Neuroprotective function of autophagy against dopamine toxicity in an in vitro model of Huntington’s disease

Huntington’s Disease (HD) is a neurodegenerative disorder characterized by the progressive loss of striatal neurons and it is caused by abnormal expansion (37Q) of the polyglutamine tract in Huntigtin protein (Htt). The HD hallmark is the accumulation of toxic aggregate-prone mutant Htt (mHtt), leading to the formation of toxic cytoplasmic and nuclear inclusions. This effect may affect autophagy. Autophagy is a lysosomal degradation process known to clear protein aggregates. Dopamine (DA)-induced oxidative stress can favour the aggregation of unfolded proteins. DA may exacerbate neuronal loss in the striatum.We hypothesised that DA could induce toxicity in Htt-expressing dopaminergic neurons dysregulating autophagy and thus precipitating neuronal cell death. The hyper-expression of mHtt per se reduced cell proliferation, cell viability and induced neuronal cell death. These effects were exacerbated after DA exposure promoting a secondary necrotic-following apoptosis toxicity. We found that DA impaired the autophagy, reducing autophagosome formation, and thus preventing the degradation of mHtt aggregates. DA toxicity led to mitochondrial damage with ROS generation. Treatments with NAC and DFO anti-oxidant agents protected DA-treated cells expressing the mHtt, preventing cell death by promoting autophagy.Next, we tested the potential of the polyphenol Resveratrol (RV) to protect the neuroblastoma cells from DA toxicity. We found that DA affects the stability and function of ATG4, a redox-sensitive cysteine-protein involved in the processing of LC3 (autophagosome formation). We found that RV prevents the ROS generation, restores ATG4 level, allows the lipidation of LC3, facilitates the degradation of mHtt aggregates and protects the cells from DA toxicity. In conclusion, our data explain DA-induced impairment of autophagy underlie the parkinsonism in HD patients, and support the therapeutic use of anti-oxidant agents, as RV, in order to slow down the HD progression.