Power, Sex, Suicide: Mitochondria and the Meaning of Life (53 page)

So where does all this leave the mitochondrial theory of ageing? Perhaps surprisingly, it’s not dead and buried, merely radically transformed. A new theory has emerged, phoenix-like, from the ashes, but it still places a premium on the free radicals generated by the mitochondria. This new theory is not attributable to any one mind in particular, but has gradually condensed out of the work of researchers in several related fields. Beyond being consistent with the data, the new theory has the immeasurable benefit that it gives a deep insight into the nature of the diseases of old age, and how modern medicine might set about curing them. Critically, the best way to tackle them is not to target each one individually, as medical research currently does, but to target all of them simultaneously.

We have seen that the mitochondria operate a sensitive feedback system, in which the leaking free radicals themselves act as signals to calibrate and adjust
performance. But the fact that free radicals play an integral part in mitochondrial function does not mean that they are not toxic too. Clearly they are, even if rather less so than is shouted about in health magazines. Lifespan
does
correlate with the rate of free-radical leakage from respiratory chains. While a good correlation doesn’t necessarily imply a causal link, it’s hard to claim causality in the absence of any correlation at all. If two factors are not linked in any way, then one can hardly be said to ‘cause’ the other; and there are remarkably few, if any, other factors which correlate with lifespan across radically different groups, such as yeast, nematodes, insects, reptiles, birds, and mammals. For the sake of argument, let’s assume that free radicals
do
cause ageing. How can we square their signalling role with a more conventional idea of their toxicity—and with the evidence to date?

Yeast accumulate mitochondrial mutations at least 100 000 times faster than nuclear mutations. People, too, accumulate particular types of mitochondrial mutation with age, notably those in the ‘control’ region. Importantly, the control region mutations can often stage a ‘take-over’ of whole tissues, so that the same mutation is found in practically all the cells. In contrast, mutations in the coding regions of the mitochondrial genome can be amplified within particular cells, but only very rarely attain levels of above 1 per cent in the tissue as a whole. I suggested that this smells suspiciously of selection acting in tissues. Can we perhaps link the signalling role of free radicals with a weeding-out of detrimental mitochondrial mutations? Indeed we can, and this is the crux of the ‘new’ mitochondrial theory of ageing.

What might happen to the calibration of mitochondrial function if there is a spontaneous mutation in mitochondrial DNA? Let’s think it out step by step. If the mutation is in the control region, it doesn’t affect gene sequence, but it might affect the binding of transcription or replication factors. If the effect is not entirely neutral, the mutant mitochondrion would tend to copy its genes either more often or less often in response to an equivalent stimulus. So what is the outcome? If the mutation makes a mitochondrion ‘fall asleep’ on duty, so it responds sluggishly to signals for replication, the likely outcome is that the mutant mitochondria would simply disappear from the population. In response to a signal to divide, ‘normal’ mitochondria would divide, but the mutant mitochondria would slumber on. Their population would fall relative to normal mitochondria, and they would eventually be displaced altogether in the normal turnover of cellular components.

In contrast, if the mutation made the mitochondrion
more
alacritous in its response to an equivalent signal, we would expect to see an expansion of its DNA. At every signal to divide, the mutant mitochondria would leap into action, and so would eventually displace the ‘normal’ mitochondria from the population. And if the mutation occurred in a stem cell (which gives rise to
replacement cells in a tissue) the mutants would be more likely to be passed on every time the stem cell divided, and so would finally take over the entire tissue. It’s important to note that such mutations are most likely to stage a tissue takeover if they’re not particularly detrimental to mitochondrial function. This is likely to be true, as there is nothing the matter with the respiratory complexes themselves. Energy generation can continue normally, if a degree out of synch with requirements; and as we’ve seen (
page 287
), Giuseppe Attardi’s group has shown that one control-region mutation is actually beneficial.

So what happens if a mutation is in the coding region of a gene? Why do such mutations take over individual cells, but not the tissue as a whole? This time it’s more likely that mitochondrial function will be altered. Let’s imagine that the mutation affects cytochrome oxidase in some way. Given the need for nanoscopic precision in the interactions of different subunits, the likelihood is that respiration will be impaired, and electrons will back up in the respiratory chains. Free-radical leakage increases, and this signals the synthesis of new respiratory chain components. This time, however, building new complexes cannot correct the deficit, for these, too, would be dysfunctional (although if the deficit is modest, it may help a little). What happens next? The outcome is
not
the error catastrophe proposed in the original version of the mitochondrial theory, but more signalling. The defective mitochondrion signals its deficiency to the nucleus, by way of a feedback pathway known as the ‘retrograde response’, which enables the cell to compensate for its deficit.

The retrograde response was originally discovered in yeast, and was so named because it seems to reverse the normal chain of command from the nucleus to the rest of the cell. In the retrograde response, it is the mitochondria that signal to the nucleus to change its behaviour—the mitochondria, not the nucleus, set the agenda. Since its discovery in yeast, some of the same biochemical pathways have been found to operate in higher eukaryotes too, including humans. While the exact signals almost certainly differ in detail and meaning, the overall intention appears to be similar—to correct the metabolic deficiency. Retrograde signalling switches energy generation towards anaerobic respiration, such as fermentation, and in the longer term stimulates the genesis of more mitochondria. It also fortifies the cell against stress, aiding survival in the more trying times ahead. Yeast, which don’t depend on their mitochondria to survive, actually live longer when the retrograde response is active. With our dependence on mitochondria, it’s unlikely that similar benefits would apply to people; for us, the purpose of the retrograde response is to correct the mitochondrial deficiency. But I suppose we could be said to live longer in the sense that, without it, we would certainly live ‘shorter’.

Paradoxically, in the long term, a cell can only correct against energetic deficiency by producing more mitochondria. If the mitochondria are defective, the
cell attempts to correct the problem by producing more mitochondria—hence the tendency of defective mitochondria to ‘take over’ cells. For many years, cells can preferentially amplify the least-damaged mitochondria. The overall mitochondrial population is in continuous flux, with a turnover time of perhaps several weeks. Mitochondria either divide, if their energy deficit is fairly mild, or they die. The mitochondria that die are broken down, and their constituents recycled by the cell. This means that the most damaged mitochondria are continuously eliminated from the population. In this way, cells can spin out their lives almost indefinitely by constantly correcting the deficit. Our neurones, for example, are usually as old as we are ourselves: they are rarely, if ever, replaced, yet their function doesn’t spiral out of control in an error catastrophe, but rather declines imperceptibly. What isn’t possible though, is any return to the fountain of youth. While the most devastating mitochondrial mutations can be eliminated from cells, there is no way of restoring their pristine function, short of not using the mitochondria at all (which is how egg cells, and to a degree adult stem cells,
do
reset their clocks).

The more a cell relies on defective mitochondria, the more oxidizing the intra-cellular conditions become (oxidizing means a tendency to steal electrons). When I say ‘oxidizing’, however, I don’t mean the cell loses control of its internal environment. It retains control by adapting its behaviour, establishing a new status quo. Most proteins, lipids, carbohydrates, and DNA are not affected by the change—again, in disagreement with the predictions of the original mitochondrial theory, which anticipated evidence of accumulating oxidation. Most studies searching for such evidence have failed to find any serious difference between young and old tissues. What
is
affected is the spectrum of operative genes, and there is plenty of evidence to support this change. The shift in operative genes hinges on the activity of transcription factors—and the activity of some of the most important of these depends on their redox state (which is to say, whether they’re oxidized or reduced, having lost or gained electrons). Many transcription factors are oxidized by free radicals, and reduced again by dedicated enzymes; the dynamic balance between the two states determines their activity.

The principle here is similar to lowering a canary down a mine-shaft, to test for poisonous gases. If the canary is dead on raising it from the shaft, then the miner can take an appropriate precaution, such as only venturing down if wearing a gas mask. Redox-sensitive transcription factors behave like the canary, warning the cell of impending danger, and enabling it to take evasive action. Rather than the fabric of the cell as a whole being oxidized, which is as much as to say
dead
, the ‘canary’ transcription factors are oxidized first. Their oxidation sets in motion the changes necessary to prevent any further oxidation. For example, NRF-1 and NRF-2 (the ‘nuclear respiratory factors’) are transcription
factors that coordinate the expression of genes needed for generating new mitochondria. Both factors are sensitive to redox state, which dictates the strength of their binding to DNA. If the conditions in the cell become more oxidizing, then NRF-1 stimulates the genesis of new mitochondria to restore the balance, and for good measure also induces the expression of a battery of other genes, which protect against stress in the interim. NRF-2 appears to do the opposite, becoming more active when the conditions are ‘reducing’, and falling inactive when oxidized.

When the cell drifts to a more oxidizing internal state, then, a small posse of redox-sensitive transcription factors shifts the spectrum of active nuclear genes. The shift is away from the normal ‘house-keeping’ genes and towards those genes that protect the cell against stress, including some mediators that summon the help of immune and inflammatory cells. I argued in
Oxygen
that their activation helps to account for the chronic low-grade inflammation that underpins the diseases of advancing age, such as arthritis and atherosclerosis. While the exact spectrum of active genes varies from tissue to tissue, and with the degree of stress, in general the tissues establish a new ‘steady-state’ equilibrium, in which more resources are directed towards self-maintenance, and so fewer can be dedicated to their original tasks. This situation is liable to be stable for decades. We may notice we have less energy, or take longer to recover from minor ailments, and so on, but we’re hardly in a state of terminal decline.

So, overall, what happens is this. If conditions become oxidizing within a particular mitochondrion, then the mitochondrial genes are actively transcribed to form more respiratory complexes. If this resolves the situation, then all is well and good. However, if this
fails
to resolve the situation, then conditions in the cell as a whole become more oxidizing, and this activates transcription factors like NRF-1. Their activation shifts the spectrum of nuclear genes in operation, which in turn stimulates the genesis of more mitochondria and protects the cell against stress. The new arrangement stabilizes the cell again, albeit in a new status quo that can influence vulnerability to inflammatory conditions. But there is little oxidation of the fabric of cells and tissues, and because only the least damaged mitochondria tend to proliferate there is little overt sign of mitochondrial mutations and damage. In other words, the use of free radicals to signal danger explains why we don’t see the spiralling, catastrophic damage predicted by the original mitochondrial theory. And this in turn explains why the cell doesn’t accumulate too many antioxidants—it needs just the right amount, so that it’s sensitive to changes in the redox state of transcription factors. That is why I said earlier that biology is more dynamic than ‘mere’ free-radical chemistry: there is little that’s accidental going on here; rather, a continuing adaptation to the metabolic undercurrents of the cell.

So how do mitochondria kill us in the end? In time, some cells run out of normal mitochondria. When the next call comes to generate more, these cells have little option but to amplify their defective mitochondria clonally, and this is why particular cells are ultimately taken over by defective clones. But why do we only see a few cells with defective mitochondria at any one time, even in tissues of elderly people? Because now another level of signalling imposes itself. When cells finally work themselves into this state they are eliminated, along with their faulty mitochondria, by apoptosis, and that’s why we don’t detect high levels of mitochondrial mutations across ageing tissues. But there is a high cost for such purification—the gradual loss of tissue function, and with it ageing and death.

The ultimate fate of the cell depends on its ability to cope with its normal energetic demands, which vary with the metabolic requirements of the tissue. As in mitochondrial diseases, if the cell is normally highly active, then any significant mitochondrial deficiency will lead to a swift execution by apoptosis. What exactly constitutes the signal for apoptosis is uncertain, and again depends on the tissue, but two mitochondrial factors are probably involved—the proportion of damaged mitochondria, and the ATP levels in the cell as a whole. Of course, these two are interlinked. Clonal expansion of dysfunctional mitochondria inevitably leads to a more general failure to match ATP production to demand. In most cells, once the ATP levels fall below a particular threshold, the cell inexorably commits itself to apoptosis. Because cells with dysfunctional mitochondria eliminate themselves, it’s rare to observe heavy loads of mitochondrial mutations, even in the tissues of elderly people.