Nonvisual Photoreception in the Chick Iris

Midnight/midday-synchronized expression of cryptochrome genes in the eyes of three teleost species, zebrafish, goldfish, and medaka

Photoperiodic responses are observed in many organisms living in the temperate zones. The circadian clock is involved in photoperiodic time measurement; however, the underlying molecular mechanism for detection of the day length remains unknown. We previously compared the expression profiles of the Cryptochrome(Cry) genes in the zebrafish eye and reported that Cry1ab has a double peak with variable expression duration depending on the photoperiod. In this study, to understand commonalities and differences in the photoperiodic responses of ocular Cry genes, we identified Cryptochrome genes in two other teleost species, goldfish and medaka, living in temperate zones, and measured ocular Cry mRNA levels in all of the three species, under different photoperiods (long-day [14 h light: 10 h dark] and short-day [10 h light: 14 h dark] and in constant darkness. Cry1ab mRNA levels did not show dual peaks in goldfish or medaka under the examined conditions; however, the mRNA expression profiles of many Crys were altered in all three species, depending on the day length and light condition. Based on their expression profiles, Cry mRNA peaks were classified into three groups that better synchronize to sunrise (light-on), midnight/midday (middle points of the dark/light periods), or sunset (light-off). These results suggest the presence of multiple oscillators that oscillate independently or a complex oscillator in which Cry expression cycles change in a photoperiod-dependent manner in the eye.

Trpm1: Novel function at the intersection of light and pain response in the iris

In mammals, the retina is the photosensitive tissue that is responsible for the capture of light and the transduction of the light-initiated signals to the brain. These visual signals help to drive image and non-image forming behaviors. The pupillary light reflex (PLR) is an involuntary non-image forming behavior which involves the constriction of the iris muscle tissue in response to ambient light intensity. A subset of photosensitive retinal ganglion cells provides the principal pathway for all light input to the olivary pretectal nucleus which directs the neuronal input to drive iris constriction. Transient receptor potential melastatin 1 (Trpm1) knockout mice have a severe defect in PLR, but it remains unclear how the Trpm1 channel contributes to this behavior. We have demonstrated that the reduced PLR in Trpm1^-/- mice at scotopic and photopic intensities is due to a functional loss of Trpm1 in the retina as well as the iris sphincter muscle. We have also tested constriction in isolated eyes and have shown that light-driven constriction independent of signaling from the brain also requires Trpm1 expression. In both the in vivo PLR and the iris photomechanical response, melanopsin is required for the light-dependent activation. Finally, pharmacological experiments using capsaicin to activate pain afferents in the eye demonstrate that Trpm1 expression is required for all sensory driven iris constriction. Our results demonstrate for the first time that Trpm1 has a novel and necessary role in iridial cells and is required for all sensory-driven constriction in the iris.

Diversity of intrinsically photosensitive retinal ganglion cells: circuits and functions

The melanopsin-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs) are a relatively recently discovered class of atypical ganglion cell photoreceptor. These ipRGCs are a morphologically and physiologically heterogeneous population that project widely throughout the brain and mediate a wide array of visual functions ranging from photoentrainment of our circadian rhythms, to driving the pupillary light reflex to improve visual function, to modulating our mood, alertness, learning, sleep/wakefulness, regulation of body temperature, and even our visual perception. The presence of melanopsin as a unique molecular signature of ipRGCs has allowed for the development of a vast array of molecular and genetic tools to study ipRGC circuits. Given the emerging complexity of this system, this review will provide an overview of the genetic tools and methods used to study ipRGCs, how these tools have been used to dissect their role in a variety of visual circuits and behaviors in mice, and identify important directions for future study.

Elucidation of Cellular Mechanisms That Regulate the Sustained Contraction and Relaxation of the Mammalian Iris

Purpose

In mammals, pupil constriction and dilation form the pupillary light reflex (PLR), which is mediated by both brain-regulated (parasympathetic) and local iris-driven reflexes. To better understand the cellular mechanisms that regulate pupil physiological dynamics via central and local photoreception, we have examined the regulation of the PLR via parasympathetic and local activation, respectively.

Methods

In this study, the PLR was examined in mouse enucleated eyes ex vivo in real-time under different ionic conditions in response to acetylcholine and/or blue light (480 nm). The use of pupillometry recordings captured the relaxation, contraction, and pupil escape (redilation) processes for 10 minutes up to 1 hour.

Results

Among others, our results show that ryanodine receptor channels are the main driver for iridal stimulation-contraction coupling, in which extracellular influx of Ca2+ is required for amplification of pupil constriction. Both local and parasympathetic iridal activations are necessary, but not sufficient for sustained pupil constriction. Moreover, the degree of membrane potential repolarization in the dark is correlated with the latency and velocity of iridal constriction. Furthermore, pupil escape is driven by membrane potential hyperpolarization where voltage-gated potassium channels play a crucial role.

Conclusions

Together, this study presents new mechanisms regulating synchronized pupil dilation and contraction, sustained pupil constriction, iridal stimulation-contraction coupling, and pupil escape.

Retinal control of lens-induced astigmatism in chicks

PURPOSE

Astigmatism is a refractive error due to meridional differences in refractive powers of lens or cornea. The resulting failure to focus image points in a single plane causes blurred vision at all distances. In this study, using an animal model of lens-induced astigmatism, we tested the hypothesis that induced astigmatism is due to processing of astigmatic retinal image information by the brain, which causes distorted growth in the anterior segment via centrifugal neural projections.

METHODS

To induce astigmatism, +4.00DS/-8.00DC crossed-cylinder-lens goggles were affixed over the right eyes of 7-day-old chicks (P7), with the -8.00DC axis oriented vertically (at 90°) or horizontally (180°) (n = 12 each); the left eyes were without goggles (non-goggled). For all experiments, refractive errors of both eyes were measured by streak retinoscopy, before and after 1 week of lens wear. To test whether neuronal pathways between retina and brain are required, axonal conduction within the eye was blocked by intravitreal injections of tetrodotoxin (TTX; 7 μL of 10^-4M) in phosphate-buffered saline (PBS), or of PBS alone (7 μL); fellow open eyes received PBS alone. Pupillary light reflex (PLR) and optokinetic response (OKR) were measured, to assess the efficacy and duration of TTX action. To test whether retinal circuitry is required, groups of chicks (n = 12 each) were treated at P7 by intravitreal injection of 20 μL of mixed excitotoxins (2 μmol N-methyl-D-aspartate, 0.2 μmol quisqualic acid, 0.2 μmol kainic acid; in water) into goggled or non-goggled eyes, to compromise retinal circuitry needed for emmetropization.

RESULTS

Crossed-cylinder goggles reliably induced refractive astigmatism. Maximum astigmatic error was induced when the cylindrical axis was oriented at 90° (vertically). TTX effectively blocked nerve conduction within the eye for 48 h after injection. Goggled eyes developed astigmatism after treatment with TTX or PBS, but not after excitotoxins.

CONCLUSION

Our hypothesis was rejected. In this model, the compensatory astigmatism induced by crossed-cylinder lenses is intrinsic to the eye, and mediated by visual processing in the retina.

Cryptochromes Mediate Intrinsic Photomechanical Transduction in Avian Iris and Somatic Striated Muscle

Irises isolated from the eyes of diverse species constrict when exposed to light. Depending on species this intrinsic photomechanical transduction response (PMTR) requires either melanopsin or cryptochrome (CRY) photopigment proteins, generated by their respective association with retinoid or flavin adenine dinucleotide (FAD) chromophores. Although developmentally relevant circadian rhythms are also synchronized and reset by these same proteins, the cell type, mechanism, and specificity of photomechanical transduction (PMT) and its relationship to circadian processes remain poorly understood. Here we show that PMTRs consistent with CRY activation by 430 nm blue light occur in developing chicken iris striated muscle, identify relevant mechanisms, and demonstrate that similar PMTRs occur in striated iris and pectoral muscle fibers, prevented in both cases by knocking down CRY gene transcript levels. Supporting CRY activation, iris PMTRs were reduced by inhibiting flavin reductase, but unaffected by melanopsin antagonism. The largest iris PMTRs paralleled the developmental predominance of striated over smooth muscle fibers, and shared their requirement for extracellular Ca²⁺ influx and release of intracellular Ca²⁺. Photo-stimulation of identified striated myotubes maintained in dissociated culture revealed the cellular and molecular bases of PMT. Myotubes in iris cell cultures responded to 435 nm light with increased intracellular Ca²⁺ and contractions, mimicking iris PMTRs and their spectral sensitivity. Interestingly PMTRs featuring contractions and requiring extracellular Ca²⁺ influx and release of intracellular Ca²⁺ were also displayed by striated myotubes derived from pectoral muscle. Consistent with these findings, cytosolic CRY1 and CRY2 proteins were detected in both iris and pectoral myotubes, and knocking down myotube CRY1/CRY2 gene transcript levels specifically blocked PMTRs in both cases. Thus CRY-mediated PMT is not unique to iris, but instead reflects a more general feature of developing striated muscle fibers. Because CRYs are core timing components of circadian clocks and CRY2 is critical for circadian regulation of myogenic differentiation CRY-mediated PMT may interact with cell autonomous clocks to influence the progression of striated muscle development.

The pupillary light responses of animals; a review of their distribution, dynamics, mechanisms and functions

The timecourse and extent of changes in pupil area in response to light are reviewed in all classes of vertebrate and cephalopods. Although the speed and extent of these responses vary, most species, except the majority of teleost fish, show extensive changes in pupil area related to light exposure. The neuromuscular pathways underlying light-evoked pupil constriction are described and found to be relatively conserved, although the precise autonomic mechanisms differ somewhat between species. In mammals, illumination of only one eye is known to cause constriction in the unilluminated pupil. Such consensual responses occur widely in other animals too, and their function and relation to decussation of the visual pathway is considered. Intrinsic photosensitivity of the iris muscles has long been known in amphibia, but is in fact widespread in other animals. The functions of changes in pupil area are considered. In the majority of species, changes in pupil area serve to balance the conflicting demands of high spatial acuity and increased sensitivity in different light levels. In the few teleosts in which pupil movements occur they do not serve a visual function but play a role in camouflaging the eye of bottom-dwelling species. The occurrence and functions of the light-independent changes in pupil size displayed by many animals are also considered. Finally, the significance of the variations in pupil shape, ranging from circular to various orientations of slits, ovals, and other shapes, is discussed.

Synergistic Signaling by Light and Acetylcholine in Mouse Iris Sphincter Muscle

The mammalian pupillary light reflex (PLR) involves a bilateral brain circuit whereby afferent light signals in the optic nerve ultimately drive iris-sphincter-muscle contraction via excitatory cholinergic parasympathetic innervation [1, 2]. Additionally, the PLR in nocturnal and crepuscular sub-primate mammals has a "local" component in the isolated sphincter muscle [3-5], as in amphibians, fish, and bird [6-10]. In mouse, this local PLR requires the pigment melanopsin [5], originally found in intrinsically photosensitive retinal ganglion cells (ipRGCs) [11-19]. However, melanopsin's presence and effector pathway locally in the iris remain uncertain. The sphincter muscle itself may express melanopsin [5], or its cholinergic parasympathetic innervation may be modulated by suggested intraocular axonal collaterals of ipRGCs traveling to the eye's ciliary body or even to the iris [20-22]. Here, we show that the muscarinic receptor antagonist, atropine, eliminated the effect of acetylcholine (ACh), but not of light, on isolated mouse sphincter muscle. Conversely, selective genetic deletion of melanopsin in smooth muscle mostly removed the light-induced, but not the ACh-triggered, increase in isolated sphincter muscle's tension and largely suppressed the local PLR in vivo. Thus, sphincter muscle cells are bona fide, albeit unconventional, photoreceptors. We found melanopsin expression in a small subset of mouse iris sphincter muscle cells, with the light-induced contractile signal apparently spreading through gap junctions into neighboring muscle cells. Light and ACh share a common signaling pathway in sphincter muscle. In summary, our experiments have provided details of a photosignaling process in the eye occurring entirely outside the retina.

Ocular Photoreception for Circadian Rhythm Entrainment in Mammals

Circadian rhythms are self-sustained, approximately 24-h rhythms of physiology and behavior. These rhythms are entrained to an exactly 24-h period by the daily light-dark cycle. Remarkably, mice lacking all rod and cone photoreceptors still demonstrate photic entrainment, an effect mediated by intrinsically photosensitive retinal ganglion cells (ipRGCs). These cells utilize melanopsin (OPN4) as their photopigment. Distinct from the ciliary rod and cone opsins, melanopsin appears to function as a stable photopigment utilizing sequential photon absorption for its photocycle; this photocycle, in turn, confers properties on ipRGCs such as sustained signaling and resistance from photic bleaching critical for an irradiance detection system. The retina itself also functions as a circadian pacemaker that can be autonomously entrained to light-dark cycles. Recent experiments have demonstrated that another novel opsin, neuropsin (OPN5), is required for this entrainment, which appears to be mediated by a separate population of ipRGCs. Surprisingly, the circadian clock of the mammalian cornea is also light entrainable and is also neuropsin-dependent for this effect. The retina thus utilizes a surprisingly broad array of opsins for mediation of different light-detection tasks.

Overexpression in yeast, photocycle, and in vitro structural change of an avian putative magnetoreceptor cryptochrome4

Cryptochromes (CRYs) have been found in a wide variety of living organisms and can function as blue light photoreceptors, circadian clock molecules, or magnetoreceptors. Non-mammalian vertebrates have CRY4 in addition to the CRY1 and CRY2 circadian clock components. Though the function of CRY4 is not well understood, chicken CRY4 (cCRY4) may be a magnetoreceptor because of its high level of expression in the retina and light-dependent structural changes in retinal homogenates. To further characterize the photosensitive nature of cCRY4, we developed an expression system using budding yeast and purified cCRY4 at yields of submilligrams of protein per liter with binding of the flavin adenine dinucleotide (FAD) chromophore. Recombinant cCRY4 dissociated from anti-cCRY4 C1 mAb, which recognizes the C-terminal region of cCRY4, in a light-dependent manner and showed a light-dependent change in its trypsin digestion pattern, suggesting that cCRY4 changes its conformation with light irradiation in the absence of other retinal factors. Combinatorial analyses with UV-visible spectroscopy and immunoprecipitation revealed that there is chromophore reduction in the cCRY4 photocycle and formation of a flavosemiquinone radical intermediate that is likely accompanied by a conformational change in the carboxyl-terminal region. Thus, cCRY4 seems to be an intrinsically photosensitive and photoswitchable molecule and may exemplify a vertebrate model of cryptochrome with possible function as a photosensor and/or magnetoreceptor.

A role for the outer retina in development of the intrinsic pupillary light reflex in mice

Mice do not require the brain in order to maintain constricted pupils. However, little is known about this intrinsic pupillary light reflex (iPLR) beyond a requirement for melanopsin in the iris and an intact retinal ciliary marginal zone (CMZ). Here, we study the mouse iPLR in vitro and examine a potential role for outer retina (rods and cones) in this response. In wild-type mice the iPLR was absent at postnatal day 17 (P17), developing progressively from P21-P49. However, the iPLR only achieved ∼ 30% of the wild-type constriction in adult mice with severe outer retinal degeneration (rd and rdcl). Paradoxically, the iPLR increased significantly in retinal degenerate mice >1.5 years of age. This was accompanied by an increase in baseline pupil tone in the dark to levels indistinguishable from those in adult wild types. This rejuvenated iPLR response was slowed by atropine application, suggesting the involvement of cholinergic neurotransmission. We could find no evidence of an increase in melanopsin expression by quantitative PCR in the iris and ciliary body of aged retinal degenerates and a detailed anatomical analysis revealed a significant decline in melanopsin-positive intrinsically photosensitive retinal ganglion cells (ipRGCs) in rdcl mice >1.5 years. Adult mice lacking rod function (Gnat1(-/-)) also had a weak iPLR, while mice lacking functional cones (Cpfl5) maintained a robust response. We also identify an important role for pigmentation in the development of the mouse iPLR, with only a weak and transient response present in albino animals. Our results show that the iPLR in mice develops unexpectedly late and are consistent with a role for rods and pigmentation in the development of this response in mice. The enhancement of the iPLR in aged degenerate mice was extremely surprising but may have relevance to behavioral observations in mice and patients with retinitis pigmentosa.

Cryptochrome expression in the eye of migratory birds depends on their migratory status

Most passerine birds are nocturnal migrants. When kept in captivity during the migratory periods, these species show a migratory restlessness, or Zugunruhe. Recent studies on Sylvia warblers have shown that Zugunruhe is an excellent proxy of migratory disposition. Passerine birds can use the Earth's geomagnetic field as a compass to keep their course during their migratory flight. Among the candidate magnetoreceptive mechanisms are the cryptochromes, flavoproteins located in the retina that are supposed to perceive the magnetic field through a light-mediated process. Previous work has suggested that expression of Cryptochrome 1 (Cry1) is increased in migratory birds compared with non-migratory species. Here we tested the hypothesis that Cry1 expression depends on migratory status. Blackcaps Sylvia atricapilla were caught before fall migration and held in registration cages. When the birds were showing robust Zugunruhe, we applied a food deprivation protocol that simulates a long migratory flight. When the birds were refed after 2 days, their Zugunruhe decreased substantially, as is expected from birds that would interrupt migration for a refuelling stopover. We found that Cry1 expression was higher at night than during daytime in birds showing Zugunruhe, whereas in birds that underwent the fasting-and-refeeding protocol and reduced their levels of Zugunruhe, night Cry1 expression decreased to daytime levels. Our work shows that Cry1 expression is dependent on the presence of Zugunruhe and not on species-specific or seasonal factors, or on the birds being active versus inactive. These results support the hypothesis that cryptochromes underlie magnetoreceptive mechanisms in birds.

Identification and characterization of cryptochrome4 in the ovary of western clawed frog Xenopus tropicalis

CRY proteins can be classified into several groups based on their phylogenetic relationships, and they function as a photoreceptor, a photolyase, and/or a transcriptional repressor of the circadian clock. In order to elucidate the expression profile and functional diversity of CRYs in vertebrates, we focused on XtCRY4, a member of the uncharacterized cryptochrome family CRY4 in Xenopus tropicalis. XtCRY4 cDNA was isolated by RT-PCR, and a phylogenetic analysis of deduced sequence of XtCRY4 suggested that the vertebrate Cry4 genes evolved at much higher evolutionary rates than mammalian-type Cry genes, such as the CRY1 and CRY2 circadian clock molecules. A transcriptional assay was performed to examine the transcriptional regulatory function as circadian repressor, and XtCRY4 had marginal effects on the transactivation of XtCLOCK/XtBMAL1 via E-box element. In situ hybridization and quantitative RT-PCR was performed to detect mRNA expression in native tissues. Quantitative RT-PCR revealed that XtCry4 mRNA was highly transcribed in the ovary. In situ hybridization showed the presence of XtCry4 transcripts in the oocytes, testis, renal tubules, the visual photoreceptors, and the retinal ganglion cells. A specific antiserum to XtCRY4 was developed to detect endogeneous expression of XtCRY4 protein in the ovary. The expression level was estimated by immunoblot analysis, and this is the first detection and estimation of endogenous expression of CRY protein in the ovary. These results suggest that X. tropicalis ovary may respond to blue-light by using XtCRY4.

Cryptochrome-mediated light responses in plants

Cryptochromes (CRYs) are photolyase-like flavoproteins that have been found in all evolutionary lineages. Plant and animal CRYs are no longer DNA-repairing enzymes but they apparently gained other biochemical functions in evolution. Plant CRYs are UV-A/blue-light photoreceptors and play a pivotal role in plant growth and development, whereas animal CRYs act as either photoreceptors or transcription regulators. The first CRY gene was isolated from Arabidopsis thaliana, which regulates stem growth, flowering time, stomatal opening, circadian clock, and other light responses. CRYs are also found in all major crops investigated, with additional functions discovered, such as seed germination, leaf senescence, and stress responses. In this chapter, we will review some aspects of CRY-mediated light responses in plants. Readers are referred to other review articles for photochemistry and signal transduction mechanism of plant CRYs (Liu et al., 2010, 2011; Fankhauser and Ulm, 2011) [1-3].

A role for the ciliary marginal zone in the melanopsin-dependent intrinsic pupillary light reflex

Maintenance of pupillary constriction in light-adapted rodents has traditionally been thought to involve a reflex between retina, brain and iris, with recent work identifying the melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) as the major conduits for retinal input to the brain. There is also a less well-understood phenomenon whereby the iris of some mammals, including mice, will constrict to light when either the eye, or the iris itself is physically isolated from the brain. The intrinsic pupillary light reflex (iPLR) is the term given to pupil constriction in the absence of retinal input to the brain. Here, using an intraocular axotomy approach, we show that the iPLR in conscious mice spans a dynamic range over 3 log units of irradiance. This iPLR response is absent in melanopsin knockout (MKO) mice and can be significantly inhibited by atropine. Immunohistochemistry for cfos and melanopsin, in combination with light exposure revealed a population of small ipRGCs in the retinal ciliary marginal zone (CMZ), which remain responsive to light in axotomised mice. We report that damage to the CMZ in a novel in vitro preparation removes a significant component of the iPLR response, while a detailed immunohistochemical analysis of the CMZ in wildtype mice revealed a melanopsin-rich plexus, which was consistently most intense in nasal retina. There were clear examples of melanopsin-positive, direct retino-ciliary projections, which appear to emanate from Brn3b negative, M1 type ipRGCs. These cells are clustered along the melanopsin-rich plexus nasally and may channel ipRGC signals from retina into the iris via ciliary body. Comparison between wildtype and MKO mice reveals that the ciliary body is also weakly stained for melanopsin. Our results show that the full extent of iPLR in mice requires cholinergic neurotransmission and intact signalling at the CMZ/ciliary body. This response may be mediated to some extent by ipRGCs, which send direct projections from the retina into ciliary body. In addition to the melanopsin-mediated iris sphincter constriction suggested by others, we propose a new mechanism, which may involve constriction of the ciliary body and ipRGC-mediated relaxation of the iris dilator muscle.

Light-dependent structural change of chicken retinal Cryptochrome4

Animals have several classes of cryptochromes (CRYs), some of which function as core elements of circadian clockwork, circadian photoreceptors, and/or light-dependent magnetoreceptors. In addition to the circadian clock genes Cry1 and Cry2, nonmammalian vertebrates have the Cry4 gene, the molecular function of which remains unknown. Here we analyzed chicken CRY4 (cCRY4) expression in the retina with in situ hybridization and found that cCRY4 was likely transcribed in the visual pigment cells, cells in the inner nuclear layer, and retinal ganglion cells. We further developed several monoclonal antibodies to the carboxyl-terminal extension of cCRY4 and localized cCRY4 protein with immunohistochemistry. Consistent with the results of in situ hybridization, cCRY4 immunoreactivity was found in visual pigment cells and cells located at the inner nuclear layer and the retinal ganglion cell layer. Among the antibodies, one termed C1-mAb had its epitope within the carboxyl-terminal 14-amino acid sequence (QLTRDDADDPMEMK) and associated with cCRY4 in the retinal soluble fraction more strongly in the dark than under blue light conditions. Immunoprecipitation experiments under various light conditions indicated that cCRY4 from the immunocomplex formed in the dark dissociated from C1-mAb during blue light illumination as weak as 25 μW/cm(2) and that the release occurred with not only blue but also near UV light. These results suggest that cCRY4 reversibly changes its structure within the carboxyl-terminal region in a light-dependent manner and operates as a photoreceptor or magnetoreceptor with short wavelength sensitivity in the retina.

Melanopsin is highly resistant to light and chemical bleaching in vivo

Melanopsin is the photopigment of mammalian intrinsically photosensitive retinal ganglion cells, where it contributes to light entrainment of circadian rhythms, and to the pupillary light response. Previous work has shown that the melanopsin photocycle is independent of that used by rhodopsin (Tu, D. C., Owens, L. A., Anderson, L., Golczak, M., Doyle, S. E., McCall, M., Menaker, M., Palczewski, K., and Van Gelder, R. N. (2006) Inner retinal photoreception independent of the visual retinoid cycle. Proc. Natl. Acad. Sci. U.S.A. 103, 10426-10431). Here we determined the ability of apo-melanopsin, formed by ex vivo UV light bleaching, to use selected chromophores. We found that 9-cis-retinal, but not all-trans-retinal or 9-cis-retinol, is able to restore light-dependent ipRGC activity after bleaching. Melanopsin was highly resistant to both visible-spectrum photic bleaching and chemical bleaching with hydroxylamine under conditions that fully bleach rod and cone photoreceptor cells. These results suggest that the melanopsin photocycle can function independently of both rod and cone photocycles, and that apo-melanopsin has a strong preference for binding cis-retinal to generate functional pigment. The data support a model in which retinal is continuously covalently bound to melanopsin and may function through a reversible, bistable mechanism.

Lunar phase-dependent expression of cryptochrome and a photoperiodic mechanism for lunar phase-recognition in a reef fish, goldlined spinefoot

Lunar cycle-associated physiology has been found in a wide variety of organisms. Recent study has revealed that mRNA levels of Cryptochrome (Cry), one of the circadian clock genes, were significantly higher on a full moon night than on a new moon night in coral, implying the involvement of a photoreception system in the lunar-synchronized spawning. To better establish the generalities surrounding such a mechanism and explore the underlying molecular mechanism, we focused on the relationship between lunar phase, Cry gene expression, and the spawning behavior in a lunar-synchronized spawner, the goldlined spinefoot (Siganus guttatus), and we identified two kinds of Cry genes in this animal. Their mRNA levels showed lunar cycle-dependent expression in the medial part of the brain (mesencephalon and diencephalon) peaking at the first quarter moon. Since this lunar phase coincided with the reproductive phase of the goldlined spinefoot, Cry gene expression was considered a state variable in the lunar phase recognition system. Based on the expression profiles of SgCrys together with the moonlight's pattern of timing and duration during its nightly lunar cycle, we have further speculated on a model of lunar phase recognition for reproductive control in the goldlined spinefoot, which integrates both moonlight and circadian signals in a manner similar to photoperiodic response.

Melanopsin signalling in mammalian iris and retina

Lower vertebrates have an intrinsically-photosensitive iris and thus a local pupillary light reflex (PLR). In contrast, it has been a dogma that the PLR in mammals generally requires neuronal circuitry connecting the eye and the brain. We report here that an intrinsic component of the PLR is actually widespread in nocturnal and crepuscular mammals. In mouse, this intrinsic PLR requires the visual pigment, melanopsin. It also requires PLCβ4, the vertebrate homolog of the Drosophila NorpA phospholipase C mediating rhabdomeric phototransduction. The Plcβ4^−/− genotype, besides removing the intrinsic PLR, also essentially eliminates the intrinsic light response of the M1-subtype of melanopsin-expressing, intrinsically-photosensitive retinal ganglion cells (M1-ipRGCs), by far the most photosensitive ipRGCs and with the largest responses. Ablating in mouse the expression of both TRPC6 and TRPC7, members of the TRP channel superfamily, likewise essentially eliminated the M1-ipRGC light response, but spared the intrinsic PLR. Thus, melanopsin signaling exists in both iris and retina, involving a PLCβ4-mediated pathway that nonetheless diverges in the two locations.

A behavioral perspective on the biophysics of the light-dependent magnetic compass: a link between directional and spatial perception?

In terrestrial organisms, sensitivity to the Earth's magnetic field is mediated by at least two different magnetoreception mechanisms, one involving biogenic ferromagnetic crystals (magnetite/maghemite) and the second involving a photo-induced biochemical reaction that forms long-lasting, spin-coordinated, radical pair intermediates. In some vertebrate groups (amphibians and birds), both mechanisms are present; a light-dependent mechanism provides a directional sense or 'compass', and a non-light-dependent mechanism underlies a geographical-position sense or 'map'. Evidence that both magnetite- and radical pair-based mechanisms are present in the same organisms raises a number of interesting questions. Why has natural selection produced magnetic sensors utilizing two distinct biophysical mechanisms? And, in particular, why has natural selection produced a compass mechanism based on a light-dependent radical pair mechanism (RPM) when a magnetite-based receptor is well suited to perform this function? Answers to these questions depend, to a large degree, on how the properties of the RPM, viewed from a neuroethological rather than a biophysical perspective, differ from those of a magnetite-based magnetic compass. The RPM is expected to produce a light-dependent, 3-D pattern of response that is axially symmetrical and, in some groups of animals, may be perceived as a pattern of light intensity and/or color superimposed on the visual surroundings. We suggest that the light-dependent magnetic compass may serve not only as a source of directional information but also provide a spherical coordinate system that helps to interface metrics of distance, direction and spatial position.

Evidence for multiple photosystems in jellyfish

Cnidarians are often used as model animals in studies of eye and photopigment evolution. Most cnidarians display photosensitivity at some point in their lifecycle ranging from extraocular photoreception to image formation in camera-type eyes. The available information strongly suggests that some cnidarians even possess multiple photosystems. The evidence is strongest within Cubomedusae where all known species posses 24 eyes of four morphological types. Physiological experiments show that each cubomedusan eye type likely constitutes a separate photosystem controlling separate visually guided behaviors. Further, the visual system of cubomedusae also includes extraocular photoreception. The evidence is supported by immunocytochemical and molecular data indicating multiple photopigments in cubomedusae as well as in other cnidarians. Taken together, available data suggest that multiple photosystems had evolved already in early eumetazoans and that their original level of organization was discrete sets of special-purpose eyes and/or photosensory cells.

Cryptochrome genes are highly expressed in the ovary of the African clawed frog, Xenopus tropicalis

Cryptochromes (CRYs) are flavoproteins sharing high homology with photolyases. Some of them have function(s) including transcription regulation in the circadian clock oscillation, blue-light photoreception for resetting the clock phase, and light-dependent magnetoreception. Vertebrates retain multiple sets of CRY or CRY-related genes, but their functions are yet unclear especially in the lower vertebrates. Although CRYs and the other circadian clock components have been extensively studied in the higher vertebrates such as mice, only a few model species have been studied in the lower vertebrates. In this study, we identified two CRYs, XtCRY1 and XtCRY2 in Xenopus tropicalis, an excellent experimental model species. Examination of tissue specificity of their mRNA expression by real-time PCR analysis revealed that both the XtCRYs showed extremely high mRNA expression levels in the ovary. The mRNA levels in the ovary were about 28-fold (XtCry1) and 48-fold (XtCry2) higher than levels in the next abundant tissues, the retina and kidney, respectively. For the functional analysis of the XtCRYs, we cloned circadian positive regulator XtCLOCK and XtBMAL1, and found circadian enhancer E-box in the upstream of XtPer1 gene. XtCLOCK and XtBMAL1 exhibited strong transactivation from the XtPer1 E-box element, and both the XtCRYs inhibited the XtCLOCK:XtBMAL1-mediated transactivation, thereby suggesting this element to drive the circadian transcription. These results revealed a conserved main feedback loop in the X. tropicalis circadian clockwork and imply a possible physiological importance of CRYs in the ovarian functions such as synthesis of steroid hormones and/or control of estrus cycles via the transcription regulation.

Animal cryptochromes mediate magnetoreception by an unconventional photochemical mechanism

Understanding the biophysical basis of animal magnetoreception has been one of the greatest challenges in sensory biology. Recently it was discovered that the light-dependent magnetic sense of Drosophila melanogaster is mediated by the ultraviolet (UV)-A/blue light photoreceptor cryptochrome (Cry). Here we show, using a transgenic approach, that the photoreceptive, Drosophila-like type 1 Cry and the transcriptionally repressive, vertebrate-like type 2 Cry of the monarch butterfly (Danaus plexippus) can both function in the magnetoreception system of Drosophila and require UV-A/blue light (wavelength below 420 nm) to do so. The lack of magnetic responses for both Cry types at wavelengths above 420 nm does not fit the widely held view that tryptophan triad-generated radical pairs mediate the ability of Cry to sense a magnetic field. We bolster this assessment by using a mutant form of Drosophila and monarch type 1 Cry and confirm that the tryptophan triad pathway is not crucial in magnetic transduction. Together, these results suggest that animal Crys mediate light-dependent magnetoreception through an unconventional photochemical mechanism. This work emphasizes the utility of Drosophila transgenesis for elucidating the precise mechanisms of Cry-mediated magnetosensitivity in insects and also in vertebrates such as migrating birds.

The evolution of irradiance detection: melanopsin and the non-visual opsins

Circadian rhythms are endogenous 24 h cycles that persist in the absence of external time cues. These rhythms provide an internal representation of day length and optimize physiology and behaviour to the varying demands of the solar cycle. These clocks require daily adjustment to local time and the primary time cue (zeitgeber) used by most vertebrates is the daily change in the amount of environmental light (irradiance) at dawn and dusk, a process termed photoentrainment. Attempts to understand the photoreceptor mechanisms mediating non-image-forming responses to light, such as photoentrainment, have resulted in the discovery of a remarkable array of different photoreceptors and photopigment families, all of which appear to use a basic opsin/vitamin A-based photopigment biochemistry. In non-mammalian vertebrates, specialized photoreceptors are located within the pineal complex, deep brain and dermal melanophores. There is also strong evidence in fish and amphibians for the direct photic regulation of circadian clocks in multiple tissues. By contrast, mammals possess only ocular photoreceptors. However, in addition to the image-forming rods and cones of the retina, there exists a third photoreceptor system based on a subset of melanopsin-expressing photosensitive retinal ganglion cells (pRGCs). In this review, we discuss the range of vertebrate photoreceptors and their opsin photopigments, describe the melanopsin/pRGC system in some detail and then finally consider the molecular evolution and sensory ecology of these non-image-forming photoreceptor systems.

Comparative photochemistry of animal type 1 and type 4 cryptochromes

Cryptochromes (CRYs) are blue-light photoreceptors with known or presumed functions in light-dependent and light-independent gene regulation in plants and animals. Although the photochemistry of plant CRYs has been studied in some detail, the photochemical behavior of animal cryptochromes remains poorly defined in part because it has been difficult to purify animal CRYs with their flavin cofactors. Here we describe the purification of type 4 CRYs of zebrafish and chicken as recombinant proteins with full flavin complement and compare the spectroscopic properties of type 4 and type 1 CRYs. In addition, we analyzed photoinduced proteolytic degradation of both types of CRYs in vivo in heterologous systems. We find that even though both types of CRYs contain stoichiometric flavin, type 1 CRY is proteolytically degraded by a light-initiated reaction in Drosophila S2, zebrafish Z3, and human HEK293T cell lines, but zebrafish CRY4 (type 4) is not. In vivo degradation of type 1 CRYs does not require continuous illumination, and a single light flash of 1 ms duration leads to degradation of about 80% of Drosophila CRY in 60 min. Finally, we demonstrate that in contrast to animal type 2 CRYs and Arabidopsis CRY1 neither insect type 1 nor type 4 CRYs have autokinase activities.

Evaluation of 9-cis-retinyl acetate therapy in Rpe65-/- mice

PURPOSE

Mice lacking retinal pigment epithelium-specific 65-kDa protein (RPE65) develop retinopathy and blindness resembling Leber congenital amaurosis. Effects of 9-cis-retinyl acetate (9-cis-R-Ac) on visual function and retinopathy progression were tested in Rpe65(-/-) mice.

METHODS

Young C57Bl/6 mice were given 9-cis-R-Ac in each of four different oil-based vehicle solutions by gastric gavage to identify the vehicle most suitable for drug delivery by measuring retinoid levels in plasma. Then doses of 9-cis-R-Ac ranging from 1 to 100 mg/kg were administered to 5- to 12-week-old Rpe65(-/-) mice by different treatment regimens, including single doses and either intermittent or daily doses for various periods up to 8 weeks. Retinoid effects on visual function were evaluated by electroretinography, retinoid analyses, histologic methods, and vision-dependent behavioral testing.

RESULTS

Soybean oil vehicle provided the highest 9-cis-R-Ac metabolite levels in plasma. Single doses of 9-cis-R-Ac (6.25-50 mg/kg) provided significant dose-dependent improvement in electroretinographic responses. Well-tolerated daily doses (1-12.5 mg/kg) for 2 weeks induced remarkable improvement of retinal function. Significant dose-dependent improvement of electroretinographic responses was observed 6 days after administration of 9-cis-R-Ac daily for 3 days at 1 to 12.5 mg/kg. Mice given either daily or intermittent 9-cis-R-Ac treatment at 1 and 4 mg/kg and evaluated 8 weeks later displayed dose-dependent improvement of retinal function and morphology, whereas retinal function deteriorated in control animals. Treated mice also performed better than control animals in vision-dependent behavioral tests.

CONCLUSIONS

Treatment with 9-cis-R-Ac improves visual function and preserves retinal morphology in Rpe65(-/-) mice.

Functional motifs in the (6-4) photolyase crystal structure make a comparative framework for DNA repair photolyases and clock cryptochromes

Homologous flavoproteins from the photolyase (PHR)/cryptochrome (CRY) family use the FAD cofactor in PHRs to catalyze DNA repair and in CRYs to tune the circadian clock and control development. To help address how PHR/CRY members achieve these diverse functions, we determined the crystallographic structure of Arabidopsis thaliana (6-4) PHR (UVR3), which is strikingly (>65%) similar in sequence to human circadian clock CRYs. The structure reveals a substrate-binding cavity specific for the UV-induced DNA lesion, (6-4) photoproduct, and cofactor binding sites different from those of bacterial PHRs and consistent with distinct mechanisms for activities and regulation. Mutational analyses were combined with this prototypic structure for the (6-4) PHR/clock CRY cluster to identify structural and functional motifs: phosphate-binding and Pro-Lys-Leu protrusion motifs constricting access to the substrate-binding cavity above FAD, sulfur loop near the external end of the Trp electron-transfer pathway, and previously undefined C-terminal helix. Our results provide a detailed, unified framework for investigations of (6-4) PHRs and the mammalian CRYs. Conservation of key residues and motifs controlling FAD access and activities suggests that regulation of FAD redox properties and radical stability is essential not only for (6-4) photoproduct DNA repair, but also for circadian clock-regulating CRY functions. The structural and functional results reported here elucidate archetypal relationships within this flavoprotein family and suggest how PHRs and CRYs use local residue and cofactor tuning, rather than larger structural modifications, to achieve their diverse functions encompassing DNA repair, plant growth and development, and circadian clock regulation.

Structure and function of animal cryptochromes

Cryptochrome (CRY) is a photolyase-like flavoprotein with no DNA-repair activity but with known or presumed blue-light receptor function. Animal CRYs have DNA-binding and autokinase activities, and their flavin cofactor is reduced by photoinduced electron transfer. In Drosophila, CRY is a major circadian photoreceptor, and in mammals, the two CRY proteins are core components of the molecular clock and potential circadian photoreceptors. In mammals, CRYs participate in cell cycle regulation and the cellular response to DNA damage by controlling the expression of some cell cycle genes and by directly interacting with checkpoint proteins.

Action Spectrum of Drosophila Cryptochrome

Cryptochromes are a highly conserved class of UV-A/blue light photoreceptors. In Drosophila, cryptochrome is required for the normal entrainment of circadian rhythms to light dark cycles. The photocycle and molecular mechanism of animal cryptochrome photoreception are presently unknown. Drosophila cryptochrome undergoes light-dependent degradation when heterologously expressed in Schneider-2 cells. We have generated Drosophila luciferase-cryptochrome fusion proteins to more precisely monitor light-dependent cryptochrome degradation. We found that the luciferase-cryptochrome fusion protein undergoes light-dependent degradation with luciferase activity declining approximately 50% within 5 min of light exposure and approximately 85% within 1 h of light exposure. Degradation is inhibited by MG-132, consistent with a proteasomal degradation mechanism. Irradiance-response curves yield an action spectrum similar to absorption spectra for prokaryotic and eukaryotic cryptochromes with highest sensitivity in the UV-A. A luciferase-cryptochrome fusion protein lacking the terminal 15 amino acids is stably expressed in the dark but demonstrates increased sensitivity to light-induced degradation. The conferral of light-dependent degradation on a heterologous protein by fusion to cryptochrome may be a useful tool for probing protein function in cell expression systems.

Evolution of melanopsin photoreceptors: discovery and characterization of a new melanopsin in nonmammalian vertebrates

In mammals, the melanopsin gene (Opn4) encodes a sensory photopigment that underpins newly discovered inner retinal photoreceptors. Since its first discovery in Xenopus laevis and subsequent description in humans and mice, melanopsin genes have been described in all vertebrate classes. Until now, all of these sequences have been considered representatives of a single orthologous gene (albeit with duplications in the teleost fish). Here, we describe the discovery and functional characterisation of a new melanopsin gene in fish, bird, and amphibian genomes, demonstrating that, in fact, the vertebrates have evolved two quite separate melanopsins. On the basis of sequence similarity, chromosomal localisation, and phylogeny, we identify our new melanopsins as the true orthologs of the melanopsin gene previously described in mammals and term this grouping Opn4m. By contrast, the previously published melanopsin genes in nonmammalian vertebrates represent a separate branch of the melanopsin family which we term Opn4x. RT-PCR analysis in chicken, zebrafish, and Xenopus identifies expression of both Opn4m and Opn4x genes in tissues known to be photosensitive (eye, brain, and skin). In the day-14 chicken eye, Opn4m mRNA is found in a subset of cells in the outer nuclear, inner nuclear, and ganglion cell layers, the vast majority of which also express Opn4x. Importantly, we show that a representative of the new melanopsins (chicken Opn4m) encodes a photosensory pigment capable of activating G protein signalling cascades in a light- and retinaldehyde-dependent manner under heterologous expression in Neuro-2a cells. A comprehensive in silico analysis of vertebrate genomes indicates that while most vertebrate species have both Opn4m and Opn4x genes, the latter is absent from eutherian and, possibly, marsupial mammals, lost in the course of their evolution as a result of chromosomal reorganisation. Thus, our findings show for the first time that nonmammalian vertebrates retain two quite separate melanopsin genes, while mammals have just one. These data raise important questions regarding the functional differences between Opn4x and Opn4m pigments, the associated adaptive advantages for most vertebrate species in retaining both melanopsins, and the implications for mammalian biology of lacking Opn4x.

A new melanopsin gene, identified in fish, bird, and amphibian genomes, is the true ortholog of the melanopsin gene previously described in mammals.

Photochemistry and photobiology of cryptochrome blue-light photopigments: the search for a photocycle

Cryptochromes are flavoproteins that exhibit high sequence and structural similarity to the light-dependent DNA-repair enzyme, photolyase. Cryptochromes have lost the ability to repair DNA; instead, they use the energy from near-UV/blue light to regulate a variety of growth and adaptive processes in organisms ranging from bacteria to humans. The photocycle of cryptochrome is not yet known, although it is hypothesized that it may share some similarity to that of photolyase, which utilizes light-driven electron transfer from the catalytic flavin chromophore. In this review, we present genetic evidence for the photoreceptive role of cryptochromes and discuss recent biochemical studies that have furthered our understanding of the cryptochrome photocycle. In particular, the role of the unique C-terminal domain in cryptochrome phototransduction is discussed.

Molecular cloning, mRNA expression, and immunocytochemical localization of a putative blue-light photoreceptor CRY4 in the chicken pineal gland

In non-mammalian vertebrates, the pineal gland contains an endogenous circadian oscillator and serves as a photosensitive neuroendocrinal organ. To better understand the pineal phototransduction mechanism, we focused on the chicken putative blue-light photoreceptive molecule, Cryptochrome4 (cCRY4). Here we report the molecular cloning of pineal cCry4 cDNA, the in vivo expression of cCry4 mRNA, and the detection of cCRY4 protein. cCry4 is transcribed in a wide variety of chick tissues out of which the pineal gland and retina contain high levels of cCry4 mRNA. In the pineal gland, under 12 h light : 12 h dark cycles, the levels of both cCry4 mRNA and cCRY4 protein showed diurnal changes, and in cultured chick pineal cells, the cCry4 mRNA level was not only up-regulated by light but also controlled by circadian signals. Immunoblot analysis with a cCRY4-specific antibody detected cCRY4 in a soluble fraction of the pineal lysate. Immunocytochemistry revealed that cCRY4 was expressed in many parenchymal cells and a limited number of stromal cells. These cCRY4 features strikingly contrast with those of the chick pineal photoreceptor pinopsin, suggesting a possible temporal and/or spatial duplicity of the pineal photoreceptive system, the opsin- and CRY-based mechanisms.

Evolving visual pigments: hints from the opsin-based proteins in a phylogenetically old "eyeless" invertebrate

Visual pigments are photosensitive receptor proteins that trigger the transduction process producing the visual excitation once they have absorbed photons. In spite of the molecular and morpho-functional complexity that has characterized the development of animal eyes and eyeless photoreceptive systems, opsin-based protein family appears ubiquous along metazoan visual systems. Moreover, in addition to classic rhodopsin photoreceptors, all Metazoa have supplementary non-visual photosensitive structures, mainly located in the central nervous system, that sense light without forming an image and that rather regulate the organism's temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. Here we propose the cnidarian Hydra as a useful tool of investigation for molecular and functional differences between these pigment families. Hydra is the first metazoan owning a nervous system and it is an eyeless invertebrate showing only an extraocular photoreception, as it has no recognized visual or photosensitive structures. In this paper we provide an overview of the molecular and functional features of the opsin-based protein subfamilies and preliminary evidences in a phylogenetically old species of both image-forming and non-visual opsins. Then we give new insights on the molecular biology of Hydra photoreception and on the evolutionary pathways of visual pigments.

Non-image-forming ocular photoreception in vertebrates

It has been accepted for a hundred years or more that rods and cones are the only photoreceptive cells in the retina. The light signals generated in rods and cones, after processing by downstream retinal neurons (bipolar, horizontal, amacrine and ganglion cells), are transmitted to the brain via the axons of the ganglion cells for further analysis. In the past few years, however, convincing evidence has rapidly emerged indicating that a small subset of retinal ganglion cells in mammals is also intrinsically photosensitive. Melanopsin is the signaling photopigment in these cells. The main function of the inner-retina photoreceptors is to generate and transmit non-image-forming visual information, although some role in conventional vision (image detection) is also possible.

Photoreceptor evolution: ancient siblings serve different tasks

Photoreceptor cells of vertebrate eyes are fundamentally different from those of invertebrate eyes. New work on the brain of a ragworm now suggests that ancestral bilaterians possessed both types of photoreceptor cell.