Mitochondrial DNA Mutations, Apoptosis, and the Misfolded Protein Response

REJUVENATION RESEARCH Volume 8, Number 4, 2005. JUSTIN L. MOTT,* DEKUI ZHANG,* and HANS PETER ZASSENHAUS

ABSTRACT
Studies of mice with accumulated mtDNA (Mitochondrial DNA) mutations in the heart lead us to propose that apoptotic signaling and cell death is central to the pathogenesis of mtDNA mutations in aging. It is the cellular response to that apoptotic signaling and the organ’s compensatory response to a loss of cells that specify the phenotype of an accumulation of mtDNA mutations. In the heart, cardiomyocytes induce a vigorous anti-apoptotic, pro-survival response to counteract mitochondrial apoptotic signaling. The heart up-regulates contractility of remaining myocytes in order to maintain cardiac output. We hypothesize that mutant mitochondrial proteins originate apoptotic signaling by interacting with proteins already in place in the mitochondrial outer membrane that regulate apoptosis, for example the pro-apoptotic protein Bak. Since it is unlikely that all mutant mitochondrial proteins have the necessary structure and localization within the inner membrane to activate Bak appropriately, only a small fraction of an age-associated burden of mtDNA mutations may be pathogenic. In this model, reactive oxygen species generated by mitochondrial respiration drive the formation of mtDNA mutations, but are not the primary mechanism for their pathogenicity.

Summary
MITOCHONDRIAL DNA (mtDNA) mutations accumulate with age and disease. These sporadic mutations randomly affect the mitochondrial genome, and thus a mutation at any particular nucleotide position is rare. Even with advanced age, the frequency of mutations generally does not exceed 1% in otherwise normal individuals. Thus, for these mutations to cause disease or senescence, the cell needs to be very sensitive to mtDNA mutations. Recent evidence suggests that the burden of point mutations in mtDNA may be higher than previously thought, so that estimates extrapolate to an average of three point mutations per genome in the aged brain. Still, given the random distribution of those mutations and the multiple copies of mtDNA per mitochondrion, their effect on respiratory function would be expected to be slight.

Despite their low frequency, mtDNA mutations are still more common than nuclear DNA mutations. Because of the compact nature of the mitochondrial genome, these mutations are also more likely to affect a coding region. Due to heterogeneity, the mutation level may be higher in a given cell than the overall average of the tissue. At the next level of compartmentalization, the mitochondrion itself will have an even higher proportion of mutant genomes. Consider a cell that has only a single mutation on a background of maybe 1000 wildtype mitochondrial genomes. The tissue mutation level may be undetectable, the cellular mutation level would be 1/1000 genomes. But the level of the mutation in the affected mitochondrion would be 10–50% (given two to 10 genomes in this prototypical mitochondrion). If mtDNA mutations contribute to disease, the signaling mechanism must sense the high proportion of mutation within a mitochondrion, not the low proportion of a cell or tissue. The central tenet of the mitochondrial theory for aging is that those signals are reactive oxygen species (ROS), natural byproducts of mitochondrial respiration. The ensuing oxidative damage mutates mtDNA, leading in turn to even more ROS because of malfunctioning respiratory enzyme complexes, all of which, with the exception of Complex II, contain protein subunits encoded within mtDNA. The “vicious cycle” of ever increasing frequencies of mtDNA mutations and oxidative damage ultimately causes cellular senescence, in part because of accruing oxidative damage and in part because of declining mitochondrial oxidative phosphorylation. We argue here that apoptotic signaling is the primary pathogenic mechanism of mtDNA mutations in aging. It is now well established that mitochondria play a central role in amplifying cellular apoptotic signals within the intrinsic pathway for apoptosis. It is within mitochondrial membranes where interactions take place among pro- and anti-apoptotic proteins such as Bak, Bax, and Bcl2 to regulate outer membrane permeabilization and cytochrome c release, unleashing the caspase cascade culminating in apoptosis. We propose that mutant mitochondrially encoded proteins tap into that machinery already in place so as to originate apoptotic signals. At any one moment, most cells sensing those signals suppress execution of apoptosis by up-regulation of antiapoptotic proteins. Nevertheless, some cells succumb. Finally, we will present a hypothesis bearing on the molecular mechanism(s) for how mutant proteins might originate apoptotic signaling—a process we view as a mitochondrial misfolded protein response.