Genetic networks and transcriptional complexes of myogenic progenitor commitment and differentiation – MYOCODES

The dermomyotome of the mouse embryo is known to give rise to skeletal muscles progenitors, back dermis, to give rise to smooth muscle cells, to brown adipogenic cells and to endothelial cells. Signals from the neural tube, the ectoderm and the lateral plate mesoderm, like Shh, Wnt and BMP induce somite differentiation, by inducing several master genes of the sclerotomal compartment, or of the dermomyotomal compartment, thus restricting the pluripotence of somitic cells. Main transcription factors, which are responsible for the commitment of these pluripotent somitic cells in the myogenic lineage have been characterized. The Muscle Regulatory Factors (MRF) myf5, Myod, Mrf4 commit the cells into the myogenic fate, while myogenin is responsible for their differentiation. At the limb level, dermomyotomal cells first need to migrate in the limb bud, and the homeo-paired Pax3 gene is responsible for the delamination of these progenitors from the epithelial dermomyotome and their further migration. Pax3 and Pax7, a related paired homeoprotein, are also required for the multiplication of uncommitted myogenic progenitors at the interlimb level, and for the genesis of the pool of satellite cells that are recruited in the adult for muscle repair. Four main muscle fiber-types have been characterized in the adult skeletal muscles that are characterized by several specific physiological properties, such as their contraction speed, their oxidative or glycolytic metabolism and their resistance to exercise. How is this muscle diversity generated during development is still controversial, with some experiments demonstrating the existence of several committed myogenic populations, with clonally transmissible properties, other experiments favoring the influence on the environment to induce the fiber-type differentiation of a single homogenous myogenic population. No slow or fast-type myogenic determinant has yet been characterized in mammals. My group is interested in the mechanisms of mouse muscle development from the early stages when myogenic progenitors arise in the embryo to the later stages when fibers diversify. We have shown that Six homeoproteins initiate a genetic cascade that controls several important steps of muscle development by activating the major activators of each of these individual steps. Thus, we have shown that Six proteins are required for the genesis of hypaxial myogenic progenitors, acting upstream of the paired homeogene Pax3. Then, with Pax3, Six proteins activate Myf5, the first muscle regulatory factor known to commit the myogenic progenitor into the myogenic fate. Then, with Myf5, Six proteins activate myogenin, the muscle regulatory factor responsible for the differentiation of myogenic cells. Then, with Myf5 or with myogenin, Six proteins are responsible for the activation of a network of fast-type muscle genes like troponin, myosin, or parvalbumin. To mediate this multiplicity of effects, Six homeoproteins bind a MEF3 site in the regulatory sequences of downstream target genes. We showed in particular, by chromatin immunoprecipitation that Six1 in the embryo was bound on the enhancers of the Pax3 and Myf5 genes. We characterized several cofactors of Six and showed that these cofactors can selectively activate a number of developmental steps in synergy with the Six proteins. Some of these cofactors are expressed mainly in uncommitted progenitors, like the cofactors Eya which are required for Pax3 expression in hypaxial myogenic progenitors. Other cofactors are expressed in differentiated myocytes, like SOBP. Three bona fide Six cofactors which play a major role in the STC to drive diverse steps of muscle development are LRRFIP2, SOBP and EYA. LRRFIP2 is known to interact with Dishevelled and links Six proteins with the Wnt signaling pathway. SOBP, which is expressed in differentiating myocytes, and was previously characterized in Drosophila as a sine oculis cofactor. Eya proteins are expressed both in myogenic progenitors and in adult myofibers. These Six cofactors that can be located in the nucleus and bind directly to Six1 co-activate transcription through MEF3 binding sites. In the next four coming years we will address three questions related to the STC properties in skeletal muscle development and physiology. These projects have two main objectives: - the mechanisms that underly the determination of the stem character of dermomyotomal cells of the mouse embryo, the transcription factors that preside to the genesis of myogenic progenitors, their self-renewal and their engagement in the myogenic fate during vertebrate embryogenesis. - the mechanisms that control the genesis and the maintenance of muscle fiber-type diversity with an emphasis on the crosstalk between the slow and the fast genetic determinants that preside these antagonist fates in adult myofibers.