FGF-2 modulates Wnt signaling in undifferentiated hESC and iPS cells through activated PI3-K/GSK3beta signaling

Signaling Pathways and Gene Regulatory Networks in Cardiomyocyte Differentiation

Strategies for harnessing stem cells as a source to treat cell loss in heart disease are the subject of intense research. Human pluripotent stem cells (hPSCs) can be expanded extensively in vitro and therefore can potentially provide sufficient quantities of patient-specific differentiated cardiomyocytes. Although multiple stimuli direct heart development, the differentiation process is driven in large part by signaling activity. The engineering of hPSCs to heart cell progeny has extensively relied on establishing proper combinations of soluble signals, which target genetic programs thereby inducing cardiomyocyte specification. Pertinent differentiation strategies have relied as a template on the development of embryonic heart in multiple model organisms. Here, information on the regulation of cardiomyocyte development from in vivo genetic and embryological studies is critically reviewed. A fresh interpretation is provided of in vivo and in vitro data on signaling pathways and gene regulatory networks (GRNs) underlying cardiopoiesis. The state-of-the-art understanding of signaling pathways and GRNs presented here can inform the design and optimization of methods for the engineering of tissues for heart therapies.

A stable chimeric fibroblast growth factor (FGF) can successfully replace basic FGF in human pluripotent stem cell culture

Fibroblast growth factors (FGFs) are essential for maintaining self-renewal in human embryonic stem cells and induced pluripotent stem cells. Recombinant basic FGF (bFGF or FGF2) is conventionally used to culture pluripotent stem cells; however, because of the instability of bFGF, repeated addition of fresh bFGF into the culture medium is required in order to maintain its concentration. In this study, we demonstrate that a heat-stable chimeric variant of FGF, termed FGFC, can be successfully used for maintaining human pluripotent stem cells. FGFC is a chimeric protein composed of human FGF1 and FGF2 domains that exhibits higher thermal stability and protease resistance than do both FGF1 and FGF2. Both human embryonic stem cells and induced pluripotent stem cells were maintained in ordinary culture medium containing FGFC instead of FGF2. Comparison of cells grown in FGFC with those grown in conventional FGF2 media showed no significant differences in terms of the expression of pluripotency markers, global gene expression, karyotype, or differentiation potential in the three germ lineages. We therefore propose that FGFC may be an effective alternative to FGF2, for maintenance of human pluripotent stem cells.

Protein kinases and associated pathways in pluripotent state and lineage differentiation

Protein kinases (PKs) mediate the reversible conversion of substrate proteins to phosphorylated forms, a key process in controlling intracellular signaling transduction cascades. Pluripotency is, among others, characterized by specifically expressed PKs forming a highly interconnected regulatory network that culminates in a finely-balanced molecular switch. Current high-throughput phosphoproteomic approaches have shed light on the specific regulatory PKs and their function in controlling pluripotent states. Pluripotent cell-derived endothelial and hematopoietic developments represent an example of the importance of pluripotency in cancer therapeutics and organ regeneration. This review attempts to provide the hitherto known kinome profile and the individual characterization of PK-related pathways that regulate pluripotency. Elucidating the underlying intrinsic and extrinsic signals may improve our understanding of the different pluripotent states, the maintenance or induction of pluripotency, and the ability to tailor lineage differentiation, with a particular focus on endothelial cell differentiation for anti-cancer treatment, cell-based tissue engineering, and regenerative medicine strategies.

Targeting novel signaling pathways for resistant acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematologic malignancy that is the most common type of acute leukemia diagnosed in adults and the second most common type in children. The overall survival is poor and treatment is associated with significant complications and even death. In addition, a significant number of patients will not respond to therapy or relapse. In this review, several new signaling proteins aberrantly regulated in AML are described, including CREB, Triad1, Bcl-2 family members, Stat3, and mTOR/MEK. Identifying more effective and less toxic agents will provide novel approaches to treat AML.

On human pluripotent stem cell control: The rise of 3D bioengineering and mechanobiology

Human pluripotent stem cells (hPSCs) provide promising resources for regenerating tissues and organs and modeling development and diseases in vitro. To fulfill their promise, the fate, function, and organization of hPSCs need to be precisely regulated in a three-dimensional (3D) environment to mimic cellular structures and functions of native tissues and organs. In the past decade, innovations in 3D culture systems with functional biomaterials have enabled efficient and versatile control of hPSC fate at the cellular level. However, we are just at the beginning of bringing hPSC-based regeneration and development and disease modeling to the tissue and organ levels. In this review, we summarize existing bioengineered culture platforms for controlling hPSC fate and function by regulating inductive mechanical and biochemical cues coexisting in the synthetic cell microenvironment. We highlight recent excitements in developing 3D hPSC-based in vitro tissue and organ models with in vivo-like cellular structures, interactions, and functions. We further discuss an emerging multifaceted mechanotransductive signaling network--with transcriptional coactivators YAP and TAZ at the center stage--that regulate fates and behaviors of mammalian cells, including hPSCs. Future development of 3D biomaterial systems should incorporate dynamically modulated mechanical and chemical properties targeting specific intracellular signaling events leading to desirable hPSC fate patterning and functional tissue formation in 3D.

Anti-inflammatory activity of Wnt signaling in enteric nervous system: in vitro preliminary evidences in rat primary cultures

Background

In the last years, Wnt signaling was demonstrated to regulate inflammatory processes. In particular, an increased expression of Wnts and Frizzled receptors was reported in inflammatory bowel disease (IBD) and ulcerative colitis to exert both anti- and pro-inflammatory functions regulating the intestinal activated nuclear factor κB (NF-кB), TNFa release, and IL10 expression.

Methods

To investigate the role of Wnt pathway in the response of the enteric nervous system (ENS) to inflammation, neurons and glial cells from rat myenteric plexus were treated with exogenous Wnt3a and/or LPS with or without supporting neurotrophic factors such as basic fibroblast growth factor (bFGF), epithelial growth factor (EGF), and glial cell-derived neurotrophic factor (GDNF). The immunophenotypical characterization by flow cytometry and the protein and gene expression analysis by qPCR and Western blotting were carried out.

Results

Flow cytometry and immunofluorescence staining evidenced that enteric neurons coexpressed Frizzled 9 and toll-like receptor 4 (TLR4) while glial cells were immunoreactive to TLR4 and Wnt3a suggesting that canonical Wnt signaling is active in ENS.

Under in vitro LPS treatment, Western blot analysis demonstrated an active cross talk between canonical Wnt signaling and NF-кB pathway that is essential to negatively control enteric neuronal response to inflammatory stimuli. Upon costimulation with LPS and Wnt3a, a significant anti-inflammatory activity was detected by RT-PCR based on an increased IL10 expression and a downregulation of pro-inflammatory cytokines TNFa, IL1B, and interleukin 6 (IL6). When the availability of neurotrophic factors in ENS cultures was abolished, a changed cell reactivity by Wnt signaling was observed at basal conditions and after LPS treatment.

Conclusions

The results of this study suggested the existence of neuronal surveillance through FZD9 and Wnt3a in enteric myenteric plexus. Moreover, experimental evidences were provided to clarify the correlation among soluble trophic factors, Wnt signaling, and anti-inflammatory protection of ENS.

Functional consequences of 17q21.31/WNT3-WNT9B amplification in hPSCs with respect to neural differentiation

Human pluripotent stem cell (hPSC) lines exhibit repeated patterns of genetic variation, which can alter in vitro properties as well as suitability for clinical use. We examined associations between copy-number variations (CNVs) on chromosome 17 and hPSC mesodiencephalic dopaminergic (mDA) differentiation. Among 24 hPSC lines, two karyotypically normal lines, BG03 and CT3, and BG01V2, with trisomy 17, exhibited amplification of the WNT3/WNT9B region and rapid mDA differentiation. In hPSC lines with amplified WNT3/WNT9B, basic fibroblast growth factor (bFGF) signaling through mitogen-activated protein kinase (MAPK)/ERK amplifies canonical WNT signaling by phosphorylating LRP6, resulting in enhanced undifferentiated proliferation. When bFGF is absent, noncanonical WNT signaling becomes dominant due to upregulation of SIAH2, enhancing JNK signaling and promoting loss of pluripotency. When bFGF is present during mDA differentiation, stabilization of canonical WNT signaling causes upregulation of LMX1A and mDA induction. Therefore, CNVs in 17q21.31, a "hot spot" for genetic variation, have multiple and complex effects on hPSC cellular phenotype.

Heterogeneous nuclear ribonucleoprotein A2/B1 regulates the self-renewal and pluripotency of human embryonic stem cells via the control of the G1/S transition

Self-renewal and pluripotency of human embryonic stem cells (hESCs) are a complex biological process for maintaining hESC stemness. However, the molecular mechanisms underlying these special properties of hESCs are not fully understood. Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP A2/B1) is a multifunctional RNA-binding protein whose expression is related to cell proliferation and carcinogenesis. In this study, we found that hnRNP A2/B1 expression was localized to undifferentiated hESCs and decreased upon differentiation of hESCs. hnRNP A2/B1 knockdown reduced the number of alkaline phosphatase-positive colonies in hESCs and led to a decrease in the expression of pluripotency-associated transcription factors OCT4, NANOG, and SOX2, indicating that hnRNP A2/B1 is essential for hESC self-renewal and pluripotency. hnRNP A2/B1 knockdown increased the expression of gene markers associated with the early development of three germ layers, and promoted the process of epithelial-mesenchymal transition, suggesting that hnRNP A2/B1 is required for maintaining the undifferentiated and epithelial phenotypes of hESCs. hnRNP A2/B1 knockdown inhibited hESC proliferation and induced cell cycle arrest in the G0/G1 phase before differentiation via degradation of cyclin D1, cyclin E, and Cdc25A. hnRNP A2/B1 knockdown increased p27 expression and induced phosphorylation of p53 and Chk1, suggesting that hnRNP A2/B1 also regulates the G1/S transition of hESC cell cycle through the control of p27 expression and p53 and Chk1 activity. Analysis of signaling molecules further revealed that hnRNP A2/B1 regulated hESC proliferation in a PI3K/Akt-dependent manner. These findings provide for the first time mechanistic insights into how hnRNP A2/B1 regulates hESC self-renewal and pluripotency.

Convergent regulation of neuronal differentiation and Erk and Akt kinases in human neural progenitor cells by lysophosphatidic acid, sphingosine 1-phosphate, and LIF: specific roles for the LPA1 receptor

The bioactive lysophospholipids lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) have diverse effects on the developing nervous system and neural progenitors, but the molecular basis for their pleiotropic effects is poorly understood. We previously defined LPA and S1P signaling in proliferating human neural progenitor (hNP) cells, and the current study investigates their role in neuronal differentiation of these cells. Differentiation in the presence of LPA or S1P significantly enhanced cell survival and decreased expression of neuronal markers. Further, the LPA receptor antagonist Ki16425 fully blocked the effects of LPA, and differentiation in the presence of Ki16425 dramatically enhanced neurite length. LPA and S1P robustly activated Erk, but surprisingly both strongly suppressed Akt activation. Ki16425 and pertussis toxin blocked LPA activation of Erk but not LPA inhibition of Akt, suggesting distinct receptor and G-protein subtypes mediate these effects. Finally, we explored cross talk between lysophospholipid signaling and the cytokine leukemia inhibitory factor (LIF). LPA/S1P effects on neuronal differentiation were amplified in the presence of LIF. Similarly, the ability of LPA/S1P to regulate Erk and Akt was impacted by the presence of LIF; LIF enhanced the inhibitory effect of LPA/S1P on Akt phosphorylation, while LIF blunted the activation of Erk by LPA/S1P. Taken together, our results suggest that LPA and S1P enhance survival and inhibit neuronal differentiation of hNP cells, and LPA1 is critical for the effect of LPA. The pleiotropic effects of LPA may reflect differences in receptor subtype expression or cross talk with LIF receptor signaling.

Activin A supplement in the hESCs culture enhances the endoderm differentiation efficiency

Activin A is a critical regulator in human embryonic stem cells (hESCs) maintenance and differentiation. Different concentrations of Activin A affect hESC maintenance and differentiation in different ways. A high concentration favors anterior primitive streak and gives rise to DE if the stimulation persists. hESCs were cultured with and without 10 ng/mL Activin A supplement respectively. The two groups of cells were differentiated into endoderm cells with 100 ng/mL Activin A and other reagents. Microarray-based DNA methylation was analyzed with the Infinium Human Methylation450 BeadChip on these two groups. There was a significant difference in endoderm differentiation efficiency (average efficiency: 71 vs. 58.5%, P < 0.05). hESCs cultured with Activin A supplement had an increased propensity to form definitive endoderm cells in response to Activin A and Wnt signal. Differentially Methylated Regions (DMRs) between these two groups were found. DMRs were related to the stem cell maintenance and gene regulation by peroxisome proliferators via PPARα, indicating that hESCs maintained with Activin A supplement had stronger "stemness."

Protein kinase C-induced early growth response protein-1 binding to SNAIL promoter in epithelial-mesenchymal transition of human embryonic stem cells

Epithelial-mesenchymal transition (EMT) has been thought to occur during early embryogenesis, and also the differentiation process of human embryonic stem (hES) cells. Spontaneous differentiation is sometimes observed at the peripheral of the hES cell colonies in conventional culture conditions, indicating that EMT occurs in hES cell culture. However, the triggering mechanism of EMT is not yet fully understood. The balance between self-renewal and differentiation of human pluripotent stem (hPS) cells is controlled by various signal pathways, including the fibroblast growth factor (FGF)-2. However, FGF-2 has a complex role for self-renewal of hES cells. FGF-2 activates phosphatidylinositol-3 kinase/AKT, mitogen-activated protein kinase/extracellular signal-regulated kinase-1/2 kinase, and also protein kinase C (PKC). Here, we showed that a PKC rapidly induced an early growth response protein-1 (EGR-1) in hES cells, which was followed by upregulation of EMT-related genes. Before the induction of EMT-related genes, EGR-1 was translocated into the nucleus, and then bound directly to the promoter region of SNAIL, which is a master regulator of EMT. SNAIL expression was attenuated by knockdown of EGR-1, but upregulated by ectopic expression of EGR-1. EGR-1 as the downstream signal of PKC might play a key role in EMT initiation during early differentiation of hES cells. This study would lead to a more robust understanding of the mechanisms underlying the balance between self-renewal and initiation of differentiation in hPS cells.

Stem cell-based tissue engineering approaches for musculoskeletal regeneration

The field of regenerative medicine and tissue engineering is an ever evolving field that holds promise in treating numerous musculoskeletal diseases and injuries. An important impetus in the development of the field was the discovery and implementation of stem cells. The utilization of mesenchymal stem cells, and later embryonic and induced pluripotent stem cells, opens new arenas for tissue engineering and presents the potential of developing stem cell-based therapies for disease treatment. Multipotent and pluripotent stem cells can produce various lineage tissues, and allow for derivation of a tissue that may be comprised of multiple cell types. As the field grows, the combination of biomaterial scaffolds and bioreactors provides methods to create an environment for stem cells that better represent their microenvironment for new tissue formation. As technologies for the fabrication of biomaterial scaffolds advance, the ability of scaffolds to modulate stem cell behavior advances as well. The composition of scaffolds could be of natural or synthetic materials and could be tailored to enhance cell self-renewal and/or direct cell fates. In addition to biomaterial scaffolds, studies of tissue development and cellular microenvironments have determined other factors, such as growth factors and oxygen tension, that are crucial to the regulation of stem cell activity. The overarching goal of stem cell-based tissue engineering research is to precisely control differentiation of stem cells in culture. In this article, we review current developments in tissue engineering, focusing on several stem cell sources, induction factors including growth factors, oxygen tension, biomaterials, and mechanical stimulation, and the internal and external regulatory mechanisms that govern proliferation and differentiation.

HEXIM1 induces differentiation of human pluripotent stem cells

Hexamethylene bisacetamide inducible protein 1 (HEXIM1) is best known as the inhibitor of positive transcription elongation factor b (P-TEFb), which is composed of cyclin-dependent kinase 9 (CDK9)/cyclin T1. P-TEFb is an essential regulator for the transcriptional elongation by RNA polymerase II. A genome-wide study using human embryonic stem cells shows that most mRNA synthesis is regulated at the stage of transcription elongation, suggesting a possible role for P-TEFb/HEXIM1 in the gene regulation of stem cells. In this report, we detected a marked increase in HEXIM1 protein levels in the differentiated human pluripotent stem cells (hPSCs) induced by LY294002 treatment. Since no changes in CDK9 and cyclin T1 were observed in the LY294002-treated cells, increased levels of HEXIM1 might lead to inhibition of P-TEFb activity. However, treatment with a potent P-TEFb inhibiting compound, flavopiridol, failed to induce hPSC differentiation, ruling out the possible requirement for P-TEFb kinase activity in hPSC differentiation. Conversely, differentiation was observed when hPSCs were incubated with hexamethylene bisacetamide, a HEXIM1 inducing reagent. The involvement of HEXIM1 in the regulation of hPSCs was further supported when overexpression of HEXIM1 concomitantly induced hPSC differentiation. Collectively, our study demonstrates a novel role of HEXIM1 in regulating hPSC fate through a P-TEFb-independent pathway.

Early development of the porcine embryo: the importance of cell signalling in development of pluripotent cell lines

Understanding the cell signalling events that govern cell renewal in porcine pluripotent cells may help improve culture conditions and allow for establishment of bona fide porcine embryonic stem cells (pESC) and stable porcine induced pluripotent stem cells (piPSC). This review investigates cell signalling in the porcine preimplantation embryo containing either the inner cell mass or epiblast, with particular emphasis on fibroblast growth factor, SMAD, WNT and Janus tyrosine kinases/signal transducers and activators of transcription signalling. It is clear that key differences exist in the cell signalling events that govern pluripotency in this species compared with similar embryonic stages in mouse and human. The fact that bona fide pESC have still not been produced and that piPSC cannot survive in culture following the silencing or downregulation of the reprogramming transgenes suggest that culture conditions are not optimal. Unravelling the factor/s that regulate pluripotency in porcine embryos will pave the way for future establishment of stable pluripotent stem cell lines.

Efficient derivation of lateral plate and paraxial mesoderm subtypes from human embryonic stem cells through GSKi-mediated differentiation

The vertebrae mesoderm is a source of cells that forms a variety of tissues, including the heart, vasculature, and blood. Consequently, the derivation of various mesoderm-specific cell types from human embryonic stem cells (hESCs) has attracted the interest of many investigators owing to their therapeutic potential in clinical applications. However, the need for efficient and reliable methods of differentiation into mesoderm lineage cell types remains a significant challenge. Here, we demonstrated that inhibition of glycogen synthase kinase-3 (GSK-3) is an essential first step toward efficient generation of the mesoderm. Under chemically defined conditions without additional growth factors/cytokines, short-term GSK inhibitor (GSKi) treatment effectively drives differentiation of hESCs into the primitive streak (PS), which can potentially commit toward the mesoderm when further supplemented with bone morphogenetic protein 4. Further analysis confirmed that the PS-like cells derived from GSKi treatment are bipotential, being able to specify toward the endoderm as well. Our findings suggest that the bipotential, PS/mesendoderm-like cell population exists only at the initial stages of GSK-3 inhibition, whereas long-term inhibition results in an endodermal fate. Lastly, we demonstrated that our differentiation approach could efficiently generate lateral plate (CD34(+)KDR(+)) and paraxial (CD34(-)PDGFRα(+)) mesoderm subsets that can be further differentiated along the endothelial and smooth muscle lineages, respectively. In conclusion, our study presents a unique approach for generating early mesoderm progenitors in a chemically directed fashion through the use of small-molecule GSK-3 inhibitor, which may be useful for future applications in regenerative medicine.

Protein kinase C regulates human pluripotent stem cell self-renewal

Background

The self-renewal of human pluripotent stem (hPS) cells including embryonic stem and induced pluripotent stem cells have been reported to be supported by various signal pathways. Among them, fibroblast growth factor-2 (FGF-2) appears indispensable to maintain self-renewal of hPS cells. However, downstream signaling of FGF-2 has not yet been clearly understood in hPS cells.

Methodology/Principal Findings

In this study, we screened a kinase inhibitor library using a high-throughput alkaline phosphatase (ALP) activity-based assay in a minimal growth factor-defined medium to understand FGF-2-related molecular mechanisms regulating self-renewal of hPS cells. We found that in the presence of FGF-2, an inhibitor of protein kinase C (PKC), GF109203X (GFX), increased ALP activity. GFX inhibited FGF-2-induced phosphorylation of glycogen synthase kinase-3β (GSK-3β), suggesting that FGF-2 induced PKC and then PKC inhibited the activity of GSK-3β. Addition of activin A increased phosphorylation of GSK-3β and extracellular signal-regulated kinase-1/2 (ERK-1/2) synergistically with FGF-2 whereas activin A alone did not. GFX negated differentiation of hPS cells induced by the PKC activator, phorbol 12-myristate 13-acetate whereas Gö6976, a selective inhibitor of PKCα, β, and γ isoforms could not counteract the effect of PMA. Intriguingly, functional gene analysis by RNA interference revealed that the phosphorylation of GSK-3β was reduced by siRNA of PKCδ, PKCε, and ζ, the phosphorylation of ERK-1/2 was reduced by siRNA of PKCε and ζ, and the phosphorylation of AKT was reduced by PKCε in hPS cells.

Conclusions/Significance

Our study suggested complicated cross-talk in hPS cells that FGF-2 induced the phosphorylation of phosphatidylinositol-3 kinase (PI3K)/AKT, mitogen-activated protein kinase/ERK-1/2 kinase (MEK), PKC/ERK-1/2 kinase, and PKC/GSK-3β. Addition of GFX with a MEK inhibitor, U0126, in the presence of FGF-2 and activin A provided a long-term stable undifferentiated state of hPS cells even though hPS cells were dissociated into single cells for passage. This study untangles the cross-talk between molecular mechanisms regulating self-renewal and differentiation of hPS cells.

Signaling networks in human pluripotent stem cells

Maintaining a finely-balanced network of signaling inputs is critical for the maintenance of pluripotent stem cells. Together, signaling pathways achieve this by maintaining a long-term, proliferative state while suppressing differentiation. Although the major pathways involved in pluripotency have been known for some time, it was not previously clear how they function in concert to maintain stem cell identity. Recent work has identified a signaling network involving cross-talk between PI3K, TGFβ, MAPK and Wnt pathways that culminate in a finely-balanced molecular switch that determines the fate of pluripotent cells.

Role of mismatch repair enzymes in GAA·TTC triplet-repeat expansion in Friedreich ataxia induced pluripotent stem cells

The genetic mutation in Friedreich ataxia (FRDA) is a hyperexpansion of the triplet-repeat sequence GAA·TTC within the first intron of the FXN gene. Although yeast and reporter construct models for GAA·TTC triplet-repeat expansion have been reported, studies on FRDA pathogenesis and therapeutic development are limited by the availability of an appropriate cell model in which to study the mechanism of instability of the GAA·TTC triplet repeats in the human genome. Herein, induced pluripotent stem cells (iPSCs) were generated from FRDA patient fibroblasts after transduction with the four transcription factors Oct4, Sox2, Klf4, and c-Myc. These cells were differentiated into neurospheres and neuronal precursors in vitro, providing a valuable cell model for FRDA. During propagation of the iPSCs, GAA·TTC triplet repeats expanded at a rate of about two GAA·TTC triplet repeats/replication. However, GAA·TTC triplet repeats were stable in FRDA fibroblasts and neuronal stem cells. The mismatch repair enzymes MSH2, MSH3, and MSH6, implicated in repeat instability in other triplet-repeat diseases, were highly expressed in pluripotent stem cells compared with fibroblasts and neuronal stem cells and occupied FXN intron 1. In addition, shRNA silencing of MSH2 and MSH6 impeded GAA·TTC triplet-repeat expansion. A specific pyrrole-imidazole polyamide targeting GAA·TTC triplet-repeat DNA partially blocked repeat expansion by displacing MSH2 from FXN intron 1 in FRDA iPSCs. These studies suggest that in FRDA, GAA·TTC triplet-repeat instability occurs in embryonic cells and involves the highly active mismatch repair system.

On the role of Wnt/β-catenin signaling in stem cells

BACKGROUND

Stem cells are mainly characterized by two properties: self-renewal and the potency to differentiate into diverse cell types. These processes are regulated by different growth factors including members of the Wnt protein family. Wnt proteins are secreted glycoproteins that can activate different intracellular signaling pathways.

SCOPE OF REVIEW

Here we summarize our current knowledge on the role of Wnt/β-catenin signaling with respect to these two main features of stem cells.

MAJOR CONCLUSIONS

A particular focus is given on the function of Wnt signaling in embryonic stem cells. Wnt signaling can also improve reprogramming of somatic cells towards iPS cells highlighting the importance of this pathway for self-renewal and pluripotency. As an example for the role of Wnt signaling in adult stem cell behavior, we furthermore focus on intestinal stem cells located in the crypts of the small intestine.

GENERAL SIGNIFICANCE

A broad knowledge about stem cell properties and the influence of intrinsic and extrinsic factors on these processes is a requirement for the use of these cells in regenerative medicine in the future or to understand cancer development in the adult. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

A feeder-free, human plasma-derived hydrogel for maintenance of a human embryonic stem cell phenotype in vitro

Background

Human embryonic stem cells (hESCs) represent a tremendous resource for cell therapies and the study of human development; however to maintain their undifferentiated state in vitro they routinely require the use of mouse embryonic fibroblast (MEF) feeder-layers and exogenous protein media supplementation.

Results

These well established requirements can be overcome and in this study, it will be demonstrated that phenotypic stability of hESCs can be maintained using a novel, human plasma protein-based hydrogel as an extracellular culture matrix without the use of feeder cell co-culture. hESCs were resuspended in human platelet poor plasma (PPP), which was gelled by the addition of calcium containing DMEM-based hESC culture medium. Phenotypic and genomic expression of the pluripotency markers OCT4, NANOG and SOX2 were measured using immunohistochemistry and qRT-PCR respectively. Typical hESC morphology was demonstrated throughout in vitro culture and both viability and phenotypic stability were maintained throughout extended culture, up to 25 passages.

Conclusions

PPP-derived hydrogel has demonstrated to be an efficacious alternative to MEF co-culture with its hydrophilicity allowing for this substrate to be delivered via minimally invasive procedures in a liquid phase with polymerization ensuing in situ. Together this provides a novel technique for the study of this unique group of stem cells in either 2D or 3D both in vitro and in vivo.

Signaling network crosstalk in human pluripotent cells: a Smad2/3-regulated switch that controls the balance between self-renewal and differentiation

A general mechanism for how intracellular signaling pathways in human pluripotent cells are coordinated and how they maintain self-renewal remain to be elucidated. In this report, we describe a signaling mechanism where PI3K/Akt activity maintains self-renewal by restraining prodifferentiation signaling through suppression of the Raf/Mek/Erk and canonical Wnt signaling pathways. When active, PI3K/Akt establishes conditions where Activin A/Smad2,3 performs a pro-self-renewal function by activating target genes, including Nanog. When PI3K/Akt signaling is low, Wnt effectors are activated and function in conjunction with Smad2,3 to promote differentiation. The switch in Smad2,3 activity after inactivation of PI3K/Akt requires the activation of canonical Wnt signaling by Erk, which targets Gsk3β. In sum, we define a signaling framework that converges on Smad2,3 and determines its ability to regulate the balance between alternative cell states. This signaling paradigm has far-reaching implications for cell fate decisions during early embryonic development.

Prostaglandin E₂ maintains mouse ESC undifferentiated state through regulation of connexin31, connexin43 and connexin45 expression: involvement of glycogen synthase kinase 3β/β-catenin

BACKGROUND INFORMATION

Although previous reports have examined the function of prostaglandin E₂ (PGE₂) on gap junctions and undifferentiated stem cells, its effects on the reciprocal action of connexin (Cx) isoforms and undifferentiation in embryonic stem cells (ESCs) are unclear. Therefore, we investigated the role of PGE₂ on Cx isoforms and maintenance of mouse ESC undifferentiated state.

RESULTS

We have analysed 10 Cx genes, but found nine of them. PGE₂ (50 μM) stimulated Cx31, Cx32, Cx40, Cx43 and Cx45 mRNA expression. Amongst them, PGE₂ maximally stimulated the Cx43 mRNA expression and gap junction inter-cellular coupling. Therefore, we investigated the effect of PGE₂ on Cx43 expression. PGE₂ activated cAMP/protein kinase A (PKA) and phosphatidylinositol 3-kinase (PI3K)/Akt phosphorylation. In addition, treatments of adenylate cyclase activators increased Cx43 expression, but not PI3K/Akt phosphorylation. PGE₂ also inactivated GSK-3β and stimulated active-β-catenin. Furthermore, a ChiP assay demonstrated the association of β-catenin with the Cx26 (as control) and Cx43 promoter. Finally, down-regulation of PGE₂-induced Cx isoforms by AH 6809, Cx31-, Cx43-, Cx45 small interfering (si)RNA and 18α-glycyrrhetinic acid decreased levels of undifferentiated markers of ESCs, including Oct4, FoxD3, Sox2 and SSEA-1, but Nanog did not be down-regulated by Cx43 siRNA.

CONCLUSIONS

PGE₂ stimulates Cx isoforms via GSK-3β/β-catenin via EP2-receptor-dependent cAMP/PKA and PI3K/Akt in mouse ESCs, thereby partially contributing to the maintenance of their undifferentiated state.

Generation of induced pluripotent stem cells from primary chronic myelogenous leukemia patient samples

Induced pluripotent stem cells (iPSCs) can be generated by the expression of defined transcription factors not only from normal tissue, but also from malignant cells. Cancer-derived iPSCs are expected to provide a novel experimental opportunity to establish the disease model. We generated iPSCs from imatinib-sensitive chronic myelogenous leukemia (CML) patient samples. Remarkably, the CML-iPSCs were resistant to imatinib although they consistently expressed BCR-ABL oncoprotein. In CML-iPSCs, the phosphorylation of ERK1/2, AKT, and JNK, which are essential for the maintenance of both BCR-ABL (+) leukemia cells and iPSCs, were unchanged after imatinib treatment, whereas the phosphorylation of signal transducer and activator of transcription (STAT)5 and CRKL was significantly decreased. These results suggest that the signaling for iPSCs maintenance compensates for the inhibition of BCR-ABL. CML-iPSC-derived hematopoietic cells recovered the sensitivity to imatinib although CD34(+)38(-)90(+)45(+) immature cells were resistant to imatinib, which recapitulated the pathophysiologic feature of the initial CML. CML-iPSCs provide us with a novel platform to investigate CML pathogenesis on the basis of patient-derived samples.

Wnt/β-catenin signalling in adrenal physiology and tumour development

Wnt/β-catenin signalling plays essential roles during embryonic development and in adult tissue homeostasis. Canonical signalling through Wnt secreted ligands relies on the control of β-catenin cytoplasmic accumulation and translocation to the nucleus. In this compartment, β-catenin serves as a transcription coactivator for transcription factors such as Lef/Tcf or some nuclear receptors. Constitutive Wnt signalling resulting from inactivation of inhibitors of the pathway or from activating mutations in β-catenin, triggers tumour development in a number of tissues. Analysis of patients' samples and genetically engineered mouse models has shown that Wnt signalling was involved in adrenal development and tumourigenesis. This review will summarise all these recent findings and will focus on some of the mechanisms that may lead to aberrant accumulation of β-catenin in adrenocortical tumours.

Controlling the stem cell compartment and regeneration in vivo: the role of pluripotency pathways

Since the realization that embryonic stem cells are maintained in a pluripotent state through the interplay of a number of key signal transduction pathways, it is becoming increasingly clear that stemness and pluripotency are defined by the complex molecular convergence of these pathways. Perhaps this has most clearly been demonstrated by the capacity to induce pluripotency in differentiated cell types, so termed iPS cells. We are therefore building an understanding of how cells may be maintained in a pluripotent state, and how we may manipulate cells to drive them between committed and pluripotent compartments. However, it is less clear how cells normally pass in and out of the stem cell compartment under normal and diseased physiological states in vivo, and indeed, how important these pathways are in these settings. It is also clear that there is a potential "dark side" to manipulating the stem cell compartment, as deregulation of somatic stem cells is being increasingly implicated in carcinogenesis and the generation of "cancer stem cells." This review explores these relationships, with a particular focus on the role played by key molecular regulators of stemness in tissue repair, and the possibility that a better understanding of this control may open the door to novel repair strategies in vivo. The successful development of such strategies has the potential to replace or augment intervention-based strategies (cell replacement therapies), although it is clear they must be developed with a full understanding of how such approaches might also influence tumorigenesis.

Wnt/β-catenin signaling in embryonic stem cell self-renewal and somatic cell reprogramming

Embryonic stem cells and induced pluripotent stem (iPS) cells are characterized by their ability to self-renew and to generate differentiated cells of all three germ layers. This potential makes them an attractive source to address question of developmental and also for use in clinical regenerative medicine. Although the culture conditions to maintain pluripotency and reprogramming technologies have been established, the underlying molecular mechanisms are incompletely understood. Accumulating evidence indicates that the Wnt/β-catenin signaling pathway plays a pivotal role in the maintenance of pluripotency as well as in the process of somatic cell reprogramming. Reciprocally, Wnt/β-catenin signaling also plays a critical role in the lineage decision/commitment process. These dramatically different outcomes upon activation of the Wnt signaling cascade has fueled enormous controversy concerning the role of Wnt signaling in the maintenance of potency and induction of differentiation in stem cells. Here, we discuss and explore the divergent roles of the Wnt signaling pathways based on findings from our lab. Accumulated results from our lab indicate the usage of a critical switching mechanism that regulates the divergent Wnt/catenin transcriptional programs associated with either maintenance of potency or initiation of differentiation.

Phosphatase and tensin homolog regulates the pluripotent state and lineage fate choice in human embryonic stem cells

Understanding the intrinsic and extrinsic signals that regulate the molecular basis of the pluripotent state may improve our understanding of mammalian embryogenesis, different states of pluripotency, and our ability to tailor lineage differentiation. Although the role of the PI3K/Akt pathway in the self-renewal and maintenance of mESCs is well-established, the specific contribution of the pathway or of its negative regulator, PTEN, in the maintenance of the human pluripotent state is less understood. To explore the PI3K/AKT pathway in human embryonic stem cell (hESC) pluripotency and differentiation, we generated stable PTEN knockdown (KD) hESCs using short hairpin RNA. Similar to mESCs, we found that PTEN KD hESCs have increased self-renewal, cell survival, and proliferation over multiple passages compared to control cells. However, in contrast to mESCs, in vitro, PTEN KD hESCs differentiated inefficiently in directed differentiation assays, in part due to the continued maintenance of OCT4 and NANOG expression. In teratoma assays, PTEN KD hESCs generated tissues from the three germ layers, although with a bias toward neuroectoderm differentiation. These results demonstrate that PTEN is a key regulator of hESC growth and differentiation, and manipulation of this pathway may improve our ability to regulate and understand the pluripotent state.

Brief report: L1 cell adhesion molecule, a novel surface molecule of human embryonic stem cells, is essential for self-renewal and pluripotency

Despite the recent identification of surface markers of undifferentiated human embryonic stem cells (hESCs), the crucial cell-surface molecules that regulate the self-renewal capacity of hESCs remain largely undefined. Here, we generated monoclonal antibodies (MAbs) that specifically bind to undifferentiated hESCs but not to mouse embryonic stem cells. Among these antibodies, we selected a novel MAb, 4-63, and identified its target antigen as the L1 cell adhesion molecule (L1CAM) isoform 2. Notably, L1CAM expressed in hESCs lacked the neuron-specific YEGHH and RSLE peptides encoded by exons 2 and 27, respectively. L1CAM colocalized with hESC-specific cell-surface markers, and its expression was markedly downregulated on differentiation. Stable L1CAM depletion markedly decreased hESC proliferation, whereas L1CAM overexpression increased proliferation. In addition, the expression of octamer-binding transcription factor 4, Nanog, sex-determining region Y-box 2, and stage-specific embryonic antigen (SSEA)-3 was markedly downregulated, whereas lineage-specific markers and SSEA-1 were upregulated in L1CAM-depleted hESCs. Interestingly, the actions of L1CAM in regulating the proliferation and differentiation of hESCs were exerted predominantly through the fibroblast growth factor receptor 1 signaling pathway. Taken together, our results suggest that L1CAM is a novel cell-surface molecule that plays an important role in the maintenance of self-renewal and pluripotency in hESCs.

Roles of FGF signaling in stem cell self-renewal, senescence and aging

The aging process decreases tissue function and regenerative capacity, which has been associated with cellular senescence and a decline in adult or somatic stem cell numbers and self-renewal within multiple tissues. The potential therapeutic application of stem cells to reduce the burden of aging and stimulate tissue regeneration after trauma is very promising. Much research is currently ongoing to identify the factors and molecular mediators of stem cell self-renewal to reach these goals. Over the last two decades, fibroblast growth factors (FGFs) and their receptors (FGFRs) have stood up as major players in both embryonic development and tissue repair. Moreover, many studies point to somatic stem cells as major targets of FGF signaling in both tissue homeostasis and repair. FGFs appear to promote self-renewing proliferation and inhibit cellular senescence in nearly all tissues tested to date. Here we review the role of FGFs and FGFRs in stem cell self-renewal, cellular senescence, and aging.

Genome-wide analysis of a Wnt1-regulated transcriptional network implicates neurodegenerative pathways

Wnt proteins are critical to mammalian brain development and function. The canonical Wnt signaling pathway involves the stabilization and nuclear translocation of β-catenin; however, Wnt also signals through alternative, noncanonical pathways. To gain a systems-level, genome-wide view of Wnt signaling, we analyzed Wnt1-stimulated changes in gene expression by transcriptional microarray analysis in cultured human neural progenitor (hNP) cells at multiple time points over a 72-hour time course. We observed a widespread oscillatory-like pattern of changes in gene expression, involving components of both the canonical and the noncanonical Wnt signaling pathways. A higher-order, systems-level analysis that combined independent component analysis, waveform analysis, and mutual information-based network construction revealed effects on pathways related to cell death and neurodegenerative disease. Wnt effectors were tightly clustered with presenilin1 (PSEN1) and granulin (GRN), which cause dominantly inherited forms of Alzheimer's disease and frontotemporal dementia (FTD), respectively. We further explored a potential link between Wnt1 and GRN and found that Wnt1 decreased GRN expression by hNPs. Conversely, GRN knockdown increased WNT1 expression, demonstrating that Wnt and GRN reciprocally regulate each other. Finally, we provided in vivo validation of the in vitro findings by analyzing gene expression data from individuals with FTD. These unbiased and genome-wide analyses provide evidence for a connection between Wnt signaling and the transcriptional regulation of neurodegenerative disease genes.

It's a lipid's world: bioactive lipid metabolism and signaling in neural stem cell differentiation

Lipids are often considered membrane components whose function is to embed proteins into cell membranes. In the last two decades, studies on brain lipids have unequivocally demonstrated that many lipids have critical cell signaling functions; they are called "bioactive lipids". Pioneering work in Dr. Robert Ledeen's laboratory has shown that two bioactive brain sphingolipids, sphingomyelin and the ganglioside GM1 are major signaling lipids in the nuclear envelope. In addition to derivatives of the sphingolipid ceramide, the bioactive lipids discussed here belong to the classes of terpenoids and steroids, eicosanoids, and lysophospholipids. These lipids act mainly through two mechanisms: (1) direct interaction between the bioactive lipid and a specific protein binding partner such as a lipid receptor, protein kinase or phosphatase, ion exchanger, or other cell signaling protein; and (2) formation of lipid microdomains or rafts that regulate the activity of a group of raft-associated cell signaling proteins. In recent years, a third mechanism has emerged, which invokes lipid second messengers as a regulator for the energy and redox balance of differentiating neural stem cells (NSCs). Interestingly, developmental niches such as the stem cell niche for adult NSC differentiation may also be metabolic compartments that respond to a distinct combination of bioactive lipids. The biological function of these lipids as regulators of NSC differentiation will be reviewed and their application in stem cell therapy discussed.

Fibroblast growth factor 2 stimulation of osteoblast differentiation and bone formation is mediated by modulation of the Wnt signaling pathway

Fibroblast growth factor 2 (FGF2) positively modulates osteoblast differentiation and bone formation. However, the mechanism(s) is not fully understood. Because the Wnt canonical pathway is important for bone homeostasis, this study focuses on modulation of Wnt/β-catenin signaling using Fgf2(-/-) mice (FGF2 all isoforms ablated), both in the absence of endogenous FGF2 and in the presence of exogenous FGF2. This study demonstrates a role of endogenous FGF2 in bone formation through Wnt signaling. Specifically, mRNA expression for the canonical Wnt genes Wnt10b, Lrp6, and β-catenin was decreased significantly in Fgf2(-/-) bone marrow stromal cells during osteoblast differentiation. In addition, a marked reduction of Wnt10b and β-catenin protein expression was observed in Fgf2(-/-) mice. Furthermore, Fgf2(-/-) osteoblasts displayed marked reduction of inactive phosphorylated glycogen synthase kinase-3β, a negative regulator of Wnt/β-catenin pathway as well as a significant decrease of Dkk2 mRNA, which plays a role in terminal osteoblast differentiation. Addition of exogenous FGF2 promoted β-catenin nuclear accumulation and further partially rescued decreased mineralization in Fgf2(-/-) bone marrow stromal cell cultures. Collectively, our findings suggest that FGF2 stimulation of osteoblast differentiation and bone formation is mediated in part by modulating the Wnt pathway.

Investigating the role of FGF-2 in stem cell maintenance by global phosphoproteomics profiling

Human embryonic stem cells (hESCs) are of immense interest for regenerative medicine as a source of tissue replacement. Expansion of hESCs as a pluripotent population requires a balance between survival, proliferation and self-renewal signals. One of the key growth factors that maintains this balance is fibroblast growth factor-2 (FGF-2). However, the underlying molecular mechanisms are poorly understood. We recently profiled specifically tyrosine phosphorylation events that occur during FGF-2 stimulation of hESCs (Ding et al., J. Cell. Physiol. 2010, 225, 417-428). Here, we complement this phosphoproteome profiling by analyzing temporal dynamics of mostly serine and threonine protein phosphorylation events. Our multi-dimensional strategy combines strong cation exchange chromatography to reduce the sample complexity followed by titanium dioxide off-line for the enrichment of phosphopeptides and dimethylation-based stable isotope labeling for quantification. This approach allowed us to identify and quantify 3261 unique proteins from which 1064 proteins were found to be phosphorylated in one or more residues (representing 1653 unique phosphopeptides). Approximately 40% of the proteins (553 unique phosphopeptides) showed differential phosphorylation upon FGF-2 treatment. Among those are several members of the canonical pathways involved in pluripotency and self-renewal (e.g. Wnt and PI3K/AKT), hESC-associated proteins such as SOX2, RIF1, SALL4, DPPA4, DNMT3B and 53 proteins that are target genes of the pluripotency transcription factors SOX2, OCT4 and NANOG. These findings complement existing pluripotency analyses and provide new insights into how FGF-2 assists in maintaining the undifferentiated state of hESCs.

Impact of WNT signaling on tissue lineage differentiation in the early mouse embryo

WNT signaling activity is involved in the regulation of many cellular functions, including proliferation, migration, cell fate specification, maintenance of pluripotency and induction of tumorigenicity. Here we summarize recent progress towards understanding the regulation of canonical WNT/β-catenin signaling activity through feedback regulatory loops involving the ligands, agonists and antagonists, the availability of intracellular pools of active β-catenin and the cross-regulation of the WNT activity by β-catenin independent pathway. We also review recent findings on the role of WNT/β-catenin signaling in tissue lineage differentiation during embryogenesis and the maintenance and self renewal of embryo-derived stem cells in vitro.

Genetic ablation of sfrp4 in mice does not affect serum phosphate homeostasis

Serum phosphate levels are regulated by PTH and the fibroblast growth factor 23 (Fgf23)/Klotho endocrine system, which both affect expression of Npt2a and thus the apical reabsorption of phosphate in the proximal renal tubules. In addition to Fgf23, secreted frizzled-related protein 4 (Sfrp4) has recently been implicated as an additional phosphate regulator in vivo and in vitro. Here we demonstrate that ablation of the Sfrp4 gene in mice does not lead to altered serum or urine phosphate levels. Furthermore, Sfrp4 is unable to compensate for the absence of Fgf23 or Klotho because double knockouts have a similar biochemical profile and phenotype as animals with ablation of Fgf23 or Klotho alone. Taken together, our data suggest that Sfrp4 does not contribute to the long-term regulation of serum phosphate levels in mice.

Tyrosine phosphorylation profiling in FGF-2 stimulated human embryonic stem cells

The role of fibroblast growth factor-2 (FGF-2) in maintaining undifferentiated human embryonic stem cells (hESC) was investigated using a targeted phosphoproteomics approach to specifically profile tyrosine phosphorylation events following FGF-2 stimulation. A cumulative total number of 735 unique tyrosine phosphorylation sites on 430 proteins were identified, by far the largest inventory to date for hESC. Early signaling events in FGF-2 stimulated hESC were quantitatively monitored using stable isotope dimethyl labeling, resulting in temporal tyrosine phosphorylation profiles of 316 unique phosphotyrosine peptides originating from 188 proteins. Apart from the rapid activation of all four FGF receptors, trans-activation of several other receptor tyrosine kinases (RTKs) was observed as well as induced tyrosine phosphorylation of downstream proteins such as PI3-K, MAPK and several Src family members. Both PI3-K and MAPK have been linked to hESC maintenance through FGF-2 mediated signaling. The observed activation of the Src kinase family members by FGF-2 and loss of pluripotent marker expression post Src kinase inhibition may point to the regulation of cytoskeletal and actin depending processes to maintain undifferentiated hESC.

Human feeder cells can support the undifferentiated growth of human and mouse embryonic stem cells using their own basic fibroblast growth factors

In the culture system using human feeder cells, the mechanism through which these cells support undifferentiated growth of embryonic stem cells (ESCs) has not been well investigated. Here, we explored the mechanisms of 3 kinds of human feeder cells, including human placental cells from the chorionic plate, human bone marrow stromal cells, and human foreskin fibroblasts. First, we determined that undifferentiated growth of 2 kinds each of human (H1 and HSF6) and mouse (D3 and CE3) ESCs was possible in all human feeder cell types tested (human placental cells, human bone marrow stromal cells, and human foreskin fibroblasts), without the need for exogenous cytokine supplementation including basic fibroblast growth factor (bFGF) and leukemia inhibitory factor. We then prepared their corresponding endogenous bFGF-knockout feeders using siRNA and tried to maintain human and mouse ESCs in their undifferentiated state; however, neither human nor mouse ESCs could be maintained in bFGF-knockout human feeder cells. The expressions of stemness markers such as Oct-4 and Nanog were significantly decreased in the bFGF-knockout group compared with those in the controls, and differentiation had already occurred, despite the undifferentiated morphologic appearance of the ESCs. In conclusion, human feeder cells are able to support the undifferentiated growth of human and mouse ESCs via bFGF synthesis. Further, a bFGF-dependent pathway might be crucial for maintaining the undifferentiated characteristics of mouse and human ESCs.

Factors regulating pluripotency and differentiation in early mammalian embryos and embryo-derived stem cells

Mammalian development relies on the cellular proliferation and precisely orchestrated differentiation processes. In preimplantation embryos preservation of the pluripotent state and timely onset of differentiation are secured by specific mechanisms involving such factors as OCT₄, NANOG, SOX₂, or SALL₄. The pluripotency-sustaining cellular machinery is operational not only in the cells of preimplantation embryos but also in embryo-derived embryonic stem cells and epiblast stem cells. However, certain variations in the execution of pluripotency exist and result in the differences not only between embryonic cells and stem cells of the same mammalian species, but also between those of different mammalian species, such as mouse, rat, bank vole, or humans. In this review we describe the involvement of exogenous stimuli (e.g., LIF, WNT, BMP, FGF, and Activin) and function of intrinsic factors (e.g., OCT₄, NANOG, SOX₂, SALL₄) in the regulation of pluripotency in mammalian preimplantation embryos and pluripotent stem cells derived from them. We also focus at the existence of species-specific differences at the level of growth factor requirements, signaling pathways, and transcription factors. Thus, we discuss differences in mechanisms which understanding is one of the necessary steps allowing establishment of methods of efficient derivation, defined in vitro culture conditions, and possible future therapeutic applications of pluripotent stem cells.

Regulation of stem cell pluripotency and differentiation by G protein coupled receptors

Stem cell-based therapeutics have the potential to effectively treat many terminal and debilitating human diseases, but the mechanisms by which their growth and differentiation are regulated are incompletely defined. Recent data from multiple systems suggest major roles for G protein coupled receptor (GPCR) pathways in regulating stem cell function in vivo and in vitro. The goal of this review is to illustrate common ground between the growing field of stem cell therapeutics and the long-established field of G protein coupled receptor signaling. Herein, we briefly introduce basic stem cell biology and discuss how several conserved pathways regulate pluripotency and differentiation in mouse and human stem cells. We further discuss general mechanisms by which GPCR signaling may impact these pluripotency and differentiation pathways, and summarize specific examples of receptors from each of the major GPCR subfamilies that have been shown to regulate stem cell function. Finally, we discuss possible therapeutic implications of GPCR regulation of stem cell function.