A Non-Mammalian Type Opsin 5 Functions Dually in the Photoreceptive and Non-Photoreceptive Organs of Birds

Abstract

A mammalian type opsin 5 (neuropsin) is a recently identified ultraviolet (UV)-sensitive pigment of the retina and other photosensitive organs in birds. Two other opsin 5-related molecules have been found in the genomes of non-mammalian vertebrates. However, their functions have not been examined as yet. Here, we identify the molecular properties of a second avian opsin 5, cOpn5L2 (chicken opsin 5-like 2), and its localization in the post-hatch chicken. Spectrophotometric analysis and radionucleotide-binding assay have revealed that cOpn5L2 is a UV-sensitive bistable pigment that couples with the Gi subtype of guanine nucleotide-binding protein (G protein). As a bistable pigment, it also shows the direct binding ability to agonist all-trans-retinal to activate G protein. The absorption maxima of UV-light-absorbing and visible light-absorbing forms were 350 and 521 nm, respectively. Expression analysis showed relatively high expression of cOpn5L2 mRNA in the adrenal gland, which is not photoreceptive but an endocrine organ, while lower expression was found in the brain and retina. At the protein level, cOpn5L2 immunoreactive cells were present in the chromaffin cells of the adrenal gland. In the brain, cOpn5L2 immunoreactive cells were found in the paraventricular and supraoptic nuclei of the anterior hypothalamus, known for photoreceptive deep brain areas. In the retina, cOpn5L2 protein was localized to subsets of cells in the ganglion cell layer and the inner nuclear layer. These results suggest that the non-mammalian type opsin 5 (Opn5L2) functions as a second UV sensor in the photoreceptive organs, while it might function as chemosensor using its direct binding ability to agonist all-trans-retinal in non-photoreceptive organs such as the adrenal gland of birds.

Evolution of Mammalian Opn5 as a Specialized UV-absorbing Pigment by a Single Amino Acid Mutation

“…In our previous reports, we succeeded in the purification of recombinant Opn5 proteins obtained by the expression of chicken Opn5 genes and reported that these proteins are UVsensitive bistable pigments that activate the G i type of G protein (4,7). We also speculated from the complete conservation of the amino acid residues surrounding the chromophore between chicken and mammalian Opn5m proteins that mammalian Opn5m proteins should be UV-sensitive pigments, which was recently confirmed in mouse and human Opn5m proteins (8).…”

“…Opn5 is the most recently identified opsin in the human and mouse genomes (3) and is categorized in an independent group on the basis of its low sequence homology to the other opsin groups. Opn5 genes have been identified in various vertebrates from fishes to primates and are classified into several subgroups (4,5). Most mammals have only one Opn5 gene (Opn5m), whereas nonmammalian vertebrates have additional Opn5 genes.…”

Neuropsin (OPN5)-mediated photoentrainment of local circadian oscillators in mammalian retina and cornea

“…The function of OPN3 in mammals is unknown despite its widespread expression in neural tissues (18). OPN5 appears to be a deep-brain photopigment in the hypothalamus of birds and is thought to contribute to seasonal reproduction (19)(20)(21)(22); it has been immunolocalized to the mammalian inner retina (13, 16) (SI Text); however, to date, no retinal function for this mammalian pigment has been identified. We did not examine two other pigments, retinal G protein-coupled receptor (RGR) opsin (23) and peropsin (RRH) (24).…”

Exploring avian deep-brain photoreceptors and their role in activating the neuroendocrine regulation of gonadal development

“…To date, however, which sites are critical and how they communicate among themselves remain unknown. Each of the 4 neural loci proposed to house DBP is in different brain structures, and 3 distinct photopigments have been discovered and thought involved in the reception of photoperiodic information: melanopsin/Opn4 (Chaurasia et al, 2005;Kang et al, 2010), neuropsin/Opn5 Ohuchi et al, 2012), and vertebrate ancient opsin (VAOpn; Foster et al, 1985Foster et al, , 1994Halford et al, 2009;Davies et al, 2012).…”

“…Brains were removed rapidly, frozen, and the septal-hypothalamic regions were dissected into 3 regions: (R1) the septal, preoptic, anterior hypothalamic (SepPre/Ant-Hypo) region including the LSO and the lateral bed nucleus of the stria terminalis (BSTL; Figure 2B); (R2) the mid-hypothalamic (Mid-Hypo) region containing the PVN and a major group of GnRH-1 neurons surrounding the nucleus of the hippocampal commissure ( Figure 2B); (R3) the posterior hypothalamic (PostHypo) region containing the PMM and PVO, PT, and median eminence, Figure 2B). The reason for making the 3 dissections is that the first region includes the LSO and lateral septum with scattered neurons containing Opn4 (Chaurasia et al, 2005) and the BSTL with Opn5 neurons (Ohuchi et al, 2012). The MidHypo contains VAOpn (Halford et al, 2009) neurons in the PVN and medial bed nucleus of the stria terminalis.…”