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Using a combination of ultrastructural and biochemical assays, these papers
further report that mitochondrial defects accompany both of these phenotypes.
The third paper reports equivalent findings using an RNAi approach to target the
PINK1
gene, although phenotypes are only documented in the indirect flight
muscle, presumably because the GAL4 line chosen for this study is not expressed
in the male germline (Yang
, 2006). Again, there is some inconsistency in
the reported effect on DA neuron integrity. Two of these studies reported DA
neuron loss in the PPM1/2 and PPL1 clusters (Park
et al.
et al.
, 2006; Yang
et al.
,
2006), while one found no difference (Clark
, 2006). Currently, it is unclear
why the third study failed to detect neuron loss although, in contrast, in a later
study they do observe a subtle decrease in PPL1 (Yun
et al.
, 2008). Interestingly,
one study also reports mitochondrial swelling in DA neurons of
et al.
mutants,
suggesting that the pathogenic mechanisms responsible for muscle degeneration,
spermatid defects and DA neuron loss all involve mitochondrial dysfunction
(Park
PINK1
, 2006). It should also be noted that an independent concurrent
study analyzing
et al.
-RNAi knockdown reported multiple pathogenic
defects including profound loss of DA neurons in all clusters even at 10 days
old and degeneration of eye tissue (Wang
PINK1
, 2006). However, these findings
must be considered distinctly unreliable since ubiquitous knockdown using this
construct caused early stage larval lethality, a phenomenon clearly discordant
with the genetic null mutants, strongly implicating confounding off-target
effects.
et al.
mutants display mitochon-
drial defects advances our knowledge, the real surprise of these studies came from
genetic analyses with
While the finding that Drosophila
PINK1
parkin
. The striking similarity in phenotypes between
PINK1
mutant animals raised the hypothesis that they may act in a
common pathway. In support of this idea, it was found that
and
parkin
double
mutants are phenotypically indistinguishable from the respective single mutants
(Clark
PINK1;parkin
, 2006), consistent with the two mutations affecting
one pathway rather than two separate or independent pathways. Furthermore,
genetic epistasis experiments showed that overexpression of either Drosophila or
human
et al.
, 2006; Park
et al.
parkin
is able to rescue the
PINK1
phenotypes, but conversely
PINK1
overexpression does not detectably influence the
parkin
phenotypes (Clark
et al.
,
2006; Park
et al.
, 2006; Yang
et al.
, 2006). Although a caveat of this finding is
that
parkin
overexpression can be generally protective, Park
et al.
(2006) showed
that
overexpression is unable to suppress the toxicity of several other
cellular insults, demonstrating at least some specificity in this interaction.
Together, these findings provide compelling evidence that parkin acts down-
stream from PINK1 in a common pathway.
These findings provoke the most obvious hypothesis that PINK1 may
phosphorylate parkin and stimulate its activity. While there is some evidence
that supports this in Drosophila (Kim
parkin
et al.
, 2008b), it has been questioned by
