Tissue depletion of taurine accelerates skeletal muscle senescence and leads to early death in mice

Abstract

Taurine (2-aminoethanesulfonic acid) is found in milimolar concentrations in mammalian tissues. One of its main functions is osmoregulation; however, it also exhibits cytoprotective activity by diminishing injury caused by stress and disease. Taurine depletion is associated with several defects, many of which are found in the aging animal, suggesting that taurine might exert anti-aging actions. Therefore, in the present study, we examined the hypothesis that taurine depletion accelerates aging by reducing longevity and accelerating aging-associated tissue damage. Tissue taurine depletion in taurine transporter knockout (TauTKO) mouse was found to shorten lifespan and accelerate skeletal muscle histological and functional defects, including an increase in central nuclei containing myotubes, a reduction in mitochondrial complex 1 activity and an induction in an aging biomarker, Cyclin-dependent kinase 4 inhibitor A (p16INK4a). Tissue taurine depletion also enhances unfolded protein response (UPR), which may be associated with an improvement in protein folding by taurine. Our data reveal that tissue taurine depletion affects longevity and cellular senescence; an effect possibly linked to a disturbance in protein folding.

Figure 1. Shortened lifespan of TauTKO mice.

Kaplan-Meier survival curves for (A,B) combined (C) male and (D) female wild-type (WT) heterozygous (hetero) and TauTKO mice. (A) n = 21 (WT), 10 (hetero), 41 (TauTKO) (B) n = 37 (WT), 20 (hetero), 58 (TauTKO). (C) n = 12 (control; WT+hetero), 20 (TauTKO) (D) n = 19 (conrol; WT+hetero), 21 (TauTKO). Data for B), but not A), C) and D), includes the mice sacrificed over 520 days after birth.

Figure 2. Accelerated aging in TauTKO muscle.

A) mRNA level of the senescence marker, p16INK4A, was quantified in several tissues of old TauTKO and WT mice by real-time RT-PCR. The marker is severely increased in old TauTKO muscle. n = 4–7. B) Body weight of TauTKO mouse is consistently lower than that of the WT mouse. n = 6–12. C), D) Histological analysis of tibial anterior muscles shows an induction of myotubes with central nuclei indicated by arrows in old TauTKO muscle. The ratio of myotubes with central nuclei to total myotubes is significantly higher in old TauTKO than WT. n = 6. Scale bar = 50 µm. E) The assays for electron transport chain complex reveal that activity of mitochondrial complex1 is decreased in old TauTKO muscle. C1–4; Complex 1–4. n = 4–5. *;p<0.05.

Figure 3. Specific changes in aged and young TauTKO skeletal muscle.

A) Biological functions analysis shows a significant activation of cell cycle progression, prostaglandin D2 release, degranulation of cells, cell movement and cell death (necrosis, apoptosis) in aged TauTKO muscle. B) Wheel network diagram showing the overlap of the major upstream regulators of genes chaged in old TauTKO muscles. Each association between genes and upstream regulators is listed in Table 1. Red shapes: upregulated gene, green shapes: downregulated gene, orange shapes: predicted activation, Orange arrows: leads to activation, yellow arrow: findings inconsistent with prediction, gray arrows: effect not predicted. C) Biological functions analysis shows a significant activation of neural amino acid transport, colony formation of cells, protein synthesis, protein folding and steroid content both in young and aged TauTKO muscle compared to age-matched control littermate.

Figure 4. Activation of the unfolded protein response in TauTKO muscle.

A) Microarray data show upregulation of UPR-related genes, including ATF3, 6, Crem, Hspa5 (Grp78) and Xbp1, in TauTKO muscle. n = 3. B) Western blot reveals an induction in Grp78 protein expression in old TauTKO muscle. n = 5. C) Western blot reveals an induction in nuclear XBP1 protein expression in old TauTKO muscle. n = 4. D) Real-time PCR analysis for unspliced and spliced XBP1 (XBP1-u and XBP1-s, respectively) mRNA in WT and TauTKO mice show a significant induction in both total and spliced (active form) of XBP1 mRNA. n = 5. *;p<0.05 vs WT. E) Our data hypothesize that taurine depletion in skeletal muscle may lead the activation of UPR due to the accumulation of unfolded proteins, which in turn accelerates tissue aging in TauTKO mice.