Cell fate is fluid, and may be altered experimentally by the forced expression of master regulators mediating cell lineage. Such nuclear reprogramming has been achieved using viral vectors encoding transcription factors. We recently discovered that the viral vectors are more than passive vehicles for transcription factors, as they participate actively in the process of nuclear reprogramming to pluripotency. Viral vectors, by activating innate immunity, cause global changes in the expression of epigenetic modifiers, favoring an open chromatin state (Cell, 151 2012). Based on the recognition that activation of innate immunity increases epigenetic plasticity, we hypothesized that small molecule activators of toll-like receptor 3 (TLR3), together with external microenvironmental cues that drive EC specification, might be sufficient to induce transdifferentiation of fibroblasts into ECs (iECs). Here we show that TLR3 agonist Poly I:C, combined with exogenous EC growth factors, transdifferentiated human fibroblasts into ECs (in the absence of viral vectors or transcription factors). These iECs were comparable to HMVEC in immunohistochemical, genetic and functional assays, including the ability to form capillary-like structures and to incorporate acetylated-LDL. Furthermore, iECs significantly improved limb perfusion and neovascularization in the murine ischemic hindlimb compared to parental fibroblasts. Finally, using genetic knockdown studies, we find that the effective transdifferentiation of human fibroblasts to endothelial cells requires innate immune activation. This study suggests that manipulation of innate immune signaling may be generally used to modify cell fate. Our observations raise the question as to whether epigenetic plasticity and cell fate fluidity generally participate in the immune defense against pathogens. Because similar signaling pathways are activated by damage associated molecular patterns, epigenetic plasticity induced by innate immunity may play a fundamental role in transdifferentiation during wound healing and regeneration. Finally, this study is a first step toward development of a small molecule strategy for therapeutic transdifferentiation in vivo.
HIF1α Regulates Early Metabolic Changes due to Activation of Innate Immunity in Nuclear Reprogramming
