Promoting reprogramming by FGF2 reveals that the extracellular matrix is a barrier for reprogramming fibroblasts to pluripotency.

Leukemia inhibitory factor and bone morphogenetic protein signaling pathways play important roles in maintaining the self-renewal of mouse embryonic stem cells (ESCs). In contrast, the supplementation of fibroblast growth factor 2 (FGF2) in culture promotes mouse ESC differentiation. It has been proposed that factors that are adverse for maintaining the self-renewal of ESCs might play detrimental roles in the transcription factor-mediated reprogramming of somatic cells to pluripotency. However, recent evidence has revealed that reprogramming efficiency could be improved by FGF2, while the underlying molecular mechanism remains unknown. In this study, we dissected the roles of FGF2 in promoting mouse fibroblast reprogramming and disclosed the molecular mechanism behind this process. We used both primary induction and secondary inducible reprogramming systems and demonstrated that supplementation with FGF2 in the early phase of induced pluripotent stem cell induction could significantly increase reprogramming efficiency. Moreover, we discovered that many extracellular matrix candidate genes were significantly downregulated in fibroblasts treated with FGF2, and in particular, the synthesis of collagen could be greatly reduced by FGF2 treatment. Subsequently, we demonstrated that collagen is a barrier for reprogramming fibroblast cells to pluripotency, and the decreasing of collagen either by collagenase treatment or downregulation of collagen gene expression could significantly improve the reprogramming efficiency. Our results reveal a novel role of the extracellular matrix in mediating fibroblasts reprogramming.