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

Understanding coenzyme Q

Coenzyme Q (CoQ), also known as ubiquinone, comprises a benzoquinone head group and a long isoprenoid side chain. It is thus extremely hydrophobic and resides in membranes. It is best known for its complex function as an electron transporter in the mitochondrial electron transport chain (ETC) but is also required for several other crucial cellular processes. In fact, CoQ appears to be central to the entire redox balance of the cell. Remarkably, its structure and therefore its properties have not changed from bacteria to vertebrates. In metazoans, it is synthesized in all cells and is found in most, and maybe all, biological membranes. CoQ is also known as a nutritional supplement, mostly because of its involvement with antioxidant defenses. However, whether there is any health benefit from oral consumption of CoQ is not well established. Here we review the function of CoQ as a redox-active molecule in the ETC and other enzymatic systems, its role as a prooxidant in reactive oxygen species generation, and its separate involvement in antioxidant mechanisms. We also review CoQ biosynthesis, which is particularly complex because of its extreme hydrophobicity, as well as the biological consequences of primary and secondary CoQ deficiency, including in human patients. Primary CoQ deficiency is a rare inborn condition due to mutation in CoQ biosynthetic genes. Secondary CoQ deficiency is much more common, as it accompanies a variety of pathological conditions, including mitochondrial disorders as well as aging. In this context, we discuss the importance, but also the great difficulty, of alleviating CoQ deficiency by CoQ supplementation.

Mitochondria is involved in combination of blueberry and apple peel extracts synergistically ameliorating lifespan and oxidative stress in Caenorhabditis elegans

Mitochondrial function is closely related to the body's oxidative stress level and lifespan. Our previous research demonstrated that the combination of blueberry extracts (BE) and apple peel extracts (APE) could...

The Complexity of Making Ubiquinone

Ubiquinone (UQ, coenzyme Q) is an essential electron transfer lipid in the mitochondrial respiratory chain. It is a main source of mitochondrial reactive oxygen species (ROS) but also has antioxidant properties. This mix of characteristics is why ubiquinone supplementation is considered a potential therapy for many diseases involving mitochondrial dysfunction. Mutations in the ubiquinone biosynthetic pathway are increasingly being identified in patients. Furthermore, secondary ubiquinone deficiency is a common finding associated with mitochondrial disorders and might exacerbate these conditions. Recent developments have suggested that ubiquinone biosynthesis occurs in discrete domains of the mitochondrial inner membrane close to ER-mitochondria contact sites. This spatial requirement for ubiquinone biosynthesis could be the link between secondary ubiquinone deficiency and mitochondrial dysfunction, which commonly results in loss of mitochondrial structural integrity.

A single biochemical activity underlies the pleiotropy of the aging-related protein CLK-1

The Caenorhabditis elegans clk-1 gene and the orthologous mouse gene Mclk1 encode a mitochondrial hydroxylase that is necessary for the biosynthesis of ubiquinone (UQ). Mutations in these genes produce broadly pleiotropic phenotypes in both species, including a lengthening of animal lifespan. A number of features of the C. elegans clk-1 mutants, including a maternal effect, particularly extensive pleiotropy, as well as unexplained differences between alleles have suggested that CLK-1/MCLK1 might have additional functions besides that in UQ biosynthesis. In addition, a recent study suggested that a cryptic nuclear localization signal could lead to nuclear localization in cultured mammalian cell lines. Here, by using immunohistochemical techniques in worms and purification techniques in mammalian cells, we failed to detect any nuclear enrichment of the MCLK1 or CLK-1 proteins and any biological activity of a C. elegans CLK-1 protein devoid of a mitochondrial localization sequence. In addition, and most importantly, by pharmacologically restoring UQ biosynthesis in clk-1 null mutants we show that loss of UQ biosynthesis is responsible for all phenotypes resulting from loss of CLK-1, including behavioral phenotypes, altered expression of mitochondrial quality control genes, and lifespan.

Dietary and microbiome factors determine longevity in Caenorhabditis elegans

Diet composition affects organismal health. Nutrient uptake depends on the microbiome. Caenorhabditis elegans fed a Bacillus subtilis diet live longer than those fed the standard Escherichia coli diet. Here we report that this longevity difference is primarily caused by dietary coQ, an antioxidant synthesized by E. coli but not by B. subtilis. CoQ-supplemented E. coli fed worms have a lower oxidation state yet live shorter than coQ-less B. subtilis fed worms. We showed that mutations affecting longevity for E. coli fed worms do not always lead to similar effects when worms are fed B. subtilis. We propose that coQ supplementation by the E. coli diet alters the worm cellular REDOX homeostasis, thus decreasing longevity. Our results highlight the importance of microbiome factors in longevity, argue that antioxidant supplementation can be detrimental, and suggest that the C. elegans standard E. coli diet can alter the effect of signaling pathways on longevity.

Age-dependent changes in mitochondrial morphology and volume are not predictors of lifespan

Mitochondrial dysfunction is a hallmark of skeletal muscle degeneration during aging. One mechanism through which mitochondrial dysfunction can be caused is through changes in mitochondrial morphology. To determine the role of mitochondrial morphology changes in age-dependent mitochondrial dysfunction, we studied mitochondrial morphology in body wall muscles of the nematode C. elegans. We found that in this tissue, animals display a tubular mitochondrial network, which fragments with increasing age. This fragmentation is accompanied by a decrease in mitochondrial volume. Mitochondrial fragmentation and volume loss occur faster under conditions that shorten lifespan and occur slower under conditions that increase lifespan. However, neither mitochondrial morphology nor mitochondrial volume of five- and seven-day old wild-type animals can be used to predict individual lifespan. Our results indicate that while mitochondria in body wall muscles undergo age-dependent fragmentation and a loss in volume, these changes are not the cause of aging but rather a consequence of the aging process.

Molecular genetics of ubiquinone biosynthesis in animals

Ubiquinone (UQ), also known as coenzyme Q (CoQ), is a redox-active lipid present in all cellular membranes where it functions in a variety of cellular processes. The best known functions of UQ are to act as a mobile electron carrier in the mitochondrial respiratory chain and to serve as a lipid soluble antioxidant in cellular membranes. All eukaryotic cells synthesize their own UQ. Most of the current knowledge on the UQ biosynthetic pathway was obtained by studying Escherichia coli and Saccharomyces cerevisiae UQ-deficient mutants. The orthologues of all the genes known from yeast studies to be involved in UQ biosynthesis have subsequently been found in higher organisms. Animal mutants with different genetic defects in UQ biosynthesis display very different phenotypes, despite the fact that in all these mutants the same biosynthetic pathway is affected. This review summarizes the present knowledge of the eukaryotic biosynthesis of UQ, with focus on the biosynthetic genes identified in animals, including Caenorhabditis elegans, rodents, and humans. Moreover, we review the phenotypes of mutants in these genes and discuss the functional consequences of UQ deficiency in general.

Restoring de novo coenzyme Q biosynthesis in Caenorhabditis elegans coq-3 mutants yields profound rescue compared to exogenous coenzyme Q supplementation

Coenzyme Q (ubiquinone or Q) is an essential lipid component of the mitochondrial electron transport chain. In Caenorhabditis elegans Q biosynthesis involves at least nine steps, including the hydroxylation of the hydroquinone ring by CLK-1 and two O-methylation steps mediated by COQ-3. We characterize two C. elegans coq-3 deletion mutants, and show that while each has defects in Q synthesis, their phenotypes are distinct. First generation homozygous coq-3(ok506) mutants are fertile when fed the standard lab diet of Q-replete OP50 Escherichia coli, but their second generation homozygous progeny does not reproduce. In contrast, the coq-3(qm188) deletion mutant remains sterile when fed Q-replete OP50. Quantitative PCR analyses suggest that the longer qm188 deletion may alter expression of the flanking nuo-3 and gdi-1 genes, located 5' and 3', respectively of coq-3 within an operon. We surmise that variable expression of nuo-3, a subunit of complex I, or of gdi-1, a guanine nucleotide dissociation inhibitor, may act in combination with defects in Q biosynthesis to produce a more severe phenotype. The phenotypes of both coq-3 mutants are more drastic as compared to the C. elegans clk-1 mutants. When fed OP50, clk-1 mutants reproduce for many generations, but show reduced fertility, slow behaviors, and enhanced life span. The coq-3 and clk-1 mutants all show arrested development and are sterile when fed the Q-deficient E. coli strain GD1 (harboring a mutation in the ubiG gene). However, unlike clk-1 mutant worms, neither coq-3 mutant strain responded to dietary supplementation with purified exogenous Q(10). Here we show that the Q(9) content can be determined in lipid extracts from just 200 individual worms, enabling the determination of Q content in the coq-3 mutants unable to reproduce. An extra-chromosomal array expressing wild-type C. elegans coq-3 rescued fertility of both coq-3 mutants and partially restored steady-state levels of COQ-3 polypeptide and Q(9) content, indicating that primary defect in both is limited to coq-3. The limited response of the coq-3 mutants to dietary supplementation with Q provides a powerful model to probe the effectiveness of exogenous Q supplementation as compared to restoration of de novo Q biosynthesis.

A proposal in relation to a genetic control of lifespan in mammals

This article proposes that behavioural advancement during mammalian evolution had been in part mediated through extension of total developmental time. Such time extensions would have resulted in increased numbers of neuronal precursor cells, hence larger brains and a disproportionate increase in the neocortex. Larger neocortical areas enabled new connections to be formed during development and hence expansion of existing behavioural circuits. To have been positively selected such behavioural advances would have required enough postdevelopmental time to enable the behaviour to be fully manifest. It is therefore proposed that the success of mammalian evolution depended on initiating a genetic control of total postdevelopmental time. This could have been mediated through the redeployment of gene regulatory networks controlling total developmental time to additionally control total postdevelopmental time. The result would be that any extension of developmental time, leading to a behavioural advancement, would be accompanied by a proportional extension to postdevelopmental time. In effect it is proposed that mammalian lifespan as a whole is genetically controlled.

Long-lived mitochondrial (Mit) mutants of Caenorhabditis elegans utilize a novel metabolism

The Caenorhabditis elegans mitochondrial (Mit) mutants have disrupted mitochondrial electron transport chain (ETC) functionality, yet, surprisingly, they are long lived. We have previously proposed that Mit mutants supplement their energy needs by exploiting alternate energy production pathways normally used by wild-type animals only when exposed to hypoxic conditions. We have also proposed that longevity in the Mit mutants arises as a property of their new metabolic state. If longevity does arise as a function of metabolic state, we would expect to find a common metabolic signature among these animals. To test these predictions, we established a novel approach monitoring the C. elegans exometabolism as a surrogate marker for internal metabolic events. Using HPLC-ultraviolet-based metabolomics and multivariate analyses, we show that long-lived clk-1(qm30) and isp-1(qm150) Mit mutants have a common metabolic profile that is distinct from that of aerobically cultured wild-type animals and, unexpectedly, wild-type animals cultured under severe oxygen deprivation. Moreover, we show that 2 short-lived mitochondrial ETC mutants, mev-1(kn1) and ucr-2.3(pk732), also share a common metabolic signature that is unique. We show that removal of soluble fumarate reductase unexpectedly increases health span in several genetically defined Mit mutants, identifying at least 1 alternate energy production pathway, malate dismutation, that is operative in these animals. Our study suggests long-lived, genetically specified Mit mutants employ a novel metabolism and that life span may well arise as a function of metabolic state.

Decreased energy metabolism extends life span in Caenorhabditis elegans without reducing oxidative damage

On the basis of the free radical and rate of living theories of aging, it has been proposed that decreased metabolism leads to increased longevity through a decreased production of reactive oxygen species (ROS). In this article, we examine the relationship between mitochondrial energy metabolism and life span by using the Clk mutants in Caenorhabditis elegans. Clk mutants are characterized by slow physiologic rates, delayed development, and increased life span. This phenotype suggests that increased life span may be achieved by decreasing energy expenditure. To test this hypothesis, we identified six novel Clk mutants in a screen for worms that have slow defecation and slow development and that can be maternally rescued. Interestingly, all 11 Clk mutants have increased life span despite the fact that slow physiologic rates were used as the only screening criterion. Although mitochondrial function is decreased in the Clk mutants, ATP levels are normal or increased, suggesting decreased energy utilization. To determine whether the longevity of the Clk mutants results from decreased production of ROS, we examined sensitivity to oxidative stress and oxidative damage. We found no evidence for systematically increased resistance to oxidative stress or decreased oxidative damage in the Clk mutants despite normal or elevated levels of superoxide dismutases. Overall, our findings suggest that decreased energy metabolism can lead to increased life span without decreased production of ROS.

Coenzyme Q supports distinct developmental processes in Caenorhabditis elegans

Coenzyme Q (Q) regulates aging in Caenorhabditis elegans, and its deficiency leads to a variety of pathologies in humans. We used a coq-8 deleted strain to study the role of Q in C. elegans development and how it influences life span. Endogenous Q(9) content of coq-8(ok840) knockouts was demonstrated to be about 7% of that found in the wild-type, indicating the basal biosynthesis rate is reduced in this strain. Knockouts abnormally developed both gonads and hypodermis, showed reduced fertility and shortened life span, and this was partially recovered by ingestion of exogenous Q. Knockouts produced embryos that showed arrested development at the time of initial expression of coq-8 in embryo. Uridine, whose biosynthesis depends on mitochondrial Q, improved both egg production and progeny under Q-rich dietary conditions. COQ-8 is a candidate protein for post-translational regulation of Q biosynthesis rate and its expression correlates with Q content during the life cycle in C. elegans. We show for the first time that a critical level of Q is necessary to support embryo development and fertility in C. elegans. These results suggest that extra-mitochondrial function of Q is a key factor linking development and bioenergetics in C. elegans.

Altered bacterial metabolism, not coenzyme Q content, is responsible for the lifespan extension in Caenorhabditis elegans fed an Escherichia coli diet lacking coenzyme Q

Coenzyme Q(n) is a fully substituted benzoquinone containing a polyisoprene tail of distinct numbers (n) of isoprene groups. Caenorhabditis elegans fed Escherichia coli devoid of Q(8) have a significant lifespan extension when compared to C. elegans fed a standard 'Q-replete'E. coli diet. Here we examine possible mechanisms for the lifespan extension caused by the Q-less E. coli diet. A bioassay for Q uptake shows that a water-soluble formulation of Q(10) is effectively taken up by both clk-1 mutant and wild-type nematodes, but does not reverse lifespan extension mediated by the Q-less E. coli diet, indicating that lifespan extension is not due to the absence of dietary Q per se. The enhanced longevity mediated by the Q-less E. coli diet cannot be attributed to dietary restriction, different Qn isoforms, reduced pathogenesis or slowed growth of the Q-less E. coli, and in fact requires E. coli viability. Q-less E. coli have defects in respiratory metabolism. C. elegans fed Q-replete E. coli mutants with similarly impaired respiratory metabolism due to defects in complex V also show a pronounced lifespan extension, although not as dramatic as those fed the respiratory deficient Q-less E. coli diet. The data suggest that feeding respiratory incompetent E. coli, whether Q-less or Q-replete, produces a robust life extension in wild-type C. elegans. We believe that the fermentation-based metabolism of the E. coli diet is an important parameter of C. elegans longevity.

The trifecta of aging in Caenorhabditis elegans

Insulin signaling, mitochondrial respiration, and dietary restriction share conserved roles not only in the regulation of lifespan, but also in the timing and control of diverse functions such as reproduction, stress resistance and metabolism. These autonomous pathways differ in their dependence on known transcription factors and in their temporal requirements, but converge to manipulate the core set of physiological systems necessary for extended lifespan in worms. Recent work suggests that components of these pleiotrophic pathways might be manipulated specifically for their effects on aging without affecting additional downstream functions. Examination of these findings will help us to understand how the molecular mechanisms of distinct pathways can unite in the regulation of longevity.

Mitochondrial respiration and reactive oxygen species in C. elegans

A powerful approach to understanding complex processes such as aging is to study longevity in organisms that are amenable to genetic dissection. The nematode Caenorhabditis elegans represents a superb model system in which to study the effects of mitochondrial function on longevity. Several mutant strains have been identified that indicate that mitochondrial function is a major factor affecting the organism's lifespan. Taken as a group, these mutant strains indicate that metabolic rate, per se, only affects longevity indirectly. Mutations causing lowered metabolic rate potential are capable of decreasing or increasing longevity.

Genetic and molecular characterization of CLK-1/mCLK1, a conserved determinant of the rate of aging

The clk-1 gene of the nematode Caenorhabditis elegans encodes an evolutionarily conserved enzyme that is necessary for ubiquinone biosynthesis. Loss-of-function mutations in clk-1, as well as in its mouse orthologue mclk1, increase lifespan in both organisms. In nematodes, clk-1 extends lifespan by a mechanism that is distinct from the insulin signaling-like pathway but might have similarities to calorie restriction. The evolutionary conservation of the effect of clk-1/mclk1 on lifespan suggests that the gene affects a fundamental mechanism of aging. The clk-1/mclk1 system could allow for the understanding of this mechanism by combining genetic and molecular investigations in worms with studies in mice, where age-dependent disease processes relevant to human health can be modeled.

Uncoupling the pleiotropic phenotypes of clk-1 with tRNA missense suppressors in Caenorhabditis elegans

clk-1 encodes a demethoxyubiquinone (DMQ) hydroxylase that is necessary for ubiquinone biosynthesis. When Caenorhabditis elegans clk-1 mutants are grown on bacteria that synthesize ubiquinone (UQ), they are viable but have a pleiotropic phenotype that includes slowed development, behaviors, and aging. However, when grown on UQ-deficient bacteria, the mutants arrest development transiently before growing up to become sterile adults. We identified nine suppressors of the missense mutation clk-1(e2519), which harbors a Glu-to-Lys substitution. All suppress the mutant phenotypes on both UQ-replete and UQ-deficient bacteria. However, each mutant suppresses a different subset of phenotypes, indicating that most phenotypes can be uncoupled from each other. In addition, all suppressors restore the ability to synthesize exceedingly small amounts of UQ, although they still accumulate the precursor DMQ, suggesting that the presence of DMQ is not responsible for the Clk-1 phenotypes. We cloned six of the suppressors, and all encode tRNA(Glu) genes whose anticodons are altered to read the substituted Lys codon of clk-1(e2519). To our knowledge, these suppressors represent the first missense suppressors identified in any metazoan. The pattern of suppression we observe suggests that the individual members of the tRNA(Glu) family are expressed in different tissues and at different levels.

Genetics of longevity and aging

Longevity, i.e., the property of being long-lived, has its natural limitation in the aging process. Longevity has a strong genetic component, as has become apparent from studies with a variety of organisms, from yeast to humans. Genetic screening efforts with invertebrates have unraveled multiple genetic pathways that suggest longevity is promoted through the manipulation of metabolism and the resistance to oxidative stress. To some extent, these same mechanisms appear to act in mammals also, despite considerable divergence during evolution. Thus far, evidence from population-based studies with humans suggests the importance of genes involved in cardiovascular disease as important determinants of longevity. The challenge is to test if the candidate longevity genes that have emerged from studies with model organisms exhibit genetic variation for life span in human populations. Future investigations are likely to involve large-scale case-control studies, in which large numbers of genes, corresponding to entire gene functional modules, will be assessed for all possible sequence variation and associated with detailed phenotypic information on each individual over extended periods of time. This should eventually unravel the genetic factors that contribute to each particular aging phenotype.

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.