Serine protease espP subtype alpha, but not beta or gamma, of Shiga toxin-producing Escherichia coli is associated with highly pathogenic serogroups

A putative cytotoxic serine protease from Salmonella typhimurium UcB5 recovered from undercooked burger

A putative virulence exoprotease designated as UcB5 was successfully purified from the bacterium Salmonella typhimurium to the electrophoretic homogeneity with 13.2-fold and 17.1% recovery by hydrophobic, ion-exchange, and gel permeation chromatography using Phenyl-Sepharose 6FF, DEAE-Sepharose CL-6B, and Sephadex G-75, respectively. By applying SDS-PAGE, the molecular weight was confirmed at 35 kDa. The optimal temperature, pH, and isoelectric point were 35 °C, 8.0, 5.6 ± 0.2, respectively. UcB5 was found to have a broad substrate specificity against almost all the tested chromogenic substrates with maximal affinity against N-Succ-Ala-Ala-Pro-Phe-pNA achieving K_m of 0.16 mM, K_cat/K_m of 3.01 × 10⁵ S^-1 M^-1, and amidolytic activity of 28.9 µmol min^-1 L^-1. It was drastically inhibited by TLCK, PMSF, SBTI, and aprotinin while, DTT, β-mercaptoethanol, 2,2'-bipyridine, o-phenanthroline, EDTA, and EGTA had no effect, which suggested a serine protease-type. Also, it has shown a broad substrate specificity against a broad range of natural proteins including serum proteins. A cytotoxicity and electron microscopy study revealed that UcB5 could cause subcellular proteolysis that finally led to liver necrosis. For this, future research should focus on using a combination of external antiproteases and antimicrobial agents for the treatment of microbial diseases instead of using drugs alone.

IgG Binds Escherichia coli Serine Protease EspP and Protects Mice From E. coli O157:H7 Infection

Shiga toxin-producing Escherichia coli O157:H7 is a virulent strain causing severe gastrointestinal infection, hemolytic uremic syndrome and death. To date there are no specific therapies to reduce progression of disease. Here we investigated the effect of pooled immunoglobulins (IgG) on the course of disease in a mouse model of intragastric E. coli O157:H7 inoculation. Intraperitoneal administration of murine IgG on day 3, or both on day 3 and 6, post-inoculation improved survival and decreased intestinal and renal pathology. When given on both day 3 and 6 post-inoculation IgG treatment also improved kidney function in infected mice. Murine and human commercially available IgG preparations bound to proteins in culture filtrates from E. coli O157:H7. Bound proteins were extracted from membranes and peptide sequences were identified by mass spectrometry. The findings showed that murine and human IgG bound to E. coli extracellular serine protease P (EspP) in the culture filtrate, via the IgG Fc domain. These results were confirmed using purified recombinant EspP and comparing culture filtrates from the wild-type E. coli O157:H7 strain to a deletion mutant lacking espP. Culture filtrates from wild-type E. coli O157:H7 exhibited enzymatic activity, specifically associated with the presence of EspP and demonstrated as pepsin cleavage, which was reduced in the presence of murine and human IgG. EspP is a virulence factor previously shown to promote colonic cell injury and the uptake of Shiga toxin by intestinal cells. The results presented here suggest that IgG binds to EspP, blocks its enzymatic activity, and protects the host from E. coli O157:H7 infection, even when given post-inoculation.

Serine Protease Autotransporters of the Enterobacteriaceae (SPATEs): Out and About and Chopping It Up

Autotransporters are secreted proteins with multiple functions produced by a variety of Gram-negative bacteria. In Enterobacteriaceae, a subgroup of these autotransporters are the SPATEs (serine protease autotransporters of Enterobacteriaceae). SPATEs play a crucial role in survival and virulence of pathogens such as Escherichia coli and Shigella spp. and contribute to intestinal and extra-intestinal infections. These high molecular weight proteases are transported to the external milieu by the type Va secretion system and function as proteases with diverse substrate specificities and biological functions including adherence and cytotoxicity. Herein, we provide an overview of SPATEs and discuss recent findings on the biological roles of these secreted proteins, including proteolysis of substrates, adherence to cells, modulation of the immune response, and virulence in host models. In closing, we highlight recent insights into the regulation of expression of SPATEs that could be exploited to understand fundamental SPATE biology.

The Fimbrial Gene z3276 in Enterohemorrhagic Escherichia coli O157:H7 Contributes to Bacterial Pathogenicity

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a zoonotic pathogen of worldwide importance that causes foodborne infections in humans. It is not capable of expressing type I fimbrial because of base deletion in the fim operon. BLAST analysis shows that the open reading frame z3276, a specific genetic marker of EHEC O157:H7, encodes a sequence with high amino acid identity to other E. coli type I fimbrial, but its definitive function in EHEC O157:H7 remains unclear. We are here to report that a z3276 mutant (Δz3276) was constructed using the reference EHEC O157:H7, the mutant Δz3276 was biologically characterized, and the pathogenicity of Δz3276 was assessed in mice in comparison with the wild-type (WT) strain. Motility and biofilm formation assays revealed a smaller twitching motility zone for Δz3276 on the agar surface and significantly decreased biofilm formation by Δz3276 compared with the parental strain. The adhesion and invasion ability of Δz3276 to HEp-2 cells showed no significant change, but the invasion ability of Δz3276 to IPEC-J2 cells was attenuated. Furthermore, in the animal study, we observed shortened and lower fecal shedding among the Δz3276 mutant-infected animals compared with those infected WT strain. The data in this study indicate that this unique gene of z3276 in EHEC O157:H7 seems to play an important role in bacterial pathogenicity.

EspP, an Extracellular Serine Protease from Enterohemorrhagic E. coli, Reduces Coagulation Factor Activities, Reduces Clot Strength, and Promotes Clot Lysis

BACKGROUND

EspP (E. coli secreted serine protease, large plasmid encoded) is an extracellular serine protease produced by enterohemorrhagic E. coli (EHEC) O157:H7, a causative agent of diarrhea-associated Hemolytic Uremic Syndrome (D+HUS). The mechanism by which EHEC induces D+HUS has not been fully elucidated.

OBJECTIVES

We investigated the effects of EspP on clot formation and lysis in human blood.

METHODS

Human whole blood and plasma were incubated with EspP(WT )at various concentrations and sampled at various time points. Thrombin time (TT), prothrombin time (PT), and activated partial thromboplastin time (aPTT), coagulation factor activities, and thrombelastgraphy (TEG) were measured.

RESULTS AND CONCLUSIONS

Human whole blood or plasma incubated with EspP(WT) was found to have prolonged PT, aPTT, and TT. Furthermore, human whole blood or plasma incubated with EspP(WT) had reduced activities of coagulation factors V, VII, VIII, and XII, as well as prothrombin. EspP did not alter the activities of coagulation factors IX, X, or XI. When analyzed by whole blood TEG, EspP decreased the maximum amplitude of the clot, and increased the clot lysis. Our results indicate that EspP alters hemostasis in vitro by decreasing the activities of coagulation factors V, VII, VIII, and XII, and of prothrombin, by reducing the clot strength and accelerating fibrinolysis, and provide further evidence of a functional role for this protease in the virulence of EHEC and the development of D+HUS.

Structural and functional characterization of cleavage and inactivation of human serine protease inhibitors by the bacterial SPATE protease EspPα from enterohemorrhagic E. coli

EspPα and EspI are serine protease autotransporters found in enterohemorrhagic Escherichia coli. They both belong to the SPATE autotransporter family and are believed to contribute to pathogenicity via proteolytic cleavage and inactivation of different key host proteins during infection. Here, we describe the specific cleavage and functional inactivation of serine protease inhibitors (serpins) by EspPα and compare this activity with the related SPATE EspI. Serpins are structurally related proteins that regulate vital protease cascades, such as blood coagulation and inflammatory host response. For the rapid determination of serpin cleavage sites, we applied direct MALDI-TOF-MS or ESI-FTMS analysis of coincubations of serpins and SPATE proteases and confirmed observed cleavage positions using in-gel-digest of SDS-PAGE-separated degradation products. Activities of both serpin and SPATE protease were assessed in a newly developed photometrical assay using chromogenic peptide substrates. EspPα cleaved the serpins α1-protease inhibitor (α1-PI), α1-antichymotrypsin, angiotensinogen, and α2-antiplasmin. Serpin cleavage led to loss of inhibitory function as demonstrated for α1-PI while EspPα activity was not affected. Notably, EspPα showed pronounced specificity and cleaved procoagulatory serpins such as α2-antiplasmin while the anticoagulatory antithrombin III was not affected. Together with recently published research, this underlines the interference of EspPα with hemostasis or inflammatory responses during infection, while the observed interaction of EspI with serpins is likely to be not physiologically relevant. EspPα-mediated serpin cleavage occurred always in flexible loops, indicating that this structural motif might be required for substrate recognition.

Biochemical characterization of the SPATE members EspPα and EspI

The activity of serine proteases is influenced by their substrate specificity as well as by the physicochemical conditions. Here, we present the characterization of key biochemical features of the two SPATE members EspPα and EspI from Shiga-toxin producing Escherichia coli (STEC) and enterohemorrhagic E. coli (EHEC). Both proteases show high activity at conditions mimicking the human blood stream. Optimal activities were observed at slightly alkaline pH and low millimolar concentrations of the divalent cations Ca²⁺ and Mg²⁺ at physiological temperatures indicating a function in the human host. Furthermore, we provide the first cleavage profile for EspI demonstrating pronounced specificity of this protease.

Bacterial serine proteases secreted by the autotransporter pathway: classification, specificity, and role in virulence

Serine proteases exist in eukaryotic and prokaryotic organisms and have emerged during evolution as the most abundant and functionally diverse group. In Gram-negative bacteria, there is a growing family of high molecular weight serine proteases secreted to the external milieu by a fascinating and widely employed bacterial secretion mechanism, known as the autotransporter pathway. They were initially found in Neisseria, Shigella, and pathogenic Escherichia coli, but have now also been identified in Citrobacter rodentium, Salmonella, and Edwardsiella species. Here, we focus on proteins belonging to the serine protease autotransporter of Enterobacteriaceae (SPATEs) family. Recent findings regarding the predilection of serine proteases to host intracellular or extracellular protein-substrates involved in numerous biological functions, such as those implicated in cytoskeleton stability, autophagy or innate and adaptive immunity, have helped provide a better understanding of SPATEs' contributions in pathogenesis. Here, we discuss their classification, substrate specificity, and potential roles in pathogenesis.

Prevalence, biogenesis, and functionality of the serine protease autotransporter EspP

Enterohemorrhagic E. coli (EHEC) causes severe diseases in humans worldwide. One of its virulence factors is EspP, which belongs to the serine protease autotransporters of Enterobacteriaceae (SPATE) family. In this review we recapitulate the current data on prevalence, biogenesis, structural properties and functionality. EspP has been used to investigate mechanistic details of autotransport, and recent studies indicate that this transport mechanism is not autonomous but rather dependent on additional factors. Currently, five subtypes have been identified (EspPα-EspPε), with EspPα being associated with highly virulent EHEC serotypes and isolates from patients with severe disease. EspPα has been shown to degrade major proteins of the complement cascade, namely C3 and C5 and probably interferes with hemostasis by cleavage of coagulation factor V. Furthermore, EspPα is believed to contribute to biofilm formation perhaps by polymerization to rope-like structures. Together with the proteolytic activity, EspPα might ameliorate host colonization and interfere with host response.

Infection strategies of enteric pathogenic Escherichia coli

Enteric Escherichia coli (E. coli) are both natural flora of humans and important pathogens causing significant morbidity and mortality worldwide. Traditionally enteric E. coli have been divided into 6 pathotypes, with further pathotypes often proposed. In this review we suggest expansion of the enteric E. coli into 8 pathotypes to include the emerging pathotypes of adherent invasive E. coli (AIEC) and Shiga-toxin producing enteroaggregative E. coli (STEAEC). The molecular mechanisms that allow enteric E. coli to colonize and cause disease in the human host are examined and for two of the pathotypes that express a type 3 secretion system (T3SS) we discuss the complex interplay between translocated effectors and manipulation of host cell signaling pathways that occurs during infection.

Enterohaemorrhagic Escherichia coli haemolysin is cleaved and inactivated by serine protease EspPα

The haemolysin from enterohaemorrhagic Escherichia coli (EHEC-Hly) and the serine protease EspPα are putative virulence factors of EHEC. We investigated the interplay between these secreted factors and demonstrate that EspPα cleaves the 107 kDa large EHEC-Hly. Degradation was observed when purified EspPα was added to a growing culture of an EHEC-Hly-expressing strain, with isolated proteins and with coexpressing strains, and was independent of the EHEC serotype. EHEC-Hly breakdown occurred as a multistage process with the formation of characteristic fragments with relative molecular masses of ∼82 kDa and/or ∼84 kDa and ∼34 kDa. The initial cleavage occurred in the N-terminal hydrophobic domain of EHEC-Hly between Leu²³⁵ and Ser²³⁶ and abolished its haemolytic activity. In a cellular infection system, the cytolytic potential of EHEC-Hly-secreting recombinant strains was abolished when EspPα was coexpressed. EHEC in contact with human intestinal epithelial cells simultaneously upregulated their EHEC-Hly and EspP indicating that both molecules might interact under physiological conditions. We propose the concept of bacterial effector molecule interference (BEMI), reflecting the concerted interplay of virulence factors. Interference between effector molecules might be an additional way to regulate virulence functions and increases the complexity of monomolecular phenotypes.

EspP, a serine protease of enterohemorrhagic Escherichia coli, impairs complement activation by cleaving complement factors C3/C3b and C5

Hemolytic-uremic syndrome (HUS) is a life-threatening disorder characterized by hemolytic anemia, thrombocytopenia, and renal insufficiency. It is caused mainly by infections with enterohemorrhagic Escherichia coli (EHEC). Recently, Shiga toxin 2, the best-studied virulence factor of EHEC, was reported to interact with complement, implying that complement may be involved in the pathogenesis of EHEC-induced HUS. The aim of the present study was to investigate whether or not the serine protease EspP, an important virulence factor of EHEC, interacts with complement proteins. EspP did not have any effect on the integrity of factor H or factor I. However, EspP was shown to cleave purified C3/C3b and C5. Cleavage of the respective complement proteins also occurred in normal human serum (NHS) as a source of C3/C3b or C5 or when purified complement proteins were added to the supernatant of an EspP-producing wild-type strain. Edman degradation allowed unequivocal mapping of all three main C3b fragments but not of the three main C5 fragments. Complement activation was significantly downregulated in all three pathways for C5-depleted serum to which C5, preincubated with EspP, was added (whereas C5 preincubated with an EspP mutant was able to fully reconstitute complement activation). This indicates that EspP markedly destroyed the functional activity, as measured by a commercial total complement enzyme-linked immunosorbent assay (Wieslab). Downregulation of complement by EspP in vivo may influence the colonization of EHEC bacteria in the gut or the disease severity of HUS.

Structure and function relationship of the autotransport and proteolytic activity of EspP from Shiga toxin-producing Escherichia coli

BACKGROUND

The serine protease autotransporter EspP is a proposed virulence factor of Shiga toxin-producing Escherichia coli (STEC). We recently distinguished four EspP subtypes (EspPalpha, EspPbeta, EspPgamma, and EspPdelta), which display large differences in transport and proteolytic activities and differ widely concerning their distribution within the STEC population. The mechanisms underlying these functional variations in EspP subtypes are, however, unknown.

METHODOLOGY/PRINCIPAL FINDINGS

The structural basis of proteolytic and autotransport activity was investigated using transposon-based linker scanning mutagenesis, site-directed mutagenesis and structure-function analysis derived from homology modelling of the EspP passenger domain. Transposon mutagenesis of the passenger domain inactivated autotransport when pentapeptide linker insertions occurred in regions essential for overall correct folding or in a loop protruding from the beta-helical core. Loss of proteolytic function was limited to mutations in Domain 1 in the N-terminal third of the EspP passenger. Site-directed mutagenesis demonstrated that His(127), Asp(156) and Ser(263) in Domain 1 form the catalytic triad of EspP.

CONCLUSIONS/SIGNIFICANCE

Our data indicate that in EspP i) the correct formation of the tertiary structure of the passenger domain is essential for efficient autotransport, and ii) an elastase-like serine protease domain in the N-terminal Domain 1 is responsible for the proteolytic phenotype. Lack of stabilizing interactions of Domain 1 with the core structure of the passenger domain ablates proteolytic activity in subtypes EspPbeta and EspPdelta.