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College aantekeningen Neurodegenerative Diseases (WMBM012)
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Aantekeningen van Neurodegenerative Diseases
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Neurodegenerative diseasesMolecular Mechanisms of Neurodegeneration: Neurotoxins and molecular mechanisms of
neuronal cell death and survival
Brain input: sensory input output: muscle contraction
Therefore we need excitatory synapses (for example glutamate, serotonin, dopamine), and
inhibitory synapses (GABA or glycine)
- Muscle is activated by acetylcholine, released at the neuromuscular endplate.
Neurotoxins – two mechanisms:
1) Disturbing neuronal functions
2) Induction of neuronal cell death
1: Disturbing neuronal functions
Tetanus – Clostridium tetani, induces a strong contraction of skeletal muscles, leading to painful
spasms. Can only grow in an anaerobe atmosphere (e.g. in a dirty wound).
= Treated with muscle relaxation, and kept in a dark and silence place to avoid any sensory input.
= We get vaccinated against tetanus.
= Taken up by the bloodstream (wound) and taken up by the muscles. Tetanus enters the neuron.
Here it will jump until it reaches the interneurons to inhibit the release of GABA. GABA would
normally f an inhibitor. So the inhibition is inhibited.
Botulism – Clostridium botulinum, leads to a flaccid paralysis (completely opposite of tetanus).
Comes from spoiled food. So it is food poisoning. We do not get vaccinated against botulism.
= Found in botox relaxes the muscles.
= Gets digested by the gut (so it has to survive), then taken up by the bloodstream to be transported
to the muscles. Here it inhibits the release of acetylcholine from the neuromuscular endplate.
Both toxins belong to the same family of protein toxins and exhibit strong homologies. Both are
produced by anaerobe spore forming bacilli.
They are not directly deadly, but indirectly.
Structure of Clostridial neurotoxins
Heavy chain is important for the binding of neurons
n-terminal = to form a hole into the plasma membrane, so that
the light chain can slip into the cytoplasm to act as a protease
on SNARE proteins
Tetanus
Tetanus neurotoxins enter clathrin-coated structured in motor neurons and transported to the CNS.
Tetanus neurotoxins exploits a pathway requiring lipid rafts, and the clathrin machinery, which is
distinct from the previously mentioned routes. At the neuromuscular junction, the tetanus
neurotoxin fragment binds to a lipid-protein receptor complex containing ganglioside GD1b. The
fragment is than sorted into clathrin-coated pits. During this sorting, GD1b is excluded from the toxin
receptor complex. Internalization of the fragment is dependent on dynamin, AP-2 and AP180. After
,uptake, the fragment is targeted to a stationary
early sorting compartment, to which other
endocytic routes may converge. This
compartment is functionally coupled to the
axonal retrograde transport pathway.
Botulism: botulinum toxin flaccid paralysis
Clostridium botulinum blocks the release of
Acetylcholine from vesicles. Stimulation of muscle
cannot occur.
Steps in the life cycle of synaptic vesicles:
1. Mobilisatie transport vesicle naar synaps
2. Tethering verbindngsinteractie tussen membraan vesicle en synaps
3. Priming Voorbereiding op fuseren in reactie op calcium instroom
4. Docking De locatie van fuseren wordt gedocked
5. Maturatie ontwikkeling tot overdraagbaar synaptische vesicle
6. Fusie
7. Endocytose heropname van vesicle in volledige contactfusiemodel
8. Recycling
,General mechanism of vesicle targeting and docking in the ER and Golgi. The assembly of coat
proteins (COPs) around budding vesicles is driven by ADP-ribosylation factors (ARFs) in a GTP-
dependent fashion. Dissociation of the coat is triggered when hydrolysis of the GTP bound to ARF is
stimulated by a GTPase-activating protein (GAP) in the Golgi membrane. The cycle of coat assembly
and disassembly can continue when the replacement of GDP on ARF by GTP is catalyzed by a guanine
nucleotide exchange factor (GEF). Fusion of the vesicles with their target membrane in the Golgi is
regulated by a series of proteins:
- NSF: N-ethylmaleimide-sensitive factor
- SNAPs: soluble NSF attachment proteins
- SNAREs: SNAP receptors
Together, they assist the vesicle in docking with its target membrane.
v-SNAREs: SNAREs on the vesicle are believed to associated with corresponding t-SNAREs on the
target membrane.
The SNARE complex on the left brings the vesicles and plasma membrane closely together and likely
represents one of the last steps in vesicle fusion. Vesicular synaptobrevin binds with the syntaxin
and SNAP-25 that are anchored to the plasma membrane. Transmitter release can be blocked by
tetanus toxin and the botulinum toxins –(proteases that cleave specific SNARE proteins).
- Botulinum toxins A & E: SNAP-25
- Botulinum toxin C1: Syntaxin and SNAP-25
- Botulinum toxins B, D, F & G: synaptobrevin
- Tetanus toxin: synaptobrevin
Clostridium is very old, when the world was still anaerobic. Now their environment changed so they
need to adapt to survive. These toxins help the cells to kill the host, so they can reproduce.
Neurodegeration
Stroke: core and penumbra, caused by clotting in blood vessel in the brain causes deficiency in
glucose and oxygen. The brain is the most energy consuming organ.
- Very local brain damage: uncontrollable cell death; content is released. There is an overkill in
glutamate, which trigger receptors secondary effect that excites other neurons, which
need to pump too much. This needs too much energy and that will overstimulate the
mitochondria internal apoptotic pathway.
= Process is halted by apoptosis
In the core cells die of necrosis, in the penumbra: apoptosis.
, The apoptotic process contains the necrotic event from targeting more cells.
Stroke is treated
with blood
thinners to
reduce the clot.
The ischemic
penumbra
Energy failure anoxic depolarization,
excitotoxicity, oxidative stress, necrosis peri-infarct depolarization, Ca2+ overload, mitochondrial
damage inflammation, programmed cell death infarcted tissue
Glutamate induced excitotoxicity is an apoptotic process
TNF-induced programmed cell death pathway
Cell death is induced by various stimuli that are recognized by specific sensors.
- These receptors recruit the complex I protein platform (TRADD, RIP1, TRAF2, AND cIAP1/2),
activating the inflammatory response and survival.
- Deubiquitination of RIP1 induces complex II (FADD, and caspase-8), activating the apoptotic
pathway.
- Inhibition of caspase-8 and increase of RIP3 expression induces complex IIb (necrosome),
activating the necroptotic pathway.
Apoptosis, necroptosis, and pyrotopsis are programmed forms of cell death. Necrosis represents an
unregulated cell death. Autophagy is a survival pathway that, if is excessive or uncontrolled,
promotes cell death.
Alzheimer Pathology
Wrong formation of APP. Beta-amyloid (Aβ) is cut by ƴ-secretase and β-secretase.
1) Aβ forms β-sheets, that are very sticky and form an aggregate.
2) Tau-aggregates are also very sticky but do not form β-sheets.
Aβ-aggregation:
The monomeric form of Aβ is not toxic. Oligomerized Aβ is most toxic to neurons. They can interact
with receptors, and can form plaques. The more plaques a patients has, the worse it goes.
If you want to measure how far the disease has progressed you want to look at the
hyperphosphorylated tau (inside the cell).
= Tau represents more the number of dead neurons that have occurred, whereas Aβ is outside of the
cells. There can be an overproduction of Aβ.
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