Absence of the γ Subunit of the Skeletal Muscle Dihydropyridine Receptor Increases L-type Ca2+ Currents and Alters Channel Inactivation Properties

Abstract

In skeletal muscle the oligomeric ␣ 1S , ␣ 2 /␦-1 or ␣ 2 /␦-2, ␤1, and ␥1 L-type Ca 2؉ channel or dihydropyridine receptor functions as a voltage sensor for excitation contraction coupling and is responsible for the L-type Ca 2؉ current. The ␥1 subunit, which is tightly associated with this Ca 2؉ channel, is a membrane-spanning protein exclusively expressed in skeletal muscle. Previously, heterologous expression studies revealed that ␥1 might modulate Ca 2؉ currents expressed by the pore subunit found in heart, ␣ 1C , shifting steady state inactivation, and increasing current amplitude. To determine the role of ␥1 assembled with the skeletal subunit composition in vivo, we used gene targeting to establish a mouse model, in which ␥1 expression is eliminated. Comparing litter-matched mice with control mice, we found that, in contrast to heterologous expression studies, the loss of ␥1 significantly increased the amplitude of peak dihydropyridine-sensitive I Ca in isolated myotubes. Whereas the activation kinetics of the current remained unchanged, inactivation of the current was slowed in ␥1-deficient myotubes and, correspondingly, steady state inactivation of I Ca was shifted to more positive membrane potentials. These results indicate that ␥1 decreases the amount of Ca 2؉ entry during stimulation of skeletal muscle.

Stac3 has a direct role in skeletal muscle-type excitation–contraction coupling that is disrupted by a myopathy-causing mutation

“…Previously, the functional effects of the γ 1 subunit on Ca V 1.1 function had been investigated by analyzing myotubes or muscle fibers in which γ 1 had been knocked out. The KO of γ 1 caused the Ca 2+ current via Ca V 1.1 in myotubes to be about one-third larger and to decay more slowly during prolonged pulses (4,23). Additionally, the voltage for half-inactivation of current was positively shifted by ∼9 mV in γ 1 KO myotubes and ∼14 mV in γ 1 KO muscle fibers.…”

“…Thus, KO of γ 1 (4,5) or knockdown of α 2 -δ 1 (6, 7) does not have major effects on the EC coupling and channel functions of Ca V 1.1, whereas KO of β 1a causes the loss of EC coupling (8). In part, this is because β 1a is required for efficient trafficking of Ca V 1.1 to the plasma membrane (9).…”

“…Thus, we hypothesized that RyR1 somehow relieves the inhibitory effect of another protein that is present in myotubes and absent in tsA201 cells. An obvious candidate is the remaining Ca V 1.1 auxiliary subunit, γ 1 , which we had omitted in the previous experiments because its KO was reported to have only modest effects on Ca 2+ currents in mouse skeletal muscle (4,5,17). Fig.…”

The β 1a subunit is essential for the assembly of dihydropyridine-receptor arrays in skeletal muscle

“…Targeted deletions of the ␣ 2 ␦-1 and ␥ subunits do not critically interfere with EC coupling function (6,7). In contrast, ␣ 1S and ␤ 1a subunit null-mutant mice display a lack of EC coupling and, thus, lethal muscle paralysis (8,9).…”

Mutations in voltage-gated L-type calcium channel: implications in cardiac arrhythmia

“…The γ subunit distinguishingly modulates the functions by altering both activation and inactivation properties of LTCC [46]. For example, targeted disruption of γ 1 subunit increased the amplitude of peak calcium current in isolated myotubes [47]; whereas coexpression with γ 2 subunit could depolarize the activation and inactivation curve of Ca V 1.2 channels [48]. In the heart tissue, four different γ subunits, γ 4 , γ 6 , γ 7 and γ 8 , are expressed, and they all physically interact with the Ca V 1.2 complex, which diversify the functions of Ca V 1.2 channels [46].…”