Overview of studies on metabolic and vascular regulatory changes in early diabetic retinopathy

Current understanding of subclinical diabetic retinopathy informed by histology and high-resolution in vivo imaging

The escalating incidence of diabetes mellitus has amplified the global impact of diabetic retinopathy. There are known structural and functional changes in the diabetic retina that precede the fundus photography abnormalities which currently are used to diagnose clinical diabetic retinopathy. Understanding these subclinical alterations is important for effective disease management. Histology and high-resolution clinical imaging reveal that the entire neurovascular unit, comprised of retinal vasculature, neurons and glial cells, is affected in subclinical disease. Early functional manifestations are seen in the form of blood flow and electroretinography disturbances. Structurally, there are alterations in the cellular components of vasculature, glia and the neuronal network. On clinical imaging, changes to vessel density and thickness of neuronal layers are observed. How these subclinical disturbances interact and ultimately manifest as clinical disease remains elusive. However, this knowledge reveals potential early therapeutic targets and the need for imaging modalities that can detect subclinical changes in a clinical setting.

The Protective Effects of Neurotrophins and MicroRNA in Diabetic Retinopathy, Nephropathy and Heart Failure via Regulating Endothelial Function

Diabetes mellitus is a common disease affecting more than 537 million adults worldwide. The microvascular complications that occur during the course of the disease are widespread and affect a variety of organ systems in the body. Diabetic retinopathy is one of the most common long-term complications, which include, amongst others, endothelial dysfunction, and thus, alterations in the blood-retinal barrier (BRB). This particularly restrictive physiological barrier is important for maintaining the neuroretina as a privileged site in the body by controlling the inflow and outflow of fluid, nutrients, metabolic end products, ions, and proteins. In addition, people with diabetic retinopathy (DR) have been shown to be at increased risk for systemic vascular complications, including subclinical and clinical stroke, coronary heart disease, heart failure, and nephropathy. DR is, therefore, considered an independent predictor of heart failure. In the present review, the effects of diabetes on the retina, heart, and kidneys are described. In addition, a putative common microRNA signature in diabetic retinopathy, nephropathy, and heart failure is discussed, which may be used in the future as a biomarker to better monitor disease progression. Finally, the use of miRNA, targeted neurotrophin delivery, and nanoparticles as novel therapeutic strategies is highlighted.

Recent trends in drug-delivery systems for the treatment of diabetic retinopathy and associated fibrosis

Diabetic retinopathy is a frequent microvascular complication of diabetes and a major cause of visual impairment. In advanced stages, the abnormal neovascularization can lead to fibrosis and subsequent tractional retinal detachment and blindness. The low bioavailability of the drugs at the target site imposed by the anatomic and physiologic barriers within the eye, requires long term treatments with frequent injections that often compromise patient's compliance and increase the risk of developing more complications. In recent years, much effort has been put towards the development of new drug delivery platforms aiming to enhance their permeation, to prolong their retention time at the target site and to provide a sustained release with reduced toxicity and improved efficacy. This review provides an overview of the etiology and pathophysiology of diabetic retinopathy and current treatments. It addresses the specific challenges associated to the different ocular delivery routes and provides a critical review of the most recent developments made in the drug delivery field.

Importance of the Use of Oxidative Stress Biomarkers and Inflammatory Profile in Aqueous and Vitreous Humor in Diabetic Retinopathy

Diabetic retinopathy is one of the leading causes of visual impairment and morbidity worldwide, being the number one cause of blindness in people between 27 and 75 years old. It is estimated that ~191 million people will be diagnosed with this microvascular complication by 2030. Its pathogenesis is due to alterations in the retinal microvasculature as a result of a high concentration of glucose in the blood for a long time which generates numerous molecular changes like oxidative stress. Therefore, this narrative review aims to approach various biomarkers associated with the development of diabetic retinopathy. Focusing on the molecules showing promise as detection tools, among them we consider markers of oxidative stress (TAC, LPO, MDA, 4-HNE, SOD, GPx, and catalase), inflammation (IL-6, IL-1ß, IL-8, IL-10, IL-17A, TNF-α, and MMPs), apoptosis (NF-kB, cyt-c, and caspases), and recently those that have to do with epigenetic modifications, their measurement in different biological matrices obtained from the eye, including importance, obtaining process, handling, and storage of these matrices in order to have the ability to detect the disease in its early stages.

Perturbed Biochemical Pathways and Associated Oxidative Stress Lead to Vascular Dysfunctions in Diabetic Retinopathy

Diabetic retinopathy (DR) is a vascular insult that accompanies the hyperglycemic state. Retinal vasculature holds a pivotal role in maintaining the integrity of the retina, and any alteration to retinal vasculature affects retinal functions. The blood retinal barrier, a prerequisite to vision acuity, is most susceptible to damage during the progression of DR. This is a consequence of impaired biochemical pathways such as the polyol, advanced end glycation products (AGE), hexosamine, protein kinase C (PKC), and tissue renin-angiotensin system (RAS) pathways. Moreover, the role of histone modification and altered miRNA expression is also emerging as a major contributor. Epigenetic changes create a link between altered protein function and redox status of retinal cells, creating a state of metabolic memory. Although various biochemical pathways underlie the etiology of DR, the major insult to the retina is due to oxidative stress, a unifying factor of altered biochemical pathways. This review primarily focuses on the critical biochemical pathways altered in DR leading to vascular dysfunctions and discusses antioxidants as plausible treatment strategies.

From pathobiology to the targeting of pericytes for the treatment of diabetic retinopathy

Pericytes, the mural cells that constitute the capillaries along with endothelial cells, have been associated with the pathobiology of diabetic retinopathy; however, therapeutic implications of this association remain largely unexplored. Pericytes appear to be highly susceptible to the metabolic challenges associated with a diabetic environment, and there is substantial evidence that their loss may contribute to microvascular instability leading to the formation of microaneurysms, microhemorrhages, acellular capillaries, and capillary nonperfusion. Since pericytes are strategically located at the interface between the vascular and neural components of the retina, they offer extraordinary opportunities for therapeutic interventions in diabetic retinopathy. Moreover, the availability of novel imaging methodologies now allows for the in vivo visualization of pericytes, enabling a new generation of clinical trials that use pericyte tracking as clinical endpoints. The recognition of multiple signaling mechanisms involved in pericyte development and survival should allow for a renewed interest in pericytes as a therapeutic target for diabetic retinopathy.

Integrin-linked kinase inhibition attenuates permeability of the streptozotocin-induced diabetic rat retina

Integrin-linked kinase (ILK), as a multi-functional regulator, has been associated with diabetic retinopathy (DR). In this study, we investigated whether inhibition of ILK could result in therapeutic effects. Diabetes mellitus's rats were induced by streptozotocin (STZ) injection. After 1 weeks induction, rats were injected intraperitoneally daily with ILK inhibitor, QLT0267, at 5 mg/kg. Then, the rats were examined by 4, 8, and 12 weeks after first STZ injection. We found that QLT0267 treatment could not only lower ILK level in retina at as early as 3 weeks after the onset of diabetes but also attenuate retina permeability, which was measured by Evan's blue. Maximum effect was found in 11 weeks treatment. Meanwhile, QLT0267 did not disturbed blood glucose concentration. Furthermore, QLT0267 inhibited Akt (Ser473) activation and reduced expression of HIF1α and VEGF which were evaluated by western blot, real time PCR, and immunohistochemistry. We conclude that ILK may be a new target for DR.

Modulation of retinal blood flow by kinin B₁ receptor in Streptozotocin-diabetic rats

The vasoactive kinin B₁ receptor (B₁R) is overexpressed in the retina of diabetic rats in response to hyperglycemia and oxidative stress. The aim of the present study was to determine whether B₁R could contribute to the early retinal blood flow changes occurring in diabetes. Male Wistar rats were rendered diabetic with a single i.p. injection of Streptozotocin (STZ) and studied 4 days or 6 weeks after diabetes induction. The presence of B₁R in the retina was confirmed by Western blot. The impact of oral administration of the B₁R selective antagonist SSR240612 (10mg/kg) was measured on alteration of retinal perfusion in awake diabetic rats by quantitative autoradiography. Data showed that B₁R was upregulated in the STZ-diabetic retina at 4 days and 6 weeks. Retinal blood flow was not altered in 4-day diabetic rats compared with age-matched controls but was significantly decreased following SSR240612 treatment. In 6-week diabetic rats, retinal blood flow was markedly reduced compared to control rats and SSR240612 did not further decrease the blood flow. These results suggest that B₁R is upregulated in STZ-diabetic retina and has a protective compensatory role on retinal microcirculation at 4 days but not at 6 weeks following diabetes induction.

The role of pulsatile flow in controlling microvascular retinal endothelial and pericyte cell apoptosis and proliferation

AIMS

Aberrant retinal blood flow is a hallmark of various retinopathies and may be a causative factor in the pathology associated with these conditions. We examined the effects of pulsatile flow on bovine retinal endothelial cell (BREC) and bovine retinal pericyte (BRP) apoptosis and proliferation.

METHODS AND RESULTS

Co-cultured BRECs and BRPs were exposed to low (0.3 mL/min) or high (25 mL/min) pulsatile flow for 72 h using a perfused transcapillary culture system. Pulsatile flow increased BREC nitric oxide synthase (eNOS) and cyclooxygenase-2 (COX-2) expression and activity concomitant with a significant decrease in pre-pro-endothelin-1 (ET-1) mRNA and peptide. BREC apoptosis was significantly attenuated following exposure to high flow. The inhibition of NOS, COX, and ET receptors significantly reduced the pro-survival effects of flow on BREC. In contrast, BRP apoptosis was significantly enhanced following exposure to high flow. The inhibition of COX and ET receptors significantly attenuated the high flow-induced increase in BRP apoptosis when compared with untreated controls. Treatment of static BREC with NO donor (S-nitroso-N-acetylpenicillamine, SNAP), ET-1, or iloprost inhibited serum deprivation-induced apoptosis, whereas treatment of BRP with ET-1 and iloprost, but not SNAP, was ineffective. High pulsatile flow decreased BRP proliferation, in the absence of any changes in BREC proliferation.

CONCLUSION

Increased pulsatile flow promotes BREC survival and enhances BRP apoptosis through the activation of endothelial-derived vasoactive substances. Altered pulsatile flow does not alter BREC proliferation in co-culture with BRP, whereas BRP proliferation was significantly decreased at high flow rates. These interactions have important implications for vessel growth and regression during retinal vascular pathogenesis.

Diabetic retinopathy and blockade of the renin-angiotensin system: new data from the DIRECT study programme

The pathogenesis and medical management of diabetic retinopathy is reviewed. The importance of good control of blood glucose and blood pressure remain key elements in the prevention and treatment of diabetic retinopathy, and a number of specific metabolic pathways have been identified that may be useful additional targets for therapeutic intervention. Trial data, however, aimed specifically to answer the questions of optimum medical management are limited, so the DIRECT study of renin-angiotensin blockade using oral candesartan 32 mg daily is a welcome addition to our knowledge. This arose from the promising improvement of retinopathy outcomes in the EUCLID study of lisinopril in type I diabetes. In DIRECT, 5 years of candesartan treatment in type I diabetes reduced the incidence of retinopathy by two or more steps (EDTRS) in severity by 18% (P=0.0508) and, in a post hoc analysis, reduced the incidence of retinopathy by three-step progression by 35% (P=0.034). In type I diabetes patients there was no effect on progression of established retinopathy. In contrast, in type II diabetes, 5 years of candesartan treatment resulted in 34% regression of retinopathy (P=0.009). Importantly, an overall significant change towards less-severe retinopathy was noted in both type I and II diabetes (P<or=0.03). Although there is still no absolute proof that these effects were specific to RAS blockade, or just an effect of lower blood pressure, it is reasonable to conclude that candesartan has earned a place in the medical management of diabetic retinopathy, to prevent the problem in type I diabetes and to treat the early stages in type II diabetes.

Magnetic resonance imaging of tissue and vascular layers in the cat retina

PURPOSE

To report the visual resolution of multiple cell and vascular "layers" in the cat retina using MRI.

MATERIALS AND METHODS

T2- and diffusion-weighted MRI at 4.7 Tesla was performed. Layer-specific thickness, T2, spin density, apparent diffusion coefficient perpendicular (ADC(perpendicular)) and parallel (ADC(parallel)) to the retinal surface were tabulated. T1-weighted MRI was acquired before and after intravenous administration of Gd-DTPA and subtraction images were obtained. Histology was performed for validation.

RESULTS

Three distinct "layers" were observed. The inner strip nearest to the vitreous (exhibiting large T2, ADC, spin density with Gd-DTPA enhancement) overlapped the ganglion cell layer, bipolar cell layer, and the embedded retinal vascular layer. The middle strip (exhibiting small T2, ADC, spin density without Gd-DTPA enhancement) overlapped the photoreceptor cell layer and the inner and outer segments. The outer strip (exhibiting large T2, ADC, spin density with Gd-DTPA enhancement) overlapped the tapetum and choroidal vascular layer. T2, spin density, ADC(perpendicular) and ADC(parallel) of different "layers" were tabulated. The inner strip was slightly thicker than the other two strips. The total thickness, including neural and nonneural retina, was 358 +/- 13 microm (N = 6) by MRI and 319 +/- 77 microm (N = 5) by histology.

CONCLUSION

MRI provides a noninvasive tool to study the retina with laminar specificity without depth limitation.

Systemic and ocular pharmacokinetics of N-4-benzoylaminophenylsulfonylglycine (BAPSG), a novel aldose reductase inhibitor

To better develop N-[4-(benzoylamino)phenylsulfonyl]glycine (BAPSG), a potent and selective aldose reductase inhibitor capable of delaying the progression of ocular diabetic complications, the objective of this study was to assess its pharmacokinetics. The plasma pharmacokinetics of BASPG was assessed in male Sprague-Dawley rats following intravenous, intraperitoneal and oral routes of administration and its distribution to various tissues including those of the eye was studied following intraperitoneal administration. In addition, rat plasma protein binding of BAPSG was studied using ultracentrifugation method and its ocular tissue disposition was assessed following topical administration in rabbits. Plasma and tissue levels of BAPSG were analysed using an HPLC assay. BAPSG exhibited dose-proportionate AUC0 --> infinity (area under the plasma concentration-time curve) following both intravenous and intraperitoneal administration over the dose range (5-50 mg kg(-1)) studied and an erratic oral absorption profile with low oral bioavailability. The fraction bioavailability following oral and intraperitoneal administration was 0.06 and 0.7-1, respectively. BAPSG exhibited short plasma elimination half-lives in the range 0.5-1.5 h. BAPSG was bound to rat plasma proteins and the percent protein binding ranged from 83 to 99.8%. BAPSG was better distributed to cornea, lens and retina than to brain, following intraperitoneal administration in rats. However, the distribution was lower compared with kidney and liver. Following topical administration in rabbits, BAPSG delivery to the surface ocular tissues, cornea and conjunctiva was higher compared with intraocular tissues, aqueous humour, iris-ciliary body and lens. Thus, BAPSG was distributed to ocular tissues following systemic and topical modes of administration.

Ocular perfusion abnormalities in diabetes

PURPOSE

To review the role of ocular perfusion in the pathophysiology of diabetic retinopathy, one of the leading causes of irreversible blindness in the industrialized world.

METHODS

We carried out a Medline search of the literature published in English or with English abstracts from 1966 to 2000 using various combinations of relevant key words.

RESULTS

Hyperglycaemia leads to a wide variety of vascular abnormalities at the microvascular and macrovascular levels, including abnormal autoregulation.

CONCLUSION

Three major aspects of ocular perfusion in diabetic retinopathy require additional investigation. Firstly, the precise mechanisms that link elevated glucose to dysfunction of retinal vascular cells need to be identified. Secondly, those factors that lead to both capillary dropout and to angiogenesis, twin processes that are linked to tissue hypoxia and lead to excess perfusion, increased risk of extravascular leakage and frank haemorrhage, must be carefully delineated. Finally, once specific knowledge of disease fundamentals has been amassed, tests of therapies to reverse or prevent these pathological processes can move forward.

The pathogenesis of diabetic retinopathy: old concepts and new questions

Hyperglycaemia appears to be a critical factor in the aetiology of diabetic retinopathy and initiates downstream events including: basement membrane thickening, pericyte drop out and retinal capillary non-perfusion. More recently, focus has been directed to the molecular basis of the disease process in diabetic retinopathy. Of particular importance in the development and progression of diabetic retinopathy is the role of growth factors (eg vascular endothelial growth factor, placenta growth factor and pigment epithelium-derived factor) together with specific receptors and obligate components of the signal transduction pathway needed to support them. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. How does hyperglycaemia regulate retinal vessels? Which growth factors are most important and at what stage of retinopathy do they operate? What is the preferred point in the growth factor signalling cascade for therapeutic intervention? Answers to these questions will provide the basis for new therapeutic interventions in a debilitating ocular condition.