Decline in respiratory control ratio of rat liver mitochondria in old age

Mitochondrial dysfunction in aging and longevity: a causal or protective role?

SIGNIFICANCE

Among the most highly investigated theories of aging is the mitochondrial theory of aging. The basis of this theory includes a central role for altered or compromised mitochondrial function in the pathophysiologic declines associated with aging. In general, studies in various organisms, including nematodes, rodents, and humans, have largely upheld that aging is associated with mitochondrial dysfunction. However, results from a number of studies directly testing the mitochondrial theory of aging by modulating oxidant production or scavenging in vivo in rodents have generally been inconsistent with predictions of the theory.

RECENT ADVANCES

Interestingly, electron transport chain mutations or deletions in invertebrates and mice that causes mitochondrial dysfunction paradoxically leads to enhanced longevity, further challenging the mitochondrial theory of aging.

CRITICAL ISSUES

How can mitochondrial dysfunction contribute to lifespan extension in the mitochondrial mutants, and what does it mean for the mitochondrial theory of aging?

FUTURE DIRECTIONS

It will be important to determine the potential mechanisms that lead to enhanced longevity in the mammalian mitochondrial mutants.

A mild impairment of mitochondrial electron transport has sex-specific effects on lifespan and aging in mice

Impairments of various aspects of mitochondrial function have been associated with increased lifespan in various model organisms ranging from Caenorhabditis elegans to mice. For example, disruption of the function of the 'Rieske' iron-sulfur protein (RISP) of complex III of the mitochondrial electron transport chain can result in increased lifespan in the nematode worm C. elegans. However, the mechanisms by which impaired mitochondrial function affects aging remain under investigation, including whether or not they require decreased electron transport. We have generated knock-in mice with a loss-of-function Risp mutation that is homozygous lethal. However, heterozygotes (Risp(+/P224S)) were viable and had decreased levels of RISP protein and complex III enzymatic activity. This decrease was sufficient to impair mitochondrial respiration and to decrease overall metabolic rate in males, but not females. These defects did not appear to exert an overtly deleterious effect on the health of the mutants, since young Risp(+/P224S) mice are outwardly normal, with unaffected performance and fertility. Furthermore, biomarkers of oxidative stress were unaffected in both young and aged animals. Despite this, the average lifespan of male Risp(+/P224S) mice was shortened and aged Risp(+/P224S) males showed signs of more rapidly deteriorating health. In spite of these differences, analysis of Gompertz mortality parameters showed that Risp heterozygosity decreased the rate of increase of mortality with age and increased the intrinsic vulnerability to death in both sexes. However, the intrinsic vulnerability was increased more dramatically in males, which resulted in their shortened lifespan. For females, the slower acceleration of age-dependent mortality results in significantly increased survival of Risp(+/P224S) mice in the second half of lifespan. These results demonstrate that even relatively small perturbations of the mitochondrial electron transport chain can have significant physiological effects in mammals, and that the severity of those effects can be sex-dependent.

Osteoarthritis in the context of ageing and evolution. Loss of chondrocyte differentiation block during ageing

Ageing is the main risk factor of primary osteoarthritis (OA) and OA is the disease most strongly correlated with ageing. Both in humans and other animals OA development appears to be not strictly time-dependent but to hold pace with ageing processes. A characteristic of OA is deviant behaviour of chondrocytes in articular cartilage. These chondrocytes resemble terminal differentiated chondrocytes in growth plates and actively produce matrix degrading enzymes. The latter results in cartilage degeneration and eventually OA. We postulate that at a young age progression of chondrocyte differentiation is actively blocked in articular cartilage. This block declines when the evolutionary pressure to maintain this block, after reproductive life, is minimized. The loss of this differentiation block, maybe as a result of changes in chondrocyte TGF beta signalling, results in combination with normal joint loading in cartilage degeneration and OA.

Age-associated decline in mitochondrial respiration and electron transport in Drosophila melanogaster

The principal objective of the present study was to identify specific alterations in mitochondrial respiratory functions during the aging process. Respiration rates and the activities of electron transport chain complexes were measured at various ages in mitochondria isolated from thoraces of the fruit fly, Drosophila melanogaster, which consist primarily of flight muscles. The rates of state 3 respiration (ADP-stimulated), RCRs (respiratory control ratios) and uncoupled respiration rates decreased significantly as a function of age, using either NAD+- or FAD-linked substrates; however, there were no differences in state 4 respiration (ADP-depleted) rates. There was also a significant age-related decline in the activity of cytochrome c oxidase (complex IV), but not of the other mitochondrial oxidoreductases examined. Exposure of mitochondria isolated from young flies to low doses of KCN or NaAz (sodium azide), complex IV inhibitors, decreased cytochrome c oxidase activity and increased the production of H2O2. Collectively, these results support the hypothesis that impairment of mitochondrial respiration may be a causal factor in the aging process, and that such impairment may result from and contribute to increased H2O2 production in vivo.

Abnormal permeability of inner and outer mitochondrial membranes contributes independently to mitochondrial dysfunction in the liver during acute endotoxemia*

OBJECTIVE

This study was designed to determine the role played by the mitochondrial permeability transition in the pathogenesis of mitochondrial damage and dysfunction in a representative systemic organ during the acute phase of endotoxemia.

DESIGN

A well-established, normotensive feline model was employed to determine whether pretreatment with cyclosporine A, a potent inhibitor of the mitochondrial permeability transition, normalizes mitochondrial ultrastructural injury and dysfunction in the liver during acute endotoxemia.

SETTING

The Ohio State University Medical Center research laboratory.

SUBJECTS

Random source, adult, male conditioned cats.

INTERVENTIONS

Hemodynamic resuscitation and maintenance of acid-base balance and tissue oxygen availability were provided, as needed, to minimize the potentially confounding effects of tissue hypoxia and/or acidosis on the experimental results. Treatment groups received isotonic saline vehicle (control; n = 6), lipopolysaccharide (3.0 mg/kg, intravenously; n = 8), or cyclosporine A (6.0 mg/kg, intravenously; n = 6) or tacrolimus (FK506, 0.1 mg/kg, intravenously; n = 4) followed in 30 mins by lipopolysaccharide (3.0 mg/kg, intravenously). Liver samples were obtained 4 hrs posttreatment, and mitochondrial ultrastructure, function, and cytochrome c, Bax, and ceramide contents were assessed.

MEASUREMENTS AND MAIN RESULTS

As expected, significant mitochondrial injury was apparent in the liver 4 hrs after lipopolysaccharide treatment, despite maintenance of regional tissue oxygen availability. Namely, mitochondria demonstrated high-amplitude swelling and exhibited altered respiratory function. Cyclosporine A pretreatment attenuated lipopolysaccharide-induced mitochondrial ultrastructural abnormalities and normalized mitochondrial respiratory control, reflecting protection against inner mitochondrial membrane damage. However, an abnormal permeability of outer mitochondrial membranes to cytochrome c was observed in all lipopolysaccharide-treated groups and was associated with increased mitochondrial concentrations of Bax and ceramide.

CONCLUSIONS

These studies confirm that liver mitochondria are early targets of injury during endotoxemia and that inner and outer mitochondrial membrane damage occurs through different mechanisms. Inner mitochondrial membrane damage appears to relate to the mitochondrial permeability transition, whereas outer mitochondrial membrane damage can occur independent of the mitochondrial permeability transition. Preliminary evidence suggests that Bax may participate in lipopolysaccharide-induced outer mitochondrial membrane damage, but further investigations are needed to confirm this.

Impaired succinic dehydrogenase activity of rat Purkinje cell mitochondria during aging

The perikaryal Purkinje cell mitochondria positive to the copper ferrocyanide histochemical reaction for succinic dehydrogenase (SDH) have been investigated by means of semiautomatic morphometric methods in rats of 3, 12 and 24 months of age. The number of organelles/microm3 of Purkinje cell cytoplasm (Numeric density: Nv), the average mitochondrial volume (V) and the mitochondrial volume fraction (Volume density: Vv) were the ultrastructural parameters taken into account. Nv was significantly higher at 12 than at 3 and 24 months of age. V was significantly decreased at 12 and 24 months of age, but no difference was envisaged between adult and old rats. Vv was significantly decreased in old animals vs. the other age groups. In young and old rats, the percentage of organelles larger than 0.32 microm3 was 13.5 and 11%, respectively, while these enlarged mitochondria accounted for less than 1% in the adult group. Since SDH activity is of critical importance when energy demand is high, the marked decrease of Vv supports an impaired capacity of the old Purkinje cells to match actual energy supply at sustained transmission of the nervous impulse. However, the high percentage of enlarged organelles found in old rats may witness a morphofunctional compensatory response.

Age and temperature related changes in behavioral and physiological performance in the Peromyscus leucopus mouse

Age-related and ambient temperature-related changes in motor activity, body temperature, body weight (b.w.), and food consumption were studied in the long-lived Peromyscus leucopus mouse at environmental temperatures of 29 and 21 degrees C. Major changes in physiological performance were observed between the young (6 months) and old (60-72 month) age groups. The number of daily activity episodes, and total activity output was significantly lower in old mice. Maximum, average and minimum daily body temperature was lower in the old mice and a significant ambient temperature-by-age interaction was found. Maximum, minimum, and average daily b.w. was higher in old mice. Motor activity was evenly distributed over the active (night) phase in young mice but in old mice activity was significantly greater in the late night partition of the active cycle than in the early night partition. Both groups were significantly more active at night than during the day. Most of the food consumption in both groups occurred at night, but young mice consumed significantly more during the late night partition than the early night partition, and the consumption rates for old mice were not significantly different between early and late night partitions. The percentage of activity episodes involved with food consumption in both groups was significantly higher during the night partition, but the percentage during the early night partition was significantly higher in old mice than in young mice. Significant episodes of circadian torpor occurred in a high percentage of old mice at 06:00, on consecutive days, at both environmental temperatures, but young mice expressed no evidence of torpor.

Age-dependent decrease in the activity of succinic dehydrogenase in rat CA1 pyramidal cells: a quantitative cytochemical study

A computer-assisted morphometric study has been carried out on the ultrastructure of perikaryal CA1 pyramidal cell mitochondria positive to the copper ferricyanide cytochemical reaction for succinic dehydrogenase (SDH) in rats of 3, 12 and 23 months of age. The cytoplasmic volume fraction occupied by the positive mitochondria (Volume density: Vv), the number of organelles/micron 3 of CA1 pyramidal cell cytoplasm (Numerical density: Nv) and the average mitochondrial volume (V) were automatically calculated by means of computer-assisted morphometry. Vv was significantly decreased in 23-month-old animals versus the other age groups. Nv was unchanged between 3 and 12 months of age, but was decreased to a significant extent in old animals. V did not undergo significant changes in the three age groups taken into account. In the old animals the percent of organelles smaller than 0.16 micron 3 is above 20%, while in the young and adult groups the same size of mitochondria accounts for 7 and 3%, respectively. Thus, a reduction in the number of medium sized organelles appears to be responsible for the decrease in Vv due to age. Since SDH activity is known to support maximum rates of respiration, quantitative estimation of the active mitochondria provides information on the metabolic competence of the cells investigated when energy demand is high. In this context, our present findings document that a significant impairment in the efficiency to match actual energy provisions occurs in old CA1 pyramidal cells.

Susceptibility of ageing kidney to quinidine

Male Fischer-344 rats aged 3-4 months and 30-32 months were used in this study. Quinidine in vitro reduced accumulation of organic ions, p-ammonium hippurate (PAH) and tetraethyl ammonium (TEA), inhibited oxygen consumption and increased LDH leakage, in renal cortical slices. High concentrations of quinidine (2 and 3 mmol l-1) produced overt toxicity and no age related differences in any of the parameters measured were observed. But at lower concentrations significant age-related differences in kidney susceptibility to quinidine were evident. Administration of 75 mg kg-1 day-1 quinidine for 4 days caused exacerbated renal damage in senescent rats compared to young adults as demonstrated by greater elevation of blood urea nitrogen and serum creatinine, and greater inhibition of TEA uptake in renal cortical slices. These results establish significant age related differences in renal damage due to quinidine.

Evidence for and against the causal involvement of mitochondrial DNA mutation in mammalian ageing

Current experimental evidence on the role of mitochondrial DNA mutation in ageing is assessed alongside reports implicating other genetic and non-genetic causes, including inter-relationships between the mitochondrial and nuclear genomes and their potential effect on mitochondrial structure and function. The role of a 5-kb mtDNA deletion, identified as age-dependent in a variety of human and other mammalian species, is specifically evaluated in the context of its functional effect in mitotic and non-mitotic adult tissue. Downstream effects of mitochondrial decline are considered in terms of the maintenance of ATP production. Associated sequelae then are discussed specifically with reference to restrictions in the supply of ribose moieties for DNA and RNA synthesis, and to disruption of NADPH production and hence cellular anti-oxidant defences.

Cardiolipins and biomembrane function

Evidence is discussed for roles of cardiolipins in oxidative phosphorylation mechanisms that regulate State 4 respiration by returning ejected protons across and over bacterial and mitochondrial membrane phospholipids, and that regulate State 3 respiration through the relative contributions of proteins that transport protons, electrons and/or metabolites. The barrier properties of phospholipid bilayers support and regulate the slow proton leak that is the basis for State 4 respiration. Proton permeability is in the range 10(-3)-10(-4) cm s-1 in mitochondria and in protein-free membranes formed from extracted mitochondrial phospholipids or from stable synthetic phosphatidylcholines or phosphatidylethanolamines. The roles of cardiolipins in proton conductance in model phospholipid membrane systems need to be assessed in view of new findings by Hübner et al. [313]: saturated cardiolipins form bilayers whilst natural highly unsaturated cardiolipins form nonlamellar phases. Mitochondrial cardiolipins apparently participate in bilayers formed by phosphatidylcholines and phosphatidylethanolamines. It is not yet clear if cardiolipins themselves conduct protons back across the membrane according to their degree of fatty acyl saturation, and/or modulate proton conductance by phosphatidylcholines and phosphatidylethanolamines. Mitochondrial cardiolipins, especially those with high 18:2 acyl contents, strongly bind many carrier and enzyme proteins that are involved in oxidative phosphorylation, some of which contribute to regulation of State 3 respiration. The role of cardiolipins in biomembrane protein function has been examined by measuring retained phospholipids and phospholipid binding in purified proteins, and by reconstituting delipidated proteins. The reconstitution criterion for the significance of cardiolipin-protein interactions has been catalytical activity; proton-pumping and multiprotein interactions have yet to be correlated. Some proteins, e.g., cytochrome c oxidase are catalytically active when dimyristoylphosphatidylcholine replaces retained cardiolipins. Cardiolipin-protein interactions orient membrane proteins, matrix proteins, and on the outerface receptors, enzymes, and some leader peptides for import; activate enzymes or keep them inactive unless the inner membrane is disrupted; and modulate formation of nonbilayer HII-phases. The capacity of the proton-exchanging uncoupling protein to accelerate thermogenic respiration in brown adipose tissue mitochondria of cold-adapted animals is not apparently affected by the increased cardiolipin unsaturation; this protein seems to take over the protonophoric role of cardiolipins in other mitochondria. Many in vivo influences that affect proton leakage and carrier rates selectively alter cardiolipins in amount per mitochondrial phospholipids, in fatty acyl composition and perhaps in sidedness; other mitochondrial membrane phospholipids respond less or not at all.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparative study of succinate-supported respiration and ATP/ADP translocation in liver mitochondria from adult and old rats

This study was undertaken to compare the rates of succinate-supported hepatic mitochondrial respiration between 12 months (adult) and 29 months (old) male Fischer 344 rats. Experiments were also performed to determine the activity of adenine nucleotide translocase and the effect of its inhibition on mitochondrial respiration. Succinate-supported state 3 mitochondrial respiration was found to decline 20% between 12 and 29 months of age in rat liver, along with a similar 25% decrease in the respiratory control ratio with age. Adenine nucleotide translocase activity is shown to decrease 39% from adult to old rat liver mitochondria. This decrease does not, however, account for the decline in state 3 respiration, since translocase activity is approximately 50% greater than state 3 respiration in both adult and old rats. Therefore, adenine nucleotide translocase is not rate-limiting for state 3 mitochondrial respiration. Neither the rate of succinate permeation into the mitochondrial nor the rate of electron transport is rate-limiting for state 3 respiration, indicated by the greatly increased oxygen consumption with addition of the uncoupler carbonyl cyanide m-chlorophenyl hydrazone (m-CCCP). These processes, therefore, are not responsible for the observed decline in state 3 respiration. The implications and possible cause of the age-related decrease in the maximal rate of ATP-synthesis are discussed.

Oxidative phosphorylation in mouse liver mitochondria during weaning

Oxidative energy metabolism in mouse liver mitochondria was examined during weaning using different substrates. During the post-natal development, from suckling to weanling stage, the respiration rates showed a temporary decrease. These altered levels recovered in the adults. Respiration rates with the three substrates studied namely, glutamate, beta-hydroxybutyrate and succinate showed the same pattern. However, the levels of primary dehydrogenase as well as that of basal adenosine triphosphatase were not significantly altered during weaning. The content of cytochromes aa3 and b significantly decreased during this period. The results indicate that cytochrome aa3 may be of primary importance in the restoration of full respiratory function in mitochondria after the weaning period.

Bioenergetics in the aging Fischer 344 rat: effects of exercise and food restriction

The capacity for energy production was evaluated in male, Fischer 344 rats as they advanced from adulthood through senescence. At 10 months of age, the animals were divided into three groups: sedentary, fed ad libitum (S); exercised by treadmill running, fed ad libitum (E); and sedentary, caloric restricted by alternate day feeding (R). Activities of selected enzymes, ADP-stimulated respiration and levels of cytochromes, were determined in homogenates of liver and gastrocnemius muscle prepared from young controls (10-month old S) and 18-, 24-, and 30-month old animals. In liver, age-linked decrements were found in the activities of 3-hydroxyacyl-CoA dehydrogenase (S, E, and R) and citrate synthase (S), and in cytochrome c content (S and E), whereas substrate-catalysed oxidations were unaffected. In the gastrocnemius muscle (S, E, and R), respiration, activities of enzymes of the Krebs cycle and glycolysis, and cytochrome content were decreased after the age of 18 months. Oxidative capacity was increased in muscle through exercise (about 40%) and in liver by food restriction (about 20%). Body and soleus muscle mass declined similarly in all groups (about 14% from 30 to 18 months of age), whereas the loss of weight in the gastrocnemius muscle was much greater (34%). The data indicate that energy metabolism in the senescent animal is competent to meet its needs and age-related declines in energy metabolism are secondary to the aging process.

Does mitochondrial ATP synthesis decline as a function of change in the membrane environment with ageing

Cardiac mitochondria isolated from rats fed purified diets of known lipid composition demonstrate a decline in the rates of oxidative phosphorylation, ATP synthesis and oligomycin sensitive ATPase activity as the animal ages. Transitions in these lipid-dependent functions of mitochondrial energy metabolism are concomitant with changes in the lipid environment of the mitochondrial membrane.