Reversal of the Mitochondrial Phenotype and Slow Development of Oxidative Biomarkers of Aging in Long-lived Mclk1+/? Mice

Although there is a consensus that mitochondrial function is somehow linked to the aging process, the exact role played by mitochondria in this process remains unresolved. The discovery that reduced activity of the mitochondrial enzyme CLK-1/MCLK1 (also known as COQ7) extends lifespan in both Caenorhabditis elegans and mice has provided a genetic model to test mitochondrial theories of aging. We have recently shown that the mitochondria of young, long-lived, Mclk1^+/− mice are dysfunctional, exhibiting reduced energy metabolism and a substantial increase in oxidative stress. Here we demonstrate that this altered mitochondrial condition in young animals paradoxically results in an almost complete protection from the age-de pend ent loss of mitochondrial function as well as in a significant attenuation of the rate of development of oxidative biomarkers of aging. Moreover, we show that reduction in MCLK1 levels can also gradually prevent the deterioration of mitochondrial function and associated increase of global oxidative stress that is normally observed in Sod2^+/− mutants. We hypothesize that the mitochondrial dysfunction observed in young Mclk1^+/− mutants induces a physiological state that ultimately allows for their slow rate of aging. Thus, our study provides for a unique vertebrate model in which an initial alteration in a specific mitochondrial function is linked to long term beneficial effects on biomarkers of aging and, furthermore, provides for new evidence which indicates that mitochondrial oxidative stress is not causal to aging.Because it is well known that the aging process is characterized by declines in basal metabolic rate and in the general performance of energy-dependent processes, many aging studies have focused on mitochondria because of their central role in producing chemical energy (ATP) by oxidative phosphorylation (1). Among the various theories of aging that have been proposed, the mitochondrial oxidative stress theory of aging is the most widely acknowledged and studied (2–4). It is based on the observation that mitochondrial energy metabolism produces reactive oxygen species (ROS),² that mitochondrial components are damaged by ROS, that mitochondrial function is progressively lost during aging, and that the progressive accumulation of global oxidative damage is strongly correlated with the aged phenotype. However, the crucial question of whether these facts mean that mitochondrial dysfunction and the related ROS production cause aging remains unproven (5–7). Furthermore, recent observations made in various species, including mammals, have begun to directly challenge this hypothesis, notably by relating oxidative stress to long (8) or increased (9) lifespans, by demonstrating that overexpression of the main antioxidant enzymes does not extend lifespan (10) as well as by showing that mitochondrial dysfunction could protect against age-related diseases (11).A direct and powerful approach to attempt to clarify this major question and to test the theory is to characterize the mitochondrial function of long-lived mutants (12). CLK-1/MCLK1 is an evolutionary conserved protein (13) and has been found to be located in the mitochondria of yeast (14), worms (15), and mice (16). The inactivation of the Caenorhabditis elegans gene clk-1 substantially increases lifespan (17). Moreover, the elimination of one functional allele of its murine orthologue also resulted in an extended longevity for Mclk1^+/− mice in three distinct genetic backgrounds (18). These findings have provided for an evolutionarily conserved pathways of animal aging that is affected by the function of a mitochondrial protein (19, 20). In mitochondria CLK1/MCLK1 acts as an hydroxylase and is implicated in the biosynthesis of ubiquinone (coenzyme Q or UQ), a lipid-like molecule primarily known as an electron carrier in the mitochondrial respiratory chain and as a membrane antioxidant but which is also associated with an increasing number of different aspects of cellular metabolism (20, 21). Taken together, these observations indicate that the long-lived Mclk1^+/− mouse is a model of choice for the understanding of the links between mitochondrial energy metabolism, oxidative stress, and the aging process in mammals.Previous analysis of Mclk1^+/− mice, which show the expected reduction of MCLK1 protein levels (22), have revealed that their tissues as well as their mitochondria contain normal levels of UQ at 3 months of age (23). Yet the same study also revealed a host of phenotypes induced by Mclk1 heterozygosity (see below). Thus, it appears that MCLK1 has an additional function that is unrelated to UQ biosynthesis but responsible for the phenotypes observed in young Mclk1^+/− mutants. This is consistent with several results from nematodes which also strongly suggest that CLK-1 has other functions (24, 25).In depth characterization of the phenotype of young Mclk1^+/− mutants has revealed that the reduction of MCLK1 levels in these animals profoundly alters their mitochondrial function despite the fact that UQ production is unaffected (23). In fact, we have shown that Mclk1 heterozygosity induces a severe impairment of mitochondrial energy metabolism as revealed by a reduction in the rates of mitochondrial electron transport and oxygen consumption as well as in ATP synthesis and ATP levels in both the mitochondria and the whole cell. ATP levels in several organs were surprisingly strongly affected with, for example, a 50% reduction of overall cellular ATP levels in the livers of Mclk1^+/− mutants (23). Moreover, we have found that the Mclk1^+/− mice sustain high mitochondrial oxidative stress by a variety of measurements, including aconitase activity, protein carbonylation, and ROS production (23). Additionally, we have shown that this early mitochondrial dysfunction is associated with a reduction in some aspects of cytosolic oxidative damage and global oxidative stress that can be measured via recognized plasma biomarkers such as 8-isoprostanes and 8-hydroxy-2-deoxyguanosine (8-OHdG). Considering that the accumulation of global oxidative damage is known to be tightly linked to the aging process (26), this latter result suggests that the anti-aging effect triggered by low MCLK1 levels might already act at a young age.To further investigate the clk-1/Mclk1-dependent mechanism of aging as well as to try to elucidate the still unclear relation between mitochondrial dysfunction, oxidative stress, and aging, we have now carefully analyzed the evolution of the phenotype of Mclk1^+/− mutants over time. We have also studied the effects of reduced MCLK1 levels on the phenotype of mice heterozygous for the mitochondrial superoxide dismutase (Sod2), which represent a well known model of mitochondrial oxidative stress (27). In addition of confirming the long lifespan phenotype of the Mclk1^+/− mutants in a mixed background (129S6 x BALB/c), we also report here a study of mutants and controls on a completely isogenic background where we find that the condition of Mclk1^+/− mutants unexpectedly results in protection against the age-dependent loss of mitochondrial function. Moreover, we found that the mutants are characterized by a significant attenuation of the age-associated increase in global oxidative stress normally observed in mammals. We also show that the Mclk1^+/− condition can gradually reverse the deterioration of mitochondrial function and the associated increase of global oxidative stress that is normally observed in Sod2^+/− mutants. Thus, this study provides for a unique vertebrate model in which reduced levels of a specific mitochondrial protein causes early mitochondrial dysfunction but has long term beneficial effects that slow down the rate of aging, as established with appropriate biomarkers, and can ultimately prolong lifespan in mice. Furthermore, in line with recent studies that have raised doubts about the validity of the mitochondrial oxidative stress theory of aging (4, 8, 10), our results, which relate to a recognized long-lived mice model, represent a novel and crucial indication that mitochondrial oxidative stress might not by itself be causal to aging.     

Enhanced lipid-but not carbohydrate-supported mitochondrial respiration in skeletal muscle of PGC-1α overexpressing mice

Skeletal muscle mitochondrial dysfunction has been linked to several disease states as well as the process of aging. A possible factor involved is the peroxisome proliferator-activated receptor (PPAR) γ co-activator 1α (PGC-1α), a major player in the regulation of skeletal muscle mitochondrial metabolism. However, it is currently unknown whether PGC-1α, besides stimulating mitochondrial proliferation, also affects the functional capacity per mitochondrion. Therefore, we here tested whether PGC-1α overexpression, besides increasing mitochondrial content, also leads to intrinsic mitochondrial adaptations. Skeletal muscle mitochondria from 10 male, muscle-specific PGC-1α overexpressing mice (PGC-1αTg) and 8 wild-type (WT) mice were isolated. Equal mitochondrial quantities were then analyzed for their oxidative capacity by high-resolution respirometry, fuelled by a carbohydrate-derived (pyruvate) and a lipid (palmitoyl-CoA plus carnitine) substrate. Additionally, mitochondria were tested for reactive oxygen species (superoxide) production and fatty acid (FA)-induced uncoupling. PGC-1αTg mitochondria were characterized by an improved intrinsic mitochondrial fat oxidative capacity as evidenced by pronounced increase in ADP-stimulated respiration (P < 0.001) and maximal uncoupled respiration (P < 0.001) upon palmitoyl-CoA plus carnitine. Interestingly, intrinsic mitochondrial capacity on a carbohydrate-derived substrate tended to be reduced. Furthermore, the sensitivity to FA-induced uncoupling was diminished in PGC-1αTg mitochondria (P = 0.02) and this was accompanied by a blunted reduction in mitochondrial ROS production upon FAs in PGC-1αTg versus WT mitochondria (P = 0.04). Uncoupling protein 3 (UCP3) levels were markedly reduced in PGC-1αTg mitochondria (P < 0.001). Taken together, in addition to stimulating mitochondrial proliferation in skeletal muscle, we show here that overexpression of PGC-1α leads to intrinsic mitochondrial adaptations that seem restricted to fat metabolism.     

Memory deficits in APP23/Abca1+/? mice correlate with the level of Aβ oligomers

ABCA1, a member of the ATP-binding cassette family of transporters, lipidates ApoE (apolipoprotein A) and is essential for the generation of HDL (high-density lipoprotein)-like particles in the CNS (central nervous system). Lack of Abca1 increases amyloid deposition in several AD (Alzheimer''s disease) mouse models. We hypothesized that deletion of only one copy of Abca1 in APP23 (where APP is amyloid precursor protein) AD model mice will aggravate memory deficits in these mice. Using the Morris Water Maze, we demonstrate that 2-year-old Abca1 heterozygous APP23 mice (referred to as APP23/het) have impaired learning during acquisition, and impaired memory retention during the probe trial when compared with age-matched wild-type mice (referred to as APP23/wt). As in our previous studies, the levels of ApoE in APP23/het mice were decreased, but the differences in the levels of Aβ and thioflavin-S-positive plaques between both groups were insignificant. Importantly, dot blot analysis demonstrated that APP23/het mice have a significantly higher level of soluble A11-positive Aβ (amyloid β protein) oligomers compared with APP23/wt which correlated negatively with cognitive performance. To confirm this finding, we performed immunohistochemistry with the A11 antibody, which revealed a significant increase of A11-positive oligomer structures in the CA1 region of hippocampi of APP23/het. This characteristic region-specific pattern of A11 staining was age-dependent and was missing in younger APP23 mice lacking Abca1. In contrast, the levels of Aβ*56, as well as other low-molecular-mass Aβ oligomers, were unchanged among the groups. Overall, the results of the present study demonstrate that in aged APP23 mice memory deficits depend on Abca1 and are likely to be mediated by the amount of Aβ oligomers deposited in the hippocampus.     

The regulation of ubiquinone synthesis in fibroblasts: The effect of modulators of β-hydroxy-β-methylglutaryl-coenzyme A reductase activity

The effect of inhibitors of β-hydroxy-β-methylglutaryl-coenzyme A (HMG-CoA) reductase such as low-density lipoprotein (LDL) and compactin were tested for their effects on the biosynthesis of ubiquinone in fibroblasts using [2-¹⁴C]acetic acid as a labeled precursor. LDL added to fibroblasts incubated in lipoprotein-deficient serum inhibited acetate incorporation into ubiquinone by 35%. Compactin, 2.5 μm, inhibited acetate incorporation by 60%. Further increases in compactin concentration up to 20 μm gradually increased the extent of inhibition but leveled off between 70 and 80%. The incorporation of ³H]mevalonic acid and 4-[U-¹⁴C]hydroxybenzoic acid into ubiquinone were determined with a range of compactin concentrations. Whereas the incorporation of [³H]mevalonate showed an apparent increase in response to compactin, the incorporation of 4-[U-¹⁴C]hydroxybenzoate into ubiquinone decreased. Both curves leveled off at concentrations of 5 μm did not significantly change with further increases in compactin concentration approaching 20 μm. Thus, the inhibition of acetate and 4-hydroxybenzoate incorporation into ubiquinone by compactin showed similar patterns. Cells incubated in lipoprotein-deficient serum compared to whole human serum showed inhibition of acetate incorporation similar to that observed previously for 4-hydroxybenzoate (9), thereby suggesting the presence of a stimulatory factor for ubiquinone biosynthesis in whole human serum. These data confirm and extend our earlier conclusions that inhibition of HMG-CoA reductase greatly affects ubiquinone synthesis in fibroblasts.     

NADH- and NADPH-dependent formation of superoxide anions by bovine heart submitochondrial particles and NADH–ubiquinone reductase preparation

1. Both NADH and NADPH supported the oxidation of adrenaline to adrenochrome in bovine heart submitochondrial particles. The reaction was completely inhibited in the presence of superoxide dismutase, suggesting that superoxide anions (O(2) (-)) are responsible for the oxidation. The optimal pH of the reaction with NADPH was at pH7.5, whereas that with NADH was at pH9.0. The reaction was inhibited by treatment of the preparation with p-hydroxymercuribenzoate and stimulated by treatment with rotenone. Antimycin A and cyanide stimulated the reaction to the same extent as rotenone. The NADPH-dependent reaction was inhibited by inorganic salts at high concentrations, whereas the NADH-dependent reaction was stimulated. 2. Production of O(2) (-) by NADH-ubiquinone reductase preparation (Complex I) with NADH or NADPH as an electron donor was assayed by measuring the formation of adrenochrome or the reduction of acetylated cytochrome c which does not react with the respiratory-chain components. p-Hydroxymercuribenzoate inhibited the reaction and rotenone stimulated the reaction. The effects of pH and inorganic salts at high concentrations on the NADH- and NADPH-dependent reactions of Complex I were essentially similar to those on the reactions of submitochondrial particles. 3. These findings suggest that a region between a mercurialsensitive site and the rotenone-sensitive site of the respiratory-chain NADH dehydrogenase is largely responsible for the NADH- and NADPH-dependent O(2) (-) production by the mitochondrial inner membranes.     

Abnormal subcellular distribution of β-glucuronidase in mice with a genetic alteration in enzyme structure

Liver -glucuronidase is structurally altered in inbred strain PAC so that a peptide subunit with a more basic isoelectric point, GUS-SN, is produced. This allele of -glucuronidase was transferred to strain C57BL/6J by 12 backcross matings to form the congenic line B6 · PAC-Gus ⁿ. Liver -glucuronidase activity was halved in males of the congenic strain compared to normal males. The lowered activity was specifically accounted for by a decrease in the lysosomal component. There was no alteration in the concentration of microsomal activity. This alteration in the subcellular distribution of -glucuronidase in Gus ⁿ/Gus ⁿ mice was confirmed by two independent gel electrophoretic systems which separate microsomal and lysosomal components. -Glucuronidase activity was likewise approximately halved in mutant spleen, lung, and brain, organs which contain exclusively or predominantly lysosomal -glucuronidase. The loss of liver lysosomal -glucuronidase activity was shown by immunotitration to be due to a decrease in the number of -glucuronidase molecules in lysosomes of the congenic strain. The Gus ⁿ structural alteration likely causes the lowered lysosomal -glucuronidase activity since the two traits remain in congenic animals. Heterozygous Gus ⁿ/Gus ^b animals had intermediate levels of liver -glucuronidase. Also, the effect was specific, in that three other lysosomal enzymes were not reproducibly lower in Gus ⁿ/Gus ⁿ mice. Gus ⁿ is, therefore, an unusual example of a mutation which causes a change in the subcellular distribution of a two-site enzyme.This work was supported by National Institutes of Health Grants GM-33559 and GM-33160 and National Science Foundation Grant PCM-8215808.     

The outcome of renal ischemia-reperfusion injury is unchanged in AMPK-β1 deficient mice

Aim

Activation of the master energy-regulator AMP-activated protein kinase (AMPK) in the heart reduces the severity of ischemia-reperfusion injury (IRI) but the role of AMPK in renal IRI is not known. The aim of this study was to determine whether activation of AMPK by acute renal ischemia influences the severity of renal IRI.

Methods

AMPK expression and activation and the severity of renal IRI was studied in mice lacking the AMPK β1 subunit and compared to wild type (WT) mice.

Results

Basal expression of activated AMPK, phosphorylayed at αThr¹⁷², was markedly reduced by 96% in AMPK-β1^−/− mice. Acute renal ischaemia caused a 3.2-fold increase in α1-AMPK activity and a 2.5-fold increase in α2-AMPK activity (P<0.001) that was associated with an increase in AMPK phosphorylation of the AMPK-α subunit at Thr¹⁷² and Ser⁴⁸⁵, and increased inhibitory phosphorylation of the AMPK substrate acetyl-CoA carboxylase. After acute renal ischemia AMPK activity was reduced by 66% in AMPK-β1^−/− mice compared with WT. There was no difference, however, in the severity of renal IRI at 24-hours between AMPK-β1^−/− and WT mice, as measured by serum urea and creatinine and histological injury score. In the heart, macrophage migration inhibitory factor (MIF) released during IRI contributes to AMPK activation and protects from injury. In the kidney, however, no difference in AMPK activation by acute ischemia was observed between MIF^−/− and WT mice. Compared with the heart, expression of the MIF receptor CD74 was found to be reduced in the kidney.

Conclusion

The failure of AMPK activation to influence the outcome of IRI in the kidney contrasts with what is reported in the heart. This difference might be due to a lack of effect of MIF on AMPK activation and lower CD74 expression in the kidney.     

Does spatial distribution of tree size account for spatial variation in soil respiration in a tropical forest?

We explored the relationship between soil processes, estimated through soil respiration (R _soil ), and the spatial variation in forest structure, assessed through the distribution of tree size, in order to understand the determinism of spatial variations in R _soil in a tropical forest. The influence of tree size was examined using an index (I _c ) calculated for each tree as a function of (1) the trunk cross section area and (2) the distance from the measurement point. We investigated the relationships between I _c and litterfall, root mass and R _soil , respectively. Strong significant relationships were found between I _c and both litterfall and root mass. R _soil showed a large range of variations over the 1-ha experimental plot, from 1.5 to 12.6 g_C m^?2 d^?1. The best relationship between I _c and R _soil only explained 17% of the spatial variation in R _soil . These results support the assumption that local spatial patterns in litter production and root mass depend on tree distribution in tropical forests. Our study also emphasizes the modest contribution of tree size distribution–which is mainly influenced by the presence of the biggest trees (among the large range size of the inventoried trees greater than 10 cm diameter at 1.30 m above ground level or at 0.5 m above the buttresses)–in explaining spatial variations in R _soil .     

α-Synuclein expression selectively affects tumorigenesis in mice modeling Parkinson's disease

Alpha Synuclein (α-Syn) is a protein implicated in mechanisms of neuronal degeneration in Parkinson's disease (PD). α-Syn is primarily a neuronal protein, however, its expression is found in various tumors including ovarian, colorectal and melanoma tumors. It has been hypothesized that neurodegeneration may share common mechanisms with oncogenesis. We tested whether α-Syn expression affects tumorigenesis of three types of tumors. Specifically, B16 melanoma, E0771 mammary gland adenocarcinoma and D122 Lewis lung carcinoma. For this aim, we utilized transgenic mice expression the human A53T α-Syn form. We found that the in vivo growth of B16 and E0771 but not D122 was enhanced in the A53T α-Syn mice. The effect on tumorigenesis was not detected in age-matched APP/PS1 mice, modeling Alzheimer's disease (AD), suggesting a specific effect for α-Syn-dependent neurodegeneration. Importantly, transgenic α-Syn expression was detected within the three tumor types. We further show uptake of exogenously added, purified α-Syn, by the cultured tumor cells. In accord, with the affected tumorigenesis in the young A53T α-Syn mice, over-expression of α-Syn in cultured B16 and E0771 cells enhanced proliferation, however, had no effect on the proliferation of D122 cells. Based on these results, we suggest that certain forms of α-Syn may selectively accelerate cellular mechanisms leading to cancer.     

The distribution of α1-antichymotrypsin and amyloid production in the brain in Alzheimer’s disease

In this immunohistopathological study α₁-antichymotrypsin, which is barely demonstrable in the normal brain, was found in amyloid fibrils, endothelial cells and the cytoplasm of astroglial cells in brains from patients with Alzheimer’s disease. Amyloid precursors stained with methenamine silver were arrayed mainly along the membranes, and amyloid fibrils, which stained densely with anti-α₁-antichymotrypsin, were in direct contact with the fibrous structures connecting with the membranes of vascular feet or astrocytic processes. From the above findings, α₁-antichymotrypsin seems to play a role in the production of amyloid fibrils in Alzheimer’s disease.     

Substrate specificity and distribution of acid β-galactosidase activities in seizure-susceptible and non-susceptible strains of mice

The properties and distribution of -galactosidase were studied in the mouse brain using the artificial substrate methylumbelliferyl--galactoside. Enzyme activities were compared between an audiogenic seizure-susceptible mouse strain (DBA/2) and three non-susceptible strains of mice (BALB/c, C3H/He and Swiss A2G). At all ages, DBA/2 mice have significantly lower -galactosidase activity compared with the three other mouse strains: this is attributed to the different alleles present at the Bgs locus. The low activity of -galactosidase is also evident when the natural substrate GMI-ganglioside is hydrolyzed. In contrast to this low GMI-ganglioside--galactosidase activity, there is no difference in the activity of the second form of acid -galactosidase, galactosylceramidase, in DBA/2 mice at 7 and 14 days. However, at 21 and 28 days the activity is significantly lower in DBA/2 mice compared with the other strains of mice. These results on -galactosidase activity in the brain of seizure-susceptible and non-susceptible mice are discussed in relation to published levels of GMI-ganglioside and galactosylceramide present in the developing mouse brain.Dedicated to Henry McIlwain.     

The control by respiration of the uptake of α-methyl glucoside in Escherichia coli K12

The uptake of methyl α-d-glucopyranoside (α-MG) by Escherichia coli K12 was decreased by the addition of substrates which stimulated the rate of oxygen consumption by the cells. The inhibition, which occurred only at non-saturating concentrations of α-MG, was not the result of a stimulation of the rate of exit of intracellular α-MG, and was abolished by the presence of carbonyl cyanide m-chlorophenylhydrazone or sodium azide. Since those drugs inhibit energy conservation at the respiratory chain and did not alter significantly the rate of oxygen consumption under the conditions for the assay of α-MG uptake, it appears that the inhibition of the transport system by respirable substrates is mediated by some form of energy derived from respiration.     

An Enhanced Immune Response of Mclk1 +/? Mutant Mice Is Associated with Partial Protection from Fibrosis,Cancer and the Development of Biomarkers of Aging

The immune response is essential for survival by destroying microorganisms and pre-cancerous cells. However, inflammation, one aspect of this response, can result in short- and long-term deleterious side-effects. Mclk1^+/− mutant mice can be long-lived despite displaying a hair-trigger inflammatory response and chronically activated macrophages as a result of high mitochondrial ROS generation. Here we ask whether this phenotype is beneficial or simply tolerated. We used models of infection by Salmonella serovars and found that Mclk1^+/− mutants mount a stronger immune response, control bacterial proliferation better, and are resistant to cell and tissue damage resulting from the response, including fibrosis and types of oxidative damage that are considered to be biomarkers of aging. Moreover, these same types of tissue damage were found to be low in untreated 23 months-old mutants. We also examined the initiation of tumour growth after transplantation of mouse LLC1 carcinoma cells into Mclk1^+/− mutants, as well as during spontaneous tumorigenesis in Mclk1^+/−Trp53^+/− double mutants. Tumour latency was increased by the Mclk1^+/− genotype in both models. Furthermore, we used the transplantation model to show that splenic CD8+ T lymphocytes from Mclk1^+/− graft recipients show enhanced cytotoxicity against LLC1 cells in vitro. Mclk1^+/− mutants thus display an association of an enhanced immune response with partial protection from age-dependent processes and from pathologies similar to those that are found with increased frequency during the aging process. This suggests that the immune phenotype of these mutants might contribute to their longevity. We discuss how these findings suggest a broader view of how the immune response might impact the aging process.