T H E J O U R N A L O F C E L L B I O L O G Y
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J. Cell Biol. Vol. 183 No. 5 757–759
Mitochondrial dysfunction has long been associated with the
onset of neurodegenerative states, including the selective loss of
dopaminergic neurons in Parkinson ’ s disease (PD; Schapira,
2008 ). However, it has been diffi cult to understand whether the
degeneration of the mitochondria in these neurons is a cause or
effect of the disease. One of the diffi culties in the study of mito-
chondria in neurodegeneration has been our limited understand-
ing of how damaged mitochondrial proteins and lipids are
degraded in steady state. There are currently at least three dis-
tinct mechanisms known for mitochondrial protein turnover:
the proteolysis of proteins within the matrix or intermembrane
space ( Arnold and Langer, 2002 ), autophagic degradation of
entire organelles ( Mijaljica et al., 2007 ), and proteasome-
dependent outer mitochondrial membrane – associated degrada-
tion (OMMAD; Neutzner et al., 2007 ). With the exception of
the mitochondrial proteases, which have been studied for some
time, the molecular mechanisms and regulation of mitochon-
drial turnover via autophagy and the proteasome are less well
characterized. There have been recent hints that mitochondrial
protein turnover is selective; e.g., the ubiquitination and protea-
some-dependent degradation of the anti-apoptotic Bcl2 family
member Mcl-1 by the E3 ligase MULE occurs upon the induc-
tion of cell death ( Warr et al., 2005 ; Zhong et al., 2005 ). Simi-
larly, in yeast, the fusion GTPase Fzo1p was found to be
selectively removed from the mitochondrial outer membrane
through a proteasome-dependent mechanism ( Neutzner et al.,
2007 ). In addition, it was shown that whole mitochondria lack-
Narendra et al. (see p. 795 of this issue) have made an ex-
citing new discovery that links the fi elds of mitochondrial
quality control and the genetics of Parkinson ’ s disease (PD).
Through an elegant series of high-resolution imaging exper-
iments, they are the fi rst to provide evidence that the PARK2
gene product Parkin is selectively recruited to damaged or
uncoupled mitochondria. This recruitment leads to the clear-
ance of the organelles through the autophagosome, demon-
strating a primary function for Parkin in the regulation of
mitochondrial turnover. This work signifi cantly increases our
understanding of PD and provides a new framework for the
development of therapeutic interventions.
Correspondence to Heidi M. McBride: email@example.com
ing electrochemical potential for extended periods will be selec-
tively cleared through steady-state autophagy, or mitophagy
( Twig et al., 2008 ). These data underscore the importance of a
tightly regulated process to control the selective destruction of
mitochondrial proteins caused by accumulated damage, but also
during specifi c cellular processes. Furthermore, it is increas-
ingly becoming evident that the clearance of cellular debris
through autophagy is critical for human health, and defects in
this process are becoming more tightly linked with neurodegen-
erative states ( Mizushima et al., 2008 ).
Narendra et al. (see p. 795 of this issue) have now found
a new function for the PARK2 gene product, Parkin, in the regu-
lation of selective mitophagy. Parkin is a primarily cytosolic
ubiquitin E3 ligase that contains a ubiquitin like domain (UBL),
two RING fi nger domains, and a conserved region between the
RING domains ( Schapira, 2008 ). The PARK2 gene has been
shown to be mutated in nearly 50% of autosomal recessive and
10 – 15% of sporadic early onset PD. There have been confl icting
reports suggesting functions for Parkin in the cytosol, in the ER,
on mitochondrial targets, and at the plasma membrane. Evidence
for a primary function at the mitochondria was strengthened by
the identifi cation of a genetic link with the mitochondrial mem-
brane – anchored kinase, and Parkinson ’ s related protein, PTEN-
induced kinase 1 (Pink1). In Drosophila melanogaster , the loss
of Pink1 was rescued upon overexpression of Parkin, whereas
loss of Parkin was not rescued by the overexpression of Pink1
( Clark et al., 2006 ; Park et al., 2006 ; Yang et al., 2006 ; Exner et al.,
2007 ). In addition, the mitochondria in D. melanogaster cells
lacking Parkin or Pink1 are highly fused, and the defects in the fl y
are rescued upon overexpression of the fi ssion GTPase Drp1, or
the loss-of-fusion factors Mfn/Marf or Opa1 ( Deng et al., 2008 ;
Poole et al., 2008 ; Yang et al., 2008 ). These data have led to an
emerging model where mitochondrial dysfunction may play a
central role in the onset of PD, and suggest possible links be-
tween the fi ssion/fusion machinery and PD genes. Narendra et al.
(2008) have now determined that Parkin is strikingly and specifi -
cally recruited to dysfunctional mitochondria. Like other studies,
these authors found that at steady state, Parkin is primarily cyto-
solic; however, careful confocal imaging allowed them to visual-
ize a handful of Parkin foci colocalizing with a few, fragmented
mitochondria. Interestingly, treatment of YFP-Parkin – over-
expressing cells with the mitochondrial uncoupler carbonyl cyanide
Parkin mitochondria in the autophagosome
Heidi M. McBride
University of Ottawa Heart Institute, Ottawa, Ontario K1Y 4W7, Canada
© 2008 McBride This article is distributed under the terms of an Attribution–Noncommercial–
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scribed at http://creativecommons.org/licenses/by-nc-sa/3.0/).
JCB • VOLUME 183 • NUMBER 5 • 2008 758
CCCP treatment, the authors showed that overexpressed Parkin
was also recruited to mitochondria upon an increase in complex 1 –
dependent reactive oxygen species (ROS) using the herbicide
paraquat, a toxin similar to MPTP used to induce a PD pheno-
type in some animal and cultured models ( Terzioglu and Galter,
2008 ). Similarly, genetic backgrounds that induce partial mito-
chondrial dysfunction also led to Parkin recruitment, further in-
dicating that this is a general response to mitochondrial stress.
Because the authors did not determine whether Parkin ’ s ubiqui-
tination activity was required, or what the mitochondrial targets
might be, the potential role of ubiquitination in targeting the
damaged mitochondrial fragments to the LC3-positive autopha-
gosomes remains unknown. In addition, this study did not de-
termine the relative contribution of endogenous levels of Parkin
to mitochondrial turnover in neurons, which will be clinically
important. Nevertheless, these results are the fi rst to demon-
strate a specifi c role for the ubiquitin E3 ligase on mitochon-
drial quality control, and provide further evidence that the
etiology of PD may indeed be caused by breeches in mitochon-
The identifi cation of Parkin as the fi rst protein to regulate
the selective removal of mitochondria provides an important mo-
lecular tool to dissect this pathway and search for novel therapeu-
tics. Parkin overexpression has already been shown to provide
some protection against toxin-induced animal models of PD
( Ulusoy and Kirik, 2008 ). If the expressed Parkin could be acti-
vated for effi cient mitochondrial recruitment, then the effi cacy of
these studies may have been higher. What factors may help to
activate Parkin? The results from the Narendra et al. (2008) study
clearly indicate that Parkin is selectively recruited to damaged
mitochondria, but it is not obvious how this ligase can distinguish
healthy from damaged mitochondria. The signal to recruit may be
m-chlorophenylhydrazone (CCCP) for 24 or 48 h led fi rst to ex-
tensive Parkin recruitment, followed by the complete loss of mi-
tochondria from the cell, an event that was not observed in the
cells lacking Parkin. This loss of mitochondria was dependent on
the presence of the autophagy-related gene Atg5, which demon-
strates that the degradation of the organelles occurred in auto-
phagosomes. Mitochondrial fragmentation has already been shown
to occur upon CCCP treatment, and inhibition of these fi ssion
events using the dominant interfering mutant of DRP1 did not af-
fect Parkin recruitment. This fi nding indicates that Parkin recruit-
ment is independent of mitochondrial fragmentation. It is tempting
to consider that Parkin may play an important role in selecting
damaged mitochondrial proteins that would be pinched away
from the healthy organelle by DRP1 ( Fig. 1 ).
that the increase in fusion observed in the Parkin null fl ies may
help to buffer the accumulating damage. In this case, the residual
fi ssion may allow the transport of some mitochondrial fragments
to the autophagosome. This could partially explain why the fur-
ther loss of DRP1 and mitochondrial fi ssion is lethal. Similarly,
the rescue of the Parkin-null fl ies by the inhibition of mitochon-
drial fusion may be caused by the preservation of individually
damaged organelles in a form that is amenable for mitophagic
degradation. That the loss of Parkin can be rescued upon induc-
tion of fragmentation suggests that the protein is not essential for
the targeting of the damaged fragments to the autophagosome;
rather, it may function upstream in the selection of proteins for
DRP1-mediated fi ssion.
It is almost certain that the overexpression of Parkin and
use of CCCP exaggerate the phenotype; however, the results
clearly implicate the mitochondrial recruitment of Parkin in the
targeted degradation of damaged organelles. To ensure that the
phenomenon of Parkin recruitment was not unique to global
[ID]FIG1[/ID] The authors suggest
Figure 1. Parkin recruitment leads to selective mitophagy. Cytosolic parkin is selectively recruited to uncoupled or dysfunctional mitochondria. The outer
membrane protein kinase Pink1 functions upstream of Parkin and may play a role in sensing mitochondrial damage and Parkin recruitment. The ubiquitina-
tion activity of Parkin was not directly examined in this study, but it will be important to determine whether it mono- or poly-ubiquinates its substrates. The
recognition of Parkin-associated mitochondrial fragments by LC3-positive autophagosome requires Atg5. Fusion with lysosomes leads to the total degrada-
tion and clearance of the damaged mitochondria. p, phosphorylation; ub, ubiquitin.
759 PARKIN AND MITOCHONDRIAL TURNOVER • McBride Download full-text
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the initiation of protein aggregation within the outer membrane,
possible second messengers like ROS or NO, or through the acti-
vation of an unidentifi ed mitochondrial receptor protein.
The most obvious candidate for a Parkin recruitment fac-
tor would be the mitochondrial outer membrane kinase Pink1
( Schapira, 2008 ). Interestingly, two of the proposed Pink1 sub-
strates are also involved in quality control: the chaperone Trap1/
Hsp75 ( Pridgeon et al., 2007 ) and the serine protease HtrA2/
Omi ( Plun-Favreau et al., 2007 ). Upon cellular stress induced
through the p38 ? – Map kinase pathway, Pink1 was shown to be
required for the phosphorylation and activation of HtrA2/Omi
( Plun-Favreau et al., 2007 ). This activation presumably led to
the degradation of unfolded or oxidized intermembrane space
proteins, although the substrates were not defi ned. Similarly,
peroxide-induced stress led to a Pink1-dependent phosphoryla-
tion of Trap1/Hsp75, whose chaperone activity appears to assist
in the refolding of damaged proteins and reduction of mito-
chondrial ROS ( Pridgeon et al., 2007 ). Pink1 was initially
thought to reside within the intermembrane space, with a func-
tionally relevant cytosolic pool ( Silvestri et al., 2005 ; Haque
et al., 2008 ). However, recent studies have provided compelling
evidence for a single, outer membrane location for the enzyme
with the kinase domain facing the cytosol ( Zhou et al., 2008 ).
Because HtrA2/Omi and Trap1/Hsp75 reside within the mito-
chondria, the likelihood of direct protein interactions between
them seems less likely. Whether or not these interactions are di-
rect, it indicates that Pink1 functions as a sensor of mitochon-
drial or cellular stress. A third effect of activated Pink1 may be
in the activation of Parkin, either directly or indirectly, trigger-
ing the initiation of mitophagy.
Other PD-related genes may play roles in mitochondrial
quality control, although the mechanisms are admittedly less
obvious. For example, DJ-1 is encoded by the PARK7 gene and
has been shown to function as a redox sensor granting protec-
tion to cells against ROS-induced toxicity ( Schapira, 2008 ).
This protection is evident in multiple models of cell death, in-
cluding stroke ( Aleyasin et al., 2007 ). Interestingly, DJ-1 was
recently reported to translocate to the mitochondria within 3 h
under conditions of oxidative stress ( Junn et al., 2008 ). By 24 h,
DJ-1 was relocalized into the nucleus, where it is proposed to
bind multiple RNAs and regulate p53 ’ s transcriptional activity.
Whether or not DJ-1 recruitment is regulated in the same man-
ner as Parkin remains to be determined.
By providing a fundamentally new function for Parkin in
mitochondrial quality control, this study opens up many new
avenues of investigation. Whether and how the PD mutations in
Parkin interfere with the maintenance of functional mitochon-
dria will be the subject of intense future investigation.
Submitted: 30 October 2008
Accepted: 4 November 2008
Aleyasin , H. , M.W. Rousseaux , M. Phillips , R.H. Kim , R.J. Bland , S. Callaghan ,
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disease gene DJ-1 is also a key regulator of stroke-induced damage. Proc.
Natl. Acad. Sci. USA . 104 : 18748 – 18753 .