Ageing of Schlemm's canal in nonglaucomatous subjects

Understanding how ageing impacts ganglion cell susceptibility to injury in glaucoma

Glaucoma is a leading cause of blindness worldwide, with a marked increase in prevalence with advancing age. Due to the multifactorial nature of glaucoma pathogenesis, dissecting how ageing impacts upon glaucoma risk requires analysis and synthesis of evidence from a vast literature. While there is a wealth of human clinical studies examining glaucoma pathogenesis and why older patients have increased risk, many aspects of the disease such as adaptations of retinal ganglion cells to stress, autophagy and the role of glial cells in glaucoma, require the use of animal models to study the complex cellular processes and interactions. Additionally, the accelerated nature of ageing in rodents facilitates the longitudinal study of changes that would not be feasible in human clinical studies. This review article examines evidence derived predominantly from rodent models on how the ageing process impacts upon various aspects of glaucoma pathology from the retinal ganglion cells themselves, to supporting cells and tissues such as glial cells, connective tissue and vasculature, in addition to oxidative stress and autophagy. An improved understanding of how ageing modifies these factors may lead to the development of different therapeutic strategies that target specific risk factors or processes involved in glaucoma.

Neutrophils and neutrophil extracellular trap components: Emerging biomarkers and therapeutic targets for age-related eye diseases

Age-related eye diseases, including dry eye, glaucoma, age-related macular degeneration, and diabetic retinopathy, represent a major global health issue based on their increasing prevalence and disabling action. Unraveling the molecular mechanisms underlying these diseases will provide novel opportunities to reduce the burden of age-related eye diseases and improve eye health, contributing to sustainable development goals achievement. The impairment of neutrophil extracellular traps formation/degradation processes seems to be one of these mechanisms. These traps formed by a meshwork of DNA and neutrophil cytosolic granule proteins may exacerbate the inflammatory response promoting chronic inflammation, a pivotal cause of age-related diseases. In this review, we describe current findings that suggest the role of neutrophils and their traps in the pathogenesis of the above-mentioned age-related eye diseases. Furthermore, we discuss why these cells and their constituents could be biomarkers and therapeutic targets for dry eye, glaucoma, age-related macular degeneration, and diabetic retinopathy. We also examine the therapeutic potential of some neutrophil function modulators and provide several recommendations for future research in age-related eye diseases.

Effect of age on the morphologies of the human Schlemm's canal and trabecular meshwork measured with swept‑source optical coherence tomography

Purpose

We aimed to measure the sizes of Schlemm’s canal (SC) and the trabecular meshwork (TM) in healthy individuals and to evaluate variations with age from childhood to old age by using swept-source optical coherence tomography (OCT).

Methods

Anterior chamber angle imaging of the superior, inferior, nasal, and temporal regions of the right and left eyes was performed with swept-source OCT. The diameter and area of SC and TM width and thickness were measured manually from OCT images.

Results

A total of 114 healthy individuals were enrolled and included 48 male subjects and 66 female subjects; their ages ranged from 7 to 83 years. Both the SC diameter and area in the four quadrants decreased significantly with aging (P < 0.001) and were wider in the nasal and temporal quadrants compared with the superior and inferior quadrants. Changes in SC size showed significant positive correlations with axial length (AL) and anterior chamber depth (P < 0.001). There was a significant positive association between age and TM thickness in the nasal and temporal quadrants (P < 0.05). The inferior quadrant TM width was the widest among the quadrants. The superior quadrant TM thickness was the thinnest among the quadrants. Changes in TM thickness in the nasal and temporal quadrants showed a significant negative correlation with AL (P < 0.05). There was no statistically significant correlation in SC and TM parameters with central corneal thickness, intraocular pressure, sex, or right or left eye (P > 0.05).

Conclusions

With aging, the SC diameter and area became smaller, TM thickness increased, and TM width seemed to remain constant. Measurements of the sizes of SC and the TM with swept-source OCT could assist in clinical assessments and treatment planning for glaucoma.

Age-Related Changes in Human Schlemm's Canal: An in Vivo Optical Coherence Tomography-Based Study

Purpose: To investigate age-related changes in human Schlemm’s canal (SC) using spectral-domain optical coherence tomography (SD-OCT).

Methods: A total of 125 normal eyes were imaged using SD-OCT nasally and temporally. The age-related variations of SC sagittal diameter and cross-sectional area (CSA) from four age groups [A (16–20 years), B (21–40 years), C (41–60 years), and D (61–80 years)] were analyzed with Spearman correlation.

Results: The positive detection rates of SC showed a significantly downward trend with age. The mean CSA was 13,296 ± 1,897 μm² nasally and 14,552 ± 2,589 μm² temporally. The mean CSA was significantly larger in the temporal than in the nasal region (P < 0.05). Nasal CSA values varied among the four age groups (P = 0.004).

Conclusion: Our study found for the first time that SC in vivo exhibits a morphological variant with age in healthy humans. Clinicians may need to consider this phenomenon when performing examinations targeting SC for glaucoma patients.

From DNA damage to functional changes of the trabecular meshwork in aging and glaucoma

Glaucoma is a degenerative disease of the eye. Both the anterior and posterior segments of the eye are affected, extensive damage being detectable in the trabecular meshwork and the inner retina-central visual pathway complex. Oxidative stress is claimed to be mainly responsible for molecular damage in the anterior chamber. Indeed, oxidation harms the trabecular meshwork, leading eventually to endothelial cell decay, tissue malfunction, subclinical inflammation, changes in the extracellular matrix and cytoskeleton, altered motility, reduced outflow facility and (ultimately) increased IOP. Moreover, free radicals are involved in aging and can be produced in the brain (as well as in the eye) as a result of ischemia, leading to oxidation of the surrounding neurons. Glaucoma-related cell death occurs by means of apoptosis, and apoptosis is triggered by oxidative stress via (a) mitochondrial damage, (b) inflammation, (c) endothelial dysregulation and dysfunction, and (d) hypoxia. The proteomics of the aqueous humor is significantly altered in glaucoma as a result of oxidation-induced trabecular damage. Those proteins whose aqueous humor levels are increased in glaucoma are biomarkers of trabecular meshwork impairment. Their diffusion from the anterior to the posterior segment of the eye may be relevant in the cascade of events triggering apoptosis in the inner retinal layers, including the ganglion cells.

Endogenous Bioactive Lipids and the Regulation of Conventional Outflow Facility

Perturbation of paracrine signaling within the human conventional outflow pathway influences tissue homeostasis and outflow function. For example, exogenous introduction of the bioactive lipids, sphingosine-1-phosphate, anandamide or prostaglandin F(2α), to conventional outflow tissues alters the rate of drainage of aqueous humor through the trabecular meshwork, and into Schlemm's canal. This review summarizes recent data that characterizes endogenous bioactive lipids, their receptors and associated signaling partners in the conventional outflow tract. We also discuss the potential of targeting such signaling pathways as a strategy for the development of therapeutics to treat ocular hypertension and glaucoma.

Anatomic alterations in aging and age-related diseases of the eye

PURPOSE

We described anatomic age-related changes in the human eye to determine potential areas of investigation that may lead to identifying eyes at risk for age-related disease.

METHODS

A descriptive review of anatomic changes in the eye related to aging was performed in the context of current areas of investigation. The review was performed specifically for differing anatomic ocular structures, including cornea, trabecular meshwork, lens, uveal tract, Bruch's membrane, retina, RPE, vitreous, sclera, and optic nerve.

RESULTS

Age-related changes occur in all ocular tissues. The cornea flattens and there is an attrition of endothelial cells. The shape of the trabecular meshwork changes and there is a loss of trabecular endothelium. The lens grows and becomes cataractous. The ciliary body becomes collagenized, there are choroidal vascular changes, and Bruch's membrane thickens. Retinal vessels become hyalinized and there is a loss of rods before cones in the macula. RPE morphometric changes occur with aging. The vitreous becomes liquefied and there is a loss of vitreous compartmentalization. The sclera becomes rigid and may become calcified. The optic nerve exhibits structural changes with age.

CONCLUSIONS

There are numerous anatomic age-related changes in the human eye. Current areas of investigation related to these changes include adaptive optics scanning laser ophthalmoscopy imaging of the RPE mosaic in the context of aging, and drug delivery devices that overcome age-related alterations to retinal and macular perfusion.

Oxidative stress impact on barrier function of porcine angular aqueous plexus cell monolayers

PURPOSE

Our goal was to investigate the effect of chronic oxidative stress on angular aqueous plexus (AAP, functional equivalent to human Schlemm's canal) endothelial cells from porcine eyes.

METHODS

AAP cells were differentially isolated from porcine outflow tissues using puromycin selection. Confluent cultures of porcine AAP cells were grown for 2 weeks in physiological (5% O2) or hyperoxic conditions (40% O2) to model elevated oxidative stress associated with ageing. Cell growth rate, size, transendothelial electrical resistance (TEER), and hydraulic conductivity (HC) were measured. The expression of senescence-associated β-galactosidase and DNA damage marker 8-hydroxy-2'-deoxyguanosine (8-OHdG) was monitored, and the levels of cytoskeletal and cell-cell adhesion proteins such as F-actin, phospho-myosin light chain (phosphor-MLC), occludin, claudin-5, ZO-1, β-catenin, and VE-cadherin were measured by immunofluorescence staining and Western blot analysis.

RESULTS

Data showed that chronic hyperoxia inhibited cell growth rate from day 3 onward, the cell size increased by 18.2%±5.1%, and cells stained positive for β-galactosidase and 8-OHdG. Hyperoxia resulted in a significant 30% increase in TEER compared with the control group (P<0.05, n=6). When perfused in the basal-to-apical direction at 4 mm Hg, HC of AAP cells was 1.97±0.12 and 1.54±0.13 μL/mm Hg/min/cm2 in control and hyperoxia groups, respectively (P<0.05, n=6). Stressed cells expressed a significantly greater abundance of F-actin, phospho-MLC, occludin, claudin-5, β-catenin, and VE-cadherin compared to the control group by both immunofluorescence and Western blot analyses.

CONCLUSIONS

Chronic exposure of AAP cells to oxidative stress decreased cell monolayer permeability and up-regulated cytoskeletal and cell-cell adhesion protein expression; suggesting that, with age and increased oxidative stress, resistance at the level of Schlemm's canal increases.

A model of giant vacuole dynamics in human Schlemm's canal endothelial cells

Aqueous humour transport across the inner wall endothelium of Schlemm's canal likely involves flow through giant vacuoles and pores, but the mechanics of how these structures form and how they influence the regulation of intraocular pressure (IOP) are not well understood. In this study, we developed an in vitro model of giant vacuole formation in human Schlemm's canal endothelial cells (HSCECs) perfused in the basal-to-apical direction (i.e., the direction that flow crosses the inner wall in vivo) under controlled pressure drops (2 or 6 mmHg). The system was mounted on a confocal microscope for time-lapse en face imaging, and cells were stained with calcein, a fluorescent vital dye. At the onset of perfusion, elliptical void regions appeared within an otherwise uniformly stained cytoplasm, and 3-dimensional reconstructions revealed that these voids were dome-like outpouchings of the cell to form giant vacuole-like structures or GVLs that reproduced the classic "signet ring" appearance of true giant vacuoles. Increasing pressure drop from 2 to 6 mmHg increased GVL height (14 ± 4 vs. 21 ± 7 μm, p < 0.0001) and endothelial hydraulic conductivity (1.15 ± 0.04 vs. 2.11 ± 0.49 μl min⁻¹ mmHg⁻¹ cm⁻²; p < 0.001), but there was significant variability in the GVL response to pressure between cell lines isolated from different donors. During perfusion, GVLs were observed "migrating" and agglomerating about the cell layer and often collapsed despite maintaining the same pressure drop. GVL formation was also observed in human umbilical vein and porcine aortic endothelial cells, suggesting that giant vacuole formation is not a unique property of Schlemm's canal cells. However, in these other cell types, GVLs were rarely observed "migrating" or contracting during perfusion, suggesting that Schlemm's canal endothelial cells may be better adapted to withstand basal-to-apical directed pressure gradients. In conclusion, we have established an in vitro model system to study giant vacuole dynamics, and we have demonstrated that this system reproduces key aspects of giant vacuole morphology and behaviour. This model offers promising opportunities to investigate the role of endothelial cell biomechanics in the regulation of intraocular pressure in normal and glaucomatous eyes.