Autophagy contributes to degradation of Hirano bodies

Hirano bodies are actin-rich inclusions reported most frequently in the hippocampus in association with a variety of conditions including neurodegenerative diseases and aging. We have developed a model system for formation of Hirano bodies in Dictyostelium and cultured mammalian cells to permit detailed studies of the dynamics of these structures in living cells. Model Hirano bodies are frequently observed in membrane-enclosed vesicles in mammalian cells consistent with a role of autophagy in the degradation of these structures. Clearance of Hirano bodies by an exocytotic process is supported by images from electron microscopy showing extracellular release of Hirano bodies, and observation of Hirano bodies in the culture medium of Dictyostelium and mammalian cells. An autophagosome marker protein Atg8-GFP was colocalized with model Hirano bodies in wild-type Dictyostelium cells, but not in atg5(-) or atg1-1 autophagy mutant strains. Induction of model Hirano bodies in Dictyostelium with a high-level expression of 34 kDa Delta EF1 from the inducible discoidin promoter resulted in larger Hirano bodies and a cessation of cell doubling. The degradation of model Hirano bodies still occurred rapidly in autophagy mutant (atg5(-)) Dictyostelium, suggesting that other mechanisms such as the ubiquitin-mediated proteasome pathway could contribute to the degradation of Hirano bodies. Chemical inhibition of the proteasome pathway with lactacystin significantly decreased the turnover of Hirano bodies in Dictyostelium, providing direct evidence that autophagy and the proteasome can both contribute to degradation of Hirano bodies. Short-term treatment of mammalian cells with either lactacystin or 3-methyl adenine results in higher levels of Hirano bodies and a lower level of viable cells in the cultures, supporting the conclusion that both autophagy and the proteasome contribute to degradation of Hirano bodies.