MSCs derived from human embryonic stem cells (MSCs from WA-09, a gift of Dr. Peiman Hematti) and HL-1 cardiomyocytes (a gift of Dr. William Claycomb) were expanded and cultured as previously described [43, 44] . Briefly, MSCs were cultured on a 0.1% gelatin (Sigma-Aldrich, St. Louis, MO, USA) pretreated flask containing α-minimum essential medium-(MEM-) complete. Alpha-MEM-complete consisted of α-MEM (Invitrogen, Carlsbad CA, USA), 10% fetal bovine serum (Hyclone, Logan UT), 0.1 mM nonessential amino acids (Invitrogen), and 2 mM Lglutamine (Invitrogen). MSC cultures were allowed to grow to 60-70% confluency and were replated at a concentration of 1,500 cells/cm 2 . CMs were cultured on fibronectin/gelatin (1.25 mg fibronectin/100 mL 0.02% gelatin) (Sigma-Aldrich) pretreated flasks containing Claycomb-complete. Claycombcomplete medium was comprised of Claycomb medium (SAFC Biosciences, St. Louis, MO, USA), 10% fetal bovine serum qualified for CMs (SAFC Biosciences), 100 U/mL: 100 μg/mL penicillin-streptomycin (Lonza, Walkersville, MD, USA), 0.1 mM norepinephrine (Sigma-Aldrich), and 2 mM L-glutamine (Invitrogen). CMs were passaged at 100% confluence and split 1 : 2. Experiments were performed using passages 7-10 and 60-110 for MSCs and CMs, respectively. All cultures were maintained at 37 • C in 5% CO 2 .