Peter J. Hume

Salmonella takes control: effector-driven manipulation of the host.

Salmonella pathogenesis relies upon the delivery of over thirty specialised effector proteins into the host cell via two distinct type III secretion systems. These effectors act in concert to subvert the host cell cytoskeleton, signal transduction pathways, membrane trafficking and pro-inflammatory responses. This allows Salmonella to invade non-phagocytic epithelial cells, establish and maintain an intracellular replicative niche and, in some cases, disseminate to cause systemic disease. This review focuses on the actions of the effectors on their host cell targets during each stage of Salmonella infection.

WAVE regulatory complex activation by cooperating GTPases Arf and Rac1

The WAVE regulatory complex (WRC) is a critical element in the control of actin polymerization at the eukaryotic cell membrane, but how WRC is activated remains uncertain. While Rho GTPase Rac1 can bind and activate WRC in vitro, this interaction is of low affinity, suggesting other factors may be important. By reconstituting WAVE-dependent actin assembly on membrane-coated beads in mammalian cell extracts, we found that Rac1 was not sufficient to engender bead motility, and we uncovered a key requirement for Arf GTPases. In vitro, Rac1 and Arf1 were individually able to bind weakly to recombinant WRC and activate it, but when both GTPases were bound at the membrane, recruitment and concomitant activation of WRC were dramatically enhanced. This cooperativity between the two GTPases was sufficient to induce WAVE-dependent bead motility in cell extracts. Our findings suggest that Arf GTPases may be central components in WAVE signalling, acting directly, alongside Rac1.

The purified Shigella IpaB and Salmonella SipB translocators share biochemical properties and membrane topology

An essential early event in Shigella and Salmonella pathogenesis is invasion of non‐phagocytic intestinal epithelial cells. Pathogen entry is triggered by the delivery of multiple bacterial effector proteins into target mammalian cells. The Shigella invasion plasmid antigen B (IpaB), which inserts into the host plasma membrane, is required for effector delivery and invasion. To investigate the biochemical properties and membrane topology of IpaB, we purified the native full‐length protein following expression in laboratory Escherichia coli. Purified IpaB assembled into trimers via an N‐terminal domain predicted to form a trimeric coiled‐coil, and is predominantly α‐helical. Upon lipid interaction, two transmembrane domains (residues 313–333 and 399–419) penetrate the bilayer, allowing the intervening hydrophilic region (334–398) to cross the membrane. Purified IpaB integrated into model, erythrocyte and mammalian cell membranes without disrupting bilayer integrity, and induced liposome fusion in vitro. An IpaB‐derived 162 residue α‐helical polypeptide (IpaB418−580) is a potent inhibitor of IpaB‐directed liposome fusion in vitro and blocked Shigella entry into cultured mammalian cells at 10−8 M. It is also a heterologous inhibitor of Salmonella invasion protein B (SipB) activity and Salmonella entry. In contrast, IpaB418−580 failed to prevent the contact‐dependent haemolytic activity of Shigella. These findings question the proposed direct link between contact‐dependent haemolysis and Shigella entry, and demonstrate that IpaB and SipB share biochemical properties and membrane topology, consistent with a conserved mode of action during cell entry.

The Salmonella Effector SptP Dephosphorylates Host AAA+ ATPase VCP to Promote Development of its Intracellular Replicative Niche

Summary Virulence effectors delivered into intestinal epithelial cells by Salmonella trigger actin remodeling to direct pathogen internalization and intracellular replication in Salmonella-containing vacuoles (SCVs). One such effector, SptP, functions early during pathogen entry to deactivate Rho GTPases and reverse pathogen-induced cytoskeletal changes following uptake. SptP also harbors a C-terminal protein tyrosine phosphatase (PTPase) domain with no clear host substrates. Investigating SptPs longevity in infected cells, we uncover a late function of SptP, showing that it associates with SCVs, and its PTPase activity increases pathogen replication. Direct SptP binding and specific dephosphorylation of the AAA+ ATPase valosin-containing protein (VCP/p97), a facilitator of cellular membrane fusion and protein degradation, enhanced pathogen replication in SCVs. VCP and its adaptors p47 and Ufd1 were necessary for generating Salmonella-induced filaments on SCVs, a membrane fusion event characteristic of the pathogen replicative phase. Thus, Salmonella regulates the biogenesis of an intracellular niche through SptP-mediated dephosphorylation of VCP.

Salmonella virulence effector SopE and Host GEF ARNO cooperate to recruit and activate WAVE to trigger bacterial invasion.

Summary Salmonella virulence effectors elicit host cell membrane ruffling to facilitate pathogen invasion. The WAVE regulatory complex (WRC) governs the underlying membrane-localized actin polymerization, but how Salmonella manipulates WRC is unknown. We show that Rho GTPase activation by the Salmonella guanine nucleotide exchange factor (GEF) SopE efficiently triggered WRC recruitment but not its activation, which required host Arf GTPase activity. Invading Salmonella recruited and activated Arf1 to facilitate ruffling and uptake. Arf3 and Arf6 could also enhance invasion. RNAi screening of host Arf-family GEFs revealed a key role for ARNO in pathogen invasion and generation of pathogen-containing macropinosomes enriched in Arf1 and WRC. Salmonella recruited ARNO via Arf6 and the phosphoinositide phosphatase effector SopB-induced PIP3 generation. ARNO in turn triggered WRC recruitment and activation, which was dramatically enhanced when SopE and ARNO cooperated. Thus, we uncover a mechanism by which pathogen and host GEFs synergize to regulate WRC and trigger Salmonella invasion.

Enteropathogenic Escherichia coli Recruits the Cellular Inositol Phosphatase SHIP2 to Regulate Actin-Pedestal Formation

Adhesion of enteropathogenic Escherichia coli to epithelial cells triggers actin-rich pedestal formation beneath the bacteria. Pedestal formation requires delivery and insertion of the bacterial translocated intimin receptor (Tir) into the host plasma membrane. The C-terminal regions in Tir, encompassing Y483 and Y511, share sequence similarity with cellular immunoreceptor tyrosine-based inhibition motifs (ITIMs), which are critical regulators of eukaryotic signaling pathways. We demonstrate that Y483 and Y511 within tandem ITIM-like sequences are essential for recruiting SHIP2, a host inositol phosphatase. SHIP2 controls condensed F-actin-pedestal formation by engaging the adaptor SHC and by generating a PI(3,4)P(2)-enriched lipid platform for recruitment of the cytoskeletal regulator lamellipodin. Therefore, mimicry of eukaryotic receptor motifs by Tir controls both the lipid and protein composition of the signaling platform necessary for pedestal formation. Further, the dual action of SHIP2s scaffolding and phosphatase functions ensures tight compartmentalization and coordination of actin dynamics during pedestal formation.

Topology of the Salmonella invasion protein SipB in a model bilayer

A critical early event in Salmonella infection is entry into intestinal epithelial cells. The Salmonellainvasion protein SipB is required for the delivery of bacterial effector proteins into target eukaryotic cells, which subvert signal transduction pathways and cytoskeletal dynamics. SipB inserts into the host plasma membrane during infection, and the purified protein has membrane affinity and heterotypic membrane fusion activity in vitro. We used complementary biochemical and biophysical techniques to inves‐tigate the topology of purified SipB in a model membrane. We show that the 593 residue SipB is predominantly α‐helical in aqueous solution, and that no significant change in secondary structural content accompanies lipid interaction. SipB contains two α‐helical transmembrane domains (residues 320–353 and 409–427), which insert deeply into the bilayer. Their integration allowed the hydrophilic region between the hydrophobic domains (354–408) to cross the bilayer. SipB membrane integration required both the hydrophobic domains and an additional helical C‐terminal region (428–593). Further spectroscopic analysis of these domains in isolation showed that the hydrophobic regions insert obliquely into the bilayer, whereas the C‐terminal domain associates with the bilayer surface, tilted parallel to the membrane. The combined data suggest a topological model for membrane‐inserted SipB.

Arf6 coordinates actin assembly through the WAVE complex, a mechanism usurped by Salmonella to invade host cells

Significance The small GTPase ADP ribosylation factor (Arf) 6 anchors to the plasma membrane, where it coordinates actin filament polymerization to remodel the membrane-associated cytoskeleton in diverse cell processes, yet how Arf6 directs actin polymerization is unknown. By reconstituting membrane-associated actin assembly, we find that Arf6 assembles actin via the WAVE regulatory complex (WRC). In contrast to other Arf family members, which directly bind and activate WRC, Arf6 signaled to WRC indirectly by recruiting the Arf1 activator ARNO to the membrane. Remarkably, we demonstrate this mechanism is hijacked by the pathogen Salmonella that usurped Arf6 and WRC to invade human host cells and establish intracellular infection. This study describes a mechanism for Arf6-driven actin polymerization. ADP ribosylation factor (Arf) 6 anchors to the plasma membrane, where it coordinates membrane trafficking and cytoskeleton remodelling, but how it assembles actin filaments is unknown. By reconstituting membrane-associated actin assembly mediated by the WASP family veroprolin homolog (WAVE) regulatory complex (WRC), we recapitulated an Arf6-driven actin polymerization pathway. We show that Arf6 is divergent from other Arf members, as it was incapable of directly recruiting WRC. We demonstrate that Arf6 triggers actin assembly at the membrane indirectly by recruiting the Arf guanine nucleotide exchange factor (GEF) ARNO that activates Arf1 to enable WRC-dependent actin assembly. The pathogen Salmonella usurped Arf6 for host cell invasion by recruiting its canonical GEFs EFA6 and BRAG2. Arf6 and its GEFs facilitated membrane ruffling and pathogen invasion via ARNO, and triggered actin assembly by generating an Arf1–WRC signaling hub at the membrane in vitro and in cells. This study reconstitutes Arf6-dependent actin assembly to reveal a mechanism by which related Arf GTPases orchestrate distinct steps in the WRC cytoskeleton remodelling pathway.

A Salmonella SipB-derived polypeptide blocks the 'trigger' mechanism of bacterial entry into eukaryotic cells.

Entry into non‐phagocytic mammalian cells by the invasive pathogens Salmonella and Shigella is triggered by the delivery of bacterial virulence effector proteins into the host cell. This is dependent upon Salmonella SipB or its Shigella homologue IpaB, which insert into the eukaryotic cell plasma membrane. Here we show that a SipB‐derived 166 residue α‐helical polypeptide is a potent inhibitor of SipB‐directed liposome fusion in vitro, preventing the membrane‐associated form of SipB from inserting deeply into the bilayer. This polypeptide blocks Salmonella entry into cultured mammalian cells at 10−10 M, and is a heterologous inhibitor of analogous IpaB activity and Shigella cell entry. These findings reveal a potential strategy to identify inhibitors of the ‘trigger’ mechanism underlying cell entry by these major invasive pathogens.

Clustering transfers the translocated Escherichia coli receptor into lipid rafts to stimulate reversible activation of c-Fyn.

Enteropathogenic Escherichia coli (EPEC) mimic a ligand–receptor interaction to induce ‘pedestal‐like’ pseudopodia on mammalian cells, providing a tractable system to study tyrosine kinase signalling to the actin cytoskeleton. EPEC delivers its own receptor (Tir), which is engaged by a bacterial surface ligand (intimin). When Tir delivery and activity are uncoupled, intimin‐induced Tir clustering stimulates TirY474 phosphorylation by the Src‐family kinase (SFK) c‐Fyn, triggering actin polymerization and pedestal formation. How c‐Fyn specifically targets Tir and is regulated remains unknown. We show that clustering transfers Tir into cholesterol‐rich detergent‐resistant microdomains (DRMs), a signal prompting transient c‐Fyn accumulation at bacterial adhesion sites. Co‐clustering of TirY474 and c‐Fyn in DRMs rapidly stimulates robust kinase activation both by induced c‐FynY531 dephosphorylation to unlock the inactive state and by reciprocal c‐FynY417 autophosphorylation to promote activity. After signal induction, c‐Fyn dissipates and the resting state restored by Csk‐dependent phosphorylation of c‐FynY531. These data illustrate a sophisticated mechanism evolved by a pathogen effector to reversibly regulate SFKs, and resolve early interactions at a model receptor initiating tyrosine kinase signalling.