Swiss Army Pathogen: The Salmonella Entry Toolkit

Abstract

Salmonella causes disease in humans and animals ranging from mild self-limiting gastroenteritis to potentially life-threatening typhoid fever. Salmonellosis remains a considerable cause of morbidity and mortality globally, and hence imposes a huge socio-economic burden worldwide. A key property of all pathogenic Salmonella strains is the ability to invade non-phagocytic host cells. The major determinant of this invasiveness is a Type 3 Secretion System (T3SS), a molecular syringe that injects virulence effector proteins directly into target host cells. These effectors cooperatively manipulate multiple host cell signaling pathways to drive pathogen internalization. Salmonella does not only rely on these injected effectors, but also uses several other T3SS-independent mechanisms to gain entry into host cells. This review summarizes our current understanding of the methods used by Salmonella for cell invasion, with a focus on the host signaling networks that must be coordinately exploited for the pathogen to achieve its goal.

Keywords: SPI1-independent entry; Salmonella invasion; Salmonella pathogenicity islands; T3SS effectors; actin cytoskeleton; membrane ruffling.

Figure 1

Salmonella infection requires invasion of host cells. Following ingestion and passage through the stomach, Salmonella encounters the intestinal epithelial layer, a barrier which must be breached. It is thought that the pathogen preferentially enters M cells, and is trancytosed and passed to underlying macrophages. Here Salmonella can evade killing by causing apoptosis leading to release, or survive and replicate long enough to allow systemic spread through the reticuloendothelial system. Salmonella can also force its own uptake by epithelial cells from the apical side, i.e., the lumen of the intestine. Following uptake, the bacteria can replicate intracellularly, or be transcytosed and released on the basolateral side, from here they can infect further epithelial cells. Additionally, dendritic cells can directly capture Salmonella from the lumen and transport them across the epithelium.

Figure 2

Salmonella subversion of the WAVE regulatory complex. The Wave regulatory complex (WRC) exists in an inactive state. Upon effector protein delivery by Salmonella, Arf6 is activated by host cell GEFs such as EFA6 and BRAG, which stimulate the exchange of GDP (white circle) bound to Arf6 for GTP (red circle). Active Arf6, along with the lipid PI(3,4,5)P₃ generated by SopB, recruits the host GEF ARNO that in turn activates Arf1. Arf1 consequently anchors via its exposed myristoylation moiety (black lines) to the plasma membrane, where active Arf1 and SopE-activated Rac1 work in cooperation to recruit and activate the WRC and induce Arp2/3-dependent polymerization of actin filaments (pink). Arf1 can subsequently be inactivated by cellular GAPs, and cycles of activation and inactivation promote invasion efficiency. Signaling can be switched off by SptP-mediated inactivation of Rac1.

Figure 3

RhoA-dependent pathways targeted by Salmonella. Following delivery of SopB into host cells RhoA becomes activated by an unknown mechanism, presumably involving the generation of phopshatidylinositol lipids. RhoA can activate Rho kinase (ROCK), which in turn may be responsible for activating Myosin II-mediated contractility, which contributes to Salmonella uptake. Rock may also activate the formin FHOD1, which can directly polymerize actin filaments. In addition, it has been reported that RhoA can directly activate the WASH complex, to promote Arp2/3-dependent actin assembly, though this link remains hypothetical.

Figure 4

Manipulation of vesicle trafficking by Salmonella. Injection of SopE leads to activation of RalA, either directly or via Rac1 and some unknown host factor. RalA may bind to both Myosin 1c and the octomeric exocyst complex, which cooperate to target vesicles to sites of Salmonella entry. SipC also contributes to the recruitment of the exocyst complex by directly binding Exo70. Rac1, activated by SopE, is maintained in the GTP-bound state by interaction with IQGAP1, which also acts as a scaffold and recruits the kinase MEK. It is possible that MEK activates ERK, which in turn phosphorylates Exo70, promoting exocyst complex assembly.

Figure 5

Role of Myosin VI during Salmonella invasion. SopE activates Rho GTPases such as Rac1, which induce the recruitment of Myosin VI (Myo VI) to the membrane via PAK-mediated phosphorylation. SopB and Myosin VI (possibly via delivering some cargo vesicle to the membrane) are required to trigger PI3K signaling to generate PIP3. SopB dephosphorylates PI(3,4,5)P₃ to generate PI3P, which in turn recruits PI3P-binding proteins, such as Frabin, that promote Salmonella uptake.