Facilitated maturation of Ca²⁺ handling properties of human embryonic stem cell-derived cardiomyocytes by calsequestrin expression

Cardiomyocytes (CMs) are nonregenerative. Self-renewable pluripotent human embryonic stem cells (hESCs) can differentiate into CMs for cell-based therapies. We recently reported that Ca²⁺ handling, crucial to excitation-contraction coupling of hESC-derived CMs (hESC-CMs), is functional but immature. Such immature properties as smaller cytosolic Ca²⁺ transient amplitudes, slower kinetics, and reduced Ca²⁺ content of sarcoplasmic reticulum (SR) can be attributed to the differential developmental expression profiles of specific Ca²⁺ handling and regulatory proteins in hESC-CMs and their adult counterparts. In particular, calsequestrin (CSQ), the most abundant, high-capacity but low-affinity, Ca²⁺-binding protein in the SR that is anchored to the ryanodine receptor, is robustly expressed in adult CMs but completely absent in hESCCMs. Here we hypothesized that gene transfer of CSQ in hESC-CMs suffices to induce functional improvement of SR. Transduction of hESC-CMs by the recombinant adenovirus Ad-CMV-CSQ-IRES-GFP (Ad-CSQ) significantly increased the transient amplitude, upstroke velocity, and transient decay compared with the control Ad-CMV-GFP (Ad-GFP) and Ad-CMV-CSQΔ-IRES-GFP (Ad-CSQΔ, which mediated the expression of a nonfunctional, truncated version of CSQ) groups. Ad-CSQ increased the SR Ca²⁺ content but did not alter L-type Ca ²⁺ current. Pharmacologically, untransduced wildtype, Ad-GFP-, Ad-CSQΔ-, and Ad-CSQ-transduced hESC-CMs behaved similarly. Whereas ryanodine significantly reduced the Ca²⁺ transient amplitude and slowed the upstroke, thapsigargin slowed the decay. Neither triadin nor junctin was affected. We conclude that CSQ expression in hESC-CMs facilitates Ca ²⁺ handling maturation. Our results shed insights into the suitability of hESC-CMs for therapies and as certain heart disease models for drug screening.