title: Muscleblind-like 1 is required for normal heart valve development in vivo.
abstract: BACKGROUND
  Development of the valves and septa of the heart depends on the formation and remodeling of the endocardial cushions in the atrioventricular canal and outflow tract. These cushions are populated by mesenchyme produced from the endocardium by epithelial-mesenchymal transition (EMT). The endocardial cushions are remodeled into the valves at post-EMT stages via differentiation of the mesenchyme and changes in the extracellular matrix (ECM). Transforming growth factor β (TGFβ) signaling has been implicated in both the induction of EMT in the endocardial cushions and the remodeling of the valves at post-EMT stages. We previously identified the RNA binding protein muscleblind-like 1 (MBNL1) as a negative regulator of TGFβ signaling and EMT in chicken endocardial cushions ex vivo. Here, we investigate the role of MBNL1 in endocardial cushion development and valvulogenesis in Mbnl1(∆E3/∆E3) mice, which are null for MBNL1 protein.
  
  
  METHODS
  Collagen gel invasion assays, histology, immunohistochemistry, real-time RT-PCR, optical coherence tomography, and echocardiography were used to evaluate EMT and TGFβ signaling in the endocardial cushions, and morphogenesis, ECM composition, and function of the heart valves.
  
  
  RESULTS
  As in chicken, the loss of MBNL1 promotes precocious TGFβ signaling and EMT in the endocardial cushions. Surprisingly, this does not lead to the production of excess mesenchyme, but later valve morphogenesis is aberrant. Adult Mbnl1(∆E3/∆E3) mice exhibit valve dysmorphia with elevated TGFβ signaling, changes in ECM composition, and increased pigmentation. This is accompanied by a high incidence of regurgitation across both inflow and outflow valves. Mbnl1(∆E3/∆E3) mice also have a high incidence of ostium secundum septal defects accompanied by atrial communication, but do not develop overt cardiomyopathy.
  
  
  CONCLUSIONS
  Together, these data indicate that MBNL1 plays a conserved role in negatively regulating TGFβ signaling, and is required for normal valve morphogenesis and homeostasis in vivo.
authors.affiliation: Department of Cellular & Molecular Medicine, Lerner Research Institute, 9500 Euclid Ave. NC10, Cleveland Clinic, Cleveland, OH, 44195, USA. ryan_coram@yahoo.com.|Present Address: Ohio University Heritage College of Osteopathic Medicine, Athens, OH, 45701, USA. ryan_coram@yahoo.com.
authors.forename: Ryan J
authors.initials: RJ
authors.lastname: Coram
authors.affiliation: Department of Cellular & Molecular Medicine, Lerner Research Institute, 9500 Euclid Ave. NC10, Cleveland Clinic, Cleveland, OH, 44195, USA. stillws@ccf.org.|Present Address: Department of Obstetrics and Gynecology, Women's Health Institute, Cleveland Clinic, Cleveland, OH, 44195, USA. stillws@ccf.org.
authors.forename: Samantha J
authors.initials: SJ
authors.lastname: Stillwagon
authors.affiliation: Department of Cellular & Molecular Medicine, Lerner Research Institute, 9500 Euclid Ave. NC10, Cleveland Clinic, Cleveland, OH, 44195, USA. guggila@ccf.org.
authors.forename: Anuradha
authors.initials: A
authors.lastname: Guggilam
authors.affiliation: Department of Pediatrics, Case Western Reserve University School of Medicine, Cleveland, OH, 44106, USA. mwj5@case.edu.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Jenkins
authors.affiliation: Department of Molecular Genetics & Microbiology, College of Medicine, Center for NeuroGenetics and the Genetics Institute, University of Florida, Gainesville, FL, 32610, USA. mswanson@ufl.edu.
authors.forename: Maurice S
authors.initials: MS
authors.lastname: Swanson
authors.affiliation: Department of Cellular & Molecular Medicine, Lerner Research Institute, 9500 Euclid Ave. NC10, Cleveland Clinic, Cleveland, OH, 44195, USA. ladda@ccf.org.
authors.forename: Andrea N
authors.initials: AN
authors.lastname: Ladd
mesh_terms: D000818:Animals; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D054089:Endocardial Cushions / Q000196:embryology* / Q000378:metabolism; D058750:Epithelial-Mesenchymal Transition; D006321:Heart / Q000196:embryology; D006351:Heart Valves / Q000166:cytology / Q000196:embryology* / Q000378:metabolism; D051379:Mice; D038081:Organogenesis; D016601:RNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D016212:Transforming Growth Factor beta / Q000378:metabolism
chemical_list: D004268:DNA-Binding Proteins; C479922:Mbnl1 protein, mouse; D016601:RNA-Binding Proteins; D016212:Transforming Growth Factor beta