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

Abstract

Summary 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.

Muscle glycogen concentrations and response to diet and exercise regimes in Warmblood horses with type 2 Polysaccharide Storage Myopathy

“…The diet and exercise regime recommended for PSSM2 horses was originally designed to decrease glycogen synthesis in skeletal muscle and improve oxidative metabolism of glycogen and fat during exercise. In PSSM1 horses, the diet effectively lowers serum insulin concentrations which could thereby decrease glucose uptake and decrease activation of glycogen synthase in skeletal muscle [ 9 ; 30 ]. Whereas glycogen concentrations are >1.5 fold normal in PSSM1 WB, a recent study did not find significantly elevated mean glycogen concentrations in the 13 PSSM2 WB studied [ 14 ].…”

Myopathies Related to Glycogen Metabolism Disorders

“…Patients with GSD 0 can present with exercise-induced arrhythmia, cardiomyopathy, and/or exercise intolerance and possibly epilepsy [46,47]. Interestingly, mutations in GYS1 are a common cause of exertional rhabdomyolysis and polysaccharide storage myopathy (PSSM) in racehorses [48,49]. A characteristic feature on muscle biopsy in humans is the absence of glycogen and polyglucosan bodies with mitochondrial proliferation that can mimic mitochondrial myopathies [46].…”

Species‐specific consequences of an E40K missense mutation in superoxide dismutase 1 (SOD1)

“…9 However, as ALS-like diseases, such as equine motor neuron disease and long motor nerve degeneration (eg, as seen in recurrent laryngeal neuropathy (RLN)) occur in horses-some with high prevalence 10,11 -it seems reasonable to speculate that wild-type K40 horse SOD1 could predispose these animals to neurodegenerative diseases. [14][15][16] If true for horse SOD1, this would shed significant light on the role that the enzyme and its associated pathways play in mammalian neurophysiology and highlight the interaction between evolutionary biology and the modern environment and domestication. Similar domestication and modern managementassociated disease exacerbation occurs in another highly prevalent equine genetic neuromuscular disease, type 1 polysaccharide storage myopathy.…”

“…Similar domestication and modern managementassociated disease exacerbation occurs in another highly prevalent equine genetic neuromuscular disease, type 1 polysaccharide storage myopathy. [14][15][16] If true for horse SOD1, this would shed significant light on the role that the enzyme and its associated pathways play in mammalian neurophysiology and highlight the interaction between evolutionary biology and the modern environment and domestication.…”