NMDA-induced accumulation of Shank at the postsynaptic density is mediated by CaMKII

IRSp53 accumulates at the postsynaptic density under excitatory conditions

IRSp53 (BAIAP2) is an abundant protein at the postsynaptic density (PSD) that binds to major PSD scaffolds, PSD-95 and Shanks, as well as to F-actin. The distribution of IRSp53 at the PSD in cultured hippocampal neurons was examined under basal and excitatory conditions by immuno-electron microscopy. Under basal conditions, label for IRSp53 is concentrated at the PSD. Upon depolarization by application of a medium containing 90 mM K+, the intensity of IRSp53 label at the PSD increased by 36±7%. Application of NMDA (50 μM) yielded 53±1% increase in the intensity of IRSp53 label at the PSD compared to controls treated with APV, an NMDA antagonist. The accumulation of IRSp53 label upon application of high K+ or NMDA was prominent at the deeper region of the PSD (the PSD pallium, lying 40-120 nm from the postsynaptic plasma membrane). IRSp53 molecules that accumulate at the distal region of the PSD pallium under excitatory conditions are too far from the plasma membrane to fulfill the generally recognized role of the protein as an effector of membrane-bound small GTPases. Instead, these IRSp53 molecules may have a structural role organizing the Shank scaffold and/or linking the PSD to the actin cytoskeleton.

CaMKII-mediated displacement of AIDA-1 out of the postsynaptic density core

Ankyrin repeat and sterile alpha motif domain-containing protein 1B (ANKS1B, also known as AIDA-1) is a major component of the postsynaptic density (PSD) in excitatory neurons where it concentrates at the electron-dense core under basal conditions and moves out during activity. This study investigates the molecular mechanism underlying activity-induced displacement of AIDA-1. Experiments with PSD fractions from brain indicate phosphorylation of AIDA-1 upon activation of endogenous CaMKII. Immuno-electron microscopy studies show that treatment of hippocampal neurons with NMDA results in an ~ 30 nm shift in the median distance of the AIDA-1 label from the postsynaptic membrane, an effect that is blocked by the CaMKII inhibitor tatCN21. CaMKII-mediated redistribution of AIDA-1 is similar to that observed for SynGAP. CaMKII-mediated removal of two abundant PSD-95-binding proteins from the PSD core during activity is expected to initiate a molecular reorganization at the PSD.

The Postsynaptic Density: There Is More than Meets the Eye

The postsynaptic density (PSD), apparent in electron micrographs as a dense lamina just beneath the postsynaptic membrane, includes a deeper layer, the “pallium”, containing a scaffold of Shank and Homer proteins. Though poorly defined in traditionally prepared thin-section electron micrographs, the pallium becomes denser and more conspicuous during intense synaptic activity, due to the reversible addition of CaMKII and other proteins. In this Perspective article, we review the significance of CaMKII-mediated recruitment of proteins to the pallium with respect to both the trafficking of receptors and the remodeling of spine shape that follow synaptic stimulation. We suggest that the level and duration of CaMKII translocation and activation in the pallium will shape activity-induced changes in the spine.

Zinc Stabilizes Shank3 at the Postsynaptic Density of Hippocampal Synapses

Shank3 is a postsynaptic density (PSD) scaffold protein of the Shank family. Here we use pre-embedding immunogold electron microscopy to investigate factors influencing the distribution of Shank3 at the PSD. In dissociated rat hippocampal cultures under basal conditions, label for Shank3 was concentrated in a broad layer of the PSD, ~20–80 nm from the postsynaptic membrane. Upon depolarization with high K⁺ (90 mM, 2 min), or application of NMDA (50 μM, 2 min), both the labeling intensity at the PSD and the median distance of label from the postsynaptic membrane increased significantly, indicating that Shank3 molecules are preferentially recruited to the distal layer of the PSD. Incubation in medium supplemented with zinc (50 μM ZnCl₂, 1 hr) also significantly increased labeling intensity for Shank3 at the PSD, but this addition of Shank3 was not preferential to the distal layer. When cells were incubated with zinc and then treated with NMDA, labeling intensity of Shank3 became higher than with either treatment alone and manifested a preference for the distal layer of the PSD. Without zinc supplementation, NMDA-induced accumulation of Shank3 at the PSD was transient, reversing within 30 min after return to control medium. However, when zinc was included in culture media throughout the experiment, the NMDA-induced accumulation of Shank3 was largely retained, including Shank3 molecules recruited to the distal layer of the PSD. These results demonstrate that activity induces accumulation of Shank3 at the PSD and that zinc stabilizes PSD-associated Shank3, possibly through strengthening of Shank-Shank association.

Differential distribution of Shank and GKAP at the postsynaptic density

Shank and GKAP are scaffold proteins and binding partners at the postsynaptic density (PSD). The distribution and dynamics of Shank and GKAP were studied in dissociated hippocampal cultures by pre-embedding immunogold electron microscopy. Antibodies against epitopes containing their respective mutual binding sites were used to verify the expected juxtapositioning of Shank and GKAP. If all Shank and GKAP molecules at the PSD were bound to each other, the distribution of label for the two proteins should coincide. However, labels for the mutual binding sites showed significant differences in distribution, with a narrow distribution for GKAP located close to the postsynaptic membrane, and a wider distribution for Shank extending deeper into the cytoplasm. Upon depolarization with high K⁺, neither the intensity nor distribution of label for GKAP changed, but labeling intensity for Shank at the PSD increased to ~150% of controls while the median distance of label from postsynaptic membrane increased by 7.5 nm. These results indicate a preferential recruitment of Shank to more distal parts of the PSD complex. Conversely, upon incubation in Ca²⁺-free medium containing EGTA, the labeling intensity of Shank at the PSD decreased to ~70% of controls and the median distance of label from postsynaptic membrane decreased by 9 nm, indicating a preferential loss of Shank molecules in more distal parts of the PSD complex. These observations identify two pools of Shank at the PSD complex, one relatively stable pool, closer to the postsynaptic membrane that can bind to GKAP, and another more dynamic pool at a location too far away to bind to GKAP.