All-trans retinoic acid promotes neural lineage entry by pluripotent embryonic stem cells via multiple pathways
© Lu et al; licensee BioMed Central Ltd. 2009
Received: 17 November 2008
Accepted: 30 July 2009
Published: 30 July 2009
All-trans retinoic acid (RA) is one of the most important morphogens with pleiotropic actions. Its embryonic distribution correlates with neural differentiation in the developing central nervous system. To explore the precise effects of RA on neural differentiation of mouse embryonic stem cells (ESCs), we detected expression of RA nuclear receptors and RA-metabolizing enzymes in mouse ESCs and investigated the roles of RA in adherent monolayer culture.
Upon addition of RA, cell differentiation was directed rapidly and exclusively into the neural lineage. Conversely, pharmacological interference with RA signaling suppressed this neural differentiation. Inhibition of fibroblast growth factor (FGF) signaling did not suppress significantly neural differentiation in RA-treated cultures. Pharmacological interference with extracellular signal-regulated kinase (ERK) pathway or activation of Wnt pathway effectively blocked the RA-promoted neural specification. ERK phosphorylation was enhanced in RA-treated cultures at the early stage of differentiation.
RA can promote neural lineage entry by ESCs in adherent monolayer culture systems. This effect depends on RA signaling and its crosstalk with the ERK and Wnt pathways.
Embryonic stem cells (ESCs) are self-renewable and show a pluripotent ability to differentiate into cellular derivatives of all three primary germ layers. They are considered to be a unique biological system for disclosing the mechanisms of pluripotency and lineage commitment, and a useful material for disease modeling, pharmacological screening, and cell therapy.
All-trans retinoic acid (RA), which is a metabolic product of vitamin A (retinol), is one of the most important morphogens but with pleiotropic actions it induces development of several lineages . RA is an established signaling molecule involved in neuronal patterning, neural differentiation and axon outgrowth . It is of great importance to understand well the mechanism of action of RA on the development of the nervous system. RA is known to be capable of inducing ESCs to differentiate into neurons within embryoid bodies (EBs) in vitro [3–5]. However, the route by which RA influences neural commitment is still obscure. Furthermore, it is difficult to dissect and manipulate differentiation within EBs because they are multicellular agglomerations of extra-embryonic endoderm and definitive ectodermal, mesodermal and endodermal derivatives .
In this study, we used a monolayer culture system of neural induction  and treated the cultures with low concentration of RA (10-8 M), which ranged between the physiological (10-9 M) and pharmacological (10-6 M) levels . We found that among RA nuclear receptors retinoic acid receptors (RARs) and retinoid × receptors (RXRs), and RA-metabolizing enzymes (CYP26a1 and Raldh2), only the mRNAs of RARα, RARγ, RXRβ, Cyp26a1 and Raldh2 were detectable in mouse ESCs. Upon stimulation with RA, the nuclear receptors and RA-metabolizing enzymes showed dynamic changes in expression. In RA-treated cultures, the specific marker for ESCs, i.e., Oct4, was down-regulated more rapidly, and markers for neural cells were up-regulated sooner than in the other two control groups. We reported that RA-promoted neural differentiation was blocked by the RARα-selective antagonist Ro 415253, which indicates that RA signaling is essential for neural specification in this monolayer culture system, and RARα pathway may play a key role in this effect. Furthermore, our data demonstrated that the process of this enhanced neural differentiation involved crosstalk between RA and the extracellular signal-regulated kinase (ERK) and Wnt pathways.
1. RA causes earlier expression of Sox1 in ESCs
To assess whether RA could promote ESCs entry to the neural lineage in a monolayer culture system, 46C ESCs were transferred into a differentiation induction regime which comprised an adherent monolayer culture without any exogenous growth factors , or with RA or solvent (0.02% ethanol). It is believed that the first conditions are permissive for neural commitment driven by autocrine signaling . During experiments, we found that high does of RA (10-6 M) showed an obvious noxious effect on cell survival (see Additional file 1 Figure S1). Therefore, we treated the cultures with low concentration of RA (10-8 M), which was between the physiological (10-9 M) and pharmacological (10-6 M) levels .
10-8 M RA increased the number of cells in the monolayer culture at the early stage of neural induction (see Additional file 2 Figure S2). Therefore, it is reasonable to speculate that RA can also act after specification to increase the survival and/or proliferation of neural precursors. We cultured cells without RA for 96 h, and then treated the adherent cultures with RA. We observed an increase in the number of Sox1GFP+ cells after treatment with RA (Figure 1D). However, it is still hard to judge whether the increase in Sox1GFP+ cells was caused by the survival and/or proliferative effects of RA on neural precursors or by its neural differentiation effects on ESCs. Therefore, we obtained some purified Sox1GFP+ cells by culturing cells without RA for 6 days and selecting Sox1GFP+ cells by treatment with 0.5 μg/ml puromycin for another 4 days. These purified Sox1GFP+ cells were treated with or without RA for several days. After treatment with RA, there was a dramatic decrease in Sox1GFP+ cells (Figure 1E). Moreover, cultures at clonal density showed that there were almost no clones that contained GFP+ cells in control and solvent control cultures, while more than 80% of clones contained GFP+ cells in RA-treated cultures (Figure 1F). These data suggested that the observed initial increase in frequency of Sox1GFP-expressing cells was not caused mainly by the survival and/or proliferative effects of RA on neural precursors but by its neural differentiation effects on ESCs.
To test further whether RA directly promoted ESC differentiation, we examined the cell-density dependence of neural specification. Neural induction of ESCs during monolayer differentiation is strongly inhibited by even modest increases in cell density . Cells treated with 10-8 M RA generated far fewer Sox1GFP+ neural precursors at high cell density (3 × 104cells/cm2) than those at low cell density (1 × 104cells/cm2), while in control and solvent control cultures, cells almost completely eliminated neural specification at high density (see Additional file 3 Figure S3). This suggested that RA affected neural specification of ESCs also in a cell-density dependent manner.
We also noted a difference in the variability of GFP intensity between cultures. Control or solvent control cultures contained a mixture of less intensely GFP+ cells [Figure 1B (Day5 N2B27+0 and Day5 N2B27+ethanol)], whereas RA-treated cells showed uniformly moderate GFP expression [Figure 1B (Day5 N2B27 + 10-8 M RA)].
The effects of RA at the onset of ESC differentiation were indicated by the kinetics and pattern of gene regulation. One of the key marker genes Oct4 was examined. Oct4, a critical determinant of the pluripotent state, is down-regulated early during somatic differentiation . Within 3 days of leukemia inhibitory factor (LIF) and serum withdrawal, Oct4 mRNA decreased dramatically in RA-treated cells, while there was a smaller decrease in control or solvent control cells; after day 3, almost no Oct4 expression was observed in RA-treated cultures (Figure 1GI).
In summary, these data indicated that RA acted at the initial stages of ESC differentiation to promote the transition into neuroectoderm.
2. RA facilitates emergence of neural progenitors, neural stem cells and neurons
We also used fluorescence microscopy and western blot to monitor the expression of markers for neural stem cells, neurons and glia. At day 3, with the rapid disappearance of Oct4+ cells, Sox1GFP+ and Nestin+ cells were found in RA-treated cultures (Figure 2D). The expression of Tuj1 was upregulated at day 6 in RA-treated cultures, while 3 days later, Tuj1+ cells appeared in control and solvent control cultures (Figure 2E and 2F). With the accelerated emergence of neural stem cells and immature neurons, the expression of Neuronal nuclei (NeuN; a vertebrate neuron-specific nuclear protein) and glial fibrillary acidic protein (GFAP; a marker of glia cells) was also upregulated, and MAP2+ and GFAP+ cells also appeared earlier in RA-treated cultures (Figure 2EG). At day 11, in control and solvent control cultures, most cells were Nestin+ and were in the rosette conformations that is typical of neuroepithelial cells (Figure 2Gg and 2Gh), whereas in RA-treated cultures, although Nestin+ cells were still distributed in the rosette pattern, the percentage of Nestin+ cells was obviously lower (Figure 2Gi).
These findings indicated clearly that RA actually facilitated the differentiation of neural progenitors and neural stem cells, and as a consequence, neurons and glial cells appeared earlier in RA-treated cultures.
3. Requirement for the RA signaling during efficient neural specification of ESCs
When 46C ESCs were exposed to the RARα-selective antagonist Ro 415253  during the monolayer differentiation, the majority of cells failed to become Sox1GFP+ neural cells. After 92 h of differentiation, <10% of inhibitor-treated cells became Sox1GFP+ compared with around 40% of cells exposed to DMSO vehicle (Figure 3D and 3E, p < 0.001). Many inhibitor-treated cells remained Oct4+ during this period (Figure 3F and 3G). Ro 415253 also inhibited neural differentiation in the culture without RA (Figure 3H and 3I).
Our observations suggested that blockade of the RARα pathway initially impeded lineage commitment, and RA signaling was required for the efficient neural specification of ESCs.
4. RA acts upstream of MAP kinase/ERK kinase (MEK) to promote neural specification
To assess whether RA affected activation of the ERK pathway during neural specification, we detected phosphorylation of ERK by western blotting. Twelve hours after differentiation, a higher level of phosphorylated ERK was observed in RA-treated cultures (Figure 4F). The expression of the transcription factor Elk1, one of the downstream targets of ERK [23, 24], was also up-regulated more in the RA-treated cultures than in the other two control cultures before day 3 (Figure 4G and 4H).
These findings indicated that RA acted upstream of MEK1/2 in the neural induction cascade. RA may play a role in enhancing ERK phosphorylation via MEK1/2 or its upstream but not in an FGF-signaling-dependent manner. The exact targets of RA remain to be determined.
5. Wnt signaling is involved in RA-promoted neural specification
These findings showed that activation of Wnt signaling blocked the effect of RA. However, the roles of RA and signaling molecules involved in Wnt signaling still remain to be elucidated (see discussion).
The pluripotent ESC is a valuable in vitro model for studying the effects of various factors on cell lineage decisions in very early embryonic stages of mammalian development. In particular, the effects of RA signaling on the differentiation of ESCs and neural induction have been studied extensively. In addition to previous studies that have shown that RA promotes neural differentiation of ESCs [3, 4, 27], the present study demonstrated that RA facilitated the emergence of neural progenitors, neural stem cells and neurons in a monolayer culture. This is a simple system that allows direct observation, analysis and manipulation of the process of neural specification without the confounding influence of cell aggregation, co-culture, uncharacterized media constituents, or cell selection . Furthermore, these results also demonstrated the novel and more precise molecular mechanisms of action of RA on neural differentiation.
1. Effects of RA on ESCs differentiation
RA and other retinoids are potent regulators of morphogenesis, growth and cell differentiation. RA, which is abundant in mammalian nervous systems , induces neurite outgrowth and neuronal differentiation from various sources, including ESCs [3, 29], dorsal root ganglia , or neuroblastoma cells . RA-induced neuronal differentiation is characterized by the expression of tissue-specific genes, proteins, ion channels, and receptors in a developmentally controlled manner .
Addition of RA promoted faster differentiation of ESCs, as indicated by the pattern of Oct4 expression, which was down-regulated more rapidly in RA-treated cultures, and could not be detected after day3. This is coincident with previous studies . The expression levels of markers for undifferentiated neural cells, i.e., Sox1, Pax6, Nestin, BLBP, Prominin, Olig2 and Musashi, were higher in RA-treated cultures. As a consequence, the expression levels of markers for neurons, i.e., Tuj1, MAP2, and NCAM, and glial cells, i.e., GFAP were also elevated in RA-treated cultures. These findings are consistent with RA-induced differentiation of neural progenitor cells in the cell aggregation system, as described previously [3, 27, 33].
The initial higher frequency of Sox1GFP+ cells in RA-treated cultures is not caused mainly by the enhanced viability or proliferation of neural precursor cells. After treatment with RA, the number of purified Sox1GFP+ neural precursor cells decreased dramatically, which indicated that RA could not maintain the Sox1GFP+ state. This observation is coincident with the previous studies . Treatment of adherent cultures with RA after the appearance of the first Sox1GFP+ cells increased the number of Sox1GFP+ cells. One explanation may be that, in monolayer culture, the speed at which RA induced ESCs into Sox1GFP+ neural precursor cells was faster than the speed of the loss of Sox1GFP+ cells.
2. Molecular mechanisms of action of RA on neural differentiation
2.1 RA signaling is involved in efficient neural specification of ESCs
It is proposed that the molecular mechanisms of action of RA during embryogenesis in vivo involve a complex signaling pathway. RA exerts its function by binding to cellular RA-binding proteins that interact with the nuclear RA receptors. RA binds to RARs, which normally act as ligand-inducible transcription factors by binding as heterodimers with RXRs to RA response elements located in regulatory regions of target genes . During in vitro differentiation of ESCs into the neuronal lineage, RA might act in the same way as it does in vivo. The hypothesis that in vitro and in vivo pathways are comparable is supported by significant up-regulation of RARα and RARβ mRNA, but rapid down-regulation of RARγ and RXRγ mRNA during RA-induced neuronal differentiation of mouse embryonal carcinoma cells . This suggests a role for RARα, -β, -γ and RXRγ during neuroectodermal differentiation [37, 38]. However, the mechanisms whereby RA induces neural differentiation of ESCs are not fully understood.
To demonstrate whether RA exerts its effects through RA-related nuclear receptors and to determine which receptors play more important roles in this efficient neural specification, we detected all six isoforms of RARs and RXRs by RT-PCR or Q-PCR. RARα, RARγ and RXRβ were detected in ESCs. During differentiation, few differences in the expression of RARγ and RXRβ were observed between the RA-treated and the two control groups, while obviously different expression of RARα was observed. In addition, the expression of RARα was up-regulated more at day 5 than at day 3. These observations hint that RARα may play an important role in RA-treated efficient neural specification. Adding the RARα-selective antagonist Ro 415253 effectively blocked the neural fate of ESCs, and many RA and Ro 415253 co-treated cells remained in an Oct4+ state. These findings suggest that RA signaling is indeed involved in promotion of this neural fate, and the nuclear receptor RARα plays a key role in this effect.
2.2 RA activates the ERK pathway during promoted neural specification
Studies in planaria, frog embryos and avian embryos [17, 18, 39, 40] have suggested a primary requirement for FGF signaling in neural specification. FGF4 is produced in an autocrine fashion by undifferentiated ESCs [41, 42] and activates the Ras-MEK-ERK intracellular signaling cascades . In our study, FGF receptor tyrosine kinase inhibitor SU5402 was used to try to block the neural specification effect of RA. This inhibitor could not significantly block the emergence of Sox1GFP+ neural progenitor cells in the RA-treated monolayer cultures, although it had the ability to inhibit the neural specification of ESCs when RA was absent from the culture (, see Additional file 6 Figure S6). On the other hand, the two inhibitors PD184352 and PD0325901, both of which can block MEK1/2, upstream of ERK1/2, were able to block neural specification of ESCs even in the RA-treated cultures, and some cells remained as Oct4+ cells. All of these indicate that RA exerts its effects not through FGF signaling but requiring the activation of MEK-ERK signaling cascades. Previous studies have reported that RA can activate the ERK pathway in an unknown manner [43–46]. In our study, we also observed that phosphorylation of ERK was up-regulated at the early stage during differentiation of ESCs in RA-treated cultures, and the transcription factor Elk-1, one of the downstream targets of ERK [23, 24], was also more up-regulated in the RA-treated cultures than in the two control cultures before day3. Activated ERK1/2 also mediates phosphorylation of the MAPK sites of Smad1 (pSmad1MAPK), which inhibits the inhibitory function of the bone morphogenetic proteins (BMP)-Smad1 pathway on neural differentiation .
2.3 Wnt signaling also plays a role in RA-promoted neural differentiation
Down-regulation of Wnt signaling has been shown to be one of the mechanisms involved in RA-induced neural differentiation of mouse ESCs. Up-regulation of Sfrp2 can inhibit the Wnt-β-catenin anti-neural pathway. Conversely, adding LiCl, which inhibits GSK3 and partially mimics Wnt signaling, can enhance the anti-neural pathway . On the other hand, GSK3 mediated-Smad1 phosphorylation can block the inhibitory effect of BMPs on neural differentiation . Therefore, inhibition of GSK3 may also make the BMP-Smad1 pathway become valid for inhibiting neural differentiation. Interestingly, β-catenin, which is a key molecule in Wnt signaling, has been shown to interact directly with RAR, and as a result retinoids decrease activation of T-cell transcription factor (TCF) in cultured cells in a dose-dependent manner .
In the present study, we found that RA up-regulated the expression of RARα and RARβ at the early stage of differentiation. We also investigated the effect of LiCl and another GSK3 inhibitor CHIR99021. Negative effects of Wnt pathways on neural differentiation were observed by adding the two inhibitors to RA-treated cultures, and most cells lost the marker of ESCs. RA also showed its effect on up-regulating the expression of Sfrp2.
Based on the present and previous studies, RA may exert its effects through the following mechanisms (Fig. 6B). (1) RA promotes neural specification by up-regulating Sfrp2. The expression of Sfrp2 is up-regulated by stimulation with RA, which blocks the message passing downstream from Wnts . Upon adding LiCl or CHIR99021, which stimulates β-catenin and/or BMP signaling through inhibition of GSK3, but not through Wnts, the β-catenin pathway and/or BMP signaling can still play a part in suppressing neural specification, even with the stimulus of RA-up-regulated Sfrp2. (2) RA may promote neural differentiation by up-regulating RAR, which may decrease β-catenin-lymphoid enhancing factor (LEF)/TCF mediated transactivation. (3) RA also may enhance the sequestration of β-catenin to suppress the anti-neural effects of the Wnt-β-catenin pathway. However, this hypothesis needs further evidence.
Our study showed that RA can promote neural lineage entry by ESCs in adherent monolayer culture systems. We also found that RA stimulation led to dramatic changes in RA nuclear receptor and RA-metabolizing enzyme expression. Further study showed that these effects of RA depended on RA signaling and its crosstalk with the ERK and Wnt pathways. Our study showed a more limpid circumscription of the effects of RA and its pathway on neural lineage choice of mouse ESCs.
Mouse ESC line 46C (passage 2535), which has one allele of Sox1 inactivated by targeted integration of a GFPirespac cassette, was cultivated as previously described . Briefly, ESCs were maintained on gelatin-coated dishes in the absence of feeder cells in GMEM (Sigma, St. Louis, MO) supplemented with 2 mM glutamine (Gibco, Grand Island, NY), 0.001% β-mercaptoethanol (Gibco), 1× nonessential amino acids (Gibco), 10% fetal bovine serum (FBS) (Gibco), and 2,000 units/ml human recombinant LIF (R & D Systems, Minneapolis, MN).
ESC adherent monoculture
Procedure was described in detail in . Briefly, ESCs were washed with N2B27 serum-free medium to remove serum and then plated onto 0.1% gelatin-coated tissue culture plastic at a density of 1 × 104 cells/cm2 in N2B27 with or without RA (Sigma). N2B27 consists of a 1:1 ratio of DMEM/F12 and Neurobasal media supplemented with 0.5% modified N2 (Gibco), 0.5% B27 (Gibco), and 2-mercaptoethanol (Gibco). Medium was changed every other day. For purifying Sox1GFP+ cells, 0.5 μg/ml puromycin (Sigma) was used.
For clonal cultures assay, dissociated ESCs were plated onto 0.1% gelatin-coated 6-well plates (Corning Incorporated, Corning, NY) at the density of 1.5 × 103 cells/well and cultured in serum-containing ESC medium (3 ml/well) plus LIF. At this density, more than 90% of colonies derived from single cells. After 2 days, medium was replaced with serum-free neural differentiation medium N2B27. Medium was renewed with fresh N2B27 every other day. After 2 days culture in N2B27 medium, total colonies and colonies containing Sox1GFP+ cells were counted.
Cells were counted by using a hemocytometer.
Immunofluorescence and FACS
Cells were fixed in 4% paraformaldehyde and incubated for 1 h in blocking buffer (PBS, 10% FBS, and 0.1% Triton X-100). Primary antibodies were diluted in blocking buffer and applied overnight at 4°C. After three washes in PBS, secondary antibodies conjugated to TRITC (tetramethylrhodamine isothiocyanate) (Vector, Burlingame, CA) were diluted at 1: 200 in blocking buffer and applied for 1 h at room temperature. The cells were washed at least three times in PBS and visualized on an Olympus inverted fluorescence microscope. For nuclear counter staining, nuclei were stained with bisbenzimide (Hoechst 33342, 10 μM, Sigma).
Primary antibodies were obtained from the following sources: Oct4 (1: 500, Santa Cruz Biotechnology, Santa Cruz, CA), nestin (1:200, Chemicon, Temecula, CA), Tuj1 (1:200, R & D Systems), MAP2 (1: 200, Sigma), GFAP (1:400, Chemicon).
FACS analysis was performed using a Becton-Dickinson (Palo Alto, CA) FACS Calibur flow cytometer.
RT-PCR and Q-PCR analysis
RT-PCR analysis of at least three independent cultures was performed in most of the experiments, and similar results were obtained.
RNA was isolated using TRIzol (Invitrogen, Carlsbad, CA), DNase I-digested (Invitrogen), and column-purified (RNeasy MinElute Cleanup; Qiagen, Valencia, CA). First-strand cDNA was synthesized using M-MLV Reverse Transcriptase (Invitrogen). To analyze relative expression of different mRNAs, the amount of cDNA was normalized based on the signals from ubiquitously expressed GAPDH mRNA. To provide negative controls and exclude contamination by genomic DNA, the reverse transcriptase was omitted in the cDNA synthesis step, and the samples were subjected to the PCR reaction in the same manner with primer sets for GAPDH, and were indicated at the bottom of each figure as RT (-). Primer sequences and PCR cycling conditions could be found in the Additional file 7 Table S1.
Q-PCR was performed with Power SYBR Green PCR Master Mix (Applied Biosystems, Foster City, CA) according to manufacturer's instructions. Signals were detected with an ABI7300 Real-Time PCR System (Applied Biosystems). The relative expression level was determine by the 2-delta Ct method and normalized against ribosomal protein L19 (RPL19). Primer sequences could be found in the Additional file 8 Table S2.
Western blot analysis
Cells and tissues were lysed in a lysis buffer (20 mM Tris-HCl, pH 7.4, 2% Triton X-100, 10 mM EDTA, 5 mM NaF and 1 mM sodium orthovanadate). Protein concentration was determined by BCA assay (Pierce, Rockford, IL). Then proteins were resolved in 12% SDS polyacrylamide gels and transferred onto nitrocellulose membranes. The membranes were then incubated overnight at 4°C in blocking solution containing 5% nonfat dry milk in PBS with 0.1% Tween-20. Subsequently the membranes were incubated with primary antibody against Oct-4 (1:1000, Santa Cruz), ERK1/2, phosphor-ERK1/2 (both 1:500, Cell Signaling Technology, MA), Neuronal Nuclei (NeuN, 1:200, Chemicon), β-actin (1:2000, Biovision, Mountain View, CA) or alpha Tubulin (1:2000, Sigma) followed by incubation with HRP (horseradish peroxidase)-conjugated second antibodies. Detection of HRP was performed by SuperSignal West Pico Chemiluminescent Substrate (Pierce).
Ro 415253 (Biomol, Plymouth Meeting, PA) was used at 1 μM. PD184352, PD0325901 and CHIR99021 (kind gifts of Professor Qilong Ying, University of Southern California, USA) was used at a concentration of 4 μM, 4 μM and 3 μM respectively. SU5402 (Calbiochem, San Diego, CA) was used at 5 μM. LiCl (Sigma) was used at 10 mM.
The statistics were analyzed by STAT 8.0 software. For comparisons of two conditions, the Student's t test was used. All data were expressed as mean ± standard error of mean (S.E.M.).
(brain lipid binding protein)
extracellular signal-regulated kinase
embryonic stem cells
fibroblast growth factor
fetal bovine serum
glial fibrillary acidic protein
green fluorescent protein
glycogen synthase kinase
lymphoid enhancing factor
leukemia inhibitory factor
microtubule associated protein 2
MAP kinase/ERK kinase
neural cell adhesion molecule
alltrans retinoic acid
retinoic acid receptor
retinoid × receptor
secreted frizzled-related protein 2
T-cell transcription factor
This project is supported by a grant from "211 Project" to Houyan Song. We are grateful to Prof. Austin Smith (Wellcome Trust Centre for Stem Cell Research, University of Cambridge, Cambridge, UK) for providing us with the 46C ESCs and Prof. Qilong Ying (Center for Stem Cell and Regenerative Medicine, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA) for providing us with PD184352, PD0325901 and CHIR99021.
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