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

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Abstract
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 arise from Ca 2+ -induced damage stemming from mutant RyR1 proteins forming “leaky” sarcoplasmic reticulum (SR) Ca 2+ release channels. A novel mutation in the C-terminal region of RyR1 (I4898T) 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. Acad. 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 (I4897T). Here we show that homozygous expression of I4897T in dyspedic myotubes results in a complete uncoupling of sarcolemmal excitation from voltage-gated SR Ca 2+ release without significantly altering resting cytosolic Ca 2+ levels, SR Ca 2+ content, or RyR1-mediated enhancement of dihydropyridine receptor (DHPR) channel activity. Coexpression of both I4897T and wild-type RyR1 resulted in a 60% reduction in voltage-gated SR Ca 2+ release, again without altering resting cytosolic Ca 2+ levels, SR Ca 2+ content, or DHPR channel activity. These findings indicate that muscle weakness suffered by individuals possessing the I4898T mutation involves a functional uncoupling of sarcolemmal excitation from SR Ca 2+ release, rather than the expression of overactive or leaky SR Ca 2+ release channels.