Sir2-independent life span extension by calorie restriction in yeast

Abstract

Calorie restriction slows aging and increases life span in many organisms. In yeast, a mechanistic explanation has been proposed whereby calorie restriction slows aging by activating Sir2. Here we report the identification of a Sir2-independent pathway responsible for a majority of the longevity benefit associated with calorie restriction. Deletion of FOB1 and overexpression of SIR2 have been previously found to increase life span by reducing the levels of toxic rDNA circles in aged mother cells. We find that combining calorie restriction with either of these genetic interventions dramatically enhances longevity, resulting in the longest-lived yeast strain reported thus far. Further, calorie restriction results in a greater life span extension in cells lacking both Sir2 and Fob1 than in cells where Sir2 is present. These findings indicate that Sir2 and calorie restriction act in parallel pathways to promote longevity in yeast and, perhaps, higher eukaryotes.

Figure 1. Regulation of Longevity by CR and Fob1

(A) Life span analysis for three genetic models of CR and deletion of FOB1. Strains shown (and mean life spans) are BY4742 (26.7), fob1Δ (37.8), gpa2Δ (34.9), gpr1 (34.4), and hxk2Δ (36.7). (B) fob1Δ and hxk2Δ increase life span additively. Strains shown (and mean life spans) are BY4742 (26.6), fob1Δ (37.3), hxk2Δ (36.7), and fob1Δ hxk2Δ (48.3). (C) fob1Δ and gpa2Δ increase life span additively. Strains shown (and mean life spans) are BY4742 (27.2), fob1Δ (37.8), gpa2Δ (36.7), and fob1Δ gpa2Δ (54.5).

Figure 2. Life Span Extension by CR Does Not Require Sir2

(A) CR fails to increase life span of a sir2Δ mutant. Strains shown (and mean life span) are BY4742 (26.7), sir2Δ (14.0), hxk2Δ sir2Δ (12.4), and gpa2Δ sir2Δ (11.7). (B) Deletion of FOB1 suppresses the short life span of a sir2Δ strain. Strains shown and mean life spans are: BY4742 (27.5), sir2Δ (14.0), sir2Δ fob1Δ (30.0). (C) Deletion of HXK2 increases the life span of a sir2Δ fob1Δ double mutant. Strains shown (and mean life spans) are BY4742 (26.5), sir2Δ fob1Δ (30.0), and sir2Δ fob1Δ hxk2Δ (45.3). (D) Deletion of GPA2 increases the life span of a sir2Δ fob1Δ double mutant. Strains shown (and mean life spans) are BY4742 (26.6), sir2Δ fob1Δ (30.0), and sir2Δ fob1Δ gpa2Δ (51.0).

Figure 3. CR Is More Effective at Enhancing Longevity in a sir2Δ fob1Δ Double Mutant than in Wild-Type Cells

Percent increase in mean life span relative to growth on 2% glucose was determined for 20 mother cells from each strain at 0.5%, 0.1%, and 0.05% glucose.

Figure 4. CR Increases the Life Span of Cells Overexpressing SIR2

(A) Neither deletion of FOB1 nor overexpression of SIR2 impact longevity in PSY316. Strains shown (and mean life spans) are PSY316 (21.1), PSY316 fob1Δ (20.7), and PSY316 SIR2-ox (21.7). (B) Overexpression of SIR2 and CR increase life span additively in BY4742. Strains shown (and mean life spans) are BY4742 on 2% glucose (26.1), BY4742 on 0.05% glucose (31.8), BY4742 SIR2-ox on 2% glucose (34.6), and BY4742 SIR2-ox on 0.05% glucose (42.2).

Figure 5. Two Pathways Determine Yeast Longevity

The longevity of mother cells can be modified by at least two independent interventions: altered ERC levels and CR. In cells lacking Sir2 but containing Fob1, senescence due to ERCs predominates, causing an extremely short life span that cannot be increased by CR. In cells lacking FOB1, ERCs are greatly reduced and the CR pathway predominates. The presence or absence of Sir2 does not impact the longevity benefits of CR under this condition.