Angiotensin II type 1 receptor signaling contributes to synaptophysin degradation and neuronal dysfunction in the diabetic retina.
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.


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. 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 the 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 electroretinography changes and reduction of synaptophysin protein, but not mRNA, levels in the diabetic retina. In agreement with the AT1R-mediated posttranscriptional 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 the AT1R blockade as a novel neuroprotective treatment for diabetic retinopathy.
Kurihara
Toshihide
T
Laboratory of Retinal Cell Biology, Keio University School of Medicine, Tokyo, Japan.
Ozawa
Yoko
Y
Nagai
Norihiro
N
Shinoda
Kei
K
Noda
Kousuke
K
Imamura
Yutaka
Y
Tsubota
Kazuo
K
Okano
Hideyuki
H
Oike
Yuichi
Y
Ishida
Susumu
S
D000804:Angiotensin II / Q000378:metabolism; D047228:Angiotensin II Type 1 Receptor Blockers / Q000494:pharmacology; D000818:Animals; D001562:Benzimidazoles / Q000494:pharmacology; D001565:Benzoates / Q000494:pharmacology; D002460:Cell Line; D003921:Diabetes Mellitus, Experimental / Q000378:metabolism / Q000503:physiopathology*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000235:genetics / Q000378:metabolism; D015151:Immunoblotting; D051379:Mice; D008810:Mice, Inbred C57BL; D009474:Neurons / Q000166:cytology / Q000378:metabolism; D051058:Oncogene Protein v-akt / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D034622:RNA Interference; D044140:Receptor, Angiotensin, Type 1 / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D012160:Retina / Q000166:cytology / Q000378:metabolism* / Q000503:physiopathology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D050796:STAT3 Transcription Factor / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D016708:Synaptophysin / Q000235:genetics / Q000378:metabolism*; D000077333:Telmisartan; D013777:Tetrazoles / Q000494:pharmacology; D054875:Ubiquitination / Q000187:drug effects; D014633:Valine / Q000031:analogs & derivatives / Q000494:pharmacology; D000068756:Valsartan
D047228:Angiotensin II Type 1 Receptor Blockers; D001562:Benzimidazoles; D001565:Benzoates; D044140:Receptor, Angiotensin, Type 1; D050796:STAT3 Transcription Factor; D016708:Synaptophysin; D013777:Tetrazoles; D000804:Angiotensin II; D000068756:Valsartan; D051058:Oncogene Protein v-akt; D048049:Extracellular Signal-Regulated MAP Kinases; D014633:Valine; D000077333:Telmisartan