SummaryThe generation of tissue-specific cell types from human embryonic stem cells (hESCs) is critical for the development of future stem cell-based regenerative therapies. Here, we identify CD13 and ROR2 as cell-surface markers capable of selecting early cardiac mesoderm emerging during hESC differentiation. We demonstrate that the CD13+/ROR2+ population encompasses pre-cardiac mesoderm, which efficiently differentiates to all major cardiovascular lineages. We determined the engraftment potential of CD13+/ROR2+ in small (murine) and large (porcine) animal models, and demonstrated that CD13+/ROR2+ progenitors have the capacity to differentiate toward cardiomyocytes, fibroblasts, smooth muscle, and endothelial cells in vivo. Collectively, our data show that CD13 and ROR2 identify a cardiac lineage precursor pool that is capable of successful engraftment into the porcine heart. These markers represent valuable tools for further dissection of early human cardiac differentiation, and will enable a detailed assessment of human pluripotent stem cell-derived cardiac lineage cells for potential clinical applications.

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Duchenne muscular dystrophy (DMD) is caused by an out-of-frame mutation in the DMD gene that results in the absence of a functional dystrophin protein, leading to a devastating progressive lethal muscle-wasting disease. Muscle stem cell-based therapy is a promising avenue for improving muscle regeneration. However, despite the efforts to deliver the optimal cell population to multiple muscles most efforts have failed. Here we describe a detailed optimized method of for the delivery of human skeletal muscle progenitor cells (SMPCs) to multiple hindlimb muscles in healthy, dystrophic and severely dystrophic mouse models. We show that systemic delivery is inefficient and is affected by the microenvironment. We found that significantly less human SMPCs were detected in healthy gastrocnemius muscle cross-sections, compared to both dystrophic and severely dystrophic gastrocnemius muscle. Human SMPCs were found to be detected inside blood vessels distinctly in healthy, dystrophic and severely dystrophic muscles, with prominent clotting identified in severely dystrophic muscles after intra arterial (IA) systemic cell delivery. We propose that muscle microenvironment and the severity of muscular dystrophy to an extent impacts the systemic delivery of SMPCs and that overall systemic stem cell delivery is not currently efficient or safe to be used in cell based therapies for DMD. This work extends our understanding of the severe nature of DMD, which should be taken into account when considering stem cell-based systemic delivery platforms.

Duchenne Muscular Dystrophy (DMD) is a devastating disease with no cure affecting approximately 1 in 3,500‐5,000 boys. Stem cell treatments using skeletal muscle stem cells (or satellite cells, SCs) provide great potential for regenerating new muscle and we have developed directed differentiation strategies to generate skeletal muscle cells from human induced pluripotent stem cells (hiPSCs). Our work has shown that hiPSCs generate PAX7+ skeletal muscle progenitor cells (SMPCs) resembling early myogenic cells that align closer to week 7‐12 in human development and are not equivalent to adult SCs. We are interested in understanding the key molecular and functional differences that control SMPC versus SC cell states. Recently, there has been an intense interest in the three dimensional (3D) organization of the genome and its involvement in cell specific gene regulation. This has led to the discovery of chromatin loops between gene enhancers and promoters as well as self‐interacting domains termed topologically associating domains (TADs). Recently published data support a role for the 3D genome in cellular differentiation, however, the differences between the 3D genome in human SMPCs compared to adult SCs is unknown. High throughput chromosome conformation capture (Hi‐C) was used to characterize the 3D genome of SMPCs. We found genome‐wide 3D configurations were different between hPSCs and SMPCs as was the number of TADs. Interestingly, TAD size was also different between cell types, suggesting dynamic control of TADs during differentiation. When focusing on the PAX7 locus, TAD boundaries differ between cell types further highlighting the dynamic nature of TADs with respect to cell specific gene expression. Not all muscle specific loci were different between cell types, however, suggesting that some TADs may be pre‐established early in the differentiation process while others are established de‐novo. With respect to chromatin looping at the PAX7 locus, we found SMPC specific looping between the PAX7 promoter and downstream sequences that were unique in SMPCs. Considering that PAX7 enhancer sequences have yet to be determined, these sequences may serve as candidate enhancers for PAX7. These data, for the first time, characterize the 3D genome of human SMPCs using Hi‐C. Moreover, these data provide candidate enhancer sequences for that could provide unique candidates for support of PAX7 SMPCs.