The pore region of the skeletal muscle ryanodine receptor is a primary locus for excitation-contraction uncoupling in central core disease

Effect of Ryanodine Receptor Mutations on Interleukin-6 Release and Intracellular Calcium Homeostasis in Human Myotubes from Malignant Hyperthermia-susceptible Individuals and Patients Affected by Central Core Disease

In this study we report for the first time the functional properties of human myotubes isolated from patients harboring the native RYR1 I4898T and R4893W mutations linked to central core disease. We examined two aspects of myotube physiology, namely excitation-contraction and excitation-secretion coupling. Our results show that upon activation of the ryanodine receptor (RYR), myotubes release interleukin-6 (IL-6); this was dependent on de novo protein synthesis and could be blocked by dantrolene and cyclosporine. Myotubes from the two patients affected by central core disease showed a 4-fold increase in the release of the inflammatory cytokine IL-6, compared with cells derived from control or malignant hyperthermia susceptible individuals. All tested myotubes released calcium from intracellular stores upon stimulation via surface membrane depolarization or direct RYR activation by 4-chloro-m-cresol. The functional impact on calcium release of RYR1 mutations linked to central core disease or malignant hyperthermia is different: human myotubes carrying the malignant hyperthermia-linked RYR1 mutation V2168M had a shift in their sensitivity to the RYR agonist 4-chloro-m-cresol to lower concentrations, whereas human myotubes harboring C-terminal mutations linked to central core disease exhibited reduced [Ca2+]i increase in response to 4-chloro-m-cresol, caffeine, and KCl. Taken together, these results suggest that abnormal release of calcium via mutated RYR enhances the production of the inflammatory cytokine IL-6, which may in turn affect signaling pathways responsible for the trophic status of muscle fibers.

Distinct effects on Ca2+ handling caused by malignant hyperthermia and central core disease mutations in RyR1

Malignant hyperthermia (MH) and central core disease (CCD) are disorders of skeletal muscle Ca2+ homeostasis that are linked to mutations in the type 1 ryanodine receptor (RyR1). Certain RyR1 mutations result in an MH-selective phenotype (MH-only), whereas others result in a mixed phenotype (MH + CCD). We characterized effects on Ca2+ handling and excitation-contraction (EC) coupling of MH-only and MH + CCD mutations in RyR1 after expression in skeletal myotubes derived from RyR1-null (dyspedic) mice. Compared to wild-type RyR1-expressing myotubes, MH + CCD- and MH-only-expressing myotubes exhibited voltage-gated Ca2+ release (VGCR) that activated at more negative potentials and displayed a significantly higher incidence of spontaneous Ca2+ oscillations. However, maximal VGCR was reduced only for MH + CCD mutants (Y4795C, R2435L, and R2163H) in which spontaneous Ca2+ oscillations occurred with significantly longer duration (Y4795C and R2435L) or higher frequency (R2163H). Notably, myotubes expressing these MH + CCD mutations in RyR1 exhibited both increased [Ca2+]i and reduced sarcoplasmic reticulum (SR) Ca2+ content. We conclude that MH-only mutations modestly increase basal release-channel activity in a manner insufficient to alter net SR Ca2+ content ("compensated leak"), whereas the mixed MH + CCD phenotype arises from mutations that enhance basal activity to a level sufficient to promote SR Ca2+ depletion, elevate [Ca2+]i, and reduce maximal VGCR ("decompensated leak").

Amino acids 1-1,680 of ryanodine receptor type 1 hold critical determinants of skeletal type for excitation-contraction coupling. Role of divergence domain D2

To identify domains of the ryanodine receptor (RyR1) that are functionally relevant for excitation-contraction (EC) coupling in vivo, we have studied the ability of RyR1/RyR3 chimera to rescue skeletal EC coupling in dyspedic myotubes. In this work we show that chimeric receptors containing amino acids 1-1,680 of RyR1 were able to render depolarization-induced Ca2+ release to RyR3. Within this region, residues 1,272-1,455, containing divergent domain D2 of RyR1, proved to be a critical element because the absence of this region selectively abolished depolarization-evoked Ca2+ transients without affecting chemically induced activation. Although the D2 domain by itself failed to restore skeletal EC coupling to RyR3, the addition of the D2 region resulted in a dramatic enhancement of EC coupling restored by an RyR3 chimera containing amino acids 1,681-3,770 of RyR1. These results suggest that although the D2 domain of RyR1 plays a key role during EC coupling, additional region(s) from the N-terminal end of RyR1 as well as previously identified regions of the central portion of the receptor are needed in order to allow normal EC coupling.