From Beat Rate Variability in Induced Pluripotent Stem Cell-derived Pacemaker Cells to Heart Rate Variability in Human Subjects

Meital Ben-Ari ^1,2,3 Revital Schick ^1,2,3 Lili Barad ^1,2,3 Atara Novak ^1,2,3 Erez Ben-Ari ^3,4 Avraham Lorber ^3,5 Joseph Itskovitz-Eldor ^1,3,6 Michael R Rosen ⁷ Amir Weissman ^3,6 Ofer Binah ^1,2,3

¹Stem Cells, The Sohnis Family Stem Cells Center, Haifa
²Medicine, The Rappaport Institute, Hifa
³Medicine, Ruth & Bruce Rappaport Faculty of Medicine, Technio, Haifa
⁴Electrical Engineering, Technion, Haifa
⁵Pediatric Cardiology, Rambam Medical Center, Haifa
⁶Department of Obstetrics and Gynecology, Rambam Medical Center, Haifa
⁷Pharmacology, College of Physicians and Surgeons of Columbia University, New York

Background: Heart Rate Variability (HRV), a key property of human sinoatrial node (SAN), exhibits fractal-like oscillations attributed to autonomic input. We previously reported induced Pluripotent Stem Cell-derived cardiomyocytes (iPSC-CM), which lack external inputs, manifest beat rate variability (BRV) resembling HRV in human SAN.We now hypothesized the BRV-HRV continuum originates in pacemaker cells.

Methods and Results: We measured BRV/HRV properties in single pacemaker cells, contracting embryoid bodies (EBs) derived from iPSC, and electrocardiograms from the same individual. Pronounced BRV/HRV and fractality were present at all three levels. The standard deviation (STD) of inter-beat intervals (IBI) and the Poincaré plot SD1 and SD2 in single cells were 20 times > EBs (P<0.05) and in situ heart (the latter two were similar, P>0.05). We also compared BRV magnitude among single cells, small EBs (~5-10 cells) and larger EBs (>10 cells): BRV indices progressively increased (P<0.05) as cell number decreased. Disrupting intracellular Ca²⁺ handling markedly augmented BRV magnitude, revealing a unique bi-modal firing pattern, suggesting intracellular mechanisms contribute to BRV/HRV and the fractal behavior of heart rhythm.

Conclusions:The significant decrease in BRV magnitude in transitioning from single cell to EB suggests HRV of hearts in situ originates from summation and integration of multiple cell-based oscillators. Hence, complex interactions among multiple pacemaker cells and intracellular Ca²⁺ handling determine HRV in humans and isolated cardiomyocyte networks.