Project description:Enhancer analysis of rat cardiac myocytes and fibroblasts reveals a collaborative control by transcription factor families [RNA-Seq]
Project description:Enhancer analysis of rat cardiac myocytes and fibroblasts reveals a collaborative control by transcription factor families [ChIP-Seq]
Project description:Scar tissue that forms in the heart after cardiac injury, comprises an abundant number of non-excitable fibroblasts in close proximity to excitable myocytes, that are embedded within the matrix of the scar. Electrical coupling of fibroblasts and myocytes is known to occur and in vitro simulation studies have demonstrated that changes in fibroblast membrane potential can lead to myocyte excitability and susceptibility to arrhythmogenesis. However, the physiologic significance of electrical coupling between myocytes and fibroblasts in scar tissue, in the regulation of cardiac excitability and arrhythmogenesis in vivo is hotly debated and has never been demonstrated. Here, we genetically engineer a mouse that expresses the optogenetic cationic channel ChR2 exclusively in cardiac fibroblasts and not in cardiac myocytes. We subject the animal to cardiac injury and demonstrate that optical stimulation of scar tissue elicits cardiac excitability and induces arrhythmias. Connexin 43 (Cx43) is a gap junctional protein that is the most abundant connexin isoform in the heart and thought to mediate electrical coupling of fibroblasts and myocytes. Using genetic loss of function approaches, we show that Cx43 is not necessary for fibroblast-myocyte electrical coupling in vivo. CRISPR/Cas 9 mediated sequential deletion of the other highly expressed connexins also did not affect electrical coupling of fibroblasts and myocytes. Using computational modeling approaches, we show that gap junctional and non-gap junctional coupling mechanisms synergize in a functionally redundant manner to excite myocytes coupled to fibroblasts. These observations demonstrate that cardiac fibroblasts in scar tissue directly regulate cardiac excitability in vivo and can induce arrhythmogenesis. Our findings throw insight into the importance of electrical coupling of fibroblasts and myocytes in the genesis of scar associated cardiac arrhythmias.
Project description:Growth differentiating factor (GDF)15 is a TGFβ superfamily cytokine and a reported biomarker of heart failure. Myocardial expression of GDF15 is increased in heart failure. Yet, the mechanisms that control synthesis and release of GDF15 as well as the autocrine/paracrine functions of GDF15 on fibroblast function is lacking. Thus, the aim of this study was to investigate signaling pathways and functions of GDF15 in cardiac fibroblasts. Cardiac fibroblasts and cardiac myocytes were isolated from adult C57/BL6 mice, maintained in primary culture and stimulated with recombinant (r)GDF15 or recombinant (r)CCN2. Short-term stimulation (30 minutes) of cardiac fibroblasts demonstrated a GDF15-induced activation of several intracellular signaling pathways including a concentration-dependent increase of phospho-Smad3(Ser423/425) (p<0.05, n=3), phospho-AKT(Ser473) (p<0.05, n=3) and phospho-IκBα(Ser32/36) (p<0.05, n=3) levels. However, rGDF15 did not phosphorylate Smad3(Ser423/425) or IκBα(Ser32/36) in cardiac myocyte. Cardiac fibroblasts exposed to rGDF15 for 48 hours displayed differentiation towards myofibroblasts reflected by increased levels of the differentiation marker α-smooth muscle actin (SMA) similar to cardiac fibroblasts stimulated with TGFβ. The effect of GDF15 on α-SMA was dose-dependent ranging from 500 nM - 20 nM rGDF15 (p<0.05, n=3). Differentiation towards a myofibroblast phenotype in the presence of GDF15 was also supported by higher matrix metalloproteinase (MMP) enzyme activity in the cell culture medium (6±1 fold increase, n=3, p<0.05) and increased expression and release of MMP-3, 9 and 13. Immunoreactive GDF15 was predominantly found in cardiac myocytes. Recombinant CCN2 substantially induced GDF15 expression in cardiac myocytes, but not in cardiac fibroblasts. In conclusion, our data demonstrate that GDF15 is a paracrine factor in myocardial tissue and specifically regulated by CCN2 in cardiac myocytes. GDF15 has similar effects as TGFβ on fibroblasts by activation of intracellular signaling pathways and differentiation to a myofibroblasts phenotype.