Control of Glycogen Content in Retina: Allosteric Regulation of Glycogen Synthase

Abstract

Retinal tissue is exceptional because it shows a high level of energy metabolism. Glycogen content represents the only energy reserve in retina, but its levels are limited. Therefore, elucidation of the mechanisms controlling glycogen content in retina will allow us to understand retina response under local energy demands that can occur under normal and pathological conditions. Thus, we studied retina glycogen levels under different experimental conditions and correlated them with glucose-6-phosphate (G-6-P) content and glycogen synthase (GS) activity.Glycogen and G-6-P content were studied in ex vivo retinas from normal, fasted, streptozotocin-treated, and insulin-induced hypoglycemic rats. Expression levels of GS and its phosphorylated form were also analyzed. Ex vivo retina from normal rats showed low G-6-P (14±2 pmol/mg protein) and glycogen levels (43±3 nmol glycosyl residues/mg protein), which were increased 6 and 3 times, respectively, in streptozotocin diabetic rats. While no changes in phosphorylated GS levels were observed in any condition tested, a positive correlation was found between G-6-P levels with GS activity and glycogen content. The results indicated that in vivo, retina glycogen may act as an immediately accessible energy reserve and that its content was controlled primarily by G-6-P allosteric activation of GS. Therefore, under hypoglycemic situations retina energy supply is strongly compromised and could lead to the alterations observed in type 1 diabetes.

Abnormal Glycogen Storage by Retinal Neurons in Diabetes

“…This may explain experiments in diabetic rats that showed increased GS activity and a 3-fold increase in retinal glycogen compared to nondiabetic controls, but without a measurable change in the level of pGS. 21 Nuclear-cytoplasmic shuttling of GS has been extensively characterized in muscle 23,24 and has been demonstrated in neurons in vitro 12 ; however, this study offers the first evidence of nuclear sequestration of pGS in CNS neurons in vivo and alteration in response to diabetes. Significantly, the amacrine cells that contained excessive glycogen stores in the diabetic retina often showed loss or reduction of nuclear pGS, but positive staining for the enzyme in their glycogen-filled cytoplasm.…”

“…Although it is not possible to completely exclude the possibility of an overall downregulation of GS, our view is supported by a recent study showing increased retinal GS activity, but with no quantitative alteration in GS expression during diabetes. 21 Historically, much attention has been paid to the phosphorylation state of GS, and recently in relation to its role in the ''silencing'' of GS in neurons 12 ; however, the bulk of available evidence suggests that phosphorylation plays only a ''fine-tuning'' role in GS regulation, and that allosteric activation by G6P represents the dominant control mechanism. 33 Indeed Roach et al 1 emphasize that the presence of G6P may overcome phosphorylationmediated inactivation of GS and restore full enzymatic activity.…”

“…13 Although primary neurodegeneration and functional hypoxia represent features of early diabetic retinopathy, [14][15][16][17][18][19] increased glycogen synthesis in the diabetic retina, although the cellular distribution of the polysaccharide has not been addressed. [20][21][22] It is therefore important to document the distribution of glycogen in retinal neurons and to consider the possible roles of this important glucose storage molecule in the context of diabetic retinopathy.…”

In the Early Stages of Diabetes, Rat Retinal Mitochondria Undergo Mild Uncoupling due to UCP2 Activity

“…Western blots were performed as previously described [ 18 ]. A retina was transferred into a lysis buffer (1:3 (p/v): RIPA-Tris buffer (2mM EGTA; 316mM NaCl; 20mM Na 2 MoO 4 ; 50mM NaF; 20mM Tris-HCl; 100mM PMSF and 100mM EDTA; 0.1% leupeptine and 0.1% aprotinine; 0.2% SDS and 2% Triton-X100) and maintained under constant shaking for 1 h at 4°C.…”

Hypoglycemia and Retinal Cell Death

“…Moreover, Müller cells can resist glucose limitation, as they use other substrates such as lactate or pyruvate to produce energy substrate by the tricarboxylic acid cycle. 12 Glucose is important since electrical activity (needed for visual functions) depends on its availability. 10 Glycogen content is regulated by glycogen synthase (GS) and glycogen phosphorylase (GP), which are mainly localized in Müller cells.…”