Myophosphorylase deficiency affects muscle mitochondrial respiration as shown by 31P-MR spectroscopy in a case with associated multifocal encephalopathy

Low aerobic capacity in McArdle disease: A role for mitochondrial network impairment?

BACKGROUND

McArdle disease is caused by myophosphorylase deficiency and results in complete inability for muscle glycogen breakdown. A hallmark of this condition is muscle oxidation impairment (e.g., low peak oxygen uptake (VO_2peak)), a phenomenon traditionally attributed to reduced glycolytic flux and Krebs cycle anaplerosis. Here we hypothesized an additional role for muscle mitochondrial network alterations associated with massive intracellular glycogen accumulation.

METHODS

We analyzed in depth mitochondrial characteristics-content, biogenesis, ultrastructure-and network integrity in skeletal-muscle from McArdle/control mice and two patients. We also determined VO_2peak in patients (both sexes, N = 145) and healthy controls (N = 133).

RESULTS

Besides corroborating very poor VO_2peak values in patients and impairment in muscle glycolytic flux, we found that, in McArdle muscle: (a) damaged fibers are likely those with a higher mitochondrial and glycogen content, which show major disruption of the three main cytoskeleton components-actin microfilaments, microtubules and intermediate filaments-thereby contributing to mitochondrial network disruption in skeletal muscle fibers; (b) there was an altered subcellular localization of mitochondrial fission/fusion proteins and of the sarcoplasmic reticulum protein calsequestrin-with subsequent alteration in mitochondrial dynamics/function; impairment in mitochondrial content/biogenesis; and (c) several OXPHOS-related complex proteins/activities were also affected.

CONCLUSIONS

In McArdle disease, severe muscle oxidative capacity impairment could also be explained by a disruption of the mitochondrial network, at least in those fibers with a higher capacity for glycogen accumulation. Our findings might pave the way for future research addressing the potential involvement of mitochondrial network alterations in the pathophysiology of other glycogenoses.

Long-Term Exercise Intervention in Patients with McArdle Disease: Clinical and Aerobic Fitness Benefits

INTRODUCTION

The long-term effects of exercise in patients with McArdle disease-the paradigm of "exercise intolerance"-are unknown. This is an important question because the severity of the disease frequently increases with time.

PURPOSE

This study aimed to study the effects of a long-term exercise intervention on clinical and fitness-related outcomes in McArdle patients.

METHODS

Seventeen patients (exercise group: n = 10, 6 male, 38 ± 18 yr; control: n = 7, 4 male, 38 ± 18 yr) participated in a 2-yr unsupervised intervention including moderate-intensity aerobic (cycle-ergometer exercise for 1 h) and resistance (high load-low repetition circuit) training on 5 and 2-3 d·wk -1 , respectively. Patients were assessed at baseline and postintervention. Besides safety, outcomes included clinical severity (e.g., exercise intolerance features) on a 0-3 scale (primary outcome), and aerobic fitness, gross muscle efficiency, and body composition (total/regional fat, muscle, and bone mass; secondary outcomes).

RESULTS

The exercise program was safe and resulted in a reduction of 1 point (-1.0; 95% confidence interval, -1.6 to -0.5; P = 0.025) in clinical severity versus the control group, with 60% of participants in the exercise group becoming virtually asymptomatic and with no functional limitation in daily life activities. Compared with controls, the intervention induced significant and large benefits (all P < 0.05) in the workload eliciting the ventilatory threshold (both in absolute (watts, +37%) and relative units (watts per kilogram of total body mass or of lower-limb muscle mass, +44%)), peak oxygen uptake (in milliliters per kilogram per minute, +28%), and peak workload (in absolute (+27%) and relative units (+33%)). However, no significant changes were found for muscle efficiency or for any measure of body composition.

CONCLUSIONS

A 2-yr unsupervised intervention including aerobic and resistance exercise is safe and induces major benefits in the clinical course and aerobic fitness of patients with McArdle disease.

13C/31P MRS Metabolic Biomarkers of Disease Progression and Response to AAV Delivery of hGAA in a Mouse Model of Pompe Disease

The development of therapeutic clinical trials for glycogen storage disorders, including Pompe disease, has called for non-invasive and objective biomarkers. Glycogen accumulation can be measured in vivo with ¹³C MRS. However, clinical implementation remains challenging due to low signal-to-noise. On the other hand, the buildup of glycolytic intermediates may be detected with ³¹P MRS. We sought to identify new biomarkers of disease progression in muscle using ¹³C/³¹P MRS and ¹H HR-MAS in a mouse model of Pompe disease (Gaa^−/−). We evaluated the sensitivity of these MR biomarkers in vivo after treatment using an adeno-associated virus vector 2/9 encoding hGAA driven by the desmin promotor. ³¹P MRS showed significantly elevated phosphomonoesters (PMEs) in Gaa^−/− compared to control at 2 (0.06 ± 0.02 versus 0.03 ± 0.01; p = 0.003), 6, 12, and 18 months of age. Correlative ¹H HR-MAS measures in intact gastrocnemius muscles revealed high glucose-6-phosphate (G-6-P). After intramuscular AAV injections, glycogen, PME, and G-6-P were decreased within normal range. The changes in PME levels likely partly resulted from changes in G-6-P, one of the overlapping phosphomonoesters in the ³¹P MR spectra in vivo. Because ³¹P MRS is inherently more sensitive than ¹³C MRS, PME levels have greater potential as a clinical biomarker and should be considered as a complementary approach for future studies in Pompe patients.

Higher oxidative stress in skeletal muscle of McArdle disease patients

McArdle disease (MCD) is an autosomal recessive condition resulting from skeletal muscle glycogen phosphorylase deficiency. The resultant block in glycogenolysis leads to an increased flux through the xanthine oxidase pathway (myogenic hyperuricemia) and could lead to an increase in oxidative stress. We examined markers of oxidative stress (8-isoprostane and protein carbonyls), NAD(P)H-oxidase, xanthine oxidase and antioxidant enzyme (superoxide dismutase, catalase and glutathione peroxidase) activity in skeletal muscle of MCD patients (N = 12) and controls (N = 12). Eight-isoprostanes and protein carbonyls were higher in MCD patients as compared to controls (p < 0.05). There was a compensatory up-regulation of catalase protein content and activity (p < 0.05), mitochondrial superoxide dismutase (MnSOD) protein content (p < 0.01) and activity (p < 0.05) in MCD patients, yet this increase was not sufficient to protect the muscle against elevated oxidative damage. These results suggest that oxidative stress in McArdle patients occurs and future studies should evaluate a potential role for oxidative stress contributing to acute pathology (rhabdomyolysis) and possibly later onset fixed myopathy.

Isoform-selective regulation of glycogen phosphorylase by energy deprivation and phosphorylation in astrocytes

Glycogen phosphorylase (GP) is activated to degrade glycogen in response to different stimuli, to support both the astrocyte's own metabolic demand and the metabolic needs of neurons. The regulatory mechanism allowing such a glycogenolytic response to distinct triggers remains incompletely understood. In the present study, we used siRNA-mediated differential knockdown of the two isoforms of GP expressed in astrocytes, muscle isoform (GPMM), and brain isoform (GPBB), to analyze isoform-specific regulatory characteristics in a cellular setting. Subsequently, we tested the response of each isoform to phosphorylation, triggered by incubation with norepinephrine (NE), and to AMP, increased by glucose deprivation in cells in which expression of one GP isoform had been silenced. Successful knockdown was demonstrated on the protein level by Western blot, and on a functional level by determination of glycogen content showing an increase in glycogen levels following knockdown of either GPMM or GPBB. NE triggered glycogenolysis within 15 min in control cells and after GPBB knockdown. However, astrocytes in which expression of GPMM had been silenced showed a delay in response to NE, with glycogen levels significantly reduced only after 60 min. In contrast, allosteric activation of GP by AMP, induced by glucose deprivation, seemed to mainly affect GPBB, as only knockdown of GPBB, but not of GPMM, delayed the glycogenolytic response to glucose deprivation. Our results indicate that the two GP isoforms expressed in astrocytes respond to different physiological triggers, therefore conferring distinct metabolic functions of brain glycogen.

Mitochondrial DNA point mutation in the COI gene in a patient with McArdle's disease

We studied a 57-year-old female patient with clinical and biochemical evidences of McArdle's disease. Her muscle biopsy also revealed signs of mitochondrial proliferation, scattered RRF, and a deficit in complex I of the respiratory chain. Molecular genetic analysis showed that the patient was heterozygous for the most common mutation at codon 49 in the myophosphorylase gene. Mitochondrial DNA analysis of muscle tissue revealed an additional G-to-A transition at nucleotide position 7444 in the cytochrome c oxidase subunit I (COI) gene.

Influence of cytosolic pH on in vivo assessment of human muscle mitochondrial respiration by phosphorus magnetic resonance spectroscopy

The authors present an in vivo phosphorus magnetic resonance spectroscopy systematic study on the effects of cytosolic pH on skeletal muscle mitochondrial respiration in human calf muscle. In 49 normal subjects, the effect of cytosolic pH on kinetics of phosphocreatine and adenosine diphosphate recovery and on maximum rate of mitochondrial adenosine triphosphate production (Q(MAX)) was evaluated. The results show a strong relationship between the rate of postexercise phosphocreatine recovery and the lowest value of cytosolic pH reached during recovery from exercise (termed minimum pH; r = 0.89); in contrast, both adenosine diphosphate recovery halftime and Q(MAX) were independent of cytosolic pH at the end of exercise.

Molecular characterization of myophosphorylase deficiency in a group of patients from northern Italy

We studied a group of 14 patients from Northern Italy with myophosphorylase deficiency. The disease presented considerable clinical and biochemical heterogeneity, which was reflected at the molecular level. The clinical presentation was typical in 3 patients, mild in 7 (exercise intolerance), and severe in 4 (fixed weakness). Enzyme activity was undetectable in 10 patients, below 3% of control in 3, and 13% of control in one. Enzymatic protein was detectable immunologically only in 1 patient. Myophosphorylase mRNA was present in 8 patients, but in 7 of them it was reduced in amount. Two patients were homozygous for the common nonsense R49X mutation, 5 were heterozygous. Two missense mutations not previously observed were identified in this group of patients. The frequency of alleles with the R49X mutation was significantly lower in this group of patients than in previously reported series. Myophosphorylase deficiency is genetically heterogeneous even among patients living in a small region and with a common ethnic background.