Cardiomyopathy in Irx4-deficient mice is preceded by abnormal ventricular gene expression

Nkx genes are essential for maintenance of ventricular identity

Establishment of specific characteristics of each embryonic cardiac chamber is crucial for development of a fully functional adult heart. Despite the importance of defining and maintaining unique features in ventricular and atrial cardiomyocytes, the regulatory mechanisms guiding these processes are poorly understood. Here, we show that the homeodomain transcription factors Nkx2.5 and Nkx2.7 are necessary to sustain ventricular chamber attributes through repression of atrial chamber identity. Mutation of nkx2.5 in zebrafish yields embryos with diminutive ventricular and bulbous atrial chambers. These chamber deformities emerge gradually during development, with a severe collapse in the number of ventricular cardiomyocytes and an accumulation of excess atrial cardiomyocytes as the heart matures. Removal of nkx2.7 function from nkx2.5 mutants exacerbates the loss of ventricular cells and the gain of atrial cells. Moreover, in these Nkx-deficient embryos, expression of vmhc, a ventricular gene, fades, whereas expression of amhc, an atrial gene, expands. Cell-labeling experiments suggest that ventricular cardiomyocytes can transform into atrial cardiomyocytes in the absence of Nkx gene function. Through suggestion of transdifferentiation from ventricular to atrial fate, our data reveal a pivotal role for Nkx genes in maintaining ventricular identity and highlight remarkable plasticity in differentiated myocardium. Thus, our results are relevant to the etiologies of fetal and neonatal cardiac pathology and could direct future innovations in cardiac regenerative medicine.

Atrial identity is determined by a COUP-TFII regulatory network

Atria and ventricles exhibit distinct molecular profiles that produce structural and functional differences between the two cardiac compartments. However, the factors that determine these differences remain largely undefined. Cardiomyocyte-specific COUP-TFII ablation produces ventricularized atria that exhibit ventricle-like action potentials, increased cardiomyocyte size, and development of extensive T tubules. Changes in atrial characteristics are accompanied by alterations of 2,584 genes, of which 81% were differentially expressed between atria and ventricles, suggesting that a major function of myocardial COUP-TFII is to determine atrial identity. Chromatin immunoprecipitation assays using E13.5 atria identified classic atrial-ventricular identity genes Tbx5, Hey2, Irx4, MLC2v, MLC2a, and MLC1a, among many other cardiac genes, as potential COUP-TFII direct targets. Collectively, our results reveal that COUP-TFII confers atrial identity through direct binding and by modulating expression of a broad spectrum of genes that have an impact on atrial development and function.

Signaling and transcriptional networks in heart development and regeneration

The mammalian heart is the first functional organ, the first indicator of life. Its normal formation and function are essential for fetal life. Defects in heart formation lead to congenital heart defects, underscoring the finesse with which the heart is assembled. Understanding the regulatory networks controlling heart development have led to significant insights into its lineage origins and morphogenesis and illuminated important aspects of mammalian embryology, while providing insights into human congenital heart disease. The mammalian heart has very little regenerative potential, and thus, any damage to the heart is life threatening and permanent. Knowledge of the developing heart is important for effective strategies of cardiac regeneration, providing new hope for future treatments for heart disease. Although we still have an incomplete picture of the mechanisms controlling development of the mammalian heart, our current knowledge has important implications for embryology and better understanding of human heart disease.

Regulation of retinal interneuron subtype identity by the Iroquois homeobox gene Irx6

Interneuronal subtype diversity lies at the heart of the distinct molecular properties and synaptic connections that shape the formation of the neuronal circuits that are necessary for the complex spatial and temporal processing of sensory information. Here, we investigate the role of Irx6, a member of the Iroquois homeodomain transcription factor family, in regulating the development of retinal bipolar interneurons. Using a knock-in reporter approach, we show that, in the mouse retina, Irx6 is expressed in type 2 and 3a OFF bipolar interneurons and is required for the expression of cell type-specific markers in these cells, likely through direct transcriptional regulation. In Irx6 mutant mice, presumptive type 3a bipolar cells exhibit an expansion of their axonal projection domain to the entire OFF region of the inner plexiform layer, and adopt molecular features of both type 2 and 3a bipolar cells, highlighted by the ectopic upregulation of neurokinin 3 receptor (Nk3r) and Vsx1. These findings reveal Irx6 as a key regulator of type 3a bipolar cell identity that prevents these cells from adopting characteristic features of type 2 bipolar cells. Analysis of the Irx6;Vsx1 double null retina suggests that the terminal differentiation of type 2 bipolar cells is dependent on the combined expression of the transcription factors Irx6 and Vsx1, but also points to the existence of Irx6;Vsx1-independent mechanisms in regulating OFF bipolar subtype-specific gene expression. This work provides insight into the generation of neuronal subtypes by revealing a mechanism in which opposing, yet interdependent, transcription factors regulate subtype identity.

Cooperative and antagonistic roles for Irx3 and Irx5 in cardiac morphogenesis and postnatal physiology

The Iroquois homeobox (Irx) homeodomain transcription factors are important for several aspects of embryonic development. In the developing heart, individual Irx genes are important for certain postnatal cardiac functions, including cardiac repolarization (Irx5) and rapid ventricular conduction (Irx3). Irx genes are expressed in dynamic and partially overlapping patterns in the developing heart. Here we show in mice that Irx3 and Irx5 have redundant function in the endocardium to regulate atrioventricular canal morphogenesis and outflow tract formation. Our data suggest that direct transcriptional repression of Bmp10 by Irx3 and Irx5 in the endocardium is required for ventricular septation. A postnatal deletion of Irx3 and Irx5 in the myocardium leads to prolongation of atrioventricular conduction, due in part to activation of expression of the Na(+) channel protein Nav1.5. Surprisingly, combined postnatal loss of Irx3 and Irx5 results in a restoration of the repolarization gradient that is altered in Irx5 mutant hearts, suggesting that postnatal Irx3 activity can be repressed by Irx5. Our results have uncovered complex genetic interactions between Irx3 and Irx5 in embryonic cardiac development and postnatal physiology.

Iroquois homeodomain transcription factors in heart development and function

Numerous cardiac transcription factors play overlapping roles in both the specification and proliferation of the cardiac tissues and chambers during heart development. It has become increasingly apparent that cardiac transcription factors also play critical roles in the regulation of expression of many functional genes in the prenatal and postnatal hearts. Accordingly, mutations of cardiac transcription factors cannot only result in congenital heart defects but also alter heart function thereby predisposing to heart disease and cardiac arrhythmias. In this review, we summarize the roles of Iroquois homeobox (Irx) family of transcription factors in heart development and function. In all, 6 Irx genes are expressed with distinct and overlapping patterns in the mammalian heart. Studies in several animal models demonstrate that Irx genes are important for the establishment of ventricular chamber properties, the ventricular conduction system, as well as heterogeneity of the ventricular repolarization. The molecular mechanisms by which Irx proteins regulate gene expression and the clinical relevance of Irx functions in the heart are discussed.

IRX4 at 5p15 suppresses prostate cancer growth through the interaction with vitamin D receptor, conferring prostate cancer susceptibility

Recent genome-wide association studies (GWAS) identified a number of prostate cancer (PC) susceptibility loci, but most of their functional significances are not elucidated. Through our previous GWAS for PC in a Japanese population and subsequent resequencing and fine mapping, we here identified that IRX4 (Iroquois homeobox 4), coding Iroquois homeobox 4, is a causative gene of the PC susceptibility locus (rs12653946) at chromosome 5p15. IRX4 is expressed specifically in the prostate and heart, and quantitative expression analysis revealed a significant association between the genotype of rs12653946 and IRX4 expression in normal prostate tissues. Knockdown of IRX4 in PC cells enhanced their growth and IRX4 overexpression in PC cells suppressed their growth, indicating the functional association of IRX4 with PC and its tumor suppressive effect. Immunoprecipitation confirmed its protein-protein interaction to vitamin D receptor (VDR), and we found a significant interaction between IRX4 and VDR in their reciprocal transcriptional regulation. These findings indicate that the PC-susceptibility locus represented by rs12653946 at 5p15 is likely to regulate IRX4 expression in prostate which could suppress PC growth by interacting with the VDR pathway, conferring to PC susceptibility.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

Echocardiographic evaluation of the single right ventricle in congenital heart disease: results of new techniques

Right ventricular (RV) function is increasingly recognized as having prognostic significance in various disease processes. The current gold standard for noninvasive measurement of RV function is cardiac magnetic resonance imaging; however, because of practical considerations, echocardiography remains the most often used modality for evaluating the RV. In the past, because of its complex morphology, echocardiographic assessment of the RV was usually qualitative in nature. Current advances in echocardiographic techniques have been able to overcome some of the previous limitations and thus quantification of RV function is increasingly being performed. In addition, recent echocardiographic guidelines for evaluating the RV have been published to aid in standardizing practice. The evaluation of RV function almost certainly has no greater importance than in the congenital heart population, especially in those patients that have a single RV acting as the systemic ventricle. As this complex population continues to increase in number, accurate and precise evaluation of RV function will be a major issue in determining clinical care.

Iroquois homeobox gene 3 establishes fast conduction in the cardiac His-Purkinje network

Rapid electrical conduction in the His-Purkinje system tightly controls spatiotemporal activation of the ventricles. Although recent work has shed much light on the regulation of early specification and morphogenesis of the His-Purkinje system, less is known about how transcriptional regulation establishes impulse conduction properties of the constituent cells. Here we show that Iroquois homeobox gene 3 (Irx3) is critical for efficient conduction in this specialized tissue by antithetically regulating two gap junction-forming connexins (Cxs). Loss of Irx3 resulted in disruption of the rapid coordinated spread of ventricular excitation, reduced levels of Cx40, and ectopic Cx43 expression in the proximal bundle branches. Irx3 directly represses Cx43 transcription and indirectly activates Cx40 transcription. Our results reveal a critical role for Irx3 in the precise regulation of intercellular gap junction coupling and impulse propagation in the heart.

Genetics of inherited cardiomyopathy

During the past two decades, numerous disease-causing genes for different cardiomyopathies have been identified. These discoveries have led to better understanding of disease pathogenesis and initial steps in the application of mutation analysis in the evaluation of affected individuals and their family members. As knowledge of the genetic abnormalities, and insight into cellular and organ biology has grown, so has appreciation of the level of complexity of interaction between genotype and phenotype across disease states. What were initially thought to be one-to-one gene-disease correlates have turned out to display important relational plasticity dependent in large part on the genetic and environmental backgrounds into which the genes of interest express. The current state of knowledge with regard to genetics of cardiomyopathy represents a starting point to address the biology of disease, but is not yet developed sufficiently to supplant clinically based classification systems or, in most cases, to guide therapy to any significant extent. Future work will of necessity be directed towards elucidation of the biological mechanisms of both rare and common gene variants and environmental determinants of plasticity in the genotype-phenotype relationship with the ultimate goal of furthering our ability to identify, diagnose, risk stratify, and treat this group of disorders which cause heart failure and sudden death in the young.

Two novel mutations of the IRX4 gene in patients with congenital heart disease

IRX4 was the first identified cardiac transcription factor that is restricted to the ventricles at all stages of heart development. Irx4-deficient mice show ventricular dysfunction and develop cardiomyopathy. To study the potential impact of sequence variations in IRX4 on congenital heart disease (CHD) in humans, we examined the coding region of IRX4 in a cohort of 698 Chinese people with congenital heart disease and 250 healthy individuals as the controls. We found two potential disease-causing mutations, p. Asn85Tyr and p. Glu92Gly. A mammalian two-hybrid assay showed that both of the mutations significantly affected the interaction between IRX4 and RXRA. It demonstrated that IRX4 had a potential causative impact on the development of congenital heart disease, particularly ventricular septal defect.

CTCF promotes muscle differentiation by modulating the activity of myogenic regulatory factors

CTCF nuclear factor regulates many aspects of gene expression, largely as a transcriptional repressor or via insulator function. Its roles in cellular differentiation are not clear. Here we show an unexpected role for CTCF in myogenesis. Ctcf is expressed in myogenic structures during mouse and zebrafish development. Gain- and loss-of-function approaches in C2C12 cells revealed CTCF as a modulator of myogenesis by regulating muscle-specific gene expression. We addressed the functional connection between CTCF and myogenic regulatory factors (MRFs). CTCF enhances the myogenic potential of MyoD and myogenin and establishes direct interactions with MyoD, indicating that CTCF regulates MRF-mediated muscle differentiation. Indeed, CTCF modulates functional interactions between MyoD and myogenin in co-activation of muscle-specific gene expression and facilitates MyoD recruitment to a muscle-specific promoter. Finally, ctcf loss-of-function experiments in zebrafish embryos revealed a critical role of CTCF in myogenic development and linked CTCF to broader aspects of development via regulation of Wnt signaling. We conclude that CTCF modulates MRF functional interactions in the orchestration of myogenesis.

WNT signaling in adult cardiac hypertrophy and remodeling: lessons learned from cardiac development

On pathological stress, the heart reactivates several signaling pathways that traditionally were thought to be operational only in the developing heart. One of these pathways is the WNT signaling pathway. WNT controls heart development but is also modulated during adult heart remodeling. This review summarizes the currently available data regarding WNT signaling during left ventricular (LV) remodeling. Upstream, soluble frizzled-related proteins (sFRPs) block WNT-dependent activation of the canonical WNT pathway. By inhibition of WNT activation, these factors also reduce β-catenin-dependent transcription by altering the ratio of cytoplasmic/nuclear β-catenin. In experimental settings, sFRPs injected into the heart attenuated LV remodeling. sFRPs are secreted from autologous bone marrow-derived mononuclear cells. Disheveled is a signaling intermediate of both the canonical and noncanonical WNT pathway. Similarly to the effect of sFRP, depletion of a disheveled isoform attenuated LV remodeling. In contrast, disheveled activation led to progressive dilated cardiomyopathy. Inhibition of nuclear β-catenin signaling downstream of the canonical WNT pathway significantly reduced postinfarct mortality and functional decline of LV function following chronic left anterior descending coronary artery ligation. WNT signaling also affects mobilization and homing of bone marrow-derived vasculogenic progenitor cells. Finally, heart-specific WNT/β-catenin interaction partners have been identified that will possibly allow targeting this pathway in a tissue-specific manner. In summary, the WNT pathway plays a pivotal role in adult cardiac remodeling and may be suitable for therapeutic interventions. Currently, several molecular and cellular mechanisms whereby WNT inhibition attenuates LV remodeling are proposed. Reactivation of the developmental program to restore functional LV myocardium from resident precursor cells may significantly contribute to this process.

Identification of candidate genes potentially relevant to chamber-specific remodeling in postnatal ventricular myocardium

Molecular predisposition of postnatal ventricular myocardium to chamber-dependent (concentric or eccentric) remodeling remains largely elusive. To this end, we compared gene expression in the left (LV) versus right ventricle (RV) in newborn piglets, using a differential display reverse transcription-PCR (DDRT-PCR) technique. Out of more than 5600 DDRT-PCR bands, a total of 153 bands were identified as being differentially displayed. Of these, 96 bands were enriched in the LV, whereas the remaining 57 bands were predominant in the RV. The transcripts, displaying over twofold LV-RV expression differences, were sequenced and identified by BLAST comparison to known mRNA sequences. Among the genes, whose expression was not previously recognized as being chamber-dependent, we identified a small cohort of key regulators of muscle cell growth/proliferation (MAP3K7IP2, MSTN, PHB2, APOBEC3F) and gene expression (PTPLAD1, JMJD1C, CEP290), which may be relevant to the chamber-dependent predisposition of ventricular myocardium to respond differentially to pressure (LV) and volume (RV) overloads after birth. In addition, our data demonstrate chamber-dependent alterations in expression of as yet uncharacterized novel genes, which may also be suitable candidates for association studies in animal models of LV/RV hypertrophy.

Specification of the cardiac conduction system by transcription factors

Diseases of the cardiovascular system that cause sudden cardiac deaths are often caused by lethal arrhythmias that originate from defects in the cardiac conduction system. Development of the cardiac conduction system is a complex biological process that can be wrought with problems. Although several genes involved in mature conduction system function have been identified, their association with development of specific subcomponents of the cardiac conduction system remains challenging. Several transcription factors, including homeodomain proteins and T-box proteins, are essential for cardiac conduction system morphogenesis and activation or repression of key regulatory genes. In addition, several transcription factors modify expression of genes encoding the ion channel proteins that contribute to the electrophysiological properties of the conduction system and govern contraction of the surrounding myocardium. Loss of transcriptional regulation during cardiac development has detrimental effects on cardiogenesis that may lead to arrhythmias. Human genetic mutations in some of these transcription factors have been identified and are known to cause congenital heart diseases that include cardiac conduction system malformations. In this review, we summarize the contributions of several key transcription factors to specification, patterning, maturation, and function of the cardiac conduction system. Further analysis of the molecular programs involved in this process should lead to improved diagnosis and therapy of conduction system disease.

The homeobox gene Mohawk represses transcription by recruiting the sin3A/HDAC co-repressor complex

Mohawk is an atypical homeobox gene expressed in embryonic progenitor cells of skeletal muscle, tendon, and cartilage. We demonstrate that Mohawk functions as a transcriptional repressor capable of blocking the myogenic conversion of 10T1/2 fibroblasts. The repressor activity is located in three small, evolutionarily conserved domains (MRD1-3) in the carboxy-terminal half of the protein. Point mutation analysis revealed six residues in MRD1 are sufficient for repressor function. The carboxy-terminal half of Mohawk is able to recruit components of the Sin3A/HDAC co-repressor complex (Sin3A, Hdac1, and Sap18) and a subset of Polymerase II general transcription factors (Tbp, TFIIA1 and TFIIB). Furthermore, Sap18, a protein that bridges the Sin3A/HDAC complex to DNA-bound transcription factors, is co-immunoprecipitated by MRD1. These data predict that Mohawk can repress transcription through recruitment of the Sin3A/HDAC co-repressor complex, and as a result, repress target genes required for the differentiation of cells to the myogenic lineage.

The Xenopus Irx genes are essential for neural patterning and define the border between prethalamus and thalamus through mutual antagonism with the anterior repressors Fezf and Arx

The Iroquois (Irx) genes encode homeoproteins conserved during evolution. Vertebrate genomes contain six Irx genes organized in two clusters, IrxA (which harbors Irx1, Irx2 and Irx4) and IrxB (which harbors Irx3, Irx5 and Irx6). To determine the precise role of these genes during development and their putative redundancies, we conducted a comparative expression analysis and a comprehensive loss-of-function study of all the early expressed Irx genes (Irx1-5) using specific morpholinos in Xenopus. We found that the five Irx genes display largely overlapping expression patterns and contribute to neural patterning. All Irx genes are required for proper formation of posterior forebrain, midbrain, hindbrain and, to a lesser an extent, spinal cord. Nevertheless, Irx1 and Irx3 seem to have a predominant role during regionalization of the neural plate. In addition, we find that the common anterior limit of Irx gene expression, which will correspond to the future border between the prethalamus and thalamus, is defined by mutual repression between Fezf and Irx proteins. This mutual repression is likely direct. Finally, we show that Arx, another anteriorly expressed repressor, also contribute to delineate the anterior border of Irx expression.

Interaction between transcription factors Iroquois proteins 4 and 5 controls cardiac potassium channel Kv4.2 gene transcription

AIMS

The homeobox transcription factor, Iroquois protein 5 (Irx5), plays an essential role in the generation of region-selective expression of Kv4.2 gene across the left ventricular wall of rodent hearts. Here, we analyse molecular mechanisms underlying the Irx5-induced regulation of the rat Kv4.2 promoter.

METHODS AND RESULTS

The mRNA levels for Irx members in various heart regions were assessed by RT-PCR. A luciferase reporter gene with the rat Kv4.2 promoter was used to test the effects of Irx members on channel promoter activity. Irx3 and Irx5 mRNAs were differentially distributed across the left ventricular wall, whereas Irx4 message was equally abundant in various ventricular regions. Irx5, but not Irx3 or Irx4, increased Kv4.2 promoter activity in 10T1/2 fibroblasts, whereas the transcription factor decreased promoter activity in neonatal ventricular myocytes. These effects were mediated by the C-terminal portion of Irx5. Irx4 appeared to inhibit the Irx5-induced increase in channel promoter activity in 10T1/2 cells. The N-terminal region of Irx4 was necessary and sufficient for this inhibition. Furthermore, when endogenous Irx4 expression was suppressed with siRNA, Irx5 increased channel promoter activity in neonatal myocytes.

CONCLUSION

These results indicate that Irx5 possesses the ability to activate the Kv4.2 promoter. The abundant Irx4 expression throughout the rat ventricle may play a role in the inverse relationship between Irx5 and Kv4.2 levels across the left ventricular wall.

Transcriptome analysis of the normal human mammary cell commitment and differentiation process

Mature mammary epithelial cells are generated from undifferentiated precursors through a hierarchical process, but the molecular mechanisms involved, particularly in the human mammary gland, are poorly understood. To address this issue, we isolated highly purified subpopulations of primitive bipotent and committed luminal progenitor cells as well as mature luminal and myoepithelial cells from normal human mammary tissue and compared their transcriptomes obtained using three different methods. Elements unique to each subset of mammary cells were identified, and changes that accompany their differentiation in vivo were shown to be recapitulated in vitro. These include a stage-specific change in NOTCH pathway gene expression during the commitment of bipotent progenitors to the luminal lineage. Functional studies further showed NOTCH3 signaling to be critical for this differentiation event to occur in vitro. Taken together, these findings provide an initial foundation for future delineation of mechanisms that perturb primitive human mammary cell growth and differentiation.

Prepatterning the Drosophila notum: the three genes of the iroquois complex play intrinsically distinct roles

The Drosophila thorax exhibits 11 pairs of large sensory organs (macrochaetes) identified by their unique position. Remarkably precise, this pattern provides an excellent model system to study the genetic basis of pattern formation. In imaginal wing discs, the achaete-scute proneural genes are expressed in clusters of cells that prefigure the positions of each macrochaete. The activities of prepatterning genes provide positional cues controlling this expression pattern. The three homeobox genes clustered in the iroquois complex (araucan, caupolican and mirror) are such prepattern genes. mirror is generally characterized as performing functions predominantly different from the other iroquois genes. Conversely, araucan and caupolican are described in previous studies as performing redundant functions in most if not all processes in which they are involved. We have addressed the question of the specific role of each iroquois gene in the prepattern of the notum and we clearly demonstrate that they are intrinsically different in their contribution to this process: caupolican and mirror, but not araucan, are required for the neural patterning of the lateral notum. However, when caupolican and/or mirror expression is reduced, araucan loss of function has an effect on thoracic bristles development. Moreover, the overexpression of araucan is able to rescue caupolican loss of function. We conclude that, although retaining some common functionalities, the Drosophila iroquois genes are in the process of diversification. In addition, caupolican and mirror are required for stripe expression and, therefore, to specify the muscular attachment sites prepattern. Thus, caupolican and mirror may act as common prepattern genes for all structures in the lateral notum.

Zebrafish homologue irx1a is required for the differentiation of serotonergic neurons

Serotonergic (5HT) neurons produce neurotransmitter serotonin, which modulates various neuronal circuits. The specification and differentiation of 5HT neurons require both extrinsic signals such as Shh and Fgf, as well as intrinsic transcription factors such as nkx2.2, mash1, phox2b, Gata2, and pet1. In this study, we show that iroquois homeodomain factor irx1a, but not irx1b, is expressed in the 5HT neurons. Knockdown of irx1a by antisense morpholino nucleotides reveals that it is a critical determinant for the differentiation of 5HT neurons in the hindbrain. However, irx1a morphants do not show a reduction of the progenitors of 5HT neurons. Hence, irx1a is not required for the initial specification but it is required for the complete differentiation of 5HT neurons.

The prepattern transcription factor Irx3 directs nephron segment identity

The nephron, the basic structural and functional unit of the vertebrate kidney, is organized into discrete segments, which are composed of distinct renal epithelial cell types. Each cell type carries out highly specific physiological functions to regulate fluid balance, osmolarity, and metabolic waste excretion. To date, the genetic basis of regionalization of the nephron has remained largely unknown. Here we show that Irx3, a member of the Iroquois (Irx) gene family, acts as a master regulator of intermediate tubule fate. Comparative studies in Xenopus and mouse have identified Irx1, Irx2, and Irx3 as an evolutionary conserved subset of Irx genes, whose expression represents the earliest manifestation of intermediate compartment patterning in the developing vertebrate nephron discovered to date. Intermediate tubule progenitors will give rise to epithelia of Henle's loop in mammals. Loss-of-function studies indicate that irx1 and irx2 are dispensable, whereas irx3 is necessary for intermediate tubule formation in Xenopus. Furthermore, we demonstrate that misexpression of irx3 is sufficient to direct ectopic development of intermediate tubules in the Xenopus mesoderm. Taken together, irx3 is the first gene known to be necessary and sufficient to specify nephron segment fate in vivo.

Genetic insights into normal and abnormal heart development

Congenital heart defects (CHDs) affect 1-2% of newborn children and are the leading cause of death in infants under 1 year of age. CHDs represent the single largest class of birth defects and account for 25% of all human congenital abnormalities. Numerous epidemiologic studies have established the heritable nature of CHDs. However, despite the remarkable progress of the past decade, very few CHD-causing genes have been identified so far. Molecular and genetic analysis of heart development--which requires the execution of specific genetic programs--has led to the identification of essential cardiac regulators and mutations that are linked to human CHD. Elucidation of the mechanisms of action of these transcription factors has also provided a molecular framework that will continue to help furthering our understanding of the molecular basis of normal and abnormal heart growth. This review will summarize present knowledge of cardiac development and illustrate how analysis of heart development has helped understand the genetic basis of some CHDs and how these advances could translate into better prevention, diagnosis, and care of congenital heart disease.

Toward a systems biology of mouse inner ear organogenesis: gene expression pathways, patterns and network analysis

We describe the most comprehensive study to date on gene expression during mouse inner ear (IE) organogenesis. Samples were microdissected from mouse embryos at E9-E15 in half-day intervals, a period that spans all of IE organogenesis. These included separate dissections of all discernible IE substructures such as the cochlea, utricle, and saccule. All samples were analyzed on high density expression microarrays under strict statistical filters. Extensive confirmatory tests were performed, including RNA in situ hybridizations. More than 5000 genes significantly varied in expression according to developmental stage, tissue, or both and defined 28 distinct expression patterns. For example, upregulation of 315 genes provided a clear-cut "signature" of early events in IE specification. Additional, clear-cut, gene expression signatures marked specific structures such as the cochlea, utricle, or saccule throughout late IE development. Pathway analysis identified 53 signaling cascades enriched within the 28 patterns. Many novel pathways, not previously implicated in IE development, including beta-adrenergic, amyloid, estrogen receptor, circadian rhythm, and immune system pathways, were identified. Finally, we identified positional candidate genes in 54 uncloned nonsyndromic human deafness intervals. This detailed analysis provides many new insights into the spatial and temporal genetic specification of this complex organ system.

Hesr1 and Hesr2 regulate atrioventricular boundary formation in the developing heart through the repression of Tbx2

The establishment of chamber specificity is an essential requirement for cardiac morphogenesis and function. Hesr1 (Hey1) and Hesr2 (Hey2) are specifically expressed in the atrium and ventricle, respectively, implicating these genes in chamber specification. In our current study, we show that the forced expression of Hesr1 or Hesr2 in the entire cardiac lineage of the mouse results in the reduction or loss of the atrioventricular (AV) canal. In the Hesr1-misexpressing heart, the boundaries of the AV canal are poorly defined, and the expression levels of specific markers of the AV myocardium, Bmp2 and Tbx2, are either very weak or undetectable. More potent effects were observed in Hesr2-misexpressing embryos, in which the AV canal appears to be absent entirely. These data suggest that Hesr1 and Hesr2 may prevent cells from expressing the AV canal-specific genes that lead to the precise formation of the AV boundary. Our findings suggest that Tbx2 expression might be directly suppressed by Hesr1 and Hesr2. Furthermore, we find that the expression of Hesr1 and Hesr2 is independent of Notch2 signaling. Taken together, our data demonstrate that Hesr1 and Hesr2 play crucial roles in AV boundary formation through the suppression of Tbx2.

4D cardiac MRI in the mouse

With the introduction of mouse models for the study of cardiac morphogenesis, there arises a need for new imaging protocols that can capture both morphological and functional information. High-resolution 2D cardiac cine MRI has often been used to quantify left and right ventricular function. In this study we propose a 3D isotropic cardiac cine MRI protocol with a voxel size of 200 microm(3) as a means of studying cardiac multi-chamber morphology and function. A black blood sequence was used to enhance blood myocardium contrast. Manual segmentation of the ventricles was used to measure ventricular volumes at end diastole and end systole. This method is demonstrated on an Irx4-deficient mouse model. We have been able to identify the volumes of both ventricles dynamically and to show differences in ejection fraction in the mutant. We have also identified an abnormality of the papillary muscle in the mutant that had been missed in previous phenotyping with ultrasound and histology.

Essential roles of the bHLH transcription factor Hrt2 in repression of atrial gene expression and maintenance of postnatal cardiac function

The basic helix-loop-helix transcriptional repressor Hairy-related transcription factor 2 (Hrt2) is expressed in ventricular, but not atrial, cardiomyocytes, and in endothelial and vascular smooth muscle cells. Mice homozygous for a null mutation of Hrt2 die perinatally from a spectrum of cardiac abnormalities, raising questions about the specific functions of this transcriptional regulator in individual cardiac cell lineages. Using a conditional Hrt2 null allele, we show that cardiomyocyte-specific deletion of Hrt2 in mice results in ectopic activation of atrial genes in ventricular myocardium with an associated impairment of cardiac contractility and a unique distortion in morphology of the right ventricular chamber. Consistent with the atrialization of ventricular gene expression in Hrt2 mutant mice, forced expression of Hrt2 in atrial cardiomyocytes is sufficient to repress atrial cardiac genes. These findings reveal a ventricular myocardial cell-autonomous function for Hrt2 in the suppression of atrial cell identity and the maintenance of postnatal cardiac function.

Aetiology-specific patterns in end-stage heart failure patients identified by functional annotation and classification of microarray data

BACKGROUND

The objective of the present study was to use gene expression profiling, functional annotations and classification to identify aetiology-specific biological processes and potential molecular markers for different aetiologies of end-stage heart failure.

METHODS AND RESULTS

Individual left ventricular myocardial samples from eleven coronary artery disease and nine dilated cardiomyopathy transplant patients were co-hybridized with pooled RNA from four non-failing hearts on custom-made arrays of 7000 human genes. Significance analysis identified differential expression of 153 and 147 genes, respectively, in coronary artery disease or dilated cardiomyopathy versus non-failing hearts. Analysis of Gene Ontology biological process annotations indicated aetiology-specific patterns, primarily related to genes involved in catabolism and in regulation of protein kinase activity. Gene expression classifiers were obtained and used for class prediction of random samples of coronary artery diseased and dilated cardiomyopathic hearts. Best classifiers frequently included matrix metalloproteinase 3, fibulin 1, ATP-binding cassette, sub-family B member 1 and iroquois homeobox protein 5.

CONCLUSION

Combining functional annotation from microarray data and classification analysis constitutes a potent strategy to identify disease-specific biological processes and gene expression markers in e.g. end-stage coronary artery disease and dilated cardiomyopathy.

Neuromuscular implications in left ventricular hypertrabeculation/noncompaction

This review focuses on recent advances in the association between left ventricular hypertrabeculation/noncompaction (LVHT), a form of unclassified cardiomyopathy, and neuromuscular disorders (NMD). So far, LVHT has been found in single patients with dystrophinopathy, dystrobrevinopathy, laminopathy, zaspopathy, myotonic dystrophy, infantile glycogenosis type II (Pompe's disease), myoadenylate-deaminase deficiency, mitochondriopathy, Barth syndrome, Friedreich ataxia, and Charcot-Marie-Tooth disease. Most frequently LVHT is found in patients with Barth syndrome and mitochondrial disorders. The prevalence of LVHT in NMD patients is not known. On the contrary, NMD can be detected in up to four fifths of the patients with LVHT. Because LVHT is associated with an increased risk of rhythm abnormalities and heart failure, it is essential to detect LVHT as soon as possible. Because of adequate therapeutic options, all patients with NMD should undergo a comprehensive cardiological examination as soon as their neurological diagnosis is established. In reverse, all patients with LVHT should undergo a comprehensive neurological investigation following the detection of LVHT.

Irxl1, a divergent Iroquois homeobox family transcription factor gene

Iroquois homeodomain (Irx) transcription factors are encoded by a conserved family of six genes that are found in two clusters of three genes each. Irx proteins are highly conserved, and their expression patterns overlap considerably during embryonic development, suggesting genetically redundant functions. We have identified a highly divergent Irx gene, which we term Iroquois homeobox-like 1 (Irxl1). The chromosomal location of Irxl1 is distinct from the Irx gene clusters. Irxl1 is conserved in most vertebrates, and the deduced amino acid sequence of its protein product predicts a homeodomain that bears significant homology to Irx homeodomains, but is clearly very divergent. We also identified in Irxl1 a divergent Iro box, a motif that is the defining feature of the Irx family. Expression of Irxl1 during mouse embryogenesis was distinct from that of most Irx genes, and was largely restricted to the epaxial and hypaxial components of the somites, limb buds, otic vescicle, craniofacial mesenchyme, retinal ganglion cell layer, and lens. We conclude that Irxl1 is a newly identified highly divergent member of the Irx gene family with specific expression patterns in mouse embryogenesis.

Cri du chat syndrome and congenital heart disease: a review of previously reported cases and presentation of an additional 21 cases from the Pediatric Cardiac Care Consortium

OBJECTIVES

To analyze the cases submitted to the Pediatric Cardiac Care Consortium (PCCC) database from 1982 to 2002 to determine the frequency and distribution of congenital heart disease (CHD) found in this population, to review the literature for previously published accounts of CHD in this population, and to review current genotype-phenotype associations for cri du chat (CDC) syndrome with CHD.

METHODS

We performed a retrospective review of the 98422 CHD cases submitted to the PCCC between 1982 and 2002, to find patients who had a noncardiac diagnosis of CDC syndrome.

RESULTS

A total of 21 patients (15 female and 6 male patients) were identified. Although some patients had multiple cardiac anomalies, they were categorized according to primary diagnoses on the basis of the most hemodynamically significant component. The patient groups were ventricular septal defect (n = 6), patent ductus arteriosus (n = 6), tetralogy of Fallot (n = 5), pulmonary valve atresia with ventricular septal defect (n = 2), pulmonary valve stenosis (n = 1), and double-outlet right ventricle (n = 1). Eighteen of the 21 patients underwent surgical repair of their defects. There was 1 late operative death. To determine whether the observed frequency of these cardiac defects among patients with CDC syndrome was comparable to that of the general population of patients with CHD, data for all cases submitted to the PCCC from 1982 to 2002 were used. Use of these numbers to determine expected frequencies for these defects showed significantly greater proportions of patients with these specific lesions among the patients with CDC syndrome.

CONCLUSIONS

Currently there is no clear understanding of the genomic cause of the prevalence of these defects in the population with CDC syndrome, although CHD has been noted among patients with other deletion syndromes.

Congenital heart diseases in small animals: part I. Genetic pathways and potential candidate genes

Proper cardiac morphogenesis requires a series of specific cell and tissue interactions driven by several cardiac transcription factors and downstream cardiac genes. To date, a number of genetic aetiologies responsible for human congenital heart defects (CHDs) have been identified, although none has been found for CHDs in small animals. Most gene mutations responsible for human CHDs exist in genetic pathways associated with cardiomorphogenesis. Insights into cardiomorphogenesis from human and mouse genetic studies will help us to identify potential genetic aetiologies in CHDs in small animals. Therefore, in this first part of a two-part review, the major genetic pathways for cardiomorphogenesis and important candidate genes for CHDs, based on mouse knock-out and human genetic studies are discussed.

Bmp2 instructs cardiac progenitors to form the heart-valve-inducing field

A hallmark of heart-valve development is the swelling and deposition of extracellular matrix in the heart-valve region. Only myocardium overlying this region can signal to underlying endothelium and cause it to lose cell-cell contacts, delaminate, and invade the extracellular space abutting myocardium and endocardium to form endocardial cushions (EC) in a process known as epithelial to mesenchymal transformation (EMT). The heart-valve myocardium expresses bone morphogenetic protein-2 (Bmp2) coincident with development of valve mesenchyme. BMPs belong to the transforming growth factor beta superfamily (TGF-beta) and play a wide variety of roles during development. We show that conditional ablation of Bmp2 in cardiac progenitors results in cell fate changes in which the heart-valve region adopts the identity of differentiated chamber myocardium. Moreover, Bmp2-deficient hearts fail to induce production and deposition of matrix at the heart-valve-forming region, resulting in the inability of the endothelium to swell and impairing the development of ECs. Furthermore, in collagen invasion assays, Bmp2 mutant endothelium is incapable of undergoing EMT, and addition of BMP2 protein to mutant heart explants rescues this phenotype. Our results demonstrate that Bmp2 is both necessary and sufficient to specify a field of cardiac progenitor cells as the heart-valve-inducing region amid developing atria and ventricles.

The homeodomain transcription factor Irx5 establishes the mouse cardiac ventricular repolarization gradient

Rhythmic cardiac contractions depend on the organized propagation of depolarizing and repolarizing wavefronts. Repolarization is spatially heterogeneous and depends largely on gradients of potassium currents. Gradient disruption in heart disease may underlie susceptibility to fatal arrhythmias, but it is not known how this gradient is established. We show that, in mice lacking the homeodomain transcription factor Irx5, the cardiac repolarization gradient is abolished due to increased Kv4.2 potassium-channel expression in endocardial myocardium, resulting in a selective increase of the major cardiac repolarization current, I(to,f), and increased susceptibility to arrhythmias. Myocardial Irx5 is expressed in a gradient opposite that of Kv4.2, and Irx5 represses Kv4.2 expression by recruiting mBop, a cardiac transcriptional repressor. Thus, an Irx5 repressor gradient negatively regulates potassium-channel-gene expression in the heart, forming an inverse I(to,f) gradient that ensures coordinated cardiac repolarization while also preventing arrhythmias.

The Iroquois homeobox gene, Irx5, is required for retinal cone bipolar cell development

In the mouse retina, at least ten distinct types of bipolar interneurons are involved in the transmission of visual signals from photoreceptors to ganglion cells. How bipolar interneuron diversity is generated during retinal development is poorly understood. Here, we show that Irx5, a member of the Iroquois homeobox gene family, is expressed in developing bipolar cells starting at postnatal day 5 and is localized to a subset of cone bipolar cells in the mature mouse retina. In Irx5-deficient mice, defects were observed in the expression of some, but not all, immunohistological markers that define mature Type 2 and Type 3 OFF cone bipolar cells, indicating a role for Irx5 in bipolar cell differentiation. The differentiation of these two bipolar cell types has previously been shown to require the homeodomain-CVC transcription factor, Vsx1. However, the defects observed in Irx5-deficient retinas do not coincide with a reduction of Vsx1 expression, and conversely, the expression of Irx5 in cone bipolar cells does not require the presence of a functional Vsx1 allele. These results indicate that there are at least two distinct genetic pathways (Irx5-dependent and Vsx1-dependent) regulating the development of Type 2 and Type 3 cone bipolar cells.

The MLC1v gene provides a transgenic marker of myocardium formation within developing chambers of the Xenopus heart

Many details of cardiac chamber morphogenesis could be revealed if muscle fiber development could be visualized directly within the hearts of living vertebrate embryos. To achieve this end, we have used the active promoter of the MLC1v gene to drive expression of green fluorescent protein (GFP) in the developing tadpole heart. By using a line of Xenopus laevis frogs transgenic for the MLC1v-EGFP reporter, we have observed regionalized patterns of muscle formation within the ventricular chamber and maturation of the atrial chambers, from the onset of chamber formation through to the adult frog. In f1 generation MLC1v-EGFP animals, promoter activity is first detected within the looping heart tube and delineates the forming ventricular chamber and proximal outflow tract throughout their development. The 8-kb MLC1v promoter faithfully reproduces the embryonic expression of the endogenous MLC1v mRNA. At later larval stages, weak patches of EGFP fluorescence are found on the atrial side of the atrioventricular boundary. Subsequently, an extensive lattice of MLC1v-expressing fibers extend across the mature atrial chambers of adult frog hearts and the transgene reveals the differing arrangement of muscle fibers in chamber versus outflow myocardium. The complete activity of the promoter resides within the proximal 4.5 kb of the MLC1v DNA fragment, whereas key elements regulating chamber-specific expression are present in the proximal-most 1.5 kb. Finally, we demonstrate how cardiac and craniofacial muscle expression of the MLC1v promoter can be used to diagnose mutant phenotypes in living embryos, using the injection of RNA encoding a Tbx1-engrailed repressor-fusion protein as an example.

Determinants of natriuretic peptide gene expression

The cardiac natriuretic peptides (NP) atrial natriuretic factor or peptide (ANF or ANP) and brain natriuretic peptide (BNP) are polypeptide hormones synthesized, stored and secreted mainly by cardiac muscle cells (cardiocytes) of the atria of the heart. Both ANF and BNP are co-stored in storage granules referred to as specific atrial granules. The biological properties of NP include modulation of intrinsic renal mechanisms, the sympathetic nervous system, the rennin-angiotensin-aldosterone system (RAAS) and other determinants, of fluid volume, vascular tone and renal function. Studies on the control of baseline and stimulated ANF synthesis and secretion indicate at least two types of regulated secretory processes in atrial cardiocytes: one is stretch-stimulated and pertussis toxin (PTX) sensitive and the other is Gq-mediated and is PTX insensitive. Baseline ANF secretion is also PTX insensitive. In vivo, it is conceivable that the first process mediates stimulated ANF secretion brought about by changes in central venous return and subsequent atrial muscle stretch as observed in acute extracellular fluid volume expansion. The second type of stimulation is brought about by sustained hemodynamic and neuroendocrine stimuli such as those observed in congestive heart failure.

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.

Lats2/Kpm is required for embryonic development, proliferation control and genomic integrity

The Drosophila melanogaster warts/lats tumour suppressor has two mammalian counterparts LATS1/Warts-1 and LATS2/Kpm. Here, we show that mammalian Lats orthologues exhibit distinct expression profiles according to germ cell layer origin. Lats2(-/-) embryos show overgrowth in restricted tissues of mesodermal lineage; however, lethality ultimately ensues on or before embryonic day 12.5 preceded by defective proliferation. Lats2(-/-) mouse embryonic fibroblasts (MEFs) acquire growth advantages and display a profound defect in contact inhibition of growth, yet exhibit defective cytokinesis. Lats2(-/-) embryos and MEFs display centrosome amplification and genomic instability. Lats2 localizes to centrosomes and overexpression of Lats2 suppresses centrosome overduplication induced in wild-type MEFs and reverses centrosome amplification inherent in Lats2(-/-) MEFs. These findings indicate an essential role of Lats2 in the integrity of processes that govern centrosome duplication, maintenance of mitotic fidelity and genomic stability.

Role of BMP signaling and the homeoprotein iroquois in the specification of the cranial placodal field

Different types of placodes originate at the anterior border of the neural plate but it is still an unresolved question whether individual placodes arise as distinct ectodermal specializations in situ or whether all or a subset of the placodes originate from a common preplacodal field. We have analyzed the expression and function of the homeoprotein Iro1 in Xenopus and zebrafish embryos, and we have compared its expression with several preplacodal and placodal markers. Our results indicate that the iro1 genes are expressed in the preplacodal region, being one of the earliest markers for this area. We show that an interaction between the neural plate and the epidermis is able to induce the expression of several preplacodal markers, including Xiro1, by a similar mechanism to that previously shown for neural crest induction. In addition, we analyzed the role of BMP in the specification of the preplacodal field by studying the expression of the preplacodal markers Six1, Xiro1, and several specific placodal markers. We experimentally modified the level of BMP activity by three different methods. First, we implanted beads soaked with noggin in early neurula stage Xenopus embryos; second, we injected the mRNA that encodes a dominant negative of the BMP receptor into Xenopus and zebrafish embryos; and third, we grafted cells expressing chordin into zebrafish embryos. The results obtained using all three methods show that a reduction in the level of BMP activity leads to an expansion of the preplacodal and placodal region similar to what has been described for neural crest regions. By using conditional constructs of Xiro1, we performed gain and loss of function experiments. We show that Xiro1 play an important role in the specification of both the preplacodal field as well as individual placodes. We have also used inducible dominant negative and activator constructs of Notch signaling components to analyze the role of these factors on placodal development. Our results indicate that the a precise level of BMP activity is required to induce the neural plate border, including placodes and neural crest cells, that in this border the iro1 gene is activated, and that this activation is required for the specification of the placodes.

Ablation of the CLP-1 gene leads to down-regulation of the HAND1 gene and abnormality of the left ventricle of the heart and fetal death

We have recently reported that cardiac lineage protein-1 (CLP-1), a nuclear protein with an acidic region that constitutes a potential protein-protein interaction domain, regulates transcription of the cardiac myosin light chain-2v (MLC-2v) gene promoter in a manner consistent with its being a transcriptional co-activator or regulator. To test the postulate that CLP-1 is a regulator of cardiac genes we ablated the CLP-1 gene in mice. Past embryonic day (E)16.5, CLP-1 null alleles did not show Mendelian inheritance suggesting that absence of CLP-1 was lethal in late fetal stages. CLP-1 (-/-) fetal hearts exhibited a reduced left ventricular chamber with thickened myocardial walls, features suggestive of cardiac hypertrophy. Electron microscopic analysis of E16.5 CLP-1 (-/-) ventricular myocardium showed a marked decline in cell density and altered nuclear and myofibril morphologies similar to that seen in animal models of hypertrophic heart. Analysis of contractile and non-contractile protein genes known to be re-expressed during cardiac hypertrophy showed them to have higher expression levels in CLP-1 (-/-) hearts thereby confirming the hypertrophic phenotype at the molecular level. Analysis of cardiac development genes showed that expression of the HAND1 transcription factor, a gene involved in patterning of the heart tube and down-regulated in hypertrophic hearts, was also significantly reduced in CLP-1 (-/-) fetal hearts. CLP-1 and HAND1 have similar expression patterns in the developing heart ventricles. These data suggest that CLP-1 and the HAND transcription factors may be part of a genetic program critical to proper heart development, perturbation of which can lead to cardiomyopathy.

Molecular Regulation of Cardiac Chamber-Specific Gene Expression

This review focuses on recent studies investigating the genetic regulatory mechanisms leading to formation of morphologically, functionally, and molecularly distinct cardiac chambers. The regulation of four representative chamber-specific genes that have been studied in detail is reviewed. These genes include the atrial-specific genes, myosin light chain-2a (MLC2a), slow myosin heavy chain-3 (slow MyHC3), and atrial natriuretic factor (ANF) and the ventricular specific gene, myosin light chain-2v (MLC2v). Comparison of these promoters reveals some generalizations about the regulatory mechanisms involved in chamber-specific gene expression but, equally, indicates the large gaps in the knowledge concerning this intriguing genetic program.

The Iroquois homeobox gene Irx2 is not essential for normal development of the heart and midbrain-hindbrain boundary in mice

The Iroquois homeobox (Irx) genes have been implicated in the specification and patterning of several organs in Drosophila and several vertebrate species. Misexpression studies of chick, Xenopus, and zebra fish embryos have demonstrated that Irx genes are involved in the specification of the midbrain-hindbrain boundary. All six murine Irx genes are expressed in the developing heart, suggesting that they might possess distinct functions during heart development, and a role for Irx4 in normal heart development has been recently demonstrated by gene-targeting experiments. Here we describe the generation and phenotypic analysis of an Irx2-deficient mouse strain. By targeted insertion of a lacZ reporter gene into the Irx2 locus, we show that lacZ expression reproduces most of the endogenous Irx2 expression pattern. Despite the dynamic expression of Irx2 in the developing heart, nervous system, and other organs, Irx2-deficient mice are viable, are fertile, and appear to be normal. Although chick Irx2 has been implicated in the development of the midbrain-hindbrain region, we show that Irx2-deficient mice develop a normal midbrain-hindbrain boundary. Furthermore, Irx2-deficient mice have normal cardiac morphology and function. Functional compensation by other Irx genes might account for the absence of a phenotype in Irx2-deficient mice. Further studies of mutant mice of other Irx genes as well as compound mutant mice will be necessary to uncover the functional roles of these evolutionarily conserved transcriptional regulators in development and disease.

Cardiac chamber formation: development, genes, and evolution

Concepts of cardiac development have greatly influenced the description of the formation of the four-chambered vertebrate heart. Traditionally, the embryonic tubular heart is considered to be a composite of serially arranged segments representing adult cardiac compartments. Conversion of such a serial arrangement into the parallel arrangement of the mammalian heart is difficult to understand. Logical integration of the development of the cardiac conduction system into the serial concept has remained puzzling as well. Therefore, the current description needed reconsideration, and we decided to evaluate the essentialities of cardiac design, its evolutionary and embryonic development, and the molecular pathways recruited to make the four-chambered mammalian heart. The three principal notions taken into consideration are as follows. 1) Both the ancestor chordate heart and the embryonic tubular heart of higher vertebrates consist of poorly developed and poorly coupled "pacemaker-like" cardiac muscle cells with the highest pacemaker activity at the venous pole, causing unidirectional peristaltic contraction waves. 2) From this heart tube, ventricular chambers differentiate ventrally and atrial chambers dorsally. The developing chambers display high proliferative activity and consist of structurally well-developed and well-coupled muscle cells with low pacemaker activity, which permits fast conduction of the impulse and efficacious contraction. The forming chambers remain flanked by slowly proliferating pacemaker-like myocardium that is temporally prevented from differentiating into chamber myocardium. 3) The trabecular myocardium proliferates slowly, consists of structurally poorly developed, but well-coupled, cells and contributes to the ventricular conduction system. The atrial and ventricular chambers of the formed heart are activated and interconnected by derivatives of embryonic myocardium. The topographical arrangement of the distinct cardiac muscle cells in the forming heart explains the embryonic electrocardiogram (ECG), does not require the invention of nodes, and allows a logical transition from a peristaltic tubular heart to a synchronously contracting four-chambered heart. This view on the development of cardiac design unfolds fascinating possibilities for future research.