Mitochondrial Antioxidants as a Contributing Cause of Naked Mole-Rat Longevity

Naked mole-rats exhibit exceptional longevity in comparison to other rodent species. They can live nine times longer than similarly sized mice, for example. There are no doubt a sizable number of distinct mechanisms that contribute to this difference in species life span, and the existence of mammals with widely divergent life spans acts as a natural laboratory for researchers interested in better understanding aging. If one species lives a much longer life than another, then using their differences in order to identify the more important aspects of cellular metabolism in the matter of aging may well be a faster approach than other strategies that aim to reverse engineer the workings of aging. Thus research groups have been energetically investigating the biochemistry of naked mole-rats for many years now.

Naked mole-rats are exceptionally resistant to cancer, to the point at which for all of the populations maintained across the years in laboratories and zoos, only a few cases of cancer have ever been reported. Of late the ability of naked mole-rats to suppress cancerous mutations and cancerous cells has become one of the primary areas of study when it comes to their metabolic peculiarities. Avoiding death by cancer probably isn't one of the most important contributions to naked mole-rat longevity, however.

Instead, it seems likely that at least some of the major determinants of longevity relate to mitochondrial function and cellular resistance to oxidative damage. The horde of mitochondria in every cell act as power plants, but also as a source of oxidative molecules. These are generated as a byproduct of the energetic chemical reactions needed to package up the adenosine triphosphate (ATP) used as fuel for cellular processes. The presence of too many oxidative molecules are harmful to cells, and mitochondria themselves can be damaged by oxidative molecules in ways that contribute to aging. The situation is far from simple, however: oxidative molecules are used as signals for cellular maintenance, and thus small or brief increases are in fact beneficial. Further, antioxidant processes in mitochondria act to clean up much of the exhaust of new oxidative molecules. This is a complex, dynamic system of oxidants and reactions to oxidants that does not lend itself to easy predictions of outcomes.

The membrane pacemaker hypothesis suggests that the important factor in all of this, when considering differences between species, is the composition of cell membranes, particularly those of mitochondria. Different cell membrane lipids are more or less vulnerable to oxidative reactions and consequent functional damage. Species like naked mole-rats, with very high levels of all of the markers of oxidative stress, yet few to no apparent consequences, are perhaps a good argument for the membrane pacemaker way of looking at things. Equally, the research here makes a different argument - that this is all about the degree to which mitochondria can direct their own antioxidant processes to consume oxidizing molecules, and naked mole-rats are much better at this than mice. It is known that raising levels of mitochondrial antioxidants, either via gene therapy or by delivering artificial antioxidants that localize to mitochondria, appears to slow aging in a number of different species. The question, as always, is the size of any specific contribution to the overall outcome.

The exceptional longevity of the naked mole-rat may be explained by mitochondrial antioxidant defenses

Naked mole-rats (NMRs; Heterocephalus glaber, Rodentia) are mouse-sized eusocial mammals native to Eastern Africa that live in large subterranean colonies. Individuals of this species can live for longer than 30 years in laboratory conditions, and also exhibit a remarkably long health span; typical signs of senescence seen in old rodent are mostly absent in NMRs. Conversely, the common mouse (Mus musculus, Rodentia) lives less than 4 years and is highly susceptible to aging-related diseases and physiological decline. As a result, comparisons between these two species are considered to be a "gold standard" in mammalian studies of aging.

According to the oxidative stress theory of aging, senescence is caused by the gradual accumulation of oxidative damage to cells, inflicted by reactive oxygen species (ROS) of mitochondrial origin. However, previous comparative studies of NMR biology mostly provided evidence that contradicted this theory. For example, comparisons of isolated heart mitochondria found no difference in the rate of H2O2 efflux (i.e., the proportion of H2O2 not consumed by the mitochondrion before detection) between NMRs and mice. In addition, extensive oxidative damage and limited antioxidant capacity have been reported in the cytosol of NMR hepatocytes. Taken together, these findings led to the conclusion that the longevity of NMRs occurs independently of enhanced protection against oxidative damage, and this conclusion has been used repeatedly to refute the oxidative stress theory of aging.

More recently, however, the mitochondrial oxidative stress hypothesis of aging has gained empirical support; however, this hypothesis remains controversial, and has not yet been investigated in NMRs. This refined hypothesis stems from the fact that mitochondrial ROS are mostly released inside the mitochondrion (i.e., within the mitochondrial matrix), thereby directly exposing mitochondrial biomolecules to oxidative damage. According to the mitochondrial stress hypothesis, cellular senescence is primarily driven by loss of mitochondrial function with age. A central step toward testing this hypothesis would be to measure the balance between internal production and internal consumption of ROS within mitochondria themselves.

We have recently shown that traditional methodologies for detecting the rate of H2O2 formation from isolated mitochondria underestimate ROS generation because of the remarkable endogenous capacity of matrix antioxidants to consume H2O2. For example, this underestimation can reach 80% or more in rat skeletal muscle with certain respiratory substrates. Moreover, mitochondria can consume far more H2O2 than they generate; therefore, this capacity of mitochondria to consume H2O2 putatively represents a novel and widely underappreciated test of the mitochondrial oxidative stress theory of aging in of itself. We hypothesized that differences in the capacity of mitochondria to eliminate H2O2 might solve the apparent NMR oxidative stress/longevity-conundrum.

To test our hypothesis, we took advantage of antioxidant inhibition methods that we developed previously to measure H2O2 formation rates without the confounding influence of internal consumption. We also compared mitochondrial H2O2 clearance (i.e., maximal consumption) rates between these two species in functional isolated mitochondria. Our results support the mitochondrial oxidative stress hypothesis of aging via a mechanism that has not been previously demonstrated: NMRs and mice do not differ in their rate of H2O2 formation, but rather in the markedly greater capacity of NMR mitochondria to consume H2O2.