Genetic analysis of tissue aging in Caenorhabditis elegans

Specific mutations in the genome of the soil nematode Caenorhabditis elegans affect lifespan, lengthening or shortening the total life of the animal. To learn more about the genetic control of lifespan, mutants with extended lifespan were characterized. Two mutants with minimal secondary phenotypes, mu372 and mu373, were identified from this analysis. As a means of understanding mechanisms of aging, an investigation of biomarkers was performed. After initial investigations using reporter fusions, we developed a simple yet powerful technique of observing age-related tissue changes by Nomarski microscopy. Two C. elegans structures were examined: the head and the distal gonad. The tissues in these regions change appearance with age, in a manner that can be evaluated and quantified by a specific protocol. Nonparametric statistical analysis demonstrated that these changes increased significantly over the life of the animal. Age-related changes occurred more slowly in long-lived mutants in the insulin/IGF1 pathway, and more rapidly in a short-lived mutant in this pathway. This simple protocol was used to identify animals with a prematurely-old appearance from a screen for short-lived animals; these animals had reduced activity of C. elegans heat-shock factor. Finally, the age-related changes were evaluated to ask whether they might have a role in causing death. Old animals near death often had mouthparts and gastrointestinal tracts that were distended with bacteria, their food. This result suggested that live bacteria might be harmful to aged C. elegans' survival. To directly test this model, animals were cultured on bacteria that had been killed or rendered incapable of proliferation. This reduced the incidence of bacterial packing in the animals. While animals on nonproliferating bacteria did not appear more youthful, they displayed a sizable increase in mean and maximum lifespan. Although a full understanding of the genetic control of lifespan is many years away, this work contributes a system for analyzing aging in C. elegans. Furthermore, it identifies a non-genetic, environmental factor that can affect C. elegans survival.