Molecular targets of myostatin involved in muscle hypertrophy in mouse and cattle | Cibles moléculaires de la myostatine impliquées dans l'hypertrophie musculaire chez la souris et le bovin

English. Muscle hypertrophy has been extensively studied in meat producing animals. It is generally due to an increase in the total number of fibres, especially, of fast glycolytic fibres and associated to lower collagen content and lower intramuscular fat level. This is favourable to high meat yield and greater tenderness but detrimental to meat flavour. In double-muscled (DM) cattle, a loss-of-function mutation in the myostatin gene (MSTN) is responsible for muscle hypertrophy. This member of Transforming Growth Factor (TGF-β) superfamily negatively regulates skeletal muscle mass growth by controlling the number of fibres through its anti-proliferative activity, and the size of individual fibres. In order to get a thorough knowledge of the molecular mechanisms of myostatin action, my PhD thesis (2006-2009) aimed to identify myostatin targets of this growth factor that may be involved in the regulation of muscle hypertrophy. In a first part of this work, I analysed the proteomic and transcriptomic profiles in the muscle of MSTN-null mice and their control littermates (collaboration with J. Reecy, Iowa University, USA). Differential expression of genes and proteins, considered as putative molecular targets of MSTN inactivation, was confirmed by Western-blot and by qPCR. This study revealed an activation of the PI3K/Akt pathway and a differential expression of genes and proteins involved in cell survival (increase in cell survival /anti-apoptotic factors) (Publication 1). In a second part of this work, I analysed the abundance of transcripts and/or proteins involved in the apoptosis (programmed cell death) pathway and quantified apoptotic nuclei by measuring the DNA fragmentation index in cross sections of the muscle. I showed a decrease of pro-apoptotic factor expression and of apoptotic nuclear index illustrating attenuation of muscle nuclei apoptosis in the context of MSTN inactivation (Publication 2). The third part of this work was the validation of mice data in DM cattle using reference techniques (Western-blot and qPCR). This study confirmed the existence of molecular signatures and mechanisms similar to those identified in mice (Publication 3). Altogether, the data presented here showed an increase of myonuclei survival and hence the size of nucleo-cytoplasmic domain, an important determinant for protein synthesis. This mechanism could contribute to muscular hypertrophy in animals which presented MSTN inactivation.

French. La myostatine, un facteur de croissance de la superfamille des TGF-béta, régule négativement la croissance et la masse du muscle squelettique en contrôlant le nombre de fibres par son action anti-proliférative, et la croissance individuelle de chacune d'entre elles. Dans le but de compléter les connaissances sur ces mécanismes d'action, cette thèse (2006-2009) avait pour objectif d'identifier de nouvelles cibles moléculaires de la myostatine impliquées dans l'hypertrophie musculaire. Ce travail de thèse a révélé de nouvelles cibles de la myostatine, notamment une activation de la cascade PI3K/Akt, une expression différentielle des gènes et des protèines impliquées dans la régulation de la survie cellulaire. L'inhibition de l'apoptose des noyaux musculaires en l'absence de myostatins fonctionnelle, et par conséquent , l'augmentation du nombre de noyaux et de la taille du domaine nucléo-cytoplasmique pourraient ainsi contribuer au phénomène d'hypertrophie