Polyglucosan neurotoxicity caused by glycogen branching enzyme deficiency can be reversed by inhibition of glycogen synthase

Abstract

Uncontrolled elongation of glycogen chains, not adequately balanced by their branching, leads to the formation of an insoluble, presumably neurotoxic, form of glycogen called polyglucosan. To test the suspected pathogenicity of polyglucosans in neurological glycogenoses, we have modeled the typical glycogenosis Adult Polyglucosan Body Disease (APBD) by suppressing glycogen branching enzyme 1 (GBE1, EC 2.4.1.18) expression using lentiviruses harboring short hairpin RNA (shRNA). GBE1 suppression in embryonic cortical neurons led to polyglucosan accumulation and associated apoptosis, which were reversible by rapamycin or starvation treatments. Further analysis revealed that rapamycin and starvation led to phosphorylation and inactivation of glycogen synthase (GS, EC 2.4.1.11), dephosphorylated and activated in the GBE1-suppressed neurons. These protective effects of rapamycin and starvation were reversed by overexpression of phosphorylation site mutant GS only if its glycogen binding site was intact. While rapamycin and starvation induce autophagy, autophagic maturation was not required for their corrective effects, which prevailed even if autophagic flux was inhibited by vinblastine. Furthermore, polyglucosans were not observed in any compartment along the autophagic pathway. Our data suggest that glycogen branching enzyme repression in glycogenoses can cause pathogenic polyglucosan buildup, which might be corrected by GS inhibition. Keywords: adult polyglucosan body disease, autophagy, glycogen branching enzyme, glycogen synthase, polyglucosan, polyglucosan bodies.

A novel image-based high-throughput screening assay discovers therapeutic candidates for adult polyglucosan body disease

“…While accumulating in multiple tissues, PB lead to cell death only in neurons, where they can clog the narrow axons(3), or dendrites(4). This PB-mediated neuronal death was also confirmed in tissue culture(5). The main forms of GSD type IV are the Andersen Disease (OMIM 232500)(6), a fatal liver disorder with progressive cirrhosis of the organ leading to its failure, and the neurodegenerative disorder Adult Polyglucosan Body Disease (APBD, OMIM 263570)(7).…”

“…As our results (Figure 1A) show, amylase resistant staining, and therefore PG and PB, were uniquely observed in APBD and not in control fibroblasts. Rapamycin, an established inhibitor of GS(5), not used as a therapeutic drug due to its toxicity in animal models (unpublished results), was used to demonstrate our ability to reduce PB pharmacologically. Furthermore, to anticipate the possible range of PB reduction by tested compounds, we have co-cultured patient and healthy control fibroblasts in different ratios (Figure 1B).…”

A Modified Enzymatic Method for Measurement of Glycogen Content in Glycogen Storage Disease Type IV

“…Mutations in the Gbe1 gene cause a complete or partial loss of GBE activity in GSD IV, which leads to an increase in the ratio of GS to GBE, a critical determinant of PB formation during the process of glycogen synthesis (Raben et al 2001;Pederson et al 2003;Kakhlon et al 2013). The Y329S is the most common mutation found in Jewish families of Ashkenazi ancestry with adult onset GSD IV, also referred to as adult polyglucosan body disease (Lossos et al 1998;Mochel et al 2012).…”

Myocardial stress and autophagy: mechanisms and potential therapies

“…In neurons, knockdown of glycogen branching enzyme inhibits autophagy, and conversely, activation of autophagy by rapamycin treatment, inhibits glycogen synthase activity. 54 In a mouse model of Lafora disease (a genetic neurodegenerative condition characterized by abnormally branched and insoluble intracellular glycogen deposits), the glycogen overload and associated autophagy impairment is mitigated when glycogen synthase deficiency is induced. 55 These observations have prompted speculation that glycogen accumulation may be a cause, not a consequence, of autophagy impairment, at least in this genetic context of glycogen storage abnormality.…”