Alternatives to standard life-span assays that accelerate the normal aging process can be useful tools for rapidly screening genes and compounds for effects on longevity. These methods suffer, however, from the fact that they are not true aging assays and putative aging effects must be verified in each case. A more satisfying, but also more difficult approach, is the development of automated, high-throughput methods for standard life-span measurements.
The system most amenable to high-throughput longevity phenotyping is the chronological life-span assay in budding yeast. "Chronological life span'' refers to the length of time a yeast cell is capable of maintaining viability in a nondividing, quiescent state (see Chapter 18: Longevity and Aging in the Budding Yeast). Traditionally, chronological life span has been measured by culturing cells in 5-50 mL volumes and periodically plating onto rich media and counting colonies (Fabrizio and Longo, 2003). A semiautomated method for chronological life-span determination has recently been developed that allows for the aging of cells in a 96-well format using submilliliter volumes (Kaeberlein, 2004). Viability as a function of age is determined by transferring a small volume of cells (~1 mL) into rich media, allowing outgrowth for a fixed period of time, and measuring the optical density at 660 nm. This method allows for the simultaneous determination of chronological life span for several thousand strains.
The first true genome-wide longevity screen to be described is an RNAi-based screen for genes that increase life span when knocked down in C. elegans (Lee et al., 2003). A library of E. coli strains carrying double-stranded RNAi constructs corresponding to nearly 17,000 unique genes (~85% of C. elegans predicted ORFs) has been partially screened, with several genes identified that increase life span when knocked down. By far, the largest class of genes that increase life span when knocked down are genes that play a role in proper mitochondrial function, suggesting that decreased mito-chondrial function increases life span in worms, perhaps due to decreased production of oxidative damage. The genome-wide RNAi approach is limited, however, by the fact that only genes that function to promote aging (genes that increase life span when knocked down) can be identified with reliability from such a screen. Efficiency of gene knock-down by RNAi is also variable from gene to gene, and the use of double-stranded mRNA for RNAi may result in the knock-down of genes with homology to the target gene. A newly developed C. elegans RNAi library using hairpin siRNA technology may circumvent some of these limitations (Rual et al., 2004), and the development of high-throughput methods for making transgenic worms would allow screening for genes that function to promote longevity.
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When over eighty years of age, the poet Bryant said that he had added more than ten years to his life by taking a simple exercise while dressing in the morning. Those who knew Bryant and the facts of his life never doubted the truth of this statement.