Specific amacrine cell changes in an induced mouse model of glaucoma

Characterizing Presumed Displaced Retinal Ganglion Cells in the Living Human Retina of Healthy and Glaucomatous Eyes

Purpose

The purpose of this study was to investigate the large somas presumed to be displaced retinal ganglion cells (dRGCs) located in the inner nuclear layer (INL) of the living human retina. Whereas dRGCs have previously been studied in mammals and human donor tissue, they have never been investigated in the living human retina.

Methods

Five young, healthy subjects and three subjects with varying types of glaucoma were imaged at multiple locations in the macula using adaptive optics optical coherence tomography. In the acquired volumes, bright large somas at the INL border with the inner plexiform layer were identified, and the morphometric biomarkers of soma density, en face diameter, and spatial distribution were measured at up to 13 degrees retinal eccentricity. Susceptibility to glaucoma was assessed.

Results

In the young, healthy individuals, mean density of the bright, large somas was greatest foveally (550 and 543 cells/mm2 at 2 degrees temporal and nasal, respectively) and decreased with increasing retinal eccentricity (38 cells/mm2 at 13 degrees temporal, the farthest we measured). Soma size distribution showed the opposite trend with diameters and size variation increasing with retinal eccentricity, from 12.7 ± 1.8 µm at 2 degrees to 15.7 ± 3.5 µm at 13 degrees temporal, and showed evidence of a bimodal distribution in more peripheral locations. Within and adjacent to the arcuate defects of the subjects with glaucoma, density of the bright large somas was significantly lower than found in the young, healthy individuals.

Conclusions

Our results suggest that the bright, large somas at the INL border are likely comprised of dRGCs but amacrine cells may contribute too. These somas appear highly susceptible to glaucomatous damage.

Localization of Calretinin, Parvalbumin, and S100 Protein in Nothobranchius guentheri Retina: A Suitable Model for the Retina Aging

Calcium-binding proteins (CaBPs) are members of a heterogeneous family of proteins able to buffer intracellular Ca2+ ion concentration. CaBPs are expressed in the central and peripheral nervous system, including a subpopulation of retinal neurons. Since neurons expressing different CaBPs show different susceptibility to degeneration, it could be hypothesized that they are not just markers of different neuronal subpopulations, but that they might be crucial in survival. CaBPs' ability to buffer Ca2+ cytoplasmatic concentration makes them able to defend against a toxic increase in intracellular calcium that can lead to neurodegenerative processes, including those related to aging. An emergent model for aging studies is the annual killifish belonging to the Nothobranchius genus, thanks to its short lifespan. Members of this genus, such as Nothobranchius guentheri, show a retinal stratigraphy similar to that of other actinopterygian fishes and humans. However, according to our knowledge, CaBPs' occurrence and distribution in the retina of N. guentheri have never been investigated before. Therefore, the present study aimed to localize Calretinin N-18, Parvalbumin, and S100 protein (S100p) in the N. guentheri retina with immunohistochemistry methods. The results of the present investigation demonstrate for the first time the occurrence of Calretinin N-18, Parvalbumin, and S100p in N. guentheri retina and, consequently, the potential key role of these CaBPs in the biology of the retinal cells. Hence, the suitability of N. guentheri as a model to study the changes in CaBPs' expression patterns during neurodegenerative processes affecting the retina related both to disease and aging can be assumed.

Parvalbumin expression changes with retinal ganglion cell degeneration

Background

Glaucoma is one of the main causes of irreversible visual field loss and blindness worldwide. Vision loss in this multifactorial neurodegenerative disease results from progressive degeneration of retinal ganglion cells (RGCs) and their axons. Identifying molecular markers that can be measured objectively and quantitatively may provide essential insights into glaucoma diagnosis and enhance pathophysiology understanding.

Methods

The chronic, progressive DBA/2J glaucomatous mouse model of glaucoma and C57BL6/J optic nerve crush (ONC) mouse model were used in this study. Changes in PVALB expression with RGC and optic nerve degeneration were assessed via gene expression microarray analysis, quantitative real-time polymerase chain reaction (qRT-PCR), Western blot and immunohistochemistry.

Results

Microarray analysis of the retinal gene expression in the DBA/2J mice at different ages showed that the expression of PVALB was downregulated as the mice aged and developed glaucoma with retinal ganglion cell loss. Analysis of qRT-PCR results demonstrated PVALB at the mRNA level was reduced in the retinas and optic nerves of old DBA/2J mice and in those after ONC compared to baseline young DBA2/J mice. PVALB protein expression measured by Western blot was also significantly reduced signal in the retinas and optic nerves of old DBA/2J mice and those eyes with crushed nerves. Immunohistochemical staining results demonstrated that there were fewer PVALB-positive cells in the ganglion cell layer (GCL) of the retina and staining pattern changed in the optic nerve from old DBA/2J mice as well as in mice eyes following ONC.

Conclusion

PVALB is abundantly expressed both by RGCs' soma in the retinas and RGCs' axons in the optic nerves of C57BL/6J. Furthermore, the expression level of PVALB decreases with RGC degeneration in the glaucomatous DBA/2J mice and after ONC injury of C57BL6/6J, indicating that PVALB is a reliable RGC molecular marker that can be used to study retinal and optic nerve degeneration.

Selective drug delivery to the retinal cells: Biological barriers and avenues

Retinal drug delivery is a challenging, but important task, because most retinal diseases are still without any proper therapy. Drug delivery to the retina is hampered by the anatomical and physiological barriers resulting in minimal bioavailability after topical ocular and systemic administrations. Intravitreal injections are current method-of-choice in retinal delivery, but these injections show short duration of action for small molecules and low target bioavailability for many protein, gene based drugs and nanomedicines. State-of-art delivery systems are based on prolonged retention, controlled drug release and physical features (e.g. size and charge). However, drug delivery to the retina is not cell-specific and these approaches do not facilitate intracellular delivery of modern biological drugs (e.g. intracellular proteins, RNA based medicines, gene editing). In this focused review we highlight biological factors and mechanisms that form the basis for the selective retinal drug delivery systems in the future. Therefore, we are presenting current knowledge related to retinal membrane transporters, receptors and targeting ligands in relation to nanomedicines, conjugates, extracellular vesicles, and melanin binding. These issues are discussed in the light of retinal structure and cell types as well as future prospects in the field. Unlike in some other fields of targeted drug delivery (e.g. cancer research), selective delivery technologies have been rarely studied, even though cell targeted delivery may be even more feasible after local administration into the eye.

Loss of optic nerve oligodendrocytes during maturation alters retinal organization

The myelin sheath facilitates signal conduction along axons in white matter tracts, and when disrupted, can result in significant functional deficits. Demyelination, observed in diseases like multiple sclerosis and optic neuritis, are associated with neural degeneration, however the extent of this damage on upstream circuitry is not well understood. Here we use the MBP-iCP9 mouse model to induce selective oligodendrocyte ablation in the optic nerve at P14 via a chemical inducer of dimerization (CID), resulting in partial demyelination of retinal ganglion cell (RGC) axons with minimal inflammation after two weeks. Oligodendrocyte loss reduced axon diameter and altered compound action potential waveforms, blocking conduction in the slowest-conducting axon populations. Demyelination resulted in disruptions to the normal composition of the retina, including reduced density of RBPMS+, Brn3a+, and OFF-transient RGCs, thinning of the IPL, and reduced density of displaced amacrine cells. The INL and ONL were unaffected by oligodendrocyte loss, suggesting that demyelination-induced deficits in this model are specific to the IPL and GCL. These results show that a partial demyelination of a subpopulation of RGC axons disrupts optic nerve function and affects the organization of the retinal network. This study highlights the significance of myelination in maintaining upstream neural connectivity and provides support for targeting neuronal degeneration in treatments of demyelinating diseases.

Potential Neuroprotective Role of Calretinin-N18 and Calbindin-D28k in the Retina of Adult Zebrafish Exposed to Different Wavelength Lights

The incidence rates of light-induced retinopathies have increased significantly in the last decades because of continuous exposure to light from different electronic devices. Recent studies showed that exposure to blue light had been related to the pathogenesis of light-induced retinopathies. However, the pathophysiological mechanisms underlying changes induced by light exposure are not fully known yet. In the present study, the effects of exposure to light at different wavelengths with emission peaks in the blue light range (400-500 nm) on the localization of Calretinin-N18 (CaR-N18) and Calbindin-D28K (CaB-D28K) in adult zebrafish retina are studied using double immunofluorescence with confocal laser microscopy. CaB-D28K and CaR-N18 are two homologous cytosolic calcium-binding proteins (CaBPs) implicated in essential process regulation in central and peripheral nervous systems. CaB-D28K and CaR-N18 distributions are investigated to elucidate their potential role in maintaining retinal homeostasis under distinct light conditions and darkness. The results showed that light influences CaB-D28K and CaR-N18 distribution in the retina of adult zebrafish, suggesting that these CaBPs could be involved in the pathophysiology of retinal damage induced by the short-wavelength visible light spectrum.

Selective vulnerability of the intermediate retinal capillary plexus precedes retinal ganglion cell loss in ocular hypertension

Introduction: Glaucoma, a disease of retinal ganglion cell (RGC) injury and potentially devastating vision loss, is associated with both ocular hypertension (OHT) and reduced ocular blood flow. However, the relationship between OHT and retinal capillary architecture is not well understood. In this project, we studied microvasculature damage in mice exposed to mild levels of induced OHT. Methods: Mild OHT was induced with the microbead model for 2 weeks. At this time point, some retinas were immunostained with CD31 (endothelium), Collagen IV (basement membrane), and RBPMS (RGCs) for z-stack confocal microscopy. We processed these confocal images to distinguish the three retinal capillary plexi (superficial, intermediate, and deep). We manually counted RGC density, analyzed vascular complexity, and identified topographical and spatial vascular features of the retinal capillaries using a combination of novel manual and automated workflows. Other retinas were dissociated and immunopanned to isolate RGCs and amacrine cells (ACs) for hypoxia gene array analysis. Results: RGC counts were normal but there was decreased overall retinal capillary complexity. This reduced complexity could be explained by abnormalities in the intermediate retinal capillary plexus (IRCP) that spared the other plexi. Capillary junction density, vessel length, and vascular area were all significantly reduced, and the number of acellular capillaries was dramatically increased. ACs, which share a neurovascular unit (NVU) with the IRCP, displayed a marked increase in the relative expression of many hypoxia-related genes compared to RGCs from the same preparations. Discussion: We have discovered a rapidly occurring, IRCP-specific, OHT-induced vascular phenotype that precedes RGC loss. AC/IRCP NVU dysfunction may be a mechanistic link for early vascular remodeling in glaucoma.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

Deletion of transcription factor AP-2β from the developing murine trabecular meshwork region leads to progressive glaucomatous changes

Glaucoma is one of the leading causes of irreversible blindness and can result from abnormalities in anterior segment structures required for aqueous humor outflow, including the trabecular meshwork (TM) and Schlemm's canal (SC). Transcription factors such as AP-2β play critical roles in anterior segment development. Here, we show that the Mgp-Cre knock-in (Mgp-Cre.KI) mouse can be used to target the embryonic periocular mesenchyme giving rise to the TM and SC. Fate mapping of male and female mice indicates that AP-2β loss causes a decrease in iridocorneal angle cells derived from Mgp-Cre.KI-expressing populations compared to controls. Moreover, histological analyses revealed peripheral iridocorneal adhesions in AP-2β mutants that were accompanied by a decrease in expression of TM and SC markers, as observed using immunohistochemistry. In addition, rebound tonometry showed significantly higher intraocular pressure (IOP) that was correlated with a progressive significant loss of retinal ganglion cells, reduced retinal thickness, and reduced retinal function, as measured using an electroretinogram, in AP-2β mutants compared with controls, reflecting pathology described in late-stage glaucoma patients. Importantly, elevated IOP in AP-2β mutants was significantly reduced by treatment with latanoprost, a prostaglandin analog that increases unconventional outflow. These findings demonstrate that AP-2β is critical for TM and SC development, and that these mutant mice can serve as a model for understanding and treating progressive human primary angle-closure glaucoma.

Retinal Contrast Gain Control and Temporal Modulation Sensitivity Across the Visual Field in Glaucoma at Photopic and Mesopic Light Conditions

Purpose

Glaucoma affects many aspects of visual performance, including adaptation, and this may depend on ambient luminance. We determine the influence of glaucoma and luminance on temporal aspects of adaptation, specifically on contrast gain control and temporal modulation sensitivity (TMS).

Methods

This case-control study included 12 glaucoma patients and 25 age-similar controls (50-70 years). Threshold perimetry was performed with a minimized testing grid (fovea and four peripheral locations). Stimuli (Goldmann size III 50 ms increment/decrement) were presented on a time-varying background with sinusoidally-modulated luminance (amplitude 60%; frequency 0-30 Hz; mean background luminance, 1 and 100 cd/m2). TMS (2.5-30 Hz) was measured in the same locations with a sinusoidally-modulated stimulus (Goldmann size IV, 334 ms) on a steady background (1 and 100 cd/m2).

Results

In healthy subjects, contrast sensitivity decreased with increasing background modulation frequency and increased again at very high frequencies, indicating contrast gain control. Minimum sensitivity was located between 2.5 and 20 Hz, depending on luminance and eccentricity. In glaucoma patients, the same frequency dependency was found (P = 0.12) but with an overall reduced sensitivity (P = 1 × 10-5), independent of luminance (P = 0.20). Decrements differentiated better between glaucoma and healthy subjects than increments (P = 0.004). TMS was reduced in glaucoma (P = 5 × 10-6) across all frequencies and luminance levels, with complete loss for high frequencies at 1 cd/m2.

Conclusions

Contrast gain control is largely unaffected in glaucoma, suggesting intact amacrine cell function. Perimetry with decrements or a high-frequency stimulus on a low-luminance background seems best to differentiate between glaucoma and healthy subjects.

Rod bipolar cells dysfunction occurs before ganglion cells loss in excitotoxin-damaged mouse retina

Progressive degeneration of retinal ganglion cells (RGCs) will cause a blinding disease. Most of the study is focusing on the RGCs itself. In this study, we demonstrate a decline of the presynaptic rod bipolar cells (RBCs) response precedes RGCs loss and a decrease of protein kinase Cα (PKCα) protein expression in RBCs dendrites, using whole-cell voltage-clamp, electroretinography (ERG) measurements, immunostaining and co-immunoprecipitation. We present evidence showing that N-methyl D-aspartate receptor subtype 2B (NR2B)/protein interacting with C kinase 1 (PICK1)-dependent degradation of PKCα protein in RBCs contributes to RBCs functional loss. Mechanistically, NR2B forms a complex with PKCα and PICK1 to promote the degradation of PKCα in a phosphorylation- and proteasome-dependent manner. Similar deficits in PKCα expression and response sensitivity were observed in acute ocular hypertension and optic never crush models. In conclusion, we find that three separate experimental models of neurodegeneration, often used to specifically target RGCs, disrupt RBCs function prior to the loss of RGCs. Our findings provide useful information for developing new diagnostic tools and treatments for retinal ganglion cells degeneration disease.

Inner Retinal Changes in Primary Open-Angle Glaucoma Revealed Through Adaptive Optics-Optical Coherence Tomography

PURPOSE

To examine the microstructural changes in the inner nuclear layer (INL) and ganglion cell layer (GCL) in a primary open-angle glaucoma (POAG) subject at 2 timepoints, 4 months apart.

PATIENTS AND METHODS

This case-control study (1 POAG subject and 1 normal control) used the single cell, 3-dimensional volumetric imaging capability of an adaptive optics-optical coherence tomography-scanning laser ophthalmoscopy system to examine the inner retina.

RESULTS

At the area of greatest glaucomatous change in the POAG subject [3-degrees temporal (T), 3-degrees inferior (I), right eye], the GCL was greatly thinned at both timepoints, yet retinal ganglion cell soma remained visible amid a meshwork of capillaries. Microcystic lesions in the INL were visible at both timepoints, ranging in diameter from 8 to 43 μm on day 1 to 11 to 64 μm at 4 months, with an average diameter increase of ∼124%. Small hyperreflective features (not seen in the contralateral eye or control subject) at a depth midway through the INL seemed correlated to the development of microcysts.

CONCLUSIONS

We demonstrate the ability to image microcystic lesions early in their development and have quantified longitudinal changes. The presence of small hyperreflective structures at a layer midway through the INL seems to be a precursor to their formation and is a potential biomarker for assessing POAG severity and progression. The adaptive optics imaging system is also able to visualize retinal ganglion cells in this subject, despite severe thinning of the GCL.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

Expression of Ca2+-Binding Buffer Proteins in the Human and Mouse Retinal Neurons

Ca2+-binding buffer proteins (CaBPs) are widely expressed by various neurons throughout the central nervous system (CNS), including the retina. While the expression of CaBPs by photoreceptors, retinal interneurons and the output ganglion cells in the mammalian retina has been extensively studied, a general description is still missing due to the differences between species, developmental expression patterns and study-to-study discrepancies. Furthermore, CaBPs are occasionally located in a compartment-specific manner and two or more CaBPs can be expressed by the same neuron, thereby sharing the labor of Ca2+ buffering in the intracellular milieu. This article reviews this topic by providing a framework on CaBP functional expression by neurons of the mammalian retina with an emphasis on human and mouse retinas and the three most abundant and extensively studied buffer proteins: parvalbumin, calretinin and calbindin.

Functional Analysis and Immunochemical Analyses of Ca2+ Homeostasis-Related Proteins Expression of Glaucoma-Induced Retinal Degeneration in Rats

The retinal degeneration resulting from elevated intraocular pressure was evaluated through functional and morphological analyses, for better understanding of the pathophysiology of glaucoma. Ocular hypertension was induced via unilateral episcleral venous cauterization in rats. Experimental time was set at 1 and 3 days, and 1, 2, 4, and 8 weeks post-operation. Retinal function was analyzed using electroretinography. For morphological analysis, retinal tissues were processed for immunochemistry by using antibodies against the calcium-sensing receptor and calcium-binding proteins. Apoptosis was analyzed using the TUNEL method and electron microscopy. Amplitudes of a- and b-wave in scotopic and photopic responses were found to be reduced in all glaucomatous retinas. Photopic negative response for ganglion cell function significantly reduced from 1-day and more significantly reduced in 2-week glaucoma. Calcium-sensing receptor immunoreactivity in ganglion cells remarkably reduced at 8 weeks; conversely, protein amounts increased significantly. Calcium-binding proteins immunoreactivity in amacrine cells clearly reduced at 8 weeks, despite of uneven changes in protein amounts. Apoptosis appeared in both photoreceptors and ganglion cells in 8-week glaucomatous retina. Apoptotic feature of photoreceptors was typical, whereas that of ganglion cells was necrotic in nature. These findings suggest that elevated intraocular pressure affects the sensitivity of photoreceptors and retinal ganglion cells, and leads to apoptotic death. The calcium-sensing receptor may be a useful detector for alteration of extracellular calcium levels surrounding the ganglion cells.

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.

Pre-treatment with vinpocetine protects against retinal ischemia

Vinpocetine has been shown to have beneficial effects for tissues of the central nervous system subjected to ischemia and other related metabolic insults. We recently showed vinpocetine promotes glucose availability, prevents unregulated cation channel permeability and regulates glial reactivity when present during retinal ischemia. Less is known however about the ability of vinpocetine to protect against future ischemic insults. This study explores the effect of vinpocetine when used as a pre-treatment in an ex vivo model for retinal ischemia using cation channel permeability of agmatine (AGB) combined with immunohistochemistry as a measure for cell functionality. We found that vinpocetine pre-treatment reduced cation channel permeability and apoptotic marker immunoreactivity in the GCL and increased parvalbumin immunoreactivity of inner retinal neurons in the inner nuclear layer following ischemic insult. Vinpocetine pre-treatment also reduced Müller cell reactivity following ischemic insults of up to 120 min compared to untreated controls. Many of vinpocetine's effects however were transient in nature suggesting the drug can protect retinal neurons against future ischemic damage but may have limited long-term applications.

Amacrine cells coupled to ganglion cells via gap junctions are highly vulnerable in glaucomatous mouse retinas

We determined whether the structural and functional integrity of amacrine cells (ACs), the largest cohort of neurons in the mammalian retina, are affected in glaucoma. Intraocular injection of microbeads was made in mouse eyes to elevate intraocular pressure as a model of experimental glaucoma. Specific immunocytochemical markers were used to identify AC and displaced (d)ACs subpopulations in both the inner nuclear and ganglion cell layers, respectively, and to distinguish them from retinal ganglion cells (RGCs). Calretinin- and γ-aminobutyric acid (GABA)-immunoreactive (IR) cells were highly vulnerable to glaucomatous damage, whereas choline acetyltransferase (ChAT)-positive and glycinergic AC subtypes were unaffected. The AC loss began 4 weeks after initial microbead injection, corresponding to the time course of RGC loss. Recordings of electroretinogram (ERG) oscillatory potentials and scotopic threshold responses, which reflect AC and RGC activity, were significantly attenuated in glaucomatous eyes following a time course that matched that of the AC and RGC loss. Moreover, we found that it was the ACs coupled to RGCs via gap junctions that were lost in glaucoma, whereas uncoupled ACs were largely unaffected. Our results suggest that AC loss in glaucoma occurs secondary to RGC death through the gap junction-mediated bystander effect. J. Comp. Neurol. 527:159-173, 2019. © 2016 Wiley Periodicals, Inc.

Changes in parvalbumin immunoreactive retinal ganglion cells and amacrine cells after optic nerve injury

Parvalbumin (PARV) is a Ca(2+)-binding protein that may offer resistance to cell death as it primarily functions to maintain Ca(2+) homeostasis. The purpose of this study was to investigate whether PARV expressing retinal ganglion cells (RGCs) would be more resistant to cell death than RGCs that do not express PARV. RGCs in Sprague-Dawley rats were retrogradely labeled with Fluorogold (FG). After 2-28 days following an optic nerve crush (ONC) injury immunohistochemistry was performed on the sections using antibodies against PARV and markers of RGCs. The proportion of retinal ganglion cell layer cells labeled with PARV colocalized with FG or Brn3a and labeled only with PARV (displaced amacrine cells; dACs) were analyzed. PARV staining intensity was measured in ACs, dACs, and RGCs. Double labeling studies revealed that 49% of RGCs and 22% of dACs expressed PARV. There was an immediate reduction in RGC PARV staining after ONC but the overall rate of cell death after 28 days was similar in PARV and non-PARV expressing RGCs. There was no change in PARV AC or dAC number or staining intensity. Although this study suggests that there is no selective survival of the subpopulation of RGCs that contain PARV, there is down-regulation of PARV expression by these RGCs. This suggests that down-regulation of PARV may contribute to RGC death due to a compromised Ca(2+) buffering capacity. Maintaining PARV expression after injury could be an important neuroprotective strategy to improve RGC survival after injury.

A laser-induced mouse model with long-term intraocular pressure elevation

Purpose

To develop and characterize a mouse model with intraocular pressure (IOP) elevation after laser photocoagulation on the trabecular meshwork (TM), which may serve as a model to investigate the potential of stem cell-based therapies for glaucoma.

Methods

IOP was measured in 281 adult C57BL/6 mice to determine normal IOP range. IOP elevation was induced unilaterally in 50 adult mice, by targeting the TM through the limbus with a 532-nm diode laser. IOP was measured up to 24 weeks post-treatment. The optic nerve damage was detected by electroretinography and assessed by semiautomatic counting of optic nerve axons. Effects of laser treatment on the TM were evaluated by histology, immunofluorescence staining, optical coherence tomography (OCT) and transmission electron microscopy (TEM).

Results

The average IOP of C57BL/6 mice was 14.5±2.6 mmHg (Mean ±SD). After laser treatment, IOP averaged above 20 mmHg throughout the follow-up period of 24 weeks. At 24 weeks, 57% of treated eyes had elevated IOP with the mean IOP of 22.5±2.5 mmHg (Mean ±SED). The difference of average axon count (59.0%) between laser treated and untreated eyes was statistically significant. Photopic negative response (PhNR) by electroretinography was significantly decreased. CD45+ inflammatory cells invaded the TM within 1 week. The expression of SPARC was increased in the TM from 1 to 12 weeks. Histology showed the anterior chamber angle open after laser treatment. OCT indicated that most of the eyes with laser treatment had no synechia in the anterior chamber angles. TEM demonstrated disorganized and compacted extracellular matrix in the TM.

Conclusions

An experimental murine ocular hypertension model with an open angle and optic nerve axon loss was produced with laser photocoagulation, which could be used to investigate stem cell-based therapies for restoration of the outflow pathway integrity for ocular hypertension or glaucoma.

Elevated intraocular pressure causes inner retinal dysfunction before cell loss in a mouse model of experimental glaucoma

PURPOSE

We assessed the relationship among intraocular pressure (IOP), histology, and retinal function changes in a mouse model of induced, chronic, mild ocular hypertension.

METHODS

IOP was elevated experimentally via anterior chamber injection of polystyrene beads and measured twice weekly with a rebound tonometer. Histology was assessed with a combination of neurobiotin (NB) retrograde labeling of retinal ganglion cells (RGCs) and TO-PRO3 staining. Retinal function was assessed with serial dark-adapted electroretinograms (ERGs) optimized for detection of the a-wave, b-wave, and positive and negative scotopic threshold responses (pSTR, nSTR). Comparisons between bead-injected and saline-injected (control) eyes were conducted.

RESULTS

IOP remained elevated for at least 3 months following a single injection of polystyrene beads. Elevated IOP resulted in a mild, progressive reduction of RGCs, and a mild increase in axial length at 6 and 12 weeks after bead injection. The raw b-wave amplitude was increased shortly after IOP elevation, but the raw a-wave, pSTR, and nSTR amplitudes were unchanged. pSTR and nSTR amplitudes were normalized to the increased b-wave. With this normalization, the pSTR amplitude was decreased shortly after IOP elevation.

CONCLUSIONS

Polystyrene bead injection results in a mild, chronic elevation of IOP that recapitulates several critical aspects of human ocular hypertension and glaucoma, and results in early changes in retinal electrical function that precede histologic changes. It is possible that glaucoma associated with elevated IOP involves the early disruption of a complex combination of retinal synapses.

Protecting the retinal neurons from glaucoma: lowering ocular pressure is not enough

The retina is theater of a number of biochemical reactions allowing, within its layers, the conversion of light impulses into electrical signals. The axons of the last neuronal elements, the ganglion cells, form the optic nerve and transfer the signals to the brain. Therefore, an appropriate cellular communication, not only within the different retinal cells, but also between the retina itself and the other brain structures, is fundamental. One of the most diffuse pathologies affecting retinal function and communication, which thus reverberates in the whole visual system, is glaucoma. This insidious disease is characterized by a progressive optic nerve degeneration and sight loss which may finally lead to irreversible blindness. Nevertheless, the progressive nature of this pathology offers an opportunity for therapeutic intervention. To better understand the cellular processes implicated in the development of glaucoma useful to envision a targeted pharmacological strategy, this manuscript first examines the complex cellular and functional organization of the retina and subsequently identifies the targets sensitive to neurodegeneration. Within this context, high ocular pressure represents a key risk factor. However, recent literature findings highlight the concept that lowering ocular pressure is not enough to prevent/slow down glaucomatous damage, suggesting the importance of combining the hypotensive treatment with other pharmacological approaches, such as the use of neuroprotectants. Therefore, this important and more novel aspect is extensively considered in this review, also emphasizing the idea that the neuroprotective strategy should be extended to the entire visual system and not restricted to the retina.