Cell death and the creation of regional differences in neuronal numbers.

Regional variations in cell death are ubiquitous in the nervous system. In the retina, cell death in retinal ganglion cells is elevated in the retinal periphery and may be important in setting up the initial conditions that produce central retinal specializations such as an area centralis or visual streak. In central visual system structures, pronounced spatial and spatiotemporal inhomogeneities in cell death are seen both in layers and regions of the lateral geniculate nucleus and superior colliculus; similar indications of inhomogeneities are seen in those nonvisual structures that have been examined. Cell death in the cortex is highly nonuniform, by layer and by cortical area. A variety of possible functions for these regional losses are proposed, in the context of a uniform mechanism for cell death that allows it to assume multiple functions.     

Gene delivery into mouse retinal ganglion cells by in utero electroporation

Background  

The neural retina is a highly structured tissue of the central nervous system that is formed by seven different cell types that are arranged in layers. Despite much effort, the genetic mechanisms that underlie retinal development are still poorly understood. In recent years, large-scale genomic analyses have identified candidate genes that may play a role in retinal neurogenesis, axon guidance and other key processes during the development of the visual system. Thus, new and rapid techniques are now required to carry out high-throughput analyses of all these candidate genes in mammals. Gene delivery techniques have been described to express exogenous proteins in the retina of newborn mice but these approaches do not efficiently introduce genes into the only retinal cell type that transmits visual information to the brain, the retinal ganglion cells (RGCs).     

Recursive features of circular receptive fields

The distribution of excitability in retinal receptive fields may be well approximated by functions with recursive features. Physiological data do not exclude an implementation of recursive structures in the visual system. It is the most remarkable advantage of a recursive visual system, that cortical receptive fields tuned to different spatial frequencies will have an identical neuronal circuitry. Structural consequences for retina, LGN and visual cortex are discussed.     

The formation of the area centralis of the retinal ganglion cell layer in the chick

In adult domestic chickens, the neurones in the retinal ganglion cell layer are very unevenly disposed such that there is a sixfold increase in neurone density from the retinal edge to the retinal centre. The formation of the high ganglion-cell-density area centralis was studied on chick retinal wholemounts from the 8th day of incubation (E8) to 4 weeks after hatching (4WAH). The density of viable neurones and the number and the distribution of pyknotic neurones in the ganglion cell layer were estimated across the whole retina. Between E8 and E10, the distribution of neurones in the ganglion cell layer was anisodensitic with 53,000 mm-2 in the centre compared to 34,000 mm-2 in the periphery of the retina. Thereafter, a progressively steeper gradient of neurone density developed, which decreased from 24,000 mm-2 in the retinal centre to 6000 mm-2 at the retinal periphery by 4WAH. Neuronal pyknosis in the ganglion cell layer was observed between E9 and E17. From E11 onwards, consistently more pyknotic neurones were found in the peripheral than in the central retina. It was estimated that over the period of cell death approximately twice as many neurones died per unit area in the retinal periphery than in the centre. Retinal area measurements and estimation of neurone densities in the ganglion cell layer after the period of neurone generation and neurone death indicated differential retinal expansion, with more expansion in the peripheral than in the central retina. These observations allow us to conclude that the formation of the area centralis of the chick retina involves (1) slightly higher cell generation in the retinal centre, (2) higher rate of cell loss in the retinal periphery and (3) differential retinal expansion.     

Visual Sensorial Impairments in Neurodevelopmental Disorders: Evidence for a Retinal Phenotype in Fragile X Syndrome

Visual sensory impairments are common in Mental Deficiency (MD) and Autism Spectrum Disorder (ASD). These defects are linked to cerebral dysfunction in the visual cortical area characterized by the deregulation of axon growth/guidance and dendrite spine immaturity of neurons. However, visual perception had not been addressed, although the retina is part of the central nervous system with a common embryonic origin. Therefore, we investigated retinal perception, the first event of vision, in a murine model of MD with autistic features. We document that retinal function is altered in Fmr1 KO mice, a model of human Fragile X Syndrome. Indeed, In Fmr1 KO mice had a lower retinal function characterized by a decreased photoreceptors neuron response, due to a 40% decrease in Rhodopsin content and to Rod Outer Segment destabilization. In addition, we observed an alteration of the visual signal transmission between photoreceptors and the inner retina which could be attributed to deregulations of pre- and post- synaptic proteins resulting in retinal neurons synaptic destabilization and to retinal neurons immaturity. Thus, for the first time, we demonstrated that retinal perception is altered in a murine model of MD with autistic features and that there are strong similarities between cerebral and retinal cellular and molecular defects. Our results suggest that both visual perception and integration must be taken into account in assessing visual sensory impairments in MD and ASD.     

Experimental retinal detachment causes widespread and multilayered degeneration in rabbit retina

Retinal detachment remains one of the most frequent causes of visual impairment in humans, even after ophthalmoscopically successful retinal reattachment. This study was aimed at monitoring (ultra-) structural alterations of retinae of rabbits after experimental detachment. A surgical procedure was used to produce local retinal detachments in rabbit eyes similar to the typical lesions in human patients. At various periods after detachment, the detached retinal area as well as neighbouring attached regions were studied by light and electron microscopy. In addition to the well-known degeneration of photoreceptor cells in the detached retina, the following progressive alterations were observed, (i) in both the detached and the attached regions, an incomplete but severe loss of ganglion cell axons occurs; (ii) there is considerable ganglion cell death, particularly in the detached area; (iii) even in the attached retina distant from the detachment, small adherent groups of photoreceptor cells degenerate; (iv) these photoreceptor cells degenerate in an atypical sequence, with severely destructed somata and inner segments but well-maintained outer segments; and (v) the severe loss of retinal neurons is not accompanied by any significant loss of Müller (glial) cells. It is noteworthy that the described progressive (and probably irreparable) retinal destructions occur also in the attached retina, and may account for visual impairment in strikingly large areas of the visual field, even after retinal reattachment.     

Human brain activity during illusory visual jitter as revealed by functional magnetic resonance imaging

One central problem in vision is how to compensate for retinal slip. A novel illusion (visual jitter) suggests the compensation mechanism is based solely on retinal motion. Adaptation to visual noise attenuates the motion signals used by the compensation stage, producing illusory jitter due to the undercompensation of retinal slip. Here, we investigated the neural substrate of retinal slip compensation during this illusion using high-field fMRI and retinotopic mapping in flattened cortical format. When jitter perception occurred, MR signal decreased in lower stages of the visual system but increased prominently in area MT+. In conclusion, visual areas as early as V1 are responsible for the adaptation stage, and MT+ is involved in the compensation stage. The present finding suggests the pathway from V1 to MT+ has an important role in stabilizing the visual world.     

The Mitochondrial Permeability Transition as a Target for Neuroprotection

Mitochondria serve as checkpoints and amplifiers on cell death pathways. In the central nervous system, mitochondrial involvement seems essential for normal expression of cell death phenotypes, and interference with these pathways thus seems a reasonable approach to neuroprotection. We have been involved in examining the potential involvement of the mitochondrial permeability transition (mPT) as one of several possible mechanisms by which mitochondria may be drawn into these death cascades. This possibility, though still controversial, is supported by evidence that factors that may stimulate mPT induction are associated with some forms of cell death (e.g., in stroke) and are modulated by diseases of the central nervous system (e.g., Huntington's). Evidence of neuroprotection seen with compounds such as N-Met-Val cyclosporine also support this possibility.     

Patterns of correlated spontaneous bursting activity in the developing mammalian retina

The adult visual system is highly organized in its patterns of connectivity. Connections between the retina and its central target, the dorsal lateral geniculate nucleus (dLGN), are remodeled during development as inappropriate synaptic inputs are eliminated by a process that requires retinal activity. Multineuronal recordings of the neonatal ferret retina reveal that during the refinement period, retinal ganglion cells spontaneously display rhythmic bursting activity in which the bursts of neighboring cells are correlated by propagating excitatory waves. These spontaneous retinal waves have temporal and spatial properties that appear instructive for the refinement of the early patterns of retinogeniculate connections prior to visual stimulation.     

Cell death in the developing vertebrate retina

Programmed cell death occurs naturally, as a physiological process, during the embryonic development of multicellular organisms. In the retina, which belongs to the central nervous system, at least two phases of cell death have been reported to occur during development. An early phase takes place concomitant with the processes of neurogenesis, cell migration and cell differentiation. A later phase affecting mainly neurons occurs when connections are established and synapses are formed, resulting in selective elimination of inappropriate connections. This pattern of cell death in the developing retina is common among different vertebrates. However, the timing and magnitude of retinal cell death varies among species. In addition, a precise regulation of apoptosis during retinal development has been described. Factors such as neurotrophins, among many others, and electrical activity influence the survival of retinal cells during the course of development. In this paper, we present a summary of these different aspects of programmed cell death during retinal development, and examine how these differ among different species.     

TGF-beta modulates programmed cell death in the retina of the developing chick embryo

Programmed cell death (PCD) is a key phenomenon in the regulation of cell number in multicellular organisms. We have shown that reduction of endogenous transforming growth factor beta (TGF-beta) prevents apoptotic PCD of neurons in the developing peripheral and central nervous system, suggesting that TGF-beta is an important mediator of ontogenetic neuron death. Previous studies suggested that there are other pro-apoptotic molecules, nerve growth factor (NGF) and brain-derived neurotrophic factor, that induce cell death in the nervous system. In the developing chick retina, NGF induces PCD by activation of the p75 receptor. We have studied the role of TGF-beta and its putative interdependence with NGF-mediated PCD in the chick retina. We found that TGF-beta is present in the developing chick retina during the period of PCD and is essentially required to regulate PCD of retinal cells. TGF-beta 2, TGF-beta 3 and the ligand-binding TGF-beta receptor can be detected immunocytochemically in the central retina, a region where apoptosis is most prominent during the early period of PCD. Application of a TGF-beta-neutralizing antibody to chick embryos in ovo resulted in a decrease in the number of TUNEL-positive cells and a reduction of free nucleosome levels. In terms of magnitude, reduction of PCD caused by the neutralization of endogenous TGF-beta was equivalent to that seen after anti-NGF application. Neutralization of both factors did not result in a further decrease in apoptosis, indicating that NGF and TGF-beta may act on the same cell population. Furthermore, neutralization of TGF-beta did not affect the expression of NGF or the p75-receptor. Our results suggest that TGF-beta and NGF are both required to regulate cell death in the chick retina in vivo.     

Position,guidance, and mapping in the developing visual system

Positional identity in the visual system affects the topographic projection of the retina onto its central targets. In this review we discuss gradients and positional information in the retina, when and how they arise, and their functional significance in development. When the axons of retinal ganglion cells leave the eye, they navigate through territory in the central nervous system that is rich in positional information. We review studies that explore the navigational cues that the growth cones of retinal axons use to orient towards their target and organize themselves as they make this journey. Finally, these axons arrive at their central targets and make a precise topographic map of visual space that is crucial for adaptive visual behavior. In the last section of this review, we examine the topographic cues in the tectum, what they are, when, and how they arise, and how retinal axons respond to them. We also touch on the role of neural activity in the refinement of this topography. © 1993 John Wiley & Sons, Inc.     

Coherence in nervous system design: the visual system of Pantodon buchholzi

One of the more unusual visual systems of the Actinopterygii is that of Pantodon buchholzi (Osteoglossomorpha: Osteoglossidae). Its adaptations associate neuroanatomy at different levels of the visual system with ecological and behavioural correlates and demonstrate that the visual system of this fish has adapted for simultaneous vision in air and water. The visual field is divided into three distinct areas: for viewing into the water column, into air, and for viewing the aquatic reflection from the underside of the water surface. Cone diameters in different retinal areas correlate with the differing physical constraints in the respective visual field. Retinal differentiation between the aquatic and aerial views is paralleled at different levels of the central nervous system. A diencephalic nucleus receives both direct and indirect (tectal) afferent input from only the aerial visual system and a specific type of cell in the optic tectum is preferentially distributed in the tectum processing aerial inputs. Distinctions within a single sensory system suggest that some behaviours may be organized according to visual field. For Pantodon, feeding is initiated by stimuli seen by the ventral hemiretina so the anatomical specializations may well play an important role as elements in a feeding circuit.     

Glutamate-Induced Excitotoxicity in Retina: Neuroprotection with Receptor Antagonist,Dextromethorphan, but Not with Calcium Channel Blockers

The purpose of our studies was to evaluate different strategies for possible neuroprotection in glutamate-induced neurotoxicity in the retina. In a first set of experiments we attempted to determine if dextrorphan antagonism of glutamate action on NMDA receptors would protect against excitotoxic injury associated with secondary damage seen after surgical laser treatment in retina. In a second set of experiments, the effects of different calcium channel blockers in an in-vitro model of N-methyl-D-aspartate (NMDA)-induced retinal ganglion cell excitotoxicity that utilized rabbit retinal explants were evaluated. Dextrorphan infusion prior to laser treatment of rabbit retina produced a significant decrease in the area of neural retinal damage. We attribute the apparent dextrorphan protection to attenuation of glutamate mediated excitotoxicity secondary to laser induced cell death. Preincubation of rabbit retinal explants with verapamil, nimodipine or -conotoxin MVIIA did not cause a significant change in NMDA induced cell death in the ganglion cell layer.     

A Critical role for Bim in retinal ganglion cell death

Optic nerve transection results in the death of retinal ganglion cells (RGCs) by apoptosis. Apoptosis is regulated by the Bcl-2 family of proteins, of which the Bcl-2 homology (BH3) -only proteins forms a subset. As BH3-only proteins have been shown to play a significant role in regulating cell death in the central nervous system, we wished to investigate the role of Bcl-2 interacting mediator of cell death (Bim), a prominent member of this protein family in the regulation of cell death in the RGC layer using in vitro retinal explants. In this study, we use an innovative retinal shaving procedure to isolate the cells of the ganglion cell layer to use for western blotting. Members of the BH3-only protein family are down-regulated during retinal development and are not normally expressed in the adult retina. Using this procedure, we demonstrate that Bim is re-expressed and its expression is increased over time following axotomy. Expression of Bad and Bik decreases over the same time course, whereas there is no indication that Bid and Puma are re-expressed. We show that explants from Bim knockout mice are resistant to axotomy-induced death when compared with their wild-type counterparts. Genetic deletion of Bim also prevents caspase 3 cleavage. The activity of Bim can be negatively regulated by phosphorylation. We show that the decrease of Bim phosphorylation correlates with a decrease in expression of survival kinases such as pAkt and pERK over the same time course. These results implicate Bim re-expression as being essential for axotomy-induced death of RGCs and that phosphorylation of Bim negatively regulates its activity in RGCs.     

The Relationship between Visuospatial Impairments and Retinal and Cortical Thickness in Parkinson’s Disease

The paper touches upon current views on the pathophysiology of visuospatial impairments in Parkinson’s disease (PD). To assess thickness of retina’s ganglionic layer, retinal nerve fiber layer, and macular map, optical coherence tomography method was used. Brain MRI was also performed followed by evaluation of the cortical thickness. Patients underwent neuropsychological tests, including those for assessment of visuospatial perception and cognitive functions. We found certain retinal regions and areas of visual cortex with significant changes in PD patients on different stages. Our findings allowed us to speculate on the role of changes in the peripheral and central structures of the visual sensory system in the pathophysiology of visuospatial impairments in PD.     

Pineal photoreceptor cells are required for maintaining the circadian rhythms of behavioral visual sensitivity in zebrafish

In non-mammalian vertebrates, the pineal gland functions as the central pacemaker that regulates the circadian rhythms of animal behavior and physiology. We generated a transgenic zebrafish line [Tg(Gnat2:gal4-VP16/UAS:nfsB-mCherry)] in which the E. coli nitroreductase is expressed in pineal photoreceptor cells. In developing embryos and young adults, the transgene is expressed in both retinal and pineal photoreceptor cells. During aging, the expression of the transgene in retinal photoreceptor cells gradually diminishes. By 8 months of age, the Gnat2 promoter-driven nitroreductase is no longer expressed in retinal photoreceptor cells, but its expression in pineal photoreceptor cells persists. This provides a tool for selective ablation of pineal photoreceptor cells, i.e., by treatments with metronidazole. In the absence of pineal photoreceptor cells, the behavioral visual sensitivity of the fish remains unchanged; however, the circadian rhythms of rod and cone sensitivity are diminished. Brief light exposures restore the circadian rhythms of behavioral visual sensitivity. Together, the data suggest that retinal photoreceptor cells respond to environmental cues and are capable of entraining the circadian rhythms of visual sensitivity; however, they are insufficient for maintaining the rhythms. Cellular signals from the pineal photoreceptor cells may be required for maintaining the circadian rhythms of visual sensitivity.     

Effects of white light‐emitting diode (LED) exposure on retinal pigment epithelium in vivo

Ageing and alteration of the functions of the retinal pigment epithelium (RPE) are at the origin of lost of vision seen in age‐related macular degeneration (AMD). The RPE is known to be vulnerable to high‐energy blue light. The white light‐emitting diodes (LED) commercially available have relatively high content of blue light, a feature that suggest that they could be deleterious for this retinal cell layer. The aim of our study was to investigate the effects of “white LED” exposure on RPE. For this, commercially available white LEDs were used for exposure experiments on Wistar rats. Immunohistochemical stain on RPE flat mount, transmission electron microscopy and Western blot were used to exam the RPE. LED‐induced RPE damage was evaluated by studying oxidative stress, stress response pathways and cell death pathways as well as the integrity of the outer blood–retinal barrier (BRB). We show that white LED light caused structural alterations leading to the disruption of the outer blood–retinal barrier. We observed an increase in oxidized molecules, disturbance of basal autophagy and cell death by necrosis. We conclude that white LEDs induced strong damages in rat RPE characterized by the breakdown of the BRB and the induction of necrotic cell death.     

Guanine and its retinal distribution in the tapetum of the bigeye tuna,thunnus obesus

The eye of the bigeye tuna (Thunnus obesus) contains a retinal tapetum composed of guanine. The total amount of the guanine in one eye of the fish (SL=120 cm) was about 88.6 mg. The mean guanine content of the tapetum was approximately 1.25 mg/cm² of the retinal surface. The highest content of guanine (2.15 mg/cm²) was observed only in the ventro-temporal part of the retina. To distinguish this area from the rest of the eye, we suggested the term ‘locus tapetalis’ for it. The visual accommodation system clearly indicated that the visual axis of the fish is upper-forward and the resulting retinal area for acute vision was suggested to be in the ventro-temporal retina. We discussed that the area centralis of the bigeye tuna may have two functions: to guarantee high visual acuity and to allow for high photo-sensitivity in dim light vision.