556miR-23b and miR-199a regulate atrioventricular cushion epithelial-to-mesenchymal transition in chickens

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Valve development is a multistep process involving the activation of the cardiac endothelium, an epithelial-mesenchymal transition (EMT) and the progressive lining and differentiation of distinct mesenchymal cell types into the fibrosa and spongiosa layers. Several pathways such as Notch/delta, Tbf-beta and/or Vegf signalling have been involved in a crucial steps of valvulogenesis. We have previously demonstrated that discrete changes on microRNAs expression occur during ventricular chamber development. Several of these microRNAs were predicted to target Bmp- and Tgf-beta signalling, which are crucial steps during valvulogenesis. We have therefore analyzed the expression profile of candidate microRNAs in the atrial, ventricular and atrioventricular canal regions at four different developmental stages, HH13, HH19, HH27 and HH32. qRT-PCR analyses of microRNAs demonstrated a highly dynamic and distinct expression profile within the atrial, ventricular and atrioventricular canal regions of the developing chicken heart. Several microRNAs, such as miR-23b displayed increased expression at early but not late AVC developmental stages as compared to the atrial and ventricular chambers, whereas others such as miR-130a display decreased expression levels. Furthermore, we have implemented an in vitro EMT model to study microRNA function. Our results demonstrated that over-expression of miR-23b or miR-199 significantly blocks EMT in a similar fashion as high glucose administration controls, yet the analyses of the distinct signalling pathways involved in valvulogenesis suggest microRNA-specific control mechanisms. We shall provide a working model as how microRNA impact EMT during atrioventricular valve development.

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