Two years ago, Leo Pallanck and Mel Feany and their colleagues succeeded in breeding fruit flies with mutations in parkin, the gene responsible for most cases of early onset PD in humans (see ARF related news story). To the disappointment of many, those flies did not seem to fully recapitulate the quintessential feature of PD, namely the gradual loss of dopaminergic neurons in the brain. Now, search-and-rescue experiments by Pallanck and colleagues at the University of Washington in Seattle reveal that parkin mutants do, in fact, suffer a subtle and progressive loss of a single subset of those neurons. What’s more, the authors have found a way to prevent that loss—increasing expression of the antioxidant enzyme glutathione S-transferase (GST).
The work, which appears online this week in PNAS, indicates that oxidative stress, long thought to cause sporadic PD in humans, can wipe out dopaminergic neurons in parkin flies, as well. The results raise the hope that antioxidants, and in particular therapies that boost GST, could offer a new advance for PD treatment. This view is in sync with a recent meta-analysis of dietary studies, which suggests that eating foods rich in vitamin E can reduce the risk of PD.
Drosophila parkin mutants, like their human counterparts, suffer from mitochondrial dysfunction, but the most obvious ramification for flies turned out to be muscle wasting, not neuronal loss. The less obvious neuronal phenotype was revealed only when the researchers, led by first author Alexander Whitworth, performed tyrosine hydroxylase staining and confocal microscopy on whole brains to look at every single dopaminergic (DA) neuron. By visualizing individual neurons, Whitworth was able to pick up one neuronal cluster—out of the dozen or so present in flies—that in mutant animals showed a significant, age-related reduction in DA neurons. Apart from that, the flies showed no gross brain abnormality and no defects in other neurons. Whitworth and colleagues found that the pinpoint lesion, occurring in the protocerebral posterior lateral (PPL1) cluster, was due to parkin deficiency, because enforced expression of parkin in the DA neurons restored that cluster.
Having identified a DA neuron loss phenotype, the researchers then screened for additional mutants that could modify the parkin effect. The strongest enhancers they found were loss-of function mutations in the Drosophila GST1 gene. Even 1-day-old parkin flies with GST1 mutations had significantly enhanced PPL1 neuron loss, while the same mutations had no effect on normal flies. Expression of GST1 targeted to DA neurons in the parkin mutants prevented neuronal loss, comparable to the level of rescue seen with parkin overexpression. GST expression driven by a muscle-specific promoter also prevented degeneration in that tissue.
From these results, the authors propose that parkin functions to protect neurons and other tissues from oxidative stress and its resulting tissue damage. They saw direct evidence for oxidative damage in the parkin flies, which had 4 times higher levels of protein carbonyls than usual. The next step will be figuring out how parkin protects cells, whether by getting rid of damaged proteins, by affecting mitochondrial integrity, or maybe even by targeting malfunctioning mitochondria for destruction, the authors speculate.
In humans, a large body of evidence links oxidative stress to both early onset and sporadic PD. Alleles of the human omega-1 GST gene influence the onset of sporadic PD (Li et al., 2004). While the flies may not be a perfect model for human PD (they lack homologs of important parkin substrates, for one thing), the striking similarities between flies and people in this study suggest that pharmacologic elevation of GST should be considered as a new approach to PD therapy.
Of course, the use of antioxidants, and particularly antioxidant vitamins, to stave off all manner of aging-related diseases is not a new principle. A meta-analysis of eight separate dietary studies looking at vitamin E and ascorbic acid, or vitamin C, concludes that moderate dietary intake of vitamin E, but not the other nutrients, is associated with a reduced risk of PD. The study, led by Mahyar Etminan of McGill University in Montreal and published in the June Lancet Neurology, looked only at vitamin E intake in food and not supplements, which have been shown to give no protection against PD (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=retrieve&db=pubmed&lis...) or AD (see ARF related news story) in prospective placebo-controlled clinical trials. So rather than pop that pill, perhaps we’d best just eat our fruits and vegetables.—Pat McCaffrey