila

We investigated the cloning, catalytic activity and anion inhibition of the β-class carbonic anhydrases (CAs, EC 4.2.1.1) from the bacterial pathogen Legionella pneumophila. Two such enzymes, lpCA1 and lpCA2, were found in the genome of this pathogen. These enzymes were determined to be efficient catalysts for CO2 hydration, with kcat values in the range of (3.4-8.3)×10(5) s(-1) and kcat/KM values of (4.7-8.5)×10(7) M(-1) s(-1). A set of inorganic anions and small molecules was investigated to identify inhibitors of these enzymes. Perchlorate and tetrafluoroborate were not acting as inhibitors (KI >200 mM), whereas sulfate was a very weak inhibitor for both lpCA1 and lpCA2 (KI values of 77.9-96.5 mM). The most potent lpCA1 inhibitors were cyanide, azide, hydrogen sulfide, diethyldithiocarbamate, sulfamate, sulfamide, phenylboronic acid and phenylarsonic acid, with KI values ranging from 6 to 94 μM. The most potent lpCA2 inhibitors were diethyldithiocarbamate, sulfamide, sulfamate, phenylboronic acid and phenylarsonic acid, with KI values ranging from 2 to 13 μM. As these enzymes seem to be involved in regulation of phagosome pH during Legionella infection, inhibition of these targets may lead to antibacterial agents with a novel mechanism of action.

Programmed cell death in Legionella infection

ABSTRACT: The causative agent of Legionnaires’ disease, Legionella pneumophila, resides within alveolar macrophages by exporting 295 bacterial virulence proteins (effectors) to modulate host cell processes. This leads to the formation of a unique vacuolar niche and the suppression of macrophage cell death pathways, which, in turn, promote bacterial survival and allow sufficient time for replication. However, once nutrients within the vacuole are depleted, Legionella must act to induce host cell death in order to facilitate bacterial egress and reinfect new cells. Intracellular Legionella also evade detection by the host cell’s innate immune system, which seeks to destroy invading pathogens by activating inflammasome complexes, thereby promoting proinflammatory cytokine activation and pyroptotic cell death. Understanding how different forms of programmed cell death contribute to Legionella infectivity and are manipulated by Legionella effector proteins will be important for identifying novel antibacterial therapeutic targets.

Hijacking the host proteasome for the temporal degradation of bacterial effectors

To establish infection, intracellular pathogens need to modulate host cellular processes. Modulation of host processes is achieved by the action of various "effector proteins" which are delivered from the bacteria to the host cell cytosol. In order to orchestrate host cell reprogramming, the function of effectors inside host cells is regulated both temporally and spatially. In eukaryotes one of the most prominent processes used to degrade proteins is the ubiquitin-proteasome system. Recently it has emerged that the intracellular pathogen Legionella pneumophila is able to achieve temporal regulation of an effector using the ubiquitin-proteasome system. After establishing its replicative niche, the L. pneumophila effector SidH is degraded by the host proteasome. Most remarkably another effector protein LubX is able to mimic the function of an eukaryotic E3 ubiquitin ligase and polyubiquitinates SidH, targeting it for degradation. In this paper we describe a method to detect the polyubiquitin-modified forms of SidH in vitro and in vivo. Analyzing the temporal profile of polyubiquitination and degradation of bacterial effectors aids towards our understanding of how bacteria hijack host systems.

A Legionella effector modulates host cytoskeletal structure by inhibiting actin polymerization

Successful infection by the opportunistic pathogen Legionella pneumophila requires the collective activity of hundreds of virulence proteins delivered into the host cell by the Dot/Icm type IV secretion system. These virulence proteins, also called effectors modulate distinct host cellular processes to create a membrane-bound niche called the Legionella containing vacuole (LCV) supportive of bacterial growth. We found that Ceg14 (Lpg0437), a Dot/Icm substrate is toxic to yeast and such toxicity can be alleviated by overexpression of profilin, a protein involved in cytoskeletal structure in eukaryotes. We further showed that mutations in profilin affect actin binding but not other functions such as interactions with poly-l-proline or phosphatidylinositol, abolish its suppressor activity. Consistent with the fact the profilin suppresses its toxicity, expression of Ceg14 but not its non-toxic mutants in yeast affects actin distribution and budding of daughter cells. Although Ceg14 does not detectably interact with profilin, it co-sediments with filamentous actin and inhibits actin polymerization, causing the accumulation of short actin filaments. Together with earlier studies, these results reveal that multiple L. pneumophila effectors target components of the host cytoskeleton.

Induction of caspase 3 activation by multiple Legionella pneumophila Dot/Icm substrates

The intracellular pathogen Legionella pneumophila is able to strike a balance between the death and survival of the host cell during infection. Despite the presence of high level of active caspase 3, the executioner caspase of apoptotic cell death, infected permissive macrophages are markedly resistant to exogenous apoptotic stimuli. Several bacterial molecules capable of promoting the cell survival pathways have been identified, but proteins involved in the activation of caspase 3 remain unknown. To study the mechanism of L. pneumophila-mediated caspase 3 activation, we tested all known Dot/Icm substrates for their ability to activate caspase 3. Five effectors capable of causing caspase 3 activation upon transient expression were identified. Among these, by using its ability to activate caspase 3 by inducing the release of cytochrome c from the mitochondria, we demonstrated that VipD is a phospholipase A2, which hydrolyses phosphatidylethanolamine (PE) and phosphocholine (PC) on the mitochondrial membrane in a manner that appears to require host cofactor(s). The lipase activity leads to the production of free fatty acids and 2-lysophospholipids, which destabilize the mitochondrial membrane and may contribute to the release of cytochrome c and the subsequent caspase 3 activation. Furthermore, we found that whereas it is not detectably defectively in caspase 3 activation in permissive cells, amutant lacking all of these five genes is less potent in inducing apoptosis in dendritic cells. Our results reveal that activation of host cell death pathways by L. pneumophila is a result of the effects of multiple bacterial proteins with diverse biochemical functions.

Genome Sequence of an Environmental Isolate of the Bacterial Pathogen Legionella pneumophila

We report here the genomic sequence of Legionella pneumophila strain LPE509 from the water distribution system of a hospital in Shanghai, China. This is the first complete genome sequence of an environmental L. pneumophila isolate. Genomic analyses identified approximately 600 genes unique to LPE509 compared to those of the 7 available L. pneumophila genomes.

Induction of rapid cell death by an environmental isolate of Legionella pneumophila in mouse macrophages

Legionella pneumophila, the etiological agent for Legionnaires' disease, is ubiquitous in the aqueous environment, where it replicates as an intracellular parasite of free-living protozoa. Our understanding of L. pneumophila pathogenicity is obtained mostly from study of derivatives of several clinical isolates, which employ almost identical virulent determinants to exploit host functions. To determine whether environmental L. pneumophila isolates interact similarly with the model host systems, we analyzed intracellular replication of several recently isolated such strains and found that these strains cannot productively grow in bone marrow-derived macrophages of A/J mice, which are permissive for all examined laboratory strains. By focusing on one strain called LPE509, we found that its deficiency in intracellular replication in primary A/J macrophages is not caused by the lack of important pathogenic determinants because this strain replicates proficiently in two protozoan hosts and the human macrophage U937 cell. We also found that in the early phase of infection, the trafficking of this strain in A/J macrophages is similar to that of JR32, a derivative of strain Philadelphia 1. Furthermore, infection of these cells by LPE509 caused extensive cell death in a process that requires the Dot/Icm type IV secretion system. Finally, we showed that the cell death is caused neither by the activation of the NAIP5/NLRC4 inflammasome nor by the recently described caspase 11-dependent pathway. Our results revealed that some environmental L. pneumophila strains are unable to overcome the defense conferred by primary macrophages from mice known to be permissive for laboratory L. pneumophila strains. These results also suggest the existence of a host immune surveillance mechanism differing from those currently known in responding to L. pneumophila infection.

Effector translocation by the Legionella Dot/Icm type IV secretion system

Legionella pneumophila is an opportunistic pathogen responsible for Legionnaires' disease. This bacterium survives and replicates within phagocytes by bypassing their bactericidal activity. Intracellular replication of L. pneumophila requires the Dot/Icm type IV secretion system made of approximately 27 proteins that presumably traverses the bacterial and phagosomal membranes. The perturbation of the host killing ability largely is mediated by the collective functions of the protein substrates injected into host cells via the Dot/Icm transporter. Proper protein translocation by Dot/Icm is determined by a number of factors, including signals recognizable by the translocator, chaperones that may facilitate the proper folding of substrates and transcriptional regulation and protein stability that determine the abundance and temporal transfer of the substrates. Although a large number of Dot/Icm substrates have been identified, investigation to understand the translocation is ongoing. Here we summarized the recent advancements in our understanding of the factors that determine the protein translocation activity of the Dot/Icm transporter.

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.