Proteomic analysis of activity-dependent synaptic plasticity in hippocampal neurons

Anti-GluA3 autoantibodies define a new sub-population of frontotemporal lobar degeneration patients with distinct neuropathological features

Autoantibodies directed against the GluA3 subunit (anti-GluA3 hIgGs) of AMPA receptors have been identified in 20%-25% of patients with frontotemporal lobar degeneration (FTLD). Data from patients and in vitro/ex vivo pre-clinical studies indicate that anti-GluA3 hIgGs negatively affect glutamatergic neurotransmission. However, whether and how the chronic presence of anti-GluA3 hIgGs triggers synaptic dysfunctions and the appearance of FTLD-related neuropathological and behavioural signature has not been clarified yet. To address this question, we developed and characterized a pre-clinical mouse model of passive immunization with anti-GluA3 hIgGs purified from patients. In parallel, we clinically compared FTLD patients who were positive for anti-GluA3 hIgGs to negative ones. Clinical data showed that the presence of anti-GluA3 hIgGs defined a subgroup of patients with distinct clinical features. In the preclinical model, anti-GluA3 hIgGs administration led to accumulation of phospho-tau in the postsynaptic fraction and dendritic spine loss in the prefrontal cortex. Remarkably, the preclinical model exhibited behavioural disturbances that mostly reflected the deficits proper of patients positive for anti-GluA3 hIgGs. Of note, anti-GluA3 hIgGs-mediated alterations were rescued in the animal model by enhancing glutamatergic neurotransmission with a positive allosteric modulator of AMPA receptors. Overall, our study clarified the contribution of anti-GluA3 autoantibodies to central nervous system symptoms and pathology and identified a specific subgroup of FTLD patients. Our findings will be instrumental in the development of a therapeutic personalised medicine strategy for patients positive for anti-GluA3 hIgGs.

Functional interdependence of the actin regulators CAP1 and cofilin1 in control of dendritic spine morphology

The vast majority of excitatory synapses are formed on small dendritic protrusions termed dendritic spines. Dendritic spines vary in size and density that are crucial determinants of excitatory synaptic transmission. Aberrations in spine morphogenesis can compromise brain function and have been associated with neuropsychiatric disorders. Actin filaments (F-actin) are the major structural component of dendritic spines, and therefore, actin-binding proteins (ABP) that control F-actin dis-/assembly moved into the focus as critical regulators of brain function. Studies of the past decade identified the ABP cofilin1 as a key regulator of spine morphology, synaptic transmission, and behavior, and they emphasized the necessity for a tight control of cofilin1 to ensure proper brain function. Here, we report spine enrichment of cyclase-associated protein 1 (CAP1), a conserved multidomain protein with largely unknown physiological functions. Super-resolution microscopy and live cell imaging of CAP1-deficient hippocampal neurons revealed impaired synaptic F-actin organization and dynamics associated with alterations in spine morphology. Mechanistically, we found that CAP1 cooperates with cofilin1 in spines and that its helical folded domain is relevant for this interaction. Moreover, our data proved functional interdependence of CAP1 and cofilin1 in control of spine morphology. In summary, we identified CAP1 as a novel regulator of the postsynaptic actin cytoskeleton that is essential for synaptic cofilin1 activity.

Rabphilin-3A as a Novel Target to Reverse α-synuclein-induced Synaptic Loss in Parkinson's Disease

Toxic aggregates of α-synuclein (αsyn) are considered key drivers of Parkinson's disease (PD) pathology. In early PD, αsyn induces synaptic dysfunction also modulating the glutamatergic neurotransmission. However, a more detailed understanding of the molecular mechanisms underlying αsyn-triggered synaptic failure is required to design novel therapeutic interventions. Here, we described the role of Rabphilin-3A (Rph3A) as novel target to counteract αsyn-induced synaptic loss in PD. Rph3A is a synaptic protein interacting with αsyn and involved in stabilizing dendritic spines and in promoting the synaptic retention of NMDA-type glutamate receptors. We found that in vivo intrastriatal injection of αsyn-preformed fibrils in mice induces the early loss of striatal synapses associated with decreased synaptic levels of Rph3A and impaired Rph3A/NMDA receptors interaction. Modulating Rph3A striatal expression or interfering with the Rph3A/αsyn complex with a small molecule prevented dendritic spine loss and rescued associated early motor defects in αsyn-injected mice. Notably, the same experimental approaches prevented αsyn-induced synaptic loss in vitro in primary hippocampal neurons. Overall, these findings indicate that approaches aimed at restoring Rph3A synaptic functions can slow down the early synaptic detrimental effects of αsyn aggregates in PD.

Rabphilin-3A Drives Structural Modifications of Dendritic Spines Induced by Long-Term Potentiation

The interaction of Rabphilin-3A (Rph3A) with the NMDA receptor (NMDAR) in hippocampal neurons plays a pivotal role in the synaptic retention of this receptor. The formation of a Rph3A/NMDAR complex is needed for the induction of long-term potentiation and NMDAR-dependent hippocampal behaviors, such as spatial learning. Moreover, Rph3A can also interact with AMPA receptors (AMPARs) through the formation of a complex with myosin Va. Here, we used a confocal imaging approach to show that Rph3A overexpression in primary hippocampal neuronal cultures is sufficient to promote increased dendritic spine density. This morphological event is correlated with an increase in GluN2A-containing NMDARs at synaptic membranes and a decrease in the surface levels of GluA1-containing AMPARs. These molecular and morphological modifications of dendritic spines are sufficient to occlude the spine formation induced by long-term potentiation, but do not prevent the spine loss induced by long-term depression. Overall, our results demonstrate a key role for Rph3A in the modulation of structural synaptic plasticity at hippocampal synapses that correlates with its interactions with both NMDARs and AMPARs.

Analysis of mRNA and Protein Levels of CAP2, DLG1 and ADAM10 Genes in Post-Mortem Brain of Schizophrenia, Parkinson's and Alzheimer's Disease Patients

Schizophrenia (SCZ) is a mental illness characterized by aberrant synaptic plasticity and connectivity. A large bulk of evidence suggests genetic and functional links between postsynaptic abnormalities and SCZ. Here, we performed quantitative PCR and Western blotting analysis in the dorsolateral prefrontal cortex (DLPFC) and hippocampus of SCZ patients to investigate the mRNA and protein expression of three key spine shapers: the actin-binding protein cyclase-associated protein 2 (CAP2), the sheddase a disintegrin and metalloproteinase 10 (ADAM10), and the synapse-associated protein 97 (SAP97). Our analysis of the SCZ post-mortem brain indicated increased DLG1 mRNA in DLPFC and decreased CAP2 mRNA in the hippocampus of SCZ patients, compared to non-psychiatric control subjects, while the ADAM10 transcript was unaffected. Conversely, no differences in CAP2, SAP97, and ADAM10 protein levels were detected between SCZ and control individuals in both brain regions. To assess whether DLG1 and CAP2 transcript alterations were selective for SCZ, we also measured their expression in the superior frontal gyrus of patients affected by neurodegenerative disorders, like Parkinson’s and Alzheimer’s disease. Interestingly, also in Parkinson’s disease patients, we found a selective reduction of CAP2 mRNA levels relative to controls but unaltered protein levels. Taken together, we reported for the first time altered CAP2 expression in the brain of patients with psychiatric and neurological disorders, thus suggesting that aberrant expression of this gene may contribute to synaptic dysfunction in these neuropathologies.

A light-gated potassium channel for sustained neuronal inhibition

Currently available inhibitory optogenetic tools provide short and transient silencing of neurons, but they cannot provide long-lasting inhibition because of the requirement for high light intensities. Here we present an optimized blue-light-sensitive synthetic potassium channel, BLINK2, which showed good expression in neurons in three species. The channel is activated by illumination with low doses of blue light, and in our experiments it remained active over (tens of) minutes in the dark after the illumination was stopped. This activation caused long periods of inhibition of neuronal firing in ex vivo recordings of mouse neurons and impaired motor neuron response in zebrafish in vivo. As a proof-of-concept application, we demonstrated that in a freely moving rat model of neuropathic pain, the activation of a small number of BLINK2 channels caused a long-lasting (>30 min) reduction in pain sensation.

Identification of long-lived synaptic proteins by proteomic analysis of synaptosome protein turnover

Memory formation is believed to result from changes in synapse strength and structure. While memories may persist for the lifetime of an organism, the proteins and lipids that make up synapses undergo constant turnover with lifetimes from minutes to days. The molecular basis for memory maintenance may rely on a subset of long-lived proteins (LLPs). While it is known that LLPs exist, whether such proteins are present at synapses is unknown. We performed an unbiased screen using metabolic pulse-chase labeling in vivo in mice and in vitro in cultured neurons combined with quantitative proteomics. We identified synaptic LLPs with half-lives of several months or longer. Proteins in synaptic fractions generally exhibited longer lifetimes than proteins in cytosolic fractions. Protein turnover was sensitive to pharmacological manipulations of activity in neuronal cultures or in mice exposed to an enriched environment. We show that synapses contain LLPs that may underlie stabile long-lasting changes in synaptic structure and function.

Anti-AMPA GluA3 antibodies in Frontotemporal dementia: a new molecular target

Frontotemporal Dementia (FTD) is a neurodegenerative disorder mainly characterised by Tau or TDP43 inclusions. A co-autoimmune aetiology has been hypothesised. In this study, we aimed at defining the pathogenetic role of anti-AMPA GluA3 antibodies in FTD. Serum and cerebrospinal fluid (CSF) anti-GluA3 antibody dosage was carried out and the effect of CSF with and without anti-GluA3 antibodies was tested in rat hippocampal neuronal primary cultures and in differentiated neurons from human induced pluripotent stem cells (hiPSCs). TDP43 and Tau expression in hiPSCs exposed to CSF was assayed. Forty-one out of 175 screened FTD sera were positive for the presence of anti-GluA3 antibodies (23.4%). FTD patients with anti-GluA3 antibodies more often presented presenile onset, behavioural variant FTD with bitemporal atrophy. Incubation of rat hippocampal neuronal primary cultures with CSF with anti-GluA3 antibodies led to a decrease of GluA3 subunit synaptic localization of the AMPA receptor (AMPAR) and loss of dendritic spines. These results were confirmed in differentiated neurons from hiPSCs, with a significant reduction of the GluA3 subunit in the postsynaptic fraction along with increased levels of neuronal Tau. In conclusion, autoimmune mechanism might represent a new potentially treatable target in FTD and might open new lights in the disease underpinnings.

Local translation in neuronal compartments: how local is local?

Efficient neuronal function depends on the continued modulation of the local neuronal proteome. Local protein synthesis plays a central role in tuning the neuronal proteome at specific neuronal regions. Various aspects of translation such as the localization of translational machinery, spatial spread of the newly translated proteins, and their site of action are carried out in specialized neuronal subcompartments to result in a localized functional outcome. In this review, we focus on the various aspects of these local translation compartments such as size, biochemical and organelle composition, structural boundaries, and temporal dynamics. We also discuss the apparent absence of definitive components of translation in these local compartments and the emerging state-of-the-art tools that could help dissecting these conundrums in greater detail in the future.

Mass Spectrometry-Based Approaches to Understand the Molecular Basis of Memory

The central nervous system is responsible for an array of cognitive functions such as memory, learning, language, and attention. These processes tend to take place in distinct brain regions; yet, they need to be integrated to give rise to adaptive or meaningful behavior. Since cognitive processes result from underlying cellular and molecular changes, genomics and transcriptomics assays have been applied to human and animal models to understand such events. Nevertheless, genes and RNAs are not the end products of most biological functions. In order to gain further insights toward the understanding of brain processes, the field of proteomics has been of increasing importance in the past years. Advancements in liquid chromatography-tandem mass spectrometry (LC-MS/MS) have enabled the identification and quantification of thousands of proteins with high accuracy and sensitivity, fostering a revolution in the neurosciences. Herein, we review the molecular bases of explicit memory in the hippocampus. We outline the principles of mass spectrometry (MS)-based proteomics, highlighting the use of this analytical tool to study memory formation. In addition, we discuss MS-based targeted approaches as the future of protein analysis.

Ring finger protein 10 is a novel synaptonuclear messenger encoding activation of NMDA receptors in hippocampus

Synapses and nuclei are connected by bidirectional communication mechanisms that enable information transfer encoded by macromolecules. Here, we identified RNF10 as a novel synaptonuclear protein messenger. RNF10 is activated by calcium signals at the postsynaptic compartment and elicits discrete changes at the transcriptional level. RNF10 is enriched at the excitatory synapse where it associates with the GluN2A subunit of NMDA receptors (NMDARs). Activation of synaptic GluN2A-containing NMDARs and induction of long term potentiation (LTP) lead to the translocation of RNF10 from dendritic segments and dendritic spines to the nucleus. In particular, we provide evidence for importin-dependent long-distance transport from synapto-dendritic compartments to the nucleus. Notably, RNF10 silencing prevents the maintenance of LTP as well as LTP-dependent structural modifications of dendritic spines.

DOI:http://dx.doi.org/10.7554/eLife.12430.001

Brain activity depends on the communication between neurons. This process takes place at the junctions between neurons, which are known as synapses, and typically involves one of the cells releasing a chemical messenger that binds to receptors on the other cell. The binding triggers a cascade of events inside the recipient cell, including the production of new receptors and their insertion into the cell membrane. These changes strengthen the synapse and are thought to be one of the ways in which the brain establishes and maintains memories.

However, in order to induce these changes at the synapse, neurons must be able to activate the genes that encode their component parts. These genes are present inside the cell nucleus, which is located some distance away from the synapse. Studies have shown that signals can be sent from the nucleus to the synapse and vice versa, enabling the two parts of the cell to exchange information. Synapses that communicate using a chemical called glutamate have been particularly well studied; but it still remains unclear how the activation of receptors at these “glutamatergic synapses” is linked to activation of genes inside the nucleus at the molecular level.

Dinamarca, Guzzetti et al. have now discovered that this process at glutamatergic synapses involves the movement of a protein messenger to the nucleus. Specifically, activation at synapses of a particularly common subtype of receptor, called NMDA, causes a protein called Ring Finger protein 10 (or RNF10 for short) to move from the synapse to the nucleus. To leave the synapse, RNF10 first has to bind to proteins called importins, which transport RNF10 into the nucleus. Once inside the nucleus, RNF10 binds to another protein that interacts with the DNA to start the production of new synaptic proteins.

Further work is required to identify the molecular mechanisms that trigger RNF10 to leave the synapse. In addition, future studies should evaluate the levels and activity of RNF10 in brain disorders in which synapses are known to function abnormally.

DOI:http://dx.doi.org/10.7554/eLife.12430.002

Proteomics of the Synapse--A Quantitative Approach to Neuronal Plasticity

The advances in mass spectrometry based proteomics in the past 15 years have contributed to a deeper appreciation of protein networks and the composition of functional synaptic protein complexes. However, research on protein dynamics underlying core mechanisms of synaptic plasticity in brain lag far behind. In this review, we provide a synopsis on proteomic research addressing various aspects of synaptic function. We discuss the major topics in the study of protein dynamics of the chemical synapse and the limitations of current methodology. We highlight recent developments and the future importance of multidimensional proteomics and metabolic labeling. Finally, emphasis is given on the conceptual framework of modern proteomics and its current shortcomings in the quest to gain a deeper understanding of synaptic plasticity.

LRRK2 phosphorylates pre-synaptic N-ethylmaleimide sensitive fusion (NSF) protein enhancing its ATPase activity and SNARE complex disassembling rate

Background

Lrrk2, a gene linked to Parkinson’s disease, encodes a large scaffolding protein with kinase and GTPase activities implicated in vesicle and cytoskeletal-related processes. At the presynaptic site, LRRK2 associates with synaptic vesicles through interaction with a panel of presynaptic proteins.

Results

Here, we show that LRRK2 kinase activity influences the dynamics of synaptic vesicle fusion. We therefore investigated whether LRRK2 phosphorylates component(s) of the exo/endocytosis machinery. We have previously observed that LRRK2 interacts with NSF, a hexameric AAA+ ATPase that couples ATP hydrolysis to the disassembling of SNARE proteins allowing them to enter another fusion cycle during synaptic exocytosis. Here, we demonstrate that NSF is a substrate of LRRK2 kinase activity. LRRK2 phosphorylates full-length NSF at threonine 645 in the ATP binding pocket of D2 domain. Functionally, NSF phosphorylated by LRRK2 displays enhanced ATPase activity and increased rate of SNARE complex disassembling. Substitution of threonine 645 with alanine abrogates LRRK2-mediated increased ATPase activity.

Conclusions

Given that the most common Parkinson’s disease LRRK2 G2019S mutation displays increased kinase activity, our results suggest that mutant LRRK2 may impair synaptic vesicle dynamics via aberrant phosphorylation of NSF.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0066-z) contains supplementary material, which is available to authorized users.

The IgLON Family Member Negr1 Promotes Neuronal Arborization Acting as Soluble Factor via FGFR2

IgLON proteins are GPI anchored adhesion molecules that control neurite outgrowth. In particular, Negr1 down-regulation negatively influences neuronal arborization in vitro and in vivo. In the present study, we found that the metalloprotease ADAM10 releases Negr1 from neuronal membrane. Ectodomain shedding influences several neuronal mechanisms, including survival, synaptogenesis, and the formation of neurite trees. By combining morphological analysis and virus-mediated selective protein silencing in primary murine cortical neurons, we found that pharmacologically inhibition of ADAM10 results in an impairment of neurite tree maturation that can be rescued upon treatment with soluble Negr1. Furthermore, we report that released Negr1 influences neurite outgrowth in a P-ERK1/2 and FGFR2 dependent manner. Together our findings suggest a role for Negr1 in regulating neurite outgrowth through the modulation of FGFR2 signaling pathway. Given the physiological and pathological role of ADAM10, Negr1, and FGFR2, the regulation of Negr1 shedding may play a crucial role in sustaining brain function and development.

Rabphilin 3A retains NMDA receptors at synaptic sites through interaction with GluN2A/PSD-95 complex

NMDA receptor (NMDAR) composition and synaptic retention represent pivotal features in the physiology and pathology of excitatory synapses. Here, we identify Rabphilin 3A (Rph3A) as a new GluN2A subunit-binding partner. Rph3A is known as a synaptic vesicle-associated protein involved in the regulation of exo- and endocytosis processes at presynaptic sites. We find that Rph3A is enriched at dendritic spines. Protein-protein interaction assays reveals that Rph3A N-terminal domain interacts with GluN2A(1349-1389) as well as with PSD-95(PDZ3) domains, creating a ternary complex. Rph3A silencing in neurons reduces the surface localization of synaptic GluN2A and NMDAR currents. Moreover, perturbing GluN2A/Rph3A interaction with interfering peptides in organotypic slices or in vivo induces a decrease of the amplitude of NMDAR-mediated currents and GluN2A density at dendritic spines. In conclusion, Rph3A interacts with GluN2A and PSD-95 forming a complex that regulates NMDARs stabilization at postsynaptic membranes.

Zinc transporter-1: a novel NMDA receptor-binding protein at the postsynaptic density

Zinc (Zn(2+) ) is believed to play a relevant role in the physiology and pathophysiology of the brain. Hence, Zn(2+) homeostasis is critical and involves different classes of molecules, including Zn(2+) transporters. The ubiquitous Zn(2+) transporter-1 (ZNT-1) is a transmembrane protein that pumps cytosolic Zn(2+) to the extracellular space, but its function in the central nervous system is not fully understood. Here, we show that ZNT-1 interacts with GluN2A-containing NMDA receptors, suggesting a role for this transporter at the excitatory glutamatergic synapse. First, we found that ZNT-1 is highly expressed at the hippocampal postsynaptic density (PSD) where NMDA receptors are enriched. Two-hybrid screening, coimmunoprecipitation experiments and clustering assay in COS-7 cells demonstrated that ZNT-1 specifically binds the GluN2A subunit of the NMDA receptor. GluN2A deletion mutants and pull-down assays indicated GluN2A(1390-1464) domain as necessary for the binding to ZNT-1. Most importantly, ZNT-1/GluN2A complex was proved to be dynamic, since it was regulated by induction of synaptic plasticity. Finally, modulation of ZNT-1 expression in hippocampal neurons determined a significant change in dendritic spine morphology, PSD-95 clusters and GluN2A surface levels, supporting the involvement of ZNT-1 in the dynamics of excitatory PSD. Zn(2+) transporter-1 (ZNT-1) pumps cytosolic Zn(2+) to the extracellular space, but its function in the central nervous system is not fully understood. We show that ZNT-1 interacts with GluN2A-containing NMDA receptors at the glutamatergic synapse. Most importantly, ZNT-1/GluN2A complex is regulated by induction of synaptic plasticity. Modulation of ZNT-1 expression in hippocampal neurons determined a shrinkage of dendritic spines and a reduction of GluN2A surface levels supporting the involvement of ZNT-1 in the dynamics of the excitatory synapse.

Novel IL1RAPL1 mutations associated with intellectual disability impair synaptogenesis

Mutations in interleukin-1 receptor accessory protein like 1 (IL1RAPL1) gene have been associated with non-syndromic intellectual disability (ID) and autism spectrum disorder. This protein interacts with synaptic partners like PSD-95 and PTPδ, regulating the formation and function of excitatory synapses. The aim of this work was to characterize the synaptic consequences of three IL1RAPL1 mutations, two novel causing the deletion of exon 6 (Δex6) and one point mutation (C31R), identified in patients with ID. Using immunofluorescence and electrophysiological recordings, we examined the effects of IL1RAPL1 mutant over-expression on synapse formation and function in cultured rodent hippocampal neurons. Δex6 but not C31R mutation leads to IL1RAPL1 protein instability and mislocalization within dendrites. Analysis of different markers of excitatory synapses and sEPSC recording revealed that both mutants fail to induce pre- and post-synaptic differentiation, contrary to WT IL1RAPL1 protein. Cell aggregation and immunoprecipitation assays in HEK293 cells showed a reduction of the interaction between IL1RAPL1 mutants and PTPδ that could explain the observed synaptogenic defect in neurons. However, these mutants do not affect all cellular signaling because their over-expression still activates JNK pathway. We conclude that both mutations described in this study lead to a partial loss of function of the IL1RAPL1 protein through different mechanisms. Our work highlights the important function of the trans-synaptic PTPδ/IL1RAPL1 interaction in synaptogenesis and as such in ID in the patients.

LRRK2 kinase activity regulates synaptic vesicle trafficking and neurotransmitter release through modulation of LRRK2 macro-molecular complex

Mutations in Leucine-rich repeat kinase 2 gene (LRRK2) are associated with familial and sporadic Parkinson's disease (PD). LRRK2 is a complex protein that consists of multiple domains executing several functions, including GTP hydrolysis, kinase activity, and protein binding. Robust evidence suggests that LRRK2 acts at the synaptic site as a molecular hub connecting synaptic vesicles to cytoskeletal elements via a complex panel of protein-protein interactions. Here we investigated the impact of pharmacological inhibition of LRRK2 kinase activity on synaptic function. Acute treatment with LRRK2 inhibitors reduced the frequency of spontaneous currents, the rate of synaptic vesicle trafficking and the release of neurotransmitter from isolated synaptosomes. The investigation of complementary models lacking LRRK2 expression allowed us to exclude potential off-side effects of kinase inhibitors on synaptic functions. Next we studied whether kinase inhibition affects LRRK2 heterologous interactions. We found that the binding among LRRK2, presynaptic proteins and synaptic vesicles is affected by kinase inhibition. Our results suggest that LRRK2 kinase activity influences synaptic vesicle release via modulation of LRRK2 macro-molecular complex.

Leucine-rich repeat kinase 2 binds to neuronal vesicles through protein interactions mediated by its C-terminal WD40 domain

Mutations in the leucine-rich repeat kinase 2 gene (LRRK2) are associated with familial and sporadic Parkinson's disease (PD). LRRK2 is a complex protein that consists of multiple domains, including predicted C-terminal WD40 repeats. In this study, we analyzed functional and molecular features conferred by the WD40 domain. Electron microscopic analysis of the purified LRRK2 C-terminal domain revealed doughnut-shaped particles, providing experimental evidence for its WD40 fold. We demonstrate that LRRK2 WD40 binds and sequesters synaptic vesicles via interaction with vesicle-associated proteins. In fact, a domain-based pulldown approach combined with mass spectrometric analysis identified LRRK2 as being part of a highly specific protein network involved in synaptic vesicle trafficking. In addition, we found that a C-terminal sequence variant associated with an increased risk of developing PD, G2385R, correlates with a reduced binding affinity of LRRK2 WD40 to synaptic vesicles. Our data demonstrate a critical role of the WD40 domain within LRRK2 function.

Elongation factor-2 phosphorylation in dendrites and the regulation of dendritic mRNA translation in neurons

Neuronal activity results in long lasting changes in synaptic structure and function by regulating mRNA translation in dendrites. These activity dependent events yield the synthesis of proteins known to be important for synaptic modifications and diverse forms of synaptic plasticity. Worthy of note, there is accumulating evidence that the eukaryotic Elongation Factor 2 Kinase (eEF2K)/eukaryotic Elongation Factor 2 (eEF2) pathway may be strongly involved in this process. Upon activation, eEF2K phosphorylates and thereby inhibits eEF2, resulting in a dramatic reduction of mRNA translation. eEF2K is activated by elevated levels of calcium and binding of Calmodulin (CaM), hence its alternative name calcium/CaM-dependent protein kinase III (CaMKIII). In dendrites, this process depends on glutamate signaling and N-methyl-D-aspartate receptor (NMDAR) activation. Interestingly, it has been shown that eEF2K can be activated in dendrites by metabotropic glutamate receptor (mGluR) 1/5 signaling, as well. Therefore, neuronal activity can induce local proteomic changes at the postsynapse by altering eEF2K activity. Well-established targets of eEF2K in dendrites include brain-derived neurotrophic factor (BDNF), activity-regulated cytoskeletal-associated protein (Arc), the alpha subunit of calcium/CaM-dependent protein kinase II (αCaMKII), and microtubule-associated protein 1B (MAP1B), all of which have well-known functions in different forms of synaptic plasticity. In this review we will give an overview of the involvement of the eEF2K/eEF2 pathway at dendrites in regulating the translation of dendritic mRNA in the context of altered NMDAR- and neuronal activity, and diverse forms of synaptic plasticity, such as metabotropic glutamate receptor-dependent-long-term depression (mGluR-LTD). For this, we draw on studies carried out both in vitro and in vivo.

A cell surface biotinylation assay to reveal membrane-associated neuronal cues: Negr1 regulates dendritic arborization

A complex and still not comprehensively resolved panel of transmembrane proteins regulates the outgrowth and the subsequent morphological and functional development of neuronal processes. In order to gain a more detailed description of these events at the molecular level, we have developed a cell surface biotinylation assay to isolate, detect, and quantify neuronal membrane proteins. When we applied our assay to investigate neuron maturation in vitro, we identified 439 differentially expressed proteins, including 20 members of the immunoglobulin superfamily. Among these candidates, we focused on Negr1, a poorly described cell adhesion molecule. We demonstrated that Negr1 controls the development of neurite arborization in vitro and in vivo. Given the tight correlation existing among synaptic cell adhesion molecules, neuron maturation, and a number of neurological disorders, our assay results are a useful tool that can be used to support the understanding of the molecular bases of physiological and pathological brain function.

Afferent regulation of chicken auditory brainstem neurons: rapid changes in phosphorylation of elongation factor 2

The relationships between protein synthesis and neuronal survival are poorly understood. In chicken nucleus magnocellularis (NM), significant alterations in overall protein synthesis precede neuronal death induced by deprivation of excitatory afferent activity. Previously we demonstrated an initial reduction in the overall rate of protein synthesis in all deprived NM neurons, followed by quick recovery (starting at 6 hours) in some, but not all, neurons. Neurons with recovered protein synthesis ultimately survive, whereas others become "ghost" cells (no detectable Nissl substance) at 12-24 hours and die within 48 hours. To explore the mechanisms underlying this differential influence of afferent input on protein synthesis and cell survival, the current study investigates the involvement of eukaryotic translation elongation factor 2 (eEF2), the phosphorylation of which reduces overall protein synthesis. Using immunocytochemistry for either total or phosphorylated eEF2 (p-eEF2), we found significant reductions in the level of phosphorylated, but not total, eEF2 in NM neurons as early as 0.5-1 hour following cochlea removal. Unexpectedly, neurons with low levels of p-eEF2 show reduced protein synthesis at 6 hours, indicated by a marker for active ribosomes. At 12 hours, all "ghost" cells exhibited little or no p-eEF2 staining, although not every neuron with a comparable low level of p-eEF2 was a "ghost" cell. These observations demonstrate that a reduced level of p-eEF2 is not responsible for immediate responses (including reduced overall protein synthesis) of a neuron to compromised afferent input but may impair the neuron's ability to initiate recovery signaling for survival and make the neuron more vulnerable to death.

The cytochrome c gene proximal enhancer drives activity-dependent reporter gene expression in hippocampal neurons

The proximal enhancer of the cytochrome c gene (Cycs) contains binding sites for both cAMP response element binding proteins (CREB) and Nuclear Respiratory Factor 1 (NRF1). To investigate how neuronal activity regulates this enhancer region, a lentivirus was constructed in which a short-lived green fluorescent protein (GFP) was placed under the transcriptional control of the Cycs proximal enhancer linked to a synthetic core promoter. Primary hippocampal neurons were infected, and the synaptic strengths of individual neurons were measured by whole-cell patch clamping. On average the amplitude of miniature postsynaptic currents (mEPSCs) was higher in brighter GFP⁺ neurons, while the frequency of mEPSCs was not significantly different. Increasing neural activity by applying a GABAA receptor antagonist increased GFP expression in most neurons, which persisted after homeostatic synaptic scaling as evidenced by a decrease in the amplitude and frequency of mEPSCs. Removing the CREB binding sites revealed that calcium influx through L-type channels and NMDA receptors, and ERK1/2 activation played a role in NRF1-mediated transcription. CREB and NRF1, therefore, combine to regulate transcription of Cycs in response to changing neural activity.

The neuropeptide PACAP38 induces dendritic spine remodeling through ADAM10/N-Cadherin signaling pathway

The neuropeptide pituitary adenylate cyclase-activating polypeptide 38 (PACAP38) has been implicated in the induction of synaptic plasticity at the excitatory glutamatergic synapse. In particular, recent studies have shown that it is involved in the regulation of NMDA and AMPA receptor activation. Here we demonstrate the effect of PACAP38 on the modulation of dendritic spine morphology through ADAM10/N-Cadherin/AMPA receptor signaling pathway. Treatment of primary hippocampal neurons with PACAP38 induces an accumulation of ADAM10 at the postsynaptic membrane. This event leads to a significant decrease of dendritic spine head width and to a concomitant reduction of GluR1 co-localization with postsynaptic markers. PACAP38-induced effect on dendritic spine head width is prevented by either treatment with ADAM10 specific inhibitor or transfection of a cleavage-defective N-Cadherin construct, mutated in the ADAM10 cleavage site. Overall, our findings reveal for the first time that PACAP38 is involved in the modulation of dendritic spine morphology in hippocampal neurons and assign to the ADAM10/N-Cadherin signaling pathway a crucial role in this modification of the excitatory glutamatergic synapse.

The X-linked intellectual disability protein IL1RAPL1 regulates excitatory synapse formation by binding PTPδ and RhoGAP2

Mutations of the Interleukin-1-receptor accessory protein like 1 (IL1RAPL1) gene are associated with cognitive impairment ranging from non-syndromic X-linked mental retardation to autism. IL1RAPL1 belongs to a novel family of IL1/Toll receptors, which is localized at excitatory synapses and interacts with PSD-95. We previously showed that IL1RAPL1 regulates the synaptic localization of PSD-95 by controlling c-Jun N-terminal kinase activity and PSD-95 phosphorylation. Here, we show that the IgG-like extracellular domains of IL1RAPL1 induce excitatory pre-synapse formation by interacting with protein tyrosine phosphatase delta (PTPδ). We also found that IL1RAPL1 TIR domains interact with RhoGAP2, which is localized at the excitatory post-synaptic density. More interestingly, the IL1RAPL1/PTPδ complex recruits RhoGAP2 at excitatory synapses to induce dendritic spine formation. We also found that the IL1RAPL1 paralog, IL1RAPL2, interacts with PTPδ and induces excitatory synapse and dendritic spine formation. The interaction of the IL1RAPL1 family of proteins with PTPδ and RhoGAP2 reveals a pathophysiological mechanism of cognitive impairment associated with a novel type of trans-synaptic signaling that regulates excitatory synapse and dendritic spine formation.

Importance of Shank3 protein in regulating metabotropic glutamate receptor 5 (mGluR5) expression and signaling at synapses

Shank3/PROSAP2 gene mutations are associated with cognitive impairment ranging from mental retardation to autism. Shank3 is a large scaffold postsynaptic density protein implicated in dendritic spines and synapse formation; however, its specific functions have not been clearly demonstrated. We have used RNAi to knockdown Shank3 expression in neuronal cultures and showed that this treatment specifically reduced the synaptic expression of the metabotropic glutamate receptor 5 (mGluR5), but did not affect the expression of other major synaptic proteins. The functional consequence of Shank3 RNAi knockdown was impaired signaling via mGluR5, as shown by reduction in ERK1/2 and CREB phosphorylation induced by stimulation with (S)-3,5-dihydroxyphenylglycine (DHPG) as the agonist of mGluR5 receptors, impaired mGluR5-dependent synaptic plasticity (DHPG-induced long-term depression), and impaired mGluR5-dependent modulation of neural network activity. We also found morphological abnormalities in the structure of synapses (spine number, width, and length) and impaired glutamatergic synaptic transmission, as shown by reduction in the frequency of miniature excitatory postsynaptic currents (mEPSC). Notably, pharmacological augmentation of mGluR5 activity using 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)-benzamide as the positive allosteric modulator of these receptors restored mGluR5-dependent signaling (DHPG-induced phosphorylation of ERK1/2) and normalized the frequency of mEPSCs in Shank3-knocked down neurons. These data demonstrate that a deficit in mGluR5-mediated intracellular signaling in Shank3 knockdown neurons can be compensated by 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)-benzamide; this raises the possibility that pharmacological augmentation of mGluR5 activity represents a possible new therapeutic approach for patients with Shank3 mutations.

Proteomic analysis of short- and intermediate-term memory in Hermissenda

Changes in cellular and synaptic plasticity related to learning and memory are accompanied by both upregulation and downregulation of the expression levels of proteins. Both de novo protein synthesis and post-translational modification of existing proteins have been proposed to support the induction and maintenance of memory underlying learning. However, little is known regarding the identity of proteins regulated by learning that are associated with the early stages supporting the formation of memory over time. In this study we have examined changes in protein abundance at two different times following one-trial in vitro conditioning of Hermissenda using two-dimensional difference gel electrophoresis (2D-DIGE), quantification of differences in protein abundance between conditioned and unpaired controls, and protein identification with tandem mass spectrometry. Significant regulation of protein abundance following one-trial in vitro conditioning was detected 30 min and 3 h post-conditioning. Proteins were identified that exhibited statistically significant increased or decreased abundance at both 30 min and 3 h post-conditioning. Proteins were also identified that exhibited a significant increase in abundance only at 30 min, or only at 3 h post-conditioning. A few proteins were identified that expressed a significant decrease in abundance detected at both 30 min and 3 h post-conditioning, or a significant decrease in abundance only at 3 h post-conditioning. The proteomic analysis indicates that proteins involved in diverse cellular functions such as translational regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to the formation of memory produced by one-trial in vitro conditioning. These findings support the view that changes in protein abundance over time following one-trial in vitro conditioning involve dynamic and complex interactions of the proteome.

MicroRNA regulation of homeostatic synaptic plasticity

Homeostatic mechanisms are required to control formation and maintenance of synaptic connections to maintain the general level of neural impulse activity within normal limits. How genes controlling these processes are co-coordinately regulated during homeostatic synaptic plasticity is unknown. MicroRNAs (miRNAs) exert regulatory control over mRNA stability and translation and may contribute to local and activity-dependent posttranscriptional control of synapse-associated mRNAs. However, identifying miRNAs that function through posttranscriptional gene silencing at synapses has remained elusive. Using a bioinformatics screen to identify sequence motifs enriched in the 3'UTR of rapidly destabilized mRNAs, we identified a developmentally and activity-regulated miRNA (miR-485) that controls dendritic spine number and synapse formation in an activity-dependent homeostatic manner. We find that many plasticity-associated genes contain predicted miR-485 binding sites and further identify the presynaptic protein SV2A as a target of miR-485. miR-485 negatively regulated dendritic spine density, postsynaptic density 95 (PSD-95) clustering, and surface expression of GluR2. Furthermore, miR-485 overexpression reduced spontaneous synaptic responses and transmitter release, as measured by miniature excitatory postsynaptic current (EPSC) analysis and FM 1-43 staining. SV2A knockdown mimicked the effects of miR-485, and these effects were reversed by SV2A overexpression. Moreover, 5 d of increased synaptic activity induced homeostatic changes in synaptic specializations that were blocked by a miR-485 inhibitor. Our findings reveal a role for this previously uncharacterized miRNA and the presynaptic protein SV2A in homeostatic plasticity and nervous system development, with possible implications in neurological disorders (e.g., Huntington and Alzheimer's disease), where miR-485 has been found to be dysregulated.

Synaptic localization and activity of ADAM10 regulate excitatory synapses through N-cadherin cleavage

N-Cadherin has an important role during dendrite arborization, axon guidance, and synaptogenesis. In particular, at synaptic sites, N-cadherin is involved in the regulation of cell-cell adhesion and in morphology and plasticity control. Recent studies have shown that N-cadherin can be cleaved by the metalloproteinase ADAM10. Here we demonstrate that impairing ADAM10 localization and activity at synaptic sites decreases its processing of N-cadherin. This leads to an accumulation of the full-length form of N-cadherin, to an increase in spine head width, and to modifications of the number and function of glutamate receptors of AMPA type, both in vitro and in vivo. Our results indicate a key role for ADAM10 in the complex sequence of events through which N-cadherin affects spine maturation and controls structure and function of glutamatergic synapses.

Synaptic activity controls dendritic spine morphology by modulating eEF2-dependent BDNF synthesis

Activity-dependent changes in synaptic structure and spine morphology are required for learning and memory, and depend on protein translation. We show that the kinase for eukaryotic elongation factor 2 (eEF2K) regulates dendritic spine stability and synaptic structure by modulating activity-dependent dendritic BDNF synthesis. Specifically RNAi knockdown of eEF2K reduces dendritic spine stability and inhibits dendritic BDNF protein expression; whereas overexpression of a constitutively activated eEF2K induces spine maturation and increases expression of dendritic BDNF. Furthermore, BDNF overexpression rescues the spine stability reduced by RNAi knockdown of eEF2K. We also show that synaptic activity-dependent spine maturation and dendritic BDNF protein expression depend on mGluR/EF2K-induced eEF2 phosphorylation. We propose that the eEF2K/eEF2 pathway is a key biochemical sensor that couple neuronal activity to spine plasticity, by controlling the dendritic translation of BDNF.

A postsynaptic signaling pathway that may account for the cognitive defect due to IL1RAPL1 mutation

BACKGROUND

Interleukin-1 receptor accessory protein-like 1 (IL1RAPL1) gene mutations are associated with cognitive impairment ranging from nonsyndromic X-linked mental retardation to autism. IL1RAPL1 belongs to a novel family of Toll/IL-1 receptors, whose expression in the brain is upregulated by neuronal activity. Currently, very little is known about the function of this protein. We previously showed that IL1RAPL1 interacts with the neuronal calcium sensor NCS-1 and that it regulates voltage-gated calcium channel activity in PC12 cells.

RESULTS

Here we show that IL1RAPL1 is present in dendritic spine where it interacts with PSD-95, a major component of excitatory postsynaptic compartment. Using gain- and loss-of-function experiments in neurons, we demonstrated that IL1RAPL1 regulates the synaptic localization of PSD-95 by controlling c-Jun terminal kinase (JNK) activity and PSD-95 phosphorylation. Mice carrying a null mutation of the mouse Il1rapl1 gene show a reduction of both dendritic spine density and excitatory synapses in the CA1 region of the hippocampus. These structural abnormalities are associated with specific deficits in hippocampal long-term synaptic plasticity.

CONCLUSION

The interaction of IL1RAPL1 with PSD-95 discloses a novel pathophysiological mechanism of cognitive impairment associated with alterations of the JNK pathway leading to a mislocalization of PSD-95 and abnormal synaptic organization and function.

Regulation of the proteasome by neuronal activity and calcium/calmodulin-dependent protein kinase II

Protein degradation via the ubiquitin proteasome system has been shown to regulate changes in synaptic strength that underlie multiple forms of synaptic plasticity. It is plausible, therefore, that the ubiquitin proteasome system is itself regulated by synaptic activity. By utilizing live-cell imaging strategies we report the rapid and dynamic regulation of the proteasome in hippocampal neurons by synaptic activity. We find that the blockade of action potentials (APs) with tetrodotoxin inhibited the activity of the proteasome, whereas the up-regulation of APs with bicuculline dramatically increased the activity of the proteasome. In addition, the regulation of the proteasome is dependent upon external calcium entry in part through N-methyl-D-aspartate receptors and L-type voltage-gated calcium channels and requires the activity of calcium/calmodulin-dependent protein kinase II (CaMKII). Using in vitro and in vivo assays we find that CaMKII stimulates proteasome activity and directly phosphorylates Rpt6, a subunit of the 19 S (PA700) subcomplex of the 26 S proteasome. Our data provide a novel mechanism whereby CaMKII may regulate the proteasome in neurons to facilitate remodeling of synaptic connections through protein degradation.

Proteomics in the study of hippocampal plasticity

Synaptic plasticity is the dynamic regulation of the strength of synaptic communication between nerve cells. It is central to neuronal development as well as experience-dependent remodeling of the adult nervous system as occurs during memory formation. Aberrant forms of synaptic plasticity also accompany a variety of neurological and psychiatric diseases, and unraveling the biological basis of synaptic plasticity has been a major goal in neurobiology research. The biochemical and structural mechanisms underlying different forms of synaptic plasticity are complex, involving multiple signaling cascades, reconfigurations of structural proteins and the trafficking of synaptic proteins. As such, proteomics should be a valuable tool in dissecting the molecular events underlying normal and disease-related forms of plasticity. In fact, progress in this area has been disappointingly slow. We discuss the particular challenges associated with proteomic interrogation of synaptic plasticity processes and outline ways in which we believe proteomics may advance the field over the next few years. We pay particular attention to technical advances being made in small sample proteomics and the advent of proteomic imaging in studying brain plasticity.