P.67Overexpression of a constitutively active glycogen synthase in the GSL30 mouse leads to a polyglucosan myopathy

Abstract

Polyglucosan accumulation is a characteristic feature of several glycogen storage diseases of mammals, including eight in humans and one in horses — the latter associated with a gain of function, through a constitutively active mutant glycogen synthase (GYS1). GSL30 mice overexpressing constitutively active rabbit glycogen synthase in skeletal muscle, accumulate 5-6 times more glycogen in skeletal muscle than wild-type (WT) controls. We hypothesised that this excessive glycogen accumulation would also be associated with a myopathic histological phenotype characterised by polyglucosan accumulation, as seen in GYS1 mutant horses. We examined serum CK activity and skeletal muscle histology from 6 week, 3 month and 8 month old GSL30 mice compared with age-matched WT controls (n=5 to 8). Cryosections from the gastrocnemius, a predominantly fast-twitch muscle, were stained with H&E and amylase-periodic acid Schiff and scored (blinded to genotype) for the presence of internalised nuclei and amylase-resistant polyglucosan within myofibres. Whilst there was no difference in serum CK activity between groups (P>0.05), GSL30 mice had significantly greater numbers of internalised myonuclei than WT controls at 8 months (13.2±6% vs. 0.5%±0.4%; mean±SD; P<0.0001). Polyglucosan was not identified in WT control muscle, but was present increasingly in GSL30 mouse muscle fibres as they aged (reaching 70±5% of myofibres at 8 months). In 6 week old GSL30 mice, amylase-resistant polyglucosan was predominantly subsarcolemmal; by 3 months of age, punctate deposits were identified within the sarcoplasm. By 8 months of age, large pools of glycogen and amylase-resistant polyglucosan, often encompassing the entire myofiber, and focal areas of necrosis were evident. This data reveals that overexpression of glycogen leads to a polyglucosan myopathy in GSL30 mice. Future work will examine the influence of diet and exercise on the phenotype.