Early Inner Retinal Astrocyte Dysfunction during Diabetes and Development of Hypoxia, Retinal Stress, and Neuronal Functional Loss

Abstract

Neuronal and glial alterations precede the overt vascular change that characterizes diabetic retinopathy. Because retinal astrocytes modulate neuronal and vascular function, this study investigated the time course of astrocyte, Müller cell, and neuronal change during diabetes to determine whether astrocytes may play an early role in diabetic retinopathy. Sprague-Dawley rats were rendered diabetic via streptozotocin and neuronal and glial changes were assessed after 2-10 weeks. Astrocyte change was investigated using connexin-26 immunolabeling, whereas connexin-26 and -43 gene expressions were quantified using real-time PCR. Hypoxia was measured by pimonidazole labeling and the expression of hypoxia-inducible factor-1 alpha (HIF-1α) was quantified using Western blot. Müller cell gliosis was assessed by glial fibrillary acidic protein immunolabeling and retinal function assessed using the electroretinogram. Astrocyte connexin-26 and -43 gene and protein expression decreased after 4 weeks of diabetes, before significant astrocyte loss. At the same time, the retina became hypoxic, with increased HIF-1α expression and pimonidazole labeling in the ganglion cell layer. This coincided with a decrease in ganglion cell function. After 6 weeks of diabetes, Müller cell gliosis became more evident and there were additional functional deficits in photoreceptoral and amacrine cell responses. These findings suggest that early changes in astrocytes are coincident with inner retinal hypoxia and ganglion cell functional deficits, whereas Müller cell gliosis and more extensive decreases in neuronal function occur later. Astrocytes may play an early and key role in changes in retinal vasculature and inner retinal dysfunction in diabetes.