Cardiomyocyte Survival Pathways

Abstract

In the present thesis, the link between the genotype of the mouse and the concurrent phenotype is investigated employing sophisticated molecular and cellular techniques combined with in vivo cardiac performance measurements. In chapter 1 we focus on the characteristics of cardiac remodeling following an ischemic event in man. The characteristics of remodeling are thoroughly discussed through the results solely derived from studies in genetically modified mice. Despite the data available, intracellular signaling is still a mystery as we lack the tools to study several pathways simultaneously and maybe even more important the temporal changes in signaling cascade interactions. In chapter 2 the molecular processes involved in cardiac hypertrophy are discussed. The concept of beneficial and maladaptive pathways is introduced and related to the detrimental transition of hypertrophy towards heart failure. Open-chest and closed-chest protocols for in vivo left ventricular pressure-volume measurement are illustrated in chapter 3.We have chosen for closed-chest cardiac function assessment protocol in our ischemia-reperfusion studies, because of better systolic and diastolic performance, normal arterial-ventricular coupling and preservation of myocardial integrity. To unravel the importance of specific signaling pathways (e.g. MEK1-ERK1/2 and calcineurin-NFAT) in ischemia-reperfusion-induced cell death techniques of transgenesis (e.g. MEK1 and GSK-3B transgenic mice) and gene targeting (e.g. ERK1 homozygous, ERK2 heterozygous and CnAB homozygous knockout mice) have been used. In chapter 4 we analyzed ERK1 homozygous knockout mice, ERK2 heterozygous knockout mice, and transgenic mice with activated MEK1-ERK1/2 signaling in the heart to determine a direct causal relationship between ERK1/2 signaling and cardioprotection. A direct cardioprotective role for ERK signaling in the heart following ischemia-reperfusion injury was demonstrated. In chapter 5 we investigate the role the calcium/calmodulin-activated protein phosphatase calcineurin plays in modulating cardiac apoptosis following acute ischemia-reperfusion injury in the heart. Calcineurin Aß (CnAB) gene targeted mice showed a greater loss of viable myocardium, more apoptosis, and a greater loss in functional performance following ischemia-reperfusion injury when compared to strain-matched wildtype control mice. Increased cell death was associated with a reduction of NFAT activity. The study presented in chapter 6 was designed to elucidate the role of GSK-3B following ischemia-reperfusion. In GSK-3B transgenic mice, ischemia led to a larger infarct area and subsequent worsened cardiac performance, while post-ischemic hypertrophic remodeling was blunted. In previous studies activation of both MEK1-ERK1/2 and calcineurin-NFAT signaling pathways were associated with hypertrophic cellular growth. The present thesis proves their involvement in anti-apoptotic protection against ischemia-reperfusion injury. The dualistic role of both resuscitative survival pathways in distinct pathogenic conditions as cardiomyocyte hypertrophy and apoptosis, is a fascinating observation. To further delineate the specific roles of individual survival pathways and investigate their potential use in clinical practice, future research should focus on studying multiple signaling cascades simultaneously and their interplay. A possible option is to cross-breed different genetically modified mice, thereby creating double knockout or knockout/transgenic mice. Another possibility could come from dynamic molecular imaging, through which protein function and interaction could be visualized. The treatment of cardiac ischemia will eventually be improved significantly through new developments in molecular science