Activity-regulated Cytoskeletal-associated Protein Research Articles

Molecular Mechanisms in Perirhinal Cortex Selectively Necessary for Discrimination of Overlapping Memories, but Independent of Memory Persistence.

Successful memory involves not only remembering over time but also keeping memories distinct. The ability to separate similar experiences into distinct memories is a main feature of episodic memory. Discrimination of overlapping representations has been investigated in the dentate gyrus of the hippocampus (DG), but little is known about this process in other regions such as the perirhinal cortex (Prh). We found in male rats that perirhinal brain-derived neurotrophic factor (BDNF) is required for separable storage of overlapping, but not distinct, object representations, which is identical to its role in the DG for spatial representations. Also, activity-regulated cytoskeletal-associated protein (Arc) is required for disambiguation of object memories, as measured by infusion of antisense oligonucleotides. This is the first time Arc has been implicated in the discrimination of objects with overlapping features. Although molecular mechanisms for object memory have been shown previously in Prh, these have been dependent on delay, suggesting a role specifically in memory duration. BDNF and Arc involvement were independent of delay—the same demand for memory persistence was present in all conditions—but only when discrimination of similar objects was required were these mechanisms recruited and necessary. Finally, we show that BDNF and Arc participate in the same pathway during consolidation of overlapping object memories. We provide novel evidence regarding the proteins involved in disambiguation of object memories outside the DG and suggest that, despite the anatomical differences, similar mechanisms underlie this process in the DG and Prh that are engaged depending on the similarity of the stimuli.

Stimulus-evoked ERK-dependent phosphorylation of activity-regulated cytoskeleton-associated protein (Arc) regulates its neuronal subcellular localization

Activity-regulated cytoskeletal-associated protein (Arc) is implicated as a master regulator of long-term synaptic plasticity and memory formation in mammalian brain. Arc acts at synapses and within the nucleus, but the mechanisms controlling Arc localization and function are little known. As Arc transcription and translation are regulated by extracellularsignal-regulated kinase (ERK) signaling, we asked whether Arc protein itself is phosphorylated by ERK. GST-fused Arc of rat origin was able to pull down endogenous ERK2 from rat hippocampal lysates. Using a peptide array, we show that ERK binds a non-canonical docking (D) motif in the C-terminal domain of Arc, and this interaction is abolished by phosphorylation of Tyr309. Activated ERK2 phosphorylated bacterially expressed Arc in vitro at all five predicted sites, as confirmed by phospho-specific protein staining and LC–MS/MS analysis. In neuroblastoma cells expressing epitope tagged-Arc, we demonstrate ERK-dependent phosphorylation of Arc in response to activation of muscarinic cholinergic receptors with carbachol. Using phosphosite-specific antibodies, this stimulus-evoked phosphorylation was shown to occur on Ser206 located within the central hinge region of Arc. In cultured hippocampal neurons expressing phosphomutant Arc under control of the activity-dependent promoter, we show that Ser206 phosphorylation regulates the nuclear:cytosolic localization of Arc. Thus, the neuronal activity-induced phosphomimic exhibits enhanced cytosolic localization relative to phosphodeficient and wild-type Arc. Furthermore, enhanced Ser206 phosphorylation of endogenous Arc was detected in the dentate gyrus cytoskeletal fraction after induction of long-term potentiation (LTP) in live rats. Taken together, this work demonstrates stimulus-evoked ERK-dependent phosphorylation and regulation of Arc protein.

Ventral hippocampal neurons inhibit postprandial energy intake.

Evidence suggests that the memory of a recently ingested meal limits subsequent intake. Given that ventral hippocampal (vHC) neurons are involved in memory and energy intake, the present experiment tested the hypothesis that vHC neurons contribute to the formation of a memory of a meal and inhibit energy intake during the postprandial period. We tested (1) whether pharmacological inactivation of vHC neurons during the period following a sucrose meal, when the memory of the meal would be undergoing consolidation, accelerates the onset of the next sucrose meal and increases intake and (2) whether sucrose intake increases vHC expression of the synaptic plasticity marker activity-regulated cytoskeletal-associated protein (Arc). Adult male Sprague-Dawley rats were trained to consume a 32% sucrose solution daily at the same time and location. On the experimental day, the rats were given intra-vHC infusions of the GABAA receptor agonist muscimol or vehicle after they finished their first sucrose meal. Compared to vehicle infusions, postmeal intra-vHC muscimol infusions decreased the latency to the next sucrose meal, increased the amount of sucrose consumed during that meal, increased the total number of sucrose meals and the total amount of sucrose ingested. In addition, rats that consumed sucrose had higher levels of Arc expression in both vHC CA1 and CA3 subfields than cage control rats. Collectively, these findings are the first to show that vHC neurons inhibit energy intake during the postprandial period and support the hypothesis that vHC neurons form a memory of a meal and inhibit subsequent intake. © 2016 Wiley Periodicals, Inc.

TRIAD3/RNF216 mutations associated with Gordon Holmes syndrome lead to synaptic and cognitive impairments via Arc misregulation.

SummaryMultiple loss‐of‐function mutations in TRIAD3 (a.k.a. RNF216) have recently been identified in patients suffering from Gordon Holmes syndrome (GHS), characterized by cognitive decline, dementia, and movement disorders. TRIAD3A is an E3 ubiquitin ligase that recognizes and facilitates the ubiquitination of its target for degradation by the ubiquitin‐proteasome system (UPS). Here, we demonstrate that two of these missense substitutions in TRIAD3 (R660C and R694C) could not regulate the degradation of their neuronal target, activity‐regulated cytoskeletal‐associated protein (Arc/Arg 3.1), whose expression is critical for synaptic plasticity and memory. The synaptic deficits due to the loss of endogenous TRIAD3A could not be rescued by TRIAD3A harboring GHS‐associated missense mutations. Moreover, we demonstrate that the loss of endogenous TRIAD3A in the mouse hippocampal CA1 region led to deficits in spatial learning and memory. Finally, we show that these missense mutations abolished the interaction of TRIAD3A with Arc, disrupting Arc ubiquitination, and consequently Arc degradation. Our current findings of Arc misregulation by TRIAD3A variants suggest that loss‐of‐function mutations in TRIAD3A may contribute to dementia observed in patients with GHS driven by dysfunctional UPS components, leading to cognitive impairments through the synaptic protein Arc.

Glucocorticoid Homeostasis in the Dentate Gyrus Is Essential for Opiate Withdrawal-Associated Memories.

Drug-withdrawal-associated aversive memories might trigger relapse to drug-seeking behavior. However, changes in structural and synaptic plasticity, as well as epigenetic mechanisms, which may be critical for long-term aversive memory, have yet to be elucidated. We used male Wistar rats and performed conditioned-place aversion (CPA) paradigm to uncover the role of glucocorticoids (GCs) on plasticity-related processes that occur within the dentate gyrus (DG) during opiate-withdrawal conditioning (memory formation-consolidation) and after reactivation by re-exposure to the conditioned environment (memory retrieval). Rats subjected to conditioned morphine-withdrawal robustly expressed CPA, while adrenalectomy impaired naloxone-induced CPA. Importantly, while activity-regulated cytoskeletal-associated protein (Arc) expression was induced in sham- and ADX-dependent animals during the conditioning phase, Arc and early growth response 1 (Egr-1) induction was restricted to sham-dependent rats following memory retrieval. Moreover, we found a correlation between Arc induction and CPA score, and Arc was selectively expressed in the granular zone of the DG in dopaminoceptive, glutamatergic and GABAergic neurons. We further found that brain-derived neurotrophic factor was regulated in the opposite way during the test phase. Our results also suggest a role for epigenetic regulation on the expression of glucocorticoid receptors and Arc following memory retrieval. Our data provide the first evidence that GC homeostasis is important for the expression of long-term morphine-withdrawal memories. Moreover, our results support the idea that targeting Arc and Egr-1 in the DG may provide important insights into the role of these signaling cascades in withdrawal-context memory re-consolidation. Together, disrupting these processes in the DG might lead to effective treatments in drug addiction thereby rapidly and persistently reducing invasive memories and subsequent drug seeking.

The role of myocardin-related transcription factor-A in Aβ25–35 induced neuron apoptosis and synapse injury

Myocardin-related transcription factor-A (MRTF-A) highly expressed in brain has been demonstrated to promote neuronal survival via regulating the transcription of related target genes as a powerful co-activator of serum response factor (SRF). However, the role of MRTF-A in Alzheimer's disease (AD) is still unclear. Here, we showed that MRTF-A was significantly downregulated in cortex of the Aβ25–35-induced AD rats, which played a key role in Aβ25–35 induced cerebral neuronal degeneration in vitro. Bilateral intracerebroventricular injection of Aβ25–35 caused significantly MRTF–A expression decline in cortex of rats, along with significant neuron apoptosis and plasticity damage. In vitro, transfection of MRTF-A into primary cultured cortical neurons prevented Aβ25–35 induced neuronal apoptosis and synapses injury. And luciferase reporter assay determined that MRTF-A could bind to and enhance the transactivity of the Mcl-1 (Myeloid cell leukemia-1) and Arc (activity-regulated cytoskeletal-associated protein) promoters by activating the key CArG box element. These data demonstrated that the decreasing of endogenous MRTF-A expression might contribute to the development of AD, whereas the upregulation MRTF-A in neurons could effectively reduce Aβ25–35 induced synapse injury and cell apoptosis. And the underlying mechanism might be partially due to MRTF-A-mediated the transcription and expression of Mcl-1 and Arc by triggering the CArG box.

Spatial Memory Impairment is Associated with Intraneural Amyloid-β Immunoreactivity and Dysfunctional Arc Expression in the Hippocampal-CA3 Region of a Transgenic Mouse Model of Alzheimer's Disease.

Dysfunction of synaptic communication in cortical and hippocampal networks has been suggested as one of the neuropathological hallmarks of the early stages of Alzheimer's disease (AD). Also, several lines of evidence have linked disrupted levels of activity-regulated cytoskeletal associated protein (Arc), an immediate early gene product that plays a central role in synaptic plasticity, with AD "synaptopathy". The mapping of Arc expression patterns in brain networks has been extensively used as a marker of memory-relevant neuronal activity history. Here we evaluated basal and behavior-induced Arc expression in hippocampal networks of the 3xTg-AD mouse model of AD. The basal percentage of Arc-expressing cells in 10-month-old 3xTg-AD mice was higher than wild type in CA3 (4.88% versus 1.77% , respectively) but similar in CA1 (1.75% versus 2.75% ). Noteworthy, this difference was not observed at 3 months of age. Furthermore, although a Morris water maze test probe induced a steep (∼4-fold) increment in the percentage of Arc+ cells in the CA3 region of the 10-month-old wild-type group, no such increment was observed in age-matched 3xTg-AD, whereas the amount of Arc+ cells in CA1 increased in both groups. Further, we detected that CA3 neurons with amyloid-β were much more likely to express Arc protein under basal conditions. We propose that in 3xTg-AD mice, intraneuronal amyloid-β expression in CA3 could increase unspecific neuronal activation and subsequent Arc protein expression, which might impair further memory-stabilizing processes.

New Insights on Retrieval-Induced and Ongoing Memory Consolidation: Lessons from Arc.

The mainstream view on the neurobiological mechanisms underlying memory formation states that memory traces reside on the network of cells activated during initial acquisition that becomes active again upon retrieval (reactivation). These activation and reactivation processes have been called “conjunctive trace.” This process implies that singular molecular events must occur during acquisition, strengthening the connection between the implicated cells whose synchronous activity must underlie subsequent reactivations. The strongest experimental support for the conjunctive trace model comes from the study of immediate early genes such as c-fos, zif268, and activity-regulated cytoskeletal-associated protein. The expressions of these genes are reliably induced by behaviorally relevant neuronal activity and their products often play a central role in long-term memory formation. In this review, we propose that the peculiar characteristics of Arc protein, such as its optimal expression after ongoing experience or familiar behavior, together with its versatile and central functions in synaptic plasticity could explain how familiarization and recognition memories are stored and preserved in the mammalian brain.

Retrieval-induced NMDA receptor-dependent Arc expression in two models of cocaine-cue memory

The association of environmental cues with drugs of abuse results in persistent drug-cue memories. These memories contribute significantly to relapse among addicts. While conditioned place preference (CPP) is a well-established paradigm frequently used to examine the modulation of drug-cue memories, very few studies have used the non-preference-based model conditioned activity (CA) for this purpose. Here, we used both experimental approaches to investigate the neural substrates of cocaine-cue memories. First, we directly compared, in a consistent setting, the involvement of cortical and subcortical brain regions in cocaine-cue memory retrieval by quantifying activity-regulated cytoskeletal-associated (Arc) protein expression in both the CPP and CA models. Second, because NMDA receptor activation is required for Arc expression, we investigated the NMDA receptor dependency of memory persistence using the CA model. In both the CPP and CA models, drug-paired animals showed significant increases in Arc immunoreactivity in regions of the frontal cortex and amygdala compared to unpaired controls. Additionally, administration of a NMDA receptor antagonist (MK-801 or memantine) immediately after cocaine-CA memory reactivation impaired the subsequent conditioned locomotion associated with the cocaine-paired environment. The enhanced Arc expression evident in a subset of corticolimbic regions after retrieval of a cocaine-context memory, observed in both the CPP and CA paradigms, likely signifies that these regions: (i) are activated during retrieval of these memories irrespective of preference-based decisions, and (ii) undergo neuroplasticity in order to update information about cues previously associated with cocaine. This study also establishes the involvement of NMDA receptors in maintaining memories established using the CA model, a characteristic previously demonstrated using CPP. Overall, these results demonstrate the utility of the CA model for studies of cocaine-context memory and suggest the involvement of an NMDA receptor-dependent Arc induction pathway in drug-cue memory interference.

Immediate early gene activity-regulated cytoskeletal-associated protein regulates estradiol-induced lordosis behavior in female rats.

Sensory feedback is an important component of any behavior, with each instance influencing subsequent activity. Female sexual receptivity is mediated both by the steroid hormone milieu and interaction with the male. We tested the influence of repeated mating on the level of sexual receptivity in ovariectomized rats treated with estradiol benzoate (EB) once every fourth day to mimic the normal phasic changes of circulating estradiol. Females were divided into two groups: naïve, which were tested for lordosis behavior once, and experienced rats, which were tested for lordosis after each EB injection. To monitor the effect of mating, the number of neurons expressing the immediate early gene activity-regulated cytoskeleton-associated protein (Arc) were counted in the mediobasal hypothalamus. Females were unreceptive following the first EB treatment, but the mating induced Arc expression. In naïve rats, each subsequent EB injection increased the levels of sexual receptivity. This ramping was not observed in experienced rats, which achieved only a moderate level of sexual receptivity. However, experienced females treated with EB and progesterone were maximally receptive and did not have Arc expression. To test whether the expression of Arc attenuated lordosis, Arc antisense oligodeoxynucleotides (asODN) were microinjected into experienced females' arcuate nuclei. Arc expression was attenuated, and the experienced EB-treated females achieved maximal sexual receptivity. These results demonstrate that Arc expression in the hypothalamus might influence future sexual receptivity and provides evidence of learning in the arcuate nucleus. The loss of Arc results in unrestrained sexual receptivity.

A Role for Dendritic mGluR5-Mediated Local Translation of Arc/Arg3.1 in MEF2-Dependent Synapse Elimination

Experience refines synaptic connectivity through neural activity-dependent regulation of transcription factors. Although activity-dependent regulation of transcription factors has been well described, it is unknown whether synaptic activity and local, dendritic regulation of the induced transcripts are necessary for mammalian synaptic plasticity in response to transcription factor activation. Neuronal depolarization activates the myocyte enhancer factor 2 (MEF2) family of transcription factors that suppresses excitatory synapse number. We report that activation of metabotropic glutamate receptor 5 (mGluR5) on the dendrites, but not cell soma, of hippocampal CA1 neurons is required for MEF2-induced functional and structural synapse elimination. We present evidence that mGluR5 is necessary for synapse elimination to stimulate dendritic translation of the MEF2 target gene Arc/Arg3.1. Activity-regulated cytoskeletal-associated protein (Arc) is required for MEF2-induced synapse elimination, where it plays an acute, cell-autonomous, and postsynaptic role. This work reveals a role for dendritic activity in local translation of specific transcripts in synapse refinement.

Long-term abstinence from developmental cocaine exposure alters Arc/Arg3.1 modulation in the rat medial prefrontal cortex.

Cocaine is a psychostimulant whose abuse causes a social and economic burden for our society. Most of the published literature deals with acute effects of cocaine or short-term abstinence in adult animals but much less information exists on neuroplastic changes following long-term abstinence. We have recently shown that the long-term abstinence following developmental exposure to cocaine results in increased Activity-Regulated Cytoskeletal-associated protein (Arc/Arg3.1) expression in the crude synaptosomal fraction (Giannotti et al. Int J Neuropsychopharmacology 7(4):625-634, 2014). Given that Arc/Arg3.1 localizes not only at active synapse but also in the nucleus (Okuno et al. Cell 149:886-898, 2012; Korb et al. Nat Neurosci 16:874-883 2013; Bloomer et al. Brain Res 1153:20-33 2007), we investigated Arc/Arg3.1 protein levels in the whole homogenate and the nuclear fraction of animals exposed to cocaine during adolescence. We observed the increased expression of Arc/Arg3.1 in both the fractions, suggesting that up-regulation of Arc/Arg3.1 protein may be partly due to the increased nuclear expression of Arc/Arg3.1 in the medial prefrontal cortex (mPFC) of rats sacrificed at postnatal day 90, following 48 days of abstinence. This effect seems to cause reduced Gria1 transcription. We also found reduced expression of fragile X mental retardation gene (FMR1) which normally inhibits Arc/Arg3.1 translation together with reduced expression of Ubiquitin-protein ligase E3A (Ube3a) that normally causes Arc/Arg3.1 protein degradation via ubiquitination. Further, we found increased expression of metabotropic glutamate receptor 5 (GRM5) which is also involved in the regulation of Arc/Arg3.1 expression. Taken together, our findings show that abstinence from developmental exposure to cocaine is associated with alterations in the finely tuned mechanisms that regulate Arc/Arg3.1 expression.

Corticosterone-induced enhancement of memory and synaptic Arc protein in the medial prefrontal cortex

Acute administration of the stress hormone corticosterone enhances memory consolidation in a manner that is dependent upon the modulatory effects of the basolateral complex of the amygdala (BLA). Posttraining administration of corticosterone increases expression of the activity-regulated cytoskeletal-associated protein (Arc) in hippocampal synaptic-enriched fractions. Interference with hippocampal Arc expression impairs memory, suggesting that the corticosterone-induced increase in hippocampal Arc plays a role in the memory enhancing effect of the hormone. Blockade of β-adrenoceptors in the BLA attenuates the corticosterone-induced increase in hippocampal Arc expression and blocks corticosterone-induced memory enhancement. To determine whether posttraining corticosterone treatment affects Arc protein expression in synapses of other areas of the brain that are involved in memory processing, a memory-enhancing dose of corticosterone was administered to rats immediately after inhibitory avoidance training. As seen in the hippocampus, Arc protein expression was increased in synaptic fractions taken from the prelimbic region of the medial prefrontal cortex (mPFC). Blockade of Arc protein expression significantly impaired memory, indicating that the protein is necessary in the mPFC for long-term memory formation. To test the hypothesis that blockade of β-adrenoceptors in the BLA would block the effect of systemic corticosterone on memory and attenuate mPFC Arc expression, as it does in the hippocampus, posttraining intra-BLA microinfusions of the β-adrenoceptor antagonist propranolol were given concurrently with the systemic corticosterone injection. Although this treatment blocked corticosterone-induced memory enhancement, it increased corticosterone-induced Arc protein expression in mPFC synaptic fractions. These findings suggest that the BLA mediates stress hormone effects on memory by participating in the negative or positive regulation of corticosterone-induced synaptic plasticity in efferent brain regions.

Maternal and postweaning high-fat diets disturb hippocampal gene expression, learning, and memory function

We tested the hypothesis that excess saturated fat consumption during pregnancy, lactation, and/or postweaning alters the expression of genes mediating hippocampal synaptic efficacy and impairs spatial learning and memory in adulthood. Dams were fed control chow or a diet high in saturated fat before mating, during pregnancy, and into lactation. Offspring were weaned to either standard chow or a diet high in saturated fat. The Morris Water Maze was used to evaluate spatial learning and memory. Open field testing was used to evaluate motor activity. Hippocampal gene expression in adult males was measured using RT-PCR and ELISA. Offspring from high fat-fed dams took longer, swam farther, and faster to try and find the hidden platform during the 5-day learning period. Control offspring consuming standard chow spent the most time in memory quadrant during the probe test. Offspring from high fat-fed dams consuming excess saturated fat spent the least. The levels of mRNA and protein for brain-derived neurotrophic factor and activity-regulated cytoskeletal-associated protein were significantly decreased by maternal diet effects. Nerve growth factor mRNA and protein levels were significantly reduced in response to both maternal and postweaning high-fat diets. Expression levels for the N-methyl-d-aspartate receptor (NMDA) receptor subunit NR2B as well as synaptophysin were significantly decreased in response to both maternal and postweaning diets. Synaptotagmin was significantly increased in offspring from high fat-fed dams. These data support the hypothesis that exposure to excess saturated fat during hippocampal development is associated with complex patterns of gene expression and deficits in learning and memory.

Sequence characterization, polymorphism, and tissue expression profile of an effector immediate-early gene: activity-regulated cytoskeletal associated protein gene (Arc/Arg3.1) in swamp and river buffalo.

The activity-regulated cytoskeletal associated protein (Arc/Arg3.1) has been implicated in experience-dependent synaptic plasticity and memory formation. However, information regarding its coding gene in buffalo remains scarce. In this study, the full-length of Arc/Arg3.1 was isolated and characterized (accession No. JX491649) and genetic variations of six river buffalo and eight swamp buffalo were investigated. A tissue expression profile was obtained using semi-quantitative reverse transcription-polymerase chain reaction. The coding region sequence of Arc/Arg3.1 contained 1191 nucleotides encoding a putative protein of 396 amino acids with a theoretical isoelectric point (pI) and molecular weight (Mw) of 5.4 and 45.2 kDa, respectively. Four polymorphisms (c.63T>C, c.228T>C, c.558G>A, and c.625G>C) were found in buffalo; however, only substitution c.625G>C was non-synonymous, leading to an amino acid change from Val to Leu at the 209th position of the Arc/Arg3.1 protein sequence. Bioinformatics analysis revealed that this substitution had no significant effect on Arc/Arg3.1 function (subPSEC = -1.4039, Pdeleterious = 0.1685), which indicated that Arc/Arg3.1 was highly conserved and functionally important in buffalo. Phylogenetic analysis revealed that the gene is closely related to that of Bos taurus and Bos grunniens. The gene was moderately expressed in the hypophysis and the placenta; it was weakly expressed in the kidney, milk, mammary gland, cerebrum, lung, heart, rumen, fat, and uterus; and it was almost silent in the muscle, liver, and skin. These findings will provide further insights into the structure and function of the immediate-early gene in buffalo.

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

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

Involvement of hippocampal Arc in amnesia and its recovery by alcoholic extract of Ashwagandha leaves

Arc (Activity-regulated cytoskeletal-associated protein) is a member of the immediate-early gene (IEG) family protein. Because of its critical role in learning and memory, it is widely considered to be an important protein in synaptic plasticity and related neurobiological functions. Alcoholic extract of Ashwagandha leaves (i-Extract) was recently shown to have preventive and therapeutic potential for scopolamine-induced amnesia and glutamate-induced excitotoxicity. In the present study, we investigated the involvement of Arc in scopolamine-induced amnesia and its recovery by i-Extract with particular focus to the changes in Arc expression in the hippocampus and cerebral cortex of mice. Morris water maze test showed that spatial learning and memory of mice were drastically reduced by scopolamine administration but improved with i-Extract treatment as compared to control and scopolamine-challenged mice. Molecular analysis revealed a remarkable decline in Arc expression in both hippocampus and cerebral cortex of amnesic mice, which was recovered after i-Extract treatment. Interestingly, Arc expression showed better recovery in the hippocampus than the cerebral cortex and the pre-treatment with i-Extract was more effective than the post-treatment. These findings suggest that Arc may be involved in i-Extract mediated recovery from amnesia.

Hippocampal Arc (Arg3.1) expression is induced by memory recall and required for memory reconsolidation in trace fear conditioning

Mounting evidence suggests that long-lasting, protein synthesis-dependent changes in synaptic strength accompany both the initial acquisition and subsequent recall of specific memories. Within brain areas thought to be important for learning and memory, including the hippocampus, learning-related plasticity is likely mediated in part by NMDA receptor activation and experience-dependent changes in gene expression. In the present study, we examined the role of activity-regulated cytoskeletal-associated protein (Arc/Arg3.1) expression in the acquisition, recall, and reconsolidation of memory in a trace fear conditioning paradigm. First, we show that the expression of Arc protein in ventral hippocampus (VH) is dramatically enhanced by memory recall 24h after the acquisition of trace fear conditioning, and that both memory recall and the associated recall-induced enhancement of Arc expression are blocked by pre-training administration of 2-amino-5-phosphonovaleric acid (APV). Next, we show that while infusion of Arc antisense oligodeoxynucleotides (ODNs) into VH prior to testing had little effect on memory recall, it significantly reduced both Arc protein expression and freezing behavior during subsequent testing sessions. Collectively, these results suggest that Arc/Arg3.1 protein plays an important functional role in both the initial acquisition of hippocampal-dependent memory and the reconsolidation of these memories after recall.

Reduced basal and novelty-induced levels of activity-regulated cytoskeleton associated protein (Arc) and c-Fos mRNA in the cerebral cortex and hippocampus of APPswe/PS1ΔE9 transgenic mice

Activity-regulated cytoskeletal-associated protein (Arc) and c-Fos are immediate early gene (IEG) products induced by novelty in the hippocampus and involved in the consolidation of synaptic plasticity and long-term memory. We investigated whether induction of arc and c-fos after exposure to a novel open field environment was compromised in different neocortical areas and the hippocampal formation in APP/PS1ΔE9 transgenic mice characterized by pronounced accumulation and deposition of beta amyloid (Aβ). Notably, the basal level of Arc and c-fos mRNA in the neocortex was significantly lower in APP/PS1ΔE9 compared to wild-type mice. Novelty exposure induced an increase in Arc and c-Fos mRNA in the medial prefrontal cortex (mPFC), parietal cortex, and hippocampal formation in both APP/PS1ΔE9 transgenic and wild-type mice. However, novelty-induced IEG expression did not reach the same levels in APP/PS1ΔE9 as in the wild-type mice. In contrast, synaptophysin levels did not differ between mutant and wild type mice, suggesting that the observed effect was not due to a general decrease in the number of presynapses. These data suggest a reduction in basal and novelty-induced neuronal activity in a transgenic mouse model of Alzheimer’s disease, which is most pronounced in cortical regions, indicating that a decreased functional response in IEG expression could be partly responsible for the cognitive deficits observed in patients with Alzheimer’s disease.

Impairment of TrkB-PSD-95 Signaling in Angelman Syndrome

Author SummaryAngelman syndrome (AS) is a debilitating neurological disorder caused by a dysfunctional Ube3A gene. Most children with AS exhibit developmental delay, movement disorders, speech impairment, and often autistic features. The Ube3A enzyme normally regulates the degradation of the synaptic protein Arc, and in its absence the resulting elevated levels of Arc weaken synaptic contacts, making it difficult to generate long-term potentiation (LTP) and to process and store memory. In this study, we show that increased levels of Arc disrupt brain-derived neurotrophic factor (BDNF) signaling through the TrkB receptor (which is important for both the induction and maintenance of LTP). We find that the association of the postsynaptic density protein PSD-95 with TrkB is critical for intact BDNF signaling, and that the high levels of Arc in AS interfere with BDNF-induced recruitment of postsynaptic density protein-95 (PSD-95) and other effectors to TrkB. By disrupting the interaction between Arc and PSD-95 with the novel cyclic peptidomimetic compound CN2097, we were able to restore BDNF signaling and improve the induction of LTP in a mouse model of AS. We propose that the disruption of TrkB receptor signaling at synapses contributes to the cognitive dysfunction that occurs in Angelman syndrome.