Exploiting the conformational regulation of ELMO proteins reveals that myoblast fusion can be exploited for regenerative therapies in muscle diseases

Tran V^1,2, Thibault M-P¹, Killoran R^2,3, Smith M J^2,3 and Côté JF^1,2,4.

¹Montreal Clinical Research Institute (IRCM).
² Université de Montréal
³ Institute for Research in Immunology and Cancer (IRIC)
⁴ McGill University

Myoblast fusion during myogenesis is central to the formation of multinucleated fibres. We recently mapped a pathway involved in this cellular process, where DOCK1 plays an evolutionarily conserved role. We also showed that BAI3 promotes fusion by engaging DOCK1-RAC signalling via ELMO. Mechanistically, we established that ELMO switch between open/closed conformations, therefore providing a molecular checkpoint for RAC activation by DOCK1. Hence, our hypothesis is that ELMO1/2 are key regulators of myoblast fusion and that manipulating their conformational state offers an opportunity to test if increasing the activity of the ELMO/DOCK1/RAC pathway can improve muscle regeneration. We investigated whether ELMO is required for embryonic myoblast fusion and we could demonstrate a severe block in fusion when ELMO1/2 were eliminated. This led us to investigate if manipulating the activity of the ELMO/DOCK1/RAC pathway may promote regeneration. We generated knock-in mice in which Elmo2 is maintained in an inactive (Elmo2^RBD) or active (Elmo2^EID) conformation. Analysis of Elmo2^EID muscle showed larger fibres, suggesting that up-regulation of the pathway activity enhances muscle growth. We also found that CTX-induced muscle damage regenerates more efficiently in these mice. Conversely, we observed that decreasing the pathway activity decreased the efficiency of regeneration. Finally,interbreeding of Elmo2^EID with Dysferlin^KO mice (mouse model of the Limb-Girdle Muscular Dystrophy type 2B) could rescue their dystrophic phenotype.Collectively, our work revealed a critical role for ELMO in myoblast fusion and provides a proof of concept that activation of signalling pathways promoting fusion may provide therapeutic opportunitiesfor the treatment of myopathies.