Laser-induced ocular hypertension in adult rats does not affect non-RGC neurons in the ganglion cell layer but results in protracted severe loss of cone-photoreceptors

Single-cell RNA sequencing analysis of the retina under acute high intraocular pressure

JOURNAL/nrgr/04.03/01300535-202419110-00032/figure1/v/2024-03-08T184507Z/r/image-tiff High intraocular pressure causes retinal ganglion cell injury in primary and secondary glaucoma diseases, yet the molecular landscape characteristics of retinal cells under high intraocular pressure remain unknown. Rat models of acute hypertension ocular pressure were established by injection of cross-linked hyaluronic acid hydrogel (Healaflow®). Single-cell RNA sequencing was then used to describe the cellular composition and molecular profile of the retina following high intraocular pressure. Our results identified a total of 12 cell types, namely retinal pigment epithelial cells, rod-photoreceptor cells, bipolar cells, Müller cells, microglia, cone-photoreceptor cells, retinal ganglion cells, endothelial cells, retinal progenitor cells, oligodendrocytes, pericytes, and fibroblasts. The single-cell RNA sequencing analysis of the retina under acute high intraocular pressure revealed obvious changes in the proportions of various retinal cells, with ganglion cells decreased by 23%. Hematoxylin and eosin staining and TUNEL staining confirmed the damage to retinal ganglion cells under high intraocular pressure. We extracted data from retinal ganglion cells and analyzed the retinal ganglion cell cluster with the most distinct expression. We found upregulation of the B3gat2 gene, which is associated with neuronal migration and adhesion, and downregulation of the Tsc22d gene, which participates in inhibition of inflammation. This study is the first to reveal molecular changes and intercellular interactions in the retina under high intraocular pressure. These data contribute to understanding of the molecular mechanism of retinal injury induced by high intraocular pressure and will benefit the development of novel therapies.

Improving rat welfare through the development of a peribulbar anaesthesia technique for ophthalmic procedures: A preliminary study

Rats are a commonly used animal model for the study of the pathogenesis and novel treatments of glaucoma, which is induced experimentally using invasive, painful procedures. Peribulbar anaesthesia (PBA) is frequently used in people and domestic animals prior to ophthalmic surgeries to provide excellent perioperative analgesia. Our goal was to develop a PBA technique adapted to rat anatomy, improving the welfare of animals used as a model for glaucoma. Eighteen rat cadavers (n = 36 eyes) were used to establish the optimal needle insertion location. Five injection techniques using 0.1 mL/100 g lidocaine 2% and a contrast agent (1:1 volume ratio) were compared via computed tomography (CT). CT images were scored for injectate distribution at four locations: extraconal, intraconal, around the optic nerve and at the orbital fissure (scale 0-8, where 0 = none and 8 = excellent). Median scores using the dorso-medial-75° (5; range 2-6) and medial-canthus (4.5; range 2-8) injection techniques were not different from the dorso-medial-45° (4; range 3-6) technique and were higher (better distribution) compared with mid-ventral (3; range 2-5) and ventro-lateral (2; range 1-3) techniques. The two superior techniques were used in two experimental rats (n = 4 eyes) to determine the volume of bupivacaine 0.5% necessary to affect corneal touch threshold (CTT) and periocular skin sensitivity (PSS). A volume of 0.05 mL/100 g decreased CTT and PSS for several hours, while a larger volume produced excessively long effects. Dorso-medial-75° or medial-canthus PBA using 0.05 mL/100 g bupivacaine are likely to provide ocular and periocular analgesia in rats, with minor transient adverse effects.

Evaluation of the neuroprotective efficacy of the gramine derivative ITH12657 against NMDA-induced excitotoxicity in the rat retina

Purpose

The aim of this study was to investigate, the neuroprotective effects of a new Gramine derivative named: ITH12657, in a model of retinal excitotoxicity induced by intravitreal injection of NMDA.

Methods

Adult Sprague Dawley rats received an intravitreal injection of 100 mM NMDA in their left eye and were treated daily with subcutaneous injections of ITH12657 or vehicle. The best dose-response, therapeutic window study, and optimal treatment duration of ITH12657 were studied. Based on the best survival of Brn3a + RGCs obtained from the above-mentioned studies, the protective effects of ITH12657 were studied in vivo (retinal thickness and full-field Electroretinography), and ex vivo by quantifying the surviving population of Brn3a + RGCs, αRGCs and their subtypes α-ONsRGCs, α-ONtRGCs, and α-OFFRGCs.

Results

Administration of 10 mg/kg ITH12657, starting 12 h before NMDA injection and dispensed for 3 days, resulted in the best significant protection of Brn3a + RGCs against NMDA-induced excitotoxicity. In vivo, ITH12657-treated rats showed significant preservation of retinal thickness and functional protection against NMDA-induced retinal excitotoxicity. Ex vivo results showed that ITH12657 afforded a significant protection against NMDA-induced excitotoxicity for the populations of Brn3a + RGC, αRGC, and αONs-RGC, but not for the population of αOFF-RGC, while the population of α-ONtRGC was fully resistant to NMDA-induced excitotoxicity.

Conclusion

Subcutaneous administration of ITH12657 at 10 mg/kg, initiated 12 h before NMDA-induced retinal injury and continued for 3 days, resulted in the best protection of Brn3a + RGCs, αRGC, and αONs-RGC against excitotoxicity-induced RGC death. The population of αOFF-RGCs was extremely sensitive while α-ONtRGCs were fully resistant to NMDA-induced excitotoxicity.

Intraocular Adeno-Associated Virus-Mediated Transgene Endothelin-1 Delivery to the Rat Eye Induces Functional Changes Indicative of Retinal Ischemia-A Potential Chronic Glaucoma Model

Endothelin-1 (ET-1) overactivity has been implicated as a factor contributing to glaucomatous neuropathy, and it has been utilized in animal models of retinal ischemia. The functional effects of long-term ET-1 exposure and possible compensatory mechanisms have, however, not been investigated. This was therefore the purpose of our study. ET-1 was delivered into rat eyes via a single intravitreal injection of 500 µM or via transgene delivery using an adeno-associated viral (AAV) vector. Retinal function was assessed using electroretinography (ERG) and the retinal expression of potentially compensatory genes was evaluated by means of qRT-PCR. Acute ET-1 delivery led to vasoconstriction and a significant reduction in the ERG response. AAV-ET-1 resulted in substantial transgene expression and ERG results similar to the acute ET-1 injections and comparable to other models of retinal ischemia. Compensatory changes were observed, including an increase in calcitonin gene-related peptide (CGRP) gene expression, which may both counterbalance the vasoconstrictive effects of ET-1 and provide neuroprotection. This chronic ET-1 ischemia model might be especially relevant to glaucoma research, mimicking the mild and repeated ischemic events in patients with long-term vascular dysfunction. The compensatory mechanisms, and particularly the role of vasodilatory CGRP in mitigating the retinal damage, warrant further investigation with the aim of evaluating new therapeutic strategies.

Multifocal electroretinography increases following experimental glaucoma in nonhuman primates with retinal ganglion cell axotomy

PURPOSE

To determine whether short-latency changes in multifocal electroretinography (mfERG) observed in experimental glaucoma (EG) are secondary solely to retinal ganglion cell (RGC) loss or whether there is a separate contribution from elevated intraocular pressure (IOP).

METHODS

Prior to operative procedures, a series of baseline mfERGs were recorded from six rhesus macaques using a 241-element unstretched stimulus. Animals then underwent hemiretinal endodiathermy axotomy (HEA) by placing burns along the inferior 180° of the optic nerve margin in the right eye (OD). mfERG recordings were obtained in each animal at regular intervals following for 3-4 months to allow stabilization of the HEA effects. Laser trabecular meshwork destruction (LTD) to elevate IOP was then performed; first-order kernel (K1) waveform root-mean-square (RMS) amplitudes for the short-latency segment of the mfERG wave (9-35 ms) were computed for two 7-hexagon groupings-the first located within the superior (non-axotomized) macula and the second within the inferior (axotomized) macula. Immunohistochemistry for glial fibrillary acidic protein (GFAP) was done.

RESULTS

By 3 months post HEA, there was marked thinning of the inferior nerve fiber layer as measured by optical coherence tomography. Compared with baseline, no statistically significant changes in 9-35 ms K1 RMS amplitudes were evident in either the axotomized or non-axotomized portions of the macula. Following LTD, mean IOP in HEA eyes rose to 46 ± 9 compared with 20 ± 2 mmHg (SD) in the fellow control eyes. In the HEA + EG eyes, statistically significant increases in K1 RMS amplitude were present in both the axotomized inferior and non-axotomized superior portions of the OD retinas. No changes in K1 RMS amplitude were found in the fellow control eyes from baseline to HEA epoch, but there was a smaller increase from baseline to HEA + EG. Upregulation of GFAP in the Müller cells was evident in both non-axotomized and axotomized retina in eyes with elevated IOP.

CONCLUSIONS

The RMS amplitudes of the short-latency mfERG K1 waveforms are not altered following axotomy but undergo marked increases following elevated IOP. This suggests that the increase in mfERG amplitude was not solely a result of RGC loss and may reflect photoreceptor and bipolar cell dysfunction and/or changes in Müller cells.

Assessment of outer retinal thickness and function in mice after experimental optic nerve trauma

BACKGROUND

Optic nerve trauma caused by crush injury is frequently used for investigating experimental treatments that protect retinal ganglion cells (RGCs) and induce axonal regrowth. Retaining outer retinal light responses is essential for therapeutic rescue of RGCs after injury. However, whether optic nerve crush also damages the structure or function of photoreceptors has not been systematically investigated. In this study, we investigated whether outer retinal thickness and visual function are altered by optic nerve crush in the mouse.

METHODS

Wildtype mice underwent optic nerve crush and intravitreal injection of a control solution in one eye with the fellow eye remaining uninjured. Two weeks after injury, the thickness of the ganglion cell region (GCL to IPL) and photoreceptor layer (bottom of the OPL to top of the RPE) were measured using OCT. Retinal function was assessed using flash ERGs. Immunodetection of RGCs was performed on retinal cryosections and RGCs and ONL nuclei rows were counted. Multiple comparison analyses were conducted using Analysis of Variance (ANOVA) with Tukey's post hoc test and P values less than 0.05 were considered statistically significant.

RESULTS

Optic nerve crush injury induced RGC death as expected, demonstrated by thinning of the ganglion cell region and RGC loss. In contrast, outer retinal thickness, photopic and scotopic a-wave and b-wave amplitudes and photoreceptor nuclei counts, were equivalent between injured and uninjured eyes.

CONCLUSIONS

Secondary degeneration of the outer retina was not detected after optic nerve injury in the presence of significant RGC death, suggesting that the retina has the capacity to compartmentalize damage. These findings also indicate that experimental treatments to preserve the GCL and rescue vision using this optic nerve injury model would not require additional strategies to preserve the ONL.

Photoreceptor density in relation to axial length and retinal location in human eyes

The purpose of the study was to examine the density of retinal photoreceptors and retinal pigment epithelium (RPE) cells in relation to myopic axial elongation in human eyes. Using light microscopy, we assessed the density of photoreceptors and RPE cells at the ora serrata, equator, and midperiphery (equator/posterior pole midpoint), and the RPE cell density additionally at the posterior pole, in enucleated human globes. The study included 78 eyes (mean age: 59.2 ± 15.6 years; range: 32-85 years) with a mean axial length of 27.3 ± 3.6 mm (range: 21.5-37.0 mm). Close to the ora serrata, at the equator and midperiphery, photoreceptor and RPE cell density was 246 ± 183, 605 ± 299 and 1089 ± 441 photoreceptors/mm and 56.1 ± 13.7, 45.2 ± 15.1, and 48.8 ± 15.6 RPE cells/mm, respectively. Densities of both cell types in all three regions were positively correlated with each other (all P < 0.001) and decreased with longer axial length (all P < 0.001) and longer distance between the ora serrata and the posterior pole (all P < 0.001), most marked at the midperiphery and least marked close to the ora serrata. The PRE cell density at the posterior pole was not significantly (P = 0.35) related to axial length. The photoreceptor density at the ora serrata (beta:- 0.33) and equator (beta: - 0.27) and RPE cell density at the ora serrata (beta: - 0.27) decreased additionally with the presence of glaucoma. The findings suggest that the axial elongation-related decrease in photoreceptor and RPE cell density is most marked at the midperiphery, followed by the equator and finally the ora serrata region. It suggests that the axial elongation-related enlargement of the eye wall predominantly takes place in the retro-equatorial region, followed by the equatorial region.

The retina of the lab rat: focus on retinal ganglion cells and photoreceptors

Albino and pigmented rat strains are widely used in models to study retinal degeneration and to test new therapies. Here, we have summarized the main topographical and functional characteristics of the rat retina focussing on photoreceptors and retinal ganglion cells (RGCs), the beginning and end of the retinal circuitry, respectively. These neurons are very sensitive to injury and disease, and thus knowing their normal number, topography, and function is essential to accurately investigate on neuronal survival and protection.

DNA and RNA oxidative damage in the retina is associated with ganglion cell mitochondria

This study examines retinas from a rat glaucoma model for oxidized nucleosides 8OHdG and 8OHG, biomarkers for oxidative damage of DNA and RNA, respectively. Immunohistochemical data indicate a predominant localization of 8OHdG/8OHG in retinal ganglion cells (RGCs). The levels for these oxidized DNA/RNA products were 3.2 and 2.8 fold higher at 1 and 2 weeks after intraocular pressure elevation compared to control retinas, respectively. 8OHdG/8OHG were almost exclusively associated with mitochondrial DNA/RNA: ~ 65% of 8OHdG/8OHG were associated with RNA isolated from mitochondrial fraction and ~ 35% with DNA. Furthermore, we analyzed retinas of the rd10 mouse, a model for retinitis pigmentosa, with severe degeneration of photoreceptors to determine whether high levels of 8OHdG/8OHG staining intensity in RGCs of control animals is related to the high level of mitochondrial oxidative phosphorylation necessary to support light-evoked RGC activity. No significant difference in 8OHdG/8OHG staining intensity between control and rd10 mouse retinas was observed. The results of this study suggest that high levels of 8OHdG/8OHG in RGCs of wild-type animals may lead to cell damage and progressive loss of RGCs observed during normal aging, whereas ocular hypertension-induced increase in the level of oxidatively damaged mitochondrial DNA/RNA could contribute to glaucomatous neurodegeneration.

A Robust Microbead Occlusion Model of Glaucoma for the Common Marmoset

Purpose

To establish a robust experimental model of glaucoma in the common marmoset (Callithrix jacchus), a New World primate, using an intracameral microbead injection technique.

Methods

Elevated intraocular pressure (IOP) was induced by an injection of polystyrene microbeads. Morphologic changes in the retina and optic nerve of glaucomatous eyes were assessed and electroretinogram (ERG) recordings were performed to evaluate functional changes.

Results

Microbead injections induced a sustained IOP elevation for at least 10 weeks in a reproducible manner. At the end of the 10-week experimental period, there was significant loss of retinal ganglion cells (RGCs) in all quadrants and eccentricities, although it was more prominent in the mid-peripheral and peripheral regions. This was consistent with a thinning of the Retinal nerve fiber layer (RNFL) seen in spectral domain optical coherence tomography scans. Surviving RGCs showed marked changes in morphology, including somatic shrinkage and dendritic atrophy. Retinas also showed significant gliosis. The amplitude of the ERG photopic negative response, with subsequent a- and b-wave changes, was reduced in glaucomatous eyes. The optic nerve of glaucomatous eyes showed expanded cupping, disorganization of the astrocytic matrix, axonal loss, and gliosis.

Conclusions

We developed a robust and reproducible model of glaucoma in the marmoset. The model exhibits both structural and functional alterations of retina and optic nerve characteristic of glaucoma in humans and animal models.

Translational Relevance

The glaucoma model in the marmoset described here forms a robust method to study the disease etiology, progression, and potential therapies in a nonhuman primate, allowing for more effective translation of animal data to humans.

7,8-Dihydroxiflavone Maintains Retinal Functionality and Protects Various Types of RGCs in Adult Rats with Optic Nerve Transection

To analyze the neuroprotective effects of 7,8-Dihydroxyflavone (DHF) in vivo and ex vivo, adult albino Sprague-Dawley rats were given a left intraorbital optic nerve transection (IONT) and were divided in two groups: One was treated daily with intraperitoneal (ip) DHF (5 mg/kg) (n = 24) and the other (n = 18) received ip vehicle (1% DMSO in 0.9% NaCl) from one day before IONT until processing. At 5, 7, 10, 12, 14, and 21 days (d) after IONT, full field electroretinograms (ERG) were recorded from both experimental and one additional naïve-control group (n = 6). Treated rats were analyzed 7 (n = 14), 14 (n = 14) or 21 d (n = 14) after IONT, and the retinas immune stained against Brn3a, Osteopontin (OPN) and the T-box transcription factor T-brain 2 (Tbr2) to identify surviving retinal ganglion cells (RGCs) (Brn3a⁺), α-like (OPN⁺), α-OFF like (OPN⁺Brn3a⁺) or M4-like/α-ON sustained RGCs (OPN⁺Tbr⁺). Naïve and right treated retinas showed normal ERG recordings. Left vehicle-treated retinas showed decreased amplitudes of the scotopic threshold response (pSTR) (as early as 5 d), the rod b-wave, the mixed response and the cone response (as early as 10 d), which did not recover with time. In these retinas, by day 7 the total numbers of Brn3a⁺RGCs, OPN⁺RGCs and OPN⁺Tbr2⁺RGCs decreased to less than one half and OPN⁺Brn3a⁺RGCs decreased to approximately 0.5%, and Brn3a⁺RGCs showed a progressive loss with time, while OPN⁺RGCs and OPN⁺Tbr2⁺RGCs did not diminish after seven days. Compared to vehicle-treated, the left DHF-treated retinas showed significantly greater amplitudes of the pSTR, normal b-wave values and significantly greater numbers of OPN⁺RGCs and OPN⁺Tbr2⁺RGCs for up to 14 d and of Brn3a⁺RGCs for up to 21 days. DHF affords significant rescue of Brn3a⁺RGCs, OPN⁺RGCs and OPN⁺Tbr2⁺RGCs, but not OPN⁺Brn3a⁺RGCs, and preserves functional ERG responses after IONT.

Endoplasmic reticulum stress is involved in retinal injury induced by repeated transient spikes of intraocular pressure

Clinically, a large proportion of glaucoma patients undergo repeated intraocular pressure (IOP) spike (Spike IOP) attacks during their sleep, which may facilitate retinopathy. In this study, we established a mouse model of repeated transient Spike IOP to investigate the direct damage to the retina following Spike IOP attacks, and elucidated the underlying molecular mechanism. We analyzed the changes in the number of retinal ganglion cells (RGCs) via immunofluorescence. Thereafter, we detected retinal cell apoptosis via terminal deoxynucleotidyl transferase deoxyuridine triphosphate (dUTP) nick-end labeling (TUNEL) staining, and performed RNA sequencing (RNA-seq) to reveal the underlying molecular mechanism. Finally, we validated the expression of key molecules in the endoplasmic reticulum (ER) stress pathway using quantitative real-time polymerase chain reaction (qRT-PCR) and western blot analysis. Results revealed a time-dependent RGC loss in Spike IOP, evidenced by a reduction in the number of Brn3a-positive RGCs in experimental eyes following a 7-d continuous treatment with Spike IOP. In addition, TUNEL staining indicated that apoptosis of retinal cells started in the outer nuclear layer (ONL), and then spread to the ganglion cell layer (GCL) with time. RNA-seq analysis revealed that ER stress might be involved in Spike IOP-induced retinal injury. This result was corroborated by western blot, which revealed upregulation of ER stress-related proteins including binding immunoglobulin protein/glucose-regulated protein 78 (BiP/GRP78), phosphorylated inositol-requiring enzyme 1 (p-IRE1), unspliced X-box-binding protein 1 (XBP1-u), spliced X-box-binding protein 1 (XBP1-s), phosphorylated c-Jun N-terminal kinase (p-JNK), C/EBP-homologous protein (CHOP), and B-cell lymphoma 2 (Bcl-2)-associated X protein (Bax). These findings indicate that repeated IOP transients are detrimental to the retina, while ER stress plays an important role in retinal cell apoptosis in this situation. Notably, repeated Spike IOP among glaucoma patients is a crucial factor for progressive retinopathy.

Establishing the ground squirrel as a superb model for retinal ganglion cell disorders and optic neuropathies

Retinal ganglion cell (RGC) death occurs after optic nerve injury due to acute trauma or chronic degenerative conditions such as optic neuropathies (e.g., glaucoma). Currently, there are no effective therapies to prevent permanent vision loss resulting from RGC death, underlining the need for research on the pathogenesis of RGC disorders. Modeling human RGC/optic nerve diseases in non-human primates is ideal because of their similarity to humans, but has practical limitations including high cost and ethical considerations. In addition, many retinal degenerative disorders are age-related making the study in primate models prohibitively slow. For these reasons, mice and rats are commonly used to model RGC injuries. However, as nocturnal mammals, these rodents have retinal structures that differ from primates - possessing less than one-tenth of the RGCs found in the primate retina. Here we report the diurnal thirteen-lined ground squirrel (TLGS) as an alternative model. Compared to other rodent models, the number and distribution of RGCs in the TLGS retina are closer to primates. The TLGS retina possesses ~600,000 RGCs with the highest density along the equatorial retina matching the location of the highest cone density (visual streak). TLGS and primate retinas also share a similar interlocking pattern between RGC axons and astrocyte processes in the retina nerve fiber layer (RNFL). In addition, using TLGS we establish a new partial optic nerve injury model that precisely controls the extent of injury while sparing a portion of the retina as an ideal internal control for investigating the pathophysiology of axon degeneration and RGC death. Moreover, in vivo optical coherence tomography (OCT) imaging and ex vivo microscopic examinations of the retina in optic nerve injured TLGS confirm RGC loss precedes proximal axon degeneration, recapitulating human pathology. Thus, the TLGS retina is an excellent model, for translational research in neurodegeneration and therapeutic neuroprotection.

Neuroprotection of the Inner Retina Also Prevents Secondary Outer Retinal Pathology in a Mouse Model of Glaucoma

Purpose

We examined structural and functional changes in the outer retina of a mouse model of glaucoma. We examined whether these changes are a secondary consequence of damage in the inner retina and whether neuroprotection of the inner retina also prevents outer retinal changes.

Methods

We used an established microbead occlusion model of glaucoma whereby intraocular pressure (IOP) was elevated. Specific antibodies were used to label rod and cone bipolar cells (BCs), horizontal cells (HCs), and retinal ganglion cells (RGCs), as well as synaptic components in control and glaucomatous eyes, to assess structural damage and cell loss. ERG recordings were made to assess outer retina function.

Results

We found structural and functional damage of BCs, including significant cell loss and dendritic/axonal remodeling of HCs, following IOP elevation. The first significant loss of both BCs occurred at 4 to 5 weeks after microbead injection. However, early changes in the dendritic structure of RGCs were observed at 3 weeks, but significant changes in the rod BC axon terminal structure were not seen until 4 weeks. We found that protection of inner retinal neurons in glaucomatous eyes by pharmacological blockade of gap junctions or genetic ablation of connexin 36 largely prevented outer retinal damage.

Conclusions

Together, our results indicate that outer retinal impairments in glaucoma are a secondary sequalae of primary damage in the inner retina. The finding that neuroprotection of the inner retina can also prevent outer retinal damage has important implications with regard to the targets for effective neuroprotective therapy.

Activation of Apoptosis in a βB1-CTGF Transgenic Mouse Model

To reveal the pathomechanisms of glaucoma, a common cause of blindness, suitable animal models are needed. As previously shown, retinal ganglion cell and optic nerve degeneration occur in βB1-CTGF mice. Here, we aimed to determine possible apoptotic mechanisms and degeneration of different retinal cells. Hence, retinae were processed for immunohistology (n = 5–9/group) and quantitative real-time PCR analysis (n = 5–7/group) in 5- and 10-week-old βB1-CTGF and wildtype controls. We noted significantly more cleaved caspase 3⁺ cells in βB1-CTGF retinae at 5 (p = 0.005) and 10 weeks (p = 0.02), and a significant upregulation of Casp3 and Bax/Bcl2 mRNA levels (p < 0.05). Furthermore, more terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL⁺) cells were detected in transgenic mice at 5 (p = 0.03) and 10 weeks (p = 0.02). Neurofilament H staining (p = 0.01) as well as Nefh (p = 0.02) and Tubb3 (p = 0.009) mRNA levels were significantly decreased at 10 weeks. GABAergic synapse intensity was lower at 5 weeks, while no alterations were noted at 10 weeks. The glutamatergic synapse intensity was decreased at 5 (p = 0.007) and 10 weeks (p = 0.01). No changes were observed for bipolar cells, photoreceptors, and macroglia. We conclude that apoptotic processes and synapse loss precede neuronal death in this model. This slow progression rate makes the βB1-CTGF mice a suitable model to study primary open-angle glaucoma.

Arrangement of the photoreceptor mosaic in a diabetic rat model imaged with multiphoton microscopy

Diabetic retinopathy (DR) is defined as a microvascular pathology. However, some data have suggested that the retinal photoreceptors (PRs) might be important in the pathogenesis of this ocular disease. In this study the organization of the PRs in control and diabetic-induced rats was compared using multiphoton microscopy. The PR mosaic was imaged at different locations in non-stained retinas. The density of PRs was directly quantified from cell counting. The spatially resolved density presents a double-slope pattern (from the central retina towards the periphery) in both healthy and pathological samples, although the values for the latter were significantly lower all across the retina. Moreover, Voronoi analysis was performed to explore changes in PR topography. In control specimens a hexagonally packed structure was dominant. However, despite the non-controlled effects of the disease in retinal structures, this PR regularity was fairly maintained in diabetic retinas.

Functional and morphological alterations in a glaucoma model of acute ocular hypertension

To study short and long-term effects of acute ocular hypertension (AOHT) on inner and outer retinal layers, in adult Sprague-Dawley rats AOHT (87mmHg) was induced for 90min and the retinas were examined longitudinally in vivo with electroretinogram (ERG) recordings and optical coherent tomography (OCT) from 1 to 90 days (d). Ex vivo, the retinas were analyzed for rod (RBC) and cone (CBC) bipolar cells, with antibodies against protein kinase Cα and recoverin, respectively in cross sections, and for cones, horizontal (HZ) and ganglion (RGC) cells with antibodies against arrestin, calbindin and Brn3a, respectively in wholemounts. The inner retina thinned progressively up to 7d with no further changes, while the external retina had a normal thickness until 30d, with a 20% thinning between 30 and 90d. Functionally, the a-wave showed an initial reduction by 24h and a further reduction from 30 to 90d. All other main ERG waves were significantly reduced by 1d without significant recovery by 90d. Radial sections showed a normal population of RBCs but their terminals were reduced. The CBCs showed a progressive decrease with a loss of 56% by 30d. In wholemount retinas, RGCs diminished to 40% by 3d and to 16% by 30d without further loss. Cones diminished to 58% and 35% by 3 and 7d, respectively and further decreased between 30 and 90d. HZs showed normal values throughout the study. In conclusion, AOHT affects both the inner and outer retina, with a more pronounced degeneration of the cone than the rod pathway.

Activation of adenosine A3 receptor protects retinal ganglion cells from degeneration induced by ocular hypertension

Glaucoma is a progressive chronic retinal degenerative disease and a leading cause of global irreversible blindness. This disease is characterized by optic nerve damage and retinal ganglion cell (RGC) death. The current treatments available target the lowering of intraocular pressure (IOP), the main risk factor for disease onset and development. However, in some patients, vision loss progresses despite successful IOP control, indicating that new and effective treatments are needed, such as those targeting the neuroprotection of RGCs. Adenosine A₃ receptor (A₃R) activation confers protection to RGCs following an excitotoxic stimulus. In this work, we investigated whether the activation of A₃R could also afford protection to RGCs in the laser-induced ocular hypertension (OHT) model, a well-characterized animal model of glaucoma. The intravitreal injection of 2-Cl-IB-MECA, a selective A₃R agonist, abolished the alterations induced by OHT in the negative and positive components of scotopic threshold response (STR) without changing a- and b-wave amplitudes both in scotopic and photopic conditions. Moreover, the treatment of OHT eyes with the A₃R agonist promoted the survival of RGCs, attenuated the impairment in retrograde axonal transport, and improved the structure of the optic nerve. Taking into consideration the beneficial effects afforded by 2-Cl-IB-MECA, we can envisage that A₃R activation can be considered a good therapeutic strategy to protect RGCs from glaucomatous damage.

Time course of bilateral microglial activation in a mouse model of laser-induced glaucoma

Microglial activation is associated with glaucoma. In the model of unilateral laser-induced ocular hypertension (OHT), the time point at which the inflammatory process peaks remains unknown. Different time points (1, 3, 5, 8, and 15 d) were compared to analyze signs of microglial activation both in OHT and contralateral eyes. In both eyes, microglial activation was detected in all retinal layers at all time points analyzed, including: i) increase in the cell number in the outer segment photoreceptor layer and plexiform layers (only in OHT eyes) from 3 d onward; ii) increase in soma size from 1 d onward; iii) retraction of the processes from 1 d in OHT eyes and 3 d in contralateral eyes; iv) increase in the area of the retina occupied by Iba-1+ cells in the nerve fiber layer/ganglion cell layer from 1 d onward; v) increase in the number of vertical processes from 1 d in contralateral eyes and 3 d in OHT eyes. In OHT eyes at 24 h and 15 d, most Iba-1+ cells were P2RY12+ and were down-regulated at 3 and 5 d. In both eyes, microglial activation was stronger at 3 and 5 d (inflammation peaked in this model). These time points could be useful to identify factors implicated in the inflammatory process.

Topical Brimonidine or Intravitreal BDNF, CNTF, or bFGF Protect Cones Against Phototoxicity

Purpose

To develop a focal photoreceptor degeneration model by blue light-emitting diode (LED)-induced phototoxicity (LIP) and investigate the protective effects of topical brimonidine (BMD) or intravitreal brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), or basic fibroblast growth factor (bFGF).

Methods

In anesthetized, dark-adapted, adult female Swiss mice, the left eye was dilated and exposed to blue light (10 seconds, 200 lux). After LIP, full-field electroretinograms (ERG) and spectral-domain optical coherence tomography (SD-OCT) were obtained longitudinally, and reactive-Iba-1⁺monocytic cells, TUNEL⁺ cells and S-opsin⁺ cone outer segments were examined up to 7 days. Left eyes were treated topically with BMD (1%) or vehicle, before or right after LIP, or intravitreally with BDNF (2.5 μg), CNTF (0.2 μg), bFGF (0.5 μg), or corresponding vehicle right after LIP. At 7 days, S-opsin⁺ cone outer segments were counted within predetermined fixed-size areas (PFA) centered on the lesion in both flattened retinas.

Results

SD-OCT showed a circular region in the superior-temporal left retina with progressive thinning (207.9 ± 5.6 μm to 160.7 ± 6.8 μm [7 days], n = 8), increasing TUNEL⁺ cells (peak at 3 days), decreasing S-opsin⁺ cone outer segments, and strong microglia activation. ERGs were normal by 3 days. Total S-opsin⁺ cones in the PFA for LIP-treated and fellow-retinas were 2330 ± 262 and 5601 ± 583 (n = 8), respectively. All neuroprotectants (n = 7–11), including topical BMD pre- or post-LIP, or intravitreal BDNF, CNTF, and bFGF, showed significantly greater S-opsin⁺ cone survival than their corresponding vehicle-treated groups.

Conclusions

LIP is a reliable, quantifiable focal photoreceptor degeneration model. Topical BMD or intravitreal BDNF, CNTF, or bFGF protect against LIP-induced cone-photoreceptor loss.

Translational Relevance

Topical BMD or intravitreal BDNF, CNTF, or bFGF protect cones against phototoxicity.

Intravitreal Therapy Against the Complement Factor C5 Prevents Retinal Degeneration in an Experimental Autoimmune Glaucoma Model

In glaucoma, studies revealed an involvement of the complement system. In an experimental autoimmune glaucoma model, immunization with an optic nerve homogenate antigen (ONA) led to retinal ganglion cell (RGC) loss, while intraocular pressure (IOP) remained unchanged. Here, we investigated the therapeutic effect of a complement system inhibition in this model. Hence, rats were immunized with ONA and compared to controls. In one eye of the ONA animals, an antibody against complement factor C5 was intravitreally injected (15 μmol: ONA+C5-I or 25 μmol: ONA+C5-II) before immunization and then every two weeks. IOP was measured weekly. After 6 weeks, spectral-domain optical coherence tomographies (SD-OCT), electroretinograms (ERG), immunohistochemistry, and quantitative real-time PCR analyses were performed. IOP and retinal thickness remained unchanged within all groups. The a-wave amplitudes were not altered in the ONA and ONA+C5-I groups, whereas a decrease was noted in ONA+C5-II animals (p < 0.05). ONA immunization provoked a significant decrease of the b-wave amplitude (p < 0.05), which could be preserved in ONA+C5-I, but not in ONA+C5-II animals. ONA animals showed a loss of RGCs (p = 0.001), while ONA+C5-I and ONA+C5-II retinae had similar cell counts as controls. A significant downregulation of apoptotic Bax/Bcl2 mRNA was noted in ONA+C5-I retinae (p = 0.02). Significantly more C3⁺ and MAC⁺ cells were observed in ONA animals (p < 0.001). The amount of C3⁺ cells in both treatment groups was significantly increased (p < 0.01), while the number of MAC⁺ cells in the treated retinas did not differ from controls. The number of activated microglia cells remained unchanged in ONA animals, but was increased in the treatment groups (p < 0.05). Recoverin⁺ cells were diminished in ONA animals (p = 0.049), but not in treated ones. Rho mRNA was downregulated in ONA and in ONA+C5-II retinas (both p = 0.014). Less opsin⁺ cones were observed in ONA animals (p = 0.009), but not in the treated groups. Our results indicate that the C5 antibody inhibits activation of the complement system, preventing the loss of retinal function as well as RGC, cone bipolar, and photoreceptor loss. Therefore, this approach might be a suitable new treatment for glaucoma patients, in which immune dysregulation plays an important factor for the development and progression of glaucoma.

A Chronic Ocular-Hypertensive Rat Model induced by Injection of the Sclerosant Agent Polidocanol in the Aqueous Humor Outflow Pathway

Background: To induce a moderate chronic ocular hypertension (OHT) by injecting polidocanol, a foamed sclerosant drug, in the aqueous humor outflow pathway. Methods: Intraocular pressure (IOP) was monitored for up to 6 months. Pattern and full-field electroretinogram (PERG and ERG) were recorded and retinal ganglion cells (RGC) and retinal nerve fiber layer (RNFL) thickness were assessed in vivo with optical coherence tomography (OCT) and ex vivo using Brn3a immunohistochemistry. Results: In the first 3 weeks post-injection, a significant IOP elevation was observed in the treated eyes (18.47 ± 3.36 mmHg) when compared with the control fellow eyes (12.52 ± 2.84 mmHg) (p < 0.05). At 8 weeks, 65% (11/17) of intervention eyes had developed an IOP increase >25% over the baseline. PERG responses were seen to be significantly reduced in the hypertensive eyes (2.25 ± 0.24 µV) compared to control eyes (1.44 ± 0.19 µV) (p < 0.01) at week 3, whereas the ERG components (photoreceptor a-wave and bipolar cell b-wave) remained unaltered. By week 24, RNFL thinning and cell loss in the ganglion cell layer was first detected (2/13, 15.3%) as assessed by OCT and light microscopy. Conclusions: This novel OHT rat model, with moderate levels of chronically elevated IOP, and abnormal PERG shows selective functional impairment of RGC.

Melanopsin+RGCs Are fully Resistant to NMDA-Induced Excitotoxicity

We studied short- and long-term effects of intravitreal injection of N-methyl-d-aspartate (NMDA) on melanopsin-containing (m⁺) and non-melanopsin-containing (Brn3a⁺) retinal ganglion cells (RGCs). In adult SD-rats, the left eye received a single intravitreal injection of 5µL of 100nM NMDA. At 3 and 15 months, retinal thickness was measured in vivo using Spectral Domain-Optical Coherence Tomography (SD-OCT). Ex vivo analyses were done at 3, 7, or 14 days or 15 months after damage. Whole-mounted retinas were immunolabelled for brain-specific homeobox/POU domain protein 3A (Brn3a) and melanopsin (m), the total number of Brn3a⁺RGCs and m⁺RGCs were quantified, and their topography represented. In control retinas, the mean total numbers of Brn3a⁺RGCs and m⁺RGCs were 78,903 ± 3572 and 2358 ± 144 (mean ± SD; n = 10), respectively. In the NMDA injected retinas, Brn3a⁺RGCs numbers diminished to 49%, 28%, 24%, and 19%, at 3, 7, 14 days, and 15 months, respectively. There was no further loss between 7 days and 15 months. The number of immunoidentified m⁺RGCs decreased significantly at 3 days, recovered between 3 and 7 days, and were back to normal thereafter. OCT measurements revealed a significant thinning of the left retinas at 3 and 15 months. Intravitreal injections of NMDA induced within a week a rapid loss of 72% of Brn3a⁺RGCs, a transient downregulation of melanopsin expression (but not m⁺RGC death), and a thinning of the inner retinal layers.

Loss of retinal ganglion cells in a new genetic mouse model for primary open-angle glaucoma

Primary open‐angle glaucoma (POAG) is one of the most common causes for blindness worldwide. Although an elevated intraocular pressure (IOP) is the main risk factor, the exact pathology remained indistinguishable. Therefore, it is necessary to have appropriate models to investigate these mechanisms. Here, we analysed a transgenic glaucoma mouse model (βB1‐CTGF) to elucidate new possible mechanisms of the disease. Therefore, IOP was measured in βB1‐CTGF and wildtype mice at 5, 10 and 15 weeks of age. At 5 and 10 weeks, the IOP in both groups were comparable (P > 0.05). After 15 weeks, a significant elevated IOP was measured in βB1‐CTGF mice (P < 0.001). At 15 weeks, electroretinogram measurements were performed and both the a‐ and b‐wave amplitudes were significantly decreased in βB1‐CTGF retinae (both P < 0.01). Significantly fewer Brn‐3a⁺ retinal ganglion cells (RGCs) were observed in the βB1‐CTGF group on flatmounts (P = 0.02), cross‐sections (P < 0.001) and also via quantitative real‐time PCR (P = 0.02). Additionally, significantly more cleaved caspase 3⁺ RGCs were seen in the βB1‐CTGF group (P = 0.002). Furthermore, a decrease in recoverin⁺ cells was observable in the βB1‐CTGF animals (P = 0.004). Accordingly, a significant down‐regulation of Recoverin mRNA levels were noted (P < 0.001). Gfap expression, on the other hand, was higher in βB1‐CTGF retinae (P = 0.023). Additionally, more glutamine synthetase signal was noted (P = 0.04). Although no alterations were observed regarding photoreceptors via immunohistology, a significant decrease of Rhodopsin (P = 0.003) and Opsin mRNA (P = 0.03) was noted. We therefore assume that the βB1‐CTGF mouse could serve as an excellent model for better understanding the pathomechanisms in POAG.

Review of rodent hypertensive glaucoma models

Glaucoma is a neurodegenerative disease characterized by the progressive loss of retinal ganglion cells (RGCs). Elevated intraocular pressure (IOP) is a primary risk factor for the development and progression of glaucoma. Rodent models of glaucoma have greatly improved our understanding of the pathophysiology of glaucoma and served as a useful tool to investigate neuroprotective agents. An ideal glaucoma animal model should be easy to induce, reproducible, biologically plausible and predictable. Of the available animal models of glaucoma, rodents are commonly studied because they have a relatively short life span and can be genetically altered. A successful hypertensive glaucoma model should induce structural glaucomatous changes: including loss of retinal nerve fibres, retinal ganglion cells and optic-disc cupping along with IOP elevation. The level and duration of IOP elevation should be titratable depending on the targeted glaucomatous damage. This review summarizes the outcomes of induced rodent hypertensive glaucoma models including intracameral injection of microbeads, laser photocoagulation, episcleral vein cauterization, injection of hypertonic saline and hyaluronic acid. We aim to provide a detailed overview of each of the models with a focus on parameters that defines a successful glaucoma model. The induced IOP elevation and duration of elevation varied among the different models and strain of rodent; nonetheless, they all achieved a sustainable raised IOP with corresponding RGC loss. The limitations of each model are discussed.

Analyzing the impact of glaucoma on the macular architecture using spectral-domain optical coherence tomography

Purpose

Using spectral domain optical coherence tomography (SD-OCT) the retina can be segmented automatically to visualize all retinal layers. In glaucoma chronically elevated intraocular pressure leads to a decline of retinal ganglion cells (RGC) which changes retinal architecture. The goal of these analyses was to gain insight into the changes induced by glaucoma within all macular layers using SD-OCT within a closely circumscribed glaucoma cohort.

Materials and methods

SD-OCT measurements with automated retinal layer segmentation were performed in both eyes of primary open-angle glaucoma patients with a defined monocular absolute visual field scotoma in the central 10° of the visual field and in an age-matched healthy control group. Thickness of single retinal layers and entire retina were compared with special attention to the localization of the visual field scotoma in the glaucomatous eyes.

Results

30 eyes of 15 glaucoma patients and 15 eyes of 15 healthy controls were included in this study. Statistical significant thickness differences were detected in the control group between superior and inferior retina for the retinal nerve fiber layer (RNFL), the outer plexiform layer (OPL) and the outer nuclear layer (ONL). In the glaucoma group thickness differences between worse and less affected eyes in the RNFL, the ganglion cell layer (GCL) and the inner plexiform layers (INL) were found. Comparison between healthy and diseased eyes revealed significant thickness differences in the RNFL, GCL, IPL and total retinal thickness but not the outer retinal layers.

Conclusion

Comparison between SD-OCT measurements of the macula between healthy and glaucomatous eyes in a closely circumscribed disease stage showed a pronounced disease impact on the inner but not the outer retina. These results provide evidence that GCL and IPL thickness seem to be good measures to discriminate between affected and unaffected eyes in testing for glaucoma.

Microglial dynamics after axotomy-induced retinal ganglion cell death

BACKGROUND

Microglial cells (MCs) are the sentries of the central nervous system. In health, they are known as surveying MCs because they examine the tissue to maintain the homeostasis. In disease, they activate and, among other functions, become phagocytic to clean the cellular debris. In this work, we have studied the behavior of rat retinal MCs in two models of unilateral complete intraorbital optic nerve axotomy which elicit a different time course of retinal ganglion cell (RGC) loss.

METHODS

Albino Sprague-Dawley rats were divided into these groups: (a) intact (no surgery), (b) fluorogold (FG) tracing from the superior colliculi, and (c) FG tracing + crush or transection of the left optic nerve. The retinas were dissected from 2 days to 2 months after the lesions (n = 4-12 group/lesion and time point) and then were subjected to Brn3a and Iba1 double immunodetection. In each intact retina, the total number of Brn3a+RGCs and Iba+MCs was quantified. In each traced retina (b and c groups), FG-traced RGCs and phagocytic microglial cells (PMCs, FG+Iba+) were also quantified. Topographical distribution was assessed by neighbor maps.

RESULTS

In intact retinas, surveying MCs are homogenously distributed in the ganglion cell layer and the inner plexiform layer. Independently of the axotomy model, RGC death occurs in two phases, one quick and one protracted, and there is a lineal and topographical correlation between the appearance of PMCs and the loss of traced RGCs. Furthermore, the clearance of FG+RGCs by PMCs occurs 3 days after the actual loss of Brn3a expression that marks RGC death. In addition, almost 50% of MCs from the inner plexiform layer migrate to the ganglion cell layer during the quick phase of RGC loss, returning to the inner plexiform layer during the slow degeneration phase. Finally, in contrast to what happens in mice, in rats, there is no microglial phagocytosis in the contralateral uninjured retina.

CONCLUSIONS

Axotomy-induced RGC death occurs earlier than RGC clearance and there is an inverse correlation between RGC loss and PMC appearance, both numerically and topographically, suggesting that phagocytosis occurs as a direct response to RGC death rather than to axonal damage.

HSP27 immunization reinforces AII amacrine cell and synapse damage induced by S100 in an autoimmune glaucoma model

Previous studies have revealed a loss of retinal ganglion cells (RGCs) and optic nerve fibers after immunization with the S100B protein. Addition of heat shock protein 27 (HSP27) also leads to a decrease of RGCs. Our present aim has been to analyze various retinal cell types after immunization with S100B or S100B + HSP27 (S100 + HSP). After 28 days, retinas were processed for immunohistology and Western blot. RGCs, immunostained for NeuN, were significantly decreased in the S100 and the S100 + HSP groups. Significantly fewer ChAT⁺ cells were noted in both groups, whereas parvalbumin⁺ cells were only affected in the S100 + HSP group. Western blot results also revealed fewer ChAT signals in both immunized groups. No changes were noted with regard to PKCα⁺ rod bipolar cells, whereas a significant loss of recoverin⁺ cone bipolar cells was observed in both groups via immunohistology and Western blot. The presynaptic marker Bassoon and the postsynaptic marker PSD95 were significantly reduced in the S100 + HSP group. Opsin⁺ and rhodopsin⁺ photoreceptors revealed no changes in either group. Thus, the inner retinal layers are affected by immunization. However, the combination of S100 and HSP27 has a stronger additive effect on the retinal synapses and AII amacrine cells.

Pupillometric evaluation of the melanopsin containing retinal ganglion cells in mitochondrial and non-mitochondrial optic neuropathies

In recent years, chromatic pupillometry is used in humans to evaluate the activity of melanopsin expressing intrinsic photosensitive retinal ganglion cells (ipRGCs). Blue light is used to stimulate the ipRGCs and red light activates the rod/cone photoreceptors. The late re-dilation phase of pupillary light reflex is primarily driven by the ipRGCs. Optic neuropathies i.e. Leber hereditary optic neuropathy (LHON), autosomal dominant optic atrophy (ADOA), nonarteritic anterior ischemic optic neuropathy (NAION), glaucoma, optic neuritis and idiopathic intracranial hypertension (IIH) are among the diseases, which have been subject to pupillometric studies. The ipRGCs are differentially affected in these various optic neuropathies. In mitochondrial optic neuropathies, the ipRGCs are protected against degeneration, whereas in glaucoma, NAION, optic neuritis and IIH the ipRGCs are damaged. Here, we will review the results of pupillometric, histopathological and animal studies evaluating the ipRGCs in mitochondrial and non-mitochondrial optic neuropathies.

Tau Accumulation, Altered Phosphorylation, and Missorting Promote Neurodegeneration in Glaucoma

Glaucoma, the leading cause of irreversible blindness worldwide, is characterized by the selective death of retinal ganglion cells (RGCs). Ocular hypertension is the most significant known risk factor for developing the disease, but the mechanism by which elevated pressure damages RGCs is currently unknown. The axonal-enriched microtubule-associated protein tau is a key mediator of neurotoxicity in Alzheimer's disease and other tauopathies. Using a well characterized in vivo rat glaucoma model, we show an age-related increase in endogenous retinal tau that was markedly exacerbated by ocular hypertension. Early alterations in tau phosphorylation, characterized by epitope-dependent hyperphosphorylation and hypophosphorylation, correlated with the appearance of tau oligomers in glaucomatous retinas. Our data demonstrate the mislocalization of tau in the somatodendritic compartment of RGCs subjected to high intraocular pressure. In contrast, tau was depleted from RGC axons in the optic nerve of glaucomatous eyes. Importantly, intraocular administration of short interfering RNA against tau effectively reduced retinal tau accumulation and promoted robust survival of RGC somas and axons, supporting a critical role for tau alterations in ocular hypertension-induced neuronal damage. Our study reveals that glaucoma displays signature pathological features of tauopathies, including tau accumulation, altered phosphorylation, and missorting; and identifies tau as a novel target to counter RGC neurodegeneration in glaucoma and prevalent optic neuropathies.

SIGNIFICANCE STATEMENT

In this study, we investigated the role of tau in retinal ganglion cell (RGC) damage in glaucoma. We demonstrate that high intraocular pressure leads to a rapid increase in endogenous retinal tau with altered phosphorylation profile and the formation of tau oligomers. Tau accumulation was primarily observed in RGC dendrites, while tau in RGC axons within the optic nerve was depleted. Attenuation of endogenous retinal tau using a targeted siRNA led to striking protection of RGC somas and axons from hypertension-induced damage. Our study identifies novel and substantial alterations of endogenous tau protein in glaucoma, including abnormal subcellular distribution, an altered phosphorylation profile, and neurotoxicity.

Shared and Differential Retinal Responses against Optic Nerve Injury and Ocular Hypertension

Glaucoma, one of the leading causes of blindness worldwide, affects primarily retinal ganglion cells (RGCs) and their axons. The pathophysiology of glaucoma is not fully understood, but it is currently believed that damage to RGC axons at the optic nerve head plays a major role. Rodent models to study glaucoma include those that mimic either ocular hypertension or optic nerve injury. Here we review the anatomical loss of the general population of RGCs (that express Brn3a; Brn3a⁺RGCs) and of the intrinsically photosensitive RGCs (that express melanopsin; m⁺RGCs) after chronic (LP-OHT) or acute (A-OHT) ocular hypertension and after complete intraorbital optic nerve transection (ONT) or crush (ONC). Our studies show that all of these insults trigger RGC death. Compared to Brn3a⁺RGCs, m⁺RGCs are more resilient to ONT, ONC, and A-OHT but not to LP-OHT. There are differences in the course of RGC loss both between these RGC types and among injuries. An important difference between the damage caused by ocular hypertension or optic nerve injury appears in the outer retina. Both axotomy and LP-OHT induce selective loss of RGCs but LP-OHT also induces a protracted loss of cone photoreceptors. This review outlines our current understanding of the anatomical changes occurring in rodent models of glaucoma and discusses the advantages of each one and their translational value.

The effects of immune protein CD3ζ development and degeneration of retinal neurons after optic nerve injury

Major histocompatibility complex (MHC) class I molecules and their receptors play fundamental roles in neuronal death during diseases. T-cell receptors (TCR) function as MHCI receptor on T-cells and both MHCI and a key component of TCR, CD3ζ, are expressed by mouse retinal ganglion cells (RGCs) and displaced amacrine cells. Mutation of these molecules compromises the development of RGCs. We investigated whether CD3ζ regulates the development and degeneration of amacrine cells after RGC death. Surprisingly, mutation of CD3ζ not only impairs the proper development of amacrine cells expressing CD3ζ but also those not expressing CD3ζ. In contrast to effects of MHCI and its receptor, PirB, on other neurons, mutation of CD3ζ has no effect on RGC death and starburst amacrine cells degeneration after optic nerve crush. Thus, unlike MHCI and PirB, CD3ζ regulates the development of RGCs and amacrine cells but not their degeneration after optic nerve crush.

Early Events in Retinal Degeneration Caused by Rhodopsin Mutation or Pigment Epithelium Malfunction: Differences and Similarities

To study the course of photoreceptor cell death and macro and microglial reactivity in two rat models of retinal degeneration with different etiologies. Retinas from P23H-1 (rhodopsin mutation) and Royal College of Surgeon (RCS, pigment epithelium malfunction) rats and age-matched control animals (Sprague-Dawley and Pievald Viro Glaxo, respectively) were cross-sectioned at different postnatal ages (from P10 to P60) and rhodopsin, L/M- and S-opsin, ionized calcium-binding adapter molecule 1 (Iba1), glial fibrillary acid protein (GFAP), and proliferating cell nuclear antigen (PCNA) proteins were immunodetected. Photoreceptor nuclei rows and microglial cells in the different retinal layers were quantified. Photoreceptor degeneration starts earlier and progresses quicker in P23H-1 than in RCS rats. In both models, microglial cell activation occurs simultaneously with the initiation of photoreceptor death while GFAP over-expression starts later. As degeneration progresses, the numbers of microglial cells increase in the retina, but decreasing in the inner retina and increasing in the outer retina, more markedly in RCS rats. Interestingly, and in contrast with healthy animals, microglial cells reach the outer nuclei and outer segment layers. The higher number of microglial cells in dystrophic retinas cannot be fully accounted by intraretinal migration and PCNA immunodetection revealed microglial proliferation in both models but more importantly in RCS rats. The etiology of retinal degeneration determines the initiation and pattern of photoreceptor cell death and simultaneously there is microglial activation and migration, while the macroglial response is delayed. The actions of microglial cells in the degeneration cannot be explained only in the basis of photoreceptor death because they participate more actively in the RCS model. Thus, the retinal degeneration caused by pigment epithelium malfunction is more inflammatory and would probably respond better to interventions by inhibiting microglial cells.

MicroRNA regulation in an animal model of acute ocular hypertension

Purpose

To analyse miRNA regulation in a rat model of acute ocular hypertension (AOH).

Methods

Acute ocular hypertension (AOH) was induced in the left eye of adult albino rats by inserting a cannula connected with a saline container into the anterior chamber. The contralateral eye served as a control. Seven days later, animals were killed. Retinas were used either for quantitative analysis of retinal ganglion cells (RGCs) and microglial cells or for miRNA array hybridization, qRT‐PCR and Western blotting.

Results

Anatomically, AOH caused axonal degeneration, a significant loss of RGCs and a significant increase in microglial cells in the ganglion cell layer. The miRNAs microarray analysis revealed 31 differentially expressed miRNAs in the AOH versus control group, and the regulation of 12 selected microRNAs was further confirmed by qRT‐PCR. Bioinformatic analysis indicates that several signalling pathways are putatively regulated by the validated miRNAs. Of particular interest was the inflammatory pathway signalled by mitogen‐activated protein kinases (MAPKs). In agreement with the in silico analysis, p38 MAP kinase, tumour necrosis factor‐alpha (TNF‐α) and iNOS proteins were significantly upregulated in the AOH retinas.

Conclusions

Acute IOP elevation led to changes in the expression of miRNAs, whose target genes were associated with the regulation of microglia‐mediated neuroinflammation or neural apoptosis. Addressing miRNAs in the process of retinal ischaemia and optic nerve damage in association with high IOP elevation may open new avenues in preventing retinal ganglion cell apoptosis and may serve as target for future therapeutic regimen in acute ocular hypertension and retinal ischaemic conditions.

Cone Integrity in Glaucoma: An Adaptive-Optics Scanning Laser Ophthalmoscopy Study

PURPOSE

To investigate photoreceptor changes in eyes with glaucoma.

DESIGN

Cross-sectional study.

METHODS

The study included 35 eyes of 35 patients with primary open-angle glaucoma who had suffered parafoveal visual field loss at least 3 years previously, as well as 21 eyes of 21 normal subjects. Eyes with an axial length ≥26.0 mm were excluded. All subjects underwent a full ophthalmologic examination, including spectral-domain optical coherence tomography (SDOCT) and prototype adaptive-optics scanning laser ophthalmoscopy (AO-SLO) imaging.

RESULTS

As determined using AO-SLO, eyes with glaucoma did not differ significantly from normal eyes in terms of either cone density (26 468 ± 3392 cones/m² vs 26 147 ± 2700 cones/m², respectively; P = .77; measured 0.5 mm from the foveal center) or cone spatial organization (ratio of hexagonal Voronoi domain: 43.7% ± 4.4% vs 44.3% ± 4.9%; P = .76; measured 0.5 mm from the foveal center). Furthermore, SDOCT showed that the 2 groups did not differ significantly in terms of the photoreceptor-related layer thickness, and that the photoreceptor ellipsoid zone band was continuous in all normal and glaucoma eyes. In glaucoma eyes with vertically asymmetric severity, the more affected side did not significantly differ from the less affected side in terms of cone density, cone spatial organization, or photoreceptor-related layer thickness. In 8 eyes (22.9%) with glaucoma, dark, partition-like areas surrounded the cones on the AO-SLO.

CONCLUSIONS

Both AO-SLO and SDOCT showed cone integrity in eyes with glaucoma, even in areas with visual field and nerve fiber loss. In AO-SLO, microcystic lesions in the inner nuclear layer may influence images of the cone mosaic.

Caffeine administration prevents retinal neuroinflammation and loss of retinal ganglion cells in an animal model of glaucoma

Glaucoma is the second leading cause of blindness worldwide, being characterized by progressive optic nerve damage and loss of retinal ganglion cells (RGCs), accompanied by increased inflammatory response involving retinal microglial cells. The etiology of glaucoma is still unknown, and despite elevated intraocular pressure (IOP) being a major risk factor, the exact mechanisms responsible for RGC degeneration remain unknown. Caffeine, which is an antagonist of adenosine receptors, is the most widely consumed psychoactive drug in the world. Several evidences suggest that caffeine can attenuate the neuroinflammatory responses and afford protection upon central nervous system (CNS) injury. We took advantage of a well characterized animal model of glaucoma to investigate whether caffeine administration controls neuroinflammation and elicits neuroprotection. Caffeine or water were administered ad libitum and ocular hypertension (OHT) was induced by laser photocoagulation of the limbal veins in Sprague Dawley rats. Herein, we show that caffeine is able to partially decrease the IOP in ocular hypertensive animals. More importantly, we found that drinking caffeine prevented retinal microglia-mediated neuroinflammatory response and attenuated the loss of RGCs in animals with ocular hypertension (OHT). This study opens the possibility that caffeine or adenosine receptor antagonists might be a therapeutic option to manage RGC loss in glaucoma.

The pro-inflammatory role of high-mobility group box 1 protein (HMGB-1) in photoreceptors and retinal explants exposed to elevated pressure

To determine the role of high-mobility group box 1 protein (HMGB-1) in cellular and tissue models of elevated pressure-induced neurodegeneration, regeneration, and inflammation. Mouse retinal photoreceptor-derived cells (661W) and retinal explants were incubated either under elevated pressure or in the presence of recombinant HMGB-1 (rHMGB-1) to investigate the mechanisms of response of photoreceptors. Immunohistochemistry, western blotting, and the quantitative real-time PCR were used to examine the expression levels of immunological factors (eg, HMGB-1, receptor for advanced glycation end products (RAGE)), Toll-like receptors 2 and 4 (TLR-2, TLR-4), apoptosis-related factors (eg, B-cell lymphoma 2 (Bcl-2), Bcl-2-associated death promoter (Bad)) as well as cytokine expression (eg, tumor necrosis factor alpha (TNF-α), interleukin (IL)-4, IL-6, and vascular endothelial growth factor (VEGF)). The data revealed increased the expression of HMGB-1 and its receptors RAGE, TLR-2, and TLR-4, and TNF-α as well as pro-apoptotic factors (eg, Bad) as well as apoptosis in 661W cells exposed to elevated pressure. Co-cultivation of 661W cells with rHMGB-1 increased the expression levels of pro-apoptotic Bad and cleaved Caspase-3 resulting in apoptosis. Cytokine array studies revealed an increased release of TNF-α, IL-4, IL-6, and VEGF after incubation of 661W cells with rHMGB-1. Upregulation of HMGB-1, TLR-2, and RAGE as well as anti-apoptotic Bcl-2 expression levels was found in the retinal explants exposed to rHMGB-1 or elevated pressure. The results suggest that HMGB-1 promotes an inflammatory response and mediates apoptosis in the pathology of photoreceptors and retinal homeostasis. HMGB-1 may have a key role in ongoing damage of retinal cells under conditions of elevated intraocular pressure.

Retinal ganglion cell neuroprotection induced by activation of alpha7 nicotinic acetylcholine receptors

The α7nAChR agonist, PNU-282987, has previously been shown to have a neuroprotective effect against loss of retinal ganglion cells (RGCs) in an in vivo glaucoma model when the agent was injected into the vitreous chamber of adult Long Evans rat eyes. Here, we characterized the neuroprotective effect of PNU-282987 at the nerve fiber and retinal ganglion cell layer, determined that neuroprotection occurred when the agonist was applied as eye drops and verified detection of the agonist in the retina, using LC/MS/MS. To induce glaucoma-like conditions in adult Long Evans rats, hypertonic saline was injected into the episcleral veins to induce scar tissue and increase intraocular pressure. Within one month, this procedure produced significant loss of RGCs compared to untreated conditions. RGCs were quantified after immunostaining with an antibody against Thy 1.1 and imaged using a confocal microscope. In dose-response studies, concentrations of PNU-282987 were applied to the animal's right eye two times each day, while the left eye acted as an internal control. Eye drops of PNU-282987 resulted in neuroprotection against RGC loss in a dose-dependent manner using concentrations between 100 μM and 2 mM PNU-282987. LC/MS/MS results demonstrated that PNU-282987 was detected in the retina when applied as eye drops, relatively small amounts of PNU-282987 were measured in blood plasma and no PNU-282987 was detected in cardiac tissue. These results support the hypothesis that eye drop application of PNU-282987 can prevent loss of RGCs associated with glaucoma, which can lead to neuroprotective treatments for diseases that involve α7nAChRs.

Retinal neurodegeneration in experimental glaucoma

In rats and mice, limbar tissues of the left eye were laser-photocoagulated (LP) and ocular hypertension (OHT) effects were investigated 1 week to 6 months later. To investigate the innermost layers, retinas were examined in wholemounts using tracing from the superior colliculi to identify retinal ganglion cells (RGCs) with intact retrograde axonal transport, melanopsin immunodetection to identify intrinsically photosensitive RGCs (m(+)RGC), Brn3a immunodetection to identify most RGCs but not m(+)RGCs, RECA1 immunodetection to examine the inner retinal vessels, and DAPI staining to detect all nuclei in the GC layer. The outer retinal layers (ORLs) were examined in cross sections analyzed morphometrically or in wholemounts to study S- and L-cones. Innervation of the superior colliculi was examined 10 days to 14 weeks after LP with orthogradely transported cholera toxin subunit B. By 2 weeks, OHT resulted in pie-shaped sectors devoid of FG(+)RGCs or Brn3a(+)RGCs but with large numbers of DAPI(+)nuclei. Brn3a(+)RGCs were significantly greater than FG(+)RGCs, indicating the survival of large numbers of RGCs with their axonal transport impaired. The inner retinal vasculature showed no abnormalities that could account for the sectorial loss of RGCs. m(+)RGCs decreased to approximately 50-51% in a diffuse loss across the retina. Cross sections showed focal areas of degeneration in the ORLs. RGC loss at 1m diminished to 20-25% and did not progress further with time, whereas the S- and L-cone populations diminished progressively up to 6m. The retinotectal projection was reduced by 10 days and did not progress further. LP-induced OHT results in retrograde degeneration of RGCs and m(+)RGCs, severe damage to the ORL, and loss of retinotectal terminals.

The Rodent Model of Glaucoma and Its Implications

Glaucoma is a group of progressive optic neuropathies, characterized by the degeneration of retinal ganglion cells related to the level of intraocular pressure and other factors. The exact pathogenesis of glaucoma is not known, and current therapeutic options are not sufficient to prevent or recover vision loss in glaucoma patients. Functional, repeatable, and easy-to-use animal models are therefore needed. Because of their inherent advantages, rodent animals, including mice and rats, have been widely developed as models to study various aspects of glaucoma and to evaluate possible novel therapies. However, no single model has been shown to emulate all aspects of glaucoma. In this review, we discuss currently available rodent animal models of glaucoma, their strengths and weaknesses, and the possible implications for current glaucoma research.

Effects of ocular hypertension in the visual system of pigmented mice

To study the effects of ocular hypertension (OHT) on the visual system of C57BL/6 pigmented mice, the limbal and episcleral veins of the left eye were laser photocoagulated (LP). LP increased the intraocular pressure during the first five days (d), reaching basal values at 7d. To investigate the effect of OHT on the retinal ganglion cell (RGC) retrograde axonal transport, hydroxistilbamidine methanesulfonate (OHSt) was applied to both superior colliculi (SCi) and the retinas were dissected 2 or 4 weeks after LP. To determine RGC survival, these same retinas were immunoreacted against Brn3a (general RGC population) and melanopsin (intrinsically photosensitive RGCs, m⁺RGCs). To study whether OHT affected non-RGC neurons in the ganglion cell layer (GCL), RGCs were immunodetected with Brn3a and all GCL nuclei counterstained with DAPI in a group of animals examined 4 weeks post-LP. Innervation of the SCi was examined at 10 days, 8 or 14 weeks after LP with the orthogradely transported cholera toxin subunit-B. OHT resulted in diffuse and sectorial loss of OHSt+RGCs (50% at 2 weeks and 62% at 4 weeks) and in a comparable loss of Brn3a+RGCs at the same time intervals. m+RGCs decreased to 59% at 2 weeks and to 46% at 4 weeks, such loss was diffuse, did not parallel the sectorial loss of the general RGC population and was more severe in the superior-temporal retina. In the GCL, cell loss is selective for RGCs and does not affect other non-RGC neurons. The retinotectal innervation appeared significantly reduced at 10 days (55.7%) and did not progress further up to 14 weeks (46.6%). Thus, LP-induced OHT results in retrograde degeneration of RGCs and m+RGCs, as well as in the loss of CTB-labelled retinotectal terminals.