'What controls aqueous humour outflow resistance?'

The regulation of connective tissue growth factor expression influences the viability of human trabecular meshwork cells

Connective tissue growth factor (CTGF) induces extracellular matrix (ECM) synthesis and contractility in human trabecular meshwork (HTM) cells. Both processes are involved in the pathogenesis of primary open-angle glaucoma. To date, little is known about regulation and function of CTGF expression in the trabecular meshwork (TM). Therefore, we analysed the effects of different aqueous humour proteins and stressors on CTGF expression in HTM cells. HTM cells from three different donors were treated with endothelin-1, insulin-like growth factor (IGF)-1, angiotensin-II, H₂O₂ and heat shock and were analysed by immunohistochemistry, real-time RT-PCR and Western blotting. Viability after H₂O₂ treatment was measured in CTGF silenced HTM-N cells and their controls. Latrunculin A reduced expression of CTGF by about 50% compared to untreated HTM cells, whereas endothelin-1, IGF-1, angiotensin-II, heat shock and oxidative stress led to a significant increase. Silencing of CTGF resulted in a delayed expression of αB-crystallin and in reduced cell viability in comparison to the controls after oxidative stress. Conversely, CTGF treatment led to a higher cell viability rate after H₂O₂ treatment. CTGF expression is induced by factors that have been linked to glaucoma. An increased level of CTGF appears to protect TM cells against damage induced by stress. The beneficial effect of CTGF for viability of TM cells is likely associated with the effects on increased ECM synthesis and higher contractility of the TM, thereby contributing to reduced aqueous humour outflow facility causing increased intraocular pressure.

Shorter scleral spur in eyes with primary open-angle glaucoma

PURPOSE

We determined whether the scleral spur is shorter in primary open-angle glaucoma (POAG) eyes compared to age-matched normal eyes and whether the collapse of Schlemm's canal (SC) is more prevalent in eyes with a shorter scleral spur.

METHODS

The anterior segments of normal (n = 20) and POAG eyes (n = 20) were fixed and processed for light microscopy. The scleral spur length, ratio of posterior trabecular meshwork (TM) insertion into the scleral spur to the posterior TM height, and the percentage of SC collapse were measured. Analysis using an existing mathematical model was conducted to estimate the distances that the scleral spur theoretically would move in vivo and to determine if these distances would be sufficient to keep SC open in POAG compared to normal eyes.

RESULTS

The mean scleral spur length was significantly shorter in POAG eyes compared to normal eyes (P < 0.0001). A higher mean percentage of SC collapse was found in POAG eyes than in normal eyes (P < 0.0001). Estimated posterior movement of scleral spur in POAG eyes was less than sufficient to prevent the collapse of SC. A significant negative correlation was found between the posterior scleral spur movement and percent collapse of SC (P < 0.0001).

CONCLUSIONS

A shorter scleral spur found in POAG eyes was associated with a higher percent of SC collapse. Our data suggest that a shorter scleral spur may be a risk factor in the development of POAG by being insufficient to hold SC open.

Aqueous outflow: segmental and distal flow

The elevated intraocular pressure (IOP) of primary open-angle glaucoma is caused by impaired outflow of aqueous humor through the trabecular meshwork. Within the juxtacanalicular region, alterations of both extracellular matrix homeostasis and the cellular tone of trabecular meshwork endothelial and the inner wall of Schlemm canal cells affect outflow. Newer pharmacologic agents that target trabecular meshwork and Schlemm canal cell cytoskeleton lower IOP. Aqueous drainage occurs nonhomogenously with greater flow going through certain portions of the TM and less going through other portions-a concept known as segmental flow, which is theoretically the result of outflow being dependent on the presence of discrete pores within Schlemm canal. The limited long-term success of trabecular meshwork bypass surgeries implicates the potential impact of resistance in Schlemm canal itself and collector channels. Additionally, others have observed that outflow occurs preferentially near collector channels. These distal structures may be more important to aqueous outflow than previously believed.

FINANCIAL DISCLOSURE

Dr. Rhee is a consultant to Aerie Pharmaceuticals, Alcon Laboratories, Inc., Allegan, Inc., Aquesys, Inc., Glaukos Corp., Ivantis, Inc., Johnson & Johnson, Merck Sharp & Dohme Corp. and Santen, Inc., and has received research funding from Alcon Laboratories, Inc., Merck Sharp & Dohme Corp., and Ivantis, Inc. No other author has a financial or proprietary interest in any material or method mentioned.

Wnt inhibition induces persistent increases in intrinsic stiffness of human trabecular meshwork cells

Wnt antagonism has been linked to glaucoma and intraocular pressure regulation, as has increased stiffness of human trabecular meshwork (HTM) tissue. We have shown culturing HTM cells on substrates that mimic the elevated stiffness of glaucomatous tissue leads to elevated expression of the Wnt antagonist secreted frizzled related protein 1 (SFRP1), suggesting a linkage between SFRP1 and HTM mechanobiology. In this study, we document biomechanical consequences of Wnt antagonism on HTM cells. Cells were treated with the Wnt antagonists (SFRP1, KY02111, and LGK-974) for 8 days and allowed to recover for 4 days. After recovery, intrinsic cell stiffness and activation of the Wnt pathway via β-catenin staining and blotting were assayed. Basal cell stiffness values were 3.71 ± 0.37, 4.33 ± 3.07, and 3.07 ± kPa (median ± S.D.) for cells derived from 3 donors. Cell stiffness increased after 0.25 μg/mL (4.32 ± 5.12, 8.86 ± 8.51, 4.84 ± 3.15 kPa) and 0.5 μg/mL (16.75 ± 5.59, 13.18 ± 7.99, and 8.54 ± 5.77 kPa) SFRP1 treatment. Stiffening was observed after 10 μM KY02111 (10.72 ± 5.63 and 6.57 ± 5.53 kPa) as well as LGK-974 (9.60 ± 7.41 and 11.40 ± 9.24 kPa) treatment compared with controls (3.79 ± 1.01 and 5.16 ± 2.14 kPa). Additionally, Wnt inhibition resulted in decreased β-catenin staining and increased phosphorylation at threonine 41 after recovery. In conclusion, this work demonstrates a causal relationship between Wnt inhibition and cell stiffening. Additionally, these findings suggest transient Wnt inhibition resulted in durable modulation of the mechanical phenotype of HTM cells. When placed in context with previous results, these findings provide a causal link between Wnt antagonism and cell stiffness and suggest a feedback loop contributing to glaucoma progression.

Colocalization of outflow segmentation and pores along the inner wall of Schlemm's canal

All aqueous humor draining through the conventional outflow pathway must cross the endothelium of Schlemm's canal (SC), likely by passing through micron-sized transendothelial pores. SC pores are non-uniformly distributed along the inner wall endothelium, but it is unclear how the distribution of pores relates to the non-uniform or segmental distribution of aqueous humor outflow through the trabecular meshwork. It is hypothesized that regions in the juxtacanalicular tissue (JCT) with higher local outflow should coincide with regions of greater inner wall pore density compared to JCT regions with lower outflow. Three pairs of non-glaucomatous human donor eyes were perfused at 8 mmHg with fluorescent tracer nanospheres to decorate local patterns of outflow segmentation through the JCT. The inner wall was stained for CD31 and/or vimentin and imaged en face using confocal and scanning electron microscopy (SEM). Confocal and SEM images were spatially registered to examine the spatial relationship between inner wall pore density and tracer intensity in the underlying JCT. For each eye, tracer intensity, pore density (n) and pore diameter (D) (for both transcellular "I" and paracellular "B" pores) were measured in 4-7 regions of interest (ROIs; 50 × 150 μm each). Analysis of covariance was used to examine the relationship between tracer intensity and pore density, as well as the relationship between tracer intensity and three pore metrics (nD, nD(2) and nD(3)) that represent the local hydraulic conductivity of the outflow pathway as predicted by various hydrodynamic models. Tracer intensity in the JCT correlated positively with local pore density when considering total pores (p = 0.044) and paracellular B pores on their own (p = 0.016), but not transcellular I-pores on their own (p = 0.54). Local hydraulic conductivity as predicted by the three hydrodynamic models all showed a significant positive correlation with tracer intensity when considering total pores and B-pores (p < 0.0015 and p < 10(-4)) but not I-pores (p > 0.38). These data suggest that aqueous humor passes through micron-sized pores in the inner wall endothelium of SC. Paracellular B-pores appear to have a dominant contribution towards transendothelial filtration across the inner wall relative to transcellular I-pores. Impaired pore formation, as previously described in glaucomatous SC cells, may thereby contribute to greater outflow heterogeneity, outflow obstruction, and IOP elevation in glaucoma.

Biomechanics of Schlemm's canal endothelium and intraocular pressure reduction

Ocular hypertension in glaucoma develops due to age-related cellular dysfunction in the conventional outflow tract, resulting in increased resistance to aqueous humor outflow. Two cell types, trabecular meshwork (TM) and Schlemm's canal (SC) endothelia, interact in the juxtacanalicular tissue (JCT) region of the conventional outflow tract to regulate outflow resistance. Unlike endothelial cells lining the systemic vasculature, endothelial cells lining the inner wall of SC support a transcellular pressure gradient in the basal to apical direction, thus acting to push the cells off their basal lamina. The resulting biomechanical strain in SC cells is quite large and is likely to be an important determinant of endothelial barrier function, outflow resistance and intraocular pressure. This review summarizes recent work demonstrating how biomechanical properties of SC cells impact glaucoma. SC cells are highly contractile, and such contraction greatly increases cell stiffness. Elevated cell stiffness in glaucoma may reduce the strain experienced by SC cells, decrease the propensity of SC cells to form pores, and thus impair the egress of aqueous humor from the eye. Furthermore, SC cells are sensitive to the stiffness of their local mechanical microenvironment, altering their own cell stiffness and modulating gene expression in response. Significantly, glaucomatous SC cells appear to be hyper-responsive to substrate stiffness. Thus, evidence suggests that targeting the material properties of SC cells will have therapeutic benefits for lowering intraocular pressure in glaucoma.

Finite element analysis of the pressure-induced deformation of Schlemm's canal endothelial cells

The endothelial cells lining the inner wall of Schlemm's canal (SC) in the eye are relatively unique in that they support a basal-to-apical pressure gradient that causes these cells to deform, creating giant vacuoles and transendothelial pores through which the aqueous humor flows. Glaucoma is associated with an increased resistance to this flow. We used finite element modeling and estimates of cell modulus made using atomic force microscopy to characterize the pressure-induced deformation of SC cells and to estimate the maximum pressure drop that SC cells can support. We examined the effects of cell geometry, cell stiffness, and the contribution of the cell cortex to support the pressure-generated load. We found that the maximum strain generated by this loading occurs at the points of cell-substrate attachment and that the cortex of the cells bears nearly all of this load. The ability of these cells to support a significant transcellular pressure drop is extremely limited (on the order of 5 mmHg or less) unless these cells either stiffen very considerably with increasing deformation or have substantial attachments to their substratum away from their periphery. This puts limits on the flow resistance that this layer can generate, which has implications regarding the site where the bulk of the flow resistance is generated in healthy and glaucomatous eyes.

RhoA GTPase-induced ocular hypertension in a rodent model is associated with increased fibrogenic activity in the trabecular meshwork

Ocular hypertension arising from increased resistance to aqueous humor (AH) outflow through the trabecular meshwork is a primary risk factor for open-angle glaucoma, a leading cause of blindness. Ongoing efforts have found little about the molecular and cellular bases of increased resistance to AH outflow through the trabecular meshwork in ocular hypertension patients. To test the hypothesis that dysregulated Rho GTPase signaling and a resulting fibrotic activity within the trabecular meshwork may result in ocular hypertension, we investigated the effects of expressing a constitutively active RhoA GTPase (RhoAV14) in the AH outflow pathway in Sprague-Dawley rats by using lentiviral vector-based gene delivery. Rats expressing RhoAV14 in the iridocorneal angle exhibited a significantly elevated intraocular pressure. Elevated intraocular pressure in the RhoAV14-expressing rats was associated with fibrotic trabecular meshwork and increased levels of F-actin, phosphorylated myosin light chain, α-smooth muscle actin, collagen-1A, and total collagen in the trabecular AH outflow pathway. Most of these changes were ameliorated by topical application of Rho kinase inhibitor. Human autopsy eyes from patients with glaucoma exhibited significant increases in levels of collagen-1A and total collagen in the trabecular AH outflow pathway. Collectively, these observations indicate that increased fibrogenic activity because of dysregulated RhoA GTPase activity in the trabecular AH outflow pathway increases intraocular pressure in a Rho kinase-dependent manner.

A laser-induced mouse model with long-term intraocular pressure elevation

Purpose

To develop and characterize a mouse model with intraocular pressure (IOP) elevation after laser photocoagulation on the trabecular meshwork (TM), which may serve as a model to investigate the potential of stem cell-based therapies for glaucoma.

Methods

IOP was measured in 281 adult C57BL/6 mice to determine normal IOP range. IOP elevation was induced unilaterally in 50 adult mice, by targeting the TM through the limbus with a 532-nm diode laser. IOP was measured up to 24 weeks post-treatment. The optic nerve damage was detected by electroretinography and assessed by semiautomatic counting of optic nerve axons. Effects of laser treatment on the TM were evaluated by histology, immunofluorescence staining, optical coherence tomography (OCT) and transmission electron microscopy (TEM).

Results

The average IOP of C57BL/6 mice was 14.5±2.6 mmHg (Mean ±SD). After laser treatment, IOP averaged above 20 mmHg throughout the follow-up period of 24 weeks. At 24 weeks, 57% of treated eyes had elevated IOP with the mean IOP of 22.5±2.5 mmHg (Mean ±SED). The difference of average axon count (59.0%) between laser treated and untreated eyes was statistically significant. Photopic negative response (PhNR) by electroretinography was significantly decreased. CD45+ inflammatory cells invaded the TM within 1 week. The expression of SPARC was increased in the TM from 1 to 12 weeks. Histology showed the anterior chamber angle open after laser treatment. OCT indicated that most of the eyes with laser treatment had no synechia in the anterior chamber angles. TEM demonstrated disorganized and compacted extracellular matrix in the TM.

Conclusions

An experimental murine ocular hypertension model with an open angle and optic nerve axon loss was produced with laser photocoagulation, which could be used to investigate stem cell-based therapies for restoration of the outflow pathway integrity for ocular hypertension or glaucoma.

Endothelial glycocalyx layer in the aqueous outflow pathway of bovine and human eyes

The glycocalyx layer on the vascular endothelium is known to have an important role as a transport barrier and in the mechanotransduction of fluid shear stress. The detailed structure and distribution of the glycocalyx in the bovine and human aqueous humor outflow pathways has not yet been reported. The purpose of this study was to determine whether this layer exists in the bovine and human aqueous outflow pathways and to compare the distribution and thickness therein. Enucleated bovine (N = 4) and human (N = 4) eyes were fixed using Alcian Blue to preserve the glycocalyx. The glycocalyx distribution and thickness (in regions where it was seen) were measured on the trabecular beams (TM), Schlemm's canal (SC)/aqueous plexus (AP), and collector channels (CC). The glycocalyx, which appears as a layer of hair-like brushes, coats the surface of the endothelium non-uniformly in the bovine and human aqueous outflow pathways with a thickness in bovine eyes of 68-122 nm and in human eyes of 52-166 nm (25th to 75th percentiles). The distribution of the glycocalyx in different regions of the outflow pathway is not the same between bovine and human eyes. In both species, the glycocalyx was most uniform in the CCs. Less coverage of glycocalyx was found in the AP than the TM in bovine eyes, while more coverage was found in SC than the TM in human eyes. Most interestingly, glycocalyx was also found filling most pores of the endothelium of AP/SC in both bovine and human eyes. Glycocalyx was usually not found coating the inner membranes of the giant vacuoles (GVs); however, in GVs with a visible pore, glycocalyx was frequently observed on the inner membranes of the GVs. Based on our findings and those from the vascular endothelium, it is likely that the glycocalyx in SC plays a role in transduction of shear stress and perhaps regulation of outflow resistance.

Altered mechanobiology of Schlemm's canal endothelial cells in glaucoma

Increased flow resistance is responsible for the elevated intraocular pressure characteristic of glaucoma, but the cause of this resistance increase is not known. We tested the hypothesis that altered biomechanical behavior of Schlemm's canal (SC) cells contributes to this dysfunction. We used atomic force microscopy, optical magnetic twisting cytometry, and a unique cell perfusion apparatus to examine cultured endothelial cells isolated from the inner wall of SC of healthy and glaucomatous human eyes. Here we establish the existence of a reduced tendency for pore formation in the glaucomatous SC cell--likely accounting for increased outflow resistance--that positively correlates with elevated subcortical cell stiffness, along with an enhanced sensitivity to the mechanical microenvironment including altered expression of several key genes, particularly connective tissue growth factor. Rather than being seen as a simple mechanical barrier to filtration, the endothelium of SC is seen instead as a dynamic material whose response to mechanical strain leads to pore formation and thereby modulates the resistance to aqueous humor outflow. In the glaucomatous eye, this process becomes impaired. Together, these observations support the idea of SC cell stiffness--and its biomechanical effects on pore formation--as a therapeutic target in glaucoma.

Anatomic changes in Schlemm's canal and collector channels in normal and primary open-angle glaucoma eyes using low and high perfusion pressures

PURPOSE

To examine the anatomy of Schlemm's canal (SC) and collector channels (CCs) in normal human and primary open-angle glaucoma (POAG) eyes under low and high perfusion pressure.

METHODS

In normal (n = 3) and POAG (n = 3) eye pairs, one eye was perfused at 10 mm Hg while the fellow eye was perfused at 20 mm Hg for 2 hours. Eyes were perfusion fixed at like pressures, dissected into quadrants, embedded in Epon Araldite, and scanned by three-dimensional micro-computed tomography (3D micro-CT). Schlemm's canal volume, CC orifice area, diameter, and number were measured using ANALYZE software.

RESULTS

Normal eyes showed a larger SC volume (3.3-fold) and CC orifice area (9962.8 vs. 8825.2 μm(2)) and a similar CC diameter (34.3 ± 17.8 vs. 32.7 ± 13.0 μm) at 10 mm Hg compared to 20 mm Hg. In POAG eyes, SC volume (2.0-fold), CC orifice area (8049.2 μm(2)-6468.4 μm(2)), and CC diameter (36.2 ± 19.1 vs. 29.0 ± 13.8 μm) were increased in 10 mm Hg compared to 20 mm Hg perfusion pressures. Partial and total CC occlusions were present in normal and POAG eyes, with a 3.7-fold increase in total occlusions in POAG eyes compared to normal eyes at 20 mm Hg. Visualization of CCs increased by 24% in normal and by 21% in POAG eyes at 20 mm Hg compared to 10 mm Hg. Schlemm's canal volume, CC area, and CC diameter were decreased in POAG eyes compared to normal eyes at like pressures.

CONCLUSIONS

Compensatory mechanisms for transient and short periods of increased pressure appear to be diminished in POAG eyes. Variable response to pressure change in SC and CCs may be a contributing factor to outflow facility change in POAG eyes.

Schlemm's canal is a unique vessel with a combination of blood vascular and lymphatic phenotypes that forms by a novel developmental process

A draining vessel in the eye arises via a novel hybrid process of vascular development and is important for understanding ocular fluid homeostasis and glaucoma.

Schlemm's canal (SC) plays central roles in ocular physiology. These roles depend on the molecular phenotypes of SC endothelial cells (SECs). Both the specific phenotype of SECs and development of SC remain poorly defined. To allow a modern and extensive analysis of SC and its origins, we developed a new whole-mount procedure to visualize its development in the context of surrounding tissues. We then applied genetic lineage tracing, specific-fluorescent reporter genes, immunofluorescence, high-resolution confocal microscopy, and three-dimensional (3D) rendering to study SC. Using these techniques, we show that SECs have a unique phenotype that is a blend of both blood and lymphatic endothelial cell phenotypes. By analyzing whole mounts of postnatal mouse eyes progressively to adulthood, we show that SC develops from blood vessels through a newly discovered process that we name “canalogenesis.” Functional inhibition of KDR (VEGFR2), a critical receptor in initiating angiogenesis, shows that this receptor is required during canalogenesis. Unlike angiogenesis and similar to stages of vasculogenesis, during canalogenesis tip cells divide and form branched chains prior to vessel formation. Differing from both angiogenesis and vasculogenesis, during canalogenesis SECs express Prox1, a master regulator of lymphangiogenesis and lymphatic phenotypes. Thus, SC development resembles a blend of vascular developmental programs. These advances define SC as a unique vessel with a combination of blood vascular and lymphatic phenotypes. They are important for dissecting its functions that are essential for ocular health and normal vision.

Schlemm's canal serves as a drainage tube for fluid from the anterior chamber of the eye and is directly relevant to glaucoma, a disease that causes vision loss in over 70 million people. Aqueous humor enters the canal and then drains into connected veins. Molecular understanding of the development of Schlemm's canal and its drainage functions has remained limited. We provide a detailed characterization of Schlemm's canal development, and in so doing discover a novel process of vascular development that we name “canalogenesis.” We show that although the process requires a functional KDR receptor, which is also critical in blood vessel development, the endothelial cells of Schlemm's canal have a unique hybrid molecular phenotype, expressing proteins that are characteristic of both blood and lymphatic vessels. Of note, the expression of Prox1, a master regulator of lymphatic fate, and other lymphatic proteins are largely restricted to specialized cells of the inner wall of Schlemm's canal through which the aqueous humor passes as it exits the eye. Thus, Prox1 and other lymphatic proteins may be critical for the functional specialization of these cells for aqueous humor drainage. Schlemm's canal is thus a unique vessel with a combination of blood vascular and lymphatic characteristics.

Ultrastructural changes associated with dexamethasone-induced ocular hypertension in mice

PURPOSE

To determine whether dexamethasone (DEX)-induced ocular hypertension (OHT) in mice mimics the hallmarks of steroid-induced glaucoma (SIG) in humans, including reduced conventional outflow facility (C), increased extracellular matrix (ECM), and myofibroblasts within the outflow pathway.

METHODS

Osmotic mini-pumps were implanted subcutaneously into C57BL/6J mice for systemic delivery of DEX (3-4 mg/kg/d, n = 31 mice) or vehicle (n = 28). IOP was measured weekly by rebound tonometry. After 3 to 4 weeks, mice were euthanized and eyes enucleated for ex vivo perfusion to measure C, for electron microscopy to examine the trabecular meshwork (TM) and Schlemm's canal (SC), or for immunohistochemistry to examine type IV collagen and α-smooth muscle actin. The length of basement membrane material (BMM) was measured along the anterior-posterior extent of SC by electron microscopy. Ultrastructural changes in BMM of DEX-treated mice were compared against archived human SIG specimens.

RESULTS

Dexamethasone increased IOP by 2.6 ± 1.6 mm Hg (mean ± SD) over 3 to 4 weeks and decreased C by 52% ± 17% versus controls. Intraocular pressure elevation correlated with decreased C. Dexamethasone treatment led to increased fibrillar material in the TM, plaque-like sheath material surrounding elastic fibers, and myofibroblasts along SC outer wall. The length of BMM underlying SC was significantly increased in mice with DEX and in humans with SIG, and in mice decreased C correlated with increased BMM.

CONCLUSIONS

Dexamethasone-induced OHT in mice mimics hallmarks of human SIG within 4 weeks of DEX treatment. The correlation between reduced C and newly formed ECM motivates further study using DEX-treated mice to investigate the pathogenesis of conventional outflow obstruction in glaucoma.

Differential gene expression in glaucoma

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

The formation of cortical actin arrays in human trabecular meshwork cells in response to cytoskeletal disruption

The cytoskeleton of human trabecular meshwork (HTM) cells is known to be altered in glaucoma and has been hypothesized to reduce outflow facility through contracting the HTM tissue. Latrunculin B (Lat-B) and Rho-associated protein kinase (ROCK) inhibitors disrupt the actin cytoskeleton and are in clinical trials as glaucoma therapeutics. We have previously reported a transient increase in HTM cell stiffness peaking at 90 min after Lat-B treatment with a return to pretreatment values after 270 min. We hypothesize that changes in actin morphology correlate with alterations in cell stiffness induced by Lat-B but this is not a general consequence of other cytoskeletal disrupting agents such as Rho kinase inhibitors. We treated HTM cells with 2 µM Lat-B or 100 µM Y-27632 and allowed the cells to recover for 30-270 min. While examining actin morphology in Lat-B treated cells, we observed striking cortical actin arrays (CAAs). The percentage of CAA positive cells (CPCs) was time dependent and exceeded 30% at 90 min and decreased after 270 min. Y-27632 treated cells exhibited few CAAs and no changes in cell stiffness. Together, these data suggest that the increase in cell stiffness after Lat-B treatment is correlated with CAAs.

An emerging treatment option for glaucoma: Rho kinase inhibitors

Rho kinase (ROCK) inhibitors are a novel potential class of glaucoma therapeutics with multiple compounds currently in Phase II and III US Food and Drug Administration trials in the United States. These selective agents work by relaxing the trabecular meshwork through inhibition of the actin cytoskeleton contractile tone of smooth muscle. This results in increased aqueous outflow directly through the trabecular meshwork, achieving lower intraocular pressures in a range similar to prostaglandins. There are also animal studies indicating that ROCK inhibitors may improve blood flow to the optic nerve, increase ganglion cell survival, and reduce bleb scarring in glaucoma surgery. Given the multiple beneficial effects for glaucoma patients, ROCK inhibitors are certainly a highly anticipated emerging treatment option for glaucoma.

The role of matricellular proteins in glaucoma

Glaucoma is an optic neuropathy affecting approximately 60million people worldwide and is the second most common cause of irreversible blindness. Elevated intraocular pressure (IOP) is the main risk factor for developing glaucoma and is caused by impaired aqueous humor drainage through the trabecular meshwork (TM) and Schlemm's canal (SC). In primary open angle glaucoma (POAG), this elevation in IOP in turn leads to deformation at the optic nerve head (ONH) specifically at the lamina cribrosa (LC) region where there is also a deposition of extracellular matrix (ECM) molecules such as collagen and fibronectin. Matricellular proteins are non-structural secreted glycoproteins that help cells communicate with their surrounding ECM. This family of proteins includes connective tissue growth factor (CTGF), also known as CCN2, thrombospondins (TSPs), secreted protein acidic and rich in cysteine (SPARC), periostin, osteonectin, and Tenascin-C and -X and other ECM proteins. All members appear to play a role in fibrosis and increased ECM deposition. Most are widely expressed in tissues particularly in the TM and ONH and deficiency of TSP1 and SPARC have been shown to lower IOP in mouse models of glaucoma through enhanced outflow facility. The role of these proteins in glaucoma is emerging as some have an association with the pathophysiology of the TM and LC regions and might therefore be potential targets for therapeutic intervention in 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.

The prostaglandin f2α analog fluprostenol attenuates the fibrotic effects of connective tissue growth factor on human trabecular meshwork cells

Abstract Purpose: The trabecular meshwork (TM) outflow pathways of the aqueous humor show an increase in extracellular matrix in patients with primary open-angle glaucoma (POAG). The increase in TM extracellular matrix appears to be caused by transforming growth factor-β signaling and its downstream mediator connective-tissue growth factor (CTGF). Here we studied whether treatment with the prostaglandin F2α analog fluprostenol modulates the CTGF-mediated increase of the TM extracellular matrix.

METHODS

Human TM cells from 3 different donors were treated with CTGF (50 ng/mL) and/or fluprostenol (10(-6) M and 10(-7) M) and were analyzed by real-time reverse transcription polymerase chain reaction and Western blotting. Cell supernatants of the treated cells were analyzed by zymography.

RESULTS

Treatment with CTGF induced the expression and synthesis of CTGF, fibronectin, collagen type IV and VI, while treatment with fluprostenol alone had no effects. The effects of CTGF were blocked by 1-h pretreatment with fluprostenol in a dose-dependent manner. Treatment with fluprostenol or combined fluprostenol/CTGF induced the activity of matrix metalloproteinase 2 (MMP2) in TM cells, whereas treatment with CTGF alone had no effects on MMP2 activity.

CONCLUSIONS

Fluprostenol blocks the fibrotic effects of CTGF on human TM cells and increases the activity of MMP2. Both effects have the distinct potential to attenuate a CTGF-mediated increase in TM extracellular matrix in patients with POAG and any effects on TM outflow resistance that may result from that.

The microfibril hypothesis of glaucoma: implications for treatment of elevated intraocular pressure

Microfibrils are macromolecular aggregates located in the extracellular matrix of both elastic and nonelastic tissues that have essential functions in formation of elastic fibers and control of signaling through the transforming growth factor beta (TGFβ) family of cytokines. Elevation of systemic TGFβ and chronic activation of TGFβ signal transduction are associated with diseases caused by mutations in microfibril-associated genes, including FBN1. A role for microfibrils in glaucoma is suggested by identification of risk alleles in LOXL1 for exfoliation glaucoma and mutations in LTBP2 for primary congenital glaucoma, both of which are microfibril-associated genes. Recent identification of a mutation in another microfibril-associated gene, ADAMTS10, in a dog model of primary open-angle glaucoma led us to form the microfibril hypothesis of glaucoma, which in general states that defective microfibrils may be an underlying cause of glaucoma. Microfibril defects could contribute to glaucoma through alterations in biomechanical properties of tissue and/or through effects on signaling through TGFβ, which is well established to be elevated in the aqueous humor of glaucoma patients. Recent work has shown that diseases caused by microfibril defects are associated with increased concentrations of TGFβ protein and chronic activation of TGFβ-mediated signal transduction. In analogy with other microfibril-related diseases, defective microfibrils could provide a mechanism for the elevation of TGFβ2 in glaucomatous aqueous humor. If glaucoma shares mechanisms with other diseases caused by defective microfibrils, such as Marfan syndrome, therapeutic interventions to inhibit chronic activation of TGFβ signaling used in those diseases may be applied to glaucoma.

Intraocular pressure homeostasis: maintaining balance in a high-pressure environment

Although glaucoma is a relatively common blinding disease, most people do not develop glaucoma. A robust intraocular pressure (IOP) homeostatic mechanism keeps ocular pressures within relatively narrow acceptable bounds throughout most peoples' lives. The trabecular meshwork and/or Schlemm's canal inner wall cells respond to sustained IOP elevation and adjust the aqueous humor outflow resistance to restore IOP to acceptable levels. It appears that the cells sense IOP elevations as mechanical stretch or distortion of the actual outflow resistance and respond by initiating a complex extracellular matrix (ECM) turnover process that takes several days to complete. Although considerable information pertinent to this process is available, many aspects of the IOP homeostatic process remain to be elucidated. Components and mechanisms beyond ECM turnover could also be relevant to IOP homeostasis, but will not be addressed in detail here. Known aspects of the IOP homeostasis process as well as possible ways that it might function and impact glaucoma are discussed.

Tissue inhibitor of metalloproteinase 4 in aqueous humor of patients with primary open angle glaucoma, pseudoexfoliation syndrome and pseudoexfoliative glaucoma and its role in proteolysis imbalance

BACKGROUND

To quantify the levels of tissue inhibitor of metalloproteinase 4 (TIMP4) and its ratios with free metalloproteinases (MMP) in the aqueous humor of patients with primary open angle glaucoma (POAG), pseudoexfoliation syndrome (PXS) and pseudoexfoliative glaucoma (PXG) and to evaluate a possible imbalance between MMPs and TIMPs in these samples.

METHODS

Free MMP2, MMP3, MMP9, TIMP1, TIMP2, TIMP4 concentrations and active levels of MMP2 and MMP3 were determined with immunoassay ELISA and activity assay kits in 168 aqueous samples.

RESULTS

TIMP4 was elevated in glaucoma patients(POAG: 0.95 ± 0.49 PXG: 1.28 ± 1.38 pg/ml. p < 0.001). POAG, PXS and PXG samples demonstrated higher MMP2, TIMP1 and TIMP2 concentrations (p < 0.001). Samples from the PXS and PXG groups had a lower total/active MMP2 ratio (p < 0.004 and p < 0.008 respectively). Stoichiometric analysis showed an overbalance of TIMPsover MMPs in both POAG & PXG groups,especially of TIMP4.

CONCLUSION

TIMP4 elevation is a novel finding in glaucomatous eyes. A disregulation of extracellular matrix homeostasis is suggested in POAG, PXS and PXG.

Clinical results of ab interno trabeculotomy using the trabectome for open-angle glaucoma: the Mayo Clinic series in Rochester, Minnesota

PURPOSE

To determine outcomes of ab interno trabeculotomy for treatment of open-angle glaucoma (OAG).

DESIGN

Retrospective interventional single-surgeon, single-center case series.

METHODS

Data were collected from 246 patients undergoing ab interno trabeculotomy between September 1, 2006, and December 1, 2010, with 3 months' follow-up or longer. Kaplan-Meier analysis was performed using Criteria A (postoperative intraocular pressure [IOP] ≤21 mm Hg or ≥20% reduction from preoperative IOP) and Criteria B (IOP ≤18 mm Hg and ≥20% reduction in IOP). Failure included increased glaucoma medications or subsequent surgery. Failure risk factors were identified using Cox proportional hazards ratio (HR).

RESULTS

Of 88 cases of ab interno trabeculotomy-only and 158 cases of ab interno trabeculotomy with cataract extraction, the retention rate was 70% for 1 year and 62% for 2 years. Preoperative mean IOP was 21.6 ± 8.6 mm Hg; the number of glaucoma medications was 3.1 ± 1.1. At 24 months postoperatively, mean IOP was reduced 29% to 15.3 ± 4.6 mm Hg (P < 0.001) and the number of glaucoma medications was reduced 38% to 1.9 ± 1.3 (P < 0.001) with a success rate of 62% (95% CI, 56%-68%) using Criteria A and 22% (95% CI, 16%-29%) using Criteria B. Failure risk factors using Criteria A included primary OAG (HR 3.14, P < 0.01, 95% CI, 1.91-5.17) and past argon laser trabeculoplasty (HR 1.81, P < 0.01, 95% CI, 1.18-2.77). Using Criteria B, the HR for pseudoexfoliative glaucoma was 0.43 (P < 0.01, 95% CI 0.27-0.67). Of the cases, 66 (26.8%) required subsequent surgery on an average of 10 months (2 days to 3.2 years) after ab interno trabeculotomy.

CONCLUSIONS

For criteria involving IOP ≤18 mm Hg, the 24-month survival of ab interno trabeculotomy is low. This surgery is appropriate for patients requiring a target IOP of 21 mm Hg or above.

Rho-kinase inhibitors offer a new approach in the treatment of glaucoma

INTRODUCTION

Primary open-angle glaucoma (POAG) is a leading cause for worldwide blindness and is characterized by progressive optic nerve damage. The etiology of POAG is unknown, but elevated intraocular pressure (IOP) and advanced age have been identified as risk factors. IOP reduction is the only known treatment for glaucoma. Recently, drugs that inhibit rho-associated protein kinase (ROCK) have been studied in animals and people for their ability to lower IOP and potentially treat POAG. ROCK inhibitors lower IOP through a trabecular mechanism and may represent a new therapeutic paradigm for the treatment of POAG.

AREAS COVERED

Exploring the place that ROCK inhibitors may occupy in our treatment of POAG requires a thorough understanding of pathophysiology and treatment. This article summarizes current research on the incidence, proposed etiologies and mechanisms of action for this drug class. ROCK inhibitor research is presented and considered in light of the current standard of pharmacologic care.

EXPERT OPINION

ROCK inhibitors alter the cell shape and extracellular matrix (ECM) of the trabecular meshwork. Preclinical studies demonstrate that these drugs have the potential to become a new therapy for glaucoma. However, ROCK inhibitors can affect multiple cell types, and their utility can be proven only after clinical studies in patients.

Rho/Rho-associated kinase pathway in glaucoma (Review)

The Rho/ROCK pathway plays important roles in the modulation of the cytoskeletal integrity of cells, the synthesis of extracellular matrix components in the aqueous humor outflow tissue and the permeability of Schlemm's canal endothelial cells. The activation of the Rho/ROCK pathway results in trabecular meshwork (TM) contraction, and the inhibition of this pathway would provoke relaxation of TM with subsequent increase in outflow facility and, thereby, decrease intraocular pressure (IOP). ROCK inhibitors also serve as potent anti‑scarring agents via inhibition of transdifferentiation of tenon fibroblasts into myofibroblasts. Furthermore, the RhoA/ROCK pathway is involved in optic nerve neuroprotection. Inactivation of Rho/ROCK signaling increase ocular blood flow, improve retinal ganglion cell (RGC) survival and promote RGC axon regeneration. Considering the IOP modulation, potent bleb anti-scarring effect and neuroprotective properties of ROCK inhibitors, the Rho/ROCK pathway is an attractive target for anti-glaucoma therapy, and it may be used for human therapy in the near future.

Effects of Y27632 on aqueous humor outflow facility with changes in hydrodynamic pattern and morphology in human eyes

PURPOSE

To determine the effect of Y27632, a Rho-kinase inhibitor on aqueous outflow facility, flow pattern, and juxtacanalicular tissue (JCT)/trabecular meshwork (TM) morphology in human eyes.

METHODS

Sixteen enucleated human eyes were perfused with PBS plus glucose (GPBS) at 15 mm Hg to establish the baseline outflow facility. Six eyes were perfused for short-duration (30 minute) with either 50 μM Y27632 or GPBS (n = 3 per group). Ten eyes were perfused for long duration (3 hours) with either 50 μM Y27632 or GPBS (n = 5 per group). Outflow pattern was labeled using fluorescent microspheres, and effective filtration length (EFL) was measured. Morphologic changes and their relationship to EFL and facility were analyzed.

RESULTS

Outflow facility significantly increased after short-duration perfusion with Y27632 compared with its own baseline (P = 0.03), but did not reach statistical significance compared with its controls (P = 0.07). Outflow facility (P = 0.01) and EFL (P < 0.05) were significantly increased after long-duration perfusion with Y27632 compared with its controls. Increases in outflow facility and EFL demonstrated a positive correlation. Morphologically, the TM and JCT of high-tracer regions were more expanded compared with low-tracer regions. A significant increase in JCT thickness was found in the long-duration Y27632 group compared with its control group (10.0 vs. 8.0 μm, P < 0.01).

CONCLUSIONS

Y27632 increases outflow facility in human eyes. This increase correlates positively with an increase in EFL, which is associated with an increased expansion in the JCT. Our data suggest that EFL could serve as a novel parameter to correlate with outflow facility.

Sources of structural autofluorescence in the human trabecular meshwork

PURPOSE

In situ 2-photon excitation fluorescence (TPEF) of the human trabecular meshwork (TM) reveals beams of heterogeneous autofluorescence (AF) comprising high intensity fluorescent fibers (AF-high) on a background of lower intensity fluorescence (AF-low). To determine the sources of this AF heterogeneity, we imaged human TM to characterize AF, second harmonic generation (SHG) for collagen, and eosin-labeled fluorescence identifying elastin.

METHODS

Corneoscleral rims retained after corneal transplantation were incubated with and without eosin, and imaged by TPEF. TPEF was collected through multiphoton bandpass filters to obtain AF, SHG (collagen bandwidth), and eosin-labeled fluorescence images. For qualitative comparisons, near-simultaneous image acquisition pairs of AF-SHG (+/-eosin coincubation), AF-eosin, and SHG-eosin were captured. For quantitative comparisons, multiple regions of interest (ROI) were defined in separate TM beam regions within the uveal and corneoscleral meshwork for image acquisition pairs of AF-SHG (without eosin coincubation) and SHG-eosin. We defined 18 ROI within each acquisition pair as the basis for Manders colocalization analysis. Perfect colocalization was defined as a Manders coefficient (Mcoeff) of 1.

RESULTS

Qualitatively and quantitatively, AF-low colocalized with SHG (Mcoeff=1), but not SHG signal-voids. AF-high colocalized with SHG signal-voids (Mcoeff=1), but not the SHG signal. Like AF-high, eosin-labeled fluorescence qualitatively and quantitatively colocalized (Mcoeff=1) with SHG signal-voids, but not the SHG signal.

CONCLUSIONS

Heterogeneous AF in human TM is comprised of high intensity signal originating from elastin fibers in beam cores and lower intensity signal originating from collagen. These findings are relevant to interpreting structural extracellular matrix signals in AF images of the TM.

Differential effects of prostaglandin E2-sensitive receptors on contractility of human ocular cells that regulate conventional outflow

PURPOSE

The goal of this study was to functionally compare prostaglandin E2 (PGE2)-sensitive receptors in human primary cells involved in conventional outflow.

METHODS

The expression profile of prostaglandin (PG) receptors in primary cultures of human trabecular meshwork (TM) and Schlemm's canal (SC) cells were determined by quantitative-PCR. The functional activities of endogenous PGE2-sensitive receptors were evaluated using subtype-selective agonists and antagonists with cell impedance technology.

RESULTS

Agonist-sensitive EP1, EP2, and EP4 receptors were present in TM cells, all increasing cell stiffness (or contractility) in a dose-dependent manner. Rank order of efficacy (Emax) for agonists in TM cells were EP1 greater than EP2 greater than EP4 with EC50 1.1 μM, 0.56 μM, and 0.1 μM, respectively, and no functional EP3 receptors were found. Of the four EP receptor subtypes active in SC cells, EP1 and EP3 receptor activation increased cell stiffness, while EP2 and EP4 agonists dose-dependently decreased cell stiffness 47% and 23% with EC50 values of 170 nM and 69 nM, respectively. Consistent with these observations, the Rho kinase inhibitor Y-27632 decreased cell impedance (stiffness) of TM and SC cells (∼60%), while Rho GTPase activator thrombin caused cell impedance to increase in both cell types (168%-190%).

CONCLUSIONS

Cell impedance positively correlates with cellular stiffness/contractility. Because EP2/4 receptors caused decreased cell stiffness in SC, but not in TM cells, both receptors appear to mediate IOP lowering via changes in SC cell stiffness in the conventional outflow pathway.

New therapeutic targets for intraocular pressure lowering

Primary open-angle glaucoma (POAG) is a leading cause of irreversible and preventable blindness and ocular hypertension is the strongest known risk factor. With current classes of drugs, management of the disease focuses on lowering intraocular pressure (IOP). Despite of their use to modify the course of the disease, none of the current medications for POAG is able to reduce the IOP by more than 25%-30%. Also, some glaucoma patients show disease progression despite of the therapeutics. This paper examines the new described physiological targets for reducing the IOP. The main cause of elevated IOP in POAG is thought to be an increased outflow resistance via the pressure-dependent trabecular outflow system, so there is a crescent interest in increasing trabecular meshwork outflow by extracellular matrix remodeling and/or by modulation of contractility/TM cytoskeleton disruption. Modulation of new agents that act mainly on trabecular meshwork outflow may be the future hypotensive treatment for glaucoma patients. There are also other agents in which modulation may decrease aqueous humour production or increase uveoscleral outflow by different mechanisms from those drugs available for glaucoma treatment. Recently, a role for the ghrelin-GHSR system in the pathophysiology modulation of the anterior segment, particularly regarding glaucoma, has been proposed.

Recreating a human trabecular meshwork outflow system on microfabricated porous structures

Glaucoma is the leading cause of irreversible blindness, resulting from an increase in intraocular pressure (IOP). IOP is the only modifiable risk factor of glaucoma and is controlled by the outflow of the aqueous humor through the human trabecular meshwork (HTM). Currently, the lack of a proper in vitro HTM model impedes advances in understanding outflow physiology and discovering effective IOP-lowering anti-glaucoma therapeutics. Therefore, we designed and constructed an in vitro HTM model using micropatterned, porous SU-8 scaffolds, which support cells to recapitulate functional HTM morphology and allow the study of outflow physiology. The pore size of SU-8 scaffolds, surface coating, cell seeding density, and culture duration were evaluated for HTM cell growth. The bioengineered HTM was characterized by F-actin staining and immunocytochemistry of HTM markers. A stand-alone perfusion chamber with an integrated pressure sensing system was further constructed and used for the investigation of the outflow facility of the bioengineered HTM treated with latrunculin B-an IOP lowering agent. Cells in the in vitro model exhibited HTM-like morphology, expression of α-smooth muscle actin, myocilin, and αß-crystallin, outflow characteristics and drug responsiveness. Altogether, we have developed an in vitro HTM model system for understanding HTM cell biology and screening of pharmacological or biological agents that affect trabecular outflow facility, expediting discovery of IOP-lowering, anti-glaucoma therapeutics.

What do mechanotransduction, Hippo, Wnt, and TGFβ have in common? YAP and TAZ as key orchestrating molecules in ocular health and disease

Cells in vivo are exposed to a complex signaling environment. Biochemical signaling modalities, such as secreted proteins, specific extracellular matrix domains and ion fluxes certainly compose an important set of regulatory signals to cells. However, these signals are not exerted in isolation, but rather in concert with biophysical cues of the surrounding tissue, such as stiffness and topography. In this review, we attempt to highlight the biophysical attributes of ocular tissues and their influence on cellular behavior. Additionally, we introduce the proteins YAP and TAZ as targets of biophysical and biochemical signaling and important agonists and antagonists of numerous signaling pathways, including TGFβ and Wnt. We frame the discussion around this extensive signaling crosstalk, which allows YAP and TAZ to act as orchestrating molecules, capable of integrating biophysical and biochemical cues into a broad cellular response. Finally, while we draw on research from various fields to provide a full picture of YAP and TAZ, we attempt to highlight the intersections with vision science and the exciting work that has already been performed.

The Juxtacanalicular Region of Ocular Trabecular Meshwork: A Tissue with a Unique Extracellular Matrix and Specialized Function

The trabecular meshwork (TM) is a filter-like tissue located in the anterior segment of the eye. It is composed of a series of fenestrated beams through which aqueous humor flows to exit the anterior chamber via Schlemm's canal. The primary function of the TM is to regulate the flow of aqueous humor in order to establish intraocular pressure (IOP). Dysregulated aqueous humor outflow causes elevated IOP, which is a primary risk factor for glaucoma. The region of the TM implicated in establishing IOP lies adjacent to Schlemm's canal and is called the juxtacanalicular tissue (JCT) or cribriform region. Recent advanced light microscopy studies suggest that the JCT can be subdivided into inner and outer zones based on the localization of certain extracellular matrix (ECM) molecules. By comparing the ECM of the JCT to other connectives tissues and disease processes, this review outlines the evidence for two new concepts in TM biology: (1) continuous maintenance ECM remodeling, which may be critical in order to preserve open aqueous humor flow channels by releasing trapped debris and associated ECM fragments from the outflow pathways; (2) the JCT ECM as a barrier to functionally isolate the aqueous outflow channels. The ECM surrounding the outflow channels in the JCT may function to sequester small active regulatory molecules and prevent them from aberrantly modulating outflow resistance. These adaptations point to a distinctive tissue that has evolved transient ECM remodeling processes in order to regulate aqueous humor outflow and maintain rigorous control of IOP.

Vitamin E reduces TGF-beta2-induced changes in human trabecular meshwork cells

AIM

To determine the effects of vitamin E on transforming growth factor-beta2 (TGF-β2)-induced cellular changes in cultured human trabecular meshwork (TM) cells.

MATERIALS AND METHODS

Human TM cells were pre-treated with different concentrations of vitamin E. Afterwards, cells were exposed to 1.0 ng/ml TGF-β2 for 24 h. Expressions of the heat shock protein αB-crystallin, the extracellular matrix (ECM) component fibronectin and the ECM-degrading enzyme matrix metalloproteinase-2 (MMP-2) were examined by real-time polymerase chain reaction (PCR) analyses. The cytoskeleton was investigated by phalloidin staining.

RESULTS

TGF-β2 increased the expressions of αB-crystallin and fibronectin and reduced the levels of MMP-2. TGF-β2 induced the formation of actin stress fibers and cross-linked actin networks. Pre-treatment with different concentrations of vitamin E reversed the TGF-β2-induced cellular changes in cultured human TM cells.

CONCLUSIONS

TGF-β2-mediated changes in human TM cells could be reduced by pre-treatment with vitamin E. Therefore, it may be speculated that increasing the antioxidative capacity may help to lower the incidence of characteristic glaucomatous changes in the TM.

24-hour intraocular pressure and ocular perfusion pressure in glaucoma

This review analyzes the currently available literature on circadian rhythms of intraocular pressure (IOP), blood pressure, and calculated ocular perfusion pressure (OPP) in patients with open-angle glaucoma. Although adequately powered, prospective trials are not available. The existing evidence suggests that high 24-hour IOP and OPP fluctuations can have detrimental effects in eyes with glaucoma. The currently emerging continuous IOP monitoring technologies may soon offer important contributions to the study of IOP rhythms. Once telemetric technologies become validated and widely available for clinical use, they may provide an important tool towards a better understanding of long- and short-term IOP fluctuations during a patient's daily routine. Important issues that need to be investigated further include the identification of appropriate surrogate measures of IOP and OPP fluctuation for patients unable to undergo 24-hour measurements, the determination of formulae that best describe the relationship between systemic blood pressure and IOP with OPP, and the exact clinical relevance of IOP and OPP fluctuation in individual patients. Despite the unanswered questions, a significant body of literature suggests that OPP assessment may be clinically relevant in a significant number of glaucoma patients.

A de novo MYOC mutation detected in juvenile open angle glaucoma associated with reduced myocilin protein in aqueous humor

MYOC mutations were originally identified in patients with juvenile open angle glaucoma (JOAG). Cell culture and mouse studies suggest that MYOC mutations cause glaucoma through a dominant-negative effect on myocilin protein secretion. We tested this hypothesis with patient samples in this study. Glaucoma and control patients underwent complete ocular examination. DNA samples from glaucoma patients, unaffected relatives and controls were used for DNA sequencing of MYOC. Aqueous humor (AH) samples from glaucoma and control patients were obtained at the time of surgery. Myocilin protein in AH was detected by quantitative Western blot analysis. A de novo Val251Ala mutation of MYOC was found to segregate with disease in a family with autosomal dominant JOAG. Myocilin protein was detected in all control AH samples but was nearly undetectable in AH samples from a patient heterozygous for the Val251Ala mutation. Our results using human patient samples are consistent with a dominant-negative effect of pathogenic MYOC mutations on myocilin secretion.

Integrins in trabecular meshwork and optic nerve head: possible association with the pathogenesis of glaucoma

Integrins are a family of membrane-spanning proteins that are important receptors for cell adhesion to extracellular matrix proteins. They also provide connections between the extracellular environment and intracellular cytoskeletons and are responsible for activation of many intracellular signaling pathways. In vitro and in vivo data strongly indicate that integrin-mediated signaling events can modulate the organization of the actin cytoskeleton in trabecular meshwork (TM) cells and are associated with astrocyte migration and microglia activation of the optic nerve head in patients with primary open angle glaucoma. Consequently, increase in resistance in the TM outflow pathways and remodeling of the optic nerve head occur, which in turn increases intraocular pressure (IOP), adds additional mechanical stress and strain to optic nerve axons, and accelerates damage of axons initially caused by optic nerve head remodeling. Integrins appear to be ideal candidates for translating physical stress and strain into cellular responses known to occur in glaucomatous optic neuropathy.

Illuminating human health through cell mechanics

Cells reside in mechanically rich and dynamic microenvironments, and the complex interplay between mechanics and biology is widely acknowledged. Recent research has yielded insights linking the mechanobiology of cells, human physiology, and pathophysiology. In particular, we have learned of the cell's astounding ability to sense and respond to its mechanical microenvironment. This seemingly innate behaviour of the cell has driven efforts to characterise precisely the cellular behaviour from a mechanical viewpoint. Here we present an overview of technologies used to probe cell mechanical and material properties, how they have led to the discovery of seemingly strange cellular mechanical behaviours, and their influential role in health and disease, including asthma, cancer, and glaucoma. The properties reviewed here have implications in physiology and pathology and raise questions that will fuel research opportunities for years to come.

Direct trabecular meshwork imaging in porcine eyes through multiphoton gonioscopy

The development of technologies to characterize the ocular aqueous outflow system (AOS) is important for the understanding of the pathophysiology of glaucoma. Multiphoton microscopy (MPM) offers the advantage of high-resolution, label-free imaging with intrinsic image contrast because the emitted signals result from the specific biomolecular content of the tissue. Previous attempts to use MPM to image the murine irido-corneal region directly through the sclera have suffered from degradation in image resolution due to scattering of the focused laser light. As a result, transscleral MPM has limited ability to observe fine structures in the AOS. In this work, the porcine irido-corneal angle was successfully imaged through the transparent cornea using a gonioscopic lens to circumvent the highly scattering scleral tissue. The resulting high-resolution images allowed the detailed structures in the trabecular meshwork (TM) to be observed. Multimodal imaging by two-photon autofluorescence and second harmonic generation allowed visualization of different features in the TM without labels and without disruption of the TM or surrounding tissues. MPM gonioscopy is a promising noninvasive imaging tool for high-resolution studies of the AOS, and research continues to explore the potential for future clinical applications in humans.

Contribution of the latent transforming growth factor-β binding protein 2 gene to etiology of primary open angle glaucoma and pseudoexfoliation syndrome

PURPOSE

To assess for the first time the possible contribution of latent transforming growth factor (TGF)-beta binding protein 2 (LTBP2), an extracellular matrix (ECM) protein that associates with fibrillin-1-containing microfibrils, to the etiology of primary open angle glaucoma (POAG) and pseudoexfoliation (PEX) syndrome. Mutations in LTBP2 have previously been shown to be the cause of primary congenital glaucoma (PCG) and other disorders that often manifest as secondary glaucoma.

METHODS

All exons of LTBP2 were sequenced in the DNA of 42 unrelated patients with POAG and 48 unrelated patients with PEX syndrome. Contribution of candidate variations to disease was assessed by screening in control individuals and use of biochemical, bioinformatics, and evolutionary criteria, and in one case by segregation analysis within the family of a proband with POAG. Microscopy was performed on the skin of a patient with PEX syndrome whose condition developed into PEX glaucoma during the course of the study and on the skin of her son previously identified with PCG who harbored the same LTBP2 mutation.

RESULTS

Among the 30 sequence variations observed in LTBP2, five found in five patients with POAG and two found in two patients with PEX glaucoma syndrome may contribute to their diseases. One of the mutations was observed in a patient with POAG and in a patient with PEX glaucoma syndrome. Light, fluorescent, and electron microscopy showed that a mutation present in one of the individuals affected with PEX glaucoma syndrome and in her son affected with PCG causes disruptions in the ECM.

CONCLUSIONS

Some LTBP2 sequence variations can contribute to the etiology of POAG and PEX glaucoma syndrome. It is not expected that in these diseases LTBP2 mutations behave in a strictly Mendelian fashion with complete penetrance. In conjunction with recent findings, the results suggest that anomalies in the ECM are among the factors that can contribute to POAG and PEX glaucoma syndrome. LTBP2 and other related ECM protein coding genes should be screened in larger cohorts with these diseases, which are common disorders and important to the public health.

Spectral-domain optical coherence tomographic assessment of Schlemm's canal in Chinese subjects with primary open-angle glaucoma

PURPOSE

To evaluate in vivo features of Schlemm's canal (SC) in patients with primary open-angle glaucoma (POAG) with spectral-domain optical coherence tomography (SD-OCT) and to investigate the relationship of SC size with intraocular pressure (IOP) and glaucoma severity.

DESIGN

Prospective, comparative study.

PARTICIPANTS

Fifty Chinese patients with newly diagnosed POAG who had not undergone surgery and 50 normal Chinese subjects from a population-based, cross-sectional study in Shanghai.

METHODS

All participants underwent SD-OCT. The diameter and area of SC were examined in the temporal and nasal sections and measured with customized software.

MAIN OUTCOME MEASURES

Patient demographics, repeatability and reproducibility assessed with the coefficient of variation (CV) and the intraclass correlation coefficient (ICC), SC parameters and their correlation with IOP, and the mean deviation (MD) of the visual field were analyzed.

RESULTS

The percentage of sections in which SC was observable was similar between eyes with POAG and normal eyes, and ranged from 78% to 86%. For intraobserver repeatability, the CV and ICC values were 7.9% and 0.97 for diameter, and 13.8% and 0.83 for area, respectively. For interobserver repeatability, the CV and ICC values were 13.6% and 0.89 for diameter, and 13.4% and 0.80 for area, respectively. Significant differences between the 2 groups were found for the average SC area (11332 ± 2015 μm(2) vs. 13991 ± 1357 μm(2); P<0.001), but not for the SC diameter (40.2 ± 7.1 μm vs. 45.2 ± 4.0 μm; P = 0.195). In addition, the mean IOP values correlated well only with the SC area (ρ = -0.674, P<0.001), not with the SC diameter (ρ = -0.103, P = 248). No significant correlations were found between the MD values and the SC parameters.

CONCLUSIONS

Eyes with POAG have a decreased SC area compared with normal eyes. A correlation between the SC area and the IOP also was observed. However, the degree of glaucoma damage was not consistently associated with the SC area. Spectral-domain OCT could be used for investigating SC changes in patients with glaucoma.

Role of substratum stiffness in modulating genes associated with extracellular matrix and mechanotransducers YAP and TAZ

PURPOSE

Primary open-angle glaucoma is characterized by increased resistance to aqueous humor outflow and a stiffer human trabecular meshwork (HTM). Two Yorkie homologues, Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif, encoded by WWTR1 (TAZ), are mechanotransducers of the extracellular-microenvironment and coactivators of transcription. Here, we explore how substratum stiffness modulates the YAP/TAZ pathway and extracellular matrix genes in HTM cells and how this may be play a role in the onset and progression of glaucoma.

METHODS

HTM cells from normal donors were cultured on hydrogels mimicking the stiffness of normal (5 kPa) and glaucomatous (75 kPa) HTM. Changes in expression of YAP/TAZ related genes and steroid responsiveness were determined. Additionally, transglutaminase-2 expression was determined after YAP silencing.

RESULTS

YAP and TAZ are both expressed in human trabecular meshwork cells. In vitro, YAP and TAZ were inversely regulated by substratum stiffness. YAP and 14-3-3σ were downregulated to different extents on stiffer substrates; TAZ, tissue transglutaminase (TGM2), and soluble frizzled-related protein-1 (sFRP-1) were significantly upregulated. CTGF expression appeared to be altered differentially by both YAP and TAZ. Myocilin and angiopoietin-like 7 expression in response to dexamethasone was more pronounced on stiffer substrates. We demonstrated a direct effect by YAP on TGM2 when YAP was silenced by small interfering RNA.

CONCLUSIONS

The expression of YAP/TAZ and ECM-related-genes is impacted on physiologically relevant substrates. YAP was upregulated in cells on softer substrates. Stiffer substrates resulted in upregulation of canonical Wnt modulators, TAZ and sFRP-1, and thus may influence the progression of glaucoma. These results demonstrate the importance of YAP/TAZ in the HTM and suggest their role in glaucoma.

Regulation of Adherens Junctions in Trabecular Meshwork Cells by Rac GTPase and their influence on Intraocular Pressure

Intercellular adherens junctions and cell-extracellular matrix interactions are presumed to influence aqueous humor (AH) drainage via the conventional route, however, their direct role in modulation of intraocular pressure (IOP) is not well understood. Here, we investigated the role of Rac GTPase signaling in basal and growth factor-induced formation of adherens junctions in human trabecular meshwork (HTM) cells as compared to human umbilical vascular endothelial cells, and evaluated the effects of inhibition of Rac GTPase activity on IOP in rabbits. Expression of a constitutively active Rac1 GTPase or treatment with platelet derived growth factor (PDGF), a known activator of Rac GTPase, induced formation of β-catenin-based adherens junctions, actin cytoskeletal reorganization and membrane ruffle in HTM cells. In contrast, treatment of HTM cells with inhibitors of Rac GTPase caused cell-cell separation, a decrease in adherens junctions, and reorganization of actin stress fibers to the cell cortical regions and focal adhesion to the cell leading edges. Both, constitutively active Rac1 and PDGF stimulated generation of Reactive Oxygen Species (ROS) in HTM cells, and ROS were found to increase adherens junction formation and transendothelial electrical resistance (TEER) in HTM cells. Topical application of Rac GTPase inhibitors (EHT1864 and NSC23766), however, only marginally influenced IOP in rabbit eyes. Taken together, these data reveal that while Rac GTPase signaling plays a significant role in regulation of adherens junctions, ROS production and TEER in cells of the AH outflow pathway, Rac inhibitors showed only a marginal influence on IOP in live rabbits.

Reversible changes in aqueous outflow facility, hydrodynamics, and morphology following acute intraocular pressure variation in bovine eyes

BACKGROUND

Elevated intraocular pressure (IOP) is primarily due to increased aqueous outflow resistance, but how aqueous outflow resistance is generated and regulated are still not fully understood. The aim of this study is to determine whether changes in outflow facility, outflow pattern, and morphology following acute IOP elevation were reversible when the IOP was returned to a normal level in bovine eyes using a two-color tracer technique to label outflow patterns within the same eye.

METHODS

Twelve fresh enucleated bovine eyes were perfused with Dulbecco's phosphate buffer saline (PBS) containing 5.5 mmol/L glucose (DBG) at 30 mmHg first to establish the baseline outflow facility followed by a fixed volume of red fluorescent microspheres (0.5 µm, 0.002% v/v). After the red tracer being replaced with DBG in the anterior chamber, perfusion was continued at 7 mmHg with the same volume of green tracer, followed by a fixative. In two control groups, the eyes were constantly perfused at either 30 mmHg (n = 6) or 7 mmHg (n = 6) using the same methods. The outflow facility (C, µl × min × (-1)mmHg(-1)), was continuously recorded. Confocal images were taken along the inner wall (IW) of the aqueous plexus (AP) in frontal sections. The percent of the effective filtration length (PEFL, PEFL = IW length exhibiting tracer labeling/total length of IW) was measured. Sections with AP were processed and examined by light microscopy. The total length of IW and the length exhibiting separation (SL) in the juxtacanalicular connective tissue (JCT) were measured. A minimum of eight collector channel (CC) ostia per eye were analyzed for herniations.

RESULTS

In the experimental (30 - 7 mmHg) group, the outflow facility was significantly higher at 7 mmHg ((4.81 - 1.33) µl × min × (-1)mmHg(-1)) than that at 30 mmHg ((0.99 ± 0.15) µl × min × (-1)mmHg(-1), P = 0.002), corresponding to a significant increase in the PEFL (P = 0.0003). The percent of CC ostia exhibiting herniations in the experimental group ((67.40 ± 8.90) µl × min × (-1)mmHg(-1)) decreased significantly compared to that in the control at 30 mmHg ((94.44 ± 3.33) µl × min × (-1)mmHg(-1), P = 0.03), but higher than that in the control at 7 mmHg ((29.43 ± 4.60) µl × min × (-1)mmHg(-1), P = 0.01). Washout-associated separation between the IW and JCT was found by light microscopy and percent separation length (PSL, PSL = SL/total length of IW) was decreased in the control at 30 mmHg compared to that in the experimental group and control at 7 mmHg.

CONCLUSIONS

The pressure-induced morphological and hydrodynamic changes were reversible. Changes (collapse of AP, separation between the JCT and IW, and herniation into CC ostia) influence the effective filtration area that regulates outflow facility.