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

Abstract

Malignant hyperthermia (MH) susceptibility is a dominantly inherited disorder in which volatile anesthetics trigger aberrant Ca 2+ release in skeletal muscle and a potentially fatal rise in perioperative body temperature. Mutations causing MH susceptibility have been identified in two proteins critical for excitation-contraction (EC) coupling, the type 1 ryanodine receptor (RyR1) and Ca V 1.1, the principal subunit of the L-type Ca 2+ channel. All of the mutations that have been characterized previously augment EC coupling and/or increase the rate of L-type Ca 2+ entry. The Ca V 1.1 mutation R174W associated with MH susceptibility occurs at the innermost basic residue of the IS4 voltage-sensing helix, a residue conserved among all Ca V channels [Carpenter D, et al. (2009) BMC Med Genet 10:104-115.]. To define the functional consequences of this mutation, we expressed it in dysgenic (Ca V 1.1 null) myotubes. Unlike previously described MH-linked mutations in Ca V 1.1, R174W ablated the L-type current and had no effect on EC coupling. Nonetheless, R174W increased sensitivity of Ca 2+ release to caffeine (used for MH diagnostic in vitro testing) and to volatile anesthetics. Moreover, in Ca V 1.1 R174W-expressing myotubes, resting myoplasmic Ca 2+ levels were elevated, and sarcoplasmic reticulum (SR) stores were partially depleted, compared with myotubes expressing wild-type Ca V 1.1. Our results indicate that Ca V 1.1 functions not only to activate RyR1 during EC coupling, but also to suppress resting RyR1-mediated Ca 2+ leak from the SR, and that perturbation of Ca V 1.1 negative regulation of RyR1 leak identifies a unique mechanism that can sensitize muscle cells to MH triggers.

Ca2+ Influx via the Na+/Ca2+ Exchanger Is Enhanced in Malignant Hyperthermia Skeletal Muscle

“…Furthermore, it is well established that up to 1 M (dose used in the present study) does not affect the forward mode (37 The increased NCX3 reverse mode activity in MHS murine muscle cells suggest a compensatory mechanism to overcome the lower SR Ca 2ϩ loading described in MHS muscle cells (19,25,64) as a result of RyR1-MH leakiness (21). The reduced SR Ca 2ϩ levels are likely to promote an increased Ca 2ϩ influx through Orai and TRPC channels (19), and possibly other Ca 2ϩ channels (34).…”

“…The calcium ionophore II, ETH-129 (Fluka, Sigma), and the Na ϩ ionophore I, ETH-227 (Fluka, Sigma), were used to back-fill the Ca 2ϩ and Na ϩ -selective microelectrodes, respectively. Each ion-selective microelectrode was individually calibrated as described previously (25,30). After making measurements of [Ca 2ϩ ] r and [Na ϩ ] r all microelectrodes were recalibrated, and if the two calibration curves did not agree within 3 mV, data from that microelectrode were discarded.…”

“…In more than 70% of humans with MH susceptibility (MHS) the trait is linked to one of Ͼ185 mutations within RYR1 located on ch19q13.1, the gene that encodes the type 1 skeletal isoform of the ryanodine receptor (RYR1), the primary Ca 2ϩ release channel in the sarcoplasmic reticulum (SR) (24). In addition, mutations in CACNA1S on ch1q31-32, which codes for Ca V 1.1 have also been found in 2% of MHS individuals (25)(26)(27).…”

Bridging the myoplasmic gap II: more recent advances in skeletal muscle excitation–contraction coupling

“…The E1014K mutation was designed to eliminate inward divalent permeation under standard patch-clamp recording conditions while sparing EC coupling (see also Bannister andBeam, 2009, 2011). Similarly, Eltit et al (2012) examined another mutant α 1S subunit carrying a mutation associated with malignant hyperthermia in humans. This mutation, an arginine to tryptophan exchange at position 174 occurred at a site intuitively involved in channel gating -the innermost positively charged residue in the voltage-sensing S4 α-helix of Repeat I (Carpenter et al, 2009).…”

Ca2+ handling abnormalities in early-onset muscle diseases: Novel concepts and perspectives

“…The RyR1 becomes leaky, resulting in an increased resting cytosolic [Ca 2+ ] and partial depletion of SR Ca 2+ stores. Though no studies on Ca 2+ influx or altered ECCE were performed on cells carrying this mutation, the increased resting [Ca 2+ ] was insensitive to nifedipine suggesting that, at least in this case, the pathological effect is due to the lack of inhibition of RyR1-mediated Ca(2+) leak by the DHPR at rest [65]. Similar conclusions were reached when studying the effects of the CACNA1S p.R1086H mutation in myotubes, although in the latter case the CACNA1S mutation results in the enhanced sensitivity of the RyR1 to activation by both endogenous and exogenous activators [66].…”