Structural and Gap Junction Remodeling

Significant morphological alterations in the architecture of the heart have been observed in the context of AF, affecting the cardiomyocytes themselves and myocardial interstitium. Frustaci et al. collected biopsy samples from the atrial septum in patients with PAF and demonstrated hypertrophy, vacuolar degeneration, necrosis of myocytes and patchy fibrosis (16). Hatem and colleagues also reported evidence of apoptotic myocyte death in the right atrial appendage associated with AF (33). In both studies, these findings were absent in biopsies from control samples. Histological analyses of LA samples have similarly demonstrated significantly increased collagen deposition surrounding muscle bundles and between individual cardiomyocytes (34). Furthermore, the degree of extracellular matrix remodeling correlates with the duration of AF persistence (figure 1a-c) (35) and is a risk factor for post-operative AF in patients undergoing coronary artery bypass surgery (36,37).
These studies provide compelling evidence of increased fibrotic atrial remodeling associated with AF. However, such histological studies to date have been correlative and do not establish a causal relationship between structural remodeling and AF persistence, nor do they serve to explain how these changes provide a substrate for arrhythmia maintenance. The loss of myocytes, through either apoptosis or necrosis, may reflect a lack of reversibility in the remodeling process, potentially contributing to the progressive and increasingly intractable nature of AF. Disruption in gap junction organization and activity is also likely to be contributory. Expression levels of connexins appear to be reduced in AF and their location being less limited to the intercalated discs (38,39). Such changes are likely to impact conduction properties, favoring re-entry (40).
Beyond these proposed effects on the coupling between cardiomyocytes, fibrotic remodeling also appears to have the potential for more direct modulation of the electrical properties of the cardiac cells. Though non-excitable, fibroblasts appear to express gap junction proteins and make heterocellular contact with cardiomyocytes in vitro (41,42). The resting membrane potential of fibroblasts is less negative than that of atrial cardiomyocytes (43), thus when electrically coupled with myocytes they may act as a current source during electrical diastole and current sink during myocyte depolarization (44). Accordingly, co-culturing of fibroblasts with cardiomyocytes in vitro results in a density-dependent depolarization of the cardiomyocyte resting membrane potential (45), thereby inactivating voltage-gated sodium channels and impeding conduction (46). In keeping with this, under experimental conditions, myofibroblast interaction with cardiomyocytes is associated with reduced conduction velocity across the tissue (47). Interestingly, the experiments also suggested passive transmission of an impulse across an area of fibrosis with significant conduction delay and block. Notably, similar passive electrotonic activity in scar zone myofibroblasts has also been reported in an ex vivo whole heart model (48,49). These results highlight the potential for conduction slowing, anisotropy and block secondary to fibrotic remodeling, all of which support re-entry.
Changes in atrial refractory properties have long been considered as key aspects of arrhythmogenesis in AF and have generally been attributed to altered ion channel activity (50). Myofibroblast-cardiomyocyte coupling may act as a current sink during the myocyte action potential peak and plateau phases, thereby abbreviating the action potential duration and contributing to the dispersion of atrial refractoriness. In laboratory preparations, myofibroblast-cardiomyocyte coupling increases the propensity for ectopic activity in a dose-dependent fashion (51). Moreover, in-silico modeling studies suggest fibrosis-induced disruption of myocyte coupling promotes automaticity and atrial ectopic activity (52). Thus, such fibrotic remodeling potentially generates non-pulmonary vein triggers for AF, in addition to the described effects of conduction and refractoriness.
Such experimental reports underline the arrhythmogenic potential of adverse structural remodeling, however there appears to be significant variability in the nature of fibrosis in AF and the precise role of these changes in its pathogenesis remains debated. For example, in a canine heart failure model, AF induced through right ventricular tachy-pacing is associated with marked atrial interstitial fibrosis and conduction heterogeneities, but no alterations in atrial refractory properties (53). In contrast, in rapid atrial pacing models, AF maintenance is primarily mediated through electrical remodeling with few structural abnormalities (54). However, structural remodeling is more evident when rapid atrial pacing is combined with mitral regurgitation. The combination confers greater vulnerability to AF (55), together suggesting that structural remodeling can contribute to AF maintenance, but that AF may also persist in its absence.
Significant variability has also been reported in the composition of fibrotic remodeling in AF. Studies evaluating the nature of gap junction remodeling have reported markedly discrepant results with increased, decreased, and unaltered expression of atrial connexin isoforms (38,56,57). Inconsistencies are also apparent in the nature of collagen deposition, further emphasizing the complex nature of fibrotic remodeling seen in AF. A two-fold increase in left atrial collagen I deposition was seen in patients with AF compared to those with sinus rhythm (34). However, patients with significant mitral valve disease also display a significant increase in collagen III deposition, which was not observed in those with ‘lone’ AF.
Conflicting reports from human studies further highlight the complexities of structural remodeling. Ho and colleagues performed morphometric analysis of post-mortem tissue samples, and described significantly increased fibrosis associated with AF (58). The extent of remodeling was more pronounced in those with a history of non-paroxysmal AF. Extracellular matrix remodeling also correlated with AF duration in patients with a background of dilated cardiomyopathy (35). However, in the study by Frustaci et al., fibrotic remodeling was evident in patients with PAF (16). Such early-onset of fibrosis was also demonstrated in patients with AF undergoing cardiac surgery, with no appreciable increase in fibrosis seen in patient with long-standing persistent AF compared to those with AF of more recent onset (34,59). Progressive fibro-fatty deposition has also been purported to underlie the increasing propensity to AF with ageing as well as a number of chronic conditions such as hypertension and diabetes. However, in histological analyses, the degree of fibrotic change has similarly failed to mirror the burden of comorbidities (58).
Fibrotic remodeling is often considered a convergent pathological end point of a multitude of conditions associated with a propensity to AF; the pattern of fibrosis is in itself not uniform. Indeed fibrosis is broadly categorized as either reparative or reactive, each with a differing composition of extracellular matrix constituents and deposition patterns (figure 1d). The former describes fibrotic replacement in zones of degenerating myocardial parenchyma, producing discontinuities in the cardiomyocyte network and potentially forming barriers to conduction. Reactive fibrosis is thought to be driven by cardiac inflammation, occurring within the interstitium remote from areas of focal injury. For example, substantial atrial myocyte death is observed in experimentally induced heart failure favoring reparative fibrosis (60).
Reactive fibrosis is associated with expansion of the interstitial space, forming thicker sheaths of fibrous tissue around muscle bundles, but significantly not disrupting the muscle bundle itself. Longitudinal conduction through the muscle remains intact, and may even be enhanced through insulation of individual muscle bundles. Chronic pressure overload is associated with progressive interstitial fibrosis, initially in the perivascular space and later becoming more diffuse (61). Histological analysis of left atrial appendage tissue from individuals undergoing surgical AF ablation revealed no difference in fibrotic burden between paroxysmal and persistent AF (62). Interestingly longitudinal conduction velocity was higher in samples with greater interstitial collagen content, although rate-dependent conduction slowing and zig-zag conduction were observed. It remains unclear which form predominates in AF, or it varies according to the underlying etiology. The fibrosis patterns need not be mutually exclusive and may co-exist within a single atrium.
Deposition of fibrotic tissue thus forms an integral component of atrial structural remodeling in AF. However, while fibrosis is commonly considered a stereotyped process with predictable effects on the electrical properties of the atria, in reality the processes involved are not quite so uniform. The relationship between AF duration and fibrosis is non-linear, and it is clear that such fibrotic remodeling is not a pre-requisite for AF to persist. Importantly, given the variability in the pattern of structural remodeling, and its effects on the electrical properties of the atria, the optimal strategy for delineating arrhythmogenic tissue through electro-anatomical mapping remains debated.