Ryanodine receptors (RyRs) are large (>500 kDa), intracellular calcium channels found in the sarcoplasmic/endoplasmic reticulum membrane and are responsible for the release of Ca2+ from intracellular stores in excitable cells, such as muscle and neurons. RyRs exist as three mammalian isoforms (RyR1-3), all of which form homotetramers regulated by phosphorylation and/or direct or indirect interaction with a variety of proteins (L-type calcium channels, PKA, FKBP12/12.6, CaMKII, calmodulin, calsequestrin, junctin, and triadin) and ions (Mg2+ and Ca2+). Regulation of the RyR channel by protein modulators occurs within the large cytoplasmic domain, whereas the carboxy terminal portion of the protein forms the ion-binding and conducting pore. RyR1 and RyR2 are predominantly expressed in skeletal and cardiac muscle, respectively, where they localize exclusively to the sarcoplasmic reticulum (SR) and facilitate calcium-mediated communication between transverse-tubules and sarcoplasmic reticulum. Contraction of skeletal muscle is triggered by release of calcium ions from the SR following depolarization of T-tubules. Research studies have shown that defects in RyR1 are the cause of malignant hyperthermia susceptibility type 1 (MHS1), central core disease of muscle (CCD), multiminicore disease with external ophthalmoplegia, and congenital myopathy with fiber-type disproportion (CFTD), each of which is manifested by defects in muscle function, metabolism, and development.
