Excitotoxicity II Flashcards
Competitive antagonists of the NMDA receptor can bind at
either the:
Glycine site OR Glutamate site
Side effects of antagonizing the NMDAR
Significant side effects: agitation, confusion, hallucination, hypertension, death
Due to poor selectively (across receptor subtypes)
T/F: antagonizing the NMDAR is a good treatment option
FALSE
Clinical trials stopped because of the serious adverse effects (agitation, confusion, hallucination, death)
How traditional channel blockers work
- Require prior activation of the receptor (as they bind within the channel and need an open pore to enter)
- Act in a voltage-dependent manner
- Do not generally discriminate between NMDAR subtypes
Examples of Traditional Channel blockers
- Dizocilpine (MK801)
- Phencyclidine (PCP)
- Ketamine
Dizocilpine (MK801)
- Potent anti-convulsant with dissociative anesthetic properties
NOT used clinically due to: - Unacceptable toxicity and side effects (cognitive disruption, psychotic-spectrum reactions)
- Low specificity. It is also an antagonist of nicotinic acetylcholine receptors, and inhibits serotonin and dopamine transporters.
Phencyclidine (PCP)
- Originally developed as dissociative anesthetic.
- Discontinued use due to serious side-effects (including delirium, psychosis and agitation)
- decreases bp, pulse rate and respiration with high doses
- increases strength and decreases inhibitions = dangerous behaviours
- Used as “street drug” (angel dust)
Ketamine
- Used for dissociative anesthesia in minor
surgical procedures in the pediatric population and in veterinary. - High incidence of dysphoria, hallucinations, psycosis etc.
- short half-life and low potency compared to PCP and MK801
- causes psychological dependence
- depresses consciousness and breathing at high doses
Ketamine vs. other traditional channel blockers
Short half-life and low potency make ketamine safer than MK801 or phencyclidine
Traditional NMDAR-based strategies fail because
… of lack of specificity across NMDAR subtypes
- Can’t discriminate between synaptic and extrasynaptic receptors
- inhibits both pro-apoptotic and pro-survival signals
Current therapies for excitotoxicity
- Voltage-gated Na+ or Ca++ channel inhibitors
- Glutamate re-uptake enhancer
- Low affinity open channel blockers
- Extrasynaptic signaling modulators
Voltage-gated Na+ or Ca++ channel inhibitors examples
Riluzole
Glutamate re-uptake enhancer examples
Riluzole
Ceftriaxone
Low affinity open channel blockers
At synaptic receptors: - Amantadine - Memantine - Riluzole At extrasynaptic sites - Memantine
Extrasynaptic signaling modulators
Tat-NR2B9c
ZL006
Riluzole mechanism
has various mechanisms of action and more
favorable and safer clinical profile than classical NMDAR antagonists:
i) Decreases presynaptic release of glutamate via inhibition of pre-synaptic Na+ channels.
ii) Blocks NMDAR activation, preventing Ca++ entry through the channel. Binding site not identified
iii) Potent activator of glutamate re-uptake.
Riluzole in ALS
- It is one of only two drugs with some (modest) beneficial effect in ALS (FDA approved).
- It slows down progression and increases survival by 3-5 months.
Riluzole in HD & PD
Not effective in clinical trials in HD and PD
Differences in efficacy of riluzole in ALS vs HD/PD
ALS target likely the SC, whereas HD/PD is the brain
Mechanism of action on the spinal cord may be different from action on brain.
OR excitotoxicity may be more relevant in ALS than HD/PD
CEFTRIAXONE mechanism
- β-lactam antibiotics (penicillin, amoxicillin, ceftriaxone, etc.) increase EAAT2 protein expression –> increase glut reuptake
- a glut reuptake enhancer (increase EAATS protein expression)
CEFTRIAXONE efficacy
Increases survival of ALS mice.
Currently in clinical trial for the treatment of ALS
Open-channel blickers
Open-channel NMDAR blockers bind to and block the receptor only during depolarization and opening of the channel.
New vs. old channel blockers
- NEW are characterized by lower affinity and faster dissociation than classic NMDAR channel blockers.
- These properties decrease side effects and toxicity
- new: amantadine, memantine
Memantine: key properties
Key therapeutic properties of memantine: UFO drug
- Uncompetitive antagonist
- Fast “Off-rate”
Low doses of memantine
At low doses memantine preferentially blocks stimulation of extrasynaptic NMDA receptors, while sparing synaptic receptors
Uncompetitive vs. noncompetitive inhibitors
Uncompetitive–> icnreased binding with increased conc of the agonist (NMDA)
Noncompetitive–doesn’t compete, binding rate is consistent/independent of angonist concentration
Memantine and glut/NMDA concentration
Uncompetitive agonist
works well when glut is raised for longer periods
Memantine at synaptic vs extrasynaptic receptors
Synaptic receptors–> glut is only high for miliseconds –> memantine doen’t work great
Extrasynaptic –> prolonged channel activity, with elevated glut over minutes = memantine is more effective allows selectivity for extrasyn receptors
Memantine binding site
Binding site is within the channel and on the NR1 subunit. Little preference for the NR2 subunit.
Binding affinity of memantine
Low affinity: it rapidly binds and dissociates from the receptor
= fewer side effects
T/F memantine has effects at sites other than the NMDAR
TRUE
Also inhibits a7 nicotinic acetylcholine receptors
Memantine efficacy for Dementia
Beneficial effects, although modest, in AD and vascular dementia.
However, it does not appear to be able to modify disease course.
Currently in clinical trials in PD, ALS, HD, frontotemporal lobar degeneration, dementia associated with traumatic brain injury
Memantine action at a7 nicotinic acetylcholine
Intially Inhibition of a7 nicotinic acetylcholine receptors (nAChR) –> worsening of cognitive function at the beginning of treatment
Over time Inhibition of nAChR induces receptor upregulation over longer time –>
might contribute to the cognitive-enhancing effects of chronic memantine treatment
Tat-NR2B9c structure
20 amino acid-long peptide that includes:
- -> the NR2B C-terminal sequence that binds PSD95
- -> the membrane transducing domain of the HIV1 Tat protein (which makes the peptide membrane permeable)
How Tat-NR2B9c works
Binds PSD95 instead of NMDARs (acts as a decoy)
In excess will compete with receptor binding for PSD95 –> inhibit NO release and block downstream signalling
ZL006
small molecule drug that interferes with the binding of nNOS to PDS95
nNOS binds to ZL006 instead of PSD95
ZL006 & Tat-NR2B9c
Both strategies have been successfully used to reduce stroke damage in animal
models by Inhibition of protein-protein interactions at extrasynaptic NMDA receptor
signaling platforms
Inhibition of protein-protein interactions at extrasynaptic NMDA receptor signaling platforms: normal
- PSD95 and nNOS bind –> release NO –> signalling of excitotoxicity
- protein-protein interactions at extrasynaptic NMDA receptor
Tat-NR2B9c in non-human primates
- Induction of stroke by surgical middle cerebral artery occlusion + Intravenous administration of Tat-NR2B9c peptide after 1 hour
- Tat-NR2B9c treated animals showed decreased infarct and lower stroke scale scores after 7, 14-days compared to placebo
Tat-NB29c peptide is neuroprotective in
patients with iatrogenic stroke
- Evaluating Neuroprotection in Aneurysm Coiling Therapy (ENACT) Phase II trial
- Decreased numbers of stroke lesions in patients taking Tat-NR2B9c–fewer lesions after TAT-NR2B9c compared to placebo
- Benefits of Tat-NB29c administration in combination with classic thrombolytic
therapies needs to be assessed.
Iatrogenic stroke
stroke caused by a medical procedure (ex. endovascular aneurysm repair)
ZL006 development
- researchers found that nNOS is in complex with PSD-95 and active NMDA NR2B subunits after stroke and that this complex is crucial for neuronal death.
- Found that peptides that destroy PSD-95 – nNOS binding are protective
- Used rationalized drug design to prevent crucial amino acids interactions for PSD-95-nNOS binding –> developed ZL006 as potent inhibitor of nNOS – PSD95 binding and excitotoxicity, without side effects
Structural underpinning of PSD95-nNOS interaction
- A beta-finger domain in nNOS is important for interaction with PSD95
- this domain is stabilized by he interaction between Arg121 and Asp62 of nNOS
- ZL006 is designed to bind Arg121, preventing its interaction with Asp62
Treatment of cerebral ischemia by disrupting ischemia induced interaction of nNOS with PSD-95
ZL006 decreased infarct size and neurological score by almost 50% compared to placebo
