Aims Coronavirus disease 2019 (COVID-19) is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and has emerged as a global pandemic. SARS-CoV-2 infection can lead to elevated markers of cardiac injury associated with higher risk of mortality. It is unclear whether cardiac injury is caused by direct infection of cardiomyocytes or is mainly secondary to lung injury and inflammation. Here, we investigate whether cardiomyocytes are permissive for SARS-CoV-2 infection. Methods and results Two strains of SARS-CoV-2 infected human induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs) as demonstrated by detection of intracellular double-stranded viral RNA and viral spike glycoprotein expression. Increasing concentrations of viral RNA are detected in supernatants of infected cardiomyocytes, which induced infections in Caco-2 cell lines, documenting productive infections. SARS-COV-2 infection and induced cytotoxic and proapoptotic effects associated with it abolished cardiomyocyte beating. RNA sequencing confirmed a transcriptional response to viral infection as demonstrated by the up-regulation of genes associated with pathways related to viral response and interferon signalling, apoptosis, and reactive oxygen stress. SARS-CoV-2 infection and cardiotoxicity was confirmed in a 3D cardiosphere tissue model. Importantly, viral spike protein and viral particles were detected in living human heart slices after infection with SARS-CoV-2. Coronavirus particles were further observed in cardiomyocytes of a patient with COVID-19. Infection of iPS-CMs was dependent on cathepsins and angiotensin-converting enzyme 2 (ACE2), and was blocked by remdesivir. Conclusions This study demonstrates that SARS-CoV-2 infects cardiomyocytes in vitro in an ACE2- and cathepsin-dependent manner. SARS-CoV-2 infection of cardiomyocytes is inhibited by the antiviral drug remdesivir. Translational Perspective Although this study cannot address whether cardiac injury and dysfunction in COVID-19 patients is caused by direct infection of cardiomyocytes, the demonstration of direct cardiotoxicity in cardiomyocytes, organ mimics, human heart slices and human hearts warrants the further monitoring of cardiotoxic effects in COVID-19 patients.

Pathological cardiac hypertrophy is a leading cause of heart failure, but knowledge of the full repertoire of cardiac cells and their gene expression profiles in the human hypertrophic heart is missing. Here, by using large-scale single-nucleus transcriptomics, we present the transcriptional response of human cardiomyocytes to pressure overload caused by aortic valve stenosis and describe major alterations in cardiac cellular crosstalk. Hypertrophied cardiomyocytes had reduced input from endothelial cells and fibroblasts. Genes encoding Eph receptor tyrosine kinases, particularly EPHB1, were significantly downregulated in cardiomyocytes of the hypertrophied heart. Consequently, EPHB1 activation by its ligand ephrin (EFN)B2, which is mainly expressed by endothelial cells, was reduced. EFNB2 inhibited cardiomyocyte hypertrophy in vitro, while silencing its expression in endothelial cells induced hypertrophy in co-cultured cardiomyocytes. Our human cell atlas of the hypertrophied heart highlights the importance of intercellular crosstalk in disease pathogenesis and provides a valuable resource.

Leukemia inhibitory factor (LIF) is a pleiotropic cytokine known to control the proliferation and survival of stem cells including primordial germ cells and gonocytes. This led us to study the origin and regulation of testicular LIF. The LIF transcript was detected in the rat testis by RT-PCR from 13.5 days postcoitum until adulthood. LIF expression was investigated further in vitro in seven different highly purified testicular cell populations using RT-PCR and bioassays combined with neutralization experiments. LIF was found to be produced by peritubular cells and, to a much lesser extent, by the other testicular somatic cell types. No LIF was detected in meiotic and postmeiotic germ cell-conditioned medium, and only low levels of LIF were detected in spermatogonia-conditioned medium. Large amounts of bioactive LIF were measured in testicular lymph. While LIF production was greatly enhanced in presence of serum, lipopolysaccharide, and TNFalpha further increased this production in peritubular and Sertoli cells, and human CG enhanced Leydig cell LIF production. In conclusion, peritubular cells are the principal source of testicular LIF, probably accounting for its high concentration in the lymph. Given the proliferative effect of LIF on immature germ cells, we suggest that peritubular LIF plays an important role in the regulation of testicular function.

Cellular specialization and interaction with other cell types in cardiac tissue is essential for the coordinated function of cell populations in the heart. The complex interplay between cardiomyocytes, endothelial cells and fibroblasts is necessary for adaptation but can also lead to pathophysiological remodeling. To understand this complex interplay, we developed 3D vascularized cardiac tissue mimetics (CTM) to study heterocellular crosstalk in hypertrophic, hypoxic and fibrogenic environments. This 3D platform responds to physiologic and pathologic stressors and mimics the microenvironment of diseased tissue. In combination with endothelial cell fluorescence reporters, these cardiac tissue mimetics can be used to precisely visualize and quantify cellular and functional responses upon stress stimulation. Utilizing this platform, we demonstrate that stimulation of α/β-adrenergic receptors with phenylephrine (PE) promotes cardiomyocyte hypertrophy, metabolic maturation and vascularization of CTMs. Increased vascularization was promoted by conditioned medium of PE-stimulated cardiomyocytes and blocked by inhibiting VEGF or upon β-adrenergic receptor antagonist treatment, demonstrating cardiomyocyte-endothelial cross-talk. Pathophysiological stressors such as severe hypoxia reduced angiogenic sprouting and increased cell death, while TGF β2 stimulation increased collagen deposition concomitant to endothelial-to-mesenchymal transition. In sum, we have developed a cardiac 3D culture system that reflects native cardiac tissue function, metabolism and morphology-and for the first time enables the tracking and analysis of cardiac vascularization dynamics in physiology and pathology.