Proteasome inhibition by chronic oxidative stress in human trabecular meshwork cells

The pathophysiologic mechanisms leading to the malfunction of the trabecular meshwork (TM)-Schlemm's canal (SC) outflow pathway in glaucoma are still unclear. We hypothesize that chronic oxidative stress may contribute to the malfunction of the outflow pathway by impairing the intracellular proteasome system of the cells, decreasing the ability of the tissue to modulate outflow resistance. To study the effects of chronic oxidative stress on proteasome function, primary cultures of human TM cells were incubated under 40% oxygen and proteasome activity was analyzed by measuring the accumulation of enhanced green fluorescent protein fused to a PEST motif. Changes in proteasome content, cellular senescence, and cell viability were also monitored. After 10 days of exposure to chronic oxidative stress, TM cells showed a marked decline in proteasome activity that was associated with premature senescence and decreased cell viability. These results suggest that proteasome failure may be involved in glaucoma pathophysiology.

Loss of heterozygosity in patients with pseudoexfoliation syndrome

PURPOSE

The purpose of the study was to evaluate the possible occurrence of loss of heterozygosity (LOH) in the anterior capsule, lens nucleus, iris, and trabeculum samples taken from patients with pseudoexfoliation syndrome (PEX). Loss of heterozygosity in a microsatellite marker locus indicates that the neighboring gene may be inactivated. Previous attempts to find a gene defect that might be responsible for pseudoexfoliation glaucoma have been unsuccessful.

METHODS

Specimens of the anterior capsule, the lens nucleus, the iris, the trabeculum, and reference blood samples were collected from 19 PEX patients. Fluorescent multiplex PCR was used to amplify the microsatellite markers located on chromosomes 2, 4, 7, 12, 18, 19, and 21.

RESULTS

LOH was found in 58% of the iris specimens and 50% of the anterior capsule specimens collected from PEX patients. The highest incidence of LOH was observed at the marker D7S820.

CONCLUSIONS

It is possible, that genetic factors may be involved in the etiology and pathogenesis of PEX.

Tonography demonstrates reduced facility of outflow of aqueous humor in myocilin mutation carriers

PURPOSE

To demonstrate the effect in vivo of the myocilin gene mutation Thr377Met on outflow facility of aqueous humor, as measured by tonography.

MATERIALS AND METHODS

Forty-two members of a pedigree known to carry the Thr377Met mutation were examined for glaucoma, evaluated with tonography, and screened for myocilin mutations. Tonography was used to calculate the coefficient of aqueous outflow facility (C), as well as the ratio of the resting intraocular pressure to C (P(0)/C). Subjects were reexamined for glaucoma 5 years after tonography.

RESULTS

Seven subjects were excluded because of previous treatment known to alter facility of aqueous outflow. The mean outflow facility of the eyes of the 12 subjects carrying the Thr377Met mutation was significantly reduced compared with the 23 non-carriers' eyes using both C (P<0.001) and P(0)/C (P<0.001). Reduced outflow facility was also demonstrated in those mutation carriers who were not yet expressing clinical signs of glaucoma or ocular hypertension when measured using C (P = 0.015) and P(0)/C (P = 0.001). After 5 years, progression towards glaucoma had occurred in 5 of the myocilin mutation-carriers, 2 of whom showed bilateral progression; 3 carriers remained completely normal. Four subjects had bilateral glaucoma at the outset and remained unchanged. The carriers' eyes that progressed towards glaucoma had reduced outflow facility compared with those that remained normal, although the difference was not statistically significant.

CONCLUSIONS

Carriers of the myocilin Thr377Met mutation have reduced outflow facility, which may be detected prior to developing glaucoma. Tonography was not seen to be clinically useful in predicting progression towards glaucoma.

Myocilin and glaucoma: A TIGR by the tail?

In 1997, Stone and 14 colleagues from 7 laboratories reported the identification of a gene (TIGR) associated with juvenile open-angle glaucoma (JOAG). Screening of adults with primary open-angle glaucoma (POAG) revealed that about 4% also carried a mutation of the coding region of this gene. The mutations were found through genetic linkage analysis of families with JOAG. Juvenile open-angle glaucoma was a logical starting point in the search for genetic causes of open-angle glaucoma: it shows a strong autosomal-dominant inheritance pattern, occurs at an early age, demonstrates obvious phenotypic signs (dramatic elevation of intraocular pressure and subsequent optic nerve damage), and is likely to be found in multiple generations as parents of affected children are still living. These factors, however, also serve to distinguish it from adult-onset POAG, which generally has a lower intraocular pressure and a less severe course. The discovery of the actual gene represented a true advance over previous studies that had mapped the gene to a segment of a chromosome but did not identify the specific gene. How the mutant gene causes glaucoma is unknown and is the subject of intense research. To date, 26 mutations in the TIGR gene sequence (the term TIGR has been replaced by the term myocilin, abbreviated MYOC) have been described, all associated with either JOAG or adult-onset POAG. A correlation between specific mutations in MYOC and the clinical course of glaucoma has been found. Not all cases of JOAG or POAG have mutations in the MYOC gene, however, indicating that more discoveries of other genes are yet to come. Arch Ophthalmol. 2000;118:974-978

Ocular blood flow in experimental glaucoma: a study in cynomolgus monkeys

Experimental glaucoma was induced in 1 eye of 6 cynomolgus monkeys by laser treatment of the trabecular meshwork. In 5 of the 6 monkeys the increased intraocular pressure (IOP) caused marked glaucomatous damage in the experimental eye. Ocular blood flow was determined with labeled microspheres 4 years after the laser treatment. IOP was regulated with an external reservoir. With the same perfusion pressure in both eyes no statistically significant difference was observed between the 2 eyes for total ocular blood flow or for blood flow through any of the ocular tissues. Total ocular blood flow was 343.5 +/- 61.4 mg/min (mean +/- SEM) in the control eye and 385.3 +/- 107.7 mg/min in the experimental eye.

Ciliary body ablation: where are we and how did we get here?

Management of intraocular pressure remains the cornerstone of glaucoma treatment. Related medical and surgical practices involve increasing aqueous outflow or decreasing aqueous production. Filtration procedures that increase aqueous outflow are the first-line surgical defense in glaucoma. However, some cases of glaucoma are resistant to such treatment. In these cases, ciliary body ablation by various methods has had substantial success. Surgical manipulation of aqueous production has been used in glaucoma management since the turn of the century. Techniques have progressed markedly as technology has produced more discrete therapies designed to decrease aqueous production by destroying ciliary body epithelium. Over the past 90 years success has been achieved with a wide range of techniques, from surgical disinsertion of the ciliary body to recent laser and ultrasound techniques. With the development of more precise contact lasers and endoscopic visualization, side effects have been reduced and clinical success rates increased.

Effects of Er:YAG laser trabecular ablation on outflow facility in cadaver porcine eyes

BACKGROUND

Mid-infrared laser technology opens new perspectives in experimental glaucoma surgery. The aim of this study was to establish the effectiveness of Er:YAG laser photoablation of the trabecular meshwork in increasing outflow facility in cadaver porcine eyes.

METHODS

Eye cups of 20 freshly enucleated cadaver porcine eyes were fixed in an anterior segment perfusion system, measuring outflow facility under constant pressure conditions (7-13 mmHg) over a period of 45 min. Trabecular ablation was performed using a pulsed Er:YAG (2.94 microns) laser. Radiation was delivered in a single 4-mJ pulse of 200 microseconds duration by means of an articulated zirconium fluoride optical fibre and a 320-microns quartz-fibre tip.

RESULTS

Er:YAG laser treatment yielded reproducible ablation areas confined to the trabecular meshwork, with a diameter corresponding to that of the fibre tip. Collateral thermal damage extended up to a depth of 30 microns. Outflow facility measurements revealed an increase (mean +/- SD) from 0.128 +/- 0.041 microliter min-1 mmHg-1 in control eyes (n = 11) to 0.308 +/- 0.093 microliter min-1 mmHg-1 in treated ones (n = 9).

CONCLUSION

Application of the photoablative Er:YAG laser technique in non-fistulating trabecular surgery produces patent trabecular perforation with minimal collateral tissue damage and effects a significant reduction (79%) in outflow resistance.

Latanoprost as a new horizon in the medical management of glaucoma

Latanoprost is a new prostaglandin F2 alpha analogue specifically developed for the treatment of glaucoma. Latanoprost is a selective FP receptor agonist, with a primary mode of action of increased uveoscleral outflow of aqueous humor. A dose of 50 micrograms/mL (0.005%) once daily has been found optimal in clinical trials. Latanoprost reduces the nocturnal intraocular pressure in addition to the diurnal, and has been shown to be additive to other glaucoma medication. In long-term phase III clinical trials, latanoprost 0.005% once daily has been proven to be at least as effective as timolol 0.5% twice a day. The main side effect of latanoprost is increased iridial pigmentation, which is relatively frequent in patients with mixed color of the iris. This unique side effect is based on the ability of prostaglandins to stimulate melanin formation in melanocytes. The advantages of latanoprost compared with other glaucoma medication comprise different mode of action, good intraocular pressure-reducing effect, once-daily dosing, and absence of systemic side effects. The long-term consequences of increased iridial pigmentation need to be further studied.

[The efficacy of a microtrabecular prosthesis in the surgery of refractory glaucomas]

A new model of intermediate artificial drainage system (called microtrabeculoprosthesis--MTP) in the surgery of refractory glaucoma is suggested. The prospective study includes 37 operated cases followed up for an average interval of 18.2 months (ranges 11 and 72 months). Surgery is not difficult, and MTP is very well tolerated. A relief of elevated intraocular pressure (IOP < or = 20 mm Hg) was obtained in 73% of the cases (with or without additional antiglaucoma drugs); the irritative phenomena have disappeared in preterminal and absolute glaucoma; the overall therapeutic success rate was 86.5%. The results obtained with MTP are definitely superior to those by conventional surgery but inferior to modern posterior artificial drainage systems (with ameliorative techniques); the rate of postoperative complications is small, and their severity is minimal. In the absence of modern posterior artificial drainage systems. MTP is an efficient solution for the treatment of refractory glaucoma.

Zonular function: a new hypothesis with clinical implications

A new hypothesis about zonular function is presented that has implications in regard to the cause and treatment of ocular hypertension and open-angle glaucoma. This hypothesis is based on the concept that the lens is under constant tension. The anterior and posterior zonules serve a different function than do the equatorial zonules. The anterior and posterior zonules are passive in terms of structural support of the lens; the equatorial zonules are the active components during accommodation.

[Behavior of the intraocular pressure after blunt eye injuries]

Elevation of intraocular pressure is often seen immediately after blunt trauma of the eye, but it can occur even years after the primary impact. A retrospective study was performed in 144 patients with contusion to the eye. After a mean observation period of 6.1 years, a significant rise in intraocular pressure was found in the engaged eye (P < 0.05). In more than 8% of the patients the level of intraocular pressure exceeded 22 mmHg, but only 3 cases developed a glaucomatous visual field and optic nerve head changes. Thus, the risk of secondary glaucoma after contusion of the eye is low.

Immunohistochemical monitoring of the effect of a synthetic fibronectin-like peptide (Arg-Gly-Asp) on the age-related changes in the isolated human corneoscleral tissue of glaucomatous eyes

Fibronectin, an adhesion glycoprotein has been detected and localized in samples of the trabecular meshwork from eight normotensive and 30 glaucomatous human eyes of various ages by means of the indirect immunoperoxidase staining technique. Fibronectin concentration in the trabecular meshwork tissue was evaluated by morphometric analysis. Deposits of the adhesion glycoprotein fibronectin were shown to be spread in the ocular drainage outflow system from patients along with progressive primary open-angle glaucoma (POAG). The fibronectin level quantitatively evaluated in serial cross-sections of trabecular meshwork, appeared to be increased during ageing and more rapidly in the event of POAG development. The active amino acid sequence in fibronectin is an arginine-glycine-aspartic acid tripeptide (Arg-Gly-Asp) and it was shown that the synthetic Arg-Gly-Asp peptide specifically inhibited the adhesive function of fibronectin in trabecular meshwork samples when incubated for 30 min at a concentration of 1-2 mg/ml. The peptide concentration necessary for a 50% decrease of the maximal fibronectin level in the trabecular meshwork specimen derived from patients with moderately advanced POAG stage, was about 1 mg/ml. Immunohistochemical staining exhibited a fainter fibronectin staining in trabecular tissues including the external trabecular layers and subendothelial region of Schlemm's canal, in samples incubated with the synthetic peptide compared with the same tissue explants before peptide treatment. It may be concluded that the adhesion control system is likely to play an important role in development and maintenance of tissue architecture and specialization of the normal human trabecular meshwork.

Argon laser trabeculoplasty: a review

A review of argon laser trabeculoplasty (ALT) indicates that it appears to be a viable tool in the treatment of many forms of open angle glaucoma. It effectively lowers intraocular pressure with few relative risks to the patient. Unfortunately, its pressure-lowering effects diminish with time. The primary risks of ALT are a transient rise in IOP (20 percent incidence) and the development of peripheral anterior synechia (20-46 percent incidence). The Glaucoma Laser Trial (GLT), a multi-institutional study, is currently evaluating the long-term effect of ALT.

Laser trabeculostimulation in open-angle glaucoma: a new trial. Part I

We describe herein a new trial to stimulate trabecular meshwork tissue and thereby increase outflow facility. The efficacy of our laser trabeculostimulation (LTS) was compared with that of conventional laser trabeculoplasty (LTP), since LTP causes shrinkage and cicatrization of the meshwork tissue, thus limiting the reduction of postoperative intraocular pressure (IOP). The mean preoperative IOP of 40 eyes was 24.9 +/- 6.8 mm Hg (mean +/- SD). LTS was applied with a 50-microns spot, a power setting of 50 mW and an exposure time of 1 s. With LTS, the IOP decreased but then increased afterward. Results indicate that low power (50 mW) is effective if used for a longer duration. Our LTS can be applied repeatedly and used in addition to conventional LTP. Although our LTS was less effective than LTP, the trial may lead to the development of another useful treatment modality.

Obstruction of the trabecular meshwork by retinal rod outer segments

Rod outer segments (ROS) have been described in the anterior chamber of patients with the Schwartz syndrome (retinal detachment, uveitis, and glaucoma). The effect of ROS on aqueous outflow was studied by intracameral injection of ROS in human eye bank eyes and in vivo in the cat. Injection of 10(8) ROS in eye bank eyes (n = 8) caused a 19.7% decrease in outflow facility. This was significantly different than in control eyes (n = 8), where the outflow facility decreased 5.2%. Injection of intracameral ROS in the cat resulted in a rise in intraocular pressure (IOP) from a baseline of 19.7 +/- 1.3 mm Hg to a peak of 30.0 +/- 0.7 mm Hg at 24 hours. By 48 hours, IOP returned to baseline. Control injection did not cause a change in IOP. Electron microscopy of the cat trabecular meshwork revealed occlusion of the intratrabecular spaces by ROS with little evidence of inflammatory activity.

[Cofactorial glaucoma--causes]

The various types of glaucoma may be classified more accurately by studying the structure and physiology of the trabecular meshwork. Seen in this light, the development of glaucoma is linked to "extrinsic" and "intrinsic" factors. The intrinsic factors represent the predisposition to glaucoma; the secondary factors intervene in a secondary phase, creating a resistance to aqueous humor outflow. In so-called primary glaucoma the predisposition plays a decisive role, while the extrinsic factors are not so important. In cofactorial glaucoma the predisposition is only of slight significance, while the extrinsic factors are extremely important. In true secondary glaucoma predisposition plays a minimal role, while the extrinsic factors are all the more important. In correlated glaucomas the predisposition may be extremely relevant, but the extrinsic factors which represent the key causes are still unknown. Several types of glaucoma are described as examples.

Pathophysiology of laser trabeculoplasty

The precise effect of argon laser trabeculoplasty on the trabecular meshwork is only partially understood. Its intraocular pressure lowering effect through enhancement of aqueous outflow is well documented. Clinically, this effect seems non-specific for laser characteristics, including wave form, wave length, burn location, clock hours treated, and even to some degree, laser power. The response is delayed in onset, diminishes over time, and in some cases can be renewed with retreatment. Laser photocoagulation of the trabecular meshwork focally destroys, but diffusely stimulates trabecular meshwork cells. Shape alterations in the trabecular meshwork after laser are likely to contribute to outflow enhancement only at very high levels of intraocular pressure. More likely, cellular stimulation activates a molecular biologic chain of events, perhaps within the trabecular extracellular matrix, that permits improved facility of aqueous outflow.

Alteration of trabecular aqueous flow after cataract extraction

Two human eyes that had undergone intracapsular cataract extraction were perfused with cationized ferritin. In both eyes segmental areas of trabecular meshwork collapse were demonstrated superiorly, adjacent to the cataract incision. Electron-microscopic study of these regions disclosed no labeling of the trabecular beams and trabecular spaces, suggesting impermeability to aqueous flow. In other regions, a diffuse labeling with cationized ferritin was demonstrated throughout the trabecular spaces, indicating aqueous flowthrough. Use of corneoscleral sutures may cause trabecular collapse and impermeability. When the collapse is extensive, subsequent increase in intraocular pressure may follow.

Pigmentary dispersion glaucoma secondary to posterior chamber intra-ocular lenses

Thirteen cases of pigmentary dispersion glaucoma are presented. These had in common heavy pigment granule deposition in the trabecular meshwork in the operated eye alone, a history of posterior chamber intra-ocular lens implantation, and mild to marked iris pigment atrophy in areas contingent with or adjacent to the lens. High intra-ocular pressures were usually transient and easily controlled. One, however, eventually required trabeculoplasty.

Corticosteroid glaucoma

Topical instillations of parabulbar injections of corticosteroids produce an increase of insoluble polymerized acid mucopolysaccharides. These glycosaminoglycans close the intratrabecular meshes by increasing the thickness of the trabeculae. Perfusion with hyaluronidase, which depolymerizes the mucopolysaccharides, diminishes the diameter of the trabeculae, the more so that after the suppression of the corticosteroid treatment the lysosomal membranes become more permeable for the acid hydrolases.

Why is intraocular pressure elevated in chronic simple glaucoma? Anatomical considerations

Ultrastructural analysis of 400 trabeculectomy specimens of glaucomatous eye revealed three types of extracellular deposits within the cribriform layer of the trabecular meshwork. One of these derives from the sheath of the subendothelial elastic-like fibres. Tissue culture and ultrahistochemical studies led to the assumption that these deposits contain glycoproteins, probably secreted by the cribriform layer cells.