The postsynaptic region, the PSD and their functions in synaptic plasticity

The postsynaptic region

• symmetric (Gray II): variable, large NT vesicles; gen. inhibitory

• asymmetric (Gray I): PSD; small, round NT vesicles; gen. excitatory

• formation

Glu-erg GABA / Gly-erg

• stabilisation

• synatic efficacy (plasticity)

regulated in excitatory and inhibitory synapses, too!

excitatory inhibitory

The postsynaptic region

mushroom

thin / filamentous

stubby

branched

~0.5 mm

~0.5 mm ~0.1 mm

PSD

Dendritic spines

Dendritic spines

• spine apparatus : SER cisternae

• PSD: postsynaptic density

• actin cytoskeleton

• vesicules, mitochondria

Dendritic spines

• mostly excitatory synapses – functions?

- increasing dendritic surface (Cajal)

- increasing connectivity: 1 axon forms 1-1 synapse/CA1 dendrites

- compartmentalisation: filtering local electric and [Ca2+] signals -> integration of incoming inputs

[neck: large electric resistance, diffusion bottleneck?]

- input-dependent plasticity: changing the head-to-neck ratio of spines

1-5 NMDAR activation/spine-> ~6000 Ca2+ influx into ~ 1 fL = ~10 mM [Ca2+] ; ~ 1 mM [Ca2+]free

Dendritic spines

Structure of the excitatory postsynaptic region

(Ras GTPase)

• adhesion molecules • cytoskeleton (actin),

scaffold proteins • membrane and organelle

transport • receptors and ion channels

(transport, localisation, regulation)

• + extracellular matrix

• + proteolytic processes

diverse

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pro

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co

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Formation and maintenance of the postsynaptic area

• activity-dependent cooperation: signals derived from nearby axons and dendrites

• ECM signals: attractive / repellent

- „unmasking”, proteolysis

Formation and maintenance of the postsynaptic area

neurexin / neuroligin

neurexin (pre) neuroligin (post)

• Ca-dependent trans-synaptic adhesion

• isoform- and splice variant-dependent functions in synaptogenesis

• PDZ binding domain: binding to PDZ/scaffold proteins, receptor clustering

Formation and maintenance of the postsynaptic area – adhesion complexes

neurexin / neuroligin

Formation and maintenance of the postsynaptic area – adhesion complexes

EphB / ephrin B

• Eph (A, B): ephrin receptor; receptor tyrosin kinases

• ephrin (A, B) : membrane-bound ligand; proteolysis also takes place!

• biderectional (pre/post) signaling

• binds to PDZ domain proteins

• EphB2: postsynapse; ephrin B: pre/postsynapse

• cis / trans effects, clustering

Rho, PAK -> actin • NMDAR, AMPAR

localisation (extra/intracell. effects)

Formation and maintenance of the postsynaptic area – adhesion complexes

Ig-superfamily

cadherins

• PDZ binding domain, Ig-like ectodomain

• SynCAM (synaptic CAM), NGL2 (netrin G2 ligand)

• Ca-dependent, homophilic adhesion

• a, b catenin, Rho -> actin

• pre- and postsynaptic effects, receptor clustering

• NCAM (neural CAM): activity-dependent poly-syalisation – to prepare for structural changes?

Formation and maintenance of the postsynaptic area – adhesion complexes

Adhesive signals and synaptic plasticity

Formation and maintenance of the postsynaptic area – scaffold proteins

• frequent multimerisation

- PDZ domain / PDZ target proteins

MAGUK family

Mei, Weng: Neuregulin 1 in neural development,

synaptic plasticity and schizophrenia

• multidomain proteins, diverse protein-protein interaction

Formation and maintenance of the postsynaptic area – scaffold proteins

• anchoring and/or transporting receptors

ProSAP/Shank: „master scaffolding” platform = PSD

Formation and maintenance of the postsynaptic area – scaffold proteins

Formation and maintenance of the postsynaptic area – actin polymerisation

• morphological changes: actin-driven process during development and plasticity

Formation and maintenance of the postsynaptic area – actin polymerisation

Synaptic plasticity and actin polymerisation

actin polymerisation actin depolymerisation

Synaptic plasticity and actin polymerisation

Formation and maintenance of the postsynaptic area – membrane microdomains and organelles

• regulated delivery of NT receptors to the synaptic membrane + regulated endocytosis + recycling

extrasynaptic membrane

perisynaptic membrane

synaptic membrane

recycling endosome

spine apparatus

Golgi

endocytotic zone

Formation and maintenance of the postsynaptic area – membrane microdomains and organelles

Membrane microdomains and synaptic plasticity

• ratio of synaptic/extrasynaptic NT receptors depends upon activity

postsynapes

GluR1 (AMPAR) trajectories

fixed

synaptic

extrasynaptic

GluR1 (AMPAR) distribution

• GluR1: higher lateral motility extrasynaptically

• AMPARs entering the synaptic area are „trapped” during activity

Formation and maintenance of the postsynaptic area – transport and localisation of NT receptors

• lateral diffusion:

- Ca2+- and phosphorylation dependent interactions with scaffold proteins

- endocytosis (EZ): prevents escape from the synaptic membranes/area

Formation and maintenance of the postsynaptic area – transport and localisation of NT receptors

• receptor recycling:

- surface / synaptic AMPAR quantity is regulated within minutes – fast activity-dependent actions

Receptor trafficking and synaptic plasticity

• regulated predominantly by the ubiquitin-proteasoma (UPS) system + extracellular proteases

• both pre- and postsynaptically

Formation and maintenance of the postsynaptic area – proteolysis

The inhibitory (symmetric) postsynaptic area

• CNS: mainly GABAAR and glycineR: hyperpolarisation

• different structure and composition, but similar functions:

- adhesion dependent synaptogenesis: neuroligin2 - neurexin

- postsynaptic protein scaffold: gephyrin

- activity-dependent NT receptor / vesicle recycling: radixin <-> HAP1

The inhibitory (symmetric) postsynaptic area

The inhibitory (symmetric) postsynaptic area

• very similar plasticity-dependent changes as in excitatory synapses

- changes in the synaptic membrane surface

- formation or elimination of synaptic connections

- changes in network activity

The inhibitory (symmetric) postsynaptic area

Nature 536, 210–214 (11 August 2016) doi:10.1038/nature19058

Trans-synaptic „nanocolumns”

• Dosophila NMJ, „ribbon” synapses: common organisation between the active zone and the postsynaptic structures

• frequently within the CNS: regulation of release likelihood within the AZ

https://www.youtube.com/watch?v=HPjADfy0O6g

Nature 536, 210–214 (11 August 2016) doi:10.1038/nature19058

Trans-synaptic „nanocolumns”

Essay questions (choose one) Describe the initiation and formation of the excitatory postsynaptic region! Are there any

similarities or differences compared to the formation and organisation of an inhbibitory

postsynaptic area? / Hol és hogyan alakulhat ki a serkentő posztszinapszis? Vesse

össze ennek főbb jellemzőit a gátló szinapszisokra jellemző folyamatokkal!

What are the characteristics of dendritic spines? How are they formed and what

functions do they play in synaptic communication or plasticity? / Mik azok a

dendrittüskék, hogyan alakulnak ki és hogyan lehet őket jellemezni? Milyen funkcionális

idegélettani szerepet tulajdonítanak ezeknek a struktúráknak?

Describe the major adhesive factors regulating synapse formation and maintenance! /

Milyen adhéziós komplexek játszhatnak szerepet a szinaptikus struktúra kialakításában

és fenntartásában? Ismertesse főbb jellegzetességeiket és funkciójukat is!

What functions do scaffold proteins have in synapse formation and maintenance? / Mik

azok az állvány (scaffold) fehérjék? Hogyan vesznek részt a szinaptikus struktúra

kialakításában és fenntartásában?

How are the transport and localisation of NT receptors regulated? How is it connected to

synaptic plasticity? / Milyen tényező(k) szabjá(k) meg a receptorok mozgását és

eloszlását a posztszinaptikus struktúrában? Hogyan befolyásolja ez a szinaptikus

plaszticitást?

Recommended literature

Enlarged figures

Dendritic spines

Ultrastructure of the dendritic spines

spine apparatus („tüske készülék”)

SER network

clathrin-dependent endocytosis

Proteolytic unmasking of ECM-resident signalling functions. a | Unmasking of previously unrecognizable receptor-binding sites may result from

conformational changes due to proteolytic cleavage within a domain of an extracellular matrix (ECM) molecule. Different molecular effects resulting from

proteolytic cleavage of an ECM protein by matrix metalloproteinase 9 (MMP9) may lead to the exposure of a previously hidden, or ‘cryptic’, binding site for

integrin receptors, such as an Arg–Gly–Asp (RGD)-containing binding site for a β1 integrin receptor. Integrin signalling increases the lateral mobility of NMDARs

(N-methyl-d-aspartate receptors) through an unknown mechanism. Receptor diffusion between synaptic and extrasynaptic domains might be a mechanism for

the regulation of synapse maturation and plasticity119,120. b | Unmasking of a cryptic ECM-resident signal may also result from the proteolytic separation of

one or multiple domains from the parent ECM molecule. The neuronal serine protease neurotrypsin is stored in presynaptic terminals and secreted in an inactive

form in association with presynaptic action potential (AP) firing. Its activation requires an NMDAR-dependent postsynaptic process. Activated neurotrypsin

cleaves agrin and yields a carboxy-terminal 22-kDa fragment (agrin 22), which is essential for the formation of dendritic filopodia96. The domain structure of

agrin is illustrated: a transmembrane segment that is present in transmembrane isoforms (shown in grey); cysteine-rich repeats that are similar to follistatin

(shown in light blue); laminin epidermal growth factor (EGF)-like domains (shown in dark blue); serine/threonine-rich regions (shown in yellow); sperm protein,

enterokinase and agrin domain (shown in orange); EGF-like domains (shown in green); and laminin globular domains (shown in pink). The neurotryps

independent α- and β-cleavage sites are indicated together with the C-terminal 90-kDa and 22-kDa fragments (shown by black bars), which result from the

cleavage of agrin.

ECM proteolysis and synaptogenesis

Integrating Models for Receptor

Trafficking and Diffusion during

Synaptic Plasticity

Induction of LTP by Ca2+ influx

through NMDA receptors leads to

activation (lightning bolt) of PKA

and CaMKII, which in turn

promotes the mobilization of

recycling endosomes (RE) into

spines, exocytosis from recycling

endosomes, and appearance of

AMPA receptors at the spine

membrane. The number of

available slots in the PSD

increases through unknown

mechanisms, which can be filled by

increased levels of extrasynaptic

AMPA receptors. Receptor

diffusion inside synapses

decreases due to stronger scaffold

interactions and/or receptor

confinement. The EZ may also

contribute to LTP by maintaining

local recycling of AMPA receptors

and preventing their escape from

the spine membrane. On the other

hand, induction of LTD leads to

activation of protein phosphatases

(lightning bolt), including PP2B and

PP1, triggering clathrin-, dynamin-,

and Rab5-dependent endocytosis of AMPA receptors, likely at the spine EZ. Receptor downregulation occurs

by trafficking through early (EE) and late endosomes (LE). Loss of synaptic slot positions through unknown

mechanisms reduces AMPA receptor capacity and increases the diffusion of synaptic AMPA receptors. EZ,

endocytic zone; P-GluR1, phosphorylated GluR1.

Formation and maintenance of the postsynaptic area – membrane microdomains and organelles