The murine cripto gene: expression during mesoderm induction and early heart morphogenesis

Spatiotemporal transcriptomic maps of whole mouse embryos at the onset of organogenesis

Spatiotemporal orchestration of gene expression is required for proper embryonic development. The use of single-cell technologies has begun to provide improved resolution of early regulatory dynamics, including detailed molecular definitions of most cell states during mouse embryogenesis. Here we used Slide-seq to build spatial transcriptomic maps of complete embryonic day (E) 8.5 and E9.0, and partial E9.5 embryos. To support their utility, we developed sc3D, a tool for reconstructing and exploring three-dimensional 'virtual embryos', which enables the quantitative investigation of regionalized gene expression patterns. Our measurements along the main embryonic axes of the developing neural tube revealed several previously unannotated genes with distinct spatial patterns. We also characterized the conflicting transcriptional identity of 'ectopic' neural tubes that emerge in Tbx6 mutant embryos. Taken together, we present an experimental and computational framework for the spatiotemporal investigation of whole embryonic structures and mutant phenotypes.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Optimization of production and characterization of a recombinant soluble human Cripto-1 protein inhibiting self-renewal of cancer stem cells

Human Cripto-1 is a member of the epidermal growth factor (EGF)-Cripto-FRL-1-Cryptic (CFC) family family and performs critical roles in cancer and various pathological and developmental processes. Recently we demonstrated that a soluble form of Cripto-1 suppresses the self-renewal and enhances the differentiation of cancer stem cells (CSCs). A functional form of soluble Cripto-1 was found to be difficult to obtain because of the 12 cysteine residues in the protein which impairs the folding process. Here, we optimized the protocol for a T7 expression system, purification from inclusion bodies under denatured conditions refolding of a His-tagged Cripto-1 protein. A concentrations of 0.2-0.4 mM isopropyl β-D-1-thiogalactopyranoside (IPTG) at 37°C was found to be the optimal concentration for Cripto-1 expression while imidazole at 0.5 M was the optimum concentration to elute the Cripto-1 protein from a Ni-column in the smallest volume. Cation exchange column chromatography of the Cripto-1 protein in the presence of 8 M urea exhibited sufficient elution profile at pH 5, which was more efficient at recovery. The recovery of the protein reached to more than 26.6% after refolding with arginine. The purified Cripto-1 exhibited high affinity to the anti-ALK-4 antibody and suppressed sphere forming ability of CSCs at high dose and induced cell differentiation.

Whence CRIPTO: The Reemergence of an Oncofetal Factor in 'Wounds' That Fail to Heal

There exists a set of factors termed oncofetal proteins that play key roles in ontogeny before they decline or disappear as the organism's tissues achieve homeostasis, only to then re-emerge in cancer. Although the unique therapeutic potential presented by such factors has been recognized for more than a century, their clinical utility has yet to be fully realized1. This review highlights the small signaling protein CRIPTO encoded by the tumor derived growth factor 1 (TDGF1/Tdgf1) gene, an oft cited oncofetal protein whose presence in the cancer literature as a tumor promoter, diagnostic marker and viable therapeutic target continues to grow. We touch lightly on features well established and well-reviewed since its discovery more than 30 years ago, including CRIPTO's early developmental roles and modulation of SMAD2/3 activation by a selected set of transforming growth factor β (TGF-β) family ligands. We predominantly focus instead on more recent and less well understood additions to the CRIPTO signaling repertoire, on its potential upstream regulators and on new conceptual ground for understanding its mode of action in the multicellular and often stressful contexts of neoplastic transformation and progression. We ask whence it re-emerges in cancer and where it 'hides' between the time of its fetal activity and its oncogenic reemergence. In this regard, we examine CRIPTO's restriction to rare cells in the adult, its potential for paracrine crosstalk, and its emerging role in inflammation and tissue regeneration-roles it may reprise in tumorigenesis, acting on subsets of tumor cells to foster cancer initiation and progression. We also consider critical gaps in knowledge and resources that stand between the recent, exciting momentum in the CRIPTO field and highly actionable CRIPTO manipulation for cancer therapy and beyond.

Ventricular, atrial, and outflow tract heart progenitors arise from spatially and molecularly distinct regions of the primitive streak

The heart develops from 2 sources of mesoderm progenitors, the first and second heart field (FHF and SHF). Using a single-cell transcriptomic assay combined with genetic lineage tracing and live imaging, we find the FHF and SHF are subdivided into distinct pools of progenitors in gastrulating mouse embryos at earlier stages than previously thought. Each subpopulation has a distinct origin in the primitive streak. The first progenitors to leave the primitive streak contribute to the left ventricle, shortly after right ventricle progenitor emigrate, followed by the outflow tract and atrial progenitors. Moreover, a subset of atrial progenitors are gradually incorporated in posterior locations of the FHF. Although cells allocated to the outflow tract and atrium leave the primitive streak at a similar stage, they arise from different regions. Outflow tract cells originate from distal locations in the primitive streak while atrial progenitors are positioned more proximally. Moreover, single-cell RNA sequencing demonstrates that the primitive streak cells contributing to the ventricles have a distinct molecular signature from those forming the outflow tract and atrium. We conclude that cardiac progenitors are prepatterned within the primitive streak and this prefigures their allocation to distinct anatomical structures of the heart. Together, our data provide a new molecular and spatial map of mammalian cardiac progenitors that will support future studies of heart development, function, and disease.

Targeting Nodal and Cripto-1: Perspectives Inside Dual Potential Theranostic Cancer Biomarkers

BACKGROUND

Elucidating the mechanisms of recurrence of embryonic signaling pathways in tumorigenesis has led to the discovery of onco-fetal players which have physiological roles during normal development but result aberrantly re-activated in tumors. In this context, Nodal and Cripto-1 are recognized as onco-developmental factors, which are absent in normal tissues but are overexpressed in several solid tumors where they can serve as theranostic agents.

OBJECTIVE

To collect, review and discuss the most relevant papers related to the involvement of Nodal and Cripto-1 in the development, progression, recurrence and metastasis of several tumors where they are over-expressed, with a particular attention to their occurrence on the surface of the corresponding sub-populations of cancer stem cells (CSC).

RESULTS

We have gathered, rationalized and discussed the most interesting findings extracted from some 370 papers related to the involvement of Cripto-1 and Nodal in all tumor types where they have been detected. Data demonstrate the clear connection between Nodal and Cripto-1 presence and their multiple oncogenic activities across different tumors. We have also reviewed and highlighted the potential of targeting Nodal, Cripto-1 and the complexes that they form on the surface of tumor cells, especially of CSC, as an innovative approach to detect and suppress tumors with molecules that block one or more mechanisms that they regulate.

CONCLUSION

Overall, Nodal and Cripto-1 represent two innovative and effective biomarkers for developing potential theranostic anti-tumor agents that target normal as well as CSC subpopulations and overcome both pharmacological resistance and tumor relapse.

Exogenous Cripto-1 Suppresses Self-Renewal of Cancer Stem Cell Model

Cripto-1 is a glycophosphatidylinositol (GPI) anchored signaling protein of epidermal growth factor (EGF)-Cripto-1-FRL1-Cryptic (CFC) family and plays a significant role in the early developmental stages and in the different types of cancer cells, epithelial to mesenchymal transition and tumor angiogenesis. Previously, we have developed cancer stem cells (miPS-LLCcm) from mouse iPSCs by culturing them in the presence of conditioned medium of Lewis Lung Carcinoma (LLC) cells for four weeks. Nodal and Cripto-1 were confirmed to be expressed in miPS-LLCcm cells by quantitative reverse transcription PCR (rt-qPCR) implying that Cr-1 was required in maintaining stemness. To investigate the biological effect of adding exogenous soluble CR-1 to the cancer stem cells, we have prepared a C-terminally truncated soluble form of recombinant human CR-1 protein (rhsfCR-1), in which the GPI anchored moiety was removed by substitution of a stop codon through site-directed mutagenesis. rhsfCR-1 effectively suppressed the proliferation and sphere forming ability of miPS-LLCcm cells in a dose-dependent manner in the range of 0 to 5 µg/mL, due to the suppression of Nodal-Cripto-1/ALK4/Smad2 signaling pathway. Frequency of sphere-forming cells was dropped from 1/40 to 1/69 by rhsfCR-1 at 1 µg/mL. Moreover, rhsfCR-1 in the range of 0 to 1 µg/mL also limited the differentiation of miPS-LLCcm cells into vascular endothelial cells probably due to the suppression of self-renewal, which should reduce the number of cells with stemness property. As demonstrated by a soluble form of exogenous Cripto-1 in this study, the efficient blockade would be an attractive way to study Cripto-1 dependent cancer stem cell properties for therapeutic application.

Neuregulin-1β induces proliferation, survival and paracrine signaling in normal human cardiac ventricular fibroblasts

Neuregulin-1β (NRG-1β) is critical for cardiac development and repair, and recombinant forms are currently being assessed as possible therapeutics for systolic heart failure. We previously demonstrated that recombinant NRG-1β reduces cardiac fibrosis in an animal model of cardiac remodeling and heart failure, suggesting that there may be direct effects on cardiac fibroblasts. Here we show that NRG-1β receptors (ErbB2, ErbB3, and ErbB4) are expressed in normal human cardiac ventricular (NHCV) fibroblast cell lines. Treatment of NHCV fibroblasts with recombinant NRG-1β induced activation of the AKT pathway, which was phosphoinositide 3-kinase (PI3K)-dependent. Moreover, the NRG-1β-induced PI3K/AKT signaling in these cells required phosphorylation of both ErbB2 and ErbB3 receptors at tyrosine (Tyr)1248 and Tyr1289 respectively. RNASeq analysis of NRG-1β-treated cardiac fibroblasts obtained from three different individuals revealed a global gene expression signature consistent with cell growth and survival. We confirmed enhanced cellular proliferation and viability in NHCV fibroblasts in response to NRG-1β, which was abrogated by PI3K, ErbB2, and ErbB3 inhibitors. NRG-1β also induced production and secretion of cytokines (interleukin-1α and interferon-γ) and pro-reparative factors (angiopoietin-2, brain-derived neurotrophic factor, and crypto-1), suggesting a role in cardiac repair through the activation of paracrine signaling.

Activin A Modulates CRIPTO-1/HNF4α+ Cells to Guide Cardiac Differentiation from Human Embryonic Stem Cells

The use of human pluripotent stem cells in basic and translational cardiac research requires efficient differentiation protocols towards cardiomyocytes. In vitro differentiation yields heterogeneous populations of ventricular-, atrial-, and nodal-like cells hindering their potential applications in regenerative therapies. We described the effect of the growth factor Activin A during early human embryonic stem cell fate determination in cardiac differentiation. Addition of high levels of Activin A during embryoid body cardiac differentiation augmented the generation of endoderm derivatives, which in turn promoted cardiomyocyte differentiation. Moreover, a dose-dependent increase in the coreceptor expression of the TGF-β superfamily member CRIPTO-1 was observed in response to Activin A. We hypothesized that interactions between cells derived from meso- and endodermal lineages in embryoid bodies contributed to improved cell maturation in early stages of cardiac differentiation, improving the beating frequency and the percentage of contracting embryoid bodies. Activin A did not seem to affect the properties of cardiomyocytes at later stages of differentiation, measuring action potentials, and intracellular Ca²⁺ dynamics. These findings are relevant for improving our understanding on human heart development, and the proposed protocol could be further explored to obtain cardiomyocytes with functional phenotypes, similar to those observed in adult cardiac myocytes.

Ectopic expression of Cripto-1 in transgenic mouse embryos causes hemorrhages, fatal cardiac defects and embryonic lethality

Targeted disruption of Cripto-1 in mice caused embryonic lethality at E7.5, whereas we unexpectedly found that ectopic Cripto-1 expression in mouse embryos also led to embryonic lethality, which prompted us to characterize the causes and mechanisms underlying embryonic death due to ectopic Cripto-1 expression. RCLG/EIIa-Cre embryos displayed complex phenotypes between embryonic day 14.5 (E14.5) and E17.5, including fatal hemorrhages (E14.5-E15.5), embryo resorption (E14.5-E17.5), pale body surface (E14.5-E16.5) and no abnormal appearance (E14.5-E16.5). Macroscopic and histological examination revealed that ectopic expression of Cripto-1 transgene in RCLG/EIIa-Cre embryos resulted in lethal cardiac defects, as evidenced by cardiac malformations, myocardial thinning, failed assembly of striated myofibrils and lack of heartbeat. In addition, Cripto-1 transgene activation beginning after E8.5 also caused the aforementioned lethal cardiac defects in mouse embryos. Furthermore, ectopic Cripto-1 expression in embryonic hearts reduced the expression of cardiac transcription factors, which is at least partially responsible for the aforementioned lethal cardiac defects. Our results suggest that hemorrhages and cardiac abnormalities are two important lethal factors in Cripto-1 transgenic mice. Taken together, these findings are the first to demonstrate that sustained Cripto-1 transgene expression after E11.5 causes fatal hemorrhages and lethal cardiac defects, leading to embryonic death at E14.5-17.5.

CRIPTO overexpression promotes mesenchymal differentiation in prostate carcinoma cells through parallel regulation of AKT and FGFR activities

Members of the EGF-CFC (Cripto, FRL-1, Cryptic) protein family are increasingly recognized as key mediators of cell movement and cell differentiation during vertebrate embryogenesis. The founding member of this protein family, CRIPTO, is overexpressed in various human carcinomas. Yet, the biological role of CRIPTO in this setting remains unclear. Here, we find CRIPTO expression as especially high in a subgroup of primary prostate carcinomas with poorer outcome, wherein resides cancer cell clones with mesenchymal traits. Experimental studies in PCa models showed that one notable function of CRIPTO expression in prostate carcinoma cells may be to augment PI3K/AKT and FGFR1 signaling, which promotes epithelial-mesenchymal transition and sustains a mesenchymal state. In the observed signaling events, FGFR1 appears to function parallel to AKT, and the two pathways act cooperatively to enhance migratory, invasive and transformation properties specifically in the CRIPTO overexpressing cells. Collectively, these findings suggest a novel molecular network, involving CRIPTO, AKT, and FGFR signaling, in favor of the emergence of mesenchymal-like cancer cells during the development of aggressive prostate tumors.

Cripto-1 ablation disrupts alveolar development in the mouse mammary gland through a progesterone receptor-mediated pathway

Cripto-1, a member of the epidermal growth factor-Cripto-1/FRL-1/Cryptic family, is critical for early embryonic development. Together with its ligand Nodal, Cripto-1 has been found to be associated with the undifferentiated status of mouse and human embryonic stem cells. Several studies have clearly shown that Cripto-1 is involved in regulating branching morphogenesis and epithelial-mesenchymal transition of the mammary gland both in vitro and in vivo and together with the cofactor GRP78 is critical for the maintenance of mammary stem cells ex vivo. Our previous studies showed that mammary-specific overexpression of human Cripto-1 exhibited dramatic morphological alterations in nulliparous mice mammary glands. The present study shows a novel mechanism for Cripto-1 regulation of mammary gland development through direct effects on progesterone receptor expression and pathways regulated by progesterone in the mammary gland. We demonstrate a strict temporal regulation of mouse Cripto-1 (mCripto-1) expression that occurs during mammary gland development and a stage-specific function of mCripto-1 signaling during mammary gland development. Our data suggest that Cripto-1, like the progesterone receptor, is not required for the initial ductal growth but is essential for subsequent side branching and alveologenesis during the initial stages of pregnancy. Dissection of the mechanism by which this occurs indicates that mCripto-1 activates receptor activator NF-κB/receptor activator NF-κB ligand, and NF-κB signaling pathways.

Tomoregulin-1 prevents cardiac hypertrophy after pressure overload in mice by inhibiting TAK1-JNK pathways

Cardiac hypertrophy is associated with many forms of heart disease, and identifying important modifier genes involved in the pathogenesis of cardiac hypertrophy could lead to the development of new therapeutic strategies. Tomoregulin-1 is a growth factor that is primarily involved in embryonic development and adult central nervous system (CNS) function, and it is expressed abnormally in a variety of CNS pathologies. Tomoregulin-1 is also expressed in the myocardium. However, the effects of tomoregulin-1 on the heart, particularly on cardiac hypertrophy, remains unknown. The aim of the study is to examine whether and by what mechanism tomoregulin-1 regulates the development of cardiac hypertrophy induced by pressure overload. In this study, we found that tomoregulin-1 was significantly upregulated in two cardiac hypertrophy models: cTnT(R92Q) transgenic mice and thoracic aorta constriction (TAC)-induced cardiac hypertrophy mice. The transgenic overexpression of tomoregulin-1 increased the survival rate, improved the cardiac geometry and functional parameters of echocardiography, and decreased the degree of cardiac hypertrophy of the TAC mice, whereas knockdown of tomoregulin-1 expression resulted in an opposite phenotype and exacerbated phenotypes of cardiac hypertrophy induced by TAC. A possible mechanism by which tomoregulin-1 regulates the development of cardiac hypertrophy in TAC-induced cardiac hypertrophy is through inhibiting TGFβ non-canonical (TAK1-JNK) pathways in the myocardium. Tomoregulin-1 plays a protective role in the modulation of adverse cardiac remodeling from pressure overload in mice. Tomoregulin-1 could be a therapeutic target to control the development of cardiac hypertrophy.

The multifaceted role of the embryonic gene Cripto-1 in cancer, stem cells and epithelial-mesenchymal transition

Cripto-1 (CR-1)/Teratocarcinoma-derived growth factor1 (TDGF-1) is a cell surface glycosylphosphatidylinositol (GPI)-linked glycoprotein that can function either in cis (autocrine) or in trans (paracrine). The cell membrane cis form is found in lipid rafts and endosomes while the trans acting form lacking the GPI anchor is soluble. As a member of the epidermal growth factor (EGF)/Cripto-1-FRL-1-Cryptic (CFC) family, CR-1 functions as an obligatory co-receptor for the transforming growth factor-β (TGF-β) family members, Nodal and growth and differentiation factors 1 and 3 (GDF1/3) by activating Alk4/Alk7 signaling pathways that involve Smads 2, 3 and 4. In addition, CR-1 can activate non-Smad-dependent signaling elements such as PI3K, Akt and MAPK. Both of these pathways depend upon the 78kDa glucose regulated protein (GRP78). Finally, CR-1 can facilitate signaling through the canonical Wnt/β-catenin and Notch/Cbf-1 pathways by functioning as a chaperone protein for LRP5/6 and Notch, respectively. CR-1 is essential for early embryonic development and maintains embryonic stem cell pluripotentiality. CR-1 performs an essential role in the etiology and progression of several types of human tumors where it is expressed in a population of cancer stem cells (CSCs) and facilitates epithelial-mesenchymal transition (EMT). In this context, CR-1 can significantly enhance tumor cell migration, invasion and angiogenesis. Collectively, these facts suggest that CR-1 may be an attractive target in the diagnosis, prognosis and therapy of several types of human cancer.

Cripto is required for mesoderm and endoderm cell allocation during mouse gastrulation

During mouse gastrulation, cells in the primitive streak undergo epithelial-mesenchymal transformation and the resulting mesenchymal cells migrate out laterally to form mesoderm and definitive endoderm across the entire embryonic cylinder. The mechanisms underlying mesoderm and endoderm specification, migration, and allocation are poorly understood. In this study, we focused on the function of mouse Cripto, a member of the EGF-CFC gene family that is highly expressed in the primitive streak and migrating mesoderm cells on embryonic day 6.5. Conditional inactivation of Cripto during gastrulation leads to varied defects in mesoderm and endoderm development. Mutant embryos display accumulation of mesenchymal cells around the shortened primitive streak indicating a functional requirement of Cripto during the formation of mesoderm layer in gastrulation. In addition, some mutant embryos showed poor formation and abnormal allocation of definitive endoderm cells on embryonic day 7.5. Consistently, many mutant embryos that survived to embryonic day 8.5 displayed defects in ventral closure of the gut endoderm causing cardia bifida. Detailed analyses revealed that both the Fgf8-Fgfr1 pathway and p38 MAP kinase activation are partially affected by the loss of Cripto function. These results demonstrate a critical role for Cripto during mouse gastrulation, especially in mesoderm and endoderm formation and allocation.

Cripto/GRP78 modulation of the TGF-β pathway in development and oncogenesis

Cripto is a small, GPI-anchored signaling protein that regulates cellular survival, proliferation, differentiation and migration during normal developmental processes and tumorigenesis. Cripto functions as an obligatory co-receptor for the TGF-β ligands Nodal, GDF1 and GDF3 but attenuates signaling of others such as activin-A, activin-B and TGF-β1. Soluble, secreted forms of Cripto also activate Src, ras/raf/MAPK and PI3K/Akt pathways via a mechanism that remains largely obscure. This review describes the biological roles and signaling mechanisms of Cripto, highlighting our identification of the 78 kDa glucose regulated protein (GRP78) as a cell surface receptor/co-factor required for Cripto signaling via both TGF-β and Src/MAPK/PI3K pathways. We discuss emerging evidence indicating that Cripto/GRP78 signaling regulates normal somatic stem cells and their tumorigenic counterparts.

Cripto-1 is required for hypoxia to induce cardiac differentiation of mouse embryonic stem cells

Cripto-1 is a membrane-bound protein that is highly expressed in embryonic stem cells and in human tumors. In the present study, we investigated the effect of low levels of oxygen, which occurs naturally in rapidly growing tissues, on Cripto-1 expression in mouse embryonic stem (mES) cells and in human embryonal carcinoma cells. During hypoxia, Cripto-1 expression levels were significantly elevated in mES cells and in Ntera-2 or NCCIT human embryonal carcinoma cells, as compared with cells growing with normal oxygen levels. The transcription factor hypoxia-inducible factor-1alpha directly regulated Cripto-1 expression by binding to hypoxia-responsive elements within the promoter of mouse and human Cripto-1 genes in mES and NCCIT cells, respectively. Furthermore, hypoxia modulated differentiation of mES cells by enhancing formation of beating cardiomyocytes as compared with mES cells that were differentiated under normoxia. However, hypoxia failed to induce differentiation of mES cells into cardiomyocytes in the absence of Cripto-1 expression, demonstrating that Cripto-1 is required for hypoxia to fully differentiate mES cells into cardiomyocytes. Finally, cardiac tissue samples derived from patients who had suffered ischemic heart disease showed a dramatic increase in Cripto-1 expression as compared with nonischemic heart tissue samples, suggesting that hypoxia may also regulate Cripto-1 in vivo.

Activin promotes differentiation of cultured mouse trophoblast stem cells towards a labyrinth cell fate

Prolonged maintenance of trophoblast stem (TS) cells requires fibroblast growth factor (FGF) 4 and embryonic fibroblast feeder cells or feeder cell-conditioned medium. Previous studies have shown that TGF-beta and Activin are sufficient to replace embryonic fibroblast-conditioned medium. Nodal, a member of the TGF-beta superfamily, is also known to be important in vivo for the maintenance of TS cells in the developing placenta. Our current studies indicate that TS cells do not express the Nodal co-receptor, Cripto, and do not respond directly to active Nodal in culture. Conversely, Activin subunits and their receptors are expressed in the placenta and TS cell cultures, with Activin predominantly expressed by trophoblast giant cells (TGCs). Differentiation of TS cells in the presence of TGC-conditioned medium or exogenous Activin results in a reduction in the expression of TGC markers. In line with TGC-produced Activin representing the active component in TGC-conditioned medium, this differentiation-inhibiting effect can be reversed by the addition of follistatin. Additional experiments in which TS cells were differentiated in the presence or absence of exogenous Activin or TGF-beta show that Activin but not TGF-beta results in the maintenance of expression of TS cell markers, prolongs the expression of syncytiotrophoblast markers, and significantly delays the expression of spongiotrophoblast and TGC markers. These results suggest that Activin rather than TGF-beta (or Nodal) acts directly on TS cells influencing both TS cell maintenance and cell fate, depending on whether the cells are also exposed to FGF4.

Human embryonic stem cells and cardiac repair

The muscle lost after a myocardial infarction is replaced with noncontractile scar tissue, often initiating heart failure. Whole-organ cardiac transplantation is the only currently available clinical means of replacing the lost muscle, but this option is limited by the inadequate supply of donor hearts. Thus, cell-based cardiac repair has attracted considerable interest as an alternative means of ameliorating cardiac injury. Because of their tremendous capacity for expansion and unquestioned cardiac potential, pluripotent human embryonic stem cells (hESCs) represent an attractive candidate cell source for obtaining cardiomyocytes and other useful mesenchymal cell types for such therapies. Human embryonic stem cell-derived cardiomyocytes exhibit a committed cardiac phenotype and robust proliferative capacity, and recent testing in rodent infarct models indicates that they can partially remuscularize injured hearts and improve contractile function. Although the latter successes give good reason for optimism, considerable challenges remain in the successful application of hESCs to cardiac repair, including the need for preparations of high cardiac purity, improved methods of delivery, and approaches to overcome immune rejection and other causes of graft cell death. This review will describe the phenotype of hESC-derived cardiomyocytes and preclinical experience with these cells and will consider strategies to overcoming the aforementioned challenges.

Cripto recruits Furin and PACE4 and controls Nodal trafficking during proteolytic maturation

The glycosylphosphatidylinositol (GPI)-anchored proteoglycan Cripto binds Nodal and its type I receptor Alk4 to activate Smad2,3 transcription factors, but a role during Nodal precursor processing has not been described. We show that Cripto also binds the proprotein convertases Furin and PACE4 and localizes Nodal processing at the cell surface. When coexpressed as in early embryonic cells, Cripto and uncleaved Nodal already associated during secretion, and a Cripto-interacting region in the Nodal propeptide potentiated the effect of proteolytic maturation on Nodal signalling. Disruption of the trans-Golgi network (TGN) by brefeldin A blocked secretion, but export of Cripto and Nodal to the cell surface was not inhibited, indicating that Nodal is exposed to extracellular convertases before entering the TGN/endosomal system. Density fractionation and antibody uptake experiments showed that Cripto guides the Nodal precursor in detergent-resistant membranes to endocytic microdomains marked by GFP-Flotillin. We conclude that Nodal processing and endocytosis are coupled in signal-receiving cells.

Long-range action of Nodal requires interaction with GDF1

GDF1 (growth/differentiation factor 1), a Vg1-related member of the transforming growth factor-beta superfamily, is required for left-right patterning in the mouse, but the precise function of GDF1 has remained largely unknown. In contrast to previous observations, we now show that GDF1 itself is not an effective ligand but rather functions as a coligand for Nodal. GDF1 directly interacts with Nodal and thereby greatly increases its specific activity. Gdf1 expression in the node was found necessary and sufficient for initiation of asymmetric Nodal expression in the lateral plate of mouse embryos. Coexpression of GDF1 with Nodal in frog embryos increased the range of the Nodal signal. Introduction of Nodal alone into the lateral plate of Gdf1 knockout mouse embryos did not induce Lefty1 expression at the midline, whereas introduction of both Nodal and GDF1 did, showing that GDF1 is required for long-range Nodal signaling from the lateral plate to the midline. These results suggest that GDF1 regulates the activity and signaling range of Nodal through direct interaction.

Cripto promotes A-P axis specification independently of its stimulatory effect on Nodal autoinduction

The EGF-CFC gene cripto governs anterior-posterior (A-P) axis specification in the vertebrate embryo. Existing models suggest that Cripto facilitates binding of Nodal to an ActRII-activin-like kinase (ALK) 4 receptor complex. Cripto also has a crucial function in cellular transformation that is independent of Nodal and ALK4. However, how ALK4-independent Cripto pathways function in vivo has remained unclear. We have generated cripto mutants carrying the amino acid substitution F78A, which blocks the Nodal-ALK4-Smad2 signaling both in embryonic stem cells and cell-based assays. In cripto(F78A/F78A) mouse embryos, Nodal fails to expand its own expression domain and that of cripto, indicating that F78 is essential in vivo to stimulate Smad-dependent Nodal autoinduction. In sharp contrast to cripto-null mutants, cripto(F78A/F78A) embryos establish an A-P axis and initiate gastrulation movements. Our findings provide in vivo evidence that Cripto is required in the Nodal-Smad2 pathway to activate an autoinductive feedback loop, whereas it can promote A-P axis formation and initiate gastrulation movements independently of its stimulatory effect on the canonical Nodal-ALK4-Smad2 signaling pathway.

Effects of thioredoxin reductase-1 deletion on embryogenesis and transcriptome

Thioredoxin reductases (Txnrd) maintain intracellular redox homeostasis in most organisms. Metazoan Txnrds also participate in signal transduction. Mouse embryos homozygous for a targeted null mutation of the txnrd1 gene, encoding the cytosolic thioredoxin reductase, were viable at embryonic day 8.5 (E8.5) but not at E9.5. Histology revealed that txnrd1-/- cells were capable of proliferation and differentiation; however, mutant embryos were smaller than wild-type littermates and failed to gastrulate. In situ marker gene analyses indicated that primitive streak mesoderm did not form. Microarray analyses on E7.5 txnrd-/- and txnrd+/+ littermates showed similar mRNA levels for peroxiredoxins, glutathione reductases, mitochondrial Txnrd2, and most markers of cell proliferation. Conversely, mRNAs encoding sulfiredoxin, IGF-binding protein 1, carbonyl reductase 3, glutamate cysteine ligase, glutathione S-transferases, and metallothioneins were more abundant in mutants. Many gene expression responses mirrored those in thioredoxin reductase 1-null yeast; however, mice exhibited a novel response within the peroxiredoxin catalytic cycle. Thus, whereas yeast induce peroxiredoxin mRNAs in response to thioredoxin reductase disruption, mice induced sulfiredoxin mRNA. In summary, Txnrd1 was required for correct patterning of the early embryo and progression to later development. Conserved responses to Txnrd1 disruption likely allowed proliferation and limited differentiation of the mutant embryo cells.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

Early dysregulation of cripto-1 and immunomodulatory genes in the cerebral cortex in a macaque model of neuroAIDS

Human immunodeficiency virus type 1 (HIV-1) and related primate lentiviruses are known to enter the central nervous system (CNS) during the primary phase of infection. Neuroinvasion by simian immunodeficiency virus and simian human immunodeficiency virus (SHIV) is characterized by transient meningitis and astrocytosis. In this report, we used targeted cytokine cDNA arrays to analyze cortical brain tissue from four pig-tailed macaques inoculated for 2 weeks with pathogenic SHIV(50OLNV) and a normal age-matched pig-tailed macaque. Our results revealed that eight genes were significantly upregulated in all four macaques. These included: leukocyte interferon inducible peptide, corticotrophin releasing factor receptor 1, interleukin 6, CDW40 antigen, cysteine-rich fibroblast growth factor, neurotrophin 3, ciliary neurotrophin factor receptor and cripto-1. The upregulation of three of these genes was confirmed by reverse transcriptase PCR (RT-PCR). Since cripto-1 had not been previously identified within specific cell types within the primate central nervous system, we performed immunohistochemical studies, which revealed the presence of cripto-1 in neurons. RT-PCR studies demonstrated that cripto-1 mRNA was widely expressed in the CNS. These results indicate that immunomodulatory genes are upregulated during the primary phase of infection of the central nervous system. Cripto-1, which acts as a survival factor in tumor cells and may be neuroprotective, is expressed in neurons within the CNS and is upregulated during viral invasion.

TGF-β superfamily expression and actions in the endometrium and placenta

Transforming growth factor β (TGFβ) superfamily members are closely associated with tissue remodelling events and reproductive processes. This review summarises the current state of knowledge regarding the expression and actions of TGFβ superfamily members in the uterus, during the menstrual cycle and establishment of pregnancy. TGFβs and activin β subunits are abundantly expressed in the endometrium, where roles in preparation events for implantation have been delineated, particularly in promoting decidualisation of endometrial stroma. These growth factors are also expressed by epithelial glands and secreted into uterine fluid, where interactions with preimplantation embryos are anticipated. Knockout models and embryo culture experiments implicate activins, TGFβs, nodal and bone morphogenetic proteins (BMPs) in promoting pre- and post-implantation embryo development. TGFβ superfamily members may therefore be important in the maternal support of embryo development. Following implantation, invasion of the decidua by fetal trophoblasts is tightly modulated. Activin promotes, whilst TGFβ and macrophage inhibitory cytokine-1 (MIC-1) inhibit, trophoblast migration in vitro, suggesting the relative balance of TGFβ superfamily members participate in modulating the extent of decidual invasion. Activins and TGFβs have similar opposing actions in regulating placental hormone production. Inhibins and activins are produced by the placenta throughout pregnancy, and have explored as a potential markers in maternal serum for pregnancy and placental pathologies, including miscarriage, Down’s syndrome and pre-eclampsia. Finally, additional roles in immunomodulation at the materno-fetal interface, and in endometrial inflammatory events associated with menstruation and repair, are discussed.

Chemical synthesis of mouse cripto CFC variants

We report for the first time the chemical synthesis of refolded CFC domain of mouse Cripto (mCFC) and of two variants bearing mutations on residues W107 and H104 involved in Alk4 binding. The domains undergo spontaneous and quantitative refolding in about 4 h, yet with very different kinetics. Disulfide linkages have been assessed by enzyme digestion and mass spectrometry analysis of resulting fragments, and the first experimental studies on structural organization have been conducted by circular dichroism spectroscopy under different pH conditions. Upon refolding, the domains considerably change their conformations, although they do not assume canonical structures, and become highly resistant to enzyme degradation. A comparative study of receptor binding shows that the CFC domain can bind Alk4 and confirms the importance of W107 and H104 for receptor recognition.

A novel Cripto-related protein reveals an essential role for EGF-CFCs in Nodal signalling in Xenopus embryos

The location, timing and intensity of Nodal signalling are all critical for proper patterning of the vertebrate embryo. Genetic evidence from mouse and zebrafish indicates that EGF-CFC family members are essential for Nodal ligands to signal. However, the Xenopus EGF-CFC, FRL1, has been implicated in Wnt signalling and in activation of Erk MAP kinase. Here, we identify two additional Xenopus EGF-CFCs, XCR2 and XCR3. We have focused on the role of XCR1/FRL1 and XCR3, which are both expressed at gastrula stages when Nodal signalling is active. We demonstrate spatial and temporal regulation of XCR1 protein expression, whereas XCR3 appears to be expressed ubiquitously. Using gain and loss of function approaches, we show that XCR1 and XCR3 are required for Nodal-related ligands to signal during early Xenopus development. Moreover, different Nodal-related ligands require different XCRs to signal. When both XCR1 and XCR3 are knocked down, activation of the Nodal intracellular signal transducer, Smad2, is severely inhibited and neither gastrulation nor mesendoderm formation occurs. Together our results indicate that the XCRs are important for modulation of the timing and intensity of Nodal signalling in Xenopus embryos.

Embryonic and adult stem cell-derived cardiomyocytes: lessons from in vitro models

For years, research has focused on how to treat heart failure by sustaining the overloaded remaining cardiomyocytes. Recently, the concept of cell replacement therapy as a treatment of heart diseases has opened a new area of investigation. In vitro-generated cardiomyocytes could be injected into the heart to rescue the function of a damaged myocardium. Embryonic and/or adult stem cells could provide cardiac cells for this purpose. Knowledge of fundamental cardiac differentiation mechanisms unraveled by studies on animal models has been improved using in vitro models of cardiogenesis such as mouse embryonal carcinoma cells, mouse embryonic stem cells and, recently, human embryonic stem cells. On the other hand, studies suggesting the existence of cardiac stem cells and the potential of adult stem cells from bone marrow or skeletal muscle to differentiate toward unexpected phenotypes raise hope and questions about their potential use for cardiac cell therapy. In this review, we compare the specificities of embryonic vs adult stem cell populations regarding their cardiac differentiation potential, and we give an overview of what in vitro models have taught us about cardiogenesis.

Left–right asymmetry and congenital cardiac defects: Getting to the heart of the matter in vertebrate left–right axis determination

Cellular and molecular left-right differences that are present in the mesodermal heart fields suggest that the heart is lateralized from its inception. Left-right asymmetry persists as the heart fields coalesce to form the primary heart tube, and overt, morphological asymmetry first becomes evident when the heart tube undergoes looping morphogenesis. Thereafter, chamber formation, differentiation of the inflow and outflow tracts, and position of the heart relative to the midline are additional features of heart development that exhibit left-right differences. Observations made in human clinical studies and in animal models of laterality disease suggest that all of these features of cardiac development are influenced by the embryonic left-right body axis. When errors in left-right axis determination happen, they almost always are associated with complex congenital heart malformations. The purpose of this review is to highlight what is presently known about cardiac development and upstream processes of left-right axis determination, and to consider how perturbation of the left-right body plan might ultimately result in particular types of congenital heart defects.

Cripto-1: a multifunctional modulator during embryogenesis and oncogenesis

It is increasingly evident that genes known to perform critical roles during early embryogenesis, particularly during stem cell renewal, pluripotentiality and survival, are also expressed during the development of cancer. In this regard, oncogenesis may be considered as the recapitulation of embryogenesis in an inappropriate temporal and spatial manner. The epidermal growth factor-Cripto-1/FRL1/cryptic family of proteins consists of extracellular and cell-associated proteins that have been identified in several vertebrate species. During early embryogenesis, epidermal growth factor-Cripto-1/FRL1/cryptic proteins perform an obligatory role as coreceptors for the transforming growth factor-beta subfamily of proteins, which includes Nodal. Cripto-1 has also been shown to function as a ligand through a Nodal/Alk4-independent signaling pathway that involves binding to glypican-1 and the subsequent activation through src of phosphoinositol-3 kinase/Akt and ras/mitogen-activated protein kinase intracellular pathways. Expression of Cripto-1 is increased in several human cancers and its overexpression is associated with the development of mammary tumors in mice. Here, we review the role of Cripto-1 during embryogenesis, cell migration, invasion and angiogenesis and how these activities may relate to cellular transformation and tumorigenesis. We also briefly discuss evidence suggesting that Cripto-1 may be involved in stem cell maintenance.

Nodal-dependant Cripto signaling in ES cells: from stem cells to tumor biology

Embryonic stem (ES) cells have provided a valid model to understand early events of mammalian lineage specification and differentiation, leading to important insights into the mechanisms that control embryogenesis at the molecular and cellular levels. Furthermore, ES cells have recently evoked great scientific interest as ideal candidates for the generation of tissues for transplantation therapies. In this respect, particular attention has been paid to the molecules and signaling pathways triggering ES cell differentiation. The EGF-CFC Cripto protein is a key regulator of ES cells fate. The cripto gene is expressed both in ES cells and during the early phases of embryo development, while, in the adult, it is reactivated in a wide range of epithelial cancers. This review will discuss recent findings on the molecular basis of Cripto signaling in ES cell differentiation, providing an intriguing link between stem cell and tumor biology.

Human Cripto-1 overexpression in the mouse mammary gland results in the development of hyperplasia and adenocarcinoma

Human Cripto-1 (CR-1) is overexpressed in approximately 80% of human breast, colon and lung carcinomas. Mouse Cr-1 upregulation is also observed in a number of transgenic (Tg) mouse mammary tumors. To determine whether CR-1 can alter mammary gland development and/or may contribute to tumorigenesis in vivo, we have generated Tg mouse lines that express human CR-1 under the transcriptional control of the mouse mammary tumor virus (MMTV). Stable Tg MMTV/CR-1 FVB/N lines expressing different levels of CR-1 were analysed. Virgin female MMTV/CR-1 Tg mice exhibited enhanced ductal branching, dilated ducts, intraductal hyperplasia, hyperplastic alveolar nodules and condensation of the mammary stroma. Virgin aged MMTV/CR-1 Tg mice also possessed persistent end buds. In aged multiparous MMTV/CR-1 mice, the hyperplastic phenotype was most pronounced with multifocal hyperplasias. In the highest CR-1-expressing subline, G4, 38% (12/31) of the multiparous animals aged 12-20 months developed hyperplasias and approximately 33% (11/31) developed papillary adenocarcinomas. The long latency period suggests that additional genetic alterations are required to facilitate mammary tumor formation in conjunction with CR-1. This is the first in vivo study that shows hyperplasia and tumor growth in CR-1-overexpressing animals.

Cripto as a target for cancer immunotherapy

One of the recent, significant advances in cancer immunotherapy is the identification of molecules as targets which regulate cell growth by induction of proliferation and survival signalling pathways. Among them, epidermal growth factor receptor and Her2 have been effectively targeted by monoclonal antibodies. Currently, the treatment of cancer has limitations and most cancer deaths result from the local invasion and distant metastasis of tumour cells. An important insight for the understanding of tumour invasion and metastasis came from the recent discovery that the phenotypic changes of increased motility and invasiveness of cancer cells are reminiscent of the epithelial-mesenchymal transition (EMT) that occurs during embryonic development. The human Cripto, a member of the epidermal growth factor-Cripto, Frl1, and Cryptic (EGF-CFC) protein family and a signalling protein during early embryonic development, plays an important role in cancers. Cripto is attached to the cell membrane through a glycosyl-phosphatidylinositol motif, and is upregulated in a wide range of epithelial cancers. In this paper the authors review the role of Cripto expression in tumourigenesis and in EMT to promote tumour invasion, with emphasis that the unique EGF-like region of Cripto plays a critical role in Cripto signalling-mediated tumour growth and EMT. Therefore, the region should be regarded as a therapeutic point for interruption of the oncogenic and metastatic potential of Cripto for cancer immunotherapy.

Cardiomyocytes derived from stem cells

One way to restore failing heart function following myocardial infarction would be to replace lost or damaged cardiac cells by local or systemic injection. The sources of replacement cells presently discussed include embryonic stem cells, hematopoietic and non-hematopoietic stem cells from bone marrow or cord blood and small stem cell populations thought to reside in the heart itself or in skeletal muscle. Here we review this area of stem cell research with focus particularly on recent laboratory advances towards producing cardiomyocytes from embryonic stem cells. We conclude that embryonic stem cells and cardiac progenitors in the heart itself are the only proven sources of cardiomyocytes and that reported clinical effects of bone marrow stem currently undergoing validation are likely mediated by other mechanisms.

Cripto-1: an oncofetal gene with many faces

Human Cripto-1 (CR-1), a member of the epidermal growth factor (EGF)-CFC family, has been implicated in embryogenesis and in carcinogenesis. During early vertebrate development, CR-1 functions as a co-receptor for Nodal, a transforming growth factor beta (TGFbeta) family member and is essential for mesoderm and endoderm formation and anterior-posterior and left-right axis establishment. In adult tissues, CR-1 is expressed at a low level in all stages of mammary gland development and expression increases during pregnancy and lactation. Overexpression of CR-1 in mouse mammary epithelial cells leads to their transformation in vitro and, when injected into mammary glands, produces ductal hyperplasias. CR-1 can also enhance migration, invasion, branching morphogenesis and epithelial to mesenchymal transition (EMT) of several mouse mammary epithelial cell lines. Furthermore, transgenic mouse studies have shown that overexpression of a human CR-1 transgene in the mammary gland under the transcriptional control of the mouse mammary tumor virus (MMTV) promoter results in mammary hyperplasias and papillary adenocarcinomas. Finally, CR-1 is expressed at high levels in approximately 50 to 80% of different types of human carcinomas, including breast, cervix, colon, stomach, pancreas, lung, ovary, and testis. In conclusion, EGF-CFC proteins play dual roles as embryonic pattern formation genes and as oncogenes. While during embryogenesis EGF-CFC proteins perform specific and regulatory functions related to cell and tissue patterning, inappropriate expression of these molecules in adult tissues can lead to cellular proliferation and transformation and therefore may be important in the etiology and/or progression of cancer.

Transcriptional profiling of initial differentiation events in human embryonic stem cells

Currently, there are no differentiation strategies for human embryonic stem cells (hESCs) that efficiently produce one specific cell type, possibly because of lack of understanding of the genes that control signaling events prior to overt differentiation. sed HepG2 cell conditioned medium (MEDII), which induces early differentiation in mouse ES cells while retaining pluripotent markers, to query gene expression in hESCs. Treatment of adherent hESCs with 50% MEDII medium effected differentiation to a cell type with gene expression similar to primitive streak stage cells of mouse embryos. MEDII treatment up-regulates TDGF1 (Cripto), a gene essential for anterior-posterior axis and mesoderm formation in mouse embryos and a key component of the TGFB1/NODAL signaling pathway. LEFTYA, an antagonist of NODAL/TDGF1 signaling expressed in anterior visceral endoderm, is down-regulated with MEDII treatment, as is FST, an inhibitor of mesoderm induction via the related INHBE1 pathway. In summary, the TGFB1/NODAL pathway is important for primitive-streak and mesoderm formation and in using MEDII, we present a means for generating an in vitro cell population that maintains pluripotent gene expression (POU5F1, NANOG) and SSEA-4 markers while regulating genes in the TGFB1/NODAL pathway, which may lead to more uniform formation of mesoderm in vitro.

Stem cell-derived angiogenic/vasculogenic cells: possible therapies for tissue repair and tissue engineering

1. The recent ability to isolate stem cells and study their specific capacity of self-renewal with the formation of different cell types has opened up exciting vistas to help the repair of damaged tissue and even the formation of new tissue. In the present review, we deal with the characteristics and sources that stem cells can be derived and cultured from. 2. We focus on the role that stem cell-derived vascular cells or endothelial progenitor cells (EPC) may play in (re)vascularization of ischaemic and engineered tissues. This so-called vasculogenesis resembles the embryological process in which 'haemangioblasts' differentiate in blood cells, as well as in primitive vessels. Although also derived from the blood-forming bone marrow, in adult life vasculogenic stem cells contribute only little to the regular vascular repair mechanisms: namely (i) angiogenesis (outgrowth of vessels from existing vessels); and (ii) arteriogenesis (monocyte-aided increase in the calibre of existing arteriolar collaterals). 3. Most attempts to increase vascular repair by stem cells involve the use of growth factors, which mobilize stem cells from bone marrow into the blood, sometimes combined with isolation and reinfusion of these cells after ex vivo expansion and differentiation into EPC. 4. Clear improved perfusion of ischaemic sites and new vasculature has been observed in vivo mostly in animal models. Specific homing or administration of these cells and regulated and quantitative expansion and (final) differentiation at these vascular (repair) sites are less studied, but are paramount for efficacy and safety. 5. In conclusion, the use of embryonic stem cells will still encounter ethical objections. Moreover, special attention and measures are needed to cope with the allogeneic barriers that these cells usually encounter. In general, the long and complicated ex vivo cultures to obtain sufficient offspring from the very small numbers of stem cells that can be obtained as starting material will be costly and cumbersome. Both basic research on conceptual matters and cost-effective development of the product itself will have to go a long way before the clinical use of some volume can be expected.

Nodal-dependent Cripto signaling promotes cardiomyogenesis and redirects the neural fate of embryonic stem cells

The molecular mechanisms controlling inductive events leading to the specification and terminal differentiation of cardiomyocytes are still largely unknown. We have investigated the role of Cripto, an EGF-CFC factor, in the earliest stages of cardiomyogenesis. We find that both the timing of initiation and the duration of Cripto signaling are crucial for priming differentiation of embryonic stem (ES) cells into cardiomyocytes, indicating that Cripto acts early to determine the cardiac fate. Furthermore, we show that failure to activate Cripto signaling in this early window of time results in a direct conversion of ES cells into a neural fate. Moreover, the induction of Cripto activates the Smad2 pathway, and overexpression of activated forms of type I receptor ActRIB compensates for the lack of Cripto signaling in promoting cardiomyogenesis. Finally, we show that Nodal antagonists inhibit Cripto-regulated cardiomyocyte induction and differentiation in ES cells. All together our findings provide evidence for a novel role of the Nodal/Cripto/Alk4 pathway in this process.

Anterior neural plate regionalization in cripto null mutant mouse embryos in the absence of node and primitive streak

The relation between the role of the organizer at the gastrula stage and the activity of earlier signals in the specification, maintenance, and regionalization of the developing brain anlage is still controversial. Mouse embryos homozygous for null mutation in the cripto gene die at about 9.0 days postcoitum (d.p.c.) and fail to gastrulate and to form the node (the primary organizer). Here, we study the presence and the distribution of anterior neural plate molecular domains in cripto null mutants. We demonstrate that, in cripto(-/-) embryos, the main prosencephalic and mesencephalic regions are present and that they assume the correct topological organization. The identity of the anterior neural domains is maintained in mutant embryos at 8.5 d.p.c., as well as in mutant explants dissected at 8.5 d.p.c. and cultured in vitro for 24 h. Our data imply the existence of a stable neural regionalization of anterior character inside the cripto(-/-) embryos, despite the failure in both the gastrulation process and node formation. These results suggest that, in mouse embryos, the specification of the anterior neural identities can be maintained without an absolute requirement for the embryonic mesoderm and the node.

The CRIPTO/FRL-1/CRYPTIC (CFC) domain of human Cripto. Functional and structural insights through disulfide structure analysis

The disulfide structure of the CRIPTO/FRL-1/CRYPTIC (CFC) domain of human Cripto protein was determined by a combination of enzymatic and chemical fragmentation, followed by chromatographic separation of the fragments, and characterization by mass spectrometry and N-terminal sequencing. These studies showed that Cys115 forms a disulfide bond with Cys133, Cys128 with Cys149, and Cys131 with Cys140. Protein database searching and molecular modeling revealed that the pattern of disulfide linkages in the CFC domain of Cripto is the same as that in PARS intercerebralis major Peptide C (PMP-C), a serine protease inhibitor, and that the EGF-CFC domains of Cripto are predicted to be structurally homologous to the EGF-VWFC domains of the C-terminal extracellular portions of Jagged 1 and Jagged 2. Biochemical studies of the interactions of ALK4 with the CFC domain of Cripto by fluorescence-activated cell sorter analysis indicate that the CFC domain binds to ALK4 independent of the EGF domain. A molecular model of the CFC domain of Cripto was constructed based on the nuclear magnetic resonance structure of PMP-C. This model reveals a hydrophobic patch in the domain opposite to the presumed ALK4 binding site. This hydrophobic patch may be functionally important for the formation of intra or intermolecular complexes.

Dynamic morphogenetic events characterize the mouse visceral endoderm

Several lines of evidence suggest that the extraembryonic endoderm of vertebrate embryos plays an important role in the development of rostral neural structures. In mice, neural inductive signals are thought to reside in an area of visceral endoderm that expresses the Hex gene. Here, we have conducted a morphological and lineage analysis of visceral endoderm cells spanning pre- and postprimitive streak stages. Our results show that Hex-expressing cells have a tall, columnar epithelial morphology, which distinguishes them from other visceral endoderm cells. This region of visceral endoderm thickening (VET) is found overlying first the distal and then one side of the epiblast at stages between 5.5 and 5.75 days post coitum (d.p.c.). In addition, we show that the epiblast has an anteroposterior-compressed appearance that is aligned with the position of the VET. Intracellular labeling of VET/Hex-expressing cells reveals an anterior and anterolateral shift from their distal epiblast position. VET/Hex-expressing cells are first localized to the anterior side of the epiblast by 5.75 d.p.c. and form a crescent on the anterior half of the embryo at the onset of gastrulation. Subsequently, VET descendants are distributed along the embryonic/extraembryonic boundary by headfold stages at 7.5 d.p.c. The morphological characteristics and position of VET/Hex-expressing cells distinguishes the future anteroposterior axis of the embryo and provide landmarks to stage mouse embryos at preprimitive streak stages. Moreover, the morphological characteristics of pregastrulation mouse embryos together with the stereotyped shift in the position of visceral endoderm cells reveal similarities among amniote embryos that suggest an evolutionary conservation of the mechanisms that pattern the rostral neurectoderm at pregastrula stages.

Heart development: molecular insights into cardiac specification and early morphogenesis

The heart develops from two bilateral heart fields that are formed during early gastrulation. In recent years, signaling pathways that specify cardiac mesoderm have been extensively analyzed. In addition, a battery of transcription factors that regulate different aspects of cardiac morphogenesis and cytodifferentiation have been identified and characterized in model organisms. At the anterior pole, a secondary heart field is formed, which in its molecular make-up, appears to be similar to the primary heart field. The cardiac outflow tract and the right ventricle to a large extent are derivatives of this anterior heart field. Cardiac mesoderm receives positional information by which it is patterned along the three body axes. The molecular control of left-right axis development has received particular attention, and the underlying regulatory network begins to emerge. Cardiac chamber development involves the activation of a transcription program that is different from the one present in the primary heart field and regulates cardiac morphogenesis in a region-specific manner. This review also attempts to identify areas in which additional research is needed to fully understand early cardiac development.

Extraembryonic proteases regulate Nodal signalling during gastrulation

During gastrulation, a cascade of inductive tissue interactions converts pre-existing polarity in the mammalian embryo into antero-posterior pattern. This process is triggered by Nodal, a protein related to transforming growth factor-beta (TFG-beta) that is expressed in the epiblast and visceral endoderm, and its co-receptor Cripto, which is induced downstream of Nodal. Here we show that the proprotein convertases Spc1 and Spc4 (also known as Furin and Pace4, respectively) are expressed in adjacent extraembryonic ectoderm. They stimulate Nodal maturation after its secretion and are required in vivo for Nodal signalling. Embryo explants deprived of extraembryonic ectoderm phenocopy Spc1(-/-); Spc4(-/-) double mutants in that endogenous Nodal fails to induce Cripto. But recombinant mature Nodal, unlike uncleaved precursor, can efficiently rescue Cripto expression. Cripto is also expressed in explants treated with bone morphogenetic protein 4 (BMP4). This indicates that Nodal may induce Cripto through both a signalling pathway in the embryo and induction of Bmp4 in the extraembryonic ectoderm. A lack of Spc1 and Spc4 affects both pathways because these proteases also stimulate induction of Bmp4.

Role of the EGF-CFC gene cripto in cell differentiation and embryo development

The EGF-CFC proteins have been recently recognized as a novel family of extracellular factors required during early vertebrate development. Cripto is the founder member of the EGF-CFC family initially related to the epidermal growth factor (EGF); its expression is increased in human colon, gastric, pancreatic and lung carcinoma and in different types of both mouse and human breast carcinomas. Genetic studies in the mouse have established an essential role of cripto in the formation and correct positioning of the anterior-posterior axis. Furthermore, the absence of cripto results in a defective precardiac mesoderm, unable to differentiate into functional cardiomyocytes. Although mouse and human Cripto have been shown to activate the ras/raf/MAP kinase pathway in mammary epithelial cell lines, genetic evidence in Zebrafish has been provided for a relationship between the EGF-CFC proteins and Nodal, a member of the transforming growth factor family. Here we review the biological role of cripto in development and differentiation, pointing out recent findings on the biochemical interactions of Cripto, Nodal and Activin-like receptors.

Cripto-1 activates nodal- and ALK4-dependent and -independent signaling pathways in mammary epithelial Cells

Cripto-1 (CR-1), an epidermal growth factor-CFC (EGF-CFC) family member, has a demonstrated role in embryogenesis and mammary gland development and is overexpressed in several human tumors. Recently, EGF-CFC proteins were implicated as essential signaling cofactors for Nodal, a transforming growth factor beta family member whose expression has previously been defined as embryo specific. To identify a receptor for CR-1, a human brain cDNA phage display library was screened using CR-1 protein as bait. Phage inserts with identity to ALK4, a type I serine/threonine kinase receptor for Activin, were identified. CR-1 binds to cell surface ALK4 expressed on mammalian epithelial cells in fluorescence-activated cell sorter analysis, as well as by coimmunoprecipitation. Nodal is coexpressed with mouse Cr-1 in the mammary gland, and CR-1 can phosphorylate the transcription factor Smad-2 in EpH-4 mammary epithelial cells only in the presence of Nodal and ALK4. In contrast, CR-1 stimulation of mitogen-activated protein kinase and AKT in these cells is independent of Nodal and ALK4, suggesting that CR-1 may modulate different signaling pathways to mediate its different functional roles.

Cripto: a tumor growth factor and more

Cripto, a growth factor with an EGF-like domain, and the first member of the EGF-CFC family of genes to be sequenced and characterized, contributes to deregulated growth of cancer cells. A role for Cripto in tumor development has been described in the human and the mouse. Members of the EGF-CFC family are found only in vertebrates: CFC proteins in zebrafish, Xenopus, chick, mouse and human have been characterized and indicate some common general functions in development. Cripto expression was first found in human and mouse embryonal carcinoma cells and male teratocarcinomas, and was demonstrated to be over-expressed in breast, cervical, ovarian, gastric, lung, colon, and pancreatic carcinomas in contrast to normal tissues where Cripto expression was invariably low or absent. Cripto may play a role in mammary tumorigenesis, since in vitro, Cripto induces mammary cell proliferation, reduces apoptosis, increases cell migration, and inhibits milk protein expression. This prediction is strengthened by observations of Cripto expression in 80% of human and mouse mammary tumors. At least three important roles for Cripto in development have created considerable interest, and each activity may be distinct in its mechanism of receptor signaling. One role is in the patterning of the anterior-posterior axis of the early embryo, a second is a crucial role in the development of the heart, and a third is in potentiating branching morphogenesis and modulating differentiation in the developing mammary gland. Whether these properties are functions of different forms of Cripto, different Cripto receptors or the distinct domains within this 15-38 kDa glycoprotein are examined here, but much remains to be revealed about this evolutionarily conserved gene product. Since all Cripto receptors have not yet been determined with certainty, future possible uses as therapeutic targets remain to be developed. Cripto is released or shed from expressing cells and may serve as an accessible marker gene in the early to mid-progressive stages of breast and other cancers. Meanwhile some speculations on possible receptor complexes for Cripto signaling in mammary cells are offered here as a spur to further discoveries.