A Multiscale Investigation Of Gap Junctions In Myocyte-Fibroblast Interactions

The adult heart is composed of a dense network of cardiomyocytes surrounded by non-myocyte cells, the most abundant of which are cardiac fibroblasts. Several cardiac diseases, such as myocardial infarction or pressure overload, are associated with an increased density of fibroblasts, i.e., cardiac fibrosis. Fibroblasts are known to play a significant role in the development of electric and mechanical dysfunction of the heart, however the exact mechanisms are only partially understood. One widely studied hypothesis suggests that fibroblasts produce excess extracellular matrix, resulting in collagenous septa. These collagenous septa slow propagation, cause zig-zag conduction paths, and decouple cardiomyocytes resulting in a substrate for cardiac arrhythmia. Another emerging hypothesis suggests that fibroblasts promote arrhythmogenesis through direct electrical interactions with cardiomyocytes via gap junctional channels. In the heart, three major connexin (Cx) isoforms, Cx40, Cx43 and Cx45 form gap junction channels in cell-type-specific combinations. Because each Cx is characterized by unique gating properties (i.e., time- and voltage-dependent gating), the hypothesis that the electrophysiological contributions of fibroblasts may vary with the specific composition of the myocyte-fibroblast gap junction channel was explored in this study. First, the role of gap junction channel gating in myocyte-fibroblast interactions was investigated by using a unique strategy of coupling fibroblasts electrophysiology models with in vitro single cell ventricular cardiomyocytes via mathematical models of gap junction channel gating using the dynamic-clamp technique. These investigations revealed that gap junction gating reduces the peak of the junctional current compared to a constant value conductance representation of gap junctions. Second, the gap junction models were incorporated into a detailed two-dimensional computational model of cardiac fibrosis. These simulations revealed that gap junction channel gating did not significantly alter impulse propagation and conduction velocity during cardiac fibrosis. Finally, a novel experimental approach using dye transfer techniques in cardiac tissue slices was developed and used to investigate myocyte-fibroblast interactions in a multicellular environment. These studies presents a framework for a multiscale approach to investigate complex myocyte-fibroblast interactions that have the potential to lead to a clearer understanding of the emerging mechanism by which cardiac fibroblasts promote cardiac arrhythmogenesis