The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

92
The Mitochondrial Free Radical Theory of Aging
mutant ones that sparked the replication pulse in the first place. A remarkably neat idea;
and moreover, it was recently shown by direct visualisation 9-11 that, at least in vitro, mtDNA
replication indeed occurs mainly around the nucleus. Unfortunately, though, this proposal
is fraught with statistical difficulties. Firstly, such amplification could not begin until quite
a lot of mutant mitochondria had arisen without selection, all of which would be equally
amplified thereafter; this seems incompatible with the apparent takeover of cells by copies
of a single mutation. Secondly, once there are very many mutant mitochondria, their random
movements will tend more and more exactly to cancel each other out, so that variations in
the energy supply near the nucleus would become negligible, as therefore would the replicative
advantage of mutant mitochondria. Finally, once there are debilitatingly few respiring
mitochondria, the cell would be replicating mitochondria at full speed all the time, irrespective
of mitochondrial distribution (because there would be a perpetual ATP shortage in the
nucleus), so mutants would have no advantage at all.
8.3. A Clue From Yeast
I mentioned in Section 6.3 that suppressiveness, the dominant deficiency of respiration
discovered by Ephrussi in the 1950s, 12 was much later to become a clue to MiFRA. It will
already be clear that there are similarities between suppressiveness in yeast and amplification
of mutant mtDNA in humans. But the big mechanistic clue from suppressiveness turned up
in the 1980s, 13 and was only identified as such in 1996. It was a study of the rate of mtDNA
replication in the context of suppressiveness. The experimenters found that, for most
suppressive strains, the heteroplasmic diploid cells made by fusion of a suppressive cell with
a wild-type one did not replicate their mutant mtDNA any more often than the normal
mtDNA.
This was, with hindsight, a sensational result. If, as one would naturally predict, this
characteristic of suppressiveness was also true of human cells that amplified mutant mtDNA,
then the search for a mechanism in human cells was starting from a false assumption: that
preferential amplification resulted from preferential replication.
The same paper 13 also reported an observation which supported the hypothesis that
suppressiveness and human mtDNA decline were driven by similar mechanisms. This was
that starvation of the culture in which heteroplasmic cells were formed, causing a temporary
suspension of cell division, markedly increased the levels of suppressiveness which the culture
then exhibited, independent of genetic factors. This bore a striking similarity to the finding
that non-dividing human cells are much more susceptible to mtDNA decline than dividing
ones 14 (see Section 2.4.6). Moreover, suppressiveness had also been found 15,16 in Neurospora
and Podospora, filamentous fungi which, like a muscle fiber, are syncytial. A subsequent
observation 17 fitted the same scheme: that even amoeba exhibit suppressiveness if they are
kept with the cell cycle arrested for a time.
8.4. Proton Leak: Boon or Bane?
Oxidative phosphorylation relies, as explained in Section 2.3.4, on the inner mitochondrial
membrane’s being impermeable to protons. But of course this is not an all-or-nothing condition:
if the membrane is just slightly leaky, so that the occasional proton gets through it, the only
harm done is that the respiratory chain must work a little harder in order to make the required
amount of ATP. Membranes are, after all, only a few nanometers thick—and indeed they have
to be that thin, since (among other reasons) otherwise they would be dysfunctionally rigid—so
it is reasonable to expect a trade-off between impermeability and other membrane properties.
And in fact, mitochondrial membranes do exhibit an easily measurable amount of this
“proton leak.” 18 Research has been done on it for many years, but its biological role is still
highly controversial. It has been popular 19 to seek reasons why proton leak is valuable to the