Dyspedic Mouse Skeletal Muscle Expresses Major Elements of the Triadic Junction but Lacks Detectable Ryanodine Receptor Protein and Function

Abstract

The ry1(53) dyspedic mouse contains two disrupted alleles for ryanodine receptor type 1 (skeletal isoform of ryanodine receptor; Ry1R) resulting in perinatal death. In the present study, whole skeletal muscle homogenates and sucrose gradient-purified junctional sarcoplasmic reticulum from neonatal wild-type and dyspedic mice were assayed for biochemical and functional markers. Equilibrium binding experiments performed with 1-120 nM [3H]ryanodine reveal saturable high and low affinity binding to membrane preparations from wild-type mice, but not to preparations from dyspedic mice. Binding experiments performed with [3H]PN200 show a 2-fold reduction in [3H]PN200 binding capacity in dyspedic muscle, compared to age-matched wild-type muscle, with no change in receptor affinity. The presence or absence of proteins known to be critical for normal ryanodine receptor/Ca2+ channel complex function was assessed by Western blot analysis. Results indicate that FKBP-12, DHPRalpha1, triadin, calsequestrin, SERCA1 (sarco(endo)plasmic reticulum Ca2+ ATPase), and skeletal muscle myosin heavy chain are present in both dyspedic and wild-type muscle. Only wild-type membranes showed immunoreactivity toward Ry1R antibody. Neither dyspedic nor wild-type mouse muscle showed detectable immunoreactivity toward Ry2R or Ry3R antibodies, even after sucrose gradient purification of sarcoplasmic reticulum. These results indicate that proteins critical for ryanodine receptor function are expressed in dyspedic skeletal muscle in the absence of Ry1R. Ca2+ transport measurements show that membranes from wild-type controls, but not dyspedic mice, release Ca2+ upon exposure to ryanodine. Dyspedic mice and cells derived from them serve as excellent homologous expression systems in which to study how Ry1R structure relates to function.