When more is less: excess and deficiency of autophagy coexist in skeletal muscle in Pompe disease

Abstract

The role of autophagy, a catabolic lysosome-dependent pathway, has recently been recognized in a variety of disorders, including Pompe disease, which results from a deficiency of the glycogen-degrading lysosomal hydrolase acid-alpha glucosidase (GAA). Skeletal and cardiac muscle are most severely affected by the progressive expansion of glycogen-filled lysosomes. In both humans and an animal model of the disease (GAA KO), skeletal muscle pathology also involves massive accumulation of autophagic vesicles and autophagic buildup in the core of myofibers, suggesting an induction of autophagy. Only when we suppressed autophagy in the skeletal muscle of the GAA KO mice did we realize that the excess of autophagy manifests as a functional deficiency. This failure of productive autophagy is responsible for the accumulation of potentially toxic aggregate-prone ubiquitinated proteins, which likely cause profound muscle damage in Pompe mice. Also, by generating muscle-specific autophagy-deficient wild-type mice, we were able to analyze the role of autophagy in healthy skeletal muscle.

Figure 1

(A) Basal level of autophagy in fast muscles (quadriceps and white gastrocnemius), liver, and brain from 5-month-old wild-type and GAA KO mice. Western blotting of protein lysates with LC3 antibody shows a dramatic increase in the levels of both LC3-I and LC3-II in muscle, but not in liver or brain from GAA KO mice. (B) Western blotting of protein lysates (white gastrocnemius) with LC3 antibody from 1-month old wild-type and GAA KO mice after 48-hour starvation. Analysis of the amount of LC3-II by densitometry showed an ∼60% increase in the GAA KO after starvation. GAPDH serves as a loading control.

Figure 2

Electron microscopy images of fast muscle (psoas) from a 5-month-old muscle-specific autophagy-deficient wild-type mouse. The upper image shows clusters of vesicles (arrows), which likely represent lysosomes; also shown are electron-light, irregularly shaped inclusions (arrowhead), which likely represent polyubiquitinated protein aggregates. A filamentous inclusion (arrowhead) is clearly seen at higher magnification (lower). Bar: 0.5 micron.