Experimental evidence against the mitochondrial theory of aging. A study of isolated human skeletal muscle mitochondria

Sarcopenia of aging and its metabolic impact

Sarcopenia contributes significantly to the morbidity, decrease in quality of life, and health care costs in the elderly. It is characterized by a decrease in muscle mass and strength, starting as early as the fourth decade of life in humans. Distinct muscle changes include a decrease in type 2 muscle fibers and a decrease in myosin heavy chains IIa and IIx mRNA levels. In addition, a decrease in whole body protein turnover, mixed muscle protein synthesis, myosin heavy chain synthesis, and mitochondrial protein synthesis have been reported. Different tissues and organs display different responses to aging, with more oxidative tissue generally having more age-related changes. Exercise has been shown to increase strength, aerobic capacity, and muscle protein synthesis, as well as to increase muscle mitochondrial enzyme activity in both young and older people; however, exercise does not reverse all age-related changes. The metabolic effects of sarcopenia include a decrease in resting metabolic rate secondary to decreased fat-free mass and decreased physical activity, leading to a higher prevalence of insulin resistance, type 2 diabetes mellitus, dyslipidemia, and hypertension. The way in which age-related changes in hormone levels affect muscle remains to be fully understood. The effect of replacing those hormones on sarcopenia has led to some conflicting results, but further investigations are ongoing.

Due to reverse electron transfer, mitochondrial H2O2 release increases with age in human vastus lateralis muscle although oxidative capacity is preserved

Age-related changes in mitochondrial H2O2 release (MHR) could be responsible for an increase in oxidative stress in skeletal muscle and participate in the development of sarcopenia. We compared MHR in vastus lateralis biopsies obtained from young (23.5+/-2.0 year, n=6) and elderly (67.3+/-1.5 year, n=6) healthy sedentary men. Isolated mitochondria were incubated in the presence of glutamate/malate/succinate, with or without rotenone. Muscle fat oxidative capacity, citrate synthase, complex II, complex III, and cytochrome c oxidase activities were also measured. In parallel, we analyzed in gastrocnemius of young male Wistar rats (n=6), the impact of lidocaine (local anesthetic used in humans) on mitochondrial respiration and MHR. In humans, muscle oxidative capacity was preserved with age but muscle MHR was markedly enhanced in elderly subjects compared to young adults (+175%, P<0.05). Rotenone abolished this increase, demonstrating that it was due to a free radical release during reverse electron transfer from complex II towards complex I. Lidocaine can interfere with MHR measurements (intra-muscular injection in rats) but it can be avoided by minimizing contact with muscle (small multiple subcutaneous injections in humans). Physiologic consequences of the observed increase in muscle MHR with aging remain to be determined.

Skeletal muscle bioenergetics: a comparative study of mitochondria isolated from pigeon pectoralis, rat soleus, rat biceps brachii, pig biceps femoris and human quadriceps

The metabolism of mitochondria isolated from five functionally different skeletal muscles is compared. Data for a single ectothermic preparation are also reported. The mitochondria were prepared in yields of 44+/-7% from 50 to 100 mg muscle. The muscle content of mitochondrial protein ranged between 2 and 40 g kg(-1). Twelve specific activities of key enzymes and metabolic systems were determined, 10 of these in functional assays with respiratory measurements. The specific activities of glutamate dehydrogenase, alpha-glycerophosphate dehydrogenase, and exo-NADH oxidase differed considerably among muscle sources. Seven specific activities, including very central reactions, showed low among-muscle variation. The activity of ATP synthesis, for instance, was 1.0-1.3 mmol min(-1) g(-1) mitochondrial protein, 25 degrees C. In vitro data were extrapolated to in vivo conditions of the muscles. The calculated rates of respiration and ATP synthesis were in accordance with reported tissue activities. Pigeon pectoralis mitochondria showed a unique cytochrome spectrum and a respiratory chain activity that might effect simultaneous carbohydrate and fatty acid respiration. In mitochondria from the other muscles, the respiratory chain activity balanced the carbohydrate oxidation capacity. In all muscles, the respiratory capacity exceeds that needed for oxidative phosphorylation. This may secure maximal mitochondrial ATP synthesis during maximal work rates and high cellular [Ca(2+)].

Mitochondrial DNA and aging

Among the numerous theories that explain the process of aging, the mitochondrial theory of aging has received the most attention. This theory states that electrons leaking from the ETC (electron transfer chain) reduce molecular oxygen to form O2•− (superoxide anion radicals). O2•−, through both enzymic and non-enzymic reactions, can cause the generation of other ROS (reactive oxygen species). The ensuing state of oxidative stress results in damage to ETC components and mtDNA (mitochondrial DNA), thus increasing further the production of ROS. Ultimately, this ‘vicious cycle’ leads to a physiological decline in function, or aging. This review focuses on recent developments in aging research related to the role played by mtDNA. Both supportive and contradictory evidence is discussed.

Falsifying falsifications: the most critical task of theoreticians in biology

Occasionally, experimental biologists obtain results which mystify them so deeply that the paradoxical nature of their finding is acknowledged in the paper reporting it. This constitutes a more-or-less explicit invitation to those who did not perform the experiments - and even those who do not perform experiments at all - to propose explanations that eluded the experimenter. A much more frequent scenario, however, is that the experimenter asserts confidently that his or her data can be explained by a particular model but are at odds with some other model. In such circumstances, it is often overlooked that the stated falsification of the latter model is error-prone: just as the mystified experimenter saw no explanation when in fact there is one, the other experimenter may see only one explanation of the data when there are two. The main reason this phenomenon is neglected is, of course, the fact that here the theoretician (or other experimenter) must take the initiative in critiquing a conclusion that, far from troubling the experimenter, may by the time of its publication be a cornerstone of his or her research program, so whose refutation may be decidedly unwelcome. For precisely this reason, such critiques - especially, perhaps, when they come from those who do not do bench work at all and thus have a complementary approach to the analysis of data - are fundamental to maximising the rate of progress in fields of biology that otherwise risk languishing in ever-better-studied cul-de-sacs for many years. Computational biology, including simulation, plays an especially important role in this, whereas its ability to contribute to biology in other ways is often less than its proponents claim. Here I discuss some representative examples of falsification-falsification, including a previously unpublished analysis of mitochondrial DNA population dynamics in cell culture, in the hope of stimulating more theoreticians - and perhaps also more experimentalists - to engage in it.

Is defective electron transport at the hub of aging?

The bulwark of the mitochondrial theory of aging is that a defective respiratory chain initiates the death cascade. The increased production of superoxide is suggested to result in progressive oxidant damage to cellular components and particularly to mtDNA that encodes subunits assembled in respiratory complexes. Earlier studies of respiration in muscle mitochondria obtained from large cohorts of patients supported this notion by showing that either singly or in combinations, the respiratory complexes exhibited decreased activity in the elderly. The following critique of the most cited publications over the past decade points out the systematic errors that put earlier work at odds with recent findings. These later investigations indicate that aging has no overt effect on either the electron transport system or oxidative phosphorylation.

Mitochondria, reactive oxygen species and longevity: some lessons from the Barja group

To demonstrate that an uncoupling of respiration and phosphorylation, measured in vitro, reflects an in vivo situation, we badly need in vivo measurements of some uncoupling-linked parameters. The importance of this assertion is illustrated by studies of Barja and co-workers. A lower rate of H(2)O(2) production by mitochondria isolated from long-lived birds compared with short-lived mammals of the same body weight (see publications by Barja's and Sohal's groups) could be explained by (i) an in vivo difference or (ii) an in vitro artefact. In both cases, the reason for lower H(2)O(2) production may well be the same, i.e. a mild uncoupling of respiration in avian mitochondria showing lowered respiratory control. Again, this should be due to an in vivo operation of some bird-specific natural uncouplers (the first case) or stronger in vitro damage to the avian mitochondria during their isolation and incubation (the second). The latter possibility seemed more probable when Barja and co-workers revealed that the level of antioxidants in birds is lower than in mammals. However, further studies by the same group showed that the degree of unsaturation of fatty acids in birds is lower than in mammals, indicating a greater resistance of avian mitochondria to oxidative damage in vitro. Indeed, it was found that lipid peroxidation in isolated avian mitochondria occurs at a much lower rate than in mammals. More importantly, the in vivo level of peroxidation of lipids and proteins appears to be lower in birds than in mammals. Thus, it seems probable that longer lifespan of birds really does correlate with a slower rate of production of H2O2 by mitochondria in vivo.

Skeletal muscle mitochondrial protein metabolism and function in ageing and type 2 diabetes

PURPOSE OF REVIEW

Mitochondria are the site of oxidative substrate utilization to produce adenosine triphosphate for normal tissue function. Tissue substrate utilization is impaired in ageing and type 2 diabetes. Defects in mitochondrial gene expression, protein synthesis and function occur with ageing in various tissues including skeletal muscle, and are emerging in individuals with type 2 diabetes. The current review will discuss advances in the understanding of skeletal muscle mitochondrial alterations associated with age and type 2 diabetes.

RECENT FINDINGS

Insulin acutely stimulates skeletal muscle mitochondrial protein synthesis and adenosine triphosphate production. These insulin effects are impaired in insulin-resistant patients with type 2 diabetes who also exhibit defective basal muscle mitochondrial function. The age-related reduction in mitochondrial adenosine triphosphate production has been confirmed in vivo in skeletal muscle in humans and rodents.

SUMMARY

The emerging concept that insulin stimulates mitochondrial protein synthesis and function indicates potential novel molecular mechanisms of metabolic defects in type 2 diabetes, particularly in the post-prandial period characterized by acute increments of plasma insulin concentrations. The potential relationship between insulin resistance and basal post-absorptive muscle mitochondrial defects should be further investigated. As ageing is characterized by insulin resistance, the hypothesis that impaired insulin action could contribute to age-related muscle mitochondrial dysfunction, and metabolic alterations should be addressed.