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Travis S. D'Cruz, Brittany N. Weibley, Scot R. Kimball, Alistair J. Barber; Diabetes Increases Mannose Rich Synaptophysin in the Rat Retina. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5426.
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Synaptophysin (Syn) is an abundant presynaptic glycoprotein that plays an important role in neurotransmitter vesicle docking and recycling. Our previous work shows that 1 month of streptozotocin (STZ)-diabetes increases the rate of translation of Syn along with several other proteins in the rat retina. During its post-translational modification and processing, Syn is N-acetylglucosamine (Glc-NAc) glycosylated and is temporarily mannose rich (immature). The aim of this study was to test the hypothesis that diabetes increases the content of mannose rich Syn in the rat retina.
Diabetes was induced in male Sprague Dawley rats by injection of streptozotocin (STZ, 65mg/ml, i.p.) and rats were housed for 1-2 months. To measure overall Syn N-glycoslation, lysates from 1 month STZ-diabetic and aged-matched controls were treated with PNGase F for 1 hour in 37°C, before detection by immunoblot analysis. Mannose rich Syn was isolated by treating retinal lysates with endoglycosidase H (Endo H). ER stress markers GRP78 and XBP1 protein and mRNA were also quantified in retinal lysates by immunoblot analysis and PCR respectively.
PNGase F deglycosylated Syn in lysates from 1 month STZ-diabetic rats and age-matched controls. Endo H sensitive Syn was significantly increased in retinas from 1 month STZ-diabetic rats compared with age-matched controls (p<0.005). There was no significant change in GRP78 protein content and XBP1 mRNA splicing after 1 month of STZ-diabetes.
The data suggest that diabetes does not change the overall N-glycosylation of Syn. Diabetes does, however, increase the content of mannose rich synaptophysin protein in the retina, suggesting a dysregulation of post-translational processing. Furthermore, the lack of significant change in GRP78 content and splicing of XBP1 indicates that ER stress may not be activated in the 1-month STZ-diabetic rat retina.
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