The formation and sculpting neuronal circuits in the brain are not specific to the course of development but occur continuously throughout life. Synapses, connection sites between neurons, are added or removed from neuronal circuits constantly. Synapses also undergo changes in morphology as well as efficiency of information transmission. These plastic properties of synapses are thought to be the fundamental cellular basis of our cognitive functions including learning and memory. In the past couple of decades, lines of evidence indicate that neuronal activity-dependent gene expression is indispensable for long-lasting synaptic plasticity and memory formation processes in the brain. Among many genes that are regulated by neuronal activities, we are especially interested in a particular gene subgroup called immediate-early genes (IEGs), whose expression is rapidly and transiently upregulated after receiving strong synaptic inputs. The goal of our research is to elucidate IEG-dependent molecular and cellular mechanisms underlying synaptic plasticity and cognitive functions. Our current research includes following projects.

1) Roles of Arc in cognitive functions Arc (aka Arg3.1) is one of the most dynamically regulated gene in the brain and tightly linked to information processing in the brain. Arc/Arg3.1 protein is enriched in the postsynapses and its synaptic localization is regulated through interaction with the beta subunit of CaMKII (Okuno et al., 2012). Synaptic Arc/Arg3.1 controls surface expression levels of AMPA-type glutamate receptors (Chowdhury et al., 2006; Okuno et al., 2012). We currently attempt to elucidate the physiological role of Arc using mouse molecular genetics and behavioral paradigms.

2) Visualization of memory-related neuronal ensembles in the brain Expression of IEGs, such as c-fos and Arc, has been widely used to detect previously activated neurons in the brain (Okuno, 2011). We previously characterized and identified an activity-dependent enhancer in the Arc promoter region (Kawashima et al., 2009). Taking advantage of the potent activity-dependency of the Arc enhancer element, we have developed several neuronal activity-dependent gene reporter systems (Okuno et al., 2012; Kawashima et al., 2013; Vousden et al., 2014). These reporter systems are useful to visualize and analyze the activated neuronal ensembles in the brain during various phases of memory formation.

3) Identification and characterization of novel synaptic plasticity-related genes Despite previous extensive screening, the complete lists of genes that are upregulated in the neurons that are activated during memory acquisition and storage processes have not yet established. Enjoying the great advance in next generation sequencing technologies, we revisit identification and characterization of novel activity-dependent genes by a single-neuron RNA-seq method, and aim to investigate their biological functions on synaptic plasticity.

References
Chowdhury, S., Shepherd, JD, Okuno, H., Lyford, G., Petralia, RS., Plath, N., Kuhl, D., Huganir, RL Worley, PF. Arc interacts with the endocytic machinery to regulate AMPA receptor trafficking. Neuron, 42:445-459, (2006).
Kawashima, T., Kitamura, K., Suzuki, K., Nonaka, M., Kamijyo, S., Takemoto-Kimura, S., Kano, M., Okuno, H., Ohki, K, and Bito, H. Functional labeling of neurons and their projections using the synthetic activity-dependent promoter E-SARE. Nat Methods, 10: 889-895 (2013).
Kawashima, T., Okuno, H., Nonaka, M., Adachi-Morishima, A., Kyo, N., Okamura, M., Takemoto-Kimura, S., Worley, P., Bito, H. A synaptic activity-responsive element in the Arc/Arg3.1 promoter essential for synapse-to-nucleus signaling in activated neurons, Proc. Natl. Acad. Sci. USA 106:316-321, (2009).
Okuno, H., Akashi, K., Ishii, Y., Yagishita-Kyo, N., Suzuki, K., Nonaka, M., Kawashima, T., Fujii, H., Takemoto-Kimura, S., Abe, M., Natsume, R., Chowdhury, S., Sakimura, K., Worley, P.F., Bito, H. Inverse synaptic tagging of inactive synapses via dynamic interaction of Arc/Arg3.1 with CaMKIIβ. Cell, 149:886-898, (2012).
Okuno, H. Regulation and function of immediate-early genes in the brain: beyond neuronal activity markers. Neuroscience Res. 69:175-186, (2011).
Vousden, DA., Epp, J., Okuno, H., Nieman, BJ., van Eede, M., Dazai, J., Ragan, T., Bito, H., Frankland, PW., Lerch, JP., Henkelman, RM. Whole-brain mapping of behaviorally-induced neural activation in mice. Brain Struc. Func., [Epub ahead of print] (2014).