Endocardial identity is established during early somitogenesis by Bmp signalling acting upstream of npas4l and etv2

The endocardium plays important roles in the development and function of the vertebrate heart; however, few molecular markers of this tissue have been identified and little is known about what regulates its differentiation. Here, we describe the Gt(SAGFF27C); Tg(4xUAS:egfp) line as a marker of endocardial development in zebrafish. Transcriptomic comparison between endocardium and pan-endothelium confirms molecular distinction between these populations and time-course analysis suggests differentiation as early as eight somites. To investigate what regulates endocardial identity, we employed npas4l, etv2 and scl loss-of-function models. Endocardial expression is lost in npas4l mutants, significantly reduced in etv2 mutants and only modestly affected upon scl loss-of-function. Bmp signalling was also examined: overactivation of Bmp signalling increased endocardial expression, whereas Bmp inhibition decreased expression. Finally, epistasis experiments showed that overactivation of Bmp signalling was incapable of restoring endocardial expression in etv2 mutants. By contrast, overexpression of either npas4l or etv2 was sufficient to rescue endocardial expression upon Bmp inhibition. Together, these results describe the differentiation of the endocardium, distinct from vasculature, and place npas4l and etv2 downstream of Bmp signalling in regulating its differentiation.

Summary: A zebrafish transgenic reporter of the endocardium is identified, permitting transcriptomic analysis and identification of new endocardial markers. Epistasis experiments demonstrate npas4l and etv2 act downstream of Bmp signalling to regulate endocardial differentiation.

Getting to the Heart of Left-Right Asymmetry: Contributions from the Zebrafish Model

The heart is laterally asymmetric. Not only is it positioned on the left side of the body but the organ itself is asymmetric. This patterning occurs across scales: at the organism level, through left-right axis patterning; at the organ level, where the heart itself exhibits left-right asymmetry; at the cellular level, where gene expression, deposition of matrix and proteins and cell behaviour are asymmetric; and at the molecular level, with chirality of molecules. Defective left-right patterning has dire consequences on multiple organs; however, mortality and morbidity arising from disrupted laterality is usually attributed to complex cardiac defects, bringing into focus the particulars of left-right patterning of the heart. Laterality defects impact how the heart integrates and connects with neighbouring organs, but the anatomy of the heart is also affected because of its asymmetry. Genetic studies have demonstrated that cardiac asymmetry is influenced by left-right axis patterning and yet the heart also possesses intrinsic laterality, reinforcing the patterning of this organ. These inputs into cardiac patterning are established at the very onset of left-right patterning (formation of the left-right organiser) and continue through propagation of left-right signals across animal axes, asymmetric differentiation of the cardiac fields, lateralised tube formation and asymmetric looping morphogenesis. In this review, we will discuss how left-right asymmetry is established and how that influences subsequent asymmetric development of the early embryonic heart. In keeping with the theme of this issue, we will focus on advancements made through studies using the zebrafish model and describe how its use has contributed considerable knowledge to our understanding of the patterning of the heart.

Culturing and Measuring Fetal and Newborn Murine Long Bones

Long bones are complex and dynamic structures, which arise from endochondral ossification via a cartilage intermediate. The limited access to healthy human bones makes particularly valuable the use of mammalian models, such as mouse and rat, to look into different aspects of bone growth and homeostasis. Additionally, the development of sophisticated genetic tools in mice allows more complex studies of long bone growth and asks for an expansion of techniques used to study bone growth. Here, we present a detailed protocol for ex vivo murine bone culture, which allows the study of bone and cartilage in a tightly controlled manner while recapitulating most of the in vivo process. The method described allows the culture of a range of bones, including tibia, femur, and metatarsal bones, but we have focused mainly on tibial culture here. Moreover, it can be used in combination with other techniques, such as time-lapse live imaging or drug treatment.

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.

The Hippo pathway effector Wwtr1 regulates cardiac wall maturation in zebrafish

Cardiac trabeculation is a highly regulated process that starts with the delamination of compact layer cardiomyocytes. The Hippo signaling pathway has been implicated in cardiac development but many questions remain. We have investigated the role of Wwtr1, a nuclear effector of the Hippo pathway, in zebrafish and find that its loss leads to reduced cardiac trabeculation. However, in mosaic animals, wwtr1^-/- cardiomyocytes contribute more frequently than wwtr1^+/- cardiomyocytes to the trabecular layer of wild-type hearts. To investigate this paradox, we examined the myocardial wall at early stages and found that compact layer cardiomyocytes in wwtr1^-/- hearts exhibit disorganized cortical actin structure and abnormal cell-cell junctions. Accordingly, wild-type cardiomyocytes in mosaic mutant hearts contribute less frequently to the trabecular layer than when present in mosaic wild-type hearts, indicating that wwtr1^-/- hearts are not able to support trabeculation. We also found that Nrg/Erbb2 signaling, which is required for trabeculation, could promote Wwtr1 nuclear export in cardiomyocytes. Altogether, these data suggest that Wwtr1 establishes the compact wall architecture necessary for trabeculation, and that Nrg/Erbb2 signaling negatively regulates its nuclear localization and therefore its activity.

Apical ectodermal ridge morphogenesis in limb development is controlled by Arid3b-mediated regulation of cell movements

The apical ectodermal ridge (AER) is a specialized epithelium located at the distal edge of the limb bud that directs outgrowth along the proximodistal axis. Although the molecular basis for its function is well known, the cellular mechanisms that lead to its maturation are not fully understood. Here, we show that Arid3b, a member of the ARID family of transcriptional regulators, is expressed in the AER in mouse and chick embryos, and that interference with its activity leads to aberrant AER development, in which normal structure is not achieved. This happens without alterations in cell numbers or gene expression in main signalling pathways. Cells that are defective in Arid3b show an abnormal distribution of the actin cytoskeleton and decreased motility in vitro. Moreover, movements of pre-AER cells and their contribution to the AER were defective in vivo in embryos with reduced Arid3b function. Our results show that Arid3b is involved in the regulation of cell motility and rearrangements that lead to AER maturation.