Alternative Routes to Induced Pluripotent Stem Cells Revealed by Reprogramming of the Neural Lineage

Abstract

During the reprogramming of mouse embryonic fibroblasts (MEFs) to induced pluripotent stem cells, the activation of pluripotency genes such as NANOG occurs after the mesenchymal to epithelial transition. Here we report that both adult stem cells (neural stem cells) and differentiated cells (astrocytes) of the neural lineage can activate NANOG in the absence of cadherin expression during reprogramming. Gene expression analysis revealed that only the NANOG+E-cadherin+ populations expressed stabilization markers, had upregulated several cell cycle genes; and were transgene independent. Inhibition of DOT1L activity enhanced both the numbers of NANOG+ and NANOG+E-cadherin+ colonies in neural stem cells. Expressing SOX2 in MEFs prior to reprogramming did not alter the ratio of NANOG colonies that express E-cadherin. Taken together these results provide a unique pathway for reprogramming taken by cells of the neural lineage.

Figure 1

Nanog+ Colonies from Neural Stem Cell and Astrocyte Reprogramming Can Emerge Independent of E-Cadherin or SSEA1 (A) Immunofluorescence (IF) images of NANOG colonies on day 10 of reprogramming NSC with E-cadherin and/or SSEA1. Scale bar, 50 μm. Insets, magnification of field. (B) Scheme of experiment presented in (C) and (D). Dox was added to cells on day 0 (d0) and reprogramming cultures were fixed on indicated days. (C) Counts of Nanog+ (N+), E-cadherin+ (E+) and Nanog+/E-cadherin+ (N+E+) colonies from (i) NSC, (ii) astrocyte, and (iii) MEF reprogramming cultures, fixed on days indicated on X axis. Counts from three independent experiments are stacked. Statistical significance of difference between N+ and N+E+ colonies by paired two-tailed t test; ^∗p < 0.05, ^∗∗p < 0.01. Non-significant differences are not marked. (D) As in (C) above except for SSEA1+. SSEA1 = S+, Nanog+/SSEA1 = N+S+. t test; ^∗p < 0.05, ^∗∗p < 0.01. (E) Combined counts of NANOG+/SSEA1+/E-cadherin. Counts from two independent experiments are stacked. Note that the number of S+E+ colonies and E+S+ colonies differ because of scoring between large fields of SSEA containing multiple E-cadherin+ colonies.

Figure 2

Nanog Colonies Emerge Independent of N-Cadherin Expression (A) Left: NSC on day 0 stained with N-cadherin or E-cadherin. Right: IF images of Nanog colonies from NSC reprogramming cultures at d7 and d11 co-stained with N-cadherin or E-cadherin. Scale bar, 50 μm; d0 N-cadherin = 100 μm. Insets, magnification of field. (B) Time course of NSC reprogramming showing relative expression of Nanog and genes involved in MET, relative to d0 NSC = 1. Error bars indicate SD of three technical replicates from one representative experiment. ESC, embryonic stem cell. (C) Quantification of d7 and d11 NANOG colonies co-stained with E-cadherin and N-cadherin. Counts from two independent experiments are stacked and average percentages presented.

Figure 3

Late Reprogramming Markers Are Enriched in the N+E+ Population (A) Flow cytometry plot of reprogramming culture stained with Nanog-647 and E-cadherin-488. (B) RNA-Seq read counts (transcripts per million [TPM]) for E-cadherin, labeled Cdh1 in figure, (left, Y axis) and Nanog (right, Y axis) for N+E+ and N+E− populations from three independent experiments. (C) Heatmap of genes 1.5-fold differentially expressed between N+E+ and N+E− populations (set to 1) ranked by average TPM from three independent experiments. (D) Functional annotation of gene expression in N+E+ and N+E− populations. (E) Left: Immunofluorescence images of NANOG, E-cadherin, and DPPA4 on d11 of NSC reprogramming. Right: Counts from two independent experiments of Dppa4+ colonies in N+E+ and N+E− colonies are stacked. Scale bar, 50 μm. Insets, magnification of field. (F) Comparison of d14 N+ and N+E+ colonies following withdrawal of dox on the day indicated on the X axis. Counts from four independent experiments are stacked. No significant difference between N+ and N+E+ colonies by paired two-tailed t test.

Figure 4

Effects of Reduced DOT1L Activity and Enhanced Sox2 Levels on the Ratio of Nanog+/E-Cadherin+ Colonies (A) Left: Scheme of experiment for “Fix” samples in (B)–(D). Dox added on d0, fixed on days indicated. Right: Scheme of experiment for “-Dox” in (B)–(D). Dox added on d0, removed on days indicated, and fixed on d14. No significant difference between N+E+ colonies in (B), (C), (D). (B) Comparison of N+E+ colonies in “Fix” and “-Dox” samples for NSCs. Counts from three independent experiments are stacked. Note the same data are presented in Figures 1C and 3C. (C) As in (B) but for astrocytes. Note: same data are presented in Figures 1C and S2B. (D) As in (B) but for MEFs. Note: same data are presented in Figures 1C and S2C. (E) Counts of N+, E+ and N+E+ colonies from NSC reprogramming cultures on d10 in the presence of DMSO (left) or DOT1L inhibitor SGC0946 (middle). Counts from three independent experiments are stacked. Western blot analysis of H3K79me2 after SGC0946 treatment for 24 hr. No significant difference in the two conditions by t test. (F) Relative expression of endogenous Oct4, Sox2, Klf4, and c-Myc in NSC, astrocytes, and MEFs. Error bars are SD of three technical replicates from one representative experiment. Inset, SOX2 expression in GFAP+ astrocytes. Scale bar, 50 μm. (G) Left: Immunofluorescence images of MEFs following infection with pMX-Sox2 retrovirus. Percentages are SOX2+ cells from 100 nuclei (DAPI). Right: counts for N+ and N+E+ colonies following 7 days doxycycline treatment after pre-infection with pMX-Flag (Ctrl) or pMX-Sox2 (Sox2). Scale bar, 50 μm.