Bacterial pathogens commandeer Rab GTPases to establish intracellular niches

Abstract

Intracellular bacterial pathogens deploy virulence factors termed effectors to inhibit degradation by host cells and to establish intracellular niches where growth and differentiation take place. Here, we describe mechanisms by which human bacterial pathogens (including Chlamydiae; Coxiella burnetii; Helicobacter pylori; Legionella pneumophila; Listeria monocytogenes; Mycobacteria; Pseudomonas aeruginosa, Salmonella enterica) modulate endocytic and exocytic Rab GTPases in order to thrive in host cells. Host cell Rab GTPases are critical for intracellular transport following pathogen phagocytosis or endocytosis. At the molecular level bacterial effectors hijack Rab protein function to: evade degradation, direct transport to particular intracellular locations and monopolize host vesicles carrying molecules that are needed for a stable niche and/or bacterial growth and differentiation. Bacterial effectors may serve as specific receptors for Rab GTPases or as enzymes that post-translationally modify Rab proteins or endosomal membrane lipids required for Rab function. Emerging data indicate that bacterial effector expression is temporally and spatially regulated and multiple virulence factors may act concertedly to usurp Rab GTPase function, alter signaling and ensure niche establishment and intracellular bacterial growth, making this field an exciting area for further study.

Figure 1. Rab GTPase regulated pathways

Over 60 Rab GTPase family members regulate membrane transport on the exocytic, endocytic, phagocytic and recycling pathways. Shown are the normal functions (arrows) and localizations (black circles) of Rab GTPases that are targeted by bacterial pathogens as detailed in the text and Table 3. Phagosome and autophagosome maturation depend on the sequential fusion with early endosomes, late endosomes and lysosomes. Additional components needed for phagosome maturation are likely recruited from interactions with the secretory pathway based on the involvement of Rab GTPases with primary functions in exocytosis, organelle biogenesis, ER and Golgi dynamics, and mitochondrial function (Rab20, Rab32, Rab38, Rab43). Over 40 Rab GTPases have been identified on phagosomes at various stages of maturation; depicted are 24 Rab GTPases whose kinetic acquisition and functions have been characterized by analyses of latex bead and non-pathogenic bacterial phagosomes (see text for detail). Some Rab GTPases may be acquired in a biphasic manner (51) and others transition gradually with small changes in concentration and phosphorylation state triggering changes in activity (12; 53; 55). MAM, mitochondria associated membrane (thought to be of ER origin); SV, synaptic vesicle; SG, secretory granule.

Figure 2. Intracellular bacterial pathogens create specialized niches in macrophage and epithelial hosts by modulating Rab GTPases

Pathogens alter Rab GTPase functions to escape degradation and obtain essential nutrients for growth and survival. (A) Macrophage host. Legionella pneumophila enters alveolar macrophages by coiling phagocytosis and creates a replicative niche in close apposition to the endoplasmic reticulum (ER) by modulating the activity of Rab1 and Rab35. Legionella evades fusion with lysosomes (L), though some exchange with endosomes may take place and pH is mildly acidic. Salmonella enterica can infect enterocytes or traverse the intestinal epithelial barrier by transcytosis, and in the subluminal Peyer’s patches be phagocytosed by macrophages and dendritic cells that can promote systemic dissemination and infection. Salmonella enterica coopts active Rab7-regulated, microtubule transport to establish a replicative niche in the peri-Golgi region and form tubules called Sifs that promote cell-to-cell spread. Mycobacterium tuberculosis and Coxiella burnetii both preferentially infect alveolar macrophages, though Mycobacteria allow only early endosome (EE) fusion and induce phagosome arrest by selective Rab GTPase recruitment to avoid fusion with late endosomes (LE) and lysosomes. Coxiella-containing phagosomes on the other hand fuse with late endosomes, lysosomes and autophagosomes (AP), therefore, Coxiella burnetii are adapted to thrive in an acidic niche. (B) Epithelial Host. Listeria monocytogenes infects macrophages, intestinal epithelia and hepatocytes; gaining entry by specific binding to and internalization with E-cadherin or Met receptors and evading degradation by blocking Rab5 before release to the cytoplasm. Helicobacter pylori infect intestinal epithelia through the apical recruitment of tight junction proteins (ZO-1 and Jam-A) and after internalization establish a replicative niche via the vacuolating toxin VacA and Rab7-mediated fusion with endosomes. Chlamydia trachomatis (Ct) or pneumonia (Cp) infect epithelia from the apical surface and utilize Rab-regulated, microtubule transport to establish a specialized inclusion in the peri-Golgi region that depends on Rab-regulated fusion with early endosomes, late endosomes and Golgi-derived vesicles. Pseudomonas aeruginosa recruits the phosphatidylinositol 3-kinase to the apical plasma membrane (PM) where it resides within plasma membrane blebs and blocks endocytosis by ribosylation of Rab5.

Figure 3. Schematic of bacterial effector proteins secreted by Legionella pneumophila to subvert Rab function

The figure illustrates the modification of Rab proteins by (left) phosphocholine mediated by AnkX and (right) adenosine monophosphate mediated by DrrA. Phosphocholination strongly inhibits GEF catalysis by connecdenn 1 while adenylylation strongly impairs binding of the human effector protein Mical-3 and inactivation by GAPs (e.g. LepB). In contrast, the ‘supereffector’ LidA can bind Rab1 also in the modified states and might act as a tethering factor. Interaction with GDI can only occur after removal of the modifications, indicating a possible role of the modifications in recruitment and entrapment of Rab proteins at the surface of endogenous membranes. All proteins encoded by Legionella pneumophila for subversion of Rab function are indicated in red letters.