Adaptation of pineal expressed teleost exo-rod opsin to non-image forming photoreception through enhanced Meta II decay

Functional diversification process of opsin genes for teleost visual and pineal photoreceptions

Most vertebrates have a rhodopsin gene with a five-exon structure for visual photoreception. By contrast, teleost fishes have an intron-less rhodopsin gene for visual photoreception and an intron-containing rhodopsin (exo-rhodopsin) gene for pineal photoreception. Here, our analysis of non-teleost and teleost fishes in various lineages of the Actinopterygii reveals that retroduplication after branching of the Polypteriformes produced the intron-less rhodopsin gene for visual photoreception, which converted the parental intron-containing rhodopsin gene into a pineal opsin in the common ancestor of the Teleostei. Additional analysis of a pineal opsin, pinopsin, shows that the pinopsin gene functions as a green-sensitive opsin together with the intron-containing rhodopsin gene for pineal photoreception in tarpon as an evolutionary intermediate state but is missing in other teleost fishes, probably because of the redundancy with the intron-containing rhodopsin gene. We propose an evolutionary scenario where unique retroduplication caused a "domino effect" on the functional diversification of teleost visual and pineal opsin genes.

Rhodopsins: An Excitingly Versatile Protein Species for Research, Development and Creative Engineering

The first member and eponym of the rhodopsin family was identified in the 1930s as the visual pigment of the rod photoreceptor cell in the animal retina. It was found to be a membrane protein, owing its photosensitivity to the presence of a covalently bound chromophoric group. This group, derived from vitamin A, was appropriately dubbed retinal. In the 1970s a microbial counterpart of this species was discovered in an archaeon, being a membrane protein also harbouring retinal as a chromophore, and named bacteriorhodopsin. Since their discovery a photogenic panorama unfolded, where up to date new members and subspecies with a variety of light-driven functionality have been added to this family. The animal branch, meanwhile categorized as type-2 rhodopsins, turned out to form a large subclass in the superfamily of G protein-coupled receptors and are essential to multiple elements of light-dependent animal sensory physiology. The microbial branch, the type-1 rhodopsins, largely function as light-driven ion pumps or channels, but also contain sensory-active and enzyme-sustaining subspecies. In this review we will follow the development of this exciting membrane protein panorama in a representative number of highlights and will present a prospect of their extraordinary future potential.

Analysis of opsin gene family of Crimson snapper (Lutjanus erythropterus)

Opsin is a fellow of the G protein-coupled receptors (GPCRs) superfamily. It can be divided into visual and non-visual opsin according to whether it is directly involved in visual imaging. Opsin plays an important role in visual image formation and the regulation of non-image forming functions such as circadian entrainment in the growth, development and evolution of fish. Crimson snapper belongs to Perciforme mainly found in the Indo-West Pacific and the South China Sea. It is one of the most influential economic fishes in the South China Sea. In order to study the existence and expression of opsin gene in Crimson snapper, we sequenced the genome and tissue sample transcriptome of Crimson snapper. In this study, 32 opsin genes were identified from the genome of Crimson snapper. The length of these genes ranged from 1061 bp to 86203 bp and were distributed on 15 different chromosomes. The analysis of opsin gene family of Crimson snapper showed that the sws2 had two extra copies as compared with that of Zebrafish. Domain and motif analysis revealed that all the 32 opsin genes have seven-(pass)-transmembrane domain receptors (7TM receptors) each, and the opsin family contained 10 common motifs. The expression level of opsin gene, confirmed by RT-qPCR, was analyzed by using nine tissues transcriptome databases of Crimson snapper. The results showed that almost all opsin genes were highly expressed in the retina and brain, except opn7a and opn7b which were expressed in intestine and red skin, and almost no expression in other tissues. Our results provide a comprehensive basic knowledge for the opsin gene family of Crimson snapper, which has significance for the study of the function of opsin in Lutjanidaes.

Evolutionary history of teleost intron-containing and intron-less rhodopsin genes

Recent progress in whole genome sequencing has revealed that animals have various kinds of opsin genes for photoreception. Among them, most opsin genes have introns in their coding regions. However, it has been known for a long time that teleost retinas express intron-less rhodopsin genes, which are presumed to have been formed by retroduplication from an ancestral intron-containing rhodopsin gene. In addition, teleosts have an intron-containing rhodopsin gene (exo-rhodopsin) exclusively for pineal photoreception. In this study, to unravel the evolutionary origin of the two teleost rhodopsin genes, we analyzed the rhodopsin genes of non-teleost fishes in the Actinopterygii. The phylogenetic analysis of full-length sequences of bichir, sturgeon and gar rhodopsins revealed that retroduplication of the rhodopsin gene occurred after branching of the bichir lineage. In addition, analysis of the tissue distribution and the molecular properties of bichir, sturgeon and gar rhodopsins showed that the abundant and exclusive expression of intron-containing rhodopsin in the pineal gland and the short lifetime of its meta II intermediate, which leads to optimization for pineal photoreception, were achieved after branching of the gar lineage. Based on these results, we propose a stepwise evolutionary model of teleost intron-containing and intron-less rhodopsin genes.

A second visual rhodopsin gene, rh1-2, is expressed in zebrafish photoreceptors and found in other ray-finned fishes

Rhodopsin (rh1) is the visual pigment expressed in rod photoreceptors of vertebrates that is responsible for initiating the critical first step of dim-light vision. Rhodopsin is usually a single copy gene; however, we previously discovered a novel rhodopsin-like gene expressed in the zebrafish retina, rh1-2, which we identified as a functional photosensitive pigment that binds 11-cis retinal and activates in response to light. Here, we localized expression of rh1-2 in the zebrafish retina to a subset of peripheral photoreceptor cells, which indicates a partially overlapping expression pattern with rh1 We also expressed, purified and characterized Rh1-2, including investigation of the stability of the biologically active intermediate. Using fluorescence spectroscopy, we found the half-life of the rate of retinal release of Rh1-2 following photoactivation to be more similar to that of the visual pigment rhodopsin than to the non-visual pigment exo-rhodopsin (exorh), which releases retinal around 5 times faster. Phylogenetic and molecular evolutionary analyses show that rh1-2 has ancient origins within teleost fishes, is under similar selective pressure to rh1, and likely experienced a burst of positive selection following its duplication and divergence from rh1 These findings indicate that rh1-2 is another functional visual rhodopsin gene, which contradicts the prevailing notion that visual rhodopsin is primarily found as a single copy gene within ray-finned fishes. The reasons for retention of this duplicate gene, as well as possible functional consequences for the visual system, are discussed.

Cysteine residue is not essential for CPM protein thermal-stability assay

A popular thermal-stability assay developed especially for the study of membrane proteins uses a thiol-specific probe, 7-diethylamino-3-(4-maleimidophenyl)-4-methylcoumarin (CPM). The fluorescence emission of CPM surges when it forms a covalent bond with the side chain of a free Cys, which becomes more readily accessible upon protein thermal denaturation. Interestingly, the melting temperatures of membrane proteins determined using the CPM assay in literature are closely clustered in the temperature range 45-55 °C. A thorough understanding of the mechanism behind the observed signal change is critical for the accurate interpretation of the protein unfolding. Here we used two α-helical membrane proteins, AqpZ and AcrB, as model systems to investigate the nature of the fluorescence surge in the CPM assay. We found that the transition temperatures measured using circular-dichroism (CD) spectroscopy and the CPM assay were significantly different. To eliminate potential artifact that might arise from the presence of detergent, we monitored the unfolding of two soluble proteins. We found that, contrary to current understanding, the presence of a sulfhydryl group was not a prerequisite for the CPM thermal-stability assay. The observed fluorescence increase is probably caused by binding of the fluorophore to hydrophobic patches exposed upon protein unfolding.

Exorhodopsin and melanopsin systems in the pineal complex and brain at early developmental stages of Atlantic halibut (Hippoglossus hippoglossus)

The complexity of the nonvisual photoreception systems in teleosts has just started to be appreciated, with colocalization of multiple photoreceptor types with unresolved functions. Here we describe an intricate expression pattern of melanopsins in early life stages of the marine flat fish Atlantic halibut (Hippoglossus hippoglossus), a period when the unpigmented brain is directly exposed to environmental photons. We show a refined and extensive expression of melanopsins in the halibut brain already at the time of hatching, long before the eyes are functional. We detect melanopsin in the habenula, suprachiasmatic nucleus, dorsal thalamus, and lateral tubular nucleus of first feeding larvae, suggesting conserved functions of the melanopsins in marine teleosts. The complex expression of melanopsins already at larval stages indicates the importance of nonvisual photoreception early in development. Most strikingly, we detect expression of both exorhodopsin and melanopsin in the pineal complex of halibut larvae. Double-fluorescence labeling showed that two clusters of melanopsin-positive cells are located lateral to the central rosette of exorhodopsin-positive cells. The localization of different photopigments in the pineal complex suggests that two parallel photoreceptor systems may be active. Furthermore, the dispersed melanopsin-positive cells in the spinal cord of halibut larvae at the time of hatching may be primary sensory cells or interneurons representing the first example of dispersed high-order photoreceptor cells. The appearance of nonvisual opsins early in the development of halibut provides an alternative model for studying the evolution and functional significance of nonvisual opsins.

Encephalic photoreception and phototactic response in the troglobiont Somalian blind cavefish Phreatichthys andruzzii

Many physiological and behavioural responses to changes in environmental lighting conditions are mediated by extraocular photoreceptors. Here we investigate encephalic photoreception in Phreatichthys andruzzii, a typical cave-dwelling fish showing an extreme phenotype with complete anophthalmy and a reduction in size of associated brain structures. We firstly identified two P. andruzzii photopigments, orthologues of rod opsin and exo-rod opsin. In vitro, both opsins serve as light-absorbing photopigments with λ(max) around 500 nm when reconstituted with an A(1) chromophore. When corrected for the summed absorption from the skin and skull, the spectral sensitivity profiles shifted to longer wavelengths (rod opsin: 521 nm; exo-rod opsin: 520 nm). We next explored the involvement of both opsins in the negative phototaxis reported for this species. A comparison of the spectral sensitivity of the photophobic response with the putative A(2) absorbance spectra corrected for skin/skull absorbance indicates that the A(2) versions of either or both of these pigments could explain the observed behavioural spectral sensitivity.