Excitotoxicity I Flashcards
How Excitotoxicity influences neurodegeneration
- Excitotoxicity contributes to neurodegeneration in stroke, brain trauma, PD, AD, HD, ALS etc.
Excess Calcium effects in excitotoxicity
NOS --> Nitric oxide release PLA2 --> arachidonic acid Proteases endonucleuses ODC --> polyamines Mitochondrial damage = neuronal death
Old view of excitotoxicity
excessive glut release/ excess NMDAR activation is linked to excessive Ca2+ in neurons which leads to excitotoxicity via a variety of pathways
NMDA (N-methyl-D-aspartate) Receptors
- Ion channels permeable to Na+ and Ca++
- Co-activated by glutamate and glycine
- Depolarization by activation of AMPA receptors is necessary to remove the Mg2+ that blocks the ion channel
NMDAR structure
- Tetramers 2NR1 + 2NR2 Encoded by - NR1: 8 splice variants - NR2: 4 genes (A, B, C, D) - NR2B enriched in forebrain and striatum
Synaptic NMDA receptors
- NR2 subunits have a regulatory role
- NR2A and NR2B subunits interact with scaffold proteins, PSD-95 and SAP-102 which link NMDARs directly or indirectly to a number of signalling molecules (activates downstream signalling)
- The composition of NMDA receptors at synapses is dynamic and regulated by neural activity and through membrane insertion/endocytosis and lateral diffusion
Most abundant subunit
NR2A
Neuroprotective pathways activated by synaptic NMDA receptors
Activation of synaptic NMDARs activates:
- AKT pathway
- ERK + MAPK pathways –> phosphorylation of CREB
= Pro-survival signal (incl. BDNF production), enhanced antioxidant defenses, improved mit function
PhosphoCREB
activated form of creb
leads to pro-survival signals, enhanced antioxidants, improved mit function
Extrasynaptic NMDA receptors–where
- Extrasynaptic NMDA receptor are present in the neck of dendritic spines, adjacent to postsynaptic density, and even in the cell body.
Extrasynaptic NMDARs–activation
Extrasynaptic NMDA receptor are not activated during low-frequency synaptic events that activate synaptic NMDA receptors
____ subunit most abundant at synaptic membrane; ____ subunit most abundant at extrasynaptic membranes (___ of all NMDAR in adult brain)
NR2A: most abundant at synaptic membrane (more pro-survival)
NR2B: most abundant at extrasynaptic membranes (1/3 of all NMDAR in the adult brain; more pro-apoptotic)
Extra-synaptic NMDA receptors
- Areas with high expression of NR2B subunits (i.e. extrasynaptic) are more sensitive to excitotoxicity
- “Physiological” role of extrasynaptic receptors not clear. More than just a
reservoir. Modulatory? Perhaps involved in LTP - Extrasynaptic receptors are likely to be activated by glutamate released
by astrocytes - Extra-synaptic NMDAR are the most important activators of excitotoxicity
Synaptic vs Extra-synaptic NMDARs
Extrasynaptic receptors have low surface density and cause apoptotic signals (via CREB inhibition)
Synaptic receptors have high surface density and causes pro-survival signals (creb activation)
Testing NMDARs in culture
Neurons in culture
- mix of glut and gaba
- gabaergic neurons in culture will inhibit glut
STUDY: Extrasynaptic NMDARs oppose synaptic NMDARs
1) Bicuculline (GABA-AR antag) triggers release of glutamate and calcium flux
through synaptic NMDA receptors, leading to phosphorylation of CREB
(pCREB)
2) Application of APV (aminophoshovalerate, NMDAR competitive inhibitor) slowly decreases pCREB by inhibiting synaptic receptors
3) When high levels of Glu are applied, both synaptic and extrasynaptic NMDAr are stimulated. Stimulation of extrasynaptic receptors leads to rapid CREB shut-off
4) If an inhibitor of NMDAR, APV, is used, pCREB decreases more gradually, because both synaptic and extrasynaptic receptors are blocked
Synaptic vs Extra-synaptic NMDA receptors
- Using biculculine –> synaptic NMDAR activation –> pCREB = pro-survival
- APV –> inhibits synaptic receptors –> slow decrease pCREB
- high glu levels –> activation of both extra and syn receptors –> extrasynaptic win over = rapid CREB shut off
- inhibit NMDARs w/ APV –> gradual pCERB decrease (slower shut off compared to when extrasynaptics can shut off)
Functional dichotomy of NMDAR signaling
The C-terminal domain of the NR2B subunit is critical for association with PSD95 and nNOS and excitotoxicity
- JACOB activation –> dephosphorylation of CREB and ERK –> inactivates these pathways
- Ca2+ activates Calpains (calcium-dependent proteases) –> active calpains cleave transporters at plasma memb –> decreased PMCA and NCX (2 main transporters for decreasing intracellular calcium conc in active neurons)
- activate calpains akso cleave STEP –> STEP usually inhibits P38 –> no step = active P38 –> P38 can activate apoptosis
- nNOS and PSD95 form a complex –> produce NO –> 1) ROS production & 2) nitrosylation of Drp1 –> activation of Drp1 –> promotes mit fragmentation and apoptosis
JACOB
JACOB activation –> dephosphorylation of CREB and ERK –> inactivates these pathways
Calpains
Ca2+ activates Calpains (calcium-dependent proteases) –> active calpains
1) active calpains cleave transporters at plasma memb –> decreased PMCA and NCX (2 main transporters for decreasing intracellular calcium conc in active neurons)
2) activate calpains akso cleave STEP –> STEP usually inhibits P38 –> no step = active P38 –> P38 can activate apoptosis
nNOS and PSD95 complex
nNOS and PSD95 form a complex –> produce NO –> 1) ROS production & 2) nitrosylation of Drp1 –> activation of Drp1 –> promotes mit fragmentation and apoptosis
Models of NMDA receptor-dependent
excitotoxicity–old vs. new view
- Ca++ overload per se is not necessarily responsible for excitotocity (as per old view)
- It is the Ca++ flow through extrasynaptic NMDA receptors that is
harmful to neurons (new view) - When the same Ca++ flow is triggered by synaptic and extrasynaptic receptors, it produces dramatically different downstream events
