Depletion of SNAP-23 and Syntaxin 4 alters lipid droplet homeostasis during Chlamydia infection

Rickettsial pathogen augments tick vesicular-associated membrane proteins for infection and survival in the vector host

Anaplasma phagocytophilum is an obligate intracellular rickettsial pathogen that infects humans and animals. The black-legged tick Ixodes scapularis acts as a vector and transmits this bacterium to the vertebrate host. Upon entry into a host cell, A. phagocytophilum resides and multiplies in a host-derived vacuole called morulae. There is not much information available on the molecules that play an important role(s) in A. phagocytophilum entry and formation of these morulae in tick cells. In this study, we provide evidence that tick vesicular-associated membrane proteins, VAMP3 and VAMP4, play important roles in this phenomenon. Quantitative real-time polymerase chain reaction (QRT-PCR) analysis showed that both vamp3 and vamp4 transcripts are significantly upregulated at early time points of A. phagocytophilum infection in tick cells. We noted that both VAMP3 and VAMP4 predominantly localized to the A. phagocytophilum-containing vacuole. RNAi-mediated silencing of vamp3 and/or vamp4 expression, followed by confocal microscopy and expression analysis, indicated an impairment in A. phagocytophilum morulae formation in tick cells. We also noted that VAMP3 and VAMP4 play a role in the A. phagocytophilum persistent infection of ticks and tick cells. Furthermore, RNAi-mediated silencing of expression of arthropod vamp3 and vamp4 affected bacterial acquisition from an infected murine host to ticks. Collectively, this study not only provides evidence on the role of arthropod vesicular-associated membrane proteins in A. phagocytophilum morulae formation in tick cells but also demonstrates that these proteins are important for bacterial acquisition from an infected vertebrate host into ticks.

IMPORTANCE

Anaplasma phagocytophilum is a tick-borne pathogen primarily transmitted by black-legged Ixodes scapularis ticks to humans and animals. This bacterium enters host cells, forms a host-derived vacuole, and multiplies within this vacuole. The molecules that are critical in the formation of host-derived vacuole in tick cells is currently not well-characterized. In this study, we provide evidence that arthropod vesicular-associated membrane proteins, VAMP3 and VAMP4, are critical for A. phagocytophilum early and persistent infection in tick cells. These arthropod proteins are important for the formation of host-derived vacuoles in tick cells. Our study also provides evidence that these proteins are important for A. phagocytophilum acquisition from the infected murine host into ticks. Characterization of tick molecules important in bacterial entry and/or survival in the vector host could lead to the development of strategies to target this and perhaps other rickettsial pathogens.

Hijacking host cell vesicular transport: New insights into the nutrient acquisition mechanism of Chlamydia

Chlamydia infection is an important cause of public health diseases, and no effective vaccine is currently available. Owing to its unique intracellular lifestyle, Chlamydia requires a variety of nutrients and substrates from host cells, particularly sphingomyelin, cholesterol, iron, amino acids, and the mannose-6-phosphate receptor, which are essential for inclusion development. Here, we summarize the recent advances in Chlamydia nutrient acquisition mechanism by hijacking host cell vesicular transport, which plays an important role in chlamydial growth and development. Chlamydia obtains the components necessary to complete its intracellular developmental cycle by recruiting Rab proteins (major vesicular trafficking regulators) and Rab effector proteins to the inclusion, interfering with Rab-mediated multivesicular trafficking, reorienting the nutrition of host cells, and reconstructing the intracellular niche environment. Consequently, exploring the role of vesicular transport in nutrient acquisition offers a novel perspective on new approaches for preventing and treating Chlamydia infection.

SNAREs: a double-edged sword for intravacuolar bacterial pathogens within host cells

In the tug-of-war between host and pathogen, both evolve to combat each other's defence arsenals. Intracellular phagosomal bacteria have developed strategies to modify the vacuolar niche to suit their requirements best. Conversely, the host tries to target the pathogen-containing vacuoles towards the degradative pathways. The host cells use a robust system through intracellular trafficking to maintain homeostasis inside the cellular milieu. In parallel, intracellular bacterial pathogens have coevolved with the host to harbour strategies to manipulate cellular pathways, organelles, and cargoes, facilitating the conversion of the phagosome into a modified pathogen-containing vacuole (PCV). Key molecular regulators of intracellular traffic, such as changes in the organelle (phospholipid) composition, recruitment of small GTPases and associated effectors, soluble N-ethylmaleimide-sensitive factor-activating protein receptors (SNAREs), etc., are hijacked to evade lysosomal degradation. Legionella, Salmonella, Coxiella, Chlamydia, Mycobacterium, and Brucella are examples of pathogens which diverge from the endocytic pathway by using effector-mediated mechanisms to overcome the challenges and establish their intracellular niches. These pathogens extensively utilise and modulate the end processes of secretory pathways, particularly SNAREs, in repurposing the PCV into specialised compartments resembling the host organelles within the secretory network; at the same time, they avoid being degraded by the host's cellular mechanisms. Here, we discuss the recent research advances on the host-pathogen interaction/crosstalk that involves host SNAREs, conserved cellular processes, and the ongoing host-pathogen defence mechanisms in the molecular arms race against each other. The current knowledge of SNAREs, and intravacuolar bacterial pathogen interactions, enables us to understand host cellular innate immune pathways, maintenance of homeostasis, and potential therapeutic strategies to combat ever-growing antimicrobial resistance.

The Chlamydia trachomatis inclusion membrane protein CT006 associates with lipid droplets in eukaryotic cells

Chlamydia trachomatis causes genital and ocular infections in humans. This bacterial pathogen multiplies exclusively within host cells in a characteristic vacuole (inclusion) and delivers proteins such as inclusion membrane proteins (Incs) into the host cell. Here, we identified CT006 as a novel C. trachomatis protein that when expressed ectopically eukaryotic cells can associate with lipid droplets (LDs). A screen using Saccharomyces cerevisiae identified two Incs causing vacuolar protein sorting defects and seven Incs showing tropism for eukaryotic organelles. Ectopic expression in yeast and mammalian cells of genes encoding different fragments of CT006 revealed tropism for the endoplasmic reticulum and LDs. We identified a LD-targeting region within the first 88 amino acid residues of CT006, and positively charged residues important for this targeting. Comparing with the parental wild-type strain, cells infected by a newly generated C. trachomatis strain overproducing CT006 with a double hemagglutinin tag showed a slight increase in the area occupied by LDs within the inclusion region. However, we could not correlate this effect with the LD-targeting regions within CT006. We further showed that both the amino and carboxy-terminal regions of CT006, flanking the Inc-characteristic bilobed hydrophobic domain, are exposed to the host cell cytosol during C. trachomatis infection, supporting their availability to interact with host cell targets. Altogether, our data suggest that CT006 might participate in the interaction of LDs with C. trachomatis inclusions.

Cross Talk between ARF1 and RhoA Coordinates the Formation of Cytoskeletal Scaffolds during Chlamydia Infection

Chlamydia trachomatis is an obligate intracellular bacterium that has developed sophisticated mechanisms to survive inside its infectious compartment, the inclusion. Notably, Chlamydia weaves an extensive network of microtubules (MTs) and actin filaments to enable interactions with host organelles and enhance its stability. Despite the global health and economic burden caused by this sexually transmitted pathogen, little is known about how actin and MT scaffolds are integrated into an increasingly complex virulence system. Previously, we established that the chlamydial effector InaC interacts with ARF1 to stabilize MTs. We now demonstrate that InaC regulates RhoA to control actin scaffolds. InaC relies on cross talk between ARF1 and RhoA to coordinate MTs and actin, where the presence of RhoA downregulates stable MT scaffolds and ARF1 activation inhibits actin scaffolds. Understanding how Chlamydia hijacks complex networks will help elucidate how this clinically significant pathogen parasitizes its host and reveal novel cellular signaling pathways.

Shifting proteomes: limitations in using the BioID proximity labeling system to study SNARE protein trafficking during infection with intracellular pathogens

We hypothesize that intracellular trafficking pathways are altered in chlamydial infected cells to maximize the ability of Chlamydia to scavenge nutrients while not overtly stressing the host cell. Previous data demonstrated the importance of two eukaryotic SNARE proteins, VAMP4 and syntaxin 10 (Stx10), in chlamydial growth and development. Although, the mechanism for these effects is still unknown. To interrogate whether chlamydial infection altered these proteins' networks, we created BirA*-VAMP4 and BirA*-Stx10 fusion constructs to use the BioID proximity labeling system. While we identified a novel eukaryotic protein-protein interaction between Stx10 and VAPB, we also identified caveats in using the BioID system to study the impact of infection by an obligate intracellular pathogen on SNARE protein networks. The addition of the BirA* altered the localization of VAMP4 and Stx10 during infection with Chlamydia trachomatis serovars L2 and D and Coxiella burnetii Nine Mile Phase II. We also discovered that BirA* traffics to and biotinylates Coxiella-containing vacuoles and, in general, has a propensity for labeling membrane or membrane-associated proteins. While the BioID system identified a novel association for Stx10, it is not a reliable methodology to examine intracellular trafficking pathway dynamics during infection with intracellular pathogens.

Got mutants? How advances in chlamydial genetics have furthered the study of effector proteins

Chlamydia trachomatis is the leading cause of infectious blindness and a sexually transmitted infection. All chlamydiae are obligate intracellular bacteria that replicate within a membrane-bound vacuole termed the inclusion. From the confines of the inclusion, the bacteria must interact with many host organelles to acquire key nutrients necessary for replication, all while promoting host cell viability and subverting host defense mechanisms. To achieve these feats, C. trachomatis delivers an arsenal of virulence factors into the eukaryotic cell via a type 3 secretion system (T3SS) that facilitates invasion, manipulation of host vesicular trafficking, subversion of host defense mechanisms and promotes bacteria egress at the conclusion of the developmental cycle. A subset of these proteins intercalate into the inclusion and are thus referred to as inclusion membrane proteins. Whereas others, referred to as conventional T3SS effectors, are released into the host cell where they localize to various eukaryotic organelles or remain in the cytosol. Here, we discuss the functions of T3SS effector proteins with a focus on how advances in chlamydial genetics have facilitated the identification and molecular characterization of these important factors.