Graduation Year

2010

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Molecular Pharmacology and Physiology

Major Professor

Eric S. Bennett, Ph.D.

Committee Member

Jahanshah Amin, Ph.D.

Committee Member

Jay B. Dean, Ph.D.

Committee Member

Andreas G. Seyfang, Ph.D.

Committee Member

Kay-Pong D. Yip, Ph.D.

Keywords

glycans, K+, arrhythmias, sugars, drug block

Abstract

Arrhythmias are often caused by aberrant ion channel activity, resulting in remodeling of the cardiac action potential. Two K

+ currents, IKs and IKr, contribute to phase III repolarization of the human cardiac action potential. Human ether-a-go-go-related gene 1 (hERG1), a voltage-gated potassium channel, underlies IKr. Alterations in the repolarization phase of the action potential, and in particular IKr, can lead to arrhythmias, long or short QT syndrome, heart disease, and sudden cardiac death. HERG1A has two putative N-glycosylation sites located in the S5-S6 linker region, one of which is N-glycosylated. The aim of the first study was to determine whether and how N-linked glycosylation modifies hERG1A channel function. Voltage-dependent gating and kinetics of hERG1A were evaluated under conditions of full glycosylation, no sialylation, in the absence of complex N-glycans, and following the removal of the full N-glycosylation structure. The hERG1A steady state activation relationship was shifted linearly along the voltage axis by a depolarizing ~9 mV under each condition of reduced glycosylation. Steady state channel availability curves were shifted by a much greater depolarizing 20–30 mV under conditions of reduced glycosylation. There was no significant difference in steady state gating parameters among the less glycosylated channels, suggesting that channel sialic acids are responsible for

most of the effect of N-glycans on hERG1A gating. A large rightward shift in hERG1A window current for the less glycosylated channels was caused by the observed depolarizing shifts in steady state activation and inactivation. The much larger shift in inactivation compared to activation leads to an increase in hERG1A window current. Together, these data suggest that there is an increase in the persistent hERG current that occurs at more depolarized potentials under conditions of reduced glycosylation. This would lead to increased hERG1A activity during the AP, effectively increasing the rate of repolarization, and reducing AP duration, as observed through in silico modeling of the ventricular AP. The data describe a novel mechanism by which hERG1A activity is modulated by physiological and pathological changes in hERG1A glycosylation, with increased channel sialylation causing a loss of hERG1A activity that would likely cause an extension of the ventricular AP. The second study was to evaluate possible changes in antibiotic drug block as a result of alterations to N-glycosylation. We determined that N-glycans play a protective role on the hERG1A channel. SMX, Erythromycin, and Penicillin G were assessed individually at three concentrations. The data showed increases in antibiotic block with decreases in N-glycans. In addition, alterations in the voltage-dependence of block with changes in N-glycans were observed. SMX block was voltage-independent at each drug concentration under conditions of reduced sialylation only. Overall, these data indicate a functional role for N-glycosylation in the modulation of hERG1A antibiotic block, suggesting that even small changes in channel N-glycosylation modulate hERG1A block, and thereby likely impact the rate of action potential repolarization. The data from these studies enhances our understanding of the role of N-glycosylation on hERG1A function and drug block, and how that role will impact the cardiac action potential and overall cardiac excitability.

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