SDPR expression in human trabecular meshwork and its potential role in racial disparities of glaucoma

In order to identify how differential gene expression in the trabecular meshwork (TM) contributes to racial disparities of caveolar protein expression, TM dysfunction and development of primary open angle glaucoma (POAG), RNA sequencing was performed to compare TM tissue obtained from White and Black POAG surgical (trabeculectomy) specimens. Healthy donor TM tissue from White and Black donors was analyzed by PCR, qPCR, immunohistochemistry staining, and Western blot to evaluate SDPR (serum deprivation protein response; Cavin 2) and CAV1/CAV2 (Caveolin 1/Caveolin 2). Standard transmission electron microscopy (TEM) and immunogold labeled studies were performed. RNA sequencing demonstrated reduced SDPR expression in TM from Black vs White POAG patients' surgical specimens, with no significant expression differences in other caveolae-associated genes, confirmed by qPCR analysis. No racial differences in SDPR gene expression were noted in healthy donor tissue by PCR analysis, but there was greater expression as compared to specimens from patients with glaucoma. Analysis of SDPR protein expression confirmed specific expression in the TM regions, but not in adjacent tissues. TEM studies of TM specimens from healthy donors did not demonstrate any racial differences in caveolar morphology, but a significant reduction of caveolae with normal morphology and immuno-gold staining of SDPR were noted in glaucomatous TM as compared to TM from healthy donors. Linkage of SDPR expression levels in TM, POAG development, and caveolar ultrastructural morphology may provide the basis for a novel pathway of exploration of the pathologic mechanisms of glaucoma. Differential gene expression of SDPR in TM from Black vs White subjects with glaucoma may further our understanding of the important public health implications of the racial disparities of this blinding disease.

Racial Disparities in Glaucoma: From Epidemiology to Pathophysiology

Among individuals of African and Latinx descent compared to those of European background, there is a higher prevalence, earlier onset, more rapid progression of primary open angle glaucoma and greater incidence of blindness. Although some suggest that outreach, education and screening programs may expand earlier diagnosis, and attention to access, cost of treatment, and adherence will improve outcomes, there is increasing evidence of genetic and physiologic differences which may be associated with these disease disparities.

Bioinspired Thermosensitive Hydrogel as a Vitreous Substitute: Synthesis, Properties, and Progress of Animal Studies

In many vitreal diseases, the surgeon removes the natural vitreous and replaces it with silicone oils, gases, or balanced salt solutions to fill the eyeball and hold the retina in position. However, these materials are often associated with complications and have properties that differ from natural vitreous. Herein, we report an extension of our previous work on the synthesis of a biomimetic hydrogel that is composed of thiolated gellan as an analogue of type II collagen and poly(methacrylamide-co-methacrylate-co-bis(methacryloyl)cystamine), a polyelectrolyte, as an analogue of hyaluronic acid. This thermosensitive hydrogel can be injected into the eye as a viscous solution at 45 °C. It then forms a physical gel in situ when it reaches body temperature, and later forms disulfide covalent crosslinks. In this article, we evaluated two different formulations of the biomimetic hydrogels for their physical, mechanical, and optical properties, and we determined their biocompatibility with several cell lines. Finally, we report on the progress of the four-month preclinical evaluation of our bio-inspired vitreous substitute in comparison to silicone oil or a balanced salt solution. We assessed the eyes with a slit-lamp examination, intraocular pressure measurements, electroretinography, and optical coherence tomography. Preliminary results are very encouraging for the continuing evaluation of our bio-inspired hydrogel in clinical trials.

Histopathology and selective biomarker expression in human meibomian glands

Background/aims

Meibomian gland dysfunction (MGD) is the most common form of evaporative dry eye disease, but its pathogenesis is poorly understood. This study examined the histopathological features of meibomian gland (MG) tissue from cadaver donors to identify potential pathogenic processes that underlie MGD in humans.

Methods

Histological analyses was performed on the MGs in the tarsal plates dissected from four cadaver donors, two young and two old adults, including a 36-year-old female (36F) and three males aged 30, 63 and 64 years (30M, 63M and 64M).

Results

The MGs of 36F displayed normal anatomy and structure, whereas the MGs of 30M showed severe ductal obstruction with mild distortion. The obstruction was caused by increased cytokeratin levels in association with hyperproliferation, but not hyperkeratinisation. In two older males, moderate to severe MG atrophy was noted. Cell proliferation was significantly reduced in the MG acini of the two older donors as measured by Ki67 labelling index (6.0%±3.4% and 7.9%±2.8% in 63M and 64M, respectively) when compared with that of the two younger donors (23.2%±5.5% and 16.9%±4.8% in 30M and 36F, respectively) (p<0.001). The expression patterns of meibocyte differentiation biomarkers were similar in the older and younger donors.

Conclusion

Our histopathological study, based on a small sample size, suggests potentially distinct pathogenic mechanisms in MGD. In the young male adult, hyperproliferation and aberrant differentiation of the central ductal epithelia may lead to the obstruction by overproduced cytokeratins. In contrast, in older adults, decreased cell proliferation in acinar basal epithelia could be a contributing factor leading to MG glandular atrophy.

Intraocular Oxygen and Antioxidant Status: New Insights on the Effect of Vitrectomy and Glaucoma Pathogenesis

PURPOSE

The purpose of this study was to investigate correlations of partial pressure of oxygen (pO₂) in the ocular anterior segment of human eyes and aqueous humor antioxidant levels of ascorbate (AsA) and total reactive antioxidant potential (TRAP) with glaucoma and vitreous status.

METHODS

This prospective, cross-sectional study stratified patients (n = 288 eyes) by lens and vitreous status and the presence of primary open-angle glaucoma for statistical analyses. Intraocular pO₂ concentrations were measured using a fiberoptic probe in patients at the beginning of planned glaucoma and/or cataract surgery. Aqueous humor specimens were obtained for antioxidant analysis of AsA and TRAP.

RESULTS

Following prior pars plana vitrectomy, pO₂ levels were significantly higher than in the reference group of cataract surgery in the anterior chamber angle (16.2 ± 5.0 vs. 13.0 ± 3.9 mm Hg; P = .0171) and in the posterior chamber (7.6 ± 3.1 vs. 3.9 ± 2.7 mm Hg; P < .0001). AsA and TRAP levels were significantly lower (1.1 ± 0.4 vs. 1.4 ± 0.5 mM, respectively; 403.3 ±116.5 vs. 479.0 ± 146.7 Trolox units, respectively; P = .004 and P = .024, respectively) in patients after vitrectomy. In patients with an intact vitreous, neither pO₂ nor antioxidant status correlated with lens status or glaucoma.

CONCLUSIONS

Increased pO₂ and antioxidant depletion following vitrectomy suggests an alteration of the intraocular oxidant-antioxidant balance. Our study links physiologic factors such as increased pO₂ in the anterior chamber angle and the posterior chamber to decreased antioxidant levels in aqueous humor following vitrectomy. Oxidative stress/damage to the trabecular meshwork in such post-vitrectomy cases may contribute to intraocular pressure elevation and increased risk of glaucoma. NOTE: Publication of this article is sponsored by the American Ophthalmological Society.

Pupillary light reaction in preclinical Alzheimer's disease subjects compared with normal ageing controls

BACKGROUND/AIMS

We wished to determine whether the pupillary light reaction can differentiate preclinical Alzheimer's disease (AD) subjects from normal ageing controls. We performed a prospective study evaluating the pupillary light reaction in a cohort of well-characterised subjects with preclinical AD versus normal ageing controls.

METHODS

We recruited 57 subjects from our institution's Memory and Aging Project, part of our Alzheimer's Disease Research Center. All subjects completed PET-PiB imaging, cerebrospinal fluid analysis and at least 1 neuropsychiatric assessment after their baseline assessment. All participants were assigned a clinical dementia rating and underwent a complete neuro-ophthalmic examination. Participants were divided into a dementia biomarker+ (preclinical AD) and biomarker- (normal ageing) group based on preclinical risk for Alzheimer's dementia. Pupillometry measurements were performed by using the NeurOptics PLR-200 Pupillometer.

RESULTS

A total of 57 subjects were recruited with 24 dementia biomarker+ and 33 dementia biomarker- individuals. A variety of pupil flash response (PLR) parameters were assessed. Comparisons between groups were analysed using generalised estimating equations. None of the pupillary parameters showed a significant difference between groups.

CONCLUSIONS

We found no significant differences in PLR between preclinical AD subjects and normal ageing controls. This suggests that the disease effect on the PLR may be small and difficult to detect at the earliest stages of the disease. Future studies could include larger sample size and chromatic pupillometry.

Effects of Vitrectomy and Lensectomy on Older Rhesus Macaques: Oxygen Distribution, Antioxidant Status, and Aqueous Humor Dynamics

Purpose

The purpose of this study is to evaluate effects of vitrectomy (PPV) and lens extraction with intraocular lens implantation (PE/IOL) on molecular oxygen (pO₂) distribution, aqueous humor antioxidant–oxidant balance, aqueous humor dynamics, and histopathologic changes in the trabecular meshwork (TM) in the older macaque monkey.

Methods

Six rhesus monkeys underwent PPV followed by PE/IOL. pO₂, outflow facility, and intraocular pressure (IOP) were measured. Aqueous and vitreous humor specimens were analyzed for antioxidant status and 8-hydroxy-2′-deoxyguanosine (8-OHdG), a marker of oxidative damage. TM specimens were obtained for immunohistochemical and quantitative PCR analysis.

Results

pO₂ at baseline revealed steep gradients in the anterior chamber and low levels in the posterior chamber (PC) and around the lens. Following PPV and PE/IOL, pO₂ significantly increased in the PC, around the IOL, and angle. IOP increased following both surgical interventions, with no change in outflow facility. Histopathologic analysis did not show changes in TM cell quantification, but there was an increase in 8-OHdG. Quantitative PCR did not reveal significant differences in glaucoma-related gene expression. Aqueous and vitreous humor analysis revealed decreased ascorbate and total reactive antioxidant potential and increased 8-OHdG in the aqueous humor only in the surgical eyes.

Conclusions

Oxygen distribution in the older rhesus monkey is similar to humans at baseline and following surgical interventions. Our findings of histopathologic changes of TM oxidative damage and alterations in the oxidant-antioxidant balance suggest a potential correlation of increased oxygen exposure with oxidative stress/damage and the development of open angle glaucoma.

Expression Profiling of Ascorbic Acid-Related Transporters in Human and Mouse Eyes

Purpose

Ascorbic acid (AsA) is an important antioxidant in the eye. Ascorbic acid is usually transported by sodium-dependent AsA transporters (SVCTs), and dehydroascorbic acid (DHA) by glucose transporters (GLUTs). This study investigates these AsA-related transporters in human compared with mouse eyes.

Methods

Five pairs of human donor eyes and 15 pairs of mouse eyes were collected. Immunofluorescence and in situ hybridization were performed to detect SVCTs and GLUTs expression in the ciliary epithelium, retina, and lens epithelial cells (LECs). These tissues were isolated with laser microdissection followed by extraction of total RNA. Quantitative PCR (qPCR) was performed to examine the mRNA level of SVCTs and GLUTs in human and mouse ocular tissues.

Results

Immunofluorescence and in situ hybridization showed SVCT2 and GLUT1 expression in human ciliary epithelium with varied distributions. Sodium-dependent AsA transporter 2 is expressed only in the pigmented epithelium (PE), and GLUT1 is predominately expressed in the nonpigmented epithelium (NPE). However, SVCT2 was not identified in mouse ciliary epithelium, whereas GLUT1 expressed in both PE and NPE. Laser microdissection and qPCR revealed high levels of SVCT2 mRNA in human RPE cells and murine neural retina. Sodium-dependent AsA transporter 1 mRNA could be detected only in human and murine LECs. Glucose transporter 3 and GLUT4 mRNA could not be detected in either the human or mouse ciliary processes or in the lens epithelium.

Conclusions

These fundamental findings indicate AsA transporter expression in eyes of humans is significantly different compared with mice. This may explain why human aqueous and vitreous humors contain higher AsA levels compared with other animals.

GDF15 is elevated in mice following retinal ganglion cell death and in glaucoma patients

Glaucoma is the second leading cause of blindness worldwide. Physicians often use surrogate endpoints to monitor the progression of glaucomatous neurodegeneration. These approaches are limited in their ability to quantify disease severity and progression due to inherent subjectivity, unreliability, and limitations of normative databases. Therefore, there is a critical need to identify specific molecular markers that predict or measure glaucomatous neurodegeneration. Here, we demonstrate that growth differentiation factor 15 (GDF15) is associated with retinal ganglion cell death. Gdf15 expression in the retina is specifically increased after acute injury to retinal ganglion cell axons and in a murine chronic glaucoma model. We also demonstrate that the ganglion cell layer may be one of the sources of secreted GDF15 and that GDF15 diffuses to and can be detected in aqueous humor (AH). In validating these findings in human patients with glaucoma, we find not only that GDF15 is increased in AH of patients with primary open angle glaucoma (POAG), but also that elevated GDF15 levels are significantly associated with worse functional outcomes in glaucoma patients, as measured by visual field testing. Thus, GDF15 maybe a reliable metric of glaucomatous neurodegeneration, although further prospective validation studies will be necessary to determine if GDF15 can be used in clinical practice.

Mitochondrial oxygen metabolism in primary human lens epithelial cells: Association with age, diabetes and glaucoma

PURPOSE

The hypoxic environment around the lens is important for maintaining lens transparency. Lens epithelial cells (LECs) play a key role in lens metabolism. We measured oxygen consumption to assess the role of human LECs in maintaining hypoxia around the lens, as well as the impact of systemic and ocular diagnosis on these cells.

METHODS

Baseline cellular respiration was measured in rabbit LECs (NN1003A), canine kidney epithelial cells (MDCK), trabecular meshwork cells (TM-5), and bovine corneal endothelial cells (CCEE) using a XF96 Extracellular Flux Analyzer (Seahorse Bioscience, North Billerica, MA), which measures oxygen consumption rate (OCR) and extracellular acidification rate (ECAR) in vitro. Following informed written consent, lens capsule epithelial cells were obtained from patients during cataract surgery and were divided into small explants in 96-well plates. Capsules were removed when LECs became confluent. OCR was normalized to the number of cells per well using rabbit LECs as a standard. The effect of patient age, sex, race, and presence of diabetes or glaucoma on oxygen consumption was assessed by using the Mann-Whitney U test and multivariate regression analysis.

RESULTS

Primary LECs were obtained from 69 patients. The OCR from donors aged 70 and over was lower than that of those under 70 years (2.21±1.037 vs. 2.86±1.383 fmol/min/cell; p<0.05). Diabetic patients had lower OCR than non-diabetic patients (2.02±0.911 vs. 2.79±1.332fmol/min/cell; p<0.05), and glaucoma patients had lower OCR than non-glaucoma patients (2.27±1.19 vs. 2.83±1.286 fmol/min/cell; p<0.05). Multivariate regression analysis confirmed that donors aged 70 and over (p<0.05), diabetic patients (p<0.01), and glaucoma patients (p<0.05) had significantly lower OCR, independent of other variables. Gender and race had no significant effect on OCR.

CONCLUSIONS

The lower oxygen consumption rate of human LECs in older donors and patients with diabetes or glaucoma could contribute to cataract development. Diabetes and glaucoma are particularly important factors associated with decreased OCR, independent of age. Ongoing studies are examining pO2 at the anterior surface of the lens in vivo and oxygen consumption in the patient's LECs.

Impact of Corneal Endothelial Dysfunctions on Intraocular Oxygen Levels in Human Eyes

PURPOSE

We studied the implications of corneal endothelial dysfunctions on oxidative stress in the anterior segment via in vivo measurements of oxygen partial pressure (pO2) in the anterior chamber (AC) of human eyes.

METHODS

We recruited 51 patients undergoing cataract surgery and/or endothelial keratoplasty (EK). Endothelial cell density (ECD; n = 33) and central corneal thickness (CCT; n = 41) were measured on patients with relatively clear corneas. Before surgery, an oxygen sensor was introduced into the AC via a peripheral corneal paracentesis. In all patients, seven measurements of pO2 were obtained by positioning the flexible tip near the endothelium at the central cornea, at four cardinal subendothelial locations near the midperipheral cornea, and in the mid-AC and AC angle. In patients with pseudophakia or eyes undergoing cataract surgery, pO2 also was measured near the lens surface and in the posterior chamber.

RESULTS

Consistent with our previous reports, a steep oxygen gradient was noted in the anterior segment of normal controls (n = 24). In patients with endothelial dysfunctions (n = 27), there was a significant increase of pO2 at all five subendothelial locations without a significant increase of pO2 in the AC angle. By regression analyses, subendothelial pO2 correlated inversely with ECD and positively with CCT in patients with endothelial dysfunctions.

CONCLUSIONS

This study demonstrates an even steeper intraocular oxygen gradient in eyes with corneal endothelial dysfunctions. It suggests that the reduced oxygen consumption in corneal endothelial cells may increase oxidative stress in the AC and the existence of an alternative aqueous inflow pathway that maintains a relatively low and constant pO2 at the AC angle.

Photoacoustic tomography imaging and estimation of oxygen saturation of hemoglobin in ocular tissue of rabbits

This study evaluated in vivo imaging capabilities and safety of qualitative monitoring of oxygen saturation of hemoglobin (sO2) of rabbit ciliary body tissues obtained with acoustic resolution (AR) photoacoustic tomography (PAT). AR PAT was used to collect trans-scleral images from ciliary body vasculature of seven New Zealand White rabbits. The PAT sO2 measurements were obtained under the following conditions: when systemic sO2 as measured by pulse oximetry was between 100% and 99% (level 1); systemic sO2 as measured by pulse oximetry was between 98% and 90% (level 2); and systemic sO2 as measured by pulse oximetry was less than 90% (level 3). Following imaging, histological analysis of ocular tissue was conducted to evaluate for possible structural damage caused by the AR PAT imaging. AR PAT was able to resolve anatomical structures of the anterior segment of the eye, viewed through the cornea or anterior sclera. Histological studies revealed no ocular damage. On average, sO2 values (%) obtained with AR PAT were lower than sO2 values obtained with pulse oximetry (all p < 0.001): 86.28 ± 4.16 versus 99.25 ± 0.28, 84.09 ± 1.81 vs. 95.3 ± 2.6, and 64.49 ± 7.27 vs. 71.15 ± 10.21 for levels 1, 2 and 3 respectively. AR PAT imaging modality is capable of qualitative monitoring for deep tissue sO2 in rabbits. Further studies are needed to validate and modify the AR PAT modality specifically for use in human eyes. Having a safe, non-invasive method of in vivo imaging of sO2 in the anterior segment is important to studies evaluating the role of oxidative damage, hypoxia and ischemia in pathogenesis of ocular diseases.

O2 -sensitive MRI distinguishes brain tumor versus radiation necrosis in murine models

PURPOSE

The goal of this study was to quantify the relationship between the (1) H longitudinal relaxation rate constant, R1 , and oxygen (O2 ) concentration (relaxivity, r1 ) in tissue and to quantify O2 -driven changes in R1 (ΔR1 ) during a breathing gas challenge in normal brain, radiation-induced lesions, and tumor lesions.

METHODS

R1 data were collected in control-state mice (n = 4) during three different breathing gas (and thus tissue O2 ) conditions. In parallel experiments, pO2 was measured in the thalamus of control-state mice (n = 4) under the same breathing gas conditions using an O2 -sensitive microprobe. The relaxivity of tissue O2 was calculated using the R1 and pO2 data. R1 data were collected in control-state (n = 4) mice, a glioma model (n = 7), and a radiation necrosis model (n = 6) during two breathing gas (thus tissue O2 ) conditions. R1 and ΔR1 were calculated for each cohort.

RESULTS

O2 r1 in the brain was 9 × 10(-4)  ± 3 × 10(-4) mm Hg(-1) · s(-1) at 4.7T. R1 and ΔR1 measurements distinguished radiation necrosis from tumor (P< 0.03 and P< 0.01, respectively).

CONCLUSION

The relaxivity of O2 in the brain is determined. R1 and ΔR1 measurements differentiate tumor lesions from radiation necrosis lesions in the mouse models. These pathologies are difficult to distinguish by traditional imaging techniques; O2 -driven changes in R1 holds promise in this regard. Magn Reson Med 75:2442-2447, 2016. © 2015 Wiley Periodicals, Inc.

A test of lens opacity as an indicator of preclinical Alzheimer Disease

Previous studies reported that characteristic lens opacities were present in Alzheimer Disease (AD) patients postmortem. We therefore determined whether cataract grade or lens opacity is related to the risk of Alzheimer dementia in participants who have biomarkers that predict a high risk of developing the disease. AD biomarker status was determined by positron emission tomography-Pittsburgh compound B (PET-PiB) imaging and cerebrospinal fluid (CSF) levels of Aβ42. Cognitively normal participants with a clinical dementia rating of zero (CDR = 0; N = 40) or with slight evidence of dementia (CDR = 0.5; N = 2) were recruited from longitudinal studies of memory and aging at the Washington University Knight Alzheimer's Disease Research Center. The age, sex, race, cataract type and cataract grade of all participants were recorded and an objective measure of lens light scattering was obtained for each eye using a Scheimpflug camera. Twenty-seven participants had no biomarkers of Alzheimer dementia and were CDR = 0. Fifteen participants had biomarkers indicating increased risk of AD, two of which were CDR = 0.5. Participants who were biomarker positive were older than those who were biomarker negative. Biomarker positive participants had more advanced cataracts and increased cortical light scattering, none of which reached statistical significance after adjustment for age. We conclude that cataract grade or lens opacity is unlikely to provide a non-invasive measure of the risk of developing Alzheimer dementia.

Central corneal thickness correlates with oxygen levels in the human anterior chamber angle

PURPOSE

To measure oxygen (pO2) in eyes of patients undergoing intraocular surgery and identify correlations with central corneal thickness (CCT).

DESIGN

Prospective, cross-sectional study.

METHODS

setting: Institutional. patient population: 124 patients undergoing cataract and/or glaucoma surgery. observation procedure: Prior to surgery, an oxygen sensor was introduced into the anterior chamber (AC) via peripheral corneal paracentesis. The tip of the flexible fiberoptic probe was positioned for 3 measurements in all patients: (1) near central corneal endothelium; (2) in mid-AC; and (3) in AC angle. In patients undergoing cataract extraction, additional measurements were taken (4) at the anterior lens surface and (5) in the posterior chamber. main outcome measures: pO2 measurements at 5 locations within the eye were compared to central corneal thickness measurements by multivariate regression analyses.

RESULTS

There was a statistically significant inverse correlation between CCT and pO2 in the anterior chamber angle (P = .048). pO2 was not significantly related to CCT at any other location, including beneath the central cornea. Regression analysis relating CCT to age, race, and oxygen levels in all 5 locations in the anterior segment revealed an association of a thinner cornea with increasing age (P = .007).

CONCLUSIONS

Physiologic correlations with central corneal thickness may provide clues to understanding why a thinner cornea increases the risk of open glaucoma. Associations between glaucoma risk, CCT, and pO2 in the AC angle suggest that exposure of the outflow system to increased oxygen or oxygen metabolites may increase oxidative damage to the trabecular meshwork cells, resulting in elevation of intraocular pressure.

Quantitative imaging of enzymatic vitreolysis-induced fiber remodeling

PURPOSE

Collagen fiber remodeling in the vitreous body has been implicated in cases of vitreomacular traction, macular hole, and retinal detachment, and also may occur during pharmacologic vitreolysis. The purpose of this study was to evaluate quantitative polarized light imaging (QPLI) as a tool for studying fiber organization in the vitreous and near the vitreoretinal interface in control and enzymatically perturbed conditions.

METHODS

Fiber alignment was measured in anterior-posterior sections of bovine and porcine vitreous. Additional tests were performed on bovine lenses and nasal-temporal vitreous sections. Effects of proteoglycan degradation on collagen fiber alignment using trypsin and plasmin were assessed at the microstructural level using electron microscopy and at the global level using QPLI.

RESULTS

Control vitreous showed fiber organization patterns consistent with the literature across multiple-length scales, including the global anterior-posterior coursing of vitreous fibers, as well as local fibers parallel to the equatorial vitreoretinal interface and transverse to the posterior interface. Proteoglycan digestion with trypsin or plasmin significantly increased fiber alignment throughout the vitreous (P < 0.01). The largest changes (3×) occurred in the posterior vitreous where fibers are aligned transverse to the posterior vitreoretinal interface (P < 0.01).

CONCLUSIONS

Proteoglycan loss due to enzymatic vitreolysis differentially increases fiber alignment at locations where tractions are most common. We hypothesize that a similar mechanism leads to retinal complications during age-related vitreous degeneration. Structural changes to the entire vitreous body (as opposed to the vitreoretinal interface alone) should be evaluated during preclinical testing of pharmacological vitreolysis candidates.

Preservation of the structure of enzymatically-degraded bovine vitreous using synthetic proteoglycan mimics

PURPOSE

Vitreous liquefaction and subsequent posterior vitreous detachment can lead to several sight-threatening diseases, including retinal detachment, macular hole and macular traction syndrome, nuclear cataracts, and possibly, open-angle glaucoma. In this study, we tested the ability of three novel synthetic chondroitin sulfate proteoglycan mimics to preserve the structure and physical properties of enzymatically-degraded bovine vitreous.

METHODS

Chondroitin sulfate proteoglycan mimics, designed to bind to type II collagen, hyaluronic acid, or both, were applied to trypsin- or collagenase-treated bovine vitreous in situ and in vitro. Rheology and liquefaction tests were performed to determine the physical properties of the vitreous, while Western blots were used to detect the presence and degradation of soluble collagen II (α1). Deep-etch electron microscopy (DEEM) identified the ultrastructure of mimic-treated and untreated enzyme-degraded bovine vitreous.

RESULTS

Proteoglycan mimics preserved the physical properties of trypsin-degraded bovine vitreous and protected against vitreous liquefaction. Although the collagen-binding mimic maintained the physical properties of collagenase-treated vitreous, liquefaction still occurred. Western blots indicated that the mimic provided only marginal protective ability against soluble collagen degradation. Deep-etch electron microscopy, however, showed increased density and isotropy of microstructural components in mimic-treated vitreous, supporting the initial result that vitreous structure was preserved.

CONCLUSIONS

Proteoglycan mimics preserved bovine vitreous physical properties after enzymatic degradation. These compounds may be useful in delaying or preventing the pathological effects of age-related, or enzymatically-induced, degradation of the vitreous body.

Enzymatic degradation identifies components responsible for the structural properties of the vitreous body

PURPOSE

Vitreous degeneration contributes to several age-related eye diseases, including retinal detachment, macular hole, macular traction syndrome, and nuclear cataracts. Remarkably little is understood about the molecular interactions responsible for maintaining vitreous structure. The purpose of this study was to measure the structural properties of the vitreous body after enzymatic degradation of selected macromolecules.

METHODS

Mechanical properties of plugs of bovine and porcine vitreous were analyzed using a rheometer. Oscillatory and extensional tests measured vitreous stiffness and adhesivity, respectively. Major structural components of the vitreous were degraded by incubation overnight in collagenase, trypsin, or hyaluronidase, singly or in combination. Vitreous bodies were also incubated in hyper- or hypotonic saline. Effects of these treatments on the mechanical properties of the vitreous were measured by rheometry.

RESULTS

Enzymatic digestion of each class of macromolecules decreased the stiffness of bovine vitreous by approximately half (P < 0.05). Differential effects were observed on the damping capacity of the vitreous (P < 0.05), which was shown to correlate with material behavior in extension (P < 0.01). Digestion of hyaluronan significantly decreased the damping capacity of the vitreous and increased adhesivity. Collagen degradation resulted in the opposite effect, whereas digestion of proteins and proteoglycans with trypsin did not alter behavior relative to controls. Osmotic perturbations and double-enzyme treatments further implicated hyaluronan and hyaluronan-associated water as a primary regulator of adhesivity and material behavior in extension.

CONCLUSIONS

Collagen, hyaluronan, and proteoglycans act synergistically to maintain vitreous stiffness. Hyaluronan is a key mediator of vitreous adhesivity, and mechanical damping is an important factor influencing dynamic vitreous behavior.

Computational model for oxygen transport and consumption in human vitreous

PURPOSE

Previous studies that measured liquefaction and oxygen content in human vitreous suggested that exposure of the lens to excess oxygen causes nuclear cataracts. Here, we developed a computational model that reproduced available experimental oxygen distributions for intact and degraded human vitreous in physiologic and environmentally perturbed conditions. After validation, the model was used to estimate how age-related changes in vitreous physiology and structure alter oxygen levels at the lens.

METHODS

A finite-element model for oxygen transport and consumption in the human vitreous was created. Major inputs included ascorbate-mediated oxygen consumption in the vitreous, consumption at the posterior lens surface, and inflow from the retinal vasculature. Concentration-dependent relations were determined from experimental human data or estimated from animal studies, with the impact of all assumptions explored via parameter studies.

RESULTS

The model reproduced experimental data in humans, including oxygen partial pressure (Po2) gradients (≈15 mm Hg) across the anterior-posterior extent of the vitreous body, higher oxygen levels at the pars plana relative to the vitreous core, increases in Po2 near the lens after cataract surgery, and equilibration in the vitreous chamber following vitrectomy. Loss of the antioxidative capacity of ascorbate increases oxygen levels 3-fold at the lens surface. Homogeneous vitreous degeneration (liquefaction), but not partial posterior vitreous detachment, greatly increases oxygen exposure to the lens.

CONCLUSIONS

Ascorbate content and the structure of the vitreous gel are critical determinants of lens oxygen exposure. Minimally invasive surgery and restoration of vitreous structure warrant further attention as strategies for preventing nuclear cataracts.

Oxidative responses induced by pharmacologic vitreolysis and/or long-term hyperoxia treatment in rat lenses

PURPOSE

The aim of the study was to investigate the protective effects of intact vitreous gel on the lens after pharmacologic vitreolysis and hyperoxia exposure in rats in vivo.

METHODS

Eyes of Sprague-Dawley rats were induced to posterior vitreous detachment (PVD) by pharmacologic vitreolysis, and the rats with and without PVD were treated with hyperoxia 3 h per day for 5 months. Lens transparency was monitored by a slit-lamp biomicroscope. A series of biochemical measurements were made in extracts of the lens cortex and nucleus. Ascorbate levels were measured in the aqueous and vitreous humors.

RESULTS

No significant differences in lens transparency or morphology were observed in all groups, and no significant biochemical changes were observed in the cortex or nucleus of lenses of the PVD group. In the lens nucleus, the values of water-soluble protein concentration in PVD + hyperoxia group were lower than that of the PVD group. The levels of water-soluble proteins, glutathione (GSH) and ascorbate decreased in the hyperoxia group with an intact vitreous body. Vitreolysis enhanced the effect of hyperoxia, decreasing soluble protein, GSH and ascorbate below the levels seen in eyes with vitreolysis alone. The levels of antioxidants and soluble proteins were lower in the lens nucleus, and the effects of vitreolysis plus hyperoxia were more significant in the nucleus. Hyperoxia and hyperoxia plus vitreolysis reduced catalase activity and increased oxidized GSH to a greater extent in the lens cortex, although these treatments increased protein-GSH mixed disulfides in both regions. Long-term hyperoxia also lowered ascorbate levels in the vitreous and aqueous humors, an effect that was enhanced by vitreolysis.

CONCLUSIONS

Exposure to excess molecular oxygen produces significant oxidative damage to the lens, especially the lens nucleus. These effects were enhanced by pharmacologic vitreolysis, indicating that intact vitreous gel protects the lens from oxidative damage.

Racial differences in ocular oxidative metabolism: implications for ocular disease

OBJECTIVE

To compare the Po(2) distribution in different regions in the eyes of patients undergoing intraocular surgery.

METHODS

Before initiation of intraocular cataract and/or glaucoma surgery, an optical oxygen sensor was introduced into the anterior chamber via a peripheral corneal paracentesis. The tip of the flexible fiberoptic probe was positioned by the surgeon for 3 measurements in all patients: (1) near the central corneal endothelium, (2) in the mid-anterior chamber, and (3) in the anterior chamber angle. In patients scheduled to undergo cataract extraction, Po(2) was also measured (4) at the anterior lens surface and (5) in the posterior chamber just behind the iris. Oxygen measurements at the 5 locations were compared using a 2-tailed unpaired t test and multivariate regression.

RESULTS

The Po(2) value was significantly higher in African American patients at all 5 locations compared with Caucasian patients. Adjusting for age increased the significance of this association. Adjusting for race revealed that age was associated with increased Po(2) beneath the central cornea.

CONCLUSIONS

Racial differences in oxygen levels in the human eye reflect an important difference in oxidative metabolism in the cornea and lens and may reflect differences in systemic physiologic function. Increased oxygen or oxygen metabolites may increase oxidative stress, cell damage, intraocular pressure, and the risk of developing glaucoma. Oxygen use by the cornea decreases with age.

Vitreoretinal influences on lens function and cataract

The lens is composed of a thin metabolically active outer layer, consisting of epithelial and superficial fibre cells. Lying within this outer shell are terminally differentiated, metabolically inactive fibre cells, which are divided into an outer cortex and central nucleus. Mature fibre cells contain a very high protein concentration, which is important for the transparency and refractive power of the lens. These proteins are protected from oxidation by reducing substances, like glutathione, and by the low-oxygen environment around the lens. Glutathione reaches the mature fibre cells by diffusing from the metabolically active cells at the lens surface. With age, the cytoplasm of the nucleus becomes stiffer, reducing the rate of diffusion and making nuclear proteins more susceptible to oxidation. Low pO(2) is maintained at the posterior surface of the lens by the physical and physiological properties of the vitreous body, the gel filling the space between the lens and the retina. Destruction or degeneration of the vitreous body increases exposure of the lens to oxygen from the retina. Oxygen reaches the lens nucleus, increasing protein oxidation and aggregation and leading to nuclear cataract. We suggest that maintaining low pO(2) around the lens should prevent the formation of nuclear cataracts.

Ischemic diabetic retinopathy may protect against nuclear sclerotic cataract

PURPOSE

To determine whether diabetes mellitus is protective for nuclear sclerotic cataract at baseline and 6 and 12 months after vitrectomy surgery.

DESIGN

Prospective, interventional cohort study.

METHODS

Phakic diabetic and nondiabetic patients undergoing vitrectomy surgery for a variety of retinal conditions underwent Scheimpflug lens photography in the operated and fellow eye at baseline and at 6 and 12 months after vitrectomy surgery.

RESULTS

Of 52 eyes included in the analysis, 23 eyes were from diabetic patients, 14 of which had surgery for ischemic retinopathy. At baseline, eyes with ischemic diabetic retinopathy had less nuclear sclerotic cataract than nonischemic diabetic and nondiabetic eyes. This was true for eyes undergoing vitrectomy surgery (P = .0001) and for fellow eyes (P = .003). Nuclear sclerotic cataract developed after vitrectomy surgery in nonischemic diabetic eyes and nondiabetic eyes at the same rate. Diabetic eyes with ischemic retinopathy showed no significant progression of nuclear opacification, and therefore had significantly less postvitrectomy nuclear cataract at 6 months (P < 1 × 10(-6)) and at 12 months (P < .001) than nondiabetic or nonischemic diabetic eyes. Normalizing to baseline opacity and adjusting for age and other comorbidities did not alter this result.

CONCLUSIONS

Ischemic diabetic retinopathy, not just systemic diabetes mellitus, protected against nuclear sclerotic cataract at baseline and after vitrectomy surgery. These findings are consistent with the hypothesis that increased exposure to oxygen is responsible for nuclear cataract formation.

Comparison of two probe designs for determining intraocular oxygen distribution

INTRODUCTION

Alterations in intraocular oxygen levels are important contributors to, or indications of, ocular disease. Polarographic electrodes and fibre-optic sensors (optodes) have been used to measure oxygen and to map the distribution of oxygen in animal models and in human eyes. A recent study reported the use of a commercial electrode to compare oxygen distribution in the vitreous of patients undergoing vitrectomy related to central retinal vein occlusion, macular hole or preretinal membrane. The results of this study were at variance with previous measures of oxygen distribution in the human vitreous using polarographic or optical sensors. To resolve this discrepancy, the present study compared measurements made in vitro or in animal eyes, using the electrode employed in the previous study or a fibre-optic sensor of a different design.

STUDY DESIGN

Comparative in vitro and in vivo measurements.

RESULTS

In vitro, the two devices reported similar levels of oxygen, although the electrode consistently detected levels above the calculated values. In rabbit eyes, the electrode had a slow response time and was unable to detect oxygen gradients that were readily measured by the smaller optode. When the electrode was inserted into an eye of similar size to the human eye, the reference thermistor measured the temperature outside the eye, not in the vitreous.

CONCLUSIONS

The design of the electrode used in the previous study makes it unsuitable for measurements of oxygen distribution in the eye.

Oxidative damage and the prevention of age-related cataracts

PURPOSE

Cataracts are often considered to be an unavoidable consequence of aging. Oxidative damage is a major cause or consequence of cortical and nuclear cataracts, the most common types of age-related cataracts.

METHODS

In this review, we consider the different risk factors, natural history and etiology of each of the 3 major types of age-related cataract, as well as the potential sources of oxidative injury to the lens and the mechanisms that protect against these insults. The evidence linking different oxidative stresses to the different types of cataracts is critically evaluated.

RESULTS

We conclude from this analysis that the evidence for a causal role of oxidation is strong for nuclear, but substantially lower for cortical and posterior subcapsular cataracts. The preponderance of evidence suggests that exposure to increased levels of molecular oxygen accelerates the age-related opacification of the lens nucleus, leading to nuclear cataract. Factors in the eye that maintain low oxygen partial pressure around the lens are, therefore, important in protecting the lens from nuclear cataract.

CONCLUSIONS

Maintaining or restoring the low oxygen partial pressure around that lens should decrease or prevent nuclear cataracts.

Oxygen distribution in the human eye: relevance to the etiology of open-angle glaucoma after vitrectomy

PURPOSE

Vitrectomy, when followed by cataract surgery, increases the risk of open-angle glaucoma. This study was conducted in patients to determine whether these procedures are associated with increased exposure of the trabecular meshwork to oxygen.

METHODS

Oxygen distribution was recorded with a fiberoptic probe in patients undergoing surgery for cataract, glaucoma, or retinal disease. pO(2) was measured beneath the central cornea, in the mid-anterior chamber, and in the anterior chamber angle. In patients who were pseudophakic or were scheduled for cataract extraction, pO(2) was also measured in the posterior chamber and near the lens.

RESULTS

Eyes with no previous cataract or vitrectomy surgery had steep oxygen gradients in the aqueous humor between the cornea and lens. pO(2) was low in the posterior chamber and near the lens. Previous vitrectomy was associated with significantly increased pO(2) in the posterior chamber. Eyes with previous cataract surgery had significantly elevated pO(2) only in the posterior chamber and in front of the intraocular lens (IOL). Eyes that had both vitrectomy and previous cataract surgery had increased pO(2) in the posterior chamber, anterior to the IOL, and in the anterior chamber angle. pO(2) in the posterior chamber and the anterior chamber angle correlated strongly.

CONCLUSIONS

Oxygen metabolism by the lens and cornea establishes oxygen gradients in the anterior segment. Vitrectomy and cataract surgery increase pO(2) in the anterior chamber angle, potentially damaging trabecular meshwork cells. We propose that oxygen levels in the anterior chamber angle are strongly influenced by oxygen derived from the ciliary body circulation.

Visualizing lens epithelial cell proliferation in whole lenses

PURPOSE

To develop a means to image cells in S-phase of the cell cycle while preserving the anatomic relationships within the lens.

METHODS

Mice were injected with the thymidine analog, EdU. Whole lenses were removed, fixed and permeabilized. Cells that had incorporated EdU into their DNA were chemically labeled using fluorescent azides and "click" chemistry. Double labeling was performed with antibodies to other antigens, like phospho-histoneH3, a marker of mitotic cells. The position of labeled cells and lens anatomy was viewed using a simple device to position and flatten the lens.

RESULTS

The nuclei of cells in S-phase of the cell cycle were intensely stained without the use of antibodies. Stained cells were readily localized with reference anatomic landmarks, like the transition zone. Whole lenses could be assayed by rotating the lens on the microscope stage. Double-labeling permitted the co-localization of markers in cycling cells.

CONCLUSIONS

EdU labeling of whole lenses provides a simple, rapid and sensitive means to analyze lens epithelial cell proliferation in the anatomic context of the whole lens.

The gel state of the vitreous and ascorbate-dependent oxygen consumption: relationship to the etiology of nuclear cataracts

OBJECTIVE

To investigate the rate and mechanism of oxygen consumption by the vitreous.

METHODS

Oxygen consumption was measured with a microrespirometer. Vitreous ascorbate was measured spectrophotometrically and by gas chromatography-mass spectrometry. Vitreous degeneration was related to the rate of oxygen consumption and ascorbate concentration in samples obtained during vitrectomy.

RESULTS

Prolonged exposure to oxygen or treatment with ascorbate oxidase eliminated oxygen consumption by the vitreous. Adding ascorbate restored oxygen consumption. Oxygen consumption persisted after boiling or treating the vitreous with the chelating agents EDTA and deferoxamine. In patients undergoing retinal surgery, liquefaction of the vitreous and previous vitrectomy were associated with decreased ascorbate concentration and lower oxygen consumption.

CONCLUSIONS

Ascorbate in the vitreous decreases exposure of the lens to oxygen. The catalyst for this reaction is not known, although free iron may contribute. The gel state of the vitreous preserves ascorbate levels, thereby sustaining oxygen consumption. Vitrectomy or advanced vitreous degeneration may increase exposure of the lens to oxygen, promoting the progression of nuclear cataracts.

CLINICAL RELEVANCE

Determining how the eye is protected from nuclear cataracts should suggest treatments to reduce their incidence.

The function of VEGF-A in lens development: formation of the hyaloid capillary network and protection against transient nuclear cataracts

A network of capillaries branches from the hyaloid vascular system and surrounds the mammalian lens throughout much of its embryonic development. These vessels are presumed to be important for the growth and maturation of the lens, although the lenses of non-mammalian vertebrates have no comparable vessels. Over expression of VEGF-A in the lens increases the extent of these capillaries, but it is not known whether VEGF-A from the lens is necessary for their formation or survival. To address this question, we deleted Vegfa in the lens. This prevented the formation of the capillary networks adjacent to the lens capsule, but did not alter nearby hyaloid vessels at the surface of the retina. Postnatal lenses lacking Vegfa were smaller than wild type and, by 1 month of age, many had mild nuclear opacities. These opacities regressed with age. The lens is hypoxic throughout most of life and VEGF-A expression is often regulated by the transcription factor, hypoxia inducible factor-1. Lenses lacking Hif1a were of apparently normal size, had markedly reduced levels of mRNA for VEGF-A and glyceraldehyde-3-phosphate dehydrogenase, but had normal-appearing capillaries covering their surface. We conclude that VEGF-A from the lens is necessary for the formation of the normal hyaloid vascular system and that lack of these capillaries was the most likely cause of growth retardation during fetal and early postnatal lens development. In the absence of HIF-1 function, sufficient VEGF-A is produced by the lens to promote capillary formation. Further study is needed to explain the formation of the mild opacities seen in some lenses lacking Vegfa and their regression later in life.

HIF-1: an age-dependent regulator of lens cell proliferation

PURPOSE

The lens grows throughout life, and lens size is a major risk factor for nuclear and cortical cataracts. A previous study showed that the hypoxic environment around the lens suppressed lens growth in older rats. The present study was conducted to investigate the mechanism responsible for the age-dependent decline in lens cell proliferation.

METHODS

Transgenic mice expressing Cre recombinase in the lens were bred to mice containing floxed Hif1a alleles. Transgenic mice expressing oxygen insensitive forms of HIF-1alpha in lens epithelial cells were exposed to room air or 60% oxygen. Proliferation was measured by BrdU labeling and cell death by using the TUNEL assay. Morphology was assessed in histologic sections. HIF-1alpha and p27(KIP1) levels were determined by Western blot. The expression of HIF-regulated genes was assessed on microarrays.

RESULTS

Lenses lacking Hif1a degenerated, precluding study in older animals. Breathing 60% oxygen reduced HIF-1alpha levels and HIF-1-regulated transcripts in lens epithelial cells from young and older lenses. Overexpression of oxygen-insensitive HIF-1alpha had no effect on lens size, but suppressed increased proliferation in response to oxygen. Systemic injection of the iron chelator, 1,10-phenanthroline prevented the degradation of HIF-1alpha and reduced oxygen-induced proliferation. Increasing oxygen decreased levels of p27(KIP1) in the epithelial cells of older mice, which was prevented by expressing oxygen-insensitive forms of HIF-1alpha.

CONCLUSIONS

HIF-1alpha is present and HIF-1 is transcriptionally active throughout life, but suppresses growth only in older lenses. Maintaining elevated levels of p27(KIP1) in older lenses requires HIF-1. p27(KIP1) and other growth regulators may selectively suppress the proliferation of older lens epithelial cells.

Age-dependent control of lens growth by hypoxia

PURPOSE

The lens grows continuously throughout life, but the factors that influence the size of the adult lens are not known. Lens thickness is a significant risk factor for age-related cataract. It has been postulated that the hypoxic environment in the eye protects the lens from nuclear cataracts. The authors sought to determine whether the Po(2) in the eye regulates lens growth.

METHODS

Lens cell proliferation was determined by counting BrdU-labeled and total nuclei in the germinative zone in flatmounts of lens epithelia. Oxygen levels in the eye were altered by having rats breathe 11%, 21% (room air), or 60% oxygen. Oxygen levels in the vitreous were measured with a fiberoptic oxygen sensor.

RESULTS

The BrdU-labeling index in the germinative zone declined from approximately 3.5% at 1 month to less than 0.7% at 8 months. Raising oxygen levels in the eyes of 1-month-old animals did not alter the rate of lens cell proliferation. Elevating intraocular oxygen in animals older than 1 month increased proliferation to the more rapid rate seen at 1 month. Decreasing oxygen levels below their normally low level did not affect the BrdU-labeling index at any age. Chronic exposure to increased oxygen led to the production of more lens fiber cells and larger lenses.

CONCLUSIONS

Normal age-related decline in lens growth requires the low oxygen level normally present in the eye. Increases in lens cell number and mass may account for some of the increase in cataract risk caused by chronic exposure of the lens to elevated oxygen levels.

SAFETY OF INTRAVITREAL INJECTION OF BEVACIZUMAB IN RABBIT EYES

PURPOSE

To evaluate the safety of intravitreal injection of bevacizumab in rabbits using electrophysiological testing and histopathologic analysis.

METHODS

New Zealand albino rabbits were injected in one eye with control antibody (n = 2), 0.05 mL of bevacizumab (n = 3), or 0.2 mL of bevacizumab (n = 3). Electroretinograms were obtained 1 week and 4 weeks after injection. Histologic analysis was performed after completion of the electroretinographic studies.

RESULTS

No statistical differences were seen in scotopic and photopic a- and b-wave amplitudes between untreated control and bevacizumab-injected eyes. No histopathologic differences were identified between untreated control and bevacizumab-injected eyes.

CONCLUSION

Our study did not find evidence of retinal toxicity from a single intravitreal injection of bevacizumab in rabbits.

Lower intraocular oxygen tension in diabetic patients: possible contribution to decreased incidence of nuclear sclerotic cataract

PURPOSE

To report intraocular oxygen tension in eyes of diabetic and nondiabetic patients.

DESIGN

A prospective, interventional consecutive case series.

METHODS

Oxygen was measured with an optical oxygen sensor in patients who were undergoing vitrectomy. Before turning on the infusion fluid, intraocular oxygen tension was measured in two locations: adjacent to the lens and in the mid vitreous cavity.

RESULTS

Fifty eyes from 50 patients were included in the study. Twenty-one eyes were from diabetic patients and 29 eyes were from nondiabetic patients. The mean oxygen tension adjacent to the lens was significantly lower in diabetic than in nondiabetic patients (8.4 +/- 0.7 mm Hg vs 10.7 +/- 0.8 mm Hg; P < .05). Similarly, the mean oxygen tension in the center of the vitreous cavity was lower in diabetic than in nondiabetic patients (5.7 +/- 0.7 mm Hg vs 8.5 +/- 0.6 mm Hg; P < .001). In subgroup analyses, previous panretinal photocoagulation or cataract surgery did not affect oxygen levels significantly in the vitreous of diabetic or nondiabetic patients.

CONCLUSION

Eyes from diabetic patients have significantly lower intraocular oxygen tension than in eyes from nondiabetic patients. Because oxidative damage to the lens nucleus and increased intraocular oxygen tension have been associated with nuclear sclerotic cataract, these findings may help explain recent reports of an apparent protective effect of diabetes mellitus against nuclear sclerotic cataract.

Oxygen distribution in the rabbit eye and oxygen consumption by the lens

PURPOSE

Excessive exposure to oxygen has been proposed to be a risk factor for nuclear cataracts. For a better understanding of the metabolism of oxygen in the eye, oxygen distribution was mapped in the intraocular fluids, and the rate of oxygen consumption by the lens in rabbits breathing different levels of oxygen was calculated.

METHODS

Young albino rabbits were anesthetized, intubated, and exposed to normoxic, hypoxic, or hyperoxic conditions. The hemoglobin saturation of the blood was monitored with a pulse oximeter, and arterial oxygen levels were measured with a blood gas analyzer. A fiberoptic optical oxygen sensor (optode) was used to determine oxygen levels in different regions of the eye. Oxygen flux across the posterior of the lens was calculated from the measured oxygen gradients in the vitreous chamber.

RESULTS

Oxygen levels in the ocular fluids changed markedly when rabbits breathed air made hypoxic or hyperoxic. Oxygen levels were highest near the retinal vasculature, the iris vasculature, and the inner surface of the central cornea. Compared with nearby regions, oxygen levels were decreased in the aqueous humor closest to the pars plicata of the ciliary body and near the anterior chamber angle. Oxygen levels were generally lower closer to the lens. From the oxygen gradients in the vitreous body, oxygen consumption by the posterior half of the lens was calculated to be 0.2 to 0.4 microL/h under normoxic conditions. Oxygen consumption by the posterior of the lens increased in proportion to the amount of oxygen supplied.

CONCLUSIONS

Intraocular oxygen is mostly derived from the retinal and iris vasculature and by diffusion across the cornea. Freshly secreted aqueous humor and the aqueous humor in the anterior chamber angle are relatively depleted of oxygen. The marked increase in oxygen consumption that occurs when the lens is exposed to increased oxygen is likely to result in the production of higher levels of reactive oxygen species and may provide a link between elevated oxygen levels and the risk of nuclear cataracts.

Vitrectomy surgery increases oxygen exposure to the lens: a possible mechanism for nuclear cataract formation

PURPOSE

To report vitreous oxygen tension before, immediately after, and at longer times after vitrectomy.

DESIGN

A prospective, interventional consecutive case series.

METHODS

Oxygen was measured using an optical oxygen sensor in patients undergoing vitrectomy. Intraoperatively, oxygen measurements were taken before and after vitrectomy in two intraocular locations: adjacent to the lens and in the mid-vitreous.

RESULTS

Sixty-nine eyes underwent oxygen tension measurements at the time of vitrectomy. In baseline eyes, oxygen tension in the vitreous was low, measuring 8.7 +/- 0.6 mm Hg adjacent to the lens and 7.1 +/- 0.5 mm Hg in the mid-vitreous. The difference between the two locations was statistically significant (P < .003), indicating that vitreous gel maintains an intraocular oxygen gradient. Immediately after vitrectomy, oxygen tension in the fluid-filled eye was higher, measuring 69.6 +/-4.8 mm Hg adjacent to the lens and 75.6 +/- 4.1 mm Hg in the mid-vitreous. There was no statistically significant oxygen gradient between the two locations. The difference in oxygen tension pre- and postvitrectomy is highly statistically significant (P < .0001 lens, P < .0001 mid-vitreous). In eyes with a history of vitrectomy and previous removal of the vitreous gel, the intraocular oxygen tension was significantly higher than in eyes with a formed vitreous gel undergoing a first vitrectomy (P < .02 lens, P < .003 mid-vitreous).

CONCLUSION

Vitrectomy surgery significantly increases intraocular oxygen tension during and for prolonged periods after surgery. This exposes the crystalline lens to abnormally high oxygen and may lead to nuclear cataract formation.

Expression and Regulation of α-, β-, and γ-Crystallins in Mammalian Lens Epithelial Cells

PURPOSE

In the mammalian lens, the expression of the beta- and gamma-crystallin families is thought to be limited to fiber cells. However, several studies detected these proteins or their mRNAs in human lens epithelial cells. To resolve this apparent discrepancy, 14 crystallin mRNAs were examined and the expression and subcellular distribution of selected crystallin proteins in lens epithelial cells determined.

METHODS

Transcript levels were analyzed by quantitative real-time PCR using mRNA from P3 rat lens epithelia cultured for 0 or 20 hours or 4 or 7 days in basal medium or with added FGF2. Antibodies to betaB1-, gammaS-, alphaA-, and alphaB-crystallins were used for Western blot analysis of proteins extracted from adult mouse, human, bovine, rabbit, and rat lens epithelial and fiber cells. Rat lenses or lens epithelia were rapidly fixed in situ, 30 minutes after death, or after dissection from the lens, and the intracellular distributions of crystallins were examined by immunostaining and confocal microscopy.

RESULTS

Four patterns of crystallin gene expression were detected in cultured lens epithelia. Transcripts encoding most beta- and gamma-crystallins were detectable and, in some cases, abundant at the time of explantation. Changes in crystallin protein levels in P3 epithelia cultured in basal or FGF-supplemented medium generally reflected the changes in their mRNAs. betaB1- and gammaS-crystallins were abundant in adult human, mouse, rat, rabbit, and bovine lens epithelial cells. The alpha-, beta- and gamma-crystallins were found in distinct subcellular locations in adult lens epithelial cells. These proteins dramatically relocalized during fiber cell differentiation and after death and/or dissection of the lens epithelium.

CONCLUSIONS

BetaB1- and gammaS-crystallins are normally abundant in adult mammalian lens epithelial cells. Complex programs of transcription and degradation regulate the accumulation of crystallin mRNAs in lens epithelial cells after stress, at different ages, and during cell differentiation. Because crystallins selectively localize in distinct subcellular compartments during differentiation or stress, they may function to protect lens cells from injury. After stress, most alphaA- and alphaB-crystallin subunits are not in the same macromolecular complexes.

Importance of vitreous liquefaction in age-related cataract

PURPOSE

Vitrectomy is associated with the rapid progression of nuclear cataracts. This study was conducted to test the hypothesis that age-related liquefaction of the vitreous gel may also be associated with lens opacification.

METHODS

Lenses from eye bank eyes were graded for nuclear, cortical, and posterior subcapsular opacities, and the amount of liquid vitreous was measured.

RESULTS

Nuclear sclerosis (NS) grade, percent liquefaction, and age all correlated highly (P < 0.0001). After adjustment was made for age, the correlation between NS and percent liquefaction was 0.37 (P < 0.0001), and the correlation between age and NS after adjustment for liquefaction was 0.57 (P < 0.0001). After the eyes were stratified into age groups, the correlation between NS and percent liquefaction was found to be highest in eyes from donors aged 51 to 70. Cortical and posterior subcapsular opacities increased with age, but scores for these cataract types did not significantly correlate with vitreous liquefaction.

CONCLUSIONS

Loss of vitreous gel during aging increases the risk of nuclear cataracts. Preservation or replacement of the vitreous gel may protect patients from nuclear cataract.

Vascular endothelial growth factor expression and signaling in the lens

PURPOSE

Previous studies have identified sequences encoding vascular endothelial growth factor (VEGF)-A and one of the VEGF receptors (VEGFR2, Flk-1, KDR) in lens fiber cells. The current study was undertaken to determine the distribution of VEGF-A protein in the lens, whether signaling through VEGF receptors occurs in lens cells, the pattern of VEGF-A expression during lens development, and the effect of hypoxia on VEGF-A expression.

METHODS

VEGF-A and VEGFR2 were localized using immunocytochemistry. VEGF-A and VEGFR2 protein were identified and quantified by Western blot analysis. Activated (tyrosine phosphorylated) VEGFR2 was detected by immunoprecipitation with an anti-phosphotyrosine antibody followed by Western blot analysis with antibody to VEGFR2. Levels of VEGF-A mRNA were measured by quantitative PCR. Suturing the lids of adult mouse or rabbit eyes for 3 days was used to induce lens hypoxia.

RESULTS

VEGFR2 sequences were present in adult human lens epithelial cells, and VEGF-A transcripts were detected in chicken embryo, adult human, and mouse lens epithelial cells. VEGF-A protein localized to the ends of mouse embryo lens fiber cells at developmental stages when the fetal vasculature was forming. At later stages, VEGF-A was distributed uniformly throughout the cytoplasm of cortical fiber cells. VEGFR2 was present in mouse lens epithelial and fiber cells and was tyrosine phosphorylated at all stages examined. VEGF-A protein was barely detectable in lens epithelial cells during the first postnatal week, but increased as the capillaries of the anterior pupillary membrane regressed. VEGF-A levels were highest in adult lenses. Suturing the eyelid caused an increase in VEGF-A mRNA and protein in lens epithelial and fiber cells.

CONCLUSIONS

VEGF-A secreted by lens cells may stimulate the formation of the fetal vasculature, but regression of these vessels is not likely to be caused by a reduction in VEGF-A production by the lens. An active VEGF-A signaling system of unknown function appears to be active in the lens. It is likely that VEGF-A expression is regulated by tissue hypoxia at all stages of lens development.

Evaluation of fibroblast growth factor signaling during lens fiber cell differentiation

PURPOSE

Previous studies have implicated members of the fibroblast growth factor (FGF) and insulin-like growth factor (IGF) families as stimulators of lens fiber cell differentiation in rodent and chicken embryo lenses, respectively. In the present study, the role of FGFs in fiber cell differentiation and epithelial cell proliferation in chicken embryos was examined.

METHODS

Lenses were injected on embryonic day (E)3 with replication-defective retroviruses that express full-length or truncated FGF receptor (FGFR)-1 or a secreted form of FGF1. Lens epithelial explants were cultured in defined medium or medium supplemented with FGFs or vitreous humor, in the presence or absence of the FGF receptor antagonist SU5402. Explants were also cultured in vitreous humor that had been depleted of heparin-binding growth factors. Cell elongation was measured optically and protein accumulation by densitometry and Western blot analysis.

RESULTS

Lens fiber cell differentiation was not inhibited in cells infected with virus expressing truncated FGFR1. Epithelial cells infected with virus encoding a secreted form of FGF1 did not differentiate into ectopic fiber cells. Viral transduction of FGFR1, truncated FGFR1, or FGF1 did not appreciably alter the proliferation of lens epithelial cells. Bovine vitreous humor stimulated chicken embryo lens epithelial cells to elongate and express markers of lens fiber cell differentiation. Bovine vitreous humor, but not FGF2, protected lens epithelial cells from apoptosis. Depleting vitreous humor of heparin-binding growth factors or treatment of lens cells with SU5402 did not inhibit the initial, rapid phase of lens cell elongation. Both treatments, used separately or together, reduced but did not prevent the expression of later markers of fiber cell differentiation.

CONCLUSIONS

Fiber differentiation factors that are not members of the FGF family are present in chicken and mammalian vitreous humor. The factors that stimulate fiber cell differentiation in avian and mammalian eyes are similar.

Alpha-tocopherol protects cultured human cells from the acute lethal cytotoxicity of dioxin

The possible protection of cultured human cells from acute dioxin injury by antioxidants was investigated. The most potent dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), caused vacuolization of the smooth endoplasmic reticulum and Golgi apparatus in cultured human conjunctival epithelial cells and cervical cancer cells. Subsequent nuclear damage included a deep irregular indentation resulting in cell death. A dosage of 30-40 ng/mL TCDD induced maximal intracellular production of H2O2 at 30 minutes and led to severe cell death (0-31% survival) at two hours. A dose of 1.7 mM alpha-tocopherol or 1 mM L-dehydroascorbic acid significantly protected human cells against acute TCDD injuries (78-97% survivals), but vitamin C did not provide this protection. These results indicate that accidental exposure to fatal doses of TCDD causes cytoplasmic free radical production within the smooth endoplasmic reticular systems, resulting in severe cytotoxicity, and that vitamin E and dehydroascorbic acid can protect against TCDD-induced cell damage.

Prevalence of primary cornea guttata and morphology of corneal endothelium in aging Japanese and Singaporean subjects

The purpose of the present study was to examine, in the aging people, racial and gender differences in the morphology of corneal endothelium as well as the incidence of cornea guttata in two Asian subject groups, one in Singapore and the other in Japan. Four hundred and sixty-five Chinese Singaporeans and 299 Japanese subjects (residents of Monzen-machi, Ishikawa Prefecture) aged 50 years and older were recruited for the study. Corneal endothelial abnormalities were diagnosed with slit-lamp biomicroscopy and specular microscopy. Primary cornea guttata appeared as beaten metal appearance in slit-lamp images and dark regions in specular images. In addition to cornea guttata, corneal endothelial morphology was also analysed with specular microscopy. The mean cell density was 2,808/mm(2) in the Japanese subjects which was significantly higher than the 2,718/mm(2) seen in the Singaporeans (p < 0.001). The incidence of cornea guttata was significantly higher in the Singaporeans than in the Japanese individuals and also higher in women than in men of both racial groups. These differences support not only a racial and a gender factor but also a possible environmental influence.