Group of Brain Function and Structure

Our research interests focus on the brain function and organization and their molecular and genetic bases.

Functional organization of the brain

Fig. 1 Mauthner (M) cells and other reticulospinal neurons in zebrafish hindbrain

A striking feature of the vertebrate brain is its division into segments along the rostrocaudal length.  This segmentation is most obvious in the hindbrain, where the reticulospinal (RS) neurons appear repeatedly in each segment (Fig. 1). In zebrafish, successive segments contain morphologically homologous RS neurons, which are functionally related to each other.  Typically, the paired giant Mauthner (M) cells and their homologs are arranged in adjacent segments, which are thought to be involved in initiation and control of fast escape behavior in zebrafish. M-cell, however, exhibits unique membrane excitability: M-cell fires with only a single spike in response to membrane depolarization, whereas the other RS neurons show multiple spiking.  We examine physiological and molecular basis for the functional differentiation between the M-cell and other RS neurons.

Development of the brain

Fig. 2 Inner ear hair cells connected to the otolith (*) and VIII nerve cells in zebrafish embryo

We are also interested in development of auditory input to M-cell, which is most effective in activating an M-cell to initiate escape behavior in zebrafish. We have shown that appearance and development of auditory responsiveness of the M-cell coincide with those of the inner ear hair cells, whereas the functional auditory circuits from the hair cells to the M-cell form before that. Now, we are studying the cellular and molecular mechanisms underlying the acquisition of auditory responsiveness in the inner ear hair cells (Fig. 2), by employing electrophysiological, morphological and molecular biological approaches to zebrafish embryos.

Semaphorin/plexin system in C. elegans

The semaphorin/plexin system plays various roles in animal development. Notably, it is one of the major regulators in the formation of vertebrate neuronal circuits. In order to fully understand the function as well as the signal transduction mechanism of the semaphorin/plexin system in vivo, we are using C. elegans, a simple model organism amenable to genetic analyses. We have revealed that semaphorin regulates morphogenetic changes of cells via stimulated mRNA translation. Currently, our interest is focused on the link between the semaphorin/plexin system and mTOR signaling. We have been also developing a novel method for manipulating the gene expression in targeted single cells of living worms. Exploiting activation of heat-shock promoter-driven transgenes by infrared-laser irradiation, the method, named IR-LEGO, serves as a powerful tool for examining cell-cell interaction in vivo.

Lab members