Skeletal muscle possesses a remarkable ability to adapt to various physiologic conditions. AMPK is a sensor of intracellular energy status that maintains energy stores by fine‐tuning anabolic and catabolic pathways. AMPK's role as an energy sensor is particularly critical in tissues displaying highly changeable energy turnover. Due to the drastic changes in energy demand that occur between the resting and exercising state, skeletal muscle is one such tissue. Here, we review the complex regulation of AMPK in skeletal muscle and its consequences on metabolism ( e.g ., substrate uptake, oxidation, and storage as well as mitochondrial function of skeletal muscle fibers). We focus on the role of AMPK in skeletal muscle during exercise and in exercise recovery. We also address adaptations to exercise training, including skeletal muscle plasticity, highlighting novel concepts and future perspectives that need to be investigated. Furthermore, we discuss the possible role of AMPK as a therapeutic target as well as different AMPK activators and their potential for future drug development.— Kjøbsted, R., Hingst, J. R., Fentz, J., Foretz, M., Sanz, M.‐N., Pehmøller, C., Shum, M., Marette, A., Mounier, R., Treebak, J. T., Wojtaszewski, J. F. P., Viollet, B., Lantier, L. AMPK in skeletal muscle function and metabolism. FASEB J. 32, 1741–1777 (2018). www.fasebj.org

Earlier studies have demonstrated that muscle insulin sensitivity to stimulate glucose uptake is enhanced several hours after an acute bout of exercise. Using AICAR, we recently demonstrated that prior activation of AMPK is sufficient to increase insulin sensitivity in mouse skeletal muscle. Here we aimed to determine whether activation of AMPK is also a prerequisite for the ability of muscle contraction to increase insulin sensitivity. We found that prior in situ contraction of m. extensor digitorum longus (EDL) and treadmill exercise increased muscle and whole-body insulin sensitivity in wild-type (WT) mice, respectively. These effects were not found in AMPKα1α2 muscle-specific knockout mice. Prior in situ contraction did not increase insulin sensitivity in m. soleus from either genotype. Improvement in muscle insulin sensitivity was not associated with enhanced glycogen synthase activity or proximal insulin signaling. However, in WT EDL muscle, prior in situ contraction enhanced insulin-stimulated phosphorylation of TBC1D4 Thr649 and Ser711. Such findings are also evident in prior exercised and insulin-sensitized human skeletal muscle. Collectively, our data suggest that the AMPK-TBC1D4 signaling axis is likely mediating the improved muscle insulin sensitivity after contraction/exercise and illuminates an important and physiologically relevant role of AMPK in skeletal muscle.

Skeletal muscle conveys several of the health-promoting effects of exercise; yet the underlying mechanisms are not fully elucidated. Studying skeletal muscle is challenging due to its different fiber types and the presence of non-muscle cells. This can be circumvented by isolation of single muscle fibers. Here, we develop a workflow enabling proteomics analysis of pools of isolated muscle fibers from freeze-dried human muscle biopsies. We identify more than 4000 proteins in slow- and fast-twitch muscle fibers. Exercise training alters expression of 237 and 172 proteins in slow- and fast-twitch muscle fibers, respectively. Interestingly, expression levels of secreted proteins and proteins involved in transcription, mitochondrial metabolism, Ca²⁺ signaling, and fat and glucose metabolism adapts to training in a fiber type-specific manner. Our data provide a resource to elucidate molecular mechanisms underlying muscle function and health, and our workflow allows fiber type-specific proteomic analyses of snap-frozen non-embedded human muscle biopsies.

Insulin resistance is a major health risk, and although exercise clearly improves skeletal muscle insulin sensitivity, the mechanisms are unclear. Here we show that initiation of a euglycemic-hyperinsulinemic clamp 4 h after single-legged exercise in humans increased microvascular perfusion (determined by contrast-enhanced ultrasound) by 65% in the exercised leg and 25% in the rested leg (P < 0.05) and that leg glucose uptake increased 50% more (P < 0.05) in the exercised leg than in the rested leg. Importantly, infusion of the nitric oxide synthase inhibitor l-NG-monomethyl-l-arginine acetate (l-NMMA) into both femoral arteries reversed the insulin-stimulated increase in microvascular perfusion in both legs and abrogated the greater glucose uptake in the exercised compared with the rested leg. Skeletal muscle phosphorylation of TBC1D4 Ser318 and Ser704 and glycogen synthase activity were greater in the exercised leg before insulin and increased similarly in both legs during the clamp, and l-NMMA had no effect on these insulin-stimulated signaling pathways. Therefore, acute exercise increases insulin sensitivity of muscle by a coordinated increase in insulin-stimulated microvascular perfusion and molecular signaling at the level of TBC1D4 and glycogen synthase in muscle. This secures improved glucose delivery on the one hand and increased ability to take up and dispose of the delivered glucose on the other hand.

Acute exercise increases glucose uptake in skeletal muscle by an insulin-independent mechanism. In the period after exercise insulin sensitivity to increase glucose uptake is enhanced. The molecular mechanisms underpinning this phenomenon are poorly understood, but appear to involve an increased cell surface abundance of GLUT4. While increased proximal insulin signaling does not seem to mediate this effect, elevated phosphorylation of TBC1D4, a downstream target of both insulin (Akt) and exercise (AMPK) signaling, appears to play a role. The main purpose of this study was to determine whether AMPK activation increases skeletal muscle insulin sensitivity. We found that prior AICAR stimulation of wild-type mouse muscle increases insulin sensitivity to stimulate glucose uptake. However, this was not observed in mice with reduced or ablated AMPK activity in skeletal muscle. Furthermore, prior AICAR stimulation enhanced insulin-stimulated phosphorylation of TBC1D4 at Thr⁶⁴⁹ and Ser⁷¹¹ in wild-type muscle only. These phosphorylation events were positively correlated with glucose uptake. Our results provide evidence to support that AMPK is sufficient to increase skeletal muscle insulin sensitivity. Moreover, TBC1D4 phosphorylation may facilitate the effect of prior AMPK activation to enhance glucose uptake in response to insulin.