RAB11-mediated trafficking in host-pathogen interactions

Abstract

Many bacterial and viral pathogens block or subvert host cellular processes to promote successful infection. One host protein that is targeted by invading pathogens is the small GTPase RAB11, which functions in vesicular trafficking. RAB11 functions in conjunction with a protein complex known as the exocyst to mediate terminal steps in cargo transport via the recycling endosome to cell-cell junctions, phagosomes and cellular protrusions. These processes contribute to host innate immunity by promoting epithelial and endothelial barrier integrity, sensing and immobilizing pathogens and repairing pathogen-induced cellular damage. In this Review, we discuss the various mechanisms that pathogens have evolved to disrupt or subvert RAB11-dependent pathways as part of their infection strategy.

Box1. Role of Rab11 and the exocyst in innate immunity

Box2. Similarities between pore-forming toxins and cAMP toxins

Figure 1. Rab small GTPase mediated endocytic recycling

a) Rab regulatory cycles. Rab-GTPases are regulated by two coupled cycles: guanine-nucleotide exchange factors (GEFs) exchange GDP for GTP to generate the active Rab•GTP bound form, whereas GTPase activating proteins (GAPs) generate the inactive Rab•GDP bound form. GEFs and GAPs can be highly selective for specific Rabs. Rabs are also regulated by extraction from vesicle membranes by cytoplasmic GDP dissociation inhibitory (GDIs) proteins that bind non-selectively to Rab•GDP forms and sequester them in the cytoplasm. GDIs have a hydrophobic pocket that binds the prenylated moiety of Rabs (blue line), which is otherwise inserted into the lipid bilayer of a vesicle. Membrane-bound GDI displacement factors (GDFs) extract Rab•GDP from complexes with GDIs and re-deliver them to the membrane compartments. b) Rab11-mediated junctional transport. Retrieval of membrane proteins into early endosomes occurs through endocytosis, which is mediated through Rab5 and Arf6, respectively. Cargo from the early endosome can be recycled back to the cell surface by a fast (that is, within minutes) Rab4-mediated pathway and a slow (that is, within hours) Rab11-mediated process via late recycling endosomes. Early endosomes can also traffic cargo via Rab5 to the multivesicular body (MVB; also known as the late endosome ) and then via Rab7 to the lysosome. Late recycling endosomes can also acquire cargo via Rab11 from the Golgi. Rab11+ late recycling endosomes are delivered to the cell surface along actin filaments via MyoV motors, where these vesicles are tethered to the plasma membrane by Rab11 binding to the exocyst component Sec15 forming a tripartite docking complex. The late recycling endosome are also decorated with vesicle SNARE (vSNARE) and the SNARE component SNAP25, which binds to the exocyst. Following docking, vSNARE forms a trimeric complex with SNAP25 and the membrane tethered tSNARE to initiate fusion of the late recycling endosome with the plasma membrane and delivery of cargo.

Figure 2. The mode of action of anthrax and cholera toxins

a) Bacillus anthracis edema toxin (comprising edema factor (EF) and protective antigen (PA)) and lethal toxin (comprising lethal factor (LF) and PA) bind to the host receptors CMG2 and TEM8, which leads to their endocytic uptake and translocation to the cytoplasm. EF, a calmodulin-dependent adenylate cyclase, remains perinuclear, where it produces cyclic AMP to activate PKA as well as EPAC and Rap1. The metalloprotease LF disperses into the cytoplasm to cleave and inactivate MEKs. These pathways then reduce levels of Rab11 (EF) on late recycling endosomes and of apical Sec15 (EF and LF), thereby blocking exocyst-mediated trafficking of junctional proteins (such as, cadherins and Notch ligands) to adherens junctions. The alignment of tight junctions with adherens junctions might also be altered. These effects of anthrax toxins on membrane junctions result in reduced endothelial barrier function and vascular leakage. b) The Vibrio cholerae toxin holoenzyme consists of the catalytic A-subunit (CtxA) and five B-subunits (CtxB). The toxin binds to the ganglioside GM1 on the plasma membrane, where it is endocytosed. CtxA, is translocated into the cytoplasm, where it binds to the Arf6 and activates host adenylate cyclases to increase cAMP levels and induce PKA as well as EPAC and Rap1. This results in the reduction of junctional levels of Rab11 and the exocyst component Sec15, which inhibits trafficking of cargo to adherens junctions and tight junctions. The junction disrupting effects of CtxA may increase paracellular efflux of Na+ and water thereby facilitating transcellular secretion of Cl− ions induced by PKA-dependent phosphorylation of CFTR. c) Cholera toxin produces apical gaps (arrows) and creates lacuna (asterisks) between intestinal epithelial cells and shortens microvilli in patients infected with V. cholerae (top: Modified/Reproduced with permission from Fig. 3A,C in ), in cholera toxin-treated CACO-2 cells (Modified/Reproduced with permission from Fig. 4A,B in ), and in the Drosophila gut (Modified/Reproduced with permission from Fig. 7O,P in – see also supplemental Fig. 7K-P in ).

Figure 3. Pathogen subversion of Rab11 and exocyst functions

The intracellular bacterial pathogen Salmonella uses a type III Secretion System (T3SS) to inject virulence factors into the host cell. The tips of the T3SS comprises two proteins that penetrate into the host cytoplasm, namely SipB and SipC. These proteins which penetrate into the cytoplasm, also interact with host proteins to manipulate membrane trafficking. SipC interacts with the exocyst component Exo70 to recruit membrane to sites of bacterial invasion (invadosomes). The Salmonella T3SS also injects the effectors SopE and SopE2. These proteins are RalA GEFs that promote an interaction between RalA and Sec5, which is essential for bacterial invasion. Shigella uses the T3SS proteins IpaB and IpaC to penetrate host cells. IpaB binds to and redirects cholesterol, to the site of bacterial entry from the Golgi, leading to Golgi fragmentation. Golgi fragmentation leads to tubulation of the Rab11 compartment and disruption of E-cadherin transport to adherens junctions and thus junctional disruption. Chlamydia, another intracellular pathogen, also induces Golgi fragmentation via interactions with Rab11 and Rab6 to disrupt anterograde (Rab11 mediated) and retrograde (Rab6-mediated) transport between the Golgi and late recycling endosomes. This promotes Chlamydia development and reproduction. Golgi fragmentation through Rab11 also requires the resident Golgi protein p155.

Figure 4. Viral exit via the recycling endosome

Andes virus (ANDV), a New World hantavirus, forms cytoplasmic inclusions and undergoes assembly in the trans-Golgi (Rab11•GDP+) and recycling endosomal (Rab11•GTP+) compartments. Mature viral particles are then trafficked via the recycling pathway in a Rab11-dependent fashion to the cell surface where they are released. Respiratory syncytial virus (RSV) enters host cells either apically or basally and is then exported to the apical cell surface via Rab11 and MyoV-dependent trafficking.