Type III effector NleH2 from Escherichia coli O157:H7 str. Sakai features an atypical protein kinase domain

α-Helices in the Type III Secretion Effectors: A Prevalent Feature with Versatile Roles

Type III Secretion Systems (T3SSs) are multicomponent nanomachines located at the cell envelope of Gram-negative bacteria. Their main function is to transport bacterial proteins either extracellularly or directly into the eukaryotic host cell cytoplasm. Type III Secretion effectors (T3SEs), latest to be secreted T3S substrates, are destined to act at the eukaryotic host cell cytoplasm and occasionally at the nucleus, hijacking cellular processes through mimicking eukaryotic proteins. A broad range of functions is attributed to T3SEs, ranging from the manipulation of the host cell's metabolism for the benefit of the bacterium to bypassing the host's defense mechanisms. To perform this broad range of manipulations, T3SEs have evolved numerous novel folds that are compatible with some basic requirements: they should be able to easily unfold, pass through the narrow T3SS channel, and refold to an active form when on the other side. In this review, the various folds of T3SEs are presented with the emphasis placed on the functional and structural importance of α-helices and helical domains.

Structural insights of macromolecules involved in bacteria-induced apoptosis in the pathogenesis of human diseases

Numbers of pathogenic bacteria can induce apoptosis in human host cells and modulate the cellular pathways responsible for inducing or inhibiting apoptosis. These pathogens are significantly recognized by host proteins and provoke the multitude of several signaling pathways and alter the cellular apoptotic stimuli. This process leads the bacterial entry into the mammalian cells and evokes a variety of responses like phagocytosis, release of mitochondrial cytochrome c, secretion of bacterial effectors, release of both apoptotic and inflammatory cytokines, and the triggering of apoptosis. Several mechanisms are involved in bacteria-induced apoptosis including, initiation of the endogenous death machinery, pore-forming proteins, and secretion of superantigens. Either small molecules or proteins may act as a binding partner responsible for forming the protein complexes and regulate enzymatic activity via protein-protein interactions. The bacteria induce apoptosis, attack the human cell and gain control over various types of cells and tissue. Since these processes are intricate in the defense mechanisms of host organisms against pathogenic bacteria and play an important function in host-pathogen interactions. In this chapter, we focus on the various bacterial-induced apoptosis mechanisms in host cells and discuss the important proteins and bacterial effectors that trigger the host cell apoptosis. The structural characterization of bacterial effector proteins and their interaction with human host cells are also considered.

Attaching and effacing pathogens: the effector ABC of immune subversion

The innate immune response resembles an essential barrier to bacterial infection. Many bacterial pathogens have, therefore, evolved mechanisms to evade from or subvert the host immune response in order to colonize, survive and multiply. The attaching and effacing pathogens enteropathogenic Escherichia coli, enterohaemorrhagic E. coli, Escherichia albertii and Citrobacter rodentium are Gram-negative extracellular gastrointestinal pathogens. They use a type III secretion system to inject effector proteins into the host cell to manipulate a variety of cellular processes. Over the last decade, considerable progress was made in identifying and characterizing the effector proteins of attaching and effacing pathogens that are involved in the inhibition of innate immune signaling pathways, in determining their host cell targets and elucidating the mechanisms they employ. Their functions will be reviewed here.

EPEC NleH1 is significantly more effective in reversing colitis and reducing mortality than NleH2 via differential effects on host signaling pathways

Enteropathogenic Escherichia coli (EPEC) is a foodborne pathogen that uses a type III secretion system to translocate effector molecules into host intestinal epithelial cells (IECs) subverting several host cell processes and signaling cascades. Interestingly, EPEC infection induces only modest intestinal inflammation in the host. The homologous EPEC effector proteins, NleH1 and NleH2, suppress the nuclear factor-κB (NF-κB) pathway and apoptosis in vitro. Increased apoptosis and activation of NF-κB and MAP kinases (MAPK) contribute to the pathogenesis of inflammatory bowel diseases (IBD). The aim of this study was to determine if NleH1 and NleH2 also block MAPK pathways in vitro and in vivo, and to compare the effects of these bacterial proteins on a murine model of colitis. Cultured IECs were infected with various strains of EPEC expressing NleH1 and NleH2, or not, and the activation of ERK1/2 and p38 was determined. In addition, the impact of infection with various strains of EPEC on murine DSS colitis was assessed by change in body weight, colon length, histology, and survival. Activation of apoptosis and MAPK signaling were also evaluated. Our data show that NleH1, but not NleH2, suppresses ERK1/2 and p38 activation in vitro. Interestingly, NleH1 affords significantly greater protection against and hastens recovery from dextran sodium sulfate (DSS)-induced colitis compared to NleH2. Strikingly, colitis-associated mortality was abolished by infection with EPEC strains expressing NleH1. Interestingly, in vivo NleH1 suppresses activation of ERK1/2 and p38 and blocks apoptosis independent of the kinase domain that inhibits NF-κB. In contrast, NleH2 suppresses only caspase-3 and p38, but not ERK1/2. We conclude that NleH1 affords greater protection against and improves recovery from DSS colitis compared to NleH2 due to its ability to suppress ERK1/2 in addition to NF-κB, p38, and apoptosis. These findings warrant further investigation of anti-inflammatory bacterial proteins as novel treatments for IBD.

Modulation of Host Cell Processes by T3SS Effectors

Two of the enteric Escherichia coli pathotypes-enteropathogenic E. coli (EPEC) and enterohaemorrhagic E. coli (EHEC)-have a conserved type 3 secretion system which is essential for virulence. The T3SS is used to translocate between 25 and 50 bacterial proteins directly into the host cytosol where they manipulate a variety of host cell processes to establish a successful infection. In this chapter, we discuss effectors from EPEC/EHEC in the context of the host proteins and processes that they target-the actin cytoskeleton, small guanosine triphosphatases and innate immune signalling pathways that regulate inflammation and cell death. Many of these translocated proteins have been extensively characterised, which has helped obtain insights into the mechanisms of pathogenesis of these bacteria and also understand the host pathways they target in more detail. With increasing knowledge of the positive and negative regulation of host signalling pathways by different effectors, a future challenge is to investigate how the specific effector repertoire of each strain cooperates over the course of an infection.

Structural insight into effector proteins of Gram-negative bacterial pathogens that modulate the phosphoproteome of their host

Invading pathogens manipulate cellular process of the host cell to establish a safe replicative niche. To this end they secrete a spectrum of proteins called effectors that modify cellular environment through a variety of mechanisms. One of the most important mechanisms is the manipulation of cellular signaling through modifications of the cellular phosphoproteome. Phosphorylation/dephosphorylation plays a pivotal role in eukaryotic cell signaling, with ∼ 500 different kinases and ∼ 130 phosphatases in the human genome. Pathogens affect the phosphoproteome either directly through the action of bacterial effectors, and/or indirectly through downstream effects of host proteins modified by the effectors. Here we review the current knowledge of the structure, catalytic mechanism and function of bacterial effectors that modify directly the phosphorylation state of host proteins. These effectors belong to four enzyme classes: kinases, phosphatases, phospholyases and serine/threonine acetylases.

Bringing down the host: enteropathogenic and enterohaemorrhagic Escherichia coli effector-mediated subversion of host innate immune pathways

Enteric bacterial pathogens commonly use a type III secretion system (T3SS) to successfully infect intestinal epithelial cells and survive and proliferate in the host. Enteropathogenic and enterohaemorrhagic Escherichia coli (EPEC; EHEC) colonize the human intestinal mucosa, form characteristic histological lesions on the infected epithelium and require the T3SS for full virulence. T3SS effectors injected into host cells subvert cellular pathways to execute a variety of functions within infected host cells. The EPEC and EHEC effectors that subvert innate immune pathways--specifically those involved in phagocytosis, host cell survival, apoptotic cell death and inflammatory signalling--are all required to cause disease. These processes are reviewed within, with a focus on recent work that has provided insights into the functions and host cell targets of these effectors.

The bacterial pathogen-ubiquitin interface: lessons learned from Shigella

Shigella species are the aetiological agents of shigellosis, a severe diarrhoeal disease that is a significant cause of morbidity and mortality worldwide. Shigellosis causes massive colonic destruction, high fever and bloody diarrhoea. Shigella pathogenesis is tightly linked to the ability of the bacterium to invade and replicate intracellularly within the colonic epithelium. Shigella uses a type 3 secretion system to deliver its effector proteins into the cytosol of infected cells. Among the repertoire of Shigella effectors, many are known to target components of the actin cytoskeleton to promote bacterial entry. An emerging alternate theme for effector function is the targeting of the host ubiquitin system. Ubiquitination is a post-translational modification restricted to eukaryotes and is involved in many essential host processes. By virtue of sheer number of ubiquitin-modulating effector proteins, it is clear that Shigella has invested heavily into subversion of the ubiquitin system. Understanding these host-pathogen interactions will inform us about the strategies used by successful pathogens and may also provide avenues for novel antimicrobial strategies.