Organization of the core structure of the postsynaptic density

Abstract

Much is known about the composition and function of the postsynaptic density (PSD), but less is known about its molecular organization. We use EM tomography to delineate the organization of PSDs at glutamatergic synapses in rat hippocampal cultures. The core of the PSD is dominated by vertically oriented filaments, and ImmunoGold labeling shows that PSD-95 is a component of these filaments. Vertical filaments contact two types of transmembrane structures whose sizes and positions match those of glutamate receptors and intermesh with two types of horizontally oriented filaments lying 10–20 nm from the postsynaptic membrane. The longer horizontal filaments link adjacent NMDAR-type structures, whereas the smaller filaments link both NMDA- and AMPAR-type structures. The orthogonal, interlinked scaffold of filaments at the core of the PSD provides a structural basis for understanding dynamic aspects of postsynaptic function.

Structural plasticity with preserved topology in the postsynaptic protein network

“…Using previous measurements of the number of PSD-95 molecules present at synapses (8,12) (see SI Text), we converted our imaging data into an estimate of the absolute density of PSD-95 molecules in the PSD. The results of this analysis predict a mean density of 4.3 Ϯ 0.4 PSD-95 molecules per 1000 nm 2 in the PSD, or a center-to-center spacing of 17 Ϯ 1 nm, remarkably consistent with the 10 nm size of purified PSD-95 (22) and the nearest-neighbor spacing of 13 nm measured by electron tomography (17), and supporting the notion of a dense arrangement of PSD-95 molecules capable of binding one another either directly or through minimal intermediates. Notably, the pattern of molecular density within PSDs changed over time, with localized concentration and dispersion of PSD-95 occurring within PSD subdomains over periods of minutes (Fig.…”

“…We reasoned that changes to the PSD itself are highly likely to reflect altered synapse function. PSD-95 is tightly associated with the membrane by N-terminal palmitoylation and by PDZ interactions with NMDA receptors, transmembrane AMPA receptor regulatory proteins (TARPs), and synaptic adhesion molecules (17,44,45). The deformation of the PSD that we have documented is thus likely accompanied by localized distortion of the postsynaptic membrane, the shape of the synaptic cleft, or the alignment of the presynaptic and postsynaptic membranes.…”

“…Indeed, recent EM tomography (17) has confirmed the distribution of PSD-95 spaced throughout the PSD, and suggested that intermolecular spacing of PSD-95 may be established through intermediate scaffolds (17). Such intermediates provide an attractive mechanism to flexibly link nodes of the matrix in Fig.…”

“…Proteomic characterization of the PSD (8,(12)(13)(14), the atomic structure of many PSD proteins (15), the 3-dimensional shape of biochemically extracted PSDs (16), and the steady-state position of molecules within the PSD (16)(17)(18) have been determined to increasing accuracy. Recent live-imaging experiments have focused on the accumulation and dispersal of PSD scaffolds during synaptogenesis or synapse loss (19)(20)(21)(22), the translocation of assembled PSD scaffolds in developing neurons as synapses form (23), or the rates of exchange of various PSD constituents (22,(24)(25)(26).…”

Palmitoylation regulates glutamate receptor distributions in postsynaptic densities through control of PSD95 conformation and orientation

“…New studies using superresolution light microscopy observed that AMPARs (21,22) and palmitoylated PSD95 (22,23) within PSDs were clustered in nanodomains. Using EM tomography, Chen et al (1,2) found NMDAR clusters were separate from AMPARs. The NMDAR nanodomain contained NMDAR-PSD95 complexes in a 1:2 stoichiometry, whereas other PSD regions with AMPARs contained AMPAR-PSD95 complexes in a 1:1 stoichiometry.…”