Fellowship for Dukhande: Regulation of glycogen phosphorylase by the malin-laforin complex: Implications for cardiac and brain glycogenoses.

Autosomal recessive mutations in genes encoding malin or laforin cause Lafora disease (LD), a progressive myoclonic neurodegenerative epilepsy. Mutations in the gamma-subunit of AMP-activated protein kinase (AMPK) result in familial Wolff-Parkinson-White (WPW) syndrome ventricular pre-excitation and cardiac hypertrophy. LD and WPW both have characteristic abnormal glycogen accumulations and could be termed as glycogenoses. Our group identified laforin as a novel glycogen phosphatase essential for maintaining glycogen in a soluble form. We also defined the protein malin as an E3 ubiquitin ligase. We and others demonstrated that malin ubiquitinates proteins involved in glycogen metabolism such as laforin, glycogen synthase, protein targeting to glycogen and glycogen debranching enzyme. Glycogen phosphorylase (GP) and glycogen debranching enzyme are vital in catabolism of cellular glycogen. Mutations in genes encoding GP cause glycogen storage disorders such as McArdle’s disease and Hers’ disease. Therefore, glycogenoses link malin, laforin, AMPK and GP. The role and alterations of GP in LD are not yet defined. In addition, our preliminary data shows that GP protein levels decrease in malin-deficient mice. In this study, we will define the mechanism of regulation of GP by the malin-laforin complex and elucidate its physiological implications for cardiac and brain glycogenoses. We employed immunoprecipitation, western analysis, ubiquitination assay, immunocytochemistry and enzyme activity measurements. Our data uncovered GP as a novel substrate of malin. The expression of GP was decreased in liver and skeletal muscle tissues from malin knockout mice and GP expression was unchanged in brain and liver tissues and was slightly decreased in heart tissue from laforin knockout mice. In addition, our preliminary data indicates that the malin-laforin complex ubiquitinates GP and changes its subcellular localization. Further exploration into the mechanisms of this regulation will unravel insights in glycogen metabolism, physiological functions of the the malin-laforin complex and will aid in the betterment of cardiac, brain and muscle glycogenoses such as LD, WPW syndrome, McArdle’s disease and Hers’ disease.