Form-Deprivation and Lens-Induced Myopia Are Similarly Affected by Pharmacological Manipulation of the Dopaminergic System in Chicks

Mechanisms of emmetropization and what might go wrong in myopia

Studies in animal models and humans have shown that refractive state is optimized during postnatal development by a closed-loop negative feedback system that uses retinal image defocus as an error signal, a mechanism called emmetropization. The sensor to detect defocus and its sign resides in the retina itself. The retina and/or the retinal pigment epithelium (RPE) presumably releases biochemical messengers to change choroidal thickness and modulate the growth rates of the underlying sclera. A central question arises: if emmetropization operates as a closed-loop system, why does it not stop myopia development? Recent experiments in young human subjects have shown that (1) the emmetropic retina can perfectly distinguish between real positive defocus and simulated defocus, and trigger transient axial eye shortening or elongation, respectively. (2) Strikingly, the myopic retina has reduced ability to inhibit eye growth when positive defocus is imposed. (3) The bi-directional response of the emmetropic retina is elicited with low spatial frequency information below 8 cyc/deg, which makes it unlikely that optical higher-order aberrations play a role. (4) The retinal mechanism for the detection of the sign of defocus involves a comparison of defocus blur in the blue (S-cone) and red end of the spectrum (L + M-cones) but, again, the myopic retina is not responsive, at least not in short-term experiments. This suggests that it cannot fully trigger the inhibitory arm of the emmetropization feedback loop. As a result, with an open feedback loop, myopia development becomes "open-loop".

Effect of isoflurane anaesthetic time on ocular a-scan ultrasonography measures and their relationship to age and OCT measures in the Guinea pig

A-scan ultrasonography enables precise measurement of internal ocular structures. Historically, its use has underpinned fundamental studies of eye development and aberrant eye growth in animal models of myopia; however, the procedure typically requires anaesthesia. Since anaesthesia affects intra-ocular pressure (IOP), we investigated changes in internal ocular structures with isoflurane exposure and compared measurements with those taken in awake animals using optical coherence tomography (OCT). Continuous A-scan ultrasonography was undertaken in tri-coloured guinea pigs aged 21 (n = 5), 90 (n = 5) or 160 (n = 5) days while anaesthetised (up to 36 min) with isoflurane (5% in 1.5L/min O2). Peaks were selected from ultrasound traces corresponding to the boundaries of the cornea, crystalline lens, retina, choroid and sclera. OCT scans (Zeiss Cirrus Photo 800) of the posterior eye layers were taken in 28-day-old animals (n = 19) and compared with ultrasound traces, with choroid and scleral thickness adjusted for the duration of anaesthesia based on the changes modelled in 21-day-old animals. Ultrasound traces recorded sequentially in left and right eyes in 14-day-old animals (n = 30) were compared, with each adjusted for anaesthesia duration. The thickness of the cornea was measured in enucleated eyes (n = 5) using OCT following the application of ultrasound gel (up to 20 min). Retinal thickness was the only ultrasound internal measure unaffected by anaesthesia. All other internal distances rapidly changed and were well fitted by exponential functions (either rise-to-max or decay). After 10 and 20 min of anaesthesia, the thickness of the cornea, crystalline lens and sclera increased by 17.1% and 23.3%, 0.4% and 0.6%, and 5.2% and 6.5% respectively, whilst the anterior chamber, vitreous chamber and choroid decreased by 4.4% and 6.1%, 0.7% and 1.1%, and 10.7% and 11.8% respectively. In enucleated eyes, prolonged contact of the cornea with ultrasound gel resulted in an increase in thickness of 9.3% after 10 min, accounting for approximately half of the expansion observed in live animals. At the back of the eye, ultrasound measurements of the thickness of the retina, choroid and sclera were highly correlated with those from posterior segment OCT images (R2 = 0.92, p = 1.2 × 10-13, R2 = 0.55, p = 4.0 × 10-4, R2 = 0.72, p = 5.0 × 10-6 respectively). Furthermore, ultrasound measures for all ocular components were highly correlated in left and right eyes measured sequentially, when each was adjusted for anaesthetic depth. This study shows that the depth of ocular components can change dramatically with anaesthesia. Researchers should therefore be wary of these concomitant effects and should employ adjustments to better render 'true' values.

Synchronous myopia development induced by bilateral form deprivation in chicks

Form deprivation (FD) is a widely employed experimental paradigm, typically used to induce unilateral myopia in animal models. This model is weakened by potential influence upon the FD eye from vision in the freely-viewing contralateral eye, which could be eliminated by imposing FD in both eyes; but while a few previous studies have explored the feasibility of inducing bilateral FD in chicks, substantial discrepancies in treatment outcomes were noted. Consequently, this study aimed to establish a bilateral FD myopia model in chicks, with validation by investigating the associated ocular growth patterns, feeding, and social behavior. Six-day-old chicks were treated with bilateral (n = 21) or unilateral (n = 10) FD for 12 days; the fellow untreated eyes in the unilateral FD group served as controls. Refractive error, corneal power, and ocular axial dimensions were measured at 4-day intervals after the onset of form deprivation, with a Hartinger refractometer, a custom-made videokeratography system, and a high-resolution A-scan ultrasonographer, respectively. Body weight was monitored to assess the chick's physical development. Our results showed that birds treated with bilateral FD grew as robustly as the unilaterally form-deprived chicks, with similar or slightly heavier body weights and mortalities. Unilateral FD induced significantly higher myopia in the treated eye, with stronger corneal power, deeper anterior and vitreous chambers, and longer axial length. Moreover, either bilaterally or unilaterally FD eyes developed similar refractive error (bilateral FD, left: -28.03 ± 9.06 D, right: -28.44 ± 9.45 D; unilateral FD: -29.48 ± 8.26 D) and ocular biometric changes; but choroidal thickness was thicker in bilaterally FD eyes, rather than thinner as in unilaterally FD eyes. In addition to the highly synchronized (symmetrical, parallel) development reported previously in bilateral FD, we found in this study that the correlations between bilaterally form-deprived eyes were highest for ocular biometric parameters directly contributing to myopia development, including corneal power (r = 0.74 to 0.93), anterior chamber depth (r = 0.60 to 0.85), vitreous chamber depth (r = 0.92 to 0.94), and axial length (r = 0.90 to 0.96). The remarkably synchronized growth pattern confirmed the feasibility of the bilateral FD paradigm for future research on myopia.

Factors associated with progressive anisometropia after bilateral intraocular lens implantation in patients with pediatric cataract

OBJECTIVES

To identify factors associated with progressive anisometropia after bilateral intraocular lens (IOL) implantation in patients with pediatric cataract.

METHODS

Clinical and standardized questionnaire data were collected for Sixty-eight patients with pediatric cataract (136 eyes) who underwent bilateral IOL implantation and at least 1 year of follow-up. Univariate and multivariate linear regression models were used to identify factors associated with postoperative anisometropia.

RESULTS

The median age at IOL implantation was 3.2 years (range: 1-12.4 years), and median follow-up time was 5.7 years (range: 1.1-14 years). At 1 month postoperatively and at the last follow-up, there were 19 (27%) and 31 (46%) cases of anisometropia ≥1 D, 9 (13%) and 15 (22%) cases of anisometropia ≥2 D, and 2 (3%) and 9 (13%) cases of anisometropia ≥3 D, respectively. Compared with 1 month postoperatively, the amount of anisometropia increased in 45 (67%) patients. Greater anisometropia one year or more after bilateral IOL implantation was associated with larger intereye difference in IOL power (P = 0.032, 95%CI 0.013 to 0.285), intereye difference in preoperative axial length (P = 0.018, 95%CI -1.247 to -0.123), presence of strabismus (P = 0.017, 95%CI 0.063-0.601), anisometropia at 1 month postoperatively (P = 0.001, 95%CI 0.126-0.478), and intereye difference in axial length at the last follow-up (P = 0.047, 95%CI 0.005-0.627).

CONCLUSION

Anisometropia might progress after bilateral IOL implantation in patients with pediatric cataract. Greater intereye difference in IOL power, presence of strabismus might increase the potential of progressive anisometropia.

Dynamic changes in ocular shape during human development and its implications for retina fovea formation

The human fovea is known for its distinctive pit-like appearance, which results from the displacement of retinal layers superficial to the photoreceptors cells. The photoreceptors are found at high density within the foveal region but not the surrounding retina. Efforts to elucidate the mechanisms responsible for these unique features have ruled out cell death as an explanation for pit formation and changes in cell proliferation as the cause of increased photoreceptor density. These findings have led to speculation that mechanical forces acting within and on the retina during development underly the formation of foveal architecture. Here we review eye morphogenesis and retinal remodeling in human embryonic development. Our meta-analysis of the literature suggests that fovea formation is a protracted process involving dynamic changes in ocular shape that start early and continue throughout most of human embryonic development. From these observations, we propose a new model for fovea development.

The role of the serotonergic system in atropine's anti-myopic effects

The muscarinic cholinergic antagonist atropine is the most widely used pharmacological treatment for the visual disorder myopia (short-sightedness), the leading cause of low-vision worldwide. This study sought to better define the mechanism by which atropine inhibits myopic growth. Although classified as a muscarinic-cholinergic antagonist, atropine has been found to bind and modulate the activity of several non-cholinergic systems (e.g., serotonin). Thus, this study investigated whether the serotonergic system could underly atropine's anti-myopic effects. Using a chick model of myopia, we report that atropine's growth-inhibitory effects can be attenuated by pharmacological stimulation of the serotonin system. This may suggest that atropine can slow the development of myopia through inhibiting serotonergic receptor activity. We also observed that pharmacological antagonism of serotonergic receptors inhibits the development of experimental myopia in a dose-dependent manner, further demonstrating that modulation of serotonergic receptor activity can alter ocular growth rates. Finally, we found that neither experimental myopia, nor atropine treatment, induced a significant change in retinal serotonergic output (i.e., synthesis, transport, release and catabolism). This may suggest that, although myopic growth can be inhibited through modulation of serotonergic receptor activity (by atropine or serotonergic antagonists), this does not require a change in serotonin levels. These findings regarding a serotonergic mechanism for atropine may have significant ramifications for the treatment of human myopia. This includes assessing the use of atropine in patients who are also undergoing treatment to upregulate serotonergic signaling (e.g., serotonergic anti-depressants).

Integrative Transcriptome and Proteome Analyses Elucidate the Mechanism of Lens-Induced Myopia in Mice

Purpose

The purpose of this study was to investigate the underlying molecular mechanism of lens-induced myopia (LIM) through transcriptome and proteome analyses with a modified mouse myopia model.

Methods

Four-week-old C57BL/6J mice were treated with a homemade newly designed -25 diopter (D) lens mounting by a 3D printing pen before right eyes for 4 weeks. Refraction (RE) and axial dimensions were measured every 2 weeks. Retinas were analyzed by RNA-sequencing and data-independent acquisition liquid chromatography tandem mass spectrometry. Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation, and STRING databases were used to identify significantly affected pathways in transcriptomic and proteomic data sets. Western blot was used to detect the expression of specific proteins.

Results

The modified model was accessible and efficient. Mice displayed a significant myopic shift (approximately 8 D) following 4 weeks' of lens treatment. Through transcriptomics and proteomics analysis, we elucidated 175 differently expressed genes (DEGs) and 646 differentially expressed proteins (DEPs) between binoculus. The transcriptomic and proteomic data showed a low correlation. Going over the mRNA protein matches, insulin like growth factor 2 mRNA binding protein 1 (Igf2bp1) was found to be a convincing biomarker of LIM, which was confirmed by Western blot. RNA-seq and proteome profiling confirmed that these two "omics" data sets complemented one another in KEGG pathways annovation. Among these, metabolic and human diseases pathways were considered to be correlated with the LIM forming process.

Conclusions

The newly constructed LIM model provides a useful tool for future myopia research. Combining transcriptomic and proteomic analysis may potentially brighten the prospects of novel therapeutic targets for patients with myopia.

Retinal Neurotransmitter Alteration in Response to Dopamine D2 Receptor Antagonist from Myopic Guinea Pigs

This study aimed to investigate the changes in retinal neurotransmitters and the role of the dopamine D2 receptor (D2R) pathway in regulating the myopic refractive state. Tricolor guinea pigs were randomly divided into two groups: the normal control group (NC) and the form-deprivation myopia group (FDM). Animals in the FDM group had their right eye covered with a balloon for 4 weeks. These two groups were further divided into two subgroups based on intravitreal injection with D2R antagonist sulpiride once a week for 3 weeks (NC, NC-Sul, FDM, and FDM-Sul groups). Ultrahigh-performance liquid chromatography-tandem mass spectrometry was used to quantitatively detect the changes in 17 retinal neurotransmitters. Compared to the NC group, the concentrations of dopamine (DA) and γ-aminobutyric acid (GABA) decreased, while those of glutamate (Glu), 3-methoxytyramine (3-MT), and glycine increased, accompanied by an increase in myopic refraction and axial length (AL) in the FDM group. In the FDM-Sul group, glycine and DA levels were upregulated, whereas 3-MT and Glu levels were downregulated, accompanied by a decrease in myopic refraction and AL. The ratio of Glu to GABA (RGG) represents the balance between excitatory and inhibitory neurotransmitters. Notably, RGG changes occurred with corresponding AL changes, which increased in the FDM group and decreased in the FDM-Sul group. Decreased retinal DA concentration, with an increase in Glu, may be involved in the myopia progression. D2R antagonists might effectively slow myopia progression by increasing retinal DA, regulating Glu concentration to match GABA, and maintaining the balance between excitatory and inhibitory neurotransmitters.

Ocular Posterior Segment Distribution and Pharmacokinetics of Brimonidine After Intravitreal Administration in Guinea Pigs

Purpose: Brimonidine is a highly alpha-2 adrenergic agonist, which provides a potential myopia control effect. This study aimed to examine the pharmacokinetics and concentration of brimonidine in the posterior segment tissue of eyes in guinea pigs. Methods: A liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was successfully used for brimonidine pharmacokinetics and tissue distribution research in guinea pigs following intravitreal administration (20 μg/eye). Results: Brimonidine concentrations in the retina and sclera were maintained at a high level (>60 ng/g) at 96 h postdosing. Brimonidine concentration peaked in the retina (377.86 ng/g) at 2.41 h and sclera (306.18 ng/g) at 6.98 h. The area under curve (AUC0-∞) was 27,179.99 ng h/g in the retina and 39,529.03 ng h/g in the sclera. The elimination half-life (T1/2e) was 62.43 h in the retina and 67.94 h in the sclera. Conclusions: The results indicated that brimonidine was rapidly absorbed and diffused to the retina and sclera. Meanwhile, it maintained higher posterior tissue concentrations, which can effectively activate the alpha-2 adrenergic receptor. This may provide pharmacokinetic evidence for the inhibition of myopia progression by brimonidine in animal experiments.

Identification of miR-671-5p and Its Related Pathways as General Mechanisms of Both Form-Deprivation and Lens-Induced Myopia in Mice

Animal models have been indispensable in shaping the understanding of myopia mechanisms, with form-deprivation myopia (FDM) and lens-induced myopia (LIM) being the most utilized. Similar pathological outcomes suggest that these two models are under the control of shared mechanisms. miRNAs play an important role in pathological development. Herein, based on two miRNA datasets (GSE131831 and GSE84220), we aimed to reveal the general miRNA changes involved in myopia development. After a comparison of the differentially expressed miRNAs, miR-671-5p was identified as the common downregulated miRNA in the retina. miR-671-5p is highly conserved and related to 40.78% of the target genes of all downregulated miRNAs. Moreover, 584 target genes of miR-671-5p are related to myopia, from which we further identified 8 hub genes. Pathway analysis showed that these hub genes are enriched in visual learning and extra-nuclear estrogen signaling. Furthermore, two of the hub genes are also targeted by atropine, which strongly supports a key role of miR-671-5p in myopic development. Finally, Tead1 was identified as a possible upstream regulator of miR-671-5p in myopia development. Overall, our study identified the general regulatory role of miR-671-5p in myopia as well as its upstream and downstream mechanisms and provided novel treatment targets, which might inspire future studies.

Studies on the interactions of retinal dopamine with choroidal thickness in the chicken

PURPOSE

Recently, an increasing number of studies relied on the assumption that visually induced changes in choroidal thickness can serve as a proxy to predict future axial eye growth. The retinal signals controlling choroidal thickness are, however, not well defined. We have studied the potential roles of dopamine, released from the retina, in the choroidal response in the chicken.

METHODS

Changes in retinal dopamine release and choroidal thickness changes were induced by intravitreal injections of either atropine (250 µg or 360 nMol), atropine combined with a dopamine antagonist, spiperone (500 µMol), or spiperone alone and were tracked by optical coherence tomography (OCT). To visually stimulate dopamine release, other chicks were exposed to flicker light of 1, 10, or 400 Hz (duty cycle 0.2) and choroidal thickness was tracked. In all experiments, dopamine and 3,4-Dihydroxyphenylacetic acid (DOPAC) were measured in vitreous, retina, and choroid by high-performance liquid chromatography with electrochemical detection (HLPC-ED). The distribution of the rate-limiting enzyme of dopamine synthesis, tyrosine hydroxylase (TH), neuronal nitric oxide synthase (nNOS), vascular endothelial growth factor (VEGF), and alpha2A adrenoreceptors (alpha2A-ADR) was studied in the choroid by immunofluorescence.

RESULTS

The choroid thickened strongly in atropine-injected eyes, less so in atropine + spiperone-injected eyes and became thinner over the day in spiperone alone-, vehicle-, or non-injected eyes. Flickering light at 20 lx, both 1 and 10 Hz, prevented diurnal choroidal thinning, compared to 400 Hz, and stimulated retinal dopamine release. Correlation analysis showed that the higher retinal dopamine levels or release, the thicker became the choroid. TH-, nNOS-, VEGF-, and alpha2A adrenoreceptor-positive nerve fibers were localized in the choroid around lacunae and in the walls of blood vessels with colocalization of TH and nNOS, and TH and VEGF.

CONCLUSIONS

Retinal DOPAC and dopamine levels were positively correlated with choroidal thickness. TH-positive nerve fibers in the choroid were closely associated with peptides known to play a role in myopia development. Findings are in line with the hypothesis that dopamine is related to retinal signals controlling choroidal thickness.

Mice Lacking Gpr179 with Complete Congenital Stationary Night Blindness Are a Good Model for Myopia

Mutations in GPR179 are one of the most common causes of autosomal recessive complete congenital stationary night blindness (cCSNB). This retinal disease is characterized in patients by impaired dim and night vision, associated with other ocular symptoms, including high myopia. cCSNB is caused by a complete loss of signal transmission from photoreceptors to ON-bipolar cells. In this study, we hypothesized that the lack of Gpr179 and the subsequent impaired ON-pathway could lead to myopic features in a mouse model of cCSNB. Using ultra performance liquid chromatography, we show that adult Gpr179^-/- mice have a significant decrease in both retinal dopamine and 3,4-dihydroxyphenylacetic acid, compared to Gpr179^+/+ mice. This alteration of the dopaminergic system is thought to be correlated with an increased susceptibility to lens-induced myopia but does not affect the natural refractive development. Altogether, our data added a novel myopia model, which could be used to identify therapeutic interventions.

Fallopia Japonica and Prunella vulgaris inhibit myopia progression by suppressing AKT and NFκB mediated inflammatory reactions

Background

The increased global incidence of myopia requires the establishment of therapeutic approaches. This study aimed to investigate the effect of Fallopia Japonica (FJ) and Prunella vulgaris (PV) extract on myopia caused by monocular form deprivation (MFD).

Methods

We used human retinal pigment epithelial cell to study the molecular mechanisms on how FJ extract (FJE) and PV extract (PVE) lowering the inflammation of the eye. The effect of FJE and PVE in MFD induced hamster model and explore the role of inflammation cytokines in myopia.

Results

FJE + PVE reduced IL-6, IL-8, and TNF-α expression in RPE cells. Furthermore, FJE and PVE inhibited inflammation by attenuating the phosphorylation of protein kinase B (AKT), and nuclear factor kappa-light-chain-enhancer of activated B (NF-κB) pathway. In addition, we report two resveratrol + ursolic acid compounds from FJ and PV and their inhibitory activities against IL-6, IL-8, and TNF-α expression levels in RPE cells treated with IL-6 and TNF-α. FJE, PVE, and FJE + PVE were applied to MFD hamsters and their axial length was measured after 21 days. The axial length showed statistically significant differences between phosphate-buffered saline- and FJE-, PVE-, and FJE + PVE-treated MFD eyes. FJE + PVE suppressed expressions of IL-6, IL-8, and TNF-α. They also inhibited myopia-related transforming growth factor-beta (TGF)-β1, matrix metalloproteinase (MMP)-2, and NF-κB expression while increasing type I collagen expression.

Conclusions

Overall, these results suggest that FJE + PVE may have a therapeutic effect on myopia and be used as a potential treatment option.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12906-022-03747-2.

Effects of air pollution on myopia: an update on clinical evidence and biological mechanisms

Myopia is one of the most common forms of refractive eye disease and considered as a worldwide pandemic experienced by half of the global population by 2050. During the past several decades, myopia has become a leading cause of visual impairment, whereas several factors are believed to be associated with its occurrence and development. In terms of environmental factors, air pollution has gained more attention in recent years, as exposure to ambient air pollution seems to increase peripheral hyperopia defocus, affect the dopamine pathways, and cause retinal ischemia. In this review, we highlight epidemiological evidence and potential biological mechanisms that may link exposure to air pollutants to myopia. A thorough understanding of these mechanisms is a key for establishing and implementing targeting strategies. Regulatory efforts to control air pollution through effective policies and limit individual exposure to preventable risks are required in reducing this global public health burden.

Candidate pathways for retina to scleral signaling in refractive eye growth

The global prevalence of myopia, or nearsightedness, has increased at an alarming rate over the last few decades. An eye is myopic if incoming light focuses prior to reaching the retinal photoreceptors, which indicates a mismatch in its shape and optical power. This mismatch commonly results from excessive axial elongation. Important drivers of the myopia epidemic include environmental factors, genetic factors, and their interactions, e.g., genetic factors influencing the effects of environmental factors. One factor often hypothesized to be a driver of the myopia epidemic is environmental light, which has changed drastically and rapidly on a global scale. In support of this, it is well established that eye size is regulated by a homeostatic process that incorporates visual cues (emmetropization). This process allows the eye to detect and minimize refractive errors quite accurately and locally over time by modulating the rate of elongation of the eye via remodeling its outermost coat, the sclera. Critically, emmetropization is not dependent on post-retinal processing. Thus, visual cues appear to influence axial elongation through a retina-to-sclera, or retinoscleral, signaling cascade, capable of transmitting information from the innermost layer of the eye to the outermost layer. Despite significant global research interest, the specifics of retinoscleral signaling pathways remain elusive. While a few pharmacological treatments have proven to be effective in slowing axial elongation (most notably topical atropine), the mechanisms behind these treatments are still not fully understood. Additionally, several retinal neuromodulators, neurotransmitters, and other small molecules have been found to influence axial length and/or refractive error or be influenced by myopigenic cues, yet little progress has been made explaining how the signal that originates in the retina crosses the highly vascular choroid to affect the sclera. Here, we compile and synthesize the evidence surrounding three of the major candidate pathways receiving significant research attention - dopamine, retinoic acid, and adenosine. All three candidates have both correlational and causal evidence backing their involvement in axial elongation and have been implicated by multiple independent research groups across diverse species. Two hypothesized mechanisms are presented for how a retina-originating signal crosses the choroid - via 1) all-trans retinoic acid or 2) choroidal blood flow influencing scleral oxygenation. Evidence of crosstalk between the pathways is discussed in the context of these two mechanisms.

Epiregulin, epigen and betacellulin antibodies and axial elongation in young guinea pigs with lens-induced myopization

Background

To examine an effect of intravitreally applied antibodies against epidermal growth factor family members, namely epiregulin, epigen and betacellulin, on ocular axial elongation.

Methods

The experimental study included 30 guinea pigs (age:3–4 weeks) which underwent bilateral lens-induced myopization and received three intraocular injections of 20 µg of epiregulin antibody, epigen antibody and betacellulin antibody in weekly intervals into their right eyes, and of phosphate-buffered saline into their left eyes. Seven days after the last injection, the animals were sacrificed. Axial length was measured by sonographic biometry.

Results

At baseline, right eyes and left eyes did not differ (all P > 0.10) in axial length in neither group, nor did the interocular difference in axial length vary between the groups (P = 0.19). During the study period, right and left eyes elongated (P < 0.001) from 8.08 ± 0.07 mm to 8.59 ± 0.06 mm and from 8.08 ± 0.07 mm to 8.66 ± 0.07 mm, respectively. The interocular difference (left eye minus right eye) in axial elongation increased significantly in all three groups (epiregulin-antibody:from 0.03 ± 0.06 mm at one week after baseline to 0.16 ± 0.08 mm at three weeks after baseline;P = 0.001); epigen-antibody group:from -0.01 ± 0.06 mm to 0.06 ± 0.08 mm;P = 0.02; betacellulin antibody group:from -0.05 ± 0.05 mm to 0.02 ± 0.04 mm;P = 0.004). Correspondingly, interocular difference in axial length increased from -0.02 ± 0.04 mm to 0.13 ± 0.06 mm in the epiregulin-antibody group (P < 0.001), and from 0.01 ± 0.05 mm to 0.07 ± 0.05 mm in the epigen-antibody group (P = 0.045). In the betacellulin-antibody group the increase (0.01 ± 0.04 mm to 0.03 ± 0.03 mm) was not significant (P = 0.24).

Conclusions

The EGF family members epiregulin, epigen and betacellulin may be associated with axial elongation in young guinea pigs, with the effect decreasing from epiregulin to epigen and to betacellulin.

Retinal Dopamine D2 Receptors Participate in the Development of Myopia in Mice

Purpose

To learn more about the locations of dopamine D2 receptors (D2Rs) that regulate form-deprivation myopia (FDM), using different transgenic mouse models.

Methods

One eye of D2R-knockout (KO) mice and wild-type littermates was subjected to four weeks of monocular FDM, whereas the fellow eye served as control. Mice in both groups received daily intraperitoneal injections of either the D2R antagonist sulpiride (8 µg/g) or vehicle alone. FDM was also induced in retina- (Six3^creD2R^fl/fl) or fibroblast-specific (S100a4^creD2R^fl/fl) D2R-KO mice. A subset of retina-specific D2R-KO mice and D2R^fl/fl littermates were also given sulpiride or vehicle injections. Refraction was measured with an eccentric infrared photorefractor, and other biometric parameters were measured by optical coherence tomography (n ≈ 20 for each group).

Results

FDM development was attenuated in wild-type littermates treated with sulpiride. However, this inhibitory effect disappeared in the D2R-KO mice, suggesting that antagonizing D2Rs suppressed myopia development. Similarly, the development of myopia was partially inhibited by retina-specific (deletion efficiency: 94.7%) but not fibroblast-specific (66.9%) D2R-KO. The sulpiride-mediated inhibitory effects on FDM also disappeared with retinal D2R-KO, suggesting that antagonizing D2Rs outside the retina may not attenuate myopia. Changes in axial length were less marked than changes in refraction, but in general the two were correlated.

Conclusions

This study demonstrates that D2Rs located in the retina participate in dopaminergic regulation of FDM in mice. These findings provide an important and fundamental basis for further exploring the retinal mechanism(s) involved in dopamine signaling and myopia development.

Advances in biomedical study of the myopia-related signaling pathways and mechanisms

Myopia has become one of the most critical health problems in the world with the increasing time spent indoors and increasing close work. Pathological myopia may have multiple complications, such as myopic macular degeneration, retinal detachment, cataracts, open-angle glaucoma, and severe cases that can cause blindness. Mounting evidence suggests that the cause of myopia can be attributed to the complex interaction of environmental exposure and genetic susceptibility. An increasing number of researchers have focused on the genetic pathogenesis of myopia in recent years. Scleral remodeling and excessive axial elongating induced retina thinning and even retinal detachment are myopia's most important pathological manifestations. The related signaling pathways are indispensable in myopia occurrence and development, such as dopamine, nitric oxide, TGF-β, HIF-1α, etc. We review the current major and recent progress of biomedicine on myopia-related signaling pathways and mechanisms.

Emmetropic, But Not Myopic Human Eyes Distinguish Positive Defocus From Calculated Blur

Purpose

Defocus blur imposed by positive lenses can induce hyperopia, whereas blur imposed by diffusers induces deprivation myopia. It is unclear whether the retina can distinguish between both conditions when the magnitude of blur is matched.

Methods

Ten emmetropic (average 0.0 ± 0.3 diopters [D]) and 10 subjects with myopia (−2.7 ± 0.9 D; 24 ± 4 years) watched a movie on a large screen (65 inches at 2 meters (m) distance. The movie was presented either unfiltered (“control”), with calculated low-pass filtering equivalent to a defocus of 2.5 D, or with binocular real optical defocus of +2.5 D. Spatial filtering was done in real-time by software written in Visual C++. Axial length was followed with the Lenstar LS-900 with autopositioning system.

Results

Watching unfiltered movies (“control”) caused no changes in axial length. In emmetropes, watching movies with calculated defocus caused axial eye elongation (+9.8 ± 7.6 µm) while watching movies with real positive defocus caused shorter eyes (−8.8 ± 9.2 µm; difference between both P < 0.0001). In addition, in myopes, calculated defocus caused longer eyes (+8.4 ± 9.0 µm, P = 0.001). Strikingly, myopic eyes became also longer with positive defocus (+9.1 ± 11.2 µm, P = 0.02). The difference between emmetropic and myopic eyes was highly significant (−8.8 ± 9.2 µm vs. +9.1 ± 11.2 µm, respectively, P = 0.001).

Conclusions

(1) In emmetropic human subjects, the retina is able to distinguish between real positive defocus and calculated defocus even when the modulation transfer function was matched, (2) in myopic eyes, the retina no longer distinguishes between both conditions because the eyes became longer in both cases. Results suggest that the retina in a myopic eye has reduced ability to detect positive defocus.

Transcriptome-based insights into gene networks controlling myopia prevention

Myopia (short-sightedness), usually caused by excessive elongation of the eye during development, has reached epidemic proportions worldwide. In animal systems including the chicken model, several treatments have been shown to inhibit ocular elongation and experimental myopia. Although diverse in their apparent mechanism of action, each one leads to a reduction in the rate of ocular growth. We hypothesize that a defined set of retinal molecular changes may underlie growth inhibition, irrespective of the treatment agent used. Accordingly, across five well-established but diverse methods of inhibiting myopia, significant overlap is seen in the retinal transcriptome profile (transcript levels and alternative splicing events) in chicks when analyzed by RNA-seq. Within the two major pathway networks enriched during growth inhibition, that of cell signaling and circadian entrainment, transcription factors form the largest functional grouping. Importantly, a large percentage of those genes forming the defined retinal response are downstream targets of the transcription factor EGR1 which itself shows a universal response to all five growth-inhibitory treatments. This supports EGR1's previously implicated role in ocular growth regulation. Finally, by contrasting our data with human linkage and GWAS studies on refractive error, we confirm the applicability of our study to the human condition. Together, these findings suggest that a universal set of transcriptome changes, which sit within a well-defined retinal network that cannot be bypassed, is fundamental to growth regulation, thus paving a way for designing novel targets for myopia therapies.

Insights into the mechanism of atropine's anti-myopia effects: evidence against cholinergic hyperactivity and modulation of dopamine release

Background and Purpose

The ability of the muscarinic cholinergic antagonist atropine to inhibit myopia development in humans and animal models would suggest that cholinergic hyperactivity may underlie myopic growth. To test this, we investigated whether cholinergic agonists accelerate ocular growth rates in chickens. Furthermore, we investigated whether atropine alters ocular growth by downstream modulation of dopamine levels, a mechanism postulated to underlie its antimyopic effects.

Experimental Approach

Muscarinic (muscarine and pilocarpine), nicotinic (nicotine) and non‐specific (oxotremorine and carbachol) cholinergic agonists were administered to chicks developing form‐deprivation myopia (FDM) or chicks that were otherwise untreated. Vitreal levels of dopamine and its primary metabolite 3,4‐dihydroxyphenylacetic acid (DOPAC) were examined using mass spectrometry MS in form‐deprived chicks treated with atropine (360, 15 or 0.15 nmol). Further, we investigated whether dopamine antagonists block atropine's antimyopic effects.

Key Results

Unexpectedly, administration of each cholinergic agonist inhibited FDM but did not affect normal ocular development. Atropine only affected dopamine and DOPAC levels at its highest dose. Dopamine antagonists did not alter the antimyopia effects of atropine.

Conclusion and Implications

Muscarinic, nicotinic and non‐specific cholinergic agonists inhibited FDM development. This indicates that cholinergic hyperactivity does not underlie myopic growth and questions whether atropine inhibits myopia via cholinergic antagonism. This study also demonstrates that changes in retinal dopamine release are not required for atropine's antimyopic effects. Finally, nicotinic agonists may represent a novel and more targeted approach for the cholinergic control of myopia as they are unlikely to cause the anterior segment side effects associated with muscarinic treatment.

Retinal Stem Cell 'Retirement Plans': Growth, Regulation and Species Adaptations in the Retinal Ciliary Marginal Zone

The vertebrate retina develops from a specified group of precursor cells that adopt distinct identities and generate lineages of either the neural retina, retinal pigmented epithelium, or ciliary body. In some species, including teleost fish and amphibians, proliferative cells with stem-cell-like properties capable of continuously supplying new retinal cells post-embryonically have been characterized and extensively studied. This region, termed the ciliary or circumferential marginal zone (CMZ), possibly represents a conserved retinal stem cell niche. In this review, we highlight the research characterizing similar CMZ-like regions, or stem-like cells located at the peripheral margin, across multiple different species. We discuss the proliferative parameters, multipotency and growth mechanisms of these cells to understand how they behave in vivo and how different molecular factors and signalling networks converge at the CMZ niche to regulate their activity. The evidence suggests that the mature retina may have a conserved propensity for homeostatic growth and plasticity and that dysfunction in the regulation of CMZ activity may partially account for dystrophic eye growth diseases such as myopia and hyperopia. A better understanding of the properties of CMZ cells will enable important insight into how an endogenous generative tissue compartment can adapt to altered retinal physiology and potentially even restore vision loss caused by retinal degenerative conditions.

Protective effects of sunlight exposure against PRK-induced myopia in infant rhesus monkeys

PURPOSE

Extensive clinical evidence suggests that time spent outdoors might reduce the risk of myopia. This study aimed to determine whether increasing sunlight exposure has a protective effect on hyperopic-defocus induced myopia in a non-human primate.

METHODS

Twelve 2-month-old rhesus monkeys were treated monocularly with photorefractive keratectomy (PRK) (4.0 D) and divided randomly into two groups: artificial light (AL; n = 6) and natural light (NL; n = 6). Monkeys in the AL group were reared under artificial (indoor) lighting (08:00-20:00 h). Monkeys in the NL group were exposed to natural (outdoor) lighting for 4 h (09:00-11:00 and 15:00-17:00 h). Ocular refraction, corneal power and axial dimensions were measured before sunlight exposure and every 10 days after PRK. At day 180, retinal histology and apoptosis activity were evaluated by hematoxylin and eosin (H&E) staining and terminal deoxynucleotidyl transferase biotin (dUTP) nick end labelling (TUNEL) assay.

RESULTS

Mean (±SD) PRK induced anisometropia was +3.11 (0.33) D. At the end of the experiment, both eyes of the NL monkeys exhibited significantly more hyperopia and shorter vitreous chamber depths (VCD), compared with AL monkeys (p < 0.05). The NL group exhibited a significantly slower rate of compensation to the induced anisometropia than the AL group (p < 0.05). The retinas of both groups exhibited normal histology and levels of apoptosis.

CONCLUSIONS

Moderate sunlight exposure exerts protective effects against the myopic shift resulting from PRK-induced defocus in monkeys. These results are consistent with current clinical findings that increased outdoor exposure protects against myopia development. Sunlight exposure should serve as an independent positive factor in human myopia control.

Coadministration With Carbidopa Enhances the Antimyopic Effects of Levodopa in Chickens

Purpose

Topical application of levodopa inhibits the development of form-deprivation myopia (FDM) and lens-induced myopia (LIM) in chicks. Here we examine whether coadministration with carbidopa enhances this protection and compare the effectiveness of topical versus systemic administration. We also investigate the degree to which topical and systemic administration of these compounds alters retinal dopamine release and examine whether this is the mechanism by which they inhibit experimental myopia.

Methods

Levodopa and levodopa:carbidopa (at a 4:1 ratio) were administered as twice-daily eye drops or once-daily intraperitoneal injections to chicks developing FDM or LIM over an ascending dose range. Axial length and refraction were measured following 4 days of treatment. Dopamine levels in the vitreous and blood were analyzed using liquid chromatography-mass spectrometry following topical or systemic administration of levodopa or levodopa:carbidopa. Finally, chicks receiving topical or systemic levodopa or levodopa:carbidopa were cotreated with the dopamine antagonist spiperone.

Results

Levodopa:carbidopa inhibited the development of FDM and LIM to a greater extent than levodopa alone (P < 0.05). Topical application was more effective than systemic administration (P < 0.001). Vitreal dopamine levels were increased to the greatest extent by topical application of levodopa:carbidopa (P < 0.001). Systemic but not topical administration significantly increased dopamine levels within the blood (P < 0.01). Cotreatment with spiperone inhibited the antimyopic effects (P < 0.05) of levodopa and levodopa:carbidopa.

Conclusions

The presence of carbidopa increases the bioavailability of levodopa within the eye, enhancing its antimyopic effects, with topical application showing the greatest efficacy. Thus levodopa:carbidopa may be a promising treatment for controlling the progression of human myopia.