The effect of vasoactive intestinal peptide on development of form deprivation myopia in the chick: a pharmacological and immunocytochemical study

Unveiling the gut microbiota and metabolite profiles in guinea pigs with form deprivation myopia through 16S rRNA gene sequencing and untargeted metabolomics

Aim

The aim of this study was to confirm the presence of the form deprivation myopia (FDM) guinea pig eye-gut axis and investigate the relationship between serum vasoactive intestinal peptide (VIP), lipopolysaccharides (LPS), specific gut microbiota and their metabolites.

Method

20 specific-pathogen-free (SPF) guinea pigs were divided into the FDM and the control(Con) group. Following model induction, serum levels of VIP and LPS were quantified. A combination of 16S ribosomal ribosomal Ribonucleic Acid (rRNA) gene sequencing, non-targeted metabolomics and bioinformatics analysis were employed to identify disparities in gut microbiota and metabolites between the two groups of guinea pigs.

Result

Compared to the control group, FDM guinea pigs exhibited a significant trend towards myopia, along with significantly elevated concentrations of LPS and VIP (p < 0.0001). Furthermore, Ruminococcus_albus emerged as the predominant bacterial community enriched in FDM (p < 0.05), and demonstrated positive correlations with 10 metabolites, including l-Glutamic acid, Additionally, Ruminococcus_albus exhibited positive correlations with VIP and LPS levels (p < 0.05).

Conclusion

The findings suggest that the Ruminococcus_Albus and glutamate metabolic pathways play a significant role in myopia development, leading to concurrent alterations in serum VIP and LPS levels in FDM guinea pigs. This underscores the potential of specific gut microbiota and their metabolites as pivotal biomarkers involved in the pathogenesis of myopia.

Retinal VIP-amacrine cells: their development, structure, and function

Amacrine cells (ACs) are the most structurally and functionally diverse neuron type in the retina. Different ACs have distinct functions, such as neuropeptide secretion and inhibitory connection. Vasoactive intestinal peptide (VIP) -ergic -ACs are retina gamma-aminobutyric acid (GABA) -ergic -ACs that were discovered long ago. They secrete VIP and form connections with bipolar cells (BCs), other ACs, and retinal ganglion cells (RGCs). They have a specific structure, density, distribution, and function. They play an important role in myopia, light stimulated responses, retinal vascular disease and other ocular diseases. Their significance in the study of refractive development and disease is increasing daily. However, a systematic review of the structure and function of retinal VIP-ACs is lacking. We discussed the detailed characteristics of VIP-ACs from every aspect across species and providing systematic knowledge base for future studies. Our review led to the main conclusion that retinal VIP-ACs develop early, and although their morphology and distribution across species are not the same, they have similar functions in a wide range of ocular diseases based on their function of secreting neuropeptides and forming inhibitory connections with other cells.

Transcriptional profiling of the chick retina identifies down-regulation of VIP and UTS2B genes during early lens-induced myopia

Gene expression of the chick retina was examined during the early development of lens-induced myopia (LIM) using whole transcriptome sequencing. Monocular treatment of the right eyes with −10 diopter (D)...

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.

Genome-wide analysis of retinal transcriptome reveals common genetic network underlying perception of contrast and optical defocus detection

BACKGROUND

Refractive eye development is regulated by optical defocus in a process of emmetropization. Excessive exposure to negative optical defocus often leads to the development of myopia. However, it is still largely unknown how optical defocus is detected by the retina.

METHODS

Here, we used genome-wide RNA-sequencing to conduct analysis of the retinal gene expression network underlying contrast perception and refractive eye development.

RESULTS

We report that the genetic network subserving contrast perception plays an important role in optical defocus detection and emmetropization. Our results demonstrate an interaction between contrast perception, the retinal circadian clock pathway and the signaling pathway underlying optical defocus detection. We also observe that the relative majority of genes causing human myopia are involved in the processing of optical defocus.

CONCLUSIONS

Together, our results support the hypothesis that optical defocus is perceived by the retina using contrast as a proxy and provide new insights into molecular signaling underlying refractive eye development.

The biological basis of myopic refractive error

Myopia is among the most common refractive errors and is associated with the greatest risk of pathological outcomes. Most animals, including humans, are born with hyperopic errors. During development, axial elongation of the eye occurs and is regulated through a vision-dependent process, known as emmetropisation The extremely rapid changes in the prevalence of myopia and the dependence of myopia on the level of education indicate that there are very strong environmental impacts on the development of myopia. This conflicts with the common occurrence of familial patterns of inheritance of myopia, which suggests a role for genetic determination. There are more than 150 defined genetic syndromes in which familial high myopia is one of the features, including some that are not associated with other syndromes. The evidence for the roles of both nature and nurture in the aetiology of myopia is discussed. This review also examines the experimentally induced refractive errors associated with form-deprivation, recovery from form deprivation and the effects of both negative and positive lenses. In addition, it looks at the local and optical control of eye growth. Finally, the various control pathways for growth are considered. These include dopamine, ZENK-glucagon, retinoic acid and retinoic acid receptors, crystallin, seratonin and melatonin, vasoactive intestinal peptide and enkephalins, nitric oxide and various growth factors.

Dysfunction of VIPR2 leads to myopia in humans and mice

Background

Myopia is the leading cause of refractive errors. As its pathogenesis is poorly understood, we determined if the retinal VIP-VIPR2 signalling pathway axis has a role in controlling signalling output that affects myopia development in mice.

Methods

Association analysis meta-study, single-cell transcriptome, bulk RNA sequencing, pharmacological manipulation and VIPR2 gene knockout studies were used to clarify if changes in the VIP-VIPR2 signalling pathway affect refractive development in mice.

Results

The SNP rs6979985 of the VIPR2 gene was associated with high myopia in a Chinese Han cohort (randomceffect model: p=0.013). After either 1 or 2 days’ form deprivation (FD) retinal VIP mRNA expression was downregulated. Retinal single-cell transcriptome sequencing showed that VIPR2 was expressed mainly by bipolar cells. Furthermore, the cAMP signalling pathway axis was inhibited in some VIPR2+ clusters after 2 days of FD. The selective VIPR2 antagonist PG99-465 induced relative myopia, whereas the selective VIPR2 agonist Ro25-1553 inhibited this response. In Vipr2 knockout (Vipr2-KO) mice, refraction was significantly shifted towards myopia (p<0.05). The amplitudes of the bipolar cell derived b-waves in 7-week-old Vipr2-KO mice were significantly larger than those in their WT littermates (p<0.05).

Conclusions

Loss of VIPR2 function likely compromises bipolar cell function based on presumed changes in signal transduction due to altered signature electrical wave activity output in these mice. As these effects correspond with increases in form deprivation myopia (FDM), the VIP-VIPR2 signalling pathway axis is a viable novel target to control the development of this condition.

The acute effect of atropine eye drops on the human full-field electroretinogram

PURPOSE

Atropine eye drops are a common and effective treatment for slowing myopia progression, but the site and mode of action of atropine in controlling myopia are unclear. We investigated the early retinal sites of action of atropine by examining its effects on the human full-field electroretinogram (ffERG).

METHOD

Baseline ffERGs were recorded in both eyes of 24 healthy subjects (mean ± SD: 21.0 ± 2.3 years; spherical equivalent refraction, range: + 1.63 to - 0.75 D) using 6 standard ISCEV protocols, 30 min after bilateral pupil dilation with 1% Tropicamide. Atropine (1 drop, 0.1%) was then instilled into the non-dominant eye. 24 h later, ffERGs were again recorded in both eyes. Ratios (post-atropine: pre-atropine) of dark-adapted (DA) and light-adapted (LA) ffERGs were compared between atropine-treated and control eyes using multivariate repeated measures general linear models.

RESULTS

Atropine-treated eyes responded with 14% lower DA3.0 OP (oscillatory potential) amplitude (p = 0.003) and 4% delay in the DA10.0 a-wave peak time (p = 0.00099) compared with control eyes. Amplitudes and peak times were not different between atropine-treated and control eyes for DA0.01, LA3.0, and LA3.0 flicker ERGs. While atropine caused a small (1.26 mm², p = 0.03) extra increase in pupil area in the treated eye, atropine-induced changes in ffERG responses bore no relationship with changes in pupil area (R²= 2-5%, p > 0.05).

CONCLUSIONS

The observed changes in oscillatory potentials corroborate previous findings that atropine affects neural activity in the inner retina. However, observed changes to the a-wave suggest that atropine also affects activity in photoreceptors.

Pathogenesis and Prevention of Worsening Axial Elongation in Pathological Myopia

PURPOSE

This review discusses the etiology and pathogenesis of myopia, prevention of disease progression and worsening axial elongation, and emerging myopia treatment modalities.

INTRODUCTION

Pediatric myopia is a public health concern that impacts young children worldwide and is associated with numerous future ocular diseases such as cataract, glaucoma, retinal detachment and other chorioretinal abnormalities. While the exact mechanism of myopia of the human eye remains obscure, several studies have reported on the role of environmental and genetic factors in the disease development.

METHODS

A review of literature was conducted. PubMed and Medline were searched for combinations and derivatives of the keywords including, but not limited to, "pediatric myopia", "axial elongation", "scleral remodeling" or "atropine." The PubMed and Medline database search were performed for randomized control trials, systematic reviews and meta-analyses using the same keyword combinations.

RESULTS

Studies have reported that detection of genetic correlations and modification of environmental influences may have a significant impact in myopia progression, axial elongation and future myopic ocular complications. The conventional pharmacotherapy of pediatric myopia addresses the improvement in visual acuity and prevention of amblyopia but does not affect axial elongation or myopia progression. Several studies have published varying treatments, including optical, pharmacological and surgical management, which show great promise for a more precise control of myopia and preservation of ocular health.

DISCUSSION

Understanding the role of factors influencing the onset and progression of pediatric myopia will facilitate the development of successful treatments, reduction of disease burden, arrest of progression and improvement in future of the management of myopia.

Association of VIPR2 and ZMAT4 with high myopia

Background: To investigate the associations of Single Nucleotide Polymorphisms (SNPs) in the VIPR2 and ZMAT4 genes with high myopia in a Han Chinese population.Materials and Methods: In this case-control genetic association study comprising 193 high myopia participants and 135 normal emmetropic controls from a Han Chinese population, 15 SNPs from the VIPR2 and ZMAT4 genes were selected for genotyping based on previous studies. Allelic frequencies of the SNPs and haplotypes were compared for association with high myopia and axial length (AL).Results: RS885863 (G-reference/A-effect) and RS7829127 (A-reference/G-effect) were significantly associated with high myopia (OR = 1.832, P = .045; OR = 0.539, P = .023 respectively). The associations of RS885863 with high myopia were observed under the dominant (GA+AA: OR = 1.972, P < .05) and co-dominant models (Heterozygous GA: OR = 1.874; Homozygous AA: OR = 5.310; P < .05) against GG (reference). The mean AL of GG was 25.94 mm, compared with that in GA and AA of 26.64 mm and 27.48 mm respectively. The associations of RS7829127 with high myopia were observed under the dominant (AG+GG: OR = 0.512, P < .05) and co-dominant models (Heterozygous AG: OR = 0.524; Homozygous GG: OR = 0.307; P < .05) against AA (reference). The mean AL of AA was 26.35 mm, compared with that in AG and GG of 25.62 mm and 25.17 mm respectively. The importance of RS885863 and RS7829127 were also highlighted by their being the constituent SNPs in the haplotypes (ACGA, P = .002; and GA, P = .008 respectively) that were significantly associated with high myopia.Conclusions: Our findings agree that RS885863 from VIPR2 and RS7829127 from ZMAT4 are significantly associated with high myopia in a Han Chinese population.

The myopia susceptibility locus vasoactive intestinal peptide receptor 2 (VIPR2) contains variants with opposite effects

Myopia is the commonest eye disorder in the world. High myopes are predisposed to ocular pathologies. The vasoactive intestinal peptide receptor 2 (VIPR2) gene was identified as a myopia susceptibility locus by our group and another group. We continued to fine-map this locus. A case-control study was performed in 4 sequential stages with a total of 941 highly myopic subjects and 846 control subjects, all unrelated Chinese. Stage 1 experimentally genotyped 64.4% of the entire cohort for 152 single-nucleotide polymorphisms (SNPs) and Stage 2 the remaining subjects for 21 SNPs. Stage 3 combined the genotypes for 21 SNPs for the entire cohort, and identified one group of high-risk haplotypes and one group of protective haplotypes significantly associated with high myopia. Stage 4 imputed genotypes for variants in the VIPR2 region and identified two independent groups of variants: one group with high-risk minor alleles and another with protective minor alleles. Variants within each group were generally in strong linkage disequilibrium among themselves while high-risk variants were in linkage equilibrium with protective variants. Therefore, the VIPR2 locus seems to contain variants with opposite effects. This is the first study that has examined the genetic architecture of a myopia susceptibility locus in detail.

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.

Circadian rhythms, refractive development, and myopia

PURPOSE

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

RECENT FINDINGS

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

SUMMARY

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

The hyperopic effect of narrow-band long-wavelength light in tree shrews increases non-linearly with duration

During postnatal refractive development, an emmetropization mechanism uses refractive error to modulate the growth rate of the eye. Hyperopia (image focused behind the retina) produces what has been described as "GO" signaling that increases growth. Myopia (image focused in front of the retina) produces "STOP" signaling that slows growth. The interaction between GO and STOP conditions is non-linear; brief daily exposure to STOP counteracts long periods of GO. In young tree shrews, long-wavelength (red) light, presented 14 h per day, also appears to produce STOP signals. We asked if red light also shows temporal non-linearity; does brief exposure slow the normal decrease in hyperopia in infant animals? At 11 days after eye opening (DVE), infant tree shrews (n = 5/group) began 13 days of daily treatment (red LEDs, 624 ± 10 or 636 ± 10 nm half peak intensity bandwidth) at durations of 0 h (normal animals, n = 7) or 1, 2, 4, or 7 h. Following each daily red period, colony lighting resumed. A 14 h red group had no colony lights. Refractive state was measured daily; ocular component dimensions at the end of the 13-day red-light period. Even 1 h of red light exposure produced some hyperopia. The average hyperopic shift from normal rose exponentially with duration (time constant 2.5 h). Vitreous chamber depth decreased non-linearly with duration (time constant, 3.3 h). After red treatment was discontinued, refractions in colony lighting recovered toward normal; the initial rate was linearly related to the amount of hyperopia. The red light may produce STOP signaling similar to myopic refractive error.

Altered gene expression in tree shrew retina and retinal pigment epithelium produced by short periods of minus-lens wear

Hyperopic refractive error is detected by retinal neurons, which generate GO signals through a direct emmetropization signaling cascade: retinal pigment epithelium (RPE) into choroid and then into sclera, thereby increasing axial elongation. To examine signaling early in this cascade, we measured gene expression in the retina and RPE after short exposure to hyperopia produced by minus-lens wear. Gene expression in each tissue was compared with gene expression in combined retina + RPE. Starting 24 days after normal eye opening, three groups of juvenile tree shrews (n = 7 each) wore a monocular -5 D lens. The untreated fellow eye served as a control. The "6h" group wore the lens for 6 h; the "24h" group wore the lens for 24 h; each group provided separate retina and RPE tissues. Group "24hC" wore the lens for 24 h and provided combined retina + RPE tissue. Quantitative PCR was used to measure the relative differences (treated eye vs. control eye) in mRNA levels for 66 candidate genes. In the retina after 6 h, mRNA levels for seven genes were significantly regulated: EGR1 and FOS (early intermediate genes) were down-regulated in the treated eyes. Genes with secreted protein products, BMP2 and CTGF, were down-regulated, whilst FGF10, IL18, and SST were up-regulated. After 24 h the pattern changed; only one of the seven genes still showed differential expression; BMP2 was still down-regulated. Two new genes with secreted protein products, IGF2 and VIP, were up-regulated. In the RPE, consistent with its role in receiving, processing, and transmitting GO signaling, differential expression was found for genes whose protein products are at the cell surface, intracellular, in the nucleus, and are secreted. After 6 h, mRNA levels for 17 genes were down-regulated in the treated eyes, whilst four genes (GJA1, IGF2R, LRP2, and IL18) were up-regulated. After 24 h the pattern was similar; mRNA levels for 14 of the same genes were still down-regulated; only LRP2 remained up-regulated. mRNA levels for six genes no longer showed differential expression, whilst nine genes, not differentially expressed at 6 h, now showed differential expression. In the combined retina + RPE after 24 h, mRNA levels for only seven genes were differentially regulated despite the differential expression of many genes in the RPE. Four genes showed the same expression in combined tissue as in retina alone, including up-regulation of VIP despite significant VIP down-regulation in RPE. Thus, hyperopia-induced GO signaling, as measured by differential gene expression, differs in the retina and the RPE. Retinal gene expression changed between 6 h and 24 h of treatment, suggesting evolution of the retinal response. Gene expression in the RPE was similar at both time points, suggesting sustained signaling. The combined retina + RPE does not accurately represent gene expression in either retina or, especially, RPE. When gene expression signatures were compared with those in choroid and sclera, GO signaling, as encoded by differential gene expression, differs in each compartment of the direct emmetropization signaling cascade.

Novel evidence for complement system activation in chick myopia and hyperopia models: a meta-analysis of transcriptome datasets

Myopia (short-sightedness) and hyperopia (long-sightedness) occur when the eye grows too long or short, respectively, for its refractive power. There are currently approximately 1.45 billion myopes worldwide and prevalence is rising dramatically. Although high myopia significantly increases the risk of developing a range of sight-threatening disorders, the molecular mechanisms underlying ocular growth regulation and its relationship to these secondary complications remain poorly understood. Thus, this study meta-analyzed transcriptome datasets collected in the commonly used chick model of optically-induced refractive error. Fifteen datasets (collected across five previous studies) were obtained from GEO, preprocessed in Bioconductor, and divided into 4 conditions representing early (≤1 day) and late (>1 day) myopia and hyperopia induction. Differentially expressed genes in each condition were then identified using Rank Product meta-analysis. The results provide novel evidence for transcriptional activation of the complement system during both myopia and hyperopia induction, and confirm existing literature implicating cell signaling, mitochondrial, and structural processes in refractive error. Further comparisons demonstrated that the meta-analysis results also significantly improve concordance with broader omics data types (i.e., human genetic association and animal proteomics studies) relative to previous transcriptome studies, and show extensive similarities with the genes linked to age-related macular degeneration, choroidal neovascularization, and cataract.

Vasoactive intestinal peptide, a promising agent for myopia?

AIM

To investigate the role of vasoactive intestinal peptide (VIP) in form-deprivation myopia (FDM).

METHODS

FDM was created in three groups of eight chicks by placing a translucent diffuser on their right eyes. Intravitreal injections of saline and VIP were applied once a day into the occluded eyes of groups 2 and 3, respectively. Retinoscopy and axial length (AL) measurements were performed on the first and 8^th days of diffuser wear. The retina mRNA levels of the VIP receptors and the ZENK protein in right eyes of the three groups and left eyes of the first group on day 8 were determined using real time polymerase chain reaction (PCR).

RESULTS

The median final refraction (D) in right eyes were -13.75 (-16.00, -12.00), -11.50 (-12.50, -7.50), and -1.50 (-4.75, -0.75) in groups 1, 2, and 3, respectively (P<0.001). The median AL (mm) in right eyes were 10.65 (10.00, 11.10), 9.90 (9.70, 10.00), and 9.20 (9.15, 9.25) in groups 1, 2, and 3, respectively (P<0.001). The median delta-delta cycle threshold (CT) values for the VIP2 receptors were 1.07 (0.82, 1.43), 1.22 (0.98, 1.65), 0.29 (0.22, 0.45) in right eyes of groups 1, 2, and 3, and 1.18 (0.90, 1.37) in left eyes of group 1, respectively (P=0.001). The median delta-delta CT values for the ZENK protein were 1.07 (0.63, 5.03), 3.55 (2.20, 5.55), undetectable in right eyes of groups 1, 2, and 3 and 1.89 (0.21, 4.73) in left eyes of group 1, respectively (P=0.001).

CONCLUSION

VIP has potential inhibitory effects in the development of FDM.

Animal models in myopia research

Our current understanding of the development of refractive errors, in particular myopia, would be substantially limited had Wiesel and Raviola not discovered by accident that monkeys develop axial myopia as a result of deprivation of form vision. Similarly, if Josh Wallman and colleagues had not found that simple plastic goggles attached to the chicken eye generate large amounts of myopia, the chicken model would perhaps not have become such an important animal model. Contrary to previous assumptions about the mechanisms of myopia, these animal models suggested that eye growth is visually controlled locally by the retina, that an afferent connection to the brain is not essential and that emmetropisation uses more sophisticated cues than just the magnitude of retinal blur. While animal models have shown that the retina can determine the sign of defocus, the underlying mechanism is still not entirely clear. Animal models have also provided knowledge about the biochemical nature of the signal cascade converting the output of retinal image processing to changes in choroidal thickness and scleral growth; however, a critical question was, and still is, can the results from animal models be applied to myopia in children? While the basic findings from chickens appear applicable to monkeys, some fundamental questions remain. If eye growth is guided by visual feedback, why is myopic development not self-limiting? Why does undercorrection not arrest myopic progression even though positive lenses induce myopic defocus, which leads to the development of hyperopia in emmetropic animals? Why do some spectacle or contact lens designs reduce myopic progression and others not? It appears that some major differences exist between animals reared with imposed defocus and children treated with various optical corrections, although without the basic knowledge obtained from animal models, we would be lost in an abundance of untestable hypotheses concerning human myopia.

Large-Scale microRNA Expression Profiling Identifies Putative Retinal miRNA-mRNA Signaling Pathways Underlying Form-Deprivation Myopia in Mice

Development of myopia is associated with large-scale changes in ocular tissue gene expression. Although differential expression of coding genes underlying development of myopia has been a subject of intense investigation, the role of non-coding genes such as microRNAs in the development of myopia is largely unknown. In this study, we explored myopia-associated miRNA expression profiles in the retina and sclera of C57Bl/6J mice with experimentally induced myopia using microarray technology. We found a total of 53 differentially expressed miRNAs in the retina and no differences in miRNA expression in the sclera of C57BL/6J mice after 10 days of visual form deprivation, which induced -6.93 ± 2.44 D (p < 0.000001, n = 12) of myopia. We also identified their putative mRNA targets among mRNAs found to be differentially expressed in myopic retina and potential signaling pathways involved in the development of form-deprivation myopia using miRNA-mRNA interaction network analysis. Analysis of myopia-associated signaling pathways revealed that myopic response to visual form deprivation in the retina is regulated by a small number of highly integrated signaling pathways. Our findings highlighted that changes in microRNA expression are involved in the regulation of refractive eye development and predicted how they may be involved in the development of myopia by regulating retinal gene expression.

The effect of intravitreal injection of vehicle solutions on form deprivation myopia in tree shrews

lntravitreal injection of substances dissolved in a vehicle solution is a common tool used to assess retinal function. We examined the effect of injection procedures (three groups) and vehicle solutions (four groups) on the development of form deprivation myopia (FDM) in juvenile tree shrews, mammals closely related to primates, starting at 24 days of visual experience (about 45 days of age). In seven groups (n = 7 per group), the myopia produced by monocular form deprivation (FD) was measured daily for 12 days during an 11-day treatment period. The FD eye was randomly selected; the contralateral eye served as an untreated control. The refractive state of both eyes was measured daily, starting just before FD began (day 1); axial component dimensions were measured on day 1 and after eleven days of treatment (day 12). Procedure groups: the myopia (treated eye - control eye refraction) in the FD group was the reference. The sham group only underwent brief daily anesthesia and opening of the conjunctiva to expose the sclera. The puncture group, in addition, had a pipette inserted daily into the vitreous. In four vehicle groups, 5 μL of vehicle was injected daily. The NaCl group received 0.85% NaCl. In the NaCl + ascorbic acid group, 1 mg/mL of ascorbic acid was added. The water group received sterile water. The water + ascorbic acid group received water with ascorbic acid (1 mg/mL). We found that the procedures associated with intravitreal injections (anesthesia, opening of the conjunctiva, and puncture of the sclera) did not significantly affect the development of FDM. However, injecting 5 μL of any of the four vehicle solutions slowed the development of FDM. NaCl had a small effect; myopia development in the last 6 days (-0.15 ± 0.08 D/day) was significantly less than in the FD group (-0.55 ± 0.06 D/day). NaCl + Ascorbic acid further slowed the development of FDM on several treatment days. H2O (-0.09 ± 0.05 D/day) and H2O + ascorbic acid (-0.08 ± 0.05 D/day) both almost completely blocked myopia development. The treated eye vitreous chamber elongation, compared with the control eye, in all groups was consistent with the amount of myopia. When FD continued (days 12-16) without injections in the water and water + ascorbic acid groups, the rate of myopia development quickly increased. Thus, it appears the vehicles affected retinal signaling rather than causing damage. The effect of water and water + ascorbic acid may be due to reduced osmolality or ionic concentration near the tip of the injection pipette. The effect of ascorbic acid, compared to NaCl alone, may be due to its reported dopaminergic activity.

Prevalence of Myopia in France: A Cross-Sectional Analysis

Refractive error (RE), particularly myopia, is the first cause of visual impairment throughout the world. This study aimed to depict the prevalence of myopia in a multicentric series of French individuals.This cross-sectional analysis was carried out between January 2012 and November 2013 in eye clinics dedicated to REs. Data collection included age, gender, best-corrected visual acuity, RE, and any relevant medical history involving laser refractive surgery and cataract surgery. Exclusion criteria consisted of monophthalm patients or those with incomplete demographic data.Prevalences in the overall population, by gender and by age groups were reported for mild myopia (-0.50 to -2.75 diopter [D]), moderate myopia (-3 to -5.75 D), high myopia (less than -6 D), and very high myopia (less than -10 D).The analysis included 100,429 individuals, mean age 38.5 years (± 16.9). Overall prevalence of myopia was 39.1% (95% CI 38.8-39.4). Prevalences of mild, moderate, high and very high myopia were respectively 25.1% (95% CI 25.4-24.9), 10.6% (95% CI 10.4-10.8), 3.4% (95% CI 3.3-3.5) and 0.5% (95% CI 0.48-0.57).Even if possible bias occurred in recruitment, our results are similar to RE data collected in nationally representative samples of Caucasians in other studies. This is to our knowledge, one of the largest European series of individuals dedicated to myopia prevalences in different age groups. These results confirm the importance of myopia as a major health issue in Western countries.

APLP2 Regulates Refractive Error and Myopia Development in Mice and Humans

Myopia is the most common vision disorder and the leading cause of visual impairment worldwide. However, gene variants identified to date explain less than 10% of the variance in refractive error, leaving the majority of heritability unexplained (“missing heritability”). Previously, we reported that expression of APLP2 was strongly associated with myopia in a primate model. Here, we found that low-frequency variants near the 5’-end of APLP2 were associated with refractive error in a prospective UK birth cohort (n = 3,819 children; top SNP rs188663068, p = 5.0 × 10⁻⁴) and a CREAM consortium panel (n = 45,756 adults; top SNP rs7127037, p = 6.6 × 10⁻³). These variants showed evidence of differential effect on childhood longitudinal refractive error trajectories depending on time spent reading (gene x time spent reading x age interaction, p = 4.0 × 10⁻³). Furthermore, Aplp2 knockout mice developed high degrees of hyperopia (+11.5 ± 2.2 D, p < 1.0 × 10⁻⁴) compared to both heterozygous (-0.8 ± 2.0 D, p < 1.0 × 10⁻⁴) and wild-type (+0.3 ± 2.2 D, p < 1.0 × 10⁻⁴) littermates and exhibited a dose-dependent reduction in susceptibility to environmentally induced myopia (F(2, 33) = 191.0, p < 1.0 × 10⁻⁴). This phenotype was associated with reduced contrast sensitivity (F(12, 120) = 3.6, p = 1.5 × 10⁻⁴) and changes in the electrophysiological properties of retinal amacrine cells, which expressed Aplp2. This work identifies APLP2 as one of the “missing” myopia genes, demonstrating the importance of a low-frequency gene variant in the development of human myopia. It also demonstrates an important role for APLP2 in refractive development in mice and humans, suggesting a high level of evolutionary conservation of the signaling pathways underlying refractive eye development.

Gene variants identified by GWAS studies to date explain only a small fraction of myopia cases because myopia represents a complex disorder thought to be controlled by dozens or even hundreds of genes. The majority of genetic variants underlying myopia seems to be of small effect and/or low frequency, which makes them difficult to identify using classical genetic approaches, such as GWAS, alone. Here, we combined gene expression profiling in a monkey model of myopia, human GWAS, and a gene-targeted mouse model of myopia to identify one of the “missing” myopia genes, APLP2. We found that a low-frequency risk allele of APLP2 confers susceptibility to myopia only in children exposed to large amounts of daily reading, thus, providing an experimental example of the long-hypothesized gene-environment interaction between nearwork and genes underlying myopia. Functional analysis of APLP2 using an APLP2 knockout mouse model confirmed functional significance of APLP2 in refractive development and implicated a potential role of synaptic transmission at the level of glycinergic amacrine cells of the retina for the development of myopia. Furthermore, mouse studies revealed that lack of Aplp2 has a dose-dependent suppressive effect on susceptibility to form-deprivation myopia, providing a potential gene-specific target for therapeutic intervention to treat myopia.

The sonic hedgehog signaling pathway induces myopic development by activating matrix metalloproteinase (MMP)-2 in Guinea pigs

Purpose

To investigate whether the Sonic hedgehog (Shh) signaling induces myopic development by increasing the expression of matrix metalloproteinase (MMP)-2 in guinea pigs.

Methods

A translucent diffuser was glued onto the right eye to induce form-deprivation myopia (FDM) in 10 guinea pigs. Four guinea pigs were served as a control group. The other 100 guinea pigs were subdivided into 5 groups (20 per group) and received a 10 µl intravitreal injection every 2 days for 4 times. Two groups were injected with 20 or 50 µg/ml Shh amino-terminal peptide (Shh-N) into the right eye and 0.1% bovine serum albumin into the other. FDM was induced in the right eyes of the three cyclopamine-treated groups and both eyes were injected with 50, 100, or 200 µg/ml cyclopamine. Retinoscopic refraction and eye dimensions were assessed on Day 14 of treatment. MMP-2 protein expression was determined in both scleras by western blotting.

Results

Both concentrations of Shh-N stimulated myopic development and axial growth as compared with control eyes. Myopia and axial elongation were significantly greater in the 50 µg/ml than in the 20 µg/ml Shh-N group (P<0.001 and P = 0.0019, respectively). All three doses of cyclopamine significantly attenuated myopic development compared with the FDM group (P<0.0001). Cyclopamine at 100 or 200 µg/ml significantly reduced axial elongation compared with the FDM group (P = 0.044 and P = 0.001, respectively). FDM-induced myopia and axial elongation were significantly greater in the 50 µg/ml than in the 200 µg/ml cyclopamine group (P<0.0001 and P = 0.008, respectively). MMP-2 expression was significantly greater in Shh-N–treated eyes than in the control eyes, and was lower in the cyclopamine plus FDM groups than in the FDM group.

Conclusions

The Shh signaling pathway induces myopic development by activating MMP-2 in guinea pigs.

Wnt signaling in form deprivation myopia of the mice retina

BACKGROUND

The canonical Wnt signaling pathway plays important roles in cellular proliferation and differentiation, axonal outgrowth, cellular maintenance in retinas. Here we test the hypothesis that elements of the Wnt signaling pathway are involved in the regulation of eye growth and prevention of myopia, in the mouse form-deprivation myopia model.

METHODOLOGY/PRINCIPAL FINDINGS

(1) One hundred twenty-five C57BL/6 mice were randomly distributed into form-deprivation myopia and control groups. Form-deprivation myopia (FDM) was induced by suturing the right eyelid, while the control group received no treatment. After 1, 2, and 4 weeks of treatment, eyes were assessed in vivo by cycloplegic retinoscopic refraction and axial length measurement by photography or A-scan ultrasonography. Levels of retinal Wnt2b, Fzd5 and β-catenin mRNA and protein were evaluated using RT-PCR and western blotting, respectively. (2) Another 96 mice were divided into three groups: control, drugs-only, and drugs+FDM (by diffuser). Experimentally treated eyes in the last two groups received intravitreal injections of vehicle or the proteins, DKK-1 (Wnt-pathway antagonist) or Norrin (Wnt-pathway agonist), once every three days, for 4 injections total. Axial length and retinoscopic refraction were measured on the 14th day of form deprivation. Following form-deprivation for 1, 2, and 4 weeks, FDM eyes had a relatively myopic refractive error, compared with contralateral eyes. There were no significant differences in refractive error between right and left eye in control group. The amounts of Wnt2b, Fzd5 and β-catenin mRNA and protein were significantly greater in form-deprived myopia eyes than in control eyes.DKK-1 (antagonist) reduced the myopic shift in refractive error and increase in axial elongation, whereas Norrin had the opposite effect in FDM eyes.

CONCLUSIONS/SIGNIFICANCE

Our studies provide the first evidence that the Wnt2b signaling pathway may play a role in the development and progression of form-deprivation myopia, in a mammalian model.

Pharmaceutical intervention for myopia control

Myopia is the result of a mismatch between the optical power and the length of the eye, with the latter being too long. Driving the research in this field is the need to develop myopia treatments that can limit axial elongation. When axial elongation is excessive, as in high myopia, there is an increased risk of visual impairment and blindness due to ensuing pathologies such as retinal detachments. This article covers both clinical studies involving myopic children, and studies involving animal models for myopia. Atropine, a nonselective muscarinic antagonist, has been studied most extensively in both contexts. Because it remains the only drug used in a clinical setting, it is a major focus of the first part of this article, which also covers the many shortcomings of topical ophthalmic atropine. The second part of this article focuses on in vitro and animal-based drug studies, which encompass a range of drug targets including the retina, retinal pigment epithelium and sclera. While the latter studies have contributed to a better understanding of how eye growth is regulated, no new antimyopia drug treatments have reached the clinical setting. Less conservative approaches in research, and in particular, the exploration of new bioengineering approaches for drug delivery, are needed to advance this field.

The effects and interactions of GABAergic and dopaminergic agents in the prevention of form deprivation myopia by brief periods of normal vision

Intravitreal injections of GABA antagonists, dopamine agonists and brief periods of normal vision have been shown separately to inhibit form-deprivation myopia (FDM). Our study had three aims: (i) establish whether GABAergic agents modify the myopia protective effect of normal vision, (ii) investigate the receptor sub-type specificity of any observed effect, and (iii) consider an interaction with the dopamine (DA) system. Prior to the period of normal vision GABAergic agents were applied either (i) individually, (ii) in combination with other GABAergic agents (an agonist with an antagonist), or (iii) in combination with DA agonists and antagonists. Water injections were given to groups not receiving drug treatments so that all experimental eyes received intravitreal injections. As shown previously, constant form-deprivation resulted in high myopia and when diffusers were removed for 2 h per day the period of normal vision greatly reduced the FDM that developed. GABA agonists inhibited the protective effect of normal vision whereas antagonists had the opposite effect. GABAA/C agonists and D2 DA antagonists when used in combination were additive in suppressing the protective effect of normal vision. A D2 DA agonist restored some of the protective effect of normal vision that was inhibited by a GABA agonist (muscimol). The protective effect of normal vision against form-deprivation is modifiable by both the GABAergic and DAergic pathways.

A genome-wide meta-analysis identifies two novel loci associated with high myopia in the Han Chinese population

High myopia, highly prevalent in the Chinese population, is a leading cause of visual impairment worldwide. Genetic factors play a critical role in the development of this visual disorder. Genome-wide association studies in recent years have revealed several chromosomal regions that contribute to its progression. To identify additional genetic variants for high myopia susceptibility, we used a genome-wide meta-analysis to examine the associations between the disease and 286 031 single-nucleotide polymorphisms (SNPs) in a combined cohort of 665 cases and 960 controls. The most significant SNPs (n = 61) were genotyped in a replication cohort (850 cases and 1197 controls), and 14 SNPs were further tested through genotyping in two additional validation cohorts (combined 1278 cases and 2486 controls). As a result of this analysis, four SNPs reached genome-wide significance (P < 2.0 × 10(-7)). The most significantly associated SNP, rs2730260 [overall P = 8.95 × 10(-14); odds ratio (95% CI) =1.33 (1.23-1.44)], is located in the VIPR2 gene, which is located in the MYP4 locus. The other three SNPs (rs7839488, rs4395927 and rs4455882) in the same linkage disequilibrium block are located in the SNTB1 gene, with -P values ranging from 1.13 × 10(-8) to 2.13 × 10(-11). The VIPR2 and SNTB1 genes are expressed in the retina and the retinal pigment epithelium and have been previously reported to have potential functions for the pathogenesis of myopia. Our results suggest that variants of the VIPR2 and SNTB1 genes increase susceptibility to high myopia in Han Chinese.

Pharmacology of myopia and potential role for intrinsic retinal circadian rhythms

Despite the high prevalence and public health impact of refractive errors, the mechanisms responsible for ametropias are poorly understood. Much evidence now supports the concept that the retina is central to the mechanism(s) regulating emmetropization and underlying refractive errors. Using a variety of pharmacologic methods and well-defined experimental eye growth models in laboratory animals, many retinal neurotransmitters and neuromodulators have been implicated in this process. Nonetheless, an accepted framework for understanding the molecular and/or cellular pathways that govern postnatal eye development is lacking. Here, we review two extensively studied signaling pathways whose general roles in refractive development are supported by both experimental and clinical data: acetylcholine signaling through muscarinic and/or nicotinic acetylcholine receptors and retinal dopamine pharmacology. The muscarinic acetylcholine receptor antagonist atropine was first studied as an anti-myopia drug some two centuries ago, and much subsequent work has continued to connect muscarinic receptors to eye growth regulation. Recent research implicates a potential role of nicotinic acetylcholine receptors; and the refractive effects in population surveys of passive exposure to cigarette smoke, of which nicotine is a constituent, support clinical relevance. Reviewed here, many puzzling results inhibit formulating a mechanistic framework that explains acetylcholine's role in refractive development. How cholinergic receptor mechanisms might be used to develop acceptable approaches to normalize refractive development remains a challenge. Retinal dopamine signaling not only has a putative role in refractive development, its upregulation by light comprises an important component of the retinal clock network and contributes to the regulation of retinal circadian physiology. During postnatal development, the ocular dimensions undergo circadian and/or diurnal fluctuations in magnitude; these rhythms shift in eyes developing experimental ametropia. Long-standing clinical ideas about myopia in particular have postulated a role for ambient lighting, although molecular or cellular mechanisms for these speculations have remained obscure. Experimental myopia induced by the wearing of a concave spectacle lens alters the retinal expression of a significant proportion of intrinsic circadian clock genes, as well as genes encoding a melatonin receptor and the photopigment melanopsin. Together this evidence suggests a hypothesis that the retinal clock and intrinsic retinal circadian rhythms may be fundamental to the mechanism(s) regulating refractive development, and that disruptions in circadian signals may produce refractive errors. Here we review the potential role of biological rhythms in refractive development. While much future research is needed, this hypothesis could unify many of the disparate clinical and laboratory observations addressing the pathogenesis of refractive errors.

The complex interactions of retinal, optical and environmental factors in myopia aetiology

Myopia is the commonest ocular abnormality but as a research topic remains at the margins of mainstream ophthalmology. The concept that most myopes fall into the category of 'physiological myopia' undoubtedly contributes to this position. Yet detailed analysis of epidemiological data linking myopia with a range of ocular pathologies from glaucoma to retinal detachment demonstrates statistically significant disease association in the 0 to -6 D range of 'physiological myopia'. The calculated risks from myopia are comparable to those between hypertension, smoking and cardiovascular disease. In the case of myopic maculopathy and retinal detachment the risks are an order of magnitude greater. This finding highlights the potential benefits of interventions that can limit or prevent myopia progression. Our understanding of the regulatory processes that guide an eye to emmetropia and, conversely how the failure of such mechanisms can lead to refractive errors, is certainly incomplete but has grown enormously in the last few decades. Animal studies, observational clinical studies and more recently randomized clinical trials have demonstrated that the retinal image can influence the eye's growth. To date human intervention trials in myopia progression using optical means have had limited success but have been designed on the basis of simple hypotheses regarding the amount of defocus at the fovea. Recent animal studies, backed by observational clinical studies, have revealed that the mechanisms of optically guided eye growth are influenced by the retinal image across a wide area of the retina and not solely the fovea. Such results necessitate a fundamental shift in how refractive errors are defined. In the context of understanding eye growth a single sphero-cylindrical definition of foveal refraction is insufficient. Instead refractive error must be considered across the curved surface of the retina. This carries the consequence that local retinal image defocus can only be determined once the 3D structure of the viewed scene, off axis performance of the eye and eye shape has been accurately defined. This, in turn, introduces an under-appreciated level of complexity and interaction between the environment, ocular optics and eye shape that needs to be considered when planning and interpreting the results of clinical trials on myopia prevention.

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.

Gene profiling in experimental models of eye growth: clues to myopia pathogenesis

To understand the complex regulatory pathways that underlie the development of refractive errors, expression profiling has evaluated gene expression in ocular tissues of well-characterized experimental models that alter postnatal eye growth and induce refractive errors. Derived from a variety of platforms (e.g. differential display, spotted microarrays or Affymetrix GeneChips), gene expression patterns are now being identified in species that include chicken, mouse and primate. Reconciling available results is hindered by varied experimental designs and analytical/statistical features. Continued application of these methods offers promise to provide the much-needed mechanistic framework to develop therapies to normalize refractive development in children.

Transforming growth factor-beta in the chicken fundal layers: an immunohistochemical study

In the chicken model of myopia, it has first been shown that imposing defocus to the retina results in active remodelling of the sclera which, in turn, results in axial length changes of the eye. Transforming growth factor-beta (TGF-beta) is one of the scleral growth modulators but its cellular localization in the fundal layers, colocalization and function are not well known. The aim of the current study was to investigate the cellular distribution of the three isoforms TGF-beta1, 2 and 3 by immunohistochemical labelling. Furthermore, the effects of visual experience that induces refractive errors on TGF-beta2 labelling were examined. Transversal cryostat sections of the fundal layers were analyzed by indirect immunofluorescent labelling and cell counts. Visual experience was changed by having the chicks wear either diffusers, or positive or negative lenses of 7D power in front of the right eyes for various periods of time. Left eyes served as uncovered controls. All TGF-beta isoforms were localized in both scleral layers. In choroid, diffuse labelling of all isoforms was found. In retina, TGF-beta1 and 3 were detected in bipolar, amacrine and ganglion cells and TGF-beta2 in amacrine and ganglion cells. To further characterize these cells, double-labelling with known amacrine and bipolar cell markers was performed (calbindin, cellular retinoic acid binding protein (CRABP), Islet1, Lim3 and protein kinase C (PKC)). TGF-beta1, 2 and 3 could be colocalized with calbindin and CRABP in single amacrine cells. TGF-beta1-positive bipolar cells were immunoreactive to Lim3. TGF-beta1 and 3 were never colocalized with PKC in bipolar cells. Also, colocalization with peptides known to be involved in myopia development in chicks, such as glucagon, or vasointestinal polypeptide and the key enzyme for dopamine synthesis, tyrosine hydroxylase, was not observed. Lenses or diffusers, worn by the chicks for various periods of time, had no effect on TGF-beta2 immunoreactivity in choroid or sclera, or on the number of TGF-beta2 (active and latent form) expressing amacrine cells. This result did not change when the two identified populations of TGF-beta2 expressing amacrine cells (one calbindin-positive and the other CRABP-positive) were separately considered. Also no modulation was seen in choroid, although an earlier study had found changes in TGF-beta2 mRNA after lens treatment. The lack of any visually-induced changes in retina or choroid suggests that TGF-beta may not represent a key molecule in the retino-choroidal signalling cascade although it has previously been shown to have a primary role in scleral remodelling.

Time-course of changes to nitric oxide signaling pathways in form-deprivation myopia in guinea pigs

The aim of this study was to investigate the time-course change of nitric oxide synthase (NOS) activity and cyclic GMP (cGMP) concentration in the posterior retina, choroid and sclera after differing periods of form-deprivation in guinea pigs. Three groups of guinea pigs were subjected to monocular FD for 7, 14 or 21 days. NOS activity and cGMP concentrations in ocular tissues of FD eyes and control eyes were analyzed by radioimmunoassay. The presence of NOS isoforms was detected by immunohistochemistry. Guinea pigs presented with considerable myopia after 14 days of FD. Retinal NOS activity in the FD group was lower than in the control group after 7 days of FD and was higher than in the control group after 14 and 21 days of FD. The choroidal and scleral NOS activities in the FD groups were higher than in the control groups after 21 days. The cGMP concentrations in the FD groups were higher than in the control groups at 21 days of the retinal, choroidal, and scleral tissues. Furthermore, the retinal cGMP concentration in the FD group was also significantly elevated at 14 days relative to the control group. We detected expression of three NOS isoforms in guinea pig ocular tissues. Our main observations were a change in NOS activity and an up-regulation in cGMP concentrations in posterior ocular tissues during the development of myopia. The function of elevated NOS activity may be mediated by cGMP.

Form-deprivation myopia in chick induces limited changes in retinal gene expression

PURPOSE

Evidence has implicated the retina as a principal controller of refractive development. In the present study, the retinal transcriptome was analyzed to identify alterations in gene expression and potential signaling pathways involved in form-deprivation myopia of the chick.

METHODS

One-week-old white Leghorn chicks wore a unilateral image-degrading goggle for 6 hours or 3 days (n = 6 at each time). Total RNA from the retina/(retinal pigment epithelium) was used for expression profiling with chicken gene microarrays (Chicken GeneChips; Affymetrix, Santa Clara, CA). To identify gene expression level differences between goggled and contralateral nongoggled eyes, normalized microarray signal intensities were analyzed by the significance analysis of microarrays (SAM) approach. Differentially expressed genes were validated by real-time quantitative reverse transcription-polymerase chain reaction (qPCR) in independent biological replicates.

RESULTS

Small changes were detected in differentially expressed genes in form-deprived eyes. In chickens that had 6 hours of goggle wear, downregulation of bone morphogenetic protein 2 and connective tissue growth factor was validated. In those with 3 days of goggle wear, downregulation of bone morphogenetic protein 2, vasoactive intestinal peptide, preopro-urotensin II-related peptide and mitogen-activated protein kinase phosphatase 2 was validated, and upregulation of endothelin receptor type B and interleukin-18 was validated.

CONCLUSIONS

Form-deprivation myopia, in its early stages, is associated with only minimal changes in retinal gene expression at the level of the transcriptome. While the list of validated genes is short, each merits further study for potential involvement in the signaling cascade mediating myopia development.

Basic fibroblast growth factor suppresses retinal neuronal apoptosis in form-deprivation myopia in chicks

PURPOSE

Chorioretinal atrophy including retinal neuronal apoptosis occurred in chronic form-deprivation myopia induced by lid suture in chicks. We investigated whether exogenous basic fibroblast growth factor (bFGF) could simultaneously inhibit the excessive axial elongation and retinal neuronal loss in the myopic chick eyes.

METHODS

Unilateral form deprivation was produced in neonatal male Hyline chicks by lid suture 24 hr after hatching. Ten microliters of solution containing 5 ng bFGF or PBS (vehicle) was injected into the vitreal chamber at 10 weeks of age, once every 3 days until week 12, when the animals were sacrificed. Ocular refraction and axial length were assessed by retinoscopy and calipers at the age of 12 weeks. Retinal apoptotic neurons and their caspase-3-like protease activity were analyzed by transmission electron microscopy, the terminal-deoxynucleotidyl transferase-mediated biotin-deoxyuridine triphosphate nick-end labeling (TUNEL) staining, and a colorimetric method using Ac-DEVD-pNA as a substrate. The number of TUNEL-positive cells was counted to analyize the survival activity of bFGF on retina.

RESULTS

After 12 weeks of lid suture, the control eyes were either emmetropic or hyperopic, whereas the deprived eyes became myopic with axial length increased. TUNEL-positive nuclei, condensed nuclear chromatin, and apoptotic bodies were observed in posterior retinal outer nuclear layer (ONL) and inner nuclear layer (INL) in the myopic chick eyes. Activity of retinal caspase-3-like protease in these eyes was elevated. In addition to ameliorating myopic ocular growth, intravitreal bFGF therapy significantly reduced the number of retinal apoptotic neurons and downregulated caspase-3 activity.

CONCLUSIONS

Exogenous bFGF effectively ameliorates the excessive axial elongation and retinal neuron apoptosis in chronic form-deprivation myopia in chicks. It is possible that bFGF will be a promising therapeutic agent for high human myopia.

Acute effects of dietary retinoic acid on ocular components in the growing chick

When the eyes of chicks are induced to grow toward myopia or hyperopia by having them wear spectacle lenses or diffusers, opposite changes take place in the retina and choroid in the synthesis and levels of all-trans Retinoic Acid (RA). To explore whether RA plays a causal role in the regulation of eye growth, we fed young chicks RA (doses 0.5 to 24 mg/kg) either twice a day or on alternate days or only once. Refractive error was measured with a Hartinger refractometer; ocular length, lens-thickness and choroidal thickness were measured by A-scan ultrasound. The amount of RA present in ocular tissues was determined using HPLC. Oral delivery of RA effectively increased RA in ocular tissues within 8h. During the first day after feeding RA at levels above 8 mg/kg, the rate of ocular elongation tripled, the choroid thickened and lens thickening was inhibited. The day following a dose of RA, the rate of ocular elongation was inhibited and the lens thickened more than normal. Nonetheless, the cumulative effect of repeated doses was that the eye became longer and the lens became thinner than normal, with no net change in refractive error. The rate of elongation was also increased by feeding 13-cis RA, and was reduced by citral, an inhibitor of RA synthesis. Surprisingly, birds fed RA while being kept in darkness also had normal refractive errors despite increased ocular elongation, and birds wearing either +6D or -6D spectacle lenses compensated normally for the lenses despite the enhanced ocular elongation caused by the RA. These results suggest that RA may act at the level of a coordinated non-visual regulatory system which controls the growth of the various ocular components, arguing that emmetropization does not depend entirely on vision.

Glucagon receptor agonists and antagonists affect the growth of the chick eye: a role for glucagonergic regulation of emmetropization?

PURPOSE

In chicks, plus defocus retards eye growth, thickens the choroid, and activates glucagonergic amacrine cells, probably releasing glucagon. Glucagon receptor antagonists (expected to inhibit compensation to plus defocus) and agonists (expected to block myopia induction by form deprivation) were administered to eyes of chicks, to test the hypothesis that glucagon mediates the induction of changes in eye growth by plus defocus.

METHODS

Seven-day-old (P7) chick eyes were injected intravitreally with peptides at concentrations of approximately 10(-9) to 10(-5) M in 20 microL (injection volume). The glucagon-receptor antagonists [des-His(1),des- Phe(6),Glu(9)]-glucagon-NH(2) (des- Phe(6)-antagonist) and [des-His(1),Glu(9)]-glucagon-NH(2) (Phe(6)-antagonist) were administered daily for 4 to 5 days to plus-defocused eyes. Agonists (porcine glucagon-[1-29] and [Lys(17,18),Glu(21)]-glucagon-NH(2)) were monocularly administered daily for 5 days to form-deprived eyes. The contralateral eye remained open and received saline. After treatment, eyes were refracted, measured, and examined for histologic changes.

RESULTS

The Phe(6)-antagonist at 10(-5) M (in the syringe) inhibited changes in both refractive error and axial length compensation induced by +7-D lens wear; however, des-Phe(6)-antagonist (10(-5) M) had weak, inconsistent effects and did not antagonize the action of exogenous glucagon. Glucagon prevented ocular elongation and myopia and induced choroidal thickening in form-deprived eyes. [Lys(17,18),Glu(21)]-glucagon-NH(2) had little effect at 10(-7) M, but at 10(-6) to 10(-5) M altered rod structure and inhibited eye growth.

CONCLUSIONS

Exogenous glucagon inhibited the growth of form-deprived eyes, whereas Phe(6)-antagonist inhibited compensation to plus defocus, as might be expected if glucagon is an endogenous mediator of emmetropization. The reason for the failure of des-Phe(6)-antagonist to counteract the effects of exogenous glucagon requires further investigation.

The role of the retinal pigment epithelium in eye growth regulation and myopia: a review

Myopia is increasing in prevalence world-wide, nearing epidemic proportions in some populations. This has led to expanded research efforts to understand how ocular growth and refractive errors are regulated. Eye growth is sensitive to visual experience, and is altered by both form deprivation and optical defocus. In these cases, the primary targets of growth regulation are the choroidal and scleral layers of the eye that demarcate the boundary of the posterior vitreous chamber. Of significance to this review are observations of local growth modulation that imply that the neural retina itself must be the source of growth-regulating signals. Thus the retinal pigment epithelium (RPE), interposed between the retina and the choroid, is likely to play a critical role in relaying retinal growth signals to the choroid and sclera. This review describes the ion transporters and signal receptors found in the chick RPE and their possible roles in visually driven changes in eye growth. We focus on the effects of four signaling molecules, otherwise implicated in eye growth changes (dopamine, acetylcholine, vasoactive intestinal peptide (VIP), and glucagon), on RPE physiology, including fluid transport. A model for RPE-mediated growth regulation is proposed.

Homeostasis of eye growth and the question of myopia

As with other organs, the eye's growth is regulated by homeostatic control mechanisms. Unlike other organs, the eye relies on vision as a principal input to guide growth. In this review, we consider several implications of this visual guidance. First, we compare the regulation of eye growth to that of other organs. Second, we ask how the visual system derives signals that distinguish the blur of an eye too large from one too small. Third, we ask what cascade of chemical signals constitutes this growth control system. Finally, if the match between the length and optics of the eye is under homeostatic control, why do children so commonly develop myopia, and why does the myopia not limit itself? Long-neglected studies may provide an answer to this last question.

Colchicine causes excessive ocular growth and myopia in chicks

Colchicine has been reported to destroy ganglion cells (GCs) in the retina of hatchling chicks. We tested whether colchicine influences normal ocular growth and form-deprivation myopia, and whether it affects cells other than GCs. Colchicine greatly increased axial length, equatorial diameter, eye weight, and myopic refractive error, while reducing corneal curvature. Colchicine caused DNA fragmentation in many GCs and some amacrine cells and photoreceptors, ultimately leading to the destruction of most GCs and particular sub-sets of amacrine cells. Colchicine-induced ocular growth may result from the destruction of amacrine cells that normally suppress ocular growth, and corneal flattening may result from the destruction of GCs whose central pathway normally plays a role in shaping the cornea.

Flicker parameters are different for suppression of myopia and hyperopia

Axial eye growth rates in the chicken are controlled by local retinal image-processing circuits. These circuits quantify the loss of contrast for different spatial frequencies and promote axial eye growth rates in correlation with the amount of retinal image degradation ("deprivation myopia"). They also distinguish whether the plane of focus lies in front of or behind the retina. How the sign of defocus is detected still remains unclear. Cues from chromatic aberration are not important. In an attempt to isolate retinal circuits controlling the development of myopia or hyperopia, young chickens were raised in flickering light of different frequencies (12 and 6 Hz) and duty cycles (4-75%) produced by rotating chopper disks. The effects of flickering light on refractive errors and change in axial growth rates induced by translucent occluders or defocusing lenses were measured by infrared retinoscopy and A-scan ultrasound, respectively. Retinal electrical activity was evaluated by flicker ERG after matching flicker parameters and stimulation brightness at retinal surface. Changes in retinal and vitreal dopamine content caused by flicker in occluded and normal eyes were determined by HPLC-ECD. Strikingly, suppression of myopia occurred for similar flicker parameters, whether induced by translucent occluders ("deprivation") or negative lenses ("defocus"). The degree to which myopia was suppressed was correlated with the duration of flicker dark phase and with the ERG amplitude. In contrast, suppression of hyperopia did not correlate with these parameters. We conclude that two different retinal circuits with different temporal characteristics are involved in the processing of hyperopic defocus/deprivation and of myopic defocus, the first one dependent on flicker ERG amplitude. However, we did not find any correlation between the rate of dopamine release and the degree of inhibition of deprivation myopia in flickering light.

Endogenous opiates in the chick retina and their role in form-deprivation myopia

In this study, the possible role of the retinal enkephalin system in form-deprivation myopia (FDM) in the chick eye was investigated. Daily intravitreal injection of the nonspecific opiate antagonist naloxone blocked development of FDM in a dose-dependent manner, while injection of the opiate agonist morphine had no effect at any dose tested. The ED50 for naloxone (calculated maximum concentration in the vitreous) was found to be in the low picomolar range. The results using receptor-subtype-specific drugs were contradictory. Drugs specific for mu and delta receptors had no effect on FDM. The kappa-specific antagonist nor-binaltorphimine (nor-BNI) reduced FDM by about 50% at maximum daily retinal doses ranging between 4 x 10(-10) and 4 x 10(-7) M, while the kappa-specific agonist U50488 blocked FDM in a dose-dependent manner with an ED50 between 5 x 10(-8) and 5 x 10(-7) M. Met-enkephalin immunoreactivity (ME-IR) was localized immunocytochemically to a subset of amacrine cells (ENSLI cells) and their neurites in the inner plexiform layer (IPL). As reported previously, ENSLI cells from untreated chick retinas showed a cyclical pattern of immunoreactivity, with increased immunoreactivity in the light compared to the dark. Form-deprivation did not appear to change this pattern. Amounts of preproenkephalin mRNA from normal or form-deprived eyes were approximately the same under all conditions. Daily injection of naloxone, however, did increase ME-IR in the dark. These results suggest that naloxone may affect release of enkephalin from the ENSLI cells. The results as presented are inconclusive with regards to the role of the enkephalin system in FDM. While the kappa receptor may participate, there is no conclusive evidence here for a direct effect of opiate receptors. The effect of naloxone on form-deprived eyes may be due to its effect on release of peptides from the ENSLI cells.

Concentrations of biogenic amines in fundal layers in chickens with normal visual experience, deprivation, and after reserpine application

Previous experiments in chickens have shown that dopamine released from the retina may be one of the messengers controlling the growth of the underlying sclera. It is also possible, however, that the apparent relationship between dopamine and myopia is secondary and artifactual. We have done experiments to assess this hypothesis. Using High Pressure Liquid Chromatography with electrochemical detection (HPLC-ED), we have asked whether changes in dopamine metabolism are restricted to the local retinal regions in which myopia was locally induced. Furthermore, we have measured the concentrations of biogenic amines separately in different fundal layers (vitreous, retina, choroid, and sclera) to find out how changes induced by "deprivation" (= removal of high spatial frequencies from the retinal image by translucent eye occluders which produce "deprivation myopia") are transmitted through these layers. Finally, we have repeated the deprivation experiments after intravitreal application of the irreversible dopamine re-uptake blocker reserpine to see how suppression of dopaminergic transmission affects these changes. We found that (1) Alterations in retinal dopamine metabolism were indeed restricted to the retinal areas in which myopia was induced. (2) The retina was the major source of dopamine release with a steep gradient both to the vitreal and choroidal side. Vitreal content was about one-tenth, choroidal content about one-third, and scleral content about one-twentieth of that of the retina. (3) There was a drop by about 40% in vitreal dopamine, DOPAC (3,4-dihydroxyphenylacetic acid) and HVA (homovanilic acid) concentrations following deprivation which occurred already at a time where little changes could yet be seen in their total retinal contents. (4) Choroidal and scleral dopamine levels were not affected by deprivation, indicating that other messengers must relay the information to the sclera. (5) A single intravitreal injection of reserpine lowered dopamine and HVA levels in retina and vitreous for at least 10 days in a dose-dependent fashion and diminished or suppressed further effects of deprivation on these compounds. DOPAC levels continued to change upon deprivation even after reserpine injection (Fig. 3). Our results suggest that the release rates of dopamine from retinal amacrine cells can be estimated from vitreal dopamine concentrations; furthermore, they are in line with the hypothesis that there is an inverse relationship between dopamine release and axial eye growth rates. Although our experiments do not ultimately prove that dopamine has a functional role in the visual control of eye growth, they are in line with this notion.