Cardiomyocyte-mediated mechanisms of ischemia



 DESCRIPTION (provided by applicant): Sudden cardiac death (SCD) from ventricular tachyarrhythmia's is a leading cause of death in adults and costs more than 300,000 lives annually in the US alone, with myocardial ischemia being the most common cause. Here we present data that strongly support the novel hypothesis that sensitizing the myofilaments to Ca (cardiomyocyte) impairs local coronary perfusion (vasculature) to generate an arrhythmia-susceptible heart. Myofilament Ca sensitization occurs in ischemic (ICM) and familial hypertrophic cardiomyopathy (HCM). We found that Ca sensitization, due to a HCM-linked mutation (TnT-I79N) or treatment with the Ca sensitizer EMD57033, increases the susceptibility to ventricular tachycardia during stress. The arrhythmias are the result of arrhythmogenic regional gap junctional uncoupling and conduction velocity slowing linked to focal energy deprivation. More specifically, Ca sensitized hearts showed focal accumulation of the P0 connexin 43 phospho-isoform (Cx43-P0) and phosphorylated AMP-kinase, both markers for cellular energy deprivation. Unknown is though how myofilament Ca sensitization causes myocardial energy deprivation and why this occurs in some regions and not others. Our preliminary data indicate that ischemia is at least in part responsible for the focal energy deprivation. Regions with increased Cx43-P0 become hypoxic during stress and this is associated with drastically reduced coronary perfusion compared to non-hypoxic regions. Our preliminary data also suggest that pannexin (Px) channels provide a critical link between myofilament Ca sensitization and focal energy deprivation. Px1 channels form large pores, release paracrine signaling molecules like ATP and are implicated in long-range signaling between different cell types. Expression of Px1 increases post-ischemia and is heterogeneously increased in TnT-I79N ventricle. Strikingly, global Px1 ablation suppressed focal energy deprivation and arrhythmias in TnT-I79N mice. Our overarching hypothesis is that myofilament Ca sensitization impairs local coronary perfusion and leads to focal ischemia during stress, mediated in part by Px1. We will test this hypothesis by quantifying the energy deficit directly, b determining if structural properties predispose certain regions to energy deprivation (Aim 1) and how myofilament Ca sensitization impairs coronary perfusion (Aim 2). We will test the contribution of paracrine signaling via Px1 for the generation of focal ischemia (Aim 3). These experiments will delineate a novel mechanistic pathway linking myocyte properties to altered local blood flow to generate an arrhythmia- prone heart. The results will advance the long-term goals of understanding mechanisms of SCD, defining strategies to prevent it and identifying novel therapeutic targets.