Topological arrangement of cardiac fibroblasts regulates cellular plasticity

Rationale: Cardiac fbroblasts do not form a syncytium but reside in the interstitium between myocytes. This topological relationship between fbroblasts and myocytes is maintained throughout postnatal life until an acute myocardial injury occurs, when fbroblasts are recruited to, proliferate and aggregate in the region of myocyte necrosis. The accumulation or aggregation of fbroblasts in the area of injury thus represents a unique event in the life cycle of the fbroblast, but little is known about how changes in the topological arrangement of fbroblasts after cardiac injury affect fbroblast function. Objective: The objective of the study was to investigate how changes in topological states of cardiac fbroblasts (such as after cardiac injury) affect cellular phenotype. Methods and Results: Using 2 and 3-dimensional (2D versus 3D) culture conditions, we show that simple aggregation of cardiac fbroblasts is suffcient by itself to induce genome-wide changes in gene expression and chromatin remodeling. Remarkably, gene expression changes are reversible after the transition from a 3D back to 2D state demonstrating a topological regulation of cellular plasticity. Genes induced by fbroblast aggregation are strongly associated and predictive of adverse cardiac outcomes and remodeling in mouse models of cardiac hypertrophy and failure. Using solvent-based tissue clearing techniques to create optically transparent cardiac scar tissue, we show that fbroblasts in the region of dense scar tissue express markers that are induced by fbroblasts in the 3D conformation. Finally, using live cell interferometry, a quantitative phase microscopy technique to detect absolute changes in single cell biomass, we demonstrate that conditioned medium collected from fbroblasts in 3D conformation compared with that from a 2D state signifcantly increases cardiomyocyte cell hypertrophy. Conclusions: Taken together, these fndings demonstrate that simple topological changes in cardiac fbroblast organization are suffcient to induce chromatin remodeling and global changes in gene expression with potential functional consequences for the healing heart. (Circ Res. 2018;123:73-85. DOI: 10.1161/CIRCRESAHA.118.312589.)