The postsynaptic organization of synapses

Abstract

The postsynaptic side of the synapse is specialized to receive the neurotransmitter signal released from the presynaptic terminal and transduce it into electrical and biochemical changes in the postsynaptic cell. The cardinal functional components of the postsynaptic specialization of excitatory and inhibitory synapses are the ionotropic receptors (ligand-gated channels) for glutamate and γ-aminobutyric acid (GABA), respectively. These receptor channels are concentrated at the postsynaptic membrane and embedded in a dense and rich protein network comprised of anchoring and scaffolding molecules, signaling enzymes, cytoskeletal components, as well as other membrane proteins. Excitatory and inhibitory postsynaptic specializations are quite different in molecular organization. The postsynaptic density of excitatory synapses is especially complex and dynamic in composition and regulation; it contains hundreds of different proteins, many of which are required for cognitive function and implicated in psychiatric illness.

Figure 1.

Categorization of proteins of the PSD of excitatory synapses. (A) The percentage of PSD proteins (as identified by mass spectrometry of purified PSD fractions) in various functional classes. Proteins with miscellaneous functions comprise ∼15% (not shown). (B) Copy number of selected proteins in an average size PSD of the forebrain.

Figure 2.

Scaffold proteins of the PSD and their domains. Major scaffold proteins and cytoskeleton-associated proteins of the PSD of excitatory synapses, scaled approximately by size. Act, actin-binding; Ank, ankyrin repeats; ArfGAP, GTPase-activating protein for Arf small GTPase; CC, coiled coil; CH, calponin homology; CK, calmodulin-dependent kinase (CaMK)-like domain; CRIB, Cdc42/Rac interactive-binding; EFh, EF-hand; EVH1, Ena/VASP homology 1; GH1, GKAP homology 1; GK, guanylate kinaselike domain; GKBD, GKAP-binding domain; GRKBD, GRK2-binding domain; GTP-CDC, GTP-cell division protein; L27, domain initially found in LIN-2 and LIN-7; LRR, leucine-rich repeat; PDZ domains are shown as red ellipses. PH, pleckstrin homology; RapGAP, GTPase-activating protein for Rap small GTPases; RasGAP, GTPase-activating protein for Ras small GTPases; RCB, Rac binding; SAM, sterile alpha motif; SH3, Src homology 3; SHD, Spa2 homology domain; Spectrin, spectrin repeat; ST, subcellular targeting domain; WW, domain with two conserved Trp (W) residues.

Figure 3.

Molecular organization of the PSD of excitatory synapses. Schematic diagram of the major proteins of the PSD, with protein interactions indicated by direct contacts or overlaps between the proteins. The relative numbers of the proteins shown correlate roughly with their relative abundance in PSDs of forebrain. Copy numbers of most adhesion molecules and ion channels are unknown (indicated by dark gray lines). Each CaMKII shape represents a dodecamer.

Figure 4.

3D organization of the excitatory PSD revealed by EM tomography. (A) EM of excitatory synapse from neuron in hippocampal culture prepared with conventional fixation and staining. (B) Section (200 nm thick)—typical of those used for tomography—through a dendritic spine of cultured hippocampal neuron prepared by freeze substitution. (C) Virtual sections (1.4 nm thick) derived from a tomographic reconstruction (tomogram) of synapse shown in (B). Fine structural details become apparent within the PSD, including vertical filaments (arrows). Asterisk indicates a synaptic vesicle. Scale bar, 100 nm. (D) Rendering of vertical filaments (colored red) from the tomogram (on-edge view of the PSD, with the postsynaptic membrane in yellow). (E) En-face view of the postsynaptic membrane (yellow), showing the regular distribution of the vertical filaments (red). (F) On-edge view of the PSD at higher magnification in which one type of transmembrane structure has been rendered in green on its extracellular side and in blue on the cytoplasmic side of the postsynaptic membrane. These putative AMPA receptorlike structures are usually contacted by one vertical filament (red). (G) A second type of transmembrane structure (putative NMDA receptors) is shown rendered in gold on its extracellular side and azure on its larger cytoplasmic side. These putative NMDAR-type structures are typically contacted by two vertical filaments (red). (H) Cross section of PSD slightly tilted to reveal its cytoplasmic side. The vertical filaments (red) contact two types of horizontal filament, rendered in purple and white. (I) Schematic diagram showing the distribution and interactions of the two types of transmembrane structures, the vertical filaments and the two types of horizontal filaments. The azure and dark blue structures represent the cytoplasmic aspects of putative NMDA receptor and AMPA receptor complexes, respectively, and the red vertical filaments are likely composed of PSD-95. The vertical filaments connect transmembrane structures to an orthogonal, interlinked scaffold at the core of the PSD. (Figure adapted, with permission, from Chen et al. 2008b and reprinted, with permission, from the Society of Neuroscience © 2008.)

Figure 5.

Postsynaptic organization of inhibitory GABAergic synapses. Diagram of major proteins at a GABAergic synapse, with protein interactions indicated by direct contacts or overlaps between the shapes. Note that distinct cytoplasmic domains of neuroligin 2 bind to both gephyrin and S-SCAM. Glycinergic synapses likely show a similar molecular organization including the interaction of glycine receptors with gephyrin. See text for details.