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The adhesion function of the sodium channel beta subunit (β1) contributes to cardiac action potential propagation.

ABSTRACT: Computational modeling indicates that cardiac conduction may involve ephaptic coupling - intercellular communication involving electrochemical signaling across narrow extracellular clefts between cardiomyocytes. We hypothesized that β1(SCN1B) -mediated adhesion scaffolds trans-activating Na_V1.5 (SCN5A) channels within narrow (<30 nm) perinexal clefts adjacent to gap junctions (GJs), facilitating ephaptic coupling. Super-resolution imaging indicated preferential β1 localization at the perinexus, where it co-locates with Na_V1.5. Smart patch clamp (SPC) indicated greater sodium current density (I_Na) at perinexi, relative to non-junctional sites. A novel, rationally designed peptide, βadp1, potently and selectively inhibited β1-mediated adhesion, in electric cell-substrate impedance sensing studies. βadp1 significantly widened perinexi in guinea pig ventricles, and selectively reduced perinexal I_Na, but not whole cell I_Na, in myocyte monolayers. In optical mapping studies, βadp1 precipitated arrhythmogenic conduction slowing. In summary, β1-mediated adhesion at the perinexus facilitates action potential propagation between cardiomyocytes, and may represent a novel target for anti-arrhythmic therapies.

SUBMITTER: Veeraraghavan R

PROVIDER: S-EPMC6122953 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Key pointsAssociation of plasma membrane BKCa channels with BK-β subunits shapes their biophysical properties and physiological roles; however, functional modulation of the mitochondrial BKCa channel (mitoBKCa ) by BK-β subunits is not established. MitoBKCa -α and the regulatory BK-β1 subunit associate in mouse cardiac mitochondria. A large fraction of mitoBKCa display properties similar to that of plasma membrane BKCa when associated with BK-β1 (left-shifted voltage dependence of activation, V1/2 = -55 mV, 12 µm matrix Ca2+ ). In BK-β1 knockout mice, cardiac mitoBKCa displayed a low Po and a depolarized V1/2 of activation (+47 mV at 12 µm matrix Ca2+ ) Co-expression of BKCa with the BK-β1 subunit in HeLa cells doubled the density of BKCa in mitochondria. The present study supports the view that the cardiac mitoBKCa channel is functionally modulated by the BK-β1 subunit; proper targeting and activation of mitoBKCa shapes mitochondrial Ca2+ handling.AbstractAssociation of the plasma membrane BKCa channel with auxiliary BK-β1-4 subunits profoundly affects the regulatory mechanisms and physiological processes in which this channel participates. However, functional association of mitochondrial BK (mitoBKCa ) with regulatory subunits is unknown. We report that mitoBKCa functionally associates with its regulatory subunit BK-β1 in adult rodent cardiomyocytes. Cardiac mitoBKCa is a calcium- and voltage-activated channel that is sensitive to paxilline with a large conductance for K+ of 300 pS. Additionally, mitoBKCa displays a high open probability (Po ) and voltage half-activation (V1/2 = -55 mV, n = 7) resembling that of plasma membrane BKCa when associated with its regulatory BK-β1 subunit. Immunochemistry assays demonstrated an interaction between mitochondrial BKCa -α and its BK-β1 subunit. Mitochondria from the BK-β1 knockout (KO) mice showed sparse mitoBKCa currents (five patches with mitoBKCa activity out of 28 total patches from n = 5 different hearts), displaying a depolarized V1/2 of activation (+47 mV in 12 µm matrix Ca2+ ). The reduced activity of mitoBKCa was accompanied by a high expression of BKCa transcript in the BK-β1 KO, suggesting a lower abundance of mitoBKCa channels in this genotype. Accordingly, BK-β1subunit increased the localization of BKDEC (i.e. the splice variant of BKCa that specifically targets mitochondria) into mitochondria by two-fold. Importantly, both paxilline-treated and BK-β1 KO mitochondria displayed a more rapid Ca2+ overload, featuring an early opening of the mitochondrial transition pore. We provide strong evidence that mitoBKCa associates with its regulatory BK-β1 subunit in cardiac mitochondria, ensuring proper targeting and activation of the mitoBKCa channel that helps to maintain mitochondrial Ca2+ homeostasis.

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The adhesion function of the sodium channel beta subunit (β1) contributes to cardiac action potential propagation.

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