In the mammalian heart a conduction system of nodes and conducting cells generates and transduces the electrical signals evoking myocardial contractions. Specialized pacemaker cells initiating and controlling cardiac contraction rhythmicity are localized in an anatomically identifiable structure of myocardial origin, the sinus node. We previously showed that in mammalian embryos sinus node cells originate from cardiac progenitors expressing the transcription factors T-box transcription factor 3 (Tbx3) and Islet-1 (Isl1). Although cardiac development and function are strikingly conserved amongst animal classes, in lower vertebrates neither structural nor molecular distinguishable components of a conduction system have been identified, questioning its evolutionary origin. Here we show that zebrafish embryos lacking the LIM/homeodomain-containing transcription factor Isl1 display heart rate defects related to pacemaker dysfunction. Moreover, 3D reconstructions of gene expression patterns in the embryonic and adult zebrafish heart led us to uncover a previously unidentified, Isl1-positive and Tbx2b-positive region in the myocardium at the junction of the sinus venosus and atrium. Through their long interconnecting cellular protrusions the identified Isl1-positive cells form a ring-shaped structure. In vivo labeling of the Isl1-positive cells by transgenic technology allowed their isolation and electrophysiological characterization, revealing their unique pacemaker activity. In conclusion we demonstrate that Isl1-expressing cells, organized as a ring-shaped structure around the venous pole, hold the pacemaker function in the adult zebrafish heart. We have thereby identified an evolutionary conserved, structural and molecular distinguishable component of the cardiac conduction system in a lower vertebrate.

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The pacemaking activity of specialized tissues in the heart and gut results in lifelong rhythmic contractions. Here we describe a new syndrome characterized by Chronic Atrial and Intestinal Dysrhythmia, termed CAID syndrome, in 16 French Canadians and 1 Swede. We show that a single shared homozygous founder mutation in SGOL1, a component of the cohesin complex, causes CAID syndrome. Cultured dermal fibroblasts from affected individuals showed accelerated cell cycle progression, a higher rate of senescence and enhanced activation of TGF-β signaling. Karyotypes showed the typical railroad appearance of a centromeric cohesion defect. Tissues derived from affected individuals displayed pathological changes in both the enteric nervous system and smooth muscle. Morpholino-induced knockdown of sgol1 in zebrafish recapitulated the abnormalities seen in humans with CAID syndrome. Our findings identify CAID syndrome as a novel generalized dysrhythmia, suggesting a new role for SGOL1 and the cohesin complex in mediating the integrity of human cardiac and gut rhythm.

Objective: To study the role of microRNAs in cardiac valve formation. Methods and Results:We show that zebrafish dicer mutant embryos, lacking mature miRNAs, form excessive endocardial cushions. By screening miRNAs expressed in the heart, we found that miR-23 is both necessary and sufficient for restricting the number of endocardial cells that differentiate into endocardial cushion cells. In addition, in mouse endothelial cells, miR-23 inhibited a transforming growth factor-␤-induced endothelial-to-mesenchymal transition. By in silico screening of expression data with predicted miR-23 target sites combined with in vivo testing, we identified hyaluronic acid synthase 2 (Has2), Icat, and Tmem2 as novel direct targets of miR-23. Finally, we demonstrate that the upregulation of Has2, an extracellular remodeling enzyme required for endocardial cushion and valve formation, is responsible for the excessive endocardial cushion cell differentiation in dicer mutants. T he development of valve structures occurs in distinguishable phases, which are highly conserved. 1 The first event during valve development is the induction of endocardial cushions (ECs) within the atrioventricular (AV) canal (AVC) and outflow tract of the primitive heart tube. It has been well established that bone morphogenetic protein (BMP), a member of the transforming growth factor (TGF)-␤ superfamily, is the major myocardial signal that initiates EC formation from human to zebrafish. [2][3][4][5] Compromised BMP signaling results in downregulation of multiple pathways including TGF-␤, Has2 (hyaluronic acid synthase 2), and Notch1, as well as the transcription factors Snail1 and Twist1. 2,4 The importance of regulating Has2 expression in the endocardium has been exemplified by the observation that in Has2 deficient mice the cardiac jelly does not expand and ECs fail to form. 6 Has2 is responsible for the production of hyaluronic acid (HA), one of the ECM components of cardiac jelly. 6 HA production leads to expansion of the extracellular space because it binds salt and water and induces PI3K and ErbB signaling (reviewed by B. Toole 7 ). Conclusions: MiR-23 in the embryonic heart is required to restrict endocardial cushion formation by inhibitingDespite differences in the cellular processes that precede valve formation the molecular signals regulating valve formation (eg, Notch, NFAT, ErbB, and TGF-␤ signaling) have been conserved between amniotes and zebrafish. 8 -11 MiRNAs are a class of 21-to 25-nucleotide single-stranded noncoding RNAs transcribed from DNA but not translated into protein. Instead, miRNAs interact with messenger RNAs in the cytosol and regulate their final output at the protein level. 12 Dicer, an RNase III endonuclease, processes hairpinlike pre-miRNAs into double-stranded miRNA molecules. MiRNA function during organogenesis has been studied mainly by generating conditional Dicer knockout mouse models because conventional Dicer mutant mice are embryonic lethal. 13 Conditional depletion of Dicer from the endocardium has been pre...