Abnormal junctions between surface membrane and sarcoplasmic reticulum in skeletal muscle with a mutation targeted to the ryanodine receptor.

Abstract

Junctions that mediate excitation-contraction (e-c) coupling are formed between the sarcoplasmic reticulum (SR) and either the surface membrane or the transverse (T) tubules in normal skeletal muscle. Two structural components of the junctions, the feet of the SR and the tetrads of T tubules, have been identified respectively as ryanodine receptors (RyRs, or SR calcium-release channels), and as groups of four dihydropyridine receptors (DHPRs, or voltage sensors of e-c coupling). A targeted mutation (skrrml) of the gene for skeletal muscle RyRs in mice results in the absence of e-c coupling in homozygous offspring of transgenic parents. The mutant gene is expected to produce no functional RyRs, and we have named the mutant mice "dyspedic" because they lack feet-the cytoplasmic domain of RyRs anchored in the SR membrane. We have examined the development of junctions in skeletal muscle fibers from normal and dyspedic embryos. Surprisingly, despite the absence of RyRs, junctions are formed in dyspedic myotubes, but the junctional gap between the SR and T tubule is narrow, presumably because the feet are missing. Tetrads are also absent from these junctions. The results confirm the identity of RyRs and feet and a major role for RyRs and tetrads in e-c coupling. Since junctions form in the absence of feet and tetrads, coupling of SR to surface membrane and T tubules appears to be mediated by additional proteins, distinct from either RyRs or DHPRs.

An Ryr1 I4895T mutation abolishes Ca 2+ release channel function and delays development in homozygous offspring of a mutant mouse line

“…3 D-F) muscles, but misalignment was more frequent in the mutants. This feature was observed previously in dysgenic (38) and dyspedic muscles (36).…”

“…1G). Similar abnormalities of the skeletal muscle development and organization were detected previously in the dysgenic (21,35) and dyspedic mouse models (22,36), suggesting that similar intracellular pathways might be affected in all three models. Skeletal Abnormalities.…”

Structural interaction between RYRs and DHPRs in calcium release units of cardiac and skeletal muscle cells

“…Surprisingly, absence of either α 1 sDHPR or RyR1 does not affect either formation of junctions or targeting of the second protein to junctional domains. In dysgenic muscle, triads containing organized arrays of feet are formed even if DHPRs are missing (33), and in dyspedic mice both triads and peripheral couplings are formed in the absence of RyRs (34). In regards to the targeting of DHPRs to junctions lacking RyRs, there have been some controversial results.…”

Triadopathies: An Emerging Class of Skeletal Muscle Diseases

“…One barrier to treatment development is the relatively small number of preclinical models, and the difficulty presented by the phenotypes of those models. The Ryr1 knockout mouse ("dyspedic") dies at birth, and thus is not suitable for most in vivo studies [38,39]. The available RyR1 "knockin" mouse models, in which identified disease mutations are engineered into the mouse genome, model these conditions well and have provided a wealth of information about MHS, core myopathy, and RyR1 pathophysiology [40].…”