Abstract
BackgroundDevelopment 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.MethodsCollagen 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. ResultsAs 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.ConclusionsTogether, 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12861-015-0087-4) contains supplementary material, which is available to authorized users.
Highlights
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
Using Mbnl1ΔE3/ΔE3 mice, which are null for muscleblind-like 1 (MBNL1) protein, we show that loss of MBNL1 promotes invasive mesenchyme production in atrioventricular canal (AVC) explants ex vivo and precocious Transforming growth factor β (TGFβ) signaling and epithelial-mesenchymal transition (EMT) in the AVC in vivo, but does not lead to an excess in total mesenchyme production
MBNL1 is expressed in the endocardial cushions and mature heart valves To determine whether MBNL1 expression in the endocardial cushions is conserved in mammals, we performed immunohistochemistry on sections from mouse hearts at different stages of development
Summary
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). 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. TGFβ1 is expressed in mouse AVC endocardium, and it is not essential for endocardial cushion EMT in vivo, Tgfb1-null mice have disorganized valves [19]
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