Delineate myogenic trajectories in development and disease. – MyoTrajectories

The myogenic commitment programme of skeletal muscle stem cells has been the subject of intensive study for over 30 years. Histology, morphometrics and more recently transcriptomic studies have been used to define myogenic trajectories, from proliferating myoblasts to fusion-competent myocytes and finally to contractile myofibres. However, the application of more advanced tools and methodologies by us and others suggests the presence of new transient cellular states during normal and pathological myogenesis. In this project, we focus on two main aspects of myogenesis: (i) the distinct fusion steps of mononucleated progenitors during the formation of multinucleated myofibre syncytia, and (ii) the different states of quiescence among dormant stem cells, pre-activated stem cells, and stem cells returning to the niche. We propose to study these critical aspects of myogenesis in different contexts: during normal postnatal development, in activated muscle stem cells (in response to injury or by targeted gene ablation), as well as in mouse models of two skeletal muscle diseases: Duchenne muscular dystrophy and LAMA2-related muscular dystrophy. In both diseases, the muscle undergoes constant cycles of degeneration-regeneration and the dystrophic MuSCs show disturbed quiescence and a deficit of asymmetric division. In addition, at an advanced stage, a large subset of muscle fibres shows longitudinal divisions, probably due to disturbed myocyte fusion. To address these aspects, we will combine murine genetics, specialised cell and nuclear isolation protocols, and state-of-the-art bioinformatics tools to systematically study transient cellular states, in normal and pathological muscle growth and regeneration. By systematically comparing these trajectories in normal and mutant tissues, we aim to reveal previously unidentified cellular states and signalling networks, and to gain new insights into disease development.