Illuminating insights into opsin 3 function in the skin

Because sunlight is essential for human survival, we have developed complex mechanisms for detecting and responding to light stimuli. The eyes and skin are major organs for sensing light and express several light-sensitive opsin receptors. These opsins mediate cellular responses to spectrally-distinct wavelengths of visible and ultraviolet light. How the eyes mediate visual phototransduction is well understood, but less is known about how the skin detects light. Both human and murine skin express a wide array of opsins, with one of the most highly expressed being the functionally elusive opsin 3 (OPN3). In this review we explore light reception, opsin expression and signaling in skin cells; we compile data elucidating potential functions for human OPN3 in skin, with emphasis on recent studies investigating OPN3 regulation of melanin within epidermal melanocytes.

Divergent projection patterns of M1 ipRGC subtypes

In addition to its well-known role in pattern vision, light influences a wide range of non-image forming, subconscious visual behaviors including circadian photoentrainment, sleep, mood, learning, and the pupillary light reflex. Each of these behaviors is thought to require input from the M1 subtype of melanopsin-expressing, intrinsically photosensitive retinal ganglion cell (ipRGC). Recent work has demonstrated that the M1 subtype of ipRGC can be further subdivided based on expression of the transcription factor Brn3b. Brn3b-positive M1 ipRGCs project to the olivary pretectal nucleus and are necessary for the pupillary light reflex, while Brn3b-negative M1 ipRGCs project to the suprachiasmatic nucleus (SCN) and are sufficient for circadian photoentrainment. However, beyond the circadian and pupil systems, little is known about the projection patterns of M1 ipRGC subtypes. Here we show that Brn3b-positive M1 ipRGCs comprise the majority of sparse M1 ipRGC inputs to the thalamus, midbrain, and hypothalamus. Our data demonstrate that very few brain targets receive convergent input from both M1 ipRGC subpopulations, suggesting that each subpopulation drives a specific subset of light-driven behaviors.

Melanopsin-expressing ganglion cells on macaque and human retinas form two morphologically distinct populations

The long-term goal of this research is to understand how retinal ganglion cells that express the photopigment melanopsin, also known as OPN4, contribute to vision in humans and other primates. Here we report the results of anatomical studies using our polyclonal antibody specifically against human melanopsin that confirm and extend previous descriptions of melanopsin cells in primates. In macaque and human retina, two distinct populations of melanopsin cells were identified based on dendritic stratification in either the inner or the outer portion of the inner plexiform layer (IPL). Variation in dendritic field size and cell density with eccentricity was confirmed, and dendritic spines, a new feature of melanopsin cells, were described. The spines were the sites of input from DB6 diffuse bipolar cell axon terminals to the inner stratifying type of melanopsin cells. The outer stratifying melanopsin type received inputs from DB6 bipolar cells via a sparse outer axonal arbor. Outer stratifying melanopsin cells also received inputs from axon terminals of dopaminergic amacrine cells. On the outer stratifying melanopsin cells, ribbon synapses from bipolar cells and conventional synapses from amacrine cells were identified in electron microscopic immunolabeling experiments. Both inner and outer stratifying melanopsin cell types were retrogradely labeled following tracer injection in the lateral geniculate nucleus (LGN). In addition, a method for targeting melanopsin cells for intracellular injection using their intrinsic fluorescence was developed. This technique was used to demonstrate that melanopsin cells were tracer coupled to amacrine cells and would be applicable to electrophysiological experiments in the future. J. Comp. Neurol. 524:2845-2872, 2016. © 2016 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

Retinofugal Projections from Melanopsin-Expressing Retinal Ganglion Cells Revealed by Intraocular Injections of Cre-Dependent Virus

To understand visual functions mediated by intrinsically photosensitive melanopsin-expressing retinal ganglion cells (mRGCs), it is important to elucidate axonal projections from these cells into the brain. Initial studies reported that melanopsin is expressed only in retinal ganglion cells within the eye. However, recent studies in Opn4-Cre mice revealed Cre-mediated marker expression in multiple brain areas. These discoveries complicate the use of melanopsin-driven genetic labeling techniques to identify retinofugal projections specifically from mRGCs. To restrict labeling to mRGCs, we developed a recombinant adeno-associated virus (AAV) carrying a Cre-dependent reporter (human placental alkaline phosphatase) that was injected into the vitreous of Opn4-Cre mouse eyes. The labeling observed in the brain of these mice was necessarily restricted specifically to retinofugal projections from mRGCs in the injected eye. We found that mRGCs innervate multiple nuclei in the basal forebrain, hypothalamus, amygdala, thalamus and midbrain. Midline structures tended to be bilaterally innervated, whereas the lateral structures received mostly contralateral innervation. As validation of our approach, we found projection patterns largely corresponded with previously published results; however, we have also identified a few novel targets. Our discovery of projections to the central amygdala suggests a possible direct neural pathway for aversive responses to light in neonates. In addition, projections to the accessory optic system suggest that mRGCs play a direct role in visual tracking, responses that were previously attributed to other classes of retinal ganglion cells. Moreover, projections to the zona incerta raise the possibility that mRGCs could regulate visceral and sensory functions. However, additional studies are needed to investigate the actual photosensitivity of mRGCs that project to the different brain areas. Also, there is a concern of "overlabeling" with very sensitive reporters that uncover low levels of expression. Light-evoked signaling from these cells must be shown to be of sufficient sensitivity to elicit physiologically relevant responses.

Opsin3 sensitizes hepatocellular carcinoma cells to 5-fluorouracil treatment by regulating the apoptotic pathway

Hepatocellular carcinoma (HCC) is the third most common cancer worldwide, causing over 0.5 million deaths per year, with approximately half of these in China. Chemotherapy is the optimal treatment for patients with advanced HCC, although chemoresistance has become a significant obstacle to successful anti-cancer therapy. The expression of opsin3 (OPN3), also called encephalopsin or panopsin, is lower in 5-fluorouracil (5-FU)-resistant Bel7402(5-FU) cells compared to 5-FU-sensitive Bel7402 cells. To explore the role of OPN3 in 5-FU resistance, OPN3 overexpressing (Bel7402(5-FU)-OPN3) and knockdown (Bel7402-RNAi-OPN3) cell lines were generated. Bel7402(5-FU)-OPN3 cells were more sensitive to 5-FU treatment than controls, while OPN3 knockdown resulted in a significant increase in 5-FU resistance. This result was replicated in a second HCC cell line, HepG2. Further investigation of the mechanism revealed that decreased OPN3 levels in Bel7402(5-FU) cells activated the anti-apoptotic pathway through increasing phospho-Akt and the Bcl2/Bax ratio, while overexpression of OPN3 inactivated this pathway. Taken together, these results suggest that OPN3 depletion is involved in 5-FU resistance, and that therapeutic strategies targeting OPN3 may improve HCC sensitivity to chemotherapy.

The ERG responses to light stimuli of melanopsin-expressing retinal ganglion cells that are independent of rods and cones

The mechanisms by which melanopsin-expressing retinal ganglion cells (mRGCs) regulate circadian rhythms in humans have not been established. To understand mRGC characteristics and their role independent of effects due to the rods and cones, mRGC responses should be induced or measured independent of cone and rod responses. In the present study, we obtained results from light stimuli which differentially induce only the mRGC response by using a receptor-silent substitution technique. The mRGCs responded linearly to contrast changes of light stimuli, whereas they showed complicated responses to frequency changes with regard to the latency of response time. These results suggest that mRGC behavior is not a simple response to the various frequencies found in solar light but may be related to intrinsic neural circuits with feedback connections in the mRGC pathway. The results in this study also demonstrated that the test stimuli affected only the mRGC response and that this could be successfully detected by using the electroretinogram (ERG).

Regulation of Melanopsin Expression

Circadian rhythms in mammals are adjusted daily to the environmental day/night cycle by photic input via the retinohypothalamic tract (RHT). Retinal ganglion cells (RGCs) of the RHT constitute a separate light-detecting system in the mammalian retina used for irradiance detection and for transmission to the circadian system and other non-imaging forming processes in the brain. The RGCs of the RHT are intrinsically photosensitive due to the expression of melanopsin, an opsin-like photopigment. This notion is based on anatomical and functional data and on studies of mice lacking melanopsin. Furthermore, heterologous expression of melanopsin in non-neuronal mammalian cell lines was found sufficient to render these cells photosensitive. Even though solid evidence regarding the function of melanopsin exists, little is known about the regulation of melanopsin gene expression. Studies in albino Wistar rats showed that the expression of melanopsin is diurnal at both the mRNA and protein levels. The diurnal changes in melanopsin expression seem, however, to be overridden by prolonged exposure to light or darkness. Significant increase in melanopsin expression was observed from the first day in constant darkness and the expression continued to increase during prolonged exposure in constant darkness. Prolonged exposure to constant light, on the other hand, decreased melanopsin expression to an almost undetectable level after 5 days of constant light. The induction of melanopsin by darkness was even more pronounced if darkness was preceded by light suppression for 5 days. These observations show that dual mechanisms regulate melanopsin gene expression and that the intrinsic light-responsive RGCs in the albino Wistar rat adapt their expression of melanopsin to environmental light and darkness.

Absence of functional short-wavelength sensitive cone pigments in hamsters (Mesocricetus)

Studies of Syrian golden hamsters (Mesocricetus auratus) have yielded contradictory evidence as to whether the retina of this species supports a population of cones containing short-wavelength sensitive pigments. We undertook a re-examination of this issue by (a) measuring lens transmission, (b) determining complete spectral sensitivity functions using electroretinogram (ERG) flicker photometry, (c) employing a sensitive chromatic-adaptation paradigm in conjunction with ERG measurements to conduct a specific search for the presence of a short-wavelength sensitive mechanism, and (d) assaying for the presence of retinal mRNA using real-time, reverse transcription polymerase chain reactions (RT-PCR). Parallel measurements were made on Turkish hamster (Mesocricetus brandtii) and control measurements were derived from recordings made on a rodent whose retina is known to contain a population of short-wavelength sensitive cones (the rat, Rattus norvegicus). Although UV opsin transcripts can be detected in the retina of the Syrian hamster, the electrophysiological measurements imply that these are not translated. Syrian hamsters thus lack a functional short-wavelength sensitive pigment, and that seems also true for the Turkish hamster. Members of this genus belong to a disparate group of mammals that have lost function of their short-wavelength sensitive cone pigments through ancestral opsin gene mutations.

Daily profile in melanopsin transcripts depends on seasonal lighting conditions in the rat retina

The retinal photopigment melanopsin (Opn4) mediates photoentrainment of the circadian system. In the present study, seasonal regulation of the melanopsin gene was investigated in comparison with the arylalkylamine N-acetyltransferase (AA-NAT) gene as an indicator of retinal pacemaker output. For this purpose, the daily profiles in the amount of melanopsin mRNA and AA-NAT mRNA were monitored under 8 : 16 h light/dark, 12 : 12 h light/dark and 16 : 8 h light/dark photoperiods using real-time polymerase chain reaction analysis. We found that, under all of the lighting regimes, melanopsin and AA-NAT expression oscillated with a peak around dark onset and the middle of the dark phase, respectively. The lighting regime influenced both genes, but in an opposing manner. Under long photoperiods, the duration of peak expression was prolonged for melanopsin, whereas it was shortened for AA-NAT. Under constant darkness, the rhythm of mRNA was abolished for melanopsin, but persisted for AA-NAT whereas, under constant light, the rhythm of mRNA was abolished for both genes. Our findings suggest that, in contrast to the AA-NAT gene, the daily and photoperiod-dependent regulation of the melanopsin gene does not rely on a circadian oscillator but is directly illumination-dependent.

Identification of novel retinal target genes of thyroid hormone in the human WERI cells by expression microarray analysis

Using the human WERI-Rb1 cell line as a model system, we performed a genome-wide search for retinal target genes of thyroid hormone (TH) via expression microarray analysis followed by quantitative real-time RT-PCR verification. We identified 12 novel retinal targets of TH, including 10 up-regulated genes (OPN1MW, OPN1LW, TIMP3, RP1L1, GNGT2, CRX, ARR3, GCAP1, IMPDH1, and PDE6C) and 2 down-regulated genes (GNGT1 and GNB3). In addition, we found a number of novel TH-targets that are not currently known to be retinal genes. This is the first report of human retinal targets regulated by thyroid hormone.

Expression pattern of the melanopsin-like (cOpn4m) and VA opsin-like genes in the developing chicken retina and neural tissues

We examined the expression pattern of melanopsin-like (cOpn4m) and VA opsin-like (cVAL) genes during chicken development. Two types of cOpn4m transcripts, distinct in their carboxyl terminals were found, as is the case for the chicken melanopsin (cOpn4) reported previously. The expression of cOpn4m was restricted to the developing retina, specifically to a subset of developing amacrine cells from embryonic day 10. VA opsin is one of the non-canonical opsins, reported to exist in fish so far. In this study, an aberrant type of VA opsin-like (cVAL) cDNA was isolated from chicken embryonic neural tissues. The expression of cVAL was observed in the ventral region of the developing brain and neural tube; however, specific signals for cVAL could not be detected in the developing retina. These results indicate that the additional melanopsin in avian identifies a subset of developing amacrine cells in the retina and that the aberrant transcript of the VA opsin-like gene are present during neural tube development in the chicken.

Cone-based vision in the aging mouse

People often experience age-related declines in cone-based visual capacities despite an absence of apparent visual pathology. Although mice are used as models of human visual pathologies associated with aging, little is known about how age impacts vision in animals with disease-free retinas since most studies have heretofore examined relatively young mice. We examined the effects of age on cone-based vision by assessing opsin gene transcription, cone densities, the flicker electroretinogram (ERG), and behavioral increment thresholds in mice. ERG measurements of cone function showed age-related declines in maximum voltage (Vmax), while opsin gene transcription, cone density, and increment thresholds were unchanged even in extremely old mice. The age-related decline in Vmax seen in mice is qualitatively similar to that documented for human subjects. It is notable that Vmax, a commonly used index of ERG activity, does not predict behavioral performance in the mouse.

Light-sensitive motile iridophores and visual pigments in the neon tetra, Paracheirodon innesi

Although motile iridophores in the longitudinal stripes of neon tetra skin are under control of the sympathetic nervous system, they also respond to light directly and show circadian color changes. Using neon tetra skin, we found that the photoresponse of iridophores depends on light intensity, and that light near 500 nm is most effective. RT-PCR demonstrated the expression of mRNAs encoding rhodopsin and two kinds of cone opsins (Pi-green1 and Pi-green2) in neon tetra skin where the light-sensitive iridophores exist. These mRNAs are also expressed in the lateral eyes. The cone opsin genes, Pi-green1 and Pi-green2, show high similarity with the g101 and g103 genes of unique green cone opsins (belonging to the MWS/LWS group) of the blind Mexican cavefish. These results show that Pi-green1, Pi-green2, and/or rhodopsin may play important roles in the photoresponse of neon tetra iridophores, which are most sensitive to light near 500 nm.

Active and passive displacement of transmembrane domains both occur during opsin biogenesis at the Sec61 translocon

We used a site-specific crosslinking approach to study the membrane integration of the polytopic protein opsin at the endoplasmic reticulum. We show that transmembrane domain 1 occupies two distinct Sec61-based environments during its integration. However, transmembrane domains 2 and 3 exit the Sec61 translocon more rapidly in a process that suggests a displacement model for their integration where the biosynthesis of one transmembrane domain would facilitate the exit of another. In order to investigate this hypothesis further, we studied the integration of the first and third transmembrane domains of opsin in the absence of any additional C-terminal transmembrane domains. In the case of transmembrane domain 1, we found that its lateral exit from the translocon is clearly dependent upon the synthesis of subsequent transmembrane domains. By contrast, the lateral exit of the third transmembrane domain occurred independently of any such requirement. Thus, even within a single polypeptide chain, distinct transmembrane domains display different requirements for their integration through the endoplasmic reticulum translocon, and the displacement of one transmembrane domain by another is not a global requirement for membrane integration.

Targeted effects of retinoic acid signaling upon photoreceptor development in zebrafish

Retinoic acid (RA) is a signaling molecule important for photoreceptor development in vertebrates. The purpose of this study was to examine the mechanisms of the effects of RA upon developing rod and cone photoreceptors in the embryonic zebrafish. Exposure to exogenous RA increased the number of photoreceptors expressing rod opsin and red cone opsin, and decreased the number of photoreceptors expressing the blue and UV cone opsins, suggesting targeted effects of RA on photoreceptor development. RA exposure also increased opsin expression in individual rods and red cones, but decreased opsin expression in individual blue and UV cones, as indicated by differences in the strength of opsin hybridization in identified photoreceptors. RA exposure did not, however, significantly alter quantitative measures of photoreceptor pattern in a manner expected for changes in photoreceptor fate. These observations collectively indicate that RA treatment does not affect photoreceptor fate, but rather differentially influences opsin transcription in determined photoreceptors. An enzyme involved in RA synthesis, RALDH2, was immunocytochemically localized to retinal progenitor cells and the retinal pigmented epithelium (RPE), suggesting the presence of RA in the vicinity of developing photoreceptors. However, expression of an RA response element-driven transgene was restricted to the RPE, retinal progenitors, and a small population of neurons in ventral retina, suggesting that the endogenous RA signaling system is spatially limited within the eye.

Photoreceptor differentiation during retinal development, growth, and regeneration in a metamorphic vertebrate

To test the hypothesis that growth and regeneration of the adult retina involves a mechanistic recapitulation of retinal development, the patterns of photoreceptor differentiation were investigated in the developing retina, as well as growing and regenerating adult retina, of a metamorphic vertebrate, the winter flounder. Only one opsin, of type RH2 (a "green" cone opsin), was expressed in premetamorphic (developing) retina, and a corresponding middle-wavelength visual pigment was observed. In premetamorphic retinas there was no evidence for any other cone opsins or pigments, rods, rod opsin expression, or rod visual pigment. In contrast, a rod opsin (RH1) and three cone opsins (SWS2, RH2, and LWS) were expressed in postmetamorphic (adult) retina, and these opsins were consistent with the observed repertoire of visual pigments. During postmetamorphic retinal growth and regeneration, cones were always produced before rods, but the different cone types were apparently produced simultaneously, suggesting that cone differentiation mechanisms might change after metamorphosis. The results support the hypothesis that photoreceptor differentiation during growth and regeneration of the adult retina involves a recapitulation of mechanisms that control the sequence of photoreceptor production during retinal development.

Classical photoreceptors regulate melanopsin mRNA levels in the rat retina

Recent studies have demonstrated that melanopsin is a key photopigment in the mammalian circadian system. This novel opsin is exclusively expressed in retinal ganglion cells that are intrinsically sensitive to light, perhaps responding via a melanopsin-based signaling pathway. Previous investigations using transgenic mice have also demonstrated that ablation of the classical photoreceptors and of melanopsin prevents entrainment of several circadian rhythms, thus demonstrating that these photoreceptors are necessary and sufficient for circadian photoreception. In this study, we investigated the effect of photoreceptor degeneration on melanopsin mRNA regulation in RCS/N-rdy rats (Royal College of Surgeons rats with a defect in the retinal dystrophy gene). We used animals at postnatal day 21 (P21), P33, P45, and P60. At P60 degeneration of the retina in RCS/N-rdy has advanced to the point where the majority of the photoreceptors have degenerated. Our data indicate that melanopsin mRNA levels were rhythmic in light/dark cycle and in constant darkness in congenic controls (RCS/N-rdy+) and in RCS/N-rdy at P21 (i.e., before the degeneration of the photoreceptors). On the other hand, in RCS/N-rdy at P60, melanopsin mRNA levels were greatly reduced (<90%) and not rhythmic. Photoreceptor degeneration did not affect the expression of pituitary adenylate cyclase-activating polypeptide mRNA (a marker for melanopsin-containing ganglion cells). Our results suggest that classical photoreceptors (rods and cones) regulate the expression of melanopsin mRNA in the rat. Because RCS/N-rdy rats are a model for studies on retinitis pigmentosa in human, our data may provide an important insight on melanopsin function in patients affected by retinitis pigmentosa.

Circadian rhythms of behavioral cone sensitivity and long wavelength opsin mRNA expression: a correlation study in zebrafish

Using a behavioral assay based on visually mediated escape responses, we measured long-wavelength-sensitive red cone (LC) sensitivities in zebrafish. In a 24 h period, the zebrafish were least sensitive to red light in the early morning and most sensitive in the late afternoon. To investigate if the fluctuation of behavioral cone sensitivity correlates with opsin gene expression, we measured LC opsin mRNA expression at different times in the day and night under different lighting conditions. Under a normal light-dark cycle, the expression of LC opsin mRNA determined by real-time RT-PCR was low in the early morning and high in the late afternoon, similar to the fluctuation of behavioral cone sensitivity. This rhythm of LC opsin mRNA expression, however, dampened out gradually in constant conditions. After 24 h of constant light (LL), the expression of LC opsin mRNA dropped to levels similar to those determined in the early morning in control animals. By contrast, when the zebrafish were kept in constant dark (DD), the expression of LC opsin mRNA increased, to levels about 30-fold higher than the expression in the early morning in control animals. This day-night fluctuation in LC opsin mRNA expression was correlated to changes in opsin density in the outer segment of cone photoreceptor cells. Microspectrophotometry (MSP) measurements found significant differences in red cone outer segment optical density with a rhythm following the behavioral sensitivity. Furthermore, dopamine modulated the circadian rhythms in expression of LC opsin mRNA. Administration of dopamine increased LC opsin mRNA expression, but only in the early morning.

Molecular cloning, localization and circadian expression of chicken melanopsin (Opn4): differential regulation of expression in pineal and retinal cell types

The avian retina and pineal gland contain autonomous circadian oscillators and photo-entrainment pathways, but the photopigment(s) that mediate entrainment have not been definitively identified. Melanopsin (Opn4) is a novel opsin involved in entrainment of circadian rhythms in mammals. Here, we report the cDNA cloning of chicken melanopsin and show its expression in retina, brain and pineal gland. Like the melanopsins identified in amphibians and mammals, chicken melanopsin is more similar to the invertebrate retinaldehyde-based photopigments than the retinaldehyde-based photopigments typically found in vertebrates. In retina, melanopsin mRNA is expressed in cells of all retinal layers. In pineal gland, expression was strong throughout the parenchyma of the gland. In brain, expression was observed in a few discrete nuclei, including the lateral septal area and medial preoptic nucleus. The retina and pineal gland showed distinct diurnal expression patterns. In pineal gland, melanopsin mRNA levels were highest at night at Zeitgeber time (ZT) 16. In contrast, transcript levels in the whole retina reached their highest levels in the early morning (ZT 0-4). Further analysis of melanopsin mRNA expression in retinal layers isolated by laser capture microdissection revealed different patterns in different layers. There was diurnal expression in all retinal layers except the ganglion cell layer, where heavy expression was localized to a small number of cells. Expression of melanopsin mRNA peaked during the daytime in the retinal pigment epithelium and inner nuclear layer but, like in the pineal, at night in the photoreceptors. Localization and regulation of melanopsin mRNA in the retina and pineal gland is consistent with the hypothesis that this novel photopigment plays a role in photic regulation of circadian function in these tissues.

Adult stemmata of the butterfly Vanessa cardui express UV and green opsin mRNAs

Adult stemmata are distinctive insect photoreceptors located on the posterior surfaces of the optic lobes. They originate as larval eyes that migrate inward during metamorphosis. We used a combination of light microscopy and in situ hybridization to examine their anatomical organization in the butterfly Vanessa cardui and to test for the presence of visual pigments, the light sensitive components of the visual transduction pathway. The bilateral cluster of six internal stemmata is located near the ventral edge of the lamina. They retain the dark screening pigment and overlying crystalline cones of the larval stemmata. We found two opsin mRNAs expressed in the stemmata that are also expressed, respectively, in UV-sensitive and green-sensitive photoreceptor cells in the compound eye. A third mRNA that is expressed in blue-sensitive photoreceptor cells of the compound eye was not expressed in the stemmata. Our results reinforce the idea that the adult stemmata are not merely developmental remnants of larval eyes, but remain functional, possibly as components of the circadian input channel.

Population variation in opsin expression in the bluefin killifish, Lucania goodei : a real-time PCR study

Quantitative genetics have not been used in vision studies because of the difficulty of objectively measuring large numbers of individuals. Here, we examine the effectiveness of a molecular technique, real-time PCR, as an inference of visual components in the bluefin killifish, Lucania goodei, to determine whether there is population variation in opsin expression. Previous work has shown that spring animals possess a higher frequency of UV and violet cones and a lower frequency of yellow and red cones than swamp animals. Here, we found a good qualitative match between the population differences in opsin expression and those found previously in cone frequency. Spring animals expressed higher amounts of SWS1 and SWS2B opsins (which correspond to UV and violet photopigments) and lower amounts of RH2 and LWS opsins (which correspond to yellow and red photopigments) than swamp animals. The counterintuitive pattern between color pattern, lighting environment, and vision remains. Males with blue anal fins are more abundant in swamps where animals express fewer SWS1 and SWS2B opsins and where transmission of UV/blue wavelengths is low. Understanding this system requires quantitative genetic studies. Real-time PCR is an effective tool for studies requiring inferences of visual physiology in large numbers of individuals.

Functional and cellular responses in a novel rodent model of anterior ischemic optic neuropathy

PURPOSE

Anterior ischemic optic neuropathy (AION) is caused by sudden loss of vascular supply to retinal ganglion cell (RGC) axons in the anterior portion of the optic nerve and is a major cause of optic nerve dysfunction. There has been no easily obtainable animal model of this disorder. The current study was conducted to design a novel model of rodent AION (rAION), to enable more detailed study of this disease.

METHODS

A novel rodent photoembolic stroke model was developed that is directly analogous to human AION. Using histologic, electrophysiological, molecular- and cell biological methods, the early changes associated with isolated RGC axonal ischemia were characterized.

RESULTS

Functional (electrophysiological) changes occurred in RGCs within 1 day after rAION, with a loss of visual evoked potential (VEP) amplitude that persisted in the long term. The retinal gene expression pattern rapidly changed after rAION induction, with an early (<1 day) initial induction of c-Fos mRNA, and loss of RGC-specific gene expression. RGC-specific protein expression declined 2 days after detectable mRNA level changes, and immunostaining suggested that multiple retinal layers react to isolated RGC axonal ischemia.

CONCLUSIONS

rAION rapidly results in electrophysiological and histologic changes similar to clinical AION, with reactive responses in primary and supporting neuronal cell layers. The rAION model can enable a detailed analysis of the individual retinal and optic nerve changes that occur after optic nerve stroke, which may be useful in determining possible therapeutic interventions for this disorder.

A broad role for melanopsin in nonvisual photoreception

The rod and cone photoreceptors that mediate visual phototransduction in mammals are not required for light-induced circadian entrainment, negative masking of locomotor activity, suppression of pineal melatonin, or the pupillary light reflex. The photopigment melanopsin has recently been identified in intrinsically photosensitive retinal ganglion cells (RGCs) that project to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus, suggesting that melanopsin might influence a variety of irradiance-driven responses. We have found novel projections from RGCs that express melanopsin mRNA to the ventral subparaventricular zone (vSPZ), a region involved in circadian regulation and negative masking, and the sleep-active ventrolateral preoptic nucleus (VLPO) and determined the subsets of melanopsin-expressing RGCs that project to the SCN, the pretectal area (PTA), and the IGL division of the lateral geniculate nucleus (LGN). Melanopsin was expressed in the majority of RGCs that project to the SCN, vSPZ, and VLPO and in a subpopulation of RGCs that innervate the PTA and the IGL but not in RGCs projecting to the dorsal LGN or superior colliculus. Two-thirds of RGCs containing melanopsin transcript projected to each of the SCN and contralateral PTA, and one-fifth projected to the ipsilateral IGL. Double-retrograde tracing from the SCN and PTA demonstrated a subpopulation of RGCs projecting to both sites, most of which contained melanopsin mRNA. Our results suggest that melanopsin expression defines a subset of RGCs that play a broad role in the regulation of nonvisual photoreception, providing collateralized projections that contribute to circadian entrainment, negative masking, the regulation of sleep-wake states, and the pupillary light reflex.

Identification of circadian brain photoreceptors mediating photic entrainment of behavioural rhythms in lizards

We have shown previously that in ruin lizards (Podarcis sicula) the ablation of all known photoreceptive structures (lateral eyes, pineal and parietal eye) in the same individual animal does not prevent entrainment of their circadian locomotor rhythms to light. The present study was aimed at identifying the circadian brain photoreceptors mediating entrainment. For this purpose, we looked for opsin expression in the brain by means of immunocytochemistry. Using anti-cone-opsin antiserum CERN 874 we have localized photoreceptors in the periventricular area of hypothalamus, near the third cerebral ventricle. We also cloned a brain opsin cDNA that, on the basis of the deduced amino acid sequence, appears to belong to the RH2 class of cone-opsins. We named the cloned cone-opsin Ps-RH2. To examine whether brain cone-opsins mediate photic entrainment of circadian locomotor rhythms, we performed post-transcriptional inactivation experiments by injecting an expression eukaryotic vector transcribing the antisense cone-opsin Ps-RH2 mRNA in the third cerebral ventricle of pinealectomized-retinectomized lizards previously entrained to a light-dark (LD) cycle. Injections of the antisense construct abolished photic entrainment of circadian locomotor rhythms of pinealectomized-retinectomized lizards to the LD cycle for 6-9 days. CERN 874 completely failed to label cells within the periventricular area of hypothalamus of brains injected with antisense construct. Thus, abolishment of photic entrainment is due to inactivation of endogenous brain cone-opsins mRNA. The present results demonstrate for the first time in a vertebrate that brain cone-opsins are part of a true circadian brain photoreceptor participating in photic entrainment of behavioural rhythms.

Melanopsin retinal ganglion cells and the maintenance of circadian and pupillary responses to light in aged rodless/coneless (rd/rd cl) mice

Melanopsin-expressing ganglion cells have been proposed as the photoreceptors mediating non-rod, non-cone ocular responses to light. Here we use the aged (approximately 2 years) rodless and coneless (rd/rd cl) mouse to assess the impact of progressive inner retinal cell loss on melanopsin expression, circadian entrainment and pupillary constriction. Aged rd/rd cl mice show substantial transneuronal retinal degeneration leaving only the ganglion cell layer and little of the inner nuclear layer. Despite this loss, quantitative reverse transcriptase-polymerase chain reaction showed normal levels of melanopsin expression, and immunocytochemistry demonstrated both the presence and normal cellular appearance of these cells. Furthermore, the optic nerves of the two genotypes (rd/rd cl and +/+) were not obviously different in animals older than 2 years. However, this massive level of retinal degeneration left both pupillary and circadian responses to light intact, even in rd/rd cl mice older than 2 years. Our data provide the first positive correlation between the persistence of melanopsin-expressing cells and the maintenance of both circadian and pupillary responses to light in the absence of rods and cones. These findings, together with recent studies on melanopsin knockout mice, are consistent with the hypothesis that melanopsin-expressing ganglion cells are photosensitive and mediate a range of irradiance-detection tasks.

An in vivo doxycycline-controlled expression system for functional studies of the retina

PURPOSE

Transgenic mice were developed that express tetracycline-controlled transactivator 1 (tTA1) specifically in photoreceptor cells. In these mice the transcription of the gene of interest can be easily inactivated in the retina in a short time frame.

METHODS

A construct was prepared containing tTA1 under control of the murine rhodopsin regulatory region. This construct was used for the generation of transgenic mice. In situ hybridization was performed to study the distribution of the transactivator in the retina. The activity of the transactivator was analyzed by mating the lines with a luciferase reporter transgenic mouse. tTA1 activity and doxycycline's ability to block it were analyzed by luciferase assay. The effects of tTA1 on the retina were assessed by histology and electrophysiology.

RESULTS

Two transgenic lines were developed that specifically express tTA1 in photoreceptor cells. The time course of transgene expression replicated transcription of endogenous rhodopsin. tTA1 was not toxic to the retina. Transactivator activity was blocked readily by doxycycline.

CONCLUSIONS

An expression system for photoreceptor cells was generated to drive transcription in a cell-specific and time-controllable manner. This system is suitable for the study of factors involved in retinal biology and of mutant forms of genes involved in retinal diseases.

Correlation of regenerable opsin with rod ERG signal in Rpe65-/- mice during development and aging

PURPOSE

RPE65 has been shown to be essential for the production of 11-cis retinal by the retinal pigment epithelium. Mutations in RPE65 are known to be associated with severe forms of early-onset retinal dystrophy. This project was designed to determine the amount of regenerable opsin in Rpe65-/- mice during development and aging, and to examine the function of this rhodopsin by electroretinography (ERG).

METHODS

Young and aged Rpe65-/- and wild-type (WT) mice were dark adapted. Endogenous rhodopsin and regenerable opsin were measured using absorption-difference spectrophotometry. Photoreceptor function was assessed with scotopic single-flash ERGs and photoreceptors were counted in histologic sections. Opsin's primary structure was analyzed by mass-spectrometric mapping.

RESULTS

Unlike WT mice, amounts of regenerable opsin in Rpe65-/- mice decreased significantly with age, which correlated with a decrease in the number of photoreceptors and a decline in ERG amplitudes. Opsin structure, however, did not change. No endogenous levels of rhodopsin were measurable in the Rpe65-/- mice (detection limit: 0.225 pmol). 11-cis Retinal injections resulted in the regeneration of similar amounts of rhodopsin and improved rod function in a comparable way, irrespective of age.

CONCLUSIONS

In the aged Rpe65-/- mouse, opsin levels decrease because of the loss of photoreceptors. The remaining opsin is structurally intact, and the components of the phototransduction cascade and the retinal circuitry remain functional, despite the absence of normal photoreceptor activity.

Loss of photic entrainment and altered free-running circadian rhythms in math5-/- mice

Mammalian free-running circadian rhythms are entrained to the external light/dark cycle by photic signaling to the suprachiasmatic nuclei via the retinohypothalamic tract (RHT). We investigated the circadian entrainment and clock properties of math5-/- mutant mice. math5 is a critical regulator of retinal ganglion cell development; math5-/- mice show severe optic nerve hypoplasia. By anterograde cholera toxin B tracing, we find that math5-/- mice do not develop an identifiable RHT pathway. This appears to be attributable to agenesis or dysgenesis of the majority of RHT-projecting retinal ganglion cells. math5-/- mice display free-running circadian rhythms with a period approximately 1 hr longer than B6/129 controls (24.43 +/- 0.10 vs 23.62 +/- 0.19 hr; p < 0.00001). The free-running period of heterozygote mice is indistinguishable from that of controls. math5-/- mice show no entrainment to light/dark cycles, whereas heterozygote mice show normal entrainment to both 12 hr light/dark cycles and to a 1 hr skeletal photoperiod. math5-/- mice show reduced ability to entrain their rhythms to the nonphotic time cue of restricted running wheel access but demonstrate both free-running behavior and entrained anticipation of wheel unlocking in these conditions, suggesting the presence of a second diurnal oscillatory system in math5-/- animals. These results demonstrate that retinal ganglion cell input is not necessary for the development of a free-running circadian timekeeping system in the suprachiasmatic nucleus but is important for both photic entrainment and determination of the free-running period.

Adenosine A(2a) receptor-mediated inhibition of rod opsin mRNA expression in tiger salamander

The neuromodulator adenosine mediates dark-adaptive changes in retinal photoreceptors through A(2a) receptors. In cold-blooded vertebrates, opsin mRNA expression is lower at night than during the day. In the present study, we tested whether adenosine could inhibit opsin mRNA expression in cultured rod cells and if endogenous adenosine acts to suppress opsin mRNA in the intact retina at night. Semi-quantitative in situ hybridization showed that treatment with 100 nm of the A(2a)/A(2b) agonist N(6)-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine (DPMA) reduced opsin mRNA 41% in cultured rod cells. The effect of DPMA was blocked by 10 microm of the A(2a) antagonist 8-(3-chlorostyryl)caffeine (CSC) but not by 10 microm of the A(2b) antagonist alloxazine. One micromolar adenosine alone had no effect on opsin mRNA. However, in the presence of the adenosine deaminase inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA), 1 microm adenosine reduced opsin mRNA 61%. EHNA alone reduced opsin mRNA by 26%. Consistent with an A(2a) receptor mechanism, 100 nm forskolin (adenylate cyclase agonist) decreased opsin mRNA 34%. Finally, northern blots showed that intravitreal injection of 10 microm CSC at night increased opsin I mRNA 38%. Thus, endogenous adenosine suppresses rod opsin I mRNA expression at night; in vitro results indicate this reduction occurs through A(2a)-like receptor binding and stimulation of adenylate cyclase activity.

The circadian photopigment melanopsin is expressed in the blind subterranean mole rat, Spalax

Despite severe degeneration of its eyes, the blind subterranean mole rat, Spalax, is able to adjust circadian rhythms to the environmental light/dark cycle due to a conserved retinohypothalamic tract (RHT). The photopigment mediating the circadian photoreception and it cellular localisation is unknown in the Spalax retina. Here we show, using in situ hybridization and immunohistochemistry, that melanopsin, a recently identified opsin, is expressed in retinal ganglion cells which also co-store PACAP, a neurotransmitter of the RHT. The melanopsin-component of retinal ganglion cells in the Spalax retina is well conserved resulting in a relatively higher density of melanopsin positive cells per area compared to the rat. The results show that the Spalax, as sighted animals expresses melanopsin in ganglion cells projecting to the circadian clock supporting a role of melanopsin as a circadian photopigment.

Multiple mechanisms of rhabdom shedding in the lateral eye of Limulus polyphemus

Rhabdom shedding in horseshoe crab lateral eye photoreceptors was studied with anti-opsin and anti-arrestin immunocytochemistry. Two, possibly three, distinct shedding mechanisms were revealed in animals maintained in natural lighting. Transient rhabdom shedding, triggered by dawn, is a brief, synchronous event that removes up to 10% of the rhabdom membrane. Whorls of rhabdomeral membrane break into vesicles and form compact multivesicular bodies. These debris particles are immunoreactive for opsin and are of a relatively uniform size, averaging approximately 2 microm(2) in area. Transient shedding requires that input from circadian efferent fibers to the retina precedes the light trigger, and cutting the optic nerve blocks efferent input and transient shedding. Light-driven rhabdom shedding is a progressive process. Rhabdomeral membrane is removed by coated vesicles that accumulate into loosely packed multivesicular bodies. These debris particles label with antibodies directed against opsin, arrestin, and adaptin, and they have a large distribution of sizes, averaging almost 6 microm(2) in area and ranging up to 25 microm(2) or more. The amount of rhabdomeral membrane removed by light-driven shedding has seasonal variation and depends on latitude. Light-driven shedding does not require circadian efferent input. A possible third shedding mechanism, light-independent shedding, is observed when transient shedding is blocked either by 48 hours of darkness or by cutting the optic nerve. Small particles, averaging 1.8 microm(2) in area, exhibiting opsin but not arrestin immunoreactivity can then be found in the cytoplasm surrounding the rhabdom. The nature of light-independent shedding is not yet clear.

The amino terminus of opsin translocates "posttranslationally" as efficiently as cotranslationally

Opsin, a member of the G-protein-coupled receptor family, is a polytopic membrane protein that does not encode a cleaved amino-terminal signal sequence. The amino terminus of opsin precedes the first known targeting information, suggesting that it translocates across the endoplasmic reticulum (ER) membrane after synthesis, uncoupled from translation. However, translocation across the mammalian ER is believed to be coupled to protein synthesis. In this study we show that opsin, within a range of nascent peptide lengths, targets and translocates equally efficiently co- and posttranslationally. Longer nascent opsin peptides have a lower efficiency of cotranslational translocation but an even lower efficiency of posttranslational translocation. We also show that SRP is required for both co- and posttranslational targeting.

Dopamine has a critical role in photoreceptor degeneration in the rd mouse

Photoreceptors receive paracrine input from dopaminergic interplexiform cells. Rod photoreceptors in the rd mouse degenerate rapidly due to a specific gene defect. We investigated the effects of dopamine on rd mouse photoreceptors in retinal organ culture. Retinas were harvested from rd or wild-type mice at postnatal day 2 and grown in organ culture for 27 days. When antagonists for either D(1)- or D(2)-family dopamine receptors were added to the media, photoreceptor degeneration was blocked. Furthermore, when dopamine was depleted by the addition of 6-hydroxydopamine and pargyline, photoreceptor survival appeared comparable to wild-type retinal cultures. The addition of a dopamine agonist induced photoreceptor degeneration in dopamine-depleted rd organ cultures. In all cases, photoreceptors maintained robust staining of opsin. These results demonstrate that dopamine antagonists or dopamine depletion blocks photoreceptor degeneration and that dopamine is necessary for photoreceptor degeneration in the rd mouse retinal organ culture model, indicating that dopamine antagonists may represent a therapeutic strategy in retinal degenerative disease.

Photoreceptive net in the mammalian retina. This mesh of cells may explain how some blind mice can still tell day from night

We have discovered an expansive photoreceptive 'net' in the mouse inner retina, visualized by using an antiserum against melanopsin, a likely photopigment. This immunoreactivity is evident in a subset of retinal ganglion cells that morphologically resemble those that project to the suprachiasmatic nucleus (SCN), the site of the primary circadian pacemaker. Our results indicate that this bilayered photoreceptive net is anatomically distinct from the rod and cone photoreceptors of the outer retina, and suggest that it may mediate non-visual photoreceptive tasks such as the regulation of circadian rhythms.

A novel isoform of vertebrate ancient opsin in a smelt fish, Plecoglossus altivelis

Vertebrate ancient (VA) opsin of nonvisual pigment in fishes was reported to exist in two isoforms, i.e., short and long variants with an unusual predicted amino acid sequence length compared to vertebrate visual opsins. Here we cloned an isoform (Pal-VAM) of VA opsin showing the usual opsin length in addition to the long type isoform (Pal-VAL) from a smelt fish, Plecoglossus altivelis. Pal-VAM and Pal-VAL were composed of 346 and 387 amino acids, respectively. The deduced amino acid sequences of these variants were identical to each other within the first 342 residues, but they showed divergence in the carboxyl-terminal sequence. Pal-VAL corresponded to the long isoform found in zebrafish and carp, and Pal-VAM was identified as a new type of VA opsin variant. Southern blotting experiments indicated that the VA opsin gene of the smelt is present as a single copy, and RT-PCR analysis revealed that Pal-VAM and Pal-VAL mRNA were expressed in both the eyes and brain. In situ hybridization showed that Pal-VAM and Pal-VAL mRNA are expressed in amacrine cells in the retina. Pal-VAM is a new probably functional nonvisual photoreceptive molecule in fish.

The photopigment melanopsin is exclusively present in pituitary adenylate cyclase-activating polypeptide-containing retinal ganglion cells of the retinohypothalamic tract

Mammalian circadian rhythms generated in the hypothalamic suprachiasmatic nuclei are entrained to the environmental light/dark cycle via a monosynaptic pathway, the retinohypothalamic tract (RHT). We have shown previously that retinal ganglion cells containing pituitary adenylate cyclase-activating polypeptide (PACAP) constitute the RHT. Light activates the RHT via unknown photoreceptors different from the classical photoreceptors located in the outer retina. Two types of photopigments, melanopsin and the cryptochromes (CRY1 and CRY2), both of which are located in the inner retina, have been suggested as "circadian photopigments." In the present study, we cloned rat melanopsin photopigment cDNA and produced a specific melanopsin antibody. Using in situ hybridization histochemistry combined with immunohistochemistry, we demonstrate that the distribution of melanopsin was identical to that of the PACAP-containing retinal ganglion cells. Colocalization studies using the specific melanopsin antibody and/or cRNA probes in combination with PACAP immunostaining revealed that melanopsin was found exclusively in the PACAP-containing retinal ganglion cells located at the surface of somata and dendrites. These data, in conjunction with published action spectra analyses and work in retinally degenerated (rd/rd/cl) mutant mice, suggest that melanopsin is a circadian photopigment located in retinal ganglion cells projecting to the biological clock.

Pinopsin mRNA levels are significantly elevated in the pineal glands of chickens carrying a null mutation in guanylate cyclase-1

The purpose of this study was to determine if the absence of guanylate cyclase-1 (RetGC1, GC1), a key visual phototransduction cascade enzyme that is expressed in both retinal photoreceptors and pinealocytes, disrupts light regulation of pinopsin mRNA levels in the chicken pineal gland. In this series of experiments, we compared levels of pinopsin and tryptophan 5-hydroxylase mRNA in the pineal glands of GUCY1*B (*B) and normal chickens housed under either cyclic light or constant dark conditions. The *B chicken carries a null mutation in the gene encoding guanylate cyclase-1 that results in blindness in these animals at hatching. The results of our experiments show (1) that the amount of pinopsin mRNA in *B pineal is significantly higher than the amount in normal pineal in both light and dark conditions, (2) that light induces an increase in pinopsin mRNA levels in *B pineal, (3) that the relative magnitude of the light-induced increase in pinopsin mRNA in *B pineal is not significantly different from that observed in normal pineal, and (4) that the changes in the regulation of pinopsin mRNA levels in *B pineal gland are not accompanied by changes in the circadian expression of tryptophan 5-hydroxylase mRNA. These results show that the absence of guanylate cyclase-1 expression in the *B pineal gland leads to a significant increase in basal levels of pinopsin mRNA in this gland but does not alter the magnitude of the increase in pinopsin mRNA levels that is observed as a result of light stimulation.

Rods are rods and cones cones, and (never) the twain shall meet

More than 100 photopigment G protein-coupled receptors (opsins) have been sequenced and organized into six classes. Rod photoreceptors in various species have been found to express an opsin from one of the two rhodopsin classes, while cones express an opsin from one of the four remaining classes. It has now been discovered that salamander short-wavelength sensitive cones and green rods express the same opsin, while manifesting other features that classically distinguish rods from cones.

Molecular cloning of Bombyx cerebral opsin (Boceropsin) and cellular localization of its expression in the silkworm brain

We have cloned a cDNA for a novel opsin from the larval brain of the silkworm Bombyx mori in which the photoperiodic photoreceptor had been supposed to reside in the cephalic central nervous system (CNS). Its deduced amino acid sequence was composed of 381 amino acids and included amino acid residues highly conserved in insect visual pigments. This opsin belonged to the long wavelength photoreceptor group of insect opsins and showed the greatest degree of homology (84%) with the green visual photoreceptor in the sphingid moth. We have designated this Bombyx cerebral opsin as Boceropsin. Southern blotting experiments indicated that the Boceropsin gene is present in a single copy, and RT-PCR analysis revealed that Boceropsin mRNA is expressed in the larval brain but not in the subesophageal ganglion (Sg) or thoracic ganglion (Tg). Immunohistochemical analyses demonstrated that Boceropsin protein is present bilaterally in some defined cells localized in the brain of Bombyx larvae. This is the first report of expression of an opsin-based protein in CNS of an insect. The possibility that the Boceropsin functions as the photoperiodic receptive pigment in the silkworm is also discussed.

Developmental patterns of protein expression in photoreceptors implicate distinct environmental versus cell-intrinsic mechanisms

The present study has examined the spatial and temporal expression patterns of various proteins associated with the structure and function of mature photoreceptor outer segments in the developing ferret's retina using immunocytochemistry and RT-PCR. One set of proteins, including rod opsin, arrestin, and recoverin, was detected progressively in photoreceptors as they became postmitotic, being expressed well before the differentiation of outer segments. A second set of proteins, including beta- and gamma-transducin, cGMP-phosphodiesterase, phosducin, rhodopsin kinase, rod cGMP-gated cation channel protein, and peripherin, displayed a contrasting temporal onset and pattern of spatial emergence. These latter proteins first became detectable either shortly before or coincident with outer segment formation, and were expressed simultaneously in both older and younger photoreceptor cells. A third set, the short wavelength-sensitive (SWS) and medium wavelength-sensitive (MWS) cone opsin proteins, was the last to be detected, but materialized in a spatio-temporal pattern reminiscent of the neurogenetic gradient of the cones. These different spatial and temporal patterns indicate that cellular maturation must play a primary role in regulating the onset of expression of some of these proteins, while extrinsic signals must act to coordinate the expression of other proteins across photoreceptors of different ages.

Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor

The photopigment in the human eye that transduces light for circadian and neuroendocrine regulation, is unknown. The aim of this study was to establish an action spectrum for light-induced melatonin suppression that could help elucidate the ocular photoreceptor system for regulating the human pineal gland. Subjects (37 females, 35 males, mean age of 24.5 +/- 0.3 years) were healthy and had normal color vision. Full-field, monochromatic light exposures took place between 2:00 and 3:30 A.M. while subjects' pupils were dilated. Blood samples collected before and after light exposures were quantified for melatonin. Each subject was tested with at least seven different irradiances of one wavelength with a minimum of 1 week between each nighttime exposure. Nighttime melatonin suppression tests (n = 627) were completed with wavelengths from 420 to 600 nm. The data were fit to eight univariant, sigmoidal fluence-response curves (R(2) = 0.81-0.95). The action spectrum constructed from these data fit an opsin template (R(2) = 0.91), which identifies 446-477 nm as the most potent wavelength region providing circadian input for regulating melatonin secretion. The results suggest that, in humans, a single photopigment may be primarily responsible for melatonin suppression, and its peak absorbance appears to be distinct from that of rod and cone cell photopigments for vision. The data also suggest that this new photopigment is retinaldehyde based. These findings suggest that there is a novel opsin photopigment in the human eye that mediates circadian photoreception.

Pinopsin expressed in the retinal photoreceptors of a diurnal gecko

Retinal cDNAs encoding the putative opsins, dg3 and dg4, were isolated from a diurnal gecko, Phelsuma madagascariensis longinsulae. dg3 mRNA is localized in about 20% of the thin members of type C double cones, and likely encodes an opsin of the ultraviolet-sensitive pigment. Surprisingly, dg4 is very similar to chicken pinopsin, a pineal-specific photoreceptive molecule. An anti-dg4 antiserum recognized a small population of photoreceptor outer segments in the retina and a large number of pinealocytes. Our results suggest that P. m. longinsulae expresses pinopsin in its retina, which usually plays a role as a photoreceptive molecule in the pineal organ.

Isolation of a zebrafish rod opsin promoter to generate a transgenic zebrafish line expressing enhanced green fluorescent protein in rod photoreceptors

To exploit zebrafish as a transgenic model, tissue-specific promoters must be identified. We isolated a 20-kilobase (kbp) zebrafish rod opsin genomic clone, which consists of 18 kbp of 5'-flanking region, the entire coding region, and 0.5 kbp of 3'-flanking sequence. Polymerase chain reaction, Southern blotting, and DNA sequencing revealed the rod opsin gene lacks introns. The transcription start site was localized 94 nucleotides upstream of the translation initiation site. Sequence alignment with orthologous promoters revealed conserved cis-elements including glass, NRE, OTX/Bat-1, Ret-1/PCE-1, Ret-4, and TATA box. A 1.2-kbp promoter fragment was cloned upstream of the enhanced green fluorescent protein (EGFP) cDNA and microinjected into 1- to 2-cell stage zebrafish embryos. EGFP expression was detected in the ventral-nasal eye at 3 days postfertilization and spread throughout the eye. Progeny of the positive founder fish, which were identified by polymerase chain reaction amplification of fin genomic DNA, exhibited EGFP expression in the retina, confirming the germline transmission of the transgene. Frozen eye sections demonstrated the EGFP expression was rod-specific and exhibited a similar developmental expression profile as the rod opsin protein. This stable transgenic line provides a novel tool for identification of genes regulating development and maintenance of rod photoreceptors.

The relationship between opsin overexpression and photoreceptor degeneration

PURPOSE

To characterize the process by which overexpression of normal opsin leads to photoreceptor degeneration.

METHODS

Three transgenic mouse lines were generated that express different levels of an opsin with three amino acid modifications at the C terminus. These modifications created an epitopic site that can be readily distinguished from the endogenous protein using a bovine opsin-specific antibody. Evidence of degeneration associated with opsin overexpression was provided by anatomic studies and electroretinogram (ERG) recordings. Western blot analysis was used to confirm the production of the transgenic opsin, and an enzyme-linked immunosorbent assay (ELISA) was used to determine the amounts of opsin overexpressed in each line. Immunocytochemistry was used to determine the cellular localization of transgenic opsin. Amounts of 11-cis retinal were determined by extraction and high-performance liquid chromatography (HPLC).

RESULTS

Opsin expression levels in the three lines were found to be 123%, 169%, and 222% of the level measured in nontransgenic animals, providing direct correlation between the level of transgene expression and the severity of the degenerative phenotype. In the lower expressing lines, ERG a-wave amplitudes were reduced to less than approximately 30% and 15% of normal values, whereas responses of the highest expressing line were indistinguishable from noise. In the lowest expressor, a 26% elevation in 11-cis retinal was observed, whereas in the medium and the high expressors, 11-cis retinal levels were increased by only 30% to 33%, well below the 69% and 122% increases in opsin levels.

CONCLUSIONS

The overexpression of normal opsin induces photoreceptor degeneration that is similar to that seen in many mouse models of retinitis pigmentosa. This degeneration can be induced by opsin levels that exceed by only approximately 23% that of the normal mouse retina. Opsin overexpression has potential implications in retinitis pigmentosa.

Tetracycline-inducible system for photoreceptor-specific gene expression

PURPOSE

To develop a system for inducible photoreceptor-specific gene expression in transgenic mice. The tetracycline regulatory system was chosen because it possesses the useful property of direct control of gene expression through use of an exogenous agent, doxycycline, a tetracycline derivative.

METHODS

Transgenic mice were generated that carried the reverse tetracycline-controlled transactivator under the control of the photoreceptor-specific promoters for rhodopsin and interphotoreceptor retinoid-binding protein. These animals were crossed with transgenic mice carrying the lacZ reporter gene under control of the tetracycline operator cassette, creating doubly transgenic mice. Doxycycline was administered to induce expression of the reporter gene. Reporter assays were then performed to evaluate lacZ expression.

RESULTS

Doxycycline administration led to photoreceptor-specific expression of the lacZ reporter gene in the doubly transgenic mice. X-gal staining was restricted to photoreceptor inner segments and synaptic termini. Induction could be achieved by addition of the drug to the animals' drinking water or by intravitreal injection. Induction was noted within 24 hours of doxcycline administration. Because of variability among animals, there was an approximate correlation, but not a clean dose-response curve relating drug dose to level of reporter expression.

CONCLUSIONS

A transgenic system for inducible photoreceptor-specific gene expression has been developed. This system is currently being exploited to study the effects of regulated expression of genes of biological interest.

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.

Expression of a recombinant human RGR opsin in Lentivirus-transduced cultured cells

PURPOSE

Our goals were to produce a functional recombinant RPE retinal G protein-coupled receptor (RGR) opsin for biochemical studies and to test the efficiency of a lentiviral vector for transgene expression of human RGR.

METHODS

A human RGR cDNA was cloned into a replication-defective lentiviral vector, and recombinant hRGR-Lentivirus was prepared for transduction of the ARPE-19, a human retinal pigment epithelium (RPE) cell line, and COS-7 cells. Recombinant RGR expression was detected by Western blot analysis, and functionality of the protein was tested by a [3H]all-trans-retinal binding assay.

RESULTS

RGR protein was detected in each cell type after transduction with recombinant virus and was not observed in untreated cells. RGR expression in ARPE-19 cells increased steadily for up to 10 days after transduction and was stable for at least 6 months. The transduced ARPE-19 cells produced approximately 100-fold higher amounts of RGR protein than the transduced COS-7 cells. When cell membranes from the ARPE-19 cells were incubated with [3H]all-trans-retinal, the chromophore bound specifically to the expressed protein. Uptake of [3H]all-trans-retinol into the ARPE-19 cells was followed by specific binding of radiolabeled retinoid to RGR.

CONCLUSIONS

Using a Lentivirus-derived gene delivery system, we were able to express high amounts of human RGR protein in the ARPE-19 human RPE cell line. The transduced ARPE-19 cells remain able to process all-trans-retinol, and the expressed protein is capable of binding to the all-trans-retinal chromophore. The Lentivirus-based expression of functional RGR can be used to study RGR in cultured cells and to test in vivo transduction of quiescent RPE cells.

Pigment epithelium-derived factor supports normal development of photoreceptor neurons and opsin expression after retinal pigment epithelium removal

Dysfunction of the retinal pigment epithelium (RPE), its loss, or separation from the underlying neural retina results in severe photoreceptor degeneration. Pigment epithelium-derived factor (PEDF) is a glycoprotein with reported neuroprotective and differentiation properties that is secreted in abundance by RPE cells. The "pooling" of PEDF within the interphotoreceptor matrix places this molecule in a prime physical location to affect the underlying neural retina. The purpose of this study was to analyze the morphogenetic activity of PEDF in a model of photoreceptor dysmorphogenesis induced by removal of the RPE. Eyes were dissected from embryonic Xenopus laevis, and the RPE was removed before culturing in medium containing PEDF, PEDF plus anti-PEDF antibodies, or medium alone. Control retinas were maintained with an adherent RPE. Light and electron microscopic analysis was used to examine retinal ultrastructure. Opsin was localized immunocytochemically and quantified as an index of outer segment membranous material and photoreceptor protein expression. Removal of the RPE resulted in an aberrant assembly of photoreceptor outer segments, loss of fine subcellular ultrastructure in photoreceptors, and a reduction in opsin protein levels when compared with control retinas. The addition of PEDF prevented the dysmorphic photoreceptor changes induced by RPE removal. In particular, photoreceptor ultrastructure, outer segment membrane assembly, and steady-state levels of opsin were equivalent to control conditions. Anti-PEDF antibodies completely blocked the morphogenetic activity of PEDF. These results indicate that PEDF is able to mimic the supportive role of the RPE on photoreceptors during the final stages of retinal morphogenesis.

The murine cone photoreceptor: a single cone type expresses both S and M opsins with retinal spatial patterning

Mice express S and M opsins that form visual pigments for the detection of light and visual signaling in cones. Here, we show that S opsin transcription is higher than that of M opsin, which supports ultraviolet (UV) sensitivity greater than midwavelength sensitivity. Surprisingly, most cones coexpress both S and M opsins in a common cone cell type throughout the retina. All cones express M opsin, but the levels are graded from dorsal to ventral. The levels of S opsin are relatively constant. However, in the far dorsal retina, S opsin is repressed stochastically, such that some cones express M opsin only. These observations indicate that two different mechanisms control M and S opsin expression. We suggest that a common cone type is patterned across the retinal surface to produce phenotypic cone subtypes.