N-terminus of IpaB provides a potential anchor to the Shigella type III secretion system tip complex protein IpaD

The sequence of events of enteropathogenic E. coli's type III secretion system translocon assembly

Many bacterial pathogens employ the type III secretion system (T3SS), a specialized complex that transports effector proteins that manipulate various cellular processes. The T3SS forms a translocon pore within the host-cell membrane consisting of two secreted proteins that transition from a soluble state into a transmembrane complex. Still, the exact sequence of events leading to the formation of a membranous functional pore remains uncertain. Here, we utilized the translocon proteins of enteropathogenic E. coli (EPEC) to investigate the sequence of those steps leading to translocon assembly, including self-oligomerization, hetero-oligomerization, interprotein interaction, and membrane insertion. We found that in EPEC, EspD (SctE) plays a dominant role in pore formation as it assembles into an oligomeric state, regardless of pH, membrane contact, or the presence of EspB (SctB). Subsequently, EspB subunits integrate into EspD homo-oligomers to create EspB-EspD hetero-oligomers that adopt a transmembrane orientation to create a functional pore complex.

Toward a Shigella Vaccine: Opportunities and Challenges to Fight an Antimicrobial-Resistant Pathogen

Shigellosis causes more than 200,000 deaths worldwide and most of this burden falls on Low- and Middle-Income Countries (LMICs), with a particular incidence in children under 5 years of age. In the last decades, Shigella has become even more worrisome because of the onset of antimicrobial-resistant strains (AMR). Indeed, the WHO has listed Shigella as one of the priority pathogens for the development of new interventions. To date, there are no broadly available vaccines against shigellosis, but several candidates are being evaluated in preclinical and clinical studies, bringing to light very important data and information. With the aim to facilitate the understanding of the state-of-the-art of Shigella vaccine development, here we report what is known about Shigella epidemiology and pathogenesis with a focus on virulence factors and potential antigens for vaccine development. We discuss immunity after natural infection and immunization. In addition, we highlight the main characteristics of the different technologies that have been applied for the development of a vaccine with broad protection against Shigella.

Transmembrane substrates of type three secretion system injectisomes

The type three secretion system injectisome of Gram-negative bacterial pathogens injects virulence proteins, called effectors, into host cells. Effectors of mammalian pathogens carry out a range of functions enabling bacterial invasion, replication, immune suppression and transmission. The injectisome secretes two translocon proteins that insert into host cell membranes to form a translocon pore, through which effectors are delivered. A subset of effectors also integrate into infected cell membranes, enabling a unique range of biochemical functions. Both translocon proteins and transmembrane effectors avoid cytoplasmic aggregation and integration into the bacterial inner membrane. Translocated transmembrane effectors locate and integrate into the appropriate host membrane. In this review, we focus on transmembrane translocon proteins and effectors of bacterial pathogens of mammals. We discuss what is known about the mechanisms underlying their membrane integration, as well as the functions conferred by the position of injectisome effectors within membranes.

Characterization of Functional B-Cell Epitopes at the Amino Terminus of Shigella Invasion Plasmid Antigen B (IpaB)

Shigella invasion plasmid antigen B (IpaB) plays an important role in causing shigellosis. While IpaB's protein structure, contribution to disease mechanism, and protective immunity against Shigella infection have been well studied, the significance of individual antigenic domains, especially at the N terminus, has not been systematically characterized. In an attempt to identify IpaB protein functional epitopes and to construct an optimized polyvalent multiepitope fusion antigen (MEFA) immunogen for development of a protein-based cross protective Shigella vaccine, in this study, we in silico identified immunodominant B-cell epitopes from the IpaB N terminus, fused each epitope to carrier protein CsaB (the major subunit of enterotoxigenic Escherichia coli CS4 adhesin) for epitope fusion proteins, immunized mice with each epitope fusion protein, examined IpaB-specific antibody responses, and assessed antibody functional activity against Shigella bacterial invasion. A total of 10 B-cell continuous epitopes were identified from IpaB N terminus, and after being fused to carrier protein CsaB, each epitope induced anti-IpaB IgG responses in the intramuscularly immunized mice. While in vitro antibody invasion inhibition assays demonstrated that antibodies derived from each identified epitope were functional, epitopes 1 (LAKILASTELGDNTIQAA), 2 (HSTSNILIPELKAPKSL), and 4 (QARQQKNLEFSDKI) induced antibodies to inhibit Shigella sonnei and Shigella flexneri invasion at levels similar to those of recombinant IpaB protein, suggesting that these three IpaB epitopes can be used potentially as IpaB-representing antigens to induce protective anti-IpaB antibodies and for construction of an epitope-based polyvalent MEFA protein immunogen for Shigella vaccine development. IMPORTANCE Currently, there are no effective measures for control or prevention of Shigella infection, the most common cause of diarrhea in children 3 to 5 years of age in developing countries. Challenges in developing Shigella vaccines include virulence heterogeneity among species and serotypes. To overcome virulence heterogeneity challenge and to develop a protein-based multivalent Shigella vaccine, we targeted a panel of virulence factors, including invasion plasmid antigens, identified functional antigenic domains or epitopes as representative antigens, and applied the novel epitope- and structure-based vaccinology platform multiepitope fusion antigen (MEFA) to integrate functional antigenic domains or epitopes into a backbone immunogen to produce a polyvalent immunogen for cross protective antibodies. Identification of functional IpaB epitopes from this study enhances our understanding of IpaB immunogenicity and allows us to directly utilize IpaB epitopes for construction of a cross protective polyvalent Shigella immunogen and to accelerate development of a protein-based Shigella vaccine.

Phosphomimetic Tyrosine Mutations in Spa47 Inhibit Type Three Secretion ATPase Activity and Shigella Virulence Phenotype

Shigella is a highly infectious human pathogen responsible for 269 million infections and 200,000 deaths per year. Shigella virulence is absolutely reliant on the injection of effector proteins into the host cell cytoplasm via its type three secretion system (T3SS). The protein Spa47 is a T3SS ATPase whose activity is essential for the proper function of the Shigella T3SS needle-like apparatus through which effectors are secreted. A phosphoproteomics study recently found several Shigella T3SS proteins, including Spa47, to be tyrosine phosphorylated, suggesting a means of regulating Spa47 enzymatic activity, T3SS function, and overall Shigella virulence. The work presented here employs phosphomimetic mutations in Spa47 to probe the effects of phosphorylation at these targeted tyrosines through in vitro radiometric ATPase assays and circular dichroism as well as in vivo characterization of T3SS secretion activity, erythrocyte hemolysis, and cellular invasion. Results presented here demonstrate a direct correlation between Spa47 tyrosine phosphorylation state, Spa47 ATPase activity, T3SS function, and Shigella virulence. Together, these findings provide a strong foundation that leads the way to uncovering the specific pathway(s) that Shigella employ to mitigate wasteful ATP hydrolysis and effector protein secretion when not required as well as T3SS activation in preparation for host infection and immune evasion.

An antibody targeting type III secretion system induces broad protection against Salmonella and Shigella infections

Salmonella and Shigella bacteria are food- and waterborne pathogens that are responsible for enteric infections in humans and are still the major cause of morbidity and mortality in the emerging countries. The existence of multiple Salmonella and Shigella serotypes as well as the emergence of strains resistant to antibiotics requires the development of broadly protective therapies. Recently, the needle tip proteins of the type III secretion system of these bacteria were successfully utilized (SipD for Salmonella and IpaD for Shigella) as vaccine immunogens to provide good prophylactic cross-protection in murine models of infections. From these experiments, we have isolated a cross-protective monoclonal antibody directed against a conserved region of both proteins. Its conformational epitope determined by Deep Mutational Scanning is conserved among needle tip proteins of all pathogenic Shigella species and Salmonella serovars, and are well recognized by this antibody. Our study provides the first in vivo experimental evidence of the importance of this common region in the mechanism of virulence of Salmonella and Shigella and opens the way to the development of cross-protective therapeutic agents.

Salmonella and Shigella are responsible for gastrointestinal diseases and continue to remain a serious health hazard in South and South-East Asia and African countries, even more with the new emergence of multi drug resistances. Developed vaccines are either not commercialized (for Shigella) or cover only a limited number of serotypes (for Salmonella). There is thus a crucial need to develop cross-protective therapies. By targeting proteins SipD and IpaD belonging respectively to the injectisome of Salmonella and Shigella and necessary to their virulence, we have shown that a monoclonal antibody (mAb) directed against a conserved common region of their apical part provides good cross-protection prophylactic efficacy. We have determined the region targeted by this mAb which could explain why it is conserved among Salmonella and Shigella bacteria.

The Shigella Type III Secretion System: An Overview from Top to Bottom

Shigella comprises four species of human-restricted pathogens causing bacillary dysentery. While Shigella possesses multiple genetic loci contributing to virulence, a type III secretion system (T3SS) is its primary virulence factor. The Shigella T3SS nanomachine consists of four major assemblies: the cytoplasmic sorting platform; the envelope-spanning core/basal body; an exposed needle; and a needle-associated tip complex with associated translocon that is inserted into host cell membranes. The initial subversion of host cell activities is carried out by the effector functions of the invasion plasmid antigen (Ipa) translocator proteins, with the cell ultimately being controlled by dedicated effector proteins that are injected into the host cytoplasm though the translocon. Much of the information now available on the T3SS injectisome has been accumulated through collective studies on the T3SS from three systems, those of Shigella flexneri, Salmonella typhimurium and Yersinia enterocolitica/Yersinia pestis. In this review, we will touch upon the important features of the T3SS injectisome that have come to light because of research in the Shigella and closely related systems. We will also briefly highlight some of the strategies being considered to target the Shigella T3SS for disease prevention.

Molecular Targets and Strategies for Inhibition of the Bacterial Type III Secretion System (T3SS); Inhibitors Directly Binding to T3SS Components

The type III secretion system (T3SS) is a virulence apparatus used by many Gram-negative pathogenic bacteria to cause infections. Pathogens utilizing a T3SS are responsible for millions of infections yearly. Since many T3SS knockout strains are incapable of causing systemic infection, the T3SS has emerged as an attractive anti-virulence target for therapeutic design. The T3SS is a multiprotein molecular syringe that enables pathogens to inject effector proteins into host cells. These effectors modify host cell mechanisms in a variety of ways beneficial to the pathogen. Due to the T3SS’s complex nature, there are numerous ways in which it can be targeted. This review will be focused on the direct targeting of components of the T3SS, including the needle, translocon, basal body, sorting platform, and effector proteins. Inhibitors will be considered a direct inhibitor if they have a binding partner that is a T3SS component, regardless of the inhibitory effect being structural or functional.

Tchagang C, Tomaro K, Campbell-Valois FX. The T3SS of Shigella: Expression, Structure, Function, and Role in Vacuole Escape

Shigella spp. are one of the leading causes of infectious diarrheal diseases. They are Escherichia coli pathovars that are characterized by the harboring of a large plasmid that encodes most virulence genes, including a type III secretion system (T3SS). The archetypal element of the T3SS is the injectisome, a syringe-like nanomachine composed of approximately 20 proteins, spanning both bacterial membranes and the cell wall, and topped with a needle. Upon contact of the tip of the needle with the plasma membrane, the injectisome secretes its protein substrates into host cells. Some of these substrates act as translocators or effectors whose functions are key to the invasion of the cytosol and the cell-to-cell spread characterizing the lifestyle of Shigella spp. Here, we review the structure, assembly, function, and methods to measure the activity of the injectisome with a focus on Shigella, but complemented with data from other T3SS if required. We also present the regulatory cascade that controls the expression of T3SS genes in Shigella. Finally, we describe the function of translocators and effectors during cell-to-cell spread, particularly during escape from the vacuole, a key element of Shigella’s pathogenesis that has yet to reveal all of its secrets.

The type III secretion system needle, tip, and translocon

Many Gram-negative bacteria pathogenic to plants and animals deploy the type III secretion system (T3SS) to inject virulence factors into their hosts. All bacteria that rely on the T3SS to cause infectious diseases in humans have developed antibiotic resistance. The T3SS is an attractive target for developing new antibiotics because it is essential in virulence, and part of its structural component is exposed on the bacterial surface. The structural component of the T3SS is the needle apparatus, which is assembled from over 20 different proteins and consists of a base, an extracellular needle, a tip, and a translocon. This review summarizes the current knowledge on the structure and assembly of the needle, tip, and translocon.

Interfacial amino acids support Spa47 oligomerization and shigella type three secretion system activation

Like many Gram-negative pathogens, Shigella rely on a type three secretion system (T3SS) for injection of effector proteins directly into eukaryotic host cells to initiate and sustain infection. Protein secretion through the needle-like type three secretion apparatus (T3SA) requires ATP hydrolysis by the T3SS ATPase Spa47, making it a likely target for in vivo regulation of T3SS activity and an attractive target for small molecule therapeutics against shigellosis. Here, we developed a model of an activated Spa47 homo-hexamer, identifying two distinct regions at each protomer interface that we hypothesized to provide intermolecular interactions supporting Spa47 oligomerization and enzymatic activation. Mutational analysis and a series of high-resolution crystal structures confirm the importance of these residues, as many of the engineered mutants are unable to form oligomers and efficiently hydrolyze ATP in vitro. Furthermore, in vivo evaluation of Shigella virulence phenotype uncovered a strong correlation between T3SS effector protein secretion, host cell membrane disruption, and cellular invasion by the tested mutant strains, suggesting that perturbation of the identified interfacial residues/interactions influences Spa47 activity through preventing oligomer formation, which in turn regulates Shigella virulence. The most impactful mutations are observed within the conserved Site 2 interface where the native residues support oligomerization and likely contribute to a complex hydrogen bonding network that organizes the active site and supports catalysis. The critical reliance on these conserved residues suggests that aspects of T3SS regulation may also be conserved, providing promise for the development of a cross-species therapeutic that broadly targets T3SS ATPase oligomerization and activation.

The Tip Complex: From Host Cell Sensing to Translocon Formation

Type III secretion systems are used by some Gram-negative bacteria to inject effector proteins into targeted eukaryotic cells for the benefit of the bacterium. The type III secretion injectisome is a complex nanomachine comprised of four main substructures including a cytoplasmic sorting platform, an envelope-spanning basal body, an extracellular needle and an exposed needle tip complex. Upon contact with a host cell, secretion is induced, resulting in the formation of a translocon pore in the host membrane. Translocon formation completes the conduit needed for effector secretion into the host cell. Control of type III secretion occurs in response to environmental signals, with the final signal being host cell contact. Secretion control occurs primarily at two sites-the cytoplasmic sorting platform, which determines secretion hierarchy, and the needle tip complex, which is critical for sensing and responding to environmental signals. The best-characterized injectisomes are those from Yersinia, Shigella and Salmonella species where there is a wealth of information on the tip complex and the two translocator proteins. Of these systems, the best characterized from a secretion regulation standpoint is Shigella. In the Shigella system, the tip complex and the first secreted translocon both contribute to secretion control and, thus, both are considered components of the tip complex. In this review, all three of these type III secretion systems are described with discussion focused on the structure and formation of the injectisome tip complex and what is known of the transition from nascent tip complex to assembled translocon pore.

Shutting Down Shigella Secretion: Characterizing Small Molecule Type Three Secretion System ATPase Inhibitors

Many important human pathogens rely on one or more type three secretion systems (T3SSs) to inject bacterial effector proteins directly into the host cell cytoplasm. Secretion of protein through the needlelike type three secretion apparatus (T3SA) is essential for pathogen virulence and relies on a highly conserved ATPase at the base of the apparatus, making it an attractive target for anti-infective therapeutics. Here, we leveraged the ability to purify an active oligomeric Shigella T3SS ATPase to provide kinetic analyses of three T3SS ATPase inhibitors of Spa47. In agreement with in silico docking simulations, the inhibitors displayed noncompetitive inhibition profiles and efficiently reduced Spa47 ATPase activity with IC₅₀s as low as 52 ± 3 μM. Two of the inhibitors functioned well in vivo, nearly abolishing effector protein secretion without significantly affecting the Shigella growth phenotype or HeLa cell viability. Furthermore, characterization of Spa47 complexes in vitro and Shigella T3SA formation in vivo showed that the inhibitors do not function through disruption of Spa47 oligomers or by preventing T3SA formation. Together, these findings suggest that inhibitors targeting Spa47 may be an effective means of combating Shigella infection by shutting down type three secretion without preventing presentation of the highly antigenic T3SA tip proteins that aid in clearing the infection and developing pan- Shigella immunological memory. In summary, this is the first report of Shigella T3SS ATPase inhibitors and one of only a small number of studies characterizing T3SS ATPase inhibition in general. The work presented here provides much-needed insight into T3SS ATPase inhibition mechanisms and provides a strong platform for developing and evaluating non-antibiotic therapeutics targeting Spa47 and other T3SS ATPases.

Using disruptive insertional mutagenesis to identify the in situ structure-function landscape of the Shigella translocator protein IpaB

Bacterial type III secretion systems (T3SS) are used to inject proteins into mammalian cells to subvert cellular functions. The Shigella T3SS apparatus (T3SA) is comprised of a basal body, cytoplasmic sorting platform and exposed needle with needle "tip complex" (TC). TC maturation occurs when the translocator protein IpaB is recruited to the needle tip where both IpaD and IpaB control secretion induction. IpaB insertion into the host membrane is the first step of translocon pore formation and secretion induction. We employed disruptive insertional mutagenesis, using bacteriophage T4 lysozyme (T4L), within predicted IpaB loops to show how topological features affect TC functions (secretion control, translocon formation and effector secretion). Insertions within the N-terminal half of IpaB were most likely to result in a loss of steady-state secretion control, however, all but the two that were not recognized by the T3SA retained nearly wild-type hemolysis (translocon formation) and invasiveness levels (effector secretion). In contrast, all but one insertion in the C-terminal half of IpaB maintained secretion control but were impaired for hemolysis and invasion. These nature of the data suggest the latter mutants are defective in a post-secretion event, most likely due to impaired interactions with the second translocator protein IpaC. Intriguingly, only two insertion mutants displayed readily detectable T4L on the bacterial surface. The data create a picture in which the makeup and structure of a functional T3SA TC is highly amenable to physical perturbation, indicating that the tertiary structure of IpaB within the TC is more plastic than previously realized.

MxiN Differentially Regulates Monomeric and Oligomeric Species of the Shigella Type Three Secretion System ATPase Spa47

Shigella rely entirely on the action of a single type three secretion system (T3SS) to support cellular invasion of colonic epithelial cells and to circumvent host immune responses. The ATPase Spa47 resides at the base of the Shigella needle-like type three secretion apparatus (T3SA), supporting protein secretion through the apparatus and providing a likely means for native virulence regulation by Shigella and a much needed target for non-antibiotic therapeutics to treat Shigella infections. Here, we show that MxiN is a differential regulator of Spa47 and that its regulatory impact is determined by the oligomeric state of the Spa47 ATPase, with which it interacts. In vitro and in vivo characterization shows that interaction of MxiN with Spa47 requires the six N-terminal residues of Spa47 that are also necessary for stable Spa47 oligomer formation and activation. This interaction with MxiN negatively influences the activity of Spa47 oligomers while upregulating the ATPase activity of monomeric Spa47. Detailed kinetic analyses of monomeric and oligomeric Spa47 in the presence and absence of MxiN uncover additional mechanistic insights into the regulation of Spa47 by MxiN, suggesting that the MxiN/Spa47 species resulting from interaction with monomeric and oligomeric Spa47 are functionally distinct and that both could be involved in Shigella T3SS regulation. Uncovering regulation of Spa47 by MxiN addresses an important gap in the current understanding of how Shigella controls T3SA activity and provides the first description of differential T3SS ATPase regulation by a native T3SS protein.

Deoxycholate-Enhanced Shigella Virulence Is Regulated by a Rare π-Helix in the Type Three Secretion System Tip Protein IpaD

Type three secretion systems (T3SS) are specialized nanomachines that support infection by injecting bacterial proteins directly into host cells. The Shigella T3SS has uniquely evolved to sense environmental levels of the bile salt deoxycholate (DOC) and upregulate virulence in response to DOC. In this study, we describe a rare i + 5 hydrogen bonding secondary structure element (π-helix) within the type three secretion system tip protein IpaD that plays a critical role in DOC-enhanced virulence. Specifically, engineered mutations within the π-helix altered the pathogen's response to DOC, with one mutant construct in particular exhibiting an unprecedented reduction in virulence following DOC exposure. Fluorescence polarization binding assays showed that these altered DOC responses are not the result of differences in affinity between IpaD and DOC, but rather differences in the DOC-dependent T3SS tip maturation resulting from binding of IpaD to translocator/effector protein IpaB. Together, these findings begin to uncover the complex mechanism of DOC-enhanced Shigella virulence while identifying an uncommon structural element that may provide a much needed target for non-antibiotic treatment of Shigella infection.

In silico design of a novel chimeric shigella IpaB fused to C terminal of clostridium perfringens enterotoxin as a vaccine candidate

This study aimed to design a novel chimeric protein in silico to serve as a serotype-independent vaccine candidate against Shigella. The chimera contains amino acid residues 240–460 of Shigella invasion plasmid antigen B (IpaB) and the C-terminus of Clostridium perfringens enterotoxin (C-CPE). Amino acid sequences of 537 peptide linkers were obtained from two protein linker databases. 3D structures of IpaB-CPE_290–319, IpaB-CPE_184–319, IpaB-CPE_194–319 and 537 newly designed IpaB-linker-CPE_290–319 constructs with varying linker regions were predicted. These predicted 3D structures were merged with the 3D structures of native IpaB_240–460, CPE_194–319, CPE_184–319 and CPE_290–319 to select the structure most similar to native IpaB and C-CPE. Several in silico tools were used to determine the suitability of the selected IpaB-C-CPE structure as a vaccine candidate. None of the 537 linkers was capable of preserving the native structure of CPE_290–319 within the IpaB-linker-CPE_290–319 structure. In silico analysis determined that the IpaB-CPE_194–319 3D structure was the most similar to the 3D structure of the respective native CPE domain and that it was a stable chimeric protein exposing multiple B-cell epitopes. IpaB-CPE_194–319 was designed for its capability to bind to human intestinal epithelial and M cells and to accumulate on these cells. The predicted B-cell epitopes are likely to be capable of inducing a mucosal antibody response in the human intestine against Shigella IpaB. This study also showed that the higher binding affinities of CPE_184–319 and CPE_194–319 to claudin molecules than those of CPE_290–319 is the result of preserving the 3D structures of CPE_184–319 and CPE_194–319 when they are linked to the C-termini of other proteins.

Characterization of Small-Molecule Scaffolds That Bind to the Shigella Type III Secretion System Protein IpaD

Many pathogens such as Shigella and other bacteria assemble the type III secretion system (T3SS) nanoinjector to inject virulence proteins into their target cells to cause infectious diseases in humans. The rise of drug resistance among pathogens that rely on the T3SS for infectivity, plus the dearth of new antibiotics require alternative strategies in developing new antibiotics. The Shigella T3SS tip protein IpaD is an attractive target for developing anti-infectives because of its essential role in virulence and its exposure on the bacterial surface. Currently, the only known small molecules that bind to IpaD are bile salt sterols. In this study we identified four new small-molecule scaffolds that bind to IpaD, based on the methylquinoline, pyrrolidine-aniline, hydroxyindole, and morpholinoaniline scaffolds. NMR mapping revealed potential hotspots in IpaD for binding small molecules. These scaffolds can be used as building blocks in developing small-molecule inhibitors of IpaD that could lead to new anti-infectives.

Single-domain antibodies pinpoint potential targets within Shigella invasion plasmid antigen D of the needle tip complex for inhibition of type III secretion

Numerous Gram-negative pathogens infect eukaryotes and use the type III secretion system (T3SS) to deliver effector proteins into host cells. One important T3SS feature is an extracellular needle with an associated tip complex responsible for assembly of a pore-forming translocon in the host cell membrane. Shigella spp. cause shigellosis, also called bacillary dysentery, and invade colonic epithelial cells via the T3SS. The tip complex of Shigella flexneri contains invasion plasmid antigen D (IpaD), which initially regulates secretion and provides a physical platform for the translocon pore. The tip complex represents a promising therapeutic target for many important T3SS-containing pathogens. Here, in an effort to further elucidate its function, we created a panel of single-VH domain antibodies (VHHs) that recognize distinct epitopes within IpaD. These VHHs recognized the in situ tip complex and modulated the infectious properties of Shigella Moreover, structural elucidation of several IpaD-VHH complexes provided critical insights into tip complex formation and function. Of note, one VHH heterodimer could reduce Shigella hemolytic activity by >80%. Our observations along with previous findings support the hypothesis that the hydrophobic translocator (IpaB in Shigella) likely binds to a region within the tip protein that is structurally conserved across all T3SS-possessing pathogens, suggesting potential therapeutic avenues for managing infections by these pathogens.

Survival of the Fittest: How Bacterial Pathogens Utilize Bile To Enhance Infection

Bacterial pathogens have coevolved with humans in order to efficiently infect, replicate within, and be transmitted to new hosts to ensure survival and a continual infection cycle. For enteric pathogens, the ability to adapt to numerous host factors under the harsh conditions of the gastrointestinal tract is critical for establishing infection. One such host factor readily encountered by enteric bacteria is bile, an innately antimicrobial detergent-like compound essential for digestion and nutrient absorption. Not only have enteric pathogens evolved to resist the bactericidal conditions of bile, but these bacteria also utilize bile as a signal to enhance virulence regulation for efficient infection. This review provides a comprehensive and up-to-date analysis of bile-related research with enteric pathogens. From common responses to the unique expression of specific virulence factors, each pathogen has overcome significant challenges to establish infection in the gastrointestinal tract. Utilization of bile as a signal to modulate virulence factor expression has led to important insights for our understanding of virulence mechanisms for many pathogens. Further research on enteric pathogens exposed to this in vivo signal will benefit therapeutic and vaccine development and ultimately enhance our success at combating such elite pathogens.

Characterization of V. cholerae T3SS-dependent cytotoxicity in cultured intestinal epithelial cells

AM-19226 is a pathogenic, non-O1/non-O139 serogroup strain of Vibrio cholerae that uses a Type 3 Secretion System (T3SS) mediated mechanism to colonize host tissues and disrupt homeostasis, causing cholera. Co-culturing the Caco2-BBE human intestinal epithelial cell line with AM-19226 in the presence of bile results in rapid mammalian cell death that requires a functional T3SS. We examined the role of bile, sought to identify the mechanism, and evaluated the contributions of T3SS translocated effectors in in vitro cell death. Our results suggest that Caco2-BBE cytotoxicity does not proceed by apoptotic or necrotic mechanisms, but rather displays characteristics consistent with osmotic lysis. Cell death was preceded by disassembly of epithelial junctions and reorganization of the cortical membrane skeleton, although neither cell death nor cell-cell disruption required VopM or VopF, two effectors known to alter actin dynamics. Using deletion strains, we identified a subset of AM-19226 Vops that are required for host cell death, which were previously assigned roles in protein translocation and colonization, suggesting that they function other than to promote cytotoxicity. The collective results therefore suggest that cooperative Vop activities are required to achieve cytotoxicity in vitro, or alternatively, that translocon pores destabilize the membrane in a bile dependent manner.

NMR identification of the binding surfaces involved in the Salmonella and Shigella Type III secretion tip-translocon protein-protein interactions

The type III secretion system (T3SS) is essential for the pathogenesis of many bacteria including Salmonella and Shigella, which together are responsible for millions of deaths worldwide each year. The structural component of the T3SS consists of the needle apparatus, which is assembled in part by the protein-protein interaction between the tip and the translocon. The atomic detail of the interaction between the tip and the translocon proteins is currently unknown. Here, we used NMR methods to identify that the N-terminal domain of the Salmonella SipB translocon protein interacts with the SipD tip protein at a surface at the distal region of the tip formed by the mixed α/β domain and a portion of its coiled-coil domain. Likewise, the Shigella IpaB translocon protein and the IpaD tip protein interact with each other using similar surfaces identified for the Salmonella homologs. Furthermore, removal of the extreme N-terminal residues of the translocon protein, previously thought to be important for the interaction, had little change on the binding surface. Finally, mutations at the binding surface of SipD reduced invasion of Salmonella into human intestinal epithelial cells. Together, these results reveal the binding surfaces involved in the tip-translocon protein-protein interaction and advance our understanding of the assembly of the T3SS needle apparatus. Proteins 2016; 84:1097-1107. © 2016 Wiley Periodicals, Inc.

Characterization of the Shigella and Salmonella Type III Secretion System Tip-Translocon Protein-Protein Interaction by Paramagnetic Relaxation Enhancement

Many Gram-negative pathogens, such as Shigella and Salmonella, assemble the type III secretion system (T3SS) to inject virulence proteins directly into eukaryotic cells to initiate infectious diseases. The needle apparatus of the T3SS consists of a base, an extracellular needle, a tip protein complex, and a translocon. The atomic structure of the assembled tip complex and the translocon is unknown. Here, we show by NMR paramagnetic relaxation enhancement (PRE) that the mixed α-β domain at the distal region of the Shigella and Salmonella tip proteins interacts with the N-terminal ectodomain of their major translocon proteins. Our results reveal the binding surfaces involved in the tip-translocon protein-protein interaction and provide insights about the assembly of the needle apparatus of the T3SS.

Spa47 is an oligomerization-activated type three secretion system (T3SS) ATPase from Shigella flexneri

Gram-negative pathogens often use conserved type three secretion systems (T3SS) for virulence. The Shigella type three secretion apparatus (T3SA) penetrates the host cell membrane and provides a unidirectional conduit for injection of effectors into host cells. The protein Spa47 localizes to the base of the apparatus and is speculated to be an ATPase that provides the energy for T3SA formation and secretion. Here, we developed an expression and purification protocol, producing active Spa47 and providing the first direct evidence that Spa47 is a bona fide ATPase. Additionally, size exclusion chromatography and analytical ultracentrifugation identified multiple oligomeric species of Spa47 with the largest greater than 8 fold more active for ATP hydrolysis than the monomer. An ATPase inactive Spa47 point mutant was then engineered by targeting a conserved Lysine within the predicted Walker A motif of Spa47. Interestingly, the mutant maintained a similar oligomerization pattern as active Spa47, but was unable to restore invasion phenotype when used to complement a spa47 null S. flexneri strain. Together, these results identify Spa47 as a Shigella T3SS ATPase and suggest that its activity is linked to oligomerization, perhaps as a regulatory mechanism as seen in some related pathogens. Additionally, Spa47 catalyzed ATP hydrolysis appears to be essential for host cell invasion, providing a strong platform for additional studies dissecting its role in virulence and providing an attractive target for anti-infective agents.

Characterization of the Binding of Hydroxyindole, Indoleacetic acid, and Morpholinoaniline to the Salmonella Type III Secretion System Proteins SipD and SipB

Many Gram-negative bacteria require the type III secretion system (T3SS) to cause infectious diseases in humans. A looming public health problem is that all bacterial pathogens that require the T3SS to cause infectious diseases in humans have developed multidrug resistance to current antibiotics. The T3SS is an attractive target for the development of new antibiotics because of its critical role in virulence. An initial step in developing anti-T3SS-based therapeutics is the identification of small molecules that can bind to T3SS proteins. Currently, the only small molecules that are known to bind to the Salmonella T3SS proteins SipD and SipB are bile salts (to SipD) and sphingolipids and cholesterol (to SipB). Herein we report the results of a surface plasmon resonance screen of 288 compounds wherein the binding of 4-morpholinoaniline to SipD, 3-indoleacetic acid to SipB, and 5-hydroxyindole to both SipD and SipB were identified. We also identified by NMR the SipD surfaces involved in binding. These three compounds represent a new class of molecules that can bind to T3SS tip (SipD) and translocon (SipB) proteins that could find use in future drug design.

The Many Faces of IpaB

The type III secretion system (T3SS) is Shigella's most important virulence factor. The T3SS apparatus (T3SA) is comprised of an envelope-spanning basal body and an external needle topped by a tip complex protein called IpaD. This nanomachine is used to deliver effector proteins into host cells to promote pathogen entry. A key component of the matured T3SS needle tip complex is the translocator protein IpaB. IpaB can exist in multiple states when prepared as a recombinant protein, however, it has also been described as having additional roles in Shigella pathogenesis. This mini-review will briefly describe some of the features of IpaB as a T3SS needle tip protein, as a pore-forming translocator protein and as an effector protein. Reflection on the potential importance of the different in vitro states of IpaB on its function and importance in serotype-independent vaccines is also provided.

Type 3 Secretion Translocators Spontaneously Assemble a Hexadecameric Transmembrane Complex

A type 3 secretion system is used by many bacterial pathogens to inject proteins into eukaryotic cells. Pathogens insert a translocon complex into the target eukaryotic membrane by secreting two proteins known as translocators. How these translocators form a translocon in the lipid bilayer and why both proteins are required remains elusive. Pseudomonas aeruginosa translocators PopB and PopD insert pores into membranes forming homo- or hetero-complexes of undetermined stoichiometry. Single-molecule fluorescence photobleaching experiments revealed that PopD formed mostly hexameric structures in membranes, whereas PopB displayed a bi-modal distribution with 6 and 12 subunits peaks. However, individually the proteins are not functional for effector translocation. We have found that when added together, the translocators formed distinct hetero-complexes containing 8 PopB and 8 PopD molecules. Thus, the interaction between PopB and PopD guide the assembly of a unique hetero-oligomer in membranes.

The bacterial type III secretion system as a target for developing new antibiotics

Antibiotic resistance in pathogens requires new targets for developing novel antibacterials. The bacterial type III secretion system (T3SS) is an attractive target for developing antibacterials as it is essential in the pathogenesis of many Gram-negative bacteria. The T3SS consists of structural proteins, effectors, and chaperones. Over 20 different structural proteins assemble into a complex nanoinjector that punctures a hole on the eukaryotic cell membrane to allow the delivery of effectors directly into the host cell cytoplasm. Defects in the assembly and function of the T3SS render bacteria non-infective. Two major classes of small molecules, salicylidene acylhydrazides and thiazolidinones, have been shown to inhibit multiple genera of bacteria through the T3SS. Many additional chemically and structurally diverse classes of small molecule inhibitors of the T3SS have been identified as well. While specific targets within the T3SS of a few inhibitors have been suggested, the vast majority of specific protein targets within the T3SS remain to be identified or characterized. Other T3SS inhibitors include polymers, proteins, and polypeptides mimics. In addition, T3SS activity is regulated by its interaction with biologically relevant molecules, such as bile salts and sterols, which could serve as scaffolds for drug design.

The roles of the virulence factor IpaB in Shigella spp. in the escape from immune cells and invasion of epithelial cells

Shigellosis is an acute invasive enteric infection by the Gram negative pathogen Shigella, which causes human diarrhea. Shigella, which are highly epidemic and pathogenic, have become a serious public health problem. The virulence plasmid is a large plasmid essential to the infected host cells. Many virulence factors are encoded in the ipa-mxi-spa region by the virulence plasmid. IpaB is a multifunctional and essential virulence factor in the infection process. In this review article, we introduce the recent studies of the effect of IpaB in Shigella-infected host cells. IpaB is involved in a type III secretion system (T3SS) structure. It also controls the secretion of virulence factors and Shigella adhesion to host cells. In addition, it forms the ion pore, destroys phagosomes, and induces the immune cell's apoptosis or necrosis. Moreover, IpaB can become a potential antigen for Shigella vaccine development.

Chlamydia Outer Protein (Cop) B from Chlamydia pneumoniae possesses characteristic features of a type III secretion (T3S) translocator protein

Background

Chlamydia spp. are believed to use a conserved virulence factor called type III secretion (T3S) to facilitate the delivery of effector proteins from the bacterial pathogen to the host cell. Important early effector proteins of the type III secretion system (T3SS) are a class of proteins called the translocators. The translocator proteins insert into the host cell membrane to form a pore, allowing the injectisome to dock onto the host cell to facilitate translocation of effectors. CopB is a predicted hydrophobic translocator protein within the chlamydial T3SS.

Results

In this study, we identified a novel interaction between the hydrophobic translocator, CopB, and the putative filament protein, CdsF. Furthermore, we identified a conserved PxLxxP motif in CopB (amino acid residues 166–171), which is required for interaction with its cognate chaperone, LcrH_1. Using a synthetic peptide derived from the chaperone binding motif of CopB, we were able to block the LcrH_1 interaction with either CopB or CopD; this CopB peptide was capable of inhibiting C. pneumoniae infection of HeLa cells at micromolar concentrations. An antibody raised against the N-terminus of CopB was able to inhibit C. pneumoniae infection of HeLa cells.

Conclusion

The inhibition of the LcrH_1:CopB interaction with a cognate peptide and subsequent inhibition of host cell infection provides strong evidence that T3S is an essential virulence factor for chlamydial infection and pathogenesis. Together, these results support that CopB plays the role of a hydrophobic translocator.

Three-dimensional electron microscopy reconstruction and cysteine-mediated crosslinking provide a model of the type III secretion system needle tip complex

Type III secretion systems are found in many Gram-negative bacteria. They are activated by contact with eukaryotic cells and inject virulence proteins inside them. Host cell detection requires a protein complex located at the tip of the device's external injection needle. The Shigella tip complex (TC) is composed of IpaD, a hydrophilic protein, and IpaB, a hydrophobic protein, which later forms part of the injection pore in the host membrane. Here we used labelling and crosslinking methods to show that TCs from a ΔipaB strain contain five IpaD subunits while the TCs from wild-type can also contain one IpaB and four IpaD subunits. Electron microscopy followed by single particle and helical image analysis was used to reconstruct three-dimensional images of TCs at ∼ 20 Å resolution. Docking of an IpaD crystal structure, constrained by the crosslinks observed, reveals that TC organisation is different from that of all previously proposed models. Our findings suggest new mechanisms for TC assembly and function. The TC is the only site within these secretion systems targeted by disease-protecting antibodies. By suggesting how these act, our work will allow improvement of prophylactic and therapeutic strategies.