Long-term high-yield skeletal muscle stem cell expansion through staged perturbation of cytokine signaling in a soft hydrogel culture platform

Muscle stem cells (MuSCs) are an essential stem cell population for skeletal muscle homeostasis and regeneration throughout adulthood. MuSCs are an ideal candidate for cell therapies for chronic and acute muscle injuries and diseases given their inherent ability to self-renew and generate progenitor cells capable of myogenic commitment and fusion. Given their rarity and propensity to lose stem-cell potential in prolonged culture, methods for MuSC expansion that achieve clinical-scale stem cell yields represent a critical unmet need in muscle cell-therapeutic development. Here, we tested a microenvironment engineering approach to achieve long-term adult mouse MuSC expansion suitable for clinical demands through the combined optimization of techniques previously reported to achieve small-yield MuSC expansion in short-term cultures. We developed an optimized protocol for high-yield MuSC expansion through the combination of inflammatory cytokine and growth factor co-stimulation, temporally-staged inhibition of the p38α/β mitogen activated protein kinase signaling pathway, and modulation of substrate rigidity in long-term hydrogel cultures. We found that, on soft, muscle-mimicking (12 kPa) hydrogel substrates, a mixture of the cytokines TNF-α, IL-1α, IL-13, and IFN-γ and the growth factor FGF2 stimulated robust exponential proliferation of adult MuSCs from both wildtype and dystrophic mice for up to five weeks of culture that was accompanied by a phenotype shift towards committed myocytes. After observing that the temporal variation in myogenic commitment coincided with an oscillatory activation of p38α/β signaling, we tested a late-stage p38α/β inhibition strategy and found that blocking p38α/β signaling after three weeks, but not earlier, substantially enhanced cell yield, stem-cell phenotypes, and, critically, preserved transplantation potential for up to five weeks of FGF2/cytokine mix culture on soft hydrogels. Notably, this retention of transplant engraftment potency was not observed on traditional plastic substrates. We estimate that this protocol achieves >10-fold yield in Pax7 stem cells from each starting MuSC, which represents a substantial improvement in stem-cell yield from long-term cultures compared to established methods.