Single-Cell RNA Sequencing Uncovers Paracrine Functions of the Epicardial-Derived Cells in Arrhythmogenic Cardiomyopathy
Authors: Yuan Ping , Cheedipudi Sirisha M. , Rouhi Leila , Fan Siyang , Simon Lukas , Zhao Zhongming , Hong Kui , Gurha Priyatansh , Marian Ali J. 
Published: March 2021

Background:
Arrhythmogenic cardiomyopathy (ACM) manifests with sudden death, arrhythmias, heart failure, apoptosis, and myocardial fibro-adipogenesis. The phenotype typically starts at the epicardium and advances transmurally. Mutations in genes encoding desmosome proteins, including DSP (desmoplakin), are major causes of ACM.

Methods:

To delineate contributions of the epicardium to the pathogenesis of ACM, the
Dsp
allele was conditionally deleted in the epicardial cells in mice upon expression of tamoxifen-inducible Cre from the
Wt1
locus. Wild type (WT) and
Wt1-Cre

ERT2

:Dsp
W/F
were crossed to Rosa26
mT/mG
(R26
mT/mG
) dual reporter mice to tag the epicardial-derived cells with the EGFP (enhanced green fluorescent protein) reporter protein. Tagged epicardial-derived cells from adult
Wt1-Cre

ERT2

:R26
mT/mG
and
Wt1-Cre

ERT2

: R26
mT/mG
:Dsp
W/F
mouse hearts were isolated by fluorescence-activated cell staining and sequenced by single-cell RNA sequencing.

Results:

WT1 (Wilms tumor 1) expression was progressively restricted postnatally and was exclusive to the epicardium by postnatal day 21. Expression of
Dsp
was reduced in the epicardial cells but not in cardiac myocytes in the
Wt1-Cre

ERT2

:Dsp
W/F
mice. The
Wt1-Cre

ERT2

:Dsp
W/F
mice exhibited premature death, cardiac dysfunction, arrhythmias, myocardial fibro-adipogenesis, and apoptosis. Single-cell RNA sequencing of ≈18 000 EGFP-tagged epicardial-derived cells identified genotype-independent clusters of endothelial cells, fibroblasts, epithelial cells, and a very small cluster of cardiac myocytes, which were confirmed on coimmunofluorescence staining of the myocardial sections. Differentially expressed genes between the paired clusters in the 2 genotypes predicted activation of the inflammatory and mitotic pathways—including the TGFβ1 (transforming growth factor β1) and fibroblast growth factors—in the epicardial-derived fibroblast and epithelial clusters, but predicted their suppression in the endothelial cell cluster. The findings were corroborated by analysis of gene expression in the pooled RNA-sequencing data, which identified predominant dysregulation of genes involved in epithelial–mesenchymal transition, and dysregulation of 146 genes encoding the secreted proteins (secretome), including genes in the TGFβ1 pathway. Activation of the TGFβ1 and its colocalization with fibrosis in the
Wt1-Cre

ERT2

:R26
mT/mG
:Dsp
W/F
mouse heart was validated by complementary methods.

Conclusions:
Epicardial-derived cardiac fibroblasts and epithelial cells express paracrine factors, including TGFβ1 and fibroblast growth factors, which mediate epithelial–mesenchymal transition, and contribute to the pathogenesis of myocardial fibrosis, apoptosis, arrhythmias, and cardiac dysfunction in a mouse model of ACM. The findings uncover contributions of the epicardial-derived cells to the pathogenesis of ACM.