Michael Fossel Michael is President of Telocyte

March 20, 2018

Aging and Disease: 2.0 – Cell senescence, Perspective

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


March 6, 2018

Aging and Disease: 1.4 – Aging, the Overview

How does aging work?

So far, in the prologue (section 0) and the section 1 posts, we have discussed a perspective, what aging isn’t (and is), and what we need to explain in any accurate model of aging. In this post, I provide an overview of how the aging process occurs, from cell division to cell disease, followed by a post on the common misconceptions about this model, which will complete section 1. Section 2 is a series of posts that provide a detailed discussion of cell aging, section 3 explores age-related disease, and section 4 maps out the potential clinical interventions in aging and age-related disease. In this post, however, I provide an outline or map of the entire aging process. This will shoehorn much of what we know about cellular aging and age-relaed disease into a single post, giving you an overview of how aging works.

Cell Division

Aging begins when cells divide. Before moving beyond this, however, we need to ask ourselves why cells divide in the first place. The impetus for cell division is itself a driving force for aging, and the rate and number of cell divisions will control the rate of aging. IF cell division “causes” aging, then what causes cell division? As with any comprehensive examination of causation, we immediately discover that if A causes B, there is always something (often ignored) that must have caused A in turn. In short, causation (and this is equally true of aging) is a cascade of causation that can be pushed back as far as you have to patience to push the question. In the case of cell division, the next upstream “cause” is often environmental and is related to daily living itself. For example, we loose skin cells because we continually slough them off and we therefore need our cells to divide and replace the cells that we lose. As with most tissues, the rate of cell division is strongly modulated by what we do (or what we’re exposed to). If we undergo repeated trauma or environmental stress, then we lose more cells (and consequently have more frequent cell divisions) than we would otherwise. In the knee joint, for example, cell division in the joint surface will be faster in those who undergo repetitive trauma (e.g., basketball players) than in those who engage in low-impact activities (e.g., yoga). In the arteries, cell divisions along the inner arterial surface will be faster in those suffering from hypertension than in those with lower blood pressure (and lower rheological stress). Not all cells divide regularly. While some cells rarely divide in the adult (muscle cells, neurons, etc.), those that do divide regularly – such as skin, endothelial cells in the vascular system, glial cells in the brain, chondrocytes in the joints, osteocytes in the bone, etc. – will vary their rate of division in response to trauma, toxic insults, malnutrition, infections, inflammation, and a host of other largely environmental factors. Putting it simply, in any particular tissue you look at, the rate of cellular aging depends on what you do to that tissue and those cells. Repeated sunburns induce more rapid skin aging, hypertension induces more rapid arterial aging, close head injuries induce more rapid brain aging, and joint impacts induce more rapid joint aging. In all of these cases, the clinical outcome is the acceleration of tissue-specific age-related disease. So while we might accurately say that aging begins when cells divide, we might equally go up one level and say that aging begins in whatever prompts cell division. Any procees that accelerates cell loss, accelerated cell division, and thus accelerates aging and age-related disease.

Telomere Loss

Cell division has limits (as Len Haylfick pointed out in the 1960’s) and tee limits on cell division are, in turn, determined by telomere loss (as Cal Harley and his colleagues pointed out in the 1990’s). Telomeres, the last several thousand base pairs at the end of nuclear chromosomes (as opposed to mitochondrial chromosomes), act as a clock, setting the pace and the limits of cell division. In fact, they determine cell aging. Telomeres are longer in young cells and shorter in old cells. Of course, it’s never quite that simple. Some cells (such as germ cells) actively replace lost telomere length regardless of chronological age, while others (such as neurons and muscle cells) divide rarely and never shorten their telomeres as the adult tissues age. Most of your body’s cells, those that routinely divide, show continued cell division over the decades of your adult life and show a orrelated shortening of their telomeres. Note (as we will in the next blog post) that it is not the absolute telomere length that is the operative variable, but the relative telomere loss that determines cell aging. Nor, in many ways, does even the relative telomere length matter, were it not for what telomeres control “downstream”: gene expression.

Gene Expression

As telomeres shorten, they have a subtle, but pervasive effect upon gene expression throughout the chromosomes and hence upon cell function. In general, we can accurately simplify most of this process as a “turning down” of gene expression. The process is not all-or-nothing, but is a step-by-step, continuum. Gene expression changes gradually, slowly, and by percent. The change is analogous to adjustments in an “volume control” rather the use of an on/off switch. Where once the expression of a particular gene resulted in a vast number of proteins in a given time interval, we now see 99% of that amount are now produced in that time interval. The difference may be one percent, it may be less, but this small deceleration in the rate of gene expression becomes more significant as the telomere shortens over time. Whereas the young cell might produce (and degrade) a pool of proteins using a high rate of molecular “recycling”, this recycling rate slows with continued cell division and telomere shortening, until older cells have a dramatically slower rate of molecular recycling. While you might suspect that a slightly slower rate of turnover wouldn’t make much difference, this is actually the single key concept in aging and age-related disease, both at the cellular and the tissue levels. We might, with accuracy and validity, say that aging is not caused by telomere loss, but that aging is caused by changes in gene expression and, even more accurately, that aging is caused by the slowing of molecular turnover.

Molecular Turnover

To understand molecular turnover is to understand aging. As we will see later in this series (including a mathematical treatment with examples), the predominant effect of slower molecular turnover is to increase the percentage of denatured or ineffective molecules. Examples would include oxidized, cross-linked, or otherwise disordered molecules due to free radicals, spontaneous thermal isomerization, or other disruptive, entropic processes. The cell’s response to such molecular disruption is not to repair damaged molecules, but to replace such molecules with new ones. This replacement process, molecular turnover, is continual and occurs regardless of whether the molecules are damaged or not. The sole exception to the use of replacement rather than repair is that of DNA, which is continuall being repaired. But even the enzymes responsible for DNA repair are themselves being continually replaced and not repaired. There are no stable molecular pools, intracellular or extracellular: all molecular pools are in dynamic equilibrium, undergoing continual turnover, albeit at varying and different rates. Some molecules are replaced rapidly (such as the aerobic enzymes within the mitochondria), others more slowly (such as collagen in the skin), but all molecular pools are in a condition of dynamic equilibrium. More importantly, if we are to understand aging, the rate of molecular turnover slows in every case as cells senesce and the result is a rise in the proportion of damage molecules. To use one example, beta amyloid microaggregates in the brain (in Alzheimer’s disease) occur not simply result because damage accrues over time (entropy). Amyloid microaggregates begin to form when the rate of glial cell turnover of beta amyloid molecules (the binding, internalization, degradation, and replacement of these molecules) becomes slower over time and is no longer keeping pace with the rate of molecular damage (maintenance versus entropy). The result is that beta amyloid molecular damage occurs faster than molecular turnover, and the the histological consequence is the advent of beta amyloid plaques. The same principle – the slowing of molecular turnover with cell aging – applies to DNA repair and the result in an exponential rise in cancer, as we will see in later sections. This general problem of slower molecular turnover applies equally within aging skin, where wrinkles and other facets of skin aging are not the result of entropy, but result from the failure of maintenance (e.g., turnover of collagen and elastin) to keep up with entropy. The incremental and gradual slowing of molecular turnover or molecular recycling is the single most central concept in aging. Aging isn’t caused by damage, but by the failure of maintenance to keep up with that damage. Aging results from insufficient molecular turnover.

Cell and Tissue Dysfunction

The slower molecular turnover and it’s outcome – an increase in dysfunctional molecules – results in a failure within and between cells. Within the cell, we see slower DNA repair, leakier mitocondrial membranes, an increase in the ratio of ROS/ATP production (creating more free radicals and less energy), decreasinly effective free radical scavengers, and a general decrease in the rate of replacement of those molecules that are damage, whether by free radicals or otherwise. For the cell itself, the outcome is a gradual loss of function and an increase in unrepaired DNA. With respect to free radicals, for example, it’s not that free radical damage causes aging, but that cellular aging causes free radical damage. As our cells age (and molecular turnover slows), our mitochondria produce more free radicals (since the aerobic enzyemes aren’t as frequently replace), the mitochondrial membranes leak more free radicals (since the lipid molecules in the mitochondrial aren’t as frequently replaced), free radicals are more common in the cytoplasm (since free radical scavenger molecules are as frequently replaced), and consequent damage becomes more common (since damaged molecules aren’t as frequently replaced). Free radicals do not cause aging: they are merely an important by-product of the aging process. As in cells, so in tissues: just as molecular turnover slows and results in cellular dysfunction, so do do we see dysfunction at higher levels: tissue, structural anatomy, and organ systems. Slowing of molecular turnover expresses itself in dysfunctional cells, an increase in carcinogenesis, and ultimately in clinical disease.

Age-Related Disesase

At the clinical level, the changes in cell and tissue function result in disease and other age-related changes. Wrinkles, for example, may not be a disease, but they result from exactly the same cellular processes outlined above. In each case, however, we see age-related changes or age-related diseases are the result of underlying “upstream” processes that follow a cascade of pathology from cell division, to telomere shortening, to epigenetic changes, to a slowing of molecular turnover, to growing cellular dysfunction. As glial cells “slow down” (in their handling of amyloid, but also in regard to mitochondrial efficiency and a host of other subtle dysfunctions), the result is Alzheimer’s and the other human dementias. As vascular endothelial cells senesce, the result is coronary artery disease, as well as heart attacks, strokes, aneursyms, peripheral vascular disease, and a dozen other age-related diseases and syndromes. As chondrocytes senesce, the result is ostoarthritis. As osteocytes senesce, the result is osteopororis. Nor are these the only manifestations. We see cell senescence in renal podocytes, in dermal and epidermal cells of the skin, in fibroblasts within the lung, and in essentially every tissue that manifests age-related changes. Age related disease and age-related changes are, at the clinical level, the predictable and ultimate outcomes of cellular aging.

The above model is accurate, consistent, and predictively valid, yet there have been a number of crucial misconceptions that have remained common in the literature, making it difficult for many people to grasp the model correctly. Next time, we will explore these errors before moving into the details of aging and disease.

Next: 1.5 – Aging, Misconceptions


March 23, 2016

Soliloquy, Requiem, and . . . Hope

Early last Saturday, I received a short, sad email from an old friend. Many of you may know Leonard Hayflick, who first pointed out that cells age, more than fifty years ago. He stood up for himself and for the truth of his data, in the face of strong opposition and irrationality, and finally proved that cells do not age because of the passage of time, but because of cell divisions. From that one observation – and his willingness to stand by his observations – we have come to realize that not only do cells age, but that it is cell aging that causes our bodies to age and causes all the myriad diseases of aging: “the thousand natural shocks that flesh is heir to.”

Len Hayflick’s wife, Ruth, died last Friday night, leaving Len alone physically, but by no means alone in spirit. He has admirers and friends throughout the world and will have them throughout time. He deserves more than we can possibly give him.

As we age, we have long suffered the slings and arrows of fortune, yet only now do we – almost, tantalizingly – have the ability to end them. The pity is, that we do so too late, not only for those who are already gone, but for those who will yet succumb before we can help. It is aging itself , and the diseases of aging, that “makes calamity of so long life”. We are so close to curing the diseases of aging, yet even that very proximity, compounded by our pressing need, makes a cure feel so tragically far away. The greater the need, the closer the goal, the more pitiable that we have yet to achieve it.

Over the next few years, many of us will suffer, and suffer needlessly. If only we could have moved faster yesterday, if only we were moving faster today, if only we will move faster tomorrow: we would have saved lives yesterday, we would be saving lives today, we will have been saving more lives tomorrow. And yet, here we are more than fifty years since one honest man stood up and – with his “native hue of resolution” — pointed out reality. Why has it been half a century and yet we still suffer from diseases that could have been cured? Why does it take so long to save the lives and souls of those who still endure the sea of troubles that comes in, seemingly inevitably, as we age?

Soon, very soon, we will cure Alzheimer’s and a host of other diseases, yet however soon it will be, it will have been too late for too many of us. And far too late for some of those we love, those we long for, those we will long remember.

May those memories soon push us to create a more compassionate world.

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