The auxiliary subunit γ ₁ of the skeletal muscle L-type Ca ²⁺ channel is an endogenous Ca ²⁺ antagonist

Abstract

Ca 2؉ channels play crucial roles in cellular signal transduction and are important targets of pharmacological agents. They are also associated with auxiliary subunits exhibiting functions that are still incompletely resolved. Skeletal muscle L-type Ca 2؉ channels (dihydropyridine receptors, DHPRs) are specialized for the remote voltage control of type 1 ryanodine receptors (RyR1) to release stored Ca 2؉ . The skeletal muscle-specific ␥ subunit of the DHPR (␥1) down-modulates availability by altering its steady state voltage dependence. The effect resembles the action of certain Ca 2؉ antagonistic drugs that are thought to stabilize inactivated states of the DHPR. In the present study we investigated the cross influence of ␥1 and Ca 2؉ antagonists by using wild-type (␥؉/؉) and ␥1 knockout (␥؊/؊) mice. We studied voltage-dependent gating of both L-type Ca 2؉ current and Ca 2؉ release and the allosteric modulation of drug binding. We found that 10 M diltiazem, a benzothiazepine drug, more than compensated for the reduction in high-affinity binding of the dihydropyridine agent isradipine caused by ␥1 elimination; 5 M devapamil [(؊)D888], a phenylalkylamine Ca 2؉ antagonist, approximately reversed the right-shifted voltage dependence of availability and the accelerated recovery kinetics of Ca 2؉ current and Ca 2؉ release. Moreover, the presence of ␥1 altered the effect of D888 on availability and strongly enhanced its impact on recovery kinetics demonstrating that ␥1 and the drug do not act independently of each other. We propose that the ␥1 subunit of the DHPR functions as an endogenous Ca 2؉ antagonist whose task may be to minimize Ca 2؉ entry and Ca 2؉ release under stress-induced conditions favoring plasmalemma depolarization.accessory subunits ͉ dihydropyridine receptor ͉ excitation-contraction coupling ͉ mouse skeletal muscle ͉ phenylalkylamine

A retrograde signal from RyR1 alters DHP receptor inactivation and limits window Ca 2+ release in muscle fibers of Y522S RyR1 knock-in mice

“…Potential targets for such modifications are the ␣ 1S polypeptide and its ancillary subunits. Notably, the ␥ 1 subunit exhibits a pronounced effect on DHPR inactivation (26). Finally, a long-term change in the expression of proteins that modify or modulate the DHPR may also be involved.…”

Three-Dimensional Localization of the α and β Subunits and of the II-III Loop in the Skeletal Muscle L-type Ca2+ Channel

“…The structure of its core region has been solved to atomic resolution for several isoforms (24 -26). The ␥ subunit (34 kDa) has four transmembrane domains with cytosolic C-and N-termini and interacts with Ca V 1.1 (27), acting as a Ca 2ϩ channel antagonist (28). The structure of the DHPR complex has been mainly studied by transmission electron microscopy (TEM) and single-particle analysis.…”

A Small Peptide Inhibitor of the Low Voltage-Activated Calcium Channel Cav3.1

“…Numerous studies have shown that auxiliary subunits ␤ and ␣ 2 ␦ modulate voltage-dependent calcium channels by promoting the membrane insertion of the ␣ 1 subunits and by enhancing channel activities (Dolphin, 2003;Klugbauer et al, 2003). Given the recent proposal that ␥ 1 is an endogenous antagonist of the Cav1.1 HVA channel (Andronache et al, 2007), our use of ␥ 6 TM1a as a drug provides strong evidence that ␥ 6 is an endogenous antagonist of the Cav3.1 channel. Because ␥ 6 displays a higher affinity toward LVA current at least in cardiomyocytes (Lin et al, 2008), perhaps the existence of ␥ 6 serves to regulate the subtle but critical amount of window current through Cav3.1 channels (Vassort et al, 2006) in pacemaker cells and atrial myocytes.…”

“…It is noteworthy that the shift in inactivation curve can be compensated by ϳ5 M phenylalkalamine D888 (Andronache et al, 2007). Furthermore, the binding of the Cav1.1 channel to its antagonist isradipine is modulated by ␥ 1 as well as a noncompetitive antagonist, diltiazem (Andronache et al, 2007), suggesting that ␥ 1 subunit acts as an endogenous antagonist of the skeletal HVA calcium current. A study by the Campbell group maps the interaction site(s) with the ␣ 1.1 subunit to the first half of the ␥ 1 molecule .…”

“…In the muscle fibers of ␥ 1 -null mice, the high voltage-activated (HVA; Cav1.1) current has increased current density and depolarized inactivation curve (Freise et al, 2000;Held et al, 2002;. It is noteworthy that the shift in inactivation curve can be compensated by ϳ5 M phenylalkalamine D888 (Andronache et al, 2007). Furthermore, the binding of the Cav1.1 channel to its antagonist isradipine is modulated by ␥ 1 as well as a noncompetitive antagonist, diltiazem (Andronache et al, 2007), suggesting that ␥ 1 subunit acts as an endogenous antagonist of the skeletal HVA calcium current.…”