Reduced Threshold for Luminal Ca2+ Activation of RyR1 Underlies a Causal Mechanism of Porcine Malignant Hyperthermia

Abstract

Malignant hyperthermia (MH)3 is an autosomal dominant, pharmacogenetic disorder of skeletal muscle. MH is triggered by volatile anesthetics (e.g. halothane) and depolarizing muscle relaxants and is characterized by muscle rigidity and a hypermetabolic state (1-5). MH also occurs in pigs, in which it is caused by stress and known as porcine stress syndrome (6, 7). A single point mutation, R615C, in the pig skeletal muscle ryanodine receptor RyR1 is responsible for all cases of porcine MH (6, 7). On the other hand, human MH has been linked to a large number of mutations in RyR1 (4,5,8). Some human RyR1 mutations have also been linked to central core disease, which is often associated with MH. Although the genetic basis of MH has been well defined, the molecular mechanisms by which RyR1 mutations confer MH susceptibility and volatile anesthetics and stress trigger MH are not completely understood.The pig model has proven invaluable in investigating the molecular basis of MH, and these studies have consistently demonstrated that Ca 2ϩ release from MH-susceptible (MHS) pig skeletal muscle or sarcoplasmic reticulum (SR) membrane vesicles is enhanced upon exposure to various stimuli (1, 4, 9 -14). However, to date no clear mechanistic basis of this enhanced responsiveness of MHS RyR1 channels to stimuli has emerged. For example, some studies reported that this enhanced activity of MHS RyR1 channels was associated with changes in the apparent sensitivity of the channel to cytosolic Ca 2ϩ or Mg 2ϩ , whereas others found no marked difference in the apparent sensitivity to cytosolic Ca 2ϩ activation between MHS and normal RyR1 channels (1,(13)(14)(15)(16)(17)(18)(19)(20). Hence, the intrinsic properties of the RyR1 channel that are altered by the MH R615C mutation have remained undefined.Increasing evidence has highlighted the importance not only of cytosolic Ca 2ϩ , but also of luminal Ca 2ϩ , in controlling the activity of the RyR channel (21-23). However, in comparison with the extensive investigations of the sensitivity of MHS RyR1 channels to cytosolic Ca 2ϩ and Mg 2ϩ , the effects of the MH R615C mutation on the luminal Ca 2ϩ sensitivity of the channel remain largely unexplored. A notable exception is found among the earliest work investigating the effect of the MH R615C mutation on Ca 2ϩ handling by isolated SR membranes (13,24,25). Nelson and colleagues (13,24,25)

Fiber hypertrophy and increased oxidative capacity can occur simultaneously in pig glycolytic skeletal muscle

“…Mechanistically, RyR1 R615C increases sensitivity to agents that stimulate channel opening, contributing to enhanced Ca 2ϩ release (36,37); RyR1 R615C also lowers the threshold for store overload-induced Ca 2ϩ release (28). This increased sensitivity and enhanced Ca 2ϩ release are generally considered to contribute to the "work-induced hypertrophy," or greater fiber CSA, in RyR1 R615C pig muscle.…”

“…The RyR1 protein assembles in tetramers to form Ca 2ϩ release channels, which modulate the frequency and amplitude of Ca 2ϩ release into the cytosol. Mechanistically, RyR1 R615C increases sensitivity to agents that stimulate channel opening and enhances luminal Ca 2ϩ activation of RyR1 (28,37). Aberrant Ca 2ϩ homeostasis profoundly influences muscle function and energy metabolism.…”

Ryanodine Receptors: Structure and Function

“…Several studies suggest a direct interaction between dantrolene and RyR1 (69,70). Dantrolene inhibits RyR1 even when expressed in a heterologous system such as HEK293 cells and appears to inhibit SOICR (71). However, the single-channel behavior of RyR1 incorporated in planar lipid bilayers appears to be unaffected, and the precise mechanism of action is therefore still unknown.…”

IP3 Receptors: Toward Understanding Their Activation

“…Persuasive evidence suggests that Ca 2þ release by RyR may be terminated before Ca 2þ stores are entirely depleted because luminal Ca 2þ is required to maintain RyR activity (Györke and Györke 1998;Launikonis et al 2006;Jiang et al 2008), possibly via its interaction with calsequestrin, a luminal high-capacity Ca 2þ -binding protein (Launikonis et al 2006;Terentyev et al 2006). A similar scheme has been proposed to account for two features of IP 3 -evoked Ca 2þ release: the initiation of Ca 2þ release after the quiescent interspike interval during repetitive Ca 2þ spikes (Berridge 2007) and quantal Ca 2þ release via IP 3 R. The latter describes the situation wherein unidirectional Ca 2þ efflux from intracellular stores terminates before the stores have fully emptied after stimulation with submaximally effective concentrations of IP 3 without loss of their ability to respond to a further increase in IP 3 concentration (Muallem et al 1989;Meyer and Stryer 1990;Taylor and Potter 1990;Oldershaw et al 1991;Bootman et al 1992;Brown et al 1992;Combettes et al 1992;Ferris et al 1992;Hirota et al 1995).…”