The roles of unfolded protein response pathways in Chlamydia pathogenesis

Abstract

Chlamydia is an obligate intracellular bacterium that relies on host cells for essential nutrients and adenosine triphosphate (ATP) for a productive infection. Although the unfolded protein response (UPR) plays a major role in certain microbial infectivity, its role in chlamydial pathogenesis is unknown. We hypothesized that Chlamydia induces UPR and exploits it to upregulate host cell uptake and metabolism of glucose, production of ATP, phospholipids, and other molecules required for its replicative development and host survival. Using a combination of biochemical and pathway inhibition assays, we showed that the 3 UPR pathway transducers-protein kinase RNA-activated (PKR)-like ER kinase (PERK), inositol-requiring enzyme-1α (IRE1α), and activating transcription factor-6α (ATF6α)-were activated during Chlamydia infection. The kinase activity of PERK and ribonuclease (RNase) of IRE1α mediated the upregulation of hexokinase II and production of ATP via substrate-level phosphorylation. In addition, the activation of PERK and IRE1α promoted autophagy formation and apoptosis resistance for host survival. Moreover, the activation of IRE1α resulted in the generation of spliced X-box binding protein 1 (sXBP1) and upregulation of lipid production. The vital role of UPR pathways in Chlamydia development and pathogenesis could lead to the identification of potential molecular targets for therapeutics against Chlamydia.

The molecular mechanism of induction of unfolded protein response by Chlamydia

“…The activation of the IRE1α arm of the UPR pathways by all chlamydial strains and serovars reflected the fact that it is the most conserved of all the three UPR arms in eukaryotes (36). We therefore confirmed and extended previous studies with primary mouse epithelial cells that all the three arms of UPR were activated by Chlamydia (19). Since UPR induction may involve cytoskeletal protein assembly and interaction with the master regulator (BiP) and transducers of UPR signaling especially IRE1α, and chlamydial T3SS effector proteins bind to cytoskeletal proteins (34), we tested the hypothesis that Tarp and CT228 are involved in UPR initiation.…”

“…We extended previous studies that the mouse agent of pneumonitis ( C. muridarum ) activated all the three arms of UPR (19), by establishing that different strains and serovars of Chlamydia induce UPR. Thus, the assessment of UPR induction in epithelial cells using the human C. trachomatis (Ct.) serovars D and L. or the murine strain C. muridarum , revealed that all the chlamydial strains activated the IRE1α arm of the UPR (Figure 1a).…”

“…Since UPR is a relatively early event during chlamydial infection (19), we predicted that the cytoskeleton-binding Chlamydia T3SS effectors proteins involved in UPR induction would be present either in the elementary bodies (EBs) and/or expressed early following host cell invasion by Chlamydia . Thus, the presence of Tarp and CT228 in EBs of C. trachomatis serovar L2 (Ct L2) and their expression pattern during infection were analyzed with western blot using anti-Tarp and anti-CT228 antibodies.…”

“…Chlamydia muridarum Nigg (the agent of mouse pneumonitis, MoPn - animal specific strain), C. trachomatis (human specific strain) serovars L2/LGV-434 and D/UW-3 were grown in HeLa 229 cells (ATCC, Rockville, MD USA) and purified elementary bodies (EBs) were tittered as infectious forming units per millimeter (IFU/ml) using standard procedures previously described (19)(38)(39). …”

“…Indeed, recent reports have demonstrated that Chlamydia , like certain other obligate intracellular microbial agents such as Brucella melitensis , Listeria monocytogenes and Hepatitis C virus, activate the UPR pathways to enhance their intracellular survival and replication (15, 17–19). Mechanistically, Shiga toxigenic strains of Escherichia coli produce AB5 subtilase cytotoxin that binds to and inactivates BiP to activate the UPR transducers (20); Hepatitis C virus activates UPR at least in part due to the accumulation of immature core protein (Core 199) in the ER lumen (15), while L. monocytogenes require its cholesterol-dependent cytolysin toxin (listeriolysin O) to induce UPR (18).…”