Cellular organization of the vertebrate retina

Birds multiplex spectral and temporal visual information via retinal On- and Off-channels

In vertebrate vision, early retinal circuits divide incoming visual information into functionally opposite elementary signals: On and Off, transient and sustained, chromatic and achromatic. Together these signals can yield an efficient representation of the scene for transmission to the brain via the optic nerve. However, this long-standing interpretation of retinal function is based on mammals, and it is unclear whether this functional arrangement is common to all vertebrates. Here we show that male poultry chicks use a fundamentally different strategy to communicate information from the eye to the brain. Rather than using functionally opposite pairs of retinal output channels, chicks encode the polarity, timing, and spectral composition of visual stimuli in a highly correlated manner: fast achromatic information is encoded by Off-circuits, and slow chromatic information overwhelmingly by On-circuits. Moreover, most retinal output channels combine On- and Off-circuits to simultaneously encode, or multiplex, both achromatic and chromatic information. Our results from birds conform to evidence from fish, amphibians, and reptiles which retain the full ancestral complement of four spectral types of cone photoreceptors.

In vitro Model Systems for Studies Into Retinal Neuroprotection

Therapy development for neurodegenerative diseases of the retina constitutes a major unmet medical need, and this may be particularly relevant for inherited diseases of the retina, which are largely untreatable to this day. Therapy development necessitates appropriate models to improve the understanding of the underlying degenerative mechanisms, as well as for the testing and evaluation of novel treatment approaches. This review provides an overview of various in vitro model systems used to study retinal neuroprotection. The in vitro methods and technologies discussed range from primary retinal cell cultures and cell lines, to retinal organoids and organotypic retinal explants, to the cultivation of whole eyeballs. The advantages and disadvantages of these methods are compared and evaluated, also in view of the 3R principles (i.e., the refinement, reduction, and replacement of live animal testing), to identify suitable in vitro alternatives for in vivo experimentation. The article further expands on the use of in vitro models to test and evaluate neuroprotective treatments and to aid the development of retinal drug delivery systems. Among the pharmacological agents tested and characterized in vitro are such that interfere with aberrant cyclic guanosine monophosphate (cGMP) -signaling or such that inhibit the activities of poly (ADP-ribose) polymerase (PARP), histone deacetylases (HDAC), calpain-type proteases, as well as unfolded protein response-related stress. We then introduce nanoparticle-based drug delivery systems and discuss how different in vitro systems may be used to assess their efficacy in the treatment of retinal diseases. The summary provides a brief comparison of available in vitro models and relates their advantages and limitations to the various experimental requirements, for instance, for studies into disease mechanisms, novel treatments, or retinal toxicity. In many cases, combinations of different in vitro models may be required to obtain a comprehensive view of the efficacy of a given retinal neuroprotection approach.

Inherited Retinal Dystrophies: Role of Oxidative Stress and Inflammation in Their Physiopathology and Therapeutic Implications

Inherited retinal dystrophies (IRDs) are a large group of genetically and clinically heterogeneous diseases characterized by the progressive degeneration of the retina, ultimately leading to loss of visual function. Oxidative stress and inflammation play fundamental roles in the physiopathology of these diseases. Photoreceptor cell death induces an inflammatory state in the retina. The activation of several molecular pathways triggers different cellular responses to injury, including the activation of microglia to eliminate debris and recruit inflammatory cells from circulation. Therapeutical options for IRDs are currently limited, although a small number of patients have been successfully treated by gene therapy. Many other therapeutic strategies are being pursued to mitigate the deleterious effects of IRDs associated with oxidative metabolism and/or inflammation, including inhibiting reactive oxygen species’ accumulation and inflammatory responses, and blocking autophagy. Several compounds are being tested in clinical trials, generating great expectations for their implementation. The present review discusses the main death mechanisms that occur in IRDs and the latest therapies that are under investigation.

Longitudinal In Vivo Changes in Retinal Ganglion Cell Dendritic Morphology After Acute and Chronic Optic Nerve Injury

Purpose

To characterize in vivo dendritic changes in retinal ganglion cells (RGCs) after acute (optic nerve transection, ONT) and chronic (experimental glaucoma, EG) optic nerve injury.

Methods

ONT and EG (microbead model) were carried out in Thy1-YFP mice in which the entire RGC dendritic arbor was imaged with confocal fluorescence scanning laser ophthalmoscopy over two weeks in the ONT group and over two and six months, respectively, in two (groups 1 and 2) EG groups. Sholl analysis was used to quantify dendritic structure with the parameters: area under the curve (AUC), radius of the dendritic field, peak number of intersections (PI), and distance to the PI (PD).

Results

Dendritic changes were observed after three days post-ONT with significant decreases in all parameters at two weeks. In group 1 EG mice, mean (SD) intraocular pressure (IOP) was 15.2 (1.1) and 9.8 (0.3) mmHg in the EG and untreated contralateral eyes, respectively, with a significant corresponding decrease in AUC, PI, and PD, but not radius. In group 2 mice, the respective IOP was 13.1 (1.0) and 8.8 (0.1) mmHg, peaking at two months before trending towards baseline. Over the first two months, AUC, PI, and PD decreased significantly, with no further subsequent changes. The rates of change of the parameters after ONT was 5 to 10 times faster than in EG.

Conclusions

Rapid dendritic changes occurred after ONT, while changes in EG were slower and associated with level of IOP increase. The earliest alterations were loss of inner neurites without change in dendritic field.

Visual Disfunction due to the Selective Effect of Glutamate Agonists on Retinal Cells

One of the causes of nervous system degeneration is an excess of glutamate released upon several diseases. Glutamate analogs, like N-methyl-DL-aspartate (NMDA) and kainic acid (KA), have been shown to induce experimental retinal neurotoxicity. Previous results have shown that NMDA/KA neurotoxicity induces significant changes in the full field electroretinogram response, a thinning on the inner retinal layers, and retinal ganglion cell death. However, not all types of retinal neurons experience the same degree of injury in response to the excitotoxic stimulus. The goal of the present work is to address the effect of intraocular injection of different doses of NMDA/KA on the structure and function of several types of retinal cells and their functionality. To globally analyze the effect of glutamate receptor activation in the retina after the intraocular injection of excitotoxic agents, a combination of histological, electrophysiological, and functional tools has been employed to assess the changes in the retinal structure and function. Retinal excitotoxicity caused by the intraocular injection of a mixture of NMDA/KA causes a harmful effect characterized by a great loss of bipolar, amacrine, and retinal ganglion cells, as well as the degeneration of the inner retina. This process leads to a loss of retinal cell functionality characterized by an impairment of light sensitivity and visual acuity, with a strong effect on the retinal OFF pathway. The structural and functional injury suffered by the retina suggests the importance of the glutamate receptors expressed by different types of retinal cells. The effect of glutamate agonists on the OFF pathway represents one of the main findings of the study, as the evaluation of the retinal lesions caused by excitotoxicity could be specifically explored using tests that evaluate the OFF pathway.

Single-Cell Transcriptomic Comparison of Human Fetal Retina, hPSC-Derived Retinal Organoids, and Long-Term Retinal Cultures

To study the development of the human retina, we use single-cell RNA sequencing (RNA-seq) at key fetal stages and follow the development of the major cell types as well as populations of transitional cells. We also analyze stem cell (hPSC)-derived retinal organoids; although organoids have a very similar cellular composition at equivalent ages as the fetal retina, there are some differences in gene expression of particular cell types. Moreover, the inner retinal lamination is disrupted at more advanced stages of organoids compared with fetal retina. To determine whether the disorganization in the inner retina is due to the culture conditions, we analyze retinal development in fetal retina maintained under similar conditions. These retinospheres develop for at least 6 months, displaying better inner retinal lamination than retinal organoids. Our single-cell RNA sequencing (scRNA-seq) comparisons of fetal retina, retinal organoids, and retinospheres provide a resource for developing better in vitro models for retinal disease.

Preparing a Single Cell Suspension from Zebrafish Retinal Tissue for Flow Cytometric Cell Sorting of Müller Glia

Preparation of a single cell suspension from solid tissue is vital for a successful flow cytometry experiment. We report a detailed and reproducible method to produce a quality cell suspension from the zebrafish retina. Zebrafish retinas, especially their Müller glia cells, are of particular interest for their inherent regenerative capacity, making them a useful model for regenerative medicine and cell therapy research. Here, we detail a papain-based dissociation that is gentle enough to keep cells intact, but strong enough to disrupt cell-cell and cell-matrix interactions to yield a cell suspension that produces clean and reliable flow cytometric cell sorting results. This procedure consistently results in over 90% viability and three populations of cells based on GFP expression. The dissociation procedure described herein has been optimized for the collection of Müller glia from Tg(apoe:gfp) zebrafish retinas; however, the overall process may be applicable to other cell types in the fish retina, additional flow cytometric techniques, or preparing cell suspensions from similar tissues. © 2019 International Society for Advancement of Cytometry.

Genetic control of cellular morphogenesis in Müller glia

Cell shape is critical for the proper function of every cell in every tissue in the body. This is especially true for the highly morphologically diverse neural and glia cells of the central nervous system. The molecular processes by which these, or indeed any, cells gain their particular cell-specific morphology remain largely unexplored. To identify the genes involved in the morphogenesis of the principal glial cell type in the vertebrate retina, the Müller glia (MG), we used genomic and CRISPR based strategies in zebrafish (Danio rerio). We identified 41 genes involved in various aspects of MG cell morphogenesis and revealed a striking concordance between the sequential steps of anatomical feature addition and the expression of cohorts of functionally related genes that regulate these steps. We noted that the many of the genes preferentially expressed in zebrafish MG showed conservation in glia across species suggesting evolutionarily conserved glial developmental pathways.

Fate-restricted retinal progenitor cells adopt a molecular profile and spatial position distinct from multipotent progenitor cells

During development, multipotent retinal progenitor cells generate a large number of unique cell types. Recent evidence suggests that there are fate-restricted progenitor cell states in addition to multipotent ones. Here we report a transcriptomic analysis of fate- restricted progenitor cells biased to produce cone photoreceptors and horizontal cells, marked by the THRB cis-regulatory element ThrbCRM1. Comparison to a control population enriched in multipotent progenitor cells identified several genes considered to be pan-progenitor, such as VSX2, LHX2, and PAX6, as downregulated in these fate- restricted retinal progenitor cells. This differential regulation occurs in chick and in a different restricted progenitor population in mouse suggesting that this is a conserved feature of progenitor dynamics during retinal development. S-phase labeling also revealed that nuclear positions of restricted progenitor populations occupy distinct spatial niches within the developing chick retina. Using a conserved regulatory element proximal to the VSX2 gene, a potential negative feedback mechanism from specific transcription factors enriched in cone/horizontal cell progenitor cells was identified. This study identifies conserved molecular and cellular changes that occur during the generation of fate restricted retinal progenitor cells from multipotent retinal progenitor cells.

Circadian rhythms, refractive development, and myopia

PURPOSE

Despite extensive research, mechanisms regulating postnatal eye growth and those responsible for ametropias are poorly understood. With the marked recent increases in myopia prevalence, robust and biologically-based clinical therapies to normalize refractive development in childhood are needed. Here, we review classic and contemporary literature about how circadian biology might provide clues to develop a framework to improve the understanding of myopia etiology, and possibly lead to rational approaches to ameliorate refractive errors developing in children.

RECENT FINDINGS

Increasing evidence implicates diurnal and circadian rhythms in eye growth and refractive error development. In both humans and animals, ocular length and other anatomical and physiological features of the eye undergo diurnal oscillations. Systemically, such rhythms are primarily generated by the 'master clock' in the surpachiasmatic nucleus, which receives input from the intrinsically photosensitive retinal ganglion cells (ipRGCs) through the activation of the photopigment melanopsin. The retina also has an endogenous circadian clock. In laboratory animals developing experimental myopia, oscillations of ocular parameters are perturbed. Retinal signaling is now believed to influence refractive development; dopamine, an important neurotransmitter found in the retina, not only entrains intrinsic retinal rhythms to the light:dark cycle, but it also modulates refractive development. Circadian clocks comprise a transcription/translation feedback control mechanism utilizing so-called clock genes that have now been associated with experimental ametropias. Contemporary clinical research is also reviving ideas first proposed in the nineteenth century that light exposures might impact refraction in children. As a result, properties of ambient lighting are being investigated in refractive development. In other areas of medical science, circadian dysregulation is now thought to impact many non-ocular disorders, likely because the patterns of modern artificial lighting exert adverse physiological effects on circadian pacemakers. How, or if, such modern light exposures and circadian dysregulation contribute to refractive development is not known.

SUMMARY

The premise of this review is that circadian biology could be a productive area worthy of increased investigation, which might lead to the improved understanding of refractive development and improved therapeutic interventions.

Expression and signaling of NGF in the healthy and injured retina

This review summarizes the present knowledge concerning the retinal localization of the nerve growth factor (NGF), its precursor proNGF, and the receptors TrkA and p75NTR in the developing and mature rodent retina. We further discuss the changes in the expression of NGF and the receptors in experimental models of retinal disorders and diseases like inherited retinitis pigmentosa, retinal detachment, glaucoma, and diabetic retinopathy. Since proNGF is now recognized as a bioactive signaling molecule which induces cell death through p75NTR activation, the role of proNGF in the induction of retinal cell loss under neurodegenerative conditions is also highlighted. In addition, we present the evidences for a potential therapeutic intervention with NGF for the treatment of retinal neurodegenerative diseases. Different strategies have been developed and experimentally tested in mice and rats in order to reduce cell loss and Müller cell gliosis, e.g., increasing the availability of endogenous NGF, administration of exogenous NGF, activation of TrkA, and inhibition of p75NTR. Here, we discuss the several lines of evidence supporting a protective effect of NGF on retinal cell loss, with specific emphasis on photoreceptor and retinal ganglion cell degeneration. A better understanding of the mechanisms underlying the effects of NGF and proNGF in the modulation of neurodegeneration and gliosis in the retina will help to develop efficient therapeutic strategies for various retinal diseases.

Extracellular Matrix Components Regulate Cellular Polarity and Tissue Structure in the Developing and Mature Retina

While genetic networks and other intrinsic mechanisms regulate much of retinal development, interactions with the extracellular environment shape these networks and modify their output. The present review has focused on the role of one family of extracellular matrix molecules and their signaling pathways in retinal development. In addition to their effects on the developing retina, laminins play a role in maintaining Müller cell polarity and compartmentalization, thereby contributing to retinal homeostasis. This article which is intended for the clinical audience, reviews the fundamentals of retinal development, extracellular matrix organization and the role of laminins in retinal development. The role of laminin in cortical development is also briefly discussed.

Correlation between SD-OCT, immunocytochemistry and functional findings in an animal model of retinal degeneration

Purpose: The P23H rhodopsin mutation is an autosomal dominant cause of retinitis pigmentosa (RP). The degeneration can be tracked using different anatomical and functional methods. In our case, we evaluated the anatomical changes using Spectral-Domain Optical Coherence Tomography (SD-OCT) and correlated the findings with retinal thickness values determined by immunocytochemistry.Methods: Pigmented rats heterozygous for the P23H mutation, with ages between P18 and P180 were studied. Function was assessed by means of optomotor testing and ERGs. Retinal thicknesses measurements, autofluorescence and fluorescein angiography were performed using Spectralis OCT. Retinas were studied by means of immunohistochemistry. Results: Between P30 and P180, visual acuity decreased from 0.500 to 0.182 cycles per degree (cyc/deg) and contrast sensitivity decreased from 54.56 to 2.98 for a spatial frequency of 0.089 cyc/deg. Only cone-driven b-wave responses reached developmental maturity. Flicker fusions were also comparable at P29 (42 Hz). Double flash-isolated rod-driven responses were already affected at P29. Photopic responses revealed deterioration after P29.A reduction in retinal thicknesses and morphological modifications were seen in OCT sections. Statistically significant differences were found in all evaluated thicknesses. Autofluorescence was seen in P23H rats as sparse dots. Immunocytochemistry showed a progressive decrease in the outer nuclear layer (ONL), and morphological changes. Although anatomical thickness measures were significantly lower than OCT values, there was a very strong correlation between the values measured by both techniques.Conclusions: In pigmented P23H rats, a progressive deterioration occurs in both retinal function and anatomy. Anatomical changes can be effectively evaluated using SD-OCT and immunocytochemistry, with a good correlation between their values, thus making SD-OCT an important tool for research in retinal degeneration.

Loss of outer retinal neurons and circuitry alterations in the DBA/2J mouse

PURPOSE

The DBA/2J mouse line develops essential iris atrophy, pigment dispersion, and glaucomatous age-related changes, including an increase of IOP, optic nerve atrophy, and retinal ganglion cell (RGC) death. The aim of this study was to evaluate possible morphological changes in the outer retina of the DBA/2J mouse concomitant with disease progression and aging, based on the reduction of both the a- and b-waves and photopic flicker ERGs in this mouse line.

METHODS

Vertically sectioned DBA/2J mice retinas were evaluated at 3, 8, and 16 months of age using photoreceptor, horizontal, and bipolar cell markers. Sixteen-month-old C57BL/6 mice retinas were used as controls.

RESULTS

The DBA/2J mice had outer retinal degeneration at all ages, with the most severe degeneration in the oldest retinas. At 3 months of age, the number of photoreceptor cells and the thickness of the OPL were reduced. In addition, there was a loss of horizontal and ON-bipolar cell processes. At 8 months of age, RGC degeneration occurred in patches, and in the outer retina overlying these patches, cone morphology was impaired with a reduction in size as well as loss of outer segments and growth of horizontal and bipolar cell processes into the outer nuclear layer. At 16 months of age, connectivity between photoreceptors and horizontal and bipolar cell processes overlying these patches was lost.

CONCLUSIONS

Retinal degeneration in DBA/2J mice includes photoreceptor death, loss of bipolar and horizontal cell processes, and loss of synaptic contacts in an aging-dependent manner.

Cellular responses following retinal injuries and therapeutic approaches for neurodegenerative diseases

Retinal neurodegenerative diseases like age-related macular degeneration, glaucoma, diabetic retinopathy and retinitis pigmentosa each have a different etiology and pathogenesis. However, at the cellular and molecular level, the response to retinal injury is similar in all of them, and results in morphological and functional impairment of retinal cells. This retinal degeneration may be triggered by gene defects, increased intraocular pressure, high levels of blood glucose, other types of stress or aging, but they all frequently induce a set of cell signals that lead to well-established and similar morphological and functional changes, including controlled cell death and retinal remodeling. Interestingly, an inflammatory response, oxidative stress and activation of apoptotic pathways are common features in all these diseases. Furthermore, it is important to note the relevant role of glial cells, including astrocytes, Müller cells and microglia, because their response to injury is decisive for maintaining the health of the retina or its degeneration. Several therapeutic approaches have been developed to preserve retinal function or restore eyesight in pathological conditions. In this context, neuroprotective compounds, gene therapy, cell transplantation or artificial devices should be applied at the appropriate stage of retinal degeneration to obtain successful results. This review provides an overview of the common and distinctive features of retinal neurodegenerative diseases, including the molecular, anatomical and functional changes caused by the cellular response to damage, in order to establish appropriate treatments for these pathologies.

Retinal proteome analysis in a mouse model of oxygen-induced retinopathy

To identify proteins that are involved in the molecular mechanisms of oxygen-induced retinopathy (OIR), a well-established model of blinding ischemic retinopathy, we quantitatively analyzed the retinal proteome in a mouse model of OIR. OIR was induced by exposing C57BL/6 mice on postnatal day 7 (P7) to 75% hyperoxia for 5 days, followed by 5 days in room air. Retinas from mice on P12 and P17, the hyperoxic and hypoxic phases, respectively, and control groups were examined using isobaric tags for relative and absolute quantitation (iTRAQ) and nano-LC-ESI-MS/MS. In total, 1422 retinal proteins were identified: 699 from the iTRAQ experiment and 1074 by nano-LC-ESI-MS/MS. Compared with control retinas in the iTRAQ study, OIR retinas upregulated and downregulated 21 and 17 proteins, respectively, in P17 retinas and 25 and 14 proteins, respectively, in P12 retinas. Of the differentially expressed proteins, the retinal expression of crystallin proteins, Müller cell-associated proteins, neurodegeneration-associated proteins, and angiogenesis-associated proteins, such as 150-kDa oxygen-regulated protein (ORP150), were analyzed. ORP150 colocalized to the neovascular tufts, and knockdown of ORP150 by siRNA decreased the levels of secreted VEGF in cultured retinal pigment epithelial cells. Moreover, intravitreal administration of siRNA targeting ORP150 significantly reduced the retinal neovascularization in OIR. In conclusion, our proteomic discovery method, coupled with targeted approaches, revealed many proteins that were differentially regulated in the mouse model of OIR. These proteins, including ORP150, are potential novel therapeutic targets for the treatment of proliferative ischemic retinopathy.

Neural regeneration

Regeneration of the nervous system requires either the repair or replacement of nerve cells that have been damaged by injury or disease. While lower organisms possess extensive capacity for neural regeneration, evolutionarily higher organisms including humans are limited in their ability to regenerate nerve cells, posing significant issues for the treatment of injury and disease of the nervous system. This chapter focuses on current approaches for neural regeneration, with a discussion of traditional methods to enhance neural regeneration as well as emerging concepts within the field such as stem cells and cellular reprogramming. Stem cells are defined by their ability to self-renew as well as their ability to differentiate into multiple cell types, and hence can serve as a source for cell replacement of damaged neurons. Traditionally, adult stem cells isolated from the hippocampus and subventricular zone have served as a source of neural stem cells for replacement purposes. With the advancement of pluripotent stem cells, including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (iPSCs), new and exciting approaches for neural cell replacement are being developed. Furthermore, with increased understanding of the human genome and epigenetics, scientists have been successful in the direct genetic reprogramming of somatic cells to a neuronal fate, bypassing the intermediary pluripotent stage. Such breakthroughs have accelerated the timing of production of mature neuronal cell types from a patient-specific somatic cell source such as skin fibroblasts or mononuclear blood cells. While extensive hurdles remain to the translational application of such stem cell and reprogramming strategies, these approaches have revolutionized the field of regenerative biology and have provided innovative approaches for the potential regeneration of the nervous system.

Differential expression of the ADAMs in developing chicken retina

The expression patterns of the seven members of the ADAM (a disintegrin and metalloprotease) family, ADAM9, ADAM10, ADAM12, ADAM13, ADAM17, ADAM22, and ADAM23 were analyzed in the developing chicken retina by in situ hybridization and immunohistochemistry. Results show that each individual ADAM is expressed and regulated spatiotemporally in the developing retinal layers. ADAM9, ADAM10 and ADAM17 are widely expressed in the differential layers of the retina throughout the whole embryonic period, while ADAM12 and ADAM13 are mainly expressed in the ganglion cell layer at a later stage. ADAM22 and ADAM23 are restricted to the inner nuclear layer and the ganglion cell layer at a later stage. Furthermore, ADAM10 protein is co-expressed with the four members of the classic cadherins, N-cadherin, R-cadherin, cadherin-6B and cadherin-7 in distinct retinal layers. Therefore, the differential expression of the investigated ADAMs in the developing retina suggests the contribution of them to the retina development.

Expression of classic cadherins and delta-protocadherins in the developing ferret retina

BACKGROUND

Cadherins are a superfamily of calcium-dependent adhesion molecules that play multiple roles in morphogenesis, including proliferation, migration, differentiation and cell-cell recognition. The subgroups of classic cadherins and delta-protocadherins are involved in processes of neural development, such as neurite outgrowth, pathfinding, target recognition, synaptogenesis as well as synaptic plasticity. We mapped the expression of 7 classic cadherins (CDH4, CDH6, CDH7, CDH8, CDH11, CDH14, CDH20) and 8 delta-protocadherins (PCDH1, PCDH7, PCDH8, PCDH9, PCDH10, PCDH11, PCDH17, PCDH18) at representative stages of retinal development and in the mature retina of the ferret by in situ hybridization.

RESULTS

All cadherins investigated by us are expressed differentially by restricted populations of retinal cells during specific periods of the ferret retinogenesis. For example, during embryonic development, some cadherins are exclusively expressed in the outer, proliferative zone of the neuroblast layer, whereas other cadherins mark the prospective ganglion cell layer or cells in the prospective inner nuclear layer. These expression patterns anticipate histogenetic changes that become visible in Nissl or nuclear stainings at later stages. In parallel to the ongoing development of retinal circuits, cadherin expression becomes restricted to specific subpopulations of retinal cell types, especially of ganglion cells, which express most of the investigated cadherins until adulthood. A comparison to previous results in chicken and mouse reveals overall conserved expression patterns of some cadherins but also species differences.

CONCLUSIONS

The spatiotemporally restricted expression patterns of 7 classic cadherins and 8 delta-protocadherins indicate that cadherins provide a combinatorial adhesive code that specifies developing retinal cell populations and intraretinal as well as retinofugal neural circuits in the developing ferret retina.

Axon-bearing and axon-less horizontal cell subtypes are generated consecutively during chick retinal development from progenitors that are sensitive to follistatin

BACKGROUND

Horizontal cells are retinal interneurons that modulate the output from photoreceptors. A rich literature on the morphological classification and functional properties of HCs in different animals exists, however, the understanding of the events underlying their development is still limited. In most vertebrates including chicken, two main horizontal cell (HC) subtypes are identified based on the presence or absence of an axon.

RESULTS

In this work we have molecularly characterized three HC subtypes based on Lim1, Isl1, GABA and TrkA, a classification that is consistent with three chick HC subtypes previously defined by morphology. The axon-bearing and axon-less HC subpopulations molecularly defined by Lim1 and Isl1, are born consecutively on embryonic day (E) 3-4 and E4-5, respectively, and exhibit temporally distinguishable periods of migration. Their relative numbers are not adjusted by apoptosis. A sharp decrease of high endogenous levels of the activin-inhibitor follistatin at E3 coincides with the appearance of the Lim1 positive cells. Extending the follistatin exposure of the HC retinal progenitor cells by injection of follistatin at E3 increased the number of both Lim1- and Isl1 positive HCs when analysed at E9.

CONCLUSION

The results imply that the axon-bearing and axon-less HC subgroups are defined early and are generated consecutively from a retinal progenitor cell population that is sensitive to the inhibitory action of follistatin. The results are consistent with a model wherein added follistatin causes HC-generating progenitors to proliferate beyond the normal period of HC generation, thus producing extra HCs of both types that migrate to the HC layer.

Modulation of extracellular proton fluxes from retinal horizontal cells of the catfish by depolarization and glutamate

Self-referencing H(+)-selective microelectrodes were used to measure extracellular proton fluxes from cone-driven horizontal cells isolated from the retina of the catfish (Ictalurus punctatus). The neurotransmitter glutamate induced an alkalinization of the area adjacent to the external face of the cell membrane. The effect of glutamate occurred regardless of whether the external solution was buffered with 1 mM HEPES, 3 mM phosphate, or 24 mM bicarbonate. The AMPA/kainate receptor agonist kainate and the NMDA receptor agonist N-methyl-D-aspartate both mimicked the effect of glutamate. The effect of kainate on proton flux was inhibited by the AMPA/kainate receptor blocker CNQX, and the effect of NMDA was abolished by the NMDA receptor antagonist DAP-5. Metabotropic glutamate receptor agonists produced no alteration in proton fluxes from horizontal cells. Depolarization of cells either by increasing extracellular potassium or directly by voltage clamp also produced an alkalinization adjacent to the cell membrane. The effects of depolarization on proton flux were blocked by 10 microM nifedipine, an inhibitor of L-type calcium channels. The plasmalemma Ca(2+/)H(+) ATPase (PMCA) blocker 5(6)-carboxyeosin also significantly reduced proton flux modulation by glutamate. Our results are consistent with the hypothesis that glutamate-induced extracellular alkalinizations arise from activation of the PMCA pump following increased intracellular calcium entry into cells. This process might help to relieve suppression of photoreceptor neurotransmitter release that results from exocytosed protons from photoreceptor synaptic terminals. Our findings argue strongly against the hypothesis that protons released by horizontal cells act as the inhibitory feedback neurotransmitter that creates the surround portion of the receptive fields of retinal neurons.

Zebrafish: a model system for the study of eye genetics

Over the last decade, the use of the zebrafish as a genetic model has moved beyond the proof-of-concept for the analysis of vertebrate embryonic development to demonstrated utility as a mainstream model organism for the understanding of human disease. The initial identification of a variety of zebrafish mutations affecting the eye and retina, and the subsequent cloning of mutated genes have revealed cellular, molecular and physiological processes fundamental to visual system development. With the increasing development of genetic manipulations, sophisticated techniques for phenotypic characterization, behavioral approaches and screening strategies, the identification of novel genes or novel gene functions will have important implications for our understanding of human eye diseases, pathogenesis, and treatment.

Membrane-initiated effects of progesterone on calcium dependent signaling and activation of VEGF gene expression in retinal glial cells

Neurosteroids, such as progesterone, influence central nervous system development and function by regulating a broad spectrum of physiological processes. Here, we investigated membrane-initiated actions of progesterone in the retina and identified the membrane-associated progesterone receptor component 1 (PGRMC1). We found PGRMC1 expressed mainly in retinal Muller glia (RMG) and retinal pigment epithelium, and localized uniquely to microsomal and plasma membrane fractions. In RMG, membrane-impermeable progesterone conjugate induced calcium influx and subsequent phosphatidylinositol 3-kinase-mediated phosphorylation of PKC and ERK-1/2. Induction by progesterone also led to PKC-dependent activation of VEGF gene expression and protein synthesis, suggesting a contribution of membrane-initiated hormone effects to VEGF induced neovascularization within retina.

Expression of alpha 7 nicotinic acetylcholine receptors by bipolar, amacrine, and ganglion cells of the rabbit retina

Cholinergic agents affect the light responses of many ganglion cells (GCs) in the mammalian retina by activating nicotinic acetylcholine receptors (nAChRs). Whereas retinal neurons that express beta2 subunit-containing nAChRs have been characterized in the rabbit retina, expression patterns of other nAChR subtypes remain unclear. Therefore, we evaluated the expression of alpha7 nAChRs in retinal neurons by means of single-, double-, and triple-label immunohistochemistry. Our data demonstrate that, in the rabbit retina, several types of bipolar cells, amacrine cells, and cells in the GC layer express alpha7 nAChRs. At least three different populations of cone bipolar cells exhibited alpha7 labeling, whereas glycine-immunoreactive amacrine cells comprised the majority of alpha7-positive amacrine cells. Some GABAergic amacrine cells also displayed alpha7 immunoreactivity; alpha7 labeling was never detected in rod bipolar cells or rod amacrine cells (AII amacrine cells). Our data suggest that activation of alpha7 nAChRs by acetylcholine (ACh) or choline may affect glutamate release from several types of cone bipolar cells, modulating GC responses. ACh-induced excitation of inhibitory amacrine cells might cause either inhibition or disinhibition of other amacrine and GC circuits. Finally, ACh may act on alpha7 nAChRs expressed by GCs themselves.

Imaging techniques in retinal research

In recent years, retinal research has benefited from major advances in optical imaging approaches. Investigations of the structural and functional organization of the vertebrate retina using live preparations have been facilitated by improvements in cell labeling methods, and by microscopy techniques that permit high-resolution of cells in vitro and in vivo. In particular, the generation of transgenic animals with fluorescently labeled retinal cells has permitted real-time visualization of cell generation, migration, differentiation and growth in the developing retina. Neuronal activity can also be examined by optical imaging using activity reporters directed to specific retinal cell types. Optical techniques such as multiphoton microscopy and total internal reflection fluorescence microscopy (TIRFM) have helped unravel the physiological properties and function of retinal cells. Here, we focus on the latest cell labeling methods that have proven highly useful in many aspects of retinal research. We also highlight several examples of how newly developed imaging technology itself has facilitated investigations that have advanced our understanding of retinal circuits and their development.

Cellular remodeling in mammalian retina: results from studies of experimental retinal detachment

Retinal detachment, the separation of the neural retina from the retinal pigmented epithelium, starts a cascade of events that results in cellular changes throughout the retina. While the degeneration of the light sensitive photoreceptor outer segments is clearly an important event, there are many other cellular changes that have the potential to significantly effect the return of vision after successful reattachment. Using animal models of detachment and reattachment we have identified many cellular changes that result in significant remodeling of the retinal tissue. These changes range from the retraction of axons by rod photoreceptors to the growth of neurites into the subretinal space and vitreous by horizontal and ganglion cells. Some neurite outgrowths, as in the case of rod bipolar cells, appear to be directed towards their normal presynaptic target. Horizontal cells may produce some directed neurites as well as extensive outgrowths that have no apparent target. A subset of reactive ganglion cells all fall into the latter category. Muller cells, the radial glia of the retina, undergo numerous changes ranging from proliferation to a wholesale structural reorganization as they grow into the subretinal space (after detachment) or vitreous after reattachment. In a few cases have we been able to identify molecular changes that correlate with the structural remodeling. Similar changes to those observed in the animal models have now been observed in human tissue samples, leading us to conclude that this research may help us understand the imperfect return of vision occurring after successful reattachment surgery. The mammalian retina clearly has a vast repertoire of cellular responses to injury, understanding these may help us improve upon current therapies or devise new therapies for blinding conditions.

A comparison of immunocytochemical markers to identify bipolar cell types in human and monkey retina

As more human retinas affected with genetic or immune-based diseases become available for morphological analysis, it is important to identify immunocytochemical markers for specific subtypes of retinal neurons. In this study, we have focused on bipolar cell markers in central retina. We have done single and double labeling using several antisera previously utilized in macaque monkey or human retinal studies and two new antisera (1) to correlate combinations of antisera labeling with morphological types of bipolar cells in human retina, and (2) to compare human labeling patterns with those in monkey retina. Human bipolar cells showed a wide range of labeling patterns with at least ten different bipolar cell types identified from their anatomy and marker content. Many bipolar cell bodies in the outer part of the inner nuclear layer contained combinations of protein kinase C alpha (PKC alpha), Islet-1, glycine, and Go alpha. Bipolar cells labeled with these markers had axons terminating in the inner half of the inner plexiform layer (IPL), consistent with ON bipolar cells. Bipolar cell bodies adjacent to the amacrine cells and with axons in the outer half of the IPL contained combinations of recoverin, glutamate transporter-1, and PKC beta, or CD15 and calbindin. Bipolar cells labeled with these markers were presumed OFF bipolar cells. Calcium-binding protein 5 (CaB5) labeled both putative ON and OFF bipolar cells. Using this cell labeling as a criteria, most cell bodies close to the horizontal cells were ON bipolar cells and almost all bipolar cells adjacent to the amacrine cells were OFF with a band in the middle 2-3 cell bodies thick containing intermixed ON and OFF bipolar cells. Differences were found between human and monkey bipolar cell types labeled by calbindin, CaB5, and CD15. Two new types were identified. One was morphologically similar to the DB3, but labeled for CD15 and CaB5. The other had a calbindin-labeled cell body adjacent to the horizontal cell bodies, but did not contain any accepted ON markers. These results support the use of macaque monkey retina as a model for human, but caution against the assumption that all labeling patterns are identical in the two primates.

Noninvasive two-photon imaging reveals retinyl ester storage structures in the eye

Visual sensation in vertebrates is triggered when light strikes retinal photoreceptor cells causing photoisomerization of the rhodopsin chromophore 11-cis-retinal to all-trans-retinal. The regeneration of preillumination conditions of the photoreceptor cells requires formation of 11-cis-retinal in the adjacent retinal pigment epithelium (RPE). Using the intrinsic fluorescence of all-trans-retinyl esters, noninvasive two-photon microscopy revealed previously uncharacterized structures (6.9 +/- 1.1 microm in length and 0.8 +/- 0.2 microm in diameter) distinct from other cellular organelles, termed the retinyl ester storage particles (RESTs), or retinosomes. These structures form autonomous all-trans-retinyl ester-rich intracellular compartments distinct from other organelles and colocalize with adipose differentiation-related protein. As demonstrated by in vivo experiments using wild-type mice, the RESTs participate in 11-cis-retinal formation. RESTs accumulate in Rpe65-/- mice incapable of carrying out the enzymatic isomerization, and correspondingly, are absent in the eyes of Lrat-/- mice deficient in retinyl ester synthesis. These results indicate that RESTs located close to the RPE plasma membrane are essential components in 11-cis-retinal production.

Development of a rod photoreceptor mosaic revealed in transgenic zebrafish

The number and distribution of neurons within the vertebrate retina are tightly regulated. This is particularly apparent in the highly ordered, crystalline-like arrangement of the cone photoreceptors in the teleost. In this report, using a transgenic line of zebrafish, a novel and developmentally regulated mosaic pattern of the rod photoreceptors is described. The spatial and temporal expression of EGFP, under the control of the Xenopus rhodopsin gene promoter, was nearly identical to the endogenous rhodopsin. EGFP was first detected in the ventral nasal retinal in an area of precocious neurogenesis referred to as the "ventral patch". Subsequent expression of EGFP was observed in isolated cells sporadically distributed across the dorsal and central retina. However, confocal microscopy and spatial analysis of larval eyes or retinal explants from adults revealed a precise arrangement to the rod photoreceptors. The rod terminals were arranged in regularly spaced rows with clearly identifiable telodendria linking neighboring cells. The rod inner segments projected through the cone mosaic in a predictable pattern. In the adult, the rod mosaic originated near the retinal margin where clusters of rods differentiated around the immature short single cone. In the embryo, the sporadic differentiation of the rods led to the gradual formation of the mosaic pattern. With the growing interest in neuronal stem cells, revisiting this model of neurogenesis provides an avenue to uncover mechanisms underlying the precise integration of new neuronal elements into a preexisting neural network.

Photoreceptor subtype specification: from flies to humans

Multiple cell types often differentiate from a pluripotent cell. These cells may then further diversify as distinct subtypes. The visual system provides an ideal model for studying subtype specification as various photoreceptors acquire different functions based on the type of opsin they express. Opsin expression is mostly controlled through transcriptional mechanisms that are evolutionary conserved from Drosophila to humans. In addition, it appears that, from a "default" developmental state, distinct "acquired" photoreceptor states develop upon receiving intrinsic or extrinsic signals. This review discusses factors involved in opsin gene regulation and how their integration may explain how subtype specificity is achieved.