GSD are a group of disorders characterized by a defect in gene expression of specific enzymes involved in glycogen breakdown or synthesis, commonly resulting in the accumulation of glycogen in various tissues (primarily the liver and skeletal muscle). Several different GSD animal models have been found to naturally present spontaneous mutations and others have been developed and characterized in order to further understand the physiopathology of these diseases and as a useful tool to evaluate potential therapeutic strategies. In the present work we have reviewed a total of 42 different animal models of GSD, including 26 genetically modified mouse models, 15 naturally occurring models (encompassing quails, cats, dogs, sheep, cattle and horses), and one genetically modified zebrafish model. To our knowledge, this is the most complete list of GSD animal models ever reviewed. Importantly, when all these animal models are analyzed together, we can observe some common traits, as well as model specific differences, that would be overlooked if each model was only studied in the context of a given GSD.

Carbohydrate availability affects fat metabolism during exercise; however, the effects of complete muscle glycogen unavailability on maximal fat oxidation (MFO) rate remain unknown. Our purpose was to examine the MFO rate in patients with McArdle disease, comprising an inherited condition caused by complete blockade of muscle glycogen metabolism, compared to healthy controls. Nine patients (three women, aged 36 ± 12 years) and 12 healthy controls (four women, aged 40 ± 13 years) were studied. Several molecular markers of lipid transport/metabolism were also determined in skeletal muscle (gastrocnemius) and white adipose tissue of McArdle (Pygm p.50R*/p.50R*) and wild‐type male mice. Peak oxygen uptake (trueV̇normalO2peak${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$), MFO rate, the exercise intensity eliciting MFO rate (FATmax) and the MFO rate‐associated workload were determined by indirect calorimetry during an incremental cycle‐ergometer test. Despite having a much lower trueV̇normalO2peak${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$ (24.7 ± 4 vs. 42.5 ± 11.4 mL kg−1 min−1, respectively; P < 0.0001), patients showed considerably higher values for the MFO rate (0.53 ± 0.12 vs. 0.33 ± 0.10 g min−1, P = 0.001), and for the FATmax (94.4 ± 7.2 vs. 41.3 ± 9.1 % of trueV̇normalO2peak${\dot V_{{{\rm{O}}_{\rm{2}}}{\rm{peak}}}}$, P < 0.0001) and MFO rate‐associated workload (1.33 ± 0.35 vs. 0.81 ± 0.54 W kg−1, P = 0.020) than controls. No between‐group differences were found overall in molecular markers of lipid transport/metabolism in mice. In summary, patients with McArdle disease show an exceptionally high MFO rate, which they attained at near‐maximal exercise capacity. Pending more mechanistic explanations, these findings support the influence of glycogen availability on MFO rate and suggest that these patients develop a unique fat oxidation capacity, possibly as an adaptation to compensate for the inherited blockade in glycogen metabolism, and point to MFO rate as a potential limiting factor of exercise tolerance in this disease. Key points Physically active McArdle patients show an exceptional fat oxidation capacity. Maximal fat oxidation rate occurs near‐maximal exercise capacity in these patients. McArdle patients’ exercise tolerance might rely on maximal fat oxidation rate capacity. Hyperpnoea might cloud substrate oxidation measurements in some patients. An animal model revealed overall no higher molecular markers of lipid transport/metabolism.

McArdle disease is an autosomal recessive disorder of muscle glycogen metabolism caused by pathogenic mutations in the PYGM gene, which encodes the skeletal muscle-specific isoform of glycogen phosphorylase. Clinical symptoms are mainly characterized by transient acute “crises” of early fatigue, myalgia and contractures, which can be accompanied by rhabdomyolysis. Owing to the difficulty of performing mechanistic studies in patients that often rely on invasive techniques, preclinical models have been used for decades, thereby contributing to gain insight into the pathophysiology and pathobiology of human diseases. In the present work, we describe the existing in vitro and in vivo preclinical models for McArdle disease and review the insights these models have provided. In addition, despite presenting some differences with the typical patient’s phenotype, these models allow for a deep study of the different features of the disease while representing a necessary preclinical step to assess the efficacy and safety of possible treatments before they are tested in patients.

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