CARDIAC FIBROBLAST HETEROGENEITY AND PLASTICITY

Globally, mortality and morbidity are increasing due to an increase in the number of patients with cardiovascular disease. Interstitial fibrosis plays an important role in this, which has a negative impact on the systolic and diastolic function of the heart. The current prescriptions for the treatment of chronic heart failure promote the quality of life of the patient but are not aimed at halting the further development of fibrosis or at the worsening of the heart function to the final stage of heart failure. The aim of this doctoral thesis is to study the pathophysiological role of cardiac fibroblasts during matrix and heart muscle cell remodeling. In the first study, we demonstrated that there is a wide range of fibroblast phenotypes that exhibit a high degree of plasticity. These phenotypes mainly include fibroblasts, proliferating myofibroblasts and non-proliferating myofibroblasts. This plasticity in phenotypes is mainly determined by the presence of mechanical stress and the growth factor TGF-β1 that work together as a synergistic entity (Chapter 3). This study shows that proliferating myofibroblasts have the capacity to return to the unchanged 'healthy' fibroblast phenotype while the non-proliferating myofibroblasts lack this property. The reversibility of the proliferating myofibroblast phenotype would therefore also be an excellent candidate for future therapy aimed at interstitial fibrosis. In the second study (Chapter 4) we established the presence of a heterogeneous population of fibroblast phenotypes in a large animal model with ischemic cardiomyopathy during the initial phase of matrix remodeling. After the occurrence of a myocardial infarction, fibroblast differentiation occurs in the infarct zone but also in the regions next to and far away from the infarct. The fibroblasts of these 3 regions can be distinguished from each other at transcriptome level on the basis of their differential gene expression pattern. Differentiation of fibroblasts is promoted by the presence of hemodynamic load and TGF-β1. Haemodynamic load had only increased in the region in addition to the infarct while TGF-β1 was prominent in all three regions. Interstitial fibrosis and cross-linking of collagen, however, had not only increased in the infarct region but also in the region in addition to the infarction. This observation confirms the heterogeneous role of fibroblasts after the onset of an infarction. The identification of molecular signals that distinguish between myofibroblasts of the scar tissue and those of the interstitium may lead to the development of a new line of drugs that induce the reversibility of interstitial myofibroblasts without affecting them in the scar tissue. In the third study (Chapter 5) we show that preferably proliferating and non-proliferating myofibroblasts enter into a functional link with adult heart muscle cells from the pig in a 2-dimensional co-culture environment. Each fibroblast phenotype causes cell death of cardiac muscle cells in both a 2-D culture environment and in a paracrine co-culture. The electrophysiological profile of the heart muscle cell was unchanged in the paracrine co-culture. However, myofibroblasts that were functionally linked to cardiomyocytes were able to alter the RMP and ADP90 of the cardiac muscle cell. In the latest study (Chapter 6) we studied the fibroblast populations that are present during the end-stage of heart failure in humans and more specifically in patients with ischemic and dilated cardiomyopathy. Non-suitable donor hearts for transplantation have been used to isolate normal unchanged fibroblasts as a control in the various experiments. The fibroblasts isolated from samples from heart failure patients, in contrast to those from healthy patients, have lost their proliferation capacity in cell culture. We have also demonstrated in cell culture that a subpopulation of myofibroblasts has the capacity to return to a less differentiated phenotype by inhibition of the TGF-β1 signaling pathway. The dedifferentiated myofibroblasts from heart failure patients showed a decrease in expression of pro-fibrotic genes and behaved functionally similarly as healthy normal fibroblasts. We can conclude that there is a high variation in fibroblast phenotypes during myocardial tissue remodeling and that these are functionally different from each other. We then identified the phenotypes that have the potential for reversibility in fibroblast differentiation. These findings can lead to an alternative strategy in anti-fibrotic therapy.status: publishe