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Fig. 8 The knockdown of GREM1 promoted mitochondrial oxidative respiration and cell contractility in hPSC-CMs. A The induced differentiation and treatment diagram of hPSC-CMs. B The knockdown efficiency of 10 nM GREM1-siRNA interference in hPSC-CMs. C Mitochondrial oxygen consumption rate of hPSC-CMs at the 8th day and D corresponding basal respiration, ATP production, maximal respiration, SRC of mitochondria measured with seahorse mitochondrial stress test (n = 10–15). E–F Mitochondria of hPSC-CMs stained with MitoTracker® Red CMXRos captured by confocal microscope (n = 3–4). G–H Mitochondria ROS stained by MitoSOX Red (n = 3–4). I–J Mitochondrial membrane potential detected through JC-1 staining (n = 3–4). Red indicates JC-1 aggregates, green indicates JC-1 monomers. The ratio of red to green indicates the mitochondrial membrane potential. K Mitochondrial oxygen consumption rate of hPSC-CMs at the 15th day and L corresponding basal respiration, ATP production, maximal respiration and SRC of mitochondria measured with seahorse mitochondrial stress test (n = 7–8). M The ultrastructure of (a–a`) hPSC-CMs and the enlarged views of (b–b`) mitochondria and (c–c`) myofibrils at the 15th day (n = 4–6). In GREM1-KD cells, mitochondria (red arrows) are increased in size and number with clear and matured cristae (blue arrows), but less and discrete myofibrils (yellow arrows). NC: negative control siRNA. *, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001 vs NC