Recurrent hypoglycemia markedly decreases local insulin biosynthesis in the diabetic retina

Abstract

Purpose : Clinical studies show that glucose-lowering interventions decrease the rate of retinopathy. Unfortunately, improved glucose control invariably increases the incidence of hypoglycemia.The brain depends on glucose for metabolic functions; therefore, recurrent hypoglycemic episodes can lead to cognitive impairment. However, effects of hypoglycemia on other peripheral tissues such as the retina are understudied. We previously discovered that like the brain, neuronal cells of the retina express insulin gene transcripts INS1 and INS2 that are found in the neuroretina and retinal pigment epithelium. The purpose of this project is to determine whether glycemic stress induces changes in local insulin production in the retina.

Methods : To investigate this normal, euglycemic mice were injected intraperitoneally with streptozotocin (STZ; 75 mg/kg in 0.01M sodium-citrate buffer, pH 4.5) to induce type 1 diabetes over a three day period. We utilized a model of recurrent, acute hypoglycemia in mice using the insulin analog, Humulin-R, to induce sustained hypoglycemia for 3 days. The genes for insulin (INS1 and INS2) were quantified by qRT-PCR. An enzyme linked immunosorbent assay (ELISA) assay was used to test for insulin and C-peptide in the retinal tissues. Retinal sections were tested for the retinal stress-associated protein glial fibrillary acidic protein (GFAP). A terminal deoxynucleodtidyl transferase (TdT) dUTP nick-end labeling (TUNEL) assay was used to test the hypoglycemic mice retinas for apoptotic cells. Additionally, cryosections were stained with hematoxylin and eosin to examine the retinal morphological changes in control, hypoglycemic, and diabetic mice.

Results : Both hypoglycemic and hyperglycemic mice had distorted retinal tissue when compared to the control mice. Retinas of hypoglycemic and hyperglycemic mice displayed increased GFAP immunoreactivity and increased apoptotic cells. Changes in retinal mRNA expression of INS1 and INS2 along with insulin and C-peptide protein levels were observed in both hyperglycemic and hypoglycemic mice.

Conclusions : We conclude that changes in local insulin production in the neuroretina could be an important mechanism of neurodegeneration that occurs as a result of glycemic stress in the retina.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.