Fig. 1

MBNL1 is highly expressed in the endocardial cushions and heart valves. Immunohistochemistry with an anti-MBNL1 antibody was performed on sagittal sections from wild type mouse embryos (a-f) and coronal sections of adult hearts (g). Representative sections from one of four embryos or one of two adult hearts are shown. a Close-up of the heart at E9 stained with the secondary antibody alone is shown as a negative control. b Close-up of the heart at E9 shows expression of MBNL1 in the early myocardium and endocardium of the nascent endocardial cushions just prior to the onset of EMT. c Close-up of the heart at E11.5 shows strong MBNL1 expression in the endocardial cushions of the AVC and OFT. Higher magnification views of the boxed regions in the endocardial cushion (d) and the ventricular wall (e) show intense nuclear staining in the cushion endocardial cells, but more diffuse staining in the myocardium. Intense nuclear MBNL1 staining persists in the fetal valves at E19.5 (f) and mature heart valves (g), while MBNL1 remains more diffusely distributed in the nucleus and cytoplasm in the myocardium. Asterisks highlight accumulations of melanin in the adult valve. h Western blot analysis confirms high MBNL1 expression in the adult heart, and suggests different cardiac tissues may express different MBNL1 isoforms. Protein integrity and loading were assessed by Ponceau S staining. A representative of three independent blots with similar results is shown. Atr = atrium, Vent = ventricle, EC = endocardial cushion, OFT = outflow tract, TC = tricuspid valve, Val = heart valves