Immunohistochemical localization of glycogen synthase and GSK3β: control of glycogen content in retina

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.