On the pathogenesis of diabetic retinopathy

Retinal capillary basement membrane thickening: Role in the pathogenesis of diabetic retinopathy

Vascular basement membrane (BM) thickening has been hailed over half a century as the most prominent histological lesion in diabetic microangiopathy, and represents an early ultrastructural change in diabetic retinopathy (DR). Although vascular complications of DR have been clinically well established, specific cellular and molecular mechanisms underlying dysfunction of small vessels are not well understood. In DR, small vessels develop insidiously as BM thickening occurs. Studies examining high resolution imaging data have established BM thickening as one of the foremost structural abnormalities of retinal capillaries. This fundamental structural change develops, at least in part, from excess accumulation of BM components. Although BM thickening is closely associated with the development of DR, its contributory role in the pathogenesis of DR is coming to light recently. DR develops over several years before clinical manifestations appear, and it is during this clinically silent period that hyperglycemia induces excess synthesis of BM components, contributes to vascular BM thickening, and promotes structural and functional lesions including cell death and vascular leakage in the diabetic retina. Studies using animal models show promising results in preventing BM thickening with subsequent beneficial effects. Several gene regulatory approaches are being developed to prevent excess synthesis of vascular BM components in an effort to reduce BM thickening. This review highlights current understanding of capillary BM thickening development, role of BM thickening in retinal vascular lesions, and strategies for preventing vascular BM thickening as a potential therapeutic strategy in alleviating characteristic lesions associated with DR.

Distinct CD40L receptors mediate inflammasome activation and secretion of IL-1β and MCP-1 in cultured human retinal pigment epithelial cells

CD40L signaling occurs in several diseases with inflammatory components, including ocular and retinal diseases. However, it has never been evaluated as a pathogenic mechanism in age-related macular degeneration (AMD) or as an inducer of inflammasome formation in any cell type. mRNA and protein levels of CD40, IL-1β, NALP1, NALP3, caspase-1, and caspase-5 were determined by RT-PCR, qPCR, and Western blot. CD40L receptor (CD40, α5β1, and CD11b) expression was determined by Western and immunofluorescent staining. IL-1β, IL-18, and MCP-1 secretions were determined by ELISA. NALP1 and NALP3 inflammasome formation were determined by Co-IP. Experiments were conducted on primary human retinal pigment epithelial (hRPE) cells from four different donors. Human umbilical vein endothelial (HUVEC) and monocytic leukemia (THP-1) cells demonstrated the general applicability of our findings. In hRPE cells, CD40L-induced NALP1 and NALP3 inflammasome activation, cleavage of caspase-1 and caspase-5, and IL-1β and IL-18 secretion. Interestingly, neutralizing CD11b and α5β1 antibodies, but not CD40, reduced CD40L-induced IL-1β secretion in hRPE cells. Similarly, CD40L treatment also induced HUVEC and THP-1 cells to secret IL-1β through CD11b and α5β1. Additionally, the CD40L-induced IL-1β secretion acted in an autocrine/paracrine manner to feed back and induce hRPE cells to secrete MCP-1. This study is the first to show that CD40L induces inflammasome activation in any cell type, including hRPE cells, and that this induction is through CD11b and α5β1 cell-surface receptors. These mechanisms likely play an important role in many retinal and non-retinal diseases and provide compelling drug targets that may help reduce pro-inflammatory processes.

Fluocinolone acetonide for the treatment of diabetic macular edema

In addition to VEGF inhibitors such as ranibizumab, aflibercept or bevacizumab, clinical and experimental investigations have revealed the great potential of steroids in the treatment of DME. At present two intravitreal steroid inserts are approved for the treatment of DME containing either dexamethasone or fluocinolone acetat (FA) as a pharmacological compound. The non degradable intravitreal FA insert releases 0.2 µg FA per day (Iluvien, Alimera Sciences). Clinical phase III studies have demonstrated the beneficial effect of the FA insert to last up to three years, especially in patients with a prolonged history of DME of at least three years at the initiation of therapy. While the treatment appears to be well tolerated over all, side effects such as cataract formation in nearly all treated phakic patients and raise of intraocular pressure need to be discussed with the patients as potential complications of the treatment.

Fluocinolone acetonide and its potential in the treatment of chronic diabetic macular edema

Diabetic macular edema (DME) is a potentially sight-threatening disease that predominantly affects patients with type 2 diabetes. The pathogenesis is complex, with many contributing factors involved. In addition to overexpression of vascular endothelial growth factor in the diabetic eye, there is an inflammatory pathway that contributes to the breakdown of the blood-retina barrier and nonperfusion. In addition to vascular endothelial growth factor inhibitors, clinical and experimental investigations underline the great potential of steroids in the treatment of DME. Fluocinolone acetonide is currently the only corticosteroid approved for the treatment of DME in Europe. It is manufactured as an intravitreal insert, releasing fluocinolone acetonide at a rate of 0.2 μg per day. Phase III clinical studies have demonstrated that the beneficial effect of the fluocinolone acetonide insert lasts up to 3 years. Improvement in visual acuity was especially remarkable in patients with a prolonged duration of DME of at least 3 years at the initiation of therapy. Cataract formation occurs in nearly all phakic eyes treated, and needs to be considered when the indication for treatment is made. Given the efficacy versus potential complications of the insert, fluocinolone acetonide represents a promising second-line treatment option in patients with DME. Fluocinolone appears to be especially beneficial for patients whose options for visual recovery have seemed limited up until now.

Retinopathy in diabetes

With the incidence, and prevalence of diabetes mellitus increasing worldwide, diabetic retinopathy is expected to reach epidemic proportions. The aim of this chapter is to introduce diabetic retinopathy, a leading cause of blindness in people of the working age. The clinical course of retinopathy, anatomical changes, its pathogenesis and current treatment are described, followed by an overview of the emerging drug therapies for the potential treatment of this sight-threatening complication of diabetes.

A three-pore model describes transport properties of bovine retinal endothelial cells in normal and elevated glucose

PURPOSE

Changes in blood vessel barrier properties contribute to retinal edema in diabetic retinopathy (DR). However, limited data are available to describe the routes of transport for fluids and solutes across the inner blood-retinal barrier (iBRB). In this study, a three-pore model was developed to characterize such routes in normal and elevated glucose levels.

METHODS

Diffusive and apparent permeabilities to TAMRA (467 Da), dextran (70 kDa), and LDL (2000 kDa), as well as hydraulic conductivity, were measured across bovine retinal endothelial cell (BREC) monolayers after exposure to normal- and high-glucose media for 6 days. The data were used to develop a model of transport dynamics. Claudin 5 and eNOS Western blot analysis were used to measure changes in expression and phosphorylation. Immunolocalization of ZO-1 and VE-cadherin demonstrated organization of the junctional complex. Apoptosis was measured by TUNEL assay.

RESULTS

A three-pore model describes the fractional transport of water and molecular tracers across the retinal endothelial barrier. No change in permeability or hydraulic conductivity was observed after exposure to high glucose, whereas VEGF increased permeability in both normal- and high-glucose environments. The transport results were consistent with ZO-1 and VE-cadherin immunocytochemistry and expression of claudin-5, which were all unaltered by high glucose.

CONCLUSIONS

The data describe, for the first time, a model for transport of various size solutes and fluids across endothelial cells of the iBRB. Further, the results support the existence of an indirect pathway by which iBRB permeability is increased through the upregulation of retinal VEGF in response to hyperglycemia.

Diabetic eNOS-knockout mice develop accelerated retinopathy

PURPOSE

Dysfunction of endothelial nitric oxide synthase (eNOS) has been implicated in the pathogenesis of diabetic vascular complications. This study was undertaken to determine the role of eNOS in the development of diabetic retinopathy (DR), by investigating the functional consequences of its deficiency in the diabetic state.

METHODS

Diabetes was induced in eNOS-knockout (eNOS(-/-)) and C57B/6 mice by streptozotocin (STZ) injection. Retinal vasculature was evaluated by albumin extravasation, to quantitatively measure vascular permeability, and by trypsin-digested retinal vascular preparations, to quantify acellular capillaries. Gliosis was evaluated by immunofluorescent techniques. Retinal capillary basement membrane thickness was assessed by transmission electron microscopy. Total retinal nitric oxide level was assessed by measuring nitrate/nitrite using a fluorometric-based assay, iNOS expression was examined by real-time PCR.

RESULTS

Diabetic eNOS(-/-) mice exhibit more severe retinal vascular permeability than age-matched diabetic C57BL/6 mice, detectable as early as 3 weeks after diabetes induction. Diabetic eNOS(-/-) mice also show earlier onset and an increased number of acellular capillaries, sustained gliosis, and increased capillary basement membrane thickness. Total nitric oxide (NO) level was also increased, concomitant with elevated iNOS expression in diabetic eNOS(-/-) retina.

CONCLUSIONS

Diabetic eNOS(-/-) mice exhibit A significantly wider range of advanced retinal vascular complications than the age-matched diabetic C57BL/6 mice, supporting the notion that eNOS-derived NO plays an essential role in retinal vascular function. This mouse model also faithfully replicates many of the hallmarks of vascular changes associated with human retinopathy, thus providing a unique model to aid in understanding the pathologic mechanisms of and to develop effective therapeutic strategies for diabetic retinopathy.

Diabetic macular edema: pathogenesis and treatment

Diabetic macular edema is a major cause of visual impairment. The pathogenesis of macular edema appears to be multifactorial. Laser photocoagulation is the standard of care for macular edema. However, there are cases that are not responsive to laser therapy. Several therapeutic options have been proposed for the treatment of this condition. In this review we discuss several factors and mechanisms implicated in the etiology of macular edema (vasoactive factors, biochemical pathways, anatomical abnormalities). It seems that combined pharmacologic and surgical therapy may be the best approach for the management of macular edema in diabetic patients.

A multifocal electroretinogram model predicting the development of diabetic retinopathy

The prevalence of diabetes has been accelerating at an alarming rate in the last decade; some describe it as an epidemic. Diabetic eye complications are the leading cause of blindness in adults aged 25-74 in the United States. Early diagnosis and development of effective preventatives and treatments of diabetic retinopathy are essential to save sight. We describe efforts to establish functional indicators of retinal health and predictors of diabetic retinopathy. These indicators and predictors will be needed as markers of the efficacy of new therapies. Clinical trials aimed at either prevention or early treatments will rely heavily on the discovery of sensitive methods to identify patients and retinal locations at risk, as well as to evaluate treatment effects. We report on recent success in revealing local functional changes of the retina with the multifocal electroretinogram (mfERG). This objective measure allows the simultaneous recording of responses from over 100 small retinal patches across the central 45 degrees field. We describe the sensitivity of mfERG implicit time measurement for revealing functional alterations of the retina in diabetes, the local correspondence between functional (mfERG) and structural (vascular) abnormalities in eyes with early nonproliferative retinopathy, and longitudinal studies to formulate models to predict the retinal sites of future retinopathic signs. A multivariate model including mfERG implicit time delays and 'person' risk factors achieved 86% sensitivity and 84% specificity for prediction of new retinopathy development over one year at specific locations in eyes with some retinopathy at baseline. A preliminary test of the model yielded very positive results. This model appears to be the first to predict, quantitatively, the retinal locations of new nonproliferative diabetic retinopathy development over a one-year period. In a separate study, the predictive power of a model was assessed over one- and two-year follow-ups. This permitted successful prediction of new retinopathy development in eyes with and without retinopathy at baseline. Finally, we briefly describe our current research efforts to (a) locally predict future sight-threatening diabetic macular edema, (b) investigate local retinal function change in adolescent patients with diabetes, and (c) better understand the physiological bases of the mfERG delays. The ability to predict the retinal locations of future retinopathy based on mfERG implicit time provides clinicians a powerful tool to screen, follow-up, and even consider early prophylactic treatment of the retinal tissue in diabetic patients. It also aids identification of 'at risk' populations for clinical trials of candidate therapies, which may greatly reduce their cost by decreasing the size of the needed sample and the duration of the trial.

Perspectives on diabetic retinopathy

PURPOSE

To review the evolution of the understanding of diabetic retinopathy (DR) and methods of treating DR, the present clinically relevant practice for eye disease in patients with diabetes mellitus, and trends in clinical patient care over the next 3 to 5 years.

DESIGN

Tabular review and presentation of clinical trials contributing to the understanding and treatment of DR and the author's philosophy of care for the patient with diabetes.

RESULTS

Diabetic retinopathy is a microvascular complication of diabetes mellitus that is a significant cause of new-onset blindness. The Diabetic Retinopathy Study was the first multicentered, randomized, clinical trial in ophthalmology. This study provided the scientific evidence for treatment of DR with scatter (panretinal) photocoagulation, and led to the funding of other multicentered clinical trials, including the Early Treatment Diabetic Retinopathy Study, which greatly elucidated the natural history of DR and firmly established laser photocoagulation as treatment for proliferative diabetic retinopathy (PDR) and diabetic macular edema. The Diabetes Control and Complications Trial and United Kingdom Prospective Diabetes Study established the value of intensive glycemic control in reducing the risk of onset and progression of DR and other microvascular complications of diabetes.

CONCLUSIONS

Severe vision loss and moderate vision loss from diabetes are essentially preventable with timely detection and treatments, careful long-term follow-up and comprehensive diabetes mellitus care firmly based on clinical evidence. Future treatments, as outgrowths of further understanding of the biochemical basis of the disease, will aim at curing or preventing retinal complications from diabetes.

Inhibition of advanced glycation end-products protects against retinal capillary basement membrane expansion during long-term diabetes

The purpose of this study was to investigate the advanced glycation end-product (AGE)-inhibitory properties of aminoguanidine and to determine whether treatment in long-term diabetic rats can prevent basement membrane lesions of diabetic retinopathy. Four groups of male Wistar rats were studied: untreated diabetics injected with 45 mg/kg streptozotocin, aminoguanidine-treated diabetics, untreated controls, and aminoguanidine-treated controls. After 12 months' diabetes, the retinas from six animals were processed for electron microscopy or the retinal microvasculature was isolated using the trypsin digest technique. Stereological analysis was used to estimate quantitative ultrastructural changes in the retinal capillary-associated basement membrane. Serum AGEs were quantified by competitive AGE-ELISA, while microvascular-associated, immunoreactive AGEs were analysed on retinal trypsin digests. Aminoguanidine significantly reduced serum AGEs in the diabetic group (p < 0.001). In the retinal capillaries, there was a marked reduction in AGE immunoreactivity in the aminoguanidine-treated diabetics when compared with untreated diabetics. The surface area and absolute volume of the retinal capillary basement membrane were significantly increased in the diabetic rats when compared with non-diabetic controls (p < 0.001 and p < 0.001, respectively). Aminoguanidine treatment of diabetic rats protected against basement membrane expansion when compared with untreated diabetic counterparts. Aminoguanidine treatment prevents the development of diabetes-induced basement membrane expansion in retinal capillaries. The AGE inhibition properties of aminoguanidine suggest that AGEs play an important role in the complex pathogenesis of basement membrane thickening during diabetic retinopathy.

Retinal nerve fibre layer loss in patients with type 1 diabetes mellitus without retinopathy

BACKGROUND/AIM

There is evidence suggesting the occurrence of neurovisual abnormalities in patients with diabetes without retinopathy. However, the determination of abnormalities in the neural and glial elements in vivo is difficult. The aim of this study was to investigate whether a retinal nerve fibre layer (RNFL) defect (as determined by scanning laser polarimetry, SLP) is present in patients without clinical manifestations of diabetic retinopathy.

METHODS

12 patients with type 1 diabetes mellitus (DM) without retinopathy or other diabetes induced microvascular complications, underwent a complete ophthalmological examination, including automated perimetry and RNFL measurements with a nerve fibre layer analyser GDx. The data were compared with a normal control group matched for age and sex.

RESULTS

The superior segment retardation in patients with diabetes was lower than in the control group, based on the superior integral (0.19 (SD 0.06) v 0.23 (0.04) mm(2), p=0.03) and the superior average (71.0 (11.05) v 84.27 (10.56) microm, p=0.007) parameters.

CONCLUSION

This finding may be indicative of significant nerve fibre loss in the superior segment of the retina in patients with type 1 diabetes mellitus but without retinopathy. The meaning of intraretinal differences in RNFL retardation, indicating asymmetric NFL loss, in patients with diabetes is yet not understood.

The effect of antioxidants on glycated albumin-induced cytotoxicity in bovine retinal pericytes

Loss of retinal pericytes is the initial deficit in the early stage of diabetic retinopathy. Glycated albumin (GA) forms under hyperglycemic conditions and exists in the retinal blood vessels of diabetic patients with retinopathy. In this study, using a 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) reduction test, we investigated whether GA induces cytotoxicity in cultured bovine retinal pericytes, and whether the antioxidants, l-ascorbic acid, Trolox, and probucol, provide any protection from GA-mediated cytotoxicity. GA induced pericyte death in a dose-dependent manner. With increasing time, GA-induced cytotoxicity also increased despite no strict time dependence. Furthermore, this cell death was found to be mediated both by apoptosis, which was confirmed by apoptosis-specific fluorescent staining of nuclei and cell membranes, and mitochondrial damage, as elucidated by electron microscopy. All three antioxidants used in this study partially protected against GA-induced pericyte death, suggesting that oxidative stress plays a role in GA-induced pericyte death. The results indicate that GA induces cell death in cultured bovine retinal pericytes, and that certain antioxidants may reduce this cytotoxicity.

Intravitreal VEGF and bFGF produce florid retinal neovascularization and hemorrhage in the rabbit

PURPOSE

Vascular endothelial growth factor (VEGF) causes widespread retinal vascular dilation, produces breakdown of the blood-retinal barrier, and is implicated in ocular neovascularization (NV). Basic fibroblast growth factor (bFGF) also has been implicated in the production of ocular NV. This study was performed to investigate the ability of simultaneous sustained intravitreal release of both VEGF and bFGF to induce robust retinal NV in the rabbit.

METHODS

Intravitreal implantation of sustained-release Hydron polymeric pellets containing both 20 microg of VEGF and 20 microg of bFGF was performed on adult male Dutch belted rabbits. In other animals either 20 microg or 50 microg bFGF-containing pellets was implanted intravitreally; also, either 20 microg VEGF or 50 microg VEGF-containing pellets was implanted. Control rabbits received either blank polymeric pellets or a pellet containing 30 microg bovine serum albumin. Eyes were examined by indirect ophthalmoscopy after surgery at 24 hrs, 48 hrs, 4 days, 7 days, 14 days, 21 days, and 28 days. Findings were documented by color fundus photography and fluorescein angiography (FA). Eyes were enucleated and prepared for histologic analysis at 28 days following intravitreal implantation of the VEGF/bFGF-containing pellets.

RESULTS

In all eyes implanted with VEGF/bFGF pellets, dilation and tortuosity of existing blood vessels were observed within 48 hrs after pellet implantation. The progression of retinal vascular changes was rapid and occurred over the entire optic disk and medullary rays between 4 and 7 days. Hemorrhage occurred as early as 14 days after VEGF/bFGF pellet implantation. In eyes with massive hemorrhage, total traction retinal detachment developed after the second week. The presence of abnormal tissues at the vitreo-retinal interface within 28 days was demonstrated by light microscopy while FA showed profuse leakage of dye from anomalous vessels within the first week. Neither bFGF-exposed eyes nor control eyes showed any vascular changes. Eyes that received only VEGF-containing pellets exhibited tortuosity of existing vessels, but neither hemorrhaging nor retinal detachment occurred.

CONCLUSIONS

These results demonstrate that retinal vascular changes leading to hemorrhaging is produced rapidly in the rabbit by simultaneous intravitreal release of both VEGF and bFGF. Understanding how these growth factors induce retinal NV may suggest novel therapeutic treatment strategies.

The pathogenesis of edema in diabetic maculopathy

Diabetic maculopathy is characterized by the accumulation of extracellular fluid in Henle's layer and the inner nuclear layer of the retina. The localization of the edema is likely to be due, in part, to the relative barrier properties of the inner and outer plexiform layers. The origin of the extracellular fluid is from the intravascular compartment. Although changes to retinal blood flow may partly explain the extravasation of fluid, the most important mechanism is breakdown of the blood retinal barriers. Both the inner blood retinal barrier formed by the retinal capillary endothelial cell tight junctions and the outer barrier formed by the retinal pigment epithelial cell tight junctions can be affected. The mechanism of breakdown of the blood retinal barriers is likely to be changes to the tight junction proteins including occludin and ZO-1. The biochemical messenger inducing these changes may be vascular endothelial growth factor. The origin of this or other cofactors may be the retinal glial cells. The underlying biochemical stimulus to the production of vascular endothelial growth factor is chronic hyperglycaemia, but it is uncertain by what pathway this is effected.

Vascular endothelial growth factor increases release of gelatinase A and decreases release of tissue inhibitor of metalloproteinases by microvascular endothelial cells in vitro

The present study was designed to determine the influences of vascular endothelial growth factor (VEGF) on cell proliferation and the release of matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) from human dermal microvascular endothelial cells. Treatment of cultures with 10 ng/ml or more of VEGF significantly increased cell proliferation. The effect of VEGF treatment on the levels of specific MMPs and TIMPs in the media was subsequently examined in cultures that were treated with 10 ng/ml VEGF. Zymography and Western blot analyses demonstrated that gelatinase A levels in the media were increased by VEGF treatment. Collagenase was detected by Western blots in both VEGF-treated and untreated culture media, but the levels were not significantly increased by the VEGF treatment. An ELISA assay confirmed that VEGF treatment significantly increased gelatinase A levels but did not significantly increase collagenase levels. Western blot and ELISA data showed that VEGF treatment significantly decreased TIMP-1 and TIMP-2 levels compared to untreated cultures. The data suggest that VEGF may modulate endothelial cell-derived MMP activity by: (1) increasing the abundance of gelatinase A; (2) disinhibiting gelatinase A by decreasing the abundance of TIMP-2; and (3) disinhibiting preexisting collagenase by reducing levels of TIMP-1. These actions could contribute to the ability of VEGF to promote endothelial cell invasion of new territory.

Intrachoroidal microvascular abnormality: a previously unrecognized form of choroidal neovascularization

PURPOSE

To evaluate the histopathologic and histochemical characteristics of intrachoroidal microvascular abnormality.

METHODS

Forty eyes obtained at autopsy from human donors ranging in age from 20 to 91 years (25 diabetics, 15 nondiabetics) were analyzed. Choroids were processed for alkaline phosphatase flat-embedding. Vascular patterns were examined and analyzed before embedding and serial sectioning.

RESULTS

Intrachoroidal microvascular abnormality had the most prominent alkaline phosphatase reaction product of choroidal vessels. These formations appeared as ameboid or cobweb-like vascular networks (area, 0.05 to 4.6 mm2) in the choroidal stroma external to the choriocapillaris. They appeared as both single or groups of formations in the posterior pole and equatorial regions in all subjects. Intrachoroidal microvascular abnormality was found in five subjects with diabetes: four with type I diabetes mellitus, and one with type II diabetes mellitus. One subject had proliferative diabetic retinopathy, three had background retinopathy, and one had no retinopathy. Intrachoroidal microvascular abnormality was connected with all levels of choroidal vasculature. Microaneurysms were observed within intrachoroidal microvascular abnormality formations in most subjects but not in other choroidal vessels.

CONCLUSIONS

Intrachoroidal microvascular abnormality is a form of intrachoroidal neovascularization. This neovascularization has features similar to intraretinal microvascular abnormalities found in diabetic subjects but seems to form independently of the status of retinopathy. The presence of microaneurysms in intrachoroidal microvascular abnormalities and not other choroidal vessels supports the view that aneurysms may be aborted attempts at neovascularization.

Heparan sulfate proteoglycan synthesis and its expression are decreased in the retina of diabetic rats

PURPOSE

Heparan sulfate proteoglycan (HSPG) is an integral component of all basement membranes and is implicated in the charge-selective properties of these basement membranes. Studies have shown a decrease in the number of HSPG anionic sites in the retinal capillary and glomerular basement membrane of diabetic animals. This study examined whether a decrease in both HSPG synthesis and its expression as perlecan are reduced in the retina of streptozotocin-diabetic rats.

METHODS

Diabetes was induced in male Wistar rats by intraperitoneal injection of streptozotocin and the rats were sacrificed 20 weeks later. Retinas were isolated and HSPG synthesis was assessed by incorporation of 35S-sulfate into heparan sulfate. The expression of mRNA for perlecan was quantified by RNase protection assay.

RESULTS

Both the synthesis of HSPG and mRNA expression for perlecan were decreased in the retina of diabetic compared to normal rats.

CONCLUSIONS

The decrease in HSPG synthesis may account for the reported decrease in retinal basement membrane anionic sites and increased capillary permeability that occurs in diabetes.

Diabetic retinopathy in pregnancy

Diabetic retinopathy is the most common chronic complication associated with diabetes mellitus. It affects 20% to 30% of diabetic women in the reproductive age group. This article reviews the course and treatment of pregnancy complicated by diabetic retinopathy.

Disappearance and formation rates of microaneurysms in early diabetic retinopathy

AIM

To analyse the formation and disappearance rates of individual microaneurysms in mild background retinopathy.

METHODS

Three fluorescein angiograms were taken at 1 year intervals during a 2 year follow up from 24 type 1 diabetics with mild background retinopathy. Microaneurysms were identified and localised twice from each angiogram using a computerised system for retrieval of the coordinates for each microaneurysm. Microaneurysms identified similarly in both sessions were then processed further to obtain rates of microaneurysm formation and disappearance, and microaneurysm count changes.

RESULTS

In the whole material the total number of microaneurysms increased from 298 to 436 from baseline to 2 years. During the 2 year follow up 395 new microaneurysms formed and 258 microaneurysms disappeared. Of the microaneurysms present at baseline 174 (58%) were still present at the 1 year and 142 (48%) at the 2 year follow up. In patients with good glucose control (HbA1c < or = 7.5 mmol) microaneurysm formation showed a trend of being decreased whereas microaneurysm disappearance did not correlate with glucose control.

CONCLUSION

Background diabetic retinopathy is a dynamic process. A significant proportion of microaneurysms present disappear within 2 years. This is compensated for by formation of new microaneurysms, the resultant net changes in microaneurysm counts being small. Microaneurysm formation and disappearance rates are new variables of diabetic retinopathy and may prove to be more sensitive indicators of the progression patterns of background diabetic retinopathy than microaneurysm count changes.

Effects of extracellular matrix and Bruch's membrane on retinal outer segment phagocytosis by cultured human retinal pigment epithelium

The goal of this study was to determine the effect of extracellular matrix components on the phagocytic function of the retinal pigment epithelial (RPE) cell. Cultured human fetal RPE cells were established in culture and plated on three commercially-prepared substrates: collagen IV, fibronectin and laminin and on three native matrices: bovine corneal endothelial cell matrix (BCEM) denuded bovine Bruch's membrane and denuded human Bruch's membrane. Cultured cells were allowed to become confluent and maintained for an additional two weeks before uptake of fluorescent bovine retinal outer segments (ROS) was measured by flow cytometry. Morphology by phase contrast microscopy and melanization was also determined as measures of differentiation. The results showed that morphology, melanization and ROS uptake by cells on collagen IV, laminin and fibronectin were not different from control cells plated on tissue culture plastic. However, ROS uptake by cells plated on BCEM was significantly less than that of cells cultured on plastic and melanization was greater. ROS uptake by cells plated on both types of Bruch's membrane was also significantly less than control cells. Treatment of cells plated on tissue culture plastic with 44 mM NaHCO3, which increased melanization, also reduced ROS uptake. We conclude that native matrices seem to contain components that significantly depress ROS uptake in culture. The inhibition is not mimicked by collagen, laminin or fibronectin coated wells. The ECM may play a significant role in controlling phagocytosis of ROS either by determining morphology, increasing differentiation or by directly influencing intracellular metabolism, and thus serve as another level of control for this RPE function which may not occur in cells plated on tissue culture plastic. These results may also have implications for the effects of aging or disease in which there are changes in the ECM.

Non-retinovascular leakage in diabetic maculopathy

BACKGROUND

Diabetic macular oedema is the leading cause of vision deterioration in diabetic retinopathy. Extracellular fluid within the retina, which distorts the retinal architecture, was assumed to be strictly of retinal vasculature origin. However, there is some experimental evidence supporting clinical observations suggesting a possible role of the retinal pigment epithelium (RPE). An unusual form of diabetic maculopathy is presented in which the RPE and the subretinal space play the main role.

METHODS

Fluorescein angiograms of 1850 non-proliferative diabetic retinopathy (NPDR) patients were examined. Nineteen eyes (14 patients, 1% of NPDR patients) met the criteria-mainly having minimal diabetic retinopathy with only a few microaneurysms and no clinically significant macular oedema (CSMO). Early phase angiograms were compared with late phase angiograms.

RESULTS

It was found that in all 19 eyes the area of diffuse RPE late phase leakage was spread around the macular area. No cystic changes or cystoid macular oedema were present in any of the eyes and the visual acuity was 6/10 or better in all the eyes.

CONCLUSIONS

Possible changes occur in the RPE that may be responsible for the late leakage in NPDR patients-namely, diabetic retinal pigment epitheliopathy. These changes are associated with a breakdown of the outer blood-retinal barrier, consisting of leakage through RPE cells. No focal or diffuse leakage across the RPE has been reported in the literature related to the fluorescein angiograms in diffuse diabetic maculopathy.

Histological and ultrastructural investigation of retinal microaneurysm development in diabetic patients

BACKGROUND

Although microaneurysms are a clinicopathological hallmark of diabetic retinopathy, there have been few ultrastructural studies of these important lesions. As a result, knowledge of the mechanisms involved in the pathogenesis of microaneurysms remains fragmentary. This study provides histological and ultrastructural evidence of various stages in microaneurysm formation within the retinal vasculature.

METHODS

The eyes of three type II diabetic patients, obtained within 24 hours of death, were studied by the trypsin digest technique. Eyes from two further type II diabetics were fixed in 2.5% glutaraldehyde within 12 hours of death and processed for electron microscopy.

RESULTS

In the trypsin digest preparations, small saccular and fusiform microaneurysms were observed in the peripheral retinal. In the central retina, the microaneurysms ranged in morphology from thin walled, cellular forms to dense, acellular, hyalinised forms. Ultrastructurally, four distinct groups of microaneurysm were observed. Type I showed an extensive accumulation of polymorphonuclear cells into the lumen. The endothelium remained intact, although pericytes were invariably absent. Type II microaneurysms were typified by large numbers of red blood cells (RBCs) in the lumen. Endothelial cells and pericytes were completely absent. The type III microaneurysm was also non-perfused and contained aggregates of irregularly shaped RBC profiles and RBC breakdown products. Recanalisation by new vessels into the occluded lumen was observed in one microaneurysm. Type IV microaneurysms were almost or completely sclerosed, with extensive fibrosis and lipid infiltration into the lumen and basement membrane wall.

CONCLUSION

This investigation describes several distinctive stages in the formation of microaneurysms during diabetic retinopathy. With reference to the pathogenesis of retinal microaneurysms, the interaction of various cell types is discussed and the significance of vascular cell death and localised hypertensive events highlighted.

Presence and further development of retinal dysfunction after 3-year follow up in IDDM patients without angiographically documented vasculopathy

Abnormalities in neuroretinal function may play a role in the development of diabetic retinopathy. The natural course of diabetic retinal dysfunction in a group of subjects with insulin-dependent diabetes mellitus and with no apparent microvascular alterations in the retina was followed-up with fluorescein angiography and a sensitive electrophysiological technique, i.e., steady-state focal electroretinogram at the macula, for 3 years. Before the beginning and throughout our study, strict glycaemic control was maintained by three or four daily insulin injections under careful monitoring. Analysis of macular electroretinogram provided information from different neural layers. At the first examination, functional activities of postreceptoral neurons were significantly decreased with respect to those of age-matched control subjects. Diabetic patients showed a functional loss of both ganglion cell (0.53 +/- 0.09 vs 0.42 +/- 0.11 microV; t = 5; p = 0.0001) and preganglion cell (0.51 +/- 0.13 vs 0.42 +/- 0.14 microV; t = 2.8; p = 0.007) layers. Diabetes did not alter photoreceptor activity. After 3 years, dysfunction was significantly greater in the preganglion cell layer (0.28 +/- 0.11 microV; t = 6.3; p = 0.0001). Although in some patients further impairment of ganglion cell function was shown, no significant difference was found in 3 years. Photoreceptor function remained unaltered. No vascular abnormalities in the retina were noted after 3 years in this group of patients. Metabolic control was not correlated to functional changes. Our findings suggest that the middle retinal layer is the most sensitive physiological locus of progressive diabetes-induced dysfunction in the absence of angiographically documented abnormalities.

Growth factors in retinal diseases: proliferative vitreoretinopathy, proliferative diabetic retinopathy, and retinal degeneration

The goal of this review is to present the current knowledge on specific growth factor involvement in posterior segment eye disease. Growth factors can be defined as multifunctional signals which modify cell growth or proliferation, alone or in concert, by binding to specific cell surface receptors. Their biological effects on cells include cell adhesion, migration, survival, differentiation, extracellular matrix secretion, protease and protease inhibitor release, production of other growth activities, and angiogenesis. Growth factors couple the cell to the microenvironment. As some growth factors are soluble mediators of wound repair and angiogenesis, it seems possible that proliferative vitreoretinopathy and proliferative diabetic retinopathy are caused or aggravated by these factors. Other factors act as survival factors and can possibly prevent retinal degeneration. The multifunctional nature of growth factors makes it probable that practical uses will be found for these agents in the future.

Aldose reductase inhibitors and diabetic complications

Aldose reductase inhibitors impede flux of glucose through the sorbitol pathway in diabetes mellitus. They therefore reduce the accumulation of the pathway metabolites, sorbitol and fructose, reduce the impact of the flux on the cofactors used by the pathway and reduce other derived phenomena, such as osmotic stress and myo-inositol depletion. As drugs, their targets are the chronic complications of diabetes--neuropathy, retinopathy, nephropathy and vasculopathy. In experimental models there is proof of activity against biochemical, functional and structural defects in all of the involved tissues, but we await full clinical verification of this potential.

Aldose reductase inhibitors as pathobiochemical probes

In tissues susceptible to damage from chronic diabetes, excess glucose is metabolized by aldose reductase (AR) to sorbitol. Originally, AR-catalyzed sorbitol formation (and accumulation) was found in the diabetic lens; the cataractogenicity of this process was proven by preventing cataract formation with an AR inhibitor (ARI). These findings were extended to the hypothesis that, in diabetic tissues, excessive intracellular sorbitol formation initiates a cascade of metabolic abnormalities which gradually progress to loss of functional and structural integrity. The pivotal role of AR as a trigger for such abnormalities was established by preventing their occurrence in diabetic animals treated with an ARI. By inference, this led to the concept that inhibition of AR should prevent, arrest, and, possibly, reverse the development of late diabetic sequelae. In addition to motivating drug-oriented research, the ARI concept provided a rationale for the use of ARIs as experimental tools to probe the pathogenesis of diabetic complications. By helping to elucidate the metabolic, functional, and structural ramifications of the AR-catalyzed disposal of excess glucose in diabetic schemes, and in addition, by helping to define the applicability of animal models for the study of early functional pathogenic alterations occurring in diabetic subjects, ARIs may enable the discrimination in diabetic tissue of arrestible and reversible from the irreversible abnormalities.

The effects of an aldose reductase inhibitor on the progression of diabetic retinopathy

The polyol pathway has long been associated with diabetic retinopathy. Glucose is converted to sorbitol with the aid of the enzyme aldose reductase. Aldose reductase inhibitors can prevent changes induced by diabetes. A total of 30 patients with minimal background retinopathy were randomly divided into a ponalrestat-taking group and a placebo-taking group. All were followed for 6 months and twenty-three were followed for 12 months. The baseline microaneurysm count was 2.6 +/- 1.9 (mean +/- SD) for the ponalrestat group and 3.5 +/- 2.9 for the placebo group. At 6 months the counts were 3.1 +/- 3.5 and 2.9 +/- 3.6 and after 12 months 3.0 +/- 4.1 and 2.9 +/- 3.4. There is no statistically significant difference between the groups at 0, 6 or 12 months of study. The change in retinopathy severity level did not significantly differ between the two groups at either 6 or 12 months. Ponalrestat administration at a dosage of 600 mg daily for 12 months has no significant effect on the course of minimal retinopathy in diabetic patients.

On the pathogenesis of diabetic retinopathy. A 1990 update

Although most investigators now agree that chronic hyperglycemia is the basis for diabetic retinopathy, this has not been proven definitively. Even if chronic hyperglycemia is the initial common pathway leading to retinopathy and other complications of diabetes, it appears to act by different mechanisms in different tissues. The enzyme, aldose reductase, may play a major role in the development of diabetic retinopathy, but contradictory evidence exists. At the present time, results of the only study of aldose reductase inhibition and diabetic retinopathy reported in humans were negative. Another mechanism worthy of consideration is nonenzymatic glycation (glycosylation) of proteins, but there is no direct evidence of a causal role in diabetic retinopathy. Several growth factors have been identified in the retina that may promote neovascularization, and at least two inhibitors may prevent the process. There is evidence to support a role for basic and, perhaps, acidic fibroblast growth factors in retinal vasoproliferation. Transforming growth-factor beta, a peptide produced by capillary pericytes and smooth muscle cells and activated by the interaction of these cells with vascular endothelial cells, appears to be an important inhibitor of neovascularization, as is the vascular basement membrane.

Human growth hormone stimulates proliferation of human retinal microvascular endothelial cells in vitro

Growth hormone (GH) has been implicated in the pathogenesis of proliferative diabetic retinopathy. We sought to determine whether this could be mediated by an effect of GH on proliferation of endothelial cells, and, for this purpose, established long-term cultures of human retinal microvascular endothelial cells (hREC) from normal postmortem human eyes. High-purity (greater than 95%) hREC preparations were selected for experiments, based on immunofluorescence with acetylated low density lipoprotein (LDL) and anti-factor VIII-related antigen. Growth requirements for these cells were complex, including serum for maintenance at slow growth rates and additional mitogens for more rapid proliferation. Exposure of hREC to physiologic doses of human GH (hGH) resulted in 100% greater cell number vs. control (P less than 0.01) but could be elicited only in the presence of serum. When differing serum conditions were compared, hGH stimulated [3H]thymidine incorporation up to 1.6- to 2.2-fold under each condition and increased DNA content significantly in the presence of human, horse, and fetal calf serum. Depending on the culture conditions used, the threshold hGH concentration for significant stimulation of hREC proliferation was 0.4-4 micrograms/liter. In contrast, proliferation of human umbilical vein endothelial cells was not significantly altered by hGH added to concentrations as high as 200 micrograms/liter. In summary, hREC respond to physiologic concentrations of hGH in vitro with enhanced proliferation. This specific effect of GH on retinal microvascular endothelial cells supports the hypothesis of a role for GH in endothelial cell biology.

The epidemiology and prevalence of diabetic retinopathy in the Veneto region of north east Italy. Veneto Group for Diabetic Retinopathy

The prevalence of diabetic retinopathy and its relationship to a number of risk factors were examined in a population-based study in the Veneto region of North East Italy. Of 1321 diabetic patients selected, 98% attended for examination. Prevalence of diabetic retinopathy was 26.2% (24.4% background and 1.8% proliferative). The prevalence of retinopathy was significantly related (p less than 0.01) to the duration of diabetes (17.3% for less than 5 years; 60.8% for greater than 20 years). Proliferative retinopathy was much more prevalent after 20 years of diabetes. After 10 years most proliferative retinopathy was found in Type 1 diabetic patients, but before 10 years from diagnosis it was most prevalent in Type 2 diabetes. The prevalence of retinopathy was significantly related (p less than 0.001) to the type of diabetes and was found predominantly in Type 1 (46.2%) and insulin-treated Type 2 (45.9%) subjects and to a lesser degree in non-insulin-treated patients (24.6%). The prevalence of retinopathy was significantly related to both fasting and post-prandial blood glucose levels (p less than 0.001), blood urea nitrogen (p less than 0.05), and systolic (p less than 0.001) and diastolic (p less than 0.01) blood pressure. No significant differences were found in the prevalence of total or proliferative retinopathy between males and females. No significant relationships were found with family history of diabetes, alcohol intake, smoking habits, cholesterol, triglycerides, and serum uric acid.

A non-mammalian in vivo model for cellular and molecular analysis of glucose-mediated thickening of basement membranes

An increase of basement membrane thickness in renal glomeruli, blood vessels, and other tissues is a consistent pathological observation in individuals with diabetes mellitus. Although a number of pathological complications of the disease are thought to result from this structural abnormality in basement membranes, the mechanism(s) responsible for this glucose-mediated process remain unknown. The current study was designed to develop a non-mammalian in vivo epithelial/basement membrane model which would facilitate detailed analysis of the cellular and molecular processes which lead to thickening of basement membrane under hyperglycaemic conditions. The system developed utilizes the Cnidarian, Hydra vulgaris. Hydra lends itself to such studies because of (1) its simplified body structure which is composed of an epithelial bilayer with an intervening basement membrane (mesoglea) and (2) its extensive regenerative capacity which allows cell pellets (Hydra cell aggregates), formed from isolated Hydra cells, to develop into adult Hydra within 72-96 h. This process involves reformation of an epithelial bilayer and de novo biosynthesis of a basement membrane. Our studies indicate that exposure of developing Hydra cell aggregates to levels of D-glucose which mimic that observed in the human diabetic patient (15 mmol/l) induces a doubling of Hydra basement membrane thickness within 72-96 h of pellet formation. The same results were obtained using 15 mmol/l D-Ribose which is a highly efficient glycating agent. The data presented support the use of the Hydra cell aggregate system as a potentially powerful non-mammalian in vivo model to investigate the cellular and molecular mechanism(s) underlying glucose-mediated basement membrane thickening.

Progression of diabetic retinopathy after pancreas transplantation. The University of Michigan Pancreas Transplant Evaluation Committee

The progression of diabetic retinopathy after combined pancreatic and kidney transplantation was studied in eight patients for 12 to 49 months. Four patients who had rapid pancreatic graft failure constituted a control group for comparison with four patients who retained functioning grafts. Using Fisher's exact probability test, the authors found no posttransplantation difference between the two groups in visual acuity lost, severity of diabetic macular edema, extent of capillary closure, progression of preretinal gliosis, development of disc or preretinal neovascularization, or worsening of the severity of the retinopathy. Achievement of normoglycemia by pancreatic transplantation is not effective in halting the progression of diabetic retinopathy in patients who already have severe diabetic microangiopathy joined the current follow-up.

Diabetic choroidal and iris vasculature scanning electron microscopy findings

Scanning Electron Microscopy (SEM) was used to study vascular casts of twenty-four autopsy eyes taken from patients with long-standing insulin-dependent Diabetes Mellitus. These casts were compared to casts of ten 'normal' autopsy eyes from patients without a history of diabetes or other vascular disease. The SEM findings in the choroidal vessels of the diabetic eyes included: increased tortuosity, focal vascular dilations and narrowings, hypercellularity, vascular loops and microaneurysm formation, 'drop-out' of choriocapillaries, and sinus-like structure formation between choroidal lobules in the equatorial area. In the iris, neovascularization was evident in the vascular casts in cases with clinically recognized rubeosis iridis. These findings indicate that there is significant involvement of the uveal tract in diabetic eyes. The present study strongly supports the Hidayat and Fine light microscopic observation that the diabetic choroid demonstrates significant vascular changes (e.g. narrowed vessels with possible 'drop-out' of capillaries and neovascularization). Changes in the diabetic choroid, especially in the choriocapillaris, may be a contributing factor in diabetic retinopathy, resulting in decreased oxygenation of the outer layer of the retina. Short reviews and updated information of diabetic eye disease provide some additional insights into the vascular problems in the eye.

Recent developments in the understanding and management of diabetic retinopathy

The natural history of diabetic retinopathy, one of the leading causes of visual impairment in the United States, is well described; its pathogenesis, however, is poorly understood. The Diabetic Retinopathy Study and the Early Treatment of Diabetic Retinopathy Study have demonstrated that timely intervention with photocoagulation prevents visual loss. However, recent studies demonstrate that many diabetic patients are not being referred to ophthalmologists for timely treatment. Suggested management and referral strategies are presented in this article.

Pancreatic islet allograft prevents basement membrane thickening in the diabetic rat retina

Progressive basement membrane thickening is a characteristic structural abnormality in diabetic tissues including the retina. We examined the effect of pancreatic islet allotransplantation on basement membrane thickening and irregularities in retinal capillaries of the streptozotocin-diabetic rat. Diabetic animals received intraportal or intracerebral pancreatic islet allografts. Animals with functioning allografts demonstrated euglycaemia and a normal body weight gain during the 400-day post-transplantation period. The characteristic thickening of capillary basement membranes was completely prevented in animals with successful transplantation. The present findings suggest that islet allotransplantation may be a rational therapeutic approach in the treatment of diabetes mellitus and the prevention of ensuing secondary complications.

Galactose and cataract

Galactosemia is a disorder caused by a deficiency of any one of three possible enzymes involved in the metabolism of galactose: galactokinase, transferase or epimerase. Any single deficient enzyme can result in cataract through the accumulation of galactitol in the lens. The ophthalmologist may play an important role in this disease, since early recognition of cataract development followed by the initiation of a galactose-free diet may lead to clearing of lenticular opacities. The clinical and laboratory findings that distinguish the three enzyme deficiency disorders of galactosemia are discussed. The biochemical genetics of each enzyme also are reviewed, along with the recent evidence linking heterozygous galactokinase deficiency and presenile cataract.

Pathogenetic heterogeneity in retinal capillary basement membrane thickening in the diabetic BB-rat

In the present study, we compared the effects of good blood glucose control and that of aldose reductase inhibitor (Statil) treatment on diabetic retinal capillary basement membrane thickening in the diabetic BB-rat. Aggressive insulin treatment maintaining euglycaemia for 4 months prevented the characteristic basement membrane thickening in both the superficial and deep capillary beds of the diabetic retina. Statil treatment of hyperglycaemic rats for the same length of time prevented basement membrane thickening in the deep capillary bed but not in the superficial capillaries.

Proteinuria and blood glucose levels in a population with diabetic retinopathy

In a population-based study of all insulin-treated diabetic patients on the Swedish island of Gotland, we compared the prevalence and severity of retinopathy with those of nephropathy as measured by proteinuria and serum creatinine levels. Of 365 diabetic patients, 66 (18%) had proteinuria. Of these 66, 39 (59%) had retinopathy. Proteinuria and serum creatinine correlated with increasing severity of retinopathy (P less than .001). Of 47 patients with proliferative retinopathy, 19 (40%) had proteinuria. Of 124 patients with retinopathy of other grades of severity, 20 (16%) had proteinuria. Visual acuity in the best eye was negatively correlated to proteinuria, which was present in 17 of 203 (8.4%) patients with a visual acuity of 20/20, compared with eight of 15 (53%) of those with a visual acuity of 20/200 or less. Blood glucose, determined two hours postprandially, was satisfactory (less than 10 mmol/l) in 162 patients (44%), unsatisfactory (10 to 14 mmol/l) in 89 (24%), and poorly regulated (greater than 14 mmol/l) in 114 (31%). Increasing mean blood glucose correlated to retinopathy (P less than .05).

Prevention of diabetic retinal capillary pericyte degeneration and loss by pancreatic islet allograft

Degeneration and loss of retinal capillary pericytes are characteristic features of diabetic retinopathy. The effect of pancreatic islet cell allotransplantation across a major histocompatibility barrier on diabetic pericyte changes was examined in streptozotocin induced diabetic rats 14 months post-transplantation. Morphological studies of the retinal microvasculature were carried out using qualitative and quantitative morphological techniques. We describe significant prevention of pericyte degeneration and loss after successful allotransplantation suggesting that pancreatic islet allograft is a promising therapeutical approach in the prevention of diabetic retinopathy.