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LE Magazine September
2002
Telomere Control & Cellular
Aging
Aging cells may not be clueless about
their life span: Recent studies show they have a "clock" that
reminds them of passing time so that they can achieve
essential goals before it is too late. 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, a structure at the end of each chromosome that
shortens with each cell division. 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.
by Carmia
Borek, Ph.D.
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."
Telomeres, telomerase and aging
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 happily 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 60 to 80 times.
The telomere, p53 and senescence
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| 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. |
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The telomere is a kind of molecular cap, made of DNA, that
protects the ends of the chromosome from damage. Telomere DNA
has over 1000 bases (building blocks), with the sequence
TTAGG, that repeats over and over. In order to divide, a
normal cell has to replicate all the DNA in its chromosomes.
But normal cells have difficulty in copying the last few bases
on the telomere. As a result, the telomere shortens with each
round of DNA replication and cell division. As a cell ages,
the telomere keeps shortening until it reaches a finite
length. At that point cells stop dividing. This halt in growth
is triggered by a gene called p53 that is activated in
response to DNA damage. A telomere that has become too short
no longer protects the chromosome from DNA damage. When the
damage takes place, p53 responds by stopping cell replication
and forcing it into senescence. As a telomere gets too short,
the finite cell growth prevents DNA-damaged cell growth that
could lead to abnormal cells and to cancer.
Telomerase and longevity
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 from old people lose
their ability to divide at a faster rate than cells from the
young, and that senescent cells increase in the body, with
age.
While telomere shortening provides replicative history-a
clock that reminds a cell how many times it has divided and
how long it yet has to live-elongation of the telomere adds
longevity to a cell. This occurs naturally in cancer cells,
where a complex protein called telomerase, which has an enzyme
component, helps build up and elongate the telomere with each
cell division. This allows the cells to continue growing and
become effectively "immortal," the hallmark of cancer cells.
If one blocks the action of telomerase in a cancer cell by
genetic manipulation, the telomere will begin to shorten with
each division, as in normal cells, and the cancer cells will
stop dividing and die.
In normal cells that are not germ cells, telomerase is
switched off at an early stage of development. Telomeres do
not elongate and cells must yield to a fate of a limited
number of divisions. If one introduces a telomerase gene into
normal cells by genetic manipulation, the cell can extend its
life span. This has been shown in several studies, including
experiments by a team that included Drs. Wright and Shay.
In these experiments telomerase was introduced into
telomerase-negative human retina and foreskin cells. The cells
began to express telomerase, as actively as cancer cells.
Their telomere 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 longer life span, compared to
the cells that were not treated with telomerase, whose
telomere shortened with each division, leading to senescence.
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 normal appearance and normal number of
chromosomes.
Telomere position effect and gene
silencing
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
functions 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.
TPE in yeast cells
In 1990, Gottschling and colleagues showed in yeast cells
that by inserting a gene next to a telomere, it was silenced.
The experiments used marker gene ADE2 that produces changes in
the color of colonies, depending on whether the gene is
expressed (white colonies) or silenced (red colonies).
Insertion of ADE2 next to the telomere produced red colonies,
(silenced gene). But the red cell colonies had sectors of
white colonies, showing the gene was switched back on. Within
the white sectors, in the largely red colonies, red sectors
appeared. This shows gene reversal; the ADE2 gene was first
silenced (red colony), then switched on (white sector), and
then silenced again (red within white). The switches may be
due in part to neighboring genes influencing the ADE2 gene.
This means that while silencing depends on the gene's
proximity to the telomere, competing regulatory factors
produced by neighboring genes may modify a gene's
behavior.
TPE in human cells
The findings that TPE exists in human cells is novel; they
show a similarity between TPE in human cells and yeast, and
offer clues to cellular aging. In the experiments reported in
Science, 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.
Telomere position effects and cellular
aging
Telomere position effect sheds light on the role of
telomere in cellular aging. According to a simple and older
telomere hypothesis of cellular aging, senescent cells have
lost an essential gene that allows them to divide. By
contrast, immortal cells, including cancer cells, have avoided
this loss because they have regained telomerase activity. They
continue to maintain their telomeres and press on with cell
division.
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The existence of telomere positioning effect in human cells
offers a different scenario, where there is no need for the
loss of a gene to push cells into senescence. It is speculated
that, for example, when the cell is young and the telomere
long, TPE silences "aging genes" that are located near the
telomere, but far away from its end. As the cell divides and
the telomere shortens, an "aging gene" would be more affected
by its position on the telomere, as it increases its proximity
to the end of the telomere. In an old cell where the telomere
has shortened to its final length, the "aging genes" are no
longer repressed. Silencing is switched off and the "aging
genes" activated.
According to Drs. Shay, Woodring and their colleagues, J.
Bauer and Dr. Ying Zou, once TPE has been discovered in human
cells, there will be a challenge: to identify genes on
chromosomes "whose expression is influenced by telomere
length, in order to determine whether TPE actually influences
the physiology of aging or cancer."
It is known that certain proteins (gene products), affect
cell behavior in different ways, depending on the age of the
cell. The genes that regulate these proteins may be important
for programming pre-senescence changes in a cell, before
telomeres reach their final length.
Take, for example, a cell that needs to alter its energy
metabolism to allow for changes in old age. TPE, which keeps
track of the "aging gene" in relation to telomere length, will
cause mobilization of regulatory genes to help make the needed
change before the telomere is too short.
Telomere, telomerase and age related
disease
Cellular aging contributes to many conditions in the
elderly. The skin wrinkles through loss of collagen production
by skin cells that have lost function, partly through free
radical damage to DNA (sun damage), and senescence.
Atherosclerosis is caused by a loss of division-capacity in
cells that line blood vessels (endothelial cells). This, in
turn, results in overloading of cell factors that increase the
risk of atherosclerotic plaques and blood clots. Active cell
division is also important in response to injury. For
instance, a damaged liver resulting from excess alcohol intake
can lead to liver cirrhosis. In this condition, rapid cell
division of the normal healthy liver cells, in response to the
injury, could replace damaged tissue by supplying functioning
liver cells. The shortening of telomeres, however, would limit
liver cell replication and prevent tissue renewal. Introducing
telomerase into the dividing liver cells, to elongate the
telomere, would exert TPE and a silencing of the "aging gene,"
allowing continuous division that may offer treatment. This
was shown experimentally, in a mouse model of chronic liver
injury, where inserting the telomerase gene into the injured
liver of the mouse prevented cirrhosis.
Possible therapies
It is thought that in normal human organs with a capacity
for cell replacement, the telomere clock allows enough
divisions for normal growth, repair and maintenance. This
setting point is not enough, however, to enable additional
cell replications needed during chronic disease. Under these
conditions, a potential remedy may be found by increasing the
life span of tissue cells, by telomerase. Another possibility
may involve taking cells from an individual, extending the
life span of the cells in vitro by telomerase, and then
re-introducing the cells into the organ that requires help.
The discovery of TPE in human cells provides a mechanism to
silence critical genes and change the pattern of cell
behavior. This finding may lead to further research that
uncovers the secrets of cellular aging.
References
Baur J, Zou Y, Shay JW, Wright WE
Telomere position in human cells. Science 2001;
292:2075-2077.
Bodnar AG et al Extension of life span by introduction of
telomerase in normal human cells. Science 1998; 279: 349-352.
Chiu CP Harley, CB Replicative senescence and cell
immortality: the role of telomeres and telomerase. Proc Soc Exp Biol Med 1977; 214:
99-106.
Harley CB, Sherwood SW Telomerase, Checkpoints and cancer
Surv 1997; 29:
263-284.
Herbert B-S, et al Inhibition of human telomerase in immortal
human cells leads to progressive telomere shortening and cell
death. Proc Nat Acad Sci
1999 96; 14276-14281.
Jiang XR et al Telomerase expression in human somatic cells
does not induce changes associated with a transformed
phenotype. Nat Gen 1999;
21: 111-114.
Position effect at S cerevisiae Telomeres Reversible
repression of POL II transcription. Cell 1990; 63: 751-762.
Weinrich SL et al. Reconstitution of telomerase with the
catalytic protein subunit hTERT. Nature Gen 1997; 17: 498-502.
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