Fibrin defines tissue stiffness and biomechanical signalling in regenerating zebrafish hearts as revealed by high-resolution stiffness mapping
The infarction of the human heart leads to irreversible scar formation, which permanently impairs its mechanical properties and physiological functions. Interestingly, the zebrafish heart is capable of resolving scar tissue and regenerating injured myocardium, offering a unique opportunity to study the interplay between scar tissue and cardiac mechanical properties during regeneration. In this study, we developed a novel method combining atomic force microscope-based nanoindentation with confocal microscopy to generate an elasticity map of the zebrafish heart. This high-resolution mapping revealed distinct regions of stiffness within the injury site, despite the overall global mechanical properties remaining unaffected. Specifically, we identified a stiff area that is cell-poor and fibrin-rich, contrasting with the center of injury, which is as soft as the surrounding myocardium. Extensive whole transcriptome analyses uncovered several components of the coagulation and fibrinolysis cascade in the regenerating heart. Pharmacological inhibition of Serpine1, a regulator of fibrinolysis, demonstrated that reduced fibrin-mediated stiffness impacts the activation of biomechanical Hippo pathway signaling in adjacent endocardial cells. Our approach characterizes the mechanical properties of different regions in the zebrafish ventricle, highlighting fibrin as the stiffest element influencing biomechanical signaling. This suggests it may play a structural role in the regeneration process. The methodology presented here offers a detailed view of the biomechanical environment and mechanotransductive signaling in regenerating cardiac tissue, which is crucial for understanding regenerative mechanisms. Overall design: RNAseq profiling of apexes of zebrafish hearts. Unijured hearts or hearts upon 1.5, 3, 7 and 14 days post cryoinjury were taken.