Heart and soul/PRKCi and nagie oko/Mpp5 regulate myocardial coherence and remodeling during cardiac morphogenesis.

Organ morphogenesis requires cellular shape changes and tissue rearrangements that occur in a precisely timed manner. Here, we show that zebrafish heart and soul (Has)/protein kinase C iota (PRKCi) is required tissue-autonomously within the myocardium for normal heart morphogenesis and that this function depends on its catalytic activity. In addition, we demonstrate that nagie oko (Nok) is the functional homolog of mammalian protein associated with Lin-seven 1 (Pals1)/MAGUK p55 subfamily member 5 (Mpp5), and we dissect its earlier and later functions during myocardial morphogenesis. Has/PRKCi and Nok/Mpp5 are required early for the polarized epithelial organization and coherence of myocardial cells during heart cone formation. Zygotic nok/mpp5 mutants have later myocardial defects, including an incomplete heart tube elongation corresponding with a failure of myocardial cells to correctly expand in size. Furthermore, we show that nok/mpp5 acts within myocardial cells during heart tube elongation. Together, these results demonstrate that cardiac morphogenesis depends on the polarized organization and coherence of the myocardium, and that the expansion of myocardial cell size contributes to the transformation of the heart cone into an elongated tube.