Retinal ion regulation in a mouse model of diabetic retinopathy: natural history and the effect of Cu/Zn superoxide dismutase overexpression.

Microvascular abnormality is the clinical hallmark of diabetic retinopathy. Yet functional losses occur in the retina between the onset of hyperglycemia and the appearance of ophthalmoscopically visible microangiopathy. In this period of emergent diabetic retinopathy, dysfunction occurs not only in vascular retina (e.g., autoregulatory impairment) but also in nonvascular retina (e.g., reduced contrast sensitivity, relatively lower retinal electrophysiological parameters, and subnormal ion regulation).1–16 The importance of such functional impairment is underscored by noting that correction of early diabetes-induced retinal dysfunctionality, such as abnormal ion regulation, is linked to the inhibition of late-stage vascular histopathology.17–19 For example, in experimental diabetes, preventive pharmaceutical treatment with captopril can inhibit early reductions in a major regulator of ion homeostasis in the retina, Na/K-ATPase activity, and can ameliorate late-stage development of vascular histopathology.18,19 These associations are suggestive, and more work is needed to better define the factors regulating diabetes-induced subnormal retinal ion activity. Pathobiochemical insults, such as oxidative stress, are also strongly linked with developing diabetic retinopathy. However, few studies have investigated the interplay between early diabetes-induced ion dysregulation and oxidative stress. Treating diabetic rats with α-lipoic acid corrected diabetes-induced reductions in retinal antioxidants such as manganese superoxide dismutase (SOD2) and Cu/Zn superoxide dismutase (SOD1).20,21 The same treatment inhibited early ion dysregulation and late-stage vascular histopathology such as the degeneration of retinal capillaries.17 Recent elegant work involving SOD2 overexpressor mice has established an important role of SOD2 in diabetic retinopathy.20,22,23 SOD1 constitutes at least 85% of the total cellular superoxide dismutase activity in many mammalian cells, has high activity in the retina, and is a key defense against superoxide-related oxidative stress in the cytosol and intermembrane space of mitochondria.24–26 Efforts to understand the role of SOD1 in nondiabetic models of retinal injury using SOD1 overexpressor mice have produced mixed results, with overexpression either increasing retinal neuronal damage, providing neuroprotection, or having no effect.27–29 Despite strong evidence that diabetes causes an environment of early oxidative stress in retinas of rats and mice23,30–32 and that antioxidant therapy inhibits the later degeneration of the retinal capillaries, a specific role of SOD1 in the diabetic retina has not been well studied. One powerful noninvasive method for studying cellular ionic activity is manganese-enhanced MRI (MEMRI).33,34 MEMRI allows for noninvasive measurement of retinal thickness and, notably, layer-specific uptake of manganese (Mn2+) ion after systemic MnCl2 injection.17,35–37 Manganese ion, an MRI contrast agent and analog of, for example, calcium, readily accumulates intracellularly as a function of membrane integrity and cellular activity, is sensitive to Na/K-ATPase activity, and is relatively slowly removed.33,35,38–40 In rat models of diabetic retinopathy and in a nondiabetic rat ischemia/reperfusion model that produces histopathology similar to that of diabetic retinopathy, impaired ion regulation developed before the appearance of neuronal and vascular degeneration.17,35–37 Such changes in retinal ion regulation in mouse models of diabetic retinopathy have not yet been reported. In this study, we evaluated the sensitivity of MEMRI to normal changes in retinal ion regulation with light and dark adaption (i.e., visual processing) in the mouse retina in vivo. Next, we described the natural history of MEMRI changes before and during the appearance of acellular capillaries in mice. In addition, we compared the effect of diabetes in wild-type (WT) mice and transgenic mice that overexpress SOD1 on intraretinal ion regulation and thickness and on vascular histopathology. If increased oxidative stress played an important role in altering ion homeostasis in diabetic retinopathy, the upregulation of SOD1 activity would have been expected to be protective against early ion dysregulation and, possibly, late-stage histopathology.29