Michael Fossel Michael is President of Telocyte

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

 

July 20, 2016

Curing Disease: More Insight Instead of Mere Effort

 

Curing disease correlates with insight, not blind effort.

There is an eternal trade-off between insight and effort. If we think carefully, understand the problem, and plan, then effort is minimized. If (as too often happens) we think carelessly, misunderstand the problem, and rely on hope instead of planning, then effort is not only maximized, but is usually a complete waste. Lacking insight, we foolishly flush both money and effort down the drain. In the case of clinical trials for Alzheimer’s disease – and in fact, all age-related diseases – this is precisely the case.

The major problem is a naïve complaisance that we already understand aging pathology.

If there was a single concept that is key to all of aging, it is the notion that everything in our organs, in our tissues, and in our cells is dynamically and actively in flux, rather than being a set of organs, tissues, cells, and molecules that statically and passively deteriorate. Aging isn’t just entropy; aging is entropy with insufficient biological response. Senescent cells no longer keep up with entropy, while young cells manage entropy quite handily. At the tissue level, the best example might be bone. We don’t form just bone and then leave it to the mercy of entropy, rather we continually recycle bony tissue throughout our lives – although more-and-more slowly as our osteocytes lose telomere length. This is equally true at the molecular level, for example the collagen and elastin molecules in our skin. We don’t finish forming collagen and elastin in our youth and then leave it to the vagaries of entropy, rather we continually recycle collagen and elastin molecules throughout our lives, although more-and-more slowly as our skin cells lose telomere length. Aging is not a process in which a fixed amount of bone, collagen, or elastin gradually erodes, denatures, or becomes damaged. Rather, aging is a process in which the rate of recycling of bone, collagen, or elastin gradually slows down as our shortening telomeres alter gene expression, slowing the rate of molecular turnover, and allowing damage to get ahead of the game. We don’t age because we are damaged, we age because cells with shortening telomeres no longer keep up with the damage.

The same is true not only of biological aging as a general process, but equally true of every age-related disease specifically. Vascular disease is not a disease in which our arteries are a static tissue that gradually gives way to an erosive entropy, but an active and dynamic set of cells that gradually slow their turnover of critical cellular components, culminating in the failure of endothelial cell function, the increasing pathology of the subendothelial layer, and the clinical outcomes of myocardial infarction, stroke, and a dozen other medical problems. Merely treating cholesterol, blood pressure, and hundreds of other specific pathological findings does nothing to reset the epigenetic changes that lie upstream and that cause those myriad changes. Small wonder that we fail to change the course of arterial disease if our only interventions are merely “stents and statins”.

Nor is Alzheimer’s a disease in which beta amyloid and tau proteins passively accumulate over time as they become denatured, resulting in neuronal death and cognitive failure. Alzheimer’s is a disease in which the turnover – the binding, the uptake, the degradation, and the replacement – of key molecules gradually slows down with telomere shortening, culminating in the failure of both glial cell and neuron function, the accumulation of plaques and tangles, and ending finally in a profound human tragedy. The cause is the change in gene expression, not the more obvious plaques and tangles.

Our lack of insight, even when we exert Herculean efforts – enormous clinical trials, immense amounts of funding, and years of work – is striking for a complete failure of every clinical trial aimed at Alzheimer’s disease. Naively, we target beta amyloid, tau proteins, phosphodiesterase, immune responses, and growth factors, without ever understanding the subtle upstream causes of these obvious downstream effects. Aging, aging diseases, and especially Alzheimer’s disease are not amenable to mere well-intended efforts. Without insight, our funding, our time, and our exertions are useless. Worse yet, that same funding time, and exertion could be used quite effectively, if used intelligently. If our target is to cure the diseases of aging, then we don’t need more effort, but more thought. However well intentioned, however much investment, however many grants, and however many clinical trials, all will be wasted unless we understand the aging process. Aging is not a passive accumulation of damage, but an active process in which damage accumulates because cells change their patterns of gene expression, patterns which can be reset.

Curing Alzheimer’s requires insight and intelligence, not naive hope and wasted effort.

 

 

February 16, 2016

Unexamined Assumptions

The problem with curing Alzheimer’s is, as with so much of our understanding of aging and age-related diseases, that we make unexamined assumptions. Let me admit that many of our unexamined assumptions are either useful or reasonable. I assume that the sun will come up again tomorrow morning and that’s a useful and reasonable assumption. Useful, in that it allows me to plan my future, reasonable in that the sun has been coming up every morning for quite a while and is therefore likely to do so tomorrow as well. Certain unexamined assumptions are equally justifiable in dealing with Alzheimer’s disease. In the strictly poetic sense, Alzheimer’s certainly is the disease that “steals our souls”, yet no physician or researcher would actually make the assumption that the mind is some vague ethereal quantity that can be stolen by demons, let alone go on to promulgate a theory of Alzheimer’s pathology based on this assumption.

Yet we make exactly that same error, using an unexamined assumption, when we blithely assume that aging is simply the accumulation of damage and, pari passu, that Alzheimer’s disease is simply the accumulation of damaged molecules, be they amyloid, tau tangles, or altered mitochondrial enzymes. This unexamined assumption lies behind almost innumerable multi-million dollar FDA trials, academic papers, and clinical interventions. We assume, without even realizing we have made the assumption, that Alzheimer’s is merely the accumulation of damaged molecules.

We make the same unexamined assumption in looking at other age-related diseases and in the broader field of aging itself. We delve into the details of advanced glycation end-products (AGE), lipofuscin, cross-linking, and other molecular pools showing “accumulative damage”, all the time never realizing that we are making the same fallacy. We are working with completely unexamined (and erroneous) assumptions about how aging works. We naively assume that aging occurs – and age-related diseases follow – merely because things “rust” over time. We age because “molecules fall apart.”

 

Yet the data and logic both say differently. Let me give you a useful analogy: the cell phone. Consider a large pool (several thousand) of people who own cell phones. We know that if we examine any SINGLE cell phone, the best predictor of failure is how long it has been since production. If, however, we want to predict the percentage of failures in any large pool of owners, the best predictor is not time-since-production, but length-of-contract, that is, how often does it get turned over and replaced? Imagine two large pools of cell phone owners. In group A, the cell phones are replaced annually, with a failure rate (at equilibrium) of approximately 1%. In group B, the cell phones are replaced every ten years, with a failure rate (at equilibrium) of approximately 80%. In both groups, the rate of failure of any individual phone is the same. Furthermore, the rate of failure is only marginally related to the “genes”, i.e., whether the phone is an Apple iPhone, an Android, or some other type (a different “allele”). As the turnover rate (contract length to replacement) lengthens, the percent of failed cell phones climbs dramatically, regardless of the failure rate of any individual cell phone. In a pool of cell phones, “aging” is not a matter of passively accumulated damage, but of how actively we replace them.

The same is occurring in molecular pools in biological systems. The key predictor of “denatured” or dysfunctional molecules (e.g., AGE, beta amyloid microaggregates, cross-linking, elastin failure, collagen stiffening, etc) is not the rate of damage but the rate of turnover. In the case of cell aging, when we reset gene expression (reset telomere length) we reset the turnover rates (anabolism and catabolism rates) of all molecular pools to those typical of “young” cells. The outcome is that molecule pool turnover is more than sufficient to deal with typical rates of damage.

Without realizing it, most of us make the mistake of thinking of molecular pools as static and damage as purely accumulative. The reality is that such pools are dynamic and the key dependent variable (as with cell phones) is not the passive rate of damage, but the active rate of turnover.

Unless we understand – and examine – our assumptions, we can never expect to cure age-related diseases. Once we start down the wrong path, all the logic and data in the world can’t make up for the fact that we are looking in the wrong place. It’s time we stopped blaming “demons” and starting thinking carefully.

August 25, 2015

Alzheimer’s: One Disease?

Most of us have wondered about what causes Alzheimer’s. As commonly happens, we stumble badly when we make assumptions, even in asking questions, let alone in trying to answer those questions. The question “what causes Alzheimer’s?” presupposes that there is a single such disease (Alzheimer’s) and that we can define it well enough to ask about “its” cause. Neither of these is probably an accurate assumption. The reality is that there is considerable difficulty in agreeing on the “hallmarks” (the pathognomonic characteristics that define AD) and the “boundaries” between AD and other somewhat similar diseases on the differential diagnosis. Comparing Alzheimer’s to many other age-related neurological diseases can be humbling – and it should be. Small wonder we have so much trouble understanding the cause, let alone finding a cure when we don’t really know what we’re looking at.

Rather than just reinforce our preconceptions, let’s look at reality a bit more closely.

One of the things that has become clearer over the past century – and especially so over the past two decades – is that there is a remarkable amount of overlap in the pathology found in what we have thought of as different age-related neurological problems. This has become grudgingly accepted as we compare not only Alzheimer’s and Parkinson’s disease, but a host of other clinical problems, such as microvascular infarcts, vascular dementia, frontotemporal dementia, hippocampal sclerosis, Huntington’s disease, amyotrophic lateral sclerosis, dementia with Lewy bodies, and mixed dementia (a term that sort of sums up the problem we’re discussing). Just to restrict ourselves to the two classic diseases – AD versus PD – Alzheimer’s tends to have primarily cognitive rather than motor problems, whereas Parkinson’s tends to have primarily motor rather than cognitive problems. In reality, however, both Alzheimer’s and Parkinson’s patients tend to have some of both, particularly as their diseases progress. At the histological level, we tend to distinguish the locations of each disease, and at the neurochemical level we likewise make distinctions, yet there still remains overlap at almost any level, once we look more carefully.

Perhaps there is a single, common, underlying causative pathology that results in BOTH of these diseases. Could both AD and PD be two different manifestations of a shared problem?

This same question surfaces when we look carefully at the vascular dementias: they overlap in many ways with the classical “non-vascular” etiologies. Again: could all of these have a common underlying factor with disparate clinical presentations? We see the same problem when we look at age-related neurological dysfunction in animal models, such as laboratory-created Alzheimer’s models in mice, as well as the “normal” decline in any wild species (such as mice or rats). We go to a lot of trouble to introduce human genes into laboratory species in order to produce a “mouse model of Alzheimer’s”, yet these animals show behavioral declines even in the wild and when we introduce human genes, it’s certainly not clear that we end up with a mouse model that teaches us anything useful when we want to find a cure.

We could put all of these clinical changes together by positing that they derive from a common cellular problem, that of cell senescence. Different patients have different genes and different patterns of gene expression, so their disease expressions differ, some having AD, some having PD, some having any number of other disease phenotypes. Different animals (humans versus mice, for example) likewise have differing genetic and epigenetic settings, so their disease expressions also differ, humans showing beta amyloid and tau protein changes, mice showing a different pattern, but all showing behavioral and cognitive decline over time, whatever the individual pathway the pathology uses to express itself.

Consider our diagram of the “Common Pathological Pathways in Age-Related CNS Failure” (see figure A). The proposition illustrated in this diagram is that of a single underlying problem, with multiple possible pathways, and a shared outcome: age-related CNS failure. One cause, multiple pathways (often defined as different diseases), but one outcome. Whatever the pathway chosen, the outcome is an increasing neurological dysfunction with age.

 

Figure A08-25-15 Figure A

In the case of particular diseases (or particular species), the clinical phenotype depends on which cells are senescing fastest (e.g., glial cells in the brain, endothelial cells in the arterial tree, etc.) and which protein products (e.g., beta amyloid, tau protein, alpha synuclein, etc.) are most likely to cause problems first, depending upon the genetic landscape and the epigenetics of the individual patient or the individual species. If we now label the common diagram with specific diseases and species, we get something like the second diagram (see figure B).

 

Figure B08-25-15 Figure B

If we really want to understand, and cure, Alzheimer’s, then we need to start by understanding (and curing) our own preconceptions. It is only when we look at not only the clinical data, but a wide panoply of species that we can truly understand any of the diseases that we see day-to-day.

One cause, multiple pathways, and a single shared outcome: CNS failure.

Curing Alzheimer’s becomes – as it has been for a century – a fool’s errand if all we target are the specific genes and proteins that we (naively) think of as the hallmarks of the disease. If we truly want cure Alzheimer’s, then it’s time we understand the disease and it’s high time we target the actual causes of not only Alzheimer’s disease, but the entire spectrum of age-related neurological diseases that should be labelled under a common rubric, diseases of cell senescence.

It’s time we understand Alzheimer’s and time we cure it.

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