Morphological and functional changes in the retina after chronic oxygen-induced retinopathy

Agonism of β3-Adrenoceptors Inhibits Pathological Retinal Angiogenesis in the Model of Oxygen-Induced Retinopathy

Purpose

In response to hypoxia, sympathetic fibers to the retina activate β-adrenoceptors (β-ARs) that play an important role in the regulation of vascular and neuronal functions. We investigated the role of β3-AR using the mouse model of oxygen-induced retinopathy (OIR).

Methods

Mouse pups were exposed to 75% oxygen at postnatal day 7 (PD7) followed by a return to room air at PD12. The β3-AR preferential agonist BRL37344 was subcutaneously administered once daily at different times after the return to room air. At PD17, the OIR mice underwent flash and pattern electroretinogram. After sacrifice, retinal wholemounts were used for vessel staining or immunohistochemistry for astrocytes, Müller cells, or retinal ganglion cells (RGCs). In retinal homogenates, the levels of markers associated with neovascularization (NV), the blood-retinal barrier (BRB), or astrocytes were determined by western blot, and quantitative reverse-transcription polymerase chain reaction was used to assess β3-AR messenger. Administration of the β3-AR antagonist SR59230A was performed to verify BRL37344 selectivity.

Results

β3-AR expression is upregulated in response to hypoxia, but its increase is prevented by BRL37344, which counteracts NV by inhibiting the pro-angiogenic pathway, activating the anti-angiogenic pathway, recovering BRB-associated markers, triggering nitric oxide production, and favoring revascularization of the central retina through recovered density of astrocytes that ultimately counteracts NV in the midperiphery. Vasculature rescue prevents dysfunctional retinal activity and counteracts OIR-associated retinal ganglion cell loss.

Conclusions

β3-AR has emerged as a crucial intermediary in hypoxia-dependent NV, suggesting a role of β3-AR agonists in the treatment of proliferative retinopathies.

Assessment of Inner Blood-Retinal Barrier: Animal Models and Methods

Proper functioning of the neural retina relies on the unique retinal environment regulated by the blood-retinal barrier (BRB), which restricts the passage of solutes, fluids, and toxic substances. BRB impairment occurs in many retinal vascular diseases and the breakdown of BRB significantly contributes to disease pathology. Understanding the different molecular constituents and signaling pathways involved in BRB development and maintenance is therefore crucial in developing treatment modalities. This review summarizes the major molecular signaling pathways involved in inner BRB (iBRB) formation and maintenance, and representative animal models of eye diseases with retinal vascular leakage. Studies on Wnt/β-catenin signaling are highlighted, which is critical for retinal and brain vascular angiogenesis and barriergenesis. Moreover, multiple in vivo and in vitro methods for the detection and analysis of vascular leakage are described, along with their advantages and limitations. These pre-clinical animal models and methods for assessing iBRB provide valuable experimental tools in delineating the molecular mechanisms of retinal vascular diseases and evaluating therapeutic drugs.

Targeting proliferative retinopathy: Arginase 1 limits vitreoretinal neovascularization and promotes angiogenic repair

Current therapies for treatment of proliferative retinopathy focus on retinal neovascularization (RNV) during advanced disease and can trigger adverse side-effects. Here, we have tested a new strategy for limiting neurovascular injury and promoting repair during early-stage disease. We have recently shown that treatment with a stable, pegylated drug form of the ureohydrolase enzyme arginase 1 (A1) provides neuroprotection in acute models of ischemia/reperfusion injury, optic nerve crush, and ischemic stroke. Now, we have determined the effects of this treatment on RNV, vascular repair, and retinal function in the mouse oxygen-induced retinopathy (OIR) model of retinopathy of prematurity (ROP). Our studies in the OIR model show that treatment with pegylated A1 (PEG-A1), inhibits pathological RNV, promotes angiogenic repair, and improves retinal function by a mechanism involving decreased expression of TNF, iNOS, and VEGF and increased expression of FGF2 and A1. We further show that A1 is expressed in myeloid cells and areas of RNV in retinal sections from mice with OIR and human diabetic retinopathy (DR) patients and in blood samples from ROP patients. Moreover, studies using knockout mice with hemizygous deletion of A1 show worsened RNV and retinal injury, supporting the protective role of A1 in limiting the OIR-induced pathology. Collectively, A1 is critically involved in reparative angiogenesis and neuroprotection in OIR. Pegylated A1 may offer a novel therapy for limiting retinal injury and promoting repair during proliferative retinopathy.

Thioredoxin-1 Ameliorates Oxygen-Induced Retinopathy in Newborn Mice Through Modulation of Proinflammatory and Angiogenic Factors

Oxygen-induced retinopathy (OIR) is an animal model for retinopathy of prematurity, which is a leading cause of blindness in children. Thioredoxin-1 (TRX) is a small redox protein that has cytoprotective and anti-inflammatory properties in response to oxidative stress. The purpose of this study was to determine the effect of TRX on OIR in newborn mice. From postnatal day 7, C57BL/6 wild type (WT) and TRX transgenic (TRX-Tg) mice were exposed to either 21% or 75% oxygen for 5 days. Avascular and neovascular regions of the retinas were investigated using fluorescence immunostaining. Fluorescein isothiocyanate-dextran and Hoechst staining were used to measure retinal vascular leakage. mRNA expression levels of proinflammatory and angiogenic factors were analyzed using quantitative polymerase chain reaction. Retinal histological changes were detected using immunohistochemistry. In room air, the WT mice developed well-organized retinas. In contrast, exposing WT newborn mice to hyperoxia hampered retinal development, increasing the retinal avascular and neovascular areas. After hyperoxia exposure, TRX-Tg mice had enhanced retinal avascularization compared with WT mice. TRX-Tg mice had lower retinal neovascularization and retinal permeability during recovery from hyperoxia compared with WT mice. In the early stages after hyperoxia exposure, VEGF-A and CXCL-2 expression levels decreased, while IL-6 expression levels increased in WT newborn mice. Conversely, no differences in gene expressions were observed in the TRX-Tg mouse retina. IGF-1 and Angpt1 levels did not decrease during recovery from hyperoxia in TRX-Tg newborn mice. As a result, overexpression of TRX improves OIR in newborn mice by modulating proinflammatory and angiogenic factors.

Limited hyperoxia-induced proliferative retinopathy: A model of persistent retinal vascular dysfunction, preretinal fibrosis and hyaloidal vascular reprogramming for retinal rescue

Background

Retinopathy of prematurity (ROP) remains the leading cause for blindness in children. Limited hyperoxia induced proliferative retinopathy (L-HIPR) was recently introduced as a potential animal model for ROP and persistent fetal vasculature; however, the detailed pathological changes remain unclear.

Methods

To model L-HIPR, we placed C57BL/6J mice in 65% oxygen from birth to post-natal day 7 (P7). We examined eyes at intervals between P12 and P30. Retinal morphometry, thickness, and preretinal fibrosis were quantified at different time points on histological sections stained with hematoxylin and eosin (H&E) and Masson Trichrome, respectively. Vascular development, angiogenesis, inflammation, and pericyte coverage were analyzed using immunohistochemistry staining in retinal flat mounts and cross sections.

Results

In L-HIPR, the hyaloidal vessels persisted until the latest time point in this study, P30 and began to invaginate the peripheral then central retina starting at P12. Central retinal distortion was noted beginning at P17, while the peripheral retina demonstrated a trend of thinning from P12 to P30. We found that L-HIPR was associated with delayed and abnormal retinal vascular development with subsequent retinal inflammation, pericyte loss and preretinal fibrosis.

Conclusion

Our study presents a detailed analysis of the L-HIPR animal model demonstrating vitreoretinal pathologic changes, preretinal fibrosis and persistent hyaloidal vessels into adulthood. Based on our findings, we suggest that the persistence and peculiar stepwise migration of the hyaloidal vessels into the retina may provide a potential rescue mechanism for inner retinal development that deserves further study.

Optimal Efficacy and Safety of Humanized Anti-Scg3 Antibody to Alleviate Oxygen-Induced Retinopathy

The retinopathy of prematurity (ROP), a neovascular retinal disorder presenting in premature infants, is the leading causes of blindness in children. Currently, there is no approved drug therapy for ROP in the U.S., highlighting the urgent unmet clinical need for a novel therapeutic to treat the disease. Secretogranin III (Scg3) was recently identified as a disease-selective angiogenic factor, and Scg3-neutralizing monoclonal antibodies were reported to alleviate pathological retinal neovascularization in mouse models. In this study, we characterized the efficacy and safety of a full-length humanized anti-Scg3 antibody (hAb) to ameliorate retinal pathology in oxygen-induced retinopathy (OIR) mice, a surrogate model of ROP, by implementing histological and functional analyses. Our results demonstrate that the anti-Scg3 hAb outperforms the vascular endothelial growth factor inhibitor aflibercept in terms of efficacy and safety to treat OIR mice. Our findings support the development of anti-Scg3 hAb for clinical application.

Thyroid Activating Enzyme, Deiodinase II Is Required for Photoreceptor Function in the Mouse Model of Retinopathy of Prematurity

Purpose

Retinopathy of prematurity (ROP) is a severe complication of premature infants, leading to vision loss when untreated. Presently, the molecular mechanisms underlying ROP are still far from being clearly understood. This study sought to investigate whether thyroid hormone (TH) signaling contributes to the neuropathology of ROP using the mouse model of ROP to evaluate longitudinal photoreceptor function.

Methods

Animals were exposed to hyperoxia from P7 to P12 to induce retinopathy, thereafter the animals were returned to room air (normoxia). The thyroid-activating enzyme type 2 deiodinases (Dio2) knockout (KO) mice and the littermate controls that were exposed to hyperoxia or maintained in room air and were then analyzed. The retinal function was evaluated using electroretinograms (ERGs) at three and seven weeks followed by histologic assessments with neuronal markers to detect cellular changes in the retina. Rhodopsin protein levels were measured to validate the results obtained from the immunofluorescence analyses.

Results

In the ROP group, the photoreceptor ERG responses are considerably lower both in the control and the Dio2 KO animals at P23 compared to the non-ROP group. In agreement with the ERG responses, loss of Dio2 results in mislocalized cone nuclei, and abnormal rod bipolar cell dendrites extending into the outer nuclear layer. The retinal function is compromised in the adult Dio2 KO animals, although the cellular changes are less severe. Despite the reduction in scotopic a-wave amplitudes, rhodopsin levels are similar in the adult mice, across all genotypes irrespective of exposure to hyperoxia.

Conclusions

Using the mouse model of ROP, we show that loss of Dio2 exacerbates the effects of hyperoxia-induced retinal deficits that persist in the adults. Our data suggest that aberrant Dio2/TH signaling is an important factor in the pathophysiology of the visual dysfunction observed in the oxygen-induced retinopathy model of ROP.

Hyaloid Vasculature as a Major Source of STAT3+ (Signal Transducer and Activator of Transcription 3) Myeloid Cells for Pathogenic Retinal Neovascularization in Oxygen-Induced Retinopathy

OBJECTIVE

Myeloid cells are critically involved in inflammation-induced angiogenesis, although their pathogenic role in the ischemic retina remains controversial. We hypothesize that myeloid cells contribute to pathogenic neovascularization in retinopathy of prematurity through STAT3 (signal transducer and activator of transcription 3) activation. Approach and Results: Using the mouse model of oxygen-induced retinopathy, we show that myeloid cells (CD45⁺IsolectinB4 [IB4]⁺) and particularly M2-type macrophages (CD45⁺ Arg1⁺), comprise a major source of STAT3 activation (pSTAT3) in the immature ischemic retina. Most of the pSTAT3-expressing myeloid cells concentrated at the hyaloid vasculature and their numbers were strongly correlated with the severity of pathogenic neovascular tuft formation. Pharmacological inhibition of STAT3 reduced the load of IB4⁺ cells in the hyaloid vasculature and significantly reduced the formation of pathogenic neovascular tufts during oxygen-induced retinopathy, leading to improved long-term visual outcomes (ie, increased retinal thickness and scotopic b-wave electroretinogram responses). Genetic deletion of SOCS3 (suppressor of cytokine signaling 3), an endogenous inhibitor of STAT3, in myeloid cells, enhanced pathological and physiological neovascularization in oxygen-induced retinopathy, indicating that myeloid-STAT3 signaling is crucial for retinal angiogenesis.

CONCLUSIONS

Circulating myeloid cells may migrate to the immature ischemic retina through the hyaloid vasculature and contribute to retinal neovascularization via activation of STAT3. Understanding how STAT3 modulates myeloid cells for vascular repair/pathology may provide novel therapeutic options in pathogenic angiogenesis.

Hyperglycemia-induced ubiquitination and degradation of β-catenin with the loss of platelet endothelial cell adhesion molecule-1 in retinal endothelial cells

OBJECTIVE

Increased retinal vascular permeability is one of the earliest manifestations of diabetic retinopathy. The aim of this study was to investigate the role of hyperglycemia-induced platelet endothelial cell adhesion molecule-1 loss on retinal vascular permeability via the β-catenin pathway.

METHODS

Type I diabetes was induced in male Wistar rats using streptozotocin injections, with age-matched non-diabetic rats as controls. Rat retinal microvascular endothelial cells were grown under normal or high glucose conditions for 6 days. Small interfering Ribonucleic Acid was used to knock down platelet endothelial cell adhesion molecule-1 in rat retinal microvascular endothelial cells for loss-of-function studies. Retinas and rat retinal microvascular endothelial cells were subjected to Western blot, immunofluorescence labeling, and co-immunoprecipitation analyses to assess protein levels and interactions. A biotinylated gelatin and fluorescein isothiocyanate-avidin assay was used for retinal endothelial cell permeability studies.

RESULTS

β-catenin, β-catenin/platelet endothelial cell adhesion molecule-1 interaction, active Src homology 2 domain-containing protein tyrosine phosphatase were significantly decreased, while β-catenin ubiquitination levels and endothelial permeability were significantly increased, in hyperglycemic retinal endothelial cells. Similar results were observed with platelet endothelial cell adhesion molecule-1 partial knockdown, where β-catenin and active Src homology 2 domain-containing protein tyrosine phosphatase levels were decreased, while phospho-β-catenin and retinal endothelial cell permeability were increased.

CONCLUSION

Platelet endothelial cell adhesion molecule-1 loss may contribute to increased retinal endothelial cell permeability by attenuating β-catenin levels under hyperglycemic conditions.

Oxidative stress upregulates Wnt signaling in human retinal microvascular endothelial cells through activation of disheveled

Abnormal retinal neovascularization associated with various retinopathies can result in irreversible vision loss. Although the mechanisms involved in this occurrence is unclear, increasing evidence suggests that aberrant Wnt signaling participates in the pathogenesis of abnormal neovascularization. Because Wnt signaling upregulation can be induced by oxidative stress through the activation of disheveled (DVL), a key molecule in the Wnt signaling pathway, we investigated whether oxidative stress can activate Wnt signaling and induce angiogenic phenotypes in human retinal microvascular endothelial cells (HRMECs). We found that increased Wnt signaling activity, as well as enhanced angiogenic phenotypes, such as tube formation and cell migration, were detected in the hydrogen peroxide-treated HRMECs. Moreover, these effects were effectively suppressed by a small-molecule Wnt inhibitor targeting the PDZ domain of DVL. Therefore, we propose that targeting abnormal Wnt signaling at the DVL level with a small-molecule inhibitor may represent a novel approach in retinal neovascularization treatment and prevention.

Simultaneous assessment of aberrant retinal vascularization, thickness, and function in an in vivo mouse oxygen-induced retinopathy model

BACKGROUND

Retinopathy of prematurity is a condition of abnormal retinal vascularization in premature infants. The effect of abnormal vascularization on retinal structure and function is unclear. In vivo studies of retinal vascularization, thickness, and function were performed in mice with oxygen-induced retinopathy (OIR mice).

METHODS

Eighteen mice were exposed to hyperoxia at postnatal day (P) 7, whereas 18 mice were raised in room air (RA). At P20 and 40, electroretinogram was performed for a-wave and b-wave amplitudes and peak times, followed by simultaneous fluorescein angiography for retinal avascular area, arterial tortuosity, and vein dilation assessments, and spectral domain optical coherence tomography for retinal thickness.

RESULTS

Capillary density appeared sparser in OIR mice, but retinal avascular area similar to RA mice. Retinal artery tortuosity was higher at P20 and P40 (P = 0.0001) in OIR than RA mice. OIR mice had dilated retinal veins at P20 and thinner inner retinas at P40. Retinal vein width positively correlated with inner retinal thickness (P = 0.008). b-wave amplitude was decreased in avascular retinal areas, and correlated with inner retinal thinning. b-wave peak time was prolonged in adult OIR mice at high intensities (P = 0.03).

CONCLUSIONS

Focal variations in retinal vascularization of OIR mice correlate with thickness and function. Adult OIR mice had increased retinal artery tortuosity, prolonged b-wave peak time, and decreased retinal vein width with inner retina attrition. These suggest abnormalities in inner retinal morphology or post-receptor signaling. Studying interactions between retinal vascular, structural, and functional changes could enhance knowledge of OIR pathogenesis and potential therapies.

Fully automated, deep learning segmentation of oxygen-induced retinopathy images

Oxygen-induced retinopathy (OIR) is a widely used model to study ischemia-driven neovascularization (NV) in the retina and to serve in proof-of-concept studies in evaluating antiangiogenic drugs for ocular, as well as nonocular, diseases. The primary parameters that are analyzed in this mouse model include the percentage of retina with vaso-obliteration (VO) and NV areas. However, quantification of these two key variables comes with a great challenge due to the requirement of human experts to read the images. Human readers are costly, time-consuming, and subject to bias. Using recent advances in machine learning and computer vision, we trained deep learning neural networks using over a thousand segmentations to fully automate segmentation in OIR images. While determining the percentage area of VO, our algorithm achieved a similar range of correlation coefficients to that of expert inter-human correlation coefficients. In addition, our algorithm achieved a higher range of correlation coefficients compared with inter-expert correlation coefficients for quantification of the percentage area of neovascular tufts. In summary, we have created an open-source, fully automated pipeline for the quantification of key values of OIR images using deep learning neural networks.

Norrin treatment improves ganglion cell survival in an oxygen-induced retinopathy model of retinal ischemia

Treatment of a mouse model of oxygen-induced retinopathy (OIR) with recombinant human Norrin (Norrie Disease Protein, gene: NDP) accelerates regrowth of the microvasculature into central ischemic regions of the neural retina, which are generated after treatment with 75% oxygen. While this reduces the average duration and severity of ischemia overall, we do not know if this accelerated recovery of the microvasculature results in any significant survival of retinal ganglion cells (RGCs). The purpose of this study was to investigate ganglion cell survival with and without the intravitreal injection of Norrin in the murine model of oxygen induced retinopathy (OIR), using two strains of mice: C57BL/6J and Thy1-YFP mice. Intravitreal injections of Norrin or vehicle were done after five days of exposure to 75% oxygen from ages P7 to P12. The C57BL/J mice were followed by Spectral-Domain Optical Coherence Tomography (SD-OCT), and the average nerve fiber layer (NFL) and inner-plexiform layer (IPL) thicknesses were measured at twenty-four locations per retina at P42. Additionally, some C57BL/J retinas were flat mounted and immunostained for the RGC marker, Brn3a, to compare the population density of surviving retinal ganglion cells. Using homozygous Thy1-YFP mice, single intrinsically fluorescent RGCs were imaged in live animals with a Micron-III imaging system at ages P21, 28 and P42. The relative percentage of YFP-fluorescent RGCs with dendritic arbors were compared. At age P42, the NFL was thicker in Norrin-injected OIR eyes, 14.4 μm, compared to Vehicle-injected OIR eyes, 13.3 μm (p = 0.01). In the superior retina, the average thickness of the IPL was greater in Norrin-injected OIR eyes, 37.7 μm, compared to Vehicle-injected OIR eyes, 34.6 μm (p = 0.04). Retinas from Norrin injected OIR mice had significantly more surviving RGCs (p = 0.03) than vehicle-injected mice. Based upon NFL thickness and counts of RGCs, we conclude that Norrin treatment, early in the ischemic phase, increased the relative population density of surviving RGCs in the central retinas of OIR mice.

Antibody-Mediated Inhibition of Tspan12 Ameliorates Vasoproliferative Retinopathy Through Suppression of β-Catenin Signaling

Background:

Anti-angiogenic biologicals represent an important concept for the treatment of vasoproliferative diseases. However, the need for continued treatment, the presence of nonresponders, and the risk of long-term side effects limit the success of existing therapeutic agents. Although Tspan12 has been shown to regulate retinal vascular development, nothing is known about its involvement in neovascular disease and its potential as a novel therapeutic target for the treatment of vasoproliferative diseases.

Methods:

Rodent models of retinal neovascular disease, including the mouse model of oxygen-induced retinopathy and the very low density lipoprotein receptor knockout mouse model were analyzed for Tspan/β-catenin regulation. Screening of a phage display of a human combinatorial antibody (Ab) library was used for the development of a high-affinity Ab against Tspan12. Therapeutic effects of the newly developed Ab on vascular endothelial cells were tested in vitro and in vivo in the oxygen-induced retinopathy and very low density lipoprotein receptor knockout mouse model.

Results:

The newly developed anti-Tspan12 Ab exhibited potent inhibitory effects on endothelial cell migration and tube formation. Mechanistic studies confirmed that the Ab inhibited the interaction between Tspan12 and Frizzled-4 and effectively modulates β-catenin levels and target genes in vascular endothelial cells. Tspan12/β-catenin signaling was activated in response to acute and chronic stress in the oxygen-induced retinopathy and very low density lipoprotein receptor mouse model of proliferative retinopathy. Intravitreal application of the Ab showed significant therapeutic effects in both models without inducing negative side effects on retina function. Moreover, combined intravitreal injection of the Ab with a known vascular endothelial growth factor inhibitor, Aflibercept, resulted in significant enhancement of the therapeutic efficacy of each monotherapy. Combination therapy with the Tspan12 blocking antibody can be used to reduce anti-vascular endothelial growth factor doses, thus decreasing the risk of long-term off-target effects.

Conclusions:

Tspan12/β-catenin signaling is critical for the progression of vasoproliferative disease. The newly developed anti-Tspan12 antibody has therapeutic effects in vasoproliferative retinopathy and can enhance the potency of existing anti- vascular endothelial growth factor agents.

Vulnerability of Dopaminergic Amacrine Cells to Chronic Ischemia in a Mouse Model of Oxygen-Induced Retinopathy

Purpose

Retinal dopamine deficiency is a potential cause of myopia and visual deficits in retinopathy of prematurity (ROP). We investigated the cellular mechanisms responsible for lowered levels of retinal dopamine in an oxygen-induced retinopathy (OIR) mouse model of ROP.

Methods

Retinopathy was induced by exposing mice to 75% oxygen from postnatal day 7 (P7) to P12. Oxygen-induced retinopathy and age-matched control mice were euthanized at P12, P17, P25, or P42 to P50. Immunohistochemistry, electrophysiology, and biochemical approaches were used to determine the effect of OIR on the structure and function of dopaminergic amacrine cells (DACs).

Results

The total number of DACs was unchanged in OIR retinas at P12 despite significant capillary dropout in the central retina. However, a significant loss of DACs was observed in P17 OIR retinas (in which neovascularization was maximal), with the cell loss being more profound in the central (avascular) than in the peripheral (neovascular) regions. Cell loss was persistent in both regions at P25, at which time retinal neovascularization had regressed. At P42, the percentage of DACs lost (54%) was comparable to the percent decrease in total dopamine content (53%). Additionally, it was found that DACs recorded in OIR retinas at P42 to P50 had a complete dendritic field and exhibited relatively normal spontaneous and light-induced electrical activity.

Conclusions

The results suggest that remaining DACs are structurally and functionally intact and that loss of DACs is primarily responsible for the decreased levels of retinal dopamine observed after OIR.

Efficacy of Intravitreal injection of 2-Methoxyestradiol in regression of neovascularization of a retinopathy of prematurity rat model

Background

Retinopathy of prematurity (ROP) is one of the targets for early detection and treatment to prevent childhood blindness in world health organization programs. The purpose of study was to evaluate the efficacy of intravitreal injection of 2-Methoxyestradiol (2-ME) nanoemulsion in regressing neovascularization of a ROP rat model.

Methods

A prospective comparative case - control animal study conducted on 56 eyes of 28 healthy new born Sprague Dawley male albino rat. ROP was induced in 21 rats then two concentrations of 2-ME nanoparticles were injected in right eyes of 14 rats (low dose; study group I, high dose; study group II). A blank nanoemulsion was injected in the right eyes of seven rats (control positive group I). No injections performed in contralateral left eyes (control positive group II). Seven rats (14 eyes) were kept in room air (control negative group). On postnatal day 17, eyeballs were enucleated. Histological structure of the retina was examined using Hematoxylin and eosin staining. Vascular endothelial growth factor (VEGF) and glial fibrillary acidic protein (GFAP) expressions were detected by immunohistochemical studies.

Results

Intravitreal injection of 2-ME (in the two concentrations) caused marked regression of the new vascular tufts on the vitreal side with normal organization and thickness of the retina especially in study group II, which also show negative VEGF immunoreaction. Positive GFAP expression was detected in the control positive groups and study group (I).

Conclusion

Intravitreal injection of 2-Methoxyestradiol nanoemulsion is a promising effective method in reduction of neovascularization of a ROP rat model.

Electronic supplementary material

The online version of this article (doi:10.1186/s12886-017-0433-3) contains supplementary material, which is available to authorized users.

In Vivo Angiography Quantifies Oxygen-Induced Retinopathy Vascular Recovery

Purpose

Retinopathy of prematurity (ROP) is a potentially blinding vasoproliferative disease. There is no standardized way to quantify plus disease (tortuous and dilated retinal vessels) or characterize abnormal recovery during ROP monitoring. This study objectively studies vascular features in live mice during development using noninvasive retinal imaging.

Methods

Using fluorescein angiography (FA), retinal vascular features were quantified in live mice with oxygen induced retinopathy (OIR). A total of 105 wild-type mice were exposed to 77% oxygen from postnatal day 7 (P7) till P12 (OIR mice). Also, 105 age-matched pups were raised in room air (RA mice). In vivo FA was performed at early (P16 to P20), mid (P23 to P27), late (P30 to P34), and mature (P47) phases of retinal vascular development. Retinal vascular area, retinal vein width, and retinal artery tortuosity were quantified.

Results

Retinal artery tortuosity was higher in OIR than RA mice at early (p < 0.0001), mid (p < 0.0001), late (p < 0.0001), and mature (p < 0.0001) phases. Retinal vascular area in OIR mice increased from early to mid-phase (p < 0.0001), but remained unchanged from mid to late (p = 0.23), and from late to mature phase (p = 0.98). Retinal vein width was larger in OIR mice compared to RA mice during early phase only. Arteries in OIR mice were more tortuous from early to mid-phase (p < 0.0001), but tortuosity remained stable from mid through mature phase. RA mice had an increase in retinal vascular area from early to late phase, but maintained uniform retinal vein width and retinal artery tortuosity in all phases.

Conclusions

In vivo FA distinguished arterial and venous features, similar to plus disease, and revealed aberrant recovery of OIR mice (arterial tortuosity, reduced capillary density, and absent neovascular buds) that persisted into adulthood. Retinal artery tortuosity may be a reliable, objective marker of severity of ROP. Infants with abnormal retinal vascular recovery may need extended monitoring.

Deficiency of aldose reductase attenuates inner retinal neuronal changes in a mouse model of retinopathy of prematurity

Retinopathy of prematurity (ROP) is a leading cause of childhood blindness where vascular abnormality and retinal dysfunction are reported. We showed earlier that genetic deletion of aldose reductase (AR), the rate-limiting enzyme in the polyol pathway, reduced the neovascularization through attenuating oxidative stress induction in the mouse oxygen-induced retinopathy (OIR) modeling ROP. In this study, we further investigated the effects of AR deficiency on retinal neurons in the mouse OIR. Seven-day-old wild-type and AR-deficient mice were exposed to 75 % oxygen for 5 days and then returned to room air. Electroretinography was used to assess the neuronal function at postnatal day (P) 30. On P17 and P30, retinal cytoarchitecture was examined by morphometric analysis and immunohistochemistry for calbindin, protein kinase C alpha, calretinin, Tuj1, and glial fibrillary acidic protein. In OIR, attenuated amplitudes and delayed implicit time of a-wave, b-wave, and oscillatory potentials were observed in wild-type mice, but they were not significantly changed in AR-deficient mice. The morphological changes of horizontal, rod bipolar, and amacrine cells were shown in wild-type mice and these changes were partly preserved with AR deficiency. AR deficiency attenuated the Müller cell gliosis induced in OIR. Our observations demonstrated AR deficiency preserved retinal functions in OIR and AR deficiency could partly reduce the extent of retinal neuronal histopathology. These findings suggested a therapeutic potential of AR inhibition in ROP treatment with beneficial effects on the retinal neurons.

Toll-like receptor 4 in bone marrow-derived cells contributes to the progression of diabetic retinopathy

Diabetic retinopathy (DR) is a major microvascular complication in diabetics, and its mechanism is not fully understood. Toll-like receptor 4 (TLR4) plays a pivotal role in the maintenance of the inflammatory state during DR, and the deletion of TLR4 eventually alleviates the diabetic inflammatory state. To further elucidate the mechanism of DR, we used bone marrow transplantation to establish reciprocal chimeric animals of TLR4 mutant mice and TLR4 WT mice combined with diabetes mellitus (DM) induction by streptozotocin (STZ) treatment to identify the role of TLR4 in different cell types in the development of the proinflammatory state during DR. TLR4 mutation did not block the occurrence of high blood glucose after STZ injection compared with WT mice but did alleviate the progression of DR and alter the expression of the small vessel proliferation-related genes, vascular endothelial growth factor (VEGF), and hypoxia inducible factor-1α (HIF-1α). Grafting bone marrow-derived cells from TLR4 WT mice into TLR4 mutant mice increased the levels of TNF-α, IL-1β, and MIP-2 and increased the damage to the retina. Similarly, VEGF and HIF-1α expression were restored by the bone marrow transplantation. These findings identify an essential role for TLR4 in bone marrow-derived cells contributing to the progression of DR.

Integral-geometry characterization of photobiomodulation effects on retinal vessel morphology

The morphological characterization of quasi-planar structures represented by gray-scale images is challenging when object identification is sub-optimal due to registration artifacts. We propose two alternative procedures that enhances object identification in the integral-geometry morphological image analysis (MIA) framework. The first variant streamlines the framework by introducing an active contours segmentation process whose time step is recycled as a multi-scale parameter. In the second variant, we used the refined object identification produced in the first variant to perform the standard MIA with exact dilation radius as multi-scale parameter. Using this enhanced MIA we quantify the extent of vaso-obliteration in oxygen-induced retinopathic vascular growth, the preventative effect (by photobiomodulation) of exposure during tissue development to near-infrared light (NIR, 670 nm), and the lack of adverse effects due to exposure to NIR light.

Cilostazol prevents retinal ischemic damage partly via inhibition of tumor necrosis factor-α-induced nuclear factor-kappa B/activator protein-1 signaling pathway

Cilostazol is a specific inhibitor of phosphodiesterase III and is widely used to treat ischemic symptoms of peripheral vascular disease. We evaluated the protective effects of cilostazol in a murine model of ocular ischemic syndrome in which retinal ischemia was induced by 5-h unilateral ligation of both the pterygopalatine artery (PPA) and the external carotid artery (ECA) in anesthetized mice. The effects of cilostazol (30 mg/kg, p.o.) on ischemia/reperfusion (I/R)-induced retinal damage were examined by histological, retinal vascular permeability, and electrophysiological analyses. Using immunoblotting, the protective mechanism for cilostazol was evaluated by examining antiinflammatory effects of cilostazol on the expression of tumor necrosis factors-α (TNF-α) and tight junction proteins (ZO-1 and claudin-5), and the phosphorylations of nuclear factor-kappa B (NF-κB) and c-Jun. The histological analysis revealed that I/R decreased the cell number in the ganglion cell layer (GCL) and the thicknesses of the inner plexiform layer (IPL) and inner nuclear layer (INL), and that cilostazol attenuated these decreases. Additionally, cilostazol prevented the hyperpermeability of blood vessels. Electroretinogram (ERG) measurements revealed that cilostazol prevented the I/R-induced reductions in a-, b-, and oscillatory potential (OP) wave amplitudes seen at 5 days after I/R. Cilostazol inhibited the increased expression of TNF-α and the phosphorylation levels of NF-κB and c-Jun in the retina after I/R. In addition, cilostazol prevented TNF-α-induced reduction of ZO-1 and claudin-5 expression in human retinal microvascular endothelial cells (HRMECs). These findings indicate that cilostazol may prevent I/R-induced retinal damage partly through inhibition of TNF-α-induced NF-κB/AP-1 signaling pathway.

Modulation of angiogenesis by genetic manipulation of ATF4 in mouse model of oxygen-induced retinopathy [corrected]

PURPOSE

The activation of the unfolded protein response (UPR) and an increase in activating transcription factor 4 (ATF4) has been previously reported in the diabetic retina. Despite this, a direct link between ATF4 and the degree of proliferative retinopathy has not been demonstrated to date. Therefore, the objective of this study was to determine whether ATF4 deficiency could reduce neovascularization in mice with oxygen-induced retinopathy (OIR).

METHODS

We induced OIR in C57BL/6, ATF4(+/-), and endoplasmic reticulum stress-activated indicator (ERAI) mice and used quantitative RT-PCR and Western blot analysis to evaluate relative gene and protein expression. Histology and microscopy were used to calculate the extent of neovascularization in flat-mounted retinas.

RESULTS

Experimental data revealed Xbp1 splicing in the retinal ganglia cells, outer plexiform layer, inner nuclear layer, and outer nuclear layer and in pericytes of postdevelopment day 17 ERAI OIR mice, confirming the activation of IRE1 UPR signaling. In naive ATF4-deficient mice, we also observed an elevation in UPR-associated and vascular-associated gene expression (Bip, Atf6, Hif1a, Pik3/Akt, Flt1/Vegfa, and Tgfb1), which may have contributed to the alleviation of hypoxia-driven neovascularization in experimental ATF4(+/-) retinas. The OIR ATF4(+/-) retinas demonstrated reprogramming of the UPR seen at both the mRNA (Atf6 and Bip) and protein (pATF6 and peIf2α) levels, as well as a reduction in vascularization-associated gene expression (Flt1, Vegf1, Hif1, and Tgb1). These changes corresponded to the decline in the rate of neovascularization.

CONCLUSIONS

Our study validates ATF4 as a prospective therapeutic target to inhibit neovascularization in proliferative retinopathy.

Arginase in retinopathy

Ischemic retinopathies, such as diabetic retinopathy (DR), retinopathy of prematurity and retinal vein occlusion are a major cause of blindness in developed nations worldwide. Each of these conditions is associated with early neurovascular dysfunction. However, conventional therapies target clinically significant macula edema or neovascularization, which occur much later. Intra-ocular injections of anti-VEGF show promise in reducing retinal edema, but the effects are usually transient and the need for repeated injections increases the risk of intraocular infection. Laser photocoagulation can control pathological neovascularization, but may impair vision and in some patients the retinopathy continues to progress. Moreover, neither treatment targets early stage disease or promotes repair. This review examines the potential role of the ureahydrolase enzyme arginase as a therapeutic target for the treatment of ischemic retinopathy. Arginase metabolizes l-arginine to form proline, polyamines and glutamate. Excessive arginase activity reduces the l-arginine supply for nitric oxide synthase (NOS), causing it to become uncoupled and produce superoxide and less NO. Superoxide and NO react and form the toxic oxidant peroxynitrite. The catabolic products of polyamine oxidation and glutamate can induce more oxidative stress and DNA damage, both of which can cause cellular injury. Studies indicate that neurovascular injury during retinopathy is associated with increased arginase expression/activity, decreased NO, polyamine oxidation, formation of superoxide and peroxynitrite and dysfunction and injury of both vascular and neural cells. Furthermore, data indicate that the cytosolic isoform arginase I (AI) is involved in hyperglycemia-induced dysfunction and injury of vascular endothelial cells whereas the mitochondrial isoform arginase II (AII) is involved in neurovascular dysfunction and death following hyperoxia exposure. Thus, we postulate that activation of the arginase pathway causes neurovascular injury by uncoupling NOS and inducing polyamine oxidation and glutamate formation, thereby reducing NO and increasing oxidative stress, all of which contribute to the retinopathic process.