Summary Skeletal muscle is a complex tissue containing tissue resident muscle stem cells (satellite cells, MuSCs) important for postnatal muscle growth and regeneration. Quantitative analysis, biological function, and the molecular pathways responsible for a potential juxtavascular niche for MuSCs is currently lacking. We utilized fluorescent reporter mice and muscle tissue clearing to investigate the proximity of MuSCs to capillaries in 3-dimensions. We show that MuSCs express abundant VEGFA, which recruits endothelial cells (ECs) in vitro, whereas both blocking VEGFA by a VEGF inhibitor and MuSC-specific VEGFA gene deletion reduce the proximity of MuSCs to capillaries. Importantly, this proximity to the blood vessels was associated with MuSC self-renewal in which EC-derived Notch ligand Dll4 induces quiescence in MuSCs. We hypothesize that MuSCs recruit capillary ECs via VEGFA, and in return ECs maintain MuSC quiescence though Dll4.

Suppression of the myogenic transcription factor MyoD is required for maintenance of muscle stem cells.

MyoD is a myogenic master transcription factor that plays an essential role in muscle satellite cell (muscle stem cell) differentiation. To further investigate the function of MyoD in satellite cells, we examined the transplantation of satellite cell-derived myoblasts lacking the MyoD gene into regenerating skeletal muscle. After injection into injured muscle, MyoD ؊/؊ myoblasts engrafted with significantly higher efficiency compared with wild-type myoblasts. In addition, MyoD ؊/؊ myoblast-derived satellite cells were detected underneath the basal lamina of muscle fibers, indicating the self-renewal property of MyoD ؊/؊ myoblasts. To gain insights into MyoD gene deficiency in muscle stem cells, we investigated the pathways regulated by MyoD by GeneChip microarray analysis of gene expression in wild-type and MyoD ؊/؊ myoblasts. MyoD deficiency led to down-regulation of many muscle-specific genes and up-regulation of some stem cell markers. Importantly, in MyoD ؊/؊ myoblasts, many antiapoptotic genes were up-regulated, whereas genes known to execute apoptosis were downregulated. Consistent with these gene expression profiles, MyoD ؊/؊ myoblasts were revealed to possess remarkable resistance to apoptosis and increased survival compared with wild-type myoblasts. Forced expression of MyoD or the proapoptotic protein Puma increased cell death in MyoD ؊/؊ myoblasts. Therefore, MyoD ؊/؊ myoblasts may preserve stem cell characteristics, including their resistance to apoptosis, expression of stem cell markers, and efficient engraftment and contribution to satellite cells after transplantation. Furthermore, our data offer evidence for improved therapeutic stem cell transplantation for muscular dystrophy, in which suppression of MyoD in myogenic progenitors would be beneficial to therapy by providing a selective advantage for the expansion of stem cells.apoptosis ͉ cell therapy ͉ microarrays ͉ muscular dystrophy ͉ satellite cell