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

Heart development: molecular insights into cardiac specification and early morphogenesis

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

TALE class homeodomain gene Irx5 is an immediate downstream target for Hoxb4 transcriptional regulation

The Hox genes are a family of homeodomain-containing transcription factors that determine anteroposterior identity early on in development. Although much is now known about their regulation and function, very little is known of their effector (downstream target) genes. Here, we show that the TALE class homeodomain transcription factor Irx5 is a direct, positively regulated target of Hoxb4.

The developing heart and congenital heart defects: a make or break situation

Congenital heart defects are common in humans, but the underlying basis for these defects is not well understood. It has been clear that abnormal heart development is at the root of these diseases, but the genes involved have remained elusive until recently. This review focuses on recent advances in our understanding of mammalian heart formation, and how some of these processes, when disrupted, lead to congenital heart defects.

Slow myosins in muscle development

Myogenesis has been a system central to investigations on mechanisms of diversification within groups of differentiating cells. Diversity among cell types has been well described in striated muscle tissue at the protein and enzymatic-function levels for decades, but it is only in recent years that some understanding of the molecular mechanisms responsible for this diversity has begun to emerge. Study of the expression of the slow isoforms of the myosin heavy chain has contributed to our understanding of how cell diversity arises within skeletal and cardiac muscle. Slow MyHc isoforms are developmentally responsive to a number of cues provided by the nervous systems, the endocrine system and, later in development, to functional demands on these developing tissues. Perhaps most informative have been studies on the mechanism for regulation of slow MyHc expression in mammals and birds where studies on the calcineurin-NF-AT pathways and nuclear hormone action have been shown to control MyHC gene expression in skeletal muscle and in the developing heart. The mechanisms involved in cell diversification in myogenesis are undoubtedly more varied and complex than those currently offered to explain cell diversification, but these recent studies have broadened our understanding of the interplay between the nervous system, the endocrine system and cell lineages in controlling cell diversification. Greater focus on the first fibers and cardiomyocytes to form in the embryo are likely to bring additional insights into the mechanism crucial for establishing the patterns of diversity required for successful formation of embryonic tissues.

Ventricular septal defect and cardiomyopathy in mice lacking the transcription factor CHF1/Hey2

Ventricular septal defects are common in human infants, but the genetic programs that control ventricular septation are poorly understood. Here we report that mice with a targeted disruption of the cardiovascular basic helix-loop-helix factor (CHF)1Hey2 gene show isolated ventricular septal defects. These defects result primarily in failure to thrive. Mice often succumbed within the first 3 wk after birth and showed pulmonary and liver congestion. The penetrance of this phenotype varied, depending on genetic background, suggesting the presence of modifier genes. Expression patterns of other cardiac-specific genes were not affected. Of the few animals on a mixed genetic background that survived to adulthood, most developed a cardiomyopathy but did not have ventricular septal defects. Our results indicate that CHF1 plays an important role in regulation of ventricular septation in mammalian heart development and is important for normal myocardial contractility. These mice provide a useful model for the study of the ontogeny and natural history of ventricular septal defects and cardiomyopathy.

Homeotic genes autonomously specify the anteroposterior subdivision of the Drosophila dorsal vessel into aorta and heart

The embryonic dorsal vessel in Drosophila possesses anteroposterior polarity and is subdivided into two chamber-like portions, the aorta in the anterior and the heart in the posterior. The heart portion features a wider bore as compared with the aorta and develops inflow valves (ostia) that allow the pumping of hemolymph from posterior toward the anterior. Here, we demonstrate that homeotic selector genes provide positional information that determines the anteroposterior subdivision of the dorsal vessel. Antennapedia (Antp), Ultrabithorax (Ubx), abdominal-A (abd-A), and Abdominal-B (Abd-B) are expressed in distinct domains along the anteroposterior axis within the dorsal vessel, and, in particular, the domain of abd-A expression in cardioblasts and pericardial cells coincides with the heart portion. We provide evidence that loss of abd-A function causes a transformation of the heart into aorta, whereas ectopic expression of abd-A in more anterior cardioblasts causes the aorta to assume heart-like features. These observations suggest that the spatially restricted expression and activity of abd-A determine heart identities in cells of the posterior portion of the dorsal vessel. We also show that Abd-B, which at earlier stages is expressed posteriorly to the cardiogenic mesoderm, represses cardiogenesis. In light of the developmental and morphological similarities between the Drosophila dorsal vessel and the primitive heart tube in early vertebrate embryos, these data suggest that Hox genes may also provide important anteroposterior cues during chamber specification in the developing vertebrate heart.

Early signals in cardiac development

The heart is the first organ to form during embryogenesis and its circulatory function is critical from early on for the viability of the mammalian embryo. Developmental abnormalities of the heart have also been widely recognized as the underlying cause of many congenital heart malformations. Hence, the developmental mechanisms that orchestrate the formation and morphogenesis of this organ have received much attention among classical and molecular embryologists. Due to the evolutionary conservation of many of these processes, major insights have been gained from the studies of a number of vertebrate and invertebrate models, including mouse, chick, amphibians, zebrafish, and Drosophila. In all of these systems, the heart precursors are generated within bilateral fields in the lateral mesoderm and then converge toward the midline to form a beating linear heart tube. The specification of heart precursors is a result of multiple tissue and cell-cell interactions that involve temporally and spatially integrated programs of inductive signaling events. In the present review, we focus on the molecular and developmental functions of signaling processes during early cardiogenesis that have been defined in both vertebrate and invertebrate models. We discuss the current knowledge on the mechanisms through which signals induce the expression of cardiogenic transcription factors and the relationships between signaling pathways and transcriptional regulators that cooperate to control cardiac induction and the formation of a linear heart tube.

Chimeric analysis of retinoic acid receptor function during cardiac looping

Retinoids (vitamin A and its derivatives) play essential roles during vertebrate development. Vitamin A deprivation leads to severe congenital malformations affecting many tissues, including diverse neural crest cell populations and the heart. The vitamin A signal is transduced by the retinoic acid receptors (RARalpha, RARbeta, and RARgamma). However, these receptors exhibit considerable functional redundancy, as judged by the mild phenotype of RAR single null mutants relative to the defects evoked by loss of multiple RARs. To circumvent this redundancy, the endogenous RARgamma2 allele was replaced with a ligand-binding RARgamma mutant (RARgammaE(305)) by gene targeting in mouse embryonic stem (ES) cells. Chimeric embryos derived from hemizygous RARgammaE(305) ES cells displayed several defects similar to those observed in certain RAR double null mutants, including hypoplasia or absence of the caudal pharyngeal arches and myocardial deficiencies. The latter defects were not due to abnormal cardiac specification as affected hearts still expressed chamber-specific markers in an appropriate manner. Chimeras also displayed cardiac looping anomalies, which were associated with a reduction of Pitx2. This work suggests a role for RAR signaling in late looping morphogenesis and illustrates the utility of using a dominant-negative gene substitution approach to circumvent the functional redundancy inherent to the RAR family.

The homeoprotein Xiro1 is required for midbrain-hindbrain boundary formation

The isthmic organizer, which patterns the anterior hindbrain and midbrain, is one of the most studied secondary organizers. In recent years, new insights have been reported on the molecular nature of its morphogenetic activity. Studies in chick, mouse and zebrafish have converged to show that mutually repressive interactions between the homeoproteins encoded by Otx and Gbx genes position this organizer in the neural primordia.

We present evidence that equivalent, in addition to novel, interactions between these and other genes operate in Xenopus embryos to position the isthmic organizer. We made use of fusion proteins in which we combined Otx2 or Gbx2 homeodomains with the E1A activation domain or the EnR repressor element which were then injected into embryos. Our results show that Otx2 and Gbx2 are likely to be transcriptional repressors, and that these two proteins repress each other transcription. Our experiments show that the interaction between these two proteins is required for the positioning of the isthmic organizer genes Fgf8, Pax2 and En2. In this study we also developed a novel in vitro assay for the study of the formation of this organizer. We show that conjugating animal caps previously injected with Otx2 and Gbx2 mRNAs recreate the interactions required for the induction of the isthmic organizer. We have used this assay to determine which cells produce and which cells receive the Fgf signal.

Finally, we have added a novel genetic element to this process, Xiro1, which encode another homeoprotein. We show that the Xiro1 expression domain overlaps with territories expressing Otx2, Gbx2 and Fgf8. By expressing wild-type or dominant negative forms of Xiro1, we show that this gene activates the expression of Gbx2 in the hindbrain. In addition, Xiro1 is required in the Otx2 territory to allow cells within this region to respond to the signals produced by adjacent Gbx2 cells. Moreover, Xiro1 is absolutely required for Fgf8 expression at the isthmic organizer. We discuss a model where Xiro1 plays different roles in regulating the genetic cascade of interactions between Otx2 and Gbx2 that are necessary for the specification of the isthmic organizer.

Transcriptional regulation of vertebrate cardiac morphogenesis

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

Developmental pattern of ANF gene expression reveals a strict localization of cardiac chamber formation in chicken

In mouse, atrial natriuretic factor (ANF) gene expression was shown to be a marker for chamber formation within the embryonic heart. To gain insight into the process of chamber formation in the chicken embryonic heart, we analyzed the expression pattern of cANF during development. We found cANF to be specifically expressed in the myocardium of the morphologically distinguishable atrial and ventricular chambers, similar to ANF in mouse. cANF expression was never detected in the myocardium of the atrioventricular canal (AVC), inner curvature, and outflow tract (OFT), which is lined by endocardial cushions. Expression was strictly excluded from the interventricular myocardium and most proximal part of the bundle branches, as identified by the expression of Msx-2, whereas the rest of the bundle branches, trabeculae, and surrounding working myocardium did express cANF. The myocardium that forms de novo within the cushions after looping did not express cANF. At HH9 cANF expression was first observed in a subset of cardiomyocytes, which was localized ventrally in the fused heart tube and laterally in the unfused cardiac sheets. Together, these results show that cANF expression can be used to distinguish differentiated chamber (working) myocardium, including the peripheral ventricular conduction system, from embryonic myocardium. We conclude that differentiation of chamber myocardium takes place already at HH9 at the ventral side of the linear heart tube, possibly preceded by latero-medial signals in the unfused cardiac sheets.

Cardiac patterning and morphogenesis in zebrafish

Development of the embryonic vertebrate heart requires the precise coordination of pattern formation and cell movement. Taking advantage of the availability of zebrafish mutations that disrupt cardiogenesis, several groups have identified key regulators of specific aspects of cardiac patterning and morphogenesis. Several genes, including gata5, fgf8, bmp2b, one-eyed pinhead, and hand2, have been shown to be relevant to the patterning events that regulate myocardial differentiation. Studies of mutants with morphogenetic defects have indicated at least six genes that are essential for cardiac fusion and heart tube assembly, including casanova, bonnie and clyde, gata5, one-eyed pinhead, hand2, miles apart, and heart and soul. Furthermore, analysis of the jekyll gene has indicated its important role during the morphogenesis of the atrioventricular valve. Altogether, these data provide a substantial foundation for future investigations of cardiac patterning, cardiac morphogenesis, and the relationship between these processes.

Retinoid signaling and cardiac anteroposterior segmentation

Establishment of anterior-posterior polarity is one of the earliest decisions in cardiogenesis. Specification of anterior (outflow) and posterior (inflow) structures ensures proper connections between venous system and inflow tract and between arterial tree and outflow tract. The last few years have witnessed remarkable progress in our understanding of cardiac anteroposterior patterning. Molecular cloning and subsequent studies on RALDH2, the key embryonic retinaldehyde dehydrogenase in retinoic acid (RA) synthesis, provided the missing link between teratogenic studies on RA deficiency and excess and normal chamber morphogenesis. We discuss work establishing the foundations of our current understanding of the mechanisms of cardiac anteroposterior segmentation, the reasons why early evidence pointing to the role of RA in anteroposterior segmentation was overlooked, and the key experiments unraveling the role of RA in cardiac anteroposterior segmentation. We have also integrated recent experiments in a model of cardiac anteroposterior patterning in which RALDH2 expression determines anteroposterior boundaries in the heart field.

A murine model of Holt-Oram syndrome defines roles of the T-box transcription factor Tbx5 in cardiogenesis and disease

Heterozygous Tbx5(del/+) mice were generated to study the mechanisms by which TBX5 haploinsufficiency causes cardiac and forelimb abnormalities seen in Holt-Oram syndrome. Tbx5 deficiency in homozygous mice (Tbx5(del/del)) decreased expression of multiple genes and caused severe hypoplasia of posterior domains in the developing heart. Surprisingly, Tbx5 haploinsufficiency also markedly decreased atrial natriuretic factor (ANF) and connexin 40 (cx40) transcription, implicating these as Tbx5 target genes and providing a mechanism by which 50% reduction of T-box transcription factors cause disease. Direct and cooperative transactivation of the ANF and cx40 promoters by Tbx5 and the homeodomain transcription factor Nkx2-5 was also demonstrated. These studies provide one potential explanation for Holt-Oram syndrome conduction system defects, suggest mechanisms for intrafamilial phenotypic variability, and account for related cardiac malformations caused by other transcription factor mutations.

Irx4 forms an inhibitory complex with the vitamin D and retinoic X receptors to regulate cardiac chamber-specific slow MyHC3 expression

The slow myosin heavy chain 3 gene (slow MyHC3) is restricted in its expression to the atrial chambers of the heart. Understanding its regulation provides a basis for determination of the mechanisms controlling chamber-specific gene expression in heart development. The observed chamber distribution results from repression of slow MyHC3 gene expression in the ventricles. A binding site, the vitamin D response element (VDRE), for a heterodimer of vitamin D receptor (VDR) and retinoic X receptor alpha (RXR alpha) within the slow MyHC3 promoter mediates chamber-specific expression of the gene. Irx4, an Iroquois family homeobox gene whose expression is restricted to the ventricular chambers at all stages of development, inhibits AMHC1, the chick homolog of quail slow MyHC3, gene expression within developing ventricles. Repression of the slow MyHC3 gene in ventricular cardiomyocytes by Irx4 requires the VDRE. Unlike VDR and RXR alpha, Irx4 does not bind directly to the VDRE. Instead two-hybrid and co-immunoprecipitation assays show that Irx4 interacts with the RXR alpha component of the VDR/RXR alpha heterodimer and that the amino terminus of the Irx4 protein is required for its inhibitory action. These observations indicate that the mechanism of atrial chamber-specific expression requires the formation of an inhibitory protein complex composed of VDR, RXR alpha, and Irx4 that binds at the VDRE inhibiting slow MyHC3 expression in the ventricles.

The Iroquois family of genes: from body building to neural patterning

The Iroquois (Iro) family of genes are found in nematodes, insects and vertebrates. They usually occur in one or two genomic clusters of three genes each and encode transcriptional controllers that posses a characteristic homeodomain. The Iro genes function early in development to specify the identity of diverse territories of the body, such as the dorsal head and dorsal mesothorax of Drosophila and the neural plate of Xenopus. In some aspects they act in the same way as classical selector genes, but they display specific properties that place them into a category of their own. Later in development in both Drosophila and vertebrates, the Iro genes function again to subdivide those territories into smaller domains.