Microbial Toxins: Molecular and Cellular Biology Chapter Abstracts
Chapter 1
Introduction
Joseph E. Alouf
Abstract
There is no abstract available for this chapter. Therefore we present the first paragraph of the Introduction : Over the past twenty years (1983-2003), great progress has been made in the depth of our understanding of microbial toxins, particularly bacterial protein toxins. This progress is mainly attributable to (i) the worldwide increasing number of research groups working in this field, (ii) the increasing discovery of novel toxins and (iii) the extraordinary development of various disciplines involved in toxin research : protein biophysics and biochmistry, molecular microbiology, molecular genetics and the advent of DNA technology (genetic engineering of toxin molecules), cell biology, cellular and molecular immunology, neurophysiology, pathophysiology of toxin-mediated bacterial diseases and more recent disciplines such as cellular microbiology, genomics and proteomics. In parallel, the development of a number of technological achievements were also instrumental for our understanding of the mechanism of action of bacterial toxins: experimental animal models (particularly transgenic and knock-out animals), tissue cultures, artificial models of biological membranes, monoclonal antibodies and other immunological tools, determination of the three-dimensional structure of many crystallised toxins. Furthermore, these achievements have armed biologists with toolkits to dissect structures, functions and metabolism of eucaryotic cells in health and disease. In this introductory chapter, I briefly highlighted some significant hallmarks in the development of toxin research and tried to offset possible tedium by kindling our attention on the nexus of the most compelling issues in this field.
Chapter 2
Activation of Secondary Messenger Pathways by ADP-Ribosylation of G-proteins
Michael Jobling and Randall K. Holmes
Abstract
Cholera toxin (CT) from Vibrio cholerae, the family of Escherichia coli heat-labile enterotoxins (LT's), and pertussis toxin (PT) of Bordetella pertussis are all protein exotoxins that increase cyclic AMP levels in affected cells. Each uses NAD to ADP-ribosylate G proteins in target cells resulting in activation of adenylate cyclase, either by constitutive activation of the stimulatory Gsa (CT and LT's) or inactivation of the inhibitory Gia (PT). These toxins share a similar overall structure and mechanism of action, formed as heteterohexameric A-B toxins with enzymatically active A subunits and a pentameric receptor-binding B oligomer. All these toxins gain access to the cytoplasm, and their target proteins, by first trafficking in a retrograde manner to the ER. CT and LT modify an arginine residue while PT modifies a cysteine in their target G proteins. The enzymatic subunits have critical arginine, serine and catalytic glutamate residues, and show limited structural homology to other ADP-ribosylating toxins. CT, LT and PT have potent immunomodulatory effects and have potential uses as vaccine adjuvants.
Chapter 3
Microbial Toxins That Modulate the Actin Cytoskeleton
Jianjun Sun, Klaus Aktories, and Joseph T. Barbieri
Abstract
Bacterial toxins modulate the actin cytoskeleton through covalent and non- covalent modification of host proteins. These modifications can have either positive or negative effects on the cytoskeleton and subsequent cellular processes, such as motility, polarity, and phagocytic processes. Bacterial toxins target several host proteins for covalent modification, including: actin, adaptor proteins, and Rho GTPases through various modifications: Adenosine dinucleotide phosphate (ADP)-ribosylation, glucosylation, deamidation, and proteolysis. The activation states of the Rho GTPases are activated by guanine nucleotide exchange factors (GEFs) and inhibited by Rho GTPase activating proteins (GAPs). Bacterial toxins mimic the action of the Rho GEFs and GAPs to modulate the nucleotide state of the Rho GTPases. This latter group of toxins provides a temporal modulation of the actin cytoskeleton. Recent advances in the structural biology of toxins and the cellular processes that regulate the organization of the actin cytoskeleton provide insight into the molecular basis of toxin action to usurp cytoskeleton organization.
Chapter 4
The Cytolethal Distending Toxin
Carol Pickett and Robert B. Lee
Abstract
Cytolethal distending toxin (CDT) was first reported about fifteen years ago, but was under appreciated until it was discovered to have the novel capability of causing eucaryotic cells of various lineages to become irreversibly blocked in the G1 or G2 phase of the cell cycle. CDT is encoded by three adjacent genes, cdtA, cdtB, and cdtC, and one copy of each gene product is present in the holotoxin. The cell cycle block has been shown to arise from the DNase activity of the cdtB subunit; double-stranded DNA cuts invoke the DNA damage checkpoint pathway, bringing about cell cycle arrest. CDT is produced by a variety of bacterial pathogens, including Campylobacter jejuni, some Escherichia coli and Shigella sp., Haemophilus ducreyi, Actinobacillus actinomycetemcomitans, and Helicobacter hepaticus. The CDTs produced by all of these pathogens are being actively investigated with the goals of understanding the toxin's contribution to pathogenesis and its use in the development of tools for disease prevention or intervention.
Chapter 5
Toxins That Interfere with Protein Synthesis
Diana Marra Oram and Randall K. Holmes
Abstract
Secretion of toxins is common tactic used by pathogenic bacterial species to gain access to nutrients, promote transmission and disable the immune system of the host. Toxins that inhibit eukaryotic protein synthesis are produced by both Gram-positive and Gram-negative bacterial species. The first bacterial protein toxin to be extensively characterised, diphtheria toxin (DT), acts by inhibiting protein synthesis as do the more recently described bacterial toxins, Pseudomonas exotoxin A (ETA) and those within the Shiga toxin family. DT and ETA act by the same mechanism to inhibit eukaryotic protein synthesis while members of the Shiga toxin family use a completely different mechanism to achieve the same effect. Although the mechanisms by which DT and ETA act at their intracellular target are identical, they bind to different receptors on the surface of susceptible cells, they traffic through eukaryotic cells using different pathways and the protein sequences of these two toxins are not closely related. The crystal structures of DT, ETA and members of the Shiga toxin family have all been solved and the structures of these protein toxins have led to many insights into their mechanisms of action.
Chapter 6
Cell-damaging/Pore-forming Toxins
Robert J. C. Gilbert
Abstract
Many microbes produce one or more pore-forming toxins. This chapter presents an overview, systematizing its discussion around the different sizes of oligomer that generate the functioning pore in different cases. Other themes developed include the intriguing homologies found between toxins produced in evolutionarily distant organisms, the usefulness of pore formation to the microbe in its infective or offensive strategy, and the biotechnological applications of membrane-damaging toxins. Overall, pore-forming protein toxins are set within the broader picture of membrane proteins in general, and indeed of protein biochemistry in general, representing as they do pointed examples of the folding problem and of proteins that possess more than one stable fold.
Chapter 7
Clostridial Neurotoxins: Mechanism of Action of Tetanus and Botulinum Neurotoxins
Ornella Rossetto and Cesare Montecucco
Abstract
Tetanus and botulinum neurotoxins are the most potent toxins known as few nanograms are sufficient to kill most mammals. Their exceptional toxicity derives from: a) the absolute specificity for the nervous tissue, whose complete functionality is essential for survival, particularly in the wilderness, and b) their enzymatic activity in the cytosol of nerve terminals where they cleave one after the other all the copies of the core proteins of the neuroexocytosis apparatus causing a persistent blockade of neurotransmitter release. The seven botulinum neurotoxins cause the flaccid paralysis of botulism by acting on peripheric cholinergic nerve terminals. At variance, tetanus neurotoxins enters inside the axon terminals of peripheral neurons and moves retroaxonally to the spinal cord where it impairs inhibitory interneurons causing the spastic paralysis of tetanus. Because of their specificity of action botulinum neurotoxins are increasingly used in the therapy of diseases caused by hyperfunction of cholinergic nerve terminals and to correct defects due to muscle hyperactivity.
Chapter 8
Superantigens - Microbial Toxins That Target the Immune System
Thomas Proft, Birgit Schrage, and John D. Fraser
Abstract
Superantigens (SAgs) are a group of proteins that share the ability to trigger excessive and aberrant activation of human and other mammalian T lymphocytes. Unlike conventional peptide antigens, SAgs bind as intact molecules to the major histocompatibility complex (MHC) class II of antigen presenting cells (APCs) outside the peptide-binding groove. Sequential binding to the Vb region of the T cell receptor (TcR) on T cells results in systemic and massive release of pro-inflammatory cytokines and T cell mediators. Bacterial SAgs are small heat-stable exotoxins and include the structurally related staphylococcal and streptococcal SAgs, the Mycoplasma arthritidis mitogen, and two variants of the Yersinia pseudotuberculosis mitogen. Bacterial SAgs have been implicated in a number of important human diseases, such as toxic shock syndrome, rheumatic fever, Kawasaki Disease and food poisoning. Viral SAgs include the minor lymphocyte stimulating (Mls) antigens expressed in thymic stromal cells of mice by the mouse mammary tumour virus (MMTV) and the human endogenous retrovirus superantigens.
Chapter 9
The Role of Toxins in Invasive Streptococcal Disease
Hugh H. Russell and Shiranee Sriskandan
Abstract
The Gram-positive human pathogen Streptococcus pyogenes causes a range of disease from relatively benign sore throats to life threatening diseases; necrotising fasciitis and toxic shock. This bacterium secretes a number of toxins which all have some theoretical or proven contribution to the pathogenesis of severe invasive streptococcal disease. These secreted toxins include the well studied streptococcal superantigens, streptolysins and a variety of proteases. The evidence for each toxin's contribution is spread across a number of areas including patient data, animal studies, and in vitro experiments. Together each line of evidence helps one to better understand the role of streptococcal pathogenesis.
Chapter 10
Toxins in Staphylococcus aureus Pathogenesis
Gilles Prévost
Abstract
The probable reason of the wide pathogenicity for humans of Staphylococcus aureus resides in the extraordinary variety of virulence factors that can be produced and secreted as different panels from strain to strain. These strains are often resistant to several antimicrobials. Staphylococcus aureus shows a very dynamic evolution regarding genes encoding these variable factors and a highly customized adaptation to unfavourable conditions produced, for example, by human healthcare. Besides the multiple adhesion factors that initiate the colonization of the bacterium, Staphylococcus aureus toxins belong to several functional families. Few of them are directly associated with clinical syndromes while other structure-related toxins exist, and might constitute pitfalls for the immune response. It can be suspected that these superantigens, pore-forming toxins, proteases or ADP-ribosylating toxins, can nonetheless avert host defence and immune response, but may exert complementary effects that contribute to the progression of the infection, thus playing an economic role for the bacterial expansion.
Chapter 11
Pathogenesis of Bacillus anthracis: The Role of Anthrax Toxins
Yogendra Singh, Xudong Liang, and Nicholas S. Duesbery
Abstract
Anthrax is caused by the Gram-positive bacterium Bacillus anthracis. The virulence of B. anthracis principally depends on the presence of two large plasmids, known as pXO1 and pXO2. pXO2 contains genes required for synthesis of an anti-phagocytic capsule. pXO1 contains genes encoding an exotoxin, called anthrax toxin, which is composed of three proteins: protective antigen (PA), lethal factor (LF), and oedema factor (EF). In this chapter we will present recent advances in our understanding of the structural and functional properties of these proteins and discuss their roles in pathogenesis of this disease. Finally, we will discuss how these advances have fueled the discovery of novel approaches towards the development of anthrax therapeutics.
Chapter 12
Clostridial Gas Gangrene: Clinical Correlations, Microbial Virulence Factors, and Molecular Mechanisms of Pathogenesis
Dennis L. Stevens and Amy E. Bryant
Abstract
The genus Clostridium encompasses over 60 species of Gram-positive anaerobic spore-forming rods that cause a variety of infections in humans and animals by virtue of a myriad of potent proteinaceous exotoxins. Strains of clostridia, such as C. perfringens, C. histolyticum, C. septicum, C. novyi and C. sordellii cause aggressive necrotizing or histotoxic infections of the soft tissues, attributable in part, to the elaboration of bacterial proteases, phospholipases and cytotoxins. Histotoxic clostridial infection is a general term coined over a century ago that referred to gas gangrene and malignant edema in humans and blackleg in cattle (Table 1). More recently, novel histotoxic infections have been described such as necrotic enteritis, neutropenic enterocolitis and spontaneous gas gangrene - all of which occur exclusively in humans - and abomasal ulceration in cattle (Table 1). Irrespective of the nomenclature, these infections are rapidly progressive, associated with gas in tissue, and manifest impressive tissue destruction, shock and frequently death. This review article will discuss the clinical characteristics of some of these infections, the important virulence factors of clostridial species and the genetic control of some of these factors, and will emphasize important new concepts in the pathogenesis of shock and tissue destruction in gas gangrene caused by Clostridium perfringens.
Chapter 13
Contribution of Shiga Toxin (Verotoxin) to the Pathogenesis of Shigella and Escherichia coli
Patty Tam and Clifford Lingwood
Abstract
Shigella, enteroinvasive E. coli and enterohemorrhagic E. coli are among the most common bacterial agents of diarrhea. Of these, Shigella dysenteriae 1 and enterohemorrhagic E. coli, are distinguished by the production of verotoxins. Though these bacteria have different mechanisms of infection, the end result of hemorrhagic colitis remains the same. In addition to hemorrhagic colitis, some patients will develop hemolytic uremic syndrome as a further complication to their infection. It is widely accepted that verotoxin is the major virulence factor responsible for development of the hemolytic uremic syndrome (HUS). In this chapter we offer a brief overview of the effects verotoxin on cells, intracellular traffic of verotoxins, clinical pathologies associated with HUS, animal models of HUS, as well as current efforts in development of HUS treatments. In particular, we focus on the impact of toxin/receptor interactions on pathogenesis and as a mechanism for treatment.
Chapter 14
Yersinia Ysc-Yop System: The Role in Pathogenesis
Indira Basu and Joan Miquel Balada-Llasat
Abstract
Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica are the three Yersinia spp that are pathogenic in humans and rodents. The virulence factors of Yersinia spp. encoded on the virulence plasmid include the structural, regulator and effector molecules of a type III secretion system, also called the Ysc-Yop system in Yersinia. The Ysc proteins encoded by this plasmid takes part in the formation of a needle-like apparatus extending from the bacterial cell in the host. At the tip of the needle, translocator proteins called "translocator Yops" in Yersinia, are thought to form a pore in host cell membranes through which bacterial effector molecules, called "effector Yops" in Yersinia, are transported into the host cell. Effector yops disrupt normal host cell functions. This enables Yersinia spp. to disarm normal host immune responses in a variety of ways including escaping internalisation by phagocytic cells, and inducing apoptosis in macrophages. Thus the arsenal of Ysc proteins and Yops, enocoded on the virulence plasmid, enables pathogenic Yersinia to survive, colonize and disseminate in the host by hijacking the host defense mechanism designed to protect against invading pathogens.
Chapter 15
Bacteriocins Associated with Cytotoxicity for Eukaryote Cells
Philip A. Wescombe, Nicholas C. K. Heng, Ralph W. Jack, and John R. Tagg
Abstract
Bacteriocins are defined as proteinaceous substances produced by bacteria that kill species or strains closely related to the producing organism. However, there is now evidence that some bacteriocins also exhibit toxicity toward eukaryotic cells. Moreover, some bacteriocins, while not themselves cytotoxic, have been shown to be associated with either bona fide toxins or with known virulence properties of their bacterial hosts. In this chapter, we will provide a brief overview of the bacteriocins, and then proceed with a more detailed discussion of bacteriocins or bacteriocin-like substances (i) that display direct cytotoxicity toward eukaryotic cells, (ii) which are associated with known toxins or virulence factors by phenotypic or genotypic linkage, and (iii) potentially act as signalling molecules to modulate cellular physiology in response to environmental stimuli. In addition, the relationship between bacteriocins and antimicrobial peptides of eukaryotic origin, as well as bacterial mechanisms to avoid killing by antimicrobial peptides, are considered. Finally, as bacteriocins are being touted for use in human or veterinary applications, the safety aspects of the use of such substances must be seriously considered.
Chapter 16
Mycotoxins and Human Disease
Ruth A. Etzel
Abstract
Mycotoxins are chemicals produced by fungi. The six major classes of mycotoxins with effects on human health are ergot alkaloids, aflatoxins, ochratoxins, zearalenone, fumonisins, and trichothecenes. Humans can be exposed to these toxic chemicals by ingestion, inhalation, or dermal exposure. Although the ergot alkaloids rarely contaminate food today, historically they caused ergotism, characterised by vasoconstriction, gangrene and limb loss. Aflatoxins, common contaminants of nuts and grains in tropical areas, are potent human carcinogens linked to hepatocellular carcinoma. Ochratoxins are associated with kidney diseases such as Balkan nephropathy and renal tumors. Zearalenone has estrogenic properties and may be associated with cervical cancer. Fumonisins are linked to oesophageal cancer, liver cancer, and neural tube defects. Trichothecenes are associated with Alimentary Toxic Aleukia, stachybotrytoxicosis, and acute pulmonary hemorrhage and sudden death in infants. Certain mycotoxins, such as aflatoxins and trichothecenes, have potential uses in bioterrorism.
Chapter 17
Host Cell Penetration and Trafficking of Protein Toxins
Kirsten Sandvig, Silje U. Lauvrak, and Bo van Deurs
Abstract
Bacterial and plant toxins with enzymatic activity and intracellular targets often use a common strategy to reach their targets. One moiety of the molecule (B) binds cell surface receptors, then the toxin is endocytosed, and subsequently the other part of the molecule, the enzymatically active part (A), is translocated to the cytosol from an intracellular compartment. These toxins seem to fall into two major groups; one that uses the low endosomal pH to obtain a conformational change allowing insertion into the membrane and subsequent translocation to the cytosol. The other group of toxins is transported retrogradely to the Golgi apparatus and all the way to the endoplasmic reticulum (ER) before translocation to the cytosol. In spite of structural similarities, the toxins exploit different pathways to enter from the cell surface and to move all the way to the ER. The internalization and intracellular pathways used by these toxins as well as their translocation to the cytosol will be described in this chapter. Furthermore, some medical applications will be discussed.
Chapter 18
Regulation of Bacterial Toxin Expression
Nathan Shankar and Michael S. Gilmore
Abstract
As few bacteria are obligate pathogens, toxin expression in most instances is a regulated process to insure that toxins are produced only in a context that is appropriate - or at least in a context that has been favored by natural selection. As our understanding of the mechanisms that control gene expression develops, with tremendous recent advances growing out of genome, microarray and proteome studies, it is also clear that regulation of gene expression usually results from a web of interactions. To review the subject 'regulation of toxin expression' it is necessary to seek an optimum between comprehensively covering all toxins, and depth of coverage. Toward that end, this review focuses on representative toxin expression systems, with the goal of illustrating the variety that exists in mechanisms of toxin regulation. This review therefore first discusses toxins for which primary mechanisms of regulation appear to be local, and finally discusses toxins subject to regulation by global regulatory elements. This distinction is likely to be proven artificial with time, however, as our knowledge is incomplete and undoubtedly biased by the order in which discoveries were made. That is, some toxins have been studied from the bottom up, others from the top down, and that undoubtedly skews our perspective. Nevertheless, it provides a logical framework in which to consider the subject.
Chapter 19
Modification and Improvement of Live Bacterial Vaccines by the Use of Bacterial Cytolysins
Guido Dietrich and Werner Goebel
Abstract
Cytolysins represent important virulence factors of Gram-positive and Gram-negative bacteria. While such cytolysins are usually the reason for morbidity and even mortality, vaccine researchers have turned them into tools for vaccine delivery. In particular, Haemolysin A of Escherichia coli and Listeriolysin of Listeria monocytogenes have found widespread application in vaccine research and are highly suitable for the elicitation of cell-mediated immunity. In this paper, we will review the use of the haemolysin A secretion system and Listeriolysin to modify and improve Live Bacterial Vaccines.
Chapter 20
Microbial Toxins as Potential Tools in Bioterrorism
Stuart C. Clarke
Abstract
Toxins provide microbes with a system of defense that is often detrimental to humans. The very versatility of toxins makes them potential tools of bioterrorism. Microbes are available to individuals with appropriate contacts and even many low grade bacterial pathogens produce toxins which can severely affect health. It must be remembered that terorrism does not always aim at killing but rather striking fear into peoples lives. Therefore, toxins such as ricin, botulinum, and cholera toxin, could be used which may not cause significant mortality, but would cause panic in the society and potentially high morbidity. The current threat of bioterrorism and the response measures which could be taken is important. In the global climate, the increased concern of bioterrorism is fully justified and, although an attack is of a low probability, an attack would be a high consequence event. Importantly, no state is fully prepared for a response and it is probable that no state ever could be. That is why biological agents are so attractive as weapons. This chapter illustrates the use of the microbial toxins described in other chapters as potential agents of bioterrorism. The impact of such bioterrorism is also discussed.
Current Books: