Promotion of Feeder-Independent Self-Renewal of Embryonic Stem Cells by Retinol (Vitamin A)

A Comprehensive Review on the Role of ZSCAN4 in Embryonic Development, Stem Cells, and Cancer

ZSCAN4 is a transcription factor that plays a pivotal role during early embryonic development. It is a unique gene expressed specifically during the first tide of de novo transcription during the zygotic genome activation. Moreover, it is reported to regulate telomere length in embryonic stem cells and induced pluripotent stem cells. Interestingly, ZSCAN4 is expressed in approximately 5% of the embryonic stem cells in culture at any given time, which points to the fact that it has a tight regulatory system. Furthermore, ZSCAN4, if included in the reprogramming cocktail along with core reprogramming factors, increases the reprogramming efficiency and results in better quality, genetically stable induced pluripotent stem cells. Also, it is reported to have a role in promoting cancer stem cell phenotype and can prospectively be used as a marker for the same. In this review, the multifaceted role of ZSCAN4 in embryonic development, embryonic stem cells, induced pluripotent stem cells, cancer, and germ cells are discussed comprehensively.

Modaline sulfate promotes Oct4 expression and maintains self-renewal and pluripotency of stem cells through JAK/STAT3 and Wnt signaling pathways

BACKGROUND

Stem cells have been extensively explored for a variety of regenerative medical applications and they play an important role in clinical treatment of many diseases. However, the limited amount of stem cells and their tendency to undergo spontaneous differentiation upon extended propagation in vitro restrict their practical application. Octamer-binding transcription factor-4 (Oct4), a transcription factor belongs to the POU transcription factor family Class V, is fundamental for maintaining self-renewal ability and pluripotency of stem cells.

METHODS

In the present study, we used the previously constructed luciferase reporters driven by the promoter and 3'-UTR of Oct4 respectively to screen potential activators of Oct4. Colony formation assay, sphere-forming ability assay, alkaline phosphatase (AP) activity assay and teratoma-formation assay were used to assess the role of modaline sulfate (MDLS) in promoting self-renewal and reinforcing pluripotency of P19 cells. Immunofluorescence, RT-PCR, and western blotting were used to measure expression changes of stem-related genes and activation of related signaling pathways.

RESULTS

We screened 480 commercially available small-molecule compounds and discovered that MDLS greatly promoted the expression of Oct4 at both mRNA and protein levels. Moreover, MDLS significantly promoted the self-renewal capacity of P19 cells. Also, we observed that the expression of pluripotency markers and alkaline phosphatase (AP) increased significantly in MDLS-treated colonies. Furthermore, MDLS could promote teratoma formation and enhanced differentiation potential of P19 cells in vivo. In addition, we found that in the presence of LIF, MDLS could replace feeder cells to maintain the undifferentiated state of OG2-mES cells (Oct4-GFP reporter gene mouse embryonic stem cell line), and the MDLS-expanded OG2-mES cells showed an elevated expression levels of pluripotency markers in vitro. Finally, we found that MDLS promoted Oct4 expression by activating JAK/STAT3 and classic Wnt signaling pathways, and these effects were reversed by treatment with inhibitors of corresponding signaling pathways.

CONCLUSIONS

These findings demonstrated, for the first time, that MDLS could maintain self-renewal and pluripotency of stem cells.

Effects of vitamin A and retinoic acid on mouse embryonic stem cells and their differentiating progeny

Embryonic development is controlled by retinoids, and one approach that has been used to investigate the mechanisms for retinoid actions in developmental processes has been to study the effects of adding retinoids to cultures of pluripotent embryonic stem cells (ESC). To date, most in vitro retinoid research has been directed at deciphering the actions of all-trans retinoic acid (atRA). atRA is a derivative of all-trans retinol (a.k.a. vitamin A, VA), which mammals can generate via an enzyme-catalyzed pathway. atRA's effects on development result from its (1) activation of receptor complexes (RARs and RXRs) in the nucleus which then bind to and activate RA response elements (RAREs) in genes and (2) its interactions with processes that are initiated in the cytoplasm. While much work has focused on the impact of atRA on cell differentiation, VA, itself, has been shown to exert effects on the maintenance of ESC identity that are not dependent upon classic RA-signaling pathways. In this chapter, we present results from our laboratory and others using well-documented approaches for investigating the effects of retinoids on the differentiation of ESC in vitro and introduce a novel method that uses chemically-defined growth conditions. The merits of each approach are discussed.

Oncostatin M Maintains Naïve Pluripotency of mESCs by Tetraploid Embryo Complementation (TEC) Assay

It has been well established that leukemia inhibitory factor (LIF) is essential for maintaining naïve pluripotency of embryonic stem cells (ESCs). Oncostatin M (OSM) is a member of the IL-6 family of cytokines which share gp130 as a receptor subunit, and the OSM-gp130 complex can recruit either LIF receptor β or OSM receptor β. Here we show that OSM can completely replace LIF to maintain naïve pluripotency of ESCs. Mouse ESCs (mESCs) cultured in the presence of LIF or OSM not only express pluripotency genes at similar levels but also exhibit the same developmental pluripotency as evidenced by the generation of germline competent chimeras, supporting previous findings. Moreover, we demonstrate by tetraploid embryo complementation assay, the most stringent functional test of authentic pluripotency that mESCs cultured in OSM produce viable all-ESC pups. Furthermore, telomere length and telomerase activity, which are also crucial for unlimited self-renewal and genomic stability of mESCs, do not differ in mESCs cultured under OSM or LIF. The transcriptome of mESCs cultured in OSM overall is very similar to that of LIF, and OSM activates Stat3 signaling pathway, like LIF. Additionally, OSM upregulates pentose and glucuronate interconversion, ascorbate and aldarate metabolism, and steroid and retinol metabolic pathways. Although the significance of these pathways remains to be determined, our data shows that OSM can maintain naïve pluripotent stem cells in the absence of LIF.

Inverse agonism of retinoic acid receptors directs epiblast cells into the paraxial mesoderm lineage

We have investigated the differentiation of paraxial mesoderm from mouse embryonic stem cells utilizing a Tbx6-EYFP/Brachyury (T)-Cherry dual reporter system. Differentiation from the mouse ESC state directly into mesoderm via Wnt pathway activation was low, but augmented by treatment with AGN193109, a pan-retinoic acid receptor inverse agonist. After five days of differentiation, T⁺ cells increased from 12.2% to 18.8%, Tbx6⁺ cells increased from 5.8% to 12.7%, and T⁺/Tbx6⁺ cells increased from 2.4% to 14.1%. The synergism of AGN193109 with Wnt3a/CHIR99021 was further substantiated by the increased expression of paraxial mesoderm gene markers Tbx6, Msgn1, Meox1, and Hoxb1. Separate to inverse agonist treatment, when mouse ESCs were indirectly differentiated into mesoderm via a transient epiblast step the efficiency of paraxial mesoderm formation markedly increased. Tbx6⁺ cells represented 65-75% of the total cell population after just 3 days of differentiation and the expression of paraxial mesoderm marker genes Tbx6 and Msgn increased over 100-fold and 300-fold, respectively. Further evaluation of AGN193109 treatment on the indirect differentiation protocol suggested that RARs have two distinct roles. First, AGN193109 treatment at the epiblast step and mesoderm step promoted paraxial mesoderm formation over other mesoderm and endoderm lineage types. Second, continued treatment during mesoderm formation revealed its ability to repress the maturation of presomitic mesoderm into somitic paraxial mesoderm. Thus, the continuous treatment of AGN193109 during epiblast and mesoderm differentiation steps yielded a culture where ~90% of the cells were Tbx6⁺. The surprisingly early effect of inverse agonist treatment at the epiblast step of differentiation led us to further examine the effect of AGN193109 treatment during an extended epiblast differentiation protocol. Interestingly, while inverse agonist treatment had no impact on the conversion of ESCs into epiblast cells based on the expression of Rex1, Fgf5, and pluripotency marker genes Oct4, Nanog, and Sox2, after three days of differentiation in the presence of AGN193109 caudal epiblast and early paraxial mesoderm marker genes, T, Cyp26a1, Fgf8, Tbx6 and Msgn were all highly up-regulated. Collectively, our studies reveal an earlier than appreciated role for RARs in epiblast cells and the modulation of their function via inverse agonist treatment can promote their differentiation into the paraxial mesoderm lineage.

Retinol Promotes In Vitro Growth of Proximal Colon Organoids through a Retinoic Acid-Independent Mechanism

Retinol (ROL), the alcohol form of vitamin A, is known to control cell fate decision of various types of stem cells in the form of its active metabolite, retinoic acid (RA). However, little is known about whether ROL has regulatory effects on colonic stem cells. We examined in this study the effect of ROL on the growth of murine normal colonic cells cultured as organoids. As genes involved in RA synthesis from ROL were differentially expressed along the length of the colon, we tested the effect of ROL on proximal and distal colon organoids separately. We found that organoid forming efficiency and the expression level of Lgr5, a marker gene for colonic stem cells were significantly enhanced by ROL in the proximal colon organoids, but not in the distal ones. Interestingly, neither retinaldehyde (RAL), an intermediate product of the ROL-RA pathway, nor RA exhibited growth promoting effects on the proximal colon organoids, suggesting that ROL-dependent growth enhancement in organoids involves an RA-independent mechanism. This was confirmed by the observation that an inhibitor for RA-mediated gene transcription did not abrogate the effect of ROL on organoids. This novel role of ROL in stem cell maintenance in the proximal colon provides insights into the mechanism of region-specific regulation for colonic stem cell maintenance.

[Effects of retinol on expressions of epidermal growth factor, stem cell factor, colony-stimulating factor 1 and leukemia inhibitory factor in human umbilical cord-derived mesenchymal stem cells]

OBJECTIVE

To investigate effects of retinol on the expressions of epidermal growth factor (EGF), stem cell factor (SCF), colony-stimulating factor 1 (CSF1) and leukemia inhibitory factor (LIF) in cultured human umbilical-derived mesenchymal stem cells (UCMSCs).

METHODS

Human UCMSCs were isolated from human umbilical cord and identified for immunophenotypes. The cells were then cultured in DMEM/F12 media supplemented with 12% fetal bovine serum (FBS), 12% FBS+1 µmol/L retinol, 15% knockout serum replacement (KSR) and 15% KSR+ 1 µmol/L retinol. The expressions of the cytokines EGF, SCF, CSF1 and LIF in the cells were detected using RT-PCR and ELISA.

RESULTS

The isolated cells exhibited characteristic immunophenotypes of human UCMSCs and expressed EGF, CSF1 and SCF at both mRNA and protein levels but not LIF protein. Retinol (1 µmol/L) significantly promoted the expressions of SCF and CSF1 at both mRNA and protein levels but did not result in changes of EGF and LIF expressions in human UCMSCs.

CONCLUSION

Retinol at the concentration of 1 µmol/L can promote expression of SCF and CSF1 in human UCMSCs in vitro.

Identification of compounds that modulate retinol signaling using a cell-based qHTS assay

In vertebrates, the retinol (vitamin A) signaling pathway (RSP) controls the biosynthesis and catabolism of all-trans retinoic acid (atRA), which regulates transcription of genes essential for embryonic development. Chemicals that interfere with the RSP to cause abnormal intracellular levels of atRA are potential developmental toxicants. To assess chemicals for the ability to interfere with retinol signaling, we have developed a cell-based RARE (Retinoic Acid Response Element) reporter gene assay to identify RSP disruptors. To validate this assay in a quantitative high-throughput screening (qHTS) platform, we screened the Library of Pharmacologically Active Compounds (LOPAC) in both agonist and antagonist modes. The screens detected known RSP agonists, demonstrating assay reliability, and also identified novel RSP agonists including kenpaullone, niclosamide, PD98059 and SU4312, and RSP antagonists including Bay 11-7085, LY294002, 3,4-Methylenedioxy-β-nitrostyrene, and topoisomerase inhibitors (camptothecin, topotecan, amsacrine hydrochloride, and idarubicin). When evaluated in the P19 pluripotent cell, these compounds were found to affect the expression of the Hoxa1 gene that is essential for embryo body patterning. These results show that the RARE assay is an effective qHTS approach for screening large compound libraries to identify chemicals that have the potential to adversely affect embryonic development through interference with retinol signaling.

Leukemia Inhibitory Factor and Fibroblast Growth Factor 2 Critically and Mutually Sustain Pluripotency of Rabbit Embryonic Stem Cells

Effects of leukemia inhibitory factor (LIF) and fibroblast growth factor 2 (FGF2) on establishment and maintenance of rabbit embryonic stem cell (rESC) lines were assessed. When grown on MEF feeders, rESC lines derived from fertilized embryos were established and maintained in medium containing paracrine factors LIF (via STAT3) and/or FGF2 (via MEK-ERK1/2 and PI3K-AKT). However, high levels of ERK1/2 and AKT activities in rESCs were crucial for maintaining their undifferentiated proliferation. Although rESCs under the influence of either LIF (500, 1,000, and 2,000 U/ml) or FGF2 (5, 10, and 20 ng/ml) alone had enhanced expression of pluripotency markers, peak expression occurred when both LIF (1,000 U/ml) and FGF2 (10 ng/ml) were applied. Induced dephosphorylation of STAT3, ERK1/2, and AKT by specific inhibitors limited growth of rESCs and caused remarkable losses of self-renewal capacity; therefore, we inferred that STAT3, ERK, and AKT had essential roles in maintaining rESC proliferation and self-renewal. We concluded that LIF and FGF2 jointly maintained the undifferentiated state and self-renewal of rESCs through an integrative signaling module.

Materials for stem cell factories of the future

Polymeric substrates are being identified that could permit translation of human pluripotent stem cells from laboratory-based research to industrial-scale biomedicine. Well-defined materials are required to allow cell banking and to provide the raw material for reproducible differentiation into lineages for large-scale drug-screening programs and clinical use. Yet more than 1 billion cells for each patient are needed to replace losses during heart attack, multiple sclerosis and diabetes. Producing this number of cells is challenging, and a rethink of the current predominant cell-derived substrates is needed to provide technology that can be scaled to meet the needs of millions of patients a year. In this Review, we consider the role of materials discovery, an emerging area of materials chemistry that is in large part driven by the challenges posed by biologists to materials scientists.

Vitamin A/retinol and maintenance of pluripotency of stem cells

Retinol, the alcohol form of vitamin A is a key dietary component that plays a critical role in vertebrate development, cell differentiation, reproduction, vision and immune system. Natural and synthetic analogs of retinol, called retinoids, have generally been associated with the cell differentiation via retinoic acid which is the most potent metabolite of retinol. However, a direct function of retinol has not been fully investigated. New evidence has now emerged that retinol supports the self-renewal of stem cells including embryonic stem cells (ESCs), germ line stem cells (GSCs) and cancer stem cells (CSCs) by activating the endogenous machinery for self-renewal by a retinoic acid independent mechanism. The studies have also revealed that stem cells do not contain enzymes that are responsible for metabolizing retinol into retinoic acid. This new function of retinol may have important implications for stem cell biology which can be exploited for quantitative production of pure population of pluripotent stem cells for regenerative medicine as well as clinical applications for cancer therapeutics.

Small molecules for cell reprogramming and heart repair: progress and perspective

Regenerative medicine for heart failure seeks to replace lost cardiomyocytes. Chemical approaches for producing ample supplies of cells, such as pluripotent stem cells and cardiomyocytes, hold promise as practical means to achieve safe, facile cell-based therapy for cardiac repair and regenerative medicine. In this review, we describe recent advances in the application of small molecules to improve the generation and maintenance of pluripotent stem cells. We also describe new directions in heart repair and regeneration in which chemical approaches may find their application.

Stem cells and small molecule screening: haploid embryonic stem cells as a new tool

Stem cells can both self-renew and differentiate into various cell types under certain conditions, which makes them a good model for development and disease studies. Recently, chemical approaches have been widely applied in stem cell biology by promoting stem cell self-renewal, proliferation, differentiation and somatic cell reprogramming using specific small molecules. Conversely, stem cells and their derivatives also provide an efficient and robust platform for small molecule and drug screening. Here, we review the current research and applications of small molecules that modulate stem cell self-renewal and differentiation and improve reprogramming, as well as the applications that use stem cells as a tool for small molecule screening. Moreover, we introduce the recent advance in haploid embryonic stem cells research. Haploid embryonic stem cells maintain haploidy and stable growth over extensive passages, possess the ability to differentiate into all three germ layers in vitro and in vivo, and contribute to the germlines of chimeras when injected into blastocysts. Androgenetic haploid stem cells can also be used in place of sperm to produce fertile progeny after intracytoplasmic injection into mature oocytes. Such characteristics demonstrate that haploid stem cells are a new approach for genetic studies at both the cellular and animal levels and that they are a valuable platform for future small molecule screening.

Ectopic pregnancy-derived human trophoblastic stem cells regenerate dopaminergic nigrostriatal pathway to treat parkinsonian rats

Background

Stem cell therapy is a potential strategy to treat patients with Parkinson’s disease (PD); however, several practical limitations remain. As such, finding the appropriate stem cell remains the primary issue in regenerative medicine today. We isolated a pre-placental pluripotent stem cell from the chorionic villi of women with early tubal ectopic pregnancies. Our objectives in this study were (i) to identify the characteristics of hTS cells as a potential cell source for therapy; and (ii) to test if hTS cells can be used as a potential therapeutic strategy for PD.

Methods and Findings

hTS cells expressed gene markers of both the trophectoderm (TE) and the inner cell mass (ICM). hTS cells exhibited genetic and biological characteristics similar to that of hES cells, yet genetically distinct from placenta-derived mesenchymal stem cells. All-trans retinoic acid (RA) efficiently induced hTS cells into trophoblast neural stem cells (tNSCs) in 1-day. Overexpression of transcription factor Nanog was possibly achieved through a RA-induced non-genomic c-Src/Stat3/Nanog signaling pathway mediated by the subcellular c-Src mRNA localization for the maintenance of pluripotency in tNSCs. tNSC transplantation into the lesioned striatum of acute and chronic PD rats not only improved behavioral deficits but also regenerated dopaminergic neurons in the nigrostriatal pathway, evidenced by immunofluorescent and immunohistological analyses at 18-weeks. Furthermore, tNSCs showed immunological advantages for the application in regenerative medicine.

Conclusions

We successfully isolated and characterized the unique ectopic pregnancy-derived hTS cells. hTS cells are pluripotent stem cells that can be efficiently induced to tNSCs with positive results in PD rat models. Our data suggest that the hTS cell is a dynamic stem cell platform that is potentially suitable for use in disease models, drug discovery, and cell therapy such as PD.

Retina, retinol, retinal and the natural history of vitamin A as a light sensor

Light is both the ultimate energy source for most organisms and a rich information source. Vitamin A-based chromophore was initially used in harvesting light energy, but has become the most widely used light sensor throughout evolution from unicellular to multicellular organisms. Vitamin A-based photoreceptor proteins are called opsins and have been used for billions of years for sensing light for vision or the equivalent of vision. All vitamin A-based light sensors for vision in the animal kingdom are G-protein coupled receptors, while those in unicellular organisms are light-gated channels. This first major switch in evolution was followed by two other major changes: the switch from bistable to monostable pigments for vision and the expansion of vitamin A's biological functions. Vitamin A's new functions such as regulating cell growth and differentiation from embryogenesis to adult are associated with increased toxicity with its random diffusion. In contrast to bistable pigments which can be regenerated by light, monostable pigments depend on complex enzymatic cycles for regeneration after every photoisomerization event. Here we discuss vitamin A functions and transport in the context of the natural history of vitamin A-based light sensors and propose that the expanding functions of vitamin A and the choice of monostable pigments are the likely evolutionary driving forces for precise, efficient, and sustained vitamin A transport.

Detection of variable levels of RARα and RARγ proteins in pluripotent and differentiating mouse embryonal carcinoma and mouse embryonic stem cells

Pluripotent mouse embryonal carcinoma (mEC) and mouse embryonic stem (mES) cells differentiate into several cell lineages upon retinoic acid (RA) addition. Differentiation is facilitated, in part, by RA activation of nuclear RA receptors (RARs) that bind to DNA response elements located in the promoters of target genes. The purpose of the studies reported here was to immunolocalize RARα and RARγ protein in mEC and mES cells and in their RA-induced differentiated progeny. Fixed cells were reacted with three different RARα antibodies and one RARγ antibody. Pluripotent and differentiated mEC and mES cells showed positive nuclear immunoreactivity with all antibodies tested. Two RARα antibodies also showed positive reactivity in the cytoplasm. Surprisingly, our results revealed variability in immunofluorescence intensity and in RARα and RARγ distribution from one cell to the other, suggesting that RARα and RARγ protein levels were not synchronous throughout the cell population. The results indicate that RARα and RARγ are present in pluripotent and differentiating mEC and mES cells and suggest that the expression of these proteins is dynamic.

Retinol (vitamin A) maintains self-renewal of pluripotent male germline stem cells (mGSCs) from adult mouse testis

Studies have shown that male germline stem cells (mGSCs), which are responsible for maintaining spermatogenesis in the male, could be obtained from mouse and human testis. However, the traditional cultural methods were mostly dependent on serum and feeder, and the initial mGSCs were either obtained from neonatal mice or the detailed description of its potency and origin was not provided. Here we reported a novel (retinol (RE) serum-free and feeder-free) system for the successful culture of adult germline stem cells from adult Kunming mice (8-24 weeks) testis. The isolated mGSCs cultured in RE serum-free and feeder-free medium maintained the typical morphology of undifferentiated embryonic stem cells (ESCs), and they proliferated well in RE medium analyzed by proliferation assay, RT-PCR, microarray, and Western blotting. These cells also showed typical properties of ESCs (alkaline phosphatase (AP) positive, expressions of Oct4, Sox2, Nanog, and SSEA1, with the capacity to form teratomas and differentiate into various types of cells within three germ layers). Taken together, we conclude that RE promotes the self-renewal of mGSCs and maintains the pluripotency of mGSCs, the RE serum-free and feeder-free system may be useful for the culture of pluripotent stem cell lines from adult testis tissues, which provides a new resource for tissue engineering and therapy for infertility.

The promotion of stemness and pluripotency following feeder-free culture of embryonic stem cells on collagen-grafted 3-dimensional nanofibrous scaffold

The components of extracellular matrix (ECM) may substitute for feeder layers that promote the self-renewal pathways in embryonic stem cells. Surface modification of electrospun nanofibrous scaffolds have been studied to closely resemble natural ECMs and support in vitro and in vivo proliferation, pluripotency and differentiation of stem cells. In this study, we analyzed the maintenance of stemness and pluripotency of the mouse embryonic stem cell (mESC) following feeder-free culture on collagen-grafted polyethersulfone (PES-COL) electrospun nanofibrous scaffold. Our results showed that, the mESCs cultured for seven passages on PES-COL scaffolds had a typical undifferentiated morphology, enhanced proliferation, stable diploid normal karyotype, and continued expression of stemness and pluripotency-associated markers, Oct-4, Nanog, SSEA-1, and Alkaline phosphatase (ALP) in comparison with PES scaffolds and gelatin-coated plate. Moreover, these cells retained their in vitro and in vivo pluripotency. Our results indicated the enhanced infiltration and teratoma formation of mESCs in PES-COL. Collagen-grafted polyethersulfone nanofibrous scaffold has potential for feeder-free culture of pluripotent stem cells because of its 3-dimensional structure and bioactivity which enhance pluripotency, proliferation, differentiation, and infiltration of embryonic stem cells.

Short-term serum-free culture reveals that inhibition of Gsk3β induces the tumor-like growth of mouse embryonic stem cells

Here, we present evidence that the tumor-like growth of mouse embryonic stem cells (mESCs) is suppressed by short-term serum-free culture, which is reversed by pharmacological inhibition of Gsk3β. Mouse ESCs maintained under standard conditions using fetal bovine serum (FBS) were cultured in a uniquely formulated chemically-defined serum-free (CDSF) medium, namely ESF7, for three passages before being subcutaneously transplanted into immunocompromised mice. Surprisingly, the mESCs failed to produce teratomas for up to six months, whereas mESCs maintained under standard conditions generated well-developed teratomas in five weeks. Mouse ESCs cultured under CDSF conditions maintained the expression of Oct3/4, Nanog, Sox2 and SSEA1, and differentiated into germ cells in vivo. In addition, when mESCs were cultured under CDSF conditions supplemented with FBS, or when the cells were cultured under CDSF conditions followed by standard culture conditions, they consistently developed into teratomas. Thus, these results validate that the pluripotency of mESCs was not compromised by CDSF conditions. Mouse ESCs cultured under CDSF conditions proliferated significantly more slowly than mESCs cultured under standard conditions, and were reminiscent of Eras-null mESCs. In fact, their slower proliferation was accompanied by the downregulation of Eras and c-Myc, which regulate the tumor-like growth of mESCs. Remarkably, when mESCs were cultured under CDSF conditions supplemented with a pharmacological inhibitor of Gsk3β, they efficiently proliferated and developed into teratomas without upregulation of Eras and c-Myc, whereas mESCs cultured under standard conditions expressed Eras and c-Myc. Although the role of Gsk3β in the self-renewal of ESCs has been established, it is suggested with these data that Gsk3β governs the tumor-like growth of mESCs by means of a mechanism different from the one to support the pluripotency of ESCs.

Membrane receptors and transporters involved in the function and transport of vitamin A and its derivatives

The eye is the human organ most sensitive to vitamin A deficiency because of vision's absolute and heavy dependence on vitamin A for light perception. Studies of the molecular basis of vision have provided important insights into the intricate mechanistic details of the function, transport and recycling of vitamin A and its derivatives (retinoid). This review focuses on retinoid-related membrane receptors and transporters. Three kinds of mammalian membrane receptors and transporters are discussed: opsins, best known as vitamin A-based light sensors in vision; ABCA4, an ATP-dependent transporter specializes in the transport of vitamin A derivative; and STRA6, a recently identified membrane receptor that mediates cellular uptake of vitamin A. The evolutionary driving forces for their existence and the wide spectrum of human diseases associated with these proteins are discussed. Lessons learned from the study of the visual system might be useful for understanding retinoid biology and retinoid-related diseases in other organ systems as well. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

The membrane receptor for plasma retinol-binding protein, a new type of cell-surface receptor

Vitamin A is essential for diverse aspects of life ranging from embryogenesis to the proper functioning of most adult organs. Its derivatives (retinoids) have potent biological activities such as regulating cell growth and differentiation. Plasma retinol-binding protein (RBP) is the specific vitamin A carrier protein in the blood that binds to vitamin A with high affinity and delivers it to target organs. A large amount of evidence has accumulated over the past decades supporting the existence of a cell-surface receptor for RBP that mediates cellular vitamin A uptake. Using an unbiased strategy, this specific cell-surface RBP receptor has been identified as STRA6, a multitransmembrane domain protein with previously unknown function. STRA6 is not homologous to any protein of known function and represents a new type of cell-surface receptor. Consistent with the diverse functions of vitamin A, STRA6 is widely expressed in embryonic development and in adult organ systems. Mutations in human STRA6 are associated with severe pathological phenotypes in many organs such as the eye, brain, heart, and lung. STRA6 binds to RBP with high affinity and mediates vitamin A uptake into cells. This review summarizes the history of the RBP receptor research, its expression in the context of known functions of vitamin A in distinct human organs, structure/function analysis of this new type of membrane receptor, pertinent questions regarding its very existence, and its potential implication in treating human diseases.

The orphan nuclear receptor TR4 is a vitamin A-activated nuclear receptor

Testicular receptors 2 and 4 (TR2/4) constitute a subgroup of orphan nuclear receptors that play important roles in spermatogenesis, lipid and lipoprotein regulation, and the development of the central nervous system. Currently, little is known about the structural features and the ligand regulation of these receptors. Here we report the crystal structure of the ligand-free TR4 ligand binding domain, which reveals an autorepressed conformation. The ligand binding pocket of TR4 is filled by the C-terminal half of helix 10, and the cofactor binding site is occupied by the AF-2 helix, thus preventing ligand-independent activation of the receptor. However, TR4 exhibits constitutive transcriptional activity on multiple promoters, which can be further potentiated by nuclear receptor coactivators. Mutations designed to disrupt cofactor binding, dimerization, or ligand binding substantially reduce the transcriptional activity of this receptor. Importantly, both retinol and retinoic acid are able to promote TR4 to recruit coactivators and to activate a TR4-regulated reporter. These findings demonstrate that TR4 is a ligand-regulated nuclear receptor and suggest that retinoids might have a much wider regulatory role via activation of orphan receptors such as TR4.

Techniques to study specific cell-surface receptor-mediated cellular vitamin A uptake

STRA6 is a multitransmembrane domain protein that was recently identified as the cell-surface receptor for plasma retinol-binding protein (RBP), the vitamin A carrier protein in the blood. STRA6 binds to RBP with high affinity and mediates cellular uptake of vitamin A from RBP. It is not homologous to any known receptors, transporters, and channels, and it represents a new class of membrane transport protein. Consistent with the diverse physiological functions of vitamin A, STRA6 is widely expressed in diverse adult organs and throughout embryonic development. Mutations in human STRA6 that abolish its vitamin A uptake activity cause severe pathological phenotypes in many human organs including the eye, brain, lung, and heart. This chapter describes functional assays for STRA6 in live cells and on cellular membranes. These assays can be employed to study the mechanism of this new membrane transport mechanism and its roles in the physiology and pathology of many organs.

Feeder-independent culture of mouse embryonic stem cells using vitamin A/retinol

Embryonic stem (ES) cells derived from the inner cell mass of a mammalian blastocyst represent unlimited source of all types of cells for regenerative medicine and for drug discovery. Mouse and human ES cells require mouse embryonic fibroblast feeder cells to maintain their undifferentiated state which involve additional time-consuming and labor-intensive steps. Recently we reported a novel function of retinol, the alcohol form of vitamin A, in preventing the differentiation of mouse ES cells. Retinol/vitamin A induces the overexpression of Nanog, a key transcription factor that is important for maintaining the pluripotency of mouse and human ES cells. Further, retinol/vitamin A also supports feeder-independent culture of ES cells in long-term cultures. The cells continue to maintain the expression of pluripotent cell-specific markers such as Nanog, Oct4, and Sox2 and form chimeric animals after injection into blastocysts. In this chapter, we describe feeder-independent cultures of mouse ES cells in the medium supplemented with retinol. The ES cells are cultured over plates coated with gelatin in ES medium with leukemia inhibitory factor (LIF) which is supplemented with 0.5 muM retinol/vitamin A. The cells are passaged every 3-5 days by trypsinization. The pluripotency of the cells is tested by different undifferentiated ES cell-specific markers.

A novel signaling by vitamin A/retinol promotes self renewal of mouse embryonic stem cells by activating PI3K/Akt signaling pathway via insulin-like growth factor-1 receptor

Pluripotent embryonic stem (ES) cells are a potential source of all types of cells for regenerative medicine. ES cells maintain pluripotency through a complex interplay of different signaling pathways and transcription factors, including leukemia inhibitory factor (LIF), Nanog, Sox2, and Oct3/4. Nanog, however, plays a key role in maintaining the pluripotency of mouse and human ES cells. Phosphoinositde 3-kinase (PI3K) signaling pathway which is activated in response to growth factors and cytokines also plays a critical role in promoting the survival and proliferation of ES cells. Our earlier studies revealed that retinol, the alcohol form of vitamin A, enhances the expression of Nanog and prevents differentiation of ES cells in long-term cultures. Normally vitamin A/retinol is associated with cell differentiation via its potent metabolite, retinoic acid. Thus far, no direct function has been ascribed to retinol itself. In this study, we demonstrate for the first time that retinol directly activates phosphoinositide three (PI3) kinase signaling pathway through IGF-1 receptor/insulin receptor substrate one (IRS-1) by engaging Akt/PKB-mTORC1 mammalian target of rapamycin-2 (mammalian target of rapamycin complex 2), indicating a growth factor-like function of vitamin A. Furthermore, ES cells do not express enzymes to metabolize retinol into retinoic acid and lack receptors for retinol transport into the cytoplasm, indicating that retinol signaling is independent of retinoic acid. This study presents a novel system to investigate how extracellular signals control the self renewal of ES cells which will be important for high-quality ES cells for regenerative medicine.

Small molecules that modulate embryonic stem cell fate and somatic cell reprogramming

Recent breakthroughs in stem cell biology, especially the development of induced pluripotent stem cell technique, have generated tremendous enthusiasm and efforts to explore the therapeutic potential of stem cells in regenerative medicine. Improved understanding of stem cell biology, in addition to better control of stem cell fate, is critical to realize this potential. Small molecules, targeting specific signaling pathways and/or mechanisms, have been shown to be useful chemical tools in manipulating cell fate, state and function. These small molecules are starting to play increasingly important roles in both elucidating the fundamental biology of stem cells and facilitating the development of therapeutic approaches toward regenerative medicine. Such approaches could involve cell replacement therapies using homogenous functional cells produced under chemically defined conditions in vitro and the development of small-molecule drugs that can stimulate patients' endogenous cells to repair and regenerate. Here, we review recent progress in using small molecules to sustain pluripotency, or induce differentiation of embryonic stem cells. We also highlight small molecules that can replace transcription factors and/or enhance efficiency during somatic cell reprogramming.

AICAR activates the pluripotency transcriptional network in embryonic stem cells and induces KLF4 and KLF2 expression in fibroblasts

Background

Pluripotency, the property of a cell to differentiate into all cellular types of a given organism, is central to the development of stem cell-based therapies and regenerative medicine. Stem cell pluripotency is the result of the orchestrated activation of a complex transcriptional network characterized by the expression of a set of transcription factors including the master regulators of pluripotency Nanog and Oct4. Recently, it has been shown that pluripotency can be induced in somatic cells by viral-mediated expression of the transcription factors Oct3/4, Sox2, Klf4, and c-Myc.

Results

Here we show that 5-Aminoimidazole-4-carboxamide-1-b-riboside (AICAR) is able to activate the molecular circuitry of pluripotency in mouse embryonic stem cells (mESC) and maintain Nanog and Oct4 expression in mESC exposed to the differentiating agent retinoic acid. We also show that AICAR is able to induce Klf4, Klf2 and Myc expression in both mESC and murine fibroblasts.

Conclusion

AICAR is able to activate the molecular circuitry of pluripotency in mESC and to induce the expression of several key regulators of pluripotency in somatic cells. AICAR is therefore a useful pharmacological entity for studying small molecule mediated induction of pluripotency.

Expression profile of differentiating serotonin neurons derived from rhesus embryonic stem cells and comparison to adult serotonin neurons

The rhesus monkey embryonic stem cell line 366.4 differentiates into serotonin neurons. We examined the genetic cascade during differentiation and compared ESC-derived serotonin neurons to adult monkey serotonin neurons. RNA was extracted from ESC colonies, embryoid bodies (EBs), neurospheres in selection (N1) and proliferation stages (N2), differentiated serotonin neurons (N3) and from laser captured (LC) serotonin neurons of spayed female macaques treated with placebo, estrogen (E), progesterone (P) or E+P. The RNA was labeled and hybridized to Rhesus Monkey Affymetrix Gene Chips (n=1 per stage and 2 per animal treatment). Gene expression was examined with GeneSifter software. 545 genes that were related to developmental processes showed a threefold or greater change between stages. TGFb, Wnt, VEGF and Hedgehog signaling pathways showed the highest percent of probe set changes during differentiation. Genes in the categories (a) homeobox binding and transcription factors, (b) growth factors and receptors, (c) brain and neural specific factors and (d) serotonin specific factors are reported. Pivotal genes were confirmed with quantitative RT-PCR. In the serotonin developmental cascade, FGFR2 was robustly expressed at each stage. GATA3 was robustly expressed in EBs. Sonic hedgehog (Shh), PTCH (Shh-R) and Fev1 transcription factor expression coincided with the induction of serotonin specific marker genes during N1-selection. A majority of the examined genes were expressed in adult serotonin neurons. However, in the ESC-derived neurons, there was significant over-representation of probe sets related to cell cycle, axon guidance & dorso-ventral axis formation. This analysis suggests that the 366.4 cell line possesses cues for serotonin differentiation at early stages of differentiation, but that ESC-derived serotonin neurons are still immature.