Increased cytosolic calcium buffering contributes to a cellular arrhythmogenic substrate in iPSC-cardiomyocytes from patients with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a major risk factor for heart failure and is associated with the development of life-threatening cardiac arrhythmias. Using a patient-specific induced pluripotent stem cell-derived cardiomyocyte (iPSC-CM) model harbouring a mutation in cardiac troponin T (R173W), we aim to examine the cellular basis of arrhythmogenesis in DCM patients with this mutation. iPSC from control (Ctrl) and DCM-TnT-R173W donors from the same family were differentiated into iPSC-CM and analysed through optical action potential (AP) recordings, simultaneous measurement of cytosolic calcium concentration ([Ca(2+)](i)) and membrane currents and separately assayed using field stimulation to detect the threshold for AP- and [Ca(2+)](i)-alternans development. AP duration was unaltered in TnT-R173W iPSC-CM. Nevertheless, TnT-R173W iPSC-CM showed a strikingly low stimulation threshold for AP- and [Ca(2+)](i)-alternans. Myofilaments are known to play a role as intracellular Ca(2+) buffers and here we show increased Ca(2+) affinity of intracellular buffers in TnT-R173W cells, indicating increased myofilament sensitivity to Ca(2+). Similarly, EMD57033, a myofilament Ca(2+) sensitiser, replicated the abnormal [Ca(2+)](i) dynamics observed in TnT-R173W samples and lowered the threshold for alternans development. In contrast, application of a Ca(2+) desensitiser (blebbistatin) to TnT-R173W iPSC-CM was able to phenotypically rescue Ca(2+) dynamics, normalising Ca(2+) transient profile and minimising the occurrence of Ca(2+) alternans at physiological frequencies. This finding suggests that increased Ca(2+) buffering likely plays a major arrhythmogenic role in patients with DCM, specifically in those with mutations in cardiac troponin T. In addition, we propose that modulation of myofilament Ca(2+) sensitivity could be an effective anti-arrhythmic target for pharmacological management of this disease. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s00395-022-00912-z.