Role of autophagy in GM1-mediated neuroprotection in Huntington's Disease

Abstract

Background: Huntington’s disease (HD) is an inherited neurodegenerative disorder caused by an expanded polyglutamine tract in the huntingtin (HTT) protein. Mutant huntingtin (mHTT) accumulates as toxic oligomers and aggregates inside the cell, when it is not removed efficiently by selective autophagy. In HD cells, this degradative pathway is impaired, due to an aberrant interaction of mHTT with p62, an adaptor and cargo recognition protein that guides cargo molecules and organelles to autophagic structures for degradation. Recently, the ganglioside GM1 was shown to protect HD cells from apoptosis, and to restore motor functions in an HD mouse model. These therapeutic effects were accompanied by phosphorylation of mHTT at amino acids Ser13 and Ser16, a post-translational modification that has been proposed to modulate mHTT toxicity and its cellular clearance. The exact mechanism underlying GM1 effects is still unclear. Previous work suggested that gangliosides may stimulate autophagy in brain cells and in other neurodegenerative models. Hypothesis: The therapeutic effects of GM1 in HD models are due, at least in part, to activation of autophagy and/or restoration of cargo recognition in HD cells, and to enhanced mHTT clearance by autophagy. Results: Soluble species of mHTT were not significantly decreased in HD cell or mouse models after GM1 treatment. However, mHTT aggregates were reduced in certain brain areas, suggesting that GM1 might affect aggregate formation or clearance. Autophagic markers such as LC3-II and p62 were not affected by GM1 treatment in both HD cell and mouse models. Autophagic flux, measured in a cell model that expressed the reporter protein RFP-EGFP-LC3, was also unaffected by GM1 in our experimental conditions. Immunoprecipitation experiments showed that GM1 treatment decreases the aberrant interaction between mHTT and p62. These data suggest that GM1 might improve cargo recognition and selective autophagy in HD models, potentially leading to increased clearance of mHTT aggregates and decreased mHTT toxicity.