Sci. Aging Knowl. Environ., 11 January 2006
Chain of Command
Study fingers abettors of life-stretching protein
When prosecutors are trying to break up a Mafia conspiracy, they usually start with the lowly perpetrators and work up to the bosses. Researchers probing how one biochemical pathway controls worm aging are following the opposite course. They know that the protein DAF-16 extends survival, but they are still rounding up the genes that do its bidding. A new study catches more than 100 genes receiving orders from DAF-16.
Nematodes wriggle longer if researchers quash the insulin/insulin-like growth factor 1 pathway. That alteration unleashes a protein called DAF-16, which in turn flips other genes on and off. To clarify how DAF-16 extends life, researchers have tried to ferret out its target genes. For example, 3 years ago molecular biologist Coleen Murphy, now at Princeton University, and colleagues compared gene activity in long-lived and short-lived worms to unmask about 500 possible DAF-16 underlings. They tested how 58 of these genes affect life span. The same year, another team pinpointed 17 potential targets by combing the worm's DNA for DAF-16-attracting sequences (see "Foundations of Longevity"). However, neither study demonstrated that DAF-16 adheres to the genes' control regions, an essential step for modifying their activity.
Molecular geneticist Heidi Tissenbaum of the University of Massachusetts Medical School in Worcester and colleagues turned to a method called chromatin immunoprecipitation to resolve the problem. Antibodies that bind DAF-16 allowed the researchers to nab worm genes that the protein hooked onto. The team identified 103 genes with docking sites for DAF-16. Although some had cropped up in previous searches for DAF-16 targets, others were new. Next, the researchers analyzed the functions of the first 33 genes they'd pinpointed. They either engineered worms to carry a faulty version of a particular gene or blocked its output using small RNA molecules. Meddling with 18 genes altered life span, fat storage, or the worms' ability to enter the dormant "dauer" stage. Because DAF-16 governs these functions, the results strengthen the evidence that these genes are immediately downstream of DAF-16. The other 15 genes might influence processes that the team didn't measure, Tissenbaum says.
The paper "is definitely a move in the right direction," says Murphy. Geneticist Pam Larsen of the University of Texas Health Sciences Center in San Antonio agrees, praising the work for providing "pretty convincing evidence that there's a physical interaction" between the genes and DAF-16. Detecting that interaction was a technical feat that required great patience and care, says geneticist Andrew Dillin of the Salk Institute for Biological Studies in San Diego, California. Because of the worms' rugged coating, isolating their innards without disrupting molecular interactions is tricky, he says. He and Murphy agree that the next step is refining the analysis to determine which genes DAF-16 latches onto at different stages in worm life. That work might provide more clues about how genes collude to control aging.
January 11, 2006
Science of Aging Knowledge Environment. ISSN 1539-6150