The effects of heavy metal toxicity and oxidative stress on genetically-compromised models of Drosophila melanogaster

Heavy metal toxicity and oxidative stress are two important factors in the aging process and in the initiation and/or progression of neurodegenerative disorders. I investigated the effects of both essential (iron, copper, and zinc) and toxic (cadmium) heavy metals on wild-type and on genetically-compromised strains of Drosophila melanogaster. I demonstrated that heavy metals act as deterrents to feeding and to egg laying at high concentrations, necessitating the use of lower concentrations for toxicity experiments. Considering that heavy metals can be both essential and toxic, it is important to maintain their homeostasis. Metal-responsive transcription factor (MTF-1) is a highly conserved zinc finger protein that regulates the activation of various metal homeostatic genes. Earlier studies have shown that MTF-1 knockout flies have an enhanced sensitivity to heavy metal stress. To further understand the role of heavy metal toxicity and MTF-1 in aging, I studied the effects of MTF-1 overexpression on Drosophila life-span under normal conditions and under stress. My results illustrate that MTF-1 overexpression in the nervous system enhances resistance to heavy metals and extends normal life-span by up to 40%, indicating that heavy metal toxicity in the nervous system is an important cause of aging in Drosophila melanogaster. Next, I constructed a Drosophila model of Menkes disease by RNAi-induced silencing of DmATP7, the sole orthologue of the mammalian ATP7A gene, in the digestive tract. RNAi-induced silencing of DmATP7 in the digestive tract limited copper delivery to the brain, interfered with normal brain development throughout the metamorphosis, and caused death in the majority of flies. Flies that survived to adulthood, however, had a normal morphology and lived significantly longer than the control flies, suggesting that RNAi-silencing results in expression level very near the threshold for normal development. Finally, I tested the effects of various dietary and enzymatic antioxidants on genetically compromised models of D. melanogaster. While supplementation of vitamin E had no significant effect on normal life-span, it significantly enhanced resistance towards hyperoxia and extended the short life-span of Cu/Zn superoxide dismutase-deficient flies. Vitamin C, on the other hand, exhibited a strong pro-oxidant activity in the presence of iron. In the presence of vitamin C, the whole-body iron pool was significantly increased and flies were short lived. With fly models of Huntington's disease, neither dietary supplementation of antioxidants nor overexpression of antioxidant enzymes could rescue the lethal phenotypes of flies, indicating that the progress of the disease cannot be inhibited by antioxidants. With fly models of Alzheimer's disease, overexpression of antioxidant enzyme, glutathione-S-transferase S1 (GstS1), partially extended the short life-span of flies and almost fully rescued their locomotory defects. I provide evidence that the protective effects of GstS1 may be attributed to its antioxidant and zinc homeostatic properties. Overexpression of catalase antioxidant enzyme, however, significantly enhanced neurodegeneration in Alzheimer's flies and caused an early death, presumably due to catalase binding to amyloid-beta peptides. Taken together, my results illustrate that oxidative stress and heavy metal toxicity are playing an important role in aging and neurodegeneration.