I have been interested in physiological roles and molecular mechanisms of photoreception in animals. In my earlier career, I pursued biochemical and molecular biological studies on the retinal photoreception in vertebrate visual cells. Then my interest extended to exploring the photoreception in avian pineal glands and later to the photic entrainment of circadian rhythms.
(i) Photoreception and visual transduction process: I started my scientific career in Prof. Yoshizawa lab (Kyoto University) by finding that metarhodopsin II is a physiologically active intermediate of rhodopsin (BBA, 1979) and that cis-trans isomerization at C11=C12 of retinylidene chromophore is indispensable for visual transduction (Biochem., 1984). Then Yoshizawa lab purified four kinds of cone opsins for color vision, UV-Violet (SWS1), Blue (SWS2), Green (RH2), and Red (LWS) opsins, from chicken retinas (Biochem, 1989). cDNA cloning of the four color opsins revealed that the divergence of the color opsin genes is ancient to that of rhodopsin, a scotopic opsin (PNAS, 1992). Recently, Dr. Daisuke Kojima and I found that sine oculis homeobox 6 (Six6) and Six7 are the transcription factors responsible for expression of two middle wavelengths-sensitive opsins, Blue and Green opsins (PNAS, 2019). Further analysis identified Foxq2 is the key transcription factor determining the blue cone identity, under Six6/Six7 regulation (Science Adv., 2021).
(ii) Visual transduction mediated by G-protein, Transducin: We found C-terminal farnesylation and carboxyl methylation of Transducin (Gt) gamma-subunit (Nature, 1990; EMBO J., 1991). I was interested in physiological significance of the difference between the C15- and C20-modifications. We developed knock-in mice, in which the C-terminal CAAX sequence, CVIS, directing farnesylation (C15) was replaced by CVIL directing geranylgeranylation (C20). Then light-adaptation of the mutant rod cells was significantly impaired because C20-modified Gt was unable to translocate from the outer segment to the inner region upon light illumination (Neuron, 2005). We also identified that the N-terminus of Gt alpha-subunit is heterogeneously modified by one of myristate and three related fatty acids. It turned out that the heterogeneous N-acylation contributes to broadening of the light-sensitive range of the rhodopsin signaling due to functional divergence among the Gt-alpha subspecies (Nature, 1992).
(iii) Pineal photoreception and photic entrainment of circadian clock: Dr. Toshiyuki Okano and I found chicken pineal photoreceptive molecule responsible for light-dependent inhibition of melatonin production, and he named the new opsin “pinopsin” after pineal opsin as the first example of opsins expressed in extra-retinal tissues (Nature, 1994). Then, we found that photoactivation of pinopsin triggers the phase-shift of the pineal circadian clock through signaling of G-protein G11 (J. Neurosci., 2002). We identified E4bp4 gene as a light-inducible gene regulating the phase-shift of the pineal clock (Curr. Biol., 2004). Then Dr. Kojima and I became interested in similarity in gene expression profiles between the pineal and retinal cells, and we found an important DNA cis-element governing the pineal-specificity in gene expression, and named it PIPE after “pineal expression-promoting element” (PNAS, 2002). Finally, we identified a homeobox protein Bsx responsible for the development of the pineal gland and pineal-specific gene expression in zebrafish (Commun. Biol., 2019).