D on 3.5 cm glass-bottom dishes (WillCo-dishH Glass Bottom Dishes, the Netherlands). The spontaneous action potentials (AP) were recorded from hiPSCCalcium Calcitonin (salmon) biological activity sparks in 4 IBP iPSC-Derived CardiomyocytesFigure 1. Characterization of hiPSCs and hiPSC-derived CMs. (A) Immunofluorescent staining of hiPSC colonies with antibodies against Oct-4, SSEA-4, TRA-1-60 and TRA-1-81. (B) The hiPSC-CMs differentia4ed from above hiPSC line. (Ba) The phase-contrast light micrograph images of a V-CM cluster. (Bb and Bc) Immunofluorescent staining hiPSC-CMs with antibodies against alpha-actinin and beta-MHC, respectively. Nuclei were 1531364 stained with DAPI. (C) Action potential traces of ventricular-, atrial- and nodal-like CMs derived from hiPSCs. (D) Response of a ventricular-like hiPSC-CM to ISO recorded with patch-clamp. Abbreviations: ISO, isoproterenol. doi:10.1371/journal.pone.0055266.gCalcium Sparks in iPSC-Derived Cardiomyocytesclusters of cardiomyocytes (15,30 cells) dissociated from contracting EBs tended to contain exclusively homogenous subtypes of V-CMs and N-CMs (See Text S1). Furthermore, hiPSC-derived V-CMs (n = 5) showed a classical response towards ISO at minimal effective dose of 1 mM that induced contractions per 100ms at baseline and post ISO treatment at 26.465.2 and 35.266.4 (p,0.001) respectively (Figure 1D). However, atrial-and nodal-like CMs were not tested due to low yield of such subtypes in the hiPSC-CM preparation. Collectively, our data confirmed that hiPSC-CMs displayed cardiac structures and physiological function of cardiomyocytes similar to those of hESC-CMs.In order to further determine the characteristics of Ca2+ sparks, we analyzed the amplitude (F/F0), spatial size (FWHM: full width at half maximum) or duration (FDHM: full duration at half maximum) of spontaneous Ca2+ sparks. Figure 4E showed the histogram for F/F0, FDHM and FWHM of Ca2+ sparks which we deduced the relationship between the amplitude and size distributions of Ca2+ sparks and the population of Ca2+ sparks from their histogram plots. The mean values for F/F0, FWHM and FDHM were 1.6460.04, 2.3160.03 mm and 30.960.6 ms, respectively. Ca2+ sparks between hiPSC-CMs and adult ventricular myocytes (nspark = 302) have similar characteristics of Ca2+ sparks (Table S2).Spontaneous Ca2+ Transients in hiPSC-CMsFigure 2Ab shows representative Ca2+ transients obtained from sequential images recorded by a frame-scan mode in single hiPSCCM. A typical line-scan image of Ca2+ transient and its average fluorescence intensity were shown in Figure 2B and 2C. The average peak amplitude of Ca2+ transients (F/F0) was 3.860.7 in hiPSC-CMs. To observe spread patterns of Ca2+ transients of hiPSC-CMs, transverse line-scan images of Ca2+ transient were performed. As shown in Figure 2Da, Ca2+ increased first at the periphery of the cell before propagating towards the centre of the cell with a mean time delay of 46615 ms (n = 7) 1317923 (Figure 2Db). Calibration of [Ca2+]i was performed as described in Text S1 and Figure S1. In contrast to hiPSC-CMs, field stimulation evoked a rapid and uniform increase in intracellular Ca2+, and then Ca2+ quickly dropped homogeneously to resting levels in adult rat cardiomyocytes (nrat = 5, ncell = 12). The average amplitude of Ca2+ transients (F/F0) was 3.560.6 (Figure S2).L-type Ca2+ Channels Contributes to Spontaneous Ca2+ Sparks and Ca2+ TransientsTo examine whether some of Ca2+ sparks were triggered by activation of RyRs associated with spontaneous L-type Ca2+ ch.D on 3.5 cm glass-bottom dishes (WillCo-dishH Glass Bottom Dishes, the Netherlands). The spontaneous action potentials (AP) were recorded from hiPSCCalcium Sparks in iPSC-Derived CardiomyocytesFigure 1. Characterization of hiPSCs and hiPSC-derived CMs. (A) Immunofluorescent staining of hiPSC colonies with antibodies against Oct-4, SSEA-4, TRA-1-60 and TRA-1-81. (B) The hiPSC-CMs differentia4ed from above hiPSC line. (Ba) The phase-contrast light micrograph images of a V-CM cluster. (Bb and Bc) Immunofluorescent staining hiPSC-CMs with antibodies against alpha-actinin and beta-MHC, respectively. Nuclei were 1531364 stained with DAPI. (C) Action potential traces of ventricular-, atrial- and nodal-like CMs derived from hiPSCs. (D) Response of a ventricular-like hiPSC-CM to ISO recorded with patch-clamp. Abbreviations: ISO, isoproterenol. doi:10.1371/journal.pone.0055266.gCalcium Sparks in iPSC-Derived Cardiomyocytesclusters of cardiomyocytes (15,30 cells) dissociated from contracting EBs tended to contain exclusively homogenous subtypes of V-CMs and N-CMs (See Text S1). Furthermore, hiPSC-derived V-CMs (n = 5) showed a classical response towards ISO at minimal effective dose of 1 mM that induced contractions per 100ms at baseline and post ISO treatment at 26.465.2 and 35.266.4 (p,0.001) respectively (Figure 1D). However, atrial-and nodal-like CMs were not tested due to low yield of such subtypes in the hiPSC-CM preparation. Collectively, our data confirmed that hiPSC-CMs displayed cardiac structures and physiological function of cardiomyocytes similar to those of hESC-CMs.In order to further determine the characteristics of Ca2+ sparks, we analyzed the amplitude (F/F0), spatial size (FWHM: full width at half maximum) or duration (FDHM: full duration at half maximum) of spontaneous Ca2+ sparks. Figure 4E showed the histogram for F/F0, FDHM and FWHM of Ca2+ sparks which we deduced the relationship between the amplitude and size distributions of Ca2+ sparks and the population of Ca2+ sparks from their histogram plots. The mean values for F/F0, FWHM and FDHM were 1.6460.04, 2.3160.03 mm and 30.960.6 ms, respectively. Ca2+ sparks between hiPSC-CMs and adult ventricular myocytes (nspark = 302) have similar characteristics of Ca2+ sparks (Table S2).Spontaneous Ca2+ Transients in hiPSC-CMsFigure 2Ab shows representative Ca2+ transients obtained from sequential images recorded by a frame-scan mode in single hiPSCCM. A typical line-scan image of Ca2+ transient and its average fluorescence intensity were shown in Figure 2B and 2C. The average peak amplitude of Ca2+ transients (F/F0) was 3.860.7 in hiPSC-CMs. To observe spread patterns of Ca2+ transients of hiPSC-CMs, transverse line-scan images of Ca2+ transient were performed. As shown in Figure 2Da, Ca2+ increased first at the periphery of the cell before propagating towards the centre of the cell with a mean time delay of 46615 ms (n = 7) 1317923 (Figure 2Db). Calibration of [Ca2+]i was performed as described in Text S1 and Figure S1. In contrast to hiPSC-CMs, field stimulation evoked a rapid and uniform increase in intracellular Ca2+, and then Ca2+ quickly dropped homogeneously to resting levels in adult rat cardiomyocytes (nrat = 5, ncell = 12). The average amplitude of Ca2+ transients (F/F0) was 3.560.6 (Figure S2).L-type Ca2+ Channels Contributes to Spontaneous Ca2+ Sparks and Ca2+ TransientsTo examine whether some of Ca2+ sparks were triggered by activation of RyRs associated with spontaneous L-type Ca2+ ch.
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