Abstract
Damage repair after myocardial infarction involves a series of complex events, which are thought to be initiated by inflammatory responses. The infarcted necrosis tissue induces production of cytokines and chemokines, which recruit immune cells and lead to the generation of several growth factors to induce and regulate angiogenesis, myocardial proliferation, and tissue remodeling. In most mammals, the disproportion of regeneration and tissue remodeling causes the injured tissue to be replaced by fibrotic scars. These non-elastic fibrosis tissues block the contraction of the heart and damage the normal cardiac muscle tissue, which may result in heart failure and death. In contrast, zebrafish has the ability to undertake scar-free healing after cardiac injury. This implies that if we could understand the mechanism of heart regeneration in zebrafish, we may adapt the knowledge to manage heart repair in human myocardial infarction. Previous studies have reported that syndecan-4 (Sdc4) regulates the inflammatory response and fibroblast activity in the vertebrates. Interestingly, Sdc4 knockout mice have higher mortality rate after myocardial infarction due to heart wall rupture. Therefore, we hypothesized that in early stage of zebrafish heart repair, Sdc4 may be a key regulator of inflammatory response and the production of extracellular matrix supportive proteins to protect heart from further damage and to promote tissue regeneration. To test this hypothesis, we first conducted a time-lapsed gene expression analysis to identify the gene expression profile of sdc4 after heart injury in zebrafish. As expected, we found the sdc4 expression increased rapidly after cryoinjury, which implies its importance in the early phase of heart regeneration. Then, we injected nano-magnetic beads coupled with sdc4 siRNA to inhibit sdc4 expression in zebrafish heart under an external magnetic field. We used the Lyz:DsRed zebrafish line to label myeloid cells to track the movement of immune cells. The results showed that loss of Sdc4 blocked the accumulation of innate immune cells in the lesion. In addition, when sdc4 was knocked down, we found the gene expression of col1a1a and tgfb1a and the protein expression of fibronectin were all down-regulated. Moreover, loss of Sdc4 might also affect the cell activation and proliferation around subepicardium. Finally, we measured the electrocardiogram in post-heart injury zebrafish with integrated video recording of ex vivo heart. After sdc4 knockdown, we observed the reduced capacity of wound repair and abnormal ventricular contraction around the cryoinjury lesion, meanwhile, the electrocardiogram showed sustained ST-elevation, the marker of myocardial infarction, in sdc4 knockdown group. In conclusion, Sdc4 might mediate the innate immune cells’ movement, the production of ECM proteins, the cell proliferation, the heart’s contractility, and the cardiac repair in the early stage of zebrafish heart repair. Sdc4 could be a critical regulator for scar-free healing in zebrafish heart regeneration.