Mapping domains and mutations on the skeletal muscle ryanodine receptor channel Jean H. Hwang 1 , Francesco Zorzato 2, 3 , Nigel F. Clarke 1, 4* , and Susan Treves 2, 3* 1 Institute for Neuroscience and Muscle Research, Children’s Hospital at Westmead, Sydney, 2145, Australia 2 Departments of Anesthesia and Biomedizin, Basel University Hospital, 4031 Basel, Switzerland 3 Department of Experimental and Diagnostic Medicine, General Pathology Section, University of Ferrara, Ferrara, 44100 Italy 4 Discipline of Paediatrics and Child Health, University of Sydney, Sydney, NSW 2006 Australia The skeletal muscle ryanodine receptor isoform 1 (RyR1) is a calcium release channel involved in excitation– contraction coupling, the process whereby an action potential is translated to a cytoplasmic Ca 2+ signal that activates muscle contraction. Dominant and recessive mutations in RYR1 cause a range of muscle disorders, including malignant hyperthermia and several forms of congenital myopathies. Many aspects of disease patho- genesis in ryanodinopathies remain uncertain, particular- ly for those myopathies due to recessive mutations. A thorough understanding of the ryanodine receptor mac- romolecular complex and its interactions with proteins and small molecular modulators is an essential starting point from which to investigate disease mechanisms. Ryanodine receptor function in normal and diseased muscle Rapid changes in the intracellular calcium concentration ([Ca 2+ ] i ) are important signaling events in most eukaryotic cells, necessitating tight regulation of cytoplasmic Ca 2+ levels. Global increases in [Ca 2+ ] i are triggered by the release of Ca 2+ from the endoplasmic reticulum (ER)/sarco- plasmic reticulum (SR) intracellular stores and/or by calci- um influx from the extracellular environment via opening of specific plasma membrane channels. The primary organelle involved in storing rapidly releasable Ca 2+ is the SR in striated muscles and the ER in most other mammalian cell types. Two families of channels are responsible for mediat- ing rapid calcium release from these intracellular stores: ryanodine receptors (RyRs) which are predominantly expressed in excitable cells and inositol 1,4,5-triphosphate receptors (IP 3 Rs) which are expressed in most cells. The skeletal muscle SR calcium release channel protein was first identified at the biochemical level in the 1980s and was named for its ability to bind the plant alkaloid ryanodine. Three RyR isoforms, encoded by separate genes sharing a high level of homology, have been identified in mammalian tissues. Ryanodine receptor type 1 (RyR1) is the primary isoform expressed in skeletal muscle. RyR1 is also expressed in some areas of the central nervous system and in some hemopoietic cells. RyR2 is the predominant isoform in cardiac muscle and is expressed in various regions of the brain, whereas RyR3 has widespread expres- sion, particularly during development. In skeletal muscle, RyR1 is a key protein involved in excitation–contraction (EC) coupling. In a simplified overview of this process, an electrical signal generated by an action potential travelling along the transverse (t)-tubules is detected by the dihy- dropyridine receptor (DHPR), a voltage-sensitive Ca 2+ channel. In response to membrane depolarization, DHPR channels undergo a conformational change and induce the opening of RyR1 channels that are closely apposed and located on the SR terminal cisternae. This leads to the release of Ca 2+ from SR stores into the cytoplasm. The cytoplasmic Ca 2+ binds to troponin and initiates muscle contraction in sarcomeres. Muscle contraction is terminat- ed upon closure of RyR1 and reuptake of Ca 2+ from the cytoplasm into the SR by sarcoplasmic/endoplasmic Ca 2+ ATPase (SERCA) pumps. Ablation of RyR1 in mice (dyspedic mice) results in a lethal phenotype, most likely due to respiratory failure. Mutant neonates also display skeletal abnormalities, in- cluding spinal curvature, arched vertebral column, thin limbs, and a thick neck, and resemble dysgenic mice that lack DHPR [1]. Primary dyspedic muscle cell cultures exhibit severely impaired Ca 2+ release after depolariza- tion, demonstrating the pivotal role RyR1 plays in skeletal muscle EC coupling. Over the past decades it has emerged that mutations in RYR1 are a major cause of muscle disease. Both dominant and recessive mutations have been identified throughout the RYR1 coding sequence and are responsible for a wide range of muscle disorders including malignant hyperthermia (MH), central core disease (CCD), multi-minicore disease, centronuclear myopathy, core–rod myopathy, and congenital fiber type disproportion (Box 1) [2–4]. Dantrolene, used in the emergency treatment of MH, is currently the only clinically relevant drug for RyR1 disorders and drugs that address the chronic, often life- threatening muscle weakness associated with many RyR1 disorders are much needed. The aim of this review is to summarize the current understanding of RyR1 structure and function and, in Review Corresponding author: Treves, S. (susan.treves@unibas.ch) Keywords: ryanodine receptor type 1; excitation–contraction coupling; calcium signaling; skeletal muscle; protein interactions; domains; disease pathogenesis. * These authors have contributed equally to the work. 644 1471-4914/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.molmed.2012.09.006 Trends in Molecular Medicine, November 2012, Vol. 18, No. 11