The identification of myocilin-associated proteins in the human trabecular meshwork

Identification of proteomic changes for dexamethasone-induced ocular hypertension using a tandem mass tag (TMT) approach

Glaucoma, characterized by ocular hypertension, is the second most common cause of vision loss worldwide. The potential mechanism, however, has yet to be elucidated. This study aimed to assess the proteomic changes in the trabecular meshwork (TM) in an observational animal model of Dexamethasone (DEX)-induced OHT. OHT was induced in Wistar rats by applying DEX topically to both eyes for 28 days. Intraocular pressure (IOP) was evaluated and TM protein expressions and protein identification were performed by a TMT-based method for comparing the changes in proteins between DEX-induced OHT and the control group. The results showed that average IOP was elevated significantly in rats of the DEX-induced OHT group compared to controls. Further, a total of 4,804 proteins in the control and DEX-induced OHT group were determined and 4,064 proteins were quantified via TMT proteomics. In total, 292 significantly abundant proteins (173 downregulated and 119 upregulated) were identified between the two groups. Proteins associated with vision, including Crystallin related proteins, filensin, rhodopsin, recoverin, phosducin were lowered in the DEX-induced OHT group relative to the control group. In summary, DEX induced extensive changes in the protein expression of TM tissue. These proteins were found to be candidate biomarkers for personalized treatment and diagnostic research in the future for improving visual health.

The role of genetic factors in the pathogenesis of primary open-angle glaucoma. Part 1. Connective tissue

The article presents an analytical review of works devoted to molecular and genetic studies in primary open-angle glaucoma from the perspective of the concept of hereditary inferiority of the connective tissue of the eye (scleral component), and the entire body as a whole, as triggers in the development of the disease. The relationship between the main theories of the pathogenesis of glaucoma optical neuropathy and the determining role of molecular and genetic mechanisms of specific changes in the eye tissue is shown. The clinical features of primary open-angle glaucoma in patients with a family history are analyzed. Potentially new directions for preclinical diagnosis of glaucoma and pathogenetically oriented therapy are proposed.

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.

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.

Epigenetic Mechanisms of the Aging Human Retina

Degenerative retinal diseases, such as glaucoma, age-related macular degeneration, and diabetic retinopathy, have complex etiologies with environmental, genetic, and epigenetic contributions to disease pathology. Much effort has gone into elucidating both the genetic and the environmental risk factors for these retinal diseases. However, little is known about how these genetic and environmental risk factors bring about molecular changes that lead to pathology. Epigenetic mechanisms have received extensive attention of late for their promise of bridging the gap between environmental exposures and disease development via their influence on gene expression. Recent studies have identified epigenetic changes that associate with the incidence and/or progression of each of these retinal diseases. Therefore, these epigenetic modifications may be involved in the underlying pathological mechanisms leading to blindness. Further genome-wide epigenetic studies that incorporate well-characterized tissue samples, consider challenges similar to those relevant to gene expression studies, and combine the genome-wide epigenetic data with genome-wide genetic and expression data to identify additional potentially causative agents of disease are needed. Such studies will allow researchers to create much-needed therapeutics to prevent and/or intervene in disease progression. Improved therapeutics will greatly enhance the quality of life and reduce the burden of disease management for millions of patients living with these potentially blinding conditions.

Cellular processing of myocilin

Background

Myocilin (MYOC) is a gene linked directly to juvenile- and adult-onset open angle glaucoma. Mutations including Pro370Leu (P370L) and Gln368stop (Q368X) have been identified in patients. In the present study, we investigated the processing of myocilin in human trabecular meshwork (TM) cells as well as in inducible, stable RGC5 cell lines.

Methodology/Principal Findings

The turnover and photoactivation experiments revealed that the endogenous myocilin in human trabecular meshwork (TM) cells was a short-lived protein. It was found that the endogenous myocilin level in TM cells was increased by treatment of lysosomal and proteasomal inhibitors, but not by autophagic inhibitor. Multiple bands immunoreactive to anti-ubiquitin were seen in the myocilin pull down, indicating that myocilin was ubiquitinated. In inducible cell lines, the turnover rate of overexpressed wild-type and mutant P370L and Q368X myocilin-GFP fusion proteins was much prolonged. The proteasome function was compromised and autophagy was induced. A decreased PSMB5 level and an increased level of autophagic marker, LC3, were demonstrated.

Conclusions/Significance

The current study provided evidence that in normal homeostatic situation, the turnover of endogenous myocilin involves ubiquitin-proteasome and lysosomal pathways. When myocilin was upregulated or mutated, the ubiquitin-proteasome function is compromised and autophagy is induced. Knowledge of the degradation pathways acting on myocilin can help in design of novel therapeutic strategies for myocilin-related glaucoma.

Tissue-based imaging model of human trabecular meshwork

We have developed a tissue-based model of the human trabecular meshwork (TM) using viable postmortem corneoscleral donor tissue. Two-photon microscopy is used to optically section and image deep in the tissue to analyze cells and extracellular matrix (ECM) within the original three-dimensional (3D) environment of the TM. Multimodal techniques, including autofluorescence (AF), second harmonic generation (SHG), intravital dye fluorescence, and epifluorescence, are combined to provide unique views of the tissue at the cellular and subcellular level. SHG and AF imaging are non-invasive tissue imaging techniques with potential for clinical application, which can be modeled in the system. We describe the following in the tissue-based model: analysis of live cellularity to determine tissue viability; characteristics of live cells based on intravital labeling; features and composition of the TM's structural ECM; localization of specific ECM proteins to regions such as basement membrane; in situ induction and expression of tissue markers characteristic of cultured TM cells relevant to glaucoma; analysis of TM actin and pharmacological effects; in situ visualization of TM, inner wall endothelium, and Schlemm's canal; and application of 3D reconstruction, modeling, and quantitative analysis to the TM. The human model represents a cost-effective use of valuable and scarce yet available human tissue that allows unique cell biology, pharmacology, and translational studies of the TM.

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.

The glaucoma-associated olfactomedin domain of myocilin is a novel calcium binding protein

Myocilin is a protein found in the trabecular meshwork extracellular matrix tissue of the eye that plays a role in regulating intraocular pressure. Both wild-type and certain myocilin variants containing mutations in the olfactomedin (OLF) domain are linked to the optic neuropathy glaucoma. Because calcium ions are important biological cofactors that play numerous roles in extracellular matrix proteins, we examined the calcium binding properties of the myocilin OLF domain (myoc-OLF). Our study reveals an unprecedented high affinity calcium binding site within myoc-OLF. The calcium ion remains bound to wild-type OLF at neutral and acidic pH. A glaucoma-causing OLF variant, myoc-OLF(D380A), is calcium-depleted. Key differences in secondary and tertiary structure between myoc-OLF(D380A) and wild-type myoc-OLF, as well as limited access to chelators, indicate that the calcium binding site is largely buried in the interior of the protein. Analysis of six conserved aspartate or glutamate residues and an additional 18 disease-causing variants revealed two other candidate residues that may be involved in calcium coordination. Our finding expands our knowledge of calcium binding in extracellular matrix proteins; provides new clues into domain structure, function, and pathogenesis for myocilin; and offers insights into highly conserved, biomedically relevant OLF domains.

Establishment of inducible wild type and mutant myocilin-GFP-expressing RGC5 cell lines

Background

Myocilin is a gene linked directly to juvenile- and adult-onset open angle glaucoma. Mutations including Gln368stop (Q368X) and Pro370Leu (P370L) have been identified in patients. The exact role of myocilin and its functional association with glaucoma are still unclear. In the present study, we established tetracycline-inducible (Tet-on) wild type and mutant myocilin-green fluorescence protein (GFP) expressing RGC5 stable cell lines and studied the changes in cell migration and barrier function upon induction.

Methodology/Principal Findings

After several rounds of selection, clones that displayed low, moderate, or high expression of wild type, Q368X or P370L myocilin-GFP upon doxycycline (Dox) induction were obtained. The levels of wild type and mutant myocilin-GFP in various clones were confirmed by Western blotting. Compared to non-induced controls, the cell migration was retarded, the actin stress fibers were fewer and shorter, and the trypsinization time needed for cells to round up was reduced when wild type or mutant myocilin was expressed. The barrier function was in addition aberrant following induced expression of wild type, Q368X or P370L myocilin. Immunoblotting further showed that tight junction protein occludin was downregulated in induced cells.

Conclusions/Significance

Tet-on inducible, stable RGC5 cell lines were established. These cell lines, expressing wild type or mutant (Q368X or P370L) myocilin-GFP upon Dox induction, are valuable in facilitating studies such as proteomics, as well as functional and pathogenesis investigations of disease-associated myocilin mutants. The barrier function was found impaired and the migration of cells was hindered with induced expression of wild type and mutant myocilin in RGC5 cell lines. The reduction in barrier function might be related to the declined level of occludin. The retarded cell migration was consistent with demonstrated myocilin phenotypes including the loss of actin stress fibers, lowered RhoA activities and compromised cell-matrix adhesiveness.

Two-photon immunofluorescence characterization of the trabecular meshwork in situ

PURPOSE

To develop an in situ model to study biological responses and glaucoma pathology in the human trabecular meshwork (TM). Characteristic TM cell- and glaucoma-associated markers were localized in situ in relation to the tissue's autofluorescent structural extracellular matrix (ECM) by two-photon excitation fluorescence optical sectioning (TPEF).

METHODS

Human donor corneoscleral (CS) tissue containing the intact aqueous drainage tract was incubated with dexamethasone (Dex) or TGF-β1, and immunostained for epifluorescence (EF) microscopy with antibodies to myocilin and alpha smooth muscle (α-SMA). Separate specimens were labeled for Type-IV collagen and fibronectin. Nuclei were stained with Hoechst 33342. Multimodal TPEF was used to visualize EF, intravital dyes, and autofluorescence (AF) in situ. Three-dimensional (3D) localization of fluorescence within the TM was analyzed using reconstruction software.

RESULTS

Autofluorescent beams, perforated sheets, and fibers, consistent with the uveal (UV), CS, and juxtacanalicular (JCT) meshwork, respectively, were captured at different depths of the TM. Type-IV collagen EF distinctly outlined the AF beams in a location consistent with basement membrane. Fibronectin EF showed a diffuse reticular pattern throughout the TM. TGF-β1-induced α-SMA expression, which was distributed perinuclearly in cells among autofluorescent structures. Dex-induced myocilin expression had both cytosolic and extracellular distributions.

CONCLUSIONS

The authors have localized markers that are characteristic of TM cells and relevant to glaucoma pathogenesis in situ using multimodal TPEF without conventional histological embedding and sectioning. Protein expression was inducible in situ and could be analyzed with respect to cells and the ECM within the 3D environment of the human TM.

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.

Myocilin and Optineurin: Differential Characteristics and Functional Consequences

Myocilin and optineurin are two genes linked to glaucoma, a major blinding disease characterized by progressive loss of retinal ganglion cells and their axons. This review describes the characteristics of myocilin and optineurin protein products and summarizes the consequences of ectopically expressed wild type and mutant myocilin and optineurin in trabecular meshwork and/or neuronal cells. Myocilin and optineurin exhibit differential characteristics and have divergent functional consequences. They contribute to the development of glaucoma likely via distinct mechanisms.

Molecular chaperone function for myocilin

PURPOSE

Myocilin is thought to be a stress response protein, but its exact molecular functions have not been established. Studies were conducted to see whether myocilin can act as a general molecular chaperone.

METHODS

Myocilin was isolated and purified from porcine trabecular meshwork (TM) cell culture media. Its ability to protect citrate synthase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and the restriction endonuclease DrdI from thermal inactivation was evaluated. Light scattering was used to evaluate thermally induced aggregation of citrate synthase. Myocilin induction was assessed after exposure of TM cells to several types of stress treatments.

RESULTS

Levels of extracellular myocilin expressed by TM cells were increased in response to mechanical stretch, heat shock, TNFα, or IL-1α. Myocilin protected citrate synthase activity against thermal inactivation for 5 minutes at 55°C in a concentration-dependent manner, with nearly full protection of 1.5 μM citrate synthase in the presence of 650 nM myocilin. Myocilin significantly reduced thermal aggregation of citrate synthase to levels 36% to 44% of control levels. Myocilin also protected GAPDH from thermal inactivation for 10 minutes at 45°C. Myocilin at 18 nM was more effective than 1 μM bovine serum albumin at protecting DrdI from thermal inactivation.

CONCLUSIONS

Myocilin is induced in response to several cellular stresses and displays general molecular chaperone activity by protecting DrdI, citrate synthase, and GAPDH from thermal inactivation. Myocilin also suppresses the thermal aggregation of citrate synthase. One function of myocilin may be to serve as a molecular chaperone.

Keeping an eye on myocilin: a complex molecule associated with primary open-angle glaucoma susceptibility

MYOC encodes a secretary glycoprotein of 504 amino acids named myocilin. MYOC is the first gene to be linked to juvenile open-angle glaucoma (JOAG) and some forms of adult-onset primary open-angle glaucoma (POAG). The gene was identified as an up-regulated molecule in cultured trabecular meshwork (TM) cells after treatment with dexamethasone and was originally referred to as trabecular meshwork-inducible glucocorticoid response (TIGR). Elevated intraocular pressure (IOP), due to decreased aqueous outflow, is the strongest known risk factor for POAG. Increasing evidence showed that the modulation of the wild-type (wt) myocilin protein expression is not causative of glaucoma while some misfolded and self-assembly aggregates of mutated myocilin may be associated with POAG in related or unrelated populations. The etiology of the disease remains unclear. Consequently, a better understanding of the molecular mechanisms underlyingPOAG is required to obtain early diagnosis, avoid potential disease progression, and develop new therapeutic strategies. In the present study, we review and discuss the most relevant studies regarding structural characterizations, expressions, molecular interactions, putative functions of MYOC gene and/or its corresponding protein in POAG etiology.

Complex genetic mechanisms in glaucoma: an overview

Glaucomas comprise a group of hereditary optic neuropathies characterized by progressive and irreversible visual field loss and damage to the optic nerve head. It is a complex disease with multiple molecular mechanisms underlying its pathogenesis. Genetic heterogeneity is the hallmark of all glaucomas and multiple chromosomal loci have been linked to the disease, but only a few genes have been characterized, viz. myocilin (MYOC), optineurin (OPTN), WDR36 and neurotrophin-4 (NTF4) in primary open angle glaucoma (POAG) and CYP1B1 and LTBP2 in congenital and developmental glaucomas. Case-control-based association studies on candidate genes involved in different stages of glaucoma pathophysiology have indicated a very limited involvement. The complex mechanisms leading to glaucoma pathogenesis indicate that it could be attributed to multiple genes with varying magnitudes of effect. In this review, we provide an appraisal of the various efforts in unraveling the molecular mystery in glaucoma and also some future directions based on the available scientific knowledge and technological developments.

Biophysical characterization of the olfactomedin domain of myocilin, an extracellular matrix protein implicated in inherited forms of glaucoma

Myocilin is an eye protein found in the trabecular extracellular matrix (TEM), within the anatomic region that controls fluid flow. Variants of myocilin, localized to its olfactomedin (OLF) domain, have been linked to inherited forms of glaucoma, a disease associated with elevated intraocular pressure. OLF domains have also been implicated in psychiatric diseases and cancers by their involvement in signaling, neuronal growth, and development. However, molecular characterization of OLFs has been hampered by challenges in recombinant expression, a hurdle we have recently overcome for the myocilin OLF domain (myoc-OLF). Here, we report the first detailed solution biophysical characterization of myoc-OLF to gain insight into its structure and function. Myoc-OLF is stable in the presence of glycosaminoglycans, as well as in a wide pH range in buffers with functional groups reminiscent of such glycosaminoglycans. Circular dichroism (CD) reveals significant β-sheet and β-turn secondary structure. Unexpectedly, the CD signature is reminiscent of α-chymotrypsin as well as another ocular protein family, the βγ-crystallins. At neutral pH, intrinsic tryptophan fluorescence and CD melts indicate a highly cooperative transition with a melting temperature of ∼55°C. Limited proteolysis combined with mass spectrometry reveals that the compact core structural domain of OLF consists of approximately residues 238-461, which retains the single disulfide bond and is as stable as the full myoc-OLF construct. The data presented here inform new testable hypotheses for interactions with specific TEM components, and will assist in design of therapeutic agents for myocilin glaucoma.

Interaction of recombinant myocilin with the matricellular protein SPARC: functional implications

PURPOSE

Myocilin is an extracellular glycoprotein with unknown function that is associated with glaucoma. Calpain II cleaves recombinant myocilin within the linker region of the protein, releasing the C-terminal olfactomedin domain from the N-terminal domain. The authors previously reported that myocilin interacts with the C-terminal region of hevin, a secretory glycoprotein belonging to the SPARC family of matricellular proteins. This study aims to investigate the interaction of myocilin with SPARC.

METHODS

Protein-protein interactions were evaluated by the yeast two-hybrid system. The positive interactions were confirmed by solid-phase binding assays using Ni-chelating HPLC purified recombinant proteins and coexpression of recombinant proteins in HEK-293T cells. Coexpression of myocilin, SPARC, and hevin in ocular tissues was identified by immunoflorescence microscopy, Western blot, and array-based gene profiling.

RESULTS

Yeast two-hybrid analyses showed that myocilin interacted with the highly conserved C-terminal extracellular calcium binding (EC) domain within SPARC and hevin. Solid-phase binding assays confirmed these interactions and showed that both myocilin and its C-terminal olfactomedin fragment interacted noncovalently with SPARC and a peptide containing the EC domain of SPARC. Full-length myocilin interacted with higher affinity with SPARC and its EC domain than the myocilin C-terminal fragment. Coexpression of the two recombinant proteins in HEK-293T cells also indicated their intracellular interaction.

CONCLUSIONS

Recombinant myocilin and SPARC interact through their C-terminal domains. The data suggest that the proteolytic processing of myocilin modulates this interaction as well as the interactions of myocilin with other extracellular matrix and matricellular proteins, further supporting a functional role for this proteolytic cleavage.

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

Mutations in the myocilin gene are associated with juvenile and adult-onset primary open-angle glaucoma. However, the pathogenic mechanisms of myocilin-induced glaucoma are still largely unknown. To investigate these mechanisms, we developed stably transfected HEK293 cell lines expressing wild-type or mutant (Y437H and I477N) myocilins under an inducible promoter. Expression of two mutant myocilins led to different levels of endoplasmic reticulum stress and increased apoptosis after treatment of cells with hydrogen peroxide. The Y437H mutant myocilin cell line showed the highest sensitivity to the oxidant treatment. Several antioxidant genes were down-regulated in the Y437H mutant myocilin cell line, but not in other cell lines. The Y437H mutant myocilin cell line also produced more reactive oxygen species than other cell lines examined. Consistent with the data obtained in cultured cells, the endoplasmic reticulum stress marker, 78 kDa glucose-regulated protein, was up-regulated, whereas antioxidant proteins, paraoxonase 2 and glutathione peroxidase 3, were down-regulated in the eye angle tissue of 18-month-old transgenic mice expressing Y437H myocilin mutant. In addition, a pro-apoptotic factor, CCAAT/enhancer-binding protein-homologous protein, was up-regulated in the aged transgenic mouse angle tissue. Our results suggest that expression of mutated myocilins may have a sensitization effect, which can lead to a severe phenotype in combination with oxidative stress. Mutant myocilins may confer different sensitivity to oxidative stress depending on the mutation.

Differential effects of myocilin and optineurin, two glaucoma genes, on neurite outgrowth

Myocilin and optineurin are two genes linked to glaucoma, a major blinding disease characterized by progressive loss of retinal ganglion cells (RGCs) and their axons. To investigate the effects of force-expressed wild-type and mutant myocilin and optineurin on neurite outgrowth in neuronal cells, we transiently transfected cells with pEGFP-N1 (mock control) as well as myocilin and optineurin plasmids including pMYOC(WT)-EGFP, pMYOC(P370L)-EGFP, pMYOC(1-367)-EGFP, pOPTN(WT)-EGFP, and pOPTN(E50K)-EGFP. PC12 cells transfected with pEGFP-N1 produced, as anticipated, long and extensive neuritis on nerve growth factor induction. The neurite length in those cells transfected with myocilin constructs was shortened and the number of neurites was also reduced. A similar inhibitory effect on neurite outgrowth was also elicited by myocilin transfection in RGC5 cells. In contrast, neither transfection of the optineurin constructs pOPTN(WT)-EGFP and pOPTN(E50K)-EGFP nor the myocilin and optineurin small-interfering RNA treatments induced significant alterations in neurite outgrowth. Transfection with the wild-type optineurin construct, but not with that of the wild-type myocilin, increased the apoptotic activity in cells. These results demonstrated that the two glaucoma genes, myocilin and optineurin, exhibited differential effects on neurite outgrowth. They may contribute to the development of neurodegenerative glaucoma via distinct mechanisms.

Glaucoma-associated myocilin: a better understanding but much more to learn

Over a decade has passed since myocilin was identified as the first gene linked to early and late-onset primary open-angle glaucoma. During this time, considerable effort has been put forth to understand the functional role myocilin has in normal and glaucomatous eyes. Myocilin is expressed in many ocular and non-ocular tissues, is found in both intracellular and extracellular spaces, and has been linked to elevations in intraocular pressure. Mutations in the myocilin gene that have been associated with glaucoma appear to confer a gain-of-functional activity rather than loss of function. Unfortunately, what the normal function of myocilin is and how alterations in the function can confer a glaucoma phenotype have yet to be elucidated. We will review the current understanding of myocilin with special emphasis on the structural makeup of the myocilin gene and protein, its possible physiological roles internal and external to ocular cells, the regulation of intraocular pressure as evidenced through the use of perfusion culture systems and animal models, and as a causative agent in some forms of glaucoma.

Myocilin is a modulator of Wnt signaling

It is well documented that mutations in the MYOCILIN gene may lead to juvenile- and adult-onset primary open-angle glaucoma. However, the functions of wild-type myocilin are still not well understood. To study the functions of human myocilin and its two proteolytic fragments, these proteins were expressed in HEK293 cells. Conditioned medium from myocilin-expressing cells, as well as purified myocilin, induced the formation of stress fibers in primary cultures of human trabecular meshwork or NIH 3T3 cells. Stress fiber-inducing activity of myocilin was blocked by antibodies against myocilin, as well as secreted inhibitors of Wnt signaling, secreted Frizzled-related protein 1 (sFRP1) or sFRP3, and beta-catenin small interfering RNA. Interaction of myocilin with sFRP1, sFRP3, and several Frizzled receptors was confirmed by immunoprecipitation experiments and by binding of myocilin to the surface of cells expressing cysteine-rich domains of different Frizzled and sFRPs. Treatment of NIH 3T3 cells with myocilin and its fragments induced intracellular redistribution of beta-catenin and its accumulation on the cellular membrane but did not induce nuclear accumulation of beta-catenin. Overexpression of myocilin in the eye angle tissues of transgenic mice stimulated accumulation of beta-catenin in these tissues. Myocilin and Wnt proteins may perform redundant functions in the mammalian eye, since myocilin modulates Wnt signaling by interacting with components of this signaling pathway.

Regulation of myocilin-associated exosome release from human trabecular meshwork cells

PURPOSE

The goal of the present study was to determine whether the release of exosomes containing MYOC from trabecular meshwork (TM) cells is constitutive or regulated.

METHODS

Conditioned media from TM cells were analyzed for MYOC-associated exosomes after treatment with IFN-gamma, porcine aqueous humor, dexamethasone, or a calcium ionophore in cells pretreated with dexamethasone. Aqueous humor was tested whole or fractionated by size exclusion filters. Exosomes from conditioned media were purified by differential centrifugation. Proteins in whole, exosome, and soluble fractions were separated by SDS-PAGE and analyzed for MYOC content by Western blot and densitometry.

RESULTS

Although treatment of TM cells with IFN-gamma increased the appearance of extracellular MYOC-associated exosomes, results were not significantly different from those of control (P = 0.13). In contrast, treatment with dexamethasone increased the appearance of MYOC in the exosome fraction by 376% (P < 0.01). The increase in MYOC-associated exosomes caused by dexamethasone was enhanced by an additional 379% after short-term exposure to ionomycin (P < 0.05). When cultured in media containing aqueous humor, MYOC-associated exosomes increased 514% over control (P < 0.01). Such an increase was diminished in cells treated with aqueous humor that was first passed through a 3-kDa or a 30-kDa, but not a 100-kDa, size exclusion filter.

CONCLUSIONS

The appearance of MYOC-associated exosomes in conditioned media from human TM cells is regulated by a corticosteroid, a calcium ionophore, and a component of aqueous humor, suggesting that TM cells respond to environmental cues by releasing MYOC-associated exosomes.

Characterization of monoclonal antibodies against the glaucoma-associated protein myocilin

Although the glaucoma-associated protein myocilin has been the focus of intensive research, its biological function is still unknown. One of the limiting factors has been the lack of well-characterized antibodies, particularly monoclonal antibodies. We describe the development of six monoclonal antibodies specific to myocilin and characterize their suitability in Western blot and immunohistochemical applications. Three of the six monoclonal antibodies recognize the N-terminus of myocilin (amino acids 33-214), two antibodies recognize the middle third of the protein (amino acids 215-368), and one antibody recognizes the C-terminus (amino acids 369-504). Isotyping revealed that all antibodies are of the IgG1 kappa class except one, which is IgG2b kappa. Purified myocilin monoclonal antibodies were able to recognize myocilin in human aqueous humor separated on denatured/reduced and native gels, and human trabecular meshwork lysate by Western blot. Myocilin was also detected by immunohistochemistry in trabecular meshwork, ciliary body, iris, cornea, sclera, choroid, and retinal pigment epithelial cells.

Immunolocalization of myocilin protein in the anterior eye of normal and primary open-angle glaucomatous dogs

OBJECTIVE

The presence of myocilin was investigated in a colony of Beagles, a canine model for inherited primary open-angle glaucoma (POAG). The myocilin protein was localized in the normal and glaucomatous canine eyes by immunohistochemistry and immunocytochemistry.

METHODS

Paraffin- and plastic-embedded specimens from the anterior uveas of 10 Beagles with inherited glaucoma (3 months to 13 years old) and 6 age-matched normal dogs were sectioned, and were then incubated with primary antibody, rabbit polyclonal antihuman MYOC IgG, overnight at 4 degrees C. Specimens were incubated with secondary antibody with one of the following: biotinylated link followed by peroxidase-labeled streptavidin and then by substrate-chromogen for light microscopy; fluorescent marker Texas red; or 18 nm colloidal gold-labeled goat antirabbit IgG for transmission electron microscopy.

RESULTS

With normal, pre- and early glaucomatous canine specimens, cell membranes of smooth muscle cells of the iris and ciliary body stained positively, as well as most resident stromal and vascular endothelial cells. The cytoplasm of cells within the nonpigmented ciliary epithelium of the ciliary body processes stained intensely, being weaker along the pars plana. Trabecular meshwork (TM) cells and surrounding extracellular matrix labeled, as well as the sclera adjacent to the angular aqueous plexus. In specimens with moderate and advanced glaucoma, greater intensity of staining was observed within TM cells and adjacent sclera, and portions of the nonpigmented epithelium of the ciliary processes. Fibrinous material labeled intensely within the posterior chamber.

CONCLUSIONS

Myocilin in the normal and glaucomatous canine eye was successfully immunolocalized. These findings with regard to the normal eye are nearly identical to those previously reported in humans, and support the original hypothesis that there is an increase in both accumulation and localization of myocilin in glaucomatous canine eyes. It also supports the possibility that changes in the activity of myocilin within the aqueous humor outflow pathway of individuals with spontaneous glaucoma are associated with the rise of intraocular pressure and subsequent development of this disease, but may not be the primary event in the initial raise in intraocular pressure in POAG in the Beagle.

Aggregated myocilin induces russell bodies and causes apoptosis: implications for the pathogenesis of myocilin-caused primary open-angle glaucoma

Primary open-angle glaucoma with elevated intraocular pressure is a leading cause of blindness worldwide. Mutations of myocilin are known to play a critical role in the manifestation of the disease. Misfolded mutant myocilin forms secretion-incompetent intracellular aggregates. The block of myocilin secretion was proposed to alter the extracellular matrix environment of the trabecular meshwork, with subsequent impediment of aqueous humor outflow leading to elevated intraocular pressure. However, the molecular pathogenesis of myocilin-caused glaucoma is poorly defined. In this study, we show that heteromeric complexes composed of wild-type and mutant myocilin were retained in the rough endoplasmic reticulum, aggregating to form inclusion bodies typical of Russell bodies. The presence of myocilin aggregates induced the unfolded protein response proteins BiP and phosphorylated endoplasmic reticulum-localized eukaryotic initiation factor-2alpha kinase (PERK) with the subsequent activation of caspases 12 and 3 and expression of C/EBP homologous protein (CHOP)/GADD153, leading to apoptosis. Our findings identify endoplasmic reticulum stress-induced apoptosis as a pathway to explain the reduction of trabecular meshwork cells in patients with myocilin-caused glaucoma. As a consequence, the phagocytotic capacity of the remaining trabecular meshwork cell population would be insufficient for effective cleaning of aqueous humor, constituting a major pathogenetic factor for the development of increased intraocular pressure in primary open-angle glaucoma.

Mitochondrial association of myocilin, product of a glaucoma gene, in human trabecular meshwork cells

The trabecular meshwork (TM), an ocular tissue next to the cornea, is a major site for regulation of the aqueous humor outflow. Malfunctioning of this tissue is believed to be responsible for development of glaucoma, a major blinding disease. Myocilin is a gene directly linked to the most common form of glaucoma. Its protein product has been localized to both intra- and extra-cellular sites in TM cells. This study was to investigate the association of myocilin with mitochondria in TM cells. In vitro mitochondrial import assays showed that myocilin was imported to the TM mitochondria, targeting to mitochondrial membranes and/or the intermembrane space. The targeting was mediated mostly via the amino-terminal region of myocilin. When myocilin expression was induced either by treatment with dexamethasone or transfection with a myocilin construct, the mitochondrial membrane potential in TM cells, as assessed by JC-1 staining, was lowered. Subcellular fractionation and Western blot analyses confirmed that a portion of myocilin sedimented with the mitochondrial fractions. Upon anti-Fas treatment to provoke apoptosis, an increase of myocilin distribution in cytosolic fraction was observed, suggesting that myocilin was partially released from mitochondrial compartments. These results confirmed the association of myocilin with TM cell mitochondria and indicated that myocilin may have a proapoptotic role in TM cells.

Mapping of glycolytic enzyme-binding sites on human erythrocyte band 3

Previous work has shown that GAPDH (glyceraldehyde-3-phosphate dehydrogenase), aldolase, PFK (phosphofructokinase), PK (pyruvate kinase) and LDH (lactate dehydrogenase) assemble into a GE (glycolytic enzyme) complex on the inner surface of the human erythrocyte membrane. In an effort to define the molecular architecture of this complex, we have undertaken to localize the binding sites of these enzymes more accurately. We report that: (i) a major aldolase-binding site on the erythrocyte membrane is located within N-terminal residues 1-23 of band 3 and that both consensus sequences D6DYED10 and E19EYED23 are necessary to form a single enzyme-binding site; (ii) GAPDH has two tandem binding sites on band 3, located in residues 1-11 and residues 12-23 respectively; (iii) a PFK-binding site resides between residues 12 and 23 of band 3; (iv) no GEs bind to the third consensus sequence (residues D902EYDE906) at the C-terminus of band 3; and (v) the LDH- and PK-binding sites on the erythrocyte membrane do not reside on band 3. Taken together, these results argue that band 3 provides a nucleation site for the GE complex on the human erythrocyte membrane and that other components near band 3 must also participate in organizing the enzyme complex.

In-vitro study of the activity of ciprofloxacin alone and in combination against strains of Pseudomonas aeruginosa with multiple antibiotic resistance

Ciprofloxacin appears to have useful activity against Pseudomonas aeruginosa. We have studied its in-vitro activity against ten strains of Ps. aeruginosa with multiple antibiotic resistance. We have confirmed that ciprofloxacin is very active against Ps. aeruginosa with minimal inhibitory concentrations ranging from 0.07 to 0.7 mg/l. Killing curves show ciprofloxacin to be rapidly bactericidal with no regrowth after 24 h. Checkerboard studies with ciprofloxacin in combination with gentamicin, azlocillin and ceftazidime show no consistent interaction. These studies suggest that ciprofloxacin should prove a useful antibiotic in treating infections caused by multiresistant Ps. aeruginosa.