Expression of Non-visual Opsins Opn3 and Opn5 in the Developing Inner Retinal Cells of Birds. Light-Responses in Müller Glial Cells

Abstract

The avian retina is composed of different types of photoreceptors responsible for image and non-image forming tasks: the visual photoreceptor cells (cones and rods), the melanopsin-expressing intrinsically photoresponsive retinal ganglion cells (ipRGCs) and horizontal cells. Furthermore, the non-visual opsins Opn3 (encephalopsin/panaopsin) and Opn5 (neuropsin) have been shown to be expressed in the vertebrate inner retina, responding to blue (BL) and UV light, respectively. Here we investigated the expression and localization of Opn3 and Opn5 in the developing chick retina at different embryonic days (E) as well as in primary cultures of retinal Müller glial cells (MCs). Opn3 and Opn5 mRNAs and proteins appeared as early as E10 although traces of Opn3- and Opn5-like proteins were seen earlier by E7 in the forming RGC layer and in glial cells extending throughout the developing nuclear layer. Later on, at postnatal days 1–10 (PN1–10) a significant expression of Opn3 was observed in inner retinal cells and processes in plexiform layers, together with expression of the glial markers glutamine synthetase (GS) and the glial fibrillary acidic protein (GFAP). Opn3 and Opn5 were found to be expressed in primary MC cultures prepared at E8 and kept for 2 weeks. In addition, significant effects of BL exposure on Opn3 expression and subcellular localization were observed in MCs as BL significantly increased its levels and modified its nuclear location when compared with dark controls, through a mechanism dependent on protein synthesis. More importantly, a subpopulation of MCs responded to brief BL pulses by increasing intracellular Ca 2+ levels; whereas light-responses were completely abolished with the retinal bleacher hydroxylamine pretreatment. Taken together, our findings show that these two opsins are expressed in inner retinal cells and MCs of the chicken retina at early developmental phases and remain expressed in the mature retina at PN days. In addition, the novel photic responses seen in MCs may suggest another important role for the glia in retinal physiology.

Wounding Induces Facultative Opn5-Dependent Circadian Photoreception in the Murine Cornea

“…17 In birds, OPN5 functions as a photoreceptor in both the retina and hypothalamic areas. [18][19][20] The murine cornea expresses robust circadian rhythms of clock gene expression in the majority of cells throughout the epithelium and endothelium both in vivo and ex vivo. 5,21,22 Synchronization of corneal phase to the lightdark cycle appears to be influenced by both melatonin and glucocorticoids, 21,23,24 suggesting circulating systemic cues set the phase of this tissue from the master clock.…”

Illuminating the Inner Retina of Vertebrates: Multiple Opsins and Non-Visual Photoreceptors

“…In addition to the melanopsin that is expressed in ipRGCs and HCs, other photopigments have been described in the inner retina of vertebrates; they are the opsins Opn3 (called encephalopsin) and Opn5 (called neuropsin), and the photoisomerase RGR (Retinal G protein coupled Receptor), whose expression we have found in rat and chicken retina (Nieto et al, 2011;Diaz et al, 2017;Rios et al, 2019) (Fig. 3).…”

“…iii-Muller's glial cells and other cells, which express Opn3 and Opn5, respond to longer-time light stimuli, mainly blue light, mobilizing intracellular calcium through a mechanism not yet elucidated (Rios et al, 2019), and collaborating potentially in processes related to neuron-glia or glia-glia interaction under prolongued light stimuli (Figs. 3).…”

“…The generic name of both opsins is due to the fact that they were first found expressed in the brain, but today it is known that they are expressed in the vertebrate retina, and in extra-retinal tissues, such as the skin, with potential functions not yet fully characterized. In chicken, both opsins are expressed in nerve cells of the embryonic inner retina and in Müller glial cells (Rios et al, 2019). Furthermore, primary cultures enriched in Müller's glial cells that express these photopigments respond to blue light of high intensity and prolonged duration (> 20 seconds), mobilizing intracellular calcium, through a cascade not completely characterized even from the biochemical point of view (Rios et al, 2019) (Fig.…”

Non-visual Opsins and Novel Photo-Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spectrum Region

“…Some vertebrates including avian species are also sensitive to UV light and have specific photoreceptors in this range, the socalled UV-sensitive or violet cones with maximum absorbance around 418 nm. In addition to vision and photopigments directly involved in image-forming activities, a new set of photopigments has been reported in the vertebrate retina-the so-called non-visual opsins such as melanopsins (Opn4M and Opn4X) (Provencio et al 2000;Chaurasia et al 2005;Bellingham et al 2006;Contin et al 2006Contin et al , 2010Verra et al 2011) encephalopsin (Opn3) (Rios et al 2019), neuropsin (Opn5) (Yamashita et al 2010;Kojima et al 2011;Nieto et al 2011;Rios et al 2019) and the photoisomerase Retinal G protein-coupled receptor (RGR) (Díaz et al 2017). The visual opsins (Opn1 and Opn2) together with the various non-visual opsins (Opn3, Opn4, Opn5), acting particularly on the violet/blue light portion of the spectrum, provide vertebrates with the evolutionary advantage of photosensitivity in a broader spectrum region (Fig.…”