2023 年 20 巻 Supplemental 号 論文ID: e201013
Animal opsin is a photoreceptor protein with its chromophore retinal, and it is a member of class A G-protein coupled receptor (GPCR) superfamily. The first cloned opsin of the class A GPCR is vertebrate rhodopsin present in the bovine retinal rod cells [1]. Since then, four cone photoreceptor proteins, violet-, blue-, green- and red-sensitive cone opsins, have been found to be expressed in vertebrate retinal cone cells, and invertebrate rhodopsin genes have also been found in the retinal visual cells of invertebrate species. In early 1990’s, animal photoreceptor opsins were believed to be present only in the retinal visual cells for the purpose of visual transduction. In 1994, however, a new member of opsin family was identified in chick pineal cells, and it was named pinopsin after pineal opsin by Okano et al. [2]. This was the first example of animal opsins expressed in extra-ocular tissues, for non-visual purpose. In a molecular phylogenetic tree, pinopsin is clustered with the members of vertebrate visual opsins forming a single subfamily. In the past three decades, it becomes evident that the opsin family in vertebrate and invertebrate species is composed of a lot of additional subfamilies which contain diverged members of opsins represented by encephalopsin (OPN3), melanopsin (OPN4), neuropsin (OPN5), Go-opsin, peropsin, and others [3]. Interestingly, vertebrate melanopsin is clustered with visual opsins of invertebrates such as mollusc and arthropod. In vertebrate species, most of the non-visual opsin members are expressed in the extra-retinal photosensitive tissues, but a few members of these opsins are expressed in a limited number of the retinal neurons other than the visual cells. Therefore, the term of “extra-retinal” opsins or “extra-ocular” opsins, is not a correct term to indicate the opsins expressed in the retinal cells other than the visual cells (rods and cones). Rather, we may refer to it as “opsins for non-visual purpose” or simply “non-visual opsins”. Most likely, non-visual opsins are involved in light-dependent physiologies with non-visual functions or non-image forming vision. This symposium session 10 (titled with “Recent advances in biological rhythm and non-visual photoreception”) at the 19th International Conference on Retinal Proteins was organized to discuss recent advances in the research of light-dependent physiologies such as photic regulation of biological rhythms and the non-visual photoreception in animals.
Melanopsin is a typical member of non-image forming opsin expressed in a small subset of ganglion cells in mammalian retinas. These cells are now termed ipRGCs, intrinsically photosensitive retinal ganglion cells, and they play an important role in a wide range of light-dependent physiologies, including circadian photoentrainment, pupillary light reflex (PLR), sleep, mood, memory and learning. Dr. Phyllis R. Robinson (University of Maryland Baltimore County, Baltimore, MD, USA) presented their recent studies on “The role of phosphorylation in the regulation melanopsin in ipRGCs.” Like other GPCRs, melanopsin signaling is regulated by its phosphorylation. Dr. Robinson and their colleagues explored the role of both the C-terminal tail phosphorylation by G-protein receptor kinases (GRKs) and phosphorylation of residues in the intracellular loops by protein kinase A (PKA) on ipRGC physiology and function [4]. They found that the C-tail phosphorylation of melanopsin regulated the recovery kinetics of the photo-response of ipRGCs, the PLR, and the speed of re-entrainment to a change in environmental light conditions. Mouse melanopsin also contains PKA-mediated phosphorylation sites in intracellular loops, and phosphorylation of these sites produced an attenuation of melanopsin signaling in vitro. They predicted that PKA activation in D1 receptor-expressing ipRGCs through neural input from dopaminergic amacrine cells will modulate both the physiology of ipRGCs and the behaviors they regulate. Their research has demonstrated the importance of melanopsin phosphorylation on the biochemistry, the physiology of ipRGCs and mouse behavior.
Dr. Daisuke Kojima (Department of Biological Sciences, School of Science, The University of Tokyo, Japan) presented their study on “Functional roles of retinal photoreceptors in non-visual physiologies of vertebrates.” Vertebrate retinas have typically rod cells expressing rhodopsin and four type of cone cells expressing violet-, blue-, green- or red-sensitive opsins, some of which have been lost in many mammalian species. The mechanism regulating expression of the middle wavelengths-sensitive opsins (i.e., blue and green opsins) has been elusive, because most of molecular biological studies have been performed in mice, in which blue and green opsin genes had been lost during the evolution. By using zebrafish retina expressing a full set of the four cone subtypes, Dr. Daisuke Kojima and colleagues found that, a transcription factor Foxq2 is indispensable for expression of the blue opsin gene in the zebrafish retina. Interestingly, Foxq2 gene is lost in many mammalian species that have lost blue opsin gene. It was demonstrated that a transcriptional network of Six6/Six7-Foxq2 plays a key role in expression of the middle wavelengths-sensitive blue and gene opsin genes in the vertebrate retina [5]. In addition to the color vision, the retinal photoreception in vertebrate species regulates various physiological functions including entrainment of the circadian rhythm and body color change. In melanopsin-expressing ipRGCs, light-activated melanopsin triggers a Gq-type G-protein signaling pathway in the ipRGCs, whereas recent findings suggest another light signaling pathway in the ipRGCs. Interestingly, non-mammalian vertebrates also use melanopsin signaling in non-visual photoreception. In teleost fish, retinal photoreception is required for a light-dependent body color change, i.e., background adaptation. To elucidate the mechanism of light signaling in the background adaptation, Dr. Kojima used zebrafish larvae as the animal model. It was strongly suggested that photoreception of melanopsin in non-rod/non-cone retinal neurons plays an essential role in the background adaptation.
In 1994, Dr. Toshiyuki Okano (Waseda University, Advanced Science and Engineering, Japan) identified pinopsin as the first example of opsin family members that function as the non-visual photoreceptor [2]. Since then, Dr. Okano became interested in another photoreceptive flavoprotein, cryptochrome, that plays various roles for non-visual functions across species in vertebrates, invertebrates and plants. In the research field of animal biological rhythms, Cryptochrome (CRY) was first reported in the fruit fly as a photoreceptor that regulates light-entrainment of 24-hr cycles of transcription-translation feedback loop composed of clock genes and their encoded proteins. Later, mouse Cryptochrome 1 (mCry1) and 2 (mCry2) were identified as clock genes encoding repressors for circadian expression of E-box-regulated genes. Vertebrate species have a varity of Cry paralogs, but their functional differentiation is poorly understood. Dr. Toshiyuki Okano presented their recent studies on “Non-circadian rhythms and blue-light-dependent magnetoreception mediated by cryptochromes.” They discovered the possibility that CRY3 is involved in lunar response in a rabbitfish, which exhibits lunar phase-synchronized spawning behavior [6]. They also demonstrated, in the eyes of zebrafish, goldfish, and medaka, the existence of circadian clocks synchronized with noon (and midnight), suggesting that the clocks may be involved in the seasonal response (photoperiodism) and/or the sun compass [7]. Interestingly, recent studies including their own raises the possibility that CRY4 in the avian retina is involved in the light-dependent geomagnetoreception. They performed spectroscopic and biochemical analyses of chicken CRY4 [8] to help understanding the molecular mechanism underlying the geomagnetoreception. CRY proteins may be responsible not only for the circadian clockwork but also for acquisition of spatiotemporal information of various vertebrate species.
Living organisms show biological rhythms with a variety of periods of not only approx. 24 hr (i.e., circadian rhythms) but also those far longer or shorter than 24 hr (called infradian or ultradian rhythms, respectively). The molecular mechanisms underlying these infradian and ultradian rhythms are far less understood than that of the circadian rhythms. Particularly, the mechanisms underlying infradian rhythms such as lunar rhythms and seasonal rhythms remains largely elusive. Dr. Takashi Yoshimura (Institute of Transformative Bio-Molecules, WPI-ITbM, and Graduate School of Bioagricultural Sciences, Nagoya University, Japan) presented their recent study on a topics, “Towards understanding molecular mechanisms of infradian rhythms.” They paid special attention to seasonal behaviors of spawning aggregation and trembling of grass puffer, Takifugu alboplumbeus, at beach. In RNA-seq analysis of the hypothalamus and pituitary from the male animal, they identified semilunar genes, including those crucial for reproduction and receptors for pheromone prostaglandin E (PGE). PGE2 was secreted into the seawater during the spawning and its administration in experimental condition triggers trembling behavior of breeding individuals, suggesting that PGE2 is a key synchronizer of the lunar-regulated beach-spawning behavior in grass puffers [9]. Exploring the regulatory mechanism underlying seasonal traits in animal models will provide insight into human seasonality and help understanding human diseases such as seasonal affective disorder.
The pineal gland, a vertebrate neuroendocrine organ, synthesizes and secretes melatonin that plays various physiological roles, such as regulation of the circadian clock, sleep, seasonal breeding and migration. In non-mammalian vertebrates, the melatonin production is regulated by intrinsic light-sensitivity due to expression of opsins. Particularly in lower vertebrates, the pineal and its related organs show chromatic response to a wide spectral region from UV to red light. Drs. Seiji Wada, Mitsumasa Koyanagi, and Akihisa Terakita (Osaka Metropolitan University, Osaka, Japan) presented their recent topics, “Visualization of neural pathways based on the molecular property of a pineal opsin and its contribution to light-dependent behavior using zebrafish larvae.” They found that parapinopsin1 (PP1) can elicit the chromatic response from single pineal cells in the zebrafish. The dark state of PP1 having UV-sensitivity is converted to a visible light-sensitive photoproduct, which is converted back to the original UV-sensitive state by absorbing visible light. Dr. Wada and colleagues identified this reversibility of PP1 as being responsible for the chromatic response of the photoreceptor cells sensitive to both UV and visible light [10]. It was reasoned that UV and visible light irradiation respectively increase and decrease the relative amount of PP1 photoproduct activating G-protein signaling. Now they are pursuing elucidation of physiological function(s) of the pineal chromatic response by determining any brain regions where the PP1-cells transmit the light information in the zebrafish larvae. A whole-brain Ca2+-imaging has successfully identified potential neurons exhibiting PP1-dependent chromatic excitation.
In the concluding remarks, the organizers of the session 10, Dr. Yoshitaka Fukada (The University of Tokyo, Japan) and Dr. Gebhard F. X. Shertler (Paul Scherrer Institute, Switzerland) summarized recent advance of the topics presented by all the speakers in this session, and emphasized important roles of non-visual photoreception in a variety of animal physiology. Our future studies are expected to pave the way for further understanding of the function and mechanism of non-visual photoreception.
Supported by JSPS KAKENHI (JP17H06096 to YF).
