Eggs have own biological clock

Aging mechanisms in worms’ reproductive cells differ compared with rest of body

DENVER — Egg cells age differently than cells in the rest of the body, a new study shows.

The finding, from experiments with roundworms presented December 5 at the annual meeting of the American Society for Cell Biology, might one day lead to ways to predict how long women will stay fertile or even to extend a woman’s fertile years.

Princeton University biologist Coleen Murphy and her colleagues study aging in the roundworm, Caenorhabditis elegans. The worms typically live for about 21 days, but fertility drops off sharply after about a week and the worms can no longer reproduce after they are about 9 days old. Even though 9-day-old worms still have plenty of eggs left, the egg cells, also called oocytes, are of such poor quality they can’t produce embryos.

Women experience a similar sharp decline in fertility starting in their late 30s. This drop-off in reproductive capability is one of the earliest signs of aging.

In earlier work, Murphy and colleagues discovered that certain mutations in biological processes regulated by insulin prolonged worms’ lives and gave them about three extra fertile days. Mutations in a different biological process, controlled by a protein called TGF-beta, extended fertility but not life span.

In the new study, the researchers examined which genes are turned on or off to prolong life and fertility in the oocytes and other body cells of the long-lived worms.

“We were really surprised to find this was a completely different mechanism” controlling aging in eggs compared with other body cells, Murphy said at the cell biology meeting. “In fact, there was almost no overlap between the genes involved in the long life of worms and those that extend fertility in the oocytes.”

Body, or somatic, cells are known to turn on stress-management genes to protect proteins and change metabolism as they age. But oocytes don’t bother with guarding proteins, Murphy and her colleagues found. Instead, eggs ramp up production of factors that protect them from or repair DNA damage and make more of proteins that help egg cells divvy up their chromosomes correctly, the researchers reported.

Because the entire job of an egg is to provide genetic information used to build a new generation, it is perhaps not so surprising that eggs devote resources to making sure the DNA stays healthy and chromosomes and are allocated properly, said Craig Blackstone, a physician and researcher at the National Institute of Neurological Disorders and Stroke in Bethesda, Md. “It makes sense that this would happen, but it hadn’t been shown before,” he said. “It’s clever of her to study this.”

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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