Regulation of mitochondrial autophagy by huntingtin
Huntington’s disease (HD) is an inherited and incurable neurodegenerative disorder caused by an abnormal polyglutamine (polyQ) expansion in the huntingtin (Htt) protein. Dysregulation of protein quality control is a key event in HD pathogenesis. Here, we determined if mutant Htt (mHtt) also affects mitochondrial quality control via dysregulation of mitophagy. To monitor mitophagy in live neurons, we developed a sensitive and quantitative image-based technique utilizing automated microscopy and photoswitchable fluorescent proteins. We showed that neurons with diffuse mHtt exhibit mitochondrial damage and accelerated mitophagy, but neurons with inclusion bodies (IBs) do not. To unravel cause-and-effect relationships between mHtt, mitophagy and neurodegeneration, we collected longitudinal single cell measures of each variable and integrated these data into a Bayesian statistical model. From this model, we learned that diffuse mHtt is associated with increased mitophagy and neurodegeneration. Moreover, the total load of mHtt is a more important determinant of mitochondrial clearance than the presence of an IB. Finally, increased mitochondrial clearance is associated with reduced survival of neurons expressing mHtt. Thus, we uncovered a surprising relationship between mHtt expression, mitophagy, and neurodegeneration that has implications for the use of mitophagy pathways as therapeutic targets in HD.