The proteolytic activity of Listeria monocytogenes HtrA

Biophysical characterization and in silico analysis of natural and synthetic compounds targeting Listeria monocytogenes HtrA protease

HtrA protein is a member of a serine protease family with dual functions as a protease and molecular chaperone. It is a virulence factor in many bacteria, including the food-borne pathogen Listeria monocytogenes (Lm), which induces listeriosis in humans. Hence, inhibitors of LmHtrA protease have great importance in the control of infection. Many natural compounds have been used in the inhibition studies of proteases; here, we have performed the inhibition studies of LmHtrA with 31 compounds from different origins. The spectrophotometric assays revealed that plant compounds are promising inhibitors of LmHtrA protease activity compared to other tested peptides and synthetic compounds. The green tea catechin, EGCG has been identified as an inhibitor of protease activity of LmHtrA with a low IC50 value of 0.754 ± 0.2 μM. The substrate cleavage analysis by SDS-PAGE and SPR experiments corroborates the spectrophotometric results by exhibiting protease inhibition and showing the micromolar affinity of EGCG with LmHtrA, respectively. The interaction between rLmHtrA and EGCG was investigated by fluorescence spectroscopy. The binding constant and the number of binding sites were determined as 1.86 × 10(5) M(-1) and 1.2, respectively. The molecular docking and dynamics results of LmHtrA-inhibitor complexes have provided new insights into the inhibition mechanism of LmHtrA compared with other serine proteases. The findings of this study may open up new avenues for the development of natural compound-based derivatives of LmHtrA inhibitors that might be more potent and less harmful to humans.

The HtrA chaperone monitors sortase-assembled pilus biogenesis in Enterococcus faecalis

Sortase-assembled pili contribute to virulence in many Gram-positive bacteria. In Enterococcus faecalis, the endocarditis and biofilm-associated pilus (Ebp) is polymerized on the membrane by sortase C (SrtC) and attached to the cell wall by sortase A (SrtA). In the absence of SrtA, polymerized pili remain anchored to the membrane (i.e. off-pathway). Here we show that the high temperature requirement A (HtrA) bifunctional chaperone/protease of E. faecalis is a quality control system that clears aberrant off-pathway pili from the cell membrane. In the absence of HtrA and SrtA, accumulation of membrane-bound pili leads to cell envelope stress and partially induces the regulon of the ceftriaxone resistance-associated CroRS two-component system, which in turn causes hyper-piliation and cell morphology alterations. Inactivation of croR in the OG1RF ΔsrtAΔhtrA background partially restores the observed defects of the ΔsrtAΔhtrA strain, supporting a role for CroRS in the response to membrane perturbations. Moreover, absence of SrtA and HtrA decreases basal resistance of E. faecalis against cephalosporins and daptomycin. The link between HtrA, pilus biogenesis and the CroRS two-component system provides new insights into the E. faecalis response to endogenous membrane perturbations.

Effects of the antimicrobial glabridin on membrane integrity and stress response activation in Listeria monocytogenes

Glabridin is a prenylated isoflavan which can be extracted from liquorice roots and has shown antimicrobial activity against foodborne pathogens and spoilage microorganisms. However, its application may be hindered due to limited information about its mode of action. In this study, we aimed to investigate the mode of action of glabridin using a combined phenotypic and proteomic approach on Listeria monocytogenes. Fluorescence and transmission electron microscopy of cells exposed to glabridin showed membrane permeabilization upon treatment with lethal concentrations of glabridin. Comparative proteomics analysis of control cells and cells exposed to sub-lethal concentrations of glabridin showed upregulation of proteins related to the two-component systems LiaSR and VirRS, confirming cell envelope damage during glabridin treatment. Additional upregulation of SigmaB regulon members signified activation of the general stress response in L. monocytogenes during this treatment. In line with the observed upregulation of cell envelope and general stress response proteins, sub-lethal treatment of glabridin induced (cross)protection against lethal heat and low pH stress and against antimicrobials such as nisin and glabridin itself. Overall, this study sheds light on the mode of action of glabridin and activation of the main stress responses to this antimicrobial isoflavan and highlights possible implications of its use as a naturally derived antimicrobial compound.

A comprehensive investigation of protein expression profiles in L. monocytogenes exposed to thermal abuse, mild acid, and salt stress conditions

Preventing L. monocytogenes infection is crucial for food safety, considering its widespread presence in the environment and its association with contaminated RTE foods. The pathogen's ability to persist under adverse conditions, for example, in food processing facilities, is linked to virulence and resistance mechanisms, including biofilm formation. In this study, the protein expression patterns of two L. monocytogenes 1/2a strains, grown under environmental stressors (mild acidic pH, thermal abuse, and high concentration of NaCl), were investigated. Protein identification and prediction were performed by nLC-ESI-MS/MS and nine different bioinformatic software programs, respectively. Gene enrichment analysis was carried out by STRING v11.05. A total of 1,215 proteins were identified, of which 335 were non-cytosolic proteins and 265 were immunogenic proteins. Proteomic analysis revealed differences in protein expression between L. monocytogenes strains in stressful conditions. The two strains exhibited unique protein expression profiles linked to stress response, virulence, and pathogenesis. Studying the proteomic profiles of such microorganisms provides information about adaptation and potential treatments, highlighting their genetic diversity and demonstrating the utility of bioinformatics and proteomics for a broader analysis of pathogens.

Inhibition of Listeria Monocytogenes HtrA Protease with Camostat, Gabexate and Nafamostat Mesylates and the Binding Mode of the Inhibitors

In many bacteria, the High Temperature requirement A (HtrA) protein functions as a chaperone and protease. HtrA is an important factor in stress tolerance and plays a significant role in the virulence of several pathogenic bacteria. Camostat, gabexate and nafamostat mesylates are serine protease inhibitors and have recently shown a great impact in the inhibition studies of SARS-CoV2. In this study, the inhibition of Listeria monocytogenes HtrA (LmHtrA) protease activity was analysed using these three inhibitors. The cleavage assay, using human fibrinogen and casein as substrates, revealed that the three inhibitors effectively inhibit the protease activity of LmHtrA. The agar plate assay and spectrophotometric analysis concluded that the inhibition of nafamostat (IC50 value of 6.6 ± 0.4 µM) is more effective compared to the other two inhibitors. Previous studies revealed that at the active site of the protease, these inhibitors are hydrolysed and one of the hydrolysates is covalently bound to the active site serine. To understand the mode of binding of these inhibitors at the active site of LmHtrA, docking of the inhibitors followed by molecular dynamics simulations were carried out. Analysis of the LmHtrA-inhibitor complex structures revealed that the covalently bound inhibitor is unable to occupy the S1 pocket of the LmHtrA which is in contrast to the previously determined camostat and nafamostat complex structures. This observation provides the first glimpse of the substrate specificity of LmHtrA which is not known. The obtained results also suggest that the development of novel inhibitors of LmHtrA and its homologs with active site architecture similar to LmHtrA can be pursued with suitable modification of these inhibitors. To date, only a very few studies have been carried out on identifying the inhibitors of HtrA proteolytic activity.

Trimer stability of Helicobacter pylori HtrA is regulated by a natural mutation in the protease domain

The human pathogen Helicobacter pylori is a major risk factor for gastric disease development. Serine protease HtrA is an important bacterial virulence factor that cleaves the cell junction proteins occludin, claudin-8 and E-cadherin, which causes gastric tissue damage. Using casein zymography, we discovered that HtrA trimer stability varies in clinical H. pylori strains. Subsequent sequence analyses revealed that HtrA trimer stability correlated with the presence of leucine or serine residue at position 171. The importance of these amino acids in determining trimer stability was confirmed by leucine-to-serine swapping experiments using isogenic H. pylori mutant strains as well as recombinant HtrA proteins. In addition, this sequence position displays a high sequence variability among various bacterial species, but generally exhibits a preference for hydrophilic amino acids. This natural L/S171 polymorphism in H. pylori may affect the protease activity of HtrA during infection, which could be of clinical importance and may determine gastric disease development.

Aeromonas sobria Serine Protease Degrades Several Protein Components of Tight Junctions and Assists Bacterial Translocation Across the T84 Monolayer

Aeromonas sobria is a Gram-negative pathogen that causes food-borne illness. In immunocompromised patients and the elderly, A. sobria opportunistically leads to severe extraintestinal diseases including sepsis, peritonitis, and meningitis. If A. sobria that infects the intestinal tract causes such an extraintestinal infection, the pathogen must pass through the intestinal epithelial barrier. In our earlier study using intestinal cultured cells (T84 cells), we observed that an A. sobria strain with higher A. sobria serine protease (ASP) production caused a marked level of bacterial translocation across the T84 intestinal epithelial monolayer. Herein, we investigated the effect of ASP on tight junctions (TJs) in T84 cells. We observed that ASP acts on TJs and causes the destruction of ZO-1, ZO-2, ZO-3, and claudin-7 (i.e., some of the protein components constituting TJs), especially in the strains with high ASP productivity. Based on the present results together with those of our earlier study, we propose that ASP may cause a disruption of the barrier function of the intestinal epithelium as a whole due to the destruction of TJs (in addition to the destruction of adherens junctions) and that ASP may assist invasion of the pathogens from the intestinal epithelium into deep sites in the human body.

High Temperature Requirement A (HtrA) protease of Listeria monocytogenes and its interaction with extracellular matrix molecules

High Temperature Requirement A (HtrA) was identified as a secreted virulence factor in many pathogenic bacteria, including Listeria monocytogenes. Recently, it was discovered that Helicobacter pylori and Campylobacter jejuni HtrAs can directly cleave the human cell-adhesion molecule E-cadherin, which facilitates bacterial transmigration. HtrAs also interact with extracellular matrix (ECM) molecules. However, only a limited number of studies have been carried out in this regard. In the present study, the protease and ECM binding properties of L. monocytogenes HtrA (LmHtrA) were studied using native rLmHtrA, catalytically inactive rLmHtrA(S343A) and rLmHtrA lacking the PDZ domain (∆PDZ) to gain more insights into HtrA-ECM molecule interaction. The results show that (1) native rLmHtrA cleaves fibrinogen, fibronectin, plasminogen and casein in a time and temperature dependent manner, (2) interaction of rLmHtrA with various host proteins was found in the micromolar to nanomolar range, (3) in the absence of PDZ domain, rLmHtrA exhibits no drastic change in binding affinity toward the host molecules when compared with native rLmHtrA and (4) the PDZ domain plays an important role in the substrate cleavage as rLmHtrA1-394∆PDZ cleaves the substrates only under certain conditions. The proteolysis of various ECM molecules by rLmHtrA possibly highlights the role of HtrA in L. monocytogenes pathogenesis involving ECM degradation.

Listeria monocytogenes Cold Shock Proteins: Small Proteins with A Huge Impact

Listeria monocytogenes has evolved an extensive array of mechanisms for coping with stress and adapting to changing environmental conditions, ensuring its virulence phenotype expression. For this reason, L. monocytogenes has been identified as a significant food safety and public health concern. Among these adaptation systems are cold shock proteins (Csps), which facilitate rapid response to stress exposure. L. monocytogenes has three highly conserved csp genes, namely, cspA, cspB, and cspD. Using a series of csp deletion mutants, it has been shown that L. monocytogenes Csps are important for biofilm formation, motility, cold, osmotic, desiccation, and oxidative stress tolerance. Moreover, they are involved in overall virulence by impacting the expression of virulence-associated phenotypes, such as hemolysis and cell invasion. It is postulated that during stress exposure, Csps function to counteract harmful effects of stress, thereby preserving cell functions, such as DNA replication, transcription and translation, ensuring survival and growth of the cell. Interestingly, it seems that Csps might suppress tolerance to some stresses as their removal resulted in increased tolerance to stresses, such as desiccation for some strains. Differences in csp roles among strains from different genetic backgrounds are apparent for desiccation tolerance and biofilm production. Additionally, hierarchical trends for the different Csps and functional redundancies were observed on their influences on stress tolerance and virulence. Overall current data suggest that Csps have a wider role in bacteria physiology than previously assumed.