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

Coxiella co-opts the Glutathione Peroxidase 4 to protect the host cell from oxidative stress-induced cell death

The causative agent of human Q fever, Coxiella burnetii, is highly adapted to infect alveolar macrophages by inhibiting a range of host responses to infection. Despite the clinical and biological importance of this pathogen, the challenges related to genetic manipulation of both C. burnetii and macrophages have limited our knowledge of the mechanisms by which C. burnetii subverts macrophages functions. Here, we used the related bacterium Legionella pneumophila to perform a comprehensive screen of C. burnetii effectors that interfere with innate immune responses and host death using the greater wax moth Galleria mellonella and mouse bone marrow-derived macrophages. We identified MceF (Mitochondrial Coxiella effector protein F), a C. burnetii effector protein that localizes to mitochondria and contributes to host cell survival. MceF was shown to enhance mitochondrial function, delay membrane damage, and decrease mitochondrial ROS production induced by rotenone. Mechanistically, MceF recruits the host antioxidant protein Glutathione Peroxidase 4 (GPX4) to the mitochondria. The protective functions of MceF were absent in primary macrophages lacking GPX4, while overexpression of MceF in human cells protected against oxidative stress-induced cell death. C. burnetii lacking MceF was replication competent in mammalian cells but induced higher mortality in G. mellonella, indicating that MceF modulates the host response to infection. This study reveals an important C. burnetii strategy to subvert macrophage cell death and host immunity and demonstrates that modulation of the host antioxidant system is a viable strategy to promote the success of intracellular bacteria.

Macrophage infectivity potentiator protein, a peptidyl prolyl cis-trans isomerase, essential for Coxiella burnetii growth and pathogenesis

Coxiella burnetii is a Gram-negative intracellular pathogen that causes the debilitating disease Q fever, which affects both animals and humans. The only available human vaccine, Q-Vax, is effective but has a high risk of severe adverse reactions, limiting its use as a countermeasure to contain outbreaks. Therefore, it is essential to identify new drug targets to treat this infection. Macrophage infectivity potentiator (Mip) proteins catalyse the folding of proline-containing proteins through their peptidyl prolyl cis-trans isomerase (PPIase) activity and have been shown to play an important role in the virulence of several pathogenic bacteria. To date the role of the Mip protein in C. burnetii pathogenesis has not been investigated. This study demonstrates that CbMip is likely to be an essential protein in C. burnetii. The pipecolic acid derived compounds, SF235 and AN296, which have shown utility in targeting other Mip proteins from pathogenic bacteria, demonstrate inhibitory activities against CbMip. These compounds were found to significantly inhibit intracellular replication of C. burnetii in both HeLa and THP-1 cells. Furthermore, SF235 and AN296 were also found to exhibit antibiotic properties against both the virulent (Phase I) and avirulent (Phase II) forms of C. burnetii Nine Mile Strain in axenic culture. Comparative proteomics, in the presence of AN296, revealed alterations in stress responses with H2O2 sensitivity assays validating that Mip inhibition increases the sensitivity of C. burnetii to oxidative stress. In addition, SF235 and AN296 were effective in vivo and significantly improved the survival of Galleria mellonella infected with C. burnetii. These results suggest that unlike in other bacteria, Mip in C. burnetii is required for replication and that the development of more potent inhibitors against CbMip is warranted and offer potential as novel therapeutics against this pathogen.

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.

Coxiella burnetii Sterol-Modifying Protein Stmp1 Regulates Cholesterol in the Intracellular Niche

Coxiella burnetii replicates in a phagolysosome-like vacuole called the Coxiella-containing vacuole (CCV). While host cholesterol readily traffics to the CCV, cholesterol accumulation leads to CCV acidification and bacterial death. Thus, bacterial regulation of CCV cholesterol content is essential for Coxiella pathogenesis. Coxiella expresses a sterol-modifying protein, Stmp1, that may function to lower CCV cholesterol through enzymatic modification. Using an Stmp1 knockout (Δstmp1), we determined that Stmp1 is not essential for axenic growth. Inside host cells, however, Δstmp1 mutant bacteria form smaller CCVs which accumulate cholesterol, preferentially fuse with lysosomes, and become more acidic, correlating with a significant growth defect. However, in cholesterol-free cells, Δstmp1 mutant bacteria grow similarly to wild-type bacteria but are hypersensitive to cholesterol supplementation. To better understand the underlying mechanism behind the Δstmp1 mutant phenotype, we performed sterol profiling. Surprisingly, we found that Δstmp1 mutant-infected macrophages accumulated the potent cholesterol homeostasis regulator 25-hydroxycholesterol (25-HC). We next determined whether dysregulated 25-HC alters Coxiella infection by treating wild-type Coxiella-infected cells with 25-HC. Similar to the Δstmp1 mutant phenotype, 25-HC increased CCV proteolytic activity and inhibited bacterial growth. Collectively, these data indicate that Stmp1 alters host cholesterol metabolism and is essential to establish a mature CCV which supports Coxiella growth. IMPORTANCE Coxiella burnetii is the causative agent of human Q fever, an emerging infectious disease and significant cause of culture-negative endocarditis. Acute infections are often undiagnosed, there are no licensed vaccines in the United States, and chronic Q fever requires a prolonged antibiotic treatment. Therefore, new treatment and preventive options are critically needed. Coxiella is an obligate intracellular bacterium that replicates within a large acidic phagolysosome-like compartment, the Coxiella-containing vacuole (CCV). We previously discovered that cholesterol accumulation in the CCV increases its acidification, leading to bacterial death. Therefore, in order to survive in this harsh environment, Coxiella likely regulates CCV cholesterol levels. Here, we found that Coxiella sterol modifying protein (Stmp1) facilitates bacterial growth by reducing CCV cholesterol and host cell 25-hydroxycholesterol (25-HC) levels, which prevents excessive CCV fusion with host lysosomes and CCV acidification. This study establishes that Stmp1-mediated regulation of host cholesterol homeostasis is essential for Coxiella intracellular survival.

Coxiella burnetii and Related Tick Endosymbionts Evolved from Pathogenic Ancestors

Both symbiotic and pathogenic bacteria in the family Coxiellaceae cause morbidity and mortality in humans and animals. For instance, Coxiella-like endosymbionts (CLEs) improve the reproductive success of ticks—a major disease vector, while Coxiella burnetii causes human Q fever, and uncharacterized coxiellae infect both animals and humans. To better understand the evolution of pathogenesis and symbiosis in this group of intracellular bacteria, we sequenced the genome of a CLE present in the soft tick Ornithodoros amblus (CLEOA) and compared it to the genomes of other bacteria in the order Legionellales. Our analyses confirmed that CLEOA is more closely related to C. burnetii, the human pathogen, than to CLEs in hard ticks, and showed that most clades of CLEs contain both endosymbionts and pathogens, indicating that several CLE lineages have evolved independently from pathogenic Coxiella. We also determined that the last common ancestorof CLEOA and C. burnetii was equipped to infect macrophages and that even though horizontal gene transfer (HGT) contributed significantly to the evolution of C. burnetii, most acquisition events occurred primarily in ancestors predating the CLEOA–C. burnetii divergence. These discoveries clarify the evolution of C. burnetii, which previously was assumed to have emerged when an avirulent tick endosymbiont recently gained virulence factors via HGT. Finally, we identified several metabolic pathways, including heme biosynthesis, that are likely critical to the intracellular growth of the human pathogen but not the tick symbiont, and show that the use of heme analog is a promising approach to controlling C. burnetii infections.