Regulation of mitochondrial and nonmitochondrial protein turnover by the PINK1-Parkin pathway

Abstract

The accumulation of damaged mitochondria has been proposed as a key factor in aging and in the pathogenesis of many common age-related diseases, including Parkinson disease (PD). Recently, <italic>in vitro</italic> studies of the PD-related proteins Parkin and PINK1 have found that these factors act in a common pathway to promote the selective autophagic degradation of damaged mitochondria (mitophagy). However, whether PINK1 and Parkin promote mitophagy <italic>in vivo</italic> is unknown. To address this question, I used a proteomic approach in <italic>Drosophila</italic> to study the effects of null mutations in <italic>parkin</italic> or <italic>PINK1</italic> on mitochondrial protein turnover. The <italic>parkin</italic> null mutants showed a significant overall slowing of mitochondrial protein turnover, similar to but less severe than the slowing seen in autophagy-deficient <italic>Atg7</italic> mutants, consistent with the model that Parkin acts upstream of Atg7 to promote mitophagy. By contrast, the turnover of many mitochondrial respiratory chain (RC) subunits showed greater impairment in <italic>parkin</italic> than in <italic>Atg7</italic> mutants, and RC turnover was also selectively impaired in <italic>PINK1</italic> mutants. These findings demonstrate that the PINK1-Parkin pathway promotes mitophagy <italic>in vivo</italic> and, unexpectedly, also promotes selective turnover of mitochondrial RC components. Furthermore, differential tissue expression analyses suggest that selective RC subunit turnover may be particularly important in neural tissue, as mitophagy appears to account for a relatively low proportion of mitochondrial protein turnover in brain. Because Parkin is an E3 ubiquitin ligase with multiple known nonmitochondrial substrates, I also examined the effects of <italic>parkin</italic> and <italic>PINK1</italic> mutations on turnover of nonmitochondrial proteins. <italic>parkin</italic> mutants had moderately impaired turnover of most types of nonmitochondrial proteins, including synaptic and plasma membrane proteins, and strikingly impaired turnover of many extracellular proteins. This finding was consistent with previous evidence that Parkin regulates the endocytic/endosomal pathway. <italic>PINK1</italic> mutants, by contrast, had significantly slowed turnover in only 3 of 10 categories of nonmitochondrial proteins, and did not show a defect in endocytic/endosomal turnover. Correlational analysis suggested that PINK1 and Parkin may work together to promote turnover of cytoplasmic proteins, but Parkin's role in endocytic/endosomal protein turnover appears to be independent of PINK1. Further investigation of the roles played by Parkin and PINK1 in both mitochondrial and nonmitochondrial protein turnover will shed light on the contribution of these proteins to human disease.