Author's School

Graduate School of Arts & Sciences

Author's Department/Program

Biology and Biomedical Sciences: Molecular Genetics and Genomics

Language

English (en)

Date of Award

1-1-2012

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Jeanne Nerbonne

Abstract

Pathologic biomechanical stresses cause cardiac hypertrophy, which is associated with QT prolongation and increased risk of life-threatening ventricular arrhythmias. Previous studies demonstrated that repolarizing K+ current densities are decreased in pressure overload-induced left ventricular hypertrophy, resulting in action potential and QT prolongation. Cardiac hypertrophy also occurs with exercise training, but this "physiological hypertrophy" is not associated with electrical abnormalities or increased arrhythmia risk, suggesting that repolarizing K+ currents are upregulated, in parallel with the increase in myocyte size, to maintain normal cardiac function. To explore this hypothesis directly, two mouse models of physiological hypertrophy, one produced by chronic exercise: swim-) training of wild type mice and the other by cardiac-specific expression of constitutively active phosphoinositide-3-kinase-p110α: caPI3Kα), were utilized. Electrophysiological experiments revealed that repolarizing K+ current amplitudes were increased and K+ current densities were normalized in hypertrophied ventricular myocytes from swim-trained or caPI3Kα animals. Molecular analyses revealed that increases in K+currents reflect the upregulation of the transcripts encoding the underlying K+ channel subunits. Importantly, additional experiments demonstrated that the transcriptional upregulation of myocardial K+ channel expression in response to exercise or augmented PI3Kα signaling is independent of cellular hypertrophy and Akt signaling. The hypothesis that increased PI3Kα signaling can counteract the adverse electrophysiological remodeling, including decreased K+ current densities and impaired repolarization associated with pathological hypertrophy and heart failure was also explored. These experiments revealed that increased PI3Kα signaling, but not renin-angiotensin system blockade, results in transcriptional upregulation of repolarizing K+ channel subunits and normalization of K+ current densities in transverse aortic constriction: TAC)-induced pathological hypertrophy, as well as in a transgenic mouse model of dilated cardiomyopathy/heart failure. Increased PI3Kα signaling, therefore, normalizes ventricular action potential durations, QT intervals and cardiac electrical functioning in the hypertrophied and failing heart. Additional studies here applied a combined miRNA- and RNA-sequencing approach to define the impact of enhanced PI3Kα signaling on myocardial transcriptome structure in the setting of pressure overload-induced pathological left ventricular hypertrophy. These analyses revealed that enhanced PI3Kα signaling normalized miRNAs and mRNAs that were aberrantly expressed in pathological hypertrophy and that increased PI3Kα signaling reduces cardiac fibrosis in pathological hypertrophy through the modulation of TGF-β signaling and miR-21 expression. In conclusion, enhanced PI3Kα signaling results in the transcriptional upregulation of K+ channel subunits and the maintenance of cardiac excitability in physiological hypertrophy, and the impact of increased PI3Kα signaling on K+ channel regulation is independent of cellular hypertrophy and Akt. In addition, enhancing PI3Kα signaling increases repolarizing K+ currents and K+ channel subunit expression in mouse models of pathological hypertrophy and heart failure, ameliorating arrhythmogenic electrical remodeling. Augmentation of PI3Kα signaling, therefore, may be a useful and unique strategy to protect against the increased risk of ventricular arrhythmias and sudden death associated with cardiomyopathy. The results here also demonstrate the power and robustness of next-generation sequencing in efforts to define the cardiac transcriptome architecture and dynamics in physiological and pathological contexts, as well as to identify novel molecular mechanisms important in cardiovascular pathophysiology.

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Permanent URL: http://dx.doi.org/10.7936/K7P848X4

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