Dr. Harley and colleagues at Geron, Woodring Wright, Jerry Shay, and colleagues at Southwestern Medical Center discovered in 1998, as reported in Science magazine, that human cells could be immortalized in a petri dish by repairing their degraded telomeres through the introduction of a gene that causes the expression of the catalytic human protein telomerase. Telomerase is composed of two essential components, an RNA molecule (Terc, telomerase RNA component) and a catalytic subunit (TERT, telomerase reverse transcriptase). Telomerase is able to compensate for telomere loss by rebuilding degraded telomere sequences. In their experiments Dr. Shay and Dr. Wright found that by lengthening chromosomal telomeres very old cells began to function and behave like young cells. Later in 1998, MIT repeated this experiment and achieved identical results. Since then more than 800 universities and corporations have repeated the study in some form. In subsequent experiments tissues and organs grown from once old cells remain perfectly young even in the context of a significant reproduction requirement.

 

Anecdotal evidence for the role of telomeres can be seen in certain age related diseases such as Progeria which causes children to become old at a young age because they are born with shortened telomeres. Likewise, famous “Dolly the Sheep” grew old quickly because she was born with “short” telomeres. 

 

The Shay Wright experiments

 

In the January 1998 issue of Science, Harley and colleagues at Geron, Woodring Wright, Jerry Shay, and colleagues at Southwestern Medical Center, reported the effects of adding the gene that encodes human telomerase reverse transcriptase (TERT) to normal human cells in culture. In these experiments, TERT was introduced into telomerase-negative human retina and foreskin cells. The cells began to express telomerase. Their telomeres elongated, and the cells divided vigorously and did not express a cell marker for senescence (beta galactosidase). Furthermore, the cells showed an increased number of cell divisions and a limitless life span, compared to the cells that were not treated with telomerase, whose telomeres shortened with each division, leading to senescence (cell death). Another important observation was that the introduction of telomerase into the cells and their continuous rapid division and longer life span did not make them cancerous. They remained with a young appearance and normal number of chromosomes.

 

These experiments used cells important in human disease and aging--retinal pigment epithelium, fibroblasts, and vascular endothelium. Slowed metabolism of retinal epithelium can cause age-related macular degeneration. Fibroblasts in aging skin make less collagen and elastin and more collagenase, causing wrinkles. And overgrowth of the endothelium that forms capillaries and lines blood vessel interiors contributes to atherosclerosis.  The results of adding telomerase to these cells were striking--the cells regained their proliferative and reparative potential, ignoring the Hayflick limits. They concluded that cells with sufficiently elongated telomeres energetically produce, in high levels, proteins like catalase, superoxide dismutase, glutathione, Ku, collagen, elastin and many other proteins important in tissue formation, cell repair, and antioxidation, that become scarce as telomeres shorten.

 

Other Important Experiments

 

Subsequent experiments established the role telomeres play in organismal aging. An experiment conducted by Geron Corp. and published in November of 2000 in the Journal of Experimental Cell Research describes the relationship between telomeres and organismal aging. Geron Corp. scientists modified aging human skin cells to express the key catalytic enzyme TERT, from these cells they developed (grew) skin grafts, which were then transplanted into mice. The results showed that TERT restored normal function to the aged cells: their ability to divide increased exponentially and the pattern of gene expression changed. The transplanted skin remained young throughout the lifespan of the mouse while the mouse’s native cells and skin aged.

 

According to Geron's Chief Executive Officer Thomas Okarma, Ph.D., M.D., ``Demonstrating that telomerase restores a youthful function to aging human cells in an animal model supports our belief that this technology can be developed for regenerative medicine.” 

 

In 2004 UCLA scientists showed that the protein telomerase prevents the premature aging of immune cells that fight HIV, enabling the cells to divide indefinitely and prolong their defense against infection. Published Nov. 15 of 2004 in the Journal of Immunology, the research suggests a future therapy for boosting the weakened immune systems of HIV-positive people.

 

"Immune cells that fight HIV are under constant strain to divide in order to continue performing their protective functions. This massive amount of division shortens these cells' telomeres prematurely," explained Dr. Rita Effros, Plott Chair in Gerontology and professor of pathology and laboratory medicine at the David Geffen School of Medicine at UCLA. "So the telomeres of a 40-year-old person infected with HIV resemble those of a healthy 90-year-old person." Effros and first author Mirabelle Dagarag, Ph.D., hypothesized that harnessing telomerase's power over telomeres may provide a potent weapon in helping the AIDS patient's exhausted immune system defend itself against HIV. The researchers extracted immune cells from the blood of HIV-infected persons and tested what would happen if telomerase remained permanently switched on in the cell. "By exploiting telomerase's growth influence on telomeres, we thought we might be able to keep the immune cells youthful and active as they replicated under attack," said Dagarag, a postgraduate researcher. "We used gene therapy to boost the immune cell's telomerase and then exposed the cell to HIV." What Dagarag and Effros saw delighted them. "We found that the immune cells could divide endlessly," said Effros, a member of the UCLA AIDS Institute. "They grew at a normal rate and didn't show any chromosomal abnormalities that might lead to cancer.” “We also saw that telomerase stabilized the telomere length," added Dagarag. "The telomere didn't shorten each time the cell divided, which left the cell able to vigorously battle HIV much longer." In another important study, in April of 2000, Advanced Cell Technologies cloned 6 cows starting with cells engineered in the lab to have extended telomeres that were much longer than normal. The now mature cows have telomeres significantly longer than regular cows of the same age. As the cows aged their cells looked far younger than expected. When the cloned cow’s cells were put in a lab dish, they divided more than 90 times before dying. Presently the cows have much younger cells and appear considerably younger than other cows of the same age. In a year 2000 article written in Science magazine Dr. Lanza MD, Ph.D of Advanced Cell Technologies indicated that they expect animals cloned with this technique to live up to 50% longer. Real world applications of telomere therapy are already in use. Researchers at the University of Tennessee-Memphis are using telomerase to give old cells new life by growing artificial corneas from real cornea cells. Telomerase allows these cells to keep replicating over and over, something they couldn't do in nature because the telomeres become too short. A Dutch pharmaceutical company is growing telomerized skin grafts for burn victims, while Geron Corp. is growing whole organs and tissues in the lab to replace dysfunctional and diseased organs and tissues.

 

In a study published in the Feb. 1 issue of The Lancet scientists from the University of Utah provide further evidence that telomere shortening is a cause of aging.

 

Researchers studied 143 people over age 60, taking blood samples to measure their telomere length. Those in the top half for telomere length lived four to five years longer than those in the bottom half. The study showed no difference in the shortening rate of telomeres between men and women. 

 

Those with shorter telomeres had higher death rates, with three times greater risk of death from heart disease and nearly a nine times higher risk of death from infectious diseases. Numerous environmental and genetic factors can affect telomere length, writes lead researcher Richard M. Cawthon, PhD.

 

Protein and Enzyme Production

 

More than just cell death is controlled by telomere expression. The length of a cell’s telomeres also determines function and behavior. Normal human cells replicate a limited number of times before they reach "replicative senescence" and stop dividing. At this point the cells are still alive, breathing and metabolizing food, sometimes for months, until they die. The "molecular clock" that informs the cell of its limited life span is the telomere. According to Carmia Borek, Ph.D. in LE Magazine, “Research shows the mechanism by which a human cell keeps track of its division, by the length of bits of DNA at the end of the chromosome, and their proximity to specific genes.”

 

A study reported in Science magazine found that in human cells, as in yeast cells, there exists a "telomere position effect" (TPE). TPE is dependent on telomere length and the position of the gene in relation to the telomere. It enables a cell to keep track of its number of divisions, and provides a way to modify gene expression during the lifetime of the cell. According to Dr. Woodring Wright, a senior co-author of the study with Dr. Jerry Shay and colleagues, the telomere position effect suggests that it can "let a cell know how old it is so that it could change its behavior before it became senescent."

 

The hallmark of aging is a gradual loss of functioning cells in the body. But not all cells age at the same rate, even in the same organ. When tested for their ability to divide, normal cells taken from a particular organ, such as the skin, are aggressively dividing. Others are incrementally slowing and dividing at a more gradual pace. And then there are those that have reached cell senescence ("old age") and no longer divide or function. On the whole, as tested in cell culture, normal human cells reach senescence after dividing around 50 to 70 times.

 

As there are 46 chromosomes in each cell, each with double strands, there are 92 telomeres that dictate its life span. Cells in most growing human tissues and organs gradually slow in growth, in proportion to the shortening of their telomeres. Studies have shown that normal cells of the elderly lose their ability to divide at a faster rate than cells from the young, and that senescent cells increase in the body, with age.Position effect is a term used to describe an event in which a gene's behavior is affected by its location on the chromosome. The changes in behavior can be expressed in various ways, such as differences in the appearance and function of cells (phenotype), relay of instructions from the gene, and in doubling time of the dividing cells. Position effects have been reported in insects, plants, yeast and mice, and more recently in human cells. The findings that TPE exists in human cells offer clues to cellular aging. In the experiments reported in Science magazine, investigators used a human cancer cell line called HeLa to investigate TPE and the relation between gene activity and telomere length. HeLa cells, which are "immortal," contain telomerase that lengthens the telomere, enabling the cells to keep dividing.In the experiments, investigators introduced into the cell a gene called luciferase (the gene that makes fire flies glow), linked to DNA. Luciferase, called a reporter gene whose location is identified in the cell by its luminescence, was inserted near a telomere. Its luminescence compared to that of the reporter inserted at internal sites of the chromosome. To test if telomere length influences gene silencing, the investigators then elongated the telomere by telomerase, and examined telomere positional effect on luciferase.The results showed that luciferase near the telomere produced 10 times less luminescence than luciferase located at internal sites in the chromosome. Increasing the length of the telomere further increased TPE, resulting in an additional two- to 10-fold decrease in luminescence. These experiments showed that the proximity of a telomere to a gene silences the gene: when the telomere is lengthened, and the gene is located further away from the critical end of the telomere, it is silenced even more.TPE is meaningful and consistent with experimentation. Old cells fail to produce key enzymes and proteins essential in tissue repair, they divide very slowly, and eventually reach cell senescence and die. However, cells treated to produce the telomerase enzyme divide indefinitely in an energetic fashion, produce the proteins that young cells produce, and repair internal cell damage while preventing genomic instability. Young cells show an incredible resistance to oxidative damage, glycation, and cell mutations of all kind that old cells do not. Because young cells replicate vigorously, demonstrate superior cell signaling, produce important proteins (such as collagen and elastin in the skin), and have more rapid cell function, they are able to maintain tissues and vital organs in a way that old cells cannot. In the picture below, human retinal cells are visible. They are marked with a dye that highlights enzyme production. The top picture shows cells of an old age that divide very slowly and fail to produce essential enzymes. On the bottom right are retinal cells that have been genetically engineered to produce telomerase. These cells continue to divide healthily and appear to be perpetually youthful.