Non-visual Opsins and Novel Photo-Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spect
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REVIEW PAPER
Non‑visual Opsins and Novel Photo‑Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spectrum Region Mario E. Guido1,2 · Natalia A. Marchese1,2 · Maximiliano N. Rios1,2 · Luis P. Morera3 · Nicolás M. Diaz4 · Eduardo Garbarino‑Pico1,2 · María Ana Contin1,2 Received: 7 August 2020 / Accepted: 3 November 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract In recent decades, a number of novel non-visual opsin photopigments belonging to the family of G protein- coupled receptors, likely involved in a number of non-image-forming processes, have been identified and characterized in cells of the inner retina of vertebrates. It is now known that the vertebrate retina is composed of visual photoreceptor cones and rods responsible for diurnal/color and nocturnal/black and white vision, and cells like the intrinsically photosensitive retinal ganglion cells (ipRGCs) and photosensitive horizontal cells in the inner retina, both detecting blue light and expressing the photopigment melanopsin (Opn4). Remarkably, these non-visual photopigments can continue to operate even in the absence of vision under retinal degeneration. Moreover, inner retinal neurons and Müller glial cells have been shown to express other photopigments such as the photoisomerase retinal G protein-coupled receptor (RGR), encephalopsin (Opn3), and neuropsin (Opn5), all able to detect blue/violet light and implicated in chromophore recycling, retinal clock synchronization, neuronto-glia communication, and other activities. The discovery of these new photopigments in the inner retina of vertebrates is strong evidence of novel light-regulated activities. This review focuses on the features, localization, photocascade, and putative functions of these novel non-visual opsins in an attempt to shed light on their role in the inner retina of vertebrates and in the physiology of the whole organism. Keywords Retina · Light responses · Opsin · Non-visual photopigment · Non-image forming activity · Circadian rhythm · Blue light
The Vertebrate Retina and Visual/Non‑visual Photoreceptors
Natalia A. Marchese and Maximiliano N. Rios are contributed equally to this work. * Mario E. Guido [email protected] 1
CIQUIBIC‑CONICET, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, 5000 Córdoba, Argentina
2
Departamento de Química Biológica “Ranwel Caputto”, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, 5000 Córdoba, Argentina
3
Present Address: Instituto de Organizaciones Saludables, Universidad Siglo 21, Córdoba, Argentina
4
Present Address: Department of Ophthalmology, University of Washington School of Medicine, 750 Republican St., Campus, Box 358058, Seattle, WA 98109, USA
Beyond the ability of the retina to detect light for visual processing and image formation, the inner retina of vertebrates has acquired a number of cells expressing non-visual photopigments potentially implicated in photoreception in the visible wavelengths of the spectrum mainly
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