Pramel7 Mediates LIF/STAT3-Dependent Self-Renewal in embryoniC Stem Cells

PRAMEL7 and CUL2 decrease NuRD stability to establish ground-state pluripotency

Pluripotency is established in E4.5 preimplantation epiblast. Embryonic stem cells (ESCs) represent the immortalization of pluripotency, however, their gene expression signature only partially resembles that of developmental ground-state. Induced PRAMEL7 expression, a protein highly expressed in the ICM but lowly expressed in ESCs, reprograms developmentally advanced ESC+serum into ground-state pluripotency by inducing a gene expression signature close to developmental ground-state. However, how PRAMEL7 reprograms gene expression remains elusive. Here we show that PRAMEL7 associates with Cullin2 (CUL2) and this interaction is required to establish ground-state gene expression. PRAMEL7 recruits CUL2 to chromatin and targets regulators of repressive chromatin, including the NuRD complex, for proteasomal degradation. PRAMEL7 antagonizes NuRD-mediated repression of genes implicated in pluripotency by decreasing NuRD stability and promoter association in a CUL2-dependent manner. Our data link proteasome degradation pathways to ground-state gene expression, offering insights to generate in vitro models to reproduce the in vivo ground-state pluripotency.

Synergistic enhancement of the mouse Pramex1 and Pramel1 in repressing retinoic acid (RA) signaling during gametogenesis

BACKGROUND

PRAME constitutes one of the largest multi-copy gene families in Eutherians, encoding cancer-testis antigens (CTAs) with leucine-rich repeats (LRR) domains, highly expressed in cancer cells and gametogenic germ cells. This study aims to elucidate genetic interactions between two members, Pramex1 and Pramel1, in the mouse Prame family during gametogenesis using a gene knockout approach.

RESULT

Single-gene knockout (sKO) of either Pramex1 or Pramel1 resulted in approximately 7% of abnormal seminiferous tubules, characterized by a Sertoli-cell only (SCO) phenotype, impacting sperm count and fecundity significantly. Remarkably, sKO female mice displayed normal reproductive functions. In contrast, Pramex1/Pramel1 double knockout (dKO) mice exhibited reduced fecundity in both sexes. In dKO females, ovarian primary follicle count decreased by 50% compared to sKO and WT mice, correlating with a 50% fecundity decrease. This suggested compensatory roles during oogenesis in Pramex1 or Pramel1 sKO females. Conversely, dKO males showed an 18% frequency of SCO tubules, increased apoptotic germ cells, and decreased undifferentiated spermatogonia compared to sKO and WT testes. Western blot analysis with PRAMEX1- or PRAMEL1-specific antibodies on sKO testes revealed compensatory upregulation of each protein (30-50%) in response to the other gene's deletion. Double KO males exhibited more severe defects in sperm count and litter size, surpassing Pramex1 and Pramel1 sKO accumulative effects, indicating a synergistic enhancement interaction during spermatogenesis. Additional experiments administering trans-retinoic acid (RA) and its inhibitor (WIN18,446) in sKO, dKO, and WT mice suggested that PRAMEX1 and PRAMEL1 synergistically repress the RA signaling pathway during spermatogenesis.

CONCLUSION

Data from sKO and dKO mice unveil a synergistic interaction via the RA signaling pathway between Pramex1 and Pramel1 genes during gametogenesis. This discovery sets the stage for investigating interactions among other members within the Prame family, advancing our understanding of multi-copy gene families involved in germ cell formation and function.

Mouse Pramel1 regulates spermatogonial development by inhibiting retinoic acid signaling during spermatogenesis

Spermatogenesis begins when cell fate-committed prospermatogonia migrate to the basement membrane and initiate spermatogenesis in response to retinoic acid (RA) in the neonatal testis. The underlying cellular and molecular mechanisms in this process are not fully understood. Here, we report findings on the involvement of a cancer/testis antigen, PRAMEL1, in the initiation and maintenance of spermatogenesis. By analyzing mouse models with either global or conditional Pramel1 inactivation, we found that PRAMEL1 regulates the RA responsiveness of the subtypes of prospermatogonia in the neonatal testis, and affects their homing process during the initiation of spermatogenesis. Pramel1 deficiency led to increased fecundity in juvenile males and decreased fecundity in mature males. In addition, Pramel1 deficiency resulted in a regional Sertoli cell-only phenotype during the first round of spermatogenesis, which was rescued by administration of the RA inhibitor WIN18,446, suggesting that PRAMEL1 functions as an inhibitor of RA signaling in germ cells. Overall, our findings suggest that PRAMEL1 fine-tunes RA signaling, playing a crucial role in the proper establishment of the first and subsequent rounds of spermatogenesis.

BET proteins are essential for the specification and maintenance of the epiblast lineage in mouse preimplantation embryos

Background

During mammalian preimplantation development, as the fertilized egg develops and differentiates, three cell lineages become specified: trophectoderm (TE), epiblast, and primitive endoderm (PrE). Through two steps of cell fate decisions, 16-cell blastomeres develop into TE and an inner cell mass (ICM), and thereafter, the latter differentiates into pluripotent epiblast and PrE. Although bromodomain and extra-terminal domain (BET) proteins, such as BRD4, are necessary for the transcriptional activation of genes involved in the maintenance of mouse embryonic stem cells by occupying their enhancers, their roles in the development of mouse preimplantation are unknown.

Results

To evaluate the effect of BET protein deficiency on cell lineage formation, we cultured preimplantation embryos in the presence of JQ1, which blocks the binding of BET bromodomains to acetylated-histones. We found BET inhibition blocked the transcriptional activation of genes, such as Nanog, Otx2, and Sox2, important for the formation of the epiblast lineage in blastocysts. Expression studies with lineage-specific markers in morulae and blastocysts revealed BET proteins were essential for the specification and maintenance of the epiblast lineage but were dispensable for the formation of primarily extraembryonic TE and PrE lineages. Additional Ingenuity Pathway Analysis and expression studies with a transcriptionally active form of signal transducer and activator of the transcription 3 (STAT3) suggested BET-dependent activation was partly associated with the STAT3-dependent pathway to maintain the epiblast lineage. To identify BET proteins involved in the formation of the epiblast lineage, we analyzed mutant embryos deficient in Brd4, Brd2, and double mutants. Abolishment of NANOG-positive epiblast cells was only evident in Brd4/Brd2 double-deficient morulae. Thus, the phenotype of JQ1-treated embryos is reproduced not by a Brd4- or Brd2-single deficiency, but only Brd4/Brd2-double deficiency, demonstrating the redundant roles of BRD2 and BRD4 in the specification of the epiblast lineage.

Conclusions

BET proteins are essential to the specification and maintenance of the epiblast lineage by activating lineage-specific core transcription factors during mouse preimplantation development. Among BET proteins, BRD4 plays a central role and BRD2 a complementary role in the specification and maintenance of epiblast lineages. Additionally, BET-dependent maintenance of the epiblast lineage may be partly associated with the STAT3-dependent pathway.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01251-0.

Subcellular localization of the mouse PRAMEL1 and PRAMEX1 reveals multifaceted roles in the nucleus and cytoplasm of germ cells during spermatogenesis

BACKGROUND

Preferentially expressed antigen in melanoma (PRAME) is a cancer/testis antigen (CTA) that is predominantly expressed in normal gametogenic tissues and a variety of tumors. Members of the PRAME gene family encode leucine-rich repeat (LRR) proteins that provide a versatile structural framework for the formation of protein-protein interactions. As a nuclear receptor transcriptional regulator, PRAME has been extensively studied in cancer biology and is believed to play a role in cancer cell proliferation by suppressing retinoic acid (RA) signaling. The role of the PRAME gene family in germline development and spermatogenesis has been recently confirmed by a gene knockout approach. To further understand how PRAME proteins are involved in germ cell development at a subcellular level, we have conducted a systematic immunogold electron microscopy (IEM) analysis on testis sections of adult mice with gene-specific antibodies from two members of the mouse Prame gene family: Pramel1 and Pramex1. Pramel1 is autosomal, while Pramex1 is X-linked, both genes are exclusively expressed in the testis.

RESULTS

Our IEM data revealed that both PRAMEL1 and PRAMEX1 proteins were localized in various cell organelles in different development stages of spermatogenic cells, including the nucleus, rER, Golgi, mitochondria, germ granules [intermitochondrial cement (IMC) and chromatoid body (CB)], centrioles, manchette, and flagellum. Unlike other germ cell-specific makers, such as DDX4, whose proteins are evenly distributed in the expressed-organelle(s), both PRAMEL1 and PRAMEX1 proteins tend to aggregate together to form clusters of protein complexes. These complexes were highly enriched in the nucleus and cytoplasm (especially in germ granules) of spermatocytes and spermatids. Furthermore, dynamic distribution of the PRAMEL1 protein complexes were observed in the microtubule-based organelles, such as acroplaxome, manchette, and flagellum, as well as in the nuclear envelope and nuclear pore. Dual staining with PRAMEL1 and KIF17B antibodies further revealed that the PRAMEL1 and KIF17B proteins were co-localized in germ granules.

CONCLUSION

Our IEM data suggest that the PRAMEL1 and PRAMEX1 proteins are not only involved in transcriptional regulation in the nucleus, but may also participate in nucleocytoplasmic transport, and in the formation and function of germ cell-specific organelles during spermatogenesis.

The PRAME family of cancer testis antigens is essential for germline development and gametogenesis†

Preferentially expressed antigen in melanoma (PRAME) belongs to a group of cancer/testis antigens that are predominately expressed in the testis and a variety of tumors, and are involved in immunity and reproduction. Much of the attention on PRAME has centered on cancer biology as PRAME is a prognostic biomarker for a wide range of cancers and a potential immunotherapeutic target. Less information is available about the PRAME family's function (s) during gametogenesis and in the overall reproduction process. Here, we review the current knowledge of the PRAME gene family and its function in germline development and gametogenesis. Members of the PRAME family are leucine rich repeat proteins, localized in nucleus and cytoplasm, with multifaceted roles in germ cells. As transcriptional regulators, the PRAME family proteins are involved in germline development, particularly in the maintenance of embryonic stem cell pluripotency, development of primordial germ cells, and differentiation/proliferation of spermatogenic and oogenic cells. The PRAME family proteins are also enriched in cytoplasmic organelles, such as rough endoplasmic reticulum, Golgi vesicle, germinal granules, centrioles, and play a role in the formation of the acrosome and sperm tail during spermiogenesis. The PRAME gene family remains transcriptionally active in the germline throughout the entire life cycle and is essential for gametogenesis, with some members specific to either male or female germ cells, while others are involved in both male and female gametogenesis. A potential molecular mechanism that underlies the function of PRAME, and is shared by gametogenesis and oncogenesis is also discussed.

Nanoparticle-Assembled Vacuolated Coacervates Control Macromolecule Spatiotemporal Distribution to Provide a Stable Segregated Cell Microenvironment

Membraneless coacervate compartments in the intracellular and pericellular space mediate critical cellular functions. Developing synthetic coacervates that emulate the morphological, physical, and functional complexity of these natural coacervates is challenging but highly desirable. Herein, a generalizable nanoparticle assembly (NPA) strategy is developed, which is applicable to interactive core-shell nanoparticles with different chemical makeups, to fabricate vacuolated coacervates. The obtained NPA coacervates contain stable internal vacuoles to provide segregated microcompartments, which can mediate the spatially heterogeneous distribution of diverse macromolecules via restricted diffusion. It is further shown that the vacuolated NPA coacervates can harbor and retain macromolecular medium supplements to regulate the functions of cells encapsulated in vacuoles. Furthermore, the restricted macromolecule diffusion can be abolished on demand via the triggered coacervate-hydrogel transition, thereby altering the exposure of encapsulated cells to environmental factors. It is believed that the NPA strategy provides new insights into the design principles of hierarchical coacervates that hold promising potential for a wide array of biomedical applications.

Sertoli cell-only phenotype and scRNA-seq define PRAMEF12 as a factor essential for spermatogenesis in mice

Spermatogonial stem cells (SSCs) have the dual capacity to self-renew and differentiate into progenitor spermatogonia that develop into mature spermatozoa. Here, we document that preferentially expressed antigen of melanoma family member 12 (PRAMEF12) plays a key role in maintenance of the spermatogenic lineage. In male mice, genetic ablation of Pramef12 arrests spermatogenesis and results in sterility which can be rescued by transgenic expression of Pramef12. Pramef12 deficiency globally decreases expression of spermatogenic-related genes, and single-cell transcriptional analysis of post-natal male germline cells identifies four spermatogonial states. In the absence of Pramef12 expression, there are fewer spermatogonial stem cells which exhibit lower expression of SSC maintenance-related genes and are defective in their ability to differentiate. The disruption of the first wave of spermatogenesis in juvenile mice results in agametic seminiferous tubules. These observations mimic a Sertoli cell-only syndrome in humans and may have translational implications for reproductive medicine.

X-Chromosome Dosage Modulates Multiple Molecular and Cellular Properties of Mouse Pluripotent Stem Cells Independently of Global DNA Methylation Levels

Reprogramming female mouse somatic cells into induced pluripotent stem cells (iPSCs) leads to X-chromosome reactivation. The extent to which increased X-chromosome dosage (X-dosage) in female iPSCs compared with male iPSCs leads to differences in the properties of iPSCs is still unclear. We show that chromatin accessibility in mouse iPSCs is modulated by X-dosage. Specific sets of transcriptional regulator motifs are enriched in chromatin with increased accessibility in XX or XY iPSCs. The transcriptome, growth and pluripotency exit are also modulated by X-dosage in iPSCs. To understand how increased X-dosage modulates the properties of mouse pluripotent stem cells, we used heterozygous deletions of the X-linked gene Dusp9. We show that X-dosage regulates the transcriptome, open chromatin landscape, growth, and pluripotency exit largely independently of global DNA methylation. Our results provide insights into how gene dosage modulates the epigenetic and genetic mechanisms that regulate cell identity.

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X-chromosome dosage modulates the pluripotent chromatin accessibility landscape

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Increased X-chromosome dosage slows down growth

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Dusp9 heterozygous female ESCs display pluripotency exit delay

STAT3-Inducible Mouse ESCs: A Model to Study the Role of STAT3 in ESC Maintenance and Lineage Differentiation

Studies have demonstrated that STAT3 is essential in maintaining self-renewal of embryonic stem cells (ESCs) and modulates ESC differentiation. However, there is still lack of direct evidence on STAT3 functions in ESCs and embryogenesis because constitutive STAT3 knockout (KO) mouse is embryonic lethal at E6.5-E7.5, prior to potential functional role in early development can be assessed. Therefore, in this study, two inducible STAT3 ESC lines were established, including the STAT3 knockout (InSTAT3 KO) and pSTAT3 overexpressed (InSTAT3 CA) using Tet-on inducible system in which STAT3 expression can be strictly controlled by doxycycline (Dox) stimulation. Through genotyping, deletion of STAT3 alleles was detected in InSTAT3 KO ESCs following 24 hours Dox stimulation. Western blot also showed that pSTAT3 and STAT3 protein levels were significantly reduced in InSTAT3 KO ESCs while dominantly elevated in InSTAT3 CA ECSs upon Dox stimulation. Likewise, it was found that STAT3-null ESCs would affect the differentiation of ESCs into mesoderm and cardiac lineage. Taken together, the findings of this study indicated that InSTAT3 KO and InSTAT3 CA ESCs could provide a new tool to clarify the direct targets of STAT3 and its role in ESC maintenance, which will facilitate the elaboration of the mechanisms whereby STAT3 maintains ESC pluripotency and regulates ESC differentiation during mammalian embryogenesis.

Cancer-testis antigens: Unique cancer stem cell biomarkers and targets for cancer therapy

Cancer-testis antigens (CTAs) are considered as unique and promising cancer biomarkers and targets for cancer therapy. CTAs are multifunctional protein group with specific expression patterns in normal embryonic and adult cells and various types of cancer cells. CTAs are involved in regulating of the basic cellular processes during development, stem cell differentiation and carcinogenesis though the biological roles and cell functions of CTA families remain largely unclear. Analysis of CTA expression patterns in embryonic germ and somatic cells, pluripotent and multipotent stem cells, cancer stem cells and their cell descendants indicates that rearrangements of characteristic CTA profiles (aberrant expression) could be associated with cancer transformation and failure of the developmental program of cell lineage specification and germ line restriction. Therefore, aberrant CTA profiles can be used as panels of biomarkers for diagnoses and the selection of cancer treatment strategies. Moreover, immunogenic CTAs are prospective targets for cancer immunotherapy. Clinical trials testing broad range of cancer therapeutic vaccines against antigens of MAGEA and NY-ESO-1 families for treating various cancers have shown mixed clinical efficiency, safety and tolerability, suggesting the requirement of in-depth research of CTA expression in normal and cancer stem cells and extensive clinical trials for improving cancer immunotherapy technologies. This review focuses on recent advancement in study of CTAs in normal and cancer cells, particularly in normal and cancer stem cells, and provides a new insight into CTA expression patterns during normal and cancer stem cell lineage development. Additionally, new approaches in development of effective CTA-based therapies exclusively targeting cancer stem cells will be discussed.

LIF-activated Jak signaling determines Esrrb expression during late-stage reprogramming

The regulatory process of naïve-state induced pluripotent stem cell (iPSC) generation is not well understood. Leukemia inhibitory factor (LIF)-activated Janus kinase/signal transducer and activator of transcription 3 (Jak/Stat3) is the master regulator for naïve-state pluripotency achievement and maintenance. The estrogen-related receptor beta (Esrrb) serves as a naïve-state marker gene regulating self-renewal of embryonic stem cells (ESCs). However, the interconnection between Esrrb and LIF signaling for pluripotency establishment in reprogramming is unclear. We screened the marker genes critical for complete reprogramming during mouse iPSC generation, and identified genes including Esrrb that are responsive to LIF/Jak pathway signaling. Overexpression of Esrrb resumes the reprogramming halted by inhibition of Jak activity in partially reprogrammed cells (pre-iPSCs), and leads to the generation of pluripotent iPSCs. We further show that neither overexpression of Nanog nor stimulation of Wnt signaling, two upstream regulators of Esrrb in ESCs, stimulates the expression of Esrrb in reprogramming when LIF or Jak activity is blocked. Our study demonstrates that Esrrb is a specific reprogramming factor regulated downstream of the LIF/Jak signaling pathway. These results shed new light on the regulatory role of LIF pathway on complete pluripotency establishment during iPSC generation.

Summary:Esrrb is a downstream target and effector of LIF during reprogramming. Forced Esrrb expression accelerates pluripotency establishment in the absence of LIF signaling. The activation of Esrrb is LIF dependent in the reprogramming process.

Opposing Roles of Acetylation and Phosphorylation in LIFR-Dependent Self-Renewal Growth Signaling in Mouse Embryonic Stem Cells

LIF promotes self-renewal of mouse embryonic stem cells (mESCs), and in its absence, the cells differentiate. LIF binds to the LIF receptor (LIFR) and activates the JAK-STAT3 pathway, but it remains unknown how the receptor complex triggers differentiation or self-renewal. Here, we report that the LIFR cytoplasmic domain contains a self-renewal domain within the juxtamembrane region and a differentiation domain within the C-terminal region. The differentiation domain contains four SPXX repeats that are phosphorylated by MAPK to restrict STAT3 activation; the self-renewal domain is characterized by a 3K motif that is acetylated by p300. In mESCs, acetyl-LIFR undergoes homodimerization, leading to STAT3 hypo- or hyper-activation depending on the presence or absence of gp130. LIFR-activated STAT3 restricts differentiation via cytokine induction. Thus, LIFR acetylation and serine phosphorylation differentially promote stem cell self-renewal and differentiation.

Sp5 induces the expression of Nanog to maintain mouse embryonic stem cell self-renewal

Activation of signal transducer and activator of transcription 3 (STAT3) by leukemia inhibitory factor (LIF) maintains mouse embryonic stem cell (mESC) self-renewal. Our previous study showed that trans-acting transcription factor 5 (Sp5), an LIF/STAT3 downstream target, supports mESC self-renewal. However, the mechanism by which Sp5 exerts these effects remains elusive. Here, we found that Nanog is a direct target of Sp5 and mediates the self-renewal-promoting effect of Sp5 in mESCs. Overexpression of Sp5 induced Nanog expression, while knockdown or knockout of Sp5 decreased the Nanog level. Moreover, chromatin immunoprecipitation (ChIP) assays showed that Sp5 directly bound to the Nanog promoter. Functional studies revealed that knockdown of Nanog eliminated the mESC self-renewal-promoting ability of Sp5. Finally, we demonstrated that the self-renewal-promoting function of Sp5 was largely dependent on its zinc finger domains. Taken together, our study provides unrecognized functions of Sp5 in mESCs and will expand our current understanding of the regulation of mESC pluripotency.

STAT3 Undergoes Acetylation-dependent Mitochondrial Translocation to Regulate Pyruvate Metabolism

Cytoplasmic STAT3, after activation by growth factors, translocates to different subcellular compartments, including nuclei and mitochondria, where it carries out different biological functions. However, the precise mechanism by which STAT3 undergoes mitochondrial translocation and subsequently regulates the tricarboxylic acid (TCA) cycle-electron transport chain (ETC) remains poorly understood. Here, we clarify this process by visualizing STAT3 acetylation in starved cells after serum reintroduction or insulin stimulation. CBP-acetylated STAT3 undergoes mitochondrial translocation in response to serum introduction or insulin stimulation. In mitochondria, STAT3 associates with the pyruvate dehydrogenase complex E1 (PDC-E1) and subsequently accelerates the conversion of pyruvate to acetyl-CoA, elevates the mitochondrial membrane potential, and promotes ATP synthesis. SIRT5 deacetylates STAT3, thereby inhibiting its function in mitochondrial pyruvate metabolism. In the A549 lung cancer cell line, constitutively acetylated STAT3 localizes to mitochondria, where it maintains the mitochondrial membrane potential and ATP synthesis in an active state.

Retinoic Acid Specifically Enhances Embryonic Stem Cell Metastate Marked by Zscan4

Pluripotency confers Embryonic Stem Cells (ESCs) the ability to differentiate in ectoderm, endoderm, and mesoderm derivatives, producing the majority of cell types. Although the majority of ESCs divide without losing pluripotency, it has become evident that ESCs culture consists of multiple cell populations with different degrees of potency that are spontaneously induced in regular ESC culture conditions. Zscan4, a key pluripotency factor, marks ESC subpopulation that is referred to as high-level of pluripotency metastate. Here, we report that in ESC cultures treated with retinoic acid (RA), Zscan4 ESCs metastate is strongly enhanced. In particular, we found that induction of Zscan4 metastate is mediated via RA receptors (RAR-alpha, RAR-beta, and RAR-gamma), and it is dependent on phosphoinositide-3-kinase (PI3K) signaling. Remarkably, Zscan4 metastate induced by RA lacks canonical pluripotency genes Oct3/4 and Nanog but retained both self-renewal and pluripotency capabilities. Finally we demonstrated that the conditional ablation of Zscan4 subpopulation is dispensable for both endoderm and mesoderm but is required for ectoderm lineage. In conclusion, our research provides new insights about the role of RA signaling during ESCs high pluripotency metastate fluctuation.

Maintenance of Self-Renewal and Pluripotency in J1 Mouse Embryonic Stem Cells through Regulating Transcription Factor and MicroRNA Expression Induced by PD0325901

Embryonic stem cells (ESCs) have the ability to grow indefinitely and retain their pluripotency in culture, and this self-renewal capacity is governed by several crucial molecular pathways controlled by specific regulatory genes and epigenetic modifications. It is reported that multiple epigenetic regulators, such as miRNA and pluripotency factors, can be tightly integrated into molecular pathways and cooperate to maintain self-renewal of ESCs. However, mouse ESCs in serum-containing medium seem to be heterogeneous due to the self-activating differentiation signal of MEK/ERK. Thus, to seek for the crucial miRNA and key regulatory genes that establish ESC properties in MEK/ERK pathway, we performed microarray analysis and small RNA deep-sequencing of J1 mESCs treated with or without PD0325901 (PD), a well-known inhibitor of MEK/ERK signal pathway, followed by verification of western blot analysis and quantitative real-time PCR verification; we found that PD regulated the transcript expressions related to self-renewal and differentiation and antagonized the action of retinoic acid- (RA-) induced differentiation. Moreover, PD can significantly modulate the expressions of multiple miRNAs that have crucial functions in ESC development. Overall, our results demonstrate that PD could enhance ESC self-renewal capacity both by key regulatory genes and ES cell-specific miRNA, which in turn influences ESC self-renewal and cellular differentiation.

Wnt/β-catenin and LIF-Stat3 signaling pathways converge on Sp5 to promote mouse embryonic stem cell self-renewal

Activation of leukemia inhibitor factor (LIF)–Stat3 or Wnt/β-catenin signaling promotes mouse embryonic stem cell (mESC) self-renewal. A myriad of downstream targets have been identified in the individual signal pathways, but their common targets remain largely elusive. In this study, we found that the LIF–Stat3 and Wnt/β-catenin signaling pathways converge on Sp5 to promote mESC self-renewal. Forced Sp5 expression can reproduce partial effects of Wnt/β-catenin signaling but mimics most features of LIF–Stat3 signaling to maintain undifferentiated mESCs. Moreover, Sp5 is able to convert mouse epiblast stem cells into a naïve pluripotent state. Thus, Sp5 is an important component of the regulatory network governing mESC naïve pluripotency.

Summary: This study reveals a new function of Sp5 in mouse embryonic stem cell (ESC) self-renewal mediated by CHIR99021 and LIF, and reprogramming of EpiSCs into naїve ESCs.

Molecular basis of embryonic stem cell self-renewal: from signaling pathways to pluripotency network

Embryonic stem cells (ESCs) can be maintained in culture indefinitely while retaining the capacity to generate any type of cell in the body, and therefore not only hold great promise for tissue repair and regeneration, but also provide a powerful tool for modeling human disease and understanding biological development. In order to fulfill the full potential of ESCs, it is critical to understand how ESC fate, whether to self-renew or to differentiate into specialized cells, is regulated. On the molecular level, ESC fate is controlled by the intracellular transcriptional regulatory networks that respond to various extrinsic signaling stimuli. In this review, we discuss and compare important signaling pathways in the self-renewal and differentiation of mouse, rat, and human ESCs with an emphasis on how these pathways integrate into ESC-specific transcription circuitries. This will be beneficial for understanding the common and conserved mechanisms that govern self-renewal, and for developing novel culture conditions that support ESC derivation and maintenance.

Stat3 signaling regulates embryonic stem cell fate in a dose-dependent manner

Stat3 is essential for mouse embryonic stem cell (mESC) self-renewal mediated by LIF/gp130 receptor signaling. Current understanding of Stat3-mediated ESC self-renewal mechanisms is very limited, and has heretofore been dominated by the view that Stat3 signaling functions in a binary "on/off" manner. Here, in contrast to this binary viewpoint, we demonstrate a contextual, rheostat-like mechanism for Stat3's function in mESCs. Activation and expression levels determine whether Stat3 functions in a self-renewal or a differentiation role in mESCs. We also show that Stat3 induces rapid differentiation of mESCs toward the trophectoderm (TE) lineage when its activation level exceeds certain thresholds. Stat3 induces this differentiation phenotype via induction of Tfap2c and its downstream target Cdx2. Our findings provide a novel concept in the realm of Stat3, self-renewal signaling, and pluripotent stem cell biology. Ultimately, this finding may facilitate the development of conditions for the establishment of authentic non-rodent ESCs.

Inferring the choreography of parental genomes during fertilization from ultralarge-scale whole-transcriptome analysis

The mechanisms underlying parental genome coordination upon mammalian fertilization remain elusive due to difficulties in preparing large numbers of high-quality preimplantation cells. Here, Park et al. collect an unprecedented number of mouse oocytes and establish detailed transcriptional profiles for four early embryonic stages and parthenogenetic development. Bioinformatic analysis identifies a distinctive gene regulatory network activated in embryos after fertilization compared with parthenotes. This large-scale profile of early mouse embryos yields a valuable resource for developmental biology and stem cell research.

Fertilization precisely choreographs parental genomes by using gamete-derived cellular factors and activating genome regulatory programs. However, the mechanism remains elusive owing to the technical difficulties of preparing large numbers of high-quality preimplantation cells. Here, we collected >14 × 10⁴ high-quality mouse metaphase II oocytes and used these to establish detailed transcriptional profiles for four early embryo stages and parthenogenetic development. By combining these profiles with other public resources, we found evidence that gene silencing appeared to be mediated in part by noncoding RNAs and that this was a prerequisite for post-fertilization development. Notably, we identified 817 genes that were differentially expressed in embryos after fertilization compared with parthenotes. The regulation of these genes was distinctly different from those expressed in parthenotes, suggesting functional specialization of particular transcription factors prior to first cell cleavage. We identified five transcription factors that were potentially necessary for developmental progression: Foxd1, Nkx2-5, Sox18, Myod1, and Runx1. Our very large-scale whole-transcriptome profile of early mouse embryos yielded a novel and valuable resource for studies in developmental biology and stem cell research. The database is available at http://dbtmee.hgc.jp.

Identification of a novel gene signature of ES cells self-renewal fluctuation through system-wide analysis

Embryonic Stem cells (ESCs) can be differentiated into ectoderm, endoderm, and mesoderm derivatives, producing the majority of cell types. In regular culture conditions, ESCs' self-renewal is maintained through molecules that inhibit spontaneous differentiation enabling long-term cellular expansion. This undifferentiating condition is characterized by multiple metastable states that fluctuate between self-renewal and differentiation balance. Here, we aim to characterize the high-pluripotent ESC metastate marked by the expression of Zscan4 through a supervised machine learning framework based on an ensemble of support vector machine (SVM) classifiers. Our study revealed a leukaemia inhibitor factor (Lif) dependent not-canonical pluripotency signature (AF067063, BC061212, Dub1, Eif1a, Gm12794, Gm13871, Gm4340, Gm4850, Tcstv1/3, and Zfp352), that specifically marks Zscan4 ESCs' fluctuation. This novel ESC metastate is enhanced by high-pluripotency culture conditions obtained through Extracellular signal Regulated-Kinase (ERK) and Glycogen synthase kinase-3 (Gsk-3) signaling inhibition (2i). Significantly, we reported that the conditional ablation of the novel ESC metastate marked by the expression of Gm12794 is required for ESCs self-renewal maintenance. In conclusion, we extend the comprehension of ESCs biology through the identification of a novel molecular signature associated to pluripotency programming.

Embryonic stem cell self-renewal pathways converge on the transcription factor Tfcp2l1

Mouse embryonic stem cell (mESC) self-renewal can be maintained by activation of the leukaemia inhibitory factor (LIF)/signal transducer and activator of transcription 3 (Stat3) signalling pathway or dual inhibition (2i) of glycogen synthase kinase 3 (Gsk3) and mitogen-activated protein kinase kinase (MEK). Several downstream targets of the pathways involved have been identified that when individually overexpressed can partially support self-renewal. However, none of these targets is shared among the involved pathways. Here, we show that the CP2 family transcription factor Tfcp2l1 is a common target in LIF/Stat3- and 2i-mediated self-renewal, and forced expression of Tfcp2l1 can recapitulate the self-renewal-promoting effect of LIF or either of the 2i components. In addition, Tfcp2l1 can reprogram post-implantation epiblast stem cells to naïve pluripotent ESCs. Tfcp2l1 upregulates Nanog expression and promotes self-renewal in a Nanog-dependent manner. We conclude that Tfcp2l1 is at the intersection of LIF- and 2i-mediated self-renewal pathways and plays a critical role in maintaining ESC identity. Our study provides an expanded understanding of the current model of ground-state pluripotency.

Cross-species genome wide expression analysis during pluripotent cell determination in mouse and rat preimplantation embryos

The transition between morula and blastocyst stage during preimplantation development represents the first differentiation event of embryogenesis. Morula cells undergo the first cellular specialization and produce two well-defined populations of cells, the trophoblast and the inner cell mass (ICM). Embryonic stem cells (ESCs) with unlimited self-renewal capacity are believed to represent the in vitro counterpart of the ICM. Both mouse and rat ESCs can be derived from the ICM cells, but their in vitro stability differs. In this study we performed a microarray analysis in which we compared the transcriptome of mouse and rat morula, blastocyst, and ICM. This cross-species comparison represents a good model for understanding the differences in derivation and cultivation of ESCs observed in the two species. In order to identify alternative regulation of important molecular mechanisms the investigation of differential gene expression between the two species was extended at the level of signaling pathways, gene families, and single selected genes of interest. Some of the genes differentially expressed between the two species are already known to be important factors in the maintenance of pluripotency in ESCs, like for example Sox2 or Stat3, or play a role in reprogramming somatic cells to pluripotency like c-Myc, Klf4 and p53 and therefore represent interesting candidates to further analyze in vitro in the rat ESCs. This is the first study investigating the gene expression changes during the transition from morula to blastocyst in the rat preimplantation development. Our data show that in the pluripotent pool of cells of the rat and mouse preimplantation embryo substantial differential regulation of genes is present, which might explain the difficulties observed for the derivation and culture of rat ESCs using mouse conditions.

Human serine protease HTRA1 positively regulates osteogenesis of human bone marrow-derived mesenchymal stem cells and mineralization of differentiating bone-forming cells through the modulation of extracellular matrix protein

Mammalian high-temperature requirement serine protease A1 (HTRA1) is a secreted member of the trypsin family of serine proteases which can degrade a variety of bone matrix proteins and as such has been implicated in musculoskeletal development. In this study, we have investigated the role of HTRA1 in mesenchymal stem cell (MSC) osteogenesis and suggest a potential mechanism through which it controls matrix mineralization by differentiating bone-forming cells. Osteogenic induction resulted in a significant elevation in the expression and secretion of HTRA1 in MSCs isolated from human bone marrow-derived MSCs (hBMSCs), mouse adipose-derived stromal cells (mASCs), and mouse embryonic stem cells. Recombinant HTRA1 enhanced the osteogenesis of hBMSCs as evidenced by significant changes in several osteogenic markers including integrin-binding sialoprotein (IBSP), bone morphogenetic protein 5 (BMP5), and sclerostin, and promoted matrix mineralization in differentiating bone-forming osteoblasts. These stimulatory effects were not observed with proteolytically inactive HTRA1 and were abolished by small interfering RNA against HTRA1. Moreover, loss of HTRA1 function resulted in enhanced adipogenesis of hBMSCs. HTRA1 Immunofluorescence studies showed colocalization of HTRA1 with IBSP protein in osteogenic mASC spheroid cultures and was confirmed as being a newly identified HTRA1 substrate in cell cultures and in proteolytic enzyme assays. A role for HTRA1 in bone regeneration in vivo was also alluded to in bone fracture repair studies where HTRA1 was found localized predominantly to areas of new bone formation in association with IBSP. These data therefore implicate HTRA1 as having a central role in osteogenesis through modification of proteins within the extracellular matrix.

Recurrent transcriptional clusters in the genome of mouse pluripotent stem cells

A number of studies have shown that transcriptome analysis in terms of chromosomal location can reveal regions of non-random transcriptional activity within the genome. Genomic clusters of differentially expressed genes can identify genomic patterns of structural organization, underlying copy number variations or long-range epigenetic regulation such as X-chromosome inactivation. Here we apply an integrative bioinformatics analysis to a collection of 315 freely available mouse pluripotent stem cell samples to discover transcriptional clusters in the genome. We show that over half of the analysed samples (56.83%) carry whole or partial-chromosome spanning clusters which recur in genomic regions previously implicated in chromosomal imbalances. Strikingly, we found that the presence of such large-clusters is linked to the differential expression of a limited number of genes, common to all samples carrying clusters irrespectively of the chromosome where the cluster is found. We have used these genes to train and test classification models that can predict samples that carry large-scale clusters on any chromosome with over 90% accuracy. Our findings suggest that there is a common downstream activation in these cells that affects a limited number of nodes. We propose that this effect is linked to selective advantage and identify potential driver genes.

Low microRNA-199a expression in human amniotic epithelial cell feeder layers maintains human-induced pluripotent stem cell pluripotency via increased leukemia inhibitory factor expression

Human-induced pluripotent stem (iPS) cells share the same key properties as embryonic stem cells, and may be generated from patient- or disease-specific sources, which makes them attractive for personalized medicine, drug screens, or cellular therapy. Long-term cultivation and maintenance of normal iPS cells in an undifferentiated self-renewing state is a major challenge. Our previous studies have shown that human amniotic epithelial cells (HuAECs) could provide a good source of feeder cells for mouse and human embryonic stem cells, or spermatogonial stem cells, as they express endogenous leukemia inhibitory factor (LIF) at high levels. Here, we examined the effect of exogenous microRNA-199a regulation on endogenous LIF expression in HuAECs, and in turn on human iPS cell pluripotency. We found that HuAECs feeder cells transfected with microRNA-199a mutant expressed LIF at high levels, allowing iPS to maintain a high level of alkaline phosphatase activity in long-term culture and form teratomas in severe combined immunodeficient mice. The expression of stem cell markers was increased in iPS cultured on HuAECs feeder cells transfected with the microRNA-199a mutant, compared with iPS cultured on HuAECs transfected with microRNA-199a or mouse embryo fibroblasts. Taken together, these results suggested that LIF expression might be regulated by microRNA-199a, and LIF was a crucial component in feeder cells, and also was required for maintenance of human iPS cells in an undifferentiated, proliferative state capable of self-renewal.

DLK1, delta-like 1 homolog (Drosophila), regulates tumor cell differentiation in vivo

The stem cell-like characteristics of tumor cells are not only essential for tumor development and malignant progression, but also significantly contribute to therapy resistance. However, it remains poorly understood how cancer cell differentiation or stemness is regulated in vivo. We investigated the role of the stem cell gene DLK1, or delta-like 1 homolog (Drosophila), in the regulation of cancer cell differentiation in vivo using neuroblastoma (NB) xenografts as a model. We found that loss-of-function mutants of DLK1 significantly enhanced NB cell differentiation in vivo likely by increasing the basal phosphorylation of MEK and ERK kinases, a mechanism that has been shown to facilitate neuronal differentiation. We also found that DLK1(+) cells are preferentially located in hypoxic regions. These results clearly demonstrate that DLK1 plays an important role in the maintenance of undifferentiated, stem cell-like phenotypes of NB cells in vivo.

The Role of the Leukemia Inhibitory Factor (LIF) - Pathway in Derivation and Maintenance of Murine Pluripotent Stem Cells

Developmental biology, regenerative medicine and cancer biology are more and more interested in understanding the molecular mechanisms controlling pluripotency and self-renewal in stem cells. Pluripotency is maintained by a synergistic interplay between extrinsic stimuli and intrinsic circuitries, which allow sustainment of the undifferentiated and self-renewing state. Nevertheless, even though a lot of efforts have been made in the past years, the precise mechanisms regulating these processes remain unclear. One of the key extrinsic factors is leukemia inhibitory factor (LIF) that is largely used for the cultivation and derivation of mouse embryonic and induced pluripotent stem cells. LIF acts through the LIFR/gp130 receptor and activates STAT3, an important regulator of mouse embryonic stem cell self-renewal. STAT3 is known to inhibit differentiation into both mesoderm and endoderm lineages by preventing the activation of lineage-specific differentiation programs. However, LIF activates also parallel circuitries like the PI3K-pathway and the MEK/ERK-pathway, but its mechanisms of action remain to be better elucidated. This review article aims at summarizing the actual knowledge on the importance of LIF in the maintenance of pluripotency and self-renewal in embryonic and induced pluripotent stem cells.