Abstract
Brucella spp. are intracellular vacuolar pathogens that causes brucellosis, a worldwide zoonosis of profound importance. We previously demonstrated that the activity of host unfolded protein response (UPR) sensor IRE1α (inositol-requiring enzyme 1) and ER-associated autophagy confer susceptibility to Brucella melitensis and Brucella abortus intracellular replication. However, the mechanism by which host IRE1α regulates the pathogen intracellular lifestyle remains elusive. In this study, by employing a diverse array of molecular approaches, including biochemical analyses, fluorescence microscopy imaging, and infection assays using primary cells derived from Ern1 (encoding IRE1) conditional knockout mice, we address this gap in our understanding by demonstrating that a novel IRE1α to ULK1, an important component for autophagy initiation, signaling axis confers susceptibility to Brucella intracellular parasitism. Importantly, deletion or inactivation of key signaling components along this axis, including IRE1α, BAK/BAX, ASK1, and JNK as well as components of the host autophagy system ULK1, Atg9a, and Beclin 1, resulted in striking disruption of Brucella intracellular trafficking and replication. Host kinases in the IRE1α-ULK1 axis, including IRE1α, ASK1, JNK1, and/or AMPKα as well as ULK1, were also coordinately phosphorylated in an IRE1α-dependent fashion upon the pathogen infection. Taken together, our findings demonstrate that the IRE1α-ULK1 signaling axis is subverted by the bacterium to promote intracellular parasitism, and provide new insight into our understanding of the molecular mechanisms of intracellular lifestyle of Brucella.
Highlights
Infectious diseases caused by bacterial pathogens contribute significantly to global disease burden
We found that RNAi-mediated depletion of these proteins reduced Brucella intracellular replication (Figure S1), thereby raising the possibility that an axis linking IRE1α and downstream autophagy proteins controls B. melitensis strain 16M (Bm16M) intracellular parasitism
This report demonstrates that disruption of IRE1α activity in host cells reduces Brucella intracellular replication by initiating a signaling cascade that regulates the maturation of Brucella–containing vacuoles (BCVs)
Summary
Infectious diseases caused by bacterial pathogens contribute significantly to global disease burden. Brucellosis has eluded systematic attempts at eradication for more than a century (Godfroid et al, 2002), and to date no approved human vaccine is available (Ficht and Adams, 2009; Pandey et al, 2016). These features contribute to the classification of Brucella as a potential bioterror agent and economic threat, and to the enormous interest expressed in this pathogen by the biosecurity and world health communities. The VirB type IV secretion system (T4SS) is a significant virulence factor that regulates Brucella intracellular trafficking (Marchesini et al, 2011; Paredes-Cervantes et al, 2011; Sá et al, 2012; Smith et al, 2012), and organisms that lack this system fail to establish an intracellular replicative niche
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