Transcriptomic Signatures of Postnatal and Adult Intrinsically Photosensitive Ganglion Cells

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.

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.

Pou4f1-Tbr1 transcriptional cascade controls the formation of Jam2-expressing retinal ganglion cells

More than 40 retinal ganglion cell (RGC) subtypes have been categorized in mouse based on their morphologies, functions, and molecular features. Among these diverse subtypes, orientation-selective Jam2-expressing RGCs (J-RGCs) has two unique morphologic characteristics: the ventral-facing dendritic arbor and the OFF-sublaminae stratified terminal dendrites in the inner plexiform layer. Previously, we have discovered that T-box transcription factor T-brain 1 (Tbr1) is expressed in J-RGCs. We further found that Tbr1 is essential for the expression of Jam2, and Tbr1 regulates the formation and the dendritic morphogenesis of J-RGCs. However, Tbr1 begins to express in terminally differentiated RGCs around perinatal stage, suggesting that it is unlikely involved in the initial fate determination for J-RGC and other upstream transcription factors must control Tbr1 expression and J-RGC formation. Using the Cleavage Under Targets and Tagmentation technique, we discovered that Pou4f1 binds to Tbr1 on the evolutionary conserved exon 6 and an intergenic region downstream of the 3'UTR, and on a region flanking the promoter and the first exon of Jam2. We showed that Pou4f1 is required for the expression of Tbr1 and Jam2, indicating Pou4f1 as a direct upstream regulator of Tbr1 and Jam2. Most interestingly, the Pou4f1-bound element in exon 6 of Tbr1 possesses high-level enhancer activity, capable of directing reporter gene expression in J-RGCs. Together, these data revealed a Pou4f1-Tbr1-Jam2 genetic hierarchy as a critical pathway in the formation of J-RGC subtype.

Pan-retinal ganglion cell markers in mice, rats, and rhesus macaques

Univocal identification of retinal ganglion cells (RGCs) is an essential prerequisite for studying their degeneration and neuroprotection. Before the advent of phenotypic markers, RGCs were normally identified using retrograde tracing of retinorecipient areas. This is an invasive technique, and its use is precluded in higher mammals such as monkeys. In the past decade, several RGC markers have been described. Here, we reviewed and analyzed the specificity of nine markers used to identify all or most RGCs, i.e., pan-RGC markers, in rats, mice, and macaques. The best markers in the three species in terms of specificity, proportion of RGCs labeled, and indicators of viability were BRN3A, expressed by vision-forming RGCs, and RBPMS, expressed by vision- and non-vision-forming RGCs. NEUN, often used to identify RGCs, was expressed by non-RGCs in the ganglion cell layer, and therefore was not RGC-specific. γ-SYN, TUJ1, and NF-L labeled the RGC axons, which impaired the detection of their somas in the central retina but would be good for studying RGC morphology. In rats, TUJ1 and NF-L were also expressed by non-RGCs. BM88, ERRβ, and PGP9.5 are rarely used as markers, but they identified most RGCs in the rats and macaques and ERRβ in mice. However, PGP9.5 was also expressed by non-RGCs in rats and macaques and BM88 and ERRβ were not suitable markers of viability.

Alpha retinal ganglion cells in pigmented mice retina: number and distribution

Purpose: To identify and characterize numerically and topographically the population of alpha retinal ganglion cells (αRGCs) and their subtypes, the sustained-response ON-center αRGCs (ONs-αRGCs), which correspond to the type 4 intrinsically photosensitive RGCs (M4-ipRGCs), the transient-response ON-center αRGCs (ONt-αRGCs), the sustained-response OFF-center αRGCs (OFFs-αRGCs), and the transient-response OFF-center αRGCs (OFFt-αRGCs) in the adult pigmented mouse retina.

Methods: The αRGC population and its subtypes were studied in flat-mounted retinas and radial sections immunodetected against non-phosphorylated high molecular weight neurofilament subunit (SMI-32) or osteopontin (OPN), two αRGCs pan-markers; Calbindin, expressed in ONs-αRGCs, and amacrines; T-box transcription factor T-brain 2 (Tbr2), a key transcriptional regulator for ipRGC development and maintenance, expressed in ipRGCs and GABA-displaced amacrine cells; OPN4, an anti-melanopsin antibody; or Brn3a and Brn3c, markers of RGCs. The total population of RGCs was counted automatically and αRGCs and its subtypes were counted manually, and color-coded neighborhood maps were used for their topographical representation.

Results: The total mean number of αRGCs per retina is 2,252 ± 306 SMI32+αRGCs and 2,315 ± 175 OPN+αRGCs (n = 10), representing 5.08% and 5.22% of the total number of RGCs traced from the optic nerve, respectively. αRGCs are distributed throughout the retina, showing a higher density in the temporal hemiretina. ONs-αRGCs represent ≈36% [841 ± 110 cells (n = 10)] of all αRGCs and are located throughout the retina, with the highest density in the temporal region. ONt-αRGCs represent ≈34% [797 ± 146 cells (n = 10)] of all αRGCs and are mainly located in the central retinal region. OFF-αRGCs represent the remaining 32% of total αRGCs and are divided equally between OFFs-αRGCs and OFFt-αRGCs [363 ± 50 cells (n = 10) and 376 ± 36 cells (n = 10), respectively]. OFFs-αRGCs are mainly located in the supero-temporal peripheral region of the retina and OFFt-αRGCs in the mid-peripheral region of the retina, especially in the infero-temporal region.

Conclusions: The combination of specific antibodies is a useful tool to identify and study αRGCs and their subtypes. αRGCs are distributed throughout the retina presenting higher density in the temporal area. The sustained ON and OFF response subtypes are mainly located in the periphery while the transient ON and OFF response subtypes are found in the central regions of the retina.

The retina of the lab rat: focus on retinal ganglion cells and photoreceptors

Albino and pigmented rat strains are widely used in models to study retinal degeneration and to test new therapies. Here, we have summarized the main topographical and functional characteristics of the rat retina focussing on photoreceptors and retinal ganglion cells (RGCs), the beginning and end of the retinal circuitry, respectively. These neurons are very sensitive to injury and disease, and thus knowing their normal number, topography, and function is essential to accurately investigate on neuronal survival and protection.

Linking transcriptomes with morphological and functional phenotypes in mammalian retinal ganglion cells

Retinal ganglion cells (RGCs) are the brain's gateway to the visual world. They can be classified into different types on the basis of their electrophysiological, transcriptomic, or morphological characteristics. Here, we characterize the transcriptomic, morphological, and functional features of 472 high-quality RGCs using Patch sequencing (Patch-seq), providing functional and morphological annotation of many transcriptomic-defined cell types of a previously established RGC atlas. We show a convergence of different modalities in defining the RGC identity and reveal the degree of correspondence for well-characterized cell types across multimodal data. Moreover, we complement some RGC types with detailed morphological and functional properties. We also identify differentially expressed genes among ON, OFF, and ON-OFF RGCs such as Vat1l, Slitrk6, and Lmo7, providing candidate marker genes for functional studies. Our research suggests that the molecularly distinct clusters may also differ in their roles of encoding visual information.

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.

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.

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.

Selective glycinergic input from vGluT3 amacrine cells confers a suppressed-by-contrast trigger feature in a subtype of M1 ipRGCs in the mouse retina

KEY POINTS

M1 intrinsically photosensitive retinal ganglion cells (ipRGCs) are known to encode absolute light intensity (irradiance) for non-image-forming visual functions (subconscious vision), such as circadian photoentrainment and the pupillary light reflex. It remains unclear how M1 cells respond to relative light intensity (contrast) and patterned visual signals. The present study identified a special form of contrast sensitivity (suppressed-by-contrast) in M1 cells, suggesting a role of patterned visual signals in regulating non-image-forming vision and a potential role of M1 ipRGCs in encoding image-forming visual cues. The study also uncovered a synaptic mechanism and a retinal circuit mediated by vesicular glutamate transporter 3 (vGluT3) amacrine cells that underlie the suppressed-by-contrast response of M1 cells. M1 ipRGC subtypes (M1a and M1b) were revealed that are distinguishable based on synaptic connectivity with vGluT3 amacrine cells, receptive field properties, intrinsic photo sensitivity and membrane excitability, and morphological features, suggesting a division of visual tasks among discrete M1 subpopulations.

ABSTRACT

The M1 type ipRGC (intrinsically photosensitive retinal ganglion cell) is known to encode ambient light signals for non-image-forming visual functions such as circadian photo-entrainment and the pupillary light reflex. Here, we report that a subpopulation of M1 cells (M1a) in the mouse retina possess the suppressed-by-contrast (sbc) trigger feature that is a receptive field property previously found only in ganglion cells mediating image-forming vision. Using optogenetics and the dual patch clamp technique, we found that vesicular glutamate transporter 3 (vGluT3) (vGluT3) amacrine cells make glycinergic, but not glutamatergic, synapses specifically onto M1a cells. The spatiotemporal and pharmacological properties of visually evoked responses of M1a cells closely matched the receptive field characteristics of vGluT3 cells, suggesting a major role of the vGluT3 amacrine cell input in shaping the sbc trigger feature of M1a cells. We found that the other subpopulation of M1 cells (M1b), which did not receive a direct vGluT3 cell input, lacked the sbc trigger feature, being distinctively different from M1a cells in intrinsic photo responses, membrane excitability, receptive-field characteristics and morphological features. Together, the results reveal a retinal circuit that uses the sbc trigger feature to regulate irradiance coding and potentially send image-forming cues to non-image-forming visual centres in the brain.

ON-bipolar cell gene expression during retinal degeneration: Implications for optogenetic visual restoration

Purpose

Retinal bipolar cells survive even in the later stages of inherited retinal degenerations (IRDs) and so are attractive targets for optogenetic approaches to vision restoration. However, it is not known to what extent the remodelling that these cells undergo during degeneration affects their function. Specifically, it is unclear if they are free from metabolic stress, receptive to adeno-associated viral vectors, suitable for opsin-based optogenetic tools and able to propagate signals by releasing neurotransmitter.

Methods

Fluorescence activated cell sorting (FACS) was performed to isolate labelled bipolar cells from dissociated retinae of litter-mates with or without the IRD mutation Pde6b^rd1/rd1 selectively expressing an enhanced yellow fluorescent protein (EYFP) as a marker in ON-bipolar cells. Subsequent mRNA extraction allowed Illumina® microarray comparison of gene expression in bipolar cells from degenerate to those of wild type retinae. Changes in four candidate genes were further investigated at the protein level using retinal immunohistochemistry over the course of degeneration.

Results

A total of sixty differentially expressed transcripts reached statistical significance: these did not include any genes directly associated with native primary bipolar cell signalling, nor changes consistent with metabolic stress. Four significantly altered genes (Srm2, Slf2, Anxa7 & Cntn1), implicated in synaptic remodelling, neurotransmitter release and viral vector entry had immunohistochemical staining colocalising with ON-bipolar cell markers and varying over the course of degeneration.

Conclusion

Our findings suggest relatively few gene expression changes in the context of degeneration: that despite remodelling, bipolar cells are likely to remain viable targets for optogenetic vision restoration. In addition, several genes where changes were seen could provide a basis for investigations to enhance the efficacy of optogenetic therapies.

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Bipolar cells are attractive targets for therapeutic optogenetics in IRDs.

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This is the first cell specific transcriptomic analysis of bipolar cells in an IRD model.

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Bipolar cells maintain expression of genes essential to act as targets for optogenetics.

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Protein staining relating to four candidate genes (Anxa7, Cntn1, Srm2, Sulf2) is confirmed using immunohistochemistry.

Intrinsically Photosensitive Retinal Ganglion Cells of the Human Retina

Light profoundly affects our mental and physical health. In particular, light, when not delivered at the appropriate time, may have detrimental effects. In mammals, light is perceived not only by rods and cones but also by a subset of retinal ganglion cells that express the photopigment melanopsin that renders them intrinsically photosensitive (ipRGCs). ipRGCs participate in contrast detection and play critical roles in non-image-forming vision, a set of light responses that include circadian entrainment, pupillary light reflex (PLR), and the modulation of sleep/alertness, and mood. ipRGCs are also found in the human retina, and their response to light has been characterized indirectly through the suppression of nocturnal melatonin and PLR. However, until recently, human ipRGCs had rarely been investigated directly. This gap is progressively being filled as, over the last years, an increasing number of studies provided descriptions of their morphology, responses to light, and gene expression. Here, I review the progress in our knowledge of human ipRGCs, in particular, the different morphological and functional subtypes described so far and how they match the murine subtypes. I also highlight questions that remain to be addressed. Investigating ipRGCs is critical as these few cells play a major role in our well-being. Additionally, as ipRGCs display increased vulnerability or resilience to certain disorders compared to conventional RGCs, a deeper knowledge of their function could help identify therapeutic approaches or develop diagnostic tools. Overall, a better understanding of how light is perceived by the human eye will help deliver precise light usage recommendations and implement light-based therapeutic interventions to improve cognitive performance, mood, and life quality.

Photosensitive ganglion cells: A diminutive, yet essential population

Our visual system has evolved to provide us with an image of the scene that surrounds us, informing us of its texture, colour, movement, and depth with an enormous spatial and temporal resolution, and for this purpose, the image formation (IF) dedicates the vast majority of our retinal ganglion cell (RGC) population and much of our cerebral cortex. On the other hand, a minuscule proportion of RGCs, in addition to receiving information from classic cone and rod photoreceptors, express melanopsin and are intrinsically photosensitive (ipRGC). These ipRGC are dedicated to non-image-forming (NIF) visual functions, of which we are unaware, but which are essential for aspects related to our daily physiology, such as the timing of our circadian rhythms and our pupillary light reflex, among many others. Before the discovery of ipRGCs, it was thought that the IF and NIF functions were distinct compartments regulated by different RGCs, but this concept has evolved in recent years with the discovery of new types of ipRGCs that innervate subcortical IF regions, and therefore have IF visual functions. Six different types of ipRGCs are currently known. These are termed M1-M6, and differ in their morphological, functional, molecular properties, central projections, and visual behaviour responsibilities. A review is presented on the melanopsin visual system, the most active field of research in vision, for which knowledge has grown exponentially during the last two decades, when RGCs giving rise to this pathway were first discovered.

Adult Mouse Retina Explants: From ex vivo to in vivo Model of Central Nervous System Injuries

In mammals, adult neurons fail to regenerate following any insult to adult central nervous system (CNS), which leads to a permanent and irreversible loss of motor and cognitive functions. For a long time, much effort has been deployed to uncover mechanisms of axon regeneration in the CNS. Even if some cases of functional recovery have been reported, there is still a discrepancy regarding the functionality of a neuronal circuit upon lesion. Today, there is a need not only to identify new molecules implicated in adult CNS axon regeneration, but also to decipher the fine molecular mechanisms associated with regeneration failure. Here, we propose to use cultures of adult retina explants to study all molecular and cellular mechanisms that occur during CNS regeneration. We show that adult retinal explant cultures have the advantages to (i) recapitulate all the features observed in vivo, including axon regeneration induced by intrinsic factors, and (ii) be an ex vivo set-up with high accessibility and many downstream applications. Thanks to several examples, we demonstrate that adult explants can be used to address many questions, such as axon guidance, growth cone formation and cytoskeleton dynamics. Using laser guided ablation of a single axon, axonal injury can be performed at a single axon level, which allows to record early and late molecular events that occur after the lesion. Our model is the ideal tool to study all molecular and cellular events that occur during CNS regeneration at a single-axon level, which is currently not doable in vivo. It is extremely valuable to address unanswered questions of neuroprotection and neuroregeneration in the context of CNS lesion and neurodegenerative diseases.

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.

Melanopsin and the Intrinsically Photosensitive Retinal Ganglion Cells: Biophysics to Behavior

The mammalian visual system encodes information over a remarkable breadth of spatiotemporal scales and light intensities. This performance originates with its complement of photoreceptors: the classic rods and cones, as well as the intrinsically photosensitive retinal ganglion cells (ipRGCs). IpRGCs capture light with a G-protein-coupled receptor called melanopsin, depolarize like photoreceptors of invertebrates such as Drosophila, discharge electrical spikes, and innervate dozens of brain areas to influence physiology, behavior, perception, and mood. Several visual responses rely on melanopsin to be sustained and maximal. Some require ipRGCs to occur at all. IpRGCs fulfill their roles using mechanisms that include an unusual conformation of the melanopsin protein, an extraordinarily slow phototransduction cascade, divisions of labor even among cells of a morphological type, and unorthodox configurations of circuitry. The study of ipRGCs has yielded insight into general topics that include photoreceptor evolution, cellular diversity, and the steps from biophysical mechanisms to behavior.

Melanopsin Carboxy-terminus phosphorylation plasticity and bulk negative charge, not strict site specificity, achieves phototransduction deactivation

Melanopsin is a visual pigment expressed in a small subset of ganglion cells in the mammalian retina known as intrinsically photosensitive retinal ganglion cells (ipRGCs) and is implicated in regulating non-image forming functions such as circadian photoentrainment and pupil constriction and contrast sensitivity in image formation. Mouse melanopsin's Carboxy-terminus (C-terminus) possesses 38 serine and threonine residues, which can potentially serve as phosphorylation sites for a G-protein Receptor Kinase (GRK) and be involved in the deactivation of signal transduction. Previous studies suggest that S388, T389, S391, S392, S394, S395 on the proximal region of the C-terminus of mouse melanopsin are necessary for melanopsin deactivation. We expressed a series of mouse melanopsin C-terminal mutants in HEK293 cells and using calcium imaging, and we found that the necessary cluster of six serine and threonine residues, while being critical, are insufficient for proper melanopsin deactivation. Interestingly, the additional six serine and threonine residues adjacent to the required six sites, in either proximal or distal direction, are capable of restoring wild-type deactivation of melanopsin. These findings suggest an element of plasticity in the molecular basis of melanopsin phosphorylation and deactivation. In addition, C-terminal chimeric mutants and molecular modeling studies support the idea that the initial steps of deactivation and β-arrestin binding are centered around these critical phosphorylation sites (S388-S395). The degree of functional versatility described in this study, along with ipRGC biophysical heterogeneity and the possible use of multiple signal transduction cascades, might contribute to the diverse ipRGC light responses for use in non-image and image forming behaviors, even though all six sub types of ipRGCs express the same melanopsin gene OPN4.

Single-Cell Profiles of Retinal Ganglion Cells Differing in Resilience to Injury Reveal Neuroprotective Genes

Neuronal types in the central nervous system differ dramatically in their resilience to injury or other insults. Here we studied the selective resilience of mouse retinal ganglion cells (RGCs) following optic nerve crush (ONC), which severs their axons and leads to death of ∼80% of RGCs within 2 weeks. To identify expression programs associated with differential resilience, we first used single-cell RNA-seq (scRNA-seq) to generate a comprehensive molecular atlas of 46 RGC types in adult retina. We then tracked their survival after ONC; characterized transcriptomic, physiological, and morphological changes that preceded degeneration; and identified genes selectively expressed by each type. Finally, using loss- and gain-of-function assays in vivo, we showed that manipulating some of these genes improved neuronal survival and axon regeneration following ONC. This study provides a systematic framework for parsing type-specific responses to injury and demonstrates that differential gene expression can be used to reveal molecular targets for intervention.