Michael Fossel Michael is President of Telocyte

June 13, 2018

Aging and Disease: 2.6 Cell Senescence, Changes In Molecular Turnover, DNA Repair

Why are we more likely to get cancer as we age?

Not only does the incidence of cancer go up with age, but it goes up exponentially. Why? Moreover, the exponential rise is seen in most species, regardless of their lifespan. It’s not the years, it’s the aging process, regardless of time. Why? The key to these questions lies with the rise of DNA damage as we age. But just as with other kinds of cell damage – free radicals in the mitochondria, for example – the issue is not the rate of damage, but the rate of maintenance. In the case of DNA, however, the key feature to maintenance isn’t the rate molecular turnover, but the rate of DNA repair. DNA is the only molecule that is repaired rather than simply replaced. We replace (i.e., recycle) all other molecules in our cells (and even outside of our cells), but we never replace DNA. Instead, we repair it with great effort and in exquisite detail. DNA carries priceless information in its structure, so rather that just recycling the molecule (breaking it down and building a new molecule), our cells go to enormous lengths (and enormous metabolic cost) to find and repair every single error. Without delving into detail, let’s look at an overview of DNA damage, DNA repair, and the clinical implications for aging cells – and aging people.

DNA damage is continual, as is repair. DNA damage occurs continually due to radiation, oxidation, toxins, viruses, and even spontaneous thermal disruption (even at normal body temperatures) with an incidence estimated at up to 106 hits per cell per day. If unrepaired, the result will not only be a dysfunctional individual cell, but a cell that divides without control, thereby harming (and even killing) the entire organism. Ultimately, uncontrolled cell division is expressed clinically as cancer. Left unrepaired, DNA damage becomes fatal. Clearly DNA repair is critical, and must be both constant and all-but-flawless for any organism to survive.

 

DNA repair is, like most biological concepts, remarkably (almost indescribably) complex. No matter how we discuss it, there will be exceptions, qualifications, and additional intricacies which remain unaddressed in our discussion. We will therefore and of necessity, present a simplified summary of DNA repair, one which presents only a high-level, conceptual view of the cell’s response to a single type of DNA damage (single-base errors), while ignoring other types of DNA damage (e.g., double-strand breaks). With this caveat in mind, we will characterize DNA repair as being handled by four basic families of DNA repair enzymes which have these functions:

  1. Identification: find the damaged DNA base and flag it for removal
  2. Excision: remove the damaged DNA base from the strand
  3. Replacement: insert the correct DNA base into place in the strand
  4. Ligation: link the new DNA base to neighboring bases in the strand

In the aging cell, and correlated with telomere shortening, the expression of all four of these types of DNA repair enzymes are down-regulated. This down regulation is typical of cell senescence and is modulated by the telomere. As the telomere shortens, all four repair processes are down-regulated. DNA repair continues, but at a slower pace. Young cells repair DNA almost instantly, older cells repair DNA but at a more lackadaisical pace. The result is that, at any given moment, older cells are more likely to have unrepaired DNA.

The result is that slower DNA repair – and the rising percentage of (as yet) unrepaired DNA damage – means a higher likelihood that such damage will affect the cell’s ability to control cell division. For example, if the damage occurs to the DNA repair genes themselves or to the genes that are central to the cell cycle braking system (which would otherwise prevent cells with DNA damage from dividing), then the cell may replicate and carry the DNA damage into the daughter cells. The result is a cascade of increasing cell damage and a decreasing ability to control cell division. In short, the stage is set for malignancy, clinical cancer, and death.

We begin to see why cancer rises with age. As cells lose telomere length, DNA repair slows, and the risk of cancer rises. Worse yet, however, each of the steps involved in DNA repair are multiplicative, that is, each step will have an impact on all subsequent steps. So if detection slows and the number of DNA errors doubles, then if excision slows, the number of DNA errors goes up another factor of two, i.e., the DNA errors go up four-fold. When you add in replacement and ligation, the effects multiply one another again, with the result that if we down-regulate all of the steps in DNA repair, the increase goes up exponentially.

Most people assume that cancer rates climb with age because of a longer lifetime means a greater cumulative exposure to carcinogens. In fact, the rate of cancer isn’t correlated with years so much as it is with percent of lifespan. For example, mice have an exponential increase in cancer, just as humans do, despite the fact that the average lifespan of a mouse is about 40 time shorter than the average lifespan of a human. It’s not the years, it’s the rate of DNA repair that determines how fast that exponential curve rises. Ultimately, the deciding factor is not cumulative exposure, but the rate of repair. Mice slow DNA repair over a short lifespan and their rate of cancer goes up exponentially in only two years; humans slow DNA repair over a long lifespan and their rate of cancer goes up exponentially over a much longer lifespan. It’s not a matter of having good DNA repair genes, nor is it a matter of chronology. The deciding factor is neither time nor genes, but gene expression and gene expression is controlled by telomere shortening.

If we take the curves for cancer in mice and humans and overlap them to show not years but lifespans, then the curves become identical. It’s not the years, it’s the rate of repair. If we want to prevent or treat cancer, we shouldn’t be focusing as much on exposure to carcinogens, but on cell senescence. Putting it bluntly, if only slightly simplistically, the reason we get more cancer as we age isn’t a matter of what we were exposed to, but the rate at which we repair the damage that is constantly in play over our lifetimes.

We get cancer because of cell senescence.

 

Next Time: 2.7 Cell Senescence, Changes In Molecular Turnover, Mitochondria

June 8, 2018

Aging and Disease: 2.5 Cell Senescence, Changes In Molecular Turnover, Most Molecules

Filed under: Aging diseases,mitochondria,senescent cells — Tags: , — webmaster @ 3:17 pm

As the human body is composed of cells, so are cells composed of molecules. It is true that the cell encompasses a plethora of organelles (membranes, mitochondria, nuclei, Golgi bodies, ribosomes, etc.), but each of these organelles is in turn composed of pools of various molecules. Just as cytoplasm is a “soup” of molecules, organelles are also collections of molecules. The precise types of molecules, their numbers, and their rates of turnover vary from organelle to organelle. The most common molecular types are lipids and proteins, but with admixtures of other molecules, such as carbohydrates (such as sugars), as well as hybrid molecules, such as glycolipids and glycoproteins. As you’d guess, the complexity not only doesn’t stop there, it barely begins there. To talk of a few simple molecular types is only a fuzzy, naïve sketch of that complexity involved in living cells. We’ll look a bit deeper at t he molecular types, then take a simpler view and focus on the only critical feature in aging cells, namely molecular turnover.

Looking at the cell as a whole, the typical human cell is (by numbers) about half lipid molecules and about half protein molecules. Most of the lipids are in membranes (such as the cell membranes, the mitochondrial membranes, and the nuclear membranes); most (but by no means all) of the proteins are in solution in the intracellular fluid. While the membranes have far more lipid molecules than protein molecules, the proteins are heavier (and larger). So while lipids are more numerous (about 50 x as numerous) than proteins in the membranes, the mass of the lipids and the proteins (as well as glycoproteins, etc.) are about equal.

While the lipids determine how the membrane acts in a general sense, it’s proteins that determine the functional (and the active) properties of the membrane. So while there aren’t as many protein molecules, but what they lack in numbers they more than make up for in their importance to cell activity. What about cytoplasm? About 40% of body weight is made of the intracellular fluid, where lipids are heavily outnumbered. In the cytoplasm, it’s the proteins, the electrolytes, and other molecules that determines the activity.

While proteins (as well as glycoproteins, etc.) come in thousands of types, even the lipids are a complex family. The most commons lipid molecules are phospholipids, but there are also cholesterol molecules (sometimes as numerous as phospholipids) and glycolipids, which are sugar-lipid molecules, especially common on the outer cellular. To make things even more complex, some molecules are complex conglomerates of both proteins and lipids.

That’s the introduction to the complexity, but from our standpoint – aging related disease – the important point is not the types of molecules or where they are, but the observation that all of them – every molecule we’ve mentioned – is in continual flux. All of the thousands of different types of molecules are being actively recycled. This is true whether we look at lipids or proteins, organelles or cytoplasm, intracellular molecules or extracellular molecules, molecules of a single type or compound molecules. They all being actively recycled. To be specific, none of them sit around for your lifetime, but all of them are being replaced on a moment-to-moment basis. With the sole exception of DNA, none of the molecules repaired. Instead, they’re simply recycled.

Some of these molecules are recycled slowly, some are recycled quickly. In the case of aerobic enzymes in your mitochondria, where the damage rate is high, these molecules are turned over rapidly; in the case of come cholesterol molecules, where the damage rate is lower, the molecules are turned over more slowly. While you might guess that “damaged” proteins are tagged and turned over more quickly, the reality is that ALL of your proteins – even those that are 100% perfect – are continually recycled. This turnover, proteolysis, is not just a passive “recycling” but is actively regulated and fine-tuned, and is part of the cell cycle and cell division, gene transcription, and general cellular quality control. Where we once viewed proteins a stable molecular pools that were subject only to “wear and tear”, active molecular turnover has been proven by isotopic studies. There is a basal rate of molecular turnover, specific to each protein and each lipid, which occurs regardless of damage: in any given molecular pool, molecules are degraded and replaced whether the molecule is normal or not. However, the rate of degradation can go up or down, depending on the rate of damage. For example, ubiquitin conjugation to globin molecules is markedly enhanced by denaturation of hemoglobin, so although hemoglobin undergoes “recycling” regardless of damage, the rate of that “recycling” goes up in the case of molecular damage.

Not only does this permit fine control of cell functions, but it is the only way to ensure quality control as well: the faster molecules are turned over, the more likely the molecules are to be undamaged and capable of doing their jobs. As we saw in the last blog, the slower the turnover, the higher the percentage of dysfunctional molecules. If we think of this recycling process as cell maintenance, then the slower the maintenance, the less functional the cell as it becomes clogged with molecules that don’t work.

Proteins, lipids and other molecules are turning over continuously and extensively. The turnover of each individual type of molecule is specifically regulated and varies with cell conditions and over time. The regulation of cell processes is not merely controlled at the transcriptional or translational levels, but is finely regulated at the level of protein degradation as well.

How fast do these molecules turnover? Proteins have half-lives varying from a few minutes to several days. The rate of turnover varies depending upon the protein, available nutrients, hormone levels, and especially by cell aging.

But if molecular recycling requires metabolic energy, then why does the cell engage in molecular turnover at all? They answer is to avoid the accumulation of damaged and dysfunctional molecules. It’s much like asking why a home owner spends money on the upkeep of their house. Both the home owner and the cell must spend (money or energy) in order to maintain function. The more they spend, the higher the quality of the house on a day-to-day basis. The less they spend, the more likely the house (or the cell) is to fall apart.

The key observation, from the standpoint of aging and age-related disease, is that almost every molecule we look at shows a deceleration of turnover as cells age. Lipids, proteins, and other molecules sit around longer. The result is leaker membranes, less effective DNA repair, dysfunctional mitochondria, and a host of other gradually increasing failures in the aging cell.

 

Next time: 2.6 Cell Senescence, Changes In Molecular Turnover, DNA Repair

May 15, 2018

Aging and Disease: 2.4 Cell Sensecence, Changes In Molecular Turnover

Effective maintenance is a product of the rate and the quality of the maintenance process. If we look at a car, for example, the long-term condition of the car depends on how often we institute maintenance (once a month or once every few years?) and the quality of the maintenance procedures (do you replace and repair everything or do you simply change the oil?). If we look at a house, the same questions apply: do you maintain it regularly (every few months?) and do you maintain it thoroughly (do you just vacuum the carpets or do you replace and repair the paint, the pipes, the roof, and the windows?). If we look at a garden, again we find the same issues: how often do you maintain it (once a day or once a year?) and how thoroughly do you care for the garden (do you merely mow the lawn or do you weed, fertilize, trim, and replant?).

Cars, houses, and gardens are not immortal and unchanging. To remain viable, they require maintenance: the more frequent the maintenance and the more detailed and careful the maintenance, then the longer-lasting they are. A well-cared for car, house, or garden can – in effect – be “immortal”. If the maintenance is sufficiently frequent and of sufficiently high-quality, then they appear to resist entropy without any apparent change.

The same is true of cells. Whether we look at proteins, lipids, or almost any other molecular pool, we discover that they are in continual equilibrium: they are continually being produced and continually broken down. There is no molecular pool in the body that remains untouched by the years; whether rapidly or slowly, every molecular pool is in the process of being recycled. The one odd exception is our DNA, which isn’t recycled, but repaired in situ. While we repair our DNA, we simply replace everything else. Even in the case of DNA, however, the molecules that do the repairing are themselves being continually replaced.

The result of all of this recycling is that the cells are generally able to functional. To use the analogy of the Red Queen from Alice In Wonderland, our cells run as quickly as they can in order to stay in one place. Moreover, the faster they run (recycle) the more they are able to stay in one place (fully functional). Or, as the French saying has it “plus ça change, plus c’est la même” (the more things change, the more they stay the same).

The problem comes about when we slow the rate of turnover. The slower this “recycling” rate, the more we tend to see damage. This occurs even if the rate of damage is unchanged. The more critical variable is not the rate of damage, but the rate of turnover. Alas, as our telomeres shorten and our cells senesce, this rate of turnover goes down. We still create molecules – the collagen and elastin molecules in our skin for example – and we still destroy molecules. The rate of creation and destruction is perfectly balanced, so the total number of molecules available at any one time remains unchanged, but the rate at which those molecules are turned over falls with cell senescence.

The upshot is that damage accrues.

Let’s take a typical intracellular molecular protein. A young cell might (hypothetically) have a thousand molecules of this protein and might every day destroy 500 of these molecules and create 500 of these molecules, so that every day it might “recyle” 50% of the molecules. The pool size doesn’t change, but the molecules are changed regularly. An old cell, however, might (hypothetically) have the same thousand molecules of this protein, but create only 50 of the molecules and destroy on 50 the molecules, so that every day it might “recycle” only 5% of the molecules. While the number of molecules available to the cell (1000) remains unchanged, the slower turnover means that any time a molecule becomes damaged, it will be replaced much more slowly. In short, the problem isn’t so much the damage per se, as it is the rate at which the cell maintains itself. The “older” the cell (i.e., the more senescent the gene expression), the slower the rate of molecular turnover and the higher the percentage of damaged molecules (see Figure 2.4a).

To invoke another analogy, if the damage is the rate at which your family produces garbage and the turnover rate (the “recyclilng” rate) is the frequency of garbage pickup, then imagine what happens if you go from once-a-week garbage pickup to once-a-year garbage pickup. Conceptually, this is much the same problem that occurs in cells as they senesce. The solution is not to adjust the rate at which you produce garbage, but the rate of garbage pick-up. To take this analogy back to the cell, the solution is not to adjust the rate of damage (through UV, spontaneous racemization, free radicals, etc.), but to adjust the rate of turnover. Young cells have high rates of turnover and low percentages of damaged molecules; old cells have low rates of turnover and high percentages of damaged molecules.

To take this into a clinical venue, this applies to wrinkles in our skin (in which, for example) collagen and elastin turnover are slower), in Alzheimer’s disease (in which, for example, beta amyloid turnover is slower), and in mitochondria (in which, for example, aerobic enzymes and molecules on the lipid bilayers have slower turnover. In every example of aging and age-related disease – with no exception – we can trace the changes to slower molecular turnover.

For those who might like to get a firmer (and more mathematical) grasp on how this works, consider the following equation and its implications (from my textbook, Cells, Aging, and Human Disease; Oxford University Press, 2004):

If the rate of damage (here arbitrarily 1% of molecules/day) and the total number of molecules in the pool (here 100%) remain constant, but the turnover rate varies (r = the percentage of molecules replaced/day), then the percentage of damaged molecules (X) on day (N) will be XN. At equilibrium, XN = XN-1. This can be calculated as the per cent damaged on a particular day, plus the number of damaged molecules remaining from the previous day (XN-1 times M), minus the number of previously damaged molecules replaced during the past day (XN-1 times r), divided by the total percentage of molecules (M) in the cell. At equilibrium:

Equilibrium protein damage:             X = 1 + [X(100 -r)/100]

If the molecular turnover rate (r) is 50%, then:

X = 1 + 0.5X

X = 2

Given a damage rate of 1%, if the turnover rate were 50%, then at equilibrium, 2% of the molecules are damaged on any given day. If the molecular turnover rate (r) is 2%, then:

X = 1 +.98X

X = 50

Given a damage rate of 1%, if the turnover rate were only 2%, then at equilibrium, 50% of available molecules have been damaged (see Fossel; Reversing Human Aging, 1996; p 260). Turnover rates – whether protein, lipid, or other molecules – have a profound effect on the burden of damaged molecules within a cell, i.e., on cell dysfunction.

In the next few blogs, we will see how this process affects: first, the most common intracellular molecules (2.5), then how it affects DNA (2.6), then mitochondrial molecules (2.7), and finally extracellular molecules (2.8)

Next Time: 2.5 Cell Senescence, Changes In Molecular Turnover, Most Molecules

 

April 24, 2018

Aging and Disease: 2.3 – Cell senescence, Changes in Gene Expression

Changes in gene expression underlie aging and age-related diseases. There is all-but-universal (and equally unwarranted) assumption that both aging and age-related diseases are genetic. We see articles on “aging genes” and “genes that cause Alzheimer’s disease” (or genes that cause heart disease, osteoarthritis, etc.). The reality is that both aging and age-related diseases are not genetic, they are epigenetic.

To get at the difference, albeit in a slightly different context, consider the difference between a skin cell and a nerve cell. These cells have the same genes, but very different gene expression. The difference between a skin cell and a nerve cell is not genetic, but epigenetic. Same genes, different gene expression.

The same is true of aging cells. The difference between a typical young cell and a typical old cell is not genes, but gene expression. The two cells – for example, a young skin cell and an old skin cell – have the same genes, but very different patterns of gene expression. What makes a cell “old” is not gene damage or altered genes, but alterations in the way those genes are expressed. To use the analogy of a symphony orchestra, both young cells and old cells have the same orchestral instruments (violins, oboes, etc.), but they’re playing slightly different scores (Mozart instead of Bach, as it were). Old cells aren’t old because their “instruments” (the genes) are “out of tune”, but they are old because they play a different tune.

This alteration in gene expression underlies all age-related diseases. The reason we have heart disease, dementia, osteoarthritis, osteoporosis, or other hallmarks of aging (including things like wrinkles, that aren’t actually diseases at all), is because certain cells have an altered pattern of gene expression. Same genes, different gene expression.

A growing number of papers have pin-pointed specific changes in gene expression that are present in old cells and old tissues, but they focus narrowly on such changes as “the” important change, then explore how they might address that single, specific change. They see a single “tree” (of a change of expression in single gene) but lack the ability to see the larger “forest” (encompassing the gamut of changes in expression in hundreds of genes). Too often, they view each change as a “cause” of aging, not realizing that each single change is an effect, caused in turn by a more fundamental process: the shortening of the telomere. In fact, there are literally hundreds (perhaps thousands) of such changes, all of which are not, by themselves, causes of disease or aging, but are the results of changes in telomere length. Aging – and age-related diseases – are not the result of one gene, nor the result of the change of expression in one gene, but rather the result of wholesale and subtle changes of expression in many genes, acting in concert. To harp back to the orchestra: the problem is the orchestral score, not the orchestral instrument.

Nor are do such epigenetic changes stop there. As the telomere influences the expression of a few local genes, these in turn influence the expression of more distant genes, which in turn influence genes on other chromosomes. Moreover, there are interactional effects between such genes: gene a1 may affect three other genes, but such “downstream” genes may well be influenced by other genes as well.

Views of aging (or disease) that focus only on one particular gene or gene product (any of the various “x’s” at the bottom of figure 2.3a) miss the complexity of the process. As examples of this, we see human trials that, in the case of Alzheimer’s disease for example, focus narrowly upon particular gene products, such as beta amyloid (or genes, such as APOE4), then express confusion and surprise when carefully thought out interventions (aimed only at beta amyloid) fail to have any impact on the progressive course of the disease. These trials my employ an effective intervention for one particular gene or gene product, but they ignore the expression of other genes and ignore the complex interactions of multiple genes, all of which are undergoing changes in gene expression as the cells age.

Such human trials remove one tree and then wonder why the forest is still there.

Moreover, as we will see, even when you restrict your focus to a particular gene, the problem is not the product itself, but the rate at which it turns over. To stretch our tree and forest analogy, even if you restrict your view to one particular tree, you find that it keeps regrowing. The question isn’t “can you cut the tree”, but “how often you need to recut the tree?” Beta amyloid, for example, is continually being turned over. Simply lowering the amount of amyloid (“cutting the tree”) won’t work – as many human trials aimed at amyloid have shown – because amyloid is a dynamic pool (a “tree that keeps regrowing”).

The problem comes back to the telomere. Not only isn’t it enough to focus on a single gene, a single protein, or a molecule, but even if you use a broader view and look at all the changes in gene expression – modulated by changes in telomere length – you must realize that every single gene, protein, or molecule is dynamic. Alzheimer’s, for example, is not JUST a matter of beta amyloid, but a matter of dynamic turnover in the amyloid pool. To account for the broad changes, you need to account for ALL the gene changes and account for the turnover rates as gene expression changes.

Trying to treat disease is much like trying to treat hundreds of dynamic processes all at once. You can try aiming at all the processes with hundreds of drugs, you can even try to find a drug that will increase the turnover rates of all these hundreds of processes with hundreds of drugs, one-by-one and with interactive side effects. The actual processes that encompass these age-related changes in gene expression are stunningly complex, encompassing DNA methylation, histone tails and other histone modifications, nucleosome positioning, micro RNA’s (miRNA’s), repressor proteins, i-motif DNA “knots”, and probably dozens of other “tools” of our epigenetic landscape, but the details of these processes lie well beyond our current discussion.

The upshot is plain, however. We could focus one-by-one on each of thousands of individual genes, we could focus one-by-one on each of dozens of different regulatory processes, and for each of these thousand genes or dozen processes attempt to develop (one-by-one!) effective interventions, then hope to combine all of these interventions (while hoping there are not interactive side effects) and use them to treat age-related disease by giving thousands of small molecule drugs.

Or, we can simply reset gene expression by addressing the change in telomere lengths.

 

Next time: 2.4 Cell Senescence, Changes in Molecular Turnover

 

April 5, 2018

Aging and Disease: 2.2 – Cell Senescence, Telomeres

Everyone seems to “know” that telomeres have something to do with aging. The internet even has pop-up ads about foods that lengthen your telomeres, with the unstated assumption that will make your younger, or at least healthier. Inquiry shows, however, that not only do most people have no understanding of the role of telomeres in aging, but neither do most researchers, academics, or clinicians. The result is that many have an unfounded faith in telomeres, while others scoff at the idea that they have any value whatsoever. In fact, both groups are naïve, albeit for different reasons.

The contrarian in me is tempted to assert that “telomeres have nothing to do with aging”, just because people expect me to say that telomeres cause aging, which they don’t. Telomeres play an important role. To say that telomeres have nothing to do with aging is inaccurate, but it’s just as inaccurate to say that telomeres cause aging. To give an analogy, we might say that your entire life is determined by your genes, which is inaccurate, or that genes play no role in your life, which isn’t true either. As with most things, the truth is complicated. Were we to be accurate we might say that telomeres play an important role in the incredibly complex cascade of pathology that we see as aging, indeed a critical and irreplaceable role, but telomeres do not cause aging any more than does any other facet in that intricate web of pathology. Aging is not simply telomeres.

Telomeres have a lot to do with how aging works, but telomeres don’t cause aging.

Causation is a slippery concept, despite the assumption that it’s concrete and well-defined. Causation might apply to billiard balls and the laws of motion, but causation becomes misleading when we apply it to multifactorial events, let alone to complex webs of biological mechanisms. This definitional fuzziness is blithely ignored by both those who ask about causation and those who provide an answer.

To move the discussion to history, for a moment, if I asked for the cause of the American Revolution, there might be a thousand answers that were relevant and appropriate (and not necessarily overlapping). We might focus on taxation, representation, the cultural and geographical distance, any number of specific “flash points”, any of several dozen key players on either side of the Atlantic, etc. Pretending there is “a” cause of the American Revolution presupposes that we already share not only a common framework for the discussion, but common assumptions about what constitutes a cause, and (probably) a great many unexamined prejudices as well. In short, most discussions about causation start with the assumptions that already presuppose a narrow answer. Not a good point to begin understanding.

This is equally true of biological causation. For example, what causes cancer? Is it your genes? Is it down-regulated DNA repair mechanisms? Is it cosmic rays, oxidative damage, or “carcinogens”? It depends on what you are asking. All of these contain an element of truth (and supportive data), but none of them are “the” cause of cancer unless you specify what you are asking and what you want to discuss. If you are a genetic counselor, genes are the focus. If you work for the EPA, carcinogens are the focus. You choose to narrow down your focus but doing so prevents an understanding of the broader question of how cancer occurs and why.

In the case of aging we find the same naiveté. The “cause” of aging depends on your assumptions, why you are asking, and how myopically you look at the process. In short, the question often presupposes the answer. As the Romans once said “Finis origine pendet”. The End hangs on the Beginning, or as too often the case (and using more modern phrasing), garbage in, garbage out. If you already presuppose the answer, then why are you asking? To truly understand how aging works, you need to erase your assumptions, step back, and look at the complexity without blinders or preconceptions. Looking at aging without preconceptions about “the” cause is almost always too much to ask.

There is, however, a more practical approach to understanding aging and the complex cascade of pathology that results from the aging process. Rather than looking for causes, look for effective interventions. If we ignore the deceptive question of causation for a moment and focus on intervention, then telomeres come to the center stage. It’s not that telomeres are in any sense the “cause” of aging, but telomeres are, without doubt, the single most effective point of intervention in the aging process and in age-related diseases.

Telomeres lie at the crossroads – from an interventional perspective – of everything going on in the aging cell. To extend the crossroads analogy, all the roads that lead to aging enter the crossroads of telomeres and all the roads leading toward age-related disease leave that same crossroads. The entire road system – that complex web of pathology that we call aging – consists of myriad highways, county roads, local by-ways, and even walking paths, but almost every one of them, eventually, passes though the same crossroads: the telomere.

Telomeres don’t cause aging and they are not the be-all-and-end-all of the aging process, but they do function as a pivot point, a sine qua non of age-related diseases, and – most importantly of all – the most efficient place to intervene.

Having put telomeres in a more reasonable perspective, what DO they do?

In an odd, but almost accurate sense, you might say that no one really knows. That’s true in two senses. The first sense is that there is simply a great deal that we’ve come to know about telomere mechanisms in the past few decades and there is doubtless a great deal more yet to find out about telomere mechanisms. That first sense, however, is true of everything: there’s a lot we don’t know and anyone who thinks otherwise is probably still in their teen years or has managed to get through life with their eyes (and their minds) closed. The second sense, however, is more specific to telomeres, the aging process, and age-related disease. This second sense is worth exploring, if only to realize the specific gaps in knowledge and how they might impinge on our ability to intervene clinically. This involves how telomeres affect gene expression. What we don’t know (for certain) is the linkage mechanisms, despite discussions about T-loops, sliding sheaths, and all the accompanying data involved over the past two decades. It’s still a bit of a black box. What we do know (for certain), is that telomere shortening changes gene expression (see figure 2.2a), and we do know (for certain) that when we reset telomere lengths we reset gene expression (see figure 2.2b).

We know that this change in gene expression is related to overall shortening and that the change in gene expression is more closely related to the shortest telomere than to the average telomere. We also know that all of this has nothing to do with telomeres “unraveling”. As we discussed before, they don’t unravel. It’s merely a pleasant myth based on the shoelace analogy. Telomeres may function a bit like aglets, but the chromosomal shoelace never unravels. Finally, we know that the absolute length doesn’t determine the changes in gene expression: it’s the relative telomere length that sets the pace of cell aging. Again, this is just the most common misconception, and one that causes inordinate confusion among researchers.

Once telomeres shorten, we know that gene expression changes not only on the same chromosome, but on other chromosomes as well. We know that the changes are progressive and subtle if you only look from one-cell-division-to-the-next (with the associated loss of base pairs). Yet over multiple cell divisions and thousands of base pair losses, these changes in gene expression add up, altering gene expression just enough to have effects upon DNA repair, mitochondrial efficiency, free radical production, lipid membrane competency, protein turnover, and myriad other processes that we associate with aging.

As we will see later in this series, it is this loss of telomere length and the crucial changes that it causes in gene expression that underlies aging and age-related disease, as well as explaining many other diseases, such as the progerias. It also explains why, when telomeres are preserved, cells gain indefinite proliferative potential whether in vitro or in vivo: they are, in common parlance if certainly not in fact, immortal.

Finally, all of this explains why, when we re-extend telomeres, whether in vitro or in vivo, we reset gene expression and not only allow cells to become fully functional again but allow the organism to become functional as well. In short, it explains why and how we may prevent and cure aging and age-related disease.

March 27, 2018

Aging and Disease: 2.1 – Cell senescence, Why Cells Divide

Why do some people age faster than others? We’ve all seen people – high school reunions come to mind – who have the same chronological age, but different biological ages: with the same “age”, one person looks ten years older (or younger) than another. If aging is related to cell senescence and cell senescence depends on cell division, then why do some people’s cells divide more than other people’s cells? Why don’t people age at the same rate?

Why does he look old, but she doesn’t, even at the same “age”?

And why do our own organs and tissues age at different rates? We’ve all seen people whose skin looks old, but they have no evidence of osteoarthritis or dementia; equally, we’ve seen other people with terrible osteoarthritis, but no heart disease or dementia. Not only do we age at different rates when we compare different people, but our tissues sometimes age at different rates even within the same person. If aging is related to cell senescence and cell senescence depends on cell division, then why do people vary internally, having some cells (in one tissue) divide more frequently than other cells (in another tissue)? Why don’t all of our tissues age in parallel?

Why does he have bad knees, but she has a bad heart, even at the same “age”?

The easy – and naïve – answer is to say the magic word “genes” and nod knowingly.

The real – and more complex – answer demands a lot more thought. It requires that we reexamine both the data and our assumptions. It requires, in a word, that we think about what’s really going on. Part of this complex answer begins easily. We notice that people who were exposed to too much sun (and too many sun burns), for example, have skin that ages faster than people who avoided sun damage to their skin, and this is true even with identical genes, as in identical twins. We have discussed the fact that aging is not simple a matter of genes, but it’s a balance between damage and maintenance. “It’s not the years, it’s the miles.” Indeed, the degree to which we pile damage onto our tissues shows a good correlation to how fast those tissues show aging and age-related disease. Most of us know this without really thinking about it. For example, we automatically assume that smoking causes COPD, “bad” diets increase your risk of heart attacks, and so forth. These assumptions are now part of our cultural baggage and (true or not) have attained the status of medical wisdom. In fact, to a large extent these are supported by a fair amount of good evidence, although it’s always a bit more complex than the current culturally accepted facts would have you believe. For example, it may or may not (depending on the decade we’re talking about) be accepted that dietary cholesterol has a direct impact on the cholesterol deposits in your coronary arteries, but the evidence that dietary intake (unspecified for the moment, but not just cholesterol) has a long-term impact on coronary artery disease is fairly good.

In short, your behavior (diet, exercise, stress, etc.) can accelerate or decelerate not only your overall rate of aging, but the rate of aging (and age-related disease) in a number of specific tissues. To give a few more examples, people engaged in high-impact activities (think basketball) have a higher incidence of osteoarthritis of the knees than do people engaged in low-impact activities (think yoga). People who get repeated head injuries (think pugilists and American football players) have a higher incidence of Alzheimer’s and other dementias. In both of these cases – osteoarthritis and dementia – those at high risk not only have a higher incidence of the age-related disease in old age, but they get the specific age-related disease at a younger age than do those at lower risk. They are both more likely to get the disease and more likely to get it earlier. What this tells us is not surprising: aging is related to what you do behaviorally, not just who you are genetically. In short, it’s not just your genes.

Genes do, of course, play a fundamental role but they do it in complex relationship with the damage that accrues over a lifetime. If you really want to avoid osteoarthritis, you not only want to have parents who never had osteoarthritis, but you want to avoid repetitive high-impacts to your joints. If you really want to avoid dementia, you not only want a double allele of APOE-2 (instead of two APOE-4 alleles), but you want to avoid boxing or playing football. But then if these sorts of behavior cause age-related disease, and cell senescence underlies age-related disease, what is the relationship?

The key relationship is the rate of cell division. If your cells are forced to divide more frequently, you force them to senesce faster. If, for example, you damage your knees (forcing your chondrocytes to divide and replace the damaged cells) then you will accelerate aging in your knees (as those cells divide, lose telomeres, and change gene expression). The more you damage your knee joints, the more rapidly your chondrocytes divide, and the more rapidly you develop osteoarthritis. If you damage your head (forcing glial cells to divide and replace the damaged cells), then you will accelerate aging in your brain (as those cells divide, lose telomeres, and change gene expression). The more you damage your brain, the more rapidly your glial cells divide, and the more rapidly you develop dementia.

The details, the pathology, the reality of these age-related diseases are wildly more complex than this cursory review suggests, but the basic theme is valid. Given equivalent genes, people who engage in a lifestyle that increases cell turnover will increase their rate of aging. Likewise, your particular lifestyle may increase cell turnover preferentially in one organ or tissue and that will accelerate the rate at which that organ or tissue develops age-related disease.

Any cell in your body (in any tissue) has a baseline “rate of cell division” (i.e., rate of tissue aging). Skin cells, gastrointestinal lining cells, and hematopoietic stem cells divide frequently, while neurons, muscle cells, etc. divide very infrequently in the adult (an in some cases, not at all). Anything that accelerates cell division, accelerates aging. Anytime you increase the rate of damage to a tissue, you increase the rate of cell division (i.e., the rate of tissue aging) and the result is increased aging and increased age-related disease. The same is true between individuals. We each (based on our own genetics) have what you might think of as a “baseline rate of aging” for our body. If you take care of yourself, you still age inexorably, but relatively slowly. If you engage in a high-risk lifestyle, you will age not only inexorably, but relatively quickly.

Aging is caused by cell senescence and cell senescence is cause by cell division, but while you need your cells to divide in order to survive, the relative rate of cell division is, to an extent, controlled by your lifestyle. Cells divide because you’re alive, but the way you live has an impact on how fact those cells divide and how fast you age.

So, let’s answer our initial question. We have been making the case that aging occurs because cells divide, shortening telomeres, which changes gene expression, which results in dysfunctional cells, dysfunctional tissues, and tissue aging (and disease). This is true, but it begs the question of “if cell division causes aging, then what causes cell division?”

The answer is that cell division is both a natural result of being you (your genes, your personality, your culture, and the simple fact that you are alive and some of your cells MUST divide to keep you alive) and the result of what you do to yourself. You have a baseline rate of cell division (and hence aging). If you have a high-risk lifestyle, you age faster; if you have a low-risk lifestyle, you age a bit more slowly. You can increase or decrease your rate of aging – to a degree – depending on what you do. There is (so far) nothing you can do to STOP aging, but can certainly make it a bit slower, or a lot faster.

Next time: 2.2 Cell senescence, Telomeres

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

 

Aging and Disease: An Index

For those interested in knowing where this blog is going (or where it has been), here is an index of all previous and planned posts for this series on Aging and Disease. Note that the planned posts may change as we progress.

0.1 Prologue

1.0 Aging, our purpose, our perspective

1.1 Aging, what is isn’t

1.2 Aging, what we have to explain

1.3 Aging, what it is

1.4 Aging, the overview

1.5 Aging, misconceptions

2.0 Cell senescence, perspective

2.1 Why cells divide

2.2 Telomeres

2.3 Changes in gene expression

2.4 Changes in molecular turnover

2.5 Changes in molecular turnover, most molecules

2.6 Changes in molecular turnover, DNA repair

2.7 Changes in molecular turnover, Mitochondria

2.8 Changes in molecular turnover, extra-cellular molecules

2.9 Cell senescence and tissue aging

3.0 Aging disease

3.1 Cancer

3.2 Direct and indirect aging

3.3 Skin

3.4 Immune system

3.5 Osteoarthritis

3.6 Osteoporosis

3.7 Arterial (vascular) disease

3.8 CNS disease

3.9 CNS: Parkinson’s disease

3.10 CNS: Alzheimer’s disease

4.0 Treating age-related disease, what doesn’t work, small molecular approaches

4.1 What doesn’t work, killing senescent cells

4.2 What works, lowering risks

4.3 What works, resetting gene expression

5.0 Telomerase in the Clinic

March 15, 2018

Aging and Disease: 1.5 – Aging, Misconceptions

Misconceptions regarding the current model of aging are rampant and they tend to fall into one of several categories. These include Straw man arguments, unfamiliarity with how age-related human pathology occurs, simplistic views cell senescence, genes, and expression, or misguided approaches to measuring telomeres (usually in the wrong cells).

Straw man arguments

          The Earth can’t possibly be round, or you’d fall off the other side.

This sort of argument attacks a position by attacking the wrong target, then claiming victory. The approach is called a “straw man argument”. Rather than facing an actual opponent (or making a logical argument), you build a man out of straw (or offer up a faulty premise), attack it and beat it (or disprove the faulty premise), then claim that you have beaten your opponent (or proven your entire argument). Straw man arguments are safer and easier but they’re dishonest and they don’t lead to clinical progress.

Several centuries ago, some clerics argued that if Copernicus was right about the sun being the center of the solar system, then he must be denying the existence of God (the straw man) and the truth of the Bible (another straw man). Never mind the astronomical data: critics focused on the religious straw man. A century ago, some people argued that humans could never fly because humans are heavier than air. You couldn’t deny the straw man (we really are heavier than air), but it didn’t affect validity of flying machines. Even the Wright brothers would be shocked senseless by the weight of the modern commercial jet. History is replete with “disproof’s” that misrepresent or make wildly erroneous straw man arguments about new thoughts, new theories, and new technologies.

Straw man arguments do nothing but prevent progress.

The telomerase theory of aging has frequently been criticized using straw man arguments. The most common example is suggesting that telomere length (instead of change in length) is important to aging, then demolishing the straw man. Cellular aging – as marked by changes in gene expression – is not modulated by telomere length but is modulated by changes in telomere length. Telomere length per se is a straw man. The fact that some young mice have 150kbp telomeres (but a 2-year lifespan) while some young humans have 15kbp telomerase (but 80-year lifespans) is irrelevant: it’s a straw man. Cell aging is determined by the gradual changes in gene expression and these are determined by relative telomere loss, not by absolute telomere length. To say that some species have longer telomeres and shorter lifespans while other species have shorter telomeres and longer lifespans is interesting but misses the point. Telomere length (the straw man) has nothing to do with lifespan or cell aging. The key factor isn’t length, but the change in length of the telomeres and – more directly – how the changing length of telomeres changes the pattern of gene expression. To focus on telomere length creates a wild goose chase. The key feature is not the telomere (and certainly not the absolute telomere length), but the patterns of gene expression as modulated by the changes in telomere length over time.

Human pathology: which cells cause the disease?

A more egregious error occurs when the straw man is due to a stunning naiveté regarding age-related pathology. In this case the error lies in misunderstanding clinical medicine rather than in misunderstanding telomere biology. This type of straw man argument has surfaced repeatedly online, in articles, and (sadly) even in academic discussions. The two most typical (and most egregious) examples aim at heart disease and dementia. The most typical false statements are:

  1. Cell aging can’t explain heart disease, since heart cells don’t divide.
  2. Cell aging can’t explain dementia, since neurons don’t divide.

These statements, as is often the case, tell us far more about the critic than they tell us about the target of the criticism. In these two examples, we discover that the critics have no understanding of the clinical pathology underlying either heart disease or dementia. The two statements are not only straw man arguments but display an extraordinary lack of clinical knowledge. While it’s true that heart cells and neurons generally don’t divide, that fact has nothing to do with the actual disease process nor the role of cell aging.

Classical “heart” disease (i.e., myocardial infarction, angina, etc.) doesn’t begin in the heart muscle (whose cells rarely divide), but in the endothelial cells that line the coronary arteries (whose cells divide regularly). The observation that heart cells don’t divide is (more or less) accurate but has nothing to do with heart disease being caused by cell aging. Heart muscle cells are the innocent bystanders. The vascular endothelial cells are where the pathology begins. To blame heart disease on heart muscle cells is like blaming the murder victim rather than the murderer. Heart cells are the victim, not the perpetrator. We might have equally (and just as foolishly) said that “cholesterol can’t explain heart disease, since heart cells don’t accumulate cholesterol.” The latter is true, but it’s hardly relevant. Cholesterol’s role (like that of cell aging) lies in the vascular lining cells, not in the heart muscle cells. Whether we are talking about cell aging or cholesterol deposits, the heart cells are the innocent bystanders and it’s the coronary arteries that are the problem. Cell aging accurately explains everything we know of human “heart disease”, as well as age-related vascular disease generally (e.g., strokes, aneurysms, peripheral vascular disease, congestive heart failure, etc.). The straw man arguments are disingenuous and largely based on a willful (a woeful) ignorance of human age-related disease.

Much the same is true for dementia. Neurons don’t divide (much, if at all, in the adult human), but glial cells (such as microglia) both divide and have been implicated in the basic pathology that underlies Alzheimer’s and many other dementias. We know, for example, that Alzheimer’s patients have shorter telomeres than do age-matched patients without Alzheimer’s. In short, cell aging explains dementia logically and accurately, while the lack of neuronal cell division has nothing to do with the argument (or the disease). In this context, such Straw man arguments display the distressing naiveté of those using them.

Cell senescence, genes, and expression

Cell senescence is often regarded as all-or-nothing: a cell is either young or old, but never anything in-between. Over the past half century, this error has often resulted in people speaking past one another, never recognizing that they have different definitions of “cell senescence”. While it’s true that there is an endpoint (a senescent cell that is incapable of division or much else), short of that extreme, cell senescence remains a relative matter. This is not only seen in the physiology (how well does the cell function?) but in terms of gene expression. Like cell senescence, gene expression is not all-or-nothing. It’s true that a particular gene at a particular time is either being transcribed or not, but if we look at the rate of gene expression over any reasonable time duration (e.g., an hour, a day, or a week), we see that the rate of gene expression looks more like a continuum. You might say that it’s “analog” rather than “digital”. More importantly, that rate of gene expression can be seen to change not only over time, but as an integral part of cell senescence. In “older” cells, while we find that the genes and gene transcription process is perfectly normal (i.e., the same quality of genes and gene transcription as a “young” cell), we find that the rate of gene expression is now quite different. Putting it simply, the rate of gene expression slows down as a cell segues from a young cell to a senescent cell. Thinking of cell senescence and gene expression as all-or-nothing is a troublesome error but is not the only error when it comes to genes and aging.

Perhaps the most rampant error lies in thinking of “aging genes”. A century ago, it wasn’t unusual to hear people talk about genes for any number of things: intelligence, beauty, compassion, etc. While there are genes that play a role in these (and myriad other characteristics), the relationship between intelligence and genes has proven to be remarkably complex, requiring input from epigenetics, environment, diet, and other factors. Even if we restrict ourselves to genes alone, there are probably hundreds of genes that play a role in determining intelligence. Moreover, these same genes also play dozens of roles at once, including roles in immunity, endocrine development, motor function, memory, and cells throughout the body and in every tissue. So are these genes really “intelligence” genes? To think of them that way is merely to expose both our ignorance and our naiveté. These are systems genes; they play dozens (hundreds?) of interacting roles in virtually every part of the body. Much the same can be said for “aging” genes. Short of a few genes that characterize some of the progerias (for example, the lamin-A gene in H-G progeria), there are no aging genes. To look at your gene scan and point to an “aging gene” is exactly like the early phrenologists who looked at your skull and pointed to a “bump of combativeness” or a “bump of sublimity”. There are no such bumps and there are no such “aging genes”. There are certainly genes that play a role (or much more likely, play multiple roles) in the aging process. Unquestionably, there are innumerable genes that increase (or decrease) your risk of age-related diseases or that increase (or decrease) the probable length of your lifespan, but there are no specific “aging genes”, unless you’d like to go to the other extreme and acknowledge that all genes are aging genes, as in some sense, they are.

Misguided approaches to measuring telomeres

About once every two weeks, I receive a research article that goes something like this. The authors measured the telomeres of several dozen volunteers, then performed an intervention (changed the diet, taught them meditation, increased their daily exercise, etc.), then measured the telomeres again in six months, and found that the telomeres had lengthened. They conclude that the intervention lengthens telomeres (and, by implication, reverses aging). While they might be right, the data prove certainly don’t justify their conclusions. If they are right, they are right despite poor design, poor analysis, poor thinking, and a very shaky knowledge of cells. There are several problems these types of study, starting with the fact that almost every one of these studies only measures telomere lengths in white blood cells, which are easy to obtain, but not particularly useful (nor are they valid or reliable, as we’ll see). A typical study of this type is summarized in Figure 1.5a.

The first problem is that even if they truly lengthened the telomeres in those white blood cells (and see below), most of us die of aging cells in our arteries or aging cells in our brains (not to mention the problems we have with our joints, our bones, our kidneys, etc.). Measuring the telomeres in white cells tells us precisely nothing about these more important cells and tissues. It’s much like using hair color (how gray is your hair?) to assess your risk for having a heart attack or Alzheimer’s disease. White cells are the wrong cells to look at. They may be easy to get, but they don’t get you anywhere.

The second problem is that white cells are a dynamic population and they respond to almost any stress by dividing (and shortening their telomeres). Once the stress is gone, the white cells get replaced by “younger” white cells (with longer telomeres) from the stem cells in your bone marrow. So, you might say that if you only measure your white cell telomeres, then you will appear older as a result of any stress and you will appear younger again once the stress goes away. For example, you will appear to have older white cells if you have an infection, if you just had a loved one die, if you lost your job, or if you are malnourished. The opposite is equally true: your white cells will appear younger if your stress resolves, since your white cells will then be replaced with “younger” cells from the stem cell compartment in your bone marrow. Note that if we actually measured your bone marrow cells (and not the circulating white cells), you would find that your hematopoietic stem cells are slowly aging almost regardless of what you do. Whether we cure your infection, improve your diet, make you exercise regularly, or have you meditate, makes little difference to your marrow cells. Almost any clinical intervention might affect your circulating white cells, but there is no evidence that any intervention can make your stem cells younger (or can increase their telomeres). To focus on the white cell telomeres is an illusion. This is not to say that these various interventions aren’t useful and may not improve your health, but there is no evidence that any of these interventions make you any younger. For that matter, there may be evidence that these interventions change the particular white cells you sample (so the new sample has longer telomeres), but there is no evidence that these interventions lengthen telomeres, let alone make you any younger.

To give you an analogy, imagine that you are trying to make people younger in a large country (the US, for example), so you measure the average age in a particular block of a major city (Boston, for example), then you perform an intervention (an urban renewal program, for example) over several decades (between 1950 and 2018, for example), then measure the average age of people living in that same block. The average age may well be lower in 2018 than it was in 1950, but that does NOT mean that you have made anyone get younger and it certainly doesn’t mean that the rest of the country is now younger. The population has changed: some people moved out, some moved in and those that moved in tended to be younger.

The same thing happens when you measure white cell telomeres: the old white cells are gone, and new white cells have “moved into the block”. To conclude that you have made the white cells (let alone the whole body) younger is silly, to say nothing of entirely unsupported by the data. This is not to say that the various interventions purported to affect telomeres and/or aging (meditation, vegetarian diets, exercise, or in one case, living in zero gravity) may not have physical benefits (or that they might actually affect telomeres or aging), but that not a single one of these various interventions has valid data to answer those questions. Measuring peripheral white cell telomere lengths is not only fraught with errors, but (at least as far as most current research goes) has approximately the same validity as casting a horoscope.

Finally, most telomere measurements are done by average length, which is relatively cheap but not particularly relevant. Tissue function is highly dependent upon the oldest (not the average) cells in the tissue and cell function is highly dependent upon the shortest (not the average) telomere in the cells. Measuring the average telomere may be cheap and easy, but it’s like trying to figure out the risk of terrorism in a city by measuring the average person. The average person isn’t a terrorist, but that’s not the point. It’s the extremes that determine the overall risk of terrorism in a community. It only takes a few terrorists to result in disaster and, in your tissues, it only takes a few senescent cells to result in disease. Within the cells, it only takes a few short telomeres to result in a dysfunctional cell. The upshot is that when we measure telomere lengths, the measurement that is most often used is the measurement that doesn’t tell you what you know. The result is that most studies measure the wrong thing and then, with perfect confidence, draw the entirely unwarranted conclusions. No wonder the literature is misleading.

Understanding aging – and understanding cell aging – is replete with pitfalls and misconceptions that are all-too-common, even in the research literature. Leaving these caveats aside for now, however, let’s delve directly into the aging process itself, starting with the cell.

How does a cell age?

 

Next time: Aging and Disease: 2.0 – Cell senescence, Perspective

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

 

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