Heterogeneity in Pkp2-Containing Junctions in the Adult Epicardium

The epicardium, a thin layer of mesothelial cells that encapsulate the heart, is a major source of cardiac fibroblasts during development as a result of epithelial to mesenchymal transition (EMT). EMT requires the loss of cell-to-cell adhesion and conditions and/or diseases that destabilize cellular junctions. Arrhythmogenic Right Ventricular Cardiomyopathy (ARVC) is an inherited disorder associated with mutations in desmosomal proteins and characterized by myocardial accumulation of fibrous or fibrofatty tissue. It has been hypothesized that expression of mutant desmosomal proteins in epicardial cells might contribute to the pathogenesis of ARVC. However, this has not been tested nor has regional variation in epicardial mechanical junctions been examined. We used ultrastructural analysis and immunohistochemistry to examine the expression of desmosomal proteins in mouse epicardium. The number of structures containing Plakophilin 2 (Pkp2), a desmosomal protein that is frequently mutated in ARVC, was quantified in whole mount preparations of epicardium and normalized to the number of epicardial cell nuclei. Ultrastructural analysis revealed mechanical junctions of indeterminate morphology were in both atrial and ventricular epicardium, but were more abundant in atrial epicardium. In frozen sections, Pkp2 was expressed in the epicardium and there was more Pkp2 immunoreactivity in atrial epicardium. In whole mount preparations, both the number and density of Pkp2 containing epicardial structures were greater in the atria than the ventricles. This suggests that the epicardium exhibits heterogeneity in the abundance of Pkp2-containing mechanical junctions. Future work will determine if these junctions are bona fide desmosomes or a different type of Pkp2-containing junction. By understanding which desmosomal proteins are expressed in the adult epicardium, we also hope to gain insight into the potential role of the epicardium in the development of fibrosis in ARVC patients with mutations in other desmosomal genes.