Retinal horizontal cells: challenging paradigms of neural development and cancer biology

Development and maintenance of vision's first synapse

Synapses in the outer retina are the first information relay points in vision. Here, photoreceptors form synapses onto two types of interneurons, bipolar cells and horizontal cells. Because outer retina synapses are particularly large and highly ordered, they have been a useful system for the discovery of mechanisms underlying synapse specificity and maintenance. Understanding these processes is critical to efforts aimed at restoring visual function through repairing or replacing neurons and promoting their connectivity. We review outer retina neuron synapse architecture, neural migration modes, and the cellular and molecular pathways that play key roles in the development and maintenance of these connections. We further discuss how these mechanisms may impact connectivity in the retina.

Mammalian Retinal Cell Quantification

Normal retina and its cell layers are essential for processing visual stimuli, and loss of its integrity has been documented in many disease processes. The numbers and the axonal processes of retinal ganglion cells are reduced substantially in glaucoma, leading to vision loss and blindness. Similarly, selective loss of photoreceptors in age-related macular degeneration and hereditary retinal dystrophies also results in the compromise of visual acuity. Development of genetically modified mice has led to increased understanding of the pathogenesis of many retinal diseases. Similarly, in this digital era, usage of modalities to quantify the retinal cell loss has grown exponentially leading to a better understanding of the suitability of animal models to study human retinal diseases. These quantification modalities provide valuable quantifiable data in studying pathogenesis and disease progression. This review will discuss the immunohistochemical markers for various retinal cells, available automated tools to quantify retinal cells, and present an example of retinal ganglion cell quantification using HALO image analysis platform. Additionally, we briefly review retinal cell types and subtypes, salient features of retina in various laboratory animal species, and a few of the main disease processes that affect retinal cell numbers in humans.

Non-visual Opsins and Novel Photo-Detectors in the Vertebrate Inner Retina Mediate Light Responses Within the Blue Spectrum Region

In recent decades, a number of novel non-visual opsin photopigments belonging to the family of G protein- coupled receptors, likely involved in a number of non-image-forming processes, have been identified and characterized in cells of the inner retina of vertebrates. It is now known that the vertebrate retina is composed of visual photoreceptor cones and rods responsible for diurnal/color and nocturnal/black and white vision, and cells like the intrinsically photosensitive retinal ganglion cells (ipRGCs) and photosensitive horizontal cells in the inner retina, both detecting blue light and expressing the photopigment melanopsin (Opn4). Remarkably, these non-visual photopigments can continue to operate even in the absence of vision under retinal degeneration. Moreover, inner retinal neurons and Müller glial cells have been shown to express other photopigments such as the photoisomerase retinal G protein-coupled receptor (RGR), encephalopsin (Opn3), and neuropsin (Opn5), all able to detect blue/violet light and implicated in chromophore recycling, retinal clock synchronization, neuron-to-glia communication, and other activities. The discovery of these new photopigments in the inner retina of vertebrates is strong evidence of novel light-regulated activities. This review focuses on the features, localization, photocascade, and putative functions of these novel non-visual opsins in an attempt to shed light on their role in the inner retina of vertebrates and in the physiology of the whole organism.

Brainwide Genetic Sparse Cell Labeling to Illuminate the Morphology of Neurons and Glia with Cre-Dependent MORF Mice

Cajal recognized that the elaborate shape of neurons is fundamental to their function in the brain. However, there are no simple and generalizable genetic methods to study neuronal or glial cell morphology in the mammalian brain. Here, we describe four mouse lines conferring Cre-dependent sparse cell labeling based on mononucleotide repeat frameshift (MORF) as a stochastic translational switch. Notably, the optimized MORF3 mice, with a membrane-bound multivalent immunoreporter, confer Cre-dependent sparse and bright labeling of thousands of neurons, astrocytes, or microglia in each brain, revealing their intricate morphologies. MORF3 mice are compatible with imaging in tissue-cleared thick brain sections and with immuno-EM. An analysis of 151 MORF3-labeled developing retinal horizontal cells reveals novel morphological cell clusters and axonal maturation patterns. Our study demonstrates a conceptually novel, simple, generalizable, and scalable mouse genetic solution to sparsely label and illuminate the morphology of genetically defined neurons and glia in the mammalian brain.

Dose-dependent regulation of horizontal cell fate by Onecut family of transcription factors

Genome duplication leads to an emergence of gene paralogs that are essentially free to undergo the process of neofunctionalization, subfunctionalization or degeneration (gene loss). Onecut1 (Oc1) and Onecut2 (Oc2) transcription factors, encoded by paralogous genes in mammals, are expressed in precursors of horizontal cells (HCs), retinal ganglion cells and cone photoreceptors. Previous studies have shown that ablation of either Oc1 or Oc2 gene in the mouse retina results in a decreased number of HCs, while simultaneous deletion of Oc1 and Oc2 leads to a complete loss of HCs. Here we study the genetic redundancy between Oc1 and Oc2 paralogs and focus on how the dose of Onecut transcription factors influences abundance of individual retinal cell types and overall retina physiology. Our data show that reducing the number of functional Oc alleles in the developing retina leads to a gradual decrease in the number of HCs, progressive thinning of the outer plexiform layer and diminished electrophysiology responses. Taken together, these observations indicate that in the context of HC population, the alleles of Oc1/Oc2 paralogous genes are mutually interchangeable, function additively to support proper retinal function and their molecular evolution does not follow one of the typical routes after gene duplication.

LKB1 coordinates neurite remodeling to drive synapse layer emergence in the outer retina

Structural changes in pre and postsynaptic neurons that accompany synapse formation often temporally and spatially overlap. Thus, it has been difficult to resolve which processes drive patterned connectivity. To overcome this, we use the laminated outer murine retina. We identify the serine/threonine kinase LKB1 as a key driver of synapse layer emergence. The absence of LKB1 in the retina caused a marked mislocalization and delay in synapse layer formation. In parallel, LKB1 modulated postsynaptic horizontal cell refinement and presynaptic photoreceptor axon growth. Mislocalized horizontal cell processes contacted aberrant cone axons in LKB1 mutants. These defects coincided with altered synapse protein organization, and horizontal cell neurites were misdirected to ectopic synapse protein regions. Together, these data suggest that LKB1 instructs the timing and location of connectivity in the outer retina via coordinate regulation of pre and postsynaptic neuron structure and the localization of synapse-associated proteins.

Single-nuclei RNA-seq on human retinal tissue provides improved transcriptome profiling

Single-cell RNA-seq is a powerful tool in decoding the heterogeneity in complex tissues by generating transcriptomic profiles of the individual cell. Here, we report a single-nuclei RNA-seq (snRNA-seq) transcriptomic study on human retinal tissue, which is composed of multiple cell types with distinct functions. Six samples from three healthy donors are profiled and high-quality RNA-seq data is obtained for 5873 single nuclei. All major retinal cell types are observed and marker genes for each cell type are identified. The gene expression of the macular and peripheral retina is compared to each other at cell-type level. Furthermore, our dataset shows an improved power for prioritizing genes associated with human retinal diseases compared to both mouse single-cell RNA-seq and human bulk RNA-seq results. In conclusion, we demonstrate that obtaining single cell transcriptomes from human frozen tissues can provide insight missed by either human bulk RNA-seq or animal models.

Semblance: An empirical similarity kernel on probability spaces

In data science, determining proximity between observations is critical to many downstream analyses such as clustering, classification, and prediction. However, when the data's underlying probability distribution is unclear, the function used to compute similarity between data points is often arbitrarily chosen. Here, we present a novel definition of proximity, Semblance, that uses the empirical distribution of a feature to inform the pair-wise similarity between observations. The advantage of Semblance lies in its distribution-free formulation and its ability to place greater emphasis on proximity between observation pairs that fall at the outskirts of the data distribution, as opposed to those toward the center. Semblance is a valid Mercer kernel, allowing its principled use in kernel-based learning algorithms, and for any data modality. We demonstrate its consistently improved performance against conventional methods through simulations and real case studies from diverse applications in single-cell transcriptomics, image reconstruction, and financial forecasting.

The Neuroprotective Peptide PACAP1-38 Contributes to Horizontal Cell Development in Postnatal Rat Retina

Purpose

PACAP1-38, a member of the secretin/glucagon superfamily, is expressed in the developing retina with documented neuroprotective effects. However, its function in retinal cell differentiation has yet to be elucidated. Our goals, therefore, were to identify PAC1 expressing cells morphologically, investigate the PACAP1-38 action functionally, and establish PACAP1-38 regulated events developmentally during the first postnatal week in rat retina.

Methods

P1 retinal sections or whole mounts of Wistar rats were used to reveal PAC1 and calbindin immunoreactive structures. P1, P3, or P7 pups were injected intravitreally with 100 pmol PACAP1-38. Tissues were harvested 24 hours post-treatment, then processed for calbindin immunohistochemistry to determine horizontal cell number, or 6, 12, 24 hours post-treatment for real-time PCR and immunoblots to detect PCNA expression. To localize proliferating cells, anti-PCNA antibody was applied.

Results

We showed various PAC1 expressing cells in RPE, NBL, and GCL in P1 retina including calbindin positive horizontal cells. We found that PACAP1-38 induced a marked cell number increase at P3 and P7 and showed upregulated cell proliferation as its mechanism; however, it was ineffective at P1. PACAP1-38 induced proliferative cells localized in the NBL, and double-marker studies demonstrated that the induced proliferative cells were horizontal cells.

Conclusions

PACAP1-38 appears to act in retinal differentiation by inducing mitosis selectively in a time and cell specific manner through PAC1. The control of horizontal cell proliferation raises the novel possibilities that (1) PACAP1-38 may be a major player in retinal patterning and (2) PACAP signaling may be critical in retinoblastoma.

An integrated transcriptional analysis of the developing human retina

The scarcity of embryonic/foetal material as a resource for direct study means that there is still limited understanding of human retina development. Here, we present an integrated transcriptome analysis combined with immunohistochemistry in human eye and retinal samples from 4 to 19 post-conception weeks. This analysis reveals three developmental windows with specific gene expression patterns that informed the sequential emergence of retinal cell types and enabled identification of stage-specific cellular and biological processes, and transcriptional regulators. Each stage is characterised by a specific set of alternatively spliced transcripts that code for proteins involved in the formation of the photoreceptor connecting cilium, pre-mRNA splicing and epigenetic modifiers. Importantly, our data show that the transition from foetal to adult retina is characterised by a large increase in the percentage of mutually exclusive exons that code for proteins involved in photoreceptor maintenance. The circular RNA population is also defined and shown to increase during retinal development. Collectively, these data increase our understanding of human retinal development and the pre-mRNA splicing process, and help to identify new candidate disease genes.

Differential expression of cyclin-dependent kinases in the adult human retina in relation to CDK inhibitor retinotoxicity

Cyclin-dependent kinases (CDKs) are a family of kinases associated predominantly with cell cycle control, making CDK inhibitors interesting candidates for anti-cancer therapeutics. However, retinal toxicity (loss of photoreceptors) has been associated with CDK inhibitors, including the pan-CDK inhibitor AG-012896. The purpose of this research was to use a novel planar sectioning technique to determine CDK expression profiles in the ex vivo human retina with the aim of identifying isoforms responsible for CDK retinotoxicity. Four CDK isoforms (CDK11, 16, 17 and 18) were selected as a result of IC₅₀ data comparing neurotoxic (AG-012986 and NVP-1) and non-neurotoxic (dinaciclib and NVP-2) CDK inhibitors, with IC₅₀s at CDK11 showing a clear difference between the neurotoxic and non-neurotoxic drugs. CDK11 was maximally expressed in the photoreceptor layer, whereas CDK16, 17 and 18 showed maximal expression in the inner nuclear layer. CDK5 (an isoform associated with retinal homeostasis) was maximally expressed in the retinal ganglion cell layer. Apart from CDK18, each isoform showed expression in the photoreceptor layer. The human Müller cell line MIO-M1 expressed CDK5, 11, 16 and 17 and AG-01298 (0.02-60 µM) caused a dose-dependent increase in MIO-M1 cell death. In conclusion, CDK11 appears the most likely candidate for mediation of photoreceptor toxicity. RNA profiling can be used to determine the distribution of genes of interest in relation to retinal toxicity in the human retina.

Cell-cell communication in diabetic retinopathy

In diabetic retinopathy, high glucose (HG)-mediated breakdown in cell-cell communication promotes disruption of retinal homeostasis. Several studies indicate that HG condition alters expression of connexin genes and subsequent gap junction intercellular communication (GJIC) in retinal vascular cells and non-vascular cells. A serious consequence of disrupted cell-cell communication is apoptosis and breakdown of the blood-retinal barrier (BRB). More recently, studies suggest adverse effects from HG on retinal Müller cells. This article focuses on HG-mediated changes in connexin expression and GJIC and their subsequent effects on the breakdown of retinal homeostasis, cell death, compromised vascular permeability, and interactions between endothelial cells, pericytes and retinal Müller cells in the pathogenesis of diabetic retinopathy. Additionally, options for rectifying disrupted homeostasis under HG condition associated with diabetic retinopathy are reviewed.

Immunocytochemical Profiling of Cultured Mouse Primary Retinal Cells

Primary retinal cell cultures and immunocytochemistry are important experimental platforms in ophthalmic research. Translation of retinal cells from their native environment to the in vitro milieu leads to cellular stress, jeopardizing their in vivo phenotype features. Moreover, the specificity and stability of many retinal immunochemical markers are poorly evaluated in retinal cell cultures. Hence, we here evaluated the expression profile of 17 retinal markers, that is, recoverin, rhodopsin, arrestin, Chx10, PKC, DCX, CRALBP, GS, vimentin, TPRV4, RBPMS, Brn3a, β-tubulin III, NeuN, MAP2, GFAP, and synaptophysin. At 7 and 18 days of culture, the marker expression profiles of mouse postnatal retinal cells were compared with their age-matched in vivo retinas. We demonstrate stable in vitro expression of all markers, except for arrestin and CRALBP. Differences in cellular expression and location of some markers were observed, both over time in culture and compared with the age-matched retina. We hypothesize that these differences are likely culture condition dependent. Taken together, we suggest a thorough evaluation of the antibodies in specific culture settings, before extrapolating the in vitro results to an in vivo setting. Moreover, the identification of specific cell types may require a combination of different genes expressed or markers with structural information.

The Zebrafish Anillin-eGFP Reporter Marks Late Dividing Retinal Precursors and Stem Cells Entering Neuronal Lineages

Monitoring cycling behaviours of stem and somatic cells in the living animal is a powerful tool to better understand tissue development and homeostasis. The tg(anillin:anillin-eGFP) transgenic line carries the full-length zebrafish F-actin binding protein Anillin fused to eGFP from a bacterial artificial chromosome (BAC) containing Anillin cis-regulatory sequences. Here we report the suitability of the Anillin-eGFP reporter as a direct indicator of cycling cells in the late embryonic and post-embryonic retina. We show that combining the anillin:anillin-eGFP with other transgenes such as ptf1a:dsRed and atoh7:gap-RFP allows obtaining spatial and temporal resolution of the mitotic potentials of specific retinal cell populations. This is exemplified by the analysis of the origin of the previously reported apically and non-apically dividing late committed precursors of the photoreceptor and horizontal cell layers.

Horizontal cells expressing melanopsin x are novel photoreceptors in the avian inner retina

In the vertebrate retina, three types of photoreceptors-visual photoreceptor cones and rods and the intrinsically photosensitive retinal ganglion cells (ipRGCs)-converged through evolution to detect light and regulate image- and nonimage-forming activities such as photic entrainment of circadian rhythms, pupillary light reflexes, etc. ipRGCs express the nonvisual photopigment melanopsin (OPN4), encoded by two genes: the Xenopus (Opn4x) and mammalian (Opn4m) orthologs. In the chicken retina, both OPN4 proteins are found in ipRGCs, and Opn4x is also present in retinal horizontal cells (HCs), which connect with visual photoreceptors. Here we investigate the intrinsic photosensitivity and functioning of HCs from primary cultures of embryonic retinas at day 15 by using calcium fluorescent fluo4 imaging, pharmacological inhibitory treatments, and Opn4x knockdown. Results show that HCs are avian photoreceptors with a retinal-based OPN4X photopigment conferring intrinsic photosensitivity. Light responses in HCs appear to be driven through an ancient type of phototransduction cascade similar to that in rhabdomeric photoreceptors involving a G-protein q, the activation of phospholipase C, calcium mobilization, and the release of the inhibitory neurotransmitter GABA. Based on their intrinsic photosensitivity, HCs may have a key dual function in the retina of vertebrates, potentially regulating nonvisual tasks together with their sister cells, ipRGCs, and with visual photoreceptors, modulating lateral interactions and retinal processing.

Horizontal Cells, the Odd Ones Out in the Retina, Give Insights into Development and Disease

Thorough investigation of a neuronal population can help reveal key aspects regarding the nervous system and its development. The retinal horizontal cells have several extraordinary features making them particularly interesting for addressing questions regarding fate assignment and subtype specification. In this review we discuss and summarize data concerning the formation and diversity of horizontal cells, how morphology is correlated to molecular markers, and how fate assignment separates the horizontal lineage from the lineages of other retinal cell types. We discuss the novel and unique features of the final cell cycle of horizontal cell progenitors and how they may relate to retinoblastoma carcinogenesis.

A Mutation in Syne2 Causes Early Retinal Defects in Photoreceptors, Secondary Neurons, and Müller Glia

PURPOSE

The purpose of this study was to identify the molecular basis and characterize the pathological consequences of a spontaneous mutation named cone photoreceptor function loss 8 (cpfl8) in a mouse model with a significantly reduced cone electroretinography (ERG) response.

METHODS

The chromosomal position for the recessive cpfl8 mutation was determined by DNA pooling and by subsequent genotyping with simple sequence length polymorphic markers in an F2 intercross phenotyped by ERG. Genes within the candidate region of both mutants and controls were directly sequenced and compared. The effects of the mutation were examined in longitudinal studies by light microscopy, marker analysis, transmission electron microscopy, and ERG.

RESULTS

The cpfl8 mutation was mapped to Chromosome 12, and a premature stop codon was identified in the spectrin repeat containing nuclear envelope 2 (Syne2) gene. The reduced cone ERG response was due to a significant reduction in cone photoreceptors. Longitudinal studies of the early postnatal retina indicated that the cone photoreceptors fail to develop properly, rod photoreceptors mislocalize to the inner nuclear layer, and both rods and cones undergo apoptosis prematurely. Moreover, we observed migration defects of secondary neurons and ectopic Müller cell bodies in the outer nuclear layer in early postnatal development.

CONCLUSIONS

SYNE2 is important for normal retinal development. We have determined that not only is photoreceptor nuclear migration affected, but also the positions of Müller glia and secondary neurons are disturbed early in retinal development. The cpfl8 mouse model will serve as an important resource for further examining the role of nuclear scaffolding and migration in the developing retina.

Neurite Mistargeting and Inverse Order of Intraretinal Vascular Plexus Formation Precede Subretinal Vascularization in Vldlr Mutant Mice

In the retina blood vessels are required to support a high metabolic rate, however, uncontrolled vascular growth can lead to impaired vision and blindness. Subretinal vascularization (SRV), one type of pathological vessel growth, occurs in retinal angiomatous proliferation and proliferative macular telangiectasia. In these diseases SRV originates from blood vessels within the retina. We use mice with a targeted disruption in the Vldl-receptor (Vldlr) gene as a model to study SRV with retinal origin. We find that Vldlr mRNA is strongly expressed in the neuroretina, and we observe both vascular and neuronal phenotypes in Vldlr^-/- mice. Unexpectedly, horizontal cell (HC) neurites are mistargeted prior to SRV in this model, and the majority of vascular lesions are associated with mistargeted neurites. In Foxn4^-/- mice, which lack HCs and display reduced amacrine cell (AC) numbers, we find severe defects in intraretinal capillary development. However, SRV is not suppressed in Foxn4^-/-;Vldlr^-/- mice, which reveals that mistargeted HC neurites are not required for vascular lesion formation. In the absence of VLDLR, the intraretinal capillary plexuses form in an inverse order compared to normal development, and subsequent to this early defect, vascular proliferation is increased. We conclude that SRV in the Vldlr^-/- model is associated with mistargeted neurites and that SRV is preceded by altered retinal vascular development.

Psychophysical testing in rodent models of glaucomatous optic neuropathy

Processing of visual information begins in the retina, with photoreceptors converting light stimuli into neural signals. Ultimately, signals are transmitted to the brain through signaling networks formed by interneurons, namely bipolar, horizontal and amacrine cells providing input to retinal ganglion cells (RGCs), which form the optic nerve with their axons. As part of the chronic nature of glaucomatous optic neuropathy, the increasing and irreversible damage and ultimately loss of neurons, RGCs in particular, occurs following progressive damage to the optic nerve head (ONH), eventually resulting in visual impairment and visual field loss. There are two behavioral assays that are typically used to assess visual deficits in glaucoma rodent models, the visual water task and the optokinetic drum. The visual water task can assess an animal's ability to distinguish grating patterns that are associated with an escape from water. The optokinetic drum relies on the optomotor response, a reflex turning of the head and neck in the direction of the visual stimuli, which usually consists of rotating black and white gratings. This reflex is a physiological response critical for keeping the image stable on the retina. Driven initially by the neuronal input from direction-selective RGCs, this reflex is comprised of a number of critical sensory and motor elements. In the presence of repeatable and defined stimuli, this reflex is extremely well suited to analyze subtle changes in the circuitry and performance of retinal neurons. Increasing the cycles of these alternating gratings per degree, or gradually reducing the contrast of the visual stimuli, threshold levels can be determined at which the animal is no longer tracking the stimuli, and thereby visual function of the animal can be determined non-invasively. Integrating these assays into an array of outcome measures that determine multiple aspects of visual function is a central goal in vision research and can be realized, for example, by the combination of measuring optomotor reflex function with electroretinograms (ERGs) and optical coherence tomography (OCT) of the retina. These structure-function correlations in vivo are urgently needed to identify disease mechanisms as potential new targets for drug development. Such a combination of the experimental assessment of the optokinetic reflex (OKR) or optomotor response (OMR) with other measures of retinal structure and function is especially valuable for research on GON. The chronic progression of the disease is characterized by a gradual decrease in function accompanied by a concomitant increase in structural damage to the retina, therefore the assessment of subtle changes is key to determining the success of novel intervention strategies.

So many pieces, one puzzle: cell type specification and visual circuitry in flies and mice

The visual system is a powerful model for probing the development, connectivity, and function of neural circuits. Two genetically tractable species, mice and flies, are together providing a great deal of understanding of these processes. Current efforts focus on integrating knowledge gained from three cross-fostering fields of research: (1) understanding how the fates of different cell types are specified during development, (2) revealing the synaptic connections between identified cell types ("connectomics") by high-resolution three-dimensional circuit anatomy, and (3) causal testing of how identified circuit elements contribute to visual perception and behavior. Here we discuss representative examples from fly and mouse models to illustrate the ongoing success of this tripartite strategy, focusing on the ways it is enhancing our understanding of visual processing and other sensory systems.

Arginase 2 deficiency prevents oxidative stress and limits hyperoxia-induced retinal vascular degeneration

Background

Hyperoxia exposure of premature infants causes obliteration of the immature retinal microvessels, leading to a condition of proliferative vitreoretinal neovascularization termed retinopathy of prematurity (ROP). Previous work has demonstrated that the hyperoxia-induced vascular injury is mediated by dysfunction of endothelial nitric oxide synthase resulting in peroxynitrite formation. This study was undertaken to determine the involvement of the ureahydrolase enzyme arginase in this pathology.

Methods and Findings

Studies were performed using hyperoxia-treated bovine retinal endothelial cells (BRE) and mice with oxygen-induced retinopathy (OIR) as experimental models of ROP. Treatment with the specific arginase inhibitor 2(S)-amino-6-boronohexanoic acid (ABH) prevented hyperoxia-induced apoptosis of BRE cells and reduced vaso-obliteration in the OIR model. Furthermore, deletion of the arginase 2 gene protected against hyperoxia-induced vaso-obliteration, enhanced physiological vascular repair, and reduced retinal neovascularization in the OIR model. Additional deletion of one copy of arginase 1 did not improve the vascular pathology. Analyses of peroxynitrite by quantitation of its biomarker nitrotyrosine, superoxide by dihydroethidium imaging and NO formation by diaminofluoroscein imaging showed that the protective actions of arginase 2 deletion were associated with blockade of superoxide and peroxynitrite formation and normalization of NOS activity.

Conclusions

Our data demonstrate the involvement of arginase activity and arginase 2 expression in hyperoxia-induced vascular injury. Arginase 2 deletion prevents hyperoxia-induced retinal vascular injury by preventing NOS uncoupling resulting in decreased reactive oxygen species formation and increased nitric oxide bioavailability.

The patterning of retinal horizontal cells: normalizing the regularity index enhances the detection of genomic linkage

Retinal neurons are often arranged as non-random distributions called “mosaics,” as their somata minimize proximity to neighboring cells of the same type. The horizontal cells serve as an example of such a mosaic, but little is known about the developmental mechanisms that underlie their patterning. To identify genes involved in this process, we have used three different spatial statistics to assess the patterning of the horizontal cell mosaic across a panel of genetically distinct recombinant inbred strains. To avoid the confounding effect of cell density, which varies twofold across these different strains, we computed the “real/random regularity ratio,” expressing the regularity of a mosaic relative to a randomly distributed simulation of similarly sized cells. To test whether this latter statistic better reflects the variation in biological processes that contribute to horizontal cell spacing, we subsequently compared the genomic linkage for each of these two traits, the regularity index, and the real/random regularity ratio, each computed from the distribution of nearest neighbor (NN) distances and from the Voronoi domain (VD) areas. Finally, we compared each of these analyses with another index of patterning, the packing factor. Variation in the regularity indexes, as well as their real/random regularity ratios, and the packing factor, mapped quantitative trait loci to the distal ends of Chromosomes 1 and 14. For the NN and VD analyses, we found that the degree of linkage was greater when using the real/random regularity ratio rather than the respective regularity index. Using informatic resources, we narrowed the list of prospective genes positioned at these two intervals to a small collection of six genes that warrant further investigation to determine their potential role in shaping the patterning of the horizontal cell mosaic.

Early appearance of nonvisual and circadian markers in the developing inner retinal cells of chicken

The retina is a key component of the vertebrate circadian system; it is responsible for detecting and transmitting the environmental illumination conditions (day/night cycles) to the brain that synchronize the circadian clock located in the suprachiasmatic nucleus (SCN). For this, retinal ganglion cells (RGCs) project to the SCN and other nonvisual areas. In the chicken, intrinsically photosensitive RGCs (ipRGCs) expressing the photopigment melanopsin (Opn4) transmit photic information and regulate diverse nonvisual tasks. In nonmammalian vertebrates, two genes encode Opn4: the Xenopus (Opn4x) and the mammalian (Opn4m) orthologs. RGCs express both Opn4 genes but are not the only inner retinal cells expressing Opn4x: horizontal cells (HCs) also do so. Here, we further characterize primary cultures of both populations of inner retinal cells (RGCs and HCs) expressing Opn4x. The expression of this nonvisual photopigment, as well as that for different circadian markers such as the clock genes Bmal1, Clock, Per2, and Cry1, and the key melatonin synthesizing enzyme, arylalkylamine N-acetyltransferase (AA-NAT), appears very early in development in both cell populations. The results clearly suggest that nonvisual Opn4 photoreceptors and endogenous clocks converge all together in these inner retinal cells at early developmental stages.

Developmental regulation of linkers of the nucleoskeleton to the cytoskeleton during mouse postnatal retinogenesis

Sun proteins and Nesprins are two families of proteins whose direct interactions across the nuclear envelope provide for the core of Linkers of the Nucleoskeleton to the Cytoskeleton (LINC complexes) that physically connect the nucleus interior to cytoskeletal networks. Whereas LINC complexes play essential roles in nuclear migration anchorage and underlie normal CNS development, the developmental regulation of their composition remains largely unknown. In this study, we examined the spatiotemporal expression of lamins, Sun proteins and Nesprins during postnatal mouse retinal development. Whereas retinal precursor cells mostly express B-type lamins, Sun1, and high molecular weight isoforms of Nesprins, post-mitotic retinal cells are characterized by a drastic downregulation of the latter, the expression of A-type lamins, and the strong induction of a specific isoform of Nesprin1 late in retinal development. Importantly, our results emphasize different spatiotemporal expression for Nesprin1 and Nesprin2 and further suggest an important role for KASH-less isoforms of Nesprin1 in the CNS. In conclusion, the transition from retinal precursor cells undergoing interkinetic nuclear migration to post-mitotic retinal cells undergoing nuclear translocation and/or anchorage is accompanied by a profound remodeling of LINC complexes composition. This remodeling may reflect different requirements of nuclear dynamics at different stages of CNS development.

Overlapping expression patterns and redundant roles for AP-2 transcription factors in the developing mammalian retina

BACKGROUND

We have previously shown that the transcription factor AP-2α (Tcfap2a) is expressed in postmitotic developing amacrine cells in the mouse retina. Although retina-specific deletion of Tcfap2a did not affect retinogenesis, two other family members, AP-2β and AP-2γ, showed expression patterns similar to AP-2α.

RESULTS

Here we show that, in addition to their highly overlapping expression patterns in amacrine cells, AP-2α and AP-2β are also co-expressed in developing horizontal cells. AP-2γ expression is restricted to amacrine cells, in a subset that is partially distinct from the AP-2α/β-immunopositive population. To address possible redundant roles for AP-2α and AP-2β during retinogenesis, Tcfap2a/b-deficient retinas were examined. These double mutants showed a striking loss of horizontal cells and an altered staining pattern in amacrine cells that were not detected upon deletion of either family member alone.

CONCLUSIONS

These studies have uncovered critical roles for AP-2 activity in retinogenesis, delineating the overlapping expression patterns of Tcfap2a, Tcfap2b, and Tcfap2c in the neural retina, and revealing a redundant requirement for Tcfap2a and Tcfap2b in horizontal and amacrine cell development.

Heterogenic final cell cycle by chicken retinal Lim1 horizontal progenitor cells leads to heteroploid cells with a remaining replicated genome

Retinal progenitor cells undergo apical mitoses during the process of interkinetic nuclear migration and newly generated post-mitotic neurons migrate to their prospective retinal layer. Whereas this is valid for most types of retinal neurons, chicken horizontal cells are generated by delayed non-apical mitoses from dedicated progenitors. The regulation of such final cell cycle is not well understood and we have studied how Lim1 expressing horizontal progenitor cells (HPCs) exit the cell cycle. We have used markers for S- and G2/M-phase in combination with markers for cell cycle regulators Rb1, cyclin B1, cdc25C and p27Kip1 to characterise the final cell cycle of HPCs. The results show that Lim1+ HPCs are heterogenic with regards to when and during what phase they leave the final cell cycle. Not all horizontal cells were generated by a non-apical (basal) mitosis; instead, the HPCs exhibited three different behaviours during the final cell cycle. Thirty-five percent of the Lim1+ horizontal cells was estimated to be generated by non-apical mitoses. The other horizontal cells were either generated by an interkinetic nuclear migration with an apical mitosis or by a cell cycle with an S-phase that was not followed by any mitosis. Such cells remain with replicated DNA and may be regarded as somatic heteroploids. The observed heterogeneity of the final cell cycle was also seen in the expression of Rb1, cyclin B1, cdc25C and p27Kip1. Phosphorylated Rb1-Ser608 was restricted to the Lim1+ cells that entered S-phase while cyclin B1 and cdc25C were exclusively expressed in HPCs having a basal mitosis. Only HPCs that leave the cell cycle after an apical mitosis expressed p27Kip1. We speculate that the cell cycle heterogeneity with formation of heteroploid cells may present a cellular context that contributes to the suggested propensity of these cells to generate cancer when the retinoblastoma gene is mutated.

Full-field electroretinography under general anesthesia in retinoblastoma

PURPOSE

To investigate the electrical responses of the retina in retinoblastoma (RB), by recording full-field electroretinography (ERG) under general anesthesia.

METHODS

The ERG was recorded using Ephios hand-held portable ERG system, according to International Standards for Clinical Electrophysiology of Vision. Forty-eight eyes of 43 cases and 33 eyes of 33 controls were enrolled. The cases were classified based on international intraocular retinoblastoma classification (IIRC). Forty-eight eyes of cases were divided into 30 cases with active RB and 18 cases with regressed RB.

RESULTS

The amplitudes of a- and b-waves were decreased as compared to controls in all subgroups. The implicit times of all RB patients from group A to C differed statistically from controls (p value < 0.05) except for single-flash rod response. The ERG waveforms in group E eyes were non-recordable. The comparison of ERG parameters between active and regressed groups (IIRC groups A and B) was statistically insignificant. Single case follow-up of unilateral RB after systemic chemotherapy showed improvement in amplitudes compared to baseline parameters.

CONCLUSIONS

Reduced amplitudes and delayed implicit times were noted in advanced disease. The ERG of RB cases did not follow any specific pattern of waveform. ERG appears to be a dynamic parameter to observe changes following treatment for RB. Although ERG is not a diagnostic test for RB, it can be used as a complementary test to assess the residual retinal function in RB eyes.

Cell fate determination in the vertebrate retina

The vertebrate retina is a well-characterized and tractable model for studying neurogenesis. Retinal neurons and glia are generated in a conserved sequence from a pool of multipotent progenitor cells, and numerous cell fate determinants for the different classes of retinal cell types have been identified. Here, we summarize several recent developments in the field that have advanced understanding of the regulation of multipotentiality and temporal competence of progenitors. We also discuss recent insights into the relative influence of lineage-based versus stochastic modes of cell fate determination. Enhancing and integrating knowledge of the molecular and genetic machinery underlying retinal development is critically important for understanding not only normal developmental mechanisms, but also therapeutic interventions aimed at restoring vision loss.

A regulatory domain is required for Foxn4 activity during retinogenesis

Foxn4, a member of the N-family forkhead transcription factors, controls fate decision in mouse retina and spinal cord as well as in zebrafish heart. Analysis of Foxn4 amino acid sequence revealed the presence of a region homologous to the activation domain of its close relative Foxn1 in between C-terminal amino acids 402 and 455 of Foxn4 protein. The requirement of Foxn4 putative activation domain remains to be elucidated. Using a gain-of function approach in rat and chick retinal explants, we report that deletion of Foxn4 putative activation domain results in a complete loss of its activity during retinogenesis. Target promoter transcription assay indicates that this domain is critical for Foxn4 transcriptional regulatory properties in vitro. Accordingly, in chick retinal explants, this domain is required for proper regulation of target retinogenic factors expression by Foxn4. Thus, our study demonstrates that the domain between amino acids 402 and 455 is necessary for Foxn4 transcriptional activity both in vitro and in the retina.

AP2 transcription factor induces apoptosis in retinoblastoma cells

The underlying cause of human retinoblastoma is complete inactivation of both copies of the RB1 gene. Other chromosome abnormalities, with the most common being extra copies of chromosome arm 6p, are also observed in retinoblastoma. The RB protein has previously been shown to interact with TFAP2 transcription factors. Here, we show that TFAP2A and TFAP2B, which map to chromosome arm 6p, are expressed in the amacrine and horizontal cells of human retina. TFAP2A RNA can readily be detected in retinoblastoma cell lines and tumors; however, the great majority of retinoblastoma cell lines and tumors are completely devoid of TFAP2A protein and TFAP2B RNA/protein. Transfection of TFAP2A and TFAP2B expression constructs into retinoblastoma cells induces apoptosis and inhibits proliferation. Our results suggest that a consequence of loss of RB1 gene function in retinoblastoma cells is inactivation of TFAP2A and TFAP2B function. We propose that inability to differentiate along the amacrine/horizontal cell lineages may underlie retinoblastoma tumor formation.

Development of the retina and optic pathway

Our understanding of the development of the retina and visual pathways has seen enormous advances during the past 25years. New imaging technologies, coupled with advances in molecular biology, have permitted a fuller appreciation of the histotypical events associated with proliferation, fate determination, migration, differentiation, pathway navigation, target innervation, synaptogenesis and cell death, and in many instances, in understanding the genetic, molecular, cellular and activity-dependent mechanisms underlying those developmental changes. The present review considers those advances associated with the lineal relationships between retinal nerve cells, the production of retinal nerve cell diversity, the migration, patterning and differentiation of different types of retinal nerve cells, the determinants of the decussation pattern at the optic chiasm, the formation of the retinotopic map, and the establishment of ocular domains within the thalamus.

Analysis of retinal cell development in chick embryo by immunohistochemistry and in ovo electroporation techniques

Background

Retinal cell development has been extensively investigated; however, the current knowledge of dynamic morphological and molecular changes is not yet complete.

Results

This study was aimed at revealing the dynamic morphological and molecular changes in retinal cell development during the embryonic stages using a new method of targeted retinal injection, in ovo electroporation, and immunohistochemistry techniques. A plasmid DNA that expresses the green fluorescent protein (GFP) as a marker was delivered into the sub-retinal space to transfect the chick retinal stem/progenitor cells at embryonic day 3 (E3) or E4 with the aid of pulses of electric current. The transfected retinal tissues were analyzed at various stages during chick development from near the start of neurogenesis at E4 to near the end of neurogenesis at E18. The expression of GFP allowed for clear visualization of cell morphologies and retinal laminar locations for the indication of retinal cell identity. Immunohistochemistry using cell type-specific markers (e.g., Visinin, Xap-1, Lim1+2, Pkcα, NeuN, Pax6, Brn3a, Vimentin, etc.) allowed further confirmation of retinal cell types. The composition of retinal cell types was then determined over time by counting the number of GFP-expressing cells observed with morphological characteristics specific to the various retinal cell types.

Conclusion

The new method of retinal injection and electroporation at E3 - E4 allows the visualization of all retinal cell types, including the late-born neurons, e.g., bipolar cells at a level of single cells, which has been difficult with a conventional method with injection and electroporation at E1.5. Based on data collected from analyses of cell morphology, laminar locations in the retina, immunohistochemistry, and cell counts of GFP-expressing cells, the time-line and dynamic morphological and molecular changes of retinal cell development were determined. These data provide more complete information on retinal cell development, and they can serve as a reference for the investigations in normal retinal development and diseases.

New mouse lines for the analysis of neuronal morphology using CreER(T)/loxP-directed sparse labeling

Background

Pharmacologic control of Cre-mediated recombination using tamoxifen-dependent activation of a Cre-estrogen receptor ligand binding domain fusion protein [CreER(T)] is widely used to modify and/or visualize cells in the mouse.

Methods and Findings

We describe here two new mouse lines, constructed by gene targeting to the Rosa26 locus to facilitate Cre-mediated cell modification. These lines should prove particularly useful in the context of sparse labeling experiments. The R26rtTACreER line provides ubiquitous expression of CreER under transcriptional control by the tetracycline reverse transactivator (rtTA); dual control by doxycycline and tamoxifen provides an extended dynamic range of Cre-mediated recombination activity. The R26IAP line provides high efficiency Cre-mediated activation of human placental alkaline phosphatase (hPLAP), complementing the widely used, but low efficiency, Z/AP line. By crossing with mouse lines that direct cell-type specific CreER expression, the R26IAP line has been used to produce atlases of labeled cholinergic and catecholaminergic neurons in the mouse brain. The R26IAP line has also been used to visualize the full morphologies of retinal dopaminergic amacrine cells, among the largest neurons in the mammalian retina.

Conclusions

The two new mouse lines described here expand the repertoire of genetically engineered mice available for controlled in vivo recombination and cell labeling using the Cre-lox system.

Ptf1a is expressed transiently in all types of amacrine cells in the embryonic zebrafish retina

Background

The vertebrate retina is composed of five major types of neurons: three excitatory (photoreceptors, bipolar cells and ganglion cells) and two inhibitory (horizontal and amacrine cells). The transcription factor Ptf1a (pancreas transcription factor 1a) is important for the normal development of the inhibitory retinal neurons.

Results

Using a transgenic Ptf1a:GFP reporter and in situ hybridization in the zebrafish retina, we show that ptf1a message is transiently expressed in all amacrine and horizontal cells within hours after the terminal division of multipotent progenitors at the apical surface of the retinal neuroepithelium, and remains on as these cells migrate to their final laminar location. The message then shuts off, but we can follow the stable Ptf1a:GFP protein for up to 120 hours post-fertilization. A variety of anatomically and neurochemically distinct subtypes of amacrine cells can already be distinguished at this embryonic time point.

Conclusion

The timing of Ptf1a expression suggests that it is involved in the very early stages or steps in the differentiation of amacrine cells, which, due to the perdurance of the Ptf1a:GFP, can be seen to rapidly diversify into a large number of subtypes. This work sets the stage for future studies looking at genetic specification of amacrine subtypes.