1Medizinische Fakultät Mannheim der Universität Heidelberg Kardiovaskuläre Physiologie Mannheim, Deutschland; 2Universitätsklinikum Heidelberg Klinik für Innere Med. III, Kardiologie, Angiologie u. Pneumologie Heidelberg, Deutschland
Introduction: Infarct scar remodelling caused by excessive fibrosis after myocardial infarction (MI) result in cardiac dilatation and reduced function, contributing to the high morbidity and mortality of heart failure. Fibroblasts migrate into the infarct area and play a major role in the excessive fibrosis after an ischemic injury. A major part of these fibroblasts is of epicardial origin. Sox9, an essential embryonic transcription factor, is a known regulator of cardiac fibrosis and inflammation and is upregulated in the scar area and epicardium after infarct. To further understand the role of Sox9, we modulate fibrotic scarring by manipulating Sox9 expression in fibroblasts and epicardial cells in a novel minimal-invasive myocardial infarction model in mice.
Methods: Sox9fl/f-FBKO-mT/mG and Sox9fl/f-EpiKO-mT/mG mice exerting either a tamoxifen-inducible fibroblast-specific or epicardial-specific Sox9 knockout, were subjected to a minimal-invasive MI. Littermate Sox9fl/fl-mT/mG mice were used as controls. MI was induced by ultrasound-guided coagulation of the left anterior descending artery in the closed chest of 8- to 10-week-old mice under isoflurane 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 performed in parasternal long and short axes and analysed via the Vevo strain software. Histology and immunohistochemistry analysis were done on 5 µm slices of paraffin-embedded hearts.
Results: Both Sox9 knockout mouse lines showed improved systolic function assessed by global longitudinal strain, left ventricular ejection fraction and reduced cardiac dilatation 4 weeks after MI. Loss of synchronized left ventricular endocardial wall movement after infarct induction was alleviated during the course of MI in both knockout mouse lines (assessed by maximum opposite wall delay) compared to control mice already 1 week after MI, which was maintained until 4 weeks after MI. Sox9 deletion in fibroblast also reduced left ventricular hypertrophy, but this was not found in the epicardial- Sox9-knockout mice. In Sox9fl/f-FBKO-mT/mG mice, scar thickness was increased after 4 weeks. Scar size as well as fibrosis in the remote area of the left ventricle were reduced in both Sox9 knock-out mouse lines versus control, despite similar initial myocardial injury compared to control mice directly after MI induction (as evaluated by echocardiography).
Conclusion: Sox9 ablation in fibroblasts, but also in epicardial cells improves cardiac functions and scar development after MI induction. Therefore, epicardial Sox9 might be an interesting and novel therapeutic target during myocardial infarct progression.