Skeletal muscle plasticity and energy metabolism

Abstract

Skeletal muscle is remarkable in its ability to adjust to our needs. It can change its energy stores and usage, as well as its total mass. Furthermore, skeletal muscle adapts to deactivate reactive oxygen species produced, which in turn can both damage cells and convey signals. The molecular mechanisms regulating skeletal muscle plasticity are many, including reactive oxygen species, AMPK, and FOXO. AMPK functions as a molecular energy sensor, while the FOXO proteins are transcription factors that bind to the DNA, and regulate gene transcription. To understand the role of reactive oxygen species in health, we investigated how an intravenous antioxidant infusion of N-acetyl-cysteine (NAC), affected exercise-modulated insulin sensitivity. We found that NAC infusion decreased whole-body insulin sensitivity and skeletal muscle p70S6K phosphorylation, indicating diminished glucose uptake and attenuated protein synthesis. We also investigated the changes occurring in the atrophying skeletal muscle of individuals with spinal cord injury. We find that AMPK signaling decreases during the first year after injury, and that protein content of the AMPK regulatory γ1 subunit decreased, and γ3 increased. Skeletal muscle energy metabolism decreased during the first year after spinal cord injury, as indicated by the decreased protein content of the mitochondrial respiration complexes I-III. The contractile myosin heavy chain proteins myosin heavy chain 1 declined, and myosin heavy chain IIa increased 12 months after spinal cord injury.

In order to understand how the changes in energy metabolizing and contractile proteins occurred, we investigated the mechanisms mediating protein degradation and synthesis, namely translation, autophagy and proteasomal degradation. We found that protein content of LC3II, as well as protein content and phosphorylation of S6 kinase, increased transiently during the first year after injury, indicating a temporary increase in autophagy and protein synthesis. We also detected stably increased levels of Lys48 poly-ubiquitinated proteins, indicating constantly increased proteasomal degradation during the first year after injury. Additionally, FOXO3 protein content, and FOXO1 phosphorylation decreased during the first year after spinal cord injury. To better understand the metabolic role of FOXO proteins, we transfected mouse skeletal muscle with FOXO proteins modified to bind to the DNA without activating transcription, leading to inhibited expression of FOXO regulated genes. We find that inhibition FOXO transcriptional activity decreased skeletal muscle glucose uptake, and increased inflammatory signaling and immune cell infiltration.

Together, these studies partly elucidate how skeletal muscle adapts to its changing environment. We find that reactive oxygen species appear to be involved in the beneficial effects of exercise, and we unravel the signals and mechanisms mediating decreased skeletal muscle mass after spinal cord injury. Finally, we find that FOXO proteins directly affect gene networks involved in regulating inflammation and glucose metabolism in skeletal muscle.