Neuronal Death and Dysfunction Flashcards
1
Q
What percentage of neurons will die due to not finding an innervation target?
A
30-50%
2
Q
What are reasons neurons can die?
A
- lack of target innervations and survival signal input
- trauma and injury
- neurodegenerative diseases
3
Q
What are the consequences of neuronal dysfunction?
A
- axonal degeneration
- programmed cell death of the neuron soma
- oligodendrocytic apoptosis and demyelination
- astroglial activation and scar formation
- synaptic changes
4
Q
What do astrocytes do?
A
provide neurons with energetic intermediates
5
Q
Why does the CNS never come into contact with the PNS?
A
due to the autographic blood-brain barrier
6
Q
What can injury to the spinal cord result in?
A
paralysis from the site of injury downwards
7
Q
What are the cell shape changes of apoptosis and necrosis respectively?
A
- apoptosis = cell shrinkage
- necrosis = cell swelling and lysis
8
Q
What are the ogranelle changes of apoptosis and necrosis respectively?
A
- apoptosis = involution and contraction
- necrosis = swelling and disruption
9
Q
What are the nuclear changes of apoptosis and necrosis respectively?
A
- apoptosis = chromatin condensation and fragmentation
- necrosis = karyolysis, pyknosis
10
Q
What happens when cells die from necrosis?
A
the plasma membrane is ruptured and immune cells cluster around the injury site due to chemoattractants
11
Q
What is necrosis?
A
non-programmed cell death
12
Q
What is apoptosis mediated by?
A
the activation of a protease cascade
13
Q
Give examples of apoptotic stimulus
A
- radiation
- physical injury
- anticancer drugs
14
Q
What is the apoptosome?
A
a multimolecular holoenzyme complex assembled around the adaptor protein Apaf1
15
Q
When is the apoptosome assembled?
A
upon mitochondria-mediated apoptosis stimulated by some type of stress signal
16
Q
What is the apoptosome in drosophila and mice respectively?
A
- drosophila = dark/dronk
- mice = Apaf1/caspase-9
17
Q
What are caspases?
A
cysteine proteases that cleave after aspartic acids
18
Q
What are the 3 components that lead to apoptosis?
A
- apoptotic stimulus
- apoptosome
- apoptosis executioner
19
Q
How is caspase-3 activated?
A
- caspase-9 cleaves aspartate 28 and 175 to produce pro-domain, p12 and p17
- p17 and p12 come together to form a dimer of dimers (tetramer) which is the active caspase-3
20
Q
What are the 5 steps of the general intrinsic apoptotic pathway?
A
- fas ligand binds to fas and forms aggregates that attract scaffold proteins that activate caspase-8
- activated caspase 8 cleaves protein Bid to produce tBid
- Bax and Bak bind to the outer mitochondrial membrane and punch a hole in it to increase its permeability
- cytochrome C (death signal) is released and binds to Apaf-1 and attracts the binding of procaspase-9
- this then activates caspase-3/7 (depending on tissue type) and the execution is cut in thousands of places to induce cell death
21
Q
What is the BCL-2 protein family characterised by?
A
BCL-2 homology domains and interactions with each other
22
Q
What activates the general intrinsic apoptotic pathway?
A
many toxins and DNA damaging agents
23
Q
Where do the intrinsic and extrinsic apoptotic general pathways converge?
A
at the mitochondria
24
Q
What do IAPs do?
A
inhibit activation of caspase-9
25
What inhibits IAPs?
Smac/Diablo
26
What 4 factors do mitochondria release that lead to apoptosis and what do they do?
- cytochrome C - apoptosome activation
- Smac/Diablo - inhibits IAPs
- AIF - regulates mitochondrial permeability
- endonuclease G - translocates into nucleus and causes DNA fragmentation
27
What is calpain?
a calcium-activated protease which exists as an inactive proenzyme in the cytosol
28
How is calpain activated?
increased intracellular calcium levels
29
What does activated calpain do?
cleave cytoplasmic and nuclear substrates, leading to apoptosis
30
How does autophagy mediate cell death?
it synergises with extra-lysosomal cathepsins
31
What is necroptosis?
a regulated inflammatory mode of cell death (a programmed form of necrosis)
32
What does necroptosis signalling originate from?
death receptors e.g. tumour necrosis factor alpha (TNFα) receptor
33
What does necroptosis signalling require?
receptor-interacting protein kinase 3 (RIPK3) and its substrate Mixed lineage kinase domain-like (MLKL)
34
How is the reprosome/ripoptosome formed?
TRADD and TRAF2 signal to RIPK1 which recruits RIPK3
35
What drives oligomerisation of MLKL?
phosphorylation by the ripoptosome
36
What does oligomerisation of MLKL allow?
insertion into and permeabilisation of the plasma membranes and organelles
37
What does integration of MLKL into the plasma membrane lead to?
the inflammatory phenotype and release of damage-associated molecular patterns (DAMPs), which elicit immune responses
38
What is ferroptosis?
a non-apoptotic form of cell death resulting from impaired iron (Fe) homeostasis
39
How can ferroptosis be triggered?
by small molecules or conditions that inhibit glutathione biosynthesis or glutathione-dependent antioxidant enzymes
40
What is ferroptosis characterised by?
iron-dependent accumulation of lipid ROS and depletion of plasma membrane polyunsaturated fatty acids by lipid peroxidation
41
What are neurotrophins?
pro-survivor molecules that specifically bind to a class of receptors known as the Trks
42
What are proprioceptive neurons responsible for?
pain perception
43
How does trophic factor removal induce apoptosis?
1. withdrawal results in JNK activation and the phosphorylation of c-Jun
2. this activates the expression of DP5
3. DP5 activates Bax which causes mitochondrial damage and the release of cytochrome C
4. the cytochrome C/Apaf1/caspase-9 complex forms and activates caspase-9
5. caspase-9 activates caspase-3 and apoptosis is induced
44
How is the survival pathway activated?
1. NGF binds to TrkA resulting in phosphorylation of its cytoplasmic domain
2. scaffold proteins are recruited which activates the pro-survivor protein pathways MAPK and PI3/AKT
3. phosphorylation of CREB and IKK stimulates transcription of pro-survival factors
4. phosphorylation of Bad, Forkhead and capsase-9 inhibits the pro-apoptotic pathway
45
What activates Rsk?
the interaction of Shc–Grb2 and SOS
46
What are 14-3-3 proteins?
a family of small chaperone-like molecules found in the cytoplasm that bind to phosphorylated proteins and keep them away from mitochondria and BCL-2
47
What does cerebral ischemia/stroke lead to?
a catastrophic cascade of events leading to massive neuronal cell death
48
What are neurons dependent on in the blood to produce ATP?
- oygxen
- glucose
49
Why do neurons require ATP?
to maintain their polarised membrane potential for action potentials
50
What would loss of ATP in neurons lead to?
1. a large depolarisation which would cause the uncontrolled release of glutamate and increased calcium concentration
2. calpain activation which degrades eIF4G
3. inhibition of protein synthesis
51
What can phospholipases do after activation by calcium?
cause brain damage via proinflammatory substances
52
What is calcineurin and what does it do?
a phosphatase that activates nitric oxide synthase which causes lipid peroxidation
53
What drug is given within 3 hours of a stroke?
an antineuropic agent
54
What is reperfusion?
the restoration of blood flow to an organ or tissue after having been blocked i.e. return of oxygen and glucose to neurons
55
How can increased calcium levels lead to apoptosis?
1. increased calcium
2. increased mitochondrial permeability
3. release of cytochrome C and caspase 9
4. activation of caspase 3
5. apoptosis
56
What can inflammation of neurons cause?
- secondary ischemia
- inflammatory cytotoxicity
57
How can calcium mobilisation during ischemic insult lead to necrosis?
µ-calpain is activated at the lysosomal membrane of CA1 neurons to induce release of cathepsins B, L and D that leads to necrosis.
58
What is the most common form of axonal degradation?
Wallerian degeneration
59
What is Wallerian degeneration?
an active process degeneration of the distal end of an axon resulting from a lesion
60
What are the 5 steps of Wallerian degeneration?
1. axotomy (separation of axon)
2. first rapid, short-term Ca2+ wave, NAD+ and ATP depletion, mitochondrial swelling, ROS production
3. second long-term Ca2+ wave, destabilised MTs, unaltered axonal morphology
4. disruption of MTs, dismantling of cytoskeleton
5. swelling and beading of the axon, catastrophic axonal fragmentation, engagement of glia and other phagocytes to clear axonal debris
61
What is the central executioner of pathological axon degeneration?
sterile alpha and TIR motif-containing protein 1 (SARM1)
62
What is NMNAT2 and what does it do?
a survival factor transported down axons from the cell body that generates NAD+ from NMN and maintains SARM1 in an inactive state
63
How is SARM1 activated?
by an increase in the ratio of NMN to NAD+
64
What does activation of SARM1 do?
trigger Wallerian degeneration
65
How do healthy axons maintain NMNAT2?
through continuous supply by axonal transport
66
How does damage to the axon impair axonal transport?
by altering the axonal cytoskeleton leading to rapid turnover and decreased levels of NMNAT2
67
What are the effects of decreased NMNAT2 levels?
- accumulation of NMN and SARM1 activation
- almost complete depletion of axonal NAD+
- decreased ATP and subsequent axonal collapse
