Diabetes downregulates presynaptic proteins and reduces basal synapsin I phosphorylation in rat retina

Diabetic retinopathy can result in vision loss and involves progressive neurovascular degeneration of the retina. This study tested the hypothesis that diabetes decreases the retinal expression of presynaptic proteins involved in synaptic function. The protein and mRNA contents for synapsin I, synaptophysin, vesicle-associated membrane protein 2, synaptosomal-associated protein of 25 kDa and postsynaptic density protein of 95 kDa were measured by immunohistochemistry, immunoblotting and real-time quantitative polymerase chain reaction in whole retinas and retinal synaptosomes from streptozotocin-diabetic and control Sprague-Dawley rats. There was less presynaptic protein immunoreactivity after 1 and 3 months of diabetes than in controls. Discrete synaptophysin-immunoreactive puncta were significantly smaller and fewer in sections from 1- and 3-month diabetic rat retinas than in those from controls. The content of presynaptic proteins was significantly less in whole retinas of 1- and 3-month diabetic rats, and in synaptosomes from 1-month diabetic rats, than in controls. Whole retinas had significantly less mRNA for these genes after 3 months but not 1 month of diabetes, as compared to controls (with the exception of postsynaptic density protein of 95 kDa). In contrast, there was significantly less mRNA for synaptic proteins in synaptosomes of 1-month diabetic rats than in controls, suggesting a localized depletion at synapses. Protein and mRNA for beta-actin and neuron-specific enolase were unchanged by diabetes. The ratio of phosphorylated to total synapsin I was also reduced in whole retina and isolated synaptosomes from 1-month diabetic rats, as compared to controls. These data suggest that diabetes has a profound impact on presynaptic protein expression in the retina, and may provide a mechanism for the well-established defects in vision and the electrophysiological response of the retina in diabetes.