Modulation of Autophagy and Cell Death by Bacterial Outer-Membrane Vesicles

Abstract

Bacteria, akin to eukaryotic cells, possess the ability to release extracellular vesicles, lipidic nanostructures that serve diverse functions in host-pathogen interactions during infections. In particular, Gram-negative bacteria produce specific vesicles with a single lipidic layer called OMVs (Outer Membrane Vesicles). These vesicles exhibit remarkable capabilities, such as disseminating throughout the entire organism, transporting toxins, and being internalized by eukaryotic cells. Notably, the cytosolic detection of lipopolysaccharides (LPSs) present at their surface initiates an immune response characterized by non-canonical inflammasome activation, resulting in pyroptotic cell death and the release of pro-inflammatory cytokines. However, the influence of these vesicles extends beyond their well-established roles, as they also profoundly impact host cell viability by directly interfering with essential cellular machinery. This comprehensive review highlights the disruptive effects of these vesicles, particularly on autophagy and associated cell death, and explores their implications for pathogen virulence during infections, as well as their potential in shaping novel therapeutic approaches.

Keywords: OMV; apoptosis; autophagy; cell death; infection; inflammation; pyroptosis; xenophagy.

Figure 1

Diversity of bacteria extravesicular vesicles (a) composition and (b) mode of internalization in eukaryotic cells. (a) Structures and compositions of different bacterial extracellular vesicles. Outer Membrane Vesicles (OMVs) are single lipid bilayer membrane vesicles from the outer membrane of Gram-negative bacteria and carry compounds present in the outer membrane (LPSs, lipids and proteins of the outer membrane) and a content originating from the periplasmic space (fragments of peptidoglycan, secondary metabolites, proteins or toxins). Outer–Inner Membrane Vesicles (OIMVs) are double lipid bilayer membrane vesicles from the outer and the inner membrane of Gram-negative bacteria and carry periplasmic and cytoplasmic content (proteins, toxins, DNA, etc.). Cytoplasmic membrane vesicles (CMVs) from Gram-positive bacteria also contain proteins, toxins or DNA. (b) Internalization of BEVs in eukaryotic cells occurs by endocytosis or direct fusion at the plasma membrane. During endocytosis, the plasma membrane folds with itself and forms an endosome. This process can be lipid-raft dependent (caveolae mediated) or lipid-raft independent (clathrin mediated); both pathways depend on dynamin to close the developing endosome. Macropinocytosis consists of an actin-dependent invagination of the plasma membrane composed of both lipid rafts and non-lipid rafts structures. Fusion occurs between the membrane of the BEV and the plasma membrane, notably lipid rafts, of the eukaryotic cells. Internalization of BEVs and release of toxins into the cytoplasm of the host cell can lead to an autophagic modulation and eventually cell death (adapted from [9]).

Figure 3

Modulation of host cell autophagy by pathogenic bacteria. The different steps of the autophagic flux and the possible bacterial subversions that lead to autophagy are depicted: (1) Increased autophagosomes formation, (2) loss of pathogen recognition, (3) inhibition of phagophore formation and (4) inhibition of lysosomal degradation (adapted from [60]).

Figure 2

Mechanisms of cell death triggered by bacterial outer-membrane vesicles (OMVs). (a) Different cell death pathways can be induced by OMVs. Apoptosis is characterized by the fragmentation of the cells into apoptotic bodies, whereas pyroptosis induces pore formation in the plasma membrane of immune cells, leading to cell lysis. OMVs are also suspected to impact many types of cellular machinery (endoplasmic reticulum, autophagy or mitochondria), leading to cellular stress and cell death. (b) Apoptosis can be triggered through two pathways, the extrinsic and the intrinsic pathways. The extrinsic pathway is initiated by a pro-death signal from outside the cell recognized by the TNF-α receptors. The intrinsic pathway is induced by intracellular injury or activation of BAX and BAK proteins and the downregulation of pro-survival BCL 2 proteins. OMVs can trigger intrinsic apoptotic cell death by impacting mitochondrial integrity directly, by the release of toxins or by the modulation of pro-survival protein synthesis. Pyroptosis can be triggered by pathogens or pathogen-associated molecular patterns (PAMPs) recognition, leading to the activation of the canonical or the non-canonical inflammasome pathways. Caspase 11 senses LPSs or OMVs directly in the cytosol of eukaryotic cells and activates GSDMD, leading to cell lysis.