Power, Sex, Suicide: Mitochondria and the Meaning of Life (56 page)

So what does all this have to do with birds and respiratory chains? Birds have adopted the strategy of the large workforce. Management preferred the high labour costs and the risk of workforce attrition, but they valued their equipment and didn’t want to see it wilfully damaged. Moreover, they were ambitious, and imagined that they would win a lot of work, and so would need a large workforce. Translated into biological terms, all this means the following. Birds rely on a large number of mitochondria, and within the individual mitochondria, they have a large number of respiratory chains. They have a high workforce, and for much of the time they have a lot of spare capacity. In molecular terms, the reduction state of complex I is low: the electrons entering into the respiratory chains have plenty of space for themselves. In contrast, mammals adopt the alternative policy: they like to employ a small workforce. This means they retain as small a number of mitochondria, and respiratory chains within the mitochondria, as they can get away with. Even when the workload is low, they still pack in the electrons quite densely. The workforce becomes rebellious, and smashes up the equipment, as if they were free radicals destroying
the fabric of the cell. With this degree of damage, it is only a matter of time before the factory has to close altogether.

Incidentally, it is worth noting that the rebelliousness of the workforce—the degree of damage to the equipment—would depend on the proportion of the time that the workforce is stressed and overworked to the point of rebelliousness. This depends on their workload, which in biological terms equates to metabolic rate. Animals with a fast resting metabolic rate, like rats, have a higher workload, and less spare capacity than mammals with a slow metabolic rate, like elephants. They therefore leak free radicals quickly—their workforce is rebellious much of the time—and suffer the penalty of a rapid accumulation of damage, a fast rate of ageing, and death. The same relationship applies to birds, except that, in this case, all birds have more space capacity than the equivalent mammals; small birds live longer than the equivalent mammals, but shorter than larger birds.

The idea of a rebellious workforce also helps to explain the benefits of calorie restriction, and many ‘longevity’ genes in nematode worms and fruit flies. In these cases, the alterations don’t affect the size of the workforce, but they may reduce the workload (lower the metabolic rate, and so increase spare capacity) or they might perhaps mollify the workers, so they become less rebellious, despite continuing to handle the same volume of work (no change in spare capacity). In this sense, the effect is like religion, which Marx referred to as the opium of the masses. To continue our analogy, the management policy to curb violence in the workforce might be to offer free opium. Either way, there are costs involved—the cost of a lower capacity for work, or of an opiated workforce. In biological terms, the costs of longevity genes are usually reflected in curtailed sexuality: a shift in resource use allows the metabolic rate to remain similar, but the cost of longevity is reduced fecundity.

Birds retain spare capacity in their mitochondria without any of these drawbacks—they have a high capacity for work without a penalty in fecundity. So how come birds retain so much spare capacity? I think the answer is that powered flight requires an aerobic capacity that outstrips anything that can be achieved by even the most athletic mammals. Just to get aloft, they require more mitochondria, and more respiratory chains. If they lose these mitochondria, they simultaneously lose the ability to fly, or to fly as skilfully. From the management strategy point of view, the factory work can
only
be accomplished by a large workforce, so there is really no choice: management can’t risk redundancies when the demand is low. So when birds are resting, their metabolic rate idles along and they have a large over-capacity. Technically, complex I is reduced to a lesser degree. Exactly the same reasoning applies to bats, too, which must also maintain a high aerobic capacity for powered flight.

Lest this sound theoretical, the hearts and flight muscles of birds and bats
do
have more mitochondria, and in them a greater density of respiratory chains,
than mammals; but do their other organs too? After all, it is the organs, not flight muscles, which contribute most to the resting metabolic rate, as we noted in
Part 4
. Surprisingly little is known about the number of mitochondria in the organs of birds and bats, but it’s feasible that they do have more mitochondria than do land-bound mammals, as the entire physiology of birds and bats is geared to maximal aerobic performance. To give just a single example, the number of glucose transporters in the intestine of a humming bird is far higher than in mammals, for they need to absorb glucose very quickly, to power their costly hovering flight. The extra transporters are powered by extra mitochondria. So the aerobic capacity of organs seemingly unrelated to flight is probably high too—far higher than needed to meet the low demands of resting metabolism.

Birds and bats are usually said to live long lives because flying away helps them avoid predators. No doubt there is truth in this, although many small birds have a high mortality rate in the wild, yet still live relatively long lives. The answer I’ve just suggested is tied in directly with the high-energy requirements of flight. High-energy expenditure demands a high density of mitochondria, not just in the flight muscles and heart but also in other organs too, to compensate. Such compensation is similar to that postulated for the origin of endothermy in the aerobic capacity hypothesis (see
Part 4
), but is stronger because the maximal aerobic demands of powered flight are greater than those of running, even at full stretch. Because the density of mitochondria is greater, the spare capacity at rest is greater, and this lowers the reduction state of complex I. Free-radical leakage is lowered by necessity, and this corresponds to a longer life.

So what happens in mammals other than bats: why don’t they maintain lots of spare capacity, in the form of high numbers of mitochondria? One reason might be that most mammals have little to gain by having more mitochondria and aerobic power—if threatened by predators their best policy is to scarper for the nearest hole. It is in the nature of things to use it or lose it. Rats are most likely to jettison unnecessary mitochondria as a costly burden, but this leads them straight back to the problem that they have fewer complexes and a higher reduction state of complex 1. They leak more free radicals, live fast and die young. Or do they?

If rats can’t gain much by hoarding more mitochondria in terms of their aerobic capacity, they ought to have another advantage: any rats that
did
hoard more mitochondria would have greater spare capacity, and so should live longer. Leaking fewer free radicals, they would not need to hoard more antioxidants or stress enzymes, and so should not be penalized under the terms of the disposable soma theory (see
page 278
). In fact, with such fine fettle—all those mitochondria, all that aerobic capacity—they should be physically impressive, sexually alluring to other rats, biologically ‘fit’, and so at an advantage in the
struggle to mate. The coupling of long lifespan with strong biological fitness means that their longevity genes should spread. But none of this has happened. Rats are still rats, and they still die quickly. Is there something else to it? I think there is, and it’s critically important to us, for if we wish to engineer ourselves a few genes that combine sexual allure with long life, we should know about the drawbacks.

The problem is this: having a low rate of free-radical leakage means that a more sensitive detection system is required to maintain respiratory efficiency. This after all, is why we have retained any genes in the mitochondria at all (see
page 141
). The costs of evolving greater refinement could explain why rats don’t restrict free-radical leakage. They have the cost of elaborating a sensitive detection system, as well as that of maintaining a lot of spare capacity. The combination of the two must have been too much for rats. In the case of birds, however, the high evolutionary costs of a more sensitive detection system are counterbalanced by the strong selective advantages of improved flight. Because flight is costly, yet pays high dividends, birds
do
benefit from packing more mitochondria in all their tissues, and so have greater superfluous capacity when at rest—they benefit from retaining a large workforce, and even investing some of the profits on the latest equipment. Their spare capacity translates into a lower free-radical leakage at rest, and a longer lifespan, but
requires
a more sensitive detection system. In this case, however, the advantages of flight do outweigh the costs in terms of survival and reproduction.

If we wish to live longer, then, and to rid ourselves of the diseases of old age, we will need more mitochondria, but also perhaps a more refined free-radical detection system. That may be problematic, and will no doubt tax the ingenuity of medical researchers. But our lifespan is already several times longer than that of equivalent mammals. If my reasoning is correct, we should already have more mitochondria than mammals with an equivalent resting metabolic rate: we should have more spare capacity, coupled with a sensitive free-radical detection system. In our own case, the sophistication was perhaps worth it for a different reason to the birds—not aerobic capacity, but for its own sake, for the benefits that longevity confers on social cohesion in kin groups. The elders of the tribe passed on knowledge and experience that gave their tribe a competitive edge; and they would even have been alluring into the bargain. Have we really done this? I don’t know, but it’s an interesting hypothesis, and easily testable. All we need to do is to measure mitochondrial density in the organs of mammals with a comparable metabolic rate, and, with a little more difficulty, to test the sensitivity of the free-radical signalling system.

There’s a tantalizing hint that we might be able to engineer longer lives in this way. Earlier on, in
Chapter 17
, I mentioned that a single letter change in the mitochondrial control region was five times more common in centenarians
than in a cross-section of the population. The mutation seems to stimulate slightly more mitochondrial genesis in response to a signal. So if a signal arrives to say ‘
Mitochondria: divide!
’, then people with the mutation may produce 110 new mitochondria, while those without would produce just 100. The effect might be for us to become a little more like the birds: we would have greater overcapacity when at rest. In principle, a similar effect could be achieved pharmacologically without modifying any genes at all, simply by amplifying each signal a little—so whenever a signal came for the mitochondria to divide, we could try to amplify it by, say, 10 per cent. In both cases, the extra mitochondria would share a lesser burden of work. The reduction state of the complexes would fall, and they would leak fewer free radicals. So long as we could still detect them well enough—a very delicate balancing act, but presumably the centenarians did—then we would have a good chance of living longer and better lives, less troubled by disease in our dying days.

More than a decade ago I spent a lot of time in the lab, trying to preserve kidneys for transplantation. The challenge was not related to rejection, the sexier end of research, but to a more pressing problem. As soon as a kidney, or any other organ, is removed from the body, the clock starts ticking—frantically. In the case of kidneys, decay renders the organ unusable within a couple of days. In the case of hearts, lungs, livers, and so on, time is even more pressing: they can’t be stored for much more than a day before they are wasted. The terrible prospect of rejection sharpens the problem. It is vital, literally, to match the immune profile of the donor organ to that of the recipient, to prevent acute rejection taking place on the operating table before your eyes. That often means transporting organs over a few hundred miles to a suitable recipient. Given the constant shortage of organs, any wastage is a crime. A stride forward in preservation, giving longer to locate the most suitable recipient, to arrange transport, and to mobilize the local transplant team, would waste fewer organs. Conversely, if we could work out exactly when an organ became unusable then we could salvage organs otherwise condemned as irretrievably damaged, for example, those taken from non-heart-beating donors.

It is practically impossible to tell, just by looking at a stored organ, whether or not it will function after transplantation, even if we take a biopsy and scrutinize it down the microscope. When an organ is removed from the body, the blood is flushed out using a carefully formulated solution, and the organ stored on ice. All looks well, but appearances can be deceptive. An apparently normal organ may become irreversibly damaged after transplantation. Paradoxically, this injury is thought to be caused by the return of oxygen. The storage period primes the organ for a disastrous loss of function upon transplantation, caused by oxygen free radicals escaping the mitochondrial respiratory chains.

One day I was in an operating theatre, fixing probes to a kidney during a transplant operation, in the hope of working out what was going on inside without physically taking a sample. The machine we were using was ingenious—a near infra-red spectrometer. It shines a beam of infra-red rays, which can penetrate several centimetres across biological tissues, and measures how much comes out the other side. From this, a complicated algorithm calculates how much radiation is absorbed or reflected on route, and how much passes through. The precise wavelength of infra-red radiation chosen is critical, as different molecules absorb different wavelengths. Choose your wavelength with care, and you can focus on haem compounds—those proteins that incorporate
a chemical entity known as a haem group, such as haemoglobin or cytochrome oxidase, the terminal enzyme of the respiratory chains, deep in mitochondria. Not only is it possible to work out the concentration of haemoglobin—both oxygenated and de-oxygenated forms—but you can calculate the redox state of cytochrome oxidase, which is to say you can work out what proportion of cytochrome molecules are in the oxidized versus the reduced state: what proportion is at that moment in possession of respiratory electrons. We paired this technique with a related form of spectroscopy that enabled us to work out the redox state of NADH, the compound that supplies the electrons entering into the respiratory chain. By combining the two techniques, we hoped to gain a dynamic idea of respiratory chain function in real time, without actually cutting into the kidney—obviously an immeasurable advantage during a major operation.