Prolonged elevation of intraocular pressure results in retinal ganglion cell loss and abnormal retinal function in mice

Neuroprotective Effect of a Nutritional Supplement Containing Spearmint Extract, Forskolin, Homotaurine and Group B Vitamins in a Mouse Model of Transient Ocular Hypertension

Glaucoma is one of the most common sight-threatening eye disorders and one of the main causes of irreversible blindness worldwide. The current therapies focusing on reducing intraocular pressure (IOP) are often insufficient to prevent the progression of the disease, so the therapeutic management of glaucoma remains a challenge. The aim of this study was to evaluate the neuroprotective, IOP-lowering independent effects of a nutritional supplement containing forskolin, homotaurine, spearmint extract and vitamins of the B group in a model of acute glaucoma developed in mice. Glaucoma was induced in adult wild-type C57BL/6J mice by transient elevation of IOP. The dietary supplement, branded as Gangliomix^® (125 mg/kg/day), was administered by oral gavage for 17 days and ocular hypertension was induced on the 10th day of treatment. A histological analysis of the retinas was performed and RGC survival was evaluated with fluorogold labeling and Brn3a immunostaining on wholemount and retinal sections. Expression of alpha-spectrin, caspase-3, PARP-1 and GFAP was studied with western blotting or immunofluorescence. A significant increase in RGC survival was reported in the retina of mice treated with the dietary supplement as compared to vehicle-treated animals. The observed neuroprotection was associated with a calpain activity decrease, reduction in caspase-3 and PARP-1 activation, and prevention of GFAP upregulation. These effects were independent from the hypotensive effects of the supplement. Altogether, our data suggest that the dietary supplementation with forskolin, homotaurine, spearmint extract and vitamins of the B group supports RGC survival and may offer beneficial effects in glaucoma patients in combination with the currently used IOP-lowering therapy.

Retinal Ganglion Cells: Global Number, Density and Vulnerability to Glaucomatous Injury in Common Laboratory Mice

How many RBPMS⁺ retinal ganglion cells (RGCs) does a standard C57BL/6 laboratory mouse have on average and is this number substrain- or sex-dependent? Do RGCs of (European) C57BL/6J and -N mice show a different intrinsic vulnerability upon glaucomatous injury? Global RGC numbers and densities of common laboratory mice were previously determined via axon counts, retrograde tracing or BRN3A immunohistochemistry. Here, we report the global RGC number and density by exploiting the freely available tool RGCode to automatically count RGC numbers and densities on entire retinal wholemounts immunostained for the pan-RGC marker RBPMS. The intrinsic vulnerability of RGCs from different substrains to glaucomatous injury was evaluated upon introduction of the microbead occlusion model, followed by RBPMS counts, retrograde tracing and electroretinography five weeks post-injury. We demonstrate that the global RGC number and density varies between substrains, yet is not sex-dependent. C57BL/6J mice have on average 46K ± 2K RBPMS⁺ RGCs per retina, representing a global RGC density of 3268 ± 177 RGCs/mm². C57BL/6N mice, on the other hand, have on average less RBPMS⁺ RGCs (41K ± 3K RGCs) and a lower density (3018 ± 189 RGCs/mm²). The vulnerability of the RGC population of the two C57BL/6 substrains to glaucomatous injury did, however, not differ in any of the interrogated parameters.

Utilization of Modified Induced Pluripotent Stem Cells as the Advance Therapy of Glaucoma: A Systematic Review

Glaucoma is an optic neuropathy disease that causes cupping of the optic disc and decreased visual field. Glaucoma is still the second leading cause of blindness globally, with a worldwide prevalence of more than 76 million people in 2020. However, no therapy can cure glaucoma completely, especially when optic nerve damage has occurred. Available treatments only play a role in keeping the intraocular pressure stable This research aims to determine the potential use of modified stem cell therapy to treat intraocular damage in glaucoma cases. Literature research was conducted by involving seven online databases, namely Pubmed, ScienceDirect^®, Proquest, EBSCOhost^®, SAGE^®, Clinicalkey^®, and Scopus, published between 2010–2020 with the keywords stem cells; therapy; glaucoma; optic nerve. Six articles were selected, and out of the six articles, all writings were experimental research. The entire literature states that modified stem cell therapy has the potential as a therapeutic option in treating intraocular damage in patients with glaucoma. Based on the systematic literature review that has been carried out, it is known that stem cell therapy has the potential to be a therapeutic option in treating glaucoma cases. Much more research is needed to assess the effectiveness of modified stem cell therapy in managing intraocular damage due to glaucoma.

Influence of Sex on Neuroretinal Degeneration: Six-Month Follow-Up in Rats With Chronic Glaucoma

Purpose

To evaluate differences by sex in the neuroretina of rats with chronic glaucoma over 24 weeks of follow-up, and to assess by sex the influence on neurodegeneration of different methods of inducing ocular hypertension.

Methods

Forty-six Long-Evans rats-18 males and 28 females-with induced chronic glaucoma were analyzed. Glaucoma was achieved via 2 models: repeatedly sclerosing the episcleral veins (9 male/14 female) or by injecting poly(lactic-co-glycolic acid) microspheres measuring 20 to 10 µm (Ms20/10) into the anterior chamber (9 male/14 female). The IOP was measured weekly by tonometer; neuroretinal function was recorded by dark/light-adapted electroretinography at baseline and weeks 12 and 24; and structure was analyzed by optical coherence tomography using the retina posterior pole, retinal nerve fiber layer and ganglion cell layer protocols at baseline and weeks 8, 12, 18, and 24.

Results

Males showed statistically significant (P < 0.05) higher IOP in both chronic glaucoma models, and greater differences were found in the episcleral model at earlier stages. Males with episclerally induced glaucoma showed a statistically higher increase in retinal thickness in optical coherence tomography recordings than females and also when comparing Ms20/10 at 12 weeks. Males showed a higher percentage of retinal nerve fiber layer thickness loss in both models. Ganglion cell layer thickness loss was only detected in the Ms20/10 model. Males exhibited worse dark/light-adapted functionality in chronic glaucoma models, which worsened in the episcleral sclerosis model at 12 weeks, than females.

Conclusions

Female rats with chronic glaucoma experienced lower IOP and structural loss and better neuroretinal functionality than males. Sex and the ocular hypertension-inducing method influenced neuroretinal degeneration.

Targeting HDAC3 in the DBA/2J spontaneous mouse model of glaucoma

High intraocular pressure (IOP) is the most common risk factor associated with glaucoma in humans. While lowering IOP is effective at reducing the rate of retinal ganglion cell (RGC) loss, to date, no treatment exists to directly preserve these cells affected by damage to the optic nerve. Recently, histone deacetylase-3 (HDAC3) has become a potential therapeutic target because it plays an important role in the early nuclear atrophic events that precede RGC death. Conditional knockout or inhibition of HDAC3 prevents histone deacetylation, heterochromatin formation, apoptosis, and eventual RGC loss following acute optic nerve injury. Using these approaches to repress HDAC3 activity, we tested whether targeting HDAC3 protects RGCs from ganglion cell-specific BRN3A expression loss, total somatic cell loss, and optic nerve degeneration in the DBA/2J mouse model of spontaneous glaucoma. Targeted ablation of Hdac3 activity did not protect RGCs from axonal degeneration or somatic cell death in the DBA/2J mouse model of glaucoma. However, inhibition of HDAC3 activity using RGFP966 conferred mild protection against somatic cell loss in the ganglion cell layer in aged DBA/2J mice. Further experimentation is necessary to determine whether other class I HDACs may serve as potential therapeutic targets in chronic models of glaucoma.

Functional analysis of mesencephalic astrocyte-derived neurotrophic factor in retinal ganglion cells under oxidative stress

Glaucoma is optic neuropathy that is characterized by progressive neurodegeneration of the retinal ganglion cells (RGCs) and axons. This condition will lead to visual impairment and bring glaucoma to become the second cause of blindness globally. Neuroprotection in glaucoma is needed to prevent the progression of optic neuropathy. In this study, we examined the effects of the superior colliculus (SC), and mesencephalic astrocyte-derived neurotrophic factor (MANF) secreted from the SC, on RGC survival after oxidative stress. SC slices and RGCs from rats (3-day old) were co-cultured using a 3D-transwell system. In addition, primary RGCs from 4 to 5-day-old rats were cultured and treated with 100 μM hydrogen peroxide (H₂ O₂ ), together with stimulation by MANF. Immunoblot and immunofluorescence analyses indicated down-regulated expression levels of several survival markers of RGCs. Extension of neurites was decreased in RGCs treated with 100 μM H₂ O₂ . Following co-culture with SC slices, or the addition of MANF, we found that both the down-regulated expression of neural markers and extension of neurites caused by oxidative stress in RGCs were blocked. Furthermore, we found a decrease in the expression of neural markers and extension of neurites after co-culture with MANF siRNA-treated SC slices compared with slices treated with mock siRNA, but, RGCs co-cultured with SC slices treated with MANF siRNA displayed no-changed about to apoptosis. These results suggest that MANF secreted from the SC may play an important role in maintenance of function and survival of RGCs. It is also possible that MANF is an important factor in neuroprotection of RGCs. SIGNIFICANCE OF THE STUDY: Glaucoma is a progressive neurodegenerative disease of the retinal ganglion cell (RGC) and their axons. Neuroprotection is aimed at protecting those neurons that are damaged glaucomatous optic neuropathy. We have now examined the effects of superior colliculus, or msencephalic astrocyte-derived neutrophic factor (MANF), secreted from superior colliculus, on RGC survival using co-culture system. Our results suggested that MANF may important key factor in neuroprotection of RGC.

Neuroprotective effect of a dietary supplement against glutamate-induced excitotoxicity in retina

AIM

To evaluate the neuroprotective effect of a dietary supplement (ClearVision EX^®; CV) against glutamate-induced excitotoxicity in retina.

METHODS

We evaluated the protective effects CV on glutamate-induced cell toxicity of an immortalized mouse hippocampal cell line (HT-22) in vitro and N-methyl-D-aspartate (NMDA) induced retinal injury in vivo. Once-daily oral administration of CV or vehicle (5% Arabic gum) was started the day before the NMDA injection and continued until the end of the study. Electroretinograms (ERGs) were recorded to evaluate the retinal function at 2d after NMDA injection. Furthermore, a histological evaluation, Western blot analysis, and immunohistochemistry were performed for assessing the signal transduction pathway.

RESULTS

HT-22 cell death was induced by the addition of glutamate and co-incubation with CV protected against it. Oral administration of CV inhibited the decrease in scotopic threshold response amplitudes induced by the intravitreal injection of NMDA and those of the thickness of the inner retinal layer in the histological evaluation. The increased phosphorylated levels of extracellular signal-regulated kinase (ERK) but not cAMP response element binding protein (CREB) or Akt were observed 1h after NMDA injection in both the vehicle- and CV-treated rats; however, pERK activation was no more upregulated at 3h after NMDA injection. pERK upregulation was observed in Müller cells.

CONCLUSION

CV shows a protective effect against both glutamate-induced HT-22 cell death and NMDA-induced retinal damage. pERK upregulation in the Müller cells plays a key role in the protective effect of CV against glutamate-induced retinal toxicity.

Phenotypic and Functional Characterization of Müller Glia Isolated from Induced Pluripotent Stem Cell-Derived Retinal Organoids: Improvement of Retinal Ganglion Cell Function upon Transplantation

Glaucoma is one of the leading causes of blindness, and there is an ongoing need for new therapies. Recent studies indicate that cell transplantation using Müller glia may be beneficial, but there is a need for novel sources of cells to provide therapeutic benefit. In this study, we have isolated Müller glia from retinal organoids formed by human induced pluripotent stem cells (hiPSCs) in vitro and have shown their ability to partially restore visual function in rats depleted of retinal ganglion cells by NMDA. Based on the present results, we suggest that Müller glia derived from retinal organoids formed by hiPSC may provide an attractive source of cells for human retinal therapies, to prevent and treat vision loss caused by retinal degenerative conditions. Stem Cells Translational Medicine 2019;8:775&784.

Methods to Induce Chronic Ocular Hypertension: Reliable Rodent Models as a Platform for Cell Transplantation and Other Therapies

Glaucoma, a form of progressive optic neuropathy, is the second leading cause of blindness worldwide. Being a prominent disease affecting vision, substantial efforts are being made to better understand glaucoma pathogenesis and to develop novel treatment options including neuroprotective and neuroregenerative approaches. Cell transplantation has the potential to play a neuroprotective and/or neuroregenerative role for various ocular cell types (e.g., retinal cells, trabecular meshwork). Notably, glaucoma is often associated with elevated intraocular pressure, and over the past 2 decades, several rodent models of chronic ocular hypertension (COH) have been developed that reflect these changes in pressure. However, the underlying pathophysiology of glaucoma in these models and how they compare to the human condition remains unclear. This limitation is the primary barrier for using rodent models to develop novel therapies to manage glaucoma and glaucoma-related blindness. Here, we review the current techniques used to induce COH-related glaucoma in various rodent models, focusing on the strengths and weaknesses of the each, in order to provide a more complete understanding of how these models can be best utilized. To so do, we have separated them based on the target tissue (pre-trabecular, trabecular, and post-trabecular) in order to provide the reader with an encompassing reference describing the most appropriate rodent COH models for their research. We begin with an initial overview of the current use of these models in the evaluation of cell transplantation therapies.

Transcriptomic profiles of retinal ganglion cells are defined by the magnitude of intraocular pressure elevation in adult mice

Elevated intraocular pressure (IOP) is the major risk factor for glaucoma, a sight threatening disease of retinal ganglion cells (RGCs) and their axons. Despite the central importance of IOP, details of the impact of IOP elevation on RGC gene expression remain elusive. We developed a 4-step immunopanning protocol to extract adult mouse RGCs with high fidelity and used it to isolate RGCs from wild type mice exposed to 2 weeks of IOP elevation generated by the microbead model. IOP was elevated to 2 distinct levels which were defined as Mild (IOP increase >1 mmHg and <4 mmHg) and Moderate (IOP increase ≥4 mmHg). RNA sequencing was used to compare the transcriptional environment at each IOP level. Differentially expressed genes were markedly different between the 2 groups, and pathway analysis revealed frequently opposed responses between the IOP levels. These results suggest that the magnitude of IOP elevation has a critical impact on RGC transcriptional changes. Furthermore, it is possible that IOP-based set points exist within RGCs to impact the direction of transcriptional change. It is possible that this improved understanding of changes in RGC gene expression can ultimately lead to novel diagnostics and therapeutics for glaucoma.

Enlarged Optic Nerve Axons and Reduced Visual Function in Mice with Defective Microfibrils

Glaucoma is a leading cause of irreversible vision loss due to retinal ganglion cell (RGC) degeneration that develops slowly with age. Elevated intraocular pressure (IOP) is a significant risk factor, although many patients develop glaucoma with IOP in the normal range. Mutations in microfibril-associated genes cause glaucoma in animal models, suggesting the hypothesis that microfibril defects contribute to glaucoma. To test this hypothesis, we investigated IOP and functional/structural correlates of RGC degeneration in mice of either sex with abnormal microfibrils due to heterozygous Tsk mutation of the fibrilin-1 gene (Fbn1^Tsk/+). Although IOP was not affected, Fbn1^Tsk/+ mice developed functional deficits at advanced age consistent with glaucoma, including reduced RGC responses in electroretinogram (ERG) experiments. While RGC density in the retina was not affected, the density of RGC axons in the optic nerve was significantly reduced in Fbn1^Tsk/+ mice. However, reduced axon density correlated with expanded optic nerves, resulting in similar numbers of axons in Fbn1^Tsk/+ and control nerves. Axons in the optic nerves of Fbn1^Tsk/+ mice were significantly enlarged and axon diameter was strongly correlated with optic nerve area, as has been reported in early pathogenesis of the DBA/2J mouse model of glaucoma. Our results suggest that microfibril abnormalities can lead to phenotypes found in early-stage glaucomatous neurodegeneration. Thinning of the elastic fiber-rich pia mater was found in Fbn1^Tsk/+ mice, suggesting mechanisms allowing for optic nerve expansion and a possible biomechanical contribution to determination of axon caliber.

Morphological and functional changes in the rat retina associated with 2 months of intermittent moderate intraocular pressure elevation

Morphological and functional changes in the rat retina and optic nerve head (ONH), associated with 8 weeks of intermittent moderately elevated intraocular pressure (IOP) were measured with a combined ultrahigh resolution optical coherence tomography (UHR-OCT) and electroretinography (ERG) system. The IOP of male Sprague-Dawley rats was raised in one eye to ~35 mmHg for 1 hour/day on 6 days each week using vascular loops. Single-flash ERG traces and volumetric UHR-OCT images of the ONH were acquired from both eyes before, during and after IOP elevations at weeks 1, 5 and 9 of the study. The UHR-OCT images showed depression of the posterior eye around the ONH during the IOP elevations, the magnitude of which increased significantly from week 1 to week 9 (p = 0.01). The ERG a-wave and b-wave amplitudes increased temporarily during IOP elevations and returned to normal ~30 minutes after loop removal. Recurrent intermittent IOP spikes caused > 30% decrease in the ERG a-wave and b-wave amplitudes measured during the IOP elevations over the course of 2 months. This study suggests that recurrent, relatively short-duration IOP spikes for extended period of time are associated with peri-ONH tissue hypercompliance and reduced retinal functional response to visual stimulation during acute IOP elevation.

Neuroprotection of Transplanting Human Umbilical Cord Mesenchymal Stem Cells in a Microbead Induced Ocular Hypertension Rat Model

PURPOSE

The purpose of this study is to investigate the potential therapeutic benefits of intravitreally transplanted human umbilical cord mesenchymal stem cells (UC-MSCs) in an animal model of microbead-injection-induced ocular hypertension (OHT).

METHODS

UC-MSCs were isolated from human umbilical cords and then cultured. The OHT model was induced via intracameral injection of polystyrene microbeads in Sprague-Dawley adult rat eyes. Fifty-four healthy adult rats were randomly divided into three groups: normal control, OHT model treated with intravitreal transplantation of UC-MSCs, or phosphate-buffered saline (PBS). Two days after OHT was induced, either 5 µl 10⁵ UC-MSCs suspension or PBS was injected into the vitreous cavity of rats. UC-MSCs localization and integration were examined via immunohistochemistry. Neuroprotection was quantified by counting retinal ganglion cells (RGCs) and axons 2 weeks following transplantation. The expression levels of glial-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and glial fibrillary acidic protein (GFAP) were assessed via immunohistochemistry and Western blot. Functional recovery was assessed 2 weeks after transplantation via scotopic threshold response (STR) electroretinography.

RESULTS

Elevated IOP levels were sustained at least 3 weeks after intracameral microbead injection and the number of β-III-tubulin⁺ RGCs significantly declined compared to PBS-injected eyes. UC-MSCs survived for at least 2 weeks after intravitreal transplantation and predominantly located in the vitreous cavity. A fraction of cells migrated into the ganglion cell layer of host retina, but without differentiation. Intravitreal UC-MSC transplantation resulted in increased number of RGCs, axons, and increased expression of GDNF and BDNF but decreased expression of GFAP. Intravitreal delivery of UC-MSCs significantly improved the recovery of the positive STR.

CONCLUSIONS

Intravitreal transplantation of UC-MSCs revealed the neuroprotection in the microbead-injection induced OHT. The effects could be related to the secretion of tropic factors (BDNF and GDNF) and the modulation of glial cell activation.

Characterization of Retinal Ganglion Cell and Optic Nerve Phenotypes Caused by Sustained Intracranial Pressure Elevation in Mice

Elevated intracranial pressure (ICP) can result in multiple neurologic sequelae including vision loss. Inducible models of ICP elevation are lacking in model organisms, which limits our understanding of the mechanism by which increased ICP impacts the visual system. We adapted a mouse model for the sustained elevation of ICP and tested the hypothesis that elevated ICP impacts the optic nerve and retinal ganglion cells (RGCs). ICP was elevated and maintained for 2 weeks, and resulted in multiple anatomic changes that are consistent with human disease including papilledema, loss of physiologic cupping, and engorgement of the optic nerve head. Elevated ICP caused a loss of RGC somas in the retina and RGC axons within the optic nerve, as well as a reduction in both RGC electrical function and contrast sensitivity. Elevated ICP also caused increased hypoxia-inducible factor (HIF)-1 alpha expression in the ganglion cell layer. These experiments confirm that sustained ICP elevation can be achieved in mice and causes phenotypes that preferentially impact RGCs and are similar to those seen in human disease. With this model, it is possible to model human diseases of elevated ICP such as Idiopathic Intracranial Hypertension and Spaceflight Associated Neuro-ocular Syndrome.

CYP1B1: A key regulator of redox homeostasis

CYP1B1 is a member of the CYP1 subfamily of CYP superfamily of enzymes, which contains three members, CYP1A1, CYP1A2, and CYP1B1. CYP1B1 is expressed in both adult and fetal human extrahepatic tissues, including the parenchymal and stromal cells of most organs. Mutations in the CYP1B1 gene are linked to the development of primary congenital glaucoma in humans. However, the underlying mechanisms remain unknown. Using Cyp1b1-deficient mice, we showed that CYP1B1 is constitutively expressed in retinal vascular cells with a significant role in retinal neovascularization during oxygen-induced ischemic retinopathy. We also showed CYP1B1 is constitutively expressed in trabecular meshwork (TM) cells and its expression plays a significant role in the normal development and function of the TM tissue. We have observed that germline deletion of Cyp1b1 is associated with increased oxidative stress in the retinal vascular and TM cells in culture, and retinal and TM tissue in vivo. We showed increased oxidative stress was responsible for altered production of the extracellular matrix proteins and had a significant impact on cellular integrity and function of these tissues. Collectively, our studies have established an important role for CYP1B1 expression in modulation of tissue integrity and function through the regulation of cellular redox homeostasis and extracellular microenvironment.

Total Outflow Facility in Live C57BL/6 Mice of Different Age

Purpose

To characterize total outflow facility across the live adult mouse lifespan as a reference for mouse glaucoma studies and the common C57BL/6 background strain.

Methods

Microperfusion was performed by single-needle cannulation and feedback-controlled coupling of pressure and flow to maintain a constant pressure in the anterior chambers of live C57BL/6NCrl mice aged 3-4 months (n = 17), 6-9 months (n = 10), and 23-27 months (n = 12). This mouse age range represented an equivalent human age range of young adult to elderly. We characterized the following across age groups in vivo: (1) outflow facility based on constant pressure perfusion in a pressure range of 15-35 mm Hg, (2) perfusion flow rates, and (3) anterior segment tissue histology after perfusion. Thirty-nine live mice underwent perfusion.

Results

Pressure-flow rate functions were consistently linear for all age groups (all R ² > 0.96). Total outflow facility in mice aged 3-4, 6-9, and 23-27 months was 0.0066, 0.0064, and 0.0077 μL/min/mm Hg, respectively. Facility was not significantly different between age groups (all p > 0.4). The groups had closely overlapping frequency distribution profiles with right-sided tails. Post hoc estimates indicated that group facility differences of at least 50% would have been detectable, with this limit set mainly by inherent variability in the strain. A trend toward higher perfusion flow rates was seen in older mice aged 23-27 months, but this was not significantly different from that of mice aged 3-4 months or 6-9 months (p > 0.2). No histological disruption or difference in iridocorneal angle or drainage tissue structure was seen following perfusion in the different age groups.

Conclusion

We did not find a significant difference in total outflow facility between different age groups across the live C57BL/6 mouse adult lifespan, agreeing with some human studies. The possibility that more subtle differences might exist ought to be judged with respect to the heterogeneity in facility at different ages. Our findings provide reference data for live perfusion studies pertaining to glaucoma involving the C57BL/6 strain.

Retinal pathology in the PPCD1 mouse

Retinal phenotypes of the PPCD1 mouse, a mouse model of posterior polymorphous corneal dystrophy, have been characterized. PPCD1 mice on the DBA/2J background (D2.Ppcd1) have previously been reported to develop an enlarged anterior chamber due to epithelialization and proliferation of the corneal endothelium and subsequent blockage of the iridocorneal angle. Results presented here show that D2.Ppcd1 mice develop increased intraocular pressure (IOP), with measurements at three months of age revealing significant increases in IOP. Significant retinal ganglion cell layer cell loss is observed at five months of age. D2.Ppcd1 animals also exhibit marked degeneration of the outer nuclear layer in association with hyperplasia of the retinal pigment epithelium. Evidence of retinal detachment is present as early as three weeks of age. By 3.5 months of age, focal areas of outer nuclear layer loss are observed. Although the Gpnmb^R150X mutation leads to increased IOP and glaucoma in DBA/2J mice, development of anterior segment and retinal defects in D2.Ppcd1 animals does not depend upon presence of the Gpnmb^R150X mutation.

Optical modelling of a supplementary tunable air-spaced goggle lens for rodent eye imaging

Aberration variations severely degrade retinal imaging in small animal eyes. Previously, the approach of a goggle lens with a matching corneal index was proposed to overcome the on-axis resolution limit of static imaging systems, which allows the use of the full eye pupil. But this technique didn’t address the problem of the large power variation, and the ensuing aberration on and off-axis, when dealing with small animal eyes. In this study, we present the concept of a tunable goggle lens, designed to compensate individual ocular aberration for different rodent eye powers. Ray tracing evidences that lens-fitted goggles permit, not only to adjust individual eyes power, but also to surpass conventional adaptive correction technique over large viewing angle, provided a minimum use of two spaced liquids. We believe that the overlooked advantage of the 3D lens function is a seminal finding for further technological advancements in widefield retinal imaging.

Establishment of an experimental glaucoma animal model: A comparison of microbead injection with or without hydroxypropyl methylcellulose

The present study aimed to compare microbead injection with and without hydroxypropyl methylcellulose (HPM) in order to establish an experimental animal model of glaucoma. This model was established in C57BL/6 mice and transgenic mice expressing cyan fluorescent protein (CFP) under the control of the Thy1 promoter in retinal ganglion cells (RGCs). C57BL/6 mice aged between 12 and 20 weeks old were randomly separated into three groups, which received different injections into the anterior chamber of the eye. Group A (microbead) received 2 µl microbeads (10×106 beads/ml) and 1 µl air. Group B (microbeads + HPM) received 2 µl microbeads and 1 µl HPM. Group C (control group) received 2 µl PBS and 1 µl air. The intraocular pressure (IOP) was measured with a tonometer under topical anesthesia daily for 1 month. A single injection of microbeads, with or without HPM, induced consistent IOP elevation when compared with the control group. Thy1-CFP mice received an injection of 2 µl microbeads and 1 µl HPM into the anterior chamber of the eyes, and the number of CFP+ RGCs was subsequently assessed in vivo by confocal scanning laser microscopy in the same area of the retina weekly for 6 weeks. The results from in vivo imaging of Thy1-CFP mice were comparable with the immunohistochemical staining results from the C57BL/6 mice. The combined injection of microbeads and HPM induced longer and higher peaks of IOP elevation when compared with the microbeads alone. The rate of RGC loss following the administration of microbeads alone was 25.0±1.3% 6 weeks after the initial IOP elevation, while it was 33.2±1.9% following the administration of microbeads + HPM. These results indicate that the injection of microbeads + HPM is a more effective method of establishing a mouse model with chronic elevation of IOP. In addition, the in vivo imaging that can be used with this technique provides an effective and noninvasive approach for monitoring the progress of RGC loss.

Effects of Chronic and Acute Intraocular Pressure Elevation on Scotopic and Photopic Contrast Sensitivity in Mice

PURPOSE

To compare the impact of intraocular pressure (IOP) elevation on scotopic and photopic contrast sensitivity in mice.

METHODS

We chronically elevated the IOP of wild-type mice via injection of polystyrene beads or acutely via injection of highly cohesive sodium hyaluronate. Some eyes with chronically elevated IOP were treated with either topical brimonidine tartrate 0.1% or brinzolamide 1%. Scotopic and photopic contrast sensitivity was assessed at peak spatiotemporal frequencies at multiple time points, with an established optokinetic technique. Retinal ganglion cell (RGC) counts were determined with an antibody to class III beta-tubulin. Correlations among IOP level, RGC count, and scotopic or photopic contrast sensitivity were performed.

RESULTS

Six weeks of IOP elevation caused a generalized reduction of photopic contrast sensitivity and a preferential reduction of scotopic contrast sensitivity at peak spatiotemporal frequencies. The administration of brinzolamide but not brimonidine caused a significant reduction in cumulative IOP, whereas brimonidine, but not brinzolamide, prevented RGC loss. Both brimonidine and brinzolamide prevented contrast sensitivity loss, but brimonidine did so at earlier time points and across a wider range of lighting conditions. Following either chronic or acute IOP elevation, scotopic contrast sensitivity was impacted most prominently by IOP level and not by RGC count, while photopic contrast sensitivity was impacted by a combination of factors.

CONCLUSIONS

It is possible that scotopic-specific retinal circuitry is altered preferentially by IOP elevation, and that changes in scotopic contrast sensitivity will assist with glaucoma detection. Brimonidine appears to prevent RGC loss via an IOP-independent mechanism.

Optic neuropathy and increased retinal glial fibrillary acidic protein due to microbead-induced ocular hypertension in the rabbit

AIM

To characterize whether a glaucoma model with chronic elevation of the intraocular pressure (IOP) was able to be induced by anterior chamber injection of microbeads in rabbits.

METHODS

In order to screen the optimal dose of microbead injection, IOP was measured every 3d for 4wk using handheld applanation tonometer after a single intracameral injection of 10 µL, 25 µL, 50 µL or 100 µL microbeads (5×10⁶ beads/mL; n=6/group) in New Zealand White rabbits. To prolong IOP elevation, two intracameral injections of 50 µL microbeads or phosphate buffer saline (PBS) were made respectively at days 0 and 21 (n=24/group). The fellow eye was not treated. At 5wk after the second injection of microbeads or PBS, bright-field microscopy and transmission electron microscopy (TEM) were used to assess the changes in the retina. The expression of glial fibrillary acidic protein (GFAP) in the retina was evaluated by immunofluorescence, quantitative real-time polymerase chain reaction and Western blot at 5wk after the second injection of microbeads.

RESULTS

Following a single intracameral injection of 10 µL, 25 µL, 50 µL or 100 µL microbead, IOP levels showed a gradual increase and a later decrease over a 4wk period after a single injection of microbead into the anterior chamber of rabbits. A peak IOP was observed at day 15 after injection. No significant difference in peak value of IOP was found between 10 µL and 25 µL groups (17.13±1.25 mm Hg vs 17.63±0.74 mm Hg; P=0.346). The peak value of IOP from 50 µL group (23.25±1.16 mm Hg) was significantly higher than 10 µL and 25 µL groups (all P<0.05). Administration of 100 µL microbead solution (23.00±0.93 mm Hg) did not lead to a significant increase in IOP compared to the 50 µL group (P=0.64). A prolonged elevated IOP duration up to 8wk was achieved by administering two injections of 50 µL microbeads (20.48±1.21 mm Hg vs 13.60±0.90 mm Hg in PBS-injected group; P<0.05). The bright-field and TEM were used to assess the changes of retinal ganglion cells (RGCs). Compared with PBS-injected group, the extended IOP elevation was associated with the degeneration of optic nerve, the reduction of RGC axons (47.16%, P<0.05) and the increased GFAP expression in the retina (4.74±1.10 vs 1.00±0.46, P<0.05).

CONCLUSION

Two injections of microbeads into the ocular anterior chamber of rabbits lead to a prolonged IOP elevation which results in structural abnormality as well as loss in RGCs and their axons without observable ocular structural damage or inflammatory response. We have therefore established a novel and practical model of experimental glaucoma in rabbits.

Short-Term Moderately Elevated Intraocular Pressure Is Associated With Elevated Scotopic Electroretinogram Responses

Purpose

Moderately elevated intraocular pressure (IOP) is a risk factor for open-angle glaucoma. Some patients suffer glaucoma despite clinically measured normal IOPs. Fluctuations in IOP may have a significant role since IOPs are higher during sleep and inversion activities. Controlled transient elevations of IOPs in rats over time lead to optic nerve structural changes that are similar to the early changes observed in constant chronic models of glaucoma. Because early intervention decreases glaucoma progression, this study was done to determine if early physiological changes to the retina could be detected with noninvasive electrophysiological and optical imaging tests during moderately elevated IOP.

Methods

Intraocular pressures were raised to moderately high levels (35 mm Hg) in one eye of Sprague-Dawley rats while the other (control) eye was untreated. One group of rats underwent scotopic threshold response (STR) and electroretinogram (ERG) testing, while another 3 groups underwent optical coherence tomography (OCT) imaging, Western blot, or histologic evaluation.

Results

The amplitudes of the STR and ERG responses in eyes with moderately elevated IOPs were enhanced compared to the values before IOP elevation, and compared to untreated contralateral eyes. Structural changes to the optic nerve also occurred during IOP elevation.

Conclusions

Although ischemic IOP elevations are well-known to globally reduce components of the scotopic ERG, acute elevation in rats to levels often observed in untreated glaucoma patients caused an increase in these parameters. Further exploration of these phenomena may be helpful in better understanding the mechanisms mediating early retinal changes during fluctuating or chronically elevated IOP.

An open-source computational tool to automatically quantify immunolabeled retinal ganglion cells

A fully automated and robust method was developed to quantify β-III-tubulin-stained retinal ganglion cells, combining computational recognition of individual cells by CellProfiler and a machine-learning tool to teach phenotypic classification of the retinal ganglion cells by CellProfiler Analyst. In animal models of glaucoma, quantification of immunolabeled retinal ganglion cells is currently performed manually and remains time-consuming. Using this automated method, quantifications of retinal ganglion cell images were accelerated tenfold: 1800 images were counted in 3 h using our automated method, while manual counting of the same images took 72 h. This new method was validated in an established murine model of microbead-induced optic neuropathy. The use of the publicly available software and the method's user-friendly design allows this technique to be easily implemented in any laboratory.

Crocin protects retinal ganglion cells against H2O2-induced damage through the mitochondrial pathway and activation of NF-κB

Glaucoma is a degenerative nerve disorder that results in irreversible blindness. It has been reported that the apoptosis of retinal ganglion cells (RGCs) is a hallmark of glaucoma. Oxidative stress is one of the major factors that cause apoptosis of RGCs. Crocin has many beneficial effects, including antioxidant and anti-apoptotic actions. However, the mechanism by which crocin protects against oxidative stress‑induced damage to RGCs remains unclear. The present study aimed to investigate the mechanism by which crocin protects RGC-5 cells against H2O2-induced damage. H2O2 was used to establish a model of oxidative stress injury in RGC-5 cells to mimic the development of glaucoma in vitro. Different concentrations (0.1 and 1 µM) of crocin were added to test whether crocin was capable of protecting RGCs from H2O2-induced damage. WST-1, lactic dehydrogenase (LDH) release and Annexin V/FITC assays were then performed. Levels of reactive oxygen species (ROS) were detected using a ROS assay kit, mitochondrial membrane potential (ΔΨm) was analyzed by JC-1 staining, caspase-3 activity was examined using a Caspase-3 assay kit, and the protein levels of Bax, Bcl-1 and cytochrome c were measured using western blot analysis. In addition, the protein level of phosphorylated nuclear factor-κB (p-NF-κB) p65 was also evaluated using western blot analysis. The results showed that crocin protected RGC-5 cells from apoptosis, decreased LDH release and enhanced cell viability. Additional experiments demonstrated that crocin decreased ROS levels, increased ΔΨm, downregulated the protein expression of Bax and cytochrome c, promoted Bcl-2 protein expression and activated NF-κB. Taken together, the findings of this study indicate that crocin prevented H2O2‑induced damage to RGCs through the mitochondrial pathway and activation of NF-κB.

Elevated intracranial pressure causes optic nerve and retinal ganglion cell degeneration in mice

The purpose of this study was to develop a novel experimental system for the modulation and measurement of intracranial pressure (ICP), and to use this system to assess the impact of elevated ICP on the optic nerve and retinal ganglion cells (RGCs) in CD1 mice. This system involved surgical implantation of an infusion cannula and a radiowave based pressure monitoring probe through the skull and into the subarachnoid space. The infusion cannula was used to increase ICP, which was measured by the probe and transmitted to a nearby receiver. The system provided robust and consistent ICP waveforms, was well tolerated, and was stable over time. ICP was elevated to approximately 30 mmHg for one week, after which we assessed changes in optic nerve structure with transmission electron microscopy in cross section and RGC numbers with antibody staining in retinal flat mounts. ICP elevation resulted in optic nerve axonal loss and disorganization, as well as RGC soma loss. We conclude that the controlled manipulation of ICP in active, awake mice is possible, despite their small size. Furthermore, ICP elevation results in visual system phenotypes of optic nerve and RGC degeneration, suggesting that this model can be used to study the impact of ICP on the visual system. Potentially, this model can also be used to study the relationship between ICP and IOP, as well diseases impacted by ICP variation such as glaucoma, idiopathic intracranial hypertension, and the spaceflight-related visual impairment intracranial pressure syndrome.