Altered quinone biosynthesis in the long-lived clk-1 mutants of Caenorhabditis elegans

Complementation of Saccharomyces cerevisiae coq7 mutants by mitochondrial targeting of the Escherichia coli UbiF polypeptide: two functions of yeast Coq7 polypeptide in coenzyme Q biosynthesis

Coenzyme Q (ubiquinone or Q) functions in the respiratory electron transport chain and serves as a lipophilic antioxidant. In the budding yeast Saccharomyces cerevisiae, Q biosynthesis requires nine Coq proteins (Coq1-Coq9). Previous work suggests both an enzymatic activity and a structural role for the yeast Coq7 protein. To define the functional roles of yeast Coq7p we test whether Escherichia coli ubiF can functionally substitute for yeast COQ7. The ubiF gene encodes a flavin-dependent monooxygenase that shares no homology to the Coq7 protein and is required for the final monooxygenase step of Q biosynthesis in E. coli. The ubiF gene expressed at low copy restores growth of a coq7 point mutant (E194K) on medium containing a non-fermentable carbon source, but fails to rescue a coq7 null mutant. However, expression of ubiF from a multicopy vector restores growth and Q synthesis for both mutants, although with a higher efficiency in the point mutant. We attribute the more efficient rescue of the coq7 point mutant to higher steady state levels of the Coq3, Coq4, and Coq6 proteins and to the presence of demethoxyubiquinone, the substrate of UbiF. Coq7p co-migrates with the Coq3 and Coq4 polypeptides as a high molecular mass complex. Here we show that addition of Q to the growth media also stabilizes the Coq3 and Coq4 polypeptides in the coq7 null mutant. The data suggest that Coq7p, and the lipid quinones (demethoxyubiquinone and Q) function to stabilize other Coq polypeptides.

The role of stress in ageing: research on the nematode, Caenorhabditis elegans

The nematode Caenorhabditis elegans has proven a robust genetic model for studies of aging and the roles of stress. In this review we focus on the genetics of select long-lived and short-lived mutants of C. elegans that have proven useful in revealing the relationships that exist between oxidative stress and life span. The former are known to be controlled by an insulin/insulin-like signaling pathway, while the latter are affected by mitochondrial functions.

Coenzyme Q is irreplaceable by demethoxy-coenzyme Q in plasma membrane of Caenorhabditis elegans

A procedure was developed to isolate fractions enriched in plasma membrane from Caenorhabditis elegans. Coenzyme Q9 (Q9) was found in plasma membrane isolated from either wild-type or long-lived qm30 and qm51 clk-1 mutant strains of Caenorhabditis elegans, along with dietary coenzyme Q8 (Q8) and the biosynthetic intermediate demethoxy-Q9 (DMQ9). NADH was able to reduce both Q8 and Q9, but not DMQ9. Our results indicate that DMQ9 cannot achieve the same redox role of Q9 in plasma membrane, suggesting that proportion of all these Q isoforms in plasma membrane must be an important factor in establishing the clk-1 mutant phenotype.

A mitochondrial paradigm of metabolic and degenerative diseases, aging, and cancer: a dawn for evolutionary medicine

Life is the interplay between structure and energy, yet the role of energy deficiency in human disease has been poorly explored by modern medicine. Since the mitochondria use oxidative phosphorylation (OXPHOS) to convert dietary calories into usable energy, generating reactive oxygen species (ROS) as a toxic by-product, I hypothesize that mitochondrial dysfunction plays a central role in a wide range of age-related disorders and various forms of cancer. Because mitochondrial DNA (mtDNA) is present in thousands of copies per cell and encodes essential genes for energy production, I propose that the delayed-onset and progressive course of the age-related diseases results from the accumulation of somatic mutations in the mtDNAs of post-mitotic tissues. The tissue-specific manifestations of these diseases may result from the varying energetic roles and needs of the different tissues. The variation in the individual and regional predisposition to degenerative diseases and cancer may result from the interaction of modern dietary caloric intake and ancient mitochondrial genetic polymorphisms. Therefore the mitochondria provide a direct link between our environment and our genes and the mtDNA variants that permitted our forbears to energetically adapt to their ancestral homes are influencing our health today.

Evolutionary conservation of the clk-1-dependent mechanism of longevity: loss of mclk1 increases cellular fitness and lifespan in mice

Inactivation of the Caenorhabditis elegans gene clk-1, which is required for ubiquinone biosynthesis, increases lifespan by an insulin signaling-independent mechanism. We find that homozygous inactivation of mclk1, the mouse ortholog of clk-1, yields ES cells that are protected from oxidative stress and damage to DNA. Moreover, in the livers of old mclk1(+/-) mice, hepatocytes that have lost mclk1 expression by loss of heterozygosity undergo clonal expansion, suggesting that their resistance to stress allows them to outcompete cells that still express the gene. mclk1(+/-) mice, whose growth and fertility are normal, also display a substantial increase in lifespan in each of three different genetic backgrounds. These observations indicate that the distinct mechanism by which clk-1/mclk1 affects lifespan is evolutionarily conserved from nematodes to mammals and is not tied to a particular anatomy or physiology.

Metabolism in the Caenorhabditis elegans Mit mutants

In many eukaryotes oxidative phosphorylation via the mitochondrial electron transport chain provides the major means of ATP production. Complete removal of this capacity often results in premature death. Recent studies using the nematode Caenorhabditis elegans are surprising because they have revealed that disruption of many of the key components of the normal mitochondrial energy-generating machinery do not result in death, rather they result in adult life span extension. Such mutants have been collectively termed Mit mutants. In this short review, the potential use of alternate metabolic pathways for energy generation by Mit mutants will be considered. The effects of using such pathways on residual mitochondrial functionality, reactive radical species production, and longevity will also be explored.

Evolutionary and mechanistic theories of aging

Senescence (aging) is defined as a decline in performance and fitness with advancing age. Senescence is a nearly universal feature of multicellular organisms, and understanding why it occurs is a long-standing problem in biology. Here we present a concise review of both evolutionary and mechanistic theories of aging. We describe the development of the general evolutionary theory, along with the mutation accumulation, antagonistic pleiotropy, and disposable soma versions of the evolutionary model. The review of the mechanistic theories focuses on the oxidative stress resistance, cellular signaling, and dietary control mechanisms of life span extension. We close with a discussion of how an approach that makes use of both evolutionary and molecular analyses can address a critical question: Which of the mechanisms that can cause variation in aging actually do cause variation in natural populations?

C. elegans knockouts in ubiquinone biosynthesis genes result in different phenotypes during larval development

Ubiquinone is an essential molecule in aerobic organisms to achieve both, ATP synthesis and antioxidant defence. Mutants in genes responsible of ubiquinone biosynthesis lead to non-respiring petite yeast. In C. elegans, coq-7/clk-1 but not coq-3 mutants live longer than wild type showing a 'slowed' phenotype. In this paper we demonstrate that absence in ubiquinone in coq-1, coq-2 or coq-8 mutants lead to larval development arrest, slowed pharyngeal pumping, eventual paralysis and cell death. All these features emerge during larval development, whereas embryo development appeared similar to that of wild type individuals. Dietary coenzyme Q did not restore any of the alterations found in these coq mutants. These phenomena suggest that coenzyme Q mutants unable to synthesize this molecule develop a deleterious phenotype leading to lethality. On the contrary, phenotype of C. elegans coq-7/clk-1 mutants may be a unique phenotype than can not generalize to mutants in ubiquinone biosynthesis. This particular phenotype may not be based on the absence of endogenous coenzyme Q, but to the simultaneous presence of dietary coenzyme Q and the its biosynthesis intermediate demethoxy-coenzyme Q.

The role of ubiquinone in Caenorhabditis elegans longevity

Aging is an irreversible physiological process that affects all living organisms. Different mutations in the insulin signaling pathway and caloric restriction have been shown to retard aging in Caenorhabditis elegans. In addition, mutations or RNAi silencing of components of the respiratory chain results in the modification of adult life span. Another class of genes that affect life span in C. elegans is the clock (clk) genes. Particularly interesting is clk-1, which encodes an enzyme required for ubiquinone (coenzyme Q, CoQ) biosynthesis. Down-regulation by RNAi silencing of the genes required for ubiquinone biosynthesis also extends life span in C. elegans, and CoQ supplied in the diet also affects nematode longevity in both clk-1 and wild-type strains. Although there are many aspects that can be considered in aging, we focus this review on the role of CoQ in the longevity of C. elegans. We will review the current information about the biosynthesis of CoQ and its dietary supplementation related to the extension of life span. We will also analyze the function of CoQ in the electron transport chain and reactive oxygen species production in the context of aging. We hypothesize that the role of CoQ on longevity of C. elegans supports the oxidative damage theory of aging.

Mitochondrial Oxidative Phosphorylation Is Defective in the Long-lived Mutant clk-1

The long-lived mutant of Caenorhabditis elegans, clk-1, is unable to synthesize ubiquinone, CoQ(9). Instead, the mutant accumulates demethoxyubiquinone(9) and small amounts of rhodoquinone(9) as well as dietary CoQ(8). We found a profound defect in oxidative phosphorylation, a test of integrated mitochondrial function, in clk-1 mitochondria fueled by NADH-linked electron donors, i.e. complex I-dependent substrates. Electron transfer from complex I to complex III, which requires quinones, is severely depressed, whereas the individual complexes are fully active. In contrast, oxidative phosphorylation initiated through complex II, which also requires quinones, is completely normal. Here we show that complexes I and II differ in their ability to use the quinone pool in clk-1. This is the first direct demonstration of a differential interaction of complex I and complex II with the endogenous quinone pool. This study uses the combined power of molecular genetics and biochemistry to highlight the role of quinones in mitochondrial function and aging.

coq7/clk-1 regulates mitochondrial respiration and the generation of reactive oxygen species via coenzyme Q

coq7/clk-1 was isolated from a long-lived mutant of Caenorhabditis elegans, and shows sluggish behaviours and an extended lifespan. In C. elegans and Saccharomyces cerevisiae, coq7/clk-1 is required for the biosynthesis of coenzyme Q (CoQ), an essential co-factor in mitochondrial respiration. The clk-1 mutant contains dietary CoQ(8) from Escherichia coli and demethoxyubiquinone 9 (DMQ9) instead of CoQ(9). In a previous study, we generated COQ7-deficient mice by targeted disruption of the coq7 gene and reported that mouse coq7/clk-1 is also essential for CoQ synthesis, maintenance of mitochondrial integrity and neurogenesis. In the present study, we rescued COQ7-deficient mice from embryonic lethality and established a mouse model with decreased CoQ level by transgene expression of COQ7/CLK-1. A biochemical analysis showed a concomitant decrease in CoQ(9), mitochondrial respiratory enzyme activity and the generation of reactive oxygen species (ROS) in the mitochondria of CoQ-insufficient mice. This implied that the depressed activity of respiratory enzymes and the depressed production of ROS may play a physiological role in the control of lifespan in mammalian species and of C. elegans.

The effects of complex I function and oxidative damage on lifespan and anesthetic sensitivity in Caenorhabditis elegans

A mutation in a subunit of complex I of the mitochondrial electron transport chain (gas-1) causes Caenorhabditis elegans to be hypersensitive to volatile anesthetics and oxygen as well as shortening lifespan. We hypothesized that changes in mitochondrial respiration or reactive oxygen species production cause these changes. Therefore, we compared gas-1 to other mitochondrial mutants to identify the relative importance of these two aspects of mitochondrial function in determining longevity. Lifespans of gas-1 and mev-1 were decreased compared with N2, while that of clk-1 was increased. Rates of oxidative phosphorylation were decreased in all three mutants, but the ROS damage was decreased only in clk-1. Suppressors of gas-1 increased rates of oxidative phosphorylation, decreased oxidative damage to mitochondrial proteins and increased lifespan. Two strains containing combinations of mutations predicted to have very decreased complex I function, had unexpectedly long lifespans. We conclude that mitochondrial changes in lifespan appear to be mediated primarily by changes in oxidative damage rather than by changes in rates of oxidative phosphorylation. In contrast, the effects of mitochondrial changes on anesthetic sensitivity appear to be mediated by both altered respiration and oxidative damage.

Demethoxy-Q, an intermediate of coenzyme Q biosynthesis, fails to support respiration in Saccharomyces cerevisiae and lacks antioxidant activity

Caenorhabditis elegans clk-1 mutants cannot produce coenzyme Q(9) and instead accumulate demethoxy-Q(9) (DMQ(9)). DMQ(9) has been proposed to be responsible for the extended lifespan of clk-1 mutants, theoretically through its enhanced antioxidant properties and its decreased function in respiratory chain electron transport. In the present study, we assess the functional roles of DMQ(6) in the yeast Saccharomyces cerevisiae. Three mutations designed to mirror the clk-1 mutations of C. elegans were introduced into COQ7, the yeast homologue of clk-1: E233K, predicted to disrupt the di-iron carboxylate site considered essential for hydroxylase activity; L237Stop, a deletion of 36 amino acid residues from the carboxyl terminus; and P175Stop, a deletion of the carboxyl-terminal half of Coq7p. Growth on glycerol, quinone content, respiratory function, and response to oxidative stress were analyzed in each of the coq7 mutant strains. Yeast strains lacking Q(6) and producing solely DMQ were respiratory deficient and unable to support (6)either NADH-cytochrome c reductase or succinate-cytochrome c reductase activities. DMQ(6) failed to protect cells against oxidative stress generated by H(2)O(2) or linolenic acid. Thus, in the yeast model system, DMQ does not support respiratory activity and fails to act as an effective antioxidant. These results suggest that the life span extension observed in the C. elegans clk-1 mutants cannot be attributed to the presence of DMQ per se.

Mitochondrial influence on aging rate in Caenorhabditis elegans

Virtually every model of mitochondrial involvement in aging shares the underlying proposition that mitochondrial dysfunction will accelerate the rate of aging. Caenorhabditis elegans is a post-mitotic organism with limited capacity for replacement and repair, and there is a great deal of evidence that interventions which decrease the induction of damage extend lifespan in this model. However, decreased availability of ubiquinone in adulthood has also been found to promote longevity and stress resistance, and evidence tentatively supports decreased mitochondrial function under these conditions. In addition, gene silencing experiments and mutations that target mitochondrial electron transport have also been found to increase lifespan and stress resistance in C. elegans, as has treatment with the mitochondrial inhibitor antimycin A. The involvement of damage by reactive oxygen species has been suggested, and yet many of these manipulations would be expected to increase the production of reactive oxygen species. The extension of lifespan by these interventions seems paradoxical and the mechanism, when it is elucidated, promises to have far-reaching significance.

Coenzyme Q10 can prolong C. elegans lifespan by lowering oxidative stress

The mev-1 gene encodes cytochrome b, a large subunit of the Complex II enzyme succinate-CoQ oxidoreductase. The mev-1(kn1) mutants are hypersensitive to oxidative stress and age precociously, probably because of elevated superoxide anion production in mitochondria. Coenzyme Q (CoQ) is essential for the mitochondrial respiratory chain. Here, we show that CoQ(10) and Vitamin E extended the life span of wild-type Caenorhabditis elegans. Conversely, only CoQ(10) recovered the life shortening effects seen in mev-1. We also show that CoQ(10) but not Vitamin E reduced superoxide anion levels in wild type and mev-1. Another previously described phenotype of mev-1 animals is the presence of supernumerary apoptotic cells. We now demonstrate that CoQ(10) (but not Vitamin E) suppressed these supernumerary apoptoses. Collectively these data suggest that exogenously supplied CoQ(10) can play a significant anti-aging function. It may do so either by acting as an antioxidant to dismutate the free radical superoxide anion or by reducing the uncoupling of reactions during election transport that could otherwise result in superoxide anion production. The latter activity has not been ascribed to CoQ(10); however, it is known that conditions that uncouple electron transport reactions can lead to elevated superoxide anion production.

Redox regulation of germline and vulval development in Caenorhabditis elegans

In vitro studies have indicated that reactive oxygen species (ROS) and the oxidation of signaling molecules are important mediators of signal transduction. We have identified two pathways by which the altered redox chemistry of the clk-1 mutants of Caenorhabditis elegans acts in vivo on germline development. One pathway depends on the oxidation of an analog of vertebrate low density lipoprotein (LDL) and acts on the germline through the Ack-related tyrosine kinase (ARK-1) kinase and inositol trisphosphate (IP3) signaling. The other pathway is the oncogenic ras signaling pathway, whose action on germline as well as vulval development appears to be modulated by cytoplasmic ROS.

Theoretical Study of the Trapping of the OOH Radical by Coenzyme Q

The reactivity of the hydroperoxyl radical with coenzyme Q, as a prototypical chemical reaction involved in biological antioxidant actions, was studied theoretically. Two pathways were analyzed: the hydrogen abstraction reaction from the phenolic hydrogen on the reduced form (ubiquinol), and OOH addition on the oxidized form (ubiquinone). Optimized geometries, harmonic vibrational frequencies, and energies of the stationary points (reactants, intermediate complexes, transition states, and products) for each pathway were calculated at the BHandHLYP/6-31G level of theory. The reaction paths for the two mechanisms were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory with multidimensional small-curvature tunneling. We found that the reactivity of the OOH radical is dominated by the hydrogen abstraction mechanism on ubiquinol, with a rate constant of 5.32 x 10(5) M(-1) s(-1), at 298 K. This result strongly contrasts with that, also obtained by our group, for the more reactive OH radical, which attacks ubiquinone by an addition mechanism, with a diffusion-controlled rate of 6.25 x 10(10) M(-1) s(-1), at 298 K.

Reproductive Fitness and Quinone Content of Caenorhabditis elegans clk-1 Mutants Fed Coenzyme Q Isoforms of Varying Length

Caenorhabditis elegans clk-1 mutants lack coenzyme Q9 and accumulate the biosynthetic intermediate demethoxy-Q9. A dietary source of ubiquinone (Q) is required for larval growth and development of the gonad and germ cells. We considered that uptake of the shorter Q8 isoform present in the Escherichia coli food may contribute to the Clk phenotypes of slowed development and reduced brood size observed when the animals are fed Q-replete E. coli. To test the effect of isoprene tail length, N2 and clk-1 animals were fed E. coli engineered to produce Q7, Q8, Q9, or Q10. Wild-type nematodes showed no change in reproductive fitness regardless of the Qn isoform fed. clk-1(e2519) fed the Q9 diet showed increased egg production; however, this diet did not improve reproductive fitness of the clk-1(qm30) animals. Furthermore, animals with the more severe clk-1(qm30) allele become sterile and their progeny inviable when fed Q7-containing bacteria. The content of Q7 in the mitochondria of clk-1 animals was decreased relative to Q8, suggesting less effective transport of Q7 to the mitochondria, impaired retention, or decreased stability. Additionally, regardless of E. coli diet, clk-1(qm30) animals contain a dysfunctional dense form of mitochondria. The gonads of clk-1(qm30) worms fed Q7-containing food were severely shrunken and disordered. The differential fertility of clk-1 mutant nematodes fed Q isoforms may result from changes in Q localization, altered recognition by Q-binding proteins, and/or potential defects in mitochondrial function resulting from the mutant CLK-1 polypeptide itself.

Sensitivity of Caenorhabditis elegans clk-1 mutants to ubiquinone side-chain length reveals multiple ubiquinone-dependent processes

Ubiquinone (coenzyme Q, or Q) is a membrane constituent, whose head group is capable of accepting and donating electrons and whose lipidic side chain is composed of a variable number of isoprene subunits. A possible role for Q as a dietary antioxidant for treating conditions that involve altered cellular redox states is being intensely studied. Mutations in the clk-1 gene of the nematode Caenorhabditis elegans affect numerous physiological rates including behavioral rates, developmental rates, reproduction, and life span. clk-1 encodes a protein associated with the inner mitochondrial membrane that is necessary for Q biosynthesis in C. elegans. clk-1 mutants do not synthesize Q but accumulate demethoxyubiquinone, a Q synthesis intermediate that is able to partially sustain mitochondrial respiration in worms as well as in mammals. Recently, we and others have found that exogenous Q is necessary for the fertility and development of clk-1 mutants. Here, we take advantage of the clk-1 genetic model to identify structural features of Q that are functionally important in vivo. We show that clk-1 mutants are exquisitely sensitive to the length of the side chain of the Q they consume. We also identified differential sensitivity to Q side-chain length between null alleles of clk-1 (qm30 and qm51) and the weaker allele e2519. This allows us to propose a model where we distinguish several types of Q-dependent processes in vivo: processes that are very sensitive to Q side-chain length and processes that are permissive to Q with shorter chains.

Molecular mechanism of maternal rescue in the clk-1 mutants of Caenorhabditis elegans

The clk-1 mutants of Caenorhabditis elegans display an average slowing down of physiological rates, including those of development, various behaviors, and aging. clk-1 encodes a hydroxylase involved in the biosynthesis of the redox-active lipid ubiquinone (co-enzyme Q), and in clk-1 mutants, ubiquinone is replaced by its biosynthetic precursor demethoxyubiquinone. Surprisingly, homozygous clk-1 mutants display a wild-type phenotype when issued from a heterozygous mother. Here, we show that this maternal effect is the result of the persistence of small amounts of maternally derived CLK-1 protein and that maternal CLK-1 is sufficient for the synthesis of considerable amounts of ubiquinone during development. However, gradual depletion of CLK-1 and ubiquinone, and expression of the mutant phenotype, can be produced experimentally by developmental arrest. We also show that the very long lifespan observed in daf-2 clk-1 double mutants is not abolished by the maternal effect. This suggests that, like developmental arrest, the increased lifespan conferred by daf-2 allows for depletion of maternal CLK-1, resulting in the expression of the synergism between clk-1 and daf-2. Thus, increased adult longevity can be uncoupled from the early mutant phenotypes, indicating that it is possible to obtain an increased adult lifespan from the late inactivation of processes required for normal development and reproduction.

A metabolic model for life span determination in Caenorhabditis elegans

Several studies with the nematode Caenorhabditis elegans have made the unexpected discovery that certain hypomorphic mutations in genes encoding mitochondrial proteins result in life span extension. These mutations appear to act independently of the other known pathway that regulates life span extension, the dauer-specifying insulin/IGF-1-like pathway. Here we present a hypothesis that unifies the effects of these two classes of genes on longevity. The central concept is that energy generation in C. elegans occurs by differential flux through two coexisting mitochondrial metabolic pathways-aerobic respiration and fermentative malate dismutation. In the latter process, fumarate is terminally reduced at complex II to succinate. We suggest that most, if not all, long-lived mutants in C. elegans utilize malate dismutation, a byproduct of which is the generation of fewer radical species.

The role of mitochondria in the life of the nematode, Caenorhabditis elegans

Mitochondria are essential organelles involved in energy metabolism via oxidative phosphorylation. They play a vital role in diverse biological processes such as aging and apoptosis. In humans, defects in the mitochondrial respiratory chain (MRC) are responsible for or associated with a bewildering variety of diseases. The nematode Caenorhabditis elegans is a simple animal and a powerful genetic and developmental model system. In this review, we discuss how the nematode model system has contributed to our understanding of mitochondrial dynamics, of the genetics and inheritance of the mitochondrial genome, and of the consequences of nuclear and mitochondrial DNA (mtDNA) mutations. Mitochondrial respiration is vital to energy metabolism but also to other aspects of multicellular life such as aging and development. We anticipate that further significant contributions to our understanding of mitochondrial function in animal biology are forthcoming with the C. elegans model system.

Is life span extension in single gene long-lived Caenorhabditis elegans mutants due to hypometabolism?

The nematode C. elegans is widely used in aging research largely because of the identification of numerous gene mutations that significantly increase worm longevity. While model organisms such as C. elegans can provide important insights into aging it is also important to consider the limitations of these systems. For example, ectothermic (poikilothermic) organisms are able to tolerate a much larger metabolic depression than humans and considering only chronological longevity when assaying for long-lived mutants provides a limited perspective on the mechanisms by which longevity is increased. In order to provide true insight into the aging process additional physiological processes, such as metabolic rate, must also be assayed. Currently it is controversial when long-lived C. elegans mutants retain normal metabolic function. Resolving this issue requires accurately measuring the metabolic rate of C. elegans under conditions that minimize environmental stress. Comparisons of metabolic rate between long-lived and wild-type C. elegans under more optimized conditions indicate that the extended longevity of at least some long-lived C. elegans mutants may be due to a reduction in metabolic rate, rather than an alteration of a metabolically-independent genetic mechanism specific to aging. Consistent with this assertion are studies showing that the disruption of mitochondrial function in C. elegans can extend worm's longevity, but typically causes worms to grow and develop more slowly than wild-type animals.

Silencing of ubiquinone biosynthesis genes extends life span in Caenorhabditis elegans

Ubiquinone (coenzyme Q; Q) is a key factor in the mitochondria electron transport chain, but it also functions as an antioxidant and as a cofactor of mitochondrial uncoupling proteins. Furthermore, Q isoforms balance in Caenorhabditis elegans is determined by both dietary intake and endogenous biosynthesis. In the absence of synthesis, withdrawal of dietary Q8 in adulthood extends life span. Thus, Q plays an important role in the aging process and understanding its synthesis acquires a new impetus. We have identified by RNA interference (RNAi) eight genes, including clk-1, involved in ubiquinone biosynthesis in C. elegans feeding animals with dsRNA-containing Escherichia coli HT115 strains. Silenced C. elegans showed lower levels of both endogenous Q9 and Q8 provided by diet, produced less superoxide without a significant modification of mitochondrial electron chain, and extended life span compared with non-interfered animals. E. coli strains harboring dsRNA also interfered with their own Q8 biosynthesis. These findings suggest that more efficient electron transport between a lower amount of Q and electron transport capacity of the mitochondrial complexes leads to less production of reactive oxygen species that contributes to extension of life span in the nematode C. elegans.

Complementation of Escherichia coli ubiF mutation by Caenorhabditis elegans CLK-1, a product of the longevity gene of the nematode worm

Caenorhabditis elegans CLK-1 was identified from long-lived mutant worms, and is believed to be involved in ubiquinone biosynthesis. The protein belongs to the eukaryotic CLK-1/Coq7p family, which is also similar to the bacterial Coq7 family, that hydroxylates demethoxyubiquinone, resulting in the formation of hydroxyubiquinone, a precursor of ubiquinone. In Escherichia coli, the corresponding reaction is catalyzed by UbiF, a member of a distinct class of hydroxylase. Although previous studies suggested that the eukaryotic CLK-1/Coq7 family is a hydroxylase of demethoxyubiquinone, there was no direct evidence to show the enzymatic activity of the eukaryotic CLK-1/Coq7 family. Here we show that the plasmid encoding C. elegans CLK-1 supported aerobic respiration on a non-fermentable carbon source of E. coli ubiF mutant strain and rescued the ability to synthesize ubiquinone, suggesting that the eukaryotic CLK-1/Coq7p family could function as bacterial UbiF.

Isolation and characterization of the stage-specific cytochrome b small subunit (CybS) of Ascaris suum complex II from the aerobic respiratory chain of larval mitochondria

We recently reported that Ascaris suum mitochondria express stage-specific isoforms of complex II: the flavoprotein subunit and the small subunit of cytochrome b (CybS) of the larval complex II differ from those of adult enzyme, while two complex IIs share a common iron-sulfur cluster subunit (Ip). In the present study, A. suum larval complex II was highly purified to characterize the larval cytochrome b subunits in more detail. Peptide mass fingerprinting and N-terminal amino acid sequencing showed that the larval and adult cytochrome b (CybL) proteins are identical. In contrast, cDNA sequences revealed that the small subunit of larval cytochrome b (CybS(L)) is distinct from the adult CybS (CybS(A)). Furthermore, Northern analysis and immunoblotting showed stage-specific expression of CybS(L) and CybS(A) in larval and adult mitochondria, respectively. Enzymatic assays revealed that the ratio of rhodoquinol-fumarate reductase (RQFR) to succinate-ubiquinone reductase (SQR) activities and the K(m) values for quinones are almost identical for the adult and larval complex IIs, but that the fumarate reductase (FRD) activity is higher for the adult form than for the larval form. These results indicate that the adult and larval A. suum complex IIs have different properties than the complex II of the mammalian host and that the larval complex II is able to function as a RQFR. Such RQFR activity of the larval complex II would be essential for rapid adaptation to the dramatic change of oxygen availability during infection of the host.

Complex II from phototrophic purple bacterium Rhodoferax fermentans displays rhodoquinol-fumarate reductase activity

It has long been accepted that bacterial quinol-fumarate reductase (QFR) generally uses a low-redox-potential naphthoquinone, menaquinone (MK), as the electron donor, whereas mitochondrial QFR from facultative and anaerobic eukaryotes uses a low-redox-potential benzoquinone, rhodoquinone (RQ), as the substrate. In the present study, we purified novel complex II from the RQ-containing phototrophic purple bacterium, Rhodoferax fermentans that exhibited high rhodoquinol-fumarate reductase activity in addition to succinate-ubiquinone reductase activity. SDS/PAGE indicated that the purified R. fermentans complex II comprises four subunits of 64.0, 28.6, 18.7 and 17.5 kDa and contains 1.3 nmol heme per mg protein. Phylogenetic analysis and comparison of the deduced amino acid sequences of R. fermentans complex II with pro/eukaryotic complex II indicate that the structure and the evolutional origins of R. fermentans complex II are closer to bacterial SQR than to mitochondrial rhodoquinol-fumarate reductase. The results strongly indicate that R. fermentans complex II and mitochondrial QFR might have evolved independently, although they both utilize RQ for fumarate reduction.

Genetics and the specificity of the aging process

The identification and study of long-lived mutant animals has provided valuable insights into the mechanisms that limit the life-span of organisms. Findings with the gene SIR2 suggest that the rate of aging can be regulated under certain conditions. Indeed, increased expression of SIR2 lengthens life-span by acting on biological processes that promote survival under conditions of scarcity. In addition, studies of mutant strains of Caenorhabditis elegans, in particular daf-2, clk-1, and isp-1 mutants, suggest that the biology of reactive oxygen species in the mitochondria and elsewhere might be the main determinant of life-span in this organism. Thus, the aging process may be more specific than previously anticipated on evolutionary grounds.

Deficiencies in C20 polyunsaturated fatty acids cause behavioral and developmental defects in Caenorhabditis elegans fat-3 mutants

Arachidonic acid and other long-chain polyunsaturated fatty acids (PUFAs) are important structural components of membranes and are implicated in diverse signaling pathways. The Delta6 desaturation of linoleic and linolenic acids is the rate-limiting step in the synthesis of these molecules. C. elegans fat-3 mutants lack Delta6 desaturase activity and fail to produce C20 PUFAs. We examined these mutants and found that development and behavior were affected as a consequence of C20 PUFA deficiency. While fat-3 mutants are viable, they grow slowly, display considerably less spontaneous movement, have an altered body shape, and produce fewer progeny than do wild type. In addition, the timing of an ultradian rhythm, the defecation cycle, is lengthened compared to wild type. Since all these defects can be ameliorated by supplementing the nematode diet with gamma-linolenic acid or C20 PUFAs of either the n6 or the n3 series, we can establish a causal link between fatty acid deficiency and phenotype. Similar epidermal tissue defects and slow growth are hallmarks of human fatty acid deficiency.

Atpenins, potent and specific inhibitors of mitochondrial complex II (succinate-ubiquinone oxidoreductase)

Enzymes in the mitochondrial respiratory chain are involved in various physiological events in addition to their essential role in the production of ATP by oxidative phosphorylation. The use of specific and potent inhibitors of complex I (NADH-ubiquinone reductase) and complex III (ubiquinol-cytochrome c reductase), such as rotenone and antimycin, respectively, has allowed determination of the role of these enzymes in physiological processes. However, unlike complexes I, III, and IV (cytochrome c oxidase), there are few potent and specific inhibitors of complex II (succinate-ubiquinone reductase) that have been described. In this article, we report that atpenins potently and specifically inhibit the succinate-ubiquinone reductase activity of mitochondrial complex II. Therefore, atpenins may be useful tools for clarifying the biochemical and structural properties of complex II, as well as for determining its physiological roles in mammalian tissues.

Caenorhabditis elegans ubiquinone biosynthesis genes

Ubiquinone (coenzyme Q, Q) is an essential lipid electron carrier in the mitochondria respiratory chain, and also functions as antioxidant and participates as a cofactor of mitochondrial uncoupling proteins. Caernorhabditis elegans synthesize Q9, but both dietary Q8 intake and endogenous Q9 biosynthesis determine Q balance. Thus, it is of current interest to know the regulatory mechanisms of Q9 biosynthesis in this nematode. Here we review results that leaded to identification of genes involved in Q9 biosynthesis in this nematode using the RNA interference technology. C. elegans coq genes were silenced and depletion of Q content was observed, indicating that the genes related here participate in Q9 biosynthesis. Silenced populations showed an extension of adult life span, probably by the decrease of endogenous oxidative stress produced in mitochondria. We also report the heterologous complementation of C. elegans coq-5 and coq-7 genes in their homologue yeast coq null mutants, leading to restore its ability to growth in non-fermentable sugars. These complemented yeast strains accumulated Q6 but also the intermediate demethoxy-Q6. These findings support the conservative functional homology of these genes.

Membrane-bound diiron carboxylate proteins

Four proteins have been identified recently as diiron carboxylate proteins on the basis of conservation of six amino acids (four carboxylate residues and two histidines) constituting an iron-binding motif. Unlike previously identified proteins with this motif, biochemical studies indicate that each of these proteins is membrane bound, although homology modeling rules out a transmembrane mode of binding. Therefore, the predicted structure of each protein [the alternative oxidase (AOX), the plastid terminal oxidase (PTOX), the diiron 5-demethoxyquinone hydroxylase (DMQ hydroxylase), and the aerobic Mg-protoporphyrin IX monomethylester hydroxylase (MME hydroxylase)] is that of a protein bound monotopically to one leaflet of the membrane bilayer. Three of these enzymes utilize a quinol substrate, with two oxidizing the quinol (AOX and PTOX) and one hydroxylating it (DMQ hydroxylase). MME hydroxylase is involved in synthesis of the isocyclic ring of chlorophyll. Two enzymes are involved in respiration (AOX and, indirectly, the diiron DMQ hydroxylase through ubiquinone biosynthesis) and two in photosynthesis, through their roles in carotenoid and chlorophyll biosynthesis (PTOX and MME hydroxylase, respectively). We discuss what is known about each enzyme as well as our expectations based on their identification as interfacially bound proteins with a diiron carboxylate active site.

Rates of behavior and aging specified by mitochondrial function during development

To explore the role of mitochondrial activity in the aging process, we have lowered the activity of the electron transport chain and adenosine 5'-triphosphate (ATP) synthase with RNA interference (RNAi) in Caenorhabditis elegans. These perturbations reduced body size and behavioral rates and extended adult life-span. Restoring messenger RNA to near-normal levels during adulthood did not elevate ATP levels and did not correct any of these phenotypes. Conversely, inhibiting respiratory-chain components during adulthood only did not reset behavioral rates and did not affect life-span. Thus, the developing animal appears to contain a regulatory system that monitors mitochondrial activity early in life and, in response, establishes rates of respiration, behavior, and aging that persist during adulthood.

Biosynthesis, bioproduction and novel roles of ubiquinone

Ubiquinone (coenzyme Q) is a well-known component of the electron transfer system in living organisms. It is known that ubiquinone transfers electrons from Complex I (or Complex II) to Complex III in the respiratory chain. However, recent evidence indicates that an involvement in respiration is not the sole role of ubiquinone, and various novel roles have been elucidated. A role as a lipid soluble antioxidant is now widely accepted. The relationship between lifespan and ubiquinone has attracted much interest based on the study of a Caenorhabditis elegans clk-1 mutant. The connection between disulfide bond formation and ubiquinone (or menaquinone) in Escherichia coli has been well studied. The production of hydrogen sulfide in a ubiquinone-deficient fission yeast is an interesting phenotype recently observed. These are some examples of the novel roles of ubiquinone and this review summarizes the recent findings relating to the biosynthesis, bioproduction and novel roles of ubiquinone.

The influence of metabolic rate on longevity in the nematode Caenorhabditis elegans

Much of the recent interest in aging research is due to the discovery of genes in a variety of model organisms that appear to modulate aging. A large amount of research has focused on the use of such long-lived mutants to examine the fundamental causes of aging. While model organisms do offer many advantages for studying aging, it also critical to consider the limitations of these systems. In particular, ectothermic (poikilothermic) organisms can tolerate a much larger metabolic depression than humans. Thus, considering only chronological longevity when assaying for long-lived mutants provides a limited perspective on the mechanisms by which longevity is increased. In order to provide true insight into the aging process additional physiological processes, such as metabolic rate, must also be assayed. This is especially true in the nematode Caenorhabditis elegans, which can naturally enter into a metabolically reduced state in which it survives many times longer than its usual lifetime. Currently it is seen as controversial if long-lived C. elegans mutants retain normal metabolic function. Resolving this issue requires accurately measuring the metabolic rate of C. elegans under conditions that minimize environmental stress. Additionally, the relatively small size of C. elegans requires the use of sensitive methodologies when determining metabolic rates. Several studies indicating that long-lived C. elegans mutants have normal metabolic rates may be flawed due to the use of inappropriate measurement conditions and techniques. Comparisons of metabolic rate between long-lived and wild-type C. elegans under more optimized conditions indicate that the extended longevity of at least some long-lived C. elegans mutants may be due to a reduction in metabolic rate, rather than an alteration of a metabolically independent genetic mechanism specific to aging.

Mitochondrial DNA repair and aging

The mitochondrial electron transport chain plays an important role in energy production in aerobic organisms and is also a significant source of reactive oxygen species that damage DNA, RNA and proteins in the cell. Oxidative damage to the mitochondrial DNA is implicated in various degenerative diseases, cancer and aging. The importance of mitochondrial ROS in age-related degenerative diseases is further strengthened by studies using animal models, Caenorhabditis elegans, Drosophila and yeast. Research in the last several years shows that mitochondrial DNA is more susceptible to various carcinogens and ROS when compared to nuclear DNA. DNA damage in mammalian mitochondria is repaired by base excision repair (BER). Studies have shown that mitochondria contain all the enzymes required for BER. Mitochondrial DNA damage, if not repaired, leads to disruption of electron transport chain and production of more ROS. This vicious cycle of ROS production and mtDNA damage ultimately leads to energy depletion in the cell and apoptosis.

Development and fertility in Caenorhabditis elegans clk-1 mutants depend upon transport of dietary coenzyme Q8 to mitochondria

The Caenorhabditis elegans clk-1 mutants lack coenzyme Q(9) and instead accumulate the biosynthetic intermediate demethoxy-Q(9) (DMQ(9)). clk-1 animals grow to reproductive adults, albeit slowly, if supplied with Q(8)-containing Escherichia coli. However, if Q is withdrawn from the diet, clk-1 animals either arrest development as young larvae or become sterile adults depending upon the stage at the time of the withdrawal. To understand this stage-dependent response to a Q-less diet, the quinone content was determined during development of wild-type animals. The quinone content varies in the different developmental stages in wild-type fed Q(8)-replete E. coli. The amounts peak at the second larval stage, which coincides with the stage of arrest of clk-1 larvae fed a Q-less diet from hatching. Levels of the endogenously synthesized DMQ(9) are high in the clk-1(qm30)-arrested larvae and sterile adults fed Q-less food. Comparison of quinones from animals fed a Q-replete or a Q-less diet establishes that the Q(8) present is assimilated from the E. coli. Furthermore, this E. coli-specific Q(8) is present in mitochondria isolated from fertile clk-1(qm30) adults fed a Q-replete diet. These results suggest that the uptake and transport of dietary Q(8) to mitochondria prevent the arrest and sterility phenotypes of clk-1 mutants and that DMQ is not functionally equivalent to Q.

No reduction of energy metabolism in Clk mutants

Mutation in any of the four clock genes (clk-1, clk-2, clk-3, gro-1) causes an average slowing down of many temporal processes, and an increase of mean life span. The latter effect has been linked to the slow phenotype, and it has been reasoned that any reduction of the rate of living would reduce the load of oxidative damage, which is thought to drive the ageing process. To test this model we measured several parameters describing metabolic output in wild type worms and all four Clk mutants. We found no gross changes in metabolic output, as assessed from oxygen consumption and heat production rates, lucigenin-mediated light production capacity, ATP content, and lipofuscin autofluorescence. Catalase and superoxide dismutase (SOD) were variably altered, but not cooperatively, as would be expected to enhance reactive oxygen species (ROS) scavenging activity. Thus we conclude that the prolonged life span of Clk mutants cannot be attributed to reduced metabolic rate or an increased activity of the major antioxidant enzymes catalase and SOD.

Metabolism and aging in the nematode Caenorhabditis elegans

Research into the causes of aging has greatly increased in recent years. Much of this interest is due to the discovery of genes in a variety of model organisms that appear to modulate aging. Studies of long-lived mutants can potentially provide valuable insights into the fundamental mechanisms of aging. While there are many advantages to the use of model organisms to study aging it is also important to consider the limitations of these systems, particularly because ectothermic (poikilothermic) organisms can survive a far greater metabolic depression than humans. As such, the consideration of only chronological longevity when assaying for long-lived mutants provides a limited perspective on the mechanisms by which longevity is increased. Additional physiological processes, such as metabolic rate, must also be assayed to provide true insight into the aging process. This is especially true in the nematode Caenorhabditis elegans, which has the natural ability to enter into a metabolically reduced state in which it can survive many times longer than its normal lifetime. The extended longevity of at least some long-lived C. elegans mutants may be due to a reduction in metabolic rate, rather than an alteration of a metabolically independent genetic mechanism specific for aging.

CLK-1 protein has DNA binding activity specific to O(L) region of mitochondrial DNA

Mutations in the clk-1 gene of Caenorhabditis elegans extend worm life span and slow down a variety of physiological processes. Here we report that C. elegans CLK-1 as well as its mouse homologue have DNA binding activity that is specific to the O(L) region of mitochondrial DNA. DNA binding activity of CLK-1 is inhibited by ADP, and is altered by mutations that extend nematode life span. Our results suggest that, in addition to its enzymatic function in ubiquinone biosynthesis, CLK-1 is involved in the regulation of mtDNA replication or transcription.

Quinones in long-lived clk-1 mutants of Caenorhabditis elegans

Ubiquinone (UQ) (coenzyme Q) is a lipophilic redox-active molecule that functions as an electron carrier in the mitochondrial electron transport chain. Electron transfer via UQ involves the formation of semiubiquinone radicals, which causes the generation of superoxide radicals upon reaction with oxygen. In the reduced form, UQ functions as a lipid-soluble antioxidant, and protects cells from lipid peroxidation. Thus, UQ is also important as a lipophilic regulator of oxidative stress. Recently, a study on long-lived clk-1 mutants of Caenorhabditis elegans demonstrated that biosynthesis of UQ is dramatically altered in mutant mitochondria. Demethoxy ubiquinone (DMQ), that accumulates in clk-1 mutants in place of UQ, may contribute to the extension of life span. Here we elucidate the possible mechanisms of life span extension in clk-1 mutants, with particular emphasis on the electrochemical property of DMQ. Recent findings on the biochemical function of CLK-1 are also discussed.

Ubiquinone is necessary for Caenorhabditis elegans development at mitochondrial and non-mitochondrial sites

Ubiquinone (UQ) is a lipid co-factor that is involved in numerous enzymatic processes and is present in most cellular membranes. In particular, UQ is a crucial electron carrier in the mitochondrial respiratory chain. Recently, it was shown that clk-1 mutants of the nematode worm Caenorhabditis elegans do not synthesize UQ(9) but instead accumulate demethoxyubiquinone (DMQ(9)), a biosynthetic precursor of UQ(9) (the subscript refers to the length of the isoprenoid side chain). DMQ(9) is capable of carrying out the function of UQ(9) in the respiratory chain, as demonstrated by the functional competence of mitochondria isolated from clk-1 mutants, and the ability of DMQ(9) to act as a co-factor for respiratory enzymes in vitro. However, despite the presence of functional mitochondria, clk-1 mutant worms fail to complete development when feeding on bacteria that do not produce UQ(8). Here we show that clk-1 mutants cannot grow on bacteria producing only DMQ(8) and that worm coq-3 mutants, which produce neither UQ(9) nor DMQ(9), arrest development even on bacteria producing UQ(8). These results indicate that UQ is required for nematode development at mitochondrial and non-mitochondrial sites and that DMQ cannot functionally replace UQ at those non-mitochondrial sites.

Role of complex II in anaerobic respiration of the parasite mitochondria from Ascaris suum and Plasmodium falciparum

Parasites have developed a variety of physiological functions necessary for existence within the specialized environment of the host. Regarding energy metabolism, which is an essential factor for survival, parasites adapt to low oxygen tension in host mammals using metabolic systems that are very different from that of the host. The majority of parasites do not use the oxygen available within the host, but employ systems other than oxidative phosphorylation for ATP synthesis. In addition, all parasites have a life cycle. In many cases, the parasite employs aerobic metabolism during their free-living stage outside the host. In such systems, parasite mitochondria play diverse roles. In particular, marked changes in the morphology and components of the mitochondria during the life cycle are very interesting elements of biological processes such as developmental control and environmental adaptation. Recent research has shown that the mitochondrial complex II plays an important role in the anaerobic energy metabolism of parasites inhabiting hosts, by acting as quinol-fumarate reductase.

Extension of life-span in Caenorhabditis elegans by a diet lacking coenzyme Q

The isoprenylated benzoquinone coenzyme Q is a redox-active lipid essential for electron transport in aerobic respiration. Here, we show that withdrawal of coenzyme Q (Q) from the diet of wild-type nematodes extends adult life-span by approximately 60%. The longevity of clk-1, daf-2, daf-12, and daf-16 mutants is also extended by a Q-less diet. These results establish the importance of Q in life-span determination. The findings suggest that Q and the daf-2 pathway intersect at the mitochondria and imply that a concerted production coupled with enhanced scavenging of reactive oxygen species contributes to the substantial life-span extension.

Model organisms as a guide to mammalian aging

Recent studies on aging in model systems such as yeast and roundworms have revealed conserved regulation of the process in response to nutrient availability and specific genes that appear to mediate this regulation. Here we review these findings with a focus on the nematode Caenorhabditis elegans and the budding yeast Saccharomyces cerevisiae and highlight general features of the regulation of aging that may have implications for mammals.

CLK-1/Coq7p is a DMQ mono-oxygenase and a new member of the di-iron carboxylate protein family

Strains of Caenorhabditis elegans mutant for clk-1 exhibit a 20-40% increase in mean lifespan. clk-1 encodes a mitochondrial protein thought to be either an enzyme or regulatory molecule acting within the ubiquinone biosynthesis pathway. Here CLK-1 is shown to be related to the ubiquinol oxidase, alternative oxidase, and belong to the functionally diverse di-iron-carboxylate protein family which includes bacterioferritin and methane mono-oxygenase. Construction and analysis of a homology model indicates CLK-1 is a 2-polyprenyl-3-methyl-6-methoxy-1,4-benzoquinone mono-oxygenase as originally predicted. Analysis of known CLK-1/Coq7p mutations also supports this notion. These findings raise the possibility of developing CLK-1-specific inhibitors to test for lifespan extension in higher organisms.