Promyelocytic Leukemia Protein Is an Essential Regulator of Stem Cell Pluripotency and Somatic Cell Reprogramming

Abstract

Promyelocytic leukemia protein (PML), the main constituent of PML nuclear bodies, regulates various physiological processes in different cell types. However, little is known about its functions in embryonic stem cells (ESC). Here, we report that PML contributes to ESC self-renewal maintenance by controlling cell-cycle progression and sustaining the expression of crucial pluripotency factors. Transcriptomic analysis and gain- or loss-of-function approaches showed that PML-deficient ESC exhibit morphological, metabolic, and growth properties distinct to naive and closer to the primed pluripotent state. During differentiation of embryoid bodies, PML influences cell-fate decisions between mesoderm and endoderm by controlling the expression of Tbx3. PML loss compromises the reprogramming ability of embryonic fibroblasts to induced pluripotent stem cells by inhibiting the transforming growth factor β pathway at the very early stages. Collectively, these results designate PML as a member of the regulatory network for ESC naive pluripotency and somatic cell reprogramming.

Keywords: differentiation; embryonic stem cells; induced pluripotent stem cells; pluripotency; promyelocytic leukemia protein.

Graphical abstract

Figure 1

PML Depletion Impairs Self-Renewal and Affects the Cell Cycle (A) PML protein level in one representative PML KD ESC clone and wild type (WT) ESC. (B) Morphology of control (WT), PML KD, and KO ESC. Scale bar, 100 μm. (C) Pluripotency factors protein levels upon depletion or deletion of PML. (D) Co-immunoprecipitation of endogenous pluripotency factors (OCT4, c-MYC, STAT3, NR0B1) with endogenous PML from PML OE ESC compared with PML KD ESC. (E) Cell-cycle analysis of WT, PML KD, and KO ESC. (F) Growth curve of WT, PML KD, and KO ESC. Error bars indicate ±SD of four independent experiments (n = 4). (G) The activity of APRE-Luc reporter and p-STAT3 protein expression levels in ESC WT, PML KD, and KO. Data represent the mean + SD of four independent experiments (n = 4). ^∗p < 0.05. (H) Relative mRNA levels of cell-cycle regulators Junb and Pim-1. Data are shown as mean + SD of three independent experiments (n = 3). (I) Protein levels of cell-cycle regulators p-pRB, pRB, and CCND1 were detected.

Figure 2

PML Reduction Promotes an Epi-like Stem Cell State (A) Heatmap with the top representative naive and primed genes. (B) OTX2, CDH1, NANOG, and KLF4 protein levels prior to and after PML loss in ESC. (C) Luciferase activity of BRE-Luc reporter upon PML depletion. Data represent the mean + SD of four independent experiments (n = 4). ^∗p < 0.05, ^∗∗p < 0.01. (D) mRNA levels of BMP signaling target genes (Id1, Id2, Id3) in WT, PML KD, and KO ESC. Data are shown as mean + SD of three independent experiments (n = 3). (E) CAGA-Luc reporter activity upon PML ablation. Error bars indicate +SD of four independent experiments (n = 4). ^∗p < 0.05. (F) Heatmap of the gene expression profiles of our WT and PML KD ESC replicates along with three EpiSC and two ESC lines.

Figure 3

PML Impedes the Conversion from Naive ESC to EpiSC (A) Proliferation of WT and PML KD ESC with or without JAKi. Western blot showing JAKi abrogation of p-STAT3. Data represent the mean ± SD of three independent experiments (n = 3). D, day. (B) Morphology of WT and PML KO ESC after 2 and 6 days of JAKi treatment. Scale bars, 100 μm. D, day. (C) WT, PML KO, and PML OE ESC morphology at 6 days under 2i/LIF or F/A culture conditions. Scale bars, 100 μm. (D) NANOG and OTX2 protein levels in WT, PML OE, and PML KO ESC. Cells were cultured either in 2i/LIF or F/A for 6 days. (E) Western blot analysis of NANOG and OTX2 in WT, PML OE, and PML KO ESC. Cells were cultured in 2i/LIF (6 days), F/A (6 days), and F/A→ 2i/LIF (6 days F/A plus 6 days 2i/LIF).

Figure 4

PML Inhibition Promotes Mesoderm and Represses Endoderm Differentiation (A) Heatmap displaying top endodermal genes and mesodermal genes differentially expressed in PML KD and WT EB D4 (fold change > 1.5, p < 0.05). (B) Validation of microarray results using RT-PCR in PML KD, KO, and WT EB. Error bars indicate +SD of four independent experiments (n = 4). (C) Western blot analysis of PML, T, and α-fetoprotein (AFP) expression in PML KD and KO compared with WT EB. (D) Nes and Pax6 mRNA levels upon differentiation of PML KD, KO, or WT ESC. Data are shown as mean + SD of four independent experiments (n = 4). (E) Tbx3, Wnt pathway genes, Eomes, and Jmjd3 mRNA levels upon differentiation of PML KD, KO, or WT ESC. Data represent the mean + SD of three independent experiments (n = 3). (F) Endodermal (Afp, Gata4) and mesodermal (T, Wnt3a) gene expression levels in the course of differentiation of WT, PML KD, and TBX3 OE PML KD ESC. Error bars indicate +SD of four independent experiments (n = 4). D, day.

Figure 5

PML Is Essential for Efficient Reprogramming of MEF to iPSC (A) Alkaline phosphatase (AP) staining of iPSC colonies 28 days after OSKM lentiviral transduction. AP-positive colony numbers are shown on the right. Data are shown as mean + SD of three independent experiments (n = 3). (B) Protein levels of pluripotency markers in WT and PML KO iPSC. (C) Teratoma formation by intramuscular injection of WT and PML KO iPSC in immunocompromised mice. Photos show the differences between teratoma size. H&E staining analysis of the three germ layers in teratoma sections demonstrated by the respective arrows (bl, blastema; ec, ectoderm; en, endoderm; me, mesoderm). Scale bars, 100 μm. (D) Protein expression levels of epithelial, mesenchymal (upper panel), and pluripotency markers (lower panel) at day (D) 12 of reprogramming process. (E) Protein levels of p-SMAD2 and ZEB1 at days (D) 0, 1.5, and 3 of reprogramming process.