and from several drug and device companies active in atrial fibrillation. PK has previously received honoraria from several such companies. PK and LF are listed as inventors on 2 patents held by the University of Birmingham ("Atrial fibrillation therapy," WO 2015140571; "Markers for atrial fibrillation," WO 2016012783).

BackgroundImproved understanding of the interconnectedness of structural remodeling processes in atrial fibrillation (AF) in patients could identify targets for future therapies.MethodsWe present transcriptome sequencing of atrial tissues of patients without AF, with paroxysmal AF, and persistent AF (total n = 64). RNA expression levels were validated in the same and an independent cohort with qPCR. Biological processes were assessed with histological and immunohistochemical analyses.ResultsIn AF patients, epicardial cell gene expression decreased, contrasting with an upregulation of epithelial‐to‐mesenchymal transition (EMT) and mesenchymal cell gene expression. Immunohistochemistry demonstrated thickening of the epicardium and an increased proportion of (myo)fibroblast‐like cells in the myocardium, supporting enhanced EMT in AF. We furthermore report an upregulation of endothelial cell proliferation, angiogenesis, and endothelial signaling. EMT and endothelial cell proliferation concurred with increased interstitial (myo)fibroblast‐like cells and extracellular matrix gene expression including enhanced tenascin‐C, thrombospondins, biglycan, and versican. Morphological analyses discovered increased and redistributed glycosaminoglycans and collagens in the atria of AF patients. Signaling pathways, including cell–matrix interactions, PI3K‐AKT, and Notch signaling that could regulate mesenchymal cell activation, were upregulated.ConclusionOur results suggest that EMT and endothelial cell proliferation work in concert and characterize the (myo)fibroblast recruitment and ECM remodeling of AF. These processes could guide future research toward the discovery of targets for AF therapy.

The visualization method addresses some of the main drawbacks of the original methodology and might bring the potential for its clinical application closer.