Background: Advancing structural and functional maturation of stem cell–derived cardiomyocytes remains a key challenge for applications in disease modeling, drug screening, and heart repair. Here, we sought to advance cardiomyocyte maturation in engineered human myocardium (EHM) toward an adult phenotype under defined conditions.Methods: We systematically investigated cell composition, matrix, and media conditions to generate EHM from embryonic and induced pluripotent stem cell–derived cardiomyocytes and fibroblasts with organotypic functionality under serum-free conditions. We used morphological, functional, and transcriptome analyses to benchmark maturation of EHM.Results: EHM demonstrated important structural and functional properties of postnatal myocardium, including: (1) rod-shaped cardiomyocytes with M bands assembled as a functional syncytium; (2) systolic twitch forces at a similar level as observed in bona fide postnatal myocardium; (3) a positive force-frequency response; (4) inotropic responses to β-adrenergic stimulation mediated via canonical β₁- and β₂-adrenoceptor signaling pathways; and (5) evidence for advanced molecular maturation by transcriptome profiling. EHM responded to chronic catecholamine toxicity with contractile dysfunction, cardiomyocyte hypertrophy, cardiomyocyte death, and N-terminal pro B-type natriuretic peptide release; all are classical hallmarks of heart failure. In addition, we demonstrate the scalability of EHM according to anticipated clinical demands for cardiac repair.Conclusions: We provide proof-of-concept for a universally applicable technology for the engineering of macroscale human myocardium for disease modeling and heart repair from embryonic and induced pluripotent stem cell–derived cardiomyocytes under defined, serum-free conditions.

A giant disruption of the heart: Certain forms of heart failure originate from genetic mutations. Understanding how the culprit mutant proteins alter normal heart function could lead to more effective treatments. One candidate is the giant protein tintin, which is mutated in a subset of patients with dilated cardiomyopathy. Through a combination of patient-derived stem cells, tissue engineering, and gene editing, Hinson et al. found that disease-associated titin mutations disrupt the function of the contractile unit in heart muscle. As a result, the heart does not respond properly to mechanical and other forms of stress. Science , this issue p. 982

Background— To support the clinical distinction between systolic heart failure (SHF) and diastolic heart failure (DHF), left ventricular (LV) myocardial structure and function were compared in LV endomyocardial biopsy samples of patients with systolic and diastolic heart failure. Methods and Results— Patients hospitalized for worsening heart failure were classified as having SHF (n=22; LV ejection fraction (EF) 34±2%) or DHF (n=22; LVEF 62±2%). No patient had coronary artery disease or biopsy evidence of infiltrative or inflammatory myocardial disease. More DHF patients had a history of arterial hypertension and were obese. Biopsy samples were analyzed with histomorphometry and electron microscopy. Single cardiomyocytes were isolated from the samples, stretched to a sarcomere length of 2.2 μm to measure passive force (F _passive ), and activated with calcium-containing solutions to measure total force. Cardiomyocyte diameter was higher in DHF (20.3±0.6 versus 15.1±0.4 μm, P <0.001), but collagen volume fraction was equally elevated. Myofibrillar density was lower in SHF (36±2% versus 46±2%, P <0.001). Cardiomyocytes of DHF patients had higher F _passive (7.1±0.6 versus 5.3±0.3 kN/m ² ; P <0.01), but their total force was comparable. After administration of protein kinase A to the cardiomyocytes, the drop in F _passive was larger ( P <0.01) in DHF than in SHF. Conclusions— LV myocardial structure and function differ in SHF and DHF because of distinct cardiomyocyte abnormalities. These findings support the clinical separation of heart failure patients into SHF and DHF phenotypes.