The transcription factor EBF1 non-cell-autonomously regulates cardiac growth and differentiation.

Reciprocal interactions between non-myocytes and cardiomyocytes regulate cardiac growth and differentiation. Here, we report that the transcription factor Ebf1 is highly expressed in non-myocytes and potently regulates heart development. Ebf1-deficient hearts display myocardial hypercellularity and reduced cardiomyocyte size, ventricular conduction system hypoplasia, and conduction system disease. Growth abnormalities in Ebf1 knockout hearts are observed as early as embryonic day 13.5. Transcriptional profiling of Ebf1-deficient embryonic cardiac non-myocytes demonstrates dysregulation of Polycomb repressive complex 2 targets, and ATAC-Seq reveals altered chromatin accessibility near many of these same genes. Gene set enrichment analysis of differentially expressed genes in cardiomyocytes isolated from E13.5 hearts of wild-type and mutant mice reveals significant enrichment of MYC targets and, consistent with this finding, we observe increased abundance of MYC in mutant hearts. EBF1-deficient non-myocytes, but not wild-type non-myocytes, are sufficient to induce excessive accumulation of MYC in co-cultured wild-type cardiomyocytes. Finally, we demonstrate that BMP signaling induces Ebf1 expression in embryonic heart cultures and controls a gene program enriched in EBF1 targets. These data reveal a previously unreported non-cell-autonomous pathway controlling cardiac growth and differentiation.

Isolation and characterization of embryonic stem cell-derived cardiac Purkinje cells.

The cardiac Purkinje fiber network is composed of highly specialized cardiomyocytes responsible for the synchronous excitation and contraction of the ventricles. Computational modeling, experimental animal studies, and intracardiac electrical recordings from patients with heritable and acquired forms of heart disease suggest that Purkinje cells (PCs) may also serve as critical triggers of life-threatening arrhythmias. Nonetheless, owing to the difficulty in isolating and studying this rare population of cells, the precise role of PC in arrhythmogenesis and the underlying molecular mechanisms responsible for their proarrhythmic behavior are not fully characterized. Conceptually, a stem cell-based model system might facilitate studies of PC-dependent arrhythmia mechanisms and serve as a platform to test novel therapeutics. Here, we describe the generation of murine embryonic stem cells (ESC) harboring pan-cardiomyocyte and PC-specific reporter genes. We demonstrate that the dual reporter gene strategy may be used to identify and isolate the rare ESC-derived PC (ESC-PC) from a mixed population of cardiogenic cells. ESC-PC display transcriptional signatures and functional properties, including action potentials, intracellular calcium cycling, and chronotropic behavior comparable to endogenous PC. Our results suggest that stem-cell derived PC are a feasible new platform for studies of developmental biology, disease pathogenesis, and screening for novel antiarrhythmic therapies.

Lipomatous Atrial Septal Hypertrophy: A Review of Its Anatomy, Pathophysiology, Multimodality Imaging, and Relevance to Percutaneous Interventions.

Lipomatous atrial septal hypertrophy (LASH) is a histologically benign cardiac lesion characterized by excessive fat deposition in the region of the interatrial septum that spares the fossa ovalis. The etiology of LASH remains unclear, though it may be associated with advanced age and obesity. Because of the sparing of the fossa ovalis, LASH has a pathognomonic dumbbell shape. LASH may be mistaken for various tumors of the interatrial septum. Histologically, LASH is composed of both mature and brown (fetal) adipose tissue, but the role of brown adipose tissue remains unclear. In interventional procedures requiring access to the left atrium, LASH may interfere with transseptal puncture, as traversing the thickened area can reduce the maneuverability of catheters and devices. This may cause the needle to enter the epicardial space, causing dangerous pericardial effusions. LASH was once considered a contraindication to percutaneous device closure of atrial septal defects because of an associated increased risk for incorrect device deployment. However, careful attention to preprocedural imaging and procedural intracardiac echocardiography enable interventional cardiologists to perform procedures in patients with LASH without serious complications. In this review article, the authors describe anatomic and functional aspects of LASH, with emphasis on their roles in percutaneous interventions.

PCP4 regulates Purkinje cell excitability and cardiac rhythmicity.

Cardiac Purkinje cells are important triggers of ventricular arrhythmias associated with heritable and acquired syndromes; however, the mechanisms responsible for this proarrhythmic behavior are incompletely understood. Here, through transcriptional profiling of genetically labeled cardiomyocytes, we identified expression of Purkinje cell protein-4 (Pcp4), a putative regulator of calmodulin and Ca2+/calmodulin-dependent kinase II (CaMKII) signaling, exclusively within the His-Purkinje network. Using Pcp4-null mice and acquired cardiomyopathy models, we determined that reduced expression of PCP4 is associated with CaMKII activation, abnormal electrophysiology, dysregulated intracellular calcium handling, and proarrhythmic behavior in isolated Purkinje cells. Pcp4-null mice also displayed profound autonomic dysregulation and arrhythmic behavior in vivo. Together, these results demonstrate that PCP4 regulates cardiac excitability through both Purkinje cell-autonomous and central mechanisms and identify this modulator of CaMKII signaling as a potential arrhythmia-susceptibility candidate.

The role of multimodality imaging in percutaneous left atrial appendage suture ligation with the LARIAT device.

Atrial fibrillation (AF), the most common cardiac arrhythmia, is a significant cause of embolic stroke. Although systemic anticoagulation is the primary strategy for preventing the thromboembolic complications of AF, anticoagulants carry major bleeding risks, and many patients have contraindications to their use. Because thromboembolism typically arises from a clot in the left atrial appendage (LAA), local therapeutic alternatives to systemic anticoagulation involving surgical or percutaneous exclusion of the LAA have been developed. Surgical exclusion of the LAA is typically performed only as an adjunct to other cardiac surgeries, thus limiting the number of eligible patients. Furthermore, surgical exclusion of the LAA is frequently incomplete, and thromboembolism may still occur. Percutaneous LAA exclusion includes two approaches: transseptal delivery of an occlusion device to the LAA and epicardial suture ligation of the LAA, the LARIAT procedure. In the LARIAT procedure, a pretied snare is placed around the epicardial surface of the LAA orifice via pericardial access. Proper snare placement is achieved with epicardial and endocardial magnet-tipped guidewires. The endocardial wire is advanced transvenously to the LAA apex after transseptal puncture. The epicardial wire, introduced into the pericardial space, achieves end-to-end union with the endocardial wire at the LAA apex. The snare is then placed over the LAA, tightened, and sutured. On the basis of early clinical experience, the LARIAT procedure has a high success rate of LAA exclusion with low risk for complications. The authors describe the indispensable role of real-time transesophageal echocardiography in the guidance of LAA epicardial suture ligation with the LARIAT device.