Hepatic S6K1 Partially Regulates Lifespan of Mice with Mitochondrial Complex I Deficiency

Abstract

The inactivation of ribosomal protein S6 kinase 1 (S6K1) recapitulates aspects of caloric restriction and mTORC1 inhibition to achieve prolonged longevity in invertebrate and mouse models. In addition to delaying normative aging, inhibition of mTORC1 extends the shortened lifespan of yeast, fly, and mouse models with severe mitochondrial disease. Here we tested whether disruption of S6K1 can recapitulate the beneficial effects of mTORC1 inhibition in the Ndufs4 knockout (NKO) mouse model of Leigh Syndrome caused by Complex I deficiency. These NKO mice develop profound neurodegeneration resulting in brain lesions and death around 50-60 days of age. Our results show that liver-specific, as well as whole body, S6K1 deletion modestly prolongs survival and delays onset of neurological symptoms in NKO mice. In contrast, we observed no survival benefit in NKO mice specifically disrupted for S6K1 in neurons or adipocytes. Body weight was reduced in WT mice upon disruption of S6K1 in adipocytes or whole body, but not altered when S6K1 was disrupted only in neurons or liver. Taken together, these data indicate that decreased S6K1 activity in liver is sufficient to delay the neurological and survival defects caused by deficiency of Complex I and suggest that mTOR signaling can modulate mitochondrial disease and metabolism via cell non-autonomous mechanisms.

Keywords: S6K1; lifespan; liver; mTORC1; mitochondrial disease.

Figure 1

Generation of tissue specific S6K1 conditional KO mice. (A) Schematic targeting strategy to create tissue specific disruption of the S6K1 gene. (B) Genotyping PCR for S6K1 floxed alleles in brain, fat, and liver collected from floxed mice crossed with Albumin-cre, Synapsin1-cre, Adiponectin-cre, and CMV-cre mice. Upper bands indicate flox-out alleles. Lower bands indicate intact flox alleles. (C) Western blot with S6K1 and actin antibody in brain, fat, and liver, collected from floxed mice crossed with Synapsin1-cre (Syn1), Adiponectin-cre (Adipoq), Albumin-cre (Alb), and CMV-cre mice.

Figure 2

Body size of S6K1 conditional KO strains. Body weight at 2 months of age of (A) whole body (CMV, n = 8–10 for female and n = 7–10 for male), (B) fat-specific (Adipoq, n = 6–7 for female and n = 4–6 for male), (C) liver-specific (Alb, n = 13–16 for female and n = 5–7 for male), and (D) brain-specific (Syn1, n = 7–8 for female and n = 8–11 for male) S6K1 conditional KO mice. Data are indicated as mean ± s.e.m. ^*p < 0.05, ^**p < 0.01, ^***p < 0.001.

Figure 3

Lifespan of S6K1 conditional KO strains in the Ndufs4 ^−/− background. (A) Lifespan of homozygous S6K1 flox-out (whole body S6K1 KO, n = 20, 6 female and 14 male) and their littermate heterozygous S6K1 flox-out (n = 21, 12 female and 9 male) by CMV-cre. P < 0.05. (B) Lifespan of Albumin-cre; S6K1^fl/fl; Ndufs4^−/− (Liver S6K1 cKO, n = 21, 9 female and 12 male) and their littermates without the cre gene (n = 20, 10 female and 10 male). p < 0.05. (C) Lifespan of Adiponectin-cre; S6K1^fl/fl; Ndufs4^−/− (Fat S6K1 cKO, n = 21, 15 female and 6 male) and their littermates without the cre gene (n = 21, 8 female and 13 male). (D) Lifespan of Synapsin1-cre; S6K1^fl/fl; Ndufs4^−/− (Brain S6K1 cKO, n = 12, 5 female and 7 male) and their littermates without the cre gene (n = 18, 11 female and 7 male).

Figure 4

Phenotype of S6K1 conditional KO strains in the Ndufs4 ^−/− background. (A) Maximum body weight each mouse reached during the lifespan analysis for whole body S6K1 KO (KO by CMV-cre) and tissue-specific S6K1 cKOs (brain cKO by Syn1-cre, fat cKO by Adipoq-cre, and liver cKO by Alb-cre) in the Ndufs4 KO strains. (B) Age at which animals first show the clasping phenotype as an indicator of neurological dysfunction. Data are indicated as mean ± s.e.m. ^*p < 0.05, ^**p < 0.01.