Novel glycogen synthase kinase 3 and ubiquitination pathways in progressive myoclonus epilepsy

Abstract

Lafora progressive myoclonus epilepsy, caused by defective laforin or malin, insidiously present in normal teenagers with cognitive decline, followed by rapidly intractable epilepsy, dementia and death. Pathology reveals neurodegeneration with neurofibrillary tangle formation and Lafora bodies (LBs). LBs are deposits of starch-like polyglucosans, insufficiently branched and hence insoluble glycogen molecules resulting from glycogen synthase (GS) overactivity relative to glycogen branching enzyme activity. We previously made the unexpected observation that laforin, in the absence of which polyglucosans accumulate, specifically binds polyglucosans. This suggested that laforin's role is to detect polyglucosan appearances during glycogen synthesis and to initiate mechanisms to downregulate GS. Glycogen synthase kinase 3 (GSK3) is the principal inhibitor of GS. Dephosphorylation of GSK3 at Ser 9 activates GSK3 to inhibit GS through phosphorylation at multiple sites. Glucose-6-phosphate is a potent allosteric activator of GS. Glucose-6-phosphate levels are high when the amount of glucose increases and its activation of GS overrides any phospho-inhibition. Here, we show that laforin is a GSK3 Ser 9 phosphatase, and therefore capable of inactivating GS through GSK3. We also show that laforin interacts with malin and that malin is an E3 ubiquitin ligase that binds GS. We propose that laforin, in response to appearance of polyglucosans, directs two negative feedback pathways: polyglucosan-laforin-GSK3-GS to inhibit GS activity and polyglucosan-laforin-malin-GS to remove GS through proteasomal degradation.

Glycogen metabolism in tissues from a mouse model of Lafora disease

“…The measurements of bulk enzyme activity, though, do not give much mechanistic insight into the cause of Lafora body formation. The increased glycogen synthase protein would be consistent with the proposal that laforin participates in glycogen synthase degradation but not that it activates GSK-3 by NH 2 -terminal dephosphorylation [23]. Branching enzyme activity is also increased in the transgenic muscle so that the ratio of glycogen synthase to branching activities is little changed.…”

“…Loss of function mutations in EPM2B should thus stabilize laforin making it hard to understand how recessive mutations in either EPM2A or EPM2B result in the disease. Lohi et al [23] found that malin interacts also with glycogen synthase and propose that the laforin-malin-glycogen synthase complex is targeted for degradation. In the same study, it was also reported that laforin could dephosphorylate the NH 2 -terminal inhibitory phosphorylation site of the protein kinase GSK-3 which should result in glycogen synthase inactivation.…”

Recent advances in the molecular basis of Lafora’s progressive myoclonus epilepsy

“…4). While the mutation did not affect the subcellular localization of the laforin mutant, the phosphatase activity of the pro- (Gentry et al 2005), and glycogen synthase kinase 3 (GSK3) (Lohi et al 2005a). Laforin-laforin interaction has also been documented (Fernandez-Sanchez et al 2003).…”

“…Malin interacts at least with four distinct E2 ubiquitin conjugating enzymes (Ubc) involved in the ubiquitination process: UbcH2, UbcH5a, UbcH5c and UbcH6 (Gentry et al 2005). GS is regulated by GSK3, and the activity of GSK3 is shown to be modulated by laforin (Lohi et al 2005a). Laforin and malin, therefore, appear to regulate critical steps in glycogen metabolism.…”

“…Malin contains the RING finger ligase domain and six NHL repeats functioning as a substrate-interacting motif (Chan et al 2003b;Gentry et al 2005). Malin has been shown to be a single subunit E3 ubiquitin ligase involved in the ubiquitin-mediated proteolysis cascade (Gentry et al 2005;Lohi et al 2005a). Additionally, malin interacts with, and ubiquitinates, laforin, leading to its degradation (Gentry et al 2005).…”