Monodansylcadaverine inhibits cytotoxicity of Vibrio parahaemolyticus thermostable direct hemolysin on cultured rat embryonic fibroblast cells

Hepatopancreatic transcriptome analysis and humoral immune factor assays in red claw crayfish (Cherax quadricarinatus) provide insight into innate immunomodulation under Vibrio parahaemolyticus infection

Red claw crayfish (Cherax quadricarinatus) is an economically and nutritionally important specie. We aimed to assess the immunostimulatory response to C. quadricarinatus infection with Vibrio parahaemolyticus. After determining the LD50, we infected C. quadricarinatus and examined the differential expression profiles of hepatopancreas transcriptional genes, and observed the temporal changes of hepatopancreas pathological sections and serum immunoenzymatic activities at different time points to reveal the infection mechanism of V. parahaemolyticus and the immune detoxification mechanism of the organism. The results showed that V. parahaemolyticus infection with C. quadricarinatus caused hepatopancreas injury and the immune enzyme activity of the organism changed with time delay. Transcriptome analysis of 47,338 single genes obtained by RNA sequencing and de nove transcriptome assembly identified a total of 3678 differentially expressed genes (P < 0.05) in the expression profiles of susceptible and normal animals for comparative analysis, and 2516 differentially expressed genes (P < 0.05) in the expression profiles of asymptomatic (infection-resistant) and normal animals. GO and KEGG and analyses revealed immune-related pathways under V. parahaemolyticus infection, including Vibrio cholerae infection, phagosome, lysozyme, oxidative phosphorylation, antigen processing and presentation, apoptosis, and Toll-like receptor signaling, as well as significant differences in the expression patterns of related immune genes at different times (P < 0.05). These new experimental results reveal the molecular response of the hepatopancreas to V. parahaemolyticus infection and the corresponding adaptive mechanisms, thus further revealing the pathogenesis due to bacterial infection in the aquatic environment, and providing a reference for further understanding of microbial-host interactions in aquatic systems.

Molecular mechanisms of Vibrio parahaemolyticus pathogenesis

Vibrio parahaemolyticus is a Gram-negative halophilic bacterium that is mainly distributed in the seafood such as fish, shrimps and shellfish throughout the world. V. parahaemolyticus can cause diseases in marine aquaculture, leading to huge economic losses to the aquaculture industry. More importantly, it is also the leading cause of seafood-borne diarrheal disease in humans worldwide. With the development of animal model, next-generation sequencing as well as biochemical and cell biological technologies, deeper understanding of the virulence factors and pathogenic mechanisms of V. parahaemolyticus has been gained. As a globally transmitted pathogen, the pathogenicity of V. parahaemolyticus is closely related to a variety of virulence factors. This article comprehensively reviewed the molecular mechanisms of eight types of virulence factors: hemolysin, type III secretion system, type VI secretion system, adhesion factor, iron uptake system, lipopolysaccharide, protease and outer membrane proteins. This review comprehensively summarized our current understanding of the virulence factors in V. parahaemolyticus, which are potentially new targets for the development of therapeutic and preventive strategies.

Structure, function and regulation of the thermostable direct hemolysin (TDH) in pandemic Vibrio parahaemolyticus

Vibrio parahaemolyticus is a leading cause of seafood-associated bacterial gastroenteritis. The pathogen produces the thermostable direct hemolysin (TDH), which is the sole cause of the Kanagawa phenomenon (KP), a special β-type haemolysis in the Wagatsuma agar. TDH also exerts several other biological activities, the major includes lethal toxicity, cytotoxicity, and enterotoxicity. The structure and roles of TDH and the transcriptional regulation of tdh genes, are summarized in this review, which will give a better understanding of the pathogenesis of V. parahaemolyticus.

Association of Vibrio parahaemolyticus thermostable direct hemolysin with lipid rafts is essential for cytotoxicity but not hemolytic activity

Thermostable direct hemolysin (TDH), a major virulence factor of Vibrio parahaemolyticus, induces cytotoxicity in cultured cells. However, the mechanism of TDH's cytotoxic effect including its target molecules on the plasma membrane of eukaryotic cells remains unclear. In this study, we identified the role of lipid rafts, cholesterol- and sphingolipid-enriched microdomains, in TDH cytotoxicity. Treatment of cells with methyl-beta-cyclodextrin (MbetaCD), a raft-disrupting agent, inhibited TDH cytotoxicity. TDH was associated with detergent-resistant membranes (DRMs), and MbetaCD eliminated this association. In contrast, there was no such association between a nontoxic TDH mutant and DRMs. The disruption of lipid rafts neither affected hemolysis nor inhibited Ca(2+) influx into HeLa cells induced by TDH. These findings indicate that the cytotoxicity but not the hemolytic activity of TDH is dependent on lipid rafts. The exogenous and endogenous depletion of cellular sphingomyelin also prevented TDH cytotoxicity, but a direct interaction between TDH and sphingomyelin was not detected with either a lipid overlay assay or a liposome absorption test. Treatment with sphingomyelinase (SMase) at 100 mU/ml disrupted the association of TDH with DRMs but did not affect the localization of lipid raft marker proteins (caveolin-1 and flotillin-1) with DRMs. These results suggest that sphingomyelin is important for the association of TDH with lipid rafts but is not a molecular target of TDH. We hypothesize that TDH may target a certain group of rafts that are sensitive to SMase at a certain concentration, which does not affect other types of rafts.

Cloning and expression of gene encoding the thermostable direct hemolysin from Vibrio alginolyticus strain HY9901, the causative agent of vibriosis of crimson snapper (Lutjanus erythopterus)

AIMS

The main aims of this study were to clone and express complete open reading frame (ORF) of thermostable direct haemolysin gene (tdh) from Vibrio alginolyticus strain HY9901 in Escherichia coli, and further evaluate the virulence of expressed TDH on mouse and crimson snapper.

METHODS AND RESULTS

A 410 bp internal fragment of the tdh gene was amplified by touchdown PCR with designed primers. Then its unknown flanking sequences of the 5'- and 3'-ends were finally characterized by inverse PCR and nested PCR. Sequence analysis showed that the tdh gene contain 570 bp ORF which encoded 189 amino acids. The deduced amino acid sequence of the ORF was in significant homology with several Vibrio TDH. The product that the tdh gene expressed in E. coli was purified by Ni(2+)-IDA Sepharose affinity column. The activity of purified TDH was 4651 U mg(-1) protein by hide powder azure digestion. The lethal toxicity test showed that LD(50) values of the purified TDH were 5.68 and 8.34 microg TDH g(-1) body weight for mouse and crimson snapper, respectively.

CONCLUSIONS

The complete ORF of tdh gene was obtained by touchdown PCR, inverse PCR and nested PCR. The ORF was perfectly expressed in E. coli. The activity and toxicity assays showed that the N-terminal signal peptide was essential to autocatalyse and fold correctly to obtain the activity and toxicity in the purified TDH. The Native-PAGE analysis showed that the activated tdh gene expressed in E. coli was a dimer with two identical subunits.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study demonstrates that the expressed activated TDH can produce the toxicity protein determined on mouse and fish, which will lead to better understandings of the identifying virulence factor that could be considered as a candidate antigen for vaccine and a diagnostic tool for vibriosis. Its use as an immunizing antigen might prevent the ability of V. alginolyticus to infect the marine aquatic animals, as a complementary measure to tick control and appropriate management in countries affected by vibriosis.

Trafficking of Streptococcus uberis in bovine mammary epithelial cells

Results from our laboratory showed that Streptococcus uberis internalized bovine mammary epithelial cells by exploiting host cell cytoskeleton and signal transduction mechanisms. It was also shown that S. uberis survived intracellularly for up to 120 h and capable of transcytose bovine mammary epithelial cells. To define mechanisms and strategies used by S. uberis to move through host cells and survive intracellularly, internalization studies using specific inhibitors, double immunofluorescence labeling and confocal laser microscopy were conducted. When bovine mammary epithelial cells were treated with inhibitors of endocytic vesicle acidification, the number of intracellular S. uberis was similar to untreated controls. When selective inhibitors of lipid rafts/caveolae or receptor-mediated endocytosis were used, a significantly lower number of intracellular S. uberis was detected compared with untreated controls. However, when the effect of inhibitors of receptor-mediated endocytosis and lipid rafts/caveolae were compared, the latter induced the lowest S. uberis internalization values suggesting a preferential exploitation of caveolae-mediated endocytosis. Since caveloae-dependent intracellular trafficking does not include intravesicular acidification or lysosome fusion; these results suggest that by exploiting preferential intracellular trafficking pathways in bovine mammary epithelial cells, S. uberis avoids intracellular bactericidal mechanisms. Such a strategy would allow S. uberis to persist intracellularly and may explain how persistent intramammary infections occur.

Cytotoxicity and enterotoxicity of the thermostable direct hemolysin-deletion mutants of Vibrio parahaemolyticus

The thermostable direct hemolysin (TDH) has been proposed to be a major virulence factor of Vibrio parahaemolyticus. We have recently completed the genome sequence of a TDH-producing V. parahaemolyticus strain, RIMD2210633. In this study, we constructed tdh-deletion mutants from the sequenced strain by homologous recombination and analyzed their phenotypes. Although the deletion of both copies of tdh completely abolished the hemolytic activity of the wild-type strain, the deletion did not affect the cytotoxicity to HeLa cells. Enterotoxicity, assayed by the rabbit ileal loop test, was lowered by tdh deletion, but the mutant still showed partial fluid accumulation in rabbit intestine. These results indicate that the cytotoxicity and enterotoxicity of TDH-producing V. parahaemolyticus are not explained by TDH alone, and suggest that an unknown virulence factor(s) could be involved in these pathogenic activities.

Effect of Vibrio parahaemolyticus haemolysin on human erythrocytes

Haemolysin Kanagawa, a toxin from Vibrio parahaemolyticus, is known to trigger haemolysis. Flux studies indicated that haemolysin forms a cation channel. In the present study, channel properties were elucidated by patch clamp and functional significance of ion fluxes by fluorescence-activated cell sorting (FACS) analysis. Treatment of human erythrocytes with 1 U ml-1 haemolysin within minutes induces a non-selective cation permeability. Moreover, haemolysin activates clotrimazole-sensitive K+ channels, pointing to stimulation of Ca2+-sensitive Gardos channels. Haemolysin (1 U ml-1) leads within 5 min to slight cell shrinkage, which is reversed in Ca2+-free saline. Erythrocytes treated with haemolysin (0.1 U ml-1) do not undergo significant haemolysis within the first 60 min. Replacement of extracellular Na+ with NMDG+ leads to slight cell shrinkage, which is potentiated by 0.1 U ml-1 haemolysin. According to annexin binding, treatment of erythrocytes with 0.1 U ml-1 haemolysin leads within 30 min to breakdown of phosphatidylserine asymmetry of the cell membrane, a typical feature of erythrocyte apoptosis. The annexin binding is significantly blunted at increased extracellular K+ concentrations and by K+ channel blocker clotrimazole. In conclusion, haemolysin Kanagawa induces cation permeability and activates endogenous Gardos K+ channels. Consequences include breakdown of phosphatidylserine asymmetry, which depends at least partially on cellular loss of K+.