It ain’t what you don’t know that gets you into trouble. It’s what you know for sure that just ain’t so. – Mark Twain Twain was right, particularly when it comes to the aging process: there is a lot we think we “know for sure that just ain’t so”. For example, most people (without even […]
Aging and Disease: 1.1 – Aging, What it Isn’t
It ain’t what you don’t know that gets you into trouble.
It’s what you know for sure that just ain’t so.
– Mark Twain
Twain was right, particularly when it comes to the aging process: there is a lot we think we “know for sure that just ain’t so”. For example, most people (without even thinking about it and with a fair amount of naïve hand-waving) assume that all organisms age and equate aging with entropy. In other words, they think that “aging is just wear-and-tear”. We assume that aging “just happens” and that nothing can be done about it. After all, we all get old, things fall apart, things rust, everything wears out, so what can you expect? But as with Twain’s remark, the trouble is that we are quite sure of ourselves and we what we think is completely obvious, turns out to be completely wrong. We are content to gloss over our faulty assumptions and move to faulty conclusions. It’s bad logic, bad science, and a bad way to intervene in the diseases of aging. Without thinking about it, we conclude that aging is as simple as our preconceptions, which turn out to be erroneous.
Aging isn’t simple and our preconceptions are wrong.
As with most concepts that we don’t examine meticulously, aging is a lot more complex than we realize. Aging isn’t just entropy, it isn’t just wear-and-tear, and it isn’t many things that people blithely believe it to be. Let’s look at a few examples that make us back up and reconsider how aging works. Let’s start with your cells, and then your mitochondria.
We could take any cell in your body, for example a skin cell on the back of your hand. How old is that skin cells? Since we shed perhaps 50 million skin cells every day, there’s a good chance that the cell we are thinking about is only a day or so old, or at least a day or so since the last cell division. But that last division was from a “mother” cell that was there before the cell division resulted in two “daughter” cells. So perhaps our skin cell, counting the age of the “mother” cell is a week or so old? But that “mother” cell, in turn, derived from a dividing cell that was there several weeks ago, backwards ad infinitum to the first cells that formed your body. In fact, every cell in your body is certainly as the whole body, so perhaps that skin cell is a few decades old. You might say that the skin cell has the same age that you see on your driver’s license. Except that your entire body is the result of a cell (ova) from your mother and a cell (sperm) from your father, and each of those cells was already a few decades old (or however old your parents were) when the sperm and ovum became “you” when they joined at fertilization. But, of course, your parent’s germ cells came from their parents, whose germ cells came from their parents, and we can trace that lineage of germ cells back to… Well, all the way back to the origin of life on Earth. So in a very real, very strictly accurate biological sense, every cell in your body is 3.5 billion years old.
But if we assume that aging is just entropy, then we have explain why that line of germ cells (that resulted in your entire body) didn’t undergo any entropy (i.e., didn’t age) for 3.5 billion years and yet your somatic cells are now undergoing entropy (i.e., aging) in your body and have been aging since you were born. Why do somatic cells suffer from entropy, if germ cells don’t? Does entropy only work in certain cells and not in others? Apparently so. And if that’s true, then we can’t just wave our hands and invoke entropy as the entire explanation, can we? We have to explain something more subtle and complicated: why entropy results in aging in some cases (the somatic cells in your body) but not in other cases (the line of 3.5 billion year-old germ cells that led up to you having a body in the first place). How interesting. So much for just invoking the concept of entropy and walking away satisfied.
Entropy almost certainly plays a key role in aging, but we can’t simply leave it at that. We need to think a bit harder. Sometimes entropy wins (your body and most of its cells age in a matter of decades) and sometimes entropy doesn’t appear to win at all (your germ cell line didn’t age for 3.5 billion years). Why sometimes and not other times?
One way that some people have tried to explain this is to invoke mitochondrial damage, but an almost identical problem surfaces in the case of mitochondrial entropy. Given the prevalence of aging explanations based on free radical theory (reactive oxygen species, etc.), mitochondrial dysfunction is an obvious suspect for an explanation of aging. We know that older mitochondria make more free radicals, leak more, and those free radicals aren’t scavenged as well, so perhaps all of aging is a mitochondrial problem? Perhaps entropy simply causes mitochondrial damage and that’s why we age. Perhaps entropy works by aging our mitochondria, right?
Except that mitochondrial entropy can’t explain aging either.
If aging were the result of “aging” mitochondria, damaged by entropy (high internal mitochondrial temperature, free radicals, loose protons and electrons, and a general accumulation of mitochondrial damage over time), then we are still left with an embarrassing conundrum. To understand the problem, let’s ask a simple question: how old are your mitochondria? Mitochondria divide fairly constantly, depending on the cell and its energy demands. In some cells (such as liver cells), with high energy demands, mitochondria are dividing all the time, in others with low energy demands, mitochondria divide much less frequently. On the other hand, since every mitochondria in every cell in your body derived from the mitochondria that were present in you as a fertilized zygote, we might reasonably say that your mitochondria are all the same age as your body, i.e., all of your mitochondria are a few decades old, and as time goes by, your mitochondria simply wear out, right?
Every mitochondria that you had as a fertilized zygote was derived from your mother’s ovum, which supplied all of your original mitochondria, so your mitochondria are as old as you are. Well, as old as you are plus as old as your mother was when you were conceived. Oh, and plus the age of her mother and her mother and so on, ad infinitum back as far as the very first mitochondrial inclusion in the very first eukaryotes (or so). So every mitochondria in your body is about 1.5 billion years old and they’re doing pretty well for their age. But that means that if we want to blame aging entirely on mitochondrial dysfunction (and mitochondria surely play a major role in aging), we are still left with a conundrum. We have to explain why all of those dividing mitochondria (which were at least 1.5 billion years old) hadn’t aged for 1.5 billion years, and now all of your mitochondria are having significant problems after only a few decades. Why do your mitochondria suddenly start aging when they were doing so well for the last 1.5 billion years? The problem is that your mitochondria really do showing aging changes, but the mitochondria from your mother clearly didn’t until you came along. Worse yet, we have to explain both of these effects (aging and non-aging) simultaneously if we want to explain aging at all. How can we do both? We can’t simply wave our hands (again) and blame entropy unless we can simultaneously explain why entropy works sometimes and in some cells (liver cells, for example), but entropy doesn’t work at other times and in other cells (the mitochondria in the germ cell line, for example). Again, why sometimes and not other times?
If entropy were an entirely sufficient explanation, they why does entropy age some cells (and some mitochondria) and not other cells (and other mitochondria)? If we restrict our explanation of aging solely to entropy, then we have a problem. We can’t just say that entropy does cause aging (because sometimes it doesn’t) nor can we say that entropy doesn’t cause aging (because sometimes it does). Entropy plays a role in aging, but not always. Why? What we have to do, if we really want to explain aging, is explain why entropy varies in biological systems. Sometimes entropy wins, sometimes it doesn’t.
Our preconception about entropy – wear-and-tear – as the sole cause for aging is a common misconception and not always noticed. It creates a subtle, but pervasive bias in our thinking about biolgy and aging. Even once we realize that entropy can’t explain all of cell or mitochondrial aging, we still find entropy creeping back into our thinking, but disguised under a different form. We tend to think of Alzheimer’s, for example, as what happens when beta amyloid, tau proteins, or mitochondria undergo entropy and cause neuronal death and clinical disease. We think of skin aging as what happens when collagen and elastin undergo entropy and cause wrinkles and aging skin. Some people blame aging on entropy of the endocrine system, concluding that all of aging comes about because of entropy in a gland or hormonal tissue. The fact that aging can occur in some organisms without endocrine systems (and that replacing hormones doesn’t stop aging) doesn’t change their misconception. But whatever guise it hides under, entropy by itself, cannot explain aging or age-related disease. There are too many odd things to explain, too many exceptions, too many cases where entropy explains one finding, but not another finding. Entropy can explain this cell, but not that cell. Entropy can explain this mitochondria, but not that mitochondria. Entropy simply can’t explain aging in toto. We have to dig a bit further.
Entropy, as an explanation of aging, only works if we close our eyes and ignore most of biology. As we’ll see in the next blog, there is a lot of biology that needs to be accounted for if we are going to explain how aging works. However we try to shoehorn entropy into being the entire explanation, aging cannot be entropy alone. As we will see, entropy does play a crucial role, but we cannot simply cite entropy, wave our hands, and say we understand aging. Aging is not entropy: aging is entropy plus something else, something subtle and complex, but something crucial to a complete understanding of aging.
As we will soon see, aging is entropy in the face of failing maintenance.
Excellent! Keep’em coming!
It’s a very good point that mitochondria don’t age on their own, as proven by bacteria, so the deterioration that occurs in them in human cells is downstream of other problems likely linked to gene expression in the nucleus.