Abstract
BackgroundRecent data indicate that inflammatory mechanisms contribute to diabetic retinopathy (DR). We have determined that serine racemase (SR) expression is increased by inflammatory stimuli including liposaccharide (LPS), amyloid β-peptide (A-beta), and secreted amyloid precursor protein (sAPP); expression is decreased by the anti-inflammatory drug, dexamethasone. We tested possibility that SR and its product, D-serine, were altered in a rat model of DR.MethodsIntraperitoneal injection of streptozotocin (STZ; 70 mg/kg body weight) to Sprague-Dawley rats produced type-I diabetic mellitus (fasting blood sugar higher than 300 mg/dL). At 3 and 5 months after STZ or saline injection, retinas from some rats were subjected to cryosectioning for immunofluorescent analysis of SR and TUNEL assay of apoptosis. Retinal homogenates were used to detect SR levels and Jun N-terminal kinase (JNK) activation by immunoblotting. Aqueous humor and retina were also collected to assay for neurotransmitters, including glutamate and D-serine, by reverse-phase HPLC.ResultsCompared to saline-injected rats, STZ-injected (diabetic) rats showed elevation of SR protein levels in retinal homogenates, attributed to the inner nuclear layer (INL) by immunofluorescence. Aqueous humor fluid from STZ-injected rats contained significantly higher levels of glutamate and D-serine compared to controls; by contrast, D-serine levels in retinas did not differ. Levels of activated JNK were elevated in diabetic retinas compared to controls.ConclusionsIncreased expression of SR in retina and higher levels of glutamate and D-serine in aqueous humor of STZ-treated rats may result from activation of the JNK pathway in diabetic sequelae. Our data suggest that the inflammatory conditions that prevail during DR result in elevation of D-serine, a neurotransmitter contributing to glutamate toxicity, potentially exacerbating the death of retinal ganglion cells in this condition.
Highlights
Recent data indicate that inflammatory mechanisms contribute to diabetic retinopathy (DR)
We examined retinal ganglion cell layer (RGCL) integrity in our rat subjects with Hematoxylin and eosin (H&E) and TUNEL staining
H&E staining indicated a reduction in the number of RGCs in some areas of RGCL in diabetic rats 3 months after STZ injection, as compared to the saline-injected group (Figure 1, A vs. 1B); similar effects were observed at 5 months after STZ injection
Summary
Recent data indicate that inflammatory mechanisms contribute to diabetic retinopathy (DR). The pathology of DR involves microvasular changes, including blood-retinal barrier (BRB) breakdown, microaneurysm, increased expression of intercellular adhesion molecule 1 (ICAM-1), and death of endothelial cells and pericytes [8,9,10,11]. Excitotoxins including homocysteine and glutamate can induce toxicity in RGCs [14]; increased retinal glutamate is found in the streptozotocin (STZ)-induced model of diabetes [15]. Whole-cell recording in rat retinas has indicated that D-serine enhances currents transmitted by N-methyl D-aspartate (NMDA) receptors, and removal of D-serine by D-amino acid oxidase (DAAOx) returned the currents to control amplitudes [20]
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