The E3 ubiquitin-protein ligase Rbx1 regulates cardiac wall morphogenesis in zebrafish

Cardiac trabeculae are muscular ridge-like structures within the ventricular wall that are crucial for cardiac function. In zebrafish, these structures first form primarily through the delamination of compact wall cardiomyocytes (CMs). Although defects in proteasomal degradation have been associated with decreased cardiac function, whether they also affect cardiac development has not been extensively analyzed. Here we report a role during cardiac wall morphogenesis in zebrafish for the E3 ubiquitin-protein ligase Rbx1, which has been shown to regulate the degradation of key signaling molecules. Although development is largely unperturbed in zebrafish rbx1 mutant larvae, they exhibit CM multi-layering. This phenotype is not affected by blocking ErbB signaling, but fails to manifest itself in the absence of blood flow/cardiac contractility. Surprisingly, rbx1 mutants display ErbB independent Notch reporter expression in the myocardium. We generated tissue-specific rbx1 overexpression lines and found that endothelial, but not myocardial, specific rbx1 expression normalizes the cardiac wall morphogenesis phenotype. In addition, we found that pharmacological activation of Hedgehog signaling ameliorates the multi-layered myocardial wall phenotype in rbx1 mutants. Collectively, our data indicate that endocardial activity of Rbx1 is essential for cardiac wall morphogenesis.

In vivo analysis of cardiomyocyte proliferation during trabeculation

Cardiomyocyte proliferation is crucial for cardiac growth, patterning and regeneration; however, few studies have investigated the behavior of dividing cardiomyocytes in vivo Here, we use time-lapse imaging of beating hearts in combination with the FUCCI system to monitor the behavior of proliferating cardiomyocytes in developing zebrafish. Confirming in vitro observations, sarcomere disassembly, as well as changes in cell shape and volume, precede cardiomyocyte cytokinesis. Notably, cardiomyocytes in zebrafish embryos and young larvae mostly divide parallel to the myocardial wall in both the compact and trabecular layers, and cardiomyocyte proliferation is more frequent in the trabecular layer. While analyzing known regulators of cardiomyocyte proliferation, we observed that the Nrg/ErbB2 and TGFβ signaling pathways differentially affect compact and trabecular layer cardiomyocytes, indicating that distinct mechanisms drive proliferation in these two layers. In summary, our data indicate that, in zebrafish, cardiomyocyte proliferation is essential for trabecular growth, but not initiation, and set the stage to further investigate the cellular and molecular mechanisms driving cardiomyocyte proliferation in vivo.

Opposite effects of Activin type 2 receptor ligands on cardiomyocyte proliferation during development and repair

Zebrafish regenerate damaged myocardial tissue very effectively. Hence, insights into the molecular networks underlying zebrafish heart regeneration might help develop alternative strategies to restore human cardiac performance. While TGF-β signaling has been implicated in zebrafish cardiac regeneration, the role of its individual ligands remains unclear. Here, we report the opposing expression response during zebrafish heart regeneration of two genes, mstnb and inhbaa, which encode TGF-β family ligands. Using gain-of-function (GOF) and loss-of-function (LOF) approaches, we show that these ligands mediate inverse effects on cardiac regeneration and specifically on cardiomyocyte (CM) proliferation. Notably, we find that Inhbaa functions as a CM mitogen and that its overexpression leads to accelerated cardiac recovery and scar clearance after injury. In contrast, mstnb GOF and inhbaa LOF both lead to unresolved scarring after cardiac injury. We further show that Mstnb and Inhbaa inversely control Smad2 and Smad3 transcription factor activities through alternate Activin type 2 receptors.

Zebrafish can regenerate damaged myocardial tissue but it is unclear how this is regulated. Here, the authors show that two TGF-β family members, Mstnb and Inhbaa, have opposite effects in regeneration, with mstnb overexpression or inhbaa loss-of-function causing cardiac scarring after injury.

In Vivo Visualization of Cardiomyocyte Apicobasal Polarity Reveals Epithelial to Mesenchymal-like Transition during Cardiac Trabeculation

Despite great strides in understanding cardiac trabeculation, many mechanistic aspects remain unclear. To elucidate how cardiomyocyte shape changes are regulated during this process, we engineered transgenes to label their apical and basolateral membranes. Using these tools, we observed that compact-layer cardiomyocytes are clearly polarized while delaminating cardiomyocytes have lost their polarity. The apical transgene also enabled the imaging of cardiomyocyte apical constriction in real time. Furthermore, we found that Neuregulin signaling and blood flow/cardiac contractility are required for cardiomyocyte apical constriction and depolarization. Notably, we observed the activation of Notch signaling in cardiomyocytes adjacent to those undergoing apical constriction, and we showed that this activation is positively regulated by Neuregulin signaling. Inhibition of Notch signaling did not increase the percentage of cardiomyocytes undergoing apical constriction or of trabecular cardiomyocytes. These studies provide information about cardiomyocyte polarization and enhance our understanding of the complex mechanisms underlying ventricular morphogenesis and maturation.

Regulation of cardiomyocyte behavior in zebrafish trabeculation by Neuregulin 2a signaling

Trabeculation is crucial for cardiac muscle growth in vertebrates. This process requires the Erbb2/4 ligand Neuregulin (Nrg), secreted by the endocardium, as well as blood flow/cardiac contractility. Here, we address two fundamental, yet unresolved, questions about cardiac trabeculation: why does it initially occur in the ventricle and not the atrium, and how is it modulated by blood flow/contractility. Using loss-of-function approaches, we first show that zebrafish Nrg2a is required for trabeculation, and using a protein-trap line, find that it is expressed in both cardiac chambers albeit with different spatiotemporal patterns. Through gain-of-function experiments, we show that atrial cardiomyocytes can also respond to Nrg2a signalling, suggesting that the cardiac jelly, which remains prominent in the atrium, represents a barrier to Erbb2/4 activation. Furthermore, we find that blood flow/contractility is required for Nrg2a expression, and that while non-contractile hearts fail to trabeculate, non-contractile cardiomyocytes are also competent to respond to Nrg2a/Erbb2 signalling.

Cardiac trabeculae (which are sponge-like muscular structures) form mostly as a result of cardiomyocyte (CM) delamination in zebrafish. Here, the authors identify Nrg2a in zebrafish as a key regulator of trabeculation, and atrial and non-contractile CMs also respond to Nrg2a despite not forming trabeculae.

Regulation of GSK-3beta and beta-Catenin by Galphaq in HEK293T cells

Recent studies have shown that heterotrimeric G proteins are involved in the regulation of the canonical Wnt/beta-Catenin pathway. However, the mechanism(s) behind this involvement is (are) poorly understood. Our previous results have shown that activation of Galphaq in Xenopus oocytes leads to inhibition of GSK-3beta and stabilization of the beta-Catenin protein, suggesting that Galphaq might stabilize beta-Catenin via inhibition of GSK-3beta. In this study, we have observed similar results in HEK293T cells. In these cells optimal activation of endogenous Galphaq by expressing M3-muscarinic acetylcholine receptor (with or without carbachol treatment), or exposing the cells to thrombin led to an increase of 2 to 3-fold in endogenous cytoplasmic beta-Catenin protein levels. In addition, expression of the activated mutant of Galphaq (GalphaqQL) dramatically enhanced accumulation of exogenous beta-Catenin with no effect on beta-catenin (CTNNB1) gene transcription. The Galphaq-mediated cellular accumulation of beta-Catenin was blocked by expression of a minigene encoding a Galphaq specific inhibitory peptide but not by a minigene encoding a Galphas blocking peptide. Also, expression of GalphaqQL led to a significant reduction in GSK-3beta kinase activity, supporting the idea that the positive role of Galphaq signaling in inducing cellular accumulation of beta-Catenin is mediated through inhibition of GSK-3beta.