Mesenchymal stem cells and their conditioned medium improve integration of purified induced pluripotent stem cell-derived cardiomyocyte clusters into myocardial tissue.

Induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) might become therapeutically relevant to regenerate myocardial damage. Purified iPS-CMs exhibit poor functional integration into myocardial tissue. The aim of this study was to investigate whether murine mesenchymal stem cells (MSCs) or their conditioned medium (MScond) improves the integration of murine iPS-CMs into myocardial tissue. Vital or nonvital embryonic murine ventricular tissue slices were cocultured with purified clusters of iPS-CMs in combination with murine embryonic fibroblasts (MEFs), MSCs, or MScond. Morphological integration was assessed by visual scoring and functional integration by isometric force and field potential measurements. We observed a moderate morphological integration of iPS-CM clusters into vital, but a poor integration into nonvital, slices. MEFs and MSCs but not MScond improved morphological integration of CMs into nonvital slices and enabled purified iPS-CMs to confer force. Coculture of vital slices with iPS-CMs and MEFs or MSCs resulted in an improved electrical integration. A comparable improvement of electrical coupling was achieved with the cell-free MScond, indicating that soluble factors secreted by MSCs were involved in electrical coupling. We conclude that cells such as MSCs support the engraftment and adhesion of CMs, and confer force to noncontractile tissue. Furthermore, soluble factors secreted by MSCs mediate electrical coupling of purified iPS-CM clusters to myocardial tissue. These data suggest that MSCs may increase the functional engraftment and therapeutic efficacy of transplanted iPS-CMs into infarcted myocardium.

Mechanical preconditioning enables electrophysiologic coupling of skeletal myoblast cells to myocardium.

OBJECTIVE: The effect of mechanical preconditioning on skeletal myoblasts in engineered tissue constructs was investigated to resolve issues associated with conduction block between skeletal myoblast cells and cardiomyocytes. METHODS: Murine skeletal myoblasts were used to generate engineered tissue constructs with or without application of mechanical strain. After in vitro myotube formation, engineered tissue constructs were co-cultured for 6 days with viable embryonic heart slices. With the use of sharp electrodes, electrical coupling between engineered tissue constructs and embryonic heart slices was assessed in the presence or absence of pharmacologic agents. RESULTS: The isolation and expansion procedure for skeletal myoblasts resulted in high yields of homogeneously desmin-positive (97.1% ± 0.1%) cells. Mechanical strain was exerted on myotubes within engineered tissue constructs during gelation of the matrix, generating preconditioned engineered tissue constructs. Electrical coupling between preconditioned engineered tissue constructs and embryonic heart slices was observed; however, no coupling was apparent when engineered tissue constructs were not subjected to mechanical strain. Coupling of cells from engineered tissue constructs to cells in embryonic heart slices showed slower conduction velocities than myocardial cells with the embryonic heart slices (preconditioned engineered tissue constructs vs embryonic heart slices: 0.04 ± 0.02 ms vs 0.10 ± 0.05 ms, P = .011), lower maximum stimulation frequencies (preconditioned engineered tissue constructs vs embryonic heart slices: 4.82 ± 1.42 Hz vs 10.58 ± 1.56 Hz; P = .0009), and higher sensitivities to the gap junction inhibitor (preconditioned engineered tissue constructs vs embryonic heart slices: 0.22 ± 0.07 mmol/L vs 0.93 ± 0.15 mmol/L; P = .0004). CONCLUSIONS: We have generated skeletal myoblast-based transplantable grafts that electrically couple to myocardium.

Contractile properties of early human embryonic stem cell-derived cardiomyocytes: beta-adrenergic stimulation induces positive chronotropy and lusitropy but not inotropy.

Human embryonic stem cell-derived cardiomyocytes (hESC-CMs) provide the unique opportunity to study the very early development of the human heart. The aim of this study was to investigate the effect of calcium and beta-adrenergic stimulation on the contractile properties of early hESC-CMs. Beating clusters containing hESC-CMs were co-cultured in vitro with noncontractile slices of neonatal murine ventricles. After 5-7 days, when beating clusters had integrated morphologically into the damaged tissue, isometric force measurements were performed during spontaneous beating as well as during electrical field stimulation. Spontaneous beating stopped when extracellular calcium ([Ca²âº](ec)) was removed or after administration of the Ca²âº channel blocker nifedipine. During field stimulation at a constant rate, the developed force increased with incremental concentrations of [Ca²âº](ec). During spontaneous beating, rising [Ca²âº](ec) increased beating rate and developed force up to a [Ca²âº](ec) of 2.5 mM. When [Ca²âº](ec) was increased further, spontaneous beating rate decreased, whereas the developed force continued to increase. The beta-adrenergic agonist isoproterenol induced a dose-dependent increase of the frequency of spontaneous beating; however, it did not significantly change the developed force during spontaneous contractions or during electrical stimulation at a constant rate. Force developed by early hESC-CMs depends on [Ca²âº](ec) and on the L-type Ca²âº channel. The lack of an inotropic reaction despite a pronounced chronotropic response after beta-adrenergic stimulation most likely indicates immaturity of the sarcoplasmic reticulum. For cell-replacement strategies, further maturation of cardiac cells has to be achieved either in vitro before or in vivo after transplantation.