Reactive oxygen species (ROS) act as intracellular compartmentalized second messengers, mediating metabolic stress-adaptation. In skeletal muscle fibers, ROS have been suggested to stimulate glucose ...transporter 4 (GLUT4)-dependent glucose transport during artificially evoked contraction ex vivo, but whether myocellular ROS production is stimulated by in vivo exercise to control metabolism is unclear. Here, we combined exercise in humans and mice with fluorescent dyes, genetically-encoded biosensors, and NADPH oxidase 2 (NOX2) loss-of-function models to demonstrate that NOX2 is the main source of cytosolic ROS during moderate-intensity exercise in skeletal muscle. Furthermore, two NOX2 loss-of-function mouse models lacking either p47phox or Rac1 presented striking phenotypic similarities, including greatly reduced exercise-stimulated glucose uptake and GLUT4 translocation. These findings indicate that NOX2 is a major myocellular ROS source, regulating glucose transport capacity during moderate-intensity exercise.
Key points
Muscle contractions increase protein synthesis in a mechanistic target of rapamycin (mTOR)‐dependent manner, yet it is unclear which/how mTOR complexes regulate muscle protein synthesis.
...We investigated the requirement of mTOR Complex 2 (mTORC2) in contraction‐stimulated muscle protein synthesis.
mTORC2 inhibition by muscle‐specific Rictor knockout (Rictor mKO) did not prevent contraction‐induced muscle protein synthesis.
Rapamycin prevented contraction‐induced muscle protein synthesis in Rictor mKO but not wild‐type mice.
Protein synthesis increases following muscle contractions. Previous studies have shown that inhibition of the mechanistic target of rapamycin complex 1 (mTORC1) suppresses the early but not late muscle protein synthesis response, while inhibition of both mTORC1 and mTORC2 abolishes the two effects. Therefore, we hypothesized that mTORC2 regulates muscle protein synthesis following muscle contractions. To test this, we investigated the effect of mTORC2 inhibition by mouse muscle‐specific Rictor knockout (Rictor mKO) on muscle protein synthesis 3 h after contraction. The right gastrocnemius muscles of Rictor mKO and wild‐type (WT) mice were isometrically contracted using percutaneous electrical stimulation, while the left gastrocnemius muscles served as controls. Vehicle or the mTORC1 inhibitor rapamycin (1.5 mg/kg) was injected intraperitoneally 1 h before contraction. Treatment of WT mice with rapamycin and Rictor mKO lowered protein synthesis in general, but the response to contractions was intact 3 h after contractions in both conditions. Rapamycin treatment in Rictor mKO mice prevented contraction‐stimulated muscle protein synthesis. Notably, signalling traditionally associated with mTORC1 was increased by muscle contractions despite rapamycin treatment. In rapamycin‐treated Rictor mKO mice, the same mTORC1 signalling was blocked following contractions. Our results indicate that although neither rapamycin‐sensitive mTOR/mTORC1 nor mTORC2 is necessary for contraction‐induced muscle protein synthesis, combined inhibition of rapamycin‐sensitive mTOR/mTORC1 and mTORC2 synergistically inhibits contraction‐induced muscle protein synthesis.
Key points
Muscle contractions increase protein synthesis in a mechanistic target of rapamycin (mTOR)‐dependent manner, yet it is unclear which/how mTOR complexes regulate muscle protein synthesis.
We investigated the requirement of mTOR Complex 2 (mTORC2) in contraction‐stimulated muscle protein synthesis.
mTORC2 inhibition by muscle‐specific Rictor knockout (Rictor mKO) did not prevent contraction‐induced muscle protein synthesis.
Rapamycin prevented contraction‐induced muscle protein synthesis in Rictor mKO but not wild‐type mice.
Key points
AMP‐activated protein kinase (AMPK)‐dependent Raptor Ser792 phosphorylation does not influence mechanistic target of rapamycin complex 1 (mTORC1)‐S6K1 activation by intense muscle ...contraction.
α2‐AMPK activity‐deficient mice have lower contraction‐stimulated protein synthesis.
Increasing glycogen activates mTORC1‐S6K1.
Normalizing muscle glycogen content rescues reduced protein synthesis in AMPK‐deficient mice.
The mechansitic target of rapamycin complex 1 (mTORC1)‐S6K1 signalling pathway regulates muscle growth‐related protein synthesis and is antagonized by AMP‐activated protein kinase (AMPK) in multiple cell types. Resistance exercise stimulates skeletal muscle mTORC1‐S6K1 and AMPK signalling and post‐contraction protein synthesis. Glycogen inhibits AMPK and has been proposed as a pro‐anabolic stimulus. The present study aimed to investigate how muscle mTORC1‐S6K1 signalling and protein synthesis respond to resistance exercise‐mimicking contraction in the absence of AMPK and with glycogen manipulation. Resistance exercise‐mimicking unilateral in situ contraction of musculus quadriceps femoris in anaesthetized wild‐type and dominant negative α2 AMPK kinase dead transgenic (KD‐AMPK) mice, measuring muscle mTORC1 and AMPK signalling immediately (0 h) and 4 h post‐contraction, and protein‐synthesis at 4 h. Muscle glycogen manipulation by 5 day oral gavage of the glycogen phosphorylase inhibitor CP316819 and sucrose (80 g L−1) in the drinking water prior to in situ contraction. The mTORC1‐S6K1 and AMPK signalling axes were coactivated immediately post‐contraction, despite potent AMPK‐dependent Ser792 phosphorylation on the mTORC1 subunit raptor. KD‐AMPK muscles displayed normal mTORC1‐S6K1 activation at 0 h and 4 h post‐exercise, although there was impaired contraction‐stimulated protein synthesis 4 h post‐contraction. Pharmacological/dietary elevation of muscle glycogen content augmented contraction‐stimulated mTORC1‐S6K1‐S6 signalling and rescued the reduced protein synthesis‐response in KD‐AMPK to wild‐type levels. mTORC‐S6K1 signalling is not influenced by α2‐AMPK during or after intense muscle contraction. Elevated glycogen augments mTORC1‐S6K1 signalling. α2‐AMPK‐deficient KD‐AMPK mice display impaired contraction‐induced muscle protein synthesis, which can be rescued by normalizing muscle glycogen content.
Key points
AMP‐activated protein kinase (AMPK)‐dependent Raptor Ser792 phosphorylation does not influence mechanistic target of rapamycin complex 1 (mTORC1)‐S6K1 activation by intense muscle contraction.
α2‐AMPK activity‐deficient mice have lower contraction‐stimulated protein synthesis.
Increasing glycogen activates mTORC1‐S6K1.
Normalizing muscle glycogen content rescues reduced protein synthesis in AMPK‐deficient mice.
Key points
Exercise is a potent physiological stimulus to clear blood glucose from the circulation into skeletal muscle.
The mammalian target of rapamycin complex 2 (mTORC2) is an important regulator ...of muscle glucose uptake in response to insulin stimulation.
Here we report for the first time that the activity of mTORC2 in mouse muscle increases during exercise.
We further show that glucose uptake during exercise is decreased in mouse muscle that lacks mTORC2 activity.
We also provide novel identifications of new mTORC2 substrates during exercise in mouse muscle.
Exercise increases glucose uptake into insulin‐resistant muscle. Thus, elucidating the exercise signalling network in muscle may uncover new therapeutic targets. The mammalian target of rapamycin complex 2 (mTORC2), a regulator of insulin‐controlled glucose uptake, has been reported to interact with ras‐related C3 botulinum toxin substrate 1 (Rac1), which plays a role in exercise‐induced glucose uptake in muscle. Therefore, we tested the hypothesis that mTORC2 activity is necessary for muscle glucose uptake during treadmill exercise. We used mice that specifically lack mTORC2 signalling in muscle by deletion of the obligatory mTORC2 component Rictor (Ric mKO). Running capacity and running‐induced changes in blood glucose, plasma lactate and muscle glycogen levels were similar in wild‐type (Ric WT) and Ric mKO mice. At rest, muscle glucose uptake was normal, but during running muscle glucose uptake was reduced by 40% in Ric mKO mice compared to Ric WT mice. Running increased muscle phosphorylated 5′ AMP‐activated protein kinase (AMPK) similarly in Ric WT and Ric mKO mice, and glucose transporter type 4 (GLUT4) and hexokinase II (HKII) protein expressions were also normal in Ric mKO muscle. The mTORC2 substrate, phosphorylated protein kinase C α (PKCα), and the mTORC2 activity readout, phosphorylated N‐myc downstream regulated 1 (NDRG1) protein increased with running in Ric WT mice, but were not altered by running in Ric mKO muscle. Quantitative phosphoproteomics uncovered several additional potential exercise‐dependent mTORC2 substrates, including contractile proteins, kinases, transcriptional regulators, actin cytoskeleton regulators and ion‐transport proteins. Our study suggests that mTORC2 is a component of the exercise signalling network that regulates muscle glucose uptake and we provide a resource of new potential members of the mTORC2 signalling network.
Key points
Exercise is a potent physiological stimulus to clear blood glucose from the circulation into skeletal muscle.
The mammalian target of rapamycin complex 2 (mTORC2) is an important regulator of muscle glucose uptake in response to insulin stimulation.
Here we report for the first time that the activity of mTORC2 in mouse muscle increases during exercise.
We further show that glucose uptake during exercise is decreased in mouse muscle that lacks mTORC2 activity.
We also provide novel identifications of new mTORC2 substrates during exercise in mouse muscle.
Reactive oxygen species (ROS) have been proposed as signaling molecules mediating exercise training adaptation, but the ROS source has remained unclear. This study aimed to investigate if increased ...NADPH oxidase (NOX)2-dependent activity during exercise is required for long-term high-intensity interval training (HIIT) in skeletal muscle using a mouse model lacking functional NOX2 complex due to absent p47phox (Ncf1) subunit expression (ncf1* mutation).
HIIT was investigated after an acute bout of exercise and after a chronic intervention (3x/week for 6 weeks) in wild-type (WT) vs. NOX2 activity-deficient (ncf1*) mice. NOX2 activation during HIIT was measured using an electroporated genetically-encoded biosensor. Immunoblotting and single-fiber microscopy was performed to measure classical exercise-training responsive endpoints in skeletal muscle.
A single bout of HIIT increased NOX2 activity measured as p47-roGFP oxidation immediately after exercise but not 1 h or 4 h after exercise. After a 6-week HIIT regimen, improvements in maximal running capacity and some muscle training-markers responded less to HIIT in the ncf1* mice compared to WT, including superoxide dismutase 2, catalase, hexokinase II, pyruvate dehydrogenase and protein markers of mitochondrial oxidative phosphorylation complexes. Strikingly, HIIT-training increased mitochondrial network area and decreased fragmentation in WT mice only.
This study suggests that HIIT exercise increases NOX2 activity in skeletal muscle and shows that NOX2 activity is required for specific skeletal muscle adaptations to HIIT relating to antioxidant defense, glucose metabolism, and mitochondria.
Growth Factor-Dependent and -Independent Activation of mTORC2 Knudsen, Jonas R.; Fritzen, Andreas M.; James, David E. ...
Trends in endocrinology and metabolism,
January 2020, 2020-Jan, 2020-01-00, 20200101, Letnik:
31, Številka:
1
Journal Article
Recenzirano
The target of rapamycin complex 2 (TORC2) was discovered in 2002 in budding yeast. Its mammalian counterpart, mTORC2, was first described in 2004. Soon thereafter it was demonstrated that mTORC2 ...directly phosphorylates Akt on Ser473, ending a long search for the elusive ‘second’ insulin-responsive Akt kinase. In this review we discuss key evidence pertaining to the subcellular localization of mTORC2, highlighting a spatial heterogeneity that relates to mTORC2 activation. We summarize current models for how growth factors (GFs), such as insulin, trigger mTORC2 activation, and we provide a comprehensive discussion focusing on a new exciting frontier, the molecular mechanisms underpinning GF-independent activation of mTORC2.
Different versions of mTORC2 are located in different subcellular compartments, allowing versatile signaling sensing and transduction.In addition to activation by classical GFs, mTORC2 can be activated by GF-independent mechanisms.GF-independent mTORC2 activation comprises adrenergic agonists, lipid species, exercise and possibly acetylation.The phosphorylation state of Akt Ser473, a good proxy for GF-dependent activation of mTORC2, is a poor readout for assessing GF-independent mTORC2 activity.
Activation of the sympathetic nervous system causes pronounced metabolic changes that are mediated by multiple adrenergic receptor subtypes. Systemic treatment with β
adrenergic receptor agonists ...results in multiple beneficial metabolic effects, including improved glucose homeostasis. To elucidate the underlying cellular and molecular mechanisms, we chronically treated wild-type mice and several newly developed mutant mouse strains with clenbuterol, a selective β
-adrenergic receptor agonist. Clenbuterol administration caused pronounced improvements in glucose homeostasis and prevented the metabolic deficits in mouse models of β-cell dysfunction and insulin resistance. Studies with skeletal muscle-specific mutant mice demonstrated that these metabolic improvements required activation of skeletal muscle β
-adrenergic receptors and the stimulatory G protein, G
. Unbiased transcriptomic and metabolomic analyses showed that chronic β
-adrenergic receptor stimulation caused metabolic reprogramming of skeletal muscle characterized by enhanced glucose utilization. These findings strongly suggest that agents targeting skeletal muscle metabolism by modulating β
-adrenergic receptor-dependent signaling pathways may prove beneficial as antidiabetic drugs.
The small molecule kinase inhibitor SBI-0206965 was originally described as a specific inhibitor of ULK1/2. More recently, it was reported to effectively inhibit AMPK and several studies now report ...its use as an AMPK inhibitor. Currently, we investigated the specificity of SBI-0206965 in incubated mouse skeletal muscle, measuring the effect on analog 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR)-stimulated AMPK-dependent glucose transport and insulin-stimulated AMPK-independent glucose uptake. Pre-treatment with 10 µM SBI-0206965 for 50 min potently suppressed AICAR-stimulated glucose transport in both the extensor digitorum longus (EDL) and soleus muscle. This was despite only a modest lowering of AICAR-stimulated AMPK activation measured as ACC2 Ser212, while ULK1/2 Ser555 phosphorylation was prevented. Insulin-stimulated glucose transport was also potently inhibited by SBI-0206965 in soleus. No major changes were observed on insulin-stimulated cell signaling. No general effect of SBI-0206965 on intracellular membrane morphology was observed by transmission electron microscopy. As insulin is known to neither activate AMPK nor require AMPK to stimulate glucose transport, and insulin inhibits ULK1/2 activity, these data strongly suggest that SBI-0206965 has a non-specific off-target inhibitory effect on muscle glucose transport. Thus, SBI-0206965 is not a specific inhibitor of the AMPK/ULK-signaling axis in skeletal muscle, and data generated with this inhibitor must be interpreted with caution.
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