16 Sep 2005

Hunting for clues to the function of the Parkinson gene DJ-1, researchers have garnered new leads by once again tapping a favorite informant, the fruit fly Drosophila. In back-to-back papers published in the September 6 Current Biology, and another in last week’s PNAS Early Edition, three groups present several versions of DJ-1-deficient flies. Although the phenotypes vary, the studies all support the same conclusion: The major function of DJ-1 is to protect cells from oxidative attack, particularly when the insult comes from without. While this may come as no surprise based on previous studies of knockout mice (Kim et al., 2005) and human cells (see ARF related news story), the new work illustrates dramatically how DJ-1 defects and oxidative insults act jointly to kill neurons, making the Drosophila model a unique opportunity to probe the interplay of genes and environment in a neurodegenerative disease.

Fruit flies have been famously useful for revealing the functions of the PD genes α-synuclein and parkin, so looking for homologs of the recently discovered familial PD gene DJ-1 was an obvious path to understanding what that protein does. Working independently, two groups, one led by Kyung-tai Min at the NIH in Bethesda, Maryland, and, the other, a collaborative venture between Leo Pallanck in Seattle and Nancy Bonini in Philadelphia, scanned the Drosophila genome and came up with 2 DJ-1 homologs. DJ-1α expression appears restricted to testes, while DJ-1β is widely expressed. Using transposon-mediated mutagenesis to knock out one or both alleles, the researchers found that single or double knockouts are fertile, viable, and apparently normal. From there, though, their observations diverged.

Under normal conditions, Pallanck’s and Bonini’s double knockout flies appeared intact. When the flies were exposed to paraquat and rotenone, two agricultural poisons suspected of causing sporadic PD in humans, the knockouts were up to ten times more sensitive to the killing effects of the chemicals than were isogenic controls. The effect was selective for oxidative toxins, since the flies were not any more susceptible to b-me, DTT, or the proteasome inhibitor MG132, but did show enhanced death after treatment with hydrogen peroxide. Analysis of single knockouts showed that it was absence of the widely expressed DJ-1β that was responsible for the phenotype. Finally, this group showed that exposure of flies to paraquat resulted in a modification of the DJ-1 protein similar to that seen in human cells (see ARF related news story), showing that the protein could directly respond to oxidative stress.

In contrast, the DJ-1β knockout flies created in Kyung-tai Min’s lab showed a mixed phenotype: While they were more sensitive to hydrogen peroxide, they displayed an increased resistance to paraquat. The resistance to paraquat was attributed to a compensatory expression of the normally testes-restricted DJ-1α in the brain of the knockouts, a situation which was not observed in Pallanck’s and Bonini’s flies. Min’s group went on to show that forced overexpression of DJ-1α in dopamine neurons conferred protection against paraquat, but not peroxide.

The Min lab flies also showed effects on dopamine neurons, which survived aging better in the DJ-1β knockouts compared to wild-type. In both these papers, the authors discuss the possible reasons for the phenotypic differences, including the use of different mutagenic protocols, where insertions or deletions create distinctly different mutant alleles. Also, the different genetic backgrounds might give rise to the apparent differences in the functions of the two DJ-1 homologs.

Adding another wrinkle, Yufeng Yang and colleagues from Bingwei Lu’s lab at Stanford weigh in with a different experimental approach, and some radically different results. When this group used RNAi to knock down DJ-1α in flies, their larvae died. Using targeted expression of DJ1α RNAi in dopamine neurons, these researchers found an age-dependent loss of cells, lower brain dopamine levels, and elevated levels of reactive oxygen species. Expressing RNAi in postmitotic neurons yielded flies that were more sensitive to peroxide toxicity.

To investigate a bit further how DJ-1 might act, Yang et al. tried genetic complementation with their RNAi transgenic flies. They found that activation of the Akt pathway suppressed the loss of dopamine neurons and ROS accumulation after DJ1α down-regulation, suggesting that loss of DJ1 compromises the survival of cells by interfering with Akt activation. The downstream involvement of Akt in DJ-1’s action jibes with previous results showing that DJ-1 delivers a survival signal by turning off the proapoptotic kinase ASK-1 in mammalian cells (see ARF related news story).

The dramatic effects of DJ-1α RNA interference in neurons might be surprising, given the apparently restricted expression pattern and mild phenotypes detected by the mutagenic analysis. But although the overall phenotypes differ between the mutants and RNAi transgenics, the end conclusion that DJ-1 somehow protects cells from oxidative stress, and particularly environmental stress, remains unchallenged, and should spur further investigation into potential environmental causes of sporadic PD and other neurodegenerative diseases.—Pat McCaffrey

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References

News Citations

1. DJ Chaperones α-Synuclein—Offers Cell Model of Parkinson Disease 8 Oct 2004
2. Protection Against Parkinson’s—How the DJ Changes Station 7 Jun 2004
3. DJ-1 Dances with Daxx to Keep Neurons Spry 5 Jul 2005

Paper Citations

1. Kim RH, Smith PD, Aleyasin H, Hayley S, Mount MP, Pownall S, Wakeham A, You-Ten AJ, Kalia SK, Horne P, Westaway D, Lozano AM, Anisman H, Park DS, Mak TW. Hypersensitivity of DJ-1-deficient mice to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyrindine (MPTP) and oxidative stress. Proc Natl Acad Sci U S A. 2005 Apr 5;102(14):5215-20. PubMed.

Further Reading

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