The Glenn A. Fry Award Lecture 1992: aqueous humor proteins: a key for unlocking glaucoma?

Local ocular renin-angiotensin-aldosterone system: any connection with intraocular pressure? A comprehensive review

The renin-angiotensin system (RAS) is one of the oldest and most extensively studied human peptide cascades, well-known for its role in regulating blood pressure. When aldosterone is included, RAAS is involved also in fluid and electrolyte homeostasis. There are two main axes of RAAS: (1) Angiotensin (1-7), angiotensin converting enzyme 2 and Mas receptor (ACE2-Ang(1-7)-MasR), (2) Angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor (ACE1-AngII-AT1R). In its entirety, RAAS comprises dozens of angiotensin peptides, peptidases and seven receptors. The first mentioned axis is known to counterbalance the deleterious effects of the latter axis. In addition to the systemic RAAS, tissue-specific regulatory systems have been described in various organs, evidence that RAAS is both an endocrine and an autocrine system. These local regulatory systems, such as the one present in the vascular endothelium, are responsible for long-term regional changes. A local RAAS and its components have been detected in many structures of the human eye. This review focuses on the local ocular RAAS in the anterior part of the eye, its possible role in aqueous humour dynamics and intraocular pressure as well as RAAS as a potential target for anti-glaucomatous drugs.KEY MESSAGESComponents of renin-angiotensin-aldosterone system have been detected in different structures of the human eye, introducing the concept of a local intraocular renin-angiotensin-aldosterone system (RAAS).Evidence is accumulating that the local ocular RAAS is involved in aqueous humour dynamics, regulation of intraocular pressure, neuroprotection and ocular pathology making components of RAAS attractive candidates when developing new effective ways to treat glaucoma.

Human aqueous humor proteome in cataract, glaucoma, and pseudoexfoliation syndrome

Twenty-nine human aqueous humor samples from patients with eye diseases such as cataract and glaucoma with and without pseudoexfoliation syndrome were characterized by LC-high resolution MS analysis. In total, 269 protein groups were identified with 1% false discovery rate including 32 groups that were not reported previously for this biological fluid. Since the samples were analyzed individually, but not pooled, 36 proteins were identified in all samples, comprising the constitutive proteome of the fluid. The most dominant molecular function of aqueous humor proteins as determined by GO analysis is endopeptidase inhibitor activity. Label-free protein quantification showed no significant difference between glaucoma and cataract aqueous humor proteomes. At the same time, we found decrease in the level of apolipoprotein D as a marker of the pseudoexfoliation syndrome. The data are available from ProteomeXchange repository (PXD002623).

Many Faces of Renin-angiotensin System - Focus on Eye

The renin-angiotensin system (RAS), that is known for its role in the regulation of blood pressure as well as in fluid and electrolyte homeostasis, comprises dozens of angiotensin peptides and peptidases and at least six receptors. Six central components constitute the two main axes of the RAS cascade. Angiotensin (1-7), an angiotensin converting enzyme 2 and Mas receptor axis (ACE2-Ang(1-7)-MasR) counterbalances the harmful effects of the angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor axis (ACE1-AngII-AT1R) Whereas systemic RAS is an important factor in blood pressure regulation, tissue-specific regulatory system, responsible for long term regional changes, that has been found in various organs. In other words, RAS is not only endocrine but also complicated autocrine system. The human eye has its own intraocular RAS that is present e.g. in the structures involved in aqueous humor dynamics. Local RAS may thus be a target in the development of new anti-glaucomatous drugs. In this review, we first describe the systemic RAS cascade and then the local ocular RAS especially in the anterior part of the eye.

Aqueous outflow - A continuum from trabecular meshwork to episcleral veins

In glaucoma, lowered intraocular pressure (IOP) confers neuroprotection. Elevated IOP characterizes glaucoma and arises from impaired aqueous humor (AH) outflow. Increased resistance in the trabecular meshwork (TM), a filter-like structure essential to regulate AH outflow, may result in the impaired outflow. Flow through the 360° circumference of TM structures may be non-uniform, divided into high and low flow regions, termed as segmental. After flowing through the TM, AH enters Schlemm's canal (SC), which expresses both blood and lymphatic markers; AH then passes into collector channel entrances (CCE) along the SC external well. From the CCE, AH enters a deep scleral plexus (DSP) of vessels that typically run parallel to SC. From the DSP, intrascleral collector vessels run radially to the scleral surface to connect with AH containing vessels called aqueous veins to discharge AH to blood-containing episcleral veins. However, the molecular mechanisms that maintain homeostatic properties of endothelial cells along the pathways are not well understood. How these molecular events change during aging and in glaucoma pathology remain unresolved. In this review, we propose mechanistic possibilities to explain the continuum of AH outflow control, which originates at the TM and extends through collector channels to the episcleral veins.

The many faces of the trabecular meshwork cell

With the combined purpose of facilitating useful vision over a lifetime, a number of ocular cells have evolved specialized features not found elsewhere in the body. The trabecular meshwork (TM) cell at the irido-corneal angle, which is a key regulator of intraocular pressure, is no exception. Examination of cells in culture isolated from the human TM has shown that they are unique in many ways, displaying characteristic features of several different cell types. Thus, these neural crest derived cells display expression patterns and behaviors typical of endothelia, fibroblasts, smooth muscle and macrophages, owing to the multiple roles and two distinct environments where they operate to maintain intraocular pressure homeostasis. In most individuals, TM cells function normally over a lifetime in the face of persistent stressors, including phagocytic, oxidative, mechanical and metabolic stress. Study of TM cells isolated from ocular hypertensive eyes has shown a compromised ability to perform their daily duties. This review highlights the many responsibilities of the TM cell and its challenges, progress in our understanding of TM biology over the past 30 years, as well as discusses unanswered questions about TM dysfunction that results in IOP dysregulation and glaucoma.

Proteome analysis of human aqueous humor

PURPOSE

Human aqueous humor (hAH) provides nutrition and immunity within the anterior chamber of the eye. Characterization of the protein composition of hAH will identify molecules involved in maintaining a homeostatic environment for anterior segment tissues. The present study was conducted to analyze the proteome of hAH.

METHODS

hAH samples obtained during elective cataract surgery were divided into three matched groups and immunodepleted of albumin, IgG, IgA, haploglobin, antitrypsin, and transferrin. Reduced and denatured proteins (20 μg) from each group were separated by gel electrophoresis. Thirty-three gel slices were excised from each of three gel lanes (n = 99), digested with trypsin, and subjected to nanoflow liquid chromatography electrospray ionization tandem mass spectrometry (nano-LC-ESI-MS/MS). The protein component of hAH was also analyzed by antibody-based protein arrays, and selected proteins were quantified.

RESULTS

A total of 676 proteins were identified in hAH. Of the 355 proteins identified by nano-LC-ESI-MS/MS, 206 were found in all three groups. Most of the proteins identified by nano-LC-ESI-MS/MS had catalytic, enzymatic, and structural properties. Using antibody-based protein arrays, 328 cytokines, chemokines, and receptors were identified. Most of the quantified proteins had concentrations that ranged between 0.1 and 2.5 ng/mL. Ten proteins were identified by both nano-LC-ESI-MS/MS and antibody protein arrays.

CONCLUSIONS

Proteomic analysis of hAH identified 676 nonredundant proteins. More than 80% of these proteins are novel identifications. The elucidation of the aqueous proteome will establish a foundation for protein function analysis and identification of differentially expressed markers associated with diseases of the anterior segment.

Direct effect of light on 24-h variation of aqueous humor protein concentration in Sprague-Dawley rats

Sprague-Dawley rats 10-12 weeks of age were entrained to a standard light-dark cycle with lights turned on at 6 am and off at 6 pm. Variations of 24-h aqueous humor protein concentration were determined. Samples were taken every 4h (N=10-14) under the standard light-dark condition at 8 pm, midnight, 4 am, 8 am, noon, and 4 pm. Under an acute constant dark condition, when lights were not turned on at 6 am, samples were collected at 8 am, noon, 4 pm, and 8 pm. Aqueous humor protein concentrations under the standard light-dark condition were found in the range of 0.305+/-0.115 mg/ml (mean+/-SD, N=10) at midnight to 1.505+/-0.342 mg/ml (N=14) at noon. The 3 light-phase protein concentrations were each higher than the 3 dark-phase concentrations. Aqueous humor protein concentrations at 8 am, noon, and 4 pm under the acute constant dark condition were each higher than the concentrations at 8 pm (after both 2h and 26 h in the dark), midnight, and 4 am, demonstrating an endogenously driven 24-h pattern. At 8 am, noon, and 4 pm, protein concentrations were 56-147% higher when exposed to light. Intraocular pressure (IOP) was monitored using telemetry in separate groups of light-dark entrained rats under the standard light-dark condition and the acute constant dark condition. The 24-h IOP pattern was inverse to the 24-h pattern of aqueous humor protein concentration under the standard light-dark condition, and this IOP pattern was not altered by the acute constant dark condition. In conclusion, an endogenously driven 24-h variation of aqueous humor protein concentration occurred in Sprague-Dawley rats with higher concentrations during the light-phase than the dark-phase. This endogenous pattern of protein concentration was accentuated by a direct effect of light, which was unrelated to the 24-h pattern of IOP.

Monkey organ-cultured anterior segments: technique and response to H-7

In the intact primate eye in vivo, it may be difficult to determine whether an outflow facility response is due to direct effects on the trabecular meshwork or indirectly via an effect on ciliary muscle tone unless complicated surgical ciliary muscle disinsertion or isotope accumulation procedures are used. We established a monkey anterior segment organ culture system modeled after the human anterior segment organ culture system and determined its characteristics and response to H-7, an agent shown to increase trabecular outflow in vivo and in other organ culture systems. Outflow facility studies conducted using the monkey organ-cultured anterior segment system showed that: baseline values were comparable to those measured in vivo; washout occurred with time; the two-level constant pressure perfusion technique gave results comparable to the constant rate technique; species differences were found comparing baseline outflow facility and intraocular pressure in rhesus and cynomolgus monkey organ-cultured anterior segments; there was no effect of age on outflow facility in either species by one-way ANOVA; anterior segment exchange perfusion increased outflow facility if measured within 1 hr of the exchange; the magnitude of the response to H-7 was comparable to those reported in vivo and in other in vitro systems. The onset of the response to H-7 was delayed in most cases (day after H-7 exchange), compared to in vivo monkey studies (onset 6-9 min) and in vitro human and porcine organ culture systems (within 1 hr). Also the duration of the response (2-3 days from the onset) was longer than that found for in vitro human studies (1 day). Thus, the monkey organ-cultured anterior segment system can be used to determine the effects of pharmacological and biological agents on trabecular outflow.

Micropurification of beta- and gamma-crystallins from rabbit aqueous humor

Soluble crystallins are normally present in the aqueous humor, originating from the lens, and their concentration may increase in certain conditions such as cataract, possibly contributing to aqueous outflow pathway obstruction, leading to glaucoma. Whether the stability and the tendency of aqueous crystallins to aggregate are different in patients with certain forms of open-angle glaucoma has not so far been established, mainly due to the lack of a suitable purification procedure from this fluid in which crystallins are present at very low concentration together with dozens of other proteins. About 4 microg each of beta- and gamma-crystallins were obtained from 20 ml of rabbit aqueous humor by C8 reversed-phase high-performance liquid chromatography (HPLC) and high-performance electrophoresis chromatography (HPEC). The identity of the proteins was confirmed by amino acid analysis following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and electrophoretic blotting onto polyvinylidene fluoride membranes, with or without previous digestion with Staphylococcus aureus protease V8.