Ankyrin B (AnkB) is an "adaptor" protein that anchors several membrane proteins to the cytoskeleton. AnkB deficiency emerged as an important pro-arrhythmic factor a few years ago, when it was found that AnkB loss- of-function mutations generate human long QT syndrome type 4 (LQT4), the only LQT produced by alterations in a protein other than an ion channel. Besides LQT4, humans with AnkB loss-of-function mutations display a complex cardiac phenotype that also includes bradycardia, stress-induced ventricular arrhythmias and sudden cardiac death. The mechanisms responsible for such a phenotype are largely unknown. AnkB protein expression and distribution are drastically altered in the infarct border zone after myocardial infarction (MI). This may result in a mechanism for post-MI remodeling and arrhythmogenesis similar to that found in patients with AnkB loss-of-function mutations. Direct interaction with AnkB is required for the membrane targeting and stability of the Na/Ca exchanger (NCX) and Na/K-ATPase (NKA), which are essential in the regulation of cardiac [Na]i and [Ca]i and thus contractility and potential for triggered arrhythmias. The severity of the human cardiac phenotype generated by various AnkB loss-of-function mutants directly relates to the inability of those mutants to target NCX and NKA correctly to the cardiac myocyte sarcolemma. Thus, altered NCX and NKA expression and membrane distribution are key to the cardiac phenotype generated by AnkB loss-of-function. This phenotype is largely reproduced in mice heterozygous for a null mutation in AnkB (AnkB mice). Myocytes from AnkB mice show modestly reduced NCX and NKA protein expression, predominantly at the T-tubules, larger cellular and sarcoplasmic reticulum (SR) Ca load and increased frequency of early afterdepolarizations (EAD) and delayed afterdepolarizations (DAD). Despite its physiological and pathophysiological significance, the role of AnkB in regulating cardiac [Ca]i and arrhythmogenesis is poorly understood. The overall goal of this proposal is to decipher the mechanisms responsible for altered cardiac Ca regulation and triggered arrhythmias induced by AnkB loss-of-function (inherited and acquired). We will combine measurements of [Na]i and [Ca]i (in the bulk and junctional cleft), patch-clamp and molecular biology techniques in isolated cardiac myocytes in three Specific Aims. First, I will test several specific hypotheses aimed at understanding how AnkB affects intracellular Ca in the heart.
Aim 2 will focus on the mechanisms responsible for the occurrence of early and delayed afterdepolarizations in myocytes with AnkB deficiency. In the final aim I will test the hypothesis that AnkB proteolysis by calpain and the ensuing remodeling in NCX and NKA is a more general mechanism for triggered ventricular arrhythmias. These studies will both advance our understanding of how AnkB affects cardiac [Ca]i regulation and will provide key mechanistic information that could lead to the development of new treatments for patients with ventricular arrhythmias associated with inherited or acquired AnkB loss-of-function.
The proposed work will identify the main factors responsible for altered Ca regulation and triggered ventricular arrhythmias generated by ankyrin B deficiency. This project will also determine to what extent calpain- mediated proteolysis of ankyrin B, which occurs in pathophysiological conditions such as ischemia/reperfusion, leads to a mechanism for cardiac dysfunction and arrhythmogenesis similar to that seen with genetic loss-of- function of ankyrin B. Thus, these studies will both advance our understanding of how ankyrin B affects cardiac Ca regulation and will provide key mechanistic information that could lead to the development of new treatments for patients with ventricular arrhythmias associated with inherited or acquired loss-of-function of ankyrin B.
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