Gene structure and properties of TIGR, an olfactomedin-related glycoprotein cloned from glucocorticoid-induced trabecular meshwork cells

Structural basis for anomalous cellular trafficking behavior of glaucoma-associated A427T mutant myocilin

Familial mutations in myocilin cause vision loss in glaucoma due to misfolding and a toxic gain of function in a senescent cell type in the anterior eye. Here we characterize the cellular behavior and structure of the myocilin (myocilin A427T) mutant, of uncertain pathogenicity. Our characterization of A427T demonstrates that even mutations that minimally perturb myocilin structure and stability can present challenges for protein quality control clearance pathways. Namely, when expressed in an inducible immortalized trabecular meshwork cell line, inhibition of the proteasome reroutes wild-type myocilin, but not myocilin A427T, from endoplasmic reticulum associated degradation to lysosomal degradation. Yet, the crystal structure of the A427T myocilin olfactomedin domain shows modest perturbations largely confined to the mutation site. The previously unappreciated range of mutant myocilin behavior correlating with variable stability and structure provides a rationale for why it is challenging to predict causal pathogenicity of a given myocilin mutation, even in the presence of clinical data for members of an affected family. Comprehending the continuum of mutant myocilin behavior in the laboratory supports emerging efforts to use genetics to assess glaucoma risk in the clinic. In addition, the study supports a therapeutic strategy aimed at enhancing autophagic clearance of mutant myocilin.

Modeling complex age-related eye disease

Modeling complex eye diseases like age-related macular degeneration (AMD) and glaucoma poses significant challenges, since these conditions depend highly on age-related changes that occur over several decades, with many contributing factors remaining unknown. Although both diseases exhibit a relatively high heritability of >50%, a large proportion of individuals carrying AMD- or glaucoma-associated genetic risk variants will never develop these diseases. Furthermore, several environmental and lifestyle factors contribute to and modulate the pathogenesis and progression of AMD and glaucoma. Several strategies replicate the impact of genetic risk variants, pathobiological pathways and environmental and lifestyle factors in AMD and glaucoma in mice and other species. In this review we will primarily discuss the most commonly available mouse models, which have and will likely continue to improve our understanding of the pathobiology of age-related eye diseases. Uncertainties persist whether small animal models can truly recapitulate disease progression and vision loss in patients, raising doubts regarding their usefulness when testing novel gene or drug therapies. We will elaborate on concerns that relate to shorter lifespan, body size and allometries, lack of macula and a true lamina cribrosa, as well as absence and sequence disparities of certain genes and differences in their chromosomal location in mice. Since biological, rather than chronological, age likely predisposes an organism for both glaucoma and AMD, more rapidly aging organisms like small rodents may open up possibilities that will make research of these diseases more timely and financially feasible. On the other hand, due to the above-mentioned anatomical and physiological features, as well as pharmacokinetic and -dynamic differences small animal models are not ideal to study the natural progression of vision loss or the efficacy and safety of novel therapies. In this context, we will also discuss the advantages and pitfalls of alternative models that include larger species, such as non-human primates and rabbits, patient-derived retinal organoids, and human organ donor eyes.

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

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

Myocilin misfolding and glaucoma: A 20-year update

Mutations in the gene MYOC account for approximately 5% of cases of primary open angle glaucoma (POAG). MYOC encodes for the protein myocilin, a multimeric secreted glycoprotein composed of N-terminal coiled-coil (CC) and leucine zipper (LZ) domains that are connected via a disordered linker to a 30 kDa olfactomedin (OLF) domain. More than 90% of glaucoma-causing mutations are localized to the OLF domain. While myocilin is expressed in numerous tissues, mutant myocilin is only associated with disease in the anterior segment of the eye, in the trabecular meshwork. The prevailing pathogenic mechanism involves a gain of toxic function whereby mutant myocilin aggregates intracellularly instead of being secreted, which causes cell stress and an early timeline for TM cell death, elevated intraocular pressure, and subsequent glaucoma-associated retinal degeneration. In this review, we focus on the work our lab has conducted over the past ∼15 years to enhance our molecular understanding of myocilin-associated glaucoma, which includes details of the molecular structure and the nature of the aggregates formed by mutant myocilin. We conclude by discussing open questions, such as predicting phenotype from genotype alone, the elusive native function of myocilin, and translational directions enabled by our work.

COBL, MKX and MYOC Are Potential Regulators of Brown Adipose Tissue Development Associated with Obesity-Related Metabolic Dysfunction in Children

Obesity is already accompanied by adipose tissue (AT) dysfunction and metabolic disease in children and increases the risk of premature death. Due to its energy-dissipating function, brown AT (BAT) has been discussed as being protective against obesity and related metabolic dysfunction. To analyze the molecular processes associated with BAT development, we investigated genome-wide expression profiles in brown and white subcutaneous and perirenal AT samples of children. We identified 39 upregulated and 26 downregulated genes in uncoupling protein 1 (UCP1)-positive compared to UCP1-negative AT samples. We prioritized for genes that had not been characterized regarding a role in BAT biology before and selected cordon-bleu WH2 repeat protein (COBL), mohawk homeobox (MKX) and myocilin (MYOC) for further functional characterization. The siRNA-mediated knockdown of Cobl and Mkx during brown adipocyte differentiation in vitro resulted in decreased Ucp1 expression, while the inhibition of Myoc led to increased Ucp1 expression. Furthermore, COBL, MKX and MYOC expression in the subcutaneous AT of children is related to obesity and parameters of AT dysfunction and metabolic disease, such as adipocyte size, leptin levels and HOMA-IR. In conclusion, we identify COBL, MKX and MYOC as potential regulators of BAT development and show an association of these genes with early metabolic dysfunction in children.

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.

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.

Prevalence of MYOC risk variants for glaucoma in different populations

PURPOSE

To assess the clinical relevance of myocilin (MYOC) gene variants as risk factors for glaucoma in literature and to estimate their prevalence in different populations.

METHODS

We reviewed the literature for published MYOC variants in glaucoma patients and estimated their prevalence in general population using gnomAD and BRAVO databases. We used several bioinformatics tools and the criteria of the American College of Medical Genetics and Genomics (ACMG) to assess the pathogenicity of the variants. We evaluated the carrier frequency of the variants in gnomAD, including its subpopulations.

RESULTS

We found 13 missense and 5 loss-of-function (LOF) reported variants in MYOC that were both probable pathogenic or risk variants and listed in gnomAD. Six likely pathogenic missense variants were p.(Cys25Arg), p.(Gln48His), p.(Gly326Ser), p.(Thr353Ile), p.(Thr377Met) and p.(Gly399Val). They were most prevalent in East and South Asia (frequency, 0.92% and 0.81%, respectively). The most common missense variants were p.(Thr353Ile) (0.91% in East Asia) and p.(Gln48His) (0.79% in South Asia). Five LOF variants were p.(Arg46Ter), p.(Arg91Ter), p.(Arg272Ter), p.(Gln368Ter) and p.(Tyr453MetfsTer11). We considered these glaucoma risk variants. They were most prevalent in the East Asian and the Finnish population (0.93% and 0.33%, respectively).

CONCLUSION

Pathogenic MYOC variants appear to be population-associated. Our results highlight allelic heterogeneity of MYOC variants in open-angle glaucoma. Many of the probable pathogenic variants are over-represented in some of the populations causing doubt of their status as monogenic disease-causing variants.

Recombinant antibodies recognize conformation-dependent epitopes of the leucine zipper of misfolding-prone myocilin

Recombinant antibodies with well-characterized epitopes and known conformational specificities are critical reagents to support robust interpretation and reproducibility of immunoassays across biomedical research. For myocilin, a protein prone to misfolding that is associated with glaucoma and an emerging player in other human diseases, currently available antibodies are unable to differentiate among the numerous disease-associated protein states. This fundamentally constrains efforts to understand the connection between myocilin structure, function, and disease. To address this concern, we used protein engineering methods to develop new recombinant antibodies that detect the N-terminal leucine zipper structural domain of myocilin and that are cross-reactive for human and mouse myocilin. After harvesting spleens from immunized mice and in vitro library panning, we identified two antibodies, 2A4 and 1G12. 2A4 specifically recognizes a folded epitope while 1G12 recognizes a range of conformations. We matured antibody 2A4 for improved biophysical properties, resulting in variant 2H2. In a human IgG1 format, 2A4, 1G12, and 2H2 immunoprecipitate full-length folded myocilin present in the spent media of human trabecular meshwork (TM) cells, and 2H2 can visualize myocilin in fixed human TM cells using fluorescence microscopy. These new antibodies should find broad application in glaucoma and other research across multiple species platforms.

Incidence and risk factors for postoperative intraocular pressure response to topical prednisolone eye drops in patients undergoing phacoemulsification

AIM/PURPOSE

To report the incidence, risk factors, and magnitude of steroid response in individuals receiving topical 1% prednisolone acetate eye drops following phacoemulsification surgery MATERIALS AND METHODS: Postoperative IOP of 1118 consecutive patients who had uneventful cataract surgery and used 1% topical prednisolone acetate were studied. Baseline ocular parameters like best-corrected visual acuity, IOP, and slit-lamp examination findings were noted preoperatively and at postoperative day 30. Incidence of postoperative intraocular pressure response to steroid was analyzed and graded as mild, moderate, or severe and risk factors studied.

RESULTS

The mean age of our study cohort was 59.49 ± 7.25 years. The overall incidence of steroid response was 3.2%, (2.8% being moderate responders, and 0.4% high responders). Mean preoperative IOP was 14.67 ± 2.2 mm Hg in the study cohort (n = 1118). Mean postoperative IOP was 21.33 ± 7.97 mm Hg in the steroid responder (SR) and 14.66 ± 2.8 mm Hg in the non-responder (NR), with a statistically significant difference from the baseline IOP in the SR group (p < 0.001) and no difference in the NR. Univariate analysis revealed younger age and high axial length as risk factors but on multiple regression analysis, only younger age < 50 years was found to be a significant risk factor for steroid response.

CONCLUSION

The overall steroid response in this population post-cataract surgery was low with most being moderate responders. Younger age and higher axial length were identified as risk factors for steroid response, and hence this warrants the judicious use of steroids in such individuals.

ITGA8 positive cells in the conventional outflow tissue exhibit Schlemm's canal endothelial cell properties

AIMS

Elevated intraocular pressure is primarily induced by the increased resistance of conventional outflow of aqueous humor. Dysfunction of the juxtacanalicular region of trabecular meshwork (TM) and Schlemm's canal (SC) endothelium, as the main conventional outflow tissue, have been implicated as the major reasons for the increased resistance. Integrins are widespread in these tissues, especially alpha8 integrin (ITGA8). We aim to investigate the properties of cells expressing ITGA8 in the conventional outflow tissue.

MAIN METHODS

Fluorescence in situ hybridization (FISH) and immunofluorescence (IF) were performed to detect the mRNA and protein levels of ITGA8 in human conventional outflow tissue. ITGA8-positive cells were isolated from the cultured human TM cells through a magnetic bead-based approach. Flow Cytometry was used to determine the purification efficiency. The expressions of TM and SC biomarkers and dexamethasone-induced myocilin secretion capacity of ITGA8-positive cells was assessed by Real-time PCR, IF and Western blot. A gel contraction assay was performed to evaluate contractility of ITGA8-positive cells after endothelin 1 treatment.

KEY FINDINGS

ITGA8 was found with robust expression near the inner wall of SC endothelium. After purification, the proportion of ITGA8-positive cells were increased by about 10%. ITGA8-positive cells were identified with the properties as SC endothelial cells, such as more robust expressions of SC biomarkers, less dexamethasone-inducible myocilin expression, and stronger contractility.

SIGNIFICANCE

This study demonstrated that cells expressing ITGA8 in SC region possess more properties as SC endothelial cells. Our data implicate a crucial role of ITGA8 in aqueous humor (AH) outflow resistance regulation.

Knockout of myoc Provides Evidence for the Role of Myocilin in Zebrafish Sex Determination Associated with Wnt Signalling Downregulation

Myocilin is a secreted glycoprotein with a poorly understood biological function and it is mainly known as the first glaucoma gene. To explore the normal role of this protein in vivo we developed a myoc knockout (KO) zebrafish line using CRISPR/Cas9 genome editing. This line carries a homozygous variant (c.236_239delinsAAAGGGGAAGGGGA) that is predicted to result in a loss-of-function of the protein because of a premature termination codon p.(V75EfsX60) that resulted in a significant reduction of myoc mRNA levels. Immunohistochemistry showed the presence of myocilin in wild-type embryonic (96 h post-fertilization) anterior segment eye structures and caudal muscles. The protein was also detected in different adult ocular and non-ocular tissues. No gross macroscopic or microscopic alterations were identified in the KO zebrafish, but, remarkably, we observed absence of females among the adult KO animals and apoptosis in the immature juvenile gonad (28 dpf) of these animals, which is characteristic of male development. Transcriptomic analysis showed that adult KO males overexpressed key genes involved in male sex determination and presented differentially expressed Wnt signalling genes. These results show that myocilin is required for ovary differentiation in zebrafish and provides in vivo support for the role of myocilin as a Wnt signalling pathway modulator. In summary, this myoc KO zebrafish line can be useful to investigate the elusive function of this protein, and it provides evidence for the unexpected function of myocilin as a key factor in zebrafish sex determination.

Assessment of differential intraocular pressure response to dexamethasone treatment in perfusion cultured Indian cadaveric eyes

The purpose of the present study was to assess the differential intraocular pressure response (IOP) to dexamethasone (DEX) treatment at two dose levels (100 or 500 nM) in perfusion cultured Indian cadaveric eyes to investigate glucocorticoid (GC) responsiveness. In a human organ-cultured anterior segment (HOCAS) set-up, the eye pressure was monitored for every 24 h post DEX infusion (100 or 500 nM) or 0.1% ethanol treatment for 7 days after baseline stabilization. The expression of DEX-inducible proteins such as myocilin and fibronectin in HOCAS-TM tissues was assessed by immunostaining. Elevated IOP was observed in 6/16 eyes [Mean ± SEM (mΔIOP): 15.50 ± 1.96 mmHg; 37.5% responders] and 3/15 eyes (Mean ± SEM mΔIOP: 10 ± 0.84 mmHg; 20% responders) in 100 nM and 500 nM dose groups respectively. Elevated IOP in GC responder eyes was substantiated with a significant increase in myocilin (11.8-fold; p = 0.0002) and fibronectin (eightfold; p = 0.04) expression as compared to vehicle-treated eyes by immunofluorescence analysis. This is the first study reporting the GC responsiveness in Indian cadaveric eyes. The observed GC response rate was comparable with the previous studies and hence, this model will enable us to investigate the relationship between differential gene expression and individual GC responsiveness in our population.

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.

Ex-vivo cultured human corneoscleral segment model to study the effects of glaucoma factors on trabecular meshwork

Glaucoma is the second leading cause of irreversible blindness worldwide. Primary open angle glaucoma (POAG), the most common form of glaucoma, is often associated with elevation of intraocular pressure (IOP) due to the dysfunction of trabecular meshwork (TM) tissues. Currently, an ex vivo human anterior segment perfusion cultured system is widely used to study the effects of glaucoma factors and disease modifying drugs on physiological parameters like aqueous humor (AH) dynamics and IOP homeostasis. This system requires the use of freshly enucleated intact human eyes, which are sparsely available at very high cost. In this study, we explored the feasibility of using human donor corneoscleral segments for modeling morphological and biochemical changes associated with POAG. Among the number of corneas donated each year, many are deemed ineligible for transplantation due to stringent acceptance criteria. These ineligible corneoscleral segments were obtained from the Lions Eye Bank, Tampa, Florida. Each human donor anterior corneoscleral segment was dissected into four equal quadrants and cultured for 7 days by treating with the glaucoma factors dexamethasone (Dex) or recombinant transforming growth factor (TGF) β2 or transduced with lentiviral expression vectors containing wild type (WT) and mutant myocilin. Hematoxylin and Eosin (H&E) staining analysis revealed that the TM structural integrity is maintained after 7 days in culture. Increased TUNEL positive TM cells were observed in corneoscleral quadrants treated with glaucoma factors compared to their respective controls. However, these TUNEL positive cells were mainly confined to the scleral region adjacent to the TM. Treatment of corneoscleral quadrants with Dex or TGFβ2 resulted in glaucomatous changes at the TM, which included increased extracellular matrix (ECM) proteins and induction of endoplasmic reticulum (ER) stress. Western blot analysis of the conditioned medium showed an increase in ECM (fibronectin and collagen IV) levels in Dex- or TGFβ2-treated samples compared to control. Lentiviral transduction of quadrants resulted in expression of WT and mutant myocilin in TM tissues. Western blot analysis of conditioned medium revealed decreased secretion of mutant myocilin compared to WT myocilin. Moreover, increased ECM deposition and ER stress induction was observed in the TM of mutant myocilin transduced quadrants. Our findings suggest that the ex-vivo cultured human corneoscleral segment model is cost-effective and can be used as a pre-screening tool to study the effects of glaucoma factors and anti-glaucoma therapeutics on the TM.

Mendelian genes in primary open angle glaucoma

Mutations in each of three genes, myocilin (MYOC), optineurin (OPTN), and TANK binding kinase 1 (TBK1), may cause primary open-angle glaucoma (POAG) that is inherited as a Mendelian trait. MYOC mutations cause 3-4% of POAG cases with IOP >21 mmHg, while mutations in OPTN, TBK1, and MYOC each cause ∼1% of POAG with IOP ≤21 mmHg, i.e. normal tension glaucoma. Identification of these disease-causing genes has provided insights into glaucoma pathogenesis. Mutations in MYOC cause a cascade of abnormalities in the trabecular meshwork including intracellular retention of MYOC protein, decreased aqueous outflow, higher intraocular pressure, and glaucoma. Investigation of MYOC mutations demonstrated that abnormal retention of intracellular MYOC and stimulation of endoplasmic reticular (ER) stress may be important steps in the development of MYOC-associated glaucoma. Mutations in OPTN and TBK1 cause a dysregulation of autophagy which may directly cause retinal ganglion cell damage and normal tension glaucoma. Discovery of these Mendelian causes of glaucoma has also provided a new set of potential therapeutic targets that may ultimately lead to novel, gene-directed glaucoma treatments.

Antibodies Used to Detect Glaucoma-Associated Myocilin: More or Less Than Meets the Eye?

Antibodies are key reagents used in vision research, indeed across biomedical research, but they often do not reveal the whole story about a sample. It is important for researchers to be aware of aspects of antibodies that may affect or limit data interpretation. Federal agencies now require funded grants to demonstrate how they will authenticate reagents used. There is also a push for recombinant antibodies, enabled by phage display technology awarded the 2018 Nobel Prize in Chemistry, which allow for thorough validation and a fixed DNA sequence. Here, we discuss how issues surrounding antibodies are pertinent to detecting myocilin, a protein found in trabecular meshwork and associated with a portion of hereditary glaucoma. Confirmation of myocilin expression in tissues and cell culture has been adopted as validation standard in trabecular meshwork research; thus, a discussion of antibody characteristics and fidelity is critical. Further, based on our basic structural understanding of myocilin architecture and its biophysical aggregation properties, we provide a wish list for the characteristics of next-generation antibody reagents for vision researchers. In the long term, well-characterized antibodies targeting myocilin will enable new insights into its function and involvement in glaucoma pathogenesis.

The LRRC8-mediated volume-regulated anion channel is altered in glaucoma

Regulation of cellular volume is an essential process to balance volume changes during cell proliferation and migration or when intracellular osmolality increases due to transepithelial transport. We previously characterized the key role of volume-regulated anion channels (VRAC) in the modulation of the volume of trabecular meshwork (TM) cells and, in turn, the aqueous humour (AH) outflow from the eye. The balance between the secretion and the drainage of AH determines the intraocular pressure (IOP) that is the major casual risk factor for glaucoma. Glaucoma is an ocular disease that causes irreversible blindness due to the degeneration of retinal ganglion cells. The recent identification of Leucine-Rich Repeat-Containing 8 (LRRC8A-E) proteins as the molecular components of VRAC opens the field to elucidate their function in the physiology of TM and glaucoma. Human TM cells derived from non-glaucomatous donors and from open-angle glaucoma patients were used to determine the expression and the functional activity of LRRC8-mediated channels. Expression levels of LRRC8A-E subunits were decreased in HTM glaucomatous cells compared to normotensive HTM cells. Consequently, the activity of VRAC currents and volume regulation of TM cells were significantly affected. Impaired cell volume regulation will likely contribute to altered aqueous outflow and intraocular pressure.

GPR158 in the Visual System: Homeostatic Role in Regulation of Intraocular Pressure

Purpose: GPR158 is a newly characterized family C G-protein-coupled receptor, previously identified in functional screens linked with biological stress, including one for susceptibility to ocular hypertension/glaucoma induced by glucocorticoid stress hormones. In this study, we investigated GPR158 function in the visual system. Methods: Gene expression and protein immunolocalization analyses were performed in mouse and human brain and eye to identify tissues where GPR158 might function. Gene expression was perturbed in mice, and in cultures of human trabecular meshwork cells of the aqueous outflow pathway, to investigate function and mechanism. Results: GPR158 is highly expressed in the brain, and in this study, we show prominent expression specifically in the visual center of the cerebral cortex. Expression was also observed in the eye, including photoreceptors, ganglion cells, and trabecular meshwork. Protein was also localized to the outer plexiform layer of the neural retina. Gpr158 deficiency in knockout (KO) mice conferred short-term protection against the intraocular pressure increase that occurred with aging, but this was reversed over time. Most strikingly, the pressure lowering effect of the acute stress hormone, epinephrine, was negated in KO mice. In contrast, no disruption of the electroretinogram was observed. Gene overexpression in cell cultures enhanced cAMP production in response to epinephrine, suggesting a mechanism for intraocular pressure regulation. Overexpression also increased survival of cells subjected to oxidative stress linked to ocular hypertension, associated with TP53 pathway activation. Conclusions: These findings implicate GPR158 as a homeostatic regulator of intraocular pressure and suggest GPR158 could be a pharmacological target for managing ocular hypertension.

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.

Mutant myocilin impacts sarcomere ultrastructure in mouse gastrocnemius muscle

Myocilin (MYOC) is the gene with mutations most common in glaucoma. In the eye, MYOC is in trabecular meshwork, ciliary body, and retina. Other tissues with high MYOC transcript levels are skeletal muscle and heart. To date, the function of wild-type MYOC remains unknown and how mutant MYOC causes high intraocular pressure and glaucoma is ambiguous. By investigating mutant MYOC in a non-ocular tissue we hoped to obtain novel insight into mutant MYOC pathology. For this study, we utilized a transgenic mouse expressing human mutant MYOC Y437H protein and we examined its skeletal (gastrocnemius) muscle phenotype. Electron micrographs showed that sarcomeres in the skeletal muscle of mutant CMV-MYOC-Y437H mice had multiple M-bands. Western blots of soluble muscle lysates from transgenics indicated a decrease in two M-band proteins, myomesin 1 (MYOM1) and muscle creatine kinase (CKM). Immunoprecipitation identified CKM as a MYOC binding partner. Our results suggest that binding of mutant MYOC to CKM is changing sarcomere ultrastructure and this may adversely impact muscle function. We speculate that a person carrying the mutant MYOC mutation will likely have a glaucoma phenotype and may also have undiagnosed muscle ailments or vice versa, both of which will have to be monitored and treated.

Binding of a glaucoma-associated myocilin variant to the αB-crystallin chaperone impedes protein clearance in trabecular meshwork cells

Myocilin (MYOC) was discovered more than 20 years ago and is the gene whose mutations are most commonly observed in individuals with glaucoma. Despite extensive research efforts, the function of WT MYOC has remained elusive, and how mutant MYOC is linked to glaucoma is unclear. Mutant MYOC is believed to be misfolded within the endoplasmic reticulum, and under normal physiological conditions misfolded MYOC should be retro-translocated to the cytoplasm for degradation. To better understand mutant MYOC pathology, we CRISPR-engineered a rat to have a MYOC Y435H substitution that is the equivalent of the pathological human MYOC Y437H mutation. Using this engineered animal model, we discovered that the chaperone αB-crystallin (CRYAB) is a MYOC-binding partner and that co-expression of these two proteins increases protein aggregates. Our results suggest that the misfolded mutant MYOC aggregates with cytoplasmic CRYAB and thereby compromises protein clearance mechanisms in trabecular meshwork cells, and this process represents the primary mode of mutant MYOC pathology. We propose a model by which mutant MYOC causes glaucoma, and we propose that therapeutic treatment of patients having a MYOC mutation may focus on disrupting the MYOC-CRYAB complexes.

Intraocular Pressure Increases After Intraarticular Knee Injection With Triamcinolone but Not Hyaluronic Acid

BACKGROUND

Intraarticular steroid injections are a common first-line therapy for severe osteoarthritis, which affects an estimated 27 million people in the United States. Although topical, oral, intranasal, and inhalational steroids are known to increase intraocular pressure in some patients, the effect of intraarticular steroid injections on intraocular pressure has not been investigated, to the best of our knowledge. If elevated intraocular pressure is sustained for long periods of time or is of sufficient magnitude acutely, permanent loss of the visual field can occur.

QUESTIONS/PURPOSES

How does intraocular pressure change 1 week after an intraarticular knee injection either with triamcinolone acetonide or hyaluronic acid?

METHODS

A nonrandomized, nonblinded prospective cohort study was conducted at an outpatient, ambulatory orthopaedic clinic. This study compared intraocular pressure elevation before and 1 week after intraarticular knee injection of triamcinolone acetonide versus hyaluronic acid for management of primary osteoarthritis of the knee. Patients self-selected to be injected in their knee with either triamcinolone acetonide or hyaluronic acid before being informed of the study. The primary endpoint was intraocular pressure elevation of ≥ 7 mm Hg 1 week after injection. This cutoff is determined as the minimum significant pressure change in the ophthalmology literature recognized as an intermediate responder to steroids. Intraocular pressure was measured using a handheld Tono-Pen® applanation device. This device is frequently used in intraocular pressure measurement in clinical and research settings; 10 sequential measurements are obtained and averaged with a confidence interval. Only measurements with a 95% confidence interval were used. Over a 6-month period, a total of 96 patients were approached to enroll in the study. Sixty-two patients out of 96 approached (65%) agreed. Thirty-one (50%) were injected with triamcinolone and 31 (50%) were injected with hyaluronic acid. Patients with osteoarthritis of the knee who were suitable candidates for either a steroid injection or hyaluronic acid injection were included in the study. Exclusion criteria included previous glaucoma surgery, previous corneal injury precluding use of a Tono-Pen, current acute or chronic steroid use, and diagnosis of glaucoma other than primary open-angle. Patients with elevated intraocular pressure at the 1-week timepoint were invited to return at 1 month for repeat measurement; however, only five of nine (55.6%) were able to do so. The mean age of the total population was 64.1 ± 11.65 years. There were 46 (74%) women and 16 men. Patient in the hyaluronic acid injection group were younger than the triamcinolone group, 59.5 ± 11.7 versus 68.7 ± 9.7 years of age (p < 0.003).

RESULTS

The mean intraocular pressure increased by 2.79 mm Hg 1 week after treatment with triamcinolone, but it did not change among those patients treated with hyaluronic acid (2.79 ± 9.9 mm Hg versus -0.14 ± 2.96 mm Hg; mean difference 2.93 mm Hg; 95% confidence interval, -0.71 to 6.57 mm Hg; p = 0.12). More patients who received triamcinolone injections developed an increase in intraocular pressure > 7 mm than did those who received hyaluronic acid (29% [nine of 29] versus 0% [zero of 31]; p = 0.002). Of the nine patients who developed elevated intraocular pressure after a triamcinolone injection, five returned for reevaluation 1 month later, and four of them had pressures that remained elevated > 7 mm Hg from baseline.

CONCLUSIONS

There appears to be an associated intraocular pressure elevation found in patients who have undergone a triamcinolone injection of the knee. Further larger scale randomized investigations are warranted to determine the longevity of this pressure elevation as well as long-term clinical implications, including optic nerve damage and visual field loss.

LEVEL OF EVIDENCE

Level II, therapeutic study.

Research progress on human genes involved in the pathogenesis of glaucoma (Review)

Glaucoma is the leading cause of irreversible blindness globally. It is known that the incidence of glaucoma is closely associated with inheritance. A large number of studies have suggested that genetic factors are involved in the occurrence and development of glaucoma, and even affect the drug sensitivity and prognosis of glaucoma. In the present review, 22 loci of glaucoma are presented, including the relevant genes (myocilin, interleukin 20 receptor subunit B, optineurin, ankyrin repeat- and SOCS box-containing protein 10, WD repeat-containing protein 36, EGF-containing fibulin-like extracellular matrix protein 1, neurotrophin 4, TANK-binding kinase 1, cytochrome P450 subfamily I polypeptide 1, latent transforming growth factor β binding protein 2 and TEK tyrosine kinase endothelial) and 74 other genes (including toll-like receptor 4, sine oculis homeobox Drosophila homolog of 1, doublecortin-like kinase 1, RE repeats-encoding gene, retinitis pigmentosa GTPase regulator-interacting protein, lysyl oxidase-like protein 1, heat-shock 70-kDa protein 1A, baculoviral IAP repeat-containing protein 6, 5,10-methylenetetrahydrofolate reductase and nitric oxide synthase 3 and nanophthalmos 1) that are more closely associated with glaucoma. The pathogenesis of these glaucoma-associated genes, glaucomatous genetics and genetic approaches, as well as glaucomatous risk factors, including increasing age, glaucoma family history, high myopia, diabetes, ocular trauma, smoking, intraocular pressure increase and/or fluctuation were also discussed.

Epitope mapping of commercial antibodies that detect myocilin

The presence of myocilin is often used in the process of validating trabecular meshwork (TM) cells and eye tissues, but the antibody reagents used for detection are poorly characterized. Indeed, for over a century, researchers have been using antibodies to track proteins of interest in a variety of biological contexts, but many antibodies remain ill-defined at the molecular level and in their target epitope. Such issues have prompted efforts from major funding agencies to validate reagents and combat reproducibility issues across biomedical sciences. Here we characterize the epitopes recognized by four commercial myocilin antibodies, aided by structurally and biochemically characterized myocilin fragments. All four antibodies recognize enriched myocilin secreted from human TM cell media. The detection of myocilin fragments by ELISA and Western blot reveal a variety of epitopes across the myocilin polypeptide chain. A more precise understanding of myocilin antibody targets, including conformational specificity, should aid the community in standardizing protocols across laboratories and in turn, lead to a better understanding of eye physiology and disease.

A novel single nucleotide polymorphism in exon 3 of MYOC enhances the risk of glaucoma

Genetic polymorphismsof MYOCalter the myocilin protein,which leads to disruption of thenormal regulation of intraocular pressure (IOP) that ultimately causes glaucoma.Theaim of the present study was to identify the polymorphism in exon 3 of the MYOC gene of theglaucoma patients in Lahore, Pakistan. We conducted a case-control study with 100 patients and 100 controls subjects. We extracted DNA from blood samples,amplified the target DNA fragmentby PCR, and identifiedpolymorphisms through sequencing. We observed that the allelic and genotypic frequencies of rs74315341 and rs879255525 were associated with glaucoma in our patient population. The polymorphism atrs74315341 led to the substitutionof serine for arginine,whereas the polymorphism at rs879255525 led to the substitution ofasparagine for lysine. The haplotype TGAAGCCATTTC was associated with disease onset, whereas the haplotype GGAAGCCATTTC was protective against disease development. In conclusion, weidentified MYOC gene polymorphisms in susceptible regions that were associated withglaucoma onset among the Lahore patient population.This is the first report to identify a novel mutation in rs879255525 in exon 3 of the MYOC genethat is associated withglaucoma.

Full-length myocilin protein is purified from mammalian cells as a dimer

Myocilin (MYOC) is a secreted protein found in human aqueous humor (AH) and mutations in the MYOC gene are the most common mutation observed in glaucoma patients. Human AH analyzed under non-reducing conditions suggests that MYOC is not normally found in a monomeric form, but rather is predominantly dimeric. Although MYOC was first reported almost 20 years ago, a technical challenge still faced by researchers is an inability to isolate full-length MYOC protein for experimental purposes. Herein we describe two methods by which to isolate sufficient quantities of human full-length MYOC protein from mammalian cells. One method involved identification of a cell line (HeLa S3) that would secrete full-length protein (15 mg/L) while the second method involved a purification approach from 293 cells requiring identification and modification of an internal MYOC cleavage site (Glu214/Leu215). MYOC protein yield from 293 cells was improved by mutation of two MYOC N-terminal cysteines (C47 and C61) to serines. Analytical size exclusion chromatography of our full-length MYOC protein purified from 293 cells indicated that it is predominantly dimeric and we propose a structure for the MYOC dimer. We hope that by providing methods to obtain MYOC protein, researchers will be able to utilize the protein to obtain new insights into MYOC biology. The ultimate goal of MYOC research is to better understand this target so we can help the patient that carries a MYOC mutation retain vision and maintain quality of life.

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

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

Pentablock copolymer dexamethasone nanoformulations elevate MYOC: in vitro liberation, activity and safety in human trabecular meshwork cells

AIM

The aim of this study is to examine the elevation of MYOC in long-term treatment of human trabecular meshwork (HTM) cells using dexamethasone (DEX) encapsulated pentablock (PB) copolymer-based nanoparticles (NPs) (DEX-PB-NPs).

MATERIALS & METHODS

PB copolymers and DEX-PB-NPs were synthesized and characterized using nuclear magnetic resonance, gel permeation chromatography, and X-ray diffraction analyses. MYOC levels secreted from HTM cells were measured by western blot (WB) analysis.

RESULTS

DEX-PB-NPs were formulated in the size range of 109 ± 3.77 nm (n = 3). A long term DEX release from the NPs was observed over three months. Cell viability and cytotoxicity were not affected up to 12 weeks of treatment with PB-copolymer or DEX-PB-NPs. WB data from five HTM cell strains showed that MYOC levels increased by 5.2 ± 1.3, 7.4 ± 4.3, and 2.8 ± 1.1-fold in the presence of DEX-PB-NPs compared with 9.2 ± 3.8, 2.2 ± 0.5, and 1.5 ± 0.3-fold at 4, 8 and 12 weeks in control-DEX treatment group, respectively (n = 5). Based on the decline in MYOC levels after withdrawal of DEX from control wells, DEX-PB-NPs released the DEX for at least 10 weeks.

CONCLUSION

The treatment of HTM cells using DEX-PB-NPs were analyzed in this study. The in vitro cell-based system developed here is a valuable tool for determining the safety and effects of steroids released from polymeric NPs.

Small-molecule inhibition of Wnt signaling abrogates dexamethasone-induced phenotype of primary human trabecular meshwork cells

Trabecular meshwork (TM) cells are the governing regulators of the TM structure. When the functionality of these cells is impaired, the structure of the TM is perturbed which often results in increased ocular hypertension. High intraocular pressure is the most significant risk factor for steroid-induced glaucoma. Dexamethasone (Dex)-induced phenotype of TM cells is widely utilized as a model system to gain insight into mechanisms underlying damaged TM in glaucoma. In this study, to assess the possible role of abnormal Wnt signaling in steroid-induced glaucoma, we analyzed the effects of small-molecule Wnt signaling modulators on Dex-induced expression extracellular matrix proteins of primary human TM cells. While Dex-treated TM cells exhibited increased collagen and fibronectin expression, we found that Wnt signaling inhibitor 3235-0367 suppressed these Dex-induced effects. We therefore propose that Wnt signaling plays an important role in Dex-mediated impairment of TM cell functions. Moreover, the use of small-molecule Wnt signaling inhibitors to treat TM cells may provide an opportunity of restoring TM tissue in steroid-induced glaucoma.

Dexamethasone-Induced Ocular Hypertension in Mice: Effects of Myocilin and Route of Administration

Glucocorticoid (GC)-induced ocular hypertension (OHT) is a serious adverse effect of prolonged GC therapy that can lead to iatrogenic glaucoma and permanent vision loss. An appropriate mouse model can help us understand precise molecular mechanisms and etiology of GC-induced OHT. We therefore developed a novel, simple, and reproducible mouse model of GC-induced OHT. GC-induced myocilin expression in the trabecular meshwork (TM) has been suggested to play an important role in GC-induced OHT. We further determined whether myocilin contributes to GC-OHT. C57BL/6J mice received weekly periocular conjunctival fornix injections of a dexamethasone-21-acetate (DEX-Ac) formulation. Intraocular pressure (IOP) elevation was relatively rapid and significant, and correlated with reduced conventional outflow facility. Nighttime IOPs were higher in ocular hypertensive eyes compared to daytime IOPs. DEX-Ac treatment led to increased expression of fibronectin, collagen I, and α-smooth muscle actin in the TM in mouse eyes. No changes in body weight indicated no systemic toxicity associated with DEX-Ac treatment. Wild-type mice showed increased myocilin expression in the TM on DEX-Ac treatment. Both wild-type and Myoc^-/- mice had equivalent and significantly elevated IOP with DEX-Ac treatment every week. In conclusion, our mouse model mimics many aspects of GC-induced OHT in humans, and we further demonstrate that myocilin does not play a major role in DEX-induced OHT in mice.

Myocilin expression is regulated by retinoic acid in the trabecular meshwork-derived cellular environment

Glaucoma is the leading cause of irreversible blindness and is usually classified as angle closure and open angle glaucoma (OAG). Primary open angle glaucoma represents the most frequent clinical presentation leading to ganglion cell death and optic nerve degeneration as a main consequence of an intraocular pressure' (IOP) increase. The mechanisms of this IOP increase in such pathology remain unclear but one protein called Myocilin could be a part of the puzzle in the trabecular meshwork (TM). Previously described to be transcriptionally regulated by glucocorticoids, the comprehension of the trabecular regulation of Myocilin' expression has only weakly progressed since 15 years. Due to the essential molecular and cellular implications of retinoids' pathway in eye development and physiology, we investigate the potential role of the retinoic acid in such regulation and expression. This study demonstrates that the global retinoids signaling machinery is present in immortalized TM cells and that Myocilin (MYOC) expression is upregulated by retinoic acid alone or combined with a glucocorticoid co-treatment. This regulation by retinoic acid acts through the MYOC promoter which contains a critical cluster of four retinoic acid responsive elements (RAREs), with the RARE-DR2 presenting the strongest effect and binding the RARα/RXRα heterodimer. All together, these results open up new perspectives for the molecular understanding glaucoma pathophysiology and provide further actionable clues on Myocilin gene regulation.

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.

Differential response and withdrawal profile of glucocorticoid-treated human trabecular meshwork cells

The goal of the study was to examine secreted protein response and withdrawal profiles from cultured human trabecular meshwork (HTM) cells following short- and long-term glucocorticoid treatment. Primary cultures of five human HTM cell strains isolated from 5 different individual donor eyes were tested. Confluent HTM cells were differentiated in culture media containing 1% FBS for at least one week, and then treated with Dexamethasone (Dex, 100 nM) 3 times/week for 1 or 4 weeks. Cell culture supernatants were collected 3 times per week for 8 weeks. Secretion profiles of myocilin (MYOC), matrix metalloproteinase-2 (MMP2) and fibronectin (FN) were determined by Western blot analysis and MMP2 activity by zymography. Dex treatment reduced MMP2 expression and activity, returning to normal levels shortly after Dex withdrawal in 5 HTM cell strains. All five cell strains significantly upregulated MYOC in response to Dex treatment by an average of 17-fold, but recovery to basal levels after Dex withdrawal took vastly different periods of time depending on cell strain and treatment duration. Dex treatment significantly increased FN secretion in all strains but one, which decreased FN secretion in the presence of Dex. Interestingly, secretion of FN and MYOC negatively correlated during a 4 week recovery period following 4 weeks of Dex treatment. Taken together, the time course and magnitude of response and recovery for three different secreted, extracellular matrix-associated proteins varied greatly between HTM cell strains, which may underlie susceptibility to glucocorticoid-induced ocular hypertension.

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

Purpose

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

Methods

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

Results

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

Conclusions

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

The many faces of the trabecular meshwork cell

With the combined purpose of facilitating useful vision over a lifetime, a number of ocular cells have evolved specialized features not found elsewhere in the body. The trabecular meshwork (TM) cell at the irido-corneal angle, which is a key regulator of intraocular pressure, is no exception. Examination of cells in culture isolated from the human TM has shown that they are unique in many ways, displaying characteristic features of several different cell types. Thus, these neural crest derived cells display expression patterns and behaviors typical of endothelia, fibroblasts, smooth muscle and macrophages, owing to the multiple roles and two distinct environments where they operate to maintain intraocular pressure homeostasis. In most individuals, TM cells function normally over a lifetime in the face of persistent stressors, including phagocytic, oxidative, mechanical and metabolic stress. Study of TM cells isolated from ocular hypertensive eyes has shown a compromised ability to perform their daily duties. This review highlights the many responsibilities of the TM cell and its challenges, progress in our understanding of TM biology over the past 30 years, as well as discusses unanswered questions about TM dysfunction that results in IOP dysregulation and glaucoma.

Hic-5 Regulates Actin Cytoskeletal Reorganization and Expression of Fibrogenic Markers and Myocilin in Trabecular Meshwork Cells

PURPOSE

To explore the role of inducible focal adhesion (FA) protein Hic-5 in actin cytoskeletal reorganization, FA formation, fibrogenic activity, and expression of myocilin in trabecular meshwork (TM) cells.

METHODS

Using primary cultures of human TM (HTM) cells, the effects of various external factors on Hic-5 protein levels, as well as the effects of recombinant Hic-5 and Hic-5 small interfering RNA (siRNA) on actin cytoskeleton, FAs, myocilin, α-smooth muscle actin (αSMA), and collagen-1 were determined by immunofluorescence and immunoblot analyses.

RESULTS

Hic-5 distributes discretely to the FAs in HTM cells and throughout the TM and Schlemm's canal of the human aqueous humor (AH) outflow pathway. Transforming growth factor-β2 (TGF-β2), endothelin-1, lysophosphatidic acid, hydrogen peroxide, and RhoA significantly increased Hic-5 protein levels in HTM cells in association with reorganization of actin cytoskeleton and FAs. While recombinant Hic-5 induced actin stress fibers, FAs, αv integrin redistribution to the FAs, increased levels of αSMA, collagen-1, and myocilin, Hic-5 siRNA suppressed most of these responses in HTM cells. Hic-5 siRNA also suppressed TGF-β2-induced fibrogenic activity and dexamethasone-induced myocilin expression in HTM cells.

CONCLUSIONS

Taken together, these results reveal that Hic-5, whose levels were increased by various external factors implicated in elevated intraocular pressure, induces actin cytoskeletal reorganization, FAs, expression of fibrogenic markers, and myocilin in HTM cells. These characteristics of Hic-5 in TM cells indicate its importance in regulation of AH outflow through the TM in both normal and glaucomatous eyes.

Tetramethylpyrazine (TMP), an Active Ingredient of Chinese Herb Medicine Chuanxiong, Attenuates the Degeneration of Trabecular Meshwork through SDF-1/CXCR4 Axis

Background

A traditional Chinese medicine, Tetramethylpyrazine (TMP), has been prescribed as a complementary treatment for glaucoma to improve patient prognosis. However, the pharmacological mechanism of action of TMP is poorly understood. In previous studies, we demonstrated that TMP exerts potent inhibitory effects on neovascularization, suppresses the tumorigenic behavior of glioma cells, and protects neural cells by regulating CXCR4 expression. Here, we further investigated whether the SDF-1/CXCR4 pathway is also involved in the TMP-mediated activity in trabecular meshwork cells.

Methodology/Principal Findings

CXCR4 expression was examined by quantitative real-time PCR in trabecular and iris specimens from 54 primary open-angle glaucoma (POAG) patients who required surgery and 19 non-glaucomatous donors. Our data revealed markedly elevated CXCR4 expression in the trabecular meshwork of POAG patients compared with that of controls. Consistently, CXCR4 expression was much higher in glaucomatous trabecular meshwork cells than in normal trabecular meshwork cells. Using RT-PCR and western blot assays, we determined that glaucoma-related cytokines and dexamethasone (DEX) also significantly up-regulated CXCR4 expression in primary human trabecular meshwork (PHTM) cells. Moreover, the TGF-β1-mediated induction of CXCR4 expression in PHTM cells was markedly down-regulated by TMP compared with control treatment (PBS) and the CXCR4 antagonist AMD3100. In addition, TMP could counteract the TGF-β1-induced effects on stress fiber accumulation and expansion of PHTM cells. TMP markedly suppressed the migration of PHTM cells stimulated by TGF-β1 in transwell and scratch wound assays. TMP also suppressed the extracellular matrix (ECM) accumulation induced by TGF-β2.

Conclusions

Our findings demonstrate that CXCR4 might be involved in the pathogenetic changes in the trabecular meshwork of patients with POAG. Additionally, TMP might exert its beneficial effects in POAG patients by down-regulating CXCR4 expression.

Elevation of intraocular pressure in rodents using viral vectors targeting the trabecular meshwork

Rodents are increasingly being used as glaucoma models to study ocular hypertension, optic neuropathy, and retinopathy. A number of different techniques are used to elevate intraocular pressure in rodent eyes by artificially obstructing the aqueous outflow pathway. Another successful technique to induce ocular hypertension is to transduce the trabecular meshwork of rodent eyes with viral vectors expressing glaucoma associated transgenes to provide more relevant models of glaucomatous damage to the trabecular meshwork. This technique has been used to validate newly discovered glaucoma pathogenesis pathways as well as to develop rodent models of primary open angle glaucoma. Ocular hypertension has successfully been induced by adenovirus 5 mediated delivery of mutant MYOC, bioactivated TGFβ2, SFRP1, DKK1, GREM1, and CD44. Advantages of this approach are: selective tropism for the trabecular meshwork, the ability to use numerous mouse strains, and the relatively rapid onset of IOP elevation. Disadvantages include mild-to-moderate ocular inflammation induced by the Ad5 vector and sometimes transient transgene expression. Current efforts are focused at discovering less immunogenic viral vectors that have tropism for the trabecular meshwork and drive sufficient transgene expression to induce ocular hypertension. This viral vector approach allows rapid proof of concept studies to study glaucomatous damage to the trabecular meshwork without the expensive and time-consuming generation of transgenic mouse lines.

Olfactomedin-1 Has a V-shaped Disulfide-linked Tetrameric Structure

Olfactomedin-1 (Olfm1; also known as noelin and pancortin) is a member of the olfactomedin domain-containing superfamily and a highly expressed neuronal glycoprotein important for nervous system development. It binds a number of secreted proteins and cell surface-bound receptors to induce cell signaling processes. Using a combined approach of x-ray crystallography, solution scattering, analytical ultracentrifugation, and electron microscopy we determined that full-length Olfm1 forms disulfide-linked tetramers with a distinctive V-shaped architecture. The base of the “V” is formed by two disulfide-linked dimeric N-terminal domains. Each of the two V legs consists of a parallel dimeric disulfide-linked coiled coil with a C-terminal β-propeller dimer at the tips. This agrees with our crystal structure of a C-terminal coiled-coil segment and β-propeller combination (Olfm1^coil-Olf) that reveals a disulfide-linked dimeric arrangement with the β-propeller top faces in an outward exposed orientation. Similar to its family member myocilin, Olfm1 is stabilized by calcium. The dimer-of-dimers architecture suggests a role for Olfm1 in clustering receptors to regulate signaling and sheds light on the conformation of several other olfactomedin domain family members.

Identification of a Novel Mucin Gene HCG22 Associated With Steroid-Induced Ocular Hypertension

PURPOSE

The pathophysiology of ocular hypertension (OH) leading to primary open-angle glaucoma shares many features with a secondary form of OH caused by treatment with glucocorticoids, but also exhibits distinct differences. In this study, a pharmacogenomics approach was taken to discover candidate genes for this disorder.

METHODS

A genome-wide association study was performed, followed by an independent candidate gene study, using a cohort enrolled from patients treated with off-label intravitreal triamcinolone, and handling change in IOP as a quantitative trait.

RESULTS

An intergenic quantitative trait locus (QTL) was identified at chromosome 6p21.33 near the 5' end of HCG22 that attained the accepted statistical threshold for genome-level significance. The HCG22 transcript, encoding a novel mucin protein, was expressed in trabecular meshwork cells, and expression was stimulated by IL-1, and inhibited by triamcinolone acetate and TGF-β. Bioinformatic analysis defined the QTL as an approximately 4 kilobase (kb) linkage disequilibrium block containing 10 common single nucleotide polymorphisms (SNPs). Four of these SNPs were identified in the National Center for Biotechnology Information (NCBI) GTEx eQTL browser as modifiers of HCG22 expression. Most are predicted to disrupt or improve motifs for transcription factor binding, the most relevant being disruption of the glucocorticoid receptor binding motif. A second QTL was identified within the predicted signal peptide of the HCG22 encoded protein that could affect its secretion. Translation, O-glycosylation, and secretion of the predicted HCG22 protein was verified in cultured trabecular meshwork cells.

CONCLUSIONS

Identification of two independent QTLs that could affect expression of the HCG22 mucin gene product via two different mechanisms (transcription or secretion) is highly suggestive of a role in steroid-induced OH.

Ser341Pro MYOC gene mutation in a family with primary open-angle glaucoma

Glaucoma is known to induce visual impairment and blindness. The aim of the present study was to determine the clinical and genetic findings of a family with primary open-angle glaucoma (POAG). A family diagnosed with glaucoma was examined clinically and followed up for five years. Genomic DNA was extracted from the venous blood of 12 family members, and of 100 healthy individuals. The mode of inheritance was determined by the pedigree analysis. The third exon and its flanking introns of myocilin (MYOC) were amplified, and quantitative polymerase chain reaction (qPCR) products were sequenced. The restriction fragment length polymorphism analysis was performed on samples from the 12 family members and 100 normal controls. The predicted effects of the detected variants on the secondary structure of the MYOC protein were analyzed by the Garnier-Osguthorpe-Robson method. In this family, three members were diagnosed with POAG, and one member with ocular hypertension. The mode of inheritance of the family was autosomal dominant with six members being genetically affected. The heterozygous mutation was identified in the third exon of MYOC that revealed a T → C transition at position 1021 (p.S341P), which switched serine (Ser) to proline (Pro). This is a missense mutation eliminating a CviKI-1 restriction site that segregated the affected members. Secondary structure prediction of p.S341P suggested that the MYOC protein was misfolded. Ser341Pro MYOC mutation was detected in the family with POAG. The clinical and genetic characteristics of this mutation require further investigation. The mutation spectrum of MYOC may be expanded for a better diagnosis and treatment for POAG patients.

Thermally labile components of aqueous humor potently induce osteogenic potential in adipose-derived mesenchymal stem cells

Adipose-derived mesenchymal stem cells (ASCs) hold promise for use in cell-based therapies. Their intrinsic anti-inflammatory properties are potentially useful for treatments of inflammatory conditions such as uveitis, while their ability to differentiate along multiple cell lineages suggests use in regenerating damaged or degenerated tissue. However, how ASCs will respond to the intraocular environment is poorly studied. We have recently reported that aqueous humor (AH), the fluid that nourishes the anterior segment of the eye, potently increases alkaline phosphatase (ALP) activity of ASCs, indicating osteogenic differentiation. Here, we expand on our previous findings to better define the nature of this response. To this end, we cultured ASCs in the presence of 0, 5, 10, and 20% AH and assayed them for ALP activity. We found ALP activity correlates with increasing AH concentrations from 5 to 20%, and that longer treatments result in increased ALP activity. By using serum free media and pretreating AH with dextran-coated charcoal, we found that serum and charcoal-adsorbable AH components augment but are not required for this response. Further, by heat-treating the AH, we established that thermally labile components are required for the osteogenic response. Finally, we showed myocilin, a protein present in AH, could induce ALP activity in ASCs. However, this was to a lesser extent than untreated 5% AH, and myocilin could only partially rescue the effect after heat treatment, documenting there were additional thermally labile constituents of AH involved in the osteogenic response. Our work adds to the understanding of the induction of ALP in ASCs following exposure to AH, providing important insight in how ASCs will be influenced by the ocular environment. In conclusion, increased osteogenic potential upon exposure to AH represents a potential challenge to developing ASC cell-based therapies directed at the eye.

NFATc1 activity regulates the expression of myocilin induced by dexamethasone

Mutations in the myocilin gene (MYOC) account for 10% of juvenile open-angle glaucoma cases and 3-4% of adult onset primary open-angle glaucoma cases. It is a secreted glycoprotein found in many ocular and non-ocular tissues and has been linked to elevated intraocular pressure. In human trabecular meshwork (HTM) cells, MYOC expression can be induced by the glucocorticoid dexamethasone (DEX). In this study we examined the role of the calcineurin/NFATc1 (Nuclear Factor of Activated T-cells) pathway in the DEX induction of MYOC in HTM cells. In post-confluent HTM cells treated with either 500 nM DEX or 0.1% ethanol (EtOH; vehicle control) for 0-6 days both protein and mRNA levels of MYOC were increased while DEX was present. The protein and mRNA levels remained elevated for an additional 12 days after the removal of DEX. Only 1 day of DEX treatment was sufficient to trigger a sustained increase in MYOC mRNA that lasted for 4 days after the removal of DEX. Similar to other studies, myocilin protein expression was not seen until the second day of DEX treatment while mRNA increased within one day of DEX indicating that this is a secondary glucocorticoid response. To determine if MYOC gene expression was regulated by calcineurin/NFATc1, HTM cells were pre-treated for 1 h with the calcineurin inhibitors cyclosporin A or INCA-6 prior to the addition of DEX or EtOH for 2 days. NFATc1 siRNA was used to determine if NFATc1 was required for MYOC mRNA expression. Cells were also treated with the ionophone ionomycin to determine if increased cytosolic calcium affected MYOC expression. These studies showed that the DEX induced increase in MYOC mRNA could be inhibited with either cyclosporin A or INCA-6 or by transfection with NFATc1 siRNA and that ionomycin was unable to increase MYOC mRNA. Immunofluorescence microscopy was also performed to determine if DEX caused the nuclear translocation of NFATc1. Immunostaining showed that NFATc1 relocated to the nucleus within 15 min of DEX treatment and remained there for up to 2 h. The data suggest that the DEX-induced increase in MYOC expression activates a calcineurin and NFATc1 pathway in a calcium independent mechanism.

Differential gene expression in glaucoma

In glaucoma, regardless of its etiology, retinal ganglion cells degenerate and eventually die. Although age and elevated intraocular pressure (IOP) are the main risk factors, there are still many mysteries in the pathogenesis of glaucoma. The advent of genome-wide microarray expression screening together with the availability of animal models of the disease has allowed analysis of differential gene expression in all parts of the eye in glaucoma. This review will outline the findings of recent genome-wide expression studies and discuss their commonalities and differences. A common finding was the differential regulation of genes involved in inflammation and immunity, including the complement system and the cytokines transforming growth factor β (TGFβ) and tumor necrosis factor α (TNFα). Other genes of interest have roles in the extracellular matrix, cell-matrix interactions and adhesion, the cell cycle, and the endothelin system.

Ocular complications in children with nephrotic syndrome on long term oral steroids

OBJECTIVES

To estimate the burden of ocular complications like posterior subcapsular cataract (PSCC) and raised intra ocular pressure (IOP) in children with nephrotic syndrome on long term steroid therapy and to assess the correlation of cumulative dosage and duration of consumption of steroids with these ocular complications.

METHODS

Children between 4-18 y with nephrotic syndrome, who received steroids for at least six months, were included in this cross sectional study. Demographic, clinical and treatment details were obtained from case records. Detailed ocular evaluation was performed to detect PSCC and to measure IOP.

RESULTS

One hundred and eighteen children were screened and 82 with a median (IQR=Interquartile range) follow up of 4.2 y (2.4, 6.3 y) were included in the final analysis. The median (IQR) age of children was 9.3 y (6, 12.5 y) at recruitment. The proportion of children with PSCC and raised IOP were 22/82 (26.8 %) and 9/82 (10.97 %) respectively. PSCC was associated with older age (p = 0.009). The median cumulative dose of steroids in those with and without cataract was 338.4 mg/kg and 343.2 mg/kg respectively (p = 0.58). The median duration of steroid theraphy was 58 wk and 59 wk in the two groups respectively (p = 0.73). Of children with PSCC, 9 (42.8 %) had mild diminution of vision.

CONCLUSIONS

One in 4 and 1 in 9 children with nephrotic syndrome in the present study had PSCC and raised IOP respectively. Cumulative dose and duration of steroid therapy were not significantly associated with PSCC or raised IOP. The present study emphasizes the need for regular ocular evaluation and also to explore additional factors in causation of steroid induced ocular complications.