Microarray Analysis and Functional Genomics Identify Novel Components of Melanopsin Signaling

The causal relationship between sleep and risk of psychiatric disorders: A two-sample mendelian randomization study

Introduction

Sleep is associated with psychiatric disorders. However, their causality remains unknown.

Methods

The study explored the causal relationship between seven sleep parameters (sleep duration, insomnia, sleep apnea, chronotype, daytime dozing, napping during the day, and snoring) and three psychiatric disorders including major depressive disorder (MDD), schizophrenia, and attention-deficit/hyperactivity disorder (ADHD) using two-sample Mendelian randomization (MR). Genome-wide association study (GWAS) summary data for sleep parameters were obtained from the United Kingdom biobank, FinnGen biobank, and EBI databases. MR-Egger, weighted median, inverse-variance weighted (IVW), simple mode, weighted mode, maximum likelihood, penalized weighted median, and IVW(fixed effects) were used to perform the MR analysis. The heterogeneity was detected by Cochran's Q statistic. The horizontal pleiotropy was detected by MR Egger. The sensitivity was investigated by the leave-one-out analysis.

Results

Insomnia (OR = 2.02, 95%CI = 1.34-3.03, p = 0.001, False-discovery rate (FDR) corrected p-value = 0.011) and napping during the day (OR = 1.81, 95%CI = 1.34-2.44, FDR corrected p-value<0.001) were associated with an increased risk of MDD. Longer sleep duration (OR = 2.20, 95%CI = 1.24-3.90, FDR corrected p-value = 0.049) had an association with the increased risk of schizophrenia, while daytime dozing (OR = 4.44, 95%CI = 1.20-16.41, corrected p-value = 0.088)and napping during the day (OR = 2.11, 95%CI = 1.11-4.02, FDR corrected p-value = 0.088) had a suggestive association with an increased risk of schizophrenia. Longer sleep duration had a suggestive association with a decreased risk of ADHD (OR = 0.66, 95%CI = 0.42-0.93, FDR corrected p-value = 0.088).

Conclusion

This study provides further evidence for a complex relationship between sleep and psychiatric disorders. Our findings highlight the potential benefits of addressing sleep problems in the prevention of psychiatric disorders.

Global genetic diversity, introgression, and evolutionary adaptation of indicine cattle revealed by whole genome sequencing

Indicine cattle, also referred to as zebu (Bos taurus indicus), play a central role in pastoral communities across a wide range of agro-ecosystems, from extremely hot semiarid regions to hot humid tropical regions. However, their adaptive genetic changes following their dispersal into East Asia from the Indian subcontinent have remained poorly documented. Here, we characterize their global genetic diversity using high-quality whole-genome sequencing data from 354 indicine cattle of 57 breeds/populations, including major indicine phylogeographic groups worldwide. We reveal their probable migration into East Asia was along a coastal route rather than inland routes and we detected introgression from other bovine species. Genomic regions carrying morphology-, immune-, and heat-tolerance-related genes underwent divergent selection according to Asian agro-ecologies. We identify distinct sets of loci that contain promising candidate variants for adaptation to hot semi-arid and hot humid tropical ecosystems. Our results indicate that the rapid and successful adaptation of East Asian indicine cattle to hot humid environments was promoted by localized introgression from banteng and/or gaur. Our findings provide insights into the history and environmental adaptation of indicine cattle.

Divergent G-protein selectivity across melanopsins from mice and humans

Melanopsin is an opsin photopigment and light-activated G-protein-coupled receptor; it is expressed in photoreceptive retinal ganglion cells (mRGCs) and can be employed as an optogenetic tool. Mammalian melanopsins can signal via G_q/11 and G_i/o/t heterotrimeric G proteins, but aspects of the mRGC light response appear incompatible with either mode of signalling. We use live-cell reporter assays in HEK293T cells to show that melanopsins from mice and humans can also signal via G_s. We subsequently show that this mode of signalling is substantially divergent between species. The two established structural isoforms of mouse melanopsin (which differ in the length of their C-terminal tail) both signalled strongly through all three G-protein classes (G_q/11, G_i/o and G_s), whereas human melanopsin showed weaker signalling through G_s. Our data identify G_s as a new mode of signalling for mammalian melanopsins and reveal diversity in G-protein selectivity across mammalian melanopsins.

Summary: The photopigment melanopsin (OPN4), which provides inner retinal photoreception in mammals, shows light-dependent activation of G_s G protein that is more pronounced for mouse than human photopigment.

Melanopsin phototransduction: beyond canonical cascades

Melanopsin is a visual pigment that is expressed in a small subset of intrinsically photosensitive retinal ganglion cells (ipRGCs). It is involved in regulating non-image forming visual behaviors, such as circadian photoentrainment and the pupillary light reflex, while also playing a role in many aspects of image-forming vision, such as contrast sensitivity. Melanopsin was initially discovered in the melanophores of the skin of the frog Xenopus, and subsequently found in a subset of ganglion cells in rat, mouse and primate retinas. ipRGCs were initially thought to be a single retinal ganglion cell population, and melanopsin was thought to activate a single, invertebrate-like Gq/transient receptor potential canonical (TRPC)-based phototransduction cascade within these cells. However, in the 20 years since the discovery of melanopsin, our knowledge of this visual pigment and ipRGCs has expanded dramatically. Six ipRGC subtypes have now been identified in the mouse, each with unique morphological, physiological and functional properties. Multiple subtypes have also been identified in other species, suggesting that this cell type diversity is a general feature of the ipRGC system. This diversity has led to a renewed interest in melanopsin phototransduction that may not follow the canonical Gq/TRPC cascade in the mouse or in the plethora of other organisms that express the melanopsin photopigment. In this Review, we discuss recent findings and discoveries that have challenged the prevailing view of melanopsin phototransduction as a single pathway that influences solely non-image forming functions.

Circadian Photoentrainment in Mice and Humans

Light around twilight provides the primary entrainment signal for circadian rhythms. Here we review the mechanisms and responses of the mouse and human circadian systems to light. Both utilize a network of photosensitive retinal ganglion cells (pRGCs) expressing the photopigment melanopsin (OPN4). In both species action spectra and functional expression of OPN4 in vitro show that melanopsin has a λ_max close to 480 nm. Anatomical findings demonstrate that there are multiple pRGC sub-types, with some evidence in mice, but little in humans, regarding their roles in regulating physiology and behavior. Studies in mice, non-human primates and humans, show that rods and cones project to and can modulate the light responses of pRGCs. Such an integration of signals enables the rods to detect dim light, the cones to detect higher light intensities and the integration of intermittent light exposure, whilst melanopsin measures bright light over extended periods of time. Although photoreceptor mechanisms are similar, sensitivity thresholds differ markedly between mice and humans. Mice can entrain to light at approximately 1 lux for a few minutes, whilst humans require light at high irradiance (>100’s lux) and of a long duration (>30 min). The basis for this difference remains unclear. As our retinal light exposure is highly dynamic, and because photoreceptor interactions are complex and difficult to model, attempts to develop evidence-based lighting to enhance human circadian entrainment are very challenging. A way forward will be to define human circadian responses to artificial and natural light in the “real world” where light intensity, duration, spectral quality, time of day, light history and age can each be assessed.

Transcriptomic Signatures of Postnatal and Adult Intrinsically Photosensitive Ganglion Cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are rare mammalian photoreceptors essential for non-image-forming vision functions, such as circadian photoentrainment and the pupillary light reflex. They comprise multiple subtypes distinguishable by morphology, physiology, projections, and levels of expression of melanopsin (Opn4), their photopigment. The molecular programs that distinguish ipRGCs from other ganglion cells and ipRGC subtypes from one another remain elusive. Here, we present comprehensive gene expression profiles of early postnatal and adult mouse ipRGCs purified from two lines of reporter mice that mark different sets of ipRGC subtypes. We find dozens of novel genes highly enriched in ipRGCs. We reveal that Rasgrp1 and Tbx20 are selectively expressed in subsets of ipRGCs, though these molecularly defined groups imperfectly match established ipRGC subtypes. We demonstrate that the ipRGCs regulating circadian photoentrainment are diverse at the molecular level. Our findings reveal unexpected complexity in gene expression patterns across mammalian ipRGC subtypes.

Molecular genetic mechanisms of influence of laser radiation with 577 nm wavelength in a microimpulse mode on the condition of the retina

THE AIM OF THE STUDY

was to investigate the molecular genetic mechanisms of the influence of laser radiation with 577 nm wavelength in a microimpulse mode on the retina in the experimental conditions after the intravitreal injection of VEGF.

MATERIALS AND METHODS

The study was performed on 4-5 week-old male mice of the line C57BL/6J. The animals were divided into 4 groups of 5 mice in each group, one eye was excremental, the contralateral eye remained intact. In the first group, intravitreal injection of PBS was performed; in the second group, intravitreal injection of 50 ng/ml of recombinant VEGF₁₆₅ in 2 μL of phosphate-buffered saline (PBS) was performed; in the third and fourth groups, a day after the intravitreal injection of recombinant VEGF₁₆₅, laser radiation with wavelength 577 nm was applied in the micropulse and continuous modes, respectively. Tissue samples (neuroepithelium, pigment epithelium) for the microarray transcription analysis in the animals from group 1 and 2 were taken 2 days after the injection of PBS and VEGF, in the animals from group 3 and 4 - a day after the retina was exposed to laser radiation.

RESULTS AND CONCLUSION

Molecular genetic mechanisms of the influence of laser radiation with wavelength 577 nm in a microimpulse mode on the retina in experimental conditions were studied and the genes that significantly changed the level of expression (the genes that take part in the regulation of neoangiogenesis, structural cell functions, processes of cells proliferation, transcription, differentiation, transmembrane transport, signaling, synaptic transmission, etc.) were identified.

Maternal circadian disruption is associated with variation in placental DNA methylation

Circadian disruption is a common environmental and occupational exposure with public health consequences, but not much is known about whether circadian disruption affects in utero development. We investigated whether maternal circadian disruption, using night shift work as a proxy, is associated with variations in DNA methylation patterns of placental tissue in an epigenome-wide association study (EWAS) of night shift work. Here, we compared cytosine-guanosine dinucleotide (CpG) specific methylation genome-wide of placental tissue (measured with the Illumina 450K array) from participants (n = 237) in the Rhode Island Child Health Study (RICHS) who did (n = 53) and did not (n = 184) report working the night shift, using robust linear modeling and adjusting for maternal age, pre-pregnancy smoking, infant sex, maternal adversity, and putative cell mixture. Statistical analyses were adjusted for multiple comparisons and results presented with Bonferroni or Benjamini and Hochberg (BH) adjustment for false discovery rate. Night shift work was associated with differential methylation in placental tissue, including CpG sites in the genes NAV1, SMPD1, TAPBP, CLEC16A, DIP2C, FAM172A, and PLEKHG6 (Bonferroni-adjusted p<0.05). CpG sites within NAV1, MXRA8, GABRG1, PRDM16, WNT5A, and FOXG1 exhibited the most hypomethylation, while CpG sites within TDO2, ADAMTSL3, DLX2, and SERPINA1 exhibited the most hypermethylation (BH q<0.10). Functional analysis indicated GO-terms associated with cell-cell adhesion and enriched GWAS results for psoriasis. Night shift work was associated with differential methylation of the placenta, which may have implications for fetal health and development. This is the first study to examine the epigenetic impacts of night shift exposure, as a proxy for circadian disruption, on placental methylation in humans, and, while results should be interpreted with caution, suggests circadian disruption may have epigenetic impacts.

An unexpected INAD PDZ tandem-mediated plcβ binding in Drosophila photo receptors

INAD assembles key enzymes of the Drosophila compound eye photo-transduction pathway into a supramolecular complex, supporting efficient and fast light signaling. However, the molecular mechanism that governs the interaction between INAD and NORPA (phospholipase Cβ, PLCβ), a key step for the fast kinetics of the light signaling, is not known. Here, we show that the NORPA C-terminal coiled-coil domain and PDZ-binding motif (CC-PBM) synergistically bind to INAD PDZ45 tandem with an unexpected mode and unprecedented high affinity. Guided by the structure of the INAD-NORPA complex, we discover that INADL is probably a mammalian counterpart of INAD. The INADL PDZ89 tandem specifically binds to PLCβ4 with a mode that is strikingly similar to that of the INAD-NORPA complex, as revealed by the structure of the INADL PDZ89-PLCβ4 CC-PBM complex. Therefore, our study suggests that the highly specific PDZ tandem - PLCβ interactions are an evolutionarily conserved mechanism in PLCβ signaling in the animal kingdom.

Genome-wide association analyses of sleep disturbance traits identify new loci and highlight shared genetics with neuropsychiatric and metabolic traits

Chronic sleep disturbances, associated with cardiometabolic diseases, psychiatric disorders and all-cause mortality, affect 25-30% of adults worldwide. Although environmental factors contribute substantially to self-reported habitual sleep duration and disruption, these traits are heritable and identification of the genes involved should improve understanding of sleep, mechanisms linking sleep to disease and development of new therapies. We report single- and multiple-trait genome-wide association analyses of self-reported sleep duration, insomnia symptoms and excessive daytime sleepiness in the UK Biobank (n = 112,586). We discover loci associated with insomnia symptoms (near MEIS1, TMEM132E, CYCL1 and TGFBI in females and WDR27 in males), excessive daytime sleepiness (near AR-OPHN1) and a composite sleep trait (near PATJ (INADL) and HCRTR2) and replicate a locus associated with sleep duration (at PAX8). We also observe genetic correlation between longer sleep duration and schizophrenia risk (rg = 0.29, P = 1.90 × 10-13) and between increased levels of excessive daytime sleepiness and increased measures for adiposity traits (body mass index (BMI): rg = 0.20, P = 3.12 × 10-9; waist circumference: rg = 0.20, P = 2.12 × 10-7).

Genetic adaptation of the human circadian clock to day-length latitudinal variations and relevance for affective disorders

Background

The temporal coordination of biological processes into daily cycles is a common feature of most living organisms. In humans, disruption of circadian rhythms is commonly observed in psychiatric diseases, including schizophrenia, bipolar disorder, depression and autism. Light therapy is the most effective treatment for seasonal affective disorder and circadian-related treatments sustain antidepressant response in bipolar disorder patients. Day/night cycles represent a major circadian synchronizing signal and vary widely with latitude.

Results

We apply a geographically explicit model to show that out-of-Africa migration, which led humans to occupy a wide latitudinal area, affected the evolutionary history of circadian regulatory genes. The SNPs we identify using this model display consistent signals of natural selection using tests based on population genetic differentiation and haplotype homozygosity. Signals of natural selection driven by annual photoperiod variation are detected for schizophrenia, bipolar disorder, and restless leg syndrome risk variants, in line with the circadian component of these conditions.

Conclusions

Our results suggest that human populations adapted to life at different latitudes by tuning their circadian clock systems. This process also involves risk variants for neuropsychiatric conditions, suggesting possible genetic modulators for chronotherapies and candidates for interaction analysis with photoperiod-related environmental variables, such as season of birth, country of residence, shift-work or lifestyle habits.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-014-0499-7) contains supplementary material, which is available to authorized users.

Melanopsins: Localization and Phototransduction in Xenopus laevis Melanophores

Xenopus laevis melanophores express two melanopsins, Opn4x and Opn4m. We identified Opn4x immunoreactivity throughout the melanophore cytoplasm and in the cell membrane. The strongest immunopositivity for Opn4m was observed in the nuclear region, and no labeling was seen in the cell membrane. This immunodistribution suggests Opn4x as the functional photopigment. In X. laevis melanophores, light triggers pigment dispersion and clock gene induction at blue wavelength, which maximally activates melanopsins. Although light stimulation activates phospholipase C and increases intracellular calcium and cGMP, this nucleotide does not participate in photo-induced melanin dispersion. Nevertheless, the guanylyl cyclase activator YC-1 stimulates Per1 expression, similar to blue light pulse, and the use of pharmacological inhibitors indicates the participation of the phosphoinositide cascade. Since cAMP levels does not change after blue light stimulation, the cAMP/PKA pathway most probably is not involved in blue light induction of Per in X. laevis melanophores. Given the localization of melanopsins and our pharmacological data, the light-induced clock gene expression seems to be mediated by Opn4x through phosphoinositide cascade and rise in cGMP, thus leading to the reset of the biological clock in our model.

TRPM3 in temperature sensing and beyond

TRPM3, also known as melastatin 2 (MLSN2), LTRPC3 (long TRPC3) and KIAA1616, is a member of the TRPM subfamily of transient receptor potential (TRP) ion channels. The channel was originally identified as a volume-regulated ion channel that can be activated upon reduction of the extracellular osmolality. Later, the channel was proposed to be involved in the modulation of insulin release in pancreatic islets. However, new evidence has uncovered a role of TRPM3 as a thermosensitive nociceptor channel implicated in the detection of noxious heat. The channel is functionally expressed in a subset of neurons of the somatosensory system and can be activated by heat. The purpose of the present review is to summarize existing knowledge of the expression, biophysics and pharmacology of TRPM3 and to serve as a guide for future studies aimed at improving the understanding of the mechanism of thermosensation and proposed physiological functions of TRPM3.

From blue light to clock genes in zebrafish ZEM-2S cells

Melanopsin has been implicated in the mammalian photoentrainment by blue light. This photopigment, which maximally absorbs light at wavelengths between 470 and 480 nm depending on the species, is found in the retina of all classes of vertebrates so far studied. In mammals, melanopsin activation triggers a signaling pathway which resets the circadian clock in the suprachiasmatic nucleus (SCN). Unlike mammals, Drosophila melanogaster and Danio rerio do not rely only on their eyes to perceive light, in fact their whole body may be capable of detecting light and entraining their circadian clock. Melanopsin, teleost multiple tissue (tmt) opsin and others such as neuropsin and va-opsin, are found in the peripheral tissues of Danio rerio, however, there are limited data concerning the photopigment/s or the signaling pathway/s directly involved in light detection. Here, we demonstrate that melanopsin is a strong candidate to mediate synchronization of zebrafish cells. The deduced amino acid sequence of melanopsin, although being a vertebrate opsin, is more similar to invertebrate than vertebrate photopigments, and melanopsin photostimulation triggers the phosphoinositide pathway through activation of a G(q/11)-type G protein. We stimulated cultured ZEM-2S cells with blue light at wavelengths consistent with melanopsin maximal absorption, and evaluated the time course expression of per1b, cry1b, per2 and cry1a. Using quantitative PCR, we showed that blue light is capable of slightly modulating per1b and cry1b genes, and drastically increasing per2 and cry1a expression. Pharmacological assays indicated that per2 and cry1a responses to blue light are evoked through the activation of the phosphoinositide pathway, which crosstalks with nitric oxide (NO) and mitogen activated protein MAP kinase (MAPK) to activate the clock genes. Our results suggest that melanopsin may be important in mediating the photoresponse in Danio rerio ZEM-2S cells, and provide new insights about the modulation of clock genes in peripheral clocks.

Blurring the boundaries of vision: novel functions of intrinsically photosensitive retinal ganglion cells

Mammalian vision consists of the classic image-forming pathway involving rod and cone photoreceptors interacting through a neural network within the retina before sending signals to the brain, and a non image-forming pathway that uses a photosensitive cell employing an alternative and evolutionary ancient phototransduction system and a direct connection to various centers in the brain. Intrinsically photosensitive retinal ganglion cells (ipRGCs) contain the photopigment melanopsin, which is independently capable of photon detection while also receiving synaptic input from rod and cone photoreceptors via bipolar cells. These cells are the retinal sentry for subconscious visual processing that controls circadian photoentrainment and the pupillary light reflex. Classified as irradiance detectors, recent investigations have led to expanding roles for this specific cell type and its own neural pathways, some of which are blurring the boundaries between image-forming and non image-forming visual processes.

Melanopsin-expressing amphioxus photoreceptors transduce light via a phospholipase C signaling cascade

Melanopsin, the receptor molecule that underlies light sensitivity in mammalian ‘circadian’ receptors, is homologous to invertebrate rhodopsins and has been proposed to operate via a similar signaling pathway. Its downstream effectors, however, remain elusive. Melanopsin also expresses in two distinct light-sensitive cell types in the neural tube of amphioxus. This organism is the most basal extant chordate and can help outline the evolutionary history of different photoreceptor lineages and their transduction mechanisms; moreover, isolated amphioxus photoreceptors offer unique advantages, because they are unambiguously identifiable and amenable to single-cell physiological assays. In the present study whole-cell patch clamp recording, pharmacological manipulations, and immunodetection were utilized to investigate light transduction in amphioxus photoreceptors. A G_q was identified and selectively localized to the photosensitive microvillar membrane, while the pivotal role of phospholipase C was established pharmacologically. The photocurrent was profoundly depressed by IP₃ receptor antagonists, highlighting the importance of IP₃ receptors in light signaling. By contrast, surrogates of diacylglycerol (DAG), as well as poly-unsaturated fatty acids failed to activate a membrane conductance or to alter the light response. The results strengthen the notion that calcium released from the ER via IP₃-sensitive channels may fulfill a key role in conveying - directly or indirectly - the melanopsin-initiated light signal to the photoconductance; moreover, they challenge the dogma that microvillar photoreceptors and phoshoinositide-based light transduction are a prerogative of invertebrate eyes.

Profound defects in pupillary responses to light in TRPM-channel null mice: a role for TRPM channels in non-image-forming photoreception

TRPM1 is a spontaneously active non-selective cation channel that has recently been shown to play an important role in the depolarizing light responses of ON bipolar cells. Consistent with this role, mutations in the TRPM1 gene have been identified as a principal cause of congenital stationary night blindness. However, previous microarray studies have shown that Trpm1 and Trpm3 are acutely regulated by light in the eyes of mice lacking rods and cones (rd/rd cl), a finding consistent with a role in non-image-forming photoreception. In this study we show that pupillary light responses are significantly attenuated in both Trpm1(-/-) and Trpm3(-/-) animals. Trpm1(-/-) mice exhibit a profound deficit in the pupillary response that is far in excess of that observed in mice lacking rods and cones (rd/rd cl) or melanopsin, and cannot be explained by defects in bipolar cell function alone. Immunolocalization studies suggest that TRPM1 is expressed in ON bipolar cells and also a subset of cells in the ganglion cell layer, including melanopsin-expressing photosensitive retinal ganglion cells (pRGCs). We conclude that, in addition to its role in bipolar cell signalling, TRPM1 is involved in non-image-forming responses to light and may perform a functional role within pRGCs. By contrast, TRPM3(-/-) mice display a more subtle pupillary phenotype with attenuated responses under bright light and dim light conditions. Expression of TRPM3 is detected in Muller cells and the ciliary body but is absent from pRGCs, and thus our data support an indirect role for TRPM3 in pupillary light responses.

Intrinsic phototransduction persists in melanopsin-expressing ganglion cells lacking diacylglycerol-sensitive TRPC subunits

In mammals, intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate various non-image-forming photic responses, such as circadian photoentrainment, pupillary light reflex and pineal melatonin suppression. ipRGCs directly respond to environmental light by activation of the photopigment melanopsin followed by the opening of an unidentified cation-selective channel. Studies in heterologous expression systems and in the native retina have strongly implicated diacylglycerol-sensitive transient receptor potential channels containing TRPC3, TRPC6 and TRPC7 subunits in melanopsin-evoked depolarization. Here we show that melanopsin-evoked electrical responses largely persist in ipRGCs recorded from early postnatal (P6-P8) and adult (P22-P50) mice lacking expression of functional TRPC3, TRPC6 or TRPC7 subunits. Multielectrode array (MEA) recordings performed at P6-P8 stages under conditions that prevent influences from rod/cone photoreceptors show comparable light sensitivity for the melanopsin-evoked responses in these mutant mouse lines in comparison to wild-type (WT) mice. Patch-clamp recordings from adult mouse ipRGCs lacking TRPC3 or TRPC7 subunits show intrinsic light-evoked responses equivalent to those recorded in WT mice. Persistence of intrinsic light-evoked responses was also noted in ipRGCs lacking TRPC6 subunits, although with significantly smaller magnitudes. These results demonstrate that the melanopsin-evoked depolarization in ipRGCs is not mediated by either TRPC3, TRPC6 or TRPC7 channel subunits alone. They also suggest that the melanopsin signaling pathway includes TRPC6-containing heteromeric channels in mature retinas.

Intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) respond to light in the absence of all rod and cone photoreceptor input. The existence of these ganglion cell photoreceptors, although predicted from observations scattered over many decades, was not established until it was shown that a novel photopigment, melanopsin, was expressed in retinal ganglion cells of rodents and primates. Phototransduction in mammalian ipRGCs more closely resembles that of invertebrate than vertebrate photoreceptors and appears to be mediated by transient receptor potential channels. In the retina, ipRGCs provide excitatory drive to dopaminergic amacrine cells and ipRGCs are coupled to GABAergic amacrine cells via gap junctions. Several subtypes of ipRGC have been identified in rodents based on their morphology, physiology and expression of molecular markers. ipRGCs convey irradiance information centrally via the optic nerve to influence several functions including photoentrainment of the biological clock located in the hypothalamus, the pupillary light reflex, sleep and perhaps some aspects of vision. In addition, ipRGCs may also contribute irradiance signals that interface directly with the autonomic nervous system to regulate rhythmic gene activity in major organs of the body. Here we review the early work that provided the motivation for searching for a new mammalian photoreceptor, the ground-breaking discoveries, current progress that continues to reveal the unusual properties of these neuron photoreceptors, and directions for future investigation.

Vertebrate ancient opsin and melanopsin: divergent irradiance detectors

Both vertebrates and invertebrates respond to light by utilising a wide-ranging array of photosensory systems, with diverse photoreceptor organs expressing a characteristic photopigment, itself consisting of an opsin apoprotein linked to a light-sensitive retinoid chromophore based on vitamin A. In the eye, the pigments expressed in both cone and rod photoreceptors have been studied in great depth and mediate contrast perception, measurement of the spectral composition of environmental light, and thus classical image forming vision. By contrast, the molecular basis for non-visual and extraocular photoreception is far less understood; however, two photopigment genes have become the focus of much study, the vertebrate ancient (va) opsin and melanopsin (opn4). In this review, we discuss the history of discovery for each gene, as well as focusing on the evolution, expression profile, functional role and broader physiological significance of each photopigment. Recently, it has been suggested independently by Arendt et al. and Lamb that an ancestral opsin bifurcated in early metazoans and evolved into two quite different photopigments, one expressed in rhabdomeric photoreceptors and the other in ciliary photoreceptors. This interpretation of the evolution of the metazoan eye has provided a powerful framework for understanding photobiological organization. Their proposal, however, does not encompass all current experimental observations that would be consistent with what we term a central "Evolution of Photosensory Opsins with Common Heredity (EPOCH)" hypothesis to explain the complexity of animal photosensory systems. Clearly, many opsin genes (e.g. va opsin) simply do not fit neatly within this scheme. Thus, the review concludes with a discussion of these anomalies and their context regarding the phylogeny of photoreceptor and photopigment development.

Intrinsically photosensitive retinal ganglion cells

Life on earth is subject to alternating cycles of day and night imposed by the rotation of the earth. Consequently, living things have evolved photodetective systems to synchronize their physiology and behavior with the external light-dark cycle. This form of photodetection is unlike the familiar "image vision," in that the basic information is light or darkness over time, independent of spatial patterns. "Nonimage" vision is probably far more ancient than image vision and is widespread in living species. For mammals, it has long been assumed that the photoreceptors for nonimage vision are also the textbook rods and cones. However, recent years have witnessed the discovery of a small population of retinal ganglion cells in the mammalian eye that express a unique visual pigment called melanopsin. These ganglion cells are intrinsically photosensitive and drive a variety of nonimage visual functions. In addition to being photoreceptors themselves, they also constitute the major conduit for rod and cone signals to the brain for nonimage visual functions such as circadian photoentrainment and the pupillary light reflex. Here we review what is known about these novel mammalian photoreceptors.

The emerging roles of melanopsin in behavioral adaptation to light

The adaptation of behavior and physiology to changes in the ambient light level is of crucial importance to life. These adaptations include the light modulation of neuroendocrine function and temporal alignment of physiology and behavior to the day:night cycle by the circadian clock. These non-image-forming (NIF) responses can function independent of rod and cone photoreceptors but depend on ocular light reception, suggesting the participation of novel photoreceptors in the eye. The discovery of melanopsin in intrinsically photosensitive retinal ganglion cells (ipRGCs) and genetic proof for its important role in major NIF responses have offered an exciting entry point to comprehend how mammals adapt to the light environment. Here, we review the recent advances in our understanding of the emerging roles of melanopsin and ipRGCs. These findings now offer new avenues to understand the role of ambient light in sleep, alertness, dependent physiologies and potential pharmacological intervention as well as lifestyle modifications to improve the quality of life.

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.

The acute light-induction of sleep is mediated by OPN4-based photoreception

Sleep is regulated by both homeostatic and circadian mechanisms. The latter, termed 'process c', helps synchronize sleep-wake patterns to the appropriate time of the day. However, in the absence of a circadian clock, overall sleep-wake rhythmicity is preserved and remains synchronized to the external light-dark cycle, indicating that there is an additional, clock-independent photic input to sleep. We found that the direct photic regulation of sleep in mice is predominantly mediated by melanopsin (OPN4)-based photoreception of photosensitive retinal ganglion cells (pRGCs). Moreover, OPN4-dependent sleep regulation was correlated with the activation of sleep-promoting neurons in the ventrolateral preoptic area and the superior colliculus. Collectively, our findings describe a previously unknown pathway in sleep regulation and identify the pRGC/OPN4 signaling system as a potentially new pharmacological target for the selective manipulation of sleep and arousal states.

Refined anatomical isolation of functional sleep circuits exhibits distinctive regional and circadian gene transcriptional profiles

Powerful new approaches to study molecular variation in distinct neuronal populations have recently been developed enabling a more precise investigation of the control of neural circuits involved in complex behaviors such as wake and sleep. We applied laser capture microdissection (LCM) to isolate precise brain nuclei from rat CNS at opposing circadian time points associated with wake and sleep. Discrete anatomical and temporal analysis was performed to examine the extent of variation in the transcriptional control associated with both identifiable anatomical nuclei and with light/dark cycle. Precise isolation of specific brain nuclei regulating sleep and arousal, including the LC, SCN, TMN, VTA, and VLPO, demonstrated robust changes in gene expression. Many of these differences were not observed in previous studies where whole brain lysates or gross dissections were used to probe for changes in gene expression. The robust and differential profiles of genomic data obtained from the approaches used herein underscore the requirement for careful anatomical refinement in CNS gene expression studies designed to understand genomic control within behaviorally-linked, but functionally isolated brain nuclei.

Melanopsin Ganglion Cells Use a Membrane-Associated Rhabdomeric Phototransduction Cascade

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are photoreceptors of the mammalian eye that drive pupillary responses, synchronization of circadian rhythms, and other reflexive responses to daylight. Melanopsin is the ipRGC photopigment, but the signaling cascade through which this invertebrate-like opsin triggers the photocurrent in these cells is unknown. Here, using patch-clamp recordings from dissociated ipRGCs in culture, we show that a membrane-associated phosphoinositide cascade lies at the heart of the ipRGC phototransduction mechanism, similar to the cascade in rhabdomeric photoreceptors of invertebrate eyes. When ipRGCs were illuminated, melanopsin activated a G protein of the Gq/11class, stimulating the effector enzyme phospholipase C. The presence of these signaling components in ipRGCs was confirmed by single-cell RT-PCR and immunofluorescence. The photoresponse was fully functional in excised inside-out patches of ipRGC membrane, indicating that all core signaling components are within or tightly coupled to the plasma membrane. The striking similarity of phototransduction in ipRGCs and invertebrate rhabdomeric photoreceptors reinforces the emerging view that these cells have a common evolutionary origin.

Functional expression, targeting and Ca2+ signaling of a mouse melanopsin-eYFP fusion protein in a retinal pigment epithelium cell line

Melanopsin, first discovered in Xenopus melanophores, is now established as a functional sensory photopigment of the intrinsically photosensitive retinal ganglion cells. These ganglion cells drive circadian rhythm and pupillary adjustments through projection to the brain. Melanopsin shares structural similarities with all known opsins. Comprehensive characterization of melanopsin with respect to its spectral properties, photochemical cascade and signaling partners requires a suitable recombinant system and high expression levels. This combination has not yet been described. To address this issue, we have expressed recombinant mouse melanopsin in several cell lines. Using enhanced yellow fluorescent protein (eYFP) as a visualization tag, expression was observed in all cell lines. Confocal microscopy revealed that melanopsin was properly routed to the plasma membrane only in retinal pigment epithelium (RPE)-derived D407 cells and in human embryonic kidney (HEK) cells. Further, we performed intracellular calcium measurements in order to probe the melanopsin signaling activity of this fusion protein. Transfected cells were loaded with the calcium indicator Fura2-AM. Upon illumination, an immediate but transient calcium response was observed in HEK as well as in D407 cells, while mock-transfected cells showed no calcium response under identical conditions. Supplementation with 11-cis retinal or all-trans retinal enhanced the response. After prolonged illumination the cells became desensitized. Thus, RPE-derived cells expressing recombinant melanopsin may constitute a suitable system for the study of the structural and functional characteristics of melanopsin.

Light-evoked calcium responses of isolated melanopsin-expressing retinal ganglion cells

A small number (<2%) of mammalian retinal ganglion cells express the photopigment melanopsin and are intrinsically photosensitive (ipRGCs). Light depolarizes ipRGCs and increases intracellular calcium levels ([Ca2+]i) but the signaling cascades underlying these responses have yet to be elucidated. To facilitate physiological studies on these rare photoreceptors, highly enriched ipRGC cultures from neonatal rats were generated using anti-melanopsin-mediated plate adhesion (immunopanning). This novel approach enabled experiments on isolated ipRGCs, eliminating the potential confounding influence of rod/cone-driven input. Light induced a rise in [Ca2+]i (monitored using fura-2 imaging) in the immunopanned ipRGCs and the source of this Ca2+ signal was investigated. The Ca2+ responses were inhibited by 2-aminoethoxydiphenyl borate, SKF-96365 (1-2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy]ethyl-1H-imidazole), flufenamic acid, lanthanum, and gadolinium, consistent with the involvement of canonical transient receptor potential (TRP) channels in ipRGC phototransduction. However, the contribution of direct Ca2+ flux through a putative TRP channel to ipRGC [Ca2+]i was relatively small, as most (approximately 90%) of the light-induced Ca2+ responses could be blocked by preventing action potential firing with tetrodotoxin. The L-type voltage-gated Ca2+ channel (VGCC) blockers verapamil and (+)-cis-diltiazem significantly reduced the light-evoked Ca2+ responses, while the internal Ca2+ stores depleting agent thapsigargin had negligible effect. These results indicate that Ca2+ influx through VGCCs, activated after action potential firing, was the primary source for light-evoked elevations in ipRGC [Ca2+]i. Furthermore, concurrent Ca2+ imaging and cell-attached electrophysiological recordings demonstrated that the Ca2+ responses were highly correlated to spike frequency, thereby establishing a direct link between action potential firing and somatic [Ca2+]i in light-stimulated ipRGCs.