Bilodeau PA, Coyne ES, Wing SS. The ubiquitin proteasome system in atrophying skeletal muscle: roles and regulation. Am J Physiol Cell Physiol 311: C392-C403, 2016. First published August 10, 2016; doi:10.1152/ajpcell.00125.2016.-Muscle atrophy complicates many diseases as well as aging, and its presence predicts both decreased quality of life and survival. Much work has been conducted to define the molecular mechanisms involved in maintaining protein homeostasis in muscle. To date, the ubiquitin proteasome system (UPS) has been shown to play an important role in mediating muscle wasting. In this review, we have collated the enzymes in the UPS whose roles in muscle wasting have been confirmed through loss-of-function studies. We have integrated information on their mechanisms of action to create a model of how they work together to produce muscle atrophy. These enzymes are involved in promoting myofibrillar disassembly and degradation, activation of autophagy, inhibition of myogenesis as well as in modulating the signaling pathways that control these processes. Many anabolic and catabolic signaling pathways are involved in regulating these UPS genes, but none appear to coordinately regulate a large number of these genes. A number of catabolic signaling pathways appear to instead function by inhibition of the insulin/IGF-I/protein kinase B anabolic pathway. This pathway is a critical determinant of muscle mass, since it can suppress key ubiquitin ligases and autophagy, activate protein synthesis, and promote myogenesis through its downstream mediators such as forkhead box O, mammalian target of rapamycin, and GSK3␤, respectively. Although much progress has been made, a more complete inventory of the UPS genes involved in mediating muscle atrophy, their mechanisms of action, and their regulation will be useful for identifying novel therapeutic approaches to this important clinical problem.hormones; muscle atrophy SKELETAL MUSCLE SERVES TWO essential functions, a contractile function for locomotion/maintenance of posture and a metabolic function as the protein reservoir of the body. The myofibers that make up the muscle consist primarily of myofibrillar proteins. The ability of the body to maintain posture or to move arises from the precise organization of myofibrillar proteins into a contractile unit called the sarcomere. The sarcomere consists of thick filaments containing primarily myosin that can slide over thin filaments containing primarily actin. Both types of filaments contain additional regulatory proteins and are linked to the ␣-actinin-containing Z disk, the thin filaments directly so and the thick filaments via titin. Vimentin and desmin are intermediate filaments that serve to anchor sarcomeres properly at the Z disks. These myofibrillar proteins, as well as the sarcoplasmic proteins, also serve as the protein reservoir of the body. Upon fasting, once hepatic glycogen stores are depleted, muscle protein degradation must be activated to provide amino acids to the liver for gluconeogenesis. These amino a...

Background: Hedgehog (Hh) signaling regulates skeletal myogenesis; however, the molecular mechanisms involved are not fully understood.Results: Gli2, a transactivator of Hh signaling, associates with MyoD gene elements, regulating MyoD expression, and binds to MyoD protein, regulating its ability to induce myogenesis.Conclusion: Hh signaling is linked to MyoD gene expression and MyoD protein function.Significance: Novel mechanistic insight is gained into the Hh-regulated myogenesis.

USP19 deubiquitinating enzyme has two isoforms, cytoplasmic and endoplasmic reticulum (ER) localized. The ER-localized isoform specifically suppresses muscle cell differentiation in vitro and appears to do so by inhibiting the unfolded-protein response that occurs during such differentiation. In vivo, loss of USP19 promotes muscle regeneration following injury.

The ubiquitin system plays a critical role in muscle wasting. Previous work has focused on the roles of ubiquitination. However, a role for deubiquitination in this process has not been established. Because ubiquitin-specific protease (USP)19 deubiquitinating enzyme is induced in skeletal muscle in many catabolic conditions, we generated USP19 knockout (KO) mice. These mice lost less muscle mass than wild-type (WT) animals in response to glucocorticoids, a common systemic cause of muscle atrophy as well as in response to denervation, a model of disuse atrophy. KO mice retained more strength and had less myofiber atrophy with both type I and type IIb fibers being protected. Rates of muscle protein synthesis were similar in WT and KO mice, suggesting that the sparing of atrophy was attributed to suppressed protein degradation. Consistent with this, expression of the ubiquitin ligases MuRF1 and MAFbx/atrogin-1 as well as several autophagy genes was decreased in the muscles of catabolic KO mice. Expression of USP19 correlates with that of MuRF1 and MAFbx/atrogin-1 in skeletal muscles from patients with lung cancer or gastrointestinal cancer, suggesting that USP19 is involved in human muscle wasting. Inhibition of USP19 may be a useful approach to the treatment of many muscle-wasting conditions.

Aims/hypothesis Elucidating the molecular mechanisms of fat accumulation and its metabolic consequences is crucial to understanding and treating obesity, an epidemic disease. We have previously observed that Usp19 deubiquitinating enzyme-null mice (Usp19 −/− ) have significantly lower fat mass than wild-type (WT) mice. Thus, this study aimed to provide further understanding of the role of ubiquitin-specific peptidase 19 (USP19) in fat development, obesity and diabetes. Methods In this study, the metabolic phenotypes of WT and Usp19 −/− mice were compared. The stromal vascular fractions (SVFs) of inguinal fat pads from WT and Usp19 −/− mice were isolated and cells were differentiated into adipocytes in culture to assess their adipogenic capacity. Mice were fed a high-fat diet (HFD) for 18 weeks. Body composition, glucose metabolism and metabolic variables were assessed. In addition, following insulin injection, signalling activity was analysed in the muscle, liver and adipose tissue. Finally, the correlation between the expression of Usp19 mRNA and adipocyte function genes in human adipose tissue was analysed. Result Upon adipogenic differentiation, SVF cells from Usp19 −/− failed to accumulate lipid and upregulate adipogenic genes, unlike cells from WT mice. Usp19 −/− mice were also found to have smaller fat pads throughout the lifespan and a higher percentage of lean mass, compared with WT mice. When fed an HFD, Usp19 −/− mice were more glucose tolerant, pyruvate tolerant and insulin sensitive than WT mice. Moreover, HFD-fed Usp19 −/− mice had enhanced insulin signalling in the muscle and the liver, but not in adipose tissue. Finally, USP19 mRNA expression in human adipose tissue was positively correlated with the expression of important adipocyte genes in abdominal fat depots, but not subcutaneous fat depots. Conclusions/interpretation USP19 is an important regulator of fat development. Its inactivation in mice exerts effects on multiple tissues, which may protect against the negative metabolic effects of high-fat feeding. These findings suggest that inhibition of USP19 could have therapeutic potential to protect from the deleterious consequences of obesity and diabetes.