A Phase 2 Trial to Test Safety and Efficacy of ST-100, a Unique Collagen Mimetic Peptide Ophthalmic Solution for Dry Eye Disease

Objective

Dry eye disease (DED) is a worldwide source of ocular discomfort. This first-in-human phase 2 clinical study determined the efficacy of treating signs and symptoms of DED using an ophthalmic solution of synthesized mimetic of human collagen (ST-100).

Design

This double-masked, randomized, study compared high (60 μg/mL) and low (22 μg/mL) dose ST-100 to vehicle utilizing the Ora, Inc. Controlled Adverse Environment (CAE) during a 28-day period.

Participants

Participants included males and females ≥ 18 years of age with signs and symptoms of DED for ≥ 6 months that worsened during CAE exposure who were not taking any topical prescription therapeutic.

Intervention

Participants applied ST-100 or vehicle placebo topically to both corneas (1 drop) twice daily via a blow-fill-sealed preservative-free container.

Main Outcome Measures

The prespecified primary efficacy sign end point was mean change from baseline (CFB) in total corneal fluorescein staining, and the primary symptom end point was mean CFB in ocular discomfort. A secondary prespecified efficacy end point was CFB in unanesthetized Schirmer's test for tear film production.

Results

Of 160 subjects in the intent-to-treat population (112 female, 48 male, median age 64), 146 completed the study. Total corneal fluorescein staining CFB improved for high-dose ST-100, with superiority over vehicle when both eyes were considered together (2-sample t test: P = 0.0394). High-dose ST-100 was superior to vehicle in Schirmer's CFB for the study eye (least squares mean difference [confidence interval] = 2.3 [0.6, 4.0], P = 0.0094). For study eyes, the proportion of Schirmer's test responders (CFB ≥ 10 mm, Schirmer's responder rate) was 12.2% for high-dose ST-100 versus 0.0% for vehicle (P = 0.0266). The CFB for ocular discomfort score improved in study eyes for high- and low-dose ST-100 (paired t test, P = 0.0133, P = 0.0151, respectively) but without superiority over vehicle (ANCOVA: P = 0.5696, P = 0.8968, respectively). ST-100 Schirmer's responders also demonstrated total elimination of worsening of corneal fluorescein stain during the stress of CAE sessions.

Conclusions

ST-100 significantly improved tear production and related outcomes in DED and was well-tolerated in reducing symptoms.

Financial Disclosures

Proprietary or commercial disclosure may be found in the Footnotes and Disclosures at the end of this article.

Addressing neurodegeneration in glaucoma: Mechanisms, challenges, and treatments

Glaucoma is the leading cause of irreversible blindness globally. The disease causes vision loss due to neurodegeneration of the retinal ganglion cell (RGC) projection to the brain through the optic nerve. Glaucoma is associated with sensitivity to intraocular pressure (IOP). Thus, mainstay treatments seek to manage IOP, though many patients continue to lose vision. To address neurodegeneration directly, numerous preclinical studies seek to develop protective or reparative therapies that act independently of IOP. These include growth factors, compounds targeting metabolism, anti-inflammatory and antioxidant agents, and neuromodulators. Despite success in experimental models, many of these approaches fail to translate into clinical benefits. Several factors contribute to this challenge. Firstly, the anatomic structure of the optic nerve head differs between rodents, nonhuman primates, and humans. Additionally, animal models do not replicate the complex glaucoma pathophysiology in humans. Therefore, to enhance the success of translating these findings, we propose two approaches. First, thorough evaluation of experimental targets in multiple animal models, including nonhuman primates, should precede clinical trials. Second, we advocate for combination therapy, which involves using multiple agents simultaneously, especially in the early and potentially reversible stages of the disease. These strategies aim to increase the chances of successful neuroprotective treatment for glaucoma.

Collagen in the central nervous system: contributions to neurodegeneration and promise as a therapeutic target

The extracellular matrix is a richly bioactive composition of substrates that provides biophysical stability, facilitates intercellular signaling, and both reflects and governs the physiological status of the local microenvironment. The matrix in the central nervous system (CNS) is far from simply an inert scaffold for mechanical support, instead conducting an active role in homeostasis and providing broad capacity for adaptation and remodeling in response to stress that otherwise would challenge equilibrium between neuronal, glial, and vascular elements. A major constituent is collagen, whose characteristic triple helical structure renders mechanical and biochemical stability to enable bidirectional crosstalk between matrix and resident cells. Multiple members of the collagen superfamily are critical to neuronal maturation and circuit formation, axon guidance, and synaptogenesis in the brain. In mature tissue, collagen interacts with other fibrous proteins and glycoproteins to sustain a three-dimensional medium through which complex networks of cells can communicate. While critical for matrix scaffolding, collagen in the CNS is also highly dynamic, with multiple binding sites for partnering matrix proteins, cell-surface receptors, and other ligands. These interactions are emerging as critical mediators of CNS disease and injury, particularly regarding changes in matrix stiffness, astrocyte recruitment and reactivity, and pro-inflammatory signaling in local microenvironments. Changes in the structure and/or deposition of collagen impact cellular signaling and tissue biomechanics in the brain, which in turn can alter cellular responses including antigenicity, angiogenesis, gliosis, and recruitment of immune-related cells. These factors, each involving matrix collagen, contribute to the limited capacity for regeneration of CNS tissue. Emerging therapeutics that attempt to rebuild the matrix using peptide fragments, including collagen-enriched scaffolds and mimetics, hold great potential to promote neural repair and regeneration. Recent evidence from our group and others indicates that repairing protease-degraded collagen helices with mimetic peptides helps restore CNS tissue and promote neuronal survival in a broad spectrum of degenerative conditions. Restoration likely involves bolstering matrix stiffness to reduce the potential for astrocyte reactivity and local inflammation as well as repairing inhibitory binding sites for immune-signaling ligands. Facilitating repair rather than endogenous replacement of collagen degraded by disease or injury may represent the next frontier in developing therapies based on protection, repair, and regeneration of neurons in the central nervous system.

Collagen Mimetic Peptides Promote Repair of MMP-1-Damaged Collagen in the Rodent Sclera and Optic Nerve Head

The structural and biomechanical properties of collagen-rich ocular tissues, such as the sclera, are integral to ocular function. The degradation of collagen in such tissues is associated with debilitating ophthalmic diseases such as glaucoma and myopia, which often lead to visual impairment. Collagen mimetic peptides (CMPs) have emerged as an effective treatment to repair damaged collagen in tissues of the optic projection, such as the retina and optic nerve. In this study, we used atomic force microscopy (AFM) to assess the potential of CMPs in restoring tissue stiffness in the optic nerve head (ONH), including the peripapillary sclera (PPS) and the glial lamina. Using rat ONH tissue sections, we induced collagen damage with MMP-1, followed by treatment with CMP-3 or vehicle. MMP-1 significantly reduced the Young's modulus of both the PPS and the glial lamina, indicating tissue softening. Subsequent CMP-3 treatment partially restored tissue stiffness in both the PPS and the glial lamina. Immunohistochemical analyses revealed reduced collagen fragmentation after MMP-1 digestion in CMP-3-treated tissues compared to vehicle controls. In summary, these results demonstrate the potential of CMPs to restore collagen stiffness and structure in ONH tissues following enzymatic damage. CMPs may offer a promising therapeutic avenue for preserving vision in ocular disorders involving collagen remodeling and degradation.

Neutral sphingomyelinase inhibition promotes local and network degeneration in vitro and in vivo

BACKGROUND

Cell-to-cell communication is vital for tissues to respond, adapt, and thrive in the prevailing milieu. Several mechanisms mediate intercellular signaling, including tunneling nanotubes, gap junctions, and extracellular vesicles (EV). Depending on local and systemic conditions, EVs may contain cargoes that promote survival, neuroprotection, or pathology. Our understanding of pathologic intercellular signaling has been bolstered by disease models using neurons derived from human pluripotent stems cells (hPSC).

METHODS

Here, we used hPSC-derived retinal ganglion cells (hRGC) and the mouse visual system to investigate the influence of modulating EV generation on intercellular trafficking and cell survival. We probed the impact of EV modulation on cell survival by decreasing the catabolism of sphingomyelin into ceramide through inhibition of neutral sphingomyelinase (nSMase), using GW4869. We assayed for cell survival in vitro by probing for annexin A5, phosphatidylserine, viable mitochondria, and mitochondrial reactive oxygen species. In vivo, we performed intraocular injections of GW4869 and measured RGC and superior colliculus neuron density and RGC anterograde axon transport.

RESULTS

Following twenty-four hours of dosing hRGCs with GW4869, we found that inhibition of nSMase decreased ceramide and enhanced GM1 ganglioside accumulation. This inhibition also reduced the density of small EVs, increased the density of large EVs, and enriched the pro-apoptotic protein, annexin A5. Reducing nSMase activity increased hRGC apoptosis initiation due to enhanced density and uptake of apoptotic particles, as identified by the annexin A5 binding phospholipid, phosphatidylserine. We assayed intercellular trafficking of mitochondria by developing a coculture system of GW4869-treated and naïve hRGCs. In treated cells, inhibition of nSMase reduced the number of viable mitochondria, while driving mitochondrial reactive oxygen species not only in treated, but also in naive hRGCs added in coculture. In mice, 20 days following a single intravitreal injection of GW4869, we found a significant loss of RGCs and their axonal recipient neurons in the superior colliculus. This followed a more dramatic reduction in anterograde RGC axon transport to the colliculus.

CONCLUSION

Overall, our data suggest that perturbing the physiologic catabolism of sphingomyelin by inhibiting nSMase reorganizes plasma membrane associated sphingolipids, alters the profile of neuron-generated EVs, and promotes neurodegeneration in vitro and in vivo by shifting the balance of pro-survival versus -degenerative EVs. Video Abstract.

NRF2/ARE mediated antioxidant response to glaucoma: role of glia and retinal ganglion cells

Glaucoma, the second leading cause of irreversible blindness worldwide, is associated with age and sensitivity to intraocular pressure (IOP). We have shown that elevated IOP causes an early increase in levels of reactive oxygen species (ROS) in the microbead occlusion mouse model. We also detected an endogenous antioxidant response mediated by Nuclear factor erythroid 2-Related Factor 2 (NRF2), a transcription factor that binds to the antioxidant response element (ARE) and increases transcription of antioxidant genes. Our previous studies show that inhibiting this pathway results in earlier and greater glaucoma pathology. In this study, we sought to determine if this endogenous antioxidant response is driven by the retinal ganglion cells (RGCs) or glial cells. We used Nrf2fl/fl mice and cell-type specific adeno-associated viruses (AAVs) expressing Cre to alter Nrf2 levels in either the RGCs or glial cells. Then, we quantified the endogenous antioxidant response, visual function and optic nerve histology after IOP elevation. We found that knock-down of Nrf2 in either cell type blunts the antioxidant response and results in earlier pathology and vision loss. Further, we show that delivery of Nrf2 to the RGCs is sufficient to provide neuroprotection. In summary, both the RGCs and glial cells contribute to the antioxidant response, but treatment of the RGCs alone with increased Nrf2 is sufficient to delay onset of vision loss and axon degeneration in this induced model of glaucoma.

Retinal astrocyte morphology predicts integration of vascular and neuronal architecture

Astrocytes are important regulators of blood flow and play a key role in the response to injury and disease in the central nervous system (CNS). Despite having an understanding that structural changes to these cells have consequences for local neurovascular physiology, individual astrocyte morphology remains largely unexplored in the retina. Here, we used MORF3 mice to capture full membranous morphology for over fifteen hundred individual astrocytes in the mouse retina, a highly metabolically active component of the CNS. We demonstrate that retinal astrocytes have been misrepresented as stellate in morphology due to marker use like GFAP and S100β which underestimates cell complexity. We also find that astrocytes contain recurring morphological motifs which are predictive of the underlying neurovascular architecture of the inner retina and suggestive of function. These motifs predict fine sampling and integration of retinal ganglion cell electrical activity with consequences for blood flow regulation. Additionally, our data shows that astrocytes participate in neurovascular interactions to a much greater degree than currently reported. 100% of cells contact the vasculature through one of three mutually exclusive classes of connections. Similarly, 100% of cells contact some neuronal element, be it an RGC axon or soma. Finally, we report that astrocyte morphology depends on retinal eccentricity, with cells appearing compressed near the nerve head and in the periphery. These results reveal a large degree of astrocyte morphological complexity that informs their contribution to neurovascular coupling in the retina.

Ocular stress enhances contralateral transfer of lenadogene nolparvovec gene therapy through astrocyte networks

Lenadogene nolparvovec (GS010) was developed to treat a point mutation in mitochondrial ND4 that causes Leber hereditary optic neuropathy. GS010 delivers human cDNA encoding wild-type ND4 packaged into an rAAV2/2 vector that transduces retinal ganglion cells, to induce allotopic expression of hybrid mitochondrial ND4. GS010 clinical trials improved best-corrected visual acuity (BCVA) up to 5 years after treatment. Interestingly, unilateral treatment improved BCVA bilaterally. Subsequent studies revealed GS010 DNA in visual tissues contralateral to the injected eye, suggesting migration. Here we tested whether unilateral intraocular pressure (IOP) elevation could influence the transfer of viral ND4 RNA in contralateral tissues after GS010 delivery to the IOP-elevated eye and probed a potential mechanism mediating translocation in mice. We found IOP elevation enhanced viral ND4 RNA transcripts in contralateral visual tissues, including retinas. Using conditional transgenic mice, we depleted astrocytic gap junction connexin 43 (Cx43), required for distant redistribution of metabolic resources between astrocytes during stress. After unilateral IOP elevation and GS010 injection, Cx43 knockdown eradicated ND4 RNA transcript detection in contralateral retinal tissues, while transcript was still detectable in optic nerves. Overall, our study indicates long-range migration of GS010 product to contralateral visual tissues is enhanced by Cx43-linked astrocyte networks.

Collagen mimetic peptide repair of the corneal nerve bed in a mouse model of dry eye disease

The intraepithelial sub-basal nerve plexus of the cornea is characterized by a central swirl of nerve processes that terminate between the apical cells of the epithelium. This plexus is a critical component of maintaining homeostatic function of the ocular surface. The cornea contains a high concentration of collagen, which is susceptible to damage in conditions such as neuropathic pain, neurotrophic keratitis, and dry eye disease. Here we tested whether topical application of a collagen mimetic peptide (CMP) is efficacious in repairing the corneal sub-basal nerve plexus in a mouse model of ocular surface desiccation. We induced corneal tear film reduction, epithelial damage, and nerve bed degradation through a combination of environmental and pharmaceutical (atropine) desiccation. Mice were subjected to desiccating air flow and bilateral topical application of 1% atropine solution (4× daily) for 2 weeks. During the latter half of this exposure, mice received topical vehicle [phosphate buffered saline (PBS)] or CMP [200 μm (Pro-Pro-Gly)₇, 10 μl] once daily, 2 h prior to the first atropine treatment for that day. After euthanasia, cornea were labeled with antibodies against βIII tubulin to visualize and quantify changes to the nerve bed. For mice receiving vehicle only, the two-week desiccation regimen reduced neuronal coverage of the central sub-basal plexus and epithelial terminals compared to naïve, with some corneas demonstrating complete degeneration of nerve beds. Accordingly, both sub-basal and epithelial βIII tubulin-labeled processes demonstrated increased fragmentation, indicative of nerve disassembly. Treatment with CMP significantly reduced nerve fragmentation, expanded both sub-basal and epithelial neuronal coverage compared to vehicle controls, and improved corneal epithelium integrity, tear film production, and corneal sensitivity. Together, these results indicate that topical CMP significantly counters neurodegeneration characteristic of corneal surface desiccation. Repairing underlying collagen in conditions that damage the ocular surface could represent a novel therapeutic avenue in treating a broad spectrum of diseases or injury.

Microfluidic Platforms Promote Polarization of Human-Derived Retinal Ganglion Cells That Model Axonopathy

Purpose

Axons depend on long-range transport of proteins and organelles which increases susceptibility to metabolic stress in disease. The axon initial segment (AIS) is particularly vulnerable due to the high bioenergetic demand of action potential generation. Here, we prepared retinal ganglion cells derived from human embryonic stem cells (hRGCs) to probe how axonal stress alters AIS morphology.

Methods

hRGCs were cultured on coverslips or microfluidic platforms. We assayed AIS specification and morphology by immunolabeling against ankyrin G (ankG), an axon-specific protein, and postsynaptic density 95 (PSD-95), a dendrite-specific protein. Using microfluidic platforms that enable fluidic isolation, we added colchicine to the axon compartment to lesion axons. We verified axonopathy by measuring the anterograde axon transport of cholera toxin subunit B and immunolabeling against cleaved caspase 3 (CC3) and phosphorylated neurofilament H (SMI-34). We determined the influence of axon injury on AIS morphology by immunolabeling samples against ankG and measuring AIS distance from soma and length.

Results

Based on measurements of ankG and PSD-95 immunolabeling, microfluidic platforms promote the formation and separation of distinct somatic-dendritic versus axonal compartments in hRGCs compared to coverslip cultures. Chemical lesioning of axons by colchicine reduced hRGC anterograde axon transport, increased varicosity density, and enhanced expression of CC3 and SMI-34. Interestingly, we found that colchicine selectively affected hRGCs with axon-carrying dendrites by reducing AIS distance from somas and increasing length, thus suggesting reduced capacity to maintain excitability.

Conclusions

Thus, microfluidic platforms promote polarized hRGCs that enable modeling of axonopathy.

Translational Relevance

Microfluidic platforms may be used to assay compartmentalized degeneration that occurs during glaucoma.

Retinal ganglion cells adapt to ionic stress in experimental glaucoma

Introduction

Identification of early adaptive and maladaptive neuronal stress responses is an important step in developing targeted neuroprotective therapies for degenerative disease. In glaucoma, retinal ganglion cells (RGCs) and their axons undergo progressive degeneration resulting from stress driven by sensitivity to intraocular pressure (IOP). Despite therapies that can effectively manage IOP many patients progress to vision loss, necessitating development of neuronal-based therapies. Evidence from experimental models of glaucoma indicates that early in the disease RGCs experience altered excitability and are challenged with dysregulated potassium (K+) homeostasis. Previously we demonstrated that certain RGC types have distinct excitability profiles and thresholds for depolarization block, which are associated with sensitivity to extracellular K+.

Methods

Here, we used our inducible mouse model of glaucoma to investigate how RGC sensitivity to K+ changes with exposure to elevated IOP.

Results

In controls, conditions of increased K+ enhanced membrane depolarization, reduced action potential generation, and widened action potentials. Consistent with our previous work, 4 weeks of IOP elevation diminished RGC light-and current-evoked responses. Compared to controls, we found that IOP elevation reduced the effects of increased K+ on depolarization block threshold, with IOP-exposed cells maintaining greater excitability. Finally, IOP elevation did not alter axon initial segment dimensions, suggesting that structural plasticity alone cannot explain decreased K+ sensitivity.

Discussion

Thus, in response to prolonged IOP elevation RGCs undergo an adaptive process that reduces sensitivity to changes in K+ while diminishing excitability. These experiments give insight into the RGC response to IOP stress and lay the groundwork for mechanistic investigation into targets for neuroprotective therapy.

The importance of unambiguous cell origin determination in neuronal repopulation studies

Neuronal repopulation achieved through transplantation or transdifferentiation from endogenous sources holds tremendous potential for restoring function in chronic neurodegenerative disease or acute injury. Key to the evaluation of neuronal engraftment is the definitive discrimination of new or donor neurons from preexisting cells within the host tissue. Recent work has identified mechanisms by which genetically encoded donor cell reporters can be transferred to host neurons through intercellular material transfer. In addition, labeling transplanted and endogenously transdifferentiated neurons through viral vector transduction can yield misexpression in host cells in some circumstances. These issues can confound the tracking and evaluation of repopulated neurons in regenerative experimental paradigms. Using the retina as an example, we discuss common reasons for artifactual labeling of endogenous host neurons with donor cell reporters and suggest strategies to prevent erroneous conclusions based on misidentification of cell origin.

Intraocular Sustained Release of EPO-R76E Mitigates Glaucoma Pathogenesis by Activating the NRF2/ARE Pathway

Erythropoietin (EPO) is neuroprotective in multiple models of neurodegenerative diseases, including glaucoma. EPO-R76E retains the neuroprotective effects of EPO but diminishes the effects on hematocrit. Treatment with EPO-R76E in a glaucoma model increases expression of antioxidant proteins and is neuroprotective. A major pathway that controls the expression of antioxidant proteins is the NRF2/ARE pathway. This pathway is activated endogenously after elevation of intraocular pressure (IOP) and contributes to the slow onset of pathology in glaucoma. In this study, we explored if sustained release of EPO-R76E in the eye would activate the NRF2/ARE pathway and if this pathway was key to its neuroprotective activity. Treatment with PLGA.EPO-E76E prevented increases in retinal superoxide levels in vivo, and caused phosphorylation of NRF2 and upregulation of antioxidants. Further, EPO-R76E activates NRF2 via phosphorylation by the MAPK pathway rather than the PI3K/Akt pathway, used by the endogenous antioxidant response to elevated IOP.

Ex Vivo Integration of Human Stem Retinal Ganglion Cells into the Mouse Retina

Cell replacement therapies may be key in achieving functional recovery in neurodegenerative optic neuropathies diseases such as glaucoma. One strategy that holds promise in this regard is the use of human embryonic stem cell and induced pluripotent stem-derived retinal ganglion cells (hRGCs). Previous hRGC transplantation studies have shown modest success. This is in part due to the low survival and integration of the transplanted cells in the host retina. The field is further challenged by mixed assays and outcome measurements that probe and determine transplantation success. Thefore, we have devised a transplantation assay involving hRGCs and mouse retina explants that bypasses physical barriers imposed by retinal membranes. We show that hRGC neurites and somas are capable of invading mouse explants with a subset of hRGC neurites being guided by mouse RGC axons. Neonatal mouse retina explants, and to a lesser extent, adult explants, promote hRGC integrity and neurite outgrowth. Using this assay, we tested whether suppmenting cultures with brain derived neurotrophic factor (BDNF) and the adenylate cyclase activator, forskolin, enhances hRGC neurite integration, neurite outgrowth, and integrity. We show that supplementing cultures with a combination BDNF and forskolin strongly favors hRGC integrity, increasing neurite outgrowth and complexity as well as the invasion of mouse explants. The transplantation assay presented here is a practical tool for investigating strategies for testing and optimizing the integration of donor cells into host tissues.

Axon hyperexcitability in the contralateral projection following unilateral optic nerve crush in mice

Optic neuropathies are characterized by degeneration of retinal ganglion cell axonal projections to the brain, including acute conditions like optic nerve trauma and progressive conditions such as glaucoma. Despite different aetiologies, retinal ganglion cell axon degeneration in traumatic optic neuropathy and glaucoma share common pathological signatures. We compared how early pathogenesis of optic nerve trauma and glaucoma influence axon function in the mouse optic projection. We assessed pathology by measuring anterograde axonal transport from retina to superior colliculus, current-evoked optic nerve compound action potential and retinal ganglion cell density 1 week following unilateral optic nerve crush or intraocular pressure elevation. Nerve crush reduced axon transport, compound axon potential and retinal ganglion cell density, which were unaffected by intraocular pressure elevation. Surprisingly, optic nerves contralateral to crush demonstrated 5-fold enhanced excitability in compound action potential compared with naïve nerves. Enhanced excitability in contralateral sham nerves is not due to increased accumulation of voltage-gated sodium channel 1.6, or ectopic voltage-gated sodium channel 1.2 expression within nodes of Ranvier. Our results indicate hyperexcitability is driven by intrinsic responses of αON-sustained retinal ganglion cells. We found αON-sustained retinal ganglion cells in contralateral, sham and eyes demonstrated increased responses to depolarizing currents compared with those from naïve eyes, while light-driven responses remained intact. Dendritic arbours of αON-sustained retinal ganglion cells of the sham eye were like naïve, but soma area and non-phosphorylated neurofilament H increased. Current- and light-evoked responses of sham αOFF-sustained retinal ganglion cells remained stable along with somato-dendritic morphologies. In retinas directly affected by crush, light responses of αON- and αOFF-sustained retinal ganglion cells diminished compared with naïve cells along with decreased dendritic field area or branch points. Like light responses, αOFF-sustained retinal ganglion cell current-evoked responses diminished, but surprisingly, αON-sustained retinal ganglion cell responses were similar to those from naïve retinas. Optic nerve crush reduced dendritic length and area in αON-sustained retinal ganglion cells in eyes ipsilateral to injury, while crush significantly reduced dendritic branching in αOFF-sustained retinal ganglion cells. Interestingly, 1 week of intraocular pressure elevation only affected αOFF-sustained retinal ganglion cell physiology, depolarizing resting membrane potential in cells of affected eyes and blunting current-evoked responses in cells of saline-injected eyes. Collectively, our results suggest that neither saline nor sham surgery provide a true control, chronic versus acute optic neuropathies differentially affect retinal ganglion cells composing the ON and OFF pathways, and acute stress can have near-term effects on the contralateral projection.

Sensitivity to extracellular potassium underlies type-intrinsic differences in retinal ganglion cell excitability

Neuronal type-specific physiologic heterogeneity can be driven by both extrinsic and intrinsic mechanisms. In retinal ganglion cells (RGCs), which carry visual information from the retina to central targets, evidence suggests intrinsic properties shaping action potential (AP) generation significantly impact the responses of RGCs to visual stimuli. Here, we explored how differences in intrinsic excitability further distinguish two RCG types with distinct presynaptic circuits, alpha ON-sustained (αON-S) cells and alpha OFF-sustained (αOFF-S) cells. We found that αOFF-S RGCs are more excitable to modest depolarizing currents than αON-S RGCs but excitability plateaued earlier as depolarization increased (i.e., depolarization block). In addition to differences in depolarization block sensitivity, the two cell types also produced distinct AP shapes with increasing stimulation. αOFF-S AP width and variability increased with depolarization magnitude, which correlated with the onset of depolarization block, while αON-S AP width and variability remained stable. We then tested if differences in depolarization block observed in αON-S and αOFF-S RGCs were due to sensitivity to extracellular potassium. We found αOFF-S RGCs more sensitive to increased extracellular potassium concentration, which shifted αON-S RGC excitability to that of αOFF-S cells under baseline potassium conditions. Finally, we investigated the influence of the axon initial segment (AIS) dimensions on RGC spiking. We found that the relationship between AIS length and evoked spike rate varied not only by cell type, but also by the strength of stimulation, suggesting AIS structure alone cannot fully explain the observed differences RGC excitability. Thus, sensitivity to extracellular potassium contributes to differences in intrinsic excitability, a key factor that shapes how RGCs encode visual information.

Absence of lenadogene nolparvovec DNA in a brain tumor biopsy from a patient in the REVERSE clinical study, a case report

BACKGROUND

Leber Hereditary Optic Neuropathy (LHON) is a rare, maternally-inherited mitochondrial disease that primarily affects retinal ganglion cells (RGCs) and their axons in the optic nerve, leading to irreversible, bilateral severe vision loss. Lenadogene nolparvovec gene therapy was developed as a treatment for patients with vision loss from LHON caused by the most prevalent m.11778G > A mitochondrial DNA point mutation in the MT-ND4 gene. Lenadogene nolparvovec is a replication-defective recombinant adeno-associated virus vector 2 serotype 2 (AAV2/2), encoding the human wild-type MT-ND4 protein. Lenadogene nolparvovec was administered by intravitreal injection (IVT) in LHON patients harboring the m.11778G > A ND4 mutation in a clinical development program including one phase 1/2 study (REVEAL), three phase 3 pivotal studies (REVERSE, RESCUE, REFLECT), and one long-term follow-up study (RESTORE, the follow-up of REVERSE and RESCUE patients).

CASE PRESENTATION

A 67-year-old woman with MT-ND4 LHON, included in the REVERSE clinical study, received a unilateral IVT of lenadogene nolparvovec in the right eye and a sham injection in the left eye in May 2016, 11.4 months and 8.8 months after vision loss in her right and left eyes, respectively. The patient had a normal brain magnetic resonance imaging with contrast at the time of diagnosis of LHON. Two years after treatment administration, BCVA had improved in both eyes. The product was well tolerated with mild and resolutive anterior chamber inflammation in the treated eye. In May 2019, the patient was diagnosed with a right temporal lobe glioblastoma, IDH-wildtype, World Health Organization grade 4, based on histological analysis of a tumor excision. The brain tumor was assessed for the presence of vector DNA by using a sensitive validated qPCR assay targeting the ND4 sequence of the vector.

CONCLUSION

ND4 DNA was not detected (below 15.625 copies/μg of genomic DNA) in DNA extracted from the brain tumor, while a housekeeping gene DNA was detected at high levels. Taken together, this data shows the absence of detection of lenadogene nolparvovec in a brain tumor (glioblastoma) of a treated patient in the REVERSE clinical trial 3 years after gene therapy administration, supporting the long-term favorable safety of lenadogene nolparvovec.

Solving neurodegeneration: common mechanisms and strategies for new treatments

Across neurodegenerative diseases, common mechanisms may reveal novel therapeutic targets based on neuronal protection, repair, or regeneration, independent of etiology or site of disease pathology. To address these mechanisms and discuss emerging treatments, in April, 2021, Glaucoma Research Foundation, BrightFocus Foundation, and the Melza M. and Frank Theodore Barr Foundation collaborated to bring together key opinion leaders and experts in the field of neurodegenerative disease for a virtual meeting titled "Solving Neurodegeneration". This "think-tank" style meeting focused on uncovering common mechanistic roots of neurodegenerative disease and promising targets for new treatments, catalyzed by the goal of finding new treatments for glaucoma, the world's leading cause of irreversible blindness and the common interest of the three hosting foundations. Glaucoma, which causes vision loss through degeneration of the optic nerve, likely shares early cellular and molecular events with other neurodegenerative diseases of the central nervous system. Here we discuss major areas of mechanistic overlap between neurodegenerative diseases of the central nervous system: neuroinflammation, bioenergetics and metabolism, genetic contributions, and neurovascular interactions. We summarize important discussion points with emphasis on the research areas that are most innovative and promising in the treatment of neurodegeneration yet require further development. The research that is highlighted provides unique opportunities for collaboration that will lead to efforts in preventing neurodegeneration and ultimately vision loss.

Dysfunctional cGMP Signaling Leads to Age-Related Retinal Vascular Alterations and Astrocyte Remodeling in Mice

The nitric oxide-guanylyl cyclase-1-cyclic guanylate monophosphate (NO-GC-1-cGMP) pathway is integral to the control of vascular tone and morphology. Mice lacking the alpha catalytic domain of guanylate cyclase (GC1^-/-) develop retinal ganglion cell (RGC) degeneration with age, with only modest fluctuations in intraocular pressure (IOP). Increasing the bioavailability of cGMP in GC1^-/- mice prevents neurodegeneration independently of IOP, suggesting alternative mechanisms of retinal neurodegeneration. In continuation to these studies, we explored the hypothesis that dysfunctional cGMP signaling leads to changes in the neurovascular unit that may contribute to RGC degeneration. We assessed retinal vasculature and astrocyte morphology in young and aged GC1^-/- and wild type mice. GC1^-/- mice exhibit increased peripheral retinal vessel dilation and shorter retinal vessel branching with increasing age compared to Wt mice. Astrocyte cell morphology is aberrant, and glial fibrillary acidic protein (GFAP) density is increased in young and aged GC1^-/- mice, with areas of dense astrocyte matting around blood vessels. Our results suggest that proper cGMP signaling is essential to retinal vessel morphology with increasing age. Vascular changed are preceded by alterations in astrocyte morphology which may together contribute to retinal neurodegeneration and loss of visual acuity observed in GC1^-/- mice.

Intraocular Delivery of a Collagen Mimetic Peptide Repairs Retinal Ganglion Cell Axons in Chronic and Acute Injury Models

Vision loss through the degeneration of retinal ganglion cell (RGC) axons occurs in both chronic and acute conditions that target the optic nerve. These include glaucoma, in which sensitivity to intraocular pressure (IOP) causes early RGC axonal dysfunction, and optic nerve trauma, which causes rapid axon degeneration from the site of injury. In each case, degeneration is irreversible, necessitating new therapeutics that protect, repair, and regenerate RGC axons. Recently, we demonstrated the reparative capacity of using collagen mimetic peptides (CMPs) to heal fragmented collagen in the neuronal extracellular milieu. This was an important step in the development of neuronal-based therapies since neurodegeneration involves matrix metalloproteinase (MMP)-mediated remodeling of the collagen-rich environment in which neurons and their axons exist. We found that intraocular delivery of a CMP comprising single-strand fractions of triple helix human type I collagen prevented early RGC axon dysfunction in an inducible glaucoma model. Additionally, CMPs also promoted neurite outgrowth from dorsal root ganglia, challenged in vitro by partial digestion of collagen. Here, we compared the ability of a CMP sequence to protect RGC axons in both inducible glaucoma and optic nerve crush. A three-week +40% elevation in IOP caused a 67% degradation in anterograde transport to the superior colliculus, the primary retinal projection target in rodents. We found that a single intravitreal injection of CMP during the period of IOP elevation significantly reduced this degradation. The same CMP delivered shortly after optic nerve crush promoted significant axonal recovery during the two-week period following injury. Together, these findings support a novel protective and reparative role for the use of CMPs in both chronic and acute conditions affecting the survival of RGC axons in the optic projection to the brain.

Bax Contributes to Retinal Ganglion Cell Dendritic Degeneration During Glaucoma

The BCL-2 (B-cell lymphoma-2) family of proteins contributes to mitochondrial-based apoptosis in models of neurodegeneration, including glaucomatous optic neuropathy (glaucoma), which degrades the retinal ganglion cell (RGC) axonal projection to the visual brain. Glaucoma is commonly associated with increased sensitivity to intraocular pressure (IOP) and involves a proximal program that leads to RGC dendritic pruning and a distal program that underlies axonopathy in the optic projection. While genetic deletion of the Bcl2-associated X protein (Bax^-/-) prolongs RGC body survival in models of glaucoma and optic nerve trauma, axonopathy persists, thus raising the question of whether dendrites and the RGC light response are protected. Here, we used an inducible model of glaucoma in Bax^-/- mice to determine if Bax contributes to RGC dendritic degeneration. We performed whole-cell recordings and dye filling in RGCs signaling light onset (αON-Sustained) and offset (αOFF-Sustained). We recovered RGC dendritic morphologies by confocal microscopy and analyzed dendritic arbor complexity and size. Additionally, we assessed RGC axon function by measuring anterograde axon transport of cholera toxin subunit B to the superior colliculus and behavioral spatial frequency threshold (i.e., spatial acuity). We found 1 month of IOP elevation did not cause significant RGC death in either WT or Bax^-/- retinas. However, IOP elevation reduced dendritic arbor complexity of WT αON-Sustained and αOFF-Sustained RGCs. In the absence of Bax, αON- and αOFF-Sustained RGC dendritic arbors remained intact following IOP elevation. In addition to dendrites, neuroprotection by Bax^-/- generalized to αON-and αOFF-Sustained RGC light- and current-evoked responses. Both anterograde axon transport and spatial acuity declined during IOP elevation in WT and Bax^-/- mice. Collectively, our results indicate Bax contributes to RGC dendritic degeneration and distinguishes the proximal and distal neurodegenerative programs involved during the progression of glaucoma.

Biodistribution of intravitreal lenadogene nolparvovec gene therapy in nonhuman primates

Lenadogene nolparvovec (Lumevoq) gene therapy was developed to treat Leber hereditary optic neuropathy (LHON) caused by the m.11778G > A in MT-ND4 that affects complex I of the mitochondrial respiratory chain. Lenadogene nolparvovec is a replication-defective, single-stranded DNA recombinant adeno-associated virus vector 2 serotype 2, containing a codon-optimized complementary DNA encoding the human wild-type MT-ND4 subunit protein. Lenadogene nolparvovec was administered by unilateral intravitreal injection in MT-ND4 LHON patients in two randomized, double-masked, and sham-controlled phase III clinical trials (REVERSE and RESCUE), resulting in bilateral improvement of visual acuity. These and other earlier results suggest that lenadogene nolparvovec may travel from the treated to the untreated eye. To investigate this possibility further, lenadogene nolparvovec was unilaterally injected into the vitreous body of the right eye of healthy, nonhuman primates. Viral vector DNA was quantifiable in all eye and optic nerve tissues of the injected eye and was detected at lower levels in some tissues of the contralateral, noninjected eye, and optic projections, at 3 and 6 months after injection. The results suggest that lenadogene nolparvovec transfers from the injected to the noninjected eye, thus providing a potential explanation for the bilateral improvement of visual function observed in the LHON patients.

Restoring the Extracellular Matrix: A Neuroprotective Role for Collagen Mimetic Peptides in Experimental Glaucoma

Optic neuropathies are a major cause of visual disabilities worldwide, causing irreversible vision loss through the degeneration of retinal ganglion cell (RGC) axons, which comprise the optic nerve. Chief among these is glaucoma, in which sensitivity to intraocular pressure (IOP) leads to RGC axon dysfunction followed by outright degeneration of the optic projection. Current treatments focus entirely on lowering IOP through topical hypotensive drugs, surgery to facilitate aqueous fluid outflow, or both. Despite this investment in time and resources, many patients continue to lose vision, underscoring the need for new therapeutics that target neurodegeneration directly. One element of progression in glaucoma involves matrix metalloproteinase (MMP) remodeling of the collagen-rich extracellular milieu of RGC axons as they exit the retina through the optic nerve head. Thus, we investigated the ability of collagen mimetic peptides (CMPs) representing various single strand fractions of triple helix human type I collagen to protect RGC axons in an inducible model of glaucoma. First, using dorsal root ganglia maintained in vitro on human type I collagen, we found that multiple CMPs significantly promote neurite outgrowth (+35%) compared to vehicle following MMP-induced fragmentation of the α1(I) and α2(I) chains. We then applied CMP to adult mouse eyes in vivo following microbead occlusion to elevate IOP and determined its influence on anterograde axon transport to the superior colliculus, the primary RGC projection target in rodents. In glaucoma models, sensitivity to IOP causes early degradation in axon function, including anterograde transport from retina to central brain targets. We found that CMP treatment rescued anterograde transport following a 3-week +50% elevation in IOP. These results suggest that CMPs generally may represent a novel therapeutic to supplement existing treatments or as a neuroprotective option for patients who do not respond to IOP-lowering regimens.

Collagen Mimetic Peptides Promote Corneal Epithelial Cell Regeneration

The cornea of the eye is at risk for injury through constant exposure to the extraocular environment. A highly collagenous structure, the cornea contains several different types distributed across multiple layers. The anterior-most layer contains non-keratinized epithelial cells that serve as a barrier to environmental, microbial, and other insults. Renewal and migration of basal epithelial cells from the limbus involve critical interactions between secreted basement membranes, composed primarily of type IV collagen, and underlying Bowman's and stromal layers, which contain primarily type I collagen. This process is challenged in many diseases and conditions that insult the ocular surface and damage underlying collagen. We investigated the capacity of a collagen mimetic peptide (CMP), representing a fraction of a single strand of the damaged triple helix human type I collagen, to promote epithelial healing following an acute corneal wound. In vitro, the collagen mimetic peptide promoted the realignment of collagen damaged by enzymic digestion. In an in vivo mouse model, topical application of a CMP-containing formulation following a 360° lamellar keratectomy targeting the corneal epithelial layer accelerated wound closure during a 24 h period, compared to vehicle. We found that the CMP increased adherence of the basal epithelium to the underlying substrate and enhanced density of epithelial cells, while reducing variability in the regenerating layer. These results suggest that CMPs may represent a novel therapeutic to heal corneal tissue by repairing underlying collagen in conditions that damage the ocular surface.

Intrinsic Morphologic and Physiologic Development of Human Derived Retinal Ganglion Cells In Vitro

Purpose

Human retinal ganglion cells (hRGC) derived from human pluripotent stem cells are promising candidates to model, protect, and replace degenerating RGCs. Here, we examined intrinsic morphologic and physiologic development of hRGCs.

Methods

We used CRISPR-Cas9 to selectively express tdTomato under the RGC-specific promoter, BRN3B. Human pluripotent stem cells were chemically differentiated into hRGCs and cultured up to 7 weeks. We measured soma area, neurite complexity, synaptic protein, axon-related messenger RNA and protein, and voltage-dependent responses.

Results

Soma area, neurite complexity, and postsynaptic density protein 95 increased over time. Soma area and neurite complexity increased proportionally week to week, and this relationship was dynamic, strengthening between 2 and 3 weeks and diminishing by 4 weeks. Postsynaptic density 95 localization was dependent on culture duration. After 1 to 2 weeks, postsynaptic density 95 localized within somas but redistributed along neurites after 3 to 4 weeks. Axon initial segment scaffolding protein, Ankyrin G, expression also increased over time, and by 7 weeks, Ankyrin G often localized within putative axons. Voltage-gated inward currents progressively developed, but outward currents matured by 4 weeks. Current-induced spike generation increased over time but limited by depolarization block.

Conclusions

Human RGCs develop up to 7 weeks after culture. Thus, the state of hRGC maturation should be accounted for in designing models and treatments for optic neuropathies.

Translational Relevance

We characterized hRGC morphologic and physiologic development towards identifying key time points when hRGCs express mechanisms that may be harnessed to enhance the efficacy of neuroprotective and cell replacement therapies.

Bilateral visual improvement with unilateral gene therapy injection for Leber hereditary optic neuropathy

REVERSE is a randomized, double-masked, sham-controlled, multicenter, phase 3 clinical trial that evaluated the efficacy of a single intravitreal injection of rAAV2/2-ND4 in subjects with visual loss from Leber hereditary optic neuropathy (LHON). A total of 37 subjects carrying the m.11778G>A (MT-ND4) mutation and with duration of vision loss between 6 to 12 months were treated. Each subject's right eye was randomly assigned in a 1:1 ratio to treatment with rAAV2/2-ND4 (GS010) or sham injection. The left eye received the treatment not allocated to the right eye. Unexpectedly, sustained visual improvement was observed in both eyes over the 96-week follow-up period. At week 96, rAAV2/2-ND4-treated eyes showed a mean improvement in best-corrected visual acuity (BCVA) of -0.308 LogMAR (+15 ETDRS letters). A mean improvement of -0.259 LogMAR (+13 ETDRS letters) was observed in the sham-treated eyes. Consequently, the primary end point, defined as the difference in the change in BCVA from baseline to week 48 between the two treatment groups, was not met (P = 0.894). At week 96, 25 subjects (68%) had a clinically relevant recovery in BCVA from baseline in at least one eye, and 29 subjects (78%) had an improvement in vision in both eyes. A nonhuman primate study was conducted to investigate this bilateral improvement. Evidence of transfer of viral vector DNA from the injected eye to the anterior segment, retina, and optic nerve of the contralateral noninjected eye supports a plausible mechanistic explanation for the unexpected bilateral improvement in visual function after unilateral injection.

Neuroprotection by WldS depends on retinal ganglion cell type and age in glaucoma

BACKGROUND

Early challenges to axonal physiology, active transport, and ultrastructure are endemic to age-related neurodegenerative disorders, including those affecting the optic nerve. Chief among these, glaucoma causes irreversible vision loss through sensitivity to intraocular pressure (IOP) that challenges retinal ganglion cell (RGC) axons, which comprise the optic nerve. Early RGC axonopathy includes distal to proximal progression that implicates a slow form of Wallerian degeneration. In multiple disease models, including inducible glaucoma, expression of the slow Wallerian degeneration (WldS) allele slows axon degeneration and confers protection to cell bodies.

METHODS

Using an inducible model of glaucoma along with whole-cell patch clamp electrophysiology and morphological analysis, we tested if WldS also protects RGC light responses and dendrites and, if so, whether this protection depends upon RGC type. We induced glaucoma in young and aged mice to determine if neuroprotection by WldS on anterograde axonal transport and spatial contrast acuity depends on age.

RESULTS

We found WldS protects dendritic morphology and light-evoked responses of RGCs that signal light onset (αON-Sustained) during IOP elevation. However, IOP elevation significantly reduces dendritic complexity and light responses of RGCs that respond to light offset (αOFF-Sustained) regardless of WldS. As expected, WldS preserves anterograde axon transport and spatial acuity in young adult mice, but its protection is significantly limited in aged mice.

CONCLUSION

The efficacy of WldS in conferring protection to neurons and their axons varies by cell type and diminishes with age.

Retinal oxidative stress activates the NRF2/ARE pathway: An early endogenous protective response to ocular hypertension

Oxidative stress contributes to degeneration of retinal ganglion cells and their axons in glaucoma, a leading cause of irreversible blindness worldwide, through sensitivity to intraocular pressure (IOP). Here, we investigated early elevations in reactive oxygen species (ROS) and a role for the NRF2-KEAP1-ARE endogenous antioxidant response pathway using microbead occlusion to elevate IOP in mice. ROS levels peaked in the retina at 1- and 2-wks following IOP elevation and remained elevated out to 5-wks. Phosphorylation of NRF2 and antioxidant gene transcription and protein levels increased concomitantly at 2-wks after IOP elevation, along with phosphorylation of PI3K and AKT. Inhibiting PI3K or AKT signaling prevented NRF2 phosphorylation and reduced transcription of antioxidant-regulated genes. Ocular hypertensive mice lacking Nrf2 had elevated ROS and a diminished increase in antioxidant gene expression. They also exhibited earlier axon degeneration and loss of visual function. In conclusion, the NRF2-KEAP1-ARE pathway is endogenously activated early in ocular hypertension due to phosphorylation of NRF2 by the PI3K/AKT pathway and serves to slow the onset of axon degeneration and vision loss in glaucoma. These data suggest that exogenous activation of this pathway might further slow glaucomatous neurodegeneration.

Insulin Signaling as a Therapeutic Target in Glaucomatous Neurodegeneration

Glaucoma is a multifactorial disease that is conventionally managed with treatments to lower intraocular pressure (IOP). Despite these efforts, many patients continue to lose their vision. The degeneration of retinal ganglion cells (RGCs) and their axons in the optic tract that characterizes glaucoma is similar to neurodegeneration in other age-related disorders of the central nervous system (CNS). Identifying the different molecular signaling pathways that contribute to early neuronal dysfunction can be utilized for neuroprotective strategies that prevent degeneration. The discovery of insulin and its receptor in the CNS and retina led to exploration of the role of insulin signaling in the CNS. Historically, insulin was considered a peripherally secreted hormone that regulated glucose homeostasis, with no obvious roles in the CNS. However, a growing number of pre-clinical and clinical studies have demonstrated the potential of modulating insulin signaling in the treatment of neurodegenerative diseases. This review will highlight the role that insulin signaling plays in RGC neurodegeneration. We will focus on how this pathway can be therapeutically targeted to promote RGC axon survival and preserve vision.

Corneal collagen as a potential therapeutic target in dry eye disease

Dry eye disease (DED) is a major cause of ocular discomfort, inflammation and dysfunction worldwide. Tear film instability in DED both causes and is exacerbated by disruption of the corneal epithelium. This tandem leads to a cycle of inflammation at the corneal surface involving immune cell dysregulation and increased chemokines and cytokines, which activate mitogen-activated protein kinases in the epithelium and elevates matrix metalloproteinases (MMPs). We review evidence suggesting that corneal collagen might be highly susceptible in DED to MMP-induced disruption, digestion, and thinning. We also summarize that collagen is far from inert and contains binding sites that serve as ligands for multiple inflammatory and immune regulators. Fragmented collagen not only challenges these receptor-ligand binding relationships, but also can promote recruitment and motility of pro-inflammatory immune cells. Current physician-directed therapies for DED focus on reducing inflammation, but do not directly ameliorate the underlying corneal damage that could exacerbate surface inflammation. We argue that an important gap in practice is lack of a direct therapeutic reparative for damaged corneal collagen, which is slow to heal, and likely amplifies sight-threatening inflammation. Healing fragmented collagen in the cornea may represent a more effective means to interrupt the "vicious cycle" of inflammation in DED and other conditions that damages, sometimes irreversibly, the ocular surface.

Adaptive responses to neurodegenerative stress in glaucoma

Glaucoma causes loss of vision through degeneration of the retinal ganglion cell (RGC) projection to the brain. The disease is characterized by sensitivity to intraocular pressure (IOP) conveyed at the optic nerve head, through which RGC axons pass unmyelinated to form the optic nerve. From this point, a pathogenic triumvirate comprising inflammatory, oxidative, and metabolic stress influence both proximal structures in the retina and distal structures in the optic projection. This review focuses on metabolic stress and how the optic projection may compensate through novel adaptive mechanisms to protect excitatory signaling to the brain. In the retina and proximal nerve head, the unmyelinated RGC axon segment is energy-inefficient, which leads to increased demand for adenosine-5'-triphosphate (ATP) at the risk of vulnerability to Ca²⁺-related metabolic and oxidative pressure. This vulnerability may underlie the bidirectional nature of progression. However, recent evidence highlights that the optic projection in glaucoma is not passive but rather demonstrates adaptive processes that may push back against neurodegeneration. In the retina, even as synaptic and dendritic pruning ensues, early progression involves enhanced excitability of RGCs. Enhancement involves depolarization of the resting membrane potential and increased response to light, independent of RGC morphological type. This response is axogenic, arising from increased levels and translocation of voltage-gated sodium channels (NaV) in the unmyelinated segment. During this same early period, large-scale networks of gap-junction coupled astrocytes redistribute metabolic resources to the optic projection stressed by elevated IOP to slow loss of axon function. This redistribution may reflect more local remodeling, as astrocyte processes respond to focal metabolic duress by boosting glycogen turnover in response to axonal activity in an effort to promote survival of the healthiest axons. Both enhanced excitability and metabolic redistribution are transient, indicating that the same adaptive mechanisms that apparently serve to slow progression ultimately may be too expensive for the system to sustain over longer periods.

TRPV1 Supports Axogenic Enhanced Excitability in Response to Neurodegenerative Stress

Early progression in neurodegenerative disease involves challenges to homeostatic processes, including those controlling axonal excitability and dendritic organization. In glaucoma, the leading cause of irreversible blindness, stress from intraocular pressure (IOP) causes degeneration of retinal ganglion cells (RGC) and their axons which comprise the optic nerve. Previously, we discovered that early progression induces axogenic, voltage-gated enhanced excitability of RGCs, even as dendritic complexity in the retina reduces. Here, we investigate a possible contribution of the transient receptor potential vanilloid type 1 (TRPV1) channel to enhanced excitability, given its role in modulating excitation in other neural systems. We find that genetic deletion of Trpv1 (Trpv1^−/−) influences excitability differently for RGCs firing continuously to light onset (αON-Sustained) vs. light offset (αOFF-Sustained). Deletion drives excitability in opposing directions so that Trpv1^−/− RGC responses with elevated IOP equalize to that of wild-type (WT) RGCs without elevated IOP. Depolarizing current injections in the absence of light-driven presynaptic excitation to directly modulate voltage-gated channels mirrored these changes, while inhibiting voltage-gated sodium channels and isolating retinal excitatory postsynaptic currents abolished both the differences in light-driven activity between WT and Trpv1^−/− RGCs and changes in response due to IOP elevation. Together, these results support a voltage-dependent, axogenic influence of Trpv1^−/− with elevated IOP. Finally, Trpv1^−/− slowed the loss of dendritic complexity with elevated IOP, opposite its effect on axon degeneration, supporting the idea that axonal and dendritic degeneration follows distinctive programs even at the level of membrane excitability.

Protect, Repair, and Regenerate: Towards Restoring Vision in Glaucoma

PURPOSE OF REVIEW

We summarize recent advances in strategies that aim to restore optic nerve function and vision in glaucoma through protective, reparative, and regenerative avenues.

RECENT FINDINGS

Neuroprotection relies on identification of early retinal ganglion cell dysfunction, which could prove challenging in the clinic. Cell replacement therapies show promise in restoring lost vision, but some hurdles remain in restoring visual circuitry in the retina and central connections in the brain.

SUMMARY

Identification and manipulation of intrinsic and extrinsic cellular mechanisms that promote axon regeneration in both resident and transplanted RGCs will drive future advances in vision restoration. Understanding the roles of multiple cell types in the retina that act in concert to promote RGC survival will aid efforts to promote neuronal health and restoration. Effective RGC transplantation, fine tuning axon guidance and growth, and synaptogenesis of transplanted and resident RGCs are still areas that require more research.

Redistribution of metabolic resources through astrocyte networks mitigates neurodegenerative stress

In the central nervous system, glycogen-derived bioenergetic resources in astrocytes help promote tissue survival in response to focal neuronal stress. However, our understanding of the extent to which these resources are mobilized and utilized during neurodegeneration, especially in nearby regions that are not actively degenerating, remains incomplete. Here we modeled neurodegeneration in glaucoma, the world's leading cause of irreversible blindness, and measured how metabolites mobilize through astrocyte gap junctions composed of connexin 43 (Cx43). We elevated intraocular pressure in one eye and determined how astrocyte-derived metabolites in the contralateral optic projection responded. Remarkably, astrocyte networks expand and redistribute metabolites along distances even 10 mm in length, donating resources from the unstressed to the stressed projection in response to intraocular pressure elevation. While resource donation improves axon function and visual acuity in the directly stressed region, it renders the donating tissue susceptible to bioenergetic, structural, and physiological degradation. Intriguingly, when both projections are stressed in a WT animal, axon function and visual acuity equilibrate between the two projections even when each projection is stressed for a different length of time. This equilibration does not occur when Cx43 is not present. Thus, Cx43-mediated astrocyte metabolic networks serve as an endogenous mechanism used to mitigate bioenergetic stress and distribute the impact of neurodegenerative disease processes. Redistribution ultimately renders the donating optic nerve vulnerable to further metabolic stress, which could explain why local neurodegeneration does not remain confined, but eventually impacts healthy regions of the brain more broadly.

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.

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.

Emixustat Reduces Metabolic Demand of Dark Activity in the Retina

Purpose

In the dark, photoreceptor outer segments contain high levels of cyclic guanosine 3'-5' monophosphate (cGMP), which binds to ion channels, holding them open and allowing an influx of cations. Ion pumping activity, which balances cation influx, uses considerable amounts of adenosine triphosphate (ATP) and oxygen. Light reduces cation influx and thereby lowers metabolic demand. Blood vessels are compromised in the diabetic retina and may not be able to meet the higher metabolic demand in darkness. Emixustat is a visual cycle modulator (VCM) that reduces chromophore levels and, therefore, may mimic light conditions. We evaluated the effect of emixustat on oxygen consumption and cation influx in dark conditions.

Methods

Cation influx was measured in rats using Mn2+-magnetic resonance imaging (MEMRI). Retinal oxygen profiles were recorded to evaluate oxygen consumption. In the MEMRI protocol, animals were treated with either emixustat or vehicle. In the oxygen protocol, animals were untreated or treated with emixustat.

Results

In vehicle-treated animals, cation channel activity increased in the dark. Emixustat treatment reduced cation channel activity; activity was comparable to vehicle-treated controls in light conditions. In vehicle-treated animals, minimum retinal oxygen tension decreased as the retina recovered from a photobleach, indicating that more oxygen was being consumed. Emixustat treatment prevented the decrease in oxygen pressure after photobleach.

Conclusions

Emixustat reduced the cation influx and retinal oxygen consumption associated with dark conditions. VCMs are a promising potential treatment for ischemic retinal neovascularization, such as that in diabetic retinopathy.

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.

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.

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.

Astrocyte remodeling without gliosis precedes optic nerve Axonopathy

Astroyctes serve myriad functions but are especially critical in white matter tracts, where energy-demanding axons propagate action potentials great distances between neurons. Axonal dependence on astrocytes for even normal function accentuates the critical role astrocytes serve during disease. In glaucoma, the most common optic neuropathy, sensitivity to intraocular pressure (IOP) challenges RGC axons early, including degradation of anterograde transport to the superior colliculus (SC). Astrocyte remodeling presages overt axon degeneration in glaucoma and thus may present a therapeutic opportunity. Here we developed a novel metric to quantify organization of astrocyte processes in the optic nerve relative to axon degeneration in the DBA/2 J hereditary mouse model of glaucoma. In early progression, as axons expand prior to loss, astrocyte processes become more parallel with migration to the nerve’s edge without a change in overall coverage of the nerve. As axons degenerate, astrocyte parallelism diminishes with increased glial coverage and reinvasion of the nerve. In longitudinal sections through aged DBA/2 J nerve, increased astrocyte parallelism reflected elevated levels of the astrocyte gap-junction protein connexin 43 (Cx43). In the distal nerve, increased Cx43 also indicated with a higher level of intact anterograde transport from retina to SC. Our results suggest that progression of axonopathy in the optic nerve involves astrocyte remodeling in two phases. In an early phase, astrocyte processes organize in parallel, likely through gap-junction coupling, while a later phase involves deterioration of organization as glial coverage increases and axons are lost.

The Microbead Occlusion Model of Ocular Hypertension in Mice

Glaucoma is a common optic neuropathy that leads to vision loss through the degeneration of retinal ganglion cells (RGCs) and their axons. RGC degeneration in glaucoma is associated with sensitivity to intraocular pressure (IOP) and elevated IOP (also known as ocular hypertension) is the primary modifiable risk factor. Ocular hypertension is the primary characteristic of rodent models for glaucoma research. Intracameral injection of microbeads has evolved as a preferred method of IOP elevation in rodents, particularly in mice. Here, we outline the protocol and method for the Microbead Occlusion Model in mice. We highlight the importance of anesthesia choice and the utilization of glass micropipettes in combination with a micromanipulator and microsyringe pump for the successful execution of the model.

Oral Delivery of a Synthetic Sterol Reduces Axonopathy and Inflammation in a Rodent Model of Glaucoma

Glaucoma is a group of optic neuropathies associated with aging and sensitivity to intraocular pressure (IOP). The disease is the leading cause of irreversible blindness worldwide. Early progression in glaucoma involves dysfunction of retinal ganglion cell (RGC) axons, which comprise the optic nerve. Deficits in anterograde transport along RGC axons to central visual structures precede outright degeneration, and preventing these deficits is efficacious at abating subsequent progression. HE3286 is a synthetic sterol derivative that has shown therapeutic promise in models of inflammatory disease and neurodegenerative disease. We examined the efficacy of HE3286 oral delivery in preventing loss of anterograde transport in an inducible model of glaucoma (microbead occlusion). Adult rats received HE3286 (20 or 100 mg/kg) or vehicle daily via oral gavage for 4 weeks. Microbead occlusion elevated IOP ~30% in all treatment groups, and elevation was not affected by HE3286 treatment. In the vehicle group, elevated IOP reduced anterograde axonal transport to the superior colliculus, the most distal site in the optic projection, by 43% (p = 0.003); HE3286 (100 mg/kg) prevented this reduction (p = 0.025). HE3286 increased brain-derived neurotrophic factor (BDNF) in the optic nerve head and retina, while decreasing inflammatory and pathogenic proteins associated with elevated IOP compared to vehicle treatment. Treatment with HE3286 also increased nuclear localization of the transcription factor NFκB in collicular and retinal neurons, but decreased NFκB in glial nuclei in the optic nerve head. Thus, HE3286 may have a neuroprotective influence in glaucoma, as well as other chronic neurodegenerations.

The challenge of regenerative therapies for the optic nerve in glaucoma

This review arose from a discussion of regenerative therapies to treat optic nerve degeneration in glaucoma at the 2015 Lasker/IRRF Initiative on Astrocytes and Glaucomatous Neurodegeneration. In addition to the authors, participants included Jonathan Crowston, Andrew Huberman, Elaine Johnson, Richard Lu, Hemai Phatnami, Rebecca Sappington, and Don Zack. Glaucoma is a neurodegenerative disease of the optic nerve, and is the leading cause of irreversible blindness worldwide. The disease progresses as sensitivity to intraocular pressure (IOP) is conveyed through the optic nerve head to distal retinal ganglion cell (RGC) projections. Because the nerve and retina are components of the central nervous system (CNS), their intrinsic regenerative capacity is limited. However, recent research in regenerative therapies has resulted in multiple breakthroughs that may unlock the optic nerve's regenerative potential. Increasing levels of Schwann-cell derived trophic factors and reducing potent cell-intrinsic suppressors of regeneration have resulted in axonal regeneration even beyond the optic chiasm. Despite this success, many challenges remain. RGC axons must be able to form new connections with their appropriate targets in central brain regions and these connections must be retinotopically correct. Furthermore, for new axons penetrating the optic projection, oligodendrocyte glia must provide myelination. Additionally, reactive gliosis and inflammation that increase the regenerative capacity must be outweigh pro-apoptotic processes to create an environment within which maximal regeneration can occur.

Loss of Fractalkine Signaling Exacerbates Axon Transport Dysfunction in a Chronic Model of Glaucoma

Neurodegeneration in glaucoma results in decline and loss of retinal ganglion cells (RGCs), and is associated with activation of myeloid cells such as microglia and macrophages. The chemokine fractalkine (FKN or Cx3cl1) mediates communication from neurons to myeloid cells. Signaling through its receptor Cx3cr1 has been implicated in multiple neurodegenerative diseases, but the effects on neuronal pathology are variable. Since it is unknown how FKN-mediated crosstalk influences RGC degeneration in glaucoma, we assessed this in a chronic mouse model, DBA/2J. We analyzed a DBA/2J substrain deficient in Cx3cr1, and compared compartmentalized RGC degeneration and myeloid cell responses to those in standard DBA/2J mice. We found that loss of FKN signaling exacerbates axon transport dysfunction, an early event in neurodegeneration, with a significant increase in RGCs with somal accumulation of the axonal protein phosphorylated neurofilament, and reduced retinal expression of genes involved in axon transport, Kif1b, and Atp8a2. There was no change in the loss of Brn3-positive RGCs, and no difference in the extent of damage to the proximal optic nerve, suggesting that the loss of fractalkine signaling primarily affects axon transport. Since Cx3cr1 is specifically expressed in myeloid cells, we assessed changes in retinal microglial number and activation, changes in gene expression, and the extent of macrophage infiltration. We found that loss of fractalkine signaling led to innate immune changes within the retina, including increased infiltration of peripheral macrophages and upregulated nitric oxide synthase-2 (Nos-2) expression in myeloid cells, which contributes to the production of NO and can promote axon transport deficits. In contrast, resident retinal microglia appeared unchanged either in number, morphology, or expression of the myeloid activation marker ionized calcium binding adaptor molecule 1 (Iba1). There was also no significant increase in the proinflammatory gene interleukin 1 beta (Il1β). We conclude that loss of fractalkine signaling causes a selective worsening of axon transport dysfunction in RGCs, which is linked to enhanced Nos-2 expression in myeloid cells. Our findings suggest that distinct mechanisms may contribute to different aspects of RGC decline in glaucoma, with axonal transport selectively altered after loss of Cx3cr1 in microglia and/or macrophages.

Activation of transient receptor potential vanilloid-1 (TRPV1) influences how retinal ganglion cell neurons respond to pressure-related stress

Our recent studies implicate the transient receptor potential vanilloid-1 (TRPV1) channel as a mediator of retinal ganglion cell (RGC) function and survival. With elevated pressure in the eye, TRPV1 increases in RGCs, supporting enhanced excitability, while Trpv1 -/- accelerates RGC degeneration in mice. Here we find TRPV1 localized in monkey and human RGCs, similar to rodents. Expression increases in RGCs exposed to acute changes in pressure. In retinal explants, contrary to our animal studies, both Trpv1 -/- and pharmacological antagonism of the channel prevented pressure-induced RGC apoptosis, as did chelation of extracellular Ca(2+). Finally, while TRPV1 and TRPV4 co-localize in some RGC bodies and form a protein complex in the retina, expression of their mRNA is inversely related with increasing ocular pressure. We propose that TRPV1 activation by pressure-related insult in the eye initiates changes in expression that contribute to a Ca(2+)-dependent adaptive response to maintain excitatory signaling in RGCs.

Glial coverage in the optic nerve expands in proportion to optic axon loss in chronic mouse glaucoma

Within the white matter, axonal loss by neurodegeneration is coupled to glial cell changes in gene expression, structure and function commonly termed gliosis. Recently, we described the highly variable expansion of gliosis alebosco@neuro.utah.edu in degenerative optic nerves from the DBA/2J mouse model of chronic, age-related glaucoma. Here, to estimate and compare the levels of axonal loss with the expansion of glial coverage and axonal degeneration in DBA/2J nerves, we combined semiautomatic axon counts with threshold-based segmentation of total glial/scar areas and degenerative axonal profiles in plastic cross-sections. In nerves ranging from mild to severe degeneration, we found that the progression of axonal dropout is coupled to an increase of gliotic area. We detected a strong correlation between axon loss and the aggregate coverage by glial cells and scar, whereas axon loss did not correlate with the small fraction of degenerating profiles. Nerves with low to medium levels of axon loss displayed moderate glial reactivity, consisting of hypertrophic astrocytes, activated microglia and normal distribution of oligodendrocytes, with minimal reorganization of the tissue architecture. In contrast, nerves with extensive axonal loss showed prevalent rearrangement of the nerve, with loss of axon fascicle territories and enlarged or almost continuous gliotic and scar domains, containing reactive astrocytes, oligodendrocytes and activated microglia. These findings support the value of optic nerve gliotic expansion as a quantitative estimate of optic neuropathy that correlates with axon loss, applicable to grade the severity of optic nerve damage in mouse chronic glaucoma.

Early astrocyte redistribution in the optic nerve precedes axonopathy in the DBA/2J mouse model of glaucoma

Glaucoma challenges the survival of retinal ganglion cell axons in the optic nerve through processes dependent on both aging and ocular pressure. Relevant stressors likely include complex interplay between axons and astrocytes, both in the retina and optic nerve. In the DBA/2J mouse model of pigmentary glaucoma, early progression involves axonopathy characterized by loss of functional transport prior to outright degeneration. Here we describe novel features of early pathogenesis in the DBA/2J nerve. With age the cross-sectional area of the nerve increases; this is associated generally with diminished axon packing density and survival and increased glial coverage of the nerve. However, for nerves with the highest axon density, as the nerve expands mean cross-sectional axon area enlarges as well. This early expansion was marked by disorganized axoplasm and accumulation of hyperphosphorylated neurofilamants indicative of axonopathy. Axon expansion occurs without loss up to a critical threshold for size (about 0.45-0.50 μm(2)), above which additional expansion tightly correlates with frank loss of axons. As well, early axon expansion prior to degeneration is concurrent with decreased astrocyte ramification with redistribution of processes towards the nerve edge. As axons expand beyond the critical threshold for loss, glial area resumes an even distribution from the center to edge of the nerve. We also found that early axon expansion is accompanied by reduced numbers of mitochondria per unit area in the nerve. Finally, our data indicate that both IOP and nerve expansion are associated with axon enlargement and reduced axon density for aged nerves. Collectively, our data support the hypothesis that diminished bioenergetic resources in conjunction with early nerve and glial remodeling could be a primary inducer of progression of axon pathology in glaucoma.