Functional Sarcoplasmic Reticulum for Calcium Handling of Human Embryonic Stem Cell‐Derived Cardiomyocytes: Insights for Driven Maturation

Cardiomyocytes (CMs) are nonregenerative. Self‐renewable pluripotent human embryonic stem cells (hESCs) can differentiate into CMs for cell‐based therapies. In adult CMs, Ca2+‐induced Ca2+ release from the sarcoplasmic reticulum (SR) via the ryanodine receptor (RyR) is key in excitation‐contraction coupling. Therefore, proper Ca2+ handling properties of hESC‐derived CMs are required for their successful functional integration with the recipient heart. Here, we performed a comprehensive analysis of CMs differentiated from the H1 (H1‐CMs) and HES2 (HES2‐CMs) hESC lines and human fetal (F) and adult (A) left ventricular (LV) CMs. Upon electrical stimulation, all of H1‐, HES2‐, and FLV‐CMs generated similar Ca2+ transients. Caffeine induced Ca2+ release in 65% of FLV‐CMs and ∼38% of H1‐ and HES2‐CMs. Ryanodine significantly reduced the electrically evoked Ca2+ transient amplitudes of caffeine‐responsive but not ‐insensitive HES2‐ and H1‐CMs and slowed their upstroke; thapsigargin, which inhibits the sarco/endoplasmic reticulum Ca2+‐ATPase (SERCA) pump, reduced the amplitude of only caffeine‐responsive HES2‐ and H1‐CMs and slowed the decay. SERCA2a expression was highest in ALV‐CMs but comparable among H1‐, HES2‐, and FLV‐CMs. The Na+‐Ca2+ exchanger was substantially expressed in both HES2‐ and H1‐CMs relative to FLV‐ and ALV‐CMs. RyR was expressed in HES2‐, H1‐, and FLV‐CMs, but the organized pattern for ALV‐CMs was not observed. The regulatory proteins junctin, triadin, and calsequestrin were expressed in ALV‐CMs but not HES2‐ and H1‐CMs. We conclude that functional SRs are indeed expressed in hESC‐CMs, albeit immaturely. Our results may lead to driven maturation of Ca2+ handling properties of hESC‐CMs for enhanced contractile functions. Disclosure of potential conflicts of interest is found at the end of this article.