The ryanodine receptor: A pivotal Ca ²⁺ regulatory protein and potential therapeutic drug target

The ryanodine receptor (RyR) calcium release channel is an essential intracellular ion channel that is central to Ca ²⁺ signaling and contraction in the heart and skeletal muscle. The rapid release of Ca ²⁺ from the internal sarcoplasmic reticulum Ca ²⁺ stores through the RyR during excitation-contraction coupling is facilitated by the unique arrangement of the surface and sarcoplasmic reticulum membrane systems. Debilitating and sometimes fatal skeletal and cardio-myopathies result from changes in RyR activity that disrupt normal Ca ²⁺ signaling. Such changes can be caused by point mutations in many different regions of the RyR protein or acquired as a result of stress associated with exercise, heart failure, age or drugs. In general, both inherited and acquired changes include an increase in RyR channel activity. Because of its central function, the RyR is a potential therapeutic target for the inherited disorders and many of the acquired disorders. The RyR is currently used as a therapeutic target in malignant hyperthermia where dantrolene is effective and to relieve ventricular arrhythmia and with the use of JTV519 and flecainide. These drugs show that the RyR is a valid therapeutic target, but have side effects that prevent their chronic use. Thus there is an urgent need for the development of skeletal and cardiac specific drugs to treat these diverse muscle disorders. In this review, we discuss the mutations that cause skeletal myopathies and cardiac arrhythmias and how these mutations pinpoint residues within the RyR protein that are functionally significant and might be developed as targets for therapeutic drugs.