Sci. Aging Knowl. Environ., 20 October 2004
DNA landing sites for worm hibernation protein might flag aging-associated genes
To turn on a lamp at midnight, an insomniac must locate the switch. Researchers trying to illuminate longevity pathways have now found potentially crucial gene switches in DNA. The results should help scientists identify life-span-governing genes.
Worms survive hard times by hibernating, resuming their development when circumstances improve. Some mutations that force worms into hibernation also extend adult life span (see "Hardy Helper" and "Hard Times Teach Life-Extending Lessons"). The hibernate-or-mature toggle is a protein called DAF-12; it belongs to a family of proteins whose members turn different genes on or off depending on whether they're bound to a hormone. DAF-12's hormone signal originates in the insulin/insulin-like growth factor-1 (IGF-1) pathway, which also controls life span. To find DAF-12-regulated genes, Shostak and colleagues wanted to identify the DNA sequence that DAF-12 grabs, which presumably composes part of such genes.
The researchers mixed the DNA-binding chunk of DAF-12 with chopped-up worm DNA and isolated 40 unique DNA pieces that stuck to the protein. The 15 bits that clung most strongly shared a similar stretch of six DNA letters--the probable DAF-12 DNA-landing site. To confirm that DAF-12 latched onto this sequence, the researchers engineered two DNAs: One contained the tightest binding snippet, which carries several copies of the six-letter landing site, adjacent to the gene that makes a green-glowing protein; the second harbored mutations that disrupted half of these landing sites. The researchers then injected these DNA molecules into larvae at a time when they produce DAF-12 in large quantities. Animals that received DNA with intact sites fluoresced more brightly than did those that carried altered DNA. These and other observations indicate that DAF-12 can use the sequence to activate a gene.
The team then identified the genes that contained the 40 sticky sequences. Only one of these, skr-8, had been directly associated with longevity. Another, sel-8a/lag-3a, participates in a pathway that influences life span (see Tatar Perspective). A handful of additional ones play roles in oxidative stress or mitochondrial metabolism, processes that have been implicated in aging. The rest are involved in a wide array of physiological activities or their functions are unknown.
"This is the first time researchers have identified targets for DAF-12," says molecular biologist Marc Tatar of Brown University in Providence, Rhode Island. But to understand the protein's role in longevity, researchers must determine which of these genes DAF-12 controls in the adult worm, Tatar says. Molecular geneticist Gordon Lithgow of the Buck Institute for Age Research in Novato, California, says that his own lab has uncovered some of the same genes in an unpublished search for genes involved in aging. DAF-12 instigates "a giant sweep of signaling" that constitutes "the circuitry behind control of life span" by the insulin/IGF-1 pathway. Researchers know much about how an IGF-1 signal reaches DAF-12, but these genes will fill in the blank space between DAF-12 and life extension, he says. Now that researchers don't have to poke around in the dark for the switches that DAF-12 flicks, perhaps they can turn the spotlight on long life.
October 20, 2004
Science of Aging Knowledge Environment. ISSN 1539-6150