Signaling network crosstalk in human pluripotent cells: a Smad2/3-regulated switch that controls the balance between self-renewal and differentiation

Abstract

A general mechanism for how intracellular signaling pathways in human pluripotent cells are coordinated and how they maintain self-renewal remain to be elucidated. In this report, we describe a signaling mechanism where PI3K/Akt activity maintains self-renewal by restraining prodifferentiation signaling through suppression of the Raf/Mek/Erk and canonical Wnt signaling pathways. When active, PI3K/Akt establishes conditions where Activin A/Smad2,3 performs a pro-self-renewal function by activating target genes, including Nanog. When PI3K/Akt signaling is low, Wnt effectors are activated and function in conjunction with Smad2,3 to promote differentiation. The switch in Smad2,3 activity after inactivation of PI3K/Akt requires the activation of canonical Wnt signaling by Erk, which targets Gsk3β. In sum, we define a signaling framework that converges on Smad2,3 and determines its ability to regulate the balance between alternative cell states. This signaling paradigm has far-reaching implications for cell fate decisions during early embryonic development.

Figure 1. Activin A promotes self-renewal or differentiation depending on the status of PI3K/AKT signaling

(A) Transcript markers of BG02 hESCs in −HI. (B) Immunostaining of BG02 hESCs in −HI. Micron bar, 50 μm. (C) Immunoblot analysis of BG02 hESC lysates in HAI or −HI for 24 hours. (D) Transcript markers after transfection of WA09 hESCs with a myr.AKT-IRES-GFP expression vector in +HAI media. 24 hours after transfection GFP⁺ and GFP⁻ cells were isolated by FACS, plated and then grown in −HI for 3 days. (E) Transcript markers in −HI in WA09 hESCs after 4 days +/− SB 431542 (20 μM). (F) Immunoblot analysis of BG02 hESC lysates from cells grown in −HI for 2 and 4 days, +/− SB 431542. (G) MixL1 promoter-luciferase assays. 24 hours after transfection, WA09 hESCs were cultured for 2 days in +HAI, −HI or −HI +SB 431542 and assayed. (H) Immunoblot analysis of BG02 hESC in +HAI media and starved of factors for 18 hours. Factors were re-added for 3 hours as indicated. H, heregulin (10 ng/ml); A, Activin A (10 or 100 ng/ml); I, Igf-1 (200 ng/ml); F₁₀, Fgf2 (10 ng/ml). (I) Model summarizing results from Figure 1. *P<0.05, **P<0.01. See also Supplementary Figure 1.

Figure 2. Activin A regulates Erk and Gsk3β in the absence of PI3K/AKT activity

(A) Immunoblot of BG02 hESCs +/− LY 294002 (50 μM) for 12 hours. (B) WA09 hESCs were starved of factors for 24 hours (−HAI) then stimulated with factors as indicated for 3 hours; +H, heregulin (10 ng/ml); +A, Activin A (10 ng/ml); +I, Igf-1 (200 ng/ml); +F₁₀, Fgf2 (10 ng/ml). (C) WA09 hESCs (+HAI) were treated and analyzed as in (B) with indicated factors for 3 hours. (D) WA09 hESCs (+HAI) were transfected with myr.AKT-IRES-GFP expression vector. GFP⁺ and GFP⁻ cells were isolated by FACS, plated in +HAI media for 24 hours then plated in −HI media for 2 days. (E) Immunoprecipitation of WA09 hESCs (+HAI) with Akt1 and Raf1, followed by immunoblotting. (F) ERK activation measured as the ratio of YFP/CFP fluorescence in WA09 hESCs (+HAI) transfected with EKAR. The ratio average ± SEM for untreated cells (10; black line) and LY 294002-treated cells (7; red line). (G) Model summarizing data presented in the Figure. *P<0.05, **P<0.01. See also Supplementary Figure 2.

Figure 3. Raf/Mek/Erk signaling promotes the differentiation of hESCs and inhibits Gsk3β

(A) Transcript markers in −HI for 3 days, with or without transfection of dnErk1 from WA09 hESCs. (B) Transcript levels from WA09 hESCs (−HI) after 4 days +/− U0126 (20 μM). (C) Immunoblot analysis of BG02 hESC lysates (−HI) for 2 and 4 days, +/− U0126. (D) Immunoblot analysis of WA09 hESC lysates in −HI, +/− U0126. (E) Transcript markers after transfection and FACS of WA09 hESCs with a constitutively active-cRAF-IRES-GFP vector after culture (+HAI) for 3 days. (F) Immunoblot analysis after transfection and FACS of WA09 hESCs with a constitutively-active cRAF-IRES-GFP vector. Cell lysates prepared after a further 3 hours culture in −HI media. (G) Immunoblot analysis of WA09 hESC lysates (+HAI) having a constitutively-active MEK-ER transgene or empty vector, +/− 100 nM 4-hydroxy-tamoxifen (4OHT) for 24 hours. (H) Transcript markers of WA09 hESCs containing a constitutively active MEK-ER transgene or vector control, +/− 100 nM of 4OHT for 3 days (+HAI). (I) Immunoprecipitation of WA09 hESCs (+HAI) with Erk1,2 and Gsk3β, followed by immunoblotting. (J) Immunoprecipitation (IP)-kinase assays for Gsk3β, of WA09 hESCs cultured in media with +HAI, −HI, −HI +U0126, or −HI +Dkk1 (150 ng/ml). (K) Luciferase assay of WA09 hESCs treated LY 294002 +/− U0126 for 2 days after transfection of Top-Flash or Fop-Flash reporters. (L) Model summarizing the data presented in Figure 3. *P<0.05, **P<0.01.

Figure 4. Loss of Gsk3β activity promotes mesendoderm differentiation of hESCs

(A) Transcript markers in BG02 hESCs in +HAI media, +/− BIO (2 μM). (B) Immunofluorescence of BG02 hESCs +/− BIO. Micron bar, 100 μm. (C) Transcript markers of WA09 hESCs with BIO +/− U0126 for 3 days. (D) Immunoblot analysis of WA09 hESCs treated with increasing doses of BIO after 24 hours. (E) Luciferase assays in WA09 hESCs treated with increasing doses of BIO, 48 hours after transfection. (F) Transcript markers of WA09 hESCs treated with increasing doses of BIO for 72 hours. (G) Immunofluorescence of WA09 hESCs treated with increasing doses of BIO for 72 hours. *P<0.05, **P<0.01. (F). *P<0.05, **P<0.01. See also Supplementary Figures 3 and 4.

Figure 5. Inhibition of Wnt signaling by Dkk1 stabilizes human pluripotent cells

(A) Immunofluorescence analysis of WA09 hESCs in +HAI, +/− Dkk1 (150 ng/ml) for 6 passages. Micron bar, 50 μm. (B) Luciferase assay of WA09 hESCs in +HAI, +/− Dkk1 for 6 passages 2 days after transfection. Values expressed as ratio of Top-Flash to Fop-Flash signals. (C) Transcript levels in WA09 hESCs (+HAI) after treatment with Dkk1 for 4 days. (D) Flow cytometry analysis of WA09 hESCs cultured +/− Dkk1 for 6 passages. (E) Transcript markers in −HI, +/− Dkk1 (F) Transcript markers after transfection and FACS of a constitutively active cRAF-IRES-GFP vector in +HAI, +/− Dkk1 for 3 days. (G) Model summarizing the data from Figures 4 and 5. *P<0.05, **P<0.01. See also Supplementary Figure 5.

Figure 6. Wnt/β-catenin and Activin/Smad signaling cooperate to regulate mesendoderm gene expression

(A) Transcript markers of WA09 hESCS grown in +HAI, +/− BIO (2 μM) +/− SB 431542 (20 μM). (B) MixL1-luciferase reporter assays in WA09 hESCs cultured in +HAI medium after addition of BIO +/− SB 431542 after 3 days. (C) Chromatin Immunoprecipitation assay of WA09 hESCs (+HAI) after the addition of BIO or BIO + SB 431542 for 12 hours with Smad2,3 and β-catenin on the MixL1 promoter. (D) MixL1-luciferase assays in +HAI media co-transfected with vector alone or caSmad3, +/− Dkk1 (150 ng/ml) for 3 days. (E) Transcript markers of WA09 hESCs following transfection and FACS for a caSmad3-IRES-GFP vector +/− Dkk1. *P<0.05, **P<0.01.

Figure 7. Model of signaling networks that regulate self-renewal and differentiation of human pluripotent stem cells

(A) Model summarizing the self-renewing state, when PI3K/Akt signaling is active. Akt first modulates the threshold of Smad2,3 activity and second, inhibits Erk and maintains Gsk3β activity, compatible with Nanog expression. (B) Model summarizing the differentiated state. Upon PI3K/Akt inactivation, Activin A/Smad2,3 signaling is enhanced, Erk is activated and when coupled with Wnt signaling, promotes Gsk3β inhibition and β-catenin activation. Subsequently, Smad2/3 and Wnt effectors co-operate to promote the activation of mesendoderm markers, such as MixL1. Nanog expression is lost after 4 days of differentiation.