Aging and Disease: 2.0 – Cell senescence, Perspective

• Aging
• Epigenetics
• Hayflick
• Telomerase
• Telomeres

Most of us – when we think of cells at all – seldom appreciate that the idea of a “cell” is a modern idea, not quite two centuries old. One of the tenets of cell theory is that cells are the “basic unit of life”. This makes some sense but note that while the components of cells (mitochondria, for example) can’t live independently but can only survive as part of a cell, it’s also true that most cells don’t do very well independently either but can only survive as part of an organism. Nevertheless, and for good reason, cells are generally thought of at the building block of life, the unit out of which organisms are made. This sort of statement has exceptions (what about viruses?) and qualifications (some muscle “cells” tend to blur together), but overall, cells do function as the “basic unit of life”.

More importantly, most diseases operate at the cellular level or are most easily discussed in cellular terms. Want to understand the immune system? The focus is white blood cells. Want to understand heart attacks? The focus is the dying cardiac muscle cells. Want to understand Alzheimer’s? We tend to focus on dying neurons. In all these cases, other cells are not only involved, but are often the source of the pathology, but regardless of the complexities, qualifications, and exceptions, if you really want to understand a disease these days, you want to look at cells. You may be looking at an organ (such as the liver) or a tissue (such as the surface of a joint), but when push comes to shove, you need to get down into the cells to really understand how a disease works and what might be done about it.

Oddly enough, however, the idea of aging cells somehow never really took off until the middle of the last century. In fact, there was an overriding acceptance of the idea that cells did NOT age. Aging was (here, much hand waving occurred) something that happened between cells and not within them. Organisms certainly aged, while cells did not. This is not surprising when you think of the fact that all organisms derive from single (fertilized) cells that have a germ cell line going back to the origin of life, so while that cell line clearly hadn’t aged, you certainly aged. Voila! Cells don’t age, but you do. There was even a large body of (faulty) data showing that you could keep cells (in this case chicken heart muscle cells) alive and dividing “forever”.

In 1960, however, Len Hayflick pointed out that cells themselves age, and that this aging is related to the number of times the cells divides. Moreover, this rate of cell aging is specific to both species and cell type. While germ cell (think ova and sperm) don’t age, the normal “somatic cells” of an organism show cell aging. By the way, this aging had no relationship to the passage of time but was strictly controlled by the number of cell divisions. In other words, entropy and the passage of years was irrelevant. The only variable that mattered was cell division itself. Entropy only triumphed as cells divided and only in somatic cells. Len had no idea of how cells could count, although he termed this mechanism (whatever it was) the “replicometer” since it measured cell replications.

A decade later, Alexey Olovnikov figured out the mechanism. He pointed out that because of the way chromosomes replicated, every time you replicated a chromosome, you would lose a tiny piece at the end of the chromosome, the telomere. Clearly that wasn’t all there was to it or – since cells and chromosomes have been replicating for billions of years – there wouldn’t be any chromosomes (or life) left on the planet. There had to be something that could replace the missing piece, at least in some cells, such as the germ cell line. That something was telomerase. At least as importantly, however, Alexey pointed out that this was probably the mechanism of Len Hayflick’s “replicometer”: the number of cell divisions was measured in telomere loss.

As it turns out, Len (about cell divisions) and Alexey (about telomeres) were both right. The connection was finally shown in 1990 by Cal Harley and his colleagues, who found that telomere length exactly predicted cell aging and vice versa: if you knew one, you knew the other. At first, this was merely correlation, if a remarkably good one, but it didn’t take more than a few more years to show that telomere loss determined cell aging. Specifically, if you reset the length of the telomere, then you reset cell aging. If, for example, you reset the telomere length in human cells, then those “old” cells now looked and acted exactly like young cells. In short: you could reverse cell aging at will.

This prompted the first book (Reversing Human Aging, 1996) and the first articles in the medical literature (published in JAMA, 1997 & 1998) to suggest that not only did cell aging underlie and explain human aging, but that cell aging could be reversed, and that the clinical potential was unprecedented in the ability to cure and prevent age-related human disease. This was rapidly followed by a set of experiments showing that if you reextended telomeres in aged human cells, you could grow young, healthy human tissues in vitro, specifically in human skin, arterial tissue, and bone. The entire area was extensively reviewed in what is still the only medical textbook on this area (Cells, Aging, and Human Disease; Oxford University Press, 2004). Since then, there have been at least three peer-reviewed publications looking at the use of telomerase activators, each of which showed intriguing and significant (if not dramatic) improvements in many age-related biomarkers (e.g., immune response, insulin response, bone density, etc.).

In a landmark paper (Nature, 2011), DePinho and his group, then at Harvard, showed that telomerase activation in aged mice resulted in impressive (and unprecedented) improvements not only in biomarkers, but (to mention CNS-related findings alone) in brain weight, neural stem cells, and behavior. This was followed by an even more impressive result (EMBO Molecular Medicine, 2012) by Blasco and her group (at the CNIO in Madrid), who showed that the same results could be accomplished using gene therapy to deliver a telomerase gene to aged mice. This result was the more impressive because precisely the same approach can be used in human trials.

Exactly this technique is planned for human Alzheimer’s disease trials next year. But to get there, we need to understand not only the background history, but how cells themselves age, the results of cell aging, and why we can intervene.

Next time: 2.1 Cell senescence, why cells divide