Research Report 2010 - MDC

Transformation of the flat heart field into anelongated heart tube. Myocardial cells (green) ofthe right heart field initiate a tissue involutionprocess which creates the ventral floor of thenascent heart (indicated by white arrow) whereascells on the left side do not change theirmonolayered organization and form the dorsalroof. Various tissues are recognized by aPCK (red)and ALCAM (blue) staining. Inserts indicate theorientation of the section planes shown.tein interaction domains and obtained several surprisingresults that change the understanding of the extensivebiochemical and in vitro protein-protein interaction datathat have previously been described for this protein complex.Therefore, alternative scaffolding interactions mustbe in place, in addition to the canonical model of proteinproteininteractions, to allow association of the largerCrumbs-Nagie oko-Patj-Lin-7-Par6-aPKC protein complexin vivo. Moreover, we provided evidence for differentmechanisms involved in the polarization of the apicalCrumbs complex within different tissues of the zebrafishembryo. These findings suggest that distinct epitheliaemploy divergently composed cell polarity complexesduring tissue polarization.Epithelial functions of aPKCi and Nagieoko/Mpp5 during cardiac morphogenesisStefan Rohr, Nana Bit-AvragimNok/Mpp5 is an essential scaffolding partner for bothCrumbs and for the Par6-aPKC protein complex. In thisstudy, we characterized early cardiogenesis in heart andsoul (has)/aPKCi or nok/mpp5 mutants. Loss of either ofthe two cell polarity regulators disrupted the epithelialorganization of myocardial cells during heart tube formationand blocked the progression of cardiac morphogenesisbeyond the heart cone stage. Has/aPKCi andNok/Mpp5 were shown to function autonomouslywithin the myocardial layer during morphogenesis andthe catalytic kinase activity of Has/aPKCi was essentialfor this process. These findings demonstrated theimportance of correct cell polarity and epithelial morphogenesisduring heart formation.Na + ,K + -ATPase interacts with the apical Crumbscomplex in maintaining myocardial polarityElena Cibrián-Uhalte(in collaboration with Adam Langenbacher, XiaodongShu, Jau-Nian Chen, University of California LosAngeles)In another study, we demonstrated the importance ofcorrect ion balance for junctional maintenance andepithelial character of myocardial cells. The Na + ,K + -ATPase, or Na-pump, is an enzyme primarily involved inthe generation of Na + and K + gradients across membranesthereby regulating ionic concentrations withinepithelial cells. The zebrafish 1B1 subunit of Na + ,K +ATPase is encoded by the heart and mind (had) locusand had mutants show a delayed heart tube elongation.This phenotype is reminiscent of the has/aPKCi ornok/mpp5 mutant phenotypes which are characterizedby a lack of epithelial cell polarity. In genetic interactionstudies, Had/Na + ,K + -ATPase and Nok/Mpp5, a componentof the apical Crumbs protein complex, interactedin the maintenance of apical myocardial junctions raisingthe intriguing possibility that the ion balance producedby the Na-pump is critical for cell polarity. Tofunctionally characterize the role of the ion pump function,we produced a mutant form of Had/Na + ,K + ATPasewhich specifically affects the ATPase activity that isessential for pumping sodium across the plasma membraneand found that it could neither rescue the hearttube elongation phenotype nor myocardial polarity.Our study suggests that the osmotic balance producedby the Na-pump contributes to the maintenance ofapical junction belts, a function that is uncovered uponloss of Nok/Mpp5.Selected PublicationsZhang, J, Piontek, J, Wolburg, H, Piehl, C, Liss, M, Otten, C, Christ, A,Willnow, TE, Blasig, IE, and Abdelilah-Seyfried, S. (2009). Establishment ofa neuroepithelial barrier by Claudin5a is essential for zebrafish brainventricular lumen expansion. Proc. Natl. Acad. Sci USA, Early EditionJanuary 2010.Rohr, S, Otten, C, Abdelilah-Seyfried, S. (2008). Asymmetric involution ofthe myocardial field drives heart tube formation in zebrafish, Circ. Res.102, e12-19.Bit-Avragim, N, Hellwig, N, Rudolph, F, Munson, C, Stainier, D, andAbdelilah-Seyfried, S. (2008). Divergent polarization mechanisms duringvertebrate epithelial development mediated by the Crumbs complexprotein Nagie oko. J. Cell Sci. 121, 2503-2510.Cibrián-Uhalte, E, Langenbacher, A, Shu, X, Chen, JN, Abdelilah-Seyfried,S. (2007). Involvement of Na,K ATPase in myocardial cell junctionmaintenance. J. Cell Biol. 176, 223-230.Rohr, S, Bit-Avragim, N, Abdelilah-Seyfried, S. (2006). Heart and soul/PRKCiand Nagie oko/Mpp5 regulate myocardial coherence and remodelingduring cardiac morphogenesis. Development 133, 107-115.Cardiovascular and Metabolic Disease Research 19