Inverse synaptic tagging: An inactive synapse-specific mechanism to capture activity-induced Arc/arg3.1 and to locally regulate spatial distribution of synaptic weights

The Diverse Evolutionary Histories of Domesticated Metaviral Capsid Genes in Mammals

Selfish genetic elements comprise significant fractions of mammalian genomes. In rare instances, host genomes domesticate segments of these elements for function. Using a complete human genome assembly and 25 additional vertebrate genomes, we re-analyzed the evolutionary trajectories and functional potential of capsid (CA) genes domesticated from Metaviridae, a lineage of retrovirus-like retrotransposons. Our study expands on previous analyses to unearth several new insights about the evolutionary histories of these ancient genes. We find that at least five independent domestication events occurred from diverse Metaviridae, giving rise to three universally retained single-copy genes evolving under purifying selection and two gene families unique to placental mammals, with multiple members showing evidence of rapid evolution. In the SIRH/RTL family, we find diverse amino-terminal domains, widespread loss of protein-coding capacity in RTL10 despite its retention in several mammalian lineages, and differential utilization of an ancient programmed ribosomal frameshift in RTL3 between the domesticated CA and protease domains. Our analyses also reveal that most members of the PNMA family in mammalian genomes encode a conserved putative amino-terminal RNA-binding domain (RBD) both adjoining and independent from domesticated CA domains. Our analyses lead to a significant correction of previous annotations of the essential CCDC8 gene. We show that this putative RBD is also present in several extant Metaviridae, revealing a novel protein domain configuration in retrotransposons. Collectively, our study reveals the divergent outcomes of multiple domestication events from diverse Metaviridae in the common ancestor of placental mammals.

Overnight neuronal plasticity and adaptation to emotional distress

Expressions such as 'sleep on it' refer to the resolution of distressing experiences across a night of sound sleep. Sleep is an active state during which the brain reorganizes the synaptic connections that form memories. This Perspective proposes a model of how sleep modifies emotional memory traces. Sleep-dependent reorganization occurs through neurophysiological events in neurochemical contexts that determine the fates of synapses to grow, to survive or to be pruned. We discuss how low levels of acetylcholine during non-rapid eye movement sleep and low levels of noradrenaline during rapid eye movement sleep provide a unique window of opportunity for plasticity in neuronal representations of emotional memories that resolves the associated distress. We integrate sleep-facilitated adaptation over three levels: experience and behaviour, neuronal circuits, and synaptic events. The model generates testable hypotheses for how failed sleep-dependent adaptation to emotional distress is key to mental disorders, notably disorders of anxiety, depression and post-traumatic stress with the common aetiology of insomnia.

Role of Post-Transcriptional Regulation in Learning and Memory in Mammals

After many decades, during which most molecular studies on the regulation of gene expression focused on transcriptional events, it was realized that post-transcriptional control was equally important in order to determine where and when specific proteins were to be synthesized. Translational regulation is of the most importance in the brain, where all the steps of mRNA maturation, transport to different regions of the cells and actual expression, in response to specific signals, constitute the molecular basis for neuronal plasticity and, as a consequence, for structural stabilization/modification of synapses; notably, these latter events are fundamental for the highest brain functions, such as learning and memory, and are characterized by long-term potentiation (LTP) of specific synapses. Here, we will discuss the molecular bases of these fundamental events by considering both the role of RNA-binding proteins (RBPs) and the effects of non-coding RNAs involved in controlling splicing, editing, stability and translation of mRNAs. Importantly, it has also been found that dysregulation of mRNA metabolism/localization is involved in many pathological conditions, arising either during brain development or in the adult nervous system.

Developmental synapse pathology triggered by maternal exposure to the herbicide glufosinate ammonium

Environmental and genetic factors influence synapse formation. Numerous animal experiments have revealed that pesticides, including herbicides, can disturb normal intracellular signals, gene expression, and individual animal behaviors. However, the mechanism underlying the adverse outcomes of pesticide exposure remains elusive. Herein, we investigated the effect of maternal exposure to the herbicide glufosinate ammonium (GLA) on offspring neuronal synapse formation in vitro. Cultured cerebral cortical neurons prepared from mouse embryos with maternal GLA exposure demonstrated impaired synapse formation induced by synaptic organizer neuroligin 1 (NLGN1)-coated beads. Conversely, the direct administration of GLA to the neuronal cultures exhibited negligible effect on the NLGN1-induced synapse formation. The comparison of the transcriptomes of cultured neurons from embryos treated with maternal GLA or vehicle and a subsequent bioinformatics analysis of differentially expressed genes (DEGs) identified "nervous system development," including "synapse," as the top-ranking process for downregulated DEGs in the GLA group. In addition, we detected lower densities of parvalbumin (Pvalb)-positive neurons at the postnatal developmental stage in the medial prefrontal cortex (mPFC) of offspring born to GLA-exposed dams. These results suggest that maternal GLA exposure induces synapse pathology, with alterations in the expression of genes that regulate synaptic development via an indirect pathway distinct from the effect of direct GLA action on neurons.

Excitation-transcription coupling, neuronal gene expression and synaptic plasticity

Excitation-transcription coupling (E-TC) links synaptic and cellular activity to nuclear gene transcription. It is generally accepted that E-TC makes a crucial contribution to learning and memory through its role in underpinning long-lasting synaptic enhancement in late-phase long-term potentiation and has more recently been linked to late-phase long-term depression: both processes require de novo gene transcription, mRNA translation and protein synthesis. E-TC begins with the activation of glutamate-gated N-methyl-D-aspartate-type receptors and voltage-gated L-type Ca2+ channels at the membrane and culminates in the activation of transcription factors in the nucleus. These receptors and ion channels mediate E-TC through mechanisms that include long-range signalling from the synapse to the nucleus and local interactions within dendritic spines, among other possibilities. Growing experimental evidence links these E-TC mechanisms to late-phase long-term potentiation and learning and memory. These advances in our understanding of the molecular mechanisms of E-TC mean that future efforts can focus on understanding its mesoscale functions and how it regulates neuronal network activity and behaviour in physiological and pathological conditions.

Arc expression regulates long-term potentiation magnitude and metaplasticity in area CA1 of the hippocampus in ArcKR mice

Expression of the immediate early gene Arc/Arg3.1 (Arc), a key mediator of synaptic plasticity, is enhanced by neural activity and then reduced by proteasome-dependent degradation. We have previously shown that the disruption of Arc degradation, in an Arc knock-in mouse (ArcKR), where the predominant Arc ubiquitination sites were mutated, reduced the threshold to induce, and also enhanced, the strength of Group I metabotropic glutamate receptor-mediated long-term depression (DHPG-LTD). Here, we have investigated if ArcKR expression changes long-term potentiation (LTP) in CA1 area of the hippocampus. As previously reported, there was no change in basal synaptic transmission at Schaffer collateral/commissural-CA1 (SC-CA1) synapses in ArcKR versus wild-type (WT) mice. There was, however, a significant increase in the amplitude of synaptically induced (with low frequency paired-pulse stimulation) LTD in ArcKR mice. Theta burst stimulation (TBS)-evoked LTP at SC-CA1 synapses was significantly reduced in ArcKR versus WT mice (after 2 h). Group 1 mGluR priming of LTP was abolished in ArcKR mice, which could also potentially contribute to a depression of LTP. Although high frequency stimulation (HFS)-induced LTP was not significantly different in ArcKR compared with WT mice (after 1 h), there was a phenotype in environmentally enriched mice, with the ratio of LTP to short-term potentiation (STP) significantly reduced in ArcKR mice. These findings support the hypothesis that Arc ubiquitination supports the induction and expression of LTP, likely via limiting Arc-dependent removal of AMPA receptors at synapses.

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.

The diverse evolutionary histories of domesticated metaviral capsid genes in mammals

Selfish genetic elements and their remnants comprise at least half of the human genome. Active transposons duplicate by inserting copies at new sites in a host genome. Following insertion, transposons can acquire mutations that render them inactive; the accrual of additional mutations can render them unrecognizable over time. However, in rare instances, segments of transposons become useful for the host, in a process called gene domestication. Using the first complete human genome assembly and 25 additional vertebrate genomes, we analyzed the evolutionary trajectories and functional potential of genes domesticated from the capsid genes of Metaviridae, a retroviral-like retrotransposon family. Our analysis reveals four families of domesticated capsid genes in placental mammals with varied evolutionary outcomes, ranging from universal retention to lineage-specific duplications or losses and from purifying selection to lineage-specific rapid evolution. The four families of domesticated capsid genes have divergent amino-terminal domains, inherited from four distinct ancestral metaviruses. Structural predictions reveal that many domesticated genes encode a previously unrecognized RNA-binding domain retained in multiple paralogs in mammalian genomes both adjacent to and independent from the capsid domain. Collectively, our study reveals diverse outcomes of domestication of diverse metaviruses, which led to structurally and evolutionarily diverse genes that encode important, but still largely-unknown functions in placental mammals. (207).

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.

Prions induce an early Arc response and a subsequent reduction in mGluR5 in the hippocampus

Synapse dysfunction and loss are central features of neurodegenerative diseases, caused in part by the accumulation of protein oligomers. Amyloid-β, tau, prion, and α-synuclein oligomers bind to the cellular prion protein (PrPC), resulting in the activation of macromolecular complexes and signaling at the post-synapse, yet the early signaling events are unclear. Here we sought to determine the early transcript and protein alterations in the hippocampus during the pre-clinical stages of prion disease. We used a transcriptomic approach focused on the early-stage, prion-infected hippocampus of male wild-type mice, and identify immediate early genes, including the synaptic activity response gene, Arc/Arg3.1, as significantly upregulated. In a longitudinal study of male, prion-infected mice, Arc/Arg-3.1 protein was increased early (40% of the incubation period), and by mid-disease (pre-clinical), phosphorylated AMPA receptors (pGluA1-S845) were increased and metabotropic glutamate receptors (mGluR5 dimers) were markedly reduced in the hippocampus. Notably, sporadic Creutzfeldt-Jakob disease (sCJD) post-mortem cortical samples also showed low levels of mGluR5 dimers. Together, these findings suggest that prions trigger an early Arc response, followed by an increase in phosphorylated GluA1 and a reduction in mGluR5 receptors.

Arc Regulates Transcription of Genes for Plasticity, Excitability and Alzheimer’s Disease

The immediate early gene Arc is a master regulator of synaptic function and a critical determinant of memory consolidation. Here, we show that Arc interacts with dynamic chromatin and closely associates with histone markers for active enhancers and transcription in cultured rat hippocampal neurons. Both these histone modifications, H3K27Ac and H3K9Ac, have recently been shown to be upregulated in late-onset Alzheimer’s disease (AD). When Arc induction by pharmacological network activation was prevented using a short hairpin RNA, the expression profile was altered for over 1900 genes, which included genes associated with synaptic function, neuronal plasticity, intrinsic excitability, and signalling pathways. Interestingly, about 100 Arc-dependent genes are associated with the pathophysiology of AD. When endogenous Arc expression was induced in HEK293T cells, the transcription of many neuronal genes was increased, suggesting that Arc can control expression in the absence of activated signalling pathways. Taken together, these data establish Arc as a master regulator of neuronal activity-dependent gene expression and suggest that it plays a significant role in the pathophysiology of AD.

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.

Autophagy coupled to translation is required for long-term memory

An increase in protein synthesis following learning is a fundamental and evolutionarily conserved mechanism of long-term memory. To maintain homeostasis, this protein synthesis must be counterbalanced by mechanisms such as protein degradation. Recent studies reported that macroautophagy/autophagy, a major protein degradation mechanism, is required for long-term memory formation. However, how learning regulates autophagy and recruits it into long-term memory formation remains to be established. Here, we show that inhibitory avoidance in rats significantly increases the levels of autophagy and lysosomal degradation proteins, including BECN1/beclin 1, LC3-II, SQSTM1/p62 and LAMP1, as well as autophagic flux in the hippocampus. Moreover, pharmacological inhibition or targeted molecular disruption of the learning-induced autophagy impairs long-term memory, leaving short-term memory intact. The increase in autophagy proteins results from active translation of their mRNA and not from changes in their total mRNA levels. Additionally, the induction of autophagy requires the immediate early gene Arc/Arg3.1. Finally, in contrast to classical regulation of autophagy in other systems, we found that the increase in autophagy upon learning is dispensable for the increase in protein synthesis. We conclude that coupling between learning-induced translation and autophagy, rather than translation per se, is an essential mechanism of long-term memory.Abbreviations: AAV: adeno-associated virus; ARC/ARG3.1: activity regulated cytoskeletal-associated protein; ATG: autophagy related; DG: dentate gyrus; GFP: green fluorescent protein; IA: inhibitory avoidance; LAMP1: lysosomal-associated membrane protein 1; MAP1LC3B/LC3B: microtubule-associated protein 1 light chain 3 beta; ODN: oligodeoxynucleotide; qPCR: quantitative polymerase chain reaction; SBI: SBI0206965; SQSTM1/p62: sequestosome 1; SUnSET: surface sensing of translation; TRAP: translating ribosome affinity purification; ULK1: unc-51 like kinase 1.

Repressor element-1 silencing transcription factor regulates glutamate receptors and immediate early genes to affect synaptic plasticity

OBJECTIVE

This study aimed to investigate the regulatory effects of repressor element-1 silencing transcription factor (REST) on the glutamate receptors and immediate early genes (IEGs) in the SH-SY5Y cells.

METHODS

The genes regulated by REST were screened by bioinformatics between AD patients and the control group. Then, SH-SY5Y cells were treated with 10 μM Aβ or REST siRNA/cDNA, and the expressions of synaptic genes and IEGs were detected. Moreover, the protein expression of synaptophysin and PSD-95 was detected by Western blotting in the primary mouse hippocampal neurons.

RESULTS

Firstly, 464 differentially expressed genes regulated by REST were identified between Alzheimer's disease (AD) patients and controls, and REST was closely related to the glutamatergic synapses and long-term potentiation. GRIA1, GRIN2A, GRIN1, and ARC showed significant variations with the changes of REST. Moreover, the loss of REST reduced the expression of synaptophysin and PSD-95, which was related to synaptic plasticity.

CONCLUSION

REST maintains synaptic plasticity by affecting both glutamate receptors and IEGs, and the imbalance between neural excitation and inhibition mediated by REST compromises neural function, contributing to cognitive impairment.

Deletion of BDNF in Pax2 Lineage-Derived Interneuron Precursors in the Hindbrain Hampers the Proportion of Excitation/Inhibition, Learning, and Behavior

Numerous studies indicate that deficits in the proper integration or migration of specific GABAergic precursor cells from the subpallium to the cortex can lead to severe cognitive dysfunctions and neurodevelopmental pathogenesis linked to intellectual disabilities. A different set of GABAergic precursors cells that express Pax2 migrate to hindbrain regions, targeting, for example auditory or somatosensory brainstem regions. We demonstrate that the absence of BDNF in Pax2-lineage descendants of Bdnf^Pax2KOs causes severe cognitive disabilities. In Bdnf^Pax2KOs, a normal number of parvalbumin-positive interneurons (PV-INs) was found in the auditory cortex (AC) and hippocampal regions, which went hand in hand with reduced PV-labeling in neuropil domains and elevated activity-regulated cytoskeleton-associated protein (Arc/Arg3.1; here: Arc) levels in pyramidal neurons in these same regions. This immaturity in the inhibitory/excitatory balance of the AC and hippocampus was accompanied by elevated LTP, reduced (sound-induced) LTP/LTD adjustment, impaired learning, elevated anxiety, and deficits in social behavior, overall representing an autistic-like phenotype. Reduced tonic inhibitory strength and elevated spontaneous firing rates in dorsal cochlear nucleus (DCN) brainstem neurons in otherwise nearly normal hearing Bdnf^Pax2KOs suggests that diminished fine-grained auditory-specific brainstem activity has hampered activity-driven integration of inhibitory networks of the AC in functional (hippocampal) circuits. This leads to an inability to scale hippocampal post-synapses during LTP/LTD plasticity. BDNF in Pax2-lineage descendants in lower brain regions should thus be considered as a novel candidate for contributing to the development of brain disorders, including autism.

CaMKII inhibitor 1 (CaMK2N1) mRNA is upregulated following LTP induction in hippocampal slices

CaMK2N1 and CaMK2N2 (also known as CaMKIINα and β) are endogenous inhibitors of calcium/calmodulin-dependent kinase II (CaMKII), an enzyme critical for memory and long-term potentiation (LTP), a form of synaptic plasticity thought to underlie learning. CaMK2N1/2 mRNAs are rapidly and differentially upregulated in the hippocampus and amygdala after acquisition or retrieval of fear memory. Moreover, CaMK2N2 protein levels increase after contextual fear conditioning. Therefore, it was proposed that CaMK2N1/2 genes (Camk2n1/2) could be immediate-early genes transcribed promptly (30-60 min) after training. As a first approach to explore a role in synaptic plasticity, we assessed a possible regulation of Camk2n1/2 during the expression phase of LTP in hippocampal CA3-CA1 connections in rat brain slices. Quantitative PCR revealed that Camk2n1, but not Camk2n2, is upregulated 60 min after LTP induction by Schaffer collaterals high-frequency stimulation. We observed a graded, significant positive correlation between the magnitude of LTP and Camk2n1 change in individual slices, suggesting a coordinated regulation of these properties. If mRNA increment actually resulted in the protein upregulation in plasticity-relevant subcellular locations, CaMK2N1 may be involved in CaMKII fine-tuning during LTP maintenance or in the regulation of subsequent plasticity events (metaplasticity).

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.

Synaptic Pruning in Alzheimer's Disease: Role of the Complement System

Alz heimer’s disease (AD) continues to threaten aged individuals and health care systems around the world. Human beings have been trying to postpone, reduce, or eliminate the primary risk factor for AD, aging, throughout history. Despite this, there is currently only symptomatic treatment for AD and this treatment is limited to only a handful of FDA approved AD drugs.

Neurotransmission through dopamine D1 receptors is required for aversive memory formation and Arc activation in the cerebral cortex

Dopaminergic neurotransmission is considered to play an important role not only in reward-based learning, but also in aversive learning. Here, we investigated the role of dopaminergic neurotransmission via dopamine D1 receptors (D1Rs) in aversive memory formation in a passive avoidance test using D1R knockdown (KD) mice, in which the expression of D1Rs can conditionally and reversibly be controlled by doxycycline (Dox) treatment. We also performed whole-brain imaging after aversive footshock stimulation in activity-regulated cytoskeleton protein (Arc)-dVenus D1RKD mice, which were crossbred from Arc-dVenus transgenic mice and D1RKD mice, to examine the distribution of Arc-controlled dVenus expression in the hippocampus and cerebral cortex during aversive memory formation. Knockdown of D1R expression following Dox treatment resulted in impaired performance in the passive avoidance test and was associated with a decrease in dVenus expression in the cerebral cortex (visual, somatosensory, and motor cortices), but not the hippocampus, compared with control mice without Dox treatment. These findings indicate that D1R-mediated dopaminergic transmission is critical for aversive memory formation, specifically by influencing Arc expression in the cerebral cortex.

Post-translational palmitoylation of ionotropic glutamate receptors in excitatory synaptic functions

In the mammalian CNS, glutamate is the major excitatory neurotransmitter. Ionotropic glutamate receptors (iGluRs) are responsible for the glutamate-mediated postsynaptic excitation of neurons. Regulation of glutamatergic synapses is critical for higher brain functions including neural communication, memory formation, learning, emotion, and behaviour. Many previous studies have shown that post-translational protein S-palmitoylation, the only reversible covalent attachment of lipid to protein, regulates synaptic expression, intracellular localization, and membrane trafficking of iGluRs and their scaffolding proteins in neurons. This modification mechanism is extremely conserved in the vertebrate lineages. The failure of appropriate palmitoylation-dependent regulation of iGluRs leads to hyperexcitability that reduces the maintenance of network stability, resulting in brain disorders, such as epileptic seizures. This review summarizes advances in the study of palmitoylation of iGluRs, especially AMPA receptors and NMDA receptors, and describes the current understanding of palmitoylation-dependent regulation of excitatory glutamatergic synapses. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc.

Cataloguing and Selection of mRNAs Localized to Dendrites in Neurons and Regulated by RNA-Binding Proteins in RNA Granules

Spatiotemporal translational regulation plays a key role in determining cell fate and function. Specifically, in neurons, local translation in dendrites is essential for synaptic plasticity and long-term memory formation. To achieve local translation, RNA-binding proteins in RNA granules regulate target mRNA stability, localization, and translation. To date, mRNAs localized to dendrites have been identified by comprehensive analyses. In addition, mRNAs associated with and regulated by RNA-binding proteins have been identified using various methods in many studies. However, the results obtained from these numerous studies have not been compiled together. In this review, we have catalogued mRNAs that are localized to dendrites and are associated with and regulated by the RNA-binding proteins fragile X mental retardation protein (FMRP), RNA granule protein 105 (RNG105, also known as Caprin1), Ras-GAP SH3 domain binding protein (G3BP), cytoplasmic polyadenylation element binding protein 1 (CPEB1), and staufen double-stranded RNA binding proteins 1 and 2 (Stau1 and Stau2) in RNA granules. This review provides comprehensive information on dendritic mRNAs, the neuronal functions of mRNA-encoded proteins, the association of dendritic mRNAs with RNA-binding proteins in RNA granules, and the effects of RNA-binding proteins on mRNA regulation. These findings provide insights into the mechanistic basis of protein-synthesis-dependent synaptic plasticity and memory formation and contribute to future efforts to understand the physiological implications of local regulation of dendritic mRNAs in neurons.

Developmental changes in plasticity, synaptic, glia, and connectivity protein levels in rat basolateral amygdala

The basolateral complex of amygdala (BLA) processes emotionally arousing aversive and rewarding experiences. The BLA is critical for acquisition and storage of threat-based memories and the modulation of the consolidation of arousing explicit memories, that is, the memories that are encoded and stored by the medial temporal lobe. In addition, in conjunction with the medial prefrontal cortex (mPFC), the BLA plays an important role in fear memory extinction. The BLA develops relatively early in life, but little is known about the molecular changes that accompany its development. Here, we quantified relative basal expression levels of sets of plasticity, synaptic, glia, and connectivity proteins in the rat BLA at various developmental ages: postnatal day 17 (PN17, infants), PN24 (juveniles), and PN80 (young adults). We found that the levels of activation markers of brain plasticity, including phosphorylation of CREB at Ser133, CamKIIα at Thr286, pERK1/pERK2 at Thr202/Tyr204, and GluA1 at Ser831 and Ser845, were significantly higher in infant and juvenile compared with adult brain. In contrast, age increase was accompanied by a significant augmentation in the levels of proteins that mark synaptogenesis and synapse maturation, such as synaptophysin, PSD95, SynCAM, GAD65, GAD67, and GluN2A/GluN2B ratio. Finally, we observed significant age-associated changes in structural markers, including MAP2, MBP, and MAG, suggesting that the structural connectivity of the BLA increases over time. The biological differences in the BLA between developmental ages compared with adulthood suggest the need for caution in extrapolating conclusions based on BLA-related brain plasticity and behavioral studies conducted at different developmental stages.

Activity-Regulated Cytoskeleton-Associated Protein (Arc/Arg3.1) is Transiently Expressed after Heat Shock Stress and Suppresses Heat Shock Factor 1

Heat shock proteins are induced by activation of heat shock factor 1 (HSF1) in response to heat shock and protect against heat stress. However, the molecular mechanisms underlying the downstream signal of heat shock have not been fully elucidated. We found that similarly to canonical Hsps, Arc/Arg3.1 is also markedly induced by heat shock and by other cellular stress inducers, including diamide, sodium arsenite and H₂O₂ in various cells. We noted that heat stress–induced Arc/Arg3.1 protein is short lived, with a half-life of <30 min, and is readily degraded by the ubiquitin–proteasome system. Arc/Arg3.1 overexpression inhibited the up-regulation of heat shock–induced Hsp70 and Hsp27, suggesting that Arc/Arg3.1 is a negative regulator of heat shock response (HSR). Studying the effect of Arc/Arg3.1 on HSF1, a major transcription factor in HSR, we found that Arc/Arg3.1 binds to HSF1 and inhibits its binding to the heat shock element in gene promoters, resulting in reduced induction of Hsp27 and Hsp70 mRNAs, without affecting HSF1′s phosphorylation-dependent activation, or nuclear localization. Arc/Arg3.1 overexpression decreased cell survival in response to heat shock. We conclude that Arc/Arg3.1 is transiently expressed after heat shock and negatively regulates HSF1 in the feedback loop of HSR.

The Arc gene: Retroviral heritage in cognitive functions

Stabilization of neuronal plastic changes is mediated by transient gene expression, including transcription of the activity-regulated cytoskeleton-associated gene (Arc), also known as Arg 3.1. Arc is implicated in several types of synaptic plasticity, including synaptic scaling, long-term potentiation, and long-term depression. However, the precise mechanisms by which Arc mediates these forms of long-term plasticity are unclear. It was recently found that Arc protein is capable of forming capsid-like structures and of transferring its own mRNA to neighboring cells. Moreover, Arc mRNA undergoes activity-dependent translation in these "transfected" cells. These new data raise unexpected possibilities for the mechanisms of the Arc action, and many intriguing questions concerning the role of Arc transcellular traffic in neuronal plasticity. In this mini-review, we discuss a possible link between the role of Arc in learning and memory and the virus-like properties of this protein. Additionally, we highlight some of the emerging questions for future neurobiological studies and translational applications of Arc transsynaptic effects.

Perturbed expression pattern of the immediate early gene Arc in the dentate gyrus of GluA1 C-terminal palmitoylation-deficient mice

BACKGROUND

AMPA receptors predominantly mediate fast excitatory synaptic transmission in the mammalian brain. Post-translational protein S-palmitoylation of AMPA receptor GluA subunits at their C-termini reversibly controls the receptors trafficking to and from excitatory glutamatergic synapses. Excitatory inputs to neurons induce the expression of immediate early genes (IEGs), including Arc, with particular spatial patterns. In the hippocampal dentate gyrus, Arc is mainly expressed in the upper (dorsal) blade at the basal state. GluA1 C-terminal palmitoylation-deficient (GluA1C811S) mice showed enhanced seizure susceptibility and disturbed synaptic plasticity without impaired gross anatomy or basal synaptic transmission. These mutant mice also exhibited an increased expression of IEG products, c-Fos and Arc proteins, in the hippocampus and cerebral cortex. In this report, we further analyzed excitability and Arc expression pattern in the dentate gyrus of GluA1C811S mice.

METHODS AND RESULTS

Electrophysiological analysis of granule neurons to measure the evoked excitatory postsynaptic current/evoked inhibitory postsynaptic current ratio revealed that excitatory/inhibitory (E/I) balance was normal in GluA1C811S mice. In contrast, immunohistochemical staining showed an abnormal distribution of Arc-positive cells between upper and lower (ventral) blades of the dentate gyrus in these mutant mice. These data suggest that deficiency of GluA1 palmitoylation causes perturbed neuronal inputs from the entorhinal cortex to the dentate gyrus, which potentially underlies the excessive excitability in response to seizure-inducing stimulation.

CONCLUSION

Our findings conclude that an appropriate regulation of Arc expression in the dentate gyrus, ensured by AMPA receptor palmitoylation, may be critical for stabilizing hippocampal neural circuits and may suppress excess excitation.

Deficiency of AMPAR-Palmitoylation Aggravates Seizure Susceptibility

Synaptic AMPAR expression controls the strength of excitatory synaptic transmission and plasticity. An excess of synaptic AMPARs leads to epilepsy in response to seizure-inducible stimulation. The appropriate regulation of AMPARs plays a crucial role in the maintenance of the excitatory/inhibitory synaptic balance; however, the detailed mechanisms underlying epilepsy remain unclear. Our previous studies have revealed that a key modification of AMPAR trafficking to and from postsynaptic membranes is the reversible, posttranslational S-palmitoylation at the C-termini of receptors. To clarify the role of palmitoylation-dependent regulation of AMPARs in vivo, we generated GluA1 palmitoylation-deficient (Cys811 to Ser substitution) knock-in mice. These mutant male mice showed elevated seizure susceptibility and seizure-induced neuronal activity without impairments in synaptic transmission, gross brain structure, or behavior at the basal level. Disruption of the palmitoylation site was accompanied by upregulated GluA1 phosphorylation at Ser831, but not at Ser845, in the hippocampus and increased GluA1 protein expression in the cortex. Furthermore, GluA1 palmitoylation suppressed excessive spine enlargement above a certain size after LTP. Our findings indicate that an abnormality in GluA1 palmitoylation can lead to hyperexcitability in the cerebrum, which negatively affects the maintenance of network stability, resulting in epileptic seizures.SIGNIFICANCE STATEMENT AMPARs predominantly mediate excitatory synaptic transmission. AMPARs are regulated in a posttranslational, palmitoylation-dependent manner in excitatory synapses of the mammalian brain. Reversible palmitoylation dynamically controls synaptic expression and intracellular trafficking of the receptors. Here, we generated GluA1 palmitoylation-deficient knock-in mice to clarify the role of AMPAR palmitoylation in vivo We showed that an abnormality in GluA1 palmitoylation led to hyperexcitability, resulting in epileptic seizure. This is the first identification of a specific palmitoylated protein critical for the seizure-suppressing process. Our data also provide insight into how predicted receptors such as AMPARs can effectively preserve network stability in the brain. Furthermore, these findings help to define novel key targets for developing anti-epileptic drugs.

Arc 3' UTR Splicing Leads to Dual and Antagonistic Effects in Fine-Tuning Arc Expression Upon BDNF Signaling

Activity-regulated cytoskeletal associated protein (Arc) is an immediate-early gene critically involved in synaptic plasticity and memory consolidation. Arc mRNA is rapidly induced by synaptic activation and a portion is locally translated in dendrites where it modulates synaptic strength. Being an activity-dependent effector of homeostatic balance, regulation of Arc is uniquely tuned to result in short-lived bursts of expression. Cis-Acting elements that control its transitory expression post-transcriptionally reside primarily in Arc mRNA 3′ UTR. These include two conserved introns which distinctively modulate Arc mRNA stability by targeting it for destruction via the nonsense mediated decay pathway. Here, we further investigated how splicing of the Arc mRNA 3′ UTR region contributes to modulate Arc expression in cultured neurons. Unexpectedly, upon induction with brain derived neurotrophic factor, translational efficiency of a luciferase reporter construct harboring Arc 3′ UTR is significantly upregulated and this effect is dependent on splicing of Arc introns. We find that, eIF2α dephosphorylation, mTOR, ERK, PKC, and PKA activity are key to this process. Additionally, CREB-dependent transcription is required to couple Arc 3′ UTR-splicing to its translational upregulation, suggesting the involvement of de novo transcribed trans-acting factors. Overall, splicing of Arc 3′ UTR exerts a dual and unique effect in fine-tuning Arc expression upon synaptic signaling: while inducing mRNA decay to limit the time window of Arc expression, it also elicits translation of the decaying mRNA. This antagonistic effect likely contributes to the achievement of a confined yet efficient burst of Arc protein expression, facilitating its role as an effector of synapse-specific plasticity.

Arc protein: a flexible hub for synaptic plasticity and cognition

Mammalian excitatory synapses express diverse types of synaptic plasticity. A major challenge in neuroscience is to understand how a neuron utilizes different types of plasticity to sculpt brain development, function, and behavior. Neuronal activity-induced expression of the immediate early protein, Arc, is critical for long-term potentiation and depression of synaptic transmission, homeostatic synaptic scaling, and adaptive functions such as long-term memory formation. However, the molecular basis of Arc protein function as a regulator of synaptic plasticity and cognition remains a puzzle. Recent work on the biophysical and structural properties of Arc, its protein-protein interactions and post-translational modifications have shed light on the issue. Here, we present Arc protein as a flexible, multifunctional and interactive hub. Arc interacts with specific effector proteins in neuronal compartments (dendritic spines, nuclear domains) to bidirectionally regulate synaptic strength by distinct molecular mechanisms. Arc stability, subcellular localization, and interactions are dictated by synaptic activity and post-translational modification of Arc. This functional versatility and context-dependent signaling supports a view of Arc as a highly specialized master organizer of long-term synaptic plasticity, critical for information storage and cognition.