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Calcium ions play critical roles in intracellular signaling of a variety of cells. In cardiac and skeletal muscle, transiently elevated Ca2+ concentrations during muscle action potentials initiate muscle contraction. In my laboratory we are studying how these Ca2+ transients are well regulated and how aberrant intracellular calcium homeostasis causes diseases in the cardiac and skeletal muscle.
(1) Heart failure is one of the leading causes of death in humans. In cardiac pathological studies, dysfunction of calcium transporting proteins is found to be implicated in cardiac hypertrophy and arrhythmia often resulting in heart failure. During an cardiac action potential Ca2+ influx through voltage-dependent L-type Ca2+ channels (Cav1.2) activates Ca2+ release channels (ryanodine receptors type2: RyR2s), which release Ca2+ from the sarcoplasmic reticulum (SR) by Ca2+-induced Ca2+ release (CICR).
I am currently interested in regulation mechanism of RyR2 and Cav1.2 by calmodulin, a ubiquitous cytoplasmic Ca2+ binding protein. During cardiac muscle contraction, elevated cytoplasmic Ca2+ and Ca2+-bound calmodulin regulate a number of proteins including these ion channels by a feedback mechanism. To address functional significance of calmodulin regulation of RyR2 and Cav1.2, I am characterizing wild type and mutant channels in vitro (heterologous cell expression) and in vivo (mutant mouse model). I have recently generated a genetically modified mouse impaired in calmodulin regulation of RyR2. Prolonged SR Ca2+ release was measured in cardiomyocytes isolated from mutant mouse hearts. In addition, cardiac hypertrophy and early death of the mutant mice were observed. This mutant mouse is a powerful model to analyze how abnormal Ca2+ homeostasis activates signaling pathways underlying cardiac hypertrophy.
(2) Intracellular Ca2+ transients in skeletal muscle are mediated by type1 ryanodine receptors calcium release channels (RyR1s). Missense mutations in RyR1 are associated with human skeletal myopathies including central core disease (CCD). A well-known molecular mechanism is that RyR1 mutations increase affinities for channel agonist, therefore causing intracellular Ca2+ overload. We hypothesize that an alternative mechanism underlying the skeletal myopathies is impairment of inhibitory regulation of RyR1. We recently have identified RyR domains involved in this Ca2+-dependent inactivation. We are characterizing biochemical and biophysical properties of the RyR1 harboring disease-associated point mutations in the identified domains. These studies are expected to provide a novel insight in dysfunctional Ca2+ homeostasis in skeletal pathology.