Lysosomal Ca2+-flux contributes to abnormal automaticity-related arrhythmia in ventricular cardiomyopathy

Abnormal automaticity contributes to ventricular arrhythmias during cardiac ischemia. This mechanism involves Ca2+ handling at the membrane and the sarcoplasmic reticulum (SR). We have described a contribution of mitochondrial Ca2+ flux to automaticity. Recent publications showed that lysosomal Ca2+ release influences SR Ca2+ release. Therefore, we determined if lysosomal Ca2+ flux could modify abnormal automaticity in ventricular cardiomyocytes. Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and cardiomyocytes (CMs) derived from infarcted mice were studied. The spontaneous beating rate of CMs was determined from action potentials or cytoplasmic Ca2+ transients. The hiPSC-CMs transfected with lysosomal transient receptor potential mucolipin Ca2+ release channel (TRPML1) siRNA had significantly reduced automaticity and a significantly increased SR Ca2+ release time constant. Moreover, the TRPML1 specific agonist, ML-SA1, accelerated the spontaneous beating while a TRPML1 specific antagonist, ML-SI1 substantially decreased this frequency. Proteasome inhibition by MG132 activated lysosomal transcription factor EB (TFEB). TFEB activation increased lysosomes and resulted in increased automaticity. Proteasome inhibition had the opposite effect. Blocking the lysosomal two-pore Ca2+ channel (TPC2) or inhibiting the V-type ATPase responsible for lysosomal Ca2+ loading had similar effects on automaticity. Furthermore, inhibition lysosomal Ca2+ release reduced the abnormal automaticity in adult cardiomyocytes isolated after myocardial infarction. As expected based upon hiPSC-CMs and mouse cardiomyocytes data, the protein level of TRPML1 in heart failure (HF) patients with tachycardia (VT) was significantly enhanced when compared with that in HF patients without VT. Lysosomes Ca2+-cycling modulated automaticity in ventricular hiPSC-CMs or cardiomyocytes from infarcted mouse hearts and contributed to HF patient VT events, implying that changes in lysosomal Ca2+ handling may play an important role in abnormal automaticity related arrhythmia.