Assembly of alpha-hemolysin on A431 cells leads to clustering of Caveolin-1

Staphylococcus aureus pore-forming toxins: The interface of pathogen and host complexity

Staphylococcus aureus is a prominent human pathogen capable of infecting a variety of host species and tissue sites. This versatility stems from the pathogen's ability to secrete diverse host-damaging virulence factors. Among these factors, the S. aureus pore-forming toxins (PFTs) α-toxin and the bicomponent leukocidins, have garnered much attention for their ability to lyse cells at low concentrations and modulate disease severity. Although many of these toxins were discovered nearly a century ago, their host cell specificities have only been elucidated over the past five to six years, starting with the discovery of the eukaryotic receptor for α-toxin and rapidly followed by identification of the leukocidin receptors. The identification of these receptors has revealed the species- and cell type-specificity of toxin binding, and provided insight into non-lytic effects of PFT intoxication that contribute to disease pathogenesis.

Oligomerization of Clostridium perfringens epsilon toxin is dependent upon caveolins 1 and 2

Evidence from multiple studies suggests that Clostridium perfringens ε-toxin is a pore-forming toxin, assembling into oligomeric complexes in the plasma membrane of sensitive cells. In a previous study, we used gene-trap mutagenesis to identify mammalian factors contributing to toxin activity, including caveolin-2 (CAV2). In this study, we demonstrate the importance of caveolin-2 and its interaction partner, caveolin-1 (CAV1), in ε-toxin-induced cytotoxicity. Using CAV2-specific shRNA in a toxin-sensitive human kidney cell line, ACHN, we confirmed that cells deficient in CAV2 exhibit increased resistance to ε-toxin. Similarly, using CAV1-specific shRNA, we demonstrate that cells deficient in CAV1 also exhibit increased resistance to the toxin. Immunoprecipitation of CAV1 and CAV2 from ε-toxin-treated ACHN cells demonstrated interaction of both CAV1 and -2 with the toxin. Furthermore, blue-native PAGE indicated that the toxin and caveolins were components of a 670 kDa protein complex. Although ε-toxin binding was only slightly perturbed in caveolin-deficient cells, oligomerization of the toxin was dramatically reduced in both CAV1- and CAV2-deficient cells. These results indicate that CAV1 and -2 potentiate ε-toxin induced cytotoxicity by promoting toxin oligomerization – an event which is requisite for pore formation and, by extension, cell death.

Gene-trap mutagenesis identifies mammalian genes contributing to intoxication by Clostridium perfringens ε-toxin

The Clostridium perfringens ε-toxin is an extremely potent toxin associated with lethal toxemias in domesticated ruminants and may be toxic to humans. Intoxication results in fluid accumulation in various tissues, most notably in the brain and kidneys. Previous studies suggest that the toxin is a pore-forming toxin, leading to dysregulated ion homeostasis and ultimately cell death. However, mammalian host factors that likely contribute to ε-toxin-induced cytotoxicity are poorly understood. A library of insertional mutant Madin Darby canine kidney (MDCK) cells, which are highly susceptible to the lethal affects of ε-toxin, was used to select clones of cells resistant to ε-toxin-induced cytotoxicity. The genes mutated in 9 surviving resistant cell clones were identified. We focused additional experiments on one of the identified genes as a means of validating the experimental approach. Gene expression microarray analysis revealed that one of the identified genes, hepatitis A virus cellular receptor 1 (HAVCR1, KIM-1, TIM1), is more abundantly expressed in human kidney cell lines than it is expressed in human cells known to be resistant to ε-toxin. One human kidney cell line, ACHN, was found to be sensitive to the toxin and expresses a larger isoform of the HAVCR1 protein than the HAVCR1 protein expressed by other, toxin-resistant human kidney cell lines. RNA interference studies in MDCK and in ACHN cells confirmed that HAVCR1 contributes to ε-toxin-induced cytotoxicity. Additionally, ε-toxin was shown to bind to HAVCR1 in vitro. The results of this study indicate that HAVCR1 and the other genes identified through the use of gene-trap mutagenesis and RNA interference strategies represent important targets for investigation of the process by which ε-toxin induces cell death and new targets for potential therapeutic intervention.

Biological relevance of natural alpha-toxin fragments from Staphylococcus aureus

Serine proteases represent an essential part of cellular homeostasis by generating biologically active peptides. In bacteria, proteolysis serves two different roles: a major housekeeping function and the destruction of foreign or target cell proteins, thereby promoting bacterial invasion. In the process, other virulence factors such as exotoxins become affected. In Staphylococcus aureus culture supernatant, the pore-forming alpha-toxin is cleaved by the coexpressed V8 protease and aureolysin. The oligomerizing and pore-forming abilities of five such spontaneously occurring N- and C-terminal alpha-toxin fragments were studied. (3)H-marked alpha-toxin fragments bound to rabbit erythrocyte membranes but only fragments with intact C termini, missing 8, 12 and 71 amino acids from their N-terminal, formed stable oligomers. All isolated fragments induced intoxication of mouse adrenocortical Y1 cells in vitro, though the nature of membrane damage for a fragment, degraded at its C terminus, remained obscure. Only one fragment, missing the first eight N-terminal amino acids, induced irreversible intoxication of Y1 cells in the same manner as the intact toxin. Four of the isolated fragments caused swelling, indicating altered channel formation. Fragments missing 12 and 71 amino acids from the N terminus occupied the same binding sites on Y1 cell membranes, though they inhibited membrane damage caused by intact toxin. In conclusion, N-terminal deletions up to 71 amino acids are tolerated, though the kinetics of channel formation and the channel's properties are altered. In contrast, digestion at the C terminus results in nonfunctional species.

Membrane bound monomer of Staphylococcal alpha-hemolysin induces caspase activation and apoptotic cell death despite initiation of membrane repair pathway

Background

Wild type Staphylococcal α-hemolysin (α-HL) assembly on target mammalian cells usually results in necrotic form of cell death; however, caspase activation also occurs. The pathways of caspase activation due to binding/partial assembly by α-HL are unknown till date.

Results

Cells treated with H35N (a mutant of α-HL that remains as membrane bound monomer), have been shown to accumulate hypodiploid nuclei, activate caspases and induce intrinsic mitochondrial apoptotic pathway. We have earlier shown that the binding and assembly of α-HL requires functional form of Caveolin-1 which is an integral part of caveolae. In this report, we show that the caveolae of mammalian cells, which undergo a continuous cycle of ‘kiss and run’ dynamics with the plasma membrane, have become immobile upon the binding of the monomer. The cells treated with H35N were unable to recover despite activation of membrane repair mechanism involving caspase-1 dependent activation of sterol regulatory element binding protein-1.

Conclusions

This is for the first time we show the range of cellular changes and responses that take place immediately after the binding of the monomeric form of staphylococcal α-hemolysin.

Aromatic residues of Caveolin-1 binding motif of alpha-hemolysin are essential for membrane penetration

We have created single cysteine Caveolin-1 binding motif mutants (SCCBMMs) of staphylococcal alpha-HL for understanding assembly and penetration. All SCCBMMs have normal folding like alpha-HL as examined by limited proteolysis, intrinsic fluorescence emission, no hemolytic activity and do not form hetero oligomers with alpha-HL indicating that the conformational changes occurred at the cell membrane are different to that of alpha-HL. While modification of SCCBMMs with a membrane impermeant reagent has resulted in reduced binding, badan modification has resulted in the enhancement of badan fluorescence with time of assembly (incubation time) indicating the change in environment of the badan and the need for the penetration of the aromatic amino acids. Our studies indicate that the conformational changes are probably initiated at the Caveolin-1 binding motif and provide a basis for differential mode of interaction of the Caveolin-1 binding motif depending upon the nature of the target cell membrane.

The leukocidin pore: evidence for an octamer with four LukF subunits and four LukS subunits alternating around a central axis

The staphylococcal alpha-hemolysin (alphaHL) and leukocidin (Luk) polypeptides are members of a family of related beta-barrel pore-forming toxins. Upon binding to susceptible cells, alphaHL forms water-filled homoheptameric transmembrane pores. By contrast, Luk pores are formed by two classes of subunit, F and S, rendering a heptameric structure displeasing on symmetry grounds at least. Both the subunit stoichiometry and arrangement within the Luk pore have been contentious issues. Here we use chemical and genetic approaches to show that (1) the predominant, or perhaps the only, form of the Luk pore is an octamer; (2) the subunit stoichiometry is 1:1; and (3) the subunits are arranged in an alternating fashion about a central axis of symmetry, at least when a fused LukS-LukF construct is used. The experimental approaches we have used also open up new avenues for engineering the arrangement of the subunits of beta-barrel pore-forming toxins.

Role of the amino latch of staphylococcal alpha-hemolysin in pore formation: a co-operative interaction between the N terminus and position 217

Staphylococcal alpha-hemolysin (alphaHL) is a beta barrel pore-forming toxin that is secreted by the bacterium as a water-soluble monomeric protein. Upon binding to susceptible cells, alphaHL assembles via an inactive prepore to form a water-filled homoheptameric transmembrane pore. The N terminus of alphaHL, which in the crystal structure of the fully assembled pore forms a latch between adjacent subunits, has been thought to play a vital role in the prepore to pore conversion. For example, the deletion of two N-terminal residues produced a completely inactive protein that was arrested in assembly at the prepore stage. In the present study, we have re-examined assembly with a comprehensive set of truncation mutants. Surprisingly, we found that after truncation of up to 17 amino acids, the ability of alphaHL to form functional pores was diminished, but still substantial. We then discovered that the mutation Ser(217) --> Asn, which was present in our original set of truncations but not in the new ones, promotes complete inactivation upon truncation of the N terminus. Therefore, the N terminus of alphaHL cannot be critical for the prepore to pore transformation as previously thought. Residue 217 is involved in the assembly process and must interact indirectly with the distant N terminus during the last step in pore formation. In addition, we provide evidence that an intact N terminus prevents the premature oligomerization of alphaHL monomers in solution.

alpha-Hemolysin-induced dephosphorylation of EGF receptor of A431 cells is carried out by rPTPsigma

Earlier we have shown that the epidermal growth factor receptor was unable to retain its phospho Tyr signal after the assembly of staphylococcal alpha-hemolysin (alpha-HL). However, the nature of the protein tyrosine phosphatase (PTPase) or its identity is not known. In this report, we demonstrate that the alpha-HL elevates the activity of receptor like protein tyrosine phosphatase sigma (rPTPsigma). The alpha-HL induced dephosphorylation is prominent only in intact A431 cells. The PTPase activity is not inhibited if the alpha-HL treatment precedes PTPase inhibitor treatments. The anti-EGFr immunoprecipitates have exhibited higher PTPase activity after alpha-HL treatment of A431 cells. Interestingly, PTPase activity of anti-EGFr immunoprecipitates from the A431 cells expressing the antisense message of rPTPsigma has not increased despite alpha-HL treatment, confirming the role of rPTPsigma in the dephosphorylation of EGFr. The studies presented here will be useful in understanding the process of signal modulation by the assembly of alpha-HL.

Functional form of Caveolin-1 is necessary for the assembly of alpha-hemolysin

The assembly of alpha-HL was shown to rapidly progress upon its interaction with Caveolin-1. Treatment of A431 cells with alpha-HL has resulted in clustering of Caveolin-1 at cell-cell contacts. Consistent with this observation, alpha-HL mutants devoid of assembly property have not induced the clustering of Caveolin-1. While cholesterol depletion of A431 cells completely arrests the assembly of alpha-HL, chelation of membrane cholesterol results in its retarded assembly. Interestingly, HT29 cells, with low Caveolin-1 levels, are resistant to alpha-HL attack. Clustering of Caveolin-1, as seen in case of A431 cells, was readily observed in case of HT29 cells transfected with Caveolin-1 construct, thus overexpressing the full length Caveolin-1, upon alpha-HL treatment. A model was constructed to visualize the interactions between alpha-HL and Caveolin-1 which suggests that facile penetration of alpha-HL's beta-barrel might occur through protein-protein interactions with the surrounding 7 alpha-helices of Caveolin-1.