Molecular Mechanism of Maternal Rescue in the clk-1 Mutants of Caenorhabditis elegans

Abstract

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.The clk-1 gene of Caenorhabditis elegans encodes a 187-amino acid protein that is orthologous to yeast Coq7p as well as to rodent and human sequences (1). CLK-1 is necessary for ubiquinone (coenzyme Q, or Q) 1 biosynthesis. Q n is a prenylated benzoquinone (the numerical subscript denotes the number of isoprene repeats, which is a species-specific trait) that is primarily involved in numerous redox reactions in the cell, including those in the mitochondrial electron transport chain. In its reduced form, Q is an antioxidant, preventing lipid peroxidation in biological membranes (2, 3). In the absence of CLK-1, worms and mice are defective in Q biosynthesis (4 -6) and accumulate the Q precursor, demethoxyubiquinone (DMQ 9 ) (5, 6). Yet, mitochondrial respiration is not strongly affected in clk-1 mutants, indicating that DMQ can substitute for some, but not all, of the Q functions in vivo (5-9).Despite the presence of DMQ, clk-1 mutants require Q from their bacterial food source to proceed through development and become fertile adults (4, 10). Indeed, when clk-1 mutants are transferred onto a Q 8 -deficient E. coli strain before the early larval stages, they arrest development at the second larval stage, and when the animals are transferred onto Q 8 -deficient bacteria later in development, they become sterile adults. This fact strongly suggests that DMQ 9 cannot functionally replace Q 9 for all of its cellular roles, and that at least some Q is required, which can be provided as Q 8 in the diet. Thus, the viability and overall good health of the clk-1 mutants seems to rely on a combination of endogenously produced DM...

Decreased Energy Metabolism Extends Life Span inCaenorhabditis elegansWithout Reducing Oxidative Damage

“…Synergistic effects of Clk mutants and daf-2 life span: Deletion of clk-1 has been shown to markedly extend the life span of daf-2 worms such that clk-1;daf-2 worms live much longer than if the already extended life spans of clk-1 and daf-2 were added together (Lakowski and Hekimi 1996;Hekimi et al 2001;Burgess et al 2003). If the synergism between clk-1 and daf-2 life spans occurs at the phenotypic level, we would expect the other Clk mutants to similarly extend the life span of daf-2 worms, while if the mechanism occurs at a molecular level, this marked extension of daf-2 life span would be expected to be specific to clk-1.…”

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

“…Previously, it was shown that the small amounts of coenzyme Q 8 assimilated by clk-1 animals fed the standard OP50 E. coli diet are sufficient to allow for growth and reproduction (Jonassen et al, 2002). The maternal effect rescue of clk-1 phenotypes is attributed to the small contribution of maternally supplied Q in the egg (Burgess et al, 2003). In addition, tRNA suppressors present in the clk-1(e2519) genetic background supplied only a low level of Q 9 biosynthesis, yet allowed these animals to have brood sizes similar to wild type, and many of the suppressed mutants also had normal lifespans (Branicky et al, 2006).…”

“…C. elegans clk-1 mutants fed Q-less E. coli (GD1 ; Table 1) show arrested or greatly slowed development and are sterile Burgess et al, 2003). GD1 E. coli harbor a deletion of the ubiG gene, encoding an O-methyltransferase in the coenzyme Q biosynthetic pathway, and fail to synthesize Q 8 (Hsu et al, 1996).…”

“…The C. elegans clk-1 mutant has a defect in Q 9 biosynthesis, accumulates the intermediate demethoxyubiquinone-9, and relies on the Q supplied via a diet of E. coli for development to adulthood and fertility Miyadera et al ., 2001;Burgess et al ., 2003;Hihi et al ., 2003). clk-1 mutants fed E. coli strains lacking Q from the time of hatching stop growing during the L2 larval stage , and after about 1 week will eventually develop into sterile adults (Burgess et al ., 2003). Similarly, clk-1 dauer larvae fed the Q-less diet recover to adulthood but are completely sterile ).…”

“…Similarly, clk-1 dauer larvae fed the Q-less diet recover to adulthood but are completely sterile ). The growth arrest and sterility phenotypes of the clk-1 mutants fed Q-less E. coli are very reproducible and can be assessed quantitatively over a period of several days Burgess et al ., 2003). The clk-1 mutants also experience growth arrest in axenic media (Braeckman et al ., 1999).…”