As a biological feat, it was the equivalent of an 80-year-old woman giving birth: Because of a mutation, Coleen Murphy's worms were still fertile and laying eggs right up until the end of their lives. The worms' impressive performance adds weight to the evidence that the biological clock that rules reproduction is separate from the one that grants us the traditional threescore and 10.
In a new study, Murphy, a molecular biologist at Princeton University, showed that long-lived bodily, or somatic, cells in Caenorhabditis elegans, a one-millimeter nematode commonly used as a model for aging studies in labs, activate genetic pathways completely separate from those found in long-lived egg, or oocyte, cells. Murphy presented her work at the American Society for Cell Biology in Denver on December 5.
"Investigators of aging in humans have been interested in studying somatic aging, and they've been interested in looking at the effects of age on fertility, but, in general, there haven't been any people trying to tie those two lines of investigation together," saysTerry Hassold, a reproductive biologist at Washington State University in Pullman who was not involved in the study. "That's an extremely important aspect of Murphy’' work, because it will help those of us that study human reproduction think about it in a different way."
Longevity researchers have long turned to C. elegans to learn more about the human aging process. Although it may seem unlikely that the 959-celled roundworms have much in common with humans, many genetic pathways were conserved during the course of evolution. As a result, many of the genes and proteins that regulate various processes are almost identical in C. elegans, mice (another animal model) and humans. Their reproductive cycles are similar, too. Middle-aged human females and C. elegans (which live two- to three-weeks) generally show few outward signs of senescence halfway through their lives. The oocytes of both the women and the worms, however, age much more rapidly, effectively ending the ability to reproduce during the second half of life, a relatively unique phenomenon in the animal kingdom.
Most mutations in C. elegans affect both life span and reproduction, which had led scientists to believe that body cells and female reproductive cells aged according to the same clock. But in Murphy's worms, a mutation in a gene known as transforming growth factor beta (TGF-β) enabled the production of high-quality eggs right up to the day they died.
While completing her postdoc, Murphy began to study C. elegans mutants that could live and reproduce twice as long as normal worms. These long-lived worms had mutations that decreased the production of a protein known as insulinlike growth factor 1 (IGF-1), which helps drive cellular growth and division. The TGF-β mutants that Murphy also studied could reproduce far longer than wild-type (nonmutant) worms—but, unlike the IGF-1 mutants, they didn’t actually live any longer. Their oocytes might have been young, but their bodies were decrepit.
"Worms, like humans, have to be in good enough shape to actually be pregnant and have kids successfully. If they're not in good enough shape, then they die while they’re trying to lay the eggs or give birth," Murphy says. "I think there are more parallels to human reproduction and the post-reproductive life span than we anticipated."
These results showed that different genes control the life span of C. elegans and the length of reproductive time. In her latest work, Murphy wanted to know whether reproduction and life span were two aspects of the same overall genetic pathway or completely separate entities. To answer this question, Murphy tracked which genes were turned on and off over time in the oocytes and somatic cells in C. elegans IGF-1 and TGF-β mutants, as well as wild-type worms.
Murphy found that the normal pattern of gene activation seen in aging wild-type C. elegans was reversed in the body cells and oocytes of IGF-1 mutants. Gene activation was also reversed in the oocytes of TGF-β mutants. With genes that helped them produce higher-quality eggs for a longer period of time, the TGF-β mutants had double the reproductive span as control worms. When Murphy compared the genes turned off and on in oocytes and body cells of the same worm, however, she saw that two completely different sets of genes controlled oocyte and body-cell aging in C. elegans. Those genes that helped somatic cells live longer and age more slowly maintained the quality of the cell's membranes and proteins. The genes linked to oocyte longevity, however, preserved the integrity of chromosomes and DNA.
"The TGF-β have a longer reproductive span by better maintaining their DNA, chromosomes and chromatin structure—all these things that have to do with the DNA components of the cell. That's a completely non-overlapping pathway with the kind of genes we see regulating somatic aging," Murphy says. Genes maintaining protein and cell membrane quality help drive body cell longevity, because the priority of these cells is to preserve existing function rather than produce large numbers of new cells.
"What drives the decline in oocyte quality in human females is chromosome abnormalities," Hassold says. "The likelihood of a woman in her twenties conceiving a chromosomally abnormal embryo is under 5 percent. For a woman in her forties, the likelihood may be as high as 50 percent." As with humans, he says, the oocytes of C. elegans also show an increase in chromosome abnormalities with aging.
With more women delaying childbirth until their thirties or even later, understanding how oocytes age is crucial to helping women have healthy babies. But the importance of Murphy's work extends beyond fertility and childbirth. By understanding how different cells age, Murphy says, researchers may one day not only extend reproduction, but also life span and organ function.