Expression of myocilin mutants sensitizes cells to oxidative stress-induced apoptosis: implication for glaucoma pathogenesis

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.

Structure‒function‒pathogenicity analysis of C-terminal myocilin missense variants based on experiments and 3D models

MYOC is a common pathogenic gene for primary open-angle glaucoma and encodes the protein named myocilin. Multiple MYOC variations have been found, with different clinical significance. However, the pathogenesis of glaucoma induced by MYOC mutations has not been fully clarified. Here, we analyze the molecular and cellular biological differences caused by multiple variant myocilins, including protein secretion characteristics, structural changes, subcellular localization, cellular autophagic activity and oxidative stress. Denaturing and nondenaturing electrophoresis showed myocilin to be a secreted protein with the tendency to self-oligomerize. The full-length myocilin and its C-terminal cleavage fragment are secreted. Secretion analysis of 23 variant myocilins indicated that secretion defects are closely related to the pathogenicity of MYOC variants. Structural analysis showed that the alteration of steric clash is associated with the secretion characteristics and pathogenicity of myocilin variants. Immunocytochemistry results demonstrated that mutated myocilins are retained in the endoplasmic reticulum and disrupt autophagy. MTT assay, MitoTracker staining, and DCFH-DA staining showed increased oxidative injury in cells expressing MYOC mutants. Taken together, MYOC mutations are able to induce cell dysfunction via secretion defects and intracellular accumulation resulting from steric clash alterations.

Calcium dysregulation potentiates wild-type myocilin misfolding: implications for glaucoma pathogenesis

Myocilin is secreted from trabecular meshwork cells to an eponymous extracellular matrix that is critical for maintaining intraocular pressure. Missense mutations found in the myocilin olfactomedin domain (OLF) lead to intracellular myocilin misfolding and are causative for the heritable form of early-onset glaucoma. The OLF domain contains a unique internal, hetero-dinuclear calcium site. Here, we tested the hypothesis that calcium dysregulation causes wild-type (WT) myocilin misfolding reminiscent of that observed for disease variants. Using two cellular models expressing WT myocilin, we show that the Ca2+ ATPase channel blocker thapsigargin inhibits WT myocilin secretion. Intracellular WT myocilin is at least partly insoluble and aggregated in the endoplasmic reticulum (ER), and stains positively with an amyloid dye. By comparing the effect of thapsigargin on WT myocilin to that on a de novo secretion-competent Ca2+-free variant D478S, we discern that non-secretion of WT myocilin is due initially to calcium dysregulation, and is potentiated further by resultant ER stress. In E. coli, depletion of calcium leads to recombinant expression of misfolded isolated WT OLF but the D478S variant is still produced as a folded monomer. Treatment of cells expressing a double mutant composed of D478S and either disease variants P370L or Y437H with thapsigargin promotes its misfolding and aggregation, demonstrating the limits of D478S to correct secretion defects. Taken together, the heterodinuclear calcium site is a liability for proper folding of myocilin. Our study suggests a molecular mechanism by which WT myocilin misfolding may contribute broadly to glaucoma-associated ER stress. This study explores the effect of calcium depletion on myocilin olfactomedin domain folding.

Analysis of Glaucoma Associated Genes in Response to Inflammation, an Examination of a Public Data Set Derived from Peripheral Blood from Patients with Hepatitis C

Introduction

Glaucoma is the second leading cause of blindness worldwide and despite its prevalence, there are still many unanswered questions related to its pathogenesis. There is evidence that oxidative stress and inflammation play a major role in disease progression. Glaucoma patients from several studies showed altered gene expression in leukocytes, revealing the possibility of using peripheral biomarkers to diagnose or stage glaucoma. The fact that glaucoma is associated with gene expression changes in tissues distant from the retina underscores the possible involvement of systemic oxidative stress and inflammation as potential contributing or compounding factors in glaucoma.

Methods

We assembled a list of oxidative stress and inflammatory markers related to glaucoma based on a review of the literature. In addition, we utilized publicly available data sets of gene expression values collected from peripheral blood mononuclear cells and macrophages from two patient groups: those chronically infected by the hepatitis C virus and those who have cleared it. Activation of the innate immune response can render cells or tissues more responsive to a second delayed proinflammatory stimulus. Additional gene expression data from these cells after subsequent polyinosinic:polycytidylic acid treatment, used to elicit an acute inflammatory response, allowed for the investigation of the acute inflammatory response in these groups. We used fold-change comparison values between the two patient groups to identify genes of interest.

Results

A comparison analysis identified 17 glaucoma biomarkers that were differentially expressed in response to HCV-mediated inflammation. Of these 17, six had significant p-values in the baseline vs treated values. Expression data of these genes were compared between patients who had cleared the Hepatitis C virus versus those who had not and identified three genes of interest for further study.

Discussion

These results support our hypothesis that inflammation secondary to Hepatitis C virus infection affects the expression of glaucoma biomarker genes related to the antioxidant response and inflammation. In addition, they provide several potential targets for further research into understanding the relationship between innate responses to viral infection and inflammatory aspects of glaucoma and for potential use as a predictive biomarker or pharmacological intervention in glaucoma.

The Intertwined Roles of Oxidative Stress and Endoplasmic Reticulum Stress in Glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide, and the burden of the disease continues to grow as the global population ages. Currently, the only treatment option is to lower intraocular pressure. A better understanding of glaucoma pathogenesis will help us to develop novel therapeutic options. Oxidative stress has been implicated in the pathogenesis of many diseases. Oxidative stress occurs when there is an imbalance in redox homeostasis, with reactive oxygen species producing processes overcoming anti-oxidant defensive processes. Oxidative stress works in a synergistic fashion with endoplasmic reticulum stress, to drive glaucomatous damage to trabecular meshwork, retinal ganglion cells and the optic nerve head. We discuss the oxidative stress and endoplasmic reticulum stress pathways and their connections including their key intermediary, calcium. We highlight therapeutic options aimed at disrupting these pathways and discuss their potential role in glaucoma treatment.

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.

Down Syndrome Critical Region 1 Reduces Oxidative Stress-Induced Retinal Ganglion Cells Apoptosis via CREB-Bcl-2 Pathway

Purpose

Irreversible retina ganglion cell (RGC) loss is a key process during glaucoma progression. Down syndrome critical region 1 (DSCR1) has been shown to have protective effects against neuronal death. In this study, we aimed to investigate the neuroprotective mechanisms of DSCR1 on RGCs.

Methods

DBA/2J mice and optic nerve crush (ONC) rat model were used for vivo assays. Oxidative stress model of primary RGCs was carried out with in vitro transduction. DSCR1 protein localization was assessed by immunofluorescence. Differential protein expression was validated by Western blot, and gene expression was detected by real-time PCR. TUNEL was used to identify cell apoptosis, and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide was used to analyze cell viability.

Results

Significant upregulation of DSCR1 was observed in DBA/2J mice, ONC rat model, and RGCs treated with H₂O₂, reaching peaks at the age of 6 months in DBA/2J mice, 5 days after ONC in rats, and 24 hours after H₂O₂ treatment in RGCs, respectively. DSCR1 was shown to be expressed in the ganglion cell layer. In vitro, overexpressed DSCR1 significantly promoted phosphorylation of cyclic AMP response element binding protein (CREB), B-cell lymphoma 2 (Bcl-2) expression, and RGC survival rate while reducing cleaved caspase 3 expression in H₂O₂-treated RGCs. On the other hand, the opposite effects were shown after knockdown of DSCR1. In addition, silencing of CREB inhibited expression of DSCR1.

Conclusions

Our results suggested that DSCR1 might protect the RGCs against oxidative stress via the CREB–Bcl-2 pathway, which may provide a theoretical basis for future treatments of glaucoma.

Neuroinflammatory Mechanisms of Mitochondrial Dysfunction and Neurodegeneration in Glaucoma

The exact mechanism of retinal ganglion cell loss in the pathogenesis of glaucoma is yet to be understood. Mitochondrial damage-associated molecular patterns (DAMPs) resulting from mitochondrial dysfunction have been linked to Leber's hereditary optic neuropathy and autosomal dominant optic atrophy, as well as to brain neurodegenerative diseases. Recent evidence shows that, in conditions where mitochondria are damaged, a sustained inflammatory response and downstream pathological inflammation may ensue. Mitochondrial damage has been linked to the accumulation of age-related mitochondrial DNA mutations and mitochondrial dysfunction, possibly through aberrant reactive oxygen species production and defective mitophagy. The present review focuses on how mitochondrial dysfunction may overwhelm the ability of neurons and glial cells to adequately maintain homeostasis and how mitochondria-derived DAMPs trigger the immune system and induce neurodegeneration.

Insights into the regulatory molecules involved in glaucoma pathogenesis

Glaucoma is an important cause of irreversible blindness, characterized by optic nerve anomalies. Increased intraocular pressure (IOP) and aging are major risk factors. Retinal ganglion cells and trabecular meshwork cells are certainly involved in the etiology of glaucoma. Glaucoma is usually a complex disease, and various genes and functions may contribute to its etiology. Among these may be genes that encode regulatory molecules. In this review, regulatory molecules including 18 transcription factors (TFs), 195 microRNAs (miRNAs), 106 long noncoding RNAs (lncRNAs), and two circular RNAs (circRNAs) that are reasonable candidates for having roles in glaucoma pathogenesis are described. The targets of the regulators are reported. Glaucoma-related features including apoptosis, stress responses, immune functions, ECM properties, IOP, and eye development are affected by the targeted genes. The targeted genes that are frequently targeted by multiple regulators most often affect apoptosis and the related features of cell death and cell survival. BCL2, CDKN1A, and TP53 are among the frequent targets of three types of glaucoma-relevant regulators, TFs, miRNAs, and lncRNAs. TP53 was itself identified as a glaucoma-relevant TF. Several of the glaucoma-relevant TFs are themselves among frequent targets of regulatory molecules, which is consistent with existence of a complex network involved in glaucoma pathogenesis.

Identification of genes involved in glaucoma pathogenesis using combined network analysis and empirical studies

Glaucoma is a leading cause of blindness. We aimed in this study to identify genes that may make subtle and cumulative contributions to glaucoma pathogenesis. To this end, we identified molecular interactions and pathways that include transcription factors (TFs) FOXC1, PITX2, PAX6 and NFKB1 and various microRNAs including miR-204 known to have relevance to trabecular meshwork (TM) functions and/or glaucoma. TM tissue is involved in glaucoma pathogenesis. In-house microarray transcriptome results and data sources were used to identify target genes of the regulatory molecules. Bioinformatics analyses were done to filter TM and glaucoma relevant genes. These were submitted to network-creating softwares to define interactions, pathways and a network that would include the genes. The network was stringently scrutinized and minimized, then expanded by addition of microarray data and data on TF and microRNA-binding sites. Selected features of the network were confirmed by empirical studies such as dual luciferase assays, real-time PCR and western blot experiments and apoptosis assays. MYOC, WDR36, LTPBP2, RHOA, CYP1B1, OPA1, SPARC, MEIS2, PLEKHG5, RGS5, BBS5, ALDH1A1, NOMO2, CXCL6, FMNL2, ADAMTS5, CLOCK and DKK1 were among the genes included in the final network. Pathways identified included those that affect ECM properties, IOP, ciliary body functions, retinal ganglion cell viability, apoptosis, focal adhesion and oxidative stress response. The identification of many genes potentially involved in glaucoma pathology is consistent with its being a complex disease. The inclusion of several known glaucoma-related genes validates the approach used.

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.

The clinical feature of myocilin Y437H mutation in a Chinese family with primary open-angle glaucoma

PURPOSE

To characterise the genotype(s), phenotype(s) and age-related penetrance of glaucoma in a Chinese family with primary open-angle glaucoma (POAG).

METHODS

Recruited from a Chinese family spanning four generations, 7 individuals with POAG, 1 with ocular hypertension (OHT) and 14 unaffected individuals were included in this study. Genotypic investigation included sequencing of mutation sites using a glaucoma panel in combination with high-throughput sequencing and validated using Sanger sequencing. Phenotypic characterisation included investigation into patient medical history and physical examination.

RESULTS

Eight (36.4%) family members harboured heterozygous Y437H mutation, of whom seven (87.5%) were glaucomatous and one (12.5%) had OHT. The mean age of POAG diagnosis was 30.85±7.13 years. The mean highest recorded intraocular pressure (IOP) was 46.57±6.53 mm Hg. They all had complained of symptoms associated with vision and pain. Four (57.1%) patients presented blindness. Five eyes (62.5%) presented with severe and three eyes with moderate visual field defects. Most of them underwent surgery on average 1.29±2.36 years after diagnosis, and the mean IOP at study was 17.95±7.23 mm Hg, with an average of 0.92±0.86 eye-drops. The patient with OHT was treated with latanoprost only and her IOP was well controlled. Age-related glaucoma penetrance was 0% in individuals under the age of 20 years, 50% at ages 20-35 years, 75% at ages 31-35 years and 87.5% over 45 years.

CONCLUSION

A novel MYOC mutation (c.1309T>C, p.Y437H) in a Chinese family with POAG was identified which was associated with a phenotype characterised by severe visual impairment, frequent surgical intervention requirement and relatively high penetrance.

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.

Bone Lead Levels and Risk of Incident Primary Open-Angle Glaucoma: The VA Normative Aging Study

BACKGROUND

Oxidative stress may play an important role in the etiology of primary open-angle glaucoma (POAG). The association between risk of POAG and lead exposure, which is an environmental source of oxidative stress, has not been fully investigated yet.

OBJECTIVE

Our objective was to determine the association between bone lead—a biomarker of cumulative lead dose (tibia lead) or an endogenous source of stored lead (patella lead)—and incident POAG.

METHODS

We examined a prospective cohort of 634 POAG-free men [mean baseline age=66.8 y of age (SD=6.7)] from the Normative Aging Study (NAS) who had tibia and patella K X-ray fluorescence lead measurements between 1 January 1991 and 31 December 1999. They also had standard ocular evaluations by NAS optometrists until 31 December 2014. POAG cases were identified by consistent reports of enlarged or asymmetric cup-to-disc ratio together with visual field defect or existence of disc hemorrhage. We used Cox proportional hazards regressions to estimate hazard ratios (HRs) of incident POAG and adjusted survival curves to examine changes in the risk of POAG during follow-up according to bone lead quartiles.

RESULTS

We identified 44 incident cases of POAG by the end of follow-up (incidence rate=74 per 10,000 person-years; median follow-up=10.6 y). In fully adjusted models, 10-fold increases in patella lead and tibia lead were associated with HRs of 5.06 (95% CI: 1.61, 15.88, p=0.005) and 3.07 (95% CI: 0.94, 10.0, p=0.06), respectively. The HRs comparing participants in the third and fourth quartiles with the lowest quartile were 3.41 (95% CI: 1.34, 8.66) and 3.24 (95% CI: 1.22, 8.62) for patella lead (p-for-trend=0.01), and 3.84 (95% CI: 1.54, 9.55) and 2.61 (95% CI: 0.95, 7.21) for tibia lead (p-for-trend=0.02).

CONCLUSIONS

Our study provides longitudinal evidence that bone lead may be an important risk factor for POAG in the U.S. population. https://doi.org/10.1289/EHP3442.

LTBP2 knockdown and oxidative stress affect glaucoma features including TGFβ pathways, ECM genes expression and apoptosis in trabecular meshwork cells

Glaucoma is the leading cause of irreversible blindness worldwide. Although the etiology of glaucoma is incompletely understood, it is known that the extracellular matrix (ECM) of the trabecular meshwork, oxidative stress, TGFβ signaling pathways, and apoptosis are important components of glaucoma pathogenesis. These components appear to be interrelated, but knowledge on their interactions remains incomplete. Relevant to this gap in knowledge, LTBP2, glaucoma causing gene, may also be related to the mentioned components of glaucoma pathogenesis because of its putative roles in TGFβ signaling and ECM functions. This background prompted us to further query interactions among some molecules and pathways thought to be important in glaucoma etiology, with emphasis on oxidative stress and LTBP2. To this end, effects of LTBP2 siRNA knockdown, oxidative stress induction, TGFβ2 and gremlin exposures on canonical TGFβ and BMP signaling pathways, expression of ECM related genes, and apoptosis were assayed in primary human trabecular meshwork cell cultures. We found that oxidative stress induction and LTBP2 knockdown both affected all the processes queried, and that their affects paralleled one another. We suggest that effects of both oxidative stress and LTBP2 knockdown on the ECM and apoptosis may be mediated by TGFβ and BMP signaling pathway activation.

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.

Isoform-selective Hsp90 inhibition rescues model of hereditary open-angle glaucoma

The heat shock protein 90 (Hsp90) family of molecular chaperones regulates protein homeostasis, folding, and degradation. The ER-resident Hsp90 isoform, glucose-regulated protein 94 (Grp94), promotes the aggregation of mutant forms of myocilin, a protein associated with primary open-angle glaucoma. While inhibition of Grp94 promotes the degradation of mutant myocilin in vitro, to date no Grp94-selective inhibitors have been investigated in vivo. Here, a Grp94-selective inhibitor facilitated mutant myocilin degradation and rescued phenotypes in a transgenic mouse model of hereditary primary open-angle glaucoma. Ocular toxicities previously associated with pan-Hsp90 inhibitors were not evident with our Grp94-selective inhibitor, 4-Br-BnIm. Our study suggests that selective inhibition of a distinct Hsp90 family member holds translational promise for ocular and other diseases associated with cell stress and protein misfolding.

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.

Mitochondrial dynamics, transport, and quality control: A bottleneck for retinal ganglion cell viability in optic neuropathies

Retinal ganglion cells, the neurons that selectively die in glaucoma and other optic neuropathies, are endowed with an exceedingly active metabolism and display a particular vulnerability to mitochondrial dysfunction. Mitochondria are exquisitely dynamic organelles that are continually responding to endogenous and environmental cues to readily meet the energy demand of neuronal networks. The highly orchestrated regulation of mitochondrial biogenesis, fusion, fission, transport and degradation is paramount for the maintenance of energy-expensive synapses at RGC dendrites and axon terminals geared for optimal neurotransmission. The present review focuses on the progress made to date on understanding the biology of mitochondrial dynamics and quality control and how dysregulation of these processes can profoundly affect retinal ganglion cell viability and function in optic nerve diseases.

Characterizing the "POAGome": A bioinformatics-driven approach to primary open-angle glaucoma

Primary open-angle glaucoma (POAG) is a genetically, physiologically, and phenotypically complex neurodegenerative disorder. This study addressed the expanding collection of genes associated with POAG, referred to as the "POAGome." We used bioinformatics tools to perform an extensive, systematic literature search and compiled 542 genes with confirmed associations with POAG and its related phenotypes (normal tension glaucoma, ocular hypertension, juvenile open-angle glaucoma, and primary congenital glaucoma). The genes were classified according to their associated ocular tissues and phenotypes, and functional annotation and pathway analyses were subsequently performed. Our study reveals that no single molecular pathway can encompass the pathophysiology of POAG. The analyses suggested that inflammation and senescence may play pivotal roles in both the development and perpetuation of the retinal ganglion cell degeneration seen in POAG. The TGF-β signaling pathway was repeatedly implicated in our analyses, suggesting that it may be an important contributor to the manifestation of POAG in the anterior and posterior segments of the globe. We propose a molecular model of POAG revolving around TGF-β signaling, which incorporates the roles of inflammation and senescence in this disease. Finally, we highlight emerging molecular therapies that show promise for treating POAG.

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.

Transplantation of iPSC-derived TM cells rescues glaucoma phenotypes in vivo

Glaucoma is a common cause of vision loss or blindness and reduction of intraocular pressure (IOP) has been proven beneficial in a large fraction of glaucoma patients. The IOP is maintained by the trabecular meshwork (TM) and the elevation of IOP in open-angle glaucoma is associated with dysfunction and loss of the postmitotic cells residing within this tissue. To determine if IOP control can be maintained by replacing lost TM cells, we transplanted TM-like cells derived from induced pluripotent stem cells into the anterior chamber of a transgenic mouse model of glaucoma. Transplantation led to significantly reduced IOP and improved aqueous humor outflow facility, which was sustained for at least 9 wk. The ability to maintain normal IOP engendered survival of retinal ganglion cells, whose loss is ultimately the cause for reduced vision in glaucoma. In vivo and in vitro analyses demonstrated higher TM cellularity in treated mice compared with littermate controls and indicated that this increase is primarily because of a proliferative response of endogenous TM cells. Thus, our study provides in vivo demonstration that regeneration of the glaucomatous TM is possible and points toward novel approaches in the treatment of this disease.

Measurement of Systemic Mitochondrial Function in Advanced Primary Open-Angle Glaucoma and Leber Hereditary Optic Neuropathy

Primary Open Angle Glaucoma (POAG) is a common neurodegenerative disease characterized by the selective and gradual loss of retinal ganglion cells (RGCs). Aging and increased intraocular pressure (IOP) are glaucoma risk factors; nevertheless patients deteriorate at all levels of IOP, implying other causative factors. Recent evidence presents mitochondrial oxidative phosphorylation (OXPHOS) complex-I impairments in POAG. Leber Hereditary Optic Neuropathy (LHON) patients suffer specific and rapid loss of RGCs, predominantly in young adult males, due to complex-I mutations in the mitochondrial genome. This study directly compares the degree of OXPHOS impairment in POAG and LHON patients, testing the hypothesis that the milder clinical disease in POAG is due to a milder complex-I impairment. To assess overall mitochondrial capacity, cells can be forced to produce ATP primarily from mitochondrial OXPHOS by switching the media carbon source to galactose. Under these conditions POAG lymphoblasts grew 1.47 times slower than controls, whilst LHON lymphoblasts demonstrated a greater degree of growth impairment (2.35 times slower). Complex-I enzyme specific activity was reduced by 18% in POAG lymphoblasts and by 29% in LHON lymphoblasts. We also assessed complex-I ATP synthesis, which was 19% decreased in POAG patients and 17% decreased in LHON patients. This study demonstrates both POAG and LHON lymphoblasts have impaired complex-I, and in the majority of aspects the functional defects in POAG were milder than LHON, which could reflect the milder disease development of POAG. This new evidence places POAG in the spectrum of mitochondrial optic neuropathies and raises the possibility for new therapeutic targets aimed at improving mitochondrial function.

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.

Mutated myocilin and heterozygous Sod2 deficiency act synergistically in a mouse model of open-angle glaucoma

Glaucoma is a multifactorial optic neuropathy characterized by retinal ganglion cell (RGC) death and axonal degeneration leading to irreversible blindness. Mutations in the myocilin (MYOC) gene are the most common genetic factors of primary open-angle glaucoma. To develop a genetic mouse model induced by the synergistic interaction of mutated myocilin and another significant risk factor, oxidative stress, we produced double-mutant mice (Tg-MYOC(Y437H/+)/Sod2(+/-)) bearing human MYOC with a Y437H point mutation and a heterozygous deletion of the gene for the primary antioxidant enzyme, superoxide dismutase 2 (SOD2). Sod2 is broadly expressed in most tissues including the trabecular meshwork (TM) and heterozygous Sod2 knockout mice exhibit the reduced SOD2 activity and oxidative stress in all studied tissues. Accumulation of Y437H myocilin in the TM induced endoplasmic reticulum stress and led to a 45% loss of smooth muscle alpha-actin positive cells in the eye drainage structure of 10- to 12-month-old Tg-MYOC(Y437H/+)/Sod2(+/-) mice as compared with wild-type littermates. Tg-MYOC(Y437H/+)/Sod2(+/-) mice had higher intraocular pressure, lost about 37% of RGCs in the peripheral retina, and exhibited axonal degeneration in the retina and optic nerve as compared with their wild-type littermates. Single-mutant littermates containing MYOC(Y437H/+) or Sod2(+/-) exhibited no significant pathological changes until 12 months of age. Additionally, we observed elevated expression of endothelial leukocyte adhesion molecule-1, a human glaucoma marker, in the TM of Tg-MYOC(Y437H/+)/Sod2(+/-) mice. This is the first reported animal glaucoma model that combines expression of a glaucoma-causing mutant gene and an additional mutation mimicking a deleterious environment factor that acts synergistically.

Structural basis for misfolding in myocilin-associated glaucoma

Olfactomedin (OLF) domain-containing proteins play roles in fundamental cellular processes and have been implicated in disorders ranging from glaucoma, cancers and inflammatory bowel disorder, to attention deficit disorder and childhood obesity. We solved crystal structures of the OLF domain of myocilin (myoc-OLF), the best studied such domain to date. Mutations in myoc-OLF are causative in the autosomal dominant inherited form of the prevalent ocular disorder glaucoma. The structures reveal a new addition to the small family of five-bladed β-propellers. Propellers are most well known for their ability to act as hubs for protein-protein interactions, a function that seems most likely for myoc-OLF, but they can also act as enzymes. A calcium ion, sodium ion and glycerol molecule were identified within a central hydrophilic cavity that is accessible via movements of surface loop residues. By mapping familial glaucoma-associated lesions onto the myoc-OLF structure, three regions sensitive to aggregation have been identified, with direct applicability to differentiating between neutral and disease-causing non-synonymous mutations documented in the human population worldwide. Evolutionary analysis mapped onto the myoc-OLF structure reveals conserved and divergent regions for possible overlapping and distinctive functional protein-protein or protein-ligand interactions across the broader OLF domain family. While deciphering the specific normal biological functions, ligands and binding partners for OLF domains will likely continue to be a challenging long-term experimental pursuit, atomic detail structural knowledge of myoc-OLF is a valuable guide for understanding the implications of glaucoma-associated mutations and will help focus future studies of this biomedically important domain family.

Myocilin is involved in NgR1/Lingo-1-mediated oligodendrocyte differentiation and myelination of the optic nerve

Myocilin is a secreted glycoprotein that belongs to a family of olfactomedin domain-containing proteins. Although myocilin is detected in several ocular and nonocular tissues, the only reported human pathology related to mutations in the MYOCILIN gene is primary open-angle glaucoma. Functions of myocilin are poorly understood. Here we demonstrate that myocilin is a mediator of oligodendrocyte differentiation and is involved in the myelination of the optic nerve in mice. Myocilin is expressed and secreted by optic nerve astrocytes. Differentiation of optic nerve oligodendrocytes is delayed in Myocilin-null mice. Optic nerves of Myocilin-null mice contain reduced levels of several myelin-associated proteins including myelin basic protein, myelin proteolipid protein, and 2'3'-cyclic nucleotide 3'-phosphodiesterase compared with those of wild-type littermates. This leads to reduced myelin sheath thickness of optic nerve axons in Myocilin-null mice compared with wild-type littermates, and this difference is more pronounced at early postnatal stages compared with adult mice. Myocilin also affects differentiation of oligodendrocyte precursors in vitro. Its addition to primary cultures of differentiating oligodendrocyte precursors increases levels of tested markers of oligodendrocyte differentiation and stimulates elongation of oligodendrocyte processes. Myocilin stimulation of oligodendrocyte differentiation occurs through the NgR1/Lingo-1 receptor complex. Myocilin physically interacts with Lingo-1 and may be considered as a Lingo-1 ligand. Myocilin-induced elongation of oligodendrocyte processes may be mediated by activation of FYN and suppression of RhoA GTPase.

Focus on molecular events in the anterior chamber leading to glaucoma

Primary open-angle glaucoma is a multifactorial disease that affects the retinal ganglion cells, but currently its therapy is to lower the eye pressure. This indicates a definite involvement of the trabecular meshwork, key region in the pathogenesis of glaucoma. This is the first target of glaucoma, and its functional complexity is a real challenge to search. Its functions are those to allow the outflow of aqueous humor and not the reflux. This article describes the morphological and functional changes that happen in anterior chamber. The "primus movens" is oxidative stress that affects trabecular meshwork, particularly its endothelial cells. In these develops a real mitochondriopaty. This leads to functional impotence, the trabecular meshwork altering both motility and cytoarchitecture. Its cells die by apoptosis, losing barrier functions and altering the aqueous humor outflow. All the morphological alterations occur that can be observed under a microscope. Intraocular pressure rises and the malfunctioning trabecular meshwork endotelial cells express proteins that completely alter the aqueous humor. This is a liquid whose functional proteomics complies with the conditions of the trabecular meshwork. Indeed, in glaucoma, it is possible detect the presence of proteins which testify to what occurs in the anterior chamber. There are six classes of proteins which confirm the vascular endothelium nature of the anterior chamber and are the result of the morphofunctional trabecular meshwork decay. It is possible that, all or in part, these proteins can be used as a signal to the posterior pole.

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

Myocilin is a secreted glaucoma-associated protein, specifically induced by dexamethasone in human trabecular meshwork cells, where it was discovered. Myocilin is expressed in several tissues of the body, but it causes disease only in the eye. The protein contains two domains: an N-terminal region with significant homologies to nonmuscle myosin, and a C-terminal region, which is similar to the olfactomedin proteins. Forty percent of myocilin undergoes an intracellular endoproteolytic cleavage by calpain II, a calcium-dependent cysteine protease, which releases the 2 domains. The protein is known to interact with intracellular and extracellular matrix proteins, and some is released into the extracellular space associated with exosomes. Myocilin mutations are linked to glaucoma and induce elevated intraocular pressure. Most of the glaucoma-causative mutations map to the olfactomedin domain, which appears to be a critical domain for the function of the protein. Myocilin mutants are misfolded, aggregate in the endoplasmic reticulum, and are not secreted. Overexpression of myocilin and of its mutants in primary human trabecular meshwork cells triggers changes in the expression of numerous genes, many of which have been known to be involved in mechanisms important for the physiology and pathology of the tissue. Here we review recent studies from our laboratory and those of others that deal with trabecular meshwork genes, which are altered by the overexpression of wild-type and glaucoma-causative mutant myocilin genes.

Myocilin regulates cell proliferation and survival

Myocilin, a causative gene for open angle glaucoma, encodes a secreted glycoprotein with poorly understood functions. To gain insight into its functions, we produced a stably transfected HEK293 cell line expressing myocilin under an inducible promoter and compared gene expression profiles between myocilin-expressing and vector control cell lines by a microarray analysis. A significant fraction of differentially expressed genes in myocilin-expressing cells was associated with cell growth and cell death, suggesting that myocilin may have a role in the regulation of cell growth and survival. Increased proliferation of myocilin-expressing cells was demonstrated by the WST-1 proliferation assay, direct cell counting, and immunostaining with antibodies against Ki-67, a cellular proliferation marker. Myocilin-containing conditioned medium also increased proliferation of unmodified HEK293 cells. Myocilin-expressing cells were more resistant to serum starvation-induced apoptosis than control cells. TUNEL-positive apoptotic cells were dramatically decreased, and two apoptotic marker proteins, cleaved caspase 7 and cleaved poly(ADP-ribose) polymerase, were significantly reduced in myocilin-expressing cells as compared with control cells under apoptotic conditions. In addition, myocilin-deficient mesenchymal stem cells exhibited reduced proliferation and enhanced susceptibility to serum starvation-induced apoptosis as compared with wild-type mesenchymal stem cells. Phosphorylation of ERK1/2 and its upstream kinases, c-Raf and MEK, was increased in myocilin-expressing cells compared with control cells. Elevated phosphorylation of ERK1/2 was also observed in the trabecular meshwork of transgenic mice expressing 6-fold higher levels of myocilin when compared with their wild-type littermates. These results suggest that myocilin promotes cell proliferation and resistance to apoptosis via the ERK1/2 MAPK signaling pathway.

Ligands for glaucoma-associated myocilin discovered by a generic binding assay

Mutations in the olfactomedin domain of myocilin (myoc-OLF) are the strongest link to inherited primary open angle glaucoma. In this recently identified protein misfolding disorder, aggregation-prone disease variants of myocilin hasten glaucoma-associated elevation of intraocular pressure, leading to vision loss. Despite its well-documented pathogenic role, myocilin remains a domain of unknown structure or function. Here we report the first small-molecule ligands that bind to the native state of myoc-OLF. To discover these molecules, we designed a general label-free, mix-and-measure, high throughput chemical assay for restabilization (CARS), which is likely readily adaptable to discover ligands for other proteins. Of the 14 hit molecules identified from screening myoc-OLF against the Sigma-Aldrich Library of Pharmacologically Active Compounds using CARS, surface plasmon resonance binding studies reveal three are stoichiometric ligand scaffolds with low micromolar affinity. Two compounds, GW5074 and apigenin, inhibit myoc-OLF amyloid formation in vitro. Structure-activity relationship-based soluble derivatives reduce aggregation in vitro as well as enhance secretion of full-length mutant myocilin in a cell culture model. Our compounds set the stage for a new chemical probe approach to clarify the biological function of wild-type myocilin and represent lead therapeutic compounds for diminishing intracellular sequestration of toxic mutant myocilin.

A molecular mechanism for glaucoma: endoplasmic reticulum stress and the unfolded protein response

Primary open angle glaucoma (POAG) is a common late-onset neurodegenerative disease. Ocular hypertension represents a major risk factor, but POAG etiology remains poorly understood. Some cases of early-onset congenital glaucoma and adult POAG are linked to mutations in myocilin, a secreted protein of poorly defined function. Transgenic overexpression of myocilin in Drosophila and experiments in mice and human populations implicate the unfolded protein response (UPR) in the pathogenesis of glaucoma. We postulate that compromised ability of the UPR to eliminate misfolded mutant or damaged proteins, including myocilin, causes endoplasmic reticulum stress, resulting in functional impairment of trabecular meshwork cells that regulate intraocular pressure. This mechanism of POAG is reminiscent of other age-dependent neurodegenerative diseases that involve accumulation of protein aggregates.

Gene expression profile in human trabecular meshwork from patients with primary open-angle glaucoma

PURPOSE

To identify the specific genes in human trabecular meshwork (TM) related to POAG.

METHODS

Primary open-angle glaucoma TM specimens were obtained from routine trabeculectomy surgery. Nonglaucomatous control TM specimens were dissected from donor eyes using the same approach as a standard trabeculectomy. All cases were screened for myocilin (MYOC) mutations. Total RNA was extracted, labeled, and hybridized to Illumina HumanWG-6 BeadChips. Expression data were normalized and analyzed using the R package limma in Bioconductor. Pathway analyses were performed using DAVID Bioinformatics Resources.

RESULTS

Our study included surgical TM specimens from 15 cases and 13 controls. One case was identified with a heterozygous Q368X MYOC mutation. If TMs were available from both eyes in an individual, the expression data were combined for analysis. The following three comparisons were performed for differential analyses: (1) MYOC POAG case versus 14 non-MYOC POAG cases, (2) MYOC POAG case versus 13 controls, and (3) 14 non-MYOC POAG cases versus 13 controls. Limited by one MYOC case in comparisons 1 and 2, expression changes were reported comparing the fold changes but without P values. Comparison 3 identified 483 genes, including 36 components of TM exosomes. Gene ontology analysis identified several enriched functional clusters, including cell adhesion, extracellular matrix, and secretion.

CONCLUSIONS

This is the largest TM expression study of POAG cases and controls performed to date and represents the first report of TM expression in a patient having POAG with a Q368X MYOC mutation. Our data suggest the potential role of endocytic and exosome pathways in the pathogenesis of POAG.

Myocilin mediates myelination in the peripheral nervous system through ErbB2/3 signaling

The glaucoma-associated gene, myocilin, is expressed in ocular and non-ocular tissues including the peripheral nervous system, but its functions in these tissues remain poorly understood. We demonstrate that in sciatic nerve, myocilin is expressed in Schwann cells with high concentrations at the nodes of Ranvier. There, myocilin interacts with gliomedin, neurofascin, and NrCAM, which are essential for node formation and function. Treatment of isolated dorsal root ganglion cultures with myocilin stimulates clustering of the nodal proteins neurofascin and sodium channel Nav1.2. Sciatic nerves of myocilin null mice express reduced levels of several myelin-associated and basal membrane proteins compared with those of wild-type littermates. They also demonstrate reduced myelin sheath thickness and partial disorganization of the nodes. Myocilin signaling through ErbB2/3 receptors may contribute to these observed effects. Myocilin binds to ErbB2/ErbB3, activates these receptors, and affects the downstream PI3K-AKT signaling pathway. These data implicate a role for myocilin in the development and/or maintenance of myelination and nodes of Ranvier in sciatic nerve.

Myocilin stimulates osteogenic differentiation of mesenchymal stem cells through mitogen-activated protein kinase signaling

Myocilin is a secreted glycoprotein that is expressed in ocular and non-ocular tissues. Mutations in the MYOCILIN gene may lead to juvenile- and adult-onset primary open-angle glaucoma. Here we report that myocilin is expressed in bone marrow-derived mesenchymal stem cells (MSCs) and plays a role in their differentiation into osteoblasts in vitro and in osteogenesis in vivo. Expression of myocilin was detected in MSCs derived from mouse, rat, and human bone marrow, with human MSCs exhibiting the highest level of myocilin expression. Expression of myocilin rose during the course of human MSC differentiation into osteoblasts but not into adipocytes, and treatment with exogenous myocilin further enhanced osteogenesis. MSCs derived from Myoc-null mice had a reduced ability to differentiate into the osteoblastic lineage, which was partially rescued by exogenous extracellular myocilin treatment. Myocilin also stimulated osteogenic differentiation of wild-type MSCs, which was associated with activation of the p38, Erk1/2, and JNK MAP kinase signaling pathways as well as up-regulated expression of the osteogenic transcription factors Runx2 and Dlx5. Finally, cortical bone thickness and trabecular volume, as well as the expression level of osteopontin, a known factor of bone remodeling and osteoblast differentiation, were reduced dramatically in the femurs of Myoc-null mice compared with wild-type mice. These data suggest that myocilin should be considered as a target for improving the bone regenerative potential of MSCs and may identify a new role for myocilin in bone formation and/or maintenance in vivo.

New mutation in the myocilin gene segregates with juvenile-onset open-angle glaucoma in a Brazilian family

Mutations in the myocilin gene (MYOC) account for most cases of autosomal dominant juvenile-onset open-angle glaucoma (JOAG), an earlier and more severe form of POAG. We accessed seven members of a Brazilian JOAG family by clinical and molecular investigation. Four out of seven family members were diagnosed with JOAG. All of these patients presented high intraocular pressure and two of them were bilaterally blind. The disease onset varied from 20 to 30years old. There was a nine-year-old family member who had not yet manifested the disease, although he was also a carrier of the mutation. Ophthalmologic examination included: evaluation of the visual field and optic disc, intraocular pressure measurement, and gonioscopy. The three exons and intron/exon junctions of the MYOC gene were screened for mutations through direct sequencing of PCR-amplified DNA fragments. Mutation screening revealed an in-frame mutation in the third exon of the MYOC gene: an insertion of six nucleotides between the cDNA positions 1187 and 1188 (c.1187_1188insCCCAGA, p.D395_E396insDP). This mutation presented an autosomal dominant pattern of inheritance, segregating with the disease in four family members for three generations, and it was absent in 60 normal controls. We also performed a computational structure modeling of olfactomedin-like domain of myocilin protein and conducted in silico analysis to predict the structural changes in the myocilin protein due to the presence of the mutation. These findings may be important for future diagnosis of other presymptomatic family members, as well as for the increase of the panel of MYOC mutations and their effects on phenotype.

Current understanding of conventional outflow dysfunction in glaucoma

PURPOSE OF REVIEW

Regulation of intraocular pressure by the conventional (trabecular) outflow pathway is complicated, involving a myriad of mechanical and chemical signals. In most, intraocular pressure is maintained within a tight range over a lifetime. Unfortunately in some, dysfunction results in ocular hypertension and open-angle glaucoma. In the context of established knowledge, this review summarizes recent investigations of conventional outflow function, with the goal of identifying areas for future inquiry and therapeutic targeting.

RECENT FINDINGS

Mechanical stimulation of conventional outflow cells due to intraocular pressure fluctuations impacts contractility, gene expression, pore formation, enzyme activity, and signaling. Numerous local signaling mediators in the conventional pathway such as bioactive lipids, cytokines, nitric oxide, and nucleotides participate in the regulation of outflow. Interestingly outflow through the conventional pathway is not uniform, but segmental, with passageways constantly changing due to focal protease activity of trabecular cells clearing extracellular matrix materials. The relationship between extracellular matrix expression and trabecular meshwork contractility appears to coordinately impact outflow resistance and is the target of a new class of drugs, the Rho kinase inhibitors.

SUMMARY

The conventional outflow pathway is a dynamic, pressure-sensitive tissue that is vulnerable to pathology on many fronts, each representing a therapeutic opportunity.

Glucose-regulated protein 94 triage of mutant myocilin through endoplasmic reticulum-associated degradation subverts a more efficient autophagic clearance mechanism

BACKGROUND

Mutant myocilin accumulates in the endoplasmic reticulum for unknown reasons.

RESULTS

Glucose-regulated protein (Grp) 94 depletion reduces mutant myocilin by engaging autophagy.

CONCLUSION

Grp94 triages mutant myocilin through ER-associated degradation, subverting autophagy.

SIGNIFICANCE

Treating glaucoma could be possible by inhibiting Grp94 and reducing its novel client, mutant myocilin. Clearance of misfolded proteins in the endoplasmic reticulum (ER) is traditionally handled by ER-associated degradation (ERAD), a process that requires retro-translocation and ubiquitination mediated by a luminal chaperone network. Here we investigated whether the secreted, glaucoma-associated protein myocilin was processed by this pathway. Myocilin is typically transported through the ER/Golgi network, but inherited mutations in myocilin lead to its misfolding and aggregation within trabecular meshwork cells, and ultimately, ER stress-induced cell death. Using targeted knockdown strategies, we determined that glucose-regulated protein 94 (Grp94), the ER equivalent of heat shock protein 90 (Hsp90), specifically recognizes mutant myocilin, triaging it through ERAD. The addition of mutant myocilin to the short list of Grp94 clients strengthens the hypothesis that β-strand secondary structure drives client association with Grp94. Interestingly, the ERAD pathway is incapable of efficiently handling the removal of mutant myocilin, but when Grp94 is depleted, degradation of mutant myocilin is shunted away from ERAD toward a more robust clearance pathway for aggregation-prone proteins, the autophagy system. Thus ERAD inefficiency for distinct aggregation-prone proteins can be subverted by manipulating ER chaperones, leading to more effective clearance by the autophagic/lysosomal pathway. General Hsp90 inhibitors and a selective Grp94 inhibitor also facilitate clearance of mutant myocilin, suggesting that therapeutic approaches aimed at inhibiting Grp94 could be beneficial for patients suffering from some cases of myocilin glaucoma.

Cystatin a, a potential common link for mutant myocilin causative glaucoma

Myocilin (MYOC) is a 504 aa secreted glycoprotein induced by stress factors in the trabecular meshwork tissue of the eye, where it was discovered. Mutations in MYOC are linked to glaucoma. The glaucoma phenotype of each of the different MYOC mutation varies, but all of them cause elevated intraocular pressure (IOP). In cells, forty percent of wild-type MYOC is cleaved by calpain II, a cysteine protease. This proteolytic process is inhibited by MYOC mutants. In this study, we investigated the molecular mechanisms by which MYOC mutants cause glaucoma. We constructed adenoviral vectors with variants Q368X, R342K, D380N, K423E, and overexpressed them in human trabecular meshwork cells. We analyzed expression profiles with Affymetrix U133Plus2 GeneChips using wild-type and null viruses as controls. Analysis of trabecular meshwork relevant mechanisms showed that the unfolded protein response (UPR) was the most affected. Search for individual candidate genes revealed that genes that have been historically connected to trabecular meshwork physiology and pathology were altered by the MYOC mutants. Some of those had known MYOC associations (MMP1, PDIA4, CALR, SFPR1) while others did not (EDN1, MGP, IGF1, TAC1). Some, were top-changed in only one mutant (LOXL1, CYP1B1, FBN1), others followed a mutant group pattern. Some of the genes were new (RAB39B, STC1, CXCL12, CSTA). In particular, one selected gene, the cysteine protease inhibitor cystatin A (CSTA), was commonly induced by all mutants and not by the wild-type. Subsequent functional analysis of the selected gene showed that CSTA was able to reduce wild-type MYOC cleavage in primary trabecular meshwork cells while an inactive mutated CSTA was not. These findings provide a new molecular understanding of the mechanisms of MYOC-causative glaucoma and reveal CSTA, a serum biomarker for cancer, as a potential biomarker and drug for the treatment of MYOC-induced glaucoma.

Myocilin interacts with syntrophins and is member of dystrophin-associated protein complex

Genetic studies have linked myocilin to open angle glaucoma, but the functions of the protein in the eye and other tissues have remained elusive. The purpose of this investigation was to elucidate myocilin function(s). We identified α1-syntrophin, a component of the dystrophin-associated protein complex (DAPC), as a myocilin-binding candidate. Myocilin interacted with α1-syntrophin via its N-terminal domain and co-immunoprecipitated with α1-syntrophin from C2C12 myotubes and mouse skeletal muscle. Expression of 15-fold higher levels of myocilin in the muscles of transgenic mice led to the elevated association of α1-syntrophin, neuronal nitric-oxide synthase, and α-dystroglycan with DAPC, which increased the binding of laminin to α-dystroglycan and Akt signaling. Phosphorylation of Akt and Forkhead box O-class 3, key regulators of muscle size, was increased more than 3-fold, whereas the expression of muscle-specific RING finger protein-1 and atrogin-1, muscle atrophy markers, was decreased by 79 and 88%, respectively, in the muscles of transgenic mice. Consequently, the average size of muscle fibers of the transgenic mice was increased by 36% relative to controls. We suggest that intracellular myocilin plays a role as a regulator of muscle hypertrophy pathways, acting through the components of DAPC.

Genetics of primary glaucoma

PURPOSE OF REVIEW

To provide an overview of the genetics of the primary open-angle glaucomas with particular attention to congenital, infantile, and juvenile forms.

RECENT FINDINGS

Mutations in CYP1B1, in addition to being the most common identifiable cause of autosomal recessive primary congenital/infantile glaucoma, can infrequently underlie juvenile and even primary adult-onset open-angle glaucoma, particularly in certain consanguineous populations. In 2009, patients diagnosed with congenital/infantile glaucoma were found to have recessive mutations in a second gene, LTBP2, with a phenotypic spectrum that includes primary megalocornea, spherophakia with ectopia lentis, and lens-related glaucoma. The most common identifiable cause of primary juvenile open-angle glaucoma across most populations remains heterozygous (autosomal dominant) MYOC mutation, underlying up to one-third of cases and possibly sometimes involved in earlier and later onset glaucomas Although primary adult-onset open-angle glaucoma usually does not follow simple Mendelian genetics and is etiologically complex, genome-wide association studies are uncovering genetic susceptibility factors. In some cases, primary adult-onset open-angle glaucoma can be caused by heterozygous mutation in MYOC, OPTN, or WDR36. In addition, in 2009, heterozygous NTF4 mutation was associated with the phenotype in a small percentage of patients from a German cohort.

SUMMARY

Seemingly unaffected siblings of children with CYP1B1-related primary congenital/infantile glaucoma should undergo genetic testing because of variable expressivity for the phenotype; such testing should also be considered for other asymptomatic relatives, especially in consanguineous families. In western populations, dominant MYOC mutation remains a common cause of primary open-angle juvenile glaucoma and infrequently can be implicated in congenital/infantile or adult-onset forms; identified families should undergo genetic counseling. Primary adult-onset open-angle glaucoma rarely follows simple Mendelian genetics, but genomic studies in different populations are revealing potential genetic risk factors for the phenotype.

Myocilin, a glaucoma-associated protein, promotes cell migration through activation of integrin-focal adhesion kinase-serine/threonine kinase signaling pathway

The MYOCILIN gene encodes a secreted glycoprotein which is highly expressed in eye drainage structures. Mutations in this gene may lead to juvenile open-angle glaucoma and adult onset primary open-angle glaucoma, one of the leading causes of irreversible blindness in the world. Functions of wild-type myocilin are still unclear. We have recently demonstrated that myocilin is a modulator of Wnt signaling and may affect actin cytoskeleton organization. Here we report that myocilin and its naturally occurring proteolytic fragments, similar to Wnt3a, are able to stimulate trabecular meshwork, NIH3T3, and FHL124 cell migration with the N-terminal proteolytic fragment of myocilin lacking the olfactomedin domain producing the highest stimulatory effect. Stimulation of cell migration occurs through activation of the integrin-focal adhesion kinase (FAK)-serine/threonine kinase (AKT) signaling pathway. Inhibition of FAK by siRNA reduced the stimulatory action of myocilin by threefold. Activation of several components of this signaling pathway was also demonstrated in the eyes of transgenic mice expressing elevated levels of myocilin in the eye drainage structures. These data extend the similarities between actions of myocilin and Wnt proteins acting through a β-catenin-independent mechanism. The modification of the migratory ability of cells by myocilin may play a role in normal functioning of the eye anterior segment and its pathology including glaucoma.

Glaucoma: genes, phenotypes, and new directions for therapy

Glaucoma, a leading cause of blindness worldwide, is characterized by progressive optic nerve damage, usually associated with intraocular pressure. Although the clinical progression of the disease is well defined, the molecular events responsible for glaucoma are currently poorly understood and current therapeutic strategies are not curative. This review summarizes the human genetics and genomic approaches that have shed light on the complex inheritance of glaucoma genes and the potential for gene-based and cellular therapies that this research makes possible.