Parkinson disease (PD) is the most prevalent neurodegenerative movement disorder [1] . Characterized by a progressive loss of dopaminergic neurons, PD pathology often includes the presence of Lewy bodies, ubiquitin- positive and α-synuclein-enriched inclusions, in the remaining neurons. Although sporadic forms of PD are believed to be more common, many familial forms share features with sporadic PD, including protein aggregation and mitochondrial dysfunction [2] . Pink1 (PTEN induced putative kinase 1) encodes a serine-threonine kinase that has been linked to autosomal recessive and some sporadic forms of Parkinson disease [3] - [5] . Targeted to the mitochondria, Pink1 is involved in mitochondrial protection, as loss of function of Pink1 results in substantial mitochondrial defects in sensitive tissues [6] - [10] . It is increasingly apparent that both Pink1 and parkin, acting in the same pathway, are necessary for proper mitochondrial integrity and function [11] [12] . The parkin E3 ubiquitin ligase acts downstream of Pink1. In mitochondrial protection, the recruitment of parkin to the mitochondria by Pink1 results in the ubiquitination of various mitochondrial proteins, promoting mitophagy [13] - [16] . In addition, Pink1 may have a protective role apart from the mitochondria, where an interaction with parkin could result in the tagging of cystolic proteins for degradation. This may be an important, but largely overlooked role, as neurodegenerative diseases are often characterized by the accumulation of toxic proteins.

Our laboratory has examined the adverse effects of expressing proteins that can produce toxicity, including α- synuclein and the Gal4 transcription factor [17] - [20] . We have shown the ability of parkin and Pink1 to rescue an α-synuclein-induced phenotype [17] [18] [21] , and the ability of parkin overexpression to rescue a Gal4-in- duced phenotype [20] . The suppression of the effects of Gal4 is presumably through its targeting for proteosomal degradation by parkin. To further investigate the role of Pink1 in protection against toxic proteins, we have performed expression studies to determine the effects of Pink1 on the Gal4-induced phenotype of developmental defects in the Drosophila eye.

Increase in Pink1 expression during eye development is able to alleviate the detrimental effects of Gal4 expression in a dose dependent manner. The ability of Pink1 to counteract the effects of GMR-Gal4 is similar to previous results in our laboratory with parkin [20] , supporting the theory that Pink1 is acting via parkin. Pink1 may interact with parkin to activate the ubiquitin-proteasomal system, resulting in the tagging of Gal4 for degradation. Alternatively, Pink1 may operate to protect the mitochondria from the effects of Gal4, recruiting parkin to the membrane to remove damage via mitophagy. Studies have shown that recruitment of parkin by Pink1 to depolarized mitochondria results in the ubiquitination of mitochondrial proteins VDAC1 [15] and mitofusin [13] [16] , leading to recruitment of autophagic proteins or decreases in mitochondrial fusion. It is hypothesized that when this process is impaired, by mutations in either parkin or Pink1, an accumulation of defective mitochondria results, leading to the neurodegeneration seen in Parkinson disease [14] . Pink1 may impart mitochondrial protection by interacting with other molecular chaperones at the mitochondrial membrane. For example, the phosphorylation of TRAP1 (Hsp75) by Pink1 has been shown to protect against oxidative stress and prevent cytochrome c release [25] . Identification of PINK1 targets is still in early stages. It is likely that PINK1 will be identified to interact with various proteins, and serves to protect against multiple stressors such as toxic proteins, oxidative stress and mitochondrial dysfunction.

The GMR-Gal4 heterozygotes exhibit a mild phenotype at 29˚C, with intermediate levels of apoptosis in the eye imaginal discs [19] . It was thought that this mild Gal4-induced effect may be increased with reductions in Pink1 or parkin, inducing a measurable rough eye phenotype. Our results show that these reductions are not sufficient to induce a rough eye phenotype, implying that either the effects of Gal4 are alleviated by an alternative mechanism, or that low levels of Pink1 or parkin may be sufficient to protect against low levels of Gal4. The lack of a phenotype observed using the Pink1^B9 mutant may suggest that a functional kinase is not necessary for Pink1 to participate in protection against the effects of Gal4. Alternatively, the lack of a phenotype may be due to the residual activity of maternally-inherited Pink1. As many proteins have roles independent of their kinase function, investigations into other attributes of the Pink1 protein may shed light on unexplored roles or interactions.

In conclusion, Pink1 was shown to act against the toxic consequences of GMR-Gal4, in a manner similar to parkin, likely through the tagging of the Gal4 protein for degradation by the ubiquitin-proteasomal system, or through protection of the mitochondria via mitophagy or, perhaps, both. The understanding of the ability of Pink1 to protect against protein toxicity, even an introduced protein, may provide extremely valuable clues on the way to the development therapeutics strategies to treat or prevent Parkinson disease.

Research was funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant and Parkinson Society Canada Pilot Project Grant to BES. Student support was received from the Department of Biology and the School of Graduate Studies at Memorial University of Newfoundland to AMT. We thank Dr. Liqiu Men (Bio-molecular Imaging Cluster, Memorial University of Newfoundland) for help with SEM and Maggie Hickey for help with editing and formatting of this manuscript.

The authors declare that there is no conflict of interests regarding the publication of this article.

Amy M.Todd,Brian E.Staveley, (2015) Pink1 Rescues Gal4-Induced Developmental Defects in the Drosophila Eye. Advances in Parkinson's Disease,04,43-48. doi: 10.4236/apd.2015.43006