Impact de la régulation conformationnelle des protéines Elmo sur le muscle squelettique et les maladies

The scaffold proteins Elmo forms a complex with guanine nucleotide exchange factors (GEFs) of the Dock family. The Elmo/Dock complex is characterized with a conformational regulation and at the basal level, the complex is found in a closed state, owing to the presence of contact sites blocking the binding of Elmo interactors and the GEF activity of Dock for the activation of Rac1. In their activated state, the complex adopts an open conformation and Elmo binding sites will be available. For example, binding of the GTPases RhoG or Arl4A to the RBD domain of Elmo has been shown to induce the recruitment of the complex at the cell membrane. Likewise, the DHR- 2 domain of Dock being available, the GTPase Rac1 will then be specifically activated by Dock and thus induce the remodeling of the actin cytoskeleton. Various cellular processes will then be triggered, such as cell migration, phagocytosis and muscle cell (named myoblast) fusion. In this thesis, we have emphasized the importance of the conformational regulation of Elmo by achieving two objectives.  For the first objective, we studied the conformational regulation of Elmo during myogenesis, i.e. during the establishment of skeletal muscle. The formation of multinucleated myofibers is fundamental for skeletal muscle. During this process, myoblast fusion is a key step to allow the development as well as the regeneration of the muscle. In order to study Elmo in this in vivo context, we generated a series of mouse models. First, through the generation of double knockout mice for Elmo1 and Elmo2 (Elmo1KOElmo2cKO), we demonstrated the essential function of Elmo during embryonic myoblast fusion. Indeed, only mononucleated myofibers are observed following the genetic inactivation of Elmo1 and Elmo2. Subsequently, we also generated knockin mouse lines, where mutations were introduced in specific domains of Elmo to induce its opened (Elmo2EID) or closed (Elmo2RBD) conformation. Thus, we have demonstrated that when Elmo2EID is expressed, the fusion capability is increased and the myofibres are larger. Moreover, muscle regeneration is more efficient in these mice. At the opposite, when Elmo2 has lost its RBD activity (Elmo1KOElmo2RBD), smaller myofibers are observed as well as a less efficient muscle regeneration. Finally, we demonstrated that increasing the Elmo-Dock1-Rac1 pathway activity, directly through the control of the conformational regulation of Elmo, leads to the improvement of the dystrophic phenotypes found in DysferlinKO mice, a mouse model of the limb-girdle muscular dystrophy type 2B (LGMD2B). Thus, for the first time, we have established the possibility of exploiting myoblast fusion as a regenerative therapy for muscle diseases.  For the second objective, we studied Elmo3 in a clinical case, as part of an international collaboration. More specifically, biallelic mutations in Elmo3 gene have been identified in a young patient with intellectual disability. Through biochemical and functional studies, we have shown that the mutations have an impact on the activation of Rac1, without however affecting the interaction between the proteins of the Elmo3/Dock1 complex. This study provides the first evidence of biological functions for Elmo3.  In conclusion, our study has emphasis the relevance of the proper control of the conformational regulation of Elmo. In fact, by manipulating this regulation in a skeletal muscle model, through the introduction of a specific mutation promoting the open conformation of Elmo, it promotes myoblast fusion and induce a more efficient muscle regeneration, thus improving the dystrophic phenotypes. In contrast, the presence of human mutations in Elmo can also affect the activation of Rac1 by Dock1, hence contributing to intellectual disability in the carrier.

Keyphrases: Elmo1, Elmo2, Elmo3, Dock1, Rac1, Conformational regulation, Myogenesis, Myoblast fusion, Muscle regeneration, Muscular dystrophy