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.