Angiotensin II Type 1 Receptor Signaling Contributes to Synaptophysin Degradation and Neuronal Dysfunction in the Diabetic Retina

Abstract

Objective: Pathogenic mechanisms underlying diabetes-induced retinal dysfunction are not fully understood. The aim of the present study was to show the relationship of the renin-angiotensin system (RAS) with the synaptic vesicle protein synaptophysin and neuronal activity in the diabetic retina.

Research Design and Methods: C57BL/6 mice with streptozotocin-induced diabetes were treated with the angiotensin II type 1 receptor (AT1R) blocker telimsartan or valsartan, and retinal function was analyzed by electroretinography (ERG). Retinal production of the RAS components and phosphorylation of ERK (extracellular-signal regulated kinase) were examined by immunoblotting. Retinal mRNA and protein levels of synaptophysin were measured by quantitative RT-PCR and immunoblot analyses, respectively. In vitro, synaptophysin levels were also evaluated using angiotensin II-stimulated PC12D neuronal cells cultured with or without the inhibition of ERK signaling or ubiquitin-proteasome system (UPS).

Results: Induction of diabetes led to a significant increase in retinal production of angiotensin II and AT1R together with ERK activation in the downstream of AT1R. AT1R blockade significantly reversed diabetes-induced ERG changes and reduction of synaptophysin protein, but not mRNA, levels in the diabetic retina. In agreement with the AT1R-mediated post-transcriptional downregulation of synaptophysin in vivo, in vitro application of angiotensin II to PC12D neuronal cells caused the UPS-mediated degradation of synaptophysin protein via AT1R, which proved to be induced by ERK activation.

Conclusions: These data indicate the first molecular evidence of the RAS-induced synaptophysin degradation and neuronal dysfunction in the diabetic retina, suggesting the possibility of AT1R blockade as a novel, neuroprotective treatment for diabetic retinopathy.