Activity-regulated cytoskeletal-associated protein is localized to recently activated excitatory synapses

AMPA receptors regulate transcription of the plasticity-related immediate-early gene Arc

Learning and memory depend critically on long-term synaptic plasticity, which requires neuronal gene expression. In the prevailing view, AMPA receptors mediate fast excitatory synaptic transmission and effect short-term plasticity, but they do not directly regulate neuronal gene expression. By studying regulation of Arc, a gene required for long-term plasticity, we uncovered a new role for AMPA receptors in neuronal gene expression. Spontaneous synaptic activity or activity induced by brain-derived neurotrophic factor (BDNF) elicited Arc expression in cultures of rat cortical neurons and in organotypic brain slices. Notably, inhibiting AMPA receptors strongly potentiated activity-dependent Arc expression. We found that AMPA receptors negatively regulate Arc transcription, but not translation or stability, through a mechanism involving a pertussis toxin-sensitive G protein. These results provide insights into the activity-dependent mechanisms of Arc expression and suggest that, in addition to effecting short-term plasticity, AMPA receptors regulate genes involved in long-term plasticity.

Fornix lesions decouple the induction of hippocampal arc transcription from behavior but not plasticity

The immediate-early gene (IEG) Arc is transcribed after behavioral and physiological treatments that induce synaptic plasticity and is implicated in memory consolidation. The relative contributions of neuronal activity and learning-related plasticity to the behavioral induction of Arc remain to be defined. To differentiate the contributions of each, we assessed the induction of Arc transcription in rats with fornix lesions that impair hippocampal learning yet leave cortical connectivity and neuronal firing essentially intact. Arc expression was assessed after exploration of novel environments and performance of a novel water maze task during which normal rats learned the spatial location of an escape platform. During the same task, rats with fornix lesions learned to approach a visible platform but did not learn its spatial location. Rats with fornix lesions had normal baseline levels of hippocampal Arc mRNA, but unlike normal rats, expression was not increased in response to water maze training. The integrity of signaling pathways controlling Arc expression was demonstrated by stimulation of the medial perforant path, which induced normal synaptic potentiation and Arc in rats with fornix lesions. Together, the results demonstrate that Arc induction can be decoupled from behavior and is more likely to indicate the engagement of synaptic plasticity mechanisms than synaptic or neuronal activity per se. The results further imply that fornix lesions may impair memory in part by decoupling neuronal activity from signaling pathways required for long-lasting hippocampal synaptic plasticity.

Topography of Arc/Arg3.1 mRNA expression in the dorsal and ventral hippocampus induced by recent and remote spatial memory recall: dissociation of CA3 and CA1 activation

The understanding of the mechanisms of memory retrieval and its deficits, and the detection of memory underlying neuronal plasticity, is greatly impeded by a lack of precise knowledge of the brain circuitry that underlies the functions of memory. The specific roles of anatomically distinct hippocampal subdivisions in recent and long-term memory retention and recall are essentially unknown. To address these questions, we mapped the expression of Arc/Arg 3.1 mRNA, a neuronal activity marker, in memory retention at multiple rostrocaudal levels of the dentate gyrus, CA3, CA1, subiculum, and lateral and medial entorhinal cortices after a platform search in a water-maze spatial task at 24 h and 1 month compared with swim and naive controls. We found that the entorhinohippocampal neuronal activity underlying the recall of recent and remote spatial memory has an anatomically distributed and time-dependent organization throughout both the dorsal and ventral hippocampus that is subdivision specific. We found a dissociation in the activity of the entorhinal cortex, CA3, and CA1 over a period of memory consolidation. Although CA3, the dorsal hippocampus, and the entorhinal cortex demonstrated the most persistent learning-specific signal during both recent and long-term memory recall, CA1 and the ventral hippocampus displayed the most dramatic signal decline. We determined the coordinates of activity clusters in the hippocampal subdivisions during the platform search and their dynamics over time. Our mapping data suggest that although the level of corticohippocampal interaction is similar during the retrieval of recent and remote spatial memories, the mnemonic function of the hippocampus may have changed, and the activity underlying remote spatial memory could be anatomically segregated within hippocampal subdivisions in small segments.

Long-term potentiation in the rat dentate gyrus is associated with enhanced Arc/Arg3.1 protein expression in spines, dendrites and glia

Electron microscopic immunocytochemical methods were used to determine the localization, subcellular distribution and expression of activity-regulated cytoskeletal protein (Arc/Arg3.1) in dentate gyrus after unilateral induction of long-term potentiation (LTP) in the perforant pathway of anaesthetized rats. At 2 h post-induction, immunoreaction product was visible in the dentate gyrus in both the granule cell and molecular layers. Arc expression was higher in the potentiated than the unstimulated contralateral hemisphere. Single-section electron microscopy analysis in unstimulated tissue and in tissue prepared 2 and 4 h after LTP induction showed Arc immunoreactivity (Arc-IR) in dendrites, dendritic spines and glia. Arc-IR was associated with synaptic and non-synaptic plasma membrane apposed to axon terminals and with cytoplasmic organelles, including the cytoskeleton. Arc-IR was also present in neuronal perikarya and there was occasional labelling of nuclei and axons. At 2 h post-LTP induction, there were significant increases in Arc-IR within the granule cell and molecular layers of the dentate gyrus and particularly within the middle molecular layer relative to the inner and outer molecular layers. This increase in Arc expression 2 h after LTP induction was blocked by the N-methyl-D-aspartate receptor antagonist (RS)-3-2-carboxypiperazin-4-yl-propyl-1-phosphonic acid. In animals killed 4 h after LTP induction, Arc expression had declined and differences between the potentiated and unpotentiated hemispheres were no longer significant. Our data provide ultrastructural evidence for a transient LTP-associated increase in the expression of Arc protein in the middle molecular layer of the dentate gyrus, with preferential targeting to dendrites, dendritic spines and glia.

Memory-influencing intra-basolateral amygdala drug infusions modulate expression of Arc protein in the hippocampus

Activation of beta-adrenoceptors in the basolateral complex of the amygdala (BLA) modulates memory storage processes and long-term potentiation in downstream targets of BLA efferents, including the hippocampus. Here, we show that this activation also increases hippocampal levels of activity-regulated cytoskeletal protein (Arc), an immediate-early gene (also termed Arg 3.1) implicated in hippocampal synaptic plasticity and memory consolidation processes. Infusions of the beta-adrenoreceptor agonist, clenbuterol, into the BLA immediately after training on an inhibitory avoidance task enhanced memory tested 48 h later. The same dose of clenbuterol significantly increased Arc protein levels in the dorsal hippocampus. Additionally, posttraining intra-BLA infusions of a memory-impairing dose of lidocaine significantly reduced Arc protein levels in the dorsal hippocampus. Increases in Arc protein levels were not accompanied by increases in Arc mRNA, suggesting that amygdala modulation of Arc protein and synaptic plasticity in efferent brain regions occurs at a posttranscriptional level. Finally, infusions of Arc antisense oligodeoxynucleotides into the dorsal hippocampus impaired performance of an inhibitory avoidance task, indicating that the changes in Arc protein expression are related to the observed changes in memory performance.

Synaptic targeting by Alzheimer's-related amyloid beta oligomers

The cognitive hallmark of early Alzheimer's disease (AD) is an extraordinary inability to form new memories. For many years, this dementia was attributed to nerve-cell death induced by deposits of fibrillar amyloid beta (Abeta). A newer hypothesis has emerged, however, in which early memory loss is considered a synapse failure caused by soluble Abeta oligomers. Such oligomers rapidly block long-term potentiation, a classic experimental paradigm for synaptic plasticity, and they are strikingly elevated in AD brain tissue and transgenic-mouse AD models. The current work characterizes the manner in which Abeta oligomers attack neurons. Antibodies raised against synthetic oligomers applied to AD brain sections were found to give diffuse stain around neuronal cell bodies, suggestive of a dendritic pattern, whereas soluble brain extracts showed robust AD-dependent reactivity in dot immunoblots. Antigens in unfractionated AD extracts attached with specificity to cultured rat hippocampal neurons, binding within dendritic arbors at discrete puncta. Crude fractionation showed ligand size to be between 10 and 100 kDa. Synthetic Abeta oligomers of the same size gave identical punctate binding, which was highly selective for particular neurons. Image analysis by confocal double-label immunofluorescence established that >90% of the punctate oligomer binding sites colocalized with the synaptic marker PSD-95 (postsynaptic density protein 95). Synaptic binding was accompanied by ectopic induction of Arc, a synaptic immediate-early gene, the overexpression of which has been linked to dysfunctional learning. Results suggest the hypothesis that targeting and functional disruption of particular synapses by Abeta oligomers may provide a molecular basis for the specific loss of memory function in early AD.

BDNF function in adult synaptic plasticity: the synaptic consolidation hypothesis

Interest in BDNF as an activity-dependent modulator of neuronal structure and function in the adult brain has intensified in recent years. Localization of BDNF-TrkB to glutamate synapses makes this system attractive as a dynamic, activity-dependent regulator of excitatory transmission and plasticity. Despite individual breakthroughs, an integrated understanding of BDNF function in synaptic plasticity is lacking. Here, we attempt to distill current knowledge of the molecular mechanisms and function of BDNF in LTP. BDNF activates distinct mechanisms to regulate the induction, early maintenance, and late maintenance phases of LTP. Evidence from genetic and pharmacological approaches is reviewed and tabulated. The specific contribution of BDNF depends on the stimulus pattern used to induce LTP, which impacts the duration and perhaps the subcellular site of BDNF release. Particular attention is given to the role of BDNF as a trigger for protein synthesis-dependent late phase LTP--a process referred to as synaptic consolidation. Recent experiments suggest that BDNF activates synaptic consolidation through transcription and rapid dendritic trafficking of mRNA encoded by the immediate early gene, Arc. A model is proposed in which BDNF signaling at glutamate synapses drives the translation of newly transported (Arc) and locally stored (i.e., alphaCaMKII) mRNA in dendrites. In this model BDNF tags synapses for mRNA capture, while Arc translation defines a critical window for synaptic consolidation. The biochemical mechanisms by which BDNF regulates local translation are also discussed. Elucidation of these mechanisms should shed light on a range of adaptive brain responses including memory and mood resilience.