Sphingolipid trafficking and purification in Chlamydia trachomatis-infected cells

The Small Molecule H89 Inhibits Chlamydia Inclusion Growth and Production of Infectious Progeny

Chlamydia is an obligate intracellular bacterium and the most common reportable cause of human infection in the United States. This pathogen proliferates inside a eukaryotic host cell, where it resides within a membrane-bound compartment called the chlamydial inclusion. It has an unusual developmental cycle, marked by conversion between a replicating form, the reticulate body (RB), and an infectious form, the elementary body (EB). We found that the small molecule H89 slowed inclusion growth and decreased overall RB replication by 2-fold but caused a 25-fold reduction in infectious EBs. This disproportionate effect on EB production was mainly due to a defect in RB-to-EB conversion and not to the induction of chlamydial persistence, which is an altered growth state. Although H89 is a known inhibitor of specific protein kinases and vesicular transport to and from the Golgi apparatus, it did not cause these anti-chlamydial effects by blocking protein kinase A or C or by inhibiting protein or lipid transport. Thus, H89 is a novel anti-chlamydial compound that has a unique combination of effects on an intracellular Chlamydia infection.

Sphingolipid Metabolism and Transport in Chlamydia trachomatis and Chlamydia psittaci Infections

Chlamydia species infect a large range of vertebral hosts and have become of major economic and public health concern over the last decades. They are obligate intracellular bacteria that undergo a unique cycle of development characterized by the presence of two distinct bacterial forms. After infection of the host cell, Chlamydia are found inside a membrane-bound compartment, the inclusion. The surrounding membrane of the inclusion contributes to the host-Chlamydia interface and specific pathogen-derived Inc proteins shape this interface allowing interactions with distinct cellular proteins. In contrast to many other bacteria, Chlamydia species acquire sphingomyelin from the host cell. In recent years a clearer picture of how Chlamydia trachomatis acquires this lipid emerged showing that the bacteria interact with vesicular and non-vesicular transport pathways that involve the recruitment of specific RAB proteins and the lipid-transfer protein CERT. These interactions contribute to the development of a new sphingomyelin-producing compartment inside the host cell. Interestingly, recruitment of CERT is conserved among different Chlamydia species including Chlamydia psittaci. Here we discuss our current understanding on the molecular mechanisms used by C. trachomatis and C. psittaci to establish these interactions and to create a novel sphingomyelin-producing compartment inside the host cell important for the infection.

Quantitative Dextran Trafficking to the Coxiella burnetii Parasitophorous Vacuole

The gram-negative bacterium Coxiella burnetii causes human Q fever, a disease characterized by a debilitating flu-like illness in acute cases and endocarditis in chronic patients. An obligate intracellular pathogen, Coxiella burnetii survives within a large, lysosome-like vacuole inside the host cell. A unique feature of the Coxiella parasitophorous vacuole (PV) is high levels of fusion with the host endocytic pathway, with PV-endosome fusion critical for Coxiella survival within the host cell. This unit describes quantitating PV-endosome fusion by measuring delivery of the fluid phase endosome marker dextran to the PV using live cell imaging. To study the effect of host cell proteins involved in PV-endosome fusion, details are provided for using siRNA knockdown host cells. This method is a powerful tool for understanding mechanisms underlying Coxiella's ability to manipulate host cell trafficking pathways. © 2017 by John Wiley & Sons, Inc.

Development of a Proximity Labeling System to Map the Chlamydia trachomatis Inclusion Membrane

Chlamydia grows within a membrane-bound vacuole termed an inclusion. The cellular processes that support the biogenesis and integrity of this pathogen-specified parasitic organelle are not understood. Chlamydia secretes integral membrane proteins called Incs that insert into the chlamydial inclusion membrane (IM). Incs contain at least two hydrophobic transmembrane domains flanked by termini, which vary in size and are exposed to the host cytosol. In addition, Incs are temporally expressed during the chlamydial developmental cycle. Data examining Inc function are limited because of (i) the difficulty in working with hydrophobic proteins and (ii) the inherent fragility of the IM. We hypothesize that Incs function collaboratively to maintain the integrity of the chlamydial inclusion with small Incs organizing the IM and larger Incs interfacing with host cell machinery. To study this hypothesis, we have adapted a proximity-labeling strategy using APEX2, a mutant soybean ascorbate peroxidase that biotinylates interacting and proximal proteins within minutes in the presence of H₂O₂ and its exogenous substrate, biotin-phenol. We successfully expressed, from an inducible background, APEX2 alone, or fusion proteins of IncA_TM (TM = transmembrane domain only), IncA, and IncF with APEX2 in Chlamydia trachomatis serovar L2. IncF-APEX2, IncA _TM -APEX2, and IncA-APEX2 localized to the IM whereas APEX2, lacking a secretion signal, remained associated with the bacteria. We determined the impact of overexpression on inclusion diameter, plasmid stability, and Golgi-derived sphingomyelin acquisition. While there was an overall impact of inducing construct expression, IncF-APEX2 overexpression most negatively impacted these measurements. Importantly, Inc-APEX2 expression in the presence of biotin-phenol resulted in biotinylation of the IM. These data suggest that Inc expression is regulated to control optimal IM biogenesis. We subsequently defined lysis conditions that solubilized known Incs and were compatible with pulldown conditions. Importantly, we have created powerful tools to allow direct examination of the dynamic composition of the IM, which will provide novel insights into key interactions that promote chlamydial growth and development within the inclusion.

The Making and Taking of Lipids: The Role of Bacterial Lipid Synthesis and the Harnessing of Host Lipids in Bacterial Pathogenesis

In order to survive environmental stressors, including those induced by growth in the human host, bacterial pathogens will adjust their membrane physiology accordingly. These physiological changes also include the use of host-derived lipids to alter their own membranes and feed central metabolic pathways. Within the host, the pathogen is exposed to many stressful stimuli. A resulting adaptation is for pathogens to scavenge the host environment for readily available lipid sources. The pathogen takes advantage of these host-derived lipids to increase or decrease the rigidity of their own membranes, to provide themselves with valuable precursors to feed central metabolic pathways, or to impact host signalling and processes. Within, we review the diverse mechanisms that both extracellular and intracellular pathogens employ to alter their own membranes as well as their use of host-derived lipids in membrane synthesis and modification, in order to increase survival and perpetuate disease within the human host. Furthermore, we discuss how pathogen employed mechanistic utilization of host-derived lipids allows for their persistence, survival and potentiation of disease. A more thorough understanding of all of these mechanisms will have direct consequences for the development of new therapeutics, and specifically, therapeutics that target pathogens, while preserving normal flora.

The trans-Golgi SNARE syntaxin 10 is required for optimal development of Chlamydia trachomatis

Chlamydia trachomatis, an obligate intracellular pathogen, grows inside of a vacuole, termed the inclusion. Within the inclusion, the organisms differentiate from the infectious elementary body (EB) into the reticulate body (RB). The RB communicates with the host cell through the inclusion membrane to obtain the nutrients necessary to divide, thus expanding the chlamydial population. At late time points within the developmental cycle, the RBs respond to unknown molecular signals to redifferentiate into infectious EBs to perpetuate the infection cycle. One strategy for Chlamydia to obtain necessary nutrients and metabolites from the host is to intercept host vesicular trafficking pathways. In this study we demonstrate that a trans-Golgi soluble N-ethylmaleimide–sensitive factor attachment protein (SNARE), syntaxin 10, and/or syntaxin 10-associated Golgi elements colocalize with the chlamydial inclusion. We hypothesized that Chlamydia utilizes the molecular machinery of syntaxin 10 at the inclusion membrane to intercept specific vesicular trafficking pathways in order to create and maintain an optimal intra-inclusion environment. To test this hypothesis, we used siRNA knockdown of syntaxin 10 to examine the impact of the loss of syntaxin 10 on chlamydial growth and development. Our results demonstrate that loss of syntaxin 10 leads to defects in normal chlamydial maturation including: variable inclusion size with fewer chlamydial organisms per inclusion, fewer infectious progeny, and delayed or halted RB-EB differentiation. These defects in chlamydial development correlate with an overabundance of NBD-lipid retained by inclusions cultured in syntaxin 10 knockdown cells. Overall, loss of syntaxin 10 at the inclusion membrane negatively affects Chlamydia. Understanding host machinery involved in maintaining an optimal inclusion environment to support chlamydial growth and development is critical toward understanding the molecular signals involved in successful progression through the chlamydial developmental cycle.

Vesicle-associated membrane protein 4 and syntaxin 6 interactions at the chlamydial inclusion

The predominant players in membrane fusion events are the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) family of proteins. We hypothesize that SNARE proteins mediate fusion events at the chlamydial inclusion and are important for chlamydial lipid acquisition. We have previously demonstrated that trans-Golgi SNARE syntaxin 6 localizes to the chlamydial inclusion. To investigate the role of syntaxin 6 at the chlamydial inclusion, we examined the localization and function of another trans-Golgi SNARE and syntaxin 6-binding partner, vesicle-associated membrane protein 4 (VAMP4), at the chlamydial inclusion. In this study, we demonstrate that syntaxin 6 and VAMP4 colocalize to the chlamydial inclusion and interact at the chlamydial inclusion. Furthermore, in the absence of VAMP4, syntaxin 6 is not retained at the chlamydial inclusion. Small interfering RNA (siRNA) knockdown of VAMP4 inhibited chlamydial sphingomyelin acquisition, correlating with a log decrease in infectious progeny. VAMP4 retention at the inclusion was shown to be dependent on de novo chlamydial protein synthesis, but unlike syntaxin 6, VAMP4 recruitment is observed in a species-dependent manner. Notably, VAMP4 knockdown inhibits sphingomyelin trafficking only to inclusions in which it localizes. These data support the hypothesis that VAMP proteins play a central role in mediating eukaryotic vesicular interactions at the chlamydial inclusion and, thus, support chlamydial lipid acquisition and chlamydial development.