Functional sarcoplasmic reticulum for calcium-handling of human embryonic stem cell-derived cardiomyocytes: Insights for driven maturation

Cardiomyocytes (CMs) are non-regenerative. 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 to excitation-contraction coupling. Therefore, proper Ca²⁺ handling properties of hESC-derived CMs are required for their successful functional integration with the recipient heart. Previously, it has been reported that hESC-CMs do NOT express functional SRs. Here we performed a comprehensive analysis of CMs differentiated from the H1 (H1-CMs) and HES2 (HES2-CMs) hESC lines, 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 that inhibits the 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. NCX 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 immature. A better understanding is crucial for improving the safety and functional efficacy of hESC-CMs.