A mutant cell line resistant to Vibrio parahaemolyticus thermostable direct hemolysin (TDH): its potential in identification of putative receptor for TDH

Role of TlyA in the Biology of Uncultivable Mycobacteria

TlyA proteins are related to distinct functions in a diverse spectrum of bacterial pathogens including mycobacterial spp. There are several annotated proteins function as hemolysin or pore forming molecules that play an important role in the virulence of pathogenic organisms. Many studies reported the dual activity of mycobacterial TlyA as 'hemolysin' and 'S-adenosylmethionine dependent rRNA methylase'. To act as a hemolysin, a sequence must have a signal sequence and transmembrane segment which helps the protein to enter the extracellular environment. Interestingly, the mycobacterial tlyA has neither a traditional signal sequences of general/sec/tat pathways nor any transmembrane segments are present. Still it can reach the extracellular milieu with the help of non-classical signal mechanisms. Also, retention of tlyA in cultivable mycobacterial pathogens (such as Mycobacterium tuberculosis and M. marinum) as well as uncultivated mycobacterial pathogens despite their extreme reductive evolution (such as M. leprae, M. lepromatosis and M. uberis) suggests its crucial role in evolutionary biology of pathogenic mycobacteria. Numerous virulence factors have been characterised from the uncultivable mycobacteria but the information of TlyA protein is still limited in terms of molecular and structural characterisation. The genomic insights offered by comparative analysis of TlyA sequences and its conserved domains reveal its pore forming activity which further confirms its role as a virulence protein, particularly in uncultivable mycobacteria. Therefore, this review presents a comparative analysis of mycobacterial TlyA family by sequence homology and alignment to improve our understanding of this unconventional hemolysin and RNA methyltransferase TlyA of uncultivable mycobacteria.

Site-directed mutations of thermostable direct hemolysin from Grimontia hollisae alter its arrhenius effect and biophysical properties

Recombinant thermostable direct hemolysin from Grimontia hollisae (Gh-rTDH) exhibits paradoxical Arrhenius effect, where the hemolytic activity is inactivated by heating at 60 °C but is reactivated by additional heating above 80 °C. This study investigated individual or collective mutational effect of Tyr53, Thr59, and Ser63 positions of Gh-rTDH on hemolytic activity, Arrhenius effect, and biophysical properties. In contrast to the Gh-rTDH wild-type (Gh-rTDH(WT)) protein, a 2-fold decrease of hemolytic activity and alteration of Arrhenius effect could be detected from the Gh-rTDH(Y53H/T59I) and Gh-rTDH(T59I/S63T) double-mutants and the Gh-rTDH(Y53H/T59I/S63T) triple-mutant. Differential scanning calorimetry results showed that the Arrhenius effect-loss and -retaining mutants consistently exhibited higher and lower endothermic transition temperatures, respectively, than that of the Gh-rTDH(WT). Circular dichroism measurements of Gh-rTDH(WT) and Gh-rTDH(mut) showed a conspicuous change from a β-sheet to α-helix structure around the endothermic transition temperature. Consistent with the observation is the conformational change of the proteins from native globular form into fibrillar form, as determined by Congo red experiments and transmission electron microscopy.

Purification, crystallization and preliminary X-ray analysis of a thermostable direct haemolysin from Grimontia hollisae

Vibrio hollisae, a halophilic species recently reclassified as Grimontia hollisae, is a causative agent of gastroenteritis and septicaemia. One important pathogenic Vibrio factor, thermostable direct haemolysin (TDH), has been purified and crystallized in two crystal forms using the vapour-diffusion method. The crystals belonged to an orthorhombic space group, with unit-cell parameters a = 104.8, b = 112.4, c = 61.3 Å and a = 122.9, b = 123.3, c = 89.8 Å. The crystals contained either four or eight molecules per asymmetric unit, with predicted solvent contents of 49.4 and 46.3% and Matthews coefficients (V(M)) of 2.4 and 2.3 Å(3) Da(-1), respectively. These crystals were suitable for structure determination, which would yield structural details related to the cytotoxicity and oligomeric structure of this pore-forming toxin.

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.

Acanthamoeba castellanii promotes the survival of Vibrio parahaemolyticus

Vibrio parahaemolyticus is a food-borne pathogen that naturally inhabits both marine and estuarine environments. Free-living protozoa exist in similar aquatic environments and function to control bacterial numbers by grazing on free-living bacteria. Protozoa also play an important role in the survival and spread of some pathogenic species of bacteria. We investigated the interaction between the protozoan Acanthamoeba castellanii and the bacterium Vibrio parahaemolyticus. We found that Acanthamoeba castellanii does not prey on Vibrio parahaemolyticus but instead secretes a factor that promotes the survival of Vibrio parahaemolyticus in coculture. These studies suggest that protozoa may provide a survival advantage to an extracellular pathogen in the environment.

Identification of two translocon proteins of Vibrio parahaemolyticus type III secretion system 2

The type III secretion system (T3SS) translocon complex is composed of several associated proteins, which form a translocation channel through the host cell plasma membrane. These proteins are key molecules that are involved in the pathogenicity of many T3SS-positive bacteria, because they are necessary to deliver effector proteins into host cells. A T3SS designated T3SS2 of Vibrio parahaemolyticus is thought to be related to the enterotoxicity of this bacterium in humans, but the effector translocation mechanism of T3SS2 is unclear because there is only one gene (the VPA1362 gene) in the T3SS2 region that is homologous to other translocon protein genes. It is also not known whether the VPA1362 protein is functional in the translocon of T3SS2 or whether it is sufficient to form the translocation channel of T3SS2. In this study, we identified both VPA1362 (designated VopB2) and VPA1361 (designated VopD2) as T3SS2-dependent secretion proteins. Functional analysis of these proteins showed that they are essential for T3SS2-dependent cytotoxicity, for the translocation of one of the T3SS2 effector proteins (VopT), and for the contact-dependent activity of pore formation in infected cells in vitro. Their targeting to the host cell membrane depends on T3SS2, and furthermore, they are necessary for T3SS2-dependent enterotoxicity in vivo. These results indicate that VopB2 and VopD2 act as translocon proteins of V. parahaemolyticus T3SS2 and hence have a critical role in the T3SS2-dependent enterotoxicity of this bacterium.

Tetrameric Structure of Thermostable Direct Hemolysin from Vibrio parahaemolyticus Revealed by Ultracentrifugation, Small-angle X-ray Scattering and Electron Microscopy

The thermostable direct hemolysin (TDH) is a major virulence factor of Vibrio parahaemolyticus. We have characterized the conformational properties of TDH by small-angle X-ray scattering (SAXS), ultracentrifugation and transmission electron microscopy. Sedimentation equilibrium and velocity studies revealed that the protein is tetrameric in aqueous solvents. The Guinier plot derived from SAXS data provided a radius of gyration of 29.0 A. The elongated pattern with a shoulder of a pair distance distribution function derived from SAXS data suggested the presence of molecules with an anisotropic shape having a maximum diameter of 98 A. Electron microscopic image analysis of the negatively stained TDH oligomer showed the presence of C(4) symmetric particles with edge and diagonal lengths of 65 A and 80 A, respectively. Shape reconstruction was carried out by ab initio calculations using the SAXS data with a C(4) symmetric approximation. These results suggested that the tetrameric TDH assumes an oblate structure. The hydrodynamic parameters predicted from the ab initio model differed slightly from the experimental values, suggesting the presence of flexible segments.

Microarray analysis identifies apoptosis regulatory gene expression in HCT116 cells infected with thermostable direct hemolysin-deletion mutant of Vibrio parahaemolyticus

The thermostable direct hemolysin (TDH) is considered as a major virulence factor of Vibrio parahaemolyticus. We observed this potential in several human cancer cell lines by using the TDH-producing wild-type (RIMD2210633) as well as tdh-deletion mutant of V. parahaemolyticus and found that the deletion of tdh did not affect cytotoxicity to any of the cell lines tested. DNA fragmentation and annexin V staining showed that both wild-type and tdh-mutant trigger apoptosis in these cells. To understand the molecular basis of cell death in the absence of TDH, gene expression profile of human colon cancer cell line HCT116 infected with tdh-deletion mutant was carried out using human cDNA microarrays consisting of 33,000 known genes. In infected cells, differentially expressed genes including genes for early growth response, growth arrest and DNA damage, and activating transcription factor that affect programmed cell death pathways were detected. Interestingly, mutant strains having a deletion in type III secretion system 1 (TTSS1) failed to elicit DNA fragmentation in HCT116 cells. Our results strongly suggest that apoptosis requires functional TTSS1 and TTSS1-dependent translocation factor(s) to be associated with the host cell death, and thus pathogenesis of V. parahaemolyticus.

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.

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

The mechanism of action of Vibrio parahaemolyticus thermostable direct hemolysin (TDH) on cultured cells still remains unclear. We show that addition of osmotic stabilizers, such as polyethylene glycol and dextran, could not protect cultured rat embryonic fibroblast cells (Rat-1) against cytotoxicity induced by TDH, unlike their protection against the hemolytic activity of TDH. By contrast, 100 microM monodansylcadaverine, as well as the presence of 1 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) in medium, protected the cells against cytotoxicity of TDH. Binding of TDH to Rat-1 cells and intracellular localization of TDH were affected by monodansylcadaverine and EGTA as analyzed by flow cytometry and confocal microscopy. On the hemolytic activity of TDH, monodansylcadaverine and EGTA had no effect. These results suggest that the mechanism of cytotoxicity of TDH on Rat-1 cells was different from that of hemolytic activity of TDH on red blood cells.

Vibrio parahaemolyticus thermostable direct hemolysin can induce an apoptotic cell death in Rat-1 cells from inside and outside of the cells

Rat-1 cells exposed to Vibrio parahaemolyticus thermostable direct hemolysin (TDH) developed morphological changes including shrinkage of the cells and reduction in the size of nuclei. Cells either microinjected with TDH or transfected with the tdh gene also showed morphological changes similar to those induced by externally added toxin. Furthermore, TDH-exposed or tdh-transfected cells both showed chromatin condensation and DNA fragmentation which suggest cells undergoing apoptosis. In contrast, expression of a TDH mutant (R7) did not reveal any cytotoxic effects. We demonstrate that expressed TDH was distributed in the cytoplasm. The interleukin-1beta-converting enzyme-related protease inhibitor ZVAD-FMK did not inhibit TDH cytotoxicity. Our results suggest that TDH can induce its cytotoxicity both from outside and from inside the cells and killed the cells through apoptosis.