AKT controls protein synthesis and oxidative metabolism via combined mTORC1 and FOXO1 signalling to govern muscle physiology

Abstract

Background Skeletomuscular diseases result in significant muscle loss and decreased performance, paralleled by a loss in mitochondrial and oxidative capacity. Insulin and insulin‐like growth factor‐1 (IGF‐1) are two potent anabolic hormones that activate a host of signalling intermediates including the serine/threonine kinase AKT to influence skeletal muscle physiology. Defective AKT signalling is associated with muscle pathology, including cachexia, sarcopenia, and disuse; however, the mechanistic underpinnings remain unresolved. Methods To elucidate the role of AKT signalling in muscle mass and physiology, we generated both congenital and inducible mouse models of skeletal muscle‐specific AKT deficiency. To understand the downstream mechanisms mediating AKT's effects on muscle biology, we generated mice lacking AKT1/2 and FOXO1 (M‐AKTFOXO1TKO and M‐indAKTFOXO1TKO) to inhibit downstream FOXO1 signalling, AKT1/2 and TSC1 (M‐AKTTSCTKO and M‐indAKTTSCTKO) to activate mTORC1, and AKT1/2, FOXO1, and TSC1 (M‐QKO and M‐indQKO) to simultaneously activate mTORC1 and inhibit FOXO1 in AKT‐deficient skeletal muscle. Muscle proteostasis and physiology were assessed using multiple assays including metabolic labelling, mitochondrial function, fibre typing, ex vivo physiology, and exercise performance. Results Here, we show that genetic ablation of skeletal muscle AKT signalling resulted in decreased muscle mass and a loss of oxidative metabolism and muscle performance. Specifically, deletion of muscle AKT activity during development or in adult mice resulted in a significant reduction in muscle growth by 30–40% (P < 0.0001; n = 12–20) and 15% (P < 0.01 and P < 0.0001; n = 20–30), respectively. Interestingly, this reduction in muscle mass was primarily due to an ~40% reduction in protein synthesis in both M‐AKTDKO and M‐indAKTDKO muscles (P < 0.05 and P < 0.01; n = 12–20) without significant changes in proteolysis or autophagy. Moreover, a significant reduction in oxidative capacity was observed in both M‐AKTDKO (P < 0.05, P < 0.01 and P < 0.001; n = 5–12) and M‐indAKTDKO (P < 0.05 and P < 0.01; n = 4). Mechanistically, activation and inhibition of mTORC1/FOXO1, respectively, but neither alone, were sufficient to restore protein synthesis, muscle oxidative capacity, and muscle function in the absence of AKT in vivo. In a mouse model of disuse‐induced muscle loss, simultaneous activation of mTORC1 and inhibition of FOXO1 preserved muscle mass following immobilization (~5–10% reduction in casted M‐indFOXO1TSCDKO muscles vs. ~30–40% casted M‐indControl muscles, P < 0.05 and P < 0.0001; n = 8–16). Conclusions Collectively, this study provides novel insights into the AKT‐dependent mechanisms that underlie muscle protein homeostasis, function, and metabolism in both normal physiology and disuse‐induced muscle wasting.

Post-translational dysregulation of glucose uptake during exhaustive cycling exercise in vastus lateralis muscle of healthy homozygous carriers of the ACE deletion allele

“…AKT-S473 phosphorylation is known with angiogenesis ( Shiojima and Walsh 2002 ) and high angiotensin 2 levels are associated with exercise-induced skeletal muscle angiogenesis in laboratory models (reviewed in Rodrigues et al, 2022 ). As well, we observed a trend for a negative correlation between the phosphorylation of AKT-S473 and COX4I1 transcript levels in m. Vastus lateralis ( Table 5 ) which relate to the recently reported association between AKT-S473 expression and phosphorylation and the protein expression of cytochrome C oxidative subunits in mouse skeletal muscle ( Jaiswal et al, 2022 ). Collectively, our findings point to AKT-S473 phosphorylation as possible gatekeeper of ACE I/D genotype-related differences in the import and combustion of blood-borne substrates in exhaustively exercised skeletal muscle.…”

Phosphoproteomic mapping reveals distinct signaling actions and activation of muscle protein synthesis by Isthmin-1

“…In this work, we observe a 60% decrease in pAkt S473 levels in the muscles of Ism1 -KO mice, which leads to an ~10% reduction in muscle protein content. Our data is consistent with previous reports using muscle-specific Akt -KO mice that demonstrate a 40% reduction in protein synthesis upon complete ablation of Akt ( Jaiswal et al, 2022 ). Furthermore, reduced Akt is associated with skeletal muscle insulin resistance, in line with our previous observation that Ism1 -KO mice fed a high-fat diet are more insulin resistant ( Jiang et al, 2021 ).…”

“…Akt has an established role in enhancing muscle hypertrophy and function ( Bodine et al, 2001 ; Glass, 2011 ; Jaiswal et al, 2022 ; Jaiswal et al, 2019 ; Lai et al, 2004 ; Mammucari et al, 2007 ; Wilson and Rotwein, 2007 ). However, there is still a need to identify other hormonal and physiological insulin/IGF-1 independent activators of Akt to avoid associated side effects such as hypoglycemia when used therapeutically.…”

“…To what extent the insulin resistance in the Ism1 -KO mice contributes to muscle function in obesity or during aging is an intriguing question that remains to be answered in future work. Moreover, increasing protein synthesis while suppressing protein degradation is important in a physiological setting, based on the findings that only a combination of FoxO1 inhibition and mTORC1 activation can restore Akt-mediated muscle loss ( Jaiswal et al, 2022 ). This suggests that hormonal regulators of Akt activity could have important biological functions because of Akt’s dual role in regulating mTORC1 and the FoxO genes.…”

Phosphoproteomic mapping reveals distinct signaling actions and activation of protein synthesis and muscle hypertrophy by Isthmin-1

“…In this work, we observe a 60 % decrease in pAkt S473 levels in the muscles of Ism1-KO mice, which leads to a ∼10 % reduction in muscle protein content. Our data is consistent with previous reports using muscle-specific Akt-KO mice that demonstrate a 40% reduction in protein synthesis upon complete ablation of Akt (Jaiswal et al, 2022). Furthermore, reduced Akt is associated with skeletal muscle insulin resistance, in line with our previous observation that Ism1-KO mice fed a high-fat diet are more insulin resistant (Jiang et al, 2021).…”