Over the last decade, the zebrafish has entered the field of cardiovascular research as a new model organism. This is largely due to a number of highly successful small- and large-scale forward genetic screens, which have led to the identification of zebrafish mutants with cardiovascular defects. Genetic mapping and identification of the affected genes have resulted in novel insights into the molecular regulation of vertebrate cardiac development. More recently, the zebrafish has become an attractive model to study the effect of genetic variations identified in patients with cardiovascular defects by candidate gene or whole-genome-association studies. Thanks to an almost entirely sequenced genome and high conservation of gene function compared with humans, the zebrafish has proved highly informative to express and study human disease-related gene variants, providing novel insights into human cardiovascular disease mechanisms, and highlighting the suitability of the zebrafish as an excellent model to study human cardiovascular diseases. In this review, I discuss recent discoveries in the field of cardiac development and specific cases in which the zebrafish has been used to model human congenital and acquired cardiac diseases.

Rationale: There are several methods to measure cardiomyocyte (CM) and muscle contraction but these require customized hardware, expensive apparatus and advanced informatics or can only be used in single experimental models. Consequently, data and techniques have been difficult to reproduce across models and laboratories, analysis is time consuming and only specialist researchers can quantify data. Objective: Here we describe and validate an automated, open source software tool (MUSCLEMOTION) adaptable for use with standard laboratory- and clinical imaging equipment that enables quantitative analysis of normal cardiac contraction, disease phenotypes and pharmacological responses. Methods and Results: MUSCLEMOTION allowed rapid and easy measurement of movement from high-speed movies in: (i) 1-dimensional in vitro models such as isolated adult and human pluripotent stem cell-derived CMs (hPSC-CMs); (ii) 2-dimensional in vitro models, such as beating CM monolayers or small clusters of hPSC-CMs; (iii) 3-dimensional multicellular in vitro or in vivo contractile tissues such as cardiac "organoids", engineered heart tissues (EHT), zebrafish- and human hearts. MUSCLEMOTION was effective under different recording conditions (bright field microscopy with simultaneous patch clamp recording, phase contrast microscopy and traction force microscopy). Outcomes were virtually identical to the current gold standards for contraction measurement such as optical flow, pole deflection, edge-detection systems or manual analyses. Finally, we used the algorithm to quantify contraction in in vitro and in vivo arrhythmia models and to measure pharmacological responses. Conclusions: Using a single open source method for processing video recordings, we obtained reliable pharmacological data and measures of cardiac disease phenotype in experimental cell-, animal- and human models.

We describe that galectin-1 (gal-1) is a receptor for the angiogenesis inhibitor anginex, and that the protein is crucial for tumor angiogenesis. gal-1 is overexpressed in endothelial cells of different human tumors. Expression knockdown in cultured endothelial cells inhibits cell proliferation and migration. The importance of gal-1 in angiogenesis is illustrated in the zebrafish model, where expression knockdown results in impaired vascular guidance and growth of dysfunctional vessels. The role of gal-1 in tumor angiogenesis is demonstrated in gal-1-null mice, in which tumor growth is markedly impaired because of insufficient tumor angiogenesis. Furthermore, tumor growth in gal-1-null mice no longer responds to antiangiogenesis treatment by anginex. Thus, gal-1 regulates tumor angiogenesis and is a target for angiostatic cancer therapy.

Amongst animal species, there is enormous variation in the size and complexity of the heart, ranging from the simple one-chambered heart of Ciona intestinalis to the complex four-chambered heart of lunged animals. To address possible mechanisms for the evolutionary adaptation of heart size, we studied how growth of the simple two-chambered heart in zebrafish is regulated. Our data show that the embryonic zebrafish heart tube grows by a substantial increase in cardiomyocyte number. Augmented cardiomyocyte differentiation, as opposed to proliferation, is responsible for the observed growth. By using transgenic assays to monitor developmental timing, we visualized for the first time the dynamics of cardiomyocyte differentiation in a vertebrate embryo. Our data identify two previously unrecognized phases of cardiomyocyte differentiation separated in time, space and regulation. During the initial phase, a continuous wave of cardiomyocyte differentiation begins in the ventricle, ends in the atrium, and requires Islet1 for its completion. In the later phase, new cardiomyocytes are added to the arterial pole, and this process requires Fgf signaling. Thus, two separate processes of cardiomyocyte differentiation independently regulate growth of the zebrafish heart. Together, our data support a model in which modified regulation of these distinct phases of cardiomyocyte differentiation has been responsible for the changes in heart size and morphology among vertebrate species.

β-Catenin controls both cadherin-mediated cell adhesion and activation of Wnt target genes. We demonstrate here that the β-catenin-binding protein BCL9-2, a homolog of the human proto-oncogene product BCL9, induces epithelial-mesenchymal transitions of nontransformed cells and increases β-catenin-dependent transcription. RNA interference of BCL9-2 in carcinoma cells induces an epithelial phenotype and translocates β-catenin from the nucleus to the cell membrane. The switch between β-catenin's adhesive and transcriptional functions is modulated by phosphorylation of Tyr 142 of β-catenin, which favors BCL9-2 binding and precludes interaction with α-catenin. During zebrafish embryogenesis, BCL9-2 acts in the Wnt8-signaling pathway and regulates mesoderm patterning.