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T. W. Gardner, M. K. Losiewicz, S. R. Kimball, L. S. Jefferson, Jr., P. E. Fort; Diabetes Perturbs Both Protein Synthesis and Degradation in the Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5632.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the mechanisms by which diabetes alters the expression of retinal proteins. The retina has a high metabolic rate and requires tight control of protein synthesis and degradation to maintain cellular integrity, visual processes and synaptic plasticity.
Protein synthesis rate was evaluated in retina, muscle and liver from streptozotocin-induced diabetic rats and Ins2Akita mice using the flooding dose method. Protein synthesis and degradation were determined in ex vivo retinas using radioactive labeling and pulse-chase techniques, respectively. The roles of insulin and glucose were studied by treating rats with either systemic or local insulin or systemic phlorizin administration. Ribosomal RNA, a marker of protein synthesis, was localized by fluorescent in situ hybridization. Protein synthesis regulation was assessed using sucrose density gradient centrifugation followed by isolation of RNA from polysomal and sub-polysomal fractions and quantitative real-time PCR.
Protein synthesis was reduced 40% in both models of diabetes as early as 4 weeks after diabetes onset, and was restored by both systemic and subconjunctival insulin administration and reduction of blood glucose levels by phlorizin. In contrast to skeletal muscle, retinal polysome profiles did not change in diabetes. Surprisingly, retinal protein degradation was markedly less in diabetic than in control retinas. Phlorizin, but not local insulin treatment, restored the protein degradation blockade induced by diabetes in the retina. Photoreceptors and ganglion cells exhibited the highest levels of ribosomal RNA.
This study demonstrates diabetes-induced alterations in retinal protein synthesis and degradation, and shows that regulation of retinal protein metabolism is distinct from other insulin-sensitive tissues. These findings may help to understand the metabolic basis of early diabetic retinopathy.
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