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Original Article

^a Department of Pharmacology and Physiology, University of Rochester School of Medicine and Dentistry, Rochester, NY 14642
Department of Pharmacology and Physiology, University of Rochester School of Medicine and Dentistry, 601 Elmwood Avenue, Rochester, NY 14642.(716) 273-2652

robert_dirksen{at}URMC.rochester.edu

Central core disease (CCD) is a human myopathy that involves a dysregulation in muscle Ca²+ homeostasis caused by mutations in the gene encoding the skeletal muscle ryanodine receptor (RyR1), the protein that comprises the calcium release channel of the SR. Although genetic studies have clearly demonstrated linkage between mutations in RyR1 and CCD, the impact of these mutations on release channel function and excitation-contraction coupling in skeletal muscle is unknown. Toward this goal, we have engineered the different CCD mutations found in the NH₂-terminal region of RyR1 into a rabbit RyR1 cDNA (R164C, I404M, Y523S, R2163H, and R2435H) and characterized the functional effects of these mutations after expression in myotubes derived from RyR1-knockout (dyspedic) mice. Resting Ca²+ levels were elevated in dyspedic myotubes expressing four of these mutants (Y523S > R2163H > R2435H R164C > I404M RyR1). A similar rank order was also found for the degree of SR Ca²+ depletion assessed using maximal concentrations of caffeine (10 mM) or cyclopiazonic acid (CPA, 30 µM). Although all of the CCD mutants fully restored L-current density, voltage-gated SR Ca²+ release was smaller and activated at more negative potentials for myotubes expressing the NH₂-terminal CCD mutations. The shift in the voltage dependence of SR Ca²+ release correlated strongly with changes in resting Ca²+, SR Ca²+ store depletion, and peak voltage–gated release, indicating that increased release channel activity at negative membrane potentials promotes SR Ca²+ leak. Coexpression of wild-type and Y523S RyR1 proteins in dyspedic myotubes resulted in release channels that exhibited an intermediate degree of SR Ca²+ leak. These results demonstrate that the NH₂-terminal CCD mutants enhance release channel sensitivity to activation by voltage in a manner that leads to increased SR Ca²+ leak, store depletion, and a reduction in voltage-gated Ca²+ release. Two fundamentally distinct cellular mechanisms (leaky channels and EC uncoupling) are proposed to explain how altered release channel function caused by different mutations in RyR1 could result in muscle weakness in CCD.

Key Words: excitation-contraction coupling • calcium channels • muscle disease • calcium homeostasis

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