Skeletal muscle aging is associated with increased risk of frailty, morbidity, and mortality and to date has few safe and efficient treatments. Although the mechanistic target of rapamycin complex 1 (mTORc1) signaling pathway in younger animals/humans, regulates cell metabolism positively, recent findings suggest mTOR becomes over-active in older age, and that this could be a cause of sarcopenia. In this study, we sought to map mTOR-related signaling as a function of age and fibre type.

This experiment included 24 rat samples collected from: “young” (3-months, N=10), “old” (24-months, N=10), and “very old” (27-months, N=4) rats. All 24 soleus muscles were analyzed, and 8 tibialis muscle (young N=4, old N=4) samples. mTOR related targets were quantified by Western blotting. Data were quantified via densitometry and normalized to Coomassie staining to correct for loading error. Data are shown as mean±SEM, analyzed using Shapiro-Wilk test to test normal distribution, and t-tests or a non-parametric equivalent to compare age. The alpha level of significance was P<0.05.

In soleus muscle, p-mTOR increased with aging; the very old group protein abundance being 2.1-fold higher than the old (P<0.01), and the old being 2.5-fold higher than the young (P<0.01). Downstream, p-rps6 in the very old was 3.1 and 2.4-fold higher than in the young (P<0.001) and old groups (P<0.01), respectively. Another mTOR target, p-4E-BP1, was 1.6-fold higher in the very old than the old (P<0.05). For mTOR upstream targets, p-AKT in the very old group was 2.9 and 2.6-fold greater than the young and the old (both P<0.01), respectively. p-AMPK abundance in the young and very old groups 2.9 and 2.8-fold higher than the old (both P<0.05). The autophagy marker, p-FoxO1a, in the very old group was 1.6-fold greater than the old (P<0.05). In the tibialis muscle, aging did not influence the phosphorylation of the AKT/mTOR/rps6 pathway. Nonetheless, p-AMPK, in young tibialis muscle was 2.5-fold more abundant than in the old group (P<0.01). Finally, p-FoxO1a and p-FoxO3a in the old group were 0.6 and 0.5-fold less abundant than in the young tibialis muscle (P<0.05 and P<0.0001).

Aging alters AKT/mTOR/rps6/FoXO pathway regulation in soleus, perhaps relating to dysregulated proteostasis/autophagy in slow muscle with ageing. Tibialis anterior illustrated altered AMPK/FoxO signaling, suggesting altered autophagy and upstream mTOR signaling. While preliminary in nature, these data support perturbed mTOR signaling with rat muscle ageing which may act in a muscle/fiber type-specific manner.