Since the discovery that mutations in the gene for DJ-1 cause early-onset Parkinson disease (PD), researchers have been puzzling over what the protein does. Does this DJ rock, march, or swing? Is it a protease, a kinase, or a redox switch (see ARF related news story)? A paper in the November PLoS Biology suggests, in fact, that it waltzes like a chaperone.
To narrow down the potential roles for DJ-1, Asa Abeliovich and colleagues at Columbia University, New York, tested the protein for protease, antioxidant, and catalase activity, finding that it had none of these properties. First author Shoshana Shendelman and colleagues then turned their attention to chaperone activity. Chaperones are produced by cells in response to a variety of stresses, including heat shock and the presence of reactive oxygen species (ROS), and they play a major role in keeping other proteins from denaturing. In bacteria, some chaperones contain an amino acid motif called the ThiJ domain, and the presence of this motif in DJ-1 had led to early speculation that it, too, is a chaperone.
Shendelman and colleagues tested this possibility by measuring DJ-1’s ability to prevent aggregation of citrate synthase and glutathione S-transferase in response to heat shock. The authors found that DJ was able to reduce aggregation by about 50 percent. Significantly, DJ-1 carrying the PD mutation (a proline for a leucine at position 166) failed to prevent aggregation, suggesting that the chaperone activity is what prevents the disease. That, plus having comparable activity to the human chaperone Hsp27, supports the contention that DJ-1 is a bona fide chaperone.
What about prior evidence that DJ-1 acts as a redox switch (see ARF related news story)? The redox state certainly seems to affect DJ-1, because in the presence of the reducing agent dithiothreitol, the protein completely failed to keep citrate synthase out of aggregates. When Shendelman then added the strong oxidizer hydrogen peroxide to the mix, DJ-1 was reactivated—though under these conditions it could only protect about 20 percent of the citrate synthase. Curiously, in contrast to previous studies showing that cysteine 106 is essential for a redox switch that translocates DJ-1 to mitochondria (see ARF related news story), the authors found that it is cysteine 53 that is crucial for the chaperone activity. In fact, a cysteine 106 mutant still protected cells from oxidative stress.
So what does DJ-1 do in vivo? Parkinson disease is characterized by the accumulation in dopaminergic neurons of dense protein aggregates called Lewy bodies (LB). α-synuclein is one of the major components of LBs. Could DJ-1 prevent aggregation of α-synuclein? Apparently it may. The authors found that wild-type DJ-1 prevented the generation of high molecular weight polymers that form when α-synuclein is warmed, whereas the PD mutant form of DJ-1 had no effect. This chaperone activity wasn’t just limited to the test tube, either. In neuroblastoma cells subjected to oxidative stress, overexpression of wild-type DJ-1, but not the mutant form, prevented accumulation of insoluble α-synuclein. And the authors found that in DJ-1-negative cells, detergent-insoluble synuclein also accumulated.
“Taken together, our data strongly support the notion that DJ-1 functions as a redox-dependent protein chaperone to mitigate molecular insults downstream of an ROS burst,” write the authors. And though they state that it is possible that DJ-1 plays additional roles in the mitochondria or nucleus, they also note that they did not observe any re-localization of the protein in cultured cells. But as Mark Cookson writes in an accompanying PLoS Primer, “it is not yet firmly established which activity of DJ-1 is most relevant to recessive parkinsonism.”
In an accompanying paper, Abeliovich and colleagues propose a cellular model for PD based on DJ-1-negative cells derived from embryonic stem (ES) cells. Joint first authors Shendelman, Cecile Martinat, and Alan Jonason, used DJ-1-negative ES cells to generate dopaminergic neurons. The authors found that DJ-1-negative ES cells had increased sensitivity to oxidative stress and differentiated into dopaminergic neurons much less efficiently than normal ES cells. In addition, those dopaminergic neurons that survived the process were much more sensitive to 6-hydroxydopamine, a toxin that leads to oxidative stress and apoptosis. The authors also found that cells lacking DJ-1 were more sensitive to inhibition of the proteasome, thus tying together two pathways that have been linked to PD—oxidative stress and protein degradation by the ubiquitin-proteasome system.
The authors write that “this study presents a novel, ES cell-based genetic approach to the study of neurodegenerative disorders.” Advantages of this would include the ability to study human rather than mouse dopaminergic neurons.—Tom Fagan
- Shendelman S, Jonason A, Martinat C, Leete T, Abeliovich A. DJ-1 is a redox-dependent molecular chaperone that inhibits alpha-synuclein aggregate formation. PLoS Biol. 2004 Nov;2(11):e362. PubMed.
- Cookson MR. Molecules that cause or prevent Parkinson's disease. PLoS Biol. 2004 Nov;2(11):e401. PubMed.
- Martinat C, Shendelman S, Jonason A, Leete T, Beal MF, Yang L, Floss T, Abeliovich A. Sensitivity to oxidative stress in DJ-1-deficient dopamine neurons: an ES- derived cell model of primary Parkinsonism. PLoS Biol. 2004 Nov;2(11):e327. PubMed.