DMD is a severe muscle wasting disease caused by the lack of dystrophin protein in cardiac and skeletal muscles and in resident satellite cells (SCs) which are required to repair damaged skeletal muscle. In DMD, muscle SCs are dysfunctional and exhaust leading to accelerated disease progression. Understanding how to support SCs or replace DMD SCs with healthy new SCs could serve as a continuous source for muscle regeneration and new dystrophin expression. SCs are supported by their SC niche, understanding how SC niches develop could improve our ability to generate de novo niches and better support SCs for cell and regenerative therapies.
My previous work has developed a robust approach to generate skeletal muscle progenitor cells using human pluripotent stem cells (hPSC-SMPCs). We showed that increased myogenic ability resides in the ERBB3+NGFR+ fraction of hPSC-SMPCs. We developed a single cell RNA-Sequencing atlas of human PAX7+ cells across fetal, juvenile and adult which identifies that hPSC-SMPCs align between human fetal weeks 8-12 using diffusion map analysis. We also demonstrated that hPSC-SMPCs behave functionally like fetal muscle cells.
We have shown hPSC-SMPCs engraft to restore hundreds of new dystrophin+ myofibers in mouse models of DMD. Using CRISPR/Cas9 reframing of the dystrophin gene in iPSCs we have shown that our approach also has the potential to serve as patient-specific therapy. Upon transplantation, hPSC-SMPCs fuse to form hundreds small human-only myofibers that resemble skeletal muscle found in development. PAX7+ hPSC-SMPCs were primarily associated with these newly regenerating human-only myofibers suggesting these provide a specific and supportive niche for transplanted muscle stem cells. We found that human-only myofibers continue to grow over 60 days in vivo, and PAX7+ hPSC-SMPC associate under the basal lamina of these emerging myofibers over time. This work demonstrates for the first time that fetal and hPSC-SMPCs can be used as a model to study human myofiber formation and niche occupancy in vivo.
Evaluating human niche formation over time will improve our understanding of how human muscle SC niches develop. This could improve our ability to generate de novo human niches and better support human PAX7 cells in vivo for cell and regenerative therapies.