Background: Cardiac fibrosis drives heart failure progression and is an independent predictor of death. In this study, we analyzed patients’ cardiac fibroblasts from different entities of longstanding heart disease at single-cell resolution to uncover putative therapeutic and diagnostic targets.

Methods: Single-nuclear RNA sequencing data of patients’ cardiac fibroblasts were utilized including healthy (2 samples), hypertrophic (5 samples), and failing (2 samples) human heart tissue. Sub-clustering of 7,110 nuclei defined 7 distinct fibroblast clusters and molecular signatures of healthy and diseased fibroblasts were obtained by GO term analysis of differentially expressed genes. 

Results: Surprisingly, hypertrophic as well as failing human heart tissue revealed common features of cardiac fibroblast plasticity and cluster composition with a transcriptional shift towards profibrotic extracellular matrix composition independent of   the   underlying   disease   entity.   Strikingly,   we   identified   a   distinct   fibroblast population of activated fibroblasts that was characterized by high expression of POSTN, COL1A1 and AEPB1. Adipocyte enhancer-binding protein 1 (AEBP1) has previously been reported to be involved in adipocyte proliferation and collagen fibrillogenesis. The induction of AEBP1 in activated fibroblasts was confirmed in vitro upon TGFbeta stimulation (1.6 ± 0.2-fold induction, p < 0.05). To investigate whether our findings might also be relevant on protein level in patients with longstanding heart disease, we sought to determine the role of AEBP1 as a putative circulating biomarker for cardiac fibrosis in an independent validation cohort. Strikingly, we observed significantly elevated circulating plasma protein levels of AEBP1 in patients with documented cardiac fibrosis based on T1 mapping in cardiac magnetic resonance imaging compared to controls (p < 0.05). To examine whether elevated AEBP1 levels directly contribute to a profibrotic response, humanized cardiac tissue organoids  or cardiac  fibroblasts  were   exposed   to  recombinant  AEBP1  protein (rhAEBP1), which led to a profibrotic phenotype with enhanced contractile (1.3 ± 0.06-fold induction, p < 0.05) and migratory capacity in vitro (1.2  ±  0.03-fold induction, p < 0.05). Since   AEBP1   expression   increased   in   activated   fibroblasts   of   patients   with hypertrophic or failing hearts, we tested the functional consequences of siRNA-mediated   AEBP1   silencing   as   a   putative   therapeutic   approach   to   counteract fibroblast activation. Indeed, AEBP1 silencing resulted in a significant reduction of fibroblast proliferation, migration, and contractile capacity (all p < 0.05) in human cardiac fibroblasts, which was accompanied by ameliorated stress marker expression (such as ACTA2 and COL3A1) and alphaSMA-formation. Mechanistically, the antifibrotic effects of AEBP1 silencing were linked to TGFbeta pathway modulation with reduced TGFbeta receptor (TGFBR1) abundance as well as phosphorylation of the downstream receptor activated SMAD proteins. 

Conclusion:  Taken together, single-cell profiling of patients’ cardiac fibroblasts uncovered a common feature of pathological fibroblast activation in patients with hypertrophic and failing hearts, which might be used as a diagnostic biomarker platform and modulated as a putative antifibrotic treatment.