MitoBK_Ca channel is functionally associated with its regulatory β1 subunit in cardiac mitochondria

Key points: Association of plasma membrane BK_Ca channels with BK-β subunits shapes their biophysical properties and physiological roles; however, functional modulation of the mitochondrial BK_Ca channel (mitoBK_Ca ) by BK-β subunits is not established. MitoBK_Ca -α and the regulatory BK-β1 subunit associate in mouse cardiac mitochondria. A large fraction of mitoBK_Ca display properties similar to that of plasma membrane BK_Ca when associated with BK-β1 (left-shifted voltage dependence of activation, V_1/2 = -55 mV, 12 µm matrix Ca²⁺ ). In BK-β1 knockout mice, cardiac mitoBK_Ca displayed a low P_o and a depolarized V_1/2 of activation (+47 mV at 12 µm matrix Ca²⁺ ) Co-expression of BK_Ca with the BK-β1 subunit in HeLa cells doubled the density of BK_Ca in mitochondria. The present study supports the view that the cardiac mitoBK_Ca channel is functionally modulated by the BK-β1 subunit; proper targeting and activation of mitoBK_Ca shapes mitochondrial Ca²⁺ handling.

Abstract: Association of the plasma membrane BK_Ca 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 (mitoBK_Ca ) with regulatory subunits is unknown. We report that mitoBK_Ca functionally associates with its regulatory subunit BK-β1 in adult rodent cardiomyocytes. Cardiac mitoBK_Ca is a calcium- and voltage-activated channel that is sensitive to paxilline with a large conductance for K⁺ of 300 pS. Additionally, mitoBK_Ca displays a high open probability (P_o ) and voltage half-activation (V_1/2 = -55 mV, n = 7) resembling that of plasma membrane BK_Ca when associated with its regulatory BK-β1 subunit. Immunochemistry assays demonstrated an interaction between mitochondrial BK_Ca -α and its BK-β1 subunit. Mitochondria from the BK-β1 knockout (KO) mice showed sparse mitoBK_Ca currents (five patches with mitoBK_Ca activity out of 28 total patches from n = 5 different hearts), displaying a depolarized V_1/2 of activation (+47 mV in 12 µm matrix Ca²⁺ ). The reduced activity of mitoBK_Ca was accompanied by a high expression of BK_Ca transcript in the BK-β1 KO, suggesting a lower abundance of mitoBK_Ca channels in this genotype. Accordingly, BK-β1subunit increased the localization of BKDEC (i.e. the splice variant of BK_Ca 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 mitoBK_Ca associates with its regulatory BK-β1 subunit in cardiac mitochondria, ensuring proper targeting and activation of the mitoBK_Ca channel that helps to maintain mitochondrial Ca²⁺ homeostasis.