Emerging Cellular Function of PINK1-Parkin Pathway in the Removal of Damaged Mitochondria by Mitophagy

Recessive Parkinson's disease (PD) genes Parkin and PINK1 have been placed genetically in the same molecular pathway that is essential to maintain the integrity and function of mitochondria (Clark et al, 2006; Park et al, 2006). Although prior studies suggest that PINK1 acts upstream of Parkin, the molecular picture of how the PINK1-Parkin pathway may regulate some aspects of mitochondria function remains obscure. A critical prior observation, by Narendra, Youle and colleagues (2008), showed that Parkin is selectively recruited to damaged mitochondria (e.g. by abolishing mitochondrial membrane potential with the uncoupler, carbonyl cyanide m-chlorophelhydrazone or CCCP) and can promote selective removal of damaged mitochondria by autophagy, a process also called mitophagy. Built upon this finding, a recent study by these investigators (Narendra et al, 2010) provides some molecular clarity on how PINK1 may regulate the Parkin-mediated mitophagy and on how PD-associated PINK1 and Parkin   mutations are defective in distinct stages of selective mitophagy.

They first demonstrate that PINK1 is selectively accumulated on the damaged mitochondria with low membrane potential. The mechanism underlying such damage-induced PINK1 accumulation appears to be a voltage-dependent inhibition of a constitutive clearance process in which PINK is proteolytically cleaved and the cleaved fragments are cleared by the proteosome. Consistent with the genetic pathway that placed PINK1 upstream of Parkin, PINK1 accumulation on the damaged mitochondria can occur in the absence of Parkin, but not vice versa. Interestingly, in the absence of mitochondrial membrane depolarization, several manipulations that increase the levels of PINK1 at the mitochondrial outer membrane (i.e. overexpression of wildtype PINK1, replace 1-110 of PINK1 with mitochondria-targeting sequences from OPA3, or inducible tethering of PINK1 to mitochondria) are sufficient to recruit Parkin to mitochondria and promote mitophagy. Finally, PD-associated PINK1 or Parkin mutants affect the recruitment of Parkin to the damaged mitochondria and subsequent mitophagy. Although a few of the mutants (e.g. Parkin R275W) were able to be recruited to the mitochondria, they could not promote mitophagy. Overall, this study reveals that the selective accumulation of PINK1 on the outer membrane of the damaged mitochondria could be the critical signal that initiates Parkin-mediated selective elimination of the damaged mitochondria, and PD-associated PINK1 or Parkin mutations affect distinct steps of mitophagy.

This study, together with two other recent studies (Vives-Bauza, 2010; Geisler, 2010), provides converging evidence that functional PINK1 is necessary for the recruitment of Parkin to activate mitophagy. Two of these studies showed overexpression of PINK1 in the context of normal mitochondrial membrane potential are sufficient to recruit Parkin to mitochondria. The study by Narendra et al (2010) provides by far the most convincing evidence that elevate the level of PINK1 on the mitochondrial outer membrane is sufficient to activate Parkin-dependent mitophagy. The studies by Geisler (2010) and Narendra (2010) also support that similar deficits at distinct steps of mitophagy by various Parkin mutants. Together, these studies represent a major step forward towards defining the cellular function of the PINK1-Parkin pathway in removal of damaged mitochondrial by mitophagy, implicating genetic or functional impairment of this molecular pathway in the pathogenesis of PD.

These studies also raise new questions that need to be addressed. First, the precise mechanism of how damaged mitochondria can stabilize PINK1 in the outer mitochondrial membrane remains unclear. Although Narendra et al suggests the inhibition of PINK1 proteolysis could be a key mechanism, the study by Vives-Bauza (2010) showed that the levels of both full-length 65kD and the cleaved 52Kd fragment of PINK1 appear to increase upon mitochondrial depolarization. Further study of the proteolysis and proteosomal-dependent clearance of PINK1 under the normal and mitochondrial-depolarization conditions is required to clarify the issue. Second, the molecular substrates of PINK1 phosphorylation and Parkin ubiquitination in the mitophagy pathway remain to be described. The study by Geisler et al (2010) suggests the ubiquitination of Vdac1 by Parkin may be necessary for mitophagy. Future studies of PINK1 and Parkin substrates upon mitochondria damage may provide the precise signaling cascade from PINK1 to Parkin to mitophagy. Third, a limitation of the current studies is the reliance on overexpression of fluorescent-tagged Parkin or PINK1 in mitotic cells. There is a clear need to re-evaluate the selective mitophagy pathway in post-mitotic neurons expressing (or missing) the endogenous wildtype or mutant Parkin and PINK1. One possible solution is to use iPS-differentiated neurons from normal individuals and PD patients with Parkin or PINK mutations. Fourth, the interesting observations of Parkin-PINK1 pathway in the cellular models beg the question of whether the same pathway, or any other parallel pathways, could be involved in selective removal of damaged mitochondria in the mature brain. Careful studies of selective mitophagy in Parkin and PINK1 null fly and mice may shed light on this question. A difficult yet critical final question is the relevance of this mitophagy pathway to PD. Because of the clinical differences between the recessive forms of PD and the dominant or sporadic forms of PD (Ahlskog, 2009), it would be important to investigate whether accumulation of damaged mitochondria is a robust feature in distinct forms of PD in the post-mortem brains and in the respective animal models.

Although there is much to be learned about the Parkin-PINK1 pathway in the cells, in the neurons and in the brain, these recent exciting findings have thrust mitophagy to the center stage of investigating the molecular pathogenesis of PD.