MesP1 and MesP2 are essential for the development of cardiac mesoderm

Sox17 is essential for the specification of cardiac mesoderm in embryonic stem cells

Early steps for cardiac specification are problematic for the study of mammalian embryos, which has favored using pluripotent cells that recapitulate cardiac myogenesis. Furthermore, circuits governing cardiac specification have relevance to the application of ES cells and other cells for heart repair. In mouse teratocarcinoma cells, canonical Wnts that inhibit heart formation in avian or amphibian embryos and explants activate cardiogenesis, paradoxically. Here, we show that the Wnt/beta-catenin pathway also is essential for cardiac myogenesis to occur in ES cells, acting at a gastrulation-like stage, mediating mesoderm formation and patterning (two prerequisites for cardiac myogenesis itself). Among genes associated temporally with this step was Sox17, encoding an endodermal HMG-box transcription factor. Using lentiviral vectors for RNA interference in differentiating ES cells, an essential role for Sox17 was proven in cardiac muscle cell formation. Sox17 short-hairpin RNA suppresses cardiac myogenesis selectively, acting subsequent to mesoderm formation yet before induction of Mesp1 and Mesp2, a pair of related basic helix-loop-helix transcription factors that together are indispensable for creating heart mesoderm. Sox17 short-hairpin RNA blocks cardiac myogenesis non-cell autonomously and impairs the induction of Hex, a homeodomain transcription factor that is known to be required for the production of endoderm-derived heart-inducing factors.

Multipotent flk-1+ cardiovascular progenitor cells give rise to the cardiomyocyte, endothelial, and vascular smooth muscle lineages

Cell-tracing studies in the mouse indicate that the cardiac lineage arises from a population that expresses the vascular endothelial growth factor receptor 2 (VEGFR2, Flk-1), suggesting that it may develop from a progenitor with vascular potential. Using the embryonic stem (ES) cell differentiation model, we have identified a cardiovascular progenitor based on the temporal expression of the primitive streak (PS) marker brachyury and Flk-1. Comparable progenitors could also be isolated from head-fold stage embryos. When cultured with cytokines known to function during cardiogenesis, individual cardiovascular progenitors generated colonies that displayed cardiomyocyte, endothelial, and vascular smooth muscle (VSM) potential. Isolation and characterization of this previously unidentified population suggests that the mammalian cardiovascular system develops from multipotential progenitors.

Disruption of the somitic molecular clock causes abnormal vertebral segmentation

Somites are the precursors of the vertebral column. They segment from the presomitic mesoderm (PSM) that is caudally located and newly generated from the tailbud. Somites form in synchrony on either side of the embryonic midline in a reiterative manner. A molecular clock that operates in the PSM drives this reiterative process. Genetic manipulation in mouse, chick and zebrafish has revealed that the molecular clock controls the activity of the Notch and WNT signaling pathways in the PSM. Disruption of the molecular clock impacts on somite formation causing abnormal vertebral segmentation (AVS). A number of dysmorphic syndromes manifest AVS defects. Interaction between developmental biologists and clinicians has lead to groundbreaking research in this area with the identification that spondylocostal dysostosis (SCD) is caused by mutation in Delta-like 3 (DLL3), Mesoderm posterior 2 (MESP2), and Lunatic fringe (LFNG); three genes that are components of the Notch signaling pathway. This review describes our current understanding of the somitic molecular clock and highlights how key findings in developmental biology can impact on clinical practice.

Ciona intestinalis as a model for cardiac development

The primitive chordate Ciona intestinalis has emerged as a significant model system for the study of heart development. The Ciona embryo employs a conserved heart gene network in the context of extremely low cell numbers and reduced genetic redundancy. Here, I review recent studies on the molecular genetics of Ciona cardiogenesis as well as classic work on heart anatomy and physiology. I also discuss the potential of employing Ciona to decipher a comprehensive chordate gene network and to determine how this network controls heart morphogenesis.

Cooperative Mesp activity is required for normal somitogenesis along the anterior–posterior axis

Mesp2 is a bHLH-type transcription factor that plays a key role during somitogenesis. Mesp2 is transiently expressed and is quickly degraded once translated. In our current study, we find that Mesp2 contains a degradation domain, which acts as a target of proteasome-mediated proteolysis and appears to play this role in vivo. We have also defined the nuclear localization signals (NLS) and constructed a minimum Mesp2 protein (P2-HD) composed of the NLS, bHLH and the degradation domains. The ability of the P2-HD as a transcription factor in vivo was examined. Some of the defects that had been previously observed in the Mesp2-null mice were rescued in the knock-in mice but only in the posterior half of the body, indicating differential effects of P2-HD along the anterior-posterior (AP) axis. In addition, quantitative analysis of the expression along the AP axis revealed that the relative levels of Mesp2 increased, whereas Mesp1 is down-regulated in the later stages of development by the activities of Mesp2 in the wild-type embryo. Moreover, we have found that somitogenesis in the early stages is more susceptible to changes in the Mesp gene dosage, indicating that a threshold level of Mesp activity must be required for the progression of normal somitogenesis.

FGF signaling delineates the cardiac progenitor field in the simple chordate, Ciona intestinalis

Comprehensive gene networks in Ciona intestinalis embryos provide a foundation for characterizing complex developmental processes, such as the initial phases of chordate heart development. The basic helix-loop-helix regulatory gene Ci-Mesp is required for activation of cardiac transcription factors. Evidence is presented that Ci-Ets1/2, a transcriptional effector of receptor tyrosine kinase (RTK) signaling, acts downstream from Mesp to establish the heart field. Asymmetric activation of Ets1/2, possibly through localized expression of FGF9, drives heart specification within this field. During gastrulation, Ets1/2 is expressed in a group of four cells descended from two Mesp-expressing founder cells (the B7.5 cells). After gastrulation, these cells divide asymmetrically; the smaller rostral daughters exhibit RTK activation (phosphorylation of ERK) and form the heart lineage while the larger caudal daughters form the anterior tail muscle lineage. Inhibition of RTK signaling prevents heart specification. Targeted inhibition of Ets1/2 activity or FGF receptor function also blocks heart specification. Conversely, application of FGF or targeted expression of constitutively active Ets1/2 (EtsVp16) cause both rostral and caudal B7.5 lineages to form heart cells. This expansion produces an unexpected phenotype: transformation of a single-compartment heart into a functional multicompartment organ. We discuss these results with regard to the development and evolution of the multichambered vertebrate heart.

Potential of stem-cell-based therapies for heart disease

The use of stem cells to generate replacement cells for damaged heart muscle, valves, vessels and conduction cells holds great potential. Recent identification of multipotent progenitor cells in the heart and improved understanding of developmental processes relevant to pluripotent embryonic stem cells may facilitate the generation of specific types of cell that can be used to treat human heart disease. Secreted factors from circulating progenitor cells that localize to sites of damage may also be useful for tissue protection or neovascularization. The exciting discoveries in basic science will require rigorous testing in animal models to determine those most worthy of future clinical trials.

The amphioxus T-box gene, AmphiTbx15/18/22, illuminates the origins of chordate segmentation

Amphioxus and vertebrates are the only deuterostomes to exhibit unequivocal somitic segmentation. The relative simplicity of the amphioxus genome makes it a favorable organism for elucidating the basic genetic network required for chordate somite development. Here we describe the developmental expression of the somite marker, AmphiTbx15/18/22, which is first expressed at the mid-gastrula stage in dorsolateral mesendoderm. At the early neurula stage, expression is detected in the first three pairs of developing somites. By the mid-neurula stage, expression is downregulated in anterior somites, and only detected in the penultimate somite primordia. In early larvae, the gene is expressed in nascent somites before they pinch off from the posterior archenteron (tail bud). Integrating functional, phylogenetic and expression data from a variety of triploblast organisms, we have reconstructed the evolutionary history of the Tbx15/18/22 subfamily. This analysis suggests that the Tbx15/18/22 gene may have played a role in patterning somites in the last common ancestor of all chordates, a role that was later conserved by its descendents following gene duplications within the vertebrate lineage. Furthermore, the comparison of expression domains within this gene subfamily reveals similarities in the genetic bases of trunk and cranial mesoderm segmentation. This lends support to the hypothesis that the vertebrate head evolved from an ancestor possessing segmented cranial mesoderm.

Mesp1-nonexpressing cells contribute to the ventricular cardiac conduction system

Previous fate mapping analysis, using Cre recombinase driven by the Mesp1 locus, revealed that Mesp1 is expressed in almost all of the precursors of the cardiovascular system, including the endothelium, endocardium, myocardium, and epicardium. Mesp1-nonexpressing cells were found to be restricted to the outflow tract cushion and along the interventricular septum (IVS), which is a location that is suggestive of specialized cardiac conduction system (CCS). In our current study, we examined the identity of these IVS cells by using the pattern of beta-galactosidase activity in CCS-lacZ mice. In addition, by crossing Mesp1-Cre and floxed GFP reporter mice with CCS-lacZ mice, we have calculated that approximately 20% of the ventricular CCS within the IVS corresponds to Mesp1-nonexpressing cells. These data suggest that the ventricular CCS is of heterocellular origin. Furthermore, we indicate a possibility that a population of the cells that contribute to the ventricular CCS might be distinguished at an early stage of development.

Building the mammalian heart from two sources of myocardial cells

Cardiogenesis is an exquisitely sensitive process. Any perturbation in the cells that contribute to the building of the heart leads to cardiac malformations, which frequently result in the death of the embryo. Previously, the myocardium was thought to be derived from a single source of cells. However, the recent identification of a second source of myocardial cells that make an important contribution to the cardiac chambers has modified the classical view of heart formation. It also has an important influence on the interpretation of mutant phenotypes in the mouse, with consequences for the classification and prognosis of human congenital heart defects.

Ciona intestinalis: chordate development made simple

Thanks to their transparent and rapidly developing mosaic embryos, ascidians (or sea squirts) have been a model system for embryological studies for over a century. Recently, ascidians have entered the postgenomic era, with the sequencing of the Ciona intestinalis genome and the accumulation of molecular resources that rival those available for fruit flies and mice. One strength of ascidians as a model system is their close similarity to vertebrates. Literature reporting molecular homologies between vertebrate and ascidian tissues has flourished over the past 15 years, since the first ascidian genes were cloned. However, it should not be forgotten that ascidians diverged from the lineage leading to vertebrates over 500 million years ago. Here, we review the main similarities and differences so far identified, at the molecular level, between ascidian and vertebrate tissues and discuss the evolution of the compact ascidian genome.

The evolutionary origin of cardiac chambers

Identification of cardiac mechanisms of retinoic acid (RA) signaling, description of homologous genetic circuits in Ciona intestinalis and consolidation of views on the secondary heart field have fundamental, but still unrecognized implications for vertebrate heart evolution. Utilizing concepts from evolution, development, zoology, and circulatory physiology, we evaluate the strengths of animal models and scenarios for the origin of vertebrate hearts. Analyzing chordates, lower and higher vertebrates, we propose a paradigm picturing vertebrate hearts as advanced circulatory pumps formed by segments, chambered or not, devoted to inflow or outflow. We suggest that chambers arose not as single units, but as components of a peristaltic pump divided by patterning events, contrasting with scenarios assuming that chambers developed one at a time. Recognizing RA signaling as a potential mechanism patterning cardiac segments, we propose to use it as a tool to scrutinize the phylogenetic origins of cardiac chambers within chordates. Finally, we integrate recent ideas on cardiac development such as the ballooning and secondary/anterior heart field paradigms, showing how inflow/outflow patterning may interact with developmental mechanisms suggested by these models.

Mutated MESP2 causes spondylocostal dysostosis in humans

Spondylocostal dysostosis (SCD) is a term given to a heterogeneous group of disorders characterized by abnormal vertebral segmentation (AVS). We have previously identified mutations in the Delta-like 3 (DLL3) gene as a major cause of autosomal recessive spondylocostal dysostosis. DLL3 encodes a ligand for the Notch receptor and, when mutated, defective somitogenesis occurs resulting in a consistent and distinctive pattern of AVS affecting the entire spine. From our study cohort of cases of AVS, we have identified individuals and families with abnormal segmentation of the entire spine but no mutations in DLL3, and, in some of these, linkage to the DLL3 locus at 19q13.1 has been excluded. Within this group, the radiological phenotype differs mildly from that of DLL3 mutation-positive SCD and is variable, suggesting further heterogeneity. Using a genomewide scanning strategy in one consanguineous family with two affected children, we demonstrated linkage to 15q21.3-15q26.1 and furthermore identified a 4-bp duplication mutation in the human MESP2 gene that codes for a basic helix-loop-helix transcription factor. No MESP2 mutations were found in a further 7 patients with related radiological phenotypes in whom abnormal segmentation affected all vertebrae, nor in a further 12 patients with diverse phenotypes.

Dll3 pudgy mutation differentially disrupts dynamic expression of somite genes

Mutations in the notch ligand delta-like 3 have been identified in both the pudgy mouse (Dll3(pu); Kusumi et al.: Nat Genet 19:274-278, 1998) and the human disorder spondylocostal dysostosis (SCD; Bulman et al.: Nat Genet 24:438-441, 2000), and a targeted mutation has been generated (Dll3(neo); Dunwoodie et al.: Development 129:1795-1806, 2002). Vertebral and rib malformations deriving from defects in somitic patterning are key features of these disorders. In the mouse, notch pathway genes such as Lfng, Hes1, Hes7, and Hey2 display dynamic patterns of expression in paraxial mesoderm, cycling in synchrony with somite formation (Aulehla and Johnson: Dev Biol 207:49-61, 1999; Forsberg et al.: Curr Biol 8:1027-1030, 1998; Jouve et al.: Development 127:1421-1429, 2000; McGrew et al.: Curr Biol 8:979-982, 1998; Nakagawa et al.: Dev Biol 216:72-84, 1999). We report here that the Dll3(pu) mutation has different effects on the expression of cycling (Lfng and Hes7) and stage-specific genes (Hey3 and Mesp2). This suggests a more complex situation than a single oscillatory mechanism in somitogenesis and provides an explanation for the unique radiological features of the human DLL3-type of SCD.

nanos1: a mouse nanos gene expressed in the central nervous system is dispensable for normal development

A mouse nanos (nanos1) gene was cloned and its function was examined by generating a gene-knockout mouse. The nanos1 gene encodes an RNA-binding protein, which contains a putative zinc-finger motif that exhibits similarity with other nanos-class genes in vertebrates and invertebrates. Although nanos1 is not detected in primordial germ cells, it is observed in seminiferous tubules of mature testis. Interestingly, maternally expressed nanos1 is observed in substantial amounts in oocytes, but the amount of maternal RNA is rapidly reduced after fertilization, and the transient zygotic nanos1 expression is observed in eight-cell embryos. At 12.5 days postcoitum, nanos1 is re-expressed in the central nervous system and the expression continues in the adult brain, in which the hippocampal formation is the predominant region. The nanos1 -deficient mice develop to term without any detectable abnormality and they are fertile. No significant neural defect is observed in terms of their behavior to date.

Heart development: molecular insights into cardiac specification and early morphogenesis

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

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Sequential targeted deficiency of SP-A and -D leads to progressive alveolar lipoproteinosis and emphysema

Surfactant proteins-A and -D (SP-A and SP-D) are members of the collectin protein family. Mice singly deficient in SP-A and SP-D have distinct phenotypes. Both have altered inflammatory responses to microbial challenges. To further investigate the functions of SP-A and SP-D in vivo, we developed mice deficient in both proteins by sequentially targeting the closely linked genes in embryonic stem cells using graded resistance to G-418. There is a progressive increase in bronchoalveolar lavage phospholipid, protein, and macrophage content through 24 wk of age. The macrophages from doubly deficient mice express high levels of the matrix metalloproteinase MMP-12 and develop intense but patchy lung inflammation. Stereological analysis demonstrates significant air space enlargement and reduction in alveolar septal tissue per unit volume, consistent with emphysema. These changes qualitatively resemble the lung pathology seen in SP-D-deficient mice. These doubly deficient mice will be useful in dissecting the potential overlap in function between SP-A and SP-D in host defense.

Patterning the vertebrate heart

The mammalian heart is crafted from a few progenitor cells that are subject to rapidly changing sets of instructions from their environment and from within. These instructions cause them to migrate, expand and diversify in lineage, and acquire form and function. Molecular information from various model systems, combined with increasingly detailed morphogenetic data, has provided insights into some of these key events. Many congenital heart abnormalities might arise from defects in the early stages of heart development, therefore it is important to understand the molecular pathways that underlie the lineage specification and patterning processes that shape this organ.

Transcriptional regulation of vertebrate cardiac morphogenesis

Transcription factors can regulate the expression of other genes in a tissue-specific and quantitative manner and are thus major regulators of embryonic developmental processes. Several transcription factors that regulate cardiac genes specifically have been described, and the recent discovery that dominant inherited transcription factor mutations cause congenital heart defects in humans has brought direct medical relevance to the study of cardiac transcription factors in heart development. Although this field of study is extensive, several major gaps in our knowledge of the transcriptional control of heart development still exist. This review will concentrate on recent developments in the field of cardiac transcription factors and their roles in heart formation.

Transcriptional regulation of Mesp1 and Mesp2 genes: differential usage of enhancers during development

Mesp1 and Mesp2 encode bHLH-type transcription factors, Mesp1 and Mesp2, respectively. The expression of both genes is observed in the nascent mesoderm, and subsequently in the rostral presomitic mesoderm. To determine the regulatory mechanism for gene expression, we attempted to identify enhancer elements by transient transgenic analysis. At least two enhancers, which are responsible for the expression of the two genes in the early mesoderm (early mesodermal enhancer, EME) and the presomitic mesoderm (PSM enhancer, PSME), and one suppressor, which is responsible for the rostrally restricted expression in the presomitic mesoderm, were identified. Deletion studies of these enhancer elements indicate that either gene may use the same enhancer for early mesoderm development, whereas both genes may utilize separate enhancers to regulate their expression in the presomitic mesoderm.

The murine winged helix transcription factors, Foxc1 and Foxc2, are both required for cardiovascular development and somitogenesis

The murine Foxc1/Mf1 and Foxc2/Mfh1 genes encode closely related forkhead/winged helix transcription factors with overlapping expression in the forming somites and head mesoderm and endothelial and mesenchymal cells of the developing heart and blood vessels. Embryos lacking either Foxc1 or Foxc2, and most compound heterozygotes, die pre- or perinatally with similar abnormal phenotypes, including defects in the axial skeleton and cardiovascular system. However, somites and major blood vessels do form. This suggested that the genes have similar, dose-dependent functions, and compensate for each other in the early development of the heart, blood vessels, and somites. In support of this hypothesis, we show here that compound Foxc1; Foxc2 homozygotes die earlier and with much more severe defects than single homozygotes alone. Significantly, they have profound abnormalities in the first and second branchial arches, and the early remodeling of blood vessels. Moreover, they show a complete absence of segmented paraxial mesoderm, including anterior somites. Analysis of compound homozygotes shows that Foxc1 and Foxc2 are both required for transcription in the anterior presomitic mesoderm of paraxis, Mesp1, Mesp2, Hes5, and Notch1, and for the formation of sharp boundaries of Dll1, Lfng, and ephrinB2 expression. We propose that the two genes interact with the Notch signaling pathway and are required for the prepatterning of anterior and posterior domains in the presumptive somites through a putative Notch/Delta/Mesp regulatory loop.

Allocation and early differentiation of cardiovascular progenitors in the mouse embryo

During gastrulation of the mouse embryo, progenitor cells of the endothelium of blood vessels are allocated to different compartments of the extraembryonic and embryonic tissues in accordance to the timing and the site of recruitment to the mesodermal layer. In the yolk sac, the endothelium and the erythropoietic progenitors are populated by different groups of mesodermal cells, suggesting that they may not be derived from a common pool of progenitors. An orderly pattern of movement of mesodermal cells and the provision of proper intercellular transforming growth factor beta (TGF beta) and vascular endothelial growth factor (VEGF) signaling by neighboring germ layer tissues are essential for normal morphogenesis of the vasculature.

Cell lineage determination in the mouse

During the peri-implantation development of the mouse embryo from the blastocyst through gastrulation, Pou5f1 (OCT-4) down-regulation is closely linked to the initial step of lineage allocation to extraembryonic and embryonic somatic tissues. Subsequently, differentiation of the lineage precursors is subject to inductive tissue interactions and intercellular signalling that regulate cell proliferation and the acquisition of lineage-specific morphological and molecular characteristics. A notable variation of this process of lineage specification is the persistence of Pou5f1 activity throughout the differentiation of the primordial germ cells, which may underpin their ability to produce pluripotent progeny either as stem cells (embryonic germ cells) in vitro or as gametes in vivo. Nevertheless, intercellular signalling still plays a critical role in the specification of the primordial germ cells. The findings that primordial germ cells can be induced from any epiblast cells and that they share common progenitors with other somatic cells provide compelling evidence for the absence of a pre-determined germ line in the mouse embryo.