Calcium channel γ 6 subunits are unique modulators of low voltage-activated (Cav3.1) calcium current

Abstract

The calcium channel gamma (γ) subunit family consists of eight members whose functions include modulation of high voltage-activated (HVA) calcium currents in skeletal muscle and neurons, and regulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propanoic acid (AMPA) receptor targeting. Cardiac myocytes express at least three γ subunits, γ 4, γ 6 and γ 7, whose function(s) in the heart are unknown. Here we compare the effects of the previously uncharacterized γ 6 subunit with that of γ 4 and γ 7 on a low voltage-activated calcium channel (Cav3.1) that is expressed in cardiac myoctyes. Co-expression of both the long and short γ 6 subunit isoforms, γ 6L and γ 6S, with Cav3.1 in HEK-293 cells significantly decreases current density by 49% and 69%, respectively. Two other γ subunits expressed in cardiac myocytes, γ 4 and γ 7, have no significant effect on Cav3.1 current. Neither γ 6L, γ 6S, γ 4 nor γ 7 significantly affect the voltage dependency of activation or inactivation or the kinetics of Cav3.1 current. Transient expression of γ 6L in an immortalized atrial cell line (HL-1) significantly reduces the endogenous low voltage-activated current in these cells by 63%. Green fluorescent protein tagged γ 6L is localized primarily in HEK-293 cell surface membranes where it is evenly distributed. Expression of γ 6L does not affect the level of Cav3.1 mRNA or the amount of total Cav3.1 protein in transfected HEK-293 cells. These results demonstrate that the γ 6 subunit has a unique ability to inhibit Cav3.1 dependent calcium current that is not shared with the γ 4 and γ 7 isoforms and is thus a potential regulator of cardiac low voltage-activated calcium current.