Genetics of Metabolism in Caenorhabditis elegans

Jujiao Kuang (2010). Genetics of Metabolism in Caenorhabditis elegans PhD Thesis, School of Biological Sciences, The University of Queensland.

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Author Jujiao Kuang
Thesis Title Genetics of Metabolism in Caenorhabditis elegans
School, Centre or Institute School of Biological Sciences
Institution The University of Queensland
Publication date 2010-08
Thesis type PhD Thesis
Total pages 100
Total colour pages 14
Total black and white pages 86
Subjects 06 Biological Sciences
Abstract/Summary Mitochondria are the organelles of oxidative respiration in eukaryotic cells that provide the majority of the chemical energy necessary for cellular reactions. Lifespan in Caenorhabditis elegans, Drosophila, and mice can be extended by a decrease in mitochondrial electron transport chain (ETC) function, but the mechanism behind this extension is still unknown. In the present study, I have used the model organism Caenorhabditis elegans to investigate mitochondrial-mediated longevity. I began by looking broadly at genes encoding subunits of all four ETC complexes, after which I focused my efforts on complex II. I firstly looked at the role of complex II in determining the metabolic state of the organism, and secondly, at its effect on the level of oxidative stress. Nematodes with disrupted ETC activity exhibited dramatic alterations in their mitochondrial physiology, metabolism, and oxidative stress levels. When complex I, III and IV genes were suppressed, the reduction in ATP level and fecundity was associated with extended lifespan. However, suppression of genes encoding subunits of complex II did not extend longevity. Interestingly, despite a lack of lifespan extension, disruption of most subunits of complex II still caused a decrease in the metabolic rate, although not as dramatically as when other complexes were suppressed. Furthermore, nematodes became more tolerant of chemically induced oxidative stress when subunits of complexes I, III or IV were suppressed. This resistance to oxidative stress was associated with induction of protective superoxide dismutase (SOD) genes, sod-3 and sod-5. The induction of the SOD genes does not seem to be the cause of the extended lifespan in these nematodes, however, as gene induction even occurred in animals without an extended lifespan. Furthermore, mutations that completely eliminate the function of sod-3 and sod-5 did not eliminate the extended lifespan, though they did shorten the lifespan associated with complex II gene suppression. The unique phenotypes associated with complex II gene suppression may be due to any of a number of unique metabolic functions. Complex II is unusual in Caenorhabditis elegans as it participates in mitochondrial electron transport associated with both aerobic and anaerobic respiration. It also differs from the other complexes in that electron transport is not linked to translocation of protons across the mitochondrial inner membrane to create the mitochondrial membrane potential (MMP). Unlike the other complexes, complex II participates in the TCA cycle; as the enzyme succinate dehydrogenase (SDH) under aerobic conditions and as fumarate reductase (FRD) under anaerobic conditions. The unique roles of complex II in the metabolism of the cell, as well as the unique phenotypes associated with gene suppression led me to examine each subunit for a specific role in metabolism and longevity. Despite a decrease in the metabolic rate associated with complex II disruption, it was not as severe as with suppression of the other complexes. Interestingly, I observed a strong inverse correlation between the MMP and ATP levels associated with complex II suppression. This suggests that metabolic homeostasis has been preserved to a much greater degree when complex II is disrupted than with the other complexes. Thus, I propose that the more severe metabolic disruption that results from suppression of complexes I, III and IV may result in a metabolic crisis that leads to activation of a diapause-like, energy-conserving longevity program. The ability of nematodes with impaired complex II to maintain physiological balance seems to prevent induction of the stress response, as well as the corresponding lifespan extension. The final part of the thesis is to investigate the link between complex II and oxidative stress, as complex II has been suggested as a significant source of ROS when its function is compromised genetically. I selected three oxidative stressors with unique proposed target sites, and then examined the response to oxidative stress following RNAi suppression. Surprisingly, only disruption of the three isoforms encoding flavoprotein SDHA resulted in significant changes in the oxidative response. Furthermore, the response to each of the three oxidative stressors was quite unique. Paraquat sensitivity was decreased by suppression of the FRD isoform, sdha-3, as well as by one of the two SDH isoforms, sdha-2. Suppression of the closely related SDH isoform, sdha-1 gave the surprising result of increased paraquat sensitivity. In contrast, sensitivity to phosphine was increased by suppression of both sdha-2 and sdha-3. While sdha-1 was interrupted, phosphine sensitivity was not significantly decreased, but it trended in that direction. In fact there was a significant inverse correlation between the sensitivities to phosphine and paraquat. Based on these results, sdha-2 and sdha-3 seem to respond in tandem as if they function together as a heterodimer. This has not been suggested previously, as the existing literature, which assumes, based on sequence homology, that sdha-1 and sdha-2 function independently of sdha-3.
Keyword Mitochondria
Electron transport chain
Oxidative stress
Additional Notes colour pages: 16, 18, 20, 37, 41, 44, 56, 58, 60, 61, 75, 77, 78, 80

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Created: Mon, 24 Jan 2011, 12:15:56 EST by Ms Jujiao Kuang on behalf of Library - Information Access Service