Ubiquitination of glycogen phosphorylase by the malin‐laforin complex regulates glycogen catabolism (663.17)

Abstract

Lafora disease (LD) is fatal progressive myoclonic epilepsy characterized by pathological aberrant glycogen accumulations in brain and various other organs. Autosomal recessive mutations in genes encoding malin or laforin cause LD. Our group identified laforin as a novel glycogen phosphatase essential for maintaining glycogen in a soluble form. Malin, a RING‐type E3 ubiquitin ligase, is known to ubiquitinate proteins involved in glycogen metabolism such as laforin, glycogen synthase, protein targeting to glycogen and glycogen debranching enzyme. However, malin’s role in LD pathogenesis is not clear. In this study, we aimed to discover substrate/s of malin and its function in LD pathogenesis. We used malin‐ and laforin knockout mice, affinity pull‐down and mass spectrometry, immunoprecipitation, western analysis, fluorescence microscopy, glucan‐binding assay, and enzyme activity measurements. We discovered glycogen phosphorylase (GP) as a novel substrate of malin. GP is vital in catabolism of cellular glycogen. The expression and phosphorylation of GP was decreased in tissues from malin knockout mice. Similarly, overexpression of LD mutations of malin in HEK‐293 cells decreased GP expression. Overexpression of malin in HEK‐293 cells increased the endogenous expression of GP. Malin interacted with and ubiquitinated GP, and presence of malin increased nuclear localization of GP. Malin also altered glucan‐binding of GP. Thus, we have identified a novel control mechanism of glycogen catabolism. Further exploration into the mechanisms of this regulation will unravel insights in glycogen metabolism, physiological functions of the malin‐laforin complex and will aid in betterment of LD pharmacotherapy.Grant Funding Source: Supported by NIH grants R00NS061803, R01NS070899 and University of Kentucky College of Medicine star