Modulation of myocardial scar remodelling by epicardial Sox9 deletion

After myocardial infarction (MI), excessive fibrosis in the ischemic area causes scar development and remodelling, leading to a reduced cardiac contractile function and pressure adaptation. This worsening cardiac performance leads to cardiac dilation and impaired function, contributing to the high morbidity and mortality in heart failure.  Epicardial cells undergo epicardial-to-mesenchymal transition (EMT) as a response to cardiac injury. They migrate into the ischemic area and differentiate into myofibroblasts. We have previously shown that Sox9, an embryonic transcription factor, is upregulated mainly in the epicardium after cardiac injury, but its impact on EMT and scar development after MI are unknown. Here, we explore the role of epicardial Sox9 in EMT and fibrotic remodelling after MI. 

For the deletion of Sox9 in epicardial cells a mouse line exerting a tamoxifen-inducible epicardial-specific Sox9 knockout (Sox9-EpiKO) was used. Littermate Sox9fl/fl mice were used as controls. MI was induced by ultrasound-guided coagulation of the left anterior descending artery in the closed chest under anaesthesia with a micromanipulator-controlled monopolar needle and high frequency electricity via an electrosurgical unit. Successful MI induction was verified by absence of blood flow distal of occlusion, akinesia in the affected part of the left ventricle and typical ECG changes. Sham mice were subjected to the same procedure without application of electricity. Echocardiography was analysed via the Vevo strain software. Histology and immunohistochemistry were conducted on 5 µm slices of paraffin-embedded hearts and on 7 µm cryo-slices embedded in Tissue Tek. RNA bulk sequencing was performed on whole-heart scar tissue from infarcted knockout and control mice 2 weeks after MI.

Echocardiography revealed improved systolic function and ventricular remodelling, indicated by enhanced global longitudinal strain, better left ventricular ejection fraction and reduced cardiac dilation in Sox9-EpiKO animals. Additionally, one week post-infarction, we observed a reduction in asynchronous wall movement, assessed by a decrease in maximum opposite wall delay. RNA bulk sequencing from scar tissue revealed significant alterations following epicardial Sox9 deletion, including decreased expression of genes associated with extracellular matrix, inflammation, cell migration and vasculature development. Histological analysis showed reduced scar size and fibrosis in the remote myocardium. We identified sex-specific differences in both RNA bulk sequencing and echocardiography data, with male animals appearing to benefit more from Sox9 deletion than females.

Epicardial Sox9 deletion enhances cardiac function and limits excessive scar formation following MI. Targeting Sox9 in the epicardium may offer a promising therapeutic approach for improving heart remodelling after myocardial infarction. Gender specific differences could provide a potential new perspective on post-MI processes in relation to Sox9.