Synaptic Plasticity Mechanisms Flashcards
What is learning?
Process of acquiring information
What is memory?
Persistence of learning (storage and retrieval)
What are the different forms of memory?
Declarative:
Semantic; words, faces, objects
Episodic; snapshots of life events
Procedural:
Learning how something works
How do we know that the hippocampus is involved with memory?
Lots of studies; however recent one involved fMRI studies on London taxi driver
What is the definition of synaptic plasticity?
Persistent upregulation or downregulation of synaptic strength - LTP or LTD
When does plasticity occur?
Development
Learning and memory
Ageing
Response to trauma/ disease
What electrophysiology techniques can be used to record LTP?
fEPSP - downward deflection
intracellular recording - upward deflection
Whole cell patch clamp - downward deflection
What are 3 classical properties of LTP?
Specificity
Cooperativity
Associativity
Describe specificity in terms of LTP
LTP is specific to the tetanized (high frequency stimulation) pathways
Non-tetanized inputs, even convergent on the same dendritic region are not potentiated
Describe cooperativity in terms of LTP
LTP exhibits an intensity threshold whereby a weak stimulus is unable to induce LTP whereas a strong input can
Describe associativity in terms of LTP
A weak input will potentiate provided a strong convergent input is activated simultaneously
This feature is equated with classical conditioning, with the weak and strong inputs corresponding to the conditioned and non conditioned stimuli respectively
How can LTP be induced experimentally?
High frequency train of stimulation
Patterned stimulation
Describe the 4 LTP inducing stimulation protocols
Tetanic - 1000 Hz for 1s
Theta burst - short bursts at 5 Hz
Primed burst - very similar to theta burst
Realistic stimulation protocols
What are the 4 distinct phases of LTP induction?
Post-tetanic potentiation
Early LTP
Intermediate LTP
Late LTP
What are the 3 post synaptic glutamate receptors?
AMPA
NMDA
mGlu; metabotropic glutamate receptors linked to G proteins
Which glutamergic receptor is key to LTP induction?
NMDA - contribute little to basal synaptic transmission
What characteristics of NMDA receptors makes them key to LTP induction?
Under basal membrane potential; NMDA is blocked by Mg2+
Mg2+ remains until the membrane is depolarized to around +20 mV.
CALCIUM INFLUX AND POTTASIUM EFFLUX
Describe the action of high frequency stimulus in LTP
Activation of NMDA receptors
This allows for calcium influx
CaMK2, PKC induction
LTP induction
Is GABA involved with LTP
Yes; it acts as an inhibitory interneuron to prevent over excitation of the glutamergic synapse…the IPSP curtails the EPSP
GABA is released at the same time as glutamate to act on a combo of GABAA and GABAB to form this inhibitory post synaptic potential to limit the excitation at the synaptic connection
GABAB receptors (presynaptically) auto receptors whereby they act to limit the release of GABA during periods of sustained activation.
Train of excitation that is not limited by GABAergic inhibition due to activation of GABAB
Are there behavioural consequences of NMDAR blockade?
Inhibition of NMDAR blocks both LTP and spatial learning (Morris water maze)
Spatial learning is a specific hippocampal behavioural task
Is synaptic plasticity bidirectional?
Yes; LTP and LTD
High train frequency; sustained increase in magnitude of recordings = LTP
Low train frequency; sustained decrease of magnitude of recordings = LTD
This bidirectional nature is governed by modification of synaptic transmission mediated by AMPARs at glutamatergic synapses in area CA1 of the hippocampus. Calcium is the KEY regulator
Where is the crossover point between LTD and LTP?
Around 7-10 Hz
With calcium influx the main driver - dependent on NMDAR activation
What are the 3 distinct temporal phases of LTP?
1: dependent on post-translational modification of existing proteins i.e. phosphorylation
2: dependent on synthesis of new protein via existing mRNA - mRNA translation
3: dependent on synthesis of new protein and new mRNA - gene transcription in a somatic localization
Describe the 3 phases of temporal LTP in terms of time
Phase 1 (0-1hr) = covalent modification of existing synaptic proteins
Phase 2 (1-2hrs) = translation of pre-existing mRNA
Phase 3 (3hrs +) = gene induction, transcription, translation
Can we show experimentally that there are differences in these phases of LTP?
Yes; specific inhibitors of translation and transcription
Anisomycin = translation (inhibition of early LTP)
Actinomycin D = transcription (inhibition of late LTP)
Describe phase 1 of LTP
Inhibition of a variety of kinases (blockade of phosphorylation) inhibits the early phase of LTP;
PKA, PKC, PKG, ERK, CaMKII, CaMKIV, PYK2, Fyn
Implies multiple parallel pathways, all of them essential for full expression of plasticity response
What drives the activation of kinases in early LTP?
NMADAR mediated Ca2+ rise engages kinases (LTP)/ phosphatase (LTD) that are key for LTP and LTD
How can phase 1 be modelled?
Inhibit the function of specific proteins; Receptor antagonists Enzyme inhibitors K/o mice Use of inactive/ dominant negative forms
Describe the mechanisms for the maintenance of early phase synaptic plasticity
Phosphorylation of receptors
Receptor trafficking
What is the composition of many AMPARs?
GluA1 and GluA2
What is the impact of phosphorylation of AMPARs on LTP/ LTD?
Phosphorylation of GluA1 at s831 and s845 both increase the conductance and affect receptor trafficking (receptor expression at plasma membrane or golgi apparatus)
Describe the differences in phosphorylation of AMPARs between LTD or LTP
LTD; dephosphorylation at ser845
LTP; phosphorylation at ser831
What is the evidence that GluA receptors are trafficked during LTP?
Indirect; studies that block post synaptic interactions with regulatory/ trafficking proteins e.g. PICK/GRIP. Evidence from silent synapses
Direct; immunohistochemically detection of surface GluA receptors (culture) and from imaging fluorescent GluA chimeras (SEP-GluA1)
What controls AMPA receptor trafficking?
TARPS (transmembrane AMPA receptor regulatory proteins) - example include stargazin
TARPS control AMPAR receptor trafficking and lateral surface diffusion of AMPARs
TARPs also modulate the electrophysiological (biophysical) properties of AMPARs
Control GluA1
What phosphorylates S831 and S845 respectively?
S831; CaMKII
S845; PKA
What are the actions of GRIP and PICK?
GRIP; AMPAR stabilization at the synapse
PICK; AMPAR receptor trafficking in endocytosis/ LTD and subunit composition regulation
What is the role of NSF?
Binds to GluA2
Disrupts PICK1 binding and synaptic insertion
What do AMPARs lacking GluA2 subunits show?
Pronounced inward rectification due to voltage dependent block of the channel by polyamines at positive membrane potentials
How can the post synaptic receptor numbers be increased?
Increase in receptor delivery
Lateral diffusion
Reduce removal of receptor
Is the k/o out of CaMKII sufficient to completely inhibit LTP?
No; LTP still occurs, albeit at a less intense amplitude
The addition of a NMDAR antagonist to this k/o mice completely prevent LTP however
Is PKM-zeta required for LTP?
Form of PKC
Pharmacological inhibition of PKM zeta with ZIP blocks LTP
PKM-zeta activity is necessary and sufficient for LTP maintenance
What AMPA receptor subunit allows for the permeability of calcium?
GluA2 lacking or GluA2 containing an R subunit
What is the role of polyamines in the inward rectification seen in AMPARs lacking GluA2 subunits?
Bind to the channel pore at positive potentials and block the flow via AMPA receptor channel
Describe the difference in current voltage relationship between GluA2 lacking or GluA2 containing AMPAR receptors
GluA2 lacking = rectifying I/V curve due to blockade via polyamines
GluA2 containing = linear I/V curve
Is it just AMPARs that are moved towards and away from the synapses during synaptic plasticity?
No; evidence that NMDARs are mobile
Selective blocker of NMDA receptors (MK-801), only blocks active, open NMDARs.
After application of MK-801, there was complete blockage of synaptic transmission
How can the movement of AMPARs during plasticity be monitored?
Fluorescent imaging - monitoring in real time
Genetically engineered receptors such as GFP
Or
Tack single molecules with cyanide dye or quantum dots. Recognize components of receptor to the tag with a marker
Are there differences in lateral diffusion of glutamate receptors during development?
Yes; very motile early on in development which reduces as time progresses and there is further neuronal maturation
Describe the receptor trafficking model
Receptors can move distant from synaptic cleft to insert into the membrane
They can also diffuse across whereby they can be internalized
Once internalised, the receptors can be degraded or recycled
The receptors can be synthesised and transported within the post synaptic membrane where they can then be inserted
Describe the molecular basis of late phase LTP
mRNA translation/ local protein synthesis - involves mRNA present in the dendrites (synapse specificity of plasticity via tetanized synapses maintained)
Which two mRNAs present in the dendrites have been demonstrated to be involved with phase 2 of synaptic plasticity?
MAP2; microtubule associated protein 2 modulates microtubule dynamics and so promotes outgrowth of processes
CaMKII; subunit involved in NMDAR dependent signalling
How is stimulation of local protein synthesis achieved from these mRNAs (MAP2, CaMKII)?
Inhibition of mRNA degradation; leading to elevated mRNA levels and increase synthesis of the corresponding protein
Phosphorylation of ribosomal proteins near the synapse leading to more efficient translation
Phosphorylation of ribosomal proteins triggered by ERK
What role does the alteration of glutamate receptor number on the post synaptic membrane have on synaptic plasticity?
Key in the strengthening of excitatory post synaptic potentials
How can the glutamate receptor number be increased on the post synaptic membrane?
Phase 1; increase protein delivery (membrane insertion/ trafficking, lateral mobility)
Phases 2/3: increase protein synthesis (local protein synthesis, somatic synthesis (general protein trafficking)
How is phase 3 of synaptic plasticity occured?
Synthesis of new protein and new mRNA
Gene transcription is activated by transcription factors
Transcription factors bind to specific target sites in genome to regulate transcription of a target gene (increase = activator, decrease = repressor)
Describe different types of transcription factors
Constitutive TFs = present in an inactive form in cytoplasm, to be activated following the stimulus
Inducible TFs; activated as target genes of constitutive TFs and they themselves then stimulate transcription of their own target “late response” genes
What constitutive TFs are involved in synaptic plasticity?
CREB
Elk1
NFkappaB
What inducible TFs are involved in synaptic plasticity?
zif268 (early growth response1 - erg1)
junB (component of AP1)
Describe the role of CREB activation in synaptic plasticity?
CREB can binds to DNA sequences called cAMP response elements (CRE) to increase or decrease transcription of a downstream gene
Describe the role of PKA in LTP
Genetic overexpression of PKA inhibition in mouse hippocampus
2x100Hz protocol to induce early LTP = no impact
4x100Hz protocol to induce late LTP = block of LTP particularly at late stages in comparison with WT mice
This had impact on spatial memory tasks (hippocampal based learning task)
How are IEGs (inducible TFs) involved in LTP?
Calcium activates kinases - this is a short term effect by phosphorylating receptors
This activated kinase will enter the nucleus to cause phosphorylation of the transcription factor
This results in binding to IEG DNA to initiate transcription
IEG product e.g. FOS acts as a transcription factor initiates synthesis of other proteins
Is zig/268 (inducible TF) a causal step in LTP?
High frequency stim will induce zif/268 about 15 mins after stimulus with a peak induction at 30-60 mins
MK801 (NMDA antagonist) will block LTP and the zif/268 induction
If you stimulate an inhibitory pathway to block LTP you also inadvertently block zif/268 induction
Sub-threshold stimulus paradigm incapable to induce LTP, there will be no induction of zif/268
At around 60 min time point in zif/286 animals there is not difference in LTP
However, around 2-3 days later, there are specific deficits in knock down animals in comparison with control with subsequent behavioural deficit in morris water maze
What CREB target late response genes act to maintain LTP in the long run
BDNF - promotes neuronal survival, neurite outgrowth and further plasticity
What NFkB target late response gene acts to maintain LTP in the long run?
Neuronal cell adhesion molecule (NCAM) which promotes neurite outgrowth and maintenance of synaptic architecture
What Zif/268 target late response genes act to maintain LTP in the long run
Proteasome genes
Agrin/ gephyrin which act to cluster receptors and stabilize synapses
What morphological changes occur to formulate long term memories?
Changes in synaptic morphology occur rapidly after triggering of synaptic plasticity
Morphological plasticity probably contributes to phases 1-3 being sustained by different mechanisms at each phase
What changes are proposed to occur throughout LTP?
Enlargement of post synaptic density
Synaptic splitting - one presynaptic area infiltrates 2 synapses which then go on to form 2 independent synapses
What neuromodulators have been linked to synaptic plasticity?
Leptin - leptin receptors DA - D1 2AG - CB1 (cannabinoid receptors) ACh - M1 (GPCR) 5-HT2c Histamine - H1
