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 Ca2+ 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.
Highlights
Opsins are photosensitive pigments that bind a retinaldehyde chromophore to form a light-sensitive G protein-coupled receptor able to sense specific wavelength ranges
photoreceptor cells (PRCs) connect through the outer plexiform layer (OPL) to the inner nuclear layer (INL), comprising horizontal cells (HCs), bipolar and amacrine cells, and the INL cells connect with the retinal ganglion cells (RGCs) through the inner plexiform layer (IPL)
To investigate expression of Opn3 and Opn5 in the developing retina of chickens, we evaluated their expression at different embryonic stages in the whole retina and in primary cultures of Müller glial cells (MCs) at early embryonic days
Summary
Opsins are photosensitive pigments that bind a retinaldehyde chromophore to form a light-sensitive G protein-coupled receptor able to sense specific wavelength ranges. The vertebrate retina contains a number of different opsins responsible for the photoreception involved in visual functions such as imageand non-image forming tasks (Guido et al, 2010; Meister and Tessier-Lavigne, 2013; Diaz et al, 2016). Classical photoreceptor cells (PRCs) – cones and rods – are responsible for color (diurnal) and black and white (nocturnal) vision, respectively (Meister and Tessier-Lavigne, 2013). RGC axons form the optic nerve and send visual information to the brain (Meister and Tessier-Lavigne, 2013). The retina contains diverse types of glial cells, the most abundant being the Müller glial cells (MCs) that extend throughout the inner retina. MCs are responsible for several important physiological activities, both during and after development, significantly contributing to homeostasis and even to more complex processes such as neuronal regeneration (Jorstad et al, 2017; Subirada et al, 2018)
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