Neuronal SIRT1 regulates endocrine and behavioral responses to calorie restriction

Abstract

Mammalian life span can be extended by both calorie restriction (CR) and mutations that diminish somatotropic signaling. Sirt1 is a mediator of many effects of CR in mammals, but any role in controlling somatotropic signaling has not been shown. Since the somatotropic axis is controlled by the brain, we created mice lacking Sirt1 specifically in the brain and examined the impacts of this manipulation on somatotropic signaling and the CR response. These mutant mice displayed defects in somatotropic signaling when fed ad libitum, and defects in the endocrine and behavioral responses to CR. We conclude that Sirt1 in the brain is a link between somatotropic signaling and CR in mammals.

Figure 1.

BSKO mice lack functional Sirt1 protein in the brain and are dwarfed. (A) Brains from wild-type (WT) and BSKO mice exhibit similar gross morphology. (B) Western blotting of Sirt1 protein in whole-brain lysates from wild-type and BSKO mice. Closed arrowheads indicate wild-type Sirt1 isoforms, and open arrowheads indicate mutant isoforms. Tubulin is shown as a loading control. (C) Western blotting of Sirt1 protein in pituitary lysates. (D) Body mass of male wild-type and BSKO animals at the indicated ages; n = 6–10 per genotype and time point. Curves are significantly different by repeated-measures ANOVA; P < 0.01. (E) Snout–anus body length of male wild-type and BSKO animals at 10 wk of age; n = 6–10 per genotype. All data are shown as mean ± SEM. (*) P < 0.05 by two-tailed Student's t-test.

Figure 2.

The somatotropic axis is disrupted in BSKO mice. (A) Serum GH concentrations in 4-wk-old BSKO mice and littermate controls; n = 15–20 per group. (B) Serum IGF-1 concentrations in 10-wk-old BSKO mice and controls; n = 6–10 per group. (C) IGF-1 mRNA levels in BSKO liver normalized to wild-type (WT) levels; n = 5 per group. (D) Pituitaries from BSKO and wild-type littermates. (E) Pituitary mass normalized to body mass of 10-wk-old BSKO and wild-type mice; n = 6–10 per group. (F) Pituitary GH stores in 10-wk-old BSKO mice; n = 5–6 per group. (G) Measurements of additional pituitary hormones in BSKO and wild-type mice. For PRL, TSH, and T4, n = 10–14 females per group. For T3 (females) and ACTH (males), n = 6–10 per group. All data are shown as mean ± SEM. (*) P < 0.05 by two-tailed Student's t-test.

Figure 3.

BSKO mice have altered glucose homeostasis with aging. (A) ITT on 3-mo-old BSKO and wild-type (WT) mice; n = 10–14 per group. Curves are significantly different by two-way ANOVA; P < 0.05. (B) GTT on 3-mo-old BSKO and wild-type mice; n = 10–14 per group. (C) ITT on 10-mo-old mice of indicated diet and genotype groups; n = 6–8 per group. (AL) Ad libitum fed; (CR) calorie restricted. WT-CR and BSKO-CR curves are significantly different by two-way ANOVA; P < 0.05. (D) GTT on 10-mo-old mice of indicated diet and genotype groups; n = 6–8 per group. All data are shown as mean ± SEM. (E) Blood glucose and insulin levels in 1-yr-old wild-type and BSKO mice 45 min after IP injection of 2 g/kg glucose in PBS; n = 6–7 per group. Insulin values are significantly different; P < 0.01. The P-value for blood glucose comparison is 0.0727 by a two-tailed Student's t-test. (F) PEPCK, G6pase, and Pcx mRNA expression in liver of 0.5- to 1-yr-old female wild-type and BSKO mice 45 min following vehicle (PBS) or 2 g/kg glucose (GLU) injection; n = 7–14 per group. (*) P < 0.05 by two-tailed Student's t-test versus WT-PBS for each gene.

Figure 4.

BSKO mice have altered endocrine and behavioral responses to CR. (A) Serum IGF-1 concentrations after 10 wk of CR; n = 6–8 per group. Interaction between diet and genotype is significant by two-way ANOVA; P < 0.05. Data are shown as mean ± SEM. (B–E) Each bar represents the average number of revolutions run per 15-min period over 15 d for mice of the indicated genotype and diet; n = 5 per group.