04 | CD90 identifies distinct fractions of muscle stem cells with different modalities of activation and quiescence maintenance

Lorenzo Marramiero^1|2, S. Di Savino^1|2, E. Kheir^1|2, M. Libergoli^1|2, S. Metti³, A. Zola^1|2, J.B. Jensen^4|5, J.Wang⁵, N. Jessen^4|5|6, F. Florio^1|2, F. Murganti^1|2, M. Belicchi ^7|8, C. Villa⁸, K. Vissing⁹, M. Verma¹⁰, L. Giordani¹¹, P. Bonaldo³, Y. Torrente^7|8, J. Farup⁴, S. Biressi^1|2 | ¹Department of Cellular, Computational and Integrative Biology (CIBIO), University of Trento, Povo, Italy; ²Dulbecco Telethon Institute at University of Trento, Povo, Italy; ³Department of Molecular Medicine, University of Padova, Italy; ⁴Department of Biomedicine, Aarhus University, Denmark; ⁵Steno Dia,5betes Center Aarhus, Denmark; ⁶Department of Clinical Pharmacology, Aarhus University Hospital, Denmark; ⁷Neurology Unit, Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico, Milan, Italy; ⁸Stem Cell Laboratory, Dino Ferrari Center, Department of Pathophysiology and Transplantation, University of Milan, Italy; ⁹Department of Public Health, Aarhus University, Denmark; ¹⁰University of Texas Southwestern Medical Center, Dallas, USA; ¹¹Centre de Recherche en Myologie, Sorbonne Universités,Paris, France.

Skeletal muscle is a dynamic organ whose regeneration depends on the transition of muscle stem cells (MuSCs) from a state of quiescence to activation. Our study has identified a key cell population within MuSCs that expresses the CD90 marker (CD90⁺ MuSCs), which orchestrates the regenerative response. Under homeostatic conditions, CD90⁺ MuSCs populate the stem cell niche and maintain their quiescence through the synthesis of Collagen VI and interaction with the calcitonin receptor. Following injury, however, CD90⁺ MuSCs are the first to activate, outpacing their CD90⁻ counterparts and guiding the initial phases of repair. Using an integrated in vitro and ex vivo approach (immunofluorescence, flow cytometry), we have characterized the functionality of this subpopulation. Transcriptional analysis by RNA-seq is revealing differences between CD90⁺ and CD90⁻ cells, as preliminary data indicate that approximately one hundred genes are differentially expressed. To investigate the physiological and pathological role of CD90⁺ MuSCs in vivo, we have developed a Thy1^flox/flox (CD90^flox/flox) murine model. This model will allow us to assess the impact of CD90 depletion on niche repopulation, MuSC regenerative capacity, and the resulting muscle fiber phenotype. In conclusion, our findings provide new mechanistic insights into the functional heterogeneity of MuSCs. Future results will elucidate the role of this subpopulation and its associated genes, holding translational potential for enhancing muscle repair in pathological conditions such as muscular dystrophy.