The M6 cell: A small-field bistratified photosensitive retinal ganglion cell

Defining spatial nonuniformities of all ipRGC types using an improved Opn4cre recombinase mouse line

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a crucial role in several physiological light responses. In this study, we generate an improved Opn4cre knockin allele (Opn4cre(DSO)), which faithfully reproduces endogenous Opn4 expression and improves compatibility with widely used reporters. We evaluated the efficacy and sensitivity of Opn4cre(DSO) for labeling in retina and brain and provide an in-depth comparison with the extensively utilized Opn4cre(Saha) line. Through this characterization, Opn4cre(DSO) demonstrated higher specificity in labeling ipRGCs with minimal recombination escape. Leveraging a combination of electrophysiological, molecular, and morphological analyses, we confirmed its sensitivity in detecting all ipRGC types (M1-M6) and defined their unique topographical distribution across the retina. In the brain, the Opn4cre(DSO) line labels ipRGC projections with minimal labeling of cell bodies. Overall, the Opn4cre(DSO) mouse line represents an improved tool for studying ipRGC function and distribution, offering a means to selectively target these cells to study light-regulated behaviors and physiology.

Dynamic endocannabinoid-mediated neuromodulation of retinal circadian circuitry

Circadian rhythms are biological rhythms that originate from the "master circadian clock," called the suprachiasmatic nucleus (SCN). SCN orchestrates the circadian rhythms using light as a chief zeitgeber, enabling humans to synchronize their daily physio-behavioral activities with the Earth's light-dark cycle. However, chronic/ irregular photic disturbances from the retina via the retinohypothalamic tract (RHT) can disrupt the amplitude and the expression of clock genes, such as the period circadian clock 2, causing circadian rhythm disruption (CRd) and associated neuropathologies. The present review discusses neuromodulation across the RHT originating from retinal photic inputs and modulation offered by endocannabinoids as a function of mitigation of the CRd and associated neuro-dysfunction. Literature indicates that cannabinoid agonists alleviate the SCN's ability to get entrained to light by modulating the activity of its chief neurotransmitter, i.e., γ-aminobutyric acid, thus preventing light-induced disruption of activity rhythms in laboratory animals. In the retina, endocannabinoid signaling modulates the overall gain of the retinal ganglion cells by regulating the membrane currents (Ca2+, K+, and Cl- channels) and glutamatergic neurotransmission of photoreceptors and bipolar cells. Additionally, endocannabinoids signalling also regulate the high-voltage-activated Ca2+ channels to mitigate the retinal ganglion cells and intrinsically photosensitive retinal ganglion cells-mediated glutamate release in the SCN, thus regulating the RHT-mediated light stimulation of SCN neurons to prevent excitotoxicity. As per the literature, cannabinoid receptors 1 and 2 are becoming newer targets in drug discovery paradigms, and the involvement of endocannabinoids in light-induced CRd through the RHT may possibly mitigate severe neuropathologies.

A pupillary contrast response in mice and humans: Neural mechanisms and visual functions

In the pupillary light response (PLR), increases in ambient light constrict the pupil to dampen increases in retinal illuminance. Here, we report that the pupillary reflex arc implements a second input-output transformation; it senses temporal contrast to enhance spatial contrast in the retinal image and increase visual acuity. The pupillary contrast response (PCoR) is driven by rod photoreceptors via type 6 bipolar cells and M1 ganglion cells. Temporal contrast is transformed into sustained pupil constriction by the M1's conversion of excitatory input into spike output. Computational modeling explains how the PCoR shapes retinal images. Pupil constriction improves acuity in gaze stabilization and predation in mice. Humans exhibit a PCoR with similar tuning properties to mice, which interacts with eye movements to optimize the statistics of the visual input for retinal encoding. Thus, we uncover a conserved component of active vision, its cell-type-specific pathway, computational mechanisms, and optical and behavioral significance.

Cell-specific localization of β-synuclein in the mouse retina

β-synuclein, a member of the synuclein family, is frequently co-expressed with α-synuclein in the neural system, where it serves to inhibit abnormal aggregation of α-synuclein in neurodegenerative diseases. Beyond its role in pathological conditions, β-synuclein plays various functions independently of α-synuclein. In our investigation, we discovered a broader expression of β-synuclein in the mouse retina compared to α-synuclein. This widespread pattern implies its potential significance in the retina. Through detailed examination via light- and electron-microscopic immunocytochemistry, we identified β-synuclein expression from the inner segment (IS) and outer segment (OS) of photoreceptor cells to the ganglion cell layer (GCL). Our findings unveiled unique features, including β-synuclein immunoreactive IS and OS of cones, higher expression in cone pedicles than in rod spherules, absence in horizontal cells, limited expression in cone bipolar dendrites and somas, higher expression in cone bipolar terminals, presence in most amacrine cells, and expression in almost majority of somas in GCL with an absence in intrinsically photosensitive retinal ganglion cell (ipRGCs) processes. Notably, all cholinergic amacrine cells express high β- but not α-synuclein, while dopaminergic amacrine cells express α-synuclein exclusively. These distinctive expression patterns offer valuable insights for further exploration into the functions of β-synuclein and its potential role in synuclein pathology within the retina.

Retinal ganglion cell type-specific expression of synuclein family members revealed by scRNA-sequencing

Synuclein family members (Snca, Sncb, and Scng) are expressed in the retina, but their precise locations and roles are poorly understood. We performed an extensive analysis of the single-cell transcriptome in healthy and injured retinas to investigate their expression patterns and roles. We observed the expression of all synuclein family members in retinal ganglion cells (RGCs), which remained consistent across species (human, mouse, and chicken). We unveiled differential expression of Snca across distinct clusters (highly expressed in most), while Sncb and Sncg displayed uniform expression across all clusters. Further, we observed a decreased expression in RGCs following traumatic axonal injury. However, the proportion of α-Syn-positive RGCs in all RGCs and α-Syn-positive intrinsically photosensitive retinal ganglion cells (ipRGCs) in all ipRGCs remained unaltered. Lastly, we identified changes in communication patterns preceding cell death, with particular significance in the pleiotrophin-nucleolin (Ptn-Ncl) and neural cell adhesion molecule signaling pathways, where communication differences were pronounced between cells with varying expression levels of Snca. Our study employs an innovative approach using scRNA-seq to characterize synuclein expression in health retinal cells, specifically focusing on RGC subtypes, advances our knowledge of retinal physiology and pathology.

Flp-recombinase mouse line for genetic manipulation of ipRGCs

Light has myriad impacts on behavior, health, and physiology. These signals originate in the retina and are relayed to the brain by more than 40 types of retinal ganglion cells (RGCs). Despite a growing appreciation for the diversity of RGCs, how these diverse channels of light information are ultimately integrated by the ~50 retinorecipient brain targets to drive these light-evoked effects is a major open question. This gap in understanding primarily stems from a lack of genetic tools that specifically label, manipulate, or ablate specific RGC types. Here, we report the generation and characterization of a new mouse line (Opn4FlpO), in which FlpO is expressed from the Opn4 locus, to manipulate the melanopsin-expressing, intrinsically photosensitive retinal ganglion cells. We find that the Opn4FlpO line, when crossed to multiple reporters, drives expression that is confined to ipRGCs and primarily labels the M1-M3 subtypes. Labeled cells in this mouse line show the expected intrinsic, melanopsin-based light response and morphological features consistent with the M1-M3 subtypes. In alignment with the morphological and physiological findings, we see strong innervation of non-image forming brain targets by ipRGC axons, and weaker innervation of image forming targets in Opn4FlpO mice labeled using AAV-based and FlpO-reporter lines. Consistent with the FlpO insertion disrupting the endogenous Opn4 transcript, we find that Opn4FlpO/FlpO mice show deficits in the pupillary light reflex, demonstrating their utility for behavioral research in future experiments. Overall, the Opn4FlpO mouse line drives Flp-recombinase expression that is confined to ipRGCs and most effectively drives recombination in M1-M3 ipRGCs. This mouse line will be of broad use to those interested in manipulating ipRGCs through a Flp-based recombinase for intersectional studies or in combination with other, non-Opn4 Cre driver lines.

A new Opn4cre recombinase mouse line to target intrinsically photosensitive retinal ganglion cells (ipRGCs)

Intrinsically photosensitive retinal ganglion cells (ipRGCs) play a crucial role in several physiological light responses. In this study we generate a new Opn4cre knock-in allele (Opn4cre(DSO)), in which cre is placed immediately downstream of the Opn4 start codon. This approach aims to faithfully reproduce endogenous Opn4 expression and improve compatibility with widely used reporters. We evaluated the efficacy and sensitivity of Opn4cre(DSO) for labeling in retina and brain, and provide an in-depth comparison with the extensively utilized Opn4cre(Saha) line. Through this characterization, Opn4cre(DSO) demonstrated higher specificity in labeling ipRGCs, with minimal recombination escape. Leveraging a combination of electrophysiological, molecular, and morphological analyses, we confirmed its sensitivity in detecting all ipRGC types (M1-M6). Using this new tool, we describe the topographical distributions of ipRGC types across the retinal landscape, uncovering distinct ventronasal biases for M5 and M6 types, setting them apart from their M1-M4 counterparts. In the brain, we find vastly different labeling patterns between lines, with Opn4cre(DSO) only labeling ipRGC axonal projections to their targets. The combination of off-target effects of Opn4cre(Saha) across the retina and brain, coupled with diminished efficiencies of both Cre lines when coupled to less sensitive reporters, underscores the need for careful consideration in experimental design and validation with any Opn4cre driver. Overall, the Opn4cre(DSO) mouse line represents an improved tool for studying ipRGC function and distribution, offering a means to selectively target these cells to study light-regulated behaviors and physiology.

Light affects the prefrontal cortex via intrinsically photosensitive retinal ganglion cells

The ventromedial prefrontal cortex (vmPFC) is a part of the limbic system engaged in the regulation of social, emotional, and cognitive states, which are characteristically impaired in disorders of the brain such as schizophrenia and depression. Here, we show that intrinsically photosensitive retinal ganglion cells (ipRGCs) modulate, through light, the integrity, activity, and function of the vmPFC. This regulatory role, which is independent of circadian and mood alterations, is mediated by an ipRGC-thalamic-corticolimbic pathway. Lack of ipRGC signaling in mice causes dendritic degeneration, dysregulation of genes involved in synaptic plasticity, and depressed neuronal activity in the vmPFC. These alterations primarily undermine the ability of the vmPFC to regulate emotions. Our discovery provides a potential light-dependent mechanism for certain PFC-centric disorders in humans.

Distribution, development, and identity of retinal ganglion cells labeled in the Sert-Cre reporter mouse

The mouse retina contains over 40 types of retinal ganglion cells (RGCs) that differ in morphology, function, or gene expression. RGCs also differ by whether their axons target the brain.s ipsilateral or contralateral hemisphere. Contralaterally projecting RGCs (contraRGCs) are widespread in mouse retina, whereas ipsilateral projecting RGCs (ipsiRGCs) are confined to the ventro-temporal (VT) crescent of retina. In this study, we employed the Sert-Cre transgenic line, which had been reported to selectively label ipsiRGCs, to study ipsiRGCs during development. Although the number of Cre-expressing ipsiRGCs did not significantly increase with postnatal age, the region of retina that they occupied did, and by adulthood represented ~30% of the retinal surface. Unexpectedly, genetic ablation of Sert-Cre cells failed to fully disrupt ipsilateral projecting retinal axons, suggesting that not all ipsiRGCs generated Cre in Sert-Cre mice. To test this hypothesis, we retrogradely labeled ipsiRGCs in Sert-Cre mice which revealed that not all ipsiRGCs are labeled in Sert-Cre mice and a small population of contraRGCs flanking the VT crescent generates Cre in this line. These results do not negate the usefulness of the Sert-Cre mouse but do raise important caveats to the interpretation of such studies.

Evaluation of the neuroprotective efficacy of the gramine derivative ITH12657 against NMDA-induced excitotoxicity in the rat retina

Purpose

The aim of this study was to investigate, the neuroprotective effects of a new Gramine derivative named: ITH12657, in a model of retinal excitotoxicity induced by intravitreal injection of NMDA.

Methods

Adult Sprague Dawley rats received an intravitreal injection of 100 mM NMDA in their left eye and were treated daily with subcutaneous injections of ITH12657 or vehicle. The best dose-response, therapeutic window study, and optimal treatment duration of ITH12657 were studied. Based on the best survival of Brn3a + RGCs obtained from the above-mentioned studies, the protective effects of ITH12657 were studied in vivo (retinal thickness and full-field Electroretinography), and ex vivo by quantifying the surviving population of Brn3a + RGCs, αRGCs and their subtypes α-ONsRGCs, α-ONtRGCs, and α-OFFRGCs.

Results

Administration of 10 mg/kg ITH12657, starting 12 h before NMDA injection and dispensed for 3 days, resulted in the best significant protection of Brn3a + RGCs against NMDA-induced excitotoxicity. In vivo, ITH12657-treated rats showed significant preservation of retinal thickness and functional protection against NMDA-induced retinal excitotoxicity. Ex vivo results showed that ITH12657 afforded a significant protection against NMDA-induced excitotoxicity for the populations of Brn3a + RGC, αRGC, and αONs-RGC, but not for the population of αOFF-RGC, while the population of α-ONtRGC was fully resistant to NMDA-induced excitotoxicity.

Conclusion

Subcutaneous administration of ITH12657 at 10 mg/kg, initiated 12 h before NMDA-induced retinal injury and continued for 3 days, resulted in the best protection of Brn3a + RGCs, αRGC, and αONs-RGC against excitotoxicity-induced RGC death. The population of αOFF-RGCs was extremely sensitive while α-ONtRGCs were fully resistant to NMDA-induced excitotoxicity.

Efficacy and specificity of melanopsin reporters for retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are specialized retinal output neurons that mediate behavioral, neuroendocrine, and developmental responses to environmental light. There are diverse molecular strategies for marking ipRGCs, especially in mice, making them among the best characterized retinal ganglion cells (RGCs). With the development of more sensitive reporters, new subtypes of ipRGCs have emerged. We therefore tested high-sensitivity reporter systems to see whether we could reveal yet more. Substantial confusion remains about which of the available methods, if any, label all and only ipRGCs. Here, we compared many different methods for labeling of ipRGCs, including anti-melanopsin immunofluorescence, Opn4-GFP BAC transgenic mice, and Opn4cre mice crossed with three different Cre-specific reporters (Z/EG, Ai9, and Ai14) or injected with Cre-dependent (DIO) AAV2. We show that Opn4cre mice, when crossed with sensitive Cre-reporter mice, label numerous ganglion cell types that lack intrinsic photosensitivity. Though other methods label ipRGCs specifically, they do not label the entire population of ipRGCs. We conclude that no existing method labels all and only ipRGCs. We assess the appropriateness of each reporter for particular applications and integrate findings across reporters to estimate that the overall abundance of ipRGCs among mouse RGCs may approach 11%.

Mapping the Retina onto the Brain

Vision begins in the retina, which extracts salient features from the environment and encodes them in the spike trains of retinal ganglion cells (RGCs), the output neurons of the eye. RGC axons innervate diverse brain areas (>50 in mice) to support perception, guide behavior, and mediate influences of light on physiology and internal states. In recent years, complete lists of RGC types (∼45 in mice) have been compiled, detailed maps of their dendritic connections drawn, and their light responses surveyed at scale. We know less about the RGCs' axonal projection patterns, which map retinal information onto the brain. However, some organizing principles have emerged. Here, we review the strategies and mechanisms that govern developing RGC axons and organize their innervation of retinorecipient brain areas.

Coexistence within one cell of microvillous and ciliary phototransductions across M1- through M6-IpRGCs

Intrinsically photosensitive retinal ganglion cells (ipRGCs) serve as primary photoceptors by expressing the photopigment, melanopsin, and also as retinal relay neurons for rod and cone signals en route to the brain, in both cases for the purpose of non-image vision as well as aspects of image vision. So far, six subtypes of ipRGCs (M1 through M6) have been characterized. Regarding their phototransduction mechanisms, we have previously found that, unconventionally, rhabdomeric (microvillous) and ciliary signaling motifs co-exist within a given M1-, M2-, and M4-ipRGC, with the first mechanism involving PLCβ4 and TRPC6,7 channels and the second involving cAMP and HCN channels. We have now examined M3-, M5-, and M6-cells and found that each cell likewise uses both signaling pathways for phototransduction, despite differences in the percentage representation by each pathway in a given ipRGC subtype for bright-flash responses (and saturated except for M6-cells). Generally, M3- and M5-cells show responses quite similar in kinetics to M2-responses, and M6-cell responses resemble broadly those of M1-cells although much lower in absolute sensitivity and amplitude. Therefore, similar to rod and cone subtypes in image vision, ipRGC subtypes possess the same phototransduction mechanism(s) even though they do not show microvilli or cilia morphologically.

WITHDRAWN: Light affects the prefrontal cortex via intrinsically photosensitive retinal ganglion cells

This manuscript has been withdrawn by bioRxiv following a formal request by the NIH Intramural Research Integrity Office owing to lack of author consent.

Melanopsin activates divergent phototransduction pathways in intrinsically photosensitive retinal ganglion cell subtypes

Melanopsin signaling within intrinsically photosensitive retinal ganglion cell (ipRGC) subtypes impacts a broad range of behaviors from circadian photoentrainment to conscious visual perception. Yet, how melanopsin phototransduction within M1-M6 ipRGC subtypes impacts cellular signaling to drive diverse behaviors is still largely unresolved. The identity of the phototransduction channels in each subtype is key to understanding this central question but has remained controversial. In this study, we resolve two opposing models of M4 phototransduction, demonstrating that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dispensable for this process and providing support for a pathway involving melanopsin-dependent potassium channel closure and canonical transient receptor potential (TRPC) channel opening. Surprisingly, we find that HCN channels are likewise dispensable for M2 phototransduction, contradicting the current model. We instead show that M2 phototransduction requires TRPC channels in conjunction with T-type voltage-gated calcium channels, identifying a novel melanopsin phototransduction target. Collectively, this work resolves key discrepancies in our understanding of ipRGC phototransduction pathways in multiple subtypes and adds to mounting evidence that ipRGC subtypes employ diverse phototransduction cascades to fine-tune cellular responses for downstream behaviors.

Modeling the Effects of Napping and Non-napping Patterns of Light Exposure on the Human Circadian Oscillator

In early childhood, consolidation of sleep from a biphasic to a monophasic sleep-wake pattern, that is, the transition from sleeping during an afternoon nap and at night to sleeping only during the night, represents a major developmental milestone. Reduced napping behavior is associated with an advance in the timing of the circadian system; however, it is unknown if this advance represents a standard response of the circadian clock to altered patterns of light exposure or if it additionally reflects features of the developing circadian system. Using a mathematical model of the human circadian pacemaker, we investigated the impact of napping and non-napping patterns of light exposure on entrained circadian phases. Simulated light schedules were based on published data from 20 children (34.2 ± 2.0 months) with habitual napping or non-napping sleep patterns (15 nappers). We found the model predicted different circadian phases for napping and non-napping light patterns: both the decrease in afternoon light during the nap and the increase in evening light associated with napping toddlers' later bedtimes contributed to the observed circadian phase difference produced between napping and non-napping light schedules. We systematically quantified the effects on phase shifting of nap duration, timing, and light intensity, finding larger phase delays occurred for longer and earlier naps. In addition, we simulated phase response curves to a 1-h light pulse and 1-h dark pulse to predict phase and intensity dependence of these changes in light exposure. We found the light pulse produced larger shifts compared with the dark pulse, and we analyzed the model dynamics to identify the features contributing to this asymmetry. These findings suggest that napping status affects circadian timing due to altered patterns of light exposure, with the dynamics of the circadian clock and light processing mediating the effects of the dark pulse associated with a daytime nap.

Distribution and synaptic organization of substance P-like immunoreactive neurons in the mouse retina

Substance P (SP), a neuroprotective peptidergic neurotransmitter, is known to have immunoreactivity (IR) localized to amacrine and/or ganglion cells in a variety of species' retinas, but it has not yet been studied in the mouse retina. Thus, we investigated the distribution and synaptic organization of SP-IR by confocal and electron microscopy immunocytochemistry in the mouse retina. SP-IR was distributed in the inner nuclear layer (INL), inner plexiform layer (IPL), and ganglion cell layer (GCL). Most of the SP-IR somas belonged to amacrine cells (2.5% of all) in the INL and their processes stratified into the S1, S3, and S5 layers of the IPL, with the most intense band in the S5 layer. Some SP-IR somas can also be observed in the GCL, which were identified as displaced amacrine cells (82%, 1269/1550) and ganglion cells (18%, 281/1550) by antibodies against AP2α and RBPMS, respectively. Such SP-IR ganglion cells (1.2% of all RGCs) can be further divided into 3 subgroups expressing SP/α-Synuclein (α-Syn), SP/GAD67, and/or SP/GAD67/α-Syn. Possible physiological and pathological roles of these ganglion cells are discussed. Further, electron microscopy evidence demonstrates that SP-IR amacrine cells receive major inputs from other SP-IR amacrine cell processes (146/242 inputs) and output mostly to SP-negative amacrine cell processes (291/673 outputs), suggesting series inhibition among amacrine cells. These results reveal for the first time an explicit distribution, novel ganglion cell features, and synaptic organization of SP-IR in the mouse retina, which is important for the future use of mouse models to study the roles of SP in healthy and diseased (including Parkinson's disease) retinal states.

Functional subtypes of rodent melanopsin ganglion cells switch roles between night and day illumination

Intrinsically photosensitive retinal ganglion cells (ipRGCs), contain the photopigment melanopsin, and influence both image and non-image forming behaviors. Despite being categorized into multiple types (M1-M6), physiological variability within these types suggests our current understanding of ipRGCs is incomplete. We used multi-electrode array (MEA) recordings and unbiased cluster analysis under synaptic blockade to identify 8 functional clusters of ipRGCs, each with distinct photosensitivity and response timing. We used Cre mice to drive the expression of channelrhodopsin in SON-ipRGCs, enabling the localization of distinct ipRGCs in the dorsal retina. Additionally, we conducted a retrospective unbiased cluster analysis of ipRGC photoresponses to light stimuli across scotopic, mesopic, and photopic intensities, aimed at activating both rod and cone inputs to ipRGCs. Our results revealed shared and distinct synaptic inputs to the identified functional clusters, demonstrating that ipRGCs encode visual information with high fidelity at low light intensities, but poorly at photopic light intensities, when melanopsin activation is highest. Collectively, our findings support a framework with at least 8 functional subtypes of ipRGCs, each encoding luminance with distinct spike outputs, highlighting the inherent functional diversity and complexity of ipRGCs and suggesting a reevaluation of their contributions to retinal function and visual perception under varying light conditions.

Enhanced human contrast sensitivity with increased stimulation of melanopsin in intrinsically photosensitive retinal ganglion cells

The intrinsically photosensitive retinal ganglion cells (ipRGCs) are known to serve non-image-forming functions, such as photoentrainment of the circadian rhythm and pupillary light reflex. However, how they affect human spatial vision is largely unknown. The spatial contrast sensitivity function (CSF), which measures contrast sensitivity as a function of spatial frequency, was used in the current study to investigate the function of ipRGCs in pattern vision. To compare the effects of different background lights on the CSF, we utilized the silent substitution technique. We manipulated the stimulation level of melanopsin (i.e., the visual pigment of ipRGCs) from the background light while keeping the cone stimulations constant, or vice versa. We conducted four experiments to measure the CSFs at various spatial frequencies, eccentricities, and levels of background luminance. Results showed that melanopsin stimulation from the background light enhances spatial contrast sensitivity across different eccentricities and luminance levels. Our finding that melanopsin contributes to CSF, combined with the receptive field analysis, suggests a role for the magnocellular pathway and challenges the conventional view that ipRGCs are primarily responsible for non-visual functions.

Glutamatergic and GABAergic neurons in the vLGN mediate the nociceptive effects of green and red light on neuropathic pain

Phototherapy is an emerging non-pharmacological treatment for depression, circadian rhythm disruptions, and neurodegeneration, as well as pain conditions including migraine and fibromyalgia. However, the mechanism of phototherapy-induced antinociception is not well understood. Here, using fiber photometry recordings of population-level neural activity combined with chemogenetics, we found that phototherapy elicits antinociception via regulation of the ventral lateral geniculate body (vLGN) located in the visual system. Specifically, both green and red lights caused an increase of c-fos in vLGN, with red light increased more. In vLGN, green light causes a large increase in glutamatergic neurons, whereas red light causes a large increase in GABAergic neurons. Green light preconditioning increases the sensitivity of glutamatergic neurons to noxious stimuli in vLGN of PSL mice. Green light produces antinociception by activating glutamatergic neurons in vLGN, and red light promotes nociception by activating GABAergic neurons in vLGN. Together, these results demonstrate that different colors of light exert different pain modulation effects by regulating glutamatergic and GABAergic subpopulations in the vLGN. This may provide potential new therapeutic strategies and new therapeutic targets for the precise clinical treatment of neuropathic pain.

Encephalopsin (OPN3) is required for normal refractive development and the GO/GROW response to induced myopia

Purpose

Myopia, or nearsightedness, is the most common form of refractive error and is increasing in prevalence. While significant efforts have been made to identify genetic variants that predispose individuals to myopia, these variants are believed to account for only a small portion of the myopia prevalence, leading to a feedback theory of emmetropization, which depends on the active perception of environmental visual cues. Consequently, there has been renewed interest in studying myopia in the context of light perception, beginning with the opsin family of G-protein coupled receptors (GPCRs). Refractive phenotypes have been characterized in every opsin signaling pathway studied, leaving only Opsin 3 (OPN3), the most widely expressed and blue-light sensing noncanonical opsin, to be investigated for function in the eye and refraction.

Methods

Opn3 expression was assessed in various ocular tissues using an Opn3eGFP reporter. Weekly refractive development in Opn3 retinal and germline mutants from 3 to 9 weeks of age was measured using an infrared photorefractor and spectral domain optical coherence tomography (SD-OCT). Susceptibility to lens-induced myopia was then assessed using skull-mounted goggles with a -30 diopter experimental and a 0 diopter control lens. Mouse eye biometry was similarly tracked from 3 to 6 weeks. A myopia gene expression signature was assessed 24 h after lens induction for germline mutants to further assess myopia-induced changes.

Results

Opn3 was found to be expressed in a subset of retinal ganglion cells and a limited number of choroidal cells. Based on an assessment of Opn3 mutants, the OPN3 germline, but not retina conditional Opn3 knockout, exhibits a refractive myopia phenotype, which manifests in decreased lens thickness, shallower aqueous compartment depth, and shorter axial length, atypical of traditional axial myopias. Despite the short axial length, Opn3 null eyes demonstrate normal axial elongation in response to myopia induction and mild changes in choroidal thinning and myopic shift, suggesting that susceptibility to lens-induced myopia is largely unchanged. Additionally, the Opn3 null retinal gene expression signature in response to induced myopia after 24 h is distinct, with opposing Ctgf, Cx43, and Egr1 polarity compared to controls.

Conclusions

The data suggest that an OPN3 expression domain outside the retina can control lens shape and thus the refractive performance of the eye. Prior to this study, the role of Opn3 in the eye had not been investigated. This work adds OPN3 to the list of opsin family GPCRs that are implicated in emmetropization and myopia. Further, the work to exclude retinal OPN3 as the contributing domain in this refractive phenotype is unique and suggests a distinct mechanism when compared to other opsins.

Expression of α‐Synuclein in the mouse retina is confined to inhibitory presynaptic elements

α-Synuclein (α-Syn) is enriched in presynaptic terminals of the central nervous system including the retina and plays a role in the synaptic vesicle cycle and synaptic transmission. Abnormal aggregation of α-Syn is considered to be the main component of the Lewy bodies that are the pathological hallmarks of Parkinson's disease. Although expression pattern of α-Syn has been described in the retinas, its precise cellular and subcellular locations are poorly understood. We investigated the precise expression of α-Syn using light microscopy (LM) and electron microscopy (EM) with antibodies against α-Syn in the mouse retina. We found that the majority of α-Syn immunoreactivity (IR) is located in GABAergic, glycinergic, and dopaminergic amacrine cells, and their processes often make a direct synapse to other labeled or unlabeled amacrine profiles, bipolar cell terminals, or ganglion cell dendrites. Further, our LM and immuno-EM results confirm the absence of α-Syn in excitatory photoreceptors, bipolar cell bodies, and their ribbon synapses, providing evidence, for the first time, that ribbon synapses do not express α-Syn. Additionally, α-Syn IR is located in the ganglion cells, some of which are intrinsically photosensitive retinal ganglion cells. These results reveal a previously unappreciated inhibitory synapse-specific expression pattern of α-Syn in the retina, suggesting that α-Syn may play a distinct role in the modulation and integration of inhibitory synaptic transmission in the retina.

Colour and melanopsin mediated responses in the murine retina

Introduction: Intrinsically photosensitive retinal ganglion cells (ipRGCs) integrate melanopsin and rod/cone-mediated inputs to signal to the brain. Whilst originally identified as a cell type specialised for encoding ambient illumination, several lines of evidence indicate a strong association between colour discrimination and ipRGC-driven responses. Thus, cone-mediated colour opponent responses have been widely found across ipRGC target regions in the mouse brain and influence a key ipRGC-dependent function, circadian photoentrainment. Although ipRGCs exhibiting spectrally opponent responses have also been identified, the prevalence of such properties have not been systematically evaluated across the mouse retina or yet been found in ipRGC subtypes known to influence the circadian system. Indeed, there is still uncertainty around the overall prevalence of cone-dependent colour opponency across the mouse retina, given the strong retinal gradient in S and M-cone opsin (co)-expression and overlapping spectral sensitivities of most mouse opsins.

Methods: To address this, we use photoreceptor isolating stimuli in multielectrode recordings from human red cone opsin knock-in mouse (Opn1mwR) retinas to systematically survey cone mediated responses and the occurrence of colour opponency across ganglion cell layer (GCL) neurons and identify ipRGCs based on spectral comparisons and/or the persistence of light responses under synaptic blockade.

Results: Despite detecting robust cone-mediated responses across the retina, we find cone opponency is rare, especially outside of the central retina (overall ~3% of GCL neurons). In keeping with previous suggestions we also see some evidence of rod-cone opponency (albeit even more rare under our experimental conditions), but find no evidence for any enrichment of cone (or rod) opponent responses among functionally identified ipRGCs.

Conclusion: In summary, these data suggest the widespread appearance of cone-opponency across the mouse early visual system and ipRGC-related responses may be an emergent feature of central visual processing mechanisms.

Adeno-associated virus (AAV)-mediated Cre recombinase expression in melanopsin ganglion cells without leaky expression in rod/cone photoreceptors

BACKGROUND

Constituting about 5 % of mouse retinal ganglion cells (RGCs), intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin (gene symbol Opn4) and drive such photoresponses as pupil constriction, melatonin suppression, and circadian photoentrainment. Opn4Cre mice with Cre recombinase-expressing ipRGCs have enabled genetic manipulation of ipRGCs; unfortunately, while Cre expression within the inner retina is ipRGC-specific, leaky expression also occurs in some outer retinal photoreceptors, so Cre-induced alterations in the latter cells may confound certain studies of ipRGC function. Methods that express Cre in ipRGCs but not rods or cones are needed.

NEW METHOD

We have constructed a recombinant serotype-2 adeno-associated virus, rAAV2-Opn4-Cre, with the improved Cre recombinase (iCre) gene under the control of a ∼3kbp Opn4 promoter sequence, and injected it intravitreally into mice to transduce inner retinal neurons while sparing the outer retina.

RESULTS

We introduced rAAV2-Opn4-Cre into Cre reporter mice in which enhanced green fluorescent protein (EGFP) expression indicates Cre expression. Single-cell electrophysiological recordings and intracellular dye fills showed that 84 % of the EGFP+ cells were ipRGCs including M1-M6 types, while 16 % were conventional RGCs.

COMPARISON WITH EXISTING METHODS

Whereas Opn4Cre mice express Cre in some rod/cone photoreceptors, intravitreally applied rAAV2-Opn4-Cre induces Cre only in the inner retina, albeit with leaky expression in some conventional RGCs.

CONCLUSIONS

rAAV2-Opn4-Cre has overcome a significant limitation of Opn4Cre mice. We recommend usage scenarios where the Cre-expressing conventional RGCs should not pose a problem.

Burning the candle at both ends: Intraretinal signaling of intrinsically photosensitive retinal ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) are photoreceptors located in the ganglion cell layer. They project to brain regions involved in predominately non-image-forming functions including entrainment of circadian rhythms, control of the pupil light reflex, and modulation of mood and behavior. In addition to possessing intrinsic photosensitivity via the photopigment melanopsin, these cells receive inputs originating in rods and cones. While most research in the last two decades has focused on the downstream influence of ipRGC signaling, recent studies have shown that ipRGCs also act retrogradely within the retina itself as intraretinal signaling neurons. In this article, we review studies examining intraretinal and, in addition, intraocular signaling pathways of ipRGCs. Through these pathways, ipRGCs regulate inner and outer retinal circuitry through both chemical and electrical synapses, modulate the outputs of ganglion cells (both ipRGCs and non-ipRGCs), and influence arrangement of the correct retinal circuitry and vasculature during development. These data suggest that ipRGC function plays a significant role in the processing of image-forming vision at its earliest stage, positioning these photoreceptors to exert a vital role in perceptual vision. This research will have important implications for lighting design to optimize the best chromatic lighting environments for humans, both in adults and potentially even during fetal and postnatal development. Further studies into these unique ipRGC signaling pathways could also lead to a better understanding of the development of ocular dysfunctions such as myopia.

Unusual phototransduction via cross-motif signaling from Gq to adenylyl cyclase in intrinsically photosensitive retinalganglion cells

Nonimage-forming vision in mammals is mediated primarily by melanopsin (OPN4)-expressing, intrinsically photosensitive retinal ganglion cells (ipRGCs). In mouse M1-ipRGCs, melanopsin predominantly activates, via Gα_q,11,14, phospholipase C-β4 to open transient receptor 6 (TRPC6) and TRPC7 channels. In M2- and M4-ipRGCs, however, a prominent phototransduction mechanism involves the opening of hyperpolarization- and cyclic nucleotide-gated channels via cyclic nucleotide, although the upstream steps remain uncertain. We report here experiments, primarily on M4-ipRGCs, with photo-uncaging of cyclic nucleotides and virally expressed CNGA2 channels to conclude that the second messenger is cyclic adenosine monophosphate (cAMP) - very surprising considering that cyclic guanosine monophosphate (cGMP) is used in almost all cyclic nucleotide-mediated phototransduction mechanisms across the animal kingdom. We further found that the upstream G protein is likewise G_q, which via its Gβγ subunits directly activates adenylyl cyclase (AC). Our findings are a demonstration in a native cell of a cross-motif GPCR signaling pathway from G_q directly to AC with a specific function.

Robust visual cortex evoked potentials (VEP) in Gnat1 and Gnat2 knockout mice

Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin, imparting to themselves the ability to respond to light in the absence of input from rod or cone photoreceptors. Since their discovery ipRGCs have been found to play a significant role in non-image-forming aspects of vision, including circadian photoentrainment, neuroendocrine regulation, and pupillary control. In the past decade it has become increasingly clear that some ipRGCs also contribute directly to pattern-forming vision, the ability to discriminate shapes and objects. However, the degree to which melanopsin-mediated phototransduction, versus that of rods and cones, contributes to this function is still largely unknown. Earlier attempts to quantify this contribution have relied on genetic knockout models that target key phototransductive proteins in rod and cone photoreceptors, ideally to isolate melanopsin-mediated responses. In this study we used the Gnat1^-/-; Gnat2^cpfl3/cpfl3 mouse model, which have global knockouts for the rod and cone α-transducin proteins. These genetic modifications completely abolish rod and cone photoresponses under light-adapted conditions, locking these cells into a "dark" state. We recorded visually evoked potentials in these animals and found that they still showed robust light responses, albeit with reduced light sensitivity, with similar magnitudes to control mice. These responses had characteristics that were in line with a melanopsin-mediated signal, including delayed kinetics and increased saturability. Additionally, we recorded electroretinograms in a sub-sample of these mice and were unable to find any characteristic waveform related the activation of photoreceptors or second-order retinal neurons, suggesting ipRGCs as the origin of light responses. Our results show a profound ability for melanopsin phototransduction to directly contribute to the primary pattern-forming visual pathway.

A Role for Thalamic Projection GABAergic Neurons in Circadian Responses to Light

The thalamus is an important hub for sensory information and participates in sensory perception, regulation of attention, arousal and sleep. These functions are executed primarily by glutamatergic thalamocortical neurons that extend axons to the cortex and initiate cortico-thalamocortical connectional loops. However, the thalamus also contains projection GABAergic neurons that do not extend axons toward the cortex. Here, we have harnessed recent insight into the development of the intergeniculate leaflet (IGL) and the ventral lateral geniculate nucleus (LGv) to specifically target and manipulate thalamic projection GABAergic neurons in female and male mice. Our results show that thalamic GABAergic neurons of the IGL and LGv receive retinal input from diverse classes of retinal ganglion cells (RGCs) but not from the M1 intrinsically photosensitive retinal ganglion cell (ipRGC) type. We describe the synergistic role of the photoreceptor melanopsin and the thalamic neurons of the IGL/LGv in circadian entrainment to dim light. We identify a requirement for the thalamic IGL/LGv neurons in the rapid changes in vigilance states associated with circadian light transitions.SIGNIFICANCE STATEMENT The intergeniculate leaflet (IGL) and ventral lateral geniculate nucleus (LGv) are part of the extended circadian system and mediate some nonimage-forming visual functions. Here, we show that each of these structures has a thalamic (dorsal) as well as prethalamic (ventral) developmental origin. We map the retinal input to thalamus-derived cells in the IGL/LGv complex and discover that while RGC input is dominant, this is not likely to originate from M1ipRGCs. We implicate thalamic cells in the IGL/LGv in vigilance state transitions at circadian light changes and in overt behavioral entrainment to dim light, the latter exacerbated by concomitant loss of melanopsin expression.

Quantitative characterisation of ipRGCs in retinal degeneration using a computation platform for extracting and reconstructing single neurons in 3D from a multi-colour labeled population

Light has a profound impact on mammalian physiology and behavior. Intrinsically photosensitive retinal ganglion cells (ipRGCs) express the photopigment melanopsin, rendering them sensitive to light, and are involved in both image-forming vision and non-image forming responses to light such as circadian photo-entrainment and the pupillary light reflex. Following outer photoreceptor degeneration, the death of rod and cone photoreceptors results in global re-modeling of the remnant neural retina. Although ipRGCs can continue signaling light information to the brain even in advanced stages of degeneration, it is unknown if all six morphologically distinct subtypes survive, or how their dendritic architecture may be affected. To answer these questions, we generated a computational platform-BRIAN (Brainbow Analysis of individual Neurons) to analyze Brainbow labeled tissues by allowing objective identification of voxels clusters in Principal Component Space, and their subsequent extraction to produce 3D images of single neurons suitable for analysis with existing tracing technology. We show that BRIAN can efficiently recreate single neurons or individual axonal projections from densely labeled tissue with sufficient anatomical resolution for subtype quantitative classification. We apply this tool to generate quantitative morphological information about ipRGCs in the degenerate retina including soma size, dendritic field size, dendritic complexity, and stratification. Using this information, we were able to identify cells whose characteristics match those reported for all six defined subtypes of ipRGC in the wildtype mouse retina (M1-M6), including the rare and complex M3 and M6 subtypes. This indicates that ipRGCs survive outer retinal degeneration with broadly normal morphology. We additionally describe one cell in the degenerate retina which matches the description of the Gigantic M1 cell in Humans which has not been previously identified in rodent.

Intrinsically photosensitive retinal ganglion cells in glaucoma

Glaucoma is a group of eye diseases afflicting more than 70 million people worldwide. It is characterized by damage to retinal ganglion cells (RGCs) that ultimately leads to the death of the cells and vision loss. The diversity of RGC types has been appreciated for decades, and studies, including ours, have shown that RGCs degenerate and die in a type-specific manner in rodent models of glaucoma. The type-specific loss of RGCs results in differential damage to visual and non-visual functions. One type of RGC, the intrinsically photosensitive retinal ganglion cell (ipRGC), expressing the photopigment melanopsin, serves a broad array of non-visual responses to light. Since its discovery, six subtypes of ipRGC have been described, each contributing to various image-forming and non-image-forming functions such as circadian photoentrainment, the pupillary light reflex, the photic control of mood and sleep, and visual contrast sensitivity. We recently demonstrated a link between type-specific ipRGC survival and behavioral deficits in a mouse model of chronic ocular hypertension. This review focuses on the type-specific ipRGC degeneration and associated behavioral changes in animal models and glaucoma patients. A better understanding of how glaucomatous insult impacts the ipRGC-based circuits will have broad impacts on improving the treatment of glaucoma-associated non-visual disorders.

Melanopsin retinal ganglion cells mediate light-promoted brain development

During development, melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) become light sensitive much earlier than rods and cones. IpRGCs project to many subcortical areas, whereas physiological functions of these projections are yet to be fully elucidated. Here, we found that ipRGC-mediated light sensation promotes synaptogenesis of pyramidal neurons in various cortices and the hippocampus. This phenomenon depends on activation of ipRGCs and is mediated by the release of oxytocin from the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) into cerebral-spinal fluid. We further characterized a direct connection between ipRGCs and oxytocin neurons in the SON and mutual projections between oxytocin neurons in the SON and PVN. Moreover, we showed that the lack of ipRGC-mediated, light-promoted early cortical synaptogenesis compromised learning ability in adult mice. Our results highlight the importance of light sensation early in life on the development of learning ability and therefore call attention to suitable light environment for infant care.

Subtype-specific survival and regeneration of retinal ganglion cells in response to injury

Retinal ganglion cells (RGCs) are a heterogeneous population of neurons that function synchronously to convey visual information through the optic nerve to retinorecipient target areas in the brain. Injury or disease to the optic nerve results in RGC degeneration and loss of visual function, as few RGCs survive, and even fewer can be provoked to regenerate their axons. Despite causative insults being broadly shared, regeneration studies demonstrate that RGC types exhibit differential resilience to injury and undergo selective survival and regeneration of their axons. While most early studies have identified these RGC types based their morphological and physiological characteristics, recent advances in transgenic and gene sequencing technologies have further enabled type identification based on unique molecular features. In this review, we provide an overview of the well characterized RGC types and identify those shown to preferentially survive and regenerate in various regeneration models. Furthermore, we discuss cellular characteristics of both the resilient and susceptible RGC types including the combinatorial expression of different molecular markers that identify these specific populations. Lastly, we discuss potential molecular mechanisms and genes found to be selectively expressed by specific types that may contribute to their reparative capacity. Together, we describe the studies that lay the important groundwork for identifying factors that promote neural regeneration and help advance the development of targeted therapy for the treatment of RGC degeneration as well as neurodegenerative diseases in general.

Distribution of prototypical primary cilia markers in subtypes of retinal ganglion cells

Loss of retinal ganglion cells (RGCs) underlies several forms of retinal disease including glaucomatous optic neuropathy, a leading cause of irreversible blindness. Several rare genetic disorders associated with cilia dysfunction have retinal degeneration as a clinical hallmark. Much of the focus of ciliopathy associated blindness is on the connecting cilium of photoreceptors; however, RGCs also possess primary cilia. It is unclear what roles RGC cilia play, what proteins and signaling machinery localize to RGC cilia, or how RGC cilia are differentiated across the subtypes of RGCs. To better understand these questions, we assessed the presence or absence of a prototypical cilia marker Arl13b and a widely distributed neuronal cilia marker AC3 in different subtypes of mouse RGCs. Interestingly, not all RGC subtype cilia are the same and there are significant differences even among these standard cilia markers. Alpha-RGCs positive for osteopontin, calretinin, and SMI32 primarily possess AC3-positive cilia. Directionally selective RGCs that are CART positive or Trhr positive localize either Arl13b or AC3, respectively, in cilia. Intrinsically photosensitive RGCs differentially localize Arl13b and AC3 based on melanopsin expression. Taken together, we characterized the localization of gold standard cilia markers in different subtypes of RGCs and conclude that cilia within RGC subtypes may be differentially organized. Future studies aimed at understanding RGC cilia function will require a fundamental ability to observe the cilia across subtypes as their signaling protein composition is elucidated. A comprehensive understanding of RGC cilia may reveal opportunities to understanding how their dysfunction leads to retinal degeneration.

Divergent outer retinal circuits drive image and non-image visual behaviors

Image- and non-image-forming vision are essential for animal behavior. Here we use genetically modified mouse lines to examine retinal circuits driving image- and non-image-functions. We describe the outer retinal circuits underlying the pupillary light response (PLR) and circadian photoentrainment, two non-image-forming behaviors. Rods and cones signal light increments and decrements through the ON and OFF pathways, respectively. We find that the OFF pathway drives image-forming vision but cannot drive circadian photoentrainment or the PLR. Cone light responses drive image formation but fail to drive the PLR. At photopic levels, rods use the primary and secondary rod pathways to drive the PLR, whereas at the scotopic and mesopic levels, rods use the primary pathway to drive the PLR, and the secondary pathway is insufficient. Circuit dynamics allow rod ON pathways to drive two non-image-forming behaviors across a wide range of light intensities, whereas the OFF pathway is potentially restricted to image formation.

The role of ipRGCs in ocular growth and myopia development

The increasing global prevalence of myopia calls for elaboration of the pathogenesis of this disease. Here, we show that selective ablation and activation of intrinsically photosensitive retinal ganglion cells (ipRGCs) in developing mice induced myopic and hyperopic refractive shifts by modulating the corneal radius of curvature (CRC) and axial length (AL) in an opposite way. Melanopsin- and rod/cone-driven signals of ipRGCs were found to influence refractive development by affecting the AL and CRC, respectively. The role of ipRGCs in myopia progression is evidenced by attenuated form-deprivation myopia magnitudes in ipRGC-ablated and melanopsin-deficient animals and by enhanced melanopsin expression/photoresponses in form-deprived eyes. Cell subtype-specific ablation showed that M1 subtype cells, and probably M2/M3 subtype cells, are involved in ocular development. Thus, ipRGCs contribute substantially to mouse eye growth and myopia development, which may inspire novel strategies for myopia intervention.

Differential effects of experimental glaucoma on intrinsically photosensitive retinal ganglion cells in mice

Glaucoma is a group of eye diseases characterized by retinal ganglion cell (RGC) loss and optic nerve damage. Studies, including this study, support that RGCs degenerate and die in a type-specific manner following the disease insult. Here we specifically examined one RGC type, the intrinsically photosensitive retinal ganglion cell (ipRGC), and its associated functional deficits in a mouse model of experimental glaucoma. We induced chronic ocular hypertension (OHT) by laser photocoagulation and then characterized the survival of ipRGC subtypes. We found that ipRGCs suffer significant loss, similar to the general RGC population, but ipRGC subtypes are differentially affected following chronic OHT. M4 ipRGCs, which are involved in pattern vision, are susceptible to chronic OHT. Correspondingly, mice with chronic OHT experience reduced contrast sensitivity and visual acuity. By comparison, M1 ipRGCs, which project to the suprachiasmatic nuclei to regulate circadian rhythmicity, exhibit almost no cell loss following chronic OHT. Accordingly, we observed that circadian re-entrainment and circadian rhythmicity are largely not disrupted in OHT mice. Our study demonstrates the link between subtype-specific ipRGC survival and behavioral deficits in glaucomatous mice. These findings provide insight into glaucoma-induced visual behavioral deficits and their underlying mechanisms.

Tools and Biomarkers for the Study of Retinal Ganglion Cell Degeneration

The retina is part of the central nervous system, its analysis may provide an idea of the health and functionality, not only of the retina, but also of the entire central nervous system, as has been shown in Alzheimer’s or Parkinson’s diseases. Within the retina, the ganglion cells (RGC) are the neurons in charge of processing and sending light information to higher brain centers. Diverse insults and pathological states cause degeneration of RGC, leading to irreversible blindness or impaired vision. RGCs are the measurable endpoints in current research into experimental therapies and diagnosis in multiple ocular pathologies, like glaucoma. RGC subtype classifications are based on morphological, functional, genetical, and immunohistochemical aspects. Although great efforts are being made, there is still no classification accepted by consensus. Moreover, it has been observed that each RGC subtype has a different susceptibility to injury. Characterizing these subtypes together with cell death pathway identification will help to understand the degenerative process in the different injury and pathological models, and therefore prevent it. Here we review the known RGC subtypes, as well as the diagnostic techniques, probes, and biomarkers for programmed and unprogrammed cell death in RGC.

Feature Detection by Retinal Ganglion Cells

Retinal circuits transform the pixel representation of photoreceptors into the feature representations of ganglion cells, whose axons transmit these representations to the brain. Functional, morphological, and transcriptomic surveys have identified more than 40 retinal ganglion cell (RGC) types in mice. RGCs extract features of varying complexity; some simply signal local differences in brightness (i.e., luminance contrast), whereas others detect specific motion trajectories. To understand the retina, we need to know how retinal circuits give rise to the diverse RGC feature representations. A catalog of the RGC feature set, in turn, is fundamental to understanding visual processing in the brain. Anterograde tracing indicates that RGCs innervate more than 50 areas in the mouse brain. Current maps connecting RGC types to brain areas are rudimentary, as is our understanding of how retinal signals are transformed downstream to guide behavior. In this article, I review the feature selectivities of mouse RGCs, how they arise, and how they are utilized downstream. Not only is knowledge of the behavioral purpose of RGC signals critical for understanding the retinal contributions to vision; it can also guide us to the most relevant areas of visual feature space. Expected final online publication date for the Annual Review of Vision Science, Volume 8 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Adult Expression of Tbr2 Is Required for the Maintenance but Not Survival of Intrinsically Photosensitive Retinal Ganglion Cells

Retinal ganglion cells expressing the photopigment melanopsin are intrinsically photosensitive (ipRGCs). ipRGCs regulate subconscious non-image-forming behaviors such as circadian rhythms, pupil dilation, and light-mediated mood. Previously, we and others showed that the transcription factor Tbr2 (EOMES) is required during retinal development for the formation of ipRGCs. Tbr2 is also expressed in the adult retina leading to the hypothesis that it plays a role in adult ipRGC function. To test this, we removed Tbr2 in adult mice. We found that this results in the loss of melanopsin expression in ipRGCs but does not lead to cell death or morphological changes to their dendritic or axonal termination patterns. Additionally, we found ectopic expression of Tbr2 in conventional RGCs does not induce melanopsin expression but can increase melanopsin expression in existing ipRGCs. An interesting feature of ipRGCs is their superior survival relative to conventional RGCs after an optic nerve injury. We find that loss of Tbr2 decreases the survival rate of ipRGCs after optic nerve damage suggesting that Tbr2 plays a role in ipRGC survival after injury. Lastly, we show that the GABAergic amacrine cell marker Meis2, is expressed in the majority of Tbr2-expressing displaced amacrine cells as well as in a subset of Tbr2-expressing RGCs. These findings demonstrate that Tbr2 is necessary but not sufficient for melanopsin expression, that Tbr2 is involved in ipRGC survival after optic nerve injury, and identify a marker for Tbr2-expressing displaced amacrine cells.

Differential susceptibility of retinal ganglion cell subtypes against neurodegenerative diseases

Retinal ganglion cells (RGCs) are essential to propagate external visual information from the retina to the brain. Death of RGCs is speculated to be closely correlated with blinding retinal diseases, such as glaucoma and traumatic optic neuropathy (TON). Emerging innovative technologies have helped refine and standardize the classification of RGCs; at present, they are classified into more than 40 subpopulations in mammals. These RGC subtypes are identified by a combination of anatomical morphologies, electrophysiological functions, and genetic profiles. Increasing evidence suggests that neurodegenerative diseases do not collectively affect the RGCs. In fact, which RGC subtype exhibits the strongest or weakest susceptibility is hotly debated. Although a consensus has not yet been reached, it is certain that assorted RGCs display differential susceptibility against irreversible degeneration. Interestingly, a single RGC subtype can exhibit various vulnerabilities to optic nerve damage in diverse injury models. Thus, elucidating how susceptible RGC subtypes are to various injuries can protect vulnerable RGCs from damage and improve the possibility of preventing and treating visual impairment caused by neurodegenerative diseases. In this review, we summarize in detail the progress and status quo of research on the type-specific susceptibility of RGCs and point out current limitations and the possible directions for future research in this field.

Photic Entrainment of the Circadian System

Circadian rhythms are essential for the survival of all organisms, enabling them to predict daily changes in the environment and time their behaviour appropriately. The molecular basis of such rhythms is the circadian clock, a self-sustaining molecular oscillator comprising a transcriptional–translational feedback loop. This must be continually readjusted to remain in alignment with the external world through a process termed entrainment, in which the phase of the master circadian clock in the suprachiasmatic nuclei (SCN) is adjusted in response to external time cues. In mammals, the primary time cue, or “zeitgeber”, is light, which inputs directly to the SCN where it is integrated with additional non-photic zeitgebers. The molecular mechanisms underlying photic entrainment are complex, comprising a number of regulatory factors. This review will outline the photoreception pathways mediating photic entrainment, and our current understanding of the molecular pathways that drive it in the SCN.

Molecular determinants of response kinetics of mouse M1 intrinsically-photosensitive retinal ganglion cells

Intrinsically-photosensitive retinal ganglion cells (ipRGCs) are non-rod/non-cone retinal photoreceptors expressing the visual pigment, melanopsin, to detect ambient irradiance for various non-image-forming visual functions. The M1-subtype, amongst the best studied, mediates primarily circadian photoentrainment and pupillary light reflex. Their intrinsic light responses are more prolonged than those of rods and cones even at the single-photon level, in accordance with the typically slower time course of non-image-forming vision. The short (OPN4S) and long (OPN4L) alternatively-spliced forms of melanopsin proteins are both present in M1-ipRGCs, but their functional difference is unclear. We have examined this point by genetically removing the Opn4 gene (Opn4-/-) in mouse and re-expressing either OPN4S or OPN4L singly in Opn4-/- mice by using adeno-associated virus, but found no obvious difference in their intrinsic dim-flash responses. Previous studies have indicated that two dominant slow steps in M1-ipRGC phototransduction dictate these cells' intrinsic dim-flash-response kinetics, with time constants (τ1 and τ2) at room temperature of ~ 2 s and ~ 20 s, respectively. Here we found that melanopsin inactivation by phosphorylation or by β-arrestins may not be one of these two steps, because their genetic disruptions did not prolong the two time constants or affect the response waveform. Disruption of GAP (GTPase-Activating-Protein) activity on the effector enzyme, PLCβ4, in M1-ipRGC phototransduction to slow down G-protein deactivation also did not prolong the response decay, but caused its rising phase to become slightly sigmoidal by giving rise to a third time constant, τ3, of ~ 2 s (room temperature). This last observation suggests that GAP-mediated G-protein deactivation does partake in the flash-response termination, although normally with a time constant too short to be visible in the response waveform.

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.

Retinal patterns and the cellular repertoire of neuropsin (Opn5) retinal ganglion cells

Obtaining a parts list of the sensory components of the retina is vital to understanding the effects of light in behavior, health, and disease. Rods, cones, and intrinsically photosensitive retinal ganglion cells (ipRGCs) are the best described photoreceptors in the mammalian retina, but recent functional roles have been proposed for retinal neuropsin (Opn5) - an atypical opsin. However, little is known about the pattern of Opn5 expression in the retina. Using cre (Opn5^cre ) and cre-dependent reporters, we uncover patterns of Opn5 expression and find that Opn5 is restricted to retinal ganglion cells (RGCs). Opn5-RGCs are non-homogenously distributed through the retina, with greater densities of cells located in the dorsotemporal quadrant. In addition to local topology of these cells, using cre-dependent AAV viral tracing, we surveyed their central targets and found that they are biased towards image-forming and image-stabilizing regions. Finally, molecular and electrophysiological profiling reveal that Opn5-RGCs comprise previously defined RGC types which respond optimally to edges and object-motion (F-mini-ONs, HD2, HD1, LEDs, ooDSRGCs, etc.). Together, these data describe the second collection of RGCs that express atypical opsins in the mouse, and expand the roles of image-forming cells in retinal physiology and function. This article is protected by copyright. All rights reserved.

Spectral tuning and deactivation kinetics of marine mammal melanopsins

In mammals, the photopigment melanopsin (Opn4) is found in a subset of retinal ganglion cells that serve light detection for circadian photoentrainment and pupil constriction (i.e., mydriasis). For a given species, the efficiency of photoentrainment and length of time that mydriasis occurs is determined by the spectral sensitivity and deactivation kinetics of melanopsin, respectively, and to date, neither of these properties have been described in marine mammals. Previous work has indicated that the absorbance maxima (λmax) of marine mammal rhodopsins (Rh1) have diversified to match the available light spectra at foraging depths. However, similar to the melanopsin λmax of terrestrial mammals (~480 nm), the melanopsins of marine mammals may be conserved, with λmax values tuned to the spectrum of solar irradiance at the water's surface. Here, we investigated the Opn4 pigments of 17 marine mammal species inhabiting diverse photic environments including the Infraorder Cetacea, as well as the Orders Sirenia and Carnivora. Both genomic and cDNA sequences were used to deduce amino acid sequences to identify substitutions most likely involved in spectral tuning and deactivation kinetics of the Opn4 pigments. Our results show that there appears to be no amino acid substitutions in marine mammal Opn4 opsins that would result in any significant change in λmax values relative to their terrestrial counterparts. We also found some marine mammal species to lack several phosphorylation sites in the carboxyl terminal domain of their Opn4 pigments that result in significantly slower deactivation kinetics, and thus longer mydriasis, compared to terrestrial controls. This finding was restricted to cetacean species previously found to lack cone photoreceptor opsins, a condition known as rod monochromacy. These results suggest that the rod monochromat whales rely on extended pupillary constriction to prevent photobleaching of the highly photosensitive all-rod retina when moving between photopic and scotopic conditions.

Structure and function of the gap junctional network of photoreceptive ganglion cells

Intrinsically photosensitive retinal ganglion cells (ipRGCs) signal not only anterogradely to drive behavioral responses, but also retrogradely to some amacrine interneurons to modulate retinal physiology. We previously found that all displaced amacrine cells with spiking, tonic excitatory photoresponses receive gap-junction input from ipRGCs, but the connectivity patterns and functional roles of ipRGC-amacrine coupling remained largely unknown. Here, we injected PoPro1 fluorescent tracer into all six types of mouse ipRGCs to identify coupled amacrine cells, and analyzed the latter's morphological and electrophysiological properties. We also examined how genetically disrupting ipRGC-amacrine coupling affected ipRGC photoresponses. Results showed that ipRGCs couple with not just ON- and ON/OFF-stratified amacrine cells in the ganglion-cell layer as previously reported, but also OFF-stratified amacrine cells in both ganglion-cell and inner nuclear layers. M1- and M3-type ipRGCs couple mainly with ON/OFF-stratified amacrine cells, whereas the other ipRGC types couple almost exclusively with ON-stratified ones. ipRGCs transmit melanopsin-based light responses to at least 93% of the coupled amacrine cells. Some of the ON-stratifying ipRGC-coupled amacrine cells exhibit transient hyperpolarizing light responses. We detected bidirectional electrical transmission between an ipRGC and a coupled amacrine cell, although transmission was asymmetric for this particular cell pair, favoring the ipRGC-to-amacrine direction. We also observed electrical transmission between two amacrine cells coupled to the same ipRGC. In both scenarios of coupling, the coupled cells often spiked synchronously. While ipRGC-amacrine coupling somewhat reduces the peak firing rates of ipRGCs' intrinsic melanopsin-based photoresponses, it renders these responses more sustained and longer-lasting. In summary, ipRGCs' gap junctional network involves more amacrine cell types and plays more roles than previously appreciated.

Regulatory Mechanisms of Retinal Photoreceptors Development at Single Cell Resolution

Photoreceptors are critical components of the retina and play a role in the first step of the conversion of light to electric signals. With the discovery of the intrinsically photosensitive retinal ganglion cells, which regulate non-image-forming visual processes, our knowledge of the photosensitive cell family in the retina has deepened. Photoreceptor development is regulated by specific genes and proteins and involves a series of molecular processes including DNA transcription, post-transcriptional modification, protein translation, and post-translational modification. Single-cell sequencing is a promising technology for the study of photoreceptor development. This review presents an overview of the types of human photoreceptors, summarizes recent discoveries in the regulatory mechanisms underlying their development at single-cell resolution, and outlines the prospects in this field.