MTOR-dependent dysregulation of autophagy contributes to the retinal ganglion cell loss in streptozotocin-induced diabetic retinopathy

Sanjar Batirovich Madrakhimov,Tae Kwann Park,Jin Ha Kim,Jung Woo Han,Jin Young Yang

doi:10.1186/s12964-020-00698-4

Abstract

BackgroundNeurodegeneration, an early event in the pathogenesis of diabetic retinopathy (DR), precedes clinically detectable microvascular damage. Autophagy dysregulation is considered a potential cause of neuronal cell loss, however underlying mechanisms remain unclear. The mechanistic target of rapamycin (mTOR) integrates diverse environmental signals to coordinate biological processes, including autophagy. Here, we investigated the role of mTOR signaling in neuronal cell death in DR.MethodsDiabetes was induced by a single intraperitoneal injection of streptozotocin and tissue samples were harvested at 1, 2, 3, 4, and 6 months of diabetes. Early-stage of DR was investigated in 1-month-diabetic mice treated with phlorizin (two daily subcutaneous injections at a dose of 200 mg/kg of body weight during the last 7 full days of the experiment and the morning of the 8th day, 3 h before sacrifice) or rapamycin (daily intraperitoneal injections, at a dose of 3 mg/kg for the same period as for phlorizin treatment). The effect of autophagy modulation on retinal ganglion cells was investigated in 3-months-diabetic mice treated with phlorizin (two daily subcutaneous injections during the last 10 full days of the experiment and the morning of the 11th day, 3 h before sacrifice) or MHY1485 (daily i.p. injections, at a dose of 10 mg/kg for the same period as for phlorizin treatment). Tissue samples obtained from treated/untreated diabetic mice and age-matched controls were used for Western blot and histologic analysis.ResultsmTOR-related proteins and glucose transporter 1 (GLUT1) was upregulated at 1 month and downregulated in the following period up to 6 months. Diabetes-induced neurodegeneration was characterized by an increase of apoptotic marker—cleaved caspase 3, a decrease of the total number of cells, and NeuN immunoreactivity in the ganglion cell layer, as well as an increase of autophagic protein. Insulin-independent glycemic control restored the mTOR pathway activity and GLUT1 expression, along with a decrease of autophagic and apoptotic proteins in 3-months-diabetic mice neuroretina. However, blockade of autophagy using MHY1485 resulted in a more protective effect on ganglion cells compared with phlorizin treatment.ConclusionCollectively, our study describes the mechanisms of neurodegeneration through the hyperglycemia/ mTOR/ autophagy/ apoptosis pathway.ETHY2cHao8m2tWY7DdTwFeVideo

Highlights

Neurodegeneration, an early event in the pathogenesis of diabetic retinopathy (DR), precedes clinically detectable microvascular damage
Collectively, our study describes the mechanisms of neurodegeneration through the hyperglycemia/ mechanistic target of rapamycin (mTOR)/ autophagy/ apoptosis pathway
Cleaved caspase-3 signals were detected in the ganglion cell layer (GCL) of the diabetic retina, which was weak at 1 month and strong at 3 and 6 months (Fig. 3c), positive signal was not observed in normal retina

Summary

Introduction

Neurodegeneration, an early event in the pathogenesis of diabetic retinopathy (DR), precedes clinically detectable microvascular damage. Autophagy dysregulation is considered a potential cause of neuronal cell loss, underlying mechanisms remain unclear. DR is classified into non-proliferative diabetic retinopathy and proliferative diabetic retinopathy [10, 11] These “early” and “advanced” stages are characterized by the level of microvascular and ischemic damage of the retina, which determines visible clinical manifestations [11]. The classic view on DR as the microcirculatory pathology evolved into the concept of disease of the retinal neurovascular unit (NVU), where neurodegeneration precedes clinically detectable microvascular damage [11, 28,29,30,31]

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