July 24, 2005
Fluorescing Lab Worms Signal Longer Life Spans
University of Colorado at Boulder scientists have used a fluorescent marker to predict the individual life spans of identical worms that were genetically engineered to illuminate stress levels, implying living organisms have "hidden physiological states" that dictate their ability to deal with the rigors of life.
According to CU-Boulder Research Associate Shane Rea, the genetically identical nematodes were engineered with a green fluorescent "reporter" protein coupled to a stress protein that is present in most multicellular organisms as a monitor of cellular health. The eyelash-sized, translucent worms that fluoresced the brightest after being subjected to high temperatures as young adults had significantly longer life expectancies than those that were less bright, the team reported.
"We have shown it's possible to predict the life span in an organism on the first day of adult life based on how it responds to stress," said CU-Boulder Professor Thomas Johnson. "This is something that has not been done before, and has implications for human longevity and health."
A paper on the subject by co-first authors Rea and Deqing Wu, in addition to James Cypser and Johnson of CU-Boulder's Institute for Behavioral Genetics and James Vaupel of the Max Planck Institute for Demographic Research in Germany was published in the July 17 advance online issue of Nature Genetics.
"We have engineered a single gene to monitor the health of an organism, which is a first," said Johnson. Funded in part by the National Institutes of Health, the effort began about eight years ago when Cypser, then a CU-Boulder graduate student, began testing a new idea of Johnson's in the university laboratory.
Most scientists believe the life span of humans and other living creatures is determined by a combination of genetic, environmental and chance factors. Twin studies in humans have suggested that genes are only about 15 percent to 30 percent responsible for a person's age at death, Rea said.
The CU-Boulder researchers attribute the life span variation observed in the genetically identical worms in the study to chance metabolic processes that involve an array of biochemical and kinetic reactions, said Rea.
"This work starts to address the question of why genetically identical organisms raised in identical environments still age at different rates," said Rea. "It suggests that chance alterations in how stress-response mechanisms are maintained dictate how individual organisms respond to specific environmental insults."
Carried out using about 100,000 popular laboratory nematodes known as C. elegans, the study indicated the brightness level triggered by the reporter protein could predict up to a four-fold variation in the life expectancy of a worm. In a typical experiment, the brightest worms had life expectancies of about 16 days, compared to about three days for those with the lowest expressed levels of fluorescent protein. The brightest worms also had the highest tolerance to extended exposure to heat.
"The ability to predict differences in longevity of this magnitude have never before been reported, especially given that the predictions are made on the first day of adult life," Johnson said.
Obtained from jellyfish and widely used in genetic experiments, the green fluorescent protein was attached to a heat-shock protein in the worms called HSP-16. "HSP-16 alone is probably not responsible for observed differences in survival, but instead is likely reflective of a hidden, heterogeneous, but now quantifiable state that dictates the ability of an organism to deal with the rigors of living," the team wrote in Nature Genetics.
An automated "worm sorter" equipped with a laser scanner to swiftly detect fluorescence levels in the nematodes and a pneumatic device that separates them into groups with short pulses of air was used during the study. One of only six such instruments in the United States, the machine can sort about 30 worms per second, Johnson said.
Genetic testing on both the bright and dim worms revealed that the levels of the green fluorescent protein expressed by parent worms were not passed down to offspring and therefore not heritable, said Rea.
Research suggests that humans possess several similar types of "stress-response" systems tuned to counter environmental insults like oxidative stress, pathogens, alcohol and heat, said Rea. Some of the stress-response systems appear to overlap with each other, while others operate completely independently, he said.
"We suspect several different biomarkers exist that represent the many different stress-response systems present in an organism," Rea said. "By finding them, we should ultimately be able to find what makes an animal robust to many different insults."
Johnson authored a milestone paper in 1988 showing that mutating a single gene in roundworms could double their life span -- the first evidence that the lifespan of an animal could be increased by genetic alteration. Since then, researchers around the world have been tinkering with proteins and genes of C. elegans in attempts to understand more about how life span can be increased.
Although interventions involving pharmaceuticals and other treatments clearly have a substantial effect on human longevity, many believe that the primary key to longer and healthier lives may involve genetic manipulation, said Rea. "But genes and environment are not the entire answer," he said. "Clearly, there is a large component of chance."
In the future, scientists could conceivably analyze human fluid samples for a variety of biomarkers similar to HSP-16 in order to determine a person's life span, said Rea. "They might even be able to tweak each stress-response system and set them for maximum longevity, which is believed to be about 120 years."
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