Opiate and N-methyl-D-aspartate receptors in form-deprivation myopia

Multimodal imaging and functional analysis of the chick NMDA retinal damage model

Objectives

The chick is rapidly becoming a standardized preclinical model in vision research to study mechanisms of ocular disease. We seek to comprehensively evaluate the N-methyl-D-aspartate (NMDA) model of excitotoxic retinal damage using multimodal imaging, functional, and histologic approaches in NMDA-damaged, vehicle-treated, and undamaged chicks.

Methods

Chicks were either left undamaged in both eyes or were injected with NMDA in the left eye and saline (vehicle) in the right eye. TUNEL assay was performed on chicks to assess levels of retinal cell death one day post-injection of NMDA or saline and on age-matched untreated chicks. Spectral domain optical coherence tomography (SD-OCT) was performed weekly on chicks and age-matched controls day 1 (D1) up to D28 post-injection. Light adapted electroretinograms (ERG) were performed alongside SD-OCT measurements on post-injection chicks along with age-matched untreated controls.

Results

Untreated and vehicle-treated eyes had no TUNEL positive cells while NMDA-treated eyes accumulated large numbers of TUNEL positive cells in the Inner Nuclear Layer (INL), but not other layers, at D1 post injection. Significant inner retina swelling or edema was found on SD-OCT imaging at D1 post-injection which resolved at subsequent timepoints. Both the INL and the inner plexiform layer significantly thinned by one-week post-injection and did not recover for the duration of the measurements. On ERG, NMDA-treated eyes had significantly reduced amplitudes of all parameters at D1 with all metrics improving over time. The b-wave, oscillatory potentials, and ON/OFF bipolar responses were the most affected with at least 70% reduction immediately after damage compared to the fellow eye control.

Conclusion

This study establishes a normative baseline on the retinal health and gross functional ability as well as intraocular pressures of undamaged, vehicle-treated, and NMDA-damaged chicks to provide a standard for comparing therapeutic treatment studies in this important animal model.

Functional integration of eye tissues and refractive eye development: Mechanisms and pathways

Refractive eye development is a tightly coordinated developmental process. The general layout of the eye and its various components are established during embryonic development, which involves a complex cross-tissue signaling. The eye then undergoes a refinement process during the postnatal emmetropization process, which relies heavily on the integration of environmental and genetic factors and is controlled by an elaborate genetic network. This genetic network encodes a multilayered signaling cascade, which converts visual stimuli into molecular signals that guide the postnatal growth of the eye. The signaling cascade underlying refractive eye development spans across all ocular tissues and comprises multiple signaling pathways. Notably, tissue-tissue interaction plays a key role in both embryonic eye development and postnatal eye emmetropization. Recent advances in eye biometry, physiological optics and systems genetics of refractive error have significantly advanced our understanding of the biological processes involved in refractive eye development and provided a framework for the development of new treatment options for myopia. In this review, we summarize the recent data on the mechanisms and signaling pathways underlying refractive eye development and discuss new evidence suggesting a wide-spread signal integration across different tissues and ocular components involved in visually guided eye growth.

Pharmacotherapeutic candidates for myopia: A review

This review provides insights into the mechanism underlying the pathogenesis of myopia and potential targets for clinical intervention. Although the etiology of myopia involves both environmental and genetic factors, recent evidence has suggested that the prevalence and severity of myopia appears to be affected more by environmental factors. Current pharmacotherapeutics are aimed at inhibiting environmentally induced changes in visual input and subsequent changes in signaling pathways during myopia pathogenesis and progression. Recent studies on animal models of myopia have revealed specific molecules potentially involved in the regulation of eye development. Among them, the dopamine receptor plays a critical role in controlling myopia. Subsequent studies have reported pharmacotherapeutic treatments to control myopia progression. In particular, atropine treatment yielded favorable outcomes and has been extensively used; however, current studies are aimed at optimizing its efficacy and confirming its safety. Furthermore, future studies are required to assess the efficacy of combinatorial use of low-dose atropine and contact lenses or orthokeratology.

Pharmacogenomic Approach to Antimyopia Drug Development: Pathways Lead the Way

Myopia is the most common eye disorder in the world which is caused by a mismatch between the optical power of the eye and its excessively long axial length. Recent studies revealed that the regulation of the axial length of the eye occurs via a complex signaling cascade, which originates in the retina and propagates across all ocular tissues to the sclera. The complexity of this regulatory cascade has made it particularly difficult to develop effective antimyopia drugs. The current pharmacological treatment options for myopia are limited to atropine and 7-methylxanthine, which have either significant adverse effects or low efficacy. In this review, we focus on the recent advances in genome-wide studies of the signaling pathways underlying myopia development and discuss the potential of systems genetics and pharmacogenomic approaches for the development of antimyopia drugs.

IMI - Report on Experimental Models of Emmetropization and Myopia

The results of many studies in a variety of species have significantly advanced our understanding of the role of visual experience and the mechanisms of postnatal eye growth, and the development of myopia. This paper surveys and reviews the major contributions that experimental studies using animal models have made to our thinking about emmetropization and development of myopia. These studies established important concepts informing our knowledge of the visual regulation of eye growth and refractive development and have transformed treatment strategies for myopia. Several major findings have come from studies of experimental animal models. These include the eye's ability to detect the sign of retinal defocus and undergo compensatory growth, the local retinal control of eye growth, regulatory changes in choroidal thickness, and the identification of components in the biochemistry of eye growth leading to the characterization of signal cascades regulating eye growth and refractive state. Several of these findings provided the proofs of concepts that form the scientific basis of new and effective clinical treatments for controlling myopia progression in humans. Experimental animal models continue to provide new insights into the cellular and molecular mechanisms of eye growth control, including the identification of potential new targets for drug development and future treatments needed to stem the increasing prevalence of myopia and the vision-threatening conditions associated with this disease.

Gene expression in response to optical defocus of opposite signs reveals bidirectional mechanism of visually guided eye growth

Myopia (nearsightedness) is the most common eye disorder, which is rapidly becoming one of the leading causes of vision loss in several parts of the world because of a recent sharp increase in prevalence. Nearwork, which produces hyperopic optical defocus on the retina, has been implicated as one of the environmental risk factors causing myopia in humans. Experimental studies have shown that hyperopic defocus imposed by negative power lenses placed in front of the eye accelerates eye growth and causes myopia, whereas myopic defocus imposed by positive lenses slows eye growth and produces a compensatory hyperopic shift in refractive state. The balance between these two optical signals is thought to regulate refractive eye development; however, the ability of the retina to recognize the sign of optical defocus and the composition of molecular signaling pathways guiding emmetropization are the subjects of intense investigation and debate. We found that the retina can readily distinguish between imposed myopic and hyperopic defocus, and identified key signaling pathways underlying retinal response to the defocus of different signs. Comparison of retinal transcriptomes in common marmosets exposed to either myopic or hyperopic defocus for 10 days or 5 weeks revealed that the primate retina responds to defocus of different signs by activation or suppression of largely distinct pathways. We also found that 29 genes differentially expressed in the marmoset retina in response to imposed defocus are localized within human myopia quantitative trait loci (QTLs), suggesting functional overlap between genes differentially expressed in the marmoset retina upon exposure to optical defocus and genes causing myopia in humans. These findings identify retinal pathways involved in the development of myopia, as well as potential new strategies for its treatment.

The worldwide prevalence of myopia is predicted to increase from the current 23% to about 50% in the next three decades. Although much effort has been directed towards elucidating the mechanisms underlying refractive eye development and myopia, treatment options for myopia are mostly limited to optical correction, which does not prevent progression of myopia nor the pathological blinding complications often associated with the disease. Several experimental optics-based treatments have had only limited effect on myopia progression, and currently available drug treatments are limited and the mechanisms of action are not well understood. The development of safe and effective pharmacological treatments for myopia is urgently needed to prevent the impending myopia epidemic. The main obstacles that prevent the development of anti-myopia drugs are the uncertainties regarding the mechanisms controlling eye growth and optical development, including the molecular signaling pathways underlying it. In this study, we show that, contrary to the conventional thinking that myopic and hyperopic defocus trigger opposite changes in the same genes and pathways to guide postnatal eye growth, defocus of opposite signs affect eye growth via largely distinct retinal pathways. Knowing that myopic and hyperopic defocus signals drive eye growth in opposite directions and propagate via different pathways provides a framework for the development of new anti-myopia drugs. Myopia can be controlled pharmacologically by stimulating pathways underlying the retinal response to positive lenses and/or by suppressing pathways underlying the retinal response to negative lenses.

Pathway analysis identifies altered mitochondrial metabolism, neurotransmission, structural pathways and complement cascade in retina/RPE/ choroid in chick model of form-deprivation myopia

Purpose

RNA sequencing analysis has demonstrated bidirectional changes in metabolism, structural and immune pathways during early induction of defocus induced myopia. Thus, the aim of this study was to investigate whether similar gene pathways are also related to the more excessive axial growth, ultrastructural and elemental microanalytic changes seen during the induction and recovery from form-deprivation myopia (FDM) in chicks and predicted by the RIDE model of myopia.

Methods

Archived genomic transcriptome data from the first three days of induction of monocularly occluded form deprived myopia (FDMI) in chicks was obtained from the GEO database (accession # GSE6543) while data from chicks monocularly occluded for 10 days and then given up to 24 h of normal visual recovery (FDMR) were collected. Gene set enrichment analysis (GSEA) software was used to determine enriched pathways during the induction (FDMI) and recovery (FDMR) from FD. Curated gene-sets were obtained from open access sources.

Results

Clusters of significant changes in mitochondrial energy metabolism, neurotransmission, ion channel transport, G protein coupled receptor signalling, complement cascades and neuron structure and growth were identified during the 10 days of induction of profound myopia and were found to correlate well with change in axial dimensions. Bile acid and bile salt metabolism pathways (cholesterol/lipid metabolism and sodium channel activation) were significantly upregulated during the first 24 h of recovery from 10 days of FDM.

Conclusions

The gene pathways altered during induction of FDM are similar to those reported in defocus induced myopia and are established indicators of oxidative stress, osmoregulatory and associated structural changes. These findings are also consistent with the choroidal thinning, axial elongation and hyperosmotic ion distribution patterns across the retina and choroid previously reported in FDM and predicted by RIDE.

The chick eye in vision research: An excellent model for the study of ocular disease

The domestic chicken, Gallus gallus, serves as an excellent model for the study of a wide range of ocular diseases and conditions. The purpose of this manuscript is to outline some anatomic, physiologic, and genetic features of this organism as a robust animal model for vision research, particularly for modeling human retinal disease. Advantages include a sequenced genome, a large eye, relative ease of handling and maintenance, and ready availability. Relevant similarities and differences to humans are highlighted for ocular structures as well as for general physiologic processes. Current research applications for various ocular diseases and conditions, including ocular imaging with spectral domain optical coherence tomography, are discussed. Several genetic and non-genetic ocular disease models are outlined, including for pathologic myopia, keratoconus, glaucoma, retinal detachment, retinal degeneration, ocular albinism, and ocular tumors. Finally, the use of stem cell technology to study the repair of damaged tissues in the chick eye is discussed. Overall, the chick model provides opportunities for high-throughput translational studies to more effectively prevent or treat blinding ocular diseases.

mTor signaling is required for the formation of proliferating Müller glia-derived progenitor cells in the chick retina

We investigate the roles of mTor signaling in the formation of Müller glia-derived progenitor cells (MGPCs) in the chick retina. During embryonic development, pS6 (a readout of active mTor signaling) is present in early-stage retinal progenitors, differentiating amacrine and ganglion cells, and late-stage progenitors or maturing Müller glia. By contrast, pS6 is present at low levels in a few scattered cell types in mature, healthy retina. Following retinal damage, in which MGPCs are known to form, mTor signaling is rapidly activated in Müller glia. Inhibition of mTor in damaged retinas prevented the accumulation of pS6 in Müller glia and reduced numbers of proliferating MGPCs. Inhibition of mTor had no effect on MAPK signaling or on upregulation of the stem cell factor Klf4, whereas Pax6 upregulation was significantly reduced. Inhibition of mTor potently blocked the MGPC-promoting effects of Hedgehog, Wnt and glucocorticoid signaling in damaged retinas. In the absence of retinal damage, insulin, IGF1 and FGF2 induced pS6 in Müller glia, and this was blocked by mTor inhibitor. In FGF2-treated retinas, in which MGPCs are known to form, inhibition of mTor blocked the accumulation of pS6, the upregulation of Pax6 and the formation of proliferating MGPCs. We conclude that mTor signaling is required, but not sufficient, to stimulate Müller glia to give rise to proliferating progenitors, and the network of signaling pathways that drive the formation of MGPCs requires activation of mTor.

Wnt/β-catenin-signaling and the formation of Müller glia-derived progenitors in the chick retina

Müller glia can be stimulated to de-differentiate, proliferate, and form Müller glia-derived progenitor cells (MGPCs) that are capable of producing retinal neurons. The signaling pathways that influence the de-differentiation of mature Müller glia and proliferation of MGPCs may include the Wnt-pathway. The purpose of this study was to investigate how Wnt-signaling influences the formation of MGPCs in the chick retina in vivo. In NMDA-damaged retinas where MGPCs are known to form, we find dynamic changes in retinal levels of potential readouts of Wnt-signaling, including dkk1, dkk3, axin2, c-myc, tcf-1, and cd44. We find accumulations of nuclear β-catenin in MGPCs that peaks at 3 days and rapidly declines by 5 days after NMDA-treatment. Inhibition of Wnt-signaling with XAV939 in damaged retinas suppressed the formation of MGPCs, increased expression of ascl1a and decreased hes5, but had no effect upon the differentiation of progeny produced by MGPCs. Activation of Wnt-signaling, with GSK3β-inhibitors, in the absence of retinal damage, failed to stimulate the formation of MGPCs, whereas activation of Wnt-signaling in damaged retinas stimulated the formation of MGPCs. In the absence of retinal damage, FGF2/MAPK-signaling stimulated the formation of MGPCs by activating a signaling network that includes Wnt/β-catenin. In FGF2-treated retinas, inhibition of Wnt-signaling reduced numbers of proliferating MGPCs, whereas activation of Wnt-signaling failed to influence the formation of proliferating MGPCs. Our findings indicate that Wnt-signaling is part of a network initiated by FGF2/MAPK or retinal damage, and activation of canonical Wnt-signaling is required for the formation of proliferating MGPCs. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 983-1002, 2016.

Reactive microglia and macrophage facilitate the formation of Müller glia-derived retinal progenitors

In retinas where Müller glia have been stimulated to become progenitor cells, reactive microglia are always present. Thus, we investigated how the activation or ablation of microglia/macrophage influences the formation of Müller glia-derived progenitor cells (MGPCs) in the retina in vivo. Intraocular injections of the Interleukin-6 (IL6) stimulated the reactivity of microglia/macrophage, whereas other types of retinal glia appear largely unaffected. In acutely damaged retinas where all of the retinal microglia/macrophage were ablated, the formation of proliferating MGPCs was greatly diminished. With the microglia ablated in damaged retinas, levels of Notch and related genes were unchanged or increased, whereas levels of ascl1a, TNFα, IL1β, complement component 3 (C3) and C3a receptor were significantly reduced. In the absence of retinal damage, the combination of insulin and Fibroblast growth factor 2 (FGF2) failed to stimulate the formation of MGPCs when the microglia/macrophage were ablated. In addition, intraocular injections of IL6 and FGF2 stimulated the formation of MGPCs in the absence of retinal damage, and this generation of MGPCs was blocked when the microglia/macrophage were absent. We conclude that the activation of microglia and/or infiltrating macrophage contributes to the formation of proliferating MGPCs, and these effects may be mediated by components of the complement system and inflammatory cytokines.

The reactivity, distribution and abundance of Non-astrocytic Inner Retinal Glial (NIRG) cells are regulated by microglia, acute damage, and IGF1

Recent studies have described a novel type of glial cell that is scattered across the inner layers of the avian retina and possibly the retinas of primates. These cells have been termed Non-astrocytic Inner Retinal Glial (NIRG) cells. These cells are stimulated by insulin-like growth factor 1 (IGF1) to proliferate, migrate distally into the retina, and become reactive. These changes in glial activity correlate with increased susceptibility of retinal neurons and Müller glia to excitotoxic damage. The purpose of this study was to further study the NIRG cells in retinas treated with IGF1 or acute damage. In response to IGF1, the reactivity, proliferation and migration of NIRG cells persists through 3 days after treatment. At 7 days after treatment, the numbers and distribution of NIRG cells returns to normal, suggesting that homeostatic mechanisms are in place within the retina to maintain the numbers and distribution of these glial cells. By comparison, IGF1-induced microglial reactivity persists for at least 7 days after treatment. In damaged retinas, we find a transient accumulation of NIRG cells, which parallels the accumulation of reactive microglia, suggesting that the reactivity of NIRG cells and microglia are linked. When the microglia are selectively ablated by the combination of interleukin 6 and clodronate-liposomes, the NIRG cells down-regulate transitin and perish within the following week, suggesting that the survival and phenotype of NIRG cells are somehow linked to the microglia. We conclude that the abundance, reactivity and retinal distribution of NIRG cells can be dynamic, are regulated by homoestatic mechanisms and are tethered to the microglia.

The combination of IGF1 and FGF2 and the induction of excessive ocular growth and extreme myopia

Different growth factors have been shown to influence the development of form-deprivation myopia and lens-induced ametropias. However, growth factors have relatively little effect on the growth of eyes with unrestricted vision. We investigate whether the combination of insulin-like growth factor 1 (IGF1) and fibroblast growth factor 2 (FGF2) influence ocular growth in eyes with unrestricted vision. Different doses of IGF1 and FGF2 were injected into the vitreous chamber of postnatal chicks. Measurements of ocular dimensions and intraocular pressure (IOP) were made during and at the completion of different treatment paradigms. Histological and immunocytochemical analyses were performed to assess cell death, cellular proliferation and integrity of ocular tissues. Treated eyes had significant increases in equatorial diameter and vitreous chamber depth. With significant variability between individuals, IGF1/FGF2-treatment caused hypertrophy of lens and ciliary epithelia, lens thickness was increased, and anterior chamber depth was decreased. Treated eyes developed myopia, in excess of 15 diopters of refractive error. Shortly after treatment, eyes had increased intraocular pressure (IOP), which was increased in a dose-dependent manner. Seven days after treatment with IGF1 and FGF2 changes to anterior chamber depth, lens thickness and elevated IOP were reduced, whereas increases in the vitreous chamber were persistent. Some damage to ganglion cells was detected in peripheral regions of the retina at 7 days after treatment. We conclude that the extreme myopia in IGF1/FGF2-treated eyes results from increased vitreous chamber depth, decreased anterior chamber depth, and changes in the lens. We propose that factor-induced ocular enlargement and myopia result from changes to the sclera, lens and anterior chamber depth.

Vision-guided ocular growth in a mutant chicken model with diminished visual acuity

Visual experience is known to guide ocular growth. We tested the hypothesis that vision-guided ocular growth is disrupted in a model system with diminished visual acuity. We examine whether ocular elongation is influenced by form-deprivation (FD) and lens-imposed defocus in the Retinopathy, Globe Enlarged (RGE) chicken. Young RGE chicks have poor visual acuity, without significant retinal pathology, resulting from a mutation in guanine nucleotide-binding protein β3 (GNB3), also known as transducin β3 or Gβ3. The mutation in GNB3 destabilizes the protein and causes a loss of Gβ3 from photoreceptors and ON-bipolar cells (Ritchey et al., 2010). FD increased ocular elongation in RGE eyes in a manner similar to that seen in wild-type (WT) eyes. By comparison, the excessive ocular elongation that results from hyperopic defocus was increased, whereas myopic defocus failed to significantly decrease ocular elongation in RGE eyes. Brief daily periods of unrestricted vision interrupting FD prevented ocular elongation in RGE chicks in a manner similar to that seen in WT chicks. Glucagonergic amacrine cells differentially expressed the immediate early gene Egr1 in response to growth-guiding stimuli in RGE retinas, but the defocus-dependent up-regulation of Egr1 was lesser in RGE retinas compared to that of WT retinas. We conclude that high visual acuity, and the retinal signaling mediated by Gβ3, is not required for emmetropization and the excessive ocular elongation caused by FD and hyperopic defocus. However, the loss of acuity and Gβ3 from RGE retinas causes enhanced responses to hyperopic defocus and diminished responses to myopic defocus.

Muller glia, vision-guided ocular growth, retinal stem cells, and a little serendipity: the Cogan lecture

Hypothesis-driven science is expected to result in a continuum of studies and findings along a discrete path. By comparison, serendipity can lead to new directions that branch into different paths. Herein, I describe a diverse series of findings that were motivated by hypotheses, but driven by serendipity. I summarize how investigations into vision-guided ocular growth in the chick eye led to the identification of glucagonergic amacrine cells as key regulators of ocular elongation. Studies designed to assess the impact of the ablation of different types of neurons on vision-guided ocular growth led to the finding of numerous proliferating cells within damaged retinas. These proliferating cells were Müller glia-derived retinal progenitors with a capacity to produce new neurons. Studies designed to investigate Müller glia-derived progenitors led to the identification of a domain of neural stem cells that form a circumferential marginal zone (CMZ) that lines the periphery of the retina. Accelerated ocular growth, caused by visual deprivation, stimulated the proliferation of CMZ progenitors. We formulated a hypothesis that growth-regulating glucagonergic cells may regulate both overall eye size (scleral growth) and the growth of the retina (proliferation of CMZ cells). Subsequent studies identified unusual types of glucagonergic neurons with terminals that ramify within the CMZ; these cells use visual cues to control equatorial ocular growth and the proliferation of CMZ cells. Finally, while studying the signaling pathways that stimulate CMZ and Müller glia-derived progenitors, serendipity led to the discovery of a novel type of glial cell that is scattered across the inner retinal layers.

Image defocus and altered retinal gene expression in chick: clues to the pathogenesis of ametropia

PURPOSE

Because of the retina's role in refractive development, this study was conducted to analyze the retinal transcriptome in chicks wearing a spectacle lens, a well-established means of inducing refractive errors, to identify gene expression alterations and to develop novel mechanistic hypotheses about refractive development.

METHODS

One-week-old white Leghorn chicks wore a unilateral spectacle lens of +15 or -15 D for 6 hours or 3 days. With total RNA from the retina/(retinal pigment epithelium, RPE), chicken gene microarrays were used to compare gene expression levels between lens-wearing and contralateral control eyes (n = 6 chicks for each condition). Normalized microarray signal intensities were evaluated by analysis of variance, using a false discovery rate of <10% as the statistical criterion. Selected differentially expressed genes were validated by qPCR.

RESULTS

Very few retina/RPE transcripts were differentially expressed after plus lens wear. In contrast, approximately 1300 transcripts were differentially expressed under each of the minus lens conditions, with minimal overlap. For each condition, low fold-changes typified the altered transcriptome. Differentially regulated genes under the minus lens conditions included many potentially informative signaling molecules and genes whose protein products have roles in intrinsic retinal circadian rhythms.

CONCLUSIONS

Plus or minus lens wear induce markedly different, not opposite, alterations in retina/RPE gene expression. The initial retinal responses to defocus are quite different from those when the eye growth patterns are well established, suggesting that different mechanisms govern the initiation and persistence or progression of refractive errors. The gene lists identify promising signaling candidates and regulatory pathways for future study, including a potential role for circadian rhythms in refractive development.

beta-Endorphin expression in the mouse retina

Evidence showing expression of endogenous opioids in the mammalian retina is sparse. In the present study we examined a transgenic mouse line expressing an obligate dimerized form of Discosoma red fluorescent protein (DsRed) under the control of the pro-opiomelanocortin promoter and distal upstream regulatory elements to assess whether pro-opiomelanocortin peptide (POMC), and its opioid cleavage product, beta-endorphin, are expressed in the mouse retina. Using double label immunohistochemistry we found that DsRed fluorescence was restricted to a subset of GAD-67-positive cholinergic amacrine cells of both orthotopic and displaced subtypes. About 50% of cholinergic amacrine cells colocalized DsRed and a large fraction of DsRed-expressing amacrine cells was positive for beta-endorphin immunostaining, whereas beta-endorphin-immunoreactive neurons were absent in retinas of POMC null mice. Our findings contribute to a growing body of evidence demonstrating that opioid peptides are an integral component of vertebrate retinas, including those of mammals.

Sonic hedgehog expression and its role in form-deprivation myopia in mice

PURPOSE

To investigate whether sonic hedgehog (Shh) plays a role in postnatal eye development and the development of experimentally induced myopia.

METHODS

Expression of Shh, Patched-1 (Ptc-1), and Gli3 was evaluated in the eyes of 13- to 14-day-old C57B/L6 mice with form-deprivation myopia (FDM) (n = 100) and controls (n = 100) using real-time PCR and Western blot analysis. In a second experiment, 336 mice were divided into two groups: the first wore a unilateral translucent diffuser to induce myopia and the second served as a control. Both groups received four intravitreal injections of either Shh-N (Sonic hedgehog amino-terminal peptide) or cyclopamine (a specific inhibitor of the Shh pathway) every other day. Retinoscopic refraction and axial length measurements were performed on the 11th day of form deprivation. Sections of the eyes were observed using a light microscope.

RESULTS

Inducing myopia caused a significant increase in expression of Shh mRNA (7 days: t = 6.09, p = 0.004; 14 days: t = 3.48, p = 0.025) and protein (7 days: t = 4.06, p = 0.015; 14 days: t = 4.25, p = 0.013). Expression of both Gli3 mRNA (t = 7.61, p = 0.002) and protein (t = 2.89, p = 0.045) increased after 7 days of occlusion. Administration of Shh-N stimulated the development of myopia and axial growth in both occluded (refraction: F = 7.49, p = 0.001; axial length: F = 9.89, p < 0.001) and non-occluded eyes (refraction: F = 14.20, p < 0.001; axial length: F = 20.37, p < 0.001). Cyclopamine reduced myopic refractive error and axial elongation in occluded eyes (refraction: F = 27.91, p < 0.001; axial length: F = 15.18, p < 0.001). It also influenced non-occluded eyes, reducing axial growth and shifting the refraction toward hyperopia (refraction: F = 14.81, p < 0.001; axial length: F = 3.99, p = 0.024). No difference in retinal thickness was found between experimental and control eyes.

CONCLUSIONS

The Shh signaling pathway may influence both form-deprivation myopia and the postnatal growth of eyes with normal visual input.

Bullwhip neurons in the retina regulate the size and shape of the eye

Bullwhip and mini-bullwhip cells are unconventional types of retinal neurons that utilize the neuropeptides glucagon, glucagon-like peptide 1 (GLP1) and substance P. These cells have been implicated in regulating the proliferation of neural progenitors in the circumferential marginal zone (CMZ) of the chicken retina. The purpose of this study was to investigate the roles of the bullwhip cells in regulating ocular size and shape. We found that intravitreal delivery of colchicine at postnatal day 7 destroys the vast majority (approximately 98%) of the bullwhip and mini-bullwhip cells and their peptidergic terminals that are concentrated in the CMZ near the equator of the eye. Interestingly, colchicine-treatment resulted in excessive ocular growth that involved the expansion of equatorial diameter, but not axial length. Intraocular injections of glucagon completely prevented the equatorial expansion that occurs with colchicine-treatment. In eyes with undamaged retinas, exogenous glucagon suppressed equatorial eye growth, whereas glucagon receptor antagonists caused excessive equatorial growth. Furthermore, visual stimuli that increase or decrease rates of ocular growth caused a down- or up-regulation, respectively, of the immediate early gene Egr1 in the bullwhip cells; indicating that the activity of the bullwhip cells is regulated by growth-guiding visual cues. We found that the glucagon receptor was expressed by cells in the fibrous and cartilaginous sclera in equatorial regions of the eye. Taken together, these findings suggest that glucagon peptide released from the terminals of the bullwhip and mini-bullwhip cells regulates the growth of the equatorial sclera in a vision-dependent manner. Although the bullwhip and mini-bullwhip cells are not abundant, less than 1000 cells per retina, their influence on the development of the eye is substantial and includes vision-guided ocular growth.

Effects of muscarinic antagonists on ZENK expression in the chicken retina

Muscarinic antagonists, particularly atropine, can inhibit myopia development in several animal models and also in children. However, the biochemical basis of the inhibition of axial eye growth remains obscure, and there are doubts whether muscarinic receptors are involved at all. Experiments in chickens and monkeys have shown that the synthesis of the transcription factor ZENK, also named Egr-1, in retinal glucagon amacrine cells is strongly associated with inhibition of axial eye growth (assumed to create a STOP signal). We have tested whether the muscarinic antagonists atropine, pirenzepine, oxyphenonium, gallamine, MT-3, himbacine, and 4-DAMP can stimulate ZENK expression so that the drugs' inhibitory effect on myopia development could be explained by an enhanced STOP signal. Because it is known that intravitreal quisqualic acid (QA) eliminates most cholinergic neurons in the retina within 6 or 7 days, in a second set of experiments, we tested whether these antagonists could still stimulate ZENK production, 6 days after QA was applied. Muscarinic antagonists, injected intravitreally at various concentrations, affected ZENK synthesis in various and unpredictable ways. Pirenzepine, oxyphenonium, and MT-3 increased the proportion of glucagon cells that were ZENK-immunoreactive, whereas himbacine decreased that proportion, and gallamine and 4-DAMP had no significant effect. Atropine caused an upregulation of ZENK only if all positive amacrine and bipolar cells were counted and therefore appeared to affect primarily cells other than glucagon amacrines. The pattern of results remained unchanged after ablation of most cholinergic neurons by QA. Our results suggest that at least some muscarinic antagonists do not activate cells that synthesize ZENK when they inhibit axial eye growth. Therefore, in line with other studies they also cast doubt on the assumption that muscarinic transmission is crucial, and they suggest that muscarinic antagonists may inhibit myopia through extraretinal target sites or through non-cholinergic retinal actions.

Ionotropic glutamate receptors during the development of the chick retina

Glutamate is the main neurotransmitter of photoreceptors, bipolar cells, and ganglion cells of the vertebrate retina. Three main classes of ionotropic glutamate receptors comprising different subunits can be distinguished: AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionate), KA (kainate), and NMDA (N-methyl-D-aspartate). This study was undertaken to characterize the AMPA (GluR1, GluR2/3, and GluR4), KA (GluR5/6/7), and NMDA (NR1) ionotropic glutamate receptor subunits and to determine their distribution during the development of the chick retina by Western blotting and immunohistochemistry. Western blotting analysis at 1 day after hatching indicated that the antibodies against GluR1, 2/3, 4, and 5/6/7 and NR1 recognized specifically a single band of 100-110 kDa. In turn, immunohistochemistry at P1 showed that all subunits were expressed in cells of the inner nuclear and ganglion cell layers of the chick retina, mostly amacrine and ganglion cells, and their processes in the inner plexiform layer. In addition, stained processes in the outer plexiform layer were observed with the antibodies against GluR2/3, GluR4, and GluR5/6/7. Although all subunits appeared around E5-E6 in the prospective ganglion cell layer, and later in the prospective inner nuclear layer, the distribution of cells containing these glutamate receptor subunits revealed distinct ontogenetic patterns. This multiplicity of glutamate receptors may contribute to different processes that occur in the chick retina during development.

Stimulus deprivation myopia in human congenital ptosis: a study of 95 patients

PURPOSE

To establish differences between the frequency of suspected deprivation myopia in unilateral and bilateral congenital ptosis with and without covered optical axis.

METHODS

Ametropia was evaluated in both eyes of 95 patients with congenital ptosis. The amount of refraction was documented as spherical equivalent (100% cycloplegia). Statistical analysis was performed using the chi-square and sign tests.

RESULTS

In unilateral ptosis, the frequency of myopia was lower (10/68: 15%) than that of hyperopia (58/68: 85%) in the ptotic eye (P <0.001). However, myopia occurred more often in the ptotic eye (10/68: 15%) than in the fellow eye (3/68: 4.4%). Myopic anisometropia was found only in the ptotic eye (5/68 vs 0/68), but was less frequent than hyperopic anisometropia (6/68 vs 8/68). In bilateral ptosis 7/54 myopia as compared with 47/54 hyperopia were observed and 1/27 myopic anisometropia vs 6/27 hyperopic anisometropia. Covered center of the pupil, in children < or = 8 years of age, was associated with myopia more frequently in bilateral than in unilateral ptosis (6/30 vs 1/27). We found a significantly higher rate of myopia <-1 diopter and hyperopia >2 diopter in comparison of children 5 to 7 years old with first-grade school children.

CONCLUSIONS

Two expected results were (1) compared with the normal population, an overall higher frequency of myopia in human congenital ptosis; (2) in unilateral ptosis, a higher frequency of myopia in the ptotic, than in the fellow eye.