Study on the neuronal subpopulation based on Fos enhancer activity in various brain regions

MasterTo orchestrate the precise gene expression patterns that give rise to the phenotypic and functional variety of complicated biological systems, the genome of a higher organism utilizes millions of regulatory elements known as enhancers. Enhancer activities are highly cell type-specific and essential for determining the spatiotemporal expression patterns of target genes. Several studies have engineered neuronal activity reporters such as E-SARE using transcription factor binding sites present in enhancer regions of immediate early genes (IEGs) to define active neuronal circuits or neuronal subpopulations. Our previous study showed that five enhancers associated with the Fos gene could be activated in various combinations depending on the types of stimuli. In the cultured neurons, the second enhancer (e2) was activated by KCl stimulation whereas the fourth enhancer (e4) were selectively activated by brain-derived neurotrophic factor (BDNF) stimulation. Here, we sought to develop a set of new genetic tools that can distinguish neuronal subpopulation based on different Fos enhancer activities, especially focusing on e2 and, e4. Using these reporters, we demonstrated brain region-specific Fos enhancer activity. In the hippocampus, reporters showed different expression patterns in the granular cell layer of IV CA1 and stratum oriens (SO). The SO region is one of the hippocampal regions containing somatostatin (som)+ and parvalbumin (PV) interneurons, which regulate the local activity of CA1. We also found that different enhancer activity in the shell region in nucleus accumbens (NAc), which regions showed that higher e4 activity than e2, while the core region showed no significant differential activity. We then used the retro-orbital technique to examine the reporter activities across the whole brain and found that the e4-based reporter was specifically expressed in ventral posteromedial nucleus (VPM) in the thalamus. Although further study is required, this preliminary data suggests that the Fos enhancer-based reporter system has the potential to identify activated neurons in a region- or cell type-specific manner