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

Retinal blood flow abnormalities following six months of hyperglycemia in the Ins2(Akita) mouse

The aim of this study was to characterize the microvascular flow abnormalities and oxygenation changes that are present following six months of hyperglycemia in the diabetic Ins2(Akita) mouse. Previous studies have shown decreased retinal blood flow in the first several weeks of hyperglycemia in rodents, similar to the decreases seen in the early stages of human diabetes. However, whether this alteration in the mouse retina continues beyond the initial weeks of diabetes has yet to be determined, as are the potential consequences of the decreased flow on retinal oxygenation. In this study, male Ins2(Akita) and age-matched C57BL/6 (non-diabetic) mice were maintained for a period of six months, at which time intravital microscopy was used to measure retinal blood vessel diameters, blood cell velocity, vascular wall shear rates, blood flow rates, and transient capillary occlusions. In addition, the presence of hypoxia was assessed using the oxygen-sensitive probe pimonidazole. The diabetic retinal microvasculature displayed decreases in red blood cell velocity (30%, p<0.001), shear rate (25%, p<0.01), and flow rate (40%, p<0.001). Moreover, transient capillary stoppages in flow were observed in the diabetic mice, but rarely in the non-diabetic mice. However, no alterations were observed in retinal hypoxia as determined by a pimonidazole assay, suggesting the possibility that the decreases seen in retinal blood flow may be dictated by a decrease in retinal oxygen utilization.

Low glucose under hypoxic conditions induces unfolded protein response and produces reactive oxygen species in lens epithelial cells

Aging is enhanced by hypoxia and oxidative stress. As the lens is located in the hypoglycemic environment under hypoxia, aging lens with diabetes might aggravate these stresses. This study was designed to examine whether low glucose under hypoxic conditions induces the unfolded protein response (UPR), and also if the UPR then generates the reactive oxygen species (ROS) in lens epithelial cells (LECs). The UPR was activated within 1 h by culturing the human LECs (HLECs) and rat LECs in <1.5 mM glucose under hypoxic conditions. These conditions also induced the Nrf2-dependent antioxidant-protective UPR, production of ROS, and apoptosis. The rat LECs located in the anterior center region were the least susceptible to the UPR, whereas the proliferating LECs in the germinative zone were the most susceptible. Because the cortical lens fiber cells are differentiated from the LECs after the onset of diabetes, we suggest that these newly formed cortical fibers have lower levels of Nrf2, and are then oxidized resulting in cortical cataracts. Thus, low glucose and oxygen conditions induce the UPR, generation of ROS, and expressed the Nrf2 and Nrf2-dependent antioxidant enzymes at normal levels. But these cells eventually lose reduced glutathione (GSH) and induce apoptosis. The results indicate a new link between hypoglycemia under hypoxia and impairment of HLEC functions.

From oxygen to erythropoietin: relevance of hypoxia for retinal development, health and disease

Photoreceptors and other cells of the retina consume large quantities of energy to efficiently convert light information into a neuronal signal understandable by the brain. The necessary energy is mainly provided by the oxygen-dependent generation of ATP in the numerous mitochondria of retinal cells. To secure the availability of sufficient oxygen for this process, the retina requires constant blood flow through the vasculature of the retina and the choroid. Inefficient supply of oxygen and nutrients, as it may occur in conditions of disturbed hemodynamics or vascular defects, results in tissue ischemia or hypoxia. This has profound consequences on retinal function and cell survival, requiring an adaptational response by cells to cope with the reduced oxygen tension. Central to this response are hypoxia inducible factors, transcription factors that accumulate under hypoxic conditions and drive the expression of a large variety of target genes involved in angiogenesis, cell survival and metabolism. Prominent among these factors are vascular endothelial growth factor and erythropoietin, which may contribute to normal angiogenesis during development, but may also cause neovascularization and vascular leakage under pathologically reduced oxygen levels. Since ischemia and hypoxia may have a role in various retinal diseases such as diabetic retinopathy and retinopathy of prematurity, studying the cellular and molecular response to reduced tissue oxygenation is of high relevance. In addition, the concept of preconditioning with ischemia or hypoxia demonstrates the capacity of the retina to activate endogenous survival mechanisms, which may protect cells against a following noxious insult. Part of these mechanisms is the local production of protective factors such as erythropoietin. Due to its plethora of effects in the retina including neuro- and vaso-protective activities, erythropoietin has gained strong interest as potential therapeutic factor for retinal degenerative diseases.

Intraocular oxygen distribution in advanced proliferative diabetic retinopathy

PURPOSE

To determine the preretinal distribution of oxygen in advanced proliferative diabetic retinopathy, and to investigate the relationship between intraocular oxygen tensions and vitreous cytokine concentrations.

DESIGN

Comparative cross-sectional study.

METHODS

Oxygen levels were measured at sites in the vitreous and at the inner retinal surface using an optical oxygen sensor in 14 control subjects and in 14 subjects with advanced proliferative diabetic retinopathy who had developed tractional retinal detachments despite previous panretinal photocoagulation. The vitreous and plasma concentrations of 42 cytokines were measured using multiplex cytokine arrays and their correlation with intraocular oxygen tension was investigated.

RESULTS

The mean oxygen tension in the mid-vitreous in diabetic retinopathy was 46% lower than that in control subjects (P = .017). However, the mean preretinal oxygen tension at the posterior pole in diabetic retinopathy was 37% higher than in controls (P = .039). We measured significant alterations in the vitreous concentrations of 9 cytokines-eotaxin, Flt-3 ligand, growth-related oncogene (GRO), interleukin (IL)-6, IL-8, IL-9, IFN-inducible protein-10 (IP-10), macrophage-derived cytokine (MDC), and vascular endothelial growth factor (VEGF)-in advanced proliferative diabetic retinopathy, and found that oxygen tension at the posterior pole was directly correlated with vitreous VEGF concentration.

CONCLUSION

We identified significant intraocular oxygen gradients in proliferative diabetic retinopathy. Our findings are consistent with the hypothesis that VEGF induces the development of neovascular complexes in the posterior retina that are richly perfused but nonetheless fail to redress hypoxia in the mid-vitreous. Upregulation of vitreous VEGF may be a consequence of retinal hypoxia at unidentified sites or of chronic inflammatory processes in advanced proliferative diabetic retinopathy.

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.

Increase in retinal hypoxia-inducible factor-2α, but not hypoxia, early in the progression of diabetes in the rat

Hypoxia and the associated hypoxia-inducible factors (HIFs) may be influential in the progression of diabetic retinopathy. However, little is known of the extent of hypoxia and the levels of HIFs early in the progression of the disease. In the current study, we injected the oxygen-dependent probe pimonidazole (Hypoxyprobe™-1) into diabetic rats, and also performed immunohistochemistry to determine the retinal levels of HIF-1α and HIF-2α. The rats were made diabetic using a single injection of streptozotocin (STZ; 60 mg/kg), with vehicle-injected rats used as non-diabetic controls. The measurements of hypoxia and HIF levels were obtained three weeks following STZ injection, at which time we have previously found significant decreases in retinal blood flow in the same model. In the current experiments, no increases in either HIF-1α or hypoxia were observed in the diabetic rats (compared with controls), and there was even a tendency for hypoxia levels to be decreased (tissue more highly oxygenated). However, we did observe an increase in HIF-2α in the retinas of the diabetic rats. Therefore, we conclude that early diabetes-induced increases in HIF-2α occur independently of hypoxia.

The tumor suppressor gene Trp53 protects the mouse lens against posterior subcapsular cataracts and the BMP receptor Acvr1 acts as a tumor suppressor in the lens

We previously found that lenses lacking the Acvr1 gene, which encodes a bone morphogenetic protein (BMP) receptor, had abnormal proliferation and cell death in epithelial and cortical fiber cells. We tested whether the tumor suppressor protein p53 (encoded by Trp53) affected this phenotype. Acvr1 conditional knockout (Acvr1^CKO) mouse fiber cells had increased numbers of nuclei that stained for p53 phosphorylated on serine 15, an indicator of p53 stabilization and activation. Deletion of Trp53 rescued the Acvr1^CKO cell death phenotype in embryos and reduced Acvr1-dependent apoptosis in postnatal lenses. However, deletion of Trp53 alone increased the number of fiber cells that failed to withdraw from the cell cycle. Trp53^CKO and Acvr1;Trp53^DCKO (double conditional knockout), but not Acvr1^CKO, lenses developed abnormal collections of cells at the posterior of the lens that resembled posterior subcapsular cataracts. Cells from human posterior subcapsular cataracts had morphological and molecular characteristics similar to the cells at the posterior of mouse lenses lacking Trp53. In Trp53^CKO lenses, cells in the posterior plaques did not proliferate but, in Acvr1;Trp53^DCKO lenses, many cells in the posterior plaques continued to proliferate, eventually forming vascularized tumor-like masses at the posterior of the lens. We conclude that p53 protects the lens against posterior subcapsular cataract formation by suppressing the proliferation of fiber cells and promoting the death of any fiber cells that enter the cell cycle. Acvr1 acts as a tumor suppressor in the lens. Enhancing p53 function in the lens could contribute to the prevention of steroid- and radiation-induced posterior subcapsular 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.

A novel role of gap junction connexin46 protein to protect breast tumors from hypoxia

Connexin proteins are the principle structural components of the gap junctions. Colocalization and tissue-specific expression of diverse connexin molecules are reported to occur in a variety of organs. Impairment of gap junctional intercellular communication, caused by mutations, gain of function or loss of function of connexins, is involved in a number of diseases including the development of cancer. Here we show that human breast cancer cells, MCF-7 and breast tumor tissues express a novel gap junction protein, connexin46 (Cx46) and it plays a critical role in hypoxia. Previous studies have shown that connexin46 is predominantly expressed in lens and our studies find that Cx46 protects human lens epithelial cells from hypoxia induced death. Interestingly, we find that Cx46 is upregulated in MCF-7 breast cancer cells and human breast cancer tumors. Downregulation of Cx46 by siRNA promotes 40% MCF-7 cell death at 24 hr under hypoxic conditions. Furthermore, direct injection of anti-Cx46 siRNA into xenograft tumors prevents tumor growth in nude mice. This finding will provide an exciting new direction for drug development for breast cancer treatment and suggests that both normal hypoxic tissue (lens) and adaptive hypoxic tissue (breast tumor) utilize the same protein, Cx46, as a protective strategy from hypoxia.

Diabetic patients with retinopathy show increased retinal venous oxygen saturation

BACKGROUND

Longstanding diabetes mellitus results in a disturbed microcirculation. A new imaging oximeter was used to investigate the effect of this disturbance on retinal vessel oxygen saturation.

METHODS

The haemoglobin oxygen saturation was measured in the retinal arterioles and venules of 41 diabetic patients (65 +/- 12.3 years) with mild non-proliferative through proliferative diabetic retinopathy (DR). Twelve individuals (61.3 +/- 6.2 years, mean +/- standard deviation) without systemic or ocular disease were investigated as controls. Measurements were taken by an imaging oximeter (oxygen module by Imedos GmbH, Jena). This technique is based on the proportionality of the oxygen saturation and ratio of the optical density of the vessel at two wavelengths (548 nm and 610 nm).

RESULTS

Whereas there were no significant differences in the arterial oxygen saturation between controls and diabetic retinopathy at any stage, the venous oxygen saturation increased in diabetic patients with the severity of the retinopathy: controls 63 +/- 5%, mild non-proliferative DR 69 +/- 7%, moderate non-proliferative DR 70 +/- 5%, severe non-proliferative DR, 75 +/- 5%, and proliferative DR 75 +/- 8%.

CONCLUSIONS

The increase of retinal vessel oxygen saturation in diabetic retinopathy points to a diabetic microvascular alteration. This may be due to occlusions and obliterations in the capillary bead and the formation of arterio-venous shunt vessels. On the other hand, hyperglycaemia-induced endothelial dysfunction, with subsequent suppression of the endothelial NO-synthase and disturbance of the vascular auto-regulation, may contribute to retinal tissue hypoxia.

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.

Measurement of PO2 during vitrectomy for central retinal vein occlusion, a pilot study

INTRODUCTION

In this pilot study the effects of vitrectomy on PO(2) in the vitreous cavity in CRVO were investigated.

STUDY DESIGN

Prospective, controlled, interventional study.

METHOD

Six patients with ischaemic CRVO in one eye (undergoing vitrectomy for radial optic neurotomy, RON) and six with either macula hole or membrane were included. An oxygen probe was inserted before removal of the vitreous (pre-vitrectomy) and after removal of the vitreous (post-vitrectomy). In the patients with CRVO, measurements were taken before RON was performed. Oxygenation recordings (PO(2)) were taken in the mid-vitreous cavity and the preretinal vitreous.

RESULTS

Mean age was 65 years. In controls, pre-vitrectomy, the mean PO(2) adjacent to the retina (15.0 mmHg S.D.5.7) was significantly less than mid-cavity (33.7 mmHg S.D.12.8). Similarly in CRVO, the pre-vitrectomy pre-retinal PO(2) (8.1 mmHg S.D. 3.5), was significantly less than mid-cavity (19.8 mmHg S.D.7.3). The mean PO(2) was significantly less in the eyes with CRVO than in control eyes. Post-vitrectomy, the PO(2) was significantly greater than pre-vitrectomy at both recording sites in the controls mid-cavity (61.5 mmHg S.D.13.9) and pre-retinal (75.8 mmHg S.D. 9.1), and CRVO eyes mid-cavity (53.7 mmHg S.D. 17.9) and pre-retinal (59.8 mmHg S.D. 15.8).

CONCLUSION

PO(2) is reduced in the vitreous cavity in CRVO. Vitrectomy may be a method of increasing oxygen availability to the retina.

Physiology of vitreous surgery

Vitreous surgery has various physiological and clinical consequences, both beneficial and harmful. Vitrectomy reduces the risk of retinal neovascularization, while increasing the risk of iris neovascularization, reduces macular edema and stimulates cataract formation. These clinical consequences may be understood with the help of classical laws of physics and physiology. The laws of Fick, Stokes-Einstein and Hagen-Poiseuille state that molecular transport by diffusion or convection is inversely related to the viscosity of the medium. When the vitreous gel is replaced with less viscous saline, the transport of all molecules, including oxygen and cytokines, is facilitated. Oxygen transport to ischemic retinal areas is improved, as is clearance of VEGF and other cytokines from these areas, thus reducing edema and neovascularization. At the same time, oxygen is transported faster down a concentration gradient from the anterior to the posterior segment, while VEGF moves in the opposite direction, making the anterior segment less oxygenated and with more VEGF, stimulating iris neovascularization. Silicone oil is the exception that proves the rule: it is more viscous than vitreous humour, re-establishes the transport barrier to oxygen and VEGF, and reduces the risk for iris neovascularization in the vitrectomized-lentectomized eye. Modern vitreous surgery involves a variety of treatment options in addition to vitrectomy itself, such as photocoagulation, anti-VEGF drugs, intravitreal steroids and release of vitreoretinal traction. A full understanding of these treatment modalities allows sensible combination of treatment options. Retinal photocoagulation has repeatedly been shown to improve retinal oxygenation, as does vitrectomy. Oxygen naturally reduces VEGF production and improves retinal hemodynamics. The VEGF-lowering effect of photocoagulation and vitrectomy can be augmented with anti-VEGF drugs and the permeability effect of VEGF reduced with corticosteroids. Starling's law explains vasogenic edema, which is controlled by osmotic and hydrostatic gradients between vessel and tissue. It explains the effect of VEGF-induced vascular permeability changes on plasma protein leakage and the osmotic gradient between vessel and tissue. At the same time, it takes into account hemodynamic changes that affect the hydrostatic gradient. This includes the influence of arterial blood pressure, and the effect oxygen (laser treatment) has in constricting retinal arterioles, increasing their resistance, and thus reducing the hydrostatic pressure in the microcirculation. Reduced capillary hydrostatic pressure and increased osmotic gradient reduce water fluxes from vessel to tissue and reduce edema. Finally, Newton's third law explains that vitreoretinal traction decreases hydrostatic tissue pressure in the retina, increases the pressure gradient between vessel and tissue, and stimulates water fluxes from vessel into tissue, leading to edema.

Hypoxia-regulated activity of PKCepsilon in the lens

PURPOSE

To show that hypoxia is necessary to prevent opacification of the lens. Protein kinase C (PKC)-epsilon serves a role that is distinct from PKC-gamma when both PKC isoforms are expressed in the lens. PKCepsilon serves a very important role in hypoxic conditions, helping to prevent opacification of the lens.

METHODS

Digital image analysis, confocal microscopy, dye transfer assay, coimmunoprecipitation, Western blot analysis, and enzyme activity assays were used, respectively, to study opacification of the lens, intercellular communications, cellular localization of connexin-43 (Cx43), and the interactions between PKCepsilon, PKCgamma, and Cx43 in the lens epithelial cells.

RESULTS

Hypoxic conditions (1%-5% of oxygen) were very important in maintaining clarity of the lenses of wild-type (WT) mice. Normoxic conditions induced opacification of the WT lens. Lenses from the PKCepsilon-knockout mice underwent rapid opacification, even in hypoxic conditions. Hypoxia did not induce apoptosis in the lens epithelial cells, judging by the absence of active caspase-3, and it did not change intercellular communication and did not affect the number and localization of junctional Cx43 plaques in the lens epithelial cell culture. Hypoxia activated PKCepsilon, whereas phorbol ester (TPA), oxidation (H(2)O(2)), and insulin-like growth factor-1 (IGF-1) activated PKCgamma and decreased the activity of PKCepsilon. Hypoxia did not induce the phosphorylation of the Cx43.

CONCLUSIONS

Hypoxia-induced activation of PKCepsilon is very important in surviving hypoxia and maintaining the clarity of the lens. However, PKCgamma is utilized in the control of Cx43 gap junctions.

Vitreoretinal management of the complications of sickle cell retinopathy by observation or pars plana vitrectomy

PURPOSE

To describe the management of vitreoretinal complications of sickle cell retinopathy.

DESIGN

A retrospective interventional case series.

METHODS

Review of an electronic patient record and clinical notes of 27 patients with vitreoretinal complications of sickle retinopathy.

RESULTS

Six male patients and 21 female patients presented with a mean age of 41 years (range 28-67), 12 left eyes and 16 right eyes. The mean follow-up was 15.5 months (range: 3-60). Two patients were SS, and the remainders were doubly heterozygous (SC). In all, 10 were observed without surgery- three with vitreous haemorrhage, four tractional retinal detachments (TRD), and three rhegmatogenous retinal detachments (RRD). Two patients demonstrated spontaneous flattening of the retina- one RRD and one TRD. Eighteen eyes had pars plana vitrectomies (PPVs)- seven with vitreous haemorrhage, three RRD, three TRD, three ERM (one bilateral), and two macular holes. In all, 15 patients (83%) had improved vision postoperatively. The mean logMar preoperative visual acuity was 1.07 (Snellen equivalent 6/70), SD 0.62 was significantly improved postoperatively (mean 0.42 (6/15), SD 0.48, P=0.001).

CONCLUSIONS

Sickle retinopathy occasionally presents with vitreoretinal complications. These can often be observed and may spontaneously regress. If surgery is required eyes respond to vitrectomy procedures with segmentation of sea fan proliferation.

Pars plana vitrectomy with internal limiting membrane peeling for diabetic macular edema

PURPOSE

To evaluate anatomic and visual acuity (VA) results of pars plana vitrectomy (PPV) with internal limiting membrane (ILM) peeling for diffuse diabetic macular edema (DME), and to review the literature on the topic.

METHODS

Retrospective noncomparative case series of patients who underwent PPV with ILM peeling for diffuse DME between January 1, 2000, and December 1, 2005, performed by three surgeons at Bascom Palmer Eye Institute. Main outcome measures included pre- and postoperative optical coherence tomography (OCT) and visual acuity. Mean follow-up period was 8 months (range, 43 days-2 years).

RESULTS

Twenty-four eyes of 23 patients meeting the criteria were evaluated. Duration of DME ranged from 1 to 93 months. Mean preoperative logMAR vision was 0.782 (range, 0.30-1.82). Mean logMAR visual acuity at final follow-up was 0.771 (range, 0.10-2.00). At last follow-up, 25% of eyes had > or =2 line increase in VA from baseline, 54% of eyes had no improvement in VA, and 21% of eyes had > or =2 line decrease in VA. Of 9 eyes with pre- and postoperative OCT, there was an overall reduction in central macular thickness of 141 microm at postoperative month 3 and 120 microm at last follow-up. Postoperative complications included progression of cataract in 6 (60%) of 10 phakic eyes, postoperative intraocular pressure > or =30 mmHg in 6 (24%) eyes, and postoperative vitreous hemorrhage in 2 (8%) eyes.

CONCLUSIONS

Pars plana vitrectomy with ILM peeling was associated with a reduction in DME when measured by OCT in the majority of eyes, but visual acuity outcomes showed minimal improvement compared to baseline. These results suggest the efficacy of PPV with ILM peeling for eyes with DME has not been well established and should be reserved for therapy with selected cases.

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.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Maintaining transparency: a review of the developmental physiology and pathophysiology of two avascular tissues

The lens and cornea are transparent and usually avascular. Controlling nutrient supply while maintaining transparency is a physiological challenge for both tissues. During sleep and with contact lens wear the endothelial layer of the cornea may become hypoxic, compromising its ability to maintain corneal transparency. The mechanism responsible for establishing the avascular nature of the corneal stroma is unknown. In several pathological conditions, the stroma can be invaded by abnormal, leaky vessels, leading to opacification. Several molecules that are likely to help maintain the avascular nature of the corneal stroma have been identified, although their relative contributions remain to be demonstrated. The mammalian lens is surrounded by capillaries early in life. After the fetal vasculature regresses, the lens resides in a hypoxic environment. Hypoxia is likely to be required to maintain lens transparency. The vitreous body may help to maintain the low oxygen level around the lens. The hypothesis is presented that many aspects of the aging of the lens, including increased hardening, loss of accommodation (presbyopia), and opacification of the lens nucleus, are caused by exposure to oxygen. Testing this hypothesis may lead to prevention for nuclear cataract and insight into the mechanisms of lens aging. Although they are both transparent, corneal pathology is associated with an insufficient supply of oxygen, while lens pathology may involve excessive exposure to oxygen.

A chronic grey matter penumbra, lateral microvascular intussusception and venous peduncular avulsion underlie diabetic vitreous haemorrhage

The landmark publications that gave such impetus to our understanding of proliferative diabetic retinopathy are reviewed in the light of more recent reports. Briefly, confluence of small areas of capillary closure in the midperipheral and peripheral retina results in arteriovenous shunting and abnormal oxygen partial pressure gradients. These gradients embrace a chronic ischaemic penumbra that stimulates neuroglial secretion of angiogenic growth factors and upregulation of their receptors in the retinal venous endothelium and adventitia. The blood shunting produces biomechanical stresses within the veins and induces microvascular intussusception near arteriovenous crossings, giving way to neovascular outgrowths and/or segmental venous lesions (such as omega loops and coils) that penetrate the inner limiting lamina. The lamellar collagenous matrix of the vitreous cortex is then exploited for integrin-dependent rete expansion along chemotactic gradients. During posterior vitreous detachment, haemorrhaging takes place from the arterialised veins as venous neovascular peduncles are avulsed.