A perda muscular ou atrofia é uma condição associada a importantes doenças sistêmicas humanas, incluindo diabetes, câncer e insuficiência renal. Há diversos estudos de perfis transcricionais mostrando que um conjunto comum de genes, denominados atrogenes, é modulado nos músculos atrofiados. No entanto, as alterações transcricionais que desencadeiam a reversão ou atenuação da atrofia muscular ainda não foram caracterizadas a nível molecular até o momento. Para identificar os principais genes envolvidos na recuperação da massa muscular esquelética, utilizamos a técnica de microarray e RT-PCR para investigar genes diferencialmente expressos durante a reversão da atrofia do músculo Elevador do ânus (EA) sensível a andrógeno, no modelo de castração e reposição de testosterona. Como esperado, a maioria dos genes expressos de maneira diferencial comportam-se como atrogenes e respondem à atrofia induzida pela castração. Porém, observou-se pela primeira vez um grupo de sete genes (APLN, DUSP5, IGF1, PIK3IP1, KLHL38, PI15 e MKL1) que não responderam à castração, mas exclusivamente à reposição de testosterona. Considerando que quase todas as proteínas codificadas por esses genes estão associadas à reversão da atrofia e podem funcionar como reguladores da proliferação/crescimento celular, nossos resultados abrem novas perspectivas sobre a existência de anti-atrogenes / Muscle wasting or atrophy is a condition associated with major human systemic diseases including diabetes, cancer, and kidney failure. There is accumulating evidence from transcriptional profiles showing that a common set of genes, termed atrogenes, is modulated in atrophying muscles. However, transcriptional changes that trigger reversion or attenuation of muscle atrophy have not been characterized at the molecular level until now. To identify key genes involved in the recovery of skeletal muscle mass, we have used cDNA microarrays and RT-PCR to investigate genes differentially expressed during the atrophy reversion of the androgen-sensitive Levator ani muscle (LA), in the well-established model of castration and testosterone replacement. As expected, most of the differential expressed genes behave as atrogenes and responded to castration-induced atrophy. Strikingly, seven genes (APLN, DUSP5, IGF1, PIK3IP1, KLHL38, PI15, and MKL1) did not respond to castration but exclusively to the testosterone replacement. Considering that almost all proteins encoded by these genes are associated to reversion of atrophy and may function as regulators of cell proliferation/growth, our results open new perspectives on the existence of a new group of genes, determined anti-atrogenes

As a front-line treatment for cardiovascular disease, statins are among some of the most widely prescribed drugs worldwide. Statins are effective at lowering cholesterol, but approximately 7-29% of patients report some form of adverse muscle effect during the course of treatment. The severity of these side effects ranges from low-level to life-threatening myopathy. The mechanism of statin myopathy remains ill-defined, but muscle-specific E3 ubiquitin ligases have been implicated. In addition, statins have been shown to activate caspase-1 (and increase IL-1β) in immune cells, which is a key effector of the NLRP3 inflammasome. The relevance of this inflammatory response in statin myopathy remains unknown. Using C2C12 myotubes, an in vitro model of statin-induced myopathy was developed to test the impact of NLRP3 inflammasome activation on markers of statin myopathy. Gene expression of the muscle-specific E3 ubiquitin ligases atrogin-1 and MuRF-1 (atrogenes) were used as markers of statin-induced myopathy. Lipopolysaccharide priming of the NLRP3 inflammasome was found to lower the effective dose of fluvastatin required to augment atrogene expression. This effect correlated with reduced phosphorylation of Akt and FOXO3a, a transcription factor regulating atrogene expression. Statin-induced atrogene expression was also found to be dependent on an isoprenoid that is required for protein prenylation rather than cholesterol biosynthesis pathways. Fluvastatin increased caspase-1 activity in a prenylation-dependent manner and selective inhibitors of NLRP3 and caspase-1 were able to prevent increased atrogene expression with fluvastatin treatment. Therefore, the NLRP3 inflammasome contributes to markers of statin-induced myopathy through a prenylation-dependant pathway in muscle cells. This work presents a novel mechanism involved in statin myopathy, and has shown that the inflammasome may represent a new drug target to mitigate muscle symptoms in patients taking statins. / Thesis / Master of Science (MSc) / Statins are a class of widely prescribed cholesterol-lowering drugs that reduce the risk of heart attack and stroke. However, many patients often complain of statin-induced muscle side effects (myopathy) that impact their quality of life. Symptoms of this statin-induced myopathy can manifest as muscle pain and weakness. The underlying biology causing this condition is still not well understood. Independent of its cholesterol-lowering effect, statins can activate an immune receptor called the NLRP3 inflammasome, indicating that inflammation may contribute to myopathy. Therefore, the primary goal of this study was to determine if this immune response contributes to statin-induced myopathy. It was found that inhibition of the NLRP3 inflammasome lowers markers of statin myopathy. Results from this study will provide further insight into mechanisms regulating this myopathy, and may lead to new treatments that can help alleviate statin side effects in muscle.

Le muscle squelettique s'adapte en réponse à diverses influences en modulant sa masse et ses propriétés contractiles et métaboliques. Il est ainsi rapporté que l'hypoxie sévère a un effet délétère sur la masse et les capacités oxydatives du muscle, et pourrait ralentir la maturation du phénotype contractile au cours du développement post-natal. Cependant, les mécanismes de contrôle de cette plasticité musculaire ne sont pas clairement identifiés. Le but de ce travail était de déterminer le rôle de l'hypoxie environnementale sur le contrôle de la masse et l'adaptation du phénotype du muscle en croissance (hypertrophie de surcharge du plantaris après ablation de ses muscles agonistes et régénération du soléaire après lésions étendues induites par la notexine). L'exposition hypoxique limite transitoirement l'hypertrophie induite par la surcharge fonctionnelle, tandis qu'elle accentue la fonte musculaire en réprimant la formation et la croissance des néo-fibres au cours des étapes précoces de la régénération. Ces résultats seraient en partie expliqués par la désactivation partielle de la principale voie de protéosynthèse, la voie mTOR, par un mécanisme indépendant d'Akt. Parmi les inhibiteurs endogènes de mTOR étudiés (REDD1, BNIP-3 et l'AMPK), nous montrons que l'activation prononcée de l'AMPK en hypoxie pourrait réprimer l'activité de mTOR au cours de la régénération, alors que le mécanisme responsable de l'inhibition de mTOR n'a pas pu être identifié dans le modèle de surcharge. Le système protéolytique ubiquitine/protéasome-dépendant, évalué à partir de l'expression des atrogènes MURF1 et MAFbx, pourrait également expliquer en partie l'altération de l'hypertrophie de surcharge en hypoxie. Nos résultats soulignent par ailleurs que l'activité des cellules satellites serait réprimée au cours des premiers jours de régénération musculaire, conduisant à réduire la formation et la croissance des myotubes. Malgré cette perturbation précoce de la croissance musculaire, l'exposition prolongée en hypoxie ne limite pas l'hypertrophie de surcharge et la récupération de la masse du muscle lésé. Ceci démontre que les signaux anaboliques induits dans ces deux situations de croissance musculaire l'emportent très largement sur les signaux cataboliques de l'hypoxie. L'analyse des propriétés métaboliques et contractiles met en évidence que l'hypoxie altère les capacités oxydatives du muscle en croissance, mais les mécanismes impliqués dans cette réponse adaptative restent à identifier. Par ailleurs, l'hypoxie ne constitue pas un stimulus métabolique suffisant pour altérer la transition du phénotype contractile du muscle en surcharge et la récupération complète du phénotype contractile du muscle lésé. Elle contribue uniquement à ralentir très modérément et transitoirement l'adaptation phénotypique du muscle en surcharge, et à modifier le profil contractile du muscle durant la phase de dégénérescence musculaire. / Skeletal muscle adapts to various influences, by modulating both its mass and contractile and metabolic properties. It was reported that severe hypoxia impairs muscle mass and oxidative capacities and could reduce the fast-to-slow fiber transition during post-natal development. However, mechanisms involved in muscle plasticity during hypoxia exposure are not clearly identified. This work aimed to determine the role played by ambient hypoxia on the control of muscle mass and muscle phenotype during muscle growth (functional overload-induced hypertrophy of plantaris after removal of its synergist muscles and regeneration of soleus after extensive injury induced by notexin injection). Hypoxia exposure transiently minimizes the overload-induced hypertrophy, while it enhances the muscle-mass loss by repressing the formation and growth of nascent fibers during the early steps of regeneration. These results could be partly due to an impairment of the mTOR signaling activation, the main pathway involved in protein synthesis, independently of Akt. Among the endogenous repressors of mTOR studied (REDD1, BNIP-3 and AMPK), we show that the marked activation of AMPK in hypoxia could repress mTOR activity during regeneration, whereas the mechanism involved in mTOR inhibition remains unknown in the overload model. The ubiquitin/proteasome-dependant system, assessed from expression of the two atrogenes MURF1 and MAFbx, could also partly explain the hypoxia-induced alteration of muscle hypertrophy. Nevertheless, our findings show that activity of satellite cells could be repressed during the first days of regeneration, leading to reduce formation and growth of myotubes. Although muscle growth is early impaired, prolonged hypoxia exposure does not limit the overload-induced hypertrophy and the muscle mass recovery of injured muscle. This demonstrates that anabolic signals induced in these models of drastic muscle growth widely prevail on hypoxia-induced catabolic signals. The analysis of metabolic and contractile properties shows that hypoxia alters oxidative capacities in growing muscle, but mechanisms involved in this adaptive response remain to be elucidated. Moreover, hypoxia is not a sufficient metabolic stimulus to impair the fast-to-slow fiber transition in overloaded muscle, and the complete recovery of the contractile phenotype in injured muscle. It only contributes to transiently and modestly slow down the fast-to-slow fiber shift in overloaded muscle, and to modify the contractile profile of muscle during the degeneration phase.