Regulation of glycogen phosphorylase by the malin‐laforin complex

Abstract

Autosomal recessive mutations in genes encoding malin or laforin cause Lafora disease (LD), a progressive myoclonic epilepsy. Pathological aberrant glycogen accumulations in brain and various other organs are a hallmark of LD. Our group identified laforin as a novel glycogen phosphatase essential for maintaining glycogen in a soluble form. Malin is a RING‐type E3 ubiquitin ligase. Malin is known to ubiquitinate proteins involved in glycogen metabolism such as laforin, glycogen synthase, protein targeting to glycogen and glycogen debranching enzyme. However, except laforin, expression of these proteins is unchanged in malin knockout mice and thus malin's role in LD pathogenesis is still unclear. Glycogen phosphorylase and glycogen debranching enzyme are vital in catabolism of cellular glycogen. In this study, we aimed to discover substrate/s of malin and its function in LD pathogenesis. We employed immunoprecipitation, western analysis, immunocytochemistry and enzyme activity measurements. Our data uncovered glycogen phosphorylase as a novel substrate of malin. The expression of glycogen phosphorylase was decreased in liver and skeletal muscle tissues from malin knockout mice and glycogen phosphorylase expression was unchanged in tissues from laforin knockout mice. Similarly, overexpression of LD mutations of malin in HEK‐293 cells decreased glycogen phosphorylase expression. In addition, transient overexpression of malin in HEK‐ 293 cells increased the endogenous expression of glycogen phosphorylase. Taken together, our results have identified a novel substrate of malin. Further exploration into the mechanisms of this regulation will unravel insights in glycogen metabolism, physiological functions of malin‐laforin complex and will aid in betterment of LD pharmacotherapy.Supported by NIH grants R00NS061803, R01NS070899 and University of Kentucky College of Medicine startup funds.