Elucidation of developmental and/or evolutionary mechanisms of cortical arealization
We are currently focusing on several genes expressed in the developing cerebral cortex with areal specificity and analyzing how those genes can establish their specific expression patterns during development and how they may eventually contribute to the arealization dynamics or not. For instance, we have already revealed the role of cadherin cell adhesion molecules in elaborating the somatosensory barrel area specific cytoarchitecture by using conditional gain-of-function studies in mice (Terakawa YW et al., 2013; Egusa SF et al., 2016).
Additionally, by utilizing enhancers/promoters from those genes, we are trying to genetically trace the dynamic cellular processes in mouse cortical arealization. For example, we have already confirmed that two distinct enhancers, activities of which are marked by red and green florescent reporters, differentially control the gene expression profiles in specific areas and/or layers respectively (Figure; Inoue YU et al., unpublished data). It would be worth while examining how these two genetically distinct populations emerge during cortical development and/or evolution.
Furthermore, we are planning to study the evolutionary traits of those genes by means of comparative genomics as well as in the context of xeno-transgenesis. At the first step of this study, we have modified human BACs from the autistic risk single nucleotide polymorphism (SNP) containing CDH9/CDH10 intergenic territory with an enhancer trap reporter cassette and injected them into fertilized mouse eggs to successfully find out the human specific traits of this territory (e.g. enhancer activities for a human specific microRNA expression within distinct cortical areas have been isolated from the autism-associated region; Inoue YU and Inoue T, 2016).
Clarification of cellular and molecular machinery for compartment boundary formation
No matter how complex the human brain structure and function might be, every cellular component of the vertebrate brain including our own can be emerged from the conserved simple epithelial cell sheet termed ‘neural plate’. The neural plate then roles up to zip its edges at the dorsal midline, generating the tube-like structure named ‘neural tube’. As the initial events for brain morphogenesis, many of bulges or sulci become evident at the anterior portion of the neural plate/tube and these bulges or sulci occasionally coincide with the stating points to fold up the neural plate/tube, eventually building up the basic organization of vertebrate brain. Noticeably, it has been confirmed by means of neuroepithelial cell labeling that some of those segmental domains subdivided by bulges or sulci in the neural plate/tube can be defined as compartment units with cell lineage restriction. For example, by taking advantage of mammalian whole embryo culture system, we have determined that the mouse forebrain and midbrain are established as compartments by the 5-somite stage (Inoue T et al., 2000) and in the mouse telencephalon, a morphologically identifiable border between the future cerebral cortex and striatum is confirmed to be a compartment boundary by E10.5 (Inoue T et al., 2001).
We are very interested in the cellular and molecular mechanisms of brain compartmentalization and have already found out that the differential expression profiles for cell adhesion molecules cadherins in the embryonic mouse telencephalon are necessary and sufficient to maintain the cell lineage restricted compartment boundary with their selective cell sorting activities (Inoue T et al., 2001). We are continuously evaluating the role of cadherins in brain compartmentalization by using various in vivo (Inoue YU et al., 2009) and in vitro models (Figure; Asami J and Inoue T, unpublished), and are currently trying to genetically visualize cadherin-related cellular and molecular dynamics during compartment boundary formation to finally close in on the common nature of ‘brain regionalization’ including the cortical arealization, if any.