Tuesday, June 14, 2011

“Human Aging: Is It an Accident of Evolution?”

“Human Aging: Is It an Accident of Evolution?”
by the Daily Galaxy

“National Institutes of Health researchers have identified a new pathway that sets the clock for programmed aging in normal cells. The study provides insights about the interaction between a toxic protein called progerin and telomeres, which cap the ends of chromosome. Elsewhere, an earlier Stanford University study suggests that aging is an accident of evolution. We'll examine both. The researchers have found that short or dysfunctional telomeres activate production of progerin, which is associated with age-related cell damage. As the telomeres shorten, the cell produces more progerin. Progerin is a mutated version of a normal cellular protein called lamin A, which is encoded by the normal LMNA gene. Lamin A helps to maintain the normal structure of a cell's nucleus, the cellular repository of genetic information.

In 2003, NHGRI researchers discovered that a mutation in LMNA causes the rare premature aging condition, progeria, formally known as known as Hutchinson-Gilford progeria syndrome. Progeria is an extremely rare disease in which children experience symptoms normally associated with advanced age, including hair loss, diminished subcutaneous fat, premature atherosclerosis and skeletal abnormalities. These children typically die from cardiovascular complications in their teens. "Connecting this rare disease phenomenon and normal aging is bearing fruit in an important way," said NIH Director Francis S. Collins, M.D., Ph.D., a senior author of the current paper. "This study highlights that valuable biological insights are gained by studying rare genetic disorders such as progeria. Our sense from the start was that progeria had a lot to teach us about the normal aging process and clues about more general biochemical and molecular mechanisms."

In a 2007 study, NIH researchers showed that normal cells of healthy people can produce a small amount of progerin, the toxic protein, even when they do not carry the mutation. The more cell divisions the cell underwent, the shorter the telomeres and the greater the production of progerin. But a nagging mystery remained: What was triggering the production of the toxic progerin protein?

The current study shows that the mutation that causes progeria strongly activates the splicing of lamin A to produce the toxic progerin protein, leading to all of the features of premature aging suffered by children with this disease. But modifications in the splicing of LMNA are also at play in the presence of the normal gene. The research suggests that the shortening of telomeres during normal cell division in individuals with normal LMNA genes somehow alters the way a normal cell processes genetic information when turning it into a protein, a process called RNA splicing.

To build proteins, RNA is transcribed from genetic instructions embedded in DNA. RNA does not carry all of the linear information embedded in the ribbon of DNA; rather, the cell splices together segments of genetic information called exons that contain the code for building proteins, and removes the intervening letters of unused genetic information called introns. This mechanism appears to be altered by telomere shortening, and affects protein production for multiple proteins that are important for cytoskeleton integrity. Most importantly, this alteration in RNA splicing affects the processing of the LMNA messenger RNA, leading to an accumulation of the toxic progerin protein.

Cells age as part of the normal cell cycle process called senescence, which progressively advances through a limited number of divisions in the cell lifetime. "Telomere shortening during cellular senescence plays a causative role in activating progerin production and leads to extensive change in alternative splicing in multiple other genes," said lead author Kan Cao, Ph.D., an assistant professor of cell biology and molecular genetics at the University of Maryland. Telomerase is an enzyme that can extend the structure of telomeres so that cells continue to maintain the ability to divide. The study supplied support for the telomere-progerin link, showing that cells that have a perpetual supply of telomerase, known as immortalized cells, produce very little progerin RNA. Most cells of this kind are cancer cells, which do not reach a normal cell cycle end point, and instead replicate out of control.

The researchers also conducted laboratory tests on normal cells from healthy individuals using biochemical markers to indicate the occurrence of progerin-generating RNA splicing in cells. The cell donors ranged in age from 10 to 92 years. Regardless of age, cells that passed through many cell cycles had progressively higher progerin production. Normal cells that produce higher concentrations of progerin also displayed shortened and dysfunctional telomeres, the tell-tale indication of many cell divisions. In addition to their focus on progerin, the researchers conducted the first systematic analysis across the genome of alternative splicing during cellular aging, considering which other protein products are affected by jumbled instructions as RNA molecules assemble proteins through splicing. Using laboratory techniques that analyze the order of chemical units of RNA, called nucleotides, the researchers found that splicing is altered by short telomeres, affecting lamin A and a number of other genes, including those that encode proteins that play a role in the structure of the cell.

The researchers suggest that the combination of telomere fraying and loss with progerin production together induces cell aging. This finding lends insights into how progerin may participate in the normal aging process. "Everyone has assumed we age by rust. But how do you explain animals that don't age? Some tortoises lay eggs at the age of 100, there are whales that live to be 200 and clams that make it past 400 years."

An earlier study by Stanford University Medical School researchers contradicts the prevailing theory that aging is a buildup of tissue damage similar to rust. The Stanford findings suggest specific genetic instructions drive the process. If they are right, science might one day find ways of switching the signals off and halting or even reversing aging. “We were really surprised,” said Stuart Kim, who is the senior author of the research. Kim’s lab examined the regulation of aging in C. elegans, a millimeter-long nematode worm whose simple body and small number of genes make it a useful tool for biologists. The worms age rapidly: their maximum life span is about two weeks. Comparing young worms to old worms, Kim’s team discovered age-related shifts in levels of three transcription factors, the molecular switches that turn genes on and off. These shifts trigger genetic pathways that transform young worms into social security candidates.

The question of what causes aging has spawned competing schools, with one side claiming that inborn genetic programs make organisms grow old. This theory has had trouble gaining traction because it implies that aging evolved, that natural selection pushed older organisms down a path of deterioration. However, natural selection works by favoring genes that help organisms produce lots of offspring. After reproduction ends, genes are beyond natural selection’s reach, so scientists argued that aging couldn’t be genetically programmed.

The alternate, competing theory holds that aging is an inevitable consequence of accumulated wear and tear: toxins, free-radical molecules, DNA-damaging radiation, disease and stress ravage the body to the point it can’t rebound. So far, this theory has dominated aging research. But the Stanford team’s findings told a different story. “Our data just didn’t fit the current model of damage accumulation, and so we had to consider the alternative model of developmental drift,” Kim said.

The scientists used microarrays—silicon chips that detect changes in gene expression—to hunt for genes that were turned on differently in young and old worms. They found hundreds of age-regulated genes switched on and off by a single transcription factor called elt-3, which becomes more abundant with age. Two other transcription factors that regulate elt-3 also changed with age. To see whether these signal molecules were part of a wear-and-tear aging mechanism, the researchers exposed worms to stresses thought to cause aging, such as heat (a known stressor for nematode worms), free-radical oxidation, radiation and disease. But none of the stressors affected the genes that make the worms get old.

So it looked as though worm aging wasn’t a storm of chemical damage. Instead, Kim said, key regulatory pathways optimized for youth have drifted off track in older animals. Natural selection can’t fix problems that arise late in the animals’ life spans, so the genetic pathways for aging become entrenched by mistake. Kim’s team refers to this slide as “developmental drift.”

“We found a normal developmental program that works in young animals, but becomes unbalanced as the worm gets older,” he said. “It accounts for the lion’s share of molecular differences between young and old worms.” Kim can’t say for sure whether the same process of drift happens in humans, but said scientists can begin searching for this new aging mechanism now that it has been discovered in a model organism. And he said developmental drift makes a lot of sense as a reason why creatures get old. “Everyone has assumed we age by rust,” Kim said. “But then how do you explain animals that don’t age?”

Some tortoises lay eggs at the age of 100, he points out. There are whales that live to be 200, and clams that make it past 400. Those species use the same building blocks for their DNA, proteins and fats as humans, mice and nematode worms. The chemistry of the wear-and-tear process, including damage from oxygen free-radicals, should be the same in all cells, which makes it hard to explain why species have dramatically different life spans. “A free radical doesn’t care if it’s in a human cell or a worm cell,” Kim said.

If aging is not a cost of unavoidable chemistry but is instead driven by changes in regulatory genes, the aging process may not be inevitable. It is at least theoretically possible to slow down or stop developmental drift. “The take-home message is that aging can be slowed and managed by manipulating signaling circuits within cells,” said Marc Tatar, PhD, a professor of biology and medicine at Brown University who was not involved in the research. “This is a new and potentially powerful circuit that has just been discovered for doing that.” Kim added, “It’s a new way to think about how to slow the aging process.”
Related: “Prevailing Theory of Aging Challenged in Stanford Worm Study”

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