NAV1.5 MUTATIONS ASSOCIATED WITH BRUGADA SYNDROME RESULT IN LOSS OF FUNCTIONAL APOCALMODULIN REGULATION

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Date
2015-05-06
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Johns Hopkins University
Abstract
The cardiac voltage-gated sodium (Na) channels NaV1.5 play a prominent role in cardiac action potential genesis and spatial propagation. Given its key role in the normal function of the heart, a variety of auxiliary proteins are primed to regulate NaV1.5 function. One such auxiliary proteins, the Ca2+ binding protein calmodulin (CaM), is known to associate with the carboxy-terminus of NaV1.5 for over a decade, yet the existence and nature of CaM regulation of NaV1.5 has so far evaded consensus. Although CaM has long been suspected to play a role in Ca2+ regulation of NaV1.5, NaV1.5 appears to feature neither steady-state nor transient responses to elevated intracellular Ca2+. Resolution to this conundrum may take inspiration from the recent discovery that Ca2+-free CaM (apoCaM) can intrinsically regulate ion channel functions such as CaV1.3 and NaV1.4. Here, using single channel patch clamp analysis, we find that apoCaM regulates open probability of NaV1.5 through association to the channel’s carboxy-terminus. Furthermore, loss of functional apoCaM regulation appears to be the cause of decreased Na current density in the Brugada syndrome disease mutant NaV1.5S1904L. Mechanistically, biophysical analysis of NaV1.5 reveals apoCaM to bias the channels to preferentially inactivate from the open state as opposed to the closed states. One therapeutic strategy for targeted delivery of CaM to mutant NaV1.5S1904L is demonstrated with CaM-linked β1 subunit. Overall, we uncovered a new CaM regulatory mechanism of NaV1.5 with promises of intimate disease relevance. This study furthers the exciting hypothesis of a unified CaM regulatory mechanism across different Na channel isoforms, and may provide a general therapeutic strategy for a diverse profile Na channelopathies related to functional loss of apoCaM regulation.
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Keywords
NaV1.5, cardiac sodium channels, calmodulin, calmodulation, apocalmodulin regulation, Brugada syndrome
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