Spatial Heterogeneity of Delta-like 4 Within a Multinucleated Niche Cell Maintains Muscle Stem Cell Diversity

The quiescent muscle stem cell (QSC) pool is heterogeneous and generally characterized by the presence and levels of intrinsic myogenic transcription factors. Whether extrinsic factors maintain the diversity of states across the QSC pool remains unknown. The muscle fiber is a multinucleated syncytium that serves as a niche to QSCs, raising the possibility that the muscle fiber regulates the diversity of states across the QSC pool. Here we show that the muscle fiber maintains a continuum of quiescent states, through a gradient of Notch ligand, Dll4, produced by the fiber and captured by QSCs. The abundance of Dll4 captured by the QSC correlates with levels of the SC identity gene, Pax7. Niche-specific loss of Dll4 decreases QSC diversity and shifts the continuum, towards more proliferative and committed states. We reveal that fiber-derived Mindbomb1 (Mib1), an E3 ubiquitin ligase activates Dll4 and controls the spatial localization of Dll4. In response to injury, with a Dll4-replenished niche, the normal continuum and diversity of SC pool is restored, demonstrating bi-directionality within the SC continuum. Our data shows that a post-translational mechanism controls spatial heterogeneity of Notch ligands in a multinucleated niche cell to maintain a continuum of diverse states within the SC pool during tissue homeostasis.