Laforin is a glycogen phosphatase, deficiency of which leads to elevated phosphorylation of glycogen in vivo.
Lafora disease is a progressive myoclonus epilepsy with onset typically in the second decade of life and death within 10 years. Lafora bodies, deposits of abnormally branched, insoluble glycogen-like polymers, form in neurons, muscle, liver, and other tissues. Approximately half of the cases of Lafora disease result from mutations in the EPM2A gene, which encodes laforin, a member of the dual-specificity protein phosphatase family that additionally contains a glycogen binding domain. The molecular basis for the formation of Lafora bodies is completely unknown. Glycogen, a branched polymer of glucose, contains a small amount of covalently linked phosphate whose origin and function are obscure. We report here that recombinant laforin is able to release this phosphate in vitro, in a time-dependent reaction with an apparent K(m) for glycogen of 4.5 mg/ml. Mutations of laforin that disable the glycogen binding domain also eliminate its ability to dephosphorylate glycogen. We have also analyzed glycogen from a mouse model of Lafora disease, Epm2a(-/-) mice, which develop Lafora bodies in several tissues. Glycogen isolated from these mice had a 40% increase in the covalent phosphate content in liver and a 4-fold elevation in muscle. We propose that excessive phosphorylation of glycogen leads to aberrant branching and Lafora body formation. This study provides a molecular link between an observed biochemical property of laforin and the phenotype of a mouse model of Lafora disease. The results also have important implications for glycogen metabolism generally.
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
Vincent S
VS
Tagliabracci
Julie
J
Turnbull
Wei
W
Wang
Jean-Marie
JM
Girard
Xiaochu
X
Zhao
Alexander V
AV
Skurat
Antonio V
AV
Delgado-Escueta
Berge A
BA
Minassian
Anna A
AA
Depaoli-Roach
Peter J
PJ
Roach
D000818:Animals; D004195:Disease Models, Animal; D054637:Dual-Specificity Phosphatases / Q000172:deficiency* / Q000235:genetics; D006003:Glycogen / Q000378:metabolism*; D006006:Glycogen Synthase / Q000032:analysis / Q000378:metabolism; D020192:Lafora Disease / Q000201:enzymology*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D009154:Mutation; D010766:Phosphorylation; D054558:Protein Tyrosine Phosphatases, Non-Receptor; D011817:Rabbits; D011994:Recombinant Proteins / Q000494:pharmacology
D011994:Recombinant Proteins; D006003:Glycogen; D006006:Glycogen Synthase; D054637:Dual-Specificity Phosphatases; C491989:Epm2a protein, mouse; D054558:Protein Tyrosine Phosphatases, Non-Receptor