Regulation of Hex gene expression by a Smads-dependent signaling pathway

Tel2 regulates redifferentiation of bipotential progenitor cells via Hhex during zebrafish liver regeneration

Upon extensive hepatocyte loss or impaired hepatocyte proliferation, liver regeneration occurs via biliary epithelial cell (BEC) transdifferentiation, which includes dedifferentiation of BECs into bipotential progenitor cells (BP-PCs) and then redifferentiation of BP-PCs to nascent hepatocytes and BECs. This BEC-driven liver regeneration involves reactivation of hepatoblast markers, but the underpinning mechanisms and their effects on liver regeneration remain largely unknown. Using a zebrafish extensive hepatocyte ablation model, we perform an N-ethyl-N-nitrosourea (ENU) forward genetic screen and identify a liver regeneration mutant, liver logan (lvl), in which the telomere maintenance 2 (tel2) gene is mutated. During liver regeneration, the tel2 mutation specifically inhibits transcriptional activation of a hepatoblast marker, hematopoietically expressed homeobox (hhex), in BEC-derived cells, which blocks BP-PC redifferentiation. Mechanistic studies show that Tel2 associates with the hhex promoter region and promotes hhex transcription. Our results reveal roles of Tel2 in the BP-PC redifferentiation process of liver regeneration by activating hhex.

Generating liver using blastocyst complementation: Opportunities and challenges

Orthotopic liver transplantation (OLT) is the only definitive treatment option for many patients with end-stage liver disease. Current supply of donor livers for OLT is not keeping up with the growing demand. To overcome this problem, a number of experimental strategies have been developed either to provide a bridge to transplant for patients on the waiting list or to bioengineer whole livers for OLT by replenishing them with fresh supplies of hepatic cells. In recent years, blastocyst complementation has emerged as the most promising approach for generating whole organs and, in combination with gene editing technology, it has revolutionized regenerative medicine. This methodology was successful in producing xenogeneic organs in animal hosts. Blastocyst complementation has the potential to produce whole livers in large animals that could be xenotransplanted in humans, thereby reducing the shortage of livers for OLT. However, significant experimental and ethical barriers remain for the production of human livers in domestic animals, such as the pig. This review summarizes the current knowledge and provides future perspectives for liver xenotransplantation in humans.

Zebrafish hhex-null mutant develops an intrahepatic intestinal tube due to de-repression of cdx1b and pdx1

The hepatopancreatic duct (HPD) system links the liver and pancreas to the intestinal tube and is composed of the extrahepatic biliary duct, gallbladder, and pancreatic duct. Haematopoietically expressed-homeobox (Hhex) protein plays an essential role in the establishment of HPD; however, the molecular mechanism remains elusive. Here, we show that zebrafish hhex-null mutants fail to develop the HPD system characterized by lacking the biliary marker Annexin A4 and the HPD marker sox9b. The hepatobiliary duct part of the mutant HPD system is replaced by an intrahepatic intestinal tube characterized by expressing the intestinal marker fatty acid-binding protein 2a (fabp2a). Cell lineage analysis showed that this intrahepatic intestinal tube is not originated from hepatocytes or cholangiocytes. Further analysis revealed that cdx1b and pdx1 are expressed ectopically in the intrahepatic intestinal tube and knockdown of cdx1b and pdx1 could restore the expression of sox9b in the mutant. Chromatin-immunoprecipitation analysis showed that Hhex binds to the promoters of pdx1 and cdx1b genes to repress their expression. We therefore propose that Hhex, Cdx1b, Pdx1, and Sox9b form a genetic network governing the patterning and morphogenesis of the HPD and digestive tract systems in zebrafish.

Blood platelets stimulate cancer extravasation through TGFβ-mediated downregulation of PRH/HHEX

Cancer cells go through a process known as epithelial–mesenchymal transition (EMT) during which they acquire the ability to migrate and invade extracellular matrix. Some cells also acquire the ability to move across a layer of endothelial cells to enter and exit the bloodstream; intra- and extravasation, respectively. The transcription factor PRH/HHEX (proline-rich homeodomain/haematopoietically expressed homeobox) controls cell proliferation and cell migration/invasion in a range of cell types. Our previous work showed that PRH activity is downregulated in prostate cancer cells owing to increased inhibitory PRH phosphorylation and that this increases cell proliferation and invasion. PRH inhibits migration and invasion by prostate and breast epithelial cells in part by activating the transcription of Endoglin, a transforming growth factor β (TGFβ) co-receptor. Here we show that depletion of PRH in immortalised prostate epithelial cells results in increased extravasation in vitro. We show that blood platelets stimulate extravasation of cells with depleted PRH and that inhibition of TGFβ signalling blocks the effects of platelets on these cells. Moreover, TGFβ induces changes characteristic of EMT including decreased E-Cadherin expression and increased Snail expression. We show that in prostate cells PRH regulates multiple genes involved in EMT and TGFβ signalling. However, both platelets and TGFβ increase PRH phosphorylation. In addition, TGFβ increases binding of its effector pSMAD3 to the PRH/HHEX promoter and downregulates PRH protein and mRNA levels. Thus, TGFβ signalling downregulates PRH activity by multiple mechanisms and induces an EMT that facilitates extravasation and sensitises cells to TGFβ.

Homeobox protein Hhex negatively regulates Treg cells by inhibiting Foxp3 expression and function

Regulatory T (Treg) cells play an essential role in maintaining immune homeostasis, but the suppressive function of Treg cells can be an obstacle in the treatment of cancer and chronic infectious diseases. Here, we identified the homeobox protein Hhex as a negative regulator of Treg cells. The expression of Hhex was lower in Treg cells than in conventional T (Tconv) cells. Hhex expression was repressed in Treg cells by TGF-β/Smad3 signaling. Retroviral overexpression of Hhex inhibited the differentiation of induced Treg (iTreg) cells and the stability of thymic Treg (tTreg) cells by significantly reducing Foxp3 expression. Moreover, Hhex-overexpressing Treg cells lost their immunosuppressive activity and failed to prevent colitis in a mouse model of inflammatory bowel disease (IBD). Hhex expression was increased; however, Foxp3 expression was decreased in Treg cells in a delayed-type hypersensitivity (DTH) reaction, a type I immune reaction. Hhex directly bound to the promoters of Foxp3 and other Treg signature genes, including Il2ra and Ctla4, and repressed their transactivation. The homeodomain and N-terminal repression domain of Hhex were critical for inhibiting Foxp3 and other Treg signature genes. Thus, Hhex plays an essential role in inhibiting Treg cell differentiation and function via inhibition of Foxp3.

Identification of transforming growth factor-beta-regulated microRNAs and the microRNA-targetomes in primary lung fibroblasts

Background

Lung fibroblasts are involved in extracellular matrix homeostasis, which is mainly regulated by transforming growth factor-beta (TGF-β), and are therefore crucial in lung tissue repair and remodeling. Abnormal repair and remodeling has been observed in lung diseases like COPD. As miRNA levels can be influenced by TGF-β, we hypothesized that TGF-β influences miRNA expression in lung fibroblasts, thereby affecting their function.

Materials and methods

We investigated TGF-β1-induced miRNA expression changes in 9 control primary parenchymal lung fibroblasts using miRNA arrays. TGF-β1-induced miRNA expression changes were validated and replicated in an independent set of lung fibroblasts composted of 10 controls and 15 COPD patients using qRT-PCR. Ago2-immunoprecipitation followed by mRNA expression profiling was used to identify the miRNA-targetomes of unstimulated and TGF-β1-stimulated primary lung fibroblasts (n = 2). The genes affected by TGF-β1-modulated miRNAs were identified by comparing the miRNA targetomes of unstimulated and TGF-β1-stimulated fibroblasts.

Results

Twenty-nine miRNAs were significantly differentially expressed after TGF-β1 stimulation (FDR<0.05). The TGF-β1-induced miR-455-3p and miR-21-3p expression changes were validated and replicated, with in addition, lower miR-455-3p levels in COPD (p<0.05). We identified 964 and 945 genes in the miRNA-targetomes of unstimulated and TGF-β1-stimulated lung fibroblasts, respectively. The TGF-β and Wnt pathways were significantly enriched among the Ago2-IP enriched and predicted targets of miR-455-3p and miR-21-3p. The miR-455-3p target genes HN1, NGF, STRADB, DLD and ANO3 and the miR-21-3p target genes HHEX, CHORDC1 and ZBTB49 were consistently more enriched after TGF-β1 stimulation.

Conclusion

Two miRNAs, miR-455-3p and miR-21-3p, were induced by TGF-β1 in lung fibroblasts. The significant Ago2-IP enrichment of targets of these miRNAs related to the TGF-β and/or Wnt pathways (NGF, DLD, HHEX) in TGF-β1-stimulated fibroblasts suggest a role for these miRNAs in lung diseases by affecting lung fibroblast function.

Misregulation of the proline rich homeodomain (PRH/HHEX) protein in cancer cells and its consequences for tumour growth and invasion

The proline rich homeodomain protein (PRH), also known as haematopoietically expressed homeobox (HHEX), is an essential transcription factor in embryonic development and in the adult. The PRH protein forms oligomeric complexes that bind to tandemly repeated PRH recognition sequences within or at a distance from PRH-target genes and recruit a variety of PRH-interacting proteins. PRH can also bind to other transcription factors and co-regulate specific target genes either directly through DNA binding, or indirectly through effects on the activity of its partner proteins. In addition, like some other homeodomain proteins, PRH can regulate the translation of specific mRNAs. Altered PRH expression and altered PRH intracellular localisation, are associated with breast cancer, liver cancer and thyroid cancer and some subtypes of leukaemia. This is consistent with the involvement of multiple PRH-interacting proteins, including the oncoprotein c-Myc, translation initiation factor 4E (eIF4E), and the promyelocytic leukaemia protein (PML), in the control of cell proliferation and cell survival. Similarly, multiple PRH target genes, including the genes encoding vascular endothelial growth factor (VEGF), VEGF receptors, Endoglin, and Goosecoid, are known to be important in the control of cell proliferation and cell survival and/or the regulation of cell migration and invasion. In this review, we summarise the evidence that implicates PRH in tumourigenesis and we review the data that suggests PRH levels could be useful in cancer prognosis and in the choice of treatment options.

The polycomb group protein L3MBTL1 represses a SMAD5-mediated hematopoietic transcriptional program in human pluripotent stem cells

Epigenetic regulation of key transcriptional programs is a critical mechanism that controls hematopoietic development, and, thus, aberrant expression patterns or mutations in epigenetic regulators occur frequently in hematologic malignancies. We demonstrate that the Polycomb protein L3MBTL1, which is monoallelically deleted in 20q- myeloid malignancies, represses the ability of stem cells to drive hematopoietic-specific transcriptional programs by regulating the expression of SMAD5 and impairing its recruitment to target regulatory regions. Indeed, knockdown of L3MBTL1 promotes the development of hematopoiesis and impairs neural cell fate in human pluripotent stem cells. We also found a role for L3MBTL1 in regulating SMAD5 target gene expression in mature hematopoietic cell populations, thereby affecting erythroid differentiation. Taken together, we have identified epigenetic priming of hematopoietic-specific transcriptional networks, which may assist in the development of therapeutic approaches for patients with anemia.

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L3MBTL1 is a chromatin-binding protein that represses SMAD5 expression

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Lack of L3MBTL1 primes the hematopoietic development of pluripotent stem cells

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L3MBTL1 regulates erythroid differentiation

Regulatory interactions between androgens, Hoxb5, and TGF β signaling in murine lung development

Androgens enhance airway branching but delay alveolar maturation contributing to increased respiratory morbidity in prematurely born male infants. Hoxb5 protein positively regulates airway branching in developing lung. In other organs, androgen regulation intersects with Hox proteins and TGFβ-SMAD signaling, but these interactions have not been studied in the lung. We hypothesized that androgen alteration of airway branching early in lung development requires Hoxb5 expression and that these androgen-Hoxb5 interactions occur partially through regional changes in TGFβ signaling. To evaluate acute effects of androgen and TGFβ on Hoxb5, E11 whole fetal mouse lungs were cultured with dihydrotestosterone (DHT) with/without Hoxb5 siRNA or TGFβ inhibitory antibody. Chronic in utero DHT exposure was accomplished by exposing pregnant mice to DHT (subcutaneous pellet) from E11 to E18. DHT's ability to enhance airway branching and alter phosphorylated SMAD2 cellular localization was partially dependent on Hoxb5. Hoxb5 inhibition also changed the cellular distribution of SMAD7 protein. Chronic in utero DHT increased Hoxb5 and altered SMAD7 mesenchymal localization. TGFβ inhibition enhanced airway branching, and Hoxb5 protein cellular localization was more diffuse. We conclude that DHT controls lung airway development partially through modulation of Hoxb5 protein expression and that this level of regulation involves interactions with TGFβ signaling.

Early development of cephalochordates (amphioxus)

The Phylum Chordata includes three groups--Vertebrata, Tunicata, and Cephalochordata. In cephalochordates, commonly called amphioxus or lancelets, which are basal in the Chordata, the eggs are small and relatively non-yolky. As in vertebrates, cleavage is indeterminate with cell fates determined gradually as development proceeds. The oocytes are attached to the ovarian follicle at the animal pole, where the oocyte nucleus is located. The cytoplasm at the opposite side of the egg, the vegetal pole, contains the future germ plasm or pole plasm, which includes determinants of the germline. After fertilization, additional asymmetries are established by movements of the egg and sperm nuclei, resulting in a concentration of mitochondria at one side of the animal hemisphere. This may be related to establishment of the dorsal/ventral axis. Patterning along the embryonic axes is mediated by secreted signaling proteins. Dorsal identity is specified by Nodal/Vg1 signaling, while during the gastrula stage, opposition between Nodal/Vg1 and BMP signaling establishes dorsal/anterior (i.e., head) and ventral/posterior (i.e., trunk/tail) identities, respectively. Wnt/β-catenin signaling specifies posterior identity while retinoic acid signaling specifies positions along the anterior/posterior axis. These signals are further modulated by a number of secreted antagonists. This fundamental patterning mechanism is conserved, with some modifications, in vertebrates.

Myosin II isoform switching mediates invasiveness after TGF-β-induced epithelial-mesenchymal transition

Despite functional significance of nonmuscle myosin II in cell migration and invasion, its role in epithelial-mesenchymal transition (EMT) or TGF-β signaling is unknown. Analysis of normal mammary gland expression revealed that myosin IIC is expressed in luminal cells, whereas myosin IIB expression is up-regulated in myoepithelial cells that have more mesenchymal characteristics. Furthermore, TGF-β induction of EMT in nontransformed murine mammary gland epithelial cells results in an isoform switch from myosin IIC to myosin IIB and increased phosphorylation of myosin heavy chain (MHC) IIA on target sites known to regulate filament dynamics (S1916, S1943). These expression and phosphorylation changes are downstream of heterogeneous nuclear ribonucleoprotein-E1 (E1), an effector of TGF-β signaling. E1 knockdown drives cells into a migratory, invasive mesenchymal state and concomitantly up-regulates MHC IIB expression and MHC IIA phosphorylation. Abrogation of myosin IIB expression in the E1 knockdown cells has no effect on 2D migration but significantly reduced transmigration and macrophage-stimulated collagen invasion. These studies indicate that transition between myosin IIC/myosin IIB expression is a critical feature of EMT that contributes to increases in invasive behavior.

Tbx20 transcription factor is a downstream mediator for bone morphogenetic protein-10 in regulating cardiac ventricular wall development and function

Bone morphogenetic protein 10 (BMP10) belongs to the TGFβ-superfamily. Previously, we had demonstrated that BMP10 is a key regulator for ventricular chamber formation, growth, and maturation. Ablation of BMP10 leads to hypoplastic ventricular wall formation, and elevated levels of BMP10 are associated with abnormal ventricular trabeculation/compaction and wall maturation. However, the molecular mechanism(s) by which BMP10 regulates ventricle wall growth and maturation is still largely unknown. In this study, we sought to identify the specific transcriptional network that is potentially mediated by BMP10. We analyzed and compared the gene expression profiles between α-myosin heavy chain (αMHC)-BMP10 transgenic hearts and nontransgenic littermate controls using Affymetrix mouse exon arrays. T-box 20 (Tbx20), a cardiac transcription factor, was significantly up-regulated in αMHC-BMP10 transgenic hearts, which was validated by quantitative RT-PCR and in situ hybridization. Ablation of BMP10 reduced Tbx20 expression specifically in the BMP10-expressing region of the developing ventricle. In vitro promoter analysis demonstrated that BMP10 was able to induce Tbx20 promoter activity through a conserved Smad binding site in the Tbx20 promoter proximal region. Furthermore, overexpression of Tbx20 in myocardium led to dilated cardiomyopathy that exhibited ventricular hypertrabeculation and an abnormal muscular septum, which phenocopied genetically modified mice with elevated BMP10 levels. Taken together, our findings demonstrate that the BMP10-Tbx20 signaling cascade is important for ventricular wall development and maturation.

Transcriptional control of hepatocyte differentiation

The liver is the largest glandular organ in the body and plays a central role in controlling metabolism. During hepatogenesis, complex developmental processes must generate an array of cell types that are spatially arranged to generate a hepatic architecture that is essential to support liver function. The processes that control the ultimate formation of the liver are diverse and complex and in many cases poorly defined. Much of the focus of research during the past three decades has been on understanding how hepatocytes, which are the predominant liver parenchymal cells, differentiate during embryogenesis. Through a combination of mouse molecular genetics, embryology, and molecular biochemistry, investigators have defined a myriad of transcription factors that combine to control formation and function of hepatocytes. Here, we will review the major discoveries that underlie our current understanding of transcriptional regulation of hepatocyte differentiation.

Homeoprotein Hex is expressed in mouse developing chondrocytes

Endochondral ossification is a complex process involving the formation of cartilage and the subsequent replacement by mineralized bone. Although the proliferation and differentiation of chondrocytes are strictly regulated, the molecular mechanisms involved are not completely understood. Here, we show that a divergent-type homeobox gene, hematopoietically expressed homeobox gene (HEX), is expressed in mouse chondrogenic cell line ATDC5. The expression of Hex protein drastically increased during differentiation. The chondrogenic differentiation-enhanced expression of Hex protein was also observed in chondrocytes in the tibia of embryonic day 15.5 (E15.5) mouse embryos. The localization of Hex protein in the chondrocytes of the tibia changed in association with maturation; namely, there was Hex protein in the cytoplasm near the endoplasmic reticulum (ER) in resting chondrocytes, which moved to the nucleus in prehypertrophic chondrocytes, and thereafter entered the ER in hypertrophic chondrocytes. These results suggest Hex expression and subcellular localization are associated with chondrocyte maturation.

Beta-catenin signaling in hepatic development and progenitors: which way does the WNT blow?

The Wnt/β-catenin pathway is an evolutionarily conserved signaling cascade that plays key roles in development and adult tissue homeostasis and is aberrantly activated in many tumors. Over a decade of work in mouse, chick, xenopus, and zebrafish models has uncovered multiple functions of this pathway in hepatic pathophysiology. Specifically, beta-catenin, the central component of the canonical Wnt pathway, is implicated in the regulation of liver regeneration, development, and carcinogenesis. Wnt-independent activation of beta-catenin by receptor tyrosine kinases has also been observed in the liver. In liver development across various species, through regulation of cell proliferation, differentiation, and maturation, beta-catenin directs foregut endoderm specification, hepatic specification of the foregut, and hepatic morphogenesis. Its role has also been defined in adult hepatic progenitors or oval cells especially in their expansion and differentiation. Thus, beta-catenin undergoes tight temporal regulation to exhibit pleiotropic effects during hepatic development and in hepatic progenitor biology.

Aggregated P19 mouse embryonal carcinoma cells as a simple in vitro model to study the molecular regulations of mesoderm formation and axial elongation morphogenesis

Because of their capacity to give rise to various types of cells in vitro, embryonic stem and embryonal carcinoma (EC) cells have been used as convenient models to study the mechanisms of cell differentiation in mammalian embryos. In this study, we explored the mouse P19 EC cell line as an effective tool to investigate the factors that may play essential roles in mesoderm formation and axial elongation morphogenesis. We first demonstrated that aggregated P19 cells not only exhibited gene expression patterns characteristic of mesoderm formation but also displayed elongation morphogenesis with a distinct anterior-posterior body axis as in the embryo. We then showed by RNA interference that these processes were controlled by various regulators of Wnt signaling pathways, namely beta-catenin, Wnt3, Wnt3a, and Wnt5a, in a manner similar to normal embryo development. We further showed by inhibitor treatments that the axial elongation morphogenesis was dependent on the activity of Rho-associated kinase. Because of the convenience of these experimental manipulations, we propose that P19 cells can be used as a simple and efficient screening tool to assess the potential functions of specific molecules in mesoderm formation and axial elongation morphogenesis.

PRH/Hex: an oligomeric transcription factor and multifunctional regulator of cell fate

The PRH (proline-rich homeodomain) [also known as Hex (haematopoietically expressed homeobox)] protein is a critical regulator of vertebrate development. PRH is able to regulate cell proliferation and differentiation and is required for the formation of the vertebrate body axis, the haematopoietic and vascular systems and the formation of many vital organs. PRH is a DNA-binding protein that can repress and activate the transcription of its target genes using multiple mechanisms. In addition, PRH can regulate the nuclear transport of specific mRNAs making PRH a member of a select group of proteins that control gene expression at the transcriptional and translational levels. Recent biophysical analysis of the PRH protein has shown that it forms homo-oligomeric complexes in vivo and in vitro and that the proline-rich region of PRH forms a novel dimerization interface. Here we will review the current literature on PRH and discuss the complex web of interactions centred on this multifunctional protein.

The Gata5 target, TGIF2, defines the pancreatic region by modulating BMP signals within the endoderm

Mechanisms underlying regional specification of distinct organ precursors within the endoderm, including the liver and pancreas, are still poorly understood. This is particularly true for stages between endoderm formation and the initiation of organogenesis. In this report, we have investigated these intermediate steps downstream of the early endodermal factor Gata5, which progressively lead to the induction of pancreatic fate. We have identified TGIF2 as a novel Gata5 target and demonstrate its function in the establishment of the pancreatic region within dorsal endoderm in Xenopus. TGIF2 acts primarily by restricting BMP signaling in the endoderm to allow pancreatic formation. Consistently, we found that blocking BMP signaling by independent means also perturbs the establishment of pancreatic identity in the endoderm. Previous findings demonstrated a crucial role for BMP signaling in determining dorsal/ventral fates in ectoderm and mesoderm. Our results now extend this trend to the endoderm and identify TGIF2 as the molecular link between dorsoventral patterning of the endoderm and pancreatic specification.

Transcriptional upregulation of human cathepsin L by VEGF in glioblastoma cells

The role of vascular endothelial growth factor (VEGF) on cathepsin L expression was investigated in human glioblastoma cells (U87MG). Our results demonstrate the transcriptional upregulation of cathepsin L expression by VEGF. Transient transfection of U87MG cells with VEGF expression vector significantly increased cathepsin L activity. These results were further corroborated by a parallel increase in the mRNA levels and promoter activity of cathepsin L by VEGF. By deletion analysis, we identified a 47 base pair VEGF response element (VRE) in human cathepsin L promoter. Site directed mutagenesis studies demonstrated that both SP-1 and AP-4 motifs present in this region contribute to VEGF responsiveness. These results prove for the first time that over-expression of VEGF in human glioblastoma cells induces cathepsin L expression at the transcriptional level. This mechanism could be involved in the enhanced tumorogenic potential of these cells.

An Nkx2-5/Bmp2/Smad1 negative feedback loop controls heart progenitor specification and proliferation

During heart development the second heart field (SHF) provides progenitor cells for most cardiomyocytes and expresses the homeodomain factor Nkx2-5. We now show that feedback repression of Bmp2/Smad1 signaling by Nkx2-5 critically regulates SHF proliferation and outflow tract (OFT) morphology. In the cardiac fields of Nkx2-5 mutants, genes controlling cardiac specification (including Bmp2) and maintenance of the progenitor state were upregulated, leading initially to progenitor overspecification, but subsequently to failed SHF proliferation and OFT truncation. In Smad1 mutants, SHF proliferation and deployment to the OFT were increased, while Smad1 deletion in Nkx2-5 mutants rescued SHF proliferation and OFT development. In Nkx2-5 hypomorphic mice, which recapitulate human congenital heart disease (CHD), OFT anomalies were also rescued by Smad1 deletion. Our findings demonstrate that Nkx2-5 orchestrates the transition between periods of cardiac induction, progenitor proliferation, and OFT morphogenesis via a Smad1-dependent negative feedback loop, which may be a frequent molecular target in CHD.

Definitive endoderm of the mouse embryo: formation, cell fates, and morphogenetic function

The endoderm is one of the primary germ layers but, in comparison to ectoderm and mesoderm, has received less attention. The definitive endoderm forms during gastrulation and replaces the extraembryonic visceral endoderm. It participates in the complex morphogenesis of the gut tube and contributes to the associated visceral organs. This review highlights the role of the definitive endoderm as a source of patterning cues for the morphogenesis of other germ-layer tissues, such as the anterior neurectoderm and the pharyngeal region, and also emphasizes the intricate patterning that the endoderm itself undergoes enabling the acquisition of regionalized cell fates.

Smad6 is a protein kinase X phosphorylation substrate and is required for HL-60 cell differentiation

To gain insight into the function of human protein kinase X (PrKX), a signal-transduction protein required for macrophage differentiation, we identified regulatory subunit I alpha of protein kinase A, T54 and Smad6 as partners for this protein using a yeast two-hybrid interaction screen. Interactions between PrKX and these proteins were substantiated by co-immunoprecipitation. Interaction between Smad6 and PrKX was also confirmed in human myeloid HL-60 cells following their phorbol 12-myristate 13-acetate (PMA)-induced differentiation into macrophages. In vitro phosphorylation assays demonstrated that PrKX phosphorylates Smad6 at a serine residue. Mutagenesis of this site resulted in abrogation of PrKX phosphorylation. Both PrKX and Smad6 were shown to be co-localized to the nuclear compartment of HL-60 cells during their macrophage differentiation where PrKX levels are induced and Smad6 protein levels remain relatively constant while levels of serine phosphorylation of Smad6 increase. By using in vitro electrophoretic mobility shift assays and in vivo chromatin immunoprecipitation, we also demonstrate that during macrophage differentiation Smad6 displays an increased binding to the human osteopontin, Id2, and Hex gene promoters, which correlates to an observed increased expression of these genes. Finally, vector-based RNA interference experiments established that both Smad6 and PrKX proteins are required for PMA-induced cell attachment and spreading.

Molecular control of liver development

Recent studies using animal models have elucidated a growing number of evolutionarily conserved genes and pathways that control liver development from the embryonic endoderm. It is increasingly clear that the genetic programs active in embryogenesis are often deregulated or reactivated in disease, cancer, and tissue repair. Understanding the molecular control of liver development should impact diagnosis and treatment of pediatric and adult liver diseases and aid in efforts to differentiate liver tissue in vitro for stem cell-based therapies.

Myocardin enhances Smad3-mediated transforming growth factor-beta1 signaling in a CArG box-independent manner: Smad-binding element is an important cis element for SM22alpha transcription in vivo

Transforming growth factor (TGF)-beta1 is an important cytokine involved in various diseases. However, the molecular mechanism whereby TGF-beta1 signaling modulates the regulatory network for smooth muscle gene transcription remains largely unknown. To address this question, we previously identified a Smad-binding element (SBE) in the SM22alpha promoter as one of the TGF-beta1 response elements. Here, we show that mutation of the SBE reduces the activation potential of a SM22alpha promoter in transgenic mice during embryogenesis. Chromatin immunoprecipitation assays reveal that TGF-beta1 induces Smad3 binding to the SM22alpha promoter in vivo. A multimerized SBE promoter responsive to TGF-beta1 signaling is highly activated by Smad3 but not by the closely related Smad2. Intriguingly, myocardin (Myocd), a known CArG box-dependent serum response factor coactivator, participates in Smad3-mediated TGF-beta1 signaling and synergistically stimulates Smad3-induced SBE promoter activity independent of the CArG box; no such synergy is seen with Smad2. Importantly, Myocd cooperates with Smad3 to activate the wild-type SM22alpha, SM myosin heavy chain, and SMalpha-actin promoters; they also activate the CArG box-mutated SM22alpha promoter as well as the CArG box-independent aortic carboxypeptidase-like protein promoter. Immunopreciptiation assays reveal that Myocd and Smad3 directly interact both in vitro and in vivo. Mutagenesis studies indicate that the C-terminal transactivation domains of Myocd and Smad3 are required for their functional synergy. These results reveal a novel regulatory mechanism whereby Myocd participates in TGF-beta1 signal pathway through direct interaction with Smad3, which binds to the SBEs. This is the first demonstration that Myocd can act as a transcriptional coactivator of the smooth muscle regulatory network in a CArG box-independent manner.

The disparate role of BMP in stem cell biology

Stem cells share several characteristics of cancer cells including loss of contact inhibition and immortality. Therefore, stem cells represent an excellent model system in which to define the molecular mechanisms underlying cancer development and progression. Several signal transduction pathways including leukemia inhibitory factor, Wnt and FGF have been demonstrated to function in stem cell self-renewal and differentiation. However, more recently bone morphogenetic proteins (BMPs) have emerged as key regulators of stem cell fate commitment. Intriguingly, BMPs have disparate roles in regulating the biology of embryonic stem (ES) cells compared with neural crest stem cells (NCSCs). Furthermore, although BMPs block neural differentiation of ES cells from both mouse and human, they contribute to self-renewal specifically in mouse ES cells. These observations strongly suggest that combinations of extracellular factors regulate stem cells, and that crosstalk between intracellular signaling pathways precisely defines stem cell fate commitment. In this review, we focus on the role of BMP signaling in mouse and human ES cells compared with NCSCs. We then discuss how the molecular effectors of BMP signaling may contribute to cancer, and thus represent potential targets for therapeutic intervention.

An Oct-1 binding site mediates activation of the gata2 promoter by BMP signaling

The gata2 gene encodes a transcription factor implicated in regulating early patterning of ectoderm and mesoderm, and later in numerous cell-specific gene expression programs. Activation of the gata2 gene during embryogenesis is dependent on the bone morphogenetic protein (BMP) signaling pathway, but the mechanism for how signaling controls gene activity has not been defined. We developed an assay in Xenopus embryos to analyze regulatory sequences of the zebrafish gata2 promoter that are necessary to mediate the response to BMP signaling during embryogenesis. We show that activation is Smad dependent, since it is blocked by expression of the inhibitory Smad6. Deletion analysis identified an octamer binding site that is necessary for BMP-mediated induction, and that interacts with the POU homeodomain protein Oct-1. However, this element is not sufficient to transfer a BMP response to a heterologous promoter, requiring an additional more proximal cooperating element. Based on recent studies with other BMP-dependent promoters (Drosophila vestigial and Xenopus Xvent-2), our studies of the gata2 gene suggest that POU-domain proteins comprise a common component of the BMP signaling pathway, cooperating with Smad proteins and other transcriptional activators.

Smad2 and Smad3 coordinately regulate craniofacial and endodermal development

Ligands of the transforming growth factor-beta (TGF-beta) superfamily are involved in numerous developmental and disease processes. TGF-beta, activins, and nodal ligands operate through the highly homologous Smad2 and Smad3 intracellular mediators. Smad2 mutants exhibit early embryonic lethality, while Smad3 mutants are viable, but show a plethora of postnatal phenotypes, including immune dysfunction and skeletal abnormalities. Previously, we have shown that the Smad2 and Smad3 genes function cooperatively during liver morphogenesis. Here we show that Smad2 and Smad3 are required at a full dosage for normal embryonic development. Animals lacking one allele of each gene exhibit a variably penetrant phenotype in which structures in the anterior and ventral midline are reduced or lost; additionally, we demonstrate that this craniofacial defect and the previously reported hepatic phenotypes are both due to defects in the definitive endoderm. A reduction of endodermal gene expression as well as a failure to displace the visceral endoderm occurs despite the formation of a normal foregut pocket. This precedes any defects in anterior patterning and likely causes the abnormalities observed in craniofacial and midline development, as well as hepatogenesis.

BMP4: Its Role in Development of the Hematopoietic System and Potential as a Hematopoietic Growth Factor

Blood formation occurs throughout the life of an individual in a process driven by hematopoietic stem cells (HSCs). The ability of bone marrow (BM) and cord blood (CB) HSC to undergo self-renewal and develop into multiple blood lineages has made these cells an important clinical resource. Transplantation with BM- and CB-derived HSCs is now used extensively for treatment of hematological disorders, malignancies, and immunodeficiencies. An understanding of the embryonic origin of HSC and the factors regulating their generation and expansion in vivo will provide important information for the manipulation of these cells ex vivo. This is critical for the further development of CB transplantation, the potential of which is limited by small numbers of HSC in the donor population. Although the origins of HSCs have become clearer and progress has been made in identifying genes that are critical for the formation and maintenance of HSCs, less is known about the signals that commit specific populations of mesodermal precursors to hematopoietic cell fate. Critical signals acting on these precursor cells are likely to be derived from visceral endoderm in yolk sac and from underlying stroma in the aorta-gonad-mesonephros region. Here we summarize briefly the origin of yolk sac and embryonic HSCs before detailing evidence that bone morphogenic protein-4 (BMP4) has a crucial role in Xenopus and mammalian HSC development. We discuss evidence that BMP4 acts as a hematopoietic growth factor and review its potential to modulate HSC in ex vivo expansion cultures from cord blood.

Regulation of Hex gene expression and initial stages of avian hepatogenesis by Bmp and Fgf signaling

The vertebrate liver and heart arise from adjacent cell layers in the anterior lateral (AL) endoderm and mesoderm of late gastrula embryos, and the earliest stages of liver and heart development are interrelated through reciprocal tissue interactions. Although classical embryological studies performed several decades ago in chick and quail defined the timing of hepatogenic induction in birds and the important role for cardiogenic mesoderm in this process, almost nothing is known about the molecular aspects of avian liver development. Here we use in vivo and explantation assays to investigate tissue interactions and signaling pathways regulating Hex, a homeobox gene required for liver development, and the earliest stages of hepatogenesis in the chick embryo. We find that explants of late gastrula anterior lateral endoderm plus mesoderm, which have been used extensively for studies relating to heart development, also produce albumin-expressing hepatoblasts. Expression of Hex, the earliest known molecular marker for the hepatogenic endoderm, and albumin, indicative of early committed hepatoblasts, requires both autocrine Bmp signaling and a specific paracrine signal from the cardiogenic (anterior lateral) mesoderm. Endodermal expression of Fox2a, in contrast, requires the mesoderm but is independent of Bmp signaling. In vivo induction assays show that the ability of BMP2 to activate Hex expression in the endoderm is restricted to a region that is only slightly larger than the endogenous domain of Hex expression. Although Fgfs can substitute for the cardiogenic mesoderm to support the expression of Hex and albumin in the endoderm, several Fgf genes are expressed in the anterior lateral endoderm but an Fgf expressed predominantly in the mesoderm was not identified. Studies also showed that Fgf gene expression in the endoderm does not require a signal from the mesoderm. Mechanisms regulating endodermal signaling pathways activated by Fgfs may therefore be more complex than previously appreciated.

Interaction between hex and GATA transcription factors in vascular endothelial cells inhibits flk-1/KDR-mediated vascular endothelial growth factor signaling

Recent evidence supports a role for GATA transcription factors as important signal intermediates in differentiated endothelial cells. The goal of this study was to identify proteins that interact with endothelial-derived GATA transcription factors. Using yeast two-hybrid screening, we identified hematopoietically expressed homeobox (Hex) as a GATA-binding partner in endothelial cells. The physical association between Hex and GATA was confirmed with immunoprecipitation in cultured cells. Hex overexpression resulted in decreased flk-1/KDR expression, both at the level of the promoter and the endogenous gene, and attenuated vascular endothelial growth factor-mediated tube formation in primary endothelial cell cultures. In electrophoretic mobility shift assays, Hex inhibited the binding of GATA-2 to the flk-1/KDR 5'-untranslated region GATA motif. Finally, in RNase protection assays, transforming growth factor beta1, which has been previously shown to decrease flk-1 expression by interfering with GATA binding activity, was shown to increase Hex expression in endothelial cells. Taken together, the present study provides evidence for a novel association between Hex and GATA and suggests that transforming growth factor beta-mediated repression of flk-1/KDR and vascular endothelial growth factor signaling involves the inducible formation of inhibitory Hex-GATA complexes.

New intracellular components of bone morphogenetic protein/Smad signaling cascades

Bone morphogenetic proteins (BMPs) regulate many processes in the embryo, including cell type specification, patterning, apoptosis, and epithelial-mesenchymal interaction. They also act in soft and hard tissues in adult life. Their signals are transduced from the plasma membrane to the nucleus through a limited number of Smad proteins. The list of Smad-interacting proteins is however growing and it is clear that these partners determine the outcome of the signal. We summarize the present status in BMP/Smad signaling, with emphasis on recently identified Smad partners and how these proteins may cooperate in the regulation of the expression of BMP target genes.