Arc Requires PSD95 for Assembly into Postsynaptic Complexes Involved with Neural Dysfunction and Intelligence

Targeting TrkB-PSD-95 coupling to mitigate neurological disorders

Tropomyosin receptor kinase B (TrkB) signaling plays a pivotal role in dendritic growth and dendritic spine formation to promote learning and memory. The activity-dependent release of brain-derived neurotrophic factor at synapses binds to pre- or postsynaptic TrkB resulting in the strengthening of synapses, reflected by long-term potentiation. Postsynaptically, the association of postsynaptic density protein-95 with TrkB enhances phospholipase Cγ-Ca2+/calmodulin-dependent protein kinase II and phosphatidylinositol 3-kinase-mechanistic target of rapamycin signaling required for long-term potentiation. In this review, we discuss TrkB-postsynaptic density protein-95 coupling as a promising strategy to magnify brain-derived neurotrophic factor signaling towards the development of novel therapeutics for specific neurological disorders. A reduction of TrkB signaling has been observed in neurodegenerative disorders, such as Alzheimer's disease and Huntington's disease, and enhancement of postsynaptic density protein-95 association with TrkB signaling could mitigate the observed deficiency of neuronal connectivity in schizophrenia and depression. Treatment with brain-derived neurotrophic factor is problematic, due to poor pharmacokinetics, low brain penetration, and side effects resulting from activation of the p75 neurotrophin receptor or the truncated TrkB.T1 isoform. Although TrkB agonists and antibodies that activate TrkB are being intensively investigated, they cannot distinguish the multiple human TrkB splicing isoforms or cell type-specific functions. Targeting TrkB-postsynaptic density protein-95 coupling provides an alternative approach to specifically boost TrkB signaling at localized synaptic sites versus global stimulation that risks many adverse side effects.

Targeted rescue of synaptic plasticity improves cognitive decline in sepsis-associated encephalopathy

Sepsis-associated encephalopathy (SAE) is a frequent complication of severe systemic infection resulting in delirium, premature death, and long-term cognitive impairment. We closely mimicked SAE in a murine peritoneal contamination and infection (PCI) model. We found long-lasting synaptic pathology in the hippocampus including defective long-term synaptic plasticity, reduction of mature neuronal dendritic spines, and severely affected excitatory neurotransmission. Genes related to synaptic signaling, including the gene for activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) and members of the transcription-regulatory EGR gene family, were downregulated. At the protein level, ARC expression and mitogen-activated protein kinase signaling in the brain were affected. For targeted rescue we used adeno-associated virus-mediated overexpression of ARC in the hippocampus in vivo. This recovered defective synaptic plasticity and improved memory dysfunction. Using the enriched environment paradigm as a non-invasive rescue intervention, we found improvement of defective long-term potentiation, memory, and anxiety. The beneficial effects of an enriched environment were accompanied by an increase in brain-derived neurotrophic factor (BDNF) and ARC expression in the hippocampus, suggesting that activation of the BDNF-TrkB pathway leads to restoration of the PCI-induced reduction of ARC. Collectively, our findings identify synaptic pathomechanisms underlying SAE and provide a conceptual approach to target SAE-induced synaptic dysfunction with potential therapeutic applications to patients with SAE.

Identifying new players in structural synaptic plasticity through dArc1 interrogation

The formation, expansion, and pruning of synapses, known as structural synaptic plasticity, is needed for learning and memory, and perturbation of plasticity is associated with many neurological disorders and diseases. Previously, we observed that the Drosophila homolog of Activity-regulated cytoskeleton-associated protein (dArc1), forms a capsid-like structure, associates with its own mRNA, and is transported across synapses. We demonstrated that this transfer is needed for structural synaptic plasticity. To identify mRNAs that are modified by dArc1 in presynaptic neuron and postsynaptic muscle, we disrupted the expression of dArc1 and performed genomic analysis with deep sequencing. We found that dArc1 affects the expression of genes involved in metabolism, phagocytosis, and RNA-splicing. Through immunoprecipitation we also identified potential mRNA cargos of dArc1 capsids. This study suggests that dArc1 acts as a master regulator of plasticity by affecting several distinct and highly conserved cellular processes.

Activity-regulated cytoskeletal-associated protein (Arc) in presynaptic terminals and extracellular vesicles in hippocampal synapses

The activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) is a neuron-specific immediate early gene (IEG) product. The protein regulates synaptic strength through modulation of spine density and morphology, AMPA receptor endocytosis, and as being part of a retrovirus-like inter-cellular communication mechanism. However, little is known about the detailed subsynaptic localization of the protein, and especially its possible presynaptic localization. In the present study, we provide novel electron microscopical data of Arc localization at hippocampal Schaffer collateral synapses in the CA1 region. The protein was found in both pre-and postsynaptic cytoplasm in a majority of synapses, associated with small vesicles. We also observed multivesicular body-like structures positive for Arc. Furthermore, the protein was located over the presynaptic active zone and the postsynaptic density. The relative concentration of Arc was 25% higher in the postsynaptic spine than in the presynaptic terminal. Notably, small extracellular vesicles labeled for Arc were detected in the synaptic cleft or close to the synapse, supporting a possible transsynaptic transmission of the protein in the brain.

Mechanisms and Functions of Activity-Regulated Cytoskeleton-Associated Protein in Synaptic Plasticity

Activity-regulated cytoskeleton-associated protein (Arc) is one of the most important regulators of cognitive functions in the brain regions. As a hub protein, Arc plays different roles in modulating synaptic plasticity. Arc supports the maintenance of long-term potentiation (LTP) by regulating actin cytoskeletal dynamics, while it guides the endocytosis of AMPAR in long-term depression (LTD). Moreover, Arc can self-assemble into capsids, leading to a new way of communicating among neurons. The transcription and translation of the immediate early gene Arc are rigorous procedures guided by numerous factors, and RNA polymerase II (Pol II) is considered to regulate the precise timing dynamics of gene expression. Since astrocytes can secrete brain-derived neurotrophic factor (BDNF) and L-lactate, their unique roles in Arc expression are emphasized. Here, we review the entire process of Arc expression and summarize the factors that can affect Arc expression and function, including noncoding RNAs, transcription factors, and posttranscriptional regulations. We also attempt to review the functional states and mechanisms of Arc in modulating synaptic plasticity. Furthermore, we discuss the recent progress in understanding the roles of Arc in the occurrence of major neurological disorders and provide new thoughts for future research on Arc.

Arc protein, a remnant of ancient retrovirus, forms virus-like particles, which are abundantly generated by neurons during epileptic seizures, and affects epileptic susceptibility in rodent models

A product of the immediate early gene Arc (Activity-regulated cytoskeleton-associated protein or Arc protein) of retroviral ancestry resides in the genome of all tetrapods for millions of years and is expressed endogenously in neurons. It is a well-known protein, very important for synaptic plasticity and memory consolidation. Activity-dependent Arc expression concentrated in glutamatergic synapses affects the long-time synaptic strength of those excitatory synapses. Because it modulates excitatory-inhibitory balance in a neuronal network, the Arc gene itself was found to be related to the pathogenesis of epilepsy. General Arc knockout rodent models develop a susceptibility to epileptic seizures. Because of activity dependence, synaptic Arc protein synthesis also is affected by seizures. Interestingly, it was found that Arc protein in synapses of active neurons self-assemble in capsids of retrovirus-like particles, which can transfer genetic information between neurons, at least across neuronal synaptic boutons. Released Arc particles can be accumulated in astrocytes after seizures. It is still not known how capsid assembling and transmission timescale is affected by seizures. This scientific field is relatively novel and is experiencing swift transformation as it grapples with difficult concepts in light of evolving experimental findings. We summarize the emergent literature on the subject and also discuss the specific rodent models for studying Arc effects in epilepsy. We summarized both to clarify the possible role of Arc-related pseudo-viral particles in epileptic disorders, which may be helpful to researchers interested in this growing area of investigation.

Visualizing Arc protein dynamics and localization in the mammalian brain using AAV-mediated in situ gene labeling

The activity-regulated cytoskeleton-associated (Arc) protein is essential for synaptic plasticity and memory formation. The Arc gene, which contains remnants of a structural GAG retrotransposon sequence, produces a protein that self-assembles into capsid-like structures harboring Arc mRNA. Arc capsids, released from neurons, have been proposed as a novel intercellular mechanism for mRNA transmission. Nevertheless, evidence for intercellular transport of Arc in the mammalian brain is still lacking. To enable the tracking of Arc molecules from individual neurons in vivo, we devised an adeno-associated virus (AAV) mediated approach to tag the N-terminal of the mouse Arc protein with a fluorescent reporter using CRISPR/Cas9 homologous independent targeted integration (HITI). We show that a sequence coding for mCherry can successfully be knocked in at the 5' end of the Arc open reading frame. While nine spCas9 gene editing sites surround the Arc start codon, the accuracy of the editing was highly sequence-dependent, with only a single target resulting in an in-frame reporter integration. When inducing long-term potentiation (LTP) in the hippocampus, we observed an increase of Arc protein highly correlated with an increase in fluorescent intensity and the number of mCherry-positive cells. By proximity ligation assay (PLA), we demonstrated that the mCherry-Arc fusion protein retains the Arc function by interacting with the transmembrane protein stargazin in postsynaptic spines. Finally, we recorded mCherry-Arc interaction with presynaptic protein Bassoon in mCherry-negative surrounding neurons at close proximity to mCherry-positive spines of edited neurons. This is the first study to provide support for inter-neuronal in vivo transfer of Arc in the mammalian brain.

Detection of Arc/Arg3.1 oligomers in rat brain: constitutive and synaptic activity-evoked dimer expression in vivo

The immediate early gene product activity-regulated cytoskeleton-associated protein (Arc or Arg3.1) is a major regulator of long-term synaptic plasticity with critical roles in postnatal cortical development and memory formation. However, the molecular basis of Arc function is undefined. Arc is a hub protein with interaction partners in the postsynaptic neuronal compartment and nucleus. Previous in vitro biochemical and biophysical analysis of purified recombinant Arc showed formation of low-order oligomers and larger particles including retrovirus-like capsids. Here, we provide evidence for naturally occurring Arc oligomers in the mammalian brain. Using in situ protein crosslinking to trap weak Arc-Arc interactions, we identified in various preparations a prominent Arc immunoreactive band on SDS-PAGE of molecular mass corresponding to a dimer. While putative trimers, tetramers and heavier Arc species were detected, they were of lower abundance. Stimulus-evoked induction of Arc expression and dimer formation was first demonstrated in SH-SY5Y neuroblastoma cells treated with the muscarinic cholinergic agonist, carbachol, and in primary cortical neuronal cultures treated with brain-derived neurotrophic factor (BDNF). In the dentate gyrus (DG) of adult anesthetized rats, induction of long-term potentiation (LTP) by high-frequency stimulation (HFS) of medial perforant synapses or by brief intrahippocampal infusion of BDNF led to a massive increase in Arc dimer expression. Arc immunoprecipitation of crosslinked DG tissue showed enhanced dimer expression during 4 h of LTP maintenance. Mass spectrometric proteomic analysis of immunoprecipitated, gel-excised bands corroborated detection of Arc dimer. Furthermore, Arc dimer was constitutively expressed in naïve cortical, hippocampal and DG tissue, with the lowest levels in the DG. Taken together the results implicate Arc dimer as the predominant low-oligomeric form in mammalian brain, exhibiting regional differences in its constitutive expression and enhanced synaptic activity-evoked expression in LTP.

Changes in working memory induced by lipopolysaccharide administration in mice are associated with metabotropic glutamate receptors 5 and contrast with changes induced by cyclooxygenase-2: Involvement of postsynaptic density protein 95 and down syndrome cell adhesion molecule

The strength and quality of the signal propagated by the glutamate synapse (Glu) depend, among other things, on the structure of the postsynaptic part and the quality of adhesion between the interacting components of the synapse. Postsynaptic density protein 95 (PSD95), mammalian target of rapamycin (mTOR), and Down syndrome cell adhesion molecule (DSCAM) are components of the proper functioning of an excitatory synapse. PSD95 is a member of the membrane-associated guanylate kinases protein family, mainly located at the postsynaptic density of the excitatory synapse. PSD95, via direct interaction, regulates the clustering and functionality of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptors at a synapse. Here, the effects of treatment with an antagonist of mGluR5 (MTEP) and NS398 (cyclooxygenase-2, COX-2 inhibitor) on PSD95, mTOR, and DSCAM in the hippocampus (HC) of C57B1/6 J mice using Western blots in the context of learning were examined. Moreover, the sensitivity of selected proteins to lipopolysaccharide (LPS) was monitored. MTEP injected for seven days induced upregulation of PSD95 in HC of mice. The observed effect was regulated by a COX-2 inhibitor and concurrently by LPS. Accompanying alterations in DSCAM protein were found, suggesting changes in adhesion strength after modulation of glutamatergic (Glu) synapse via tested compounds.

Cognitive outcomes caused by low-level lead, cadmium, and mercury mixture exposure at distinct phases of brain development

Contaminated water and food are the main sources of lead, cadmium, and mercury in the human body. Long-term and low-level ingestion of these toxic heavy metals may affect brain development and cognition. However, the neurotoxic effects of exposure to lead, cadmium, and mercury mixture (Pb + Cd + Hg) at different stages of brain development are rarely elucidated. In this study, different doses of low-level Pb + Cd + Hg were administered to Sprague-Dawley rats via drinking water during the critical stage of brain development, late stage, and after maturation, respectively. Our findings showed that Pb + Cd + Hg exposure decreased the density of memory- and learning-related dendritic spines in the hippocampus during the critical period of brain development, resulting in hippocampus-dependent spatial memory deficits. Only the density of learning-related dendritic spines was reduced during the late phase of brain development and a higher-dose of Pb + Cd + Hg exposure was required, which led to hippocampus-independent spatial memory abnormalities. Exposure to Pb + Cd + Hg after brain maturation revealed no significant change in dendritic spines or cognitive function. Further molecular analysis indicated that morphological and functional changes caused by Pb + Cd + Hg exposure during the critical phase were associated with PSD95 and GluA1 dysregulation. Collectively, the effects of Pb + Cd + Hg on cognition varied depending on the brain development stages.

A brain atlas of synapse protein lifetime across the mouse lifespan

The lifetime of proteins in synapses is important for their signaling, maintenance, and remodeling, and for memory duration. We quantified the lifetime of endogenous PSD95, an abundant postsynaptic protein in excitatory synapses, at single-synapse resolution across the mouse brain and lifespan, generating the Protein Lifetime Synaptome Atlas. Excitatory synapses have a wide range of PSD95 lifetimes extending from hours to several months, with distinct spatial distributions in dendrites, neurons, and brain regions. Synapses with short protein lifetimes are enriched in young animals and in brain regions controlling innate behaviors, whereas synapses with long protein lifetimes accumulate during development, are enriched in the cortex and CA1 where memories are stored, and are preferentially preserved in old age. Synapse protein lifetime increases throughout the brain in a mouse model of autism and schizophrenia. Protein lifetime adds a further layer to synapse diversity and enriches prevailing concepts in brain development, aging, and disease.

Synapse-specific changes in Arc and BDNF in rat hippocampus following chronic temporal lobe epilepsy

Expression of immediate early genes (IEGs) in the brain is important for synaptic plasticity, and probably also in neurodegenerative conditions. To understand the cellular mechanisms of the underlying neuropathophysiological processes in epilepsy, we need to pinpoint changes in concentration of synaptic plasticity-related proteins at subsynaptic levels. In this study, we examined changes in synaptic expression of Activity-regulated cytoskeleton-associated (Arc) and Brai Derived Neurotrophic Factor (BDNF) in a rat model of kainate-induced temporal lobe epilepsy (TLE). Western blotting showed reduced concentrations of Arc and increased concentrations of BDNF in hippocampal synaptosomes in chronic TLE rats. Then, using quantitative electron microscopy, we found corresponding changes in subsynaptic regions in the hippocampus. Specifically, we detected significant reductions in the concentrations of Arc in the presynaptic terminal of Schaffer collateral glutamatergic synapses in the stratum radiatum of the CA1 area in TLE, as well as in their adjacent postsynaptic spines. In CA3, there was a significant reduction of Arc only in the presynaptic terminal cytoplasm. Conversely, in CA3, there was a significant increase in the expression of BDNF in the presynaptic terminal, but not in the postsynaptic spine. Significant increase in BDNF concentration in the CA1 postsynaptic density was also obtained. We hypothesize that the observed changes in Arc and BDNF may contribute to both cognitive impairment and increased excitotoxic vulnerability in chronic epilepsy.

Hippocampal synaptic dysfunction and spatial memory impairment in omeprazole-treated rats

Although the association of prolonged use of proton pump inhibitors, such as omeprazole, with memory impairment has been reported more than two decades ago, its underlying molecular mechanism is yet to be determined. Thus, in this study, we aimed to determine the mechanisms underlying the effect of prolonged omeprazole treatment on hippocampal synaptic function and spatial memory in male rats. Adult rats were subcutaneously administered with omeprazole for 12 or 24 weeks. Spatial memory was assessed using the Morris water maze (MWM) test. We examined the hippocampal protein expression of synaptic plasticity proteins, including the AMPA receptor subunit GluA1, postsynaptic density-95 (PSD-95), and activity-regulated cytoskeleton-associated protein (Arc), and the hippocampal expression and localization of androgen receptor (AR). In the MWM test, the escape latency was found to be significantly higher, and the number of platform crossings and the time spent in the target quadrant were significantly lower in the rats treated with omeprazole compared to the control rats. Hypomagnesemia and lower bone and brain Mg²⁺ content were also detected in the omeprazole-treated groups compared with the control group. The expression of GluA1, PSD-95, and Arc in the hippocampus and the expression of AR in the dentate gyrus and CA1 of the hippocampus were significantly lower in the omeprazole-treated groups than in the control group. These results suggest that prolonged omeprazole treatment might lead to memory deficit by impairing glutamate receptor trafficking or synaptic anchoring. Hypomagnesemia and brain Mg²⁺ deficiency may be, at least in part, involved in omeprazole-induced memory impairment.

Developmental disruption and restoration of brain synaptome architecture in the murine Pax6 neurodevelopmental disease model

Neurodevelopmental disorders of genetic origin delay the acquisition of normal abilities and cause disabling phenotypes. Nevertheless, spontaneous attenuation and even complete amelioration of symptoms in early childhood and adolescence can occur in many disorders, suggesting that brain circuits possess an intrinsic capacity to overcome the deficits arising from some germline mutations. We examined the molecular composition of almost a trillion excitatory synapses on a brain-wide scale between birth and adulthood in mice carrying a mutation in the homeobox transcription factor Pax6, a neurodevelopmental disorder model. Pax6 haploinsufficiency had no impact on total synapse number at any age. By contrast, the molecular composition of excitatory synapses, the postnatal expansion of synapse diversity and the acquisition of normal synaptome architecture were delayed in all brain regions, interfering with networks and electrophysiological simulations of cognitive functions. Specific excitatory synapse types and subtypes were affected in two key developmental age-windows. These phenotypes were reversed within 2-3 weeks of onset, restoring synapse diversity and synaptome architecture to the normal developmental trajectory. Synapse subtypes with rapid protein turnover mediated the synaptome remodeling. This brain-wide capacity for remodeling of synapse molecular composition to recover and maintain the developmental trajectory of synaptome architecture may help confer resilience to neurodevelopmental genetic disorders.

Molecular physiology of Arc/Arg3.1: The oligomeric state hypothesis of synaptic plasticity

The immediate early gene, Arc, is a pivotal regulator of synaptic plasticity, memory, and cognitive flexibility. But what is Arc protein? How does it work? Inside the neuron, Arc is a protein interaction hub and dynamic regulator of intra-cellular signaling in synaptic plasticity. In remarkable contrast, Arc can also self-assemble into retrovirus-like capsids that are released in extracellular vesicles and capable of intercellular transfer of RNA. Elucidation of the molecular basis of Arc hub and capsid functions, and the relationship between them, is vital for progress. Here, we discuss recent findings on Arc structure-function and regulation of oligomerization that are giving insight into the molecular physiology of Arc. The unique features of mammalian Arc are emphasized, while drawing comparisons with Drosophila Arc and retroviral Gag. The Arc N-terminal domain, found only in mammals, is proposed to play a key role in regulating Arc hub signaling, oligomerization, and formation of capsids. Bringing together several lines of evidence, we hypothesize that Arc function in synaptic plasticity-long-term potentiation (LTP) and long-term depression (LTD)-are dictated by different oligomeric forms of Arc. Specifically, monomer/dimer function in LTP, tetramer function in basic LTD, and 32-unit oligomer function in enhanced LTD. The role of mammalian Arc capsids is unclear but likely depends on the cross-section of captured neuronal activity-induced RNAs. As the functional states of Arc are revealed, it may be possible to selectively manipulate specific forms of Arc-dependent plasticity and intercellular communication involved in brain function and dysfunction.

'Arc'-hitecture of normal cognitive aging

Arc is an effector immediate-early gene that is critical for forming long-term memories. Since its discovery 25 years ago, it has repeatedly surprised us with a number of intriguing properties, including the transport of its mRNA to recently-activated synapses, its master role in bidirectionally regulating synaptic strength, its evolutionary retroviral origins, its ability to mediate intercellular transfer between neurons via extracellular vesicles (EVs), and its exceptional regulation-both temporally and spatially. The current review discusses how Arc has been used as a tool to identify the neural networks involved in cognitive aging and how Arc itself may contribute to cognitive outcome in aging. In addition, we raise several outstanding questions, including whether Arc-containing EVs in peripheral blood might provide a noninvasive biomarker for memory-related synaptic failure in aging, and whether rectifying Arc dysregulation is likely to be an effective strategy for bending the arc of aging toward successful cognitive outcomes.

Chr21 protein-protein interactions: enrichment in proteins involved in intellectual disability, autism, and late-onset Alzheimer's disease

Down syndrome (DS) is caused by human chromosome 21 (HSA21) trisomy. It is characterized by a poorly understood intellectual disability (ID). We studied two mouse models of DS, one with an extra copy of the <i>Dyrk1A</i> gene (189N3) and the other with an extra copy of the mouse Chr16 syntenic region (Dp(16)1Yey). RNA-seq analysis of the transcripts deregulated in the embryonic hippocampus revealed an enrichment in genes associated with chromatin for the 189N3 model, and synapses for the Dp(16)1Yey model. A large-scale yeast two-hybrid screen (82 different screens, including 72 HSA21 baits and 10 rebounds) of a human brain library containing at least 10⁷ independent fragments identified 1,949 novel protein-protein interactions. The direct interactors of HSA21 baits and rebounds were significantly enriched in ID-related genes (<i>P</i>-value &lt; 2.29 × 10^-8). Proximity ligation assays showed that some of the proteins encoded by HSA21 were located at the dendritic spine postsynaptic density, in a protein network at the dendritic spine postsynapse. We located HSA21 DYRK1A and DSCAM, mutations of which increase the risk of autism spectrum disorder (ASD) 20-fold, in this postsynaptic network. We found that an intracellular domain of DSCAM bound either DLGs, which are multimeric scaffolds comprising receptors, ion channels and associated signaling proteins, or DYRK1A. The DYRK1A-DSCAM interaction domain is conserved in <i>Drosophila</i> and humans. The postsynaptic network was found to be enriched in proteins associated with ARC-related synaptic plasticity, ASD, and late-onset Alzheimer's disease. These results highlight links between DS and brain diseases with a complex genetic basis.

Is PSD-95 entangled in the side effects of antidepressants?

PSD-95 is a component and a building block of an excitatory synapse. PSD-95 is a specialized protein that is part of a "combination lock" system responsible for plastic events at the synapse, such as receptor expression, which consequently induces changes in the PSD structure and thus affects synaptic plasticity. The possible involvement of PSD-95 in antidepressant side effects related to cognitive function and psychosis will be considered. An attempt will be made to trace the sequence of events in the proposed mechanism leading to these disorders, focusing mainly on NMDA receptors. Understanding the mechanisms of action of compounds with antidepressant potential may facilitate the design of safer drugs.

Arc and Homer1 are involved in comorbid epilepsy and depression: A microarray data analysis

BACKGROUND

Depression is one of the most common comorbid psychiatric condition associated with epilepsy. It has a negative impact on the patient's quality of life. However, the underlying molecular mechanisms leading to depression are currently unclear. The aim of this study was to determine the hub genes associated with epilepsy and depression.

METHODS

Gene expression profiles (GSE47752 and GSE20388) were downloaded from the gene expression omnibus (GEO) database. Differentially expressed genes (DEGs) for epilepsy and depression groups were separately searched. Subsequently, network analyses methods were employed to establish protein-protein interaction (PPI) networks, and to perform Gene Ontology (GO) terms and pathway enrichment analyses for co-expressed DEGs.

RESULTS

A total of 772 genes were upregulated in patients with epilepsy whereas 91 genes were up-regulated in patients with depression. In addition, 1304 genes were down-regulated in epilepsy whereas 141 genes were down-regulated in patients with depression. Among co-expressed DEGs, 5 DEGs were up-regulated and 19 were down-regulated. Further analysis revealed that the co-expressed DEGs were involved in regulation of vasculature development, regulation of angiogenesis, glutamate receptor signaling pathway, cellular response to interleukin-1 and positive regulation of protein kinase B signaling. The Arc and Homer1 genes were identified as the common candidate genes involved in the pathogenesis of epilepsy and depression.

CONCLUSIONS

Arc and Homer1 may contribute to the comorbidity of epilepsy and depression.

High-affinity anti-Arc nanobodies provide tools for structural and functional studies

Activity-regulated cytoskeleton-associated protein (Arc) is a multidomain protein of retroviral origin with a vital role in the regulation of synaptic plasticity and memory formation in mammals. However, the mechanistic and structural basis of Arc function is poorly understood. Arc has an N-terminal domain (NTD) involved in membrane binding and a C-terminal domain (CTD) that binds postsynaptic protein ligands. In addition, the NTD and CTD both function in Arc oligomerisation, including assembly of retrovirus-like capsids involved in intercellular signalling. To obtain new tools for studies on Arc structure and function, we produced and characterised six high-affinity anti-Arc nanobodies (Nb). The CTD of rat and human Arc were both crystallised in ternary complexes with two Nbs. One Nb bound deep into the stargazin-binding pocket of Arc CTD and suggested competitive binding with Arc ligand peptides. The crystallisation of the human Arc CTD in two different conformations, accompanied by SAXS data and molecular dynamics simulations, paints a dynamic picture of the mammalian Arc CTD. The collapsed conformation closely resembles Drosophila Arc in capsids, suggesting that we have trapped a capsid-like conformation of the human Arc CTD. Our data obtained with the help of anti-Arc Nbs suggest that structural dynamics of the CTD and dimerisation of the NTD may promote the formation of capsids. Taken together, the recombinant high-affinity anti-Arc Nbs are versatile tools that can be further developed for studying mammalian Arc structure and function, as well as mechanisms of Arc capsid formation, both in vitro and in vivo. For example, the Nbs could serve as a genetically encoded tools for inhibition of endogenous Arc interactions in the study of neuronal function and plasticity.

The Effect of Sericin on the Cognitive Impairment, Depression, and Anxiety Caused by Learned Helplessness in Male Mice

Learned helplessness (LH) induces cognitive and emotional abnormalities via alteration of synaptic and apoptotic markers in the hippocampus. Given the sericin's neuroprotective effects on different experimental models, this study aimed to address whether sericin is able to reduce LH-induced behavioral and molecular changes in the mouse model. Sixty male mice (3 months old) were randomly divided into control, normal saline (NS), and/or different doses of sericin (Ser [100, 200, and 300 mg/kg]) for 21 days. Accordingly, the animals in NS and sericin-treated groups were subjected to 1 day learned helplessness protocol. Behavioral deficits were evaluated and alterations in both synaptic and apoptotic factors were evaluated in the hippocampus. Induction of LH was associated with behavioral changes (depression and cognitive impairment). On the other hand, the administration of sericin effectively normalized these deficits. At molecular levels, sericin increased the levels of synaptophysin, synapsin-1, and PSD-95, and decreased apoptosis in the hippocampus. Although the exact mechanisms underlying the neuroprotective effects of sericin are not fully understood, our results showed that this effect mediated via modulation of the synaptic and apoptotic proteins in the hippocampus of LH-subjected mice.

Abnormal synaptic plasticity and impaired cognition in schizophrenia

Schizophrenia (SCZ) is a severe mental illness that affects several brain domains with relation to cognition and behaviour. SCZ symptoms are typically classified into three categories, namely, positive, negative, and cognitive. The etiology of SCZ is thought to be multifactorial and poorly understood. Accumulating evidence has indicated abnormal synaptic plasticity and cognitive impairments in SCZ. Synaptic plasticity is thought to be induced at appropriate synapses during memory formation and has a critical role in the cognitive symptoms of SCZ. Many factors, including synaptic structure changes, aberrant expression of plasticity-related genes, and abnormal synaptic transmission, may influence synaptic plasticity and play vital roles in SCZ. In this article, we briefly summarize the morphology of the synapse, the neurobiology of synaptic plasticity, and the role of synaptic plasticity, and review potential mechanisms underlying abnormal synaptic plasticity in SCZ. These abnormalities involve dendritic spines, postsynaptic density, and long-term potentiation-like plasticity. We also focus on cognitive dysfunction, which reflects impaired connectivity in SCZ. Additionally, the potential targets for the treatment of SCZ are discussed in this article. Therefore, understanding abnormal synaptic plasticity and impaired cognition in SCZ has an essential role in drug therapy.

Transcriptomic and Proteomic Analysis of CRISPR/Cas9-Mediated ARC-Knockout HEK293 Cells

Arc/Arg3.1 (activity-regulated cytoskeletal-associated protein (ARC)) is a critical regulator of long-term synaptic plasticity and is involved in the pathophysiology of schizophrenia. The functions and mechanisms of human ARC action are poorly understood and worthy of further investigation. To investigate the function of the ARC gene in vitro, we generated an ARC-knockout (KO) HEK293 cell line via CRISPR/Cas9-mediated gene editing and conducted RNA sequencing and label-free LC-MS/MS analysis to identify the differentially expressed genes and proteins in isogenic ARC-KO HEK293 cells. Furthermore, we used bioluminescence resonance energy transfer (BRET) assays to detect interactions between the ARC protein and differentially expressed proteins. Genetic deletion of ARC disturbed multiple genes involved in the extracellular matrix and synaptic membrane. Seven proteins (HSPA1A, ENO1, VCP, HMGCS1, ALDH1B1, FSCN1, and HINT2) were found to be differentially expressed between ARC-KO cells and ARC wild-type cells. BRET assay results showed that ARC interacted with PSD95 and HSPA1A. Overall, we found that ARC regulates the differential expression of genes involved in the extracellular matrix, synaptic membrane, and heat shock protein family. The transcriptomic and proteomic profiles of ARC-KO HEK293 cells presented here provide new evidence for the mechanisms underlying the effects of ARC and molecular pathways involved in schizophrenia pathophysiology.

Bioinformatics and Network-based Approaches for Determining Pathways, Signature Molecules, and Drug Substances connected to Genetic Basis of Schizophrenia etiology

Knowledge of heterogeneous etiology and pathophysiology of schizophrenia (SZP) is reasonably inadequate and non-deterministic due to its inherent complexity and underlying vast dynamics related to genetic mechanisms. The evolution of large-scale transcriptome-wide datasets and subsequent development of relevant, robust technologies for their analyses show promises toward elucidating the genetic basis of disease pathogenesis, its early risk prediction, and predicting drug molecule targets for therapeutic intervention. In this research, we have scrutinized the genetic basis of SZP through functional annotation and network-based system biology approaches. We have determined 96 overlapping differentially expressed genes (DEGs) from 2 microarray datasets and subsequently identified their interconnecting networks to reveal transcriptome signatures like hub proteins (FYN, RAD51, SOCS3, XIAP, AKAP13, PIK3C2A, CBX5, GATA3, EIF3K, and CDKN2B), transcription factors and miRNAs. In addition, we have employed gene set enrichment to highlight significant gene ontology (e.g., positive regulation of microglial cell activation) and relevant pathways (such as axon guidance and focal adhesion) interconnected to the genes associated with SZP. Finally, we have suggested candidate drug substances like Luteolin HL60 UP as a possible therapeutic target based on these key molecular signatures.

Maternal Cigarette Smoke Exposure Exaggerates the Behavioral Defects and Neuronal Loss Caused by Hypoxic-Ischemic Brain Injury in Female Offspring

Objective

Hypoxic-ischemic encephalopathy affects ∼6 in 1,000 preterm neonates, leading to significant neurological sequela (e.g., cognitive deficits and cerebral palsy). Maternal smoke exposure (SE) is one of the common causes of neurological disorders; however, female offspring seems to be less affected than males in our previous study. We also showed that maternal SE exaggerated neurological disorders caused by neonatal hypoxic-ischemic brain injury in adolescent male offspring. Here, we aimed to examine whether female littermates of these males are protected from such insult.

Methods

BALB/c dams were exposed to cigarette smoke generated from 2 cigarettes twice daily for 6 weeks before mating, during gestation and lactation. To induce hypoxic-ischemic brain injury, half of the pups from each litter underwent left carotid artery occlusion, followed by exposure to 8% oxygen (92% nitrogen) at postnatal day (P) 10. Behavioral tests were performed at P40–44, and brain tissues were collected at P45.

Results

Maternal SE worsened the defects in short-term memory and motor function in females with hypoxic-ischemic injury; however, reduced anxiety due to injury was observed in the control offspring, but not the SE offspring. Both hypoxic-ischemic injury and maternal SE caused significant loss of neuronal cells and synaptic proteins, along with increased oxidative stress and inflammatory responses.

Conclusion

Oxidative stress and inflammatory response due to maternal SE may be the mechanism of worsened neurological outcomes by hypoxic-ischemic brain injury in females, which was similar to their male littermates shown in our previous study.

Genetic variation, brain, and intelligence differences

Individual differences in human intelligence, as assessed using cognitive test scores, have a well-replicated, hierarchical phenotypic covariance structure. They are substantially stable across the life course, and are predictive of educational, social, and health outcomes. From this solid phenotypic foundation and importance for life, comes an interest in the environmental, social, and genetic aetiologies of intelligence, and in the foundations of intelligence differences in brain structure and functioning. Here, we summarise and critique the last 10 years or so of molecular genetic (DNA-based) research on intelligence, including the discovery of genetic loci associated with intelligence, DNA-based heritability, and intelligence's genetic correlations with other traits. We summarise new brain imaging-intelligence findings, including whole-brain associations and grey and white matter associations. We summarise regional brain imaging associations with intelligence and interpret these with respect to theoretical accounts. We address research that combines genetics and brain imaging in studying intelligence differences. There are new, though modest, associations in all these areas, and mechanistic accounts are lacking. We attempt to identify growing points that might contribute toward a more integrated 'systems biology' account of some of the between-individual differences in intelligence.

Molecular Mechanisms of Memory Consolidation That Operate During Sleep

The role of sleep for brain function has been in the focus of interest for many years. It is now firmly established that sleep and the corresponding brain activity is of central importance for memory consolidation. Less clear are the underlying molecular mechanisms and their specific contribution to the formation of long-term memory. In this review, we summarize the current knowledge of such mechanisms and we discuss the several unknowns that hinder a deeper appreciation of how molecular mechanisms of memory consolidation during sleep impact synaptic function and engram formation.

A single-synapse resolution survey of PSD95-positive synapses in twenty human brain regions

Mapping the molecular composition of individual excitatory synapses across the mouse brain reveals high synapse diversity with each brain region showing a distinct composition of synapse types. As a first step towards systematic mapping of synapse diversity across the human brain, we have labelled and imaged synapses expressing the excitatory synapse protein PSD95 in twenty human brain regions, including 13 neocortical, two subcortical, one hippocampal, one cerebellar and three brainstem regions, in four phenotypically normal individuals. We quantified the number, size and intensity of individual synaptic puncta and compared their regional distributions. We found that each region showed a distinct signature of synaptic puncta parameters. Comparison of brain regions showed that cortical and hippocampal structures are similar, and distinct from those of cerebellum and brainstem. Comparison of synapse parameters from human and mouse brain revealed conservation of parameters, hierarchical organization of brain regions and network architecture. This work illustrates the feasibility of generating a systematic single-synapse resolution atlas of the human brain, a potentially significant resource in studies of brain health and disease.

Activation of α7 nicotinic acetylcholine receptor ameliorates HIV-associated neurology and neuropathology

HIV-associated neurocognitive disorders (HAND) in the era of combination antiretroviral therapy are primarily manifested as impaired behaviours, glial activation/neuroinflammation and compromised neuronal integrity, for which there are no effective treatments currently available.

In the current study, we used doxycycline-inducible astrocyte-specific HIV Tat transgenic mice (iTat), a surrogate HAND model, and determined effects of PNU-125096, a positive allosteric modulator of α7 nicotinic acetylcholine receptor (α7 nAChR) on Tat-induced behavioural impairments and neuropathologies.

We showed that PNU-125096 treatment significantly improved locomotor, learning and memory deficits of iTat mice while inhibited glial activation and increased PSD-95 expression in the cortex and hippocampus of iTat mice. Using α7 nAChR knockout mice, we showed that α7 nAChR knockout eliminated the protective effects of PNU-125096 on iTat mice. In addition, we showed that inhibition of p38 phosphorylation by SB239063, a p38 MAPK-specific inhibitor exacerbated Tat neurotoxicity in iTat mice. Last, we used primary mouse cortical individual cultures and neuron-astrocytes co-cultures and in vivo staining of iTat mouse brain tissues and showed that glial activation was directly involved in the interplay among Tat neurotoxicity, α7 nAChR activation and the p38 MAPK signalling pathway.

Taken together, these findings demonstrated for the first time that α7 nAChR activation led to protection against HAND and suggested that α7 nAChR modulator PNU-125096 holds significant promise for development of therapeutics for HAND.

Structural properties and peptide ligand binding of the capsid homology domains of human Arc

The activity-regulated cytoskeleton-associated protein (Arc) is important for synaptic plasticity and the normal function of the brain. Arc interacts with neuronal postsynaptic proteins, but the mechanistic details of its function have not been fully established. The C-terminal domain of Arc consists of tandem domains, termed the N- and C-lobe. The N-lobe harbours a peptide binding site, able to bind multiple targets. By measuring the affinity of human Arc towards various peptides from stargazin and guanylate kinase-associated protein (GKAP), we have refined its specificity determinants. We found two sites in the GKAP repeat region that bind to Arc and confirmed these interactions by X-ray crystallography. Phosphorylation of the stargazin peptide did not affect binding affinity but caused changes in thermodynamic parameters. Comparison of the crystal structures of three high-resolution human Arc-peptide complexes identifies three conserved C-H…π interactions at the binding cavity, explaining the sequence specificity of short linear motif binding by Arc. We further characterise central residues of the Arc lobe fold, show the effects of peptide binding on protein dynamics, and identify acyl carrier proteins as structures similar to the Arc lobes. We hypothesise that Arc may affect protein-protein interactions and phase separation at the postsynaptic density, affecting protein turnover and re-modelling of the synapse. The present data on Arc structure and ligand binding will help in further deciphering these processes.

Phosphorylation-dependent control of Activity-regulated cytoskeleton-associated protein (Arc) protein by TNIK

Activity-regulated cytoskeleton-associated protein (Arc) is an immediate early gene product that support neuroplastic changes important for cognitive function and memory formation. As a protein with homology to the retroviral Gag protein, a particular characteristic of Arc is its capacity to self-assemble into virus-like capsids that can package mRNAs and transfer those transcripts to other cells. Although a lot has been uncovered about the contributions of Arc to neuron biology and behavior, very little is known about how different functions of Arc are coordinately regulated both temporally and spatially in neurons. The answer to this question we hypothesized must involve the occurrence of different protein post-translational modifications acting to confer specificity. In this study, we used mass spectrometry and sequence prediction strategies to map novel Arc phosphorylation sites. Our approach led us to recognize serine 67 (S67) and threonine 278 (T278) as residues that can be modified by TNIK, which is a kinase abundantly expressed in neurons that shares many functional overlaps with Arc and has, along with its interacting proteins such as the NMDA receptor, and been implicated as a risk factor for psychiatric disorders. Furthermore, characterization of each residue using site-directed mutagenesis to create S67 and T278 mutant variants revealed that TNIK action at those amino acids can strongly influence Arc's subcellular distribution and self-assembly as capsids. Together, our findings reveal an unsuspected connection between Arc and TNIK. Better understanding of the interplay between these two proteins in neuronal cells could lead to new insights about apparition and progression of psychiatric disorders. Cover Image for this issue: https://doi.org/10.1111/jnc.15077.

Phosphorylation-dependent control of Activity-regulated cytoskeleton-associated protein (Arc) protein by TNIK

Activity-regulated cytoskeleton-associated protein (Arc) is an immediate early gene product that support neuroplastic changes important for cognitive function and memory formation. As a protein with homology to the retroviral Gag protein, a particular characteristic of Arc is its capacity to self-assemble into virus-like capsids that can package mRNAs and transfer those transcripts to other cells. Although a lot has been uncovered about the contributions of Arc to neuron biology and behavior, very little is known about how different functions of Arc are coordinately regulated both temporally and spatially in neurons. The answer to this question we hypothesized must involve the occurrence of different protein post-translational modifications acting to confer specificity. In this study, we used mass spectrometry and sequence prediction strategies to map novel Arc phosphorylation sites. Our approach led us to recognize serine 67 (S67) and threonine 278 (T278) as residues that can be modified by TNIK, which is a kinase abundantly expressed in neurons that shares many functional overlaps with Arc and has, along with its interacting proteins such as the NMDA receptor, and been implicated as a risk factor for psychiatric disorders. Furthermore, characterization of each residue using site-directed mutagenesis to create S67 and T278 mutant variants revealed that TNIK action at those amino acids can strongly influence Arc's subcellular distribution and self-assembly as capsids. Together, our findings reveal an unsuspected connection between Arc and TNIK. Better understanding of the interplay between these two proteins in neuronal cells could lead to new insights about apparition and progression of psychiatric disorders.

A unified resource and configurable model of the synapse proteome and its role in disease

Genes encoding synaptic proteins are highly associated with neuronal disorders many of which show clinical co-morbidity. We integrated 58 published synaptic proteomic datasets that describe over 8000 proteins and combined them with direct protein-protein interactions and functional metadata to build a network resource that reveals the shared and unique protein components that underpin multiple disorders. All the data are provided in a flexible and accessible format to encourage custom use.

Intercellular Communication in the Nervous System Goes Viral

Viruses and transposable elements are major drivers of evolution and make up over half the sequences in the human genome. In some cases, these elements are co-opted to perform biological functions for the host. Recent studies made the surprising observation that the neuronal gene Arc forms virus-like protein capsids that can transfer RNA between neurons to mediate a novel intercellular communication pathway. Phylogenetic analyses showed that mammalian Arc is derived from an ancient retrotransposon of the Ty3/gypsy family and contains homology to the retroviral Gag polyproteins. The Drosophila Arc homologs, which are independently derived from the same family of retrotransposons, also mediate cell-to-cell signaling of RNA at the neuromuscular junction; a striking example of convergent evolution. Here we propose an Arc 'life cycle', based on what is known about retroviral Gag, and discuss how elucidating these biological processes may lead to novel insights into brain plasticity and memory.

Herpes Simplex Virus Type 1 Neuronal Infection Triggers the Disassembly of Key Structural Components of Dendritic Spines

Herpes simplex virus type 1 (HSV-1) is a widespread neurotropic virus. Primary infection of HSV-1 in facial epithelium leads to retrograde axonal transport to the central nervous system (CNS) where it establishes latency. Under stressful conditions, the virus reactivates, and new progeny are transported anterogradely to the primary site of infection. During the late stages of neuronal infection, axonal damage can occur, however, the impact of HSV-1 infection on the morphology and functional integrity of neuronal dendrites during the early stages of infection is unknown. We previously demonstrated that acute HSV-1 infection in neuronal cell lines selectively enhances Arc protein expression - a major regulator of long-term synaptic plasticity and memory consolidation, known for being a protein-interaction hub in the postsynaptic dendritic compartment. Thus, HSV-1 induced Arc expression may alter the functionality of infected neurons and negatively impact dendritic spine dynamics. In this study we demonstrated that HSV-1 infection induces structural disassembly and functional deregulation in cultured cortical neurons, an altered glutamate response, Arc accumulation within the somata, and decreased expression of spine scaffolding-like proteins such as PSD-95, Drebrin and CaMKIIβ. However, whether these alterations are specific to the HSV-1 infection mechanism or reflect a secondary neurodegenerative process remains to be determined.

Proteomic insights into synaptic signaling in the brain: the past, present and future

Chemical synapses in the brain connect neurons to form neural circuits, providing the structural and functional bases for neural communication. Disrupted synaptic signaling is closely related to a variety of neurological and psychiatric disorders. In the past two decades, proteomics has blossomed as a versatile tool in biological and biomedical research, rendering a wealth of information toward decoding the molecular machinery of life. There is enormous interest in employing proteomic approaches for the study of synapses, and substantial progress has been made. Here, we review the findings of proteomic studies of chemical synapses in the brain, with special attention paid to the key players in synaptic signaling, i.e., the synaptic protein complexes and their post-translational modifications. Looking toward the future, we discuss the technological advances in proteomics such as data-independent acquisition mass spectrometry (DIA-MS), cross-linking in combination with mass spectrometry (CXMS), and proximity proteomics, along with their potential to untangle the mystery of how the brain functions at the molecular level. Last but not least, we introduce the newly developed synaptomic methods. These methods and their successful applications marked the beginnings of the synaptomics era.

The synapse in traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability worldwide and is a risk factor for dementia later in life. Research into the pathophysiology of TBI has focused on the impact of injury on the neuron. However, recent advances have shown that TBI has a major impact on synapse structure and function through a combination of the immediate mechanical insult and the ensuing secondary injury processes, leading to synapse loss. In this review, we highlight the role of the synapse in TBI pathophysiology with a focus on the confluence of multiple secondary injury processes including excitotoxicity, inflammation and oxidative stress. The primary insult triggers a cascade of events in each of these secondary processes and we discuss the complex interplay that occurs at the synapse. We also examine how the synapse is impacted by traumatic axonal injury and the role it may play in the spread of tau after TBI. We propose that astrocytes play a crucial role by mediating both synapse loss and recovery. Finally, we highlight recent developments in the field including synapse molecular imaging, fluid biomarkers and therapeutics. In particular, we discuss advances in our understanding of synapse diversity and suggest that the new technology of synaptome mapping may prove useful in identifying synapses that are vulnerable or resistant to TBI.

Synergistic inhibition of histone modifiers produces therapeutic effects in adult Shank3-deficient mice

Autism spectrum disorder (ASD) is a lifelong developmental disorder characterized by social deficits and other behavioral abnormalities. Dysregulation of epigenetic processes, such as histone modifications and chromatin remodeling, have been implicated in ASD pathology, and provides a promising therapeutic target for ASD. Haploinsufficiency of the SHANK3 gene is causally linked to ASD, so adult (3-5 months old) Shank3-deficient male mice were used in this drug discovery study. We found that combined administration of the class I histone deacetylase inhibitor Romidepsin and the histone demethylase LSD1 inhibitor GSK-LSD1 persistently ameliorated the autism-like social preference deficits, while each individual drug alone was largely ineffective. Another behavioral abnormality in adult Shank3-deficient male mice, heightened aggression, was also alleviated by administration of the dual drugs. Furthermore, Romidepsin/GSK-LSD1 treatment significantly increased transcriptional levels of NMDA receptor subunits in prefrontal cortex (PFC) of adult Shank3-deficient mice, resulting in elevated synaptic expression of NMDA receptors and the restoration of NMDAR synaptic function in PFC pyramidal neurons. These results have offered a novel pharmacological intervention strategy for ASD beyond early developmental periods.

Molecular architecture of postsynaptic Interactomes

The postsynaptic density (PSD) plays an essential role in the organization of the synaptic signaling machinery. It contains a set of core scaffolding proteins that provide the backbone to PSD protein-protein interaction networks (PINs). These core scaffolding proteins can be seen as three principal layers classified by protein family, with DLG proteins being at the top, SHANKs along the bottom, and DLGAPs connecting the two layers. Early studies utilizing yeast two hybrid enabled the identification of direct protein-protein interactions (PPIs) within the multiple layers of scaffolding proteins. More recently, mass-spectrometry has allowed the characterization of whole interactomes within the PSD. This expansion of knowledge has further solidified the centrality of core scaffolding family members within synaptic PINs and provided context for their role in neuronal development and synaptic function. Here, we discuss the scaffolding machinery of the PSD, their essential functions in the organization of synaptic PINs, along with their relationship to neuronal processes found to be impaired in complex brain disorders.

Arc/Arg3.1 function in long-term synaptic plasticity: Emerging mechanisms and unresolved issues

Arc (activity-regulated cytoskeleton-associated protein) is posited as a critical regulator of long-term synaptic plasticity at excitatory synapses, including long-term potentiation, long-term depression, inverse synaptic tagging and homoeostatic scaling, with pivotal roles in memory and postnatal cortical development. However, the mechanisms underlying the bidirectional regulation of synaptic strength are poorly understood. Here we review evidence from different plasticity paradigms, highlight outstanding issues and discuss stimulus-specific mechanisms that dictate Arc function. We propose a model in which Arc bidirectionally controls synaptic strength by coordinate regulation of AMPA-type glutamate receptor (AMPAR) trafficking and actin cytoskeletal dynamics in dendritic spines. Key to this model, Arc is proposed to function as an activity-dependent regulator of AMPAR lateral membrane diffusion and trapping at synapses.

Cocaine-Induced Synaptic Redistribution of NMDARs in Striatal Neurons Alters NMDAR-Dependent Signal Transduction

The consequence of repeated cocaine exposure and prolonged abstinence on glutamate receptor expression in the nucleus accumbens has been extensively studied. However, the early effects of cocaine on NMDAR signaling remain unknown. NMDAR signaling depends on the subunit composition, subcellular localization, and the interaction with proteins at the postsynaptic density (PSD), where NMDARs and other proteins form supercomplexes that are responsible for the signaling pathways activated by NMDAR-induced Ca²⁺ influx. Here, we investigated the effect of cocaine on NMDAR subunit composition and subcellular localization after both intraperitoneal non-contingent cocaine and response-contingent intravenous cocaine self-administration in mice. We found that repeated cocaine exposure, regardless of the route or contingency of drug administration, decreases NMDAR interactions with the PSD and synaptic lipid rafts in the accumbens shell and dorsal striatum. We provide evidence that cocaine triggers an early redistribution of NMDARs from synaptic to extrasynaptic sites, and that this adaptation has implications in the activation of downstream signaling pathways. Thus, consistent with a loss of NMDAR function, cocaine-induced ERK phosphorylation is attenuated. Because early NMDAR activity contributes to the initiation of lasting addiction-relevant neuroadaptations, these data may hold clues into cellular mechanisms responsible for the development of cocaine addiction.

Loss of Arc attenuates the behavioral and molecular responses for sleep homeostasis in mice

The activity-regulated cytoskeleton-associated protein (Arc) gene is a neural immediate early gene that is involved in synaptic downscaling and is robustly induced by prolonged wakefulness in rodent brains. Converging evidence has led to the hypothesis that wakefulness potentiates, and sleep reduces, synaptic strengthening. This suggests a potential role for Arc in these and other sleep-related processes. However, the role of Arc in sleep remains unknown. Here, we demonstrated that Arc is important for the induction of multiple behavioral and molecular responses associated with sleep homeostasis. Arc knockout (KO) mice displayed increased time spent in rapid eye movement (REM) sleep under baseline conditions and marked attenuation of sleep rebound to both 4 h of total sleep deprivation (SD) and selective REM deprivation. At the molecular level, the following homeostatic sleep responses to 4-h SD were all blunted in Arc KO mice: increase of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor GluA1 and its phosphorylation in synaptoneurosomes; induction of a subset of SD-response genes; and suppression of the GluA1 messenger RNA in the cortex. In wild-type brains, SD increased Arc protein expression in multiple subcellular locations, including the nucleus, cytoplasm, and synapse, which is reversed in part by recovery sleep. Arc is critical for these behavioral and multiple molecular responses to SD, thus providing a multifunctional role for Arc in the maintenance of sleep homeostasis, which may be attributed by the sleep/wake-associated changes in subcellular location of Arc.

ADHD symptoms and their neurodevelopmental correlates in children born very preterm

This study investigated the association between attention-deficit/hyperactivity disorder (ADHD) symptomatology in preschool-aged children who were born very preterm (<33 weeks) and cognitive outcomes, clinical risk and socio-demographic characteristics. 119 very preterm children who participated in the Evaluation of Preterm Imaging Study at term-equivalent age were assessed at a mean age of 4.5 years. Parents completed the ADHD Rating Scale IV, a norm-referenced checklist that evaluates ADHD symptomatology according to diagnostic criteria, and the Behavior Rating Inventory of Executive Function-Preschool version. Children completed the Wechsler Preschool and Primary Scales of Intelligence and the Forward Digit Span task. Longitudinal data including perinatal clinical, qualitative MRI classification, socio-demographic variables and neurodevelopmental disabilities were investigated in relation to ADHD symptomatology. All results were corrected for multiple comparisons using false discovery rate. Results showed that although the proportion of very preterm children with clinically significant ADHD did not differ from normative data after excluding those with neurodevelopmental disabilities, 32.7% met criteria for subthreshold ADHD inattentive type and 33.6% for combined type, which was higher than the expected 20% in normative samples. Higher ADHD symptom scores (all) were associated with greater executive dysfunction (inhibitory self-control, flexibility, and emergent metacognition, corrected p<0.001 for all tests). Higher inattentive ADHD symptom scores were associated with lower IQ (ρ = -0.245, p = 0.011) and higher perinatal clinical risk (more days on mechanical ventilation (ρ = 0.196, p = 0.032) and more days on parenteral nutrition (ρ = 0.222, p = 0.015). Higher hyperactive ADHD symptom scores instead were associated with lower socio-economic status (ρ = 0.259, p = 0.004). These results highlight the importance of monitoring and supporting the development of very preterm children throughout the school years, as subthreshold ADHD symptoms represent risk factors for psychosocial problems and for receiving a future clinical diagnosis of ADHD.

The proteome and its dynamics: A missing piece for integrative multi-omics in schizophrenia

The complex and heterogeneous pathophysiology of schizophrenia can be deconstructed by integration of large-scale datasets encompassing genes through behavioral phenotypes. Genome-wide datasets are now available for genetic, epigenetic and transcriptomic variations in schizophrenia, which are then analyzed by newly devised systems biology algorithms. A missing piece, however, is the inclusion of information on the proteome and its dynamics in schizophrenia. Proteomics has lagged behind omics of the genome, transcriptome and epigenome since analytic platforms were relatively less robust for proteins. There has been remarkable progress, however, in the instrumentation of liquid chromatography (LC) and mass spectrometry (MS) (LCMS), experimental paradigms and bioinformatics of the proteome. Here, we present a summary of methodological innovations of recent years in MS based proteomics and the power of new generation proteomics, review proteomics studies that have been conducted in schizophrenia to date, and propose how such data can be analyzed and integrated with other omics results. The function of a protein is determined by multiple molecular properties, i.e., subcellular localization, posttranslational modification (PTMs) and protein-protein interactions (PPIs). Incorporation of these properties poses additional challenges in proteomics and their integration with other omics; yet is a critical next step to close the loop of multi-omics integration. In sum, the recent advent of high-throughput proteome characterization technologies and novel mathematical approaches enable us to incorporate functional properties of the proteome to offer a comprehensive multi-omics based understanding of schizophrenia pathophysiology.

Transcriptomics of Gabra4 knockout mice reveals common NMDAR pathways underlying autism, memory, and epilepsy

Autism spectrum disorder (ASD) is a neuronal developmental disorder with impaired social interaction and communication, often with abnormal intelligence and comorbidity with epilepsy. Disturbances in synaptic transmission, including the GABAergic, glutamatergic, and serotonergic systems, are known to be involved in the pathogenesis of this disorder, yet we do not know if there is a common molecular mechanism. As mutations in the GABAergic receptor subunit gene GABRA4 are reported in patients with ASD, we eliminated the Gabra4 gene in mice and found that the Gabra4 knockout mice showed autistic-like behavior, enhanced spatial memory, and attenuated susceptibility to pentylenetetrazol-induced seizures, a constellation of symptoms resembling human high-functioning autism. To search for potential molecular pathways involved in these phenotypes, we performed a hippocampal transcriptome profiling, constructed a hippocampal interactome network, and revealed an upregulation of the NMDAR system at the center of the converged pathways underlying high-functioning autism-like and anti-epilepsy phenotypes.

The epilepsy and intellectual disability-associated protein TBC1D24 regulates the maintenance of excitatory synapses and animal behaviors

Perturbation of synapse development underlies many inherited neurodevelopmental disorders including intellectual disability (ID). Diverse mutations on the human TBC1D24 gene are strongly associated with epilepsy and ID. However, the physiological function of TBC1D24 in the brain is not well understood, and there is a lack of genetic mouse model that mimics TBC1D24 loss-of-function for the study of animal behaviors. Here we report that TBC1D24 is present at the postsynaptic sites of excitatory synapses, where it is required for the maintenance of dendritic spines through inhibition of the small GTPase ARF6. Mice subjected to viral-mediated knockdown of TBC1D24 in the adult hippocampus display dendritic spine loss, deficits in contextual fear memory, as well as abnormal behaviors including hyperactivity and increased anxiety. Interestingly, we show that the protein stability of TBC1D24 is diminished by the disease-associated missense mutation that leads to F251L amino acid substitution. We further generate the F251L knock-in mice, and the homozygous mutants show increased neuronal excitability, spontaneous seizure and pre-mature death. Moreover, the heterozygous F251L knock-in mice survive into adulthood but display dendritic spine defects and impaired memory. Our findings therefore uncover a previously uncharacterized postsynaptic function of TBC1D24, and suggest that impaired dendritic spine maintenance contributes to the pathophysiology of individuals harboring TBC1D24 gene mutations. The F251L knock-in mice represent a useful animal model for investigation of the mechanistic link between TBC1D24 loss-of-function and neurodevelopmental disorders.

Synapse diversity and synaptome architecture in human genetic disorders

Over 130 brain diseases are caused by mutations that disrupt genes encoding the proteome of excitatory synapses. These include neurological and psychiatric disorders with early and late onset such as autism, schizophrenia and depression and many other rarer conditions. The proteome of synapses is highly complex with over 1000 conserved proteins which are differentially expressed generating a vast, potentially unlimited, number of synapse types. The diversity of synapses and their location in the brain are described by the synaptome. A recent study has mapped the synaptome across the mouse brain, revealing that synapse diversity is distributed into an anatomical architecture observed at scales from individual dendrites to the whole systems level. The synaptome architecture is built from the hierarchical expression and assembly of proteins into complexes and supercomplexes which are distributed into different synapses. Mutations in synapse proteins change the synaptome architecture leading to behavioral phenotypes. Mutations in the mechanisms regulating the hierarchical assembly of the synaptome, including transcription and proteostasis, may also change synapse diversity and synaptome architecture. The logic of synaptome hierarchical assembly provides a mechanistic framework that explains how diverse genetic disorders can converge on synapses in different brain circuits to produce behavioral phenotypes.

Spatial Coupling Tunes NMDA Receptor Responses via Ca2+ Diffusion

In the CNS, NMDA receptors generate large and highly regulated Ca2+ signals, which are critical for synaptic development and plasticity. They are highly clustered at postsynaptic sites and along dendritic arbors, but whether this spatial arrangement affects their output is unknown. Synaptic NMDA receptor currents are subject to Ca2+-dependent inactivation (CDI), a type of activity-dependent inhibition that requires intracellular Ca2+ and calmodulin (CaM). We asked whether Ca2+ influx through a single NMDA receptor influences the activity of nearby NMDA receptors, as a possible coupling mechanism. Using cell-attached unitary current recordings from GluN1-2a/GluN2A receptors expressed in human HEK293 cells and from NMDA receptors native to hippocampal neurons from male and female rats, we recorded unitary currents from multichannel patches and used a coupled Markov model to determine the extent of signal coupling (κ). In the absence of extracellular Ca2+, we observed no cooperativity (κ < 0.1), whereas in 1.8 mm external Ca2+, both recombinant and native channels showed substantial negative cooperativity (κ = 0.27). Intracellular Ca2+ chelation or overexpression of a Ca2+-insensitive CaM mutant, reduced coupling, which is consistent with CDI representing the coupling mechanism. In contrast, cooperativity increased substantially (κ = 0.68) when overexpressing the postsynaptic scaffolding protein PSD-95, which increased receptor clustering. Together, these new results demonstrate that NMDA receptor currents are negatively coupled through CDI, and the degree of coupling can be tuned by the distance between receptors. Therefore, channel clustering can influence the activity-dependent reduction in NMDA receptor currents.SIGNIFICANCE STATEMENT At central synapses, NMDA receptors are a major class of excitatory glutamate-gated channels and a source of activity-dependent Ca2+ influx. In turn, fluxed Ca2+ ions bind to calmodulin-primed receptors and reduce further entry, through an autoinhibitory mechanism known as Ca2+ -dependent inactivation (CDI). Here, we show that the diffusion of fluxed Ca2+ between active channels situated within submicroscopic distances amplified receptor inactivation. Thus, calmodulin-mediated gating modulation, an evolutionarily conserved regulatory mechanism, endows synapses with sensitivity to both the temporal sequence and spatial distribution of Ca2+ signals. Perturbations in this mechanism, which coordinates the activity of NMDA receptors within a cluster, may cause signaling alterations that contribute to neuropsychiatric conditions.

Disturbed Prefrontal Cortex Activity in the Absence of Schizophrenia-Like Behavioral Dysfunction in Arc/Arg3.1 Deficient Mice

Arc/Arg3.1, an activity regulated immediate early gene, is essential for learning and memory, synaptic plasticity, and maturation of neural networks. It has also been implicated in several neurodevelopmental disorders, including schizophrenia. Here, we used male and female constitutive and conditional Arc/Arg3.1 knock-out (KO) mice to investigate the causal relationship between Arc/Arg3.1 deletion and schizophrenia-linked neurophysiological and behavioral phenotypes. Using in vivo local field potential recordings, we observed dampened oscillatory activity in the prefrontal cortex (PFC) of the KO and early conditional KO (early-cKO) mice, in which Arc/Arg3.1 was deleted perinatally. Whole-cell patch-clamp recordings from neurons in PFC slices revealed altered synaptic properties and reduced network gain in the KO mice as possible mechanisms underlying the oscillation deficits. In contrast, we measured normal oscillatory activity in the PFC of late conditional KO (late-cKO) mice, in which Arc/Arg3.1 was deleted during late postnatal development. Our data show that constitutive Arc/Arg3.1 KO mice exhibit no deficit in social engagement, working memory, sensorimotor gating, native locomotor activity, and dopaminergic innervation. Moreover, adolescent social isolation, an environmental stressor, failed to induce deficits in sociability or sensorimotor gating in adult KO mice. Thus, genetic removal of Arc/Arg3.1 per se does not cause schizophrenia-like behavior. Prenatal or perinatal deletion of Arc/Arg3.1 alters cortical network activity, however, without overtly disrupting the balance of excitation and inhibition in the brain and not promoting schizophrenia. Misregulation of Arc/Arg3.1 rather than deletion could potentially tip this balance and thereby promote emergence of schizophrenia and other neuropsychiatric disorders.SIGNIFICANCE STATEMENT The activity-regulated and memory-linked gene Arc/Arg3.1 has been implicated in the pathogenesis of schizophrenia, but direct evidence and a mechanistic link are still missing. The current study asks whether loss of Arc/Arg3.1 can affect brain circuitry and cause schizophrenia-like symptoms in mice. The findings demonstrate that genetic deletion of Arc/Arg3.1 before puberty alters synaptic function and prefrontal cortex activity. Although brain networks are disturbed, genetic deletion of Arc/Arg3.1 does not cause schizophrenia-like behavior, even when combined with an environmental insult. It remains to be seen whether misregulation of Arc/Arg3.1 might critically imbalance brain networks and lead to emergence of schizophrenia.

Pleiotropic Meta-Analysis of Cognition, Education, and Schizophrenia Differentiates Roles of Early Neurodevelopmental and Adult Synaptic Pathways

Susceptibility to schizophrenia is inversely correlated with general cognitive ability at both the phenotypic and the genetic level. Paradoxically, a modest but consistent positive genetic correlation has been reported between schizophrenia and educational attainment, despite the strong positive genetic correlation between cognitive ability and educational attainment. Here we leverage published genome-wide association studies (GWASs) in cognitive ability, education, and schizophrenia to parse biological mechanisms underlying these results. Association analysis based on subsets (ASSET), a pleiotropic meta-analytic technique, allowed jointly associated loci to be identified and characterized. Specifically, we identified subsets of variants associated in the expected ("concordant") direction across all three phenotypes (i.e., greater risk for schizophrenia, lower cognitive ability, and lower educational attainment); these were contrasted with variants that demonstrated the counterintuitive ("discordant") relationship between education and schizophrenia (i.e., greater risk for schizophrenia and higher educational attainment). ASSET analysis revealed 235 independent loci associated with cognitive ability, education, and/or schizophrenia at p < 5 × 10-8. Pleiotropic analysis successfully identified more than 100 loci that were not significant in the input GWASs. Many of these have been validated by larger, more recent single-phenotype GWASs. Leveraging the joint genetic correlations of cognitive ability, education, and schizophrenia, we were able to dissociate two distinct biological mechanisms-early neurodevelopmental pathways that characterize concordant allelic variation and adulthood synaptic pruning pathways-that were linked to the paradoxical positive genetic association between education and schizophrenia. Furthermore, genetic correlation analyses revealed that these mechanisms contribute not only to the etiopathogenesis of schizophrenia but also to the broader biological dimensions implicated in both general health outcomes and psychiatric illness.