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Mechanism suppressing glycogen synthesis in neurons and its demise in progressive myoclonus epilepsy

Abstract

Glycogen synthesis is normally absent in neurons. However, inclusion bodies resembling abnormal glycogen accumulate in several neurological diseases, particularly in progressive myoclonus epilepsy or Lafora disease. We show here that mouse neurons have the enzymatic machinery for synthesizing glycogen, but that it is suppressed by retention of muscle glycogen synthase (MGS) in the phosphorylated, inactive state. This suppression was further ensured by a complex of laforin and malin, which are the two proteins whose mutations cause Lafora disease. The laforin-malin complex caused proteasome-dependent degradation both of the adaptor protein targeting to glycogen, PTG, which brings protein phosphatase 1 to MGS for activation, and of MGS itself. Enforced expression of PTG led to glycogen deposition in neurons and caused apoptosis. Therefore, the malin-laforin complex ensures a blockade of neuronal glycogen synthesis even under intense glycogenic conditions. Here we explain the formation of polyglucosan inclusions in Lafora disease by demonstrating a crucial role for laforin and malin in glycogen synthesis.

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Figure 1: Neurons express MGS, but do not accumulate glycogen.
Figure 2: Effects of increased intracellular levels of G6P or overexpression of MGS.
Figure 3: PTG expression activates neuronal MGS and results in glycogen accumulation.
Figure 4: Accumulation of glycogen promotes apoptosis in primary cultured neurons.
Figure 5: Blockade of glycogen synthesis by laforin and malin.
Figure 6: The significance of the interaction between laforin and malin in glycogen metabolism.

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Acknowledgements

We thank J. Massagué for providing a critical review of the manuscript, P. Sanz and J.M. Serratosa for their advice, A. Adrover and E. Veza for their technical support, and T. Yates for correcting the manuscript. We also thank R.R. Gomis for the AdCMV-PTG virus, O. Baba for the monoclonal antibody to glycogen and D.G. Hardy for the gift of the PGSser7/10 antibody. This study was supported by grants from the Fundació La Caixa, Fundació La Marató de TV3, Fundación Marcelino Botín, the Spanish Ministry of Education and Science (SAF2005-00913; BFU2005-02253) and the Instituto de Salud Carlos III (CIBER-ER; RD06/0015/0030).

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D.V. conducted most of the experiments, data analysis and interpretation. S.R. generated the AdCMV-laf, AdCMV-malin and AdCMV-malinD146N recombinant adenoviruses. D.C. carried out the RT-PCR experiments. L.P. contributed to the primary neuron cultures and the apoptosis assays. J.V. carried out the analysis of glycogen branching. S.R., D.C. and J.V. also contributed to other experiments. B.G.-F., O.C.-G., E.F.-S. and I.M.-F. generated the monoclonal laforin antibody, pCINeo-Laforin vector and pcDNA3-Malin-HA vector. J.D. supervised several experiments and the data analysis, and contributed to writing the manuscript. M.G.-R. supervised the western blot and immunofluorescence experiments. E.S. contributed with his knowledge of the nervous system. S.R.d.C. and J.J.G. planned and supervised the project, co-wrote the manuscript and contributed to every aspect of the project.

Corresponding authors

Correspondence to Santiago Rodríguez de Córdoba or Joan J Guinovart.

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Vilchez, D., Ros, S., Cifuentes, D. et al. Mechanism suppressing glycogen synthesis in neurons and its demise in progressive myoclonus epilepsy. Nat Neurosci 10, 1407–1413 (2007). https://doi.org/10.1038/nn1998

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