Hypoxia Epigenetically Confers Astrocytic Differentiation Potential on Human Pluripotent Cell-Derived Neural Precursor Cells

Abstract

Human neural precursor cells (hNPCs) derived from pluripotent stem cells display a high propensity for neuronal differentiation, but they require long-term culturing to differentiate efficiently into astrocytes. The mechanisms underlying this biased fate specification of hNPCs remain elusive. Here, we show that hypoxia confers astrocytic differentiation potential on hNPCs through epigenetic gene regulation, and that this was achieved by cooperation between hypoxia-inducible factor 1α and Notch signaling, accompanied by a reduction of DNA methylation level in the promoter region of a typical astrocyte-specific gene, Glial fibrillary acidic protein. Furthermore, we found that this hypoxic culture condition could be applied to rapid generation of astrocytes from Rett syndrome patient-derived hNPCs, and that these astrocytes impaired neuronal development. Thus, our findings shed further light on the molecular mechanisms regulating hNPC differentiation and provide attractive tools for the development of therapeutic strategies for treating astrocyte-mediated neurological disorders.

Keywords: Rett syndrome; astrocytic differentiation; epigenetics; human neural stem cells; human pluripotent stem cells; hypoxia.

Graphical abstract

Figure 1

Impairment of Astrocytic Differentiation Is Inversely Correlated with DNA Methylation Level in the GFAP Promoter (A) Representative images of staining for TUBB3 (green) and GFAP (red) after 28 days of differentiation of four hNPCs: CB660 (from fetal brain), AF22 (from hiPSCs established from human adult fibroblasts), AF23 (from hESCs), and AF24 (from iPSCs reprogrammed from CB660). Scale bar, 200 μm. (B) Quantification of GFAP-positive cells for assessing differentiation of hNPCs in (A). (C) Diagram showing the human GFAP promoter region including the STAT3 recognition site and seven other CpG sites. The red bar of CG dinucleotide indicates a methylation site of STAT3 binding site. (D) Methylation status of the GFAP promoter region in the indicated hNPCs cultured under maintenance conditions. Open and filled circles represent unmethylated and methylated CpG sites, respectively. The red rectangles of CG dinucleotide indicate STAT3 binding sites. (E) Methylation frequency within the STAT3 binding site and total CpG sites in GFAP promoters. Solid bars depict methylation levels in total CpG sites, and white bars depict those in the STAT3 binding site (n = 3 independent experiments; error bars are mean ± SD; ^∗∗∗p < 0.001; one-way ANOVA and Tukey's test). See also Figure S1.

Figure 2

Hypoxia Increases Astrocytic Differentiation of hNPCs via DNA Demethylation (A) Representative images of staining for TUBB3 (green) and GFAP (red) after 28 days of differentiation of the indicated hNPCs under hypoxia. Scale bar, 200 μm. (B) Quantification of GFAP-positive cells for assessing differentiation under normoxic or hypoxic conditions of hNPCs (n = 3 independent experiments). (C) Experimental scheme to examine the expression of HIF1α and HIF2α and methylation status of the GFAP promoter during differentiation of AF22 under normoxic or hypoxic conditions. AF22 cells were harvested at the indicated times. (D) Time-course western blot analysis of HIF1α and HIF2α expression in AF22 under 2% O₂ differentiation conditions. GAPDH was also blotted as a control. (E) Expression levels of HIF1α and HIF2α in AF22 in normoxic (solid line) or hypoxic (broken line) differentiation conditions (n = 3 independent experiments). (F) Methylation frequency within total CpG sites (left panel) and the STAT3 binding site (right panel) in the GFAP promoter in normoxic (solid line) or hypoxic (broken line) differentiation conditions (n = 3 independent experiments; error bars are mean ± SD; ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001; Student's t test). See also Figure S2.

Figure 3

A Hypoxic Environment inside the Aggregation Accounts for Enhanced Astrocytic Differentiation of hNPCs (A) Experimental scheme for differentiation of AF22 through sphere-forming culture. (B) Representative images of staining for Hypoxiprobe (pimonidazole adducts, green) and Hoechst (blue) of AF22 cultured in aggregation forms under 21% or 30% O₂ conditions. Pimonidazole adduct formation was clearly observed under 21% but not 30% O₂. Scale bar, 300 μm. (C) Representative images of staining for TUBB3 (green) and GFAP (red), indicating that efficient GFAP-positive astrocytic differentiation of AF22 was induced through 21% O₂ but not 30% O₂ aggregation culture. Scale bar, 50 μm. (D) Quantification of GFAP-positive cells cultured as in (C) (n = 3 independent experiments; error bars are mean ± SD; ^∗∗p < 0.01; Student's t test). See also Figure S3.

Figure 4

Cooperation between Notch Signaling and HIF1α Mediates Astrocytic Differentiation under Hypoxia (A) Experimental scheme for culturing AF22 to examine the effect of Notch signal activation using DAPT (γ-secretase inhibitor; 10 μM) under the hypoxic differentiation condition. (B) Representative images of staining for TUBB3 (green) and GFAP (red) 19 days after differentiation under hypoxia in the presence or absence of DAPT. LIF was added to the culture medium for 4 days to stimulate astrocytic differentiation. Scale bar, 50 μm. (C) Quantification of GFAP-positive cells for assessing the effect of Notch signaling (n = 3 independent experiments; Student's t test). (D) Representative images of staining for TUBB3 (green) and GFAP (red) at 28 days after overexpression of NICD with or without HIF1α-CA expression under the normoxic differentiation condition. Nuclei were counterstained with Hoechst (gray). Scale bar, 50 μm. (E) Quantification of GFAP-positive cells for assessing differentiation of hNPCs in (D) (n = 4 independent experiments; one-way ANOVA and Tukey's test). (F) Methylation status of the GFAP promoter in AF22 (left) and in AF22 expressing HIF1α-CA and NICD (right), after 28 days of differentiation. Open and filled circles represent unmethylated and methylated CpG sites, respectively. The red bar of CG dinucleotide indicates a methylation site of STAT3 binding site. (G) Methylation frequency within the STAT3 binding site and total CpG sites in the GFAP promoter in (F). Solid bars depict methylation levels in total CpG sites, and white bars depict that in the STAT3 binding site (n = 3 independent experiments; error bars are mean ± SD; N.S., not statistically significant, ^∗p < 0.05, ^∗∗∗p < 0.001; Student's t test).

Figure 5

Hypoxia Enables RTT Patient-Derived hNPCs to Rapidly Differentiate into Astrocytes (A) Schematic representation of the protocol for generating astrocytes from RS2-65M and -62P. (B) Representative images of staining for TUBB3 (green) and GFAP (red) after 28 days of astrocytic differentiation of RS2-65M and -62P hNPCs. Scale bar, 200 μm. (C) Quantification of GFAP-positive cells in (B) for assessing differentiation of hNPCs (n = 3 independent experiments; Student's t test). (D) Methylation status of the GFAP promoter region in RS2-65M and -62P cultured in undifferentiated or differentiated conditions as in (A). Open and filled circles represent unmethylated and methylated CpG sites, respectively. The red bar of CG dinucleotide indicates a methylation site of STAT3 binding site. (E) Methylation frequency within the STAT3 binding site and total CpG sites in the GFAP promoter in (D). Solid bars depict methylation levels in total CpG sites, and white bars depict those in the STAT3 binding site (n = 3 independent experiments; error bars are mean ± SD; N.S., not statistically significant, ^∗p < 0.05, ^∗∗p < 0.01; one-way ANOVA and Tukey's test). See also Figure S4.

Figure 6

Astrocytes Derived from RTT-hiPSCs Impair Neuronal Maturation (A) Experimental scheme for assessing soma size and neurite length in EGFP-labeled AF22 with neuronal morphology. Astrocytic differentiation was induced in RS2-65M or -62P with FBS (from day 14 to 28) under hypoxia (1% O₂). EGFP-labeled AF22 were then seeded onto these differentiated astrocytes and cultured under normoxia in the absence of FBS for 7 days. (B) Representative images of staining for EGFP (green) and GFAP (red) of AF22 after 7 days of co-culture with differentiated RS2-65M or -62P (more than 60% of these cells differentiated into astrocytes; see also Figure 5). Boxed areas in the upper images are enlarged below. Scale bar, 20 μm. (C) Quantification of soma area and neurite length of EGFP-labeled AF22 with neuronal morphology after 7 days of co-culture with differentiated RS2-65M or -62P (n = 3 independent experiments). (D) Experimental scheme for neuronal culture with conditioned medium of astrocyte-enriched cells (CMA) collected from differentiated RS2-65M or -62P, which were cultured as in Figure 5. Each CMA was added to neuronal culture as 30% of the medium volume at day 12 and the neurons were cultured for an additional 3 days. (E) Neurites of mouse hippocampal neurons were immunostained for DLG4 (green), SLC17A7 (red), and MAP2 (blue) after 3 days of culture with CMA from differentiated RS2-65M or -62P. Representative images are shown. Scale bar, 10 μm. (F) Quantification of co-localized DLG4 and SLC17A7 signals in (E) for assessing normalized synaptic densities (n = 3 independent experiments; 30 neurons in each sample; error bars are mean ± SD; ^∗p < 0.05; Student's t test).