Genetic and environmental conditions that increase longevity in Caenorhabditis elegans decrease metabolic rate

Abstract

Mutations that increase the longevity of the soil nematode Caenorhabditis elegans could define genes involved in a process specific for aging. Alternatively, these mutations could reduce animal metabolic rate and increase longevity as a consequence. In ectotherms, longevity is often negatively correlated with metabolic rate. Consistent with these observations, environmental conditions that reduce the metabolic rate of C. elegans also extend longevity. We found that the metabolic rate of long-lived C. elegans mutants is reduced compared with that of wild-type worms and that a genetic suppressor that restored normal longevity to long-lived mutants restored normal metabolic rate. Thus, the increased longevity of some long-lived C. elegans mutants may be a consequence of a reduction in their metabolic rate, rather than an alteration of a genetic pathway that leads to enhanced longevity while maintaining normal physiology. The actual mechanism responsible for the inverse correlation between metabolic rate and longevity remains unknown.

Figure 1

Survivorship, metabolic rate, and metabolic output of wild-type worms grown at 10–25°C. (a) Survivorship of wild-type worms at 10–25°C. Each bar is the mean (± SEM) of ≈100 worms. (b) Metabolic rate of wild-type worms at different temperatures. Metabolic rates were compared between worms of comparable developmental age (young, egg-laying adults). (c) Lifetime metabolic output of wild-type worms at different growth temperatures. Bars represent the means (± SEM) of four measurement values per temperature.

Figure 2

Survivorship, metabolic rate, and metabolic output of wild-type C. elegans and long-lived C. elegans mutants. (a) Survivorship of wild-type (n = 168), age-1 (n = 202), daf-2 (n = 167), and clk-1 daf-2 worms (n = 106) maintained at 20°C. (b) Metabolic rate of wild-type and long-lived worms. Metabolic rate was measured over a 9-day period with worms maintained at 20°C. Worms from each strain were measured when 3, 6, 9, and 12 days old. The double age-1 fer 15 mutant was used in all of these experiments. Each bar represents the mean (± SEM) of six separate measurements with 50 worms used per sample measurement. The metabolic rate of daf-2 and clk-1 daf-2 worms is significantly less than wild-type worms at every time point (P < 0.02, Mann–Whitney U test). (c) Metabolic output of wild-type and long-lived worms over a 12-day period. Metabolic output was calculated by integrating the area under a plot of metabolic rate for 9 days. Each bar represents the mean (± SEM) of four integration values. Metabolic output of daf-2 and clk-1 daf-2 is significantly less than wild type (P < 0.01; unpaired Student’s t test). Although reduced, the metabolic output of age-1 was not significantly less than wild type (P = 0.09). Further experiments showed that, despite its longer life span, the lifetime metabolic output of age-1 is not significantly different from that of wild type (data not shown). (d) Metabolic rate comparison between wild-type, daf-2, and clk-1 daf-2 worms when worms are at equivalent body size and development. Each bar plots the mean (± SEM) of two measurements.

Figure 3

Metabolic rate and metabolic output of wild-type, long-lived daf-2 worms and of daf-2 worms genetically suppressed by a mutation in the daf-16 gene. daf-2;daf-16 worms are identical to daf-2 worms, except that they have an additional mutation in the daf-16 gene. This gene is required for worms to form dauer larvae. (a) Metabolic rate of wild-type, daf-2;daf-16, and daf-2 worms. Bars represent the means (± SEM) of four measurement values per temperature, with 50 worms used per measurement. (b) Metabolic output of wild-type, daf-2;daf-16, and daf-2 worms. Metabolic output was calculated as described in Fig. 2c. Each bar represents the mean (± SEM) of four integration values with each integration value calculated from metabolic rates measured at five time points.

Figure 4

Life-history traits of long-lived mutant and wild-type hermaphrodites. (a) Worm size is the area of worms measured from an video image of a worm. (b) First reproduction is the time taken for an egg to hatch and develop into an egg-laying adult. (c) Lay rate is the rate of egg laying for young adult worms. (d) Progeny number is the total number of offspring. The large reduction in age-1 fertility is probably caused by the fer-15 mutation, which is also present in this strain. All traits were assayed at 20°C. Means (± SEM) for 15 individuals per strain are plotted.