The Drosophila homolog of vertebrate Islet1 is a key component in early cardiogenesis

Drosophila as a Model to Understand Second Heart Field Development

The genetic model system Drosophila has contributed fundamentally to our understanding of mammalian heart specification, development, and congenital heart disease. The relatively simple Drosophila heart is a linear muscular tube that is specified and develops in the embryo and persists throughout the life of the animal. It functions at all stages to circulate hemolymph within the open circulatory system of the body. During Drosophila metamorphosis, the cardiac tube is remodeled, and a new layer of muscle fibers spreads over the ventral surface of the heart to form the ventral longitudinal muscles. The formation of these fibers depends critically upon genes known to be necessary for mammalian second heart field (SHF) formation. Here, we review the prior contributions of the Drosophila system to the understanding of heart development and disease, discuss the importance of the SHF to mammalian heart development and disease, and then discuss how the ventral longitudinal adult cardiac muscles can serve as a novel model for understanding SHF development and disease.

Tailup expression in Drosophila larval and adult cardiac valve cells

In Drosophila larvae, the direction of blood flow within the heart tube, as well as the diastolic filling of the posterior heart chamber, is regulated by a single cardiac valve. This valve is sufficient to close the heart tube at the junction of the ventricle and the aorta and is formed by only two cells; both are integral parts of the heart tube. The valve cells regulate hemolymph flow by oscillating between a spherical and a flattened cell shape during heartbeats. At the spherical stage, the opposing valve cells close the heart lumen. The dynamic cell shape changes of valve cells are supported by a dense, criss-cross orientation of myofibrils and the presence of the valvosomal compartment, a large intracellular cavity. Both structures are essential for the valve cells' function. In a screen for factors specifically expressed in cardiac valve cells, we identified the transcription factor Tailup. Knockdown of tailup causes abnormal orientation and differentiation of cardiac muscle fibers in the larval aorta and inhibits the formation of the ventral longitudinal muscle layer located underneath the heart tube in the adult fly and affects myofibrillar orientation of valve cells. Furthermore, we have identified regulatory sequences of tup that control the expression of tailup in the larval and adult valve cells.

The extended analogy of extraembryonic development in insects and amniotes

It is fascinating that the amnion and serosa/chorion, two extraembryonic (EE) tissues that are characteristic of the amniote vertebrates (mammals, birds and reptiles), have also independently evolved in insects. In this review, we offer the first detailed, macroevolutionary comparison of EE development and tissue biology across these animal groups. Some commonalities represent independent solutions to shared challenges for protecting the embryo (environmental assaults, risk of pathogens) and supporting its development, including clear links between cellular properties (e.g. polyploidy) and physiological function. Further parallels encompass developmental features such as the early segregation of the serosa/chorion compared to later, progressive differentiation of the amnion and formation of the amniotic cavity from serosal-amniotic folds as a widespread morphogenetic mode across species. We also discuss common developmental roles for orthologous transcription factors and BMP signalling in EE tissues of amniotes and insects, and between EE and cardiac tissues, supported by our exploration of new resources for global and tissue-specific gene expression. This highlights the degree to which general developmental principles and protective tissue features can be deduced from each of these animal groups, emphasizing the value of broad comparative studies to reveal subtle developmental strategies and answer questions that are common across species. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Development and evolution of the metazoan heart

The mechanisms of the evolution and development of the heart in metazoans are highlighted, starting with the evolutionary origin of the contractile cell, supposedly the precursor of cardiomyocytes. The last eukaryotic common ancestor is likely a combination of several cellular organisms containing their specific metabolic pathways and genetic signaling networks. During evolution, these tool kits diversified. Shared parts of these conserved tool kits act in the development and functioning of pumping hearts and open or closed circulations in such diverse species as arthropods, mollusks, and chordates. The genetic tool kits became more complex by gene duplications, addition of epigenetic modifications, influence of environmental factors, incorporation of viral genomes, cardiac changes necessitated by air‐breathing, and many others. We evaluate mechanisms involved in mollusks in the formation of three separate hearts and in arthropods in the formation of a tubular heart. A tubular heart is also present in embryonic stages of chordates, providing the septated four‐chambered heart, in birds and mammals passing through stages with first and second heart fields. The four‐chambered heart permits the formation of high‐pressure systemic and low‐pressure pulmonary circulation in birds and mammals, allowing for high metabolic rates and maintenance of body temperature. Crocodiles also have a (nearly) separated circulation, but their resting temperature conforms with the environment. We argue that endothermic ancestors lost the capacity to elevate their body temperature during evolution, resulting in ectothermic modern crocodilians. Finally, a clinically relevant paragraph reviews the occurrence of congenital cardiac malformations in humans as derailments of signaling pathways during embryonic development.

The cardiac regulatory toolkit contains many factors including epigenetic, genetic, viral, hemodynamic, and environmental factors, but also transcriptional activators, repressors, duplicated genes, redundancies and dose‐dependancies.

Numerous toolkits regulate mechanisms including cell‐cell interactions, EMT, mitosis patterns, cell migration and differentiation and left/right sidedness involved in the development of endocardial cushions, looping, septum complexes, pharyngeal arch arteries, chamber and valve formation and conduction system.

Evolutionary development of the yolk sac circulation likely preceded the advent of endothermy in amniotes.

Parallel evolutionary traits regulate the development of contractile pumps in various taxa often in conjunction with the gut, lungs and excretory organs.

Live imaging reveals hub cell assembly and compaction dynamics during morphogenesis of the Drosophila testis niche

Adult stem cells are often found in specialized niches, where the constituent cells direct self-renewal of their stem cell pool. The niche is therefore crucial for both normal homeostasis and tissue regeneration. In many mammalian tissues, niche cells have classically been difficult to identify, which has hampered any understanding of how tissues first construct niches during development. Fortunately, the Drosophila germline stem cell (GSC) niche is well defined, allowing for unambiguous identification of both niche cells and resident stem cells. The testis niche first forms in the early embryo, during a late stage of gonadogenesis. Here, using live-imaging both in vivo and ex vivo, we follow pro-niche cells as they assemble and assume their final form. We show that after ex vivo culture the niche appears fully functional, as judged by enrichment of adhesion proteins, the ability to activate STAT in adjacent GSCs, and to direct GSCs to divide orthogonally to the niche, just as they would in situ. Collectively, our imaging has generated several novel insights on niche morphogenesis that could not be inferred from fixed images alone. We identify dynamic processes that constitute an assembly phase and a compaction phase during morphogenesis. The compaction phase correlates with cell neighbor exchange among the assembled pro-niche cells, as well as a burst of divisions among newly recruited stem cells. Before compaction, an assembly phase involves the movement of pro-niche cells along the outer periphery of the gonad, using the extracellular matrix (ECM) to assemble at the anterior of the gonad. Finally, live-imaging in integrin mutants allows us to define the role of pro-niche cell-ECM interaction with regard to the new assembly and compaction dynamics revealed here.

Distinct subsets of Eve-positive pericardial cells stabilise cardiac outflow and contribute to Hox gene-triggered heart morphogenesis in Drosophila

The Drosophila heart, composed of discrete subsets of cardioblasts and pericardial cells, undergoes Hox-triggered anterior-posterior morphogenesis, leading to a functional subdivision into heart proper and aorta, with its most anterior part forming a funnel-shaped cardiac outflow. Cardioblasts differentiate into Tin-positive 'working myocytes' and Svp-expressing ostial cells. However, developmental fates and functions of heart-associated pericardial cells remain elusive. Here, we show that the pericardial cells that express the transcription factor Even Skipped adopt distinct fates along the anterior-posterior axis. Among them, the most anterior Antp-Ubx-AbdA-negative cells form a novel cardiac outflow component we call the outflow hanging structure, whereas the Antp-expressing cells differentiate into wing heart precursors. Interestingly, Hox gene expression in the Even Skipped-positive cells not only underlies their antero-posterior diversification, but also influences heart morphogenesis in a non-cell-autonomous way. In brief, we identify a new cardiac outflow component derived from a subset of Even Skipped-expressing cells that stabilises the anterior heart tip, and demonstrate non-cell-autonomous effects of Hox gene expression in the Even Skipped-positive cells on heart morphogenesis.

On the Evolution of the Cardiac Pacemaker

The rhythmic contraction of the heart is initiated and controlled by an intrinsic pacemaker system. Cardiac contractions commence at very early embryonic stages and coordination remains crucial for survival. The underlying molecular mechanisms of pacemaker cell development and function are still not fully understood. Heart form and function show high evolutionary conservation. Even in simple contractile cardiac tubes in primitive invertebrates, cardiac function is controlled by intrinsic, autonomous pacemaker cells. Understanding the evolutionary origin and development of cardiac pacemaker cells will help us outline the important pathways and factors involved. Key patterning factors, such as the homeodomain transcription factors Nkx2.5 and Shox2, and the LIM-homeodomain transcription factor Islet-1, components of the T-box (Tbx), and bone morphogenic protein (Bmp) families are well conserved. Here we compare the dominant pacemaking systems in various organisms with respect to the underlying molecular regulation. Comparative analysis of the pathways involved in patterning the pacemaker domain in an evolutionary context might help us outline a common fundamental pacemaker cell gene programme. Special focus is given to pacemaker development in zebrafish, an extensively used model for vertebrate development. Finally, we conclude with a summary of highly conserved key factors in pacemaker cell development and function.

Dissecting the Role of the Extracellular Matrix in Heart Disease: Lessons from the Drosophila Genetic Model

The extracellular matrix (ECM) is a dynamic scaffold within organs and tissues that enables cell morphogenesis and provides structural support. Changes in the composition and organisation of the cardiac ECM are required for normal development. Congenital and age-related cardiac diseases can arise from mis-regulation of structural ECM proteins (Collagen, Laminin) or their receptors (Integrin). Key regulators of ECM turnover include matrix metalloproteinases (MMPs) and their inhibitors, tissue inhibitors of matrix metalloproteinases (TIMPs). MMP expression is increased in mice, pigs, and dogs with cardiomyopathy. The complexity and longevity of vertebrate animals makes a short-lived, genetically tractable model organism, such as Drosophila melanogaster, an attractive candidate for study. We survey ECM macromolecules and their role in heart development and growth, which are conserved between Drosophila and vertebrates, with focus upon the consequences of altered expression or distribution. The Drosophila heart resembles that of vertebrates during early development, and is amenable to in vivo analysis. Experimental manipulation of gene function in a tissue- or temporally-regulated manner can reveal the function of adhesion or ECM genes in the heart. Perturbation of the function of ECM proteins, or of the MMPs that facilitate ECM remodelling, induces cardiomyopathies in Drosophila, including cardiodilation, arrhythmia, and cardia bifida, that provide mechanistic insight into cardiac disease in mammals.

Direct nkx2-5 transcriptional repression of isl1 controls cardiomyocyte subtype identity

During cardiogenesis, most myocytes arise from cardiac progenitors expressing the transcription factors Isl1 and Nkx2-5. Here, we show that a direct repression of Isl1 by Nkx2-5 is necessary for proper development of the ventricular myocardial lineage. Overexpression of Nkx2-5 in mouse embryonic stem cells (ESCs) delayed specification of cardiac progenitors and inhibited expression of Isl1 and its downstream targets in Isl1(+) precursors. Embryos deficient for Nkx2-5 in the Isl1(+) lineage failed to downregulate Isl1 protein in cardiomyocytes of the heart tube. We demonstrated that Nkx2-5 directly binds to an Isl1 enhancer and represses Isl1 transcriptional activity. Furthermore, we showed that overexpression of Isl1 does not prevent cardiac differentiation of ESCs and in Xenopus laevis embryos. Instead, it leads to enhanced specification of cardiac progenitors, earlier cardiac differentiation, and increased cardiomyocyte number. Functional and molecular characterization of Isl1-overexpressing cardiomyocytes revealed higher beating frequencies in both ESC-derived contracting areas and Xenopus Isl1-gain-of-function hearts, which associated with upregulation of nodal-specific genes and downregulation of transcripts of working myocardium. Immunocytochemistry of cardiomyocyte lineage-specific markers demonstrated a reduction of ventricular cells and an increase of cells expressing the pacemaker channel Hcn4. Finally, optical action potential imaging of single cardiomyocytes combined with pharmacological approaches proved that Isl1 overexpression in ESCs resulted in normally electrophysiologically functional cells, highly enriched in the nodal subtype at the expense of the ventricular lineage. Our findings provide an Isl1/Nkx2-5-mediated mechanism that coordinately regulates the specification of cardiac progenitors toward the different myocardial lineages and ensures proper acquisition of myocyte subtype identity.

A new heart for a new head in vertebrate cardiopharyngeal evolution

It has been more than 30 years since the publication of the new head hypothesis, which proposed that the vertebrate head is an evolutionary novelty resulting from the emergence of neural crest and cranial placodes. Neural crest generates the skull and associated connective tissues, whereas placodes produce sensory organs. However, neither crest nor placodes produce head muscles, which are a crucial component of the complex vertebrate head. We discuss emerging evidence for a surprising link between the evolution of head muscles and chambered hearts - both systems arise from a common pool of mesoderm progenitor cells within the cardiopharyngeal field of vertebrate embryos. We consider the origin of this field in non-vertebrate chordates and its evolution in vertebrates.

Cellular Mechanisms of Drosophila Heart Morphogenesis

Many of the major discoveries in the fields of genetics and developmental biology have been made using the fruit fly, Drosophila melanogaster. With regard to heart development, the conserved network of core cardiac transcription factors that underlies cardiogenesis has been studied in great detail in the fly, and the importance of several signaling pathways that regulate heart morphogenesis, such as Slit/Robo, was first shown in the fly model. Recent technological advances have led to a large increase in the genomic data available from patients with congenital heart disease (CHD). This has highlighted a number of candidate genes and gene networks that are potentially involved in CHD. To validate genes and genetic interactions among candidate CHD-causing alleles and to better understand heart formation in general are major tasks. The specific limitations of the various cardiac model systems currently employed (mammalian and fish models) provide a niche for the fly model, despite its evolutionary distance to vertebrates and humans. Here, we review recent advances made using the Drosophila embryo that identify factors relevant for heart formation. These underline how this model organism still is invaluable for a better understanding of CHD.

Signaling molecules, transcription growth factors and other regulators revealed from in-vivo and in-vitro models for the regulation of cardiac development

Several in-vivo heart developmental models have been applied to decipher the cardiac developmental patterning encompassing early, dorsal, cardiac and visceral mesoderm as well as various transcription factors such as Gata, Hand, Tin, Dpp, Pnr. The expression of cardiac specific transcription factors, such as Gata4, Tbx5, Tbx20, Tbx2, Tbx3, Mef2c, Hey1 and Hand1 are of fundamental significance for the in-vivo cardiac development. Not only the transcription factors, but also the signaling molecules involved in cardiac development were conserved among various species. Enrichment of the bone morphogenic proteins (BMPs) in the anterior lateral plate mesoderm is essential for the initiation of myocardial differentiation and the cardiac developmental process. Moreover, the expression of a number of cardiac transcription factors and structural genes initiate cardiac differentiation in the medial mesoderm. Other signaling molecules such as TGF-beta, IGF-1/2 and the fibroblast growth factor (FGF) play a significant role in cardiac repair/regeneration, ventricular heart development and specification of early cardiac mesoderm, respectively. The role of the Wnt signaling in cardiac development is still controversial discussed, as in-vitro results differ dramatically in relation to the animal models. Embryonic stem cells (ESC) were utilized as an important in-vitro model for the elucidation of the cardiac developmental processes since they can be easily manipulated by numerous signaling molecules, growth factors, small molecules and genetic manipulation. Finally, in the present review the dynamic role of the long noncoding RNA and miRNAs in the regulation of cardiac development are summarized and discussed.

An Org-1-Tup transcriptional cascade reveals different types of alary muscles connecting internal organs in Drosophila

The T-box transcription factor Tbx1 and the LIM-homeodomain transcription factor Islet1 are key components in regulatory circuits that generate myogenic and cardiogenic lineage diversity in chordates. We show here that Org-1 and Tup, the Drosophila orthologs of Tbx1 and Islet1, are co-expressed and required for formation of the heart-associated alary muscles (AMs) in the abdomen. The same holds true for lineage-related muscles in the thorax that have not been described previously, which we name thoracic alary-related muscles (TARMs). Lineage analyses identified the progenitor cell for each AM and TARM. Three-dimensional high-resolution analyses indicate that AMs and TARMs connect the exoskeleton to the aorta/heart and to different regions of the midgut, respectively, and surround-specific tracheal branches, pointing to an architectural role in the internal anatomy of the larva. Org-1 controls tup expression in the AM/TARM lineage by direct binding to two regulatory sites within an AM/TARM-specific cis-regulatory module, tupAME. The contributions of Org-1 and Tup to the specification of Drosophila AMs and TARMs provide new insights into the transcriptional control of Drosophila larval muscle diversification and highlight new parallels with gene regulatory networks involved in the specification of cardiopharyngeal mesodermal derivatives in chordates.

Tailup plays multiple roles during cardiac outflow assembly in Drosophila

The Drosophila LIM-homeodomain transcription factor Tailup and its vertebrate counterpart Islet1 are expressed in cardiac progenitor cells where they play a specification role. Loss of function of Islet1 leads to a complete absence of the right ventricle and affects the development of the cardiac outflow tract in mouse embryos. Similarly, tailup mutant embryos display a reduced number of cardiac cells but the role of tailup in cardiac outflow formation in Drosophila remains unknown. Here, we show that tailup is expressed in the main Drosophila cardiac outflow components, i.e., heart anchoring cells (HANC) and cardiac outflow muscles (COM) and that loss of its function and/or tissue-specific knockdowns dramatically affect cardiac outflow morphogenesis. Our data demonstrate that tailup plays many roles and is required for the acquisition of HANC and COM properties. We also show that tailup regulates HANC motility, COM shapes and their attachment to the heart tip and genetically interacts with ladybird, shotgun and slit, which are known to be involved in cardiac outflow assembly. Furthemore, using tissue-specific overexpression of dominant negative tailup constructs lacking sequences encoding either the homeodomain or the LIM domain, we demonstrate that tailup can exert its function not only in transcription factor mode but also via its protein-protein interaction domain. We identify Tailup as an evolutionarily-conserved regulator of cardiac outflow formation and provide further evidence for its conserved role in heart development.

Methods to assess Drosophila heart development, function and aging

In recent years the Drosophila heart has become an established model for many different aspects of human cardiac disease. This model has allowed identification of disease-causing mechanisms underlying congenital heart disease and cardiomyopathies and has permitted the study of underlying genetic, metabolic and age-related contributions to heart function. In this review we discuss methods currently employed in the analysis of the Drosophila heart structure and function, such as optical methods to infer heart function and performance, electrophysiological and mechanical approaches to characterize cardiac tissue properties, and conclude with histological techniques used in the study of heart development and adult structure.

The Iroquois complex is required in the dorsal mesoderm to ensure normal heart development in Drosophila

Drosophila heart development is an invaluable system to study the orchestrated action of numerous factors that govern cardiogenesis. Cardiac progenitors arise within specific dorsal mesodermal regions that are under the influence of temporally coordinated actions of multiple signaling pathways. The Drosophila Iroquois complex (Iro-C) consists of the three homeobox transcription factors araucan (ara), caupolican (caup) and mirror (mirr). The Iro-C has been shown to be involved in tissue patterning leading to the differentiation of specific structures, such as the lateral notum and dorsal head structures and in establishing the dorsal-ventral border of the eye. A function for Iro-C in cardiogenesis has not been investigated yet. Our data demonstrate that loss of the whole Iro complex, as well as loss of either ara/caup or mirr only, affect heart development in Drosophila. Furthermore, the data indicate that the GATA factor Pannier requires the presence of Iro-C to function in cardiogenesis. Furthermore, a detailed expression pattern analysis of the members of the Iro-C revealed the presence of a possibly novel subpopulation of Even-skipped expressing pericardial cells and seven pairs of heart-associated cells that have not been described before. Taken together, this work introduces Iro-C as a new set of transcription factors that are required for normal development of the heart. As the members of the Iro-C may function, at least partly, as competence factors in the dorsal mesoderm, our results are fundamental for future studies aiming to decipher the regulatory interactions between factors that determine different cell fates in the dorsal mesoderm.

Tup/Islet1 integrates time and position to specify muscle identity in Drosophila

The LIM-homeodomain transcription factor Tailup/Islet1 (Tup) is a key component of cardiogenesis in Drosophila and vertebrates. We report here an additional major role for Drosophila Tup in specifying dorsal muscles. Tup is expressed in the four dorsal muscle progenitors (PCs) and tup-null embryos display a severely disorganized dorsal musculature, including a transformation of the dorsal DA2 into dorsolateral DA3 muscle. This transformation is reciprocal to the DA3 to DA2 transformation observed in collier (col) mutants. The DA2 PC, which gives rise to the DA2 muscle and to an adult muscle precursor, is selected from a cluster of myoblasts transiently expressing both Tinman (Tin) and Col. The activation of tup by Tin in the DA2 PC is required to repress col transcription and establish DA2 identity. The transient, partial overlap between Tin and Col expression provides a window of opportunity to distinguish between DA2 and DA3 muscle identities. The function of Tup in the DA2 PC illustrates how single cell precision can be reached in cell specification when temporal dynamics are combined with positional information. The contributions of Tin, Tup and Col to patterning Drosophila dorsal muscles bring novel parallels with chordate pharyngeal muscle development.

Genes and networks regulating cardiac development and function in flies: genetic and functional genomic approaches

The Drosophila heart has emerged as a powerful model system for cardiovascular research. This simple organ, composed of only 104 cardiomyocytes and associated pericardiac cells, has been the focus of numerous candidate gene approaches in the last 2 decades, which have unraveled a number of transcription factors and signaling pathways involved in the regulation of early cardiac development. Importantly, these regulators seem to have largely conserved functions in mammals. Recent studies also demonstrated the usefulness of the fly circulatory system to investigate molecular mechanisms involved in the control of the establishment and maintenance of the cardiac function. In this review, we have focused on how new technological and conceptual advances in the field of functional genomics have impacted research on the cardiovascular system in Drosophila. Genome-scale genetic screens were conducted taking advantage of recently developed ribonucleic acid interference transgenic lines and molecularly defined genetic deficiencies, which have provided new insights into the genetics of both the developmental control of heart formation and cardiac function. In addition, a comprehensive picture of the transcriptional network controlling heart formation is emerging, thanks to newly developed genomic approaches which allow global and unbiased identification of the underlying components of gene regulatory circuits.

An unauthorized biography of the second heart field and a pioneer/scaffold model for cardiac development

The identification of subpharyngeal cardiac precursors has had a strong influence on the way we think about early cardiac development. From this discovery was born the concept of multiple heart fields. Early support for the concept came from gene expression, genetic retrospective fate mapping, and gene targeting studies, which collectively suggested the existence of a second heart field (SHF) on the basis of specific Islet-1 (Isl-1) expression, presence of two cardiac ancestral lineages, and compatible cardiac knockout phenotypes, respectively. A decade after the original studies, support for the SHF concept is dwindling. This is because in all bilaterian models studied, Isl expression in heart progenitors is not SHF-specific, because lineage data are best explained by alternative models including an older, truly ancestral, lineage of cardiac pioneers with unrestricted contribution to all cardiac segments and, finally, because the inflow-to-outflow segmental nature of the early vertebrate peristaltic heart has been reaffirmed with novel, less invasive, methodologies. Altogether, the paradigms derived from the discovery of subpharyngeal cardiac progenitors helped us shift from relatively simple models, which rely predominantly either on patterning, gene expression patterns or lineages, to a much more sophisticated body of knowledge in which all these parameters must be accounted. Thus, it is well possible that due consideration of the key elements contained in the inflow/outflow, pioneer/scaffold, ballooning, and SHF hypotheses may provide us with a unified framework of the early stages of cardiac development. Here, we advance into this direction by suggesting an intuitive model of early heart development based on the concept of an inflow/outflow scaffold erected by cardiac pioneers, one that is required to assemble all the subsequent cell contribution that emigrates from cardiac progenitor areas.

Islet1-expressing cardiac progenitor cells: a comparison across species

Adult mammalian cardiac stem cells express the LIM-homeodomain transcription factor Islet1 (Isl1). They are considered remnants of Isl1-positive embryonic cardiac progenitor cells. During amniote heart development, Isl1-positive progenitor cells give rise mainly to the outflow tract, the right ventricle, and parts of the atria. This led to the hypothesis that the development of the right ventricle of the amniote heart depends on the recruitment of additional cells to the primary heart tube. The region from which these additional, Isl1-positive cells originate is called second heart field, as opposed to the first heart field whose cells form the primary heart tube. Here, we review the available data about Isl1 in different species, demonstrating that Isl1 is an important component of the core transcription factor network driving early cardiogenesis in animals of the two clades, deuterostomes, and protostomes. The data support the view of a single cardiac progenitor cell population that includes Isl1-expressing cells and which differentiates into the various cardiac lineages during embryonic development in vertebrates but not in other phyla of the animal kingdom.

Genome-wide polycomb target gene prediction in Drosophila melanogaster

As key epigenetic regulators, polycomb group (PcG) proteins are responsible for the control of cell proliferation and differentiation as well as stem cell pluripotency and self-renewal. Aberrant epigenetic modification by PcG is strongly correlated with the severity and invasiveness of many types of cancers. Unfortunately, the molecular mechanism of PcG-mediated epigenetic regulation remained elusive, partly due to the extremely limited pool of experimentally confirmed PcG target genes. In order to facilitate experimental identification of PcG target genes, here we propose a novel computational method, EpiPredictor, that achieved significantly higher matching ratios with several recent chromatin immunoprecipitation studies than jPREdictor, an existing computational method. We further validated a subset of genes that were uniquely predicted by EpiPredictor by cross-referencing existing literature and by experimental means. Our data suggest that multiple transcription factor networking at the cis-regulatory elements is critical for PcG recruitment, while high GC content and high conservation level are also important features of PcG target genes. EpiPredictor should substantially expedite experimental discovery of PcG target genes by providing an effective initial screening tool. From a computational standpoint, our strategy of modelling transcription factor interaction with a non-linear kernel is original, effective and transferable to many other applications.

CIP, a cardiac Isl1-interacting protein, represses cardiomyocyte hypertrophy

RATIONALE

Mammalian heart has minimal regenerative capacity. In response to mechanical or pathological stress, the heart undergoes cardiac remodeling. Pressure and volume overload in the heart cause increased size (hypertrophic growth) of cardiomyocytes. Whereas the regulatory pathways that activate cardiac hypertrophy have been well-established, the molecular events that inhibit or repress cardiac hypertrophy are less known.

OBJECTIVE

To identify and investigate novel regulators that modulate cardiac hypertrophy.

METHODS AND RESULTS

Here, we report the identification, characterization, and functional examination of a novel cardiac Isl1-interacting protein (CIP). CIP was identified from a bioinformatic search for novel cardiac-expressed genes in mouse embryonic hearts. CIP encodes a nuclear protein without recognizable motifs. Northern blotting, in situ hybridization, and reporter gene tracing demonstrated that CIP is highly expressed in cardiomyocytes of developing and adult hearts. Yeast two-hybrid screening identified Isl1, a LIM/homeodomain transcription factor essential for the specification of cardiac progenitor cells in the second heart field, as a cofactor of CIP. CIP directly interacted with Isl1, and we mapped the domains of these two proteins, which mediate their interaction. We show that CIP represses the transcriptional activity of Isl1 in the activation of the myocyte enhancer factor 2C. The expression of CIP was dramatically reduced in hypertrophic cardiomyocytes. Most importantly, overexpression of CIP repressed agonist-induced cardiomyocyte hypertrophy.

CONCLUSIONS

Our studies therefore identify CIP as a novel regulator of cardiac hypertrophy.

Spire, an actin nucleation factor, regulates cell division during Drosophila heart development

The Drosophila dorsal vessel is a beneficial model system for studying the regulation of early heart development. Spire (Spir), an actin-nucleation factor, regulates actin dynamics in many developmental processes, such as cell shape determination, intracellular transport, and locomotion. Through protein expression pattern analysis, we demonstrate that the absence of spir function affects cell division in Myocyte enhancer factor 2-, Tinman (Tin)-, Even-skipped- and Seven up (Svp)-positive heart cells. In addition, genetic interaction analysis shows that spir functionally interacts with Dorsocross, tin, and pannier to properly specify the cardiac fate. Furthermore, through visualization of double heterozygous embryos, we determines that spir cooperates with CycA for heart cell specification and division. Finally, when comparing the spir mutant phenotype with that of a CycA mutant, the results suggest that most Svp-positive progenitors in spir mutant embryos cannot undergo full cell division at cell cycle 15, and that Tin-positive progenitors are arrested at cell cycle 16 as double-nucleated cells. We conclude that Spir plays a crucial role in controlling dorsal vessel formation and has a function in cell division during heart tube morphogenesis.

Excessive Dpp signaling induces cardial apoptosis through dTAK1 and dJNK during late embryogenesis of Drosophila

BACKGROUND

To identify genes involved in the heart development of Drosophila, we found that embryos lacking raw function exhibited cardial phenotypes. raw was initially identified as a dorsal open group gene. The dorsal open phenotype was demonstrated to be resulted from the aberrant expression of decapentaplegic (dpp), a member of the tumor growth factor beta (TGF-β), signaling pathway. Despite the role of dpp in pattering cardioblasts during early embryogenesis of Drosophila have been demonstrated, how mutation in raw and/or excessive dpp signaling involves in the differentiating heart of Drosophila has not been fully elaborated at late stages.

RESULTS

We show that raw mutation produced a mild overspecification of cardial cells at stage 14, but these overproduced cells were mostly eliminated in late mutant embryos due to apoptosis. Aberrant dpp signaling is likely to contribute to the cardial phenotype found in raw mutants, because expression of dpp or constitutively activated thickven (tkvCA), the type I receptor of Dpp, induced a raw-like phenotype. Additionally, we show that dpp induced non-autonomous apoptosis through TGFβ activated kinase 1 (TAK1), because mis-expression of a dominant negative form of Drosophila TAK1 (dTAK1DN) was able to suppress cell death in raw mutants or embryos overexpressing dpp. Importantly, we demonstrated that dpp induce its own expression through dTAK1, which also leads to the hyperactivation of Drosophila JNK (DJNK). The hyperactivated DJNK was attributed to be the cause of Dpp/DTAK1-induced apoptosis because overexpression of a dominant negative DJNK, basket (bskDN), suppressed cell death induced by Dpp or DTAK1. Moreover, targeted overexpression of the anti-apoptotic P35 protein, or a dominant negative proapoptotic P53 (P53DN) protein blocked Dpp/DTAK1-induced apoptosis, and rescued heart cells under the raw mutation background.

CONCLUSIONS

We find that ectopic Dpp led to DJNK-dependent cardial apoptosis through the non-canonical TGF-β pathway during late embryogenesis of Drosophila. This certainly will increase our understanding of the pathogenesis of cardiomyopathy, because haemodynamic overload can up-regulate TGF-β and death of cardiomyocytes is observed in virtually every myocardial disease. Thus, our study may provide possible medical intervention for human cardiomyopathy.

Screening for mutations in the ISL1 gene in patients with thyroid dysgenesis

CONTEXT

Congenital hypothyroidism (CH) is a common endocrine disorder with an incidence of 1:3000- 4000 newborns. In 80-85% of cases, CH is caused by defects in thyroid organogenesis, resulting in absent, ectopically located, and/or severely reduced gland, all conditions indicated as "thyroid dysgenesis" (TD). A higher prevalence of congenital heart diseases has been documented in children with CH compared to the general population. This association suggests a possible pathogenic role of genes involved in both heart and thyroid development. Among these, it can be included Isl1, a transcription factor containing a LIM homeodomain that is expressed in both thyroid and heart during morphogenesis.

OBJECTIVE

In the present study, we investigate the role of ISL1 in the pathogenesis of TD.

SETTINGS AND PATIENTS

By single stranded conformational polymorphism, we screened for mutations the entire ISL1 coding sequence in 96 patients with TD and in 96 normal controls.

RESULTS

No mutations have been found in patients and controls.

CONCLUSION

Our data indicate that, despite the relevant role of ISL1 in thyroid and heart morphogenesis, mutations in its coding region are not associated with TD in our group of patients.

An eye on the head: the development and evolution of craniofacial muscles

Skeletal muscles exert diverse functions, enabling both crushing with great force and movement with exquisite precision. A remarkably distinct repertoire of genes and ontological features characterise this tissue, and recent evidence has shown that skeletal muscles of the head, the craniofacial muscles, are evolutionarily, morphologically and molecularly distinct from those of the trunk. Here, we review the molecular basis of craniofacial muscle development and discuss how this process is different to trunk and limb muscle development. Through evolutionary comparisons of primitive chordates (such as amphioxus) and jawless vertebrates (such as lampreys) with jawed vertebrates, we also provide some clues as to how this dichotomy arose.

A method to measure myocardial calcium handling in adult Drosophila

RATIONALE

Normal cardiac physiology requires highly regulated cytosolic Ca(2+) concentrations and abnormalities in Ca(2+) handling are associated with heart failure. The majority of approaches to identifying the components that regulate intracellular Ca(2+) dynamics rely on cells in culture, mouse models, and human samples. However, a genetically robust system for unbiased screens of mutations that affect Ca(2+) handling remains a challenge.

OBJECTIVE

We sought to develop a new method to measure myocardial Ca(2+) cycling in adult Drosophila and determine whether cardiomyopathic fly hearts recapitulate aspects of diseased mammalian myocardium.

METHODS AND RESULTS

Using engineered transgenic Drosophila that have cardiac-specific expression of Ca(2+)-sensing fluorescent protein, GCaMP2, we developed methods to measure parameters associated with myocardial Ca(2+) handling. The following key observations were identified: (1) Control w(1118) Drosophila hearts have readily measureable Ca(2+)-dependent fluorescent signals that are dependent on L-type Ca(2+) channels and SR Ca(2+) stores and originate from rostral and caudal pacemakers. (2) A fly mutant, held-up(2) (hdp(2)), that has a point mutation in troponin I and has a dilated cardiomyopathic phenotype demonstrates abnormalities in myocardial Ca(2+) handling that include increases in the duration of the 50% rise in intensity to peak intensity, the half-time of fluorescence decline from peak, the full duration at half-maximal intensity, and decreases in the linear slope of decay from 80% to 20% intensity decay. (3) Hearts from hdp(2) mutants had reductions in caffeine-induced Ca(2+) increases and reductions in ryanodine receptor (RyR) without changes in L-type Ca(2+) channel transcripts in comparison with w(1118).

CONCLUSIONS

Our results show that the cardiac-specific expression of GCaMP2 provides a means of characterizing propagating Ca(2+) transients in adult fly hearts. Moreover, the adult fruit fly heart recapitulates several aspects of Ca(2+) regulation observed in mammalian myocardium. A mutation in Drosophila that causes an enlarged cardiac chamber and impaired contractile function is associated with abnormalities in the cytosolic Ca(2+) transient as well as changes in transcript levels of proteins associated with Ca(2+) handling. This new methodology has the potential to permit an examination of evolutionarily conserved myocardial Ca(2+)-handing mechanisms by applying the vast resources available in the fly genomics community to conduct genetic screens to identify new genes involved in generated Ca(2+) transients and arrhythmias.

Mef2cb regulates late myocardial cell addition from a second heart field-like population of progenitors in zebrafish

Two populations of cells, termed the first and second heart field, drive heart growth during chick and mouse development. The zebrafish has become a powerful model for vertebrate heart development, partly due to the evolutionary conservation of developmental pathways in this process. Here we provide evidence that the zebrafish possesses a conserved homolog to the murine second heart field. We developed a photoconversion assay to observe and quantify the dynamic late addition of myocardial cells to the zebrafish arterial pole. We define an extra-cardiac region immediately posterior to the arterial pole, which we term the late ventricular region. The late ventricular region has cardiogenic properties, expressing myocardial markers such as vmhc and nkx2.5, but does not express a full complement of differentiated cardiomyocyte markers, lacking myl7 expression. We show that mef2cb, a zebrafish homolog of the mouse second heart field marker Mef2c, is expressed in the late ventricular region, and is necessary for late myocardial addition to the arterial pole. FGF signaling after heart cone formation is necessary for mef2cb expression, the establishment of the late ventricular region, and late myocardial addition to the arterial pole. Our study demonstrates that zebrafish heart growth shows more similarities to murine heart growth than previously thought. Further, as congenital heart disease is often associated with defects in second heart field development, the embryological and genetic advantages of the zebrafish model can be applied to study the vertebrate second heart field.

The pronotum LIM-HD gene tailup is both a positive and a negative regulator of the proneural genes achaete and scute of Drosophila

Early in the development of the imaginal wing disc of Drosophila, the LIM-HD gene tailup (islet), together with the HD genes of the iroquois complex, specify the notum territory of the disc. Later, tailup has been shown to act as a prepattern gene that antagonizes formation of sensory bristles on the notum of this fly. It has been proposed that Tailup downregulates the expression of the proneural genes achaete and scute by interfering with factors needed to activate these genes in the dorsocentral and scutellar regions of the disc. By means of a clonal analysis performed with tailup null alleles, here we show that, on the one hand, tailup is necessary to prevent formation of extra macrochaetae on most of the 11 sites where these landmark bristles arise on the fly notum. On the other hand, tailup is required to activate achaete and scute at the dorsocentral region, probably by acting as an hexameric complex with the cofactor Chip and the transcriptional activator Sspd on the dorsocentral enhancer of the achaete-scute complex. In contrast, in the scutellar region Tailup acts downstream of achaete-scute, antagonizing the proneural function of these genes probably in cooperation with Chip. We conclude that tailup acts on bristle development by several, even antagonistic, mechanisms.

Genetic and genomic dissection of cardiogenesis in the Drosophila model

The linear heart tube of the fruit fly Drosophila has served as a very valuable model for studying the regulation of early heart development. In the past, regulatory genes of Drosophila cardiogenesis have been identified largely through candidate approaches. The vast genetic toolkit available in this organism has made it possible to determine their functions and build regulatory networks of transcription factors and signaling inputs that control heart development. In this review, we summarize the major findings from this study and present current approaches aiming to identify additional players in the specification, morphogenesis, and differentiation of the heart by forward genetic screens. We also discuss various genomic and bioinformatic approaches that are currently being developed to extend the known transcriptional networks more globally which, in combination with the genetic approaches, will provide a comprehensive picture of the regulatory circuits during cardiogenesis.