Expression of transcription factors divides retinal ganglion cells into distinct classes

Individual thalamic inhibitory interneurons are functionally specialized toward distinct visual features

Inhibitory interneurons in the dorsolateral geniculate nucleus (dLGN) are situated at the first central synapse of the image-forming visual pathway, but little is known about their function. Given their anatomy, they are expected to be multiplexors, integrating many different retinal channels along their dendrites. Here, using targeted single-cell-initiated rabies tracing, we found that mouse dLGN interneurons exhibit a degree of retinal input specialization similar to thalamocortical neurons. Some are anatomically highly specialized, for example, toward motion-selective information. Two-photon calcium imaging performed in vivo revealed that interneurons are also functionally specialized. In mice lacking retinal horizontal direction selectivity, horizontal direction selectivity is reduced in interneurons, suggesting a causal link between input and functional specialization. Functional specialization is not only present at interneuron somata but also extends into their dendrites. Altogether, inhibitory interneurons globally display distinct visual features which reflect their retinal input specialization and are ideally suited to perform feature-selective inhibition.

POU6F2, a risk factor for glaucoma, myopia and dyslexia, labels specific populations of retinal ganglion cells

Pou6f2 is a genetic connection between central corneal thickness (CCT) in the mouse and a risk factor for developing primary open-angle glaucoma. POU6F2 is also a risk factor for several conditions in humans, including glaucoma, myopia, and dyslexia. Recent findings demonstrate that POU6F2-positive retinal ganglion cells (RGCs) comprise a number of RGC subtypes in the mouse, some of which also co-stain for Cdh6 and Hoxd10. These POU6F2-positive RGCs appear to be novel of ON-OFF directionally selective ganglion cells (ooDSGCs) that do not co-stain with CART or SATB2 (typical ooDSGCs markers). These POU6F2-positive cells are sensitive to damage caused by elevated intraocular pressure. In the DBA/2J mouse glaucoma model, heavily-labeled POU6F2 RGCs decrease by 73% at 8 months of age compared to only 22% loss of total RGCs (labeled with RBPMS). Additionally, Pou6f2-/- mice suffer a significant loss of acuity and spatial contrast sensitivity along with an 11.4% loss of total RGCs. In the rhesus macaque retina, POU6F2 labels the large parasol ganglion cells that form the magnocellular (M) pathway. The association of POU6F2 with the M-pathway may reveal in part its role in human glaucoma, myopia, and dyslexia.

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.

The ISL LIM-homeobox 2 transcription factor is negatively regulated by circadian adrenergic signaling to repress the expression of Aanat in pinealocytes of the rat pineal gland

Melatonin is synthesized in the pineal gland during nighttime in response to nocturnal increase in the activity of the enzyme aralkylamine N-acetyltransferase (AANAT), the transcription of which is modulated by several homeodomain transcription factors. Recent work suggests that the homeodomain transcription factor ISL LIM homeobox 2 (ISL2) is expressed in the pineal gland, but its role is currently unknown. With the purpose of identifying the mechanisms that control pineal expression of Isl2 and the possible function of Isl2 in circadian pineal biology, we report that Isl2 is specifically expressed in the pinealocytes of the rat pineal gland. Its expression exhibits a 24 h rhythm with high transcript and protein levels during the day and a trough in the second half of the night. This rhythm persists in darkness, and lesion studies reveal that it requires intact function of the suprachiasmatic nuclei, suggesting intrinsic circadian regulation. In vivo and in vitro experiments show that pineal Isl2 expression is repressed by adrenergic signaling acting via cyclic AMP; further, Isl2 is negatively regulated by the nocturnal transcription factor cone-rod homeobox. During development, pineal Isl2 expression is detectable from embryonic day 19, preceding Aanat by several days. In vitro knockdown of Isl2 is accompanied by an increase in Aanat transcript levels suggesting that ISL2 represses its daytime expression. Thus, rhythmic expression of ISL2 in pinealocytes is under the control of the suprachiasmatic nucleus acting via adrenergic signaling in the gland to repress nocturnal expression, while ISL2 itself negatively regulates daytime pineal expression of Aanat and thereby suggestively enhances the circadian rhythm in melatonin synthesis.

Neuroprotective Effect of a Nutritional Supplement Containing Spearmint Extract, Forskolin, Homotaurine and Group B Vitamins in a Mouse Model of Transient Ocular Hypertension

Glaucoma is one of the most common sight-threatening eye disorders and one of the main causes of irreversible blindness worldwide. The current therapies focusing on reducing intraocular pressure (IOP) are often insufficient to prevent the progression of the disease, so the therapeutic management of glaucoma remains a challenge. The aim of this study was to evaluate the neuroprotective, IOP-lowering independent effects of a nutritional supplement containing forskolin, homotaurine, spearmint extract and vitamins of the B group in a model of acute glaucoma developed in mice. Glaucoma was induced in adult wild-type C57BL/6J mice by transient elevation of IOP. The dietary supplement, branded as Gangliomix^® (125 mg/kg/day), was administered by oral gavage for 17 days and ocular hypertension was induced on the 10th day of treatment. A histological analysis of the retinas was performed and RGC survival was evaluated with fluorogold labeling and Brn3a immunostaining on wholemount and retinal sections. Expression of alpha-spectrin, caspase-3, PARP-1 and GFAP was studied with western blotting or immunofluorescence. A significant increase in RGC survival was reported in the retina of mice treated with the dietary supplement as compared to vehicle-treated animals. The observed neuroprotection was associated with a calpain activity decrease, reduction in caspase-3 and PARP-1 activation, and prevention of GFAP upregulation. These effects were independent from the hypotensive effects of the supplement. Altogether, our data suggest that the dietary supplementation with forskolin, homotaurine, spearmint extract and vitamins of the B group supports RGC survival and may offer beneficial effects in glaucoma patients in combination with the currently used IOP-lowering therapy.

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.

Retinal self-organization: a model of retinal ganglion cells and starburst amacrine cells mosaic formation

Individual retinal cell types exhibit semi-regular spatial patterns called retinal mosaics. Retinal ganglion cells (RGCs) and starburst amacrine cells (SACs) are known to exhibit such layouts. Mechanisms responsible for the formation of mosaics are not well understood but follow three main principles: (i) homotypic cells prevent nearby cells from adopting the same type, (ii) cell tangential migration and (iii) cell death. Alongside experiments in mouse, we use BioDynaMo, an agent-based simulation framework, to build a detailed and mechanistic model of mosaic formation. We investigate the implications of the three theories for RGC's mosaic formation. We report that the cell migration mechanism yields the most regular mosaics. In addition, we propose that low-density RGC type mosaics exhibit on average low regularities, and thus we question the relevance of regular spacing as a criterion for a group of RGCs to form a RGC type. We investigate SAC mosaics formation and interactions between the ganglion cell layer (GCL) and inner nuclear layer (INL) populations. We propose that homotypic interactions between the GCL and INL populations during mosaics creation are required to reproduce the observed SAC mosaics' characteristics. This suggests that the GCL and INL populations of SACs might not be independent during retinal development.

Vision-Dependent and -Independent Molecular Maturation of Mouse Retinal Ganglion Cells

The development and connectivity of retinal ganglion cells (RGCs), the retina's sole output neurons, are patterned by activity-independent transcriptional programs and activity-dependent remodeling. To inventory the molecular correlates of these influences, we applied high-throughput single-cell RNA sequencing (scRNA-seq) to mouse RGCs at six embryonic and postnatal ages. We identified temporally regulated modules of genes that correlate with, and likely regulate, multiple phases of RGC development, ranging from differentiation and axon guidance to synaptic recognition and refinement. Some of these genes are expressed broadly while others, including key transcription factors and recognition molecules, are selectively expressed by one or a few of the 45 transcriptomically distinct types defined previously in adult mice. Next, we used these results as a foundation to analyze the transcriptomes of RGCs in mice lacking visual experience due to dark rearing from birth or to mutations that ablate either bipolar or photoreceptor cells. 98.5% of visually deprived (VD) RGCs could be unequivocally assigned to a single RGC type based on their transcriptional profiles, demonstrating that visual activity is dispensable for acquisition and maintenance of RGC type identity. However, visual deprivation significantly reduced the transcriptomic distinctions among RGC types, implying that activity is required for complete RGC maturation or maintenance. Consistent with this notion, transcriptomic alternations in VD RGCs significantly overlapped with gene modules found in developing RGCs. Our results provide a resource for mechanistic analyses of RGC differentiation and maturation, and for investigating the role of activity in these processes.

Case report: ISL2 is involved in malignant transformation in a patient with multiple relapsed oligodendroglioma

The majority of oligodendrogliomas exhibit an intrinsic tendency to develop into malignant high-grade tumors. Angiogenesis is a major factor contributing to the malignant transformation of oligodendroglioma, and its molecular regulatory mechanism needs further study. We provide a case report of an oligodendroglioma patient with two recurrences whose disease progressed from WHO grade II to grade III. We showed that the expression of insulin gene enhancer protein (ISL2) and its angiogenic ability were positively correlated with the progression of oligodendroglioma. In Low-grade glioma (LGG) patients, including oligodendroglioma patients, overexpression of ISL2 was correlated with poor prognosis, and this correlation was not affected by gender or isocitrate dehydrogenase 1(IDH1) mutation status. ISL2 expression and ISL2-mediated angiogenic pathway activity are ideal biomarkers for the malignant transformation of oligodendroglioma. Anti-ISL2 therapy is also a potential treatment option for malignantly transformed oligodendroglioma.

Satb1 expression in retinal ganglion cells of marmosets, macaques, and humans

Recent advances in single-cell RNA sequencing have enabled the molecular distinction of ganglion cell populations in mammalian retinas. Here we used antibodies against the transcription factor special AT-rich binding protein 1 (Satb1, a protein which is expressed by on-off direction-selective ganglion cells in mouse retina) to study Satb1 expression in the retina of marmosets (Callithrix jacchus), macaques (Macaca fascicularis), and humans. In all species, Satb1 was exclusively expressed in retinal ganglion cells. The Satb1 cells made up ∼2% of the ganglion cell population in the central retina of all species, rising to a maximum ∼7% in peripheral marmoset retina. Intracellular injections in marmoset and macaque retinas revealed that most Satb1 expressing ganglion cells are widefield ganglion cells. In marmoset, Satb1 cells have a densely branching dendritic tree and include broad and narrow thorny, recursive bistratified, and parasol cells, all of which show some costratification with the outer or inner cholinergic amacrine cells. The recursive bistratified cells showed the strongest costratification but did not show extensive cofasciculation as reported for on-off direction-selective ganglion cells in rabbit and rodent retinas. In macaque, Satb1 was not expressed in recursive bistratified cells, but in large sparsely branching cells. Our findings further support the idea that the expression of transcription factors in retinal ganglion cells is not conserved across Old World (human and macaque) and New World (marmoset) primates and provides a further step to link a molecular marker with specific cell types.

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.

Identification of long noncoding RNAs in injury-resilient and injury-susceptible mouse retinal ganglion cells

BACKGROUND

Emerging evidence indicates that long noncoding RNAs (lncRNAs) are important regulators of various biological processes, and their expression can be altered following certain pathological conditions, including central nervous system injury. Retinal ganglion cells (RGCs), whose axons form the optic nerve, are a heterogeneous population of neurons with more than 40 molecularly distinct subtypes in mouse. While most RGCs, including the ON-OFF direction-selective RGCs (ooDSGCs), are vulnerable to axonal injury, a small population of RGCs, including the intrinsically photosensitive RGCs (ipRGCs), are more resilient.

RESULTS

By performing systematic analyses on RNA-sequencing data, here we identify lncRNAs that are expressed in ooDSGCs and ipRGCs with and without axonal injury. Our results reveal a repertoire of different classes of lncRNAs, including long intergenic noncoding RNAs and antisense ncRNAs that are differentially expressed between these RGC types. Strikingly, we also found dozens of lncRNAs whose expressions are altered markedly in response to axonal injury, some of which are expressed exclusively in either one of the types. Moreover, analyses into these lncRNAs unraveled their neighboring coding genes, many of which encode transcription factors and signaling molecules, suggesting that these lncRNAs may act in cis to regulate important biological processes in these neurons. Lastly, guilt-by-association analysis showed that lncRNAs are correlated with apoptosis associated genes, suggesting potential roles for these lncRNAs in RGC survival.

CONCLUSIONS

Overall, the results of this study reveal RGC type-specific expression of lncRNAs and provide a foundation for future investigation of the function of lncRNAs in regulating neuronal type specification and survival.

Morphology, Molecular Characterization, and Connections of Ganglion Cells in Primate Retina

The eye sends information about the visual world to the brain on over 20 parallel signal pathways, each specialized to signal features such as spectral reflection (color), edges, and motion of objects in the environment. Each pathway is formed by the axons of a separate type of retinal output neuron (retinal ganglion cell). In this review, we summarize what is known about the excitatory retinal inputs, brain targets, and gene expression patterns of ganglion cells in humans and nonhuman primates. We describe how most ganglion cell types receive their input from only one or two of the 11 types of cone bipolar cell and project selectively to only one or two target regions in the brain. We also highlight how genetic methods are providing tools to characterize ganglion cells and establish cross-species homologies.

Glycogen Synthase Kinase 3 Regulates the Genesis of Displaced Retinal Ganglion Cells3

Glycogen synthase kinase 3 (GSK3) proteins (GSK3α and GSK3β) are key mediators of signaling pathways, with crucial roles in coordinating fundamental biological processes during neural development. Here we show that the complete loss of GSK3 signaling in mouse retinal progenitors leads to microphthalmia with broad morphologic defects. A single wild-type allele of either Gsk3α or Gsk3β is able to rescue this phenotype. In this genetic context, all cell types are present in a functional retina. However, we unexpectedly detected a large number of cells in the inner nuclear layer expressing retinal ganglion cell (RGC)-specific markers (called displaced RGCs, dRGCs) when at least one allele of Gsk3α is expressed. The excess of dRGCs leads to an increased number of axons projecting into the ipsilateral medial terminal nucleus, an area of the brain belonging to the non-image-forming visual circuit and poorly targeted by RGCs in wild-type retina. Transcriptome analysis and optomotor response assay suggest that at least a subset of dRGCs in Gsk3 mutant mice are direction-selective RGCs. Our study thus uncovers a unique role of GSK3 in controlling the production of ganglion cells in the inner nuclear layer, which correspond to dRGCs, a rare and poorly characterized retinal cell type.

NMDA Receptor Expression by Retinal Ganglion Cells Is Not Required for Retinofugal Map Formation nor Eye-Specific Segregation in the Mouse

Retinal ganglion cells (RGCs) project topographically to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Spontaneous activity plays a critical role in retinotopic mapping in both regions; however, the molecular mechanisms underlying activity-dependent refinement remain unclear. Previous pharmacologic studies implicate NMDA receptors (NMDARs) in the establishment of retinotopy. In other brain regions, NMDARs are expressed on both the presynaptic and postsynaptic side of the synapse, and recent work suggests that presynaptic and postsynaptic NMDARs play distinct roles in retinotectal developmental dynamics. To directly test the role of NMDARs expressed by RGCs in retinofugal map formation, we took a conditional genetic knock-out approach to delete the obligate GluN1 subunit of NMDARs in RGCs. Here, we demonstrate reduced GluN1 expression in the retina of Chrnb3-Cre;GluN1^flox/flox (pre-cKO) mice without altered expression in the SC. Anatomical tracing experiments revealed no significant changes in termination zone size in the SC and dLGN of pre-cKO mice, suggesting NMDAR function in RGCs is not an absolute requirement for topographic refinement. Further, we observed no change in the eye-specific organization of retinal inputs to the SC nor dLGN. To verify that NMDA induces activity in RGC terminals, we restricted GCaMP5 expression to RGCs and confirmed induction of calcium transients in RGC terminals. Together, these findings demonstrate that NMDARs expressed by RGCs are not required for retinofugal topographic map formation nor eye-specific segregation in the mouse.

Expression of cell markers and transcription factors in the avian retina compared with that in the marmoset retina

In the vertebrate retina, amacrine and ganglion cells represent the most diverse cell classes. They can be classified into different cell types by several features, such as morphology, light responses, and gene expression profile. Although birds possess high visual acuity (similar to primates that we used here for comparison) and tetrachromatic color vision, data on the expression of transcription factors in retinal ganglion cells of birds are largely missing. In this study, we tested various transcription factors, known to label subpopulations of cells in mammalian retinae, in two avian species: the common buzzard (Buteo buteo), a raptor with exceptional acuity, and the domestic pigeon (Columba livia domestica), a good navigator and widely used model for visual cognition. Staining for the transcription factors Foxp2, Satb1 and Satb2 labeled most ganglion cells in the avian ganglion cell layer. CtBP2 was established as marker for displaced amacrine cells, which allowed us to reliably distinguish ganglion cells from displaced amacrine cells and assess their densities in buzzard and pigeon. When we additionally compared the temporal and central fovea of the buzzard with the fovea of primates, we found that the cellular organization in the pits was different in primates and raptors. In summary, we demonstrate that the expression of transcription factors is a defining feature of cell types not only in the retina of mammals but also in the retina of birds. The markers, which we have established, may provide useful tools for more detailed studies on the retinal circuitry of these highly visual animals.

Identification of retinal ganglion cell types expressing the transcription factor Satb2 in three primate species

In primates, the retinal ganglion cells contributing to high acuity spatial vision (midget cells and parasol cells), and blue-yellow color vision (small bistratified cells) are well understood. Many other ganglion cell types with large dendritic fields (named wide-field ganglion cells) have been identified, but their spatial density and distribution are largely unknown. Here we took advantage of the recently established molecular diversity of ganglion cells to study wide-field ganglion cell populations in three primate species. We used antibodies against the transcription factor Special AT-rich binding protein 2 (Satb2) to explore its expression in macaque (Macaca fascicularis, M. nemestrina), human and marmoset (Callithrix jacchus) retinas. In all three species, Satb2 cells make up a low proportion (1.5-4%) of the ganglion cell population, with a slight increase from central to peripheral retina. Intracellular dye injections revealed that in macaque and human retinas, the large majority (over 80%) of Satb2 cells are inner and outer stratifying large sparse cells. By contrast, in marmoset retina the majority (over 60%) of Satb2 expressing cells were broad thorny cells, with smaller proportions of recursive bistratified (putative direction-selective), large bistratified, and outer stratifying narrow thorny cells. Our findings imply that Satb2 expression has undergone rapid species specific adaptations during primate evolution, because expression is not conserved across Old World (macaque, human) and New World (marmoset) suborders.

Cell types and cell circuits in human and non-human primate retina

This review summarizes our current knowledge of primate including human retina focusing on bipolar, amacrine and ganglion cells and their connectivity. We have two main motivations in writing. Firstly, recent progress in non-invasive imaging methods to study retinal diseases mean that better understanding of the primate retina is becoming an important goal both for basic and for clinical sciences. Secondly, genetically modified mice are increasingly used as animal models for human retinal diseases. Thus, it is important to understand to which extent the retinas of primates and rodents are comparable. We first compare cell populations in primate and rodent retinas, with emphasis on how the fovea (despite its small size) dominates the neural landscape of primate retina. We next summarise what is known, and what is not known, about the postreceptoral neurone populations in primate retina. The inventories of bipolar and ganglion cells in primates are now nearing completion, comprising ~12 types of bipolar cell and at least 17 types of ganglion cell. Primate ganglion cells show clear differences in dendritic field size across the retina, and their morphology differs clearly from that of mouse retinal ganglion cells. Compared to bipolar and ganglion cells, amacrine cells show even higher morphological diversity: they could comprise over 40 types. Many amacrine types appear conserved between primates and mice, but functions of only a few types are understood in any primate or non-primate retina. Amacrine cells appear as the final frontier for retinal research in monkeys and mice alike.

Neurogenesis and Specification of Retinal Ganglion Cells

Across all species, retinal ganglion cells (RGCs) are the first retinal neurons generated during development, followed by the other retinal cell types. How are retinal progenitor cells (RPCs) able to produce these cell types in a specific and timely order? Here, we will review the different models of retinal neurogenesis proposed over the last decades as well as the extrinsic and intrinsic factors controlling it. We will then focus on the molecular mechanisms, especially the cascade of transcription factors that regulate, more specifically, RGC fate. We will also comment on the recent discovery that the ciliary marginal zone is a new stem cell niche in mice contributing to retinal neurogenesis, especially to the generation of ipsilateral RGCs. Furthermore, RGCs are composed of many different subtypes that are anatomically, physiologically, functionally, and molecularly defined. We will summarize the different classifications of RGC subtypes and will recapitulate the specification of some of them and describe how a genetic disease such as albinism affects neurogenesis, resulting in profound visual deficits.

VEGFD Protects Retinal Ganglion Cells and, consequently, Capillaries against Excitotoxic Injury

In the central nervous system, neurons and the vasculature influence each other. While it is well described that a functional vascular system is trophic to neurons and that vascular damage contributes to neurodegeneration, the opposite scenario in which neural damage might impact the microvasculature is less defined. In this study, using an in vivo excitotoxic approach in adult mice as a tool to cause specific damage to retinal ganglion cells, we detected subsequent damage to endothelial cells in retinal capillaries. Furthermore, we detected decreased expression of vascular endothelial growth factor D (VEGFD) in retinal ganglion cells. In vivo VEGFD supplementation via neuronal-specific viral-mediated expression or acute intravitreal delivery of the mature protein preserved the structural and functional integrity of retinal ganglion cells against excitotoxicity and, additionally, spared endothelial cells from degeneration. Viral-mediated suppression of expression of the VEGFD-binding receptor VEGFR3 in retinal ganglion cells revealed that VEGFD exerts its protective capacity directly on retinal ganglion cells, while protection of endothelial cells is the result of upheld neuronal integrity. These findings suggest that VEGFD supplementation might be a novel, clinically applicable approach for neuronal and vascular protection.

Molecular Classification and Comparative Taxonomics of Foveal and Peripheral Cells in Primate Retina

High-acuity vision in primates, including humans, is mediated by a small central retinal region called the fovea. As more accessible organisms lack a fovea, its specialized function and its dysfunction in ocular diseases remain poorly understood. We used 165,000 single-cell RNA-seq profiles to generate comprehensive cellular taxonomies of macaque fovea and peripheral retina. More than 80% of >60 cell types match between the two regions but exhibit substantial differences in proportions and gene expression, some of which we relate to functional differences. Comparison of macaque retinal types with those of mice reveals that interneuron types are tightly conserved. In contrast, projection neuron types and programs diverge, despite exhibiting conserved transcription factor codes. Key macaque types are conserved in humans, allowing mapping of cell-type and region-specific expression of >190 genes associated with 7 human retinal diseases. Our work provides a framework for comparative single-cell analysis across tissue regions and species.

Molecular Fingerprinting of On-Off Direction-Selective Retinal Ganglion Cells Across Species and Relevance to Primate Visual Circuits

The ability to detect moving objects is an ethologically salient function. Direction-selective neurons have been identified in the retina, thalamus, and cortex of many species, but their homology has remained unclear. For instance, it is unknown whether direction-selective retinal ganglion cells (DSGCs) exist in primates and, if so, whether they are the equivalent to mouse and rabbit DSGCs. Here, we used a molecular/circuit approach in both sexes to address these issues. In mice, we identify the transcription factor Satb2 (special AT-rich sequence-binding protein 2) as a selective marker for three RGC types: On-Off DSGCs encoding motion in either the anterior or posterior direction, a newly identified type of Off-DSGC, and an Off-sustained RGC type. In rabbits, we find that expression of Satb2 is conserved in On-Off DSGCs; however, it has evolved to include On-Off DSGCs encoding upward and downward motion in addition to anterior and posterior motion. Next, we show that macaque RGCs express Satb2 most likely in a single type. We used rabies virus-based circuit-mapping tools to reveal the identity of macaque Satb2-RGCs and discovered that their dendritic arbors are relatively large and monostratified. Together, these data indicate Satb2-expressing On-Off DSGCs are likely not present in the primate retina. Moreover, if DSGCs are present in the primate retina, it is unlikely that they express Satb2.SIGNIFICANCE STATEMENT The ability to detect object motion is a fundamental feature of almost all visual systems. Here, we identify a novel marker for retinal ganglion cells encoding directional motion that is evolutionarily conserved in mice and rabbits, but not in primates. We show in macaque monkeys that retinal ganglion cells (RGCs) that express this marker comprise a single type and are morphologically distinct from mouse and rabbit direction-selective RGCs. Our findings indicate that On-Off direction-selective retinal neurons may have evolutionarily diverged in primates and more generally provide novel insight into the identity and organization of primate parallel visual pathways.

Tbr1 instructs laminar patterning of retinal ganglion cell dendrites

Visual information is delivered to the brain by >40 types of retinal ganglion cells (RGCs). Diversity in this representation arises within the inner plexiform layer (IPL), where dendrites of each RGC type are restricted to specific sublaminae, limiting the interneuronal types that can innervate them. How such dendritic restriction arises is unclear. We show that the transcription factor Tbr1 is expressed by four mouse RGC types with dendrites in the outer IPL and is required for their laminar specification. Loss of Tbr1 results in elaboration of dendrites within the inner IPL, while misexpression in other cells retargets their neurites to the outer IPL. Two transmembrane molecules, Sorcs3 and Cdh8, act as effectors of the Tbr1-controlled lamination program. However, they are expressed in just one Tbr1⁺ RGC type, supporting a model in which a single transcription factor implements similar laminar choices in distinct cell types by recruiting partially non-overlapping effectors.

Retinal Ganglion Cell Diversity and Subtype Specification from Human Pluripotent Stem Cells

Retinal ganglion cells (RGCs) are the projection neurons of the retina and transmit visual information to postsynaptic targets in the brain. While this function is shared among nearly all RGCs, this class of cell is remarkably diverse, comprised of multiple subtypes. Previous efforts have identified numerous RGC subtypes in animal models, but less attention has been paid to human RGCs. Thus, efforts of this study examined the diversity of RGCs differentiated from human pluripotent stem cells (hPSCs) and characterized defined subtypes through the expression of subtype-specific markers. Further investigation of these subtypes was achieved using single-cell transcriptomics, confirming the combinatorial expression of molecular markers associated with these subtypes, and also provided insight into more subtype-specific markers. Thus, the results of this study describe the derivation of RGC subtypes from hPSCs and will support the future exploration of phenotypic and functional diversity within human RGCs.

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Unique transcriptional profiles demonstrate diversity among hPSC-derived RGCs

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Numerous RGC subtypes characterized from hPSC-derived RGCs

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Molecular markers identified for RGC subtypes through single-cell RNA-seq analysis

Large scale matching of function to the genetic identity of retinal ganglion cells

Understanding the role of neurons in encoding and transmitting information is a major goal in neuroscience. This requires insight on the data-rich neuronal spiking patterns combined, ideally, with morphology and genetic identity. Electrophysiologists have long experienced the trade-offs between anatomically-accurate single-cell recording techniques and high-density multi-cellular recording methods with poor anatomical correlations. In this study, we present a novel technique that combines large-scale micro-electrode array recordings with genetic identification and the anatomical location of the retinal ganglion cell soma. This was obtained through optogenetic stimulation and subsequent confocal imaging of genetically targeted retinal ganglion cell sub-populations in the mouse. With the many molecular options available for optogenetic gene expression, we view this method as a versatile tool for matching function to genetic classifications, which can be extended to include morphological information if the density of labelled cells is at the correct level.

A Novel Retinal Ganglion Cell Promoter for Utility in AAV Vectors

Significant advances in gene therapy have enabled exploration of therapies for inherited retinal disorders, many of which are in preclinical development or clinical evaluation. Gene therapy for retinal conditions has led the way in this growing field. The loss of retinal ganglion cells (RGCs) is a hallmark of a number of retinal disorders. As the field matures innovations that aid in refining therapies and optimizing efficacy are in demand. Gene therapies under development for RGC-related disorders, when delivered with recombinant adeno associated vectors (AAV), have typically been expressed from ubiquitous promoter sequences. Here we describe how a novel promoter from the murine Nefh gene was selected to drive transgene expression in RGCs. The Nefh promoter, in an AAV2/2 vector, was shown to drive preferential EGFP expression in murine RGCs in vivo following intravitreal injection. In contrast, EGFP expression from a CMV promoter was observed not only in RGCs, but throughout the inner nuclear layer and in amacrine cells located within the ganglion cell layer (GCL). Of note, the Nefh promoter sequence is sufficiently compact to be readily accommodated in AAV vectors, where transgene size represents a significant constraint. Moreover, this promoter should in principle provide a more targeted and potentially safer alternative for RGC-directed gene therapies.

A dual-fluorescence reporter in the Eomes locus for live imaging and medium-term lineage tracing

The T-box transcription factor Eomes (also known as Tbr2) shows short-lived expression in various localized domains of the embryo, including epiblast cells during gastrulation and intermediate progenitor cells in the cerebral cortex. In these tissues Eomes fulfills crucial roles for lineage specification of progenitors. To directly observe Eomes-dependent cell lineages in the living embryo, we generated a novel dual-fluorescence reporter allele that expresses a membrane-bound tdTomato protein for investigation of cell morphology and a nuclear GFP for cell tracing. This allele recapitulates endogenous EOMES protein expression and is suitable for live imaging. We found that the allele can also be used as a short-to-medium-term lineage tracer, as GFP persists in cells longer than EOMES protein and marks Eomes-dependent lineages with a timeframe of days to weeks depending on the proliferation rate. In summary, we present a novel genetic tool for investigation of Eomes-dependent cell types by live imaging and lineage tracing.