CLOCKS & CAPS
From UCSF Magazine, March 2000, "What is Aging"
A recent wave of attention in the news media, biotechnology industry and research labs has focused on the ends of chromosomes, where structures called telomeres act like the plastic tips on shoelaces, protecting the ends of gene-bearing strands of DNA from fraying and decaying, especially as a cell replicates itself.
The exact nucleotide sequence of telomeres was first described in a pond-dwelling protozoan named tetrahymena by Elizabeth Blackburn, UCSF professor of microbiology and immunology, back when she was a postdoctoral fellow at Yale in the late 1970s. (Tetrahymena were the ideal subject for her study because they had thousands of chromosomes and thus lots of telomeres.) Specifically, Blackburn found that tetrahymena telomeres consisted of a particular sequence of nucleotides: Two thymines, then four guanines (or T2G4), repeated over and over. (Humans were later found to have a very similar repeat unit, T2AG3, the A standing for the nucleotide adenine.)
Even before Blackburn had described them, scientists theorized that these chromosome ends might shrink with each cell division because enzymes that replicate chromosomal DNA cannot do so all the way to the tip. As early as 1971, Russian scientist Alexey Olovnikov speculated that when telomeres became increasingly short, mechanisms concerned with genome stability might cause cells to stop splitting.
But such terminal shrinkage could not occur in Blackburn's single-celled tetrahymena, which reproduce by dividing. If their chromosomes shrank each time their bodies split, they would have become extinct. So how did they keep their telomeres intact?
Having moved on to UC Berkeley, Blackburn and her then-student Carol Greider discovered an enzyme they named telomerase that built up and maintained the telomeres. What makes telomeres and telomerase central to some current analyses of aging is this: Around the time Blackburn arrived at UCSF in 1990, it was found that in those human fibroblasts where UCSF's Leonard Hayflick found limited life spans, there is no active telomerase in normal adult cells.
In other words, those cells' telomeres are not routinely rebuilt but rather melt down like internal birthday candles with each division. The burning question is: In the human body, do they eventually reach a point at which they evince genomic damage and cause the cell to slow and eventually halt its division, as they do in the lab?
To hear some tell it, Blackburn's and others' discoveries had set the stage for stopping aging: "Clocks tell time in every cell in our bodies," writes Michael Fossel, in his book Reversing Human Aging. "Stop them and we stop the aging process, turn them back and we actually turn back aging. Those clocks are telomeres."
Such fiery proclamations gained fuel three years ago when researchers at the University of Texas Southwest Medical Center and Geron, a Menlo Park-based biotech company, caused Hayflick-limited human tissue cells to continue splitting by introducing telomerase. "Cellular fountain of youth works," cried one newspaper headline.
Blackburn herself doesn't completely close the door on theories that link her basic science work to aging theory and interventions. "It's very much a 'maybe' with respect to human aging," she says. But her UCSF research, and that of others in the field of telomere molecular biology she helped establish, demonstrate that matters are much more complex than merely adding telomerase to our cells and making 120th birthday plans.
To begin with, some of our 210 types of normal adult cells do generate telomerase, including germ-line cells that product sperm and eggs as well as high-turnover cells of the immune system, such as T- and B cells. "The cell types that have telomerase on - they can keep dividing pretty much indefinitely. The telomeres may shorten a bit, but not enough to matter," Blackburn says.
Still, she suggests that "the ones that don't have telomerase could be contributing to the organismal aging in some fashion." In a modern twist on that 19th-century theory of aging as the dissipation of a mysterious "vital essence," Blackburn and Greider suggested in a Scientific American article that "the functioning of the older body may at times be compromised by the senescence of a subset of cells....Cells at repeatedly injured sites could finally 'use up' their replicative capacity."
As for what causes those cells to become aged, several findings indicate that the shortening of their telomeres may not tell the whole story:
* Blackburn notes, "Mice only live for about two or three years and die with enormously long telomeres. So telomere shortening alone doesn't look like a very good theory" of aging.
* Telomere changes in some cells don't act at all like steadily ticking clocks. In a study published with UCSF pediatrics researcher Kevin Shannon, Blackburn found that the telomeres in white blood cells "shorten very fast from birth to kindergarten, then stay much the same until you finish college, and then they gradually shorten."
* One set of experiments found that adding telomerase does not stop senescence in human keratinocyte or mammary epithelial cells, "so it doesn't generalize that telomerase by itself is enough to immortalize all cell types," says Blackburn.
And yet in vitro, Blackburn's lab certainly finds telomerase can extend the mitotic life of certain otherwise Hayflick-limited cells. Thus, she says, "It is a lack of telomerase that makes us age in a programmed kind of fashion. Notice I didn't say it's only shortening of telomeres, I'd say it's the lack of telomerase."
Indeed, studies of mice in which a key gene for producing telomerase was knocked out, while not finding all of the classic signs of aging, did find an association with shortened life spans, as well as such aging hallmarks as a reduced capacity for wound healing and an increased incidence of spontaneous malignancies.
In recent years, Blackburn's lab has focused on the complex interaction between telomeres and telomerase, from analyzing binding proteins to finding that telomeres may change structurally as they lengthen or shorten in ways that block or allow access by telomerase. In a study published two years ago in conjunction with Chancellor J. Michael Bishop's lab, Blackburn and postdoctoral fellows He Wang and Jiyue Zhu found that distinct from its role as an agent for telomere lengthening, the induced presence of telomerase allowed cells to keep on splitting even as telomeres kept getting shorter.
In fact, the telomeres shortened well beyond a crisis point that, in the absence of functional telomerase, would normally send out signals of genomic instability and cause the cell to self-destruct. As Blackburn now envisions it, telomerase can act as a protective cap atop the telomeric cap.
While this ability to radically extend cellular life in vitro might seem just a step removed from the fountain of youth, there's a big catch. Besides germ-line cells and highly replicative somatic cells, the other human cells known to naturally express active telomerase are cancerous ones. If they didn't switch on telomerase, wildly over-dividing tumor cells would only run their telomeres down and reach early senescence. Indeed, agents that inhibit telomerase's activity are seen as potential cancer therapies.
Studies so far on telomerase-extended cell mitosis of normal cells have not shown carcinogenic changes. But Blackburn stresses that those cells were healthy to start with. In the case of pre-cancerous cells, telomere shortening may act as one of several key brakes on their further proliferation.
"Put telomerase into those cells, and what you're doing is saving the bad guys," she says. In healthy cells, however, telomerase would help protect the genome by protecting the telomeres. "Telomerase will either have a cancer-promoting effect or a cancer-preventing effect, depending on what kind of cell you [activate] it in."
Ultimately, that is but one of many leaps that have not been made from the petri dish to in vivo complexity. Studies in Blackburn's lab have shown that telomere length is crucial to the life span of one-celled organisms like yeast. But, she notes, "It's not been clear that people who have long telomeres live longer than people with short telomeres. When you look at telomere lengths in the fibroblasts of people at different ages, what you see is an incredible scatter." Moreover, she adds, "You could find plenty of older people with telomeres longer than young people."
And yet it is true that in the body, even a little bit of telomere shortening might be sufficient to cause aging, whereas in the tissue-culture dish those cells could go to shorter lengths. As Blackburn reminds us, "Cells in the body get all these signals that play in from the outside, so the environment in the body and in the tissue-culture dish is different. No one at this stage has any idea what the critical difference would be."
By David Jacobson
To receive a copy of the March 2000 UCSF Magazine, "What is Aging?", send an email (cburleson@pubaff.ucsf.edu).
