Michael Fossel Michael is President of Telocyte

June 20, 2018

Aging and Disease: 2.7 Cell Senescence, Changes In Molecular Turnover, Mitochondria

Filed under: mitochondria,senescent cells,Telomeres — Tags: , , — webmaster @ 9:28 am

Many people assume that mitochondria (and free radicals) lie at the heart of aging or that they are “the cause” of aging. Despite having a central role in aging, the assumption that mitochondria cause aging is both simplistic and – not to put too fine a point on it – is totally at odds with both logic and evidence.

Mitochondria do not cause aging.

This is not to say that mitochondria don’t play a central role in aging cells, but playing a central role is not the same as being the cause nor the same as an overarching explanation for aging, let alone an explanation for age-related disease. By analogy, fevers and elevated white blood cell counts (leukocytoses) play a central role in the clinical presentation of many viral infections, but they neither cause nor explain any viral infection. To describe the presentation of a viral infection, you should include the clinical symptoms and signs, including fevers and high white counts, but while they are an integral part of the process of infection, they don’t cause infection. With that in mind, let’s consider mitochondria, starting with a high-level perspective and then looking at the way mitochondria change in aging cells. In so doing, we will see how mitochondrial aging plays a role in cell aging, but we will also see that mitochondrial aging is secondary to changes within the nucleus, and specifically to changes in gene expression as modulated by telomere shortening.

Previously, we pointed out that every cell in your body is – biologically speaking – several billion years old, yet those cells only show evidence of aging during the several decades of a typical human lifespan. We each inherit a line of cells that didn’t age for billions of years, and yet begin aging during the several decades after birth. The same is true for mitochondria. Every mitochondria in the human body is at least 1.5 billion years old, the time at which the first mitochondria and eukaryotic cell joined forces. If you suggest that all aging is due to aging mitochondria, then you are immediately brought up short by the biological provenance of mitochondria. You end up forced to admit that mitochondria didn’t age for 1.5 billion years and yet (as with your cells themselves) begin aging during the decades after birth.

You inherit mitochondria that have never aged and that then age rapidly during your lifespan. If mitochondria cause aging, then why didn’t they age for the past 1.5 billion years? Why suddenly now? In short, if you want to explain aging by pointing to mitochondria, then you are going to have to back up a step and explain mitochondrial aging itself.

To put it bluntly: if mitochondrial aging causes human aging, then what causes mitochondrial aging?

As we’re just seen, you can’t very well blame “wear and tear”, since your mitochondria are all 1.5 billion years old and they did perfectly well until you came along. If mitochondria play a key role (and they do), you still have to ask why mitochondria sometimes do age and sometimes don’t, or why mitochondria didn’t age for billions of years, but then suddenly age after conception. We can’t simply ignore the question, wave our hands, and blame “free radical damage”.

Yet it is clear that mitochondria show changes during cell aging. There are two clear changes in metabolism: they become less efficient at creating energy (ATP) and they become more likely to produce free radicals (ROS). Putting it graphically, we see that as telomeres shorten, as gene expression changes, and as cells progress toward senescence, we find that the mitochondria show a decrease in ATP production and an increase in ROS production. Putting it differently, the ratio of ATP/ROS goes down with age. As our cells age, they get sloppier, taking more effort to produce usable cellular energy, while simultaneously creating more damaging ROS as they do so. To be specific, young cells typically produce about 1-4% ROS for every mole of ATP, but this percentage increases as our cells senesce. Nor, as we will see, is ROS production the only problem.

To understand why mitochondria lose efficiency, and why it is the result of telomere shortening, we need to understand a bit about mitochondria and their dependence upon the nucleus. It’s true that the mitochondria have their own 37 genes and that these code for 13 proteins that are critical to aerobic metabolism, but it is equally true that most mitochondrial enzymes are imported from the nucleus. Likewise, while the rate of mitochondrial DNA damage is 3 to 20-fold higher than that of nuclear DNA damage (due to the high concentration of mitochondrial ROS, the lack of protective histones, the lack of introns, and less effective mitochondrial DNA repair), mitochondrial DNA repair, replication, and transcription, as well as mitochondrial maintenance generally (including membrane maintenance) are entirely dependent upon the nuclear DNA. Moreover, most of the aerobic enzymes (including those required for the citric acid cycle) have components that are coded for in nuclear genes. In other words, mitochondrial function is almost entirely dependent upon gene expression in the nucleus, which is modulated by telomere shortening. The mitochondria and mitochondrial energy metabolism (including both ATP and ROS production) are completely at the mercy of nuclear gene expression, and are not independent. Aging occurs because changes in nuclear gene expression result in mitochondrial dysfunction, not the other way around.

Looking at this visually, almost all the enzymes required for normal mitochondrial function are based in the nucleus, not in the mitochondria. The mitochondria, far from being an island unto itself, is entirely dependent upon nuclear DNA, the rates of nuclear gene expression, and telomere modulation of that gene expression.

We have already pointed out that, as cells age, the mitochondria become less adept at producing energy, as well as increasingly prone to producing ROS. By implication, these two changes in energy metabolism are both the result of changes in gene expression within the nucleus, but these are far from the only changes that occur with cell aging. A young, fully functional cell has four processes which address the dangers of ROS. These four processes are:

 

  1. Production:           ROS are produced within the mitochondria
  2. Sequestration:     Membranes restrict ROS within the mitochondria
  3. Trapping:             Scavenger molecules “capture” ROS outside the mitochondria
  4. Repair:                 DNA repair and molecular turnover deal with ROS damage

 

The bulk (perhaps 93%) of ROS are produced within the mitochondria. In young cells, the rate of production is relatively low and even those ROS that are produced are generally kept within the mitochondrial membranes. As cells age (and as telomeres shorten and gene expression alters), the enzymes required for efficient energy production are turned over less rapidly, are less available, and are more likely to be denatured: the outcome is that the rate of ROS production climbs. Moreover, a similar problem is occurring within the membranes that wall off the mitochondria: the enzymes responsible for cell membrane molecules – mostly lipid molecules – are also down regulated, with the result that membrane molecular turnover slows, the percent of damaged (oxidized) lipids increases, and the membranes are increasingly porous, allowing ROS leakage to increase. Adding to the membrane problem is that membrane leakage also rises in the nuclear membranes, so the rate of DNA damage rises even as the rate of DNA repair begins to fall. In short, as telomeres shorten and gene expression changes, the mitochondria not only produce more ROS, but are more likely to leak ROS into the cytoplasm and the nucleus.

In the young cell, such ROS are rapidly trapped by multiple mechanisms, including SOD (superoxide dismutase), catalase, and other enzymes coded for in the nucleus (as well as trapping due to dietary scavenger agents, such as urate, tocopherols, etc.). As cells age, however, these scavenger enzymes are recycled more slowly and at any given time the older cell has an increasing percentage of dysfunctional ROS scavenger molecules. Worse yet, the rate of DNA repair declines, as does the rate of recycling of damaged molecules generally.

The result of these four processes is the “perfect storm”, a multiplicative set of four effects causing an exponential increase in overall cell damage.

  1. Mitochondrial ROS production increases,
  2. Mitochondrial and nuclear membrane leakage increases,
  3. ROS damage increases as scavenger efficiency declines, and
  4. Cellular responses to damage (molecular recycling and DNA repair) decline.

It is little wonder that antioxidants have little practical effect upon cell aging or age-related diseases. An increase in dietary anti-oxidants shows almost no effect upon the third of these four effects (scavenging) and no effect whatsoever upon the other three effects (production, leakage, or recycling/repair of damage.

As cells age, there are critical changes within the mitochondria as well as in the cell’s ability to respond to the increasingly dysfunctional mitochondria. However, all of these changes – in production, sequestration, scavenging, and repair – are the result of changes occurring within the nucleus. Much of what happens in cell aging can be seen in the increasing dysfunction of the mitochondria, but all of these changes are secondary to changes within the nucleus.

Specifically, it is telomere shortening and its effect upon gene expression within the nucleus that results in mitochondrial aging. Mitochondrial changes are a key part of the tragedy that plays out on the stage of cellular aging, but that tragedy is directed by the nuclear chromosomes – and by our telomeres.

Next Time: 2.8 Cell Senescence, Changes In Molecular Turnover, Extra-Cellular Molecules

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

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 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

 

February 20, 2018

Aging and Disease: 1.3 – Aging, What it IS

What IS aging?

An explanation of aging must account for all cells, all organisms, and – if we are candid – all of biology and isn’t merely entropy. Prior posts defined our boundaries: what we must include – and exclude. We know that we cannot simply point to entropy, wash our hands of any further discussion, and walk away with our eyes closed. Likewise, an honest explanation can’t simply consider humans and a few common mammals but ignore the entire gamut of Earth’s biology.

So, what IS aging? As a start, we might acknowledge that life has been on Earth for more than four billion years and during that entire time, life has resisted entropy. This serves as an excellent starting point: life might be defined as the ability to maintain itself in the face of entropy. In that case, we might rough out our initial definition: aging is the gradual failure of maintenance in the face of entropy.

We miss the point, however, unless we realize that aging is an active, dynamic process. Aging is not simply a matter of a failure of maintenance in the passive sense. To use an analogy, if entropy were an escalator carrying us downwards, then it is not the only process involved. It is countered by cell maintenance, which is precisely like walking upwards on the same escalator (see Figure 1.3a). Young cells are entirely capable, as are germ cells and many other cells, of indefinitely maintaining their position at the top of the escalator. Entropy and maintenance are equally balanced. Older cells, however, have a subtle (and sometimes not so subtle) imbalance, in which maintenance is less than entropy.

As aging occurs, the problem is not that the escalator (entropy) carries us downwards, but that we are no longer walking upwards (maintenance) at the same rate as the escalator. To view aging as the descending escalator alone is to miss the essential point of biology: life remains on this planet because cells and organisms “walk upwards” and maintain themselves indefinitely in the face of being “carried downwards” by entropy. The process is a dynamic balancing act. To explain aging, it is not enough to cite the escalator, but requires that we explain why maintenance fails, and then only in certain cells and at certain times, while remaining functional in other cells and at other times. Aging is far from universal. A valid explanation of aging must account for why aging occurs in some cases yet does not occur in other cases.

 

Aging is not the escalator but is a combination of two forces: entropy carrying cells into dysfunction and maintenance ensuring that cells remain functional. Aging occurs only when maintenance is down-regulated. If maintenance is not down-regulated, then the cells and the organism do not age. Aging cells, such as many somatic cells, age because they down-regulate maintenance. “Immortal” cells, such as germ cells, do no age because they do not down-regulate maintenance.

We might try an analogy to see where it takes us, comparing biological aging to “aging” in a car. We could say that aging in a car is not simply what happens as the car undergoes weathering and degradation over time. Rather, car aging would be what happens if we fail to maintain the car on a regular and detailed basis. There are exceptional antique cars that have been in active use longer than most human lifetimes, but they are in excellent shape not because they had better parts (i.e., have the right genes) or were made by a better manufacturer (i.e., are part of the right species), but because they were maintained scrupulously and carefully on an almost daily basis by generations of owners. Such cars are oiled, painted, repaired, realigned, and cared for on an almost daily basis, compared to most cars that are lucky to be cared for annually. The critical difference is not the chronological age of the car nor the amount of wear-and-tear, but the frequency and excellence of their maintenance. Given frequent and excellent maintenance, sufficient to keep up with entropy, a car can last indefinitely, while with sloppy and merely annual maintenance, cars typically last only a few years before “aging” takes them off the road.

In a sense, organisms are no different: the degree of aging is not just a matter of time or entropy, but of the quality and frequency of maintenance. Likewise, aging is not purely a matter of which genes or what species pertain to that organism. Rather, aging is a matter of the rate of repair and recycling within cells, that is, maintenance in the face of entropy.
It’s not the genes, it’s the gene expression.

Let’s use another example, that of water recycling. Every molecule of water that you ingest has been recycled endlessly, but the speed and efficiency of that recycling determines the quantity and quality of the water you drink. Imagine that we plan a trip to Mars. If the average astronaut needs 2 liters per day and 4 astronauts are on a 2.5-year roundtrip to Mars, we might calculate that we need to bring 7 tons of water. But that (incorrectly) assumes no recycling. We can get by on a lot less water, depending on how we recycle. The amount we need to bring with us depends not only on the amount the astronauts use daily, but on the quality and rate of recycling (from urine, for example). The faster the recycling, the less water we need to carry along. The better the quality of our recycling, the longer we can stay healthy.

In a “young” and efficient cell, we recycle molecular pools rapidly and effectively. In an old cell, however, the rate and effectiveness of the recycling decreases. The analogy for our Mars trip would be slower recycling, along with an increasing percent of contaminants that are not being removed in our water recycling unit. The outcome, whether in aging cells or a mission to Mars, is gradually increasing dysfunction. Aging cells no longer function normally (as when they were young cells) and our sickening astronauts no longer function normally either (as when they started out on Earth).
As another example, you oversee a huge office building with multiple daily customers and hundreds of employees. Every night, your cleaning crew comes through, mopping the solid floors, vacuuming the carpets, cleaning the windows, and (when necessary) repainting the walls. Maintenance is frequent and excellent; as a result, the building always looks new (i.e., young). Now let’s radically cut back on your maintenance budget. Instead of daily maintenance, the carpets are vacuumed once every two weeks, the floors are mopped once a month, the windows are cleaned once a year, and repainting occurs once a decade. The resulting problem is not due to the amount of dirt (the entropy), nor the quality of the vacuum, the mop, the washer fluids, or the paint (think of these as the quality of your genes). The problem isn’t the dirt nor is it the cleaning crew, but the rate of maintenance. The outcome is that your building looks dirty and is increasingly incapable of attracting clients or customers – or for that matter, incapable of retaining employees. This parallels the changes in aging cells: the genes (the cleaning products) are excellent and the quality of repair (the cleaning staff) are both excellent, but the frequency of maintenance is too low to maintain the quality of the building. In aging cells, molecular turnover is too slow to keep up with entropic change.

This same analogy could be applied to home repairs, garden weeding, or professional education. The problem is not entropy, but our ability to resist entropy and maintain function. Aging occurs because cell maintenance becomes slower. The quality of gene expression is fine, but molecular turnover (see figure 1.3b) – the “recycling rate” – declines. This effect is subtle but pervasive and the result is increasing dysfunction. This concept – the failure of maintenance to keep up with entropy — is not only central to aging but can account for all of aging and in all organisms, whether at the genetic level, the cellular level, the tissue level, or the clinical outcome – age-related disease.

Aging is a dynamic process, in which entropy begins to gain as maintenance processes become gradually down-regulated.

In subsequent posts, we will explore the detailed mathematics of this change, reviewing the formula and the primary variables, letting us see the remarkable results that occur in terms of denatured molecules and cellular dysfunction. For now, however, let’s look at a few specific clinical examples in human aging, all of which we’ll return to in later posts, when we consider age-related diseases in great detail.

In human skin, between cells, we see changes in collagen and elastin (among dozens of other proteins) as we age. Many people mistakenly assume that these changes are a simple, static accumulation of damage over a lifetime, but these changes are anything but static. These molecules are in dynamic equilibrium, in which the molecules (and their complex structures) are constantly being produced (anabolism) and broken down (catabolism). The overall rate of recycling (the overall metabolism) is high in young skin, with the result that at any given time, most molecules are undamaged and functional (and relatively new). This rate slows with aging, however, with the result that molecules remain longer before being “recycled” and the percentage of damaged and dysfunctional molecules rises, slowly but inexorably. In old skin, molecules “sit around” too long before being recycled. Old skin isn’t old because of damage, but because the rate of maintenance becomes slower and slower. Naïve cosmetic attempts to “replace” skin collagen, elastin, moisture, or other molecules fail because they are transient interventions. By analogy, these cosmetic interventions would be like – in the case of our old, dirty office building – suggesting that we will send in one person, one night, to clean one window pane. Even if you notice a small, transient improvement, the problem isn’t resolved by bringing in one person for a single visit, it requires that we resume having the entire cleaning crew come in every night. Intervening in skin aging is not a matter of providing a few molecules, but of increasing the rate of turnover of all the molecules.

The same problem occurs in aging bones. The problem that lies at the heart of osteoporosis is not “low calcium”, but the rate at which we turnover our bony matrix. Looking solely at calcium as one example, osteoporosis not a static problem (add calcium), but a dynamic problem (increase the rate of calcium turnover). Moving our attention from minerals to cells, young bone is constantly being taken apart (by osteoclasts) and rebuilt (by osteoblasts). The result is continual remodeling (recycling) and repair. Bone turnover is a continual process that slows with age. Young fractures heal quickly and thoroughly. In old bone, however, the rate of remodeling falls steadily, and rebuilding falls slightly behind. The result is that we have decreased matrix, decreased mineralization, decreased bone mass, and an increasing risk of fractures. The fundamental problem underlying osteoporosis is not “a loss of bone mineral density”, but an inability to maintain bony replacement. It’s not the calcium or the phosphorous, but the osteocytes themselves. Loss of bone mineralization is a symptom, not the cause of osteoporosis.

A more tragic and more fatal example is Alzheimer’s disease. Until relatively recently, the leading pathological target was beta amyloid, a molecule which (like tau proteins and other candidates) shows increasing damage and denaturation (plaques in the case of amyloid) in older patients, especially in patients with Alzheimer’s disease. Again, however, amyloid is not a static molecule that is produced, sits around, and slowly denatures over a lifetime. Amyloid is continually produced and continually broken down, but the rate of recycling falls as we age. The result is that the percentage of damaged amyloid (plaque) rises with age, solely because the rate of turnover is slowing down. As we will see, the cells that bind, internalize, and breakdown this molecule become slower as we age. To address Alzheimer’s, we don’t need to remove amyloid or prevent its production, we need to increase the rate of turnover. Beta amyloid plaques are a symptom, not the cause of Alzheimer’s disease.

Wherever we look — an aging cell, an aging tissue, or an aging organism – we see that aging is not a static, linear loss of function due to entropy. Rather, aging is a dynamic process in which the rate of recycling – whether of intracellular enzymes, extracellular proteins, aging cells, or aging tissues – becomes slower as cells senesce. Aging is a programmed failure of maintenance at all biological levels. This is equally true of DNA repair, mitochondrial function, lipid membranes, proteins, and everything else we can measure in an aging system.

We’ve had a glimpse at the core of aging. Let’s explore an overview of how changes in gene expression translate into cell dysfunction, tissue failure, clinical disease, and aging itself.

Next time: Aging, the Overview

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