Drosophila Fruit Flies
One way to identify genes that directly regulate aging is to experimentally increase or decrease their expression, and then assay for effects on life span. Decreased life span is problematic, as it is likely to result from novel pathologies that do not normally limit life span. In contrast, increased life span can only result from alterations in limiting processes, and is more likely to identify genes directly related to aging. A strength of the D. melanogaster model system is that there are a variety of transgenic methods for increasing or decreasing the expression of specific genes under well-controlled conditions.
Extensive correlative evidence suggests that, for most organisms, oxidative damage may be a primary cause of aging and functional decline. Reactive oxygen species (ROS) are toxic forms of oxygen that are generated as a byproduct of normal metabolism. One of the most common is superoxide, produced as a byproduct of the mitochondria. ROS can damage cellular components, and such oxidatively damaged molecules and organelles have been found to accumulate in all aging organisms, at least those that have been examined, including D. melanogaster. Not surprisingly, the genes tested for effects on life span in D. melanogaster have been ones involved in preventing or repairing oxidative damage. The gene hsp70 was originally identified as a gene induced in response to heat and oxidative stress. Hsp70-family proteins can help prevent or repair protein damage caused by heat or ROS by preventing protein aggregation, facilitating protein refolding, and facilitating breakdown of damaged proteins. The enzymes superoxide dismutase (SOD) and catalase work together to detoxify ROS in cells. SOD exists in two forms: cytoplasmic (Cu/ZnSOD) and mitochondrial (Mn-SOD). SOD converts superoxide to hydrogen peroxide, and catalase converts hydrogen peroxide to water and oxygen. Another important defense against ROS involves the enzyme glutathione reductase. This enzyme generates reduced glutathione, which is an abundant small molecule that detoxifies ROS.
If increased expression of a gene increases life span, that gene is, by definition, a positive regulator of life span. Transgenic D. melanogaster containing an extra copy of the catalase, CuZnSOD, MnSOD, hsp70, or glutathione reductase genes generally exhibit increased gene expression, but have not been found to exhibit any consistent increase in life span under normal culture conditions. However, extra copies of hsp70 have produced small increases in life span after mild heat stress, and extra glutathione reductase has increased survival in an atmosphere of increased oxygen concentration—a condition known to increase oxidative stress.
Relatively large increases in life span have recently been achieved using more complex methods to control the expression of transgenes. The GAL4/UAS system was used to express human Cu/ZnSOD in a tissue-specific pattern during D. melanogaster development and aging, with expression in the adult occurring primarily in motorneurons. In other studies a system called FLP-out was used to express Cu/ZnSOD specifically in the adult fly. These experiments yielded increases in average life span of up to 48 percent.
At least two negative regulators of D. melanogaster life span have also been identified. In these cases, life span is increased when the gene is disrupted or its expression is decreased. A mutation in the methuselah gene increases life span by up to 35 percent, and also increases body size and stress resistance. Mutation of the Indy gene also increases life span.
The success in identifying genes regulating aging in D. melanogaster, each of which is related to genes in humans, suggests that the fruit fly will continue to be a leading model for aging research.
DEEPAK BHOLE JOHN TOWER
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