Role of platelets in pathogenesis of diabetic retinopathy

Microvascular remodeling and wound healing: a role for pericytes

Physiologic wound healing is highly dependent on the coordinated functions of vascular and non-vascular cells. Resolution of tissue injury involves coagulation, inflammation, formation of granulation tissue, remodeling and scarring. Angiogenesis, the growth of microvessels the size of capillaries, is crucial for these processes, delivering blood-borne cells, nutrients and oxygen to actively remodeling areas. Central to angiogenic induction and regulation is microvascular remodeling, which is dependent upon capillary endothelial cell and pericyte interactions. Despite our growing knowledge of pericyte-endothelial cell crosstalk, it is unclear how the interplay among pericytes, inflammatory cells, glia and connective tissue elements shape microvascular injury response. Here, we consider the relationships that pericytes form with the cellular effectors of healing in normal and diabetic environments, including repair following injury and vascular complications of diabetes, such as diabetic macular edema and proliferative diabetic retinopathy. In addition, pericytes and stem cells possessing "pericyte-like" characteristics are gaining considerable attention in experimental and clinical efforts aimed at promoting healing or eradicating ocular vascular proliferative disorders. As the origin, identification and characterization of microvascular pericyte progenitor populations remains somewhat ambiguous, the molecular markers, structural and functional characteristics of pericytes will be briefly reviewed.

Increased levels of platelet-derived microparticles in patients with diabetic retinopathy

Diabetic retinopathy is caused by capillary occlusions. Platelet-derived microparticles (PMPs) stimulate the coagulation cascade and increase leukocyte and endothelial cell adhesions, both of which are key events in the development of diabetic retinopathy. However, the correlation between the levels of PMPs and diabetic retinopathy has not been precisely determined. The PMPs levels and the expression of platelet CD62P and CD63 were measured in 92 diabetic patients. The level of PMPs was significantly correlated with the expression of CD62P (r = 0.76, P < 0.0001) and CD63 (r = 0.71, P < 0.0001). The mean level of PMPs in diabetics (507+/-15/10(4) platelets (plt), mean+/-S.E.) was significantly higher than that in normal. The PMPs levels increased with the progression of the diabetic retinopathy; 480+/-28/10(4) plt in diabetic patients without retinopathy (n = 25), 504+/-40/10(4) plt with mild or moderate non-proliferative diabetic retinopathy (n = 13), 512+/-29/10(4) plt with severe non-proliferative diabetic retinopathy (n = 25), and 528+/-25/10(4) plt with proliferative diabetic retinopathy (n=29). The PMPs level in patients with non-perfused retinal areas (582+/-27/10(4) plt, n = 24) was significantly higher than patients without non-perfused areas (469+/-23/10(4) plt, n = 30; P = 0.0096) and without diabetic retinopathy (P = 0.024). These high correlations indicate that increased levels of PMPs may accelerate diabetic retinopathy.

Increased prevalence of microthromboses in retinal capillaries of diabetic individuals

In diabetic retinopathy, capillary nonperfusion and eventual obliteration can lead to retinal ischemia and sight-threatening neovascularization. The occurrence of retinal microthrombosis in human diabetes has long been suspected and occasionally observed but never systematically demonstrated. We used trypsin digestion to isolate the intact vascular network from retinas obtained postmortem from nine diabetic donors (age 64 +/- 11 years, duration of diabetes 6 +/- 4 years; mean +/- SD) and eight age-matched nondiabetic donors. Topographically matched sectors (each one-sixth of retina) of diabetic and nondiabetic retinas were tested sequentially with antibodies to fibrin cross-linking factor XIII and platelet glycoprotein (GP)-IIIa to identify fibrin-platelet thrombi. In some trypsin digests, we also examined vascular cell apoptosis. The retina from a nondiabetic donor, 24 years of age, who had died of trauma, was used to exclude confounding influences caused by the postmortem period. When compared with those of nondiabetic donors, the retinas of diabetic donors showed double the number of capillary segments with colocalized immunostaining for factor XIII and GPIIIa (P = 0.02). The total area of the positive segments was fourfold greater in the diabetic than in the nondiabetic donors (P = 0.02) and correlated with the duration of diabetes (r = 0.71, P < 0.05). Large thrombi were six times more frequent in the diabetic donors (P = 0.03), and there was a significant topographical association of microthrombosis with apoptotic cells in both diabetic and nondiabetic vessels (P = 0.0001). Hence, diabetes of short duration was found to be associated with a greater than normal number and size of platelet-fibrin thrombi in the retinal capillaries. These thrombi can contribute to capillary obliteration and retinal ischemia and may be a practical target for early drug intervention.

Platelet hyperactivity and abnormal Ca(2+) homeostasis in diabetes mellitus

We sought to determine the mechanisms for hyperactivity and abnormal platelet Ca(2+) homeostasis in diabetes. The glycosylated Hb (HbA(1c)) level was used as an index of glycemic control. Human platelets were loaded with Ca- green-fura red, and cytosolic Ca(2+) ([Ca(2+)](i)) and aggregation were simultaneously measured. In the first series of experiments, the platelets from diabetic and normal subjects were compared for the ability to release Ca(2+) or to promote Ca(2+) influx. A potent and relatively specific inhibitor of Na(+)/Ca(2+) exchange, 5-(4-chlorobenzyl)-2',4'-dimethylbenzamil (CB-DMB), increased the second phase of thrombin-induced Ca(2+) response, suggesting that the Na(+)/Ca(2+) exchanger works in the forward mode to mediate Ca(2+) efflux. In contrast, in the platelets from diabetics, CB-DMB decreased the Ca(2+) response, indicating that the Na(+)/Ca(2+) exchanger works in the reverse mode to mediate Ca(2+) influx. In the second series of experiments we evaluated the direct effect of hyperglycemia on platelets in vitro. We found that thrombin- and collagen-induced increases in [Ca(2+)](i) and aggregation were not acutely affected by high glucose concentrations of 45 mM. However, when the platelet-rich plasma was incubated with a high glucose concentration at 37 degrees C for 24 h, the second phase after thrombin activation was inhibited by CB-DMB. In addition, collagen-stimulated [Ca(2+)](i) response and aggregation were also increased. Thus in diabetes the direction and activity of the Na(+)/Ca(2+) exchanger is changed, which may be one of the mechanisms for the increased platelet [Ca(2+)](i) and hyperactivity. Prolonged hyperglycemia in vitro can induce similar changes, suggesting hyperglycemia per se may be the factor responsible for the platelet hyperactivity in diabetes.

The visual status of diabetics on renal replacement therapy (RRT)

The visual status of 48 patients with end-stage diabetic nephropathy who were undergoing treatment either by dialysis or transplantation was assessed. At the time of starting dialysis, the vision was good in 27 (56 per cent), impaired in 7 (15 per cent) and of navigating standard only in 12 (25 per cent) while the remaining 2 (4 per cent) had no useful vision. During the time on dialysis and/or following transplantation, vision improved in 6 patients (12 per cent), deteriorated in 9 (19 per cent) and remained unchanged in 33 (69 per cent). Changes in the degree of retinopathy in the 96 eyes resulting either from treatment of the renal failure or laser therapy were noted. The reasons for any changes in vision and the effect of ophthalmic treatment will be discussed.

Arachidonic acid metabolism by retinas of rats with streptozotocin-induced diabetes

Arachidonic acid metabolism via the cyclooxygenase pathway and the effects of aspirin and indomethacin were studied in whole retinas of rats with streptozotocin-induced diabetes. Diabetes was induced by intravenous injection of streptozotocin (60 mg/kg) and the animals were examined 5-6 weeks later. Whole retinas of nondiabetic and diabetic animals were incubated for 1 1/2 hours, and the amounts of prostacyclin and prostaglandin E2 (PGE2) accumulated in the media were measured. The amounts of PGE2 in the media of diabetic retinas was significantly lower than levels in media of nondiabetic group. However, the amounts of prostacyclin accumulated in the media of nondiabetic and diabetic retinas did not differ significantly. Addition of arachidonic acid (A.A.) to the incubation media caused an enhancement in the amounts of PGE2 and prostacyclin in the incubation media of both diabetic and nondiabetic retinas. Addition of indomethacin or aspirin to the incubation media caused a reduction of prostacyclin and PGE2 levels in the media of both groups.

Clinical and experimental studies on retinal neovascularization. XXXIX Edward Jackson Memorial Lecture

Retinal neovascularization is a serious complication of the retinopathy associated with diabetes, branch vein occlusion, sickle cell anemia, and retrolental fibroplasia. Retinal capillary nonperfusion, demonstrated on fluorescein angiography, precedes the development of neovascularization in each of these conditions. Our working hypothesis is that the nonperfused (ischemic or hypoxic) retina liberates a vasoproliferative or angiogenic substance. Although I have delineated the clinical and experimental observations relating to the hypothesis of an ischemic-mediated angiogenesis substance, other postulated mechanisms for the development of retinal neovascularization may be involved. Recent observations on the experimental model of retrolental fibroplasia have demonstrated the markedly abnormal persistence and apparent proliferation of the hyaloid vessels in mice following oxygen-induced retinal vascular closure.

Alterations in blood elements in the pathogenesis of diabetic retinopathy

The role of altered blood elements in the pathogenesis of retinal ischemia and diabetic retinopathy and the relationship to abnormal carbohydrate metabolism and to elevated levels of growth hormone are discussed. These changes involve red blood cells, platelets, plasma proteins, fibrinolytic response, and vascular endothelium. The significance of blood elements mediated by plasma is noted with aggregation of normal red cells when cross-matched with diabetic plasma and with intensive plasmapheresis, which caused red cell disaggregation and improvement of retinopathy. The relationship of metabolic control to diabetic retinopathy is reviewed and is evident by improvement of retinopathy occurring eight weeks after continuous subcutaneous infusion of insulin. A hypothesis is presented which integrates the multifactorial processes involved in the pathogenesis of diabetic retinopathy. Only through future research can one prove the implicated mechanisms in the pathogenesis of diabetic retinopathy and the role of strict metabolic control in altering the progression of retinopathy.

New aspects of the pathogenesis of atherosclerosis: possible approaches to prevention and treatment

New techniques in cell biology and recent advances in rheology have opened up fresh perspectives in the understanding of the pathogenesis of atherosclerosis. There is now an increasing amount of experimental evidence to support the hypothesis that arterial thrombi are the first stage in atherogenesis in the context of endothelial lesions and not merely secondary factors without any causal significance. Factors influencing platelet aggregation and adhesiveness are discussed, and the animal and clinical studies carried out with the platelet aggregation inhibitor pentoxifylline and its effects on the microcirculation are reviewed.

Diabetic retinopathy

Retinopathy is present in 60%-80% of long-term diabetics, and 5%-10% of diabetics surviving 20 yr from the time of diagnosis will be blind, mostly from retinopathy, which is now the commonest cause of newly diagnosed blindness in the 30-65 yr age group. The mean survival time after a diagnosis of retinopathy is only 5 yr. The natural history of diabetic retinopathy is now being understood more clearly. Mild background retinopathy, characterised by microaneurysms or scattered hard exudates, may progress to maculopathy, with macular vascular pathology leading to exudates and edema at the macula; this is most common in older patients. It may also lead to proliferative retinopathy, which may progress slowly with new vessels and fibrous-tissue formation, or rapidly with widespread capillary closure and soft-exudate formation, extensive neovascularisation, hemorrhages, and blindness. The changes of diabetic retinopathy can be documented using retinal photography, and several grading methods have been devised that are useful for evaluation treatment. The cause of diabetic retinopathy is still unclear. Evidence for incriminating genetic factors, growth-hormone excess, and hypoxia associated with changes in blood flow and retinal metabolism are reviewed. Insulin responses and plasma triglyceride seem to be different in maturity-onset diabetics with retinopathy when compared with those in whom this complication is absent. Most physicians agree that good diabetic control may both lower the incidence of retinopathy and reduce the speed of its progress. While there is little evidence that drugs are ever of much value in this condition, the role of photocoagulation both by laser and xenon arc is becoming clearer with increased experience of these techniques, and the current situation is reviewed. Pituitary ablation is a very drastic method of treatment and should never be used as a desperate measure in a patient with advanced proliferative disease. It is, however, the treatment of choice in early florid retinopathy, when it proves the only chance to arrest the condition. Some new techniques of vitreous surgery are now being developed and the possible role of these in the management of patients with advanced proliferative disease is briefly reviewed.