Mouse homologue of coq7/clk-1, longevity gene in Caenorhabditis elegans, is essential for coenzyme Q synthesis, maintenance of mitochondrial integrity, and neurogenesis.

coq7/clk-1 was isolated from a long-lived mutant of Caenorhabditis elegans, which showed sluggish behavior and an extended life span. Mouse coq7 is homologous to Saccharomyces cerevisiae coq7/cat5 that is required for biosynthesis of coenzyme Q (CoQ), an essential cofactor in mitochondrial respiration. Here we generated COQ7-deficient mice to investigate the biological role of COQ7 in mammals. COQ7-deficient mouse embryos failed to survive beyond embryonic day 10.5, exhibiting small-sized body and delayed embryogenesis. Morphological studies showed that COQ7-deficient neuroepithelial cells failed to show the radial arrangement in the developing cerebral wall, aborting neurogenesis at E10.5. Electron microscopic analysis further showed the enlarged mitochondria with vesicular cristae and enlarged lysosomes filled with disrupted membranes, which is consistent with mitochondriopathy. Biochemical analysis demonstrated that COQ7-deficient embryos failed to synthesize CoQ(9), but instead yielded demethoxyubiquinone 9 (DMQ(9)). Cultured embryonic cells from COQ7-deficient mice were rescued by adding bovine fetal serum in vitro, but exhibited slowed cell proliferation, which resembled to the phenotype of clk-1 with delayed cell divisions. The result implied the essential role of coq7 in CoQ synthesis, maintenance of mitochondrial integrity, and neurogenesis in mice.     

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.     

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

Mutations in the clk-1 gene of Caenorhabditis elegans result in an extended life span and an average slowing down of developmental and behavioral rates. However, it has not been possible to identify biochemical changes that might underlie the extension of life span observed in clk-1 mutants, and therefore the function of CLK-1 in C. elegans remains unknown. In this report, we analyzed the effect of clk-1 mutation on ubiquinone (UQ(9)) biosynthesis and show that clk-1 mutants mitochondria do not contain detectable levels of UQ(9). Instead, the UQ(9) biosynthesis intermediate, demethoxyubiquinone (DMQ(9)), is present at high levels. This result demonstrates that CLK-1 is absolutely required for the biosynthesis of UQ(9) in C. elegans. Interestingly, the activity levels of NADH-cytochrome c reductase and succinate-cytochrome c reductase in mutant mitochondria are very similar to those in the wild-type, suggesting that DMQ(9) can function as an electron carrier in the respiratory chain. To test this possibility, the short side chain derivative DMQ(2) was chemically synthesized. We find that DMQ(2) can act as an electron acceptor for both complex I and complex II in clk-1 mutant mitochondria, while another ubiquinone biosynthesis precursor, 3-hydroxy-UQ(2), cannot. The accumulation of DMQ(9) and its use in mutant mitochondria indicate, for the first time in any organism, a link between the alteration in the quinone species used in respiration and life span.     

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.     

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.     

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.     

Bacterium-induced internal egg hatching frequency is predictive of life span in Caenorhabditis elegans populations

Internal egg hatching in Caenorhabditis elegans, "worm bagging," is induced by exposure to bacteria. This study demonstrates that the determination of worm bagging frequency allows for advanced insight into the degree of bacterial pathogenicity and is highly predictive of the survival of worm populations. Therefore, worm bagging frequency can be regarded as a reliable population-wide stress reporter.     

Mitochondrial electron transport is a key determinant of life span in Caenorhabditis elegans.

Increased protection from reactive oxygen species (ROS) is believed to increase life span. However, it has not been clearly demonstrated that endogenous ROS production actually limits normal life span. We have identified a mutation in the Caenorhabditis elegans iron sulfur protein (isp-1) of mitochondrial complex III, which results in low oxygen consumption, decreased sensitivity to ROS, and increased life span. Furthermore, combining isp-1(qm150) with a mutation (daf-2) that increases resistance to ROS does not result in any significant further increase in adult life span. These findings indicate that both isp-1 and daf-2 mutations increase life span by lowering oxidative stress and result in the maximum life span increase that can be produced in this way.     

Paralogy and orthology of tyrosine kinases that can extend the life span of Caenorhabditis elegans

Modification of any one of three transmembrane protein tyrosine kinase (PTK) genes, old-1, old-2 (formerly tkr-1 and tkr-2, respectively), and daf-2 can extend the mean and maximum life span of the nematode Caenorhabditis elegans. To identify paralogs and orthologs, we delineated relationships between these three PTKs and all known transmembrane PTKs and all known mammalian nontransmembrane PTKs using molecular phylogenetics. The tree includes a number of invertebrate receptor PTKs and a novel mammalian receptor PTK (inferred from the expressed-sequence tag database) that have not previously been analyzed. old-1 and old-2 were found to be members of a surprisingly large C. elegans PTK family having 16 members. Interestingly, only four members of this transmembrane family appeared to have receptor domains (immunoglobulin-like in each case). The C-terminal domain of this family was found to have a unique sequence motif that could be important for downstream signaling. Among mammalian PTKs, the old-1/old-2 family appeared to be most closely related to the Pdgfr, Fgfr, Ret, and Tie/Tek families. However, these families appeared to have split too early from the old-1/old-2 family to be orthologs, suggesting that a mammalian ortholog could yet be discovered. An extensive search of the expressed-sequence tag database suggested no additional candidate orthologs. In contrast to old-1 and old-2, daf-2 had no C. elegans paralogs. Although daf-2 was most closely related to the mammalian insulin receptor family, a hydra insulin receptor-like sequence suggested that daf-2 might not be an ortholog of the insulin receptor family. Among PTKs, the old-1/old-2 family and daf-2 were not particularly closely related, raising the possibility that other PTK families might extend life span. On a more general note, our survey of the expressed-sequence tag database suggested that few, if any, additional mammalian PTK families are likely to be discovered. The one novel family that was discovered could represent a novel oncogene family, given the prevalence of oncogenes among PTKs. Finally, the PTK tree was consistent with nematodes and fruit flies being as divergent as nematodes and mammals, suggesting that life extension mechanisms shared by nematodes and fruit flies would be reasonable candidates for extending mammalian life spans.     

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.     

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.     

Manganous ion supplementation accelerates wild type development, enhances stress resistance, and rescues the life span of a short-lived Caenorhabditis elegans mutant

Relative to iron and copper we know very little about the cellular roles of manganese. Some studies claim that manganese acts as a radical scavenger in unicellular organisms, while there have been other reports that manganese causes Parkinson's disease-like syndrome, DNA fragmentation, and interferes with cellular energy production. The goal of this study was to uncover if manganese has any free radical scavenging properties in the complex multicellular organism, Caenorhabditis elegans. We measured internal manganese in supplemented worms using inductively coupled plasma mass spectrometry (ICP-MS) and the data obtained suggest that manganese supplemented to the growth medium is taken up by the worms. We found that manganese did not appear to be toxic as supplementation did not negatively effect development or fertility. In fact, supplementation at higher levels accelerated development and increased total fertility of wild type worms by 16%. Manganese-supplemented wild type worms were found to be thermotolerant and, under certain conditions, long-lived. In addition, the oxidatively challenged C. elegans strain mev-1's short life span was significantly increased after manganese supplementation. Although manganese appears to be beneficial to C. elegans, the mode of action remains unclear. Manganese may work directly as a free radical scavenger, as it has been postulated to do so in unicellular organisms, or may work indirectly by up regulating several protective factors.     

Ginkgo biloba extract EGb 761 increases stress resistance and extends life span of Caenorhabditis elegans.

EGb 761, a standardized extract of Ginkgo biloba leaves, has been used in clinical trials for its beneficial effects on brain functions. In mammals, EGb 761 has been shown to enhance cognition, stress resistance, and longevity, but its molecular and cellular mechanisms are not known. In the present investigation, we used the model organism Caenorhabditis elegans to evaluate pharmacological effects of EGb 761 on aging. We tested the theory that EGb 761 augments the natural antioxidant system of C elegans, and thus increases stress resistance and longevity. We found that treatment of the wild-type worms with EGb 761 extended their median life span by 8%. Amongst several purified components of EGb 761, the flavonoid tamarixetin showed the most dramatic effect: it extended the median life span by 25%. Furthermore, EGb 761 increased the wild type's resistance to acute oxidative and thermal stress by 33% and 25%, respectively. Treatment of the prematurely aging mutant worms mev-1 with EGb 761 increased their resistance to acute oxidative and thermal stress by 33% and 11%, respectively. It appears that oxidative stress, a major determinant of life span, as well as other types of stress, can be successfully counteracted by the Ginlkgo biloba extract EGb 761.     

Deficiency of Caenorhabditis elegans RecQ5 homologue reduces life span and increases sensitivity to ionizing radiation

Gene expression and RNA interference phenotypes were investigated for a Caenorhabditis elegans homologue (Ce-RCQ-5) of human RecQ5 protein. Expression of the mRNA was observed by in situ hybridization from earliest embryogenesis and gradually decreased during late embryogenesis. Ce-RCQ-5 was immuno-localized in the nuclei of embryos, germ cells, and oocytes and also in the nuclei of various somatic cells of larvae and adults. Despite ubiquitous expression in postembryonic cells, RCQ-5 protein expression was highest in intestinal cells, which was confirmed by tagging the gene expression with green fluorescence protein. When endogenous Ce-rcq-5 gene expression was inhibited by RNA interference, no clear phenotypes were observed during development. However, C. elegans life span was reduced by 37% due to RNA interference of rcq-5 gene, suggesting its possible role in maintenance of genomic stability, as has been ascribed to other RecQ family DNA helicases. In addition, C. elegans became significantly more sensitive to ionizing radiation after inhibition of rcq-5 gene expression, indicating an involvement of C. elegans RCQ-5 in a cellular response to DNA damage, possibly in DNA repair.     

The Caenorhabditis elegans orthologue of mammalian puromycin-sensitive aminopeptidase has roles in embryogenesis and reproduction

Mammals possess membrane-associated and cytosolic forms of the puromycin-sensitive aminopeptidase (PSA; EC 3.4.11.14). Increasing evidence suggests the membrane PSA is involved in neuromodulation within the central nervous system and in reproductive biology. The functional roles of the cytosolic PSA are less clear. The genome of the nematode Caenorhabditis elegans encodes an aminopeptidase, F49E8.3 (PAM-1), that is orthologous to PSA, and sequence analysis predicts it to be cytosolic. We have determined the spatio/temporal gene expression pattern of pam-1 by using the promoter region of F49E8.3 to control expression in the nematode of a second exon translational fusion of the aminopeptidase to green fluorescent protein. Cytosolic fluorescence was observed throughout development in the intestine and nerve cells of the head. Neuronal expression was also observed in the tail of adult males. Recombinant PAM-1, expressed and purified from Escherichia coli, hydrolyzed the N-terminal amino acid from peptide substrates. Favored substrates had positively charged or small neutral amino acids in the N-terminal position. Peptide hydrolysis was inhibited by the metal-chelating agent 1,10-phenanthroline and by the aminopeptidase inhibitors actinonin, amastatin, and leuhistin. However, the enzyme was approximately 100-fold less sensitive toward puromycin (IC50, 135 mum) than other PSA homologues. Following inactivation of the enzyme, aminopeptidase activity was recovered with Zn2+, Co2+, and Ni2+. Silencing expression of pam-1 by RNA interference resulted in 30% embryonic lethality. Surviving adult hermaphrodites deposited large numbers of oocytes throughout the self-fertile period. The overall brood size was, however, unaffected. We conclude that pam-1 encodes an aminopeptidase that clusters phylogenetically with the PSAs, despite attenuated sensitivity toward puromycin, and that it functions in embryo development and reproduction of the nematode.