Pathogens, toxins, and lipid rafts

Summary The plasma membrane is not a uniform two-dimensional space but includes various types of specialized regions containing specific lipids and proteins. These include clathrin-coated pits and caveolae. The existence of other cholesterol- and glycosphingolipid-rich microdomains has also been proposed. The aim of this review is to illustrate that these latter domains, also called lipid rafts, may be the preferential interaction sites between a variety of toxins, bacteria, and viruses and the target cell. These pathogens and toxins have hijacked components that are preferentially found in rafts, such as glycosylphosphatidylinositol-anchored proteins, sphingomyelin, and cholesterol. These molecules not only allow binding of the pathogen or toxin to the proper target cell but also appear to potentiate the toxic action. We briefly review the structure and proposed functions of cholesterol- and glycosphingolipid-rich microdomains and then describe the toxins and pathogens that interact with them. When possible the advantage conferred by the interaction with microdomains will be discussed.Abbreviation GPI glycosylphosphatidylinositol     

Anthrax toxins

Bacillus anthracis, the etiological agent of anthrax, secretes three polypeptides that assemble into toxic complexes on the cell surfaces of the host it infects. One of these polypeptides, protective antigen (PA), binds to the integrin-like domains of ubiquitously expressed membrane proteins of mammalian cells. PA is then cleaved by membrane endoproteases of the furin family. Cleaved PA molecules assemble into heptamers, which can then associate with the two other secreted polypeptides: edema factor (EF) and/or lethal factor (LF). The heptamers of PA are relocalized to lipid rafts where they are quickly endocytosed and routed to an acidic compartment. The low pH triggers a conformational change in the heptamers, resulting in the formation of cation-specific channels and the translocation of EF/LF. EF is a calcium- and calmodulin-dependent adenylate cyclase that dramatically raises the intracellular concentration of cyclic adenosine monophosphate (cAMP). LF is a zinc-dependent endoprotease that cleaves the amino terminus of mitogen-activated protein kinase kinases (Meks). Cleaved Meks cannot bind to their substrates and have reduced kinase activity, resulting in alterations of the signaling pathways they govern. The structures of PA, PA heptamer, EF, and LF have been solved and much is now known about the molecular details of the intoxication mechanism. The in vivo action of the toxins, on the other hand, is still poorly understood and hotly debated. A better understanding of the toxins will help in the design of much-needed anti-toxin drugs and the development of new toxin-based medical applications.Abbreviations CMG2 Capillary morphogenesis protein 2 - DTA Diphtheria toxin A chain - EF Edema factor - EFn N-terminal fragment of EF - ETx Edema toxin - GR Glucocorticoid receptors - GSK3 Glycogen synthase kinase 3 - I domain Integrin-like domain - iNOS Inducible nitric oxide synthase - LF Lethal factor - LFn N-terminal fragment of LF - LTx Lethal toxin - MAPK Mitogen-activated protein kinase - Mek MAPK kinases - PA Protective antigen - PA₂₀ 20-kDa N-terminal fragment of PA - PA₆₃ 63-kDa C-terminal fragment of PA - TEM8 Tumor endothelial marker 8     

Nutritional PharmEcology: Doses, nutrients, toxins, and medicines

The synthesis of pharmacological techniques and concepts into ecology holds considerable promise for gaining new insights into old questions, uncovering new priorities for research and, ultimately, for consolidating a new sub-discipline within the ecological sciences-PharmEcology. We argue that this potential will best be realized if the boundaries of PharmEcology are drawn broadly to encompass not only toxins and medicines, but also nutrients. The hub of our argument is that PharmEcology shares with the established discipline of nutritional ecology an organismal focus, at the core of which is the notion of evolutionary function. From this functional viewpoint the dividing lines between chemicals traditionally considered as "toxins," "medicines," and "nutrients" are often thin, vague, heavily contingent and non-stationary, and thus provide a poor footing for an emerging sub-discipline. We build our argument around three points: nutrients and toxins are not so different, medicines and nutrients are not so different, and even in cases in which nutrients, medicines and toxins can be categorically distinguished, the biological actions of these compounds are heavily interdependent.     

Yersinia pseudotuberculosis toxins

The review of publications about protein toxins Y. pseudotuberculosis are presented. It includes the main data obtained by domestic and foreign investigators as well as the results of our own elaboration in the study of Y. pseudotuberculosis protein toxins. The guestions of isolation, purification, characterization of physico-chemical and biological properties, the mechanism action and role of toxins on pathogenesis of infection were discussed.     

Bacterial glycosyltransferase toxins

Mono‐glycosylation of host proteins is a common mechanism by which bacterial protein toxins manipulate cellular functions of eukaryotic target host cells. Prototypic for this group of glycosyltransferase toxins are Clostridium difficile toxins A and B, which modify guanine nucleotide‐binding proteins of the Rho family. However, toxin‐induced glycosylation is not restricted to the Clostridia. Various types of bacterial pathogens including Escherichia coli, Yersinia, Photorhabdus and Legionella species produce glycosyltransferase toxins. Recent studies discovered novel unexpected variations in host protein targets and amino acid acceptors of toxin‐catalysed glycosylation. These findings open new perspectives in toxin as well as in carbohydrate research.     

Microbial toxins, their functional role and phylogenetic validity

Microbially produced toxins, which appear to lack a role in microbial survival, may be antimicrobial compounds of significance to the producers. These toxin/antibiotics may act against cell metabolism shared by man or animals and other microorganisms. Protein toxin/antibiotics are produced by single species of bacteria. Those from fungi and algae are nonprotein secondary metabolites and several microorganisms may make the same or similar toxin/antibiotics.     

Thiol-dependent cytolytic bacterial toxins: streptolysin O and prominent toxins

Streptolysin O is the prototype of fifteen bacterial cytolytic protein toxins elaborated by gram-positive bacteria of species Streptococcus, Clostridium, Bacillus and Listeria. These toxins share a number of common properties: they are antigenically related as shown by cross-neutralization and immunoprecipitation; their cytolytic and other reducing agents; these toxins are inactivated by cholesterol and certain related sterols. This group of oxygen-labile cytolytic toxins has been named sulfyhdryl-activated toxins or thiol-activated cytolysins. The mechanism of action of these toxins is very likely identical or at least closely similar.     

Bacteria, molds, and toxins in water-damaged building materials.

Microbial toxins and eukaryotic cell toxicity from indoor building materials heavily colonized by fungi and bacteria were analyzed. The dominant colonizers at water-damaged sites of the building were Stachybotrys chartarum (10(3) to 10(5) visible conidia cm-2), Penicillium and Aspergillus species (10(4) CFU mg-1), gram-negative bacteria (10(4) CFU mg-1), and mycobacteria (10(3) CFU mg-1). The mycobacterial isolates were most similar to M. komossense, with 98% similarity of the complete 16S rDNA sequence. Limulus assay of water extracts prepared from a water-damaged gypsum liner revealed high contents of gram-negative endotoxin (17 ng mg-1 of E. coli lipopolysaccharide equivalents) and beta-D-glucan (210 ng mg-1 of curdlan equivalents). High-performance liquid chromatography analysis of the methanol extracts showed that the water-damaged gypsum liner also contained satratoxin (17 ng mg-1). This methanol-extracted substance was 200 times more toxic to rabbit skin and fetus feline lung cells than extract of gypsum liner sampled from a non-water-damaged site. The same extract contained toxin(s) that paralyzed the motility of boar spermatozoa at extremely low concentrations; the 50% effective concentration was 0.3 microgram of dry solids per ml. This toxicity was not explainable by the amount of bacterial endotoxin, beta-D-glucan, or satratoxin present in the same extract. The novel in vitro toxicity test that utilized boar spermatozoa as described in this article is convenient to perform and reproducible and was a useful tool for detecting toxins of microbial origin toward eukaryotic cells not detectable in building materials by the other methods.     

Cytolethal distending toxins

The cytolethal distending toxins (CDTs) constitute the most recently discovered family of bacterial protein toxins. CDTs are unique among bacterial toxins as they have the ability to induce DNA double strand breaks (DSBs) in both proliferating and nonproliferating cells, thereby causing irreversible cell cycle arrest or death of the target cells. CDTs are encoded by three linked genes (cdtA, cdtB and cdtC) which have been identified among a variety of Gram-negative pathogenic bacteria. All three of these gene products are required to constitute the fully active holotoxin, and this is in agreement with the recently determined crystal structure of CDT. The CdtB component has functional homology with mammalian deoxyribonuclease I (DNase I). Mutation of the conserved sites necessary for this catalytic activity prevents the induction of DSBs as well as all subsequent intoxication responses of target cells. CDT is endocytosed via clathrin-coated pits and requires an intact Golgi complex to exert the cytotoxic activity. Several issues remain to be elucidated regarding CDT biology, such as the detailed function(s) of the CdtA and CdtC subunits, the identity of the cell surface receptor(s) for CDT, the final steps in the cellular internalization pathway, and a molecular understanding of how CDT interacts with DNA. Moreover, the role of CDTs in the pathogenesis of diseases still remains unclear.     

藻毒素的类型、危害和防治

介绍了赤潮藻毒素：麻痹性贝毒、短裸甲藻毒素、溶血性毒素、细胞性毒素和氨毒：水华藻毒素：肝毒素、神经毒素、脂多糖内毒素的化学结构、性质和毒害机理等,以及藻毒素的防治方法。     

Three toxins,two receptors,one mechanism: Mode of action of Cry1A toxins from Bacillus thuringiensis in Heliothis virescens

Insecticidal crystal (Cry) proteins from Bacillus thuringiensis (Bt) are highly active against Lepidoptera. However, field-evolved resistance to Bt toxins is on the rise. The 12-cadherin domain protein HevCaLP and the ABC transporter HevABCC2 are both genetically linked to Cry toxin resistance in Heliothis virescens. We investigated their interaction using stably expressing non-lytic clonal Sf9 cell lines expressing either protein or both together. Untransfected Sf9 cells are innately sensitive to Cry1Ca toxin, but not to Cry1A toxins; and quantitative PCR revealed negligible expression of genes involved in Cry1A toxicity such as cadherin, ABCC2, alkaline phosphatase (ALP) and aminopeptidase N (APN). Cry1Aa, Cry1Ab or Cry1Ac caused swelling of Sf9 cells expressing HevABCC2, and caused faster swelling, lysis and up to 86% mortality in cells expressing both proteins. No such effect was observed in control Sf9 cells or in cells expressing only HevCaLP. The results of a mixing experiment demonstrated that both proteins need to be expressed within the same cell for high cytotoxicity, and suggest a novel role for HevCaLP. Binding assays showed that the toxin-receptor interaction is specific. Our findings confirm that HevABCC2 is the central target in Cry1A toxin mode of action, and that HevCaLP plays a supporting role in increasing Cry1A toxicity.