Muscle-specific GSK-3ß ablation accelerates regeneration of disuse-atrophied skeletal muscle.

Muscle wasting impairs physical performance, increases mortality and reduces medical intervention efficacy in chronic diseases and cancer. Developing proficient intervention strategies requires improved understanding of the molecular mechanisms governing muscle mass wasting and recovery. Involvement of muscle protein- and myonuclear turnover during recovery from muscle atrophy has received limited attention. The insulin-like growth factor (IGF)-I signaling pathway has been implicated in muscle mass regulation. As glycogen synthase kinase 3 (GSK-3) is inhibited by IGF-I signaling, we hypothesized that muscle-specific GSK-3ß deletion facilitates the recovery of disuse-atrophied skeletal muscle. Wild-type mice and mice lacking muscle GSK-3ß (MGSK-3ß KO) were subjected to a hindlimb suspension model of reversible disuse-induced muscle atrophy and followed during recovery. Indices of muscle mass, protein synthesis and proteolysis, and post-natal myogenesis which contribute to myonuclear accretion, were monitored during the reloading of atrophied muscle. Early muscle mass recovery occurred more rapidly in MGSK-3ß KO muscle. Reloading-associated changes in muscle protein turnover were not affected by GSK-3ß ablation. However, coherent effects were observed in the extent and kinetics of satellite cell activation, proliferation and myogenic differentiation observed during reloading, suggestive of increased myonuclear accretion in regenerating skeletal muscle lacking GSK-3ß. This study demonstrates that muscle mass recovery and post-natal myogenesis from disuse-atrophy are accelerated in the absence of GSK-3ß.

Pharmacological inhibition of GSK-3 in a guinea pig model of LPS-induced pulmonary inflammation: II. Effects on skeletal muscle atrophy.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is accompanied by pulmonary inflammation and associated with extra-pulmonary manifestations, including skeletal muscle atrophy. Glycogen synthase kinase-3 (GSK-3) has been implicated in the regulation of muscle protein- and myonuclear turnover; two crucial processes that determine muscle mass. In the present study we investigated the effect of the selective GSK-3 inhibitor SB216763 on muscle mass in a guinea pig model of lipopolysaccharide (LPS)-induced pulmonary inflammation-associated muscle atrophy. METHODS: Guinea pigs were pretreated with either intranasally instilled SB216763 or corresponding vehicle prior to each LPS/saline challenge twice weekly. Pulmonary inflammation was confirmed and indices of muscle mass were determined after 12 weeks. Additionally, cultured skeletal muscle cells were incubated with tumor necrosis factor α (TNF-α) or glucocorticoids (GCs) to model the systemic effects of pulmonary inflammation on myogenesis, in the presence or absence of GSK-3 inhibitors. RESULTS: Repeated LPS instillation induced muscle atrophy based on muscle weight and muscle fiber cross sectional area. Intriguingly, GSK-3 inhibition using SB216763 prevented the LPS-induced muscle mass decreases and myofiber atrophy. Indices of protein turnover signaling were unaltered in guinea pig muscle. Interestingly, inhibition of myogenesis of cultured muscle cells by TNF-α or synthetic GCs was prevented by GSK-3 inhibitors. CONCLUSIONS: In a guinea pig model of LPS-induced pulmonary inflammation, GSK-3 inhibition prevents skeletal muscle atrophy without affecting pulmonary inflammation. Resistance to inflammation- or GC-induced impairment of myogenic differentiation, imposed by GSK-3 inhibition, suggests that sustained myogenesis may contribute to muscle mass maintenance despite persistent pulmonary inflammation. Collectively, these results warrant further exploration of GSK-3 as a potential novel drug target to prevent or reverse muscle wasting in COPD.

Recovery of muscle mass and muscle oxidative phenotype following disuse does not require GSK-3 inactivation.

BACKGROUND: Physical inactivity contributes to muscle wasting and reductions in mitochondrial oxidative phenotype (OXPHEN), reducing physical performance and quality of life during aging and in chronic disease. Previously, it was shown that inactivation of glycogen synthase kinase (GSK)-3ß stimulates muscle protein accretion, myogenesis, and mitochondrial biogenesis. Additionally, GSK-3ß is inactivated during recovery of disuse-induced muscle atrophy. AIM: Therefore, we hypothesize that GSK-3 inhibition is required for reloading-induced recovery of skeletal muscle mass and OXPHEN. METHODS: Wild-type (WT) and whole-body constitutively active (C.A.) Ser21/9 GSK-3α/ß knock-in mice were subjected to a 14-day hind-limb suspension/14-day reloading protocol. Soleus muscle mass, fiber cross-sectional area (CSA), OXPHEN (abundance of sub-units of oxidative phosphorylation (OXPHOS) complexes and fiber-type composition), as well as expression levels of their main regulators (respectively protein synthesis/degradation, myogenesis and peroxisome proliferator-activated receptor-γ co-activator-1α (PGC-1α) signaling) were monitored. RESULTS: Subtle but consistent differences suggesting suppression of protein turnover signaling and decreased expression of several OXPHOS sub-units and PGC-1α signaling constituents were observed at baseline in C.A. GSK-3 versus WT mice. Although soleus mass recovery during reloading occurred more rapidly in C.A. GSK-3 mice, this was not accompanied by a parallel increased CSA. The OXPHEN response to reloading was not distinct between C.A. GSK-3 and WT mice. No consistent or significant differences in reloading-induced changes in the regulatory steps of protein turnover, myogenesis or muscle OXPHEN were observed in C.A. GSK-3 compared to WT muscle. CONCLUSION: This study indicates that GSK-3 inactivation is dispensable for reloading-induced recovery of muscle mass and OXPHEN.

Regulation of skeletal muscle plasticity by glycogen synthase kinase-3ß: a potential target for the treatment of muscle wasting.

Muscle wasting is a prevalent and disabling condition in chronic disease and cancer and has been associated with increased mortality and impaired efficacy of surgical and medical interventions. Pharmacological therapies to combat muscle wasting are currently limited but considered as an important unmet medical need. Muscle wasting has been attributed to increased muscle proteolysis, and in particular ubiquitin 26S-proteasome system (UPS)-dependent protein breakdown. However, rates of muscle protein synthesis are also subject to extensive (patho) physiological regulation, and the balance between synthesis and degradation ultimately determines net muscle protein turnover. As multinucleated muscle fibers accommodate threshold changes in muscle protein content by the accretion and loss of muscle nuclei, myonuclear turnover may additionally determine muscle mass. Current insights in the mechanisms dictating muscle mass plasticity not only reveal intricate interactions and crosstalk between these processes, but imply the existence of signaling molecules that act as molecular switchboards, which coordinate and integrate cellular responses upon conditions that evoke changes in muscle mass. These "master regulators" of skeletal muscle mass plasticity are preferred targets for pharmacological modulation of skeletal muscle wasting. In this review Glycogen synthase kinase-3ß (GSK-3ß) is highlighted as a master regulator of muscle mass plasticity since, in addition to its role in UPS-mediated muscle protein degradation, it also controls protein synthesis, and influences myonuclear accretion and cell death. Moreover, the regulation of GSK-3ß activity as well as currently available pharmacological inhibitors are described and discussed in the context of multimodal treatment strategies aimed at the inhibition of GSK-3ß, and optimal exploitation of its potential role as a central regulator of skeletal muscle mass plasticity for the treatment of muscle wasting.