Against the oxidative damage theory of aging: superoxide dismutases protect against oxidative stress but have little or no effect on life span in Caenorhabditis elegans

Abstract

The superoxide radical (O(2)(-)) has long been considered a major cause of aging. O(2)(-) in cytosolic, extracellular, and mitochondrial pools is detoxified by dedicated superoxide dismutase (SOD) isoforms. We tested the impact of each SOD isoform in Caenorhabditis elegans by manipulating its five sod genes and saw no major effects on life span. sod genes are not required for daf-2 insulin/IGF-1 receptor mutant longevity. However, loss of the extracellular Cu/ZnSOD sod-4 enhances daf-2 longevity and constitutive diapause, suggesting a signaling role for sod-4. Overall, these findings imply that O(2)(-) is not a major determinant of aging in C. elegans.

Figure 1.

Oxidative stress resistance and longevity of MnSOD mutants. (A) Survival in 40 mM paraquat. (B) Survival under hyperoxia (60% O₂). (C,D) Life span of MnSOD single (C) and double (D) mutants under normoxia (20°C). (E) Effect of sod-2(0) or sod-3(0) on daf-2(m577) longevity (25°C). (F) Effect of sod-2(0) or sod-3(0) on daf-2(m577) Daf-c.

Figure 2.

Oxidative stress resistance and longevity of Cu/ZnSOD mutants. (A) Survival in 40 mM paraquat. (B) Survival under hyperoxia (60% O₂). (C,D) Life span of Cu/ZnSOD single (C) and double (D) mutants under normoxia (20°C). (E) Effect of loss of Cu/ZnSOD on daf-2(m577) longevity (20°C). (F) Effect of loss of Cu/ZnSOD on daf-2(m577) Daf-c. (**) P < 0.01; (***) P < 0.0001, Student’s t-tests.

Figure 3.

Overexpression of sod-1 cytosolic Cu/ZnSOD increases life span. (A) Western blots of protein extracts of wild-type, sod-1 mutant, and sod-1 overexpresser lines using anti-Cu/ZnSOD antibodies. Cytochrome C (Cyt-C) and actin were used as mitochondrion- and cytosol-specific markers, respectively. Increased SOD-1 protein was also detected in wuIs154 transgenics (data not shown). (B) Total SOD activity in protein extracts from lines overexpressing sod-1. (**) P < 0.01, Student's t-test. (C) Total catalase activity in line overexpressing catalase. (**) P < 0.01, Student’s t-test. (D) Catalase overexpression causes high levels of mortality from internal hatching of larvae (bagging), and this is suppressed by overexpression of sod-1. (***) P < 0.0001, Student’s t-test. (E) sod-1 overexpression increases sensitivity to oxidative stress (40 mM paraquat), and this is suppressed by overexpression of catalase. (F) Overexpression of sod-1 increases life span. (G) Elevated catalase does not further extend longevity of the sod-1 overexpresser.

Figure 4.

Influence of SOD and O₂⁻ on aging. This scheme shows a synthesis of conclusions drawn from the present study. Different SOD isoforms (and by deduction, the corresponding O₂⁻ pools) have different effects on aging. Extracellular Cu/ZnSOD weakly inhibits dauer formation and promotes aging, potentially by generating H₂O₂, which crosses into the cell and stimulates insulin/IGF-1 signaling by inhibiting redox-sensitive phosphatases. Cytosolic Cu/ZnSOD weakly promotes longevity, perhaps by protecting against molecular damage (sod-1 does not influence daf-2 Daf-c). Mitochondrial MnSOD has no detectable effect on aging. Arrow with dotted line implies a weak effect.