Enhancement of dynamin polymerization and GTPase activity by Arc/Arg3.1

Mimicking Protein Kinase C Phosphorylation Inhibits Arc/Arg3.1 Palmitoylation and Its Interaction with Nucleic Acids

Activity-regulated cytoskeleton-associated protein (Arc) plays essential roles in diverse forms of synaptic plasticity, including long-term potentiation (LTP), long-term depression (LTD), and homeostatic plasticity. In addition, it assembles into virus-like particles that may deliver mRNAs and/or other cargo between neurons and neighboring cells. Considering this broad range of activities, it is not surprising that Arc is subject to regulation by multiple types of post-translational modification, including phosphorylation, palmitoylation, SUMOylation, ubiquitylation, and acetylation. Here we explore the potential regulatory role of Arc phosphorylation by protein kinase C (PKC), which occurs on serines 84 and 90 within an α-helical segment in the N-terminal domain. To mimic the effect of PKC phosphorylation, we mutated the two serines to negatively charged glutamic acid. A consequence of introducing these phosphomimetic mutations is the almost complete inhibition of Arc palmitoylation, which occurs on nearby cysteines and contributes to synaptic weakening. The mutations also inhibit the binding of nucleic acids and destabilize high-order Arc oligomers. Thus, PKC phosphorylation of Arc may limit the full expression of LTD and may suppress the interneuronal transport of mRNAs.

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.

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.

Palmitoylation-regulated interactions of the pseudokinase calmodulin kinase-like vesicle-associated with membranes and Arc/Arg3.1

Calmodulin kinase-like vesicle-associated (CaMKv), a pseudokinase belonging to the Ca²⁺/calmodulin-dependent kinase family, is expressed predominantly in brain and neural tissue. It may function in synaptic strengthening during spatial learning by promoting the stabilization and enrichment of dendritic spines. At present, almost nothing is known regarding CaMKv structure and regulation. In this study we confirm prior proteomic analyses demonstrating that CaMKv is palmitoylated on Cys5. Wild-type CaMKv is enriched on the plasma membrane, but this enrichment is lost upon mutation of Cys5 to Ser. We further show that CaMKv interacts with another regulator of synaptic plasticity, Arc/Arg3.1, and that the interaction between these two proteins is weakened by mutation of the palmitoylated cysteine in CamKv.

Differential Mobility and Self-Association of Arc/Arg3.1 in the Cytoplasm and Nucleus of Living Cells

Arc, also known as Arg3.1, is an activity-dependent immediate-early gene product that plays essential roles in memory consolidation. A pool of Arc is located in the postsynaptic cytoplasm, where it promotes AMPA receptor endocytosis and cytoskeletal remodeling. However, Arc is also found in the nucleus, with a major portion being associated with promyelocytic leukemia nuclear bodies (PML-NBs). Nuclear Arc has been implicated in epigenetic control of gene transcription associated with learning and memory. In this study, we use a battery of fluorescence nanoimaging approaches to characterize the behavior of Arc ectopically expressed in heterologous cells. Our results indicate that in the cytoplasm, Arc exists predominantly as monomers and dimers associated with slowly diffusing particles. In contrast, nuclear Arc is almost exclusively monomeric and displays a higher diffusivity than cytoplasmic Arc. We further show that Arc moves freely and rapidly between PML-NBs and the nucleoplasm and that its movement within PML-NBs is relatively unobstructed.

Development and Validation of Arc Nanobodies: New Tools for Probing Arc Dynamics and Function

Activity-regulated cytoskeleton-associated (Arc) protein plays key roles in long-term synaptic plasticity, memory, and cognitive flexibility. However, an integral understanding of Arc mechanisms is lacking. Arc is proposed to function as an interaction hub in neuronal dendrites and the nucleus, yet Arc can also form retrovirus-like capsids with proposed roles in intercellular communication. Here, we sought to develop anti-Arc nanobodies (ArcNbs) as new tools for probing Arc dynamics and function. Six ArcNbs representing different clonal lines were selected from immunized alpaca. Immunoblotting with recombinant ArcNbs fused to a small ALFA-epitope tag demonstrated binding to recombinant Arc as well as endogenous Arc from rat cortical tissue. ALFA-tagged ArcNb also provided efficient immunoprecipitation of stimulus-induced Arc after carbachol-treatment of SH-SY5Y neuroblastoma cells and induction of long-term potentiation in the rat dentate gyrus in vivo. Epitope mapping showed that all Nbs recognize the Arc C-terminal region containing the retroviral Gag capsid homology domain, comprised of tandem N- and C-lobes. ArcNbs E5 and H11 selectively bound the N-lobe, which harbors a peptide ligand binding pocket specific to mammals. Four additional ArcNbs bound the region containing the C-lobe and C-terminal tail. For use as genetically encoded fluorescent intrabodies, we show that ArcNbs fused to mScarlet-I are uniformly expressed, without aggregation, in the cytoplasm and nucleus of HEK293FT cells. Finally, mScarlet-I-ArcNb H11 expressed as intrabody selectively bound the N-lobe and enabled co-immunoprecipitation of full-length intracellular Arc. ArcNbs are versatile tools for live-cell labeling and purification of Arc, and interrogation of Arc capsid domain specific functions.

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.

Membrane Remodeling by Arc/Arg3.1

The activity-regulated cytoskeletal-associated protein (Arc, also known as Arg3.1) is an immediate early gene product induced by activity/experience and required for multiple modes of synaptic plasticity. Both long-term potentiation (LTP) and long-term depression (LTD) are impaired upon Arc deletion, as well as the ability to form long-term spatial, taste and fear memories. The best-characterized cellular function of Arc is enhancement of the endocytic internalization of AMPA receptors (AMPARs) in dendritic spines. Solution of the crystal structure of a C-terminal segment of Arc revealed a striking similarity to the capsid domain of HIV Gag. It was subsequently shown that Arc assembles into viral capsid-like structures that enclose Arc mRNA, are released into the extracellular space, and are internalized by neighboring cells. Thus, Arc is unique in participating in plasma membrane budding both into and out of the cell. In this report we study the interaction of Arc with membranes using giant unilamellar vesicles (GUVs). Using the fluorescent lipid probe LAURDAN, we find that Arc promotes the formation of smaller vesicles that penetrate into the GUV interior. Our results suggest that Arc induces negative membrane curvature and may therefore facilitate the formation of mRNA-containing extracellular vesicles from the plasma membrane.

Membrane Remodeling by Arc/Arg3.1

The activity-regulated cytoskeletal-associated protein (Arc, also known as Arg3.1) is an immediate early gene product induced by activity/experience and required for multiple modes of synaptic plasticity. Both long-term potentiation (LTP) and long-term depression (LTD) are impaired upon Arc deletion, as well as the ability to form long-term spatial, taste and fear memories. The best-characterized cellular function of Arc is enhancement of the endocytic internalization of AMPA receptors (AMPARs) in dendritic spines. Solution of the crystal structure of a C-terminal segment of Arc revealed a striking similarity to the capsid domain of HIV Gag. It was subsequently shown that Arc assembles into viral capsid-like structures that enclose Arc mRNA, are released into the extracellular space, and are internalized by neighboring cells. Thus, Arc is unique in participating in plasma membrane budding both into and out of the cell. In this report we study the interaction of Arc with membranes using giant unilamellar vesicles (GUVs). Using the fluorescent lipid probe LAURDAN, we find that Arc promotes the formation of smaller vesicles that penetrate into the GUV interior. Our results suggest that Arc induces negative membrane curvature and may therefore facilitate the formation of mRNA-containing extracellular vesicles from the plasma membrane.

Arc self-association and formation of virus-like capsids are mediated by an N-terminal helical coil motif

Activity-regulated cytoskeleton-associated protein (Arc) is a protein interaction hub with diverse roles in intracellular neuronal signaling, and important functions in neuronal synaptic plasticity, memory, and postnatal cortical development. Arc has homology to retroviral Gag protein and is capable of self-assembly into virus-like capsids implicated in the intercellular transfer of RNA. However, the molecular basis of Arc self-association and capsid formation is largely unknown. Here, we identified a 28-amino-acid stretch in the mammalian Arc N-terminal (NT) domain that is necessary and sufficient for self-association. Within this region, we identified a 7-residue oligomerization motif, critical for the formation of virus-like capsids. Purified wild-type Arc formed capsids as shown by transmission and cryo-electron microscopy, whereas mutant Arc with disruption of the oligomerization motif formed homogenous dimers. An atomic-resolution crystal structure of the oligomerization region peptide demonstrated an antiparallel coiled-coil interface, strongly supporting NT-NT domain interactions in Arc oligomerization. The NT coil-coil interaction was also validated in live neurons using fluorescence lifetime FRET imaging, and mutation of the oligomerization motif disrupted Arc-facilitated endocytosis. Furthermore, using single-molecule photobleaching, we show that Arc mRNA greatly enhances higher-order oligomerization in a manner dependent on the oligomerization motif. In conclusion, a helical coil in the Arc NT domain supports self-association above the dimer stage, mRNA-induced oligomerization, and formation of virus-like capsids. DATABASE: The coordinates and structure factors for crystallographic analysis of the oligomerization region were deposited at the Protein Data Bank with the entry code 6YTU.

Characterization of the C-terminal tail of the Arc protein

The activity-regulated cytoskeleton-associate protein Arc (or Arg3.1) is specifically linked to memory formation and a number of cognitive disorders, including Alzheimer's disease and schizophrenia. Since the discovery of Arc in 1995, extensive research has been conducted on the protein to identify its function and mechanisms of action, with solving the structure of Arc as a major goal. However, the Arc protein tends to self-oligomerize in vitro, and is difficult to crystallize. These properties have hindered efforts to obtain the structure of the full-length, whole protein Arc. As an alternative approach, we and others, have sought to solve the structures of various subdomain proteins of Arc, including the N-lobe, C-lobe, and capsid domain (N-lobe + C-lobe). In this study, we characterized the C-terminal tail of Arc using integrated bioinformatic and structural biology techniques. We compared the sequences of Arc proteins in different mammal species and found that the amino-acid composition in the C-terminal tail region has a significantly higher degree of variation rate than the rest of the protein. Structural prediction programs suggested that the C-terminal tail is structurally disordered. Chemical shift analysis based on solution NMR spectra confirmed that the C-terminal tail has a random coil (disordered) structure, and the tail starts from the residue D357. Furthermore, the NMR spectra showed that the C-terminal tail has minimum (if any) interaction with its neighboring capsid domain in Arc. This study fills gaps in our specific understanding of the structural nature and functional contributions of the Arc C-terminus.

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.

Palmitoylated Proteins in Dendritic Spine Remodeling

Activity-responsive changes in the actin cytoskeleton are required for the biogenesis, motility, and remodeling of dendritic spines. These changes are governed by proteins that regulate the polymerization, depolymerization, bundling, and branching of actin filaments. Thus, processes that have been extensively characterized in the context of non-neuronal cell shape change and migration are also critical for learning and memory. In this review article, we highlight actin regulatory proteins that associate, at least transiently, with the dendritic plasma membrane. All of these proteins have been shown, either in directed studies or in high-throughput screens, to undergo palmitoylation, a potentially reversible, and stimulus-dependent cysteine modification. Palmitoylation increases the affinity of peripheral proteins for the membrane bilayer and contributes to their subcellular localization and recruitment to cholesterol-rich membrane microdomains.

Metastasis Suppressors NME1 and NME2 Promote Dynamin 2 Oligomerization and Regulate Tumor Cell Endocytosis, Motility, and Metastasis

NM23 (NME) is a metastasis suppressor that significantly reduces metastasis without affecting primary tumor size, however, the precise molecular mechanisms are not completely understood. We examined the role of dynamin (DNM2), a GTPase regulating membrane scission of vesicles in endocytosis, in NME1 and NME2 regulation of tumor cell motility and metastasis. Overexpression of NMEs in MDA-MB-231T and MDA-MB-435 cancer cell lines increased endocytosis of transferrin and EGF receptors (TfR and EGFR) concurrent with motility and migration suppression. The internalized vesicles, costained with Rab5, had AP2 depleted from the cell surface and exhibited increased Rab5-GTP levels, consistent with endocytosis. Dynamin inhibitors Iminodyn-22 and Dynole-34-2, or shRNA-mediated downregulation of DNM2, impaired NME's ability to augment endocytosis or suppress tumor cell motility. In a lung metastasis assay, NME1 overexpression failed to significantly suppress metastasis in the DNM2 knockdown MDA-MB-231T cells. Using the EGF-EGFR signaling axis as a model in MDA-MB-231T cells, NME1 decreased pEGFR and pAkt expression in a DNM2-dependent manner, indicating the relevance of this interaction for downstream signaling. NME-DNM2 interaction was confirmed in two-way coimmunoprecipitations. Transfection of a NME1 site-directed mutant lacking histidine protein kinase activity but retaining nucleoside diphosphate kinase (NDPK) activity showed that the NDPK activity of NME was insufficient to promote endocytosis or inhibit EGFR signaling. We show that addition of NME1 or NME2 to DNM2 facilitates DNM2 oligomerization and increases GTPase activity, both required for vesicle scission. NME-DNM2 interaction may contribute to metastasis suppression by altering tumor endocytic and motility phenotypes. SIGNIFICANCE: NME1 suppresses metastasis via changes in tumor endocytosis and motility, mediated by dynamin (DNM2) GTPase activity.

Arc Oligomerization Is Regulated by CaMKII Phosphorylation of the GAG Domain: An Essential Mechanism for Plasticity and Memory Formation

Arc is a synaptic protein essential for memory consolidation. Recent studies indicate that Arc originates in evolution from a Ty3-Gypsy retrotransposon GAG domain. The N-lobe of Arc GAG domain acquired a hydrophobic binding pocket in higher vertebrates that is essential for Arc's canonical function to weaken excitatory synapses. Here, we report that Arc GAG also acquired phosphorylation sites that can acutely regulate its synaptic function. CaMKII phosphorylates the N-lobe of the Arc GAG domain and disrupts an interaction surface essential for high-order oligomerization. In Purkinje neurons, CaMKII phosphorylation acutely reverses Arc's synaptic action. Mutant Arc that cannot be phosphorylated by CaMKII enhances metabotropic receptor-dependent depression in the hippocampus but does not alter baseline synaptic transmission or long-term potentiation. Behavioral studies indicate that hippocampus- and amygdala-dependent learning requires Arc GAG domain phosphorylation. These studies provide an atomic model for dynamic and local control of Arc function underlying synaptic plasticity and memory.

Spatial and temporal resolution of mORC4 fluorescent variants reveals structural requirements for achieving higher order self-association and pronuclei entry

The Origin Replication Complex (ORC), which is a multi-subunit protein complex composed of six proteins ORC1-6, is essential for initiating licensing at DNA replication origins. We have previously reported that ORC4 has an alternative function wherein it forms a cage surrounding the extruded chromatin in female meiosis and is required for polar body extrusion (PBE). As this is a highly unexpected finding for protein that normally binds DNA, we tested whether ORC4 can actually form larger, higher order structures, which would be necessary to form a cage-like structure. We generated two fluorescent constructs of mouse ORC4, mORC4-EGFP and mORC4-FlAsH, to examine its spatial dynamics during oocyte activation in live cells. We show that both constructs were primarily monomeric throughout the embryo but self-association into larger units was detected with both probes. However, mORC4-FlAsH clearly showed higher order self-association and unique spatial distribution while mORC4-EGFP failed to form large structures during Anaphase II. Interestingly, both variants were found in the pronuclei suggesting that its role in DNA licensing is still functional. Our results with both constructs support the prediction that ORC4 can form higher order structures in the cytoplasm, suggesting that it is possible to form a cage-like structure. The finding that FlAsH labeled ORC4 formed demonstrably larger higher order structures than ORC4-GFP suggests that ORC4 oligomerization is sensitive to the bulky addition of GFP at its carboxy terminus.

Herpes Simplex Virus Type 1 Enhances Expression of the Synaptic Protein Arc for Its Own Benefit

Herpes simplex virus type 1 (HSV-1) is a neurotropic virus able to reach the central nervous system (CNS) after primary infection in oronasal mucosa. HSV-1 establishes latency inside neurons due the repression of its gene expression process, which is related to periodic reactivations in response to cellular stress conditions, constituting a risk factor for neurodegenerative diseases such as Alzheimer’s disease (AD). The immediate-early gene Arc plays an essential role in neuronal morphology, synaptic plasticity and memory formation. Arc acts as a hub protein, interacting with components of the endocytic machinery required for AMPA receptor (AMPAR) recycling as well as with proteins of the post-synaptic density and actin cytoskeleton. However, to date, no studies have evaluated whether persistent neurotropic HSV-1 infection modulates the expression or function of Arc protein in brain tissue. Here, we report that neuronal in vivo and in vitro infection of HSV-1 significantly increases Arc protein levels, showing a robust perinuclear distribution in neuronal cell lines, a process that is dependent on an active HSV-1 replication cycle. Finally, we found that silencing Arc protein caused a decrease in HSV-1 proteins and viral progeny, suggesting that Arc is involved in the lifecycle of HSV-1. Our studies strongly suggest that pathogenicity of HSV-1 neuronal reactivations in humans could be mediated in part by Arc neuronal upregulation and its potential role in endocytic trafficking and AMPA-neuronal function impairment. Further studies are necessary to define whether this phenomenon could have repercussions in cognition and learning processes in infected individuals.

Arc/Arg3.1 has an activity-regulated interaction with PICK1 that results in altered spatial dynamics

Activity-regulated cytoskeleton-associated protein (Arc; also known as Arg3.1) is an immediate early gene product that is transcribed in dendritic spines and, to date, has been best characterized as a positive regulator of AMPAR endocytosis during long-term depression (LTD) through interaction with endocytic proteins. Here, we show that protein interacting with C terminal kinase 1 (PICK1), a protein known to bind to the GluA2 subunit of AMPARs and associated with AMPAR trafficking, was pulled-down from brain homogenates and synaptosomes when using Arc as immobilized bait. Fluctuation and FLIM-FRET-Phasor analysis revealed direct interaction between these proteins when co-expressed that was increased under depolarizing conditions in live cells. At the plasma membrane, Arc-mCherry oligomerization was found to be concentration dependent. Additionally, co-expression of Arc-mCherry and EGFP-PICK1 followed by depolarizing conditions resulted in significant increases in the number and size of puncta containing both proteins. Furthermore, we identified the Arc binding region to be the first 126 amino acids of the PICK1 BAR domain. Overall, our data support a novel interaction and model where PICK1 mediates Arc regulation of AMPARs particularly under depolarizing conditions.

Palmitoylation and Membrane Binding of Arc/Arg3.1: A Potential Role in Synaptic Depression

Activity-regulated cytoskeletal-associated protein (Arc, also known as activity-regulated gene 3.1 or Arg3.1) is induced in neurons in response to salient experience and neural activity and is necessary for activity-induced forms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), cellular substrates of learning and memory. The best-characterized function of Arc is enhancement of the endocytic internalization of AMPA receptors in dendritic spines, a process associated with LTD. Arc has also been implicated in the proteolytic processing of amyloid precursor protein on the surface of endosomes. To mediate these activities, Arc must associate with cellular membranes, but it is unclear whether Arc binds directly to the lipid bilayer or requires protein-protein interactions for membrane recruitment. In this study, we show that Arc associates with pure phospholipid vesicles in vitro and undergoes palmitoylation in neurons, a modification that allows it to insert directly into the hydrophobic core of the bilayer. The palmitoylated cysteines are clustered in a motif, ₉₄CLCRC₉₈, located in the N-terminal half of the protein, which has not yet been structurally characterized. Expression of Arc with three mutated cysteines in that motif cannot support synaptic depression induced by the activity-dependent transcription factor, MEF2 (myocyte enhancer factor 2), in contrast to wild-type Arc. Thus, it appears that palmitoylation regulates at least a subset of Arc functions in synaptic plasticity.

Roles for Arc in metabotropic glutamate receptor-dependent LTD and synapse elimination: Implications in health and disease

The Arc gene is robustly transcribed in specific neural ensembles in response to experience-driven activity. Upon induction, Arc mRNA is transported to dendrites, where it can be rapidly and locally translated by activation of metabotropic glutamate receptors (mGluR1/5). mGluR-induced dendritic synthesis of Arc is implicated in weakening or elimination of excitatory synapses by triggering endocytosis of postsynaptic AMPARs in both hippocampal CA1 and cerebellar Purkinje neurons. Importantly, CA1 neurons with experience-induced Arc mRNA are susceptible, or primed for mGluR-induced long-term synaptic depression (mGluR-LTD). Here we review mechanisms and function of Arc in mGluR-LTD and synapse elimination and propose roles for these forms of plasticity in Arc-dependent formation of sparse neural representations of learned experience. We also discuss accumulating evidence linking dysregulation of Arc and mGluR-LTD in human cognitive disorders such as intellectual disability, autism and Alzheimer's disease.

Arc Interacts with the Integral Endoplasmic Reticulum Protein, Calnexin

Activity-regulated cytoskeleton-associated protein, Arc, is a major regulator of long-term synaptic plasticity and memory formation. Here we reveal a novel interaction partner of Arc, a resident endoplasmic reticulum transmembrane protein, calnexin. We show an interaction between recombinantly-expressed GST-tagged Arc and endogenous calnexin in HEK293, SH-SY5Y neuroblastoma and PC12 cells. The interaction was dependent on the central linker region of the Arc protein that is also required for endocytosis of AMPA-type glutamate receptors. High-resolution proximity-ligation assays (PLAs) demonstrate molecular proximity of endogenous Arc with the cytosolic C-terminus, but not the lumenal N-terminus of calnexin. In hippocampal neuronal cultures treated with brain-derived neurotrophic factor (BDNF), Arc interacted with calnexin in the perinuclear cytoplasm and dendritic shaft. Arc also interacted with C-terminal calnexin in the adult rat dentate gyrus (DG). After induction of long-term potentiation (LTP) in the perforant path projection to the DG of adult anesthetized rats, enhanced interaction between Arc and calnexin was obtained in the dentate granule cell layer (GCL). Although Arc and calnexin are both implicated in the regulation of receptor endocytosis, no modulation of endocytosis was detected in transferrin uptake assays. Previous work showed that Arc interacts with multiple protein partners to regulate synaptic transmission and nuclear signaling. The identification of calnexin as a binding partner further supports the role of Arc as a hub protein and extends the range of Arc function to the endoplasmic reticulum, though the function of the Arc/calnexin interaction remains to be defined.

Infant avoidance training alters cellular activation patterns in prefronto-limbic circuits during adult avoidance learning: II. Cellular imaging of neurons expressing the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1)

Positive and negative feedback learning is essential to optimize behavioral performance. We used the two-way active avoidance (TWA) task as an experimental paradigm for negative feedback learning with the aim to test the hypothesis that neuronal ensembles activate the activity-regulated cytoskeletal (Arc/Arg3.1) protein during different phases of avoidance learning and during retrieval. A variety of studies in humans and other animals revealed that the ability of aversive feedback learning emerges postnatally. Our previous findings demonstrated that rats, which as infants are not capable to learn an active avoidance strategy, show improved avoidance learning as adults. Based on these findings, we further tested the hypothesis that specific neuronal ensembles are "tagged" during infant TWA training and then reactivated during adult re-exposure to the same learning task. Using cellular imaging by immunocytochemical detection of Arc/Arg3.1, we observed that, compared to the untrained control group, (1) only in the dentate gyrus the density of Arc/Arg3.1-expressing neurons was elevated during the acquisition phase of TWA learning, and (2) this increase in Arc/Arg3.1-expressing neurons was not specific for the TWA learning task. With respect to the effects of infant TWA training we found that compared to the naïve non-pretrained group (a) the infant pretraining group displayed a higher density of Arc/Arg3.1-expressing neurons in the anterior cingulate cortex during acquisition on training day 1, and (b) the infant pretraining group displayed elevated density of Arc/Arg3.1-expressing neurons in the dentate gyrus during retrieval on test day 5. Correlation analysis for the acquisition phase revealed for the ACd that the animals which showed the highest number of avoidances and the fastest escape latencies displayed the highest density of Arc/Arg3.1-expressing neurons. Taken together, we are the first to use the synaptic plasticity protein Arc/Arg3.1 to label neuronal ensembles which are involved in different phases of active avoidance learning and whose activity patterns are changing in response to previous learning experience during infancy. Our results indicate (1) that, despite the inability to learn an active avoidance response in infancy, lasting memory traces are formed encoding the subtasks that are learned in infancy (e.g., the association of the CS and UCS, escape strategy), which are encoded in the infant brain by neuronal ensembles, which alter their synaptic connectivity via activation of specific synaptic plasticity proteins such as Arc/Arg3.1 and Egr1, and (2) that during adult training these memories can be retrieved by reactivating these neuronal ensembles and their synaptic circuits and thereby accelerate learning.

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.

The mechanistic link between Arc/Arg3.1 expression and AMPA receptor endocytosis

The activity-regulated cytoskeleton associated protein (Arc/Arg3.1) plays a key role in determining synaptic strength through facilitation of AMPA receptor (AMPAR) endocytosis. Although there is considerable data on the mechanism by which Arc induction controls synaptic plasticity and learning behaviours, several key mechanistic questions remain. Here we review data on the link between Arc expression and the clathrin-mediated endocytic pathway which internalises AMPARs and discuss the significance of Arc binding to the clathrin adaptor protein 2 (AP-2) and to endophilin/dynamin. We consider which AMPAR subunits are selected for Arc-mediated internalisation, implications for synaptic function and consider Arc as a therapeutic target.

c-Fos and Arc/Arg3.1 expression in auditory and visual cortices after hearing loss: Evidence of sensory crossmodal reorganization in adult rats

Cross-modal reorganization in the auditory and visual cortices has been reported after hearing and visual deficits mostly during the developmental period, possibly underlying sensory compensation mechanisms. However, there are very few data on the existence or nature and timeline of such reorganization events during sensory deficits in adulthood. In this study, we assessed long-term changes in activity-dependent immediate early genes c-Fos and Arc/Arg3.1 in auditory and neighboring visual cortical areas after bilateral deafness in young adult rats. Specifically, we analyzed qualitatively and quantitatively c-Fos and Arc/Arg3.1 immunoreactivity at 15 and 90 days after cochlea removal. We report extensive, global loss of c-Fos and Arc/Arg3.1 immunoreactive neurons in the auditory cortex 15 days after permanent auditory deprivation in adult rats, which is partly reversed 90 days after deafness. Simultaneously, the number and labeling intensity of c-Fos- and Arc/Arg3.1-immunoreactive neurons progressively increase in neighboring visual cortical areas from 2 weeks after deafness and these changes stabilize three months after inducing the cochlear lesion. These findings support plastic, compensatory, long-term changes in activity in the auditory and visual cortices after auditory deprivation in the adult rats. Further studies may clarify whether those changes result in perceptual potentiation of visual drives on auditory regions of the adult cortex.

Dynamic Arc SUMOylation and Selective Interaction with F-Actin-Binding Protein Drebrin A in LTP Consolidation In Vivo

Activity-regulatedcytoskeleton-associated protein (Arc) protein is implicated as a master regulator of long-term forms of synaptic plasticity and memory formation, but the mechanisms controlling Arc protein function are little known. Post-translation modification by small ubiquitin-like modifier (SUMO) proteins has emerged as a major mechanism for regulating protein-protein interactions and function. We first show in cell lines that ectopically expressed Arc undergoes mono-SUMOylation. The covalent addition of a single SUMO1 protein was confirmed by in vitro SUMOylation of immunoprecipitated Arc. To explore regulation of endogenous Arc during synaptic plasticity, we induced long-term potentiation (LTP) in the dentate gyrus of live anesthetized rats. Using coimmunoprecipitation of native proteins, we show that Arc synthesized during the maintenance phase of LTP undergoes dynamic mono-SUMO1-ylation. Levels of unmodified Arc increase in multiple subcellular fractions (cytosol, membrane, nuclear and cytoskeletal), whereas enhanced Arc SUMOylation was specific to the synaptoneurosomal and the cytoskeletal fractions. Dentate gyrus LTP consolidation requires a period of sustained Arc synthesis driven by brain-derived neurotrophic factor (BDNF) signaling. Local infusion of the BDNF scavenger, TrkB-Fc, during LTP maintenance resulted in rapid reversion of LTP, inhibition of Arc synthesis and loss of enhanced Arc SUMO1ylation. Furthermore, coimmunoprecipitation analysis showed that SUMO1-ylated Arc forms a complex with the F-actin-binding protein drebrin A, a major regulator of cytoskeletal dynamics in dendritic spines. Although Arc also interacted with dynamin 2, calcium/calmodulindependentprotein kinase II-beta (CaMKIIβ), and postsynaptic density protein-95 (PSD-95), these complexes lacked SUMOylated Arc. The results support a model in which newly synthesized Arc is SUMOylated and targeted for actin cytoskeletal regulation during in vivo LTP.

In vivo and in vitro effects of vortioxetine on molecules associated with neuroplasticity

Neuroplasticity is fundamental for brain functions, abnormal changes of which are associated with mood disorders and cognitive impairment. Neuroplasticity can be affected by neuroactive medications and by aging. Vortioxetine, a multimodal antidepressant, has shown positive effects on cognitive functions in both pre-clinical and clinical studies. In rodent studies, vortioxetine increases glutamate neurotransmission, promotes dendritic branching and spine maturation, and elevates hippocampal expression of the activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) at the transcript level. The present study aims to assess the effects of vortioxetine on several neuroplasticity-related molecules in different experimental systems. Chronic (1 month) vortioxetine increased Arc/Arg3.1 protein levels in the cortical synaptosomes of young and middle-aged mice. In young mice, this was accompanied by an increase in actin-depolymerizing factor (ADF)/cofilin serine 3 phosphorylation without altering the total ADF/cofilin protein level, and an increase in the GluA1 subunit of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor phosphorylation at serine 845 (S845) without altering serine 831 (S831) GluA1 phosphorylation nor the total GluA1 protein level. Similar effects were detected in cultured rat hippocampal neurons: Acute vortioxetine increased S845 GluA1 phosphorylation without changing S831 GluA1 phosphorylation or the total GluA1 protein level. These changes were accompanied by an increase in α subunit of Ca²⁺/calmodulin-dependent kinase (CaMKIIα) phosphorylation (at threonine 286) without changing the total CaMKIIα protein level in cultured neurons. In addition, chronic (1 month) vortioxetine, but not fluoxetine, restored the age-associated reduction in Arc/Arg3.1 and c-Fos transcripts in the frontal cortex of middle-aged mice. Taken together, these results demonstrated that vortioxetine modulates molecular targets that are related to neuroplasticity.

Amyloid-β-Induced Dysregulation of AMPA Receptor Trafficking

Evidence from neuropathological, genetic, animal model, and biochemical studies has indicated that the accumulation of amyloid-beta (Aβ) is associated with, and probably induces, profound neuronal changes in brain regions critical for memory and cognition in the development of Alzheimer's disease (AD). There is considerable evidence that synapses are particularly vulnerable to AD, establishing synaptic dysfunction as one of the earliest events in pathogenesis, prior to neuronal loss. It is clear that excessive Aβ levels can disrupt excitatory synaptic transmission and plasticity, mainly due to dysregulation of the AMPA and NMDA glutamate receptors in the brain. Importantly, AMPA receptors are the principal glutamate receptors that mediate fast excitatory neurotransmission. This is essential for synaptic plasticity, a cellular correlate of learning and memory, which are the cognitive functions that are most disrupted in AD. Here we review recent advances in the field and provide insights into the molecular mechanisms that underlie Aβ-induced dysfunction of AMPA receptor trafficking. This review focuses primarily on NMDA receptor- and metabotropic glutamate receptor-mediated signaling. In particular, we highlight several mechanisms that underlie synaptic long-term depression as common signaling pathways that are hijacked by the neurotoxic effects of Aβ.

Arc: building a bridge from viruses to memory

Arc (activity-regulated cytoskeleton-associated protein) is a neuron-specific immediate early gene that is required for enduring forms of synaptic plasticity and memory in the mammalian brain. Arc expression is highly dynamic, and tightly regulated by neuronal activity and experience. Local translation of Arc protein at synapses is critical for synaptic plasticity, which is mediated by Arc-dependent trafficking of AMPA (α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid)-type glutamate receptors. To date, few structural or biophysical properties of Arc protein have been investigated. Recent studies, including that of Myrum et al. published in the 468:1 issue of the Biochemical Journal, now shed light on some intriguing biophysical properties of Arc. These findings show that Arc contains large N- and C-terminal domains around a flexible linker region and that purified Arc protein is capable of self-oligomerization. Intriguingly, these domains show homology with the viral capsid protein found in the gag polypeptide of most retroviruses. These studies provide insight into how Arc may regulate multiple critical cell biological processes in neurons and reveals unanticipated biology that resembles viral trafficking in cells.