A new resorufin-based α-glucosidase assay for high-throughput screening

Mutations in α-glucosidase cause accumulation of glycogen in lysosomes, resulting in Pompe disease, a lysosomal storage disorder. Small molecule chaperones that bind to enzyme proteins and correct the misfolding and mistrafficking of mutant proteins have emerged as a new therapeutic approach for the lysosomal storage disorders. In addition, α-glucosidase is a therapeutic target for type II diabetes, and α-glucosidase inhibitors have been used in the clinic as alternative treatments for this disease. We have developed a new fluorogenic substrate for the α-glucosidase enzyme assay, resorufin α-d-glucopyranoside. The enzyme reaction product of this new substrate emits at a peak of 590 nm, reducing the interference from fluorescent compounds seen with the existing fluorogenic substrate, 4-methylumbelliferyl-α-d-glucopyranoside. Also, the enzyme kinetic assay can be carried out continuously without the addition of stop solution due to the lower pK_a of the product of this substrate. Therefore, this new fluorogenic substrate is a useful tool for the α-glucosidase enzyme assay and will facilitate compound screening for the development of new therapies for Pompe disease.     

New insights into therapeutic options for Pompe disease

Glycogen storage disease type II or Pompe disease (GSD II, MIM 232300) is a rare inherited metabolic myopathy caused by a deficiency of lysosomal acid α-glucosidase or acid maltase (GAA; EC 3.2.1.20), resulting in a massive lysosomal glycogen accumulation in cardiac and skeletal muscles. Affected individuals exhibit either severe hypotonia associated with hypertrophic cardiomyopathy (infantile forms) or progressive muscle weakness (late-onset forms). Even if enzyme replacement therapy has recently become a standard treatment, it suffers from several limitations. This review will present the main results of enzyme replacement therapy and the recent findings concerning alternative treatments for Pompe disease, such as gene therapy, enzyme enhancement therapy, and substrate reduction therapy.     

AAV载体介导的蓬佩病模型小鼠体内基因治疗研究*

Objective: Pompe disease is a lysosomal glycogen storage disease caused by acid α-glucosidase (GAA) deficiency, which is characterized by glycogen accumulation in the heart, skeletal muscle, and central nervous system (CNS). AAV vector-mediated gene therapy is expected to be a breakthrough in the treatment of Pompe disease. In this study, AAV9 vector was used to mediate GAA gene transfer in Pompe disease model mice, and the changes of GAA protease activity, glycogen accumulation in tissues and pathological changes in mice after transgenic intervention were evaluated. Methods: Codon optimized GAA gene (coGAA) was carried by AAV9 vector, and the AAV vector was packaged by baculovirus production process. Adult Pompe model mice were given a single intravenous injection at the dose of 1.1×10¹³, 3.0×10¹³, 1.2×10¹⁴ vg/kg, and aged Pompe model mice were given a single intravenous injection at the dose of 3.0×10¹³ vg/kg. After reaching the end point of the experiment, the mice were euthanized, GAA protease activity was determined by fluorescence spectrophotometry, glycogen accumulation was observed by PAS staining, and pathological changes were detected by HE staining. Results: Five weeks after administration, GAA protein was widely expressed in all tissues of adult model mice, with higher expression levels in heart and liver, and lower expression levels in brain and spinal cord. After rAAV9-coGAA treatment, glycogen content in myocardium, skeletal muscle and brain decreased, and vacuolar degeneration in myocardium and skeletal muscle decreased significantly. After treatment, the tissue enzyme activity of the aged animals was significantly increased compared with that of the model mice. The vacuolar degeneration and inflammatory cell infiltration of the myocardium were decreased, but the pathological improvement of skeletal muscle was limited. Conclusion: A single intravenous injection of rAAV9-coGAA can enhance GAA enzyme activity, reduce glycogen accumulation and improve pathology in Pompe model mice. The therapeutic effect was dose-dependent, and the injection also had certain therapeutic effect on aged animals. This study laid a theoretical foundation for the clinical application of AAV9 mediated gene therapy via intravenous route in Pompe disease.     

High-resolution light microscopy (HRLM) and digital analysis of Pompe disease pathology.

Pompe disease is an autosomal recessive lysosomal storage disorder caused by a deficiency of the lysosomal enzyme acid alpha-glucosidase, responsible for the degradation of lysosomal glycogen. Absent or low levels of the enzyme leads to lysosomal glycogen accumulation in cardiac and skeletal muscle cells, resulting in progressive muscle weakness and death from cardiac or respiratory failure. Recombinant enzyme replacement and gene therapy are now being investigated as treatment modalities for this disease. A knockout mouse model for Pompe disease, induced by the disruption of exon 6 within the acid alpha-glucosidase gene, mimics the human disease and has been used to evaluate the efficacy of treatment modalities for clearing glycogen. However, for accurate histopathological assessment of glycogen clearance, maximal preservation of in situ lysosomal glycogen is essential. To improve retention of glycogen in Pompe tissues, several fixation and embedding regimens were evaluated. The best glycogen preservation was obtained when tissues fixed with 3% glutaraldehyde and postfixed with 1% osmium tetroxide were processed into epon-araldite. Preservation was confirmed by staining with the Periodic acid-Schiff's reaction and by electron microscopy. This methodology resulted in high-resolution light microscopy (HRLM) sections suitable for digital quantification of glycogen content in heart and skeletal muscle. Combining this method of tissue fixation with computer-assisted histomorphometry has provided us with what we believe is the most objective and reproducible means of evaluating histological glycogen load in Pompe disease.     

Immortalization of murine muscle cells from lysosomal α-glucosidase deficient mice: A new tool to study pathophysiology and assess therapeutic strategies for Pompe disease

Glycogen storage disease type II (GSDII) is an autosomal recessive disorder caused by defects in the acid α-glucosidase (GAA) gene leading to lysosomal glycogen accumulation, mainly in cardiac and muscle tissues. In order to facilitate biological investigation on this disease and to avoid time-consuming direct cell isolation and culture, we have established murine myogenic GSDII cell lines. Lentiviral/retroviral expression of SV40 T antigen, Bmi-1 or cyclin-dependent kinase 4 (CDK4) genes was used to induce the immortalization of primary satellite cells from GSDII mice. The resulting immortalized myoblasts exhibit phenotypic characteristics of their parental cells, including profound GAA deficiency, glycogen accumulation and the ability to fully differentiate into myotubes when placed in proper culture conditions. These cell lines will constitute a powerful tool for both basic and applied studies focused on a better understanding of the pathophysiological mechanisms involved in GSDII and for assessing putative therapeutic strategies.     

Hyaluronidase increases the biodistribution of acid alpha-1,4 glucosidase in the muscle of Pompe disease mice: an approach to enhance the efficacy of enzyme replacement therapy

Pompe disease (glycogen storage disease type II) is a glycogen storage disease caused by a deficiency of the lysosomal enzyme, acid maltase/acid alpha-1,4 glucosidase (GAA). Deficiency of the enzyme leads primarily to intra-lysosomal glycogen accumulation, primarily in cardiac and skeletal muscles, due to the inability of converting glycogen into glucose. Enzyme replacement therapy (ERT) has been applied to replace the deficient enzyme and to restore the lost function. However, enhancing the enzyme activity to the muscle following ERT is relatively insufficient. In order to enhance GAA activity into the muscle in Pompe disease, efficacy of hyaluronidase (hyase) was examined in the heart, quadriceps, diaphragm, kidney, and brain of mouse model of Pompe disease. Administration of hyase 3000 U/mouse (intravenous) i.v. or i.p. (intraperitoneal) and 10 min later recombinant human GAA (rhGAA) 20 mg/kg i.v. showed more GAA activity in hyase i.p. injected mice compared to those mice injected with hyase via i.v. Injection of low dose of hyase (3000 U/mouse) or high dose of hyase (10,000 U/mouse) i.p. and 20 min or 60 min later 20 mg/kg rhGAA i.v. increased GAA activity into the heart, diaphragm, kidney, and quadriceps compared to hyase untreated mice. These studies suggest that hyase enhances penetration of enzyme into the tissues including muscle during ERT and therefore hyase pretreatment may be important in treating Pompe disease.     

The role of immune tolerance induction in restoration of the efficacy of ERT in Pompe disease

Pompe disease is a lysosomal storage disorder caused by deficiency in the enzyme acid α-glucosidase (GAA). Pompe disease is characterized by the accumulation of glycogen, predominantly in muscle tissue, leading to progressive muscle weakness, loss of motor, respiratory, and, in the infantile-onset form, cardiac function. Disease progression is highly variable depending on phenotype, but premature death due to respiratory complications occurs in most patients. Beginning in 2006, approved alglucosidase alfa enzyme replacement therapies [recombinant human (rh) GAA] have been available to treat Pompe patients. Treatment of classic infantile-onset patients, who manifest the severest form of the disease, with alglucosidase alfa (Myozyme?) has led to extended survival and an evolving understanding of the pathophysiology and course of the disease. Moreover, such treatment has brought to light the role of the immune response in abrogating the efficacy of rhGAA in classic infantile-onset patients with severe genetic mutations. Thus, optimization of treatment for such patients includes development and utilization of strategies to prevent or eliminate immune responses, including modulating the immune system (prophylactic and therapeutic immune tolerance induction regimens) and engineering the enzyme to be less immunogenic and more effective. Future research is also critical for evaluating and mitigating novel disease-associated pathologies uncovered by prolonged survival of infantile-onset patients including development of novel therapeutics, and for protein design strategies to increase delivery of enzyme replacement therapy to critical target tissues. Such efforts would be greatly bolstered by further development of predictive animal models and biomarkers to facilitate clinical trials and patient management. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.     

Optimized preservation of CNS morphology for the identification of glycogen in the Pompe mouse model.

Pompe disease (glycogenosis type II) is a rare lysosomal disorder caused by a mutational deficiency of acid alpha-glucosidase (GAA). This deficiency leads to glycogen accumulation in multiple tissues: heart, skeletal muscles, and the central nervous system. A knockout mouse model mimicking the human condition has been used for histological evaluation. Currently, the best method for preserving glycogen in Pompe samples uses epon-araldite resin. Although the preservation by this method is excellent, the size of the tissue is limited to 1 mm(3). To accurately evaluate brain pathology in the Pompe mouse model, a modified glycol methacrylate (JB-4 Plus) method was developed. This approach allowed the production of larger tissue sections encompassing an entire mouse hemisphere (8 x 15 mm) while also providing a high level of morphological detail and preservation of glycogen. Application of the JB-4 Plus method is appropriate when a high level of cellular detail is desired. A modified paraffin method was also developed for use when rapid processing of multiple samples is a priority. Traditional paraffin processing results in glycogen loss. The modified paraffin method with periodic acid postfixation resulted in improved tissue morphology and glycogen preservation. Both techniques provide accurate anatomic evaluation of the glycogen distribution in Pompe mouse brain.     

The role of autophagy in the pathogenesis of glycogen storage disease type II (GSDII)

Regulated removal of proteins and organelles by autophagy-lysosome system is critical for muscle homeostasis. Excessive activation of autophagy-dependent degradation contributes to muscle atrophy and cachexia. Conversely, inhibition of autophagy causes accumulation of protein aggregates and abnormal organelles, leading to myofiber degeneration and myopathy. Defects in lysosomal function result in severe muscle disorders such as Pompe (glycogen storage disease type II (GSDII)) disease, characterized by an accumulation of autophagosomes. However, whether autophagy is detrimental or not in muscle function of Pompe patients is unclear. We studied infantile and late-onset GSDII patients and correlated impairment of autophagy with muscle wasting. We also monitored autophagy in patients who received recombinant α-glucosidase. Our data show that infantile and late-onset patients have different levels of autophagic flux, accumulation of p62-positive protein aggregates and expression of atrophy-related genes. Although the infantile patients show impaired autophagic function, the late-onset patients display an interesting correlation among autophagy impairment, atrophy and disease progression. Moreover, reactivation of autophagy in vitro contributes to acid α-glucosidase maturation in both healthy and diseased myotubes. Together, our data suggest that autophagy protects myofibers from disease progression and atrophy in late-onset patients.     

A newly identified c.1824_1828dupATACG mutation in exon 13 of the GAA gene in infantile-onset glycogen storage disease type II (Pompe disease)

Pompe disease or glycogen storage disease type II is a glycogen storage disorder associated with malfunction of the acid α-glucosidase enzyme (GAA; EC.3.2.1.3) leading to intracellular aggregations of glycogenin muscles. The infantile-onset type is the most life-threatening form of this disease, in which most of patients suffer from cardiomyopathy and hypotonia in early infancy. In this study, a typical case of Pompe disease was reported in an Iranian patient using molecular analysis of the GAA gene. Our results revealed a new c.1824_1828dupATACG mutation in exon 13 of the GAA gene. In conclusion, with the finding of this novel mutation, the genotypic spectrum of Iranian patients with Pompe disease has been extended, facilitating the definition of disease-related mutations.     

The pharmacological chaperone AT2220 increases recombinant human acid α-glucosidase uptake and glycogen reduction in a mouse model of Pompe disease

Pompe disease is an inherited lysosomal storage disease that results from a deficiency in the enzyme acid α-glucosidase (GAA), and is characterized by progressive accumulation of lysosomal glycogen primarily in heart and skeletal muscles. Recombinant human GAA (rhGAA) is the only approved enzyme replacement therapy (ERT) available for the treatment of Pompe disease. Although rhGAA has been shown to slow disease progression and improve some of the pathophysiogical manifestations, the infused enzyme tends to be unstable at neutral pH and body temperature, shows low uptake into some key target tissues, and may elicit immune responses that adversely affect tolerability and efficacy. We hypothesized that co-administration of the orally-available, small molecule pharmacological chaperone AT2220 (1-deoxynojirimycin hydrochloride, duvoglustat hydrochloride) may improve the pharmacological properties of rhGAA via binding and stabilization. AT2220 co-incubation prevented rhGAA denaturation and loss of activity in vitro at neutral pH and 37°C in both buffer and blood. In addition, oral pre-administration of AT2220 to rats led to a greater than two-fold increase in the circulating half-life of intravenous rhGAA. Importantly, co-administration of AT2220 and rhGAA to GAA knock-out (KO) mice resulted in significantly greater rhGAA levels in plasma, and greater uptake and glycogen reduction in heart and skeletal muscles, compared to administration of rhGAA alone. Collectively, these preclinical data highlight the potentially beneficial effects of AT2220 on rhGAA in vitro and in vivo. As such, a Phase 2 clinical study has been initiated to investigate the effects of co-administered AT2220 on rhGAA in Pompe patients.     

Correction of glycogenosis type 2 by muscle-specific lentiviral vector

Glycogen storage disease type II (GSDII) or Pompe disease is an inherited disease of glycogen metabolism caused by a lack of functional lysosomal acid α-glucosidase (GAA). Affected individuals store glycogen in lysosomes resulting in fatal hypertrophic cardiomyopathy and respiratory failure in the most severe form. We investigated for the first time the use of lentiviral vectors to correct the GSDII phenotype in human and murine GAA-deficient cells. Fibroblasts from infantile and adult GSDII patients were efficiently transduced by a GAA-expressing lentiviral vector placed under the control of the strong MND promoter, leading to a complete restoration of enzymatic activity. We also developed a muscle-specific lentiviral vector based on the synthetic C5–12 promoter and tested it on deficient myogenic satellite cells derived from a GSDII mouse model. GAA was expressed as a correctly processed protein allowing a complete enzymatic and metabolic correction in myoblasts and differentiated myotubes, as well as a significant mannose-6-phosphate (M6P)-dependent secretion reuptake by naive cells. Transduced cells showed lysosomal glycogen clearance, as demonstrated by electron microscopy. These results form the basis for a therapeutic approach of GSDII using lentiviral vector-mediated gene transfer into muscle stem cells.     

Glycogen storage in multiple muscles of old GSD-II mice can be rapidly cleared after a single intravenous injection with a modified adenoviral vector expressing hGAA

BACKGROUND: Glycogen storage disease II (GSD-II) is an autosomal recessive lysosomal storage disease, due to acid-alpha-glucosidase (GAA) deficiency. The disease is characterized by massive glycogen accumulation in the cardiac and skeletal muscles. There is early onset (infantile, also known as Pompe disease) as well as late onset (juvenile and adult) forms of GSD-II. Few studies have been published to date that have explored the consequences of delivering a potential therapy to either late onset GSD-II subjects, and/or early onset patients with long-established muscle pathology. One recent report utilizing GAA-KO mice transgenically expressing human GAA (hGAA) suggested that long-established disease in both cardiac and skeletal muscle is likely to prove resistant to therapies. To investigate the potential for disease reversibility in old GSD-II mice, we studied their responsiveness to exogenous hGAA exposure via a gene therapy approach that we have previously shown to be efficacious in young GAA-KO mice. METHODS: An [E1-, polymerase-] adenoviral vector encoding hGAA was intravenously injected into two groups of aged GAA-KO mice; GAA expression and tissue glycogen reduction were evaluated. RESULTS: After vector injection, we found that extremely high amounts of hepatically secreted hGAA could be produced, and subsequently taken up by multiple muscle tissues in the old GAA-KO mice by 17 days post-injection (dpi). As a result, all muscle groups tested in the old GAA-KO mice showed significant glycogen reductions by 17 dpi, relative to that of age-matched, but mock-injected GAA-KO mice. For example, glycogen reduction in heart was 84%, in quadriceps 46%, and in diaphragm 73%. Our data also showed that the uptake and the subsequent intracellular processing of virally expressed hGAA were not impaired in older muscles. CONCLUSIONS: Overall, the previously reported 'resistance' of old GAA-KO muscles to exogenous hGAA replacement approaches can be rapidly overcome after a single intravenous injection with a modified adenoviral vector expressing hGAA.     

The Pharmacological Chaperone AT2220 Increases the Specific Activity and Lysosomal Delivery of Mutant Acid Alpha-Glucosidase,and Promotes Glycogen Reduction in a Transgenic Mouse Model of Pompe Disease

Pompe disease is an inherited lysosomal storage disorder that results from a deficiency in acid α-glucosidase (GAA) activity due to mutations in the GAA gene. Pompe disease is characterized by accumulation of lysosomal glycogen primarily in heart and skeletal muscles, which leads to progressive muscle weakness. We have shown previously that the small molecule pharmacological chaperone AT2220 (1-deoxynojirimycin hydrochloride, duvoglustat hydrochloride) binds and stabilizes wild-type as well as multiple mutant forms of GAA, and can lead to higher cellular levels of GAA. In this study, we examined the effect of AT2220 on mutant GAA, in vitro and in vivo, with a primary focus on the endoplasmic reticulum (ER)-retained P545L mutant form of human GAA (P545L GAA). AT2220 increased the specific activity of P545L GAA toward both natural (glycogen) and artificial substrates in vitro. Incubation with AT2220 also increased the ER export, lysosomal delivery, proteolytic processing, and stability of P545L GAA. In a new transgenic mouse model of Pompe disease that expresses human P545L on a Gaa knockout background (Tg/KO) and is characterized by reduced GAA activity and elevated glycogen levels in disease-relevant tissues, daily oral administration of AT2220 for 4 weeks resulted in significant and dose-dependent increases in mature lysosomal GAA isoforms and GAA activity in heart and skeletal muscles. Importantly, oral administration of AT2220 also resulted in significant glycogen reduction in disease-relevant tissues. Compared to daily administration, less-frequent AT2220 administration, including repeated cycles of 4 or 5 days with AT2220 followed by 3 or 2 days without drug, respectively, resulted in even greater glycogen reductions. Collectively, these data indicate that AT2220 increases the specific activity, trafficking, and lysosomal stability of P545L GAA, leads to increased levels of mature GAA in lysosomes, and promotes glycogen reduction in situ. As such, AT2220 may warrant further evaluation as a treatment for Pompe disease.     

Glycogen storage disease type III: A novel Agl knockout mouse model

Glycogen storage disease type III is an autosomal recessive disease characterized by a deficiency in the glycogen debranching enzyme, encoded by AGL. Essential features of this disease are hepatomegaly, hypoglycemia, hyperlipidemia, and growth retardation. Progressive skeletal myopathy, neuropathy, and/or cardiomyopathy become prominent in adults. Currently, there is no available cure. We generated an Agl knockout mouse model by deletion of the carboxy terminus of the protein, including the carboxy end of the glucosidase domain and the glycogen-binding domain. Agl knockout mice presented serious hepatomegaly, but we did not observe signs of cirrhosis or adenomas. In affected tissues, glycogen storage was higher than in wild-type mice, even in the central nervous system which has never been tested in GSDIII patients. The biochemical findings were in accordance with histological data, which clearly documented tissue impairment due to glycogen accumulation. Indeed, electron microscopy revealed the disruption of contractile units due to glycogen infiltrations. Furthermore, adult Agl knockout animals appeared less prompt to move, and they exhibited kyphosis. Three-mo-old Agl knockout mice could not run, and adult mice showed exercise intolerance. In addition, older affected animals exhibited an accelerated respiratory rate even at basal conditions. This observation was correlated with severe glycogen accumulation in the diaphragm. Diffuse glycogen deposition was observed in the tongues of affected mice. Our results demonstrate that this Agl knockout mouse is a reliable model for human glycogenosis type III, as it recapitulates the essential phenotypic features of the disease.     

Induction of Tolerance to a Recombinant Human Enzyme,Acid Alpha-Glucosidase,in Enzyme Deficient Knockout Mice

When knockout mice are used to test the efficacy of recombinant human proteins, the animals often develop antibodies to the enzyme, precluding long-term pre-clinical studies. This has been a problem with a number of models, for example, the evaluation of gene or enzyme replacement therapies in a knockout model of glycogen storage disease type II (GSDII; Pompe syndrome). In this disease, the lack of acid alpha-glucosidase (GAA) results in lysosomal accumulation of glycogen, particularly in skeletal and cardiac muscle. Here, we report that in a GAA-deficient mouse model of GSDII, low levels of transgene-encoded human GAA expressed in skeletal muscle or liver dramatically blunt or abolish the immune response to human recombinant protein. Of two low expression transgenic lines, only the liver-expressing line exhibited a profound GAA deficiency in skeletal muscle and heart indistinguishable from that in the original knockouts. The study suggests that the induction of tolerance in animal models of protein deficiencies could be achieved by restricting the expression of a gene of interest to a particular, carefully chosen tissue.     

Identification of the first deletion–insertion involving the complete structure of GAA gene and part of CCDC40 gene mediated by an Alu element

Pompe disease is an uncommon autosomal recessive glycogen storage disorder caused by deficiency of acid α-glucosidase. Classic infantile form triggers severe cardiomyopathy, hypotonia, and respiratory failure, leading to death within the first two years of life. The majority of patients with Pompe disease have been reported to have point mutations in the GAA gene. We report the first complex deletion–insertion encompassing the complete structure of GAA gene and a large fragment of the gene CCDC40 in a patient with very severe form of Pompe disease. Sequencing analysis of breakpoints allowed us to determine the potential implication of an Alu repeat in the pathogenic mechanism. We suggest that molecular strategy of Pompe disease should include systematic analysis of large rearrangements.     

Glycosylation-independent Lysosomal Targeting of Acid α-Glucosidase Enhances Muscle Glycogen Clearance in Pompe Mice

We have used a peptide-based targeting system to improve lysosomal delivery of acid α-glucosidase (GAA), the enzyme deficient in patients with Pompe disease. Human GAA was fused to the glycosylation-independent lysosomal targeting (GILT) tag, which contains a portion of insulin-like growth factor II, to create an active, chimeric enzyme with high affinity for the cation-independent mannose 6-phosphate receptor. GILT-tagged GAA was taken up by L6 myoblasts about 25-fold more efficiently than was recombinant human GAA (rhGAA). Once delivered to the lysosome, the mature form of GILT-tagged GAA was indistinguishable from rhGAA and persisted with a half-life indistinguishable from rhGAA. GILT-tagged GAA was significantly more effective than rhGAA in clearing glycogen from numerous skeletal muscle tissues in the Pompe mouse model. The GILT-tagged GAA enzyme may provide an improved enzyme replacement therapy for Pompe disease patients.