Immunohistochemical Localization of Glycogen Synthase and GSK3β: Control of Glycogen Content in Retina

Abstract

Glycogen has an important role in energy handling in several brain regions. In the brain, glycogen is localized in astrocytes and its role in several normal and pathological processes has been described, whereas in the retina, glycogen metabolism has been scarcely investigated. The enzyme glycogen phosphorylase has been located in retinal Müller cells; however the cellular location of glycogen synthase (GS) and its regulatory partner, glycogen synthase kinase 3β (GSK3β), has not been investigated. Our aim was to localize these enzymes in the rat retina by immunofluorescence techniques. We found both GS and GSK3β in Müller cells in the synaptic layers, and within the inner segments of photoreceptor cells. The presence of these enzymes in Müller cells suggests that glycogen could be regulated within the retina as in other tissues. Indeed, we showed that glycogen content in the whole retina in vitro was increased by high glucose concentrations, glutamate, and insulin. In contrast, retina glycogen levels were not modified by norepinephrine nor by depolarization with high KCl concentrations. Insulin also induced an increase in glycogen content in cultured Müller cells. The effect of insulin in both, whole retina and cultured Müller cells was blocked by inhibitors of phosphatidyl-inositol 3-kinase, strongly suggesting that glycogen content in retina is modulated by the insulin signaling pathway. The expression of GS and GSK3β in the synaptic layers and photoreceptor cells suggests an important role of GSK3β regulating glycogen synthase in neurons, which opens multiple feasible roles of insulin within the retina.

Glucose, lactate, and shuttling of metabolites in vertebrate retinas

“…Müller cells also synthesize, store, and degrade glycogen. The enzymes that synthesize and degrade glycogen are present in Müller cells (Pfeiffer‐Guglielmi et al, ; Rothermel et al, ; Perezleon et al, ). The amount of glycogen stored in the retina varies across species.…”

“…In another study, the concentration of glycogen in rabbit retinas was found to be ∼100 times higher at the side of the retina with the Müller cell endfeet compared with the photoreceptor layer (Matschinsky et al, ). Dark adaptation decreases the amount of glycogen stored in rat retinas by ∼60% (Coffe et al, ), and glycogen metabolism in retinas appears to be regulated by the insulin signaling pathway (Perezleon et al, ).…”

Abnormal Glycogen Storage by Retinal Neurons in Diabetes

“…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.…”

“…12,23 Although the known phosphorylation sites are not involved in its nuclear translocation, 24 we considered that the intranuclear enzyme was likely to be inactive and phosphorylated. As previous studies have shown that cytoplasmic GS is widely distributed in the retina, chiefly in the Müller glia, 20 we sought to examine the distribution of the inactive phosphorylated form of the enzyme, to assess its nuclear sequestration in retinal neurons, and how such compartmentalization may be affected by diabetes.…”

Vps35 Deficiency Impairs Cdk5/p35 Degradation and Promotes the Hyperphosphorylation of Tau Protein in Retinal Ganglion Cells

“…GSK-3β is the most frequently reported kinase of tau, which is localized in Müller cells in the synaptic layers and within the inner segments of photoreceptor cells. 21 Our previous study demonstrated that GSK-3β was downregulated in the retinas of Vps35 +/m mice and that GSK-3β colocalized with glial cells but not with RGCs (unpublished data), suggesting that GSK-3β might not be involved in the upregulation of p-tau induced by Vps35 deficiency or needs to be further explored.…”