Counter stained with DAPI.Cardiomyocyte Differentiation of hiPSCsUnder cardiac differentiation condition, spontaneously contracting EBs were derived from hiPSC lines after 15 days. Dissociated hiPSC-CMs in the small clusters containing 15,30 CMs with uniformed subtypes (Figure 1Ba), were found to express sarcomeric alpha-actinin (a-actinin) and beta-myosin heavy chain (b-MHC) with cross striations that were typical of CMs derived from hESCs (Figure 1Bb, c). Moreover, three subtypes of CMs were identified including ventricular-, atrial- and nodal-like CMs (V-CMs, A-CMs and N-CMs) were identified in hiPSC-CMs (Figure 1C). The subtypes of hiPSC-CMs were determined by their 
typical AP properties including, action potential amplitude (APA), action potential duration (APD) and dV/dtmax. From a total of 100 cardiomyocytes examined, the percentages of V-CMs, A-CMs and N-CMs were about 61 , 17.4 and 21.6 , IQ1 cost respectively (Table S1). It was noted that smallRecording of Action PotentialDissociated hiPSC-CMs were cultured 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) 1662274 Immunofluorescent staining hiPSC-CMs with antibodies against alpha-actinin and beta-MHC, respectively. Nuclei were 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 MedChemExpress AKT inhibitor 2 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+ tr.Counter stained with DAPI.Cardiomyocyte Differentiation of hiPSCsUnder cardiac differentiation condition, spontaneously contracting EBs were derived from hiPSC lines after 15 days. Dissociated hiPSC-CMs in the small clusters containing 15,30 CMs with uniformed subtypes (Figure 1Ba), were found to express sarcomeric alpha-actinin (a-actinin) and beta-myosin heavy chain (b-MHC) with cross striations that were typical of CMs derived from hESCs (Figure 1Bb, c). Moreover, three subtypes of CMs were identified including ventricular-, atrial- and nodal-like CMs (V-CMs, A-CMs and N-CMs) were identified in hiPSC-CMs (Figure 1C). The subtypes of hiPSC-CMs were determined by their typical AP properties including, action potential amplitude (APA), action potential duration (APD) and dV/dtmax. From a total of 100 cardiomyocytes examined, the percentages of V-CMs, A-CMs and N-CMs were about 61 , 17.4 and 21.6 , respectively (Table S1). It was noted that smallRecording of Action PotentialDissociated hiPSC-CMs were cultured 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) 1662274 Immunofluorescent staining hiPSC-CMs with antibodies against alpha-actinin and beta-MHC, respectively. Nuclei were 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+ tr.
