INTRODUCTION TO DIABETIC RETINOPATHY

In humans the central retinal artery branches into a unique microvascular system that plays a major role in embryological development of the retina and other intraocular structures (1). In maturity this microvasculature serves the peculiar metabolic requirements of the unique neuroglial configuration of the retina. Within the vascular unit itself, there is complex crosstalk between the component cells which maintains cell function/survival, blood flow, and the inner blood-retinal barrier (iBRB). For example, the end-artery vascular network of the retina lacks any obvious autonomic nerve supply, and blood flow into the capillary beds is tightly regulated in response to the metabolic needs of the retinal parenchyma. This is achieved, in large part, by the regulatory capacity of the component smooth muscle cells in the retinal arteries and arterioles which are highly sensitive to endothelial-generated vasodilators and vasoconstrictors (2). When these cell relationships are disrupted, as in some pathological situations such as diabetes, the vasculature can become dysfunctional, losing the ability to tightly regulate flow and maintain barrier properties. Vascular cells may eventually die and this leads to progressive nonperfusion of the retina.

Diabetic retinopathy is widely regarded as a quintessential disease of the intraretinal microvasculature, although as diabetes progresses changes may also occur in the choroidal vessels. The vascular-centric view of this disease has led to its clinical classification into two forms: nonproliferative diabetic retinopathy (NPDR) and proliferative diabetic retinopathy (PDR). The nonproliferative form is by far the most common, and in a significant number of cases it progresses to sight-threatening PDR (3). The greatest risk of vision loss occurs in the later phases of diabetic retinopathy with the development of macular edema and/or retinal neovascularization, the former being a direct consequence of iBRB breakdown and the latter to widespread retinal ischemia

(4). Retinopathy is the most common microvascular complication suffered by patients with diabetes, and it remains a major cause of visual impairment worldwide (5).

Measurable dysfunction of the retinal microvasculature commences within weeks of diabetes onset in both patients and animal models of diabetic retinopathy. This is characterized by changes to retinal blood flow, impaired autoregulation, and abnormal vasopermeability to plasma proteins (2, 6). As disease duration increases, the nonproliferative phase of diabetic retinopathy is associated with excessive capillary permeability leading to iBRB dysfunction (7), capillary basement membrane (BM) thickening (8), and pericyte/smooth muscle depletion (9). Weakness of capillary walls (perhaps because of pericyte loss) and increased intraluminal pressure probably lead to the formation of microaneurysms which provide a clinically visible, quantifiable lesion in the fundus of diabetic patients. These are often associated with large areas of nonperfusion at the arterial side of the circulation (10, 11) (Fig. 1). Progression to the proliferative stage of diabetic retinopathy is linked to widespread ischemia and subsequent upregulation of potent angiogenic growth factors such as VEGF that drive preretinal neovascularization. Presence of new vessels on the retinal surface may lead to retraction of the vitreous from sites of firm fibrovascular adhesion and, if left untreated, can lead to tractional retinal detachment. Also associated with retinal vasodegeneration, diabetic macular edema (DME) is caused by wholesale breakdown of the iBRB and constitutes a major cause of vision loss associated with diabetic retinopathy.

endproducts (AGEs) and activation of receptors for AGEs constitute the main focus of this chapter, although it should be appreciated that hyperglycemia can simultaneously provoke several other pathogenic pathways in retinal cells. One such mechanism is linked to increased flux through the polyol or hexosamine pathways which is associated with subsequent alterations in the redox state of pyridine nucleotides (17). Accumulation of sorbitol in retinal cells is dependent on activity of aldose reductase, and this may impinge on a range of pathways and contribute to diabetic retinopathy (18). Also, de novo synthesis of diacylglycerol (DAG) leading to the overactivation of several isoforms of protein kinase C (PKC) (19), excessive production of free radicals leading to oxidative stress (20, 21), changes in blood rheology and hemodynamics (2, 22), and overactivation of the renin-angiotensin system (RAS) (23) contribute significantly to retinopathy as diabetes progresses. These mechanisms have formed a basis for therapeutic intervention, and the PKC β inhibitor ruboxistaurin holds promise for preventing progression of diabetic retinopathy. While protection was not observed for some aspects of pathology in a recent clinical trial, ruboxistaurin did achieve significant reduction in the vision loss through DME (24).

Pathogenic pathways to diabetic retinopathy continue to be elucidated, but these should not necessarily be viewed as independent phenomena. As an exemplar for this, Brownlee has proposed a unifying concept whereby hyperglycemia increases superoxide production (via the mitochondrial electron transport chain) which in turn initiates accelerated AGE formation and also exacerbates many of the aforementioned pathogenic mechanisms (25). This hypothesis has been reinforced in the field of retinopathy, in which three biochemical abnormalities involving AGE formation, flux through the hexosamine pathway, and DAG-mediated activation of PKC-β have been attenuated using the thiamine derivative benfotiamine. Treatment of diabetic animals with benfotiamine showed a convergent protective effect on three pathways and also inhibited proinflammatory NFκB activation, culminating in effective prevention of key diabetesrelated retinal lesions (26). More recently, benefits of this drug have also been demonstrated in retinal microvascular cells exposed to high glucose in vitro (27).

BIOCHEMISTRY OF AGE FORMATION

Excess glucose in cells leads to enhanced nonenzymatic glycation reactions between reducing sugars and the free amino groups on proteins, lipids, and DNA. This is an inevitable consequence of the reactivity of aldehydes and as a consequence nearly all body proteins carry some “burden” of chemically attached carbohydrate. The nature of this chemistry was established as early as 1912 when the food chemist Louis Camille Maillard reported formation of yellow brown products on heating mixtures of amino acids and sugars. The so-called Maillard reaction begins with the formation of a Schiff base between glucose and ε-amino groups (e.g., lysine) that slowly rearranges to relatively stable Amadori adducts (Fig. 2). The most widely known Amadori product is a modification of hemoglobin (HbA1c) which is used clinically as an indicator for cumulative exposure to elevated blood glucose. Both the Schiff base and the Amadori compound can undergo further oxidation and dehydration so that their concentrations ultimately depend on both forward and reverse reactions. The forward reactions give rise to additional protein-bound compounds collectively termed AGEs. During diabetes