Hippocampus LTD

What is LTD?

LTD = long-lasting, activity-dependent reduction in the efficacy of synaptic communication following a certain patterned stimulus

The theory of learning + memory: Hebb’s rule (1949) - neurons that fire together wire together - what does the theory state?

Learning and memory would involve synaptic strengthening and weakening by activity (experience) between input-output neurons in the brain

Synapse: the function of learning + the structure of memory - postsynaptic-dendritic spine structure + expression of glutamate AMPA Rs

Dendritic spine structure/size depends on how much current transferred

Mushroom spines on glutamate-sensitivity map: more sensitive, more reacting, better learning capacity

How is the maximum glutamate sensitivity is significantly correlated with the volume of dendritic spine head?

-       Spines with large heads are stable, express large numbers of AMPA-type glutamate Rs + contribute to strong synaptic connections

-       Spines with small heads are motile + unstable + contribute to weak or silent connections

How are AMPARs the primary conduits of excitatory synaptic transmission?

-       A single release event of glutamate is predicted to activate postsynaptic glutamate Rs within a subsynaptic hotspot

-       Most AMPARs are localised within the central post synaptic density (PSD), AMPAR in middle due to higher chance (shooting at empty goal)

Mechanism of Ca2+ elevation leading to mEPSCs

1.    Ca2+ elevation occurs in microdomain

2.    Ca2+ binds to synaptotagmin, causing opening of fusion pore

3.    Glutamate passes through fusion pore + diffuses in the cleft

4.    Opening of AMPA channel generates EPSC

5.    Minatare excitatory postsynaptic AMPAR-mediated currents (mEPSCs)

What are mEPSCs?

spontaneous synaptic current in a postsynaptic neuron caused by the release of single vesicle of excitatory NT (e.g. glutamate) from presynaptic terminal

What do AMPARs consist of, how can they form tetrameric ion channels + types of subunits?

AMPARs – consist of 4 different subunits (GluA1, 2, 3 + 4) which can also be alternatively spliced + RNA edited to form tetrameric ion channels

(2 types of subunits: long + short tails)

è Short tails interact with kinase + AMPAR

è Movement of AMPAR

è Need trafficking molecules or endocytes

Process of how LTD means a decrease in the number of surface AMPAR by endocytotic mechanism?

-       The C-terminal tail of GluA2 AMPA receptor interacts with the PDZ domain-containing proteins AP2 (adaptor protein 2), NSF (N-ethylmaleimide-sensitive factor), GRIP (glutamate receptor-interacting protein), ABP (AMPAR binding protein) and PICK1 (protein interacting with C-kinase)

o   PICK1 = endocytic molecule, bring to surface

o   GRIP = helping PCIK1

-       AMPA receptor interactomes play key roles in receptor surface expression / internalisation

Mechanism of Ca2+ in LTD (4) hard

1.    Ca2+ rise leads the dissociation of NSF (N-ethylmaleimide-sensitive factor) binding from GluA2 receptor which is replace by AP2 (adaptor protein 2) and initiates clathrin-mediated endocytosis of the AMPA receptor.

2.    In some circumstances, GluA2-PICK1 (protein interacting with C-kinase1) interaction causes dissociation of ABP (AMPA-binding protein)-GRIP (glutamate receptor interacting protein)-ABP (AMPA-binding protein) from GluA2, this regulates endocytic process

3.    During LTD (synapse weakening), AMPARs are subtracted from synapse

4.    Several studies have demonstrated that phosphorylation of GluA2 S880 regulates the interaction of GluA2 + PICK1, destabilising GluA2 within the PDZ + promoting GluA2 internalisation

How do changes in AMPAR number affect strenth?

AMPA Rs are the primary conduits of excitatory synaptic transmission

Change in AMPAR number = strength

→ LFS (low freq stimulation) -> LTD

→ HFS (high freq stimulation) -> LTP

How does bidirectional synaptic plasticity encode memories?

Bidirectional synaptic plasticity, LTP + LTD, is considered the cellular foundation for encoding memories by strengthening or weakening neural connections based on activity

LTP acts to strengthen synapses for memory formation, while LTD weakens unused connections, aiding in memory refinement, reversibility, and storage capacity

Bidirectional synaptic plasticity encodes memories: core mechanism

Core Mechanism: The brain encodes memories through activity-dependent changes, where persistent strengthening (LTP) or weakening (LTD) of synaptic connections, driven by neurotransmitters like glutamate, forms lasting memory traces.

Bidirectional synaptic plasticity encodes memories: LTP + memory formation

LTP and Memory Formation: High-frequency stimulation strengthens synapses (LTP), creating long-term memories in areas like the hippocampus, often involving the increased expression of receptors.

Bidirectional synaptic plasticity encodes memories: LTD + memory refinement

LTD and Memory Refinement: Long-term depression (LTD) weakens synapses, allowing for the pruning of irrelevant pathways, reducing interference between memories, and enabling the erasure or reversal of previously encoded information.

Bidirectional synaptic plasticity encodes memories: systemic balance

Systemic Balance: This bidirectional process is regulated by homeostasis, balancing excitatory and inhibitory interactions to prevent saturation and maintain efficient memory storage networks.

Bidirectional synaptic plasticity encodes memories: reversibility

Reversibility: Bidirectional plasticity provides a mechanism for rapid reversibility in behavioural learning, such as in the cerebellum, where motor learning can be updated or reversed via these changes

What do LTD and LTP stimuli each do?

 

-       An LTD-inducing stimulus activates protein phosphatase 1 (PP1) and casapse, which dephosphorylates GSK3β to activate it and permit the induction of LTD

-       An LTP stimulus activates the phosphoinositide 3-kinase (PI3K)-Akt pathway, which phosphorylates GSK3β to inhibit it, thus preventing LTD

How is physiological + pathophysiological plasticity in LTP compared to LTD?

Physiological plasticity: higher in LTD than LTP

Pathophysiological plasticity: higher in LTP than LTP

What is LTF expressed by?

-       Long-term depression (LTD) is expressed by a long-lasting decrease in the efficiency of synaptic transmission, in particular synaptic transmission that is mediated by the synaptic activation of glutamate AMPARs (α-amino-3-hydroxy-5- methylisoxazole-4-propionic acid receptors). It may involve presynaptic and postsynaptic mechanisms

How can LTD be triggered?

-       LTD can be triggered by the synaptic activation of glutamate receptors — in particular NMDARs (N-methyl-D-aspartate receptors) and metabotropic glutamate receptors (mGluRs) — or receptors for other neurotransmitters (eg., mAChR)

What signalling cascades link the induction of LTD to its induction + expression?

-       Complex signalling cascades link the induction of LTD to its induction and expression. Key cascades involve molecular mechanism of LTD: Ca2+ sensors, protein–protein interactions (eg., receptor trafficking, endocytotic protein), protein kinases and phosphatases, proteases and other signalling molecules (eg., synapse weakening related)

What does LTD have roles in?

-       LTD has diverse roles in cognition, particularly in some forms of learning and memory and in circumstances in which a flexible response is required

-       LTD also seems to be involved in pathological states, including drug addiction, mental retardation and neurodegenerative diseases such as Alzheimer's disease

How does Object Recognition in Anterior-Inferior Temporal (AIT) Cortex work?

-       a critical, final stage of the ventral visual pathway for identifying complex objects and faces

-       processes visual information by extracting complex features, organizing them into spatial domains, and linking them with memory and reward, often coding objects via combined activation of specific cell populations

How does Object Recognition in the Perihirinal (PRH) Cortex work?

-       involved in recognition memory for objects (e.g. object recognition memory for novel/familiarity discrimination)

-       response decrements in the activity of PRH neurons during familiarity discrimination suggest cellular mechanisms that are consistent with LTD-like synapse weakening in the PRH may be involved in object recognition memory

What is reversal learning?

The ability to form new memory + functional plasticity to erase/suppress old memory

How does reversal learning work in the hippocampus?

Hippocampus = essential centre for encoding the initial information + reversal learning for new memory

LTD-like synapse weakening may require reversal learning

How does aberrant synapse weakening cause synapse dysfunction + dementia? (+ risk factors)

Aberrant synapse weakening causes synapse dysfunction + dementia

Weakening of neuronal integrity = synapse defecits, closely related to disease progression

risk factors: amyloid plaque + neurofibrillary Tau tangle

è pathophysiology

è synapse weakening

è synapse dysfunction

è dementia (memory defecit)

What are neurofibrillary tangles (NFTs)+ what are they composed of?

-       Neurofibrillary Tangles (NFTs) are abnormal, insoluble intracellular aggregates of hyperphosphorylated tau protein that serve as a primary hallmark of Alzheimer’s disease and other neurodegenerative diseases, collectively known as tauopathies

Neurofibrillary Tangles (NFTs) are composed of hyperphosphorylated, aggregated tau and lead to neurodegeneration in the brain

What do neurofibrillary tangles do + how much are found in normal ageing vs AD?

-       disrupt neuronal structure and function, leading to synaptic destruction, cognitive impairment, and neuronal death, with their spread through the brain correlating strongly with dementia severity

Normal ageing – small number/no neurofibrillary tangles

Late stage of Alzheimer – very many tangles

How do neurofibrillary tangles lead to neurodegeneration?

(microtubules -> kinases with phosph group attached leave MTs + bind Tau -> hyperphosphorylated tau -> aggregate -> Tau deposition -> neurodegeneration)

What is the composition of NFTs?

• Composition: They are formed when tau, a protein that usually stabilizes microtubules, becomes hyperphosphorylated and misfolds into paired helical filaments.

What is the pathogenesis of NFTs?

• Pathogenesis: Before forming large, visible tangles, intermediate "tau oligomers" are thought to be particularly toxic, damaging synapses and causing neuronal loss.

What is the disease association of NFTs?

• Disease Association: While classic in Alzheimer’s, NFTs are found in numerous tauopathies, including frontotemporal dementia, progressive supranuclear palsy, and corticobasal degeneration.

What is the progression of NFTs?

• Progression: NFTs appear first in memory-related regions, such as the entorhinal cortex, before spreading throughout the cerebral cortex as the disease progresses.

What is the therapeutic targeting of NFTs?

• Therapeutic Targeting: Research is focused on reducing these tangles by inhibiting the kinase enzymes that cause hyperphosphorylation, encouraging clearance, and using immunotherapies to stop their progression

Putative model for the role of MT-associated protein tau (Tau) + tau phosphorylation (pTau) during the synapse weakening

-       Human Tau proteins are encoded by the MAPT gene (chromosome 17)

-       Tau protein isoforms are generally denoted by their combination of N-terminal inserts + C-terminal repeats, MTBD = MT-binding domain

-       Tau contains multiple phosphorylation sites

How does an increase of Ca2+ affect Tau?

1.    During the basal conditions tau is present in dendrites + dendritic spines (low levels of pTau)

2.    Brief rising Ca2+ activates GSK-3b-mediated pTau at serine 396, which is critical event for subsequent AMPA R endocytosis through GluA2-PICK1 protein interaction

3.    Synapse weakening mechanisms may involve changes to T stability and/or the regulation of NMDA Rs via Fyn trafficking

(GSK-3b: glycogen synthase kinase 3b)(PICK-1: protein interacting with C kinase 1)

What are the consequences of pTau on synaptic function? (4 step process)

1.    Phosphorylation of Tau residues by GSK3b

2.    Tau phosphorylation at serine 396 (S396) residue is critical molecular event during LTD

3.    S396 is phosphorylated by GSK-3b

4.    Causes synapse weakening (LTD)

How does aberrant synapse weakening cause synaptic dysfunction + dementia?

-       Neurotoxic pathogens aberrantly activate a synapse weakening Caspase-3- GSK-3β signalling cascade

-       Simultaneously inhibit synapse strengthening pathways

-       Activated GSK-3β causes the phosphorylation of tau (pTau)

-       pTau at the synapse drives excessive synapse weakening, resulting in a shift from the basal state toward aberrant synaptic plasticity (eg., synapse loss)

How do system-based proteomics identify biological targets + discover therapeutic candidates?

System-based proteomics leveraging mass spectrometry (MS) enables comprehensive, unbiased identification of biological targets and discovery of therapeutic candidates by analyzing the entire protein landscape (proteome) rather than single targets

What do Mass Spectrometry-Based Proteomics Techniques provide?

(take human post-mortem brain -> quantitative proteomics)

→ MS-based proteomics provides direct, quantitative measurements of proteins and post-translational modifications (PTMs), capturing functional biology that RNA or DNA methods often miss

Mass Spectrometry-Based Proteomics Techniques (4) + explain each

1.   Quantitative Proteomics (TMT and Label-Free)

a.    Isobaric labelling (e.g., Tandem Mass Tags - TMT) or label-free quantification (LFQ) is used for high-throughput protein profiling across diseased and healthy samples to identify differentially expressed proteins (targets)

2.   Chemical Proteomics

a.    This approach identifies direct protein targets of small molecules by using active drug probes to isolate binding partners in complex biological samples.

3.   Thermal Proteome Profiling (TPP):

a.    A popular, non-labelled technique that identifies drug-target interactions by measuring changes in protein stability upon binding to a compound, often combined with data-independent acquisition (DIA) for high throughput.

4.   Targeted Proteomics (SRM/PRM):

a.    Techniques like Selected Reaction Monitoring (SRM) or Parallel Reaction Monitoring (PRM) offer high sensitivity and reproducibility for absolute quantification of specific target proteins and validation of therapeutic candidates

What does the integration of MS data allow?

allows researchers to map novel druggable spaces and discover promising therapeutic candidates

What does mass spec-based proteomic techniques allow for? (4) - how does it help in pharma/med

1.   Novel Target Identification:

a.    Proteomics identifies proteins that are disease-modifying, druggable, and expressed in specific tissues. This includes finding targets for protein degraders (PROTACs or molecular glues)

2.   Mechanism of Action (MoA) Elucidation: 

a.    MS-based proteomics helps define the exact mechanism of drug action by analyzing cellular, subcellular, or organismal protein changes, including on-target and off-target effects.

3.   Candidate Selection & Validation: 

a.    By comparing proteomes, researchers can identify proteins whose modulation corrects a disease phenotype. These candidates are then validated through functional assays and clinical studies.

4.   Overcoming Undruggable Targets:

a.     Next-generation proteomics platforms are enabling the discovery of targets in previously inaccessible areas of the proteome (e.g., protein-protein interactions)

Which responses are decreased in AD? (3)

1. Synaptic (neuronal cell type) → NT release, calmodium binding, dendrite, endocytosis

2. Mitochondrial (neuronal cell type) → hydrolase activity, oxi phosphorylation, ATP catabolic process

3. Cytoskeleton (no enrichment) → MT, protein targeting, protein complex assembly

Which responses are increased in AD? (3)

1. Inflammation (microglia, astrocyte + endothelial cell types) → carbohydrate + nucleoside metabolism

2. Myelination (oligodendrocyte cell type) → carbohydrate + nucleoside metabolism

3. RNA binding/splicing (no enrichment) → nucleoplasm, chromosome, DNA binding + metabolism

summary of LTD lecture (5)

• Changes to the synaptic expression of AMPA receptors (AMPAR) are considered a major component of synaptic plasticity and synaptic connectivity

• Long-term depression (LTD) is a physiological form of synaptic plasticity which leads to a reduction of synaptic transmission

• AMPAR endocytosis is a key molecular mechanism of LTD

• The glycogen synthase kinase-3 beta (GSK-3β)-mediated phosphorylation of Tau (pTau) may be a general pathway to regulate physiological LTD

• Alzheimer’s disease associated pathogens can cause pathological plasticity (eg., aberrant-excess LTD expression), this leads inhibition of long-term potentiation (LTP) and a simultaneous increase in LTD in the hippocampus resulting in synapse weakening