Generation of skeletal myogenic progenitors from human pluripotent stem cells using non-viral delivery of minicircle DNA

Currently, the most efficient and promising approach for generating large numbers of engraftable human skeletal myogenic progenitors from pluripotent stem cells requires the conditional in vitro overexpression of PAX7 using lentiviral vectors. Because a non-integrating approach would be preferable to eliminate or minimize the risk associated with random genomic integration, here we investigate whether transient expression of PAX7 using minicircle DNA would enable the generation of functional pluripotent stem cell-derived myogenic progenitors, equivalent to those generated by lentivirus. Our results demonstrate that upon multiple transfections, the minicircle approach allows for the scalable generation of myogenic progenitors and these undergo efficient terminal differentiation in vitro. However, transplantation of minicircle-generated myogenic progenitors resulted in limited engraftment. This is probably due to less efficient delivery and more transient PAX7 expression in these cultures since PAX7 downregulation is accompanied by high level of spontaneous differentiation. Thus, although the in vitro data shows that the minicircle approach could potentially replace the use of lentivirus, improvements in the transfection/expression system will be necessary before it will be a feasible strategy for the generation of myogenic progenitors for cell replacement therapy

Expansion and Purification Are Critical for the Therapeutic Application of Pluripotent Stem Cell-Derived Myogenic Progenitors

Recent reports have documented the differentiation of human pluripotent stem cells toward the skeletal myogenic lineage using transgene- and cell purification-free approaches. Although these protocols generate myocytes, they have not demonstrated scalability, safety, and in vivo engraftment, which are key aspects for their future clinical application. Here we recapitulate one prominent protocol, and show that it gives rise to a heterogeneous cell population containing myocytes and other cell types. Upon transplantation, the majority of human donor cells could not contribute to myofiber formation. As a proof-of-principle, we incorporated the inducible PAX7 lentiviral system into this protocol, which then enabled scalable expansion of a homogeneous population of skeletal myogenic progenitors capable of forming myofibers in vivo. Our findings demonstrate the methods for scalable expansion of PAX7+ myogenic progenitors and their purification are critical for practical application to cell replacement treatment of muscle degenerative diseases