Heat shock proteins as virulence factors of pathogens

Temperature-regulated expression of outer membrane proteins in Shigella flexneri

BACKGROUND

Bacteria exist widely in a diversity of natural environments. In order to survive adverse conditions such as nutrient depletion, biochemical and biological disturbances, and high temperature, bacteria have developed a wide variety of coping mechanisms. Temperature is one of the most important factors that can enhance the expression of microbial proteins. This study was conducted to investigate how outer membrane proteins (OMPs) of the bacterium Shigella flexneri respond to stress, especially during fever when the host's body temperature is elevated.

METHODS

OMPs of S. flexneri ATCC 12022 and clinical isolate SH057 were extracted from an overnight culture grown at 37, 38.5, and 40°C. Comparisons of the expressed proteins under the different growth conditions were based on equal numbers of bacterial cells loaded in the SDS-PAGE gels. Separated proteins were stained with Coomassie brilliant blue. Selected proteins showing increased expression at 38.5 and 40°C were characterized by performing MALDI-ToF-ToF.

RESULTS

Different degrees of expression were demonstrated for different proteins expressed at 37°C compared to 38.5 and 40°C. The proteins with molecular sizes of 18.4, 25.6, and 57.0 kDa showed increased expression level at increasing temperature and were identified as Dps, WrbA, and PepA, respectively.

CONCLUSION

This study revealed that strains of S. flexneri respond at the proteomic level during stress caused by elevated temperature by decreasing the expression of proteins, maintaining the level of important proteins, or enhancing the levels of proteins presumably involved in survival and virulence.

Mast cell chymase degrades the alarmins heat shock protein 70, biglycan, HMGB1, and interleukin-33 (IL-33) and limits danger-induced inflammation

During infection and tissue damage, virulence factors and alarmins are pro-inflammatory and induce activation of various immune cells including macrophages and mast cells (MCs). Activated MCs instantly release preformed inflammatory mediators, including several proteases. The chymase mouse mast cell protease (MCPT)-4 is thought to be pro-inflammatory, whereas human chymase also degrades pro-inflammatory cytokines, suggesting that chymase instead limits inflammation. Here we explored the contribution of MCPT4 and human chymase to the control of danger-induced inflammation. We found that protein extracts from wild type (WT), carboxypeptidase A3-, and MCPT6-deficient mice and MCs and recombinant human chymase efficiently degrade the Trichinella spiralis virulence factor heat shock protein 70 (Hsp70) as well as endogenous Hsp70. MC-(W(sash))-, serglycin-, NDST2-, and MCPT4-deficient extracts lacked this capacity, indicating that chymase is responsible for the degradation. Chymase, but not MC tryptase, also degraded other alarmins, i.e. biglycan, HMGB1, and IL-33, a degradation that was efficiently blocked by the chymase inhibitor chymostatin. IL-7, IL-22, GM-CSF, and CCL2 were resistant to chymase degradation. MCPT4-deficient conditions ex vivo and in vivo showed no reduction in added Hsp70 and only minor reduction of IL-33. Peritoneal challenge with Hsp70 resulted in increased neutrophil recruitment and TNF-α levels in the MCPT4-deficient mice, whereas IL-6 and CCL2 levels were similar to the levels found in WT mice. The rapid and MC chymase-specific degradation of virulence factors and alarmins may depend on the presence of accessible extended recognition cleavage sites in target substrates and suggests a protective and regulatory role of MC chymase during danger-induced inflammation.

Hsp90 inhibitors: small molecules that transform the Hsp90 protein folding machinery into a catalyst for protein degradation

The 90 kDa heat shock proteins (Hsp90) are responsible for the conformational maturation of nascent polypeptides and the renaturation of denatured proteins. In transformed cells, numerous mutated and overexpressed proteins rely on the Hsp90 protein folding machinery for tumor progression. The Hsp90-mediated protein folding process is dependent upon ATP, and when inhibitors of ATP are present, the Hsp90 machinery is unable to fold client proteins into their biologically active form, which results in the degradation of protein substrates via the ubiquitin-proteasome pathway. Consequently, Hsp90 has evolved into a promising anti-cancer target because multiple oncogenic proteins can be simultaneously degraded as a consequence of Hsp90 inhibition. This review serves to explain the Hsp90 protein folding process, the impact of Hsp90 inhibition, the identification of natural product inhibitors, and the development of rationally designed inhibitors of the Hsp90 protein folding machinery.

Monitoring of dnaK gene expression in Porphyromonas gingivalis by oxygen stress using DNA microarray

Porphyromonas gingivalis, a Gram-negative anaerobe associated with adult periodontitis, expresses numerous potential virulence factors. dnaK, a member of the heat shock protein family, functions as a molecular chaperone and plays a role in microbial pathogenicity. However, little is known regarding its gene expression caused by oxygen stress in P. gingivalis. In the present study, a custom-made DNA microarray was designed and used to monitor dnaK gene expression in P. gingivalis caused by oxygen stress. The results demonstrated that dnaK mRNA was up-regulated in a short time, and the DNA microarray results were confirmed by real-time polymerase chain reaction analysis. These findings suggest that oxygen stress stimulates gene expression of dnaK and may have a relationship to the aerotolerance activity of this organism as well as its expression of pathogenesis.

Mapping and identification of Mycobacterium tuberculosis proteins by two-dimensional gel electrophoresis, microsequencing and immunodetection

Mycobacterium tuberculosis is the infectious agent giving rise to human tuberculosis. The entire genome of M. tuberculosis, comprising approximately 4000 open reading frames, has been sequenced. The huge amount of information released from this project has facilitated proteome analysis of M. tuberculosis. Two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) was applied to fractions derived from M. tuberculosis culture filtrate, cell wall, and cytosol, resulting in the resolution of 376, 413, and 395 spots, respectively, in silver-stained gels. By microsequencing and immunodetection, 38 culture filtrate proteins were identified and mapped, of which 12 were identified for the first time. In the same manner, 23 cell wall proteins and 19 cytosol proteins were identified and mapped, with 9 and 10, respectively, being novel proteins. One of the novel proteins was not predicted in the genome project, and for four of the identified proteins alternative start codons were suggested. Fourteen of the culture filtrate proteins were proposed to possess signal sequences. Seven of these proteins were microsequenced and the N-terminal sequences obtained confirmed the prediction. The data presented here are an important complement to the genetic information, and the established 2-D PAGE maps (also available at: www.ssi.dk/publichealth/tbimmun) provide a basis for comparative studies of protein expression.

Identification of a 71-kilodalton surface-associated Hsp70 homologue in Coxiella burnetii

A Coxiella burnetii Hsp70 homologue was identified by using an acid activation in vitro system in which protein synthesis has been followed by [35S]methionine labeling, autoradiography, and immunoblotting. The protein was one of those predominantly labeled, and the immunoblots revealed that it was recognized by anti-DnaK antibodies. The corresponding gene was isolated, and its nucleotide sequence was determined and analyzed. A single open reading frame (ORF) with a size of 1,968 bp was identified. The ORF encodes a protein containing 656 residues and having a molecular weight of 70, 800. The -10 promoter sequence was shown to be identical with the consensus heat shock sigma32 promoter sequence. The base composition at the presumed -35 region revealed an EcoRI site in the expected region, which is assumed to be located at the border of the cloned fragment. The gene was expressed in Escherichia coli as an intact protein. The C. burnetii 71-kDa protein sequence has a high degree of homology to sequences of the Hsp70 family. A comparison of sequences revealed that the similarity with Hsp70s from other intracellular bacteria, e.g., Legionella pneumophila and Francisella tularensis, as well as E. coli DnaK, is more than 80%. The homologous regions are found in the N-terminal and central parts of the protein sequence, and they include the signature patterns of the Hsp70 family of proteins. The presence of the 71-kDa protein in association with the cell wall as well as in the cytoplasm was demonstrated by the use of immunoelectron microscopy. The dual localization was verified by Western blot analysis of proteins in C. burnetii cell fractions, using purified antibodies directed to the 71-kDa protein.

Identification and characterization of the Yersinia enterocolitica gsrA gene, which protectively responds to intracellular stress induced by macrophage phagocytosis and to extracellular environmental stress

Yersinia enterocolitica is able to resist the microbicidal mechanisms of macrophages and to grow within phagocytic cells. Some bacteria including Y. enterocolitica have been shown to respond to the hostile environment in macrophages by producing a set of stress proteins which are also induced by environmental stresses. To understand the role of stress proteins in intracellular survival of bacteria, we identified and cloned a Y. enterocolitica gene, called gsrA (global stress requirement). The gsrA gene was identified because its insertional inactivation by a transposon resulted in the inability of the organism to grow at an elevated temperature and to survive within macrophages after phagocytosis. The gsrA gene was sequenced and shown to encode a basic, 49,500-Da protein. The GsrA protein shows significant amino acid sequence homology to the HtrA stress protein which was originally identified in Escherichia coli. Furthermore, the genetically defined Y. enterocolitica gsrA mutant was constructed and characterized. The insertional mutation of gsrA resulted in inhibition of growth at temperatures above 39 degrees C and greatly increased susceptibility to oxidative and osmotic stresses. The mutant additionally lost the ability to survive and replicate within macrophages. These results, taken together, indicate that the gsrA gene is an essential component of the protection mechanism employed by Y. enterocolitica, allowing it to respond to the intracellular stress in macrophages as well as extracellular environmental stress.

Heat stress alters the virulence of a rifampin-resistant mutant of Francisella tularensis LVS

We have studied the stress response of a rifampin-resistant mutant of Francisella tularensis LVS. This mutant, Rif 7, was avirulent with an intraperitoneally administered 50% lethal dose greater than 10(7) CFU in a murine model of infection. Exposure of Rif 7 to heat stress for 5 h in vitro resulted in a 2-log decrease in its 50% lethal dose (P < 0.02). The increase in virulence was dependent on the time of exposure to high temperature and was maximal at 5 h. Envelope preparations from heat-stressed cells showed increased levels of several proteins. Notable among these were polypeptides with approximate molecular masses of 16, 60, and 75 kDa. Increases in both virulence and envelope protein levels were reversed when heat-treated cells were subsequently grown at 37 degrees C. Inhibition of protein synthesis by actinomycin D during heat stress blocked the increase in virulence of Rif 7. Cell-free media from the heat-stressed Rif 7 reacted with the whole spectrum of bacterial proteins were not toxic to mice. Hyperimmune serum against Rif 7 reacted with the whole spectrum of bacterial proteins in Western blots (immunoblots), although its reaction with 34- and 45-kDa proteins and two 60- and 75-kDa proteins upregulated during heat stress was weak. Other stress conditions, low iron and low pH, caused similar increases in the virulence of Rif 7. However, examination of the protein profile did not reveal any major common polypeptides induced by different stresses. Heat-treated Rif 7 bacteria were fully able to replicate in macrophages in vitro and in the host tissues, even though heat treatment only partially restored virulence.

The variable C-terminal region of the Mycobacterium leprae 70-kilodalton heat shock protein is the target for humoral immune responses

The 70-kDa heat shock protein of Mycobacterium leprae has a high degree of homology with the human hsp70 protein, yet it still elicits T-lymphocyte responses in subjects infected with M. leprae or vaccinated with the related Mycobacterium bovis BCG. We examined the serological responses to this protein by using recombinant protein fragments expressed from mutants with deletions of the M. leprae p70 gene. Monoclonal antibodies raised against either M. bovis or M. leprae p70 reacted with the C-terminal fragments but not the N-terminal fragments in a solid-phase enzyme-linked immunosorbent assay and an immunoblot assay. Inhibition enzyme-linked immunosorbent assays confirmed that two separate epitopes were defined by these monoclonal antibodies. Murine polyclonal sera also showed stronger binding to the C-terminal fragments. Sera from 33 and 48% of lepromatous leprosy patients reacted with M. leprae and M. bovis p70. This reactivity was mycobacterium specific, since few sera from control subjects in the same leprosy-endemic region were seropositive. The levels of anti-mycobacterial hsp70 antibodies were higher in patients with lepromatous leprosy than in those with tuberculoid leprosy or tuberculosis. The reactivity of sera from patients with leprosy was maximal with the C-terminal fragments. Therefore the C-terminal portion of M. leprae hsp70, which includes the region of maximum divergence from human hsp70, is the major target for the humoral immune response to the protein.

Characterization of the heat shock response in Mycobacterium bovis BCG

We have for the first time characterized the heat shock response in mycobacteria both at the level of transcription, by RNA extraction, Northern (RNA) blotting, and hybridization with gene-specific probes for the Mycobacterium tuberculosis 65- and 71-kDa heat shock proteins (HSPs), and at the level of translation, by [35S]methionine labelling, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and autoradiography. We observed increased synthesis of 40-, 65-, 71-, and 90-kDa proteins, which appear to be major HSPs in mycobacteria. The 40-, 71-, and 90-kDa HSPs are coordinately regulated in terms of temperature requirements and kinetics of induction but differ in the levels of expression. The 65- and 71-kDa HSPs are differentially regulated in response to temperature, with different kinetics and levels of induction. mRNA transcript sizes for the 71-, 65-, 40-, and 30-kDa proteins were found to be broadly consistent with DNA sequence open reading frames. A maximum increase of about 69-fold in the levels of mRNA for the 71-kDa HSP after 45 min of heat shock at 45 degrees C was observed, whereas the 65-kDa HSP mRNA increased only 5-fold. It was also found that in M. bovis BCG, as in Escherichia coli, a major control mechanism of the heat shock response is operative at the level of transcription. An ability to characterize the heat shock response in mycobacteria provides an experimental model with which to study environmentally regulated gene expression and an opportunity to identify virulence genes, which may coregulate as part of the heat shock regulon.

Hydrogen peroxide-mediated toxicity for Leishmania donovani chagasi promastigotes. Role of hydroxyl radical and protection by heat shock

Leishmania must survive despite exposure to the toxic oxidant hydrogen peroxide (H2O2) during phagocytosis by macrophages. We investigated the mechanism of H2O2 toxicity for L. donovani chagasi promastigotes, and factors responsible for their relative H2O2 resistance. There was a dose-dependent toxic effect of H2O2 for promastigotes isolated during logarithmic phase of growth. In contrast, stationary phase promastigotes were less susceptible to H2O2 toxicity, and more infectious for BALB/c mice. By spin trapping we found that hydroxyl radical (.OH) was generated after exposure of promastigotes to H2O2, and the amount of .OH was greater with log-phase than with stationary-phase promastigotes. .OH was generated after the addition of H2O2 to the cytosol but not the membranes of fractionated promastigotes, and the magnitude of .OH was greater in log than in stationary promastigote cytosol. Deferoxamine inhibition suggested that intracellular promastigote iron catalyzes .OH formation via the Fenton reaction. Furthermore, exposure of log-phase promastigotes to heat shock induced a relative H2O2-resistant state, which was not associated with a decrease in .OH formation but which required ongoing transcription. Thus, growth to stationary phase and heat shock both induce a state of relative H2O2 resistance, but these are probably due to different resistance mechanisms.