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Abstract: 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. [Copyright &y& Elsevier]

The human trabecular meshwork (TM) is the major site of resistance for aqueous humor outflow. The purpose of this investigation was to determine the suitability of TMs harvested from donor corneoscleral buttons (buttons) for laboratory studies.Histologic examination using light and electron microscopy was performed. Additionally, the effect of incubation in serum free media on histologic appearance was studied. Total RNA extraction was performed on 45 buttons and four whole eyes. Some TMs were used as a source for establishing primary cell cultures of TM endothelial cells.Compared with donor whole eyes, light microscopy showed comparable TM cellularity in the juxtacanalicular and corneoscleral TM; there was some loss of cells in the inner portion of uveal TM that was adjacent to the anterior chamber. The TM cells appeared healthy and structurally normal on transmissive electron microscopy. Usable quantities of total RNA could be extracted from buttons that had been stored up to 5 weeks. The amount of total RNA extracted correlated well with the histologic appearance. Incubation in serum free media did not have an effect on the histologic appearance. All attempts at establishing primary cell cultures were successful.Unused, donor corneoscleral buttons are an excellent source of TM. [Copyright &y& Elsevier]

Mutations in the MYOC gene that encodes for myocilin are causative for some forms of juvenile and adult-onset primary open-angle glaucoma (POAG). Myocilin is a secreted 55–57 kDa glycoprotein that forms dimers and multimers. Characteristic structural motifs include a myosin-like domain, a leucine zipper region and an olfactomedin domain. Most of the mutations that have been identified in patients with POAG are localized in the olfactomedin domain, which is highly conserved among species. In the eye, myocilin is expressed in high amounts in the trabecular meshwork (TM), sclera, ciliary body and iris, and at considerable lower amounts in retina and optic nerve head. Secreted myocilin is present in the aqueous humor. In the TM, myocilin is found within the cytoplasm of TM cells and in the juxtacanalicular region in association with fibrillar extracellular matrix components. Since patients with mutations in myocilin may have high intraocular pressures, the role of myocilin for aqueous humor outflow has been investigated and conflicting results have been obtained. Recombinant myocilin increases outflow resistance in perfused anterior segment organ cultures, while overexpression of myocilin after viral gene transfer appears to reduce outflow resistance. In TM cells, the expression of myocilin is induced upon treatment with dexamethasone at a time course similar to that observed in steroid-induced glaucoma. Other factors that induce myocilin expression are transforming growth factor-β and mechanical stretch. Promoter elements that are important for the glucocorticoid induction have not been identified, but it has been shown that upstream stimulatory factor is critical for the basal promoter activity of MYOC. Mice with a targeted disruption of the myocilin gene do not express a phenotype, indicating that the glaucomatous phenotype in humans is not because of a loss-of-function effect. Experimental studies show that mutated myocilin is not secreted, but appears to accumulate in the cells. Such an accumulation might interfere with TM function and lead to impaired outflow resistance, but, so far, experimental evidence for such a scenario is lacking. In addition, the normal function(s) of myocilin is (are) still elusive. [Copyright &y& Elsevier]

Abstract: Intraocular pressure, the most critical risk factor for primary open-angle glaucoma is generated in the trabecular meshwork outflow pathways, which provide resistance to aqueous humor outflow. The resistance is increased in primary open-angle glaucoma, and changes in the quality and amount of the extracellular matrix in the juxtacanalicular region of the trabecular meshwork appear to be causatively involved. The extracellular matrix changes are very likely under control of transforming growth factor-β2 (TGF-β2), which is found at high concentrations in the aqueous humor of patients with primary open-angle glaucoma. Additional factors are thrombospondin-1, which activates TGF-β2 in vivo, and connective tissue growth factor, which is an important downstream mediator of the effects of TGF-β2 on trabecular meshwork extracellular matrix turnover. In contrast, bone morphogenetic protein-7 (BMP-7) strongly antagonizes fibrogenic actions of TGF-β2 on human trabecular meshwork cells, indicating that a pharmacological modulation of BMP-7 signalling might be a promising strategy to treat primary open-angle glaucoma. [Copyright &y& Elsevier]

CCN2/CTGF is a matricellular protein that is known to enhance transforming growth factor-β signaling and to induce a myofibroblast-like phenotype in a variety of cell types. Here, we investigated Ccn2/Ctgf promotor activity during development and in the adult mouse eye, using CTGFLacZ/+ mice in which the β-galactosidase reporter gene LacZ had been inserted into the open reading frame of Ccn2/Ctgf. Promotor activity was assessed by staining for β-galactosidase activity and by immunolabeling using antibodies against β-galactosidase. Co-immunostaining using antibodies against glutamine synthetase, glial fibrillary acidic protein, choline acetyltransferase, and CD31 was applied to identify specific cell types. Ccn2/Ctgf promotor activity was intense in neural crest-derived cells differentiating to corneal stroma and endothelium, and to the stroma of choroid, iris, ciliary body, and the trabecular meshwork during development. In the adult eye, a persistent and very strong promotor activity was present in the trabecular meshwork outflow pathways. In addition, endothelial cells of Schlemm's canal, and of retinal and choroidal vessels, retinal astrocytes, Müller glia, and starburst amacrine cells were stained. Very strong promoter activity was seen in the astrocytes of the glial lamina at the optic nerve head. We conclude that CCN2/CTGF signaling is involved in the processes that govern neural crest morphogenesis during ocular development. In the adult eye, CCN2/CTGF likely plays an important role for the trabecular meshwork outflow pathways and the glial lamina of the optic nerve head. [ABSTRACT FROM AUTHOR]

• Decorin deficiency leads to elevated intraocular pressure in mice. • TGF-β target genes are enriched in the conventional outflow pathways of decorin-deficient mice. • Decorin attenuates the expression of TGF-β2 in cultured HTM cells. • Decorin levels are reduced in the chamber angle of glaucomatous eyes. Primary open-angle glaucoma, a neurodegenerative disorder characterized by degeneration of optic nerve axons, is a frequent cause of vision loss and blindness worldwide. Several randomized multicenter studies have identified intraocular pressure as the major risk factor for its development, caused by an increased outflow resistance to the aqueous humor within the trabecular meshwork. However, the molecular mechanism for increased outflow resistance in POAG has not been fully established. One of the proposed players is the pro-fibrotic transforming growth factor (TGF)-β2, which is found in higher amounts in the aqueous humor of patients with POAG. In this study we elucidated the role of decorin, a small leucine-rich proteoglycan and known antagonist of TGF-β, in the region of aqueous humor outflow tissue. Utilizing decorin deficient mice, we discovered that decorin modulated TGF-β signaling in the canonical outflow pathways and the lack of decorin in vivo caused an increase in intraocular pressure. Additionally, the Dcn −/− mice showed significant loss of optic nerve axons and morphological changes in the glial lamina, typical features of glaucoma. Moreover, using human trabecular meshwork cells we discovered that soluble decorin attenuated TGF-β2 mediated synthesis and expression of typical downstream target genes including CCN2/CTGF, FN and COL IV. Finally, we found a negative reciprocal regulation of decorin and TGF-β, with a dramatic downregulation of decorin in the canonical outflow pathways of patients with primary open-angle glaucoma. Collectively, our results indicate that decorin plays an important role in the pathogenesis of primary open-angle glaucoma and offers novel perspectives in the treatment of this serious disease. [ABSTRACT FROM AUTHOR]

Mutations in myocilin, a 55–57 kDa secreted glycoprotein, are causative for some forms of primary open-angle glaucoma (POAG). In vitro studies indicate that myocilin can modulate the hydrodynamic outflow resistance in the trabecular meshwork (TM) and that elevated amounts of myocilin can obstruct the TM outflow system in POAG. In this study, we analyzed the localization of myocilin in the trabecular meshwork (TM) of eyes with primary open-angle glaucoma (POAG), and compared it with that of normal eyes. Immunohistochemistry for myocilin was performed in the eyes of human donors (nine normal and 14 with POAG, including one with steroid-induced glaucoma). Staining for myocilin was observed in the extracellular spaces of the juxtacanalicular tissue (JCT) in all normal eyes. Some normal eyes did also show cytoplasmic staining for myocilin in TM cells. In the eyes of six donors with POAG, staining of the JCT was more widespread and intense than in normal eyes. In the other eyes with POAG, immunoreactivity for myocilin in the JCT was not markedly different to that of normal eyes. Staining intensity in the JCT of POAG eyes did not obviously correlate with intraocular pressure or clinical severity. In the eyes of one patient with steroid-induced POAG, cells of the TM, Schlemm’s canal endothelium, and the anterior stroma of the iris showed an immunoreactivity for myocilin which was considerably more intense than in normal eyes, or in the eyes with other forms of POAG. In some cases of POAG, the structural changes in the JCT include an increase in myocilin in the extracellular pathways of aqueous humor. Treatment with steroids appears to increase myocilin synthesis in TM and iris of human eyes in situ. [ABSTRACT FROM AUTHOR]

Abstract: 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)-β2 signaling may be involved, as TGF-β2 is significantly increased in the aqueous humor of patients with POAG. In cultured human TM cells, TGF-β2 causes an increase in ECM deposition, an effect that is blunted or prevented, if BMP7 is added in combination with TGF-β2. In order to know more about the signaling network that is induced in HTM cells treated with BMP7, TGF-β2 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-β2 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-β 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-β2 treatment, the antagonizing effect of BMP7 on TGF-β2-induced CTGF expression was abolished. We conclude that Smad7 is the key molecular switch that inhibits TGF-β2 signaling, and mediates the blunting effects of BMP7 on TGF-β2 in TM cells. A therapeutic modulation of Smad7 might be a promising approach to influence ECM turnover in the TM and to treat POAG. [Copyright &y& Elsevier]

Abstract: Myocilin is a 55–57kDa secreted glycoprotein and member of the olfactomedin family, which is mutated in some forms of primary open-angle glaucoma. To assess the effects of elevated amounts of myocilin on aqueous humor outflow dynamics in an in vivo system, transgenic βB1-crystallin-MYOC mice have been developed that strongly overexpress myocilin in their eyes. The transgenic overexpression of myocilin results in an almost five-fold increase of secreted normal myocilin in the aqueous humor of βB1-crystallin-MYOC mice. In the present study, we wanted to use βB1-crystallin-MYOC as a tool to identify the response of ocular tissues to the presence of higher than normal amounts of myocilin, and to identify changes in gene expression that could help to shed light on the functional in vivo properties of myocilin. RNA was isolated from ocular tissues of βB1-crystallin-MYOC mice and wild-type littermates. Changes in gene expression were determined by hybridization of gene microarrays and confirmed by real time RT-PCR and Western blotting. The expression of genes that had been found to be differentially regulated in βB1-crystallin-MYOC mice was further analyzed in cultured human trabecular meshwork (HTM) cells treated with recombinant myocilin. Although βB1-crystallin-MYOC mice do not have an obvious phenotype, a statistically significant up- and downregulation of several distinct genes was found when compared to gene expression in wild-type littermates. Among the genes that were found to be differentially regulated were Wasl, Ceacam1, and Spon2, which are involved in cell adhesion and cell–matrix interactions. Differences in expression were also found for Six1 which encodes for a transcription factor, and for Pftk1 whose gene product is a cdc2-related protein kinase. The expression of these genes was also found to be regulated in vitro in HTM cells treated with recombinant myocilin. Substantially higher amounts in ocular tissues of βB1-crystallin-MYOC mice were found for connexin 46 and αB-crystallin. In addition, several genes that encode for olfactomedin proteins showed distinct changes in expression. Olfml3 was significantly downregulated, while Lphn1, Lphn2, and Lphn3 were significantly upregulated. Our findings support a role for myocilin in modulating cellular adhesion, and suggest functional processes that involve other proteins of the olfactomedin family. [Copyright &y& Elsevier]

Transforming growth factor-β2 (TGF-β2) is elevated in the aqueous humor of patients with primary open-angle glaucoma (POAG), and high levels of TGF-β2 are thought to contribute to the pathogenesis of POAG. Most TGF-β2 in the eye is present in a latent, inactive form and the mechanisms of its in vivo activation are unclear. Since thrombospondin-1 (TSP-1) is one of the most potent in vivo activating molecules of TGF-βs, we investigated the localization and expression of TSP-1 in the aqueous humor outflow pathways. TSP-1 immunohistochemistry was performed in the eyes of human donors (8 normal and 17 with glaucoma). In addition, the eyes of Tsp-1-/--deficient mice and normal Tsp-1+/+ mice were investigated. TSP-1 mRNA expression was assessed by reverse transcription-polymerase chain reaction and Northern blotting of RNA from fresh trabecular meshwork (TM), and human and mouse TM cells in vitro. In addition, Northern and Western blot analyses of TM cells after incubation with TGF-β and dexamethasone were performed. In most of the eyes, TSP-1 immunolabeling was predominately observed in extracellular areas of the juxtacanalicular (cribriform) part of the TM. Some focal staining was observed in the corneoscleral and uveal parts of the TM. In the eyes of six glaucoma patients (including one with steroid-induced glaucoma), TSP-1 immunoreactivity was considerably more intense and all regions of the TM were positively labeled. In double labeling experiments, staining for TSP-1 did not overlap with that of fibronectin or type VI collagen. mRNA for TSP-1 was detected in both fresh and cultured TM cells. Incubation of TM cells with TGF-β1 and dexamethasone caused a marked increase in TSP-1 expression. TSP-1 in the TM might act as a potent local endogenous activator of TGF-βs in the aqueous humor and mediate any local effects of TGF-β and/or dexamethasone on the outflow of aqueous humor. [Copyright &y& Elsevier]

The development of the chamber angle was studied in the eyes of heterozygous Pax6lacZ/+ mutant mice (Nature 387 (1997) 406). Mutations in PAX6 cause aniridia, a condition that is frequently associated with glaucoma, a blinding disease that may be associated with chamber angle defects. Mesenchymal cells were seen in the chamber angle at P1–P5. In wild-type mice, these cells differentiated into typical trabecular meshwork (TM) cells next to Schlemm''s canal. In Pax6lacZ/+ mice, TM cells remained undifferentiated and Schlemm''s canal was absent. From P1 to P4, staining for β-galactosidase and immunoreactivity for Pax6 were observed in chamber angle mesenchyme, but were absent later. Cultured murine TM cells expressed Pax6. The defects in chamber angle and TM differentiation were associated with a wide spectrum of other anterior eye defects, which included various degrees of iris hypoplasia and corneal haze, isolated iridocorneal adhesions and atypical coloboma, and a vascularized cornea in all adult animals. A third of the animals showed Peters'' anomaly including corneal opacity and iridocorneal adhesions. The separation of the lens from the cornea was incomplete, and epithelial layers of lens and cornea were continuous. Pax6 activity is directly required for differentiation of the chamber angle. Variations in phenotype of Pax6lacZ/+ mice appear not to involve direct dominant-negative or dose-dependent effects. [Copyright &y& Elsevier]