Structural and Mechanistic Bases of Nuclear Calcium Signaling in Human Pluripotent Stem Cell-Derived Ventricular Cardiomyocytes

The loss of nonregenerative, terminally differentiated cardiomyocytes (CMs) due to aging or diseases is generally considered irreversible. Human pluripotent stem cells (hPSCs) can self-renew while maintaining their pluripotency to differentiate into all cell types, including ventricular (V) cardiomyocytes (CMs), to provide a potential unlimited ex vivo source of CMs for heart disease modeling, drug/cardiotoxicity screening, and cell-based therapies. In the human heart, cytosolic Ca(2+) signals are well characterized but the contribution of nuclear Ca(2+) is essentially unexplored. The present study investigated nuclear Ca(2+) signaling in hPSC-VCMs. Calcium transient or sparks in hPSC-VCMs were measured by line scanning using a spinning disc confocal microscope. We observed that nuclear Ca(2+), which stems from unitary sparks due to the diffusion of cytosolic Ca(2+) that are mediated by RyRs on the nuclear reticulum, is functional. Parvalbumin- (PV-) mediated Ca(2+) buffering successfully manipulated Ca(2+) transient and stimuli-induced apoptosis in hPSC-VCMs. We also investigated the effect of Ca(2+) on gene transcription in hPSC-VCMs, and the involvement of nuclear factor of activated T-cell (NFAT) pathway was identified. The overexpression of Ca(2+)-sensitive, nuclear localized Ca(2+)/calmodulin-dependent protein kinase II δ (B) (CaMKIIδ (B)) induced cardiac hypertrophy through nuclear Ca(2+)/CaMKIIδB/HDAC4/MEF2 pathway. These findings provide insights into nuclear Ca(2+) signal in hPSC-VCMs, which may lead to novel strategies for maturation as well as improved systems for disease modeling, drug discovery, and cell-based therapies.