No abstract

The second heart field (SHF), progenitor cells that are initially sequestered outside the heart, migrates into the heart and gives rise to endocardium, myocardium, and smooth muscle. Because of its distinct developmental history, the SHF is likely subjected to different signals from that of the first heart field. Previous experiments revealed that canonical Wnt signaling negatively regulated first heart field specification. We inactivated the obligate canonical Wnt effector ␤-catenin using a ␤-catenin conditional null allele and the Mef2c AHF cre driver that directs cre activity specifically in SHF. We also expressed a stabilized form of ␤-catenin to model continuous Wnt signaling in SHF. Our data indicate that Wnt signaling acts in a positive fashion to promote right ventricular and interventricular myocardial expansion. Cyclin D2 and Tgf␤2 expression was drastically reduced in ␤-catenin loss-of-function mutants, indicating that Wnt signaling is required for patterning and expansion of SHF derivatives. Our findings reveal that Wnt signaling plays a major positive role in promoting growth and diversification of SHF precursors into right ventricular and interventricular myocardium.conditional genetics ͉ cardiac progenitor ͉ mouse ͉ development

PDGFR is an important target for novel anticancer therapeutics because it is overexpressed in a wide variety of malignancies. Recently, however, several anticancer drugs that inhibit PDGFR signaling have been associated with clinical heart failure. Understanding this effect of PDGFR inhibitors has been difficult because the role of PDGFR signaling in the heart remains largely unexplored. As described herein, we have found that PDGFR-β expression and activation increase dramatically in the hearts of mice exposed to load-induced cardiac stress. In mice in which Pdgfrb was knocked out in the heart in development or in adulthood, exposure to loadinduced stress resulted in cardiac dysfunction and heart failure. Mechanistically, we showed that cardiomyocyte PDGFR-β signaling plays a vital role in stress-induced cardiac angiogenesis. Specifically, we demonstrated that cardiomyocyte PDGFR-β was an essential upstream regulator of the stress-induced paracrine angiogenic capacity (the angiogenic potential) of cardiomyocytes. These results demonstrate that cardiomyocyte PDGFR-β is a regulator of the compensatory cardiac response to pressure overload-induced stress. Furthermore, our findings may provide insights into the mechanism of cardiotoxicity due to anticancer PDGFR inhibitors.

The development of the anterior segment of the mammalian eye is critical for normal ocular function, whereas abnormal development can cause glaucoma, a leading cause of blindness in the world. We report that orphan G protein-coupled receptor 48 (Gpr48/LGR4) plays an important role in the development of the anterior segment structure. Disruption of Gpr48 causes a wide spectrum of anterior segment dysgenesis (ASD), including microphthalmia, iris hypoplasia, irdiocorneal angle malformation, cornea dysgenesis, and cataract. Detailed analyses reveal that defective iris myogenesis and ocular extracellular matrix homeostasis are detected at early postnatal stages of eye development, whereas ganglion cell loss, inner nuclear layer thinness, and early onset of glaucoma were detected in 6-month-old Gpr48 ؊/؊ mice. To determine the molecular mechanism of ASD caused by the deletion of Gpr48, we performed gene expression analyses and revealed dramatic down-regulation of Pitx2 in homozygous knockout mice. In vitro studies with the constitutively active Gpr48 mutant receptor demonstrate that Pitx2 is a direct target of the Gpr48-mediated cAMP-CREB signaling pathway in regulating anterior segment development, suggesting a role of Gpr48 as a potential therapeutic target of ASD.eye development ͉ glaucoma ͉ cataract ͉ Lgr4

Current models of left-right asymmetry hold that an early asymmetric signal is generated at the node and transduced to lateral plate mesoderm in a linear signal transduction cascade through the function of the Nodal signaling molecule. The Pitx2 homeobox gene functions at the final stages of this cascade to direct asymmetric morphogenesis of selected organs including the heart. We previously showed that Pitx2 regulated an asymmetric pathway that was independent of cardiac looping suggesting a second asymmetric cardiac pathway. It has been proposed that in the cardiac outflow tract Pitx2 functions in both cardiac neural crest, as a target of canonical Wnt-signaling, and in the mesoderm-derived cardiac second lineage. We used fate mapping, conditional loss of function, and chimera analysis in mice to investigate the role of Pitx2 in outflow tract morphogenesis. Our findings reveal that Pitx2 is dispensable in the cardiac neural crest but functions in second lineage myocardium revealing that this cardiac progenitor field is patterned asymmetrically.