Excitation-contraction uncoupling by a human central core disease mutation in the ryanodine receptor

Central core disease (CCD) is a human congenital myopathy characterized by fetal hypotonia and proximal muscle weakness that is linked to mutations in the gene encoding the type-1 ryanodine receptor (RyR1). CCD is thought to a rise from Ca2+-induced damage stemming from mutant RyR1 proteins forming " leaky " sarcoplasmic reticulum (SR) Ca2+ release channels, A novel mutation in the C-terminal region of RyR1 (14898T) accounts for an unusually severe and highly penetrant form of CCD in humans [Lynch, P, J., Tong, J., Lehane, M.. Mallet, A., Giblin, L., Heffron, J. J., Vaughan, P,, Zafra, G., MacLennan, D. H. & McCarthy, T. V. (1999) Proc. Natl. Add. Sci. USA 96, 4164-4169]. We expressed in skeletal myotubes derived from RyR1-knockout (dyspedic) mice the analogous mutation engineered into a rabbit RyR1 cDNA (14897T). Here we show that homozygous expression of 14897T in dyspedic myotubes results in a complete uncoupling of sarcolemmal excitation from voltage-gated SR Ca2+ release without significantly altering resting cytosolic Ca2+ levels, SR Ca2+ content, or RyR1-mediated enhancement of dihydropyridine receptor (DHPR) channel activity. Coexpression of both 14897T and wild-type RyR1 resulted in a 60% reduction in voltage-gated SR Ca2+ release, again without altering resting cytosolic Ca2+ levels, SR Ca2+ content, or DHPR channel activity. These findings indicate that muscle weakness suffered by individuals possessing the 14898T mutation involves a functional uncoupling of sarcolemmal excitation from SR Ca2+ release, rather than the expression of overactive or leaky SR Ca2+ release channels.