Lysosomes and arrhythmia - Sudden cardiac death (SCD) resulting from malignant ventricular arrhythmia accounts for as much as one- third of all cardiac deaths in high-risk populations. Recently, we established a novel concept that organelles that take up and release Ca2+ could contribute to arrhythmic risk, especially during cardiomyopathy. Specifically, we showed that mitochondrial Ca2+ influx contributes to arrhythmic risk in nonischemic cardiomyopathy and that inhibiting mitochondrial Ca2+ influx prevents action potential prolongation, triggered activity, and arrhythmia. The ability of mitochondria to contribute to arrhythmic risk depended on: 1) organelle Ca2+ uptake and release, especially during cardiomyopathy, 2) approximation of mitochondria and the sarcoplasmic reticulum (SR), 3) amplification of mitochondrial diastolic Ca2+ release by activating SR Ca2+ release, and 4) increased sodium- calcium exchanger (NCX) current during diastole where the current effects are maximized by high membrane resistance. Cardiac lysosomes may contribute to arrhythmic risk during cardiomyopathy in a similar manner to mitochondria. Cardiac lysosomes are best known for their degradative role, but they possess all four criteria above for participation in arrhythmic risk. They maintain an intraluminal free Ca2+ concentration of ∼500 µmol/L. Lysosomal activity and interaction with SR increases in cardiomyopathy. Finally, lysosomal Ca2+ release can trigger diastolic SR Ca2+ release in noncardiac tissues. Lysosomal Ca2+ release is mediated by a two-pore channel (TPC2) and a transient receptor potential mucolipin channel (TRPML1) 13. We will present data that cardiac lysosomal TRPML1 but not TPC2 is increased in ischemic heart failure (HF). TRPML1 activation can initiate SR Ca2+ sparks and triggered activity. Conversely, TRPML1 inhibition can prevent triggered activity in HF. Finally, SR-lysosomal contact increases during HF. Hypotheses to be tested: The novel hypothesis is that lysosomes contribute to arrhythmic risk in cardiomyopathy by approximating the SR and by releasing Ca2+ during diastole causing SR diastolic Ca2+ release. We will test this hypothesis in three aims. Specific Objectives. Specific Aim 1: Determine whether a lysosomal-SR microdomain exists in ischemic HF. Specific Aim 2: Determine whether TRPML1 Ca2+ release can mediate SR diastolic Ca2+ release in ischemic HF. Specific Aim 3: Determine whether lysosomal Ca2+ release contributes to arrhythmic risk in ischemic HF. Significance. If correct, this application will establish the novel concept that lysosomes can contribute to arrhythmic risk, will reinforce the concept that Ca2+ handling organelles are important in arrhythmic risk, and will provide novel targets for antiarrhythmic therapy.
