Approximate dimensions of membrane lesions produced by streptolysin S and streptolysin O

Group A Streptococcus induces lysosomal dysfunction in THP-1 macrophages

The human-specific bacterial pathogen group A Streptococcus (GAS) is a significant cause of morbidity and mortality. Macrophages are important to control GAS infection, but previous data indicate that GAS can persist in macrophages. In this study, we detail the molecular mechanisms by which GAS survives in THP-1 macrophages. Our fluorescence microscopy studies demonstrate that GAS is readily phagocytosed by macrophages, but persists within phagolysosomes. These phagolysosomes are not acidified, which is in agreement with our findings that GAS cannot survive in low pH environments. We find that the secreted pore-forming toxin Streptolysin O (SLO) perforates the phagolysosomal membrane, allowing leakage of not only protons but also large proteins including the lysosomal protease cathepsin B. Additionally, GAS recruits CD63/LAMP-3, which may contribute to lysosomal permeabilization, especially in the absence of SLO. Thus, although GAS does not inhibit fusion of the lysosome with the phagosome, it has multiple mechanisms to prevent proper phagolysosome function, allowing for persistence of the bacteria within the macrophage. This has important implications for not only the initial response but also the overall functionality of the macrophages, which may lead to the resulting pathologies in GAS infection. Our data suggest that therapies aimed at improving macrophage function may positively impact patient outcomes in GAS infection.

Mechanism of Action of Ribosomally Synthesized and Post-Translationally Modified Peptides

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a natural product class that has undergone significant expansion due to the rapid growth in genome sequencing data and recognition that they are made by biosynthetic pathways that share many characteristic features. Their mode of actions cover a wide range of biological processes and include binding to membranes, receptors, enzymes, lipids, RNA, and metals as well as use as cofactors and signaling molecules. This review covers the currently known modes of action (MOA) of RiPPs. In turn, the mechanisms by which these molecules interact with their natural targets provide a rich set of molecular paradigms that can be used for the design or evolution of new or improved activities given the relative ease of engineering RiPPs. In this review, coverage is limited to RiPPs originating from bacteria.

Engineered Liposomes Protect Immortalized Immune Cells from Cytolysins Secreted by Group A and Group G Streptococci

The increasing antibiotic resistance of bacterial pathogens fosters the development of alternative, non-antibiotic treatments. Antivirulence therapy, which is neither bacteriostatic nor bactericidal, acts by depriving bacterial pathogens of their virulence factors. To establish a successful infection, many bacterial pathogens secrete exotoxins/cytolysins that perforate the host cell plasma membrane. Recently developed liposomal nanotraps, mimicking the outer layer of the targeted cell membranes, serve as decoys for exotoxins, thus diverting them from attacking host cells. In this study, we develop a liposomal nanotrap formulation that is capable of protecting immortalized immune cells from the whole palette of cytolysins secreted by Streptococcus pyogenes and Streptococcus dysgalactiae subsp. equisimilis—important human pathogens that can cause life-threatening bacteremia. We show that the mixture of cholesterol-containing liposomes with liposomes composed exclusively of phospholipids is protective against the combined action of all streptococcal exotoxins. Our findings pave the way for further development of liposomal antivirulence therapy in order to provide more efficient treatment of bacterial infections, including those caused by antibiotic resistant pathogens.

Structural studies of Streptococcus pyogenes streptolysin O provide insights into the early steps of membrane penetration

Cholesterol-dependent cytolysins (CDCs) are a large family of bacterial toxins that exhibit a dependence on the presence of membrane cholesterol in forming large pores in cell membranes. Significant changes in the three-dimensional structure of these toxins are necessary to convert the soluble monomeric protein into a membrane pore. We have determined the crystal structure of the archetypical member of the CDC family, streptolysin O (SLO), a virulence factor from Streptococcus pyogenes. The overall fold is similar to previously reported CDC structures, although the C-terminal domain is in a different orientation with respect to the rest of the molecule. Surprisingly, a signature stretch of CDC sequence called the undecapeptide motif, a key region involved in membrane recognition, adopts a very different structure in SLO to that of the well-characterized CDC perfringolysin O (PFO), although the sequences in this region are identical. An analysis reveals that, in PFO, there are complementary interactions between the motif and the rest of domain 4 that are lost in SLO. Molecular dynamics simulations suggest that the loss of a salt bridge in SLO and a cation-pi interaction are determining factors in the extended conformation of the motif, which in turn appears to result in a greater flexibility of the neighboring L1 loop that houses a cholesterol-sensing motif. These differences may explain the differing abilities of SLO and PFO to efficiently penetrate target cell membranes in the first step of toxin insertion into the membrane.

Understanding fertilization through intracytoplasmic sperm injection (ICSI)

Since the establishment of in vitro fertilization, it became evident that almost half of the couples failed to achieve fertilization and this phenomenon was attributed to a male gamete dysfunction. The adoption of assisted fertilization techniques particularly ICSI has been able to alleviate male factor infertility by granting the consistent ability of a viable spermatozoon to activate an oocyte. Single sperm injection, by pinpointing the beginning of fertilization, has been an invaluable tool in clarifying the different aspects of early fertilization and syngamy. However, even with ICSI some couples fail to fertilize due to ooplasmic dysmaturity in relation to the achieved nuclear maturation marked by the extrusion of the first polar body. More uncommon are cases where the spermatozoa partially or completely lack the specific oocyte activating factor. In this work, we review the most relevant aspects of fertilization and its failure through assisted reproductive technologies. Attempts at diagnosing and treating clinical fertilization failure are described.

Embryonic stem cell extracts: use in differentiation and reprogramming

Stem cells have been studied extensively for decades and they have the inherent capacity to self-renew as well as to generate one or more types of specialized cells. The current focus of research on stem cells, particularly on embryonic stem cells, is on directed differentiation of these cells into specific cell types for future regenerative medicine. For the past few years, the process of reprogramming, which mediates convertion of somatic cells to their pluripotent state, has been given much attention, as it provides a possible source of autologous stem cells. In addition, understanding the molecular mechanism of differentiation and reprogramming has long been a subject of interest. In this article, we have briefly introduced stem cells and discussed the use of embryonic stem cells in reprogramming of somatic cells and differentiation to different lineages. The application of embryonic stem cells extracts in inducing reprogramming and transdifferentiation has also been described and discussed. Should this approach be successful, patient-specific cells will be produced safely and the likelihood of rejection will be decreased when used in cell therapy for many debilitating human diseases for which there is no cure such as Parkinson’s disease, Alzheimer’s disease, diabetes and others.

Molecular dissection of HCl secretion in gastric parietal cells using streptolysin O permeabilization

Histamine-stimulated gastric acid secretion involves a transient elevation of intracellular Ca(2+) and the cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) cascade through phosphorylation, the actions of which ultimately result in the fusion of vesicles containing H,K-ATPase (adenosine triphosphatase) to the apical plasma membrane of parietal cells. To dissect the signaling events underlying gastric acid secretion, we have developed a permeabilized gastric gland model using streptolysin O (SLO). The advantage of this model is its ability to retain cytosolic components that are required for the secretory machinery while granting accessibility for the introduction of macromolecules into the cytoplasm. Our studies showed that acid secretion in SLO-permeabilized glands is a cAMP-dependent process and involves the recruitment of H,K-ATPase-rich tubulovesicles into the apical plasma membrane as judged by biochemical assays. These studies established a functional permeabilized gland model in which the resting-to-secreting transition can be triggered by second messengers, while the manipulation of the cytoplasmic environment can be achieved with ease.

Selective permeabilization of the host cell membrane of Plasmodium falciparum-infected red blood cells with streptolysin O and equinatoxin II

Plasmodium falciparum develops within the mature RBCs (red blood cells) of its human host in a PV (parasitophorous vacuole) that separates the host cell cytoplasm from the parasite surface. The pore-forming toxin, SLO (streptolysin O), binds to cholesterol-containing membranes and can be used to selectively permeabilize the host cell membrane while leaving the PV membrane intact. We found that in mixtures of infected and uninfected RBCs, SLO preferentially lyses uninfected RBCs rather than infected RBCs, presumably because of differences in cholesterol content of the limiting membrane. This provides a means of generating pure preparations of viable ring stage infected RBCs. As an alternative permeabilizing agent we have characterized EqtII (equinatoxin II), a eukaryotic pore-forming toxin that binds preferentially to sphingomyelin-containing membranes. EqtII lyses the limiting membrane of infected and uninfected RBCs with similar efficiency but does not disrupt the PV membrane. It generates pores of up to 100 nm, which allow entry of antibodies for immunofluorescence and immunogold labelling. The present study provides novel tools for the analysis of this important human pathogen and highlights differences between Plasmodium-infected and uninfected RBCs.

Syntaxin 3 is required for cAMP-induced acid secretion: streptolysin O-permeabilized gastric gland model

Gastric gland stimulation triggers H(+),K(+)-ATPase translocation from cytoplasmic tubulovesicles to apical plasma membrane in parietal cells, resulting in HCl secretion. We studied the mechanisms involved in tubulovesicle translocation with a permeabilized gland system. Streptolysin O (SLO)-treated glands were permeabilized such that exogenous fluorescently labeled actin incorporated into cytoskeleton in a pattern mimicking endogenous F-actin. As shown by accumulation of the weak base aminopyrine (AP), SLO-permeabilized glands are stimulated to secrete acid by addition of cAMP and ATP and inhibited by proton pump inhibitors. Direct visualization with the fluorescent pH probe Lysosensor showed acid accumulation in glandular lumen and parietal cell canaliculi. ME-3407, an antiulcer drug with inhibitory action implicated to involve ezrin, inhibited AP uptake in and effectively released ezrin from intact and SLO-permeabilized glands. In contrast, wortmannin, an effective secretion inhibitor in intact glands, had minimal effects on ezrin or AP accumulation in SLO-permeabilized glands. The finding that SNARE protein syntaxin 3 is associated with H(+),K(+)-ATPase-containing tubulovesicles suggested that it is involved in membrane fusion. Addition of recombinant syntaxin 3, but not syntaxin 5 or heat-denatured syntaxin 3, dose-dependently inhibited acid secretion. Our studies are consistent with a membrane recycling hypothesis that activation of protein kinase cascades leads to SNARE-mediated fusion of H(+),K(+)-ATPase-containing tubulovesicles to apical plasma membrane.

Holins: the protein clocks of bacteriophage infections

Two proteins, an endolysin and a holin, are essential for host lysis by bacteriophage. Endolysin is the term for muralytic enzymes that degrade the cell wall; endolysins accumulate in the cytosol fully folded during the vegetative cycle. Holins are small membrane proteins that accumulate in the membrane until, at a specific time that is "programmed" into the holin gene, the membrane suddenly becomes permeabilized to the fully folded endolysin. Destruction of the murein and bursting of the cell are immediate sequelae. Holins control the length of the infective cycle for lytic phages and so are subject to intense evolutionary pressure to achieve lysis at an optimal time. Holins are regulated by protein inhibitors of several different kinds. Holins constitute one of the most diverse functional groups, with >100 known or putative holin sequences, which form >30 ortholog groups.

A role for C-protein in the regulation of contraction and intracellular Ca2+ in intact rat ventricular myocytes

1. C-protein is a major component of muscle thick filaments whose function is unknown. We have examined for the first time the role of the regulatory binding domain of C-protein in modulating contraction and intracellular Ca2+ concentration ([Ca2+]i) in intact cardiac myocytes. 2. Rat ventricular myocytes were reversibly permeabilised with the pore-forming toxin streptolysin O. Myosin S2 (which binds to the regulatory domain of C-protein) was introduced into cells during permeabilisation to compete with the endogenous C-protein-thick filament interaction. 3. Introduction of S2 into myocytes increased contractility by approximately 30%, significantly lengthened the time to peak of the contraction and the time to half-relaxation, but had no effect on [Ca2+]i transient amplitude. 4. Our data are consistent with increased myofilament Ca2+ sensitivity when there is reduced binding of C-protein to myosin near the head-tail junction. 5. We propose that the effects of introducing S2 into intact cardiac cells can be equated with the consequences of selectively phosphorylating C-protein in vivo, and that the regulation of contraction by C-protein is mediated by the effects of crossbridge cycling on the Ca2+ affinity of troponin C.

Swelling-induced taurine release without chloride channel activity in Xenopus laevis oocytes expressing anion channels and transporters

Taurine is an important osmolyte involved in cell volume regulation. During regulatory volume decrease it is released via a volume-sensitive organic osmolyte/anion channel. Several molecules have been suggested as candidates for osmolyte release. In this study, we chose three of these, namely ClC-2, ClC-3 and ICln, because of their expression in rat astrocytes, a cell type which is known to release taurine under hypotonic stress, and their activation by hypotonic shock. As all three candidates were also suggested to be chloride channels, we investigated their permeability for both chloride and taurine under isotonic and hypotonic conditions using the Xenopus laevis oocyte expression system. We found a volume-sensitive increase of chloride permeability in ClC-2-expressing oocytes only. Yet, the taurine permeability was significantly increased under hypotonic conditions in oocytes expressing any of the tested candidates. Further experiments confirmed that the detected taurine efflux does not represent unspecific leakage. These results suggest that ClC-2, ClC-3 and ICln either participate in taurine transport themselves or upregulate an endogenous oocyte osmolyte channel. In either case, the taurine efflux of oocytes not being accompanied by an increased chloride flux suggests that taurine and chloride can be released via two separate pathways.

Involvement of an F-actin skeleton on the acrosome reaction in guinea pig spermatozoa

The acrosome reaction (AR) is a regulated exocytotic process. In several cell types, an actin network situated under the plasma membrane (PM) acts as a physical barrier to prevent this exocytosis. In seeking a function for a cortical skeleton in guinea pig spermatozoa, the PM and the outer acrosomal membrane (OAM) were investigated for the presence of F-actin and spectrin, proteins generally found in cell cortical skeletons. Both membrane types were visualized in whole-mount preparations by electron microscopy. PM proteins gave positive reaction to the Na(+),K(+)-ATPase antibody and the OAM proteins did not react to the antibody. Furthermore, a Triton X-100-resistant skeleton was obtained from both membrane types. Using gold immunoelectron microscopy, F-actin was visualized in the PM and in the OAM skeletons, while spectrin was only detected in the PM skeleton. The presence of an F-actin cortical skeleton in the sperm PM suggests that F-actin may be involved in the AR. The significantly higher number of AR elicited by cytochalasin D (Cyt-D) treatment(P<0.005) and data showing a significant (P>0.03) decrease in F-actin relative concentration in capacitating spermatozoa, agree with this suggestion. Furthermore, the proposal is strengthened by the fact that stabilization of F-actin by phalloidin (Ph) significantly (P>0.01) diminished AR induced by Ca(2+) in a streptolysin O (SLO)-permeabilized sperm model.

Is streptolysin S of group A streptococci a virulence factor?

The possible role played by streptolysin S (SLS) of group A streptococci in the pathophysiology of streptococcal infections and in post-streptococcal sequelae is discussed. The following properties of SLS justify its definition as a distinct virulence factor: 1) its presence on the streptococcus surface in a cell-bound form, 2) its continuous and prolonged synthesis by resting streptococci, 3) its non-immunogenicity, 4) its extractability by serum proteins (albumin, alpha lipoprotein), 5) its ability to become transferred directly to target cells while being protected from inhibitory agents in the milieu of inflammation, 6) its ability to bore holes in the membrane phospholipids in a large variety of mammalian cells, 7) its ability to synergize with oxidants, proteolytic enzymes, and with additional host-derived proinflammatory agonists, and 8) its absence in streptococcal mutants associated with a lower pathogenicity for animals. Because tissue damage in streptococcal and post-streptococcal sequelae might be the end result of a distinct synergism between streptococcal and host-derived proinflammatory agonists it is proposed that only cocktails of anti-inflammatory agents including distinct inhibitors of SLS (phospholipids), gamma globulin, inhibitors of reactive oxygen species, proteinases, cationic proteins cytokines etc., will be effective in inhibiting the multiple synergistic interactions which lead to fasciitis, myositis and the flesh-eating syndromes, and often develop into sepsis, septic shock and multiple organ failure. The creation of mutants deficient in SLS and in proteases will help shed light on the specific role played by SLS in the virulence of group A hemolytic streptococci.

Protein kinase C promotes spontaneous relaxation of streptolysin-O-permeabilized smooth muscle cells from the guinea-pig stomach

Isolated single smooth muscle cells from the fundus of a guinea-pig stomach were permeabilized by use of streptolysin-O (0.5 U/ml). Most of the permeabilized cells responded to 0.6 microM Ca2+, but not to 0.2 microM Ca2+, with a resulting maximal cell shortening to approximately 71% of the resting cell length. These cells were relaxed again by washing with the Ca2+-free solution (2.5 nM free Ca2+) for 3-5 min. Addition of 10 microM acetylcholine (ACh) resulted in both a marked decrease in the concentration of Ca2+ required to trigger a threshold response and an increase in the maximal cell shortening, indicating that the cells retained the muscarinic receptor function. When the cell treated with a protein kinase C (PKC) inhibitor, K-252b (1 microM), for 3 min was exposed to 10 microM ACh in the presence of K-252b, the cell shortened within 2 min with a maximal cell shortening. When the cell shortening was induced by 10 microM ACh plus 1 microM Ca2+ in the presence of K-252b (1 microM) or more selective PKC inhibitors, such as calphostin C (1 microM) or PKC pseudosubstrate peptide (100 microM), the extension of the shortened cells, by washing with the Ca2+-free solution, was significantly inhibited. In contrast, K-252b (1 microM) did not inhibit the relaxation of Ca2+-induced shortened cells. These results suggest that the receptor-mediated activation of PKC in the process of ACh-induced cell shortening plays a role in the subsequent relaxation of the shortened cells.

Engagement of spectrin and actin in capping of FcgammaRII revealed by studies on permeabilized U937 cells

Plasma membrane receptors can undergo translocation in the plane of plasma membrane after binding of polyvalent ligands. Ligand/receptor clusters, named patches, can collect into a polar cap, presumably due to their association with the submembrane actin-based cytoskeleton. We found that the assembly of Fcgamma receptor II caps in human monocytic U937 cells was accompanied by the accumulation of spectrin and actin in the cap region. Permeabilization of cells with streptolysin O rendered capping sensitive to inhibition by phalloidin, an actin filament stabilizing agent. A rabbit antibody directed against the chicken erythrocyte alpha-subunit of spectrin, an actin- and membrane-binding protein, also blocked the capping in a dose dependent manner. The inhibition reached approximately 50% after 20 minutes of cell treatment with the antibody. Anti-alpha-spectrin targeted specifically its submembrane antigen, in contrast to unspecific antibodies which remained dispersed in the cell interior and had no influence on the cap assembly. Our results indicate an active engagement of spectrin and actin filaments in the capping of Fcgamma receptor II.

Two structural transitions in membrane pore formation by pneumolysin, the pore-forming toxin of Streptococcus pneumoniae

The human pathogen Streptococcus pneumoniae produces soluble pneumolysin monomers that bind host cell membranes to form ring-shaped, oligomeric pores. We have determined three-dimensional structures of a helical oligomer of pneumolysin and of a membrane-bound ring form by cryo-electron microscopy. Fitting the four domains from the crystal structure of the closely related perfringolysin reveals major domain rotations during pore assembly. Oligomerization results in the expulsion of domain 3 from its original position in the monomer. However, domain 3 reassociates with the other domains in the membrane pore form. The base of domain 4 contacts the bilayer, possibly along with an extension of domain 3. These results reveal a two-stage mechanism for pore formation by the cholesterol-binding toxins.

Maintenance of motility in mouse sperm permeabilized with streptolysin O

One approach to studying the mechanisms governing sperm motility is to permeabilize sperm and examine the regulation of motility by manipulating the intracellular milieu of the cell. The most common method of sperm permeabilization, detergent treatment, has the disadvantage that the membranes and many proteins are extracted from the cell. To avoid this problem, we have developed a method that uses streptolysin O to create stable pores within the plasma membrane while leaving internal membranes intact. Sperm were permeabilized, preincubated, and then treated with 0.6 U/ml of streptolysin O. Permeabilization was assessed by fluorescent dye technologies and endogenous protein phosphorylation using exogenously added [gamma-32P]ATP. Streptolysin O-induced permeabilization rendered the sperm immotile, and the effect was Ca2+-dependent. When the cells were treated simultaneously with a medium containing ATP, streptolysin O-treated sperm maintained flagellar movement. These results demonstrate that the streptolysin O permeabilization model system is a useful experimental method for studying the mechanisms that regulate sperm motility since it allows the flagellar apparatus to be exposed to various exogenously added molecules.

Evidence for pore-forming ability by Legionella pneumophila

Legionella pneumophila is the cause of Legionnaires' pneumonia. After Internalization by macrophages, it bypasses the normal endocytic pathway and occupies a replicative phagosome bound by endoplasmic reticulum. Here, we show that lysis of macrophages and red blood cells by L. pneumophila was dependent on dotA and other loci known to be required for proper targeting of the phagosome and replication within the host cell. Cytotoxicity occurred rapidly during a high-multiplicity infection, required close association of the bacteria with the eukaryotic cell and was a form of necrotic cell death accompanied by osmotic lysis. The differential cytoprotective ability of high-molecular-weight polyethylene glycols suggested that osmotic lysis resulted from insertion of a pore less than 3 nm in diameter into the plasma membrane. Results concerning the uptake of membrane-impermeant fluorescent compounds of various sizes are consistent with the osmoprotection analysis. Therefore, kinetic and genetic evidence suggested that the apparent ability of L. pneumophila to insert a pore into eukaryotic membranes on initial contact may play a role in altering endocytic trafficking events within the host cell and in the establishment of a replicative vacuole.

Diethyldithiocarbamate and nitric oxide synergize with oxidants and with membrane-damaging agents to injure mammalian cells

The effect of diethyldithiocarbamate (DDC) and sodium nitroprusside (SNP) on the killing of endothelial cells and on the release of arachidonate by mixtures of oxidants and membrane-damaging agents was studied in a tissue culture model employing bovine aortic endothelial cells labeled either with 51Chromium or 3arachidonic acid. While exposure to low, subtoxic concentrations of oxidants (reagent H2O2, glucose-oxidase generated peroxide, xanthine xanthine oxidase, AAPH-generated peroxyl radical, menadione-generated oxidants) did not result either in cell death or in the loss of membrane-associated arachidonic acid, the addition of subtoxic amounts of a variety of membrane-damaging agents (streptolysin S, PLA2, histone, taurocholate, wheatgerm agglutinin) resulted in a synergistic cell death. However, no significant amounts of arachidonate were released unless proteinases were also present. The addition to these reaction mixtures of subtoxic amounts of DDC (an SOD inhibitor and a copper chelator) not only very markedly enhanced cell death but also resulted in the release of large amounts of arachidonate (in the complete absence of added proteinases). Furthermore, the inclusion in DDC-containing reaction mixtures of subtoxic amounts of SNP, a generator of NO, further enhanced, in a synergistic manner, both cell killing and the release of arachidonate. Cell killing and the release of arachidonate induced by the DDC and SNP-containing mixtures of agonists were strongly inhibited by catalase, glutathione, N-acetyl cysteine, vitamin A, and by a nonpenetrating PLA2 inhibitor as well as by tetracyclines. A partial inhibition of cell killing was also obtained by 1,10-phenanthroline and by antimycin. It is suggested that DDC might amplify cell damage by forming intracellular, loosely-bound complexes with copper and probably also by depleting antioxidant thiols. It is also suggested that "cocktails" containing oxidants, membrane-damaging agents, DDC, and SNP might be beneficial for killing of tumor cells in vivo and for the assessment of the toxicity of xenobiotics in vitro.

Control of cholesterol access to cytochrome P450scc in rat adrenal cells mediated by regulation of the steroidogenic acute regulatory protein

Cholesterol conversion to pregnenolone by cytochrome P450scc in steroidogenic cells, including those of the adrenal cortex, is determined by hormonal control of cholesterol availability. Intramitochondrial cholesterol movement to P450scc, which retains hormonal activation in isolated mitochondria, is apparently dependent on peripheral benzodiazepine receptor and the recently cloned steroidogenic acute regulatory (StAR) protein. In rat adrenal cells, StAR is formed as a 37-kDa precursor that is transferred to the mitochondrial inner membrane following phosphorylation by hormonally activated protein kinase A, and processed to multiple forms, some of which turn over very rapidly. In bovine cells, StAR undergoes three modifications forming a set of eight proteins seen in both glomerulosa and fasciculata cells. In the former, cyclic AMP and angiotensin II each decrease two forms and elevate six forms. Significantly, the major change seen after activation may not involve phosphorylation of StAR. Cholesterol transfer across mitochondrial membranes is also activated in isolated mitochondria by GTP and low concentrations of Ca2+, apparently prior to activation by StAR. Depletion of StAR by cycloheximide inhibits cholesterol transfer but is overcome by uptake of Ca2+ into the matrix. This activation of cellular cholesterol transport is sustained in adrenal cells permeabilized by Streptolysin O. In rat adrenal cells cAMP elevates 3.5- and 1.6-kb mRNA, hybridized by a 1.0-kb StAR cDNA. A 3.5-kb rat adrenal cDNA that encodes all except the 5' end of the longest StAR mRNA has been characterized. The corresponding gene sequence is distributed across seven exons. The shorter mRNA may arise from polyadenylation signals early in exon 7. However, the 3.5-kb mRNA comprises 80-90% of untreated rat adrenal StAR mRNA and may therefore provide the prime source for in vivo translation of StAR protein.

Molecular mechanisms of action of bacterial exotoxins

Toxins are one of the inventive strategies that bacteria have developed in order to survive. As virulence factors, they play a major role in the pathogenesis of infectious diseases. Recent discoveries have once more highlighted the effectiveness of these precisely adjusted bacterial weapons. Furthermore, toxins have become an invaluable tool in the investigation of fundamental cell processes, including regulation of cellular functions by various G proteins, cytoskeletal dynamics and neural transmission. In this review, the bacterial toxins are presented in a rational classification based on the molecular mechanisms of action.

Regulation of exocytosis by the small GTP-binding protein Rho in rat basophilic leukemia (RBL-2H3) cells

1. We investigated the effect of Clostridium botulinum C3 ADP-ribosyltransferase upon beta-hexosaminidase release induced by various stimuli from streptolysin-O (0.5-1 U/ml)-permeabilized rat basophilic leukemia (RBL-2H3) cells. 2. The C3 transferase inhibited beta-hexosaminidase release induced by Ca2+ or by guanosine-5'-(3-thiotriphosphate) (GTP gamma S) plus Ca2+. 3. The C3 transferase also inhibited beta-hexosaminidase release induced by stimulating high affinity IgE and m3 muscarinic acetylcholine receptors. 4. The substrate for the C3 transferase was present in cytosol of RBL-2H3 cells, indicating the presence of rho p21. About 60% of the total cellular substrate protein remained within the cells permeabilized by 1 U/ml of streptolysin-O. 5. The protein rho p21 appears to be regulated by several pathways and it may function as an integration point for exocytosis.

Kinetics of streptolysin O self-assembly

Streptolysin O is a member of a family of membrane-damaging toxins that bind to cell membranes containing cholesterol and then polymerize to form large pores. We have examined the kinetics of toxin action using 125I-labelled streptolysin O. Binding of toxin monomers to membranes displays first-order kinetics and is reversible; the rate of desorption from red cells shows a marked dependence on temperature. To study oligomerization, toxin was bound to erythrocytes at 0 degrees C. Oligomer formation was then triggered by a sudden temperature shift and stopped by solubilization of membranes with deoxycholate. While at moderately high streptolysin O concentrations oligomerization behaves as a reaction of second order, the kinetic pattern changes with increasing toxin concentration. We show that this can be accounted for by the assumption of a two-step reaction mechanism: two membrane-bound monomers first associate into a start complex, which then is rapidly extended by the sequential addition of further monomers up to the final oligomer size.

Binding, oligomerization, and pore formation by streptolysin O in erythrocytes and fibroblast membranes: detection of nonlytic polymers

Streptolysin O (SLO) is a representative of the family of cholesterol-binding cytolysins that form large pores in target cell membranes. Aggregation of the toxin to polymeric structures is required for pore formation. However, it is not known whether, vice versa, polymers may under certain circumstances remain nonfunctional, and whether this might be the cause underlying the relative resistance of certain cells towards toxin action. In the present study, we applied radioiodinated, functionally active SLO to human, rabbit, and mouse erythrocytes and to human fibroblasts and keratinocytes. Binding and polymerization were quantified and correlated with membrane damage. At low toxin concentrations, human and rabbit but not mouse erythrocytes were lysed, but binding and polymerization of SLO were essentially identical in all cases. Nonlytic polymers were also detected on human fibroblasts and keratinocytes treated with subcytotoxic concentrations of SLO, and quantitative estimates indicated that nonpermeabilized cells could carry hundreds of polymers on their surface. When applied at low concentrations to fibroblasts, much of the toxin remained in monomer form and was subsequently shed from the cells. This was shown by monitoring the fate of radioiodinated toxin and also by using a sensitive cell enzyme-linked immunosorbent assay that permitted immunological detection of surface-exposed SLO. Thus, relative resistance of cells towards the permeabilizing action of SLO may be due to their ability to tolerate formation of a limited number of SLO polymers and to shedding of nonoligomerized toxin from their surface.

Breakdown of the round window membrane permeability barrier evoked by streptolysin O: possible etiologic role in development of sensorineural hearing loss in acute otitis media

Sensorineural hearing loss is a common sequela of acute and chronic otitis media, and the round window membrane (RWM) is currently being considered as a major route for noxious agents to pass from the middle ear cavity to the cochlea. Streptococcus pneumoniae, a major causative agent of otitis media, and Streptococcus pyogenes A produce molecularly related toxins, pneumolysin and streptolysin O (SLO), that form large pores in target membranes. In this study, we analyzed the effects of SLO on the permeability of the RWM. Resected RWMs from a total of 104 guinea pigs were embedded between two chambers of an in vitro system. One chamber was designated as the tympanal (cis) compartment, and the other was designated as the inner ear (trans) compartment. The permeability of normal and SLO-damaged RWMs towards Na+, [14C]mannitol, and proteins was investigated. SLO evoked permeability defects dose dependently in the RWM with fluxes of both Na+ and [14C]mannitol being demonstrable over a time span of up to 8 h. Serum proteins and radioiodinated SLO were also shown to pass through the damage RWM. Scanning electron microscopy revealed the morphological correlates to these results. We propose that damage to the RWM by potent pore-forming cytolysins leads to leakage of ions from the perilymph. Ionic disequilibrium and passage of noxious macromolecules to the cochlea could contribute to disturbances of the inner ear function.

Substance P-related antagonists inhibit vasopressin and bombesin but not 5'-3-O-(thio)triphosphate-stimulated inositol phosphate production in Swiss 3T3 cells

The substance P (SP) analogues [DArg1, DPhe5, DTrp7,9, Leu11] SP (AntD) and [Arg6, DTrp7,9, MePhe8] SP (6-11) (AntG) inhibit the action of many different neuropeptides including SP. These analogues might be useful in the treatment of small cell lung cancer but their mechanism of action is unclear. Here, we analyzed the effect of AntD and AntG on neuropeptide vs. guanosine 5'-3-O-(thio) triphosphate (GTP gamma S)-stimulated inositol phosphate generation in permeabilized Swiss 3T3 cells. AntD inhibited vasopressin and bombesin stimulated inositol phosphate formation (IC50 of 0.75 microM and 2 microM, respectively). Similarly, AntG inhibited vasopressin-stimulated inositol phosphate generation with an IC50 of 1 microM. Strikingly, neither AntD up to 10 microM nor AntG up to 20 microM was able to inhibit GTP gamma S-stimulated inositol phosphate generation. Dose-response curves of neuropeptide-induced inositol phosphate generation were dramatically displaced to the right by either 10 microM AntD or 20 microM AntG. However, neither antagonist affected the dose response of GTP gamma S-stimulated inositol phosphate generation. Furthermore, 20 microM AntD had no effect on AIF-4-induced inositol phosphates in COS-1 cells transfected with G alpha q. AntD inhibited [3H]vasopressin binding competitively in intact Swiss 3T3 cells and both AntD and AntG inhibited [3H]vasopressin binding in Swiss 3T3 and rat liver membranes. Scatchard analysis revealed that AntD inhibited vasopressin binding by reducing receptor affinity without affecting receptor number in both intact and membrane preparations of Swiss 3T3 cells. The results strongly suggest that SP analogues AntD and AntG block the action of the Ca2+ mobilizing neuropeptides at the receptor level, rather than inhibiting G protein-stimulated inositol phosphate production.

Guanosine 5'-3-O-(thio)triphosphate stimulates tyrosine phosphorylation of p125FAK and paxillin in permeabilized Swiss 3T3 cells. Role of p21rho

Addition of guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) to streptolysin O-permeabilized Swiss 3T3 cells induced tyrosine phosphorylation of M(r) 110,000-130,000 and 70,000-80,000 bands. Specifically, GTP gamma S stimulated tyrosine phosphorylation of both focal adhesion kinase (p125FAK) and paxillin. GTP gamma S induced tyrosine phosphorylation was dose-dependent (EC50 of 2.5 microM) and reached maximum levels after 1.5 min for the M(r) 110,000-130,000 band and 2 min for the M(r) 70,000-80,000 paxillin band. Guanosine 5'-O-(2-thiodiphosphate) inhibited GTP gamma S-induced tyrosine phosphorylation with an IC50 of 100 microM. Protein kinase C did not mediate GTP gamma S-induced tyrosine phosphorylation. Varying the Ca2+ concentration from 0 to 6 microM did not increase tyrosine phosphorylation above basal levels and did not affect the ability of GTP gamma S to induce tyrosine phosphorylation. GTP gamma S was able to stimulate tyrosine phosphorylation in the presence of nanomolar concentrations of Mg2+. Furthermore, 30 microM AlF4- only weakly induced tyrosine phosphorylation in permeabilized cells. Pretreatment with the Clostridium botulinum C3 exoenzyme which inactivates p21rho, markedly reduced the ability of GTP gamma S to stimulate tyrosine phosphorylation of M(r) 110,000-130,000 and 70,000-80,000 bands including p125FAK and paxillin in permeabilized Swiss 3T3 cells. Furthermore, a peptide of p21rho (p21rho17-44) inhibited GTP gamma S-induced tyrosine phosphorylation in a dose-dependent manner (IC50 1 microM). This peptide also inhibited tyrosine phosphorylation of p125FAK and paxillin. In contrast, 20 microM p21ras17-44 peptide failed to inhibit GTP gamma S-induced tyrosine phosphorylation. Using permeabilized cells, our findings demonstrate that GTP gamma S stimulates tyrosine phosphorylation of p125FAK and paxillin and that a functional p21rho is implicated in this process.

Inhibitory mechanism of Ca2+ on the hemolysis caused by Vibrio vulnificus cytolysin

Calcium in millimolar concentrations protected mouse erythrocytes from hemolysis caused by Vibrio vulnificus cytolysin without affecting the release of intracellular K+ from the cells. This effect was maximal at 25 mM CaCl2. The protection was not absolute and could be partially overcome by increased concentrations of cytolysin. Calcium failed to block both the binding and oligomer formation of cytolysins on the erythrocyte membrane. After pore formation, the continued presence of calcium is required for the prevention of hemolysis. There was hardly any inflow of calcium into the erythrocytes through pores as measured by 45Ca2+ uptake. The presence of calcium after the abolition of Ca2+ gradient by ionomycin cannot inhibit the hemolysis caused by cytolysin. These results suggest that calcium exerts its major inhibitory effect on V. vulnificus cytolysin-induced hemolysis as an osmotic protectant, and that cytolysin may become an useful tool for permeabilizing cells selectively for small ions such as potassium or sodium while preventing the Ca2+ flow.

Electroporation and streptolysin O--a comparison of poration techniques

CHO (Chinese hamster ovary), xrs5 (X-ray sensitive Chinese hamster) and HF19 (untransformed human fibroblast) cells, were exposed to a lethal dose of the restriction enzyme Pvu II during electroporation or poration with the bacterial toxin streptolysin O. The uptake of the exclusion dye trypan blue was used as a measure of poration and compared with survival as measured by subsequent colony formation. It was assumed that any surviving cells had not been permeabilized and therefore did not receive any restriction enzyme. Electroporation alone proved to be more cytotoxic to the cells, whilst streptolysin O was more efficient at permeabilizing both hamster and human cells.

Permeabilization of MDCK cells with cholesterol binding agents: dependence on substratum and confluency

We studied systematically the susceptibility of Madin-Darby canine kidney (MDCK) cells to permeabilization by two cholesterol binding agents, digitonin and streptolysin-O (SLO), under different culture conditions. Monolayers grown on polycarbonate filter chambers (Transwells) required twice the concentration of digitonin effective on monolayers grown on glass or plastic (80 vs. 40 micrograms/ml) to allow antibody penetration or the release of 90% of the cytosolic protein lactate dehydrogenase (LDH). Neither the apical nor the basolateral surface showed preferential susceptibility to digitonin. Confluent MDCK cells, cultured either on filters or on impermeable substrates, showed poor antibody permeability after addition of commercial SLOs, even when used at concentrations 100 times higher (20 U/ml) than those effective on nonepithelial Chinese hamster ovary cells. Surprisingly, culture conditions that prevent tight junction formation and the acquisition of a polarized phenotype (< 10 microM Ca2+) increased dramatically the susceptibility to permeabilization by SLO. On restoration of normal Ca2+ levels, susceptibility to SLO quickly decreased. Thus conditions that lead to the full establishment of polarity result in decreased sensitivity to disruption by digitonin and SLO.

C-terminal topology of gastric H+,K(+)-ATPase

An antibody was prepared against a peptide corresponding to residues 1024-1034 (the putative C-terminus) of the alpha-subunit of hog gastric H+,K(+)-ATPase. The antibody bound to a 95 kDa band of H+,K(+)-ATPase that was solubilized in SDS, but not to that of Na+,K(+)-ATPase. It also bound to products of tryptic digestion that included C-terminal fragments of the H+,K(+)-ATPase alpha-subunit. The same amount of the antibody bound to both intact (tight) and lyophilized (leaky) inside-out gastric vesicles, indicating that its epitope is present on the cytosolic side of the vesicles. This finding was further confirmed by using fluorescence-immunolocalization techniques and streptolysin-O to permeabilize newt oxyntic cells. Stimulation of isolated newt oxyntic cells with dibutyryl cyclic AMP induces fusion of tubulovesicles with the apical membrane, so that the luminal domains of the H+,K(+)-ATPase alpha-subunit directly face the cell-suspension medium. The antibody did not bind to the stimulated intact cell, but bound to cells permeabilized with streptolysin-O, indicating that it binds from the cytoplasmic side to the C-terminus of the H+,K(+)-ATPase alpha-subunit in apical and tubulovesicular membrane, and also that the H+,K(+)-ATPase alpha-subunit has an even number of transmembrane domains.

Disparate modulation of plasma membrane protein lateral mobility by various cell permeabilizing agents

The mobility of a cell surface protein on cells osmotically swollen by treatment with several different cell permeabilizing agents retains specific restraints despite detachment of the plasma membrane from the cortical cytoskeleton. Fluorescence photobleaching recovery experiments indicate that the lateral diffusion constants of immunoglobulin E (IgE)-receptor complexes on the surface of rat basophilic leukemia cells increase 2-5x following permeabilization with streptolysin O or digitonin, with little change in their mobile fractions. Swelling by hypo-osmotic treatment in water enhances lateral diffusion of IgE-receptor complexes and raises the mobile fractions to near 100%. In contrast, swelling by treatment with filipin arrests lateral diffusion, although rotational mobility remains unhindered. Lateral mobility of a fluorescent lipid analogue remains unchanged under these conditions. Crosslinking by anti-IgE antibodies redistributes the IgE-receptor complexes into large patches on untreated cells and on cells swollen by permeabilization with streptolysin O or digitonin, but not on cells swollen by treatment with filipin. The results indicate a diversity of effects of the various permeabilizing agents on the mobility of membrane proteins. In particular, treatment with filipin appears to reorganize the plasma membrane into a network of fluid domains on a scale smaller than the bleaching spot size used (approximately 1.5 microns).

Chromosome damage induced by nanomolar concentrations of bleomycin in porated mammalian cells

We have examined chromosome damage caused by a wide range of bleomycin (BLM) concentrations in Chinese hamster ovary (CHO-K1) cells reversibly porated by the bacterial cytotoxin streptolysin-O (SLO). Chromosome damage was measured using the micronucleus cytokinesis block technique (employing cytochalasin-B). Treatment of exponentially growing cells with 0.045 IU/mL SLO for 5 min resulted in up to a thousand-fold and a million-fold increase in biological effectiveness, compared to treatment in the absence of SLO for 24 hr and 5 min, respectively. Increases in micronuclei of 4-5 times background level were observed after only 5 min exposure to the drug in the presence of SLO at doses as low as 100 pg/mL (approximately 70 pmol/L). These results indicate that the use of SLO may facilitate the treatment of cells with BLM for periods of time resembling acute exposure to ionizing radiations.

Purification and characterization of a pore-forming protein from the marine sponge Tethya lyncurium

A pore-forming protein was detected and purified for the first time from a marine sponge (Tethya lyncurium). The purified protein has a polypeptide molecular mass of 21 kDa and a pI of 6.4. Tethya pore-forming protein (also called Tethya hemolysin) rapidly lysed erythrocytes from a variety of organisms. After binding to target membranes, the hemolysin resisted elution with EDTA, salt or solutions of low ionic strength and hence resembled an integral membrane protein. Erythrocytes could be protected from hemolysis induced by Tethya hemolysin by addition of 30 mM dextran 4 (4-6 kDa; equivalent hydrodynamic diffusion radius, 1.75-2.3 nm) to the extracellular medium, but not by addition of uncharged molecules of smaller size [sucrose, raffinose and poly(ethylene glycol) 1550; equivalent hydrodynamic diffusion radii, 0.46, 0.57 and 1.2 nm, respectively]. This result indicates that hemolysin is able to form stable transmembrane pores with an effective diameter of about 2-3 nm. Treatment of osmotically protected erythrocytes with Tethya hemolysin caused a rapid efflux of intracellular K+ and ATP, and a rapid influx of extracellularly added Ca2+ and sucrose. In negative-staining electron microscopy, target erythrocyte membranes exposed to purified Tethya hemolysin displayed ultrastructural lesions but without visible pores.

Retrograde transport from the Golgi region to the endoplasmic reticulum is sensitive to GTP gamma S

The involvement of GTP-binding proteins in the intracellular transport of the secretory glycoprotein alpha 1-antitrypsin was investigated in streptolysin O-permeabilized HepG2 cells. This permeabilization procedure allows ready access to the intracellular milieu of the membrane-impermeant, nonhydrolyzable GTP analog GTP gamma S. In streptolysin O-permeabilized HepG2 cells, the constitutive secretory pathway remains functional and is sensitive to GTP gamma S. Exposure of HepG2 cells to brefeldin A resulted in redistribution of Golgi-resident glycosyltransferases (including both alpha 2----3 and alpha 2----6 sialyltransferases) to the ER. This redistribution was sensitive to GTP gamma S. Our results suggest that GTP-binding proteins are involved in the regulation not only of the anterograde, but also of the retrograde, pathway.

An in vitro model giving access to adhesion plaques

A new approach was investigated to study the interaction between integrins and actin via intracytoplasmic proteins. Because intracellular processes are hampered by the limiting plasma membrane, we developed an in vitro model with cells perforated by a bacterial toxin, streptolysin O. The specific conditions for the use of permeabilized cells to study the intramolecular associations occurring at adhesion plaques are described. The two cell types used, HUVEC and CHO, showed that the choice of the perforation method is of great importance. After perforation of cells in a monolayer, 75 +/- 10% of the cells remained adherent to a fibronectin substrate; after perforation of cells in suspension, only 25 +/- 10% of the cells readhered. Specific conditions were required however to maintain these adhesive properties up to 4 h: the presence of 1 mM Mg++ in the medium was crucial, and it was necessary to layer the cells on a specific coat rather than a substitute such as gelatin. Immunofluorescence investigations of actin, talin and vinculin, and Normarsky differential interference contrast microscopy showed retention of focal adhesion plaques in perforated cells. Moreover, in perforated cells antibodies directed against actin led to actin disorganization, showing that our model of perforated cells in a monolayer can give new insight to adhesion study.