L12 - SNAREs 2 - membrane fusion machinery in health and disease

VAMP2 knockout mice

Syntaxin 1B knockout

SNAP-25 knockout

mice with any of these knockout mutations die at birth due to loss of synaptic transmission

—> shows these components are essential for synaptic transmission

what are the two isoforms of Syntaxin?

• Syntaxin 1A

• Syntaxin 1B

Syntaxin 1A knockout

• no gross abnormalities

• subtle defects in synaptic transmission

what disease does VAMP2 mutation cause?

• neurodevelopmental disorders

• with hypotonia (muscle weakness) and autistic features

• sometimes with hyperkinetic movements (involuntary movements)

how does mutation in the VAMP2 molecule lead to neurodevelopmental disorders?

• the genetic mutations are located specifically in the SNARE domain of the VAMP2 protein

• mutations there can disrupt SNARE complex formation = leading to such disorders

S75P mutation in VAMP2

• dominant negative mutation

• S75P blocks/slows down liposome fusion

• this mutation reduces the speed and efficiency of membrane fusion

what disease does SNAP25b mutation cause?

• neurodevelopmental disorders

• with seizures, intellectual disability, severe speech delay and cerebellar ataxia

what disease do mutations in Syntaxin 11 cause?

• familial hemophagocytic lymphohistiocytosis (FHL4)

what is FHL4 (familiar hemophagocytic lymphohistiocytosis)?

• rare disease of the immune system

• effects infants

• over proliferation of T cells, B cells, natural killer cells and macrophages

• cytokine storm

• patients can die due to infection due to defective killing in T cells - the T cells kill infected cells by secreting cytotoxic granules but defects block this function

explain how mutations in Syntaxin 11 cause FHL4

• Syntaxin 11 is a Q-SNARE

• patients with FHL4 have reduced levels of Syntaxin 11

• loss of Syntaxin 11 causes defective degranulation from cytotoxic T cells

• which means T cells still recognise infected cells but cannot release their cytotoxic granules so they fail to kill infected cells

what do mutations in Munc18-2 cause?

• FHL5 which reduces the level of Syntaxin 11

• less Syntaxin 11 = less vesicle fusion = weaker immune system

what are clostridial neurotoxins?

• most powerful bacterial toxin

• they are neuron specific - they target the nervous system - neuromuscular junction or inhibitory neurons

what are the two clostridial neurotoxins?

1. botulinum toxin produced by clostridium botulinum

2. tetanus toxin produced by clostridium tetani

what disease does clostridium tetani bacterium cause?

• toxin - tetanus toxin

• causes tetanus (lock jaw)

• the toxin blocks neurotransmitter release in the spinal cord

• which leads to uncontrolled muscle contractions and severe spasms in the jaw and neck

• rigid form of paralysis

what disease does clostridium botulinum bacterium cause?

• toxin - botulinum toxin

• causes botulism

• the toxin blocks acetylcholine release at neuromuscular junctions

• which leads to muscle paralysis - starts with face and spreads downwards

what is the most toxic potent neurotoxin?

botulinum toxin produced by clostridium botulinum

what is infant botulism?

• floppy baby syndrome

• hypotonia - low muscle tone

• caused by ingesting the clostridium botulinum toxin (from honey/dust)

structure of clostridial neurotoxins

• targeting domain

• translocation domain

• protease domain (molecular scissors - cleaves molecules)

what do clostridial neurotoxins do?

• toxins are endocytosed and go into endosomes escape and then cleave SNARE proteins

• they cleave different SNAREs as they have selectivity for the SNAREs

• BONT A (Botulin A) cleaves SNAP-25

• BONT B (Botulin B) cleaves VAMP

• tetanus cleaves VAMP

—> tetanus and botulinum toxins have similar mode of action but intoxicate different neurons

what are the two clostridial neurotoxins that most commonly cause botulism?

Botulin A and Botulin B

what is the clinical use of botulinum neurotoxins?

• cosmetic uses - Botox

• clinical conditions can be treated using the botulinum toxins like:

  1. strabismus (eyes point in different directions)

  2. blepharospasm (involuntary blinking/eye twitching)

• many products are based around Botulinum A and target SNAP-25

• also used to treat neurological conditions associated with neuronal hyperactivity

what is Botulinum A used for?

• cosmetic purposes - Botox

• nonsurgical treatments for strabismus

what is Botulinum B used for?

used to treat cervical dystonia

similarity between botulinum and tetanus toxins

• have conserved protein structure

• potent inhibitors of SNARE function