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 a 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 by Ca2+ and functional consequence of arrhythmogenic syndrome-associated human RyR2 missense mutation. Toward this, we construct a series of recombinant mutant RyR2 proteins and characterize their ion channel function. Those mutations are designed in the potential regulatory domain of the channel including the Ca2+ binding site. Alternatively, we introduce the corresponding RyR2 mutations in the genome of human induced pluripotent stem cells (hiPSC) by CRISPR/Cas9 gene editing. The gene-edited hiPSCs are differentiated into the cardiomyocytes, and their cellular Ca2+ signaling pathology are studied. Combining protein and cell studies will address how human disease-associated mutations alter RyR2 channel function, thereby CICR mechanisms. Also, large amounts of hiPSC-derived cells will be useful for drug screening for cardiac myopathies.
(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.