'What controls aqueous humour outflow resistance?'

Inhibition of hyaluronan synthesis reduces versican and fibronectin levels in trabecular meshwork cells

Hyaluronan (HA) is a major component of the extracellular matrix (ECM) and is synthesized by three HA synthases (HAS). Similarities between the HAS2 knockout mouse and the hdf mutant mouse, which has a mutation in the versican gene, suggest that HA and versican expression may be linked. In this study, the relationship between HA synthesis and levels of versican, fibronectin and several other ECM components in trabecular meshwork cells from the anterior segment of the eye was investigated. HA synthesis was inhibited using 4-methylumbelliferone (4MU), or reduced by RNAi silencing of each individual HAS gene. Quantitative RT-PCR and immunoblotting demonstrated a reduction in mRNA and protein levels of versican and fibronectin. Hyaluronidase treatment also reduced versican and fibronectin levels. These effects could not be reversed by addition of excess glucose or glucosamine or exogenous HA to the culture medium. CD44, tenascin C and fibrillin-1 mRNA levels were reduced by 4MU treatment, but SPARC and CSPG6 mRNA levels were unaffected. Immunostaining of trabecular meshwork tissue after exposure to 4MU showed an altered localization pattern of HA-binding protein, versican and fibronectin. Reduction of versican by RNAi silencing did not affect HA concentration as assessed by ELISA. Together, these data imply that HA concentration affects synthesis of certain ECM components. Since precise regulation of the trabecular meshwork ECM composition and organization is required to maintain the aqueous humor outflow resistance and intraocular pressure homeostasis in the eye, coordinated coupling of HA levels and several of its ECM binding partners should facilitate this process.

The role of plasmalemma vesicle-associated protein (PLVAP) in endothelial cells of Schlemm's canal and ocular capillaries

Plasmalemma vesicle-associated protein (PLVAP, PV-1) is an endothelial protein that specifically localizes to diaphragms of fenestrae in fenestrated capillaries, and to stomatal diaphragms of caveolae. Here we investigated the localization of PLVAP in Schlemm's canal endothelium and ocular capillaries, and studied the structural effects of PLVAP deficiency. In mouse, pig and human eyes, immunoreactivity for PLVAP was present in fenestrated capillaries of choroid and ciliary processes, but not in the continuous capillaries of retina and ciliary muscle. In all three species staining for PLVAP was seen in the endothelia of the outflow vessels of aqueous humor e.g. Schlemm's canal (SC, mouse and human), aqueous plexus (AP, pig) and the scleral collector channels. Essentially comparable findings were observed when the expression of β-galactosidase was investigated in mutant heterozygous and homozygous PLVAP-deficient mice with LacZ inserted into the Plvap locus. By transmission electron microscopy, the vast majority of caveolae in SC endothelial cells showed a stomatal diaphragm. In addition, solitary fenestrae or minipores with a diaphragm were occasionally observed in SC or AP of all three species. In contrast, mutant Plvap(-/-) mice showed a complete absence of stomatal diaphragms in SC caveolae while no SC minipores were observed. Moreover, diaphragms were absent in fenestrae of endothelial cells in the capillaries of the ciliary processes or the choriocapillaris, findings which were associated with a substantial decrease in the number of fenestrae. PLVAP is expressed in endothelial cells of Schlemm's canal and is essential for the formation of diaphragms in vascular endothelial cells of the eye.

Endogenous production of extracellular adenosine by trabecular meshwork cells: potential role in outflow regulation

PURPOSE

To investigate the mechanisms for endogenous production of extracellular adenosine in trabecular meshwork (TM) cells and evaluate its physiological relevance to the regulation of aqueous humor outflow facility.

METHODS

Extra-cellular levels of adenosine monophosphate (AMP) and adenosine in porcine trabecular meshwork (PTM) cells treated with adenosine triphosphate (ATP), AMP, cAMP or forskolin with or without specific inhibitors of phosphodiesterases (IBMX) and CD73 (AMPCP) were determined by high-pressure liquid chromatography fluorometry. Extracellular adenosine was also evaluated in cell cultures subjected to cyclic mechanical stress (CMS) (20% stretching; 1 Hz) and after disruption of lipid rafts with methyl-β-cyclodextrin. Expression of CD39 and CD73 in porcine TM cells and tissue were examined by Q-PCR and Western blot. The effect of inhibition of CD73 on outflow facility was evaluated in perfused living mouse eyes.

RESULTS

PTM cells generated extracellular adenosine from extracellular ATP and AMP but not from extracellular cAMP. Increased intracellular cAMP mediated by forskolin led to a significant increase in extracellular adenosine production that was not prevented by IBMX. Inhibition of CD73 resulted, in all cases, in a significant decrease in extracellular adenosine. CMS induced a significant activation of extracellular adenosine production. Inhibition of CD73 activity with AMPCP in living mouse eyes resulted in a significant decrease in outflow facility.

CONCLUSIONS

These results support the concept that the extracellular adenosine pathway might play an important role in the homeostatic regulation of outflow resistance in the TM, and suggest a novel mechanism by which pathologic alteration of the TM, such as increased tissue rigidity, could lead to abnormal elevation of IOP in glaucoma.

Galectin-8 promotes cytoskeletal rearrangement in trabecular meshwork cells through activation of Rho signaling

Purpose

The trabecular meshwork (TM) cell-matrix interactions and factors that influence Rho signaling in TM cells are thought to play a pivotal role in the regulation of aqueous outflow. The current study was designed to evaluate the role of a carbohydrate-binding protein, galectin-8 (Gal8), in TM cell adhesion and Rho signaling.

Methods

Normal human TM cells were assayed for Gal8 expression by immunohistochemistry and Western blot analysis. To assess the role of Gal8 in TM cell adhesion and Rho signaling, the cell adhesion and spreading assays were performed on Gal8-coated culture plates in the presence and the absence of anti-β₁ integrin antibody and Rho and Rho-kinase inhibitors. In addition, the effect of Gal8-mediated cell-matrix interactions on TM cell cytoskeleton arrangement and myosin light chain 2 (MLC2) phosphorylation was examined.

Principal Findings

We demonstrate here that Gal8 is expressed in the TM and a function-blocking anti-β₁ integrin antibody inhibits the adhesion and spreading of TM cells to Gal8-coated wells. Cell spreading on Gal8 substratum was associated with the accumulation of phosphorylated myosin light chain and the formation of stress fibers that was inhibited by the Rho inhibitor, C3 transferase, as well as by the Rho-kinase inhibitor, Y27632.

Conclusions/Significance

The above findings present a novel function for Gal8 in activating Rho signaling in TM cells. This function may allow Gal8 to participate in the regulation of aqueous outflow.

Delayed-onset symptomatic hyphema after ab interno trabeculotomy surgery

PURPOSE

To describe patients who have experienced delayed-onset hyphema after ab interno trabeculotomy surgery with the Trabectome (Neomedix Corp) for open-angle glaucoma.

DESIGN

Retrospective case series.

METHODS

study population: Patients at Mayo Clinic, Rochester, Minnesota, who underwent Trabectome surgery between September 1, 2006, and December 31, 2010, and who had symptomatic hyphema at least 2 months after surgery. observation procedure: Patients with blurred vision at least 2 months after Trabectome surgery were examined for the presence of hyphema using a slit lamp and gonioscopy. main outcome measures: Proportion of patients experiencing delayed-onset symptomatic hyphema after Trabectome surgery. Associated factors and clinical course for these patients.

RESULTS

Of 262 cases of Trabectome surgery, there were 12 cases of delayed-onset symptomatic hyphema (4.6%). The average age was 74.3 years (range, 66 to 82 years). Median time to onset of hyphema was 8.6 months (range, 2 to 31 months) after surgery. Symptom onset commonly occurred on awakening. The most common characteristic was maintaining a sleep position on the surgical side. Most hyphemas resolved within 1 to 2 weeks, except in 1 patient, who required trabeculectomy for a refractory intraocular pressure spike.

CONCLUSIONS

This is a series of patients with symptomatic delayed-onset hyphema after Trabectome surgery in the absence of further surgeries or trauma. Likely mechanisms are exertion-related increase in episcleral venous pressure or ocular compression from sleeping on the surgical side, followed by sudden decompression and blood reflux. Symptomatic patients should identify and avoid associated triggers because delayed-onset hyphema may be associated with intermittent intraocular pressure spikes that may require medical or surgical treatment.

Perturbation of hyaluronan synthesis in the trabecular meshwork and the effects on outflow facility

PURPOSE

Hyaluronan (HA) is a major component of the aqueous outflow pathway. However, the contribution of HA to human outflow resistance remains unclear. Three HA synthase genes (HAS1-3) have been identified. Here, we evaluate the contribution of each of the HAS proteins to outflow facility in anterior segment perfusion culture.

METHODS

Two methods were used to reduce HA synthesis: 1 mM 4-methylumbelliferone (4MU) was used to inhibit all HAS synthases and shRNA silencing lentivirus was generated to knock down expression of each HAS individually. Quantitative RT-PCR, Western immunoblotting and an HA ELISA assay were used to assess HAS mRNA and protein levels and HA concentration, respectively. The effects of 4MU treatment and HAS gene silencing on outflow facility were assessed in human and porcine perfusion culture.

RESULTS

Quantitative RT-PCR and Western immunoblotting showed a reduction of each HAS in response to their respective silencing and 4MU treatment. HA concentration was concomitantly reduced. Treatment with 4MU decreased outflow facility in human anterior segments but increased outflow facility in porcine eyes. Lentiviral delivery of HAS1 and HAS2 silencing vectors caused similar opposite effects on outflow facility. Silencing of HAS3 did not significantly affect outflow resistance in either species.

CONCLUSIONS

This is the first conclusive evidence for a significant role of HA in the human outflow pathway. HA chains synthesized by HAS1 and HAS2 contribute to outflow resistance, while hyaluronan produced by HAS3 does not appear to play a significant role.

A multiphoton microscope platform for imaging the mouse eye

PURPOSE

To demonstrate the ability of multiphoton microscopy to obtain full three-dimensional high-resolution images of the intact mouse eye anterior chamber without need for enucleation.

METHODS

A custom multiphoton microscope was constructed and optimized for deep tissue imaging. Simultaneous two-photon autofluorescence (2PAF) and second harmonic generation (SHG) imaging were performed. A mouse holder and stereotaxic platform were designed to access different parts of the eye for imaging. A reservoir for keeping the eye moist was used during imaging sessions.

RESULTS

Non-invasive multiphoton images deep inside the anterior chamber of the mouse eye were obtained without the need for enucleation. The iris, corneal epithelium and endothelium, trabecular meshwork region and conjunctiva were visualized by the 2PAF and SHG signals. Identification of the anatomy was achieved by the intrinsic properties of the native tissue without any exogenous labeling. Images as deep as 600 microns into the eye were clearly demonstrated. Full three-dimensional image reconstructions of the entire anterior chamber were performed and analyzed using custom software.

CONCLUSIONS

Multiphoton imaging is a highly promising tool for ophthalmic research. We have demonstrated the ability to image the entire anterior chamber of the mouse eye in its native state. These results provide a foundation for future in vivo studies of the eye.

Connective tissue growth factor causes glaucoma by modifying the actin cytoskeleton of the trabecular meshwork

The most critical risk factor for optic nerve damage in cases of primary open-angle glaucoma (POAG) is an increased intraocular pressure (IOP) caused by a resistance to aqueous humor outflow in the trabecular meshwork (TM). The molecular pathogenesis of this increase in outflow resistance in POAG has not yet been identified, but it may involve transforming growth factor TGF-β2, which is found in higher amounts in the aqueous humor of patients with POAG. Connective tissue growth factor (CTGF) is a TGF-β2 target gene with high constitutive TM expression. In this study, we show that either adenoviral-mediated or transgenic CTGF overexpression in the mouse eye increases IOP and leads to optic nerve damage. CTGF induces TM fibronectin and α-SMA in animals, whereas actin stress fibers and contractility are both induced in cultured TM cells. Depletion of CTGF by RNA interference leads to a marked attenuation of the actin cytoskeleton. Rho kinase inhibitors cause a reversible decline in the IOP of CTGF-overexpressing mice to levels seen in control littermates. Overall, the effects of CTGF on IOP appear to be caused by a modification of the TM actin cytoskeleton. CTGF-overexpressing mice provide a model that mimics the essential functional and structural aspects of POAG and offer a molecular mechanism to explain the increase of its most critical risk factor.

Conjunctival findings in hyperbaric and low-tension glaucoma: an in vivo confocal microscopy study

PURPOSE

To analyse the epithelial features of the bulbar conjunctiva in hyperbaric and low-tension glaucoma (LTG) using in vivo confocal microscopy (IVCM).

METHODS

Thirty-six eyes of 36 patients [18 affected by primary open-angle glaucoma (POAG) and 18 with LTG] were studied; control group was constituted by 28 eyes of 28 healthy subjects. All eyes were examined using digital confocal laser-scanning microscopy (HRT II Rostock Cornea Module). The main IVCM outcome measurements were mean density (MMD: cysts/mm(2)) and mean total area (MMA: μm(2)) of the epithelial microcysts.

RESULTS

The mean intraocular pressure level (mmHg ± SD) was 15.1 ± 1.7, 16.3 ± 3.1 and 12.6 ± 1.8 in healthy, POAG and LTG eyes, respectively. Conjunctival microcysts were found in all patients and subjects: for healthy subjects, MMD = 10.9 ± 11.1 cysts/mm(2) and MMA = 1501.9 ± 1191.1 μm(2); for patients infected with POAG, MMD = 36.8 ± 28.6 cysts/mm(2) and MMA = 7904.8 ± 7050.5 μm(2); and for patients infected with LTG MMD = 45.6 ± 29.0 cysts/mm(2) and MMA =7946.9 ± 5227.5 μm(2). MMD and MMA were not significantly different between patients infected with POAG and those with LTG, whereas they were significantly greater in patients (fourfold and fivefold, respectively) than healthy subjects.

CONCLUSIONS

The present study demonstrated that conjunctival microcysts represent an in vivo feature in all eyes with medically controlled POAG and LTG. Therefore, conjunctiva deserves careful analysis, because its accurate microscopic definition could help clarify the pathophysiology of aqueous outflow in glaucoma.

Non-housekeeping genes expressed in human trabecular meshwork cell cultures

PURPOSE

To identify non-housekeeping genes definitively expressed in the human trabecular meshwork (TM).

METHODS

Microarray gene expression data on TM cultured cells from four studies were compared. Genes that were queried in at least three studies and assessed to be expressed in at least three studies were considered definitively expressed genes of the human TM. Housekeeping genes were removed from this set of genes. The non-housekeeping TM gene profile was analyzed for pathway enrichment and microRNA targeting, using bioinformatics tools. The results were compared with results of previous non-array based studies.

RESULTS

Nine hundred and sixty-two genes were identified as non-housekeeping TM expressed genes. Analysis of these by Kyoto Encyclopedia of Genes and Genomes led to identification of two enriched biologic pathways that achieved a highly significant Bonferroni p-value (p≤0.01): focal adhesion and extracellular matrix (ECM)-receptor interaction. Many of the genes were previously implicated in TM-related functions and the TM-associated disease glaucoma; however, some are novel. MicroRNAs known to be expressed in the trabecular meshwork were predicted to target some of the genes. Ten genes identified here, ALDH1A1 (aldehyde dehydrogenase 1 family, member A1), CDH11 (cadherin 11, type 2, OB-cadherin), CXCR7 (chemokine (C-X-C motif) receptor 7), CHI3L1 (chitinase 3-like 1), FGF2 (fibroblast growth factor 2), GNG11 (guanine nucleotide binding protein [G protein], gamma 11), IGFBP5 (insulin-like growth factor binding protein 5), PTPRM (protein tyrosine phosphatase, receptor type, M), RGS5 (regulator of G-protein signaling 5), and TUSC3 (tumor suppressor candidate 3), were also reported as TM expressed genes in three earlier non-microarray based studies.

CONCLUSIONS

A transcriptome consisting of 962 non-housekeeping genes definitively expressed in the human TM was identified. Multiple genes and microRNAs are proposed for further study for a better understanding of TM physiology.

Mechanical responsiveness of the endothelial cell of Schlemm's canal: scope, variability and its potential role in controlling aqueous humour outflow

Primary open-angle glaucoma is associated with elevated intraocular pressure, which in turn is believed to result from impaired outflow of aqueous humour. Aqueous humour outflow passes mainly through the trabecular meshwork (TM) and then through pores formed in the endothelium of Schlemm's canal (SC), which experiences a basal-to-apical pressure gradient. This gradient dramatically deforms the SC endothelial cell and potentially contributes to the formation of those pores. However, mechanical properties of the SC cell are poorly defined. Using optical magnetic twisting cytometry and traction force microscopy, here we characterize the mechanical properties of primary cultures of the human SC cell, and for the first time, the scope of their changes in response to pharmacological agents that are known to modulate outflow resistance. Lysophosphatidic acid, sphingosine-1-phosphate (S1P) and thrombin caused an increase in cell stiffness by up to 200 per cent, whereas in most cell strains, exposure to latrunculin A, isoproterenol, dibutryl cyclic-AMP or Y-27632 caused a decrease in cell stiffness by up to 80 per cent, highlighting that SC cells possess a remarkably wide contractile scope. Drug responses were variable across donors. S1P, for example, caused 200 per cent stiffening in one donor strain but only 20 per cent stiffening in another. Isoproterenol caused dose-dependent softening in three donor strains but little or no response in two others, a finding mirrored by changes in traction forces and consistent with the level of expression of β(2)-adrenergic receptors. Despite donor variability, those drugs that typically increase outflow resistance systematically caused cell stiffness to increase, while in most cases, those drugs that typically decrease outflow resistance caused cell stiffness to decrease. These findings establish the endothelial cell of SC as a reactive but variable mechanical component of the aqueous humour outflow pathway. Although the mechanism and locus of increased outflow resistance remain unclear, these data suggest the SC endothelial cell to be a modulator of outflow resistance.

Substratum compliance regulates human trabecular meshwork cell behaviors and response to latrunculin B

PURPOSE

To determine the impact of substratum compliance and latrunculin-B (Lat-B), both alone and together, on fundamental human trabecular meshwork (HTM) cell behavior. Lat-B is a reversible actin cytoskeleton disruptor that decreases resistance to aqueous humor outflow and decreases intraocular pressure.

METHODS

HTM cells were cultured on polyacrylamide hydrogels possessing values for compliance that mimic those reported for normal and glaucomatous HTM, or tissue culture plastic (TCP). Cells were treated with 0.2 μM or 2.0 μM Lat-B in dimethyl sulfoxide (DMSO) or DMSO alone. The impact of substratum compliance and/or Lat-B treatment on cell attachment, proliferation, surface area, aspect ratio, and migration were investigated.

RESULTS

HTM cells had profoundly decreased attachment and proliferation rates when cultured on hydrogels possessing compliance values that mimic those found for healthy HTM. The effect of Lat-B treatment on HTM cell surface area was less for cells cultured on more compliant hydrogels compared with TCP. HTM cell migration was increased on stiffer hydrogels that mimic the compliance of glaucomatous HTM and on TCP in comparison with more compliant hydrogels. Lat-B treatment decreased cellular migration on all surfaces for at least 7 hours after treatment.

CONCLUSIONS

Substratum compliance profoundly influenced HTM cell behaviors and modulated the response of HTM cells to Lat-B. The inclusion of substratum compliance that reflects healthy or glaucomatous HTM results in cell behaviors and responses to therapeutic agents in vitro that may more accurately reflect in vivo conditions.

The role of TGF-β in the pathogenesis of primary open-angle glaucoma

Transforming growth factor-β2 (TGF-β2) is found in increasing amounts in aqueous humor and reactive optic nerve astrocytes of patients with primary open-angle glaucoma (POAG), a major cause of blindness worldwide. The available data strongly indicate that TGF-β2 is a key player contributing to the structural changes in the extracellular matrix (ECM) of the trabecular meshwork and optic nerve head as characteristically seen in POAG. The changes involve an induction in the expression of various ECM molecules and are remarkably similar in trabecular meshwork cells and optic nerve head astrocytes. The ECM changes in the trabecular meshwork most probably play a role in the increase of aqueous humor outflow resistance causing higher intraocular pressure (IOP). In the optic nerve head, TGF-β2-induced changes might contribute to deformation of the optic nerve axons causing impairment of axonal transport and neurotrophic supply and leading to their continuous degeneration. The increase in IOP further adds mechanical stress and strain to optic nerve axons and accelerates degenerative changes. In addition, high IOP might induce the expression of activated TGF-β1 in trabecular meshwork cells and optic nerve head astrocytes; this again might significantly lead to the progress of axonal degeneration. The action of TGF-β2 in POAG is largely mediated through the connective tissue growth factor, whereas the activities of TGF-β1 and -β2 are modulated by the blocking effects of bone morphogenetic protein-4 (BMP-4) and BMP-7, by gremlin that inhibits BMP signaling and by several species of microRNAs.

Segmental versican expression in the trabecular meshwork and involvement in outflow facility

PURPOSE

Versican is a large proteoglycan with numerous chondroitin sulfate (CS) glycosaminoglycan (GAG) side chains attached. To assess versican's potential contributions to aqueous humor outflow resistance, its segmental distribution in the trabecular meshwork (TM) and the effect on outflow facility of silencing the versican gene were evaluated.

METHODS

Fluorescent quantum dots (Qdots) were perfused to label outflow pathways of anterior segments. Immunofluorescence with confocal microscopy and quantitative RT-PCR were used to determine versican protein and mRNA distribution relative to Qdot-labeled regions. Lentiviral delivery of shRNA-silencing cassettes to TM cells in perfused anterior segment cultures was used to evaluate the involvement of versican and CS GAG chains in outflow facility.

RESULTS

Qdot uptake by TM cells showed considerable segmental variability in both human and porcine outflow pathways. Regional levels of Qdot labeling were inversely related to versican protein and mRNA levels; versican levels were relatively high in sparsely Qdot-labeled regions and low in densely labeled regions. Versican silencing decreased outflow facility in human and increased facility in porcine anterior segments. However, RNAi silencing of ChGn, an enzyme unique to CS GAG biosynthesis, increased outflow facility in both species. The fibrillar pattern of versican immunostaining in the TM juxtacanalicular region was disrupted after versican silencing in perfusion culture.

CONCLUSIONS

Versican appears to be a central component of the outflow resistance, where it may organize GAGs and other ECM components to facilitate and control open flow channels in the TM. However, the exact molecular organization of this resistance appears to differ between human and porcine eyes.

Two-photon imaging of the mouse eye

PURPOSE

To image the conventional aqueous outflow pathway and adjacent structures within the intact enucleated mouse eye using a noninvasive microscopy technique.

METHODS

Two-photon microscopy (2PM) techniques, including two-photon autofluorescence (2PAF) and second harmonic generation (SHG), were used to obtain images of the trabecular meshwork (TM) region within an intact mouse eye. Cardiac perfusion of fluorescein-conjugated dextran was used to label blood vessels within the eye to serve as an anatomic reference. Eyes were subsequently fixed, paraffin embedded, sectioned, and stained for comparison to the 2PM images.

RESULTS

Three-dimensional analyses of multiple 2PM images revealed a well-defined region adjacent to the iris and cornea that is free of SHG signal and consistent with the location of Schlemm's canal. This open region is continuous with smaller tube structures consistent with collector channels. These structures do not label in mice perfused with the vascular probe dextran, supporting the hypothesis that the enclosed spaces are filled with aqueous humor rather than circulating blood. The TM region in the mouse eye was also visible, with a clear SHG signal representing collagen fibers.

CONCLUSIONS

These results support the hypothesis that 2PM may be useful for noninvasively imaging the conventional aqueous outflow pathway in mouse models of glaucoma. Studies are ongoing to validate our methodology in live animals.

Novel ocular antihypertensive compounds in clinical trials

Introduction:

Glaucoma is a multifactorial disease characterized by progressive optic nerve injury and visual field defects. Elevated intraocular pressure (IOP) is the most widely recognized risk factor for the onset and progression of open-angle glaucoma, and IOP-lowering medications comprise the primary treatment strategy. IOP elevation in glaucoma is associated with diminished or obstructed aqueous humor outflow. Pharmacotherapy reduces IOP by suppressing aqueous inflow and/or increasing aqueous outflow.

Purpose:

This review focuses on novel non-FDA approved ocular antihypertensive compounds being investigated for IOP reduction in ocular hypertensive and glaucoma patients in active clinical trials within approximately the past 2 years.

Methods:

The mode of IOP reduction, pharmacology, efficacy, and safety of these new agents were assessed. Relevant drug efficacy and safety trials were identified from searches of various scientific literature databases and clinical trial registries. Compounds with no specified drug class, insufficient background information, reformulations, and fixed-combinations of marketed drugs were not considered.

Results:

The investigational agents identified comprise those that act on the same targets of established drug classes approved by the FDA (ie, prostaglandin analogs and β-adrenergic blockers) as well as agents belonging to novel drug classes with unique mechanisms of action. Novel targets and compounds evaluated in clinical trials include an actin polymerization inhibitor (ie, latrunculin), Rho-associated protein kinase inhibitors, adenosine receptor analogs, an angiotensin II type 1 receptor antagonist, cannabinoid receptor agonists, and a serotonin receptor antagonist.

Conclusion:

The clinical value of novel compounds for the treatment of glaucoma will depend ultimately on demonstrating favorable efficacy and benefit-to-risk ratios relative to currently approved prostaglandin analogs and β-blockers and/or having complementary modes of action.

MicroRNA-24 regulates the processing of latent TGFβ1 during cyclic mechanical stress in human trabecular meshwork cells through direct targeting of FURIN

Cyclic mechanical stress (CMS) leads to alterations of cellular functions in the trabecular meshwork (TM), including the up-regulation of transforming growth factor beta 1 (TGFβ1), that can potentially contribute to the pathogenesis of glaucoma. Although microRNAs (miRNAs) are known to play important roles in many biological functions, little is known about their potential involvement in the cellular responses elicited by mechanical stress. Here we analyzed changes in miRNA expression induced by CMS, and examined the possible role of miR-24 in the response of human TM cells to CMS. CMS induced the expression of miR-24 that led to the down regulation of the subtilisin-like proprotein convertase FURIN, which is known to play a major role in the processing of TGFβ1. FURIN was confirmed as a novel target of miR-24 by 3' UTR luciferase assay and western blot. Overexpression of miR-24 resulted in a significant decrease in activated TGFβ1. This effect was mimicked by down regulation of FURIN by siRNA. Conversely, inhibition of miR-24 expression with a specific antagomir led to a small but significant increase in TGFβ1. Furthermore, the increase in active TGFβ1 induced by CMS in HTM cells was prevented by miR-24. Altogether, our results suggest that miRNAs might contribute to the regulation of responses to CMS in TM cells. Specifically, miR-24 might play an important role in modulating the induction of TGFβ1 mediated by CMS through direct targeting of FURIN.

Atomic force microscopy-based antibody recognition imaging of proteins in the pathological deposits in pseudoexfoliation syndrome

The phenomenon of protein aggregation is of considerable interest to various disciplines, including the field of medicine. A range of disease pathologies are associated with this phenomenon. One of the ocular diseases hallmarked by protein aggregation is the Pseudoexfoliation (PEX) Syndrome. This condition is characterized by the deposition of insoluble proteinaceous material on the anterior human lens capsule. Genomic and proteomic analyses have revealed an association of specific genetic markers and various proteins, respectively, with PEX syndrome. However, the ultrastructure of the protein aggregates is poorly characterized. This study seeks to build capacity to determine the molecular nature of PEX aggregates on human lens capsules in their native state by AFM-based antibody recognition imaging. Lysyl oxidase-Like 1 (LOXL1), a protein identified as a component of PEX aggregates, is detected by an antibody-modified AFM probe. Topographical AFM images and antibody recognition images are obtained using three AFM-based techniques: TREC, phase and force-volume imaging. LOXL1 is found to be present on the lens capsule surface, and is localized around fibrous protein aggregates. Our evaluation shows that TREC imaging is best suited for human tissue imaging and holds significant potential for imaging of human disease tissues in their native state.

Mapping of the disease locus and identification of ADAMTS10 as a candidate gene in a canine model of primary open angle glaucoma

Primary open angle glaucoma (POAG) is a leading cause of blindness worldwide, with elevated intraocular pressure as an important risk factor. Increased resistance to outflow of aqueous humor through the trabecular meshwork causes elevated intraocular pressure, but the specific mechanisms are unknown. In this study, we used genome-wide SNP arrays to map the disease gene in a colony of Beagle dogs with inherited POAG to within a single 4 Mb locus on canine chromosome 20. The Beagle POAG locus is syntenic to a previously mapped human quantitative trait locus for intraocular pressure on human chromosome 19. Sequence capture and next-generation sequencing of the entire canine POAG locus revealed a total of 2,692 SNPs segregating with disease. Of the disease-segregating SNPs, 54 were within exons, 8 of which result in amino acid substitutions. The strongest candidate variant causes a glycine to arginine substitution in a highly conserved region of the metalloproteinase ADAMTS10. Western blotting revealed ADAMTS10 protein is preferentially expressed in the trabecular meshwork, supporting an effect of the variant specific to aqueous humor outflow. The Gly661Arg variant in ADAMTS10 found in the POAG Beagles suggests that altered processing of extracellular matrix and/or defects in microfibril structure or function may be involved in raising intraocular pressure, offering specific biochemical targets for future research and treatment strategies.

Primary open angle glaucoma (POAG) is a leading cause of vision loss and blindness affecting tens of millions of people. Ocular hypertension is a strong risk factor for the disease and the only effective target of treatment. Ocular hypertension results from increased resistance to outflow of aqueous humor through the trabecular meshwork, a specialized filtration tissue consisting of alternating layers of cells and connective tissue, but the specific reasons for the increased resistance are not known. The animal model for human POAG used in this study was a colony of Beagle dogs that carry an inherited form of the disease in which ocular hypertension is the primary manifestation. We have found a variant in ADAMTS10 that belongs to a family of genes that contribute to formation of extracellular matrix and may itself be involved in formation of elastic microfiber structures. We found that the ADAMTS10 protein is expressed at particularly high levels in the trabecular meshwork. The candidate variant in ADAMTS10 found in the POAG–affected Beagles suggests that altered processing of connective tissue and/or elastic microfiber defects may be involved in raising eye pressure, offering specific biochemical targets for future research and treatment strategies.

The effect of biophysical attributes of the ocular trabecular meshwork associated with glaucoma on the cell response to therapeutic agents

Glaucoma is a devastating neurodegenerative disease, which can lead to vision loss and is associated with irreversible damage to retinal ganglion cells. Although the mechanism of disease onset remains unknown, we have recently demonstrated that the stiffness of the ocular trabecular meshwork (HTM) increases dramatically in human donor eyes with a history of glaucoma. Here we report that polyacrylamide hydrogels, which mimic the compliant conditions of normal and glaucomatous HTM, profoundly modulate cytoskeletal dynamics and the elastic modulus of the overlying HTM cells. Substratum compliance also modulates HTM cell response to Latrunculin-B, a cytoskeletal disrupting agent currently in human clinical trials for the treatment of glaucoma. Additionally, we observed a compliance-dependent rebound effect of Latrunculin-B with an unexpected increase in HTM cell elastic modulus being observed upon withdrawal of the drug. The results predict that cytoskeletal disrupting drugs may be more potent in advanced stages of glaucoma.

Detecting protein aggregates on untreated human tissue samples by atomic force microscopy recognition imaging

We apply topography and recognition (TREC) imaging to the analysis of whole, untreated human tissue for what we believe to be the first time. Pseudoexfoliation syndrome (PEX), a well-known cause of irreversible blindness worldwide, is characterized by abnormal protein aggregation on the anterior lens capsule of the eye. However, the development of effective therapies has been hampered by a lack of detailed knowledge of the protein constituents in these pathological deposits and their distribution. Using both TREC and immunofluorescence, one of the proteins implicated in the PEX pathology--the apolipoprotein clusterin--was detected, and differences in its distribution pattern on the surface of untreated human lens capsule tissue in both PEX and normal control samples were investigated. Our study shows the potential of TREC imaging for the analysis of whole, untreated human tissue samples.

Duration of anesthesia affects intraocular pressure, but not outflow facility in mice

PURPOSE

The study of aqueous humor dynamics (AHD) in mice is becoming more prevalent as more strains with elevated intraocular pressure (IOP) are developed. High IOP is usually associated with reduced outflow facility making this one of the more important AHD parameters to evaluate. Ocular measurements in mice require anesthesia that has profound effects on IOP but unknown effects on outflow facility. This study evaluates the effects of anesthesia duration and latanoprost treatment on outflow facility and IOP in BALB/c mice.

METHODS

IOPs were measured in conscious and anesthetized mice by tonometry. Outflow facility was evaluated in 15-min intervals at three pressure levels over two 45-min periods. Comparisons were made between latanoprost-treated eyes and untreated contralateral eyes. To determine the effect of anesthesia duration on IOP, a microneedle method was used to follow IOP for 120 min in separate mice.

RESULTS

IOP was 9.7 +/- 0.3 mmHg (mean +/- SEM) in conscious mice and 7.1 +/- 0.02 within 10 min of anesthesia initiation (p < 0.01). IOP changed significantly between but not within assessment periods. IOP at 75 min was significantly (p = 0.004) reduced compared to IOP at 15 min after initial anesthesia. In control eyes, outflow facility did not change between the two 45-min assessment periods during the 120 min test (p = 0.80). In latanoprost-treated eyes, outflow facility increased compared with control eyes during both assessment periods (p = 0.03). A test of filters in series with known resistance found that the method was sensitive enough to detect a change in outflow facility of 0.001 microl/min/mmHg.

CONCLUSIONS

Administration of ketamine/xylazine anesthesia for 120 min did not alter outflow facility or lessen the effect of latanoprost on outflow facility in mice as determined by a new analysis system. Accurate IOP measurements must be made within minutes of anesthesia administration but outflow facility measurements can be made with less haste.

Development of a versatile two-photon endoscope for biological imaging

We describe a versatile, catheter-type two-photon probe, designed for in vivo and ex vivo imaging of the aqueous outflow pathway in the eye. The device consists of a silica double cladding fiber used for laser delivery and fluorescence collection, a spiral fiber scanner driven by a miniature piezoelectric tube, and an assembly of three micro-size doublet achromatic lenses used for focusing the laser and collecting the two-photon excitation signal. All the components have a maximum diameter of 2 mm and are enclosed in a length of 12-gauge stainless steel hypodermic tubing having an outer diameter of 2.8 mm. The lateral and axial resolutions of the probe are measured to be 1.5 μm and 9.2 μm, respectively. Different lens configurations and fibers are evaluated by comparing their spatial resolutions and fluorescence signal collection efficiencies. Doublet achromatic lenses and a double cladding fiber with a high inner cladding numerical aperture are found to produce a high signal collection efficiency, which is essential for imaging live tissues. Simple methods for reducing image distortions are demonstrated. Images of human trabecular meshwork tissue are successfully obtained with this miniature two-photon microscope.

The pathogenic role of transforming growth factor-β2 in glaucomatous damage to the optic nerve head

In patients with primary open angle glaucoma (POAG), the optic nerve head (ONH) shows characteristic cupping correlated with visual field defects. The progressive optic neuropathy is characterized by irreversible loss of retinal ganglion cells (RGC). The critical risk factor for axonal damage at the ONH is an elevated intraocular pressure (IOP). The increase in IOP correlates with axonal loss in the ONH, which might be due to an impaired axoplasmatic flow leading to the loss of RGCs. Damage to the optic nerve is thought to occur in the lamina cribrosa (LC) region of the ONH, which is composed of characteristic sieve-like connective tissue cribriform plates through which RGC axons exit the eye. The cupping of the optic disc, and the compression and excavation of LC are characteristic signs of glaucomatous ONH remodelling. In ONH of POAG patients a disorganized distribution and deposition of elastic fibers and a typical pronounced thickening of the connective tissue septae surrounding the optic nerve fibers is found. Transforming growth factor (TGF)-β2 could be one of the pathogenic factors responsible for the structural alterations in POAG patients as the TGF-β2 levels in the ONH of glaucomatous eyes are elevated as well as in the aqueous homour. TGF-β2 leads to an increased synthesis of extracellular matrix (ECM) molecules mediated by connective tissue growth factor and to an impaired ECM degradation in cultured ONH astrocytes. Bone morphogenetic protein (BMP)-4 effectively antagonizes the effects of TGF-β2 on matrix deposition. The BMP antagonist gremlin blocks this inhibition, allowing TGF-β2 stimulation of ECM synthesis. Overall, the ECM in the ONH is kept in balance in the OHN by factors that augment or block the activity of TGF-β2.

Characterization of cytoskeleton-enriched protein fraction of the trabecular meshwork and ciliary muscle cells

PURPOSE

To understand the molecular basis for the known distinct contractile characteristics of trabecular meshwork (TM) and ciliary muscle (CM) cells, the cytoskeleton-enriched protein fractions of the TM and CM cells were isolated and characterized.

METHODS

The nonionic surfactant insoluble fraction enriched for cytoskeletal proteins was isolated from human and porcine TM tissue and cells and from CM cells and was characterized by SDS-PAGE, mass spectrometry, and immunoblotting techniques.

RESULTS

The cytoskeleton-enriched protein fraction derived from both human and porcine TM cells contained Plectin 1, Filamin A, non-muscle myosin IIA, clathrin, α-actinin, vimentin, actin, caldesmon, myosin IC, and annexin A2 as major proteins and was noted to exhibit compositional similarity with the cytoskeletal protein fraction isolated from TM tissue. Importantly, the cytoskeletal protein composition of the TM cells was also found to be similar to that noted for CM and vascular endothelial cells. Although the activity of myosin II, a crucial regulator of cellular contraction and a major component of the cytoskeletal protein fraction in TM and CM cells, was regulated predominantly by Rho kinase in both cell types, myosin light chain kinase (MLCK) also appeared to control myosin II activity in CM cells.

CONCLUSIONS

These data reveal that the activity of non-muscle myosin II, a critical molecule of cellular contraction, was found to be regulated differentially in TM and CM cells by the Rho kinase and the MLCK pathways despite their compositional similarity in cytoskeletal protein profile.

Reduced expression of Pax6 in lens and cornea of mutant mice leads to failure of chamber angle development and juvenile glaucoma

Heterozygous mutations in PAX6 are causative for aniridia, a condition that is frequently associated with juvenile glaucoma. Defects in morphogenesis of the iridocorneal angle, such as lack of trabecular meshwork differentiation, absence of Schlemm's canal and blockage of the angle by iris tissue, have been described as likely causes for glaucoma, and comparable defects have been observed in heterozygous Pax6-deficient mice. Here, we employed Cre/loxP-mediated inactivation of a single Pax6 allele in either the lens/cornea or the distal optic cup to dissect in which tissues both alleles of Pax6 need to be expressed to control the development of the tissues in the iridocorneal angle. Somatic inactivation of one allele of Pax6 exclusively from epithelial cells of lens and cornea resulted in the disruption of trabecular meshwork and Schlemm's canal development as well as in an adhesion between iris periphery and cornea in juvenile eyes, which resulted in the complete closure of the iridocorneal angle in the adult eye. Structural changes in the iridocorneal angle presumably caused a continuous increase in intraocular pressure leading to degenerative changes in optic nerve axons and to glaucoma. In contrast, the inactivation of a single Pax6 allele in the distal optic cup did not cause obvious changes in iridocorneal angle formation. We conclude that the defects in iridocorneal angle formation are caused by non-autonomous mechanisms due to Pax6 haploinsufficiency in lens or corneal epithelial cells. Pax6 probably controls the expression of signaling molecules in lens cells that regulate the morphogenetic processes during iridocorneal angle formation.

Two-photon imaging of the trabecular meshwork

PURPOSE

To image the trabecular meshwork (TM) in its native unfixed state using a non-invasive, non-destructive technique.

METHODS

Two-photon microscopy (2PM), including two-photon excitation fluorescence (2PEF) and second harmonic generation (SHG), was used to image flat-mounted trabecular meshwork samples from human cadaver eyes. Multiple images were analyzed along the tissue axis (z-axis) to generate a three-dimensional (3D) model of the region.

RESULTS

A lattice of large collagen fibers (approximately 10 microm in diameter) were detected by inherent fluorescence (2PEF) and SHG. There are regions of both tightly overlapping bundles as well as fluid-filled regions visible from the surface of the TM. 3D analysis of multiple images reveals that the open regions deep in the TM penetrate the juxtacanalicular TM (JTM) and connect to the inner wall of Schlemm's canal (IWSC). These open regions may represent low-resistance fluid pathways between the anterior chamber and Schlemm's canal (SC).

CONCLUSIONS

2PM imaging of the outflow system of the human eye documented collagenous structures solely from inherent optical properties, without addition of an exogenous fluorescent label. 2PM successfully imaged into the TM without the need for fixation, embedding, or histological processing. Deep penetration using advanced optical techniques revealed regions likely representing pores in the IWSC that have been documented by multiple electron microscope studies. Our work reveals that 2PM imaging has potential as a new metric for evaluating the aqueous outflow region of the human eye and is worthy of further exploration.

Advances in imaging the blood and aqueous vessels of the ocular limbus

The vessels of the limbus play a pivotal role in the drainage of the major portion of aqueous humour from the anterior chamber. Aberrations in the limbal architecture can lead to raised intraocular pressure, which in turn can lead to blinding conditions such as glaucoma. Imaging these vessels in the normal eye, in development, and in conditions where there is anterior segment dysgenesis remains a challenge. Here we review the progress in limbal vessel imaging in the past 50 years and provide key information on their strengths and limitations. Included is an analysis of serial histological sectioning, ultrathin sections, microvascular perfusion with plastics and corrosion casting, X-ray microcomputed tomography, in vivo imaging including analysis of transgenic mice expressing GFP-vascular endothelium fusion proteins, in vivo microscopy imaging using fluorescent-labelled antibodies, slit-lamp microscopy and gonioscopy, fluorescein angiography, optical coherence tomography, and various labelling procedures for the vascular endothelium and the various forms of microscopy used to view these.

Young's modulus of elasticity of Schlemm's canal endothelial cells

Schlemm's canal (SC) endothelial cells are likely important in the physiology and pathophysiology of the aqueous drainage system of the eye, particularly in glaucoma. The mechanical stiffness of these cells determines, in part, the extent to which they can support a pressure gradient and thus can be used to place limits on the flow resistance that this layer can generate in the eye. However, little is known about the biomechanical properties of SC endothelial cells. Our goal in this study was to estimate the effective Young's modulus of elasticity of normal SC cells. To do so, we combined magnetic pulling cytometry of isolated cultured human SC cells with finite element modeling of the mechanical response of the cell to traction forces applied by adherent beads. Preliminary work showed that the immersion angles of beads attached to the SC cells had a major influence on bead response; therefore, we also measured bead immersion angle by confocal microscopy, using an empirical technique to correct for axial distortion of the confocal images. Our results showed that the upper bound for the effective Young's modulus of elasticity of the cultured SC cells examined in this study, in central, non-nuclear regions, ranged between 1,007 and 3,053 Pa, which is similar to, although somewhat larger than values that have been measured for other endothelial cell types. We compared these values to estimates of the modulus of primate SC cells in vivo, based on images of these cells under pressure loading, and found good agreement at low intraocular pressure (8-15 mm Hg). However, increasing intraocular pressure (22-30 mm Hg) appeared to cause a significant increase in the modulus of these cells. These moduli can be used to estimate the extent to which SC cells deform in response to the pressure drop across the inner wall endothelium and thereby estimate the extent to which they can generate outflow resistance.

Reduced endothelial progenitor cells and brachial artery flow-mediated dilation as evidence of endothelial dysfunction in ocular hypertension and primary open-angle glaucoma

PURPOSE

This study aimed to assess vascular endothelial function in patients with ocular hypertension (OHT) or primary open-angle glaucoma (POAG) by measuring: (a) endothelium-dependent flow-mediated vasodilation (FMD) of the brachial artery, and (b) circulating endothelial progenitor cells, which are believed to support the integrity of the vascular endothelium.

METHODS

We enrolled 25 patients with OHT, 23 with POAG and 26 control subjects, all of whom were aged < 65 years and had no medical history of cardiovascular disease or cardiovascular risk factors. All subjects underwent a complete ophthalmological examination, biochemistry study, assessment of cardiovascular parameters, brachial artery ultrasound assessment of endothelium-dependent FMD, generic circulating progenitor cell (CPC) and circulating endothelial progenitor cell (EPC) count with the use of flow cytometry.

RESULTS

Flow-mediated vasodilation values differed significantly in OHT (4.5 +/- 1.1%; p = 0.021) and POAG (4.0 +/- 0.9%; p = 0.003) patients compared with controls (7.7 +/- 0.8%). The CD34(+) KDR(+) EPC count was markedly lower in OHT (28.0 +/- 5.0; p < 0.001) and POAG (24.3 +/- 3.4; p < 0.001) patients compared with controls (73.1 +/- 8.1). Neither FMD not EPCs differed significantly between OHT and POAG patients. No significant differences in CPC count or cardiovascular parameters were found among OHT or POAG patients and controls. The levels of CD34(+) KDR(+) EPCs were directly correlated (p = 0.043) with FMD, and inversely correlated (p = 0.032) with baseline intraocular pressure in OHT and POAG patients.

CONCLUSIONS

Both OHT and POAG patients without cardiovascular risk factors have previously unreported severely reduced circulating EPCs and reduced FMD, both of which are indicators of endothelial dysfunction and increased risk of cardiovascular events.

Mechanistic basis of Rho GTPase-induced extracellular matrix synthesis in trabecular meshwork cells

Elevated intraocular pressure arising from impaired aqueous humor drainage through the trabecular pathway is a major risk factor for glaucoma. To understand the molecular basis for Rho GTPase-mediated resistance to aqueous humor drainage, we investigated the possible interrelationship between actomyosin contractile properties and extracellular matrix (ECM) synthesis in human trabecular meshwork (TM) cells expressing a constitutively active form of RhoA (RhoAV14). TM cells expressing RhoAV14 exhibited significant increases in fibronectin, tenascin C, laminin, alpha-smooth muscle actin (alpha-SMA) levels, and matrix assembly in association with increased actin stress fibers and myosin light-chain phosphorylation. RhoAV14-induced changes in ECM synthesis and actin cytoskeletal reorganization were mimicked by lysophosphatidic acid and TGF-beta(2), known to increase resistance to aqueous humor outflow and activate Rho/Rho kinase signaling. RhoAV14, lysophosphatidic acid, and TGF-beta(2) stimulated significant increases in Erk1/2 phosphorylation, paralleled by profound increases in fibronectin, serum response factor (SRF), and alpha-SMA expression. Treatment of RhoA-activated TM cells with inhibitors of Rho kinase or Erk, on the other hand, decreased fibronectin and alpha-SMA levels. Although suppression of SRF expression (both endogenous and RhoA, TGF-beta(2)-stimulated) via the use of short hairpin RNA decreased alpha-SMA levels, fibronectin was unaffected. Conversely, fibronectin induced alpha-SMA expression in an SRF-dependent manner. Collectively, data on RhoA-induced changes in actomyosin contractile activity, ECM synthesis/assembly, and Erk activation, along with fibronectin-induced alpha-SMA expression in TM cells, reveal a potential molecular interplay between actomyosin cytoskeletal tension and ECM synthesis/assembly. This interaction could be significant for the homeostasis of aqueous humor drainage through the pressure-sensitive trabecular pathway.

Extracellular release of ATP mediated by cyclic mechanical stress leads to mobilization of AA in trabecular meshwork cells

PURPOSE

To investigate the mechanisms that mediate the release of ATP induced by cyclic mechanical stress (CMS) and the role of extracellular ATP in the mobilization of arachidonic acid (AA) and prostaglandin secretion.

METHODS

Porcine trabecular meshwork (pTM) cells were subjected to CMS. Extracellular ATP was detected with a luciferin-luciferase assay in the presence or absence of transport inhibitors and a lipid raft disrupter. ATP vesicles were visualized with quinacrine. The release of AA (AA 1-14C) was measured with and without ATP, ATP inhibitors, and phospholipase-A and -C inhibitors. Prostaglandin E2 (PGE2) and viability were measured with ELISA and a lactate dehydrogenase assay, respectively.

RESULTS

CMS induced ATP release that was inhibited by the vesicle inhibitors N-ethylmaleimide (NEM) and monensin. Lipid raft disruption significantly increased the extracellular ATP induced by CMS. CMS induced AA release (1-4-fold increase) and its metabolic product PGE2 (3.9-fold increase). The AA mobilization induced by CMS could be mimicked by the addition of extracellular ATP and was partially inhibited by a P2 antagonist, by an ATP inhibitor, and by inhibitors of phospholipase-A2 and -C. Addition of PGE2 (10 microM) to the media exerted cytoprotective effects against long-term CMS.

CONCLUSIONS

Extracellular release of ATP induced by CMS in TM cells is mediated by exocytosis of ATP-enriched vesicles into lipid rafts. The resulting activation of purinergic receptors leads to mobilization of AA from the plasma membrane. The subsequent release of PGE could exert protective effects by preventing TM cell loss that may result from chronic exposure to CMS.

Modulation of extracellular matrix turnover in the trabecular meshwork

Intraocular pressure (IOP) is the most critical risk factor for primary open angle glaucoma (POAG). In most cases of POAG, IOP is increased because of an abnormally high aqueous humor outflow resistance in the juxtacanalicular region of the trabecular meshwork. A distinct structural change in the trabecular meshwork of patients with POAG is the increase in fibrillar extracellular matrix in the juxtacanalicular region of the trabecular meshwork. Our knowledge on the molecular factors that govern turnover of the extracellular matrix in the trabecular meshwork has increased considerably in recent years. It has become clear that quality and quantity of the extracellular matrix in the trabecular meshwork are regulated by several signaling molecules that interact with each other to promote its synthesis, degradation, or extracellular modification. Transforming growth factor-beta1 and beta2 (TGF-beta1 and TGF-beta2) which derive from the aqueous humor or may be locally expressed induce in cultured trabecular meshwork cells the expression of a variety of extracellular matrix molecules. The action of TGF-betas very likely requires local activation by thrombospondin-1 and is partly mediated by its downstream mediator connective tissue growth factor, both of which are constitutively expressed in the trabecular meshwork. Bone morphogenetic proteins (BMP)-7 and -4 effectively antagonize the effects of TGF-beta2 on matrix deposition. The antagonizing effects of BMP-7 are mediated in trabecular meshwork cells through Smad7. Smad7 is a key molecular switch to inhibit TGF-beta2 signaling in the trabecular meshwork.

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.

The trabecular meshwork outflow pathways: structural and functional aspects

The major drainage structures for aqueous humor (AH) are the conventional or trabecular outflow pathways, which are comprised of the trabecular meshwork (made up by the uveal and corneoscleral meshworks), the juxtacanalicular connective tissue (JCT), the endothelial lining of Schlemm's canal (SC), the collecting channels and the aqueous veins. The trabecular meshwork (TM) outflow pathways are critical in providing resistance to AH outflow and in generating intraocular pressure (IOP). Outflow resistance in the TM outflow pathways increases with age and primary open-angle glaucoma. Uveal and corneoscleral meshworks form connective tissue lamellae or beams that are covered by flat TM cells which rest on a basal lamina. TM cells in the JCT are surrounded by fibrillar elements of the extracellular matrix (ECM) to form a loose connective tissue. In contrast to the other parts of the TM, JCT cells and ECM fibrils do not form lamellae, but are arranged more irregularly. SC inner wall endothelial cells form giant vacuoles in response to AH flow, as well as intracellular and paracellular pores. In addition, minipores that are covered with a diaphragm are observed. There is considerable evidence that normal AH outflow resistance resides in the inner wall region of SC, which is formed by the JCT and SC inner wall endothelium. Modulation of TM cell tone by the action of their actomyosin system affects TM outflow resistance. In addition, the architecture of the TM outflow pathways and consequently outflow resistance appear to be modulated by contraction of ciliary muscle and scleral spur cells. The scleral spur contains axons that innervate scleral spur cells or that have the ultrastructural characteristics of mechanosensory nerve endings.

Connective tissue growth factor induces extracellular matrix deposition in human trabecular meshwork cells

The major structural change in the human trabecular meshwork (TM) of eyes with primary open-angle glaucoma (POAG) is an increase in extracellular matrix (ECM) in the juxtacanalicular region of the TM. There is evidence that treatment with TGF-beta2 causes an induction of ECM deposition in cultured human TM cells and that TGF-beta2 is causatively involved in the JCT ECM increase in POAG. In the present study, we investigated the effects of connective tissue growth factor (CTGF) on the biology of cultured human TM cells. CTGF is a downstream mediator of TGF-beta2-signaling, which is expressed at high amounts in the human TM in situ. HEK293 cells were transfected with an eukaryotic expression plasmid containing the coding sequences of human CTGF. Secreted CTGF was isolated and purified by chromatography. Primary human TM cells were incubated for 24 h with CTGF at concentrations of 2.5-100 ng/ml. Following treatment with CTGF, the expression of various ECM components that are expressed in the JCT, matrix metalloproteinases (MMPs) and integrins was investigated by real-time RT-PCR and western blot analyses. In addition, the activity of MMPs was investigated by gelatine zymography. The effect of CTGF silencing on TGF-beta2-induced gene expression was investigated by transfection of immortalized HTM cells with CTGF-specific small interfering (si)RNA before TGF-beta2 treatment. CTGF-treated human TM cells showed an increase in the expression of fibronectin, collagen types I, III, IV and VI, as well as in the integrin subunits aV and beta1. Lower concentrations of CTGF caused an autoinduction of CTGF expression. No effects were observed on the expression and activity of MMP-2, MMP-9 and plasminogen activator inhibitor-1 (PAI-1). Transfection with CTGF-specific siRNA inhibited the TGF-beta2-induced upregulation of CTGF and fibronectin. Our results indicate that treatment of human TM cells with recombinant CTGF causes distinct changes in gene expression and that CTGF is a critical mediator of the effects of TGF-beta2 on ECM synthesis in human TM cells. An intriguing aspect supported by the data of the present work is that the pharmacologic modulation of CTGF might be a useful approach to develop novel therapeutic strategies to prevent or to reverse the structural changes that occur in the TM of eyes with POAG.

Dynamic contrast-enhanced MRI of ocular biotransport in normal and hypertensive eyes

This study aims to employ in vivo dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) to evaluate the ocular transport following an induction of ocular hypertension in a rat model of chronic glaucoma. Upon systemic administration of Gd-DTPA solution, T1-weighted signal increase was observed in the vitreous body of the glaucomatous eye but not the control eye. This increase occurred earlier in the anterior vitreous body than the preretinal vitreous. Further, there was an earlier Gd-DTPA transport into the anterior chamber in the majority of glaucomatous eyes. Our DCE-MRI findings revealed the leakage of Gd-DTPA at the aqueous-vitreous interface, which was likely resulted from increased permeability of blood-aqueous or aqueous-vitreous barrier. These may explain the sources of changing biochemical compositions in the chamber components, which may implicate the neurodegenerative processes in the glaucomatous visual components.

Gene expression profiling of TGFbeta2- and/or BMP7-treated trabecular meshwork cells: Identification of Smad7 as a critical inhibitor of TGF-beta2 signaling

A distinct structural change in the trabecular meshwork (TM) of patients with primary open-angle glaucoma (POAG) is the increase in fibrillar extracellular matrix (ECM) in the juxtacanalicular region of the TM. Transforming growth factor (TGF)-beta2 signaling may be involved, as TGF-beta2 is significantly increased in the aqueous humor of patients with POAG. In cultured human TM cells, TGF-beta2 causes an increase in ECM deposition, an effect that is blunted or prevented, if BMP7 is added in combination with TGF-beta2. In order to know more about the signaling network that is induced in HTM cells treated with BMP7, TGF-beta2 or the combination of both factors, we identified differentially regulated genes by microarray analysis, and confirmed selected genes by quantitative RT-PCR, Western blotting, or immunohistochemistry. We observed multiple effects of both TGF-beta2 and BMP7 on the expression of a considerable number of genes involved in growth factor signaling, ECM structure and turnover, and modification of the cytoskeleton. Among the genes that were found to be regulated were CAPZA1, CDC42BPB, EFEMP1, FGF5, FSTL3, HBEGF, LTBP1, LTBP2, MATN2, NRP1, SERPINE1, SH3MD1, SMTN, SMAD7, TFPI2, TNFAIP6, and VEGF. Since SMAD7 encodes for Smad7, an inhibitory Smad that acts in a negative-feedback loop to inhibit TGF-beta activity, we silenced Smad7 mRNA in cultured human TM cells by a specific small interfering RNA. Silencing of its mRNA caused a substantial knock down of Smad7 in TM cells. Following combined BMP7/TGF-beta2 treatment, the antagonizing effect of BMP7 on TGF-beta2-induced CTGF expression was abolished. We conclude that Smad7 is the key molecular switch that inhibits TGF-beta2 signaling, and mediates the blunting effects of BMP7 on TGF-beta2 in TM cells. A therapeutic modulation of Smad7 might be a promising approach to influence ECM turnover in the TM and to treat POAG.

The changing paradigm of outflow resistance generation: towards synergistic models of the JCT and inner wall endothelium

Aqueous humor outflow resistance is the primary determinant of intraocular pressure (IOP), and increased outflow resistance is the basis for elevated IOP associated with glaucoma. Experimental evidence suggests that the bulk of outflow resistance is generated in the vicinity of the inner wall endothelium of Schlemm's canal, its basement membrane and the juxtacanalicular connective tissue (JCT). However, attempts to sort out the contribution of each of these tissues to total outflow resistance have not been successful. Conventional understanding of outflow resistance assumes that the resistance of each tissue strata (i.e., the inner wall endothelium, its basement membrane and JCT) in the outflow pathway adds in series to contribute to total outflow resistance generation. However, this perspective leads to a paradox where the apparent resistances of all tissues in the outflow pathway are much lower than the measured total resistance. To resolve this paradox, we explore synergistic models of outflow resistance generation where hydrodynamic interactions between different tissue strata lead to a total resistance that is greater than the sum of the individual tissue resistances. We closely examine the "funneling" hypothesis that has emerged as a leading synergistic model, and we review the basis of funneling, mechanical and biological requirements for funneling and evidence in support of this hypothesis. We also propose refinements to the funneling model and describe how funneling may relate to segmental variability of aqueous humor outflow patterns observed within the trabecular meshwork. Pressure gradients across the JCT and inner wall endothelium will generate mechanical loads that influence the morphology of these tissues. Because tissue morphology may in turn affect outflow resistance, there exists the potential for a two-way coupling or a "fluid-solid interaction" between outflow hydrodynamics and the mechanical behavior of the inner wall and JCT. Furthermore, the adhesions and tethers between the inner wall and JCT must be physically capable of supporting such loads. We examine the structure and mechanical strength of these adhesions, and provide evidence that these adhesions and tethers are unable to support the full load imposed by the bulk of outflow resistance generation unless a substantial fraction of outflow resistance is generated within the JCT, consistent with the funneling model. This indicates that these attachments between the inner wall and JCT have considerable physiological importance for outflow resistance regulation, by maintaining the proximity between these two tissues to facilitate funneling. Further study is greatly needed to better characterize these important interactions.

The aqueous humor outflow pathway of zebrafish

PURPOSE

The structures of the ocular anterior segment responsible for aqueous humor secretion and absorption have been well characterized in mammals. However, the underlying molecular and cellular mechanisms that regulate aqueous humor flow have remained elusive. Experimental analysis in Danio rerio, the zebrafish, is providing mechanistic insights into many cellular processes relevant to normal human physiology and disease. To facilitate studies on the molecular and cellular mechanisms of aqueous humor dynamics using this species, the authors have characterized the anatomy of aqueous secretion and outflow in adult zebrafish eyes.

METHODS

Analyses by light and transmission electron microscopy, coupled with molecular tracers of fluid flow, were used to identify and study the sites of aqueous humor secretion and absorption in adult zebrafish eyes.

RESULTS

Zebrafish eyes show aqueous humor secretion primarily from the dorsal ciliary region and outflow through a ventral canalicular network that connects with an aqueous plexus and veins of the choroidal rete.

CONCLUSIONS

Vectorial flow of zebrafish aqueous humor is in contrast to that in mammals in which secretion and absorption of aqueous humor are circumferential around and through the iridocorneal angle. However, local anatomy and ultrastructure of the tissues and cells specialized for aqueous humor dynamics in zebrafish show conservation with that of mammals. These observations suggest that zebrafish can serve as a useful genetic model to help understand the regulation and cellular basis of normal and abnormal aqueous humor dynamics in humans.

Mechanobiology of trabecular meshwork cells

Trabecular meshwork (TM) cells likely play a key role in regulating outflow facility and hence intraocular pressure. They function in a dynamic environment subjected to variations in mechanical and fluid shear forces. Because the extent of mechanical stress on the trabecular meshwork is dependent on the intraocular pressure, the behavior of TM cells under mechanical strain may suggest mechanisms for how outflow facility is regulated. Studies have demonstrated that TM cells respond in a variety of ways to mechanical loads, including increased extracellular matrix turnover, altered gene expression, cytokine release, and altered signal transduction. This review highlights some of the considerations and limitations of studying the mechanobiology of TM cells.

The role of the actomyosin system in regulating trabecular fluid outflow

Abnormally high resistance to aqueous humor drainage via the trabecular meshwork and Schlemm's canal is highly correlated with the development of primary open-angle glaucoma. Contractility of the actomyosin system in the trabecular cells or inner wall endothelium of Schlemm's canal is an important factor in the regulation of outflow resistance. Cytoskeletal agents, affecting F-actin integrity or actomyosin contractility, or gene therapies, employing overexpression of caldesmon or Rho-A inhibition, can decrease outflow resistance in the drainage pathway. In this review, we discuss the mechanisms underlying these and similar effects on trabecular outflow resistance in living animals and/or in cultured ocular anterior segments from enucleated animal or human eyes.