Human Hand1 basic helix-loop-helix (bHLH) protein: extra-embryonic expression pattern, interaction partners and identification of its transcriptional repressor domains

Homodimeric and Heterodimeric Interactions among Vertebrate Basic Helix-Loop-Helix Transcription Factors

The basic helix–loop–helix transcription factor (bHLH TF) family is involved in tissue development, cell differentiation, and disease. These factors have transcriptionally positive, negative, and inactive functions by combining dimeric interactions among family members. The best known bHLH TFs are the E-protein homodimers and heterodimers with the tissue-specific TFs or ID proteins. These cooperative and dynamic interactions result in a complex transcriptional network that helps define the cell’s fate. Here, the reported dimeric interactions of 67 vertebrate bHLH TFs with other family members are summarized in tables, including specifications of the experimental techniques that defined the dimers. The compilation of these extensive data underscores homodimers of tissue-specific bHLH TFs as a central part of the bHLH regulatory network, with relevant positive and negative transcriptional regulatory roles. Furthermore, some sequence-specific TFs can also form transcriptionally inactive heterodimers with each other. The function, classification, and developmental role for all vertebrate bHLH TFs in four major classes are detailed.

Conditional Mutation of Hand1 in the Mouse Placenta Disrupts Placental Vascular Development Resulting in Fetal Loss in Both Early and Late Pregnancy

Congenital heart defects (CHD) affect approximately 1% of all live births, and often require complex surgeries at birth. We have previously demonstrated abnormal placental vascularization in human placentas from fetuses diagnosed with CHD. Hand1 has roles in both heart and placental development and is implicated in CHD development. We utilized two conditionally activated Hand1^A126fs/+ murine mutant models to investigate the importance of cell-specific Hand1 on placental development in early (Nkx2-5^Cre) and late (Cdh5^Cre) pregnancy. Embryonic lethality occurred in Nkx2-5^Cre/Hand1^A126fs/+ embryos with marked fetal demise occurring after E10.5 due to a failure in placental labyrinth formation and therefore the inability to switch to hemotrophic nutrition or maintain sufficient oxygen transfer to the fetus. Labyrinthine vessels failed to develop appropriately and vessel density was significantly lower by day E12.5. In late pregnancy, the occurrence of Cdh5^Cre+;Hand1^A126fs/+ fetuses was reduced from 29% at E12.5 to 20% at E18.5 and remaining fetuses exhibited reduced fetal and placental weights, labyrinth vessel density and placenta angiogenic factor mRNA expression. Our results demonstrate for the first time the necessity of Hand1 in both establishment and remodeling of the exchange area beyond early pregnancy and in patterning vascularization of the placental labyrinth crucial for maintaining pregnancy and successful fetal growth.

Mechanisms of Binding Specificity among bHLH Transcription Factors

The transcriptome of every cell is orchestrated by the complex network of interaction between transcription factors (TFs) and their binding sites on DNA. Disruption of this network can result in many forms of organism malfunction but also can be the substrate of positive natural selection. However, understanding the specific determinants of each of these individual TF-DNA interactions is a challenging task as it requires integrating the multiple possible mechanisms by which a given TF ends up interacting with a specific genomic region. These mechanisms include DNA motif preferences, which can be determined by nucleotide sequence but also by DNA’s shape; post-translational modifications of the TF, such as phosphorylation; and dimerization partners and co-factors, which can mediate multiple forms of direct or indirect cooperative binding. Binding can also be affected by epigenetic modifications of putative target regions, including DNA methylation and nucleosome occupancy. In this review, we describe how all these mechanisms have a role and crosstalk in one specific family of TFs, the basic helix-loop-helix (bHLH), with a very conserved DNA binding domain and a similar DNA preferred motif, the E-box. Here, we compile and discuss a rich catalog of strategies used by bHLH to acquire TF-specific genome-wide landscapes of binding sites.

Mechanisms of early placental development in mouse and humans

The importance of the placenta in supporting mammalian development has long been recognized, but our knowledge of the molecular, genetic and epigenetic requirements that underpin normal placentation has remained remarkably under-appreciated. Both the in vivo mouse model and in vitro-derived murine trophoblast stem cells have been invaluable research tools for gaining insights into these aspects of placental development and function, with recent studies starting to reshape our view of how a unique epigenetic environment contributes to trophoblast differentiation and placenta formation. These advances, together with recent successes in deriving human trophoblast stem cells, open up new and exciting prospects in basic and clinical settings that will help deepen our understanding of placental development and associated disorders of pregnancy.

Human placenta and trophoblast development: key molecular mechanisms and model systems

Abnormal placentation is considered as an underlying cause of various pregnancy complications such as miscarriage, preeclampsia and intrauterine growth restriction, the latter increasing the risk for the development of severe disorders in later life such as cardiovascular disease and type 2 diabetes. Despite their importance, the molecular mechanisms governing human placental formation and trophoblast cell lineage specification and differentiation have been poorly unravelled, mostly due to the lack of appropriate cellular model systems. However, over the past few years major progress has been made by establishing self-renewing human trophoblast stem cells and 3-dimensional organoids from human blastocysts and early placental tissues opening the path for detailed molecular investigations. Herein, we summarize the present knowledge about human placental development, its stem cells, progenitors and differentiated cell types in the trophoblast epithelium and the villous core. Anatomy of the early placenta, current model systems, and critical key regulatory factors and signalling cascades governing placentation will be elucidated. In this context, we will discuss the role of the developmental pathways Wingless and Notch, controlling trophoblast stemness/differentiation and formation of invasive trophoblast progenitors, respectively.

Comparison of syncytiotrophoblast generated from human embryonic stem cells and from term placentas

Human embryonic stem cells (ESCs) readily commit to the trophoblast lineage after exposure to bone morphogenetic protein-4 (BMP-4) and two small compounds, an activin A signaling inhibitor and a FGF2 signaling inhibitor (BMP4/A83-01/PD173074; BAP treatment). During differentiation, areas emerge within the colonies with the biochemical and morphological features of syncytiotrophoblast (STB). Relatively pure fractions of mononucleated cytotrophoblast (CTB) and larger syncytial sheets displaying the expected markers of STB can be obtained by differential filtration of dispersed colonies through nylon strainers. RNA-seq analysis of these fractions has allowed them to be compared with cytotrophoblasts isolated from term placentas before and after such cells had formed syncytia. Although it is clear from extensive gene marker analysis that both ESC- and placenta-derived syncytial cells are trophoblast, each with the potential to transport a wide range of solutes and synthesize placental hormones, their transcriptome profiles are sufficiently dissimilar to suggest that the two cell types have distinct pedigrees and represent functionally different kinds of STB. We propose that the STB generated from human ESCs represents the primitive syncytium encountered in early pregnancy soon after the human trophoblast invades into the uterine wall.

Deciphering the molecular mechanisms underlying the binding of the TWIST1/E12 complex to regulatory E-box sequences

The TWIST1 bHLH transcription factor controls embryonic development and cancer processes. Although molecular and genetic analyses have provided a wealth of data on the role of bHLH transcription factors, very little is known on the molecular mechanisms underlying their binding affinity to the E-box sequence of the promoter. Here, we used an in silico model of the TWIST1/E12 (TE) heterocomplex and performed molecular dynamics (MD) simulations of its binding to specific (TE-box) and modified E-box sequences. We focused on (i) active E-box and inactive E-box sequences, on (ii) modified active E-box sequences, as well as on (iii) two box sequences with modified adjacent bases the AT- and TA-boxes. Our in silico models were supported by functional in vitro binding assays. This exploration highlighted the predominant role of protein side-chain residues, close to the heart of the complex, at anchoring the dimer to DNA sequences, and unveiled a shift towards adjacent ((-1) and (-1*)) bases and conserved bases of modified E-box sequences. In conclusion, our study provides proof of the predictive value of these MD simulations, which may contribute to the characterization of specific inhibitors by docking approaches, and their use in pharmacological therapies by blocking the tumoral TWIST1/E12 function in cancers.

Intrauterine trophoblast migration: A comparative view of humans and rodents

Trophoblast migration and invasion through the decidua and maternal uterine spiral arteries are crucial events in placentation. During this process, invasive trophoblast replace vascular endothelial cells as the uterine arteries are remodeled to form more permissive vessels that facilitate adequate blood flow to the growing fetus. Placentation failures resulting from either extensive or shallow trophoblastic invasion can cause pregnancy complications such as preeclampsia, intrauterine growth restriction, placenta creta, gestational trophoblastic disease and even maternal or fetal death. Consequently, the use of experimental animal models such as rats and mice has led to great progress in recent years with regards to the identification of mechanisms and factors that control trophoblast migration kinetics. This review aims to perform a comparative analysis of placentation and the mechanisms and factors that coordinate intrauterine trophoblast migration in humans, rats and mice under physiological and pathological conditions.

Signaling pathways in mouse and human trophoblast differentiation: a comparative review

The mouse is often used as a model for understanding human placentation and offers multiple advantages, including the ability to manipulate gene expression in specific compartments and to derive trophoblast stem cells, which can be maintained or differentiated in vitro. Nevertheless, there are numerous differences between the mouse and human placentas, only the least of which are structural. This review aims to compare mouse and human placentation, with a focus on signaling pathways involved in trophoblast lineage-specific differentiation.

Homeobox genes and down-stream transcription factor PPARγ in normal and pathological human placental development

The placenta provides critical transport functions between the maternal and fetal circulations during intrauterine development. Formation of this interface is controlled by nuclear transcription factors including homeobox genes. Here we summarize current knowledge regarding the expression and function of homeobox genes in the placenta. We also describe the identification of target transcription factors including PPARγ, biological pathways regulated by homeobox genes and their role in placental development. The role of the nuclear receptor PPARγ, ligands and target genes in human placental development is also discussed. A better understanding of these pathways will improve our knowledge of placental cell biology and has the potential to reveal new molecular targets for the early detection and diagnosis of pregnancy complications including human fetal growth restriction.

Transcriptional responses of cultured rat sympathetic neurons during BMP-7-induced dendritic growth

BACKGROUND

Dendrites are the primary site of synapse formation in the vertebrate nervous system; however, relatively little is known about the molecular mechanisms that regulate the initial formation of primary dendrites. Embryonic rat sympathetic neurons cultured under defined conditions extend a single functional axon, but fail to form dendrites. Addition of bone morphogenetic proteins (BMPs) triggers these neurons to extend multiple dendrites without altering axonal growth or cell survival. We used this culture system to examine differential gene expression patterns in naïve vs. BMP-treated sympathetic neurons in order to identify candidate genes involved in regulation of primary dendritogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

To determine the critical transcriptional window during BMP-induced dendritic growth, morphometric analysis of microtubule-associated protein (MAP-2)-immunopositive processes was used to quantify dendritic growth in cultures exposed to the transcription inhibitor actinomycin-D added at varying times after addition of BMP-7. BMP-7-induced dendritic growth was blocked when transcription was inhibited within the first 24 hr after adding exogenous BMP-7. Thus, total RNA was isolated from sympathetic neurons exposed to three different experimental conditions: (1) no BMP-7 treatment; (2) treatment with BMP-7 for 6 hr; and (3) treatment with BMP-7 for 24 hr. Affymetrix oligonucleotide microarrays were used to identify differential gene expression under these three culture conditions. BMP-7 significantly regulated 56 unique genes at 6 hr and 185 unique genes at 24 hr. Bioinformatic analyses implicate both established and novel genes and signaling pathways in primary dendritogenesis.

CONCLUSIONS/SIGNIFICANCE

This study provides a unique dataset that will be useful in generating testable hypotheses regarding transcriptional control of the initial stages of dendritic growth. Since BMPs selectively promote dendritic growth in central neurons as well, these findings may be generally applicable to dendritic growth in other neuronal cell types.

Phosphorylation of the Twist1-family basic helix-loop-helix transcription factors is involved in pathological cardiac remodeling

Background

The Twist1-family basic helix-loop-helix (bHLH) transcription factors including Twist1, Hand1 and Hand2, play an essential role in heart development and are implicated in pathological heart remodeling. Previously, it was reported that these bHLH transcription factors can be regulated by phosphorylation within the basic-helix I domain, which is involved in developmental processes such as limb formation and trophoblast differentiation. However, how phosphorylation of Twist1 family functions in post-natal heart is elusive.

Principal Findings

Here, we generated transgenic mice with over-expression of Hand1 and Twist1 mutants (to mimic or to abolish phosphorylation) in cardiomyocytes and found pathological cardiac remodeling leading to heart failure and sudden death. Gene expression profile analysis revealed up-regulation of growth-promoting genes and down-regulation of metabolic genes. It is well known that aberrant activation of Akt signaling causes pathological cardiac remodeling and results in heart failure. The basic-helix I domain of Twist1 family members contain Akt substrate consensus motif and may be downstream targets of Akt signaling. Using biochemical analysis, we demonstrated that Hand1 and Twist1 were phosphorylated by Akt in the basic-helix I domain. Phosphorylation of Hand1 regulated its transcriptional activation of luciferase reporter genes and DNA binding ability.

Conclusions

This study provides novel insights into the regulation of Twist1 family in cardiac remodeling and suggests that the Twist1 family can be regulated by Akt signaling.

A bHLH code for cardiac morphogenesis

Cell specification and differentiation of cardiomyocytes from mesodermal precursors is orchestrated by epigenetic and transcriptional inputs throughout heart formation. Of the many transcription factor super families that play a role in this process, the basic Helix-loop Helix (bHLH) family of proteins is well represented. The bHLH protein by design allows for dimerization-both as homodimers and heterodimers with other proteins within the family. Although DNA binding is mediated via a short variable cis-element termed an E-box, it is clear that DNA-affinity for these elements as well as the transcriptional input conveyed is dictated largely by the transcriptional partners within the dimer complex. Dimer partner choice has a number of inputs requiring co-expression within a given cell nucleus and dimerization modulation by the level of protein present, and post-translational modifications that can both enhance or reduce protein-protein interactions. Due to these complex interrelationships, it has been difficult to identity bona-fide downstream transcriptional targets and define the molecular pathways regulated of bHLH factors within cardiogenesis, despite the clear roles suggested via loss-of-function animals models. This review focuses on the Hand bHLH proteins-key members of the Twist-family of bHLH factors. Despite over a decade of investigation, questions regarding functional redundancy, downstream targets, and biological role during heart specification and differentiation have still not been fully addressed. Our goal is to review what is currently known and address strategies for gaining further understanding of Hand/Twist gene dosage and functional redundancy relationships within the developing heart that may underlie congenital heart defect pathogenesis.

Critical growth factors and signalling pathways controlling human trophoblast invasion

Invasion of placental trophoblasts into uterine tissue and vessels is an essential process of human pregnancy and fetal development. Due to their remarkable plasticity invasive trophoblasts fulfil numerous functions, i.e. anchorage of the placenta, secretion of hormones, modulation of decidual angiogenesis/lymphangiogenesis and remodelling of maternal spiral arteries. The latter is required to increase blood flow to the placenta, thereby ensuring appropriate transfer of nutrients and oxygen to the developing fetus. Since failures in vascular changes of the placental bed are associated with pregnancy diseases such as preeclampsia or intrauterine growth restriction, basic research in this particular field focuses on molecular mechanisms controlling trophoblast invasion under physiological and pathological conditions. Throughout the years, an increasing number of growth factors, cytokines and angiogenic molecules controlling trophoblast motility have been identified. These factors are secreted from numerous cells such as trophoblast, maternal epithelial and stromal cells, as well as uterine NK cells and macrophages, suggesting that a complex network of cell types, mediators and signalling pathways regulates trophoblast invasiveness. Whereas essential features of the invasive trophoblast such as expression of critical proteases and adhesion molecules have been well characterised, the interplay between different cell types and growth factors and the cross-talk between distinct signalling cascades remain largely elusive. Similarly, key-regulatory transcription factors committing and differentiating invasive trophoblasts are mostly unknown. This review will summarise our current understanding of growth factors and signal transduction pathways regulating human trophoblast invasion/migration, as well as give insights into novel mechanisms involved in the particular differentiation process.

Expression of genes involved in early cell fate decisions in human embryos and their regulation by growth factors

Little is understood about the regulation of gene expression in human preimplantation embryos. We set out to examine the expression in human preimplantation embryos of a number of genes known to be critical for early development of the murine embryo. The expression profile of these genes was analysed throughout preimplantation development and in response to growth factor (GF) stimulation. Developmental expression of a number of genes was similar to that seen in murine embryos (OCT3B/4,CDX2,NANOG). However,GATA6is expressed throughout preimplantation development in the human. Embryos were cultured in IGF-I, leukaemia inhibitory factor (LIF) or heparin-binding EGF-like growth factor (HBEGF), all of which are known to stimulate the development of human embryos. Our data show that culture in HBEGF and LIF appears to facilitate human embryo expression of a number of genes:ERBB4(LIF) andLIFRandDSC2(HBEGF) while in the presence of HBEGF no blastocysts expressedEOMESand when cultured with LIF only two out of nine blastocysts expressedTBN. These data improve our knowledge of the similarities between human and murine embryos and the influence of GFs on human embryo gene expression. Results from this study will improve the understanding of cell fate decisions in early human embryos, which has important implications for both IVF treatment and the derivation of human embryonic stem cells.

Regulation of trophoblast invasion - a workshop report

Trophoblast invasion during placental development helps to establish efficient physiological exchange between maternal and fetal circulatory systems. Trophoblast stem cells differentiate into multiple subtypes, including some that are highly invasive. Signalling to the trophoblast from decidua, uterine natural killer cells and vascular smooth muscle can regulate extravillous trophoblast differentiation. Important questions remain about how these cellular interactions promote trophoblast invasion and the signalling pathways that are involved. New and established biological models are being used to experimentally examine these interactions and the underlying molecular mechanisms.

Mutations within helix I of Twist1 result in distinct limb defects and variation of DNA binding affinities

Twist1 is a basic helix-loop-helix (bHLH) factor that plays an important role in limb development. Haploinsufficiency of Twist1 results in polydactyly via the inability of Twist1 to antagonistically regulate the related factor Hand2. The mechanism modulating Twist1-Hand2 antagonism is via phosphoregulation of conserved threonine and serine residues in helix I of the bHLH domain. Phosphoregulation alters the dimerization affinities for both proteins. Here we show that the expression of Twist1 and Twist1 phosphoregulation mutants results in distinct limb phenotypes in mice. In addition to dimer regulation, Twist1 phosphoregulation affects the DNA binding affinities of Twist1 in a partner-dependent and cis-element-dependent manner. In order to gain a better understanding of the specific Twist1 transcriptional complexes that function during limb morphogensis, we employ a series of Twist1-tethered dimers that include the known Twist1 partners, E12 and Hand2, as well as a tethered Twist1 homodimer. We show that these dimers behave in a manner similar to monomerically expressed bHLH factors and result in distinct limb phenotypes that correlate well with those observed from the limb expression of Twist1 and Twist1 phosphoregulation mutants. Taken together, this study shows that the Twist1 dimer affinity for a given partner can modulate the DNA binding affinity and that Twist1 dimer choice determines phenotypic outcome during limb development.

Activation of the canonical wingless/T-cell factor signaling pathway promotes invasive differentiation of human trophoblast

The molecular mechanisms governing invasive differentiation of human trophoblasts remain largely elusive. Here, we investigated the role of Wnt-beta-catenin-T-cell factor (TCF) signaling in this process. Reverse transcriptase-polymerase chain reaction and Western blot analyses demonstrated expression of Wnt ligands, frizzled receptors, LRP-6, and TCF-3/4 transcription factors in total placenta and different trophoblast cell models. Immunohistochemistry of placental tissues and differentiating villous explant cultures showed that expression of TCF-3/4 strongly increased in invading trophoblasts. Some of these cells also accumulated dephosphorylated beta-catenin in the nucleus. Wnt3A treatment of primary cytotrophoblasts and SGHPL-5 cells induced activity of TCF-luciferase reporters. Accordingly, the ligand provoked interaction of TCF-3/4 with beta-catenin as assessed in electrophoretic mobility shift assays (EMSAs) and up-regulation of Wnt/TCF target genes as observed by Western blot analyses. Wnt3A stimulated trophoblast migration and invasion through Matrigel, which could be blocked by addition of Dickkopf-1, mediating in-hibition of canonical Wnt signaling. Dickkopf-1 also reduced basal migration, invasion, and proliferation of cytotrophoblasts, suggesting expression of endogenous Wnt ligand(s). Immunohistochemistry revealed that the percentage of extravillous trophoblasts containing nuclear beta-catenin was significantly higher in placentas of complete hydatidiform mole pregnancies as compared to normal placentas. Thus, canonical Wnt signaling may promote invasive trophoblast differentiation, and exaggerated activation of the path-way could contribute to trophoblastic hyperplasia and local invasion.

How to make a placenta: Mechanisms of trophoblast cell differentiation in mice – A Review

The word placenta is derived from the Latin term meaning 'flat cake'. Despite the rather humble name, the placenta is an amazing organ that forms both the interface for selective delivery of nutrients from the mother to the fetus and also re-directs maternal metabolic, endocrine, cardiovascular and immune functions to promote fetal survival and growth. These two functions are fulfilled by different specialized trophoblast cell subtypes, and my laboratory has been studying how their formation and functions are regulated during placental development. Through molecular studies in cultured cells and tissues, genetic studies in mice, and comparative analysis of placentas from humans, rodents and farm animals, it is now possible to describe molecular pathways that control the development of all major trophoblast cell subtypes and structures of the placenta. The work has revealed an intricate complexity of cell-cell interactions, environmental factors, and molecular networks that control normal development.

Molecular and morphologic analyses of expression of ESX1L in different stages of human placental development

The mRNA expression of the ESX1L gene was analyzed by RT-PCR and in situ hybridization in human normal cytogenetically placentas, of different gestational ages. Our RT-PCR analysis showed that ESX1L mRNA is expressed from 5 weeks of gestation until term, suggesting a role not only in trophoblast differentiation but also in the maintenance of the villi and microvasculature. We also observed, by in situ hybridization, that ESX1L mRNA is expressed by cytotrophoblast from chorionic plate, syncytiotrophoblast and stromal cells of all terminal, intermediate and stem villi of term placentas. ESX1L mRNA expression was more pronounced in trophoblast cells of terminal villi than in intermediate and stem villi. In conclusion, ESX1L is expressed during all stages of placental development and is localized to sparse areas of trophoblast in terminal villi in association with cytotrophoblastic cells.

Differential regulation of Hand1 homodimer and Hand1-E12 heterodimer activity by the cofactor FHL2

The basic helix-loop-helix (bHLH) factor Hand1 plays an essential role in cardiac morphogenesis, and yet its precise function remains unknown. Protein-protein interactions involving Hand1 provide a means of determining how Hand1-induced gene expression in the developing heart might be regulated. Hand1 is known to form either heterodimers with near-ubiquitous E-factors and other lineage-restricted class B bHLH proteins or homodimers with itself in vitro. To date, there have been no reported Hand1 protein interactions involving non-bHLH proteins. Heterodimer-versus-homodimer choice is mediated by the phosphorylation status of Hand1; however, little is known about the in vivo function of these dimers or, importantly, how they are regulated. In an effort to understand how Hand1 activity in the heart might be regulated postdimerization, we have investigated tertiary Hand1-protein interactions with non-bHLH factors. We describe a novel interaction of Hand1 with the LIM domain protein FHL2, a known transcriptional coactivator and corepressor expressed in the developing cardiovascular system. FHL2 interacts with Hand1 via the bHLH domain and is able to repress Hand1/E12 heterodimer-induced transcription but has no effect on Hand1/Hand1 homodimer activity. This effect of FHL2 is not mediated either at the level of dimerization or via an effect of Hand1/E12 DNA binding. In summary, our data describe a novel differential regulation of Hand1 heterodimers versus homodimers by association of the cofactor FHL2 and provide insight into the potential for a tertiary level of control of Hand1 activity in the developing heart.

Id2 is a primary partner for the E2-2 basic helix-loop-helix transcription factor in the human placenta

We screened a term placental cDNA library by the yeast two-hybrid approach with Id2, a negative regulator of basic helix-loop-helix (bHLH) factors. Of the clones obtained, approximately one-third were the E2-2 bHLH transcription factor. Id2 and E2-2 were shown to interact in direct two-hybrid assays in yeast cells, as well as immunoprecipitation assays in mammalian cells. Immunohistochemical analysis demonstrated co-localization of both Id2 and E2-2 in placental trophoblasts. Co-transfection of JEG-3 cells with E2-2 and Id2, and a luciferase reporter construct under the control of the human chorionic gonadotropin alpha-subunit promoter revealed that E2-2 had a negative effect on CGalpha-subunit transcription, which could be relieved by overexpression of Id2. The library was in turn rescreened with E2-2, and Id2 and Id1 were essentially the only clones obtained. We conclude that Id2 is a primary binding partner for the bHLH transcription factor E2-2 in the human placenta.

Genetic and genomic approaches to study placental development

Recent technological advances in genetic manipulation and expression profiling offer excellent opportunities to elucidate the molecular mechanisms controlling developmental processes during embryogenesis. Thus, this revolution also strongly benefits studies of the molecular genetics of placental development. Here we review the findings of several expression profiling analyses in extraembryonic tissues and assess how this work can contribute to the identification of essential components governing placental development. We further discuss the relevance of these components in the context of genetic manipulation experiments. In conclusion, the intelligent combination of genetic and genomic approaches will substantially accelerate the progress in identifying the key molecular pathways of placental development.

Trophoblast gene expression: transcription factors in the specification of early trophoblast

Azone of trophoblast specification is established when the embryo is a morula, presumably reflecting a unique combination of transcription factors in that zone of cells and the influence of various environmental cues and growth factors on them. A key first step in this process of specification is the down-regulation of Oct4, a transcription factor that acts as a negative regulator of trophoblast specification and of genes normally up-regulated as the trophectoderm first forms. The transcription factors believed to have a positive association with trophectoderm specification have been inferred primarily in two ways: by their expression patterns in embryos, ES cells and TS cells and by the consequences of gene disruption on embryonic development. Many of these transcription factors also control the expression of genes characteristically expressed in trophoblast but not in the epiblast, primitive endoderm and their derivatives. ES and TS cells from the mouse and other species are beginning to provide insights into the changes in gene expression that accompany lineage specification and the subsequent post-specification events that lead to functional trophoblast derivatives.

Complex Patterns of GCM1 mRNA and Protein in Villous and Extravillous Trophoblast Cells of the Human Placenta

The Gcm1 gene encodes a transcription factor that is essential for both syncytiotrophoblast differentiation and formation of chorionic villi in mice. Its early expression is very unusual in that it defines a subset of trophoblast cells in the chorion, a layer that otherwise contains trophoblast stem cells. While Gcm1 mRNA expression initiates independently within the chorion, the subsequent maintenance of mRNA expression as well as the onset of protein accumulation is dependent on contact with allantoic mesoderm. Previous studies have shown that human GCM1 mRNA and protein are detectable in the placenta, but their patterns have not been compared nor precisely localized. We, therefore, conducted the present study to determine if the human mRNA and protein are subject to the same complexities of regulation as the mouse. In situ hybridization studies showed that the GCM1 mRNA was expressed in villous cytotrophoblast cells, but only a subset and never within cells immediately at the base of columns. Interestingly, the mRNA was detected throughout the cytotrophoblast columns. GCM1 protein expression studies demonstrated that the transcription factor was present mainly within the nuclei of a subset of cytotrophoblast cells, consistent with its role as a transcription factor. Feint cytoplasmic staining of the transcription factor was found in the syncytiotrophoblast but not in aggregated syncytial nuclei. Nuclear immuno-reactivity for the GCM1 protein was detected in occasional nuclei in the distal part of the column. Therefore, GCM1 expression is regulated both at the transcriptional and translational level. Overall, these studies show that the general features of GCM1 mRNA and protein expression in the human placenta are conserved with the mouse. They also highlight the fact that villous cytotrophoblast cells are extremely heterogeneous with respect to GCM1 expression, a factor that should be considered when using isolated cytotrophoblast cells for culture studies.

Expression of connexins in human preimplantation embryos in vitro

Intercellular communication via gap junctions is required to coordinate developmental processes in the mammalian embryo. We have investigated if the connexin (Cx) isoforms known to form gap junctions in rodent preimplantation embryos are also expressed in human embryos, with the aim of identifying species differences in communication patterns in early development. Using a combination of polyA PCR and immunocytochemistry we have assessed the expression of Cx26, Cx31, Cx32, Cx40, Cx43 and Cx45 which are thought to be important in early rodent embryos. The results demonstrate that Cx31 and Cx43 are the main connexin isoforms expressed in human preimplantation embryos and that these isoforms are co-expressed in the blastocyst. Cx45 protein is expressed in the blastocyst but the protein may be translated from a generally low level of transcripts: which could only be detected in the PN to 4-cell embryos. Interestingly, Cx40, which is expressed by the extravillous trophoblast in the early human placenta, was not found to be expressed in the blastocyst trophectoderm from which this tissue develops. All of the connexin isoforms in human preimplantation embryos are also found in rodents pointing to a common regulation of these connexins in development of rodent and human early embryos and perhaps other species.

NEUROD1 acts in vitro as an upstream regulator of NEUROD2 in trophoblast cells

The basic helix-loop-helix (bHLH) transcription factors NEUROD1, NEUROD2 and ATH2 are expressed during first trimester human placental development. We determined the transactivation potential of each of these factors in trophoblasts by measuring changes in the endogenous gene activity using absolute quantification by real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) after transient transfection. In these assays, NEUROD1 was found to transiently transactivate NEUROD2 in trophoblast cells. Promotor truncation assays, using luciferase constructs, showed the presence of two domains in the NEUROD2 promotor, which showed increased activity after NeuroD1 transfection. Each of these NeuroD1-responsive domains contains an E-box sequence. The NEUROD2 transactivation data fit with the spatial expression pattern of NEUROD1 and NEUROD2, since they are expressed in endovascular trophoblasts. This expression pattern, as well as the present transactivation results, might suggest the presence of a NEUROD differentiation cascade during first trimester human placental development.

A HANDful of questions: the molecular biology of the heart and neural crest derivatives (HAND)-subclass of basic helix-loop-helix transcription factors

The HAND subclass of basic Helix-loop-helix factors is comprised of two members HAND1 and HAND2. HAND genes are present within the genomes of organisms ranging from flies to man. Experiments employing chick embryology, tissue culture, and gene targeting in mice show that HAND function is critical for the specification and/or differentiation of extraembryonic structures that include the yolk sac, placenta, and the cells of the trophoblast lineages. HAND factors also play key roles in cardiac, gut, sympathetic neuronal development and in the proper development of tissues populated by HAND-expressing neural crest cells, including regions of the developing vasculature, the limbs, the jaw, and teeth. Surprisingly, nearly 10 years after their initial identification and characterization, little is understood about the nature of the downstream target genes which HAND1 and HAND2 regulate, whether the nature of their transcriptional regulation is positive or negative, or if they modulate genetic programs common to these diverse tissue types or if they drive unique subsets of genes that contribute to tissue identity. At the core of these questions is by which mechanisms do HAND factors modulate biological activity? Do they behave like classical class B bHLH factors or is their function more complex requiring a rethinking of the dogma? What follows is a review of what is currently known about HAND factors and a reflection on why elucidating their role in the biological programs within which they participate has been so difficult.

Functional analysis of the endogenous retroviral promoter of the human endothelin B receptor gene

We previously reported that the long terminal repeats (LTRs) of retroviral elements belonging to the HERV-E family contribute to the expression of the human apolipoprotein C1 (APOC1) and endothelin B receptor (EDNRB) genes by providing alternative promoters. While both LTRs were shown to promote transcription in vivo and in vitro, their respective activity and tissue specificity appeared to differ even though they shared a high degree of sequence identity. In the present study, we further characterized the promoter of the EDNRB LTR and delineated the regions and motifs required for strong activity. We confirmed the placenta-restricted expression of the LTR by transient transfections and quantitative real-time PCR and determined that the retroviral promoter contributes significantly to the level of EDNRB transcripts in placenta, where chimeric mRNAs were found to represent 15% of overall EDNRB mRNAs. Transient transfection of 5' deletion constructs in cells of placental origin identified a motif, named LPE1, between positions 111 and 122 of the EDNRB LTR necessary for transcriptional activity. Removal of this region, which contains a putative SP1 binding site, abolished promoter activity. A second enhancing region resides between positions 175 and 215 of the LTR and was termed LPE2. Interestingly, this section contained three binding sites that were not present in the APOC1 LTR due to minor nucleotide differences. The predicted motifs in the EDNRB LTR were found to likely act in symbiosis as modifications to any of the three sites reduced transcription by one-third while alterations to all three eliminated promoter activity. The results from this study illustrate how slight variations in transcriptional regulatory sequences can have a profound effect on promoter activity and demonstrate the complex regulatory effects of human endogenous retrovirus elements on human gene expression.

Genes, development and evolution of the placenta

Through studies of transgenic and mutant mice, it is possible to describe molecular pathways that control the development of all major trophoblast cell subtypes and structures of the placenta. For example, the proliferation of trophoblast stem cells is dependent on FGF signalling and downstream transcription factors Cdx2, Eomes and Err2. Several bHLH transcription factors regulate the progression from trophoblast stem cells to spongiotrophoblast and to trophoblast giant cells (Id1/2, Mash2, Hand1, Stra13). Intercellular actions critical for maintaining stable precursor cell populations are dependent on the gap junction protein Cx31 and the growth factor Nodal. Differentiation towards syncytiotrophoblast as well as the initiation of chorioallantoic (villous) morphogenesis is regulated by the Gcm1 transcription factor, and subsequent labyrinth development is dependent on Wnt, HGF and FGF signalling. These insights suggest that most of the genes that evolved to regulate placental development are either identical to ones used in other organ systems (e.g., FGF and epithelial branching morphogenesis), were co-opted to take on new functions (e.g., AP-2gamma, Dlx3, Hand1), or arose via gene duplication to take on a specialized placental function (e.g., Gcm1, Mash2). Many of the human orthologues of these critical genes show restricted expression patterns that are consistent with a conserved function. Such information is aiding the comparison of the human and mouse placenta. In addition, the prospect of a conserved function clearly suggests potential mechanisms for explaining complications of human placental development.

Expression of the human Hand1 gene in trophoblastic cells is transcriptionally regulated by activating and repressing specificity protein (Sp)-elements

The tissue-specific basic helix-loop-helix protein Hand1 is essential for the formation of trophoblast giant cells of the murine placenta. In humans, Hand1 is detectable in trophoblastic tumour cells suggesting an equivalent role in trophoblast differentiation. To understand its mode of expression we have cloned and characterized the human Hand1 gene promoter. Primer extension analyses suggest that transcription initiates 19 nucleotides downstream of the TATA element of the proximal 5' flanking region. Expression of luciferase reporter constructs harboring deletions of the 9.5 kb Hand1 5' flanking sequence defines a promoter region within 274 bp upstream of the transcriptional start site. Compared to a reporter bearing only the TATA box, the proximal promoter activates transcription up to 30-fold. However, transcriptional activity of the region was observed in both Hand1-expressing and non-expressing cell lines. Sequencing, DNAseI footprint analyses and electrophoretic mobility shift assays reveal the presence of four GC-rich sequences, which show different affinities to the endogenous specificity proteins (Sp), and a CCAAT box. In vitro, the Sp-elements mainly interact with Sp1 and Sp3 while the CCAAT element is recognized by the alpha CAAT binding factor protein. Mutant luciferase reporters bearing single active or inactive recognition sites demonstrate that two of the four Sp-binding sites (I and IV) contribute little to the overall transcription rate. The two other Sp-cognate sequences, II and III, downregulate and activate reporter expression 2.3- and 2.6-fold, respectively. Co-transfections of Sp1/Sp3 expression vectors and mutated reporter constructs in Sp-deficient SL2 cells indicate that the Sp-binding site II and III indeed function as repressing and activating enhancer sequences. In summary, the data suggest that constitutive expression of the Hand1 gene in cultured cells is regulated by a complex interplay of Sp-proteins interacting with activator and repressor elements.