The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
Direct visualization of fused mitochondria (“megamitochondria”) has been widely
reported in many cell types: in some, such as muscle, they form a reticulated network that
appears to span large regions of a fiber, 36 whereas in others, such as kidney, there is an
intermediate situation in which mitochondria form filamentous* chains but not more
complex networks. 37 However, this is not always present: generally mitochondria appear as
separate, small ellipsoids, and the reticulation appears to be induced under conditions of
high oxidative stress. 38 The reason for this behaviour is unknown, but it has been suggested
that ATP-synthesising proton-motive force can be transmitted along such filaments. 39
We must be cautious, however, because this visualisation does not tell us that fusion is
complete, with the matrix compartments being merged: it is possible that, for example,
only the outer membranes fuse while the inner membranes remain separate. Evidence that
something like this is indeed the case comes from studies of the mitochondrial proton
gradient using the dye JC-1, 40,41 in which different parts of a filamentous mitochondrion
exhibit clearly distinct fluorescence characteristics. A single aqueous compartment is always
at uniform electrochemical potential throughout (except when there is a continuous current
within it, as will be discussed in Section 11.3.7), due to the fantastically rapid conduction
of protons in water, so the apparent heterogeneity of proton gradient indicates that the
mitochondrion is not fully fused. The phenomenon could, alternatively, be because JC-1
fluorescence is affected by unidentified factors other than the proton gradient, but
the face-value interpretation is that the matrix compartments are still separate.
Electron-microscope visualisation of such filaments also suggests that their matrix
compartment is not continuous, 42 and the electron-microscope evidence with regard to
muscle-fiber reticulations is even more unambiguous. 36,43
A completely different way to assay for mitochondrial fusion is by genetic means. The
most compelling experiments showing that it definitely does happen are quite recent. 44 Cell
lines were constructed which had mitochondria carrying a particular mutation, and these
were fused to other cells whose mitochondria carried a different mutation, such that
mitochondria carrying DNA of both types would be able to make all their proteins but the
rest would not (or not nearly so rapidly). Initially no mitochondria had both types of mtDNA,
because that was how the progenitor cell lines started out; but the fusion cells were rapidly
taken over by mitochondria which were capable of OXPHOS, and which thus had both
types. This could not have come about other than by complete mitochondrial fusion—not
simply fusion of the outer membranes.
Again, however, there is a difficulty in translating these results to the in vivo situation.
These experiments applied such enormous selective pressure in favour of the descendants
of fusion events that they do not tell us how common the events are: even if they are quite
rare, their descendants will rapidly take over the culture. One assay 44 appeared to show that
fusion was very common, because it demonstrated the sharing across all mitochondria of a
protein that only half the mtDNA present could encode; but this was contradicted by another
assay in the same study 44 which indicated that a mutation on one of the mtDNA species,
which was known to be recessive, behaved as a dominant mutation. These results could
both be explained if a small amount of mtDNA recombination occurs following fusion
events. mtDNA recombination certainly happens occasionally, though probably only as an
“accidental” side-effect of replication and/or transcription—we known this from the
occurrence of mtDNA deletions flanked by direct repeats (see Section 2.4.5), and also because
of the detectability of double- and triple-length mtDNA molecules, which has been known
* The filamentous morphology has been known since the earliest days of the study of mitochondria—indeed,
it gives them their name, which is derived from the Greek mitos, meaning thread.