Bioluminescence imaging of Arc expression enables detection of activity-dependent and plastic changes in the visual cortex of adult mice

All IEGs Are Not Created Equal-Molecular Sorting Within the Memory Engram

When neurons are recruited to form the memory engram, they are driven to activate the expression of a series of immediate-early genes (IEGs). While these IEGs have been used relatively indiscriminately to identify the so-called engram neurons, recent research has demonstrated that different IEG ensembles can be physically and functionally distinct within the memory engram. This inherent heterogeneity of the memory engram is driven by the diversity in the functions and distributions of different IEGs. This process, which we call molecular sorting, is analogous to sorting the entire population of engram neurons into different sub-engrams molecularly defined by different IEGs. In this chapter, we will describe the molecular sorting process by systematically reviewing published work on engram ensemble cells defined by the following four major IEGs: Fos, Npas4, Arc, and Egr1. By comparing and contrasting these likely different components of the memory engram, we hope to gain a better understanding of the logic and significance behind the molecular sorting process for memory functions.

Visualization of activity-regulated BDNF expression in the living mouse brain using non-invasive near-infrared bioluminescence imaging

Altered levels of brain-derived neurotrophic factor (BDNF) have been reported in neurologically diseased human brains. Therefore, it is important to understand how the expression of BDNF is controlled under pathophysiological as well as physiological conditions. Here, we report a method to visualize changes in BDNF expression in the living mouse brain using bioluminescence imaging (BLI). We previously generated a novel transgenic mouse strain, Bdnf-Luciferase (Luc), to monitor changes in Bdnf expression; however, it was difficult to detect brain-derived signals in the strain using BLI with d-luciferin, probably because of incomplete substrate distribution and light penetration. We demonstrate that TokeOni, which uniformly distributes throughout the whole mouse body after systematic injection and produces a near-infrared bioluminescence light, was suitable for detecting signals from the brain of the Bdnf-Luc mouse. We clearly detected brain-derived bioluminescence signals that crossed the skin and skull after intraperitoneal injection of TokeOni. However, repeated BLI using TokeOni should be limited, because repeated injection of TokeOni on the same day reduced the bioluminescence signal, presumably by product inhibition. We successfully visualized kainic acid-induced Bdnf expression in the hippocampus and sensory stimulation-induced Bdnf expression in the visual cortex. Taken together, non-invasive near-infrared BLI using Bdnf-Luc mice with TokeOni allowed us to evaluate alterations in BDNF levels in the living mouse brain. This will enable better understanding of the involvement of BDNF expression in the pathogenesis and pathophysiology of neurological diseases.

Secreted Amyloid Precursor Protein-Alpha Promotes Arc Protein Synthesis in Hippocampal Neurons

Secreted amyloid precursor protein-α (sAPPα) is a neuroprotective and memory-enhancing molecule, however, the mechanisms through which sAPPα promotes these effects are not well understood. Recently, we have shown that sAPPα enhances cell-surface expression of glutamate receptors. Activity-related cytoskeletal-associated protein Arc (Arg3.1) is an immediate early gene capable of modulating long-term potentiation, long-term depression and homeostatic plasticity through regulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor localization. Accordingly, we hypothesized that sAPPα may enhance synaptic plasticity, in part, by the de novo synthesis of Arc. Using primary cortical and hippocampal neuronal cultures we found that sAPPα (1 nM, 2 h) enhances levels of Arc mRNA and protein. Arc protein levels were increased in both the neuronal somata and dendrites in a Ca²⁺/calmodulin-dependent protein kinase II-dependent manner. Additionally, dendritic Arc expression was dependent upon activation of mitogen-activated protein kinase and protein kinase G. The enhancement of dendritic Arc protein was significantly reduced by antagonism of N-methyl-D-aspartate (NMDA) and nicotinic acetylcholine (α7nACh) receptors, and fully eliminated by dual application of these antagonists. This effect was further corroborated in area CA1 of acute hippocampal slices. These data suggest sAPPα-regulated plasticity within hippocampal neurons is mediated by cooperation of NMDA and α7nACh receptors to engage a cascade of signal transduction molecules to enhance the transcription and translation of Arc.

The Arc of cognition: Signaling cascades regulating Arc and implications for cognitive function and disease

The activity-regulated cytoskeletal (Arc) gene is implicated in numerous synaptic plasticity paradigms, including long-term potentiation and depression and homeostatic plasticity, and is critical for consolidating memory. How Arc facilitates these forms of plasticity is not fully understood. Unlike other neuronal immediate-early genes, Arc encodes a protein that shuttles between the somatodendritic and nuclear compartments to regulate synaptic plasticity. Little attention has been paid to Arc's role in the nucleus. Here, we highlight the regulatory elements and signaling cascades required to induce Arc transcription and discuss the significance of Arc nuclear localization for synaptic plasticity and scaling. We integrate these findings into the context of cognitive function and disease and propose a model in which Arc mediates an effect on memory as a "chaser" of synaptic activity through homeostatic scaling.

A transgenic mouse for imaging activity-dependent dynamics of endogenous Arc mRNA in live neurons

Localized translation plays a crucial role in synaptic plasticity and memory consolidation. However, it has not been possible to follow the dynamics of memory-associated mRNAs in living neurons in response to neuronal activity in real time. We have generated a novel mouse model where the endogenous Arc/Arg3.1 gene is tagged in its 3' untranslated region with stem-loops that bind a bacteriophage PP7 coat protein (PCP), allowing visualization of individual mRNAs in real time. The physiological response of the tagged gene to neuronal activity is identical to endogenous Arc and reports the true dynamics of Arc mRNA from transcription to degradation. The transcription dynamics of Arc in cultured hippocampal neurons revealed two novel results: (i) A robust transcriptional burst with prolonged ON state occurs after stimulation, and (ii) transcription cycles continue even after initial stimulation is removed. The correlation of stimulation with Arc transcription and mRNA transport in individual neurons revealed that stimulus-induced Ca²⁺ activity was necessary but not sufficient for triggering Arc transcription and that blocking neuronal activity did not affect the dendritic transport of newly synthesized Arc mRNAs. This mouse will provide an important reagent to investigate how individual neurons transduce activity into spatiotemporal regulation of gene expression at the synapse.

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

Long-lasting forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD) are fundamental cellular mechanisms underlying learning and memory. The synaptic tagging and capture (STC) hypothesis has provided a theoretical framework on how products of activity-dependent genes may interact with potentiated synapses to facilitate and maintain such long-lasting synaptic plasticity. Although Arc/arg3.1 was initially assumed to participate in STC processes during LTP, accumulating evidence indicated that Arc/arg3.1 might rather contribute in weakening of synaptic weights than in their strengthening. In particular, analyses of Arc/Arg3.1 protein dynamics and function in the dendrites after plasticity-inducing stimuli have revealed a new type of inactivity-dependent redistribution of synaptic weights, termed "inverse synaptic tagging". The original synaptic tagging and inverse synaptic tagging likely co-exist and are mutually non-exclusive mechanisms, which together may help orchestrate the redistribution of synaptic weights and promote the enhancement and maintenance of their contrast between potentiated and non-potentiated synapses during the late phase of long-term synaptic plasticity. In this review, we describe the inverse synaptic tagging mechanism that controls synaptic dynamics of Arc/Arg3.1, an immediate early gene product which is captured and preferentially targeted to non-potentiated synapses, and discuss its impact on neuronal circuit refinement and cognitive function.

A novel serine racemase inhibitor suppresses neuronal over-activation in vivo

Serine racemase (SRR) is an enzyme that produces d-serine from l-serine. d-Serine acts as an endogenous coagonist of NMDA-type glutamate receptors (NMDARs), which regulate many physiological functions. Over-activation of NMDARs induces excitotoxicity, which is observed in many neurodegenerative disorders and epilepsy states. In our previous works on the generation of SRR gene knockout (Srr-KO) mice and its protective effects against NMDA- and Aβ peptide-induced neurodegeneration, we hypothesized that the regulation of NMDARs' over-activation by inhibition of SRR activity is one such therapeutic strategy to combat these disease states. In the previous study, we performed in silico screening to identify four compounds with inhibitory activities against recombinant SRR. Here, we synthesized 21 derivatives of candidate 1, one of four hit compounds, and performed screening by in vitro evaluations. The derivative 13J showed a significantly lower IC₅₀ value in vitro, and suppressed neuronal over-activation in vivo.

Application of hairless mouse strain to bioluminescence imaging of Arc expression in mouse brain

BACKGROUND

Bioluminescence imaging (BLI) is a powerful technique for monitoring the temporal and spatial dynamics of gene expression in the mouse brain. However, the black fur, skin pigmentation and hair regrowth after depilation of mouse interfere with BLI during developmental and daily examination. The aim of this study was to extend the application of Arc-Luc transgenic (Tg) mice to the BLI of neuronal activity in the mouse brain by introducing the hairless (HL) gene and to examine Arc-Luc expression at various developmental stages without interference from black fur, skin pigmentation, and hair regrowth.

RESULTS

The Arc-Luc Tg HL mice were established by crossing the Tg C57BL/6 mouse strain with the HL mouse strain. Under physiological and pathological conditions, BLI was performed to detect the signal intensity changes at various developmental stages and at an interval of <7 days. The established Arc-Luc Tg HL mice exhibited clear and stable photon signals from the brain without interference during development. After surgical monocular deprivation during visual-critical period, large signal intensity changes in bioluminescence were observed in the mouse visual cortex. Exposure of mice to a novel object changed the photon distribution in the caudal and rostral cerebral areas. The temporal pattern of kainic-acid-induced Arc-Luc expression showed biphasic changes in signal intensity over 24 h.

CONCLUSIONS

This study showed the advantages of using the mutant HL gene in BLI of Arc expression in the mouse brain at various developmental stages. Thus, the use of the Arc-Luc Tg HL mice enabled the tracking of neuronal-activity-dependent processes over a wide range from a focal area to the entire brain area with various time windows.

In vivo imaging of CREB phosphorylation in awake-mouse brain

The cyclic adenosine monophosphate response element binding protein (CREB) is a phosphorylation-dependent transcription factor that plays important roles in memory consolidation and several neuropsychological disorders. Although analyzing the spatiotemporal pattern of CREB phosphorylation is required for elucidating the mechanism of memory consolidation, imaging of phosphorylation of a particular protein in the brain of live animals is impossible at present. Here, we developed a method for visualizing the CREB phosphorylation in the cerebral cortex of an awake mouse using a split luciferase technique. Using this technique, we demonstrated the correlation between the change in CREB phosphorylation at a particular region in the brain and behavioral consequences induced by the administration of reserpine, a psychotropic agent.

A new era for functional labeling of neurons: activity-dependent promoters have come of age

Genetic labeling of neurons with a specific response feature is an emerging technology for precise dissection of brain circuits that are functionally heterogeneous at the single-cell level. While immediate early gene mapping has been widely used for decades to identify brain regions which are activated by external stimuli, recent characterization of the promoter and enhancer elements responsible for neuronal activity-dependent transcription have opened new avenues for live imaging of active neurons. Indeed, these advancements provided the basis for a growing repertoire of novel experiments to address the role of active neuronal networks in cognitive behaviors. In this review, we summarize the current literature on the usage and development of activity-dependent promoters and discuss the future directions of this expanding new field.

Measuring CREB activation using bioluminescent probes that detect KID-KIX interaction in living cells

The cyclic adenosine monophosphate response element-binding protein (CREB) is a transcription factor that contributes to memory formation. The transcriptional activity of CREB is induced by its phosphorylation at Ser-133 and subsequent interaction with the CREB-binding protein (CBP)/p300. We designed and optimized firefly split luciferase probe proteins that detect the interaction of the kinase-inducible domain (KID) of CREB and the KIX domain of CBP/p300. The increase in the light intensity of the probe proteins results from the phosphorylation of the responsible serine corresponding to Ser-133 of CREB. Because these proteins have a high signal-to-noise ratio and are nontoxic, it has become possible for the first time to carry out long-term measurement of KID-KIX interaction in living cells. Furthermore, we examined the usefulness of the probe proteins for future high-throughput cell-based drug screening and found several herbal extracts that activated CREB.

Arc in synaptic plasticity: from gene to behavior

The activity-regulated cytoskeletal (Arc) gene encodes a protein that is critical for memory consolidation. Arc is one of the most tightly regulated molecules known: neuronal activity controls Arc mRNA induction, trafficking and accumulation, and Arc protein production, localization and stability. Arc regulates synaptic strength through multiple mechanisms and is involved in essentially every known form of synaptic plasticity. It also mediates memory formation and is implicated in multiple neurological diseases. In this review, we will discuss how Arc is regulated and used as a tool to study neuronal activity. We will also attempt to clarify how its molecular functions correspond to its requirement in various forms of plasticity, discuss Arc's role in behavior and disease, and highlight critical unresolved questions.