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 source of CMs for heart disease modeling, drug/cardiotoxicity screening, and cell-based therapies. In the human heart, cytosolic Ca signals are well characterized but the contribution of nuclear Ca is essentially unexplored. The present study investigated nuclear Ca 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, which stems from unitary sparks due to the diffusion of cytosolic Ca that are mediated by RyRs on the nuclear reticulum, is functional. Parvalbumin- (PV-) mediated Ca buffering successfully manipulated Ca transient and stimuli-induced apoptosis in hPSC-VCMs. We also investigated the effect of Ca on gene transcription in hPSC-VCMs, and the involvement of nuclear factor of activated T-cell (NFAT) pathway was identified. The overexpression of Ca-sensitive, nuclear localized Ca/calmodulin-dependent protein kinase II (CaMKII ) induced cardiac hypertrophy through nuclear Ca/CaMKIIB/HDAC4/MEF2 pathway. These findings provide insights into nuclear Ca 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.