Autophagy and Mistargeting of Therapeutic Enzyme in Skeletal Muscle in Pompe Disease

Abstract

Enzyme replacement therapy (ERT) became a reality for patients with Pompe disease, a fatal cardiomyopathy and skeletal muscle myopathy caused by a deficiency of glycogen-degrading lysosomal enzyme acid alpha-glucosidase (GAA). The therapy, which relies on receptor-mediated endocytosis of recombinant human GAA (rhGAA), appears to be effective in cardiac muscle, but less so in skeletal muscle. We have previously shown a profound disturbance of the lysosomal degradative pathway (autophagy) in therapy-resistant muscle of GAA knockout mice (KO). Our findings here demonstrate a progressive age-dependent autophagic buildup in addition to enlargement of glycogen-filled lysosomes in multiple muscle groups in the KO. Trafficking and processing of the therapeutic enzyme along the endocytic pathway appear to be affected by the autophagy. Confocal microscopy of live single muscle fibers exposed to fluorescently labeled rhGAA indicates that a significant portion of the endocytosed enzyme in the KO was trapped as a partially processed form in the autophagic areas instead of reaching its target--the lysosomes. A fluid-phase endocytic marker was similarly mistargeted and accumulated in vesicular structures within the autophagic areas. These findings may explain why ERT often falls short of reversing the disease process and point toward new avenues for the development of pharmacological intervention.

Deconstructing Pompe Disease by Analyzing Single Muscle Fibers: “To See a World in a Grain of Sand…”

“…Furthermore, we have shown that autophagic accumulation in muscle fibers was associated with resistance to enzyme replacement therapy. [21][22][23] We now demonstrate that the underlying skeletal muscle pathology and the extent of autophagy in Pompe patients are similar to those in mice. Autophagy appears to play a major role in the pathogenesis of the disease, a finding that has important implications for the effectiveness of enzyme replacement therapy with the recombinant human acid a-glucosidase.…”

Defects in calcium homeostasis and mitochondria can be reversed in Pompe disease

“…The defective autophagy in Pompe disease, which is blatantly evident in muscle fibers and to a certain extent in a cell model, may stem from reduced availability of energy source due to lysosomal glycogen sequestration. 44,45 A disturbed energy metabolism has recently been documented in Pompe patients with the adult form of the disease. 46 The autophagic defect contributes to the pathogenesis of muscle damage and alters trafficking of the replacement enzyme to the lysosome in Pompe disease.…”

“…46 The autophagic defect contributes to the pathogenesis of muscle damage and alters trafficking of the replacement enzyme to the lysosome in Pompe disease. 27,45,47 In this study we have addressed mitochondrial abnormalities and mitophagy in Pompe skeletal muscle-the major site of pathology and a challenge for therapy.…”

Why should autophagic flux be assessed?

“…Lysosomal-associated membrane protein 2 (LAMP-2) deficiency is considered to be caused by genetic mutations and to lead to the extensive accumulation of autophagosomes in the muscles of mice and patients with Danon disease [22] . The impairment of autophagic flux by defective lysosomal function has been observed in an inherited disorder of skeletal and cardiac muscle, Pompe disease, which is caused by the deficiency of the glycogen-degrading lysosomal enzyme acid alpha-glucosidase [23] .…”