Heterologous expression of limulus rhodopsin

Vitamin A deficiency and the retinal "double carrot" sign with optical coherence tomography

BACKGROUND

Spectral-domain optical coherence tomography (SD-OCT) and full-field electroretinography (ERG) allow retinal assessment with vitamin A deficiency (VAD). Using SD-OCT, this study aimed to characterize and follow a novel retinal abnormality in patients with VAD and intramuscular supplementation.

METHODS

Patients with VAD were retrospectively reviewed, including SD-OCT and electroretinography.

RESULTS

Three patients had VAD following bariatric or colon surgery and varying supplementation. All had nyctalopia, extinguished scotopic rod-specific function with ERG, and decreased serum vitamin A. None demonstrated surface abnormalities. All received intramuscular vitamin A with subjective resolution of symptoms. On SD-OCT, four of six eyes exhibited homogenous foveal hyperreflectivity anterior to retinal pigment epithelium-Bruch complex, reminiscent of a "double carrot", which improved following supplementation. ERG findings demonstrated improved scotopic rod-specific function in all cases; however, photopic function remained diminished in two cases.

CONCLUSIONS

Structural improvement of the proposed "double carrot" sign occurs soon after vitamin A supplementation. While scotopic function improves rapidly following supplementation, cone function recovers more slowly. Therefore, foveal changes such as the "double carrot" sign suggest that structural recovery of cones precedes functional recovery.

The diversity of invertebrate visual opsins spanning Protostomia, Deuterostomia, and Cnidaria

Across eumetazoans, the ability to perceive and respond to visual stimuli is largely mediated by opsins, a family of proteins belonging to the G protein-coupled receptor (GPCR) superclass. Lineage-specific gains and losses led to a striking diversity in the numbers, types, and spectral sensitivities conferred by visual opsin gene expression. Here, we review the diversity of visual opsins and differences in opsin gene expression from well-studied protostome, invertebrate deuterostome, and cnidarian groups. We discuss the functional significance of opsin expression differences and spectral tuning among lineages. In some cases, opsin evolution has been linked to the detection of relevant visual signals, including sexually selected color traits and host plant features. In other instances, variation in opsins has not been directly linked to functional or ecological differences. Overall, the array of opsin expression patterns and sensitivities across invertebrate lineages highlight the diversity of opsins in the eumetazoan ancestor and the labile nature of opsins over evolutionary time.

Molecular advances to study the function, evolution and spectral tuning of arthropod visual opsins

Visual opsins of vertebrates and invertebrates diversified independently and converged to detect ultraviolet to long wavelengths (LW) of green or red light. In both groups, colour vision largely derives from opsin number, expression patterns and changes in amino acids interacting with the chromophore. Functional insights regarding invertebrate opsin evolution have lagged behind those for vertebrates because of the disparity in genomic resources and the lack of robust in vitro systems to characterize spectral sensitivities. Here, we review bioinformatic approaches to identify and model functional variation in opsins as well as recently developed assays to measure spectral phenotypes. In particular, we discuss how transgenic lines, cAMP-spectroscopy and sensitive heterologous expression platforms are starting to decouple genotype-phenotype relationships of LW opsins to complement the classical physiological-behavioural-phylogenetic toolbox of invertebrate visual sensory studies. We illustrate the use of one heterologous method by characterizing novel LW Gq opsins from 10 species, including diurnal and nocturnal Lepidoptera, a terrestrial dragonfly and an aquatic crustacean, expressing them in HEK293T cells, and showing that their maximum absorbance spectra (λmax) range from 518 to 611 nm. We discuss the advantages of molecular approaches for arthropods with complications such as restricted availability, lateral filters, specialized photochemistry and/or electrophysiological constraints. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Light control of G protein signaling pathways by a novel photopigment

Channelopsins and photo-regulated ion channels make it possible to use light to control electrical activity of cells. This powerful approach has lead to a veritable explosion of applications, though it is limited to changing membrane voltage of the target cells. An enormous potential could be tapped if similar opto-genetic techniques could be extended to the control of chemical signaling pathways. Photopigments from invertebrate photoreceptors are an obvious choice—as they do not bleach upon illumination -however, their functional expression has been problematic. We exploited an unusual opsin, pScop2, recently identified in ciliary photoreceptors of scallop. Phylogenetically, it is closer to vertebrate opsins, and offers the advantage of being a bi-stable photopigment. We inserted its coding sequence and a fluorescent protein reporter into plasmid vectors and demonstrated heterologous expression in various mammalian cell lines. HEK 293 cells were selected as a heterologous system for functional analysis, because wild type cells displayed the largest currents in response to the G-protein activator, GTP-γ-S. A line of HEK cells stably transfected with pScop2 was generated; after reconstitution of the photopigment with retinal, light responses were obtained in some cells, albeit of modest amplitude. In native photoreceptors pScop2 couples to G_o; HEK cells express poorly this G-protein, but have a prominent Gq/PLC pathway linked to internal Ca mobilization. To enhance pScop2 competence to tap into this pathway, we swapped its third intracellular loop—important to confer specificity of interaction between 7TMDRs and G-proteins—with that of a G_q-linked opsin which we cloned from microvillar photoreceptors present in the same retina. The chimeric construct was evaluated by a Ca fluorescence assay, and was shown to mediate a robust mobilization of internal calcium in response to illumination. The results project pScop2 as a potentially powerful optogenetic tool to control signaling pathways.

Drosophila Rhodopsin 7 can partially replace the structural role of Rhodopsin 1, but not its physiological function

Rhodopsin 7 (Rh7), a new invertebrate Rhodopsin gene, was discovered in the genome of Drosophila melanogaster in 2000 and thought to encode for a functional Rhodopsin protein. Indeed, Rh7 exhibits most hallmarks of the known Rhodopsins, except for the G-protein-activating QAKK motif in the third cytoplasmic loop that is absent in Rh7. Here, we show that Rh7 can partially substitute Rh1 in the outer receptor cells (R1–6) for rhabdomere maintenance, but that it cannot activate the phototransduction cascade in these cells. This speaks against a role of Rh7 as photopigment in R1–6, but does not exclude that it works in the inner photoreceptor cells.

Ectopic Expression of Mouse Melanopsin in Drosophila Photoreceptors Reveals Fast Response Kinetics and Persistent Dark Excitation

The intrinsically photosensitive M1 retinal ganglion cells (ipRGC) initiate non-image-forming light-dependent activities and express the melanopsin (OPN4) photopigment. Several features of ipRGC photosensitivity are characteristic of fly photoreceptors. However, the light response kinetics of ipRGC is much slower due to unknown reasons. Here we used transgenic Drosophila, in which the mouse OPN4 replaced the native Rh1 photopigment of Drosophila R1-6 photoreceptors, resulting in deformed rhabdomeric structure. Immunocytochemistry revealed OPN4 expression at the base of the rhabdomeres, mainly at the rhabdomeral stalk. Measurements of the early receptor current, a linear manifestation of photopigment activation, indicated large expression of OPN4 in the plasma membrane. Comparing the early receptor current amplitude and action spectra between WT and the Opn4-expressing Drosophila further indicated that large quantities of a blue absorbing photopigment were expressed, having a dark stable blue intermediate state. Strikingly, the light-induced current of the Opn4-expressing fly photoreceptors was ∼40-fold faster than that of ipRGC. Furthermore, an intense white flash induced a small amplitude prolonged dark current composed of discrete unitary currents similar to the Drosophila single photon responses. The induction of prolonged dark currents by intense blue light could be suppressed by a following intense green light, suggesting induction and suppression of prolonged depolarizing afterpotential. This is the first demonstration of heterologous functional expression of mammalian OPN4 in the genetically emendable Drosophila photoreceptors. Moreover, the fast OPN4-activated ionic current of Drosophila photoreceptors relative to that of mouse ipRGC, indicates that the slow light response of ipRGC does not arise from an intrinsic property of melanopsin.

Structural differences and differential expression among rhabdomeric opsins reveal functional change after gene duplication in the bay scallop, Argopecten irradians (Pectinidae)

Background

Opsins are the only class of proteins used for light perception in image-forming eyes. Gene duplication and subsequent functional divergence of opsins have played an important role in expanding photoreceptive capabilities of organisms by altering what wavelengths of light are absorbed by photoreceptors (spectral tuning). However, new opsin copies may also acquire novel function or subdivide ancestral functions through changes to temporal, spatial or the level of gene expression. Here, we test how opsin gene copies diversify in function and evolutionary fate by characterizing four rhabdomeric (G_q-protein coupled) opsins in the scallop, Argopecten irradians, identified from tissue-specific transcriptomes.

Results

Under a phylogenetic analysis, we recovered a pattern consistent with two rounds of duplication that generated the genetic diversity of scallop G_q-opsins. We found strong support for differential expression of paralogous G_q-opsins across ocular and extra-ocular photosensitive tissues, suggesting that scallop G_q-opsins are used in different biological contexts due to molecular alternations outside and within the protein-coding regions. Finally, we used available protein models to predict which amino acid residues interact with the light-absorbing chromophore. Variation in these residues suggests that the four G_q-opsin paralogs absorb different wavelengths of light.

Conclusions

Our results uncover novel genetic and functional diversity in the light-sensing structures of the scallop, demonstrating the complicated nature of G_q-opsin diversification after gene duplication. Our results highlight a change in the nearly ubiquitous shadow response in molluscs to a narrowed functional specificity for visual processes in the eyed scallop. Our findings provide a starting point to study how gene duplication may coincide with eye evolution, and more specifically, different ways neofunctionalization of G_q-opsins may occur.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0823-9) contains supplementary material, which is available to authorized users.

Determination of Photoreceptor Cell Spectral Sensitivity in an Insect Model from In Vivo Intracellular Recordings

Intracellular recording is a powerful technique used to determine how a single cell may respond to a given stimulus. In vision research, intracellular recording has historically been a common technique used to study sensitivities of individual photoreceptor cells to different light stimuli that is still being used today. However, there remains a dearth of detailed methodology in the literature for researchers wishing to replicate intracellular recording experiments in the eye. Here we present the insect as a model for examining eye physiology more generally. Insect photoreceptor cells are located near the surface of the eye and are therefore easy to reach, and many of the mechanisms involved in vision are conserved across animal phyla. We describe the basic procedure for in vivo intracellular recording of photoreceptor cells in the eye of a butterfly, with the goal of making this technique more accessible to researchers with little prior experience in electrophysiology. We introduce the basic equipment needed, how to prepare a live butterfly for recording, how to insert a glass microelectrode into a single cell, and finally the recording procedure itself. We also explain the basic analysis of raw response data for determining spectral sensitivity of individual cell types. Although our protocol focuses on determining spectral sensitivity, other stimuli (e.g., polarized light) and variations of the method are applicable to this setup.

Spectral Tuning of Phototaxis by a Go-Opsin in the Rhabdomeric Eyes of Platynereis

Phototaxis is characteristic of the pelagic larval stage of most bottom-dwelling marine invertebrates. Larval phototaxis is mediated by simple eyes that can express various types of light-sensitive G-protein-coupled receptors known as opsins. Since opsins diversified early during metazoan evolution in the marine environment, understanding underwater light detection could elucidate this diversification. Opsins have been classified into three major families, the r-opsins, the c-opsins, and the Go/RGR opsins, a family uniting Go-opsins, retinochromes, RGR opsins, and neuropsins. The Go-opsins form an ancient and poorly characterized group retained only in marine invertebrate genomes. Here, we characterize a Go-opsin from the marine annelid Platynereis dumerilii. We found Go-opsin1 coexpressed with two r-opsins in depolarizing rhabdomeric photoreceptor cells in the pigmented eyes of Platynereis larvae. We purified recombinant Go-opsin1 and found that it absorbs in the blue-cyan range of the light spectrum. To characterize the function of Go-opsin1, we generated a Go-opsin1 knockout Platynereis line by zinc-finger-nuclease-mediated genome engineering. Go-opsin1 knockout larvae were phototactic but showed reduced efficiency of phototaxis to wavelengths matching the in vitro Go-opsin1 spectrum. Our results highlight spectral tuning of phototaxis as a potential mechanism contributing to opsin diversity.

Opsin clines in butterflies suggest novel roles for insect photopigments

Opsins are ancient molecules that enable animal vision by coupling to a vitamin-derived chromophore to form light-sensitive photopigments. The primary drivers of evolutionary diversification in opsins are thought to be visual tasks related to spectral sensitivity and color vision. Typically, only a few opsin amino acid sites affect photopigment spectral sensitivity. We show that opsin genes of the North American butterfly Limenitis arthemis have diversified along a latitudinal cline, consistent with natural selection due to environmental factors. We sequenced single nucleotide (SNP) polymorphisms in the coding regions of the ultraviolet (UVRh), blue (BRh), and long-wavelength (LWRh) opsin genes from ten butterfly populations along the eastern United States and found that a majority of opsin SNPs showed significant clinal variation. Outlier detection and analysis of molecular variance indicated that many SNPs are under balancing selection and show significant population structure. This contrasts with what we found by analysing SNPs in the wingless and EF-1 alpha loci, and from neutral amplified fragment length polymorphisms, which show no evidence of significant locus-specific or genome-wide structure among populations. Using a combination of functional genetic and physiological approaches, including expression in cell culture, transgenic Drosophila, UV-visible spectroscopy, and optophysiology, we show that key BRh opsin SNPs that vary clinally have almost no effect on spectral sensitivity. Our results suggest that opsin diversification in this butterfly is more consistent with natural selection unrelated to spectral tuning. Some of the clinally varying SNPs may instead play a role in regulating opsin gene expression levels or the thermostability of the opsin protein. Lastly, we discuss the possibility that insect opsins might have important, yet-to-be elucidated, adaptive functions in mediating animal responses to abiotic factors, such as temperature or photoperiod.

Opsin expression in Limulus eyes: a UV opsin is expressed in each eye type and co-expressed with a visible light-sensitive opsin in ventral larval eyes

The eyes of the horseshoe crab, Limulus polyphemus, are a model for studies of visual function and the visual systems of euarthropods. Much is known about the structure and function of L. polyphemus photoreceptors, much less about their photopigments. Three visible-light-sensitive L. polyphemus opsins were characterized previously (LpOps1, 2 and 5). Here we characterize a UV opsin (LpUVOps1) that is expressed in all three types of L. polyphemus eyes. It is expressed in most photoreceptors in median ocelli, the only L. polyphemus eyes in which UV sensitivity was previously detected, and in the dendrite of eccentric cells in lateral compound eyes. Therefore, eccentric cells, previously thought to be non-photosensitive second-order neurons, may actually be UV-sensitive photoreceptors. LpUVOps1 is also expressed in small photoreceptors in L. polyphemus ventral larval eyes, and intracellular recordings from these photoreceptors confirm that LpUVOps1 is an active, UV-sensitive photopigment. These photoreceptors also express LpOps5, which we demonstrate is an active, long-wavelength-sensitive photopigment. Thus small photoreceptors in ventral larval eyes, and probably those of the other larval eyes, have dual sensitivity to UV and visible light. Interestingly, the spectral tuning of small ventral photoreceptors may change day to night, because the level of LpOps5 in their rhabdoms is lower during the day than during the night, whereas LpUVOps1 levels show no diurnal change. These and previous findings show that opsin co-expression and the differential regulation of co-expressed opsins in rhabdoms is a common feature of L. polyphemus photoreceptors.

Rhodopsin coexpression in UV photoreceptors of Aedes aegypti and Anopheles gambiae mosquitoes

Differential rhodopsin gene expression within specialized R7 photoreceptor cells divides the retinas of Aedes aegypti and Anopheles gambiae mosquitoes into distinct domains. The two species express the rhodopsin orthologs Aaop8 and Agop8, respectively, in a large subset of these R7 photoreceptors that function as ultraviolet receptors. We show here that a divergent subfamily of mosquito rhodopsins, Aaop10 and Agop10, is coexpressed in these R7 photoreceptors. The properties of the A. aegypti Aaop8 and Aaop10 rhodopsins were analyzed by creating transgenic Drosophila expressing these rhodopsins. Electroretinogram recordings, and spectral analysis of head extracts, obtained from the Aaop8 strain confirmed that Aaop8 is an ultraviolet-sensitive rhodopsin. Aaop10 was poorly expressed and capable of eliciting only small and slow light responses in Drosophila photoreceptors, and electroretinogram analysis suggested that it is a long-wavelength rhodopsin with a maximal sensitivity near 500 nm. Thus, coexpression of Aaop10 rhodopsin with Aaop8 rhodopsin has the potential to modify the spectral properties of mosquito ultraviolet receptors. Retention of Op10 rhodopsin family members in the genomes of Drosophila species suggests that this rhodopsin family may play a conserved role in insect vision.

Synthetic biology of phenotypic adaptation in vertebrates: the next frontier

For over the last 2 decades, positively selected amino acid sites have been inferred almost exclusively by showing that the number of nonsynonymous substitutions per nonsynonymous site (dn) is greater than that of synonymous substitutions per synonymous site (ds). However, virtually none of these statistical results have been experimentally tested and remain as hypotheses. To perform such experimental tests, we must connect genotype and phenotype and relate the phenotypic changes to the environmental and behavioral changes of the organism. The genotype-phenotype relationship can be established only by synthesizing and manipulating "proper" ancestral phenotypes, whereas the actual functions of adaptive mutations can be learned by studying their chemical roles in phenotypic changes.

Coexpression of spectrally distinct rhodopsins in Aedes aegypti R7 photoreceptors

The retina of the mosquito Aedes aegypti can be divided into four regions based on the non-overlapping expression of a UV sensitive Aaop8 rhodopsin and a long wavelength sensitive Aaop2 type rhodopsin in the R7 photoreceptors. We show here that another rhodopsin, Aaop9, is expressed in all R7 photoreceptors and a subset of R8 photoreceptors. In the dorsal region, Aaop9 is expressed in both the cell body and rhabdomere of R7 and R8 cells. In other retinal regions Aaop9 is expressed only in R7 cells, being localized to the R7 rhabdomere in the central and ventral regions and in both the cell body and rhabdomere within the ventral stripe. Within the dorsal-central transition area ommatidia do not show a strict pairing of R7–R8 cell types. Thus, Aaop9 is coexpressed in the two classes of R7 photoreceptors previously distinguished by the non-overlapping expression of Aaop8 and Aaop2 rhodopsins. Electroretinogram analysis of transgenic Drosophila shows that Aaop9 is a short wavelength rhodopsin with an optimal response to 400–450 nm light. The coexpressed Aaop2 rhodopsin has dual wavelength sensitivity of 500–550 nm and near 350 nm in the UV region. As predicted by the spectral properties of each rhodopsin, Drosophila photoreceptors expressing both Aaop9 and Aaop2 rhodopsins exhibit a uniform sensitivity across the broad 350–550 nm light range. We propose that rhodopsin coexpression is an adaptation within the R7 cells to improve visual function in the low-light environments in which Ae. aegypti is active.

Dynamin- and Rab5-dependent endocytosis is required to prevent Drosophila photoreceptor degeneration

In Drosophila photoreceptors, Rhodopsin 1 (ninaE, Rh1) is required for proper morphogenesis and maintenance of the apical light-gathering organelle, the rhabdomere. It has been proposed that Rh1, coupled to the Rho GTPases Rac1 and Cdc42, promotes the morphogenesis of a sub-rhabdomeric F-actin meshwork or rhabdomere terminal web (RTW). The RTW provides mechanical support to the apical microvilli and is likely to guide Rab11-dependent delivery of Rh1-rich membrane to the rhabdomere from the trans Golgi network. However, the nature and function of the molecular pathway involved in RTW morphogenesis remains incomplete. Here, we show that Rh1 function in promoting RTW morphogenesis is light-independent and is conserved throughout evolution. This Rh1 function does not require G(q)α(e), which is required for phototransduction. Finally, we show that interfering with Dynamin- and Rab5-dependent endocytosis leads to a phenotype that is undistinguishable from that of the ninaE-null mutant. Importantly, the corresponding endocytic activity is essential to prevent early onset of rhabdomere degeneration. In conclusion, we propose that Rh1 function in promoting RTW morphogenesis is not only needed to sustain apical membrane delivery but is also required for proper rhabdomeric membrane endocytosis and turnover.

Opsin co-expression in Limulus photoreceptors: differential regulation by light and a circadian clock

A long-standing concept in vision science has held that a single photoreceptor expresses a single type of opsin, the protein component of visual pigment. However, the number of examples in the literature of photoreceptors from vertebrates and invertebrates that break this rule is increasing. Here, we describe a newly discovered Limulus opsin, Limulus opsin5, which is significantly different from previously characterized Limulus opsins, opsins1 and 2. We show that opsin5 is co-expressed with opsins1 and 2 in Limulus lateral and ventral eye photoreceptors and provide the first evidence that the expression of co-expressed opsins can be differentially regulated. We show that the relative levels of opsin5 and opsin1 and 2 in the rhabdom change with a diurnal rhythm and that their relative levels are also influenced by the animal's central circadian clock. An analysis of the sequence of opsin5 suggests it is sensitive to visible light (400-700 nm) but that its spectral properties may be different from that of opsins1 and 2. Changes in the relative levels of these opsins may underlie some of the dramatic day-night changes in Limulus photoreceptor function and may produce a diurnal change in their spectral sensitivity.

Diversity and functional properties of bistable pigments

Rhodopsin and related opsin-based pigments, which are photosensitive membrane proteins, have been extensively studied using a wide variety of techniques, with rhodopsin being the most understood G protein-coupled receptor (GPCR). Animals use various opsin-based pigments for vision and a wide variety of non-visual functions. Many functionally varied pigments are roughly divided into two kinds, based on their photoreaction: bistable and monostable pigments. Bistable pigments are thermally stable before and after photo-activation, but monostable pigments are stable only before activation. Here, we review the diversity of bistable pigments and their molecular characteristics. We also discuss the mechanisms underlying different molecular characteristics of bistable and monostable pigments. In addition, the potential of bistable pigments as a GPCR model is proposed.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.