Throughout your life, your cells are continually replaced. The replacements are newly differentiated stem cells: cells which have somehow come to a decision regarding their fate in your body, and have thus undergone a transformation from a ready-to-become-something state, to a liver cell, a retinal cell, a skin cell, a brain cell (neuron). This further means that your stem cells (and others) are constantly dividing, a process called mitosis. The most fundamental operation in mitotic division is the replication (copying) of DNA. This is a multifaceted operation involving a large cast of molecular players including (amongst others) enzymes, cofactors, and nucleic acid building blocks.
When DNA is replicated, complementary nucleic acid bases are added one by one to each strand of the double helix as it is unwound, resulting in two copies of the original piece of DNA (chromosome). However, this presents a slight problem. The enzyme complex responsible for adding these base pairs (the process is called elongation) requires a bit of RNA at the end of the strand to get started. The solution is that part of the end of the DNA strand is snipped off and replaced with a chunk of RNA as a jumping off point. This would be an issue if these enzymes were chopping off pieces of “important” (functional) DNA; cells have so called “proofreading” mechanisms to look for the kinds of erroneous DNA sequences that would result from trimming parts off the end, which if found in large enough numbers, result in cellular suicide (apoptosis). Instead, and rather ingeniously, there are long sequences that cap the ends of the important parts of DNA called telomeres. The only function of these sequences is to be snipped apart, slowly ground down over time, by replication so that the important stuff in the middle doesn’t get messed with in this process.
Telomeric DNA has been seen as a potential target for anti-aging therapy. The idea is simple: make telomeres longer, the animal lives longer because the animal’s cells can replicate forever without ever being forced to snip “important” DNA during the replication part and ultimately apoptose. Furthermore, there is an enzyme whose job it is to lengthen telomeric DNA called, unsurprisingly, telomerase. Indeed, it is known that increased telomerase activity extends the lifespan of most human cell types in vitro1. Sadly, high telomerase activity is correlated with the development of cancer, and the exploration of its use as an anti-aging therapy has thus been attenuated.
A recent study, published in the journal Cell, however, has explored the possibility of pumping-up telomerase activity in mice2. The authors of this study enhanced telomerase activity in mice who had also been given a number of tumor-suppressing agents (p53, p16, and p19ARF) to render the rodents cancer resistant. This enhancement “improves the fitness of epithelial barriers, particularly the skin and the intestine, and produces a systemic delay in aging accompanied by extension of the median life span.” (See figure, below.) The real significance of this study is that it is the first to demonstrate the ability of enhanced telomerase activity in extending lifespan in vivo, in a living organism.
Cells can only divide so many times before they become inviable (around 53 for humans). This hints at a fundamental limitation to the replication process. One component of this is surely that described herein of telomere shortening. However, there are many other ways that errors can accrue in DNA. On another note, it will be extremely intriguing to see how these sorts of aging stories regarding DNA and cell division relate to those concerning calorie restriction and associated metabolic pathways.
While the enhancement of telomerase activity is a piece of the aging puzzle, we have a long way to go in understanding the finite quality of cell division, and it may truly be inescapable, to say nothing of aging itself.
1. Bodnar AG, Ouellette M, Frolkis M, Holt SE, Chiu CP, Morin GB, Harley CB, Shay JW, Lichtsteiner S, Wright WE. Extension of life-span by introduction of telomerase into normal human cells. Science 279: 349-352, 1998.
2. Tomás-Loba A, Flores I, Fernández-Marcos PJ, Cayuela ML, Maraver A, Tejera A, Borrás C, Matheu A, Klatt P, Flores JM, Viña J, Serrano M, Blasco MA. Telomerase reverse transcriptase delays aging in cancer-resistant mice. Cell 135: 609-622, 2008.