Caenorhabditis elegans: decay of isocitrate lyase during larval development

Starvation Responses Throughout the Caenorhabditiselegans Life Cycle

Caenorhabditis elegans survives on ephemeral food sources in the wild, and the species has a variety of adaptive responses to starvation. These features of its life history make the worm a powerful model for studying developmental, behavioral, and metabolic starvation responses. Starvation resistance is fundamental to life in the wild, and it is relevant to aging and common diseases such as cancer and diabetes. Worms respond to acute starvation at different times in the life cycle by arresting development and altering gene expression and metabolism. They also anticipate starvation during early larval development, engaging an alternative developmental program resulting in dauer diapause. By arresting development, these responses postpone growth and reproduction until feeding resumes. A common set of signaling pathways mediates systemic regulation of development in each context but with important distinctions. Several aspects of behavior, including feeding, foraging, taxis, egg laying, sleep, and associative learning, are also affected by starvation. A variety of conserved signaling, gene regulatory, and metabolic mechanisms support adaptation to starvation. Early life starvation can have persistent effects on adults and their descendants. With its short generation time, C. elegans is an ideal model for studying maternal provisioning, transgenerational epigenetic inheritance, and developmental origins of adult health and disease in humans. This review provides a comprehensive overview of starvation responses throughout the C. elegans life cycle.

Knock-down of heat-shock protein 90 and isocitrate lyase gene expression reduced root-knot nematode reproduction

Crop losses caused by nematode infections are estimated to be valued at USD 157 billion per year. Meloidogyne incognita, a root-knot nematode (RKN), is considered to be one of the most important plant pathogens due to its worldwide distribution and the austere damage it can cause to a large variety of agronomically important crops. RNA interference (RNAi), a gene silencing process, has proven to be a valuable biotechnology alternative method for RKN control. In this study, the RNAi approach was applied, using fragments of M. incognita genes that encode for two essential molecules, heat-shock protein 90 (HSP90) and isocitrate lyase (ICL). Plant-mediated RNAi of these genes led to a significant level of resistance against M. incognita in the transgenic Nicotiana tabacum plants. Bioassays of plants expressing HSP90 dsRNA demonstrated a delay in gall formation and up to 46% reduction in eggs compared with wild-type plants. A reduction in the level of HSP90 transcripts was observed in recovered eggs from plants expressing dsRNA, indicating that gene silencing persisted and was passed along to first progeny. The ICL knock-down had no clear effect on gall formation but resulted in up to 77% reduction in egg oviposition compared with wild-type plants. Our data suggest that both genes may be involved in RKN development and reproduction. Thus, in this paper, we describe essential candidate genes that could be applied to generate genetically modified crops, using the RNAi strategy to control RKN parasitism.

SAGE surveys C. elegans carbohydrate metabolism: evidence for an anaerobic shift in the long-lived dauer larva

The dauer larva, a non-feeding and developmentally arrested stage of the free-living nematode Caenorhabditis elegans, is morphologically and physiologically specialized for survival and dispersal during adverse growth conditions. The ability of dauer larvae to live several times longer than the continuous developmental life span has been attributed in part to a repressed metabolism. We used serial analysis of gene expression (SAGE) profiles from dauer larvae and mixed growing stages to compare expression patterns for genes with known or predicted roles in glycolysis, gluconeogenesis, glycogen metabolism, the Krebs and glyoxylate cycles, and selected fermentation pathways. Ratios of mixed:dauer transcripts indicated non-dauer enrichment that was consistent with previously determined adult:dauer enzyme activity ratios for hexokinase (glycolysis), phosphoenolpyruvate carboxykinase and fructose 1,6-bisphosphatase (gluconeogenesis), isocitrate dehydrogenase (NADP-dependent), and isocitrate lyase-malate synthase (glyoxylate cycle). Transcripts for the majority of Krebs cycle components were not differentially represented in the two profiles. Transcript abundance for pyruvate kinase, alcohol dehydrogenase, a putative cytosolic fumarate reductase, two pyruvate dehydrogenase components, and a succinyl CoA synthetase alpha subunit implied that anaerobic pathways were upregulated in dauer larvae. Generation of nutritive fermentation byproducts and the moderation of oxidative damage are potential benefits of a hypoxic dauer interior.

Modeling human peroxisome biogenesis disorders in the nematode Caenorhabditis elegans

Peroxisomes are ubiquitous eukaryotic organelles. The proteins required for peroxisome biogenesis are called peroxins, and mutations in the peroxin genes cause the devastating human developmental syndromes called the peroxisome biogenesis disorders. Our interest is in elaborating the roles that peroxisomes play in Caenorhabditis elegans development, and in establishing an invertebrate model system for the human peroxisome biogenesis disorders. The genome of C. elegans encodes homologs of 11 of the 13 human peroxins. We disrupted five nematode peroxins using RNA interference (RNAi) and found that RNAi knockdown of each one causes an early larval arrest at the L1 stage. Using a green fluorescent protein reporter targeted to the peroxisome, we establish that peroxisomal import is impaired in prx-5(RNAi) nematodes. prx-5(RNAi) animals are blocked very early in the L1 stage and do not initiate normal postembryonic cell divisions, similar to starvation-arrested larvae. Cell and axonal migrations that normally occur during the L1 stage also appear blocked. We conclude that peroxisome function is required for C. elegans postembryonic development and that disruption of peroxisome assembly by prx-5(RNAi) prevents scheduled postembryonic cell divisions. Defects in the cellular localization of peroxisomal proteins and in development are shared features of human and nematode peroxisome biogenesis disorders. In setting up a C. elegans model of peroxisomal biogenesis disorders, we suggest that genetic screens for suppression of the Prx developmental block will facilitate identification of novel intervention strategies and may provide new insights into human disease pathogenesis.

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.

Metabolic adaptations to environmental changes in Caenorhabditis elegans

Metabolic adaptations to environmental changes were studied in Caenorhabditis elegans. To assess adjustments in enzyme function, maximum activities of key enzymes of main metabolic pathways were determined. After a 12 h incubation at varying temperatures (10, 20 degrees C) and oxygen supplies (normoxia or anoxia), the activities of the following enzymes were determined at two measuring temperatures in tissue extracts: lactate dehydrogenase (LDH; anaerobic glycolysis), 3-hydroxyacyl-CoA-dehydrogenase (HCDH; fatty acid oxidation), isocitrate dehydrogenases (NAD-IDH, NADP-IDH; tricarboxylic acid cycle) and isocitrate lyase (ICL; glyoxylate cycle). Incubation at 20 degrees C induced a strong increase in maximum LDH activity. Anoxic incubation caused maximum HCDH and NADP-IDH activities and, at 10 degrees C incubation, LDH activity to increase. Maximum NAD-IDH and ICL activities were not influenced by any type of incubation. In order to study the time course of metabolic adaptations to varying oxygen supplies, relative quantities of free and protein-bound NADH were determined in living C. elegans using time-resolved fluorescence spectroscopy. During several hours of anoxia, free and protein-bound NADH showed different time courses. One main result was that just at the moment when the protein-bound NADH had reached a constant level, and the free NADH started to increase rapidly, the worms fell into a rigor state. The data on enzyme activity and NADH fluorescence can be interpreted on the basis of a two-stage model of anaerobiosis.

Ammonia, respiration, and longevity in nematodes: insights on metabolic regulation of life span from temporal rescaling

To better understand metabolic correlates of longevity, we used a graphical technique to compare the adult temporal patterns of several markers of metabolic activity - ammonia elimination, oxygen consumption rate, ATP levels, and (in freeze-permeabilized worms) the rate of NADPH-activated, lucigenin-mediated superoxide formation - as observed by us and others in normal and long-lived mutant Caenorhabditis elegans strains. All of these traits declined with time, and when their logarithms were plotted against time, appeared reasonably linear for most of the duration of the experiments. The profiles for ammonia output conformed well to parallel regression lines; those for the other metabolic parameters varied widely in slope as originally plotted by the authors, but much less so when replotted as logarithms against adjusted time, scaled by the reciprocal of strain longevity. This is consistent with coregulation of life span, respiration rate, ATP levels, lucigenin reactivity, but not ammonia excretion, by a physiological clock distinguishable from chronologic time. Plots, scaled appropriately for equalized slopes, highlighted y-axis intercept differences among strains. On rescaled plots, these constitute deviations from the expectation based on 'strain-specific clock' differences alone. With one exception, y-intercept effects were observed only for mutants in an insulin-like signaling pathway.

Type-II 3-oxoacyl-CoA thiolase of the nematode Caenorhabditis elegans is located in peroxisomes, highly expressed during larval stages and induced by clofibrate

We examined the expression and localization of type-II 3-oxoacyl-CoA thiolase in the nematode Caenorhabditis elegans. Type-II thiolase acts on 3-oxoacyl-CoA esters with a methyl group at the alpha carbon, whereas conventional thiolases do not. Mammalian type-II thiolase, which is also termed sterol carrier protein x (SCPx) or SCP2/3-oxoacyl-CoA thiolase, is located in the peroxisomes and involved in phytanic acid degradation and most probably in bile acid synthesis. The nematode enzyme lacks the SCP2 domain, which carries the peroxisomal-targeting signal, but produces bile acids in a cell-free system. Northern and Western blot analyses demonstrated that C. elegans expressed type-II thiolase throughout its life cycle, especially during the larval stages, and that the expression was significantly enhanced by the addition of clofibrate at 5 mM or more to the culture medium. Whole-mount in situ hybridization and immunostaining of L4 larvae revealed that the enzyme was mainly expressed in intestinal cells, which are multifunctional like many of the cell types in C. elegans. Subcellular fractionation and indirect immunoelectron microscopy of the nematode detected the enzyme in the matrix of peroxisomes. These results indicate the fundamental homology between mammalian SCPx and the nematode enzyme regardless of whether the SCP2 part is fused, suggesting their common physiological roles.

Caenorhabditis elegans: purification of isocitrate lyase and the isolation and cell-free translation of poly(A+)RNA

Isocitrate lyase (EC 4.1.3.1) from mixed larval populations of Caenorhabditis elegans was stabilized in crude extracts by centrifugation over a 0.2-0.6 M sucrose gradient for 2.5 hr in a vertical rotor (VTi 50) at 210,000g. The peak fractions from this sucrose gradient showed a half-life of 33 hr at 30 C and 225 hr at 4 C in contrast to 2.5 and 52 hr, respectively, for the crude extract. A purification scheme involving (NH4)2SO4 precipitation and chromatography on Sepharose 6B and diethylaminoethyl-cellulose yielded isocitrate lyase that gave one band after electrophoresis in a sodium dodecyl sulfate-gel polymerized from 12% acrylamide. The purified enzyme with a molecular weight of 250,000 and subunit molecular weight of 61,600, had a specific activity of 2 mumoles glyoxylate formed min-1 mg protein-1, and was obtained in a 4% yield. Isocitrate lyase from C. elegans lost 80-85% of its activity in the precipitation by 33-55% (NH4)2SO4, but this step appeared to be necessary for purification to homogeneity. The use of fast protein liquid chromatography appeared to be promising in that it provided an enzyme preparation that was about 50% pure with a specific activity as high as 3 mumoles glyoxylate formed min-1 mg protein-1. Poly(A+)RNA was isolated from C. elegans and translated in wheat germ cell-free system. Analysis on a 10% sodium dodecyl sulfate-polyacrylamide gel showed varied translation products including one or more 60,000-Da polypeptides.