Cardiac forces regulate zebrafish heart valve delamination by modulating Nfat signaling.

In the clinic, most cases of congenital heart valve defects are thought to arise through errors that occur after the endothelial-mesenchymal transition (EndoMT) stage of valve development. Although mechanical forces caused by heartbeat are essential modulators of cardiovascular development, their role in these later developmental events is poorly understood. To address this question, we used the zebrafish superior atrioventricular valve (AV) as a model. We found that cellularized cushions of the superior atrioventricular canal (AVC) morph into valve leaflets via mesenchymal-endothelial transition (MEndoT) and tissue sheet delamination. Defects in delamination result in thickened, hyperplastic valves, and reduced heart function. Mechanical, chemical, and genetic perturbation of cardiac forces showed that mechanical stimuli are important regulators of valve delamination. Mechanistically, we show that forces modulate Nfatc activity to control delamination. Together, our results establish the cellular and molecular signature of cardiac valve delamination in vivo and demonstrate the continuous regulatory role of mechanical forces and blood flow during valve formation.

klf2a couples mechanotransduction and zebrafish valve morphogenesis through fibronectin synthesis.

The heartbeat and blood flow signal to endocardial cell progenitors through mechanosensitive proteins that modulate the genetic program controlling heart valve morphogenesis. To date, the mechanism by which mechanical forces coordinate tissue morphogenesis is poorly understood. Here we use high-resolution imaging to uncover the coordinated cell behaviours leading to heart valve formation. We find that heart valves originate from progenitors located in the ventricle and atrium that generate the valve leaflets through a coordinated set of endocardial tissue movements. Gene profiling analyses and live imaging reveal that this reorganization is dependent on extracellular matrix proteins, in particular on the expression of fibronectin1b. We show that blood flow and klf2a, a major endocardial flow-responsive gene, control these cell behaviours and fibronectin1b synthesis. Our results uncover a unique multicellular layering process leading to leaflet formation and demonstrate that endocardial mechanotransduction and valve morphogenesis are coupled via cellular rearrangements mediated by fibronectin synthesis.

Developmental Alterations in Heart Biomechanics and Skeletal Muscle Function in Desmin Mutants Suggest an Early Pathological Root for Desminopathies.

Desminopathies belong to a family of muscle disorders called myofibrillar myopathies that are caused by Desmin mutations and lead to protein aggregates in muscle fibers. To date, the initial pathological steps of desminopathies and the impact of desmin aggregates in the genesis of the disease are unclear. Using live, high-resolution microscopy, we show that Desmin loss of function and Desmin aggregates promote skeletal muscle defects and alter heart biomechanics. In addition, we show that the calcium dynamics associated with heart contraction are impaired and are associated with sarcoplasmic reticulum dilatation as well as abnormal subcellular distribution of Ryanodine receptors. Our results demonstrate that desminopathies are associated with perturbed excitation-contraction coupling machinery and that aggregates are more detrimental than Desmin loss of function. Additionally, we show that pharmacological inhibition of aggregate formation and Desmin knockdown revert these phenotypes. Our data suggest alternative therapeutic approaches and further our understanding of the molecular determinants modulating Desmin aggregate formation.

Oscillatory Flow Modulates Mechanosensitive klf2a Expression through trpv4 and trpp2 during Heart Valve Development.

In vertebrates, heart pumping is required for cardiac morphogenesis and altering myocardial contractility leads to abnormal intracardiac flow forces and valve defects. Among the different mechanical cues generated in the developing heart, oscillatory flow has been proposed to be an essential factor in instructing endocardial cell fate toward valvulogenesis and leads to the expression of klf2a, a known atheroprotective transcription factor. To date, the mechanism by which flow forces are sensed by endocardial cells is not well understood. At the onset of valve formation, oscillatory flows alter the spectrum of the generated wall shear stress (WSS), a key mechanical input sensed by endothelial cells. Here, we establish that mechanosensitive channels are activated in response to oscillatory flow and directly affect valvulogenesis by modulating the endocardial cell response. By combining live imaging and mathematical modeling, we quantify the oscillatory content of the WSS during valve development and demonstrate it sets the endocardial cell response to flow. Furthermore, we show that an endocardial calcium response and the flow-responsive klf2a promoter are modulated by the oscillatory flow through Trpv4, a mechanosensitive ion channel specifically expressed in the endocardium during heart valve development. We made similar observations for Trpp2, a known Trpv4 partner, and show that both the absence of Trpv4 or Trpp2 leads to valve defects. This work identifies a major mechanotransduction pathway involved during valve formation in vertebrates.

Endothelial cilia mediate low flow sensing during zebrafish vascular development.

VIDEO ABSTRACT: The pattern of blood flow has long been thought to play a significant role in vascular morphogenesis, yet the flow-sensing mechanism that is involved at early embryonic stages, when flow forces are low, remains unclear. It has been proposed that endothelial cells use primary cilia to sense flow, but this has never been tested in vivo. Here we show, by noninvasive, high-resolution imaging of live zebrafish embryos, that endothelial cilia progressively deflect at the onset of blood flow and that the deflection angle correlates with calcium levels in endothelial cells. We demonstrate that alterations in shear stress, ciliogenesis, or expression of the calcium channel PKD2 impair the endothelial calcium level and both increase and perturb vascular morphogenesis. Altogether, these results demonstrate that endothelial cilia constitute a highly sensitive structure that permits the detection of low shear forces during vascular morphogenesis.