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. 2015 Jun 15;593(12):2693-706.
doi: 10.1113/JP270085. Epub 2015 May 18.

Phenotype Consequences of Myophosphorylase Dysfunction: Insights From the McArdle Mouse Model

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Phenotype Consequences of Myophosphorylase Dysfunction: Insights From the McArdle Mouse Model

Astrid Brull et al. J Physiol. .
Free PMC article

Abstract

Key points: This is the first study to analyse the effect of muscle glycogen phosphorylase depletion in metabolically different muscle types. In McArdle mice, muscle glycogen phosphorylase is absent in both oxidative and glycolytic muscles. In McArdle mice, the glycogen debranching enzyme (catabolic) is increased in oxidative muscles, whereas the glycogen branching enzyme (anabolic) is increased in glycolytic muscles. In McArdle mice, total glycogen synthase is decreased in both oxidative and glycolytic muscles, whereas the phosphorylated inactive form of the enzyme is increased in both oxidative and glycolytic enzymes. In McArdle mice, glycogen content is higher in glycolytic muscles than in oxidative muscles. Additionally, in all muscles analysed, the glycogen content is higher in males than in females. The maximal endurance capacity of the McArdle mice is significantly lower compared to heterozygous and wild-type mice.

Abstract: McArdle disease, caused by inherited deficiency of the enzyme muscle glycogen phosphorylase (GP-MM), is arguably the paradigm of exercise intolerance. The recent knock-in (p.R50X/p.R50X) mouse disease model allows an investigation of the phenotypic consequences of muscle glycogen unavailability and the physiopathology of exercise intolerance. We analysed, in 2-month-old mice [wild-type (wt/wt), heterozygous (p.R50X/wt) and p.R50X/p.R50X)], maximal endurance exercise capacity and the molecular consequences of an absence of GP-MM in the main glycogen metabolism regulatory enzymes: glycogen synthase, glycogen branching enzyme and glycogen debranching enzyme, as well as glycogen content in slow-twitch (soleus), intermediate (gastrocnemius) and glycolytic/fast-twitch (extensor digitorum longus; EDL) muscles. Compared with wt/wt, exercise capacity (measured in a treadmill test) was impaired in p.R50X/p.R50X (∼48%) and p.R50X/wt mice (∼18%). p.R50X/p.R50X mice showed an absence of GP-MM in the three muscles. GP-MM was reduced in p.R50X/wt mice, especially in the soleus, suggesting that the function of 'slow-twitch' muscles is less dependent on glycogen catabolism. p.R50X/p.R50X mice showed increased glycogen debranching enzyme in the soleus, increased glycogen branching enzyme in the gastrocnemius and EDL, as well as reduced levels of mucle glycogen synthase protein in the three muscles (mean ∼70%), reflecting a protective mechanism for preventing deleterious glycogen accumulation. Additionally, glycogen content was highest in the EDL of p.R50X/p.R50X mice. Amongst other findings, the present study shows that the expression of the main muscle glycogen regulatory enzymes differs depending on the muscle phenotype (slow- vs. fast-twitch) and that even partial GP-MM deficiency affects maximal endurance capacity. Our knock-in model might help to provide insights into the importance of glycogen on muscle function.

Figures

Figure 1
Figure 1
Levels of enzymes of glycogen metabolism enzymes in the muscles of healthy mice A, western blot analyses. B and C, levels of the two catabolic (GP-MM and GDE) and anabolic enzymes (GS-M and GDE), respectively. Data are shown as the mean ± SD and individual values, with males indicated by black coloured squares, dots or triangles. Significant post hoc differences are indicated in parenthesis (*P < 0.01).
Figure 2
Figure 2
Comparison of protein and transcript levels of GP-MM according to muscle type and Pygm genotype A, western blot analyses in gastrocnemius muscle. B and C, protein and transcript levels, respectively, in muscles. Data are shown as the mean ± SD and individual values, with males indicated by black coloured squares, dots or triangles. GAPDH, glyceraldehyde 3-phosphate dehydrogenase. Significant post hoc differences are indicated in parenthesis (*P < 0.01). In heterozygous mice, GP-MM levels were significantly lower in the soleus muscle compared to the gastrocnemius and EDL (both P < 0.001).
Figure 3
Figure 3
Effect of Pygm genotype and muscle type on GBE and GDE A, western blot analyses in the gastrocnemius muscle. B, effect of Pygm genotype and muscle type on GBE. C, effect of Pygm genotype and muscle type on GDE. Data are shown as the mean ± SD and individual values, with males indicated by black coloured squares, dots or triangles. GAPDH, glyceraldehyde 3-phosphate dehydrogenase. Significant post hoc differences are indicated in parenthesis (*P < 0.01; **P < 0.05).
Figure 4
Figure 4
Effect of Pygm genotype and muscle type on GS-M and pGS-M A, western blot analyses in the gastrocnemius muscle. B, effect of Pygm genotype and muscle type on GS-M. C, effect of Pygm genotype and muscle type on pGS-M. Data are shown as the mean ± SD and individual values, with males indicated by black coloured squares, dots or triangles. GAPDH, glyceraldehyde 3-phosphate dehydrogenase. Significant post hoc differences are indicated in parenthesis (*P < 0.01).
Figure 5
Figure 5
Effect of Pygm genotype and muscle type on muscle glycogen content Data are shown as the mean ± SD. Significant post hoc differences are indicated in parenthesis (*P < 0.01; **P < 0.05).
Figure 6
Figure 6
Histochemical analysis of a gastrocnemius muscle from a McArdle mouse Haematoxylin and eosin (A), Gomory’s trichrome (B), PAS (C) and succinate dehydrogenase (D) staining are shown. Scale bar = 50 μm.

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