A study of the spatial protein organization of the postsynaptic density isolated from porcine cerebral cortex and cerebellum

Non-microtubule tubulin-based backbone and subordinate components of postsynaptic density lattices

This study proposes a postsynaptic density (PSD) lattice model comprising a non-microtubule tubulin-based backbone structure and its associated proteins, including various PSD scaffold/adaptor proteins and other PSD proteins.

A purification protocol was developed to identify and analyze the component proteins of a postsynaptic density (PSD) lattice, a core structure of the PSD of excitatory synapses in the central nervous system. “Enriched”- and “lean”-type PSD lattices were purified by synaptic plasma membrane treatment to identify the protein components by comprehensive shotgun mass spectrometry and group them into minimum essential cytoskeleton (MEC) and non-MEC components. Tubulin was found to be a major component of the MEC, with non-microtubule tubulin widely distributed on the purified PSD lattice. The presence of tubulin in and around PSDs was verified by post-embedding immunogold labeling EM of cerebral cortex. Non-MEC proteins included various typical scaffold/adaptor PSD proteins and other class PSD proteins. Thus, this study provides a new PSD lattice model consisting of non-microtubule tubulin-based backbone and various non-MEC proteins. Our findings suggest that tubulin is a key component constructing the backbone and that the associated components are essential for the versatile functions of the PSD.

The study of microtubule dynamics and stability at the postsynaptic density in a rat pilocarpine model of temporal lobe epilepsy

Background

The recurrence and drug resistance of temporal lobe epilepsy (TLE) has been ceaselessly challenging scientists and epilepsy experts. There has been an accumulation of evidence linking the dysregulation of postsynaptic proteins etiology and the pathology of epilepsy. For example, NMDA receptors, AMPA receptors, and metabotropic glutamate receptors (mGluRs). Furthermore, our earlier proteomic analysis proved there to be differential expressions of cytoskeletons like microtubules among rat groups. These differential expressions were shown in TLE-spontaneous recurrent seizures (TLE-SRS), TLE without SRS (TLE-NSRS) and control groups. Therefore, we aimed to understand how the microtubule system of the hippocampal postsynaptic density (PSD) regulates the development of TLE.

Methods

In this study, a pilocarpine-induced Sprague-Dawley rat TLE model were used, and Western blot, Nissl staining, and the immunoelectron microscopic method were utilized to determine the dynamic change of microtubules (α- and β-tubulin) in PSD and the extent of hippocampal neuron loss respectively in acute SE, and latent and chronic (spontaneous seizures) periods. Animal models were then stereotactically treated using colchicine, a microtubule depolymerizer, and paclitaxel, a microtubule polymerization agent, after each animal's acute SE period so as to further explore the function of PSD microtubules.

Results

Our study revealed 3 principal findings. One, both α- and β-tubulin were decreased from the 3^rd to the 30^th day (lowest at the 7^th day) in the seizure group compared with the controls. Two, both α- and β-tubulin were found to be more downregulated in the TLE-SRS and the TLE-NSRS group than in the control group (especially in the TLE-SRS group). The same trend was also noticed for hippocampal neuron loss. Three, the paclitaxel lowered the chronic SRS rate and increased the expression of PSD β-tubulin in the hippocampus.

Conclusions

Altogether, these results indicate that the microtubule system of PSD may play an essential role in the development and recurrence of epilepsy, and it may be used as a new target for the prevention and treatment of this refractory disease.

Protein components of post-synaptic density lattice, a backbone structure for type I excitatory synapses

It is essential to study the molecular architecture of post-synaptic density (PSD) to understand the molecular mechanism underlying the dynamic nature of PSD, one of the bases of synaptic plasticity. A well-known model for the architecture of PSD of type I excitatory synapses basically comprises of several scaffolding proteins (scaffold protein model). On the contrary, 'PSD lattice' observed through electron microscopy has been considered a basic backbone of type I PSDs. However, major constituents of the PSD lattice and the relationship between the PSD lattice and the scaffold protein model, remain unknown. We purified a PSD lattice fraction from the synaptic plasma membrane of rat forebrain. Protein components of the PSD lattice were examined through immuno-gold negative staining electron microscopy. The results indicated that tubulin, actin, α-internexin, and Ca²⁺ /calmodulin-dependent kinase II are major constituents of the PSD lattice, whereas scaffold proteins such as PSD-95, SAP102, GKAP, Shank1, and Homer, were rather minor components. A similar structure was also purified from the synaptic plasma membrane of forebrains from 7-day-old rats. On the basis of this study, we propose a 'PSD lattice-based dynamic nanocolumn' model for PSD molecular architecture, in which the scaffold protein model and the PSD lattice model are combined and an idea of dynamic nanocolumn PSD subdomain is also included. In the model, cytoskeletal proteins, in particular, tubulin, actin, and α-internexin, may play major roles in the construction of the PSD backbone and provide linker sites for various PSD scaffold protein complexes/subdomains.

Correlation profiling of brain sub-cellular proteomes reveals co-assembly of synaptic proteins and subcellular distribution

Protein correlation profiling might assist in defining co-assembled proteins and subcellular distribution. Here, we quantified the proteomes of five biochemically isolated mouse brain cellular sub-fractions, with emphasis on synaptic compartments, from three brain regions, hippocampus, cortex and cerebellum. We demonstrated the expected co-fractionation of canonical synaptic proteins belonging to the same functional groups. The enrichment profiles also suggested the presence of many novel pre- and post-synaptic proteins. Using super-resolution microscopy on primary neuronal culture we confirmed the postsynaptic localization of PLEKHA5 and ADGRA1. We further detected profound brain region specific differences in the extent of enrichment for some functionally associated proteins. This is exemplified by different AMPA receptor subunits and substantial differences in sub-fraction distribution of their potential interactors, which implicated the differences of AMPA receptor complex compositions. This resource aids the identification of proteins partners and subcellular distribution of synaptic proteins.

Actinin-4 Governs Dendritic Spine Dynamics and Promotes Their Remodeling by Metabotropic Glutamate Receptors

Dendritic spines are dynamic, actin-rich protrusions in neurons that undergo remodeling during neuronal development and activity-dependent plasticity within the central nervous system. Although group 1 metabotropic glutamate receptors (mGluRs) are critical for spine remodeling under physiopathological conditions, the molecular components linking receptor activity to structural plasticity remain unknown. Here we identify a Ca(2+)-sensitive actin-binding protein, α-actinin-4, as a novel group 1 mGluR-interacting partner that orchestrates spine dynamics and morphogenesis in primary neurons. Functional silencing of α-actinin-4 abolished spine elongation and turnover stimulated by group 1 mGluRs despite intact surface receptor expression and downstream ERK1/2 signaling. This function of α-actinin-4 in spine dynamics was underscored by gain-of-function phenotypes in untreated neurons. Here α-actinin-4 induced spine head enlargement, a morphological change requiring the C-terminal domain of α-actinin-4 that binds to CaMKII, an interaction we showed to be regulated by group 1 mGluR activation. Our data provide mechanistic insights into spine remodeling by metabotropic signaling and identify α-actinin-4 as a critical effector of structural plasticity within neurons.

Dendritic spine alterations in schizophrenia

Schizophrenia is a chronic illness affecting approximately 0.5-1% of the world's population. The etiology of schizophrenia is complex, including multiple genes, and contributing environmental effects that adversely impact neurodevelopment. Nevertheless, a final common result, present in many subjects with schizophrenia, is impairment of pyramidal neuron dendritic morphology in multiple regions of the cerebral cortex. In this review, we summarize the evidence of reduced dendritic spine density and other dendritic abnormalities in schizophrenia, evaluate current data that informs the neurodevelopment timing of these impairments, and discuss what is known about possible upstream sources of dendritic spine loss in this illness.

Chemical crosslinkers enhance detection of receptor interactomes

Receptor function is dependent on interaction with various intracellular proteins that ensure the localization and signaling of the receptor. While a number of approaches have been optimized for the isolation, purification, and proteomic characterization of receptor-protein interaction networks (interactomes) in cells, the capture of receptor interactomes and their dynamic properties remains a challenge. In particular, the study of interactome components that bind to the receptor with low affinity or can rapidly dissociate from the macromolecular complex is difficult. Here we describe how chemical crosslinking (CC) can aid in the isolation and proteomic analysis of receptor-protein interactions. The addition of CC to standard affinity purification and mass spectrometry protocols boosts the power of protein capture within the proteomic assay and enables the identification of specific binding partners under various cellular and receptor states. The utility of CC in receptor interactome studies is highlighted for the nicotinic acetylcholine receptor as well as several other receptor types. A better understanding of receptors and their interactions with proteins spearheads molecular biology, informs an integral part of bench medicine which helps in drug development, drug action, and understanding the pathophysiology of disease.

Glutamate receptor subunit and calmodulin kinase II expression, with and without T maze training, in the rat hippocampus following bilateral vestibular deafferentation

Many previous studies have shown that lesions of the peripheral vestibular system result in spatial memory deficits and electrophysiological dysfunction in the hippocampus. Given the importance of glutamate as a neurotransmitter in the hippocampus, it was predicted that bilateral vestibular deafferentation (BVD) would alter the expression of NMDA and AMPA receptors in this area of the brain.