Spectral tuning of rhodopsin and metarhodopsin in vivo

Full-length transcriptome reveals the circularly polarized light response-related molecular genetic characteristics of Oratosquilla oratoria

The mantis shrimp is the only animal that can recognize circularly polarized light (CPL), but its molecular genetic characteristics are unclear. Multi-tissue level full-length (FL) transcriptome sequencing of Oratosquilla oratoria, a representative widely distributed mantis shrimp, was performed in the present study. We used comparative transcriptomics to explore the critical genes of O. oratoria selected by CPL and the GNβ gene associated with CPL signal transduction was hypothesized to be positively selected. Furthermore, the FL transcriptomes of O. oratoria compound eyes under five light conditions were sequenced and used to detect alternative splicing (AS). The ASs associated with CPL recognition mainly occurred in the LWS, ARR and TRPC regions. The number of FL transcripts with AS events and annotation information also provided evidence that O. oratoria could recognize LCPL. Additionally, 51 sequences belonging to the LWS, UV and Peropsin gene families were identified based on conserved 7tm domains. The LWS, UV and Peropsin opsins have similar 3D structures with seven domains across the cell membrane and conserved KSLRTPSN, DRY, and QAKK motifs. In conclusion, these results are undoubtedly valuable for perfecting the vision theory of O. oratoria and other mantis shrimp.

Colour vision in ants (Formicidae, Hymenoptera)

Ants are ecologically one of the most important groups of insects and exhibit impressive capabilities for visual learning and orientation. Studies on numerous ant species demonstrate that ants can learn to discriminate between different colours irrespective of light intensity and modify their behaviour accordingly. However, the findings across species are variable and inconsistent, suggesting that our understanding of colour vision in ants and what roles ecological and phylogenetic factors play is at an early stage. This review provides a brief synopsis of the critical findings of the past century of research by compiling studies that address molecular, physiological and behavioural aspects of ant colour vision. With this, we aim to improve our understanding of colour vision and to gain deeper insights into the mysterious and colourful world of ants.

This article is part of the theme issue ‘Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods’.

Endocannabinoids produced in photoreceptor cells in response to light activate Drosophila TRP channels

Drosophila phototransduction is a model for signaling cascades that culminate in the activation of transient receptor potential (TRP) cation channels. TRP and TRPL are the canonical TRP (TRPC) channels that are regulated by light stimulation of rhodopsin and engagement of Gαq and phospholipase Cβ (PLC). Lipid metabolite(s) generated downstream of PLC are essential for the activation of the TRPC channels in photoreceptor cells. We sought to identify the key lipids produced subsequent to PLC stimulation that contribute to channel activation. Here, using genetics, lipid analysis, and Ca2+ imaging, we found that light increased the amount of an abundant endocannabinoid, 2-linoleoyl glycerol (2-LG), in vivo. The increase in 2-LG amounts depended on the PLC and diacylglycerol lipase encoded by norpA and inaE, respectively. This endocannabinoid facilitated TRPC-dependent Ca2+ influx in a heterologous expression system and in dissociated ommatidia from compound eyes. Moreover, 2-LG and mechanical stimulation cooperatively activated TRPC channels in ommatidia. We propose that 2-LG is a physiologically relevant endocannabinoid that activates TRPC channels in photoreceptor cells.

Optogenetic Neuronal Silencing in Drosophila during Visual Processing

Optogenetic channels and ion pumps have become indispensable tools in neuroscience to manipulate neuronal activity and thus to establish synaptic connectivity and behavioral causality. Inhibitory channels are particularly advantageous to explore signal processing in neural circuits since they permit the functional removal of selected neurons on a trial-by-trial basis. However, applying these tools to study the visual system poses a considerable challenge because the illumination required for their activation usually also stimulates photoreceptors substantially, precluding the simultaneous probing of visual responses. Here, we explore the utility of the recently discovered anion channelrhodopsins GtACR1 and GtACR2 for application in the visual system of Drosophila. We first characterized their properties using a larval crawling assay. We further obtained whole-cell recordings from cells expressing GtACR1, which mediated strong and light-sensitive photocurrents. Finally, using physiological recordings and a behavioral readout, we demonstrate that GtACR1 enables the fast and reversible silencing of genetically targeted neurons within circuits engaged in visual processing.

Microsaccadic sampling of moving image information provides Drosophila hyperacute vision

Small fly eyes should not see fine image details. Because flies exhibit saccadic visual behaviors and their compound eyes have relatively few ommatidia (sampling points), their photoreceptors would be expected to generate blurry and coarse retinal images of the world. Here we demonstrate that Drosophila see the world far better than predicted from the classic theories. By using electrophysiological, optical and behavioral assays, we found that R1-R6 photoreceptors’ encoding capacity in time is maximized to fast high-contrast bursts, which resemble their light input during saccadic behaviors. Whilst over space, R1-R6s resolve moving objects at saccadic speeds beyond the predicted motion-blur-limit. Our results show how refractory phototransduction and rapid photomechanical photoreceptor contractions jointly sharpen retinal images of moving objects in space-time, enabling hyperacute vision, and explain how such microsaccadic information sampling exceeds the compound eyes’ optical limits. These discoveries elucidate how acuity depends upon photoreceptor function and eye movements.

Fruit flies have five eyes: two large compound eyes which support vision, plus three smaller single lens eyes which are used for navigation. Each compound eye monitors 180° of space and consists of roughly 750 units, each containing eight light-sensitive cells called photoreceptors. This relatively wide spacing of photoreceptors is thought to limit the sharpness, or acuity, of vision in fruit flies. The area of the human retina (the light-sensitive surface at back of our eyes) that generates our sharpest vision contains photoreceptors that are 500 times more densely packed.

Despite their differing designs, human and fruit fly eyes work via the same general principles. If we, or a fruit fly, were to hold our gaze completely steady, the world would gradually fade from view as the eye adapted to the unchanging visual stimulus. To ensure this does not happen, animals continuously make rapid, automatic eye movements called microsaccades. These refresh the image on the retina and prevent it from fading. Yet it is not known why do they not also cause blurred vision.

Standard accounts of vision assume that the retina and the brain perform most of the information processing required, with photoreceptors simply detecting how much light enters the eye. However, Juusola, Dau, Song et al. now challenge this idea by showing that photoreceptors are specially adapted to detect the fluctuating patterns of light that enter the eye as a result of microsaccades. Moreover, fruit fly eyes resolve small moving objects far better than would be predicted based on the spacing of their photoreceptors.

The discovery that photoreceptors are well adapted to deal with eye movements changes our understanding of insect vision. The findings also disprove the 100-year-old dogma that the spacing of photoreceptors limits the sharpness of vision in compound eyes. Further studies are required to determine whether photoreceptors in the retinas of other animals, including humans, have similar properties.

Rhodopsin 7-The unusual Rhodopsin in Drosophila

Rhodopsins are the major photopigments in the fruit fly Drosophila melanogaster. Drosophila express six well-characterized Rhodopsins (Rh1–Rh6) with distinct absorption maxima and expression pattern. In 2000, when the Drosophila genome was published, a novel Rhodopsin gene was discovered: Rhodopsin 7 (Rh7). Rh7 is highly conserved among the Drosophila genus and is also found in other arthropods. Phylogenetic trees based on protein sequences suggest that the seven Drosophila Rhodopsins cluster in three different groups. While Rh1, Rh2 and Rh6 form a “vertebrate-melanopsin-type”–cluster, and Rh3, Rh4 and Rh5 form an “insect-type”-Rhodopsin cluster, Rh7 seem to form its own cluster. Although Rh7 has nearly all important features of a functional Rhodopsin, it differs from other Rhodopsins in its genomic and structural properties, suggesting it might have an overall different role than other known Rhodopsins.

A cure for the blues: opsin duplication and subfunctionalization for short-wavelength sensitivity in jewel beetles (Coleoptera: Buprestidae)

Background

Arthropods have received much attention as a model for studying opsin evolution in invertebrates. Yet, relatively few studies have investigated the diversity of opsin proteins that underlie spectral sensitivity of the visual pigments within the diverse beetles (Insecta: Coleoptera). Previous work has demonstrated that beetles appear to lack the short-wavelength-sensitive (SWS) opsin class that typically confers sensitivity to the “blue” region of the light spectrum. However, this is contrary to established physiological data in a number of Coleoptera. To explore potential adaptations at the molecular level that may compensate for the loss of the SWS opsin, we carried out an exploration of the opsin proteins within a group of beetles (Buprestidae) where short-wave sensitivity has been demonstrated. RNA-seq data were generated to identify opsin proteins from nine taxa comprising six buprestid species (including three male/female pairs) across four subfamilies. Structural analyses of recovered opsins were conducted and compared to opsin sequences in other insects across the main opsin classes—ultraviolet, short-wavelength, and long-wavelength.

Results

All nine buprestids were found to express two opsin copies in each of the ultraviolet and long-wavelength classes, contrary to the single copies recovered in all other molecular studies of adult beetle opsin expression. No SWS opsin class was recovered. Furthermore, the male Agrilus planipennis (emerald ash borer—EAB) expressed a third LWS opsin at low levels that is presumed to be a larval copy. Subsequent homology and structural analyses identified multiple amino acid substitutions in the UVS and LWS copies that could confer short-wavelength sensitivity.

Conclusions

This work is the first to compare expressed opsin genes against known electrophysiological data that demonstrate multiple peak sensitivities in Coleoptera. We report the first instance of opsin duplication in adult beetles, which occurs in both the UVS and LWS opsin classes. Through structural comparisons of known insect opsins, we suggest that opsin duplication and amino acid variation within the chromophore binding pocket explains sensitivity in the short-wavelength portion of the visible light spectrum in these species. These findings are the first to reveal molecular complexity of the color vision system within beetles.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0674-4) contains supplementary material, which is available to authorized users.

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.

Parallel evolution of angiosperm colour signals: common evolutionary pressures linked to hymenopteran vision

Flowering plants in Australia have been geographically isolated for more than 34 million years. In the Northern Hemisphere, previous work has revealed a close fit between the optimal discrimination capabilities of hymenopteran pollinators and the flower colours that have most frequently evolved. We collected spectral data from 111 Australian native flowers and tested signal appearance considering the colour discrimination capabilities of potentially important pollinators. The highest frequency of flower reflectance curves is consistent with data reported for the Northern Hemisphere. The subsequent mapping of Australian flower reflectances into a bee colour space reveals a very similar distribution of flower colour evolution to the Northern Hemisphere. Thus, flowering plants in Australia are likely to have independently evolved spectral signals that maximize colour discrimination by hymenoptera. Moreover, we found that the degree of variability in flower coloration for particular angiosperm species matched the range of reflectance colours that can only be discriminated by bees that have experienced differential conditioning. This observation suggests a requirement for plasticity in the nervous systems of pollinators to allow generalization of flowers of the same species while overcoming the possible presence of non-rewarding flower mimics.

Multiple spectral inputs improve motion discrimination in the Drosophila visual system

Color and motion information are thought to be channeled through separate neural pathways, but it remains unclear whether and how these pathways interact to improve motion perception. In insects, such as Drosophila, it has long been believed that motion information is fed exclusively by one spectral class of photoreceptor, so-called R1 to R6 cells; whereas R7 and R8 photoreceptors, which exist in multiple spectral classes, subserve color vision. Here, we report that R7 and R8 also contribute to the motion pathway. By using electrophysiological, optical, and behavioral assays, we found that R7/R8 information converge with and shape the motion pathway output, explaining flies' broadly tuned optomotor behavior by its composite responses. Our results demonstrate that inputs from photoreceptors of different spectral sensitivities improve motion discrimination, increasing robustness of perception.

From Drosophila to humans: reflections on the roles of the prolyl isomerases and chaperones, cyclophilins, in cell function and disease

Despite remarkable advances in human genetics and other genetic model systems, the fruit fly, Drosophila melanogaster, remains a powerful experimental tool to probe with ease the inner workings of a myriad of biological and pathological processes, even when evolutionary forces impart apparent divergences to some of such processes. The understanding of such evolutionary differences provides mechanistic insights into genotype-phenotype correlations underpinning biological processes across metazoans. The pioneering work developed by the William Pak laboratory for the past four decades, and the work of others, epitomize the notion of how the Drosophila system breaks new fertile ground or complements research fields of high scientific and medical relevance. Among the three major genetic complementation groups produced by the Pak's laboratory and impairing distinct facets of photoreceptor neuronal function, the nina group (ninaA, …., ninaJ) selectively affects the biogenesis of G protein-coupled receptors (GPCRs), mediating the photoconversion and transduction of light stimuli. Among the nina genes identified, ninaA arguably assumes heightened significance for several reasons. First, it presents unique physiological selectivity toward the biogenesis of a subset of GPCRs, a standalone biological manifestation yet to be discerned for most mammalian homologues of NinaA. Second, NinaA belongs to a family of proteins, immunophilins, which are the primary targets for immunosuppressive drugs at the therapeutic forefront of a multitude of medical conditions. Third, NinaA closest homologue, cyclophilin B (CyPB/PPIB), is an immunophilin whose loss-of-function was found recently to cause osteogenesis imperfecta in the human. This report highlights advances made by studies on some members of immunophilins, the cyclophilins. Finally, it reexamines critically data and dogmas derived from past and recent genetic, structural, biological, and pathological studies on NinaA and few other cyclophilins that support some of such paradigms to be less than definite and advance our understanding of the roles of cyclophilins in cell function, disease, and therapeutic interventions.

Edge detection depends on achromatic channel in Drosophila melanogaster

Edges represent important information in object recognition, and thus edge detection is crucial for animal survival. Various types of edges result from visual contrast, such as luminance contrast and color contrast. Thus far, the molecular and neural mechanisms underlying edge detection and the relationship between different edge information-processing pathways have largely been undemonstrated. In the present study, using a color-light-emitting diode (LED)-based Buridan's paradigm, we demonstrated that a blue/green demarcation is able to generate edge-orientation behavior in the adult fly. There is a blue/green intensity ratio, the so-called point of equal-luminance (POE), at which wild-type flies did not show obvious orientation behavior towards edges. This suggests that orientation behavior towards edges is dependent on luminance contrast in Drosophila. The results of mutants ninaE17 and sevLY3;rh52;rh61 demonstrated that achromatic R1-R6 photoreceptor cells, but not chromatic R7/R8 photoreceptor cells, were necessary for orientation behavior towards edges. Moreover, ectopic expression of Rh4, Rh5 or Rh6 could efficiently restore the edge-orientation defect in the ninaE17 mutant. Altogether, our results show that R1-R6 photoreceptor cells are both necessary and sufficient for orientation behavior towards edges in Drosophila.

Molecular genetics of retinal degeneration: A Drosophila perspective

Inherited retinal degeneration in Drosophila has been explored for insights into similar processes in humans. Based on the mechanisms, I divide these mutations in Drosophila into three classes. The first consists of genes that control the specialization of photoreceptor cells including the morphogenesis of visual organelles  (rhabdomeres) that house the visual signaling proteins. The second class contains genes that regulate the activity or level of the major rhodopsin, Rh1, which is the light sensor and also provides a structural role for the maintenance of rhabdomeres. Some mutations in Rh1 (NinaE) are dominant due to constitutive activity or folding defects, like autosomal dominant retinitis pigmentosa (ADRP) in humans. The third class consists of genes that control the Ca ( 2+) influx directly or indirectly by promoting the turnover of the second messenger and regeneration of PIP 2, or mediate the Ca ( 2+) -dependent regulation of the visual response. These gene products are critical for the increase in cytosolic Ca ( 2+ ) following light stimulation to initiate negative regulatory events. Here I will focus on the signaling mechanisms underlying the degeneration in norpA, and in ADRP-type NinaE mutants that produce misfolded Rh1. Accumulation of misfolded Rh1 in the ER triggers the unfolded protein response (UPR), while endosomal accumulation of activated Rh1 may initiate autophagy in norpA. Both autophagy and the UPR are beneficial for relieving defective endosomal trafficking and the ER stress, respectively. However, when photoreceptors fail to cope with the persistence of these stresses, a cell death program is activated leading to retinal degeneration.

Peripheral visual circuits functionally segregate motion and phototaxis behaviors in the fly

Like the mammalian visual cortex, the fly visual system is organized into retinotopic columns. A widely accepted biophysical model for computing visual motion, the elementary motion detector proposed nearly 50 years ago posits a temporal correlation of spatially separated visual inputs implemented across neighboring retinotopic visual columns. Whereas the inputs are defined, the neural substrate for motion computation remains enigmatic. Indeed, it is not known where in the visual processing hierarchy the computation occurs. Here, we combine genetic manipulations with a novel high-throughput dynamic behavioral analysis system to dissect visual circuits required for directional optomotor responses. An enhancer trap screen of synapse-inactivated neural circuits revealed one particularly striking phenotype, which is completely insensitive to motion yet displays fully intact fast phototaxis, indicating that these animals are generally capable of seeing and walking but are unable to respond to motion stimuli. The enhancer circuit is localized within the first optic relay and strongly labels the only columnar interneuron known to interact with neighboring columns both in the lamina and medulla, spatial synaptic interactions that correspond with the two dominant axes of elementary motion detectors on the retinal lattice.

Molecular characterization and expression of the UV opsin in bumblebees: three ommatidial subtypes in the retina and a new photoreceptor organ in the lamina

Ultraviolet-sensitive photoreceptors have been shown to be important for a variety of visual tasks performed by bees, such as orientation, color and polarization vision, yet little is known about their spatial distribution in the compound eye or optic lobe. We cloned and sequenced a UV opsin mRNA transcript from Bombus impatiens head-specific cDNA and, using western blot analysis, detected an eye protein band of approximately 41 kDa, corresponding to the predicted molecular mass of the encoded opsin. We then characterized UV opsin expression in the retina, ocelli and brain using immunocytochemistry. In the main retina, we found three different ommatidial types with respect to the number of UV opsin-expressing photoreceptor cells, namely ommatidia containing two, one or no UV opsin-immunoreactive cells. We also observed UV opsin expression in the ocelli. These results indicate that the cloned opsin probably encodes the P350 nm pigment, which was previously characterized by physiological recordings. Surprisingly, in addition to expression in the retina and ocelli, we found opsin expression in different parts of the brain. UV opsin immunoreactivity was detected in the proximal rim of the lamina adjacent to the first optic chiasm, which is where studies in other insects have found expression of proteins involved in the circadian clock, period and cryptochrome. We also found UV opsin immunoreactivity in the core region of the antennal lobe glomeruli and different clusters of perikarya within the protocerebrum, indicating a putative function of these brain regions, together with the lamina organ, in the entrainment of circadian rhythms. In order to test for a possible overlap of clock protein and UV opsin spatial expression, we also examined the expression of the period protein in these regions.

Molecular basis for ultraviolet vision in invertebrates

Invertebrates are sensitive to a broad spectrum of light that ranges from UV to red. Color sensitivity in the UV plays an important role in foraging, navigation, and mate selection in both flying and terrestrial invertebrate animals. Here, we show that a single amino acid polymorphism is responsible for invertebrate UV vision. This residue (UV: lysine vs blue:asparagine or glutamate) corresponds to amino acid position glycine 90 (G90) in bovine rhodopsin, a site affected in autosomal dominant human congenital night blindness. Introduction of the positively charged lysine in invertebrates is likely to deprotonate the Schiff base chromophore and produce an UV visual pigment. This same position is responsible for regulating UV versus blue sensitivity in several bird species, suggesting that UV vision has arisen independently in invertebrate and vertebrate lineages by a similar molecular mechanism.

Heterologous expression of limulus rhodopsin

Invertebrates such as Drosophila or Limulus assemble their visual pigment into the specialized rhabdomeric membranes of photoreceptors where phototransduction occurs. We have investigated the biosynthesis of rhodopsin from the Limulus lateral eye with three cell culture expression systems: mammalian COS1 cells, insect Sf9 cells, and amphibian Xenopus oocytes. We extracted and affinity-purified epitope-tagged Limulus rhodopsin expressed from a cDNA or cRNA from these systems. We found that all three culture systems could efficiently synthesize the opsin polypeptide in quantities comparable with that found for bovine opsin. However, none of the systems expressed a protein that stably bound 11-cis-retinal. The protein expressed in COS1 and Sf9 cells appeared to be misfolded, improperly localized, and proteolytically degraded. Similarly, Xenopus oocytes injected with Limulus opsin cRNA did not evoke light-sensitive currents after incubation with 11-cis-retinal. However, injecting Xenopus oocytes with mRNA from Limulus lateral eyes yielded light-dependent conductance changes after incubation with 11-cis-retinal. Also, expressing Limulus opsin cDNA in the R1-R6 photoreceptors of transgenic Drosophila yielded a visual pigment that bound retinal, had normal spectral properties, and coupled to the endogenous phototransduction cascade. These results indicate that Limulus opsin may require one or more photoreceptor-specific proteins for correct folding and/or chromophore binding. This may be a general property of invertebrate opsins and may underlie some of the functional differences between invertebrate and vertebrate visual pigments.

Functional diversification of lepidopteran opsins following gene duplication

A comparative approach was taken for identifying amino acid substitutions that may be under positive Darwinian selection and are correlated with spectral shifts among orthologous and paralogous lepidopteran long wavelength-sensitive (LW) opsins. Four novel LW opsin fragments were isolated, cloned, and sequenced from eye-specific cDNAs from two butterflies, Vanessa cardui (Nymphalidae) and Precis coenia (Nymphalidae), and two moths, Spodoptera exigua (Noctuidae) and Galleria mellonella (Pyralidae). These opsins were sampled because they encode visual pigments having a naturally occurring range of lambda(max) values (510-530 nm), which in combination with previously characterized lepidopteran opsins, provide a complete range of known spectral sensitivities (510-575 nm) among lepidopteran LW opsins. Two recent opsin gene duplication events were found within the papilionid but not within the nymphalid butterfly families through neighbor-joining, maximum parsimony, and maximum likelihood phylogenetic analyses of 13 lepidopteran opsin sequences. An elevated rate of evolution was detected in the red-shifted Papilio Rh3 branch following gene duplication, because of an increase in the amino acid substitution rate in the transmembrane domain of the protein, a region that forms the chromophore-binding pocket of the visual pigment. A maximum likelihood approach was used to estimate omega, the ratio of nonsynonymous to synonymous substitutions per site. Branch-specific tests of selection (free-ratio) identified one branch with omega = 2.1044, but the small number of substitutions involved was not significantly different from the expected number of changes under the neutral expectation of omega = 1. Ancestral sequences were reconstructed with a high degree of certainty from these data. Reconstructed ancestral sequences revealed several instances of convergence to the same amino acid between butterfly and vertebrate cone pigments, and between independent branches of the butterfly opsin tree that are correlated with spectral shifts.

A conserved regulatory element present in all Drosophila rhodopsin genes mediates Pax6 functions and participates in the fine-tuning of cell-specific expression

The Drosophila rhodopsin genes (rh's) represent a unique family of highly regulated cell-specific genes, where each member has its own expression pattern in the visual system. Extensive analysis of the rh's has revealed several functional elements that are involved in cell-specificity. We have investigated the functional role of the RCSI/P3 site that is found in the proximal promoter of all Drosophila rh genes. This sequence is remarkably conserved in evolution and is located 15-30 bp upstream of the TATA box. We have previously shown that, in the context of the rh1 promoter, this element is recognized in vivo by a Pax6 protein, the master regulator of eye development. Thus, rh regulation might represent the ancestral function of Pax6. Here, we investigated the role of the RCSI/P3 sequence in the other rh genes and show that they also mediate Pax6 function. We also tested the potential impact of the various RCSI/P3 sequences on the precise cell-specific expression of rh genes. Our results demonstrate that, even though all RCSI/P3 sequences bind Pax6, they are clearly distinct in various rh promoters and these differences are conserved throughout evolution: RCSI/P3 appears to participate in the fine-tuning of cell-specificity. We also show that Pax6 or a related Pax protein may be involved in the regulation of olfactory genes. Therefore, in addition to performing a global photoreceptor-specific function, RCSI also appears to mediate the combined action of Pax6 and other factors and to contribute to rh regulation in subsets of photoreceptors.

The evolution of color vision in insects

We review the physiological, molecular, and neural mechanisms of insect color vision. Phylogenetic and molecular analyses reveal that the basic bauplan, UV-blue-green-trichromacy, appears to date back to the Devonian ancestor of all pterygote insects. There are variations on this theme, however. These concern the number of color receptor types, their differential expression across the retina, and their fine tuning along the wavelength scale. In a few cases (but not in many others), these differences can be linked to visual ecology. Other insects have virtually identical sets of color receptors despite strong differences in lifestyle. Instead of the adaptionism that has dominated visual ecology in the past, we propose that chance evolutionary processes, history, and constraints should be considered. In addition to phylogenetic analyses designed to explore these factors, we suggest quantifying variance between individuals and populations and using fitness measurements to test the adaptive value of traits identified in insect color vision systems.

Characterization of the primary photointermediates of Drosophila rhodopsin

Invertebrate opsins are unique among the visual pigments because the light-activated conformation, metarhodopsin, is stable following exposure to light in vivo. Recovery of the light-activated pigment to the dark conformation (or resting state) occurs either thermally or photochemically. There is no evidence to suggest that the chromophore becomes detached from the protein during any stage in the formation or recovery processes. Biochemical and structural studies of invertebrate opsins have been limited by the inability to express and purify rhodopsins for structure-function studies. In this study, we used Drosophila to produce an epitope-tagged opsin, Rh1-1D4, in quantities suitable for spectroscopic and photochemical characterization. When expressed in Drosophila, Rh1-1D4 is localized to the rhabdomere membranes, has the same spectral properties in vivo as wild-type Rh1, and activates the phototransduction cascade in a normal manner. Purified Rh1-1D4 visual pigment has an absorption maximum of the dark-adapted state of 474 nm, while the metarhodopsin absorption maximum is 572 nm. However, the metarhodopsin state is not stable as purified in dodecyl maltoside but decays with kinetics that require a double-exponential fit having lifetimes of 280 and 2700 s. We investigated the primary properties of the pigment at low temperature. At 70 K, the pigment undergoes a temperature-induced red shift to 486 nm. Upon illumination with 435 nm light, a photostationary state mixture is formed consisting of bathorhodopsin (lambda(max) = 545 nm) and isorhodopsin (lambda(max) = 462 nm). We also compared the spectroscopic and photochemical properties of this pigment with other vertebrate pigments. We conclude that the binding site of Drosophila rhodopsin is similar to that of bovine rhodopsin and is characterized by a protonated Schiff base chromophore stabilized via a single negatively charged counterion.

Role of asparagine-linked oligosaccharides in rhodopsin maturation and association with its molecular chaperone, NinaA

Many proteins require N-linked glycosylation for conformational maturation and interaction with their molecular chaperones. In Drosophila, rhodopsin (Rh1), the most abundant rhodopsin, is glycosylated in the endoplasmic reticulum (ER) and requires its molecular chaperone, NinaA, for exit from the ER and transport through the secretory pathway. Studies of vertebrate rhodopsins have generated several conflicting proposals regarding the role of glycosylation in rhodopsin maturation. We investigated the role of Rh1 glycosylation and Rh1/NinaA interactions under in vivo conditions by analyzing transgenic flies expressing Rh1 with isoleucine substitutions at each of the two consensus sites for N-linked glycosylation (N20I and N196I). We show that Asn(20) is the sole site for glycosylation. The Rh1(N20I) protein is retained within the secretory pathway, causing an accumulation of ER cisternae and dilation of the Golgi complex. NinaA associates with nonglycosylated Rh1(N20I); therefore, retention of nonglycosylated rhodopsin within the ER is not due to the lack of Rh1(N20I)/NinaA interaction. We further show that Rh1(N20I) interferes with wild type Rh1 maturation and triggers a dominant form of retinal degeneration. We conclude that during maturation Rh1 is present in protein complexes containing NinaA and that Rh1 glycosylation is required for transport of the complexes through the secretory pathway. Failure of this transport process leads to retinal degeneration.

Blue- and green-absorbing visual pigments of Drosophila: ectopic expression and physiological characterization of the R8 photoreceptor cell-specific Rh5 and Rh6 rhodopsins

Color discrimination requires the input of different photoreceptor cells that are sensitive to different wavelengths of light. The Drosophila visual system contains multiple classes of photoreceptor cells that differ in anatomical location, synaptic connections, and spectral sensitivity. The Rh5 and Rh6 opsins are expressed in nonoverlapping sets of R8 cells and are the only Drosophila visual pigments that remain uncharacterized. In this study, we ectopically expressed Rh5 and Rh6 in the major class of photoreceptor cells (R1-R6) and show them to be biologically active in their new environment. The expression of either Rh5 or Rh6 in "blind" ninaE(17) mutant flies, which lack the gene encoding the visual pigment of the R1-R6 cells, fully rescues the light response. Electrophysiological analysis showed that the maximal spectral sensitivity of the R1-R6 cells is shifted to 437 or 508 nm when Rh5 or Rh6, respectively, is expressed in these cells. These spectral sensitivities are in excellent agreement with intracellular recordings of the R8p and R8y cells measured in Calliphora and Musca. Spectrophotometric analyses of Rh5 and Rh6 in vivo by microspectrophotometry, and of detergent-extracted pigments in vitro, showed that Rh5 is reversibly photoconverted to a stable metarhodopsin (lambda(max) = 494 nm), whereas Rh6 appears to be photoconverted to a metarhodopsin (lambda(max) = 468 nm) that is less thermally stable. Phylogenetically, Rh5 belongs to a group of short-wavelength-absorbing invertebrate visual pigments, whereas Rh6 is related to a group of long-wavelength-absorbing pigments and is the first member of this class to be functionally characterized.

Honeybee blue- and ultraviolet-sensitive opsins: cloning, heterologous expression in Drosophila, and physiological characterization

The honeybee (Apis mellifera) visual system contains three classes of retinal photoreceptor cells that are maximally sensitive to light at 440 nm (blue), 350 nm (ultraviolet), and 540 nm (green). We performed a PCR-based screen to identify the genes encoding the Apis blue- and ultraviolet (UV)-sensitive opsins. We obtained cDNAs that encode proteins having a high degree of sequence and structural similarity to other invertebrate and vertebrate visual pigments. The Apis blue opsin cDNA encodes a protein of 377 amino acids that is most closely related to other invertebrate visual pigments that are thought to be blue-sensitive. The UV opsin cDNA encodes a protein of 371 amino acids that is most closely related to the UV-sensitive Drosophila Rh3 and Rh4 opsins. To test whether these novel Apis opsin genes encode functional visual pigments and to determine their spectral properties, we expressed them in the R1-6 photoreceptor cells of blind ninaE mutant Drosophila, which lack the major opsin of the fly compound eye. We found that the expression of either the Apis blue- or UV-sensitive opsin in transgenic flies rescued the visual defect of ninaE mutants, indicating that both genes encode functional visual pigments. Spectral sensitivity measurements of these flies demonstrated that the blue and UV visual pigments are maximally sensitive to light at 439 and 353 nm, respectively. These maxima are in excellent agreement with those determined previously by single-cell recordings from Apis photoreceptor cells and provide definitive evidence that the genes described here encode visual pigments having blue and UV sensitivity.

Identification of a novel Drosophila opsin reveals specific patterning of the R7 and R8 photoreceptor cells

The function of the compound eye is dependent upon a developmental program that specifies different cell fates and directs the expression of spectrally distinct opsins in different photoreceptor cells. Rh5 is a novel Drosophila opsin gene that encodes a biologically active visual pigment that is expressed in a subset of R8 photoreceptor cells. Rh5 expression in the R8 cell of an individual ommatidium is strictly coordinated with the expression of Rh3, in the overlying R7 cell. In sevenless mutant files, which lack R7 photoreceptor cells, the expression of the Rh5 protein in R8 cells is disrupted, providing evidence for a specific developmental signal between the R7 and R8 cells that is responsible for the paired expression of opsin genes.

Molecular properties of chimerical mutants of gecko blue and bovine rhodopsin

In spite of the high similarity in amino acid sequence between rod visual pigment rhodopsin and gecko blue-sensitive pigment (gecko blue), not only the spectral sensitivities but also the thermal decay rates of the meta II- and III-intermediates are noticeably different from one another [Kojima et al. (1995) Biochemistry 34, 1096-1106]. In order to identify the protein region(s) that contain(s) key residue being responsible for the functional difference, we constructed six chimerical mutants derived from gecko blue and bovine rhodopsin, with the aid of protein production in a human embryonic kidney cell line (293S). While the absorption maximum of every mutant was located in between gecko blue (466 nm) and bovine rhodopsin (500 nm), a large blue-shift (18 nm) was observed when the helices I-III of rhodopsin were replaced with those of gecko blue. A time resolved spectroscopic study demonstrated that this replacement also accelerated the decay rate of the meta II-intermediate. The decay of the meta III-intermediate of the mutants became faster as the compartment of gecko blue was increased. Thus, the faster decay of the meta II-intermediate of gecko blue is largely attributed to residues within helices I-III, while the decay of the meta III-intermediate apparently depends on the overall structure of the protein.

Defective intracellular transport is the molecular basis of rhodopsin-dependent dominant retinal degeneration

Retinitis pigmentosa (RP) is a group of hereditary human diseases that cause retinal degeneration and lead to eventual blindness. More than 25% of all RP cases in humans appear to be caused by dominant mutations in the gene encoding the visual pigment rhodopsin. The mechanism by which the mutant rhodopsin proteins cause dominant retinal degeneration is still unclear. Interestingly, the great majority of these mutants appear to produce misfolded rhodopsin. We now report the isolation and characterization of 13 rhodopsin mutations that act dominantly to cause retinal degeneration in Drosophila; four of these correspond to identical substitutions in human autosomal dominant RP patients. We demonstrate that retinal degeneration results from interference in the maturation of wild-type rhodopsin by the mutant proteins.

Receptor demise from alteration of glycosylation site in Drosophila opsin: electrophysiology, microspectrophotometry, and electron microscopy

In the delta Asn20 Drosophila stock, the N-linked glycosylation site of opsin in R1-6 receptors (Rh1) is absent. We used electroretinography (ERG), microspectrophotometry (MSP), and electron microscopy (EM) to quantify visual cell defects. Positive controls, w9, had wild type Rh1. MSP revealed minimal photopigment in delta Asn20 for 6 days posteclosion; w9 had near normal visual pigment. ERG sensitivity and prolonged depolarizing afterpotential (PDA) were compared for delta Asn20 and w9. Delta Asn20's R1-6 function is decreased 100-fold at eclosion and diminishes until only R7/8 functions at 11 days. What little rhodopsin is routed to the rhabdomere functions. Morphometry showed smaller R1-6 rhabdomeres in delta Asn20 for 8 days posteclosion. Rhabdomeres in w9 were normal. A negative control, ninaE(ol17), a deletion of the Rh1 gene, also has small rhabdomeres. Delta Asn20 and ninaE(ol17) lack the extreme rhabdomere elimination of ora (outer rhabdomeres absent), a nonsense mutant interrupting Rh1's coding sequence. Delta Asn20 and ora have surplus membrane while ninaE(ol17) does not. Freeze fracture reveals that delta Asn20's rhabdomeric P-face particle count is as low as for vitamin A deprivation, consistent with an opsin defect. High particle density, organized into rows, is present in adjacent plasmalemma where surplus membrane accumulates. In summary, delta Asn20 interferes with either synthesis, deployment, or maintenance of opsin.