Our lab studies the principles of neocortical development. 「How is the elaborate neural network of the brain organized?」There are more questions than answers. Although organic dysfunctions of neocortical structure cause various neurological and psychiatric disorders, the development mechanism remains unknown. We have been tackling this problem with the standpoint of “understanding the principle of brain construction”. The cerebral neocortex is responsible for higher brain functions, such as conscious thought and language in humans. In the neocortex, neurons are arranged in a precise 6-layered structure formed by the sequential generation of neurons and their accurate migration toward the brain surface during the fetal period. How is this migration controlled? We study subplate neurons, which develop and mature extremely early during cortical development, to answer this question. Subplate neurons play an essential role in establishing neuronal connections between the thalamus and the neocortex. However, we recently found that subplate neurons also play a crucial role in radial neuronal migration by interacting directly with young migrating neurons. Moreover, the subplate layer is surrounded by a rich extracellular matrix (ECM), suggesting a vital signalling center for mammalian corticogenesis. Understanding the various functions of subplate neurons will lead to a better understanding of brain development.

The subplate layer lies beneath the cortical plate and is where subplate neurons, the first neurons to differentiate in the neocortex, are distributed (Fig. 2, Refs. 1, 3 ). Although subplate neurons have been known to play an essential role in thalamocortical projection, we recently found that subplate neurons facilitate mode conversion of neuronal migration through the transient synaptic transmission to migrating neurons (Fig. 4, Ref. 4). To create an elaborate neocortex within the limited developmental time of the fetal period, it is necessary for the neogenesis of neurons, migration, and the formation of neural circuits by axonal projection to proceed simultaneously and in synchrony with each other, but the overall control mechanism has remained unclear. In addition to the new functions found in this study, subplate neurons may perform various brain construction processes and play a commanding role in forming the neocortex.

In addition, the neocortex is a unique region acquired by mammals during the process of brain evolution. The cerebrum of birds and reptiles, which do not have this region, does not have a subplate layer. This suggests that the subplate layer played an essential role in the evolution of the neocortex (Fig. 5). To comprehensively identify the signalling pathways that contribute to neuronal migration, we search for a group of genes selectively expressed in migrating neurons and subplate neurons using DNA microarrays, single-cell RNAseq, and other methods.

We are also interested in abundant proteoglycans and other extracellular matrix components expressed in the subplate layer. Although chondroitin sulfate proteoglycans are expressed in the cerebral cortex, including the subplate layer, during development when neuronal migration is actively ongoing, their functions have not been clarified. We have been focusing on the role of chondroitin sulfate proteoglycans in the formation of the cerebral cortex. Chondroitin sulfate is a linear polysaccharide consisting of a large number of polymerized disaccharide units of glucuronic acid and N-acetylgalactosamine, and the sulfation of the constituent sugars and the C5 epimerization of glucuronic acid gives rise to a great variety of structures in the sugar chain (Fig. 6). Among them, the region with high sulfate density, rich in polysulfated forms, is assumed to contribute to the binding of various proteins and is expected to be functionally important. We have investigated the role of the glycosylation modifying enzymes; GalNAc4S-6ST and C2ST using in utero electroporation. As a result, when these enzymes were knocked down, the migration of neurons was impaired, and they became stagnant in the subventricular zone (Ref. 5). The polysulfated structure of chondroitin sulfate is necessary for the morphological change of neurons from multipolar to bipolar during radial migration. In addition, hippocampal neurons were cultured in chondroitin sulfate-degrading enzyme (chondroitinase ABC), and projection elongation was observed. As a result, degradation and removal of chondroitin sulfate caused abnormalities in nerve polarization, and many neurons grew multiple axons (Ref. 6). Moreover, by observing radial migration in slice cultures, it was found that the leading processes of migrating cells were difficult to determine and that multipolar-bipolarity conversion was impaired (Fig. 7). In the future, we would like to analyze how the process of cortical construction is regulated by proteoglycans and to discuss the role of extracellular matrix regulation in neocortical development and evolution.