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Biochim Biophys Acta Gen Subj. 2017 Jan;1861(1 Pt A):3388-3398. doi: 10.1016/j.bbagen.2016.08.021. Epub 2016 Aug 31.

A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase.

Author information

1
Comparative Neuromuscular Diseases Laboratory, Department of Clinical Sciences and Services, Royal Veterinary College, London, UK.
2
Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Denmark.
3
Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, USA.
4
School of Natural Sciences and Psychology, Liverpool John Moores University, Liverpool, UK.
5
Department of Comparative Biomedical Sciences, Royal Veterinary College, London, UK.
6
Veterinary Biomedical Sciences Department, University of Minnesota, St. Paul, MN, USA.
7
Veterinary Population Medicine Department, University of Minnesota, St. Paul, MN, USA.
8
Comparative Neuromuscular Diseases Laboratory, Department of Clinical Sciences and Services, Royal Veterinary College, London, UK. Electronic address: rpiercy@rvc.ac.uk.

Abstract

BACKGROUND:

Equine type 1 polysaccharide storage myopathy (PSSM1) is associated with a missense mutation (R309H) in the glycogen synthase (GYS1) gene, enhanced glycogen synthase (GS) activity and excessive glycogen and amylopectate inclusions in muscle.

METHODS:

Equine muscle biochemical and recombinant enzyme kinetic assays in vitro and homology modelling in silico, were used to investigate the hypothesis that higher GS activity in affected horse muscle is caused by higher GS expression, dysregulation, or constitutive activation via a conformational change.

RESULTS:

PSSM1-affected horse muscle had significantly higher glycogen content than control horse muscle despite no difference in GS expression. GS activity was significantly higher in muscle from homozygous mutants than from heterozygote and control horses, in the absence and presence of the allosteric regulator, glucose 6 phosphate (G6P). Muscle from homozygous mutant horses also had significantly increased GS phosphorylation at sites 2+2a and significantly higher AMPKα1 (an upstream kinase) expression than controls, likely reflecting a physiological attempt to reduce GS enzyme activity. Recombinant mutant GS was highly active with a considerably lower Km for UDP-glucose, in the presence and absence of G6P, when compared to wild type GS, and despite its phosphorylation.

CONCLUSIONS:

Elevated activity of the mutant enzyme is associated with ineffective regulation via phosphorylation rendering it constitutively active. Modelling suggested that the mutation disrupts a salt bridge that normally stabilises the basal state, shifting the equilibrium to the enzyme's active state.

GENERAL SIGNIFICANCE:

This study explains the gain of function pathogenesis in this highly prevalent polyglucosan myopathy.

KEYWORDS:

Glycogen; Glycogen storage disease; Glycogen synthase; Muscle; PSSM1; Polyglucosan

PMID:
27592162
PMCID:
PMC5148651
DOI:
10.1016/j.bbagen.2016.08.021
[Indexed for MEDLINE]
Free PMC Article

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