The marbling of skeletal muscle by ectopic adipose tissue is a hallmark of many muscle diseases, including sarcopenia and muscular dystrophies, and generally associates with impaired muscle regeneration. Although the etiology and the molecular mechanisms of ectopic adipogenesis are poorly understood, fatty regeneration can be modeled in mice using glycerol-induced muscle damage. Using comprehensive molecular and histological profiling, we compared glycerol-induced fatty regeneration to the classical cardiotoxin (CTX)-induced regeneration model previously believed to lack an adipogenic response in muscle. Surprisingly, ectopic adipogenesis was detected in both models, but was stronger and more persistent in response to glycerol. Importantly, extensive differential transcriptomic profiling demonstrated that glycerol induces a stronger inflammatory response and promotes adipogenic regulatory networks while reducing fatty acid β-oxidation. Altogether, these results provide a comprehensive mapping of gene expression changes during the time course of two muscle regeneration models, and strongly suggest that adipogenic commitment is a hallmark of muscle regeneration, which can lead to ectopic adipocyte accumulation in response to specific physio-pathological challenges.

e Brown adipose tissue (BAT) is a key tissue for energy expenditure via fat and glucose oxidation for thermogenesis. In this study, we demonstrate that the myostatin/activin receptor IIB (ActRIIB) pathway, which serves as an important negative regulator of muscle growth, is also a negative regulator of brown adipocyte differentiation. In parallel to the anticipated hypertrophy of skeletal muscle, the pharmacological inhibition of ActRIIB in mice, using a neutralizing antibody, increases the amount of BAT without directly affecting white adipose tissue. Mechanistically, inhibition of ActRIIB inhibits Smad3 signaling and activates the expression of myoglobin and PGC-1 coregulators in brown adipocytes. Consequently, ActRIIB blockade in brown adipose tissue enhances mitochondrial function and uncoupled respiration, translating into beneficial functional consequences, including enhanced cold tolerance and increased energy expenditure. Importantly, ActRIIB inhibition enhanced energy expenditure only at ambient temperature or in the cold and not at thermoneutrality, where nonshivering thermogenesis is minimal, strongly suggesting that brown fat activation plays a prominent role in the metabolic actions of ActRIIB inhibition.

Metallothioneins are proteins that are involved in intracellular zinc storage and transport. Their expression levels have been reported to be elevated in several settings of skeletal muscle atrophy. We therefore investigated the effect of metallothionein blockade on skeletal muscle anabolism in vitro and in vivo. We found that concomitant abrogation of metallothioneins 1 and 2 results in activation of the Akt pathway and increases in myotube size, in type IIb fiber hypertrophy, and ultimately in muscle strength. Importantly, the beneficial effects of metallothionein blockade on muscle mass and function was also observed in the setting of glucocorticoid addition, which is a strong atrophy-inducing stimulus. Given the blockade of atrophy and the preservation of strength in atrophy-inducing settings, these results suggest that blockade of metallothioneins 1 and 2 constitutes a promising approach for the treatment of conditions which result in muscle atrophy.KEYWORDS muscle metabolism S keletal muscle hypertrophy is characterized in the adult mammal by an increase in the size of preexisting myofibers. The induction of hypertrophy involves an activation of the pathways that increase protein synthesis and inhibition of cellular signaling, which induces protein degradation. Hypertrophy can be induced by the activation of Akt, through multiple potential inputs (1). Akt induces hypertrophy in part by activating the mTOR/70S6 kinase pathway. In addition, Akt inhibits protein degradation, by phosphorylating and therefore blocking Foxo1 and Foxo3-transcription factors which are required for the upregulation of the E3 ubiquitin ligases MuRF1 and MAFbx, which help mediate protein turnover during muscle atrophy (2-4). Therefore, activation of Akt constitutes a critical signaling node to increase muscle hypertrophy and block muscle atrophy (1).Mammalian metallothioneins (MTs) belong to a family of cysteine-rich, metalbinding proteins. In rodents, four MT isoforms have been identified: the two major isoforms, MT-1 and MT-2, are ubiquitously expressed, while MT-3 and MT-4 show tissue specific expression in the central nervous system and squamous epithelia, respectively. In humans, multiple isoforms have been reported for MT-1 (MT-1A, MT-1B, MT-1E, MT-1F, MT-1G, MT-1H, MT-1M, and MT-1X), while no splice variants are documented for MT-2, MT-3, or MT-4 (5; for a review, see reference 6).MTs play a role in cellular zinc homeostasis, mitochondrial function (7), defense against oxidative stress (8), and defense against inflammation (5). Moreover, several reports and a recent review highlight a role of metallothioneins in cancer (9), aging (10), and the onset of particular central nervous system diseases (11).

Highlights d Aged microglia have an activated phenotype that affects motor neuron (MN) health d MN damage leads to age-related defects at the neuromuscular junction d An age-related decline in motor units (MUs) results in loss of muscle innervation d Modulation of microglia by exercise or CSF1R inhibition preserves MUs in aged mice

Musculoskeletal diseases are a leading contributor to mobility disability worldwide. Since the majority of patients with musculoskeletal diseases present with associated muscle weakness, treatment approaches typically comprise an element of resistance training to restore physical strength. The health-promoting effects of resistance exercise are mediated via complex, multifarious mechanisms including modulation of systemic and local inflammation. Here we investigated whether targeted inhibition of the chemerin pathway, which largely controls inflammatory processes via chemokine-like receptor 1 (CMKLR1), can improve skeletal muscle function. Using genetically modified mice, we demonstrate that blockade of CMKLR1 transiently increases maximal strength during growth, but lastingly decreases strength endurance. In-depth analyses of the underlying long-term adaptations revealed microscopic alterations in the number of Pax7-positive satellite cells, as well as molecular changes in genes governing myogenesis and calcium handling. Taken together, these data provide evidence of a critical role for CMKLR1 in regulating skeletal muscle function by modulating the regenerative and contractile properties of muscle tissue. CMKLR1 antagonists are increasingly viewed as therapeutic modalities for a variety of diseases (e.g., psoriasis, metabolic disorders, and multiple sclerosis). Our findings thus have implications for the development of novel drug substances that aim at targeting the chemerin pathway for musculoskeletal or other diseases.