Pro-inflammatory signaling originates from epicardium-derived fibroblasts in a PKP2 deficient model of arrhythmogenic cardiomyopathy

Introduction: Arrhythmogenic Cardiomyopathy (AC) is an inherited disease that is caused by desmosome protein mutation with plakophilin 2 (PKP2) mutation being most common. AC is known to cause sporadic ventricular arrhythmias, myocyte damage, and subepicardial fibrosis. Given the origin of fibrosis from epicardium, we wanted to understand how epicardium-derived cells (EDCs) contribute to the AC pathogenesis. Hypothesis: EDCs propagate the pro-inflammatory signaling that contributes to AC pathogenesis. Methods: We developed transgenic mice that lack PKP2 in cardiomyocytes (PKP2-cKO) or in both cardiomyocyte and epicardial cells (PKP2-ceKO) via the tissue-specific expression of tamoxifen-inducible Cre recombinase. Tamoxifen injected mice expressing Cre in cardiomyocytes, but with no floxed PKP2 gene, was used as a control. EDCs were traced with Cre-dependent Green Fluorescence Protein (GFP). Non-myocyte populations were isolated 21 days post-tamoxifen injection for single cell RNA-sequencing (scRNA-seq) using 10X Genomics platform. Perfused hearts were separated into left and right ventricle for qRT-PCR. Immunohistochemistry was used for cardiac structure and cellular composition. Echocardiography was performed to measure cardiac physiology. Results: The scRNA-seq analysis identified an epicardium-derived fibroblast population that secretes pro-inflammatory cytokines, including CCL2, CCL7, Thbs1, and PTX3. While pro-inflammatory EDCs are found in both PKP2-cKO and PKP2-ceKO mice, they were most numerous in PKP2-ceKO mice. Importantly, macrophages and B cells accumulated in both PKP2-cKO and PKP2-ceKO mice compared to controls. qRT-PCR confirmed that the expression of genes encoding fibrosis markers (POSTN, Col1A1, Col1A2, and Col3A1), pro-inflammatory fibroblast markers (CCL2, CCL7, Thbs1, PTX3), macrophage markers (F4/80, Cx3cr1, Spp1, Trem2, and CCR2), and B cell markers (CD19 and CD79b) were significantly enriched in both PKP2-cKO and PKP2-ceKO mice compared to control. Our expression and histologic data also revealed an exaggerated pro-inflammatory response in PKP2-ceKO mice, that progresses from the right to bi-ventricular predominance. However, echocardiography showed no significant difference in cardiac function between PKP2-cKO and PKP2-ceKO, and B cell depletion did not alter disease progression. Conclusion: A subset of epicardium-derived fibroblasts in PKP2 KO mice secretes pro-inflammatory cytokines that contributes to cardiomyocyte damage and cardiac fibrosis in AC.