Dependency of Coxiella burnetii Type 4B Secretion on the Chaperone IcmS

Role of Type 4B Secretion System Protein, IcmE, in the Pathogenesis of Coxiella burnetii

Coxiella burnetii is an obligate intracellular Gram-negative bacterium that causes Q fever, a life-threatening zoonotic disease. C. burnetii replicates within an acidified parasitophorous vacuole derived from the host lysosome. The ability of C. burnetii to replicate and achieve successful intracellular life in the cell cytosol is vastly dependent on the Dot/Icm type 4B secretion system (T4SSB). Although several T4SSB effector proteins have been shown to be important for C. burnetii virulence and intracellular replication, the role of the icmE protein in the host-C. burnetii interaction has not been investigated. In this study, we generated a C. burnetii Nine Mile Phase II (NMII) mutant library and identified 146 transposon mutants with a single transposon insertion. Transposon mutagenesis screening revealed that disruption of icmE gene resulted in the attenuation of C. burnetii NMII virulence in SCID mice. ELISA analysis indicated that the levels of pro-inflammatory cytokines, including interleukin-1β, IFN-γ, TNF-α, and IL-12p70, in serum from Tn::icmE mutant-infected SCID mice were significantly lower than those in serum from wild-type (WT) NMII-infected mice. Additionally, Tn::icmE mutant bacteria were unable to replicate in mouse bone marrow-derived macrophages (MBMDM) and human macrophage-like cells (THP-1). Immunoblotting results showed that the Tn::icmE mutant failed to activate inflammasome components such as IL-1β, caspase 1, and gasdermin-D in THP-1 macrophages. Collectively, these results suggest that the icmE protein may play a vital role in C. burnetii virulence, intracellular replication, and activation of inflammasome mediators during NMII infection.

Recent advances in genetic systems in obligate intracellular human-pathogenic bacteria

The ability to genetically manipulate a pathogen is fundamental to discovering factors governing host-pathogen interactions at the molecular level and is critical for devising treatment and prevention strategies. While the genetic "toolbox" for many important bacterial pathogens is extensive, approaches for modifying obligate intracellular bacterial pathogens were classically limited due in part to the uniqueness of their obligatory lifestyles. Many researchers have confronted these challenges over the past two and a half decades leading to the development of multiple approaches to construct plasmid-bearing recombinant strains and chromosomal gene inactivation and deletion mutants, along with gene-silencing methods enabling the study of essential genes. This review will highlight seminal genetic achievements and recent developments (past 5 years) for Anaplasma spp., Rickettsia spp., Chlamydia spp., and Coxiella burnetii including progress being made for the still intractable Orientia tsutsugamushi. Alongside commentary of the strengths and weaknesses of the various approaches, future research directions will be discussed to include methods for C. burnetii that should have utility in the other obligate intracellular bacteria. Collectively, the future appears bright for unraveling the molecular pathogenic mechanisms of these significant pathogens.

Identification of Type 4B Secretion System Substrates That Are Conserved among Coxiella burnetii Genomes and Promote Intracellular Growth

Coxiella burnetii is a Gram-negative pathogen that infects a variety of mammalian hosts. Infection of domesticated ewes can cause fetal abortion, whereas acute human infection normally manifests as the flu-like illness Q fever. Successful host infection requires replication of the pathogen within the lysosomal Coxiella-containing vacuole (CCV). The bacterium encodes a type 4B secretion system (T4BSS) that delivers effector proteins into the host cell. Disruption of C. burnetii T4BSS effector export abrogates CCV biogenesis and bacterial replication. Over 150 C. burnetii T4BSS substrates have been designated often based on heterologous protein translocation by the Legionella pneumophila T4BSS. Cross-genome comparisons predict that many of these T4BSS substrates are truncated or absent in the acute-disease reference strain C. burnetii Nine Mile. This study investigated the function of 32 proteins conserved among diverse C. burnetii genomes that are reported to be T4BSS substrates. Despite being previously designated T4BSS substrates, many of the proteins were not translocated by C. burnetii when expressed fused to the CyaA or BlaM reporter tags. CRISPR interference (CRISPRi) indicated that of the validated C. burnetii T4BSS substrates, CBU0122, CBU1752, CBU1825, and CBU2007 promote C. burnetii replication in THP-1 cells and CCV biogenesis in Vero cells. When expressed in HeLa cells tagged at its C or N terminus with mCherry, CBU0122 localized to the CCV membrane and the mitochondria, respectively. Collectively, these data further define the repertoire of bona fide C. burnetii T4BSS substrates. IMPORTANCE Coxiella burnetii secretes effector proteins via a T4BSS that are required for successful infection. Over 150 C. burnetii proteins are reported to be T4BSS substrates and often by default considered putative effectors, but few have assigned functions. Many C. burnetii proteins were designated T4BSS substrates using heterologous secretion assays in L. pneumophila and/or have coding sequences that are absent or pseudogenized in clinically relevant C. burnetii strains. This study examined 32 previously reported T4BSS substrates that are conserved among C. burnetii genomes. Of the proteins tested that were previously designated T4BSS substrates using L. pneumophila, most were not exported by C. burnetii. Several T4BSS substrates that were validated in C. burnetii also promoted pathogen intracellular replication and one trafficked to late endosomes and the mitochondria in a manner suggestive of effector activity. This study identified several bona fide C. burnetii T4BSS substrates and further refined the methodological criteria for their designation.

The endogenous Coxiella burnetii plasmid encodes a functional toxin-antitoxin system

Coxiella burnetii is the causative agent of Q fever. All C. burnetii isolates encode either an autonomously replicating plasmid (QpH1, QpDG, QpRS, or QpDV) or QpRS-like chromosomally integrated plasmid sequences. The role of the ORFs present in these sequences is unknown. Here, the role of the ORFs encoded on QpH1 was investigated. Using a new C. burnetii shuttle vector (pB-TyrB-QpH1ori), we cured the C. burnetii Nine Mile Phase II strain of QpH1. The ΔQpH1 strain grew normally in axenic media but had a significant growth defect in Vero cells, indicating QpH1 was important for C. burnetii virulence. We developed an inducible CRISPR interference system to examine the role of individual QpH1 plasmid genes. CRISPRi of cbuA0027 resulted in significant growth defects in axenic media and THP-1 cells. The cbuA0028/cbuA0027 operon encodes CBUA0028 (ToxP) and CBUA0027 (AntitoxP), which are homologous to the HigB2 toxin and HigA2 antitoxin, respectively, from Vibrio cholerae. Consistent with toxin-antitoxin systems, overexpression of toxP resulted in a severe intracellular growth defect that was rescued by co-expression of antitoxP. ToxP inhibited protein translation. AntitoxP bound the toxP promoter (PtoxP) and ToxP, with the resulting complex binding also PtoxP. In summary, our data indicate that C. burnetii maintains an autonomously replicating plasmid because of a plasmid-based toxin-antitoxin system.

The Coxiella burnetii T4SS effector protein AnkG hijacks the 7SK small nuclear ribonucleoprotein complex for reprogramming host cell transcription

Inhibition of host cell apoptosis is crucial for survival and replication of several intracellular bacterial pathogens. To interfere with apoptotic pathways, some pathogens use specialized secretion systems to inject bacterial effector proteins into the host cell cytosol. One of these pathogens is the obligate intracellular bacterium Coxiella burnetii, the etiological agent of the zoonotic disease Q fever. In this study, we analyzed the molecular activity of the anti-apoptotic T4SS effector protein AnkG (CBU0781) to understand how C. burnetii manipulates host cell viability. We demonstrate by co- and RNA-immunoprecipitation that AnkG binds to the host cell DExD box RNA helicase 21 (DDX21) as well as to the host cell 7SK small nuclear ribonucleoprotein (7SK snRNP) complex, an important regulator of the positive transcription elongation factor b (P-TEFb). The co-immunoprecipitation of AnkG with DDX21 is probably mediated by salt bridges and is independent of AnkG-7SK snRNP binding, and vice versa. It is known that DDX21 facilitates the release of P-TEFb from the 7SK snRNP complex. Consistent with the documented function of released P-TEFb in RNA Pol II pause release, RNA sequencing experiments confirmed AnkG-mediated transcriptional reprogramming and showed that expression of genes involved in apoptosis, trafficking, and transcription are influenced by AnkG. Importantly, DDX21 and P-TEFb are both essential for AnkG-mediated inhibition of host cell apoptosis, emphasizing the significance of the interaction of AnkG with both, the DDX21 protein and the 7SK RNA. In line with a critical function of AnkG in pathogenesis, the AnkG deletion C. burnetii strain was severely affected in its ability to inhibit host cell apoptosis and to generate a replicative C. burnetii-containing vacuole. In conclusion, the interference with the activity of regulatory host cell RNAs mediated by a bacterial effector protein represent a novel mechanism through which C. burnetii modulates host cell transcription, thereby enhancing permissiveness to bacterial infection.

For intracellular replication, Coxiella burnetii depends on a functional type IV secretion system, which is utilized to inject ~150 virulence factors, so called effector proteins, into the host cell cytosol. Activities have only been established for few of them. These effector proteins interfere with vesicular trafficking, autophagy, lipid metabolism, apoptosis, and transcription by binding and manipulating the activity of host cell proteins. Here, we report that the C. burnetii T4SS effector protein AnkG (CBU0781, Q83DF6) binds to the host cell DExD box helicase 21 (DDX21) as well as to several host cell RNAs, including the small regulatory 7SK RNA, which is an important regulator of the positive elongation factor b (pTEFb). AnkG interferes with the function of the 7SK small nuclear ribonucleoprotein (7SK snRNP) complex, leading to significant changes in host cell transcription and ensuring host cell survival. AnkG activity is essential for efficient intracellular replication of C. burnetii and its ability to inhibit apoptosis. In summary, we identified a novel process by which a bacterial effector protein manipulates the host cell for its own benefit.

Coxiella burnetii encodes an LvgA-related protein important for intracellular replication

Coxiella burnetii is a bacterial pathogen that replicates in a specialised lysosome-derived organelle called the Coxiella-containing vacuole (CCV). Establishment of the CCV requires the Dot/Icm type IVB secretion system. A previous transposon mutagenesis screen identified the gene cbu1754 as being important for the intracellular replication of C. burnetii. To understand the function of the protein encoded by cbu1754, CCV maturation and intracellular replication phenotypes of a cbu1754 mutant were analysed. In contrast to vacuoles containing wild-type C. burnetii Nine Mile phase II, vacuoles containing the isogenic cbu1754 mutant were smaller and did not display detectible amounts of the autophagy protein LC3, which indicated a CCV biogenesis defect. The Cbu1754 protein was not efficiently delivered into the host cell cytosol during infection, which indicated this protein is not a Dot/Icm-translocated effector protein. Secondary structure predictions suggested that Cbu1754 could be similar to the Legionella pneumophila LvgA protein, which is a component of the Dot/Icm apparatus. Consistent with this hypothesis, production of Cbu1754 in an L. pneumophila ∆lvgA mutant restored LvgA-dependent activities. The L. pneumophila proteins LvgA, IcmS and IcmW are interacting partners that comprise a subassembly of the coupling protein complex that mediates Dot/Icm-dependent effector translocation. Similarly, the Cbu1754 protein was found to be a component of the chaperone complex containing the C. burnetii proteins IcmS and IcmW. Thus, the Cbu1754 protein is an LvgA-related protein important for Dot/Icm function and intracellular replication of C. burnetii.

The Coxiella burnetii T4SS Effector AnkF Is Important for Intracellular Replication

Coxiella burnetii is an obligate intracellular pathogen and the causative agent of the zoonotic disease Q fever. Following uptake by alveolar macrophages, the pathogen replicates in an acidic phagolysosomal vacuole, the C. burnetii-containing vacuole (CCV). Effector proteins translocated into the host cell by the type IV secretion system (T4SS) are important for the establishment of the CCV. Here we focus on the effector protein AnkF and its role in establishing the CCV. The C. burnetii AnkF knock out mutant invades host cells as efficiently as wild-type C. burnetii, but this mutant is hampered in its ability to replicate intracellularly, indicating that AnkF might be involved in the development of a replicative CCV. To unravel the underlying reason(s), we searched for AnkF interactors in host cells and identified vimentin through a yeast two-hybrid approach. While AnkF does not alter vimentin expression at the mRNA or protein levels, the presence of AnkF results in structural reorganization and vesicular co-localization with recombinant vimentin. Ectopically expressed AnkF partially accumulates around the established CCV and endogenous vimentin is recruited to the CCV in a time-dependent manner, suggesting that AnkF might attract vimentin to the CCV. However, knocking-down endogenous vimentin does not affect intracellular replication of C. burnetii. Other cytoskeletal components are recruited to the CCV and might compensate for the lack of vimentin. Taken together, AnkF is essential for the establishment of the replicative CCV, however, its mode of action is still elusive.

EirA Is a Novel Protein Essential for Intracellular Replication of Coxiella burnetii

The zoonotic bacterial pathogen Coxiella burnetii is the causative agent of Q fever, a febrile illness which can cause a serious chronic infection. C. burnetii is a unique intracellular bacterium which replicates within host lysosome-derived vacuoles. The ability of C. burnetii to replicate within this normally hostile compartment is dependent on the activity of the Dot/Icm type 4B secretion system. In a previous study, a transposon mutagenesis screen suggested that the disruption of the gene encoding the novel protein CBU2072 rendered C. burnetii incapable of intracellular replication. This protein, subsequently named EirA (essential for intracellular replication A), is indispensable for intracellular replication and virulence, as demonstrated by infection of human cell lines and in vivo infection of Galleria mellonella The putative N-terminal signal peptide is essential for protein function but is not required for localization of EirA to the bacterial inner membrane compartment and axenic culture supernatant. In the absence of EirA, C. burnetii remains viable but nonreplicative within the host phagolysosome, as coinfection with C. burnetii expressing native EirA rescues the replicative defect in the mutant strain. In addition, while the bacterial ultrastructure appears to be intact, there is an altered metabolic profile shift in the absence of EirA, suggesting that EirA may impact overall metabolism. Most strikingly, in the absence of EirA, Dot/Icm effector translocation was inhibited even when EirA-deficient C. burnetii replicated in the wild type (WT)-supported Coxiella containing vacuoles. EirA may therefore have a novel role in the control of Dot/Icm activity and represent an important new therapeutic target.