Molecular determinants of cardiac fibroblast electrical function and therapeutic implications for atrial fibrillation

Abstract

Cardiac fibroblasts account for about 75% of all cardiac cells, but because of their small size contribute only ∼10-15% of total cardiac cell volume. They play a crucial role in cardiac pathophysiology. For a long time, it has been recognized that fibroblasts and related cell types are the principal sources of extracellular matrix (ECM) proteins, which organize cardiac cellular architecture. In disease states, fibroblast production of increased quantities of ECM proteins leads to tissue fibrosis, which can impair both mechanical and electrical function of the heart, contributing to heart failure and arrhythmogenesis. Atrial fibrosis is known to play a particularly important role in atrial fibrillation (AF). This review article focuses on recent advances in understanding the molecular electrophysiology of cardiac fibroblasts. Cardiac fibroblasts express a variety of ion channels, in particular voltage-gated K(+) channels and non-selective cation channels of the transient receptor potential (TRP) family. Both K(+) and TRP channels are important determinants of fibroblast function, with TRP channels acting as Ca(2+)-entry pathways that stimulate fibroblast differentiation into secretory myofibroblast phenotypes producing ECM proteins. Fibroblasts can couple to cardiomyocytes and substantially affect their cellular electrical properties, including conduction, resting potential, repolarization, and excitability. Co-cultured preparations of cardiomyocytes and fibroblasts generate arrhythmias by a variety of mechanisms, including spontaneous impulse formation and rotor-driven reentry. In addition, the excess ECM proteins produced by fibroblasts can interrupt cardiomyocyte-bundle continuity, leading to local conduction disturbances and reentrant arrhythmias. A better understanding of the electrical properties of fibroblasts should lead to an improved comprehension of AF pathophysiology and a variety of novel targets for antiarrhythmic intervention.

Figure 1

Schematic diagram of cardiac fibrogenesis cascade and AF. In response to a variety of stimuli, cardiac fibroblasts proliferate, differentiate, synthesize extracellular matrix (ECM) proteins, and produce cytokines that in turn stimulate fibroblasts, thereby providing positive feedback and perpetuating the fibrogenesis cascade. ECM accumulation causes fibrosis that favours the occurrence and maintenance of AF. There are two types of fibrosis, reactive and reparative, which can be caused by many common stimuli, with cardiomyocyte death required for reparative fibrosis. TRPM7-mediated Ca²⁺ signals are critical for fibroblast proliferation, differentiation, and ECM production in fibroblasts from AF patients. Fibroblast Ca²⁺ signalling may be an effective target for the prevention of fibrogenesis.

Figure 2

A schematic illustration of the effects of fibroblast coupling to cardiomyocytes on cardiomyocyte action potentials (APs). (A) Intracellular electrical recordings from uncoupled fibroblasts (blue) and cardiomyocytes (red). (B) Changes resulting from coupling between fibroblasts and cardiomyocytes. Transmembrane recordings from uncoupled cells are reproduced in dotted lines; solid lines show recordings resulting from the cell-to-cell current flow that occurs with cell coupling. The dashed horizontal black line in the lower panel indicates the normal fibroblast resting potential (−30 mV). Directions of (positive) current flow between cardiomyocytes and fibroblasts through gap junctions are shown with red arrows, at cardiomyocyte maximum diastolic potential (a) and peak overshoot (b).

Figure 3

A schematic illustration of the expected effects of tissue fibrosis on conduction in bundles of cardiomyocyte (red cylinders). (A) Normal tissue. (B) Fibrosis occurring parallel to cardiomyocyte-bundle sheaths (as would occur in reactive fibrosis). Longitudinal cell-to-cell connections are intact and longitudinal conduction is unaffected. (C) Fibrosis parallel to and across cardiomyocyte bundles, as would occur in reparative fibrosis. End-to-end connections are prevented by the physical separation resulting from replacement of dead cardiomyocytes by fibroblasts and fibrotic extracellular matrix proteins, producing local impairments in longitudinal conduction.

Figure 4

(A) Longitudinal sections of Masson trichrome-stained atrial tissue from a control dog (top), a dog in which 2-week tachypacing at 240b.p.m. led to CHF (middle) and a dog tachypaced into CHF and then allowed to recover for 4 weeks without tachypacing (REC), which permitted full haemodynamic recovery (bottom). Collagen is stained blue, cardiomyocytes are pink. Arrows point to regions in which transverse fibrosis interrupts cardiomyocytes in their longitudinal direction. (B) Optical maps of electrical conduction in right atrial tissue preparations from a control (left) and a REC dog (right). Tissues were paced at 2 Hz at the upper left corner. The activation colour scale is in ms. (C) Results of mathematical modelling of conduction in simulated tissues with the same fibrosis distribution as the preparations shown immediately above, assuming that fibrosis acts as a non-conductive barrier. The mathematical model accounted well for the electrical propagation changes observed experimentally. Reproduced from the reference with permission of the American Heart Association.