In Vivo Satellite Cell Niche Emergence from Human Pluripotent Stem Cells


Pre-Clinical Research

Poster Number: 13


Michael Hicks PhD


1. University of California, Irvine

Skeletal muscle stem and progenitor cells fulfill different functional needs during myogenic development and postnatal maintenance. Understanding how stem and progenitor cells behave across developmental states is critical to understanding their regenerative potential and for benchmarking maturation of human pluripotent stem cell derived skeletal muscle progenitor cells (hPSC SMPCs). Following directed differentiation, hPSC SMPCs engraft to form human-mouse chimeric myofibers and hundreds of human-only myofibers. PAX7+ SMPCs routinely associated with human-only myofibers and less with chimeric myofibers in vivo. We profiled hPSC and fetal SMPCs, and adult SCs by RNA-Seq and identified uniquely expressed niche components that are retained by stem and progenitor cells after transplantation. We show adult SCs are better able to reside in chimeric niches and repopulate new muscle after injury. Overtime, SMPCs derived human-only myofibers form myobundles in vivo and PAX7+ cells associate within the basal lamina of emerging myobundles, which resemble the formation of fetal-like niches during development. We hypothesized human SMPCs inefficiently home to chimeric SC niches due to competition with mouse SCs. Using mdx-NSG PAX7-DTA mice, we evaluated whether population-specific ablation of mouse SCs would enable homing of human SMPCs to chimeric niches. We found increased retention of human PAX7 cells in SC-ablated mice, but surprisingly these were more associated with human-only niches and not chimeric niches. To identify regulators of SC niche formation, we profiled the regenerative response of SC-ablated and non-ablated mdx-NSG PAX7-DTA mice following injury using single nucleus RNA-Seq. We identified a transient Type 2b myofiber at the onset of SC niche formation marked by Actc1 and Eno1. These same markers were expressed on human-only myofibers that supported PAX7 SMPC association during early niche formation. Our work suggests emerging SC niche form through a similar route as newly regenerating fibers, as opposed to homing to empty SC niches of established myofibers upon endogenous SC ablation. Evaluating the regulators of emerging human niche formation during regeneration and development will improve our ability to generate de novo human niches and better support human PAX7 cells in vivo for cell therapy.