Orientia tsutsugamushi Modulates Endoplasmic Reticulum-Associated Degradation To Benefit Its Growth

Abstract

Orientia tsutsugamushi, an obligate intracellular bacterium that is auxotrophic for the aromatic amino acids and histidine, causes scrub typhus, a potentially deadly infection that threatens 1 billion people. O. tsutsugamushi growth is minimal during the first 24 to 48 h of infection but its growth becomes logarithmic thereafter. How the pathogen modulates cellular functions to support its growth is poorly understood. The unfolded protein response (UPR) is a cytoprotective pathway that relieves endoplasmic reticulum (ER) stress by promoting ER-associated degradation (ERAD) of misfolded proteins. Here, we show that O. tsutsugamushi invokes the UPR in the first 48 h and benefits from ER stress in an amino acid-dependent manner. O. tsutsugamushi also impedes ERAD during this time period. By 72 h, ER stress is alleviated and ERAD proceeds unhindered. Sustained inhibition of ERAD using RNA interference results in an O. tsutsugamushi growth defect at 72 h that can be rescued by amino acid supplementation. Thus, O. tsutsugamushi temporally stalls ERAD until ERAD-derived amino acids are needed to support its growth. The O. tsutsugamushi effector Ank4 is linked to this phenomenon. Ank4 interacts with Bat3, a eukaryotic chaperone that is essential for ERAD, and is transiently expressed by O. tsutsugamushi during the infection period when it inhibits ERAD. Ectopically expressed Ank4 blocks ERAD to phenocopy O. tsutsugamushi infection. Our data reveal a novel mechanism by which an obligate intracellular bacterial pathogen modulates ERAD to satisfy its nutritional virulence requirements.

The Obligate Intracellular Bacterium Orientia tsutsugamushi Targets NLRC5 To Modulate the Major Histocompatibility Complex Class I Pathway

“…As a first step in determining if O. tsutsugamushi modulates the MHC-I pathway, the levels of MHC-I molecules on the surfaces of infected and uninfected HeLa cells were compared. HeLa cells were ideal for this purpose because they are models for studying O. tsutsugamushi-host cell interactions and constitutively express MHC-I (21)(22)(23)(24)(25)(26)(27). Also, because they are nonprofessional APCs, HeLa cells express NLRC5 but not CIITA and are therefore useful for analyzing MHC-I gene expression exclusively in the context of NLRC5 function.…”

Orientia tsutsugamushi uses two Ank effectors to modulate NF-κB p65 nuclear transport and inhibit NF-κB transcriptional activation

“…Evidence for the importance of these Type 1 secretion system effectors to O . tsutsugamushi pathobiology continues to mount, as the bacterium transcriptionally expresses its entire Ank repertoire during infection of mammalian cells and select Anks have been shown to co-opt or modulate SCF1 ubiquitin ligase assembly, Golgi-to-endoplasmic reticulum (ER) retrograde trafficking, ER stress, ER-associated degradation, and protein secretion [ 22 – 26 ].…”

Hostile Takeover: Hijacking of Endoplasmic Reticulum Function by T4SS and T3SS Effectors Creates a Niche for Intracellular Pathogens

“…Like B. abortus, O. tsutsugamushi, the obligate intracellular agent of scrub typhus, activates the UPR. (9, 27). Two T1SS-secreted effectors have been implicated (Table 1): Ank4, an ankyrin repeat protein, interacts at the cytosolic face of the ER with Bat3, a host cytosolic chaperone involved in ERAD (80), to inhibit UPR-induced ERAD during the early (non-replicative) phase of O. tsutsugamushi infection.…”

“…(5, 6), and Chlamydia trachomatis and its relatives (7, 8) with the ER has been recognized relatively recently. However, manipulation of ER function is not limited to pathogens that replicate within a vacuole, as cytosolic pathogens such as Orientia tsutsugamushi (9, 10) and Rickettsia rickettsii (11) also target ER-based functions via secreted effectors to promote their intracellular growth.…”