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.
Tagliabracci
Vincent S
VS
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
Turnbull
Julie
J
Wang
Wei
W
Girard
Jean-Marie
JM
Zhao
Xiaochu
X
Skurat
Alexander V
AV
Delgado-Escueta
Antonio V
AV
Minassian
Berge A
BA
Depaoli-Roach
Anna A
AA
Roach
Peter J
PJ
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