Alternatives To Standard Lifespan Assays

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Even in simple eukaryotes, such as yeast, worms, and flies, the length of time that it takes to measure life span can be a rate-limiting step in high-throughput longevity phenotyping. New approaches are being developed that decrease the time and effort necessary to predict whether a particular mutation or environmental intervention is likely to alter aging rate. For example, in C. elegans a high-throughput method has been developed that combines automated worm-handling technology with a fluorescent dye exclusion phenotype to screen for small molecules that impact oxidative stress resistance and aging (Gill et al., 2003). While the utility of this method has yet to be proven, the approach appears promising.

A new method has been proposed that shortens the length of time required to carry out life-span analysis in flies by 80% (Bauer et al., 2004). By coupling the expression of a lethal toxin to expression of a putative age-dependent biomarker, flies are prematurely killed. In theory, any intervention that alters the rate of aging should similarly alter the rate of toxin expression and, hence, life span of the toxin-expressing animals. The authors do a good job of validating their system by showing that several examples known to increase longevity in wild-type flies also increase life span in the toxin-expressing strain. Whether expression of the biomarker correlates with aging rate in every situation has yet to be determined, and it will be important to validate any putative longevity-altering intervention discovered with this system using a standard life span assay.

In yeast, where replicative life span is measured by the number of daughter cells produced by each mother cell (see Chapter 18: Longevity and Aging in the Budding Yeast), methods have also been developed to screen for increased replicative life span. In one case, a strain has been constructed such that an essential gene is produced in mother cells but fails to be expressed in daughter cells (Jarolim et al., 2004). In theory, only mother cells should be able to divide under these conditions, greatly simplifying and accelerating measurement of replicative life span. Although this system, as described in the initial report, fails to recapitulate the standard life-span assay, this represents a first step toward a workable high-throughput replicative life-span assay.

Another variation on high-throughput screening for long-lived yeast strains has been developed in which the number of chitin bud scars, a surface marker of repli-cative age, is quantified fluorescently by FACS (Chen et al., 2003). This study reported that expression of human ferritin light chain in yeast increases the number of cells with elevated bud scar counts. The authors failed to confirm their findings using a standard replicative lifespan assay, however, and no further validation of this method has been published to date. These examples illustrate, yet again, that high-throughput approaches, while potentially useful for screening, should never be used as a surrogate for life-span analysis under standard conditions. Any ''hits'' identified from a high-throughput screen must be validated using traditional methods to avoid false positive identifications.

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