Neurotoxins and other toxins Flashcards
what causes Tetanus?
Clostridium tetani, Gram positive, anaerobic
how does Tetanus cause death?
• Spores enter wounds, germinate at low oxygen tension
• Toxin synthesised at approach to stationary phase
i.e. toxico-infection
• Death from exhaustion, respiratory failure
what are the Clinical symptoms of tetanus?
muscular spasms, fluctuation in blood pressure, irregular heartbeat, sweating dehydration
describe the Tetanus toxin
• Synthesised as a 150-kDa pro-toxin, protease cleaved to give:
- 50-kDa light chain
- 100-kDa heavy chain - disulphide bond
• Heavy chain mediates binding to GT1 gangliosides on peripheral nerve endings
• Conformational changes allow internalisation into nerve cells
• Retrograde axonal transport
whats the Action of tetanus toxin
- Normally signals across synapses are controlled by excitatory and inhibitory neurotransmitters
- Tetanustoxin blocks release of inhibitory neurotransmitters (e.g. glycine, g-amino-butyric acid)
- Continuous stimulation - SPASTIC PARALYSIS
what causes Botulism
Clostridium botulinum, Gram positive, anaerobic, spore forming
how does botulism cause infection?
• Spores present in food, germinate in anaerobic conditions
• Toxin released into food
• If not destroyed by heating, toxin is ingested, absorbed in gut, enters bloodstream
i.e. intoxication
what are the symptoms of botulism
vomiting, headaches, slurred speech, generalised flaccid paralysis, progressive weakness of muscles, impaired organ functions, death
describe the movement of the toxin
- PROGENITOR TOXIN- ingested, passes through stomach,
subunits dissociate in small intestine - DERIVATIVE TOXIN- proteolytically cleaved in small intestine
- ACTIVE TOXIN- enters blood, targets peripheral neurones and enters synaptic vesicles
what happens after the botulism toxin is activated?
- Toxin binds ganglioside receptors on neurone surface via C-terminus of large subunit
• pH changes at cell surface enhance toxin hydrophobicity, penetrates membrane
• N-terminus of large subunit inserts into membrane of synaptic vesicle, forms pores
• pH changes disrupt disulphide bond
• small (toxic) subunit passes into synaptic vesicle
describe the Action of botulinum toxin?
• Normally stimuli cause release of stimulatory neurotransmitter acetylcholine (A)
• Acetylcholine crosses synaptic space,
interacts withreceptors, trans-stimulus to neurones or to muscle
• Botulinum toxin prevents release of acetylcholine
• Inhibits stimulatory activity - FLACCID PARALYSIS
what is the Botulinum toxin used for?
• Used to treat disorders that involve muscle hyperactivity
- e.g. laryngeal dystonia, tremors, facial tics
- induces paralysis of target hyperactive muscle
• Cosmetic use - reduce deep wrinkles caused by contraction of facial muscles (Botox)
what are the similarities between Tetanus and botulinum toxins?
Both are ‘zinc endopeptidases’ and cleave synaptobrevins (vesicle- associated membrane proteins, VAMPS), inhibiting release of neurotransmitters
why are Tetanus and botulinum toxins Effects different?
- tetanus toxin acts at nerve junctions
- botulinum toxin acts at junctions between nerves and muscles
name an enzyme toxin
• Clostridium perfringens a-toxin - phospholipase C
• Cleaves phospholipids at phosphodisester linkage - phosphatidylcholine to phosphorylcholine + a diglyceride
- sphingomyelin to phosphorylcholine + ceramide
• Causes localised cell and tissue destruction gas gangrene
• Bacteria enter bloodstream, release more toxin (toxaemia)
• Only treatment is radical surgery
explain the action of Hyaluronidase enzyme toxin?
- produced by Group A streptococci, Bacteroides gingivalis, staphylococci
• Catalyses breakdown of hyaluronic acid, a component of connective tissue
• Sub-epidermal injection of cell free hyaluronidase causes a diffuse area of tissue damage
• Called ‘spreading factor’ – allows pathogens greater access to colonise tissues
what is the enzyme toxin of Pseudomonas aeruginosa?
- causes lung infections in immunocompromised patients, particular problem in cystic fibrosis
• Produces elastase, a protease specific for elastin, major component of connective tissue - disrupts ciliary function of respiratory epithelium and hence clearance of mucus/bacteria
whats the enzyme toxin associated with Streptococcus pneumoniae
- produces proteases specific for secretory IgA
• Degrade antibodies, prevent recognition by macrophages, so reducing phagocytosis
what are the 4 types of Membrane damaging toxins
- Non-enzymatic proteins cause lysis of erythrocytes and other cell types
- Immunologically cross-reactive, similar structures
- Toxins bind to cholesterol in target cell membranes,
insert as rings of oligomers forming pores - Efflux of small molecules, breakdown of membrane electrical potential
name 2 Non-enzymatic proteins cause lysis of erythrocytes
- pneumolysin of Streptococcus pneumonia
- streptolysin of Streptococcus pyogenes
describe the action of a-hemolysin (HlyA) of E. coli
- One of the first pore-forming toxins described
- Synthesised as pro-toxin acylated (i.e. attachment of fatty acids) and secreted (107-kDa)
- Inserts into eukaryotic membranes of erythrocytes, leukocytes, lymphocytes, epithelial and endothelial cells
- Small amounts induce cytokine production and inflammatory response
- Large amounts cause cell lysis
what happens at Low HlyA
Interaction with host proteins
e.g. GTPase RhoA
Changes Ca2+ levels
Causes cytokine induction (IL-1, TNF)
what happens at High HlyA
Pore formation
Alters osmotic balance Causes cell lysis
what are the Functions of HlyA
+ In UTI isolates work in combination with cytotoxic necrotizing factor 1 to cause sloughing of uroepithlium, bladder hemorrhage and inflammation
+ May enhance spread by disrupting epithelial cells in gut lining, providing nutrients and inactivating immune effector cells
what is Streptococcus pyogenes
- Human pathogen
- Mild diseases - pharyngitis and impetigo
- Severe, life-threatening diseases - necrotizing fasciitis
- Post-infection sequelae - glomerulonephritis and rheumatic fever
- Rheumatic fever is often associated with heart disease and is a hypersensitive reaction mediated by cross-reactive antibodies
what are S. pyogenes virulence factors
• M-protein – fibronectin-binding protein for
adhesion and inhibition of phagocytosis
• Hyaluronic acid capsule – inhibits phagocytosis
• Streptokinase, steptodornase, hyaluronidase and streptolysins – invasion, tissue damage and spread
• Pyrogenic toxin – causes scarlet fever rash and toxic shock
what is the role of Streptolysin O and SPN in Virulence
- SLO induces pro-inflammatory cytokines (IL1b, IL6 and IL8)
- the absence of SPN helps to resolve the infection.
- double-mutant lacks mitigating effect is more virulent.
- In wt, the combination of both genes results in strong pathogenic effect.
summarise Neurotoxins and other toxins
- Neurotoxins cause tetanus and botulisum, two component molecules, inhibit release of neurological signalling molecules
- Enzymatic toxins disrupt host tissues facilitating spread or inhibiting clearance
- Membrane-damaging (pore forming) toxins perturb host immune responses influencing clearance, competition and transmission
