Calmodulin kinase II cascade in cardiac (patho)physiology

Heart failure is a common disease with a prevalence of 1-2% in the overall population and above 10% in people older than 75 years. The early course of heart failure is often clinically silent over years and current guidelines for diagnosis are based on overt symptoms and evidence of structural and functional impairment of the heart. Accordingly, current guidelines urge for better understanding of remodeling processes underlying the disease initiation which may lead to development of novel and effective therapeutic strategies. However, to date, no specific therapy is available for the early stage of heart failure. Growing evidence suggests that disturbed Ca2+ handling is an early event in myocardial remodeling and that it may be causally involved in the development of heart failure by Ca2+- mediated regulation of gene expression. The enzyme calmodulin kinase II (CaMKII) is able to translate a diverse set of Ca2+ signaling events into the regulation of gene expression and it has become one of the most promising therapeutic drug targets in cardiac biology. While a range of different modes of CaMKII activation have been identied, relatively little is known about subcellular localization and isoform-specific CaMKII modications and in what way these factors interact to impact on the overall outcomes of gene expression regulation. Due to the large spectrum of CaMKII functions in the heart, a non-selective pharmacological inhibition of global CaMKIId may not be a maximally effective approach. Hence, a more nuanced understanding of context-specic effects for development of cell location-targeted and isoform-specic therapeutic interventions holds a great promise. Using the most powerful quantification and visualization techniques (electron and confocal microscopy), we will test the hypothesis that stimuli released during cardiac stress and high beating frequency can raise cellular [Ca2+] levels and activate elements of the CaMKII signaling cascade. With this technical know-how, we will study tissue and isolated cardiomyocytes from non-failing and failing human hearts and mouse models with genetic deletion or overexpression of specific CaMKII isoforms. In particular, we will look in which cellular regions the activation happens and which sets of downstream targets are affected by specific CaMKII variants. Our results should lead to the development of novel therapeutic strategies to treat early stages of heart failure, thereby leading to attenuated cardiac remodeling and less frequent hospitalizations. By acting on a limited subset of downstream targets this interventions would improve efficacy and minimize off-target effects in patients with heart failure.

Heart failure represents a growing problem worldwide due to the ageing population and a lack of curative therapies. In Austria, over 300,000 patients suffer from symptomatic heart failure, and about 15,000 are newly diagnosed each year. The estimated costs in excess of EUR 15,000 per hospitalization contribute to the high socio-economic burden of the disease. Despite being among the largest unmet medical needs in cardiovascular medicine, no specific therapy is available to halt the progression of heart failure in an early stage. The development of heart failure features a large number of changes in the heart, commonly termed as cardiac remodeling. Cardiac remodeling is initially characterized by thickening of the heart muscle - hypertrophy - which may help when an increased pumping strength is needed. However, on the long run it leads to structural and functional damage of the heart and its functional failure. Accumulating evidence positions dysregulation in calcium homeostasis (calcium is the main regulator of cardiac contraction) as an early promoter of cardiac remodelling via activation of CaMKII (calcium/calmodulin-dependent protein kinase II), cellular enzyme which can regulate transcription of multiple proteins involved in hypertrophy of heart muscle cells. On the other hand, some of the effects mediated by CaMKII are considered adaptive, making global CaMKII inhibition suboptimal approach to developing therapeutic interventions. Therefore, the aim of this project was to provide a more nuanced understanding of CaMKII activation profile to be able to target CaMKII specifically where it is detrimental. Using experimental models of cardiac remodeling and failure, we demonstrated that there is over-proportional CaMKII activation in different cellular compartments as cardiomyocytes are becoming more and more dysfunctional. In the early phase of cardiac remodeling, active CaMKII accumulates around the cell nucleus where it speeds up removal of calcium during diastole and thereby prevents nuclear calcium overload. In contrast, as the remodeling progresses, active CaMKII accumulates inside the cell nuclei (rather than just around it) and it activates transcription of genes implicated hypertrophic cellular growth and functional decay. Detailed information on CaMKII activation pattern obtained in this project will pave the way to designing subcellularly targeted inhibitors with improved efficacy and minimal off-target effects for better management of adverse cardiac remodeling.