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

Abstract

Malignant hyperthermia (MH) is a life-threatening hypermetabolic condition caused by dysfunctional Ca 2؉ homeostasis in skeletal muscle, which primarily originates from genetic alterations in the Ca 2؉ release channel (ryanodine receptor, RyR1) of the sarcoplasmic reticulum (SR). Owing to its physical interaction with the dihydropyridine receptor (DHPR), RyR1 is controlled by the electrical potential across the transverse tubular (TT) membrane. The DHPR exhibits both voltage-dependent activation and inactivation. Here we determined the impact of an MH mutation in RyR1 (Y522S) on these processes in adult muscle fibers isolated from heterozygous RyR1 Y522S -knock-in mice. The voltage dependence of DHPRtriggered Ca 2؉ release flux was left-shifted by Ϸ8 mV. As a consequence, the voltage window for steady-state Ca 2؉ release extended to more negative holding potentials in muscle fibers of the RyR1 Y522S -mice. A rise in temperature from 20°to 30°C caused a further shift to more negative potentials of this window (by Ϸ20 mV). The activation of the DHPR-mediated Ca 2؉ current was minimally changed by the mutation. However, surprisingly, the voltage dependence of steady-state inactivation of DHPR-mediated calcium conductance and release were also shifted by Ϸ10 mV to more negative potentials, indicating a retrograde action of the RyR1 mutation on DHPR inactivation that limits window Ca 2؉ release. This effect serves as a compensatory response to the lowered voltage threshold for Ca 2؉ release caused by the Y522S mutation and represents a novel mechanism to counteract excessive Ca 2؉ leak and store depletion in MH-susceptible muscle.dihydropyridine receptor ͉ excitation-contraction coupling ͉ malignant hyperthermia ͉ mouse skeletal muscle ͉ ryanodine receptor C ontraction and relaxation of skeletal muscle fibers involve a carefully controlled release and reuptake of Ca 2ϩ ions stored in the sarcoplasmic reticulum (SR). Malignant hyperthermia (MH), a life-threatening hypermetabolic state accompanied by hyperthermia, hypoxia, hypercapnia, acidosis and muscle rigidity (1Ϫ3), results from uncontrolled release of Ca 2ϩ in skeletal muscle. Episodes of MH are triggered in genetically predisposed individuals by certain pharmaceutical agents, particularly volatile anesthetics and depolarizing muscle relaxants. A large number of different mutations leading to MH susceptibility has been identified (4). These mutations primarily reside in the gene coding for the skeletal muscle isoform of the ryanodine receptor (RyR1), the major Ca 2ϩ release channel of the SR. Activation of intracellular Ca 2ϩ release results from a conformational interaction between RyR1 channels in the SR and a specialized voltage-sensitive Ca 2ϩ channel (L-type Ca 2ϩ channel), the dihydropyridine receptor (DHPR), located in the adjacent membrane of the transverse tubular (TT) system. Upon membrane depolarization, the DHPR exhibits a fast reaction leading to Ca 2ϩ release (Ϸ10 milliseconds), a slower gating transition that activates the L-type Ca 2ϩ inward curre...

Malignant hyperthermia susceptibility arising from altered resting coupling between the skeletal muscle L-type Ca 2+ channel and the type 1 ryanodine receptor

“…To determine whether the R174W mutation altered EC coupling, as has been demonstrated for other MH-causing mutations in either RyR1 (14)(15)(16)(17)(18)(19)(20)(25)(26)(27) or Ca V 1.1 (21), we measured myoplasmic Ca 2+ transients in the whole-cell configuration. As shown in Fig.…”

“…Previously characterized mutations in either channel have been found to promote activation of RyR1 during EC coupling and/or to enhance Ca 2+ entry via Ca V 1.1 (13)(14)(15)(16)(17)(18)(19)(20)(21). Thus, it has been proposed that augmented Ca 2+ movements via the EC coupling pathway or via the L-type current underlie the fulminant response to MH triggers.…”

New factors contributing to dynamic calcium regulation in the skeletal muscle triad—a crowded place

“…An important pathophysiological implication of altered bidirectional coupling was reported recently in the context of malignant hyperthermia using RyR1 Y522S knock-in mice (Andronache et al 2009). Animals with this mutation suffer from potentially lethal episodes of hyperthermia triggered by elevated ambient temperatures.…”

“…This result indicates a mechanism different from a direct physical link between both molecules. Other mechanisms of interaction discussed involve covalent modification of the DHPR, reactive oxygen species, and nitrosylation (Durham et al 2008;Andronache et al 2009). Thus, indirect models of interaction will have to be tested, possibly involving other molecular "players" in the crowded field of the triad junction and especially in the macromolecular complex of the tetrade.…”

Excitation-contraction coupling and mechano-sensitivity in denervated skeletal muscles

“…The idea to be considered is that manipulations of the RyR should reciprocally influence the behavior of any charge components with which it makes allosteric contact [4] and should spare only charge components that are not supposed to originate in the L-CaC and/or that are scarcely involved in ECC. Indeed, experimental results suggests a more complex functional communication.…”