Investigation of Runx1 and Cathepsin-L as potential therapeutic targets for myocardial infarction and cardiac ischaemia-reperfusion injury

Myocardial infarction (MI) is a leading cause of death worldwide. The fundamental treatment for MI is reperfusion of the ischaemic myocardium by primary percutaneous intervention (PPCI). Advances in PPCI techniques in recent years has led to an increased survival rate in patients undergoing MI. A larger proportion of patients surviving post- MI following PPCI face a later risk of developing heart failure due to the substantial left ventricular (LV) injury caused by MI. Post MI heart failure has become a frequent complication of MI and is associated with an extremely high mortality. To develop novel therapies for MI, two phases of the disease can be targeted. One is the period following PPCI in which ischaemia-reperfusion (IR) injury contributes to cardiac damage. At this stage, reducing IR injury can reduce cardiac damage and thereby improve cardiac function and survival. The other is the period of developing heart failure post MI, where adverse cardiac remodelling contributes to worsening cardiac function and progression of heart failure. Therefore, therapies targeting the remodelling process following MI may prevent deterioration of cardiac function and protect against heart failure. Development of novel therapy for MI and IR injury requires experimental models which model the human disease. The clinical relevance of the animal model is associated with successful translation of novel therapies. Mouse models have become commonly used in translational research due to their cost efficiency and the ease of developing genetically-modified mice. However, inducing MI and IR injury in mice is a complex technique due to their small size and thus application of mouse models of MI and IR injury has been limited. The work in this thesis, has focussed on the MI and IR injury models in mice. MI and IR injury were successfully induced by performing left anterior descending coronary artery ligation in open-chest microsurgery and cardiac function following MI and IR injury was characterized using echocardiography and intra-left ventricular pressure-volume (PV) loops. Using these models and phenotyping techniques, two potential therapeutic targets were evaluated, one is cathepsin-L inhibition by a pharmacological inhibitor CAA0225, while the other was Runx1 deficiency which was assessed in a cardiomyocyte-specific Runx1-deficient mouse. Cathespin-L is a cysteine protease typically localized in lysosomes. In patients with coronary heart diseases (CHD), cathepsin-L has been found at increased levels in the serum and plasma, and the elevation of cathepsin-L is correlated with disease severity. In this study, using a cathepsin-L inhibitor CAA0225, we tested the effects of cathepsin-L inhibition during IR injury in both Langendorff perfused ex vivo isolated rat hearts and an in vivo mouse model. In the Langendorff isolated heart model of IR injury, applying CAA0225 significantly improved systolic and diastolic cardiac function during IR injury and this improvement was paralleled by reduced infarct size. An inhibitor (CA074Me) of a separate member of the cathepsin family (cathepsin-B) was also tested in the Langendorff perfused hearts and although it improved the speed at which systolic function recovered post I/R injury, no other changes where noted in cardiac function. The beneficial effect of CAA0225 was further tested in in vivo mouse models of MI and IR injury. Intravenous injection of CAA0225 during the ischaemic period significantly improved cardiac systolic function following MI and IR injury at both 2 and 4 weeks, respectively. In vivo administration of CAA0225 reduced infarct size, as measured by a double-dye staining technique and may relate to normalization of adverse calcium handling in cardiomyocytes by CAA0225 during IR injury. The Runx1 gene is known to be related to lineage differentiation and function within the hematopoietic system. RUNX1 has been reported to be activated in border zone cardiomyocytes in humans and mice post MI. This finding indicated that the Runx1 gene may play a role in cardiac remodelling post MI. To investigate the role of Runx1, MI and IR injury was induced in a tamoxifen-inducible cardiomyocyte-specific Runx1-deficient mouse model. Runx1 deficiency significantly preserved cardiac function 8 weeks post MI and post IR injury injury and prevented LV dilation post MI. The underlying mechanism may relate to improved cardiomyocyte calcium handling. In conclusion, the work in this thesis has assessed two separate targets, cathepsin-L and Runx1 for their therapeutic potential in MI and IR injury. This work has significantly enhanced our knowledge on these novel targets and will inform the development of translational strategies for treatment of patients with MI