Michael Fossel Michael is President of Telocyte

July 4, 2018

Aging and Disease: 2.8 Cell Senescence, Changes In Molecular Turnover, Extracellular Molecules

The human body contains perhaps a bit short of 40 trillion cells, which is an impressive number, yet a large part of our body – a quarter to a third, depending how you measure it – isn’t intracellular, but extracellular. This includes not only the fluids within the blood and lymphatic spaces, but the space that lies between our cells, even in “solid” tissue. This extracellular space is just as critical – and as it turns out, just as dynamic – as our intracellular space.

The extracellular space has cells within it, for example the fibroblasts in our dermis, the lymphocytes wandering about in our lymphatic system, and the red and white (and other) cells circulating in our blood streams, but if we ignore all of these cells for a moment, we find that the extracellular space is still a complex place. It is replete with important molecules, including electrolytes and proteins (and many others), and these molecules are continually being “recycled”, much as the intracellular molecules are.

The extracellular space is not a quiet place and certainly not a place where protein molecules can quietly “retire” for a few decades. To the contrary, the molecules come and go, subject to continual degradation and replacement. Aging doesn’t occur simply because molecules “sit around and fall apart”. Aging occurs because molecules aren’t turned over as quickly as we age.

Looking solely at human skin – and then solely at a few of the dozens of important molecules that play a role – we find two well-known molecules that are worth focusing on: collagen and elastin. We will simplify our discussion by looking just at the skin, just at collagen and elastin, and just at both proteins generically, intentionally ignoring the multiple subtypes of both collagen and elastin. We will also simplify our discussion by ignoring the water, electrolytes, immune proteins, enzymes, hormones, and various other structural proteins (keratin, muscle, bony matrix, fibronectin, laminins, etc.) that we might discuss.

Let’s focus on what happens to the collagen and elastin in our skin as we age.

Both collagen and elastin are familiar to most of us, as well as to anyone who has ever watched advertisements for skin care products. Collagen is a long, chain-like protein that provides strength and some cushioning throughout the body, including the skin. It is collagen that keeps your skin from pulling apart, providing resistance to stress. In addition to skin, collagen is also found in cartilage, tendons, bones, ligaments, and just about everywhere else. Elastin is – as the name suggests – and elastic molecule that allows skin (and other tissues) to return to its original position when it has been deformed. You might think of collagen as chain that has strength and elastin as a rubber band that stretches. Collagen prevents too much deformation, while elastin pulls skin back after slight deformations.

As we age, both of these fail. Collagen breaks and our skin becomes more fragile and prone to damage from slight impacts or friction. Elastin breaks and our skin sags and no longer “bounces back”. As both of these fail over time, we form wrinkles, although these are only one of the obvious cosmetic changes that occur. Skin loses both strength (collagen) and elasticity (elastin) over time. Why?

Whether you are six or sixty, your collagen and elastin molecules are steadily breaking down and failing. The difference is not the rate of damage, but the rate of turnover. This is the rate at which molecules – such as collagen and elastin — are recycled and replaced. In young skin, collagen turnover can be as high as 10% per day, but the rate of turnover falls steadily with chronological age, or more specifically, with cell aging. As cells are lost and replaced by cell division, the telomeres shorten, gene expression changes, and molecular turnover slows down. The older your cells, the slower the rate at which they replace damaged extracellular proteins, whether collagen, elastin, or any other protein (such as beta amyloid in the elderly patient with Alzheimer’s disease). No wonder our skin becomes fragile, loses elasticity, and develops wrinkles.
Despite the advertising world, none of these changes are amenable to moisturizers, protein injections, serums, creams, or a host of other “miracle anti-aging products” that tout the ability to erase wrinkles, rejuvenate skin, and restore lost beauty.

There is, however, one intervention that would be effective: to reset gene expression and upregulate molecular turnover, so that key molecules, such as collagen and elastin, are more rapidly turned over, with the result that damaged molecules no longer accumulate, but are replaced more quickly. The key to extracellular aging isn’t the damage, but the rate of turnover. The practical implication is that whether we are talking about collagen, elastin, beta amyloid, or dozens of other types of extracellular protein, we can effectively intervene by resetting gene expression. Whether we are looking at skin, joints, bone, or brains, the potential is an innovative and effective intervention for age-related problems.

Next Time: 2.9 Cell Senescence And Tissue Aging

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

 

March 20, 2018

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

 

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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