Understanding the role of mTORC1 and mTORC2 in embryonic and adult myogenesis

Myogenesis describes the formation of skeletal muscle fibers during embryogenesis and their regeneration of injury in the adult. The formation of myofibers includes the commitment of cell progenitors into the muscle lineage, their amplification and subsequent differentiation and fusion into multi-nucleated myotubes. The mammalian target of rapamycin (mTOR) assembles into two distinct complexes, termed complex 1 (mTORC1) and 2 (mTORC2), and controls cellular growth and metabolism, in response to nutrients and extracellular signals. The mTOR signaling pathway is crucial for homeostasis of mature skeletal muscle and mTOR deregulation in muscle results in progressive myopathies. Since myogenesis is determined by a complex regulatory network involving growth factors and external stimuli, I investigated the function of mTOR signaling in embryonic and adult myogenesis. This PhD thesis describes the role of mTORC1 and mTORC2 in embryonic and adult myogenesis using genetically modified mice. Depletion of raptor, an essential protein of mTORC1, in muscle progenitors caused the mice to die perinatally because of severe defects in muscle development. I observed that mTORC1 was highly active in embryonic muscle progenitors and precursors and became downregulated in differentiating and fusing myocytes, suggesting a predominant role in muscle cell commitment and proliferation. Accordingly, raptor-depleted myoblasts showed severe defects in proliferation, most probably caused by reduced rates of protein synthesis. Furthermore, loss of mTORC1 reduced, but did not abolish differentiation of myoblasts. Thus, the myogenic process was still completed, but less efficiently, in the absence of mTORC1. To investigate the role of mTORC1 in adult myogenesis, depletion of raptor was induced in adult muscle stem cells, called satellite cells. mTORC1 depletion did not affect the quiescence of satellite cells but delayed their activation upon external stimuli. Furthermore, I established that satellite cells deficient for raptor proliferated and differentiated less efficiently, resulting in poor regeneration following muscle injury. Mice deficient for mTORC2 signaling in developing muscle were viable and showed no histological and functional alterations of skeletal muscle. Moreover, depletion of rictor in embryonic muscle progenitors did not affect the number of satellite cells and their myogenic function in adult skeletal muscle upon injury. In particular, rictor-depleted satellite cells did not differ from control cells in their proliferation, differentiation and fusion capacity. However, the number of satellite cells decreased following repeated muscle injuries in the absence of mTORC2. Furthermore, the number of quiescent satellite cells declined during physiological aging in mutant mice, causing an impairment in the regenerative capacity at progressed age. In conclusion, I established that mTORC1, but not mTORC2 signaling is required for the formation of skeletal muscle during embryogenesis and for the regeneration of the tissue following severe muscle damage. I found that loss of mTORC1 reduces protein synthesis and thereby limits the proliferation and differentiation capacity of myoblasts during embryonic and adult myogenesis. In contrast, mTORC2 is dispensable for the myogenic function of myoblasts to proliferate, differentiate and fuse, but is required for the maintenance of the muscle stem cell pool during aging and after muscle injury. Overall, these results are of major importance as they extent our knowledge about the distinct roles of mTORC1 and mTORC2 in the myogenic process and the maintenance of the muscle stem cell pool. As mTOR is a central regulatory hub, integrating the metabolic status of a cell and translating those signals into proteostatic processes, my work has established that these mTOR-controlled functions are important in muscle precursors. These results may open new avenues regarding pathological conditions, such as aging or metabolic muscle disorders, which have also been related to mTOR deregulation

Targeting deregulated AMPK/mTORC1 pathways improves muscle function in myotonic dystrophy type I

Myotonic dystrophy type I (DM1) is a disabling multisystemic disease that predominantly affects skeletal muscle. It is caused by expanded CTG repeats in the 3'-UTR of the dystrophia myotonica protein kinase (DMPK) gene. RNA hairpins formed by elongated DMPK transcripts sequester RNA-binding proteins, leading to mis-splicing of numerous pre-mRNAs. Here, we have investigated whether DM1-associated muscle pathology is related to deregulation of central metabolic pathways, which may identify potential therapeutic targets for the disease. In a well-characterized mouse model for DM1 (HSALR mice), activation of AMPK signaling in muscle was impaired under starved conditions, while mTORC1 signaling remained active. In parallel, autophagic flux was perturbed in HSALR muscle and in cultured human DM1 myotubes. Pharmacological approaches targeting AMPK/mTORC1 signaling greatly ameliorated muscle function in HSALR mice. AICAR, an AMPK activator, led to a strong reduction of myotonia, which was accompanied by partial correction of misregulated alternative splicing. Rapamycin, an mTORC1 inhibitor, improved muscle relaxation and increased muscle force in HSALR mice without affecting splicing. These findings highlight the involvement of AMPK/mTORC1 deregulation in DM1 muscle pathophysiology and may open potential avenues for the treatment of this disease

mTORC1 and PKB/Akt control the muscle response to denervation by regulating autophagy and HDAC4

Loss of innervation of skeletal muscle is a determinant event in several muscle diseases. Although several effectors have been identified, the pathways controlling the integrated muscle response to denervation remain largely unknown. Here, we demonstrate that PKB/Akt and mTORC1 play important roles in regulating muscle homeostasis and maintaining neuromuscular endplates after nerve injury. To allow dynamic changes in autophagy, mTORC1 activation must be tightly balanced following denervation. Acutely activating or inhibiting mTORC1 impairs autophagy regulation and alters homeostasis in denervated muscle. Importantly, PKB/Akt inhibition, conferred by sustained mTORC1 activation, abrogates denervation-induced synaptic remodeling and causes neuromuscular endplate degeneration. We establish that PKB/Akt activation promotes the nuclear import of HDAC4 and is thereby required for epigenetic changes and synaptic gene up-regulation upon denervation. Hence, our study unveils yet-unknown functions of PKB/Akt-mTORC1 signaling in the muscle response to nerve injury, with important implications for neuromuscular integrity in various pathological conditions

LncRNA-encoded peptides: More than translational noise?

Long non-coding RNAs (lncRNAs) belong to the ever-increasing number of transcripts that are thought not to encode proteins. A recent study has now identified a small polypeptide encoded by the lncRNA LINC00961 that inhibits amino acid-induced mTORC1 activation in skeletal muscle

mTORC2 affects the maintenance of the muscle stem cell pool

Abstract Background The mammalian target of rapamycin complex 2 (mTORC2), containing the essential protein rictor, regulates cellular metabolism and cytoskeletal organization by phosphorylating protein kinases, such as PKB/Akt, PKC, and SGK. Inactivation of mTORC2 signaling in adult skeletal muscle affects its metabolism, but not muscle morphology and function. However, the role of mTORC2 in adult muscle stem cells (MuSCs) has not been investigated. Method Using histological, biochemical, and molecular biological methods, we characterized the muscle phenotype of mice depleted for rictor in the Myf5-lineage (RImyfKO) and of mice depleted for rictor in skeletal muscle fibers (RImKO). The proliferative and myogenic potential of MuSCs was analyzed upon cardiotoxin-induced injury in vivo and in isolated myofibers in vitro. Results Skeletal muscle of young and 14-month-old RImyfKO mice appeared normal in composition and function. MuSCs from young RImyfKO mice exhibited a similar capacity to proliferate, differentiate, and fuse as controls. In contrast, the number of MuSCs was lower in young RImyfKO mice than in controls after two consecutive rounds of cardiotoxin-induced muscle regeneration. Similarly, the number of MuSCs in RImyfKO mice decreased with age, which correlated with a decline in the regenerative capacity of mutant muscle. Interestingly, reduction in the number of MuSCs was also observed in 14-month-old RImKO muscle. Conclusions Our study shows that mTORC2 signaling is dispensable for myofiber formation, but contributes to the homeostasis of MuSCs. Loss of mTORC2 does not affect their myogenic function, but impairs the replenishment of MuSCs after repeated injuries and their maintenance during aging. These results point to an important role of mTORC2 signaling in MuSC for muscle homeostasis

mTOR controls embryonic and adult myogenesis via mTORC1

The formation of multi-nucleated muscle fibers from progenitors requires the fine-tuned and coordinated regulation of proliferation, differentiation and fusion, both during development and after injury in the adult. Although some of the key factors that are involved in the different steps are well known, how intracellular signals are coordinated and integrated is largely unknown. Here, we investigated the role of the cell-growth regulator mTOR by eliminating essential components of the mTOR complexes 1 (mTORC1) and 2 (mTORC2) in mouse muscle progenitors. We show that inactivation of mTORC1, but not mTORC2, in developing muscle causes perinatal death. In the adult, mTORC1 deficiency in muscle stem cells greatly impinges on injury-induced muscle regeneration. These phenotypes are because of defects in the proliferation and fusion capacity of the targeted muscle progenitors. However, mTORC1-deficient muscle progenitors partially retain their myogenic function. Hence, our results show that mTORC1 and not mTORC2 is an important regulator of embryonic and adult myogenesis, and they point to alternative pathways that partially compensate for the loss of mTORC1

Dumas amoureux

L’Éros proprement amoureux de Dumas, qui le poussa à accumuler sa vie durant conquêtes et maîtresses, est la figure emblématique d’une énergie vitale et d’un désir de littérature et d’action qui n’avaient pas, en tant que tels, fait l’objet d’assez d’attention. Proposant une « éro-poétique » de l’œuvre dumasienne, l’ouvrage y aborde le désir selon plusieurs directions privilégiées. Il explore le désir amoureux et érotique, sa représentation, sa productivité et sa portée dans les différents genres illustrés par Dumas (théâtre, romans, contes, récits de voyage, autobiographie et écrits intimes, causeries…). Il envisage plus largement le désir comme une origine et un foyer de la création dumasienne, permettant d’en comprendre la fécondité, la variété, mais aussi les modes d’énonciation et de réception. Il s’intéresse enfin aux empreintes du désir chez les descendants biologiques et littéraires de Dumas, et à la façon dont les motifs sentimentaux et l’érotisme façonnent l’imaginaire qui se déploie dans les réécritures, les adaptations ou les suites de ses œuvres, telles que le D’Artagnan amoureux de Nimier

Sustained activation of mTORC1 in skeletal muscle inhibits constitutive and starvation-induced autophagy and causes a severe, late-onset myopathy

Autophagy is a catabolic process that ensures homeostatic cell clearance and is deregulated in a growing number of myopathological conditions. Although FoxO3 was shown to promote the expression of autophagy-related genes in skeletal muscle, the mechanisms triggering autophagy are unclear. We show that TSC1-deficient mice (TSCmKO), characterized by sustained activation of mTORC1, develop a late-onset myopathy related to impaired autophagy. In young TSCmKO mice, constitutive and starvation-induced autophagy is blocked at the induction steps via mTORC1-mediated inhibition of Ulk1, despite FoxO3 activation. Rapamycin is sufficient to restore autophagy in TSCmKO mice and improves the muscle phenotype of old mutant mice. Inversely, abrogation of mTORC1 signaling by depletion of raptor induces autophagy regardless of FoxO inhibition. Thus, mTORC1 is the dominant regulator of autophagy induction in skeletal muscle and ensures a tight coordination of metabolic pathways. These findings may open interesting avenues for therapeutic strategies directed toward autophagy-related muscle diseases