Listeriolysin O Is Necessary and Sufficient to Induce Autophagy during Listeria monocytogenes Infection

Abstract

BackgroundRecent studies have suggested that autophagy is utilized by cells as a protective mechanism against Listeria monocytogenes infection.Methodology/Principal FindingsHowever we find autophagy has no measurable role in vacuolar escape and intracellular growth in primary cultured bone marrow derived macrophages (BMDMs) deficient for autophagy (atg5−/−). Nevertheless, we provide evidence that the pore forming activity of the cholesterol-dependent cytolysin listeriolysin O (LLO) can induce autophagy subsequent to infection by L. monocytogenes. Infection of BMDMs with L. monocytogenes induced microtubule-associated protein light chain 3 (LC3) lipidation, consistent with autophagy activation, whereas a mutant lacking LLO did not. Infection of BMDMs that express LC3-GFP demonstrated that wild-type L. monocytogenes was encapsulated by LC3-GFP, consistent with autophagy activation, whereas a mutant lacking LLO was not. Bacillus subtilis expressing either LLO or a related cytolysin, perfringolysin O (PFO), induced LC3 colocalization and LC3 lipidation. Further, LLO-containing liposomes also recruited LC3-GFP, indicating that LLO was sufficient to induce targeted autophagy in the absence of infection. The role of autophagy had variable effects depending on the cell type assayed. In atg5−/− mouse embryonic fibroblasts, L. monocytogenes had a primary vacuole escape defect. However, the bacteria escaped and grew normally in atg5−/− BMDMs.Conclusions/SignificanceWe propose that membrane damage, such as that caused by LLO, triggers bacterial-targeted autophagy, although autophagy does not affect the fate of wild-type intracellular L. monocytogenes in primary BMDMs.

The arsenal of virulence factors deployed byListeria monocytogenesto promote its cell infection cycle

“…157 LLO was shown to be necessary and sufficient for SLAP formation, revealing a role for LLO in the vacuolar replication of L. monocytogenes, and suggesting that this bacterium is able to induce chronic infection of host macrophages. 157,158 Intracellular motility and cell-to-cell spread. The polarized bacterial surface protein ActA is one of the major virulence factors of L. monocytogenes.…”

Crystal structure of listeriolysin O reveals molecular details of oligomerization and pore formation

“…It has been known for some time that low concentrations of intracellular toxin affect host cell signalling in various ways, including activation of the p38 mitogen-activated protein kinase 8 , the c-Jun N-terminal pathways 9 and induction of pattern recognition receptors such as the NALP3 inflammasome 10 . In addition, LLO has been implicated in inducing autophagy 11 as well as in the suppression of reactive oxygen species produced by the NOX2 NADPH oxidase 12 . Over the past few years it has emerged that LLO activates diverse cellular processes including the dysregulation of post-translational modifications mediated by SUMOlyation 13 , induction of endoplasmic reticulum stress and reversible fragmentation of mitochondria 14 as well as regulatory epigenetic changes due to the post-translational modification of histone tails 15 .…”

Autophagy targeting ofListeria monocytogenesand the bacterial countermeasure

“…In addition, these authors confirmed that autophagy does not affect the fate of wild-type L. monocytogenes. 22 In other studies, Travassos et al used mouse embryonic fibroblasts (MEFs) to show that NOD1 and ATG16L are recruited to the plasma membrane at the site of L. monocytogenes entry, and that levels of LC3-positive bacteria in NOD1 -/-MEFs are significantly lower than in NOD +/+ MEFs. 23 Since L. monocytogenes deficient in LLO production are not decorated with LC3 in either NOD1 +/+ or NOD1 -/-MEFs, this suggests a functional link between the NOD1 and autophagy pathways, in which the plasma membrane ruptured by LLO becomes a target for autophagy.…”