Beta-catenin regulates Cripto- and Wnt3-dependent gene expression programs in mouse axis and mesoderm formation

DNMT1 can induce primary germ layer differentiation through de novo DNA methylation

DNA methyltransferases and Ten-Eleven Translocation (TET) proteins regulate the DNA methylation and demethylation cycles during mouse embryonic development. Although DNMT1 mainly plays a role in the maintenance of DNA methylation after DNA replication, it is also reported to possess de novo methyltransferase capacity. However, its physiological significance remains unclear. Here, we demonstrate that full-length DNMT1 (FL) and a mutant lacking the N-terminus necessary for its maintenance activity (602) confer the differentiation potential of mouse Dnmt1, Dnmt3a, and Dnmt3b (Dnmts-TKO) embryonic stem cells (ESCs). Both FL and 602 inhibit the spontaneous differentiation of Dnmts-TKO ESCs in the undifferentiated state. Dnmts-TKO ESCs showed loss of DNA methylation and de-repression of primitive endoderm-related genes, but these defects were partially restored in Dnmts-TKO + FL and Dnmts-TKO + 602 ESCs. Upon differentiation, Dnmts-TKO + FL ESCs show increased 5mC and 5hmC levels across chromosomes, including pericentromeric regions. In contrast, Dnmts-TKO + 602 ESCs didn't accumulate 5mC, and sister chromatids showed 5hmC asynchronously. Furthermore, in comparison with DNMT1_602, DNMT1_FL effectively promoted commitment to the epiblast-like cells and beyond, driving cell-autonomous mesendodermal and germline differentiation through embryoid body-based methods. With precise target selectivity achieved by its N-terminal region, DNMT1 may play a role in gene regulation leading to germline development.

Brain enlargement with rostral bias in larvae from a spontaneously occurring female variant line of Xenopus; role of aberrant embryonic Wnt/β-catenin signaling

Increased brain size and its rostral bias are hallmarks of vertebrate evolution, but the underlying developmental and genetic basis remains poorly understood. To provide clues to understanding vertebrate brain evolution, we investigated the developmental mechanisms of brain enlargement observed in the offspring of a previously unrecognized, spontaneously occurring female variant line of Xenopus that appears to reflect a genetic variation. Brain enlargement in larvae from this line showed a pronounced rostral bias that could be traced back to the neural plate, the primordium of the brain. At the gastrula stage, the Spemann organizer, which is known to induce the neural plate from the adjacent dorsal ectoderm and give it the initial rostrocaudal patterning, was expanded from dorsal to ventral in a large proportion of the offspring of variant females. Consistently, siamois expression, which is required for Spemann organizer formation, was expanded laterally from dorsal to ventral at the blastula stage in variant offspring. This implies that the active region of the Wnt/β-catenin signaling pathway was similarly expanded in advance on the dorsal side, as siamois is a target gene of this pathway. Notably, the earliest detectable change in variant offspring was in fertilized eggs, in which maternal wnt11b mRNA, a candidate dorsalizing factor responsible for activating Wnt/β-catenin signaling in the dorsal embryonic region, had a wider distribution in the vegetal cortical cytoplasm. Since lateral spreading of wnt11b mRNA, and possibly that of other potential maternal dorsalizing factors in these eggs, is expected to facilitate lateral expansion of the active region of the Wnt/β-catenin pathway during subsequent embryonic stages, we concluded that aberrant Wnt/β-catenin signaling could cause rostral-biased brain enlargement via expansion of siamois expression and consequent expansion of the Spemann organizer in Xenopus. Our studies of spontaneously occurring variations in brain development in Xenopus would provide hints for uncovering genetic mutations that drive analogous morphogenetic variations during vertebrate brain evolution.

3D-cultured blastoids model human embryogenesis from pre-implantation to early gastrulation stages

Naive human pluripotent stem cells have the remarkable ability to self-organize into blastocyst-like structures ("blastoids") that model lineage segregation in the pre-implantation embryo. However, the extent to which blastoids can recapitulate the defining features of human post-implantation development remains unexplored. Here, we report that blastoids cultured on thick three-dimensional (3D) extracellular matrices capture hallmarks of early post-implantation development, including epiblast lumenogenesis, rapid expansion and diversification of trophoblast lineages, and robust invasion of extravillous trophoblast cells by day 14. Extended blastoid culture results in the localized activation of primitive streak marker TBXT and the emergence of embryonic germ layers by day 21. We also show that the modulation of WNT signaling alters the balance between epiblast and trophoblast fates in post-implantation blastoids. This work demonstrates that 3D-cultured blastoids offer a continuous and integrated in vitro model system of human embryonic and extraembryonic development from pre-implantation to early gastrulation stages.

The role of Wnt signaling in the development of the epiblast and axial progenitors

Understanding how the body plan is established during embryogenesis remains a fundamental biological question. The Wnt/β-catenin signaling pathway plays a crucial and highly conserved role in body plan formation, functioning to polarize the primary anterior-posterior (AP) or head-to-tail body axis in most metazoans. In this chapter, we focus on the roles that the mammalian Wnt/β-catenin pathway plays to prepare the pluripotent epiblast for gastrulation, and to elicit the emergence of multipotent axial progenitors from the caudal epiblast. Interactions between Wnt and retinoic acid (RA), another powerful family of developmental signaling molecules, in axial progenitors will also be discussed. Gastrulation movements and somitogenesis result in the anterior displacement of the RA source (the rostral somites and lateral plate mesoderm (LPM)), from the posterior Wnt source (the primitive streak (PS)), leading to the establishment of antiparallel gradients of RA and Wnt that control the self-renewal and successive differentiation of neck, trunk and tail progenitors.

Overview of junctional complexes during mammalian early embryonic development

Cell-cell junctions form strong intercellular connections and mediate communication between blastomeres during preimplantation embryonic development and thus are crucial for cell integrity, polarity, cell fate specification and morphogenesis. Together with cell adhesion molecules and cytoskeletal elements, intercellular junctions orchestrate mechanotransduction, morphokinetics and signaling networks during the development of early embryos. This review focuses on the structure, organization, function and expressional pattern of the cell-cell junction complexes during early embryonic development. Understanding the importance of dynamic junction formation and maturation processes will shed light on the molecular mechanism behind developmental abnormalities of early embryos during the preimplantation period.

Evolution of Somite Compartmentalization: A View From Xenopus

Somites are transitory metameric structures at the basis of the axial organization of vertebrate musculoskeletal system. During evolution, somites appear in the chordate phylum and compartmentalize mainly into the dermomyotome, the myotome, and the sclerotome in vertebrates. In this review, we summarized the existing literature about somite compartmentalization in Xenopus and compared it with other anamniote and amniote vertebrates. We also present and discuss a model that describes the evolutionary history of somite compartmentalization from ancestral chordates to amniote vertebrates. We propose that the ancestral organization of chordate somite, subdivided into a lateral compartment of multipotent somitic cells (MSCs) and a medial primitive myotome, evolves through two major transitions. From ancestral chordates to vertebrates, the cell potency of MSCs may have evolved and gave rise to all new vertebrate compartments, i.e., the dermomyome, its hypaxial region, and the sclerotome. From anamniote to amniote vertebrates, the lateral MSC territory may expand to the whole somite at the expense of primitive myotome and may probably facilitate sclerotome formation. We propose that successive modifications of the cell potency of some type of embryonic progenitors could be one of major processes of the vertebrate evolution.

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.

Cripto-1 as a Key Factor in Tumor Progression, Epithelial to Mesenchymal Transition and Cancer Stem Cells

Cripto-1 is an essential protein for human development that plays a key role in the early phase of gastrulation in the differentiation of an embryo as well as assists with wound healing processes. Importantly, Cripto-1 induces epithelial to mesenchymal transition to turn fixed epithelial cells into a more mobile mesenchymal phenotype through the downregulation of epithelial adhesion molecules such as E-cadherin, occludins, and claudins, and the upregulation of mesenchymal, mobile proteins, such as N-cadherin, Snail, and Slug. Consequently, Cripto-1’s role in inducing EMT to promote cell motility is beneficial in embryogenesis, but detrimental in the formation, progression and metastasis of malignant tumors. Indeed, Cripto-1 is found to be upregulated in most cancers, such as breast, lung, gastrointestinal, hepatic, renal, cervical, ovarian, prostate, and skin cancers. Through its role in EMT, Cripto-1 can remodel cancer cells to enable them to travel through the extracellular matrix as well as blood and lymphatic vessels to metastasize to different organs. Additionally, Cripto-1 promotes the survival of cancer stem cells, which can lead to relapse in cancer patients.

New Insights into Cancer Targeted Therapy: Nodal and Cripto-1 as Attractive Candidates

The transforming growth factor beta (TGF-β) signaling is fundamental for correct embryonic development. However, alterations of this pathway have been correlated with oncogenesis, tumor progression and sustaining of cancer stem cells (CSCs). Cripto-1 (CR-1) and Nodal are two embryonic proteins involved in TGF-β signaling. Their expression is almost undetectable in terminally differentiated cells, but they are often re-expressed in tumor cells, especially in CSCs. Moreover, cancer cells that show high levels of CR-1 and/or Nodal display more aggressive phenotypes in vitro, while in vivo their expression correlates with a worse prognosis in several human cancers. The ability to target CSCs still represents an unmet medical need for the complete eradication of certain types of tumors. Given the prognostic role and the selective expression of CR-1 and Nodal on cancer cells, they represent archetypes for targeted therapy. The aim of this review is to clarify the role of CR-1 and Nodal in cancer stem populations and to summarize the current therapeutic strategy to target CSCs using monoclonal antibodies (mAbs) or other molecular tools to interfere with these two proteins.

Rab7-Mediated Endocytosis Establishes Patterning of Wnt Activity through Inactivation of Dkk Antagonism

Endocytosis has been proposed to modulate cell signaling activities. However, the role of endocytosis in embryogenesis, which requires coordination of multiple signaling inputs, has remained less understood. We previously showed that mouse embryos lacking a small guanosine triphosphate (GTP)-binding protein Rab7 implicated in endocytic flow are defective in gastrulation. Here, we investigate how subcellular defects associated with Rab7 deficiency are related to the observed developmental defects. Rab7-deficient embryos fail to organize mesodermal tissues due to defects in Wnt-β-catenin signaling. Visceral endoderm (VE)-specific ablation of Rab7 results in patterning defects similar to systemic Rab7 deletion. Rab7 mutants accumulate the Wnt antagonist Dkk1 in the extracellular space and in intracellular compartments throughout the VE epithelium. These data indicate that Rab7-dependent endocytosis regulates the concentration and availability of extracellular Dkk1, thereby relieving the epiblast of antagonism. This intercellular mechanism therefore organizes distinct spatiotemporal patterns of canonical Wnt activity during the peri-gastrulation stages of embryonic development.

Deletion of the Dishevelled family of genes disrupts anterior-posterior axis specification and selectively prevents mesoderm differentiation

The Dishevelled proteins transduce both canonical Wnt/β-catenin and non-canonical Wnt/planar cell polarity (PCP) signaling pathways to regulate many key developmental processes during embryogenesis. Here, we disrupt both canonical and non-canonical Wnt pathways by targeting the entire Dishevelled family of genes (Dvl1, Dvl2, and Dvl3) to investigate their functional roles in the early embryo. We identified several defects in anterior-posterior axis specification and mesoderm patterning in Dvl1+/-; Dvl2-/-; Dvl3-/- embryos. Homozygous deletions in all three Dvl genes (Dvl TKO) resulted in defects in distal visceral endoderm migration and a complete failure to induce mesoderm formation. To identify potential mechanisms that lead to the defects in the developmental processes preceding gastrulation, we generated Dvl TKO mouse embryonic stem cells (mESCs) and compared the transcriptional profile of these cells with wild-type (WT) mESCs during germ lineage differentiation into 3D embryoid bodies (EBs). While the Dvl TKO mESCs displayed similar morphology, self-renewal properties, and minor transcriptional variation from WT mESCs, we identified major transcriptional dysregulation in the Dvl TKO EBs during differentiation in a number of genes involved in anterior-posterior pattern specification, gastrulation induction, mesenchyme morphogenesis, and mesoderm-derived tissue development. The absence of the Dvls leads to specific down-regulation of BMP signaling genes. Furthermore, exogenous activation of canonical Wnt, BMP, and Nodal signaling all fail to rescue the mesodermal defects in the Dvl TKO EBs. Moreover, endoderm differentiation was promoted in the absence of mesoderm in the Dvl TKO EBs, while the suppression of ectoderm differentiation was delayed. Overall, we demonstrate that the Dvls are dispensable for maintaining self-renewal in mESCs but are critical during differentiation to regulate key developmental signaling pathways to promote proper axis specification and mesoderm formation.

Pleiotropic roles of tankyrase/PARP proteins in the establishment and maintenance of human naïve pluripotency

Tankyrase 1 (TNKS1; PARP-5a) and Tankyrase 2 (TNKS2; PARP-5b) are poly-ADP-ribosyl-polymerase (PARP)-domain-containing proteins that regulate the activities of a wide repertoire of target proteins via post-translational addition of poly-ADP-ribose polymers (PARylation). Although tankyrases were first identified as regulators of human telomere elongation, important and expansive roles of tankyrase activity have recently emerged in the development and maintenance of stem cell states. Herein, we summarize the current state of knowledge of the various tankyrase-mediated activities that may promote human naïve and 'extended' pluripotency'. We review the putative role of tankyrase and PARP inhibition in trophectoderm specification, telomere elongation, DNA repair and chromosomal segregation, metabolism, and PTEN-mediated apoptosis. Importantly, tankyrases possess PARP-independent activities that include regulation of MDC1-associated DNA repair by homologous recombination (HR) and autophagy/pexophagy, which is an essential mechanism of protein synthesis in the preimplantation embryo. Additionally, tankyrases auto-regulate themselves via auto-PARylation which augments their cellular protein levels and potentiates their non-PARP tankyrase functions. We propose that these non-PARP-related activities of tankyrase proteins may further independently affect both naïve and extended pluripotency via mechanisms that remain undetermined. We broadly outline a hypothetical framework for how inclusion of a tankyrase/PARP inhibitor in small molecule cocktails may stabilize and potentiate naïve and extended pluripotency via pleiotropic routes and mechanisms.

TGFβ Family Signaling Pathways in Pluripotent and Teratocarcinoma Stem Cells' Fate Decisions: Balancing Between Self-Renewal, Differentiation, and Cancer

The transforming growth factor-β (TGFβ) family factors induce pleiotropic effects and are involved in the regulation of most normal and pathological cellular processes. The activity of different branches of the TGFβ family signaling pathways and their interplay with other signaling pathways govern the fine regulation of the self-renewal, differentiation onset and specialization of pluripotent stem cells in various cell derivatives. TGFβ family signaling pathways play a pivotal role in balancing basic cellular processes in pluripotent stem cells and their derivatives, although disturbances in their genome integrity induce the rearrangements of signaling pathways and lead to functional impairments and malignant transformation into cancer stem cells. Therefore, the identification of critical nodes and targets in the regulatory cascades of TGFβ family factors and other signaling pathways, and analysis of the rearrangements of the signal regulatory network during stem cell state transitions and interconversions, are key issues for understanding the fundamental mechanisms of both stem cell biology and cancer initiation and progression, as well as for clinical applications. This review summarizes recent advances in our understanding of TGFβ family functions in naїve and primed pluripotent stem cells and discusses how these pathways are involved in perturbations in the signaling network of malignant teratocarcinoma stem cells with impaired differentiation potential.

WNT/β-catenin modulates the axial identity of embryonic stem cell-derived human neural crest

WNT/β-catenin signaling is crucial for neural crest (NC) formation, yet the effects of the magnitude of the WNT signal remain ill-defined. Using a robust model of human NC formation based on human pluripotent stem cells (hPSCs), we expose that the WNT signal modulates the axial identity of NCs in a dose-dependent manner, with low WNT leading to anterior OTX⁺ HOX^- NC and high WNT leading to posterior OTX^- HOX⁺ NC. Differentiation tests of posterior NC confirm expected derivatives, including posterior-specific adrenal derivatives, and display partial capacity to generate anterior ectomesenchymal derivatives. Furthermore, unlike anterior NC, posterior NC exhibits a transient TBXT⁺/SOX2⁺ neuromesodermal precursor-like intermediate. Finally, we analyze the contributions of other signaling pathways in posterior NC formation, which suggest a crucial role for FGF in survival/proliferation, and a requirement of BMP for NC maturation. As expected retinoic acid (RA) and FGF are able to modulate HOX expression in the posterior NC. Surprisingly, early RA supplementation prohibits NC formation. This work reveals for the first time that the amplitude of WNT signaling can modulate the axial identity of NC cells in humans.

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.

Capacitation of human naïve pluripotent stem cells for multi-lineage differentiation

Human naïve pluripotent stem cells (PSCs) share features with the pre-implantation epiblast. They therefore provide an unmatched opportunity for characterising the developmental programme of pluripotency in Homo sapiens Here, we confirm that naïve PSCs do not respond directly to germ layer induction, but must first acquire competence. Capacitation for multi-lineage differentiation occurs without exogenous growth factor stimulation and is facilitated by inhibition of Wnt signalling. Whole-transcriptome profiling during this formative transition highlights dynamic changes in gene expression, which affect many cellular properties including metabolism and epithelial features. Notably, naïve pluripotency factors are exchanged for postimplantation factors, but competent cells remain devoid of lineage-specific transcription. The gradual pace of transition for human naïve PSCs is consistent with the timespan of primate development from blastocyst to gastrulation. Transcriptome trajectory during in vitro capacitation of human naïve cells tracks the progression of the epiblast during embryogenesis in Macaca fascicularis, but shows greater divergence from mouse development. Thus, the formative transition of naïve PSCs in a simple culture system may recapitulate essential and specific features of pluripotency dynamics during an inaccessible period of human embryogenesis.

Sulforaphane Suppresses the Growth of Triple-negative Breast Cancer Stem-like Cells In vitro and In vivo

Triple-negative breast cancer (TNBC) represents the poorest prognosis among all of breast cancer subtypes with no currently available effective therapy. In this study, we hypothesized that sulforaphane, a dietary component abundant in broccoli and its sprouts, can inhibit malignant cell proliferation and tumor sphere formation of cancer stem-like cells (CSC) in TNBC. CSC population was isolated using FACS analysis with the combined stem cell surface markers, CD44⁺/CD24^-/CD49f⁺ The effect of sulforaphane on a stem-related embryonic oncogene CRIPTO-1/TDGF1 (CR1) was evaluated via ELISA. In vivo, BalbC/nude mice were supplemented with sulforaphane before and after TNBC cell inoculation (daily intraperitoneal injection of 50 mg sulforaphane/kg for 5 and 3 weeks, respectively), and the effects of sulforaphane during mammary tumor initiation and growth were accessed with NanoString gene analysis. We found that sulforaphane can inhibit cell proliferation and mammosphere formation of CSCs in TNBC. Further analysis of gene expression in these TNBC tumor cells revealed that sulforaphane significantly decreases the expression of cancer-specific CR1, CRIPTO-3/TDGF1P3 (CR3, a homologue of CR1), and various stem cell markers including Nanog, aldehyde dehydrogenase 1A1 (ALDH1A1), Wnt3, and Notch4. Our results suggest that sulforaphane may control the malignant proliferation of CSCs in TNBC via Cripto-mediated pathway by either suppressing its expression and/or by inhibiting Cripto/Alk4 protein complex formation. Thus, the use of sulforaphane for chemoprevention of TNBC is plausible and warrants further clinical evaluation.

Divergent early mesoderm specification underlies distinct head and trunk muscle programmes in vertebrates

Head and trunk muscles have discrete embryological origins and are governed by distinct regulatory programmes. Whereas the developmental route of trunk muscles from mesoderm is well studied, that of head muscles is ill defined. Here, we show that, unlike the myogenic trunk paraxial mesoderm, head mesoderm development is independent of the T/Tbx6 network in mouse. We reveal that, in contrast to Wnt and FGF-driven trunk mesoderm, dual inhibition of Wnt/β-catenin and Nodal specifies head mesoderm. Remarkably, the progenitors derived from embryonic stem cells by dual inhibition efficiently differentiate into cardiac and skeletal muscle cells. This twin potential is the defining feature of cardiopharyngeal mesoderm: the head subtype giving rise to heart and branchiomeric head muscles. Therefore, our findings provide compelling evidence that dual inhibition specifies head mesoderm and unravel the mechanism that diversifies head and trunk muscle programmes during early mesoderm fate commitment. Significantly, this is the first report of directed differentiation of pluripotent stem cells, without transgenes, into progenitors with muscle/heart dual potential. Ability to generate branchiomeric muscle in vitro could catalyse efforts in modelling myopathies that selectively involve head muscles.

Micropattern differentiation of mouse pluripotent stem cells recapitulates embryo regionalized cell fate patterning

During gastrulation epiblast cells exit pluripotency as they specify and spatially arrange the three germ layers of the embryo. Similarly, human pluripotent stem cells (PSCs) undergo spatially organized fate specification on micropatterned surfaces. Since in vivo validation is not possible for the human, we developed a mouse PSC micropattern system and, with direct comparisons to mouse embryos, reveal the robust specification of distinct regional identities. BMP, WNT, ACTIVIN and FGF directed mouse epiblast-like cells to undergo an epithelial-to-mesenchymal transition and radially pattern posterior mesoderm fates. Conversely, WNT, ACTIVIN and FGF patterned anterior identities, including definitive endoderm. By contrast, epiblast stem cells, a developmentally advanced state, only specified anterior identities, but without patterning. The mouse micropattern system offers a robust scalable method to generate regionalized cell types present in vivo, resolve how signals promote distinct identities and generate patterns, and compare mechanisms operating in vivo and in vitro and across species.

TGF-β family co-receptor function and signaling

Transforming growth factor-β (TGF-β) family members, which include TGF-βs, activins and bone morphogenetic proteins, are pleiotropic cytokines that elicit cell type-specific effects in a highly context-dependent manner in many different tissues. These secreted protein ligands signal via single-transmembrane Type I and Type II serine/threonine kinase receptors and intracellular SMAD transcription factors. Deregulation in signaling has been implicated in a broad array of diseases, and implicate the need for intricate fine tuning in cellular signaling responses. One important emerging mechanism by which TGF-β family receptor signaling intensity, duration, specificity and diversity are regulated and/or mediated is through cell surface co-receptors. Here, we provide an overview of the co-receptors that have been identified for TGF-β family members. While some appear to be specific to TGF-β family members, others are shared with other pathways and provide possible ways for signal integration. This review focuses on novel functions of TGF-β family co-receptors, which continue to be discovered.

Vertebrate Axial Patterning: From Egg to Asymmetry

The emergence of the bilateral embryonic body axis from a symmetrical egg has been a long-standing question in developmental biology. Historical and modern experiments point to an initial symmetry-breaking event leading to localized Wnt and Nodal growth factor signaling and subsequent induction and formation of a self-regulating dorsal "organizer." This organizer forms at the site of notochord cell internalization and expresses primarily Bone Morphogenetic Protein (BMP) growth factor antagonists that establish a spatiotemporal gradient of BMP signaling across the embryo, directing initial cell differentiation and morphogenesis. Although the basics of this model have been known for some time, many of the molecular and cellular details have only recently been elucidated and the extent that these events remain conserved throughout vertebrate evolution remains unclear. This chapter summarizes historical perspectives as well as recent molecular and genetic advances regarding: (1) the mechanisms that regulate symmetry-breaking in the vertebrate egg and early embryo, (2) the pathways that are activated by these events, in particular the Wnt pathway, and the role of these pathways in the formation and function of the organizer, and (3) how these pathways also mediate anteroposterior patterning and axial morphogenesis. Emphasis is placed on comparative aspects of the egg-to-embryo transition across vertebrates and their evolution. The future prospects for work regarding self-organization and gene regulatory networks in the context of early axis formation are also discussed.

Increasing β-catenin/Wnt3A activity levels drive mechanical strain-induced cell cycle progression through mitosis

Mechanical force and Wnt signaling activate β-catenin-mediated transcription to promote proliferation and tissue expansion. However, it is unknown whether mechanical force and Wnt signaling act independently or synergize to activate β-catenin signaling and cell division. We show that mechanical strain induced Src-dependent phosphorylation of Y654 β-catenin and increased β-catenin-mediated transcription in mammalian MDCK epithelial cells. Under these conditions, cells accumulated in S/G2 (independent of DNA damage) but did not divide. Activating β-catenin through Casein Kinase I inhibition or Wnt3A addition increased β-catenin-mediated transcription and strain-induced accumulation of cells in S/G2. Significantly, only the combination of mechanical strain and Wnt/β-catenin activation triggered cells in S/G2 to divide. These results indicate that strain-induced Src phosphorylation of β-catenin and Wnt-dependent β-catenin stabilization synergize to increase β-catenin-mediated transcription to levels required for mitosis. Thus, local Wnt signaling may fine-tune the effects of global mechanical strain to restrict cell divisions during tissue development and homeostasis.

DOI:http://dx.doi.org/10.7554/eLife.19799.001

Tissues and organs can both produce and respond to physical forces. For example, the lungs expand and contract; the heart pumps blood; and bones and muscles grow or shrink depending on how much they are used. These responses are possible because cells contain proteins that can respond to physical forces. One of the best studied of these is a protein called β-catenin, which increases the activity of genes that trigger cells to divide to promote the expansion of tissues. β-catenin is over-active in many types of cancer cells where it contributes to tumor growth. In addition to being switched on by mechanical force, β-catenin is also activated when cells detect a signal molecule called Wnt.

Cells cycle through a series of stages known as the cell cycle to ensure that they only divide when they are fully prepared to do so. Benham-Pyle et al. investigated if physical force and Wnt activate β-catenin in the same way or if they have different effects on cell division. The experiments were conducted on dog kidney cells that had left the cell cycle and had therefore temporarily stopped dividing. Physical forces, such as stretching, resulted in β-catenin being modified by an enzyme called SRC kinase, which allowed the cells to re-enter the cell cycle. On the other hand, Wnt stabilized β-catenin and temporarily increased the number of cell divisions.

When mechanical stretch and Wnt signaling were combined, the cells were more likely to re-enter the cell cycle and divide compared to either stimulus alone. These data suggest that physical force and Wnt signaling affect β-catenin differently and that they can therefore have a greater effect on cell or tissue growth when they act together than on their own.

The findings of Benham-Pyle et al. show that β-catenin is not simply switched on or off, but can have different levels of activity depending on the input the cells are receiving. Future experiments will test whether these mechanisms also exist in three-dimensional tissues, which will help us understand how organs develop.

DOI:http://dx.doi.org/10.7554/eLife.19799.002

β-Catenin-Independent Roles of Wnt/LRP6 Signaling

Wnt/LRP6 signaling is best known for the β-catenin-dependent regulation of target genes. However, pathway branches have recently emerged, including Wnt/STOP signaling, which act independently of β-catenin and transcription. We review here the molecular mechanisms underlying β-catenin-independent Wnt/LRP6 signaling cascades and their implications for cell biology, development, and physiology.

Resolving early mesoderm diversification through single-cell expression profiling

In mammals, specification of the three major germ layers occurs during gastrulation, when cells ingressing through the primitive streak differentiate into the precursor cells of major organ systems. However, the molecular mechanisms underlying this process remain unclear, as numbers of gastrulating cells are very limited. In the mouse embryo at embryonic day 6.5, cells located at the junction between the extra-embryonic region and the epiblast on the posterior side of the embryo undergo an epithelial-to-mesenchymal transition and ingress through the primitive streak. Subsequently, cells migrate, either surrounding the prospective ectoderm contributing to the embryo proper, or into the extra-embryonic region to form the yolk sac, umbilical cord and placenta. Fate mapping has shown that mature tissues such as blood and heart originate from specific regions of the pre-gastrula epiblast, but the plasticity of cells within the embryo and the function of key cell-type-specific transcription factors remain unclear. Here we analyse 1,205 cells from the epiblast and nascent Flk1(+) mesoderm of gastrulating mouse embryos using single-cell RNA sequencing, representing the first transcriptome-wide in vivo view of early mesoderm formation during mammalian gastrulation. Additionally, using knockout mice, we study the function of Tal1, a key haematopoietic transcription factor, and demonstrate, contrary to previous studies performed using retrospective assays, that Tal1 knockout does not immediately bias precursor cells towards a cardiac fate.

MicroRNA-142-3p Negatively Regulates Canonical Wnt Signaling Pathway

Wnt/β-catenin signaling pathway plays essential roles in mammalian development and tissue homeostasis. MicroRNAs (miRNAs) are a class of regulators involved in modulating this pathway. In this study, we screened miRNAs regulating Wnt/β-catenin signaling by using a TopFlash based luciferase reporter. Surprisingly, we found that miR-142 inhibited Wnt/β-catenin signaling, which was inconsistent with a recent study showing that miR-142-3p targeted Adenomatous Polyposis Coli (APC) to upregulate Wnt/β-catenin signaling. Due to the discordance, we elaborated experiments by using extensive mutagenesis, which demonstrated that the stem-loop structure was important for miR-142 to efficiently suppress Wnt/β-catenin signaling. Moreover, the inhibitory effect of miR-142 relies on miR-142-3p rather than miR-142-5p. Further, we found that miR-142-3p directly modulated translation of Ctnnb1 mRNA (encoding β-catenin) through binding to its 3’ untranslated region (3’ UTR). Finally, miR-142 was able to repress cell cycle progression by inhibiting active Wnt/β-catenin signaling. Thus, our findings highlight the inhibitory role of miR-142-3p in Wnt/β-catenin signaling, which help to understand the complex regulation of Wnt/β-catenin signaling.

The roles of ERAS during cell lineage specification of mouse early embryonic development

Eras encodes a Ras-like GTPase protein that was originally identified as an embryonic stem cell-specific Ras. ERAS has been known to be required for the growth of embryonic stem cells and stimulates somatic cell reprogramming, suggesting its roles on mouse early embryonic development. We now report a dynamic expression pattern of Eras during mouse peri-implantation development: its expression increases at the blastocyst stage, and specifically decreases in E7.5 mesoderm. In accordance with its expression pattern, the increased expression of Eras promotes cell proliferation through controlling AKT activation and the commitment from ground to primed state through ERK activation in mouse embryonic stem cells; and the reduced expression of Eras facilitates primitive streak and mesoderm formation through AKT inhibition during gastrulation. The expression of Eras is finely regulated to match its roles in mouse early embryonic development during which Eras expression is negatively regulated by the β-catenin pathway. Thus, beyond its well-known role on cell proliferation, ERAS may also play important roles in cell lineage specification during mouse early embryonic development.

Developmental signaling pathways regulating mammary stem cells and contributing to the etiology of triple-negative breast cancer

Cancer has been considered as temporal and spatial aberrations of normal development in tissues. Similarities between mammary embryonic development and cell transformation suggest that the underlying processes required for mammary gland development are also those perturbed during various stages of mammary tumorigenesis and breast cancer (BC) development. The master regulators of embryonic development Cripto-1, Notch/CSL, and Wnt/β-catenin play key roles in modulating mammary gland morphogenesis and cell fate specification in the embryo through fetal mammary stem cells (fMaSC) and in the adult organism particularly within the adult mammary stem cells (aMaSC), which determine mammary progenitor cell lineages that generate the basal/myoepithelial and luminal compartments of the adult mammary gland. Together with recognized transcription factors and embryonic stem cell markers, these embryonic regulatory molecules can be inappropriately augmented during tumorigenesis to support the tumor-initiating cell (TIC)/cancer stem cell (CSC) compartment, and the effects of their deregulation may contribute for the etiology of BC, in particular the most aggressive subtype of BC, triple-negative breast cancer (TNBC). This in depth review will present evidence of the involvement of Cripto-1, Notch/CSL, and Wnt/β-catenin in the normal mammary gland morphogenesis and tumorigenesis, from fMaSC/aMaSC regulation to TIC generation and maintenance in TNBC. Specific therapies for treating TNBC by targeting these embryonic pathways in TICs will be further discussed, providing new opportunities to destroy not only the bulk tumor, but also TICs that initiate and promote the metastatic spread and recurrence of this aggressive subtype of BC.

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.

Wnt/ß-catenin signalling and the dynamics of fate decisions in early mouse embryos and embryonic stem (ES) cells

Wnt/ß-catenin signalling is a widespread cell signalling pathway with multiple roles during vertebrate development. In mouse embryonic stem (mES) cells, there is a dual role for ß-catenin: it promotes differentiation when activated as part of the Wnt/ß-catenin signalling pathway, and promotes stable pluripotency independently of signalling. Although mES cells resemble the preimplantation epiblast progenitors, the first requirement for Wnt/ß-catenin signalling during mouse development has been reported at implantation [1,2]. The relationship between ß-catenin and pluripotency and that of mES cells with epiblast progenitors suggests that ß-catenin might have a functional role during preimplantation development. Here we summarize the expression and function of Wnt/ß-catenin signalling elements during the early stages of mouse development and consider the reasons why the requirement in ES cells do not reflect the embryo.

BMP Inhibition in Seminomas Initiates Acquisition of Pluripotency via NODAL Signaling Resulting in Reprogramming to an Embryonal Carcinoma

Type II germ cell cancers (GCC) can be subdivided into seminomas and non-seminomas. Seminomas are similar to carcinoma in situ (CIS) cells, the common precursor of type II GCCs, with regard to epigenetics and expression, while embryonal carcinomas (EC) are totipotent and differentiate into teratomas, yolk-sac tumors and choriocarcinomas. GCCs can present as seminomas with a non-seminoma component, raising the question if a CIS gives rise to seminomas and ECs at the same time or whether seminomas can be reprogrammed to ECs. In this study, we utilized the seminoma cell line TCam-2 that acquires an EC-like status after xenografting into the murine flank as a model for a seminoma to EC transition and screened for factors initiating and driving this process. Analysis of expression and DNA methylation dynamics during transition of TCam-2 revealed that many pluripotency- and reprogramming-associated genes were upregulated while seminoma-markers were downregulated. Changes in expression level of 53 genes inversely correlated to changes in DNA methylation. Interestingly, after xenotransplantation 6 genes (GDF3, NODAL, DNMT3B, DPPA3, GAL, AK3L1) were rapidly induced, followed by demethylation of their genomic loci, suggesting that these 6 genes are poised for expression driving the reprogramming. We demonstrate that inhibition of BMP signaling is the initial event in reprogramming, resulting in activation of the pluripotency-associated genes and NODAL signaling. We propose that reprogramming of seminomas to ECs is a multi-step process. Initially, the microenvironment causes inhibition of BMP signaling, leading to induction of NODAL signaling. During a maturation phase, a fast acting NODAL loop stimulates its own activity and temporarily inhibits BMP signaling. During the stabilization phase, a slow acting NODAL loop, involving WNTs re-establishes BMP signaling and the pluripotency circuitry. In parallel, DNMT3B-driven de novo methylation silences seminoma-associated genes and epigenetically fixes the EC state.

β-Catenin Regulates Primitive Streak Induction through Collaborative Interactions with SMAD2/SMAD3 and OCT4

Canonical Wnt and Nodal signaling are both required for induction of the primitive streak (PS), which guides organization of the early embryo. The Wnt effector β-catenin is thought to function in these early lineage specification decisions via transcriptional activation of Nodal signaling. Here, we demonstrate a broader role for β-catenin in PS formation by analyzing its genome-wide binding in a human embryonic stem cell model of PS induction. β-catenin occupies regulatory regions in numerous PS and neural crest genes, and direct interactions between β-catenin and the Nodal effectors SMAD2/SMAD3 are required at these regions for PS gene activation. Furthermore, OCT4 binding in proximity to these sites is likewise required for PS induction, suggesting a collaborative interaction between β-catenin and OCT4. Induction of neural crest genes by β-catenin is repressed by SMAD2/SMAD3, ensuring proper lineage specification. This study provides mechanistic insight into how Wnt signaling controls early cell lineage decisions.

Repairing quite swimmingly: advances in regenerative medicine using zebrafish

Regenerative medicine has the promise to alleviate morbidity and mortality caused by organ dysfunction, longstanding injury and trauma. Although regenerative approaches for a few diseases have been highly successful, some organs either do not regenerate well or have no current treatment approach to harness their intrinsic regenerative potential. In this Review, we describe the modeling of human disease and tissue repair in zebrafish, through the discovery of disease-causing genes using classical forward-genetic screens and by modulating clinically relevant phenotypes through chemical genetic screening approaches. Furthermore, we present an overview of those organ systems that regenerate well in zebrafish in contrast to mammalian tissue, as well as those organs in which the regenerative potential is conserved from fish to mammals, enabling drug discovery in preclinical disease-relevant models. We provide two examples from our own work in which the clinical translation of zebrafish findings is either imminent or has already proven successful. The promising results in multiple organs suggest that further insight into regenerative mechanisms and novel clinically relevant therapeutic approaches will emerge from zebrafish research in the future.

Rho kinase activity controls directional cell movements during primitive streak formation in the rabbit embryo

During animal gastrulation, the specification of the embryonic axes is accompanied by epithelio-mesenchymal transition (EMT), the first major change in cell shape after fertilization. EMT takes place in disparate topographical arrangements, such as the circular blastopore of amphibians, the straight primitive streak of birds and mammals or in intermediate gastrulation forms of other amniotes such as reptiles. Planar cell movements are prime candidates to arrange specific modes of gastrulation but there is no consensus view on their role in different vertebrate classes. Here, we test the impact of interfering with Rho kinase-mediated cell movements on gastrulation topography in blastocysts of the rabbit, which has a flat embryonic disc typical for most mammals. Time-lapse video microscopy, electron microscopy, gene expression and morphometric analyses of the effect of inhibiting ROCK activity showed – besides normal specification of the organizer region – a dose-dependent disruption of primitive streak formation; this disruption resulted in circular, arc-shaped or intermediate forms, reminiscent of those found in amphibians, fishes and reptiles. Our results reveal a crucial role of ROCK-controlled directional cell movements during rabbit primitive streak formation and highlight the possibility that temporal and spatial modulation of cell movements were instrumental for the evolution of gastrulation forms.

Summary: ROCK regulates cell motility, polarisation, intercalation and division in gastrulating rabbit embryos, revealing a role for Wnt-PCP signalling in primitive streak formation.

Autoregulation and heterogeneity in expression of human Cripto-1

Cripto-1 (CR-1) is involved in various processes in embryonic development and cancer. Multiple pathways regulate CR-1 expression. Our present work demonstrates a possible positive feedback circuit where CR-1 induces its own expression. Using U-87 MG cells treated with exogenous CR-1, we show that such induction involves ALK4/SMAD2/3 pathway. Stochasticity in gene expression gives rise to heterogeneity in expression in genetically identical cells. Positive feedback increases such heterogeneity and often gives rise to two subpopulations of cells, having higher and lower expression of a gene. Using flow cytometry, we show that U-87 MG cells have a minuscule subpopulation with detectable expression of CR-1. Induction of CR-1 expression, by exogenous CR-1, increases the size of this CR-1 positive subpopulation. However, even at very high dose, most of the cells remain CR-1 negative. We show that population behavior of CR-1 induction has a signature similar to bimodal expression expected in a transcriptional circuit with positive feedback. We further show that treatment of U-87 MG cells with CR-1 leads to higher expression of drug efflux protein MDR-1 in the CR-1 positive subpopulation, indicating correlated induction of these two proteins. Positive feedback driven heterogeneity in expression of CR-1 may play crucial role in phenotypic diversification of cancer cells.

Gene regulatory networks mediating canonical Wnt signal-directed control of pluripotency and differentiation in embryo stem cells

Canonical Wnt signaling supports the pluripotency of embryonic stem cells (ESCs) but also promotes differentiation of early mammalian cell lineages. To explain these paradoxical observations, we explored the gene regulatory networks at play. Canonical Wnt signaling is intertwined with the pluripotency network comprising Nanog, Oct4, and Sox2 in mouse ESCs. In defined media supporting the derivation and propagation of ESCs, Tcf3 and β-catenin interact with Oct4; Tcf3 binds to Sox motif within Oct-Sox composite motifs that are also bound by Oct4-Sox2 complexes. Furthermore, canonical Wnt signaling upregulates the activity of the Pou5f1 distal enhancer via the Sox motif in ESCs. When viewed in the context of published studies on Tcf3 and β-catenin mutants, our findings suggest Tcf3 counters pluripotency by competition with Sox2 at these sites, and Tcf3 inhibition is blocked by β-catenin entry into this complex. Wnt pathway stimulation also triggers β-catenin association at regulatory elements with classic Lef/Tcf motifs associated with differentiation programs. The failure to activate these targets in the presence of a mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK) inhibitor essential for ESC culture suggests MEK/ERK signaling and canonical Wnt signaling combine to promote ESC differentiation.

Symmetry breaking, germ layer specification and axial organisation in aggregates of mouse embryonic stem cells

Mouse embryonic stem cells (mESCs) are clonal populations derived from preimplantation mouse embryos that can be propagated in vitro and, when placed into blastocysts, contribute to all tissues of the embryo and integrate into the normal morphogenetic processes, i.e. they are pluripotent. However, although they can be steered to differentiate in vitro into all cell types of the organism, they cannot organise themselves into structures that resemble embryos. When aggregated into embryoid bodies they develop disorganised masses of different cell types with little spatial coherence. An exception to this rule is the emergence of retinas and anterior cortex-like structures under minimal culture conditions. These structures emerge from the cultures without any axial organisation. Here, we report that small aggregates of mESCs, of about 300 cells, self-organise into polarised structures that exhibit collective behaviours reminiscent of those that cells exhibit in early mouse embryos, including symmetry breaking, axial organisation, germ layer specification and cell behaviour, as well as axis elongation. The responses are signal specific and uncouple processes that in the embryo are tightly associated, such as specification of the anteroposterior axis and anterior neural development, or endoderm specification and axial elongation. We discuss the meaning and implications of these observations and the potential uses of these structures which, because of their behaviour, we suggest to call ‘gastruloids’.

Signaling mechanisms of the epithelial-mesenchymal transition

The epithelial-mesenchymal transition (EMT) is an essential mechanism in embryonic development and tissue repair. EMT also contributes to the progression of disease, including organ fibrosis and cancer. EMT, as well as a similar transition occurring in vascular endothelial cells called endothelial-mesenchymal transition (EndMT), results from the induction of transcription factors that alter gene expression to promote loss of cell-cell adhesion, leading to a shift in cytoskeletal dynamics and a change from epithelial morphology and physiology to the mesenchymal phenotype. Transcription program switching in EMT is induced by signaling pathways mediated by transforming growth factor β (TGF-β) and bone morphogenetic protein (BMP), Wnt-β-catenin, Notch, Hedgehog, and receptor tyrosine kinases. These pathways are activated by various dynamic stimuli from the local microenvironment, including growth factors and cytokines, hypoxia, and contact with the surrounding extracellular matrix (ECM). We discuss how these pathways crosstalk and respond to signals from the microenvironment to regulate the expression and function of EMT-inducing transcription factors in development, physiology, and disease. Understanding these mechanisms will enable the therapeutic control of EMT to promote tissue regeneration, treat fibrosis, and prevent cancer metastasis.

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.

Molecular mechanisms of epithelial-mesenchymal transition

The transdifferentiation of epithelial cells into motile mesenchymal cells, a process known as epithelial-mesenchymal transition (EMT), is integral in development, wound healing and stem cell behaviour, and contributes pathologically to fibrosis and cancer progression. This switch in cell differentiation and behaviour is mediated by key transcription factors, including SNAIL, zinc-finger E-box-binding (ZEB) and basic helix-loop-helix transcription factors, the functions of which are finely regulated at the transcriptional, translational and post-translational levels. The reprogramming of gene expression during EMT, as well as non-transcriptional changes, are initiated and controlled by signalling pathways that respond to extracellular cues. Among these, transforming growth factor-β (TGFβ) family signalling has a predominant role; however, the convergence of signalling pathways is essential for EMT.

CRIPTO/GRP78 signaling maintains fetal and adult mammary stem cells ex vivo

Little is known about the extracellular signaling factors that govern mammary stem cell behavior. Here, we identify CRIPTO and its cell-surface receptor GRP78 as regulators of stem cell behavior in isolated fetal and adult mammary epithelial cells. We develop a CRIPTO antagonist that promotes differentiation and reduces self-renewal of mammary stem cell-enriched populations cultured ex vivo. By contrast, CRIPTO treatment maintains the stem cell phenotype in these cultures and yields colonies with enhanced mammary gland reconstitution capacity. Surface expression of GRP78 marks CRIPTO-responsive, stem cell-enriched fetal and adult mammary epithelial cells, and deletion of GRP78 from adult mammary epithelial cells blocks their mammary gland reconstitution potential. Together, these findings identify the CRIPTO/GRP78 pathway as a developmentally conserved regulator of fetal and adult mammary stem cell behavior ex vivo, with implications for the stem-like cells found in many cancers.

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CRIPTO/GRP78 signaling activates PI3K/AKT in fetal mammary epithelial cells ex vivo

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Cell-surface GRP78 marks a CRIPTO-responsive adult mammary stem cell population

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An antagonist, ALK4^L75A-Fc, blocks soluble CRIPTO growth-factor-like activity

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CRIPTO promotes and ALK4^L75A-Fc inhibits mammary stem cell maintenance ex vivo

Embryonic origins of human vascular smooth muscle cells: implications for in vitro modeling and clinical application

Vascular smooth muscle cells (SMCs) arise from multiple origins during development, raising the possibility that differences in embryological origins between SMCs could contribute to site-specific localization of vascular diseases. In this review, we first examine the developmental pathways and embryological origins of vascular SMCs and then discuss in vitro strategies for deriving SMCs from human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). We then review in detail the potential for vascular disease modeling using iPSC-derived SMCs and consider the pathological implications of heterogeneous embryonic origins. Finally, we touch upon the role of human ESC-derived SMCs in therapeutic revascularization and the challenges remaining before regenerative medicine using ESC- or iPSC-derived cells comes of age.

MUC1 cytoplasmic tail: a potential therapeutic target for ovarian carcinoma

Ovarian cancer is often a lethal disease, since the occult progression of the tumor within the peritoneal cavity results in late diagnosis and treatment failure. The identification of molecular events specific to metastasis is critical for the development of effective therapies. MUC1 is aberrantly overexpressed by most ovarian cancer and regarded as a molecular target for ovarian cancer. This review focuses on the latest advances regarding a signaling region in the MUC1 C-terminal subunit-mediated c-Src signaling pathways in malignant transformation, invasion and metastasis. Disruption of MUC1-C-terminal subunit-associated c-Src signaling by targeting the specific sites might represent a novel immunotherapeutic approach for the treatment of ovarian cancer.

Rapidly cleaving bovine two-cell embryos have better developmental potential and a distinctive mRNA pattern

Mammalian embryos that rapidly reach the two-cell stage in culture have a higher probability of becoming viable blastocysts. Our goal was to separate two-cell bovine embryos based on their zygotic cleavage timing, and to assess their global mRNA levels. Following in vitro fertilization, all embryos that cleaved by 29.5 hpi (early) were cultured separately from those that divided at 46 hpi (late). The blastocyst rates were 46.1 ± 3.7% and 6.1 ± 3.4% for early- and late-cleavers, respectively (P < 0.01). Seven replicates of selected two-cell embryos were collected at each time point for microarray characterization (n = 4) and quantitative reverse-transcriptase PCR (n = 3); the rest were left in culture for blastocyst evaluation. A total of 774 and 594 probes were preferentially present in early- and late-cleaving embryos, respectively (fold change ± 1.5, P < 0.05), with important contrasts related to cell cycle, gene expression, RNA processing, and protein degradation functions. A total of 12 transcripts were assessed by quantitative PCR, of which ATM, ATR, CTNNB1, MSH6, MRE11A, PCNA, APC, CENPE, and GRB2 were in agreement with the hybridization results. Since most of these molecules are directly or indirectly associated with cell-cycle regulation, DNA damage response, and transcription control, our results strongly suggest key roles for those biological functions in mammalian preimplantation development.

Functional analysis of Tcl1 using Tcl1-deficient mouse embryonic stem cells

Tcl1 is highly expressed in embryonic stem (ES) cells, but its expression rapidly decreases following differentiation. To assess Tcl1’s roles in ES cells, we generated Tcl1-deficient and -overexpressing mouse ES cell lines. We found that Tcl1 was neither essential nor sufficient for maintaining the undifferentiated state. Tcl1 is reported to activate Akt and to enhance cell proliferation. We found that Tcl1 expression levels correlated positively with the proliferation rate and negatively with the apoptosis of ES cells, but did not affect Akt phosphorylation. On the other hand, the phosphorylation level of β-catenin decreased in response to Tcl1 overexpression. We measured the β-catenin activity using the TOPflash reporter assay, and found that wild-type ES cells had low activity, which Tcl1 overexpression enhanced 1.8-fold. When the canonical Wnt signaling is activated by β-catenin stabilization, it reportedly helps maintain ES cells in the undifferentiated state. We then performed DNA microarray analyses between the Tcl1-deficient and -expressing ES cells. The results revealed that Tcl1 expression downregulated a distinct group of genes, including Ndp52, whose expression is very high in blastocysts but reduced in the primitive ectoderm. Based on these results, we discuss the possible roles of Tcl1 in ES cells.

Targeted mutations of genes reveal important roles in palatal development in mice

The process of palatal development is regulated by growth factors, extracellular matrix (ECM) protein, and cell adhesion molecules, of which disturbance may result in cleft palate. Knockout mice are important animal models for studying the role of genes during palatal development. Therefore, in this review, we will describe genes knockout in mice to reveal the biological mechanisms of these genes in the formation of the cleft palate.

Porcn-dependent Wnt signaling is not required prior to mouse gastrulation

In mice and humans the X-chromosomal porcupine homolog (Porcn) gene is required for the acylation and secretion of all 19 Wnt ligands and thus represents a bottleneck for all Wnt signaling. We have generated a mouse line carrying a floxed allele for Porcn and used zygotic, oocyte-specific and visceral endoderm-specific deletions to investigate embryonic and extra-embryonic requirements for Wnt ligand secretion. We show that there is no requirement for Porcn-dependent secretion of Wnt ligands during preimplantation development of the mouse embryo. Porcn-dependent Wnts are first required for the initiation of gastrulation, where Porcn function is required in the epiblast but not the visceral endoderm. Heterozygous female embryos, which are mutant in both trophoblast and visceral endoderm due to imprinted X chromosome inactivation, complete gastrulation but display chorio-allantoic fusion defects similar to Wnt7b mutants. Our studies highlight the importance of Wnt3 and Wnt7b for embryonic and placental development but suggest that endogenous Porcn-dependent Wnt secretion does not play an essential role in either implantation or blastocyst lineage specification.

Epiblast ground state is controlled by canonical Wnt/β-catenin signaling in the postimplantation mouse embryo and epiblast stem cells

Epiblast stem cells (EpiSCs) are primed pluripotent stem cells and can be derived from postimplantation mouse embryos. We now show that the absence of canonical Wnt/β-catenin signaling is essential for maintenance of the undifferentiated state in mouse EpiSCs and in the epiblast of mouse embryos. Attenuation of Wnt signaling with the small-molecule inhibitor XAV939 or deletion of the β-catenin gene blocked spontaneous differentiation of EpiSCs toward mesoderm and enhanced the expression of pluripotency factor genes, allowing propagation of EpiSCs as a homogeneous population. EpiSCs were efficiently established and propagated from single epiblast cells in the presence of both XAV939 and the Rho kinase (ROCK) inhibitor Y27632. Cell transplantation revealed that EpiSCs were able to contribute to primordial germ cells and descendants of all three germ layers in a host embryo, suggesting that they maintained pluripotency, even after prolonged culture with XAV939. Such an improvement in the homogeneity of pluripotency achieved with the use of a Wnt inhibitor should prove advantageous for manipulation of primed pluripotent stem cells.

Mouse Tenm4 is required for mesoderm induction

Background

Tenm4 is a mouse homolog of the Drosophila gene Tenascin-m (Ten-m (Odd oz)), which functions in motor neuron routing. Recently, a genome-wide association analysis for bipolar disorder identified a new susceptibility locus at TENM4 increasing the importance of understanding Tenm4. A series of Tenm4 mouse alleles showing a broad range of phenotypes were isolated after ENU mutagenesis. Here, we examine the timing and features of gastrulation failure in a loss of function allele.

Results

Embryonic mesoderm did not form in loss of function Tenm4^m1/m1 mutant embryos. Genes normally expressed in embryonic mesoderm were not expressed in the mutant, the primitive streak did not form, and markers of the anteroposterior axis were not expressed or were mislocalized. The lack of embryonic mesoderm could not be attributed to poor proliferation of the epiblast, as normal numbers of dividing cells were observed. Epiblast cells maintained expression of Pou5f1 suggesting that they remain pluripotent, but they did not have the capacity to form any germ layer derivatives in teratomas, showing that the inability to induce mesoderm is cell autonomous. Misexpression of E-cadherin and N-cadherin suggest that the embryos did not undergo an epithelial-to-mesenchymal transition. In addition, Wnt signaling did not occur in the mutants, as assessed by the TOPGAL reporter assay, while a GSK3β inhibitor partially rescued the mutant embryos, and rescued TOPGAL reporter expression.

Conclusions

These data demonstrate that Tenm4 mutants fail to form a primitive streak and to induce embryonic mesoderm. Markers of anterior posterior patterning fail to be expressed or are mislocalized. Further, Tenm4 mutants lack the ability to differentiate in a cell autonomous manner. Together, our data suggest that embryos become impaired prior to E6.5 and as a result, Wnt signaling fails to occur; however, the involvement of other signaling pathways remains to be examined.

Dicer deficiency reveals microRNAs predicted to control gene expression in the developing adrenal cortex

MicroRNAs (miRNAs) are small, endogenous, non-protein-coding RNAs that are an important means of posttranscriptional gene regulation. Deletion of Dicer, a key miRNA processing enzyme, is embryonic lethal in mice, and tissue-specific Dicer deletion results in developmental defects. Using a conditional knockout model, we generated mice lacking Dicer in the adrenal cortex. These Dicer-knockout (KO) mice exhibited perinatal mortality and failure of the adrenal cortex during late gestation between embryonic day 16.5 (E16.5) and E18.5. Further study of Dicer-KO adrenals demonstrated a significant loss of steroidogenic factor 1-expressing cortical cells that was histologically evident as early as E16.5 coincident with an increase in p21 and cleaved-caspase 3 staining in the cortex. However, peripheral cortical proliferation persisted in KO adrenals as assessed by staining of proliferating cell nuclear antigen. To further characterize the embryonic adrenals from Dicer-KO mice, we performed microarray analyses for both gene and miRNA expression on purified RNA isolated from control and KO adrenals of E15.5 and E16.5 embryos. Consistent with the absence of Dicer and the associated loss of miRNA-mediated mRNA degradation, we observed an up-regulation of a small subset of adrenal transcripts in Dicer-KO mice, most notably the transcripts coded by the genes Nr6a1 and Acvr1c. Indeed, several miRNAs, including let-7, miR-34c, and miR-21, that are predicted to target these genes for degradation, were also markedly down-regulated in Dicer-KO adrenals. Together these data suggest a role for miRNA-mediated regulation of a subset of genes that are essential for normal adrenal growth and homeostasis.

Tcf7l1 prepares epiblast cells in the gastrulating mouse embryo for lineage specification

The core gene regulatory network (GRN) in embryonic stem cells (ESCs) integrates activities of the pro-self-renewal factors Oct4 (Pou5f1), Sox2 and Nanog with that of an inhibitor of self-renewal, Tcf7l1 (Tcf3). The inhibitor function of Tcf7l1 causes dependence on extracellular Wnt/β-catenin signaling activity, making its embryonic role within the ESC GRN unclear. By analyzing intact mouse embryos, we demonstrate that the function of Tcf7l1 is necessary for specification of cell lineages to occur concomitantly with the elaboration of a three-dimensional body plan during gastrulation. In Tcf7l1(-/-) embryos, specification of mesoderm is delayed, effectively uncoupling it from the induction of the primitive streak. Tcf7l1 repressor activity is necessary for a rapid switch in the response of pluripotent cells to Wnt/β-catenin stimulation, from one of self-renewal to a mesoderm specification response. These results identify Tcf7l1 as a unique factor that is necessary in pluripotent cells to prepare them for lineage specification. We suggest that the role of Tcf7l1 in mammals is to inhibit the GRN to ensure the coordination of lineage specification with the dynamic cellular events occurring during gastrulation.