Targeting the ER-autophagy system in the trabecular meshwork to treat glaucoma

Global research trends on endoplasmic reticulum stress in retinal diseases from 2000 to 2024

PURPOSE

To comprehensively explore global research trends on endoplasmic reticulum stress (ERS) in retinal diseases over the past 24 years.

METHODS

An analysis of 917 publications from the Web of Science Core Collection, spanning from 1 January 2000 to 15 April 2024, was conducted to explore ERS research in retinal diseases. Bibliometric and visualisation software was employed to identify key contributors and research trends.

RESULTS

Hideaki Hara and Sarah X. Zhang were identified as the most published and most cited authors, respectively. The United States led in both publications and citations. Sun Yat-sen University ranked highest in publications, while the University of Oklahoma received the most citations. Investigative Ophthalmology & Visual Science was the leading journal in both publications and citations. A total of 108 of the 1385 author keywords, each occurring five or more times, clustered into four major themes: retinal photoreceptor degeneration, glaucoma and optic nerve damage, diabetic retinopathy and age-related macular degeneration. Keywords such as "in vivo", "dominant retinitis pigmentosa", "endothelial growth factor", "molecular", and "quality control" displayed the strongest citation bursts. ERS research (2000 ~ 2024) has evolved from retinal neuroprotection to include specific cell types and diseases, explored signalling pathways and therapeutic mechanisms, and more recently has focused on molecular insights and gene therapy applications.

CONCLUSION

This bibliometric analysis highlighted significant growth in publications on ERS in retinal diseases, reflecting an increasing scholarly focus. ERS represents a potential target for exploring pathological changes in retinal neuro-microvascular and related disorders, warranting further investigation.

Aging-enhanced autophagy activity promotes fibrotic progression via the TGF-β2/Smad signaling pathway in trabecular meshwork cells-a new insight from POAG

Introduction

Glaucoma, a leading cause of irreversible blindness, is characterized by optic neuropathy and retinopathy, with primary open-angle glaucoma (POAG) being the most prevalent form. The primary pathogenic mechanism of POAG involves elevated intraocular pressure caused by chronic fibrosis of the trabecular meshwork (TM). Autophagy, a critical process for maintaining cellular homeostasis, has been implicated in fibrosis across various organs. However, its precise role in the fibrosis associated with POAG pathogenesis remains unclear. This study investigates the involvement of autophagy in TM fibrosis and explores its potential impact on POAG development, aiming to provide insights into new therapeutic targets.

Methods

To assess autophagy activity and its relationship with fibrosis, we analyzed TM tissues from POAG patients and healthy donors. Autophagic activity in human TM tissues was measured through immunohistochemical analyses. An in vitro aging model using chronic H2O2 treatment was established to investigate the change of fibrosis in TM cells. Additionally, we used dexamethasone-treated TM cells as a POAG model to explore the role of autophagy in fibrotic progression. The involvement of the TGF-β2/Smad signaling pathway was investigated through western blot analysis and quantitative real-time PCR.

Results

This study reveals increased autophagic activity in tissues from POAG patients and an age-related upregulation of autophagy in healthy human TM tissues. In the H2O2-induced aging model, TM cells displayed both elevated autophagic activity and fibrosis. Further investigation showed that enhanced autophagy activity promoted fibrotic progression via activation of the TGF-β2/Smad signaling pathway. Similarly, in the dexamethasone-treated TM cell model, autophagy was found to exacerbate fibrosis, aligning with observations in the aging model.

Discussion

In this study, we uncover the interplay between autophagy and the TGF-β2/Smad pathway in the pathogenesis of POAG. We observed increased autophagic activity in TM tissues from POAG patients and in TM tissues of aging healthy individuals. In human primary TM cells, we confirmed that autophagy becomes activated in the context of cellular senescence and the development of POAG, which further facilitates fibrotic progression via the TGF-β2/Smad signaling pathway. These findings underscore the important role of autophagy in POAG pathogenesis and confirm senescence as a pivotal risk factor.

Antibody-mediated clearance of an ER-resident aggregate that causes glaucoma

Recombinant antibodies are a promising class of therapeutics to treat protein misfolding associated with neurodegenerative diseases, and several antibodies that inhibit aggregation are approved or in clinical trials to treat Alzheimer's disease. Here, we developed antibodies targeting the aggregation-prone β-propeller olfactomedin (OLF) domain of myocilin, variants of which comprise the strongest genetic link to glaucoma and cause early onset vision loss for several million individuals worldwide. Mutant myocilin aggregates intracellularly in the endoplasmic reticulum (ER). Subsequent ER stress causes cytotoxicity that hastens dysregulation of intraocular pressure, the primary risk factor for most forms of glaucoma. Our antibody discovery campaign yielded two recombinant antibodies: anti-OLF1 recognizes a linear epitope, while anti-OLF2 is selective for natively folded OLF and inhibits aggregation in vitro. By binding OLF, these antibodies engage autophagy/lysosomal degradation to promote degradation of two pathogenic mutant myocilins. This work demonstrates the potential for therapeutic antibodies to disrupt ER-localized protein aggregates by altering the fate of folding intermediates. This approach could be translated as a precision medicine to treat myocilin-associated glaucoma with in situ antibody expression. More generally, the study supports the approach of enhancing lysosomal degradation to treat proteostasis decline in glaucoma and other diseases.

Extracellular vesicles as emerging players in glaucoma: Mechanisms, biomarkers, and therapeutic targets

In recent years, extracellular vesicles (EVs) have attracted significant scientific interest due to their widespread distribution, their potential as disease biomarkers, and their promising applications in therapy. Encapsulated by lipid bilayers these nanovesicles include small extracellular vesicles (sEV) (30-150 nm), microvesicles (100-1000 nm), and apoptotic bodies (100-5000 nm) and are essential for cellular communication, immune responses, biomolecular transport, and physiological regulation. As they reflect the condition and functionality of their originating cells, EVs play critical roles in numerous physiological processes and diseases. Therefore, EVs offer valuable opportunities for uncovering disease mechanisms, enhancing drug delivery systems, and identifying novel biomarkers. In the context of glaucoma, a leading cause of irreversible blindness, the specific roles of EVs are still largely unexplored. This review examines the emerging role of EVs in the pathogenesis of glaucoma, with a focus on their potential as diagnostic biomarkers and therapeutic agents. Through a thorough analysis of current literature, we summarize key advancements in EV research and identify areas where further investigation is needed to fully understand their function in glaucoma.

Investigation of the site 2 pocket of Grp94 with KUNG65 benzamide derivatives

Glucose-regulated protein 94 (Grp94) is an isoform of the heat shock protein 90 kDa (Hsp90) family of molecular chaperones. Inhibiting Grp94 has been implicated for many diseases. Co-crystal structures of two generations of Grp94 inhibitors revealed the importance of investigating the ester group, which is projected into the site 2 pocket unique to Grp94. Therefore, a series of KUNG65 benzamide analogs was designed and synthesized to evaluate their impact on the affinity and selectivity for Grp94. The data demonstrated that substituents with small and saturated ring systems that contain hydrogen bond acceptors exhibited increased affinity for Grp94, whereas larger saturated ring system manifested increased selectivity for Grp94 over Hsp90α.

The Delayed Turnover of Proteasome Processing of Myocilin upon Dexamethasone Stimulation Introduces the Profiling of Trabecular Meshwork Cells' Ubiquitylome

Glaucoma is chronic optic neuropathy whose pathogenesis has been associated with the altered metabolism of Trabecular Meshwork Cells, which is a cell type involved in the synthesis and remodeling of the trabecular meshwork, the main drainage pathway of the aqueous humor. Starting from previous findings supporting altered ubiquitin signaling, in this study, we investigated the ubiquitin-mediated turnover of myocilin (MYOC/TIGR gene), which is a glycoprotein with a recognized role in glaucoma pathogenesis, in a human Trabecular Meshwork strain cultivated in vitro in the presence of dexamethasone. This is a validated experimental model of steroid-induced glaucoma, and myocilin upregulation by glucocorticoids is a phenotypic marker of Trabecular Meshwork strains. Western blotting and native-gel electrophoresis first uncovered that, in the presence of dexamethasone, myocilin turnover by proteasome particles was slower than in the absence of the drug. Thereafter, co-immunoprecipitation, RT-PCR and gene-silencing studies identified STUB1/CHIP as a candidate E3-ligase of myocilin. In this regard, dexamethasone treatment was found to downregulate STUB1/CHIP levels by likely promoting its proteasome-mediated turnover. Hence, to strengthen the working hypothesis about global alterations of ubiquitin-signaling, the first profiling of TMCs ubiquitylome, in the presence and absence of dexamethasone, was here undertaken by diGLY proteomics. Application of this workflow effectively highlighted a robust dysregulation of key pathways (e.g., phospholipid signaling, β-catenin, cell cycle regulation) in dexamethasone-treated Trabecular Meshwork Cells, providing an ubiquitin-centered perspective around the effect of glucocorticoids on metabolism and glaucoma pathogenesis.

Intraocular pressure across the lifespan of Tg-MYOCY437H mice

Transgenic C57BL/6 mice expressing human myocilinY437 (Tg-MYOCY437H) are a well-established model for primary open-angle glaucoma (POAG). While the reduced trabecular meshwork (TM) cellularity due to severe endoplasmic reticulum (ER) stress has been characterized as the etiology of this model, there is a limited understanding of how glaucomatous phenotypes evolve over the lifespan of Tg-MyocY437H mice. In this study, we compiled the model's intraocular pressure (IOP) data recorded in our laboratory from 2017 to 2023 and selected representative eyes to measure the outflow facility (Cr), a critical parameter indicating the condition of the conventional TM pathway. We found that Tg-MYOCY437H mice aged 4-12 months exhibited significantly higher IOPs than age-matched C57BL/6 mice. Notably, a decline in IOP was observed in Tg-MYOCY437H mice at 17-24 months of age, a phenomenon not attributable to the gene dosage of mutant myocilin. Measurements of the Cr of Tg-MYOCY437H mice indicated that the age-related IOP reduction was not a result of ongoing TM damage. Instead, Hematoxylin and Eosin staining, immunohistochemistry analysis, and transmission electron microscopic examination revealed that this reduction might be induced by degenerations of the non-pigmented epithelium in the ciliary body of aged Tg-MYOCY437H mice. Overall, our findings provide a comprehensive profile of mutant myocilin-induced ocular changes over the Tg-MYOCY437H mouse lifespan and suggest a specific temporal window of elevated IOP that may be ideal for experimental purposes.

Somatic Mutations within Myocilin due to Aging May Be a Potential Risk Factor for Glaucoma

Glaucoma is a chronic optic neuropathy that leads to irreversible vision loss. Aging and family history are the two most important risk factors of glaucoma. One of the most studied genes involved in the onset of open-angle glaucoma is myocilin (MYOC). About 105 germline mutations within MYOC are known to be associated with glaucoma and result in endoplasmic reticulum (ER) stress, which leads to trabecular meshwork (TM) cell death and subsequent intraocular pressure (IOP) elevation. However, only about 4% of the population carry these mutations. An analysis of MYOC somatic cancer-associated mutations revealed a notable overlap with pathogenic glaucoma variants. Because TM cells have the potential to accumulate somatic mutations at a rapid rate due to ultraviolet (UV) light exposure, we propose that an accumulation of somatic mutations within MYOC is an important contributor to the onset of glaucoma.

Serine to proline mutation at position 341 of MYOC impairs trabecular meshwork function by causing autophagy deregulation

Glaucoma is a highly heritable disease, and myocilin was the first identified causal and most common pathogenic gene in glaucoma. Serine-to-proline mutation at position 341 of myocilin (MYOCS341P) is associated with severe glaucoma phenotypes in a five-generation primary open-angle glaucoma family. However, the underlying mechanisms are underexplored. Herein, we established the MYOCS341P transgenic mouse model and characterized the glaucoma phenotypes. Further, we systematically explored the functional differences between wild-type and MYOCS341P through immunoprecipitation, mass spectrometry, and RNA-seq analyses. We found that MYOCS341P transgenic mice exhibit glaucoma phenotypes, characterized by reduced aqueous humor outflow, elevated intraocular pressure, decreased trabecular meshwork (TM) cell number, narrowed Schlemm's canal, retinal ganglion cell loss, and visual impairment. Mechanistically, the secretion of dysfunctional MYOCS341P accumulated in the endoplasmic reticulum (ER), inducing ER stress and dysregulation of autophagy, thereby promoting TM cell death. We describe an effective transgenic model for mechanistic studies and the screening of therapeutic targets. Our data generated from high-throughput analyses help elucidate the mechanism underlying mutant MYOC-related glaucoma.

Genetics and Glaucoma: the state of the art

Glaucoma is the second leading cause of irreversible blindness worldwide. Although genetic background contributes differently to rare early-onset glaucoma (before age 40) or common adult-onset glaucoma, it is now considered an important factor in all major forms of the disease. Genetic and genomic studies, including GWAS, are contributing to identifying novel loci associated with glaucoma or to endophenotypes across ancestries to enrich the knowledge about glaucoma genetic susceptibility. Moreover, new high-throughput functional genomics contributes to defining the relevance of genetic results in the biological pathways and processes involved in glaucoma pathogenesis. Such studies are expected to advance significantly our understanding of glaucoma's genetic basis and provide new druggable targets to treat glaucoma. This review gives an overview of the role of genetics in the pathogenesis or risk of glaucoma.

Endoplasmic reticulum stress: molecular mechanism and therapeutic targets

The endoplasmic reticulum (ER) functions as a quality-control organelle for protein homeostasis, or "proteostasis". The protein quality control systems involve ER-associated degradation, protein chaperons, and autophagy. ER stress is activated when proteostasis is broken with an accumulation of misfolded and unfolded proteins in the ER. ER stress activates an adaptive unfolded protein response to restore proteostasis by initiating protein kinase R-like ER kinase, activating transcription factor 6, and inositol requiring enzyme 1. ER stress is multifaceted, and acts on aspects at the epigenetic level, including transcription and protein processing. Accumulated data indicates its key role in protein homeostasis and other diverse functions involved in various ocular diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, achromatopsia, cataracts, ocular tumors, ocular surface diseases, and myopia. This review summarizes the molecular mechanisms underlying the aforementioned ocular diseases from an ER stress perspective. Drugs (chemicals, neurotrophic factors, and nanoparticles), gene therapy, and stem cell therapy are used to treat ocular diseases by alleviating ER stress. We delineate the advancement of therapy targeting ER stress to provide new treatment strategies for ocular diseases.

Autophagy in glaucoma pathogenesis: Therapeutic potential and future perspectives

Glaucoma is a common blinding eye disease characterized by progressive loss of retinal ganglion cells (RGCs) and their axons, progressive loss of visual field, and optic nerve atrophy. Autophagy plays a pivotal role in the pathophysiology of glaucoma and is closely related to its pathogenesis. Targeting autophagy and blocking the apoptosis of RGCs provides emerging guidance for the treatment of glaucoma. Here, we provide a systematic review of the mechanisms and targets of interventions related to autophagy in glaucoma and discuss the outlook of emerging ideas, techniques, and multidisciplinary combinations to provide a new basis for further research and the prevention of glaucomatous visual impairment.

Degeneration of retina-brain components and connections in glaucoma: Disease causation and treatment options for eyesight preservation

Eyesight is the most important of our sensory systems for optimal daily activities and overall survival. Patients who experience visual impairment due to elevated intraocular pressure (IOP) are often those afflicted with primary open-angle glaucoma (POAG) which slowly robs them of their vision unless treatment is administered soon after diagnosis. The hallmark features of POAG and other forms of glaucoma are damaged optic nerve, retinal ganglion cell (RGC) loss and atrophied RGC axons connecting to various brain regions associated with receipt of visual input from the eyes and eventual decoding and perception of images in the visual cortex. Even though increased IOP is the major risk factor for POAG, the disease is caused by many injurious chemicals and events that progress slowly within all components of the eye-brain visual axis. Lowering of IOP mitigates the damage to some extent with existing drugs, surgical and device implantation therapeutic interventions. However, since multifactorial degenerative processes occur during aging and with glaucomatous optic neuropathy, different forms of neuroprotective, nutraceutical and electroceutical regenerative and revitalizing agents and processes are being considered to combat these eye-brain disorders. These aspects form the basis of this short review article.

A Simple Secretion Assay for Assessing New and Existing Myocilin Variants

PURPOSE

A lack of sufficient functional information exists for appropriately categorizing a large number of myocilin (MYOC) variants and their involvement in primary open angle glaucoma, hindering their clinical significance classification. Most glaucoma-causing MYOC mutations result in protein non-secretion and intracellular insoluble aggregate formation in cultured cells. Herein, we generated a Gaussia luciferase-based MYOC fusion protein to quickly and sensitively track the secretion of MYOC variants and compared these results to the better-established western blotting assay for MYOC.

METHODS

Fourteen clinically-derived MYOC variants with varying degrees of predicted pathogenicity were transfected into HEK-293A cells and analyzed by either a luciferase assay or western blotting.

RESULTS

Eight of the variants (G12R, V53A, T204T, P254L, T325T, D380H, D395_E396insDP, and P481S) had not been biochemically assessed previously. Of these, P254L and D395_E396insDP demonstrated significant secretion defects reminiscent of glaucoma-causing mutations. The luciferase assay results agreed with western blotting for thirteen of the fourteen variants (93%), suggesting a strong concordance.

CONCLUSIONS

These results suggest that the Gaussia luciferase assay may be used as a complementary or standalone assay for quickly assessing MYOC variant behavior and we anticipate that these results will be useful in MYOC variant curation and reclassification.

An Inhibitor of Grp94 Inhibits OxLDL-Induced Autophagy and Apoptosis in VECs and Stabilized Atherosclerotic Plaques

Background: Oxidized low-density lipoprotein (oxLDL) induces vascular endothelial cell (VEC) injury and atherosclerosis through activating endoplasmic reticulum stress. Expression of glucose-regulated protein 94 (Grp94) is induced by endoplasmic reticulum stress and Grp94 is involved in cardiovascular diseases. This study aimed to determine the role of Grp94 in oxLDL-induced vascular endothelial cell injury and atherosclerosis.

Methods and Results: An inhibitor of Grp94, HCP1, was used to investigate the role of Grp94 in oxLDL-induced VEC injury in human umbilical vein endothelial cells and atherosclerosis in apolipoprotein E^−/− mice. Results showed that HCP1 inhibited autophagy and apoptosis induced by oxLDL in VECs. And we found that Grp94 might interact with adenosine monophosphate-activated protein kinase (AMPK) and activate its activity. HCP1 inhibited AMPK activity and overexpression of Grp94 blocked the effect of HCP1. Besides, HCP1 activated the activity of mechanistic target of rapamycin complex 1 (mTORC1), co-treatment with AMPK activator acadesine eliminated the effect of HCP1 on mTORC1 activity as well as autophagy. In apolipoprotein E^−/− mice, HCP1 suppressed autophagy and apoptosis of atherosclerotic plaque endothelium. In addition, HCP1 increased the content of collagen, smooth muscle cells, and anti-inflammatory macrophages while reducing the activity of MMP-2/9 and pro-inflammatory macrophages in the atherosclerotic lesion.

Conclusion: HCP1 inhibited oxLDL-induced VEC injury and promoted the stabilization of atherosclerotic plaque in apoE^−/− mice. Grp94 might be a potential therapeutic target in the clinical treatment of atherosclerosis.

Characterizing the metabolic profile of dexamethasone treated human trabecular meshwork cells

The trabecular meshwork (TM) is the leading site of aqueous humor outflow in the eye and plays a critical role in maintaining normal intraocular pressure. When the TM fails to maintain normal intraocular pressure, glaucoma may develop. Mitochondrial damage has previously been found in glaucomatous TM cells; however, the precise metabolic activity of glaucomatous TM cells has yet to be quantitatively assessed. Using dexamethasone (Dex) treated primary human TM cells to model glaucomatous TM cells, we measure the respiratory and glycolytic activity of Dex-treated TM cells with an extracellular flux assay. We found that Dex-treated TM cells had quantifiably altered metabolic profiles, including increased spare respiratory capacity and ATP production rate from oxidative phosphorylation. Therefore, we propose that reversing or preventing these metabolic changes may represent an avenue for future research.

Association of systemic inflammation indices with visual field loss progression in patients with primary angle-closure glaucoma: potential biomarkers for 3P medical approaches

RELEVANCE

Accumulating evidence suggests a dysfunction of the para-inflammation in the retinal ganglion cell layer and the optic nerve head in patients with glaucoma. Currently, circulating blood platelet-to-lymphocyte ratio (PLR), neutrophil-to-lymphocyte ratio (NLR), and lymphocyte-to-monocyte ratio (LMR) are regarded as novel indicators of systemic inflammation. Biomarkers allow early identification of patients with visual field (VF) loss progression and timely implementation of replacement therapies.

OBJECTIVE

This study aimed to investigate whether higher inflammatory indices (PLR, NLR, and LMR) were associated with VF loss progression in patients with primary angle-closure glaucoma (PACG) for the predictive diagnostics, targeted prevention, and personalization of medical services.

METHODS

This prospective cohort study followed up 277 patients with PACG for at least 24 months, with clinical examination and VF testing every 6 months. Inflammatory cell quantification, including platelets, neutrophils, lymphocytes, and monocytes, was measured using the Sysmex XN-A1 automated inflammatory cells quantification system. Three systemic inflammatory indices, PLR, NLR, and LMR, were determined on the basis of baseline neutrophil, lymphocyte, monocyte, and platelet counts in patients with PACG. The risk factors for PACG were analyzed using logistic regression, Cox proportional hazards regression, and the Kaplan-Meier curve.

RESULTS

Our results revealed that 111 (40.07%) patients showed VF loss progression. The PLR was significantly higher (P = 0.046) in the progression group than in the non-progression group. A higher PLR (OR 1.05, 95% CI 1.01-1.08, P = 0.004) was a risk factor for PACG progression. In multivariate analyses, PLR independently predicted VF loss progression (HR 1.01, 95% CI 1.00-1.01, P = 0.04). Kaplan-Meier curve analysis showed that higher PLR indicated significantly higher rates of VF loss progression (66.91% vs. 52.90%, P = 0.03). Comparable results were observed in the male and female subgroups.

CONCLUSION

Our findings revealed the significant association between a high PLR and a greater risk of VF loss progression in patients with PACG. PLR may be highly recommended as a novel predictive/diagnostic tool for the assessment of VF loss progression from the perspectives of predictive, preventive, and personalized medicine in vulnerable populations and for individual screening.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13167-021-00260-3.

Age at Glaucoma Diagnosis in Germline Myocilin Mutation Patients: Associations with Polymorphisms in Protein Stabilities

Glaucoma is the leading cause of irreversible blindness worldwide, with elevated intraocular pressure (IOP) as the only known modifiable risk factor. Trabecular meshwork (TM)-inducible myocilin (the MYOC gene) was the first to be identified and linked to juvenile and primary open-angle glaucoma. It has been suggested that mutations in the MYOC gene and the aggregation of mutant myocilin in the endoplasmic reticulum (ER) of TM may cause ER stress, resulting in a reduced outflow of aqueous humor and an increase in IOP. We selected 20 MYOC mutations with experimentally determined melting temperatures of mutated myocilin proteins. We included 40 published studies with at least one glaucoma patient with one of these 20 MYOC mutations and information on age at glaucoma diagnosis. Based on data from 458 patients, we found that a statistically significant but weak correlation was present between age and melting temperature based on various assumptions for age. We therefore conclude that genetic analysis of MYOC mutations alone cannot be used to accurately predict age at glaucoma diagnosis. However, it might be an important prognostic factor combined with other clinical factors for critical and early detection of glaucoma.

Therapeutic Drugs and Devices for Tackling Ocular Hypertension and Glaucoma, and Need for Neuroprotection and Cytoprotective Therapies

Damage to the optic nerve and the death of associated retinal ganglion cells (RGCs) by elevated intraocular pressure (IOP), also known as glaucoma, is responsible for visual impairment and blindness in millions of people worldwide. The ocular hypertension (OHT) and the deleterious mechanical forces it exerts at the back of the eye, at the level of the optic nerve head/optic disc and lamina cribosa, is the only modifiable risk factor associated with glaucoma that can be treated. The elevated IOP occurs due to the inability of accumulated aqueous humor (AQH) to egress from the anterior chamber of the eye due to occlusion of the major outflow pathway, the trabecular meshwork (TM) and Schlemm’s canal (SC). Several different classes of pharmaceutical agents, surgical techniques and implantable devices have been developed to lower and control IOP. First-line drugs to promote AQH outflow via the uveoscleral outflow pathway include FP-receptor prostaglandin (PG) agonists (e.g., latanoprost, travoprost and tafluprost) and a novel non-PG EP2-receptor agonist (omidenepag isopropyl, Eybelis^®). TM/SC outflow enhancing drugs are also effective ocular hypotensive agents (e.g., rho kinase inhibitors like ripasudil and netarsudil; and latanoprostene bunod, a conjugate of a nitric oxide donor and latanoprost). One of the most effective anterior chamber AQH microshunt devices is the Preserflo^® microshunt which can lower IOP down to 10–13 mmHg. Other IOP-lowering drugs and devices on the horizon will be also discussed. Additionally, since elevated IOP is only one of many risk factors for development of glaucomatous optic neuropathy, a treatise of the role of inflammatory neurodegeneration of the optic nerve and retinal ganglion cells and appropriate neuroprotective strategies to mitigate this disease will also be reviewed and discussed.

Myocilin-associated Glaucoma: A Historical Perspective and Recent Research Progress

Glaucoma a debilitating disease, is globally the second most common kind of permanent blindness. Primary open-angle glaucoma (POAG) is its most prevalent form and is often linked with alterations in the myocilin gene (MYOC). MYOC encodes the myocilin protein, which is expressed throughout the body, but primarily in trabecular meshwork (TM) tissue in the eyes. TM is principally involved in regulating intraocular pressure (IOP), and elevated IOP is the main risk factor associated with glaucoma. The myocilin protein's function remains unknown; however, mutations compromise its folding and processing inside TM cells, contributing to the glaucoma phenotype. While glaucoma is a complex disease with various molecules and factors as contributing causes, the role played by myocilin has been the most widely studied. The current review describes the present understanding of myocilin and its association with glaucoma and aims to shift the focus toward developing targeted therapies for treating glaucoma patients with variations in MYOC.

Switch to Autophagy the Key Mechanism for Trabecular Meshwork Death in Severe Glaucoma

Purpose

The key differences in cell death mechanisms in the trabecular meshwork (TM) in adult moderate and severe primary glaucoma remain still unanswered. This study explored key differences in cell death mechanisms in the trabecular meshwork (TM) in adult moderate and severe primary glaucoma.

Design

In-vitro laboratory study on surgical specimens and primary cell lines.

Methods

Select cell death-related proteins differentially expressed on mass spectrometric analysis in ex-vivo dissected TM specimens patients with severe adult primary open-angle (POAG) or angle-closure glaucoma (PACG) compared to controls (cadaver donor cornea) were validated for temporal changes in cell death-related gene expression on in-vitro primary human TM cell culture after 48 hours (moderate) or 72 hours (severe) oxidative stress with H₂O₂ (400–1000 uM concentration). These were compared with histone modifications after oxidative stress in human TM (HTM) culture and peripheral blood of patients with moderate and severe glaucoma.

Results

Autophagy-related proteins seemed to be the predominant cell-death mechanism over apoptosis in ex-vivo dissected TM specimens in severe glaucoma. Analyzing HTM cell gene expression at 48 hours and 72 hours of oxidative stress, autophagy genes were up-regulated at 48–72 hours of exposure in contrast to apoptosis-related genes, showing down-regulation at 72 hours. There was associated increased expression of H3K14ac in HTM after 72 hours of oxidative stress and in peripheral blood of severe POAG and PACG.

Conclusion

A preference of autophagy over apoptosis may underlie stage transition from moderate to severe glaucoma in the trabecular meshwork or peripheral blood, which may be tightly regulated by epigenetic modulators.

Common and rare myocilin variants: Predicting glaucoma pathogenicity based on genetics, clinical, and laboratory misfolding data

Rare variants of the olfactomedin domain of myocilin are considered causative for inherited, early-onset open-angle glaucoma, with a misfolding toxic gain-of-function pathogenic mechanism detailed by 20 years of laboratory research. Myocilin variants are documented in the scientific literature and identified through large-scale genetic sequencing projects such as those curated in the Genome Aggregation Database (gnomAD). In the absence of key clinical and laboratory information, however, the pathogenicity of any given variant is not clear, because glaucoma is a heterogeneous and prevalent age-onset disease, and common variants are likely benign. In this review, we reevaluate the likelihood of pathogenicity for the ~100 nonsynonymous missense, insertion-deletion, and premature termination of myocilin olfactomedin variants documented in the literature. We integrate available clinical, laboratory cellular, biochemical and biophysical data, the olfactomedin domain structure, and population genetics data from gnomAD. Of the variants inspected, ~50% can be binned based on a preponderance of data, leaving many of uncertain pathogenicity that motivate additional studies. Ultimately, the approach of combining metrics from different disciplines will likely resolve outstanding complexities regarding the role of this misfolding-prone protein within the context of a multifactorial and prevalent ocular disease, and pave the way for new precision medicine therapeutics.

Dexamethasone Downregulates Autophagy through Accelerated Turn-Over of the Ulk-1 Complex in a Trabecular Meshwork Cells Strain: Insights on Steroid-Induced Glaucoma Pathogenesis

Steroid-induced glaucoma is a severe pathological condition, sustained by a rapidly progressive increase in intraocular pressure (IOP), which is diagnosed in a subset of subjects who adhere to a glucocorticoid (GC)-based therapy. Molecular and clinical studies suggest that either natural or synthetic GCs induce a severe metabolic dysregulation of Trabecular Meshwork Cells (TMCs), an endothelial-derived histotype with phagocytic and secretive functions which lay at the iridocorneal angle in the anterior segment of the eye. Since TMCs physiologically regulate the composition and architecture of trabecular meshwork (TM), which is the main outflow pathway of aqueous humor, a fluid which shapes the eye globe and nourishes the lining cell types, GCs are supposed to trigger a pathological remodeling of the TM, inducing an IOP increase and retina mechanical compression. The metabolic dysregulation of TMCs induced by GCs exposure has never been characterized at the molecular detail. Herein, we report that, upon dexamethasone exposure, a TMCs strain develops a marked inhibition of the autophagosome biogenesis pathway through an enhanced turnover of two members of the Ulk-1 complex, the main platform for autophagy induction, through the Ubiquitin Proteasome System (UPS).

The Potential Role of Small-Molecule PERK Inhibitor LDN-0060609 in Primary Open-Angle Glaucoma Treatment

Primary open-angle glaucoma (POAG) constitutes the most common type of glaucoma. Emerging evidence suggests that Endoplasmic Reticulum (ER) stress and the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-mediated Unfolded Protein Response (UPR) signaling pathway play a key role in POAG pathogenesis. Thus, the main aim of the study was to evaluate the effectiveness of the PERK inhibitor LDN-0060609 in cellular model of glaucoma using primary human trabecular meshwork (HTM) cells. To evaluate the level of the ER stress marker proteins, Western blotting and TaqMan gene expression assay were used. The cytotoxicity was measured by XTT, LDH assays and Giemsa staining, whereas genotoxicity via comet assay. Changes in cell morphology were assessed by phase-contrast microscopy. Analysis of apoptosis was performed by caspase-3 assay and flow cytometry (FC), whereas cell cycle progression by FC. The results obtained have demonstrated that LDN-0060609 triggered a significant decrease of ER stress marker proteins within HTM cells with induced ER stress conditions. Moreover, LDN-0060609 effectively increased viability, reduced DNA damage, increased proliferation, restored normal morphology, reduced apoptosis and restored normal cell cycle distribution of HTM cells with induced ER stress conditions. Thereby, PERK inhibitors, such as LDN-0060609, may provide an innovative, ground-breaking treatment strategy against POAG.

Glaucoma and neuroinflammation: An overview

Glaucoma is an optic neuropathy characterized by well-defined optic disc morphological changes (i.e., cup enlargement, neuroretinal border thinning, and notching, papillary vessel modifications) consequent to retinal ganglion cell loss, axonal degeneration, and lamina cribrosa remodeling. These modifications tend to be progressive and are the main cause of functional damage in glaucoma. Despite the latest findings about the pathophysiology of the disease, the exact trigger mechanisms and the mechanism of degeneration of retinal ganglion cells and their axons have not been completely elucidated. Neuroinflammation may play a role in both the development and the progression of the disease as a result of its effects on retinal environment and retinal ganglion cells. We summarize the latest findings about neuroinflammation in glaucoma and examine the connection between risk factors, neuroinflammation, and retinal ganglion cell degeneration.

Stable calcium-free myocilin olfactomedin domain variants reveal challenges in differentiating between benign and glaucoma-causing mutations

Nonsynonymous gene mutations can be beneficial, neutral, or detrimental to the stability, structure, and biological function of the encoded protein, but the effects of these mutations are often not readily predictable. For example, the β-propeller olfactomedin domain of myocilin (mOLF) exhibits a complex interrelationship among structure(s), stability, and aggregation. Numerous mutations within mOLF are linked to glaucoma; the resulting variants are less stable, aggregation-prone, and sequestered intracellularly, causing cytotoxicity. Here, we report the first stable mOLF variants carrying substitutions in the calcium-binding site that exhibit solution characteristics indistinguishable from those of glaucoma variants. Crystal structures of these stable variants at 1.8-2.0-Å resolution revealed features that we could not predict by molecular dynamics simulations, including loss of loop structure, helix unwinding, and a blade shift. Double mutants that combined a stabilizing substitution and a selected glaucoma-causing single-point mutant rescued in vitro folding and stability defects. In the context of full-length myocilin, secretion of stable single variants was indistinguishable from that of the WT protein, and the double mutants were secreted to varying extents. In summary, our finding that mOLF can tolerate particular substitutions that render the protein stable despite a conformational switch emphasizes the complexities in differentiating between benign and glaucoma-causing variants and provides new insight into the possible biological function of myocilin.

Differential Misfolding Properties of Glaucoma-Associated Olfactomedin Domains from Humans and Mice

Mutations in myocilin, predominantly within its olfactomedin (OLF) domain, are causative for the heritable form of open angle glaucoma in humans. Surprisingly, mice expressing Tyr423His mutant myocilin, corresponding to a severe glaucoma-causing mutation (Tyr437His) in human subjects, exhibit a weak, if any, glaucoma phenotype. To address possible protein-level discrepancies between mouse and human OLFs, which might lead to this outcome, biophysical properties of mouse OLF were characterized for comparison with those of human OLF. The 1.55 Å resolution crystal structure of mouse OLF reveals an asymmetric 5-bladed β-propeller that is nearly indistinguishable from previous structures of human OLF. Wild-type and selected mutant mouse OLFs mirror thermal stabilities of their human OLF counterparts, including characteristic stabilization in the presence of calcium. Mouse OLF forms thioflavin T-positive aggregates with a similar end-point morphology as human OLF, but amyloid aggregation kinetic rates of mouse OLF are faster than human OLF. Simulations and experiments support the interpretation that kinetics of mouse OLF are faster because of a decreased charge repulsion arising from more neutral surface electrostatics. Taken together, phenotypic differences observed in mouse and human studies of mutant myocilin could be a function of aggregation kinetics rates, which would alter the lifetime of putatively toxic protofibrillar intermediates.

Visual Defects and Ageing

Many diseases are related to age, among these neurodegeneration is particularly important. Alzheimer's disease Parkinson's and Glaucoma have many common pathogenic events including oxidative damage, Mitochondrial dysfunction, endothelial alterations and changes in the visual field. These are well known in the case of glaucoma, less in the case of neurodegeneration of the brain. Many other molecular aspects are common, such as the role of endoplasmic reticulum autophagy and neuronal apoptosis while others have been neglected due to lack of space such as inflammatory cytokine or miRNA. Moreover, the loss of specific neuronal populations, the induction of similar mechanisms of cell injury and the deposition of protein aggregates in specific anatomical areas are very similar events between these diseases. Intracellular and/or extracellular accumulation of protein aggregates is a key feature of many neurodegenerative disorders. The existence of abnormal protein aggregates has been documented in the RGCs of glaucomatous patients such as the anomalous Tau protein or the β-amyloid accumulations. Intra-cell catabolic processes also appear to be common in both glaucoma and neurodegeneration. They also help us to understand how the basis between these diseases is common and how the visual aspects can be a serious problem for those who are affected.

Physiological function of myocilin and its role in the pathogenesis of glaucoma in the trabecular meshwork (Review)

Myocilin is highly expressed in the trabecular meshwork (TM), which plays an important role in the regulation of intraocular pressure (IOP). Myocilin abnormalities may cause dysfunction of the TM, potentially leading to increased IOP. High IOP is a well‑known primary risk factor for glaucoma. Myocilin mutations are common among glaucoma patients, and they are implicated in juvenile‑onset open‑angle glaucoma (JOAG) and adult‑onset primary open‑angle glaucoma (POAG). Aggregation of aberrant mutant myocilins is closely associated with glaucoma pathogenesis. The aim of the present review was to discuss the recent findings regarding the major physiological functions of myocilin, such as intra‑ and extracellular proteolytic processes. We also aimed to discuss the risk factors associated with myocilin and the development of glaucoma, such as misfolded/mutant myocilin, imbalance of myocilin and extracellular proteins, and instability of mutant myocilin associated with temperature. Finally, we further outlined certain issues that are yet to be resolved, which may represent the basis for future studies on the role of myocilin in glaucoma.

Trifunctional High-Throughput Screen Identifies Promising Scaffold To Inhibit Grp94 and Treat Myocilin-Associated Glaucoma

Gain-of-function mutations within the olfactomedin (OLF) domain of myocilin result in its toxic intracellular accumulation and hasten the onset of open-angle glaucoma. The absence of myocilin does not cause disease; therefore, strategies aimed at eliminating myocilin could lead to a successful glaucoma treatment. The endoplasmic reticulum Hsp90 paralog Grp94 accelerates OLF aggregation. Knockdown or pharmacological inhibition of Grp94 in cells facilitates clearance of mutant myocilin via a non-proteasomal pathway. Here, we expanded our support for targeting Grp94 over cytosolic paralogs Hsp90α and Hsp90β. We then developed a high-throughput screening assay to identify new chemical matter capable of disrupting the Grp94/OLF interaction. When applied to a blind, focused library of 17 Hsp90 inhibitors, our miniaturized single-read in vitro thioflavin T -based kinetics aggregation assay exclusively identified compounds that target the chaperone N-terminal nucleotide binding site. In follow up studies, one compound (2) decreased the extent of co-aggregation of Grp94 with OLF in a dose-dependent manner in vitro, and enabled clearance of the aggregation-prone full-length myocilin variant I477N in cells without inducing the heat shock response or causing cytotoxicity. Comparison of the co-crystal structure of compound 2 and another non-selective hit in complex with the N-terminal domain of Grp94 reveals a docking mode tailored to Grp94 and explains its selectivity. A new lead compound has been identified, supporting a targeted chemical biology assay approach to develop a protein degradation-based therapy for myocilin-associated glaucoma by selectively inhibiting Grp94.

Structure and Misfolding of the Flexible Tripartite Coiled-Coil Domain of Glaucoma-Associated Myocilin

Glaucoma-associated myocilin is a member of the olfactomedins, a protein family involved in neuronal development and human diseases. Molecular studies of the myocilin N-terminal coiled coil demonstrate a unique tripartite architecture: a Y-shaped parallel dimer-of-dimers with distinct tetramer and dimer regions. The structure of the dimeric C-terminal 7-heptad repeats elucidates an unexpected repeat pattern involving inter-strand stabilization by oppositely charged residues. Molecular dynamics simulations reveal an alternate accessible conformation in which the terminal inter-strand disulfide limits the extent of unfolding and results in a kinked configuration. By inference, full-length myocilin is also branched, with two pairs of C-terminal olfactomedin domains. Selected variants within the N-terminal region alter the apparent quaternary structure of myocilin but do so without compromising stability or causing aggregation. In addition to increasing our structural knowledge of naturally occurring extracellular coiled coils and biomedically important olfactomedins, this work broadens the scope of protein misfolding in the pathogenesis of myocilin-associated glaucoma.

Myocilin Regulates Metalloprotease 2 Activity Through Interaction With TIMP3

Purpose

To elucidate functions of wild-type myocilin, a secreted glycoprotein associated with glaucoma.

Methods

Lysates of mouse eyes were used for immunoprecipitation with affinity-purified antibodies against mouse myocilin. Shotgun proteomic analysis was used for the identification of proteins interacting with myocilin. Colocalization of myocilin and tissue inhibitor of metalloproteinases 3 (TIMP3) in different eye structures was investigated by a multiplex fluorescent in situ hybridization and immunofluorescent labeling with subsequent confocal microscopy. Matrix metalloproteinase 2 (MMP2) activity assay was used to test effects of myocilin on TIMP3 inhibitory action.

Results

TIMP3 was identified by a shotgun proteomic analysis as a protein that was coimmunoprecipitated with myocilin from eye lysates of wild-type and transgenic mice expressing elevated levels of mouse myocilin but not from lysates of transgenic mice expressing mutated mouse myocilin. Interaction of myocilin and TIMP3 was confirmed by coimmunoprecipitation of myocilin and TIMP3 from HEK293 cells transiently transfected with cDNAs encoding these proteins. The olfactomedin domain of myocilin is essential for interaction with TIMP3. In the eye, the main sites of myocilin and TIMP3 colocalization are the trabecular meshwork, sclera, and choroid. Using purified proteins, it has been shown that myocilin markedly enhanced the inhibitory activity of TIMP3 toward MMP2.

Conclusions

Myocilin may serve as a modulator of TIMP3 activity via interactions with the myocilin olfactomedin domain. Our data imply that in the case of MYOCILIN null or some glaucoma-causing mutations, inhibitory activity of TIMP3 toward MMP2 might be reduced, mimicking deleterious mutations in the TIMP3 gene.

Role of nuclear factor (erythroid-derived 2)-like 2 in the age-resistant properties of the glaucoma trabecular meshwork

Glaucoma is a major cause of irreversible blindness. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) regulates the expression of numerous antioxidants within cells and is therefore a focus of current ophthalmic research. To determine the roles of Nrf2 in mediating the glaucoma trabecular meshwork (GTM), the present study evaluated the levels of Nrf2 expression in GTM and human trabecular meshwork (HTM) cells by reverse-transcription-quantitative polymerase chain reaction and western blot analysis. It was principally observed that Nrf2 expression was downregulated in GTM cells. In addition, to determine the influence of Nrf2 on the apoptosis and proliferation of GTM and HTM cells, transfection assays and western blotting were performed to evaluate the expression of apoptosis-related proteins. The results of the current study indicated that Nrf2 may promote viability and reduce apoptosis in GTM and HTM cells. Collectively, these data suggest that Nrf2 may be a novel therapeutic target to treat glaucoma.

A Novel Luciferase Assay For Sensitively Monitoring Myocilin Variants in Cell Culture

Purpose

Primary open angle glaucoma–associated mutations in myocilin (MYOC) cause protein “nonsecretion,” rendering secreted MYOC difficult to detect using conventional techniques. This study focused on developing and using an assay that can quickly and easily detect mutant MYOC secretion.

Methods

We fused Gaussia luciferase (eGLuc2) to MYOC variants and expressed the constructs in HEK-293T and NTM-5 cells. Secreted and intracellular levels of MYOC eGLuc2 variants were evaluated by Western blotting and compared to untagged and FLAG-tagged MYOC constructs. Secreted and soluble intracellular MYOC eGLuc2 were measured by a GLuc assay. The secretion of nine additional MYOC mutants was assayed in conditioned media from transfected cells to test the applicability of the assay for monitoring other MYOC variants.

Results

Myocilin eGLuc2 behaved similarly to untagged and FLAG-tagged MYOC with respect to secretion, soluble intracellular levels, and in response to drug treatment. The GLuc assay could sensitively detect Y437H MYOC secretion 30 minutes after media change. Gaussia luciferase fused variants followed anticipated trends; nonpathogenic variants (D208E, G244V) were secreted at wild-type (WT) levels, whereas predicted disease-causing variants (C245Y, G246R, E300K, Y437H, I477N) demonstrated substantial secretion defects. Secretion defects caused by the C245Y, G246R, and Y437H mutations were partially rescued by permissive growth temperature. Interestingly, however, this increase in secretion was independent of newly synthesized protein.

Conclusions

Fusion of eGLuc2 to MYOC does not significantly change the behavior of MYOC. This newly developed MYOC reporter system can be used to study engineered MYOC variants and potentially to identify modulators of MYOC secretion and function.

Working your SOCS off: The role of ASB10 and protein degradation pathways in glaucoma

Evidence is accumulating to suggest that mutations in the Ankyrin and SOCS Box-containing protein-10 (ASB10) gene are associated with glaucoma. Since its identification in a large Oregon family with primary open-angle glaucoma (POAG), ASB10 variants have been associated with disease in US, German and Pakistani cohorts. ASB10 is a member of the ASB family of proteins, which have a common structure including a unique N-terminus, a variable number of central ankyrin (ANK) repeat domains and a suppressor of cytokine signaling (SOCS) box at the C-terminus. Mutations in ASB10 are distributed throughout the entire length of the gene including the two alternatively spliced variants of exon 1. A homozygous mutation in a Pakistani individual with POAG, which lies in the center of the SOCS box, is associated with a particularly severe form of the disease. Like other SOCS box-containing proteins, ASB10 functions in ubiquitin-mediated degradation pathways. The ANK repeats bind to proteins destined for degradation. The SOCS box recruits ubiquitin ligase proteins to form a complex to transfer ubiquitin to a substrate bound to the ANK repeats. The ubiquitin-tagged protein then enters either the proteasomal degradation pathway or the autophagic-lysosomal pathway. The choice of pathway appears to be dependent on which lysine residues are used to build polyubiquitin chains. However, these reciprocal pathways work in tandem to degrade proteins because inhibition of one pathway increases degradation via the other pathway. In this publication, we will review the literature that supports identification of ASB10 as a glaucoma-associated gene and the current knowledge of the function of the ASB10 protein. In addition, we present new data that indicates ASB10 expression is up-regulated by the inflammatory cytokines tumor necrosis factor-α and interleukin-1α. Finally, we will describe the emerging role of other SOCS box-containing proteins in protein degradation pathways in ocular cells.

Development of Glucose Regulated Protein 94-Selective Inhibitors Based on the BnIm and Radamide Scaffold

Glucose regulated protein 94 (Grp94) is the endoplasmic reticulum resident of the heat shock protein 90 kDa (Hsp90) family of molecular chaperones. Grp94 associates with many proteins involved in cell adhesion and signaling, including integrins, Toll-like receptors, immunoglobulins, and mutant myocilin. Grp94 has been implicated as a target for several therapeutic areas including glaucoma, cancer metastasis, and multiple myeloma. While 85% identical to other Hsp90 isoforms, the N-terminal ATP-binding site of Grp94 possesses a unique hydrophobic pocket that was used to design isoform-selective inhibitors. Incorporation of a cis-amide bioisostere into the radamide scaffold led to development of the original Grp94-selective inhibitor, BnIm. Structure-activity relationship studies have now been performed on the aryl side chain of BnIm, which resulted in improved analogues that exhibit better potency and selectivity for Grp94. These analogues also manifest superior antimigratory activity in a metastasis model as well as enhanced mutant myocilin degradation in a glaucoma model compared to BnIm.