Week 7: Postsynaptic Density, Neuroligins, PSD-95, and Related Proteins
The Postsynaptic Density: Overview
The postsynaptic density (PSD) is the postsynaptic specialization at synapses, comprising membrane and cytoplasmic proteins clustered immediately opposite release sites or active zones.
It includes ligand-gated ion channels (), anchoring proteins, cytoskeleton components, and regulatory proteins.
It is called a "density" because it appears particularly dense in electron microscopy images.
PSDs differ according to the type of synapse (excitatory vs inhibitory).
Core organizing molecules and CAMs play key roles in signal transmission and structural organization across the synapse.
Neuroligins (postsynaptic CAMs) form trans-synaptic interactions with neurexins (presynaptic CAMs), helping couple pre- and post-synaptic specialization.
PSD Organization and Architecture
The PSD comprises two main layers:
PSD core (superficial): high concentration of PSD-95; oriented so that the N-terminus is near the postsynaptic membrane and the C-terminus extends into the spine; positions PSD-95 to bind neurotransmitter receptors.
PSD pallium (deeper layer): scaffold of Shank and Homer proteins; becomes denser during intense synaptic activity due to reversible recruitment of CaMKII and other proteins.
The pallium thickens with activity, acting as a dynamic signaling scaffold.
In excitatory synapses, the PSD contains hundreds to thousands of proteins; in inhibitory synapses, there are substantial but fewer proteins (see alphabet soup below).
The Alphabet Soup of PSD Proteins
Excitatory synapses probably have more than >1000 different proteins.
Inhibitory synapses have at least different proteins.
Many PSD proteins are CAMs, so their names do not always reflect function (examples: Homer, Shank, PSD-95, PSD-93, gephyrin).
The large diversity underpins the complexity of synaptic signaling and plasticity.
Major Protein Families in the PSD
MAGUKs (membrane-associated guanylate kinases):
PSD-95 (DLG4, SAP90) is the most abundant MAGUK at the mammalian PSD.
Other MAGUKs include SAP97 (DLG1), PSD-93 (DLG2, Chapsyn-110), and SAP102 (DLG3).
GKAP (guanylate kinase-associated protein): also called PSD-95-associated protein or DAP-1; binds to Shank and PSD-95.
Shank family: also called ProSAP (Proline-rich synapse-associated protein); other names include Synamon and CortBP; SH3-containing and multiple ankyrin-repeat domain proteins; links to the actin cytoskeleton and other scaffolds.
Homer family: also called Vesl, Cupidin, PSD-Zip45; links Shank to metabotropic glutamate receptors and IP3 receptors on the smooth endoplasmic reticulum (SER).
PSD as a Signaling Hub
PSD proteins are not just structural scaffolds; they organize and propagate signaling cascades, including:
NMDA receptors and AMPA receptors (glutamate receptors) anchored within the PSD.
Tyrosine kinase receptors (RTKs) and various intracellular signal-transduction molecules.
Calcium/calmodulin-dependent protein kinase II (CaMKII), a central signaling node that autophosphorylates and acts as a molecular memory device important for long-term potentiation (LTP).
The PSD hosts a wide array of signaling interactions, enabling rapid and coordinated synaptic responses.
Neuroligins, Neurexins, and Trans-synaptic Organization
Neuroligins are type I postsynaptic membrane proteins that act as adhesion molecules and ligands for presynaptic neurexins, thereby linking the two sides of the synapse.
Neurexins (NRXN) are presynaptic CAMs with a single transmembrane domain and roles in connecting neurons at the synapse.
Neuroligins organize the PSD by binding across the synaptic cleft to neurexins, helping to specify synaptic type and maintain synapse structure.
Types of Neuroligins:
Neuroligin 1 (NLGN1): characteristically found in all glutamatergic synapses.
Neuroligin 2 (NLGN2): preferentially at certain inhibitory (GABAergic) synapses.
Neuroligin 3 (NLGN3): found in both excitatory and inhibitory synapses.
Neuroligin 4 (NLGN4): preferentially at glycinergic synapses in the retina.
Neuroligins bind to PSD-95 via the 3rd PDZ domain; PSD-95 binds AMPA receptors via the 1st PDZ domain, helping to stabilize AMPA receptors at the postsynaptic density.
The Neuroligin/Neurexin interaction helps anchor AMPA receptors and coordinates synaptic signaling, but other CAMs can also contribute to synapse formation and maintenance.
PSD-95: PDZ Domains and Interaction Network
PSD-95 contains 3 PDZ domains (PDZ1, PDZ2, PDZ3) plus SH3 and GK (guanylate kinase) domains.
PDZ domains are ~ amino acids in length.
Neuroligins bind to PSD-95 via the 3rd PDZ domain; PSD-95 binds AMPA receptors via the 1st PDZ domain; this arrangement helps stabilize AMPA receptors at the synapse.
PSD-95 also binds NMDA receptors via PDZ interactions and can associate with CaMKII and neuronal nitric oxide synthase (nNOS), integrating excitatory signaling and plasticity.
PSD-95 interacts with several other proteins, including GKAP (to connect to Shank) and Homer, forming a dense signaling network at the PSD's superficial layer.
The PSD-95 scaffold thus coordinates receptor anchoring and downstream signaling to shape synaptic strength.
Beyond PDZ: Other Domains of PSD-95 and Interacting Partners
PSD-95 has additional domains (SH3 and GUK) that mediate interactions with other partners such as:
NRXN1B (neurexin 1B)
NLGN1 (neuroligin 1)
Additional partners through PDZ-independent interactions.
The PDZ-based interactions and SH3/GUK-mediated associations together organize excitatory synapses and organize a network of signaling across the PSD.
Signaling Molecules in the PSD
NMDA receptors: critical for calcium influx and synaptic plasticity.
CaMKII: autophosphorylates, acts as a molecular memory device, essential for LTP.
Neuronal nitric oxide synthase (nNOS): implicated in synaptic plasticity and neuroprotection.
Shank proteins: connect to GKAP and, via Homer, to metabotropic glutamate receptors (mGluRs) and IP3 receptors on the SER, enabling Ca2+ signaling from internal stores.
The interplay between NMDA receptors, CaMKII, Shank, and Homer governs calcium signaling, receptor trafficking, and plasticity.
Shank, GKAP, and Homer: Structural Coupling and Signaling Bridges
Shank proteins couple to the actin cytoskeleton via interactions with GKAP and PSD-95; they act as a bridge from the core PSD to signaling receptors.
Shank proteins can bind directly to neuroligins, contributing to trans-synaptic signaling and possibly coordinating presynaptic changes through neurexins.
GKAP (DAP-1) binds to Shank and PSD-95, linking the core scaffolds to the larger pallial scaffold.
Homer connects Shank to IP3 receptors and thus to intracellular Ca2+ signaling, providing a route from surface receptors to internal Ca2+ stores.
The combined Shank–GKAP–Homer network underpins a versatile scaffold for receptor organization and signaling.
Inhibitory Synapses: Gephyrin and GABA/Glycine Clusters
In inhibitory synapses, gephyrin is the key organizing molecule.
Gephyrin self-assembles into a hexagonal lattice and interacts with various inhibitory postsynaptic density proteins.
GABA and glycine receptors (both pentameric transmembrane receptor families) cluster at inhibitory synapses via gephyrin.
These receptors are part of the same superfamily as nicotinic acetylcholine receptors (pentameric transmembrane proteins with at least two α-subunits).
A key question remains whether nicotinic synapses have PSDs analogous to excitatory PSDs or inhibitory PSDs, highlighting ongoing research gaps in PSD organization across synapse types.
Trans-synaptic Regulation of Presynaptic Specializations
Post-synaptic specializations regulate pre-synaptic specializations via CAMs, establishing trans-synaptic coherence.
The major example is the neurexin–neuroligin interaction, which links pre- and post-synaptic machinery.
Deletion of neuroligin does not completely prevent synapse formation; other cell adhesion molecules can substitute and preserve synapse formation, indicating redundancy in synaptic assembly.
Post-synaptic Density: Composition and Core Functions
PSDs include dozens of signal transduction molecules, such as:
Glutamate receptors (NMDA-R, mGluR)
Tyrosine kinase receptors (RTKs)
Many intracellular signaling molecules, notably CaMKII (autophosphorylation supports LTP)
The PSD acts as a hub for converting receptor activity into intracellular signaling and reorganizing presynaptic elements.
Summary of Key Points
Neuroligins on the postsynaptic membrane bind to presynaptic neurexins, organizing synapse formation and maintenance.
There are four neuroligin types with distinct synaptic localizations: NLGN1 (glutamatergic), NLGN2 (inhibitory), NLGN3 (excitatory and inhibitory), NLGN4 (glycinergic retina).
PSD-95 (DLG4) is the major MAGUK scaffold in the PSD, containing 3 PDZ domains (PDZ1-3) plus SH3 and GUK domains, and binds multiple partners including NMDA receptors, AMPA receptors, neuroligins, CaMKII, and nNOS.
The PDZ domains organize receptor anchoring and signal transduction; Neuroligins bind via the 3rd PDZ domain, while AMPA receptors bind via the 1st PDZ domain of PSD-95.
The PSD core (high PSD-95) and pallium (Shank/Homer scaffolds) coordinate receptor organization and signaling; pallium density increases with intense activity due to CaMKII recruitment.
Shank, GKAP, and Homer form a critical signaling scaffold linking surface receptors to intracellular Ca2+ signaling through IP3 receptors and actin cytoskeleton interactions.
Inhibitory synapses rely on gephyrin to organize GABA and glycine receptor clusters, with gephyrin forming a lattice structure at these synapses.
Trans-synaptic CAMs (neurexin–neuroligin and related molecules) coordinate presynaptic active zones with postsynaptic densities; however, disruption of specific CAMs can be compensated by other CAMs, indicating redundancy in synapse formation.
The PSD functions as a central signaling hub with components like NMDA receptors, CaMKII, nNOS, and RTKs, underpinning synaptic plasticity, memory formation, and neuroprotection.
Autism-linked mutations frequently affect PSD proteins and CAMs, underscoring the importance of PSD organization for normal synaptic function and neurodevelopment.
References to specific slide details (for exam familiarity):
PSD layers: core vs pallium; CaMKII-dependent densification during activity.
Alphabet soup: excitatory >>1000 proteins; inhibitory > proteins; examples include Homer, Shank, PSD-95, PSD-93, gephyrin.
MAGUKs and GKAP/DAP-1 as scaffolding connectors between Shank and PSD-95.
Neuroligins (NLGN1-4) and Neurexins (NRXN) as central trans-synaptic organizers.
PDZ-domain architecture of PSD-95 and implications for receptor anchoring and signaling.
Shank–Homer–mGluR–IP3 receptor signaling axis and its link to intracellular Ca2+ stores.
Gephyrin as the organizing molecule for inhibitory synapses and its lattice-like assembly.