Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.     

Anatomic features of the interventricular septum in persistent atrioventricular canal

Twenty-two preparations of hearts with preserved atrioventricular canal have been investigated. Primary defect of the interatrial septum is a constant component of this pathological state. The size of the defect defines participation of the atria in formation of the atrioventricular canal. At the same time it is possible to distinguish the following anatomical varieties of abnormal development of the interatrial septum; oblique cleft in the oval fossa, partial absence of the valve in the oval orifice, perforation of the valve in the oval orifice, complete absence of the valve in the oval orifice and certain defects of the interatrial septum situating beyond the oval fossa limits.     

Cor Triatriatum Sinister diagnosed in adult life with three dimensional transesophageal echocardiography

Background

Cor triatriatum is a very rare congenital abnormality, usually symptomatic during childhood, diagnosis in adult age is less common.

Case Presentation

We report the case of a 40 years old woman referred to our hospital for atrial flutter ablation, transthoracic cardiac bidimensional echocardiography showed an abnormal membrane bisecting the left atrium, the diagnosis of cor triatriatum was fully made via three dimensional transesophageal echocardiography. More interstingly three other cardiac anomalies were associated: ostium secundum atrial septal defect, dilated coronary sinus due probably to persistent left superior vena cava and normally functioning bicuspid aortic valve.

Conclusions

Cor triatriatum sinister in adult life is important to recognize because it may be easily surgically correctable when hemodynamically significant. Three Dimensional transesophageal echocardiography is a minimally invasive and highly sensitive diagnostic modality.     

The atypical Rho GTPase,RhoU, regulates cell-adhesion molecules during cardiac morphogenesis

The vertebrate heart undergoes early complex morphologic events in order to develop key cardiac structures that regulate its overall function (Fahed et al., 2013). Although many genetic factors that participate in patterning the heart have been elucidated (Tu and Chi, 2012), the cellular events that drive cardiac morphogenesis have been less clear. From a chemical genetic screen to identify cellular pathways that control cardiac morphogenesis in zebrafish, we observed that inhibition of the Rho signaling pathways resulted in failure to form the atrioventricular canal and loop the linear heart tube. To identify specific Rho proteins that may regulate this process, we analyzed cardiac expression profiling data and discovered that RhoU was expressed at the atrioventricular canal during the time when it forms. Loss of RhoU function recapitulated the atrioventricular canal and cardiac looping defects observed in the ROCK inhibitor treated zebrafish. Similar to its family member RhoV/Chp (Tay et al., 2010), we discovered that RhoU regulates the cell junctions between cardiomyocytes through the Arhgef7b/Pak kinase pathway in order to guide atrioventricular canal development and cardiac looping. Inhibition of this pathway resulted in similar underlying cardiac defects and conversely, overexpression of a PAK kinase was able to rescue the loss of RhoU cardiac defect. Finally, we found that Wnt signaling, which has been implicated in atrioventricular canal development (Verhoeven et al., 2011), may regulate the expression of RhoU at the atrioventricular canal. Overall, these findings reveal a cardiac developmental pathway involving RhoU/Arhgef7b/Pak signaling, which helps coordinate cell junction formation between atrioventricular cardiomyocytes to promote cell adhesiveness and cell shapes during cardiac morphogenesis. Failure to properly form these cell adhesions during cardiac development may lead to structural heart defects and mechanistically account for the cellular events that occur in certain human congenital heart diseases.     

The sinus venosus myocardium contributes to the atrioventricular canal: potential role during atrioventricular node development?

The presence of distinct electrophysiological pathways within the atrioventricular node (AVN) is a prerequisite for atrioventricular nodal reentrant tachycardia to occur. In this study, the different cell contributions that may account for the anatomical and functional heterogeneity of the AVN were investigated. To study the temporal development of the AVN, the expression pattern of ISL1, expressed in cardiac progenitor cells, was studied in sequential stages performing co‐staining with myocardial markers (TNNI2 and NKX2‐5) and HCN4 (cardiac conduction system marker). An ISL1+/TNNI2+/HCN4+ continuity between the myocardium of the sinus venosus and atrioventricular canal was identified in the region of the putative AVN, which showed a pacemaker‐like phenotype based on single cell patch‐clamp experiments. Furthermore, qPCR analysis showed that even during early development, different cell populations can be identified in the region of the putative AVN. Fate mapping was performed by in ovo vital dye microinjection. Embryos were harvested and analysed 24 and 48 hrs post‐injection. These experiments showed incorporation of sinus venosus myocardium in the posterior region of the atrioventricular canal. The myocardium of the sinus venosus contributes to the atrioventricular canal. It is postulated that the myocardium of the sinus venosus contributes to nodal extensions or transitional cells of the AVN since these cells are located in the posterior region of the AVN. This finding may help to understand the origin of atrioventricular nodal reentrant tachycardia.     

The novel transmembrane protein Tmem2 is essential for coordination of myocardial and endocardial morphogenesis

Coordination between adjacent tissues plays a crucial role during the morphogenesis of developing organs. In the embryonic heart, two tissues - the myocardium and the endocardium - are closely juxtaposed throughout their development. Myocardial and endocardial cells originate in neighboring regions of the lateral mesoderm, migrate medially in a synchronized fashion, collaborate to create concentric layers of the heart tube, and communicate during formation of the atrioventricular canal. Here, we identify a novel transmembrane protein, Tmem2, that has important functions during both myocardial and endocardial morphogenesis. We find that the zebrafish mutation frozen ventricle (frv) causes ectopic atrioventricular canal characteristics in the ventricular myocardium and endocardium, indicating a role of frv in the regional restriction of atrioventricular canal differentiation. Furthermore, in maternal-zygotic frv mutants, both myocardial and endocardial cells fail to move to the midline normally, indicating that frv facilitates cardiac fusion. Positional cloning reveals that the frv locus encodes Tmem2, a predicted type II single-pass transmembrane protein. Homologs of Tmem2 are present in all examined vertebrate genomes, but nothing is known about its molecular or cellular function in any context. By employing transgenes to drive tissue-specific expression of tmem2, we find that Tmem2 can function in the endocardium to repress atrioventricular differentiation within the ventricle. Additionally, Tmem2 can function in the myocardium to promote the medial movement of both myocardial and endocardial cells. Together, our data reveal that Tmem2 is an essential mediator of myocardium-endocardium coordination during cardiac morphogenesis.     

Selective His-bundle pacing in an adult with atrioventricular canal defect via retrograde His localization

Adult congenital heart disease patients may undergo numerous fluoroscopically guided procedures including pacemaker implantation during their lifetime. One alternative to traditional pacemaker setup which may improve long-term pacing outcomes is His bundle pacing. Given the altered His-bundle location, and given increased radiation exposure over a lifetime, we used 3-dimensional mapping to locate the His and to minimize fluoroscopy for placement of a His-bundle pacemaker system in a 31-year old patient with atrioventricular canal defect and complete heart block with 100% RV pacing and epicardial lead fracture.MethodsAn Octapolar Livewire catheter (Abbott, Minneapolis, USA) was used for mapping and location of the His bundle from a right femoral venous access on the EnSite Precision system 3-dimensional mapping system (Abbott Medical, Abbott Park, IL). The same map was used to guide 3830 lead placement into the posterior-inferior His-bundle position.ResultsSuccessful placement of a His-bundle pacing system with thresholds of 1Volt@0.4ms for both the atrial and ventricular leads with selective His-bundle pacing noted. Ten-month follow-up demonstrated His-bundle capture at 0.75V@0.4ms with stable impedance, sensing and with 100% right ventricular pacing a projected longevity of 12 years total.ConclusionsSuccessful placement of selective His-bundle pacing can be achieved in an adult patient with atrioventricular canal defect using 3-dimensional mapping.     

Atrioventricular cushion transformation is mediated by ALK2 in the developing mouse heart

Developmental abnormalities in endocardial cushions frequently contribute to congenital heart malformations including septal and valvular defects. While compelling evidence has been presented to demonstrate that members of the TGF-beta superfamily are capable of inducing endothelial-to-mesenchymal transdifferentiation in the atrioventricular canal, and thus play a key role in formation of endocardial cushions, the detailed signaling mechanisms of this important developmental process, especially in vivo, are still poorly known. Several type I receptors (ALKs) for members of the TGF-beta superfamily are expressed in the myocardium and endocardium of the developing heart, including the atrioventricular canal. However, analysis of their functional role during mammalian development has been significantly complicated by the fact that deletion of the type I receptors in mouse embryos often leads to early embryonal lethality. Here, we used the Cre/loxP system for endothelial-specific deletion of the type I receptor Alk2 in mouse embryos. The endothelial-specific Alk2 mutant mice display defects in atrioventricular septa and valves, which result from a failure of endocardial cells to appropriately transdifferentiate into the mesenchyme in the AV canal. Endocardial cells deficient in Alk2 demonstrate decreased expression of Msx1 and Snail, and reduced phosphorylation of BMP and TGF-beta Smads. Moreover, we show that endocardial cells lacking Alk2 fail to delaminate from AV canal explants. Collectively, these results indicate that the BMP type I receptor ALK2 in endothelial cells plays a critical non-redundant role in early phases of endocardial cushion formation during cardiac morphogenesis.     

Invasion of mesenchyme into three-dimensional collagen gels: a regional and temporal analysis of interaction in embryonic heart tissue

In normal heart development the endothelium of the atrioventricular canal, but not the ventricle, produces mesenchymal cells which seed (invade) into the intervening extracellular matrix toward the myocardium at around 64-69 hr of development. We have utilized three-dimensional collagen substrates to examine the initiation of seeding by atrioventricular canal endothelia in vitro and to compare and contrast the responses of the ventricular endothelia. Explants of atrioventricular canals and ventricles from staged embryos were placed on the surfaces of collagen gels prior to the onset of seeding in situ. At varied intervals of incubation, the explant was removed, leaving behind a monolayer on the surface of the gel which consisted of endothelial cells. Subsequently, the endothelial outgrowths were examined for seeded cells. The results confirm the regional endothelial differences seen in vivo. They also show that invasion of the collagen gels is due to an alteration in phenotype mediated by interaction with other components of embryonic heart explant. Lastly, the time course of this tissue interaction in vitro mimics the onset of seeding in vivo.     

A critical role for the EphA3 receptor tyrosine kinase in heart development

Eph proteins are receptor tyrosine kinases that control changes in cell shape and migration during development. We now describe a critical role for EphA3 receptor signaling in heart development as revealed by the phenotype of EphA3 null mice. During heart development mesenchymal outgrowths, the atrioventricular endocardial cushions, form in the atrioventricular canal. This morphogenetic event requires endocardial cushion cells to undergo an epithelial to mesenchymal transformation (EMT), and results in the formation of the atrioventricular valves and membranous portions of the atrial and ventricular septa. We show that EphA3 knockouts have significant defects in the development of their atrial septa and atrioventricular endocardial cushions, and that these cardiac abnormalities lead to the death of approximately 75% of homozygous EphA3(-/-) mutants. We demonstrate that EphA3 and its ligand, ephrin-A1, are expressed in adjacent cells in the developing endocardial cushions. We further demonstrate that EphA3(-/-) atrioventricular endocardial cushions are hypoplastic compared to wildtype and that EphA3(-/-) endocardial cushion explants give rise to fewer migrating mesenchymal cells than wildtype explants. Thus our results indicate that EphA3 plays a crucial role in the development and morphogenesis of the cells that give rise to the atrioventricular valves and septa.     

Migratory behavior of cardiac cushion tissue cells in a collagen-lattice culture system

A culture system was devised for the study of factors which influence the migration of cardiac cushion tissue cells. Explants of isolated chick atrioventricular canal cushions were placed on hydrated collagen lattices. Cells grew out of the explants from the endocardium and across the surface of the collagen lattices. During further incubation, mesenchyme-type cells seeded from the surface population into the underlying collagen matrix. These cells were morphologically similar to the mesenchymal cushion tissue cells which are derived from the endocardium and which migrate into and through the cardiac jelly matrix in the embryonic heart. The events observed in the culture system mimicked those occurring in the developing chick atrioventricular cushion.     

Fine structure of postgonopodial glands of a myriapod Glomeris marginata (Villers)

The postgonopodial gland of the myriapod Glomeris marginata (Villers), which produces a pheromone, is an integumentary gland comprising numerous functional secretory units. Each secretory unit consists of two proximal secretory cells, an intermediary cell lacking secretory characteristics and a canal cell surrounding the canal, which is secretory in nature. Secretory proximal cells exhibit a zone of small channels originating from invaginations of the plasma membrane and through which secreted material is released. Apposing each invagination of these cells is a corresponding invagination of the intermediary cell: the two units in the centre of the intermediary cell join another which communicates with the canal. Secretion produced by the latter passes through the canal wall and blends with secretion of the two proximal cells. The most striking feature of all these cells is the abundance of tubules and fibrils in the small canal zone in the proximal cells, which also exhibits a centriole; in the intermediary cell around cytoplasmic membrane invaginations where a diplosome is present, and in almost the entire canal cell.     

High-speed imaging of developing heart valves reveals interplay of morphogenesis and function

Knowing how mutations disrupt the interplay between atrioventricular valve (AVV) morphogenesis and function is crucial for understanding how congenital valve defects arise. Here, we use high-speed fluorescence microscopy to investigate AVV morphogenesis in zebrafish at cellular resolution. We find that valve leaflets form directly through a process of invagination, rather than first forming endocardial cushions. There are three phases of valve function in embryonic development. First, the atrioventricular canal (AVC) is closed by the mechanical action of the myocardium, rolls together and then relaxes. The growing valve leaflets serve to block the canal during the roll and, depending on the developmental stage, either expand or hang down as a leaflet to block the canal. These steps are disrupted by the subtle morphological changes that result from inhibiting ErbB-, TGFbeta-or Cox2 (Ptgs2)-dependent signaling. Cox2 inhibition affects valve development due to its effect on myocardial cell size and shape, which changes the morphology of the ventricle and alters valve geometry. Thus, different signaling pathways regulate distinct aspects of the behavior of individual cells during valve morphogenesis, thereby influencing specific facets of valve function.     

Linkage analysis of autosomal dominant atrio ventricular canal defects: exclusion of chromosome 21

The association between trisomy 21 and a high incidence of atrioventricular canal defects (AVCDs) indicates that a locus on chromosome 21 is involved in this congenital heart defect. We have investigated whether a genetic locus on chromosome 21 is also involved in familial nonsyndromic AVCDs. Short tandem repeat polymorphisms (STRPs) from chromosome 21 were used for linkage analysis of a family having multiple members affected with AVCDs. In this family, the gene for AVCDs is transmitted as an autosomal dominant with incomplete penetrance. The affected family members are nonsyndromic and have normal karyotypes. Two-point and multipoint linkage analyses produced significantly negative LOD scores for all informative markers. A comparison of the overlapping exclusion distances obtained for each marker at LOD equal -2.0 with the 1000:1 consensus genetic map of the markers, excludes chromosome 21 as the genetic location for AVCDs in this family. The exclusion of chromosome 21 indicates that another gene, not located on chromosome 21, is involved in atrioventricular canal defect formation.     

The development of the conduction system in the mouse embryo heart: I. The first embryonic A-V conduction pathway

In the 8-, 9-, and 10-day-old mouse embryos, the primitive atria are interconnected with the ventricles via the atrioventricular (A-V) canal. Due to the twisting process of the tubular heart, the wall of the A-V canal establishes continuity not only with the left ventricle but also with the bulbus and truncus arteriosus. At this stage of heart development, the A-V node and bundle have not yet appeared, and, thus, the atrial impulse must be conveyed to the ventricle by the muscle tissue of the wall of the A-V canal, in which two muscle cell layers have been observed. The inner layer extends deep into the left ventricular cavity and is interconnected with both the trabecular system and the ventricular (IV) septum, which begins to develop on the tenth day. In the inner dorsal wall of the A-V canal, the cells are large (～ 20 μm in diameter) and show a strong PAS reaction. It is likely that these large glycogen-rich cells from which the A-V node primordium develops on the eleventh day play the main role in the A-V impulse conduction. The muscle cells at the ventrolateral walls of the canal are small and form a loose spongy myocardium into which the connective tissue cells begin to penetrate on the tenth day, ultimately to form the annulus fibrosus. At the same time, the outer cell layer of the dorsal wall begins to deteriorate; the cells show vacuolar degeneration, myolysis, and shrinkage necrosis. This process appears to represent a programmed cell death, as was described in the bird heart (Pexieder, 1975). On the basis of morphological data, the sequence of atrioventricular activation before the appearance of the A-V node and bundle is discussed.     

Development of the Hearts of Lizards and Snakes and Perspectives to Cardiac Evolution

Birds and mammals both developed high performance hearts from a heart that must have been reptile-like and the hearts of extant reptiles have an unmatched variability in design. Yet, studies on cardiac development in reptiles are largely old and further studies are much needed as reptiles are starting to become used in molecular studies. We studied the growth of cardiac compartments and changes in morphology principally in the model organism corn snake (Pantherophis guttatus), but also in the genotyped anole (Anolis carolinenis and A. sagrei) and the Philippine sailfin lizard (Hydrosaurus pustulatus). Structures and chambers of the formed heart were traced back in development and annotated in interactive 3D pdfs. In the corn snake, we found that the ventricle and atria grow exponentially, whereas the myocardial volumes of the atrioventricular canal and the muscular outflow tract are stable. Ventricular development occurs, as in other amniotes, by an early growth at the outer curvature and later, and in parallel, by incorporation of the muscular outflow tract. With the exception of the late completion of the atrial septum, the adult design of the squamate heart is essentially reached halfway through development. This design strongly resembles the developing hearts of human, mouse and chicken around the time of initial ventricular septation. Subsequent to this stage, and in contrast to the squamates, hearts of endothermic vertebrates completely septate their ventricles, develop an insulating atrioventricular plane, shift and expand their atrioventricular canal toward the right and incorporate the systemic and pulmonary venous myocardium into the atria.