Hippo Signaling Mediates TGFβ-Dependent Transcriptional In-Puts in Cardiac Cushion Mesenchymal Cells to Regulate Extracel-Lular Matrix Remodeling

Abstract

The transforming growth factor beta (TGFβ) and Hippo signaling pathways are evolutionarily conserved pathways that play a critical role in cardiac fibroblasts during embryonic development, tissue repair, and fibrosis. TGFβ and Hippo signaling is also important for cardiac cushion remodeling and septation during embryonic development. Loss of TGFβ2 in mice causes cardiac cushion remodeling defects resulting in congenital heart disease. In this study, we used in vitro molecular and pharmacologic approaches in the cushion mesenchymal cell line (tsA58-AVM) and investigated if Hippo pathway acts as a mediator of TGFβ2 signaling. Immunofluorescence staining showed that TGFβ2 induced nuclear translocation of activated SMAD3 in the cushion mesenchymal cells. In addition, the results indicated increased nuclear localization of Yes associated protein 1 (YAP1) following a similar treatment of TGFβ2. In collagen lattice formation assays, TGFβ2 treatment of cushion cells resulted in an enhanced collagen contraction compared to the untreated cushion cells. Interestingly, verteporfin, a YAP1 inhibitor, significantly blocked the ability of cushion cells to contract collagen gel in absence or presence of by exogenously added TGFβ2. To confirm the molecular mechanisms of verteporfin induced inhibition of TGFβ2-dependent extra-cellular matrix (ECM) reorganization we performed gene expression analysis of key mesenchymal genes involved in ECM remodeling in heart development and disease. Our results confirmed that verteporfin significantly decreased the expression of α-smooth muscle actin (Acta2), collagen 1a1 (Col1a1), Ccn1 (i.e., Cyr61), and Ccn2 (i.e., Ctgf). Western blot analysis indicated that verteporfin treatment significantly blocked TGFβ2-induced activation of SMAD2/3 in cushion mesenchymal cells. Collectively, these results indicate that TGFβ2 regulation of cushion mesenchymal cell be-havior and ECM remodeling is mediated by YAP1. Thus, TGFβ2 and Hippo pathway integration represents an important step in understanding the etiology of congenital heart disease.