Intracardiac flow dynamics regulate atrioventricular valve morphogenesis.

AIMS: Valvular heart disease is responsible for considerable morbidity and mortality. Cardiac valves develop as the heart contracts, and they function throughout the lifetime of the organism to prevent retrograde blood flow. Their precise morphogenesis is crucial for cardiac function. Zebrafish is an ideal model to investigate cardiac valve development as it allows these studies to be carried out in vivo through non-invasive imaging. Accumulating evidence suggests a role for contractility and intracardiac flow dynamics in cardiac valve development. However, these two factors have proved difficult to uncouple, especially since altering myocardial function affects the intracardiac flow pattern. METHODS AND RESULTS: Here, we describe novel zebrafish models of developmental valve defects. We identified two mutant alleles of myosin heavy chain 6 that can be raised to adulthood despite having only one functional chamber-the ventricle. The adult mutant ventricle undergoes remodelling, and the atrioventricular (AV) valves fail to form four cuspids. In parallel, we characterized a novel mutant allele of southpaw, a nodal-related gene involved in the establishment of left-right asymmetry, which exhibits randomized heart and endoderm positioning. We first observed that in southpaw mutants the relative position of the two cardiac chambers is altered, affecting the geometry of the heart, while myocardial function appears unaffected. Mutant hearts that loop properly or exhibit situs inversus develop normally, whereas midline, unlooped hearts exhibit defects in their transvalvular flow pattern during AV valve development as well as defects in valve morphogenesis. CONCLUSION: Our data indicate that intracardiac flow dynamics regulate valve morphogenesis independently of myocardial contractility.

Synergistic regulation of p53 by Mdm2 and Mdm4 is critical in cardiac endocardial cushion morphogenesis during heart development.

Congenital heart defects (CHDs) are the most prevalent human birth defects. More than 85% of CHDs are thought to result from a combination of genetic susceptibilities and environmental stress. However, the stress-related signalling pathways involved remain largely unknown. The p53 transcription factor is a key tumour suppressor and a central regulator of the cellular stress responses. p53 activities are tightly regulated by its inhibitors Mdm2 and Mdm4 at the post-translational level. Here we used the Cre-loxP system to delete Mdm2 (Tie2Cre;Mdm2(FM/FM) ) or one copy of both Mdm2 and Mdm4 (Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) ) in endothelial/endocardial cells and their derivatives in mice to examine the regulation of the p53/Mdm2-Mdm4 pathway during vascular and cardiovascular development. The Tie2Cre;Mdm2(FM/FM) mice died before embryonic day 10.5 (E10.5) and displayed severe vascular defects. On the other hand, the Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) mice displayed atrial and ventricular septal defects (ASD, VSD) of the heart, leading to severe heart dysfunction and postnatal death. During cardiac endocardial cushion morphogenesis, p53 activation was associated with defects in both the epithelial-mesenchymal transition (EMT) of the endocardial cells and the post-EMT proliferation of the mesenchymal cells, and the valvuloseptal phenotypes of the Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) mice were fully rescued by deletion of one copy of p53. Strikingly, maternal exposure to low-dose X-rays in C57BL/6 mice mimicked the congenital heart malformations seen in the Tie2Cre;Mdm2(FM/+) ; Mdm4(+/-) model, which was also dependent on p53 status, establishing a link between maternal exposures and CHD susceptibility through the p53 pathway. These data revealed a new regulatory mechanism in cardiac endocardial cushion morphogenesis and suggested a possible cause of CHDs due to environmental stress.

Two-photon microscopy-guided femtosecond-laser photoablation of avian cardiogenesis: noninvasive creation of localized heart defects.

Embryonic heart formation is driven by complex feedback between genetic and hemodynamic stimuli. Clinical congenital heart defects (CHD), however, often manifest as localized microtissue malformations with no underlying genetic mutation, suggesting that altered hemodynamics during embryonic development may play a role. An investigation of this relationship has been impaired by a lack of experimental tools that can create locally targeted cardiac perturbations. Here we have developed noninvasive optical techniques that can modulate avian cardiogenesis to dissect relationships between alterations in mechanical signaling and CHD. We used two-photon excited fluorescence microscopy to monitor cushion and ventricular dynamics and femtosecond pulsed laser photoablation to target micrometer-sized volumes inside the beating chick hearts. We selectively photoablated a small (∼100 µm radius) region of the superior atrioventricular (AV) cushion in Hamburger-Hamilton 24 chick embryos. We quantified via ultrasound that the disruption causes AV regurgitation, which resulted in a venous pooling of blood and severe arterial constriction. At 48 h postablation, quantitative X-ray microcomputed tomography imaging demonstrated stunted ventricular growth and pronounced left atrial dilation. A histological analysis demonstrated that the laser ablation produced defects localized to the superior AV cushion: a small quasispherical region of cushion tissue was completely obliterated, and the area adjacent to the myocardial wall was less cellularized. Both cushions and myocardium were significantly smaller than sham-operated controls. Our results highlight that two-photon excited fluorescence coupled with femtosecond pulsed laser photoablation should be considered a powerful tool for studying hemodynamic signaling in cardiac morphogenesis through the creation of localized microscale defects that may mimic clinical CHD.