Date Published: September 24, 2004
Publisher: Public Library of Science
Author(s): Matt Kaeberlein, Kathryn T Kirkland, Stanley Fields, Brian K Kennedy
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.
Partial Text: The budding yeast Saccharomyces cerevisiae has served as a useful model for aging research, leading to the identification of new longevity genes and pathways whose counterparts can be examined in higher eukaryotes (Kaeberlein et al. 2001). One measure of aging in yeast is the finite replicative life span (RLS) of mother cells, defined as the number of mitotic cycles completed prior to senescence (Mortimer and Johnston 1959). Alternatively, the survival of nondividing yeast cells over time can be monitored and has been termed chronological aging (Fabrizio and Longo 2003). It has been proposed that replicative aging in yeast may be a suitable model for the aging of dividing cells in mammals, such as germ cells; whereas, chronological aging of yeast may be related to the aging of postmitotic tissues.
We recently carried out a large-scale study of more than 40 single-gene deletions reported to affect aging in yeast (unpublished data). This analysis was performed in the BY4742 genetic background, which has a mean life span significantly longer than most other yeast strains commonly used for aging research (Table 1). Included in this analysis were three genetic models of CR (hxk2Δ, gpa2Δ, and gpr1Δ) and fob1Δ. As previously reported for shorter-lived strain backgrounds (Defossez et al. 1999; Lin et al. 2000), each of these single-gene deletions resulted in a 30%–40% increase in life span in BY4742 (Figure 1A).
We present substantial genetic evidence that CR and Sir2 act in different genetic pathways to promote longevity. The combination of CR with SIR2 overexpression results in an additive life span increase, as expected for two genetic interventions acting in parallel pathways. Further, in the context of FOB1 deletion, CR results in a larger relative increase in life span in the absence of Sir2 than in cells where Sir2 is expressed. Finally, the ability of CR to promote longevity in a strain lacking Sir2 definitively demonstrates the existence of a Sir2-independent aging pathway responsive to CR.