Breakdown of lysosomal glycogen in cultured fibroblasts from glycogenosis type II patients after uptake of acid alpha-glucosidase

Satellite cells maintain regenerative capacity but fail to repair disease-associated muscle damage in mice with Pompe disease

Pompe disease is a metabolic myopathy that is caused by glycogen accumulation as a result of deficiency of the lysosomal enzyme acid alpha glucosidase (GAA). Previously, we showed that adult muscle stem cells termed satellite cells are present at normal levels in muscle from patients with Pompe disease, but that these are insufficiently activated to repair the severe muscle pathology. Here we characterized the muscle regenerative response during disease progression in a mouse model of Pompe disease and investigated the intrinsic capacity of Gaa^-/- satellite cells to regenerate muscle damage. Gaa^-/- mice showed progressive muscle pathology from 15 weeks of age as reflected by increased lysosomal size, decreased fiber diameter and reduced muscle wet weight. Only during the first 15 weeks of life but not thereafter, we detected a gradual increase in centrally nucleated fibers and proliferating satellite cells in Gaa^-/- muscle, indicating a mild regenerative response. The levels of Pax7-positive satellite cells were increased in Gaa^-/- mice at all ages, most likely as result of enhanced satellite cell activation in young Gaa^-/- animals. Surprisingly, both young and old Gaa^-/- mice regenerated experimentally-induced muscle injury efficiently as judged by rapid satellite cell activation and complete restoration of muscle histology. In response to serial injury, Gaa^-/- mice also regenerated muscle efficiently and maintained the satellite cell pool. These findings suggest that, similar to human patients, Gaa^-/- mice have insufficient satellite cell activation and muscle regeneration during disease progression. The initial endogenous satellite cell response in Gaa^-/- mice may contribute to the delayed onset of muscle wasting compared to human patients. The rapid and efficient regeneration after experimental muscle injury suggest that Gaa^-/- satellite cells are functional stem cells, opening avenues for developing muscle regenerative therapies for Pompe disease.

GAA Deficiency in Pompe Disease Is Alleviated by Exon Inclusion in iPSC-Derived Skeletal Muscle Cells

Pompe disease is a metabolic myopathy caused by deficiency of the acid α-glucosidase (GAA) enzyme and results in progressive wasting of skeletal muscle cells. The c.-32-13T>G (IVS1) GAA variant promotes exon 2 skipping during pre-mRNA splicing and is the most common variant for the childhood/adult disease form. We previously identified antisense oligonucleotides (AONs) that promoted GAA exon 2 inclusion in patient-derived fibroblasts. It was unknown how these AONs would affect GAA splicing in skeletal muscle cells. To test this, we expanded induced pluripotent stem cell (iPSC)-derived myogenic progenitors and differentiated these to multinucleated myotubes. AONs restored splicing in myotubes to a similar extent as in fibroblasts, suggesting that they act by modulating the action of shared splicing regulators. AONs targeted the putative polypyrimidine tract of a cryptic splice acceptor site that was part of a pseudo exon in GAA intron 1. Blocking of the cryptic splice donor of the pseudo exon with AONs likewise promoted GAA exon 2 inclusion. The simultaneous blocking of the cryptic acceptor and cryptic donor sites restored the majority of canonical splicing and alleviated GAA enzyme deficiency. These results highlight the relevance of cryptic splicing in human disease and its potential as therapeutic target for splicing modulation using AONs.

Remarkably low fibroblast acid α-glucosidase activity in three adults with Pompe disease

INTRODUCTION

Most adults with Pompe disease are compound heterozygotes in which one acid α-glucosidase (GAA) allele harbors the c.-32-13T>G mutation, causing partial loss of GAA, and the other allele harbors a fully deleterious mutation. The fibroblast GAA activity in these patients is usually between 5% and 25% of the average in healthy individuals. In some adult patients, however, the fibroblast GAA activity is much lower and is in the range that is normally observed in classic-infantile Pompe disease. We investigated the genotype-phenotype correlation in three such adult patients and measured the GAA activity as well as the glycogen content in muscle and fibroblasts in order to better understand the clinical course.

METHODS

DNA was sequenced and GAA activity and glycogen content were measured in leukocytes, fibroblasts and muscle. Muscle biopsies were microscopically analyzed and the biosynthesis of GAA in fibroblasts was analyzed by immunoblotting. GAA activity and glycogen content in fibroblasts and muscle tissue in healthy controls, adult patients with Pompe disease and classic-infantile patients were compared with those of the three index patients.

RESULTS

One patient had genotype c.525delT/c.671G>A (r.0/p.Arg224Gln). Two affected brothers had genotype c.569G>A/c.1447G>A (p.Arg190His/p.Gly483Arg). In all three cases the GAA activity and the glycogen content in fibroblasts were within the same range as in classic-infantile Pompe disease, but the activity and glycogen content in muscle were both within the adult range. In fibroblasts, the first step of GAA synthesis appeared unaffected but lysosomal forms of GAA were not detectable with immunoblotting.

CONCLUSION

Some adult patients with mutations other than c.-32-13T>G can have very low GAA activity in fibroblasts but express higher activity in muscle and store less glycogen in muscle than patients with classic-infantile Pompe disease. This might explain why these patients have a slowly progressive course of Pompe disease.

Correlation of acid α-glucosidase and glycogen content in skin fibroblasts with age of onset in Pompe disease

BACKGROUND

Pompe disease is an autosomal recessive disorder of glycogen metabolism resulting from a deficiency of acid alpha-glucosidase. Pompe disease can present within a broad clinical spectrum, from the severe infantile to the attenuated adult onset phenotypes. Early diagnosis, in the form of newborn screening has been proposed. However, in the absence of clinical symptoms, prediction of disease severity and progression will be critical to provide appropriate management and treatment of affected individuals.

METHODS

We have used sensitive immune-assays to measure levels of acid alpha-glucosidase protein and activity in cultured skin fibroblasts and a new glycogen assay to specifically determine the lysosomal accumulation of glycogen in the same cells. These markers were assessed for their ability to predict age of onset.

RESULTS

Acid alpha-glucosidase activity and specific activity as well as lysosomal glycogen showed significant correlations with age of onset, with acid alpha-glucosidase activity having the highest Spearman correlation coefficient (0.887, p<0.001). Lysosomal glycogen accumulated only in cells from infantile and juvenile patients but not from adult-onset patients. However, cells from adult-onset patients had relatively low cytoplasmic glycogen compared to control individuals and other forms of the disease.

CONCLUSION

Acid-alpha-glucosidase activity and specific activity, and lysosomal glycogen content are useful predictors of age of onset in Pompe disease.

Morphological changes in muscle tissue of patients with infantile Pompe's disease receiving enzyme replacement therapy

Pompe's disease (glycogen storage disease type II) is an autosomal recessive myopathy caused by lysosomal alpha-glucosidase deficiency. Enzyme replacement therapy (ERT) is currently under development for this disease. We evaluated the morphological changes in muscle tissue of four children with infantile Pompe's disease who received recombinant human alpha-glucosidase from rabbit milk for 72 weeks. The patients were 2.5-8 months of age at entry. Prior to treatment, all patients showed lysosomal glycogen storage in skeletal and smooth muscle cells, vascular endothelium, Schwann cells, and perineurium. The first response to treatment was noticed in vascular endothelium and in peripheral nerves after 12 weeks of treatment at an enzyme dose of 15-20 mg/kg. Increasing the dose to 40 mg/kg led, after 72 weeks of treatment, to a reduction of glycogen storage and substantial improvement of muscle architecture in the least affected patient. Not all patients responded equally well, possibly due to differences in degree of glycogen storage and concomitant muscle pathology at the start of treatment. We conclude that intravenous administration of recombinant human alpha-glucosidase from rabbit milk can improve muscle morphology in classic infantile Pompe's disease when treatment is started before irreversible damage has occurred.

Correction of the enzymatic and functional deficits in a model of Pompe disease using adeno-associated virus vectors

Pompe disease is a lysosomal storage disease caused by the absence of acid alpha-1,4 glucosidase (GAA). The pathophysiology of Pompe disease includes generalized myopathy of both cardiac and skeletal muscle. We sought to use recombinant adeno-associated virus (rAAV) vectors to deliver functional GAA genes in vitro and in vivo. Myotubes and fibroblasts from Pompe patients were transduced in vitro with rAAV2-GAA. At 14 days postinfection, GAA activities were at least fourfold higher than in their respective untransduced controls, with a 10-fold increase observed in GAA-deficient myotubes. BALB/c and Gaa(-/-) mice were also treated with rAAV vectors. Persistent expression of vector-derived human GAA was observed in BALB/c mice up to 6 months after treatment. In Gaa(-/-) mice, intramuscular and intramyocardial delivery of rAAV2-Gaa (carrying the mouse Gaa cDNA) resulted in near-normal enzyme activities. Skeletal muscle contractility was partially restored in the soleus muscles of treated Gaa(-/-) mice, indicating the potential for vector-mediated restoration of both enzymatic activity and muscle function. Furthermore, intramuscular treatment with a recombinant AAV serotype 1 vector (rAAV1-Gaa) led to nearly eight times normal enzymatic activity in Gaa(-/-) mice, with concomitant glycogen clearance as assessed in vitro and by proton magnetic resonance spectroscopy.

Cardiac remodeling and contractile function in acid alpha-glucosidase knockout mice

Pompe's disease is an autosomal recessive and often fatal condition, caused by mutations in the acid alpha-glucosidase gene, leading to lysosomal glycogen storage in heart and skeletal muscle. We investigated the cardiac phenotype of an acid alpha-glucosidase knockout (KO) mouse model. Left ventricular weight-to-body weight ratios were increased 6.3 +/- 0.8 mg/g in seven KO compared with 3.2 +/- 0.2 mg/g in eight wild-type (WT) mice (P < 0.05). Echocardiography under ketamine-xylazine anesthesia revealed an increased left ventricular (LV) wall thickness (2.17 +/- 0.16 in KO vs. 1.18 +/- 0.10 mm in WT mice, P < 0.05) and a decreased LV lumen diameter (2.50 +/- 0.32 in KO vs. 3.21 +/- 0.14 mm in WT mice, P < 0.05), but LV diameter shortening was not different between KO and WT mice. The maximum rate of rise of left ventricular pressure (LV dP/dt(max)) was lower in KO than in WT mice under basal conditions (2,720 +/- 580 vs. 4,440 +/- 440 mmHg/s) and during dobutamine infusion (6,220 +/- 800 vs. 8,730 +/- 790 mmHg/s, both P < 0.05). Similarly, during isoflurane anesthesia LV dP/dt(max) was lower in KO than in WT mice under basal conditions (5,400 +/- 670 vs. 8,250 +/- 710 mmHg/s) and during norepinephrine infusion (10,010 +/- 1,320 vs. 14,710 +/- 220 mmHg/s, both P < 0.05). In conclusion, the markedly increased LV weight and wall thickness, the encroachment of the LV lumen, and LV dysfunction reflect cardiac abnormalities, although not as overt as in humans, of human infantile Pompe's disease and make these mice a suitable model for further investigation of pathophysiology and of novel therapies of Pompe's disease.

Intercellular transfer of the virally derived precursor form of acid alpha-glucosidase corrects the enzyme deficiency in inherited cardioskeletal myopathy Pompe disease

Pompe disease is a lethal cardioskeletal myopathy in infants and results from genetic deficiency of the lysosomal enzyme acid alpha-glucosidase (GAA). Genetic replacement of the cDNA for human GAA (hGAA) is one potential therapeutic approach. Three months after a single intramuscular injection of 10(8) plaque-forming units (PFU) of E1-deleted adenovirus encoding human GAA (Ad-hGAA), the activity in whole muscle lysates of immunodeficient mice is increased to 20 times the native level. Direct transduction of a target muscle, however, may not correct all deficient cells. Therefore, the amount of enzyme that can be transferred to deficient cells from virally transduced cells was studied. Fibroblasts from an affected patient were transduced with AdhGAA, washed, and plated on transwell culture dishes to serve as donors of recombinant enzyme. Deficient fibroblasts were plated as acceptor cells, and were separated from the donor monolayer by a 22-microm pore size filter. Enzymatic and Western analyses demonstrate secretion of the 110-kDa precursor form of hGAA from the donor cells into the culture medium. This recombinant, 110-kDa species reaches the acceptor cells, where it can be taken up by mannose 6-phosphate receptor-mediated endocytosis. It then trafficks to lysosomes, where Western analysis shows proteolytic processing to the 76- and 70-kDa lysosomal forms of the enzyme. Patient fibroblasts receiving recombinant hGAA by this transfer mechanism reach levels of enzyme activity that are comparable to normal human fibroblasts. Skeletal muscle cell cultures from an affected patient were also transduced with Ad-hGAA. Recombinant hGAA is identified in a lysosomal location in these muscle cells by immunocytochemistry, and enzyme activity is transferred to deficient skeletal muscle cells grown in coculture. Transfer of the precursor protein between muscle cells again occurs via mannose 6-phosphate receptors, as evidenced by competitive inhibition with 5 mM mannose 6-phosphate. In vivo studies in GAA-knockout mice demonstrate that hepatic transduction with adenovirus encoding either murine or human GAA can provide a depot of recombinant enzyme that is available to heart and skeletal muscle through this mechanism. Taken together, these data show that the mannose 6-phosphate receptor pathway provides a useful strategy for cell-to-cell distribution of virally derived recombinant GAA.

Human acid alpha-glucosidase from rabbit milk has therapeutic effect in mice with glycogen storage disease type II

Pompe's disease or glycogen storage disease type II (GSDII) belongs to the family of inherited lysosomal storage diseases. The underlying deficiency of acid alpha-glucosidase leads in different degrees of severity to glycogen storage in heart, skeletal and smooth muscle. There is currently no treatment for this fatal disease, but the applicability of enzyme replacement therapy is under investigation. For this purpose, recombinant human acid alpha-glucosidase has been produced on an industrial scale in the milk of transgenic rabbits. In this paper we demonstrate the therapeutic effect of this enzyme in our knockout mouse model of GSDII. Full correction of acid alpha-glucosidase deficiency was obtained in all tissues except brain after a single dose of i.v. enzyme administration. Weekly enzyme infusions over a period of 6 months resulted in degradation of lysosomal glycogen in heart, skeletal and smooth muscle. The tissue morphology improved substantially despite the advanced state of disease at the start of treatment. The results have led to the start of a Phase II clinical trial of enzyme replacement therapy in patients.

Recombinant human acid alpha-glucosidase: high level production in mouse milk, biochemical characteristics, correction of enzyme deficiency in GSDII KO mice

Glycogen storage disease type II (GSDII) is caused by lysosomal acid alpha-glucosidase deficiency. Patients have a rapidly fatal or slowly progressive impairment of muscle function. Enzyme replacement therapy is under investigation. For large-scale, cost-effective production of recombinant human acid alpha-glucosidase in the milk of transgenic animals, we have fused the human acid alpha-glucosidase gene to 6.3 kb of the bovine alphaS1-casein gene promoter and have tested the performance of this transgene in mice. The highest production level reached was 2 mg/ml. The major fraction of the purified recombinant enzyme has a molecular mass of 110 kDa and resembles the natural acid alpha-glucosidase precursor from human urine and the recombinant precursor secreted by CHO cells, with respect to pH optimum, Km, Vmax, N-terminal amino acid sequence and glycosylation pattern. The therapeutic potential of the recombinant enzyme produced in milk is demonstrated in vitro and in vivo. The precursor is taken up in a mannose 6-phosphate receptor-dependent manner by cultured fibroblasts, is converted to mature enzyme of 76 kDa and depletes the glycogen deposit in fibroblasts of patients. When injected intravenously, the milk enzyme corrects the acid alpha-glucosidase deficiency in heart and skeletal muscle of GSDII knockout mice.

Retroviral transfer of acid alpha-glucosidase cDNA to enzyme-deficient myoblasts results in phenotypic spread of the genotypic correction by both secretion and fusion

Myoblasts have properties that make them suitable vehicles for gene replacement therapy, and lysosomal storage diseases are attractive targets for such therapy. Type II Glycogen Storage Disease, a deficiency of acid alpha-glucosidase (GAA), results in the abnormal accumulation of glycogen in skeletal and cardiac muscle lysosomes. The varied manifestations of the enzyme deficiency in affected patient are ultimately lethal. We used a retroviral vector carrying the cDNA encoding for GAA to replace the enzyme in deficient myoblasts and fibroblasts and analyzed the properties of the transduced cells. The transferred gene was efficiently expressed, and the de novo-synthesized enzyme reached lysosomes where it digested glycogen. In enzyme-deficient myoblasts after transduction, enzyme activity rose to more than 30-fold higher than in normal myoblasts and increased about five-fold more when the cells were allowed to differentiate into myotubes. The transduced cells secreted GAA that was endocytosed via the mannose-6-phosphate receptor into lysosomes of deficient cells and digested glycogen. Moreover, the transduced myoblasts were able to fuse with and provide enzyme for GAA-deficient fusion partners. Thus, the gene-corrected cells, which appear otherwise normal, may ultimately provide phenotypic correction to neighboring GAA-deficient cells by fusion and to distant cells by secretion and uptake mechanisms.

Glycogenosis type II (acid maltase deficiency)

Glycogen storage disease type II (GSD II/glycogenosis type II/Pompe's disease/acid maltase deficiency) is caused by the deficiency of lysosomal alpha-glucosidase resulting in lysosomal accumulation of glycogen. The disease is inherited as an autosomal recessive trait and is clinically heterogeneous. Early and late onset phenotypes are distinguished. Insight in the molecular nature of the lysosomal alpha-glucosidase deficiency and the underlying genetic defect has increased significantly during the past decade. This minireview on GSD II was written at the occasion of The International Symposium on Glycolytic and Mitochondrial Defects in Muscle and Nerve, held in Osaka, Japan, July 1994. It is an update of current literature, but also includes original data from the collaborating authors on mutations occurring in the lysosomal alpha-glucosidase gene and on prenatal diagnosis by chorionic villus sampling. The genotype-phenotype correlation and the prospects for therapy are addressed.

Synthesis and in situ localization of lysosomal alpha-glucosidase in muscle of an unusual variant of glycogen storage disease type II

The lysosomal alpha-glucosidase activity is reduced to 10% to 25% of the average control value in most late-onset cases of glycogen storage disease type II (GSDII). Some adult patients, however, have been identified with an exceptionally low (< 5%) residual enzyme activity. We have investigated one such unusual variant. The rate of alpha-glucosidase synthesis appeared normal but the residual enzyme activity was only approximately 3% in cultured fibroblasts, cultured muscle cells, and muscle tissue of the patient. It appeared that fully matured enzyme molecules were more abundantly present in muscle tissue than in cultured cells. The acid phosphatase activity of affected muscle fibers was enhanced due to an increased number of lysosomes. Lysosomes were particularly abundant in vacuolated areas and they contained, as judged by immunoelectron microscopy, even more alpha-glucosidase molecules than usual. An excessive amount of enzyme molecules were also observed in the endoplasmic reticulum, the site of lysosomal enzyme synthesis, and the cisternae were dilated. These observations suggest that the lysosomal system is stimulated in response to intralysosomal glycogen storage and onset of cellular injury. We hypothesize that the onset of gross pathologic abnormalities is delayed in this particular case of adult GSDII by an increased synthesis of lysosomal alpha-glucosidase, and as a consequence, an increased residual activity in storage-prone muscle fibers.

Recombinant human acid alpha-glucosidase generated in bacteria: antigenic, but enzymatically inactive

Genetic deficiency of acid alpha-glucosidase (GAA) results in glycogen storage disease type II. To investigate whether we could generate a functional recombinant human GAA protein for future enzyme replacement therapy, we subcloned the GAA cDNA into the bacterial expression plasmid pMaI and analyzed the recombinant protein produced. This nonglycosylated recombinant human GAA was found to be antigenic by reacting with polyclonal rabbit antibody to human placental GAA using ELISA and Western techniques. However, the protein was not enzymatically active, suggesting that glycosylation may play a role in enzymatic function.

Intravenous administration of phosphorylated acid alpha-glucosidase leads to uptake of enzyme in heart and skeletal muscle of mice

The lysosomal storage disorder glycogenosis type II is caused by acid alpha-glucosidase deficiency. In this study we have investigated the possible applicability of mannose 6-phosphate receptor-mediated enzyme replacement therapy to correct the enzyme deficiency in the most affected tissues. Bovine testes acid alpha-glucosidase containing phosphorylated mannose residues was intravenously administered to mice and found to be taken up by heart (70% increase of activity) and skeletal muscle (43% increase); the major target organs. The uptake of nonphosphorylated human placenta acid alpha-glucosidase by heart and skeletal muscle appeared to be significantly less efficient, whereas uptake of dephosphorylated bovine testes enzyme was not detectable. The phosphorylated bovine testes acid alpha-glucosidase remained present in mouse skeletal muscle up to 9-15 d after administration, with a half-life of 2-4 d. Besides being measured in skeletal muscle and heart, uptake of phosphorylated bovine testes and nonphosphorylated human placenta acid alpha-glucosidase was measured in several other organs, but not in brain. The increase of acid alpha-glucosidase activity was highest in liver and spleen. We concluded that application of mannose 6-phosphate receptor-mediated enzyme replacement therapy may offer new perspectives for treatment of glycogenesis type II.

Quantitative relationship of lysosomal glycogen accumulation to lysosomal alpha-glucosidase inhibition in castanospermine-treated rats

To quantitatively examine the relationship between lysosomal acid alpha-glucosidase (LAAG, alpha-D-glucoside glucohydrolase, EC 3.2.1.20) inhibition and glycogen accumulation, rats were treated with castanospermine (CS), and liver lysosomal/mitochondrial fractions were analyzed for glycogen content and LAAG activity. Liver lysosomal glycogen accumulation positively correlated (r = 0.90) with the amount of LAAG inhibition when inhibition was about 50% or greater. Glycogen did not accumulate when LAAG inhibition was less than 50%. The route of CS administration had little effect on the amount of LAAG inhibition observed. In rats killed 17 hr after CS administration, the doses estimated to cause 50% LAAG inhibition were 0.77, 0.11, and 0.22 mg/kg for i.p., i.v., and oral administration respectively. After 89% inhibition of LAAG activity with a single oral dose of 10 mg CS/kg, LAAG activity returned to 50% of normal value in about 2.5 days. Accumulated glycogen disappeared as LAAG activity recovered. Surprisingly, twelve daily CS doses of 1 mg/kg had only a small cumulative effect on LAAG inhibition and did not cause more glycogen accumulation than a single dose.

Effects of N-hydroxyethyl-1-deoxynojirimycin (BAY m 1099) on the activity of neutral- and acid alpha-glucosidases in human fibroblasts and HepG2 cells

The effect of the glucose analogue N-hydroxyethyl-1-deoxynojirimycin (BAY m 1099) on the activity of alpha-glucosidases was studied in human fibroblasts and HepG2 cells. BAY m 1099 inhibits neutral and acid alpha-glucosidase activities of both cell types in a dosage-dependent and reversible manner. Inhibition of endoplasmic reticulum glucosidases I and/or II is suggested by delayed processing of lysosomal (acid) alpha-glucosidase. Competitive inhibition of mature acid alpha-glucosidase leads to lysosomal accumulation of glycogen as in glycogenosis type II. There seems to be little risk, however, of inducing this storage disorder when using the drug in a dose of 50 mg per os for treatment of type II diabetes. In high doses, the drug may prove useful for studying the pathogenesis of glycogenosis type II in vitro or in animal models.

An investigation of the properties and possible clinical significance of the lysosomal alpha-glucosidase GAA*2 allele

Properties of the acid alpha-glucosidase, GAA2, the product of the GAA*2 allele have been compared with those of the common allele product GAA1, GAA2 has an altered affinity for glycogen but resembles GAA1 in its affinity for low molecular weight substrates, and also in its processing, as judged by immunoblot analysis of the denatured polypeptides. Starch gel electrophoretic analysis of fibroblasts from 15 patients with late onset glycogen storage disease type II (GSDII) failed to reveal either homozygotes or heterozygotes for the GAA*2 allele (GAA2-2 or GAA2-0) providing evidence that neither of these genotypes lead to late onset GSDII despite the impaired activity of the enzyme towards glycogen.

Prospect for enzyme therapy in glycogenosis II variants: a study on cultured muscle cells

Impairment of skeletal muscle function is the common feature of distinct clinical forms of glycogenosis type II. In the present study, muscle cultures from different patients were used to investigate the cause of clinical heterogeneity and the feasibility of enzyme replacement therapy. The activity of acid alpha-glucosidase appears to be the primary factor in determining the extent of lysosomal glycogen storage in muscle, and thereby the clinical severity of the disease. Neutral alpha-glucosidases do not seem influential. Correction of the enzymatic defect was achieved in skeletal muscle cultures from patients by administration of a "high-uptake" form of acid alpha-glucosidase, purified from human urine. The enzyme reaches the lysosomes, including the glycogen storage vacuoles, and the lysosomal glycogen content is reduced to control level. In normal muscle cells 20% of the total cellular glycogen pool is segregated in lysosomal compartments. This percentage is higher than in fibroblasts, which may partly explain why muscles are more prone to store glycogen. The relevance of this study for enzyme therapy is discussed.