Critical pathogenic events underlying progression of neurodegeneration in glaucoma

Ocular Hypertension Drives Remodeling of AMPA Receptors in Select Populations of Retinal Ganglion Cells

AMPA-type glutamate receptors in the CNS are normally impermeable to Ca2+, but the aberrant expression of Ca2+-permeable AMPA receptors (CP-AMPARs) occurs in pathological conditions such as ischemia or epilepsy, or degenerative diseases such as ALS. Here, we show that select populations of retinal ganglion cells (RGCs) similarly express high levels of CP-AMPARs in a mouse model of glaucoma. CP-AMPAR expression increased dramatically in both On sustained alpha and Off transient alpha RGCs, and this increase was prevented by genomic editing of the GluA2 subunit. On sustained alpha RGCs with elevated CP-AMPAR levels displayed profound synaptic depression, which was reduced by selectively blocking CP-AMPARs, buffering Ca2+ with BAPTA, or with the CB1 antagonist AM251, suggesting that depression was mediated by a retrograde transmitter which might be triggered by the influx of Ca2+ through CP-AMPARs. Thus, glaucoma may alter the composition of AMPARs and depress excitatory synaptic input in select populations of RGCs.

Effects of Adult Müller Cells and Their Conditioned Media on the Survival of Stem Cell-Derived Retinal Ganglion Cells

Retinal neurons, particularly retinal ganglion cells (RGCs), are susceptible to the degenerative damage caused by different inherited conditions and environmental insults, leading to irreversible vision loss and, ultimately, blindness. Numerous strategies are being tested in different models of degeneration to restore vision and, in recent years, stem cell technologies have offered novel avenues to obtain donor cells for replacement therapies. To date, stem cell-based transplantation in the retina has been attempted as treatment for photoreceptor degeneration, but the same tools could potentially be applied to other retinal cell types, including RGCs. However, RGC-like cells are not an abundant cell type in stem cell-derived cultures and, often, these cells degenerate over time in vitro. To overcome this limitation, we have taken advantage of the neuroprotective properties of Müller glia (one of the main glial cell types in the retina) and we have examined whether Müller glia and the factors they secrete could promote RGC-like cell survival in organoid cultures. Accordingly, stem cell-derived RGC-like cells were co-cultured with adult Müller cells or Müller cell-conditioned media was added to the cultures. Remarkably, RGC-like cell survival was substantially enhanced in both culture conditions, and we also observed a significant increase in their neurite length. Interestingly, Atoh7, a transcription factor required for RGC development, was up-regulated in stem cell-derived organoids exposed to conditioned media, suggesting that Müller cells may also enhance the survival of retinal progenitors and/or postmitotic precursor cells. In conclusion, Müller cells and the factors they release promote organoid-derived RGC-like cell survival, neuritogenesis, and possibly neuronal maturation.

Tissue-Engineered Models for Glaucoma Research

Glaucoma is a group of optic neuropathies characterized by the progressive degeneration of retinal ganglion cells (RGCs). Patients with glaucoma generally experience elevations in intraocular pressure (IOP), followed by RGC death, peripheral vision loss and eventually blindness. However, despite the substantial economic and health-related impact of glaucoma-related morbidity worldwide, the surgical and pharmacological management of glaucoma is still limited to maintaining IOP within a normal range. This is in large part because the underlying molecular and biophysical mechanisms by which glaucomatous changes occur are still unclear. In the present review article, we describe current tissue-engineered models of the intraocular space that aim to advance the state of glaucoma research. Specifically, we critically evaluate and compare both 2D and 3D-culture models of the trabecular meshwork and nerve fiber layer, both of which are key players in glaucoma pathophysiology. Finally, we point out the need for novel organ-on-a-chip models of glaucoma that functionally integrate currently available 3D models of the retina and the trabecular outflow pathway.

The Neurovascular Unit in Glaucomatous Neurodegeneration

Glaucoma is a neurodegenerative disease of the visual system and leading cause of blindness worldwide. The disease is associated with sensitivity to intraocular pressure (IOP), which over a large range of magnitudes stresses retinal ganglion cell (RGC) axons as they pass through the optic nerve head in forming the optic projection to the brain. Despite clinical efforts to lower IOP, which is the only modifiable risk factor for glaucoma, RGC degeneration and ensuing loss of vision often persist. A major contributor to failure of hypotensive regimens is the multifactorial nature of how IOP-dependent stress influences RGC physiology and structure. This stress is conveyed to the RGC axon through interactions with structural, glial, and vascular components in the nerve head and retina. These interactions promote pro-degenerative pathways involving biomechanical, metabolic, oxidative, inflammatory, immunological and vascular challenges to the microenvironment of the ganglion cell and its axon. Here, we focus on the contribution of vascular dysfunction and breakdown of neurovascular coupling in glaucoma. The vascular networks of the retina and optic nerve head have evolved complex mechanisms that help to maintain a continuous blood flow and supply of metabolites despite fluctuations in ocular perfusion pressure. In healthy tissue, autoregulation and neurovascular coupling enable blood flow to stay tightly controlled. In glaucoma patients evidence suggests these pathways are dysfunctional, thus highlighting a potential role for pathways involved in vascular dysfunction in progression and as targets for novel therapeutic intervention.

Retinal Tissue Bioengineering, Materials and Methods for the Treatment of Glaucoma

BACKGROUND

Glaucoma, a characteristic type of optic nerve degeneration in the posterior pole of the eye, is a common cause of irreversible vision loss and the second leading cause of blindness worldwide. As an optic neuropathy, glaucoma is identified by increasing degeneration of retinal ganglion cells (RGCs), with consequential vision loss. Current treatments only postpone the development of retinal degeneration, and there are as yet no treatments available for this disability. Recent studies have shown that replacing lost or damaged RGCs with healthy RGCs or RGC precursors, supported by appropriately designed bio-material scaffolds, could facilitate the development and enhancement of connections to ganglion cells and optic nerve axons. The consequence may be an improved retinal regeneration. This technique could also offer the possibility for retinal regeneration in treating other forms of optic nerve ailments through RGC replacement.

METHODS

In this brief review, we describe the innovations and recent developments in retinal regenerative medicine such as retinal organoids and gene therapy which are specific to glaucoma treatment and focus on the selection of appropriate bio-engineering principles, biomaterials and cell therapies that are presently employed in this growing research area.

RESULTS

Identification of optimal sources of cells, improving cell survival, functional integration upon transplantation, and developing techniques to deliver cells into the retinal space without provoking immune responses are the main challenges in retinal cell replacement therapies.

CONCLUSION

The restoration of visual function in glaucoma patients by the RGC replacement therapies requires appropriate protocols and biotechnology methods. Tissue-engineered scaffolds, the generation of retinal organoids, and gene therapy may help to overcome some of the challenges in the generation of clinically safe RGCs.

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.

Of Mice and Monkeys: Neuroprotective Efficacy of the p38 Inhibitor BIRB 796 Depends on Model Duration in Experimental Glaucoma

Glaucoma is a group of optic neuropathies associated with aging and sensitivity to intraocular pressure (IOP). Early progression involves retinal ganglion cell (RGC) axon dysfunction that precedes frank degeneration. Previously we demonstrated that p38 MAPK inhibition abates axonal dysfunction and slows degeneration in the inducible microbead occlusion model of glaucoma in rat. Here, we assessed the neuroprotective effect of topical eye delivery of the p38 MAPK inhibitor BIRB 796 in three models of glaucoma (microbead occlusion in rat and squirrel monkey and the genetic DBA/2 J mouse model) with distinct durations of IOP elevation. While BIRB 796 did not influence IOP, treatment over four weeks in rats prevented degradation of anterograde axonal transport to the superior colliculus and degeneration in the optic nerve. Treatment over months in the chronic DBA/2 J model and in the squirrel monkey model reduced expression and activation of p38 downstream targets in the retina and brain but did not rescue RGC axon transport or degeneration, suggesting the efficacy of BIRB 796 in preventing associated degeneration of the RGC projection depends on the duration of the experimental model. These results emphasize the importance of evaluating potential therapeutic compounds for neuroprotection in multiple models using elongated treatment paradigms for an accurate assessment of efficacy.

TRPV1 Tunes Optic Nerve Axon Excitability in Glaucoma

The transient receptor potential vanilloid member 1 (TRPV1) in the central nervous system may contribute to homeostatic plasticity by regulating intracellular Ca²⁺, which becomes unbalanced in age-related neurodegenerative diseases, including Alzheimer’s and Huntington’s. Glaucomatous optic neuropathy – the world’s leading cause of irreversible blindness – involves progressive degeneration of retinal ganglion cell (RGC) axons in the optic nerve through sensitivity to stress related to intraocular pressure (IOP). In models of glaucoma, genetic deletion of TRPV1 (Trpv1^–/–) accelerates RGC axonopathy in the optic projection, whereas TRPV1 activation modulates RGC membrane polarization. In continuation of these studies, here, we found that Trpv1^–/– increases the compound action potential (CAP) of optic nerves subjected to short-term elevations in IOP. This IOP-induced increase in CAP was not directly due to TRPV1 channels in the optic nerve, because the TRPV1-selective antagonist iodoresiniferatoxin had no effect on the CAP for wild-type optic nerve. Rather, the enhanced CAP in Trpv1^–/– optic nerve was associated with increased expression of the voltage-gated sodium channel subunit 1.6 (NaV1.6) in longer nodes of Ranvier within RGC axons, rendering Trpv1^–/– optic nerve relatively insensitive to NaV1.6 antagonism via 4,9-anhydrotetrodotoxin. These results indicate that with short-term elevations in IOP, Trpv1^–/– increases axon excitability through greater NaV1.6 localization within longer nodes. In neurodegenerative disease, native TRPV1 may tune NaV expression in neurons under stress to match excitability to available metabolic resources.

Retinal ganglion cell loss in kinesin-1 cargo Alcadein α deficient mice

Maintenance of retinal ganglion cells (RGCs) activity is relied on axonal transport conveying materials required for their survival such as neurotrophic factors. Kinesin-1 undergoes anterograde transport in axons, and Alcadein α (Alcα; also called calsyntenin-1) is a major cargo adaptor protein that can drive kinesin-1 to transport vesicles containing Alcα. The long-term effects of Alcα-deficiency on retinal morphology and survival of RGCs during postnatal development were examined in Alcα knockout mice. At 1.5, 3, 6, and 15 months postnatal, the number of retrogradely labeled RGCs was determined in flat-mounted retinas of Alcα-deficient and wild-type mice. Retinal damage was assessed histologically by determining the retinal thickness. Intraocular pressure (IOP) was measured with a Tonolab tonometer. At 1.5 months postnatal, the number of retrogradely labeled RGCs was not different between wild-type and Alcα-deficient mice. However, at 3, 6, and 15 months postnatal, the number of RGCs was significantly lower in Alcα deficient mice than those of wild-type mice (143 ± 41.1 cells/mm2 vs. 208 ± 28.4 cells/mm2, respectively, at 3 months; P < 0.01). No differences were seen in retinal thickness or IOP between the two types of mice at any postnatal age. Alcα-deficient mice showed spontaneous loss of RGCs but no elevation in IOP. These mice mimic normal-tension glaucoma and will be useful for investigating the mechanism of neurodegeneration in this disorder and for developing treatments for RGC loss that does not involve changes in IOP.

A neuroglia-based interpretation of glaucomatous neuroretinal rim thinning in the optic nerve head

Neuroretinal rim thinning (NRR) is a characteristic glaucomatous optic disc change. However, the precise mechanism of the rim thinning has not been completely elucidated. This review focuses on the structural role of the glioarchitecture in the formation of the glaucomatous NRR thinning. The NRR is a glia-framed structure, with honeycomb geometry and mechanically reinforced astrocyte processes along the transverse plane. When neural damage selectively involves the neuron and spares the glia, the gross structure of the tissue is preserved. The disorganization and loss of the glioarchitecture are the two hallmarks of optic nerve head (ONH) remodeling in glaucoma that leads to the thinning of NRR tissue upon axonal loss. This is in contrast to most non-glaucomatous optic neuropathies with optic disc pallor where hypertrophy of the glioarchitecture is associated with the seemingly absent optic disc cupping. Arteritic anterior ischemic optic neuropathy is an exception where pan-necrosis of ONH tissue leads to NRR thinning. Milder ischemia indicates selective neuronal loss that spares glia in non-arteritic anterior ischemic optic neuropathy. The biological reason is the heterogeneous glial response determined by the site, type, and severity of the injury. The neuroglial interpretation explains how the cellular changes underlie the clinical findings. Updated understandings on glial responses illustrate the mechanical, microenvironmental, and microglial modulation of activated astrocytes in glaucoma. Findings relevant to the possible mechanism of the astrocyte death in advanced glaucoma are also emerging. Ultimately, a better understanding of glaucomatous glial response may lead to glia-targeting neuroprotection in the future.

Elevated ocular pressure reduces voltage-gated sodium channel NaV1.2 protein expression in retinal ganglion cell axons

Glaucoma is an age-related neurodegenerative disease that is commonly associated with sensitivity to intraocular pressure. The disease selectively targets retinal ganglion cells (RGCs) and constituent axons. RGC axons are rich in voltage-gated sodium channels, which are essential for action potential initiation and regeneration. Here, we identified voltage-dependent sodium channel, NaV1.2, in the retina, examined how this channel contributes to RGC light responses, and monitored NaV1.2 mRNA and protein expression in the retina during progression of modeled glaucoma. We found NaV1.2 is predominately localized in ganglion cell intraretinal axons with dispersed expression in the outer and inner plexiform layers. We showed Phrixotoxin-3, a potent NaV1.2 channel blocker, significantly decreased RGC electrical activity in a dose-dependent manner with an IC50 of 40 nM. Finally, we found four weeks of raised intraocular pressure (30% above baseline) significantly increased NaV1.2 mRNA expression but reduced NaV1.2 protein level in the retina up to 57% (p < 0.001). Following prolonged intraocular pressure elevation, NaV1.2 protein expression particularly diminished at distal sections of ganglion cell intraretinal axons (p ≤ 0.01). Our results suggest NaV1.2 might be a therapeutic target during disease progression to maintain RGC excitability, preserving presynaptic connections through action potential backpropagation.

Impairment of Membrane Repolarization Accompanies Axon Transport Deficits in Glaucoma

Glaucoma is a leading cause of blindness worldwide, resulting from degeneration of retinal ganglion cells (RGCs), which form the optic nerve. In glaucoma, axon transport deficits appear to precede structural degeneration of RGC axons. The period of time between the onset of axon transport deficits and the structural degeneration of RGC axons may represent a therapeutic window for the prevention of irreversible vision loss. However, it is unclear how deficits in axon transport relate to the electrophysiological capacity of RGCs to produce and maintain firing frequencies that encode visual stimuli. Here, we examined the electrophysiological signature of individual RGCs in glaucomatous retina with respect to axon transport facility. Utilizing the Microbead Occlusion Model of murine ocular hypertension, we performed electrophysiological recordings of RGCs with and without deficits in anterograde axon transport. We found that RGCs with deficits in axon transport have a reduced ability to maintain spiking frequency that arises from elongation of the repolarization phase of the action potential. This repolarization phenotype arises from reduced cation flux and K+ dyshomeostasis that accompanies pressure-induced decreases in Na/K-ATPase expression and activity. In vitro studies with purified RGCs indicate that elevated pressure induces early internalization of Na/K-ATPase that, when reversed, stabilizes cation flux and prevents K+ dyshomeostasis. Furthermore, pharmacological inhibition of the Na/K-ATPase is sufficient to replicate pressure-induced cation influx and repolarization phase phenotypes in healthy RGCs. These studies suggest that deficits in axon transport also likely reflect impaired electrophysiological function of RGCs. Our findings further identify a failure to maintain electrochemical gradients and cation dyshomeostasis as an early phenotype of glaucomatous pathology in RGCs that may have significant bearing on efforts to restore RGC health in diseased retina.

Early-Stage Ocular Hypertension Alters Retinal Ganglion Cell Synaptic Transmission in the Visual Thalamus

Axonopathy is a hallmark of many neurodegenerative diseases including glaucoma, where elevated intraocular pressure (ocular hypertension, OHT) stresses retinal ganglion cell (RGC) axons as they exit the eye and form the optic nerve. OHT causes early changes in the optic nerve such as axon atrophy, transport inhibition, and gliosis. Importantly, many of these changes appear to occur prior to irreversible neuronal loss, making them promising points for early diagnosis of glaucoma. It is unknown whether OHT has similarly early effects on the function of RGC output to the brain. To test this possibility, we elevated eye pressure in mice by anterior chamber injection of polystyrene microbeads. Five weeks post-injection, bead-injected eyes showed a modest RGC loss in the peripheral retina, as evidenced by RBPMS antibody staining. Additionally, we observed reduced dendritic complexity and lower spontaneous spike rate of On-αRGCs, targeted for patch clamp recording and dye filling using a Opn4-Cre reporter mouse line. To determine the influence of OHT on retinal projections to the brain, we expressed Channelrhodopsin-2 (ChR2) in melanopsin-expressing RGCs by crossing the Opn4-Cre mouse line with a ChR2-reporter mouse line and recorded post-synaptic responses in thalamocortical relay neurons in the dorsal lateral geniculate nucleus (dLGN) of the thalamus evoked by stimulation with 460 nm light. The use of a Opn4-Cre reporter system allowed for expression of ChR2 in a narrow subset of RGCs responsible for image-forming vision in mice. Five weeks following OHT induction, paired pulse and high-frequency stimulus train experiments revealed that presynaptic vesicle release probability at retinogeniculate synapses was elevated. Additionally, miniature synaptic current frequency was slightly reduced in brain slices from OHT mice and proximal dendrites of post-synaptic dLGN relay neurons, assessed using a Sholl analysis, showed a reduced complexity. Strikingly, these changes occurred prior to major loss of RGCs labeled with the Opn4-Cre mouse, as indicated by immunofluorescence staining of ChR2-expressing retinal neurons. Thus, OHT leads to pre- and post-synaptic functional and structural changes at retinogeniculate synapses. Along with RGC dendritic remodeling and optic nerve transport changes, these retinogeniculate synaptic changes are among the earliest signs of glaucoma.

Towards A Microbead Occlusion Model of Glaucoma for a Non-Human Primate

Glaucoma is a group of optic neuropathies associated with aging and sensitivity to intraocular pressure (IOP). The disease causes vision loss through the degeneration of retinal ganglion cell neurons and their axons in the optic nerve. Using an inducible model of glaucoma, we elevated IOP in the squirrel monkey (Saimiri boliviensis) using intracameral injection of 35 μm polystyrene microbeads and measured common pathogenic outcomes in the optic projection. A 42% elevation in IOP over 28 weeks reduced anterograde transport of fluorescently-labeled cholera toxin beta from retina to the lateral geniculate nucleus (60% decrease), and to the superior colliculus (49% decrease). Pressure also reduced survival of ganglion cellaxons in the optic nerve by 22%. The same elevation caused upregulation of proteins associated with glaucomatous neurodegeneration in the retina and optic nerve, including complement 1q, interleukin 6, and brain-derived neurotrophic factor. That axon degeneration in the nerve lagged deficits in anterograde transport is consistent with progression in rodent models, while the observed protein changes also occur in tissue from human glaucoma patients. Thus, microbead occlusion in a non-human primate with a visual system similar to our own represents an attractive model to investigate neurodegenerative mechanisms and therapeutic interventions for glaucoma.

Progression and Pathology of Traumatic Optic Neuropathy From Repeated Primary Blast Exposure

Indirect traumatic optic neuropathy (ITON) is a condition that is often associated with traumatic brain injury and can result in significant vision loss due to degeneration of retinal ganglion cell (RGC) axons at the time of injury or within the ensuing weeks. We used a mouse model of eye-directed air-blast exposure to characterize the histopathology of blast-induced ITON. This injury caused a transient elevation of intraocular pressure with subsequent RGC death and axon degeneration that was similar throughout the length of the optic nerve (ON). Deficits in active anterograde axon transport to the superior colliculus accompanied axon degeneration and first appeared in peripheral representations of the retina. Glial area in the ON increased early after injury and involved a later period of additional expansion. The increase in area involved a transient change in astrocyte organization independent of axon degeneration. While levels of many cytokines and chemokines did not change, IL-1α and IL-1β increased in both the ON and retina. In contrast, glaucoma shows distal to proximal axon degeneration with astrocyte remodeling and increases in many cytokines and chemokines. Further, direct traumatic optic neuropathies have a clear site of injury with rapid, progressive axon degeneration and cell death. These data show that blast-induced ITON is a distinct neuropathology from other optic neuropathies.

Axon injury signaling and compartmentalized injury response in glaucoma

Axonal degeneration is an active, highly controlled process that contributes to beneficial processes, such as developmental pruning, but also to neurodegeneration. In glaucoma, ocular hypertension leads to vision loss by killing the output neurons of the retina, the retinal ganglion cells (RGCs). Multiple processes have been proposed to contribute to and/or mediate axonal injury in glaucoma, including: neuroinflammation, loss of neurotrophic factors, dysregulation of the neurovascular unit, and disruption of the axonal cytoskeleton. While the inciting injury to RGCs in glaucoma is complex and potentially heterogeneous, axonal injury is ultimately thought to be the key insult that drives glaucomatous neurodegeneration. Glaucomatous neurodegeneration is a complex process, with multiple molecular signals contributing to RGC somal loss and axonal degeneration. Furthermore, the propagation of the axonal injury signal is complex, with injury triggering programs of degeneration in both the somal and axonal compartment. Further complicating this process is the involvement of multiple cell types that are known to participate in the process of axonal and neuronal degeneration after glaucomatous injury. Here, we review the axonal signaling that occurs after injury and the molecular signaling programs currently known to be important for somal and axonal degeneration after glaucoma-relevant axonal injuries.

Pressure-dependent modulation of inward-rectifying K+ channels: implications for cation homeostasis and K+ dynamics in glaucoma

Glaucoma is the leading cause of blindness worldwide, resulting from degeneration of retinal ganglion cells (RGCs), which form the optic nerve. Prior to structural degeneration, RGCs exhibit physiological deficits. Müller glia provide homeostatic regulation of ions that supports RGC physiology through a process called K⁺ siphoning. Recent studies suggest that several retinal conditions, including glaucoma, involve changes in the expression of K⁺ channels in Müller glia. To clarify whether glaucoma-related stressors directly alter expression and function of K⁺ channels in Müller glia, we examined changes in the expression of inwardly rectifying K⁺ (Kir) channels and two-pore domain (K2P) channels in response to elevated intraocular pressure (IOP) in vivo and in vitro in primary cultures of Müller glia exposed to elevated hydrostatic pressure. We then measured outcomes of cell health, cation homeostasis, and cation flux in Müller glia cultures. Transcriptome analysis in a murine model of microbead-induced glaucoma revealed pressure-dependent downregulation of Kir and K2P channels in vivo. Changes in the expression and localization of Kir and K2P channels in response to elevated pressure were also found in Müller glia in vitro. Finally, we found that elevated pressure compromises the plasma membrane of Müller glia and induces cation dyshomeostasis that involves changes in ion flux through cation channels. Pressure-induced changes in cation flux precede both cation dyshomeostasis and membrane compromise. Our findings have implications for Müller glia responses to pressure-related conditions, i.e., glaucoma, and identify cation dyshomeostasis as a potential contributor to electrophysiological impairment observed in RGCs of glaucomatous retina.

Silicone oil-induced ocular hypertension and glaucomatous neurodegeneration in mouse

Understanding the molecular mechanism of glaucoma and development of neuroprotectants is significantly hindered by the lack of a reliable animal model that accurately recapitulates human glaucoma. Here, we sought to develop a mouse model for the secondary glaucoma that is often observed in humans after silicone oil (SO) blocks the pupil or migrates into the anterior chamber following vitreoretinal surgery. We observed significant intraocular pressure (IOP) elevation after intracameral injection of SO, and that SO removal allows IOP to return quickly to normal. This simple, inducible and reversible mouse ocular hypertension model shows dynamic changes of visual function that correlate with progressive retinal ganglion cell (RGC) loss and axon degeneration. It may be applicable with only minor modifications to a range of animal species in which it will generate stable, robust IOP elevation and significant neurodegeneration that will facilitate selection of neuroprotectants and investigating the pathogenesis of ocular hypertension-induced glaucoma.

Microtubule Imaging Reveals Cytoskeletal Deficit Predisposing the Retinal Ganglion Cell Axons to Atrophy in DBA/2J

Purpose

Glaucoma is characterized by progressive loss of the retinal ganglion cells (RGCs) and their axons. Here we test an outstanding notion that microtubules (MTs) within RGC axons degrade before the loss of morphology (“MT hypothesis”).

Methods

The integrity of axonal MTs was interrogated by intrinsic second-harmonic generation (SHG) microscopy. Using DBA/2J mice as a model of glaucoma and DBA/2J-Gpnmb+ as a nonglaucomatous control, the relationship between MT disruption and morphology was quantitatively examined as a function of age and sex in the fresh retinal wholemounts.

Results

The mean SHG density (i.e., the mean SHG intensity per thickness) was significantly lower in DBA/2J than in DBA/2J-Gpnmb+ and also depended on sex and age. The loss of SHG density, indicating MT disruption within intact RGC axons, occurred in a sectorial manner near the loss of the retinal nerve fiber bundles. The decay rate of SHG density was approximately 97% higher than that of thickness.

Conclusions

Collectively, the results indicate that the breakdown of MTs is pathology of glaucoma and likely a precursor of morphological atrophy. Based on a new finding that SHG density is highly variable and spatially discrete, a new model of RGC degeneration is proposed. This study validates SHG retinal imaging for elucidating the role and mechanism of MT deficiency in the course of glaucoma pathogenesis.

Evaluation of dynamic thiol-disulfide homeostasis in glaucoma patients and the correlation with retinal nerve fiber layer analysis

PURPOSE

The aim of this study was to evaluate thiol/disulfide homeostasis and ischemia-modified albumin levels with primary open-angle glaucoma, ocular hypertension, and control group; also to interpret the correlation between these biochemical parameters and retinal nerve fiber layer analysis.

MATERIAL AND METHODS

In a prospective cross-sectional study, 30 primary open-angle glaucoma cases, 30 ocular hypertension cases, and 30 control subjects were included in the study. Native thiol, total thiol, and disulfide measurements and disulfide/native thiol, disulfide/total thiol, and native thiol/total thiol ratios were evaluated as thiol-disulfide homeostasis. Albumin and ischemia-modified albumin parameters were also evaluated. All cases underwent detailed ophthalmologic examination including visual acuity, retinal nerve fiber layer via optical coherence tomography, intraocular pressure, and central corneal thickness measurements and visual field analysis by 24-2 Swedish Interactive Threshold Algorithm (SITA) Standard visual field test.

RESULTS

Primary open-angle glaucoma group had significantly higher ischemia-modified albumin values than ocular hypertension and control group (p < 0.001). Native thiol and total thiol values of control group were statistically higher than those of primary open-angle glaucoma and ocular hypertension groups. The correlation between the temporal retinal nerve fiber layer value and ischemia-modified albumin, disulfide/native thiol, and disulfide/total thiol values of the primary open-angle glaucoma patients included in the study was moderate correlation in negative direction (r = -0.46, r = -0.39, r = -0.39, respectively), whereas there was a statistically significant moderate correlation in positive direction between the native thiol/total thiol values (r = 0.39) (p < 0.05).

CONCLUSION

These findings have reinforced the role of oxidative stress in the etiopathogenesis of primary open-angle glaucoma, suggesting that the thinning retinal nerve fiber layer may be associated with oxidative stress in favor of prooxidant shift.

Optogenetic Stimulation of the Superior Colliculus Confers Retinal Neuroprotection in a Mouse Glaucoma Model

Glaucoma is characterized by a progressive loss of retinal ganglion cells (RGCs) in the eye, which ultimately results in visual impairment or even blindness. Because current therapies often fail to halt disease progression, there is an unmet need for novel neuroprotective therapies to support RGC survival. Various research lines suggest that visual target centers in the brain support RGC functioning and survival. Here, we explored whether increasing neuronal activity in one of these projection areas could improve survival of RGCs in a mouse glaucoma model. Prolonged activation of an important murine RGC target area, the superior colliculus (SC), was established via a novel optogenetic stimulation paradigm. By leveraging the unique channel kinetics of the stabilized step function opsin (SSFO), protracted stimulation of the SC was achieved with only a brief light pulse. SSFO-mediated collicular stimulation was confirmed by immunohistochemistry for the immediate-early gene c-Fos and behavioral tracking, which both demonstrated consistent neuronal activity upon repeated stimulation. Finally, the neuroprotective potential of optogenetic collicular stimulation was investigated in mice of either sex subjected to a glaucoma model and a 63% reduction in RGC loss was found. This work describes a new paradigm for optogenetic collicular stimulation and a first demonstration that increasing target neuron activity can increase survival of the projecting neurons.SIGNIFICANCE STATEMENT Despite glaucoma being a leading cause of blindness and visual impairment worldwide, no curative therapies exist. This study describes a novel paradigm to reduce retinal ganglion cell (RGC) degeneration underlying glaucoma. Building on previous observations that RGC survival is supported by the target neurons to which they project and using an innovative optogenetic approach, we increased neuronal activity in the mouse superior colliculus, a main projection target of rodent RGCs. This proved to be efficient in reducing RGC loss in a glaucoma model. Our findings establish a new optogenetic paradigm for target stimulation and encourage further exploration of the molecular signaling pathways mediating retrograde neuroprotective communication.

Translational Preclinical Pharmacologic Disease Models for Ophthalmic Drug Development

Preclinical models of human diseases are critical to our understanding of disease etiology, pathology, and progression and enable the development of effective treatments. An ideal model of human disease should capture anatomical features and pathophysiological mechanisms, mimic the progression pattern, and should be amenable to evaluating translational endpoints and treatment approaches. Preclinical animal models have been developed for a variety of human ophthalmological diseases to mirror disease mechanisms, location of the affected region in the eye and severity. These models offer clues to aid in our fundamental understanding of disease pathogenesis and enable progression of new therapies to clinical development by providing an opportunity to gain proof of concept (POC). Here, we review preclinical animal models associated with development of new therapies for diseases of the ocular surface, glaucoma, presbyopia, and retinal diseases, including diabetic retinopathy and age-related macular degeneration (AMD). We have focused on summarizing the models critical to new drug development and described the translational features of the models that contributed to our understanding of disease pathogenesis and establishment of preclinical POC.

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

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

Health Benefits of Polyphenols and Carotenoids in Age-Related Eye Diseases

Oxidative stress and inflammation play a critical role in the initiation and progression of age-related ocular abnormalities as cataract, glaucoma, diabetic retinopathy, and macular degeneration. Therefore, phytochemicals with proven antioxidant and anti-inflammatory activities, such as carotenoids and polyphenols, could be of benefit in these diseases. We searched PubMed and Web of Science databases for original studies investigating the benefits of different carotenoids and polyphenols in age-related ophthalmic diseases. Our results showed that several polyphenols (such as anthocyanins, Ginkgo biloba, quercetin, and resveratrol) and carotenoids (such as lutein, zeaxanthin, and mezoxanthin) have shown significant preventive and therapeutic benefits against the aforementioned conditions. The involved mechanisms in these findings include mitigating the production of reactive oxygen species, inhibiting the tumor necrosis factor-α and vascular endothelial growth factor pathways, suppressing p53-dependent apoptosis, and suppressing the production of inflammatory markers, such as interleukin- (IL-) 8, IL-6, IL-1a, and endothelial leucocyte adhesion molecule-1. Consumption of products containing these phytochemicals may be protective against these diseases; however, adequate human data are lacking. This review discusses the role and mechanisms of polyphenols and carotenoids and their possible synergistic effects on the prevention and treatment of age-related eye diseases that are induced or augmented by oxidative stress and inflammation.

Voluntary running delays primary degeneration in rat retinas after partial optic nerve transection

Running is believed to be beneficial for human health. Many studies have focused on the neuroprotective effects of voluntary running on animal models. There were both primary and secondary degeneration in neurodegenerative diseases, including glaucoma. However, whether running can delay primary or secondary degeneration or both of them was not clear. Partial optic nerve transection model is a valuable glaucoma model for studying both primary and secondary degeneration because it can separate primary (mainly in the superior retina) from secondary (mainly in the inferior retina) degeneration. Therefore, we compared the survival of retinal ganglion cells between Sprague-Dawley rat runners and non-runners both in the superior and inferior retinas. Excitotoxicity, oxidative stress, and apoptosis are involved in the degeneration of retinal ganglion cells in glaucoma. So we also used western immunoblotting to compare the expression of some proteins involved in apoptosis (phospho-c-Jun N-terminal kinases, p-JNKs), oxidative stress (manganese superoxide dismutase, MnSOD) and excitotoxicity (glutamine synthetase) between runners and non-runners after partial optic nerve transection. Results showed that voluntary running delayed the death of retinal ganglion cells vulnerable to primary degeneration but not those to secondary degeneration. In addition, voluntary running decreased the expression of glutamine synthetase, but not the expression of p-JNKs and MnSOD in the superior retina after partial optic nerve transection. These results illustrated that primary degeneration of retinal ganglion cells might be mainly related with excitotoxicity rather than oxidative stress; and the voluntary running could down-regulate excitotoxicity to delay the primary degeneration of retinal ganglion cells after partial optic nerve transection.

Glaucoma - Next Generation Therapeutics: Impossible to Possible

The future of next generation therapeutics for glaucoma is strong. The recent approval of two novel intraocular pressure (IOP)-lowering drugs with distinct mechanisms of action is the first in over 20 years. However, these are still being administered as topical drops. Efforts are underway to increase patient compliance and greater therapeutic benefits with the development of sustained delivery technologies. Furthermore, innovations from biologics- and gene therapy-based therapeutics are being developed in the context of disease modification, which are expected to lead to more permanent therapies for patients. Neuroprotection, including the preservation of retinal ganglion cells (RGCs) and optic nerve is another area that is actively being explored for therapeutic options. With improvements in imaging technologies and determination of new surrogate clinical endpoints, the therapeutic potential for translation of neuroprotectants is coming close to clinical realization. This review summarizes the aforementioned topics and other related aspects.

Cyanin Chloride Inhibits Hyperbaric Pressure-Induced Decrease of Intracellular Glutamate-Aspartate Transporter in Rat Retinal Müller Cells

Purpose

Glaucoma is the leading cause of irreversible blindness throughout the world. The pathogenesis of glaucoma is complex, and neuroprotection is a crucial aspect of therapy. High concentrations of extracellular glutamate are toxic to the optic nerve. The glutamate-aspartate transporter (GLAST) in retinal Müller cells is involved in the development of glaucoma. Anthocyanin has been reported to protect retinal neurons. We hypothesize that cyanin chloride, a type of anthocyanin, can inhibit hyperbaric pressure-induced GLAST decreases in cultured rat retinal Müller cells and may serve as a potential neuroprotective agent in glaucoma treatment.

Materials and Methods

Sprague Dawley rat Müller cells were cultured in a hyperbaric pressure device at 60 mmHg additional pressure and treated with cyanin chloride (10 μmol/L, 30 μmol/L, or 50 μmol/L) or vehicle for 2 hours. Cell survival rates (SRs) were evaluated by an MTT assay. GLAST mRNA and protein expression were determined by western blot and RT-PCR analyses, respectively.

Results

Cell SR was significantly decreased in the 60 mmHg additional hyperbaric pressure group compared to the control group (P < 0.01). Cyanin chloride treatment significantly improved SR under 60 mmHg additional pressure (P < 0.01). GLAST mRNA and protein expression levels in Müller cells were significantly reduced in the 60 mmHg hyperbaric pressure group compared to the control group (P < 0.01), but cyanin chloride significantly inhibited hyperbaric pressure-induced decreases in GLAST expression (P < 0.01).

Conclusion

Our results support our hypothesis and demonstrate that cyanin chloride can protect rat retinal Müller cells from hyperbaric pressure-induced decreases of GLAST.

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

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

Assembly and repair of eye-to-brain connections

Vision is the sense humans rely on most to navigate the world and survive. A tremendous amount of research has focused on understanding the neural circuits for vision and the developmental mechanisms that establish them. The eye-to-brain, or 'retinofugal' pathway remains a particularly important model in these contexts because it is essential for sight, its overt anatomical features relate to distinct functional attributes and those features develop in a tractable sequence. Much progress has been made in understanding the growth of retinal axons out of the eye, their selection of targets in the brain, the development of laminar and cell type-specific connectivity within those targets, and also dendritic connectivity within the retina itself. Moreover, because the retinofugal pathway is prone to degeneration in many common blinding diseases, understanding the cellular and molecular mechanisms that establish connectivity early in life stands to provide valuable insights into approaches that re-wire this pathway after damage or loss. Here we review recent progress in understanding the development of retinofugal pathways and how this information is important for improving visual circuit regeneration.

Increased bioavailability of cyclic guanylate monophosphate prevents retinal ganglion cell degeneration

The nitric oxide - guanylyl cyclase-1 - cyclic guanylate monophosphate (NO-GC-1-cGMP) pathway has emerged as a potential pathogenic mechanism for glaucoma, a common intraocular pressure (IOP)-related optic neuropathy characterized by the degeneration of retinal ganglion cells (RGCs) and their axons in the optic nerve. NO activates GC-1 to increase cGMP levels, which are lowered by cGMP-specific phosphodiesterase (PDE) activity. This pathway appears to play a role in both the regulation of IOP, where reduced cGMP levels in mice leads to elevated IOP and subsequent RGC degeneration. Here, we investigated whether potentiation of cGMP signaling could protect RGCs from glaucomatous degeneration. We administered the PDE5 inhibitor tadalafil orally (10 mg/kg/day) in murine models of two forms of glaucoma - primary open angle glaucoma (POAG; GC-1^-/- mice) and primary angle-closure glaucoma (PACG; Microbead Occlusion Model) - and measured RGC viability at both the soma and axon level. To determine the direct effect of increased cGMP on RGCs in vitro, we treated axotomized whole retina and primary RGC cultures with the cGMP analogue 8-Br-cGMP. Tadalafil treatment increased plasma cGMP levels in both models, but did not alter IOP or mean arterial pressure. Nonetheless, tadalafil treatment prevented degeneration of RGC soma and axons in both disease models. Treatment of whole, axotomized retina and primary RGC cultures with 8-Br-cGMP markedly attenuated both necrotic and apoptotic cell death pathways in RGCs. Our findings suggest that enhancement of the NO-GC-1-cGMP pathway protects the RGC body and axon in murine models of POAG and PACG, and that enhanced signaling through this pathway may serve as a novel glaucoma treatment, acting independently of IOP.

Complement C3-Targeted Gene Therapy Restricts Onset and Progression of Neurodegeneration in Chronic Mouse Glaucoma

Dysregulation of the complement system is implicated in neurodegeneration, including human and animal glaucoma. Optic nerve and retinal damage in glaucoma is preceded by local complement upregulation and activation, but whether targeting this early innate immune response could have therapeutic benefit remains undefined. Because complement signals through three pathways that intersect at complement C3 activation, here we targeted this step to restore complement balance in the glaucomatous retina and to determine its contribution to degeneration onset and/or progression. To achieve this, we combined adeno-associated virus retinal gene therapy with the targeted C3 inhibitor CR2-Crry. We show that intravitreal injection of AAV2.CR2-Crry produced sustained Crry overexpression in the retina and reduced deposition of the activation product complement C3d on retinal ganglion cells and the inner retina of DBA/2J mice. This resulted in neuroprotection of retinal ganglion cell axons and somata despite continued intraocular pressure elevation, suggesting a direct restriction of neurodegeneration onset and progression and significant delay to terminal disease stages. Our study uncovers a damaging effect of complement C3 or downstream complement activation in glaucoma, and it establishes AAV2.CR2-Crry as a viable therapeutic strategy to target pathogenic C3-mediated complement activation in the glaucomatous retina.

Association between Open-Angle Glaucoma and the Risks of Alzheimer's and Parkinson's Diseases in South Korea: A 10-year Nationwide Cohort Study

We aimed to investigate the risks of Alzheimer’s (AD) and Parkinson’s disease (PD) in the 10 years following diagnosis of open-angle glaucoma (OAG) using a nationwide cohort. This propensity score-matched retrospective cohort study included 1,025,340 subjects from the Korean National Health Insurance Service National Sample Cohort database. The OAG group (n = 1,469) included patients who were initially diagnosed with OAG between 2004 and 2007, and the subjects in the comparison group were matched in a 1:5 ratio using propensity scores. Cox regression analyses were performed to investigate the risks of developing AD or PD. The diagnosis of OAG was significantly associated with an increased incidence of AD (hazard ratio [HR] = 1.403, 95% confidence interval [CI] 1.180–1.669, p < 0.001), but not PD (HR = 0.995, 95% CI 0.620–1.595, p = 0.983) after adjusting for possible confounding factors. In subgroup analyses, participants with OAG aged ≥65 years were more likely to develop AD compared with those aged <65 years, and female OAG patients had a greater risk of developing AD than males. Patients diagnosed with OAG have a higher risk of developing AD, but not PD, and the risk differed according to age and sex.

Injectable Neurotrophic Factor Delivery System Supporting Retinal Ganglion Cell Survival and Regeneration Following Optic Nerve Crush

In general, neurons belonging to the central nervous system (CNS), such as retinal ganglion cells (RGCs), do not regenerate. Due to this, strategies have emerged aimed at protecting and regenerating these cells. Neurotrophic factor (NTF) supplementation has been a promising approach but is limited by length of delivery and delivery vehicle. For this study, we tested a polymeric delivery system (sulfonated reverse thermal gel or SRTG) engineered to deliver cilliary neurotrophic factor (CNTF), while also being injectable. A rat optic nerve crush (ONC) model was used to determine the neuroprotective and regenerative capacity of our system. The results demonstrate that one single intravitreal injection of SRTG-CNTF following ONC showed significant protection of RGC survival at both 1 and 2 week time points, when compared to the control groups. Furthermore, there was no significant difference in the RGC count between the eyes that received the SRTG-CNTF following ONC and a healthy control eye. Intravitreal injection of the polymer system also induced noticeable axon regeneration 500 μm downstream from the lesion site compared to all other control groups. There was a significant increase in Müller cell response in groups that received the SRTG-CNTF injection following optic nerve crush also indicative of a regenerative response. Finally, higher concentrations of CNTF released from SRTG-CNTF showed a protective effect on RGCs and Müller cell response at a longer time point (4 weeks). In conclusion, we were able to show a neuroprotective and regenerative effect of this polymer SRTG-CNTF delivery system and the viability for treatment of neurodegenerations.

Real-Time Imaging of Retinal Ganglion Cell Apoptosis

Monitoring real-time apoptosis in-vivo is an unmet need of neurodegeneration science, both in clinical and research settings. For patients, earlier diagnosis before the onset of symptoms provides a window of time in which to instigate treatment. For researchers, being able to objectively monitor the rates of underlying degenerative processes at a cellular level provides a biomarker with which to test novel therapeutics. The DARC (Detection of Apoptosing Retinal Cells) project has developed a minimally invasive method using fluorescent annexin A5 to detect rates of apoptosis in retinal ganglion cells, the key pathological process in glaucoma. Numerous animal studies have used DARC to show efficacy of novel, pressure-independent treatment strategies in models of glaucoma and other conditions where retinal apoptosis is reported, including Alzheimer’s disease. This may forge exciting new links in the clinical science of treating both cognitive and visual decline. Human trials are now underway, successfully demonstrating the safety and efficacy of the technique to differentiate patients with progressive neurodegeneration from healthy individuals. We review the current perspectives on retinal ganglion cell apoptosis, the way in which this can be imaged, and the exciting advantages that these future methods hold in store.

The nitric oxide-guanylate cyclase pathway and glaucoma

Glaucoma is a prevalent optic neuropathy characterized by the progressive dysfunction and loss of retinal ganglion cells (RGCs) and their optic nerve axons, which leads to irreversible visual field loss. Multiple risk factors for the disease have been identified, but elevated intraocular pressure (IOP) remains the primary risk factor amenable to treatment. Reducing IOP however does not always prevent glaucomatous neurodegeneration, and many patients progress with the disease despite having IOP in the normal range. There is increasing evidence that nitric oxide (NO) is a direct regulator of IOP and that dysfunction of the NO-Guanylate Cyclase (GC) pathway is associated with glaucoma incidence. NO has shown promise as a novel therapeutic with targeted effects that: 1) lower IOP; 2) increase ocular blood flow; and 3) confer neuroprotection. The various effects of NO in the eye appear to be mediated through the activation of the GC- guanosine 3:5'-cyclic monophosphate (cGMP) pathway and its effect on downstream targets, such as protein kinases and Ca²⁺ channels. Although NO-donor compounds are promising as therapeutics for IOP regulation, they may not be ideal to harness the neuroprotective potential of NO signaling. Here we review evidence that supports direct targeting of GC as a novel pleiotrophic treatment for the disease, without the need for direct NO application. The identification and targeting of other factors that contribute to glaucoma would be beneficial to patients, particularly those that do not respond well to IOP-dependent interventions.

Axogenic mechanism enhances retinal ganglion cell excitability during early progression in glaucoma

Identifying new therapies for neurodegenerative disease requires understanding how neurons respond to stress and whether this response includes adaptation to slow progression. Because neurodegeneration affects both axons and dendrites, with their synaptic contacts, adaptation could involve both compartments. We investigated this question in experimental glaucoma, the world’s leading cause of irreversible vision loss. Glaucoma attacks retinal ganglion cell neurons and their axons, which comprise the optic nerve. We found that elevations in ocular pressure, a prominent risk factor for glaucoma, caused a paradoxical increase in ganglion cell excitability, including response to light, even in cells with substantial dendritic pruning. This adaptation arose from voltage-dependent mechanisms in the axon and may help maintain signaling to the brain to preserve vision.

Diseases of the brain involve early axon dysfunction that often precedes outright degeneration. Pruning of dendrites and their synapses represents a potential driver of axonopathy by reducing activity. Optic nerve degeneration in glaucoma, the world’s leading cause of irreversible blindness, involves early stress to retinal ganglion cell (RGC) axons from sensitivity to intraocular pressure (IOP). This sensitivity also influences survival of RGC dendrites and excitatory synapses in the retina. Here we tested in individual RGCs identified by type the relationship between dendritic organization and axon signaling to light following modest, short-term elevations in pressure. We found dendritic pruning occurred early, by 2 wk of elevation, and independent of whether the RGC responded to light onset (ON cells) or offset (OFF cells). Pruning was similarly independent of ON and OFF in the DBA/2J mouse, a chronic glaucoma model. Paradoxically, all RGCs, even those with significant pruning, demonstrated a transient increase in axon firing in response to the preferred light stimulus that occurred on a backdrop of generally enhanced excitability. The increased response was not through conventional presynaptic signaling, but rather depended on voltage-sensitive sodium channels that increased transiently in the axon. Pruning, axon dysfunction, and deficits in visual acuity did not progress between 2 and 4 wk of elevation. These results suggest neurodegeneration in glaucoma involves an early axogenic response that counters IOP-related stress to excitatory dendritic architecture to slow progression and maintain signaling to the brain. Thus, short-term exposure to elevated IOP may precondition the neural system to further insult.

Microspheres as intraocular therapeutic tools in chronic diseases of the optic nerve and retina

Pathologies affecting the optic nerve and the retina are one of the major causes of blindness. These diseases include age-related macular degeneration (AMD), diabetic retinopathy (DR) and glaucoma, among others. Also, there are genetic disorders that affect the retina causing visual impairment. The prevalence of neurodegenerative diseases of the posterior segment is increased as most of them are related with the elderly. Even with the access to different treatments, there are some challenges in managing patients suffering retinal diseases. One of them is the need for frequent interventions. Also, an unpredictable response to therapy has suggested that different pathways may be playing a role in the development of these diseases. The management of these pathologies requires the development of controlled drug delivery systems able to slow the progression of the disease without the need of frequent invasive interventions, typically related with endophthalmitis, retinal detachment, ocular hypertension, cataract, inflammation, and floaters, among other. Biodegradable microspheres are able to encapsulate low molecular weight substances and large molecules such as biotechnological products. Over the last years, a large variety of active substances has been encapsulated in microspheres with the intention of providing neuroprotection of the optic nerve and the retina. The purpose of the present review is to describe the use of microspheres in chronic neurodegenerative diseases affecting the retina and the optic nerve. The advantage of microencapsulation of low molecular weight drugs as well as therapeutic peptides and proteins to be used as neuroprotective strategy is discussed. Also, a new use of the microspheres in the development of animal models of neurodegeneration of the posterior segment is described.

iDrugs and iDevices Discovery Research: Preclinical Assays, Techniques, and Animal Model Studies for Ocular Hypotensives and Neuroprotectants

Discovery ophthalmic research is centered around delineating the molecular and cellular basis of ocular diseases and finding and exploiting molecular and genetic pathways associated with them. From such studies it is possible to determine suitable intervention points to address the disease process and hopefully to discover therapeutics to treat them. An investigational new drug (IND) filing for a new small-molecule drug, peptide, antibody, genetic treatment, or a device with global health authorities requires a number of preclinical studies to provide necessary safety and efficacy data. Specific regulatory elements needed for such IND-enabling studies are beyond the scope of this article. However, to enhance the overall data packages for such entities and permit high-quality foundation-building publications for medical affairs, additional research and development studies are always desirable. This review aims to provide examples of some target localization/verification, ocular drug discovery processes, and mechanistic and portfolio-enhancing exploratory investigations for candidate drugs and devices for the treatment of ocular hypertension and glaucomatous optic neuropathy (neurodegeneration of retinal ganglion cells and their axons). Examples of compound screening assays, use of various technologies and techniques, deployment of animal models, and data obtained from such studies are also presented.

Inhibition of retinal ganglion cell apoptosis: regulation of mitochondrial function by PACAP

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an endogenous peptide with neuroprotective effects on retinal neurons, but the precise mechanism underlying these effects remains unknown. Considering the abundance of mitochondria in retinal ganglion cells (RGCs), we postulate that the protective effect of PACAP is associated with the regulation of mitochondrial function. RGC-5 cells were subjected to serum deprivation for 48 hours to induce apoptosis in the presence or absence of 100 nM PACAP. As revealed with the Cell Counting Kit-8 assay, PACAP at different concentrations significantly increased the viability of RGC-5 cells. PACAP also inhibited the excessive generation of reactive oxygen species in RGC-5 cells subjected to serum deprivation. We also showed by flow cytometry that PACAP inhibited serum deprivation-induced apoptosis in RGC-5 cells. The proportions of apoptotic cells and cells with mitochondria depolarization were significantly decreased with PACAP treatment. Western blot assays demonstrated that PACAP increased the levels of Bcl-2 and inhibited the compensatory increase of PAC1. Together, these data indicate protective effects of PACAP against serum deprivation-induced apoptosis in RGCs, and that the mechanism of this action is associated with maintaining mitochondrial function.

Retrograde interferon-gamma signaling induces major histocompatibility class I expression in human-induced pluripotent stem cell-derived neurons

Objective

Injury-associated axon-intrinsic signals are thought to underlie pathogenesis and progression in many neuroinflammatory and neurodegenerative diseases, including multiple sclerosis (MS). Retrograde interferon gamma (IFN γ) signals are known to induce expression of major histocompatibility class I (MHC I) genes in murine axons, thereby increasing the susceptibility of these axons to attack by antigen-specific CD8⁺ T cells. We sought to determine whether the same is true in human neurons.

Methods

A novel microisolation chamber design was used to physically isolate and manipulate axons from human skin fibroblast-derived induced pluripotent stem cell (iPSC)-derived neuron-enriched neural aggregates. Fluorescent retrobeads were used to assess the fraction of neurons with projections to the distal chamber. Axons were treated with IFN γ for 72 h and expression of MHC class I and antigen presentation genes were evaluated by RT-PCR and immunofluorescence.

Results

Human iPSC-derived neural stem cells maintained as 3D aggregate cultures in the cell body chamber of polymer microisolation chambers extended dense axonal projections into the fluidically isolated distal chamber. Treatment of these axons with IFN γ resulted in upregulation of MHC class I and antigen processing genes in the neuron cell bodies. IFN γ-induced MHC class I molecules were also anterogradely transported into the distal axon.

Interpretation

These results provide conclusive evidence that human axons are competent to express MHC class I molecules, suggesting that inflammatory factors enriched in demyelinated lesions may render axons vulnerable to attack by autoreactive CD8⁺ T cells in patients with MS. Future work will be aimed at identifying pathogenic anti-axonal T cells in these patients.

Primary Open-Angle Glaucoma Genetics in African Americans

PURPOSE OF REVIEW

Individuals of African descent are at highest risk for developing primary open-angle glaucoma (POAG), a devastating disease and major contributor of blindness worldwide. Currently, there is a large dearth of knowledge in this area despite a critical need for better understanding the underlying genetic and environmental factors afflicting this population. Here we highlight the current literature exploring the genetics of POAG in African Americans.

RECENT FINDINGS

Current studies have yet to replicate European POAG index variants (i.e. CDKN2B-AS1 and SIX1/SIX6) in African Americans or to definitely exclude that these loci contribute to risk in African descent populations. Recent studies have evaluated clinical features that may account for some differences in POAG risk between African Americans and European Americans.

SUMMARY

In summary, little headway has been made in elucidating the genetics of primary open-angle glaucoma in African Americans and other individuals of African descent.

Electroretinography in idiopathic intracranial hypertension: comparison of the pattern ERG and the photopic negative response

PURPOSE

To evaluate the relationship between electrophysiological measures of retinal ganglion cell (RGC) function in patients who have idiopathic intracranial hypertension (IIH).

METHODS

The pattern electroretinogram (pERG) and photopic negative response (PhNR) were recorded from 11 IIH patients and 11 age-similar controls. The pERG was elicited by a contrast-reversing checkerboard. The PhNR, a slow negative component following the flash ERG b-wave, was recorded in response to a long-wavelength flash presented against a short-wavelength adapting field. The PhNR was elicited using full-field (ffPhNR) and focal macular (fPhNR) stimuli. Additionally, Humphrey visual field mean deviation (HVF MD) was measured and ganglion cell complex volume (GCCV) was obtained by optical coherence tomography.

RESULTS

The ffPhNR, fPhNR, and pERG amplitudes were outside of the normal range in 45, 9, and 45% of IIH patients, respectively. However, only mean ffPhNR amplitude was reduced significantly in the patients compared to controls (p < 0.01). The pERG amplitude correlated significantly with HVF MD and GCCV (both r > 0.65, p < 0.05). There were associations between ffPhNR amplitude and HVF MD (r = 0.58, p = 0.06) and with GCCV (r = 0.52, p = 0.10), but these did not reach statistical significance. fPhNR amplitude was not correlated significantly with HVF MD or GCCV (both r < 0.40, p > 0.20).

CONCLUSIONS

Although the fPhNR is generally normal in IIH, other electrophysiological measures of RGC function, the ffPhNR and pERG, are abnormal in some patients. These measures provide complementary information regarding RGC dysfunction in these individuals.

Together JUN and DDIT3 (CHOP) control retinal ganglion cell death after axonal injury

BACKGROUND

Optic nerve injury is an important pathological component in neurodegenerative diseases such as traumatic optic neuropathies and glaucoma. The molecular signaling pathway(s) critical for retinal ganglion cell (RGC) death after axonal insult, however, is/are not fully defined. RGC death after axonal injury is known to occur by BAX-dependent apoptosis. Two transcription factors JUN (the canonical target of JNK) and DDIT3 (CHOP; a key mediator of the endoplasmic reticulum stress response) are known to be important apoptotic signaling molecules after axonal injury, including in RGCs. However, neither Jun nor Ddit3 deficiency provide complete protection to RGCs after injury. Since Jun and Ddit3 are important apoptotic signaling molecules, we sought to determine if their combined deficiency might provide additive protection to RGCs after axonal injury.

METHODS

To determine if DDIT3 regulated the expression of JUN after an axonal insult, mice deficient for Ddit3 were examined after optic nerve crush (ONC). In order to critically test the importance of these genes in RGC death after axonal injury, RGC survival was assessed at multiple time-points after ONC (14, 35, 60, and 120 days after injury) in Jun, Ddit3, and combined Jun/Ddit3 deficient mice. Finally, to directly assess the role of JUN and DDIT3 in axonal degeneration, compound actions potentials were recorded from Jun, Ddit3, and Jun/Ddit3 deficient mice after ONC.

RESULTS

Single and combined deficiency of Jun and Ddit3 did not appear to alter gross retinal morphology. Ddit3 deficiency did not alter expression of JUN after axonal injury. Deletion of both Jun and Ddit3 provided significantly greater long-term protection to RGCs as compared to Jun or Ddit3 deficiency alone. Finally, despite the profound protection to RGC somas provided by the deficiency of Jun plus Ddit3, their combined loss did not lessen axonal degeneration.

CONCLUSIONS

These results suggest JUN and DDIT3 are independently regulated pro-death signaling molecules in RGCs and together account for the vast majority of apoptotic signaling in RGCs after axonal injury. Thus, JUN and DDIT3 may represent key molecular hubs that integrate upstream signaling events triggered by axonal injury with downstream transcriptional events that ultimately culminate in RGC apoptosis.

Age-related accumulation of phosphorylated mitofusin 2 protein in retinal ganglion cells correlates with glaucoma progression

Dysregulation of axonal bioenergetics is likely a key mechanism in the initiation and progression of age-related neurodegenerative diseases. Glaucoma is a quintessential neurodegenerative disorder characterized by progressive deterioration of the optic nerve (ON) and eventual death of retinal ganglion cells (RGCs). Age and elevation of intraocular pressure are key risk factors in glaucoma, but the common early hallmarks of decreased axonal transport and increased bioenergetic vulnerability likely underlie disease initiation. We examined the correlation between bioenergetics and axonal transport with mitochondrial mutation frequency and post-translational modifications of mitofusin 2 (Mfn2) in RGCs during glaucoma progression. No increase in the frequency of mtDNA mutations was detected, but we observed significant shifts in mitochondrial protein species. Mfn2 is a fusion protein that functions in mitochondrial biogenesis, maintenance, and mitochondrial transport. We demonstrate that Mfn2 accumulates selectively in RGCs during glaucomatous degeneration, that two novel states of Mfn2 exist in retina and ON, and identify a phosphorylated form that selectively accumulates in RGCs, but is absent in ON. Phosphorylation of Mfn2 is correlated with higher ubiquitination, and failure of the protein to reach the ON. Together, these data suggest that post-translational modification of Mfn2 is associated with its dysregulation during a window of metabolic vulnerability that precedes glaucomatous degeneration. Future work to either manipulate expression of Mfn2 or to prevent its degradation could have therapeutic value in the treatment of neurodegenerative diseases where long-tract axons are vulnerable.

Sigma 1 receptor regulates ERK activation and promotes survival of optic nerve head astrocytes

The sigma 1 receptor (S1R) is a unique transmembrane protein that has been shown to regulate neuronal differentiation and cellular survival. It is expressed within several cell types throughout the nervous system and visceral organs, including neurons and glia within the eye. S1R ligands are therapeutic targets for diseases ranging from neurodegenerative conditions to neoplastic disorders. However, effects of S1R activation and inhibition within glia cells are not well characterized. Within the eye, the astrocytes at the optic nerve head are crucial to the health and survival of the neurons that send visual information to the brain. In this study, we used the S1R-specific agonist, (+)-pentazocine, to evaluate S1R activation within optic nerve head-derived astrocytes (ONHAs). Treatment of ONHAs with (+)-pentazocine attenuated the level and duration of stress-induced ERK phosphorylation following oxidative stress exposure and promoted survival of ONHAs. These effects were specific to S1R activation because they were not observed in ONHAs that were depleted of S1R using siRNA-mediated knockdown. Collectively, our results suggest that S1R activation suppresses ERK1/2 phosphorylation and protects ONHAs from oxidative stress-induced death.