Connecting the eye to the brain: the molecular basis of ganglion cell axon guidance

Factors Affecting Stem Cell-Based Regenerative Approaches in Retinal Degeneration

Inherited and age-associated vision loss is often associated with degeneration of the cells of the retina, the light-sensitive layer at the back of the eye. The mammalian retina, being a postmitotic neural tissue, does not have the capacity to repair itself through endogenous regeneration. There has been considerable excitement for the development of cell replacement approaches since the isolation and development of culture methods for human pluripotent stem cells, as well as the generation of induced pluripotent stem cells. This has now been combined with novel three-dimensional organoid culture systems that closely mimic human retinal development in vitro. In this review, we cover the current state of the field, with emphasis on the cell delivery challenges, role of the recipient immunological microenvironment, and challenges related to connectivity between transplanted cells and host circuitry both locally and centrally to the different areas of the brain.

Human Retinal Ganglion Cells Respond to Evolutionarily Conserved Chemotropic Cues for Intra Retinal Guidance and Regeneration

Retinal ganglion cells (RGCs) connect the retina with the higher centers in the brain for visual perception. Their degeneration leads to irreversible vision loss in glaucoma patients. Since human RGCs (hRGCs) are born during fetal development and connections with the central targets are established before birth, the mechanism underlying their axon growth and guidance remains poorly understood. Here, using RGCs directly generated from human embryonic stem cells, we demonstrate that hRGCs express a battery of guidance receptors. These receptors allow hRGCs to read the spatially arrayed chemotropic cues in the developing rat retina for the centripetal orientation of axons toward the optic disc, suggesting that the mechanism of intra-retinal guidance is conserved in hRGCs. The centripetal orientation of hRGCs axons is not only in response to chemo-repulsion but also involves chemo-attraction, mediated by Netrin-1/DCC interactions. The spatially arrayed chemotropic cues differentially influence hRGCs physiological responses, suggesting that neural activity of hRGCs may facilitate axon growth during inter-retinal guidance. Additionally, we demonstrate that Netrin-1/DCC interactions, besides promoting axon growth, facilitate hRGCs axon regeneration by recruiting the mTOR signaling pathway. The diverse influence of Netrin-1/DCC interactions ranging from axon growth to regeneration may involve recruitment of multiple intracellular signaling pathways as revealed by transcriptome analysis of hRGCs. From the perspective of ex-vivo stem cell approach to glaucomatous degeneration, our findings posit that ex-vivo generated human RGCs are capable of reading the intra-retinal cues for guidance toward the optic disc, the first step toward connecting with the central target to restore vision.

Harnessing the Neuroprotective Behaviors of Müller Glia for Retinal Repair

Progressive and irreversible vision loss in mature and aging adults creates a health and economic burden, worldwide. Despite the advancements of many contemporary therapies to restore vision, few approaches have considered the innate benefits of gliosis, the endogenous processes of retinal repair that precede vision loss. Retinal gliosis is fundamentally driven by Müller glia (MG) and is characterized by three primary cellular mechanisms: hypertrophy, proliferation, and migration. In early stages of gliosis, these processes have neuroprotective potential to halt the progression of disease and encourage synaptic activity among neurons. Later stages, however, can lead to glial scarring, which is a hallmark of disease progression and blindness. As a result, the neuroprotective abilities of MG have remained incompletely explored and poorly integrated into current treatment regimens. Bioengineering studies of the intrinsic behaviors of MG hold promise to exploit glial reparative ability, while repressing neuro-disruptive MG responses. In particular, recent in vitro systems have become primary models to analyze individual gliotic processes and provide a stepping stone for in vivo strategies. This review highlights recent studies of MG gliosis seeking to harness MG neuroprotective ability for regeneration using contemporary biotechnologies. We emphasize the importance of studying gliosis as a reparative mechanism, rather than disregarding it as an unfortunate clinical prognosis in diseased retina.

Cell-Matrix Interactions in the Eye: From Cornea to Choroid

The extracellular matrix (ECM) plays a crucial role in all parts of the eye, from maintaining clarity and hydration of the cornea and vitreous to regulating angiogenesis, intraocular pressure maintenance, and vascular signaling. This review focuses on the interactions of the ECM for homeostasis of normal physiologic functions of the cornea, vitreous, retina, retinal pigment epithelium, Bruch's membrane, and choroid as well as trabecular meshwork, optic nerve, conjunctiva and tenon's layer as it relates to glaucoma. A variety of pathways and key factors related to ECM in the eye are discussed, including but not limited to those related to transforming growth factor-β, vascular endothelial growth factor, basic-fibroblastic growth factor, connective tissue growth factor, matrix metalloproteinases (including MMP-2 and MMP-9, and MMP-14), collagen IV, fibronectin, elastin, canonical signaling, integrins, and endothelial morphogenesis consistent of cellular activation-tubulogenesis and cellular differentiation-stabilization. Alterations contributing to disease states such as wound healing, diabetes-related complications, Fuchs endothelial corneal dystrophy, angiogenesis, fibrosis, age-related macular degeneration, retinal detachment, and posteriorly inserted vitreous base are also reviewed.

Proteomic Profiling of Retinoblastoma-Derived Exosomes Reveals Potential Biomarkers of Vitreous Seeding

Retinoblastoma (RB) is the most common tumor of the eye in early childhood. Although recent advances in conservative treatment have greatly improved the visual outcome, local tumor control remains difficult in the presence of massive vitreous seeding. Traditional biopsy has long been considered unsafe in RB, due to the risk of extraocular spread. Thus, the identification of new biomarkers is crucial to design safer diagnostic and more effective therapeutic approaches. Exosomes, membrane-derived nanovesicles that are secreted abundantly by aggressive tumor cells and that can be isolated from several biological fluids, represent an interesting alternative for the detection of tumor-associated biomarkers. In this study, we defined the protein signature of exosomes released by RB tumors (RBT) and vitreous seeding (RBVS) primary cell lines by high resolution mass spectrometry. A total of 5666 proteins were identified. Among these, 5223 and 3637 were expressed in exosomes RBT and one RBVS group, respectively. Gene enrichment analysis of exclusively and differentially expressed proteins and network analysis identified in RBVS exosomes upregulated proteins specifically related to invasion and metastasis, such as proteins involved in extracellular matrix (ECM) remodeling and interaction, resistance to anoikis and the metabolism/catabolism of glucose and amino acids.

Associations between Environmental Tobacco Smoke Exposure in Early Life and Astigmatism among Chinese Preschool Children

This study aimed to investigate the association between environmental exposure to tobacco smoke (ETS) during early life and astigmatism in Chinese preschool children. In this cross-sectional study, information concerning prenatal and postnatal ETS exposure at three stages of early life (during pregnancy, from birth to one year and from one to three years), visual problems of children and parents (including a confirmed diagnosis of astigmatism), socio-demographics and perinatal characteristics were obtained from 27,890 parent-reported questionnaires. Logistic regression analyses were undertaken to yield adjusted odds ratios (OR) for assessing their associations. After adjusting for the potential confounders, children were more likely to exhibit astigmatism when they were exposed to ETS during pregnancy + from one to three years [OR (95% CI) = 1.37 (1.02, 1.84)], or from birth to one year + from one to three years [OR (95% CI) = 1.36 (1.11, 1.66)], or during pregnancy + from birth to one year + from one to three years old [OR (95% CI) = 1.29 (1.16, 1.45)], compared to children without ETS exposure at any stage of early life. In Chinese preschool children, prenatal and postnatal astigmatism was associated with ETS exposure; the greater the ETS dose, the greater the astigmatism risk.

Epigenetically dysregulated genes and pathways implicated in the pathogenesis of non-syndromic high myopia

Myopia, commonly referred to as nearsightedness, is one of the most common causes of visual disability throughout the world. It affects more people worldwide than any other chronic visual impairment condition. Although the prevalence varies among various ethnic groups, the incidence of myopia is increasing in all populations across globe. Thus, it is considered a pressing public health problem. Both genetics and environment play a role in development of myopia. To elucidate the epigenetic mechanism(s) underlying the pathophysiology of high-myopia, we conducted methylation profiling in 18 cases and 18 matched controls (aged 4–12 years), using Illumina MethylationEPIC BeadChips array. The degree of myopia was variable among subjects, ranging from −6 to −15D. We identified 1541 hypermethylated CpGs, representing 1745 genes (2.0-fold or higher) (false discovery rate (FDR) p ≤ 0.05), multiple CpGs were p < 5 × 10⁻⁸ with a receiver operating characteristic area under the curve (ROC-AUC) ≥ 0.75 in high-myopia subjects compared to controls. Among these, 48 CpGs had excellent correlation (AUC ≥ 0.90). Herein, we present the first genome-wide DNA methylation analysis in a unique high-myopia cohort, showing extensive and discrete methylation changes relative to controls. The genes we identified hold significant potential as targets for novel therapeutic intervention either alone, or in combination.

Prevalence and Risk Factors of Amblyopia among Refractive Errors in an Eastern European Population

Background and objective: Amblyopia is the leading cause of visual impairment in children and adults and is very common during childhood. The aim of this study was to identify the prevalence and the risk factors of amblyopia in a pediatric population with refractive errors from an Eastern European country. Materials and methods: A total of 1231 children aged 5–16 years, who had refractive errors and were examined from January to August 2017, were enrolled in a cross-sectional population-based study. Every child underwent a complete ophthalmological exam. Amblyopia was defined as a visual acuity (VA) of less than 0.63. The study respected the Multi-Ethnic Pediatric Eye Disease Study (MEPEDS) criteria for defining amblyopia (MEPEDS, 2008). Parents participated in a face-to-face interview. The questionnaire contained details about their family history of amblyopia; the child’s maternal nutritional status in the preconception period; their history of maternal smoking or work in a toxic environment; the child’s birth, and the child’s history of congenital naso-lacrimal duct obstruction (CNLDO). Results: Amblyopia was identified in 2.8% of the participants. The ocular conditions hyperopia (p = 0.0079), astigmatism (p = 0.046), anisometropia (p < 0.001), esotropia (p < 0.001), exotropia (p = 0.0195), and CNLDO (p < 0.001), as well as a family history of amblyopia (p < 0.001), were associated with amblyopia. The non-ocular risk factors for amblyopia that were found in the study included low birth weight (p < 0.0009), prematurity (p < 0.001), an Apgar score under 7 (p = 0.0008), maternal age, maternal smoking history or work in toxic environment (p < 0.001), and maternal body mass index in the preconception period (p < 0.003). Conclusions: Some of the risk factors we identified for amblyopia are modifiable factors. This is an important observation as an adequate health education program can provide the relevant information for future mothers that will allow for a better management of the condition. We also wanted to highlight the need for amblyopia screening starting from the age of 3 years in case of significant parental refractive errors, strabismus, prematurity, and maternal risk factors.

A subset of ipRGCs regulates both maturation of the circadian clock and segregation of retinogeniculate projections in mice

The visual system consists of two major subsystems, image-forming circuits that drive conscious vision and non-image-forming circuits for behaviors such as circadian photoentrainment. While historically considered non-overlapping, recent evidence has uncovered crosstalk between these subsystems. Here, we investigated shared developmental mechanisms. We revealed an unprecedented role for light in the maturation of the circadian clock and discovered that intrinsically photosensitive retinal ganglion cells (ipRGCs) are critical for this refinement process. In addition, ipRGCs regulate retinal waves independent of light, and developmental ablation of a subset of ipRGCs disrupts eye-specific segregation of retinogeniculate projections. Specifically, a subset of ipRGCs, comprising ~200 cells and which project intraretinally and to circadian centers in the brain, are sufficient to mediate both of these developmental processes. Thus, this subset of ipRGCs constitute a shared node in the neural networks that mediate light-dependent maturation of the circadian clock and light-independent refinement of retinogeniculate projections.

Anomalous Vascular Patterns of the Optic Disc in Amblyopia

PURPOSE

To evaluate the presence of anomalies of the optic disc vascular patterns in subjects with amblyopia.

DESIGN

Case controlled study. The outcome measure is temporal deviation of central retinal vessels and / or vascular pattern asymmetry.

METHODS

A total of 342 hyperopic amblyopic patients were evaluated. An additional 88 subjects with equal visual acuity with strabismus, and 108 subjects with equal visual acuity and anisometropia were evaluated.

RESULTS

In this study, more than 50% of amblyopic patients had optic disc deformities. Sixty-five percent of subjects with amblyopia, orthophoria, and anisometropia exceeding 1.5 D had these anomalies compared with less than 7% of the groups without amblyopia or strabismus.

CONCLUSIONS

These disc anomalies may be associated with disorganization of optic nerve axons. This finding indicates the possibility of a peripheral cause for impaired vision and could provide an explanation for the variability in visual defects associated with amblyopia. Optic nerve imaging should be incorporated into clinical protocols for amblyopia research.

Early onset West syndrome with severe hypomyelination and coloboma-like optic discs in a girl with SPTAN1 mutation

Recent study has shown that mutations in the alpha-II-spectrin (SPTAN1) gene cause early onset intractable seizures, severe developmental delay, diffuse hypomyelination, and widespread brain atrophy. We report a Slovene girl with hypotonia, lack of visual attention, early onset epileptic encephalopathy, and severe developmental delay. The patient presented with segmental myoclonic jerks at the age of 6 weeks, and infantile spasms at the age of 3.5 months. Her seizures were resistant to treatment. Multiple electroencephalography recordings showed deterioration of the background activity, followed by multifocal abnormalities before progressing to hypsarrhythmia. Ophthalmologic examination revealed bilateral dysplastic, coloboma-like optic discs. Brain magnetic resonance imaging showed diffusely reduced white matter and brainstem volumes with hypomyelination. A de novo heterozygous in-frame deletion was detected in SPTAN1: c.6619_6621delGAG (p.E2270del). This report supports the causative relationship between SPTAN1 mutations and early onset intractable seizures with severe hypomyelination and widespread brain volume reduction. Coloboma-like optic discs might be an additional feature observed in patients with SPTAN1 mutations.

The Challenge of Axonal Path-Finding

Congenital syndromes of altered nervous system connectivity are reviewed along with recent findings on axonal growth: achiasma, congenital nystagmus, congenital horizontal gaze palsy, mirror movements and the syndromes of Kallmann, Wildervanck, Duane and Marcus Gunn. Identical guidance molecules are most likely involved in making axonal connections after injury and during development. Thus, investigations into variants of connectivity may help develop strategies to treat disconnections of axons in the adult.

Optic disc area and retinal area in amblyopia

PURPOSE

To compare retinal area to optic disc rim area ratios of hyperopic normal, strabismic eyes with equal acuity, amblyopic, and fellow eyes.

METHODS

Neuroretinal rim areas of 293 amblyopic and fellow eyes, and 77 non-amblyopic hyperopic right eyes, and 84 non-amblyopic strabismic right eyes were measured by magnification corrected retinal photography and planimetry. Retinal area estimates were based on axial lengths.

INCLUSION CRITERIA

All subjects had bilateral hyperopia. Patients with glaucoma or known optic nerve atrophy were excluded. The normal and strabismic groups had equal visual acuity in each eye better than 20/40. The amblyopic group had acuity worse than 20/40 in one eye uncorrectable with lenses and without gross anatomic defects.

RESULTS

The amblyopic group included 137 with strabismus and 89 with anisometropia exceeding 1.5 diopters. There were highly significant differences between the ratio of retinal area to optic disc rim area of the amblyopic and of normal eyes (unpaired t-test, p = 8.6 x 10(-6)), the amblyopic and strabismic right eyes (unpaired t-test, p = 4.22 x 10(-8)) as well between the fellow and amblyopic eyes (paired t-test, p = 2.13 x 10(-5)). The difference between the normal and strabismic eyes without amblyopia was not significant (p = 0.82). There was a 20 percent increase in the retinal receptor areas of hyperopic amblyopic eyes as compared to hyperopic eyes without amblyopia despite reduced retinal areas in the amblyopic eyes. Dysplastic and/or asymmetric optic discs were present in 163 of 293 (56 percent) amblyopic patients, 47 of 84 (56 percent) strabismic, and 10 of 77 (13 percent) normal patients.

CONCLUSIONS

The increase in the receptor area may be an explanation for diminished acuity and impaired visual function in amblyopic eyes.

Retinal area and optic disc rim area in amblyopic, fellow, and normal hyperopic eyes: a hypothesis for decreased acuity in amblyopia

PURPOSE

Defects in visual functions in amblyopic eyes may have a neuroretinal explanation. The retinal area to optic disc rim area ratios of hyperopic normal, amblyopic, and fellow eyes were evaluated.

DESIGN

Case-controlled study.

PARTICIPANTS

A total of 293 patients with amblyopia and bilateral hyperopia and 77 non-amblyopic bilaterally hyperopic patients without strabismus.

METHODS

Disc areas were measured using magnification correction formulas developed by Bengtsson and Krakau. Axial lengths were determined by ultrasound biometry or laser interferometry with a Zeiss AOL Master (Carl Zeiss Co., Oberkochen, Germany). The visual area of the retina was calculated using axial length measurements.

MAIN OUTCOME MEASURES

Optic disc rim areas, corrected for magnification, retinal areas, and a derived ratio, retinal area/disc rim area (RetA/DRimA).

RESULTS

The RetA/DRimA for the amblyopic eyes was significantly greater than that of the fellow and normal eyes, indicating that amblyopic eyes have larger retinal receptor areas than fellow or normal eyes. The RetA/DRimA of the fellow eyes was smaller than for the amblyopic but larger than that of the normal eyes. These differences were due to smaller optic disc rim areas in the amblyopic and fellow eyes.

CONCLUSIONS

Amblyopic and their fellow eyes, when compared with normal eyes, have reduced innervations of comparable retinal areas. These differences can be attributed to a paucity of nerve fibers, as indicated by the smaller neuroretinal rim areas.

FINANCIAL DISCLOSURE(S)

The authors have no proprietary or commercial interest in any materials discussed in this article.

Expression of unconventional myosin genes during neuronal development in zebrafish

Neuronal migration and growth cone motility are essential aspects of the development and maturation of the nervous system. These cellular events result from dynamic changes in the organization and function of the cytoskeleton, in part due to the activity of cytoskeletal motor proteins such as myosins. Although specific myosins such as Myo2 (conventional or muscle myosin), Myo1, and Myo5 have been well characterized for roles in cell motility, the roles of the majority of unconventional (other than Myo2) myosins in cell motility events have not been investigated. To address this issue, we have undertaken an analysis of unconventional myosins in zebrafish, a premier model for studying cellular and growth cone motility in the vertebrate nervous system. We describe the characterization and expression patterns of several members of the unconventional myosin gene family. Based on available genomic sequence data, we identified 18 unconventional myosin- and 4 Myo2-related genes in the zebrafish genome in addition to previously characterized myosin (1, 2, 3, 5, 6, 7) genes. Phylogenetic analyses indicate that these genes can be grouped into existing classifications for unconventional myosins from mouse and man. In situ hybridization analyses using EST probes for 18 of the 22 identified genes indicate that 11/18 genes are expressed in a restricted fashion in the zebrafish embryo. Specific myosins are expressed in particular neuronal or neuroepithelial cell types in the developing zebrafish nervous system, spanning the periods of neuronal differentiation and migration, and of growth cone guidance and motility.

Spatiotemporal definition of neurite outgrowth, refinement and retraction in the developing mouse cochlea

The adult mammalian cochlea receives dual afferent innervation: the inner sensory hair cells are innervated exclusively by type I spiral ganglion neurons (SGN), whereas the sensory outer hair cells are innervated by type II SGN. We have characterized the spatiotemporal reorganization of the dual afferent innervation pattern as it is established in the developing mouse cochlea. This reorganization occurs during the first postnatal week just before the onset of hearing. Our data reveal three distinct phases in the development of the afferent innervation of the organ of Corti: (1) neurite growth and extension of both classes of afferents to all hair cells (E18-P0); (2) neurite refinement, with formation of the outer spiral bundles innervating outer hair cells (P0-P3); (3) neurite retraction and synaptic pruning to eliminate type I SGN innervation of outer hair cells, while retaining their innervation of inner hair cells (P3-P6). The characterization of this developmental innervation pattern was made possible by the finding that tetramethylrhodamine-conjugated dextran (TMRD) specifically labeled type I SGN. Peripherin and choline-acetyltransferase immunofluorescence confirmed the type II and efferent innervation patterns, respectively, and verified the specificity of the type I SGN neurites labeled by TMRD. These findings define the precise spatiotemporal neurite reorganization of the two afferent nerve fiber populations in the cochlea, which is crucial for auditory neurotransmission. This reorganization also establishes the cochlea as a model system for studying CNS synapse development, plasticity and elimination.

Molecular regulation of visual system development: more than meets the eye

Vertebrate eye development has been an excellent model system to investigate basic concepts of developmental biology ranging from mechanisms of tissue induction to the complex patterning and bidimensional orientation of the highly specialized retina. Recent advances have shed light on the interplay between numerous transcriptional networks and growth factors that are involved in the specific stages of retinogenesis, optic nerve formation, and topographic mapping. In this review, we summarize this recent progress on the molecular mechanisms underlying the development of the eye, visual system, and embryonic tumors that arise in the optic system.

Visual functions without the occipital lobe or after cerebral hemispherectomy in infancy

This paper investigates whether and to what extent vision with awareness is still possible in the whole visual field after loss of the occipital lobe of one or both cerebral hemispheres or after hemispherectomy in childhood. The visual functions of four children who suffered from unilateral or bilateral loss of the occipital lobe or who had been hemispherectomized were examined. The results show that even after unilateral loss of the striate and prestriate cortex the extent of the visual field may still be in the normal range. The residual visual functions may be mediated by intact extrastriate areas such as V5 and LO of the damaged cerebral hemisphere. It is also shown that even after complete hemispherectomy in early life the visual field may have a normal extent and that conscious visual perception in the whole visual field may be preserved. In hemispherectomized children, the remaining cerebral hemisphere or neural structures in the midbrain, including the superior colliculi and the praetectum, may be able to mediate these visual functions.

Getting axons onto the right path: the role of transcription factors in axon guidance

The normal function of the nervous system requires that the constituent neurons are precisely 'wired together'. During embryogenesis, each neuron extends an axonal process, which can navigate a considerable distance to its target. Although a number of the receptors and guidance signals that direct axonal growth have been identified, less is known about the transcription factors that regulate the expression of these molecules within the neuron and its environment. This review examines recent studies in vertebrates and Drosophila that address the identity of the transcription factors that either control the repertoire of guidance receptors and signals that permits an axon to take a particular trajectory or act themselves as novel extracellular guidance factors.

Localized gene expression of axon guidance molecules in neuronal co-cultures

Axonal growth cones are guided to their targets by contact-dependent mechanisms or by diffusible chemotropic factors. Axon guidance by these factors typically involves culturing neurons on an acellular substrate which may not represent the in vivo biological environment. We developed two novel in vitro methods to create patterned gene expression of guidance molecules in a physiologically-relevant cellular environment. In the Matrigel assay, a droplet of adenovirus-Matrigel suspension was placed on astrocytes grown in Matrigel. The adenovirus diffused through the gel and transduced underlying astrocytes, creating a radial infection gradient within a localized area. In the second model, recombinant adenovirus was bound to an anti-hexon antibody adsorbed onto stripe patterns of nitrocellulose. Once the cells were added, only those contacting the adenovirus were infected. The outgrowth pattern of chick DRG neurons on NGF, semaphorin 3A and brevican were studied. As expected, results showed robust axonal growth toward NGF as opposed to either secreted Sema 3A or membrane bound brevican, however subtle differences in axonal growth responses were observed in comparison to those obtained with less physiologically-relevant methods. Novel to this technology, the location and area of molecule expression can be controlled and manipulated in an intricate cellular environment.

Amblyopia prevalence and cigarette smoking by women

The relationship between cigarette smoking by young women and amblyopia in children was investigated in 13 population groups. Two searches were conducted on Medline using the following keywords for the first search: amblyopia, prevalence or incidence, and the names of specific countries. The keywords for the second search were smoking, cigarettes, women and the names of the same specific countries. Relevant articles were reviewed. A positive relationship between the rate of smoking among women and the prevalence of amblyopia in school-aged children and military recruits was found. The Pearson correlation coefficient was 0.73. The occurrence of decreased visual acuity attributed to amblyopia in disparate population groups is directly related to rates of cigarette smoking by young women.

Emergence of realistic retinal networks in culture promoted by the superior colliculus

The developing retina is characterized by 'retinal waves', spontaneous depolarizations that propagate through a developing network of interneurons and retinal ganglion cells. Although the circuitry underlying retinal waves is well characterized, the secreted factors that are critical for its normal development are not defined. Dissociated cell culture provides an ideal system for defining these factors; however, it is difficult to recapitulate retinal circuitry in culture. Here we demonstrate that by culturing dissociated retinal neurons in the presence of cells from the superior colliculus (SC), retinal neurons form networks that are similar to those described in the intact retina. Whole-cell voltage clamp recordings reveal the presence of a spontaneously active network of interneurons. In addition, we observed spontaneous, propagating activity reminiscent of that observed in the intact retina. We propose that the presence of factors secreted from the SC results in the development of networks that reproduce critical features of the intact retina.

Stem cells to replace the optic nerve

Methods that exist now and that might be developed are suggested to replace retinal ganglion cells and their axons in the optic nerve, ultimately to re-establish functional vision in eyes blind from glaucoma.

Optic nerve axons: life and death before birth

In the 2004 Bowman Lecture, I give a panegyric for Sir William Bowman, an estimate of the importance and the epidemiology of anterior visual pathway developmental disorders, followed by a history of the anterior visual system. I review the normal development of the optic nerve and chiasm and the main developmental disorders: Optic Nerve Aplasia, Optic Nerve Hypoplasia and Achiasmia.

Activation of epidermal growth factor receptors directs astrocytes to organize in a network surrounding axons in the developing rat optic nerve

In postnatal developing optic nerves, astrocytes organize their processes in a cribriform network to group axons into bundles. In neonatal rat optic nerves in vivo, the active form of EGFR tyrosine kinase is abundantly present when the organization of astrocytes and axons is most actively occurring. Blocking activity of EGFR tyrosine kinase during the development of rat optic nerves in vivo inhibits astrocytes from extending fine processes to surround axons. In vitro, postnatal optic nerve astrocytes, stimulated by EGF, organize into cribriform structures which look remarkably like the in vivo structure of astrocytes in the optic nerve. In addition, when astrocytes are co-cultured with neonatal rat retinal explants in the presence of EGF, astrocytes that are adjacent to the retinal explants, re-organize to an astrocyte-free zone into which neurites grow out from the retinal tissue. We hypothesize that in the developing optic nerve, EGFR activity directs the formation of a histo-architectural structure of astrocytes which surrounds axons and provides a permissive environment for axon development.

Disruption of gradient expression of Zic3 resulted in abnormal intra-retinal axon projection

The targeting of retinal ganglion axons toward the optic disc is the first step in axon pathfinding in the visual system. The molecular mechanisms involved in guiding the retinal axons to project towards the optic disc are not well understood. We report that a gene encoding a zinc-finger transcription factor, Zic3, is expressed in a periphery-high and center-low gradient in the retina at the stages of active axon extension inside the retina. The gradient expression of Zic3 recedes towards the periphery over the course of development, correlating with the progression of retinal cell differentiation and axonogenesis. Disruption of gradient expression of Zic3 by retroviral overexpression resulted in mis-targeting of retinal axons and some axons misrouted to the sub-retinal space at the photoreceptor side of the retina. Misexpression of Zic3 did not affect neurogenesis or differentiation inside the retina, or grossly alter retinal lamination. By stripe assay, we show that misexpression of Zic3 may induce the expression of an inhibitory factor to the retinal axons. Zic3 appears to play a role in intra-retinal axon targeting, possibly through regulation of the expression of specific downstream genes involved in axon guidance.

Retinal axon guidance: novel mechanisms for steering

Axons from the retina traverse different molecular territories as they navigate to the tectum. A single territory might span only a few cell diameters and harbour multiple guidance cues, many of which are beginning to be characterized. Also present in the pathway are 'modulators' that influence a growth cone's response to a coincident signal but do not guide growth directly. An emerging principle is that the growth cone, itself, changes molecularly as it journeys through the visual pathway. Growing retinal axons contain mRNAs, ubiquitinating and apoptotic enzymes, translation and degradation machinery. Guidance cues can trigger rapid and local synthesis, degradation and endocytosis of proteins, providing a fast and flexible way for growth cones to respond to cues in their microenvironment and to alter their responsiveness. The data raise the idea that the localized synthesis and downregulation of proteins might help to steer retinal axon growth and, further, might contribute to the changing character of a growth cone as it ages.

Axonal Regeneration of Fish Optic Nerve after Injury

Since Sperry's work in the 1950s, it has been known that the central nervous system (CNS) neurons of lower vertebrates such as fish and amphibians can regenerate after axotomy, whereas the CNS neurons of mammals become apoptotic after axotomy. The goldfish optic nerve (ON) is one of the most studied animal models of CNS regeneration. Morphological changes in the goldfish retina and tectum after ON transection were first researched in the 1970s-1980s. Many biochemical studies of neurite outgrowth-promoting substances were then carried out in the 1980s-1990s. Many factors have been reported to be active substances that show increased levels during fish ON regeneration, as shown by using various protein chemistry techniques. However, there are very few molecular cloning techniques for studying ON regeneration after injury. In this review article, we summarize the neurite outgrowth-promoting factors reported by other researchers and describe our strategies for searching for ON regenerating molecules using a differential hybridization technique in the goldfish visual system. The process of goldfish ON regeneration after injury is very long. It takes about half a year from the start of axonal regrowth to complete restoration of vision. The process has been classified into three stages: early, middle and late. We screened for genes with increased expression during regeneration using axotomized goldfish retinal and tectal cDNA libraries and obtained stage-specific cDNA clones that were upregulated in the retina and tectum. We further discuss functional roles of these molecules in the regeneration processes of goldfish ON.