BY MARK SHWARTZ
A new form of nanotechnology developed at Stanford may lead to a better understanding of the life and death of human cells.
Writing in the Nov. 18 Proceedings of the National Academy of Sciences (PNAS), Stanford researchers described how newly created circles of synthetic DNA -- called "nanocircles" -- could help researchers learn more about the aging process in cells.
"In the long run, we have this dream of making laboratory cells live longer," said Eric Kool, a professor of chemistry at Stanford and co-author of the PNAS study. "We thought of this pie-in-the-sky idea several years ago, and we've been working toward it ever since."
All cells carry chromosomes -- large molecules of double-stranded DNA that are capped off by single-strand sequences called telomeres. In their study, the research team successfully used synthetic nanocircles to lengthen telomeres in the test tube.
In living cells, chromosomes (molecules of double-stranded DNA stained orange) are capped with telomeres (single strands of DNA stained yellow). Each time a chromosome divides, its telomeres shorten until they reach a critical size that ultimately leads to cell death. CREDIT: Robert Moyzis, UC-Irvine; U.S.Department of Energy Human Genome Program
"The telomere is the time clock that tells a cell how long it can divide before it dies," Kool noted. "The consensus is that the length of the telomere helps determine how long a cell population will live, so if you can make telomeres longer, you could have some real biological effect on the lifespan of the cell. These results suggest the possibility that, one day, we may be able to make cells live longer by this approach."
Human telomeres consist of chemical clusters called "base pairs" that are strung together in a specific sequence known by the initials TTAGGG. This sequence is repeated several thousand times along the length of the telomere. But each time a cell divides during its normal lifecycle, its telomeres are shortened by about 100 base pairs until all cell division finally comes to a halt.
"Suddenly there's a switch in the cell that says, 'It's time to stop dividing,'" Kool explained. "It's still not completely clear how that works, but it is clear that once telomeres reach the critically short length of 3,000 to 5,000 base pairs, they enter senescence and die."
In nature, a chromosome can be lengthened by the enzyme telomerase, which adds new TTAGGG sequences to the end of the telomere. But because telomerase is difficult to produce in the lab, Kool and his co-workers decided to create synthetic nanocircles that mimic the natural enzyme.
Each nanocircle consists of DNA base pairs arranged in a sequence that is complementary to the telomere. When placed in a test tube, the nanocircles automatically lengthen the telomeres by repeatedly adding new TTAGGG sequences.
"Nanocircles are so simple they're amazing," Kool observed. "Each nanocircle acts like a template that says, 'Copy more of that sequence.' In the test tube, we start with very short telomeres and end up with long ones that are easy to see under the microscope with fluorescent labeling. This suggests the possibility that one day we may be able to make cells live indefinitely and divide indefinitely, so they essentially become refreshed, as if they were younger."
Aging and cancer
Kool pointed out that most cells have a limited lifespan, which is part of the normal aging process.
"The link between organism aging and cell aging is less clear, but there very likely is a link," he noted. "On the other hand, it is pretty clear that telomere length governs how long an individual cell lives."
In some diseases, such as premature aging (progeria) and cirrhosis, patients have cells with unusually short telomeres, Kool said. Cancer is another disease closely associated with telomere size.
"In order for a cell to become cancerous, one of the things it has to do is switch on the telomerase gene which makes the telomeres longer," he said. "The body has decided that the best way to keep an organism alive is to keep telomerase turned off, because otherwise you can get mutations and cancer too easily."
Because researchers need to study cells that live a long time, many labs rely on tumor-derived cells, which continuously divide and therefore are immortal. Kool predicted that nanocircle technology could one day provide an alternative method that would allow researchers to use healthy cells in their experiments instead of cancerous ones.
"If you could study normal cells in a convenient way, it would be a major boon for biomedical research," he noted. "You could go to the store and buy liver cells, pancreatic cells and skin cells and have them live indefinitely -- if you could find a way to refresh their telomeres every couple of weeks or so. That has been our dream for this project: to find a way to refresh telomeres but without permanently turning on telomerase, which may increase the likelihood of cancer."
Kool thinks nanocircle technology may prove useful in transplantation science and organogenesis.
"Perhaps some day researchers could grow new livers, new pancreas cells, new skin for burn victims," he said. "Instead of waiting for new donors to die, we could grow normal tissue in the lab. Maybe we wouldn't need stem cells; we wouldn't need to get into the controversy of where stem cells come from, if you could just take normal cells and grow them."
Kool and his colleagues also have begun research into the structure of single-strand telomeres, which are strikingly different from double-stranded DNA found in the rest of the chromosome.
lead author of the PNAS study is Ulf M. Lindstrom, a former
postdoctoral fellow in the Stanford Department of Chemistry now at
Lund University in Sweden. Other Stanford co-authors are former
Stanford undergraduate Ravi A. Chandrasekaran, now at the
University of California-Berkeley; Stanford graduate students
Lucian Orbai, Sandra A. Helquist and Gregory P. Miller; and Emin
Oroudjev and Helen G. Hansma of the University of California-Santa
Barbara Department of Physics. The study was supported by grants
from the National Science Foundation and the Swedish Research
Stanford Report, November 20, 2002