Paying the Toll in Nuclear Reprogramming

A protocol for transdifferentiation of human cardiac fibroblasts into endothelial cells via activation of innate immunity

Endothelial cells (ECs) have emerged as key pathogenic players in cardiac disease due to their proximity with cardiomyocytes. Induced pluripotent stem cells (iPSCs) have been employed to generate ECs. However, it may be more clinically relevant to transdifferentiate fibroblasts into ECs directly without introducing pluripotent or virally driven transcription factors. Here, we present a protocol that describes the direct conversion of human cardiac fibroblasts into ECs by leveraging the innate immune system. Our protocol produces bona fide human ECs with 95%-98% purity by first passage. For complete details on the use and execution of this protocol, please refer to Liu et al. (2020) and Sayed et al. (2015).

Generation of Human iPSCs by Protein Reprogramming and Stimulation of TLR3 Signaling

The discovery of induced pluripotent stem cells (iPSCs) allows for establishment of human embryonic stem-like cells from various adult human somatic cells (e.g., fibroblasts), without the need for destruction of human embryos. This provides an unprecedented opportunity where patient-specific iPSCs can be subsequently differentiated to many cell types, e.g., cardiac cells and neurons, so that we can use these iPSC-derived cells to study patient-specific disease mechanisms and conduct drug testing and screening. Critically, these cells have unlimited therapeutic potentials, and there are many ongoing clinical trials to investigate the regenerative potentials of these iPSC-derivatives in humans. However, the traditional iPSC reprogramming methods have problem of insertional mutagenesis because of use of the integrating viral vectors. While a number of advances have been made to mitigate this issue, including the use of chemicals, excisable and non-integrating vectors, and use of the modified mRNA, safety remains a concern. Both integrating and non-integrating methods also suffer from many other limitations including low efficiency, variability, and tumorigenicity. The non-integrating mRNA reprogramming is of high efficiency, but it is sensitive to reagents and need approaches to reduce the immunogenic reaction. An alternative non-integrating and safer way of generating iPSCs is via direct delivery of recombinant cell-penetrating reprogramming proteins into the cells to be reprogrammed, but reprogramming efficiency of the protein-based approach is extremely low compared to the conventional virus-based nuclear reprogramming. Herein, we describe detailed steps for efficient generation of human iPSCs by protein-based reprogramming in combination with stimulation of the Toll-like receptor 3 (TLR3) innate immune pathway.

HIF1α Regulates Early Metabolic Changes due to Activation of Innate Immunity in Nuclear Reprogramming

Innate immune signaling has recently been shown to play an important role in nuclear reprogramming, by altering the epigenetic landscape and thereby facilitating transcription. However, the mechanisms that link innate immune activation and metabolic regulation in pluripotent stem cells remain poorly defined, particularly with regard to key molecular components. In this study, we show that hypoxia-inducible factor 1α (HIF1α), a central regulator of adaptation to limiting oxygen tension, is an unexpected but crucial regulator of innate immune-mediated nuclear reprogramming. HIF1α is dramatically upregulated as a consequence of Toll-like receptor 3 (TLR3) signaling and is necessary for efficient induction of pluripotency and transdifferentiation. Bioenergetics studies reveal that HIF1α regulates the reconfiguration of innate immune-mediated reprogramming through its well-established role in throwing a glycolytic switch. We believe that results from these studies can help us better understand the influence of immune signaling in tissue regeneration and lead to new therapeutic strategies.

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HIF1α is dramatically upregulated as a consequence of TLR3 signaling

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HIF1α is necessary for efficient induction of pluripotency and transdifferentiation

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HIF1α regulates innate immune-mediated reprogramming by inducing a glycolytic switch

Glycolytic Switch Is Required for Transdifferentiation to Endothelial Lineage

BACKGROUND

We have previously shown that activation of cell-autonomous innate immune signaling facilitates the transdifferentiation of fibroblasts into induced endothelial cells, and is required to generate induced endothelial cells with high fidelity for endothelial lineage. Recent studies indicate that a glycolytic switch plays a role in induced pluripotent stem cell generation from somatic cells.

METHODS

Seahorse and metabolomics flux assays were used to measure the metabolic changes during transdifferentiation in vitro, and Matrigel plug assay was used to assess the effects of glycolysis modulators on transdifferentiation in vivo.

RESULTS

The metabolic switch begins rapidly after activation of innate immunity, before the expression of markers of endothelial lineage. Inhibiting glycolysis impaired, whereas facilitating glycolysis enhanced, the generation of induced endothelial cells. The toll-like receptor 3 agonist poly I:C increased expression of the mitochondrial citrate transporter Slc25A1, and the nuclear ATP-citrate lyase, in association with intracellular accumulation of citrate, the precursor for acetyl coenzyme A. These metabolic changes were coordinated with increased histone acetylation during transdifferentiation.

CONCLUSION

Innate immune signaling promotes a glycolytic switch that is required for transdifferentiation, both processes being attenuated by ATP-citrate lyase knockdown. These data shed light on a novel link between metabolism and epigenetic modulation in transdifferentiation.