Nanog-independent reprogramming to iPSCs with canonical factors

Abstract

It has been suggested that the transcription factor Nanog is essential for the establishment of pluripotency during the derivation of embryonic stem cells and induced pluripotent stem cells (iPSCs). However, successful reprogramming to pluripotency with a growing list of divergent transcription factors, at ever-increasing efficiencies, suggests that there may be many distinct routes to a pluripotent state. Here, we have investigated whether Nanog is necessary for reprogramming murine fibroblasts under highly efficient conditions using the canonical-reprogramming factors Oct4, Sox2, Klf4, and cMyc. In agreement with prior results, the efficiency of reprogramming Nanog (-/-) fibroblasts was significantly lower than that of control fibroblasts. However, in contrast to previous findings, we were able to reproducibly generate iPSCs from Nanog (-/-) fibroblasts that effectively contributed to the germline of chimeric mice. Thus, whereas Nanog may be an important mediator of reprogramming, it is not required for establishing pluripotency in the mouse, even under standard conditions.

Figure 1

Nanog Null MEFs Are Reprogrammed Using KSOM and Activate the Pluripotency Transcriptional Network (A) The number of iPSC colonies generated from Nanog^−/− MEFs with four factors (KSOM) or three factors (KSO). Colonies were scored at day 21 post transduction with reprogramming factors. Error bars represent the SD between two biological replicates. (B) Primary Nanog^−/− iPSC colony 17 days post transduction with KSOM. Scale bars represent 500 μm. (C) Nanog^−/− iPSCs growing on gelatin (top panels) or on irradiated feeder cells (bottom panels). Scale bars represent 500 μm. (D) Nanog^−/− iPSCs activate the endogenous Nanog locus. Cells were treated with 400 ng/ml G418 for 4 days, and representative images were taken at days 0 and 4. Scale bars represent 500 μm. See also Figure S1.

Figure 2

Nanog Null iPSCs Express Endogenous Pluripotency Genes (A) qPCR for expression of endogenous KSO. Levels are normalized to GAPDH and plotted relative to control V6.5 mESCs (=1). y axis shows the fold change in expression as determined by the comparative C_T method. qPCR was performed in duplicate. Error bars represent the SD between two biological replicates. (B) qPCR for expression of pluripotency-related genes. Levels are normalized to GAPDH and plotted relative to control V6.5 mESCs (=1). y axis shows the fold change in expression as determined by the comparative C_T method. qPCR was performed in duplicate. Error bars represent the SD between two biological replicates. See also Figure S2.

Figure 3

Nanog Null iPSCs Recapitulate the Global Transcriptome Profile of ESCs (A) Unsupervised hierarchical clustering of global gene expression obtained by RNA-seq. Biological replicates were analyzed for each sample, and the composite result is shown. piPSC, partially reprogrammed iPSC line B1. JS, Jenson Shannon. (B) The expression of selected pluripotency-associated factors, as well as early lineage and fibroblast markers, is shown. FPKM, fragments per kilobase of transcript per million fragments mapped. In the case of Oct4 (Pou5f1) and Sox2, RNA-seq does not distinguish between endogenous and exogenous viral transcripts. (C) The overlap of genes significantly altered (FDR <0.05) more than 2-fold between indicated pluripotent stem cells and MEFs is shown. Genes altered in either independent Nanog^−/− iPSC clone (G2 or G5) are included in the Nanog^−/− iPSC category. See also Figure S3.

Figure 4

Nanog Null iPSCs Are Pluripotent and Extensively Contribute to Chimeras (A) Representative images of brain, heart, lung, and liver from postnatal day 33 (P33) Nanog^−/− chimeras generated from injection of Nanog^−/− iPSC line 3.1 into WT blastocysts. Scale bars represent 5 mm. (B) Representative E12.5 chimeras generated from injection of Nanog^−/− iPSC line 3.1 into WT blastocysts. Scale bars represent 2 mm. (C) Four-week-old chimera generated from injection of Nanog^−/− iPSC line 3.1 into WT blastocysts. (D) Summary of chimera generation results from three and four factor lines (3.1, 3.2, G5). Numbers in boxes represent number of chimeras and number of embryos or animals recovered and total number of injected embryos. ND, not determined in this experiment. F, female. (E) Genotyping results of adult progeny (P90) from chimeric and C57BL/6 cross to determine germline transmission of Nanog^−/− iPSCs. neg, negative control GFP expression (C57BL/6 uncrossed animal). (F) Representative images of tissues from adult chimera progeny (P90) genotyped as positive (#4) or negative (#3) GFP transgene, as well as a chimera parent as a positive control. Scale bars represent 2 mm. See also Figure S4.