Oxygen

by Nick Lane

  All this is very well, but what can we do about it? A few researchers talk glibly of transplanting foetal mitochondria into adult cells (or more technically ‘gene therapy by mitochondrial transfer’) but the idea is absurd as a ‘cure’ for ageing. We have an average of 100 mitochondria in each cell, so each of us harbours around 1.5 million billion mitochondria. I find it hard to imagine that we could make a real difference to such a large population simply by injecting a few new mitochondria. On the other hand, it is quite feasible to inject mitochondria into an egg cell. This has already been done as a fertility treatment, by injecting the contents of an egg from a fertile woman, along with a donor sperm, into the egg of an infertile woman — a procedure known as ooplasmic transfer. At least 30 babies have already been born using this technique, of whom the eldest celebrated his fourth birthday in June 2001. Even given the personal happi-ness that fertility treatments can bring, however, I find it hard to welcome

  ‘reprogenetic technologies to shape future children’, let alone to shape the elderly.

  Setting aside the ethical objections to ooplasmic transfer there are still some difficult technical considerations. Egg cells are subject to natural selection. Of the 7 million eggs that develop in the female foetus, only a few hundred ever come close to ovulation in sexual maturity: one in 20 000. The basis of this selection is shrouded in mystery, but there seems to be a sophisticated cross-talk between the nucleus and the mitochondria, which is even influenced by the spatial distribution of mitochondria within the egg. Essentially, if the mitochondria aren’t right, the egg never makes it.

  If an egg is forced to develop artificially, the offspring frequently suffer from bioenergetic diseases. This problem may go some way towards explaining the disturbingly high failure rate of cloning, in which an alien nucleus is inserted into an egg from which the nucleus has been removed, and development is stimulated by an electric shock. John Allen, whom

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  we met in Chapter 13, and his wife Carol develop this argument. They attribute the premature ageing of cloned animals like Dolly the sheep to a contamination of the egg cell with mitochondria. Dolly was cloned by fusing a whole somatic cell, including its mitochondria, with an enucleate egg. According to this argument, Dolly is ageing prematurely (she developed arthritis, for example, by the age of five) because many of her mitochondria came from a cell taken from a sheep that was already six years old. Dolly is therefore mutton dressed as lamb. Her biological age is probably closer to 11 than to 5. The Allens spelled out many practical ways of testing this theory in a paper published in 1999 (see Further Reading).

  The amazing fact is that ooplasmic transfer and cloning ever work at all. No doubt many of the technical problems can be ironed out in time, although as far as preventing ageing is concerned we must ask ourselves, as a society, whether we would even wish to try. But turning away from such genetic manipulations, what else can we do? We are learning all the time about how mitochondria differ between species, and how our own mitochondria change in the course of our lives. Such differences are controlled not just by genes, but also by diet, activity and hormones.

  One difference between species that correlates well with longevity is the lipid composition of mitochondrial membranes. All biological membranes are composed of a lipid bilayer, in which the water-hating tails of the lipid molecules in both layers point to the inside of the membrane.

  The bilayer is studded with proteins, which float like islands of pumice in a fatty sea. The inner mitochondrial membrane is especially rich in proteins — these make up the hundreds of respiratory chains that generate energy for the cell. Sixty per cent of the mitochondrial membrane is made of protein. As in an engine, the function of the respiratory chains depends on their ‘lubrication’. This is provided by the lipid components of the membrane. The exact composition of these lipids has a profound effect on the function of mitochondria, just as the oil alters the behaviour of an engine. If the lubricant is not effective, the mitochondria leak more free radicals and generate less energy, which contributes to cell damage and metabolic insufficiency. In mitochondria, the lubricant of choice is called cardiolipin.

  Each cardiolipin molecule incorporates four fatty acids, which can be unsaturated (containing double bonds) or saturated (not containing double bonds). Unsaturated fats keep the membrane fluid (in the same way that unsaturated oils are more fluid than saturated lards). This is

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  because the double bonds kink the fatty-acid chains, which prevents them from lining up in neat arrays (making it harder for them to set). There is a price for fluidity, however: double bonds are easily oxidized. Some sort of compromise is necessary. The best compromise varies according to the kind of performance required. For example, a high metabolic rate requires a fluid membrane, while a long life demands resistance to oxidation.

  With this in mind, Reinald Pamplona, Gustavo Barja, and their colleagues at the University of Lleida, in Spain, compared the fatty acid composition of mitochondria from different species, from rats to horses and pigeons to parakeets. They found a striking relationship. Animals with long lifespans had low levels of highly unsaturated fatty acids, such as docosahexanoic acid (with six double bonds) and arachidonic acid (with four double bonds) but much higher levels of slightly unsaturated fatty acids with two or three double bonds, such as linoleic acid. In other words, the longer the lifespan, the lower the level of unsaturation. The exact lipid composition varies somewhat with diet, but is largely refractory to change: animals convert one fatty acid into another to meet the requirements of their mitochondria. For example, the staple diet of laboratory mice contains no docosahexanoic acid (it is easily oxidized), yet their mitochondria contain 8 per cent. In contrast, horse fodder is rich in the precursors of docosahexanoic acid, but horse mitochondria contain only 0.4 per cent.

  We are left with a problem: the composition of mitochondria affects their function and our lifespan, but is not easy to alter by diet.

  Worse follows. Animals get ‘more unsaturated’ as they age. Old rats double their content of highly unsaturated fatty acids, while the proportion of less-unsaturated fatty acids falls correspondingly. As a result, mitochondria become more vulnerable to oxidation with age, and lose their lubricant, cardiolipin. The cardiolipin content of rat mitochondria halves by old age. Similar changes probably take place in us. Thus, for a long life we must restrict the proportion of highly unsaturated lipids in our mitochondria, yet as we get older the proportion, contrarily, increases. If diet can only help a little, is there anything else we can do about it?

  The answer is almost certainly yes. The composition of mitochondria is only partly influenced by diet, but equally, the changes that take place as we age are only partly controlled by changes in our genes. By this, I mean that the sequence of genes often remains inviolate, but their activity

  — whether or not they are expressed, or how much they are expressed —

  almost invariably changes. To reverse the changes that take place as we age, we need to reverse the changes in gene expression, and this is much easier

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  than altering the sequence of the genes themselves. In rats, for example, calorie restriction can reverse the age-related changes in mitochondrial composition and function, making mitochondria less vulnerable to oxidation. In other words, the inexorable decline in mitochondrial function with age is partly physiological, and not purely pathological. Whether calorie restriction can orchestrate similar changes in people is unknown, but I see no reason why not.

  Intriguingly, carnitine may exert similar effects. We met carnitine in Chapter 9 in relation to vitamin C. We need it to shuttle fats into the mitochondria for use as fuel, and to remove the left-over organic acids.

  We can synthesize carnitine ourselves, using vitamin C, but we also eat some in our food. One of the symptoms of scurvy is general lassitude, wh
ich may be explained by carnitine deficiency. Carnitine supplements have been used for many years (with regulatory approval) as energy-boosters, and to protect against heart weakness and muscle wasting. Its effects go beyond a shuttle-bus service: carnitine alters the lipid composition of mitochondrial membranes, restoring the cardiolipin content to youthful levels. These effects are not just cosmetic: old rats gain energy and are twice as active when fed carnitine.

  Carnitine is no panacea, however: it also increases free-radical leakage and oxidative stress. This may help explain its disappointing record in age-related diseases such as Alzheimer’s disease. Even so, the pro-oxidant effects can be suppressed using antioxidants such as lipoic acid, and this particular combination holds promise. In a series of papers published in the Proceedings of the National Academy of Sciences of the USA in February 2002, Bruce Ames and his colleagues at the University of California, Berkeley, reported that carnitine, given together with lipoic acid, improved the mitochondrial function and integrity of old rats, and boosted their energy levels. As Bruce Ames put it, “These old rats got up and did the Macarena.” The rats also performed better in various tests of memory and intelligence. How much benefit we might gain from carnitine is another open question, but is at least beginning to attract serious research interest, and clinical trials are now underway. Presumably, high-dose vitamin C

  might boost carnitine synthesis in old age too, although surprisingly little is known about this: perhaps we have focused too tightly on its antioxidant properties.

  Exercise itself benefits mitochondria. We saw in Chapter 13 that the health of a population of mitochondria reflects the rates of replication and breakdown. Damaged mitochondria are broken down more slowly

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  than healthy mitochondria in old tissues. Because the rate of mitochondrial replication is very slow in such tissues, the damaged mitochondria ultimately take over. This vicious circle can be broken by gentle exercise.

  When we exercise, the higher demand for energy stimulates mitochondrial replication. The healthiest mitochondria now replicate fastest, and this regenerates the stock of viable mitochondria. As usual, there is a catch: vigorous exercise often causes more oxidative damage than it cures, and it is hard to know at what point we begin to do harm; gentle aerobic exercise, like walking or swimming, is probably about right. I wonder whether something similar applies to mental exercise. Education and mental activity tend to protect against Alzheimer’s disease; why is unknown.

  It is feasible that intellectual exercise might keep the brain’s mitochondrial population turning over, rejuvenating stock.

  Mitochondrial medicine is a dynamic field set to expand in the coming years, and the tangible excitement is offset only by the humbling experience of antioxidant interventions in the past. We have learned a hard lesson: it is not good enough just to ‘throw in’ an antioxidant and hope for the best. We need to find a way of targeting the mitochondrial membranes, whether it be through metabolic boosters such as carnitine, antioxidants such as lipoic acid or coenzyme Q, hormones such as melatonin or thyroxine, or some factor I cannot even begin to imagine. It will almost certainly require an integrated physiological approach. We have a lot to learn about the way in which mitochondria work, and must expect setbacks, but I believe that here we are finally getting close to the heart of the problem. If we ever succeed in extending our lifespan to a healthy 130, I would be surprised if the big strides forward had not begun in mitochondrial medicine.

  Viewing evolution through the prism of oxygen gives us some surprising perspectives on our own lives and deaths. If water is the foundation of life, then oxygen is its engine. Without oxygen, life on Earth would never have got beyond a slime in the oceans, and the Earth would probably have ended its days in the sterility of Mars or Venus. With oxygen, life has flourished in all its wonderful variety: animals, plants, sex, sexes, consciousness itself. With it, too, came the evolution of ageing and death.

  We cannot hope to understand the complex degenerative diseases of old age unless we have an evolutionary grasp of their cause. Evolutionary

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  theory can take us so far, but will fail unless backed by empirical evidence.

  In the same way, the sixteenth-century scientist Francis Bacon famously argued that philosophy could never answer the great questions of life and death without the guiding light of experiments. We should not forget that science was born from philosophy, in other words from a system of ideas about the world. Experiments allow us to weigh the value of competing ideas that cannot be discriminated on a logical basis; but for science to be meaningful, experiments must be conducted within the framework of an idea — a hypothesis — about how the world works. Science does not work by induction — by trawling piles of miscellaneous data in the hope of finding patterns or facts — but by hypothesis and refutation.

  Today, medical research is in danger of becoming too empirical, of accumulating tremendous piles of data without giving them due thought.

  There is an uncomfortable gap between the hundreds of crazy theories about ageing and disease, which are rarely supported by coherent data, and the headlong rush of medical research, which rarely finds time to interpret new findings in a wider context. In this age of excessive healthcare spending and failing healthcare systems we need to ask whether medical research is taking us in the right direction.

  Genetic research has transformed our understanding of biology, health and disease. Many of the ideas in this book would have been unthinkable without the great advances in molecular genetics. But we should not mistake the tool for the solution. Insofar as there is any guiding philosophy behind medical research, it is that genes go wrong and cause disease. We celebrate completion of the human genome project because it tells us far more about which genes might go wrong. The time and money spent chasing defective genes for particular diseases dwarfs research into the underlying processes of ageing itself: there are thousands of specialized disciplinary journals, but just a handful devoted to the science of ageing. We get frustrated with the slow pace of research — a

  ‘breakthrough’ now may come to fruition in 20 years — but accept that this is so because the effects of genes are complex and intractable: we must just wait. Will the promises ever come to fruition, or are we being sold a line? The only way we can hazard a guess is by thinking in evolutionary terms, and this has the added bonus of giving us a clearer idea of what kind of approach might actually work.

  The idea that oxygen might accelerate ageing is not new: it was implicit in Joseph Priestley’s suggestion that we might ‘burn out’ faster, like a candle, if we breathed his pure oxygen. On the basis of experiments

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  alone, we might reasonably claim that oxygen free radicals contribute to ageing and to some diseases, and are perhaps a consequence of others.

  From an empirical point of view, the failure of antioxidants to extend life or cure diseases suggests that the role of free radicals is limited, just one factor among many. An evolutionary perspective opens up quite a different vista. We see that life has learned to cope with oxygen through a myriad of adaptations, from behaviour to size to sex. The logic of the evolutionary view can be tested in unexpected ways, through predictions about the evolution of two sexes and the development of egg cells in the follicle, to the failure of cloning experiments or the impact of malaria on the diseases of old age. I hope I have convinced you, from this perspective, that oxygen is not just the engine of evolution and life, but also the single most important cause of ageing and age-related disease.

  The crispness of this view is satisfying and helps us to see our place in nature. It is hopeful, as it shows us that ageing is neither programmed nor inevitable, even if it cannot easily be put off. It is corrective, for it shows us the fallacy of chasing ‘susceptibility’ genes for the diseases of old age. It is constructive, in that it points us to the fields of rese
arch that might best tackle the problem of ageing — immune modulation and mitochondrial medicine. And it is practical, for it offers us a rational guide to good health in old age: eat widely, but not too much, don’t be obsessively clean or get overly stressed, don’t smoke, take regular exercise, and keep an active mind. Start now! If all the advances of biology and medicine can do no more than explain the wisdom of our grandparents, may that restore some lost dignity to wise old age.

  Further Reading

  G E N E R A L T E X T S

  Brown, G. The Energy of Life. HarperCollins, London, 1999.

  Cairns-Smith, G. Seven Clues to the Origin of Life. Cambridge University Press, Cambridge, 1985.

  Cowen, R. History of Life. Blackwells, New York, 2000.

  Davies, P. The Fifth Miracle. The Search for the Origin of Life. Penguin Books, London, 1998.

  Dawkins, R. The Selfish Gene. Oxford University Press, Oxford, 1989.

  Djerassi, C. and Hoffman, R. Oxygen. Wiley-VCH, Weinheim, 2001.

  Dyson, F. Origins of Life. Revised Edition. Cambridge University Press, Cambridge, 1999.

  Emsley, J. Molecules at an Exhibition. Oxford University Press, Oxford, 1998.

  Fenchal, T. and Finlay, B. J. Ecology and Evolution in Anoxic Worlds. Oxford University Press, Oxford, 1995.

  Fortey, R. Life: An Unauthorised Biography. HarperCollins, London, 1997.

  Fortey, R. Trilobite! Flamingo, London, 2001.

  Gould, S. J. Wonderful Life. The Burgess Shale and the Nature of History. Penguin Books, London, 1989.

  Hager, T. Linus Pauling and the Chemistry of Life. Oxford University Press, Oxford, 2000.

  Halliwell, B. and Gutteridge, J. M. C. Free Radicals in Biology and Medicine, Third Edition. Oxford University Press, Oxford, 1999.