The effects of myocilin expression on functionally relevant trabecular meshwork genes: a mini-review

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.

Genes as drugs for glaucoma: latest advances

PURPOSE OF REVIEW

To provide the latest advances on the future use of gene therapy for the treatment of glaucoma.

RECENT FINDINGS

In preclinical studies, a number of genes have been shown to be able to reduce elevated intraocular pressure (IOP), and to exert neuroprotection of the retinal ganglion cells. These genes target various mechanisms of action and include among others: MMP3 , PLAT, IκB, GLIS, SIRT, Tie-2, AQP1. Some of these as well as some previously identified genes ( MMP3, PLAT, BDNF, C3, TGFβ, MYOC, ANGPTL7 ) are starting to move onto drug development. At the same time, progress has been made in the methods to deliver and control gene therapeutics (advances in these areas are not covered in this review).

SUMMARY

While preclinical efforts continue in several laboratories, an increasing number of start-up and large pharmaceutical companies are working on developing gene therapeutics for glaucoma ( Sylentis, Quetera/Astellas, Exhaura, Ikarovec, Genentech, Regeneron, Isarna, Diorasis Therapeutics ). Despite the presence of generic medications to treat glaucoma, given the size of the potential world-wide market (∼$7B), it is likely that the number of companies developing glaucoma gene therapies will increase further in the near future.

A temporal single cell transcriptome atlas of zebrafish anterior segment development

Anterior segment dysgenesis (ASD), resulting in vision impairment, stems from maldevelopment of anterior segment (AS) tissues. Incidence of ASD has been linked to malfunction of periocular mesenchyme cells (POM). POM cells specify into anterior segment mesenchyme (ASM) cells which colonize and produce AS tissues. In this study we uncover ASM developmental trajectories associated with formation of the AS. Using a transgenic line of zebrafish that fluorescently labels the ASM throughout development, Tg[foxc1b:GFP], we isolated GFP+ ASM cells at several developmental timepoints (48-144 hpf) and performed single cell RNA sequencing. Clustering analysis indicates subdifferentiation of ASM as early as 48 hpf and subsequent diversification into corneal epithelium/endothelium/stroma, or annular ligament (AL) lineages. Tracking individual clusters reveals common developmental pathways, up to 72 hpf, for the AL and corneal endothelium/stroma and distinct pathways for corneal epithelium starting at 48 hpf. Spatiotemporal validation of over 80 genes found associated with AS development demonstrates a high degree of conservation with mammalian trabecular meshwork and corneal tissues. In addition, we characterize thirteen novel genes associated with annular ligament and seven with corneal development. Overall, the data provide a molecular verification of the long-standing hypothesis that POM derived ASM give rise to AS tissues and highlight the high degree of conservation between zebrafish and mammals.

Case Report: Reversal and subsequent return of optic disc cupping in a myocilin (MYOC) gene-associated severe Juvenile Open-Angle Glaucoma (JOAG) patient

To our knowledge, this case report describes the first instance of reversal of glaucomatous optic nerve cupping in a young adult with a rare form of juvenile open-angle glaucoma (JOAG) associated with a novel variant of the myocilin gene (MYOC). This 25-year-old woman with severe-stage MYOC-associated JOAG presented with blurry vision and intermittent pain in her left eye. She had a strong family history of glaucoma in multiple first-degree relatives with an identified novel variant of MYOC. Examination revealed intraocular pressures (IOPs) of 10 mmHg OD and 46 mmHg OS, with cup-to-disc ratios of 0.90 and 0.80. The patient experienced substantial reversal of optic disc cupping OS following dramatic IOP reduction with trabeculectomy, and subsequently experienced a return of cupping after an IOP spike 15 months postoperatively. The reversal of cupping did not correspond to any changes in the patient's visual field. After an initial decrease in retinal nerve fiber layer (RNFL) thickness, RNFL remained stable for over 2 years after trabeculectomy as seen on Optical Coherence Tomography (OCT). This case suggests reversal of cupping can occur well into adulthood in a MYOC-associated JOAG patient, and it demonstrates the potential bidirectionality of this phenomenon. Moreover, it suggests that these structural changes may not correspond to any functional changes in visual fields or RNFL thickness.

Amyloid fibrillation of the glaucoma associated myocilin protein is inhibited by epicatechin gallate (ECG)

Inherited glaucoma is a recent addition to the inventory of diseases arising due to protein misfolding. Mutations in the olfactomedin (OLF) domain of myocilin are the most common genetic cause behind this disease. Disease associated variants of m-OLF are predisposed to misfold and aggregate in the trabecular meshwork (TM) tissue of the eye. In recent years, the nature of these aggregates was revealed to exhibit the hallmarks of amyloids. Amyloid aggregates are highly stable structures that are formed, often with toxic consequences in a number of debilitating diseases. In spite of its clinical relevance the amyloidogenic nature of m-OLF has not been studied adequately. Here we have studied the amyloid fibrillation of m-OLF and report ECG as an inhibitor against it. Using biophysical and biochemical assays, coupled with advanced microscopic evaluations we show that ECG binds and stabilizes native m-OLF and thus prevents its aggregation into amyloid fibrils. Furthermore, we have used REMD simulations to delineate the stabilizing effects of ECG on the structure of m-OLF. Collectively, we report ECG as a molecular scaffold for designing and testing of novel inhibitors against m-OLF amyloid fibrillation.

Short and long-term effect of dexamethasone on the transcriptome profile of primary human trabecular meshwork cells in vitro

In the quest of identifying newer molecular targets for the management of glucocorticoid-induced ocular hypertension (GC-OHT) and glaucoma (GCG), several microarray studies have attempted to investigate the genome-wide transcriptome profiling of primary human trabecular meshwork (TM) cells in response to dexamethasone (DEX). However, no studies are reported so far to demonstrate the temporal changes in the expression of genes in the cultured human TM cells in response to DEX treatment. Therefore, in the present study, the time-dependent changes in the genome-wide expression of genes in primary human TM cells after short (16 hours: 16 h) and long exposure (7 days: 7 d) of DEX was investigated using RNA sequencing. There were 199 (118 up-regulated; 81 down-regulated) and 525 (119 up-regulated; 406 down-regulated) DEGs in 16 h and 7 d treatment groups respectively. The unique genes identified in 16 h and 7 d treatment groups were 152 and 478 respectively. This study found a distinct gene signature and pathways between two treatment regimes. Longer exposure of DEX treatment showed a dys-regulation of Wnt and Rap1 signaling and so highlighted potential therapeutic targets for pharmacological management of GC-OHT/glaucoma.

Clusterin, other extracellular chaperones, and eye disease

Proteostasis refers to all the processes that maintain the correct expression level, location, folding and turnover of proteins, essential to organismal survival. Both inside cells and in body fluids, molecular chaperones play key roles in maintaining proteostasis. In this article, we focus on clusterin, the first-recognized extracellular mammalian chaperone, and its role in diseases of the eye. Clusterin binds to and inhibits the aggregation of proteins that are misfolded due to mutations or stresses, clears these aggregating proteins from extracellular spaces, and facilitates their degradation. Clusterin exhibits three main homeostatic activities: proteostasis, cytoprotection, and anti-inflammation. The so-called "protein misfolding diseases" are caused by aggregation of misfolded proteins that accumulate pathologically as deposits in tissues; we discuss several such diseases that occur in the eye. Clusterin is typically found in these deposits, which is interpreted to mean that its capacity as a molecular chaperone to maintain proteostasis is overwhelmed in the disease state. Nevertheless, the role of clusterin in diseases involving such deposits needs to be better defined before therapeutic approaches can be entertained. A more straightforward case can be made for therapeutic use of clusterin based on its proteostatic role as a proteinase inhibitor, as well as its cytoprotective and anti-inflammatory properties. It is likely that clusterin works together in this way with other extracellular chaperones to protect the eye from disease, and we discuss several examples. We end this article by predicting future steps that may lead to development of clusterin as a biological drug.

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.

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.

Tissue plasminogen activator attenuates outflow facility reduction in mouse model of juvenile open angle glaucoma

Tissue plasminogen activator (tPA) has been shown to prevent steroid-induced reduction in aqueous humor outflow facility via an upregulation in matrix metalloproteinase (Mmp) expression. The purpose of this study was to determine whether tPA can rescue outflow facility reduction in the Tg-MYOC^Y437H mouse model, which replicates human juvenile open angle glaucoma. Outflow facility was measured in Tg-MYOC^Y437H mice following: periocular steroid exposure and intraocular protein treatment with enzymatically active or enzymatically inactive tPA. Effects of tPA on outflow facility were compared to those of animals treated with topical sodium phenylbutarate (PBA), a modulator of endoplasmic reticulum stress. Gene expression of fibrinolytic pathway components (Plat, Plau, and Pai-1) and matrix metalloproteinases (Mmp-2, -9, and -13) was determined in angle ring tissues containing the trabecular meshwork. Tg-MYOC^Y437H mice did not display further outflow facility reduction following steroid exposure. Enzymatically active and enzymatically inactive tPA were equally effective in attenuating outflow facility reduction in Tg-MYOC^Y437H mice and caused enhanced expression of matrix metalloproteinases (Mmp-9 and Mmp-13). tPA was equally effective to topical PBA treatment in ameliorating outflow facility reduction in Tg-MYOC^Y437H mice. Both treatments were associated with an upregulation in Mmp-9 expression while tPA also upregulated Mmp-13 expression. tPA increases the expression of matrix metalloproteinases and may cause extracellular matrix remodeling at the trabecular meshwork, which results in reversal of outflow facility reduction in Tg-MYOC^Y437H mice.

A Biomimetic, Stem Cell-Derived In Vitro Ocular Outflow Model

Age-related human trabecular meshwork (HTM) cell loss is suggested to affect its ability to regulate aqueous humor outflow in the eye. In addition, disease-related HTM cell loss is suggested to lead to elevated intraocular pressure in glaucoma. Induced pluripotent stem cell (iPSC)-derived trabecular meshwork (TM) cells are promising autologous cell sources that can be used to restore the declining TM cell population and function. Previously, an in vitro HTM model is bioengineered for understanding HTM cell biology and screening of pharmacological or biological agents that affect trabecular outflow facility. In this study, it is demonstrated that human iPSC-derived TM cells cultured on SU-8 scaffolds exhibit HTM-like cell morphology, extracellular matrix deposition, and drug responsiveness to dexamethasone treatment. These findings suggest that iPSC-derived TM cells behave like primary HTM cells and can thus serve as reproducible and scalable cell sources when using this in vitro system for glaucoma drug screening and further understanding of outflow pathway physiology, leading to personalized medicine.

What information can be obtained from the tears of a patient with primary open angle glaucoma?

Since tears are a biological fluid, they have a potential diagnostic value for ophthalmic diseases. The aim of this study was to compare tear supernatants and pellets obtained from patients suffering from primary open angle glaucoma (POAG) and healthy persons (HPs) using transmission electron microscopy (TEM) and molecular biological methods. Tear supernatants and pellets were prepared using ultrafiltration and ultracentrifugation and were examined by negative staining and immunogold labelling TEM. DNA of the pellets was isolated, quantified and sequenced using a MiSeq (Illumina, USA) genomic sequencer with the Reagent Kit v3 (600 cycles, Illumina, USA). MicroRNA was isolated and quantified from the pellets; miR-146b, miR-16 and miR-126 were detected using TaqMan MicroRNA Assays (Applied Biosystems, USA). TEM of tear supernatants from both POAG patients and HPs revealed identical constituents: spherical or cup-shaped vesicles, "non-vesicles", cell debris and macromolecular aggregates. Pellets of POAG patients and HPs contained small extracellular vesicles (sEVs) non-labelled vesicles and "non-vesicles"; pellets of sick persons also contained sEVs with "a capsule". POAG-patient tear pellets showed elevated concentrations of genomic ds-DNA and SINE-repeats, and different expressions of miR-146b, miR-16 and miR-126 and a different set of bacterial DNA in comparison with pellets obtained from the tears of HPs. The data obtained indicate that the tears of HPs and POAG patients could serve as an object for TEM studies and as a source of sEV-containing preparations (pellets), which, in turn, could be used for the isolation and study of genomic ds-DNA and RNA. Our data provide the basis for using tears for diagnostic applications.

An In-Depth View of the Porcine Trabecular Meshwork Proteome

The house swine (Sus scrofa domestica Linnaeus 1758) is an important model organism regarding the study of neurodegenerative diseases, especially ocular neuropathies such as glaucoma. This is due to the high comparability of the porcine and human eye regarding anatomy and molecular features. In the pathogenesis of glaucoma, the trabecular meshwork (TM) forms a key ocular component in terms of intraocular pressure (IOP) elevation. Thereby, functional TM abnormalities are correlated with distinct proteomic alterations. However, a detailed analysis of the TM proteome has not been realized so far. Since the porcine eye has high potential as a model system to study ocular diseases such as glaucoma, the present study focuses on the in-depth analysis of the porcine TM proteome. By use of a bottom-up (BU) mass spectrometric (MS) platform utilizing electrospray ionization liquid chromatography tandem MS (LC-ESI-MS/MS) considering database-dependent and peptide de novo sequencing, more than 3000 TM proteins were documented with high confidence (FDR < 1%). A distinct number of proteins with neuronal association were revealed. To the best to our knowledge, many of these protein species have not been reported for TM tissue before such as reelin, centlein and high abundant neuroblast differentiation-associated protein AHNAK (AHNAK). Thereby, AHNAK might play a superordinate role in the TM regarding proposed tissue involvement in barrier function. Also, a high number of secretory proteins could be identified. The generated TM proteomic landscape underlines a multifunctional character of the TM beyond representing a simple drainage system. Finally, the protein catalogue of the porcine TM provides an in-depth view of the TM molecular landscape and will serve as an important reference map in terms of glaucoma research utilizing porcine animal models, porcine TM tissues and/or cultured TM cells.

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.

Pro-fibrotic pathway activation in trabecular meshwork and lamina cribrosa is the main driving force of glaucoma

While primary open-angle glaucoma (POAG) is a leading cause of blindness worldwide, it still does not have a clear mechanism that can explain all clinical cases of the disease. Elevated IOP is associated with increased accumulation of extracellular matrix (ECM) proteins in the trabecular meshwork (TM) that prevents normal outflow of aqueous humor (AH) and has damaging effects on the fine mesh-like lamina cribrosa (LC) through which the optic nerve fibers pass. Applying a pathway analysis algorithm, we discovered that an elevated level of TGFβ observed in glaucoma-affected tissues could lead to pro-fibrotic pathway activation in TM and in LC. In turn, activated pro-fibrotic pathways lead to ECM remodeling in TM and LC, making TM less efficient in AH drainage and making LC more susceptible to damage from elevated IOP via ECM transformation in LC. We propose pathway targets for potential therapeutic interventions to delay or avoid fibrosis initiation in TM and LC tissues.

Steroid-induced ocular hypertension/glaucoma: Focus on pharmacogenomics and implications for precision medicine

Elevation of intraocular pressure (IOP) due to therapeutic use of glucocorticoids is called steroid-induced ocular hypertension (SIOH); this can lead to steroid-induced glaucoma (SIG). Glucocorticoids initiate signaling cascades ultimately affecting expression of hundreds of genes; this provides the potential for a highly personalized pharmacological response. Studies attempting to define genetic risk factors were undertaken early in the history of glucocorticoid use, however scientific tools available at that time were limited and progress stalled. In contrast, significant advances were made over the ensuing years in defining disease pathophysiology. As the genomics age emerged, it appeared the time was right to renew investigation into genetics. Pharmacogenomics is an unbiased discovery approach, not requiring an underlying hypothesis, and provides a way to pinpoint clinically significant genes and pathways that could not have been discovered any other way. Results of the first genome-wide association study to identify polymorphisms associated with SIOH, and follow-up on two novel genes linked to the disorder, GPR158 and HCG22, is discussed in the second half of the article. However, knowledge of genetic variants determining response to steroids in the eye also has value in its own right as a predictive and diagnostic tool. This article concludes with a discussion of how the Precision Medicine Initiative®, announced by U.S. President Obama in his 2015 State of the Union address, is beginning to touch the practice of ophthalmology. It is argued that SIOH/SIG may provide one of the next opportunities for effective application of precision medicine.

From DNA damage to functional changes of the trabecular meshwork in aging and glaucoma

Glaucoma is a degenerative disease of the eye. Both the anterior and posterior segments of the eye are affected, extensive damage being detectable in the trabecular meshwork and the inner retina-central visual pathway complex. Oxidative stress is claimed to be mainly responsible for molecular damage in the anterior chamber. Indeed, oxidation harms the trabecular meshwork, leading eventually to endothelial cell decay, tissue malfunction, subclinical inflammation, changes in the extracellular matrix and cytoskeleton, altered motility, reduced outflow facility and (ultimately) increased IOP. Moreover, free radicals are involved in aging and can be produced in the brain (as well as in the eye) as a result of ischemia, leading to oxidation of the surrounding neurons. Glaucoma-related cell death occurs by means of apoptosis, and apoptosis is triggered by oxidative stress via (a) mitochondrial damage, (b) inflammation, (c) endothelial dysregulation and dysfunction, and (d) hypoxia. The proteomics of the aqueous humor is significantly altered in glaucoma as a result of oxidation-induced trabecular damage. Those proteins whose aqueous humor levels are increased in glaucoma are biomarkers of trabecular meshwork impairment. Their diffusion from the anterior to the posterior segment of the eye may be relevant in the cascade of events triggering apoptosis in the inner retinal layers, including the ganglion cells.

Glaucoma related Proteomic Alterations in Human Retina Samples

Glaucoma related proteomic changes have been documented in cell and animal models. However, proteomic studies investigating on human retina samples are still rare. In the present work, retina samples of glaucoma and non-glaucoma control donors have been examined by a state-of-the-art mass spectrometry (MS) workflow to uncover glaucoma related proteomic changes. More than 600 proteins could be identified with high confidence (FDR < 1%) in human retina samples. Distinct proteomic changes have been observed in 10% of proteins encircling mitochondrial and nucleus species. Numerous proteins showed a significant glaucoma related level change (p < 0.05) or distinct tendency of alteration (p < 0.1). Candidates were documented to be involved in cellular development, stress and cell death. Increase of stress related proteins and decrease of new glaucoma related candidates, ADP/ATP translocase 3 (ANT3), PC4 and SRFS1-interacting protein 1 (DFS70) and methyl-CpG-binding protein 2 (MeCp2) could be documented by MS. Moreover, candidates could be validated by Accurate Inclusion Mass Screening (AIMS) and immunostaining and supported for the retinal ganglion cell layer (GCL) by laser capture microdissection (LCM) in porcine and human eye cryosections. The workflow allowed a detailed view into the human retina proteome highlighting new molecular players ANT3, DFS70 and MeCp2 associated to glaucoma.

Glaucomatous MYOC mutations activate the IL-1/NF-κB inflammatory stress response and the glaucoma marker SELE in trabecular meshwork cells

PURPOSE

Activation of the IL-1/NF-κB inflammatory stress pathway and induction of SELE expression in the trabecular meshwork (TBM) is a marker for high-tension glaucomas of diverse etiology. Pathway activation stimulates aqueous outflow and protects against oxidative stress, but may be damaging in the long-term. MYOC mutations have been causally linked to high-tension forms of primary open angle glaucoma (POAG). This study investigated a possible link between MYOC mutations and activation of the IL-1/NF-κB pathway and expression of SELE.

METHODS

We constructed MYOC expression vectors with mutations at sites that cause POAG. Mutations (Q368X, Y437H, A427T) were selected to represent proteins with differing POAG-causing potency (Q368X > Y437H > A427T) and intracellular retention behavior (Q368X and Y437H retained, A427T released). The constructs were made in two different kinds of vectors; one a plasmid designed for transient transfection (pCMV6), and one a doxycycline-inducible lentiviral vector (pSLIK) for stable cell transduction. The immortalized human trabecular meshwork line TM-1 was used for all expression studies. Expression of IL1A mRNA was determined by reverse transcription (RT)-PCR, as well as a set of five other genes associated with signaling pathways linked to glaucoma: IL1B and IL6 (NF-κB pathway), TGFB2 and ACTA2 (TGF-β pathway) and FOXO1 (E2F1 apoptotic pathway). An ELISA was used to quantify IL1A protein released into culture media. To quantify intracellular NF-κB activity, we transiently transfected stably transduced cell lines with a luciferase expression vector under control of the IL8 promoter (containing an NF-κB response element).

RESULTS

Transiently expressed wild-type MYOC was released into cell culture media, whereas mutant MYOCs Q368X and Y437H remained within cells. Both mutant MYOCs activated the IL-1/ NF-κB pathway, significantly stimulating expression of IL1A and IL1B. However Y437H, which causes a severe glaucoma phenotype, was less effective than Q368X, which causes a moderate glaucoma phenotype. In addition, the retained mutants stimulated expression of stress response genes ACTA2 and FOXO1. Unexpectedly, wild-type MYOC significantly decreased expression of IL6 and TGFB2, to approximately half of the control levels, and expression of IL1B and ACTA2 was also slightly decreased. Induction of MYOC mutants Q368X and Y437H in stably transduced cell lines significantly stimulated the level of IL1A protein released into culture media. Once again however, the effect of the severe MYOC mutant Y437H was less than the effect of the moderate MYOC mutant Q368X. In contrast, induced expression of the intracellularly retained mutant MYOC A427T or wild-type MYOC did not change the amount of IL1A protein in culture media. Induction of Y437H MYOC plus IL1A treatment increased NF-κB activity by 25% over IL1A alone. In contrast, induction of Q368X or A427T plus IL1A treatment did not significantly affect NF-κB activity over IL1A alone. However, wild-type MYOC expression inhibited IL1A-stimulated NF-κB activity. We also observed that endogenous MYOC expression was induced by IL1A in TM-1 cells and primary TBM cell cultures. SELE was co-expressed with MYOC in the primary cell lines.

CONCLUSIONS

These results indicate that POAG-causing MYOC mutants activate the IL-1/NF-κB pathway, with activation levels correlated with intracellular retention of the protein, but not POAG-causing potency. Unexpectedly, it was also discovered that wild-type MYOC inhibits activation of the IL-1/NF-κB pathway, and that activation of the IL-1/NF-κB pathway stimulates expression of MYOC. This is the first evidence that glaucoma-causing MYOC mutants can activate the inflammatory response and that wild-type MYOC has anti-inflammatory activity.

Extracellular matrix in the trabecular meshwork: intraocular pressure regulation and dysregulation in glaucoma

The trabecular meshwork (TM) is located in the anterior segment of the eye and is responsible for regulating the outflow of aqueous humor. Increased resistance to aqueous outflow causes intraocular pressure to increase, which is the primary risk factor for glaucoma. TM cells reside on a series of fenestrated beams and sheets through which the aqueous humor flows to exit the anterior chamber via Schlemm's canal. The outer trabecular cells are phagocytic and are thought to function as a pre-filter. However, most of the outflow resistance is thought to be from the extracellular matrix (ECM) of the juxtacanalicular region, the deepest portion of the TM, and from the inner wall basement membrane of Schlemm's canal. It is becoming increasingly evident that the extracellular milieu is important in maintaining the integrity of the TM. In glaucoma, not only have ultrastructural changes been observed in the ECM of the TM, and a significant number of mutations in ECM genes been noted, but the stiffness of glaucomatous TM appears to be greater than that of normal tissue. Additionally, TGFβ2 has been found to be elevated in the aqueous humor of glaucoma patients and is assumed to be involved in ECM changes deep with the juxtacanalicular region of the TM. This review summarizes the current literature on trabecular ECM as well as the development and function of the TM. Animal models and organ culture models targeting specific ECM molecules to investigate the mechanisms of glaucoma are described. Finally, the growing number of mutations that have been identified in ECM genes and genes that modulate ECM in humans with glaucoma are documented.