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Background: The mesenchymal stem cell (MSC), known to remodel in disease and have an extensive secretome, has recently been isolated from the human heart. However, the effects of normal and diseased cardiac MSCs on myocyte electrophysiology remain unclear. We hypothesize that in disease the inflammatory secretome of cardiac human MSCs (hMSCs) remodels and can regulate arrhythmia substrates., Methods: hMSCs were isolated from patients with or without heart failure from tissue attached to extracted device leads and from samples taken from explanted/donor hearts. Failing hMSCs or nonfailing hMSCs were cocultured with normal human cardiac myocytes derived from induced pluripotent stem cells. Using fluorescent indicators, action potential duration, Ca2+ alternans, and spontaneous calcium release (SCR) incidence were determined., Results: Failing and nonfailing hMSCs from both sources exhibited similar trilineage differentiation potential and cell surface marker expression as bone marrow hMSCs. Compared with nonfailing hMSCs, failing hMSCs prolonged action potential duration by 24% ( P <0.001, n=15), increased Ca2+ alternans by 300% ( P <0.001, n=18), and promoted spontaneous calcium release activity (n=14, P <0.013) in human cardiac myocytes derived from induced pluripotent stem cells. Failing hMSCs exhibited increased secretion of inflammatory cytokines IL (interleukin)-1β (98%, P <0.0001) and IL-6 (460%, P <0.02) compared with nonfailing hMSCs. IL-1β or IL-6 in the absence of hMSCs prolonged action potential duration but only IL-6 increased Ca2+ alternans and promoted spontaneous calcium release activity in human cardiac myocytes derived from induced pluripotent stem cells, replicating the effects of failing hMSCs. In contrast, nonfailing hMSCs prevented Ca2+ alternans in human cardiac myocytes derived from induced pluripotent stem cells during oxidative stress. Finally, nonfailing hMSCs exhibited >25× higher secretion of IGF (insulin-like growth factor)-1 compared with failing hMSCs. Importantly, IGF-1 supplementation or anti-IL-6 treatment rescued the arrhythmia substrates induced by failing hMSCs., Conclusions: We identified device leads as a novel source of cardiac hMSCs. Our findings show that cardiac hMSCs can regulate arrhythmia substrates by remodeling their secretome in disease. Importantly, therapy inhibiting (anti-IL-6) or mimicking (IGF-1) the cardiac hMSC secretome can rescue arrhythmia substrates.

The inward rectifier potassium current (I K1 ) is generally thought to suppress cardiac automaticity by hyperpolarizing membrane potential (MP). We recently observed that I K1 could promote the spontaneously-firing automaticity induced by upregulation of pacemaker funny current (I f ) in adult ventricular cardiomyocytes (CMs). However, the intriguing ability of I K1 to activate I f and thereby promote automaticity has not been explored. In this study, we combined mathematical and experimental assays and found that only I K1 and I f , at a proper-ratio of densities, were sufficient to generate rhythmic MP-oscillations even in unexcitable cells (i.e. HEK293T cells and undifferentiated mouse embryonic stem cells [ESCs]). We termed this effect I K1 -induced I f activation. Consistent with previous findings, our electrophysiological recordings observed that around 50% of mouse (m) and human (h) ESC-differentiated CMs could spontaneously fire action potentials (APs). We found that spontaneously-firing ESC-CMs displayed more hyperpolarized maximum diastolic potential and more outward I K1 current than quiescent-yet-excitable m/hESC-CMs. Rather than classical depolarization pacing, quiescent mESC-CMs were able to fire APs spontaneously with an electrode-injected small outward-current that hyperpolarizes MP. The automaticity to spontaneously fire APs was also promoted in quiescent hESC-CMs by an I K1 -specific agonist zacopride. In addition, we found that the number of spontaneously-firing m/hESC-CMs was significantly decreased when I f was acutely upregulated by Ad-CGI-HCN infection. Our study reveals a novel role of I K1 promoting the development of cardiac automaticity in m/hESC-CMs through a mechanism of I K1 -induced I f activation and demonstrates a synergistic interaction between I K1 and I f that regulates cardiac automaticity.