Myogenesis in Development and Repair

Principal investigators: Christophe MARCELLE & Fabien LE GRAND

Skeletal muscle | stem cells | primary and secondary myogenesis | regeneration | Duchenne muscular dystrophy | gene and cell therapy | single-cell analyses | in vivo imaging | models: embryonic and adult mouse, chicken embryo, human cells

Our work focuses on understanding how skeletal muscles are formed and repaired in vertebrates.

We are using chick and mouse to address two main lines of investigation.
• Understanding how individual stem cells engage into differentiation or remain in a non-differentiated, quiescent and/or self-renewing state.
• To characterize the gene networks underlying the fusion of myoblasts into muscle fibers during embryonic development and muscle repair. This in turn will allow developing the tools and concepts to utilize cell fusion as a mean to repair ailing muscles in heritable muscle diseases (myopathies).

In recent years, our laboratory has focused much effort on understanding the molecular and cellular mechanisms regulating muscle cell fusion. The fusion of differentiating muscle cells to existing muscle fibers is a crucial step of muscle formation and repair that is poorly understood. We have undertaken a genome-wide functional screen on a mouse muscle cell line and identified hundreds of molecules implicated in the fusion of this cell line, with no effect on their proliferation or differentiation. Inhibitors and activators of fusion, members of various signaling pathways, genes that when mutated, lead to muscle dystrophies in human: there are many surprises within this list of putative modulators of muscle fusion. To test their function during fusion, we use the chick embryo as a model. The amenability of the chick embryo to manipulation and imaging, combined with the powerful technique of in vivo electroporation and the strong similarities of muscle formation in birds and mammals provide a unique paradigm to characterize this process in amniotes.

A second line of research is to use skeletal muscle formation in the chick embryo as a model to understand how cells within tissues display complex behaviours while being exposed to an ever-changing cellular environment. We have recently shown that in avian embryos, muscle formation is initiated by Delta1-positive neural crest cells migrating from the dorsal neural tube that, in passing, trigger NOTCH signalling and myogenesis in selected epithelial somite progenitor cells, allowing them to migrate into the nascent muscle to differentiate.

Preliminary data we have now obtained further indicate that in somite cells, the activation of the NOTCH pathway triggers a “signalling module” that couples the initiation of myogenesis with the epithelial-mesenchymal transition (EMT) that allows them to migrate into the growing muscle. This is a significant discovery: in many cellular contexts, essential cell fate decisions are associated with an EMT. This is true at many stages of embryonic development (e.g. the formation of the three germ layers during gastrulation, the formation of neural crest, etc.), but also during pathologies like the metastatic progression of carcinomas. Inhibiting EMT arrests cell fate decision in these experimental models, suggesting a mechanistic link between both processes that has never been understood. Our working hypothesis is that the signalling module we have uncovered underlies the coupling cell fate changes to EMT in a variety of developmental and pathological processes.

Chicken embryo at 5.5 days of development, clarified by the “3DISCO” technique, observed with a light sheet microscope (Z1 Zeiss, CIQLE). Green: neural crest and peripheral nervous system (anti-HNK1); Blue: dermomyotome, muscle progenitors and dorsal neural tube (anti-PAX7); Red: differentiated muscles (anti-Myosin Heavy Chain). Marie-Julie Dejardin & Christophe Marcelle.

This animation movie shows the morphogenesis and growth of the early myotome (i.e. the primitive muscle) in a chicken embryo. All muscles of the body and limbs derive from somites, which are epithelial balls of cells that form sequentially on both sides of the neural tube as the embryo develops. Shown here is the dorsal compartment of somites, named the dermomyotome, from which trunk muscles derive. In a first stage, cells from the medial, the posterior, the anterior and finally the lateral borders of the somite translocate below the dermomyotome, where they elongate parallel to the antero-posterior axis of the embryo. These elongated, mono-nucleated, post-mitotic cells are called myocytes and together they form what we have named the primary myotome. In a second stage, the central portion of the epithelial dermomyotome undergoes and epithelial-mensenchymal transition (EMT). As a result, part of the dermomyotomal cells can migrate towards the ectoderm to later form the dermis, while other cells are “parachuted” into the primary myotome. Unlike myocytes that do not divide, the parachuted cells are true muscle progenitors, and they can either differentiate or self-renew. Through this process, the muscles can grow during embryonic and fetal life. The muscle stem cells of the adult (named satellite cells) derive from the same population of progenitors identified here. It is important to realize that the same morphogenetic process takes places in mice, and therefore presumably in human. This animation movie was created in 2005 by Jérôme Gros with the free open source 3D software Blender. Publications associated with this movie: Gros, Scaal & Marcelle, Developmental Cell, 2004. Gros, Manceau, Thomé & Marcelle, Nature, 2005. Gros, Serralbo & Marcelle, Nature, 2009.