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MIC3032: Toxins - Coggle Diagram

- - - - Clostridium perfringens
        
        Infection of wounds or tissue without blood supply (anaerobic)
        
        Pore-forming toxins
        
        Small pore-forming toxins (SPFT): α toxin OF S. AUREUS
        
        Large pore-forming toxins (LPFT): perfringolysin O
        
        Overall process:
        
        Multiple monomers join to form an oligomer at cell surface
        
        Oligomer punctures cell membrane
        
        Water or solutes leak in/out
        
        Lysis of host cell
        
        SPFT:
        
        7 monomers polymerise to from oligomer
        
        Beta-barrel domain forms
        
        Influx of small solutes into host cell via pore
        
        LPFT:
        
        30-50 monomers form the oligomer
        
        Permeability increases for both solutes and macromolecules
        
        Perfringolysin O:
        
        It is a Cholesterol-dependent cytolysin (CDC) - i.e. can only bind to cells with cholesterol on them)
        
        Not essential for gas gangrene but prevents leukocytes from coming to address the infection
        
        Toxin comprised of four domains (D1-D4)
        
        D3 alters from α-helicies to B-sheets when D4 binds to cholesterol
        
        B-sheets unfold and pierce membrane
        
        Phospholipases (α toxin)
        
        α toxin cleaves phospholipid heads (hydrolyses them)
        
        MacFarlane and Knight (1941)
        
        To determine mode of action of α toxin due to war wounds
        
        After addition of α toxin to egg yolk, found N and P were cleaved off of lipids, as shown by the lipids now being soluble
        
        Also showed this hydrolysis was inhibited if antitoxin to α toxin was included.
        
        Williamson and Titball (1993)
        
        α toxin possesses N and C terminal domain
        
        Found that using C- domain as an immunogen meant mice survived the toxin
        
        Thus C-terminal domain had great potential as a vaccine candidate
        
        Awad et al. (1995)
        
        Proved essential role of α toxin as mutated version of the toxin made perfringens avirulent
        
        Mutation: knocked out plc gene and replaced with erythromycin reisstance via homologous recombination. W/out plc gene, the chromosome was avirulent.
        
        Showed virulence was restored when plc gene was put back into mutant (demonstrates Koch's postulates)
        
        Naylor et al. (1998)
        
        Determined crystal structure of α toxin
        
        N terminal domain: α-helical
        
        C- terminal domain: Beta sheets in sandwich structure
        
        Bacillus cereus with similar structure but no C domain is non-toxic
        
        Showed that C domain can bind to eukaryotic lipids, thus can interfere with mammalian cell membrane
      - Superantigens:
        
        Staphylococcus aureus and Streptococcus pyogenes
        
        TSST-1 of S. aureus
        
        SpeB of S. pyogenes + SAg crosslinks MHC class II molecules with T-cell receptors, causing non-specific activation of T cells
        
        Major polyclonal expansion
        --> massive release of cytokines
        --> toxic shock
- - - - T4SS: Helicobacter pylori cag
        
        Highly virulent strains encode PAI encoding cytotoxin-associated gene (cag)
        
        CagA and other factors injected into host cells. Perturbs intracellular signalling
        
        CagA dependent and independent phosphorylation
        
        T6SS:
        
        Similar to bacteriophage delivery system but going outside instead of inside.
        
        T6SS built inside of bacterium
        
        Then tail sheath contracts
        
        Phosphorylation of ATP drives effector molecules up the tail sheath equivalent
        
        Tip of T6SS falls apart so effectors can be released
        
        Effectors injected either covalently bound to a core component of the tip, or non-covalently bound (i.e. without the tip)
        
        T6SS often used to kill bacteiral competition in the gut
- - - - Diphtheria toxin (DT)
        
        Encoded by Corynebacterium diphtheriae. Virulent strains carry a lysogenic bacteriophage that encodes the toxin!
        
        ABCDEF: AB toxin is Cleaved for Diphtheria; EF2
        
        Mode of action:
        1. B component binds to heparin receptor on host cell
        2. This induces endocytosis
        3. Furin cleaves A from B at amino acid 193
        4. pH of vesicle lowered via influx of H+ as per normal
        5. Lowered pH causes conformational change in DT
        6. Transmembrane domain of B component forms pore in vesicle
        7. A component is translocated into cytosol
        8. A binds to Elongation Factor 2 (EF2), inactivating it (EF2 is essential for protein synthesis inside the cell
        NB: A doesn't remain bound to EF2. It irreversibly inactivates it, then goes on to do the same thing to other EF2 molecules. Does this to EVERY EF2 molecule in the host cell, completely stopping protein synthesis, resulting in cell death.
        NB2: DT crystal structure: 3 domains: Catalytic domain (A component); Transmembrane domain and Receptor-binding domain (component B).
        
        Receptor binding domain: binds to heparin receptor
        
        Transmembrane domain: forms α-helical pore in endocytic vesicle to move A component into cytosol
        
        Catalytic domain: contains ADP ribosyl transferase that replaces Nicotinamide (top part of NAD molecule) with EF2, rendering EF2 irreversibly inactivated.
        
        NB3: DT only affects EF2 because it is the only protein that has a diphthamide residue, which can be modified by DT
      - Tetanus Neurotoxin (TeNT)
        Singular type of toxin that occurs in deep wound infections
        Respiratory failure due to permanent contraction of muscles (esp. diaphragm)
        TeNT acts at the spinal cord, blocking the relaxation pathway
        Mode of action:
        1. Bind to cell surface receptor and enter neuron via endocytosis
        2. TeNT stays within the endocytic vesicle and migrates retrogradedly through the neuron to the spinal cord
        3. Exposure to low pH within vesicle results in change in conformation of TeNT; disulphide bond between L chain and N terminal H chain cleaved.
        4. H chains form ion channel for L chain to translocate through vesicle membrane into cytoplasm of host neuron.
      - Botulinum Neurotoxin
        
        7 serologically distinct toxins
        
        All cleave SNAREs but at different sites
        
        Acts at neuromuscular junctions by blocking acetycholine release; muscles can't contract; flaccid paralysis, can't breathe
        
        Mode of action:
        
        Ingested via food
        1. Bind to cell surface receptor, internalisation into endocytic vesicle
        2. Acidification of vesicle results in conformational change; disulphide bond broken; L chain free to migrate thru to cytoplasm (see TeNT explanation)
        3. Proteolysis of SNARE proteins;
        
        Synaptic vesicle needs to fuse with synaptic membrane to release acetylcholine to contract adjacent muscle. To do this, v-SNARE and t-SNARE proteins need to form a complex on the target membrane, but BoNT breaks down SNARE proteins, preventing this entire process.
        
        Crystal structure:
        
        Catalytic domain containing Zinc
        
        Translocation domain (N terminal of Heavy chain; α helicies)
        
        Binding domains (C terminal of Heavy chain; beta-sheets+
      - Commonalities & differences b/w TeNT and BoNT:
        
        Both zinc endopeptidases; means they cleave in the middle of the target proteins
        
        They bind different receptors, thus end up in different types of vacuoles, and act at different sites in the body, meaning that despite being almost identical toxins, they have opposite effects on the body
        + All 8 types of the toxins cleave proteins essential to bringing the synaptic vesicle close to the membrane at the end of the axon so neurotransmitters can't be released, rendering the body in one form of paralysis.
    - - Cholera Toxin (CT)
        
        A plus 5B = die in 6 hours!
        
        ctx = cholera toxin
        
        Food borne infection by Vibrio cholerae; causes fluid accumulation in GIT and diarrhoea - death
        
        encoded by ctxA and ctxB located on lysogenic integrated bacteriophage
        
        Toxin is assembled in periplasm. A subunit is nicked extracellularly into A1 and A2
        
        Crystal structure:
        
        A1 and A2 domain
        
        A2 domain dips into the 5 B domains; this indicates it will be used to form a pore so A1 can leave the vesicle and get into the cell's cytoplasm
        
        Mode of action:
        
        B subunit binds to Gm1 ganglioside receptor on intestinal epithelial cell
        
        Toxin is endocytosed
        
        Vesicle travels retrogradely through Golgi to ER
        
        Disulphide bond b/w A1 and A2 breaks; A1 thru to cytoplasm
        
        A1 of CT binds NAD, hydrolysing it. Gs forms complex with ADPR, but this can't be hydrolysed. Without GTPase, the cAMP synthesis pathway is locked 'on'.
        
        Excess production of cAMP --> fluid secretion into gut --> diarrhoea
    - - Typhoid Toxin:
        
        Systemic, potentially fatal infection caused by Salmonella enterica serovar Typhi.
        
        Contains TT which has 2 A components and 5B
        
        A components:
        PltA - homologue of A subunit of pertussis toxin
        CdtB - homologue of A subunit of cytolethal distending toxin
        
        B component - PltB - also homologue of pentameric binding subunit of pertussis.
        
        Crystal structure:
        
        Top-down: CdtB, PltA (covalently linked via disulphide bond), PltB (5B components)
        
        CdtB: hydrolyses DNA
        
        PltA: ADP ribosyl-transferase
        
        PltB: binding subunit
        
        Role of Typhoid Toxin (TT):
        
        Covalent linkage of CdtB & PltA ensures simultaneuous delivery to surface of host cell
        
        CdtB appears to be ky active componenet
        
        PltA knockout still results in significant disease
        
        CtdB knockout = no weight/water loss (i.e. disappearance of CtdB means TT is avirulent)
        
        BUT PltA still essential as otherwise CtdB can't get into cell. PltA is just not essential for enzymatic activity.