ACTIVATED PLURIPOTENT STEM CELL, AND PREPARATION METHOD THEREFOR AND USE THEREOF

Abstract

A pluripotent stem cell, a method for producing the pluripotent stem cell, and the use of the pluripotent stem cell for stem cell differentiation, cell transplantation, tissue repair, and/or tissue regeneration.

Claims

1. An isolated pluripotent stem cell, having the following characteristics: (i) having the pluripotency of a mammalian embryonic epiblast cell from post-implantation to pre-gastrulation (early post-implantation epiblast); (ii) having one or more of formative differentiation function, morphological characteristic, genomic characteristic, and epigenomics characteristic; and (iii) being capable of stable passage for at least 5 times.

2. (canceled)

3. The isolated pluripotent stem cell of claim 1, which is capable of differentiating into an endoderm cell, an ectoderm cell, or a mesoderm cell under a condition that allows the differentiation of the isolated pluripotent stem cell, (i) wherein the differentiation efficiency is at least 50%; and/or (ii) wherein differentiating into the endoderm cell, the ectoderm cell, or the mesoderm cell is within 5 days after applying the condition that allows the differentiation.

4. The isolated pluripotent stem cell of claim 1, which is capable of differentiating into a germ cell or a germ precursor cell under a condition that allows the differentiation of the isolated pluripotent stem cell, and the differentiation efficiency is at least 10%.

5. (canceled)

6. The isolated pluripotent stem cell of claim 1, which is capable of forming an epiblast-like embryoid (EpiBlastoid) with a rosette-like structure.

7. The isolated pluripotent stem cell of claim 1, wherein the isolated pluripotent stem cell comprises at least 200 super bivalent genes.

8. The isolated pluripotent stem cell of claim 1, wherein the mRNA and/or protein expression level of one or more genes selected from the group consisting of CLDN6, WNT3, MIXL1, SOX4, LIN28B, EPHA1, JPH4, MMP25, MOXD1, MYC, NECTIN1, SLC39A8, SLC7A8, TMEM125, TMEM59L, ZIC2, EOMES, TCFL5, FOXH1, GRHL2, and ZIC5 in the isolated pluripotent stem cell is at least about 2 times of that in an embryonic stem cell (ESC).

9-10. (canceled)

11. The isolated pluripotent stem cell of claim 1, which has super bivalency in the promoter region of one or more genes selected from the group consisting of: ABLIM2, ANKRD33B, BARX1, DMRT2, DMRT3, EVX1, FGFR3, FLT1, FOXC1, FOXF2, GDNF, HOXA11, HOXA3, HOXA5, HOXA6, IRX1, IRX2, KISS1R, MSX1, NFIB, NFIC, NKX3-2, NRN1, OTP, PAX5, PHOX2B, PITX3, PTGER4, SLIT2, TBX1, and VLDLR.

12. (canceled)

13. The isolated pluripotent stem cell of claim 1, wherein the promoter region of one or more genes selected from the group consisting of HAND1, T, FOXA2, NKX2-5, PAX6, PDX1, ISL1, TCF21, LHX5, PAX2, DLX5, NR4A2, OTX2, ZIC5, UTF1, FGF5, ZFP13, ZSCAN10, ZIC2, and ESRP1 in the isolated pluripotent stem cell has lower DNA methylation level compared to the promoter region of the corresponding gene in an ESC.

14-15. (canceled)

16. The isolated pluripotent stem cell of claim 1, wherein the isolated pluripotent stem cell is derived from an ESC, an induced pluripotent stem cell (iPSC), an epiblast stem cell (EpiSC), an epiblast cell (Epiblast), a blastocyst inner cell mass, a pluripotent stem cell induced from a somatic cell or an early embryo from post-implantation to pre- or post-gastrulation.

17. (canceled)

18. The isolated pluripotent stem cell of claim 1, wherein the isolated pluripotent stem cell is a mammalian pluripotent stem cell.

19. The isolated pluripotent stem cell of claim 1, wherein when the isolated pluripotent stem cell is a female isolated pluripotent stem cell, it comprises two X chromosomes, one of which is a normally activated X chromosome (Xa), and the other is a normally silent X chromosome (Xi).

20-21. (canceled)

22. A method for producing the isolated pluripotent stem cell of claim 1, comprising inhibiting epithelium-to-mesenchymal transition of an pluripotent stem cell pre-gastrulation, wherein the pluripotent stem cell pre-gastrulation is derived from an ESC, an iPSC, an EpiSC, a blastocyst inner cell mass, an epiblast cell, or an early embryo from post-implantation to pre- or post-gastrulation.

23. The method of claim 22, comprising carrying out three-dimensional culture of the pluripotent stem cell pre-gastrulation.

24. The method of claim 23, wherein the three-dimensional culture is carried out in a formation medium, wherein the formation medium comprises a basal medium supplemented with a serum substitute, Activin A, bFGF, and a Wnt/β-catenin signal transduction inhibitor.

25. The method of claim 24, wherein: (i) the serum substitute is selected from the group consisting of: KnockOut™ SR, N-2, B-27, Physiologix™ XF SR, StemSure™ serum substitute supplement, and any combination thereof; (ii) the Wnt/β-catenin signal transduction inhibitor is selected from the group consisting of IWP-2, XAV939, Wnt-059, IWP-L6, IWR-1-endo, ICG-001, KY1220, and iCRT14; and (iii) the basal medium is selected from the group consisting of KnockOut DMEM, KnockOut DMEM/F12, DMEM, DMEM/F12, neurobasal medium, and any combination thereof.

26. The method of claim 24, wherein: (i) the serum substitute is in the amount of 0.1-20% (v/v); (ii) Activin A is in the amount of 1-100 ng/mL; (iii) bFGF is in the amount of 1-100 ng/mL; and (iv) the Wnt/β-catenin signal transduction inhibitor is in the amount of 1-50 μM.

27. The method of claim 23, comprising culturing the pluripotent stem cell pre-gastrulation in the presence of a three-dimensional scaffold to obtain the three-dimensional culture.

28. The method of claim 27, wherein the three-dimensional scaffold is hydrogel.

29. The method of claim 28, wherein the method comprises the steps of: (1) mixing a single cell suspension of the pluripotent stem cell pre-gastrulation with the hydrogel to obtain a gel-like mixture embedded with the pluripotent stem cell pre-gastrulation; and (2) cultivating the gel-like mixture in step (1) in a formation medium to obtain the three-dimensional culture, wherein the formation medium comprises a basal medium supplemented with a serum substitute, Activin A, bFGF, and a Wnt/β-catenin signal transduction inhibitor.

30. The method of claim 23, wherein the method further comprises isolation of the cultured cells from the three-dimensional culture, thereby obtaining the isolated pluripotent stem cell.

31. The method of claim 30, wherein the method further comprises passage of the obtained isolated pluripotent stem cells.

32. A pharmaceutical composition comprising the isolated pluripotent stem cell of claim 1 and a pharmaceutically acceptable carrier.

33. A method for in vitro production of an endoderm cell, an ectoderm cell a mesoderm cell, a tissue, or an organ, comprising: culturing the isolated pluripotent stem cell of claim 1 under a condition that allows differentiation of the isolated pluripotent stem cell.

34. The method of claim 33 preferably, wherein the in vitro produced cell is selected from the group consisting of a germ cell, an epithelial cell, a connective tissue cell, a nerve cell, an adipocyte, a pancreatic cell, a liver cell, a renal cell, a bone cell, a hematopoietic cell, an endothelial cell, a retinal cell, and a muscle cell.

35. A disease model or a drug screening model comprising the isolated pluripotent stem cell of claim 1.

36. A method for regeneration or repair of a tissue or an organ, or for transplantation of a cell, a tissue, or an organ in a subject, comprising: i) administering the isolated pluripotent stem cell of claim 1, the to the subject; or ii) inducing in vitro differentiation and/or proliferation of the isolated pluripotent stem cell of claim 1, and administering the differentiated and/or proliferated pluripotent stem cell to the subject.

37. (canceled)

38. The isolated pluripotent stem cell of claim 1, wherein the promoter region of one or more genes selected from the group consisting of OTX2, ZIC5, UTF1, FGF5, ZFP13, ZSCAN10, ZIC2, and ESRP1 in the isolated pluripotent stem cell has increased H3K4me3 level and/or decreased H3K27me3 level compared to the promoter region of the corresponding gene in an ESC.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0150] FIGS. 1A-1J show the identification results of the basic characteristics of fPSCs in Example 2. (A) Morphology of mouse naive embryonic stem cells (mESCs, the figure shows 2i/lif is the mESCs cultured under the condition of 2i/lif), mouse formative pluripotent stem cell line (fPSCs, in the figure it is shown as mfPSC) and epiblast stem cells (EpiSCs). (B) Immunofluorescent co-staining results of pluripotent stem cell markers OCT4, SOX2, and NANOG with polar molecular marker EZRIN or cytoskeletal molecule ACTIN; pluripotent factor OCT4 is highly expressed in fPSCs, SOX2 and NANOG is expressed in a mosaic style. (C) Western blotting shows the expression of various marker molecules in mESCs (two conditions of 2i/Lif and serum S/Lif), epiblast-like cells (EpiLCs, a transient epiblast-like cell), fPSCs and EpiSCs. (D) Quantitative RT-PCR results show comparison of mRNA expression levels of putative formative markers in mESCs, fPSCs, and EpiSCs; fPSCs highly express the formative pluripotent markers of OTX2, DNMT3B, SOX4, and FGF5 relative to mESCs, EpiLCs, and EpiSCs. (E) Immunofluorescence results of co-staining of formative markers OTX2 and Cadherin in fPSCs; it shows that fPSCs highly express formative pluripotent factor OTX2 and E-cadherin. (F) H3K27me3 immunofluorescence results showed that one of the two chromosomes in female fPSCs was inactivated. (G) An fPSCs cell line in which OCT4 distal enhancer drives GFP expression shows that, unlike mESCs where OCT4 utilizes a distal enhancer, OCT4 in fPSCs utilizes a proximal enhancer. (H) Immunofluorescence staining results of pluripotent factors OCT4, SOX2 and NANOG and polar molecule EZRIN in human formative pluripotent stem cells (hfPSCs); it shows that hfPSCs highly express pluripotent factors OCT4, SOX2 and NANOG, and staining results of EZRIN show that hfPSC has a polar rosette structure. (1) Immunofluorescent staining results of the expression of the formative marker OTX2 in hfPSCs shows that hfPSCs highly express the formative pluripotent factor OTX2. FIG. 1J shows the pluripotent marker OCT4 staining results of fPSCs established from the inner cell mass extracted from mouse E3.5 blastocysts, showing that the fPSCs established from the inner cell mass extracted from mouse E3.5 blastocysts have a rosette structure.

[0151] FIGS. 2A-2H show the results of genome-level characteristic analysis of mouse fPSCs in Example 3. (A) PCA analysis reveals that the genome-wide expression of fPSCs (shown as E5.5-fPSCs and E6.5-fPSCs in the figure) was similar to that of E6.0-E6.5 epiblast cells in vivo, but significantly different from mESCs and EpiSCs. (B) Genome-wide spatiotemporal transcriptome analysis reveals that fPSCs are most similar to E6.0 mouse epiblast cells and part of E6.5 mouse epiblast cells. (C) Differential gene analysis reveals that fPSCs specifically highly express 535 genes, including previously predicted formative genes and many genes that have important functions in ESCs leaving the native state, and these genes are marker molecules of fPSCs. (D) Quantitative RT-PCR verification of marker molecules of fPSCs, ZIC2, ZIC5, LIN28B, ESRP1, UTF1, ETV1, ETV4, GRHL2, CLDN6, CLDN7 are highly expressed in fPSCs, these are marker molecules of fPSCs. (E) Compared with naive mESCs, formative genes MIXL1, EOMES, TCFL5, and WNT3 are highly expressed in mouse fPSCs. The values are transcriptome FPKM values. (F-H) shows the results of single-cell sequencing analysis of mouse fPSCs and the results of integrated analysis with single-cell sequencing data of various other types of pluripotent cells. FIG. F shows that fPSCs single cells derived from 4 independent clones (shown as clone-1 #, clone-2 #, clone-3 #, clone-4 # in the figure) are relatively homogenous at the transcriptome level, with no obvious grouping phenomenon. In FIG. (G), the combined analysis of single-cell sequencing results of multiple types of pluripotent cells further confirmed that naive ESCs had no obvious grouping phenomenon at the single-cell level, while primed EpiSCs were divided into two subpopulations, which is consistent with the reported results: FS cells were clearly divided into 3 cell subpopulations, namely subpopulations 3, 6 and 7 in FIG. H, compared to fPSCs without obvious grouping. fPSC grouping is marked by dashed lines. log 2FPKM refers to the base 2 logarithm value after normalizing the gene expression value FPKM.

[0152] FIGS. 3A-3J show the results of characteristic analysis of mouse fPSCs in Example 4 at the epigenetic level. (A) Distribution of H3K4me3 and H3K27me3 epigenetic modifications on promoters (+2.5 Kb) at the genome-wide level in various pluripotent cells, showing that the epigenetic modifications in the promoter region of fPSCs are similar to those of E6.5 epiblast cells (shown as E6.5 epi in the figure) isolated in vivo. (B) Distribution of H3K4me3 on promoters (20 Kb of upstream or downstream, shown as −10 kb and +10 kb in the figure) at the genome-wide level in super bivalent genes, housekeeping genes and bivalent genes, showing that fPSCs were most similar to E6.5 embryonic epiblast cells isolated in vivo in super bivalent genes. Wherein fPSCs are marked by a dotted line. (C) super bivalent genes identified in all stem cells, showing that the number of super bivalent genes in fPSCs (n=329) is significantly higher than that of other in vitro stem cell lines (n=68-197), which is the closest to the in vivo E6.5 epiblast cells (n=440). (E-F) Comparison of methylation levels at the genome-wide level, the results show that compared with other cells, fPSCs are closest to the in vivo E6.5 epiblast cells (shown as E6.5 epi in the Figure)(the overall discrimination is limited, and probably no good comparison method has yet been found, or DNA methylation itself is not necessarily a good way to compare these cells). (G) The epigenetic modification of H3K4me3 and H3K27me3 at the position of formative gene OXT2 and ZIC5 shows that fPSCs are closer to E6.5 epiblast cells. (H) Epigenetic modification of three germ layer cell lineage marker molecule (mostly super bivalent genes), especially H3K3me3 in the basic presence or absence relationship in the promoter region (continuous strong H3K4me3 signal) shows that fPSCs are closer to E6.5 epiblast cells. (1) DNA methylation and H3K4me3/H3K27me3 enrichment at the promoter of super bivalent/covalent modification genes (UTF1, FGF5, ZFP13, ZSCAN10, ZIC2, ESRP1, T, GATA6, LHX5, HAND1, ISL1, NKX2-5, NR4A2, PDX1) in mESC, EpiLC, EpiSC, fPSC and E6.5 epiblast cells, the results show that compared with other cells, fPSCs are closest to in vivo E6.5 epiblast cells. (J) Super bivalent gene list of fPSCs. log 2RPKM refers to the base 2 logarithm value after normalizing the gene expression value RPKM; TSS refers to the transcription start site; HK.ave refers to the average normalized RPKM value of the housekeeping gene; R refers to the correlation coefficient; mCG/CG refers to the ratio of methylated level to unmethylated level in genome.

[0153] FIGS. 4A-4J show the detection results of in vitro differentiation of mouse fPSCs in Example 5. (A) Results of differentiation of fPSCs into primordial germ cell-like cell (PGCLCs) at 8th (P8), 15th, and 20th passages, BV (green) and SC (blue) showed successful differentiation. (B) The differentiation efficiency of fPSCs into potentially functional PGCLCs (SSEA1 and CD61 double positive cells) was 11%, which was close to that of the unstable epiblast-like cells (EpiLCs) with the best known differentiation efficiency (12.3%). (C) The differentiation efficiency of fPSCs to neural precursor cells (SOX1 positive) (indicated by GFP fluorescence in the Figure) was as high as 94% on the second day, which was significantly higher than 72.6% of EpiLCs; and compared with the ESCs monolayer differentiation system (68%), not only the efficiency is high, but also the time is much shorter (5-6 days), and it is more synchronized. (D) After 2 days of differentiation, the neural precursor cells derived from fPSCs can form neuron-like cells (multiple nerve cell marker molecules ZO1, Nestin, Tuj1). (E) The differentiation efficiency of fPSCs to endoderm precursor cells (FOXA2) reached 88%. (F) The endodermal precursor cells derived from fPSCs can differentiate into liver-like cells after 7 days (AFP is a liver cell marker). (G) The differentiation efficiency of fPSCs to mesodermal cells (the proportion of beating cardiomyocyte like cells, i.e., the proportion of beating clones) reached 90%, which was significantly higher than that of ESCs differentiated by commonly used EB (˜60%). (H) Cardiomyocyte-like cells derived from fPSCs express proliferation marker Ki67, and simultaneously express cardiomyocyte markers (MLC, α-actinin, cTnT and gATA4); in addition, sarcomeres are clearly visible. (I-J) After 3-4 days of maturation of fPSCs-derived cardiomyocyte-like cells, calcium ion release (1) and electrophysiological activity (J) can be detected: at the same time, action potentials (AP), typical ventricle-like traces and atrium-like AP were observed in the cardiomyocyte-like cells differentiated from fPSCs after 7-8 days of differentiation.

[0154] FIGS. 5A-5D. FIG. 5A shows that the genome-wide expression pattern of hfPSCs is between H9_HT (original state hESC (naive hESC) and H9 KSR (primed hESC)), the pluripotent state is an activated state (a formative state), and is quite different from FS (Smith et al., 2020) and FTW_hiPSC (Wu et al., 2020). FIG. 5B shows that hfPSCs specifically highly expressed genes; there are 84 formative genes in the intersection of hfPSCs and fPSCs, and these genes are conserved marker molecules shared by fPSCs and hfPSCs. FIG. 5C shows that the high level of cholesterol, sterol metabolism and fat metabolism of hfPSCs is unique to the activated state (the formative state) through cluster analysis of biological processes of genes highly expressed in hfPSCs. FIG. 5D shows a list of 84 formative genes in the intersection of hfPSC formative genes and fPSC formative genes, and these genes are conserved marker molecules shared by fPSCs and hfPSCs. log 2TPM refers to the base 2 logarithm value after normalizing the gene expression value RPKM.

[0155] FIGS. 6A-6C shows the number of super bivalent genes (n=174) identified in hfPSC (shown as 3D_hfPSC in the figure), where all genes in quadrant I are super bivalent genes, and the genes in quadrants 11, III and IV are non-super bivalent genes; there are 31 super bivalent genes in the intersection of hfPSC super bivalent genes and fPSC super bivalent genes. FIG. 6B shows H3K4me3 and H3K27me3 super covalent modification patterns of H3K4me3 and H3K27me3 at the promoter regions of HOXA cluster, FOXA2, PAX6, FOXF2, GDNF, IRX1, MSX1, NFIB, NKX3-2, NRN1, PAX5, PTGER4, PITX3, SLIT2, TBX1, VLDLR, EVX1, PHOX2B genes in hfPSCs (H9hfpscs). FIG. 6C shows a list of 31 super bivalent genes shared by the intersection of hfPSCs and fPSCs.

DETAIL DESCRIPTION OF THE PRESENT APPLICATION

[0156] The present application will now be described with reference to the following examples, which are intended to illustrate the present application, (but not to limit it).

[0157] Unless otherwise specified, the molecular biology experiment methods and immunoassay methods used in the present application are basically with reference to the methods described in J. Sambrook et al., Molecular Cloning: A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, 1989, and F. M. Ausubel et al., Short Protocols in Molecular Biology, 3rd Edition, John Wiley & Sons, Inc., 1995; the use of restriction endonucleases is in accordance with the conditions recommended by the product manufacturer. Those skilled in the art understand that the Examples describe the present application by way of example and are not intended to limit the scope of the present application.

[0158] The source of the main reagent involved in the following examples is as follows:

[0159] 1. Reagents for cell culture of mESCs, iPSC and EpiSCs:

[0160] Feeder-free dish (Millipore, #901771), N2B27 medium with MPD035901 (LC Laboratories, #P-9688), Chir99021 (LC Laboratories, #C-6556), hLIF (human leukemia inhibitory factor, Millipore, #ESG1107). Mitomycin C (Sigma-Aldrich, #M0503), KSR (Gibco, #10828028), Activin A (PeproTech, #100-18B), bFGF (R&D Systems, #233-FB), XAV939 (Sigma-Aldrich, #X3004). Collagenase IV (Gibco, #17104-019), trypsin (Gibco, #12605010) Y-27632 (TOCRIS, #1254).

[0161] 2. Reagents for immunofluorescence staining analysis:

[0162] Paraformaldehyde (PFA) (Sigma-Aldrich, #158127), Triton X-100 (Amresco, #0694), BSA (Sigma-Aldrich, #A4378), Hoechst 33342 (Invitrogen, #H3570). Primary antibodies included: rabbit anti-Ezrin (Abcam, #Ab76247), mouse anti-GM130 (BD Biosciences, #BD610822), rat anti-Nestin (Millipore, #MAB353), rabbit anti-Zo-1 (Abcam. #Ab214228), rabbit anti-Tuj1 (Sigma-Aldrich, #T2200), goat anti-Oct4 (Santa Cruz Biotechnology, #sc-8628), rabbit anti-Nanog (Abcam; #ab80892), mouse anti-Sox2 (CST; #4900S), rabbit anti-Ki-67 (Abcam, #ab15580), mouse anti-cardiac troponin T (cTnT, Thermo Fisher Scientific, #MS-295-P0), mouse anti-α-actin (Sigma-Aldrich, #A7732), rabbit anti-myosin light chain (MLC) 2v (ProteinTech Group, #10906-1-AP), rabbit anti-Foxa2 (CST, #8186S), goat anti-Gata6 (R&D systems, #AF1700), goat anti-Otx2 (R&D systems, #AF1979), goat anti-Gata4 (Santa Cruz Biotechnology, #sc-1237), rabbit anti-E-cadherin (Santa Cruz Biotechnology, #sc-7870), mouse anti-α-SMA (Santa Cruz Biotechnology, #sc-53142), mouse anti-GFAP (Sigma-Aldrich, #G3893), mouse anti-AFP (CST, #3903S). Secondary antibodies were from Jackson ImmunoReseach and included: 488 AffiniPure donkey anti-rabbit IgG (H+L) (#711-545-152), AlexaFluor@594 AffiniPure donkey anti-rabbit IgG (H+L)(#711-585-152), Cy™5 AffiniPure donkey anti-rabbit IgG (H+L) (#711-175-152), AlexaFluor@488 AffiniPure donkey anti-mouse IgG (H+L) (#711-545-150), AlexaFluor@647 AffiniPure donkey anti-mouse IgG (H+L) (#715-605-151), AlexaFluor®594 AffiniPure donkey anti-mouse IgG (H+L) (#715-585-150), AlexaFluor®488 AffiniPure donkey anti-goat IgG (H+L) (#705-545-003), AlexaFluor®488-AffiniPure donkey anti-mouse IgG (H+L) (#712-545-153).

[0163] 3. Reagents used in quantitative RT-PCR:

[0164] RNAzol (Mrcgene, #RN190), PrimeScript RT Reagent Kit (TaKaRa, #RR037A), EvaGreen 2qPCR MasterMix (ABM, MasterMix-S).

[0165] 4. Reagents for western blot analysis:

[0166] SuperSignal™ West Femto Maximum Sensitivity Substrate (Thermo Fisher Scientific, #34095). Primary antibodies used included: mouse anti-β-actin (Yeasen, #30101 ES50), goat anti-Oct4 (Santa Cruz Biotechnology, #sc-8628), mouse anti-Sox2 (CST, #4900S), rabbit anti-Nanog (Abcam, #ab80892), goat anti-T (CST, #81694), goat anti-Otx2 (R&D Systems, #AF1979), rabbit anti-Foxa2 (CST, #8186S), rabbit anti-E-cadherin (Santa Cruz Biotechnology, #sc-7870), mouse anti-N-cadherin (BD, #610920). Secondary antibodies were from Jackson ImmunoResearch and included: peroxidase AffiniPure goat anti-rabbit IgG (#111-035-003), peroxidase AffiniPure goat anti-mouse IgG (#115-035-003), peroxidase and rabbit anti-goat IgG (#305-035-003).

[0167] 5. Alkaline phosphatase staining: BCIP/NBT Alkaline Phosphatase Color Development Kit (Beyotime, #C3206).

[0168] 6. Induction of PGCLC and three germ layers:

[0169] G-MEM medium (1) (Gibco, #11710-035). sodium pyruvate (Gibco, #11360070), non-essential amino acids (Gibco, #11140050), b-mercaptoethanol (Invitrogen, #21985-023), L-glutamine (Gibco, #35050061), streptomycin (Gibco, #15140122), BMP4 (R&D Systems. #315-27), SCF (R&D Systems. #455-MC-010), BMP8a (R&D Systems, #1073-BP-010), EGF (R&D Systems, #2028-EG), RPMI medium 1640(1) (Gibco, #11875-093), VC (Vitamin C, Sigma-Aldrich, #A7506), B27 supplement minus insulin (Gibco, #A1895601), dexamethasone (Sigma-Aldrich, #D4902), HGF (PeproTech, #100-39), oncostatin M (R&D Systems, #295-OM).

[0170] 7. Electrophysiology and intracellular Ca.sup.2+ measurement:

[0171] Fluo-3AM (Beyotime, #S1056).

[0172] 8. Chimera experiments of fPSC and culture of embryos after implantation:

[0173] M2 medium (Sigma-Aldrich, #M7167), FBS (Millipore, #ES 009B), CMRL 1066 medium (Gibco, #11530037), sodium pyruvate (Invitrogen, #11360070), glutamine (Invitrogen #25030081), N2 (Invitrogen, #17502048), B27 (Invitrogen, #17504044).

[0174] 9. Antibodies used in STAR ChIP-seq: H3K4me3 (in-house), H3K27me3 (Diagenode, pAb-069-050), rSAP (NEB, #M0371).

[0175] The sources of known pluripotent stem cells involved in the following Examples are as follows:

[0176] 1. Mouse naive embryonic stem cells (mESCs):

[0177] Purchased from Millipore Corporation.

[0178] 2. Epiblast stem cells (EpiSCs):

[0179] Prepared according to literature (Brons I G, Smithers L E, Trotter M W et al. Derivation of pluripotent epiblast stem cells from mammalian embryos. Nature 2007; 448:191-195).

[0180] 3. Epiblast-like cells (EpiLCs, a kind of transiently differentiated cells):

[0181] Prepared according to literature (Hayashi K, Ohta H, Kurimoto K, Aramaki S, Saitou M. Reconstitution of the mouse germ cell specification pathway in culture by pluripotent stem cells).

[0182] 4. Rosette-like stem cells (RSCs):

[0183] Data were obtained according to literature (Neagu A. van Genderen E, Escudero I et al. In vitro capture and characterization of embryonic rosette-stage pluripotency between naive and primed states. Nature Cell Biology 2020).

Example 1: Establishment of Formative Pluripotent Stem Cells (fPSCs) and Human Formative Pluripotent Stem Cells (hfPSCs)

[0184] 1. Transformation from mESCs to fPSCs or from hESCs to hfPSCs:

[0185] Preparation of fPSCs (hfPSC) medium: basal medium (KnockOut DMEM, KnockOut DMEM/F12, DMEM, DMEM/F12, neurobasal medium, or any combination thereof) supplemented with N-2 and B-27, KSR, Activin A, bFGF, a Wnt/β-catenin signal transduction inhibitor. [0186] 1) mESCs (hESCs) cell suspension was obtained; [0187] 2) mESCs (hESCs) and extracellular matrix (such as collagen) were mixed evenly and inoculated into petri dish; [0188] 3) fPSCs (hfPSCs) medium was added into the petri dish and cultured in an incubator at 37° C. to achieve the conversion of mESCs (hESCs) to fPSCs (hfPSCs).

[0189] 2. Establish fPSCs from epiblast cells before gastrulation in vivo: [0190] 1) According to the known EpiSCs establishment method (Brons I G, Smithers L E, Trotter M W et al. Derivation of pluripotent epiblast stem cells from mammalian embryos. Nature 2007; 448:191-195.) E5.5-6.5 epiblast cells were isolated from mouse embryos before and after gastrulation. [0191] 2) Epiblast cells were resuspended in fPSCs medium, mixed with extracellular matrix (such as collagen), and inoculated in petri dishes. [0192] 3) After 3-5 days, the fPSCs could be digested and passaged after forming clones.

[0193] 3. Passage of IPSCs (hfPSCs):

[0194] Medium for passage of fPSCs (hfPSC): basal medium (KnockOut DMEM, KnockOut DMEM/F12, DMEM, DMEM/F12, neurobasal medium, or any combination thereof) supplemented with N-2 and B-27, KSR, Activin A, bFGF, a Wnt/β-catenin signal transduction inhibitor. [0195] (1) Before passage of fPSCs (hfPSC), the petri dishes were coated with extracellular matrix; [0196] (2) The cells and extracellular matrix were transferred to a centrifuge tube; [0197] (3) fPSCs (hfPSCs) were collected after centrifugation; [0198] (4) The supernatant was sucked using a vacuum pump; [0199] (5) The fPSCs (hfPSCs) were digested into single cells; [0200] (6) After digestion, the fPSCs (hfPSCs) were collected by centrifugation. [0201] (8) The fPSCs (hfPSCs) were mixed with extracellular matrix (ECM) and inoculated into coated petri dishes. [0202] (9) The fPSCs (hfPSCs) medium was added into the petri dishes and cultured in an incubator at 37° C. for passage.

[0203] 3) Establish fPSCs from the inner cell mass [0204] (1) ICM was extracted from mouse E3.5 blastocysts by immunosurgery. [0205] (2) Refer to the process of establishing fPSCs from epiblast cells.

[0206] E5.5-6.5 epiblast cells were isolated from mouse embryos.

[0207] 2) Epiblast cells were resuspended with fPSCs medium, mixed with extracellular matrix (such as collagen), and inoculated in petri dishes. [0208] 3) After 3-5 days, the fPSCs could be digested and passaged after forming clones.

[0209] 3. Passage of fPSCs:

[0210] Medium for passage of fPSCs: basal medium (KnockOut DMEM, KnockOut DMEM/F12, DMEM, DMEM/F12, neurobasal medium, or any combination thereof) supplemented with N-2 and B-27, KSR, Activin A, bFGF, a Wnt/ft-catenin signal transduction inhibitor. [0211] (1) Before passage of fPSCs, the petri dishes were coated with extracellular matrix; [0212] (2) The cells and extracellular matrix were transferred to a centrifuge tube; [0213] (3) The iPSCs were collected after centrifugation; [0214] (4) The supernatant was sucked using a vacuum pump; [0215] (5) The fPSCs were digested into single cells; [0216] (6) After digestion, the fPSCs were collected by centrifugation. [0217] (7) The fPSCs were mixed with extracellular matrix (ECM) and inoculated into coated petri dishes. [0218] (8) The fPSCs medium were added into the petri dishes and cultured in an incubator at 37° C. for passage.

Example 2: Identification of Basic Characteristics of fPSCs

[0219] In this example, the basic characteristics of the fPSCs prepared in Example 1 were identified.

[0220] The morphology of fPSCs, mouse naive embryonic stem cells (mESCs) and epiblast stem cells (EpiSCs) prepared in Example 1 is shown in FIG. 1A, wherein fPSCs could form spherical, rosette clones with smooth margins, clearly distinguishing them from the smaller domed clones of mESCs and the larger flat clones of EpiSCs.

[0221] The results of immunofluorescence staining of pluripotent stem cell markers OCT4, SOX2 and NANOG are shown in FIG. 1B, the immunoblot results of the expression of various marker molecules in mESCs (under the two conditions of 2i/Lif and serum/Lif), epiblast-like cells (EpiLCs, a kind of transiently differentiated cells). fPSCs and EpiLCs are shown in FIG. 1C, wherein, unlike mESCs and EpiSCs, fPSCs specifically highly expressed the putative activation state marker Otx2. Quantitative RT-PCR results of mRNA expression levels of conjectured formative markers in mESCs, fPSCs and EpiSCs are shown in FIG. 1D, fPSCs also highly expressed possible formative markers DNMT3B, SOX4 and FGF5. In addition, FIG. 1E also shows the immunofluorescence results of co-staining of the conjectured formative OTX2 and cadherin in fPSCs. The above results show that the fPSCs prepared in Example 1 possess the pluripotency and immortality.

[0222] The epigenetic modification state of fPSCs was further analyzed. To detect X chromosome state, female fPSCs were established from female mESCs and stained for H3K27me3 (Plath, K., J. Fang, S. K. Mlynarczyk-Evans, R. Cao, K. A. Worringer, H. Wang, C. C. de la Cruz, A. P. Otte, B. Panning, and Y. Zhang. 2003. ‘Role of histone H3 lysine 27 methylation in X inactivation’, Science, 300:131-5), the results are shown in FIG. 1F. H3K27me3 immunofluorescence results show that one of the two chromosomes in female fPSCs cells is in inactivated state. The utilization of Oct4 enhancers by fPSC was evaluated through the Oc14-ΔPE-GEP reporting system (Illich D J, Zhang M, Ursu A et al. Distinct Signaling Requirements for the Establishment of ESC Pluripotency in Late-Stage EpiSCs. Cell Rep2016; 15: 787-800) and the results are shown in FIG. 1G, the Oct4-GFP promoter-marked fPSCs cell line showed that, unlike OCT4 utilizes a distal enhancer in mESCs, OCT4 in fPSCs utilizes a proximal enhancer.

[0223] The morphology of the hfPSC prepared in Example 1 is shown in FIGS. 1H and 1G. The hfPSC could form spherical and rosette clones with smooth margins. The results of immunofluorescent staining of pluripotent factors OCT4, SOX2 and NANOG and polar molecule EZRIN in human formative pluripotent stem cells (hfPSCs) are shown in FIG. 1H. The immunofluorescent staining results of the expression of the formative marker Otx2 in hfPSCs are shown in FIG. 1I.

Example 3: Characteristic Analysis of fPSCs at Genome Level

[0224] 1) Sequencing: After the total RNA was extracted using the Trizol kit, it was treated with DNase I to remove residual genomic DNA. The NanoPhotometer spectrophotometer instrument, and the RNA analysis kit (Qubit RNA Assay Kit) of Qubit® 2.0 Fluorometer and the RNA analysis kit (RNA 6000 Nano Kit) of the Bioanalyzer 2100 biochip analysis system were used to analyze the purity, concentration and integrity of the extracted RNA. After the quality of the sample is qualified, an average of 2 g of total RNA was prepared per sample for cDNA library construction using the NEBNext® Ultram RNA Library Prep Kit. Then, Illumina Hi-Seq 2500 was used for sequencing analysis, and each sample generated 10G of raw data.

[0225] 2) RNA-seq data processing: first, the reads obtained by sequencing were mapped to the mouse mm9 genome through TopHat v2.0.11 (Trapnell et al., 2009). Only reads that could not be mapped to the genome were removed. Then, based on the genome annotation information provided by refFlat, cufflinks v2.0.2 (Trapnell et al., 2009) was used to calculate the gene expression level (Fragments per kilobase per million of sequenced reads, FPKM). Data visualization was achieved by using online software provided by UCSC University (http://genome.ucsc.edu/).

[0226] In order to detect the gene expression of the whole genome, RNA sequencing was performed on different passages of fPSCs (P1, P10, P20, P30). The results of PCA analysis are shown in FIG. 2A. The results showed that genome-wide expression of fPSCs was similar to that of E6.25-E6.5 epiblast cells in vivo, but significantly different from mESCs and EpiSCs. The results of genome-wide spatiotemporal transcriptome analysis are shown in FIG. 2B, and the results showed that fPSCs were most similar to E6.0 epiblast cells and part of E6.5 epiblast cells.

[0227] In order to obtain specific genes that can distinguish fPSCs from mESCs and EpiSCs, these cells were analyzed for differential gene expression. The results are shown in FIG. 2C, fPSCs specifically highly expressed 535 genes, including previously predicted formative genes (e.g., DNMT3B, OTX2, FGF5) and many genes that have important functions in ESCs leaving the naive state, e.g., transcription factors: ZIC2/5, ETV1/4, GRH1/2, pluripotent regulators: LIN28B, UTF1, and epithelial regulators: ESRP1, CLDN6/7, etc. These genes may be marker molecules for fPSCs. The possible marker molecules of the above fPSCs were subsequently verified by quantitative RT-PCR, and the results were shown in FIG. 2D and FIG. 2E. FIG. 2F showed transcriptome-wide association analysis of the differentiation of mESCs, EpiLCs, EpiSCs, P1fPSCs (first generation fPSCs), P10 fPSCs, P20 fPSCs, P30 fPSCs, P10 from E5.5 epiblast cells to fPSCs, P10 from E6.5 epiblast cells to fPSCs and E6. 5 epiblast cells.

Example 4: Characteristic Analysis of IPSCs at the Epigenetic Level

[0228] 4.1 Analysis of Epigenetic Modifications (H3K4Me3 and H3K27Me3)

[0229] 1) STAR ChIP-seq library construction and sequencing of H3K4me3 and H3K27me3

[0230] (1) the embryo and cell samples were transferred to 0.2 mL RNase- and DNase-free Ep tubes, and the droplet size should not exceed 0.5 μL.

[0231] (2) 48 μL of embryo lysate was taken, 2 μL of protease inhibitor (50×, Roche, Cat. No. 04693116001) was added and mixed well. 20 μl of mixed lysate was added to each reaction system, after gently pipetting using a pipette until sample lysed, and placed on ice.

[0232] (3) MNase was diluted to 0.01U with MNase working solution, 2 μL prepared MNase and 9 μL 2×MNase working solution was added to the previous step reaction, and incubated at 37° C. for 5 minutes. Note that the reaction time should not be too long or too short, so as to avoid excessive digestion of chromatin by MNase or incomplete digestion reaction.

[0233] (4) 5 μL of 10×MNase stop buffer was added, pipetted gently and placed on ice.

[0234] (5) 2 μL of 10× protease inhibitor and 45 μL of pre-cooled 2×RIPA modified buffer on ice were added to the reaction in the previous step. Centrifuge at 13000 rpm at 4° C. for 10 minutes.

[0235] (6) The supernatant was transferred to a new 0.2 mL Ep tube, avoiding aspiration of pellets or cell debris. Then 50 μL of RIPA buffer was added.

[0236] (7) 1 μg primary antibody (self-made H3K4me3 antibody (Zhang B, Zheng H, Huang B et al. Allelic reprogramming of the histone modification H3K4me3 in early mammalian development. Nature 2016; 537:553-557); H3K27me3 antibody from Diagenode, Cat. No. C15410069) was added, and mixed by inverting overnight at 4° C.

[0237] (8) The next day, Protein A magnetic beads (Invitrogen, Cat. No. 10001D) were washed with RIPA buffer, and 100 μg of Protein A magnetic beads were added to each reaction and continued to mix by inverting at 4° C. for 3 hours.

[0238] (9) The supernatant was removed on a magnet, and washed 5 times with 150 μL RIPA buffer, 5 minutes each time, afterwards, washed once with 150 μL lithium chloride washing buffer and the supernatant was discarded.

[0239] (10) 27 μL of ddH.sub.2O, 1 μL of 10×ExTaq buffer and 1 μL of proteinase K (Roche, Cat. No. 10910000) were added, and digested with shaking at 55° C. for 90 minutes. After absorbing the supernatant on the magnet, it was treated at 80° C. for 40 minutes to inactivate proteinase K.

[0240] (11) the reaction product of the previous step was taken out, and 1 μL of shrimp alkaline phosphatase (rSAP) was added to repair the DNA ends, and was inactivated at 85° C. for 20 minutes after a hot bath at 37° C. for 1 hour.

[0241] (12) 0.5 μL dCTP (1 mM) was added, and react at 95° C. for 3 minutes to denature DNA, and placed on ice. Then 1 μL of terminal transferase (TDT) was added, mixed with shaking, centrifuged, and the mixture was reacted at 37° C. in a PCR instrument for 35 minutes to add the nucleotide C. Then 0.5 μL dATP (1 mM) was added and incubated at 37° C. for a further 5 minutes. After completion of the reaction, the TDT was inactivated at 80° C. for 20 minutes.

[0242] (13) the extension reaction system was prepared according to the table below.

TABLE-US-00001 TABLE 1-1 Extension reaction system Reagent Volume (μL) 5× KAPA 2G buffer A 12 dNTP (2.5 mM) 4.8 Biotin MP24-G9H (2 μM) 2 KAPA 2G polymerase 0.8 H.sub.2O 10.2 Total volume 29.8

[0243] (14) 29.8 μL of extension reaction system mixture was added to the product of the previous step and PCR amplification was performed as shown in the table below.

TABLE-US-00002 TABLE 1-2 Extension reaction program Cycle Temperature(° C.) Time(min) 1 95 3 2-17 47 1 68 2 18 72 10 19 4 Hold

[0244] (15) After the amplification, 6 μL of exonuclease I (Exo I) buffer and 2 μL of Exo I were added, reacted at 37° C. for 50 minutes, and the residual extension primers were digested. Then Exo I was inactivated at 80° C.

[0245] (16) 22 μL 4×BW buffer was added to the product of the previous step, added 10 μL of BW-resuspended Streptavidin C1 magnetic beads (Invitrogen, Cat. No. 65001), took 5 μL from each IP reaction, washed 3 times with 1×BW after sucking the residual liquid on magnet, and re-suspended to 10 μL). It was then transferred to a 1.5 mL Ep tube and shaken at 1100 rpm for 25 minutes at 23° C. to bind DNA to C1 magnetic beads.

[0246] (17) After removing the waste liquid on the magnet, 150 μL of 1×BW buffer was added to wash once, and then washed three times with EBT buffer. Then the DNA was eluted by using 8.8 μL EB buffer.

[0247] (18) The ligation reaction system was prepared as shown in the table below, 11.2 μL of the ligation reaction system mixture was added to the eluted DNA, and inverted overnight at 4° C.

TABLE-US-00003 TABLE 1-3 Ligation reaction system Reagent Volume(μL) 2× quick ligation buffer (NEB) 10 TA adapter (10 mM) 0.2 Quick ligase 1 Total volume 11.2

[0248] (19) The next day, the overnight ligation product was taken out, after 10 minutes at room temperature, the liquid was removed on a magnet, washed once with 1×BW and three times with EBT, then 30 μL H.sub.2O was added, shaken at 72° C. 1400 rpm for 30 minutes, and then the DNA product was eluted on a magnet.

[0249] (20) the amplification reaction system was prepared according to the table below.

TABLE-US-00004 TABLE 1-4 Amplification reaction system Reagent Volume(μL) 10× Ex Taq buffer 5 dNTP (2.5 mM) 5 P1_FL (20 μM) 0.7 Ex Taq HS polymerase 0.5 H.sub.2O 8.1 Index (20 μM) 0.7 Total volume 20

[0250] (21) 20 μL of the amplification system mixture was added to the DNA product of the previous step, and the PCR amplification reaction was carried out as shown in the table below.

TABLE-US-00005 TABLE 1-5 Amplification PCR program Cycle Temperature(° C.) Time(min) 1 95 3 2-17 95 30 s 58 30 s 70 1 18 72 7 19 4 Hold

[0251] (22) Ampure magnetic beads (Beckman, product number A63882) were used to purify DNA at a ratio of 1:1, and the concentration was measured using Qbit after dissolving in 40 μL H.sub.2O. Then sequencing was carried out on HiSeq2500 or XTen.

[0252] 2. STAR ChIP-seq data processing and identification of super bivalent genes Data processing: First, the reads (Reads) obtained by sequencing were mapped to the mouse mm9 genome by Bowtie2v2.2.2 (Langmead B, Salzberg S L. Fast gapped-read alignment with Bowtie 2. Nat Methods 2012; 9:357-359), the parameters were set to -t -q -N 1 -L 25. Second, reads that cannot be mapped to the genome, reads that can map to multiple positions, and duplicated reads caused by PCR were removed. After verifying the reproducibility between biological replicates, the data from different replicates were integrated. Afterwards, the whole genome was divided into 100 bp length sequence frames, the signal enrichment in each 100 bp region was counted, and the signal was normalized using RPKM (Reads per kilobase per million of sequenced reads). Data visualization was achieved by using online software provided by UCSC University (http://genome.ucsc.edu/).

[0253] Identification of super bivalent genes: refer to our previously published papers (Xiang Y, Zhang Y, Xu Q et al. Epigenomic analysis of gastrulation identifies a unique chromatin state for primed pluripotency. Nat Genet 2020; 52:95-105). Briefly, firstly, based on the published distribution of H3K4me3 and H3K27me3 in 27 different tissue samples, combined with gene expression, 3992 bivalent genes (co-labeled by H3K4me3 and H3K27me3 in at least half of the samples, and gene expression FPKM value <2) and 3181 housekeeping genes (expressed in all samples, FPKM>2) were screened out; secondly, the enrichment degree of H3K4me3 and H3K27me3 signals within the 10-kb range of the promoter region in each sample was calculated and normalized by using the Z-score method; finally, among all bivalent genes, super bivalent genes were screened according to H3K4me3 and H3K27me3 signals, and the screening criteria were: 1) the enrichment signal of H3K4me3 in the promoter region was stronger than that of housekeeping gene; 2) the promoter region has a strong H3K27me3 modification (RPKM>1 after normalization).

[0254] 3. DNA Methylation Sequencing and Analysis

[0255] 10.sup.7 cells were collected and sent to Novogene after quick frozen in liquid nitrogen, then performed DNA methylation library construction and genome-wide disulfide bond sequencing (30G data volume) according to the company's standard process. Using the BSseeker2 software developed by Michael Zhang's lab (Guo et al., 2013), reads were mapped to the mouse mm9 genome with the parameters --bt2-p 8 -XS 0.2, 3-a CCCCCC -m 4. Reads that could not be mapped to the genome, reads that could map to multiple locations, and duplicated reads caused by PCR removed. Afterwards, the methylation ratio of a single base C on the genome was calculated separately, and only the C sites that were detected to be more than 3 on the genome were retained, and the average DNA methylation degree per 100 bp of the whole genome is calculated.

[0256] 4. Results:

[0257] The results of epigenetic modification (H3K4me3 and H3K27me3) and DNA methylation sequencing and analysis are shown in FIG. 3. FIG. 3A shows the distribution of H3K4me3 and H3K27me3 epigenetic modifications on promoters (−2.5 Kb) at genome-wide level in various stem cells. Wherein, for fPSC, RSC, and E6.5 epiblast cells, the distribution of H3K27me3 in the promoter region of all genes is stronger than that of other pluripotent stem cells, but the distribution of H3K4me3 in the promoter region of all genes does not show obvious difference. FIG. 3B shows the distribution of H3K4me3 of the promoters (upstream or downstream 20 Kb) at the genome-wide level among super bivalent genes, housekeeping genes and bivalent genes. The results show that fPSCs were most similar to the epiblast cells isolated in vivo in terms of super bivalent genes. FIG. 3C shows the number of super bivalent genes identified in different pluripotent stem cells, and the results show that the number of super bivalent genes in fPSCs (334) was more than twice that of other in vitro stem cell lines (94-151), and was closest to in vivo E6.5 epiblast cells (440). 94% of the super bivalent genes identified in fPSCs were also identified in E6.5 epiblast cells, for example, super bivalency of Hoxa cluster, Evx1, Gata2, Pax6 were only found in fPSCs and E6.5 epiblast cells, but not in other pluripotent stem cells (FIG. 3D). FIGS. 3E-F show the detection results of methylation level at the genome-wide level, and the results showed that fPSCs were closest to in vivo E6.5 epiblast cells compared with other cells.

[0258] The epigenetic modification (H3K4me3 and H3K27me3) and DNA methylation of specific genes were further analyzed. FIGS. 3G-3I show the distribution of epigenetic modifications of formative genes and cell lineage marker molecules of the three germ layers. FIG. 3G shows DNA methylation and H3K4me3/H3K27me3 enrichment at promoters of formative maker (Otx2, Zic5). FIG. 3H shows DNA methylation and enrichment of H3K4me3 and H3K27me3 at the promoters of different germ layer makers (Pax6, Foxf1 and Foxa2) and Oct4. FIG. 3I shows DNA methylation and H3K4me3/H3K27me3 enrichment at promoters of Utf1, Fgf5, Zfp13, Zscan10, Zic2, Esrp1, T, Gata6, Lhx5, Hand1. Isl1, Nkx2-5, Nr4a2, Pdx1. FIG. 3M shows the list of super bivalent genes of fPSCs.

[0259] The results show that in fPSCs, the promoter regions of formative genes (Otx2, Zic5, Utf1, Fgf5, Zfp13, Zscan10, Zic2, Esrp1, etc.) all had higher levels of transcriptional activation markers H3K4me3 and lower levels of transcriptional repression markers H3K27me3, and low levels of DNA methylation, similar to E6.5 day epiblast cells; the promoter region of lineage-specific genes (Hand1, T, Foxa2, Nkx2-5, Pax6, PDX1, Isl1, Tcf21, Lhx5, Pax2, Dlx5, Nr4a2, etc.) has high levels of H3K4me3 and H3K27me3, and low levels of DNA methylation.

[0260] The above results showed that, compared with other pluripotent stem cells (including RSCs), the fPSC of the present application is closest to E6.5 day epiblast cells at the epigenetic level.

Example 5: Differentiation of fPSCs

[0261] 1. Differentiation of fPSCs to primordial germ cell-like cells (PGCLCs):

[0262] Preparation of PGCLCs medium: GMEM basal medium, 15% KSR. 1 mM pyruvic acid, 0.1 mM non-essential amino acids, 0.1 mM β-mercaptoethanol, 2 mM L-glutamine, 100 U/ml penicillin, 0.1 mg/ml streptomycin, BMP4: 500 ng/ml, SCF: 100 ng/ml, BMP8a: 500 ng/ml, EGF: 50 ng/ml, and hLIF: 1000u/ml; [0263] 1) The clones of fPSCs were collected and digested into single cells by TrypLE; [0264] 2) The cells were washed twice with 15% KSR medium, and after cell counting, the cells were resuspended and inoculated in a 96-well plate with PGCLCs medium at a density of 2000 cells/96-well plate.

[0265] 2. Differentiation of fPSCs to ectoderm: [0266] 1) When the continuously passaged fPSCs were cultured to the third day, the medium was replaced with N2B27 medium, and 6 μM PD0325901 was added thereto; [0267] 2) After 24 hours, PD0325901 was removed and the medium was replaced with N2B27 basal medium. After continuous culture for 24 hours, the neural precursor cells of Sox1-GFP could be obtained, and then replaced with fresh N2B27 medium every day; [0268] 3) Neural stem cells (Nestin+) could be obtained on the fourth day after differentiation, and terminally differentiated neurons (TujlV) could be obtained on the seventh day.

[0269] 3. Differentiation of fPSCs to mesoderm:

[0270] BM1 medium: RPMI 1640 medium; 4 mM Vitamin C, B27 supplement minus insulin; BM2 medium: 10 μM XAV939 was add on the basis of BM1; [0271] 1) When the continuously passaged fPSCs were cultured to the third day, the medium was replaced with EpiLCs medium, and cultured for 24 hours, so that the fPSCs underwent EMT, and the clones changed from a regular spherical shape to an irregular shape; [0272] 2) On the second day after induction of cardiomyocyte differentiation, the medium was replaced with BM1 medium; [0273] 3) On the third day of differentiation, replaced with BM2 medium; [0274] 4) After 24 hours, BM2 was replaced with BM1, and the clones began to beat one after another after replacement with BM1. On the fifth day of differentiation, more than 90% of the clones have started to beat.

[0275] 4. Differentiation of fPSCs to endoderm: [0276] 1) When the continuously passaged fPSCs were cultured to the third day, the medium was replaced with the EpiLCs medium, and cultured for two days; [0277] 2) After 48 hours, the medium was replaced with RPMI 1640 medium, B27 supplement minus insulin and 100 ng/ml Activin A were added to it to continue culturing for 2 days. At this time, a high proportion of Foxa2-positive primitive endoderm cells could be obtained; [0278] 3) Then, the medium was changed to fresh RPMI 1640 medium, and B27 supplement, 10 ng/ml bFGF and 20 ng/ml BMP4 were added to it, the cells were transferred to a low-oxygen (5% CO.sub.2/4% O.sub.2) environment and cultured for 2 days, replacing with fresh medium every day; [0279] 4) For the next 5 days, the medium was replaced with fresh RPMI 1640 medium, to which 2.5% fetal bovine serum (ES-009-B) and B27 supplement, as well as 100 nM Dexamethasone, 10 ng/ml HGF, and 20 ng/ml Oncostatin M were added, and the culture was continued in a hypoxic environment. Thus, liver precursor cells with positive AFP antibody staining were obtained.

[0280] 5. Results

[0281] To investigate the ability of fPSCs to differentiate into PGCLCs, BVSC fPSCs were constructed from BVSC mESCs according to literature (Hayashi K, Ohta H, Kurimoto K, Aramaki S, Saitou M. Reconstitution of the mouse germ cell specification pathway in culture by pluripotent stem cells. Cell 2011; 146:519-532., Ohinata Y, Payer B, O'Carroll D et al. Blimp1 is a critical determinant of the germ cell lineage in mice. Nature 2005; 436:207-213), all of which were negative for BVSCs, subsequently, BVSC fPSCs were induced to differentiate into PGCLCs by the method described in 1 above, and unstable epiblast-like cells (EpiLCs) were used as control. Differentiation results are shown in FIGS. 4A-B. The results in FIG. 4A show that fPSCs at the 20.sup.1h passage were still able to differentiate into BV+SC+ cells, indicating that long-term proliferation hardly affected the ability of fPSCs to differentiate into PGCLCs. The results in FIG. 4B show that the differentiation efficiency of fPSCs into PGCLCs (SSEA1 and CD61 double-positive cells) with potential function is 11%, which is close to the unstable epiblast-like cells (EpiLCs) with the best differentiation efficiency known at present (12.3%).

[0282] To investigate the ability of fPSCs to differentiate into neuroectoderm, GFP-negative Sox1-GFP fPSCs were obtained from mESCs reported by 46C (Yu Y, Wang X X, Zhang X X et al. ERK inhibition promotes neuroectodermal precursor commitment by blocking self-renewal and primitive streak formation of the epiblast. Stem Cell Res Ther 2018; Ying Q L, Stavridis M, Griffiths D, Li M, Smith A. Conversion of embryonic stem cells into neuroectodermal precursors in adherent monoculture. Nature biotechnology 2003; 21:183-186), followed by induction of differentiation into neuroectoderm by the method described in 2 above, with Sox1-GFP EpiLC and ESC monolayer differentiation systems as controls, GFP-positive cells were determined by flow cytometry (Yu Y. Wang X X, Zhang X X et al. ERK inhibition promotes neuroectodermal precursor commitment by blocking self-renewal and primitive streak formation of the epiblast. Stem Cell Res Ther 2018; Ying Q L, Stavridis M, Griffiths D, Li M, Smith A. Conversion of embryonic stem cells into neuroectodermal precursors in adherent monoculture. Nature biotechnology 2003; 21:183-186). The differentiation results are shown in FIGS. 4C-D. The results in FIG. 4C show that the differentiation efficiency of fPSCs to neural precursor cells (Sox1 positive) was as high as 94% the next day, which was significantly higher than the 72.6% of EpiLCs; and compared with the ESC monolayer differentiation system (68%), not only the efficiency was high, but also the time was much shorter (5-6 days), in addition to being more synchronized. The results in FIG. 4D show that the neural precursor cells derived from fPSCs could form neuron-like cells after 2 more days of differentiation.

[0283] To investigate the ability of fPSCs to differentiate into endoderm, the differentiation into hepatocyte-like cells was induced by the method described in 4 above, then endoderm marker Foxa2 and liver cell marker AFP were stained. The differentiation results are shown in FIGS. 4E-F. The results in FIG. 4E show that the differentiation efficiency of fPSCs to endoderm precursor cells (Foxa2) reached 88%. The results in FIG. 4F show that fPSCs-derived endoderm precursor cells could differentiate into hepatocyte-like cells after another 7 days.

[0284] In order to investigate the ability of fPSCs to differentiate into mesoderm, the method described in 3 above was used to induce differentiation into cardiomyocyte-like cells, and the differentiation of embryonic stem cells to EB was used as control. The results are shown in FIGS. 4G-J. The results in FIG. 4G show that the differentiation efficiency of fPSCs differentiating into mesoderm cells (beating cardiomyocyte-like cells) reached 90%, significantly higher than the differentiation efficiency of ESCs into cardiomyocyte-like cells through conventional EB differentiation (˜60%). The results in FIG. 4H show that cardiomyocyte-like cells derived from fPSCs expressed the proliferation marker molecule Ki67, and also expressed cardiomyocyte marker molecules (MLC, α-actinin, cTnT, and Gata4); in addition, sarcomeres were clearly visible. The results in FIGS. 4I-J show that after 3-4 days of maturation of fPSCs-derived cardiomyocyte-like cells, calcium ion release (I) and electrophysiological activity (J) could be detected.

[0285] The above results indicate that fPSCs of the present application have a greatly improved ability to efficiently differentiate into all cell lineages of the embryo compared to embryonic stem cells. Without being bound by theory, it is believed that ESCs are in a state that is not ready to differentiate, and similar to pre-implantation epiblast cells, these cells need to go through a maturation process in the genomic, epigenomic, metabolic and other regulatory pathways to achieve the “high potential energy” cell state (formative state) immediately before the initiation of gastrulation, and the fPSCs of the present application may correspond to this state. Therefore, fPSCs may be the only way to efficiently differentiate ESCs and iPSCs into functional cells in the future, and they have important application value in directed differentiation of stem cells and organoids.

Example 6: Analysis of Characteristics of hfPSCs at Genome Level

[0286] 1) Sequencing: hfPSCs were subjected to RNA-Seq by the method of Example 3

[0287] 2) RNA-seq data processing: first, the reads obtained by sequencing were mapped to the human hg38 genome through Hisat2v2.2.1. Only reads that could not be mapped to the genome were removed.

[0288] Then, based on the genome annotation information provided by refFlat, counts of gene expression levels were calculated using featureCount. The relative expression of sample genes was calculated by TPM (Transcripts PerKilobase Million) standardization method. Data visualization was achieved by using online software provided by UCSC University (http://genome.ucsc.edu/).

[0289] In order to detect the gene expression of the whole genome, hfPSCs were subjected to RNA sequencing, and the results of PCA analysis are shown in FIG. 5A. The results show that the genome-wide expression pattern of fPSCs is between naive hESCs and primed hESCs, and the pluripotent state is activated state (formative state), which is significantly different from FS (Smith et al., 2020) and XPSC (Wu et al., 2020).

[0290] In order to obtain specific genes that can distinguish hfPSCs from naive hESCs and primed hESCs, differential gene expression analysis was performed on these cells. As shown in FIG. 5B, hfPSCs specifically highly expressed 2378 genes, which may be marker molecules of hfPSCs. There are 84 formative genes in the intersection of hfPSCs and fPSCs, and these genes may be the conserved marker molecules shared by fPSCs and hfPSCs. The biological process cluster analysis of the genes highly expressed in hfPSCs is shown in FIG. 5C. The high level of cholesterol, sterol metabolism and fat metabolism of hfPSCs are unique to the formative state, which is consistent with the metabolic characteristics of mouse fPSCs: suggesting that this metabolic level of formative pluripotency may be conserved in humans and mice.

Example 7: Analysis of Characterization of hfPSCs at the Epigenetic Level

[0291] 7.1 Analysis of epigenetic modifications (H3K4me3 and H3K27me3)

[0292] 1. The STAR ChIP-seq library construction and sequencing of H3K4me3 and H3K27me3 were carried out on hfPSC by the method of Example 4.

[0293] 2. STAR ChIP-seq data processing and identification of super bivalent genes

[0294] Data processing: first, the reads obtained by sequencing were mapped to the human hg38 genome by Bowtie2 v2.2.2 (Langmead B, Salzberg S L. Fast gapped-read alignment with Bowtie 2. Nat Methods 2012; 9:357-359). Second, reads that could not be mapped on the genome, reads that could map to multiple positions, and duplicate reads caused by PCR were removed. After verifying the reproducibility between biological replicates, the data from different replicates were integrated. Afterwards, the whole genome was divided into 100 bp length sequence frames, and the signal enrichment in each I00 bp region was counted, and the signal was normalized using RPKM (Reads per kilobase per million of sequenced reads). The data visualization was achieved by using online software (https://epigenomegateway.wustl.edu/browser/) provided by wash u. The identification of super bivalent genes was carried out by the method of Example 4.

[0295] 3. Results:

[0296] FIG. 6A shows the number of super bivalent genes identified in hfPSCs. The results show that the number of super bivalent genes in hfPSCs is 174, and there are 31 super bivalent modification genes in the intersection of hfPSCs and fPSCs. These genes may be the super bivalent modification genes shared by fPSCs and hfPSC. FIG. 6B shows the super covalent modification patterns of H3K4me3 and H3K27me3 at promoter regions of HOXA cluster, FOXA2, PAX6, FOXF2, GDNF, IRX1, MSX1, NFIB, NKX3-2, NRN1, PAX5, PTGER4, PITX3, SLIT2, TBX1, VLDLR, EVX1, PHOX2B genes in hfPSCs.

[0297] Although the specific implementation of the present application has been described in detail, those skilled in the art will understand that: according to all the data and methods that have been published, various modifications and changes can be made to the details, and these changes are all within the protection scope of the present application. The full scope of the present application is given by the claims appended hereto and any equivalents thereof.