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P5382Geometry and strain sensing dictate YAP-dependent evolution of human cardiac stromal cells toward myofibroblasts in the cardiosphere organoid model


 
 
 Cardiac stromal cells are the most abundant cell type present in the heart, where they have an important homeostatic function in matrix renewal and electromechanical coupling. We have shown that YAP transcription factor (a component of the Hippo pathway) is important for the proliferation of these cells and, potentially, pathologic cardiac matrix stiffening in heart failure.
 
 
 
 Given the role of Hippo pathway in cell mechanosensation, we assessed the conditions for activation of the YAP-dependent transcriptional pathway related to geometry/strain sensing using human “cardiosphere”-derived cells.
 
 
 
 Human cardiospheres (C-Sp) and C-Sp-derived cells (CDCs) were obtained from fragments of atrial appendages of patients admitted for AoC bypass. C-Sps and CDCs were cultured using published methods. To analyze cell/nuclear geometry in 2D/3D culture, we employed an adapting thresholding algorithm allowing segmentation of cell membrane/nuclei shape and the relative abundance of nuclear/cytoplasmic fluorescence intensities from z-stacked confocal immunofluorescence images. Inhibitors of signaling cascades converging onto actin cytoskeleton polymerization or of YAP/TEADs interaction were used to assess YAP target genes transcription.
 
 
 
 Previous results showed stiffness sensitivity of YAP nuclear translocation process. Namely, CDCs plated onto stiff (E >50kPa) culture plates exhibited high ratios of nuclear/cytoplasmic YAP, as compared to cells plated on substrates with lower stiffness (E ∼ 10kPa). Analogously, cells with a stretched cytoplasm/nuclear shape in the border regions of the 3D organoids exhibited a higher YAP nuclear localization level and expression of fibrotic markers compared with cells with a more round geometry in the center of the spheres. To uncouple the cytoskeleton tensioning-dependent YAP nuclear shuttling, we treated CDCs plated onto stiff substrates with blebbistatin and Y27632. This showed a fully reversible YAP relocation from the nucleus to the cytoplasm and a reversion of canonical target genes (CYR61, ANKRD1 and CTGF) expression. In parallel, a “softening” process of the nucleus, likely due to release from cytoskeleton traction forces, was observed by confocal imaging along the cellular z-axis. Interestingly, use of Verteporfin, a drug directly interfering with the interaction of YAP/TAZ complex with TEADs DNA binding proteins showed reversion of pro-fibrotic YAP targets independent of high cytoskeleton tensioning.
 
 
 
 These results establish, for the first time, a deterministic relationship between pro-pathologic evolution of human adult cardiac stromal cells and activation of mechanosensing-dependent pathways. They also provide quantitative criteria for interpreting fibrotic evolution of the heart based on modifications of myocardial architecture and mechanical compliance, as well as the ability of the cells to generate tension forces affecting the nuclear structure.
