Terminal diversification of the myocyte lineage generates Purkinje fibers of the cardiac conduction system

Monitoring clonal growth in the developing ventricle

Understanding the etiology of congenital heart defects depends on a detailed knowledge of the morphogenetic events underlying cardiac development. Deciphering the developmental processes and cell behaviors resulting in the formation of a four-chambered heart requires techniques by which the destiny of individual cells can be traced during development. Ideally, such approaches provide information on progenitor cells and growth properties of clonally related myocytes. In the avian system, clonal analysis based on the use of replication-defective retroviral labeling led to a model for growth of the ventricular wall from polyclonal transmural cones of myocardial cells. In the mouse, the nlaacZ retrospective clonal analysis system has proved to be a powerful technique for studying different aspects of cardiac morphogenesis. Morphologic and histologic analyses of clonally related myocytes at early stages of development have provided genetic evidence for the formation of the heart tube from two cell lineages. Additional aspects of cardiac morphogenesis, including formation of the interventricular septum and myocardial outflow tract, and more recently, the origin of the ventricular conduction system, have been studied using this system. This brief review discusses how the nlaacZ system has provided new insights into the divergent properties of clonally related cells in these different regions of the developing heart.

Alpha actin isoforms expression in human and rat adult cardiac conduction system

In the adult heart, cardiac muscle comprises the working myocardium and the conduction system (CS). The latter includes the sinoatrial node (SAN), the internodal tract or bundle (IB), the atrioventricular node (AVN), the atrioventricular bundle (AVB), the bundle branches (BB) and the peripheral Purkinje fibers (PF). Most of the information concerning the phenotypic features of CS tissue derives from the characterization of avian and rodent developing hearts; data concerning the expression of actin isoforms in adult CS cardiomyocytes are scarce. Using specific antibodies, we investigated the distribution of alpha-skeletal (alpha-SKA), alpha-cardiac (alpha-CA), alpha-smooth muscle (alpha-SMA) actin isoforms and other muscle-typical proteins in the CS of human and rat hearts at different ages. SAN and IB cardiomyocytes were characterized by the presence of alpha-SMA, alpha-CA, calponin and caldesmon, whereas alpha-SKA and vimentin were absent. Double immunofluorescence demonstrated the co-localisation of alpha-SMA and alpha-CA in I-bands of SAN cardiomyocytes. AVN, AVB, BB and PF cardiomyocytes were alpha-SMA, calponin, caldesmon and vimentin negative, and alpha-CA and alpha-SKA positive. No substantial differences in actin isoform distribution were observed in human and rat hearts, except for the presence of isolated subendocardial alpha-SMA positive cardiomyocytes co-expressing alpha-CA in the ventricular septum of the rat. Aging did not influence CS cardiomyocyte actin isoform expression profile. These findings support the concept that cardiomyocytes of SAN retain the phenotype of a developing myogenic cell throughout the entire life span.

Cardiac expression patterns of endothelin-converting enzyme (ECE): implications for conduction system development

The spatiotemporal distribution of the endothelin-converting enzyme (ECE) protein in the embryonic chick heart and the association of this polypeptide with the developing cardiac conduction system is described here for the first time. Further, we show how cardiac hemodynamic load directly affects ECE level and distribution. Endothelin (ET) is a cytokine involved in the inductive recruitment of Purkinje fibers. ET is produced by proteolytic cleavage of Big-ET by ECE. We generated an antibody against chick ECE recognizing a single band at approximately 70 kD to correlate the cardiac expression of this protein with that reported previously for its mRNA. ECE protein expression was more widespread compared to its mRNA, being present in endothelial cells, mesenchymal cells, and myocytes, and particularly enriched in the trabeculae and nascent ventricular conduction system. The myocardial expression was significantly modified under experimentally altered hemodynamic loading. In vivo, ET receptor blockade with bosentan delayed activation sequence maturation. These data support a role for ECE in avian cardiac conduction system differentiation and maturation.

Mouse models of congenital cardiovascular disease

Congenital heart defects occur in nearly 1% of human live births and many are lethal if not surgically repaired. In addition, the genetic contribution to congenital or acquired cardiovascular diseases that are silent at birth, but progress to cause significant disease in later life is being increasingly appreciated. Heart development and structure are highly conserved between mouse and human. The discoveries that are being made in this model system are highly relevant to understanding the pathogenesis of human heart defects whether they occus in isolation, or in the context of a syndrome. Many of the genes required for cardiovascular development were discovered fortuitously when early lethality or structural defects were observed in mouse mutants generated for other purposes, and relevant genes continue to be defined in this manner. Candidate genes for this process are being identified by their roles other species, or by their expression in pertinent tissues in mice. In this review, I will briefly summarize heart development as currently understood in the mouse, and then discuss how complementary studies in mouse and human have identified genes and pathways that are critical for normal cardiovascular development, and for maintaining the structure and function of this organ system throughout life.

Cells migrating from the neural crest contribute to the innervation of the venous pole of the heart

Cells migrating from the neural crest are known to septate the outflow tract of the developing heart, and to contribute to the formation of the arterial valves, their supporting sinuses, the coronary arteries and cardiac neural ganglia. Neural crest cells have also been suggested to contribute to development of the venous pole of the heart, but the extent and fate of such cells remains unclear. In this study, in the mouse, it is shown that cells from the neural crest contribute to the parasympathetic and, to a lesser extent, the sympathetic innervation of the venous pole of the heart. Nerves within the venous pole of the heart are shown to be of mixed origin, with some being derived from the neural crest, while others have an alternative origin, presumably placodal. The neurons innervating the nodal tissue, which can exert chronotropic effects on cardiac conduction, are shown not to be derived from the neural crest. In particular, no evidence was found to support previous suggestions that cells from the neural crest make a direct contribution to the myocardial atrioventricular conduction axis, although a small subset of these cells do co-localize with the developing left bundle branch. We have therefore confirmed that cells from the neural crest migrate to the venous pole of the heart, and that their major role is in the development of the parasympathetic innervation. In addition, in some embryos, a population of cells derived from the neural crest persist in the leaflets of the atrioventricular valves, but their role in subsequent development remains unknown.

Epicardium-derived cells in cardiogenesis and cardiac regeneration

During cardiogenesis, the epicardium grows from the proepicardial organ to form the outermost layer of the early heart. Part of the epicardium undergoes epithelial-mesenchymal transformation, and migrates into the myocardium. These epicardium- derived cells differentiate into interstitial fibroblasts, coronary smooth muscle cells, and perivascular fibroblasts. Moreover, epicardium-derived cells are important regulators of formation of the compact myocardium, the coronary vasculature, and the Purkinje fiber network, thus being essential for proper cardiac development. The fibrous structures of the heart such as the fibrous heart skeleton and the semilunar and atrioventricular valves also depend on a contribution of these cells during development. We hypothesise that the essential properties of epicardium-derived cells can be recapitulated in adult diseased myocardium. These cells can therefore be considered as a novel source of adult stem cells useful in clinical cardiac regeneration therapy.

A molecular pathway including Id2, Tbx5, and Nkx2-5 required for cardiac conduction system development

The cardiac conduction system is an anatomically discrete segment of specialized myocardium that initiates and propagates electrical impulses to coordinate myocardial contraction. To define the molecular composition of the mouse ventricular conduction system we used microdissection and transcriptional profiling by serial analysis of gene expression (SAGE). Conduction-system-specific expression for Id2, a member of the Id gene family of transcriptional repressors, was identified. Analyses of Id2-deficient mice demonstrated structural and functional conduction system abnormalities, including left bundle branch block. A 1.2 kb fragment of the Id2 promoter proved sufficient for cooperative regulation by Nkx2-5 and Tbx5 in vitro and for conduction-system-specific gene expression in vivo. Furthermore, compound haploinsufficiency of Tbx5 and Nkx2-5 or Tbx5 and Id2 prevented embryonic specification of the ventricular conduction system. We conclude that a molecular pathway including Tbx5, Nkx2-5, and Id2 coordinates specification of ventricular myocytes into the ventricular conduction system lineage.

Nkx2.5 cell-autonomous gene function is required for the postnatal formation of the peripheral ventricular conduction system

The ventricular conduction system is responsible for rapid propagation of electrical activity to coordinate ventricular contraction. To investigate the role of the transcription factor Nkx2.5 in the morphogenesis of the ventricular conduction system, we crossed Nkx2.5(+/-) mice with Cx40(eGFP/+) mice in which eGFP expression permits visualization of the His-Purkinje conduction system. Major anatomical and functional disturbances were detected in the His-Purkinje system of adult Nkx2.5(+/-)/Cx40(eGFP/+) mice, including hypoplasia of eGFP-positive Purkinje fibers and the disorganization of the Purkinje fiber network in the ventricular apex. Although the action potential properties of the individual eGFP-positive cells were normal, the deficiency of Purkinje fibers in Nkx2.5 haploinsufficient mice was associated with abnormalities of ventricular electrical activation, including slowed and decremented conduction along the left bundle branch. During embryonic development, eGFP expression in the ventricular trabeculae of Nkx2.5(+/-) hearts was qualitatively normal, with a measurable deficiency in eGFP-positive cells being observed only after birth. Chimeric analyses showed that maximal Nkx2.5 levels are required cell-autonomously. Reduced Nkx2.5 levels are associated with a delay in cell cycle withdrawal in surrounding GFP-negative myocytes. Our results suggest that the formation of the peripheral conduction system is time- and dose-dependent on the transcription factor Nkx2.5 that is cell-autonomously required for the postnatal differentiation of Purkinje fibers.

Characterization of sinoatrial node in four conduction system marker mice

The specialized cardiac conduction system (CCS) consists of the sinoatrial node (SAN) and the atrioventricular (AV) conduction system (AVCS), which includes proximal (AV node, bundle of His and bundle branches) and distal (Purkinje fibers) components. In four CCS marker mice [two transgenic (cGATA6|lacZ, CCS|lacZ) and two targeted gene knock-in (minK|lacZ, Hop|lacZ)] the expression of the lacZ gene (beta-gal) has been reported to mark portions of the proximal and distal AVCS; the expression of this marker in the adult SAN is unknown. The primary objective of this study was to analyze the utility of these marker mice in the identification of the SAN. Intercaval and interventricular septal regions, containing all the components of the CCS, were freshly dissected from adult mice based on the anatomical landmarks and sectioned. Immunohistochemical characterization was performed with SAN markers (Cx45, HCN4), compared to the reporter expression (beta-gal) and markers of the working myocardium (Cx40 and Cx43). In all four of the CCS marker mice, we found that beta-gal expression is consistently observed in the proximal and distal AVCS. However, the presence of lacZ gene expression in the working myocardium outside the CCS and/or the absence of this reporter expression in the SAN prevent the effective use of these mice to identify the SAN, leading us to conclude that none of the four CCS marker mice we studied specifically mark the SAN.

Epicardium-derived cells are important for correct development of the Purkinje fibers in the avian heart

During embryonic development, the proepicardial organ (PEO) grows out over the heart surface to form the epicardium. Following epithelial-mesenchymal transformation, epicardium-derived cells (EPDCs) migrate into the heart and contribute to the developing coronary arteries, to the valves, and to the myocardium. The peripheral Purkinje fiber network develops from differentiating cardiomyocytes in the ventricular myocardium. Intrigued by the close spatial relationship between the final destinations of migrating EPDCs and Purkinje fiber differentiation in the avian heart, that is, surrounding the coronary arteries and at subendocardial sites, we investigated whether inhibition of epicardial outgrowth would disturb cardiomyocyte differentiation into Purkinje fibers. To this end, epicardial development was inhibited mechanically with a membrane, or genetically, by suppressing epicardial epithelial-to-mesenchymal transformation with antisense retroviral vectors affecting Ets transcription factor levels (n=4, HH39-41). In both epicardial inhibition models, we evaluated Purkinje fiber development by EAP-300 immunohistochemistry and found that restraints on EPDC development resulted in morphologically aberrant differentiation of Purkinje fibers. Purkinje fiber hypoplasia was observed both periarterially and at subendocardial positions. Furthermore, the cells were morphologically abnormal and not aligned in orderly Purkinje fibers. We conclude that EPDCs are instrumental in Purkinje fiber differentiation, and we hypothesize that they cooperate directly with endothelial and endocardial cells in the development of the peripheral conduction system.

Development of the cardiac conduction system

The cardiac conduction system (CCS) is a specialized tissue network that initiates and maintains a rhythmic heartbeat. The CCS consists of several functional subcomponents responsible for producing a pacemaking impulse and distributing action potentials across the heart in a coordinated manner. The formation of the distinct subcomponents of the CCS occurs within a precise temporal and spatial framework; thereby assuring that as the system matures from a tubular to a complex chambered organ, a rhythmic heartbeat is always maintained. Therefore, a defect in differentiation of any CCS component would lead to severe rhythm disturbances. Recent molecular, cell biological and physiological approaches have provided fresh and unexpected perspectives of the relationships between cell fate, gene expression and differentiation of specialized function within the developing myocardium. In particular, biomechanical forces created by the heartbeat itself have important roles in the inductive patterning and functional integration of the developing conduction system. This new understanding of the cellular origin and molecular induction of CCS tissues during embryogenesis may provide the foundation for tissue engineering, replacement and repair of these essential cardiac tissues in the future.

Developmental origin of a bipotential myocardial and smooth muscle cell precursor in the mammalian heart

Despite recent advances in delineating the mechanisms involved in cardiogenesis, cellular lineage specification remains incompletely understood. To explore the relationship between developmental fate and potential, we isolated a cardiac-specific Nkx2.5(+) cell population from the developing mouse embryo. The majority of these cells differentiated into cardiomyocytes and conduction system cells. Some, surprisingly, adopted a smooth muscle fate. To address the clonal origin of these lineages, we isolated Nkx2.5(+) cells from in vitro differentiated murine embryonic stem cells and found approximately 28% of these cells expressed c-kit. These c-kit(+) cells possessed the capacity for long-term in vitro expansion and differentiation into both cardiomyocytes and smooth muscle cells from a single cell. We confirmed these findings by isolating c-kit(+)Nkx2.5(+) cells from mouse embryos and demonstrated their capacity for bipotential differentiation in vivo. Taken together, these results support the existence of a common precursor for cardiovascular lineages in the mammalian heart.

Left ventricular myocardal activation under ventricular paced beats in chickens Gallus gallus domesticus

The aim of the study was to advance our knowledge regarding the activation process of the ventricular myocardium in birds in which Purkinje fibres penetrate into the ventricular wall to reach the epicardium. A depolarization pattern of the left ventricular free wall was studied in chickens (Gallus gallus) during ventricular paced beats. Duration of the activation process of the left ventricular free wall is significantly increased during ventricular ectopic excitation as compared with sinus rhythm. Its lowest increase occurs during subendocardial pacing of the middle part of the left ventricle, but its greatest increase is observed during subepicardial pacing of the left ventricular base. Multifocality and mosaicity of depolarization of the left ventricular free wall myocardium in chicken are expressed in a considerably less degree during ventricular paced beats in comparison with sinus rhythm. During ventricular paced beats, excitation of the left ventricular free wall is mostly due to the successive spreading of the depolarization wave from pacing sites.

The time course of engraftment of human mesenchymal stem cells in fetal heart demonstrates that Purkinje fiber aggregates derive from a single cell and not multi-cell homing

OBJECTIVE

To study the early time course of engraftment of human mesenchymal stem cells in fetal sheep heart and determine the relative roles of proliferation and homing in formation of aggregates of human Purkinje fiber cells.

METHODS

The human sheep xenograft model was utilized for these studies. Prior to injection in the preimmune fetus, human cells were labeled with fluorescent dyes to be able to track human cells at early times of engraftment.

RESULTS

Human stem cells were detected in fetal hearts between 29 and 39 hours after intraperitoneal injection. Engraftment was primarily in the Purkinje fiber system. By 45 hours engrafted human cells had a cardiac phenotype. When two groups of human mesenchymal stem cells, each labeled with a different fluorescent dye, were combined prior to injection, aggregates of human Purkinje fiber cells contained cells labeled with either one dye or the other, no aggregate contained cells labeled with both dyes.

CONCLUSIONS

Human mesenchymal stem cells introduced into fetal sheep rapidly enter the myocardium. The swift differentiation into a cardiac phenotype indicates that the cardiac milieu has a strong influence on the fate of engrafting human mesenchymal stem cells. The absence of any aggregates of human Purkinje fiber cells containing both fluorescent dyes demonstrates that each aggregate of human Purkinje fiber cells is derived from a single mesenchymal stem cell and not from homing of multiple cells to a hotspot.

Role of homeodomain-only protein in the cardiac conduction system

Diseases of the cardiac conduction system (CCS) are a significant health issue in adult patients where few therapeutic options exist outside of expensive, device-based procedures. An evolving paradigm pointing toward several key transcription factors required for CCS development and maintenance may be a group of potential targets for reversing or treating degenerative conduction system disease. Recently, a small homeodomain-only protein (Hop) involved with regulating cardiac development has been identified, which is highly expressed in the adult murine CCS. Targeted disruption of the Hop locus leads to infra-nodal conduction defects with downregulation of connexin40 expression within the confines of the CCS. Loss of Hop does not appear to affect the size or distribution of the mature murine CCS and further studies will be required to determine whether Hop is associated with conduction system disease in humans.

Notch signaling plays a key role in cardiac cell differentiation

Results from lineage tracing studies indicate that precursor cells in the ventricles give rise to both cardiac muscle and conduction cells. Cardiac conduction cells are specialized cells responsible for orchestrating the rhythmic contractions of the heart. Here, we show that Notch signaling plays an important role in the differentiation of cardiac muscle and conduction cell lineages in the ventricles. Notch1 expression coincides with a conduction marker, HNK-1, at early stages. Misexpression of constitutively active Notch1 (NIC) in early heart tubes in chick exhibited multiple effects on cardiac cell differentiation. Cells expressing NIC had a significant decrease in expression of cardiac muscle markers, but an increase in expression of conduction cell markers, HNK-1, and SNAP-25. However, the expression of the conduction marker connexin 40 was inhibited. Loss-of-function study, using a dominant-negative form of Suppressor-of-Hairless, further supports that Notch1 signaling is important for the differentiation of these cardiac cell types. Functional studies show that the expression of constitutively active Notch1 resulted in abnormalities in ventricular conduction pathway patterns.

Neural crest cells retain multipotential characteristics in the developing valves and label the cardiac conduction system

Multipotent neural crest cells (NCCs) are a major extracardiac component of cardiovascular development. Although recognized as contributing cells to the arterial valves at early developmental stages, NCC persistence in the valves at later times or in the adult heart is controversial. We analyzed NCC persistence and contributions to both semilunar and atrioventricular (AV) valves in the mature heart. Two NCC-specific promoters driving Cre recombinase, Wnt1-Cre and P0-Cre, were mated with floxed reporter mice, R26R or CAG-CAT-EGFP, to map NCC fate. Hearts were analyzed before aorticopulmonary (AP) septation through adult stages. As previously demonstrated, strong NCC labeling was detected in ventral and dorsal outflow cushions before AP septation. In contrast to previous reports, we found that substantial numbers of labeled cells persisted in the semilunar valves in late fetal, neonatal, and adult hearts. Furthermore, NCCs were also found in the AV valves, almost exclusively in the septal leaflets. NCCs in the AV valves expressed melanocytic and neurogenic markers. However, cells labeled in the proximal cardiac conduction system exhibited neurogenic and gliagenic markers, whereas some NCCs expressed no differentiation specific markers. These results suggest that cardiac NCCs contribute to the mature valves and the cardiac conduction system and retain multipotent characteristics late in development.

Mesp1-nonexpressing cells contribute to the ventricular cardiac conduction system

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

Expression of a two-pore domain K+ channel (TASK-1) in developing avian and mouse ventricular conduction systems

In this study, we report the identification and amino acid sequence of a novel two-pore domain potassium channel (TASK-1) in chicken. This protein, cTASK-1, is highly similar to mouse and human TASK-1 particularly within the pore regions. We describe the expression profile of both chicken and mouse TASK-1 in the embryonic heart as the ventricular conduction system develops. The developmental distribution of TASK-1 is similar in chicken and mouse. Initially, TASK-1 is expressed throughout the myocardium of the early heart tube. However, as cardiogenesis proceeds, ventricular expression becomes restricted to the trabeculated myocardium and eventually the bundle of His, bundle branches, and Purkinje fibers of the mature conduction system. This finding suggests that components of the ventricular conduction system differentiate from TASK-1-positive myocytes of the early heart tube that retain TASK-1 expression as they mature. Our results are consistent with a common mechanism for ventricular conduction system development in avians and mammals, despite differences in the anatomy of the mature conduction systems of these organisms.

Distinct roles of HF-1b/Sp4 in ventricular and neural crest cells lineages affect cardiac conduction system development

The heterogeneous cell types of the cardiac conduction system are responsible for coordinating and maintaining rhythmic contractions of the heart. While it has been shown that the cells of the conduction system are derived from myocytes, additional cell types, including neural crest cells, may play a role in the development and maturation of these specialized cell lineages. Previous work has shown that the expression of the hf-1b gene is required for specification of the cardiac conduction system. Using Cre-Lox technology, we conditionally mutated the hf-1b gene in the ventricular and the neural crest cell lineages. Cx40 immunohistochemistry on HF-1b tissue-restricted knockouts revealed a requirement for HF-1b in the cardiomyogenic lineage. Electrophysiological studies identified a second requirement for HF-1b in the neural crest-derived cells. Absence of HF-1b in the neural crest led to atrial and atrioventricular dysfunction resulting from deficiencies in the neurotrophin receptor trkC. Therefore, in this study, we document that a single transcription factor, HF-1b, acts through two separate cell types to direct distinct functions of the cardiac conduction system.

Differentiation of cardiac Purkinje fibers requires precise spatiotemporal regulation of Nkx2-5 expression

Nkx2-5 gene mutations cause cardiac abnormalities, including deficits of function in the atrioventricular conduction system (AVCS). In the chick, Nkx2-5 is elevated in Purkinje fiber AVCS cells relative to working cardiomyocytes. Here, we show that Nkx2-5 expression rises to a peak as Purkinje fibers progressively differentiate. To disrupt this pattern, we overexpressed Nkx2-5 from embryonic day 10, as Purkinje fibers are recruited within developing chick hearts. Overexpression of Nkx2-5 caused inhibition of slow tonic myosin heavy chain protein (sMHC), a late Purkinje fiber marker but did not affect Cx40 levels. Working cardiomyocytes overexpressing Nkx2-5 in these hearts ectopically up-regulated Cx40 but not sMHC. Isolated embryonic cardiomyocytes overexpressing Nkx2-5 also displayed increased Cx40 and suppressed sMHC. By contrast, overexpression of a human NKX2-5 mutant did not effect these markers in vivo or in vitro, suggesting one possible mechanism for clinical phenotypes. We conclude that a prerequisite for normal Purkinje fiber maturation is precise regulation of Nkx2-5 levels.

Reference guide to the stages of chick heart embryology

Cardiac progenitors of the splanchnic mesoderm (primary and secondary heart field), cardiac neural crest, and the proepicardium are the major embryonic contributors to chick heart development. Their contribution to cardiac development occurs with precise timing and regulation during such processes as primary heart tube fusion, cardiac looping and accretion, cardiac septation, and the development of the coronary vasculature. Heart development is even more complex if one follows the development of the cardiac innervation, cardiac pacemaking and conduction system, endocardial cushions, valves, and even the importance of apoptosis for proper cardiac formation. This review is meant to provide a reference guide (Table 1) on the developmental timing according to the staging of Hamburger and Hamilton (1951) (HH) of these important topics in heart development for those individuals new to a chick heart research laboratory. Even individuals outside of the heart field, who are working on a gene that is also expressed in the heart, will gain information on what to look for during chick heart development. This reference guide provides complete and easy reference to the stages involved in heart development, as well as a global perspective of how these cardiac developmental events overlap temporally and spatially, making it a good bench top companion to the many recently written in-depth cardiac reviews of the molecular aspects of cardiac development.

Absence of Msx2 Does Not Affect Cardiac Conduction or Rescue Conduction Defects Associated with Nkx2-5 Mutation

INTRODUCTION

The transcription factors governing embryonic development of the AV conduction system are largely unknown. Heterozygous mutations of the cardiac transcription factor Nkx2-5 cause AV conduction defects, which are associated with anatomic hypoplasia of the conduction system. In situ expression patterns of Msx2 in the mouse and chick embryonic heart have suggested a developmental function for this transcription factor. Homozygous Nkx2-5 knockout mouse embryos express Msx2 ectopically throughout the myocardium, suggesting Msx2 affects conduction system development through a transcriptional cascade starting with Nkx2-5. Several observations support a model in which Msx2 negatively regulates formation of the conduction system and inappropriate Msx2 up-regulation causes the conduction defects associated with Nkx2-5 mutation.

METHODS AND RESULTS

We obtained surface ECGs and performed intracardiac electrophysiologic studies in Msx2 knockout mice and in Nkx2-5 wild-type and heterozygous null mutant mice in an Msx2 null mutant background. Msx2 null mutant mice had normal cardiac conduction and no increased vulnerability to inducible arrhythmia. Absence of Msx2 did not alter the conduction defects observed in heterozygous Nkx2-5 knockout mice.

CONCLUSION

Msx2 likely does not contribute to development of the conduction system. Abnormal Msx2 expression likely does not cause the AV conduction defects present in Nkx2-5 knockout mice.

Function follows form: cardiac conduction system defects in Nkx2-5 mutation

Mutations of Nkx2-5 cause congenital heart disease and atrioventricular block in man. The altered expression of an electrophysiologic protein regulated by Nkx2-5 was originally presumed to cause the conduction defect, but when no such protein was found, an alternative hypothesis was considered. In pediatric patients, the association of certain cardiac malformations with congenital atrioventricular block suggests that errors in specific developmental pathways could cause both an anatomic and a physiologic defect. We therefore hypothesized that Nkx2-5 insufficiency perturbs the conduction system during development, which in turn manifests as a postnatal conduction defect. Experimental results from Nkx2-5 knockout mouse models support the developmental hypothesis. Hypoplasia of the atrioventricular node, His bundle, and Purkinje system can explain in whole or in part specific conduction and electrophysiologic defects present in Nkx2-5 haploinsufficiency.

Endothelin induces differentiation of ANP-EGFP expressing embryonic stem cells towards a pacemaker phenotype

Currently, only limited insight into mechanisms promoting the differentiation and specification of the mammalian cardiac conduction system is available. Therefore, we established a murine embryonic stem (ES) cell line stably expressing the enhanced green fluorescent protein (EGFP) under the transcriptional control of the human atrial natriuretic peptide (ANP) promoter to further characterize the development of very early stages of the mammalian cardiac conduction tissue. The cardiac nature of ANP-EGFP positive cells was confirmed by immunostaining. In ANP-EGFP expressing ES cell-derived cardiomyocytes, a distinct sublineage of pacemaker cells could be identified. Pacemaker cells displayed a spindle shape and exhibited a higher spontaneous beating rate, faster If current activation and larger If current densities compared with triangular atrial-like cardiocytes. Exposure to endothelin-1 significantly increased the percentage of pacemaker-like cells without affecting their electrophysiological properties. These findings were corroborated by immunostaining with antibodies against connexin 40 and connexin 45, known markers for cardiac conduction tissue. Conversely, treatment of ANP-EGFP expressing ES cells with neuregulin-1 exhibited no effect on differentiation. These results indicate that ANP-EGFP expression enables the identification of ES cell-derived pacemaker cells by their fluorescence and morphology and that endothelin-1 promotes the development of ANP-EGFP positive cardiomyocytes to a pacemaker-like phenotype.

Nkx2-5 mutation causes anatomic hypoplasia of the cardiac conduction system

Heterozygous mutations of the cardiac transcription factor Nkx2-5 cause atrioventricular conduction defects in humans by unknown mechanisms. We show in KO mice that the number of cells in the cardiac conduction system is directly related to Nkx2-5 gene dosage. Null mutant embryos appear to lack the primordium of the atrioventricular node. In Nkx2-5 haploinsufficiency, the conduction system has half the normal number of cells. In addition, an entire population of connexin40(-)/connexin45(+) cells is missing in the atrioventricular node of Nkx2-5 heterozygous KO mice. Specific functional defects associated with Nkx2-5 loss of function can be attributed to hypoplastic development of the relevant structures in the conduction system. Surprisingly, the cellular expression of connexin40, the major gap junction isoform of Purkinje fibers and a putative Nkx2-5 target, is unaffected, consistent with normal conduction times through the His-Purkinje system measured in vivo. Postnatal conduction defects in Nkx2-5 mutation may result at least in part from a defect in the genetic program that governs the recruitment or retention of embryonic cardiac myocytes in the conduction system.

Human mesenchymal stem cells form Purkinje fibers in fetal sheep heart

BACKGROUND

We have investigated the usefulness of a model of cardiac development in a large mammal, sheep, for studies of engraftment of human stem cells in the heart.

METHODS AND RESULTS

Adult and fetal human mesenchymal stem cells were injected intraperitoneally into sheep fetuses in utero. Hearts at late fetal development were analyzed for engraftment of human cells. The majority of the engrafted cells of human origin formed segments of Purkinje fibers containing exclusively human cells. There were no differences in engraftment of human mesenchymal stem cells from adult bone marrow, fetal brain, and fetal liver. On average, 43.2% of the total Purkinje fibers in random areas (n=11) of both ventricles were of human origin. In contrast, approximately 0.01% of cardiomyocytes were of human origin.

CONCLUSIONS

Human mesenchymal stem cells preferentially engraft at high levels in the ventricular conduction system during fetal development in sheep. These findings raise the possibility that stem cells contribute to normal development of the fetal heart.

Hemodynamic-dependent patterning of endothelin converting enzyme 1 expression and differentiation of impulse-conducting Purkinje fibers in the embryonic heart

Impulse-conducting Purkinje fibers differentiate from myocytes during embryogenesis. The conversion of contractile myocytes into conduction cells is induced by the stretch/pressure-induced factor, endothelin (ET). Active ET is produced via proteolytic processing from its precursor by ET-converting enzyme 1 (ECE1) and triggers signaling by binding to its receptors. In the embryonic chick heart, ET receptors are expressed by all myocytes, but ECE1 is predominantly expressed in endothelial cells of coronary arteries and endocardium along which Purkinje fiber recruitment from myocytes takes place. Furthermore, co-expression of exogenous ECE1 and ET-precursor in the embryonic heart is sufficient to ectopically convert cardiomyocytes into Purkinje fibers. Thus, localized expression of ECE1 defines the site of Purkinje fiber recruitment in embryonic myocardium. However, it is not known how ECE1 expression is regulated in the embryonic heart. The unique expression pattern of ECE1 in the embryonic heart suggests that blood flow-induced stress/stretch may play a role in patterning ECE1 expression and subsequent induction of Purkinje fiber differentiation. We show that gadolinium, an antagonist for stretch-activated cation channels, downregulates the expression of ECE1 and a conduction cell marker, Cx40, in ventricular chambers, concurrently with delayed maturation of a ventricular conduction pathway. Conversely, pressure-overload in the ventricle by conotruncal banding results in a significant expansion of endocardial ECE1 expression and Cx40-positive putative Purkinje fibers. Coincident with this, an excitation pattern typical of the mature heart is precociously established. These in vivo data suggest that biomechanical forces acting on, and created by, the cardiovascular system during embryogenesis play a crucial role in Purkinje fiber induction and patterning.

Somatic transgenesis using retroviral vectors in the chicken embryo

The avian embryo is an excellent model system for experimental studies because of its accessibility and ease of microsurgical manipulations. While the complete chicken genome sequence will soon be determined, a comprehensive germ cell transmission-based genetic approach is not available for this animal model. Several techniques of somatic cell transgenesis have been developed in the past decade. Of these, the retroviral shuttle vector system provides both (1) stable integration of exogenous genes into the host cell genome, and (2) constant expression levels in a target cell population over the course of development. This review summarizes retroviral vectors available for the avian model and outlines the uses of retroviral-mediated gene transfer for cell lineage analysis as well as functional studies of genes and proteins in the chick embryo.

Transitions in ventricular activation revealed by two-dimensional optical mapping

While cardiac function in the mature heart is dependent on a properly functioning His-Purkinje system, the early embryonic tubular heart efficiently pumps blood without a distinct specialized conduction system. Although His-Purkinje system precursors have been identified using immunohistological techniques in the looped heart, little is known whether these precursors function electrically. To address this question, we used high-resolution optical mapping and fluorescent dyes with two CCD cameras to describe the motion-corrected activation patterns of 76 embryonic chick hearts from tubular stages (stage 10) to mature septated hearts (stage 35). Ventricular activation in the tubular looped heart (stages 10-17) using both calcium-sensitive fluo-4 and voltage-sensitive di-4-ANEPPS shows sequentially uniform propagation. In late looped hearts (stages 18-22), domains of the dorsal and lateral ventricle are preferentially activated before spreading to the remaining myocardium and show alternating regions of fast and slow propagation. During stages 22-26, action potentials arise from the dorsal ventricle. By stages 27-29, action potential breakthrough is also observed at the right ventricle apex. By stage 31, activation of the heart proceeds from foci at the apex and dorsal surface of the heart. The breakthrough foci correspond to regions where putative conduction system precursors have been identified immunohistologically. To date, our study represents the most detailed electrophysiological characterization of the embryonic heart between the looped and preseptated stages and suggests that ventricular activation undergoes a gradual transformation from sequential to a mature pattern with right and left epicardial breakthroughs. Our investigation suggests that cardiac conduction system precursors may be electrophysiologically distinct and mature gradually throughout cardiac morphogenesis in the chick.

Roles of JUMONJI in mouse embryonic development

Cardiac development is a complex biological process requiring the integration of cell specification, differentiation, migration, proliferation, and morphogenesis. Although significant progress has been made recently in understanding the molecular basis of cardiac development, mechanisms of transcriptional control of cardiac development remain largely unknown. In search for the developmentally important genes, the jumonji gene (jmj) was identified by gene trap technology and characterized as a critical nuclear factor for mouse embryonic development. Jmj has been shown to play important roles in cardiovascular development, neural tube fusion process, hematopoiesis, and liver development in mouse embryos. The amino acid sequence of the JUMONJI protein (JMJ) reveals that JMJ belongs to the AT-rich interaction domain transcription factor family and more recently has been described as a member of the JMJ transcription factor family. Here, we review the roles of jmj in multiple organ development with a focus on cardiovascular development in mice.

Transcriptional regulation of cardiac conduction system development: 2004 FASEB cardiac conduction system minimeeting, Washington, DC

The development of the complex network of specialized cells that form the atrioventricular conduction system (AVCS) during cardiac morphogenesis occurs by progressive recruitment within a multipotent cardiomyogenic lineage. Understanding the molecular control of this developmental process has been the focus of recent research. Transcription factors representative of multiple subfamilies have been identified and include members of zinc-finger subfamilies (GATA4, GATA6 HF-1b), skeletal muscle transcription factors (MyoD), T-box genes (Tbx5), and also homeodomain transcription factors (Msx2 and Nkx2.5). Mutations in some of these transcription factors cause congenital heart disease and are associated with cardiac abnormalities, including deficits within the AVCS. Mouse models that closely phenocopy known human heart disease provide powerful tools for the study of molecular effectors of AVCS development. Indeed, investigations of the Nkx2.5 haploinsufficient mouse have shown that peripheral Purkinje fibers are significantly underrepresented. This piece of data corroborates our previous work showing in chick, mouse, and humans that Nkx2.5 is elevated in the differentiating AVCS relative to adjacent working ventricular myocardial tissues. Using the chick embryo as a model, we show that this elevation of Nkx2.5 is transient in the network of conduction cells comprising the peripheral Purkinje fiber system. Functional studies using defective adenoviral constructs, which disrupt the normal variation in level of this gene, result in perturbations of Purkinje fiber phenotype. Thus, the precise spatiotemporal regulation of Nkx2.5 levels during development may be required for the progressive emergence of gene expression patterns specific to differentiated Purkinje fiber cells.

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.

Wnt11 and Wnt7a are up-regulated in association with differentiation of cardiac conduction cells in vitro and in vivo

The heart beat is coordinated by a precisely timed sequence of action potentials propagated through cells of the conduction system. Previously, we have shown that conduction cells in the chick embryo are derived from multipotent, cardiomyogenic progenitors present in the looped, tubular heart. Moreover, analyses of heterogeneity within myocyte clones and cell birth dating have indicated that elaboration of the conduction system occurs by ongoing, localized recruitment from within this multipotent pool. In this study, we have focused on a potential role for Wnt signaling in development of the cardiac conduction system. Treatment of embryonic myocytes from chick with endothelin-1 (ET-1) has been shown to promote expression of markers of Purkinje fiber cells. By using this in vitro model, we find that Wnt11 are Wnt7a are up-regulated in association with ET-1 treatment. Moreover, in situ hybridization reveals expression, although not temporal coincidence of, Wnt11 and Wnt7a in specialized tissues in the developing heart in vivo. Specifically, whereas Wnt11 shows transient and prominent expression in central elements of the developing conduction system (e.g., the His bundle), relative increases in Wnt7a expression emerge at sites consistent with the location of peripheral conduction cells (e.g., subendocardial Purkinje fibers). The patterns of Wnt11 and Wnt7a expression observed in vitro and in the embryonic chick heart appear to be consistent with roles for these two Wnts in differentiation of cardiac conduction tissues.

Hemodynamics is a key epigenetic factor in development of the cardiac conduction system

The His-Purkinje system (HPS) is a network of conduction cells responsible for coordinating the contraction of the ventricles. Earlier studies using bipolar electrodes indicated that the functional maturation of the HPS in the chick embryo is marked by a topological shift in the sequence of activation of the ventricle. Namely, at around the completion of septation, an immature base-to-apex sequence of ventricular activation was reported to convert to the apex-to-base pattern characteristic of the mature heart. Previously, we have proposed that hemodynamics and/or mechanical conditioning may be key epigenetic factors in development of the HPS. We thus hypothesized that the timing of the topological shift marking maturation of the conduction system is sensitive to variation in hemodynamic load. Spatiotemporal patterns of ventricular activation (as revealed by high-speed imaging of fluorescent voltage-sensitive dye) were mapped in chick hearts over normal development, and following procedures previously characterized as causing increased (conotruncal banding, CTB) or reduced (left atrial ligation, LAL) hemodynamic loading of the embryonic heart. The results revealed that the timing of the shift to mature activation displays striking plasticity. CTB led to precocious emergence of mature HPS function relative to controls whereas LAL was associated with delayed conversion to apical initiation. The results from our study indicate a critical role for biophysical factors in differentiation of specialized cardiac tissues and provide the basis of a new model for studies of the molecular mechanisms involved in induction and patterning of the HPS in vivo.

Development of the cardiac pacemaking and conduction system

The heartbeat is initiated and coordinated by a heterogeneous set of tissues, collectively referred to as the pacemaking and conduction system (PCS). While the structural and physiological properties of these specialized tissues has been studied for more than a century, distinct new insights have emerged in recent years. The tools of molecular biology and the lessons of modern embryology are beginning to uncover the mechanisms governing induction, patterning and developmental integration of the PCS. In particular, significant advances have been made in understanding the developmental biology of the fast conduction network in the ventricles--the His-Purkinje system. Although this progress has largely been made by using animal models such as the chick and mouse, the insights gained may help explain cardiac disease in humans, as well as lead to new treatment strategies.

Connexin 43 (GJA1) mutations cause the pleiotropic phenotype of oculodentodigital dysplasia

Gap junctions are assemblies of intercellular channels that regulate a variety of physiologic and developmental processes through the exchange of small ions and signaling molecules. These channels consist of connexin family proteins that allow for diversity of channel composition and conductance properties. The human connexin 43 gene, or GJA1, is located at human chromosome 6q22-q23 within the candidate region for the oculodentodigital dysplasia locus. This autosomal dominant syndrome presents with craniofacial (ocular, nasal, and dental) and limb dysmorphisms, spastic paraplegia, and neurodegeneration. Syndactyly type III and conductive deafness can occur in some cases, and cardiac abnormalities are observed in rare instances. We found mutations in the GJA1 gene in all 17 families with oculodentodigital dysplasia that we screened. Sixteen different missense mutations and one codon duplication were detected. These mutations may cause misassembly of channels or alter channel conduction properties. Expression patterns and phenotypic features of gja1 animal mutants, reported elsewhere, are compatible with the pleiotropic clinical presentation of oculodentodigital dysplasia.

Cardiac endothelial-myocardial signaling: its role in cardiac growth, contractile performance, and rhythmicity

Experimental work during the past 15 years has demonstrated that endothelial cells in the heart play an obligatory role in regulating and maintaining cardiac function, in particular, at the endocardium and in the myocardial capillaries where endothelial cells directly interact with adjacent cardiomyocytes. The emerging field of targeted gene manipulation has led to the contention that cardiac endothelial-cardiomyocytal interaction is a prerequisite for normal cardiac development and growth. Some of the molecular mechanisms and cellular signals governing this interaction, such as neuregulin, vascular endothelial growth factor, and angiopoietin, continue to maintain phenotype and survival of cardiomyocytes in the adult heart. Cardiac endothelial cells, like vascular endothelial cells, also express and release a variety of auto- and paracrine agents, such as nitric oxide, endothelin, prostaglandin I(2), and angiotensin II, which directly influence cardiac metabolism, growth, contractile performance, and rhythmicity of the adult heart. The synthesis, secretion, and, most importantly, the activities of these endothelium-derived substances in the heart are closely linked, interrelated, and interactive. It may therefore be simplistic to try and define their properties independently from one another. Moreover, in relation specifically to the endocardial endothelium, an active transendothelial physicochemical gradient for various ions, or blood-heart barrier, has been demonstrated. Linkage of this blood-heart barrier to the various other endothelium-mediated signaling pathways or to the putative vascular endothelium-derived hyperpolarizing factors remains to be determined. At the early stages of cardiac failure, all major cardiovascular risk factors may cause cardiac endothelial activation as an adaptive response often followed by cardiac endothelial dysfunction. Because of the interdependency of all endothelial signaling pathways, activation or disturbance of any will necessarily affect the others leading to a disturbance of their normal balance, leading to further progression of cardiac failure.

Molecular embryogenesis of the heart

Development of the heart is a complex process involving primary and secondary heart fields that are set aside to generate myocardial and endocardial cell lineages. The molecular inductions that occur in the primary heart field appear to be recapitulated in induction and myocardial differentiation of the secondary heart field, which adds the conotruncal segments to the primary heart tube. While much is now known about the initial steps and factors involved in induction of myocardial differentiation, little is known about induction of endocardial development. Many of the genes expressed by nascent myocardial cells, which then become committed to a specific heart segment, have been identified and studied. In addition to the heart fields, several other "extracardiac" cell populations contribute to the fully functional mature heart. Less is known about the genetic programs of extracardiac cells as they enter the heart and take part in cardiogenesis. The molecular/genetic basis of many congenital cardiac defects has been elucidated in recent years as a result of new insights into the molecular control of developmental events.

Neuregulin-1 promotes formation of the murine cardiac conduction system

The cardiac conduction system is a network of cells responsible for the rhythmic and coordinated excitation of the heart. Components of the murine conduction system, including the peripheral Purkinje fibers, are morphologically indistinguishable from surrounding cardiomyocytes, and a paucity of molecular markers exists to identify these cells. The murine conduction system develops in close association with the endocardium. Using the recently identified CCS-lacZ line of reporter mice, in which lacZ expression delineates the embryonic and fully mature conduction system, we tested the ability of several endocardial-derived paracrine factors to convert contractile cardiomyocytes into conduction-system cells as measured by ectopic reporter gene expression in the heart. In this report we show that neuregulin-1, a growth and differentiation factor essential for ventricular trabeculation, is sufficient to induce ectopic expression of the lacZ conduction marker. This inductive effect of neuregulin-1 was restricted to a window of sensitivity between 8.5 and 10.5 days postcoitum. Using the whole mouse embryo culture system, neuregulin-1 was shown to regulate lacZ expression within the embryonic heart, whereas its expression in other tissues remained unaffected. We describe the electrical activation pattern of the 9.5-days postcoitum embryonic mouse heart and show that treatment with neuregulin-1 results in electrophysiological changes in the activation pattern consistent with a recruitment of cells to the conduction system. This study supports the hypothesis that endocardial-derived neuregulins may be the major endogenous ligands responsible for inducing murine embryonic cardiomyocytes to differentiate into cells of the conduction system.

Spatiotemporal and tissue specific distribution of apoptosis in the developing chick heart

To investigate spatial and temporal distributions of apoptosis in the embryonic chick heart and its relation to different tissue types, we examined apoptosis in the embryonic chick heart from Hamburger and Hamilton stage 17 through 3 days after hatching. MF20 antibody, alpha-smooth muscle actin (SMA) antibody and EAP-300 antibody were applied to delineate specific cell types. During early development of the embryonic chick heart, very few apoptotic cells were detected. The first distinctive zone of apoptosis was observed in the outflow tract at stage 25. This focus was most prominent during septation of the pulmonary artery from the aorta (i.e., between stages 28 and 29), and diminished to virtually background level by stage 32, except in the subconal regions. Subsequently, remarkable apoptosis appeared in the atrioventricular cushions by stage 26, peaked at stages 29-31, and dropped significantly thereafter. Characteristic distribution patterns of apoptotic cells were also detected in the cardiac conduction tissues, including the His bundle, the bundle branches, and the ventricular trabeculae. After stage 36, cell death dropped to background level, except in developing coronary vessels. MF20 and TUNEL double staining revealed that apoptosis in cardiomyocytes was limited to a few specific regions, much less than in cushion tissues. SMA and TUNEL double staining demonstrated that vascular structures were the major foci of apoptosis from stage 40 to 44, whereas adjacent perivascular Purkinje cells displayed significantly less cell death at these stages. The characteristic spatiotemporal locations of apoptosis parallel the morphologic changes and tissue differentiation during heart development, suggesting that apoptosis is crucial to the transformation of the heart from a simple tube to a complex multichambered pump.

A GATA-6 gene heart-region-specific enhancer provides a novel means to mark and probe a discrete component of the mouse cardiac conduction system

The transcriptional programs that specify the distinct components of the cardiac conduction system are poorly understood, in part due to a paucity of definitive molecular markers. In the present study we show that a cGATA-6 gene enhancer can be used to selectively express transgenes in the atrioventricular (AV) conduction system as it becomes manifest in the developing multichambered mouse heart. Furthermore, our analysis of staged cGATA-6/lacZ embryos revealed that the activity of this heart-region-specific enhancer can be traced back essentially to the outset of the cardiogenic program. We provide evidence that this enhancer reads medial/lateral and anterior/posterior positional information before the heart tube forms and we show that the activity of this enhancer becomes restricted at the heart looping stage to AV myocardial cells that induce endocardial cushion formation. We infer that a deeply-rooted heart-region-specific transcriptional program serves to coordinate AV valve placement and AV conduction system formation. Lastly, we show that cGATA-6/Cre mice can be used to delete floxed genes in the respective subsets of specialized heart cells.