Specifics of Endotoxins and Endotoxemia - Essay Example

RUNNING HEAD: BACTERIAL TOXINS Bacterial Toxins [Name of the Writer] [Name of the Institution] Bacterial Toxins Introduction A hundred years ago Pfeiffer coined the term "endotoxin" to emphasise the fact that this substance was not, like exotoxins, excreted by bacterial cells but released only after bacteriolysis. Ingress and presence of endotoxin in human blood are, almost without exception, called endotoxaemia rather than endotoxinaemia. (Ingmundson et al., 2007 p. 365-369). The distinction between these two terms is at first glance too trivial to be worthy of any special comment. Nevertheless, the abbreviated form may change the sense, leading to a distorted clinical view that the condition is primarily harmful: clinicians unfamiliar with the origin of the term endotoxin tend to split the word incorrectly into misleading etymological roots "endo" and "toxaemia". Thus endotoxaemia has come to imply a serious morbid state associated with a hazardous and acute course, much like the related septicaemia and bacteraemia. It is true that circulating endotoxin plays a fatal part in the terminal scene of adult respiratory distress syndrome; it is likewise true that endotoxin in blood is evidence of the gastrointestinal disorders that are the leading cause of death in horses of all ages; but it is equally true that the sole presence of endotoxin in the blood may be innocuous. (Ingmundson et al., 2007 p. 365-369). The bacterial toxins are a main cause of diseases as they are accountable for the majority of indications and lesion throughout infection (Von Pawel-Rammingen et al., 2000 p. 737-748). They can be cateroised into two groups; (i) exotoxins, a soluble material produced by microbes in the host tissues, and (ii) endotoxins, normally living within the cell wall and discharged into host tissues upon cell expiration. The exotoxins act at a space from the place of infection and can disperse through the organism. The illumination of the cellular mechanism of strokes of the bacterial exotoxins remains a compound problem, but they emerge to share a common method of action such as (i) binding to definite receptors on the plasma membranes of the receptive cells, (ii) pore-formation, (iii) internalization or translocation crosswise the membrane wall and (iv) direct discharge (Prehna et al., 2006 p. 869-880) The exotoxins have an unusual similarity for particular tissues and may be separated into three groups on the origin of the site influenced: (i) neurotoxins work on nervous system, (ii) cytotoxins on general tissue, and (iii) neurotoxins on intestinal mucosa. (Kenny, 2002 p. 1095-1107) The neurotoxins identify specific receptors on the unmyelinated regions of the presynaptic membrane and restrain acetylcholine discharge. The enterotoxins act by starting adenylate cyclase or guanylate cyclase. Some staphylococcal enterotoxins source the food poisoning disease. The cytotoxins act on common tissues; for example, vacuolating cytotoxin is one of the mainly vital virulence factors formed by Helicobactor pylori, a contributory agent of strict gastric illnesses like ulcers and cancers. (Prehna et al., 2006 p. 869-880) The bacterial toxins have a broad range of use today. For e.g. the cholera toxin (CT) and the heat-labile toxin from E. coli (LT) have been utilized as strong mucosal adjuvant in new form; bacterial pore-forming hemolysins, like listeriolysin O, have the possible for use in cytosolic medicine delivery structure. (Provoda and Lee, 2000) Bacterial Cell Wall Components and Septic Shock The exotoxins produced by some bacteria (such as exotoxin A produced by Pseudomonas aeruginosa, or the toxic shock syndrome toxin produced by some Staphylococcus aureus) can initiate septic shock, but it is the bacteria themselves, in particular their cell wall components, that are primarily responsible for the development of septic shock. These components are potent activators of numerous humoral pathways, and they also activate macrophages and other cell types involved in the inflammatory processes. The prime initiator of gram-negative bacterial septic shock is endotoxin, an LPS component of the bacterial outer membrane. Circulating endotoxin in the blood appears to be a predictor of poor outcome in some clinical settings such as meningococcemia, (Ingmundson et al., 2007 p. 365-369) but the levels of endotoxin required to trigger the cascade of events in septic shock may vary greatly. Indeed, it has been observed that bacterial products such as cell wall components, including endotoxin itself and staphylococcal and streptococcal toxins may greatly increase host sensitivity to endotoxin, thus rendering toxic otherwise harmless levels of endotoxin. (Ingmundson et al., 2007 p. 365-369) For these reasons measurement of endotoxin has not yet become standard clinical practice. The outermost part of the endotoxin molecule consists of a series of oligosaccharides that are structurally and antigenically diverse and are responsible for the O serotype of gram-negative bacteria. Internal to the O side chains are the core oligosaccharides and these are structurally rather similar in common gram-negative bacteria. To the core oligosaccharide is bound a lipid part, lipid A. The structure of lipid A is highly conserved, and it is lipid A that is responsible for most of the toxicity of endotoxin. Some natural lipid As and synthetic lipid A analogues with different sugar and acyl residues are less or not at all endotoxically active in vitro and in vivo. This observation has led to development of lipid A analogues that can block the toxic effects of endotoxin or act as endotoxin antagonists. (Prehna et al., 2006 p. 869-880) Activation Of Humoral Pathways The alternative complement pathway can be activated experimentally by LPS and gram-positive cell-wall components. The classical pathway is mainly activated by complexes of cell-wall components and antibodies. The anaphylatoxins C3a and C5a that result from activation of these pathways are responsible for a series of inflammatory events that have been implicated in the pathophysiology of septic shock. These events include vasodilatation and increased vascular permeability, which may be partly responsible for hemodynamic changes, platelet aggregation, and aggregation and activation of granulocytes, processes that have been implicated in the pathogenesis of the adult respiratory distress syndrome. (Katoh, 2003 p. 461-464) An important effect of the stimulation of complement is activation of neutrophils. Activated neutrophils become adherent to each other and to vascular endothelium. The subsequent release of arachidonic acid derivatives, cytotoxic products of molecular oxygen and lysosomal enzymes, produces additional local vasoactive effects on the microvasculature and endothelial cell cytotoxicity, resulting in capillary leakage. Increased concentrations of activated complement have been associated with fatal outcome in septic shock of gram-positive and gram-negative origin. (Katoh, 2003 p. 461-464) It is well known that the derivatives of arachidonic acid metabolism that cause vasodilatation, platelet aggregation, and neutrophils activation may contribute to the pathogenesis of septic shock. Such derivatives are found in increased concentrations after experimental endotoxin challenge and in humans with septic shock. The role of inhibitors/antagonists of the pathways of arachidonic acid metabolism in the prevention and treatment of septic shock is under investigation. Activated neutrophils, a key element in the inflammatory response, probably play an important part in the pathogenesis of septic shock in that they contribute to vascular and tissue injuries. Activated leucocytes adhere to each other, to endothelial cells, and to tissues through interactions of receptors (on endothelial cells) and ligands (on inflammatory cells) that are mediated by specific adhesion molecules. The adhesion process is essential for most functions of leucocytes, such as chemotaxis, phagocytosis, and cytotoxicity, (Nagai et al., 2005 p.2998-3011) and blocking of the adhesion process by monoclonal antibodies prevents tissue injury and improves survival in animal models of septic shock. Factor XII (Hageman factor) of the coagulation cascade has long been known to have a central role in the pathogenesis of septic shock. It is activated by peptidoglycan residues and teichoic acid from the cell wall of gram-positive organisms (S aureus, streptococci, pneumococci) as efficiently as by LPS and lipid A from gram-negative bacilli. (Nagai et al., 2005 p.2998-3011) Activated factor XII triggers both the intrinsic coagulation pathway, through activation of factor XI, and endothelial cells and macrophages to produce tissue factor, which in turn activates the extrinsic coagulation pathway. The activation of these pathways may lead to the consumption of coagulation factors and to disseminated intravascular coagulation (DIC). Tissue factor produced upon stimulation of macrophages and endothelial cells by LPS has been shown to have a major role in inducing DIC, since anti-tissue factor prevented LPS-induced DIC in rabbits. (Jaffe, 2005 p. 247-269) TNF is also an activator of the extrinsic pathway of coagulation, and therefore may contribute to the perturbations of coagulation in septic shock.(Jaffe, 2005 p. 247-269) In addition to being induced by activation of complement and the arachidonic acid cascade, hypotension in septic shock may also result from LPS-activated factor XII that converts prekallikrein into kallikrein. Kallikrein in turn cleaves high-molecular-weight kininogen to release bradykinin, a potent hypertensive agent. (Kenny, 2002 p. 1095-1107) Hypotension also results from release of another potent vasodilator, endothelium-derived relaxing factor, recently identified as nitric oxide." Generation of nitric oxide occurs in macrophages and in cultured endothelial cells. While it appears that LPS-induced nitric oxide release by macrophages takes several hours, endothelial cells recta within minutes, a phenomenon that might contribute to the rapid fall in blood pressure associated with endotoxic shock. Endogenous opiod peptides may play a part in septic shock, because opioid peptide secretion can be induced by endotoxin, and the administration of an opioid antagonist, naloxone, and reverses endotoxin-induced hypotension under some experimental conditions. However, the importance of endorphins in the pathophysiology of shock is still incompletely understood. (Prehna et al., 2006 p. 869-880) Small G-proteins: a template for bacterial virulence mechanisms Because small G-protein are involved in almost every aspect of cell biology, they are common targets of microbial virulence factors. The unique structural characteristics and extensive regulatory mechanisms of small G-proteins have served as templates for the design of potent pathogenic toxins and effector proteins. For example, toxins and effectors selectively activate or inhibit the guanine nucleotide molecular switch to promote pathogenesis. In addition, several studies suggest that bacteria may actually express functional mimics of small G-proteins. As suggested, bacteria have evolved ingenious mechanisms to activate small G-protein signaling cascades. The pathogenic Escherichia coli toxin, cytotoxic necrotizing factor (CNF), directly activates Rho proteins through enzymatic deamidation of Glutamine 63, a key residue involved in GTP hydrolysis (Kenny, 2002 p. 1095-1107). Through this mechanism, CNF converts inactive Rho to a constitutively GTP-bound form resulting in unregulated act in cytoskeletal changes. In addition, several bacterial type III and Type IV ‘effector’ proteins can function as GEFs to stimulate guanine nucleotide exchange. SopE and SopE2, two Type III effectors from Salmonella spp., possess Rac1 and Cdc42 GEF activity to facilitate Salmonella invasion into non-phagocytic cells (Von Pawel-Rammingen et al., 2000 p. 737-748). Likewise, Legionella pneumophila RalF, a Type IV effector with a Sec7-like GEF domain, functions as an ARF GEF to ensure Legionella’s intracellular survival in host phagosomes (Nagai et al., 2002 p. 679–682). In addition, Legionella DrrA/SidM is a combined GEF/GDF (GDI-displacement factor) that activates Rab1 during phagosome maturation (Ingmundson et al., 2007 p. 365-369). Besides activating small G-proteins, several bacterial toxins and effector proteins inhibit GTPase signaling. The Clostridium botulinum C3-like exoenzymes comprise a family of bacterial ADP-ribosyltransferases, which selectively modify Rho at Asparagines 41 (). This modification blocks GTP binding, thus irreversibly inhibits Rho proteins. In addition to the C3 toxins, the Type III effector proteins YopE, SptP, LepB and ExoS (from Yersinia spp., Salmonella spp., Legionella and Pseudomonas aeruginosa respectively), functionally mimic GAP proteins by stimulating intrinsic GTPase activity (Von Pawel-Rammingen et al., 2000 p. 737-748; Ingmundson et al., 2007 p. 365-369). This biochemical action inactivates small G-proteins. Finally, several Type III effectors employ unique mechanisms to displace GTPase from the plasma membrane. Yersinia YopT irreversibly blocks small G-protein signaling by cleaving the C-terminal lipids of Rho, Rac and Cdc42, leading to membrane detachment and inactivation of the GTPase (Shao et al., 2002 p. 575-588). Another Yersinia effector, YpkA (YopO) structurally mimics GDIs and functionally sequesters lipid-modified Rho away from cellular membranes (Prehna et al., 2006 p. 869-880). Diphtheria Toxin (DT) DT is the primary virulence factor of Corynebacterium diphtheriae, the etiologic agent of clinical diphtheria. (Shao et al., 2002 p. 575-588) This bacterium infects mucous membranes, particularly in the nose and throat, but it can also infect the eyes, the genital mucosa, and the skin. If it gains access to the blood stream it is distributed systemically and it may damage the heart, the kidneys, and the CNS [88]. DT is a 535 aa protein of approximately 58, 300 kDa composed of 3 domains. The Nterminal catalytic domain C (also called fragment A of 201 aa), a transmembrane (TM) domain T (176 aa), undergoing a conformational change at low pH responsible for the translocation across endosomal membranes of the fragment A to the cytosol, and a R-binding domain “R” (149 aa) which binds to a heparin-binding epidermal growth factor (EGF)-like precursors on eukaryotic cell surface. (Kenny, 2002 p. 1095-1107) The toxin is internalized by classical R-mediated endocytosis, then it is cleared by the endoprotease furin in the 14 aa protease-sensitive loop localized between the catalytic and the TM domains. (Shao et al., 2002 p. 575-588) The following acidification of the endosome allows the insertion of the TM domain of the toxin into the vesicle membrane and the subsequent delivery of the catalytic domain to the cytosol. The fragment A is an enzyme that catalyses in the cytosolic compartment the NAD+-dependent ADP-ribosylation of EF-2 resulting in the inhibition of protein synthesis and consequent cell death. Conclusion Natural toxins display a heterogeneous array of functions encompassing the ability to interrupt or hyper stimulate many essential pathways of eukaryotic cells. Although these molecules are generally toxic for the host (thus justifying their common definition) many activities of these molecules can be exploited for medical application. Since the development of enigmatically inactive toxins has been formally proven, many progresses have been made in the field of vaccine development, either directly against the toxin or its related pathogen or as adjuvant for induction of specific CTL against heterogonous pathogens. Therefore, enigmatically inactive toxins retaining their adjuvanticity or ability to induce CTL responses could be tested in the effort of developing anti-HIV vaccines. Of note, the B-oligomer or the enigmatically inactive mutant of PTX have been proven to own both adjuvant as well as anti-HIV activities, two features that would make them suitable for testing in HIV infected individuals, also as a potential microbicide acting by preventing HIV infection and favoring a local state of immunity. Since PTX derived molecules have been shown to affect cellular rather than viral life steps viral mutations are unlikely to occur. References Ingmundson, A., Delprato, A., Lambright, D.G., and Roy, C.R. (2007) Legionella pneumophila proteins that regulate Rab1 membrane cycling. Nature 450: 365–369. Jaffe, A.B., and Hall, A. (2005) Rho GTPase: biochemistry and biology. Annu Rev Cell Dev Biol 21: 247–269. Katoh, H., and Negishi, M. (2003) RhoG activates Rac1 by direct interaction with the Dock180-binding protein Elmo. Nature 424: 461–464. 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