Mitochondrial dysfunction and perturbed degradation of proteins have been 
implicated in Parkinson's disease (PD) pathogenesis. Mutations in the Parkin and 
PINK1 genes are a cause of familial PD. PINK1 is a putative kinase associated 
with mitochondria, and loss of PINK1 expression leads to mitochondrial 
dysfunction, which increases with time. Parkin is suggested to be downstream of 
PINK1 and also mediates the removal of damaged mitochondria by macroautophagy 
(mitophagy). We investigated whether mitochondrial dysfunction in dopaminergic 
SH-SY5Y cells following decreased PINK1 expression by RNAi may in part be due to 
the inhibition of mitophagy. Reduced flux through the macroautophagy pathway was 
found to be coincident with the inhibition of ATP synthesis following 12 days of 
PINK1 silencing. Overexpression of parkin in these cells restored both 
autophagic flux and ATP synthesis. Overexpression and RNAi studies also 
indicated that PINK1 and parkin were required for mitophagy following 
CCCP-induced mitochondrial damage. The ubiquitination of several mitochondrial 
proteins, including mitofusin 1 and mitofusin 2, were detected within 3 h of 
CCCP treatment. These post-translational modifications were reduced following 
the silencing of parkin or PINK1. The ubiquitination of mitochondrial proteins 
appears to identify mitochondria for degradation and facilitate mitophagy. PINK1 
and parkin are thus required for the removal of damaged mitochondria in 
dopaminergic cells, and inhibition of this pathway may lead to the accumulation 
of defective mitochondria which may contribute to PD pathogenesis.