12 January 2008. Using the roundworm Caenorhabditis elegans as a model, researchers have fished out several new genes that might guard against certain forms of Parkinson disease (PD). Writing in this week’s PNAS online, Guy Caldwell and colleagues at the University of Alabama in Tuscaloosa and Birmingham report five genes that help worms wriggle out of α-synuclein toxicity. α-synuclein is known to cause rare inherited forms of PD, and the research could help scientists identify new therapeutic strategies for treating the disorder.
Parkinson disease can be divided into more common sporadic forms and rarer inherited ones. Though causes for sporadic PD are largely unknown, environmental factors, including chemical toxicity, have been implicated. Just recently, in fact, investigators led by Don Gash at the University of Kentucky reported that long-term exposure to trichloroethylene, a chemical widely used in industrial cleaner, dramatically increases the risk for Parkinson’s (see Gash et al., 2007). By contrast, mutations in various genes, including that for α-synuclein, a protein with a propensity for self-association, cause inherited forms of PD. α-synuclein is the major component of Lewy bodies, dense protein aggregates found in Parkinson disease brains.
Caldwell and colleagues developed a C. elegans model of α-synuclein toxicity that, like Parkinson disease, worsens with age. In the worms, aggregates of green fluorescent protein (GFP)-tagged α-synuclein are gradually dumped into body wall muscles. By coexpressing this α-synuclein chimera with the molecular chaperone torsinA, which protects against α-synuclein aggregation (see ARF related news story), the researchers could maintain the worms at levels of α-synuclein teetering near aggregation. This, they reasoned, gives a more sensitive read of factors that might push aggregation over the edge.
First author Shusei Hamamichi and colleagues used these animals in an RNA interference screen for factors that aggravate α-synuclein aggregation. Their hypothesis-based screen focused on genes previously linked to PD through protein networks or previous aggregation screens (see ARF related news story). For example, they knocked down worm orthologs of all the familial PD genes and of genes that coexpress with or are associated with them. Of 868 genes in the dragnet, 125 enhanced α-synuclein aggregation when knocked down in adult worms (44-48 hours after eggs laid). The genes included worm homologs of parkin, DJ-1, PINK1, NURR1, and PARK9. In 20 knockdowns, α-synuclein aggregation also worsened in younger worms (32-36 hours after eggs laid). Of these 20 genes, only three (DJ-1, PINK1, and UNC-51, the worm homolog of ULK2; see ARF related news story) have been linked to Parkinson disease and one (the torsinA gene, tor-2) to α-synuclein aggregation. The remaining 16 might be novel neuroprotective genes.
To test this idea, Hamamichi and colleagues turned to a second worm model, one where α-synuclein is overexpressed in the eight C. elegans dopaminergic (DA) neurons. These animals show progressive dopaminergic neurodegeneration. The researchers tested seven of the 16 candidates they deemed most likely to be protective. When they coexpressed these genes with human α-synuclein, they found that five of them significantly protected dopaminergic neurons.
Of the five genes, VPS41, the worm ortholog of a vacuolar assembly/sorting protein involved in lysosomal biogenesis, offered the best protection. This is in keeping with the role of the lysosome in α-synuclein degradation and the growing evidence linking the lysosomal/autophagy system to PD (see ARF related news story), AD, and other neurodegenerative diseases (see ARF related news story). An ortholog of ATG7, an autophagy-associated regulatory gene, was also one of the five. Loss of this gene causes rampant neurodegeneration in animals (see ARF related news story).
The other three genes were GIPC, a PDZ-domain containing protein that is involved in G protein signaling; an ortholog of yeast Sec22p, which is involved in vesicular trafficking; and F16A11.2, which codes for a protein of unknown function. Because GIPC (GAIP interacting protein, C terminus) may interact with DA receptors, “it is interesting to speculate that GIPC serves to modulate a presynaptic protein-coupled pathway that can somehow combat the effects of α-syn misfolding and accumulation, perhaps by a DA or DA receptor-regulated manner,” write the authors. Sec22p protection may also be dopamine related. The authors speculate that reduced vesicular trafficking could limit dopamine transporter activity, causing a toxic buildup of cytosolic dopamine.
The exact protective mechanisms need to be worked out. In the meantime, the authors suggest that these genes “represent putative genetic susceptibility loci and potential therapeutic targets for PD.”—Tom Fagan.
Hamamichi S, Rivas RN, Knight AL, Cao S, Caldwell KA, Caldwell GA. Hypothesis-based RNAi screening identifies neuroprotective genes in a Parkinson’s disease model. PNAS. 2008 Jan 7 online. Abstract