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

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Multiplex quantification of C-terminal alpha-1-antitrypsin peptides provides a novel approach for characterizing systemic inflammation

C-terminal peptides (CAAPs) of the highly abundant serine protease alpha-1-antitrypsin (A1AT) have been identified at various lengths in several human materials and have been proposed to serve as putative biomarkers for a variety of diseases. CAAPs are enzymatically formed and these enzymatic activities are often associated with excessive immune responses (e.g. sepsis, allergies). However, most of those CAAPs have been either detected using in vitro incubation experiments or in human materials which are not easily accessible. To gain a comprehensive understanding about the occurrence and function of CAAPs in health and disease, a LC-MS/MS method for the simultaneous detection of nine CAAPs was developed and validated for human plasma (EDTA and lithium-heparin) and serum. Using this newly developed method, we were able to detect and quantify five CAAPs in healthy individuals thereby providing an initial proof for the presence of C36, C37, C40 and C44 in human blood. Concentrations of four CAAPs in a clinical test cohort of patients suffering from sepsis were significantly higher compared to healthy controls. These results reveal that in addition to C42 other fragments of A1AT seem to play a crucial role during systemic infections. The proposed workflow is simple, rapid and robust; thus this method could be used as diagnostic tool in routine clinical chemistry as well as for research applications for elucidating the diagnostic potential of CAAPs in numerous diseases. To this end, we also provide an overview about the current state of knowledge for CAAPs identified in vitro and in vivo.

SepA Enhances Shigella Invasion of Epithelial Cells by Degrading Alpha-1 Antitrypsin and Producing a Neutrophil Chemoattractant

Shigella spp. are highly adapted pathogens that cause bacillary dysentery in human and nonhuman primates. An unusual feature of Shigella pathogenesis is that this organism invades the colonic epithelia from the basolateral pole. Therefore, it has evolved the ability to disrupt the intestinal epithelial barrier to reach the basolateral surface. We have shown previously that the secreted serine protease A (SepA), which belongs to the family of serine protease autotransporters of Enterobacteriaceae, is responsible for the initial destabilization of the intestinal epithelial barrier that facilitates Shigella invasion. However, the mechanisms used by SepA to regulate this process remain unknown. To investigate the protein targets cleaved by SepA in the intestinal epithelium, we incubated a sample of homogenized human colon with purified SepA or with a catalytically inactive mutant of this protease. We discovered that SepA targets an array of 18 different proteins, including alpha-1 antitrypsin (AAT), a major circulating serine proteinase inhibitor in humans. In contrast to other serine proteases, SepA cleaved AAT without forming an inhibiting complex, which resulted in the generation of a neutrophil chemoattractant. We demonstrated that the products of the AAT-SepA reaction induce a mild but significant increase in neutrophil transepithelial migration in vitro. Moreover, the presence of AAT during Shigella infection stimulated neutrophil migration and dramatically enhanced the number of bacteria invading the intestinal epithelium in a SepA-dependent manner. We conclude that by cleaving AAT, SepA releases a chemoattractant that promotes neutrophil migration, which in turn disrupts the intestinal epithelial barrier to enable Shigella invasion. IMPORTANCE Shigella is the second leading cause of diarrheal death globally. In this study, we identified the host protein targets of SepA, Shigella's major protein secreted in culture. We demonstrated that by cleaving AAT, a serine protease inhibitor important to protect surrounding tissue at inflammatory sites, SepA releases a neutrophil chemoattractant that enhances Shigella invasion. Moreover, SepA degraded AAT without becoming inhibited by the cleaved product, and SepA catalytic activity was enhanced at higher concentrations of AAT. Activation of SepA by an excess of AAT may be physiologically relevant at the early stages of Shigella infection, when the amount of synthesized SepA is very low compared to the concentration of AAT in the intestinal lumen. This observation may also help to explain the adeptness of Shigella infectivity at low dose, despite the requirement of reaching the basolateral side to invade and colonize the colonic epithelium.

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.

Enterohemorrhagic E. coli (EHEC)-Secreted Serine Protease EspP Stimulates Electrogenic Ion Transport in Human Colonoid Monolayers

One of the characteristic manifestations of Shiga-toxin-producing Escherichia coli (E. coli) infection in humans, including EHEC and Enteroaggregative E. coli O104:H4, is watery diarrhea. However, neither Shiga toxin nor numerous components of the type-3 secretion system have been found to independently elicit fluid secretion. We used the adult stem-cell-derived human colonoid monolayers (HCM) to test whether EHEC-secreted extracellular serine protease P (EspP), a member of the serine protease family broadly expressed by diarrheagenic E. coli can act as an enterotoxin. We applied the Ussing chamber/voltage clamp technique to determine whether EspP stimulates electrogenic ion transport indicated by a change in short-circuit current (Isc). EspP stimulates Isc in HCM. The EspP-stimulated Isc does not require protease activity, is not cystic fibrosis transmembrane conductance regulator (CFTR)-mediated, but is partially Ca²⁺-dependent. EspP neutralization with a specific antibody reduces its potency in stimulating Isc. Serine Protease A, secreted by Enteroaggregative E. coli, also stimulates Isc in HCM, but this current is CFTR-dependent. In conclusion, EspP stimulates colonic CFTR-independent active ion transport and may be involved in the pathophysiology of EHEC diarrhea. Serine protease toxins from E. coli pathogens appear to serve as enterotoxins, potentially significantly contributing to watery diarrhea.

Corticosteroid-binding globulin cleavage may be pathogen-dependent in bloodstream infection

OBJECTIVE

The process of enzymatic cleavage of high- to low-affinity corticosteroid-binding globulin (haCBG to laCBG) by neutrophil elastase leads to local tissue release of cortisol. Recently Pseudomonas aeruginosa was shown to instigate CBG cleavage with release of free cortisol in vitro. Hence, CBG cleavage with release of anti-inflammatory cortisol in infection may be pathogen-dependent. Our objective was to determine whether haCBG and laCBG levels are altered in infected patients compared with controls, and whether these alterations were particular to causative bacteria.

DESIGN

An observational, cross-sectional study at a public pathology institution and tertiary hospital in Adelaide, South Australia.

METHODS

100 positive blood culture samples and 100 healthy control samples were analysed for serum total CBG, haCBG, laCBG, total and free cortisol, leukocyte and neutrophil count, C-reactive protein and Pitt severity score.

RESULTS

Patients with infection had lower serum total CBG, haCBG and laCBG, all P<0.0001, than healthy controls. This was true in patients with and without a systemic inflammatory response and in those with culture-positive and culture-negative infections. Pseudomonas aeruginosa infection was associated with the lowest total and laCBG levels of the pathogen groups despite having the lowest inflammatory markers.

CONCLUSIONS

There was evidence of CBG cleavage in early infection both in patients with and without systemic inflammation and regardless of culture status. Pseudomonas infection appeared to enhance cleavage. This observation, along with cleavage in severe neutropenia suggests mechanisms other than neutrophil elastase may be involved in CBG cleavage and local tissue cortisol release in infection.

HUS and the case for complement

Hemolytic-uremic syndrome (HUS) is a thrombotic microangiopathy that is characterized by microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. Excess complement activation underlies atypical HUS and is evident in Shiga toxin-induced HUS (STEC-HUS). This Spotlight focuses on new knowledge of the role of Escherichia coli-derived toxins and polyphosphate in modulating complement and coagulation, and how they affect disease progression and response to treatment. Such new insights may impact on current and future choices of therapies for STEC-HUS.