Excitotoxicity Flashcards
What is excitotoxicity?
cell death resulting from the toxic actions of excitatory amino acids
What is the main excitatory neurotransmitter in the brain?
Glutamate
What sort of processes is Glutamate essential for?
LTP and neurodevelopment
What are some other excitotoxicity causative amino acids?
cysteine, cysteine sulfonate, cysteic acid and aspartate
Why does glutamate not cause endogenous toxicity?
because the levels are too low
What removes glutamate from the EC space?
ATP-dependent glutamate transporters
How is cytosolic glutamate transported into vesicles?
vesicular ATP-dependent glutamate transporters
What can happen to glutamate uptake during an ischemic attack?
uptake can stop and reverse causing glutamate to spill out into the EC space or cytoplasm
What is the influence of glutamate on the post-synaptic neuron?
influx of Na
Which receptors does glutamate activate on the post-synaptic neuron?
mGluRs
AMPA
NMDA
Kainate
Why do NMDA receptors play a key role in glutamate excitation?
NMDAs are highly permeable to calcium
Pathological activation of NMDAs are associated with neuronal death in ischemia
NMDA R excessive activation associated with neurodegeneration
What are the components of the hippocampal EPSC?
Fast AMPA activation
Slow NMDA activation
What sort of malfunctions may occur at the glutamatergic synapse?
excessive or deficient
What is domoic acid (DA)?
a glutamate analogue associated with harmful algal blooms
What is amnesic shellfish poisoning?
where high concentrations of DA have accumulated in shell fish - causes short term memory loss, brain damage and in severe cases, death in mammals
What does amnesic shellfish poisoning do in mammals?
cause STM loss, brain damage and potentially death
How does DA exert its effects?
activates non-NMDA receptors which increases IC Ca which promotes the release of glutamate
-this then promotes further release of glutamate through NMDAs
Why is DA so toxic?
it is not readily removed by glutamate transporters
What does BMAA stand for?
B-Methylamino-L-Alanine
What is BMAA?
a neurotoxin produced by cyanobacteria which could be considered the cause of ALS or parkinsonism-dementia complex
What can BMAA be considered the cause of?
ALS or Parkinsonism dementia
How does BMAA act as a neurotoxin?
kills motor neurons by activating AMPA/Kainate glutamate receptors and boosting production of ROS
How high can EC glutamate rise to during ischemia?
100uM
What reduces ischemic neuronal damage in vivo?
NMDAR antagonists
What are the potential pre-synaptic abnormalities that can lead to excessive excitation?
- AP increased frequenct
- Ca influx - altered voltage-dependence/spontaneous activation of VGCC
- Glutamate release: altered requirement for Ca influx/altered fusion/altered vesicle loading
What is toxicity commonly due to?
lack of ability to remove glutamate
What is glutamate toxicity enhanced by?
reversal of uptake by plasma membrane glutamate transporters
Why might plasma membrane glutamate transporters reverse their uptake?
due to cytosolic ATP depletion
What are the potential post-synaptic abnormalities that can lead to excessive excitation?
- increased affinity of the glutamate binding site (not common)
- increased density of glutamate receptors
- deficient cation selectivity of the ionophore (R/Q editing)
Under what conditions do AMPA Rs become permeable to Ca?
When RNA editing fails and GluA(Q)2 channels are formed instead of GluA(R)2
What are the mechanisms of neuronal death?
Apoptosis
Shrinking of the cytoplasm and condensation of the nucleus
Necrosis
Describe apoptosis
destroys individual cells
induced by physiological stimuli
What sort of physiological stimuli may induce apoptosis?
lack of growth factors
hormonal changes
Describe what happens when the cytoplasm shrinks and the nucleus condenses?
blebbing of the plasma membrane - no integrity
energy dependent active process using cell death pathways
Describe necrosis
affects groups of cells
evoked by non-physiological events
inflammation
swelling of the cytoplasm and the mitochondria
no energy requirement, passive process by Ca overload
What are the non-physiological events that may lead to necrosis?
virsuses, ischemia, poisons
Why is there no energy requirement for necrosis?
the process is passive via Ca overload
Describe the process of apoptosis
- mild excitotoxic insults allow NMDAR activation by ambient conc. of glutamate
- leads to increased mitochondrial Ca and ROS production (still relatively preserved ATP production)
- the mitochondria may then release cytochrome c, caspase 9 or apoptosis inducing factor
What factors may mitochondria produce to initiate apoptosis?
caspase 9, cytochrome c or apoptosis inducing factor
Describe the process of necrosis
- severe insult leads to enhanced NMDAR activation which leads to increased IC Ca levels
- this activates NOS which increases mitochondrial Ca and superoxide generation and formation of peroxynitrite (ONOO-)
- results in cellular damage including DNA damage leading to activation of poly-ADP-ribose polymerase (PARS)
- mitochondrial Ca accumulation and oxidative damage lead to activation of the permeability transition pore (PTP) that is linked to excitotoxic cell death
Why are NMDA Rs considered essential?
when blockaded or KO there is apoptosis of neurons and the viability of neurons is reduced
Which NMDA Rs promote survival?
synaptic NMDAR
What happens when EC glutamate levels are chronically elevated?
Extra-synaptic NMDA Rs are activated which is coupled to pro-death signalling
How do extrasynaptic NMDA Rs signal for cell death?
shut down CREB signalling and triggers mitochondrial depolarisation
Can synaptic NMDA Rs contribute to cell death?
Yes but not as efficiently
What are the neuroprotective mechanisms of glutamergic signalling?
Suppressed apoptotic pathway
Enhanced Antioxidant Defences
What mechanisms suppress the apoptotic pathway?
reduction in p53
PI3K cascade
expression of survival genes i.e CREB pathway
In what way does PI3K cascade suppress apoptosis?
Activating Akt
What does Akt influence?
suppression of p53 -> decrease in Bax
suppression of GSKb
suppression of BAD -> decrease in Bcl2
What does suppression of p53 result in?
decrease in Bax
What does suppression of BAD result in?
decreased in Bcl 2
What CREB target genes are activated to suppress apoptosis?
AID - activity-dependent inhibitors of death
NFAT - nuclear factor of Activated T cells
What is significant about NFAT?
It is a Ca responsive TF
What does Ca influx via NMDARs enable?
communication between the synapse and the nucleus -> CREB
Why is NMDAR activation important in antioxidant defences?
neuronal vulnerability to oxidative damage is regulated by synaptic NMDAR activity
What are the key mechanisms involved in boosting anti-oxidant defences?
thioredoxin-peroxiredoxin system
gene expression changes
What happens in the thioredoxin-peroxiredoxin system?
enhanced thioredoxin activity -> reduction of hyperoxidised peroxiredoxins
What are peroxiredoxins?
antioxidant enzymes
What gene expression changes that boost anti-oxidant defences?
suppression of thioredoxin inhibitior - TXNIP
What is TXNIP?
A FOXO target gene - a thioredoxin inhibitor
What key elements for neuronal health are regulated by mitochondria?
Ca homeostasis and energy demands
What does neuronal activity regulate in terms of mitochondria?
mitochondrial fission
reduced mitochondrial mobility
localisation of mitochondria to dendritic spines
What is mitochondrial fission?
the ability of mitochondrial cells to divide
How does synaptic activity enhance neuroprotective mechanisms?
regulating the expression and release of neurotrophic factors
What examples of neurotrophic factors may synaptic activity activate?
BDNF
pro-NGF
upregulation of FGF2
What does excessive Ca uptake do to mitochondria?
causes depolarisation of the mitochondrial membrane which inhibits ATP production
What does mitochondrial dysfunction result in?
inability of neurons to regulate Ca
ROS production
What pathways may be activated in NMDAR mediated cell death?
mitochondrial dysfunction
calpain activation
Stress activated protein kinases
What is the result of excessive Ca uptake in the neuron?
impairs Ca efflux mechanisms
What is normally responsible for Ca efflux under regular conditions?
plasma membrane Ca-ATPase (PMCAs)
NCX3
How does excessive Ca uptake affect calcium efflux?
calpains are activated and cleave the major NCX3 isoform and may also inactivated PMCAs
What is the effect of calpain activation?
cleavage of major NCX3 isoform
inactivation of PMCAs
What are the Stress Activated Protein Kinases activated?
p38 MAPK and JAK
What does p38 MAPK activation involve in cerebellar and cortical neurons?
nNOS
What determines in NMDAR activity is neuroprotective or excitotoxic?
stimulus intensity
NMDAR locus
NMDAR subunit composition
What are the differential effects of stimulus intensity?
Too much or too little causes cell death pathways
What are the pathways are activated by synaptic NMDARs?
ERK pathways
CREB-dependent gene expression
PI3K-Akt pathways
What are the pathways activated by extrasynaptic NMDARs?
ERK inactivation
CREB dephosphorylation
no activation of PI3K-Akt
What are the general NMDAR subunit compositions?
one NR1, one or NR2 subunits (A/B/C/D) with or without an NR3
What is the importance NR2 subunits?
determine biophysics and pharmacology of the cell
Which NMDAR subunits dominate in the hippocampus?
NR2A (synaptic) and NR2B (Extra-Synaptic)
What is the significance of NMDARs in AD?
synaptic activation reduces Ab production and release and increases the components of non-amylodogenic pathway APPa
What is the significance of NMDARs in SMA?
administration of NMDA daily prevents MN death and improves life span and motor behaviour
What is the significance of NMDARs in ALS?
ALS mouse model has decreased NR2A subunits which is coincident with alterations in synaptic plasticity
What is the significance of NMDARs in HD?
synaptic activity increases non-toxic mutant huntington inclusions
YAC-128 mouse has increased extrasynaptic NMDARs leading to pro death signalling
memantine improves neuropathology and behaviour
