Sci. Aging Knowl. Environ., 16 April 2003
Foundations of Longevity
Genome search turns up targets of life-extending worm protein
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/sageke;2003/15/nw56
Key Words: DAF-2 FOXO RNAi Forkhead MnSOD dauer
To understand how a contractor builds a house, a person needs to know what each worker does. Similarly, to grasp how a regulator protein governs longevity, scientists must uncover the roles of its target genes. Toward that goal, researchers have now identified a host of genes that are turned on or off by a protein that prolongs life.
Some nematodes live dramatically longer than normal, thanks to mutations in their insulin/insulin-like growth factor-1 (IGF-1) pathway. Disabling the pathway activates a protein called DAF-16, enabling it to bind to DNA sequences and control the activity of nearby genes. Scientists have identified a few such genes--for instance, DAF-16 spurs production of superoxide dismutase, a protein that defuses the destructive free radical superoxide (see "The Two Faces of Oxygen"). But they haven't uncovered all of DAF-16's targets, and identifying the complete repertoire would provide a clearer view of how DAF-16 extends life.
In the new study, Lee and colleagues embarked on an exhaustive search for Caenorhabditis elegans genes controlled by DAF-16. They scanned the nematode genome for genes that carry the DAF-16-binding sequence within 1000 base pairs of their starting points. The researchers winnowed the number of genes by focusing only on those with relatives in the fruit fly that also carry the DAF-16-binding sequence. The hunt produced 17 candidates, including genes predicted to encode enzymes that alter DNA packing, influence fat storage or metabolism, and control the secretion of insulin or antioxidants.
The scientists next determined whether DAF-16 regulates each gene's activity. They compared the amounts of each gene's messenger RNA (mRNA) from two kinds of mutant worms: One group harbored permanently activated DAF-16, whereas the other carried crippled DAF-16. Three genes produced more mRNA when DAF-16 was on than when it was off, and three genes produced substantially less. Activity of the other genes didn't change, suggesting that DAF-16 doesn't control them. Alternatively, DAF-16 might subtly alter the amounts of mRNA or do so only in certain tissues, making differences hard to detect. Interfering with the activity of two of the genes suppressed by DAF-16 prolonged life span, demonstrating that the approach could yield genes important for DAF-16-promoted longevity.
This first hunt probably hasn't uncovered all of the possible DAF-16 target genes: A preliminary search that extended 1500 base pairs--rather than 1000--upstream of genes revealed an additional 66 candidates. In addition to expanding the region of DNA in which they search for the DAF-16-binding sequence, the researchers are looking for binding sites with slight sequence variations.
"It's a clever approach," says molecular geneticist Cynthia Kenyon of the University of California, San Francisco, because it reveals genes controlled by DAF-16 that researchers might not find with more targeted strategies. Scientists will need more information about the genes' functions and regulation to fit them into a cohesive view of longevity, says Kenyon, but the results provide an "entry point" from which to tackle the problem. Fleshing out the behavior of DAF-16's underlings could reveal how to construct an animal able to live overtime.
--R. John Davenport
April 16, 2003
Science of Aging Knowledge Environment. ISSN 1539-6150