title: Epicardium and myocardium separate from a common precursor pool by crosstalk between bone morphogenetic protein- and fibroblast growth factor-signaling pathways.
abstract: BACKGROUND
  The epicardium contributes to the majority of nonmyocardial cells in the adult heart. Recent studies have reported that the epicardium is derived from Nkx2.5-positive progenitors and can differentiate into cardiomyocytes. Not much is known about the relation between the myocardial and epicardial lineage during development, whereas insights into these embryonic mechanisms could facilitate the design of future regenerative strategies.
  
  
  OBJECTIVE
  Acquiring insight into the signaling pathways involved in the lineage separation leading to the differentiation of myocardial and (pro)epicardial cells at the inflow of the developing heart.
  
  
  RESULTS
  We made 3D reconstructions of Tbx18 gene expression patterns to give insight into the developing epicardium in relation to the developing myocardium. Next, using DiI tracing, we show that the (pro)epicardium separates from the same precursor pool as the inflow myocardium. In vitro, we show that this lineage separation is regulated by a crosstalk between bone morphogenetic protein (BMP) signaling and fibroblast growth factor (FGF) signaling. BMP signaling via Smad drives differentiation toward the myocardial lineage, which is inhibited by FGF signaling via mitogen-activated protein kinase kinase (Mek)1/2. Embryos exposed to recombinant FGF2 in vivo show enhanced epicardium formation, whereas a misbalance between FGF and BMP by Mek1/2 inhibition and BMP stimulation causes a developmental arrest of the epicardium and enhances myocardium formation at the inflow of the heart.
  
  
  CONCLUSIONS
  Our data show that FGF signaling via Mek1/2 is dominant over BMP signaling via Smad and is required to separate the epicardial lineage from precardiac mesoderm. Consequently, myocardial differentiation requires BMP signaling via Smad and inhibition of FGF signaling at the level of Mek1/2. These findings are of clinical interest for the development of regeneration-based therapies for heart disease.
authors.affiliation: Academic Medical Center, Department Anatomy and Embryology, Meibergdreef 15, 1105AZ Amsterdam, The Netherlands.
authors.forename: Bram
authors.initials: B
authors.lastname: van Wijk
authors.forename: Gert
authors.initials: G
authors.lastname: van den Berg
authors.forename: Radwan
authors.initials: R
authors.lastname: Abu-Issa
authors.forename: Phil
authors.initials: P
authors.lastname: Barnett
authors.forename: Saskia
authors.initials: S
authors.lastname: van der Velden
authors.forename: Martina
authors.initials: M
authors.lastname: Schmidt
authors.forename: Jan M
authors.initials: JM
authors.lastname: Ruijter
authors.forename: Margaret L
authors.initials: ML
authors.lastname: Kirby
authors.forename: Antoon F M
authors.initials: AF
authors.lastname: Moorman
authors.forename: Maurice J B
authors.initials: MJ
authors.lastname: van den Hoff
mesh_terms: D000818:Animals; D017209:Apoptosis; D055396:Bone Morphogenetic Protein 2 / Q000378:metabolism; D019485:Bone Morphogenetic Proteins / Q000378:metabolism*; D002070:Butadienes / Q000494:pharmacology; D002232:Carbocyanines; D002454:Cell Differentiation; D002460:Cell Line; D019070:Cell Lineage / Q000187:drug effects / Q000235:genetics; D049109:Cell Proliferation; D002642:Chick Embryo; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D016222:Fibroblast Growth Factor 2 / Q000378:metabolism; D005346:Fibroblast Growth Factors / Q000378:metabolism*; D005456:Fluorescent Dyes; D018507:Gene Expression Regulation, Developmental; D006321:Heart / Q000187:drug effects / Q000196:embryology*; D007091:Image Processing, Computer-Assisted; D021621:Imaging, Three-Dimensional; D048369:MAP Kinase Kinase 1 / Q000037:antagonists & inhibitors / Q000378:metabolism; D048370:MAP Kinase Kinase 2 / Q000037:antagonists & inhibitors / Q000378:metabolism; D008856:Microscopy, Fluorescence; D009206:Myocardium / Q000378:metabolism*; D009570:Nitriles / Q000494:pharmacology; D010496:Pericardium / Q000187:drug effects / Q000196:embryology* / Q000378:metabolism*; D010641:Phenotype; D010766:Phosphorylation; D047428:Protein Kinase Inhibitors / Q000494:pharmacology; D051381:Rats; D011994:Recombinant Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects / Q000235:genetics; D051785:Smad Proteins / Q000378:metabolism; D020825:T-Box Domain Proteins / Q000235:genetics
chemical_list: D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins; D002070:Butadienes; D002232:Carbocyanines; D005456:Fluorescent Dyes; D009570:Nitriles; D047428:Protein Kinase Inhibitors; D011994:Recombinant Proteins; D051785:Smad Proteins; D020825:T-Box Domain Proteins; C485324:Tbx18 protein, chicken; C113580:U 0126; D016222:Fibroblast Growth Factor 2; C024286:3,3'-dioctadecylindocarbocyanine; D005346:Fibroblast Growth Factors; D048049:Extracellular Signal-Regulated MAP Kinases; D048369:MAP Kinase Kinase 1; D048370:MAP Kinase Kinase 2