Date of Award
Doctor of Science (DSc)
Cardiac arrhythmias affects millions of Americans and can lead to sudden cardiac death, accounting for more than 300,000 deaths annually. Despite the vast knowledge available for cardiac disease and associated arrhythmias, very few effective therapies exist. Current interventions include cardioverter defibrillators and antiarrhythmic drugs targeting ion channels or the _-adrenergic pathway. In most acquired and inherited arrhythmias, molecular signaling pathways are perturbed. However, little is known about the underlying mechanism of how these signaling pathways regulate cardiac electrophysiology. Therefore, a better understanding of major signaling pathways governing cardiac development or dysregulated in cardiac disease could lead to novel therapeutics for the treatment of arrhythmias. In my thesis dissertation, I show that the Wnt signaling pathway directly regulates genes relevant for cardiac electrophysiology in both the embryonic and adult murine heart. Additionally, I found that pharmacologic inhibition of the glycogen synthase kinase 3 pathway in the adult human heart results in electrophysiological changes that could contribute to an arrhythmogenic substrate.
In the first part of my thesis, I show that developmental perturbation of Wnt signaling leads to chamber-specific transcriptional regulation of genes important in cardiac conduction that persists into adulthood. Transcriptional profiling of right versus left ventricles in mice deficient in Wnt transcriptional activity reveals global chamber differences, including genes regulating cardiac electrophysiology such as Gja1 and Scn5a. In addition, the transcriptional repressor Hey2, a gene associated with Brugada syndrome, is a direct target of Wnt signaling in the right ventricle only. These transcriptional changes lead to perturbed right ventricular cardiac conduction and cellular excitability. Ex vivo and in vivo stimulation of the right ventricle is sufficient to induce ventricular tachycardia in Wnt transcriptionally inactive hearts, while left ventricular stimulation has no effect. These data show that embryonic perturbation of Wnt signaling in cardiomyocytes leads to right ventricular arrhythmia susceptibility in the adult heart through chamber-specific regulation of genes regulating cellular electrophysiology.
In the second part of my thesis, I use a human cardiac slice culture platform to show that inhibition of glycogen synthase kinase 3 (GSK-3) pathway alters the cardiac electrical substrate. Glycogen synthase kinase 3 (GSK-3) is a multifunctional regulatory kinase that has emerged as a potential therapeutic target for several diseases, including cancer, diabetes, and bipolar disorder. In the heart, dysregulation of GSK-3 has been implicated in pathological conditions such as cardiac hypertrophy, ischemic injury, and heart failure, which are often accompanied by arrhythmias. Despite growing evidence that dysregulation of GSK-3 is associated with arrhythmias, the precise role for GSK-3 regulation of human cardiac electrophysiology remains poorly defined. Human cardiac slices cultured with the GSK-3 inhibitor SB216763 showed reduced conduction velocity at 3 and 24 hours in culture compared to vehicle controls. Action potential measurements revealed decreased excitability in GSK-3 treated slices, as measured by decreased dVm/dtmax. Computational simulations demonstrated that decreased sodium channel conductance and tissue conductivity are plausible causes for observed reduction in conduction velocity and dVm/dtmax. _-catenin, a target of GSK-3 and the transcriptional effector of Wnt signaling, increased within cardiomyocyte nuclei as expected. However, transcription of Wnt/_-catenin target genes and SCN5A were unchanged, while NaV1.5 protein, the major cardiac sodium channel subunit, decreased after 3 hours in culture with SB216763. This suggests that acute GSK-3 inhibition-mediated regulation of cardiac electrophysiology can occur in part through post-transcriptional mechanisms, and that therapeutic strategies targeting the GSK-3 pathway may be associated with an increase in adverse cardiovascular effects.
I demonstrate here that two inter-related signaling pathways, Wnt and GSK-3, regulate cardiac electrophysiology through modulation of ion channels regulating cardiac conduction via transcriptional and post-transcriptional mechanisms. In addition to the contribution to basic research through advancing our overall understanding of molecular signaling pathways in cardiac conduction and arrhythmias, these results may provide insight for translational pre-clinical research to aid in the development of novel antiarrhythmic drugs.
Colin Nichols, Jonathan Silva, Nathaniel Huebsch, Richard Schuessler,