When does a person start aging? Actually, very young, say scientists at Northwestern University who studied roundworms and found that adult cells begin aging when the animal reaches reproductive maturity.

The aging process starts when a genetic switch turns off factors that keep important proteins folded and functional, and protect the cell from stress.

The switch is thrown by germline stem cells (cells responsible for making eggs and sperm that transmit genetic information from generation to generation) after the animal starts to reproduce, ensuring the genetic line will continue.

Even though the study was done in the transparent roundworm C. elegans, the researchers say their results have implications for humans since the roundworm has a biochemical environment similar to humans, and is used as a model for studying human disease.

Knowing how aging begins could help researchers learn how to delay degenerative diseases related to aging.

"Wouldn't it be better for society if people could be healthy and productive for a longer period during their lifetime?" said senior author Richard I. Morimoto "Our findings suggest there should be a way to turn this genetic switch back on and protect our aging cells by increasing their ability to resist stress."

In animals, including C. elegans and humans, the so-called cellular "heat-shock response," which is a built-in protective response to stress that helps cells repair themselves, is essential for proper protein folding and cellular health. Aging is associated with a decline in quality control, so researchers looked at the heat-shock response in the life of the roundworm.

"We saw a dramatic collapse of the protective heat-shock response beginning in early adulthood," Morimoto said.

The study found that aging isn't a series of events, like scientists have previously believed, but a specific signal to cell tissues to turn off protective mechanisms, which starts the decline of the adult animal. In roundworms, the switch was thrown only eight hours after the animal reached maturity.

"We discovered a switch that is very precise for aging," Morimoto said. "All these stress pathways that insure robustness of tissue function are essential for life, so it was unexpected that a genetic switch is literally thrown eight hours into adulthood, leading to the simultaneous repression of the heat-shock response and other cell stress responses."

At one point, the researchers blocked the germline from sending the signal to turn off cellular quality control. They found the adult animal's tissues remained robust and stress resistant.

"This was fascinating to see," Morimoto said. "We had, in a sense, a super stress-resistant animal that is robust against all kinds of cellular stress and protein damage. This genetic switch gives us a target for future research."

The study was published in the journal Molecular Cell.