Cell metabolism

Aging is a multifactoral biological process of progressive and deleterious changes partially attributed to a build up of oxidatively damaged biomolecules resulting from attacks by free radicals. Methionine sulfoxide reductases (Msrs) are enzymes that repair oxidized methionine (Met) residues found in proteins. Oxidized Met produces two enantiomers, Met-S-(o) and Met-R-(o), reduced by MsrA and MsrB respectively. Unlike other model organisms, our MsrA null fly mutant did not display increased sensitivity to oxidative stress or shortened lifespan, suggesting that in Drosophila, having either a functional copy of either Msr is sufficient. Here, two Msr mutant types were phenotypically assayed against isogenic controls. Results suggest that only the loss of both MsrA and MsrB produces increased sensitivity to oxidative stress and shortened lifespan, while locomotor defects became more severe with the full Msr knockout fly.

Mitochondrial disorders resulting from defects in oxidative phosphorylation are the most common form of inherited metabolic disease. Mutations in the human mitochondrial translation elongation factor GFM1 have recently been shown to cause the lethal pediatric disorder Combined Oxidative Phosphorylation Deficiency Syndrome (COXPD1). Children harboring mutations in GFM1 exhibit severe developmental, metabolic and neurological abnormalities. This work describes the identification and extensive characterization of the first known mutations in iconoclast (ico), the Drosophila orthologue of GFM1. Expression of human GFM1 can rescue ico null mutants, demonstrating functional conservation between the human and fly proteins. While point mutations in ico result in developmental defects and death during embryogenesis, animals null for ico survive until the second or third instar larval stage. These results indicate that in addition to loss-of-function consequences, point mutations in ico appear to produce toxic proteins with antimorphic or neomorphic effects. Consistent with this hypothesis, transgenic expression of a mutant ICO protein is lethal when expressed during development and inhibits growth when expressed in wing discs. In addition, animals with a single copy of an ico point mutation are more sensitive to acute hyperthermic or hypoxic stress. Removal of the positively-charged tail of the protein abolishes the toxic effects of mutant ICO, demonstrating that this domain is necessary for the harmful gain-of-function phenotypes observed in ico point mutants.

Most living organisms transduce electron transport chains in order to obtain energy. Flavin adenine dinucleotide (FAD) is a common electron transfer cofactor found in electron transport proteins referred to as flavoproteins. In this study, the different ionization and oxidation states of this cofactor found in cytochrome b5 reductase were identified spectroscopically and quantified as a function of solution potential and pH. The large data sets obtained from these experiments were analyzed and the acid dissociation constant for reduced cytochrome b5 reductase was determined.

Aconitase is an important enzyme in the citric Acid Cycle, is needed for maintenance of mitochondrial DNA, is a key regulator of iron in the cell, and is very sensitive to oxidative stress. We have isolatd the yeast ACO1 gene, which codes for aconitase, and randomly mutated it to create a mutant library of cells each expressing a different version of ACO1. We will select for oxidative stress resistant aconitase in S. cerevisiae by subjecting strains to successive rounds of heat shock and competitive growth against other mutants. The "winner" of this competition will then be analyzed for which version of aconitase it is expressing, which may lead to increased longevity.

In my thesis work, I attempted to construct a plasmid that would allow stable integration of genes into the Saccharomyces cerevisiae yeast genome under the control of the repressible TetO promoter. The yeast ACO1 gene was cloned under the control of the TetO operator and the tTA transactivator. This construct was inserted into yeast cells in order to observe the effects of aconitase overexpression on aging. Unfortunately, the transformed cells appeared incapable of aconitase expression as determined by glutamic acid auxptrophy, a phenotype of aconitase mutants. We have sequenced the pIT1ACO1 plasmid and have found many abnormalities in the promoter region. If the plasmid can be made to function as intended, the resulting yeast strain can be used in the future to determine if aconitase plays an important role in cellular aging.

Telomerase is associated with telomere production and nDNA protection. However, studies by Santos et al. have demonstrated that human telomerase has a mitochondrial entry sequence and in the presence of hydrogen peroxide it has been found inside the mitochondrion and may cause mitochondrial DNA mutations. Saccharomyces cerevisiae contains telomerase, but it does not have the mitochondrial entry sequence. To determine if the presence of telomerase in the mitochondria can induce mutations an experiment was developed in which a mitochondrion entry sequence would be fused to the S. cerevisiae telomerase enzyme. This fusion could then be screened in S. cerevisiae with an ade2 mutation for a simple color assay of mitochondrial activity. To date, no successful transformant has been identified. The frequency of incorrect ligations has been recognized and may indicate that the desired fusion is lethal to E. coli cells.