FRUIT FLIES, DROSOPHILA

The fruit fly, Drosophila melanogaster, has been a leading model for aging research since early in the twentieth century. The benefits of using D. melanogaster for research include its short life span (1 to 2 months), ease of culture, and the availability of powerful genetic and molecular biological tools. The latter includes the Drosophila "P element," which is a transposable element. Transposable elements are pieces of DNA that can insert into the DNA of a chromosome, and can move from one place in the DNA to another under appropriate conditions. The P element has been engineered so that scientists can control its movement in D. melanogaster. For example, it can be used to carry modified or foreign genes into the D. melanogaster genome, where they will then be inherited by succeeding generations. Such introduced genes are called transgenes, and the resultant strain is said to be transgenic. One disadvantage of D. melanogaster for aging research is that its small size precludes detailed study of pathology and cause of death. For this reason, life span is still the most reliable measure of the D. melanogaster aging rate.

The use of D. melanogaster as a model is supported by the numerous similarities between aging in fruit flies and mammals, including a decline in performance of functions such as reproduction, learning, behavior, and locomotion. At the ultrastructural level, similarities include deterioration of muscle and nervous tissue, as well as accumulation of intracellular inclusions such as pigments (lipofuscin), abnormal mitochondria, and virus-like particles. At the molecular level, similarities include the accumulation of damaged DNA, proteins, and mitochondria. Finally, both fruit flies and mammals exhibit a tight link between stress responses and aging.

An important difference between D. melanogaster and mammals is the fact that fruit flies are cold-blooded. Raising the environmental temperature increases the rate of D. melanogaster metabolism and aging, and decreases its life span. The ability to manipulate life span in this way has proven to be useful in many studies, and has provided some the first evidence of a link between metabolic activity and life span.

Selection experiments and quantitative trait loci

Current theory suggests that aging exists due to the decreasing force of natural selection as a function of age, and that it has an underlying genetic basis. Much of the experimental support for this theory has come from study of D. melanogaster. When an appropriate population of D. melanogaster is cultured in the laboratory using only the oldest individuals to reproduce the next generation, the force of selection now acts on the older individuals. Over many generations, this selection results in populations with significantly increased fertility at older ages and with significantly increased life span relative to control populations. In other words, by experimentally altering "natural" selection in the laboratory, D. melanogaster is forced to evolve into a long-lived strain. Such long-lived strains also exhibit increased stress resistance and an increased expression of stress response genes, suggesting that life span and stress resistance are related.

Life span varies quantitatively—either shorter or longer—and for this reason is called a quantitative trait. The chromosomal loci, or genes, that affect life span are called quantitative trait loci, or QTLs. QTLs affecting life span have been identified and genetically mapped using appropriate crosses between strains having different life spans.

Changes in gene expression during aging

Aging in D. melanogaster is associated with characteristic changes in gene expression. During aging, the expression of certain stress response genes is increased in age-specific and tissue-specific patterns. At least part of this increase appears to be a response to oxidative stress. In contrast, as flies age there is a decreased ability to further increase the expression of these genes and survive acute stresses such as heat shock. Recent data suggests that a similar pattern of stress-response gene expression occurs in aging mammals. In addition, a number of other D. melanogaster genes exhibit characteristic dynamic expression patterns during aging. These include several genes with important developmental functions, though the significance of their altered expression during aging is currently unknown.

Transgenics

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

See also Accelerated Aging: Animal Models; Genetics; Genetics: Gene Expression; Life-Span Extension.

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