Cardiac contraction activates endocardial Notch signaling to modulate chamber maturation in zebrafish

Abstract

Congenital heart disease often features structural abnormalities that emerge during development. Accumulating evidence indicates a crucial role for cardiac contraction and the resulting fluid forces in shaping the heart, yet the molecular basis of this function is largely unknown. Using the zebrafish as a model of early heart development, we investigated the role of cardiac contraction in chamber maturation, focusing on the formation of muscular protrusions called trabeculae. By genetic and pharmacological ablation of cardiac contraction, we showed that cardiac contraction is required for trabeculation through its role in regulating notch1b transcription in the ventricular endocardium. We also showed that Notch1 activation induces expression of ephrin b2a (efnb2a) and neuregulin 1 (nrg1) in the endocardium to promote trabeculation and that forced Notch activation in the absence of cardiac contraction rescues efnb2a and nrg1 expression. Using in vitro and in vivo systems, we showed that primary cilia are important mediators of fluid flow to stimulate Notch expression. Together, our findings describe an essential role for cardiac contraction-responsive transcriptional changes in endocardial cells to regulate cardiac chamber maturation.

Real-time 3D visualization of cellular rearrangements during cardiac valve formation

“…Similarly, Notch signaling could not be detected in endocardial cells in tnnt2a morphants at 75 hpf. These observations are in line with previous reports showing that notch1b expression is undetectable in tnnt2a morphants at 48-50 hpf (Samsa et al, 2015) and is also downregulated in gata2 morphants, which have a lower blood viscosity (Vermot et al, 2009). Taken together, these results indicate that both Notch and Wnt/β-catenin signaling depend on cardiac contraction and/or blood flow.…”

“…After these injections, the signal of the Notch reporter was no longer detectable in endocardial cells at 75 hpf (n=7 larvae) (Fig. 5F,G) in agreement with a recent report (Samsa et al, 2015). Similarly, the strong signal of the Wnt/β-catenin signaling reporter in AVC endocardial cells was downregulated in tnnt2a morphants (n=8 larvae) (Fig.…”

N-cadherin relocalization during cardiac trabeculation

“…Previous studies have shown that Erbb2 signaling is essential for trabeculation (3,6,7,10,11). Moreover, disturbing blood flow or cardiac contractility in the ventricle perturbs trabeculation (4,12,13), suggesting an important role for mechanical forces in this process. Our understanding of the cellular mechanisms regulating trabeculation remains fairly limited.…”

The force within: endocardial development, mechanotransduction and signalling during cardiac morphogenesis

“…Some evidence suggests that Notch signalling may play a role in mechanosensitive pathways (Jahnsen et al, 2015). During zebrafish trabeculation, cilia play a role in sensing fluid forces and in inducing notch1b expression within the ventricular endocardium which, in turn, is required for correct trabeculation (Samsa et al, 2015). Studies in different model systems show that Notch expression defines regions within the embryonic endocardium that correspond with valve and chamber formation, and with ventricular trabeculation (De Luxán et al, 2015).…”

“…Since endocardial Neuregulin is known to instruct myocardial cells to form trabeculae, it is possible that myocardial cells stochastically extend protrusions into the lumen, where they receive cues that determine their fates (Staudt et al, 2014). Shear forces sensed by the endocardium, as well as myocardial stretch forces, are also likely to affect pattern formation during trabeculation (Peshkovsky et al, 2011;Samsa et al, 2015). Thus, although the precise molecular and cellular mechanisms underlying trabeculation remain unclear, this process provides an excellent model for exploring the mechanisms underlying the interactions between endocardial and myocardial cells.…”