Postsynaptic densities (PSDs) in the forebrain contain ~ 100 – 400 proteins, including receptors (NMDA; AMPA) scaffolding molecules (DLG4, DLG1; HOMER1; SHANK2), kinases, phosphatases, and cytoskeletal components. They are typically 20–30 nm thick and 300 nm long.
In order to build a model of the PSD using DLG4*, Chen et al (here) cultured hippocampal neurons, incubated primary antibodies to DLG4, conjugated the antibody to secondary 1.4-nm gold bodies, and silver enhanced the gold. They then visualized 8 dendritic spines free from ice damage with EM tomography with pixel sizes of 0.48 – 0.75 nm. Finally, they built a model of the PSD in excitatory (glutamate-based) synapses based on imaging each component.
For example, via the following an EM image of 120 nm thick section (A), they were able to create a 1.5 nm thick virtual image without so much overlap (B), and then render the vertical filaments in a 3d reconstruction (C):
The vertical filaments are typically 5 nm in diameter (4.9 ± 0.2 nm, n = 22) and 20 nm long (21 ± 3 nm, range 16–25 nm, n = 38). They are also uniformly spaced. On the basis of the immunolabeling, it appears that these filaments are DLG4 proteins, with their N-termini attached to the membrane.
After doing this for each component of the PSD, they propose the following model, where red is vertical filaments of DLG4, cyan is NMDA-like receptors, blue is AMPA-like receptors, purple is horizontal filaments associated with NMDA / AMPA – like receptors, and white cross-links the vertical filament meshwork, especially near NMDA receptors:
It’s possible that some of the vertical filaments are not DLG4 proteins but instead some other type. However, there are ~ 300 DLG4 proteins and ~ 30 NMDA receptors per PSD (see here) so it is certainly possible that there could be 1 vertical filament for each NMDA receptor with spares left over for AMPA receptors. Regardless, this is a useful mental map for the molecular structure of the PSD, with implications for the mechanisms of AMPA / NMDA receptor-dependent synaptic plasticity.
* DLG4 is AKA PSD-95, but I’m going with the wikipedia definition.
Chen X, et al. 2008 Organization of the core structure of the postsynaptic density. PNAS doi: 10.1073/pnas.0800897105 .
Chen X, et al. 2005 Mass of the postsynaptic density and enumeration of three key molecules. PNAS doi: 10.1073/pnas.0505359102